Fabrication and characterization of high-temperature microreactors with thin film heater and sensor patterns in silicon nitride tubes

Abstract

In this paper the fabrication and electrical characterization of a silicon microreactor for high-temperature catalytic gas phase reactions, like Rh-catalyzed catalytic partial oxidation of methane into synthesis gas, is presented. The microreactor, realized with micromachining technologies, contains silicon nitride tubes that are suspended in a flow channel. These tubes contain metal thin films that heat the gas mixture in the channel and sense its temperature. The metal patterns are defined by using the channel geometry as a shadow mask. Furthermore, a new method to obtain Pt thin films with good adhesive properties, also at elevated temperatures, without adhesion metal is implemented in the fabrication process. Based on different experiments, it is concluded that the electrical behaviour at high temperatures of Pt thin films without adhesion layer is better than that of Pt/Ta films. Furthermore, it is found that the temperature coefficient of resistance (TCR) and the resistivity of the thin films are stable for up to tens of hours when the temperature-range during operation of the microreactor is below the so-called "burn-in" temperature. Experiments showed that the presented suspended-tube microreactors with heaters and temperature sensors of Pt thin films can be operated safely and in a stable way at temperatures up to 700 degrees C for over 20 h. This type of microreactor solves the electrical breakdown problem that was previously reported by us in flat-membrane microreactors that were operated at temperatures above 600 degrees C.

Reliability improvement of suspended platinum-based micro-heating elements

“…The studies performed on the reliability of platinum-based heating elements are limited. Most of the studies dealing with the stability and reliability of the platinum-based metallizations referred to the Pt-Ti system, with only a few on the Pt-Ta system [14][15][16][17][18][19]. They report on the materials aspect of the platinum metallization for specific adhesion layers.…”

“…The use of Ta as adhesion layer has the advantage over Ti to be compatible with the deposition of LPCVD Si 3 N 4 , providing hotplates with more robust membranes, and to diffuse less in platinum. The influence of the temperature during the processing or post-processing on the microstructural, electrical and mechanical properties of Pt-Ta-based metallizations has already been presented [15,17,18], as well as the degradation of Pt-Ta-based micro-heater operating at high temperatures [7,8,10,18].…”

Fabrication of high-aspect-ratio arrayed structures using Si electrochemical etching

“…For example, ordered arrays of uniform pores are applicable to micro-channels for micro-fluidic and filtering devices [5][6][7]. Microscale tube and chamber structures are also required for micro reactors [8][9]. Micro and nanoscale needles can be applied to precise injector for cell [10][11][12][13][14], painless injectors [11][12][13][14], and microelectrode for electric measurement of targeted microscopic area [15][16][17], etc.…”

“…High-aspect-ratio micro and nano structures are key components for various microscale devices and systems, such as MEMS and mTAS [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19]. For example, ordered arrays of uniform pores are applicable to micro-channels for micro-fluidic and filtering devices [5][6][7].…”

Low-power silicon-based thermal sensors and actuators for chemical applications

“…The response time of the temperature sensor itself to an external heat excitation was estimated to be in a few millisecond range which made the analysis of fast catalytic reactions possible. This was also demonstrated in the past for the catalytic microreactors comprising the integrated heaters [112]. In Figure 3.10 a similar, long-term test is indicated.…”