Lpcvd Polycrystalline Silicon Thin Films: The Evolution of Structure, Texture and Stress

An investigation of undoped LPCVD polycrystalline silicon films deposited at temperatures ranging from 605 to 700 C and silane pressures from 300 to 550 mTorr revealed large variations in stress with processing conditions and a correlation between stress and texture. TEM and HRTEM analysis show that morphology differences also exist. At lower temperatures (≈605 C) and higher pressures (≈400 mTorr), the films appear to deposit in an amorphous state and crystallize during the deposition to form microstructures characterized by equi-axed grains, tensile residual stress, and a texture with {110} and {11/} (/=2 or 3) components. Higher temperatures (between 620 and 650 C) produce films that nucleate at the Si02 interface, creating a {110} oriented columnar microstructure. At 700 C, the grains are still columnar, but the dominant texture is {100}. Films deposited at temperatures greater than 620 C exhibit compressive stress, and some contain regions of hexagonal silicon. This paper proposes possible causes of the varying stresses, textures, and microstructures in the films.

Stress and Microstructure in Lpcvd Polycrystalline Silicon Films: Experimental Results and Closed Form Modeling of Stresses

Characterization of undoped polycrystalline silicon films indicates that correlations exist between stress and microstructure. Films of thickness between 0.5–3.6 μm were deposited onto SiO2-covered single crystal silicon wafers between 605 and 700°C using low pressure chemical vapor deposition (LPCVD). The average in-plane film stress and the stress gradient through the film thickness were determined from wafer curvature measurements, and film microstructure was studied with cross-sectional TEM. Films deposited near 605°C exhibit overall tensile stresses that result from an amorphous to crystalline phase change. At deposition temperatures exceeding 630°C, a columnar grain structure evolves out of a transition region of small grains at the SiO2 interface. The columnar films are compressive, with the source of compression linked to the region of small grains. Stress is modeled using a closed form solution ihat considers a linearly elastic contracting ellipsoidal inclusion near the surface of a half space. Several applications of the stress model are discussed.

Investigation of Texture and Stress in Undoped Polysilicon Films

Undoped LPCVD polysilicon films deposited on thermal oxide and prepared under various process conditions have been investigated for their texture and stress characteristics. Pole figures measured by X-ray diffraction in reflection geometry were used to determine the orientation. distribution function which provides a quantitative description of the texture. Stresses were determined using wafer curvature measurements. Both texture and stress show substantial variation with deposition condition. Textures typically exhibit axial symmetry and appear to be correlated with the sign and magnitude of the stress. Under certain deposition conditions, the stress varies considerably along the tube, with the stress in the wafers at the upstream end being tensile and at the downstream end being compressive.

Stress and Microstructure in Phosphorus Doped Polycrystalline Silicon

The research presented here is an investigation of the effects of phosphorus doping on residual stresses and microstructure in polycrystalline silicon. Undoped polycrystalline silicon films were deposited on phosphosilicate glass layers and annealed at 1050 °C for 5 to 60 minutes. The stress gradient through the film thickness was measured from wafer curvature, and microstructure was examined with cross-sectional TEM. A 20 minute anneal is sufficient for stress relief in initially tensile films and produces a uniform microstructure consisting of 0.1-0.2 µm equi-axed grains.

Stress-Corrosion Cracking and Blistering of Thin Polycrystalline Silicon Films in Hydrofluoric Acid

Thin (0.1 μm) 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 phosphosilicate glass (PSG) at 605ºC 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 SiO2 (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 650ºC onto PSG do not show any evidence of attack.

Curvature of A Cantilever Beam Subjected to An Equi-Biaxial Bending Moment

Results from a finite element analysis of a cantilever beam subjected to an equi-biaxial bending moment demonstrate that the biaxial modulus E/(1-v) must be used even for narrow beams.

Microdevices in medicine

The application of microelectromechanical systems (MEMS) to medicine is described. Three types of biomedical devices are considered, including diagnostic microsystems, surgical microsystems, and therapeutic microsystems. The opportunities of MEMS miniaturization in these emerging disciplines are considered, with emphasis placed on the importance of the technology in providing a better outcome for the patient and a lower overall health care cost. Several case examples in each of these areas are described. Key aspects of MEMS technology as it is applied to these three areas are described, along with some of the fabrication challenges.

Development of fiber optic and electrochemical pH sensors to monitor brain tissue

This article describes fiber optic and electrochemical pH sensors that could become part of a therapeutic arsenal to quickly and aggressively treat stroke victims as well as people who have suffered brain trauma. The fiber optic sensor design was based on the immobilization of a pH-sensitive dye, seminaphthorhodamine-1 carboxylate (SNARF-1C), onto the end of a 125-microm-diameter silica optical fiber using the sol-gel method. A miniature bench-top fluorimeter system was developed for use with the optical fiber to obtain pH measurements. The electrochemical sensor was based on sputter-coated iridium oxide thin films. Linear and reproducible responses for both sensors were obtained in human blood with pH varying between 6.8 ato 8.0, which encompasses the clinically relevant range. In vivo studies were also performed and results indicated that both types of sensors tracked pH with very little drift.