Annals of Biomedical Engineering, Voi. 20, pp. 423-437, 1992 Printed in the USA. All rights reserved.
0090-6964/92 $5.00 + .00 Copyright 9 1992 Pergamon Press Ltd.
A Glass/Silicon Composite Intracortical Electrode Array Kelly E. Jones, Patrick K. Campbell,* and Richard A. Normann Department of Bioengineering University of Utah Salt Lake City, UT (Received 1/4/91; Revised 6/27/91) A new manufacturing technique has been developed f o r creating silicon-based, penetrating electrode arrays intended f o r implantation into cerebral cortex. The arrays consist o f a 4.2 m m x 4.2 mm glass~silicon composite base, f r o m which project 100 silicon needle-type electrodes in a I0 • 10 array. Each needle is approximately 1,500 #m long, 80 ;tin in diameter at the base, and tapers to a sharp point at the metalized tip. The technique used to manufacture these arrays differs f r o m our previous method in that a glass dielectric, rather than a p-n-p junction, provides electrical isolation between the individual electrodes in the array. The new electrode arrays exhibit superior electrical properties to those described previously. We have measured interelectrode impedances o f at least 1013 D, and interelectrode capacitances o f approximately 50 f F f o r the new arrays. In this paper, we describe the manufacturing techniques used to create the arrays, focusing on the dielectric isolation technique, and discuss the electrical and mechanical characteristics o f these arrays. Keywords--Micromachining, electrodes.
Electrode array, Neural interface, Intracortical
INTRODUCTION In a previous p a p e r , we discussed m a n u f a c t u r i n g techniques for a novel t y p e o f int r a c o r t i c a l e l e c t r o d e a r r a y (2). This a r r a y was m i c r o m a c h i n e d f r o m a m o n o c r y s t a l line b l o c k o f silicon using a d i a m o n d dicing saw f o l l o w e d b y c h e m i c a l etching. T h e s t r u c t u r e o f the a r r a y c o n s i s t e d o f a 4.2 m m b y 4.2 m m s u b s t r a t e , a p p r o x i m a t e l y 200/~m thick, f r o m which p r o j e c t e d 100 needle-like electrodes spaced o n 400/~m centers in a l 0 x 10 a r r a y . E a c h needle was a p p r o x i m a t e l y 1.5 m m long, a n d t a p e r e d f r o m a d i a m e t e r o f 80/zm at its base to a p o i n t at its tip, O n the b a c k side o f the substrate, o p p o s i t e each needle, was a n a l u m i n u m p a d used to m a k e c o n t a c t to the electrode. T h e tips were c o a t e d with either p l a t i n u m o r g o l d , a n d the entire a r r a y (except the tips) was e n c a p s u l a t e d in p o l y i m i d e . I n this p a p e r , we h a v e r e f e r r e d to this entire
*Currently with Advanced Cardiovascular Systems, Santa Clara, CA. Acknowledgments-This work was supported by a grant from the National Science Foundation (BCS8808859), and by a National Science Foundation Graduate Fellowship to K.E.J. Address correspondence to Richard A. Normann, Department of Bioengineering, University of Utah, Salt Lake City, UT 84112. 423
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needle shaped structure, including pad, conductive needle, and metalized tip, as an electrode. An array of such electrodes may be useful in stimulation of localized groups of neurons, deep within the cortex, at low current levels. Such an array could also be used to record electrical activity from these cortical neurons. To electrically isolate the individual electrodes in the array from each other in our original manufacturing method, we employed a novel technique known as thermomigration (1) prior to any micromachining steps. The thermomigration process allowed us to create arrays of electrodes in such a manner that any two electrodes in the array were separated from each other by a pair of opposed p-n junctions. These p-n junctions were found to have leakage currents averaging 3.3 nA at 1 V. Thus, the electrodes in the array were effectively electrically isolated from each other. Although this scheme looked promising, we reported on a number of problems or potential problems associated with the manufacturing process, including poor yield, sensitivity of interelectrode impedance to surface conditions, and asymmetry of interelectrode impedance (3). In the present paper, we report on some manufacturing innovations that have addressed many of the problems associated with the previous method of manufacture. The current technique for manufacturing the arrays allows us to create a structure in which long, tapered electrodes are essentially suspended in a "sea of glass" substrate. This is done by melting a frit glass into shallow saw kerfs on one side of a silicon substrate, and removing much of the silicon material via deep saw kerfs cut from the opposite side of the silicon chip. This sawing procedure creates tall columns of silicon, which are held together at their bases by the glass. As all of the individual electrodes in the array are isolated from each other by this glass region, a very effective dielectric insulating layer is formed between adjacent electrodes. This method appears to give the array several advantages over the thermomigrated arrays, including a simplified manufacturing process, higher yield of functional electrodes, negligible interelectrode current leakage, and lower interelectrode capacitance. In addition to the dielectric isolation technique, we report briefly on our ongoing work regarding the encapsulation of these structures. METHODS Manufacturing Techniques
The starting material for the manufacturing technique presented here is monocrystalline semiconductor grade silicon. The material used is boron doped, p type, 0.01 ~-cm (100) wafers, 1.83 mm thick and 5.7 cm in diameter. The wafer is first cut into squares approximately 1.9 cm on a side. Each of these squares will eventually produce 9 arrays of 100 electrodes each. Next, a diamond dicing saw, mounted with a 50 tzm wide blade, is used to create a pattern of grooves on the surface of the square. These grooves are 300 #m deep, are spaced on 400/zm centers, and run in two orthogonal directions parallel to the sides o f the square (Figs. la, lb). The square is ultrasonically cleaned in methanol, rinsed in DI water, and blown dry. To create the insulating regions, a frit sealing glass (Corning, Inc. glass code 7070) is used. This glass has a coefficient of thermal expansion which is close to that of silicon, and exhibits high volume resistivity (see Table 1). The powdered glass is mixed to a slurry in methanol, and applied to the grooved surface o f the silicon square, where it flows into and fills the grooves. The slurry is deposited, dropwise, until the
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FIGURE 1. Drawing of silicon die: |a) top view, showing orthogonal rows of grooves cut into the surface; (b) side view of the die with 50/~m grooves; (c) with glass melted into the grooves and the surface ground smooth; (d) with columns cut into the die using a diamond dicing saw.
surface of the slurry is several hundred/zm above that of the silicon. After a brief air drying time, the silicon is placed into a covered ceramic boat and loaded into a dental vacuum furnace (J. M. Ney Company). A mechanical vacuum pump is used to evacuate the furnace chamber, and the glass coated silicon is held under vacuum for 20 min. to degas. After this degassing, the piece is subjected to a firing cycle. The furnace temperature is ramped up to 1,150~ over 12 min., and held there for 1 h. This allows the glass to melt and completely fill the saw kerfs, while any remaining bub-
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K.E. Jones, P.K. Campbell, and R.A. Normann TABLE 1. Corning Inc. glass code 7 0 7 0 properties. Young's Modulus: Coeff. of thermal exp. (0-300~ Coeff. of thermal exp. (25~ to set pt. 461 ~ Volume Resistivity Dielectric Constant (20~ 1 MHz) Loss Tangent (20~ 1 MHz)
51 GPa 32 x 10-7/~ 39 x 10-7/~ 1017+ fl-cm 4.1 .06%
bles rise to the surface. The surface tension and Viscosity of the glass keeps it from flowing out of the kerfs. After this one hour hold under vacuum, the vacuum is released, returning the oven chamber to ambient pressure. This repressurizes any bubbles in the glass, reducing their volume. After 10 min. at ambient pressure at 1,150~ the furnace temperature is reduced to 650~ safely above the annealing point of the glass. The furnace temperature is then ramped down to 400~ a temperature below the strain point of the glass, at a rate o f 2.5~ This slow cooling rate prevents the formation of stresses associated with any disparity in the coefficients of thermal expansion between the glass and the silicon. After slow cooling to 400~ the piece can be quickly cooled to room temperature. This glassing procedure leaves an uneven layer of glass on top of the silicon square. This is removed by grinding with a coarse grit diamond wheel, and polishing with alumina abrasive. This leaves a square o f silicon, with a crosshatch pattern of insulating glass embedded on one side (Fig. 1c). At this point, aluminum is deposited by electron beam evaporation onto the glassed side o f the piece, and photolithography is used to create a square aluminum contact pad on the back side of each 'island' o f silicon. These pads will later be used to make electrical contact to the individual electrode shanks in the array. The remaining process steps used to micromachine the silicon block into the desired geometry are very similar to those outlined in our previous paper (2) and are briefly reviewed here. A diamond dicing saw is used to cut two orthogonal sets of deep kerfs into the nonglassed side o f the silicon die. Each cut is made directly opposite from a glassed kerf, and the depth of the cut is set so that the blade just barely cuts into the glass. This leaves a number of tall square silicon columns, each separated from its neighbors by a layer of dielectrically insulating glass (Fig. ld). The 1.9 cm square piece is then cut into nine individuals arrays, each 6.35 mm per side. Figure 2 shows an electron micrograph of an area near the base of the array at this stage of the process, showing the square columns produced by the sawing process. Clearly visible are the marks and damage produced by the saw blade. To create the sharp, tapered geometries desirable in such an array, each individual array is subjected to a two-step acid etching procedure. The first step, termed a "dynamic" etch, involves a slow rotation of the array while mounted in a fixture suspended in a stirred acid bath. The composition of the acid is 5% HF, 95% HNO3. This etch step reduces the thickness o f the columns, while not significantly changing their shape nor length. After this etch, the array undergoes a "static" etch, wherein the array, positioned points-up in the bottom of a container of acid, undergoes a sharpening of the tips, without a significant change in diameter at the base or in length.
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FIGURE 2. Electron micrograph of a sawed, unetched array,
FIGURE 3. Electron micrograph of array after acid etching. The glass insulating regions are evident as raised bumps between columns. Note also that saw damage has been removed, and stressrelieving fillets have been formed at base of columns.
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Figure 3 shows an electron micrograph of an area near the base of one of the arrays at this point in the process. This picture shows the smooth, tapered silicon columns that form the electrode shanks, and the thin regions o f glass separating the needles. The magnification is the same as Fig. 2, allowing one to see the amount of silicon removed from the shank during etching. Here, the glass regions protrude slightly f r o m the silicon as a result of differences in etch rate between silicon and glass. Subsequently, the sharpened needle tips are coated with a metal suitable for charge transfer, such as gold, platinum, or iridium. It is then necessary to apply an encapsulant to the array, which will prevent current passage out of the array anywhere but at the electrode tips. The encapsulant used to prepare the arrays described here is polyimide. Previous work by Leyden and Basiulis (4) and McHardy et al. (5) indicates that polyimide, applied over a P I Q coupler (Hitachi Ltd.), adheres well to a variety of surfaces, including silicon and m a n y metals. This adhesion is maintained after long-term exposure to body temperature saline. The front side of the array (the electrodes and the side of the substrate to which they are attached) is coated with polyimide via the following process: First, P I Q coupler (Hitachi Ltd.) is applied to the array by dripping the liquid coupler onto the array, then wicking o f f with a paper wipe. The coupler is baked onto the array for 30 min. at 350~ The array is then mounted, points up, approximately 5 m m from the center of a silicon wafer. A polyimide/solvent mixture (DuPont's 2550 polyimide; solvent is n-methyl 2-pyrrolidone) in the ratio of 1.5:1 is dripped onto the array, until all of the needles are wetted. The wafer and array are then mounted on a spinner, and spun at 4,000 r p m for 20 s to remove the excess polyimide. Three such coatings are applied, with a 30 min. 'softcure' in a 90~ oven between each coat. Before the final imidization, the polyimide is removed from the tips of the array by inserting the tips through a thin metal or Teflon foil, and "floating" the array and protective film in a beaker of positive photoresist developer with the points projecting into the solution. The remaining polyimide is then fully cured by baking for 1 h in a 200~ oven. At this point, six to 10 polyimide-insulated gold wires are ultrasonically bonded to a subset of the aluminum contact pads on the back of the array. The back side of the array is encapsulated with several coats o f polyimide by dripping the polyimide onto the back side, and soft curing and imidizing as before. In between the first and final back side polyimide coatings, a narrow strip of Kapton film is used to cover and protect the wire bonds. The lead wires are then attached to a percutaneous connector, and the array is ready for sterilization and implantation. Evaluation Techniques
A number of tests were used to evaluate the suitability o f the array for use as an implantable stimulating or recording device. These methods are described here: Electrical tests: O f primary importance to us was the interelectrode impedance associated with the dielectric isolation technique. An attempt was made to quantify the interelectrode resistance and capacitance values in air. To measure the interelectrode resistance, a Keithley electrometer (model 610B) was used in the ammeter mode. A potential of 26V was dropped across an adjacent pair of electrodes, and the resulting current directed through the meter. To measure the interelectrode capacitance, an H P 4332A LCR meter was used on its lowest capacitance scale (3 pF). On this scale,
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the resolution of the meter is on the order of 10 fF. These electrical characteristics were measured on electrode arrays "as-is", that is, the arrays had not received any special cleaning or surface treatment before testing. The impedance of the electrodes in saline is another important characteristic of the electrodes. This impedance is a function of many factors, including stimulating waveform frequency, type of tip metallization, electrode geometry, etc. Our platinum coated electrodes were tested by submersing the front side only (electrode side) of the array in 0.9~ saline solution. The back side of the array was kept dry to eliminate any measurement error, which m a y be contributed by current leakage through the back side contacts, lead wires, etc. A small (1/~A), 1 kHz sine-wave current f r o m a constant current source was passed between the electrode and a large platinum ground electrode. The resulting voltage drop between the ground electrode and the test electrode was measured and used to compute the electrode impedance. A scanning electron microscope was used to evaluate the thickness of the polyimide encapsulation. A diamond dicing saw was used to dissect a single row of 10 needles from an array, and this " c o m b " of needles was laid flat. A cut was then made through the 10 needles, the cut forming approximately a 20 ~ angle with the base o f the array. This cut intersected each o f the 10 needles at a different height above the base, ranging f r o m about 200 #m above the base for the shortest needle, to about 1,300 #m above the base for the tallest. By looking at the cut ends of the needles, we could see and measure the polyimide thickness around the needle, and get an idea of the change in polyimide thickness with distance f r o m the base of the needle. The integrity of the polyimide coatings was also determined using "bubble testing." T w o polyimide-coated arrays, which had aluminum contact pads on the back and platinum coated tips, were used. A contact wire was attached to the back side of each array using silver paint. The back side was then insulated with several drops of fingernail polish. This method allowed us to make simultaneous contact with all 100 electrodes in the array with a single wire. The array was immersed in 0.9% saline, and a negative DC potential applied. During this test, the array was observed under a 40• dissecting microscope~ and any signs of electrolytic bubbling noted. To test the mechanical strength of the glass/silicon composite, test structures were made by breaking the needles o f f of several arrays. The remaining substrates, consisting of squares of silicon held in a sea of glass, were then ground to a flat surface using the diamond dicing saw. The samples were cut into pieces approximately 2 m m wide by 5 m m long. Eight such samples were prepared, six of them cut with their long axes aligned with the glassed kerfs, and two of them cut at a 45 ~ diagonal to the glassed kerfs. An acid etch step, similar to that used to f o r m the needle geometry, served to reduce saw damage. The pieces were then tested on an lnstron Universal Test Instrument, model number 1130. A special fixture was used to apply a threepoint bending stress to the sample, and the ultimate strength (break strength) of the sample was recorded. RESULTS Electrical Characteristics
The glass used to provide the interelectrode isolation has excellent insulating properties. D a t a supplied by Corning for this glass are listed in Table 1. Using these con-
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stants, one can calculate that the interelectrode resistance should be greater than 1017 fl, while the interelectrode capacitance should be on the order of 65 fF. When an attempt was made to measure the interelectrode DC impedance using the Keithley electrometer, no current was measured above the detection level of 10 -12 Ao Thus, the interelectrode resistance is greater than 1013 f]. When the interelectrode capacitance was measured using the L C R meter, a capacitance of 50 fF was measured (detection threshold was 10 fF). The impedance of each electrode in the array, when measured in saline, was found to be on the order o f 10 to 20 kf~ at 1 kHz. This impedance is primarily due to the spreading resistance; the resistance of the electrode shank itself is less than 50 ft. Insulation Integrity
An SEM of three combs of needles cut from an encapsulated array is shown in Fig. 4. Clearly evident in the foremost comb is the thick layer of polyimide near the base of the array. Figure 5 shows the cut end of a typical needle. The polyimide coating appears as a thin ring around the silicon needle. Typical thickness of this layer for the coating technique presented here is 3 to 6/~m around the shaft, and 20/~m along the substrate and near the base of the needles. Also apparent in some of the electron micrographs are "blobs" of polyimide that form around some of the needle shafts (Fig. 6). Although m a n y of our arrays, coated using the method described herein, are blob-free, others may have as m a n y as 15 or 20 blobs. Except for the blobs, the coatings are fairly uniform, and the thickness varies little f r o m top to bottom. At this time, we are uncertain as to the cause of these blobs, and the reason they occur on some needles or arrays but not others. During bubble testing, when the hydrogen reduction potential was exceeded, bub-
FIGURE 4. Electron micrograph of several polyimide coated combs. Each comb has been cut at an angle across the width of the array to allow determination of the polyimide thickness along the needles. Polyimide coating can be seen near the base of the foremost comb.
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FIGURE 5. Electron mierograph of the cut end of a silicon needle tip. Polyimide can be seen surrounding the cut end. The roughness on the sides of the needle is typical of the polyimide coating.
bles could be seen to appear at places where the polyimide coating was not continuous. For the two arrays tested, approximately six locations each showed signs of bubbling. The majority of these locations were near the tips of the needles, under the area that had been coated with platinum (this area would normally have had the polyimide stripped to expose the metalized surface). Strength Testing The average breaking strength of the eight pieces tested was 82.0 MPa, with a standard deviation of 23 MPa. The strength of the diagonally cut samples was not significantly higher than that for the orthogonally cut samples. It was also observed that for the six samples broken along the axes of the glass, the fracture plane remained in the glass along its entirety, and did not include the glass-silicon interface nor enter into the silicon. In samples broken at a 45 ~ angle to the axes of the glass, the fracture plane followed a more or less straight line, showing no preference for either the glass, the silicon, or the glass-silicon interface. These results are encouraging, for they indicate that the bonding between the glass and silicon is good, and does not introduce a major weakness into the system (Fig. 7). DISCUSSION Electrical Characteristics In a previous paper, we described a novel set of fabrication technologies that allowed us to create a three-dimensional structure well suited for use as an interface to
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K.E. Jones, P.K. Campbell, and R.A. Normann
FIGURE 6. Electron micrograph of a portion of an array showing the polyimide "blobs,'" which are formed on the needles. Note that polyimide has not been removed from the tips of the needles.
the central nervous system (2). We reported on the electrical characteristics of this electrode array, in which interelectrode isolation was achieved via a process known as thermomigration. Thermomigration was used to create a pair of back-to-back p-n junctions between each pair of needles in the array. While this method allowed us to create arrays with interelectrode impedances of 300 MI~, several drawbacks to this method were present. We have shown that most of these drawbacks have been addressed with the novel array fabrication method presented here. One of the problems with the thermomigration technique concerns yield. During the thermomigration process, adjacent trails of p-type material tended to merge, thus shorting out adjacent electrodes. We were able to produce arrays in which as few as 15% of the electrodes were thus shorted, but this might still be regarded as unacceptably high. In the present method, the interelectrode isolation is created with glassfilled saw kerfs in the silicon. As we can very accurately control the indexing of the saw, the isolation zones always are created as intended. We have not observed interelectrode shorting. Another advantage of the dielectrically insulated over the thermomigrated arrays is the magnitude and character o f the interelectrode isolation. Although the latter ex-
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(b)
FIGURE 7. Photographs of the back sides of two of the glassed substrates after break strength testing. The brightest areas are the aluminum contact pads, the darkest areas are silicon, and the grayish areas are the glass dielectric separating the silicon needles. In both photographs, the fracture runs horizontally near the center of the photo. (a| Substrate broken with the line of stress running at approximately a 45 ~ angle to the glassed regions. Note that the fracture follows a more or less straight path. (b) Substrate broken with the line of stress running parallel and orthogonal to the glass insulating regions. Note that the substrate fractures in the glass, not at the glass/silicon interface.
hibited interelectrode impedances averaging 300 Ml], this number was highly dependent upon the surface condition. To get an impedance this high, it was necessary to put the array through a rigorous cleaning and passivation procedure, the effects of which were nullified by contact with skin, or upon prolonged exposure to the ambient atmosphere. This would place great demands upon an encapsulation system to be used in vivo. By contrast, the dielectrically insulated arrays show an impedance higher than 1013 f~ even on uncleaned, unprepared arrays. Thus, the dielectrically insulated arrays show both higher interelectrode impedance and an insensitivity to surface condition. In addition, the glassing technique is easier and less time consuming than the therrnomigration technique. Because these arrays will most likely be used to deliver stimulating current pulses with high frequency components, another important characteristic of the arrays is the capacitive coupling between adjacent electrodes. Here, too, the dielectrically insulated electrode arrays exhibit superiority to the thermomigrated arrays. Because the thickness of the glass dielectric is many times greater than the depletion region of a thermomigrated array's p-n junction, the interelectrode capacitance of the dielectrically insulated arrays is lower. We have measured this capacitance at 50 fF for the dielectrically insulated arrays, as compared to 4 pF for the thermomigrated arrays (80 times greater than the dielectrically insulated arrays). Thus, we expect the dielectrically insulated arrays to show much lower cross talk and current leakage than the thermomigrated arrays. Another problem with the thermomigrated arrays, which we have discussed in a previous publication (3) is an asymmetry of interelectrode impedance with respect to
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stimulus polarity. When a given electrode in a thermomigrated array is used cathodically, the p-n junction associated with that electrode is back biased, and serves as the isolation between that electrode and all the others in the array. Thus, the total current leakage or cross talk from that electrode is due to the leakage across a single p-n junction. However, when the same electrode is used anodically, the p-n junction associated with it is forward biased, and, hence, presents little barrier to current flow. The source of interelectrode isolation is now the back biased p-n junctions surrounding the other 99 electrodes in the array, and, hence, there are now 99 parallel current leakage paths. The leakage or cross talk current is thus 99 times greater in the anodic mode than in the cathodic mode. This may require the use o f cathodic-only pulses, and would certainly require complex circuitry to deliver charge-balanced pulses to each electrode. One way of circumventing some of this difficulty would be to anodically bias the substrate to a voltage greater than the stimulus voltage. This would cause the diode associated with the stimulated electrode always to be back biased, regardless of stimulus polarity. While this does not reduce the overall amount of leakage, it does force the leakage to come from the bias source rather than the stimulus source. By contrast, the dielectrically insulated arrays have interelectrode impedances, which are many orders of magnitude higher than those for the thermomigrated arrays. Because the impedance was too high to be measured with our instrumentation, we are not able to state definitively whether or not the impedance is symmetric. However, as the nature of the isolation is simply dielectric, one would certainly expect it to be symmetric. Additionally, the sheer magnitude of the interelectrode impedance makes any DC leakage currents negligible, regardless of their symmetry or lack thereof. Hence, we expect to experience no such problems with these new dielectrically isolated arrays. Strength
An important characteristic of the electrode arrays is their mechanical strength. To ensure easy insertion into cerebral tissue, and minimal displacement of this tissue, the electrodes are micromachined into extremely thin, sharp needles. As silicon is a brittle material, these structures have the appearance of being very delicate. To insert the array into the cerebral cortex uniformly, without dimpling the cortex, we have found it necessary to use an "impact" insertion technique, which injects the array into the cortex at a high velocity (7). This places a stress upon the arrays, as does normal handling of the arrays. As reported in our previous papers, such arrays (when machined solely from a monocrystalline block o f silicon) exhibit excellent mechanical properties. They will easily withstand the forces associated with impact insertion of the arrays into cortical tissue (7). In fact, adjacent needles can be pushed together until their tips touch, without breaking. It was thus desirable to learn whether the new fabrication technique produced sufficiently strong structures, or whether the glass or glass/ silicon interface introduced significant weaknesses into the array. As mentioned previously, the break strength of the glass/silicon substrates was found to be 82 MPa (_+23 MPa). This is a fairly good strength for glass, although it is well below our measured value of 507 MPa (+ 130 MPa) for similarly prepared samples of pure silicon. However, the strength of the silicon/glass composite is reasonably good, and the glass
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seems to exhibit excellent bonding to the silicon. In use, we expect the highest stresses seen by the array will be experienced during cortical insertion. To date, we have inserted scores of arrays into cortical tissue, including many dielectrically insulated arrays, and have not observed any stress failures in either type of array. Thus, we conclude that the glass/silicon composite arrays are sufficiently strong for our purposes. It may be noted that the published number for the ultimate strength of silicon is 7 GPa (6) for highly polished, monocrystalline silicon. This is significantly higher than our measured value o f 507 (+_ 130) MPa, a difference most likely due to differences in surface condition.
Polyimide Encapsulation Chronic encapsulation of such an electrode array is an extremely challenging task, one that a number of investigators are working on. Because we have only recently begun working on encapsulation, we cannot report having solved all o f the inherent problems. The requirements for the encapsulant for our application are quite demanding, and include the following as a minimum: 9 the coating must be "biocompatible" for long periods of time; 9 the coating must remain 100% adherent to the substrate during chronic exposure to a warm saline environment; 9 the coating must be ion-impermeable; 9 the coating must be pinhole-free; 9 the coating must be thick enough to minimize capacitive shunting between the electrode shafts and the surrounding ionic environment. We have not yet succeeded in developing a polyimide coating process that meets all of the above mentioned requirements. For example, some electrolytic bubbling is noted along the electrode shanks. Whether this can be remedied by more coatings or by altering the curing cycles remains to be seen. Another critical property is the adherence of the coating after long-term saline exposure. Leyden and Basiulis (4) and McHardy et al. (5) have indicated that the polyimide adherence should be quite good, and we are currently conducting accelerated life tests in saline to verify this finding. Another imperfection associated with our coating is the blobs o f polyimide observed on the electrode shafts. Although we feel the blobs will have little effect on the performance of the completed array, we would like to eliminate them if possible. The occurrence of these blobs prevents us from using a more viscous polyimide formulation. Use o f a more viscous polyimide formulation has been shown to result in a thicker layer of cured polyimide. Furthermore, much of our process development has focused on minimizing the implanted volume of these arrays. It is our belief that this may help to minimize implantation trauma. The blobs increase the volume of the array, and may also result in a slight increase in the force necessary to insert the array into cerebral tissue. Thus, while the blobs will probably not seriously affect array performance, it may be worthwhile to eliminate them. At this point, polyimide appears to be the encapsulant most readily adaptable to our application. However, as can be seen from the above discussion, we have not yet finalized a method for creating a chronically encapsulated electrode array. Additional
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development and verification will be required before we can report having successfully achieved the criteria outlined above. Drawbacks o f the Composite-Type Arrays
The improvement in properties produced by the glass process are accompanied by a few potential drawbacks. For instance, the basic structure produced is now a composite material of silicon and glass, rather than monocrystalline silicon. This introduces an interfacial region, a potential site for mechanical failure. In addition, the glass has a lower service temperature than the silicon, which may preclude certain processing options. For example, the glass will flow at a temperature below that which is necessary for a reasonable rate of growth of thermal oxide on silicon. Also, the heterogeneity of the array means that more care must be exercised in choosing etchants, etc. for the process. We have been fortunate in that the 5% H F / 9 5 % HNO3 etchant used to micromachine the silicon seems to attack the glass at a similar rate as it does the silicon. Another concern involved with use of the glass is that its biocompatibility is as yet unproven. The glass contains the oxides of lithium, boron, and aluminum, for example. Leaching of these species, in ionic form, could cause undesirable effects; however, we expect all these species to remain bound in the glass matrix or to be contained by the encapsulant. We will be conducting animal biocompatibility studies to address these concerns. While the thermomigration fabrication method required the use of a semiconducting material such as silicon to produce the isolating p-n junctions, the present fabrication method could work with a variety of conducting materials. Frit sealing glasses can be obtained, which will bond to almost any material capable of handling the firing temperature. Thus, one could start with a block of platinum alloy, for example, and produce the same type of array using the glassing technique. This would produce an array with no need for thin film metallization of the tips, and eliminate the problems associated with thin films. In light of this, silicon may not be the ideal material for such an array; however, we have chosen to continue using it because of our familiarity with silicon processing techniques such as sawing, etching, coating, etc.; as well as the excellent mechanical properties of silicon. CONCLUSIONS We have devised a new fabrication scheme for penetrating electrode arrays that overcomes many of the problems of our previously reported method. These arrays possess many geometrical features that may make them ideally suited for use as an intracortical stimulating device. The arrays are fairly easy to produce, and are strong and rigid. A number of tasks must be accomplished before these arrays can successfully be used as chronic intracortical stimulating arrays. We are currently working on methods of encapsulation that will ensure chronic protection of the nonstimulating portions of the array. We are also working towards development of a demultiplexer chip, which could be bonded directly to the back of the array. Another need is for a tip metallization scheme, which will exhibit good bonding with the silicon, and safe
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charge transfer to neural tissue over months, or even years, o f stimulation. Despite these needs, we feel that the fabrication method described in this paper provides an excellent structural and electrical foundation that solves many o f the problems crucial to the development of a chronic intracortical electrode array. REFERENCES 1. Anthony, T.R.; Cline, H.E. Deep-diode arrays. J. Appl. Phys. 47(6):2550-2557; 1976. 2. Campbell, P.K.; Jones, K.E.; Huber, R.J.; Horch, K.W.; Normann, R.A. A silicon-based, three dimensional-neural interface: Manufacturing processes for an intracortical electrode array. IEEE Trans. Biomed. Eng. 38(8):758-768; 1991. 3. Jones, K.E.; Campbell, P.K.; Normann, R.A. Interelectrode isolation in a penetrating intracortical electrode array. Proc. Annu. Int. Conf. IEEE Eng. Med. Biol. Soc. 12:496-497; 1990. 4. Leyden, R.N.; Basiulis, D.I. Adhesion and electrical insulation of thin polymeric coatings under saline exposure. Mat. Res. Soc. Symp. Proc. 110:627-633; 1989. 5. McHardy, J.; Basiulis, D.I.; Angsten, G.; Higley, L.R.; Leyden, R.N. Accelerated testing of polyimide coatings for neural prostheses. In: Lupinski, J.H.; Moore, R.S., eds. Polymeric materials for electronics packaging and interconnection. ACS Symposium Series 407. 1989: pp. 168-175. 6. Peterson, K.E. Silicon as a mechanical material. Proc. IEEE 70(5):420-457; 1982. 7. Rousche, P.J.; Normann, R.A. A method for pneumatically inserting an array of penetrating electrodes into cortical tissue. Ann. Biomed, Eng. 20:413-422; 1992.
0090-6964/92 $5.00 + .00 Copyright 9 1992 Pergamon Press Ltd.
A Glass/Silicon Composite Intracortical Electrode Array Kelly E. Jones, Patrick K. Campbell,* and Richard A. Normann Department of Bioengineering University of Utah Salt Lake City, UT (Received 1/4/91; Revised 6/27/91) A new manufacturing technique has been developed f o r creating silicon-based, penetrating electrode arrays intended f o r implantation into cerebral cortex. The arrays consist o f a 4.2 m m x 4.2 mm glass~silicon composite base, f r o m which project 100 silicon needle-type electrodes in a I0 • 10 array. Each needle is approximately 1,500 #m long, 80 ;tin in diameter at the base, and tapers to a sharp point at the metalized tip. The technique used to manufacture these arrays differs f r o m our previous method in that a glass dielectric, rather than a p-n-p junction, provides electrical isolation between the individual electrodes in the array. The new electrode arrays exhibit superior electrical properties to those described previously. We have measured interelectrode impedances o f at least 1013 D, and interelectrode capacitances o f approximately 50 f F f o r the new arrays. In this paper, we describe the manufacturing techniques used to create the arrays, focusing on the dielectric isolation technique, and discuss the electrical and mechanical characteristics o f these arrays. Keywords--Micromachining, electrodes.
Electrode array, Neural interface, Intracortical
INTRODUCTION In a previous p a p e r , we discussed m a n u f a c t u r i n g techniques for a novel t y p e o f int r a c o r t i c a l e l e c t r o d e a r r a y (2). This a r r a y was m i c r o m a c h i n e d f r o m a m o n o c r y s t a l line b l o c k o f silicon using a d i a m o n d dicing saw f o l l o w e d b y c h e m i c a l etching. T h e s t r u c t u r e o f the a r r a y c o n s i s t e d o f a 4.2 m m b y 4.2 m m s u b s t r a t e , a p p r o x i m a t e l y 200/~m thick, f r o m which p r o j e c t e d 100 needle-like electrodes spaced o n 400/~m centers in a l 0 x 10 a r r a y . E a c h needle was a p p r o x i m a t e l y 1.5 m m long, a n d t a p e r e d f r o m a d i a m e t e r o f 80/zm at its base to a p o i n t at its tip, O n the b a c k side o f the substrate, o p p o s i t e each needle, was a n a l u m i n u m p a d used to m a k e c o n t a c t to the electrode. T h e tips were c o a t e d with either p l a t i n u m o r g o l d , a n d the entire a r r a y (except the tips) was e n c a p s u l a t e d in p o l y i m i d e . I n this p a p e r , we h a v e r e f e r r e d to this entire
*Currently with Advanced Cardiovascular Systems, Santa Clara, CA. Acknowledgments-This work was supported by a grant from the National Science Foundation (BCS8808859), and by a National Science Foundation Graduate Fellowship to K.E.J. Address correspondence to Richard A. Normann, Department of Bioengineering, University of Utah, Salt Lake City, UT 84112. 423
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needle shaped structure, including pad, conductive needle, and metalized tip, as an electrode. An array of such electrodes may be useful in stimulation of localized groups of neurons, deep within the cortex, at low current levels. Such an array could also be used to record electrical activity from these cortical neurons. To electrically isolate the individual electrodes in the array from each other in our original manufacturing method, we employed a novel technique known as thermomigration (1) prior to any micromachining steps. The thermomigration process allowed us to create arrays of electrodes in such a manner that any two electrodes in the array were separated from each other by a pair of opposed p-n junctions. These p-n junctions were found to have leakage currents averaging 3.3 nA at 1 V. Thus, the electrodes in the array were effectively electrically isolated from each other. Although this scheme looked promising, we reported on a number of problems or potential problems associated with the manufacturing process, including poor yield, sensitivity of interelectrode impedance to surface conditions, and asymmetry of interelectrode impedance (3). In the present paper, we report on some manufacturing innovations that have addressed many of the problems associated with the previous method of manufacture. The current technique for manufacturing the arrays allows us to create a structure in which long, tapered electrodes are essentially suspended in a "sea of glass" substrate. This is done by melting a frit glass into shallow saw kerfs on one side of a silicon substrate, and removing much of the silicon material via deep saw kerfs cut from the opposite side of the silicon chip. This sawing procedure creates tall columns of silicon, which are held together at their bases by the glass. As all of the individual electrodes in the array are isolated from each other by this glass region, a very effective dielectric insulating layer is formed between adjacent electrodes. This method appears to give the array several advantages over the thermomigrated arrays, including a simplified manufacturing process, higher yield of functional electrodes, negligible interelectrode current leakage, and lower interelectrode capacitance. In addition to the dielectric isolation technique, we report briefly on our ongoing work regarding the encapsulation of these structures. METHODS Manufacturing Techniques
The starting material for the manufacturing technique presented here is monocrystalline semiconductor grade silicon. The material used is boron doped, p type, 0.01 ~-cm (100) wafers, 1.83 mm thick and 5.7 cm in diameter. The wafer is first cut into squares approximately 1.9 cm on a side. Each of these squares will eventually produce 9 arrays of 100 electrodes each. Next, a diamond dicing saw, mounted with a 50 tzm wide blade, is used to create a pattern of grooves on the surface of the square. These grooves are 300 #m deep, are spaced on 400/zm centers, and run in two orthogonal directions parallel to the sides o f the square (Figs. la, lb). The square is ultrasonically cleaned in methanol, rinsed in DI water, and blown dry. To create the insulating regions, a frit sealing glass (Corning, Inc. glass code 7070) is used. This glass has a coefficient of thermal expansion which is close to that of silicon, and exhibits high volume resistivity (see Table 1). The powdered glass is mixed to a slurry in methanol, and applied to the grooved surface o f the silicon square, where it flows into and fills the grooves. The slurry is deposited, dropwise, until the
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FIGURE 1. Drawing of silicon die: |a) top view, showing orthogonal rows of grooves cut into the surface; (b) side view of the die with 50/~m grooves; (c) with glass melted into the grooves and the surface ground smooth; (d) with columns cut into the die using a diamond dicing saw.
surface of the slurry is several hundred/zm above that of the silicon. After a brief air drying time, the silicon is placed into a covered ceramic boat and loaded into a dental vacuum furnace (J. M. Ney Company). A mechanical vacuum pump is used to evacuate the furnace chamber, and the glass coated silicon is held under vacuum for 20 min. to degas. After this degassing, the piece is subjected to a firing cycle. The furnace temperature is ramped up to 1,150~ over 12 min., and held there for 1 h. This allows the glass to melt and completely fill the saw kerfs, while any remaining bub-
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K.E. Jones, P.K. Campbell, and R.A. Normann TABLE 1. Corning Inc. glass code 7 0 7 0 properties. Young's Modulus: Coeff. of thermal exp. (0-300~ Coeff. of thermal exp. (25~ to set pt. 461 ~ Volume Resistivity Dielectric Constant (20~ 1 MHz) Loss Tangent (20~ 1 MHz)
51 GPa 32 x 10-7/~ 39 x 10-7/~ 1017+ fl-cm 4.1 .06%
bles rise to the surface. The surface tension and Viscosity of the glass keeps it from flowing out of the kerfs. After this one hour hold under vacuum, the vacuum is released, returning the oven chamber to ambient pressure. This repressurizes any bubbles in the glass, reducing their volume. After 10 min. at ambient pressure at 1,150~ the furnace temperature is reduced to 650~ safely above the annealing point of the glass. The furnace temperature is then ramped down to 400~ a temperature below the strain point of the glass, at a rate o f 2.5~ This slow cooling rate prevents the formation of stresses associated with any disparity in the coefficients of thermal expansion between the glass and the silicon. After slow cooling to 400~ the piece can be quickly cooled to room temperature. This glassing procedure leaves an uneven layer of glass on top of the silicon square. This is removed by grinding with a coarse grit diamond wheel, and polishing with alumina abrasive. This leaves a square o f silicon, with a crosshatch pattern of insulating glass embedded on one side (Fig. 1c). At this point, aluminum is deposited by electron beam evaporation onto the glassed side o f the piece, and photolithography is used to create a square aluminum contact pad on the back side of each 'island' o f silicon. These pads will later be used to make electrical contact to the individual electrode shanks in the array. The remaining process steps used to micromachine the silicon block into the desired geometry are very similar to those outlined in our previous paper (2) and are briefly reviewed here. A diamond dicing saw is used to cut two orthogonal sets of deep kerfs into the nonglassed side o f the silicon die. Each cut is made directly opposite from a glassed kerf, and the depth of the cut is set so that the blade just barely cuts into the glass. This leaves a number of tall square silicon columns, each separated from its neighbors by a layer of dielectrically insulating glass (Fig. ld). The 1.9 cm square piece is then cut into nine individuals arrays, each 6.35 mm per side. Figure 2 shows an electron micrograph of an area near the base of the array at this stage of the process, showing the square columns produced by the sawing process. Clearly visible are the marks and damage produced by the saw blade. To create the sharp, tapered geometries desirable in such an array, each individual array is subjected to a two-step acid etching procedure. The first step, termed a "dynamic" etch, involves a slow rotation of the array while mounted in a fixture suspended in a stirred acid bath. The composition of the acid is 5% HF, 95% HNO3. This etch step reduces the thickness o f the columns, while not significantly changing their shape nor length. After this etch, the array undergoes a "static" etch, wherein the array, positioned points-up in the bottom of a container of acid, undergoes a sharpening of the tips, without a significant change in diameter at the base or in length.
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FIGURE 2. Electron micrograph of a sawed, unetched array,
FIGURE 3. Electron micrograph of array after acid etching. The glass insulating regions are evident as raised bumps between columns. Note also that saw damage has been removed, and stressrelieving fillets have been formed at base of columns.
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Figure 3 shows an electron micrograph of an area near the base of one of the arrays at this point in the process. This picture shows the smooth, tapered silicon columns that form the electrode shanks, and the thin regions o f glass separating the needles. The magnification is the same as Fig. 2, allowing one to see the amount of silicon removed from the shank during etching. Here, the glass regions protrude slightly f r o m the silicon as a result of differences in etch rate between silicon and glass. Subsequently, the sharpened needle tips are coated with a metal suitable for charge transfer, such as gold, platinum, or iridium. It is then necessary to apply an encapsulant to the array, which will prevent current passage out of the array anywhere but at the electrode tips. The encapsulant used to prepare the arrays described here is polyimide. Previous work by Leyden and Basiulis (4) and McHardy et al. (5) indicates that polyimide, applied over a P I Q coupler (Hitachi Ltd.), adheres well to a variety of surfaces, including silicon and m a n y metals. This adhesion is maintained after long-term exposure to body temperature saline. The front side of the array (the electrodes and the side of the substrate to which they are attached) is coated with polyimide via the following process: First, P I Q coupler (Hitachi Ltd.) is applied to the array by dripping the liquid coupler onto the array, then wicking o f f with a paper wipe. The coupler is baked onto the array for 30 min. at 350~ The array is then mounted, points up, approximately 5 m m from the center of a silicon wafer. A polyimide/solvent mixture (DuPont's 2550 polyimide; solvent is n-methyl 2-pyrrolidone) in the ratio of 1.5:1 is dripped onto the array, until all of the needles are wetted. The wafer and array are then mounted on a spinner, and spun at 4,000 r p m for 20 s to remove the excess polyimide. Three such coatings are applied, with a 30 min. 'softcure' in a 90~ oven between each coat. Before the final imidization, the polyimide is removed from the tips of the array by inserting the tips through a thin metal or Teflon foil, and "floating" the array and protective film in a beaker of positive photoresist developer with the points projecting into the solution. The remaining polyimide is then fully cured by baking for 1 h in a 200~ oven. At this point, six to 10 polyimide-insulated gold wires are ultrasonically bonded to a subset of the aluminum contact pads on the back of the array. The back side of the array is encapsulated with several coats o f polyimide by dripping the polyimide onto the back side, and soft curing and imidizing as before. In between the first and final back side polyimide coatings, a narrow strip of Kapton film is used to cover and protect the wire bonds. The lead wires are then attached to a percutaneous connector, and the array is ready for sterilization and implantation. Evaluation Techniques
A number of tests were used to evaluate the suitability o f the array for use as an implantable stimulating or recording device. These methods are described here: Electrical tests: O f primary importance to us was the interelectrode impedance associated with the dielectric isolation technique. An attempt was made to quantify the interelectrode resistance and capacitance values in air. To measure the interelectrode resistance, a Keithley electrometer (model 610B) was used in the ammeter mode. A potential of 26V was dropped across an adjacent pair of electrodes, and the resulting current directed through the meter. To measure the interelectrode capacitance, an H P 4332A LCR meter was used on its lowest capacitance scale (3 pF). On this scale,
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the resolution of the meter is on the order of 10 fF. These electrical characteristics were measured on electrode arrays "as-is", that is, the arrays had not received any special cleaning or surface treatment before testing. The impedance of the electrodes in saline is another important characteristic of the electrodes. This impedance is a function of many factors, including stimulating waveform frequency, type of tip metallization, electrode geometry, etc. Our platinum coated electrodes were tested by submersing the front side only (electrode side) of the array in 0.9~ saline solution. The back side of the array was kept dry to eliminate any measurement error, which m a y be contributed by current leakage through the back side contacts, lead wires, etc. A small (1/~A), 1 kHz sine-wave current f r o m a constant current source was passed between the electrode and a large platinum ground electrode. The resulting voltage drop between the ground electrode and the test electrode was measured and used to compute the electrode impedance. A scanning electron microscope was used to evaluate the thickness of the polyimide encapsulation. A diamond dicing saw was used to dissect a single row of 10 needles from an array, and this " c o m b " of needles was laid flat. A cut was then made through the 10 needles, the cut forming approximately a 20 ~ angle with the base o f the array. This cut intersected each o f the 10 needles at a different height above the base, ranging f r o m about 200 #m above the base for the shortest needle, to about 1,300 #m above the base for the tallest. By looking at the cut ends of the needles, we could see and measure the polyimide thickness around the needle, and get an idea of the change in polyimide thickness with distance f r o m the base of the needle. The integrity of the polyimide coatings was also determined using "bubble testing." T w o polyimide-coated arrays, which had aluminum contact pads on the back and platinum coated tips, were used. A contact wire was attached to the back side of each array using silver paint. The back side was then insulated with several drops of fingernail polish. This method allowed us to make simultaneous contact with all 100 electrodes in the array with a single wire. The array was immersed in 0.9% saline, and a negative DC potential applied. During this test, the array was observed under a 40• dissecting microscope~ and any signs of electrolytic bubbling noted. To test the mechanical strength of the glass/silicon composite, test structures were made by breaking the needles o f f of several arrays. The remaining substrates, consisting of squares of silicon held in a sea of glass, were then ground to a flat surface using the diamond dicing saw. The samples were cut into pieces approximately 2 m m wide by 5 m m long. Eight such samples were prepared, six of them cut with their long axes aligned with the glassed kerfs, and two of them cut at a 45 ~ diagonal to the glassed kerfs. An acid etch step, similar to that used to f o r m the needle geometry, served to reduce saw damage. The pieces were then tested on an lnstron Universal Test Instrument, model number 1130. A special fixture was used to apply a threepoint bending stress to the sample, and the ultimate strength (break strength) of the sample was recorded. RESULTS Electrical Characteristics
The glass used to provide the interelectrode isolation has excellent insulating properties. D a t a supplied by Corning for this glass are listed in Table 1. Using these con-
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stants, one can calculate that the interelectrode resistance should be greater than 1017 fl, while the interelectrode capacitance should be on the order of 65 fF. When an attempt was made to measure the interelectrode DC impedance using the Keithley electrometer, no current was measured above the detection level of 10 -12 Ao Thus, the interelectrode resistance is greater than 1013 f]. When the interelectrode capacitance was measured using the L C R meter, a capacitance of 50 fF was measured (detection threshold was 10 fF). The impedance of each electrode in the array, when measured in saline, was found to be on the order o f 10 to 20 kf~ at 1 kHz. This impedance is primarily due to the spreading resistance; the resistance of the electrode shank itself is less than 50 ft. Insulation Integrity
An SEM of three combs of needles cut from an encapsulated array is shown in Fig. 4. Clearly evident in the foremost comb is the thick layer of polyimide near the base of the array. Figure 5 shows the cut end of a typical needle. The polyimide coating appears as a thin ring around the silicon needle. Typical thickness of this layer for the coating technique presented here is 3 to 6/~m around the shaft, and 20/~m along the substrate and near the base of the needles. Also apparent in some of the electron micrographs are "blobs" of polyimide that form around some of the needle shafts (Fig. 6). Although m a n y of our arrays, coated using the method described herein, are blob-free, others may have as m a n y as 15 or 20 blobs. Except for the blobs, the coatings are fairly uniform, and the thickness varies little f r o m top to bottom. At this time, we are uncertain as to the cause of these blobs, and the reason they occur on some needles or arrays but not others. During bubble testing, when the hydrogen reduction potential was exceeded, bub-
FIGURE 4. Electron micrograph of several polyimide coated combs. Each comb has been cut at an angle across the width of the array to allow determination of the polyimide thickness along the needles. Polyimide coating can be seen near the base of the foremost comb.
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FIGURE 5. Electron mierograph of the cut end of a silicon needle tip. Polyimide can be seen surrounding the cut end. The roughness on the sides of the needle is typical of the polyimide coating.
bles could be seen to appear at places where the polyimide coating was not continuous. For the two arrays tested, approximately six locations each showed signs of bubbling. The majority of these locations were near the tips of the needles, under the area that had been coated with platinum (this area would normally have had the polyimide stripped to expose the metalized surface). Strength Testing The average breaking strength of the eight pieces tested was 82.0 MPa, with a standard deviation of 23 MPa. The strength of the diagonally cut samples was not significantly higher than that for the orthogonally cut samples. It was also observed that for the six samples broken along the axes of the glass, the fracture plane remained in the glass along its entirety, and did not include the glass-silicon interface nor enter into the silicon. In samples broken at a 45 ~ angle to the axes of the glass, the fracture plane followed a more or less straight line, showing no preference for either the glass, the silicon, or the glass-silicon interface. These results are encouraging, for they indicate that the bonding between the glass and silicon is good, and does not introduce a major weakness into the system (Fig. 7). DISCUSSION Electrical Characteristics In a previous paper, we described a novel set of fabrication technologies that allowed us to create a three-dimensional structure well suited for use as an interface to
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FIGURE 6. Electron micrograph of a portion of an array showing the polyimide "blobs,'" which are formed on the needles. Note that polyimide has not been removed from the tips of the needles.
the central nervous system (2). We reported on the electrical characteristics of this electrode array, in which interelectrode isolation was achieved via a process known as thermomigration. Thermomigration was used to create a pair of back-to-back p-n junctions between each pair of needles in the array. While this method allowed us to create arrays with interelectrode impedances of 300 MI~, several drawbacks to this method were present. We have shown that most of these drawbacks have been addressed with the novel array fabrication method presented here. One of the problems with the thermomigration technique concerns yield. During the thermomigration process, adjacent trails of p-type material tended to merge, thus shorting out adjacent electrodes. We were able to produce arrays in which as few as 15% of the electrodes were thus shorted, but this might still be regarded as unacceptably high. In the present method, the interelectrode isolation is created with glassfilled saw kerfs in the silicon. As we can very accurately control the indexing of the saw, the isolation zones always are created as intended. We have not observed interelectrode shorting. Another advantage of the dielectrically insulated over the thermomigrated arrays is the magnitude and character o f the interelectrode isolation. Although the latter ex-
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(b)
FIGURE 7. Photographs of the back sides of two of the glassed substrates after break strength testing. The brightest areas are the aluminum contact pads, the darkest areas are silicon, and the grayish areas are the glass dielectric separating the silicon needles. In both photographs, the fracture runs horizontally near the center of the photo. (a| Substrate broken with the line of stress running at approximately a 45 ~ angle to the glassed regions. Note that the fracture follows a more or less straight path. (b) Substrate broken with the line of stress running parallel and orthogonal to the glass insulating regions. Note that the substrate fractures in the glass, not at the glass/silicon interface.
hibited interelectrode impedances averaging 300 Ml], this number was highly dependent upon the surface condition. To get an impedance this high, it was necessary to put the array through a rigorous cleaning and passivation procedure, the effects of which were nullified by contact with skin, or upon prolonged exposure to the ambient atmosphere. This would place great demands upon an encapsulation system to be used in vivo. By contrast, the dielectrically insulated arrays show an impedance higher than 1013 f~ even on uncleaned, unprepared arrays. Thus, the dielectrically insulated arrays show both higher interelectrode impedance and an insensitivity to surface condition. In addition, the glassing technique is easier and less time consuming than the therrnomigration technique. Because these arrays will most likely be used to deliver stimulating current pulses with high frequency components, another important characteristic of the arrays is the capacitive coupling between adjacent electrodes. Here, too, the dielectrically insulated electrode arrays exhibit superiority to the thermomigrated arrays. Because the thickness of the glass dielectric is many times greater than the depletion region of a thermomigrated array's p-n junction, the interelectrode capacitance of the dielectrically insulated arrays is lower. We have measured this capacitance at 50 fF for the dielectrically insulated arrays, as compared to 4 pF for the thermomigrated arrays (80 times greater than the dielectrically insulated arrays). Thus, we expect the dielectrically insulated arrays to show much lower cross talk and current leakage than the thermomigrated arrays. Another problem with the thermomigrated arrays, which we have discussed in a previous publication (3) is an asymmetry of interelectrode impedance with respect to
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stimulus polarity. When a given electrode in a thermomigrated array is used cathodically, the p-n junction associated with that electrode is back biased, and serves as the isolation between that electrode and all the others in the array. Thus, the total current leakage or cross talk from that electrode is due to the leakage across a single p-n junction. However, when the same electrode is used anodically, the p-n junction associated with it is forward biased, and, hence, presents little barrier to current flow. The source of interelectrode isolation is now the back biased p-n junctions surrounding the other 99 electrodes in the array, and, hence, there are now 99 parallel current leakage paths. The leakage or cross talk current is thus 99 times greater in the anodic mode than in the cathodic mode. This may require the use o f cathodic-only pulses, and would certainly require complex circuitry to deliver charge-balanced pulses to each electrode. One way of circumventing some of this difficulty would be to anodically bias the substrate to a voltage greater than the stimulus voltage. This would cause the diode associated with the stimulated electrode always to be back biased, regardless of stimulus polarity. While this does not reduce the overall amount of leakage, it does force the leakage to come from the bias source rather than the stimulus source. By contrast, the dielectrically insulated arrays have interelectrode impedances, which are many orders of magnitude higher than those for the thermomigrated arrays. Because the impedance was too high to be measured with our instrumentation, we are not able to state definitively whether or not the impedance is symmetric. However, as the nature of the isolation is simply dielectric, one would certainly expect it to be symmetric. Additionally, the sheer magnitude of the interelectrode impedance makes any DC leakage currents negligible, regardless of their symmetry or lack thereof. Hence, we expect to experience no such problems with these new dielectrically isolated arrays. Strength
An important characteristic of the electrode arrays is their mechanical strength. To ensure easy insertion into cerebral tissue, and minimal displacement of this tissue, the electrodes are micromachined into extremely thin, sharp needles. As silicon is a brittle material, these structures have the appearance of being very delicate. To insert the array into the cerebral cortex uniformly, without dimpling the cortex, we have found it necessary to use an "impact" insertion technique, which injects the array into the cortex at a high velocity (7). This places a stress upon the arrays, as does normal handling of the arrays. As reported in our previous papers, such arrays (when machined solely from a monocrystalline block o f silicon) exhibit excellent mechanical properties. They will easily withstand the forces associated with impact insertion of the arrays into cortical tissue (7). In fact, adjacent needles can be pushed together until their tips touch, without breaking. It was thus desirable to learn whether the new fabrication technique produced sufficiently strong structures, or whether the glass or glass/ silicon interface introduced significant weaknesses into the array. As mentioned previously, the break strength of the glass/silicon substrates was found to be 82 MPa (_+23 MPa). This is a fairly good strength for glass, although it is well below our measured value of 507 MPa (+ 130 MPa) for similarly prepared samples of pure silicon. However, the strength of the silicon/glass composite is reasonably good, and the glass
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seems to exhibit excellent bonding to the silicon. In use, we expect the highest stresses seen by the array will be experienced during cortical insertion. To date, we have inserted scores of arrays into cortical tissue, including many dielectrically insulated arrays, and have not observed any stress failures in either type of array. Thus, we conclude that the glass/silicon composite arrays are sufficiently strong for our purposes. It may be noted that the published number for the ultimate strength of silicon is 7 GPa (6) for highly polished, monocrystalline silicon. This is significantly higher than our measured value o f 507 (+_ 130) MPa, a difference most likely due to differences in surface condition.
Polyimide Encapsulation Chronic encapsulation of such an electrode array is an extremely challenging task, one that a number of investigators are working on. Because we have only recently begun working on encapsulation, we cannot report having solved all o f the inherent problems. The requirements for the encapsulant for our application are quite demanding, and include the following as a minimum: 9 the coating must be "biocompatible" for long periods of time; 9 the coating must remain 100% adherent to the substrate during chronic exposure to a warm saline environment; 9 the coating must be ion-impermeable; 9 the coating must be pinhole-free; 9 the coating must be thick enough to minimize capacitive shunting between the electrode shafts and the surrounding ionic environment. We have not yet succeeded in developing a polyimide coating process that meets all of the above mentioned requirements. For example, some electrolytic bubbling is noted along the electrode shanks. Whether this can be remedied by more coatings or by altering the curing cycles remains to be seen. Another critical property is the adherence of the coating after long-term saline exposure. Leyden and Basiulis (4) and McHardy et al. (5) have indicated that the polyimide adherence should be quite good, and we are currently conducting accelerated life tests in saline to verify this finding. Another imperfection associated with our coating is the blobs o f polyimide observed on the electrode shafts. Although we feel the blobs will have little effect on the performance of the completed array, we would like to eliminate them if possible. The occurrence of these blobs prevents us from using a more viscous polyimide formulation. Use o f a more viscous polyimide formulation has been shown to result in a thicker layer of cured polyimide. Furthermore, much of our process development has focused on minimizing the implanted volume of these arrays. It is our belief that this may help to minimize implantation trauma. The blobs increase the volume of the array, and may also result in a slight increase in the force necessary to insert the array into cerebral tissue. Thus, while the blobs will probably not seriously affect array performance, it may be worthwhile to eliminate them. At this point, polyimide appears to be the encapsulant most readily adaptable to our application. However, as can be seen from the above discussion, we have not yet finalized a method for creating a chronically encapsulated electrode array. Additional
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K.E. Jones, P.K. Campbell, and R.A. Normann
development and verification will be required before we can report having successfully achieved the criteria outlined above. Drawbacks o f the Composite-Type Arrays
The improvement in properties produced by the glass process are accompanied by a few potential drawbacks. For instance, the basic structure produced is now a composite material of silicon and glass, rather than monocrystalline silicon. This introduces an interfacial region, a potential site for mechanical failure. In addition, the glass has a lower service temperature than the silicon, which may preclude certain processing options. For example, the glass will flow at a temperature below that which is necessary for a reasonable rate of growth of thermal oxide on silicon. Also, the heterogeneity of the array means that more care must be exercised in choosing etchants, etc. for the process. We have been fortunate in that the 5% H F / 9 5 % HNO3 etchant used to micromachine the silicon seems to attack the glass at a similar rate as it does the silicon. Another concern involved with use of the glass is that its biocompatibility is as yet unproven. The glass contains the oxides of lithium, boron, and aluminum, for example. Leaching of these species, in ionic form, could cause undesirable effects; however, we expect all these species to remain bound in the glass matrix or to be contained by the encapsulant. We will be conducting animal biocompatibility studies to address these concerns. While the thermomigration fabrication method required the use of a semiconducting material such as silicon to produce the isolating p-n junctions, the present fabrication method could work with a variety of conducting materials. Frit sealing glasses can be obtained, which will bond to almost any material capable of handling the firing temperature. Thus, one could start with a block of platinum alloy, for example, and produce the same type of array using the glassing technique. This would produce an array with no need for thin film metallization of the tips, and eliminate the problems associated with thin films. In light of this, silicon may not be the ideal material for such an array; however, we have chosen to continue using it because of our familiarity with silicon processing techniques such as sawing, etching, coating, etc.; as well as the excellent mechanical properties of silicon. CONCLUSIONS We have devised a new fabrication scheme for penetrating electrode arrays that overcomes many of the problems of our previously reported method. These arrays possess many geometrical features that may make them ideally suited for use as an intracortical stimulating device. The arrays are fairly easy to produce, and are strong and rigid. A number of tasks must be accomplished before these arrays can successfully be used as chronic intracortical stimulating arrays. We are currently working on methods of encapsulation that will ensure chronic protection of the nonstimulating portions of the array. We are also working towards development of a demultiplexer chip, which could be bonded directly to the back of the array. Another need is for a tip metallization scheme, which will exhibit good bonding with the silicon, and safe
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charge transfer to neural tissue over months, or even years, o f stimulation. Despite these needs, we feel that the fabrication method described in this paper provides an excellent structural and electrical foundation that solves many o f the problems crucial to the development of a chronic intracortical electrode array. REFERENCES 1. Anthony, T.R.; Cline, H.E. Deep-diode arrays. J. Appl. Phys. 47(6):2550-2557; 1976. 2. Campbell, P.K.; Jones, K.E.; Huber, R.J.; Horch, K.W.; Normann, R.A. A silicon-based, three dimensional-neural interface: Manufacturing processes for an intracortical electrode array. IEEE Trans. Biomed. Eng. 38(8):758-768; 1991. 3. Jones, K.E.; Campbell, P.K.; Normann, R.A. Interelectrode isolation in a penetrating intracortical electrode array. Proc. Annu. Int. Conf. IEEE Eng. Med. Biol. Soc. 12:496-497; 1990. 4. Leyden, R.N.; Basiulis, D.I. Adhesion and electrical insulation of thin polymeric coatings under saline exposure. Mat. Res. Soc. Symp. Proc. 110:627-633; 1989. 5. McHardy, J.; Basiulis, D.I.; Angsten, G.; Higley, L.R.; Leyden, R.N. Accelerated testing of polyimide coatings for neural prostheses. In: Lupinski, J.H.; Moore, R.S., eds. Polymeric materials for electronics packaging and interconnection. ACS Symposium Series 407. 1989: pp. 168-175. 6. Peterson, K.E. Silicon as a mechanical material. Proc. IEEE 70(5):420-457; 1982. 7. Rousche, P.J.; Normann, R.A. A method for pneumatically inserting an array of penetrating electrodes into cortical tissue. Ann. Biomed, Eng. 20:413-422; 1992.
