Synthesis of KNN Thin Films Through Pulsed Laser Deposition for Piezoelectric Micromachined Ultrasonic Transducers
Open Access
- Author:
- Sterling, Kaleb
- Area of Honors:
- Materials Science and Engineering
- Degree:
- Bachelor of Science
- Document Type:
- Thesis
- Thesis Supervisors:
- Susan E Trolier-Mckinstry, Thesis Supervisor
Robert Allen Kimel, Thesis Honors Advisor - Keywords:
- KNN
PLD
pMUT
MEMs
Piezoelectric
Thin Film - Abstract:
- This thesis explores optimization of a pulsed laser deposition process to synthesize high-quality potassium sodium niobate (KNN) thin films for use in piezoelectric micromachined ultrasonic transducers (pMUTs). The chemical and thermal interactions during calcination were investigated as well as sintering of a bulk KNN target from powder precursors. The implementation of two calcination processes at 750C and a sintering process at 1075C produced bulk KNN targets of high density. KNN thin films were deposited onto platinized silicon substrates through pulsed laser deposition. The microstructure and phase purity were evaluated in the resultant films. Significant parameter variation was enacted onto the pulsed laser deposition process, based on parameter effects documented in literature and observation, to produce thin film characteristics that predict high piezoelectric performance. The platinized silicon substrate was aligned to the KNN target at varied target-to-substrate distance of 5.7 cm. The deposition ambient reached a base pressure of 2.2 ∙ 10-7 Torr before being increased to a deposition ambient pressure of 400 mTorr. The substrate temperature was set to 550C and the pulse rate of the excimer laser was set to 5 Hz for a deposition duration of 6 minutes. The substrate was allowed to cool to room temperature at a cooling ambient pressure of 400 mTorr. The resultant pulsed laser deposition parameters yielded KNN thin films with high phase purity, but exhibited suboptimal microstructures featuring high surface roughness and porosity. Post-deposition thermal annealing processes were subsequently added to improve the thin film microstructure while retaining phase purity. Initial investigation into annealing optimization yielded phase pure polycrystalline thin films with improved microstructure at 700C for 5 minutes, suggesting that further optimization of the annealing process, along with investigation into KNN doping, could produce high quality KNN thin films suitable for implementation into pMUTs. The electrical characteristics of the resultant film were as follows: the remanent polarization (𝑃𝑟) was 10.7 μC/cm2, the maximum polarization (𝑃 ) was 12.7 μC/cm2, the dielectric constant (𝜀𝑟) was 605. The loss tangent (𝑡𝑎𝑛𝛿) was 2.8%.