Thin Solid Fihns, 229 (1993) 93-100 93

Growth and characterization of thin SiN films grown on Si by electron cyclotron resonance nitrogen plasma treatment

Jong Moon, Toshimichi Ito and Akio Hiraki Department of Electrical Engineering, Faculty of Engineering, Osaka University, Suita, Osaka 565 (Japan)

(Received July 29, 1992; accepted December 22, 1992)

Abstract

Thin silicon nitride films have been fabricated on Si at a low substrate temperature (about 450 °C) by means of the direct exposure of Si wafers to an electron cyclotron resonance nitrogen plasma generated at 2.45 GHz. The film thickness can be controlled in the range 40-400 A by suitably changing the nitrogen pressure, microwave power and substrate bias voltage. The activation energy characteristic of the growth is 0.42 + 0.11 eV. The refractive indices of the films are 1.9-2.0 and the N-to-Si atomic ratio obtained by Rutherford backscattering spectrometry ranged from 1.35 to 1.43, indicating a nitrogen-rich film (stoichiometric value of 1.33). The average breakdown voltage of AI-SiN-Si capacitors is 11.9_ 0.2 MV cm -~ and a typical leakage current density is about 10 -9 A cm -z at a 2 V positive bias stressint~ for films 16 nm thick. Ion channelling experiments show that silicon nitride films form sharper interfaces (about 5 A) on the Si substrate than chemically vapour-deposited nitrides do. Optical emission spectra measured during the film growth indicate that the nitrogen molecular ions are closely related to the growth rate and film quality.

1. Introduction

Silicon nitride is a prospective material for utilization in many technological areas f rom specialized industries to microelectronics. This is a consequence of the attrac- tive features of this material such as a high thermal stability, chemical inertness, hardness and good dielec- tric properties. For very-large-scale integration applica- tions, in particular, thin silicon nitride films have recently been used as potential gates and as a cell capacitor dielectric. However, it is well known that silicon nitride films deposited directly on silicon sub- strates by means of chemical vapour deposition (CVD) and sputtering methods cause interfacial instabilities between the silicon nitride and silicon substrate. There- fore attempts to improve the properties of the silicon nitride film have been made [1-4]. Of the various formation methods, direct nitridation of the silicon substrate is one of the promising methods because of the relatively low growth temperatures employed, leading to good interfacial characteristics and electrical properties of the silicon nitride film. Thus many reports have been published on silicon nitride films fabricated by the direct reaction of the silicon substrate [5-8]. Knoop and Sticker [9] studied thermal nitridation in a quartz am- poule containing nitrogen gas. Paloura et al. [5, 7] reported the growth of silicon nitride films at a low temperature of about 300 °C in a pure nitrogen plasma generated by an r.f. discharge.

In the present study we report the growth of silicon nitride films at a low temperature of 450 °C, in a highly pure nitrogen plasma generated by electron cyclotron resonance (ECR) using 2.45 G H z microwaves. This pro- cess can be expected to form high quality silicon nitride films because the ECR plasma permits large transporta- tions of highly excited species and less damage generation to the silicon substrate at the low temperature (about 450 °C), which may, in turn, lead to good electrical pro- perties. In this paper we present data on the activation energy characteristic of the growth with the key plasma parameters of the growth and also describe the effect of a pre-cleaning treatment by ECR hydrogen plasma on the subsequent film growth as well as on the electric proper- ties of the films. In addition, we also discuss the growth mechanism in relation to plasma conditions determined by optical emission spectroscopy (OES). In plasma processing, light emission measurements from excited species by OES provide information about the relative densities of ions and radicals so that one can discuss the possible effects of the excited species on various properties of the Si3N 4 films. For film characterization, the film composition is analysed by means of Rutherford back- scattering spectrometry (RBS) and X-ray photoelectron spectroscopy (XPS) and the amounts of hydrogen in films was evaluated by means of elastic recoil detection analysis (ERDA). Electrical examinations, e.g. breakdown volt- age and leakage current, were carried out using ramp- voltage-stressed current -vol tage ( I - V ) measurements.

Elsevier Sequoia

94 J. Moon et al. / Growth and characterization o f ECR N2-plasma-treated SiN on Si

2. Experimental details

Boron-doped p-type Czochralski Si(111) wafers with a resistivity of about 0.1 f~ cm were used as the starting material. The specimens were chemically cleaned by etching native oxide on the silicon surface in dilute hydrofluoric acid, rinsing with deionized water and drying in flowing nitrogen gas. Figure 1 shows a sche- matic drawing of the microwave plasma apparatus with a typical magnetic field distribution used in the present experiment. An ECR nitrogen plasma was generated at pressures ranging from 0.01 to 0.5 Torr using 2.45 GHz microwaves. After a cleaned specimen had been loaded into the reaction chamber, the chamber was pumped down to a pressure of less than 2 x 10 -5 Torr, and then semiconductor-grade hydrogen or nitrogen gases were introduced into the chamber. After the pressure became stable, the plasma discharge was turned on by applying the microwave power. Direct nitridation was sequen- tially performed in the same reaction chamber. First, the specimen was exposed for 1 h to the ECR hydrogen plasma to remove the surface native silicon oxide and to prevent the specimen surface from further oxidation by hydrogen passivation [10]. The microwave power applied was 1.0 kW and the pressure measured was 0.1 Tort. Secondly, after pumping down to the base

3 3

Heater

Substrat~

(a)

\Coi ls /

R o u n d t ~ Wovegu]de

(Discharge Area)

Gas Inlet

I Rectangular Wave~uide

J j i

f r i

300 200 100 Distance (mm)

(b)

2.0~ 0 ~ IT

1.0 c a u

Fig. I. (a) A schematic diagram of the ECR plasma CVD system used; (b) a representative distribution of magnetic flux density to be applied.

TABLE 1. Process conditions for samples prepared by means of the direct nitridation of Si wafers using an electron cyclotron resonance nitrogen plasma

Process parameter Value

Microwave power (W) 100-700 N 2 pressure (mTorr) 10-500 Substrate temperature (°C) 450 900 Substrate bias voltage (V) - 5 0 to + 150 Nitridation time (h) 6

pressure, direct nitridation of the specimen was per- formed at a temperature of 450-900 °C for 6 h by exposing the cleaned specimen surface to the nitrogen plasma. The specimen temperature was monitored with a thermocouple, calibrated by a pyrometer. The process conditions used in the experiment were summarized in Table 1.

Emission from the ECR nitrogen plasma was trans- mitted through a fused silica window in the vacuum vessel focused on the edge of an optical fibre with a cut-off wavelength of 3000 A, and were introduced into a polychromator with an optical multichannel detector. The focal point of the emission light was located at the top of the specimen with a wavelength resolution of 1.6 &.

The thickness and refractive index of nitride films were determined using an ellipsometer with incident light of wavelength 6328 ]k at an angle of 70 °. XPS was performed to evaluate the specimen surfaces using Mg Kct radiation while the film compositions were evalu- ated by means of RBS. For RBS and channelling measurements, the energy of the 4He2 + beam used was fixed at 2.4 MeV and the ion dosage irradiated was 20 pC. A grazing-incidence detection method (with a detection angle of 80 ° ) was applied to increase the depth sensitivity for very thin film analysis. E R D A was applied to evaluate the relative amounts of hydrogen atoms included in the specimens. The energy of the 4He2+ beam for E R D A was fixed at 2.1 MeV and the ion dosage was 100 ~tC with a typical beam current of 30 nA. The experimental arrangement used for E R D A is illustrated schematically in Fig. 2. The angle between

f l Sample 2.1MeV He A \ \ - - )

• I I ° n B eam "-k---~_ ~X%..,/ . . . . . . . . . . . . . . . . . . . .

[ . ~--~-130"/,R,e. c°it ~,flO pm Thie k o,mo,o, ,on, A' S,opper

Forward Scattered/z" ~ Z'X/' He Ions J " ~ . . ~ /..~

/V " ~ z ~ ~ Detector

Fig. 2. Schematic illustration of the experimental arrangement for profiling H in films using ERDA.

J. Moon et al. / Growth and characterization o f ECR Nz-plasma-treated SiN on Si 95

the detector and the incident beam was fixed at 150 °. A 10 ~tm aluminium foil worked as a particle filter to shield the detector from forward-scattered probing He particles.

The electrical properties of the grown films were determined from current-vol tage ( I - V ) characteristics of aluminium nitride-silicon capacitors fabricated by means of AI evaporation on the films. The gate area obtained was 0.2 cm 2. The ramp-voltage-stressed I - V technique was employed to measure the breakdown voltage and leakage current using a Keithley 617 elec- trometer and an HP 9826 instrument controller.

- <

i n

E

r . -

E U_

-50 "6 5b lC)O 150 DC bias voltage(V)

Fig. 4. Film thickness dependence on substrate d.c. bias voltage for films grown at 10 mTorr with 500 W microwave power.

3. Results and discussion

3. I. Fi lm growth We shall present various dependences of film growth

on temperature, microwave power, substrate bias and nitrogen pressure, as well as relative amounts of excited species in the ECR N2 plasma measured using OES. We shall also discuss the possible effect of the pre-cleaning treatment by the hydrogen plasma in relation to the hy- drogen content in the Si substrate determined by ERDA.

The temperature dependence of the film thickness of silicon nitrides fabricated at a constant pressure of 100 mTorr with a microwave power of 500W for a growth duration of 6 h is shown in Fig. 3. From the plots of film thickness vs. reciprocal temperature (103T-i), one can derive the activation energy involved in the growth kinetics. The activation energy obtained for film growth is 0.42 + 0.11 eV, This value is a little higher than those reported for thermal growth, 0 .2- 0.35 eV [11, 12] and those for similar direct growth on Si surface by an r.f. (13.56MHz) N2 plasma, 0.18- 0.42 eV [7]. Differences in the activation energy for the

10(1

.3 v

u l

5 ~o( ._u _C F -

E It_

Temperature (K) 1250 1003 833 714

lO o.b ' 11o ' 112 ' 1.4

lO00/T ( K )

Fig. 3. Film thickness vs. inverse temperature plots for films grown at 100mTorr with 5 0 0 W microwave power. The activation energy deduced for the growth is 0.42 + 0.11 eV.

growth rate can be explained as due to real surface temperature and the characteristics of plasma such as the ion flux, ion energy, electron temperature and ek.q- tron densities. In particular, the difference in the activa- tion energy for direct-nitridation methods of the Si surface using a nitrogen plasma has a large effect because significant amounts of highly activated nitrogen species can be easily generated under the condition of a higher electron temperature (about 8 eV) of the ECR N2 plasma [13] than that (3 eV) of the r.f. (13.56 MHz) N2 plasma [ 14].

The dependence of film thickness on the substrate d.c. bias voltage is shown in Fig. 4. The nitridation temperature, power, pressure and time employed were 450 °C, 500W, 100 mTorr and 6 h respectively. It is clear that there is a remarkable enhancement in the growth rate at bias voltages positively applied to the sample holder. On the contrary, we observe that a negative bias voltage of - 5 0 V has little effect on the film growth, which is in good agreement with the results of Hirayama et al. [6] who investigated similar effects of d.c. bias voltage from - 150 to + 150 V using an r.f. N2 plasma. The effect of growth kinetics on the significant increase in growth rate in positive d.c. bias conditions may be explained as follows: (1) the positive d.c. bias is able to collect larger amounts of highly activated elec- trons, which lead to an increase in the temperature of real reaction regions due to bombardments onto the Si surface; (2) the increase in temperature in the reaction region enhances the nitrogen diffusivity in silicon. The enhancement of nitrogen diffusivity was also shown in the Auger depth profile results of Hirayama et al. [6].

Figure 5 shows typical optical emission spectra of N-related species in the ECR plasma generated at an N2 pressure of 10 mTorr with a microwave power of 700 W. In Fig. 5(a) the first and second positive systems of N 2 radicals are identified at wavelengths ranging from 580 to 780 nm and from 300 to 500 nm respec- tively. We can easily detect the emission species of N z and N2 + from the second positive system while the light

96 J. Moon et al. / Growth and characterization o f ECR N2-plasma-treated SiN on Si

d

(a)

z

4000

3000

2000

1000

0L-- 200

i , , n

Positive System

First Positive System

400 600 800 WAVELENGTH (nm)

1000

4000

3000

2000

1000

N2

N2

!:I II!

! i:

350

NZ +

,i i I ~ £ / . ' " •

400 450 (b) WAVELENGTH (nm)

Fig. 5. (a) A representative optical emission spectrum of N 2 plasma created by ECR (a.u., arbitrary units). (b) A precise spectrum of (a) at short wavelengths (a.u., arbitrary units). N= indicates emissions at 337.1 and 357.7nm from neutral nitrogen molecules while Nz + indicates an emission at 391.4 nm from ionized nitrogen molecules.

intensity from the first system is complex and weak. Similar results of ECR N2 plasma analysis have been reported [13, 15-17]. Manabe and Mitsuyu [13] have reported that much greater amounts of N2 can be activated in an ECR plasma than in an r.f. plasma. The emissions from nitrogen molecules and nitrogen molecule ions are shown in Fig. 5(b), where N2 and N2 + means emission from neutral and ionized nitrogen molecules respectively. From the relation between the emission intensities of the neutral (i.e. radical) nitrogen molecules (at 337.1 nm and 357.7 nm) and the ionized nitrogen molecules (at 391.4nm), we can conjecture which species are more dominant for the silicon nitride film growth.

Figure 6 illustrates the microwave power depen- dences of the emission intensity ratio Rra d of the N 2

radicals at 357.7 nm to the N 2 radicals at 337.1 nm and of the emission intensity ratio Rion of the N2 ions at 391.4 nm to the N 2 radicals at 337.1 nm. The pressure and the temperature during the growth were 100 mTorr

2

,e.-

E - 1

>

n- (a)

° <

(/1

c

u

c"

E

0

200

0 0

Q

t o !

/ ' 4 o '

(b) Microwave Power(W)

Fig. 6. Microwave power dependences of (a) the relative emission intensity ratios Rr. d ( l ) and Rion (©) and (b) the film thickness, Rra d is the intensity ratio of N2 radicals at 357.7 nm to those at 337.1 nm, and R~o n is the intensity ratio of N 2 ions at 391.4 nm to N= radicals at 337,1 nm.

and 450 °C respectively. From Fig. 6(a), we can see that Rrad does not change even if the microwave power increases while R~o, increases with increasing microwave power, and that the power dependence of Rio . is very similar to that of the film thickness, as shown in Fig. 6(b). The thickness ranges from 40 to 180/~ for powers from 100 to 700 W. At powers below 100 W, we could not find any reliable growth condition because of un- stable plasma generation. As is evident in the figure, it is obvious that the growth rate increases with increasing microwave power in the above power range. These power dependences suggest that nitridant species highly activated by higher microwave powers enhance the nitridation reaction on the Si surface to yield a thicker nitride film on the assumption that N= ions in the ECR plasma are more dominant species for the reaction than are the neutral molecules.

Figure 7 shows the pressure dependences of the emis- sion intensity ratios Rr,d and R~o, and of the growth rate. These data were taken at a substrate temperature of 450 °C with a microwave power of 500W. The reacting nitrogen pressure was changed from 10 to 500 mTorr by controlling the exhaust pipe conductance. The emission intensities of N2 radicals and ions tend to decrease with increasing pressure. The ratio Rrad has an almost constant value of 1.5 while the ratio Rion de- creases markedly with increasing pressure, as shown in Fig. 7(a). Comparing these optical emission data with the dependence of film growth rate, we see that the fact

J. Moon et al. / Growth and characterization o f ECR N2-plasma-treated S iN on Si 97

1.5

,.o

~" 0.5

(a) 0

~ o.8

a: 04 c-

o

0

(b)

I

, , , , l l , i I I I I l l l I I I I

o 10 10o

P r e s s u r e (roTor r)

Fig. 7. Nitrogen pressure dependences of (a) the relative emission intensity ratios, Rra d (O) and Rio" (0) and (b) growth rates with (O) and without (O) a hydrogen pre-treatment.

that Rion increases with decreasing nitrogen pressure can lead to higher growth rates of the silicon nitride films. The pressure dependences suggest that the reac- tive nitridant species at lower pressures have longer mean free path lengths and affect the growth kinetics more effectively than those at higher pressures.

The most interesting results include the pressure de- pendences of the growth rate of silicon nitride films fabricated with and without a pre-cleaning treatment by H2 plasma exposures. The pre-cleaning treatment was performed for 1 h under the following conditions: pres- sure, 100 mTorr; temperature, 850 °C; microwave power, 1 kW. In Fig. 7(b) the growth rate was plotted against the pressure by using closed and open cycles for the cases with and without the pre-cleaning treatment respectively. Each growth rate dependence on pressure can be represented by a straight line on a semilogarith- mic scale. In the case of samples without the pre-clean- ing treatment, the growth rate is very low at the pressure of 400 mTorr. These trends are similar to those obtained at 600 mTorr [7]. The growth rates of 0.36- 0.8 ]~ min -~ obtained in the case with the pre-cleaning treatment are markedly larger than those (0.12- 0.18]kmin -~) in the case without the pre-cleaning treatment. The enhancement of the growth rate due to the pre-cleaning treatment of H 2 plasma comes (1) from the removal of substantial amounts of the unetched native oxide layer, making the nitridant species easily migrate on the whole substrate surface and (2) from the possible creation of more stable N - H bondings due to increased hydrogen content below the Si substrate sur- face. Figure 8 shows the hydrogen depth profiles for

4

Z 2

0

1

0 o 40 80 120 160

C H A N N E L N U M B E R

I

200

Fig. 8. Comparison of recoiling hydrogen depth profiles taken from silicon substrates with (0) and without (O) pre-treatment using ECR hydrogen plasma (a.u., abitrary units). The pre-treatment was accomplished at 100 mTorr for 1 h with I kW microwave power.

specimens with and without the pre-cleaning treatment by ECR H2 plasma for 1 h. Peaks located around the channel number of 140 for both spectra are not so meaningful since the peaks usually originate from hy- drogen adsorbed during handling of the specimens in air and measuring in a relatively poor vacuum chamber (10 -8 Torr) for the ERDA, but the spectral heights at lower channel numbers than 120 originate from hydro- gen existing in Si bulk regions and this bulk content of hydrogen should be considered from the viewpoint of the growth mechanism. Point (2) above can be sup- ported by the ERDA data, demonstrating a significant increase in hydrogen content in the bulk region for the pre-treated specimens in comparison with the untreated specimens as shown in the figure.

3.2. Film composition Figure 9 is an X-ray photoelectron spectrum of a

silicon nitride film grown at 100mTorr with 500W microwave power. The presence of a Si 2p photoemis- sion peak at 102.8 eV and an N ls peak at 397.8 eV is evidence of the successful nitridation of the Si substrate top surface. The atomic N-to-Si ratios measured using the RBS technique ranged from 1.35 to 1.43, showing nitrogen-rich films with a refractive index of 1.95 + 0.05. Figure 10 shows a typical ion channelling spectrum of the directly nitrided silicon nitride film. Ion chan- nelling is a useful technique for detecting the low mass elements on a high mass substrate since the background yields from the substrate are minimally suppressed, compared with that of the corresponding random spec- trum. From peaks of the spectrum, we can detect that three kinds of element i.e. nitrogen at the channel number of 210, oxygen at 235 and silicon at 340, exist in the films.

Oxygen contamination in the films during plasma processing is an inevitable problem. Therefore the rela- tive oxygen concentration can be used as a means of

98 J. Moon et al, / Growth and characterization of ECR Nz-plasma-treated SiN on Si

u a

z

_ N I s

Si2sSi2p

500 400 300 200 100 0

BINDING ENERGY (eV)

Fig. 9. A wide-range X-ray photoelectron spectrum for an SiN film grown at 100 mTorr with microwave power of 500 W.

E- :3

{3

"0

~ . ~ 2.4MeV He'"

SiN Si 1.1

' N O ? " I I :t

...................... :~ + i I

o 26o do Chonnel number

Fig. 10. A typical ion channelling spectrum of a SiN film by direct ECR nitridation. The atomic ratios of oxygen to nitrogen can be deduced.

evaluation for optimizing the process condition. In Figs. l l (a ) and l l (b ) we show the variation in the N-to-(N + O) ratios determined by RBS with the mi- crowave power and with the nitrogen pressure respec- tively. These data indicate that the N-to-(N + O) ratio increases with increasing power below 500 W and satu- rates at powers above 600 W while it increases with decreasing nitrogen pressure and tends to saturate be- low 50 mTorr. From the data presented in Fig, 11, it can be concluded that reactor pollution and reactor wall sputtering play significant roles in the increase in oxygen concentration in the film, especially in the case of relatively higher nitrogen pressures and lower mi- crowave powers. The oxygen contribution due to wall sputtering to the film growth can be ignored under the present ECR plasma process conditions for powers above 500 W and nitrogen pressures below 50 mTorr since the oxygen concentration shows minimum values in these ranges.

The location of oxygen atoms in the films was evalu- ated by sequential experiments of wet etching in dilute

0S

c)

07 Z

0E (a)

1.0

O

z

0.5

o f o -

J . / "

/

760 Power(W)

" \

i i i I ~ 1 1 1 1 J ~ i i i i r t

IO Ioo 1ooo (b) Pressure (mTorr)

Fig. 11. Variation in the N-to-(N + O) ratio with (a) microwave power and (b) nitrogen pressure. The ratios are deduced.by means of the ion scat ter ing-channell ing technique.

hydrofluoric acid and of oxygen peak positions in chan- nelling spectra using grazing-incidence detection. The etch rate experiment is very sensitive for detecting the oxygen contents of the silicon nitride films. On the one hand, the etch rates of nitride films obtained by low pressure chemical vapour deposition (LPCVD) are nearly constant with etching time while those of directly nitrided films include two stages where the etch rates are high in the initial stage and then very low after a rapid drop. On the other hand, it is very difficult to separate the nitrogen and oxygen peaks in Rutherford backscattering spectra because of the poor mass resolu- tion. Nevertheless, we can separate these two peaks successfully as shown in Fig. 10. The reason for the separation is that the nitrogen peak for the grazing-inci- dence detection shifts towards a lower energy position because of the large energy loss of He ions while the oxygen peak does not shift. From these two results, we can conclude that oxygen atoms incorporated in the directly nitrided film are located near the surface region of the film and that a good quality silicon nitride layer with little oxygen is present near the Si -SiN interface. From these conclusions, we can draw the schematic cross-sections of the two dielectric structures fabricated by LPCVD (Fig. 12(a)) and by direct nitridation (Fig. 12(b)). The lower native oxide shown in Fig. 12(a) in-

J. Moon et al. / Growth and characterization of ECR N2-plasrna-treated SiN on Si 99

(a)

SiN I ~ . ' f f l l

Nat ive ox ide

CVD-SiN SiON

N2 plasma SiON

(b)

Fig. 12. Schematic cross-sections of the two different dielectric struc- tures obtained by the different formation methods: (a) LPCVD; (b) Direct-nitridation.

10-I | A, which is also lower than that for the LPCVD nitride film (10-~°A) in the applied bias range up to 2 V. These results show that the silicon nitride films formed by ECR nitrogen plasma have better bulk prop- erties for a dielectric than the LPCVD films do. We also performed the sequential loop experiment using mega- electronvolt ion channelling technique and etching of the silicon nitride films on the sharpness of the S i -S iN interfaces. From the results, we found that the S i -S iN interface of the film formed by the ECR nitrogen plasma was estimated to be about 5 A and is sharper than that of LPCVD silicon nitride film (about 14,~). This is consistent with the I - V characteristic obtained. The detailed experimental procedure and results will be published in another paper [18].

4. Conclusions

cludes the uncontrollable oxidation layer during the insertion of the samples into the hot LPCVD reactor.

3.3. Electrical properties o f the f i lm Figure 13 shows the ramp-voltage-stressed I - V char-

acteristics of silicon nitride directly formed by ECR nitrogen plasma as well as that obtained by LPCVD for comparison. The breakdown voltage is defined as the voltage at which a current of 10 -5 A (corresponding to 10 -3 A cm -z) flows through the flat A 1 - S i N - S i capaci- tor. The thicknesses were 160 A for the nitride formed by ECR nitrogen plasma and 300 A for the LPCVD nitride. It was found that the breakdown voltage for the nitride formed by ECR plasma is 11.9 + 0.3 MV c m - ], which is rather high in comparison with that of the LPCVD nitride, 9.6 + 0.2 MV cm- l , obtained under a positive bias at room temperature. Leakage currents across the A 1 - S i N - S i capacitors have a value of

lO-S I

v 10 -~'

L..

u 10 -9

t Room Temp.

Posit ive St reg~

-11 1 1 I I I J i i

10 ' 10 20 30 40 50

V o l t s ( V )

Fig. 13. I - V curves for a directly nitrided SiN film grown at 10 mTorr with 500 W microwave power (©) and an LPCVD Si3N 4 film (O). The thicknesses are 160A for directly nitrided SiN and 300 A, for LPCVD Si3N4.

We have successfully grown SiN films by means of direct exposure of Si wafers to ECR nitrogen plasma at a low temperature of 450 °C. The activation energy obtained for the growth is 0.42 + 0.11 eV. The refrac- tive indices of the grown films range f rom 1.9 to 2.0 and the atomic N-to-Si ratio based on RBS analysis ranges from 1.35 to 1.43, indicating nitrogen-rich films, The microwave power and substrate bias voltage show posi- tive dependences on the growth rate while the nitrogen pressure shows a negative growth rate dependence. The film thickness can be controlled for a constant reaction time of 6 h in the range from 40 to 400 ~ by changing the above process parameters. The breakdown voltage and leakage current obtained were 11.9 + 0.2 MV c m - and 10 -~l A respectively. Lower nitrogen pressures or higher microwave powers result in lower oxygen con- centrations in the films. The results on optical plasma analysis indicate that the concentration of nitrogen molecular ions rather than nitrogen radicals is closely related to the film growth. The pre-cleaning treatment by ECR hydrogen plasma before the nitridation can enhance the film growth rate possible because unetched and/or regrown silicon oxide are almost eliminated and because hydrogen in the Si substrate can accelerate the diffusion of activated nitrogen species. It seems that the excellent properties of the SiN films fabricated at the low temperature of 450 °C are related to the existence of a high concentration of nitrogen molecular ions in the ECR plasma.

Acknowledgments

The authors wish to thank Dr. J. S. Ma at the Case Western Reserve University, Cleveland, OH, USA, for his support of this work. We also wish to thank Dr. T. Tohda at the Central Laboratory, Matsushita Electric

100 J. Moon et al. / Growth and characterization of ECR N2-plasma-treated SiN on Si

Corporation, for his help during the measurements of refractive indices. This work was partly supported by a Grant-in-Aid for Scientific Research on Priority Areas of Metal-Semiconductor Interfaces from the Ministry of Education, Science and Culture, Japan.

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