The number of medical applications for semiconductors is increasing as more designers take advantage of what single chip, small form factor, mixed-signal electronics have to offer. For example, combining the performance of analog and digital circuitry in close proximity alongside wireless communication has spurred development of new implantable cardiac rhythm management (CRM) products, neurostimlutators, drug pumps, and glucose and pressure sensors. Outside the body, similar technology has led to a new generation of digital hearing aids that offers performance superior to their analog predecessors. These are housed in packages so small they can be fitted discreetly into the ear canal.

New process technologies

All single-chip ASICs and ASSPs (application-specific integrated circuits and application-specific standard products) for implantable medical applications are required to consume as little power as possible to extend battery life and lengthen the time between invasive replacement procedures. However, an additional challenge in many applications is to use the lowest possible power while delivering high voltage signals. In CRM applications, control output stages are likely to involve voltages ranging from 10 V for pacemakers to almost 1,000 V for defibrillators.

In the past, the conflicting requirements of low power and high voltage have meant developing two different chips. Size constraints are another driver forcing the two chips together. Meeting the electronics size constraints requires developing new manufacturing processes to let low power and high-voltage circuits coexist in a single device.

Addressing the problem calls for a special isolation scheme: a deep-trench barrier that physically separates on-chip low-power circuits from the high-power voltages. The deep-trench barrier is added to a modified version of a CMOS semiconductor manufacturing process that is commonly used in medical electronics.

Adding low-power radio

The number of implantable devices that include a low-power radio transmitter or transmitter-receiver is rapidly increasing thanks to the radio frequency band standard called MICS (medical implant communication service). Low-power radio lets physicians download recorded data and reprogram implanted devices.

In the case of CRM devices, radio frequency (RF) communication is expected to replace the established but cumbersome inductive-sensing technology currently employed. The relatively low data rate (between 10 to 20 kbits/s) and short-range requirements of medical applications make relatively small demands on power. However, semiconductor companies are taking additional steps to further reduce power consumption. AMI Semiconductor, for example, uses a feature called “sniff mode.” This keeps the radio in sleep mode, waking periodically to poll the appropriate radio frequencies for the presence of an inquiring RF signal. The acquisition poll frequency can be pre-set to suit the needs of a specific application.