Date of Award
Doctor of Philosophy
Materials Science and Engineering
Philip D. Rack
Michael L. Simpson, David C. Joy, Thomas T. Meek
Materials and process integration of a thin film transistor array for intra/extracellular probing are described in this study. A combinatorial rf magnetron sputter deposition technique was employed to investigate the electrical characteristics and micro-structural properties of molybdenum tungsten (MoW) high temperature electrodes as a function of the binary composition. In addition to the composition, the effect of substrate bias and temperature was investigated. The electrical resistivity of MoW samples deposited at room temperature with zero bias followed the typical Nordheim’s rule as a function of composition. The resistivity of samples deposited with substrate bias is uniformly lower and obeyed the rule of mixtures as a function of composition. The metastable β-W phase was not observed in the biased films even when deposited at room temperature. High resolution scanning electron microscopy revealed a more dense structure for the biased films, which correlated to the significantly lower film resistivity.
In order to overcome deficiencies in sputtered silicon dioxide (SiO2) films the rf magnetron sputtering process was optimized by using a full factorial design of experiment (DOE). The optimized SiO2 film has a 5.7 MV/cm breakdown field and a 6.2 nm/min deposition rate at 10 W/cm2 RF power, 3 mTorr pressure, 300 °C substrate temperature, and 56 V substrate bias. Thin film transistors (TFTs) were also fabricated and characterized to show the prospective applications of the optimized SiO2 films.
The effect that direct current (DC) substrate bias has on radio frequency (RF)-sputter-deposited amorphous silicon (a-Si) films was also investigated. The substrate bias produces a denser a-Si film with fewer defects compared to unbiased films. The reduced number of defects results in a higher resistivity because defect-mediated conduction paths are reduced. Thin film transistors (TFT) that were completely sputter-deposited were fabricated and characterized. The TFT with the biased a-Si film showed lower leakage (off-state) current, higher on/off current ratio, and higher transconductance (field effect mobility) than the TFT with the unbiased a-Si film.
The crystallization properties of amorphous silicon (a-Si) thin film deposited by rf magnetron sputter deposition with substrate bias have been thoroughly characterized. The crystallization speed can be increased and the crystallization temperature can be drastically lowered relative to unbiased a-Si even though the stress state of biased a-Si film is highly compressive. The substrate bias enhances defect formation (vacancies, dislocations, stacking faults) via ion bombardment during the film growth, which effectively increases the driving force for crystallization of the films.
The electrical and optical properties of sputter-deposited silicon nitride (SiNx) and n+ amorphous silicon (n+ a-Si) films as a function of substrate bias during sputter deposition were investigated. The breakdown voltage of sputter-deposited SiNx with 20 W (125 V) substrate bias is 7.65 MV/cm which is equivalent to that of plasma enhanced chemical vapor deposition (PECVD) SiNx films. The conductivity of n+ a-Si films are also enhanced by applying substrate bias during the sputter deposition. To verify the effect of substrate bias, amorphous silicon thin film transistors (TFTs) were fabricated with substrate biased thin films and compared their electrical properties with conventional sputter deposited TFTs.
Lastly, electrochemical measurements were analyzed using gold and pyrrole solution to verify the active addressability of the TFT array fabricated by entirely by sputter deposited thin films below 200 °C temperature.
Jun, Seung-Ik, "Materials Integration and Device Fabrication of Active Matrix Thin Film Transistor Arrays for Intracellular Gene Delivery. " PhD diss., University of Tennessee, 2006.