Flexible MEMS Sensors And Pyroelectric Thin Films

Abstract

MEMS sensors on flexible substrates were developed and fabricated utilizing conventional lithography, deposition and etching tools. All of these sensors were fabricated in a MESA structure on the flexible substrates for tactile sensing, robotic, and biomedical applications. In addition, a new processing technique was developed to release the flexible substrate more easily from the rigid silicon carrier wafer to increase fabrication yield. Temperature sensors were fabricated on flexible substrates; utilizing amorphous silicon as the sensing material. After releasing from the rigid silicon carrier wafer, the sensors were packaged and characterized for their I-V, thermal response and 1/f noise properties. The amorphous silicon showed a temperature coefficient of resistance of 2.88 %/K at room temperature. The average value of normalized Hooge coefficient, K1/f was found to be 1.2 × 10-11 giving a noise equivalent temperature of 5.5 mK over the bandwidth of 1 to 941 Hz.Relative pressure sensors were fabricated on a flexible substrate using nichrome (Ni-80%/Cr-20%) as the sensing material. The sensors were characterized for I-V, pressure sensing, and noise behavior. The average value of normalized Hooge coefficient K1/f was found to be 1.89 × 10-10. The average sensitivity was calculated to be 0.44 mV/MPa yielding a noise equivalent pressure (NEPr) of 58.23 kPa in a bandwidth of 1-8 Hz.Absolute pressure sensors were fabricated on a flexible substrate utilizing nichrome (Ni-80%/Cr-20%) as a sensing material. The sensors were packaged to create a reference vacuum cavity. The sensors were characterized for I-V, pressure sensitivity, and noise characteristics. The average value of normalized Hooge coefficient K1/f was found to be 4.64 × 10-11. The average sensitivity was calculated to be 1.25 nV/Pa producing a noise equivalent pressure (NEPr) of 7.8 kPa in the bandwidth of 1-8 Hz. Modified lead titanate (lead zirconium titanate and lead calcium titanate) was deposited on gold electrodes and characterized for pyroelectric sensor applications. The materials were characterized to analyze the variation of dielectric constant with temperature and the pyroelectric current measurement. The lead calcium titanate showed a pyroelectric coefficient of 40 × 10-5 C/m2-K and lead zirconium titanate demonstrated a pyroelectric coefficient of 28 × 10-5 C/m2-K after poling. It was observed that the poling caused an increase of the pyroelectric coefficient by 10 times. The incorporation of a low adhesion strength release layer in the fabrication process was investigated. It was observed that depositing a polyimide as a release layer helps to detach flexible substrate from the rigid silicon wafer after fabrication. The release layer can also withstand high processing temperature and survive during lift-off processes. Microbolometers were designed, fabricated on flexible substrates, and characterized. The design includes several new features such as a nanomesh absorber, a double layer absorber for infrared radiation, a long-pass optical filter, and µm-sized device-level vacuum packaging. The microbolometers showed a maximum responsivity of 3.78 × 103 V/W, detectivity of 4.44 × 106 cmHz1/2/W and a minimum NEP of 2.78 × 10-9 V/Hz1/2 at a chopper frequency of 40 Hz and 1 V dc bias.