The correlations between properties of plasma-enhanced chemically vapour deposited silicon nitride and the deposition conditions

134

Thin Solid

Films,

215 (1992)

134

141

The correlations between properties of plasma-enhanced chemically vapour deposited silicon nitride and the deposition conditions Xiumiao Depurimmt

Zhang,

Guohua

of Elecironic

Engineering,

Shi, Ailing Yang and Diling Shao Hungzhou

Uniurrsit,v,

H~mg~hou,

Zh+ung

(Chinu)

(Received November 20. I99 1: accepted February 28. 1992)

Abstract Silicon nitride films were produced from an SiH,-NH, gas mixture in a Chinese-built plasma-enhanced chemical vapour deposition system. Their composition and electrical properties were investigated by varying the deposition conditions. The silicon nitride films exhibit the following characteristics: (1) the composition is approximately uniform in the direction of the film thickness, and the oxygen content is less than 3%; (2) the densities of fixed positive charges and trap states in the interface have the same order of 10” cm -I; (3) the conduction mode appears to be FrenkelLPoole emission; (4) the breakdown strength ranges from 3.0 to 8.5 MV cm-’ as the Si:N ratio ranges from 1.25 to 0.81.

1. Introduction Since silicon nitride film has a high dielectric constant and can protect integrated circuits from contamination (e.g. alkaline ions and moisture), it has been widely used in modern semiconductor technology as a final passivation layer or interlayer insulation [ 11. It is well known that the film properties such as composition, fixed positive charges and breakdown strength strongly depend on its deposition conditions [2, 31. In this work, we prepared samples in a Chinese-built plasma-enhanced chemical vapour deposition (PECVD) system (DD-P250), and then systematically investigated the properties of the films in terms of deposition conditions. As a result, fundamental knowledge on the characteristics of silicon nitride deposited from an SiH,NH, gas mixture in the DD-P250 system is obtained.

2. Deposition

system and experiments

The silicon nitride films were deposited on 10 R cm n-type silicon with (100) crystal orientation in a capacitively coupled PECVD system (DD-P250). The apparatus is shown schematically in Fig. 1; the reaction chamber and the electrodes are made of stainless steel. The substrate holder (anode electrode) was baked out for 1 h at 2.50 “C before placing the sample substrate in the system. Then, the system was evacuated to about 5 x lo-’ Torr by a pump to remove residual gases. Silane (3.0% silane mixed with 97.0% nitrogen) and ammonia were used as reactants for this deposition process. Four process parameters, i.e. substrate temper-

0040-6090/92/$5.00

Fig. I. A sketch of the plasma reactor.

ature, total gas pressure, SiH,-NH, flow rate, and r.f. power, were systematically varied according to Table 1, while r.f. frequency and deposition time were set constant at 13.56 MHz and 250 s respectively. The film thicknesses and refractive indices were determined by ellipsometry. Auger electron spectroscopy was used to determine the film composition and its uniformity. The hydrogen density was estimated by IR

(

1992

Elsevier Sequoia. All rights reserved

X. Zhang et al. / Properties o f PECVD Si nitride

135

T A B L E I. Deposition conditions

SiH4:NH ~ flow rate ratio ~

Substrate temperature ('C)

R.f. power ( W ( W cm 2))

Pressure (Torr)

Type

Sample

A

A1 A2 A3 A4 A5

480:24 = 20:1

200 250 300 350 400

80 (0.16)

0.35

B

BI B2 B3 B4 B5

480:24 --- 20:1

250

40 (0.08) 80 (0.16) 100 (0.20) 140 (0.28) 175(0.35)

0.38 0.35 0.35 0.35 0.32

C

CI C2 C3 C4 C5

400:40 600:40 800:40 1000:40 1200:40

=- 10:1 --- 15:1 =- 20:1 -: 25:1 =- 30:1

250

80(0.16)

0.35 0.35 0.35 0.31 0.31

D

D1 D2 D3 D4 D5

400:40 600:40 800:40 1000:40 1200:40

= 10:1 -= 15:1 :- 20:1 -= 25:1 -= 30:1

350

80 (0.16)

0.30 0.32 0.34 0.32 0.30

~'The flow rate values on the left-hand side of each identity are in millilitres per minute.

absorption. From the current-voltage characteristics of the metal-nitride silicon ( M N S ) structure, the breakdown strength of films was measured and the conduction mode was determined. Finally, the capacitancevoltage characteristics were measured to investigate the interface properties between silicon and silicon nitride.

3. Results and discussion

3. I. Refractive index and composition Refractive index and composition were investigated in terms of the deposition conditions. Figure 2 shows the effects of substrate temperature on these properties. As the substrate temperature is increased, the refractive index slightly increases, but the Si:N ratio decreases and approaches the value of stoichiometric nitride. Since the plasma temperature becomes higher, the reaction proceeds more completely and fewer excess silicon atoms would be incorporated in the film. Moreover, the Si:N ratio decreases with the increase in the r.f. power and the refractive index increases, as shown in Fig. 3. It is well known that the Si-H bond energy is 70.4 kcal g-1 and the N - H bond energy is 93.4kcal g-~; the Si-H bond is broken more easily than the N - H bond. When the r.f. power is increased, more N H bonds can be

broken and more nitrogen is incorporated with silicon; therefore, the S i - N ratio decreases and the refractive index increases. Figure 4 shows that the Si:N ratio and the refractive index increase with increasing SiH4:NH3 flow rate ratio. All films exhibit a slightly silicon-rich composition, i.e. S i : N > 0 . 7 5 , and the smallest Si:N ratio achieved is 0.77. The Auger electron spectrum in Fig. 5 shows that the silicon nitride films contain the impurities carbon and oxygen. The amounts of oxygen and carbon in all samples were evaluated to be about 0.6%-2.5% and 3.0%-7.0% respectively, according to the method proposed by Holloway and Stein [4]. We also analysed oxygen and carbon contaminations in the direction of the film thickness. A sample was prepared in the same way as sample D1 in Table 1 for Auger depth profile analysis. Ar + ions were used to sputter the film surface, and the sputtering velocity was about 1/6 A s 1. A representative in-depth profile of composition for an SiN/Si structure is shown in Fig. 6. Oxygen and carbon densities at the film surface appear very large, but rapidly decrease into the film. This indicates that a silicon oxynitride film of 3 - 4 nm thickness exists in the surface, as has been shown by many previous workers [3, 5]. The compositional ratio Si:N is approximately constant in the direction of the film

136

X. Zhang et al. / Properties q[ PECVD Si nitride 1.1

N

1.0

2.1

o -H

.r4

% .rq 40

8 A

1.9

O.9

~-7

o.8

!

1.5

I

40

80

L

120

~

0.7

i

I 60

200

240

Rf-power (\7) Fig. 2. Composition and refractive index as a function of substrate temperature.

1.2

2.1 si/n

o

H o ~s

1.1

1.9

Z

-,-I

4J o

1.0

1.7

I 5O

20O

25O Substrate

300

350

temperature

(°C)

40O

o.9 45o

gig. 3. Composition and refractive index as a function of r.f. power. thickness. M o r e o v e r , we did n o t observe any oxygen p e a k at the interface between silicon a n d silicon nitride. Hezel [5] r e p o r t e d that in silicon nitride films d e p o s i t e d in a resistance-heated a t m o s p h e r i c pressure r e a c t o r by the s i l a n e - a m m o n i a reaction there existed a silicon n i t r i d e - s i l i c o n oxide silicon interface layer. The interface layer was a b o u t 3 - 4 nm in thickness. T h e maxim u m oxygen c o n t e n t was 20 at.%. Y o k o y a m a et al. [3] also d e m o n s t r a t e d t h a t some oxygen was d e t e c t a b l e in the films d e p o s i t e d by their P E C V D system. In their films the silicon oxide layer was 4 n m in thickness, b u t the o x y g e n c o n t e n t was only 5 at."/,,. T h e y all a t t r i b u t e d the oxygen to a native layer f o r m e d on the s u b s t r a t e silicon surface. In o u r e x p e r i m e n t we a n a l y s e d five

samples a n d did n o t find a n y oxygen p e a k in the interface layer. The a b o v e - m e n t i o n e d difference could be explained as follows. Hezel did not remove the native oxide layer f o r m e d on the substrate silicon surface a l t h o u g h the oxygen a t o m s in the interface diffused into the SiN m a t r i x a n d the c o n c e n t r a t i o n o f these in the interface was r e d u c e d by increasing the film thickness d u r i n g the d e p o s i t i o n , the oxygen c o n c e n t r a t i o n was up to 20 at.%. Y o k o y a m a et al. d e p o s i t e d the silicon nitride film with a pressure o f 0.75 torr, a n d the r e a c t i o n gases were ionized. W h e n the ions were accelerated to a certain velocity, some oxygen a t o m s in the native oxide layer were sputtered off, a n d the oxygen c o n c e n t r a t i o n in the interface was reduced to 5 at."/,,. F o r o u r samples, the

X. Zhang et al. / Properties of PECVD Si nitride 2.1

x

137

1.8

•

2.0

s~-/

@

o

,H 4~ o

1.2

] --9

~n co

0.9

i

1.7

i

10:1

15:1

~

i

20:1

o.6

i

25:1

30:1

SiH4/.~,TH3 flow rates

Fig. 4. Composition and retYactive index as a function of SiH4:NH~ flow rate ratio.

0 510eV

N 585eV

C 272eV

Si 92 eV

Fig. 5. Auger electron spectrum for impurity contamination in PECVD

silicon nitride.

ity and then more oxygen atoms are sputtered off, so that the oxygen content at the interface is o f the same order as that in the layer. Figure 7 shows typical IR transmission spectra: spectrum a is the spectrum of the substrate; after a 20 0 0 0 / ~ thick silicon nitride film had been deposited on the substrate, spectrum b was obtained. By comparing the two spectra, the optical absorption peak around 1100 cm i (caused by Si O bonds) can be understood to occur mainly in the substrate. The optical absorption peak S i - N bonds, peaks by N - H and Si-H bonds are observed. The absorption coefficient ~ is calculated from e = ln(To/T)/d, where d is the thickness of the

Si-O

8O

I

a

60

8o

N

j

o .H

}

40

e

20

g~

0

S±

o

0.0

-

4.8

.-

9.6

14-4

19.2

-,

24.0

. .---,

f. 40

N-H

28.8

Sputtering time (x102s) 20 F i g . 6. Auger electron spectroscopy depth profile for the elemental

concentrations o f silicon, nitrogen, carbon and oxygen in P E C V D silicon nitride.

S i - L~

0

native oxide layer seems to disappear. This may be the result of our deposition with a pressure o f 0 . 3 0.35 Torr, which is lower than that used in ref. 3. At lower pressure the ions are accelerated to higher veloc-

r 320O

i 2O00

i 1500 Wavenumber

J 1000

i 500

(cm -I)

F i g . 7. IR transmission spectra: spectrum a, the silicon substrate; spectrum b, after a 20 000,~, thick silicon nitride film had been deposited on the silicon substrate.

X. Zhang eta/. / Properties o]' PECVD Si nitride

138

film, T the transmittance, and To the background transmittance. The m a x i m u m absorption coefficient is calculated using the above equation: ~..... = l n ( 7 9 / 5 7 ) / d = 1.6 x 103 cm ~ for N - H bonds, and ~..... = ln(79/67)d = 8.2 x 102 cm -~ for Si H bonds. The densities of N - H and Si H bonds are evaluated by the method of Brods k y e t a l . [6]: NN u = 1021 cm 3, Ns i H = 1019 c m - 3 - T h i s implies that P E C V D SiN films from a silane and ammonia mixture could be regarded as an Si:N:H ternary system. The chemistry of the deposition process for SiN~ H,. was explained by Smith et al. [7]. 3.2. Etch rate

The etch rate in is shown in Fig. 8 ture. The etch rate strate temperature.

A

diluted H F ( H F : H 2 0 = 1 ml:50 ml) as a function of substrate temperadecreases with the increase in subFigure 9 shows that the etch rate

2.2

oj

3.3. Fixed eharges and trap states

o~

z.o M

ta?

1.8

~o 4~ •~

[D

initially decreases and then increases while the r.f. power is increased from 40 W to 175 W. The etch rate increases when the r.f. power reaches 175 W, since at such a high r.f. power the deposition rate is so fast that the film density decreases, and the decrease in the film density makes the film structure relax. Therefore, we have shown that the r.f. power range 4 0 W 160 W is suitable for depositing silicon nitride films. F r o m the above, we also deduce that the etch rate tends to decrease as the composition Si:N approaches from the silicon-rich to the stoichiometric value; however, in our results shown in Fig. 10, the etch rate increases although the Si:N ratio approaches the stoichiometric value, as the SiH4:NH3 flow rate ratio is increased. As the hydrogen contamination changes with the variation in the SiH4:NH3 flow rate ratio, the hydrogen contamination has more influence on the etch rate than the compositional Si:N ratio, as reported by Lanford and Rand [8]. Thus, the etch rate decreases with increase in the SiH4:NH3 flow rate ratio.

I .6

0) N ]*/4

I

l

200

250

i

I

300

i

350

High frequency (1 MHz) capacitance-voltage characteristics of the M N S capacitors were measured to investigate the interface properties between silicon and silicon nitride. The flat band voltage VFB is more negative than the theoretical value. According to the C V technique, positive charges exist at the interface or in the silicon nitride layer; the positive charges are evaluated as a function of deposition conditions in Figs. 11 13. The injection type of hysteresis is also observed, and this hysteresis is caused by the charges trapping into the trap states in the silicon nitride. The hysteresis width A VvB varies between 2 V and 8 V with the change

400

Substrate temperature (°C) 2O

Fig. 8. E t c h r a t e as a f u n c t i o n o f s u b s t r a t e t e m p e r a t u r e .

8O

"7 .r-I 0

o4 ~J o

15

6o %

io %

4o 5 zo

0

0

l 40

80

,

i

120

160

J

I0:I

,

15:1

I

20:1

i

25:1

175

SiH4/~H 3 flow rates R f-power

(W)

Fig. 9. E t c h r a t e as a f u n c t i o n o f r.f. p o w e r .

Fig. 10. E t c h r a t e as a f u n c t i o n o f S i H 4 : N H 3 flow r a t e ratio.

i

30:1

X. Zhang et al. / Properties

of PECVD

139

Si nitride

10:1 80

40

120

160

15:l

2O:l

SiH@iH3 Rf-power Fig. I I. Density temperature.

of fixed positive

charges

25:

i

JO:1

200 flow rates

(Ww) as a function

of substrate

Fig. 13. Density flow rate ratio.

of fixed positive

charges

as a function

of SiH,:NH,

contend that the positive charges are silicon dangling bonds. In order to control trap states it is necessary for us to study the trap states further later. 3.4. Breakdown strength, conductive mode Figure 14 shows typical current-voltage characteristics at 25 “C for silicon nitride sample Bl. Figure 15 shows a plot of leakage current at a constant field of 1.3 x lo6 V cm-‘. The plots shown here are for the metal electrode biased positive. The linear relationship between the logarithm of the current and the square root of the field is similar to the conduction which occurs through Frenkel-Poole emission (internal Schottky effect). This involves field200

250 Substrate

Fig. 12. Density

of fixed positive

300

350

temperature charges

as a function

400

(‘0) of r.f. power.

in deposition conditions; the trap states are at densities of the order of 1 x lo’* cm-2-5 x lOI cm-*. As to the long-term stability of the interface charges at elevated temperatures, we annealed five samples for 120 min at 375 “C in a nitrogen ambient. The result shows that no change in the quantity of positive charges could be observed. This demonstrates the excellent stability of the fixed positive charges. However, the density of trap states decreases from lOI* cm-* to 10” cm-* after annealing. Comparing Figs. 11-13 with Figs. 2-4, we note that the number of positive charges decreases as the compositional Si:N ratio decreases, i.e. fewer excess silicon atoms would result in fewer positive charges. We would

!z

-2 10-11 i?

10-J 0.4

0.6

0.8 s

Fig. 14. Current-voltage

1 .O

1.2

(x 103Js)

characteristic

of sample

Bl.

1.4

140

X. Zhang et al. / Properties (~/I PECVD Si nitri~h,

number of excess silicon atoms, as call be seen from Figs. 2 4. When the film is deposited at 175 W r.f. power, Sill4 and NH~ flow rates of 4 8 0 m l m i n ~ and 2 4 0 m l m i n respectively and 250 C substrate temperature, the compositional ratio Si:N and breakdown strength are 0.77 a n d 10 7 V cm i respectively. Ordinarily, the compositional ratio Si:N is variable between 1.25 and 0.81, and the breakdown strength changes in the range (3.08.5) × 10~'Vcm

I0-5 : 1.]Sx106V/cm

10-6

10-7 =o

10-8

~q 10-9

E

~Z

~-9

2.1

2.3

0238ev

2.5

2.7

2.9

3.I

IO00/T (K -I) Fig. I5. L e a k a g e curl-enl as 1.3 × 10¢ , V c m i for s a m p l e B1.

a

['unction

of

temperature

at

enhanced thermal excitation of electrons from traps into the conduction band of the dielectric. According to this mechanism [9], J = c I E exp{

( q / k T ) [ q ~ B -- (qE/Tr¢OCd) I/~-] }

with 77.087 ¢d = T2[( slope)(ln J / E l / 2 ) ] 2 ~PB = e ~ / q + (5.746 X 10

7E/¢d)l/2

where J is the current density, E the electric field, ¢p~ the barrier height or depth of trap potential well, ~o permittivity of free space, ed a dynamic dielectric constant, c~ a function of the density of traps, and 6~ the slope of the activation energy plot of In J t:s. I / T . For the sample BI investigated in Figs. 14 and 15, E,~ is 5.8, and the leakage current has an appreciable temperature dependence ( ( ; ~ - 0 . 3 8 V above 100 C). Both of these observations would favour the Frenkel--Poole emission model, at least at temperatures above 100 C . However, the In J Ps. T i plot exhibits a significant deviation from linearity below 100 C . Thus, ~d is not really constant at various temperatures. At 6 0 C , the temperature dependence of the leakage current becomes very weak, and it might be taken as an indication of possible tunnelling components in the leakage current. The breakdown strength is determined as the electric field at which the current density exceeds 10 4 A cm ~-. The breakdown strengths are increased as the substrate temperature increases, the r.f. power increases or the SiH4:NH~ flow rate ratio decreases. These improvements of the breakdown strength can be attributed to the approach of the composition Si:N to the stoichiometric value [10, 11], i.e. a decrease in the

4. Conclusions We have invesngated the properties of silicon nitride films deposited from the Sill4 NH~ gas mixture in a Chinese-built PECVD system. Since diluted silane (a mixture of 3.0% silane and 97.0% nitrogen) was used as the reactant, the reaction gas in fact is a mixture of nitrogen, silane and ammonia. The properties of the films deposited in this system have been reported and discussed carefully. The results are summarized as follows. (1) A higher substrate temperature results in higher refractive index, higher breakdown strength, lower Si:N ratio, lower wet etch rate, and fewer fixed positive charges. (2) A higher r.f. power results in higher refractive index, higher breakdown strength, lower Si:N ratio, lower wet etch rate, and fewer fixed positive charges. (3) Higher SiH4:NH3 flow rate ratios result in higher refractive index, higher Si:N ratio, lower wet etch rate, more fixed positive charges, and lower breakdown strength. (4) An increase in total gas flow results in an increase in refractive index, a decrease in Si:N ratio, a decrease in wet etch rate, a decrease in the number of fixed positive charges, and an increase in breakdown strength. (5) The fixed positive charges are demonstrated to be very stable through an annealing process for 120 rain at 375 ~'C in a nitrogen ambient, but the trap states are easily changed. (6) All films exhibit a slightly silicon-rich composition. The electric and interface properties are rather improved as the compositional ratio Si:N approaches the stoichiometric value. (7) No oxygen peak was observed at the interface betwen silicon and silicon nitride. This is a result of the lower pressure of the reaction gas in our system.

Acknowledgment This work was supported by the National Nature Science Foundation of China.

X. Zhang et al. / Properties of PECVD Si nitride References 1 D. Frohmann-Bentchkowsky and M. Lenzlinger, J. Appl. Phys., 40 (1969) 3307. 2 G. M. Samuelon and K. M. Mar, J. Electrochem. Soc., 129(1982) 1773. 3 S. Yokoyama, N. Kajihara, M. Hirose and Y. Osaka, J. Appl. Phys., 51 (1980) 5470. 4 P. H. Holloway and H. J. Stein, J. Electrochem. Soc., 123 (1976) 723.

141

5 R. Hezel, Solid-State Electron., 24 (1981) 863. 6 M. H. Brodsky, M. Cardona and J. J. Guomo, Phys. Rev. B, 16 (1977) 3556. 7 D. L. Smith, A. S. Alimonda, C. C. Chert, S. E. Ready and B. Waeker, J. Electrochem. Soc., 137 (2) (1990) 614. 8 W. A. Lanford and M. J. Rand, J. Appl. Phys., 49(1978) 2473. 9 S. M. Sze, J. Appl. Phys., 38 (1967) 2951. 10 A. K. Sinha and T. E. Smith, J. Appl. Phys., 49 (1978) 2756. 11 H. Dun, P. Pan, F. R. White and R. W. Douse, J. Electrochem. Soc., 128, (1981) 1555.