Laser-Chemical 3-D Micromachining
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LASER-CHEMICAL 3-D MICROMACHINING T. M. BLOOMSTEIN AND D. J. EHRLICH Lincoln Laboratory, Massachusetts Institute of Technology Lexington, Massachusetts 02173-9108
ABSTRACT Three-dimensional parts are machined in silicon using laser-induced chlorine etching reactions. Structures are created directly from solid-modeling computer-aideddesign/computer-aided-manufacturing software. Removal rates exceeding 2 x 104 and 105 pm 3 /s, several orders of magnitude faster than electrodischarge machining methods, are achieved at 1-jgm and 15-gtm x-y resolution, respectively. Laser-induced metallization of resulting structures as well as replication through compression molding have been demonstrated. INTRODUCTION Laser-induced microchemical reactions [1] offer an attractive option as primary patterning methods for the emerging field of microelectromechanical systems (MEMS) [2]. Conventional lithographic techniques for fabricating these inherently threedimensional (3-D) mechanical devices are limited to unidirectional extensions of twodimensional patterns, for example, by x-ray lithography in thick photoresist [3] or by crystallographic selective etching. More truly 3-D structures are required to optimize structural characteristics such as mechanical function, strength, fracture resistance, flow properties, and out-of-plane coupling. Laser microchemical processing utilizes highly localized laser microchemical reactions and accurate scanning of the laser beam. Depth contouring for 3-D patterning is accomplished by dynamically refocusing the laser beam. Laser-induced metallization of resulting structures as well as replication through compression molding have been demonstrated. Below we present a brief summary of progress to date. More details can be found in Ref. 2. RESULTS Initial results have been obtained using an acousto-optic scanner (illustrated in Fig. 1) to digitally access a 256 x 256-pixel scanning field with 0.1-pixel accuracy. Random access rates up to 50,000 pixels/s are achieved and are limited by acoustic wave stabilization in the crystals. In the results presented the address grid is spaced at 1-gtm increments in the x-y plane. Intensity variations due to the nonlinear transmission of the deflection crystals over the scanning field are compensated using closed-loop feedback by an additional acousto-optic crystal and a photodiode that senses a portion of the output beam. Focusing of the 900-mW 488-nm laser light is through a 0.4-NA, 20x flat-field corrected objective to -1.0-gim beam size although the effective reaction zone may be slightly larger because of heat transfer in the highly conducting bulk [4]. The beam is introduced through a quartz glass cover into a stainless steel vapor cell containing the sample. Circularly polarized 488-nm light from an 15-W argon-ion laser is used as the source, and a helium-neon laser beam is also introduced into the system for autofocusing Mat. Res. Soc. Symp. Proc. Vol. 282. @1993 Materials Research Society
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on the surface using a spot minimization algorithm. The reaction
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