Selective epitaxial growth of silicon in a barrel reactor

Chemical Engwerzng Science, Vol. 44, No. 9, pp. M49-2062, Printed in Great Briton.

SELECTIVE

1989 0

EPITAXIAL

GROWTH OF SILICON REACTOR

C. G. TAKOUDIS

ooo9~2509/89 %3.X + 0.00 1989 Pergamoo Press plc

IN A BARREL

and M. M. KASTELIC

School of Chemical Engineering, Purdue University, West Lafayette, IN 47907, U.S.A.

(Receioed 3 October 1988; accepted 31 January Abstract-Selective

allows isolation among devices to be. vastly improved device structures. In this work, selective epitaxial silicon deposition is carried out in a radiantly heated barrel reactor from SiH,Cl,-H,-HCI gas mixtures. Substrate temperatures are between 850 and 1000°C whilereactor pressuresare between25 and 100 torr. All patterned silicon substrates are 3 in. diameter n-type polished wafers of (100) crystal orientation masked with a thermally grown oxide, 0.08-0.87 pm thick. A new quantity that characterizes severai features of silicon SEC in such systems is proposed. This quantity is the ratio of inlet reactor partial pressures P&,/P,. The SEC rate of silicon is found to decrease monotonically (and almost linearly) with increasing P&,/P,. An inversion in the local loading effects is observed as the characteristic quantity P&,/P, increases. The apparent activation energy of silicon deposition from SiH,CI, (DCS) is found to be 33 kcal/mol, while the overall apparent activation energy of silicon SEC is found to be between 50 and 85 kcal/mol for the conditions studied. A detailed mathematical model of SEG in a barrel reactor is also presented. It includes all mass, energy and momentum equations coupled with the special geometry and inlet and exhaust configurations of this reactor. Predictions from the mathematical model developed are shown to be in satisfactory agreement with data obtained in the radiantly heated barrel reactor used. and

offers

numerous

epitaxial

growth

possibilities

(SEG) of

1989)

for

silicon

novel

(SEC?) of silicon makes use of a mask, usually oxide, on a

INTRODUCTION

Epitaxy is a process by which material is deposited onto a crystalline substrate or seed and the crystalline configuration of the structure is maintained. Epitaxy can be classified into four different kinds: molecular beam epitaxy (MBE), vapor phase epitaxy (VPE), liquid phase epitaxy (LPE) and solid phase epitaxy (SPE) (Wolf and Tauber, 1986). VPE has found the widest acceptance because of the possible good control of impurities and the crystalline perfection obtainable. MBE allows for an even better control of layer thicknesses, but suffers from low throughput and high system cost due to the required high vacuum levels. It has, however, drawn considerable attention lately, particularly for the growth of III-V and II-VI structures. LPE is mostly employed for the growth of III-V compounds such as InP and GaAs. SPE has been used to recrystallize layers that have been amorphized by ion implantation or to recrystallize polysilicon (Drosd and Washburn, 1982, Kunii et al., 1983, 1984; lshiwara et al., 1986). VPE has been used from the early stages of device fabrication since it was the only way by which one could obtain a thin layer of lightly doped material on a heavily doped substrate with a sharp transition. Problems associated with VPE, such as autodoping, pattern shift and washout, have been remedied to a certain degree with the introduction of reduced pressure epitaxy (Ghandhi, 1983; Sze, 1988; Wolf and Tauber, 1986). A relatively new branch in VPE is selective epitaxy where the deposition of material is inhibited on certain areas of the wafer (Tanno et aZ., 1982; Endo et al., 1984, Liaw, 1984, Liaw et al., 1986; Borland and Drowley, 1985; Borland, l987a). The selective epitaxial growth

silicon wafer. Seed holes are etched through this mask to the monocrystalline silicon. In SEG, the epitaxial deposition conditions are adjusted to prevent deposition on the oxide mask regions while epitaxial growth occurs on the exposed silicon in the seed holes. If the epitaxial deposition continues long enough, the height of the silicon growth front rises above the top of the mask and starts to grow laterally over the mask. This is typically referred to as epitaxial lateral overgrowth (ELO). Figure 1 illustrates schematically the basic concepts of SEC and ELO. The main advantage, perhaps, of SEG is its ability to achieve small-area dielectric isolation for highdensity circuits (Voss and Kurten, 1983; Kitajima et al., 1983; Ishitani et al., 1984, Pagliaro et al., 1987; Borland and Drowley, 1985; Drowley, 1987; Jastrzebski et al., 1987; Liaw and Seiter, 1987; Borland, 1987a; Borland et al., 1988; Industry News, 1988). Yet, relatively little is known about the chemistry involved in SEG of silicon, and even less seems to be known about the reactor design and reaction engineering of SEG (Kastelic, 1988; Borland, 1987a). This paper includes some new results of SEG of silicon as well as one of the first studies on the reaction engineering aspects of SEG of silicon in a chemical vapor deposition (CVD) reactor, and, in particular, in a radiantly heated barrel reactor.

BACKGROUND

Chemical reactions Four sources are available for silicon epitaxy, namely SiCl,, SiHCl,, SiH,Cl, (DCS) and SiH,. The first three belong to the group of chlorosilanes and the

2049

2050

C.

G.

TAKOUDIS

and M. M.

KASTELIC

Mask Materlal

Si substrate

I Fig. I.

Schematic

cross

sections of selective epitaxial growth (SEG) and epitaxial (Schubert, 1988).

reduction processes are quite similar for these three gases. Silane has not been used widely for epitaxy due to its highly unstable behavior and its tendency to reduce in the gas phase, resulting in the formation of silica dust and successive contamination of the wafers. On the other hand, it does allow one to perform epitaxy at temperatures considerably lower than for any of the chlorosilanes. A silane system is also easier to model since the decomposition of silane is basically irreversible under normal conditions. No HCI is freed by the decomposition of silane, so that one can operate at higher reactant concentrations than with the other gases without going into the etching regime. The most commonly used reactant in the past was SiCl,, which has a rather low vapor pressure, is quite stable and easy to handle. However, DCS and SiHCI, make it possible to operate at about IO&200°C lower temperatures for comparable crystalline morphology and growth rate compared to SiCI, systems. It has been shown that the efficiency of the reaction, defined as the ratio of deposited silicon to the amount of reactant gas entering the reactor, is highest for DCS and lowest for SiCI, (Ban, 1975). The overall reaction for the deposition of silicon from DCS can be written as SiHzClt*Si

+ 2HCl.

(1)

Several researchers (Ban and Gilbert, 1975; Bloem and Claassen, 1980; Claassen and Bloem, 1980b, 1981a, b; Bloem et al., 1982; Claassen et al., 1982; Bylander, 1979; Cadoret, 1982; 1962; Bryant,

lateral overgrowth (ELO)

Duchemin et al., 1978; Farrow, 1974) have investigated the nature of epitaxial silicon deposition with mass spectroscopy. They have found that this reaction goes perhaps through many intermediate steps. It is believed that for the silicon incorporation into a silicon substrate, SiCI, molecules may have to be present. DCS and, to a lesser extent, SiCl, readily dissociate at temperatures of about 1000°C to form SiCI,. The dissociation of DCS forms H, and SiCl,. The SiCl, is believed to be adsorbed at the surface, preferably at kink sites and steps in the surface. The adsorption is reversible and one can characterize the typical time a molecule is adsorbed without undergoing chemical reactions by a time constant. During this time, the adsorbed molecules can move on the surface, a process which can be described by a difTusion constant (Farrow, 1974), till they find an energetically favorable site where they can further dissociate to form silicon and get incorporated into the lattice. In thermodynamic considerations this process is thought of being in a state of heterogeneous equilibrium, i.e. an equihbrium between the solid silicon and all siliconcontaining species in the vapor phase. However, experiments point to the fact that, under typical operating conditions, epitaxial processes are removed quite a bit from equilibrium {Langlais et al., 1982). It has been reported that there has to be a higher degree of supersaturation for the nucleation of silicon on SiO, and Si,N, as compared to that for nucleation on silicon surfaces. Thus by keeping the supersaturation below a critical value, it is possible to selectively

Selective epitaxial growth of silicon in a barrel reactor deposit silicon on silicon substrates masked by either silicon nitride or silicon oxide (e.g. Fig. 1). Crystal growth theories as discussed by Bennema and Van Leeuwen (1975) explain the initiation of growth by the adsorption of silicon at the growth interface. Adsorbed atoms form little clusters which are thermodynamically unstable until they reach a certain critical size. Thereafter it is energetically more favorable for them to remain in the solid phase than in the vapor phase (Claassen and Bloem, 1980a). The adsorption energy on foreign substrates is generally higher than that for silicon. Thus it becomes possible to operate at a point where the nucleus size on the Toreign material is held below the critical value while nuclei of overcritical size can form on the silicon-growth interface. The process is a delicate balance between reasonable growth rates and polynucleation on the masking material, most often SiO,. The onset of nucleation on the mask is a function of temperature, pressure, mask material and the Cl/Si ratio in the vapor phase. The ratio of masked to unmasked area on a wafer can have an indirect effect on the extent of nucleation (Nagao et al., 1986). SEG of silicon is most often carried out by employing a DCS-HCI system at reduced pressure and temperatures of about 9CM&looo”C. Other methods make use of silicon-iodine (Braun and Kosak, 1978) or silicon-bromine transport systems. Pagliaro et al. (1987) reported the growth of good crystalline films down to temperatures of 850°C. SEG leads to structures exhibiting distinct faceting which depends on substrate orientation and seed window alignment relative to crystal planes. It was found that (100) substrates and pattern alignment along [lOO] directions give the best results for application purposes (Ishitani et al., 1985; Kitajima et al., 1983). An imCl/S ratio provement in film quality with increased was also observed (Kitajima ef al., 1983). Growth rates in selective epitaxial systems appear to depend rather strongly on the amount of exposed silicon area at CVD reactor pressures greater than about 20 torr. This loading effect can be reduced by increasing the CI/Si ratio (Ishitani et al., 1984) or by reducing the reactor pressure below 20 torr (Reif, 1988). However, these remedies result in smaller growth rates, which because of reduced throughput in may be undesirable a fabrication line. If the film is grown longer than necessary to fill the void created by the etching of the SiO, mask, it will not only grow vertically but horizontally as well. This (SOI) type structure leads to a silicon-on-insulator which is very desirable from a device application point of view. It was initially believed that it would not be difficult to achieve higher horizontal growth rates than vertical ones, or, in other words, to obtain aspect ratios of much greater than unity. It was assumed that the enhanced diffusion of adatoms on the mask material close to the seed area would increase the amount of silicon available for incorporation into the lattice compared to that available to the top surface. This expectation has not appeared to become a reality CES 34: Q-‘I

2051

though, despite reports of aspect ratios between 6 : 1 to 100: 1 (Rathman et al., 1982). It may be generally assumed that the thin masking oxides that were necessary to achieve such ratios were degraded during the prebake preceding the epitaxial growth, thereby enlarging the seed area. A different technique to grow epitaxial silicon over SiO, was reported by Jastrzebski et al. (1983). They achieved almost nucleation-free growth by growing without any HCl for a short time and then etching with HCl for about the same amount of time. These, steps were repeated until the desired film thickness was achieved. The aspect ratio was about 1: 1. Large aspect ratios (much greater than 1: 1) are typically of great interest for advanced dielectric isolation and the design of new three-dimensional integrated circuits. One promising avenue for such a high aspect ratio is lateral SEG (LSEG), a schematic representation of which is depicted in Fig. 2 (Schubert and Neudeck, 1989). The top structure is a cavity with prepared wing layers of different etch-rate materials and with a seed hole deep inside. In the center structure, selective growth extends up into the cavity, and is technically EL0 at this stage. As the top of the EL0 meets the cavity ceiling, growth is now constrained to proceed only laterally, as shown in the bottom structure. This lateral growth is referred to as LSEG. The main advantages of LSEG are the ability to produce locally a single-crystal SOI semiconductor film with well-controlled dimensions, and a large effective aspect ratio of lateral distance to vertical height. Another advantage of LSEG is the ability to create novel geometries with high-quality material. The technique of selective epitaxial growth is of great interest to very large scale integration (VLSI) because it allows for novel device isolation techniques with higher densities as well as new device structures like silicon on insulator arrangements (Friedrich, 1987; Zingg et al., 1986; Blumenfeld, 1971; Neudeck, 1987; Shinchi and Sakutai, 1976).

Reactor models Today more than 70% of all integrated circuits fabricated employ epitaxy in one way or another. The requirements made on the quality of the epitaxially grown layers are stringent: less than *5% thickness variation over a wafer and from wafer to wafer, less than +5% doping nonuniformity and high growth rates to suppress dopant redistribution. As for cost, one looks for reactors with large wafer handling capacities and good efficiency, both in usage of electrical power as well as of the chemicals. Selective epitaxy is even more sensitive to the variation of parameters than is full wafer epitaxy; one has only a limited operating range in which nucleation does not occur and local depletion effects can significantly alter growth rates. Figure 3 illustrates typical CVD reactors used in epitaxy (Hess et al., 1985; Wolf and Taubet, 1986; Sze, 1988; Oh, 1988). The first reactor, the vertical, usually

C. G. TAKOUDISand M. M. KA~TELIC

2052

exposed sillcon

silicon

seed area

substrate

Fig. 2. Schematic diagram of lateral selective epitaxial growth (Schubert and Neudeck, 1989).

refers to a single-wafer reactor, while the other three reactors are those normally used in silicon epitaxy: the horizontal, the barrel, and the pancake reactor. The terms barrel and pancake arise from the configuration of the susceptor, which is used to hold the wafers during epitaxy. The pancake and barrel reactors are commercially viable, whereas the horizontal reactor is rarely used in VLSI production. The vertical, horizontal and barrel reactors are displacement reactors in which the incoming gas constantly sweeps out reacting gases. The pancake reactor operates in the “mixing flow mode” where the incoming gas partially mixes with reacting gases. This may be one of the reasons why a number of modeling studies have been done particularly on the previous three displacement reactors, while very little modeling of the pancake system has been done. Summaries and comprehensive reviews of CVD reactor modeling have been presented by several researchers [e.g. Ban (1978, 1984) and Hess et al. (1985)], the most recent one being by Sherman (1988). Hess er al., in particular, summarize modeling efforts of horizontal, barrel, rotating disk, impinging jet, stagnation point flow and tubular low-pressure chemical vapor deposition (LPCVD) reactors. Among these, the rotating disk, the impinging jet, and the

stagnation point ffow reactors can be considered as variations of the laboratory vertical reactor, while the tubular LPCVD reactor is usually used to prepare amorphous or polycrystalline thin films. For the barrel reactor geometry, which is of particular interest here, the phenomenological model proposed by Juza and Cermak (1982) perhaps stands out. Their model makes use of fundamental physicochemical principles from which a set of partial differential equations describing the CVD process is formulated based on the assumptions that:

(1) gas flow is laminar, no bulk gas phase chemical reaction takes place, forces) (3) external forces (including the gravitation are negligible, of (4) the channel width is infinite in the direction the coordinate z (thus the barrel reactor is treated precisely as a horizontal reactor), mixture is considered as a two(5) the reaction component system, of the flow profile in the prcdeposi(6) development tion regime is neglected, and is heated through RF heating so (7) the substrate that high-temperature gradients in the gas phase are possible.

(2)

Selective epitaxial growth of silicon in a barrel reactor -GAS

RADIANT HEATERS

2053 INLET-

QUARTZ

m

BELLJAR EXHAUST

Fig. 4. Schematic

Fig. 3. Schematic representation of: (a) vertical, (b) horizontal, (c) barrel, and(d) pancake chemical vapor deposition epitaxial reactors.

Additional stemming

introduced assumptions are then from the character of the system modeled

(Juza and Cermak, 1982). Although this is a detailed reactor model, it makes use of a CVD rate expression [eq. (la) in Juza and Cermak (1982)] that at near atmospheric pressures appears to indicate that growth rate increases as pressure decreases, contrary to eq. (1) of the same reference. Furthermore, radiant heating of the susceptor, in the absence of any cooling, significantly diminishes temperature gradients in the gas phase

(Borland,

1987b). EXPERIMENTAL

The system used in this study is a modified radiantly heated barrel reactor, in which instead of the two typical jets of feed gas mixtures (Hess et al., 1985) the reactor inlet is at the top of the reactor (Fig. 4) (Kastelic, 1988). The hydrogen main flow enters the reactor through an annulus with an inside diameter of 5 cm and an outside diameter of 7 cm, impinges on the top of the susceptor which is hexagonal, 12.2 Cm on each side, moves upwards through an annulus of an inside diameter of 7 cm and an outside diameter of 10 cm,

diagram of the radiantly reactor used.

heated barrei

and finally travels downward between the hexagonal susceptor, which is about 3” off the vertical direction, and the bell jar, which is 33 cm in diameter. A supplemental amount of H, is also continuously purged through the quartz hanger assembly, 5 cm in diameter, and it flows inside the susceptor and out of a small hole at the bottom plate of the susceptor (Kastelic, 1988). The silicon source gas, DCS, is varied between 150 and 300 seem, while the HCl gas flow is varied between 300 and 800 seem so that nucleation of silicon on SiO, is prevented. Silicon SEG temperatures between 850 and 1000°C are investigated at reduced pressures of 25-100 torr. At 1 atm and similar other conditions, nonselective growth of silicon is always observed. The Sic-coated graphite susceptor (a hexagonal configuration with three tiers accommodating wafers 4 in. in diameter) upon which silicon substrates are placed is coated once with a polysilicon film to provide a uniform surface for deposition and to help minimize outgassing of impurities from the susceptor itself. The susceptor is rotating at about 5 rpm. The patterned silicon substrates used in these experiments are all 3 in. diameter n-type polished wafers of (100) crystal orientation masked with a thermally grown oxide (0.08 0.87 pm thick) and patterned along the [OlO] direction (45” with respect to the primary fiducial). The oxide coverage on the wafer surface area is about 95%. Prior to loading in the reactor, the patterned substrates are cleaned in a H,SO*-H,O, solution followed by a brief BHF dip, rinsed thoroughly in DI water and dried in a spin rinse dryer. Prior to the actual deposition, wafers in the reactor are exposed to a hydrogen prebake at 1ooo”C and reduced pressure (28 torr). SEM and TEM micrographs show that such a hydrogen prebake removes all native oxide (Friedrich et al., 1988). The number of

C. G. TAKOUDIS and M. M. KASTELIC

2054

ill ~1111 ‘ii11IllI iIll :i I~Iidi III illi 1111 . I- --._. -2 ..g -2.

.,

‘:’

;

,

ZZZ :SE:-mnrmmls. .---/--..-ZZ -z---w ;:-:::-- :-=-=-.:--

Fig. 5. Epitaxy mask layout (Friedrich,

wafers used in a given run varies from one to six with the remaining susceptor pockets left empty. Total deposition time is tailored between 3 and 20 min for each run to provide a minimum of a 0.5 pm thick SEG layer. Growth rates are calculated with a step profilometer (Tencor Alpha Step) in a 20x 140bm window in the oxide mask. The standard experimental error for absolute growth measurements with this profilometer is estimated to be 0.01 pm. A consistent set of six locations are measured on each wafer and averaged to obtain the results reported. The growth rates among these consistent six locations is typically seen to vary by less than 5%. Figures 5 and 6 are enlarged reproductions of the mask layouts used to pattern wafers. The mask pictured in Fig. 5 is actually a composite composed of two individual masks. Although not readily discernible from Fig. 5, the patterns are drawn so that some of the seed windows of one mask overlay those of the other. This mask set allows the fabrication of recessed . and raised seed windows (Friedrich, 1987; Kastelic,

:,. .: :, l

mmuu

umumm

umumu 111

mmumm ummmm

-

1987; Kastelic,

1988; Vedala, 1988).

1988; Vedala, 1988). The overall design and layout of the seed windows with respect to spacing and width of openings (the smallest dimensions of any geometry on the seed window mask in Fig. 5 measures about 3.0 pm) was based on both previous experience with SEG on other device test patterns in the Purdue pancake reactor as well as descriptions of masti geometries in the literature. One of the objectives of the mask design was to create a pattern which would allow the study of local loading effects, as well as the influence of oxide thickness on growth rates (Friedrich, 1987; Kastelic, 1988).

MODEI> DlWEI.OPMENT There have been a few attempts to model a CVD barrel reactor (Juza and Cermak, 1982; Hess et ul., 1985; Sherman, 1988; Oh, lY118), most notably the detailed model proposed by Juza and Cermak (1982). However, in addition to the inconsistency of the deposition rate used in that model, as was discussed

Selective epitaxial growth

Fig. 6. Epitaxy mask layout

in the center

earlier, there seem to be some further issues that the model of Juza and Cermak does not address. First, strictly speaking, their model is well suited for a horizontal reactor and not for a barrel reactor. Second, no susceptor rotation and no development of the flow profile are accounted for. Third, no gas phase reactions are assumed to take place. Finally, their model does not address multicomponent reacting mixtures and is not suited for SEG of electronic materials. However, Juza and Cermak’s model does form one of the first models of a horizontal epitaxial CVD reactor developed from fundamental physicochemical principles. The model equations needed for the selective epitaxial growth of silicon in a radiantly heated barrel CVD reactor are developed next. The equation of mass conservation for the mixture yields v.pv=o while the equation

of momentum

(2) conservation

yields

pv.vv = -VP-V-t+pg

(3)

2= -p[vv+(vV)TJ+JpIv~v

(4)

where

where

P is the pressure,

g is the gravitational

of silicon

acceler-

in a barrel

2055

reactor

of a wafer (Kastelic,

1988).

ation and 7 is the viscous stress tensor. Because of the actual inlet and exhaust configurations used, a pressure drop along the axis of the susceptor would be expected and it is actually observed. Thus, eq. (18) of Juza and Cermak’s (1982) model is inappropriate. If Dufour effects (energy flux resulting from diffusion), energy radiation and the heat generated by deposition reactions are neglected, the energy balance for a radiantly heated barrel reactor becomes PC,V

.VT=V(kVT)---:Vv-PV.v.

(5)

Viscous energy dissipation may be ignored in this case. Because of the typical use of dilute reactants, the heat of deposition reactions will be negligible. Jenkinson and Pollard (1984) have shown that Dufour effects are not important in CVD systems. Furthermore, in this study radiation is neglected because of the relatively low susceptor temperatures used. The potential effect of natural convection, if any, in the barrel reactor used is estimated by the ratio Gr/Re2, where Gr and Re are the Grashof and Reynolds numbers, respectively (Curtis, 1981; Bird et al., 1960). At the conditions of silicon SEC used throughout this study, the value of Gr/Re’ is less than 0.04. One criterion typically used (Curtis, 1981) is that if Gr/Re*> 16, then free convection dominates,

C. G.

2056

TAKOUDIS

and M. M.

whereas if Gr/Re’ < 0.3 forced convection prevails in a system. Therefore, free convection is not expected to play any role in this work (Oh, 1988). Once the temperatures and velocity profiles are determined from eqs (2W5), the species mass balances may then be solved to yield all concentration profiles. Since the only gas phase reactions considered are (Borland, 1987b; Kastelic, 1988)

KASTELIC

that is, no-slip boundary condition for the velocity components, while at the relatively less hot reactor wall, no silicon deposition is assumed. In our silicon SEC runs, a little silicon thin film growth is occasionally observed on the stationary bell jar so that the boundary condition dw,/&=O seems to be justified. At the reactor inlet, which is the annulus of 5 cm inside diameter and 7 cm outside diameter, we have o,=O,

SiCl,,,,

+ HCl,,,+SiHC1,(,l

the Fick’s law formulation DCS and HCI yields

of the mass

pv-Vwj=V~[pD,(Vw,+k,Vln

balances

T)]+CvzRH, i

of

(8)

where ti is the molecular weight of the gas mixture and R, is the universal gas constant. coefficients, viscosity and thermal conDiffusion ductivity are used as the following functions of temperature and pressure:

T 0.7 [I r

(10)

0

where the diffusivities of HCI and DCS are taken to be 0.596 and 0.245 cm’/s at 1 atm and 0°C (Van der Putte et al., 1975) while the thermal conductivity and viscosity of the gas mixture are taken to be equal to the respective properties of hydrogen, since hydrogen is present in a large excess. Equations (2)(5) and (SHlO) form the modeling equations we propose for the SEG of silicon in a radiantly heated barrel reactor. Solution of these equations requires appropriate boundary conditions. Hence, appropriate boundary conditions are presented next. At the stationary reactor wall we have

v=o,

T=T,

and

and

T=Ti,

wi=wi,,.

(12)

The distance between the barrel reactor inlet and the top of the susceptor is 12 cm. At the reactor outlet, which is 48 cm away from the top of the bell jar of the barrel system and 10 cm in diameter, the boundary conditions will be

au, -_=o,

L=o,

az

a-

z=O Finally, boundary

a%

-&l,

t3Z C?Wi

and

dz=O

at the rotating susceptor, conditions will be 11,= 0,

vg = Qr,

v,=o,

(13) the

I-= T,

respective

(14)

and

a

PM

p=R,

k=k,

u,=oin,

ao

where i is DCS or HCI. The concentration profile of hydrogen, the carrier gas, is determined by the mass fractions of DCS and HCI, with the assumption that all other species are present in very small quantities. The second term on the right-hand side of eq. (8) represents Soret diffusion (thermal diffusion), while the third term accounts for gas phase reactions [e.g. Kastelic (1988)]. The equation of state used is the ideal gas law, which should be a good approximation at pressures between 25 and 100 torr and temperatures between 300 and 1000°C (Borland, 1987b; Kastclic, 1988). Therefore

and

u,=O,

$=O

where-is the derivative along the outward normal to an the surface. Since epitaxial growth of silicon occurs selectively in seed silicon windows only, the growth rates R,, are taken to be equal to zero on the areas of the wafers covered with silicon dioxide (Kastelic, 1988; Kastelic et al., 1988). Also, the susceptor itself is considered as a part of a cone, with the radius of its top surface 12.2 cm and the radius of its bottom surface 13.7 cm. The reactor pressure is specified at the outlet. The SEC rate of silicon, which is needed in eq. (15), is based on very basic deposition steps, fairly widely accepted by many investigators (e.g. see background section of this paper). These deposition steps are eq. (6), which reaches equilibrium very fast (Kastehc, 1988), and the elementary surface reactions *1

Sic&.,

SiCl,

+ S h

S + H,(,,

SiCl,.S

2 Si,,,, + 2HCl,,, L4

(161

(17)

From this very simple phenomenalogical deposition mechanism it is seen that the role of HCI is to etch silicon substrates through the reverse reaction of eq. (17), and to lower the availability of SiCl,c,j through the reaction step shown in eq. (7). A simple analysis of eqs (6), (16) and (17) results in the following SEG rate

Selective epitaxial growth of silicon in a barrel reactor

of silicon (Kastelic, 1988): R,=k,----

k&, Pi&, k,

(18)

Pm

where P,,, and P,,, are the partial pressures of HCl and DCS, respectively, next to the substrate. From low-pressure data, the two parameters k, and k,k,/k, are found to depend on temperature through an Arrhenius type of dependence (Fig. 7). This Arrhenius temperature dependence is also found to be consistent with basic kinetic considerations of rate constants (Kastelic, 1988; Borland, 1987b). The apparent activation energy of the reduction of SiCl, with H, is found to be 33 kcaljmol which is in very good agreement with the activation energy determined for silicon deposition from DCS and H, in the same type of reactor (Kastelic, 1988). It is eventually found that lo”exp(--F)pm/min

(19)

W, 7=53.2exp(-~~?$!)pmjminjtorr.

(20)

k,=2x

1

Because of the axial symmetry of the radiantly heated barrel reactor and the slow rotation of the susceptor, half of the actual geometry is used in the numerical solutions of the modeling eqs (2)-(S) and (Q-o-(O) along with their respective boundary conditions, eqs (1 lHl5) and (18H20). The complete set of modeling equations of a barrel CVD reactor, eqs (2)-(5), (SH15) and (l&(20), is solved primarily with a collocation finite-element method available in the code ELLPACK (Rice and Boisvert, 1985). In typical

a \

n

.

-1.0 -

k,

o Experimental

k.k, ’k,

0.2 -

-

-1.6 -

RESULTSAND DISCUSSION Figure 8 shows SEG rates of silicon as a function of the ratio of the inlet partial pressures P&,/P, at 950°C and 28 torr. As this ratio of inlet partial pressures increases, SEG rates of silicon decrease monotonically. On the other hand, a plot of SEG rates vs the ratio of feed partial pressures P&P,, which has been reported as a characteristic quantity of silicon SEC [e.g. Borland (1987a) and Drowley (1987)], rcsults in a nonmonotonic relationship (Kastelic, 1988). A monotonic relationship between SEG rates of silicon and the ratio P&,/P, proposed in this work has also been observed in an RF-heated pancake CVD reactor (Kastelic er al., 1988). The experimental data presented in Fig. 8 indicate that the growth rate decrease with increasing P&JPDcs is almost linear. Furthermore, this ratio at the reactor inlet is varied over a rather limited range because, at values less than about 0.3 torr, silicon thin film growth is not selective, while at values greater than about 1.6 the net result is found to be silicon etching in the seed windows. The specific characteristic quantity Pi,JP, proposed seems to be indicative of the stoichiometry of the overall reaction shown in eq. (1). However, in spite of its simplicity, the

Ea = 33 Kealtmcle

\

-1.6 -

simulations, 1000 mesh points were used over the length of a 7.5 cm diameter sample. This corresponds to an average feature length of about 75 p which is on the order of magnitude of the patterned seed on the substrates used in this study (Figs 5 and 6). Most reactor simulations were run on the Purdue CYBER 205, while the rest were run on the Computer Science Department computing system.

0.3 -

-1.2-

-1.4

2057

0.1 Ea = 12.5

Kcalimcle

0.0 -

-0.11

28 Torr H, main = 45 slm

-2.o! 8.0

I 8.2

I 8.4

.

I 8.6

\

I 8.8

0.4

.‘? 9

IO4 x l/T (K-l) Fig. 7. Temperature

Hz moin = 45 slm

dependence of kinetic rate constants k, and k,k,/k,.

0.6

0.8

1.0

0

1.2

1.4

1.6

p*tic,

/P PCS ( TOW)

Fig. 8. Selective epitaxial growth rates of silicon vs P&/P, in a barrel reactor, Experimental data and predictions (solid line) of the model eqs (2)-(5), (Q--o-(S) and (18H20). Oxide thickness = 80 nm.

2058

C. G.

TAKOUDIS

and M.

M. KASTFLIC

ratio P&,JP, at the barrel reactor inlet has served as an important guide in determining regimes over which good selectivity is obtained (Kastelic, 1988). The solid Iine in Fig. 8 corresponds to predictions of the model developed in the previous section. It is seen that the agreement between experimental observations and model predictions is satisfactory. Furthermore, theoretical predictions of the model eqs (2H5), @t-(15) and (18)-(20) show that SEG rates of silicon are decreasing monotonically with increasing J%,Ip,, (Fig. 8), whereas they vary nonmonotoniin agreement with experimental tally with PHc,JPDcs, observations. Naturally, though, since silicon nucleation on the oxide mask is not considered in the model proposed, mode1 predictions extend to values of even below 0.3 torr or above 1.6 torr. 4LJPDcs The almost linear relationship between silicon growth rates and P&,/P,. at the barrel reactor inlet is clearly depicted in Fig. 9. Here experimentally observed SEC rates arc shown at four different substrate temperatures, 850, 900, 950 and IOCWC, while the barrel reactor pressure is maintained at 28 torr and the hydrogen main flow rate is 45 standard liters per minute (slm). Two points are worth emphasizing in Fig. 9: (a) at each substrate temperature, the individual feed partial pressures P,,, and P,, can assume any value resulting from the regimes of HCI and DCS flow rates mentioned in the experimental section, yet the quantity of interest here is only P&,/P,,; and (b) the straight solid lines in Fig. 9 indicate linear leastsquares fits of the data shown at each substrate temperature. Indeed, the decrease of growth rate with pi&IIpDcs is seen to be almost linear (Fig. 9). Furthermore Fig. 9 indicates that, at lower substrate tempera-

28 Torr

H, main = 45 stm A A

\

0.0

0.4

0.8

P2

HC,

/

I

850°C

0

900°C

A

950°C

A

1000°C

1.2

pDcs

1 .6

2.0

(TOW

F‘ig. 9. Selectwe epitaxial growth rates of silicon observed at substrate temperatures of 850, 900, 950 and 1000°C as functions of feed composition.

0

,

-

4Ll‘%lcs

V

0.30

Torr

-+-0.46Torr

;<<

10 ‘\ -4

1, o’\.

\ A

-5

, I , , I , I I , I I , I 1 , I 1 7.8 8.0 8.2 8.4 8.6 8.8 ’0 104x I/T

(K-‘I

Selective epitaxial growth rates vs substrate temobserved and predIcted (solid lines) by the model eqs (Z+(5), (8x15) and (18H20). Oxide thickness-80 nm.

Fig. 10. perature,

tures,

data

P&JP,;

are presented over smaller ranges of this is because the regime over which the

selectivity of epitaxial silicon growth in seed windows is good is shrinking with decreasing substrate temperature. Growth rates predicted by the model eqs (2HS), (Sj(l5) and (18H20) are found to be in as satisfactory agreement with growth rates observed at all substrate temperatures as with the ones indicated in Fig. 8.

Figure 10 shows the SEG rates of silicon as a function of substrate temperature at a barrel reactor pressure of 28 torr, H, main flow rate of 45 slm and at three different values of the parameter P&,/P,, 0.30, 0.46 and 0.67 torr. Again, P,$.,/Pwsis evaluated at reactor inlet conditions. It is seen that apparent activalues of vation energies are higher as the parameter are increasing. Furthermore, our experJ’:c Jprm imental data suggest that for the same value of P~c,IPDcs I there appears to be a change in the overall activation energy as the temperature increases with higher activation energies at lower temperatures. A close look at Fig. 10 reveals that the average apparent activation energy for silicon SEG at P&,/P, =0.30 torr is about 50 kcal/mol, while at P,$.JP,,=O.67 torr it is about 85 kcal/mol. Activation energies of silicon deposition from DCS-H,-HCI mixtures have been reported to be 24.8 kcal/mol, at flow rates of H, = 80 slm, DCS = 0.30 slm, and HCI = 1 slm, at SO torr (Liaw et a[., 1986), and 63 kcal/mol (Tanno et al., 1982). Thus the apparent activation energies obtained from our data seem to be rather larger than others reported to date; yet the overall apparent activation energies obtained qeem to increase as SEG rates decrease, through either higher P&,/P,,, or lower substrate temperatures, when the first and second terms on the right-hand side of eq. (18) tend to balance

Selective epitaxial growth of silicon in a barrel reactor each other (Kastelic, 1988), in qualitative agreement with similar general trends of kinetic systems. Such a trend was also noted by Drowley and Turner (1987), who observed that the temperature sensitivity of growth rates in their pancake CVD reactor increased as the ratio, in this case of [HCl]/[DCS], at the system inlet increased. Similarly, Rundle (1971) has reported that, for epitaxial growth from SiCI, and H,, higher input concentrations of SiCl, result in a greater temperature sensitivity of the deposition process. The solid lines in Fig. 10 show silicon growth rates predicted by the model eqs (2H5), (SHlS) and (18H20). Predicted SEG rates become increasingly more temperature sensitive as substrate temperatures increases, in satisfactory agreedecrease or P&,/Pms ment with experimental data obtained. Figures 11 and 12 show the relative SEG rates on patterned and patternless wafers as functions of CVD reactor inlet composition, substrate temperature and local fractional area of Si. This last parameter refers to the fraction of a local area of a sample that is available for silicon SEG, the remaining area being covered by silicon oxide or silicon nitride, and thus unavailable for silicon growth. Figure 11 contains data in the form of a plot of the growth rate difference observed between bulk and pattern wafers (CR,,,, - GRpalte,,) vs the characteristic parameter P,?,,JP,,, evaluated at the system inlet, at substrate temperatures of 850,900 and 1000°C. For a given temperature, there is a ratio of P&,/P,, at which growth rates on patternless wafers change from being less than to greater than SEG rates measured on patterned ones. In all cases, such a change is observed to occur at CVD inlet conditions at which selectivity of the epitaxial silicon growth is very good. Furthermore, as the substrate

0.2 28 H,

Torr main=

45

slm

-c+ * -o-

0. I E al z x g 0.0 I 5 3 i? c3 -0. I

850-C 900°C IOOOY

3

-0.2

0

2 2

P,,,/P,cs(Torr) Fig. 11. Relative growth rate differences on patterned and patternless wafers as functions of feed composition at 850, 900 and 1000°C. ( n), (A) and (0) are respective model predictions of the points at which GR,,,,= GRpPffSrn.

0.40

2059

i

$ o*30.

-

1.02

-+-

I. IOTorr

;\

--u

0

i

o-20-

f 3 s

-----0

<-z----

O.lO-

Torr

0

1000 “C 28

4

0.00

0.0

0.2 Local

3

-

I

I

I

0.4 Fractional

Torr

,

I

0.6 Area

I C 8

of Si

Fig. 12. Selective epitaxial growth rate vs local silicon area fraction as a function of feed composition. Experimental data and predictions (solid lines) by the model eqs (2H5), (@-o-(5) and (18) (20). Oxide thickness of areas unavailable for growth = 80 nm.

temperature is increased, the ratio of P&JP,, at which the difference GR,,,, - GRpattern changes sign is increased. Similar data were reported by Drowley (1987) in a reduced-pressure CVD pancake reactor, although the parameter used in that study was different, [HCl]/[DCS]. Figure 11 also contains the points at which GRbulr is predicted to be equal to GRprttcrn by the model eqs (2HS), @HIS) and (lSH20). The agreement between experimental data and theoretical predictions is seen to be satisfactory, since our model predictions can adequately match quantitatively all trends observed. Figure 12 shows SEG rates vs local fractional area of silicon available for growth at three different values of P&/PI,. As the local area of seed windows increases, the growth rate profiles change from monotonically decreasing to monotonically increasing from 1.02 to 1.95 torr. The when Pi,-,/PDcs increases apparent inversion of the growth rate profile with respect to seed window area is seen to occur between P&-,/P,= 1.10 and 1.95, at a substrate temperature of 1OOOYZ (Fig. 12). Furthermore, at lOOO”C, the change in the relative magnitudes of growth rates on bulk and patterned wafers also occurred between these same values of P&JP,, (Fig. 11). Hence, such an inversion of the growth rate profile appears to be a result of a local loading effect (Ishitani ez al., 1984, Kastelic, 1988) along with the change of sign of the difference GRbulr - GRpatIFrn as the characteristic parameter P&,/P,, assumes increasingly higher values. Predicted SEG rates as functions of the local fractional area of silicon are depicted with solid lines in Fig. 12. The agreement between predicted and observed SEG rates is seen to remain satisfactory for all feed compositions studied.

C. G. TAKOUDIS and M. M.

2060 CONCLUSIONS

Based on a comprehensive experimental program on the selective epitaxial growth of silicon at substrate temperatures between 850 and lOOO”C, and from gas mixtures of DCS, H2 and HCI in a modified radiantly heated barrel reactor, it is found that a new characteristic quantity, the ratio of feed partial pressures of P_&,IP,cs9 which reflects the overall stoichiometry the DCS decomposition reaction, characterizes several features of silicon SEG. Thus, as P&,/P,, increases, the SEG of silicon is found to decrease almost linearly with it. Also, the overall apparent activation energy of silicon SEG is found to increase with increasing P& JPDcs, hence resulting in a higher temperature sensitivity at higher values of this ratio of reactor inlet partial pressures. The growth rate of silicon on a patternless wafer is seen to become greater than that on a patterned wafer increases and while a DCS-H,-HCI as p&,Jp,s system is run at conditions of high-quality SEG. at which CR,,,, Furthermore, the value of P&,/P, becomes greater than GRpat,_” is found to increase as the substrate temperature increases from 850 to 1000°C. An inversion of the SEG rate profile of silicon as a function of the total local area of seed windows is observed when the ratio of inlet pressures P&/P,,, increases. Thus monotonically decreasing SEG rates vs local fractional area of silicon are seen to become monotonically increasing with respect to the same local fractional area of silicon, as P&,/P,, increases. Such an inversion of the SEG rate profile appears to be a result of local loading effects as well as the relative magnitudes of GR,,,, and GRpaltern, although it is actually observed on patterned samples. The apparent activation energy of the silicon deposition from DCS with hydrogen as the carrier gas is determined to be about 33 kcal/mol, which is in very good agreement with activation energies reported in the literature. The overall apparent activation energy of SEG of silicon is found to bc between 50 and 85 kcal/mol. The higher activation energies found experimentally correspond to higher parameter values of P&-,/P,, and lower substrate temperatures, that is for typical conditions under which the rates of epitaxial silicon growth and epitaxial silicon etching tend to have similar values, the net result being growth, though. A detailed model of the SEG of silicon in a radiantly heated pancake reactor is also developed. This model includes all mass, energy and momentum equations coupled with the special geometry, inlet and exhaust configurations associated with the CVD reactor used. It accounts for typical feature sizes on a patterned wafer down to about 75 pm. The silicon SEG rate expression used is derived from reduced-pressure and low-temperature studies. The agreement between theoretical predictions of the model developed and experimental data obtained is found to be satisfactory. Finally, another important result of our modeling efforts

is that gas partial

pressures

in the vicinity

of a

KASTELIC

susceptor, where SEG of silicon occurs, may be very different from the ones at the barrel reactor inlet. Hence, growth rate expressions based on feed partial pressures may be misleading and should be used with caution. Acknowledgments-We are very grateful for the generous support from Applied Materials, CVD Applications Labvratory, in Santa Clara, CA. We are particulafly grateful to John Ogawa Borland who provided comprehensive help throughout the experimental work of this study. We are also grateful to Joerg Friedrich and Gerry W. Neudeck for their numerous helpful comments and discussions. We like lo thank David M&s, Douglas Yoder and Paul Stevenson from Delco Electronics in Kokomo, IN, for many helpful discussions. Partial financial support from Delco Elt&ronics is also greatly appreciated. Finally, we are thankful to the Purdue University Computer Center and the Computer Science Department at Purdue University for their immense help in the computational work of this study. One of the authors (C.G.T.) is particularly thankful to Manolis Vavalis who provided extensive help in the early stages of the implementation of ELLPACK in this work.

NOTATION

Di DO

heat capacity of gas binary diffusivity of species i binary diffusivity under reference tions

f

acceleration vector due to gravity identity tensor thermal conductivity

C”

k

condi-

thermal conductivity under reference conditions rate constants of elementary reactions

k, k,, k,, k,, k,

thermal diffusion coefficient molecular weight of the gas mixture absolute reactor pressure ideal gas law constant radial coordinate from the center of the rotating susceptor

k, M P R, r

rate of gas phase reactionj rate of surface reaction j of the substrate absolute temperature absolute temperature at the inlet absolute temperature of the barrel reactor wall velocity vector mass fraction of species i mass fraction of species i at the inlet axial coordinate from the inlet

R R:: T Tim T, Y Wi wii, z

Greek v P. a “ii Vfj P r

letters gas phase viscosity gas phase viscosity at reference conditions stoichiometric coefficient of species i in gas phase reaction j stoichiometric coefficient of species i in surface reaction ,j gas phase density stress tensor

Selective

epitaxial

growth

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