Processing and morphology of permeable polycrystalline silicon thin films
- PDF / 413,771 Bytes
- 8 Pages / 612 x 792 pts (letter) Page_size
- 71 Downloads / 249 Views
A.P. Pisano Department of Mechanical Engineering, University of California, Berkeley, California, 94720
T. Sands Department of Materials Science and Engineering, University of California, Berkeley, California, 94720 (Received 4 March 2002; accepted 30 May 2002)
It is known that thin films of polycrystalline silicon, deposited under the right conditions, can be permeable to HF-based etching solutions. While these films offer unique capabilities for microfabrication, both the poor reproducibility of the permeable film properties and the lack of a detailed physical understanding of the material have limited their application. This work provides a methodical study of the relationship between process, microstructure, and properties of permeable polycrystalline silicon thin films. It is shown that the permeability is a result of small pores, on the order of 10 nm, between the 100–200-nm hemispherical grains characteristic of the permeable film morphology. This morphology occurs only in nearly stress-free films grown in a narrow temperature range corresponding to the transition between tensile and compressive film growth regimes. This result strongly suggests that the monitoring of residual film stress can provide the process control needed to reliably produce permeable films. A simple kinetic model is proposed to explain the evolution of the morphology of the permeable films. I. INTRODUCTION
Developers of microelectromechanical systems (MEMS) often wish to produce device designs that require the fabrication of completely closed hollow structures. These hollow structures are difficult to realize using traditional micromachining processes, because to create a hollow cavity by the removal of a sacrificial layer, access holes must be left by which etchant can reach the sacrificial material. If these holes are not to compromise the structure, they must be placed far apart, making it necessary to etch for a long time to remove the sacrificial material. For structures such as sealed vacuum cavities and immersible ultrasonic transducers, these etch holes must be sealed by a subsequent material deposition. This requirement precludes the use of standard etch windows, instead requiring the fabrication of very thin slits that can be more readily sealed.1–3 This complicates the fabrication process and places restrictions on the device design. The sealing deposition itself can deposit unwanted material inside the hollow spaces, degrading internal structures.3 It was first reported in 1993 that thin layers of polycrystalline silicon (polysilicon) can sometimes be permeable to HF etching solutions, raising the possibility of simply removing a sacrificial oxide layer through a layer J. Mater. Res., Vol. 17, No. 9, Sep 2002
of overlying polysilicon.4 Subsequent work has demonstrated the use of permeable polysilicon etch windows to create hollow vacuum cavities.5–7 The simple process is shown in Fig. 1. Related work has investigated the HF resistance of thin polysilicon films used for metal-oxidesemiconductor field effect transistor (M
Data Loading...
