2-2-5 Surface free energy
3.3 OTFTs with patterned pentacene
A patterning method for pentacene film is proposed. This method was realized by self-assembled layer (SAM) and ultra-violet (UV) light post-exposure. The surface characteristic on the gate dielectric is controlled. The difference of the interfacial binding energy between the pentacene film and the dielectric enables the pentacene film to be removed selectively. The remaining pentacene film was fabricated as a top-contact OTFT to confirm the practicability of this experiment.
Figure 3.6 (a) showed the AFM images of the pentacene deposited on the non-UV-exposed and on 30-mins UV-exposed regions. The latter had larger grain size than the former; this agrees with the results in Ref 70 that pentacene formed larger grain on the surface with higher surface energy. The difference of the surface energy also leaded to different results after the water dipping. As shown by the AFM image in Figure 3.6 (b), after dipping, pentacene film on the non-UV-exposed area was almost unchanged while it on the 30-mins UV-exposed area was removed. The step of this two area was about 100 nm, similar to the thickness of the pentacene film. This implied that the pentacene film was almost completely removed.
An attempt was made to characterize the adhesive properties between the pentacene film and the substrate surface to study the patterning mechanism. The adhesion energy between
2( p p d d)
A pe s pe s
E = γ γ + γ γ ( 3.4 )
where EA was the adhesion energy between pentacene and substrate before water dipping;
pep
γ and γ were the polar component and the dispersion component of the surface energy dpe
for pentacene; γspand γsd were the polar component and the disperse component of the surface energy for the substrate. As shown in Figure 3.7, the adhesion energy before water dipping was slightly increased after UV-light exposure. This was an interesting result since increased adhesion energy could not explain the water-removable property.
Therefore, we calculated and compared the intrusion energy EI caused by the interaction between water, pentacene and the dielectric surface. This intrusion energy would cause a change of adhesion energy by:
E
A'= E
A− E
I ( 3.5 ) where the EA’ was the adhesion energy after water dipping.The intrusion energy could be calculated by the method of D.H. Kaelble. The calculated intrusion energy EI and the adhesion energy after water dipping EA’ were also depicted in Fig.
3.7. The EI increased drastically after UV light exposure, as a result, the EA’ decreased to be less than zero after UV exposure. Specifically, the EA’ of non-UV-exposed region was 58 mJ/m2 and that of 30-mins UV-exposed region was -38 mJ/m2. This explained the experimental results in Figure 3.6 (b). Pentacene film on non-UV-exposed area was almost unaffected due to the large EA’; pentacene film on 30-mins UV exposed area was removed by
water dipping due to the negative EA’. Also, the large difference of the EA’ between non-UV-exposed area and UV-exposed area explained the capability to successfully pattern the pentacene film.
Finally, the electric characteristics were demonstrated to confirm that the patterning method was feasible for the OTFTs. Figure 3.8 (a) compared the transfer characteristics of OTFTs under different conditions. The performance of ODMS-treated OTFTs was greatly improved when compared with conventional devices without ODMS treatment. Next, when comparing pentacene patterning - OTFTs and the ODMS-treated OTFTs, the former showed a bit right-hand side shift characteristics and a higher minimum off state current. This influence of some residual water molecules might be the plausible reason. However, After pentacene patterning, performance of ODMS treated OTFTs nearly all still maintain good electric characteristics.The mobility as 0.264 cm2/V.s and Ion/Ioff ratio higher than 6 orders were still obtained.
Typical parameters such as mobility, threshold voltage, on/off current ratio and sub-threshold swing of these devices were listed in Table Ⅳ. The output characteristics of pentacene patterning - OTFTs was also depicted in Figure 3.8 (b). No hysteresis was observed when the drain voltage was scanned from 0 (V) to -30 (V) and then from -30 (V) to 0 (V).
Chapter 4 Conclusion
A novel technology for patterning pentacene-OTFTs was proposed. ODMS-SAM and the following UV-light exposure were used to modulate the surface energy of the dielectric. The SiO2 gate dielectric with ODMS treatment exhibited low surface energy as 41mJ/m2. The surface energy increased drastically when the ODMS-treated dielectric surface was irradiated by UV light. After 30-mins UV exposure, the surface energy increased to be 155mJ/cm2.
The pentacene film was therefore controlled and patterned by the difference of the dielectric surface energy. The mobility, the threshold voltage, the on/off current ratio and the sub-threshold swing were better than those of the control sample, even following solution process. Technology that combines patterning and surface treatment may be applied to future OTFTs arrays.
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Table I Comparisons of TFTs using different materials.
Amorphous Si Poly-Si Organic Status Mature Development Research TFT type N-TFT N-TFT or P-TFT P-TFT or N-TFT Mobility (cm2/Vs) 0.1-1.0 50-200 0.005-3 Uniformity Good Poor Unknown Stability Poor Good Unknown Cost Low High Very low Ion/Ioff >106 >106 103-108 Size and voltage to
drive 10μA W=92μm W=10μm W=181μm (Gate dielectric is (VGS-VTH)=7V (VGS-VTH)=1.5V (VGS-VTH)=25V 300nm and channel
length is 5μm) (W=channel width)