Next-Gen Ultrasound

We construct one of our transducers by connecting many small units in parallel. Each contains a thin membrane, separated from an under­lying substrate by a tiny gap. In our latest designs, the membrane is made of silicon, possibly covered with a metal electrode. Silicon is desirable for several reasons. One is that it has good mechanical properties--it doesn’t fatigue, for example--and as long as it is thin, it will flex sufficiently. The substrate is silicon as well, doped with a sprinkling of other atoms to make it highly conductive.

We’ve developed different recipes for making these devices over the years, but the best scheme we’ve found uses two different wafers: a garden-variety silicon wafer for the substrate and one that’s slightly more exotic for the membrane, something known in the semiconductor industry as silicon-on-­insulator. The two are bonded together using nothing more than a modest amount of pressure and heat. This two-wafer approach permits us to add the membrane after the pockets that serve as the gaps are already formed, so we can sculpt the membrane and substrate as we wish--they don’t have to be just flat planes.

Building transducers from silicon makes it a snap to connect them with the front-end electronics of an ultrasound imager. Although it’s possible to fabricate a transducer directly over the associated electronic components on the very same silicon wafer, doing so creates a number of troublesome complications. The better tactic, we’ve found, is to bond the finished transducer array to a separate wafer containing the electronic circuitry.

Connecting each transducer element may be tricky for tightly packed 2-D arrays, because there isn’t much free real estate on the front surface to route a lot of electrical leads. But here again the microelectronics industry has a good solution: Make the connections to the electronics by burrowing down through the transducer substrate and creating vertical conductive channels, which are known in the trade as through-silicon vias.

Given the many wonderful things we’ve said about them, you might think that capacitive micromachined ultrasonic transducers would already be in use in medical imaging equipment. Many of the companies that make these systems have indeed embraced this technology, but it hasn’t yet reached vendors’ shelves. Most of the remaining technical issues are minor, though. Some stem from the electric fields these transducers must contain.

Although there is no chance of arcing across the evacuated gaps, the enormous electric fields can stress the insulating layers to the breakdown point. And even without that, these large fields can inject static electric charge into those layers, which reduces the electric field in the gap, making it necessary to keep adjusting the dc bias field to compensate.

Another challenge with capacitive transducers is that they do not respond as linearly to drive voltages as PZT transducers do. Nonlinearity of the transducer becomes an issue when an ultrasound system is used to image the nonlinear response of biological tissues. Fortunately, there are ways to circumvent this problem, such as purposefully distorting the drive signal to compensate for the nonlinearity of the transducer.

We--along with a slew of engineers at Canon, General Electric, LG Electronics, National Semi­conductor, Siemens, and elsewhereare working to solve these nagging problems and to confront the many other practical realities you have to deal with in any new product. That’ll take some time, but it’s clear to us that there are no showstoppers here.

It won’t be long before this new breed of trans­ducers arrives at hospitals all over the world. So expect those first baby pictures you’re shown, among other sorts of ultrasound images, soon to become even more stunning.