Fabrication and Packaging of CMUT Using Low Temperature Co-Fired Ceramic

Abstract

This paper presents fabrication and packaging of a capacitive micromachined ultrasonic transducer (CMUT) using anodically bondable low temperature co-fired ceramic (LTCC). Anodic bonding of LTCC with Au vias-silicon on insulator (SOI) has been used to fabricate CMUTs with different membrane radii, 24 µm, 25 µm, 36 µm, 40 µm and 60 µm. Bottom electrodes were directly patterned on remained vias after wet etching of LTCC vias. CMUT cavities and Au bumps were micromachined on the Si part of the SOI wafer. This high conductive Si was also used as top electrode. Electrical connections between the top and bottom of the CMUT were achieved by Au-Au bonding of wet etched LTCC vias and bumps during anodic bonding. Three key parameters, infrared images, complex admittance plots, and static membrane displacement, were used to evaluate bonding success. CMUTs with a membrane thickness of 2.6 µm were fabricated for experimental analyses. A novel CMUT-IC packaging process has been described following the fabrication process. This process enables indirect packaging of the CMUT and integrated circuit (IC) using a lateral side via of LTCC. Lateral side vias were obtained by micromachining of fabricated CMUTs and used to drive CMUTs elements. Connection electrodes are patterned on LTCC side via and a catheter was assembled at the backside of the CMUT. The IC was mounted on the bonding pad on the catheter by a flip-chip bonding process. Bonding performance was evaluated by measurement of bond resistance between pads on the IC and catheter. This study demonstrates that the LTCC and LTCC side vias scheme can be a potential approach for high density CMUT array fabrication and indirect integration of CMUT-IC for miniature size packaging, which eliminates problems related with direct integration.

Piezoelectric Micromachined Ultrasonic Transducers with a Cost-Effective Bottom-Up Fabrication Scheme for Millimeter-Scale Range Finding

“…As the fabrication process is relatively uniform, it is possible to integrate arrays with a high element density, small element size, MUT arrays, and external circuits in a chip for packaging. Currently, capacitive micromachined ultrasonic transducers (CMUTs) and piezoelectric micromachined ultrasonic transducers are being studied by an increasing number of research groups [10,11,12,13]. The deflection of the flexural mode of the piezoelectric micromachined ultrasonic transducer (PMUT) mainly depends on the lateral strain of its piezoelectric film while an AC voltage is applied [14,15,16].…”

Mass Sensors Based on Capacitive and Piezoelectric Micromachined Ultrasonic Transducers—CMUT and PMUT

“…As selectivity is a critical component for sensory performances, the sensing material cross-sensitivity can be addressed by using advanced microfabrication technology, wherein, individual CMUT cells can be configured in an array format [30]. In order to use CMUT configuration as a gas sensor, the top membrane can be coated by a polymer using different techniques including spin coating, inkjet dispensing, dip coating, inkjet printing, layer-by-layer deposition and electrodeposition [16,[31][32][33][34][35].…”

Comparison between a new ultrasound probe with a capacitive micromachined transducer (CMUT) and a traditional one in musculoskeletal pathology

“…This transducer does not have any crystals but uses the new-generation silicon wafer technology composed of innumerable tiny vibration drums cells formed on the μm scale. Once submitted to electrical signals, each cell membrane vibrates, generating an ultrasonic signal with the same acoustic impedance of the human body (3). The inverse phenomenon is observed when an echo contacts the cell membrane.…”