IEEE ELECTRON DEVICE LEITERS, VOL. 16, NO. 11, NOVEMBER 1995 473
Thinner Liquid Phase Deposited Oxide
for Polvsilicon Thin-Film Transistors
Ching-Fa Yeh, Shyue-Shyh Lin, and Ching-Lin Fan
Abstract-To scale down the gate insulator thickness of polysil- icon thin-film transistors (poly-Si TFT’s), a thinner oxide is developed by liquid-phase deposition with a small quantity of HzO added, producing a rather high-quality oxide. Poly-Si TFT with such a thin oxide reveals good performances in electric characteristics. Thus, the novel thinner oxide is a good candidate as a poly-Si TFT gate insulator in the near future.
I. INTRODUCTION
HERE has been great interest recently in liquid-phase
T
deposition (LPD) oxide films because of their potential application as gate insulators in polysilicon thin-film tran- sistors (poly-Si TFT’s) [l], [2]. Scaling down the thickness of the gate insulator improves electrical properties of poly- Si TFT’s; it reduces the effects of trapped charges on the threshold-voltage increases [3] and reduces the gate swing factor [4]. Thus, a thinner LPD oxide with high quality will be indispensable for improved poly-Si TFT’s.In conventional LPD processing [l], [2], both H20 and boric acid (H3B03) were added into saturated HzSiF6 solution to grow LPD oxide. The deposition rate dependent on the quantity of boric acid added is fast (80
-
120 nm/hr [5]). Also, we found that a thinner LPD oxide deposited with such a high deposition rate exhibited poor quality. According to a newly developed LPD method [6], deposition rate can be controlled by H20 addition only. In this letter, we clarify the influences of adding quantities of H20 to develop a thinner high-quality film for the first time. Then, the poly-Si TFT’s with such a thin-film as gate insulator are characterized to investigate the feasibility of thin LPD oxide.11. EXPERIMENTAL
The detailed apparatus and experimental flow for LPD process were the same as reported in [7]. N-type, (100) 1
-
5 R cm silicon substrates were used to prepare LPD oxide. Twenty-five ml to 150 ml of H20 was added to the 100 ml saturated HzSiFG solution at 15OC to investigate the effects of H20 on the properties of LPD oxide.Conventional poly-Si TFT’s (L/W = 8 pm/lO pm) with different LPD-oxide thickness as gate insulator were prepared. All the fabrication and characterization methods were the Manuscript received April 28, 1995; revised August 2, 1995. This work was supported by the National Science Council, R.O.C., under Contract NSC 84-0404-E-009-0 10.
The authors are with the Department of Electronic Engineering, National Chiao Tung University, Hsinchu 300, Taiwan, R.O.C.
IEEE Log Number 9414961.
TABLE I
CHARACTERISTICS OF LPD OXIDE DEPOSITED WITH DIFFERENT Q U A N T I T I E S OF
H 2 0 ADDED (HzSiF6 = 100 ml, D E ~ ~ S I T T O N TEMPERATURE = 15OC) H 2 0 Quantity Deposition Refractive P-etch Rate Electric Breakdown
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(ml) Rate (n&) Index (n) (&sec) Field (MVkm)
25 4.9 1.438 1.71 8.3 50 20 1.436 1.76 7.2 15 25.9 1.429 1.81 4.3 100 31.8 1.417 1.95 2.9 125 31.4 1.410 2.01 < 2
-
-same as in our previous studies [ 13. The maximum processing temperature was 625 O C
.
111. RESULTS AND DISCUSSION
The physicochemical and the electrical properties of LPD oxides deposited with various H2O quantities were investi- gated and summarized in Table I. The different H20 quantities yielded lower deposition rate than boric acid addition in all cases. The deposition rate can be made as low as possible because H20 is a kind of reactant. The refractive indexes decrease a little with increasing H20 quantity and are lower than the 1.46 of thermal oxide. The lower refractive index can be attributed to the dual effects of a less dense structure [8] and fluorine incorporation in the film [9]. Because the LPD oxide deposited with larger H20 quantities will incorporate fewer fluorine atoms in the film [lo], the decreases in refractive index with increasing quantities of H20 is due to the less dense structure. The fact that the p-etch rates increase with increasing H20 quantities further implies that the LPD oxide deposited with larger quantities of H20 added will be less dense in structure.
The electric breakdown field (E) also reveals strong depen- dence on H20 quantity; that is, E decreases with increasing H2O quantity. When adding H2O to saturated HzSiF6 so- lution, the intermediate polysilicic acids will be formed by the polymerization of the silicic monomer Si(0H)B and then absorbed onto the substrate surface. Acid-catalytic dehydration occurs between these absorbed polysilicic acids, followed by Si-0-Si bond formation. The larger the H20 quantity is, the faster the polysilicic acid formed and absorbed onto the substrate surface. In that case, the dehydration reaction cannot be completely finished in time, and many residual Si-OH bonds may exist in the film. On the contrary, the LPD oxide prepared with less H2O added will have fewer residual Si-OH bonds and will exhibit better properties.
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LIEEE ELECTRON DEVICE LElTERS, VOL. 16, NO. 1 1 , NOVEMBER 1995
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Electric Field (MVlcm)
Fig. 1. I-V characteristics of MOS capacitors with different thicknesses of LPD oxide. The most severe thermal condition during capacitor fabrications was 40O0C, 20 min for sintering. The size of capacitor is 1.77 x l o r 4 cm’. The voltage ramp rate was 0.67 V/sec.
From the above discussion, we can conclude that the best way to prepare a thin high-quality LPD oxide is to reduce the quantity of HzO added to saturated HzSiFs solution. By this method, a 30 nm thick LPD oxide was successfully developed. Its I-V characteristics are shown in Fig. 1. The I-V curves for a thick (100 nm) and a thin (50 nm) film deposited with boric acid addition are also shown for comparison. The electrical characteristics of the new thin oxide show comparable leakage current and higher breakdown electric field than the thick ones. It is evidently worth applying such a thin LPD oxide as the gate insulator in poly-Si TIT’S.
The IDS-VGS characteristics of Poly-Si TFT with 30 nm LPD oxide as gate insulator was shown in Fig. 2. An ON/OFF current ratio of 1 x lo6, threshold voltage of 7.2 V, and subthreshold swing of 1.08 V/decade were obtained. In Fig. 2, the IDS-VGS characteristics of the devices with 50 and 80 nm oxides were also shown for comparison. For the device with 30 nm-oxide, because the grain barrier height can be lowered much more, both ON- and OFF-currents become larger. However, the ON/OFF current ratio (1 x lo6) has been achieved, while they are 6.45 x lo5 and 2 x lo5 for the devices with 50 nm and 85 nm thick, respectively. The dependence of the threshold voltage and subthreshold slope on the thickness of the gate insulator were also characterized, as shown in Fig. 3. Both the threshold voltage and the subthreshold slope show nearly linear decreases as the thickness of LPD gate oxide is reduced. This is because scaling down the oxide thickness effectively increases the inversion carrier density, which consequently lowers the grain barrier height. The lower the grain barrier height is, the steeper the subthreshold slope will be [4]. The linear decreases also indicate that the trap- state densities are constant in these devices [ I I]. Because the trap-state densities are independent of gate oxide thickness, the field effect mobility, which is mainly influenced by the trap-state density, is nearly the same (30 cm2/V
.
sec) for the three samples.20 30 40 50 60 70 80 90
Gate Oxide Thickness (nm)
Fig. 3.
T l T s on thickness of LPD gate oxide.
Dependence of threshold voltage and subthreshold slope of poly-Si
IV. CONCLUSION
A thinner high-quality oxide has been developed using the LPD method with addition of small quantities of HzO. Poly- Si TFT with a thin LPD oxide shows good performances. It reveals the feasibility of applying the thin LPD oxide as gate-insulators in small-geometry poly-Si TFT’s in near future.
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