Stefan A. Nikles

A modular micromachined high-density connector system for biomedical applications

This paper presents a high-density, modular, low profile, small, and removable connector system developed using micromachining technologies for biomedical applications. This system consists of a silicon or polyimide electrode with one end in contact with the biological tissue and its back-end supported in a titanium base (12.5 mm in diameter and 2.5 mm in height) that is fixed on the test subject. An external glass substrate (6/spl times/6/spl times/0.75 mm/sup 3/), which supports a flexible polyimide diaphragm and CMOS buffers, is attached to the titanium base whenever electrical contact is required. The polyimide flexible diaphragm contains high-density gold electroplated pads (32 pads, each having an area of 100/spl times/100 /spl mu/m/sup 2/ and separated by 150 /spl mu/m) which match similar pads on the electrode back-end. When vacuum is applied between the two, the polyimide diaphragm deflects and the corresponding gold pads touch, therefore, establishing electrical connection. In vitro electrical tests in saline solution have been performed on a 32-site connector system demonstrating <5 /spl Omega/ contact resistance, which remained stable after 70 connections, and -55 dB crosstalk at 1 kHz between adjacent channels. In vivo experiments have also confirmed the establishment of multiple contacts and have produced simultaneous biopotential recordings from the guinea pig occipital cortex.

Analytical extraction of residual stresses and gradients in MEMS structures with application to CMOS-layered materials

Accurate thin-film characterization is a key requirement in the MEMS industry. Residual stresses determine both the final shape and the functionality of released micromachined structures, and should therefore be accurately assessed. To date, a number of techniques to characterize thin-film materials have been developed, from substrate curvature measurement to methods that exploit the post-release deformation of test structures. These techniques have some major drawbacks, from high implementation costs to accuracy limitations due to improper boundary condition modeling. Here, we present a new technique for the characterization of multilayered, composite MEMS structures that uses easily accessible experimental information on the post-release deformation of microbridges only, with no need for multiple beam lengths. The method is based on an analytical solution of the (post-)buckling problem of microbridges, including the effect of residual stresses (both mean and gradient) and non-ideal clamping (boundary flexibility). The method allows simultaneous characterization of both the mean and the gradient residual stress components, as well as the effective boundary condition associated with the fabrication process, yielding approximately one order of magnitude improvement in resolution compared to extant methods using the same type and number of test structures. The higher resolution is largely attributable to proper accounting for boundary flexibility by our method, with the boundary condition for the structures in this work being ~90% as stiff in bending relative to the commonly assumed perfectly clamped condition. Additional enhancement can be achieved with post-release deformation measurements of simple cantilevers in addition to the microbridges. The method is useful as it ensures very low stress extraction uncertainty using a limited number of microbridge test structures, and it is transferrable to package-stress characterization. The analytical approach can also be extended to device design, quantifying the effect of residual stresses and boundary flexibility on a structures post-release state.

Long-term hermeticity and biological performance of anodically bonded glass-silicon implantable packages

This paper reviews long-term test results obtained from a series of tests on glass-silicon (Si) hermetically sealed packages. Results are presented from 1) a 9.9-year ongoing room temperature phosphate-buffered saline (PBS) soak test of four packages; 2) accelerated soak tests in high temperature saline of 28 samples resulting in an extrapolated mean-time-to-failure (MTTF) at 37/spl deg/C of 177 years; 3) a 2.7-year in vitro 97/spl deg/C PBS soak test of a single package; and 4) in situ hermeticity and biocompatibility tests from 12 packages implanted in four guinea pigs-three packages implanted in two guinea pigs (each) for 1 month and another two guinea pigs for 20 and 22 months. All of the packages remained hermetically sealed over the lifetime of the implant. A detailed histological report of the implants is provided suggesting that they elicit no profound adverse reaction from the body.

Long-term recordings from afferent taste fibers

The receptor cells of taste buds have a life span of about 10 days but it is not known if response characteristics of these receptors alter during the turnover cycle. To examine taste cell responses over time, a micromachined polyimide sieve electrode array was implanted between the cut ends of the rat chorda tympani nerve, which then regenerated through the electrode array. Long-term stable recordings from regenerated single afferent fibers innervating taste buds were possible using this technique for up to 21 days. Responses to taste stimuli recorded from the same fiber changed with time. The changes occurred in both the magnitude of response and the relative response profiles to four chemical stimuli, NaCl, sucrose, HCl, and quinine HCl. These changes in response characteristics were hypothesized to result from changes in the taste receptor cells as the receptor cells turnover in the taste buds.

Mechanical characterization and design of flexible silicon microstructures

A variety of different silicon structures has been fabricated and characterized mechanically to optimize the design of silicon ribbon cables used in neural probes and multichip packaging structures. Boron-doped 3-/spl mu/m-thick silicon beams were tested in three modes: bending in plane, twisting (along beam axis), and pushing. Various cable configurations were investigated (straight beams, curved beams, meandered beams, etc.) as well the effects of length, width, cable termination, and the presence of reinforcing spans between multistranded cables. The results along with finite element modeling indicated that many simple modifications could be made to increase the strength and flexibility of silicon ribbon cables. One structure, a meandered beam 200-/spl mu/m wide and 2-mm long could be twisted up to 712/spl deg/. It also was seen that structures having multiple 20-/spl mu/m-wide beams were generally more robust than those with a single 500-/spl mu/m-wide beam. Finally, a method for easy determination of the bending fracture strain is analyzed and verified. It was seen that the silicon structures tested broke after a strain slightly above 2%.

Long term in vitro monitoring of polyimide microprobe electrical properties

In vitro soak tests lasting over 400 days were performed on polyimide sieve electrodes (PI2611D) using a method that allows in situ measurement of the impedance and relative dielectric constant of polyimide or alternate polymers. It was found that following an initial period of change, both values stabilized after a period of about 100 days. The dielectric constant was measured at 1 kHz and found to increase by 18% over 406 days. A short-term comparison (23 days) between polyimide cured on a hotplate in air versus using an oven with N/sub 2/ was also performed. No difference could be seen between the 2 methods.

Mechanics of Out-of-Plane MEMS via Postbuckling: Model-Experiment Demonstration Using CMOS

A novel approach to out-of-plane microelectromechanical systems (MEMS) is demonstrated where elements are designed in the postbuckling regime, exploiting buckling phenomena and residual-stress control to create functional elements that extend significantly out of the wafer plane. An analytical tool for out-of-plane MEMS design is presented, based on nonlinear postbuckling of layered structures, including boundary nonideality. The analytical design tool is applied to several MEMS designs where low-order elements (e.g., beams) are controllably formed into out-of-plane shapes. Various architectures are experimentally demonstrated using CMOS processes, including one that could find application in three-axis single-heater thermal accelerometers. The on chip approach is compatible with several MEMS fabrication techniques (e.g., CMOS and micromachining), thus providing a new extension of state-of-the-art microfabrication techniques to out-of-plane elements.

Design and testing of conductive polysilicon beam leads for use in a high-density biomedical connector

A study was conducted to measure and characterize the reliability of polysilicon cantilever beams with electroplated gold pads for use in a high-density biomedical connector. In this design, an array of beams is brought into contact with a corresponding array of 30 ?m high gold bumps, forming electrical connection. Analytical computations of multi-layer beams were performed, including the effects of residual stresses. Beam leads of various lengths and widths with electroplated gold contact pads on their ends were tested over 1000 cycles to determine their mechanical reliability, and to measure their contact resistance with gold bumps on a separate substrate. The dimensions of the polysilicon beam that produce the least breakage were determined to be 400 ?m long by 125 ?m wide. For a beam having a calculated contact force of ~100 ?N, the initial contact resistance was 764 m?. After 1000 connect/disconnect cycles, beams of this type had an average final contact resistance of 1.598 ?. These results demonstrate that very high-density connectors with high mechanical reliability and low-contact resistance can be fabricated.

Reliability and contact resistance of polysilicon beam leads for use in a high-density connector

A study was conducted to measure and characterize the reliability of polysilicon cantilever beams with electroplated gold pads for use in a high-density connector. Beam leads of various lengths and widths with electroplated gold contact pads on their ends were tested over 1000 cycles to determine their mechanical reliability, and to measure their contact resistance with gold bumps on a separate substrate. The optimum length and width of polysilicon beams, insulated on top and bottom with CVD oxide and nitrides, were determined to be 400 /spl mu/m and 125 /spl mu/m, respectively. The contact resistance was measured to be about 1.22/spl plusmn/0.68 /spl Omega/ after 1000 cycles. These results indicate very high-density connectors with high mechanical reliability and low contact resistance can be fabricated.

Mechanical Testing of Flexible Silicon Carbide Interconnect Ribbons

This paper explores polycrystalline 3C-silicon carbide (poly-SiC) deposited by LPCVD for fabricating flexible ribbon cable interconnects for micromachined neural probes. While doped silicon is used currently, we hypothesized that poly-SiC will provide enhanced mechanical robustness due to SiC’s superior mechanical properties. Paralleling prior work in silicon, forty-two different designs were fabricated from nitrogen-doped poly-SiC films deposited by LPCVD at 900°C using dichlorosilane and acetylene as precursors. The different designs were then tested in bending and twisting modes. Curved beams were found to bend nearly 250% more than straight beams before fracture. Longer beams withstood greater bending and twisting due to greater compliance. Longer and narrower beams generally outperformed shorter beams irrespective of design. Also, doped poly-SiC beams had, on average, breaking angles that were greater than those of identical doped silicon beams by ~50% in bending and ~20% in twisting modes. The paper details the designs studied, describes the fabrication process for the test structures and compares/contrasts the testing and simulation results related to the different designs to identify best design practices.