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A superconducting microbolometer employing a DyBaCuO film deposited upon a silicon microstructure was found to have a responsivity of 800 V/W at 89 K and a response time of 1 ms.

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Small micromachined structures (typically 10-5cm2) have been fabricated with very small thermal mass (C) of about IO-' JK, which are suspended from the underlying silicon substrate by supports of such delicacy that the structures are extremely well thermally isolated from the substrate having a thermal conductance to the substrate of about IO-' W K . This thermal conductance (G) is close to the smallest value possible, about W/K, due to radiative energy exchange (1). This high thermal isolation allows the microstructure temperature to be readily controlled by very small heating currents, or very small amounts of infrared (IR) incident flux. Large arrays of such microstructures have been fabricated on silicon wafers, with complex integrated electronic circuits, and operated as (1)sensitive room-temperature IR sensors ("microbolometers") and (2)large arrays of individually controllable IR microemitters. Figures 1 and 2 show the basic microstructure design common to both microbolometers and microemitters. We refer to this as a "two-level structure," as it consists of an upper silicon nitride plate, about 51 pm square and 0.5 pm thick, suspended over an underlying silicon IC substrate with a gap of about 1-2 pm between the upper plate and the substrate. A resistor material is formed within each upper microstructure plate and acts as a temperature-dependent resistor in the case of a microbolometer or a heater resistor in the case of a microemitter. Microstructure DesignThe optimum structural design of a microstructure (microbolometer or microemitter) is influenced by many practical system requirements and, as a result, is surprisingly complex. Broadly speaking, the desired features are a large fill factor, high thermal isolation, and a thermal time constant (C/G) fast enough for the application (e.g., 30-to 500-Hz frame rate operation). Microstructure FabricationThe basic process for building an efficient microstructure for infrared applications is illustrated in Figure 3. The process can be separated into four major processing steps. The first processing step involves the formation of transistors in the silicon with multilevel metallization connect circuitry, as well as providing connection points for wirebonds and the subsequent microstructure. Critical to this step is the IC compatibility with subsequent thermal, chemical, and radiation environments necessary for microstructure fabrication. After IC fabrication, the wafer must be planarized for the subsequent formation of flat microstructures. The second step involves the deposition and patterning of an IR reflecting layer followed by a "sacrificial" spacer layer. The spacer layer contains contact vias patterned as necessary for electrical and mechanical contact to the future overlying pixel structures. 2-Level Mic cos truc tureThema laoiation ""h". . SliebnNitnde 'Undedylnp ' T m~i s t~i Cirmilry ILine P l X d CMOS Pixel " Z Q 1 Fig. 1. Two-Iteve1 drawing of microemitter showing schematic of cell drive electronics:(a) microemitters and (b) microbolometers.Fig. 2. ...

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