The Influence of Fabrication Governed Surface Conditions on the Mechanical Strength of Thin Film Materials
- PDF / 1,845,360 Bytes
- 6 Pages / 612 x 792 pts (letter) Page_size
- 110 Downloads / 196 Views
The Influence of Fabrication Governed Surface Conditions on the Mechanical Strength of Thin Film Materials
Ioannis Chasiotis and Wolfgang G. Knauss Graduate Aeronautical Laboratories California Institute of Technology, Pasadena, CA 91125, U.S.A.
ABSTRACT The mechanical strength of thin polysilicon films has been examined in connection with exposure to Hydrofluoric Acid (HF). It is found that surface roughness as characterized by groove formation at grain boundaries depends on the HF release time. Surface undulations and crevasses related to grain structure result in reduced fracture strength and induced errors into the effective elastic modulus measurements - especially when the latter is determined from flexure configurations. Extensive exposure to HF results in material degradation, as evidenced by a transition from a transgranular to an intergranular failure manner and a corresponding drastic drop of the film strength with attendant increase of porosity.
INTRODUCTION Manufacturing and post-processing of thin films in MicroElectroMechanical Systems (MEMS) are critical for their mechanical response and life during applications. To better understand the geometric and boundary properties of a microstructure and their effect on the mechanical response of these materials under stress, we examine here the surface morphology of thin films processed under different conditions. We report on observations that accompany our work on microtensile testing of thin films [1,2], and are intended to alert the reader to certain aspects of the behavior of polycrystalline silicon (polysilicon) films that are crucial for their performance reliability. An important step in MEMS fabrication by surface micromachining is the postprocess wet release with 49% HF. Observations in the context of mechanical properties determinations in our laboratories have shown that this step is important because of the influence which the attendant variation of surface roughness and internal film structure have on the failure response. Accordingly, this paper reports observations made on specimens prepared at the Microelectronics Center of North Carolina (MCNC, now Cronos) for tensile testing. The tensile strength of these specimens has been recorded for different exposure times to HF and the failure cross-sections have been examined to illustrate a possible association between the material microstructure and the measured “properties”.
ROUGHNESS EVOLUTION WITH EXPOSURE TO HF SOLUTIONS The specimens were manufactured at MCNC in runs MUMPs19 (Multi User MEMS Processes), MUMPs21 and MUMPs35 primarily for determining Young’s modulus and tensile strengths [1,2]. The part of the dog-bone shaped specimens on which our surface observations are made is EE2.2.1
the large end paddle depicted in figure 1 (MUMPs19). All specimens were released at MCNC with a 49% HF wet etch for different times followed by a 15 minute de-ionized water rinse with subsequent water substitution by alcohol. They were dried using either a 10 minute oven bake (MUMPs19), or the supercritic
Data Loading...
