Monolithic arrays of micromachined pixels for infrared applications

Abstract

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. ...

Planarization of CMOS ROIC dies for uncooled detectors

“…However, little literature is available on the planarization of CMOS ROIC dies for bolometer applications. Though Cole has pointed out that the surface of ROIC wafer must be first planarized for the subsequent formation of flat microstructure, he didnÕt describe the detailed fabrication process [6,7]. There are numerous techniques to artificially planarize the nonplanar surfaces of ROIC [8].…”

Why CMOS-integrated transducers? A review

“…In the early`90s, Honeywell started developing Focal plan arrays (FPAs) of bolometers using vanadium oxide (VOx) as the active element [Wood (1995)]. A 240´336 array with CMOS read-out reaches nowadays Noise Equivalent Temperature Differences (NETDs) down to 40 mK [Cole et al (1998)]. Lockheed Martin used the same technology to produce a 327´245 FPA with a frame rate of 60 Hz.…”

Fabrication and characterization of integrated uncooled infrared sensor arrays using a-Si thin-film transistors as active elements

“…A wide variety of materials have been used for the active elements of uncooled infrared sensors. Vanadium oxide is widely known and used as resistive bolometer materials because of its high temperature coefficient of resistance (TCR) of 2-3% at 300 K [1]. However, is not a standard material in IC fabrication; thin-film undergoes a semiconductor-to-metal transition near 68 , accompanied by dramatic drop of resistance; the depositing of thin film with a suitable mixed phase of , , and is challenging due to the narrowness of the stability range of oxides.…”

“…They usually show a relatively flat response over a wide range of wavelengths and are sensitive up to wavelengths longer than 100 . There are several kinds of available uncooled IR sensors such as resistive or dielectric microbolometers [1]- [7], pyroelectric detectors [8]- [11], and thermopiles [12]- [14]. In essence, all these sensors consist of a two-dimension array of thermosensitive pixels, whose temperature is increased by the impinging IR radiation.…”