Thin Solid Films, 162 (1988) 129-143. METALLURGICAL AND PROTECTIVE COATINGS. 129. CORRELATION BETWEEN STRESS AND STRUCTURE IN.
Thin Solid Films, 162 (1988) 129-143 METALLURGICAL AND PROTECTIVE COATINGS
129
CORRELATION BETWEEN STRESS AND STRUCTURE IN CHEMICALLY VAPOUR DEPOSITED SILICON NITRIDE FILMS A. G. NOSKOV, E. B. GOROKHOV, G. A. SOKOLOVA, E. M. TRUKHANOV AND S. I. STENIN
Institute of Semiconductor Physics, Academy of Sciences of the U.S.S.R., Siberian Branch, 630090 Novosibirsk (U.S.S.R.,) (Received August 26, 1986; revised August 4, 1987; accepted March 8, 1988)
Silicon nitride films chemically vapour deposited onto silicon and germanium substrates by the reaction of silane with ammonia at atmospheric pressure within the temperature range 650-850 °C have been studied. A decrease in the concentration of hydrogen chemically bound to silicon and nitrogen atoms in films during deposition and annealings has been found to result in a decrease in film bulk and an increase in mechanical stress. During annealing, the film dimensions perpendicular to the heterosystem interface are found to decrease by 1-2 orders of magnitude more than those parallel to the interface. This effect may be attributed to a viscous relaxation of stress arising during film deposition or annealing of the heterosystem. The viscous flow of the Si3N 4 film within the temperature range 700-1000 °C is due to the presence of thermally unstable Si--H and N---H bonds in it. Comparison of the mechanical properties of the films prepared by both atmospheric pressure chemical vapour deposition and low pressure chemical vapour deposition was briefly carried out.
1. INTRODUCTION
Amorphous SiaN 4 films produced by chemical vapour deposition (CVD) widely used in fabricating semiconductor devices are a source of high mechanical stress a in heterostructures 1'2. This stress evolves during the formation and annealing of Si3N4 films and cannot be attributed to a difference in the thermal expansion coefficients (TECs) of the individual materials. For the Si-Si3N 4 system, for example, the fraction of the stress due to the above difference at room temperature is not more than 15% of the total amount of stress 1. Dielectric CVD Si3N 4 films are known to have a substantial amount of hydrogen (10%-20%) bound to silicon and nitrogen 3. Many physical and chemical properties of the given coatings, such as etch resistance4, optical density in the IR region 5'6, density of electron traps and conduction 6' 7, are affected by the concentration Cn of hydrogen chemically bound to the silicon and nitrogen atoms in the films which, in its turn, depends on the synthesis conditions. In some cases, when silicon nitride layers are used as active layers for recording and storing an electric charge, the availability of 0040-6090/88/$3.50
© ElsevierSequoia/Printedin The Netherlands
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A.G. NOSKOVe t al.
S i - - H and N - - H bonds is the necessary condition for the development of the m e m o r y units. In other cases, when the Si3N 4 is used as a passivating coating, the presence of S i - - H and N - - H bonds deleteriously affects the film's protective properties. However, there are few data on the effect of chemically bound hydrogen concentration and its variation on stress, although the latter is one of the factors influencing strongly the reliability, stability and electronic characteristics of fabricated devices. It should be noted that the data connecting the concentration of S i - - H and N - - H bonds and stress are related only to Si3N 4 films deposited in plasma s. The deposition conditions for a m o r p h o u s CVD Si3N 4 films are far from thermodynamic equilibrium, and so their composition and structure as well as stress should be determined by the parameters of the synthesis process, i.e. deposition temperature, deposition rate and reagent concentration in the vapour phase. In addition, because of the thermal instability of the S i - - H and N - - H bonds, the composition and mechanical properties of Si3N 4 films will be affected by a subsequent thermal treatment. , In connection with the above the present paper deals with the influence of the conditions of synthesis and annealing on the structure and stress of CVD Si3N 4 films on silicon and germanium substrates. 2.
EXPERIMENTAL DETAILS
The technology used for obtaining the heterostructures in question is presented elsewhere 4'6. The atmospheric pressure C V D (APCVD) Si3N 4 films 30°220 nm in thickness were deposited in a quartz horizontal reactor in an argon flow using the reaction of Sill 4 with N H a within the temperature range 650-850°C. The deposition rate vd at all temperatures was usually kept constant at vd ~ 25 n m m i n -1 by changing the SiH4:NH 3 ratio in the vapour phase from 6 x 10- 3 at 650 °C to 2 x 10- 3 at 850 °C. D a t a on the effect of deposition rate on the film properties were also obtained using a 2 nm m i n - 1 deposition rate at 700 °C and an SiH4:NH 3 ratio of 5 x 10 -4. According to Bean et al. 9 for A P C V D of SiaN4, the deposition rate is in direct relation to the Sill4 concentration for SiH4:NH3 ~ 10-2-10-3. This was confirmed by deposition rate measurements at 700 and 800 °C. N o change in the film deposition rate was observed for a variation in the N H a flow rate by a factor of 3. In this case, the total gas flow of S i H 4 + N H 3 + Ar, flow of Sill 4 and deposition temperature did not change. There was no apparent variation in the etch rate of these films either. Deposition of low pressure C V D (LPCVD) SiaN 4 films was carried out by the decomposition of an S i H , r N H 3 - A r (the ratio R = SiH4:NH 3 = 1:100) gas mixture at 850°C and an SiCI4-NH3-Ar gas mixture at 750°C (R = 1:5) and 900°C (R = 1:25). In all cases, the system pressure was about 133 Pa and the deposition rate was approximately 2 nm min-1. Substrate thicknesses were 0.3-0.5 m m and 0.20.4 m m and diameters were 2-3 cm and 4 cm for germanium and silicon respectively. The heterostructures studied were annealed in helium at temperatures up to 1000 °C. Film thickness and refractive index were measured with a laser ellipsometer (2 = 632.8 nm) with an accuracy of 0.2-0.3 nm and 0.005 respectively. Depending on their deposition and annealing conditions, structural changes of Si3N4 layers were
STRESS AND STRUCTURE IN CVD S i 3 N 4 FILMS
131
determined by the etch rate re of films in a solution including NH4F, H F and H 2 0 in amounts of 30 g, 18 ml and 60 ml (etchant A) or 30 g, 9 ml and 60 ml (etchant B) respectively. The relative accuracy of etch rate measurements was about 5~-10~o unless otherwise noted. In certain cases IR transmission spectroscopy and the multiple internal reflection (MIR) technique were used to investigate chemical composition and to determine Si--H and N---H bond concentrations in the Si3N 4 films. A DS-402G spectrophotometer and an MIR attachment were employed for MIR measurements. The silicon plate geometry and treatment of IR data were similar to those used by Kapoor et al. 7 Deposition of CVD Si3N 4 films of 100 nm thickness at 700 and 800 °C was carried out on both sides of the silicon MIR plate simultaneously. X-ray topography was applied to determine the plastic deformation of the substrates. The treatment conditions for the substrates before film deposition prevented them from being plastically deformed. The stress in the system was determined from a plate radius of curvature R, which was measured with a doublecrystal X-ray spectrometer 1o. In this case, Cu K s radiation was used and the sample was arranged to give the Bragg diffraction of the (111) planes which are parallel to the surface. All the measurements were carried out at room temperature. 3.
EXPERIMENTAL RESULTS
The influence of film deposition parameters and the conditions for subsequent thermal treatments of the films on stress trf, chemical etch rate v~ and shrinkage, given as a relative change Ah/h in film thickness, are shown in Fig. 1. In all cases the 110 nm films were deposited onto silicon substrates at 700 °C. Figures l(a) and l(b) demonstrate the dependences of stress and etch rate (etchant A) on the annealing time, with the deposition rate and annealing temperature being additional parameters. A decrease in the deposition rate decreases the etch rate and stress. Higher annealing temperatures increase stress and decrease etch rate. As can be seen from Fig. l(b), the etch rate variation for films with low deposition rates (curves 1 and 2) is far less than that for films with high deposition rates (curves 3 and 4). Structural rearrangement in the films is accompanied by decreasing thickness Ah/h, as illustrated in Fig. l(c). In this case the synthesized specimens were subjected to single annealing temperatures between 700 and 1000 °C. Shown in Fig. l(a) and l(b) are data for the specimens annealed first at 700 °C (curves 1 and 3) and then at 800 °C (curves 2 and 4). The kinetic dependences of Fig. 1 are characterized by a rapid initial change (up to 60 min) in Ah/h, ve and trf except for curves 1 and 2 in Fig. l(b) where there is little variation in film etch rate. The refractive index n of the films increased from 1.95 to 1.96 with the increase in deposition temperature from 700 to 850 °C. The decreasing film thickness was accompanied by an increasing refractive index so that a relative change in the film thickness of 2~o corresponded to a refractive index change of about 0.01. After annealing at 1000 °C, the refractive index increased in general up to 1.98-1.99 but did not reach the value of 2.00 typical of stoichiometric Si3N411. This seemed to indicate that some oxygen from the NH3 and argon gases was incorporated in the film during deposition. The volume fraction of SiO 2 in the film estimated from the
132
A.G. NOSKOVet al.
min
'E Z
T = BOO°C T : 700"C
~? 1.2
•
T : 6000C 00 C
Z T
~3
c5~ 2 .T. m 1 "1"
(a) T :700°C v w
Imm
w
(b)
~
~-~
~
q
= 800 "C
O0°C 00"C Va :ZSnmlmin
~ I
0
(c)
T
l
- I000 "C I
t.o 80 ANNEALING
J
..-L.----
120 160 TiME (rain}
Fig. 1. Effect of annealing time on (a) stress, (b) etch rate and (c) thickness of films. The deposition rate vd and annealing temperature are the parameters.
refractive index of 1.98-1.99 using equations given by Kuiper et al. 12 was calculated to be no more than 3~o-4~o. Figure 2(a) shows the dependences of etch rate in etchant B and stress on the deposition temperature for films deposited on germanium substrates. This and subsequent figures illustrate the data for the specimens produced at a deposition rate of 25 n m m i n -1. The thickness for all the films grown at all the deposition temperatures was about 100 nm. Higher deposition temperatures for films deposited on the germanium substrates result in a lower film stress such as that for Si3N 4 films on silicon substrates 1 and in a lower etch rate. In a similar manner, the tensile stress in L P C V D films deposited on silicon wafers from the SiCI4-NH3-Ar gas mixture decreases from 1.4 × 10 9 to 1.0 x 10 9 N m - 2 with increasing deposition temperature from 750 to 950 °C. A comparison of Figs. l(a) and 2(a) shows that the stress for films on germanium substrates at room temperature is 30%--40% less than that for films on silicon. It should be noted that the nature of the dependences of af, re and Ah/h on annealing time for the films grown at different temperatures qualitatively agrees with the results of Fig. 1. Annealing-caused effects are enhanced with increasing difference between annealing and deposition temperatures. To compare the effect of deposition and annealing temperatures on stress and etch rate, Fig. 2(b) shows the corresponding dependences on annealing temperature taken from Figs. l(a) and l(b) for 60 min.
STRESS
AND
STRUCTURE
IN
CVD
Si3N 4 F I L M S
133
c E ~9
~E Z
O.7
~6 as,~ w
t~
3 O3
0
(a)
I
I
650
I
I
I
750 850 OEPOSITION TEMPERATURE(*C ) 1.t, z
~9 ,~ 6
H 20
I
I
700 800 TEMPERATURE (*C)
(b) ANNEALING Fig. 2. Dependences of stress and etch rate of films on (a) deposition temperature and (b) annealing temperature.
The film stress measured after deposition depends on the film thickness, which is illustrated by solid circles in Fig. 3 for the specimens prepared at 700 °C. The stress in thin films (h ~ 50rim) is seen to be 10%-15% less than that in thicker films (h ,~ 200 rim), indicating a change in the Si3n4 structure during film deposition. After annealing at 700 °C for 60 min the level of stress increases and becomes about the same for thin and thick films (see open circles in Fig. 3). The relative change Aaf/crf in the stress decreases significantly with thicker films. It should be noted that an estimation of Aaf/af from the radius of heterosystem curvature characterizes structural processes occurring in the bulk of the film only with h ~< 140 rim, since thicker films reveal cracks during annealing. Analysis of the experimental results obtained by Isomae etal. 13 for the Si-Si3N 4 system with 5 0 - 2 1 0 n m films (these workers 13 did not observe cracks) has also revealed the relative variation in system curvature radius R (before substrate plastic flow IAR/RI oc IAcrf/afl) during thermal treatment with increasing Si3N 4 film thickness. The stresses estimated from these data t3 are presented in Fig. 3. Stress is distributed non-uniformly throughout the depth Z of the film. Figure 4 shows the data for a film with initial thickness h = 214 nm. Curve 1 is the change in average stress a. in the SiaN 4 layer with thickness Z., which is the layer available after the nth etch. The stress a(Z) for a layer of thickness Z. - Z. + 1 is equal to ,r(z) =
anZn--(Tn+ 1Zn+ 1
Z.-Zn+l
(1)
A.G. NOSKOV et al.
134
_ t3 'E z 2
o
o
o
o
z~
z
--11
O9 2
I
I
SO
J
100 FILM
I
r
150 THICKNESS
200
S0 DISTANCE
(nml
I
I
L
100 150 200 FROM INTERFACE ( n m )
Fig. 3. Dependence of stress on the film thickness: line 1, after deposition without annealing; line 2, after deposition followed by annealing; A, stresses obtained in ref. 13. Fig. 4. Dependence of stress on the distance from the interface: curve 1, change in average stress level while etching a film; curve 2, profile of the film stress estimated from curve I. tr(Z) determined by eqn. (1) is s h o w n by curve 2 in Fig. 4. We should emphasize the coincidence of the stress in the upper layers of the thick Si3N4 film (see curve 2 in Fig. 4) with the stress of thin films (curve 1 in Fig. 3). The above dependences were obtained for a film not subjected to thermal treatment after deposition at 700 °C. Annealing at 700 °C for 60 min of the SiaN 4 film with a residual thickness h = 35 nm after etching results in an increase in tr(Z) from 1.2 x 109 to 1.3 × 109 N m - 2 , with Aa(Z)/a(Z) being less than Aae/trf for a film with an initial thickness of 35 nm and not subject to etching. Inhomogeneity of properties for the Si3N 4 film as a result of its thickness is also revealed in the etch rate. Figure 5 shows the results of film etching with etchant B after deposition (curves 1 and 3) and annealing (curves 2 and 4). The etch rate of the surface layers of as-deposited films is 1.2-1.4 times that of the layers near the interface, Annealing results in lower etch rate of the film, with large changes being observed in the upper layers. IR transmission spectra in the region of the S i - - N stretching band at 850 c m - 1 for the films deposited at 700 and 800 °C on germanium substrates were similar to
AS-OEPOSITEO
c
3
TG: 700
g~ 2
1
I 0
I
I
DISTANCE
FROM
I
100
50
iNTERFACE
(nm)
Fig. 5. Change in etch rate through the film depth. Curves have been obtained at different deposition temperatures Ta and annealing temperatures.
STRESS
AND
STRUCTURE
Si3N4
IN CVD
135
FILMS
t h o s e of typical C V D SiaNa films 9 (Fig. 6(a)). T h e M I R I R a b s o r p t i o n s at 2200 c m - 1 a n d 3300 c m - 1 usually assigned to S i - - H a n d N - - H stretching b a n d s respectively are s h o w n in Fig. 6(b). T h e effect o f 700 a n d 800 °C d e p o s i t i o n t e m p e r a t u r e s a n d a n n e a l i n g at 950 °C for 5 h on S i - - H a n d N - - H b o n d c o n c e n t r a t i o n s is s h o w n in T a b l e I. -
100
-
90*C ANITA
/
8O
I:-
~o
20 I
l
1200
I
i
i
900
i
I
I
i
60O WAVE
(a)
I
3600
]
I
3000 NUMBER
I
I
I
I
2~00 {cm -~)
(b) Fig. 6. IR transmission spectra of the films deposited at 700 °C (MIR measurements): (a) Si--N stretching band of as-deposited film on germanium substrate; (b) effect of annealing on Si--H and N--H bands. TABLE I Si--H AND N--H BONDCONCENTRATIONSIN ATMOSPHERICPRESSURECHEMICALLYVAPOURDEPOSITED Si3N4 FILMS
Deposition temperature (oc)
Hydrogen bond Si--H N--H Si--H N--H
700 800
Bond concentration ( x 1021cm- 3) After deposition
After annealing at 950 °Cfor 5 h
1.2 7.1 0.4 5.3
-2.4 -2.8
4. DISCUSSION 4.1. Elastic constants and intrinsic stress o f a f i l m T h e stress trf of the film m e a s u r e d at r o o m t e m p e r a t u r e Tr is the s u m o f two c o m p o n e n t s , i.e. a n intrinsic stress tri t h a t arises d u r i n g d e p o s i t i o n a n d c a n be c h a n g e d b y further t h e r m a l t r e a t m e n t s a n d a t h e r m a l stress trr arising as a result of the different T E C s of the film a n d substrate. U s i n g the m e a s u r e m e n t s o f the film stress o n silicon a n d g e r m a n i u m s u b s t r a t e s o n e can e s t i m a t e such film c h a r a c t e r istics as Ef/(1 --vf), tri a n d the T E C % where Ef a n d vf a r e the elastic m o d u l u s a n d P o i s s o n ' s r a t i o respectively o f the film. O n e c a n write1 O'f :
O"i "q- O"T
(2)
T h e stresses trf a n d aT o n g e r m a n i u m a n d silicon s u b s t r a t e s will be d e n o t e d b y trfa=,
136
A.G. NOSKOVet al.
~Ge and o'f, Si O"T Si respectively. For films deposited on germanium and silicon under the
same conditions in the absence of plastic flow in the substrate and cracks in the film the stresses ai are assumed to be equal. The thermal stress aT is expressed as follows1: Ef ~"Tr l O'T -- 1 ~ Vf JTd [0~s-- ~f) d T
(3)
where ~s is the T E C of the substrate material. Using eqn. (3) and eqn. (2) twice for silicon and germanium we can find Ef __ (o.fGe __ o.Sl) 1 - vf
CCsi(T)} d T
(4)
Thus two measurements of the film stress at r o o m temperature on two different substrates are sufficient to determine a value of E t / ( 1 - v f ) . In our case af6e =-0.75 × 109N m -2 and a s t = 1.05 × 109N m -2, for films of 100nm thickness deposited at 700 °C (see Figs. 2(a) and 3). Using ~st(T) and ~6e(T) from ref. 14 within the temperature range from 700 to 20 °C, we obtain Ef/(1 - v f ) = 1.5 x 101 t N m - 2 , which agrees well with the data 15 on the bulk material of Si3N 4 (E = 1.2 × 1011 N m - 2). A considerable deviation from the elastic modulus obtained in ref. 16 may be due to the fact that T o k u y a m a et al. 16 have not taken account of the different levels of ai for films grown at 800 and 1000 °C, having attributed the difference in film stress observed to the difference in TECs of the materials Si3N 4 and silicon only. Knowing Er/(1 - vf) it is impossible to define at and ~f independently of eqns. (2) and (3) without additional mechanical stress measurements at elevated temperatures. Since Ef/(1 --vf) agrees well with the value for the bulk material Si3N4, let ~t = (2-3) × 1 0 - 6 K - l which is close to ~ = 2.75 x 1 0 - 6 K - 1 for the bulk material 15. The value of~f for the Si3N 4 films estimated in refs. 1 and 16 is also in the interval of ~f chosen by us. Then from eqns. (2) and (3) we obtain ai = (1.1-1.2)x 1 0 9 N m -2, which is in a good agreement with the values obtained in refs. 1, 13, 17 and 18. Figure l(a) shows that the intrinsic stress increases during thermal treatment. The longer the annealing time and the larger the difference between deposition and annealing temperatures, the greater is the stress. It follows from the data obtained that structural and compositional properties of the investigated films are similar to those of A P C V D and L P C V D Si3N 4 films used in silicon devices. The features of the film deposition process cause only quantitative variations in the film properties. This is reflected in the change in the film etch rate, which, in general, depends on (a) the stoichiometry of the film composition, i.e. Si:N ratio, (b) the presence of such defects as silicon and nitrogen dangling bonds and S i - - H and N - - H bonds, and (c) oxygen contamination. As shown by the variation of the Sill4 and N H 3 reactant gas flows at the 700 and 800 °C deposition temperatures, the etch rate depended on the SiH4:NH3 ratio at a given deposition temperature only if the film deposition rate varied. This means that the kinetics of the heterogeneous reactions on the film surface determines the film structure rather than the ratio of reactant gases in the vapour is. It is known 6'7 that for a ratio SiH4:NH 3 < 10 -2 the deposition of silicon nitride under the present conditions does not lead to the appearance of an excess
STRESS AND STRUCTURE IN CVD S i 3 N 4 FILMS
137
silicon content in the film. This is in agreement with the fact that the refractive index of the films under study does not exceed the value of 2.0 typical for stoichiometric SiaN411. The silicon excess is usually found in films with a refractive index exceeding 2.09,18
The refractive index of 1.98-1.99 for the films annealed at 1000 °C seems to be due to oxygen contamination from NH3 and argon gases. It is likely that all the oxygen directed to the film surface is incorporated into the film volume. This is due to two factors: (1) the high reactivity of oxygen and silane and (2) significantly greater Sill4 than O2 in the vapour. Therefore the films deposited at the same rates must contain identical amounts of oxygen whatever the deposition temperature because the NH3, argon and, consequently, 0 2 concentrations in gas mixture were slightly varied. An increase in oxygen contamination is possible with a decrease in the film deposition rate. However, the content of oxygen in the film increased by no more than twofold in all cases because no shift of the main absorption band at 850 cm-1 has been observed. This is confirmed by the same values of the refractive indices for the films under study. The refractive index of 1.95-1.96 before annealing, which is lower than that for stoichiometric Si3N4, can be related to the presence of a considerable quantity of S i - - H and N - - H bonds in the film. According to data of Stein and coworkers 3'6, Smirnova etal. 5 and the present results, the amount of bonded hydrogen is 1021-1022 c m - a (Table I). The decrease in film stress with decreasing deposition rate and with increasing deposition temperature could be due to reasons (a)-(c) above. From the results shown in Fig. l(a) the film stress decreases with decreasing deposition rate owing to a reduction in Sill4 flow rate and, consequently, cannot be attributed to excess silicon content 1s. As mentioned above, oxygen contamination of the film is possible, but in this case the 0 2 content seems not to vary practically with changing deposition temperature for a constant film deposition rate. Therefore the observed decrease in film stress with the increase in deposition temperature (Fig. 2(a)) is unlikely to be due to oxygen contamination. Also, variations in the stress depending on the annealing of the film cannot be attributed to changes in either the Si:N ratio or the oxygen content, as neither of these values varies under annealing. According to Irene Is, a decreasing stress was also observed with an increase in temperature and decrease of in rate of CVD Si3N 4 film deposition when other deposition parameters are held constant. In this case no oxygen contamination was detected in the film. From a comparison of the results of changing intrinsic tensile stress and densification Ah/h depending on annealing time (Fig. 1) we conclude that the increasing intrinsic stress in the film is due to film bulk shrinkage: Av/v ,~ Ah/h. In its turn, the change in film volume can be attributed to a change in its structure, i.e. transformation of the SixNyh~ containing N---H and Si---H bonds into the SisN 4. The change in film volume is possibly also due to the disappearance of other amorphous network disorders in silicon nitride, of which silicon and nitrogen dangling bond sites are the most probable19.20. However, the density of such defects abruptly falls in CVD silicon nitride near stoichiometry to about 10 is cm-3 as hydrogen passivates dangling bonds in this system 19. Therefore the changes in
138
A.G. NOSKOVet al.
S i - - H and N - - H bond concentrations are more likely to affect the structure and investigated properties of the films than changes in silicon and nitrogen dangling bond concentrations. It should be noted that the initial stage of structural rearrangement is the rupture of S i - - H and H - - N bonds followed by the formation and cross-linking of silicon and nitrogen dangling bonds ~'6. To estimate the effect of a loss of bound hydrogen on film shrinkage we compare the activation energy E of a shrinkage process with the dissociation energies of N - - H and S i - - H Chemical bonds. As can be seen in Fig. 7, which is based on the data of Fig. l(c), the temperature dependence of the shrinkage rate during annealing is well described by the Arrhenius equation W= ~
= Aexp
(5)
where W is a rate of film thickness change during thermal treatment. To obtain the rates Wshown in Fig. 7 changes in Ah/h from 0 to - 0.01 were used. The dependence of the shrinkage rate on annealing temperature on the Arrhenius plot has a similar slope if the change in Ah/h is taken as the annealing time from 0 to 60 min.
'c
1000
900
800
700~
i
i
i
I
-1
163 10~
j
0.8
09 IO001T
1.0 (K "1)
Fig. 7. Dependenceof filmshrinkage rate on annealingtemperature.The data of Fig. l(c) wereused. The activation energy E of the shrinkage process during thermal treatments proved to be equal to 2.1 + 0.4 eV. The dissociation energies of S i - - H and N - - H chemical bonds are 2.9 eV and 3.1 eV respectively4'21. The difference between the activation energy E and the dissociation energies can be explained by the transformation of incomplete bond breaking due to the interchange of valence electrons 22. The activation energy of dehydrogenation calculated by Smirnova et al. 5 for silicon nitride obtained under conditions close to ours also proved to be somewhat lower (2.6 eV) than the dissociation energy values. Consequently, the film shrinkage is assumed to be determined by dissociation of S i - - H and N - - H bonds followed by a further structural rearrangement. The fact that the shrinkage process is determined by dissociating the S i - - H and N - - H bonds can be explained by the kinetic dependence on Ah/h on annealing time which coincides qualitatively with the kinetic dependences of the thermal dissociation of N - - H and S i - - H bonds in the
STRESS AND STRUCTURE IN CVD S i a N 4 FILMS
139
SiaN 4 films 5'6. A quantitative comparison requires further investigations of the change in Si--H and N - - H bond concentrations and film thickness change during thermal treatments. In our case, the change in N - - H and Si--H bond concentrations was judged by lower film etch rates after annealing and sometimes by the change in the corresponding IR absorption bands in the film (Table I). As shown in refs. 4 and 23, the values for Ca and V~ are directly dependent, i.e. a lower concentration of chemically bonded hydrogen results in a lower etch rate, and thus
dV~/dC. > O. Furthermore, it is necessary to investigate structural rearrangements caused by defects in CVD Si3N4 films for the contribution of each kind of defect to film shrinkage and associated effects to be distinguished. The film dehydrogenation with an increase in deposition and annealing temperatures also results in a higher refractive index n. During annealing the change in n is directly proportional to Ah/h, which provides evidence of hydrogen release and an approach of the film structure to a stoichiometric Si3N4 with n = 2.(~3-2.0511. Using the above facts one can conclude that the intrinsic stress increases during annealing and the changes in film thickness and etch rate are due to a lower concentration of the bonded hydrogen in the CVD Si3N4 films. The film thickness and etch rate changes during annealing are directly dependent on Ca. At the same time dtrl/dt has the opposite sign to dCn/dt (ref. 5) as well as to dh/dt and to dVddt (see Fig. 1); that means an increase in film stress with falling Ca. On the basis of the above one can write the following relationships: dV~ -->0 dCa
dh -->0
dCn
do" i
--
