Proceedings of 8th Thin Film Materials & Devices Meeting
November 4-5, 2011, Kyoto, Japan
http://www.tfmd.jp/Improvement in Electrical Characteristics of Microcrystalline Silicon Thin
Film Transistors by High-pressure H2O vapor Heat Treatment
A. Shuku1, E. Takahashi1, Y. Andoh1, Y. Nagatomi2, and T.Sameshima2
12

NISSIN Electric Co., Ltd, 47, Umezu-Takase-cho, Ukyo-ku, Kyoto, 615-8686, JapanTokyo University of Agriculture and Technology, 2-24-16, Naka-cho, Koganei, Tokyo, 184-8588

Keywords: uc-Si, ICP, PECVD, TFT
Abstract. Thin film transistors (TFTs) with no doped source and drain regions were fabricatedin microcrystalline silicon (µc-Si) films formed at 125, 150, and 200 oC using inductively
coupled plasma chemical vapor deposition (ICP-CVD). Transfer characteristics for as
fabricated TFTs with µc-Si in every deposition condition showed typically n-type and depletion
type behavior. 1x106 Pa H2O vapor heat treatment at 260 oC for 3 h changed transfer
characteristics to enhancement type ones for TFTs fabricated in µc-Si films formed at 150 and200 oC, respectively. The threshold voltages for TFT with µc-Si formed at 125, 150, and 200 oC
were -6 ,0.5 and -0.5 V, respectively. Furthermore, the high-pressure H2O vapor heat treatment
markedly decreased the hysteresis voltage width in transfer characteristics into 0.5, 1.0 and 0.5
V for µc-Si formed at 125, 150, and 200 oC, respectively.
Introduction
Microcrystalline silicon (µc-Si) films have high field effect mobility compared withamorphous silicon (a-Si) films. They can be formed at a low temperature by using plasma
enhancement chemical vapor deposition (PECVD). µc-Si films have been therefore researchedand developed for a long time for application of them to fabrication of thin film transistors(TFTs) at low temperatures [1, 2]. The excimer laser annealing (ELA) method has beenstandardized for forming polycrystalline silicon films. It has been applied to production ofpoly-Si TFT electronic circuits for liquid crystal display (LCD) and organic
electroluminescence (EL) display devices [3]. We believe that µc-Si still is a promising material
because of advantage of the simply direct deposition for fabricating TFTs with rather high field
effect mobility for large-size-display-device production.
We have developed inductively coupled plasma (ICP) CVD equipment which has the potential
to synthesize good quality µc-Si film thanks to low plasma damage resulting from a highplasma density (1x1012 cm-3) and a low plasma potential (<40 V).
Moreover, we have also developed high-pressure H2O vapor heat treatment to improve TFT
characteristics. High-pressure H2O vapor heat treatment has improved laser-crystallized TFT
characteristics at low temperatures below 300 oC [4, 5, 6]. The defect reduction of the µc-Si
films has been achieved via a hydrothermal synthesis reaction under high pressure and hightemperature.
In this paper, we report on µc-Si TFT fabrication at low temperatures of 125, 150 and 200 oC
as temperatures of µc-Si formation. We also report improvement in TFT characteristics using
high-pressure H2O vapor heat treatment at 260 oC. In particular, we concentrate changes in thethreshold voltage and the hysteresis behaviors in transfer characteristics.
Experimental procedure
Undoped 50-nm-thick µc-Si films were synthesized by the ICP-CVD method on 0.02 Ωcm n- 120223112-01 -type crystalline silicon substrates with both surfaces coated with 100-nm-thick thermally grownsilicon dioxide (SiO2) layers. The substrate temperature was set at 125, 150 and 200 oC. A
13.56-MHz radio-frequency (RF) power was applied at 7.5 kW. A mixed gas of SiH4 at 100
sccm and H2 at 150 sccm was flowed with a pressure of 0.67 Pa. Contact holes weresubsequently opened in the µc-Si and SiO2 layers by reactive-ion etching (RIE) to use
crystalline silicon substrate as the gate electrode, as shown in Fig. 1(a). The 300-nm-thick Al
metal films were then deposited and defined as source, drain, and gate electrodes using theliftoff method. µc-Si islands were finally defined as channel areas by RIE, as shown in Fig. 1.
As shown in a plane and top view of our µc-Si TFT in Fig. 2 all of Al electrodes were formedon the top surface because of experimental measurement matter of TFT characteristics.
µc-Si TFTs were then annealed by 1x106 Pa H2O vapor heat treatment at 260 oC for 3 h withequipment shown in Fig. 3. The pressure was determined from a process temperature, achamber volume and a water volume.
TFT characteristics were measured in the dark field at a step time of 0.2 s and a drain voltage
of 0.1 V.
SiO2

Al(G)
μc-Si

(a)gate electrode

Al(S)

Al(D)

(b) source-drain electrodes
Fig.1 Schematic cross section of bottom gate µc-Si TFT

Fig.2 Plane view of bottom gate µc-Si TFT

Fig.3 High-pressure H2O vapor heat treatment

Results and Discussion
TFTs with µc-Si films formed at 125, 150 and 200 oC showed typical n-type transfer
characteristics, as shown in Fig. 3(a), (b), and (c), respectively, while TFTs had no doped sourceand drain regions, as shown in Fig. 1. Hole current did not appear in negative gate voltage
regions. Those characteristics shown in Fig. 3 suggest that the effective hole mobility might be
extremely low if hole carriers were accumulated via change in the band potential of the µc-Si- 120223112-02 -caused by the negative gate voltage according to simple MOSFET theory [7].
Substantial drain current flowed by applying the gate voltage above -10V. The negative fixedcharges probably existed at SiO2/Si interfaces or µc-Si layers. Furthermore, the drain currents inthe case of gate voltage application from negative to positive (forward direction) were lower
than that in the case of gate voltage application from positive to negative (reverse direction).
Those hysteresis phenomena indicate transient negative charge accumulation in µc-Si layer
caused by applying the gate voltage [8].
On the other hand, 1x106-Pa-H2O-vapor heat treatment at 260 oC for 3 h markedly changedtransfer characteristics. The drain current curves shifted to the positive voltage direction, as
shown in Fig. 3 (b) and (c). The shift of transfer characteristics was evaluated as the change inthe threshold voltage Vt, in the case of forward gate voltage application. The initial Vt for TFTs
with µc-Si films formed at 125, 150 and 200 oC were -3, -2, and -2 V, respectively. The
high-pressure H2O vapor heat treatment changed Vt to -6, 0.5, and -0.5 V, respectively. Thechanges in Vt were therefore -3, 2.5 and 1.5 V, respectively, as shown in Table I. It isinteresting that 1x106-Pa-H2O-vapor heat treatment at 260 oC for 3 h was effective in changing
transfer characteristics to enhancement type ones only for µc-Si deposition temperatures of 150
and 200 oC.
Moreover, 1x106-Pa-H2O vapor heat treatment increased the drain current in the high negativegate bias region for every TFT, as shown in Fig. 3. We interpret that high-pressure H2O vapor
heat treatment increased the effective hole mobility.
Furthermore, 1x106-Pa-H2O-vapor heat treatment at 260 oC for 3 h reduced hysteresis
characteristics in transfer characteristics for every TFT, as shown in Fig. 3. The transfer
characteristics in forward and reverse directions became almost the same. We evaluatedhysteresis voltage width ( Δ V) as the voltage difference between Vt (forward voltageapplication case) and Vr, which was a voltage giving the same drain current for reverse voltage
application as that at Vt (ΔV=Vt-Vr). ΔV of initial TFTs with µc-Si films formed at 125, 150
and 200 oC was 2.0 V. 1x106-Pa-H2O-vapor heat treatment at 260 oC for 3 h decreased ΔV to0.5, 1.0, and 0.5 V, respectively, as shown in Table I. Those results indicate that
1x106-Pa-H2O-vapor heat treatment at 260 oC for 3 h reduced the transient charge accumulationin µc-Si films.

10-8

10-7

H2O vaportreatment

10-9
10-10
10-11

asfabricated

10-12

10-7

10-9
10-10

H2O vaportreatment

10-11
10-12
10-13

10-13
10-14
-20

10-8

-10

0

1020

10-14
-20

10-6(b)

Vd=0.1
asfabricatedDrain Current [A]Drain Current [A]10-7

10-6(a)Vd=0.1VDrain Current [A]10-6

10-8(c)

Vd=0.1V
asfabricated

10-9
10-10

H2O vaportreatment

10-11
10-12
10-13-10010

Gate Voltage [V]2010-14-20-1001020

Gate Voltage [V]Fig.3 Transfer characteristics of as-fabricated and 1x106 Pa H2O vapor heat treatment at 260 oC for 3
hours for TFTs with µc-Si films formed (a) at 125 oC, (b) at 150 oC, and (c) at 200 oC.

- 120223112-03 -Table I. Vt and ΔV in transfer characteristics at 125, 150, 200 oC.
Substrate
H2O vapor heat
As-fabricatedtemperature[℃]
treatment
Thresholdvoltage[V]Hysteresis
voltage width[V]125-3.0-6.0

150-2.00.5200-2.0-0.5125

2.00.5

150

2.0

1.0

200

2.0

0.5

Figures 4, 5, and 6 show output characteristics of as-fabricated TFTs for µc-Si formed at 125,
150 and 200 oC, respectively. (a) and treated with 1x106-Pa-H2O-vapor heating at 260 oC for 3h(b). Output characteristics showed n-type ones with good ohmic behaviors for every TFT. The
drain current was highest in the case of µc-Si formation at 200 oC. The 1x106-Pa-H2O-vapor
heat treatment at 260 oC for 3 h increased the drain current at 125 and 150 oC by 278 and 130 %Fig.4 Output characteristic of as-fabricated (a) and
1x106 Pa H2O vapor heat treatment at 260 oC for 3
hours (b) for TFTs with µc-Si films formed at 125oCFig.5 Output characteristic of as-fabricated (a) and
1x106 Pa H2O vapor heat treatment at 260 oC for 3
hours (b) for TFTs with µc-Si films formed at 150 oC- 120223112-04 -at gate and drain voltages of 20 V. The highest drain current shows that high electricalconductance was achieved for high temperature deposition. We believe that high deposition
temperature resulted in a low density of crystalline defects associated with a low potential
barrier height.

Summary
Bottom-gate type TFTs were fabricated inµc-Si films formed at 125, 150, and 200 oC
using ICP-CVD. Vt at a drain voltage of 0.1
V for TFTs with µc-Si formed at 125, 150
and 200oC were -3, -2 and -2 V, respectively.
1x106-Pa-H2O-vapor heat treatment at 260oC
for 3 h changed Vt to -6, 0.5, and -0.5 V,respectively. Furthermore, the treatment
markedly decreased hysteresis voltage width
in transfer characteristics from 2.0 V (initial)
to 0.5, 1.0 and 0.5 V for µc-Si formed at 125,
150 and 200 oC, respectively. The
high-pressure H2O vapor heat treatment waseffectivefor
reducing
hysteresis
characteristics.

Fig.6 Output characteristic of as-fabricated and 1x106 Pa
H2O vapor heat treatment at 260 oC for 3 hours (b) for
TFTs with µc-Si films formed at 200 oC

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