ISSN 10637397, Russian Microelectronics, 2014, Vol. 43, No. 4, pp. 299–307. © Pleiades Publishing, Ltd., 2014. Original Russian Text © R.K. Yafarov, S.A. Klimova, 2014, published in Mikroelektronika, 2014, Vol. 43, No. 4, pp. 305–314.
Modification of Electronic Properties of the Surface of (100) Silicon Crystals under Microwave Plasma Micromachining R. K. Yafarov and S. A. Klimova Saratov State University, Saratov, Russia email:
[email protected] Received August 8, 2013
Abstract—The behavior of the effect of microwave plasma micromachining on electronic properties of (100) silicon crystal surfaces at weak adsorption was investigated. Model process mechanisms and factors providing stable modification of the electronic properties of the silicon surface by the formation of builtin surface potentials determined by chemical reactivity of the used working gases and modes of microwave plasma treat ment were considered. The possibility in principle of the active formation of electronic properties of the sur face of semiconductor crystals to extend their electrophysical and functional properties was shown. DOI: 10.1134/S1063739714030068
INTRODUCTION The topical problems of modern solidstate mate rial science comprise preparation of atomically clean surfaces, formation of quantumsized layers, and het eroboundaries of the required composition and struc tural perfection. It is determined by the high sensitivity of the electronic properties of the materials to defects and structural imperfections. The increased degree of integration of advanced microchips is accompanied by a decrease of the thicknesses of the epitaxial structures used for their manufacturing. As a result of this, the surfacetovolume ratio increases and, subsequently, a part of the surface phenomena increases. It make the problem of perfection of atomic structure of surfaces and transition layers of semiconductor systems created with the use of various manufacturing methods and processing techniques more severe. The retention of strong electrophysical properties of semiconductor systems during processing treat ments requires the mitigation of their effect on the structure and electronic properties near their surface that are significantly different from the electronic properties in the bulk of raw semiconductor materials. A surface reconstruction brings additional modifica tions. The modifications concern redistribution of charge density in the nearsurface region and arising, in addition to intrinsic surface states depending on the interruption of the periodicity of the semiconductor crystal lattice, of the other, extrinsic, surface states. The last ones depend on the perturbation of the perfect surface potential, are concentrated near the defects generating them, and are associated with the used methods of surface treatment. Such modification of surfaces displays itself, first, in weakening the phe nomena associated with the use of field effects, fluo rescence, photo and surface conduction, the varia
tion of the work function, etc. [1, 2]. However, again, it enables forming actively, to some extent, the elec tronic properties of the surface of the treated semicon ductor crystals to impart new functional properties to them. At the present time, lowenergy ion and plasma chemical etching appears the most promising method of preparation of atomically clean surfaces [1, 3]. Dur ing preparation of atomically clean surfaces of crystals in plasmachemical media, it is obviously necessary to take into account the possibility of the formation of chemical compounds on the surface that can substan tially affect the processes of modification of the sur face structure and electronic properties of semicon ductors. For example, the successful results of the treatment of gallium arsenide structures in lowenergy hydrogen plasma, bringing about an improvement of the optoelectronic properties of the devices manufac tured based on them, are widely known [4, 5]. In manufacturing silicon integration circuits (ICs), except surface structure quality, selection of crystal lattice orientation of crystalssubstrates is important, which is predetermined by the structural features of the lattice of solidstate material and devices manu factured based on it. Thus, bipolar circuits are tradi tionally formed on (111) silicon substrates, and MOS devices, on substrates of (100) orientation. It is caused by the fact that the surfacestate density on (100) sub strates is about one order less compared to the (111) orientation [6]. It appears reasonable that plasma treatment in the processes of preparation of atomically clean surfaces and manufacturing ICs can markedly affect the modification of their electronic structure. The purpose of this work was to investigate the influence of lowenergy microwave plasma microma chining in various gas media of silicon wafers of (100)
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crystallattice orientation on the electronic properties of their surface. 1. METHODS AND EXPERIMENTAL RESULTS Investigation of the effect of lowenergy microwave plasma micromachining in various gas media on the electronic properties of the surface of silicon wafers of (100) crystallattice orientation was carried out on the structures usually used for field effect measurements [1, 2], with the difference that not the longitudinal but transverse charge carrier transport was studied. Such a measurement plan was chosen because, according to the studies carried out earlier [7], microwave micro machining in various plasmasupporting media does not provide the uniform nanomorphology of the sur face of silicon wafers. As a result, with the traditional measurement plan of field effects in MIS structures, the contribution of surface states and charges to the charge carrier transport, depending on the type of plasma treatment, might not be estimated sufficiently objectively because of the various diffuse scattering of carriers on the surface nanomorphology of the inter faces. The experiments related to obtaining samples of structures for measurements were carried out in a vac uumpumping assembly with use of the microwave ionplasma source described in [8]. The microwave radiation power and magnetic field induction, corre sponding to the occurrence of electron cyclotron res onance (ECR) in the area of gas discharge, were 250 W and 875 Gs respectively. The pressure of working gases during plasma treatment was equal to 0.1 Pa and ensured that ECR conditions, with the degree of plasma ionization of about 5% [9], were attained. Tetrafluoromethane, argon, and hydrogen were used as working gases to prepare an atomically clean surface of ptype singlecrystal (100) silicon wafers passivated by the thin film of natural oxide. The values of accelerating voltages on the substrate holder in the processes of plasma etching of silicon in all plasma supporting media were static and equal to –100 V. An etching mode with accelerating voltage of –300 V was also used during the hydrogen plasma treatment. According to the nature of the working gas used, the treatment of silicon wafer with natural oxide was car ried out in ionplasma etching modes in the case of argon and in reactive ionplasma etching in the case of tetrafluoromethane or hydrogen. The sputtered sur face atoms of silicon during ionplasma etching the and the siliconcontaining volatile reaction products that appeared in the case of ionplasma etching were exhausted by a vacuumpumping system. After plasma cleaning of the silicon wafers from natural silicon oxide in a single vacuum technological cycle, a deposition was carried out in microwave plasma of mixed ethanol vapor and protective monosi lane (capping) film structures of hydrogenated amor phous silicon carbide (αSiC:H) 10 nm thick. Then,
metal contacts 2 mm in diameter were deposited on the surface of the heterostructure by thermal vapor deposition in vacuum. To carry out comparative eval uations of the influence of the treatment in plasma supporting media on the surface electronic properties of silicon wafers, a control heterostructure, wherein a capping layer (αSiC:H) was deposited on silicon whose surface was not cleaned by plasma treatment, was manufactured. In the obtained structures, transverse electron transport of direct polarity connected to a power sup ply was studied. The currentvoltage relationships (CVRs) were measured by use of an Agilent B1500A Semiconductor Device Analyzer (Agilent Technolo gies, United States). The currentvoltage relationships of the samples were measured under variation of exter nal voltage within the range from 0 to 10 V forward and backward with adjustable duration of carrier injection at every point of the measured CVRs from 1 × 10–5 to 5 s, and also in the steady state in which the CVR val ues were registered after complete stabilization of the readings of the measuring device. In order to avoid thermal electric breakdowns, the maximum current through the structures was limited on 0.1 A. This cor responds to a current density of 3.18 A/cm2. Figure 1 shows the logarithmic CVRs of transverse electron transport at measurements during the com plete stabilization of the readings of the measuring device in film structures αSiC:H obtained in the same modes of deposition on (100) silicon crystals after etching in various plasmasupporting media. The following behavior was observed: when voltage varied within the same range, the maximum current through a heterostructure of silicon–thin film of αSiC:H increased by an order of magnitude in the row of argon–hydrogen–tetrafluoromethane, but the voltage under which the beginning of the current increase is observed, decreased from about 10 V during the etch ing of a natural oxide coating of silicon in argon plasma; 1.3–1.4 V, during hydrogen plasma etching at Ucm = –100 V; and 0.1 V, during etching in plasma of CF4 and hydrogen at Ucm = –300 V; moreover, the parts of the beginning of the increased current after the treatment in argon and hydrogen are violent (almost steplike). Moreover, the current taper was four orders for plasma treatment in argon, more than five orders for the treatment in hydrogen plasma at Ucm = –100 V, and about nine orders from the initial level at the treat ment in plasma of CF4 and hydrogen at Ucm = –300 V. The current densities in the last case measured up to 15 A/cm2 [10]. When the voltage changes backward, the currents through the structures in all cases are higher than at the forward change of the voltage. Figure 2 shows the currentvoltage curves of the film structures on (100) silicon wafers with natural oxide coating and treated in various plasmasupport ing media at various durations of carrier injection in the processes of electrical measurements. It is evident that in the structures in which the oxide coating was
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Fig.1. CVRs of transverse electron transport obtained at forward (1) and backward (2) variation of voltage in film structures αSiC:H on ptype silicon (100), after treatment in microwave plasma of various gas media: argon (a), hydrogen at Ucm = –100 V (b), hydrogen at Ucm = –300 V (c), tetrafluoromethane (on the coordinate axes of the curves, currents are represented in μA, voltages, in V).
not removed on silicon the currents measure the max imum values permissible in the experiments, equal to 0.1 A, and at etching in microwave plasma of tetraflu oromethane and hydrogen when Ucm = –300 V, and when the silicon oxide was removed in argon and hydrogen at Ucm = –100 V, they were several orders smaller. The effect of the duration of the carrier injection during CVR measurements also depends on the media of plasma treatment. For treatments in argon and hydrogen at Ucm = –100 V, the current densities decrease by a factor of 5–6 with duration of injection increasing from 1 × 10–5 s to 5 s. Based on the obtained CVRs in Fig. 3, the dependence of conduction are constructed on the duration of the injection for a volt age of 5.6 V, achieved at CVR measurements for all cases of treatment. It is evident that the conductivity of the structures on silicon without its cleaning from nat ural oxide coating is by a factor of about 5 higher than RUSSIAN MICROELECTRONICS
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at etching in hydrogen at Ucm = –300 V and tetrafluo romethane, and almost does not depend on the dura tion of injection. On removal of the oxide coating in hydrogen at Ucm = –100 V and argon, the conductivity is several orders less. The conductivity is observed to decrease at an increased duration of injection. The effect of the treatment of silicon wafers on the dependences of the conductivities on the duration of the injection depends on the external electric field inten sity. For instance, for a power supply voltage of 10 V, in contrast to the voltage of 5.6 V, the conductivity of the structures treated in hydrogen at Ucm = –100 V is higher than in argon but falls faster when the duration of injection increases. 2. RESULTS AND DISCUSSION As is known [2], the types of CVRs given in Fig. 1 are typical for spacechargelimited currents (SCL 2014
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Fig. 2. Influence of duration of injection in CVRs of heterostructures on (100) silicon with no and at microwave plasma treatments in various gas media (1 × 10–5 s (1), 1 s (2), 3 s (3), 5 s (4)): without treatment (a), in hydrogen at Ucm = –300 V (b), in tetraflu oromethane (c), in hydrogen at Ucm = –100 V (d), in argon (e).
currents) that take place in dielectrics and highresis tance semiconductor materials with carrier capture traps. The theory supposes that these traps form deep lying levels in a forbidden band (so electron thermal kickback can be neglected) and are uniformly distrib
uted through the volume of a semiconductor. The part of the CVRs with a sharp increase of the current, which, in the terminology of SCL currents theory, is referred to as “full trap occupation” (FTO) is the boundary between the mode of low ohmic currents
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existing due to the presence of a quantity of equilib rium conduction electrons n0 in a semiconductor structure, and high currents, relevant to the part of the characteristic with the linear growth of current, which, in the terminology of SCL currents, is called “trapping quadratic law” (TQL). In the part of low ohmic cur rents, the electrons injected into the heterostructure from metal contact are captured by the traps, the spacecharge region is formed, and its induced electric field inhibits the transfer of electrons from the contact to the heterostructure. In the part of the FTO, in the reference time of the sharp increase of the current on voltage, all the traps turn out to be occupied by injected electrons, and then their concentration in the conduction band begins to increase, which results in a sharp increase of the current. In the next part of the CVRs, in the area of the formed spatial charge, the path of the current flow is described by Mott’s law. In this case, the current is proportional to the voltage squared and inversely proportional to the film thick ness cubed:
j = 9 τ M σ 0μU 2 L−3, 8 where τM is the Maxwell relaxation time, μ is the chargecarrier mobility, σ0 is electroconductivity with no injection, U is applied voltage, and L is thickness of the film structure. The voltage of the beginning of the FTO part is related to the concentration of the traps initially unoc cupied by electrons (pо) by the following ratio 2
qp0 L , ε where q is the modulus of the elementary electric charge and ε is the static dielectric capacitivity of the insulator. It follows from this ratio that the long dura U ПЗЛ =
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tion of the parts of low ohmic currents for the hetero structures after plasma micromachining of silicon in argon at Ucm = –100 V is caused by, as compared to the treatment in tetrafluoromethane and hydrogen at Ucm = –300 V, the appearance of additional electron trapping centers that are placed deep in the forbidden band of silicon and cannot be activated at room tem perature. According to modern concepts [6, 7], silicon diox ide on the surface of atomically pure silicon is repre sented by a robust ≡Si–O–Si≡ siliconoxygen scaffold in which the unsaturated bonds of the surface atoms of silicon are bonded with oxygen and, by so doing, the density of surface states on silicon is reduced. It should be appreciated, however, that a natural oxide layer is formed on silicon under atmospheric pressure in con ditions of excess oxygen and uncontrolled chaotic for mation of the siliconoxygen scaffold. This results in the distribution of bridge bond angles Si–O–Si within a wide range, and is accompanied by the formation of high mechanical stresses and structural impurity defects (pores, crystal and foreign inclusions) that transform into additional electronic states placed deep in the forbidden band of the semiconductor. At plasma removal of the oxide coatings, the effi ciencies of both physical sputtering of silicon by ions of argon and hydrogen, as well as the physicochemical interaction in the cases of the use of ions of tetrafluo romethane and hydrogen, are determined by an accommodation coefficient А = 4М1М2/(М1 + М2)2 (at М1 = М2 the accommodation coefficient is maxi mal and equal to 1) that defines a part of the bombard ing ion energy, transferred to a surface ion in elastic collision. In accordance with this, the energy transfer from the bombarding ions to the surface atoms of sili con at the treatment of silicon by argon ions happens 2014
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most effectively, and also in the case of the use of tet rafluoromethane, by CFn+ ions, where n assume the largest values. The least effective treatment, from the point of view of energy transfer, is plasma treatment in hydro gen whose ion mass is smaller by a factor of 40 than argon ion’s. It is confirmed by the fact that a sputtering threshold yield can be estimated by the ratio
AE n ≥ U 0, where U0 is the bond energy of the surface atoms of a material, equal to its sublimation (atomization) energy. А = 0.97 at the treatment of silicon by argon ions, but a sputtering threshold yield Еп = 15 eV. А = 0.13 at the treatment of silicon in hydrogen plasma. As a result of this, the sputtering threshold yield at the hydrogen plasma treatment increases by a factor of 7.5. Therefore, at the same accelerating bias potentials at microwave plasma treatment by hydrogen ions, the efficiency of the energy transfer and physical sputter ing of silicon atoms is almost one order less than in argon plasma. Accordingly, the rate of etching of the same wafers in hydrogen plasma is almost one order less than in argon [3, 7]. At micromachining in lowenergy plasma of argon, with respect to its inertness, no chemical bonds with surface silicon atoms are formed. As a result of this, the silicon surface obtains the structure and density of dangling (unsaturated) bonds, typical for an atomi cally clean surface of silicon of the specified orienta tion of the crystallattice [1]. During the hydrogen lowenergy microwave plasma micromachining, the presence of atomic hydrogen on the Si–SiО2 interface results in its inter action with silicon dioxide, with the following genera tion and desorbtion of water vapour: ≡Si–O–Si≡ + H → ≡Si–OH + Si≡.
(1)
The process of the reduction of silicon dioxide occurs in conditions of highheat generation (4.5 eV/mole) as a result of the generation of molecular hydrogen and deionisation of its ions (13.6 eV). After removal of the oxide coating, the interaction of atomic hydrogen with surface atoms of silicon with the unsaturated chemical bond is the main process: ≡Si + H → ≡Si–H.
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The result of this is valency saturation and a decrease of the surface state density on silicon. The energy necessary for the formation of a chemical com pound SiH is 3.13 eV. That is why the reaction yield depends on the energy of bombarding hydrogen ions. In accordance with the abovementioned informa tion, it is reasonable to expect that at the increase of the bias during plasma treatment in hydrogen from – 100 to –300 V, the yield increases; however, the surface state density decreases.
At ionplasma treatment of silicon in CF4 plasma, the chemicallyactive particles are ions of С+ and CFn+, where n = 1–4, and also radicals CFn and neutral atoms of fluorine [8]. The transfer of two types of par ticles, neutral and ionized, from the plasma, deter mines also two types of their interaction with the sur face atoms: heterogeneous chemical reactions with the generation of volatile compounds and physical effects, related to ion bombardment. At the microwave plasma treatment with the pressures of working gases of about 0.1 Pa, a weak adsorption mode that is defined by a low degree of occupation of the surface by adsorbed particles takes place. In this case, the most probable process is the etching process, wherein a molecular ion, for example CF3+, accelerated by the electrical bias on impact with the surface dissociates into atoms of carbon and fluorine (ioninduced (impact) dissociation of molecular ion): +
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In the modes of microwave plasma treatment, at ion energies of about 100 eV, when the processes of physical sputtering can be neglected, the etching of sil icon in fluorinecontaining plasma can start only as a result of the formation of the Si–F bonds: R ≡ Si + F0, R = Si–F, (4) where R is the crystal lattice of silicon. Removal of chemisorbed complexes Si–F from the silicon surface is a difficult problem because of the high chemicalbond energy (5.6 eV). We note the most prob able mechanisms of their removal from surface [9]: 1. Generation of chemical intermediates— adcomplexes SiF2, which can comparatively easily (the threshold energy of radiationenhanced desorb tion for them is a fraction of eV) be desorbed from the surface of the silicon by ion impact: (5) SiF2surf + E i → SiF2 ↑ , where Еi is the ion energy necessary for desorbtion from the surface of the adcomplexes SiF2surf. 2. Formation of an easilyvolatile compound SiF4 as a result of the interaction of two adcomplexes SiF2 between each other. It is evident from the reaction (3) that, during etch ing in the plasma of CF4, carbon atoms arise on the sil icon surface and conduce the reduction and removal of natural oxide on silicon as CO in the early stages of the process. After removal of the silicon oxide, the car bon adatoms, due to surface migration and chemi sorption, can form Si–C adcomplexes that, as Si–F, have high chemicalbond energy (4.55 eV) and inhibit the etching process, by slowing it down [8, 9]. Chemisorbed complexes SiC and SiF formed as a result of plasmachemical micromachining of silicon in the tetrafluoromethane medium, in the same way as at plasma micromachining in the hydrogen medium, passivate dangling (unsaturated) chemical bonds of
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surface atoms of silicon and decrease the surface elec tronic state density. From a comparison of the dissociation energies of the adcomplexes SiH, SiC, SiF, and SiO, which are equal to 3.13, 4.55, 5.6, and 8.29 eV, respectively, it is arguable that, after removal of the oxide coating of sil icon in microwave plasma and the following deposi tion of amorphous silicon carbide in a single vacuum cycle, the density of the unsaturated bonds and surface electronic states on the heteroboundary must have a tendency towards decreasing on transition from micromachining in argon plasma, to micromachining in a hydrogen and tetrafluoromethane medium with Ucm from –100 to –300 V. This is determined by the increase of the concentrations and strengths of the chemical bonds of the corresponding adcomplexes that passivate unsaturated bonds of surface atoms of silicon during cleaning from film contaminations. The maximal surface density of the unsaturated bonds, corresponding to an atomically clean surface of sili con, is implemented, as it was evident in plasma micromachining in an argon medium, as mentioned above. The considered impuritystructural changes of the surface densities of unsaturated bonds of silicon atoms at microwave plasma micromachining in various chemically active media are correlated to the corre sponding type of the obtained CVRs and conclusions that follow from the theory of SCL currents. In partic ular, this applies to the determined behavior of the variation of the durations of the parts of low ohmic currents, which depend on the appearance of addi tional electrontrapping centers in the forbidden band of silicon after the respective plasma micromachining. The formation of unsaturated electron bonds on the surface of рtype silicon during etching with the purpose of removing the natural oxide coating in microwave plasma of various plasmasupporting media results in the appearance of electronic states localized on the surface of the semiconductor. Depending on the degree of electron and hole affinity and position of the Fermi level, on the surface, these states can prove themselves as donor and acceptor traps or recombination centers [2]. The strength of the unsaturated electron bonds is substantially less than the strength of the saturated covalent bonds taking place in equilibrium electronic configurations of vol ume atoms. Again, the presence of surface electronic states on the surface of the semiconductor perturbs the electronic structure in the volume and makes it ener getically less beneficial. That is why the surface atoms of silicon with unsaturated chemical bond ≡Si act as hole traps, taking the positive charge at the loss of an electron to conduction band of the semiconductor (such surface states, as is known, are referred to as a “donor state”): ≡Si + h+ → ≡Si+. (6) This leads to the formation, on the surface, of positive charge trivalent silicon whose surface density is deter RUSSIAN MICROELECTRONICS
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mined by the unsaturated electron bonds’ density and depends on the type of plasmasupporting media as indicated earlier. In the presence of a positive charge in the localized surface states, electroneutrality in the nearsurface region of a semiconductor is provided because the internal electric field occasioned by the charge Qss conduces the rearrangement of the mobile carriers and initiation of a double electrical layer shading the bulk of the semiconductor from the action of this field. As for highlyalloyed acceptortype semiconductors, the presence of a positive surface conduces, as is known, the depletion of the nearsurface region by the main charge carriers and, hence, it takes a negative charge. The nonuniform charge distribution causes an elec trostatic builtin potential with the electric field directed to the bulk of the semiconductor. The larger the surface charge, which in the case of a free surface and with no external fields is equal and opposite in sign to the charge in the surface states, the larger the width of the spacecharge Qsp region and resistance of deple tion layer. The strength of electrostatic fields generated by the singly ionized surface atoms of silicon is in proportion to the surface density of the charges and can be, for example, for Ns = 1015 cm–2 equal to about 108 V/cm [2]. At the direct connection of the studied hetero structure on рtype silicon to a powersupply, the internal field initiated by surface charges is directed opposite to the external field that conduce the weak ening of the latter. As a result of this, a transverse cur rent through the structure at variation of the external voltage arises only after the intensity from the external field exceeds the field intensity from builtin charge. At microwave plasma treatment of silicon in hydro gen and tetrafluoromethane at Ucm = –300 V, as noted above, as a result of the formation of adcomplexes SiH, SiC, and SiF, according to the reactions (2–4), the value of surface charge Qss decreases compared to the treatment in argon and hydrogen at Ucm = –100 V, and the internal electrostatic field and degree of deple tion of the nearsurface region by majority carriers decrease as a consequence. As a consequence, the threshold of the engagement of conductivity in the heterostructure after plasma treatment of the silicon substrate in a argon and hydrogen medium at Ucm = ⎯100 V is higher than at the treatment in tetrafluo romethane and hydrogen at Ucm = –300 V. The need to cross a builtin potential to implement the transverse electron transport in heterostructure results in a decrease of the effective voltage. This decrease is higher the higher the builtin potential. As a consequence, on supplying the same value of the external electric field, the electrical conductivity of the heterostructures will increase at the changeover from argon treatment with the maximal density of unsaturated surface bonds and builtin potential to the treatment in hydrogen and tetrafluoromethane (Fig. 3). After crossing the internal electrostatic field on the boundary of the semiconductor and αSiC:H, and 2014
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2×1
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Fig. 4. Schematic view of a structure of ordered surface phases H/Si (100): Si (100) 2 × 1H (dihydride) (hydrogen atoms are shown as black circles) [1].
occupation of all traps, the thermodynamic equilib rium for the specified conditions the charge carrier’s transport, at which the injected charge of the carriers compensates all the bound electrostatic charges, is established inside the structure volume. This state is achieved after a sharp increase of the current on the CVRs. At the change of the external voltage backward, the gradual decrease of the spatial charge of mobile carriers happens since that moment, on the studied structure, as a result of the decrease of the external injection. This leads to a decrease of the current pass ing through the structure that however remains higher than at the same voltage but at the forward change, when not all the traps were occupied. At even lower external voltages and mobile carrier concentrations, when their spatial charge becomes comparable with the electrostatic builtin potential, the carrier mobility sharply falls due to the change of the scattering condi tions. The voltage at which the current is cut off defines the value of the internal electrostatic field. This explains why during the treatment of silicon in argon, where the surfacecharge densities are highest, the volt ages of the switch to the initial nonconducting condi tion are higher than at the treatment in hydrogen. A decrease of the densities of the surface electronic states on the Si–SiC heteroboundary at the changeover from the micromachining of silicon wafers in the plasma of argon and hydrogen with Ucm = –300 V and tetrafluoromethane reduces the reaction of the heterostructure to the external voltage. Thus, it is evi dent in Fig. 2 that in the structures with natural oxide coating and removed oxide coating under plasma treatments in tetrafluoromethane and hydrogen at Ucm = –300 V, the type of CVRs remains almost con stant on variation of the duration of the injection but varies significantly after the treatment in argon and
hydrogen at Ucm = –100 V. In particular, the currents through the heterostructure decrease by a factor of 5–6 (Figs. 2d and 2e) at an increase of the duration of the injection from 1 × 10–5 to 5 s. This is determined by the processes of the reconstruction of the electronicatomic structure of the silicon surface, under the action of the external electrostatic potential, that are more intensive than during the treatments in other plasma media. When the density of unsaturated bonds is high, to reduce the free surface energy, atoms shift from their initial positions to form bonds with each other and sat urate them [1]. The continued decrease of surface energy happens due to the charge transfer between the remaining unsaturated bonds (such a mechanism is called selfcompensation). So, for (100) silicon crystals, as a result of the reconstruction, the atoms of the upper layer are grouped into rows (dimers) with the formation of σbonds parallel to the surface and a halving of the quantity of the surface atoms. The rows are formed by the dimers and the surface has periodicity of 2 × 1. The bonds remained dangling, as a rule, though weaker but also react to each other with the formation of πbonds, which further decreases the surface energy. The selfcompensa tion processes are implemented most intensively after plasma treatment in an argon medium. Three different ordered phases H–Si (100) can be formed under the plasma treatment of silicon in an argon medium, depending on the temperature of the substrate, as well as the energy and doses of atomic hydrogen [1]. The monohydride phase Si(100)2 × 1–H is formed on the unsaturated bonds of silicon at low tem peratures and light doses. Moreover, the structure of the Si–Si dimers remains unchanged (Fig. 4). It is evi dent that the optimum conditions of its formation is plasma treatment at low hydrogen ion energy that is implemented at Ucm = –100 V. A dihydride phase Si(100)1 × 1–H is formed when the entering hydrogen atoms rupture the bonds in the dimers Si–Si and saturate the formed unsaturated bonds. In this case, etching of silicon is possible due to the formation of silicon tetrahydride vapor. The reac tion yield of the formation of the dihydride surface phase increases at the plasma treatment at Ucm = –300 V. In this case, almost all the weak surface bonds on silicon are ruptured and become saturated by hydrogen atoms. Moreover, the surface electronic states have minimal density and builtin surface charge. A mixed phase Si(100)3 × 1–H is formed under average treatment con ditions. In this phase, the rows of the monohydride phase interchanges with the rows of the dihydride phase. That is why the mixed phase, as well as the monohydride phase, have the property of changing the electrophysical proper ties of the surface layers of silicon on applying sufficiently strong external electric fields (Fig. 2d). Therefore, the surface unsaturated bonds, at argon and hydrogen plasma treatment at Ucm = –100 V, form relatively weak surface electron bonds on the surface of silicon by means of selfcompensation. At sufficiently strong external electric fields and durations of injec tion, these bonds can be ruptured with the formation
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of additional surface electronic states. This results in an increase of the surface charge and builtin potential and, as a consequence, in a decrease of the transverse conductance in the heterostructures (Fig. 3). The sur face state density and electronic properties of the structures provide further proof against the action of external fields and durations of carrier injection (Fig. 2) after plasma treatment of silicon (100) in tetrafluo romethane due to the formation of sufficiently strong SiC and SiF bonds, and also at the plasma treatment in hydrogen at Ucm = –300 V, by means of the rupture of relatively weak dimer bonds and the formation of dihydride surface phase Si(100)1 × 1–H. CONCLUSIONS Lowenergy microwave plasma micromachining of singlecrystal silicon wafers of (100) crystallattice ori entation in various chemically active gas media allows the electronic structure and properties of the surface to be modified (restructured) in different ways. The modification of the electronic structure is related to the redistribution of the charge density in the near surface region and is determined by the nature of the used plasmachemical media. Their activity in rela tion to the atomically clean surface and strength of the formed chemical bonds conduce the formation of steady saturated bonds of surface atoms of silicon. The unsaturated surface bonds take part in the restructur ing of the electronic structure of the nearsurface region of a semiconductor. It results in the occurrence of electrostatic builtin potential and a change of the nearsurface electrophysical properties that, except for the chemical activity of a plasmasupporting medium, depend on the treatment mode and external electrostatic potential in the processes of testing or operation of heterostructures on silicon. The latter, depending on its value, can induce the rupture of rela tively weak selfcompensation of electron bonds between the surface atoms of silicon with the forma tion of new unsaturated bonds. These factors in the sequel determine the behaviors of transverse electron transport in the film heterostructures manufactured on silicon wafers after the respective plasmachemical treatments. At lowenergy plasma micromachining in an argon medium as a result of its chemical inertness on the sur face of silicon wafers, unsaturated electron bonds with maximal surface density are formed on the surface of the silicon wafers. This induces the strongest changes of the nearsurface electrophysical properties of sili con. Thus, transverse transport in the αSiC:H het erostructure on ptype silicon (100) after plasma micromachining in an argon medium is implemented under direct the switching of power supplies only at intensities of about 1 V/nm, which corresponds to ion ization of almost all surface atoms of silicon [2]. Plasma micromachining in a tetrafluoromethane medium and in a hydrogen medium at Ucm = –300 V cre ates and passivates the major part of the dangling bonds of RUSSIAN MICROELECTRONICS
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the surface atoms of silicon as a result of the formation of strong and steady chemical bonds SiC, SiF and SiH. The builtin potential and nearsurface spatial charge on silicon, in these cases, is less than under hydrogen treatment at Ucm = –100 V and argon, and pro vides further proof against the action of external electric fields. The CVRs of the transverse electron transport for such cases have a smaller size of the parts of low ohmic currents and higher stability of electrophysical properties under the action of external electric fields. RECOMMENDATIONS The work was supported by the Ministry of Educa tion and Science of the Russian Federation. REFERENCES 1. Oura, K., Lifshits, V.G., Saranin, A.A., Zotov, A.V., and Katayama, M., Vvedenie v fiziku poverkhnosti (Introduc 1 tion to Surface Physics), Moscow: Nauka, 2006. 2. BonchBruevich, V.L. and Kalashnikov, S.G., Fizika poluprovodnikov (Semiconductor Physics), Moscow: Nauka, 1977. 3. Danilin, B.S. and Kireev, V.Yu., Primenenie nizkotem peraturnoi plazmy dlya travleniya i ochistki materialov (LowTemperature Plasma Applications for Etching and Cleaning of Materials), Moscow: Energoatomiz dat, 1987. 4. Dvoryankina, G.G., Telegin, A.A., Dvoryankin, V.F., Petrov A.G., Kyarginskaya L.G., Ushakov N.M., Averin S.F., Vydutz V.E., Petrosyan V.I., Elenkrig B.B., Electric and Photoelectric Properties of Schottky Bar riers on the AlnGaAs/nInGaAsP/iInP Structures, Mikroelektronika, 1989, vol. 18, no. 5, pp. 412–415. 5. Ushakov, N.M., Terent’ev, S.A., and Yafarov, R.K., Photoelectric Properties of Planar Structures with Double Schottky Barrier, Treated in HighVacuum Microwave Discharge, Pis’ma Zh. Tekh. Fiz., (Techni cal Physics Letters), 2002, vol. 28, no. 15, pp. 10–16. 6. Tekhnologiya SBIS (VLSI Technology), Ed. by S.M. Sze, McGrawHill book Company, 1983 7. Shanygin, V.Ya. and Yafarov, R.K., Relaxation Self organizing of Silicon Crystals Surface under the Action of Microwave Plasma Micromachining, Fiz. Tekh. Polupro vodn. (St.Peterburg), 2013, vol. 47, no. 4, pp. 447–459. 8. Shanygin, V.Ya. and Yafarov, R.K., Preparation of Atomically Clean Surfaces of Silicon in lowenergy microvawe lowpressure plasma, Zh. Tekh. Fiz., 2009, vol. 79, no. 12, pp. 73–78. 9. Yafarov, R.K., Fizika SVChvakuumnoplazmennykh nanotekhnologii (Physics of Microvawe Vacuum Plasma Nanotechnologies), Moscow: Fizmatlit, 2009. 10. Budko, D.V. and Yafarov, R.K., Anomalous Transverse Electroconductibility in TunnelThin semiconductor structures based on αSiC:H, Tezisy dokladov VI Vse rossiiskoi konferentsii molodykh uchenykh “Nanoelek tronika, nanofotonika i nelineinaya fizika” (Abstracts of VI AllRussia YoungScientists Conference “Nano electronics, Nanofotonics, and Nonlinear Physics”), Saratov.
Translated by G. Levina 2014
