• PDF / 1,932,666 Bytes
• 12 Pages / 612 x 792 pts (letter) Page_size
• 46 Downloads / 219 Views

DOWNLOAD

REPORT

---

Laser Processing of Amorphous Silicon for Polysilicon Devices, Circuits and Flat-Panel Imagers J. B. Boyce, R. T. Fulks, J. Ho, J. P. Lu, P. Mei, R. A. Street, K. F. Van Schuylenbergh and Y. Wang Xerox Palo Alto Research Center, 3333 Coyote Hill Road, Palo Alto, CA 94304 ABSTRACT Pulsed excimer-laser processing of amorphous silicon on non-crystalline substrates allows for the fabrication of high-quality polysilicon thin-film transistors (TFTs). It also provides procedures for doping self-aligned amorphous silicon TFTs. In addition, laser-crystallized polysilicon exhibits some interesting materials properties, such as, large lateral grain growth with a corresponding enhancement in the electron mobility. Under optimized processing conditions, excellent polysilicon TFTs with high mobilities, sharp turn on, low off-state leakage currents and good spatial uniformity have been achieved. These improved parameters, particularly the low off-state leakage currents and good uniformity, enable not only displays but also the moredemanding flat-panel imaging arrays to be fabricated in polysilicon. Results on both polysilicon CMOS circuits and a polysilicon flat-panel imager are presented. INTRODUCTION Interest in polycrystalline-silicon (poly-Si) devices has been driven largely by the rapid growth of large area electronic systems, specifically, flat-panel active-matrix displays and twodimensional imagers [1,2]. Higher performance, beyond that which can be achieved by using standard hydrogenated amorphous Si (a-Si:H) technology, is required in order to get high resolution flat-panel displays and imagers, to improve the noise performance of flat-panel imagers, and to integrate peripheral circuits, which include gate line drivers, data line multiplexers, and pixel-level electronics. Poly-Si can satisfy many of these requirements due to its higher carrier mobility (about 100 cm2/V-sec versus 1 cm2/V-sec for a-Si) and the availability of good p-type polysilicon TFTs, enabling high-performance CMOS circuits. The preferred method for creating this good-quality poly-Si is excimer laser processing of amorphous silicon [3, 4]. Short wavelength, high intensity, and narrow temporal pulse width have made excimer lasers the appropriate tool for crystallizing and doping amorphous Si thin films on glass or plastic substrates. The short wavelength (e.g., 308 nm for a XeCl excimer) ensures that high intensity radiation is absorbed in a thin surface layer, about 7 nm for 308 nm. The high intensity (> 1 J/cm2) and short pulse width (20 - 200 ns) enable thin Si films to be melted and solidified rapidly before sufficient heat can be transferred to the substrate to result in thermal damage. These properties allow laser crystallization of Si thin films and laser doping to form low resistance contacts for amorphous and polycrystalline Si devices on substrates which are incapable of withstanding a sustained high temperature process. High quality poly-Si thin films and devices can and have been produced by pulsed laser crystallization on glass [3] and even