Void-Free Direct Bonding of CMUT Arrays with Single Crystalline Plates and Pull- In Insulation

Thomas Lehrmann Christiansen, Ole Hansen, Mathias Dahl Johnsen, Jeppe Nyskjold Lohse, Jørgen Arendt Jensen, Erik Vilain Thomsen

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The implications on direct bonding quality, when using a double oxidation step to fabricate capacitive micromachined ultrasonic transducers (CMUTs), is analyzed. The protrusions along the CMUT cavity edges created during the second oxidation are investigated using simulations, AFM measurements, and a proposed analytical model, which is in good agreement with the simulated results. The results demonstrate protrusion heights in the order of 10 nm to 40 nm, with higher oxidation temperatures giving the highest protrusions. Isotropically wet etched cavities exhibit significantly smaller protrusions than anisotropically plasma etched cavities after the second oxidation. It is demonstrated that the protrusions will prevent good wafer bonding without subsequent polishing or etching steps. A new fabrication process is therefore proposed, allowing protrusionfree bonding surfaces with no alteration of the final structure and no additional fabrication steps compared to the double
oxidation process. Two identical CMUT arrays with circular and square cavities having diameter/side lengths of 72 μm/65 μm and a 20 μm interdistance are fabricated with the two processes,
demonstrating void-free bonding and 100 % yield from the proposed process compared to poor bonding and 7 % yield using the double oxidation process.
Original languageEnglish
Title of host publication2013 IEEE International Ultrasonics Symposium Proceedings
Publication date2013
ISBN (Print)9781467356862
Publication statusPublished - 2013
Event2013 IEEE International Ultrasonics Symposium - Prague Convention Center , Prague, Czech Republic
Duration: 21 Jul 201325 Jul 2013


Conference2013 IEEE International Ultrasonics Symposium
LocationPrague Convention Center
Country/TerritoryCzech Republic
Internet address


  • Fields, Waves and Electromagnetics
  • Anisotropic magnetoresistance
  • Bonding
  • Cavity resonators
  • Oxidation
  • Standards
  • Temperature measurement


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