Effect of Polyimide Variation and its Curing Temperature on CMOS ...

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Sensors & Transducers Journal, Vol. 103, Issue 4, April 2009, pp. 144-154Typically there is a variation <strong>of</strong> around 3 % in effective permittivity as RH value increases from40 % RH to 100 % at a particular temperature. This effect is used to realize the capacitive sensor.Capacitive sensor can be realized by laying the electrodes horizontally or vertically. Verticalconfiguration can be either in stacked form or in pillar form. Horizontal capacitor structure as shown inFig. 1 (a), is the simplest as only one layer <strong>of</strong> metallization patterning is required .The verticalstructure shown in Fig. 1(b), gives better results in terms <strong>of</strong> nominal capacitance <strong>and</strong> sensitivity. Thisstructure involves two metal layers <strong>and</strong> capacitance is governed by the thickness <strong>of</strong> the polyimide <strong>and</strong>the perforation in the top electrode.Bonding PadInterdigital Electrodes<strong>Polyimide</strong>Silicon wafer(a)Upper Electrode<strong>Polyimide</strong>Lower ElectrodesBonding PadSilicon wafer(b)Upper Metal PadUpper ElectrodeLower ElectrodeLower Metal Pad<strong>Polyimide</strong> Pillars(c)Fig. 1. Types <strong>of</strong> capacitive sensors: (a) Horizontal; (b) Vertical; (c) Pillar.The pillar structure as shown in Fig. 1(c) is not very popular due to complexity in fabrication <strong>and</strong> alsorequirement <strong>of</strong> tight tolerances. In this work stacked vertical humidity sensor as shown in Fig.1 (b) has146
Sensors & Transducers Journal, Vol. 103, Issue 4, April 2009, pp. 144-154been selected. It is worth mentioning here that optimization has to be done for nominal capacitance<strong>and</strong> sensitivity for having better performance.Fig. 2. Comparison <strong>of</strong> capacitance variation on output capacitance.2.2. Micro Heater DesignMicro heater is incorporated in the humidity sensor to improve the response time. The heater elementsunderneath the sensor are fired to remove the absorbed <strong>and</strong> frozen moisture from the sensor. Majorchallenges in micro-heater are the thermal uniformity, low cost <strong>and</strong> compatibility with the st<strong>and</strong>ard ICprocess. Most popular used geometry is a simple me<strong>and</strong>ered poly-silicon micro heater resistor onSiO 2 /Si 3 N 4 [14-15]. The design incorporates by authors having TiN as the heating element on thedielectric membrane realized using KOH etching from backside. This material has been used as TiNformation is compatible to IC technology <strong>and</strong> also known to have excellent thermal uniformity <strong>and</strong>adhesion with nitride. The membrane consists <strong>of</strong> two stacked dielectric layers <strong>of</strong> SiO 2 (5000 A 0 ) <strong>and</strong>Si 3 N 4 (2500A 0 ). The thickness <strong>and</strong> deposition parameters are chosen carefully to compensate stresses<strong>of</strong> the deposited film. The back side <strong>of</strong> the micro-heater is subjected to bulk micromachining usingKOH for thermal isolation from the bulk. Keeping structure design <strong>and</strong> area <strong>of</strong> micro-heater equal, thewidth <strong>and</strong> corresponding spacing is varied to get three different variants as shown in Fig. 3.(a)W=S=40 um (b) W=S=20 um (c) W=S=10 umFig. 3. Three variants <strong>of</strong> micro heater with total length: (a) 11470 um; (b) 20390 um; (c) 41825 um.147
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