Designing MEMS driver ASIC for contact lens sensor
AnSem is in the business of translating real world problems into ASIC solutions by matching up RF, analog and digital processing elements to transform real world inputs into the required outputs. Medical applications often have particular needs for ultra-low power consumption, high integration, small size and novel powering techniques. The choice of process is often make or break for such a project which is why AnSem works with a number of leading fabs in order to support its customers.
Sensimed AG, a Swiss company specialized in design, development and commercialization of integrated micro-systems for medical devices has developed a soft silicone single use contact lens called Sensimed Triggerfish for Glaucoma management and treatment – see figure 1.
Figure 1: The Sensimed Triggerfish is a soft silicone single-use contact lens for Glaucoma management and treatment.
Glaucoma is an eye disease affecting about 4 percent of the population over 40 years of age which can lead to blindness unless treated early. One of the symptoms of glaucoma is an increased intraocular pressure. It has long been recognized that, since intraocular pressure varies during the day, continuous monitoring of glaucoma patients is needed. The static measurements currently performed by ophthalmologists during office hours are not able to detect pressure peak variations.
Passive and active strain gauges embedded in the Sensimed Triggerfish silicone lens monitor fluctuations in intraocular pressure through variations in diameter of the eye. The patient wears the Sensimed Triggerfish for up to 24 hours and undertakes normal activities including sleep periods. When the patient returns to his doctor, the data is transferred from the recorder to the practitioner’s computer via Bluetooth technology for immediate analysis – see figure 2.
Figure 2: The lens monitors the fluctuations in intraocular pressure through variations in diameter of the eye.
Figure 3: The patch antenna transmits RF power to the lens.
The ASIC was designed to mount directly within the contact lens. Other than the RF antenna; a coil of wire with a specified inductance, and the strain gauge, there are no other electrical components in the system. The bumped die is attached directly onto copper traces within the lens during manufacture, connecting it to the antenna and MEMS array.
Being powered from an RF source meant that the device had to first bridge rectify the AC signal coming from the antenna. The obvious rectification method using schottky diodes would heavily constrain the process choice. Normal PN diodes on the other hand would exhibit too much power loss. Instead, AnSem designed a patent pending RF speed, voltage multiplying, low dropout active rectifier with superior dropout performance even than schottky diodes and that could be implemented on virtually any mainstream 5v tolerant process. Power supply decoupling had to be achieved using only on-chip capacitance. This capacitance consumed a significant fraction of the silicon area. Behind this capacitance multiple voltage domains were established using internal LDOs, again using only internal capacitance to ensure stability.
By choosing ON Semiconductor’s I3T50 0.35 μm process, AnSem was able to incorporate power capture and conditioning, RF signaling, high linearity ADC and digital control functionality on a single die within the meager power budget and at low cost. Sensimed also benefits from the security of supply afforded by the automotive qualified process, which is so important to companies operating in the medical electronics field. As the lenses are single-use devices, cost was also an important factor. The ASIC digitizes the MEMS sensor reading and transmits the measurements back to the recorder via the same RF link used to power the device using load modulation techniques. For maximum RF power coupling an internal tuning capacitor had to be matched to the antenna inductance.
The +5-percent tolerance of this capacitor was an important parameter when selecting the process. The I3T50 process is well-controlled, and we were able to work closely with On Semiconductor to meet this requirement.
The actual coupling between the lens antenna and the patch antenna can vary by a factor of 3 or more depending upon the relative orientation of the two coils. This wide variation in input power meant that the circuit needed to operate reliably and safely over a large input power range. The low dropout of the active rectifier helped minimize losses at low power levels, while a current shunt safety circuit avoided overvoltage within the device when high power levels were received. As power levels were in the μW range there was no danger of any self-heating related discomfort in the eye.
The signals coming from the strain gauge are very small such that the LSB of the ADC is at the microvolt level. Keeping the switching noise of the rectifier away from the ADC was a problem both at circuit level and at layout level. Noise coupling through the substrate had to be mitigated through clever use of the high voltage pockets available in the I3T50 process.
In this application, the die is mounted directly onto a transparent lens structure. Care had to be taken with light sensitivity of the circuitry. The back die had to be metal coated to prevent light penetration. Meeting specifications over temperature is usually a large part of our design work. It is well known that transistor parameters vary widely over temperature. Thankfully in this application the patient’s eye maintains the device at around 34ºC (temperature of the surface of the eye).
This saved both in design time and simulation time as we were able to run simulations with a much reduced corner set. AnSem will now manage the complete ASIC life cycle for Sensimed, including industrialization and supply chain for volume production, leaving Sensimed to concentrate on serving their market. For the medical market the most important thing is to translate the customer’s requirements into a complete and physically realizable specification.
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