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Shrinking silicon integrated passives for implantable heart monitors

In addition to the progress already made over the last twenty years, insertable/implantable medical device manufacturers have to push the envelope further, to extend people’s lives, improve quality of life and even allow the recovery of lost functions.


In addition to the progress already made over the last twenty years, insertable/implantable medical device manufacturers have to push the envelope further, to extend people’s lives, improve quality of life and even allow the recovery of lost functions. Size, weight, reliability and lifetime are all important when developing electronic medical devices. And the job includes passive components. To illustrate the role of passive components on the final structure and on the performance of an electronic medical device,we will detail in this article a concrete example of a miniaturized ‘implantable’ heart monitor device.

Engineering and design challenges

For the last decade, the medical community has been aware of the need for heart monitor implants to detect heart beat abnormalities.The device is inserted under the skin of the patient’s chest. It records every beat of the heart and the information is transmitted wirelessly to the medical center. The heart activity can therefore be monitored remotely so as to assess unforeseeable and abnormal cardiac events which would indicate the need to implant a pacemaker.The medical industry is strongly encouraging the development of smaller and longer-lasting devices to simplify surgical operations, and this has become a key challenge for the main medical device manufacturers. In the ‘implantable” device we consider as an example, the objectives were clearly defined by the final device manufacturer: to reduce the size of the electronics by a factor of 10 and optimize the power consumption to achieve a battery lifetime of 3 years.

Silicon-based 3D integrated passives

To achieve these objectives, large discrete components had to be replaced. The solution offered was based on silicon integrated passive device (IPD) technology.

One of the most critical passive components in an electronic

medical device - and one of the most difficult to integrate - is the capacitor, especially if its value is above 1μF. The development of 3D high-density capacitor technology has addressed these issues. The newest generation uses innovative 3D
structures and reaches densities of 250nF/mm2, enabling the manufacture of miniature silicon capacitors with a capacitance measuring several μF. The materials used in this technology are similar to those used to manufacture ICs. The primary benefits are high reliability and minimal leakage within the capacitors, mainly obtained thanks to the high purity dielectric layer generated during the high temperature curing.

Different steps with IPDs

Returning to our specific example, some multi project wafers were first proposed with single silicon capacitors with several values and silicon capacitor arrays,in order to qualify the technology and check the yield. This first step passed, two bundles of capacitors were designed. The first one was an array of large silicon capacitors totaling more
than 3μF in a single die. As specified above, the silicon-based IPD technology enables high capacitance in a small case size,with a thickness which can be as low as 100μm. A first step towards miniaturization was therefore achieved.

The second bundle was composed of another array of isolated silicon capacitors of 100nF each. The main drawback of this
design was the interference caused by the side-by-side position of these silicon capacitors,which would have had a negative influence on the leakage current and therefore on the power consumption. The challenge was to adjust the technology to get a leakage current under 10nA from one node to another. The design of this capacitor array has been revised using a ‘High Isolation’ technology mainly consisting in controlling the behavior of the diode coupled with the array. As a result, the integrated system had a total capacitance exceeding 7μF, relying on a high isolation
design to reach a final device size ten times smaller than the previous generation of ‘cardiac monitors’.

Leakage current was below 0.2nA/μF at

3.2V/25°C/120s. To achieve a faster and more reliable transition from prototyping to industrialization,prototypes were then developed under the same conditions and environment as for the industrialization step.

New horizons in medical technology

Integrated passive components with silicon in ‘insertable’ cardiac monitor devices offer new opportunities to improve the final product reliability, lifetime and performance. In this context, this leads directly to fewer device replacements for the patient. We can therefore imagine that this solution can be adapted to other implantable devices that have the same goals in terms of reliability, lifetime and performance. We are only at the dawn of an era of new possibilities. The R&D programs and technology roadmaps allow us to imagine many new functions and improvements to existing functions brought by integration.