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Barium strontium titanate thin film varactors for room-temperature microwave device applications
This article has been downloaded from IOPscience. Please scroll down to see the full text article. 2008 J. Phys. D: Appl. Phys. 41 063001 (http://iopscience.iop.org/0022-3727/41/6/063001) View the table of contents for this issue, or go to the journal homepage for more
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IOP PUBLISHING
JOURNAL OF PHYSICS D: APPLIED PHYSICS
J. Phys. D: Appl. Phys. 41 (2008) 063001 (21pp)
doi:10.1088/0022-3727/41/6/063001
TOPICAL REVIEW
Barium strontium titanate thin film varactors for room-temperature microwave device applications P Bao, T J Jackson, X Wang and M J Lancaster Department of Electronic, Electrical and Computer Engineering, University of Birmingham, Birmingham, B15 2TT, UK E-mail:
[email protected]
Received 10 October 2007, in final form 10 December 2007 Published 29 February 2008 Online at stacks.iop.org/JPhysD/41/063001 Abstract Recent progress in the development of barium strontium titanate thin film varactors for room temperature tunable microwave devices applications is reviewed, with emphasis on efforts towards the improvement in the quality of BST thin films and the fabrication issues crucial for the performance of microwave devices based on BST varactors. The paper provides examples of tunable microwave devices employing BST varactors. Other thin film materials currently competing with BST thin films are discussed. Topics which deserve further investigation are suggested.
loss tangent and increasing the tuning range, and this review focuses on the material needs and outstanding issues pertinent to microwave applications. Ferroelectric materials are attractive for microwave devices due to two factors: the strong electric-field-dependent dielectric permittivity and the relatively low dielectric loss at microwave frequency. Most ferroelectric materials have high dielectric permittivity, especially at temperatures near the ferroelectric phase transition. The electric-field-dependent dielectric permittivity is commonly described by either of the two quantities. The tunability n is defined as the ratio of the dielectric permittivity at zero electric field bias to the permittivity under electric field bias, E. The relative tunability nr is defined as the relative change of the permittivity between zero bias and bias in a field E with respect to the zero-bias value. These quantities are both given in equation (1):
1. Introduction The concept of the application of ferroelectric materials for microwave devices can be traced back forty years to the 1960s [1]. However, only in the last twenty years have intensive efforts been made in this area, due to the enhancements of both the device electronics and material technology [2]. The driving force is ferroelectric thin film technology which enables the miniaturization of microwave components and the potential integration with semiconductor microelectronic circuits [2]. The potential applications for ferroelectric thin film varactors arise across the communications technology sector. For example, greater flexibility, lower cost and lower size could be achieved in handheld devices incorporating tunable circuits. The use of tunable capacitors in matching networks in antenna and amplifiers results in more efficient use of power. Electronically steerable systems may save space and weight in satellite communications and radar systems. There are several reviews covering different aspects of tunable ferroelectric materials and their applications [2–8]. Barium strontium titanate (BST) continues to be one of the most intensively studied materials. There has been recent progress in material technology, particularly in lowering the dielectric 0022-3727/08/063001+21$30.00
n = ε (0)/ε (E), nr = (ε (0) − ε (E))/ε (0).
(1)
The symbol ε is used because the permittivity is a complex quantity: the real part is used in equation (1). A schematic of the field-dependent dielectric permittivity of a BST thin film 1
© 2008 IOP Publishing Ltd
Printed in the UK
Normalized permittivity
J. Phys. D: Appl. Phys. 41 (2008) 063001
Topical Review
Table 1. Comparison of the properties of semiconductor GaAs, MEMS and ferroelectric BST thin film varactors. 1.0
GaAs Tunability (n) RF Loss (Q)
0.5
0.0 -1.0
-0.5
0.0 0.5 Normalized bias
Control voltage Tuning speed Reliability Cost Power handling
1.0
Figure 1. Calculated normalized dielectric permittivity of a BST thin film as a function of the bias electrical field, using the formulae described in [9]. The electrical bias field has been normalized to the maximum applied field.
∼2–6 : 1 ∼20–50 at 10 GHza 10−5 s Poor High Good
2, (3) low cost of production on inexpensive substrates, and (4) reliability and reproducibility [14]. In fact, there are other varactor technologies currently competing with the ferroelectric thin film varactors, such as GaAs semiconductor varactors and micro-electrical-mechanical systems (MEMS) varactors. MEMS varactors are the closest functional equivalent to ferroelectric thin film varactors and at a similar level of development maturity [8]. The most important feature of MEMS varactors is their very high Q value. However, their response is relatively slow compared with BST and GaAs varactors and a very high operating voltage is required. The reliability of MEMS devices should also be considered. Though MEMS varactors may be used for more than 10 billion operations, they are sensitive to environmental conditions such as air moisture, temperature and vibrations.
2. Deposition of BST films BST thin film varactors have been fabricated on different substrates by various deposition methods, including pulsed laser deposition (PLD), magnetron sputtering, chemical solution deposition (CSD) and metal organic chemical vapour deposition (MOCVD) [15–29]. A typical film thickness is of the order of 500 nm. During the PLD process, material is removed from the BST target by an intense UV laser pulse within a very short time interval; therefore, the stoichiometry of the BST films is maintained to be the same as that of the target. Experiments have proved that BST thin films can be easily grown epitaxially on a wide variety of crystalline substrates [15, 16]. The microstructure and physical properties of BST films can be tailored by the growth parameters during the PLD process, such as the growth temperature, oxygen ambient pressure, substrate–target distance, laser repetition rate and the laser energy. For BST thin films, the typical growth conditions are a growth temperature of 750 ◦ C, a laser fluence at the target of 1.5 J cm−2 and oxygen ambient pressure of 0.1 mbar. The fact that PLD can operate at high ambient pressure is an advantage for the in situ deposition of multicomponent oxides [17]. Given that the laser may be shared between several deposition systems, the PLD method is widely regarded as one of the cheapest deposition methods for research purposes [17]. 2
J. Phys. D: Appl. Phys. 41 (2008) 063001
Topical Review
Figure 2. Illustration of (a) a coplanar BST varactor. Top: 3D view. Bottom: side view. (b) A parallel plate BST varactor. Top: 3D view. Bottom: side view.
Compared with PLD, RF magnetron sputtering has the advantage of large-area deposition capability and precise thickness control [18,19]. Nevertheless, it is worth considering what is exactly meant by a ‘large area’. For example, PLD is one process under consideration as a method for producing long lengths of tapes of high-temperature superconductor coated conductors where a large area really refers to a long lengths of a narrow substrate which may be spooled through the deposition environment [20]. In the RF magnetron sputtering method, the stoichiometry of BST films sputtered is normally different from that of the target and therefore can be difficult to control precisely. The chemical solution deposition method (CSD) has also been used for the deposition of BST thin films for tunable microwave device applications and dynamic random access memory (DRAM) applications [21–25]. The lowcost, effectiveness and large-area deposition capability make it attractive [26]. The stoichiometry of the film can be well controlled and dopants can be introduced into the films easily. In contrast to the methods discussed above, in CSD the film is grown from precursor molecules and is not grown directly from a BST target. Thin films fabricated by the CSD method are often polycrystalline. The films often suffer from thickness non-uniformity, surface roughness and the existence of cracks and polycrystalline voids. The voids in BST thin films will lower both the dielectric constant and tunability, both of which can be understood using a model of the dielectric behaviour of composites [2]. A recent encouraging result was the deposition of highly (0 0 1)-oriented, Mn-doped BST thin films onto (0 0 1) MgO single crystal substrates using the sol-gel technique, one of the commonly used CSD methods [27]. BST films with very good crystalline and dielectric properties have also been fabricated by metal organic chemical vapour deposition (MOCVD) [28]. MOCVD is a chemical vapour deposition method, suitable for the deposition of multilayer thin films on certain substrates, with precisely controlled thickness and properties. It offers good material distribution capabilities and reliable quality [29]. However, the cost and the high deposition temperature have impeded widespread application. To date, therefore, most of the work reported in the literature concerns films prepared by PLD and sputtering.
3. Characterization of BST thin films at microwave frequencies BST thin film varactors are most commonly used in two forms: in the coplanar type varactor and in the parallel plate type varactors. Interdigital capacitors are also sometimes used [30–33]. Figure 2 shows the layout of coplanar and parallel plate devices. Different methods should be employed to characterize the dielectric properties of each over different frequency ranges. In the development of memory technology, measurements of the capacitance, polarization loops and leakage currents of the films in the ferroelectric state have been made. The capacitance and polarization measurements are generally performed at frequencies in the kilohertz–megahertz range. Microwave devices generally, but not always, utilize films in the paraelectric state, to achieve high tunability while avoiding hysteresis. The most common parameters to be determined are the permittivity, the tunability and the loss tangent of the film in the microwave frequency range. At these high frequencies, there are several extra considerations to be taken over the low-frequency electrical characterization. Measurements are generally made using a two port vector network analyser, which determines the scattering (S) parameters for waves launched onto the device [34]: after calibration of the measurement using accurate standards, the transmitted and reflected powers are measured for signals launched from port 1 and port 2. The amplitude and phase of the S parameters are recorded. Transmission-line structures, of which the coplanar type varactors is a simple example, as shown in figure 2(a), will be physically large compared with the microwave wavelength so the capacitance and inductance are distributed. They are therefore generally modelled using an electromagnetic simulation software as well as an equivalent circuit analysis, because it can be difficult to accurately identify the components of the equivalent circuit with precise parts of the structure. It is also important to design the lines (using the gap width and the centre conductor width as parameters) to be close to 50 impedance, to reduce the reflection of signals. The typical dimensions of the gap and the centre conductor are several tens of micrometres. The insertion loss of a device or a test structure includes both reflection ‘losses’ and attenuation due to electrical loss in the conductor and in the dielectric: the use of S parameters ensures that both are taken into account. 3
J. Phys. D: Appl. Phys. 41 (2008) 063001
Topical Review
Figure 3. The RLGC equivalent circuit of a transmission line.
Once the S parameters have been determined the coplanar line may be modelled as an RLGC transmission line, as shown in figure 3. The resistance per unit length, R, is identified with the conductor loss, the inductance per unit length, L, with the layout of the conductors, the capacitance per unit length, C, involves electric field energy stored in the air gap above the lines, in the film and in the substrate and the conductance per unit length, G, is associated with the dielectric loss. The permittivity and the loss tangent of the BST thin film itself in these structures can be obtained using the conformal mapping method [30]. The analysis just outlined assumes that the waves propagate in a TEM mode, so that the static and timevarying field distributions are equivalent. If the conductor is particularly lossy, a significant component of the electric field may exist along the direction of propagation which invalidates the TEM assumption. Resonant methods can also be used for the dielectric characterization of BST thin films, especially for those with small dielectric loss, where measurement of the quality factor of the resonance allows a precise determination of the loss tangent. However, resonant methods can only provide information at the resonant frequency and the frequencydependent dielectric properties of BST thin films are not available using resonant methods. A detailed discussion of the merits of various measurement methods may be found in [35]. For parallel plate BST varactors, the loss from the electrode is often negligibly small at low frequency, so the loss tangent of the BST thin film is identical to the measured loss tangent of the device under test (DUT). At low frequencies, electrode areas or ‘pad sizes’ of the order of 1 mm2 are common. At microwave frequencies these lead to a very low impedance, essentially a short circuit. Thus much smaller electrode dimensions, of the order of tens of microns, are required. This leads to a further requirement on the permittivity of useful material: ideally this should not be too high as it reduces the impedance, requiring a smaller electrode area and thus creating more conductor loss. Nevertheless, a high tunability is required. The measurement of dielectric properties at microwave frequency in the parallel plate structure is complicated because the loss from the metal electrodes and the lead strips connecting to the electrodes will be significant and the inductance L in the DUT will affect the apparent capacitance. A ‘Schottky-like’ structure has been used for the dielectric measurement of parallel plate BST thin film varactors [14]. ‘Open’ and ‘short’ circuit structures were used to remove the parallel and series stray admittance [36]. The methods for the extraction of dielectric parameters of BST thin films in parallel plate varactors structures at microwave frequency have been discussed by several authors [14, 36, 37]. Circuit modelling methods can also be used to remove the parasitic L and series R in the test structure. However, there is always some arbitrariness existing in the circuit modelling
Figure 4. Schematic diagram of the simplest parallel plate BST varactor. Top: top view; bottom: cross-sectional view.
method because the L and R are not exactly known. It should be stressed that very careful calibration should be performed to reduce the measurement error for the highfrequency measurement of parallel plate capacitors. It was found that the error in the absolute value of L introduced in the calibration process, for the compensation of L from the probe tips, can dramatically affect the frequency dependence of the measured capacitance and dielectric loss [37]. The simplest test structure, as shown in figure 4, was found to be the best for the characterization of the dielectric properties of BST thin films used in parallel plate type varactors [37]. One of the advantages of this test structure is the minimized parasitic R and L. It is also the easiest for fabrication. The microwave and audio frequency ranges may be connected by the use of a high-frequency impedance analyser. Instruments with an upper frequency limit of 3 GHz are available. Thus it is possible to use the same measurement structures to measure the dielectric properties of a film from dc to 110 GHz. Recently, near-field microwave microscopy has been used to measure the permittivity, loss tangent and tunability of ferroelectric thin films [38, 39]. This method can ‘map’ the dielectric response to sub-micrometre spatial resolution, but careful modelling is required to separate out the dielectric properties of the film from the substrate. This technique is particularly useful in the rapid screening of material ‘libraries’ prepared through combinational synthesis [40]. As with all other microwave measurements, calibration, in this case, against known standard materials is required throughout the frequency range so that truly quantitative data can be obtained. In microwave devices and test structures, metallic electrodes are used to provide a low electrical resistance. Oxide conductors, frequently used in memory-type structures and which provide good lattice matching between film and substrate, are generally too resistive for these applications. As mentioned above, the dielectric tunability and dielectric loss tangent are two important parameters of BST thin films for microwave applications. It is found that in BST thin films a high dielectric tunability is often accompanied by a large permittivity and a high dielectric loss. Therefore, a trade-off between dielectric tunability and dielectric loss is necessary to evaluate the quality of BST thin films in microwave applications. 4
J. Phys. D: Appl. Phys. 41 (2008) 063001
Topical Review
The figure of merit K, defined by the ratio of the relative tunability and the dielectric loss tangent, is often used for the comparison of the quality of BST thin films. K is used because it has a simple form and can reflect the trade-off between tunability and dielectric loss. However, K does not have a well-defined physical meaning. An alternative measure of the performance of a material or device, named the ‘commutation quality factor’ (CQF), was proposed by Vendik et al to characterize a two-state, one port switchable network [41]. The derivation of the CQF of a ferroelectric capacitor is based upon a series resistor model of the capacitor. The CQF is important because it can be used for determining the available minimum insertion loss of a switching microwave component. The CQF for a ferroelectric varactor can be calculated as CQF = (n − 1)2 /(n · tan δ1 · tanδ2 ).
(3)
In equation (3), n is the tunability of the varactors defined in equation (1); tan δ1 and tan δ2 are the loss tangents of the ferroelectric varactor under zero and non-zero bias voltage, respectively. Clearly, both K and CQF are strongly related to the applied bias voltage and to the gap (the distance between conductors in figure 2(a) or the thickness of the film in figure 2(b)) across which the voltage is applied. A small voltage across the thickness of the film in a parallel plate structure presents a stronger electric field bias than the same voltage across the gap between electrodes in a coplanar structure. For the device engineer, the voltage required for tuning is most significant, and in the device literature, often only the voltage is specified. However for purposes of comparing materials, the electrical field strength used in a measurement must be clearly stated. To evaluate the quality of the BST thin films, the dielectric tunability, dielectric loss and figure of merit under a defined electrical field bias are all important. Both K and the CQF as originally defined may be quoted either for the whole device (test structure or circuit) or for the BST film itself, after separation of the conductor and dielectric loss. The former version is more helpful to the device engineer but the latter is more useful in comparison of materials. The confusion arises because tunability may be related simply to capacitance and because the loss tangent is defined in circuit theory as the ratio of the real and the imaginary parts of the impedance. Thus it is also important to be clear about whether or not the electrode or conductor loss has been separated and removed from the loss tangent used in equation (2). When calculating the electric field used for the bias, it may be assumed without significant loss of accuracy [42] that the macroscopic electric field in the ferroelectric is spatially uniform: in the coplanar varactors the field is parallel to the plane of the film; in the parallel plate varactors it is perpendicular to the plane. Only when the gap becomes comparable to the thickness of the ferroelectric layer will edge-effects and non-uniformities become quantitatively significant. The tunability, dielectric loss tangent and commutation quality factor of BST thin films with different barium (Ba) contents from 0% to 50% may be predicted from a model based on damped, coupled oscillators [43]. The results of such
Figure 5. Calculated (a) tunability, (b) dielectric loss tangent and (c) CQF for STO, BST10/10, BST20/80, BST30/70, BST40/60, BST50/50 in the paraelectric phase under an electric field bias of 250 KV cm−1 , using the method described in [43]. In the calculation, ξs is set as 0.8 and other parameters are same as those used in [43].
calculations are shown in figure 5. Note that the calculations leading to figure 5 were performed for the paraelectric state only, the lowest temperature on the curve for each composition is the Curie temperature, TC . It is found that, for all calculated compositions, BST thin films have similarly sized maximum tunability near the phase transition temperature. The tunability and the loss tangent decrease away from the phase transition. The maximum in the CQF occurs at a temperature higher than TC . At room temperature, the dielectric tunablility, dielectric loss and CQF increase with the increase in the Ba content. Ba50% Sr50% TiO3 (BST50/50) has the maximum tunability and CQF, as well as the maximum dielectric loss at room temperature. These calculations are a useful guide to the desired compositions. The choice of the composition depends on the operating temperature and the specific requirements of the particular microwave application. Note that these conclusions will also be modified by the discrepancies between the calculated and the real dielectric properties of the BST thin films. In real films, effects from strains and defects may be incorporated into the calculations empirically [44]. 5
J. Phys. D: Appl. Phys. 41 (2008) 063001
Topical Review
[47,50], microstructural features including charged defects and structural imperfections which cause local strains and polar regions [51], a ‘dead layer’ near the interface between the substrate and the electrodes [46] and the stoichiometry. In what follows, efforts to understand and improve the dielectric properties will be discussed in the context of the general categories given above.
4. Efforts to improve the quality of BST films The physical properties of ferroelectric thin films have been found to be substantially different from those of bulk materials in most cases [45–48]. The dielectric constant tends to be significantly lower in thin films compared with the bulk materials and the dielectric loss tends to be substantially higher. An example of the temperature dependence of dielectric properties of BST ceramics and thin films is shown in figure 6 [49]. The dielectric constant of the BST ceramic is higher and exhibits a sharp peak at the phase transition temperature, while the dielectric constant of the BST thin film is lower and is almost constant over a wide temperature interval. Many reasons have been suggested for the degradation of the dielectric properties of thin films. These include substrate-induced strain due to the mismatch with the film
4.1. The importance of the substrate BST films have been deposited onto different substrates, including oxide single crystal substrates, silicon wafers and metallized substrates. The physical properties of BST films may change significantly when grown on different substrates due to the different internal stress states and the different interfacial properties. It was suggested that the lattice parameters at the deposition temperature are much more important for the layer growth and for development of residual stresses in the film than the mismatch at room temperature [52]. Table 2 lists some important parameters of the substrates often used for BST thin films in microwave applications. The most frequently used are single crystal substrates, such as MgO, SrTiO3 , LaAlO3 , sapphire, NdGaO3 and 0.29LaAlO3 : 0.35(Sr2 TaAlO6 ), known as LSAT. The lattices of these substrates present a close match to BST, enabling epitaxial growth with an island growth mode. With the exception of SrTiO3 , these substrates have a low permittivity and low dielectric loss at microwave frequencies, which are favourable for the fabrication of high performance microwave devices. The drive for microwave devices, as opposed to studies on film growth in their own right, dictates the choice of the substrate and has led to a concentration of efforts on MgO even though it is easier to achieve ‘high quality’ films on alternative substrates. The strain in the thin films is fundamentally significant because it affects the soft phonon mode, which is closely related to the dielectric constant of ferroelectric materials
Figure 6. Variation of the dielectric constant of a BST ceramic and a thin film as a function of temperature [49]. (Reused with permission from Shaw T M, Suo Z, Huang M, Liniger E, Laibowitz R B and Baniecki J D 1999 Appl. Phys. Lett. 75 2129. Copyright 1999, American Institute of Physics.)
Table 2. Physical parameters of substrate materials commonly used in BST varactors. Note that the data are the room-temperature values unless specified otherwise.
Substrate
Space group
Lattice parameter (A)
Relative permittivity εr
Dielectric loss tan δ (at 10 GHz)
Thermal expansion coefficient α (K−1 )
MgO SrTiO3
P m3m P m3m
a = 0.42 a = 0.390 59
R − 3c
LSAT LaAlO3
R − 3m R − 3c
a = 0.475 78 c = 12.99 29 a = 0.3868 a = 0.3793
