In the past few years, inkjet printing has been emerging as a cost effective, environment friendly, net-shape microfabrication technique. This non-contact deposition technique facilitated the deposition of metallic and polymeric inks, biological proteins, and cells. The present work investigates the inkjet printing of microtransducers, with a focus on stress-sensing and movable microstructures. Piezoresistive and interdigitated capacitor based strain gauges were printed and
tested. The inexpensive conductive polymer, poly(3,4-ethylenedioxythiophene) oxidized with poly(styrenesulfonate) (PEDOT:PSS), was used as base material. We have performed measurements on several test structures to show that PEDOT:PSS does preserve its piezoresistive properties after printing. As we were relying further on PEDOT:PSS as a base material for printed transducers, the mechanical and electrical properties of this commercially available ink were comprehensively investigated. A dedicated experimental setup, which was used for the mechanical and electrical characterization of test structures, and micro-topography measurements were combined in order to extract the parameters of the PEDOT:PSS thin film: a zero-stress electrical conductivity of G=201 S/cm and gauge factor of 3.63. The longitudinal and transversal piezoresistive coefficients were estimated to be, = -5.110×10-10 ±13.7%Pa-1 L pi and = 3.45×10-10 ± 4.34%Pa-1 T pi respectively, which denote a piezoresistive material in a similar range of piezoresistivity as n-doped silicon and conventionally fabricated PEDOT:PSS.
A second explored direction was using inkjet microprinting technology for the fabrication of movable microstructures. An inkjet-printed CMUT (capacitive micromachined ultrasound transducer) was the target device, using ZnO as sacrificial layer and PEDOT:PSS as structural layer. The printed ZnO sacrificial layer was too rough, non-uniform and with a high porosity, so that printing a conductive membrane on top of it was unsuccessful. An alternative solution approach used kapton tape, a polyimide, as movable membrane; experimental characterization
has shown that the structure is not properly clamped along its rim, yielding a vibraion at a frequency of 1.1 KHz, when actuated, compared to a resonant frequency of 9.3 KHz achieved by finite element analysis of the CMUT structure. The approach shows enough promise for further investigations, along directions suggested at the end of the thesis.
University of British Columbia