Back-Channel Surface Modifications for a-Si:H Thin Film Transistors, TFTs, by Exposure to Plasma Excited N 2 O
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489 Mat. Res. Soc. Symp. Proc. Vol. 318. ©1994 Materials Research Society
2. TFT Fabrication Glass had been used as a substrate for thin film transistors structures. 0.2 gim of tungsten was sputtered on to a glass substrate and patterned to form the gate electrode. Following this sandwich structures were formed by a 250 0 C remote plasma-enhanced chemical-vapor deposition, remote PECVD, technique. 450A of silicon oxide (with no IR detectable bond-H), and 2500A of silicon nitride, (with no AES detectable Si-Si bonding, but with IR absorption for Si-H and Si-NH bonding groups) were deposited by remote PECVD. a-Si:H channel layers of different thickness were then deposited on top of the silicon nitride layers. Finally 250A of PH3 doped n+ a-Si:H was deposited for the source and drain contact layers. These films were deposited successively in a single pump-down, and at a chamber pressure of 0.3 Torr. The 0- and N-atoms source gases, N 20 and NH3, respectively, were mixed with He, and injected through the plasma generation tube, while the Si-atom source gas silane, SiH4, was introduced down-stream, and not subjected to direct plasma excitation. Following these depositions, 0.5 pim of Al was sputtered on to these multi-layer stacks, and standard photo-lithographic and dry etching techniques were used to form the source and drain regions. Table 1 displays the deposition conditions for these films. Table 1: Deposition process conditions for TFT thin film depositions. Silicon oxide -Silicon nitride
SiH 4 .2 sccm I1sccm
Other Gas 20 sccm of N20 2.5-12.5 sccm of NH 3
Plasma Power 25 W 50W
He Dilution 00 sccm 200 scc
50 W
200 sccmr
n÷ a-Si:H
1 sccm
0.1
50 W
20 scm
a-Si:H
I sccm
non
oof PH 3
We have performed two sets of experiments to compare back side channel effects that derive from native and plasma-generated oxides, and plasma-generated oxynitrides using an N2 0 source gas: (i) transistors with different channel thickness of 500A, 1000A, 1500A and 2000A, were treated by plasma 02 and plasma N 20 process, and (ii) transistors with a channel thickness of 1000A were formed with different nitride films ranging from the so-called silicon-richto nitrogen-richcompositions. The different types of nitride films were prepared by controlling R, the gas flow rate ratio of NH 3 to SiH 4 , in a range from 2.5 to 12.5. We then compared device performance before and after an plasma N 2 0 treatment. In particular we focused on the saturation field-effect mobility and threshold voltage of TFTs, which were obtained by plotting the square root of drain current as a function of Vgs = Vds, where Vgs is the gate source bias, and Vds is the drain source bias. 3. Back Side Channel Plasma 02 and a Plasma N2 0 Treatments All of the TFTs were initially annealed in vacuum at 200'C for 15 minutes. Following this the plasma back side passivation treatments were for also performed at 200*C, but for 2 minutes. The plasma 02 treatment consisted of exposing the back side of the channel region to species generated from a 200 sccm flow of He
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