The substrate effects on the performance of thin film PZT pyroelectric detectors are theoretically and experimentally investigated. Detectors with thin pyroelectric films and thin substrates show better performace than those of thick films and thick substrates. Micron-scaled thin films and thin substrates cna be easily produced by thin film ferroelectric and MEMS technologies.
PZT ferroelectric thin films were prepared by a modified sol-gel solid precursor technique, which used PZT precursor powder. Oxides of 66weight percent and phase transition temperature of 430℃ were determined in this solid precursor powder by TGA (thermogravimetric analysis)and DTA (differential thermal analysis), respectively. The crystalline phases and micro-crystal structrues of PZT thin films on Pt/Ti/$Si_3N_4$/$SiO_2$/Si substrates were identified by XRD (X-ray diffraction) and TEM (transmission electron microscope). The film annealed at 600℃ for 30 minutes showed a randomly oriented pure pervoskite structure. The pyroelectric coefficient of the PZT thin film at room temperature was about 250μC/㎡K.
To improve the performance of the PZT thin films, each PZT and PT layer was alternately deposited on a Pt/Ti/$Si_3N_4$/$SiO_2$/Si substrates by a modified sol-gel solid precursor technique. For comparison, PZT thin films were also prepared with an identical method under the same conditions. XRD measurements revealed that the diffraction pattern of the alternately deposited PZT/PT thin film was due to the superimposition of the PZT and PT patterns. The alternately deposited PZT/PT thin films showed a better voltage response.
A nickel layer was deposited onto the PZT thin film, serving both as a selective radiation absorption layer and as a top electrode. The absorption coefficient of a 10㎚ thick nickel layer was 0.7 at 632㎚ wavelength.
A theoretical analysis on the pyroelectric current responsivity versus the chopping frequency with respect to the substrate thickness was performed. The theoretical current responsivity curves had a flat region and a ramp region over the frequency. The thinner the silicon substrate, the less the thermal loss by conduction, and thus, the higher the responsivity. The pyroelectric current responsivity of a detector without a silicon substrate was about two orders higher than that of a detector with a 450㎛ thick silicon substrate. At a fixed chopping frequency of 100㎐, as the silicon substrate thickness increased up to 50㎛, the pyroelectric current responsivity decreased exponentially, and then the decrease became relatively slow. When all other parameters except thickness were the same, a 1㎛ thick PZT detector without a silicon substrate showed a 6.66 times higher pyroelectric current responsivity than a 10㎛ thick PZT without a silicon substrate.
To verify the theoretical analysis, micromachined PZT thin film pyroelectric detectors with different silicon substrate thicknesses were fabricated and characterized. The experimental points followed quite closely the theoretical curve in terms of frequency. The experimental results showed the same tendency as the theoretical analysis for all the thicknesses used.
