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

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Sensors & Transducers Journal, Vol. 103, Issue 4, April 2009, pp. 144-1544.1. Characterization <strong>of</strong> the Micro HeaterTwo widely reported methods for temperature extractions are commonly employed. [17] The firstmethod employs a calibration <strong>of</strong> the resistance versus temperature curve <strong>of</strong> the heating element in auniform temperature environment [18]. The other method explo<strong>its</strong> two different metallic resistors, aheating element <strong>and</strong> a temperature sensor, lithographically defined very close to each other, in such away that they share approximately the same temperature [19]. In this paper, former topology which issimpler is used. Equations (5)-(8) have been used for selecting the dimensions. The heaters <strong>and</strong>sensing element are realized using TiN layer. Another micro-heater using doped poly silicon (PPD)has also been fabricated <strong>and</strong> studied. The comparison <strong>of</strong> the measured results <strong>of</strong> different variants [asrepresented (a), (b), (c) in Fig. 3] is shown below in Fig. 4.Resistances values <strong>of</strong> Microheater25Element Value(k-ohms)2015105TiNPPD01 2 3VariantsFig. 6. Values <strong>of</strong> resistances for different types <strong>and</strong> material.The poly-silicon resistor has higher resistance due to <strong>its</strong> higher sheet resistivity compared to TiN. Thesensitivity is therefore better. However, most <strong>of</strong> the other parameters, such as linearity, currentcarrying capacity, etc are found to be better. The not so good linearity in poly is due to the fact that it isa semiconductor <strong>and</strong> therefore carrier concentration (n) <strong>and</strong> mobility (µ) will vary due to temperature.TiN is a metal <strong>and</strong> the temperature effect on resistivity will be determined by mobility alone.The sensors were wire bonded to a printed circuit board, <strong>and</strong> were tested inside an environmentalchamber. The chamber has the humidity range <strong>of</strong> 20-90 % <strong>and</strong> temperature ranges from -25 0 C to+55 0 C. Capacitance measurement is performed using HP4284A capacitance meter. Micro heater isfirst characterized using st<strong>and</strong>ard sensors as well as IR testing employing infrared camera (8-14 micronb<strong>and</strong>). The temperature pr<strong>of</strong>ile <strong>and</strong> power dissipation curves are shown in Fig. 7.The heater temperature depends on the dissipated power <strong>and</strong> geometry position on the membrane. Atlow temperatures, heat loss is primarily due to conduction, so a linear relationship exists. This relationcan be shown to beP = G (T-Ta), (10)where P is the power dissipation, G is the thermal loss coefficient (W/ 0 K) <strong>and</strong> Ta is the ambienttemperature. The value <strong>of</strong> G is temperature dependent <strong>and</strong> at high temperatures as convective lossesovertakes the conduction losses giving rise to, non-linearity. This can be seen in Fig. 8 which can bedivided into three sub-areas. The first part marked as A, which shows linear response, is due toconduction losses. The transition part from conduction to convection loss marked as B, which is150
Sensors & Transducers Journal, Vol. 103, Issue 4, April 2009, pp. 144-154responsible for non-linearity <strong>and</strong> can be approximated as step-wise linear. The third part, shown as C,is again linear where majority <strong>of</strong> losses are due to convection.(a)(b)Fig. 7. Micro-heater results (a) <strong>Temperature</strong> pr<strong>of</strong>ile (b) power dissipation curve.4.2. Characterization <strong>of</strong> the SensorTwo types <strong>of</strong> polyimide have been used. PI-2723 polyimide (photo-pattern able) from Dupont [24]with the thickness <strong>of</strong> 2.7 µm is used as one <strong>of</strong> them. <strong>Polyimide</strong> exhib<strong>its</strong> a thermal expansion coefficient(α P = 70 х 10 -6 0 C -1 ) which is larger than those <strong>of</strong> silicon (α si = 2.6 х 10 -6 0 C -1 ) as well as aluminum(α AL = 0.23 х 10 -6 0 C -1 ). The resulting strain can be written as∫( α ( T ) −α( T )ε th =P AL)) dT(9)This relation shows presence <strong>of</strong> strain as α P >> α AL, thereby generating tensile stress in the film. Thishas been removed by annealing at elevated temperature (≈400 0 C).<strong>Polyimide</strong>, P1 2555 (non photo pattern able) is also used to fabricate vertical capacitors sensors. Theywere developed <strong>and</strong> rinsed using DE6180 <strong>and</strong> RI -9180 solutions. The difference in outputcapacitances <strong>of</strong> 4 pF comes out using photo pattern <strong>and</strong> non-photo pattern able polyimide. Fig. 8shows the complete structure integrated with the micro-heater. As seen in the Fig. 9 the response torelative humidity is fairy linear in the range <strong>of</strong> 30-80%. In higher ranges, the surface absorption effectcreeps in which can be minimized by turning the heater on before doing any measurement on thesensors.The general expression <strong>of</strong> capacitance as a function <strong>of</strong> relative humidity has been shown by Peter et al[3] using Looyenga’s equation. This formula depends upon the topology <strong>and</strong> in our case the generalexpression can be writtenC=2.351.725⎛⎞⎜0.2873.55⎟× RH +100(11)⎝⎠[using the dielectric constant <strong>of</strong> polyimide <strong>and</strong> water as 3.0 <strong>and</strong> 80].151
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