Lithographic process on glass and plastic substrates

I developed several processes for building ZnO thin-film transistors (TFTs) on glass, freestanding polyimide, and spin-cast, ultra thin polyimide. A schematic cross section of the general device structure is shown below. 

ZnO TFTs on ultra-thin, spin-cast polyimide substrate

Substrate in picture below is just 3.5 microns thick. Bending radius is < 500 microns, but TFTs experience so little strain they remain electrically unaffected.

In-situ passivated, self-aligned ZnO TFTs

Self-aligned TFTs can improve circuits by reducing parasitic capacitances from gate-to-source/drain overlaps. In this process, I took advantage of the transparent nature of the oxide films I use to perform a simple self-alignment trick: backside exposure-- in which the opaque metal of the gate acts as a mask for photoresist, allowing an overlap from gate to source/drain of ~0.6nm. 

Material Characterization

During initial process development, I made use of a range of imaging and analysis tools to understand properties of my oxide films. In particular, I've relied on SEM, XRD, ellipsometry, reflectometry, and profilometry. 

Electrical Characterization

I have characterized thousands of TFTs over the years -- performing current-voltage and capacitance-voltage sweeps and extracting all relevant parameters, but also performing high-frequency measurements of current and power gain to extract fT and fMAX. I used these parameters as design tools, changing device layouts and fabrication steps as needed to boost high-frequency performance. 

Mechanical Characterization

It is important to assess bending limits of flexible TFTs. I subjected TFTs on polyimide substrates to compressive and tensile bending tests, and determined at what bending strain the TFTs failed.