Integration of Transducer Arrays with Integrated Circuits

It is advantageous to have the front-end circuitry, such as the preamplifier, to be close to the transducer element to mitigate the problem of signal degradation due to added noise and parasitics from the lengthy cables. Fabricating the transducer array and the electronic circuits separately reduces the cost, relaxes the process parameters for both components, and enables independent optimizations for both the IC and the CMUT chips. The IC and the CMUT are conveniently integrated using the method of flip-chip bonding. The through-wafer interconnects bring the flip-chip bond pads to the back side of the transducer array. These pads possess the appropriate under bump metallurgy (UBM), typically a stack of Ti/Pt/Au for flip-chip bonding to the electronics. We have tried two different flip-chip bonding techniques, both yielded working CMUT arrays integrated with IC.

 

Flip-chip bonding based on anisotropic conductive film (ACF)

 

In this approach, gold studs are placed on the bond pads of the IC using a wire-bonder. The ACF film is placed between the CMUT and IC dies. Following alignment, heat and pressure are applied. The ACF film conducts only in the bond pad area and in the vertical direction. For reliable connections, heat and pressure levels during bonding are both critical. No under-filling is required.

 

FIGURE 1. (a) Gold stud – ACF flip chip bonding technique. (b) Stud-bumped IC pads (c) Backside bondpads on the CMUT array. (d) Cross-section of the flip-chip bonded sample.

 

 

Flip-chip bonding based on solder bumping and Ni/Au plating

 

In order to make the aluminum pads on the IC chip solder wetable, electroless Ni/Au plating is performed on these pads. Small solder balls, typically with a diameter of less than 80 microns, are solder-jetted onto the bond pads on either the CMUT chip or the IC chip. Following the solder-ball placement, the chips are aligned, and placed in an inert gas environment at an elevated temperature for solder reflow. Solder joints are formed when the temperature is ramped down to room temperature. For added mechanical stability, the gap between the two chips is underfilled with appropriate materials. With this process, we achieved 256/256 working elements in a 16x16 array. Long-term reliability is better than the devices bonded with ACF.

Regardless of the flip-chip bonding technique, it is important to keep the IC chips at an electro-static-discharge (ESD) safe environment throughout the process.

 

FIGURE 2. (a) Solder bump – NiAu plating flip chip bonding technique. (b) Solder-bumped backside bondpads on the CMUT array (c) Electroless NiAu plated IC pads. (d) Cross-section of the flip-chip bonded sample.

 

 

 

References

 

[1] I. O. Wygant, et al., “An endoscopic ultrasound imaging system based on a two-dimensional CMUT array: real-time imaging results,” presented at 2005 IEEE Ultrasonics Symposium, Rotterdam, The Netherlands, September 19-21, 2005.

[2] X. Zhuang, et al, “Through-wafer trench-isolated electrical interconnects for CMUT arrays,” presented at 2005 IEEE Ultrasonics Symposium, Rotterdam, The Netherlands, September 19-21, 2005.

 

Acknowledgements

 

This project was funded by National Institutes of Health under grant CA99059.