and Bryan Murray, and Professor John Orton and. Professor Brian Tuck in the Electrical and Electronic. Engineering Department, colleagues in the depart-.
J O U R N A L ,OF M A T E R I A L S C I E N C E : M A T E R I A L S I N E L E C T R O N I C S
5 (1994) 1 2 9 - 1 4 6
Review Radio frequency sputtering and the deposition of high-temperature superconductors M. S. RAVEN Department of Electrical and Electronic Engineering, University of Nottingham, Nottingham NG7 2RD, UK This paper reviews the background to glow-discharge sputter deposition of thin films and the deposition of YBCO superconducting thin films in particular. The background to sputtering is briefly reviewed with reference to the recent literature on analytical and numerical techniques for investigating radiofrequency (r.f.) plasmas, magnetron Sputtering and hysteretic behaviour i n reactive sputtering. Low-energy, ion-assisted deposition techniques are briefly reviewed, and the effect of ion-beam interactions on film nucleation and growth is also discussed. The background to the sputter deposition of high-temperature superconductors (HTS) is given along with the choice of sputtering system for HTS deposition. Resputtering effects, off-axis and in-situ/ex-situ processing are also discussed. The sputter deposition of YBa2Cu30x is considered in detail along with the P-T-x oxidation conditions and the tetragonal/orthorhombic line. Typical experimental arrangements and results for YBCO sputtered onto SrTi03 and MgO are given. The problem of producing high-critical-currentdensity polycrystalline films by sputtering is also discussed.
1. IntrOduction. During the past few years there has been a sharp increase in the use of ionized plasmas and ion beams for the processing and fabrication of new materials. ApplicatiOns are extremely diverse, ranging from the deposition of high-temperature superconductors [1]; deposition of new alloys, superlattices and metastable structures [2]; deposition and etching of new materials such as diamond-like carbon [3]; modification and improvement of engineering surfaces [4, 5]; corrosion resistant, wear resistant and decorative coatings; architectural glass; diffusion barriers in integrated circuits and selectively transparent coatings [6, 7]; high reflectivity ( > 99%) dielectric mirrors for verticalcavity, surface-emitting lasers [8]; and novel applications such as high-resolution radiotracer depth profiling of diffusion samples [9]. Sputtering is essentially a high-energy vaporization technique in which the impinging atoms are accelerated to energies varying from a few eV to several keV. Using this technique, virtually any material can he vaporized. Depending on the growth thermodynamics, the condensation of this highly energetic vapour produces various growth features ranging from amorphous glassy metallic layers, polycrystalline layers with nanometre grain size, new crystalline phases, preferred orientations and epitaxy. One of the features Of the sputtering technique is the ease with which i0n-beam processing can be incorporated. Using unbalanced magnetron sputtering, highly dense alloys ha~,e been deposited yielding bulk layers several 0957-4522 9 1994 Chapman & Hall
millimetres thick; the process thus goes beyond the confines o f a film-deposition technique [10]. The layers produced are usually superior in quality to thermally evaporated films with higher density and higher adhesion strength. This leads to materials with a diverse range of applications, including, hard coatings for machine tools, high-bit-density computerdisc memories and superconducting thin films. In techniques such as radio frequency (r.f.) sputtering both the target and the substrate may be sputtered simultaneously. By adjusting the substrate sputtering rate to be less than the target sputtering rate, a film of the target material condenses on the substrate. The condensation of the target material on the substrate is therefore a dynamic process; a film condenses only if the substrate sputtering rate is less than the target sputtering rate. Because the substrate can be sputter etched during film condensation, a clean atomic surface is continuously presented to the impinging vapour. Usually, the generation of an atomically clean surface is only achieved in an ultra-high vacuum (UHV). By using sputtering techniques, it is possible to produce a dynamically clean surface in an inert-gas atmosphere where the base pressures are typically 10-5 Pa, and the technology is simpler and less expensive than UHV technology. The overall sputter-deposition process may be divided into three subprocesses as shown in Fig. 1. This comprises: I, decomposition of a target material by sputtering; II, transmission of the vapour through the plasma and system gas to the substrate; and III, 1 29
Target
Plasma
Film/Substrete
to make wider practical use of these materials, polycrystalline films are required. However, the critical currents in polycrystalline films are too low at present, mainly due to high-angle grain boundaries and associated defects. The detection and control of these defects represents a great challenge to materials science.
2. Sputtering background
I
II
III
Figure 1 The complete sputter-deposition process divided into three subprocesses: (I) sputtering of the target, (II) particle transport, (III) film deposition. Each subprocess is modelled using various schemes, including Monte-Carlo methods, in an attempt to produce an overall system-process model.
condensation, nucleation and growth of the vapour on a substrate. The overall process involves a solid-vapour-solid reaction: ( A B C D . . . ~ A* + B* +C*+D* +...~(ABCD..)withafreeenergy of compound formation AGf(Ts) where the asterisks represent the complex sputtered vapour species in their excited states. In this paper the background and techniques of sputtering (subprocesses I and II) are reviewed. Because of limited space, it has been necessary to be selective on topics so that only those techniques which are the most widely used, such as glow-discharge sputtering sources and magnetron sputtering, are discussed in detail. Ion-assisted deposition is considered since it is a rapidly developing technology which is being applied to a wide range of deposition processes, including molecular-beam epitaxy (MBE). It is simply achieved in sputtering systems by applying a negative substrate bias or by unbalancing the magnetron target field, and novel materials are being developed using this technique. The primary objective of the thin-film deposition techniques is, of course, to produce high-quality materials. Although the development of plasma processing technology has been rapid and diverse, there has been little development in the theory of heterogeneous nucleation and growth of energetic atomic species within an ionized plasma (subprocess III). The early growth processes determine the film-grain orientation and structure. The film-grain structure, grain-boundary structure and composition are critical in most applications, particularly where high current densities are required. The fundamental growth mechanisms determine the final product and these are discussed in relation to the film-growth kinetics and thermodynamics. The overall sputtering process is complex, and it involves a large number of variables. However, attempts are being made to model each of the subprocesses using analytical models and Monte-Carlo techniques, and these are briefly reviewed. The deposition of high-temperature superconductors has been one of the great successes of the sputtering technique, and this is discussed with particular reference to the sputter deposition of YBa2Cu30= (YBCO). In order
130
Sputtering is the mechanism by which atoms are ejected from a material during atomic or ionic bombardment. The effect is only significant at low pressures where the mean free path of particles is large compared with the dimensions of the vacuum chamber. Sputtering was first reported in 1852 by Sir W. R. Grove, an English judge and "man of science" [11, 12]. It is now used widely in science and technology as an analytical tool in secondary-ion mass spectroscopy (SIMS), in the design of sputter ion pumps and thinfilm deposition and in controlled etching on a nanometre scale. However, sputtering can also cause problems contributing to failure of gas-discharge tubes due to sputtering of the electrode material; this effect is commonly observed in fluorescent light tubes where the ends of the tube become coated and blackened by material sputtered from the electrodes. Highvoltage electron microscopes and particle accelerators also suffer electrode damage due to sputtering effects. There are interesting natural occurrences of sputtering. Charged particles are believed to bombard the surface of Jupiter's satellite, Io, with sputtering atoms, which form thin toroidal clouds around Jupiter [13]. The development of sputtering-theory and practice, however, is strongly linked to vacuum technology with major interest occurring in the 1960-70s as semiconductor process technology developed [14-17]. This was followed by a second-step increase in activity during the past decade with the sputter deposition and plasma processing of new materials [18, 19].
2.1. Sputtering mechanism The physical process of the type of sputtering considered here involves a collision between an incident particle and atoms within a few monolayers of a solid surface (the selvage). After collision, the primary-recoil atoms collide with other atoms in the surface in a collision cascade [20]. A surface atom is then ejected or sputtered if it receives sufficient energy to overcome its binding energy. This is approximately equal to the sublimation energy. The sputtering rate is characterized by a sputtering yield, Y, defined as the average number of emitted atoms per incident particle. The yield depends on the incident particle's energy and mass and on the angle of incidence, and on the atomic mass, surface binding energy and crystallinity of the target. The yield is nearly independent of the target temperature, which is strong evidence for the above momentum-transfer model [21]. Below a threshold energy, Eth, of 20-40 eV no sputtering occurs. For E > Et,, the yield rises to a maximum over a range 5 to 50 keV, (Fig. 2). Anderson and Bay [22] give an
101
+1
-~ 102 E o 9~ 10-1
-
A 84
9 /
>- 10-2
i
10~( 2
i
]lhi
10-1
10 0 Energy, E (keV)
101
10 2
Figure 2 A typical sputtering-yield curve.
extensive collection of sputtering-yield data for a wide range of targets and ions. In addition they provide useful details on the techniques of sputtering-yield measurements. Chapman [16] also provides useful data. An important aspect of sputtering is the dependence of Y on the angle of incidence of the particles impinging on the target. It is generally found that for polycrystalline targets Y follows a 1/(cos 0) law, with a peak occurring at an angle of incidence of about 80 ~. In single crystals, the yield decreases in directions perpendicular to low-index planes due to channelling and shadowing effects, although the results are not simply related to the bulk crystallographic structure due to the restructuring of atoms at a free surface [23]. The impiantation and mixing of bombarding ions with the surface is exploited in ion-assisted-deposition techniques, as discussed further in Section 3.
2.2. Sputter-deposition sources The simplest and oldest method of generating sputtering atoms is to use a glow discharge. The positive ions Created in the glow-discharge sputter material are placed on the cathode, Fig. 3. The electrodes may be planar and parallel as in Planar diode sputtering or coaxial as in cylindrical-diode sputtering. The energy and density of the ions can easily be altered by changing the supply voltage and current. The ionization rate can also be enhanced considerably by applying a magnetic field perpendicular to the electric field as in magnetron sputtering. In sputtering, the glow discharge is usually operated in the "abnormal glow" region of the discharge characteristic [!4. 16], Fig. 4. In this region the discharge extends over the whole of the cathode and the current density increases with applied voltage. Both ions and electrons are present within the normal negative glow region; the mean charge density is zero; and the resistance of the plasma is low. Electrons are repelled by the negatNe cathode potential giving rise to an equilibrium sheath of positive space charge over the cathode Surface. The analysis of glow discharges has been extensive [24 26] and it is only briefly discussed here. Analytical and numerical solutions for the field and particle densities use either the Boltzmann transport equation or the electron and ion continuity
Figure 3 A planar-diode-atomic-sputtering arrangement. The gas discharge is operated in the 'abnormal glow' region of the 1IV characteristics (see Fig. 4). Material placed on the cathode or target is bombarded by positive ions; the ejected material condenses on the chamber walls and the anode surface. A: anode; C: cathode; P: plasma; T: target; S: substrate; V~: supply voltage.
350 300 250 ~200
sDc~rkge
150 100 50 o 0-12 .............. 10:;o ........................ 10-8 10-6
10-4
10-2
10o
I(A)
Figure 4 Glow-discharge-characterisitic
11/1 curve. Atomicsputtering systems are usually operated in the region of the abnormal glow.
equations in combination with Maxwell's equations and Poisson's equation for the potential [19, 27, 28]. Computer techniques and particle-trajectory modelling are used extensively, as discussed further below. Experimental current-voltage results have also been compared with more basic space-charge-limited transport equations, with a Child:-Langmuir three-halves power law with J oc V 3/2 but with non-zero injection energies [29], and with a mobility-limited square law with J oc V2, [14, 16]. Recent numerical techniques have shown that the latter square-law holds for distances close to the cathode where ionization effects are low [28]. If the target material is an insulator, then a positive charge accumulates on the target and sputtering eventually ceases. This may be overcome by flooding the target with electrons emitted from a separate filament as in triode sputtering or by using radio frequency alternating currents. In the r.f. sputtering methods, similar.potentials and currents are used as in the d.c. method but at frequencies in the MHz range. The coupling of the field to the discharge is complicated because of the large impedance mismatch between the generator and the glow-discharge load and also be-
131
cause of the need to avoid electromagnetic interference. However, the r.f. method has greatly increased the versatility of the sputtering technique, allowing thin films of a wide range of insulators and semiconductors to be deposited. Fig. 5 shows the various routes used in the sputter deposition of thin films; the route frequently used for HTS deposition is shown in bold. Although the glow-discharge methods are relatively simple they suffer the disadvantage that deposition takes place at the discharge pressure, which leads to attenuation of the sputtered atoms and trapping of residual gas in the deposited film. This may be overcome by using a separate source of ions to sputter the target and to pump the space between the target and substrate so that deposition takes place at lower pressures. Resputtering effects are also reduced, and the plasma is removed to within the ion source [30]. However, glow-discharge magnetron sputtering is simpler and can yield much higher deposition rates. In addition, the presence of the plasma is exploited in some processes such as unbalanced magnetron sputtering and off-axis sputter deposition of HTS films. Both d.c. and r.f. sputtering techniques use potentially lethalpotentials, and high r.f. fields present an additional hazard.
2.3. D.c. s p u t t e r i n g In the d.c. diode sputtering technique the material to be sputtered is mounted on the cathode, and the substrate is usually fixed to the anode as shown in Fig. 3. The space between the electrodes is first evacuated to about 10 .4 Pa, and then filled with an inert gas or gas mixture to a pressure of about 2 Pa. A negative voltage in the range 1-5 kV is then applied to the target electrode, with current control or via a series current-limiting resistor, and the substrate is held positive. The voltage is increased until the gas breaks down and a visible glow discharge appears above t h e cathode dark space or sheath. The voltage on the target drops to a sustaining level depending on the gas pressure and electrode geometry. Positive ions from
the plasma bombard the cathode; material from the target is sputtered and condenses on the substrate and chamber walls. In addition, negative ions and electrons bombard the substrate surface. During sputtering, the cathode dissipates heat due to ion impact and the anode becomes heated due to electron and negative-ion impact so these electrodes are usually water cooled.
2.4. R.f. s p u t t e r i n g In the r.f. sputtering technique, a high-frequency voltage is applied across the diode electrodes with sufficient amplitude to produce a glow discharge. High rates of sputtering are obtained for frequencies in the MHz region. A particular legal frequency of 13.56 MHz has been adopted in order to minimize radiofrequency interference. The r.f. plasma has a low ohmic impedance and it makes contact with the chamber walls and ground potential. If the r.f. potential is equally balanced between each electrode, then both the target and substrate are bombarded by positive ions and negative ions plus electrons during alternate half-cycles, see Fig. 6. However, because the electron velocities are much higher than the ion velocities, a negative charge accumulates on both electrodes. This leads to the formation of negative steady-state d.c. potentials on the anode and cathode relative to the plasma and ground potential. This effectively produces automatic d.c. bias potentials, whose magnitudes are determined by the applied r.f. potential, by the intrinsic properties of the discharge and by the external impedance of the anode and cathode circuits. A space-charge sheath forms between both electrodes and the plasma, and positive ions bombard the target and substrate. The variation of these d.c. potentials close to the anode and cathode is given approximately by the solution of the one-dimensional Poisson equation, as in the d.c. sputtering case. The magnitude of the d.c. potentials arising from the discharge only may be obtained by analysing the current flux across the sheath modulated by the r.f. potential [32]. Recent investigations have used analytical and equivalent circuit models to investigate the sheath motion
G l o w - d i s c h a r g e sputtering route
l
Microwave?.
Iv
n
D.c.? .
n
Iy
Diode? ~ iy n R.f.? .
Triode?
iv
Iv
f o
Planar? ~ C y l . ? ~ ~,y n [y Reactive?,
ly
. Magnetron?
HTS
Bias? Iy n [y Singletarget? n
Iv
O f f - a x i s ? ' - - 2 On-axis? ' IY " I y
Inv.cyl.? ly
n
ly
Dt
n1
Multiple target?
Jr
Figure 5 Glow-discharge sputtering routes. The route commonly taken in the deposition of high-temperature superconductors is shown .in bold.
132
Vs9
, Unbal. mag.? --,
t t ...... Figure 6 A planar-diode-r.f.-sputtering arrangement. The floating negative d.c. potentials on the cathode and anode are determined by the cathode and anode surface areas and by their impedances to the ground potential.
[33, 34] and particle-in-cell/Monte-Carlo metho4s to simulate particle behaviour in r.f. discharges [35]. The negative d.c. potentials may also be expressed approximately by the ratio of the charge to the capacitance of the target and substrate electrodes, Vt = Q / C t and Vs = Q / C s respectively where the subscript t denotes the target or cathode the subscript s denotes the substrate or anode. If the electrode capacitances are equal and Q is the same for each electrode, then the d.c. potentials are identical. Sputtering occurs equally from both target and substrate, and no layer forms on the substrate. If the target potential is made larger than the substrate potential the target sputters faster than the substrate, a n d a layer of the target material condenses on the substrate. This may be achieved by: (i) grounding the substrate to the r.f. potential, (ii) making the substrate area greater than the target area, and (iii) applying a bias potential. The first condition reduces the r.f. voltage on the substrate to zero, which forces the steady-state voltage to zero. However, this does not prevent bombardment of the substrate by energetic neutral atoms, negative ions and electrons. Since V s / V t = Ct/Cs, making C~ > Ct, by making the area of the substrate larger than t h e target area ensures Vt > Vs. The first two conditions are normally sufficient to ensure that sputtering occurs predominantly from the target. However, under conditions which involve the creation of a large number of negative ions, which occur when sputtering superconducting materials based on Y - B a - C u - O compounds, then conditions (i) and (ii) may not be sufficient to prevent excessive sputter etching of the substrate. Under these circumstances an external bias may be necessary to control the d.c. levels independently of the sputtering system variables. A direct relationship between the electrode potentials and their respective areas may be obtained by assuming parallel-plate capacitances. This yields V J V t = ( A t D s / A s D t ) where A is the area of the electrodes and D is the sheath thickness. This leads to V ~ / V t = ( A t / A ~ ) 4 for Child-Langmuir three-halves power-law space-charge-limited currents and V J V t = (At/A~) 3 for square-law space-charge currents. Experimential results, however, give an area-ratio power dependence close to unity [16], which suggests that, in this case, the sheath effect is mainly capacitive. A more detailed analysis carried out by Keller and Pennebaker [32] includes the effect of the wall area and shows that the fourth-power rule holds only for lowplasma forages. G o d y a k et al. [36] recently carried out accuirate and systematic measurements of the r.f. discharge in a capacitively coupled parallel-plate system at 13.56 MHz, which is typical of experimental systems. They used a radially confined symmetrically driven (push2pull) parallel-plate system with equalarea electrodes. In this arrangement, the harmonic content was sufficiently low to enable the arrangement to be considered as a linear device. Their main conclusions were that the experimental results deviate from the Child-Langmuir law and its collisional analogue,
and that the proportional relationship between the d.c. voltages, the sheath voltage and the r.f. voltage only occur at very high discharge voltages near the upper limit of voltages encountered in applications. This appears to be particularly true at pressures of 102 Pa. They also provide their results in tabular format suggesting that they may be used for comparison with existing models and for verification of scaling laws.
2.5. Magnetron sputtering 2.5. 1. Glow discharges in a magnetic field The effect of a magnetic field on a gas discharge has been studied since the early days of research in this area, [25]. The magnetic field applies a force, F = - qv x B, which forces the electrons to make circular orbits about the field lines at a cyclotron angular frequency coc = q B / m , where v is the velocity of the electrons, q / m is their charge-to-mass ratio and B is the magnetic flux density. If an electric field, E, is also present, the resultant Lorentz force is F = q E + qv x B, and the electrons take helical paths, see Fig. 7. In gas discharges and sputtering systems, the fields are usually non-uniform, the electrons suffer scattering, recombination and attachment, and the E field may vary sinusoidally as in r.f. sputtering. Analytic solutions of the equations of motion are possible only for approximate cases. Numerical techniques and modelling of the particle dynamics have therefore been in use for a l o n g time, and they continue to develop as computing power increases 1-37, 38]. Analytic solutions however, provide physical insight to back up modelling and simulation. As the particles make helical orbits, there is an increased probability of collisions, the mean-free path decreases and the effective pressure, p~, of the gas increases to p[1 + (f.0c'17)2]112, where p is the actual pressure and r is the mean time between collisions,
B
T
s1
s2
s3
Figure 7 Electrons in an E x Bfield follow helical paths modulated by the moving sheath boundary, sl, s2 and s3 in r.f. sputtering [34].
133
[39, 25]. The gas-breakdown striking potential is then expected to increase with B for pressures to the right of the minimum in the Paschen curve and to decrease with B for pressures below the minimum. In both d.c. and r.f. sputtering the rate of ionization is enhanced by applying a magnetic field perpendicular to the electric field. The E x B field forces the electrons to take helical orbits, and the electron-molecule collision rate is increased~ This increases the ionization rate and hence the sputtering rate. If the magnetic field is confined to the target area, the effect is to concentrate the plasma near to the target, which leads to more efficient sputtering and higher film-deposition rates. The target potential is reduced for a given sputtering rate and the energy of the sputtered particles is lowered. In addition, secondary electrons emitted from the target are confined to the target area by the cyclotron paths, and this reduces electron bombardment and heating of the substrate.
(a)
2.5.2. Magnetrons In the early use of magnetic fields for sputtering, the fields were produced by solenoids placed outside the vacuum chamber. However, by localizing the field to the target area it soon became clear that very high sputtering rates could be achieved. In addition, the plasma was localized to the target area and the electrons were trapped near the target surface. This arises because the distance travelled by the charged particles above the target scales as 1/1/2 so that even moderate //-fields confine the cyclotron paths to the target. High values o f / / a r e desirable, since the cyclotron frequency and ionization rate increases with the magnetic field. Hence, nearly all commercial sputtering systems employ sources or magnetrons with permanent magnets mounted beneath the targets. The efficiencies and sputtering rates of such sources have been increased considerably in recent years by the use of high-energydensity magnetic materials such as neodymiumi r o n - b o r o n or samarium-cobalt alioys. Exposure of the magnet to reactive gases and outgassing of the magnet to the vacuum system is avoided by enclosing the magnet in a water-cooled copper cavity over which the targets are placed. The size of the magnetrons vary considerably from small, modular, 2 inch diameter sources which can be mounted in clusters on U H V flanges, to large 4 foot long rectangular sources. Detailed information on planar magnetron design is given by Waits [40], and a brief description follows.
2.5.3. Circularand rectangular planar magnetrons Circular planar magnetrons consist of a cylindrical permanent magnet and a central magnet, both having magnets mounted on a flat pole piece, Fig. 8a. The design is similar to loudspeaker magnets, which are convenient high-field magnets for planar magnetron sources [41]. The circular types produce an intense field between the outer ring and the centre pole, with a horizontal radial component of the field forming a ring in the plane of the target, Fig. 9. During opera134
(b) Figure 8 (a) A circular magnetron, and (b) a rectangular magnetron.
Substrate
[
--J
Substrate
B
~~
~[~P~
et
m
:@
:@
Figure 9 A planar magnetron showing a section through the magnet and plasma. This etches the 'race track' into the target. The use of Monte-Carlo methods provides simulations of the complex plasma field [38]. (B is the perpendicular magnetic-field component).
tion, electrons are trapped in this ring producing an intense circular plasma a few millimetres from the target surface. In this region, sputtering is a maximum. This eventually etches a groove or "race-track" in the target surface. The rectangular design is similar, ex-
cept that the outer and central magnets are arranged around the rectangular pole piece, Fig. 8b. This effectively produces a line source of sputtered material, compared with a ring source for the circular magnetron. Both sources suffer from non-uniform erosion of the target surfaces, and various designs have been suggested t o obtain uniform fields. However, the requirement for a high-intensity magnetic field is not easily achieved by other methods.
away from the cathode, producing a plasma beam directed at the substrate surface [42, 43]. This technique has resulted in the deposition of a variety of novel films and hard coatings [2, 42]. However, in the growth of HTS films, it is desirable to reduce the amount of ion bombardment of the growing film, which can lead to resputtering effects. This can also be achieved by using the unbalanced-magnetron technique (see Section 5).
2.5.4. Cylindrical m a g n e t r o n s In this coaxial cylinder design the inner cylinder supports the target, and the outer cylinder supports the substrates. The magnetic field is parallel to the axis of the inner cylinder as shown in Fig. 10. Cylindrical magnetrons have the advantage that many substrates can be placed around the outer cylinder and the coating is more uniform than the planar magnetrons. In the off-axis sputtering arrangement used in the deposition of HTS material, the substrates are mounted perpendicular to, but off the axis of, the target cylinder, as shown. If the outer cylinder is used as the cathode target and the inner cylinder is used as the anode, then substrates mounted on the anode receive material sputtered from a wide range of angles. This technique is used for coating complex curved surfaces a n d is referred to as an inverted magnetron.
2.6. Reactive sputtering Reactive gases, such as oxygen or nitrogen, in addition to inert gases, are frequently used in the sputter deposition of compounds [14, 44]. In the deposition of oxides and nitrides from a metal target, oxygen or nitrogen is added to the sputtering gas to give stoichiometric films. In this case sputtering can occur in two modes depending on the partial pressure of the reactive gas. Sputtering occurs either from the puremetal target or from a compound film formed by the reactive gas with the target. The sputtering yield of the compound is usually less than the yield of the pure metal. Hence, if the sputtering changes between the two modes, a pronounced hysteresis effect can occur between the gas pressure and the deposition rate. A finite-element model has been developed for a r.f.magnetron, reactive-sputter-deposition process concerned primarily with the diffusion of oxygen during the deposition of ZrO2; the calculations agree with experimental results to within 15% [45]. Vossen et al. 1-46] used a neat method to observe the direct effect of reactive gases on film deposition in sputtering A1, Zn and Ti targets. The reactive gas was passed through a tube positioned in contact with, and on top of, the substrates. The direct effect of the gas on the film growth could then be observed. A similar method is commonly employed in the deposition of HTS films where a high degree of film oxidation is required, although the tube is not usually placed in contact with the substrate. In the sputter deposition of multicomponent compounds it is also common to employ a target of the same composition and stoichiometry as that of the required film. In this case, the reactive-gas pressure is adjusted to yield the required film composition and stoichiometry. The ratio of the inert-gas pressure to the reactive-gas pressure is usually determined empirically, since it depends on the desired film properties, the materials sputtered, the sputtering-system route (Fig. 5) and the system variables. For the r.f.-sputter deposition of ZnO thin films, the Ar/O 2 ratio was found not to be critical, and a ratio of unity produced excellent films [47]. Similarly, in the r.f.-sputter deposition of HTS films, wide variations in the inert/reactive gas ratios are reported. Moradi et al. [48] have produced mathematical models for single-element and multi-element reactive sputtering which predict the hysteretic behaviour observed. A significant prediction from these calculations is that in sputtering from separate elemental targets there is a critical dependence of the film composition on the reactive-gas flow. But if a single-alloy target is employed then the metal
2.5.5. U n b a l a n c e d m a g n e t r o n s p u t t e r i n g If, in magnetron sputtering, the magnetic field is weak, or a fringing field exists, a fraction of the target's secondary electrons accelerate towards the anode, which also leads to ion bombardment of the substrate a n d the growing film. This effect is exploited in unbalanced magnetron sputtering in which the magnetic field is no longer symmetrical as shown in Figs 8 and 9 but is arranged so that a fraction of the field leaks
S
~ S
Figure 10 A cylindricalmagnetron showingthe on-axis and the offaxis substra~e positions. (S is the substrate position, T is the target, and B is axial magnetic field).
135
composition of the deposited film "will always be identical to the alloy target bulk composition"; this is a remarkable result.
3. Low-energy, ion-assisted deposition This involves the bombardment of the substrate by energetic ions during deposition, and it has been reviewed by Martin [49]. It has been common practice for many years to use glow discharges and ion beams to clean up vacuum systems and to sputterclean substrates prior to film deposition [50]. Mattox [51, 52] showed some years ago that film adhesion could be improved considerably if the substrate was exposed to energetic argon ions during film deposition; this technique is referred to as ion plating. The energy distribution of the ions impacting the substrate was, however, considered to be much less than the applied potential due to the ions making multiple collisions with neutral atoms in the gas 1-53]. Ionbeam cleaning of substrates has not been used widely in high quality film growth due to the damage caused by the energetic atoms or ions. This has an adverse effect on the material properties and the electrical characteristics of semiconductors and insulators in particular. Nevertheless, it is now recognized that, in addition to improvements in film adhesion, ion-assisted deposition increases the film density and improve the film corrosion and catalytic properties, and the technique is now widely used [49, 4]. In addition to ion plating, ion-assisted deposition is also being applied to other thin-film-deposition techniques such as ionbeam-assisted deposition (IBAD) [4], MBE and sputtering. It has recently been shown that in the doping of MBE Si (1 00) films with Sb, the dopingincorporation probability increases by up to five orders of magnitude when using low-energy Sb ions (50-400 eV) compared with thermal Sb beams [54]. Ion bombardment during film growth can also change the film-growth kinetics. Chason et al. 1-55] used in-situ, reflection high-energy electron diffraction (RHEED) to observe the MBE growth of Ge on Ge(001). They observed enhancement in surface smoothing during simultaneous Xe-ion bombardment and film growth which they interpreted as being due to ion-induced vacancy-like defects annihilating with surface atoms and/or stabilizing small clusters. By irradiating the substrate with 28 eV Ar ions, threedimensional island nucleation is reportedly suppressed during MBE of GaAs on Si (100) [56]. In r.f.-sputter deposition the presence of a substrate bias voltage can have a significant effect on the film-growth kinetics and crystallographic structure. In the deposition of Z n O by r.f. sputtering the growth mode changes from island growth (Fig. 11) to two-dimensional layer growth when bias sputtering is used [57], Fig. 12. Apart from reducing the deposition rate, substrate bombardment by energetic ions leads to lattice damage and implantation of the bombarding ions into the growing film. Surface damage can be reduced by heat treatment and surface-implanted ions may be desorbed by heating the material to temperatures of about 250 ~ for low-energy bombardment
136
Figure 11 Z n O film r.f. sputtered on SiO 2 with zero substrate bias voltage. (a) TEM bright-field micrograph, (b) Electron-diffraction pattern of (a), Corresponding to the hexagonal phase of ZnO.
( < 500 eV). In addition, the analysis of the desorbed flux of atoms can provide information about the desorption sites [58]. In glow-discharge sputter deposition the plasma is a readily available source of ions for mixing or chemically activating the substrate surface and film by application of a substrate bias, Vs. This is the biassputtering technique which has been described extensively in the literature [-14, 16]. If the substrate is insulating, then in a d.c. discharge, it will rapidly charge to a floating potential, and the bias potential has no effect. By using r.f. sputtering, a negative d.c. bias may be maintained even on an insulating substrate. This is because the electrons, which are attracted to the Substrate during the positive half-cycle of the applied a.c., have a higher mobility than the ions, and a negative bias rapidly builds up on the insulating substrate. This is similar to the r.f.sputtering mechanism described previously. The magnitude of the self-bias depends on the surface area of the anode and the impedance of the anode (substrate) circuit. A particularly simple method of introducing a negative bias on the substrate is to alter the position of the earth point between the r.f. generator and the anode. A more controlled method is to insert a transparent grid near to the substrate and to supply this with an independent d.c. bias voltage [57].
Figure 12 ZnO film r.f. sputtered on SiO 2 with a - 80 V substrate
bias. (a) TEM bright-field micrograph; and (b) electron diffraction pattern of (a), corresponding to the hexagonal phase of ZnO but with additional 1/3 and 2/3 (11.0) superlattice diffraction spots [57].
4. M a t e r i a l c o n d e n s a t i o n and g r o w t h in a plasma Sputtered atoms impinging on a substrate have mean energies of several eV and peak energies of hundreds of eV. This compares with particle energies of about 0.1 eV during thermal evaporation. In addition, the substrate is bombarded by electrons and possibly ions during sputter deposition. Ion bombardment during film growth is known to change the film-growth kinetics, as discussed in Section 3. These higher-energy deposition processes are expected to give a higher surface mobility of the adatoms. This is equivalent to raising the substrate temperature or decreasing the supersaturation ratio 1,59]. Thus as the vapour supersaturation is raised, either by lowering the substrate temperature or raising the impingement rate, the size of the critical nucleus decreases, the grain size is small
and the film may be either amorphous or small grained with columnar three-dimensional growth. If the supersaturation is low, then single-crystal growth may occur, Fig. 13. However, this simple picture does not take into account the complex interaction of energetic particles with the substrate and film, as in sputtering or ion-assisted deposition. Nucleation sites are created by energetic sputtered particles, which raise the critical nucleus density and produce films with small grains even at low supersaturations. Thornton 1-60] investigated the structure and topography of a range of metal coatings prepared by hollow-cathode sputter deposition and compared his results with a structure-zone model proposed by Movchan and Demchishin 1-61]. In this model, the ratio of the substrate temperature to the film melting temperature, T/Tm, is compared for three zones. Zone 1, T/Tm < 0.3, and the structure grows as tapered crystallites with domed tops and a width which increases with temperature. Zone 2, 0.3 < T I T m < 0.45, the film has full density with columnar grains and a smooth matt surface. Zone 3, 0.45 < TIT m < 1, equiaxed grains occur with bright surfaces. Thornton found general agreement with this model but detected a transition zone between zone 1 and zone 2 at low argon sputtering pressures (0.1 Pa), consisting of densely packed fibrous grains which generally did not pass through the coating thickness. Craig and Harding 1-62] carried out similar measurements on purecopper films and found general agreement with Thornton's results and the three-zone model for sputter deposition at argon pressures up to 100 Pa. Although the structure-zone model is mainly qualitative and phenomenological, it is highly relevant to practical applications of films and it reflects the process conditions required to obtain highly dense films, films with low resistivity and smooth surfaces. The grainstructure dependence on T/Tm may also be helpful in optimizing the Jc of HTS polycrystalline films (see Section 6). Recently, Stone and Ghoniem [63] modelled the nucleation and island formation of condensing energetic particles (100 eV) using a system of kinetic rate equations. Their mathematical model uses a comprehensive range of interactions, including the generation of point defects by the impinging particles, re-
Amorphous
Single / ~olycrystal/crystal/ ~
J
Liquid
-----
Vapour T
Figure 13 P - T diagram for thin-film growth.
137
sputtering effects, cluster growth and decay, mobile single traps and atoms, thermal desorption of single atoms, and dissociation of clusters by particle bombardment. The model indicates that the surface defect density increases the nucleation rate and cluster density but reduces the cluster size and size distribution. However, at temperatures above 75 K, thermal processes dominate. Because of the complexity of plasma film deposition it is essential to investigate film growth experimentally and to view as far as possible the early-growth features which occur for a given set of growth conditions. Recently both AFM and STM have been used to observe the early stages of sputtered film growth with atomic resolution, [64, 65].
4.1. C o m p o u n d g r o w t h thermodynamics In the condensation of a vapour o n a surface, three general growth types can be identified and related to the surface, the over-growth and interface energies Ys, Yo and Yi, respectively, through the Young equation % cos 0 = [Ys - Yil [66]: (a) Volmer-Weber or threedimensional nucleation and island growth with Yo >> IYs - Yil, (b) Frank and van der Merwe lateral or two-dimensional layer growth with y, ~< I% - Yil, and (c) Stranski-Krastanov growth which refers to the condensation of a few monolayers of layer growth followed by three-dimensional island growth, Fig. 14. In case (a), the critical nucleation density, n*; is expected to be high, so that growth by direct impingement from the vapour dominates over growth by surface diffusion. Lateral layer growth is expected to dominate for low values of n*, so that growth by surface diffusion dominates. This leads to the formation of large flat grains. In practice, three thermodynamic parameters, temperature, pressure and composition, are measured, so that equations relating to these variables are the most useful. The nucleation rate of
% (a)
(b)
(c) Figure 14 (a) Three-dimensional island growth (Volmer-Weber growth). (b) Two-dimensionallayer growth (Frank-van-der-Merwe growth). (c) Layer and island growth (Stranski-Krastanov growth). 138
the species i is
Ji = Z(on* where Z is the Zeldovich factor which corrects for deviations from equilibrium, co is the frequency or rate factor. This is proportional to exp(AGaes - AGsd)/kTs where Ts is the substrate temperature, AGdes and AQd are free energy-changes of desorption and diffusion, respectively. These must be computed for each adsorbing species. The mobility of the adsorbing species increases as the substrate temperature increases, and lateral grain growth increases over that of vertical growth. The critical nucleus density is n* =
n~exp( -- AG*)/kTs
where AG* is the critical free energy of formation, and n~ is the density of available nucleation sites. For heterogeneous growth on a surface, AG* oc %f(0)/AGv. f(O)/AGy is the ratio of the classical contact-angle function (a function of y) and the nucleus volume free energy of formation. For a compound consisting of the elements ABCD, AGv is given by the sum of the free-energy changes of the elements and the freeenergy changes of compound formation AG(A), AG(B), AG(C), AG(D), and AG(ABCD). These can be determined from their respective vapour pressures and equilibrium constants K i through AGI = - k T l n K~ [67]. Hence, experimentally, we seek particular values of the substrate temperature and vapour pressures which yield a minimum in AG* corresponding to t h e formation of the precise phase. For large flat grains, n*(L) >> n*(V) and AG*(V) >> AG*(L). The ratio of vertical to lateral critical free energy changes is then AG*(V)/AG*(L) oc yof(O)/hAGv, where h is the island height. Hence, for large grains and lateral growth, small values of AGv are required. This implies a low supersaturation, i.e. a low ratio of impinging vapour pressure to equilibrium pressure at the substrate temperature. It is observed experimentally that lateral grain growth increases with increasing substrate temperatures. This follows from the above theory since the supersaturation decreases with increasing substrate temperature. The application of the above "classical" theory of nucleation has been questioned frequently [68] and this stimulated the simplified atomistic nucleation models of Walton [69]. However, Sigsbee [70] has shown that the classical and atomistic models are essentially the same except for small embryos consisting of a few atoms, and that the nucleation rates are identical in each model. The classical thermodynamic models are also useful when considering complex multicomponent phases and comparing published thermodynamic data, and in particular oxidation data for HTS materials, as discussed further in Section 5. Nevertheless, MonteCarlo models which consider discrete growth units have made impressive progress recently [71], although they refer directly to classical supersaturation as a growth parameter. Epitaxial thin-film growth is achieved by choosing a substrate material which has a small lattice-constant mismatch between film and substrate, f = (ao - a~)/a~, where ao and as are the lattice constants of film over-growth and the substrates respectively, [72]. Mismatches of, typically, one per cent
permit epitaxial growth with the mismatch accommodated by strain or dislocations at the interface. This aspect is of great interest because of its importance in semiconductor crystal growth, in MBE and in basic science. A good working text has recently been produced by Tu, Mayer and Feldman [73], a review of misfit dislocation theory has been produced by van der Merwe [74] and the background science has been addressed by Tiller [75].
C
m
t
W
~D
5. Sputter deposition of high-temperature superconductors Thin films of superconductors with critical temperatures in the region of 20 K have been prepared by sputtering for many years. Gavaler and co-workers [76, 77] produced thin films of Nb3Ge with a Tc of 22 to 23 K by low-energy sputtering ( < 1000 V) and high pressure ( > 10 Pa). Wu et al. [78] used magnetron sputtering with relatively low target voltages to produce NbsSn films with Tr = 18.3 K and Jc = 15 MA cm -2. Sputtering is also widely used in the preparation of low-Tr films and Nb/A10~/Nb-based Josephson tunnel junctions [79]. The breakthrough to the present HTS materials came in 1986 when Bednorz and Muller discovered superconductivity at about 30 K in a L a - B a - C u - O system [80]. This was soon followed by the discovery of superconductivity at 93 K in Y - B a - C u - O [811, B i - C a - S r - C u - O with Tc = 110 Ki [823 and T1-Ca-Ba-Cu-O with Tc = 125K [83]. This history has now been widely reviewed [84]. The first papers on sputtered HTS copper oxides also appeared soon after the discoveries in the bulk copper oxides. A comprehensive bibliography of' the work up to about August 1988 is given in [85]. This contains a total of 215 publications on sputtered copper-oxide HTS films including (Lal _xSr~:)zCuO4, YBa2Cu307 and YBa2Cu30= with substitutions for Y. This also includes references to work on B i - S r - C a - C u - O and T I - C a - B a - C u - O , but the majority of papers are on YBCO. More recently, B i - S r - C a - C u - O films have been produced by d.c. magnetron sputtering [86, 87, 881 and T 1 - C a - B a - C u - O films using multitarget magnetron sputtering [89, 90].
5.1. Choice of sputter deposition process A wide variety of sputtering techniques have been used to deposit HTS thin films as shown in Fig. 5. This illustrates only the choice of the sputtering-system route. There are of course the sputtering system variables: gas pressures, gas mixtures, current densities and potentials, substrate temperature, position of the substrate a n d choice of substrate materials and target etc. Probably the most widely used sputtering route is r.f. magnetron reactive sputtering (bold in Fig. 5), with substrates mounted off-axis to avoid resputte~ng effects, see Fig. 15. This effect is discussed further ~elow. However, there are also numerous other approaches used successfully to deposit HTS films incl!~ding cylindrical magnetron sputtering [86] d.c. planar magnetron sputtering [91-93] and d.c.
Figure 15 Planar-magnetron-diode sputtering System used for depositing HTS films. Key: T, Target; S, Substrate positions; LN, liquid-nitrogen cooling, Sh, target shutter; B, pump secondary baffle; V, micrometer leak valves; F, flow meters. W, water cooling of cathode and anode, r.f., radiofrequency power source and C, coupling capacitors.
triode sputtering [94], d.c. magnetron sputtering using separate alloy targets [95], facing target magnetrons [961, ion-beam sputtering [31], r.f. plasma flash evaporation [97]. Foley et al. [98] compare ion-beam deposition and r.f. and d.c. unbalanced magnetron sputtering in the preparation of YBCO thin films and SQUIDs (superconducting quantum interference devices). They found little significant difference between these three techniques except that the r.f. unbalancedmagnetron sputtering did not deplete the YBCO target of barium as rapidly as the other techniques and that the target lasted up to three times longer. Considering the price of commercial targets (ca. s sterling), this is a useful result.
5.2. Re-sputtering effects, off-axis deposition, in-situ and ex-situ processing Soon after the first attempts to produce thin films of YBCO by sputtering, it emerged that sputtering of the film during growth (re-sputtering) created significant problems. Large variations in layer uniformity and reduction in the deposition rate perpendicular to the target surface were reported [991. In our planar diode arrangement, we found that a film could not be formed directly above the sputtering target, but a good uniform film condensed on the system walls perpendicular to the target surface. We therefore placed the substrates at an optimum angle to the target surface and perpendicular to the sputtered vapour diffusion front [1001 Fig. 15. This is generally referred to as offaxis deposition. The lack of film condensation directly above the target surface is believed to be due to the sputtering of the anode by negative oxygen ions, although sputtering due to energetic neutral atoms may also be a problem, particularly by heavy atoms such as barium. R.f. sputtering is a complex process, however; negative potentials may appear on the substrafe surface, or its holder, if they are not well groun-
139
ded to the r.f. potential, and this m a y cause sputter etching of the film or re-sputtering [32]. Other methods employed to overcome the re-sputtering problem include unbalanced magnetron sputtering as previously mentioned, and high pressure (20-60 Pa). on-axis r.f. magnetron sputtering [101]. In the unbalanced-magnetron-sputtering arrangement, (Foley et al. [98] and Savvides and Katsoros [102]) the magnetic field lines are such as to direct the weak plasma away from the substrate, reducing resputtering effects. For the on-axis sputter deposition of B i - S r - C a - C u - O films, Grace et al. [88] found that ozone increased the resputtering rate significantly compared with oxygen. They also found that the application of a negative substrate bias of - 40 V also reduced the re-sputtering effect. Many of the early experimenters concentrated on producing ex-situ films, that is, films deposited onto a suitable substrate at the ambient temperature of the system and then post-heat-treating the samples in a separate furnace. YBCO films deposited at the system ambient temperature (25-100 ~ are amorphous. The subsequent heat treatment must be sufficient to crystallize the material, and this requires temperatures in excess of 900 ~ The superconducting transition temperatures are usually found to be poor using this method. In addition, at high temperature, the film may react with the substrates (Si for example), or film adhesion may be lost due to differences in thermalexpansion coefficients between film and substrate. If the heat-treatment period is too long, mass loss due to sublimation becomes significant, particularly for thin films. For these reasons, it is desirable to grow the superconducting phase in situ within the sputtering system. This poses new problems, such as the design of high-temperature substrate heaters capable of operating over long periods at temperatures up to about 800 ~ in a highly oxidizing plasma. This is reflected in the high cost of commercial heaters, which leads many researchers to construct their own [103, 104] as discussed further in the experimental section below. For normal growth temperatures, the films condense with the tetragonal semiconducting phase. The process immediately following deposition, both in sputtering and laser ablation, is critical for the successful growth of HTS. Hence, these procedures are given in detail in the following examples.
5.3. YBCO film oxidation The pressure, temperature and oxygen concentration, x, are critical parameters for in situ growth of HTS films. P - T - x for bulk YBa2Cu3Ox has been measured by several groups [105-114] and typical results are given in Fig. 16, reproduced using data from Schleger et al. [109]. This shows the orthorhombic superconducting region and the tetragonal region separated by the line x = 6.6. Appended to this diagram are the oxygenation paths used in sputter-deposition experiments discussed further below. Measurements of the equilibrium oxygen pressure above YBa2Cu30 ~ also show that Tc increases with x over the range 6.4-7, with a constant temperature shoulder at 60 K between
140
x = 6.5 and x = 6.7, [112-114], as shown in Fig. 17. Hence, to produce HTS films in situ with the maximum To, the film must be grown with the required YBa2Cu3Ox stoichiometry with x close to 6.9. Hammond and B o r m a n n [115] produced a useful P - T - x diagram, similar to Fig. 16, which illustrated the thermodynamic regions over which sputtering, laser ablation, and other film-deposition techniques operate. In general, good epitaxial films are obtained at temperatures in the region of 700 ~ and pressures of less than 10 Pa of oxygen. Thus the films are grown in a region of the phase diagram where the tetragonal phase is expected to form. Kwok and Ying [116] used pulsed laser ablation and carried out in situ resistivity measurement and X-ray diffraction (XRD) measurements which indicated fairly conclusively that for a deposition temperature of 640~ and an oxygen pressure of 1 P a YBa2Cu30 x films condense with the tetragonal semiconducting phase. After deposition, the films were cooled from 640 ~ to room temperature in 50 Pa of oxygen. After a few seconds, the substrate heater was turned off and the film was allowed to cool to room temperature (path 1, Fig. 16). During oxidation, a sharp decrease in resistivity was observed, and the formation of the orthorhombic phase was detected by XRD. To obtain the optimum amount of oxygen take up in the film during sputter deposition, various methods have been used. These include injection of oxygen close to the substrate surface, excitation of atomic oxygen in a separate source and the use of ozone [117]. In glow-discharge sputtering the plasma is known to yield a supply of atomic oxygen which allows oxygenated films to be obtained. Although the methods used to oxygenate films vary, the two-stepcool-down process illustrated in Fig. 16 is commonly used in practice.
5.4. Typical results for YBCO Sputter deposition was one of the first techniques to yield high-temperature superconducting YBCO thin films [118, 119] soon after the discovery of the bulk material. These early films were generally of poor quality compared with bulk material having broad superconducting transitions and low critial-current densities, [120-122]. This was partly due to ex-situ heat treatment and resputtering effects. It was not until the use of in-situ off-axis deposition that the film quality improved [1, 123, 124]. Probably one of the most extensive investigations of r.f.-sputtered YBCO films was carried out by Eom et al. [1]. They used in-situ, 90 ~ off-axis, single magnetron sputtering from single stoichiometric YBCO targets and deposited films on a number of different substrates, including SrTiO 3, LaA10 3, MgO, YSZ and R-plane sapphire single crystals. In this paper, they provided, detailed characterization results including surface impedance measurements. Epitaxial or mosaic single-crystal YBCO films deposited on single-crystal substrates are now obtained using a wide range of sputtering variations which yield transition temperatures close to the
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Figure 16 Variation of the equilibrium oxygen pressure above YBazCu30 ~ [114]. Also shown are the oxygenation paths for: (1) laser ablation [ 116], (2) on-axis sputtering [102], and (3) off-axis sputtering [129].
100 + ~
106 A cm-2 and resistance ratios of R3oo/Rto o = 2.0-3.1. Figs 18-21 show results obtained for off-axis r.f. sputtering of a single YBCO target using the system 141
Mg 20
03
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20 20
25
006
007
0O4
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(a)
35
40 20
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Sr-fi03 (110) --
Sr~O 3 --
(220)
45
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YBCO
-(026)
Figure 18 (a) Scanning electron micrograph of a YBa2CuaO ~ thin film sputtered on a MgO (1 00) substrate. Ts. b = 735~
p(Ar + 02) = 28.0 Pa, p(O2) = 5.6 Pa, t = 360 min, T~o.= 83 K, T~.... = 76 K, (Y70M). (b) Conditions as for (a) but the substrate was mounted further away from the plasma. Too,= 89 K, T~.... = 82 K, (Y67M).
I
142
I
67
(b)
i l l u s t r a t e d in Fig. 15 [-129]. T h e g r o w t h c o n d i t i o n s were: t o t a l a r g o n + o x y g e n pressure of 28 Pa, oxygen p a r t i a l pressure 2 0 % (5.6 Pa), T s = 735~ growth time 3 6 0 m i n . T h e m e a s u r e d film thickness was 6 8 - 1 0 O h m . This yielded g r o w t h rates of a b o u t 0.3 n m r a i n - 1. At the e n d of the d e p o s i t i o n , the A r gas was t u r n e d off a n d the o x y g e n pressure increased to 800 Pa. T h e s u b s t r a t e s were then c o o l e d r a p i d l y from the g r o w t h t e m p e r a t u r e to a b o u t 430 ~ a n d held at this t e m p e r a t u r e for 1 h, after which the s u b s t r a t e h e a t e r s u p p l y was t u r n e d off (path 3, Fig. 16). T h e films o b t a i n e d were generally glossy b l a c k with s m o o t h surfaces. Fig. 18a shows a S E M m i c r o g r a p h of Y B C O s p u t t e r e d on M g O ( 1 0 0 ) using the a b o v e conditions. This shows a dense p a c k i n g of grains with a m e a n d i a m e t e r of a b o u t 200 nm, a n d a density of 25 x l 0 s cm - z a n d there also a p p e a r s to be a s u b g r a i n structure. H i g h - r e s o l u t i o n S T M a n d A F M analysis of similar films shows t h a t each g r a i n consists of a spiral g r o w t h of Y B C O a r o u n d a d i s l o c a t i o n [64, 65]. Fig. 18b shows a similar S E M m i c r o g r a p h of the surface of Y B C O b u t with the s u b s t r a t e p o s i t i o n e d further a w a y from the p l a s m a a n d target. T h e critical t e m p e r a t u r e has increased slightly from 83 K in Fig. 18a to 8 9 K in Fig. 18b. This m a y be due to a r e d u c t i o n in i o n - b e a m d a m a g e . H o w e v e r , the d e p o s ition rate also decreases, which p r o b a b l y leads to an increase in l a t e r a l g r a i n g r o w t h a n d i m p r o v e d film
,
,
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68
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,
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69
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32
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33
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Figure 19 (a) An XRD trace of YBa2Cu30 = sputtered onto
MgO (1 0 0) with the same sputtering conditions as in Fig. 18b but for t = 600 min. The film was c-axis aligned with YBa2Cu30 ~(0 0 1) parallel to MgO (1 00) (YRF68M). (b) An XRD trace of YBazCuaO ~ sputtered onto SrTiO a (1 1 0) with the same sputtering conditions as in Fig. 18b. The film was aligned with YBa2CuaOx (0 1 3) parallel to SrTiO3 (1 1 0) (YRF67Sr).
properties. Fig. 19 shows X R D traces for YBa2CuaO~ s p u t t e r e d o n t o M g O (I 00) a n d SrTiO3 (110). In b o t h cases, the films a r e w e l l o r i e n t e d with respect to the substrates, with YBaECU30 x ( 0 0 1 ) p a r a l l e l to M g O ( 1 0 0 ) a n d with Y B a 2 C u 3 0 ~ ( 0 1 3 ) parallel to S r T i O 3 (110). T h e resistivity versus t e m p e r a t u r e results for these films are s h o w n in Fig. 20. F o r g r o w t h on M g O ( 1 0 0 ) the resistivity in the (ab)-plane of the film was 85.3 x 10 - 6 f~cm. at 100K, Fig. 20a. The critical temp e r a t u r e s were Tc ( o n s e t ) = 8 9 K , T c ( z e r o ) = 8 0 K . F o r Y B C O o n SrTiO3 (1 10) the resistivity is m e a s ured in the ( 0 1 3 ) p l a n e with a value of 455 • 10 - 6 ~ c m . at 1 0 0 K , Fig. 20b, a n d critical values, Too, = 92 K, Tc. . . . = 87 K. T y p i c a l critical-current densities were 2 . 9 x 1 0 5 A c m - 2 a n d 2.73 x 105 A c m - 1 for g r o w t h on ( 1 0 0 ) a n d (1 1 0) SrTiO3 s u b s t r a t e s , respectively, m e a s u r e d at 7 7 K using a 5 g V limit. Fig. 21a shows a S E M m i c r o g r a p h of Y B a 2 C u 3 0 ~ s p u t t e r e d o n t o S r T i O 3 (1 1 0) b y m e a n s of d i o d e s p u t t e r i n g (not m a g n e t r o n sputtering). T h e
o6
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e
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fie
,0
i
0
r
4O
r
,
80
120 160 200 Temperature (K)
240
280
Figure 20 R - T traces of." (a) an (0 0 1) oriented YBa2CuaO ~ thin film sputtered on a MgO (1 0 0) substrate as in Fig. 18b (Y67M). (b) A (0 1 3) oriented YBa2Cu3Ox thin-film sputtered onto SrTiO 3 (1 10), as iia Fig. 19b (YRF67Sr).
The control of the growth direction is of particular interest for both fundamental studies and device applications, and this is readily achieved by epitaxial sputter deposition. Linker et al. [130], using in-situ hollow-cathode magnetron sputtering of YBCO, obtained oriented films with YBCO (01 3) II SrTiO3 (1 10) at Ts = 820~ YBCO (1 10) II SrTiO3 (1 10) at Ts = 800~ and YBCO (1 10)I[ SrTiO3 (1 10) at Ts = 700 ~ Film resistivities at 100 K were 80, 150 and 1900 gf~ cm respectively. The high electrical anisotropy in YBCO films is also observed in the normal film impedance. For YBCO (001)]l SrTiO 3 (100) or YBCO (00 1) [I MgO (1 00), the films were purely inductive in the normal state. For YBCO (0 1 3) II SrTiOa (1 10) the films were capacitive in the normal state [ 131-134]. There is also increasing interest in the deposition of HTS films on more practical metal-alloy substrates such as heat-resisting Hastelloy-C. However, the transport critical-current densities, Jet, are much less in polycrystalline films due to grain-boundary defects, but some progress is being made by the use of buffer layers. Because of the large interest in YBCO, the techniques of sputtering epitaxial and buffered-layer polycrystalline material are considered in further detail in Section 6.
6. Polycrystalline films and buffer layers
Figure 21 (a) A SEM micrograph o f a YBa2Cu30 x diode sputtered (not magnetron) onto SrTiO3 (1 0 0). p(Ar + 02) = 35 Pa, p(O2) = 1.8Pa, t = 4 3 2 m i n , l g m thick, T c o n = 8 9 K , T c.... = 6 8 K (YRF52). (b) R H E E D pattern of (a) indexed as YBa2Cu30 x (0 1 3) parallel t o SrTiO a (1 10).
RHEED pattern in Fig. 21b shows that the film is also oriented with YBaaCu3Ox (0 1 3) parallel to SrTiO 3 (1 1 0). The near vertical lateral-growth-like features were also observed on similar films grown by magnetron sputtering. These were quite different from the rectangular features observed in (1 00) oriented film, Fig. 18.
One of the central problems in the application of hightemperature superconductors is the dependence of the critical transport current density, Jet, o n grain-boundary crystallography and defects, as reviewed recently [135, 136]. In these materials, the superconducting coherence length varies between 0.3 to 1.5 nm which is ,similar to the transition width of grain boundaries. The presence of high-angle grain boundaries, dislocations and defects have a marked effect on the criticalcurrent density and generally lead to a reduction in the intergranular, Jet compared with that of the single grain intragranular critical-current density, Jet. Conversely, in order to maintain a high Jet in high magnetic fields, strong pinning is necessary. The location of the intragranular defects and pinning sites occur on an atomic scale s o that instruments with the highest spatial resolution, such as AFM/STM and highresolution TEM, are required to investigate these effects. Because of the critical nature of the grain orientation and perfection, much thin-film work has been carried out by deposition on single-crystal substrates which are lattice matched to the film. This ensures epitaxially oriented grains (mosaic single crystals) with low-angle grain boundaries. However, single-crystal substrates are useful only in certain specialist applications in electronics. For broader applications in tapes and wires for magnets and motors and in lowcost microwave and electronic applications there is a need to deposit films on more practical polycrystalline substrates. The significant problems here are the chemical reactivity and interdiffusion between the film and substrate which occurs at the elevated growth 143
temperatures (600-700 ~ Secondly, high-angle grain boundaries and defects limit the critical-current density. The effect of chemical reactivity can be reduced by the use of buffer layers and optimum grain growth is achieved by adjusting the deposition growth parameters. Some of the best results to date have been reported by Iijima et al. [137], who obtained Jc~ = 22 kA cm-2 at 77 K for YBazCu30 x (YBCO) laser ablated onto a YSZ- (yttria stabilized zirconia) coated Hastelloy C-276. This is a heat-resisting Ni-based alloy which has a thermal-expansion coefficient similar to that of YBCO. This value of J~t is much less than that obtained by r.f.-magnetron-sputtered epitaxial YBCO films, which can yield Jc of more than 1 MA cm -2 at 77 K. There is clearly room for improvement for films deposited on metal substrates. Fortunately there are several process parameters which can be adjusted in sputtering, including the substrate temperature, the substrate bias potential, the vapour impingement rate and vapour supersaturation, the position of the substrate relative to the target and the A r / O 2 ratio. This gives plenty of latitude for optimizing the deposition process. In addition, there are also a number of alternative heat-resisting alloys and buffer layers which can be tried. Diamond-like films act as an excellent diffusion barrier. In addition, the misfit between diamond and YBCO is about seven per cent. This compares with nine per cent for MgO which has been used successfully as a substrate for epit~ixial YBCO films. It may therefore be possible to obtain epitaxial growth on diamond-like films.
7. C o n c l u s i o n s The field of atomic sputtering has been reviewed with particular emphasis placed on the sputter deposition and ion-assisted deposition of thin films. It is noted that atomic sputtering also occurs extraterrestrially on a grand scale, and that laboratory experiments assist in this understanding. The atomic sputtering source can be one of many different types, as summarized in Fig. 5. From the film-growth-theory point of view, the source details are not essential, since it is only necessary to define precisely the parameters of the particles arriving at the substrate. In practical systems, such as glow-discharge sputtering, the source a n d substrate are usually related electrically through the target cathode and substrate anode. In off-axis sputtering, the substrate system can ideally be separated from the sputtering-source electrical system and only acts to adsorb a fraction of the sputtered vapour. However, it is difficult in practice to separate the substrate system from the source, since the particle energies and fluxes arriving at the substrate depend on the position of the substrate relative to the target, and grounding or isolating the substrate perturbs the electric-field distribution. The effect could be minimized by moving the substrate to a distance of at least twice the anode-cathode distance from the plasma, but at a cost of reducing the deposition rate. In the ion-beamsputtering techniques [30], the source of sputtered atoms is independent of the substrate, so this may be 144
a better system for fundamental growth studies. But glow-discharge magnetron sputtering can achieve very high deposition rates. In addition, the plasma provides a large source of positive ions for use in ionassisted deposition as in unbalanced magnetron sputtering or biased sputtering. These techniques are yielding exciting new materials and beneficial effects such as higher film densities and high film-substrate adhesion strength and the formation of new phases not obtainable by any other technique [4]. The growth mechanism of such material is highly complex due to the interaction of energetic particles, ions, electrons and neutral atoms with the condensing film. Nevertheless, attempts are being made to include these interactions in a theory of the early stages of film formation [63]. In general, film formation in sputter deposition follows that in other processes that is, island or layer growth depending on vapour supersaturation. But ion interactions can have a marked effect. In both MBE [56] and r.f. sputtering [57] the growth mode has been observed to change from island growth to layer growth after ion bombardment. This may be explained by an effective reduction in supersaturation arising from higher surface mobility and enhanced equivalent equilibrium vapour pressure. The use of classical thermodynamics in film growth is questionable because of deviations from thermodynamic equilibrium and the small size of the critical nucleus. However, the variables measured are those o f thermodynamics ( P - V - T - x ) , and when the deposited materials are multicomponent the thermodynamic approach is invariably taken [73, 75]. The deposition of HTSs represents one of the great successes of the sputtering technique. High-quality epitaxial HTS material with critical currents of over 1 MA cm -2 can now be achieved using a variety of sputtering approaches as well as other deposition techniques. When considering the complexity of the growth processes, it is rather surprising that goodquality material is obtained from such a wide range of differing techniques. This probably implies more about the skills of the growers and the strong desire to get a result than about the relative merits of the various techniques. However, the high critical-current densities are mostly achieved for well-oriented epitaxial films grown on substrates which are not particularly useful. The present objective is to produce high-quality films on more useful substrates, and to increase the critical-current density in polycrystalline films; this is a great challenge to materials science [135]. The future task is to seek new materials with higher To-values and for this research r.f. sputter deposition is well placed.
Acknowledgements I would like to thank my collaborators and students at Nottingham, in particular Dr Eyo Inametti, Yuk Wan and Bryan Murray, and Professor John Orton and Professor Brian Tuck in the Electrical and Electronic Engineering Department, colleagues in the department of Materials Engineering and Materials Design, Dr ,Peter King and Mr Bill Roys in Physics, D r
Andrew Phillips of GEC., Hirst Research LabQratories and Professor Saburo Iwama and Dr Yoshihisa Sekiya both at DIT, Nagoya, Japan. Part of this research was supported by a grant from the Science and Engineering Research Council.
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