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Vacuum/volume 45/number IO/l l/pages 1023 to 1025/1994 Elsevier Science Ltd

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Electron cyclotron resonance plasma deposition as a promising technique for low-temperature hard coatings M J Hernandez, F J Gomez, J Garrido and J Piqueras, Laboratorio de Microelectrdnica, Departamento de Fisica Aplicada, Universidad Autdnoma de Madrid, Cantoblanco, 28049 Madrid, Spain

and

C Gomez-Aleixandre lnstituto de Ciencia de Materiales, Cantoblanco, Madrid, Spain

S&N, and Sic films have been deposited by electron cyclotron resonance (ECR) plasma at low substrate temperatures /T,< 150°C). Films prepared by this new technique exhibited better properties than those deposited by other plasma methods.

Different gas ratios and microwave powers have been used. The films have been analysed by infrared spectrometry and spectroscopic ellipsometry. SisN, and Sic films show in general a good thickness homogeneity (< 4% in 3 ic. wafers) and reproducibility. Depending on the process parameters we have found growth rates up to 3500 Amin-’ for S&N, and 400 Amin-’ for Sic. The infrared spectra of these layers have shown no signal of Si-H and N-H bonds in the SisN, films and low traces of Si-H and C-H, bonds in the Sic films.

Introduction Experimental

Due to their inertness and hardness, S&N4 and SIC are attractive materials for passivation layers against aggressive ambients and hard coatings for mechanical engines. The high thermal stability of both materials should enable the operation at high tem- peratures of coated devices.

Silicon nitride is a useful insulating material in VLSI appli- cations, as final passivation, mechanical protective layers, and masks for selective oxidation of silicon. Depending on its appli- cation, several deposition techniques are used. Limitations in process temperature, film uniformity and processing costs are the main factors to consider when choosing the appropriate technique.

In contrast to other usual techniques for deposition, ECR enhanced silicon nitride deposited at temperatures below 15O”C, exhibits a quality comparable to high-temperature CVD nitride films.

Silicon carbide is a promising material for the LSI package because of its physical and electronic properties. This material would meaningfully increase device performance over products fabricated from currently available semiconductor materials.

The experiments were performed in a low-profile configuration ECR plasma reactor (Plasma Quest Model 357). A schematic diagram of the apparatus is shown in Figure 1. In the standard configuration, an electromagnet located near the microwave quartz window entrance confines the plasma in a column, and the ions are thrown towards the sample region by the slightly divergent magnetic field. In the low-profile configuration, a second collimating electromagnet close to the substrate holder zone is used to focus the ions from the resonance zone down to the sample. In such a way the confinement of the plasma may be easily controlled, and the working pressure extended towards the several torr range, where superior performances of the deposited materials have been found’. Further, due to precise control of the column shape, wide areas of homogeneous ion current may be obtained, enabling good thickness uniformity. Good uniformities with electromagnetic currents around Z,, = 185 A and I,,,.,, = 60 A, are normally obtained with this system’.

The properties of Sic that make it the best candidate for extreme working conditions are its high temperature operation, high power and high voltage ratings with power densities 100 times those of silicon, high-power microwave operation, with power levels 5 times those of silicon, enhanced charge storage capability, and high-energy blue-light emission. Other advan- tages offered by Sic include high thermal conductivity and good electric insulation.

SIC and S&N4 layers were deposited on p-type Si slices. The (l,O,O) oriented substrates have resistivities between 4-6 Rem. Samples polished on both faces were used for later infrared measurements. Prior to being loaded in the vacuum chamber the substrates were degreased, and the native oxide removed with a buffer etch. During deposition the sample holder was maintained at a temperature below 150°C.

Two gas inlets are provided in the system (see Figure 1). By the upper access the gases are introduced directly into the res- onance zone, and so become highly ionized. Far from the res- onance region, a tube ring enables the access of weakly ionized

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MJ Hernandez et al: ECR plasma deposition for low-temperature coatings

Mw ASSEMBLY

GAS lhll ET -_c II.YI A UPPER

MAGNET COIL .:;;. . . :/. ;,. J

RESONANCE ZONE

GAS -c_ RING SUBSTRATE 0

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HOLOER r-

Figure 1. Schematic ECR plasma apparatus.

species. In this way, reactions between the different precursors inside the plasma are considerably avoided and so highly compact layers may be obtained.

For S&N4 deposition, pure nitrogen is introduced by the upper entrance and 5% argon-diluted SiH, by the lower one. In the case of Sic, both reacting gases SiH, and CH, are introduced by the lower inlet, whereas the plasma is formed by pure argon introduced by the upper access. A nitrogen flow of 200 seem was used for deposition of all the Si,N4 sample series, while silane flow was varied between 100 and 700 seem. The effect of a varying microwave power from 100 to 1500 W on the growth rate and S&N, composition was also studied. SIC was deposited at the maximum power of 1500 W, with 10 to 20 seem of CH4 and 50 to 300 seem of SiH,. In all cases the reflected microwave power was less than 5% of the total applied power. The residual vacuum was better than lo-’ torr.

Infrared spectroscopy was used to identify the vibrational modes of the different bonds present in the material, and the relative band areas taken as a qualitative indication of the layer composition. The dielectric constant of the layers was measured by spectroscopic ellipsometry between 1.5 and 4.5 eV. The layer/ substrate structures were simulated using available data from the library*, varying composition and layer thicknesses to fit the experimental data. The thicknesses obtained in this way were checked with step heights measured by a profilometer on some of the samples of both materials. A good agreement between direct measurements and simulated layer thicknesses was obtained in all the checked samples.

Results and discussion

Silicon nitride. The S&N, growth rates varied from 180 to 3500 8, min-’ for the different experimental conditions.

Uniformity of some films was tested with a Dektak pro- filometer and it was better than 4% variation for films grown on 3 in. wafers.

Figure 2 shows the variation of the growth rate with the micro- wave power supplied to the plasma. In these depositions the flow of N2 gas was maintained constant to a value of 200 seem and the SiH, flow was changed from 100 up to 700 seem.

As can be seen, except for the highest silane flow and the lowest microwave power, the growth rate remained nearly constant, independent of the power density. The constant rate value increased with the silane flow. This behaviour shows that the density of nitrogen ions generated by the ECR plasma is large enough to react with the SiH, molecules. In these cases the S&N4 growth rate is limited by the SiH, flow. For the highest silane flow, 700 seem, growth rate saturation appear for microwave powers larger than 800 W. Up to this power, the growth rate is limited by the density of nitrogen ions, not by the SiH, flow. The same effect can be observed for every silane flow at the microwave power of 100 W.

Ellipsometric measurements allow us to check the quality of the ECR S&N, layers. We have tried to get the best fit of the experimental spectra with theoretical ones in structures S&N&. We model the S&N, layers with a mixed composition, including amorphous silicon because of the results in Figure 2, and some voids, to check how compact the layers are. Figure 3 shows the film quality (%Si,NJ vs pure SiH, flow with the microwave power as a parameter. The layer became around 95% pure S&N, when increasing the microwave power and decreasing the SM, flow. A wide range of power gives an acceptable composition of the film. The films exhibited compositions up to 75% of Si,N, for most of the powers when the silane flow was under 20 seem. Only for the worst conditions of both power and silane gas flow, ellipsometric fits detected the presence of amorphous silicon.

Because of the presence of some voids in the layers, we have tried to improve them with a thermal treatment at 900°C for 1 h. After this annealing no differences in thickness could be detected. Therefore, ECR plasma gives highly dense silicon nitride films deposited at low temperature.

Silicon carbide. Infrared spectroscopy has enable us to identify the layer composition, Figure 4 shows the IR spectra of SIC layers deposited with 0.66, 1, 2 and 8 CH,/SiH, flow ratios. All the spectra exhibit the main peak at 780 cm-’ of the Si-C bond. On the large wavenumber side of this band, two supplementary peaks occur around 907 and 975 cm-‘, whose importance depends on the deposition conditions. The first one, at 907 cm-‘.

4,000 -

Qr N2: 200 seem

.

.

.

. l .

A . .

* * *

A . .

* 1 *

J 0 200400 600 800 1,000 1,200 1,400 1,600

MICROWAVE POWER (W)

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7

Figure 2. Silicon nitride films growth rate versus microwave power for several silane flows (QN, = 200 seem).

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M J Hernandez et a/: ECR plasma deposition for low-temperature coatings

I 1 , I I I I I I 0 5 10 15 20 25 30 35 40

SILANE FLOW (seem)

Figure 3. Silicon nitride films richness versus silane flow for several micro- wave powers.

0.7 4 Si-C

_ 0.6 CH41 SiH,

zi 0.66 -

. l---l 2% 0.5 : ___.__

6

a

I

O.’ 3BM)28w WAVENUMBER (cm-‘)

Figure 4. Infrared spectra of SIC layers deposited with 0.66, 1, 2 and 8 CH4/SiH4 flow ratios.

may be related to a degenerated mode of Si-H, bond3, whereas the second one, at 975 cm-‘, is normally associated with wagging modes of the C-H, bond4. Stretching modes of the Si-H, bond also appear around 2100 cm-‘. Only in the samples with methane excess, do traces of C-H, band around 2850 cm-’ appear5. In the samples deposited with the lowest CH,/SiH, ratio a shoulder around 660 cm-’ might be related to rocking and wagging modes of the Si-H bond. The narrow peak at 610 cm-’ comes from the Si-Si bonds of the substrates.

Another observed effect (see Figure 4) is a certain shift of the Si-C band towards the low wavenumber region as the flow ratio decreases. The relative Si enrichment in the neighbourhood of each C-Si bond may give rise to larger apparent effective masses and so the vibration frequency decreases.

To estimate the relative contribution of each band to the over- all spectrum, and thus the concentration of the different com- ponents of the layers, fittings of the spectra have been made by

0 2

FiAT104CH4 /Sil-f4 8

Figure 5. Normalized area for Si-C, C-H. and Si-H, ir bands versus CH,/SiH, ratio. Also included percentage of Sic and a-Si composition obtained from ellipsometric data.

adding gaussians at fixed positions. Each band area has been normalized to the total area and represented as a function of the CH,/SiH, flow ratios, as shown in Figure 5. In the same figure we have included the percentages of each component obtained from the best fits of the ellipsometric data.

In the sample deposited with 0.66 ratio the relative lack of CH, gives rise to an enhancement of the Si-H, bonds’ contribution. It is indicative that an appreciable part of the layer consist of hydrogenated amorphous silicon. In fact, if we add amorphous silicon as a component of the layer, both the SiCjSi and Si,NJSi simulated ellipsometric spectra may be more accurately fitted to the experimental spectra than when disregarding this contri- bution. An added void contribution also improves the fittings. Both estimations, from infrared and ellipsometric spectra, give percentages of each component in reasonable agreement. The a- Si fraction in the layer seems to be about 20% for the 0.66 flow ratio and decreases for increasing flow ratio.

Unreacted methane gives rise to an increasing contribution of the C-H,, bonds with the flow ratio. In view of the above results we can conclude that the best quality of the layers is obtained for flow ratios of around two. Also, from the ellipsometric measure- ments in thick samples, we have observed a trend of a-Si enrich- ment as the layers grow. This effect becomes more apparent in the layer deposited with the lowest flow ratio.

A value of dynamic hardness of about twice that of silicon has been obtained from the early measurements.

References

‘J E Spencer, R Rausch, B Presley, B Shu Mercer, B Small and S Singh, 38th Nat Symp Am Vacuum Sot, Seattle, WA, November (1991) ‘Uvisel System Library, Jobin Yvon, France. ‘G Lucovsky and T M Hayes, Topics in Applied Physics (Edited by M H Brodsky) Vol36, p 235 (1974). 4A Chayahara, A Masuda, T Imura and Y Osaka, Jap J Appl Phys, 25, L564 (1986). ‘G Ramis, P Quintard, M Cauchetier, G Busca and V Lorenzelli, J Am Ceram Sot, 72, 1692 (1989).

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