Advanced processes and characterization of amorphous silicon thin film transistors.

摘要

In this work, an advanced thin film optical characterization technique is first developed to overcome the distortion in conventional spectra. This technique utilizes a Brewster angle incidence of p-polarized probe beam to achieve an interference fringe-free characterization. For vibrational absorption spectroscopy, an ideal spectrum is obtained if a straight and horizontal baseline is observed. While for optical transmission spectroscopy, the optimal spectrum is indicated by the matching of optical bandgaps defined at absorption coefficient of 104 and defined by the linear extrapolation.; To increase the throughput of TFT manufacturing, the a-Si:H plasma-enhanced chemical vapor deposition (PECVD) rate is enhanced by the process optimizations at both standard (13.56 MHz) and high frequencies (40 MHz) excitations. It is shown in this study that a-Si:H deposition rate was successfully increased from conventional 15 run/min to as high as 375 nm/min with an acceptable device characteristics. The increase alone should result in more than 20 times reduction of TFT process time.; The evaluation of material properties indicates a general adverse trend of a-Si:H quality to its deposition rate. In particular, the mono-hydride density decreases with the increase of deposition rate. Consequently, in a-Si:H, the disorder is increased, and flattening of valence and conduction bandtails is resulted. For example, the Urbach edge of a-Si:H changes from 50 ± 5 to 87 ± 5 meV when the deposition rate varies from 14 to 120 nm/min.; The a-Si:H deposition rate influence on TFTs' electrical performance is mainly manifested in TFT's electron field-effect mobility and threshold voltage, which generally decreases and increases with the a-Si:H deposition rate, respectively. The field-effect mobilities and threshold voltage in saturation region of TFT with W/L ratio of 80/100 change from the 1.14 ± 0.07 to 0.27 ± 0.02 cm2/Vs and from 3.1 ± 0.2 to 6.1 ± 0.2 V, respectively, while a-Si:H deposition rate varies from 14 to 120 nm/min.; On the other hand, TFT's long-term instability in terms of threshold voltage shift does not show significant a-Si:H deposition rate dependence. It is suggested in this study that TFT's instability be mainly affected by the quality of a-SiN_x:H and the interface of a-Si:H and a-SiN_x:H, while it is not sensitive to a-Si:H bulk quality.