ENHANCED H-2-PLASMA EFFECTS ON POLYSILICON THIN-FILM TRANSISTORS WITH THIN ONO GATE-DIELECTRICS

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228 IEEE ELECTRON DEVICE LETTERS, VOL. 16, NO. 6, JUNE 1995

Enhanced Ha-Plasma Effects on

Poly silicon Thin-Film Transistors

with Thin

ONO Gate-Dielectrics

Chien

KUO

Yang, Student Member, ZEEE, Chung Len Lee, Senior Member, IEEE, and

Tan Fu Lei

Abstract- This letter reports that passivation effects of the H2-plasma on the polysilicon thin-film transistors (TFT’s) were greatly enhanced if the TFT’s have a thin Si3N4 film on their gate- dielectrics. Compared to the conventional devices with only the Si02 gate dielectric, the TFT’s with Si3N4 have much more im- provement on their subthreshold swing and field-effect mobility after €It-plasma treatment.

I. INTRODUCTION

2-PLASMA passivation has become a well-accepted

H

method to improve the electrical properties of polysilicon thin-film transistors [l], [2]. Recently, there has been increasing interest in studying the mechanism of Hz-plasma passivation [2]-[5]. In addition, how to enhance the passivation effect of Hz-plasma on TFT’s has also become an important issue [6], [7]. Although it had been reported that Hz-plasma had a better passivation effect on polysilicon TFT’s with gate oxynitrides formed by NH3 [71, the reason of the enhanced hydrogenation has not been clearly found yet. Also the stacked SiOz/Si3Nd/SiOz (ONO) structure has been proposed to reduce the gate leakage [SI. It had been confirmed that an LPCVD silicon nitride layer could act as an effective barrier against hydrogen diffusion [4]. Therefore, it is interesting to study the Hz-plasma effects on the polysilicon TFT’s without and with an thin Si3N4 film on their gate- dielectrics. In this work, it is found that the Hz-plasma passivation effect is dramatically enhanced on the TFT’s with the thin ONO gate dielectric as compared with the devices with only the thermally grown thin gate-oxide.

11. EXPERIMENTAL

PROCEDURES

The polysilicon TFT’s were fabricated on thermally oxi- dized silicon wafers. A 60 nm amorphous-silicon film was initially deposited by an LPCVD system at 550°C and then annealed at 600°C for 24 hr. The active regions were defined by plasma etching. After a 19.2

nm

bottom oxide was grown in wet 0 2 at 850°C, a 10 nm Si3N4 film was deposited by a conventional LPCVD reactor with the source gases of SiHzClz and NH3 at 750°C followed by reoxidation in wet 0 2 at

Manuscript received November 15, 1994; revised January 20, 1995. This work was supported by the National Science Council of the Republic of China through Contract NSC-83-0404-E009-0 17

The authors are with the Department of Electronics Engincering and Institute of Electronics, National Chiao Tung University, and National Nano Device Laboratory, Hsinchu 300, Taiwan, R.O.C.

IEEE Log Number 941 1768.

-,

ONO-22.1 nm t 0 - 1 9 . 2 n m

- 5 0 5 10 1 5

Gate Voltage Vg (V)

Fig. I. I,i-Vg characteristics at

vi

= 5 V of the TFT’s without and with a 60 min Ha-plasma treatment. All devices were measured at the dimension of WIL = 40pm/lOtiin.

85OOC for 5 min. The equivalent oxide thickness of the ONO film is 22.1 nm. Then, another 300 nm polysilicon film was deposited and patterned to be the gate of the device. After the dielectrics above source/drain regions were stripped off, a self- aligned

Poc13

doping was performed at 850°C to form the source, drain, and gate electrodes. For comparison, devices with only Si02 as the gate dielectric of the thicknesses of 19.2 and 25.8 nm were also made. The gate-oxides were also thermally grown in wet 0 2 at 850OC. After contact holes were

opened, AI was deposited and then patterned. Finally, plasma hydrogenation was applied with an

Hz

and Nz gas mixture in a commercial 13.5-MHz parallel-plate plasma reactor at 300°C for 20-80 min.

111. RESULTS AND DISCUSSIONS

Fig. 1 shows the typical

&-VY

characteristics of the TFT‘s with and without the H2-plasma treatment. Table I compiles the values of the subthreshold swing S, the threshold voltage Vthr the field effect mobility p , and the effective trap density

Nt

[9], derived from the corresponding devices of Fig. 1 at

V d = 0.1 V. It is seen that for the device with the ONO 0741-3106/95$04.00 0 1995 IEEE

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YANG et al.: ENHANCED Hz-PLASMA EFFECTS ON POLYSlLlCON THIN-FILM TRANSISTORS

~

229

TABLE I

TIIE VALUES OF S, Vt,, , p , AND N L OF THE

POLYSlLlCON TFT’S WITH DIWERENT GATE-DIELECTRICS S (mV/decade) Vth (V) Cc (cm2/ Vsec) Nt &lO’*cm2eV.’) Conditions e ore afler before after before &er before after

@plasma 1 hr th-plasma 1 hr &-plasma 1 hr &-plasma 1 hr

ONO-22 I nm 642 187 4 75 0 4 7 4 6 3 2 2 9 5 3 2 0 8 0 - 1 9 2 n m 495 302 3 4 6 1 9 0 4 5 9 3 8 2 2 4 5 8 o -25 nm 631 397 4 6 2 2 3 3 3 3 I O I 8 0 3 4 7 7 h

”

4

s

e

c) el el

.*

ii

Fig. 2 h

“

s

5. d

8 P

LI 0 U c

.“

c

f

The ON- and OFF-state drain currents and the N , reduction rate as-a function of the Hz-plasma-exposure time for the devices with different gate-dielectrics.

gate-dielectric, the Hz-plasma effect was drastically enhanced, especially on its improvement of S and p. Fig. 2 shows the ON- and OFF-state drain currents and the

Nt

reduction rate as

a

function of the Hz-plasma-exposure time for the devices with different gate-dielectrics, respectively. It is found that the ONO device had a much better effect in responding to the Hz-plasma treatment. The drivability and leakage of the ONO devices were more effectively improved. Moreover, the Nt reduction rate of the ONO device was also considerably larger than those without

a

thin Si3N4 film on their gate-dielectrics. Fig. 2 also shows that after

a

20 min Hz-plasma treatment, the improvements of all the devices almost reached

a

saturation state and further increasing the Hz-plasma-exposure time did not improve the devices too much.

’ h o

possible mechanisms could be used to explain the above results. One is that for the ONO device, the Si3N4 film prevented hydrogen out-diffusion during the plasma treatment. This made the hydrogenation effect more effective. On the other hand, since it is generally recognized that the strain-bond band-tail states influence the field effect mobility [2], the large improvement on p suggests that the thin Si3N4 in the ONO gate-dielectric played an

important role in reducing the band-tail states during the

Hz-

plasma. Hence, the other possible mechanism is the relaxation of the strained bonds due to the existence of Si3N4. It is speculated that the mechanical stress

[lo],

[ l l ] caused by the existence of Si3N4 might be helpful to break the strained bonds at the poly-Si/SiOz interface and in the polysilicon film to relax the network. This speculation can be supported further by the fact that for the ONO devices, they had higher Nt before hydrogenation as shown in Table I. During hydrogenation, the Hz-plasma passivated the dangling bonds and relaxed the silicon network more effectively.

IV. CONCLUSION

In this letter, it is reported that the effect of the Hz-plasma hydrogenation on improving the electrical characteristics of the polysilicon TFT’s is greatly enhanced if the TFT’s have a thin Si3N4 film on their gate dielectrics. Compared to the TFT’s with only the Si02 gate dielectric, the ONO device has

a

larger Nt before hydrogenation but

a

much more improved

p after hydrogenation. This experimental result suggests that the deposited thin Si3N4 film plays

a

very important role in further reducing the strained bonds of the active layer during the Hz-plasma treatment.

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