Performance of PZT Based MEMS Deviceds With Integrated ZnO Electronics

Open Access
Author:
Wallace, Margeaux Louise
Graduate Program:
Materials Science and Engineering
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
March 25, 2016
Committee Members:
  • Susan E Trolier Mckinstry, Dissertation Advisor
  • Clive A Randall, Committee Member
  • Joshua Alexander Robinson, Committee Member
  • Thomas Nelson Jackson, Committee Member
  • Raegan Johnson Wilke, Special Member
Keywords:
  • thin films
  • piezoelectrics
  • integrated electronics
Abstract:
This thesis describes routes to enable increased understanding and performance of lead zirconate titanate-based microelectromechanical systems (MEMS). Emphasis was placed on monolithic integration with interposer electronics, and in understanding the role of mechanical boundary conditions on the ferroelectric/ferroelastic response. Co-processing of ZnO thin film transistors (TFTs) with Pb(Zr0.52Ti0.48)0.98Nb0.02O3 (PZT)-based piezoelectrics was investigated to assess whether interposer electronics on PZT can serve as a control scheme for large area arrays of sensors or actuators. ZnO TFT processing produced no measured changes in remanent polarization, dielectric constant, loss tangent, or aging rates. The TFT performance also did not degrade when fabricated on top of the PZT, the mobility (> 24 cm/Vs) remaining comparable to TFTs deposited on glass. To show ZnO array integration, a 5x5 array of PZT capacitors on glass was fabricated as a prototype for an adjustable X-ray mirror, where the ZnO TFT were used for row-column addressing of the actuators. 1.5 m thick sputter deposited PZT on glass patterned with large area (cm2) electrodes had a dielectric constant of >1200, tan ~ 2% and an average remanent polarization >23 μC/cm2. Photoreactive benzocyclobutene (BCB) electrically isolated the ZnO TFTs from the top electrodes of the piezoelectric. Flex cables were bonded to the wafer using anisotropic conductive film (ACF) to connect the gates (row control) and the drains (column control) in the TFT array to a control box. It was found that when actuating the PZT cells through the TFT array, the glass mirror experienced approximately 1.5 µm of deflection for a 10 V application. Studies on ferroelectric/ferroelastic domain reorientation were also performed on 1.9 m thick tetragonal {001} oriented PbZr0.3Ti0.7O3 films doped with 1% Mn. Different mechanical boundary constraints were investigated and domain reorientation was quantified through the intensity changes in the 002/200 Bragg reflections as a function of applied electric field. As the silicon substrate was systematically removed from beneath the electrode film and the mechanical boundary constraints of the substrate are relaxed, it was found that domains have an increased ability to reorient under applied fields. When 75% of the substrate was removed, more than a six fold increase in domain reorientation was observed. This approaches values for bulk ceramics and suggests that the piezoelectric coefficients should be enhanced upon declamping. The geometry of the released structures was also found to influence the domain reorientation. An untextured 1.77 µm thick PbZr0.3Ti0.7O3 thin film doped with 1% Mn was fabricated with electrodes which were 75% released in two different geometries, a circular diaphragm and an elongated beam diaphragm. It was found that when the electrodes were locally released (unable to freely deform and curl upon actuation) the amount of domain reorientation observed in clamped electrodes and the electrodes with circularly released diaphragms was approximately the same. Two times more domain reorientation was seen in locally released electrodes with elongated diaphragms for the same field excursion. This disparity between the two types of release state was shown to be due to the presence of in-plane stress which was not fully relieved upon the removal of the Si substrate. In release geometries where the perimeter of the free boundary is small, such as the case with the hole geometry, the in-plane stress suppresses domain reorientation. To further investigate the role of the substrate, in situ synchrotron X-ray diffraction was used to study intensity exchanges on 101/110 reflections in unoriented 950 nm thick PbZr0.3Ti0.7O3 thin film doped with 1% Mn on two substrates with different elastic properties: silicon and a borosilicate glass. The film on glass showed 32.1% domain reorientation at 3 Vc while the film on silicon showed 28.7%. Moreover, the amount of backswitching the films experienced upon release in field was shown to occur in a similar ratio to the difference in substrate elastic moduli. Films on glass which showed better saturated domain alignment (from less backswitching after poling), had smaller irreversible Rayleigh coefficients, α. Additionally, while the films were untextured, there was a higher {001} texture in the film on silicon. While this might have predisposed the film on silicon to have a higher a e31,f than the film on glass, it was found that due to the higher remanence in the film on glass, the measured piezoelectric coefficients between the two films were within error (e_(31,f)^glass~ –5.9 C/m2, e_(31,f)^silicon~ –5.7 C/m2). Finally, a process is presented for transferring a 850 nm thick Pb(Zr0.52Ti0.48)0.98Nb0.02O3 thin film from a silicon substrate, where it was grown, to a polymer substrate using a ZnO sacrificial layer. The developed process enabled measurements of PZT thin films with only 5 µm of polyimide as the substrate. Released electrodes were tested and compared to the response of a clamped film deposited concurrently. It was found that the film on polymer shows a slight increase in maximum and average remanent polarization as a result of imprint produced during the release process. Therefore, the films on polymer had a more saturated domain alignment after poling. When the films were electrically deaged, the Rayleigh behavior of both the clamped and released films was similar.