High-performance long wave infrared bolometer fabricated by wafer bonding

Abstract

A novel microbolometer with peak responsivity in the longwave infrared region of the electromagnetic radiation is under development at Sensonor Technologies. It is a focal plane array of pixels with a 25µm pitch, based on monocrystalline Si/SiGe quantum wells as IR sensitive material. The novelty of the proposed 3D process integration comes from the choice of several of the materials and key processes involved, which allow a high fill factor and provide improved transmission/absorption properties. Together with the high TCR and low 1/f noise provided by the thermistor material, they will lead to bolometer performances beyond those of existing devices. The thermistor material is transferred from the handle wafer to the read-out integrated circuit (ROIC) by wafer bonding. The low thermal conductance legs that connect the thermistor to the ROIC are fabricated prior to the transfer bonding and are situated under the pixel. Depending on the type of the transfer bonding used, the plugs connecting the legs to the thermistor are made before or after this bonding, resulting in two different configurations of the final structure. Using a low temperature oxide bonding and subsequent plugs formation result in through-pixel plugs. Pre-bonding plugs formation followed by thermo-compression bonding result in under-pixel plugs. The pixels are subsequently released by anhydrous vapor HF of the sacrificial oxide layer. The ROIC wafer containing the released FPAs is bonded in vacuum with a silicon cap wafer, providing hermetic encapsulation at low cost. Antireflection coatings and a thin layer getter are deposited on the cap wafer prior to bonding, ensuring high performance of the bolometer.

Toward 17µm pitch heterogeneously integrated Si/SiGe quantum well bolometer focal plane arrays

“…For a given pixel pitch, there are several design issues relating to both the sensor material and the sensor-to-ROIC integration process that must be considered to reach overall good performance. We have previously reported on some aspects of sensor material design and optimization [6,7] and on the integration process for 25x25 µm pitch pixels [8][9][10]. In this paper we will address further design aspects of the infrared absorbing internal quarter wave cavity and present the latest results on scaling of the heterogeneous integration process down to 17x17 µm pitch bolometer arrays.…”

Intermetallic Bonding for High-Temperature Microelectronics and Microsystems: Solid-Liquid Interdiffusion Bonding

“…All our work has been on Si substrates with electroplated layers, typically 5 μm Cu and 1-2 μm Sn. For wafer-level bonding, equal layer thicknesses on both bonding partners have been used, and the bonding profile (temperature and pressure vs. time) has been carefully selected to allow flux-free bonding [2,18]. For the initial chip-bonding, Sn has been electroplated only on the chip side, in order to allow maintaining the substrate hot (no Cu-Sn interdiffusion prior to bonding) while mounting successive Sn-coated chips.…”

Temperature performances of bulk MOS transistors for use as temperature sensitive element for bolometer applications

“…Their TCR (Temperature Coefficient of Resistance) is usually between -2%/K and -4%/K [1-2]. It was shown that quantum wells can be used as thermistors with TCR of 3%1K and 978-1-4799-3570-3/13/$31.00 ©2013 IEEE with values of over 4%/K considered attainable after optimization of the structure [3]. Diodes can also be used as sensitive element with even higher temperature dependency, up to 7%/K [4].…”