Effect of W coating on microengine performance

Abstract

Two major problems associated with Si-based MEMS (MicroE1ectroMechanical 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 conformal 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 WFc. The self-limiting nature of the process ensures consistent process control. The tungsten is deposited after the removal of the sacrificial oxides to minimize stress and process integration problems. The tungsten coating adheres well and is hard and conducting, which enhances performance for numerous devices. 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 adhered parts that are contacted over small areas such as dimples. The wear resistance of tungsten coated parts has been shown to be significantly improved by rnicroengine test structures.

Nanotribology and nanomechanics of MEMS/NEMS and BioMEMS/BioNEMS materials and devices

“…Humidity was shown to be a strong factor in the wear of rubbing surfaces. In order to improve the wear characteristics of rubbing surfaces, 20-nm thick tungsten (W) coating deposited at 450°C using chemical vapor deposition (CVD) technique was used [75]. Tungsten-coated microengines tested for reliabil- ity showed improved wear characteristics with longer lifetimes than polysilicon microengines.…”

Performance results of MEMS coated with a conformal DLC

“…Hard, low friction coatings offer the potential of extended life. A few hard coating depositions have also been performed directly on MEMS such as tungsten by chemical vapor deposition (CVD) [30,31], alumina by atomic layer deposition [32], silicon carbide by CVD [33,34], and a Teflon coating using a plasma reactor [35]. One of the difficulties with hard coating depositions is that they are usually line of sight techniques.…”

“…Two groups have shown the ability to coat the underside structures of MEMS with a hard coating. Mani et al [30,36] used a CVD process to perform a selective deposition of tungsten through the silicon reduction of WF 6 at 450 • C. This coating process is self limiting and very conformal [30,31]. The coating was shown to improve MEMS lifetime by reducing wear.…”

Atomistic simulations of the formation and destruction of nanoindentation contacts in tungsten

“…Tungsten was chosen as representative for a bcc metal not only because of its elastic isotropy but also because it is the most important probe tip material and because its use as wear improving coating in microelectromechanical system ͑MEMS͒. 26 The outline of the present paper is as follows. Section II gives details about the simulation method.…”