Nadim I. Maluf

Bulk micromachining of silicon

Bulk silicon etching techniques, used to selectively remove silicon from substrates, have been broadly applied in the fabrication of micromachined sensors, actuators, and structures. Despite the more recent emergence of higher resolution, surface-micromachining approaches, the majority of currently shipping silicon sensors are made using bulk etching. Particularly in light of newly introduced dry etching methods compatible with complementary metal-oxide-semiconductors, it is unlikely that bulk micromachining will decrease in popularity in the near future. The available etching methods fall into three categories in terms of the state of the etchant: wet, vapor, and plasma. For each category, the available processes are reviewed and compared in terms of etch results, cost, complexity, process compatibility, and a number of other factors. In addition, several example micromachined structures are presented.

Process for in-plane and out-of-plane single-crystal-silicon thermal microactuators

Abstract A process to manufacture single-crystal thermal actuators using silicon fusion bonding and electrochemical etch stop is presented. The process permits the simultaneous creation of in-plane and out-of-plane thermal actuators together with levers suitable for both directions of actuation. A final dry-release step is used, permitting the manufacture of MOS or bipolar devices in conjunction with actuators. Out-of-plane actuation of vertically levered devices has been demonstrated. The −3 dB response frequency of out-of-plane actuators is approximately 1000 Hz in air. Novel levered in-plane devices which achieve deflections of up to 200 μm have been fabricated. An estimate of the upper bound of thermal actuator efficiency is presented.

Silicon fusion bonding and deep reactive ion etching: a new technology for microstructures

Abstract New developments in deep reactive ion etching (DRIE) technology, when combined with silicon fusion bonding (SFB), make it possible, for the first time, to span nearly the entire range of microstructure thicknesses between surface and bulk micromachining, using only single-crystal silicon. The combination of these two powerful micromachining tools forms a versatile new technology for the fabrication of micromechanical devices. The two techniques are described and a process technology is presented. Some of the experimental structures and devices that have been demonstrated using this new process technology are discussed.

Silicon-substrate microelectrode arrays for parallel recording of neural activity in peripheral and cranial nerves

A new process for the fabrication of regeneration microelectrode arrays for peripheral and cranial nerve applications is presented. This type of array is implanted between the severed ends of nerves, the axons of which regenerate through via holes in the silicon and are thereafter held fixed with respect to the microelectrodes. The process described is designed for compatibility with industry-standard CMOS or BiCMOS processes (it does not involve high-temperature process steps nor heavily-doped etch-stop layers), and provides a thin membrane for the via holes, surrounded by a thick silicon supporting rim. Many basic questions remain regarding the optimum via hole and microelectrode geometries in terms of both biological and electrical performance of the implants, and therefore passive versions were fabricated as tools for addressing these issues in on-going work. Versions of the devices were implanted in the rat peroneal nerve and in the frog auditory nerve. In both cases, regeneration was verified histologically and it was observed that the regenerated nerves had reorganized into microfascicles containing both myelinated and unmyelinated axons and corresponding to the grid pattern of the via holes. These microelectrode arrays were shown to allow the recording of action potential signals in both the peripheral and cranial nerve settings, from several microelectrodes in parallel.<<ETX>>

Plasma-etched neural probes

Abstract A new method is presented for microfabricating silicon-based neural probes that are designed for neurobiology research. Such probes provide unique capabilities to record high-resolution signals simultaneously from multiple, precisely defined locations within neural tissue. The fabrication process utilizes a plasma etch to define the probe outline, resulting in sharp tips and compatibility with standard CMOS processes. A low-noise amplifier array has been fabricated through the MOSIS service to complete a system that has been used in multiple successful physiological experiments.

Microelectrode arrays for stimulation of neural slice preparations

A planar 6 x 6 array of iridium electrodes with four reference electrodes has been developed for use with neural tissue preparations. Precise knowledge of the relative locations of the array elements allows for spatial neurophysiological analyses. The 10 microns diameter platinized iridium electrodes on a 100 microns pitch have been used to stimulate acutely prepared slices of spinal cord from free-ranging rodents. An intracellular recording from a single neuron in the substantia gelatinosa (SG) using the whole-cell, tight-seal technique allowed low noise, high resolution studies of excitatory or inhibitory electrical responses of a given neuron to inputs from the primary afferent fibers or from stimulation by individual electrodes of the array. The resulting maps of responses provide an indication of the interconnectivity of neural processes. The pattern emerging is that of limited interconnectivity in the SG from areas surrounding a recorded neuron but with strong excitatory or inhibitory effects from those oriented in a longitudinal (rostral-caudal) direction relative to the neuron. The observations to date suggest the neurons of the SG are arranged in sets of independent networks, possibly related to sensory modality and input from particular body regions.

Miniature spectrometers for biochemical analysis

Miniature spectrometers were demonstrated by mounting micromachined diffraction gratings onto CCD imaging devices. Two implementations were tested: one for high dispersion and high sensitivity applications, and the other for low-cost consumer applications. The first system showed a dispersion of 1.7 nm/pixel and a resolution of 74.4 for the bandwidth of interest. The free spectral range of these devices was designed to be in the visible range for this particular application. The diffraction efficiency of the system was 63%. The second, low-cost system demonstrated a dispersion and resolution of 2.55 nm/pixel and 69.8 respectively. These specifications are comparable to that of a conventional, low-end commercial spectrometer. Results are shown for their applications in biochemical analysis. Optimization was sought by adding micromachined lenses and creating specialized, computer generated gratings to compress and shape the spectral signal.

Fabrication of SOI wafers with buried cavities using silicon fusion bonding and electrochemical etchback

Abstract This paper describes a new technique for batch fabrication of silicon-on-insulator (SOI) wafers for microelectromechanical systems (MEMS) applications by silicon wafer bonding techniques. The process permits the inclusion of buried cavities in the SOI wafers, providing a useful tool for sensor and actuator fabrication using the resulting wafers. A low-cost electrochemical etchback step is used to define accurately the thickness of the remaining single-crystal material even though the two wafers are bonded with an intermediate insulating oxide layer. The results presented include guidelines for backside contact definition which maximize the useful silicon area as a function of doping level. The final single-crystal silicon thickness is uniform to within 0.05 μm (standard deviation) and does not require any costly high-accuracy polishing steps.

Packaging a piezoresistive pressure sensor to measure low absolute pressures over a wide sub-zero temperature range

The packaging, calibration, and testing of a commercial silicon fusion bonded piezoresistive pressure sensor for a Martian deployment are described. Detail is provided on the sensor mounting and electronic instrumentation required for this environment. Flight testing procedures and the residual errors for packaged pressure sensors are presented. Pressure and temperature hysteresis are investigated as sources of this error. The data illustrates the potential high performance of off-the-shelf silicon fusion bonded piezoresistive pressure sensors, which are carefully packaged, instrumented, and individually calibrated.

Low voltage alternative for electron beam lithography

The current trend in electron beam lithography for patterning submicron features is towards the use of higher beam voltages (20–100 keV). Among the problems often perceived to be associated with the use of low voltages are the poorer resolution, the lower brightness, and the greater sensitivity to electric and magnetic interference. Both by simulation and by experiment at 2 kV it is shown: (1) features of less than 100 nm are clearly resolved in resist of about the same thickness; (2) such features are clearly resolved in both sparse and dense pattern; (3) such features in sparse and dense areas are clearly resolved over a twofold range of exposure doses; (4) such delineation is largely independent of substrate material; (5) there is no evidence of alternating‐current magnetic interference; (6) the lower beam brightness at low voltages is compensated by the increased sensitivity of resists to lower energy electrons. The remaining concerns about low voltage lithography are the reliability of resist with an...