100-μm-Scale High-Detectivity Infrared Detector With Thermopile/Absorber Double-Deck Structure Formed in (111) Silicon

This article presents a 96 $\mu \text{m}\,\,\times $ 106 $\mu \text{m}$ sized single-crystalline silicon (SC-Si) /Au thermopile infrared (IR) detector, with the thermopile and IR absorber located at different layers of a double-deck micromechanical structure for improving detectivity. In order to enhance IR-heat absorption within such a tiny device size, an umbrella-shaped SiN IR absorbing membrane instead of traditional plane IR-absorbing film occupies the whole area of the top layer structure. The umbrella-shaped IR absorber is suspended on top of the IR-detecting thermopile layer, with a central umbrella-stick to support the suspending and conducting the absorbed IR-heat to the bottom thermopile layer. The bottom thermopile layer consists of six pairs of spiral-shaped SC-Si/Au thermocouples that feature several times higher Seebeck coefficient compared to the traditional polysilicon/metal thermocouples. By combining surface-micromachining technique with a specific bulk-micromachining process performed in (111) silicon wafer, the double-deck structured IR-detector is successfully fabricated only from the front side of a single (111) silicon wafer for IC-foundry compatible low-cost manufacturing. Testing results show that this device of about 100- $\mu \text{m}$ scale achieves an ultrahigh IR detectivity of 1.01 $\times \,\,10^{{8}}$ cm $\cdot $ Hz $^{{{1}/{2}}}\cdot \text{W}^{-{1}}$ , which is two times improvement compared to the recently reported thermopile IR detectors, though the counterparts had larger device area. Featuring tiny size and batch fabrication capability, the proposed high-performance IR-detector is promising in both single-point detection and multipixel arrayed temperature imaging applications.