• PDF / 3,961,868 Bytes
• 6 Pages / 414.72 x 648 pts Page_size
• 39 Downloads / 182 Views

DOWNLOAD

REPORT

---

STRESS-CORROSION CRACKING AND BLISTERING OF THIN POLYCRYSTALLINE SILICON FILMS IN HYDROFLUORIC ACID D.J. MONK', P. KRULEVITCHt, R.T. HOWE*, and G.C. JOHNSONt Berkeley Sensor & Actuator Center, University of California, Berkeley, CA 94720 *Department of Chemical Engineering tDepartment of Mechanical Engineering tDepartment of Electrical Engineering and Computer Sciences ABSTRACT Thin (0.1 pm) LPCVD polycrystalline silicon (poly-Si) films are shown to exhibit permeability and suffer structural degradation when exposed to concentrated hydrofluoric acid. Analysis by TEM and SEM reveals two attack mechanisms. (1) Poly-Si films deposited onto phosphosiicate glass (PSG) at 6050C exhibit tensile residual stress and degrade immediately upon exposure to HF, a phenomenon which we attribute to stress-corrosion cracking. Phosphorus from the PSG layer enters the poly-Si during the deposition, resulting in a microstructural gradient which contributes to the cracking mechanism. (2) As-deposited tensile poly-Si films on phosphorus-free LPCVD SiO 2 (LTO) and annealed films on LTO and PSG blister at different rates in HF due to penetration at foreign particle inclusion sites and other film defects. Unannealed compressive films deposited at 650'C onto PSG do not show any evidence of attack. INTRODUCTION Silicon based micro-sensors and actuators, made using thin film and photolithographic techniques developed for integrated circuit technology, are gradually advancing from academic investigations to practical applications. Because the field is relatively new, a great deal remains to be learned about the effects of processing techniques on device characteristics, including structural integrity. One fabrication method, known as surface micromachining, employs a series of steps including thin film deposition, photolithographic patterning combined with plasma etching, and wet chemical sacrificial layer removal to produce structures with free-standing or partially freed members [1]. Polycrystalline silicon (poly-Si), the most commonly used structural thin film, is often deposited on a sacrificial layer of silicon dioxide, which is subsequently etched away using hydrofluoric acid (HF) [2]. Adding phosphorus to the SiO 2 to produce phosphosilicate glass (PSG) enhances the etch rate [2,3] and also serves as a diffusion doping source to make the poly-Si layers electrically conductive and to control residual stresses. The HF etch rate selectivity of PSG to poly-Si generally is regarded as very high; however degradation of poly-Si in HF has been reported by Lober and Howe [4] and Walker et al. [5], although the cause was not investigated. In this paper we present evidence, based on analysis by transmission and scanning electron microscopy (TEM and SEM), of stress-corrosion cracking and blistering in thin (0. 1pm) poly-Si films exposed to HF. EXPERIMENTAL PROCEDURES Undoped poly-Si films 0.12-0.13 pm thick were deposited onto Si0 2 -coated 1100) oriented single crystal silicon substrates by low pressure chemical vapor deposition (LPCVD) using pure