Thin-Film Resistive Heater

A sophisticated integrated flow-through PCR chip was fabricated (see Figure 9.11). Three reaction chambers, connection channels, thin-film resistive heaters, temperature sensors, and optical detectors are fabricated on a Si wafer. A thin... 

Key words PCR, Thermocycler, Peltier, Thin-Film Resistive Heater, Data acquisition system... 

Fig. 1. Prototype thin-film resistive heating PCR thermocycler (TFRT). (I) Overall TFRT design schematic. (II) Schematic and views of the TFRT capillary cartridge assembly. The device components are 4 Thin-film resistive heater B1 and B2fans Cthermocou-ple D PCR capillaries E1 and E2 relays F computer interface 6 control software H power source (batteries or power supply).

The capillary cassette was placed directly on the heating surface of the thin-film resistive heater to minimize thermal capacitance. A Plexiglas frame was used to hold the assembly and allow access of the cooling air flow from the fans to the capillary cassette. 

The prototype TFRT instrument consists of a thin-film resistive heater, two fans, thermocouple, controller, software, and a computer. The heating element has an effective face area of 15 x 15 mm and a resistance of 15 2 (Fig. 1A). The PCR capillaries are coupled directly to the heater. For cooling, two fans are positioned on opposite sides of the test section. The fans and the heating element are powered by 9VDC (see Note 1). The system temperature is sensed by a thermocouple (see Notes 2 and 3) mounted inside a capillary and mounted on the heater in a symmetrical fashion to the capillaries that hold the PCR mixtures. For prototype development, thermocouples were mounted in each of the capillaries to map the temperature distribution on the surface of the heating element. 

Note that the electronic industry produces smd components called thin-film resistors of the same shape and size of Ru02 thermistors. They are usually made of a thin layer of Ni-Cr. Their resistance is extremely stable with temperature and hence they find application as inexpensive (punctual) heaters (see e.g. Chapter 11). 

A cross-sectional schematic of a monolithic gas sensor system featuring a microhotplate is shown in Fig. 2.2. Its fabrication relies on an industrial CMOS-process with subsequent micromachining steps. Diverse thin-film layers, which can be used for electrical insulation and passivation, are available in the CMOS-process. They are denoted dielectric layers and include several silicon-oxide layers such as the thermal field oxide, the contact oxide and the intermetal oxide as well as a silicon-nitride layer that serves as passivation. All these materials exhibit a characteristically low thermal conductivity, so that a membrane, which consists of only the dielectric layers, provides excellent thermal insulation between the bulk-silicon chip and a heated area. The heated area features a resistive heater, a temperature sensor, and the electrodes that contact the deposited sensitive metal oxide. An additional temperature sensor is integrated close to the circuitry on the bulk chip to monitor the overall chip temperature. The membrane is released by etching away the silicon underneath the dielectric layers. Depending on the micromachining procedure, it is possible to leave a silicon island underneath the heated area. Such an island can serve as a heat spreader and also mechanically stabihzes the membrane. The fabrication process will be explained in more detail in Chap 4. 

Fig. 12. A resistive metal thin film heater is formed on the upper glass. Using the liquid-gas phase system of methyl chloride, the generated pressure is increased and the response time reduced. The dynamic range of this type of valve can be broad by adjusting cavity shape, quantity of gas and liquid molecules.

Another key element in the PLD apparatus is the substrate heater. Thin film synthesis of oxide materials may be carried out at temperatures as high as 800 °C in oxygen. These highly oxidizing conditions place severe demands on heating elements, although two general types of heaters, radiant and resistive, have been used successfully. Radiant heaters utilize lamps as a heat source. A simpler solution to this problem is to utilize simple resistance heaters. Resistance heaters are easily... 

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