Mechanical Properties of Boron Doped Si and Si/SiO 2 Membranes
- PDF / 900,828 Bytes
- 6 Pages / 612 x 792 pts (letter) Page_size
- 102 Downloads / 216 Views
Mechanical Properties of Boron Doped Si and Si/SiO2 Membranes Gabe Kuhn, Todd Myers, Susmita Bose and Amit Bandyopadhyay, School of Mechanical and Materials Engineering Washington State University, Pullman, WA 99164-2920. Abstract In our research, PZT film actuated micro-machined Si substrates are being developed for numerous applications in which membranes are actuated primarily in flexural mode. Silicon wafers, 3-inches in diameter, underwent boron doping in order to act as an etch stop. Approximately 200-nm of SiO2 was grown on the boron-doped side of the wafers. Photolithography and backside etching using EDP resulted in 2-µm thick membranes. Using reactive ion etching (RIE), beam structures resulted from the membranes. Nano-mechanical testing of the beams indicated that there were substantial residual tensile stresses in these structures. Initial calculations reveal a tensile stress of 57.7 MPa in the Si/SiO2 beams. The residual tensile stress subsequently caused the overall beam stiffness to be two orders of magnitude higher than it would be without stress. After stripping the oxide with a buffered oxide etchant (BOE), a residual stress of 26.5 MPa was measured, which is presumably caused from the remaining boron concentration. The aim of this paper is to understand influences of boron doping and processing variables on residual stresses. Introduction Mechanical properties of materials used in microelectromechanical systems (MEMS) are highly influenced by their processing steps. Residual stresses arising from processing can lead to performance critical problems. Compressive residual stresses may cause delamination, while high levels of tensile stresses lead to cracking of the films [1]. In the case of Si/SiO2 membranes of a few microns thick, compressive residual stresses lead to buckling. There have been a number of studies relating to residual stress of SiO2 films deposited with electron-gun evaporation [1], ion-assisted deposition (IAD) [2], and various forms of chemical vapor deposition (CVD) [3]. Earlier studies have tested the residual stress in thermally grown SiO2 as well [4,5]. The residual stress in silicon films have also been studied [6,7]. Various beam-bending tests have been discussed in the literature [8]. Beam-bending offers ease of testing of a micro-specimen as an advantage compared to direct tensile tests. On the other hand, the analysis of the experimental data depends greatly on theoretical models, each with certain assumptions. The total residual stress (σr) between substrate and film are the sum of three components: σi for intrinsic stress due to film formation, σth due to thermal expansion mismatch, and σw due to water absorption [1]. Values of σth are easily calculated with the following equation [1,3]:
E ba 1 −ν f
σ th =
α
(
f
− α s )∆T
(1)
where Eba is the film’s biaxial modulus, νf is poisson’s ratio of the film, αf and αs are the coefficient of thermal expansion of the film and the substrate, respectively, and ∆T is the change U3.12.1 Downloaded from https://w
Data Loading...
