3.3. Metal Adhesion
To get reliable heaters, four possible options are investigated for their adhesion properties to the COC substrate. The adhesion of Au and Pt deposited by either evaporation or DC magnetron sputtering is investigated using the Scotch tape test [60,61] before and after temperature cycling up to 100 °C. Test patterns consisting of rectangular metal strips of 2 mm by 14 mm are fabricated by depositing 100 nm of metal using a hand-made shadow mask made out of DuPont Kapton® HN polyimide film of 0.05 mm thickness (RS Components B.V., Haarlem, The Netherlands). See Table 4 for the results of the Scotch tape test.
Normally, heating up a glass or Si substrate with thin metal strips while measuring the resistance (RT) in these metal strips at certain temperature intervals (T) yields directly a linear relation, which can be fitted with RT/R0=1+αT−T0 [64], in which α is the the temperature coefficient of resistance (TCR) value. The thin-film TCR values have to be measured as they differ from the bulk TCR values due to its dependency on layer purity, grain size, and deposition method [65,66]. Belser and Hicklin also lists other attributes, such as surface roughness, porosity, and adsorbed materials present in or on the substrate which could influence the TCR value [64]. The bulk TCR values are 0.0034 K−1 and 0.0037 K−1 for Au and Pt [67].
The TCR characterizations of the metal strips on a COC substrate did not yield trustworthy TCR values at the first cycle. The first temperature cycle can be seen as a kind of thermal annealing, and therefore gives an hysteresis in the graphs, as can be seen in Figure A2 in Appendix C.1. After this first cycle, the values more or less show the linear behavior. The resulting TCR of this linear part is in agreement with the TCR ranges of Belser and Hicklin [64] and is given in Table 4. Belser and Hicklin used for their experiments substrates with coefficients of linear thermal expansion lower than 1.2 × 10−5 °C−1 [64]. The coefficient of linear thermal expansion for Au and Pt are 1.42 × 10−5 K−1 and 0.88 × 10−5 K−1, respectively [68]. COC of the grade TOPAS 6017 has a coefficient of linear thermal expansion of 6.0 × 10−5 K−1 [42]. This mismatch in coefficients of linear thermal expansion can give strain in the metal layers. Both Au [69,70,71] and Pt [72,73,74] are used as strain-sensitive gauges, and thus are sensitive to strain-induced geometry changes due to thermal expansion.
Another effect influencing the TCR value of the metal layer is aging. As can be seen in Figure A3 in Appendix C.2, the TCR value already changes after two weeks storing in ambient conditions. This could be due to adsorbed materials present on the surface [64].
However, in this device, the TCR is not of importance as the metal structure will not be used as temperature sensor. Real-time temperature sensing is done using a thermocouple in the temperature monitor chamber. The resistance of the heater structure changes with temperature; thus, the dissipated power changes when a fixed voltage or current is used. However, the results in Section 3.4 show a 25 h stability test with a constant input potential and only a ±1.5 °C deviation. The TCR can become more important when other (higher) temperatures are required for the amplification.
Based on the results in Table 4, the choice of heater material and deposition method to be used in the actual device is Au deposited using sputtering. Sputtering is an industrial-scale technique that is already being used in, for example, the car mirror and headlight industry [75].