應用於氣體感測之局域表面電漿增強式CMOS-MEMS共振式感測平台

Abstract

本論文研究為開發適用於非色散式(Nondispersive infrared, NDIR)紅外線氣體分析儀之局域表面電漿共振(Localized Surface Plasmon Resonance, LSPR)感測平台。透過局域表面電漿共振結構設計，此平台能針對不同紅外線光波長產生加強性吸收效果，並具有微小化、可攜式之特點。本研究為首次於CMOS製程平台上驗證LSPR結構，除理論模擬外，透過國家晶片中心所提供TSMC 0.35-μm 2-poly-4-metal CMOS-MEMS平台下線晶片，並在校內的北區微機電中心進行後製程步驟，包括金屬濕蝕刻釋放結構，及反應式離子蝕刻將保護層去除。元件利用METAL4與METAL3層建構MIM (Metal-Insulator-Metal)式的LSPR吸收層，並以傅立葉紅外線轉換光譜儀(Fourier Transform Infrared spectroscopy, FTIR)證明了LSPR結構之頻率選擇性，但推估可能由於製程誤差，元件經FTIR量測之光譜有Q值過低、波長飄離原本設計、非原始設計考慮到的偽模態等非理想效應產生。本研究進一步嘗試將LSPR吸收層與微機電共振器結合，共振器共振頻率高低可反映不同波長紅外線吸收。然而，由於共振器驗證結構設計上未考慮固定端結構應力之影響，具有表面電漿結構之元件在紅外光下雖有較大的共振頻率變化，對不同波長入射紅外光訊號並無明顯至足以區分選擇性，本論文亦提出共振器設計改善方案。 另一方面，以共振頻率做為感測平台的輸出物理量，元件需具有溫度補償特性，以減低環境溫度變化下後造成的頻率變化。本研究使用另一種後製程方式研發了一個被動式溫度頻率補償平台，適合用於偶極(dipole)形式的LSPR吸收層使用。此溫度補償兩端自由樑共振器利用U型補償電極的被動式電剛性頻率補償方法，使得共振器之頻率溫度係數(Temperature Coefficient of Frequency, TCF)由原本未經補償的-70 ppm下降為+0.43 ppm、在0-85°C範圍整體頻率飄移由5800 ppm下降為496 ppm，有近12倍之改善。另外，本研究亦提出預測頻率偏差的理論模型，經修正後可以準確預測頻率偏差，並實際演示了最佳化補償電極長度設計，免除了需要另一額外的補償電壓，可僅使用單一電壓同時供予共振器直流偏壓及補償偏壓以達到補償效果。

This thesis validated the possibility of applying localized surface plasmon reso-nance (LSPR) structures onto a commercially available CMOS-MEMS platform. Due to the wavelength-selectivity property of LSPR, one can embed the LSPR structures in a CMOS-MEMS resonator and realize a non-dispersive infrared (NDIR) based gas ana-lyzer. Although the LSPR structures have been widely studied in recent years, none of them has implanted LSPR structures on a standard CMOS platform yet. The LSPR structure used by this work consists of metal-insulator-metal (MIM) absorber based on METAL4 and METLA3 layers in TSMC 0.35-μm 2-poly-4-metal CMOS-MEMS plat-form and in-house post fabrication process of metal wet-etching and reactive ion etching steps. The Fourier transform infrared spectroscopy (FTIR) validates the wave-length-selectivity of the fabricated LSPR structure. Possibly due to fabrication process variation, however, the spectrum measured by FTIR presents some issues such as lower quality factor, wavelength drift and spurious modes. In addition, a clamped-clamped beam (CC-beam) resonator combined with the LSPR structure aims to yield selective responses of resonance frequency to different IR wavelengths. Although the CC-beam resonator with LSPR structures indeed shows larger frequency shift com-pared with that with no LSPR structures, the device does not produce selective fre-quency drifts in response to the IR with varying wavelengths due to improper device design. Work continues to fix this issue. On the other hand, the change of ambient temperature would affect the resonance frequency. In order to realize a NDIR sensor based on the LSPR platform, one would need to reduce the frequency drift induced by ambient temperature. This thesis utilizes a passive compensation scheme to realize a temperature insensitive resonator based on an alternative post fabrication process suitable for dipole LSPR structures. The temperature coefficient of frequency (TCF) of the temperature compensated resonator reduces from -70 ppm to +0.43 ppm, and the overall frequency drift from 5800 ppm to 496 ppm, which is 12× improved through electrical stiffness compensating technique by introduc-ing a U-shaped compensating electrode. Besides, this thesis details a mathematical model that predicts the frequency deviation well. The model also leads to an optimized compensating electrode length that eliminates the need for a second compensating DC bias. This technique presents as a great candidate for low power frequency compensa-tion.