s10_Thick and Thermally Isolated Si Microheaters for Microfabricated Preconcentrators

Thick and Thermally Isolated Si Microheaters for Microfabricated Preconcentrators

S. W. Pang and W. C. Tian,

University of Michigan, Ann Arbor, Michigan 48109-2122, USA

Freestanding, high aspect ratio microstructures in Si were micromachined as thick microheaters. These microheaters, combined with adsorbents, can be used as preconcentrators in a micromachined gas chromatography system. Dry etching of Si using etch and passivation cycles has been developed to produce 240 µm thick Si microheaters with 3 µm wide wires, achieving a high aspect ratio of 80:1. This optimized dry etching technology results in high etch rates with vertical profiles for thick Si microheaters up to 535 µm. Microheaters with 40 µm wide wires, 110 µm gaps, 400 µm thick, and an area of 9 mm² have been fabricated. With the heater on a 140 µm thick Si membrane, it takes 1320 mW to increase the temperature by 290 °C. The power consumption is reduced to 447 mW for the same temperature raise with a freestanding Si microheater. Heating response for freestanding Si microheaters with different thicknesses is also studied. These Si microheaters have fast response times and reach 75% of the final temperature in 5 s. Microheaters consisting of Si wires and posts show similar response time and heating uniformity. In addition, power consumption is reduced by thermal isolation of the microheaters. These thick, freestanding high aspect ratio Si microheaters can provide high power efficiency, large adsorbent capacity, and high mechanical strength.

Thick thermally isolated microheaters in Si are fabricated with large surface area to provide the large adsorbent capacity needed for high sensitivity microfabricated preconcentrators in a microgas chromatography system. Air gaps around the microheaters provide good thermal isolation to lower the power consumption. A 520 µm thick Si microheater with good thermal isolation has been made and its back side is anodically bonded to a pyrex glass substrate. To provide good thermal isolation, a 500 µm wide air gap around the microheater, thin poly-Si interconnects on top of dielectric membrane, and air gap isolation from the bottom glass substrate are used. Microheaters with a 500 µm air gap can be heated up 50% faster to a temperature of 270 °C compared to those with a 100 µm air gap, while the power consumption is 25% less and there is a larger temperature difference between the microheater and the bonding area. Operating the microheater in a vacuum results in lower power consumption. At 250 °C, 35% less power is needed at 1.2 Torr compared to atmospheric pressure. The power consumption is further reduced by minimizing the contact area with the heater support substrate. Up to 34% power reduction has been demonstrated by placing heaters on a thin membrane or etching trenches in the supporting substrate. These thick thermally isolated Si microheaters can provide good power efficiency, large adsorbent capacity, and high mechanical strength for microfabricated preconcentrators.

High aspect ratio microheaters can be formed by etching Si to various depths. With through wafer etching, thick, freestanding microheaters were generated after oxide removal. The freestanding microheaters without membranes allow power consumption to be reduced. Figure 1 shows a freestanding microheater consists of 40 µm wide, 400 µm thick wires with 110 µm gaps.

Figure 1. Freestanding Si microheater with 40 µm wide wires, 110 µm gaps, and 400 µm thick. The freestanding microheater is placed on top of a microscope holder with no membrane below.

The power needed to heat up Si microheaters with and without membranes was investigated, as shown in Figure 2(a). Heating responses of freestanding Si microheaters with different thicknesses was also investigated, as shown in Figure 2(b).

Figure 2. (a) Power needed to heat up 400 µm thick microheaters with and without membranes. (b) Heating responses for freestanding 400 and 435 µm thick microheaters. These Si microheaters consist of 40 µm wide wires with 110 µm gaps.

As shown in Figure 3(a), a 520 µm thick Si microheater anodically bonded to a 520 µm thick pyrex glass has been designed and fabricated. To provide a good thermal isolation, a 500 µm wide air gap around the heating elements and a 40 µm deep air gap in the bottom glass substrate are formed, as illustrated in Figure 3(b). 0.5 µm thick poly-Si interconnections sitting on 0.6/0.1/0.6 µm thick oxide/nitride/oxide membrane cross over the 500 µm air gap and connect the microheater to the bonding area, as shown in Figure 3(c). A built-in resistive poly-Si temperature sensor, as shown in Figure 3(d), is used to monitor the microheater responses.

Figure 3. (a) 520 µm thick Si microheater bonded on 520 µm thick pyrex glass substrate. The heater consists of 50 µm wide slats, 220 µm gaps, and 500 µm air gap. (b) Etched trenches in glass substrate to provide thermal isolation. (c) 0.5 µm thick poly-Si interconnection on 0.6/0.1/0.6 µm thick oxide/nitride/oxide membrane. It crosses over the air gap and connects the microheater to the bonding area. (d) Built-in resistive temperature sensor on microheater.

As shown in Figure 4(a), microheaters with a 500 µm air gap have faster heating responses than the ones with a 100 µm air gap. The required input power for microheaters with different air gaps between the heater and the bonding area are shown in Figure 4(b). Figure 4(c) shows the temperature difference between the heater and the bonding area by etching trenches in the pyrex glass substrate.

Figure 4. (a) Heating and cooling rates of microheaters with different air gaps between the heater and the bonding area. (b) Lower power is required for microheaters with larger air gap due to better thermal isolation. (c) Larger temperature difference between microheater and bonding area by etching trenches in glass substrate to reduce contact area.

To quantify the amount of convective heat loss and ultimately to improve power efficiency, the microheaters were operated in a vacuum chamber. As shown in Figure 5, in order to heat the microheaters to 250 °C, 1050 mW is needed at atmospheric pressure while 675 mW is required at 1.2 Torr. Thus, convective heat loss accounts for 35% of the heating power.

Figure 5. Better power efficiency can be obtained by operating the microheaters in a 1.2 Torr ambience.

References

Last Updated: November 19, 2007

E-Mail: pang@umich.edu

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