Plasma-enhanced silicon nitride deposition for thin film transistor applications

I O U R N A b OF'

ELSEVIER

Journal of Non-Crystalline Solids 187 (1995) 347-352

Plasma-enhanced silicon nitride deposition for thin film transistor applications L.J. Quinn*, S.J.N. Mitchell, B.M. Armstrong, H.S. Gamble Northern Ireland Semiconductor Research Centre, Department of Electrical and Electronic Engineering, The Queen "s University of Belfast, Ashby Building, Stranmillis Road, Belfast, BT9 5AH, UK

Abstract The characteristics of silicon nitride films deposited in a multiprocess reactor have been investigated to determine the most suitable layers for dielectric and passivation applications. Process parameters such as rf power, temperature and gas flow ratios have been varied to control the stoichiometry of the films, and associated parameters such as refractive index, BHF etch rate, relative permittivity and breakdown field. Using these results, silicon nitride films with favourable characteristics have been deposited and used successfully in thin film transistors.

1. Introduction Plasma-enhanced silicon nitride (SiNH) layers have a number of important applications in the semiconductor industry. They are used extensively for the passivation of devices, i.e., protection of completed devices from a hostile environment, and as a dielectric in non-volatile m e m o r y elements in M N O S devices or high efficiency inversion layer solar cells [ I - 4 ] . Excellent step coverage, good adhesion to underlying layers and a diffusion barrier to water vapour and sodium ions, make SiNH ideal for encapsulating devices after the final metallisation layer. It also gives particle and scratch protection to devices during mounting operations. For the production of thin film transistors (TFTs)

*Corresponding author. Tel: +44-232 245 133. Telefax: +44-232 667 023. E-mail: [email protected].

on glass, SiNH can be used for two purposes. Glass plates provide a low cost substrate on which T F T s may be fabricated for L C D panels [5]. However, glass contains m a n y alkali impurities detrimental to device performance. SiNH can be deposited over the glass substrate to prevent diffusion of the impurities into the active layers. In addition it has been shown that good quality SiNH can be deposited and used as a reliable gate dielectric in a T F T [6]. This paper will include the characterisation of SiNH layers deposited in a multiprocessing environment and their application to TFTs.

2. Experimental details Deposition experiments were performed in a custom-built multiprocess reactor which has been described previously 1-7]. Silicon substrates ([1 00] 2-5 f~ cm, n-type) were given a H202/H2SO, chemical

0022-3093/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI 0 0 2 2 - 3 0 9 3 ( 9 5 ) 0 0 1 6 2 - X

L.J. Quinn et al. / Journal of Non-Crystalline Solids 187 (1995) 347 352

348

Breakdown Field ( M V / c m )

BHF Etoh Rate (A/min) 400

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100

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300

350

400

450

500

550

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Temperature C Fig. 1. Breakdown field and BHF etch rate versus temperature at a rf power of 100 W and NH3/SiH4 ratio of 8 : 1. The lines are drawn as guides for the eye.

clean, 10:1 H F dip, DI rinse and spin dry before loading into the reactor. SiNH was deposited by ionising silane (diluted in nitrogen) and ammonia in a glow discharge created by an inductively coupled antenna. The maximum rf power was 1 kW, at a frequency of 13.56 MHz. The process conditions investigated were reactive gas flow ratio (NH3/SiH4 varied 8 : 1 - 2 : 1), temperature (range 300-550°C), and rf power (100-200 W). In each experiment a SiNH layer of approximately 1000 was deposited. For each process condition, deposition rate, BHF (7%) etch rate and refractive index were measured. In addition MNS capacitors were fabricated and used to obtain further information about the layers, namely the relative permittivity and the breakdown field.

3. Results and discussion

It is widely known that the hydrogen content in a SiNH layer increases dramatically when deposited at temperatures below 300°C [8]. This temperature has traditionally been used to deposit SiNH for passivation applications where the layer does not experience further high temperature processing. It has been shown that hydrogen which leaves the

SiNH layer during subsequent high temperature processing can be detrimental to device performance [9]. Fig. 1 shows the breakdown voltage and BHF etch rate of SiNH layers deposited in the temperature range 300 550°C. The reduction in BHF etch rate indicates a decreased oxygen content and higher density. The hydrogen content is also lower in films deposited at higher temperatures. It is also favourable to work at these relatively high temperatures for SiNH deposition since this minimises time between SiNH deposition and polysilicon gate deposition for in situ sequential processing in T F T fabrication. The benefits of the multiprocess are that the SiNH layer is never exposed to atmosphere and interface contamination is minimised. For these reasons all subsequent SiNH depositions were performed at 550°C. The dependence of refractive index and BHF etch rate on the active gas flow ratio at different rf powers is given in Figs. 2 and 3. For an increase in rf power the refractive index decreases, this can be interpreted as a reduction in the Si/N ratio in the film. As the NH3/SiH 4 ratio is decreased the films become increasingly silicon rich which results in a higher refractive index. This is correlated with a lower BHF etch rate. SiNH films with a higher silicon content are more suited to passivation

L.J. Quinn et al. / Journal of Non-Crystalline Solids 187 (1995) 347 352

349

R e f r a c t i v e Index 2.1

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Fig. 2. Refractive index versus gas flow ratio at three levels of rf power and a temperature of 550°C. The lines are drawn as guides for the eye.

B H F Etch r a t e (A/min)

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Fig. 3. BHF etch rate versus gas flow ratio at three levels of rf power and a temperature of 550°C. The lines are drawn as guides for the eye.

applications since these layers have a high density but have a relatively low breakdown field. Mean electron energy or electron temperature in the plasma is much higher than the substrate temperature so it may be concluded that the electron impact dissociation rate is nearly independent of the substrate temperature. In fact the plasma

enhanced deposition rate was found to have a low thermal activation energy of 0.01 eV. From Figs. 4 and 5 it can be seen that the SiNH deposition rate depends only slightly on gas phase composition and total pressure. Slight changes in the deposition rate for different gas compositions can be explained by the small variations in the

L.J. Quinn et al. / Journal of Non-Crystalline Solids 187 (1995) 347 352

350

Deposition Rate (A/min) 220

200W 200 +

150W 180 100W

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NH3/SiH4 Gas Ratio Fig. 4. Deposition rate versus NH3/SiH4 gas flow ratio at a temperature of 550°C. The lines are drawn as guides for the eye.

Deposition Rate (Almin) 160 150 140 130 120 110 100 0.2

i 0.25

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0.35

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0.45

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Pressure (mBar) Fig. 5, Deposition rate versus pressure at a rf power of 100 W and temperature of 550°C. The line is drawn as a guide for the eye.

cross-section for ionisation and dissociation for the different gases since mean electron energy is also dependent on composition. Pressure has little effect on the deposition rate since at lower pressures the mean electron energy is higher but ionisation and dissociation are proportional to total pressure [10]. The relative permittivity, er, is also dependent on the process conditions and results indicate that

~r increases as the NH3/SiH4 gas flow ratio increases, which suggests a reduction in the Si/N ratio giving a nitrogen rich film. The breakdown field follows a similar pattern with the most insulating layer, i.e., highest breakdown field, corresponding to the highest NH3/SiH4 ratio. These results were obtained from measurements on M N S capacitors.

351

L.J. Quinn et al. / Journal o f Non-Crystalline Solids 187 (1995) 347 352 .

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Fig. 6. RBS depth profile of optimised SiNH sample.

4. Process conditions and RBS/SIMS analysis of the SiNH layer F r o m the data above it was possible to select deposition parameters to deposit SiNH as a dielectric and a passivation layer. A passivation layer requires a dense film to prevent the diffusion of impurities. As a gate insulator the film should also have a minimal H and O content, low pinhole density and a high breakdown field. A SiNH layer with the above properties can be deposited using the following conditions, NH3/SiH4 ratio of 8 : 1, rf power 150W and a substrate temperature of 550°C. This process has a deposition rate of 185 A/min, and the SiNH film has a relative permit-

tivity of 6.75 and a breakdown field of 7.7 MV/cm. RBS and SIMS analysis of the layers were carried out to verify the initial experimental work. The RBS spectra were acquired at backscattering angles of 160 ° and 111 ° with a primary He 2÷ ion beam operating at 2.275 MeV. The results for a 3100,~ thick SiNH sample deposited on a silicon ([100] n-type) substrate are shown in Fig. 6. The layer has a Si/N ratio of 0.73 and there are 2.43 x t018 atoms/cm 2 present in the samples giving a density of 2.7 g/cm 3. N o variations in the Si or N concentrations were detected with respect to depth in the film and no impurities were detected. The detection limit for C and O was approximately 5 at.% while for atoms heavier than P the detection

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L.J. Quinn et al. / Journal of Non-Crystalline Solids 187 (1995) 347 352

limit was 0.05 at.% or less. H y d r o g e n c a n n o t be detected by RBS so to complete the analysis a S I M S profile of a T F T type structure was obtained. F r o m this it was calculated that the layer contains 11% of atomic hydrogen.

5. T F T fabrication Using the above process conditions for the S i N H layer, T F T s were fabricated by the procedure reported previously [11]. The S i N H films were deposited in a multiprocess procedure, such that the gate dielectric layer was never directly exposed to atmosphere. The characteristics of these devices were significantly better than those fabricated with a high temperature thermal gate oxide. The improvements in the characteristics will be discussed elsewhere [12]. Devices fabricated yielded a mobility of 30cmZ/Vs, threshold voltages of 2.7 V, I ~ ( V g = 10)/loff(Vg = 0 ) of 10 4 and the sub-threshold slope at Vg = 5 V was 2 V/decade.

6. Conclusions P E C V D deposition of S i N H was investigated in the temperature range of 300-550°C. The deposition process was optimised and incorporated in a multiple in situ layer deposition schedule for the

p r o d u c t i o n of TFTs. F o r this application considerable advantages are gained by depositing the films at temperatures above 300°C, where the H content is reduced and dielectric properties are improved. Experimental w o r k showed that a slightly nitrogen rich film yields the best characteristics for use as a dielectric. N o impurities were detected in the layers from RBS analysis and S I M S results showed that the layers had a H content of less than 11%. T F T s fabricated using these layers have given excellent results with high mobilities and low threshold voltages.

References [1] V. Dharmadhikari, Thin Solid Films 153 (1987) 459. [2] R. Schomer and R. Hezel, IEEE Trans. Electron Dev, ED-28 (1981) 1466. [3] A.K. Sinha, J. Electrochem. Soc. 125 (1978) 601. [4] H.A. Mar and J.G. Simmons, IEEE Trans. Electron Dev. ED-24 (1977) $40. [5] A. Mumumura, IEEE Trans. Electron Dev. ED-36. (2) (1989) 351. [6] I. Kobayashi, Jpn. J. Appl. Phys. 31 (1992) 336. [7] L.J. Quinn, EMRS Symposium D, Paper D-I.4, Spring 1993. 1-8] H. Dun, J. Elechochem. Soc. 128 (1981) 1555. [-9] W.A. Land and M.J. Rand, J. Appl. Phys. 49 (1978) 2473. [I0] W.A.P. Claassen, J. Electrochem. Soc. 132 (1985) 893. [11] L.J. Quinn, Proc. IEEE Melecon 2 (1994) 599. [12] L.J. Quinn, in: Proc. ESSDERC'94,Vol. 24, ed. C. Hill and P. Ashburn (Editions Fronti~res, Gif-sur-Yvette, 1994) p. 581.