Laser Dehydrogenation of PECVD Amorphous Silicon

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LASER DEHYDROGENATION OF PECVD AMORPHOUS SILICON PING MEI, J. B. BOYCE, M. HACK, R. A. LUJAN, R. I. JOHNSON, G. B. ANDERSON, D. K. FORK, S. E. READY, AND D. L. SMITH XEROX PALO ALTO RESEARCH CENTER, 3333 Coyote Hill Road, Palo Alto, CA 94304 ABSTRACT A low temperature process for laser dehydrogenation and crystallization of hydrogenated amorphous silicon has been studied. The key feature of this process is the removal of hydrogen from the amorphous silicon thin films while crystallizin the films at the same time. Studies of transient phenomena, hydrogen loss, and crystallinity, using transient reflectivity analyses, transmission electron microscopy and quadrupole mass spectrometry, find that hydrogen out-diffusion depends strongly on film structure and the melt duration controlled by the laser energy density. Utilizing this process, for which the maximum temperature is 350 °C, both high quality polycrystalline and amorphous silicon TFT's have been fabricated on the same Corning 7059 glass substrate. Introduction Hydrogen removal is important in the laser crystallization of amorphous silicon thin films deposited by plasma enhanced chemical vapor deposition (PECVD). The PECVD deposition temperature for amorphous silicon is about 250 TC. The low temperature deposition is a great advantage for the fabrication of devices on low temperature substrates such as Corning 7059 glass. A complication, however, is that due to the low substrate temperature, the amorphous silicon deposited by PECVD contains large amounts of hydrogen (- 10 atomic %) [1]. During laser crystallization, these films ablate due to explosive hydrogen evolution at laser energy densities below the level necessary to crystallize an entire film which is thicker than 50 nm [2]. In order to minimize this explosive hydrogen evolution, a

450 'C conventional furnace anneal is normally applied to the PECVD films for several hours prior to laser crystallization which removes most of the hydrogen [3]. This therefore raises the maximum process temperature to 450 C . In addition, the thermal hydrogen removal process makes it very difficult to hybridize amorphous silicon and polysilicon devices on the same chip due to hydrogen loss in the amorphous silicon devices at 450 'C [4]. Also, there are reports that dehydrogenated a-Si reacts with oxygen and other contaminants, causing degradation of the electrical properties of the crystallized films [5]. In this paper, we report our investigation of a laser hydrogen removal process for PECVD amorphous silicon. The advantages of this method are three fold. First, it reduces the maximum device fabrication temperature to the temperature for depositing amorphous silicon and dielectric films. Second, the laser removal of hydrogen can be performed on selective areas on an amorphous silicon film, which is a simple process to produce hybrid amorphous silicon and polysilicon on the same chip. Third, the hydrogen removal and crystallization are performed in the same high vacuum chamber under pulsed laser irradiations which limits cont