The influence of wafer elasticity on acoustic waves during LIGA development.

Ting, Aili

Sandia National Laboratories

关键词: Sound Waves;    Acoustic Models.;    Liga.;    Incidence Angle;    Elasticity;   

DOI  :  10.2172/918213
RP-ID  :  SAND2003-8737
RP-ID  :  AC04-94AL85000
RP-ID  :  918213

美国|英语

来源: UNT Digital Library

【 摘 要 】

During acoustically stimulated LIGA development, a wafer receives sound waves from both sides at a wide variety of incidence angles that vary in time depending on the orientation of the wafer relative to the multiple transducers that are typically actuated in a periodic sequence. It is important to understand the influence of these variables on the transmission of energy through the wafer as well as the induced motion of the wafer itself because these processes impact the induced acoustic streaming of the fluid within features, the mechanism presently thought responsible for enhanced development of LIGA features. In the present work, the impact of wafer elasticity on LIGA development is investigated. Transmission waves, wafer bending waves, and the related concepts such as critical bending frequency, mechanical impedance, coincidence, and resonance, are discussed. Supercritical-frequency incident waves induce supersonic bending waves in the wafer. Incident wave energy is channeled into three components, transmitted, reflected and energy deposited to the wafer, depending on the wafer material, thickness and wave incidence angle. Results show at normal incidence for a 1-mm PMMA wafer, about 47% of the wave energy is deposited in the wafer. The wafer gains almost half of the incident energy, a result that agrees well with the Bankert et a1 measurements. In LIGA development, transmitted waves may sometimes produce strong acoustic motion of the developer on the wafer backside, especially for the so-called coincidence case in which almost all incident wave energy transfers to the backside. Wafer bending waves cause wafer oscillation at high frequency, promoting the development process, but features shaking may weaken their attachments to the substrate. Resonance is not likely for the entire wafer, but may occur in short and wide wafer feature columns, which are least likely to break away from the substrate, perhaps resulting in good agitation of the fluid in adjacent feature cavities.