CUSTOM SILICON CHIP TECHNOLOGY FOR IMPLANTABLE FES MICROSTIMULATORS

Primoþ Strojnik*, Joseph Schulman*, Philip Troyk**,Gerald Loeb***, and Paul Meadows*

* Alfred E. Mann Foundation, Sylmar, CA, U.S.A. ** Pritzker Institute, IIT, Chicago, IL, U.S.A. *** Queen's University, Kingston, Ontario, Canada

ABSTRACT

A microminiature multichannel implantable stimulator is being developed for the use in therapeutic and functional electrical stimulation (FES) . The stimulation system uses multiple distributed stimulation modules instead of a single unit with multiple electrode leads. Up to 256 implanted modules can be individually programmed for current amplitude and pulse width for each stimulating pulse from a single external transmitter/controller. Initially, the micro-stimulators were designed as one chip devices with one receiving coil and one electrode at each end of a hermetic, biocompatible glass cylinder. However, design and chip manufacturing errors compelled additional components, a diode and a resistor, to be added as off-the-chip parts. Also, the micro-stimulator assembly procedure changed which necessitated transposition of bonding pads on the chip using a process known as gold-bumping.

BACKGROUND

Research by Guyton and Hambrecht (1) demonstrated the biocompatibility of sintered and anodized tantalum as well as the fact that mammalian extracellular liquid can serve as the liquid electrode of a tantalum capacitor. Robblee et al (2) described the ability of activated iridium to pass high current densities without electrochemical side effects. Heetderks (3) in a theoretical study showed the feasibility of an inductively powered implant with a very small receiving coil. As a consequence, the National Institutes of Health initiated a contract that has resulted in development of extremely small implantable and addressable stimulators.

APPROACH

A development of a micro-stimulator with a minimal number of components was proposed: an integrated circuit, a self resonant receiving coil, a protective glass capsule and two stimulating electrodes. One electrode is made of activated iridium, the other is a slug of anodized sintered tantalum. The latter constitutes a wet tantalum capacitor and serves both as a power storage and a charge balance capacitor. The powering and stimulation parameters are supplied by an external, high efficiency class-E transmitter. An amplitude modulated 2 MHz carrier powers and transfers stimulation data to the micro-stimulator and provides the basic clock for the digital part of the micro-stimulator circuitry. Since the microstimulators are addressable they represent another approach to multichannel stimulation in which each stimulation channel is one stimulator implanted directly at the stimulation site (4,5).

METHODS

An electronic circuit has been designed using a minimal number of components needed to make a implantable stimulator. A single chip circuit was developed that had only four connections: two for the receiving coil, two for the electrodes. The electronic block diagram is shown in Fig.1. The receiving coil continuously charges the tantalum capacitor/electrode through a rectifying diode. The charging current with a sub-threshold amplitude passes through the tissue between the electrodes. At a given command sent from the powering transmitter the charged tantalum capacitor is shorted and releases a stimulation pulse going through the tissue in the opposite direction as the charging current. A number of such commands, sent at a certain rate, generates a stimulation train of pulses.

Fig. 1

The rectified RF signal also passes through a series resistor. Any amplitude modulation on the RF signal is observed on this resistor as a voltage variation. By amplifying and cleaning the signal, the information hidden in the amplitude modulation can be extracted. Typically, the information contains idle pulses, device address (0-255), pulse-width (3.5 æs-257.5 æs in 256 equal steps), current amplitude (0.2 mA-60mA in 32 steps) and capacitor recharge current (10 æA and 100 æA). The maximal stimulation voltage is 7.5 V and the maximal pulse rate is 637 Hz per unit. Theoretically, 256 individual addresses can be accommodated with the number of bits reserved for addresses. In the chip production, seventeen distinctive chips were designed that differ only in their "address number". Stimulator "address" identifies the stimulation channel. A 3 micron P-well double poly CMOS technology was selected for the chip development.This technology makes possible combination of analog and digital circuits in the same process on the same chip.

RESULTS

When trying to manufacture a micro-stimulator according to the design, we came across several difficulties. First of all, the silicon chip did not perform according to the design. The on-the-chip rectifying diode had to be bypassed by an external diode. This enabled powering of the device, however the retrieval of stimulation information became less reliable since the series resistor was bypassed with the external diode as well. In spite of that we were able to assemble several tens of microstimulators and test them in vitro and in experimental animals to verify their biocompatibility and functionality. Fig. 2 shows a schematic assembly drawing of such a micro-stimulator. A micro-stimulator is 16 mm long and 2 mm in diameter.

Fig.2 : 1-iridium ball; 2-Coil; 3-Ferrite; 4-Bond wire; 5-Glass tubing; 6-Feed-through; 7-Tantalum electrode; 8-Extension stem; 9-Custom chip and external diode; 10-Metal shim; 11-Glass bead.

To improve the micro-stimulator's reliability, an external resistor was added in series with the external diode which required cutting of the diode connection on the chip itself. A trimming laser was used for this purpose. Because the external diode and resistor could accept only the first bond (ball bond), an intermediate land was created that could accept the second bond, (wedge bond), and thus enable the series connection of the diode and the resistor (Fig.3). Even though the scheme

Fig.3

worked very well on a bench, due to inaccurate aim and water vapor ingress into the active silicon through the broken surface chip protection layer, these microstimulators failed to work a few weeks after encapsulation. As it can be seen, the initial idea of having only two components inside the glass capsule is severely compromised. Therefore it was decided to change completely the assembly procedure, fix the errors on the chip and add components which would make the assembly easier and the device more manufacturable. The new chips performed even worse than the first ones, so we returned to the first chip while the problems with the second chip were being identified. We intended to again use the approach with the diode trace cutting and an external diode and resistor configuration. This time, the cut in the chip protective layer would be sealed and protected by a layer of silicon nitride. The new assembly procedure required all bonding pads to be on one side of the chip and have a certain sequence to make short bonds to the underlying micro PC Board. For that reason the old bonding pads had to be repositioned using a process called gold- bumping. The gold-bump process can only be done on an uncut wafer and requires several operations, described below. The processed wafer is plasma cleaned and a 10,000 Æ thick layer of low temperature silicon nitride is deposited over the wafer. Photo-resist is spun over the nitride and the old bond pads are exposed. The nitride is etched off at the pads and the rest of the photo-resist is washed off. A 2,000 Æ thick layer of titanium-tungsten is sputtered on the wafer. It serves as a barrier between aluminum and gold and also promotes gold adhesion. It is followed by a 1,000 Æ seed layer of gold. Again the photo-resist is spun on the wafer, its thickness defines the thickness of the future gold traces. The photo-resist is exposed to UV light through a mask showing the traces and subsequently developed. The wafer is plasma etched (few minutes in oxygen or argon) for one more time to remove the remnants of photo-resist. This leaves the seed layer of gold exposed. The gold-bumps are then electroplated on the seed layer to the desired thickness. Then the unexposed photo- resist is removed. The wafer is dipped into an etch solution which removes the layer of titanium- tungsten. Cleaning and rinsing is required at the end of the process. This time the diode trace was cut manually, using an ultrasonic trace cutter. It took 50 cutting probe tips and two weeks to cut diode traces on more than 1,200 chips on the wafer. A wafer with gold-bump chips delivered from a vendor was 100% electrically tested. There were 1,003 good chips and 136 bad chips on the wafer, which represents a 88% yield. Transposition of bonding pads enabled us to use the previously manufactured æPC Board as the æstim chip substrate. To accommodate the additional resistor the conducive areas on the æPC Board were reassigned. This modification required one long bond wire going from the chip pad to a distant conductive area on the æPC board. To stabilize this and other bond wires, a glob top epoxy has to be applied over the exposed chip and bond wire area.

DISCUSSION

Custom made silicon chips with combined analog-digital circuits are expensive to design and manufacture. Very often the first or even the second attempt to make a fully working chip results in a failure. In our case we were dealing with a chip that had a defective diode on board, which made the chip useless. Luckily, the position of the diode in the circuit and physically on the chip was such that it could be effectively removed from the chip and replaced by an external diode. Moreover, the resistor, essential for the data demodulation, was also conveniently placed and could be replaced by an external device. Having two additional devices in a small package required redesign of the package which in turn demanded repositioning of the bonding pads using a relatively inexpensive gold- bumping process.

REFERENCES

1. Guyton, D.L. and Hambrecht, F.T.: Theory and design of capacitor electrodes for chronic stimulation, Med. & Biol. Eng., 12, pp.613-619, 1974.

2. Robblee, L.S., Lefko, J.L., and Brummer, S.B.: Activated Ir: An electrode suitable for reversible charge injection in saline solution, J. Electrochem. Soc., 130, pp 731-733, 1983.

3. Heetderks, W.J.: RF Powering of Millimeter-and Submillimeter-Sized Neural Prosthetic Implants, IEEE Trans. Biomed. Eng. Vol BME-35, pp.323-327,1988.

4. Hildebrandt, J., Delere, K,, Richter, Ch., Seitz, W., and Uhrmeister, M.: Neuromuscular functional stimulation by miniaturized implantable electric simulators, Proc. 7th Int. Symp., ECHE, Dubrovnik, pp.283-295, 1981.

5. Strojnik, P., Schulman, J., Loeb, G., Troyk, P.: Multichannel FES Stimulator With Distributed Implantable Modules, Proc. 4th Vienna Int. Workshop FES, Baden/Vienna, 1992, pp 97-100;

ACKNOWLEDGEMENTS

This work has been supported by US NIH contracts N01-NS-9-2327, N01-NS-2-2322, N01-NS-5-2325, and the Alfred E. Mann Foundation.

AUTHOR'S ADDRESS:

Primoþ Strojnik, D.Sc. Alfred E. Mann Foundation 12744 San Fernando Rd Sylmar, CA 91342