Selective W for Coating and Releasing MEMS Devices

Abstract

Nation~Laborato~es, Albuquerque, NM 8'7 185-m$4~~F=g $p~,s smani @sandia.gov JAlll~1; $$;; ' ABSTRACT cil~~jTwo major problems associated with Si-based MEMS (MicroElectro~echanical Systems) " devices are stiction and wear. Surface modifications are needed to reduce both adhesion and friction in rnicromechanical structures to solve these problems. In this paper, we will present a CVD (Chemical Vapor Deposition) process that selectively coats MEMS devices with tungsten and significantly enhances device durability. Tungsten CVD is used in the integrated-circuit industry, which makes this approach manufacturable. This selective deposition process results in a very conforrnal coating and can potentially address both stiction and wear problems confronting MEMS processing. The selective deposition of tungsten is accomplished through the silicon reduction of Wl%. The self-limiting nature of this selective W deposition process ensures the consistency necessary for process-control. The tungsten is deposited after the removal of the sacrificial oxides to minimize stress and process integration problems. Tungsten coating adheres well and is hard and conducting, requirements for device performance. Furthermore, since the deposited tungsten infiltrates-under adhered silicon parts and the volume of W deposited is less than the amount of Si consumed, it appears to be possible to release stuck parts that are contacted over small areas such as dimples. The wear resistance of selectively coated W parts has been shown to be significantly improved on microengine test structures.

Mechanical properties and tribological behavior at micro and macro-scale of WC/WCN/W hierarchical multilayer coatings

“…Tungsten based hard coatings have many interesting applications such as space nuclear power, propulsion for heat pipes, nuclear fuel cladding and radiation shields [11], and in semiconductor substrates for microelectronics [12]. In wear applications; tungsten is used for protection of bores of large caliber tank-guns [13] and in microelectromechanical systems (MEMS), tungsten coatings are used to improve their wear resistance [12].…”

Molecular dynamics simulations of friction between alkylsilane monolayers

“…Several studies have been conducted to develop solid and liquid lubricants and hard films to minimize friction and wear in MEMS. The use of tungsten films as wear resistant coatings for silicon based MEMS has been investigated by Mani et al [5,6]. Self-assembled monolayer films of alkylsilanes have also been investigated as lubricants on smooth silicon surfaces [7][8][9].…”

Effect of W coating on microengine performance

“…The reduced strength does not effect the device performance and yields extended lifetime parts [21,22].…”

“…The presence of defects can cause a drop in the modulus from that predicted from the elastic constants, however the volume fraction of such flaws has to be a reco=mizable percentage of the sample volume. Considering that this material has a rlacture toughness on the order of 1-3 MPadm, flaws approximately 5-20 nm in size are responsible for initiating fracture [21,23], which is a very small percentage of the cross section. Efforts to measure the fracture toughness of polysilicon produced at Sandia arc undenvay.…”