Assembled Fourier transform micro-spectrometer

Abstract

Microassembly process plays a key role in building 3-dimensional heterogeneous microsystems. This paper presents a miniaturized Fourier transform spectrometer (FTS) implemented by combining silicon micromachining and microassembly techniques. The FTS is based on a Michelson interferometer where a scanning mirror mechanism creates an interferogram, and the recorded interferogram is converted to a spectrum by Fourier transform. The miniaturized Michelson interferometer is integrated on a microoptical bench, which is fabricated using Deep RIE (Reactive Ion Etching) process on a SOI (Silicon On Insulator) wafer. Key components of the FTS optical bench are a linear translation stage, mechanical assembly sockets, a beam splitter, and assembled mirrors. An electrothermal actuator with stroke amplification mechanisms provides the amplified scanning motion of a scanning mirror. The sockets are female mechanical flexure structures that allow a precise snap-fit assembly with micromachined silicon mirrors. The dimension of the FTS optical bench is 1cm2, and its embedded thermal actuator has a couple of V-beam structures whose beam length is 1mm. The mirrors are Deep RIE micromachined structures with reflection area 500x450μm2 and 750μm long flexure structures for pick & place assembly. The flexure structure allows large deflection so that a microgripper can pick up the mirror by inserting the gripper tip into the structure, and snap-fit assembles it into the mechanical socket of the bench. The linear translation stage generates up to 30μm scanning stroke at 22V input, which corresponds to a spectral resolution of 10nm at 775nm wavelength. While this microassembly method is designed to self-align the mirror in the socket, the mirror slightly tilts after assembly due to the slope of side wall of DRIE processed structures. The measured tilting angles of assembled mirrors range from -2.5° to 0.8° from several assembly trials. The tilting angle combined with beam divergence can cause the loss of power and resolution, spectrum shift and phase error. A He-Ne laser was used as a light source to create interferogram with the assembled microspectrometer. Formation of fringe patterns was successfully conducted with a prototype. Mirrors with a large tilting misalignment resulted in stripe pattern fringes, whereas an improved alignment generated circular pattern fringes. A detector was used to measure light power with respect to input voltage, and the displacement of a scanning mirror was measured and curve-fitted. The relationship between light power changes versus the displacement of a scanning mirror represents interferogram. Spectrum profiles showed a peak around 632nm with FWHM (Full Width Half Magnitude) 25nm approximately. While further research is on going to improve spectrum quality and microassembly technique for the integration of various components with heterogeneous materials and shapes, this approach is expected to facilitate the design and manufacturing of MOEMS from the constraints of micromachining processes.