Stress Time
0 2 4 6 8 10
Fig. 3-15
The threshold voltage shift under initial flat status and tensile strain Rp = 10mm, 30mm, and 50mm.
0 2000 4000 6000 8000 10000 12000
Vth Shift
Flat Initial R=50mm R=30mm R=10mm
Fig. 3-16
The transfer curves show the comparison of the ID-VG figure
0 2 4 6 8 10
Fig. 3-17
The VTH shift as a function of stress time under flat-tensile strain-reflatten conditions with and without SiNx passivation.
Stress Time
Vth Shift
Flat Tensile Reflatten Flat(SiNx) Teneile(SiNx) Reflatten(SiNx)
0 2000 4000 6000 8000 10000 12000
0 2 4 6 8 10
Stress Time
Vth Shift
Flat (SiNx)
Compressive (SiNx) Reflatten (SiNx) Flat
Compressive Reflatten
0 2000 4000 6000 8000 10000 12000
Fig. 3-19
The threshold voltage shift under three different conditions, including flat, tensile, and compressive status comparing the sample with and without post-annealing process.
Stress Time
0 2000 4000 6000 8000 10000 12000
Vth Shift
0 2 4 6 8 10
Flat (SiNx) Tensile(SiNx) Compressive(SiNx) Flat(SiNx_190c_60min) Tensile(SiNx_190c_60min) Compressive(SiNx_190c_60min)
Chapter 4 Conclusions
The reliability of mechanically strained a-Si:H TFTs was studied in this work. We start from proposing a flexible a-Si TFT, which is inverted staggered structure with back channel etching method, on stainless steel foil under 190℃ process temperature. The strain stress was applied cylindrically on TFTs and parallel to the active channel path of it.
Experimental results indicated the influence of mechanical strain was permanent, which can obviously impact the threshold voltage, sub-threshold swing and mobility of TFTs. We could find the variation of threshold voltage shift (△Vth) in the ID-VG results. When the sample was bent at the first time, a-Si:H material would suffer from a mechanical stress, and was easy to break the weak Si-Si bond in it. The phenomenon of △Vth was related to the creation of dangling bonds in acceptor-like state which originated from the broken of weak bonds in donor-like state.
That explained the permanent strain effect on device, and had seldom Vth variation under bending strain afterward. The stability measurement was performed by DC gate bias stress and lasted up to 104 seconds By
more degradation on flexible a-Si TFT than tension.
Via extracting the parasitical resistance and activation energy, we have some probing conclusions about the change of electrical performance when flexible a-Si TFT is applied bending force. The mechanism of strained degradation is independent of parasitical resistance. And then the activation energy analysis supports the conclusions we made for variation of the mobility and threshold voltage under mechanical strain.
In this thesis, we also have investigated the electrical stability of flexible a-Si TFT under mechanical strain with and without silicon nitride passivating layer. The process temperature of flexible a-Si:H TFTs, including the passivating silicon nitride layer, was well-controlled below 200 . The bias stress result indicated the a℃ -Si:H TFTs with passivation layer was improved, and the threshold voltage (Vth) have less variation under both outward and inward bending. By following the industrial process, we performed 190 post℃ -annealing process while the TFTs were completely fabricated. The Vth was shifted left and the reliability of flexible TFTs become better than that without post-annealing process.
That's related to the passivating effect of hydrogen ion under passivation layer and post-annealing process.
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