High-density silicon nitride thin film in PECVD

303

Sensors and Actuators A, 32 (1992) 303-306

High-density Brigitte

Reynes

silicon nitride and jean

Laboratoire d’Etudes des Propriktb CPdex (France)

Abstract :

Claude

thiii film in PECVD

Bruyere

EIectroniques des Solides, CNRS associated with UniwrsitP Joseph Fourier, BP 166X, 38042 Grenoble

L

The aim of this work,is to report the preparation and some chemical and physical properties of high-density silicon nitride thin films. The films are prepared by plasma-enhanced’chemical vapor deposition (PECVD) at low frequency (50 kHz) with helium dilution and different silane/nitrogen/ammonia gas mixtures. In the best case the atomic hydrogen density is less than 7 x 102’ atoms/cm’, mainly bound in NH sites, with a very low etch rate of 10 A/min. The thermal stability and the diffusion barrier properties have been checked. This material must be a good candidate for chemical grafting and as a protective layer in chemical sensors.

ihrodu&ion Silicon nitride thin films are currently used in microelectronic devices. In sensor technology, silicon nitride has attractive properties because of its mechanical hardness and high chemical resistance in addition to its classical electronic and optical properties. Silicon nitride has been used to improve the stability and detection properties of ISFETs [l]. Moreover, recently, authors have shown that SiNH/NH* sites of silicon oxynitride are responsible for the Nernstian pH sensitivity of these layers and can be chemically grafted in order to obtain a pH-insensitive surface (REFET) [2]. From these results, silicon nitride could be the better candidate for chemical grafting if the hydrogenated site density is kept to a low value. Plasma-enhanced chemical vapor deposition (PECVD) presents many advantages in the preparation of silicon nitride films. Unfortunately, without special attention, this technique sometimes yields a ‘low-density material with poor thermal or chemical stability. In the preparation of amorphous silicon nitride it is necessary to avoid the wrong chemical bonds (SiSi) and the less thermally stable bonds (lower bond energies for SiSi and SiH and for SIN and NH bonds). In these conditions, the general formula of the silicon nitride alloy will be Si,N,_ =(NH)=. With predictions based on the standard tetrahedron model [3], there exists an 0924-4247/92/$5.00

optimum hydrogen content in these alloys with respect to the N/Si ratio. So, in the near stoichiometric or slightly nitrogen-rich. films, the challenge is to keep a very low hydrogen content. Here we propose a way to produce high-density silicon nitride films with low hydrogen content (7 x lo*’ atoms/cm3) and a low etch rate comparable with that of high-temperature CVD nitride. The experimental results concern the correlations between the preparation conditions, the controlled chemical bonding, and the mechanical and stability properties of the films. All the films are almost stoichiometric or slightly nitrogen rich. Experimental Thin films were prepared in a parallel-plate reactor with a 50 kHz generator for the plasma generation. The gases, silane ( 1% SiH4 in helium, which is an inflammable mixture), nitrogen (N2), and ammonia (NH,), were introduced into the reactor at a total pressure of 135 Pa. The main parameter is the gas mixture composition controlled by the ratio R = N2/(N2 + NH,). For higher nitrogen concentrations the helium dilution ensures the uniformity of the films by suppressing the gas-phase reactions. During the deposition, the total flow rate was kept at 130 sscm with a power density of 65 mW/cm’. The substrate temperature T, varied from room temperature to 450 “C. @

1992 -

Elsevier

Sequoia.

All rights

reserved

304

The chemical bonding and the composition of the films were obtained by transmission infrared spectroscopy by a procedure described in a previous paper [4]. Three absorptions bands are considered for SiN, NH (stretching) and SiH bonds. We used a Perkin Elmer 683 apparatus range from 4000 to 200 cm-‘. The thicknesses and the refractive index were determined from ellipsometric measurements at Iz = 633 nm. The mechanical stress was determined by a Newton rings method. The profile of the chemical elements was checked by SIMS in some samples. The standard buffered oxide solution (7/l of NH4F, HF at 28 “C) was used to determine the etch rate.

Results and discussion Infrared

analysis

For all the samples, the [SiN] bond density is larger than 1.3 x 1p3 cmp3 and increases by 8% when R increases from 0.75 to 1; the maximum [SiN] density in CVD silicon nitride (without hydrogen) is 1.6 x 1O23cmm3 [5]. In Fig. 1 we observe a significant decrease of [NH] sites, until 5 x 10” cme3, with increase in the ratio R, while the [SiH] sites remain at a lower value. This decrease of [NH] bonds is specially marked in the range 0.85 < R -C 1. For R = 1 and T, higher than 200 “C, we have already reported a drastic decrease of [ SiH] bonds [4]. So, by the temperature of the substrate and the composition of the gas mixture, we are able to control the nature and the densitites of the chemical bonds in the silicon nitride. A knowledge of the chemical bond types

t

I

0

(SiH)

P

0

a

0-J 0

0

0

1 -““““,““““,p....,..,”

0.7 0.75 0.8 0.85 0.9 0.95

1

1.05

Fig. 1. [NH] and [SiH] bond densities versus R with r, = 350 “C.

30 _

ExperImental

1.3

1.4

1.5

1.6

1.7

1.8

N/Si

Fig. 2. Hydrogen content versus N/B. The full line represents the theoretical predictions. The open circles are the experimental values deduced from infrared analysis.

and their distribution is very important to obain more efficient chemical grafting, in general on NH sites. From infrared analysis and with the assumption of quasi-constant density for all samples, we determined the N/Si ratio. In Fig. 2, we can see that the hydrogen content, which is defined by H (%) = [H]/([Nj + [Si] + [HI), is slightly higher than the predicted value [3]. This discrepancy between the model of Si, N,_JNH), and the as-prepared silicon nitride is due to the [ SiSi] and the [ SiH] bonds which are not completely excluded. Mechanical

stress

When used as active or passivation layers in devices, the silicon nitride must maintain good mechanical, thermal, and chemical stabilities. These properties depend on the chemical bonds and the atomic composition of the film. With regard to the mechanical stress, it is well known that strong ionic bombardment in the lowfrequency plasma deposition process induces a compressive stress in the silicon nitride, which is usually preferred to a tensile stress, to avoid cracks in the film. The variations of the stress with the gas composition are shown in Fig. 3. Annealing at 700 “C induces a slight decrease in the stress (about 30% for R = 0.75). Assuming a linear dependence between the stress and the [NH] and the [SiH] bond densities [6], the total mechanical stress can be described by a semi-quantitative model: CJ( dyn/cm2) = a[NH] + b[ SiH] + c

(1)

where a, b, and c are experimental constants. Fairly

305

flat at the as-deposited value and there is no evolution of the alkaline element front near the silicon nitride/glass interface. These results demonstrate the good stability of the bulk silicon nitride, but more accurate measurements would be necessary, to show surface effects. Chemical 0.7

0.75

Fig. 3. Mechanical

0.8 0.85 0.9 R = N#NH,+

0.95 N3)

1

1.05

stress versus R for T, = 350 “C.

good agreement between eqn. (1) and experimental points is observed with the following dyncm, b = -2.8 x values: a= -4.4 x lo-‘* lo-‘* dyn cm, and c = 5.7 x lOi dyn/cm2. The large network distortion in the vicinity of the NH bonds explains the large absolute value of a compared with b, and we conclude that the role of the larger NH bonds in our materials is dominant. In infrared spectroscopy, the SiN peak frequency decreases with increasing R; this suggests a stretching of the SiN bonds and explains the increase of the mechanical stress with R. However, we note that the value of the compressive stress of our silicon nitride remains between those of 300 “C-LPCVD and HT-CVD silicon nitride, as indicated in Fig. 3.

18

0.75

X x a

14

1

0.8

0.85

0.9

0.95

1

1.05

N,I(NH,+N,)

stability

The thermal stability and barrier properties were checked first on two samples prepared with R = 1 and post-annealed under the following conditions: (a) 1 hour/l00 “C ‘step by step’ up to 750 “C in dry oxygen gas-the silicon nitride film was deposited on crystalline silicon, (b) 3 hours at 600 “C, in dry nitrogen gas-the silicon nitride film was deposited on soda glass substrate, In sample (a), no change is observed in the infrared spectra and SIMS profile of the silicon and nitrogen elements before and after annealing, and we note the absence of oxygen atoms in the film. The alkaline element profiles (for sodium, calcium and potassium) of sample (b) were also observed in the same way. All the profiles remain

16

6 0.7

Thermal

stability

The chemical stability depends on the composition and mainly on the hydrogen content [7] in the silicon nitride. Figure 4 shows the correlation between the etch rate and the ratio R above R = 0.85, for T, = 350 “C. Figure 5 also indicates the decrease of the etch rate of the silicon nitride, forR=l, when T, increases from the room temperature to 450 “C. Above 350 “C, the lower etch rate is close to that of CVD silicon nitride (10 A/ min) and remains constant with the thickness down to 200 A.

Fig. 4. Etch rate and hydrogen

content versus R forT,= 350 “C.

104

0

100

200

300

400

500

Ts WI

Fig. 5. Etch rate and hydrogen ture T, for R = 1.

content versus the substrate

tempera-

306

Conclusion

References

conclusion, by varying the ratio R = N&N, + NH3) and using the helium dilution of silane in low-frequency plasma, we can obtain PECVD silicon nitride films with different chemical compositions and chemical bonds. The compressive mechanical stress and the low hydrogen content result in a low etch rate of these thin films of silicon nitride, while the thermal stability of the bulk material remains very good. In chemical sensor technology, the way to prepare high-density silicon nitride appears to have interesting applications in chemical grafting and as a passivation layer.

1 T. Matsuo and M. Eshahi, Methods of ISFET fabrication, Sensors and Actuators, I (1981) 77-96. 2 J. M. Chovelon, N. Jaffrezic~Renault, P. Clechet, Y. Cros, J. J. Fombon, M. I. Baraton and P. Quintard, PECVD silicon oxynitride: a new insulator for ISFETs with insulator surface modified by chemical grafting, Sensors and Acfuators, 84 (1991) 385-389. 3 Z. Yin and F. W Smith, Tetrahedron model for the optical dielectric function of hydrogenated amorphous silicon nitride alloys, Phys. Rev. B, 42 (1990) 3658-3665. 4 B. Reynes, C. Ante, J. P. Stoquert and J. C. Bruyere, Composition, etching and optical properties of silicon nitride films deposited by PECVD in variable NH,-N, gas mixtures diluted in helium, Thin Solid Films, 203 (1991) 87-94. 5 S. Hasegawa, H. Ambutu and Y. Kurata, Connection between Si-N and SiH vibrational properties in amorphous SiN, :H films,

Acknowledgements

6 D.

In

The authors would like to thank Y. Cros for fruitful discussions and C. Bianchin for technical assistance.

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365-375.

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H. Yu and J. A. Taylor, Correlation between processing, composition and mechanical properties of PRCVD-SIN, thin films, Marer. Res. Sot. Symp. Proc., 188 (1990) 79-84. 7 R. Chow, W. A. Landford, W. Ke-Ming and R. S. Rosier, Hydrogen content of a variety of plasma-deposited silicon nitrides, J. Appl. Phys.,

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