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IEEE TRANSACTIOKS O N BIOMEDICAL ENGINEERING. VOL 37. KO. I . JANUARY 1990

Scaling Limitations of Silicon Multichannel Recording Probes

Ahstract-Thi\ paper describes the scaling limitations of multichannel recording prohe5 fabricated for use in neurophysiologj using silicon integrated circuit technologic\. Scaled d i c o n probe substrates 8 pni thick and 16 pm wide can he fabricated using boron etch-stop technique\. Theoretical expresGon5 for calculating the thickness and width of silicon wbstrates hape been derited a n d agree closel) with euperimental rewlts. The effects of scaling probe dimensions on its strength and stiffnes5 a r e described. The probe \hank diniensions can be designed to t a r ? the strength and \tiffnes5 for different application\. The waled Glicon w b \ t r a t e \ h a r e a fracture 5tre-s~of about 2 x 10“’ d\n/cm’, which I \ about six time5 that of bulk silicon, and a r e strong and \er? flexible. Scaling the feature \izes of recording electrode a r rays down to 1 p m i\ possible with le55 than 1 percent electrical cro5stalh b e h e e n channel\.

1. INTRODLICTION ONG-TERM simultaneous recording of biopotentials individual cell discharges) from many cortical neurons is essential to understanding neural system architectures and information processing techniques. It is also required for the successful development of a variety of closed-loop neural prostheses to aid the handicapped. In order to overcome the shortcomings of present recording electrodes, which have not permitted such measurements in the past, a multielectrode intracortical microprobe for the simultaneous multiunit recording of electrical discharges from neural structures has recently been fabricated using the boron etch-stop technique [ I ] . (21. Fig. 1 shows the structure of the probe along with a SEM photograph of one of the actual devices. The probe consists of a micromachined silicon substrate which supports an array of thin-film conductors which are insulated above and below by chemical vapor deposited (CVD) silicon dioxide and silicon nitride dielectric layers [ I ] . Openings in the upper dielectric are used to define recording sites at the ends of the conductors. The exposed recording sites are connected to signal processing circuitry located at the rear of the probe through deposited conductors of either polysilicon or refractory silicides. The probe is the first of its kind to integrate amplifying /multiplexing circuitry and recording electrodes on one monolithic chip using a simple, high-yield fabrication process. Its small size and

-CHIP SIGNAL PROCESSING CIRCUITRY

SUPPORTING SUBSTRATE INTERCONNECTING LEADS

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Manuscript received October 27, 1988; revised June 6, 1989. This work was supported by the National Institutes of Health under contract number NIH-NINCDS-NO1-NS-2384. The authors are with the Center for Integrated Sensors and Circuits, Solid-State Electronics Laboratory. Department of Electrical Enginccring and Computer Science, University of Michigan, Ann Arbor, MI 48109. IEEE Log Number 893 153 1 .

00 18-9294/90/0100-0001$01 .OO O 1990 IEEE

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IEEE TRANSACTIONS ON BIOMEDICAL ENGINEERING, VOL. 37, NO. I , JANUARY 1990

reproducible electrical and physical characteristics make the structure a versatile tool for a variety of neurophysiological applications. This first-generation active multichannel recording probe supports ten recording electrodes on a 160 pm wide silicon shank. The recording sites are 100 pm apart in depth and span a distance of 900 pm. Although these probes minimize tissue damage and provide the ability to simultaneously record from several neurons, future multichannel recording probes need to be scaled to still smaller shank sizes and should carry more electrodes on the smaller shank to minimize tissue damage and allow optimization of electrode-cell position. It has been shown [3] that probe shanks wider than 100 pm appreciably alter extracellular current flow, enhancing recorded signals along the front of the probe but almost totally shielding the recording sites from cells behind the shank. Only when the shank width is scaled to less than about 30 pm (or a cell diameter) does the recording field of a site become more nearly spherical. Consequently, the fundamental scaling limits associated with the probe structure need to be addressed and analyzed in detail. These limits are provided by the basic substrate formation process itself, by the strength of the scaled probes (which must penetrate a variety of tissues and membranes), and by electrical crosstalk between conductor channels as dimensions are reduced. Although this paper addresses these issues as they relate to multielectrode recording probes, the conclusions derived will likely have a much broader impact on a variety of integrated sensors which use the same basic technologies. Section I1 will briefly review the overall fabrication process of the multichannel microprobes and will discuss the scaling limits associated with it. Section I11 will discuss the scaling limits associated with the formation of the probe shank using deep boron diffusion and boron etch-stop techniques. Section IV will address the effects of scaling probe dimensions on the strength and stiffness of the silicon microprobe substrates and will analyze the fundamental mechanical limits of these structures. Section V will discuss the effects of electrical crosstalk as lithographic features are scaled down and will present a finite-element analysis of the interchannel crosstalk, and Section VI presents an example of probe scaling based on the results discussed in this paper. Finally, Section VI1 will draw conclusions from the results derived from the present study and discuss the implications of these results for other sensors using the same basic technology. 11. MICROPROBE FABRICATION Fig. 2 shows the fabrication process sequence developed for these probes. Fabrication begins with a silicon wafer of standard thickness, doping, and orientation. The wafer is first oxidized, and the oxide is patterned to define the intended probe areas. Next, these areas are subjected to a deep boron diffusion to heavily dope the probe substrate to concentrations above l O I 9 cmP3 [l]. The temperature and time of this diffusion determine the final thickness of the probe shank. Then the lower CVD di-

Grow and Pattern Thermal Oxide, Deep Boron Diffuse to Form Substrate

Deposit Bottom Dielectric Films, Deposit & Pattern Electrode Conductor, Deposit Top Dielectric Films

Pattern and Etch Top Dielectric Films, to Define Recordinging Sites, Deposit and Liftoff Site Material

Etch Field Dielectric Films Outside of Probe Areas, Etch and Free Probes in Silicon (EDP) Etchant Fig. 2. Fabrication sequence for multichannel silicon microprobes

electric layers (composed of 400 nm silicon dioxide, 200 nm silicon nitride, and 400 nm of silicon dioxide) are deposited, and the electrode conductor material is deposited and patterned to define the conductors. Next, the top dielectric layers, similar in all respects to the lower dielectric layers, are deposited and patterned using a dry etching process to open the recording sites and bonding areas. With the masking resist still in place, gold is inlayed in the exposed areas and is removed through a lift-off process everywhere except from the recording and bonding areas. Thus, the recording sites and the bonding pads are self-aligned to the dielectric openings. The field dielectrics outside the intended probe areas are now removed. Finally, the wafer is subjected to an unmasked etch in a mixture of ethylenediamine-pyrocatechol-water (EDP) [4] to separate the individual probes. The EDP etch stops at the boundary where the boron concentration in silicon exceeds a level of 5 x lOI9 cmP3and does not attack any of the other materials used. The completed probes are now

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removed from the etch, ready for lead attachment and mounting. This process is capable of high yields ( > 80 percent), results in very small structures, and requires only singlesided processing on wafers of normal thickness. Probe features can be controlled to within 1 pm or better. The finished substrates can be as thick as 15-20 pm and of arbitrary two-dimensional shape. For applications demanding larger thickness, the probes can be withdrawn from the final etch before a complete etch-stop is achieved, leaving a self-aligned support rib backing the shank. This rib forms naturally and results in no degradation of the lateral shank dimensions since these are still controlled by the boron layer and the top-side portion of the final etch. The same technique is used to create a lightly-doped silicon well at the rear of the probe substrate to house the on-chip circuitry [2]. Typical shank lengths have been 1.5-3 mm and are usually tapered from a width of 15 pm or less near the tip to 100-160 pm near the base. Although these probe substrate dimensions are smaller than most metal microelectrodes in terms of total volume of tissue displaced by the shank (e.g., the ten-channel active probe has a volume smaller by a factor of four as compared with a single conical metal microelectrode of the same length and having a base diameter of 100 pm), it is desirable to reduce the width of the probe shanks still further while keeping the thickness essentially constant to maintain overall probe strength. The scaling of substrate dimensions reduces damage to the surrounding tissue, improves the quality of recorded signals, and reduces the chances of rejection of the probe by the tissue. In addition, by scaling the lithographic feature sizes, more electrodes can be supported on a narrower shank, allowing a more detailed mapping of cell responses in depth. Scaling of the substrate width is fundamentally determined by the ability to fabricate narrow yet relatively thick substrates using deep boron diffusion and boron etch-stop techniques. This paper reports the limitations in both of these areas. The second area of importance is the strength and stiffness of scaled silicon substrates and their suitability for neurophysiological applications. The fracture limits of these silicon microstructures have been analyzed and experimentally measured. Scaling of the lithographic feature sizes is primarily limited not by available technology but by the maximum acceptable crosstalk between adjacent channels. This crosstalk is capacitive and increases rapidly as the spacing between electrode conductors on the probe shank is reduced. Scaling of the recording site areas is also not limited by photolithographic techniques but rather by the recording capability of the electrode and the increase in intrinsic thermal noise associated with the recording site. The intrinsic thermal noise of a recording site is a function of the recording impedance, which is inversely dependent on the electrode surface area [ 5 ] . In order to be able to record biopotentials of 25 pV peak amplitude, the minimum recording site size that creates a noise level below 20 pV rms is calculated to be 16 pm2 (or about 4 pm

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on a side assuming a surface roughness factor of unity) [2], [ 5 ] . This is still much larger than the limits of the fabrication technology. 111. SCALING THE PROBE SUBSTRATE As explained above, microprobe substrates are fabricated by selectively doping a silicon wafer through oxide masks and by etching the wafer in an EDP solution which dissolves the wafer, leaving the heavily boron-doped silicon intact. Therefore, the thickness and cross-sectional profile of the substrate are determined by the diffusion characteristics of boron into silicon. It is desirable to further reduce the shank width to minimize tissue damage and enhance recording ability, while keeping the shank thickness constant to maintain strength. However, as the width of the shank decreases, the boron doping process departs from classical one-dimensional diffusion. When the width of the diffusion window (mask opening) is large, the diffusion process can be characterized as a constant planar-source diffusion with an infinite amount of impurity available for diffusion. On the other hand, as the width of the diffusion window becomes narrower, the diffusion process should be characterized as a line-source diffusion into an infinite volume. In addition, as the width of the diffusion opening decreases, lateral diffusion of boron underneath the mask becomes more important and eventually limits the minimum achievable shank width for a given substrate thickness. In order to study these effects experimentally, probe shanks with mask openings ranging from 5 to 30 pm were fabricated. After performing a deep boron diffusion for 15 h at 1175"C, the shanks were etched in EDP to delineate the boron etch-stop profiles. Fig. 3(a) shows a SEM photograph of the cross-section of three probe shanks formed through mask openings 5 , 7.5, and 10 pm wide, respectively. Fig. 3(b) shows a magnified view of the smallest shank, fabricated from a 5 pm opening in the masking oxide. The shank is 16 pm wide and - 8 pm thick, and its cross-section is approximately semi-circular. Fig. 4(a) shows a plot of the measured substrate thickness for all the shanks as a function of the mask width (width of the diffusion window), normalized to the thickness for a large mask width, while Fig. 4(b) shows the measured shank width as a function of the mask width. Several important conclusions can be drawn from these results. First, it is seen that the substrate thickness decreases exponentially as the mask opening is reduced below 10-15 pm, or as the mask opening becomes comparable to the etch-stop diffusion depth. This is mainly due to the fact that the diffusion process is no longer onedimensional but rather a two-dimensional process since the lateral diffusion of boron underneath the masking oxide becomes critical. Therefore, the constant source diffusion through a narrow opening must be treated as a line source (rather than a planar source) and is approximately cylindrical in nature. The cylindrical nature of the diffusion is clearly observed in the experimental results and is

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IEEE TRANSACTIONS ON BIOMEDICAL ENGINEERING, VOL. 37. NO. I , J A N U A R Y 1990 ,-

0

Xsrn=Xs-( I -Exp(-Wm/4.2))

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--. 0

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10

15

20

25

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0

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Fig. 4. Plot of (a) the normalized substrate (.r,,,,/xTa), and (b) the final shank width W , as a function of mask opening W,,,.

the data as (1 -

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Xsm -

DIFFUSION PROFILES AT THE CORNER REGION FOR TWO DIFFUSION MODELS

Linear D i f f u s i o n Non-Linear Diffusion (Increased Curvature)

so0

e-w!t,/w/

1

(1)

where W , is a constant factor obtained experimentally and is equal to 4.2 pm for the set of data points plotted in Fig. 4(a). The junction depth for a high-temperature, concentration-dependent diffusion process using large mask openings, as in deep boron diffusion, can be approximately written as [6]

(b)

Fig 3 ( a ) C r w \ection ot three probe shanks after EDP etch The shanks were diffused through ma\k openings of 5 . 7 5 . and I O pm wide, re5pectively (b) A magnified view of the smallest shank The shank is - 16 prn wide and 8 prn thick

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indeed advantageous for the present application, where a circular cross-section is desirable. The diffusion depth and profile are determined by the limited impurity availability due to the narrow mask opening. Based on the experimental results shown above, the depth of the boron etchstop x,,, as a function of the width of the mask opening W,,,, and of the depth of the boron etch-stop for a wide mask opening x,, can be obtained by empirically fitting

where Cs, is the surface concentration, D j is the intrinsic diffusion coefficient of boron in silicon, n j is the intrinsic carrier concentration, t is the diffusion time, and K is a constant factor. Fair [6] calculated K to be -2.45 when the junction is defined at a concentration of 10" (two orders of magnitude below the surface concentration). However, in the case of the boron etch-stop, the EDP etch stops at a concentration of - 5 x lOI9 - 10'' cmP3 [4]. In this case the factor K needs to be reevaluated theoretically and experimentally. Based on Fair's [6] analysis we can calculate K to be = 2.17 if the boron etch-

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stop concentration is assumed to be 5 X lOI9 cm-' and if the surface concentration at diffusion temperatures greater than 1100°C is taken to be 2.3 x lo2' cm-' [ 7 ] . It should be noted that the value of the factor K is a strong function of both the surface concentration and the boron etch-stop concentration and should be experimentally verified for each specific diffusion and etching process. Our experimental results obtained at a temperature of 1 175°C result in a K factor of = 2.1. This number agrees fairly well with the theoretical results obtained above. In addition, it is experimentally observed that the deep boron diffusion depth and the resulting boron etchstop profiles do have a square root relationship with time f. Based on the above relationships, it is possible to determine the thickness of the scaled silicon substrates as the width of the diffusion mask is scaled down. The second conclusion that can be drawn from Fig. 4 is that the lateral diffusion of boron underneath the masking oxide essentially limits the final width of the substrate. It is experimentally shown in Fig. 4(b) that the extent of the lateral diffusion is 0.55 times the diffusion depth, which is smaller than the typical value of -0.8 obtained for a simple, linear, constant-coefficient, shallow diffusion [8]. However, in their theoretical treatment of diffusion for VLSI devices, Warner er al. [9] investigated the combined effects of nonlinear and geometric factors on the impurity atom distribution by studying a two-dimensional, concentration-dependent diffusion model in a region containing the edge of the diffusion mask. They concluded that for the case when the surface concentration is much larger than the background concentration, there was a very noticeable decrease in the ratio of the lateral-to-vertical diffusion penetrations. In fact, they calculated and measured a lateral-to-vertical ratio of 0.65 instead of 0.8, which they attributed to a nonlinear diffusion coefficient. They further concluded that as the ratio of the impurity concentration to the intrinsic carrier concentration became larger, these nonlinear effects became more pronounced. Their assumptions and results agree very well with our experimental observations. The decrease in the extent of lateral diffusion in our case is due to the fact that the diffusion front of interest is estimated to be at a concentration of - 5 x lOI9 cmP3, i.e. the boron etch-stop concentration, and that the surface concentration is larger by about five orders of magnitude than the background concentration (which is lOI5 cm-'). Based on these results we can write the final width of a scaled substrate W, as a function of the mask opening as

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W, = W,

+ 2(0.55)x,,.

(3)

Using the above equations one can estimate the final width of a scaled substrate as a function of diffusion temperature, diffusion time, and the width of the mask opening. It should be noted that more experimental results covering a wider temperature range may be required to obtain K values for temperatures other than the 1175°C that we have used in our experiments.

The third point of interest is that the profile of the diffusion front at the edge of the shank is seen to be pushed out laterally creating a semicircular cross section, as shown in Fig. 3(b). This also agrees well with the theoretical results obtained by Warner er al. [9]. They showed that the combination of nonlinear, concentration-dependent diffusion, and two-dimensional effects for the constant-source diffusion causes the curvature of the diffusion front in the comer region to be increased, as opposed to the results for linear, one-dimensional, concentration-independent diffusion theory, which assumes an error-function profile for the diffusion front. This is schematically shown in the inset in Fig. 3(b). These theoretical observations fit the deep boron diffusion quite well. This third feature is also of great benefit to our application since it provides a smooth rounded comer for the shank which reduces tissue damage and eases probe penetration into tissue. These results clearly show that narrow and relatively thick structures can be fabricated in silicon using deep boron diffusion. In particular, silicon substrates 16 km wide and 8 pm thick can be fabricated for cortical applications. Narrower and thinner substrates can also be fabricated; however, for these scaled silicon substrates to be practical in cortical applications, their mechanical scaling limits with respect to strength must also be known. IV. SILICONMICROPROBE STRENGTH The second scaling issue concerns the strength and the stiffness of the probe as its shank width decreases. It is required not only that the probes be strong enough to avoid breaking, but that they should also be stiff enough to avoid excessive bending. Our experience with probe shanks 23 mm long, -100 pm wide, and 10-15 pm thick has shown that such silicon substrates are strong enough for cortical applications. However, the scaling limits due to the strength and stiffness of probe substrates as the width of the shank decreases need to be well understood. When the shank width decreases, one expects the probe to become more flexible. In neurophysiological applications, the mounted probe is advanced against the tissue (e.g., dura mater, pia arachnoid, or exposed cortex) until penetration is achieved. So long as the probe is straight, all of the force exerted on the probe is transferred to the tip and to the tissue. However, if the probe buckles, only the normal component of the exerted force will act on the tissue to achieve penetration. After the probe shank buckles, the stress level at the rear of the shank increases rapidly as it is pressed further against the tissue and the shank continues to bend. If the stress on the shank increases beyond the fracture stress of silicon, the shank breaks at the point of maximum stress. In summary, as the probe is pressed against tissue, it typically buckles first, when the buckling load limit is reached. The stress then continues building up, and if penetration is not achieved first it eventually fractures. It should be mentioned that in cortical tissue since the mechanical resistance of cortex is much less than the surface membranes over it, the probe

IEEE TRANSACTIONS ON BIOMEDICAL ENGINEERING, VOL. 37. NO. I . JANUARY 1990

shank relaxes to become straight after penetration and the stress drops to a very low level. Therefore, in order to quantitatively analyze these probes, two parameters have to be determined: the maximum load (force) required to buckle the probe shank, which determines the stiffness of the probe shank; and the maximum fracture stress of thin silicon substrates, which determines the strength of the shank after it buckles. For most physiological applications, large buckling loads and fracture stresses are desirable. The buckling load can be obtained using mechanical theory of columns; for a column which is pinned at one end and free to rotate at the other it is shown to be [IO]

where U is the maximum amount of lateral deflection of the buckled shank. It is seen that the stress scales linearly with thickness and inversely as the square of the shank length. It is theoretically independent of the width. In designing a scaled probe for a given application, we would like to minimize the shank volume while maximizing the buckling load, typically for a fixed shank length. Since once the shank buckles, the stress as given by (7) builds up rapidly, we must also take care to remain below the fracture stress, limiting the lateral deflection before penetration is achieved. Fig. 5 shows the buckling load and the buckled stress on a probe shank as a function of the shank thickness. The shank is taken to be 100 pm wide and 3 mm long, and the stress is calculated for two 2 EZT p=( 5 ) different values of deflection U . For a 10 pm thick shank, L2 the calculated buckling load is about 300 dyn. If we scale where E is the Young’s modulus (which is 1.7 x 10I2 the shank width to 25 pm, the same buckling load is maindyns/cm2 for silicon), Z is the moment of inertia of the tained if we also scale the thickness from 10 pm to about 16 pm. This can be achieved using a mask opening ( W,) area, L is the length of the column, and P is the buckling load. The moment of inertia of the area is different for of 7.4 pm, and a boron etch stop depth (xsm)of 19.3 pm. different cross sections. For the silicon substrates fabri- The volume of tissue displaced by the scaled shank is a cated using diffusion, the cross section can be taken to be factor of 2.5 less than that for the non-scaled shank. If approximately rectangular with side dimensions of W the shank is allowed to buckle, then the stress is easily (shank width) and h (shank thickness). This is a good ap- calculated from (7) as shown in Fig. 5. At a shank thickproximation for wide shanks since as mentioned above the ness of 16 pm with a lateral deflection of 0.1 mm and an diffusion boundary has a rapid curvature at the edge of the overall shank length of 3 mm, the stress reaches 1.8 x lo9 dyn/cm2, which is still an order of magnitude below shank under the masking oxide. For narrower shanks, on the other hand, the cross section is approximately semi- the fracture limit. In fact, at the fracture limit the lateral circular with a radius of r where 2 r = 2h = W . For these deflection would be about 1 mm in this case and the probe curvature would exceed a semicircle. two cases the moments of inertia can be written as [lo] The above equations can be used to calculate the probe wh3 - 0.083wh3 dimensions associated with an acceptable buckling load I(Rectangular) 12 or, if this is exceeded, the stress associated with a given Z(Semicircular) = 0.1 l r 4 = 0.055wh3. ( 6 ) buckling level (lateral deflection). Since the required It is seen from equations (5), (6) that the buckling load probe length is typically set by the application, the apscales linearly with shank width, inversely as the square proach to scaling is normally to reduce the shank width of the shank length, and as the cube of the shank thick- to the minimum allowed by the lithographic feature size ness. Therefore, in order to keep P fairly constant, it is and the number of electrode conductors which must be critical to maintain the shank thickness as the width is accommodated and then to increase the shank thickness decreased. It should also be noted that the linear reduction to achieve the needed buckling load. The ability to retain in P with W can be compensated for by slightly decreasing a self-aligned support rib on the probe without changing the length, or by increasing the thickness (or a combina- the basic process or degrading control over the width is tion of both). These formulae for the buckling load illus- thus an essential ingredient in achieving the minimum ditrate the stiffness of the probe shank. It should be noted mensions. For a given application, it is obviously importhat although smaller than before buckling, the normal tant to understand the silicon fracture limit and the force force exerted on the tissue surface after buckling will con- required to penetrate the structure of interest. Although the stress equation is independent of shank tinue to increase as the probe is pressed further against width, it has been experimentally shown that silicon bethe tissue. The strength of a probe shank is defined by the fracture comes very flexible as its cross-sectional area decreases, stress of the material from which it is formed. As the and its fracture stress increases by a factor of = 6 comshank deflects, the stress will be distributed nonuniformly pared to bulk silicon when the cross-sectional area drops along the substrate as it bends, with the maximum stress below 2000 pm2 [ 111. This situation results because the occurring at the fixed end (rear) of the shank. This max- fracture of crystalline materials is usually initiated on the imum stress urnax for a buckled probe shank can be ap- surface at some type of surface defect of a critical size, and as the surface area decreases the probability of ocproximated by [lo] currence of such a defect decreases as well. Therefore, 6 Ehu smaller structures can typically endure higher stresses than urn,, = ( 7 ) larger structures. In order to experimentally measure the L2

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SILICON MULTICHANNEL RECORDING PROBES

3000

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Fig. 5. The maximum stress and the buckling load as a function of shank thickness for a probe shank 100 pm wide and 3 mm long. The stress is calculated for deflections of 0.5 and 1 mm.

fracture stress of silicon substrates, a number of probes with shanks having different lengths and widths were fabricated utilizing the process described in Section 11. These probes were then subjected to fracture tests, and the fracture stress was measured. The probe substrates are very flexible and can bend through angles exceeding 90". Fig. 6 shows a scaled probe substrate under deflection. The probe has two shanks, each being -30 pm wide, 10 pm thick, and 1.4 mm long. In Fig. 6(a) both shanks are pressed against a block and it is seen that the substrate bends significantly before breaking. In Fig. 6(b) one shank is pressed against the block as in (a) whereas the other is deflected in a quarter-circle. Fig. 7 shows the measured fracture stress of silicon substrates as a function of the cross-sectional area (or the shank width since all substrates have the same thickness). It is seen that the fracture stress significantly increases for substrates having widths less than 150 pm. As the width decreases further, the fracture stress stays essentially constant with only a minor increase. Shown also is the fracture stress of bulk silicon; the boron-doped silicon substrates have a fracture stress higher by a factor of 6 than bulk silicon. In order to quantitatively analyze silicon probe substrate strength and stiffness, several miniature test probes with on-chip strain gauges and with varying width, length, and thickness have been fabricated for use in in vivo experiments. The data obtained from these experiments should provide us with a database concerning the toughness of different tissues and the implications that scaling shank widths will have on probe penetration into these tissues. Early results show that probe shanks 80 pm wide and 15 pm thick can easily penetrate guinea pig and rat pia-arachnoid layers. Probes 30 pm thick easily penetrate guinea pig dura mater which is -20 times tougher than pia. Based on these experiments and the maximum tolerable silicon fracture stress ( - 2 X 10" dyn/cm2), it appears possible to scale shank widths by at least an order of magnitude for penetration into guinea pig pia (to a width of less than 20 pm). For other types of tissue, however, more experimental measurements are required on the relative toughness of the different types of tissue so

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Fig. 6. A two-pronged scaled silicon probe substrate under deflection. The shanks are 1.4 mm long, 10 pm thick, and -30 ,urn wide.

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E U 3

. U)

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C r

1010 U)

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Fig. 7. Measured fracture stress as a function of substrate cross-sectional area (or shank width). All of the substrates for these measurements were 15 um thick.

that the probe shanks can be scaled down appropriately. These results will be presented under a separate publication. We feel that the scaled probe substrates will have sufficient strength for most cortical applications. V. ELECTRICAL CROSSTALK The third issue concerning the scaling of multielectrode probes is the increase of electrical crosstalk between adjacent electrodes. For multielectrode neural applications it is desirable to keep the crosstalk level to about 1 percent of the recorded signal, below which it is negligible compared to background noise. When more electrodes are to be accommodated on a narrower shank, the width of interconnect lines and the spacing between the lines must be reduced. However, the thickness of the dielectric layers above and below the electrodes will typically remain constant. As a result, the shunt capacitance of each electrode becomes smaller because of the reduced line width, while the coupling capacitance between electrodes becomes larger because of the reduced spacing. Therefore, as the feature sizes for the recording electrodes are reduced, one expects that the electrical crosstalk due to capacitive coupling will increase.

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IEEE TRANSACTIONS ON BIOMEDICAL ENGINEERING. VOL. 37, NO. I . JANUARY 1990

In order to characterize the scaling effects on crosstalk, a finite element simulation program has been developed. In the simulation program, the situation treated is that of three conductors insulated above and below by dielectric layers which in turn are backed by ground planes, as shown in Fig. 7(a). This situation closely approximates the probe structure where the boron-doped substrate and the extracellular fluid form the ground planes. The neural tissue together with a pool of extracellular fluid around the probe shank does indeed form an effective ground plate on top due to the small dimensions of the electrodes, i.e., a few microns, and the nearly ideal dielectric characteristics of the insulating films compared to the neural tissue [12]. In addition, a grounded metallic sheet could be formed on top of the dielectrics if needed to provide a true ground. Fig. 7(b) shows the equivalent circuit model used in calculating the crosstalk. The following assumptions are made in the simulation model: 1) For each set of data points, the recording site size is kept constant. Thus, the double layer capacitance between the electrode surface and the electrolyte is constant. (Two values are used in our calculations in order to cover the range of recording site impedances of interest: i) a small-area site with a capacitance of 12 pF and an impedance of 13 M a at 1 kHz, and ii) a large-area site with a capacitance of 32 pF and an impedance of 5 MQ at 1 kHz ) . Platinizing the sites could further reduce the recording impedance by as much as an order of magnitude; however, for long-term measurements the stability of such sites is questionable. 2) The top and bottom dielectric layer thicknesses are kept constant at 1 pm. 3) The relative dielectric constant used is 4.25, which is equivalent to an 80-20 percent thickness ratio for the Si02/Si3N, layers which make up the composite dielectric. The model used, however, allows for different dielectric constant values for each element in the mesh. Based on these assumptions, the two-dimensional potential field around the interconnect lines is first calculated and the coupling capacitances to the substrate and between lines are then obtained using finite-element modeling. Fig. 8 shows the potential field isolines around the three interconnect lines for a 1 pm thick conductor for two cases: 1) a 6 pm minimum feature process, and 2) a 1 pm minimum feature process. The center electrode is at 1 V , while the other two are at 0 V . The decrement on each of the lines is 0.1 V and the dimensions are in microns. In the 6 pm situation, which corresponds to our first design [2], the 40 dB ( 1 percent) equipotential line is confined to a space within 3 pm of the source conductor. In contrast, when the feature size is reduced to 0.25 pm, the 40 dB line surrounds adjacent lines. The crosstalk change as the line width and spacing are scaled down can be computed using SPICE utilizing the circuit model shown in Fig. 7(b). Fig. 9 shows the simulated values of the coupling and shunt capacitances for a 3 mm long 1 pm thick interconnect and for different electrode widthdspacings,

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(b) Fig. 8. (a) Cross section of a multielectrode probe used in crosstalk simulation. (b) Equivalent circuit model used for calculating interelectrode electrical crosstalk.

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Fig 9 Potential field isolines around the three interconnect lines for two cases a 6 pm minimum feature process, and a 1 pm minimum feature process The center electrode is at 1 V , while the other two are at 0 V The decrement of the lines is 0 1 V and the dimensions are in microns

9

NAJAFI e / al.: SILICON MULTICHANNEL RECORDING PROBES

-

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Fig. 10. Coupling and shunt capacitance values for a 3 rnm long 1 pm thick electrode interconnect for different electrode widthsispacings.

cult. It is believed that 1 pm feature sizes are an appropriate compromise between fabrication difficulty and crosstalk. Such features would allow a reduction in width by nearly an order of magnitude over present structures. In addition, the crosstalk level can be reduced by decreasing the conductor thickness, perhaps to 0.5 pm, which is stili adequate here since the resulting series conductor resistance is still far less than the amplifier input impedance. Based on these results, crosstalk is not expected to be a limiting factor in probe scaling, at least for purely recording probes. For structures containing stimulating channels (where voltage bounces can reach 1 V ) , the data presented in Fig. 9 can be used to guide the selection of conductor spacings to limit crosstalk between recording and stimulating channels to acceptable levels.

VI. DISCUSSION PERCENT CROSSTALK FOR A 32pF RECORDING SITE 10

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WIDTHISPACING, pm Fig. 11. Electrical crosstalk as a function of the interconnect thickness for various electrode widths/spacings and for two different recording site impedances.

while Fig. 10 shows the crosstalk as a function of the interconnect thickness for various electrode widthdspacings and for two different recording site impedances. It is seen from these simulations that, for a 1 pm thick conductor, as the electrode interconnect line width and inter-electrode spacing approach 1 pm, the crosstalk approaches 1 percent and increases rapidly thereafter. For a line width and spacing as small as 0.25 pm, the crosstalk is still less than 4 percent. For most practical applications, this level of crosstalk is still acceptable; however, fabrication and process challenges become increasingly diffi-

As an example of probe scaling, consider a probe 3 mm long which must penetrate guinea pig pia arachnoid. For probes measuring 2.5 mm long X 80 pm wide X 15 pm thick, the measured stress at penetration is about 5 X 10' dyn/cm2 [13]. The lateral deflection for such a probe at penetration is approximately 20 pm. Suppose we wish to scale the shank dimensions to reduce the width while maintaining a length of 3 mm. We assume that the same force is required for penetration. This is conservative, and it is experimentally observed that the penetration force decreases with the width. Assume a scaled probe 30 pm wide, 3 mm long, and 15 pm thick is needed. In order to obtain a shank 30 pm wide by 15 pm thick, a mask opening of 13.5 pm and a boron etch-stop depth of 15.6 pm are needed based on (1)-(3). Due to the reduction in width and the increase in length, this scaled probe has a buckling load which is a factor of 5.5 lower than the nonscaled probe [based on (5)]. The lateral deflection at penetration is therefore approximately 120 pm, and the maximum stress at penetration is about 3 X lo9 dyn/cm2. This is still a factor of six lower than the fracture stress of silicon. Thus, the scaled probe should penetrate easily without breakage, although the stress and buckling loads are becoming significant. From the standpoint of crosstalk, we can stay well below the one percent level even for submicron features. If we choose a conductor pitch (line plus space) on the shank of 2 pm, consistent with readily achievable optical lithography, then the above shank could support about 15 electrodes. The volume of tissue displaced by the scaled probe has been reduced by almost a factor of 3 compared with a nonscaled probe, approaching lop4mm3/electrode for a 3 mm length. This compares to a volume of about mm3/electrode for a typical conical microelectrode of the same length. It is clear that when scaling these silicon probes, the ultimate size limits are set by strength considerations in combination with the limits imposed by the boron diffusion process. Crosstalk presents no problem so long as the array is used only for recording; however, if some elec-

10

IEEE TRANSACTIONS ON BIOMEDICAL ENGINEERING, VOL. 37. NO. I . JANUARY 1990

trodes are used for stimulation, then crosstalk becomes a more serious concern, with desired coupling capacitances no larger than 1 fF and careful shielding required (probably with interleaved ground lines). The attainable sizes are very small for the overall probe technology, implying that dense sampling of the electrical activity in volume of tissue may be possible. For the case of 15-electrode shanks measuring 3 mm x 30 pm x 15 pm, it would be possible to completely sample the single-unit activity (based on a 100 pm recording field) of a 2 mm X 2 mm X 2 mm volume of cortex using 1000 points (70 shanks) displacing about one percent of the total volume. Whether such dense monitoring is useful (for purposes of physiological study or for deriving control signals) and whether it would be tolerated by the neural system and tissue remains to be explored.

VII. CONCLUSION Microprobe scaling limitations with regard to substrate formation by deep boron diffusion and boron etch-stop techniques, silicon substrate strength and stiffness, and electrical crosstalk have been discussed. It has been shown that silicon substrates 16 pm wide and 8 pm thick can be fabricated using the boron etch-stop technique. These substrates have a semicircular cross sections and are strong and flexible enough for most cortical applications. The scaled silicon substrates have a higher fracture stress than bulk silicon due to reduced surface area. These substrates can bend to angles larger than 90” and show no sign of hysteresis. The electrical crosstalk between adjacent channels of a multichannel recording electrode has been calculated through finite element simulations. For 1 pm thick conductor lines spaced 1 pm apart and separated above and below by 1 pm thick dielectrics, the calculated crosstalk is less than 1 percent. Based on these results we feel that it will be possible to fabricate multielectrode recording arrays containing 32 electrodes on a silicon shank less than 40 pm wide and 10-15 pm thick. This structure should be strong enough for most neurophysiological situations. Ultimately, the strength requirements of a given application are likely to quantify the minimum shank dimensions that can be used, and these need to be quantitatively documented through additional studies.

ACKNOWLEDGMENT The authors wish to extend their appreciation to Dr. F. T . Hambrecht and Dr. W. Heetderks of the National Institutes of Health for their encouragement throughout this project.

REFERENCES [ I ] K. Najafi, K. D. Wise, and T . Mochizuki, “ A high-yield IC-compatible multichannel recording array,” IEEE Trans. Electron Devices. vol. ED-22, pp. 1206-1211. July 1985.

[2] K. Najafi and K. D. Wise, ”An implantable multielectrode recording array with on-chip signal processing,” IEEE J . Solid-Srare Circuits, vol. SC-21, pp. 1035-1044, Dec. 1986. [3] K. L. Drake, K. D. Wise, J. Hetke, D. J. Anderson, and S . L . BeMent, “Performance of planar multisite microprobes in recording extracellular single-unit intracortical activity,” IEEE Trans. Biomed. Eng., vol. 35, pp. 719-732, Sept. 1988. [4] E. D. Palik, V. M . Bermudez, and 0. J . Glembocki, “Ellipsometric study of the etch-stop mechanism in heavily-doped silicon,” J . Electrochem. Soc., vol. 133, pp. 135-141, Jan. 1985. [SI R . C. Gesteland, B. Howland, J . Y. Lettvin, and W . H. Pitts, “Comments on microelectrodes,” Proc. IRE, vol. 47, pp. 1856-1862, Nov. 1959. [6] R. B . Fair, “Boron diffusion in silicon-concentration and orientation dependence, background effects, and profile estimation,” J . Elecrrochern. Soc., vol. 122, pp. 800-805, June 1975. [7] G . L. Vick and K. M. Whittle, “Solid-solubility and diffusion coefficients of boron in silicon,” J . Elecrrochem. Soc., vol. 116, pp. 1142-1 144, Aug. 1969. [8] D. P. Kennedy and R. R. O’Brien, “Analysis of the impurity atom distribution near the diffusion mask for a planar p-n junction,” IEM J . , no. 9, pp. 179-186, May 1965. 191 D. D. Warner and C. L. Wilson, “Two-dimensional concentration dependent diffusion,” The Bell Sysr. Tech. J . , vol. 59, no. 1 , pp. 141, Jan. 1980. [IO] S . Timoshenko, D. H. Young, Elemenrs of Srrength ojMureriuls. D. Van Nostrand Company, 1968. [ I l l G . L. Pearson, W. T . Read Jr., and W . L. Feldman, “Deformation and fracture of small silicon crystals,” Acta Merullurgica, vol. 5, pp. 181-191, Apr. 1957. [ 121 R. Plonsey, Bioelecrric Phenomena. New York: McGraw Hill, 1969, p. 205. [I31 K. Najafi and J . F. Hetke, “Strength characterization of silicon microprobes in neurophysiological tissues,” IEEE Trans. Eiomed. Eng., to be published.

Khalil Najafi (S’84-M’86) was born in Iran in 1958 He received the B S E E. degree in 1980 and the M.S.E.E. degree in 1981, both with highest honors, and the Ph.D. degree in electncal engineering in 1986 (working on “multielectrode intracortical recording arrays with on-chip signal processing”) all from the University of Michigan, Ann Arbor. From 1986 to 1988 he was employed as a Research Fellow and from 1988 to present he is employed as an Assistant Research Scientist at the Center for Integrated Sensors and Circuits, Department of Electrical Engineering and Computer Science, University of Michigan His rewarch interests are in the development, design fabrication, and testing of solidstate integrated sensors and actuators, analog and digital integrated circuits, implantable microtelemetry systems and transducers for biomedical applications, technologies and structures for micro electromechanical systems and microstructures, and packaging techniques for implantable transducers Dr. Najafi was the recipient of the Beatrice Winner Award for Editorial Excellence at the 1986 International Solid-state Circuits Conference He is a member of Tau Beta Pi, Eta Kappa Nu, and Electrochemical Societies

NAJAFI

ct

al.: SILICON MULTICHANNEL RECORDING PROBES

Jin Ji (S’88) received the B S E E degree from Qinghua University, Beijing, China, in 1982, and the M S E E degree from the University of Michigan, Ann Arbor, i n 1984 He is currently working toward the Ph D degree in electricdl engineering in the Department of Electrical Engineering and Computer Science at the UniverWy of Michigan His research activities involve silicon analog and digital integrated circuit de\ign, process development, and the design and fabricdtion of silicon multichannel neural signal recording arrays Mr J i I \ a member of Tdu Betd Pi dnd

d

student member of IEEE

Kensall D. Wise (S’61-M’69-SM’83-F’86) received the B.S.E.E. degree with highest distinction from Purdue University, Lafayette, IN, in 1963 and the M.S. and Ph.D. degrees in electrical engineering from Stanford University, Stanford, CA, in 1964 and 1969, respectively. From 1963 to 1965 (on leave 1965-1969) and from 1972 to 1974, he was a Member of Technical Staff at Bell Telephone Laboratories, where his work was concerned with the exploratory development of integrated electronics for use in tele-

II phone communications From 1965 to 1972 he was a Research ASSi\tdnt and then a Research As\ociate and Lecturer in the Department of Electrical Engineering at Stanford, working on the development ot integrated circuit technology and its application to solid-state sensors In 1974 he joined the Department of Electrical Engineering and Computer Science at the Uni ver\ity of Michigan, Ann Arbor, where he is now serving as Professor and Director of the Center for Integrated Sensors and Circuits. His present research interests focus on the development of solid-state sen\ors for use in health care and industrial process control Dr Wise has served on many program committees for both the International Electron Devices Meeting and the International Solid-state Circuits Conference He also served as General Chairman of the 1984 IEEE Solid-State Sensor Conference, as Technical Program Chairman of the 1985 International Conference on Solid-state Sensors and Actuators, and as IEEE-EDS National Lecturer for 1986 He is a member of the Electrochemical Society, the AVS, Tau Beta Pi, Eta Kappa Nu, and Sigma Xi