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Characterization of low pressure chemically vapor deposited silicon nitride using experimental design
Thin Solid Films, 206 ( 1991)
I I - I7
II
Characterization of low pressure chemically vapor deposited silicon nitride using experimental design J. A. Gregory, Lincoln Laboratory,
Douglas Massachusetts
J. Young, Institute
R. W. Mountain
of Technology,
Lexington,
and C. L. Doherty,
Jr.
MA 02173 (U.S.A)
Abstract During the low pressure chemical vapor deposition of SisN,, significant interactions occur between the temperature, time, and gas flows, and thus a designed set of experiments has been used to establish the optimum deposition conditions, The conditions employed in this study were dictated by a full factorial matrix of the independent variables over 20 deposition runs. This procedure allowed us to vary the flow rates of NH, and SiH,CI, as well as the temperature and time of deposition. In this way we were able to determine the effects of these variables and their interactions on the electrical, physical, and chemical properties of the S&N,. Deposition conditions can then be chosen to optimize film properties. Two sets of samples were prepared: Si,N, was deposited either directly on silicon substrates or on silicon covered with 25 nm of SiO,; the nitride films deposited on SiO, were subjected to a wet oxidation to simulate the structure in working devices, Following the formation of polysilicon electrodes on both types of samples, capacitance-voltage measurements were performed to obtain data on fixed charge in the insulating layers and on interface state density. The properties investigated were thickness and composition of the S&N,; profiles of the silicon, nitrogen, oxygen, and chlorine content were obtained by secondary ion mass spectrometry and Rutherford backscattering analysis. We discovered that a significant amount of chlorine was incorporated in the nitride layers during deposition, that this chlorine was able to diffuse through an underlying SiO, layer and to accumulate at the Si&SiO, interface, and that the chlorine had a strong effect on the amount of fixed charge in the structure. The deposition rate increased with the ratio of SiH,CI, to NH, flow, as did the amount of chlorine in the nitride, while the magnitude of the negative fixed charge in the dielectric layer decreased. Following an induction period that depended on temperature and gas flow rates, the deposition rate was thermally activated and constant.
1. Introduction Silicon nitride deposited by low pressure chemical vapor deposition (LPCVD) is a dielectric that is widely of used, most often as a layer to mask the oxidation underlying silicon layers. In many applications the Si,N, is removed after an oxidation step, before the active device is completed, so the effects of the S&N, on the properties of the device are not fully characterized. For charge-coupled devices (CCDs) and memory devices such as electrically programmable read-only memories (EPROMs), the common practice is to leave the nitride in place during device fabrication. In the case of CCDs, the gate oxide must be masked during several distinct oxidation steps [ 11. Consequently, the nitride forms an integral part of the dielectric, separating the CCD channel from the polysilicon electrode, and it can influence the behavior of the threshold voltage and other properties of the metal-oxide-semiconductor (MOS) structure. The inclusion of Si,N, in the gate dielectric composed of oxide-nitride-oxide layers on silicon, referred to as the equivalent gate oxide (EGO), can introduce substantial amounts of negative fixed charge in the dielectric with a resulting shift in threshold voltage towards more positive values
[2]. In an effort to understand and control this negative fixed charge, a set of designed experiments were performed to characterize the effects of the deposition temperature and time and the ammonia and dichlorosilane flow rates on properties of the LPCVD Si,N, layers.
2. Experimental
details
The properties of thin films are influenced by many variables, which may interact with each other in ways that are more complicated than a simple linear superposition of their individual effects. If one is willing to apply mathematical models, however, many combinations of experimental conditions are possible, and designed experiments [3,4] may be applied to formulate a problem of manageable proportions. In the present case, a full factorial, central composite design (ref. 3, p. 324) explained below, was applied to generate the conditions employed during LPCVD of silicon nitride. The depositions were performed in an evacuated quartz tube, with a pressure of about 15 Pa ( 110 mTorr); the steady state pressure was determined by the flow rate of the gases and the speed of the unthrottled vacuum pump.
Elsevier
Sequoia,
Lausanne
J. A. Gregor,v et 01. 1 Charcrcterixrtion
12
The deposition conditions are shown in Table 1. In order to cover all combinations of high and low settings for four independent process variables, it was necessary to perform 24 = 16 runs. In the arrangement shown in the table, the temperature alternates between 748 and 828 “C, while the deposition time alternates between 300 and 900 s (5 and 15 min) in groups of two runs. The ammonia flow rate alternates between 49 and 77 pmol SK’ (70 and 110 standard cm3 min-‘) in groups of four runs, and the dichlorosilane flow rate alternates between 7 and 35 pmol s- ’ ( 10 and 50 standard cm3 min-‘) in groups of eight runs. The order of the runs, except for those made under standard conditions, was randomized to block the influence of extraneous factors. Runs 1, 7, 13 and 20 in the table are replicates of a standard deposition process employed at Lincoln Laboratory and are referred to as centerpoints of the design. The design assumes that a particular property depends linearly on the independent variables and on a first-order interaction of these variables; for a dependent variable Y, the relationship to the independent variables Xi is Y = CO+
1 CiXj + C C I
i
CgxiXj
j#i
where c0 is the value of Y obtained at the centerpoints, ci = a Y/13x,, and cij = a2 Y/ax, ax,. The coefficients are derived by fitting the measured responses to the values of the independent variables with a multivariable regression program [5]. The centerpoint runs are not included in this analysis; they were used to test the fit of the model. The samples were fabricated on 3 in n-Si substrates. Sixty of the substrates used for deposition of the Si,N, had 25 nm of SiO, grown in a dry ambient at 1000 “C, while another 60 were bare. For each run in Table 1, one bare silicon and one oxide-coated silicon wafer were inserted at the front, the middle, and the back of the boat. The remaining slots in the boat were filled with 44 nitride-coated dummy wafers. The boat was then loaded into the furnace for deposition of the nitride layer. The thickness of the nitride layer deposited on either the bare silicon or the oxide was determined by ellipsometry and by optical interferometry. After all 20 nitride depositions were performed, the 60 samples with an oxide layer below the nitride were inserted in a furnace and subjected to wet oxidation at 1000 “C for 35 min. These conditions convert about 15 nm of Si,N, to 25 nm of SiO,. After oxidation all samples were coated with 500 nm of polysilicon at 626 “C. The polysilicon was doped at 1000 “C with POCl,, patterned with photoresist, and etched to form 1.1 mm diameter dots of polysilicon. The back surface of each
of LPCVD
Si, N,
wafer was etched down to bare silicon, coated with aluminum, and sintered to complete the formation of MOS structures. High frequency capacitance-voltage C-V measurements were performed with the bias swept from - 10 to 10 V with a 1 MHz test signal superimposed. Low frequency measurements were made in the quasi-static mode with a bias ramp rate of 0.1 V s-‘. The composition of the films were determined using two techniques. Secondary ion mass spectrometry (SIMS) was used to obtain a composition profile for selected elements over a large range of concentrations, and Rutherford backscattering (RBS) analysis was performed to determine the absolute amounts of the predominant constituents. The SIMS measurements, which were obtained using an 0, ion beam, were performed by Evans East in Princeton, NJ, and the RBS analysis was carried out by Charles Evans Associates in Redwood City, CA.
3. Results 3.1. Nitride thickness The thickness of the nitride films is shown in Table 1. It is immediately apparent that the layers deposited at 828 “C and with a dichlorosilane flow rate of 35 pmol s-’ (50 standard cm3 min-‘) have the highest average thickness. A comparison of the results for the four centerpoint runs shows excellent reproducibility of the deposition thickness. The differences in the thicknesses of the nitride deposited on silicon and SiO, are not shown in the table, but these were generally less than I nm for a given set of deposition conditions. 3.2. Charge in dielectric Table 2 presents data on the amount of fixed charge in the MOS structures. The fixed charge in the EGO samples is calculated as if it were either a sheet at the Si-SiO, interface or a sheet at the Si02 -Si,N, interface located closest to the silicon substrate. For the nitride films deposited on silicon substrates, the charge is calculated as a sheet at the Si-Si,N, interface and also as if it were distributed uniformly throughout the S&N,. It is apparent that the charge in the nitride samples is of opposite sign and about 10 times larger in magnitude than that in the EGO samples. 3.3. Composition Profiles obtained by SIMS for an EGO sample are displayed in Fig. 1, showing the number of counts us. the sample depth (the depth scale is only approximate and assumes equal sputtering rates in all phases). The oxygen signal clearly marks the presence of the ttio Si02 layers, while the plateau for the nitrogen signal indicates the intervening layer of Si,N,. Note that the
300 300 900 900 300 300 900 900 300 300 900 900 300 300 900 900 600 600 600 600
748 828 748 828 748 828 748 828 748 828 748 828 748 828 748 828
788 788 788 788
16 I2 5 I7 8 4 11 2 IO 3 19 I8 14 15 6 9
I 7 I3 20
Run order (s)
vapor
(
21
(90) (90) (90) (90)
63 63 63 63
(10) (10) (IO) (IO) (10) (10) (10) (10) ( 50) (50) (50) (50) (50) (50) (50) (50)
7 7 7 7 7 7 7 7 35 35 35 35 35 35 35 35
(70) ( 70) (70) (70) (110) (110) (110) (110) (70) (70) ( 70) (70) (110) (110) (110) (110)
49 49 49 49 77 77 77 77 49 49 49 49 77 77 77 77
films
cm3 min- ‘)
silicon nitride
(standard
flow
deposited
(urn01 s-‘)
SiH2CI,
chemically
cm3 min-‘)
of low pressure
(standard
flow
residuals
(urn01 ss’)
NH,
rate, and statistical Time
deposition
(“C)
1. Thickness,
Temperature
TABLE
.I
1.0 21.7 15.9 74.9 2.5 23.8 16.8 81.8 6.2 37.3 36.9 136.7 6.5 36.0 38.0 141.5
(nm)
Thickness
0.100 0.089 0.026 0.089 0.250 0.097 0.028 0.097 0.052 0.166 0.051 0.166 0.054 0.160 0.053 0.172
Deposition (nm s-‘)
rate
-0.374 0.133 0.011 0.098 0.266 - 0.074 -0.280 - 0.084 0.138 -0.118 0.024 -0.086 0.118 -0.206 0.054 ~ 0.022
Normalized
residual
h
s
748 828 748 828 748 828 748 828 748 828 748 828 748 828 748 828
788 788 788 788
I 7 13 20
600 600 600 600
300 300 900 900 300 300 900 900 300 300 900 900 300 300 900 900
(s)
16 12 5 17 8 4 II 2 IO 3 19 18 14 15 6 9
Time
flow
63 63 63 63
49 49 49 49 77 77 77 77 49 49 49 49 77 77 77 77 (90) (90) (90) (90)
(70) (70) (70) (70) (110) (110) (110) (110) (70) (70) (70) (70) (110) (110) (110) (110)
(standard
cm’ min
gate oxide and %Si,N,
(pm01 s ‘)
NH,
in equivalent
( C)
distribution
Temperature
2. Charge
Run order
TABLE
‘)
(50) (30) (30) (30) (30)
-3.6 -4.0 -3.7 -3.9
-3.6 -3.3 -2.5
-2.2 -2.0 -2.0 -2.2 -2.5 -2.3 -2.4
-2.5 -2.7 -2.1
-1.5 - 1.6 -1.6
-4.0
-2.9
(50) (50) (50) (50) (50) (50) (50)
-4.6
-2.6
(10) (10)
-4.7
Centroid at SiO,-Si,N, interface ( x 10’Se m-‘)
in EGO films
-4.2
-2.6
Centroid at Si-SiOz interface ( x 1015e m-‘)
Fixed charge
-3.0
21 21 21 21
films
cm’ min- ‘)
deposited
(10) (10) (10)
(10) (10) ( 10)
7 7 7 7 7 7 7 7 35 35 35 35 35 35 35 35
vapor
(standard
flow
chemically
(pm01 s-‘)
SiH,Cl,
low pressure
films
8.4 Il.44 -0.02 2.7 5.8 11.01 -0.04 -4.07 3.63 5.35 0.1 -7.67 3.48 5.34 0.04 2.95 2.81 2.74 3.16
25 24 23 25
(10’4em-‘)
Charge density in Si,N,
in Si-Si,N,
40 35 -0.31 13 30 35 -0.73 -5.6 29 39 2.9 -10 25 39 1.2
Centroid at Si-Si,N, interface ( x IO”e mm’)
Fixed charge
6
J. A. Gregory et al. / Characterkation
of LPCVD
Si, N4
15
basic assumption is that any coefficients for a quantitative model can be derived by analyzing the data from the 16 runs, where the independent variables were set at their extreme values. Subsequently, the values predicted for the centerpoint conditions can be compared with the observed values for these runs. The replicates of the centerpoints define a confidence interval for acceptance of the model, based on the standard deviation of the measurements and Student’s t distribution (ref. 4, p. 110). We attempted to fit various models to the data. The one that provided the best fit, while also being physically plausible, is d = 2.72 x 10” exp[ - AE/k( T + 273)](t - T) X ( -0.0033 - 2.35 x DEPTH (nm)
Fig. I. Secondary
ion mass intensity
vs. depth
for an EGO sample.
DEPTH (nm)
Fig. 2. RBS composition
us. depth
for an EGO
sample
chlorine signal follows the nitrogen signal and rises in the Si,N, layer, falls in the SiO, layer, and rises again at the SiO,-Si interface. Similar overall behavior is seen in the RBS in Fig. 2. In this EGO sample, the outer layer of SiO, was removed by wet etching to improve depth resolution for RBS; the nitrogen and chlorine signals are high in the nitride layer (the chlorine signal has been magnified by 1000) and then fall off as the oxygen signal rises in the SiO, layer. The chlorine content in the nitride layer is quite large, about 0.1 at.% or 5 x 1O25me3. No peak of nitrogen or chlorine is detectable at the SiO,-Si interface in the RBS analysis.
4. Discussion 4.1. Nitride thickness The data in Table 1 on nitride thickness tion conditions can be modeled in several
vs. deposiways. The
- 711 QNH, + 3022QsiHZclZ
107Q,t+,QsiH2c12)
(2)
where d(nm) is the thickness of the deposit, AE is the activation energy, T is the temperature, t is the deposition time, r is the induction time necessary to start deposition, and Qr.,n, and QsiHzclz are the flow rates for ammonia and dichlorosilane respectively. The induction period depends on temperature and gas flows (interactions occur between the independent variables) and varies between 55 and 290 s. Unfortunately, these values include electromechanical delays in the gas delivery system, so all that we can say with certainty is that the induction period decreases as the temperature increases and that it decreases with SiH,Cl, flow at low temperature and increases with the flow at high temperature. Table 1 indicates the range of values for steady state deposition rate encountered during the runs. The regression program Strategy was used to generate a residual, that is the difference between the predicted and observed thicknesses. The residual values, normalized by the observed deposition rate, are included in Table 1. The agreement between the predicted and measured thicknesses is quite good, with the absolute value of the normalized residual averaging 0.11. The agreement is especially good for the centerpoint runs, which were not used in computing the model coefficients, and is well within the confidence interval limits ( f 8%) of the predicted value. As a further test of the model, historical data were collected for nitride thicknesses between 10 and 300 nm deposited at 788 “C, from which an induction time of 40 s and a steady state deposition rate of 0.072 nm s-’ were inferred, in excellent agreement with the centerpoint rates. Although the model based on the full factorial approach is a means of fitting the data and cannot pinpoint the physical mechanism responsible for controlling the deposition rate, the activation energy derived ( 151 kJ mol-‘) is comparable with other values
J. A. Gregory et cd. / Charcccterizution
16
obtained in kinetic studies of LPCVD Si,N, [6, 71. The presence of an induction period suggests that an island growth and coalescence mechanism [8] is responsible for the early stages of deposition. Further, as eqn. (2) makes clear, an antagonistic relationship exists between the deposition rates with ammonia and dichlorosilane. Increasing the flow of dichlorosilane increases the deposition rate, but increasing the ammonia flow increases the deposition rate at low dichlorosilane flows and decreases it at high dichlorosilane flows. This interaction might be due to competition for surface sites on the growing nitride film. 4.2, Charge in dielectric and composition As in previous work on fixed charge in nitride [9], it is not possible to determine from the data for EGO samples whether the charge is a sheet at the SiOZ-Si interface, a sheet at the Si,N,-SiO, interface, or distributed evenly throughout the nitride layer. This is a moot point for most applications since the operational characteristics of the device, such as the threshold voltage, do not depend on the model, giving some latitude in assigning a mechanism to the generation of charge. Regardless of the precise nature of the charge, a fit with small residuals can be obtained to the first of these models using a simple formula: qsl_sio, = -3.44 -7.02
+ 6700QNu, + 8.42 X 104QsiH2ci2 x
l@QNH, QSiH>ClZ
(3)
where qsi~sio, ( x 10”e m-*( x 10”e cm-*)) is the fixed charge in the EGO, assuming that the centroid is at the Si-SiO, interface. It is not necessary to include the effects of temperature or time on the fixed charge, since the flow rates are dominant. It should be noted that increasing either gas decreases the magnitude of the net negative charge. Although it is generally held that fixed charge in nitride is positive [9], as seen in Table 2 for the samples with Si,N, deposited directly on silicon there is evidence that both positive and negative defects can arise [2], albeit at much higher ratios of ammonia to dichlorosilane flow than encountered here. It is unlikely that this negative charge is due solely to isolated Cl- ions in the nitride lattice, since the amount of chlorine tends to increase as the dichlorosilane flow rate increases and the amount of negative charge decreases. The RBS data indicate that the films are silicon rich with an Si:N ratio around 0.83, so the increase in charge could be due to an increased concentration of Si” centers, offsetting the negative charge of some unspecified defect. The presence of chlorine in nitride following deposition from a chlorosilane is known from the ceramics literature [ 10, 1 l] and has also been detected following plasma-enhanced CVD of nitride films [ 121, but it ap-
c~f LPCVD
Si, N,,
pears to have been overlooked in LPCVD applications. Residual gas analysis performed here and elsewhere [ 131 suggests the presence of gas phase precursors of the form SiHCl., NH_,.. It may be that these molecules are adsorbed on the growing nitride film, and although the NH, tends to displace adsorbed chlorine [7] some chlorine is trapped in the S&N, structure. Figure 1 clearly indicates the diffusion of chlorine through the underlying SiO, film. The diffusion of NH, [ 14, 151 is also evident in Fig. 1 from the nitrogen signal accumulating at the Si-SiO, interface. The quasi-static data suggest that the density of surface states at the Si-SiO, interface, which is on the order of 10” eV-’ me2 (10” eV-’ cm-‘), decreases slightly as the chlorine concentration in the film increases. It is known that in the oxidation of silicon the presence of chlorine at the Si-Si02 interface can reduce the density of interface states [ 161. Consequently, it appears beneficial for both the fixed charge and the interface states that chlorine be incorporated in the EGO. This incorporation does not seem to affect adversely either the breakdown voltage or leakage current of the Si,N, layer. Altering the gas flow conditions can decrease the magnitude of the fixed charge, but the flow of dichlorosilane that is predicted by the model to eliminate the fixed charge would lead to an Si,N, nitride layer that is extremely silicon rich and be in a regime where mechanical vacuum pumps can operate for only short periods before maintenance is required. Thus, our study suggests that the deposition conditions cannot be altered to eliminate the fixed charge. Another implication of this work is that the amount of fixed charge will vary with position in the quartz tube, since the SiH,Cl, concentration will decrease and the amount of negative charge will consequently increase. The fixed charge in the S&N,-Si samples indicated in Table 2 can be fitted by the Strategy program. Our best fit was obtained by simulating a uniform charge density in the nitride, given in units of 1024e m-j (10”e cm-“), with the equation Psi,N, = 46.7 + 0.232t - 2.44 X 106Qs,H2c,2 - 5.31 x 105QN+ - 3.06 x 10-4tT + (2.67 x 103T + 217t)QSiH,C12
(4)
It is interesting to note that, in contrast to the case for the EGO samples, increasing the flow of either gas makes the net charge more negative and greatly increases the effects of time and temperature. At higher temperature or longer time the net charge tends to decrease as the film thickness increases. There are also strong interactions of time and temperature with dichlorosilane flow. With longer deposition time or higher temperature, at high flow rates the charge becomes more positive and at low flow rates it becomes more negative.
J. A. Gregory et ul. 1 Charucterixtion
5. Conclusions
Altering the conditions for deposition of S&N, based on a full factorial composite design has led to better understanding of the deposition process. Our work suggests that the nitride growth is initiated by the coalescence of islands, that it is thermally activated, and that an antagonism exists between ammonia and dichlorosilane for adsorption on the nitride surface. LPCVD films may contain significant amounts of chlorine, which can diffuse through an underlying silicon film and accumulate at the Si-SiOz interface. Inclusion of chlorine in the film decreases the magnitude of fixed charge and the density of interface states, which could be an important factor when trying to perfect EPROM memory devices.
Funding for this project was provided by the Department of the Air Force and the Defense Advanced Research Projects Agency. The authors acknowledge useful conversations with A. M. Chiang and B. B. Kosicki and the skillful assistance of J. S. Ciampi, S. J. Brown and A. M. Reinold.
Si,N,
17
References I B. E. Burke, R. W. Mountain, 2 3 4 5 6
7 8 9 IO II
Acknowledgments
of LPCVD
I2 I3 I4 I5 I6
P. J. Daniels and D. C. Harrison, Opt. Eng., 26 ( 1987) 890. B. Y. Nguyen. P. J. Tobin, K. W. Teng, H. G. Tompkins and K. M. Chang, J. Elecrrochem. Sot., 135 (1986) 776. G. E. P. Box, W. G. Hunter and J. S. Hunter, Stufistics for Esperimenters, Wiley, New York, 1978. T. B. Barker, Qualify by Esperimenrul Design, Dekker, New York, 1985. D. H. Doehlert. Strulegy. Experiment Strategies Foundation, Seattle, WA, 1990. R. C. Taylor and B. A. Scott, in S. T. Pantelides and G. Lucovsky (eds.), SiO, und i/s Inferfaces, Materials Research Society, Pittsburgh, PA, 1988, p. 319. K. F. Roenigk and K. F. Jensen, J. Elecfrochem. Sot., 134 ( 1987) 1777. C. A. Neugebauer. in L. 1. Maissel and R. Clang (eds.), Handbook ofThin Film Technology. McGraw-Hill, New York, 1970, pp. 8-32. R. Hezel and E. W. Hearn, J. Electrorhem. Sot., 125 ( 1978) 1848. D. R. Clarke, J. Am. Ceram. Sot.. 65 ( 1982) C2l. 0. 1. Semenova, L. A. Nenasheva and S. M. Repinskii. Inorg. Muter.. 20 (1984) 681. Y. Ron, A. Raveli. U. Carmi, A. Inspektor and R. Avni, T/tin Sotic/ Films, /07( 1983) 181. S.-S. Lin. J. Elecfroclrem. Sot., 12.5 (1978) 1877. M. M. Moslehi, C. J. Han, K. C. Saraswat, C. R. Helms and S. Shatas. J. Electrochrm. Sot.. 132 ( 1985) 2189. G. J. Dunn, R. Jayaraman. W. Yang and C. G. Sodini, Appl. Phys. Left.. 52 (1988) 1713. S. K. Ghandhi, VLSI Fdwicdion Principles, Wiley. New York, 1983. p. 389.
I I - I7
II
Characterization of low pressure chemically vapor deposited silicon nitride using experimental design J. A. Gregory, Lincoln Laboratory,
Douglas Massachusetts
J. Young, Institute
R. W. Mountain
of Technology,
Lexington,
and C. L. Doherty,
Jr.
MA 02173 (U.S.A)
Abstract During the low pressure chemical vapor deposition of SisN,, significant interactions occur between the temperature, time, and gas flows, and thus a designed set of experiments has been used to establish the optimum deposition conditions, The conditions employed in this study were dictated by a full factorial matrix of the independent variables over 20 deposition runs. This procedure allowed us to vary the flow rates of NH, and SiH,CI, as well as the temperature and time of deposition. In this way we were able to determine the effects of these variables and their interactions on the electrical, physical, and chemical properties of the S&N,. Deposition conditions can then be chosen to optimize film properties. Two sets of samples were prepared: Si,N, was deposited either directly on silicon substrates or on silicon covered with 25 nm of SiO,; the nitride films deposited on SiO, were subjected to a wet oxidation to simulate the structure in working devices, Following the formation of polysilicon electrodes on both types of samples, capacitance-voltage measurements were performed to obtain data on fixed charge in the insulating layers and on interface state density. The properties investigated were thickness and composition of the S&N,; profiles of the silicon, nitrogen, oxygen, and chlorine content were obtained by secondary ion mass spectrometry and Rutherford backscattering analysis. We discovered that a significant amount of chlorine was incorporated in the nitride layers during deposition, that this chlorine was able to diffuse through an underlying SiO, layer and to accumulate at the Si&SiO, interface, and that the chlorine had a strong effect on the amount of fixed charge in the structure. The deposition rate increased with the ratio of SiH,CI, to NH, flow, as did the amount of chlorine in the nitride, while the magnitude of the negative fixed charge in the dielectric layer decreased. Following an induction period that depended on temperature and gas flow rates, the deposition rate was thermally activated and constant.
1. Introduction Silicon nitride deposited by low pressure chemical vapor deposition (LPCVD) is a dielectric that is widely of used, most often as a layer to mask the oxidation underlying silicon layers. In many applications the Si,N, is removed after an oxidation step, before the active device is completed, so the effects of the S&N, on the properties of the device are not fully characterized. For charge-coupled devices (CCDs) and memory devices such as electrically programmable read-only memories (EPROMs), the common practice is to leave the nitride in place during device fabrication. In the case of CCDs, the gate oxide must be masked during several distinct oxidation steps [ 11. Consequently, the nitride forms an integral part of the dielectric, separating the CCD channel from the polysilicon electrode, and it can influence the behavior of the threshold voltage and other properties of the metal-oxide-semiconductor (MOS) structure. The inclusion of Si,N, in the gate dielectric composed of oxide-nitride-oxide layers on silicon, referred to as the equivalent gate oxide (EGO), can introduce substantial amounts of negative fixed charge in the dielectric with a resulting shift in threshold voltage towards more positive values
[2]. In an effort to understand and control this negative fixed charge, a set of designed experiments were performed to characterize the effects of the deposition temperature and time and the ammonia and dichlorosilane flow rates on properties of the LPCVD Si,N, layers.
2. Experimental
details
The properties of thin films are influenced by many variables, which may interact with each other in ways that are more complicated than a simple linear superposition of their individual effects. If one is willing to apply mathematical models, however, many combinations of experimental conditions are possible, and designed experiments [3,4] may be applied to formulate a problem of manageable proportions. In the present case, a full factorial, central composite design (ref. 3, p. 324) explained below, was applied to generate the conditions employed during LPCVD of silicon nitride. The depositions were performed in an evacuated quartz tube, with a pressure of about 15 Pa ( 110 mTorr); the steady state pressure was determined by the flow rate of the gases and the speed of the unthrottled vacuum pump.
Elsevier
Sequoia,
Lausanne
J. A. Gregor,v et 01. 1 Charcrcterixrtion
12
The deposition conditions are shown in Table 1. In order to cover all combinations of high and low settings for four independent process variables, it was necessary to perform 24 = 16 runs. In the arrangement shown in the table, the temperature alternates between 748 and 828 “C, while the deposition time alternates between 300 and 900 s (5 and 15 min) in groups of two runs. The ammonia flow rate alternates between 49 and 77 pmol SK’ (70 and 110 standard cm3 min-‘) in groups of four runs, and the dichlorosilane flow rate alternates between 7 and 35 pmol s- ’ ( 10 and 50 standard cm3 min-‘) in groups of eight runs. The order of the runs, except for those made under standard conditions, was randomized to block the influence of extraneous factors. Runs 1, 7, 13 and 20 in the table are replicates of a standard deposition process employed at Lincoln Laboratory and are referred to as centerpoints of the design. The design assumes that a particular property depends linearly on the independent variables and on a first-order interaction of these variables; for a dependent variable Y, the relationship to the independent variables Xi is Y = CO+
1 CiXj + C C I
i
CgxiXj
j#i
where c0 is the value of Y obtained at the centerpoints, ci = a Y/13x,, and cij = a2 Y/ax, ax,. The coefficients are derived by fitting the measured responses to the values of the independent variables with a multivariable regression program [5]. The centerpoint runs are not included in this analysis; they were used to test the fit of the model. The samples were fabricated on 3 in n-Si substrates. Sixty of the substrates used for deposition of the Si,N, had 25 nm of SiO, grown in a dry ambient at 1000 “C, while another 60 were bare. For each run in Table 1, one bare silicon and one oxide-coated silicon wafer were inserted at the front, the middle, and the back of the boat. The remaining slots in the boat were filled with 44 nitride-coated dummy wafers. The boat was then loaded into the furnace for deposition of the nitride layer. The thickness of the nitride layer deposited on either the bare silicon or the oxide was determined by ellipsometry and by optical interferometry. After all 20 nitride depositions were performed, the 60 samples with an oxide layer below the nitride were inserted in a furnace and subjected to wet oxidation at 1000 “C for 35 min. These conditions convert about 15 nm of Si,N, to 25 nm of SiO,. After oxidation all samples were coated with 500 nm of polysilicon at 626 “C. The polysilicon was doped at 1000 “C with POCl,, patterned with photoresist, and etched to form 1.1 mm diameter dots of polysilicon. The back surface of each
of LPCVD
Si, N,
wafer was etched down to bare silicon, coated with aluminum, and sintered to complete the formation of MOS structures. High frequency capacitance-voltage C-V measurements were performed with the bias swept from - 10 to 10 V with a 1 MHz test signal superimposed. Low frequency measurements were made in the quasi-static mode with a bias ramp rate of 0.1 V s-‘. The composition of the films were determined using two techniques. Secondary ion mass spectrometry (SIMS) was used to obtain a composition profile for selected elements over a large range of concentrations, and Rutherford backscattering (RBS) analysis was performed to determine the absolute amounts of the predominant constituents. The SIMS measurements, which were obtained using an 0, ion beam, were performed by Evans East in Princeton, NJ, and the RBS analysis was carried out by Charles Evans Associates in Redwood City, CA.
3. Results 3.1. Nitride thickness The thickness of the nitride films is shown in Table 1. It is immediately apparent that the layers deposited at 828 “C and with a dichlorosilane flow rate of 35 pmol s-’ (50 standard cm3 min-‘) have the highest average thickness. A comparison of the results for the four centerpoint runs shows excellent reproducibility of the deposition thickness. The differences in the thicknesses of the nitride deposited on silicon and SiO, are not shown in the table, but these were generally less than I nm for a given set of deposition conditions. 3.2. Charge in dielectric Table 2 presents data on the amount of fixed charge in the MOS structures. The fixed charge in the EGO samples is calculated as if it were either a sheet at the Si-SiO, interface or a sheet at the Si02 -Si,N, interface located closest to the silicon substrate. For the nitride films deposited on silicon substrates, the charge is calculated as a sheet at the Si-Si,N, interface and also as if it were distributed uniformly throughout the S&N,. It is apparent that the charge in the nitride samples is of opposite sign and about 10 times larger in magnitude than that in the EGO samples. 3.3. Composition Profiles obtained by SIMS for an EGO sample are displayed in Fig. 1, showing the number of counts us. the sample depth (the depth scale is only approximate and assumes equal sputtering rates in all phases). The oxygen signal clearly marks the presence of the ttio Si02 layers, while the plateau for the nitrogen signal indicates the intervening layer of Si,N,. Note that the
300 300 900 900 300 300 900 900 300 300 900 900 300 300 900 900 600 600 600 600
748 828 748 828 748 828 748 828 748 828 748 828 748 828 748 828
788 788 788 788
16 I2 5 I7 8 4 11 2 IO 3 19 I8 14 15 6 9
I 7 I3 20
Run order (s)
vapor
(
21
(90) (90) (90) (90)
63 63 63 63
(10) (10) (IO) (IO) (10) (10) (10) (10) ( 50) (50) (50) (50) (50) (50) (50) (50)
7 7 7 7 7 7 7 7 35 35 35 35 35 35 35 35
(70) ( 70) (70) (70) (110) (110) (110) (110) (70) (70) ( 70) (70) (110) (110) (110) (110)
49 49 49 49 77 77 77 77 49 49 49 49 77 77 77 77
films
cm3 min- ‘)
silicon nitride
(standard
flow
deposited
(urn01 s-‘)
SiH2CI,
chemically
cm3 min-‘)
of low pressure
(standard
flow
residuals
(urn01 ss’)
NH,
rate, and statistical Time
deposition
(“C)
1. Thickness,
Temperature
TABLE
.I
1.0 21.7 15.9 74.9 2.5 23.8 16.8 81.8 6.2 37.3 36.9 136.7 6.5 36.0 38.0 141.5
(nm)
Thickness
0.100 0.089 0.026 0.089 0.250 0.097 0.028 0.097 0.052 0.166 0.051 0.166 0.054 0.160 0.053 0.172
Deposition (nm s-‘)
rate
-0.374 0.133 0.011 0.098 0.266 - 0.074 -0.280 - 0.084 0.138 -0.118 0.024 -0.086 0.118 -0.206 0.054 ~ 0.022
Normalized
residual
h
s
748 828 748 828 748 828 748 828 748 828 748 828 748 828 748 828
788 788 788 788
I 7 13 20
600 600 600 600
300 300 900 900 300 300 900 900 300 300 900 900 300 300 900 900
(s)
16 12 5 17 8 4 II 2 IO 3 19 18 14 15 6 9
Time
flow
63 63 63 63
49 49 49 49 77 77 77 77 49 49 49 49 77 77 77 77 (90) (90) (90) (90)
(70) (70) (70) (70) (110) (110) (110) (110) (70) (70) (70) (70) (110) (110) (110) (110)
(standard
cm’ min
gate oxide and %Si,N,
(pm01 s ‘)
NH,
in equivalent
( C)
distribution
Temperature
2. Charge
Run order
TABLE
‘)
(50) (30) (30) (30) (30)
-3.6 -4.0 -3.7 -3.9
-3.6 -3.3 -2.5
-2.2 -2.0 -2.0 -2.2 -2.5 -2.3 -2.4
-2.5 -2.7 -2.1
-1.5 - 1.6 -1.6
-4.0
-2.9
(50) (50) (50) (50) (50) (50) (50)
-4.6
-2.6
(10) (10)
-4.7
Centroid at SiO,-Si,N, interface ( x 10’Se m-‘)
in EGO films
-4.2
-2.6
Centroid at Si-SiOz interface ( x 1015e m-‘)
Fixed charge
-3.0
21 21 21 21
films
cm’ min- ‘)
deposited
(10) (10) (10)
(10) (10) ( 10)
7 7 7 7 7 7 7 7 35 35 35 35 35 35 35 35
vapor
(standard
flow
chemically
(pm01 s-‘)
SiH,Cl,
low pressure
films
8.4 Il.44 -0.02 2.7 5.8 11.01 -0.04 -4.07 3.63 5.35 0.1 -7.67 3.48 5.34 0.04 2.95 2.81 2.74 3.16
25 24 23 25
(10’4em-‘)
Charge density in Si,N,
in Si-Si,N,
40 35 -0.31 13 30 35 -0.73 -5.6 29 39 2.9 -10 25 39 1.2
Centroid at Si-Si,N, interface ( x IO”e mm’)
Fixed charge
6
J. A. Gregory et al. / Characterkation
of LPCVD
Si, N4
15
basic assumption is that any coefficients for a quantitative model can be derived by analyzing the data from the 16 runs, where the independent variables were set at their extreme values. Subsequently, the values predicted for the centerpoint conditions can be compared with the observed values for these runs. The replicates of the centerpoints define a confidence interval for acceptance of the model, based on the standard deviation of the measurements and Student’s t distribution (ref. 4, p. 110). We attempted to fit various models to the data. The one that provided the best fit, while also being physically plausible, is d = 2.72 x 10” exp[ - AE/k( T + 273)](t - T) X ( -0.0033 - 2.35 x DEPTH (nm)
Fig. I. Secondary
ion mass intensity
vs. depth
for an EGO sample.
DEPTH (nm)
Fig. 2. RBS composition
us. depth
for an EGO
sample
chlorine signal follows the nitrogen signal and rises in the Si,N, layer, falls in the SiO, layer, and rises again at the SiO,-Si interface. Similar overall behavior is seen in the RBS in Fig. 2. In this EGO sample, the outer layer of SiO, was removed by wet etching to improve depth resolution for RBS; the nitrogen and chlorine signals are high in the nitride layer (the chlorine signal has been magnified by 1000) and then fall off as the oxygen signal rises in the SiO, layer. The chlorine content in the nitride layer is quite large, about 0.1 at.% or 5 x 1O25me3. No peak of nitrogen or chlorine is detectable at the SiO,-Si interface in the RBS analysis.
4. Discussion 4.1. Nitride thickness The data in Table 1 on nitride thickness tion conditions can be modeled in several
vs. deposiways. The
- 711 QNH, + 3022QsiHZclZ
107Q,t+,QsiH2c12)
(2)
where d(nm) is the thickness of the deposit, AE is the activation energy, T is the temperature, t is the deposition time, r is the induction time necessary to start deposition, and Qr.,n, and QsiHzclz are the flow rates for ammonia and dichlorosilane respectively. The induction period depends on temperature and gas flows (interactions occur between the independent variables) and varies between 55 and 290 s. Unfortunately, these values include electromechanical delays in the gas delivery system, so all that we can say with certainty is that the induction period decreases as the temperature increases and that it decreases with SiH,Cl, flow at low temperature and increases with the flow at high temperature. Table 1 indicates the range of values for steady state deposition rate encountered during the runs. The regression program Strategy was used to generate a residual, that is the difference between the predicted and observed thicknesses. The residual values, normalized by the observed deposition rate, are included in Table 1. The agreement between the predicted and measured thicknesses is quite good, with the absolute value of the normalized residual averaging 0.11. The agreement is especially good for the centerpoint runs, which were not used in computing the model coefficients, and is well within the confidence interval limits ( f 8%) of the predicted value. As a further test of the model, historical data were collected for nitride thicknesses between 10 and 300 nm deposited at 788 “C, from which an induction time of 40 s and a steady state deposition rate of 0.072 nm s-’ were inferred, in excellent agreement with the centerpoint rates. Although the model based on the full factorial approach is a means of fitting the data and cannot pinpoint the physical mechanism responsible for controlling the deposition rate, the activation energy derived ( 151 kJ mol-‘) is comparable with other values
J. A. Gregory et cd. / Charcccterizution
16
obtained in kinetic studies of LPCVD Si,N, [6, 71. The presence of an induction period suggests that an island growth and coalescence mechanism [8] is responsible for the early stages of deposition. Further, as eqn. (2) makes clear, an antagonistic relationship exists between the deposition rates with ammonia and dichlorosilane. Increasing the flow of dichlorosilane increases the deposition rate, but increasing the ammonia flow increases the deposition rate at low dichlorosilane flows and decreases it at high dichlorosilane flows. This interaction might be due to competition for surface sites on the growing nitride film. 4.2, Charge in dielectric and composition As in previous work on fixed charge in nitride [9], it is not possible to determine from the data for EGO samples whether the charge is a sheet at the SiOZ-Si interface, a sheet at the Si,N,-SiO, interface, or distributed evenly throughout the nitride layer. This is a moot point for most applications since the operational characteristics of the device, such as the threshold voltage, do not depend on the model, giving some latitude in assigning a mechanism to the generation of charge. Regardless of the precise nature of the charge, a fit with small residuals can be obtained to the first of these models using a simple formula: qsl_sio, = -3.44 -7.02
+ 6700QNu, + 8.42 X 104QsiH2ci2 x
l@QNH, QSiH>ClZ
(3)
where qsi~sio, ( x 10”e m-*( x 10”e cm-*)) is the fixed charge in the EGO, assuming that the centroid is at the Si-SiO, interface. It is not necessary to include the effects of temperature or time on the fixed charge, since the flow rates are dominant. It should be noted that increasing either gas decreases the magnitude of the net negative charge. Although it is generally held that fixed charge in nitride is positive [9], as seen in Table 2 for the samples with Si,N, deposited directly on silicon there is evidence that both positive and negative defects can arise [2], albeit at much higher ratios of ammonia to dichlorosilane flow than encountered here. It is unlikely that this negative charge is due solely to isolated Cl- ions in the nitride lattice, since the amount of chlorine tends to increase as the dichlorosilane flow rate increases and the amount of negative charge decreases. The RBS data indicate that the films are silicon rich with an Si:N ratio around 0.83, so the increase in charge could be due to an increased concentration of Si” centers, offsetting the negative charge of some unspecified defect. The presence of chlorine in nitride following deposition from a chlorosilane is known from the ceramics literature [ 10, 1 l] and has also been detected following plasma-enhanced CVD of nitride films [ 121, but it ap-
c~f LPCVD
Si, N,,
pears to have been overlooked in LPCVD applications. Residual gas analysis performed here and elsewhere [ 131 suggests the presence of gas phase precursors of the form SiHCl., NH_,.. It may be that these molecules are adsorbed on the growing nitride film, and although the NH, tends to displace adsorbed chlorine [7] some chlorine is trapped in the S&N, structure. Figure 1 clearly indicates the diffusion of chlorine through the underlying SiO, film. The diffusion of NH, [ 14, 151 is also evident in Fig. 1 from the nitrogen signal accumulating at the Si-SiO, interface. The quasi-static data suggest that the density of surface states at the Si-SiO, interface, which is on the order of 10” eV-’ me2 (10” eV-’ cm-‘), decreases slightly as the chlorine concentration in the film increases. It is known that in the oxidation of silicon the presence of chlorine at the Si-Si02 interface can reduce the density of interface states [ 161. Consequently, it appears beneficial for both the fixed charge and the interface states that chlorine be incorporated in the EGO. This incorporation does not seem to affect adversely either the breakdown voltage or leakage current of the Si,N, layer. Altering the gas flow conditions can decrease the magnitude of the fixed charge, but the flow of dichlorosilane that is predicted by the model to eliminate the fixed charge would lead to an Si,N, nitride layer that is extremely silicon rich and be in a regime where mechanical vacuum pumps can operate for only short periods before maintenance is required. Thus, our study suggests that the deposition conditions cannot be altered to eliminate the fixed charge. Another implication of this work is that the amount of fixed charge will vary with position in the quartz tube, since the SiH,Cl, concentration will decrease and the amount of negative charge will consequently increase. The fixed charge in the S&N,-Si samples indicated in Table 2 can be fitted by the Strategy program. Our best fit was obtained by simulating a uniform charge density in the nitride, given in units of 1024e m-j (10”e cm-“), with the equation Psi,N, = 46.7 + 0.232t - 2.44 X 106Qs,H2c,2 - 5.31 x 105QN+ - 3.06 x 10-4tT + (2.67 x 103T + 217t)QSiH,C12
(4)
It is interesting to note that, in contrast to the case for the EGO samples, increasing the flow of either gas makes the net charge more negative and greatly increases the effects of time and temperature. At higher temperature or longer time the net charge tends to decrease as the film thickness increases. There are also strong interactions of time and temperature with dichlorosilane flow. With longer deposition time or higher temperature, at high flow rates the charge becomes more positive and at low flow rates it becomes more negative.
J. A. Gregory et ul. 1 Charucterixtion
5. Conclusions
Altering the conditions for deposition of S&N, based on a full factorial composite design has led to better understanding of the deposition process. Our work suggests that the nitride growth is initiated by the coalescence of islands, that it is thermally activated, and that an antagonism exists between ammonia and dichlorosilane for adsorption on the nitride surface. LPCVD films may contain significant amounts of chlorine, which can diffuse through an underlying silicon film and accumulate at the Si-SiOz interface. Inclusion of chlorine in the film decreases the magnitude of fixed charge and the density of interface states, which could be an important factor when trying to perfect EPROM memory devices.
Funding for this project was provided by the Department of the Air Force and the Defense Advanced Research Projects Agency. The authors acknowledge useful conversations with A. M. Chiang and B. B. Kosicki and the skillful assistance of J. S. Ciampi, S. J. Brown and A. M. Reinold.
Si,N,
17
References I B. E. Burke, R. W. Mountain, 2 3 4 5 6
7 8 9 IO II
Acknowledgments
of LPCVD
I2 I3 I4 I5 I6
P. J. Daniels and D. C. Harrison, Opt. Eng., 26 ( 1987) 890. B. Y. Nguyen. P. J. Tobin, K. W. Teng, H. G. Tompkins and K. M. Chang, J. Elecrrochem. Sot., 135 (1986) 776. G. E. P. Box, W. G. Hunter and J. S. Hunter, Stufistics for Esperimenters, Wiley, New York, 1978. T. B. Barker, Qualify by Esperimenrul Design, Dekker, New York, 1985. D. H. Doehlert. Strulegy. Experiment Strategies Foundation, Seattle, WA, 1990. R. C. Taylor and B. A. Scott, in S. T. Pantelides and G. Lucovsky (eds.), SiO, und i/s Inferfaces, Materials Research Society, Pittsburgh, PA, 1988, p. 319. K. F. Roenigk and K. F. Jensen, J. Elecfrochem. Sot., 134 ( 1987) 1777. C. A. Neugebauer. in L. 1. Maissel and R. Clang (eds.), Handbook ofThin Film Technology. McGraw-Hill, New York, 1970, pp. 8-32. R. Hezel and E. W. Hearn, J. Electrorhem. Sot., 125 ( 1978) 1848. D. R. Clarke, J. Am. Ceram. Sot.. 65 ( 1982) C2l. 0. 1. Semenova, L. A. Nenasheva and S. M. Repinskii. Inorg. Muter.. 20 (1984) 681. Y. Ron, A. Raveli. U. Carmi, A. Inspektor and R. Avni, T/tin Sotic/ Films, /07( 1983) 181. S.-S. Lin. J. Elecfroclrem. Sot., 12.5 (1978) 1877. M. M. Moslehi, C. J. Han, K. C. Saraswat, C. R. Helms and S. Shatas. J. Electrochrm. Sot.. 132 ( 1985) 2189. G. J. Dunn, R. Jayaraman. W. Yang and C. G. Sodini, Appl. Phys. Left.. 52 (1988) 1713. S. K. Ghandhi, VLSI Fdwicdion Principles, Wiley. New York, 1983. p. 389.