366 JOURNAL OF MICROELECTROMECHANICAL SYSTEMS, VOL. 8, NO. 4, DECEMBER 1999 Germanium as a Versatile Material for Low-Temperature Micromachining Biao Li, Bin Xiong, Linan Jiang, Yitshak Zohar, and Man Wong, Member, IEEE Abstract—Though germanium (Ge) shares many similar phys- ical properties with silicon (Si), it also possesses unique charac- teristics that are complementary to those of Si. The advantages of Ge include its compatibility with Si microfabrication, its excellent gas and liquid phase etch selectivity to other materials commonly used in Si micromachining, and its low deposition temperature (<350 C) that potentially allows Ge to be used after the completion of a standard CMOS run. Wider applications of Ge as a structural, sacrificial, and sensor material require a more systematic investigation of its processing and properties. The results of such an undertaking are presently reported. The topics covered are the formation of Ge thin films and novel application of the selective deposition of Ge to etch hole filling, characteri- zation of the effects of thermal treatment on the evolution of the residual stress in Ge thin films, etch selectivity for etch mask and sacrificial layer applications, and gas phase release technique for stiction elimination. [431] Index Terms—Germanium, micromachining, stiction. I. INTRODUCTION D ESIGNING an integrated micromachining process typ- ically involves resolving material and process incom- patibility between the fabrication of the electronic and the nonelectronic components. Simultaneous realization of both kinds of devices, while desirable, often presents difficult process integration issues. An alternative approach is to re- alize the nonelectronic components either before or after the fabrication of the microelectronic circuits. In this sequential approach, if the nonelectronic components were to be realized first, they should not be affected either by the multiple addition and removal of thin films or by the high process temperature required for the fabrication of the microelectronic devices. If the sequence were reversed, high temperature would no longer be feasible because of the presence of low melting point metal interconnections in the microelectronic circuits. Though silicon (Si) integrated-circuit technology has been successfully applied to micromachining, there are inherent lim- Manuscript received March 18, 1999; revised June 30, 1999. This work was supported by a grant from the Hong Kong Research Grants Council. Subject Editor, R. T. Howe. B. Li, L. Jiang, and Y. Zohar are with the Department of Mechanical En- gineering, The Hong Kong University of Science and Technology, Kowloon, Hong Kong. B. Xiong is with the Department of Electrical and Electronic Engineering, The Hong Kong University of Science and Technology, Kowloon, Hong Kong, on leave from the Shanghai Institute of Metallury, Chinese Academy of Sciences, Shanghai, China. M. Wong is with the Department of Electrical and Electronic Engineering, The Hong Kong University of Science and Technology, Kowloon, Hong Kong (eemwong@eeu.ust.hk). Publisher Item Identifier S 1057-7157(99)09608-0. itations, particularly arising from the narrow choice of materi- als. Germanium (Ge), being a Group IV semiconductor, shares many similar physical properties with Si and is considered compatible with Si microfabrication. Ge also possesses unique complementary characteristics. These include its low deposi- tion (<350 C), dopant activation, and crystallization tempera- ture [1], [2], as well as its excellent etch selectivity to materials commonly used in Si micromachining, such as SiO Si N and aluminum (Al). All of these potentially allow Ge to be used after the microelectronic devices have been fully realized. Wider applications of Ge to MEMS require a more system- atic investigation of its physical properties and its processing. The results of such an undertaking are presently reported. The formation of Ge thin film is first discussed, and a novel application of the selective deposition of Ge to etch hole filling is proposed. This is followed by a characterization of the effects of thermal treatment on the evolution of the residual stress in Ge thin films. Etch selectivity is considered next for etch mask and sacrificial layer applications. A novel gas phase release technique is proposed for stiction [3] elimination. Like Si, Ge can be used to realize a variety of sensors. This is demonstrated by the fabrication of Ge thermistors. Lastly, issues related to the material combination of the (Al, Ge) pair as the (structural, sacrificial) material pair are discussed. II. DEPOSITION OF Ge FILMS The polycrystalline (poly-) and amorphous ( -) Ge thin films used in the present work were deposited in a conventional low-pressure chemical vapor deposition (LPCVD) system at respective temperatures of 325 C and 300 C, process pres- sures of 300 and 600 mtorr, and germane flow rates of 25 and 45 sccm [1]. Similar to the LPCVD of tungsten [4], as shown in Fig. 1, Ge can be deposited directly on the surface of Si, Ge, or SiGe alloys, but not readily on SiO [5]. This unique “selective” deposition, “self-aligned” to the exposed regions of Si in an otherwise SiO surface, allows many novel applications, such as selective Ge island formation [6] and self-aligned etching- hole filling, as shown schematically in Fig. 2. If Ge deposition on an SiO surface were desired, it would suffice to first put down a thin layer of Si “nucleation” layer [1], [5] prior to the deposition of Ge. III. Ge AS A STRUCTURAL MATERIAL Since residual stress is one of the most important physical parameters affecting the mechanical behavior of any thin-film 1057–7157/99$10.00  1999 IEEE