Effect of Substrate-Thickness on Voltage Responsivity of MEMS-Based ZnO Pyroelectric Infrared Sensors

Abstract

Pyroelectric infrared sensors incorporating suspended zinc oxide (ZnO) pyroelectric films and thermally insulated silicon substrates are fabricated using conventional MEMS-based thin-film deposition, photolithography, and etching techniques. The responsivity of the pyroelectric films is improved through annealing at a temperature of 500 °C for 4 h. The temperature variation and voltage responsivity of the fabricated sensors are evaluated numerically and experimentally for substrate thickness in the range of 1 to 500 μm. The results show that the temperature variation and voltage responsivity both increase with a reducing substrate thickness. For the lowest film thickness of 1 μm, the sensor achieves a voltage sensitivity of 3880 mV/mW at a cutoff frequency of 400 Hz. In general, the results presented in this study provide a useful source of reference for the further development of MEMS-based pyroelectric infrared sensors.

Positioning System of Infrared Sensors Based on ZnO Thin Film

“…Following the annealing process, the grain size increases while the voids and gaps decease in both size and number, and the crystal structure becomes even denser. As a result, the thermal conduction performance of the ZnO sensing layer improves, leading to enhancement of the pyroelectric current signal transmission capability, as discussed in [ 17 ].…”

“…The total etching time was 12 min with a power of 100 W. Wet etching was then performed in a 30 wt% potassium hydroxide (KOH) aqueous solution at a temperature of 80 °C for 6 h to produce a final suspended film thickness of 1 μm. The average wet etching rate was 1.5 µm/min [ 17 ].…”

“…aqueous solution at a temperature of 80 °C for 6 h to produce a final suspended film thickness of 1 µm. The average wet etching rate was 1.5 µm/min [17].…”

“… Process flowchart showing main steps in sensor fabrication process: ( a ) LPCVD deposition of Si 3 N 4 , ( b ) E-beam deposition of Au/Cr bottom electrode, ( c ) photolithography, ( d ) patterning of bottom electrode by wet etching, ( e ) RF sputter deposition of ZnO thin film, ( f ) photolithography, ( g ) patterning of sensing layer by wet etching, ( h ) photolithography, ( i ) E-beam deposition of Au/Cr top electrode, ( j ) lift-off, ( k ) dry etching of back Si 3 N 4 layer, and ( l ) KOH wet etching of silicon substrate [ 17 ]. …”

“…It was observed that the ZnO layer with 200 • C and 300 • C annealing temperatures showed improved UV detection properties in comparison to 100 • C. The ZnO layer with a higher annealing temperature performs better in the field of UV photosensing applications. Lee [17] fabricated a pyroelectric infrared sensor incorporating a ZnO sensing layer, a thermally insulated Si substrate, and Au upper and lower electrodes. The backside of the wafer was selectively etched to create a suspended sensing structure.…”