Thermally and electrically isolated single crystal silicon structures in CMOS technology

Abstract

Thermally and electrically isolated single crystal silicon structures have been fabricated using a post-processing anisotropic tetramethyl ammonium hydroxide (TMAH) electrochemical etch. The process was carried out on CMOS circuits fabricated by a commercial foundry. Since the etch consists of a single micromachining step performed on packaged and bonded dice, this technique has the potential for cost-effective prototyping and production of integrated sensors and circuits.Suspended N-Well Collect I. I NTRODUCTI ON UCH recent work has been done to create CMOS M or bipolar compatible thermal transducers using postprocessing techniques [ 11-[7]. These devices are either realized with suspended oxide membranes [l] or cantilever-style single crystal silicon structures [2]. The thermally isolated A B structures can sense a localized change in temperature, making them useful for applications such as infrared detection, gas flow monitoring and ac power measurement [1]-[5], [SI. While oxide membranes are useful for building thermopiles and polysilicon heating resistors, they do not allow for the formation Of active devices such as diodes or transistors. Silicon cantilevers, while providing diodes for highly sensitive temperature transducers, are connected to the substrate with low thermal resistance single crystal silicon. Moreover, the processes used for the fabrication of these cantilevers are either not CMOS compatible [2] or require backside alignment 151. In either Case PrOteCtiOn of the metaliZatiOn layers is required during the electrochemical etch.In contrast to previous work, the technique presented here consists of an electrochemical post-processing etch on an mercial CMOS process. ~~~~~t h~l ammonium hydroxide Of a p-type silicon substrate, leaving n-type wells suspended from oxide beams. circuits in these n-wells thermally and electrically insulated from the substrate. This technique combines the advantages of the oxide membrane and silicon cantilever by providing the high thermal isolation of oxide beams and the excellent sensitivity of silicon diode temperature sensors. ne etchant in this process is also safer, Oxide ta P+ Aluminum N-Well N+ E I P-Substrate EI P-Base 61 Emitter Fig. 1. (a) Schematic view of a CMOS vertical bipolar transistor inside a fully suspended n-well. (b) Schematic cross-section Of the Structure.manly used for CMOS compatible anisotropic silicon etching. Since no special processing steps, masks or materials are needed, this process has the potential for simple, cost-effective prototyping and production of integrated Sensor systems. DESIGN AND FABRICATIONunprotected, packaged and bonded IC fabricated using a cornThe packaged Parts used in this experiment were fabricated in a 2 pm commercial CMOS process. Regions of bare silicon cut, via cut, and Pad opening in the layout file 171. TWO different device structures were fabricated. The first is a pair of ratioed vertical bipolar transistors inside an n-well. The second consists of two series connected diodes in two closely spaced n-wells. ...

Thermal CMOS Sensors―an Overview

“…By design, the passivation layer serves as an etching mask and no further mask is required. Dielectric beams supporting thermal sensor elements [5,6], circuits [7], or radiation sources [8] are made in this way. Alternative etchants based on QAH (quaternary ammonium hydroxides) are currently being investigated [9].…”

CMOS Thermal Sensor Arrays for Enzymatic Glucose Detection

“…Other than biosensing applications, CMOS thermal sensors based on micro-bolometers [16]- [18], MOS transistors [19]- [21], and thermopiles [22]- [24] have been developed for uncooled infrared detection. Based on our prior study [21], the temperature coefficient of resistance (TCR) of a MOS sensing transistor is the largest among CMOS materials (e.g., metal and n-well resistor).…”

Transistor heater for microhotplate-based metal-oxide microsensors

“…To date, active transistor heating elements on a thermally isolated microstructure have been used for circuit temperature stabilization up to 90 C [12], [13]. High-temperature microhot- plates have been fabricated in silicon-on-insulator (SOI) CMOS technology [14], [15].…”