Znd. Eng. Chem. Res. 1996,34, 1859-1867
1869
An Experimental Study of B-Sic Epitaxial Film Growth in a Horizontal Chemical Vapor Deposition Reactor B. Bahavar and R. J. McCluskey" Department of Chemical Engineering, Clarkson University, Potsdam, New York 13699
M. I. Chaudhryt Department of Electrical and Computer Engineering, Clarkson University, Potsdam, New York 13699
Single crystal P-Sic films with thicknesses up to 10 pm were grown on Si (100)substrates using the H z - C ~ H B - S ~ H system. ~ Two-level factorial design experiments were used to study crystal growth conditions by considering three parameters in the chemical vapor, deposition (0) reactor: temperature, reactor free height, and substrate tilt. There was a critical value determined for reactor free height which corresponded to a Grashof number of 155. Smaller free heights produced films with flat growth rate profiles and larger useable areas but with about a 50%reduction in growth rates. Temperature had the strongest effect on crystal quality; increasing the temperature from 1320 to 1360 "C produced films having 65% lower carrier concentrations and 56% higher Hall mobilities. When the carrier gas was changed from H2 to H d h equimolar mixtures, the average growth rate of P-Sic films was reduced by a factor of 2.4. Particulate deposition on the reactor wall was greater for larger free heights and for HdAr carrier gas mixtures. 1. Introduction
2. Experimental Section
Of all the wide band gap semiconductors under development (Sic, diamond, ,#-BN, AN, GaN), the technology for Sic devices is the most advanced [131. Cold-wall chemical vapor deposition (CVD) reactors have been extensively used to grow ,#-Sic epitaxial films at atmospheric and lower pressures [2,17,20,24,30].The growth temperatures of 1300-1400 "C a t near atmospheric pressure highlight the importance of mixed flow, i.e., forced flow and natural convection. Substantial convective flows arise due t o the steep temperature gradients between the substrate and the cold reactor wall. The recirculating flow patterns caused by bouyancy effects influence the growth uniformity and crystal quality of the epitaxial films. Numerical modeling of epitaxial reactors a t growth temperatures of -600-900 "C which are used for the more common compound semiconductors (e.g., GaAs) has shown that the superposition of free convection rolls on the forced flow results in great variations in the extent of the useable area, film composition, and film thickness [6,11,12,291.Among the parameters in the CVD reactor, the free height, i.e., the gap between the substrate and the reactor's upper wall, is the most important in determining the degree of free convection flows. Recently, there have been specific modeling studies on the overall growth of ,#-Sic using a CVD reactor [21and a rotating disk reactor [ I ] . In this paper, we present results of two-level factorial design experiments that were used for the systematic study of the effects of temperature, reactor free height, and substrate tilt on the growth rate and its uniformity, on the extent of the useable film area, on surface morphology, and on the electrical quality of ,#-Sicfilms. The factorial method allows for identification of any interactions between the three parameters. In addition, the effect of the carrier gas on the growth rate, reactor wall deposition, and homogeneous gas phase nucleation was studied through the use of HP and equimolar mixtures of HdAr.
All experiments were carried out in a cold-wall horizontal CVD reactor operated a t atmospheric pressure. p-Sic films were grown on Si (100)substrates, 32 mm x 48 mm x 0.38mm, using C3H8 (99.95%, MG Industries) and SiH4 (99.995%, MG Industries) as source gases and palladium-purified H2 (89.999%,Air Products) and Ar (99.999%, Air Products) as carrier gases. Flow rates for H2 (or HdAr), C3H8, and Si& were 3000,0.64, and 1.00SCCM (standard cubic centimeter per minute), respectively. The reactor was a quartz tube with an inside diameter of 50 mm and an inside length of 500 mm. Details of the crystal growth procedure have been reported elsewhere [31. The susceptor temperature was monitored with an automatic optical pyrometer (IRCON, Inc., Model R-14C05)that sighted through an optical quartz window in the reactor end cap and into a 5 mm deep hole in the center of the downstream end of the susceptor. The accuracy of the pyrometer reading was tested by melting a Si substrate under actual growth conditions. Typically, the pyrometer's indicated temperature was 2030 "C below the Si melting temperature of 1410 f 5 "C. Substrate temperatures in this paper were corrected by this difference. Nonetheless, there is still uncertainty regarding the pertinent surface temperature. The emmisivity of siliocn carbide is 0.8is compared to 0.7 for silicon. Furthermore, the emmisivity of the surface varies with time as the morphology of the surface film changes. Experiments performed to judge the effect of surface temperature were always based upon two target temperatures of 1320 and 1360 "C. The difference between these temperature levels was consistently 40 "C, based on the pyrometer readings and the input electrical power settings of the induction oven.
+
Present address: CHEMI Labs, Watervliet, NY 12189.
3. The 2sFactorial Design Experiments
Table 1 shows the setup for a Z3 factorial design experiment in which there are three parameters, each
0888-5885/95/2634-1859$09.00/00 1995 American Chemical Society
1860 Ind. Eng. Chem. Res., Vol. 34, No. 5, 1995 Table 2. Factorial Effects Analysis of Growth Rates for the 5 min Growth Runsa L
L
Growth Rate, pumk T + ~ + o(2.56) + T-h+e+(2.57) T-h+e- (1.58) T+h+e-(1.69) T+M+ (4.39) m - e + (4.41) ~ ~ h - (4.23) 8T + ~ - o (4.05) -
\
f
v
Factorial Effects Analvsis 0:
effect
6
average main effects T = substrate temperature h = substrate height 8 = substrate tilt two-factor interactions Txh
5
E4
0 2 0
Width of the Substrate Figure 1. Growth rates across the width of the film for the 5 min factorial growth runs. Curve numbers, 5, 15, and 25, refer to the position (in mm) from the leading edge of the film. Table 1. Setup for the 23Factorial Design Experiments T h 8 T" 1320°C 1360 "C hb F H = 2 0 m m FH=lOmm Bc 6" 10"
+
T x B hx8 three-factor interaction
Txhxe a
estimate i standard error @mh) 3.19 i 0.02 -0.03 f 0.05
-2.17 zk 0.05 0.60 f 0.05
0.08 f 0.05 0.01 f 0.05 0.34 i0.05
-0.07 f 0.05
See sections 4.1 and 5.1 for the significance of the bold items. 0.06 p m h . MSF = 0.15 p " h .
Spooled =
4. Results
causes an apparent nonsymmetry in the growth rate as shown in Figure 1. Throughout the paper, the results for the eight 23 factorial experiment runs are displayed in the following arrangement so that easy comparisons may be made visually: h+ = top row samples, the corresponding h= the bottom row sample; T+ = corner samples, the corresponding T- = the neighboring inside sample on the same row; and 8+ = left two columns, the corresponding 8- = the mirror image through the vertical midplane. Table 2 presents the results of a quantitative factorial analysis of growth rates for the samples shown in Figure 1. The main effect for each parameter was obtained by subtracting the average growth rate at the (-1 level from the average growth rate a t the (+) level. The twofactor interaction effect is a measure of the sensitivity of one parameter to the level of the second parameter. The thickness values used for this analysis were the average of all 24 thickness measurements made on each sample, eight thickness measurements evenly spaced across the substrate width at each of the three positions along the length of the substrate. Within each factorial design of eight experiments, the order in which the experiments were performed was chosen at random. Comparison of the estimates with their standard errors suggests that the bold items (h, 8, h x 8 ) are statistically significant and require interpretation while all other effects could be due to noise. The significance of the main and interaction effects is determined as follows [321. If the absolute value of an effect is found to be larger than the minimum significant factor effect, MSF, it can be concluded that the effect is indeed nonzero. The MSF is computed from
4.1. Growth Rate And Its Uniformity. The thicknesses of the film across the width of the substrate were measured by visible light interferometry for three positions along the length of the substrate; 1 = 5, 15, and 25 mm from the leading edge of the substrate. A total of 24 measurements, eight at each longitudinal position, were made for each sample. The growth rate results are plotted and show n in Figure 1. Measurements were made at the left edge and in steps of about 4 mm from that edge. No measurements were made at the extreme right edge due to the characteristics of the automated interferometry system (Prometrix). This
where e is the value of the t distribution at 99% probability with eight degrees of freedom and Spooled is the pooled standard deviation of any measured result, e.g., average thickness. In each set of eight conditions, one run was replicated and, hence, the eight degrees of freedom. The MSF for the main effects and interactions in Table 2 is f0.15 p m k . In Table 2, there is a large effect due to the substrate height, -2.17 f 0.05 pmlh, and a moderate effect due to the substrate tilt, 0.60 & 0.05 p m k . However, h and
+ +++
+ +-
+ +
-
+ + + +
a T = substrate temperature. h = substrate height. FH = free height = the gap between the substrate and the reactor's upper wall. 8 = substrate tilt.
having two levels designated by (-1 and (+I. The three parameters are T = substrate temperature (T-= 1320 f 5 "C, T+ = 1360 k 5 "C), h = substrate height (FH @ h- = 20 k 1 mm, FH @ h+ = 10 f 1 mm), and 8 = substrate tilt (8- = 6 f lo, 8+ = 10 f 1")where hcorresponds t o a free height of FH = 20 mm (the gap between the substrate and the reactor's upper wall) and h+ corresponds to a free height of FH = 10 mm. For both tilt angles, FH is measured at the downstream end of the substrate and at its midplane (reactor tube center). The Z3 design gives rise to eight different growth conditions. The two-level factorial design estimates not only the main effects, but also the interactions, with maximum precision [8,321. The major advantages of the factorial method over the "one-factor-at-a-time"method are that (a) if the parameters do act additively, the factorial design gives more precision and (b) if the parameters do not act additively, the factorial, unlike the one-factor-at-a-timedesign, can detect and estimate interactions that measure the nonadditivity.
Ind. Eng. Chem. Res., Vol. 34, No. 5, 1995 1861 M=15 mm
M=20 mm
LAJ Width of the Substrate Figure 2. Growth rate across the width of the film 15 mm from the leading edge of the film for values of FH from 10 to 20 mm. Growth temperature is T+ = 1360 "C and substrate tilt is 8- = 6".
0 parameters interact with each other as shown by the small interaction effect h x 0, 0.34 f 0.05. This interaction effect indicates that a hh+ change reduces the thickness more at 0- than at O+. Similarly, a 0O+ change increases the thickness more a t h+ than at h-. Finally, it is clear that changing the temperature from 1320 to 1360 "C does not have any effect on the growth rate. The highest and the lowest growth rates are obtained using the T&O+ and the T*h+O-conditions, respectively. Figure 1shows that growth rate uniformity across the width of the film increases in going from h- to h+. At h+,changing from O+ to 8- improves growth uniformity. Unfortunately, changes which improve growth uniformity cause a significant reduction in the growth rate. To further explore the role of FH on the growth rate and its uniformity, additional 5 min growth runs (at T+ = 1360 "C, 0- = 6", and three h values intermediate to h- and h+) were carried out using the H z - C ~ H B - S ~ H ~ system. Figure 2 shows the growth rate across the width of the film at a distance 15 mm from the leading edge for several different heights. It is clear that a sharp transition in the growth rate and its uniformity occurs when FH is increased from 13.5 to 15 mm. Additional thickness measurements were made on the samples with FH's from 13.5 to 20 mm. Growth rate profiles were measured at distances of 5 and 25 mm from the leading edge. These data along with the 15 mm growth profiles are shown in Figure 3. The plots show that as one moves away from the leading edge, the growth rate decreases while the growth rate profile flattens. Again, a sharp transition between a FH of 13.5 and 15 mm is evident for every distance from the leading edge. 4.2. Useable Area of the Film. To produce thicker films and t o study the extent of the useable film area, a second set of factorial experiments with 2 h growth runs was carried out. Growth runs of 5 min give films with thicknesses on the order of 0.1 pm, and the entire film surface is mirror-like. For films having a thickness of about 1pm or higher, there is a visible haze on the thicker portions of the films. This section presents data on how growth conditions influence the extent of the mirror-like surface area for samples produced by the 2 h growth runs. As-grown film surfaces for the 2 h factorial samples are shown in Figure 4. In this figure, the dark area depicts a mirror-like and smooth surface whereas the gray area indicates a hazy and rough surface. The direction of gas flow is toward the top of the page. Table 3 contains the results of a factorial analysis on the effects of the three parameters on the extent of the
-
-
FH=20 mm
\
Width of the Substrate Figure 3. Growth rates across the width of the film for free heights of 13.5, 15, and 20 mm. Curve numbers, 5, 15, and 25, refer to the position (in mm) from the leading edge of the film.
Table 3. Factorial Effects Analysis of the Extent of the Useable Area for the 2 h Growth Runsa T+h+e+(1067) T+h-e+(654)
Useable Film Area, mm2 T-h+e- (goo) T-h-e+ (1.23) T-h-8-
m + e +(423)
ww
T+h+e-(1445) T+h-e- (523)
Factorial Effects Analysis estimate f standard effect error ("2) average main effects T = substrate temperature h = substrate height 8 = substrate tilt two-factor interactions T x h TxB hx8 three-factor interaction Txhxe
657 & 25 530 f 50 606 f 50 -239 f 50
61 f 50 115 f 50 -186 3z 50
-69 f 50
See sections 4.2 and 5.4 for the significance of the bold items. Spwled = 70 mm2. MSF = 163 mmz. a
useable area (smooth surface). In this table, there is clear evidence of the presence of all three main effects. The substrate height has the largest effect, 606 f 50 mm2. There is also a slight two-factor interaction effect of h x 8, -186 f 50 mm2. This interaction effect indicates that a h- h+ change increases the useable area more at 0- than at O+. Similarly, a 8- 8+ change
-
-
1862 Ind. Eng. Chem. Res., Vol. 34, No. 5, 1995
I
h- (free height of CVD reactor = 20 m m )
]
,___
r
I I I1 ”. , I 11 (1 _Figure 4. As-grown surfaces of the 2 h factorial growth runs showing the effects of T,h, and 8 on the extent of the useable film area. The dark area depicts a mirror-like and smooth surface, whereas the gray area indicates a hazy and rough surface. -l ‘ l L / ~ ,
reduces the useable area more at h+ than a t h-. The largest and smallest extents of useable film area are obtained using the T+h+8-and the T-h-8+ conditions, respectively. The average surface roughness, measured b a stylus profilometer, was in the range of 127-307 for the useable area and in the range of 875-1413 A for the rough and hazy area. This profilometer gave an average surface roughness of 51 A for the untreated Si substrate. When the effects of the three parameters on the growth rate and the extent of the useable area are considered together, it is seen that the substrate height consistently has the highest effect. Furthermore, h x 8 is the only two-factor interaction effect which is important, even though its magnitude is smaller than any main effect. The most significant insights drawn from Table 3 are (a) the effed of increasing the substrate temperature is to increase the extent of the useable area, even though temperature has no effect on the growth rate, and (b) for both h and 8, changes which decrease the growth rate improve the extent of the usable area. 4.3. Inside-WallDeposition of the CVD Reactor. At the end of all growth runs, there was always a brownish-yellow powdery film deposited on the inside wall of the CVD reactor. The extent of this deposition depended on such factors as the nature and flow rate of the process gases, the temperature, the length of growth, and the relative position of the susceptor inside the reactor. This deposition tended to concentrate on the upper surface of the CVD reactor and in the vicinity of the susceptor, especially toward the downstream end of the susceptor. The two factorial experiments provided an opportunity to correlate the wall deposition with the parameters T,h, and 8. The wall deposition was visually observed at the end of all individual growth runs. In addition, kimwipe was used to clean the reactor wall, and this provided another way of assessing the extent of wall
1
L t I L O L
.
_ _ - I
Table 4. Factorial Analysis of the Inside-Wall Deposition of the CVD React0l.a
T+h+8+(4.5) T+h-8+(6.5)
Wall Deposition, Scale of 0-10 T-h+8+(2.0) T-h+8-(4.0) T-h-8+(5.0) T-h-8-(5.5)
T+h+8-(5.0) T+h-8-(6.0)
Factorial Effects Analysis estimate f standard error (scale of 0-10) effect 4.8 f 0.3 average main effects 1.4 f 0.5 T = substrate temperature -1.9 f 0.5 h = substrate height -0.6 f 0.5 8 = substrate tilt two-factor interactions 0.4 f 0.5 Txh 0.6 f 0.5 Tx8 -0.6 f 0.5 hx8 three-factorinteraction 0.1 f 0.5 Txhx8 a
See sections 4.3 and 5.3 for the significance of the bold items.
Spooled = 0.7. MSF = 1.6.
deposition. However, no attempt was made to collect and weigh the collected particles. Thus, to provide a relative measure of wall deposition within each set of runs, the extent of particulate deposition for each run was described in the laboratory notebook and compared to the previous run(s). Later, a numerical value was assigned to each run on a roughly linear scale with 5 designating an average amount of deposition, 0 implying no. visible deposition, and 10 corresponding to the highest deposition observed. Table 4 shows the factorial analysis of wall deposition averaged over the two sets of the factorial growth runs. The trends shown in this table were consistent for both sets. The only significant h+ change effect is that of the h main effect. A hreduces wall deposition by about 33%. 4.4. Determination of Crystal Quality by Hall Measurements. The electrical characteristics of the
-
Ind. Eng. Chem. Res., Vol. 34, No.5, 1995 1863 Table 5. Factorial Effects Analysis of the Hall Mobility for the 2 h Growth Runsa T+h+e+(313) T+h-e+ (297)
Hall Mobility, cm2N s T-h+B+(124) T-h+O- (200) T-h-O+ (157) T-h-8- (216)
T+h+O-(239) T+h-e- (240)
Factorial Effects Analysis estimate f standard effect error (cmW s) average main effects T = substrate temperature h = substrate height 0 = substrate tilt two-factor interactions Txh Txe hx8 three-factor interaction Txhxe
223 f 4.4 98 f 8.8 -8.5 f 8.8 -1 f 8.8
16 f 8.8 66.5 f 8.8 0.00 f 8.8 8.5 f 8.8
See sections 4.4 and 5.5 for the significance of the bold items. Spwled = 12.5 cm2N s. MSF = 29 cm2N s. a
films were determined by making Hall measurements using the van der Pauw technique [311. The carrier concentration, n, is indicative of the resistivity and chemical purity of a given material, whereas the mobil, a good indication of its crystalline perfecity, p ~gives tion. Lattice defects and impurity scattering mechanisms affect the values of both p~ and n. Desirable properties are lower values for n and higher results for p ~ Ranges . of values reported in the literature for n and p~ of p-Sic films grown using the Hz-CsHa-SiH4 system are 6.5 x 1015 t o 2.0 x 1Ol8 cm-3 and 88-370 cm2/(VSI, respectively [I7,20,24,25,261. Film samples from the 2 h growth runs were thick enough for their isolation from the substrate without developing cracks. Thus, Hall measurements were made on free-standing films so that measurement uncertainties arising from the Si substrate were avoided. The systematic errors arising from the spacing of ohmic contacts on the test samples have been accounted for by following the procedure proposed by Koon [231. Analysis of the Hall mobility data is presented in Table 5. The only significant main effect is that of temperature. However, the two-factor interaction effect, T x 8, requires interpretation. This interaction effect indicates that a T- T+ change increases Hall mobility more a t 8+ than at e-. Table 6 shows the factorial analysis for carrier concentration. All films were determined to be n-type. The effects of the parameters on carrier concentration parallel those observed for Hall mobility; the only significant main effect is that of temperature. Again, the two-factor interaction effect, T x 8, requires interpretation. This interaction effect indicates that a T- T+change reduces carrier concentration more at 8+ than a t e-. The implications are that superior electrical properties are obtained using T+and 8+, with no evident dependence on the free height. For both Hall mobility and carrier concentration, higher temperatures produce films with superior electrical properties; increasing the growth temperature from 1320 to 1360 "C enhanced the electrical properties of films as indicated by an average increase of 56% in Hall mobility and an average decrease of 65% in carrier concentration. These trends are also evident from X-ray rocking curve results which show a 35%increase in peak intensity when increasing the growth temperature from 1320 t o 1360 "C [41.
-
-
Table 6. Factorial Effects Analysis of the Carrier Concentration for the 2 h Growth R u n e Carrier Concentration, W6cm-3 T+h+B+(7.41) T-h+O+ (50.3) T-h+B- (9.4) T+h+e- (11.6) T+h-fI+ (14.2) T-h-O+ (37.6) T-h-8- (34.5) T+h-e- (13.5) Factorial Effects Analysis estimate f standard effect error (1016 cm-3) average 22.3 f 1.7 main effects T = substrate temperature -21.3 k 3.4 h = substrate height -5.3 f 3.4 0 = substrate tilt 10.1 f 3.4 two-factor interactions Txh 0.93 f 3.4 Txe -11.9 f 3.4 hxe 8.23 f 3.4 three-factor interaction Txhxe -10.7 f 3.4 a
See sections 4.4 and 5.5 for the significance of the bold items. = 4.7 x 10l6 ~ m - ~MSF . = 11.1 x 10l6 ~ m - ~ .
Spooled
Table 7. Summary of the Significant Effects of T, h, 8, and Carrier Gas change in % change in a parameter measured response response T- T+ useable area f135 Hall mobility f56 carrier concentration -65 h- h+ useable area +171 growth rate -51 reactor wall deposition -33 e- e+ useable area -31 growth rate f20 Hz HdAr growth rate -58 reactor wall deposition f65
-.
-
4.5. HdAr Equimolar Mixture as the Carrier Gas. When the H:! carrier gas was replaced with equimolar mixtures of HdAr, the growth rate was decreased by a factor of 2.4, and wall deposition was increased by a factor of 1.6. The lower growth rate may be due to smaller gas phase diffusivities in HdAr mixtures and also due to the depletion of reactive species arising from enhanced homogeneous nucleation in the gas, i.e., higher wall deposition. The growth rate is expected to fall as gas phase diffisivity decreases for any mass transfer limited process. These effects are discussed in more detail in the following sections. A summary of the significant effects of T,h, 8, and carrier gas on the measured quantities is presented in Table 7. 5. Discussion
5.1. Growth Rate and Its Uniformity. The drastic effect of reactor FH on the growth rate was shown in Figures 2 and 3. It is clear that a sharp transition in the growth rate and its uniformity occurs when the FH is increased from 13.5 to 15 mm. Thus, the FH of -14.3 mm may be considered as a threshold value below which the growth rate is decreased by almost 50% and the growth rate uniformity is greatly enhanced. An explanation for this may be found in the gas flow hydrodynamics. The gas flow is strictly laminar. Mean axial flow velocity upstream of the substrate is 1.53 cmls, and hence, Reynolds numbers, Re, at temperatures of 300, 1000, and 1400 K are 3.5, 0.5, and 0.3, respectively. There exists a temperature difference of about 1200 "C
1864 Ind. Eng. Chem. Res., Vol. 34, No. 5, 1995
between the hot susceptor and the cold wall of the CVD reactor. This steep temperature gradient is of sufficient magnitude to produce convective roll cells. In a horizontal CVD reactor, natural convection causes the cooler gas near the wall to fall, while the gas being warmed by the centrally located susceptor will rise. Depending on the carrier gas velocity and the reactor FH, recirculating flows may develop above the reactor [11,15,291. The Grashof number, Gr, is a measure of natural convection:
Gr =
e2Pg(FHl3AT P2
where e = density (g/cm3),,8 = the coefficient of thermal expansion = (l/Z'), g = gravitational acceleration (981 cm/s2),FH = the free height above substrate (cm), AT = the temperature difference between the substrate and the reactor wall (K),and p = viscosity (g/cm SI. Only the Hz carrier gas was considered for calculation of Gr. All physical properties were evaluated at temperatures midway between the substrate and the reactor wall. Numerical values for e and p are computed (at atmospheric pressure) from [lll
e= p = 8.8 x
exp
2.44 x
T 0.61732 T
At low Gr, the flow is said to be in the forced convection mode, where stream lines are curved but noncirculating. At high Gr, a mixed convection mode occurs in which both longitudinal and traverse roll cells are present. For correlating the transition between these modes, both GrlRe2 and GrlRe have been used [11,12,14,15,18,19,291. Since our Re is of order unity, we will consider only Gr. Our experimental results show that at a FH of -14.3 mm, a sharp transition takes place in the growth rate and its uniformity. This is believed to be due to a transition from a flow dominated by forced convection at small Gr values to that of a mixed convection a t Gr values higher than -155. Gr numbers at FH's of 10,13.5, 15, and 20 mm are 53,130, 180, and 425, respectively. The mixed flow regime is described as spirals which form bilaterally about the midplane of the reaction [15,29]. These spiral flows are stabilized by thermal expansion causing upward flow along the midplane of the reactor and by thermal contraction at the side walls of the flow channel causing downward flow. As shown in this study and others [9,251,epitaxial growth of B-SiC using the H z - C ~ H B - S ~ H system ~ at temperatures above 1300 "C is a diffusion-controlled process where the growth rate is determined by the rate at which the reactants reach the substrate. In the absence of mixed convection and recirculation flows, diffusion is the only mechanism by which the reactants reach the substrate. The convective transport of gas species via convection vortices or roll cells enhances the transport of reactants to the substrate. This can explain the sudden increase in the growth rate when reactor FH was increased from 13.5 to 15 mm as shown in Figure 2. For FH values equal to or higher than 15 mm, the higher growth rates at the edges of the substrate,
Table 8. Binary and Effective Mixture Diffusion Coefficients silane diffusivity (cm2/s) temperature ("C) A: Hz B: HdAr 1100 8.05 3.30 1150 8.54 3.50 1200 9.05 3.71 1250 9.57 3.91 1300 10.1 4.14 1350 10.6 4.36
AIB 2.43 2.44 2.44 2.45 2.44 2.43
as seen in Figures 2 and 3, can be explained by the recirculating flow pattern described above. Support for the existence of such roll cells also comes from the experimental results on reactor wall deposition above the substrate (see Table 4) and from modeling studies on horizontal CVD reactors [14,18,281. 5.2. Effect of Carrier Gas on Growth Rate. We noted that when the carrier gas was changed from HZ to HdAr equimolar mixtures, the average growth rate of p-Sic films was reduced by a factor of 2.4. This effect of the carrier gas on the growth rate may be explained as follows. In the H z - C ~ H B - S ~ H system, ~ it has been noted that the /?-Sic growth rate is proportional to the feed gas concentration of silane [16,21,251.The propane concentration does not strongly influence the growth rate but does determine whether the resulting p-Sic film is a mirror-like single crystal or a rough polycrystalline film. As mentioned earlier, the growth rate of p-Sic at temperatures of interest to this discussion (> 1300 "C) does not exhibit a temperature dependence, so the assumption of a diffusion-controlled process is reasonable. This implies that growth rates should be sensitive to the gas phase diffusivity of the film or "boundary layer" adjacent to the substrate. The diffisivity of silane in a pure gas can be estimated by the Chapman-Enskog theory [71with the aid of a consistent set of Lennard-Jones parameters [271. An effective binary diffusivity can also be calculated easily for the case of argon and hydrogen [71. Table 8 summarizes the calculated values of the diffusivity of silane in hydrogen and in HdAr. The temperatures in this table reflect the temperatures existing between the substrate (1320-1360 "C) and a few millimeters above the substrate [21.At all temperatures, the ratio of diffusivity in pure hydrogen is about 2.4 times that of the effective mixture diffusivity. This is surprisingly a perfect match to the experimentally observed growth rate ratio. Of course, things cannot be that simple. Thermal gradients will be larger in pure hydrogen. Thermal diffusion due to the steep temperature gradients present in the CVD reactor, the Soret effect, should be important when hydrogen is the carrier gas but would be reduced in the presence of the higher molecular weight argon (the Soret coefficient has an inverse proportionality to molecular weight) [29,7J.Also, enhanced homogeneous gas phase nucleation due to the presence of argon, discussed in the next section, may reduce the growth rate by depleting the reactive species in the gas phase and producing particulates which deposit on the reactor wall. Despite these uncertainties, the large difference in silane diffusivity in H2 and HdAr must explain in part why the growth rate is altered. Finally, it is quite possible that there will be some degree of compensation among the secondary effects as pointed out by Rosenberger [29].This latter study shows that the film thickness or growth rate distribution in a CVD reactor appears to be insensitive to temperature. With constant
Ind. Eng. Chem. Res., Vol. 34, No. 5, 1995 1865 physical properties (evaluated at the average temperature), the species diffusivities in the vicinity of the substrate are often estimated too low, but at the same time, the (local) Grashof number is too high. Whereas the first inaccuracy leads to a reduction of the interfacial diffusion fluxes, the second error tends to increase convective transport contributions and, thus, to compensate for the first error. The temperature dependence of the Grashof number and mass diffusivity are Gr = T-4.3and Du P,7, respectively. It is worthwhile to mention that an equimolar mixture of HdAr as the carrier gas allows for monocrystalline growth of p-Sic at atmospheric pressure and a t substrate temperatures as low as 1150 "C [9,101. 5.3. Particulate Deposition inside the CVD Reactor. A summary of wall deposition results is presented in Table 4. The powdery wall deposition tended to concentrate on the upper surface of the CVD reactor and in the vicinity of the susceptor, especially toward the downstream end of the susceptor. The origin of wall deposition is particulate formation due to homogeneous nucleation in the gas. The main radical species suspected of initiating the gas phase nucleation is SiH2 produced in the decomposition of silane:
SiH, = SiH,
+ H2
When the carrier gas was changed from Hz t o HdAr equimolar mixtures, the extent of wall deposition increased by about a factor of 2. A lower hydrogen partial pressure will influence the homogeneous gas phase reactions by promoting the formation of SiH2. Higher concentrations of SiH2, a very reactive radical species, therefore enhances gas phase reactions and particle nucleation which deplete the reactive species needed for heterogeneous surface reactions of film growth. This phenomenon is partly responsible for the lower growth rate observed for runs using HdAr as the carrier gas. The brownish-yellow wall deposit can be dissolved in silicon etchant (60%HF:30% HN03:10% H20). However, a brown particulate would float in this solution. We speculate that the yellow portion of the deposit is some structure of silicon (e.g., polysilicon) and that the smaller brown portion contains silicon carbide formed by reaction with propane. Support for the above speculation comes from a simple experiment; when the flow rate of only silane was increased by a factor of 10, the amount of the yellow deposit increased proportionally. Thus, the main source of the deposit is the siliconcontaining gas, and the deposit must be a mixture of Si and Sic. Of the three parameters studied in the factorial design, only h had a significant effect on particulate deposition. The effect of susceptor height is explained by the presence of mixed convection and recirculation flows a t larger reactor FHs. In addition to poorer film thickness uniformity, recirculation is detrimental to good epitaxy since it increases the probability of gas phase nucleation with the resultant loss in layer quality [11,261.Moreover, the presence of roll cells brings back surface reaction products onto the surface. This will reduce reactant concentration near the surface. Also, roll cells can sweep fresh reactants toward the sides of the substrate, resulting in higher rates of surface impaction and, hence, growth rate in the region. This suggests that a higher ratio of surface impaction to surface diffusion leads to a rougher surface morphology. These points are discussed in more detail in the next
section. The extent of recirculation flows and mixed convection may be reduced by growth at small FH's, lower pressures, high carrier gas velocities, and low g (microgravity). 5.4. Extent of the Useable Area of the Film. Figure 4 shows the as-grown surfaces of 2 h samples where the dark area depicts the useable area of the p-Sic films (mirror-like and smooth film surface) and the gray area indicates a hazy and rough surface. Generally, the film surface at the upstream end and along the sides appears rough and gray. The extent of this rough area is variable and, in one case (T-h-8+), covers all of the substrate. Substrate height has the greatest effect on the useable area; smaller F H s increase the extent of the useable area. The effect of increasing the substrate temperature is to increase the extent of the useable area, even though temperature has no effect on growth rate. Susceptor tilt has the smallest effect; steeper angles decrease the extent of the useable area. For both h and 8, changes which decrease the growth rate improve the extent of the useable area. These effects may be due to two main factors which are (a) mixed convection and (b) the rate of surface diffusion relative to the rate of surface impaction of gas species. In the following paragraphs, the contribution of these two factors will be discussed separately. Mixed Convection. As discussed in the previous section, one result of homogeneous nucleation in the gas is the particulate deposition on the wall of the CVD reactor. A second manifestation of homogeneous gas phase reaction is particulate deposition on the growing film. It was observed that growth at the larger FH is accompanied by recirculating flow currents near the sides of the substrate and that growth rates along the sides were higher than in the middle of the film. Furthermore, particulate deposition on the reactor wall was also higher for growth at the larger FH. These observations suggest a direct dependence of the rough film surface along the sides of the film on the presence of mixed convection. Thus, conditions that suppress mixed convection also suppress the extent of wall deposition in the CVD reactor and product films with a larger useable area and higher thickness uniformity. Surface Diffusion. A temperature increase from 1320 t o 1360 "C was shown to increase the useable area considerably. Powell et al. [261also reported the formation of polycrystalline p-Sic clumps on the film. They observed that clump formation increased at lower temperatures (less than 1350 "C) and lower SYC ratios in the input gases (less than 0.45). They also observed that more clumps occurred near the sides of the susceptor. It is obvious that the temperature of the upstream end of the substrate is reduced by the input gases which are at room temperature. The same condition also exists, to a lesser degree, for the sides of the substrate. This corresponds to the presence of a higher growth rate a t the upstream end of the substrate and, to a lesser degree, for the sides of the substrate, especially for the growth runs having the larger FH (see Figure 3). The higher flux of the gas species along with the lower surface diffusion (adatom mobility) at lower temperatures will lead to a rough polycrystalline or amorphous film surface. This is similar to the growth regimes observed for the deposition of Si film on Si substrate: (a) a t low temperature and high deposition rates, the deposits are amorphous; (b) at high substrate temperatures and low deposition rates, they tend t o be
1866 Ind. Eng. Chem. Res., Vol. 34, No. 5 , 1995
single crystal; and (c) at intermediate conditions, polycrystalline films tend to form [331. In the growth process of p-Sic by CVD, it is observed that an increase in the density of critical nuclei on the growing surface and/or a decrease in temperature leads to a rougher surface morphology [5,221. In a study by Kong et al. [221, the ratio of the sum of the flow rates of SiH4 and C2H4 to the flow rate of Ha was varied from 1:3000to 3:3000. At the lowest sourceharrier gas ratio, 1:3000,monocrystalline p-Sic films were obtained with smooth as-grown surfaces. When the flow ratio was increased t o 1.5:3000, monocrystalline films having a pyramidal surface morphology formed. The flow ratio of 3:3000 resulted in a polycrystalline film having a very rough surface. In the same study, p-Sic films were grown on 6H-Sic substrates a t 1400, 1450, and 1550 "C with a gas flow ratio of 1.5:3000. It was clear that the higher growth temperature yielded a smoother surface. In a study of Bahavar et al. [51, C3H8 was replaced with CHI as the carbon source (both gas flow rates supplying the same molar flow rate of carbon atoms), increasing the sourceharrier gas ratio from 1.6: 3000 to 3:3000. Although both C3H8 and CH4 produced monocrystalline p-Sic films, the average values for surface roughness (for 5 pm films) were 130 and 1235 A, respectively. Growth rates were almost identical for both carbon sources. The results of both of the above studies are explained in terms of surface diffusion or surface mobility of adatoms; for a growing film surface with a relatively low density of critical nuclei, sufficient time and surface area are available to allow for enhanced lateral growth of the film via adatom migration and incorporation into the edges of the reduced number of nuclei. Higher growth temperatures also increase surface mobility and produce smoother film surfaces. By the same reasoning, higher densities of critical nuclei and/or lower temperatures reduce the adatom migration which, in turn, causes a decreased lateral growth rate and thus a rougher film surface. The analysis of Stinespring et al. [301of the gas phase kinetics of p-Sic CVD indicates that the decomposition of hydrocarbons (e.g., C3H8) is more kinetically limited than that of silane. Also, the surface reactivity and sticking coefficients of the decomposition products are different [I]. Thus, one would expect that the Si/C ratio at any given point on the growing p-Sic film surface is sensitive to temperature, average flow rate, and recirculating flow currents caused by mixed convection. "his study demonstrates that the extent of useable area is almost equally affected by reactor FH and temperature, both factors being varied in a relatively narrow but critical range. In general, increasing the rate of surface diffusion relative to the rate of surface impaction of gas species leads to a more uniform surface film. 5.5. Film Characterization by Hall Measurements. Because of the potential for electronic device applications of @-Sic crystals, the Hall measurements were the most important characterization in our study. Analyses of Hall measurements indicate that temperature is the main parameter of interest. This is most probably due to the fact that all samples for these analyses were cut from the smooth and shiny parts (useable area) of the as-grown films. Thus, one would expect that recirculating flow currents were not as important in the growth of these samples despite the variation of FH.
The effect of temperature may be explained in terms of surface diffusion or adatom mobility and variation of the Si/C ratio. As discussed earlier, formation of rough surface increases at the lower temperature (1320 "C), even though temperature has no effect on growth rate. In addition, the structural quality of the film, as shown by Hall mobility and X-ray rocking curve intensities, deteriorates when the growth temperature is reduced from 1360 to 1320 "C. The lower surface diffusion at the lower temperature can lead to a higher defect density. Point defects (e.g., antisite defects, vacancies) could act as donors or acceptors [20,261. As indicated in the previous section, one would expect that the Si/C ratio at any given point on the growing p-Sic film surface is sensitive to temperature among other factors. We noted that for both carrier concentration and Hall mobility, there was a strong interaction between the temperature and the susceptor tilt; increasing the temperature from 1320 to 1360 "C improved crystal quality more at the susceptor tilt of 10" than a t the tilt of 6". As discussed previously, the ratio of surface diffusion to surface impaction is an important parameter for uniform growth. Therefore, it is not surprising that the higher temperature, meaning higher surface diffusion, is more important at higher impaction ratios (higher susceptor tilt). 6. Summary The growth system of Hz-C~HS-S~H~ was used to produce P-SiC films on silicon (100) substrates using chemical vapor deposition. Thickness ranged from 30 nm to 10pm. "he effects of the CVD reactor parameters on the film quality and uniformity were studied using a two-level factorial design with three parameters: growth temperature, reactor FH, and susceptor tilt. Of the three parameters, reactor FH had the most significant effect on the film growth rate and its uniformity. At an estimated FH threshold value of 14.3 mm, a sharp transition in the growth rate and its uniformity occurs. For FH's of 13.5 mm or smaller (corresponding t o Gr I 1301, the growth rate is lower and its uniformity is greatly improved. For F H s of 15 mm or larger (corresponding t o Gr L 1801, the growth rate is higher but there is less uniformity in film thickness. The extent of the useable film area is strongly dependent on the reactor FH and the growth temperature. A FH of 10 mm, a growth temperature of 1360 "C, and a susceptor tilt of 6" gave the highest useable area, whereas a FH of 20 mm, a growth temperature of 1320 "C, and a susceptor tilt of 10" gave the least useable area. When the carrier gas was changed from H2 to HdAr equimolar mixtures, the average growth rate of ,&Sic films was reduced by a factor of 2.4. This was explained by the lower silane diffusivity in the HdAr mixture. Particulate deposition on the reactor wall, originating from the homogeneous gas phase nucleation, was greater for larger F H s and for HdAr carrier gas mixtures. The electrical properties of the useable areas of the films were examined using Hall measurements. Temperature had the strongest effect on crystal quality; increasing the temperature from 1320 to 1360 "C produced films having 65%lower carrier concentrations and 56% higher Hall mobilities. Also, for both carrier concentration and Hall mobility, there was a strong interaction between the temperature and the susceptor tilt; an increase in temperature improved the crystal quality more at the susceptor tilt of 10" than at the tilt of 6". Collaborative efforts are in progress on a comprehensive modeling of the CVD reactor that takes into account
Ind. Eng. Chem. Res., Vol. 34, No. 5, 1995 1867 full transport phenomena and reaction chemistry in both the gas phase and on the film surface.
Literature Cited (1) Allendorf, M. D.; Kee, R. J. A Model of Silicon Carbide 1991,138,841. Chemical Vapor Deposition. J.Electrochem. SOC. (2) h e n , K. D.; Stinespring, C. D.; Kuczmarski, M. A.; Powell, J. A. Modeling of the S i c Chemical Vapor Deposition Process and Comparison with Experimental Results. J . Vac. Sci. Technol. 1990,A8,2970. (3) Bahavar, B.; Chaudhry, M. I.; McCluskey, R. J. In Wide Band-Gap Semiconductors; Moustakas, T. D., Pankove, J. I., Hamakawa, Y., Eds.; Materials Research Society: Pittsburgh, PA, 1992; p 555. (4) Bahavar, B. Growth and Characterization of beta-Silicon Carbide Thin Films. Ph.D. Dissertation, Clarkson University, Potsdam, NY,1993; p 101. (5) Bahavar, B.; Chaudhry, M. I.; McCluskey, R. J. Effects of Propane and Methane on Carbonization and Surface Morphology in Hetero-epitaxial Growth of,!?-SiC Fims on Si (100) via Chemical Vapor Deposition. Appl. Phys. Lett. 1993,63,914. (6) Ban, V. S. Transport Phenomena Measurements in Epitaxial Reactors. J . Electrochem. Sac. 1978,125,317. (7) Bird, R. B.; Stewart, W. E.; Lightfoot, E. N. Transport Phenomena; Wiley: New York, 1960; pp 510,567. (8) Box, G. E. P.; Hunter, W. G.; Hunter, J. S. Statistics for Experiments; Wiley: New York, 1978; p 306. (9) Chaudhry, M. I.; McCluskey, R. J.; Wright, R. L. The Role of Carrier Gases in the Epitaxial Growth of &Sic on Si by CVD. J. Cryst. Growth 1991,113,120. (10) Chaudhry, M. I.; Wright, R. L. Epitaxial Growth of ,??-Sic on Si by Low-Temperature Chemical Vapor Deposition. J.Mater. Res. 1990,5,1595. (11) Chinoy, P. B.; Agnello, P. D.; Ghandhi, S. K. An Experimental and Theoretical Study of Growth in Horizontal Epitaxial Reactors. J. Electron. Mater. 1988,17,493. (12) Donaghey, L. F. In Crystal Growth; Pamplin, B., Ed.; Pergamon Press: New York, 1979; p 72. (13) Edgar, J. H. Prospects for Device Implementation of Wide Band Gap Semiconductors. J.Mater. Res. 1992,7,235. (14) Evans, G.; Greif, R. A Study of Traveling Wave Instabilities in a Horizontal Channel Flow with Applications to Chemical Vapor Deposition. Intl. J.Heat Mass Transfer 1989,32,895. (15) Giling, L. J. Gas Flow Patterns in Horizontal Reactor Cells Observed by Interference Holography. J.Electrochem. SOC.1982, 129,634. (16) Hattori, Y.; Suzuki, T.; Murata, T.; Yabumi, T.; Yasuda, K.; Saji, M. Growth Mechanism of 3C-Sic Layers at a Low Temperature Region in Low-Pressure CVD. J. Cryst. Growth 1991,115,607. (17) Ikoma, K.; Yamanaka, M.; Yamaguchi, H.; Shichi, Y. Heteroepitaxial Growth of B-Sic on Si (111)by CVD Using a CH3C1-siH4-H~Gas System. J.Electrochem. SOC.1991,138,3028.
(18) Jensen, K. F.; Einset, E. 0.;Fotiadis, D. I. Flow Phenomena in Chemical Vapor Deposition of Thin Films. Annu. Rev. Fluid Mech. 1991,23,197. (19) Kern,W.; Ban, V. S. In Thin Film Processes; Vossen, J. L., Kern, W., Eds.; Academic Press: New York, 1978; p 258. (20) Kong, H. S.; Wang, Y. C.; Glass, J. T.; Davis, R. F. The Effect of off-Axis Si (100) Substrates on the Defect Structure and Electrical Properties of B-Sic Thin Films. J.Mater. Res. 1988,3, 521. (21) Kong, H. S.; Glass, J. T.; Davis, R. F. Epitaxial Growth of B-Sic Thin Films on 6H a-SiC Substrates via Chemical Vapor Deposition. Appl. Phys. Lett. 1986,49,1074. (22) Kong, H. S.;Glass, J. T.; Davis, R. F. Growth Rate, Surface Morphology, and Defect Microstructure of Films Chemically Vapor Deposited on 6H-Sic Substrates. J . Mater. Res. 1989,4, 204. (23) Koon, D. W. Effect of Contact Size and Placement, and of Resistive Inhomogeneities on van der Pauw Measurements. Rev. Sci. Znstrum. 1989,60,271. (24) Liaw, P.; Davis, R. F. Epitaxial Growth and Characterization of B-Sic Thin Films. J . Electrochem. Soc. 1985,132,642. (25) Powell, J. A.; Matus, L. G.; Kuczmarski, M. A. Growth and Characterization of Cubic S i c Single-Crystal Films on Si. J. Electrochem. SOC.1987,134,1558. (26) Powell, J. A.; Matus, L. G. In Amorphous and Crystalline Silicon Carbide II; Rahman, M. M., Yang, C. Y., Harris, G. L., Eds.; Springer-Verlag: Berlin, 1989; p 14. (27) Reid, R. C.; Prausnitz, J. M.; Poling, B. E. The Properties of Gases and Liquids; McGraw-Hill: New York, 1987; p 733. (28) Rhee, S.;Szekely, J.; Ilegbusi, 0.J. On Three-Dimensional transport Phenomena in CVD Processes. J. Electrochem. SOC. 1987,134,2552. (29) Rosenberger, F. In Proc. Znt. Conf.Chem. Vap.Deposition, 10th 1987,11. (30) Stinespring, C. D.; Warmhoudt, J . C. Surface Studies Relevant to Silicon Carbide Chemical Vapor Deposition. J.Appl. Phys. 1989,65,1733. (31) van der Pauw, L. J. A Method of Measuring Specific Resistivity and Hall Effect of Discs of Arbitrary Shape. Philips Res. Rep. 1958,13,1. (32) Wolf, S.; Tauber, R. N. Silicon Processing for the VLSI Era; Lattice Press: Sunset Beach, CA, 1986; Vol. 1: Process Technology, p 618. (33) Wolf, S.; Tauber, R. N. Silicon Processing for the VLSI Era; Lattice Press: Sunset Beach, CA, 1986; Vol. 1: Process Technology, p 124. Received for review August 12, 1994 Revised manuscript received January 26, 1995 Accepted February 9, 1995@
IE940488A Abstract published in Advance ACS Abstracts, April 1, 1995. @
