CHAPTER 6 T-MEMS AS TEMPERATURE SENSORS 6.1 Theoretical Performance Original Specifications The thermophysical properties of thin films, determined in Chapter 4, was used to calculate the resolution and temperature ranges for T-MEMS. T-MEMS die used for this analysis consists of beams between 100 and 50 m long (at 1 m increment). The width ratios were varied between 0.85 (column 1) and 0.2 (column 14), at increment of 0.05. This totaled in 51 × 14, or 714 beams on the die. Film structure of the beams were identical to the actual beams: from top, 0.19 m SiO2, 0.54 m poly-Si, and 1.03 m SiO2. The beams were assigned an initial deflection of zero, and the distance between beam and Si substrate was 6 m. These specifications roughly correspond to the original design specifications of the T-MEMS. Figure 6.1 shows the progression of T-MEMS as temperature is increased, calculated theoretically based on materials properties found earlier. The y-axis in the plot indicates the number of beams within each column that have adhered to the substrate at any given temperature. The figure shows that as temperature increases, fewer beams change states per 100 °C change in temperature. This indicates a better temperature resolution at lower temperature range for these T-MEMS. The original specifications of 14 columns, each varying between 100 and 50 m long, results in a temperature range of 460 °C to over 2000 °C. Thermal processes in microelectronics industry rarely take place above 1200 58 # of beams adhered 40 1100 °C 1000 °C 900 °C 800 °C 700 °C 600 °C 30 20 10 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 column Figure 6.1 Temperature response of original design of T-MEMS °C; therefore, much of the shorter beams in the original design do not offer any additional information. Furthermore, Modified Specifications Based on these findings, a new T-MEMS die was designed with a narrower target temperature range. The sensor is to be effective in 900 – 1100 °C temperature range, as this is the most commonly used temperature range in RTP. The thermal expansion coefficients found were used to determine the temperature response for beams with width ratios ranging between 1.0 and 0.2, having lengths between 100 and 50 m. Of the 867 combinations (17 widths × 51 lengths), 97 beams were found to fall within the specified temperature range. Width ratios of 0.65, 0.6, and 0.55 were omitted from the die due to the similarity in temperature response to widths 0.7, 0.75, and 0.8, respectively. The resulting combination of beam lengths, widths, and resulting temperatures are summarized in Table 6.1. When the 97 beams are arranged on a die such that the fill factors remain as described in Table 3.1, they will cover an area of approximately 1.3 mm 59 × 1.3 mm. This configuration is optimal when T-MEMS are to be used on silicon wafer. For use on other substrate materials, beam spacing may need to be altered to produce more favorable temperature distribution over the wafer. Table 6.1 Beam dimensions for T-MEMS die with 900 – 1100 °C target temperature width ratio 1.0 0.95 0.9 0.85 0.8 0.75 0.7 0.5 0.45 0.4 0.35 0.3 0.25 0.2 beam lengths (m, at 1 m increment) 62 – 68 62 – 68 62 – 68 62 – 68 62 – 68 62 – 68 62 – 68 62 – 68 62 – 68 63 – 69 63 – 69 64 – 70 65 – 71* 67 – 73 temperature range (°C) 1099.7 – 923.1 1095.3 – 919.4 1091.4 – 916.1 1088.1 – 913.3 1085.5 – 910.9 1083.3 – 909.2 1082.1 – 908.1 1090.9 – 914.6 1098.7 – 920.6 1076.2 – 904.5 1092.6 – 917.6 1083.1 – 911.8 1085.4 – 915.5 1074.9 – 910.6 * - 70 m beam is omitted due to similar response to another beam Figure 6.2 shows the temperature at which each of the 97 beams make contact the substrate. The figure shows a non-uniform distribution in temperatures. In fact, the temperature resolution within the 900 – 1100 °C temperature range varied from less than 1100 1080 1060 1040 1020 1000 980 960 940 920 900 0.1 °C to up to 9 °C. Improving the temperature resolution requires the design of beams temperature (°C) Figure 6.2 Temperature response of modified T-MEMS die 60 that would adhere to the substrate at temperatures where current set of beam fails (for example, at ~ 980 °C from Figure 6.2). Fabrication processes of T-MEMS beams require that the film thicknesses of all beams on a die to be the same; therefore, this is not a feasible method of customizing beam response. One possibility of adjusting beam curvature is to vary the length of top and bottom layers. Thermally induced curvature affects only the layered portion of the beams. However, if the bottom layer were made substantially longer than the top layer, the extended bottom layer will continue the downward deflection at a constant slope (matching the slope at the end of layered portion). The total beam deflection can then be controlled simply by adjusting the length of the bottom layer that extends out to the substrate. This concept is illustrated in Figure 6.3. This modification will not require any additional processing steps; furthermore, the relation between deflection and curvature can be related by simple geometry for modeling. layered portion straight portion Figure 6.3 Proposed method of improving temperature resolution of T-MEMS 6.2 Repeatability The beams were found to undergo permanent change at prolonged exposure to temperatures above 800 °C. The change in curvature with time was studied for exposure of up to 50 minutes, and the behavior is shown in Figure 6.4 along with corresponding 61 0.002 -10 0.0015 -8 -6 0.001 -4 0.0005 deflection ( m) curvature (m -1 ) -12 -2 0 0 0 15 30 45 time (min) 60 Figure 6.4 Change in T-MEMS tip deflection after exposure to temperatures above 800 °C. values of tip deflection. The sample used in this study had an initial deflection of zero (flat). The beam deflections were measured after cooling to room temperature. Change in curvature per minute of exposure was 3.94 × 10-5 m-1, corresponding to a change in tip deflection of 0.2 m for a 100 m beam. The cause of the change in beam curvature is not understood. Relaxation in residual stress at high temperature will produce an opposite effect from what has been observed; the larger thermal expansion coefficient of poly-Si layer will cause the beams to bend up upon cooling to room temperature. A possible cause is oxidation of beam layers; the experiment was conducted in ambient air which may lead to formation of thermal oxide on the top layer of the beam. Since thermal oxides generally have compressive stress [39], this may contribute to the negative curvature of the beam following exposure to high temperature. Although geometric change at high temperature is not desirable for application, an understanding and characterization of the mechanism may allow the TMEMS to be annealed prior to use to ensure a zero initial deflection. An initially-flat 62 beam is beneficial since it will more closely follow the numerical models used to design the beam dimensions. 6.3 Adhesion In order for T-MEMS to function as proposed, the deflected beams must adhere to the Si substrate when they come in contact. This has not been tested yet due to the large air gap thickness of the present samples (~23 m). The maximum temperature reached by the samples under experimental conditions is approximately 850 °C, and at this temperature, the beams do not deflect enough to make contact with the substrate. However, preliminary study of adhesion was done using loose beams, scraped off the T-MEMS die, on a clean Si wafer. Loose beams consisting of silicon nitride and SiO2, curved to a radius of approximately 200 m, were placed on the Si wafer, which was heated by conduction using a compact heater. The wafer reached a maximum temperature of approximately 600 °C. After cooling to room temperature, the wafer surface was lightly rubbed by a cotton swab, and was examined under the microscope. Several broken beams were found on the wafer surface, indicating that some portion of the curved beams had adhered to the wafer and the portion extending from the wafer surface was rubbed off. This experiment showed that the beams adhere to the substrate with a force larger than the fracture strength of the beams at room temperature. 63