Low Coherence Interferometric Metrology for Ultra-thin MEMs Structures
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Low Coherence Interferometric Metrology for Ultra-thin MEMs Structures Wojciech Walecki, Frank Wei, Phuc Van, Kevin Lai, Tim Lee, Vitali Souchkov, SH Lau, and Ann Koo Frontier Semiconductor, 1631 North 1st Street San Jose CA 95112, USA ABSTRACT We present application of low coherence interferometry to characterization of ultra-thin MEMs structures such as membranes in micro-machined devices. INTRODUCTION Low coherence optical interferometry [1] has been proven to be an effective tool for characterization of thin and ultra-thin semiconductor Si wafers [2], and deep etched trenches [3]. In this paper we explain extension of this method to characterization of ultra-thin structures such as membranes in micro-machined devices (MEMs). Monitoring thickness and uniformity of the thickness of ultra-thin membranes defined in silicon matrix is still a difficult and not resolved issue. Since thickness of such membranes exceeds tens microns it is not possible to use common thin film tools such as elipsometers or dipersive or diffractive spectrally resolved reflectometers. The commonly employed tools for wafer thickness metrology are based on capacitance or air pressure methods. The first of these two competing technologies has been proven to be extremely powerful tool for measurement of thickness, bow, warp and related parameters in thick, and well conducting materials. Capacitance method however is not suitable for measuring thickness of semi-insulating, and fails to measure insulating materials. It does not provide accurate results for very thin wafers (thinner than 200 µm). Furthermore it has relatively low spatial resolution limited by physical size of probes, and may not be suitable for direct measurement of wafers mounted on insulating carriers or MEMs structures. Air pressure based metrologies are able to measure insulating materials however they also cannot be directly applied to wafers mounted on carriers. Both these competing techniques require access from both sides of the wafer. In this paper we present alternative technique, which does not suffer from above discussed limitations, and discuss it applications for metrology of ultra-thin membranes. APPARATUS AND EXPERIMENTAL Experimental apparatus used in the reported measurement is based on fiber optics interferometer shown in Figure 1, [2]. Light emitted by low coherence source A is split by means of beam-splitter 50% transmitting, and 50% reflecting beam-splitter C into two beams: first called later reference beam propagates in the reference arm of the interferometer D, second portion of the beam later called signal beam propagates in the signal arm E. The polarization of the reference beam is controlled by means of
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polarization controller F, and is collimated by means of lens G on mirror H. Mirror H resides on delay stage such that the length of the optical path of the reference beam is controlled by means of optical delay stage. The reference beam is reflected from the reflective element, passes again through polarization controller F is partiall
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