PhD oral defense (Tue, 2/19 2pm), Steve Zhuang

Steve Zhuang xzhuang at
Thu Feb 14 09:36:48 PST 2008

Stanford University Ph.D. Oral Examination

Xuefeng (Steve) Zhuang

Department of Electrical Engineering
Advisor: Professor Butrus T. Khuri-Yakub
Date: Tuesday, February 19, 2008
Time: 2 PM (Refreshments served at 1:45 PM)
Location: Packard room 202

Title: Capacitive Micromachined Ultrasonic Transducers (CMUTs) with  
Through-Wafer Interconnects


Capacitive micromachined ultrasonic transducer (CMUT) technology is a  
promising candidate for making 2D ultrasound transducer arrays for  
applications such as 3D medical ultrasound, non-destructive evaluation  
and chemical sensing.  Advantages of CMUTs over traditional  
piezoelectric transducers include low-cost batch fabrication, wide  
bandwidth, and ability to fabricate arrays with broad operation  
frequency range and different geometric configurations on a single  
wafer.  When incorporated with through-wafer interconnects, a CMUT  
array can be directly integrated with front-end IC to mitigate the  
effects of the parasitic capacitance from the connection cables.   
Through-wafer via is the existing interconnect scheme for 2D CMUT  
arrays, and many other types of MEMS devices.  However, efforts to  
integrate the direct fusion bonding process with through-wafer via  
interconnects have been unsuccessful.  The interconnect fabrication  
steps degrade the surface conditions of the wafer, making it difficult  
to bond.

In this talk I present a new through-wafer interconnect technique that  
is compatible with MEMS device fabrication in general, including both  
the surface-micromachining and the direct fusion bonding processes.   
The new technique uses through-wafer trenches to separate array  
elements and utilizes the silicon substrate as the conductive  
electrode.  Two exemplary implementations incorporating CMUT arrays  
are presented, one with a carrier wafer and the other with a built-in  
silicon frame structure for mechanical support.  Both implementations  
reduce fabrication complexity compared to the through-wafer via  
process, and result in low series resistance and small parasitic  
capacitance.  2D CMUT arrays integrated with trench-isolated  
interconnects show high output pressure (2.9 MPa), wide bandwidth  
(95%), small pulse-echo amplitude variation (sigma = 6.6%), and  
excellent yield (100%).  An important added benefit of the  
trench-isolated interconnect is the capability to realize flexible  
arrays, which is also discussed in the presentation.  I will also  
demonstrate 3D ultrasound imaging based on a trench-isolated 2D array  
integrated with an IC.

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