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

Sensors & Transducers Journal, Vol. 103, Issue 4, April 2009, pp. 144-154Various materials such as porous ceramics, hygroscopic polymers, <strong>and</strong> electrolytes are used ashumidity sensitive materials [8-9]. <strong>Polyimide</strong> is a type <strong>of</strong> hygroscopic material having the inherentadvantages <strong>of</strong> high sensitivity, high thermal stability (> 400 0 C), high resistance to most chemicals, <strong>and</strong>most importantly compatibility with the st<strong>and</strong>ard IC fabrication technology [10]. But polyimidehumidity sensors suffered from slow response <strong>and</strong> substantial long-term drift. This can be overcomeusing micro-heater <strong>and</strong> passivation layers. Humidity sensors reported by Qiu et al [11] has largerresponse time. Lei Gu et al [12] has integrated a poly-silicon heater to improve response time. Kang<strong>and</strong> Wise reported the sensors with high speed [13] by changing the shape <strong>and</strong> dimensions <strong>of</strong> themoisture sensing film. The reported structures, available in literature, so far have not detailed the effect<strong>of</strong> curing temperature <strong>and</strong> ion-implantation on the performances <strong>of</strong> the humidity sensors. This articlereports different variants <strong>of</strong> capacitive humidity sensors, comparison between vertical <strong>and</strong> capacitivehumidity sensor along with the description <strong>of</strong> the vertical capacitive sensors integrated with titaniumnitride(TiN) micro heater using polyimide as a sensing element. The effect <strong>of</strong> curing temperature <strong>and</strong>polyimide variation on the output capacitance has been detailed. Also the effect <strong>of</strong> different variants(keeping width <strong>and</strong> spacing as variable) <strong>of</strong> me<strong>and</strong>ered micro heater made with TiN as well asphosphorous doped poly-silicon on heater resistance have been presented in this paper. The design <strong>of</strong>sensor is detailed in section II A. Section II B describes the design <strong>and</strong> analysis <strong>of</strong> micro-heater. Thefabrication aspect compatible with CMOS foundry is detailed in section III followed by the results <strong>and</strong>discussions.2. Design <strong>of</strong> Sensor <strong>and</strong> Micro-heater2.1 Design <strong>of</strong> SensorCapacitive RH sensors consist <strong>of</strong> two electrodes <strong>and</strong> a dielectric layer between them. The dielectriclayer acts as active layer <strong>and</strong> absorbs the moisture thereby changing the dielectric constant, nominalcapacitance <strong>and</strong> consequently the measured relative humidity. This absorption changes the relativedielectric constant <strong>and</strong> thus changes nominal capacitance .The sensing capacitance can be described bythe following formula:Cs=ε εA 0 s(1)dThe relative dielectric constant <strong>of</strong> the polyimide film can be described using Looyenga’s empiricalequation [12]1/ 3 1/ 3 1/ 3 3s= [ γ ( ε w −εp) εp] , (2)ε +where γ is the fractional volume <strong>of</strong> water in the film, ε p the dielectric constant <strong>of</strong> polyimide <strong>and</strong> ε w isthe dielectric constant <strong>of</strong> water given as:where T is the temperature in K.ε w = 78.54{1-4.6 х 10 -4 (T-298)+8.8 х 10 -6 (T-298) 2 }, (3)In simplified terms the dielectric constant <strong>of</strong> the polyimide can be written as0.836 /(1+0.0049)333[ 0 .0404x(78.54] 1/ − 2.71/ ) + 2.71/ 3ε =, where x= %RH/100 (4)s145