Structural Properties of Polycrystalline Silicon Films Formed by Pulsed Rapid Thermal Processing

A novel pulsed rapid thermal processing (PRTP) method has been used for realizing the solid-phase crystallization of amorphous silicon films prepared by PECVD. The microstructure and surface morphology of the crystallized films are investigated by X-ray diffraction (XRD) and atomic force microscopy (AFM). The results indicate that this PRTP is a suitable postcrystallization technique for fabricating large-area polycrystalline silicon films with good structural qualities such as large grain size, small lattice microstain and smooth surface morphology on low-cost substrate. INTRODUCTION Polycrystalline silicon (poly-Si) is a material of great importance in microelectronics, photovoltaics and display technologies [1-5]. In recent years, there is growing interest in developing practical techniques for fabricating larger-area and high-quality poly-Si films on lowcost glass substrate at a low temperature. Generally, post-crystallization of amorphous silicon is thought to be a preferred technique for this purpose [6,7]. Compared to those as-grown, the poly-Si films prepared by post-crystallization have perfect structure and smooth surface morphology that are required for critical applications. Several post-crystallization methods have been reported. Among them, zone melt recrystallization (ZMR) can produce large-grain poly-Si films [8], but the very high process temperature (>1000 'C) makes the use of low-cost substrate actually impossible; Conventional furnace annealing requires several tens of hours at the temperature around 600 TC (softening point of glass) [9,10], which means high thermal budget and low throughput, diseconomic for large-scale production; Rapid thermal processing (RTP) can greatly shorten the process time [11,12], but due to the continuous thermal exposure to high temperature, the damage to the glass substrate is inevitable; Although laser beam annealing can realize the crystallization at lower temperature [13,14], it is very expensive and also needs to take a long time to scan over a large area; Post-crystallization using a metal as seed can also be carried out at the temperature lower than 600 C [ 15-17], but the contamination of metal impurity to the Si film is induced. So developing a new effective method to prepare poly-Si films on lowcost substrate is still an attractive and challenging target. Currently, we have presented an improved rapid thermal processing method---pulsed rapid thermal processing (PRTP), which involves a number of cycles of thermal process. In each cycle, a 60-s thermal bias of 550 TC is used for preheating the films to a "critical state" of crystallization, so that the rapid nucleation and grain growth can be triggered by a following 1-s high-temperature thermal pulse of 850 TC. By using this annealing method, solid-phase crystallization in amorphous silicon fihns can be completed in several minutes, without need for a metal seed. Moreover, the thermal bias temperature of 550 TC is well below the softening point of Corning 7059 glass, and 975 Mat. Res. Soc. Symp. Proc. Vol. 507 © 1998 Materials Research Society the samples are exposed to the high temperature pulse of 850TC for a very short time (only l-s) in each cycle, thus the thermal damage to the glass substrate can be effectively minimized. Many of the electrical properties of the poly-Si depend upon its structural quality since carrier transport is sensitive to the lattice imperfection. Perfect structure and smooth surface are desirable for device-quality poly-Si films. In this paper, using X-ray diffraction and atomic force microscopy, we investigate the microstructure and surface morphology of the poly-Si films obtained by PRTP. The structural properties of the poly-Si films are characterized in terms of crystalline orientation, grain size, lattice microstrain and surface roughness. The effect of the deposition parameters of the precursor amorphous silicon on the structural qualities of the crystallized films is also discussed. EXPERIMENT The precursor amorphous silicon films for post-crystallization were prepared by the plasma enhanced chemical vapor deposition (PECVD) method at different deposition rates by changing the ratio of gas flow (SiH-4/H 2). Typical deposition parameters are summarized in Table 1. The substrates are Coming 7059 glass and fused quartz. The thicknesses of these films are determined by the measurements of the optical transmission spectra, and the deposition rates are calculated by dividing film thicknesses by the deposition time. The crystallization by PRTP was carried out in a KST-2 rapid thermal processor with thirteen tungsten halogen lamps as heat source and N2/Ar as ambient atmosphere. The heating rate is about 300 °C/s. The samples were placed on a silicon wafer holder, on which a Pt-PtRh thermocouple was set beside the samples, used for measuring and controlling process temperature. The time-temperature pattern of the PRTP was programmed, as shown in Figure 1. Table. I Deposition parameters of amorphous silicon films Sample T,(°C) H2/SiH 4 Time(hour) Thickness(ýtm) rd (A/s) sl 400 10 4 0.932 0.647 s2 400 8 4 1.213 0.842 s3 400 5 4 1.435 0.995 S4 400 3 4 1.773 1.231