High rate deposition of a-Si:H and a-SiNx:H byVHF PECVDT Takagi,a K Takechi,b Y Nakagawa,a Y Watabea and S Nishidab, aANELVACorporation, 5-8-1 Yotsuya,Fuchu-shi, Tokyo, 183-8508 Japan., and bFunctional Devices Research Laboratories, NECCorporation, 4-1-1Miyazaki, Miyamae-ku, Kawasaki, Kanagawa, 216-8555 Japan.

Very High Frequency (VHF) plasma enhanced chemical vapour deposition (PECVD) has been applied to hydrogenatedamorphous silicon (a-Si:H) and hydrogenated amorphous silicon nitride (a-SiNx:H) films for thin film transistors(TFTs) fabrication. The effect of the excitation frequency on the deposition rate and the film quality of both films hasbeen investigated. The filmswere prepared by VHF (30 MHz050MHz) andHF (13.56 MHz) plasma enhancedCVD.High deposition rates were achieved in the low pressure region for both a-Si:H and a-SiNx:H depositions by the useof VHF plasma. Themaximum deposition rates were 180 nm/min for a-Si:H at 50 MHz and 340 nm/min for a-SiNx:Hat 40 MHz. For a-SiNx:H films deposited in VHF plasma, the optical bandgap, the hydrogen content and the [Si^H]/[N^H] ratio remain almost constant regardless of an increase in deposition rate. The increase of film stress could be lim-ited to a lower value even at a high deposition rate. The TFTs fabricatedwith VHF PECVD a-Si:H and a-SiNx:H filmsshowed applicable field effect mobility. It is concluded that VHF plasma is useful for high rate deposition of a-Si:Hand a-SiNx:H films for TFT LCD application. ã 1998 Elsevier Science Ltd. All rights reserved

Introduction

In order to lower the panel production cost of TFT-LCDs, it

is essential to achieve a high rate deposition of the ®lms in

PECVD process (a-Si:H, a-SiNx:H and n + a-Si:H) uniformly

on large-area substrates without sacri®cing the TFT perform-

ance. For this purpose, a-Si:H ®lms with low Si±H2 bond den-

sities are required to give high TFT performance,1 and slightly

nitrogen-rich a-SiNx:H ®lms with large optical bandgap are

required to give satisfactory TFT characteristics and high re-

liability of the device.2 Also the internal stress of both ®lms is

important as excessive tensile stress may cause cracking while

excessive compressive stress may cause peeling of the deposited

thin ®lms.2

Usually the industrial frequency of 13.56 MHz is used as the

plasma excitation frequency in PECVD. A high deposition

rate of the ®lms may be achieved by increasing the power den-

sity and/or the gas pressure, but then ®lm properties tend to

deteriorate and the internal stress increases at high depositionrates. It has been reported by many authors that high quality

a-Si:H ®lms can be achieved at high deposition rates by the

use of an excitation frequency in the VHF band

(300300 MHz).3±6 This has been explained by an enhance-

ment of generation rates of radicals and the lower ion impact

energy in VHF plasma.7, 8 However, there have been only a

few reports on the application of VHF PECVD to a-SiNx:H

deposition, including our recent reports.9±11

In this paper, we investigate the e�ect of the excitation fre-quency on the deposition rate and the ®lm properties of a-Si:H and a-SiNx:H ®lms including the TFT performance.

Experiment

A parallel-plate PECVD system (ANELVA ILV-9100) with aremodeled electrode of 500�500 mm2 was used for the depo-sition of a-Si:H, a-SiNx:H and n + a-Si:H ®lms. The distancebetween the electrode and the substrate was ®xed at 30 mm

and the substrate temperature at 3008C. The ®lms were pre-pared using VHF plasma (at the frequency of 30 MHz and50 MHz for a-Si:H, and 40 MHz for a-SiNx:H) and

13.56 MHz plasma. 50 MHz was the upper limit of the exci-tation frequency to achieve uniform plasma on a large area inour equipment. A gas mixture of SiH4 and H2 was used for a-

Si:H deposition and SiH4, NH3 and N2 for a-SiNx:H. The gas¯ow-rate ratios were SiH4/H2=2/3 for a-Si:H and SiH4/NH3/N2=1/3/10 for a-SiNx:H, unless stated otherwise.

The deposition rate and the ®lm properties such as hydro-gen contents, internal stress and TFT performance were evalu-ated for the ®lms deposited under various conditions, whichwere varied by changing the excitation frequency, the pressure,

the ¯ow rate and the power density. The hydrogen contentswere investigated by FT-IR absorption spectroscopy from the®lms deposited on silicon wafers (observation of Si±H and Si±

Vacuum/volume 51/number 4/pages 751to 755/1998ã 1998 Elsevier Science Ltd

All rights reserved. Printed in Great Britain0042-207X/98 Sl - see front matter

Pergamon

PII: S0042-207X(98)00284-X

751

H2 stretching modes at 2000 cmÿ1 and 2090 cmÿ1 for a-Si:H,12

and N±H and Si±H stretching modes at 3350 cmÿ1 and

2160 cmÿ1 for a-SiNx:H.)13 The internal stress was calculatedfrom the change in curvature of the silicon wafer before andafter ®lm deposition. The optical bandgap of a-SiNx:H wasevaluated from the ®lms deposited on quartz substrates by ob-

servation of the optical absorption spectrum in the ultra-violet(UV) range.Channel-etched inverted-staggered TFTs were fabricated to

investigate the TFT performance. P-doped a-Si:H (n+) layerwas deposited under the same deposition conditions in13.56 MHz plasma for all TFTs. In order to estimate the e�ect

of deposition frequency on the TFT performance, either a-Si:H or a-SiNx:H layer was deposited in VHF plasma, and theother layer in 13.56 MHz plasma. As an evaluation of TFT

performance, the ®eld e�ect mobility was calculated from theVg±Id characteristics by using gradual-channel approximationtheory.

Results and discussion

a-Si:H. Figure 1 shows the pressure dependence of the depo-sition rate of a-Si:H ®lms deposited at gas ¯ow rates of SiH4/

H2=200/300 sccm and a power density of 0.06 W/cm2. At13.56 MHz, the deposition rate increases with pressure andthen saturates over 120 Pa. At higher excitation frequencies,

the deposition rate shows a maximum, which shifts positiontoward lower pressure and higher deposition rates are achievedat higher frequencies.

The deposition rate of a-Si:H increases with the gas press-ure, the power density and the excitation frequency, due to theenhancement in the number density of the ®lm-growth precur-

sors in the gas-phase. When the pressure is increased, the col-lision of the ®lm-growth precursors with the source gas wouldincrease at the same time. It is suggested that there is a certainlimit in the pressure above which the collision of the ®lm-

growth precursors with the source gas exceeds the number ofthe ®lm-growth precursors reaching the ®lm surface, resultingin the decrease in deposition rate. When the excitation fre-

quency is increased, as the number density of the ®lm-growth

precursors is enhanced, the pressure at which the deposition

rate reach its maximum value would decrease.

In the following experiments, the pressures for each of the

frequencies were set to those values giving the maximum depo-

sition rates in Figure 1, that is, 180 Pa for 13.56 MHz, 90 Pa

for 30 MHz and 60 Pa for 50 MHz. By increasing the power

density to 0.14 W/cm2, the deposition rate rises up to 75 nm/

min at 13.56 MHz, 100 nm/min at 30 MHz and 130 nm/min at

50 MHz. The maximum deposition rate of 180 nm/min was

achieved at a power density of 0.14 W/cm2 in 50 MHz plasma

by increasing the SiH4 ¯ow-rate ratio to 60% (i.e. SiH4/

H2=300/200 sccm).

Figure 2 shows the Si-H2 fraction observed as [Si±H2]/([Si±

H] + [Si±H2]) versus the deposition rate of a-Si:H ®lms. Here,

[Si±H] and [Si±H2] denotes the hydrogen contents bonded as

Si±H and Si±H2, respectively. The deposition rate was varied

by changing the pressure and the power density at constant

Figure 1. The pressure dependence of the deposition rate of a-Si:Hfilms at a power density of 0.06 W/cm2 and gas flow rates of SiH4/H2 = 200/300 sccm.

Figure 2. [Si±H2]/([Si±H] + [Si±H2]) ratio versus the deposition rate ofa-Si:H films at gas flow rates of SiH4/H2=200/300 sccm. The depo-sition rate was varied by changing the pressure and the power density.

Figure 3. The field effect mobility of TFTs versus the deposition rateof a-Si:H films. The a-SiNx:H and P-doped a-Si:H (n+) layers weredeposited in 13.56 MHz plasma at 12.5 nm/min and 2.2 nm/min, re-spectively.

T Takagi et al:High rate deposition of a-Si:Hand a-SiNx:H byVHFPECVD

752

gas ¯ow rates of SiH4/H2=200/300 sccm. The Si±H2 fractionincreases with deposition rate for both 13.56 MHz and50 MHz, but a lower fraction is obtained in high deposition

rates at 50 MHz.The ®eld e�ect mobility of TFTs versus the deposition rate

of a-Si:H layer is shown in Figure 3. Both a-SiNx:H and P-

doped a-Si:H (n+) layers were deposited at 13.56 MHz atconstant deposition rates of 12.5 nm/min and 2.2 nm/min, re-spectively, and the deposition rate of a-Si:H layer was variedby changing the power density at constant gas ¯ow rates of

SiH4/H2=200/300 sccm. The ®eld e�ect mobility at a low de-position rate is similar for all frequencies, but then decreaseswith an increase in the deposition rate depending on the fre-

quency. For example, ®eld e�ect mobility of 0.4 cm2/V.s wasobserved at deposition rates of 40 nm/min for 13.56 MHz,85 nm/min for 30 MHz, and 125 nm/min for 50 MHz. Thus it

can be said that a-Si:H ®lms of equivalent quality for the usein TFTs are obtained at higher deposition rates when higherexcitation frequencies are used.

It is generally said that the TFT performance deteriorateswith an increase in the Si±H2 density in a-Si:H layer.1 In ourexperiment, a consistent correlation between the TFT mobilityand Si±H2 fraction in the a-Si:H layer was observed, indepen-

dent of the excitation frequency.

a-SiNx:H. The deposition rate of a-SiNx:H ®lms increasedwith pressure for both 13.56 MHz and 40 MHz plasma in the

range of 90 Pa to 200 Pa, but higher deposition rates wereobserved at 40 MHz. The pressure in the following exper-iments were set to those values that correspond to a deposition

rate of about 70 nm/min at gas ¯ow rates of SiH4/NH3/N2=70/200/700 sccm and a power density of 0.32 W/cm2, thatis, 200 Pa for 13.56 MHz and 150 Pa for 40 MHz.

Figure 4 shows the total gas ¯ow rate dependence of the de-position rate of a-SiNx:H ®lms at a power density of 0.48 W/cm2. The deposition rate increases linearly in the low ¯ow rateregion and tends to level o� at higher ¯ow rates, but higher

values are achieved at higher frequencies. The deposition rateat the maximum ¯ow rate is 340 nm/min for 40 MHz and stag-nates at 120 nm/min for 13.56 MHz.

Figure 5 shows the dependence of the optical bandgap on

the total gas ¯ow rate for the same deposition conditions as in

Figure 4. The optical bandgap remains in the range of

4.905.2 eV for 40 MHz, whereas it decreases rapidly to

3.0 eV for 13.56 MHz.

The decrease in the optical bandgap can be interpreted as

an increase of the Si/N ratio in the ®lm2. Instead of a direct

measurement, the [Si±H]/[N±H] ratio, which is known to show

a linear correlation with Si/N ratio14, was evaluated from FT-

IR absorption spectra. Here, [Si±H] and [N±H] represents the

hydrogen contents bonded as Si±H and N±H, respectively. It

has also been reported that excessive hydrogen content and

Si±H bonds in a-SiNx:H layer can be the source of instability

of the devices15.

Figures 6 and 7 show the hydrogen content and the [Si±H]/

[N±H] ratio, respectively, as a function of total gas ¯ow rate

at a power density of 0.48 W/cm2. Both the hydrogen content

and the [Si±H]/[N±H] ratio increase with total ¯ow rate in

13.56 MHz plasma, while they remain almost constant in

40 MHz plasma. A linear correlation of high optical bandgap

at low [Si±H]/[N±H] ratio was observed for all deposition con-

Figure 4. Total gas flow rate dependence of the a-SiNx:H depositionrate at a power density of 0.48 W/cm2 and gas flow-rate ratio of SiH4/NH3/N2=1/3/10.

Figure 5. Optical bandgap of a-SiNx:H films as a function of total gasflow rate, for a power density of 0.48 W/cm2 and gas flow-rate ratioof SiH4/NH3/N2=1/3/10.

Figure 6. Hydrogen content of a-SiNx:H films as a function of totalgas flow rate, for a power density of 0.48 W/cm2 and gas flow-rateratio of SiH4/NH3/N2=1/3/10.

T Takagi et al:High rate deposition of a-Si:Hand a-SiNx:H byVHFPECVD

753

ditions in our experiment. From the results above, it is clear

that nitrogen rich ®lms with high stability are deposited at

higher deposition rates when excitation frequencies higher

than the conventional 13.56 MHz are used.

Generally, high quality a-SiNx:H ®lms with high optical

bandgaps and low Si/N ratios can be obtained at high depo-sition rates by increasing the power density, but then the in-

ternal stress of the ®lms tends to change from tensile stress to

compressive.2 Figure 8 shows the internal stress of a-SiNx:H

®lms versus the deposition rate. The deposition rate was varied

by changing the power density. The ®lms deposited at

13.56 MHz show a compressive stress which increases in mag-

nitude with the power density, whereas ®lms deposited at

40 MHz show a tensile stress within the power density evalu-

ated. This is due to the lower ion bombardment energy at

higher frequencies.7, 16 This allows us to adjust the internal

stress of a-SiNx:H ®lms deposited at high deposition rates

almost arbitrarily, as tensile stress becomes compressive either

by increasing the power density or by adding H2 to the gas

source.2, 16

Figure 9 shows the ®eld e�ect mobility of TFTs as a func-

tion of the deposition rate of a-SiNx:H layer. Both a-Si:H and

P-doped a-Si:H (n+) layers were deposited in 13.56 MHz

plasma in the same deposition conditions at the rates of

25.5 nm/min and 2.2 nm/min, respectively. The deposition rateof a-SiNx:H layer was varied by changing the total ¯ow rateunder constant power densities of 0.48 W/cm2 at 13.56 MHz

and 0.64 W/cm2 at 40 MHz. The ®eld e�ect mobility of TFTsdeposited at 13.56 MHz decreases with an increase in the de-position rate, while it remains almost constant for ®lms depos-

ited at 40 MHz.As shown in Figures 5 and 6 and Figure 7, the ®lm proper-

ties of a-SiNx:H ®lms deposited at 13.56 MHz deteriorate with

the increase in the deposition rate, while they are almost inde-pendent of the deposition rate at 40 MHz. This correlates wellwith the relationship between the TFT mobility and the depo-sition rate shown in Figure 9. Therefore, it can be said that

the TFT performance, within the same excitation frequencyconditions, is determined by the property of a-SiNx:H layer.The ®lm properties of a-SiNx:H ®lms were higher for 40 MHz

than for 13.56 MHz in the whole range, however, the TFTmobility at low deposition rate was lower for 40 MHz. This isconsidered to be resulting from the higher defect density in the

a-SiNx:H layer due to higher input power introduced for40 MHz, as higher input power would increase the defect den-sity in the a-SiNx:H layer.

Conclusions

We have investigated the e�ect of the excitation frequency onthe deposition rate and the ®lm quality of a-Si:H and a-SiNx:H for TFT applications. An applicable ®eld e�ect mobi-

lity of 0.4 cm2/V.s was obtained by depositing a-Si:H layer in50 MHz plasma at a deposition rate of 130 nm/min, and nosigni®cant change in the mobility was observed in TFTs with

a-SiNx:H layers deposited in 40 MHz plasma up to a depo-sition rate of 300 nm/min.The TFT performance obtained by the use of a-Si:H and a-

SiNx:H layers deposited by VHF PECVD at high deposition

rates promises that VHF PECVD technique can help to lowerthe production costs in the TFT fabrication without degradingthe device performance.

Figure 7. [Si±H]/[N±H] ratio of a-SiNx:H films as a function of totalgas flow rate, for a power density of 0.48 W/cm2 and gas flow-rateratio of SiH4/NH3/N2=1/3/10.

Figure 8. The internal stress of a-SiNx:H films versus the depositionrate at gas flow rates of SiH4/NH3/N2=100/300/1000 sccm for40 MHz and SiH4/NH3/N2=120/600/2800 sccm for 13.56 MHz. Thepower density was varied in the range of 0.3200.64 W/cm2 for40 MHz and 0.3200.48 W/cm2 for 13.56 MHz.

Figure 9. The field effect mobility of TFTs versus the deposition rateof a-SiNx:H films. The a-Si:H and P-doped a-Si:H (n+) layers weredeposited in 13.56 MHz plasma at 25.5 nm/min and 2.2 nm/min, re-spectively.

T Takagi et al:High rate deposition of a-Si:Hand a-SiNx:H byVHFPECVD

754

Acknowledgements

We would like to acknowledge the TFT processinggroup at NEC Corp. for the contribution to the presentwork.

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