This overview paper will cover the miniaturization technologies as applied to microelectromechanical systems (MEMS) or micromanufacturing. Technologies reviewed will include bulk and surface micromachining of silicon, the high-aspect ratio technologies including deep X-ray lithography (LIGA) and photo sensitive polyimide, and the complementary processes which include micro-drilling, milling, turning, and electrical discharge machining, laser based micromachining and focussed ion beam micromachining. Examples of each of the process technologies will be given and a capabilities comparison among the technologies will be presented. A historical comparison of MEMS with the vlsi industry will be made and the current status and market forecast for these technologies will be presented. A brief comparison of US research with current research in Japan and Europe will be made along with comments about the status of US research, including current research projects at the Institute for Micromanufacturing.

Microelectromechanical systems (MEMS) are integrally-fabricated hybrids of micromechanical and microelectronic elements which serve together as sensors, actuators or both. While there are proprietary issues in MEMS products currently in or near commercial production, there are also a number of generic technology, measurement and standards issues which may be addressed.

Natural and synthetic microstructures with micrometer- to nanometer-scale features present a significant challenge to chemical analysis techniques. As the dimensions of features are reduced, the number of atoms and molecules to be analyzed becomes so small that useful analytical signals can only be obtained through optimization of the entire measurement process: e.g., the use of high brightness radiation sources, high efficiency spectrometers, and long counting times. Techniques based upon beams of electrons, photons, ions, and neutrons and generally incorporating some form of microscopy are available. The suite of characterization techniques can provide a wide variety of information on elemental and molecular composition, morphology, and crystal structure on a scale ranging from micrometers to nanometers.

A new x-ray micro-lithography exposure system has been designed and built at CAMD to meet specific demands of synchrotron radiation assisted high aspect ratio micromachining. The system consists of a broad-band transmission (1.2 angstroms to 6.5 angstroms) beamline and a multi-chamber X-ray Exposure Station. The beam line accepts radiation emitted in a bending magnet of the CAMD 1.3 - 1.5 GeV synchrotron storage ring. The beam line is approximately 10 m long and terminates with 125 micrometers thick Be window which defines an X-ray beam of 50 X 10 mm² at the exposure plane. The beam line is configured to provide 1.0 W/cm² at 1.5 GeV and 100 mA storage ring operation. The Exposure Station is designed to control different exposure conditions and can handle a variety of mask/sample assemblies. The first camber of the exposure tool is designated as a radiation filter, it controls x-ray spectra using foils and inert gases, and optimizes dose delivered to a sample with a thick resist (in excess of 1 mm). The second chamber is equipped with a multi-axes scanning mechanism to provide designed orientation and exposure of the mask/sample assembly. The two chamber can be separated by a thin foil to facilitate the use of reactive atmospheres for radiation induced chemical processes during exposure. An expansion of the Exposure Station providing large area exposures (up to 300 X 300 mm²) is described.

We have fabricated two microelectromechanical scanning tunneling microscopes (Micro- STMs) with 3D (xyz) actuators and integrated high aspects ratio tips. The reduction in the size of scanning probe microscopes allows for faster scanning speeds, array architectures, and massively parallel operation. The two Micro-STMs are fabricated from single crystal silicon using the high-aspect-ratio SCREAM process and are small enough to be used in array architectures. The torsional cantilever design used for out-of-plane (z) motion can be easily be adapted to scanning force microscopy. Typical atomic force microscope cantilevers have spring constants on the order of 0.01 - 10 N/m. To produce cantilevers with lower spring constants, ordinary thin film techniques would require longer (several mm) and thinner (sub- micrometers ) cantilevers. A mechanical analysis of torsional cantilevers reveals that high aspect ratio rectangular beams, such as the ones we fabricate, are easily twisted. By using the torsional design, we can achieve lower spring constants (10^-1 - 10^-7 N/m) without having to make a very thin film cantilever. We have demonstrated torsional cantilevers with spring constants on the order of 10^-2 N/m. These cantilevers can be used as extremely sensitive force sensors for atomic force microscopy.

Micro-electromechanical system (MEMS) devices are small compared to normal mechanical devices, but they are still large compared to the wavelength of visible light. Thus, simple low- cost optical measurement techniques can be adapted for precise characterization of the motions of these small objects. The results of such measurements are important for verification of simulations, especially for devices in which nonlinear effects such as squeeze film damping play a significant role. The advantages and challenges of optical metrology for MEMS are examined using an electrostatically-actuated microgripper structure as an example device. Interferometric measurements of static rotation and of small-signal sinusoidal and impulse responses are presented.

Channel electron multipliers are vacuum-electron devices for detection of charged particles and energetic photons. Several manufacturing technologies for microelectronics and microelectromechanical systems hold significant promise for the development of a new generation of silicon-based channel electron multipliers with greater capabilities and broader applications than conventional glass-based ones. This paper describes several approaches to microfabrication of such devices using surface and bulk micromachining techniques. Results are presented for fully micromachined channel electron multipliers with signal gains in the range of 10² - 10⁶.

The fabrication of surface profiles may become an interesting technology in the field of micromachining. Recently, surface profiles are known and widely used in optics, especially in diffractive optics. E-beam lithography is a suitable technology for the fabrication of such profiles. This paper gives an overview about the experiences and results achieved at the Friedrich-Schiller-University Jena, Germany, on this field for some years. At first we describe the challenges and obstacles of typical optical profiles with regard to e-beam writing technology. After introducing our two different e-beam writers ZBA 23H (variable shaped beam) and LION LV1 (high resolution gaussian beam) we demonstrate different writing strategies for the fabrication of binary, multilevel and continuous surface profiles. Variable energy writing is a new technology extending the abilities of the well-known variable dose writing. Some selected examples of interesting patterns and profiles, as holograms, gear wheels, lenses, gratings and encoder discs, demonstrate different aspects to be considered and the possible solutions for some problems.

The possibility of implementing Feynman's proposal for achieving ultraminiaturization by an iterative process of 3D machines making even-smaller 3D machines is considered in the nature of a `thought experiment.' A large array of Stewart platform machines is proposed as the `machine/factory' which possibly could have the capability to make ever-smaller versions of itself.

The automotive industry faces the challenge of creating cost-effective improvements in sensor technologies to increase vehicle quality and reliability, and to meet customer needs and wants. This paper, surveys the present, and offers a view of the future technologies and sensors required the future for the automobile.

Focused ion-beams have proved to be a superior means of fabricating holes between 5 and 20 microns in diameter, through gold and tungsten sheets over 40 microns thick. These holes are milled with an FEI FIB800 Focused Ion Beam Workstation using a Ga liquid metal source, with and without an enhancement gaseous etchant. They will be used as apertures for detectors that probe the point response function of the X-ray optics (Wolter Type-I mirror pairs), which form part of the NASA Advanced X-ray Astrophysical Facility. It is found that both pure milling and gas-assisted etching produced micron-sized holes of a quality superior to those produced by laser beam sputtering.

We report on the design and fabrication of a very high aspect ratio, entirely released single crystal silicon (SCS) micro-cantilever `on-a-frame' for z-motion applications. Motions of the micro-cantilever in the x-, y- and z-directions can be independently controlled by varying the spring constants of the SCS mechanical beams from about 10<sup>3</sup> N/m to approximately 10<sup>9</sup> N/m. We develop a new technology called Scream for High Aspect Ratio Proportions which increases the aspect ratio of the Single Crystal Reactive Etching And Metallization fabrication process. As an example, this novel technique based on a sequence of reactive ion etchings and thermal oxidations offers the capability of building a high aspect ratio wafer-free micro-cantilever `on-a-frame' with vertical dimensions exceeding 100 micrometers . The releasable grid consists of a large surface-to-volume ratio square-shaped `frame- within-a-frame' structure connected by z-motion springs. We have achieved intrinsic stress- based vertical deflections ranging from 60 micrometers to 125 micrometers with respect to the substrate floor for the large surface area (1 mm<sup>2</sup>) inner frame forming the z-stage. At the end of the fabrication process, the micro-cantilever `on-a-frame' can be fully released from the SCS substrate, thus resulting in a z-motion stage which can be entirely lifted off the wafer to be integrated with other micromechanical actuators.

This paper reports on a single coat process optimization of a new positive thick film resist originally described by Renaldo et al. from IBM at SPIE in 1995. The positive diazonapthoquinone photoresist system, SJR-3000, can achieve uniform coatings of greater than 28 microns in a single coat. In addition the process can produce images with wall profiles greater than 80 degrees and is compatible with traditional etch baths as well as gold, copper and permalloy plating baths without exhibiting cracking. Process latitude over a wide range exposure and development conditions will be demonstrated at a 20 (mu) coating thickness using a stepper exposure system.

We report on the fabrication and design of micro-machined electron guns (MEGs). The fabrication relies entirely on standard integrated circuit technology and allows the fabrication of large MEG arrays. Silicon field emitters are formed by thermal oxidation and the self- aligned submicron aperture gate electrode is fabricated using selective chemical vapor deposition of tungsten. Additional levels of tungsten electrodes are integrated using the selective tungsten multiple-level planar process. We present a brief review of the fabrication sequence, discuss the electro-mechanical and electron-optical design issues and present specific designs.

The first NIST-traceable SEM magnification calibration standard designed to meet the particular needs of the micromanufacturing industry has been fabricated and characterized in production prototype form. The SRM 2090A samples contain structures ranging in pitch from 3000 micrometers to 0.2 micrometers and are useful at both high and low accelerating voltages. The samples are fabricated using electron beam lithography and metal liftoff on a silicon substrate. Since the low-accelerating voltage, critical-dimension measurement scanning electron microscope has assumed an important role in modern semiconductor process control, the use and performance of the samples in a representative instrument is investigated. For all types of scanning electron microscopes, magnification calibration depends on several operating conditions, including magnification, accelerating voltage, and working distance. Implementation and application of the calibration factors within the SEM computer operating system can facilitate routine magnification calibration processes.

Micro-channel heat-exchanger test articles were fabricated and performance tested. The heat exchangers are being developed for innovative applications, and have been shown to be capable of handling heat loads of up to 100 W/cm². The test articles were fabricated to represent two different designs for the micro-channel portion of the heat exchanger. One design consists of 166 micro-channels etched in silicon substrate, and a second design consists of 54 micro-channels machined in copper substrate. The devices were tested in an experimental loop designed for performance testing in single- and two-phase flow with water and R124. Pressure and liquid subcooling can be regulated over the range of interest, and a secondary heat removal loop provides stable loop performance for steady-state tests. The selected operating pressures are approximately 0.344 MPa for distilled water and 0.689 MPa for R124. The temperature ranges are 15.5 to 138 C for distilled water and 15.5 to 46 C for R-124. The mass flow range 7.6 X 10^-8 to 7.6 X 10^MIN5 kg/min for both distilled water and R124.

At the macroscale, the milling process is very versatile and capable of creating 3D features and structures. Adaptation of this process at the microscale could lead to the rapid and direct fabrication of micromolds and masks to aid in the development of microcomponents. This task has been undertaken and results of the process indicate it can become an increasingly useful method. The micromilling process is currently characterized by milling tools with a diameter as small as 22 micrometers. The micromilling process can create trench-like features with vertical sidewalls and good smoothness. External corners are sharp and stepped features can be machined simply by programming those shapes. The process is direct and therefore dimensional errors do not accumulate as can occur with serial fabrication processes.

The reliable, high resolution concentration measurement of carbon dioxide is of critical importance in several life sciences and advanced life support related applications ranging from cabin air quality on extended duration space flights to monitoring and controlling plant growth and efficiency in closed life support systems. The design of a LIGA (German acronym for lithography, electrodeposition and plastic molding) micromachined, integrated optical bench for a carbon dioxide concentration sensor, based on the principle of infrared absorption, is presented in this paper as a compact solution to the need for high resolution CO₂ instrumentation. The micromachining approach takes advantage of the superior performance of optical infrared absorption sensing technology. In addition, creating an integral, micro- fabricated optical bench along with the source, detector and other necessary components on the same substrate will eliminate the size and alignment problems of the current designs. The design of the CO₂ sensor uses a folded optical system consisting of five parallel micro- mirrors placed 1.6 cm apart. A parametric evaluation of the beam divergence shows that the use of 1000 micrometers mirrors and a laser beam with a spot radius of 300 micrometers would result in a sensor design that can easily be fabricated by the LIGA process.

We present a low temperature single mask process for the fabrication of submicron very high aspect ratio silicon microstructures using a novel SOG (Spin On Glass) scheme for the RIE etch mask. SOG for the planarization of trenches in VLSI processing is extended to produce high aspect ratio oxide etch mask for the dry RIE etching of single crystal silicon. This work extends the height of silicon microstructures from what is currently achievable (10 - 20 microns) to over 50 microns, creating ultra high aspect ratio (greater than 50:1) structures.

Design, fabrication, and testing of thermal micro-sensors suitable for miniature and microscopic systems, for application on thin films (free standing or on substrates) as temperature sensors are presented in this paper. The sensors utilize the electrical resistivity temperature dependence of a metal. Using micro-lithography methods, several sets of gold resistors were fabricated in the form of flat 30 to 250 nm thick wires, 7 - 10 micrometers wide, and several cm long in a serpentine shape covering approximately 1.0 mm². These sensors have demonstrated better than 0.001 degree(s) C sensitivity. The electrical resistivity and its thermal coefficient of a thin gold metal film were compared with those of bulk material. Temperature measurements on Si wafers were performed in situations corresponding to x-ray lithography exposure conditions suitable for micromachining. The temperature rise and relaxation time of a silicon wafer during x-ray exposure were measured in vacuum and different He gas pressures.

A Confined Selective Epitaxial Growth (CSEG) technique is optimized to produce 0.9 micrometers thick and up to 15 micrometers wide local SOI slabs, isolated from the (001) substrate by a low-stress silicon nitride. In the fabrication process, a cavity is formed above the silicon substrate with access to the monocrystalline silicon using surface micromachining techniques. Subsequently, during the selective epitaxial growth this cavity is filled with single crystal silicon. In previous works, oxide was used for the isolation layers, and amorphous silicon as the sacrificial material. Here, an improved method, where low-stress nitride layers are used as structural layers to confine the epitaxial growth and PSG is used as a sacrificial material, is presented. The lateral growth rates of up to 260 nm/min were used and a horizontal to vertical aspect ratio 16:1 achieved. In these local SOI silicon slabs, a standard BiFET process has been merged to investigate technical feasibility of the vertical on-ship integration of a silicon sensor and a microelectronic cavity. The emitter-base ideality factor of 1.03 and overall performance of the fabricated transistors are very promising and indicate excellent material quality of CSEG silicon, where the [100] seed window orientation is used.

We are currently investigating the fabrication of high precision, miniaturized, electrostatic deflectors for use in electron or ion beam micro-columns. These columns can be used in a broad array of applications including microscopy, spectroscopy and lithography. Typically, micro-columns consist of a field emitter tip, a set of micromachined miniaturized lenses and one or more electrostatic deflectors. Miniaturization of the column allows the use of simple electrostatic lenses to achieve very high performance in a package that is just a few millimeters in length. Presently, all reported microcolumns have included miniaturized but conventionally-machined octupole deflector plates. If micromachined plates are used instead, lower deflection voltage is required for deflection, and the system becomes more amenable to very high speed operation. In addition, some reduction in scan field distortion is expected. These improvements results directly from the higher degree of miniaturization, tighter dimensional control, better placement accuracy, and smoother facets offered by micromachining. Given the dimensions (100 micrometers - 1000 micrometers thick) and tolerances (1 - 10 micrometers ) required, LIGA is well suited to fabricate such miniature deflectors. This paper will describe the fabrication of the deflectors using LIGA. The Center for X-ray Optics has built an endstation at Lawrence Berkeley National Laboratory's Advanced Light Source suitable for LIGA X-ray exposures.

We are fabricating sub-collimating X-ray grids that are to be used in an instrument for the High Energy Solar Spectroscopic Imager (HESSI), a proposed NASA mission. The HESSI instrument consists of twelve rotating pairs of high aspect ratio, high Z grids, each pair of which is separated by 1.7 meters and backed by a single Ge detector. The pitch for these grid pairs ranges from 34 micrometers to 317 micrometers with the grid slit openings being 60% of the pitch. For maximum grid X-ray absorbing with minimum loss of the solar image, the grid thickness-to-grid-slit ratio must be approximately 50:1, resulting in grid thicknesses of 1 to 10 millimeters. For our proof-of-concept grids we are implementing a design in which a 34 micrometers pitch, free-standing PMMA grid is fabricated with 20 micrometers wide slits and an 800 micrometers thickness. Stiffeners that run perpendicular to the grid are placed every 500 micrometers . After exposure and developing, metal, ideally gold, is electrodeposited into the free-standing PMMA grid slits. The PMMA is not removed and the metal in the slits acts as the X-ray absorber grid while the PMMA holds the individual metal pieces in place, the PMMA being nearly transparent to the X-rays coming from the sun. For optimum imaging performance, the root-mean-square pitch of the two grids of each pair must match to within 1 part in 10000 and simultaneous exposures of stacked sheets of PMMA have insured that this requirement is met.

We have developed fine pitch, sub-collimating X-ray grids for an instrument in the High Energy Solar Spectroscopic Imager (HESSI), a proposed NASA mission. In addition to high- energy X-rays, the instrument requires collimation of photons with energies of less than 4 keV such that free-standing grids are required that have no material between the grid slats. We have fabricated 25 micrometer thick gold grids that can collimate photons from visible light up to 30 keV X-rays. They are 55 millimeters in diameter and have 200 micrometer thick silicon support structures. The fabrication process starts with 200 micrometer thick 3 inch wafers onto which a 50 angstrom chrome, 300 angstrom gold electroplating strike is e-beam evaporated. A 25 micrometer thick optical resist is deposited on the wafers using a low spin rate. The resist is exposed and developed and an oxygen plasma clean is performed to fully strip resist residue from the strike. 25 micrometers of gold is then plated in the resist mold, resulting in a gold grid with photoresist between each gold slat. The wafer is turned over and a 50 micrometer dry resist is patterned such that it has a array of 1 by 4 millimeter openings to the silicon. The silicon is etched through to the chrome/gold strike using a xenon difluoride etching process. Both types of photoresist are removed with acetone followed by a piranha clean and the chrome/gold strike is removed with a hydrochloric acid and hydrogen peroxide chrome etch which also slowly etches gold.

The theory of electrostatic forces on doped semiconductor cantilevers, and their dynamic, mechanical response, is presented. Effects on constant, and time-varying, voltages between the cantilever and a mechanically fixed reference potential are studied. Surface changes are considered, since they can screen electrostatic forces significantly. The results show work function differences between the cantilever and the reference electrode must be included in calculating the mechanical response of semiconductor cantilevers to electrostatic forces. Surface charges, as long as their sheet densities are below 10¹¹ cm^-2, will not present especial difficulties for either analysis or behavior. Small-signal analysis of the mechanical response is complicated by both large-signal applied biases, giving rise to displacement currents, and penetration of the electric field into non-degenerate semiconductors.

Projection displays and microelectromechanical systems (MEMS) have evolved independently, occasionally crossing paths as early as the 1950s. But the commercially viable use of MEMS for projection displays has been illusive until the recent invention of Texas Instruments Digital Light Processing ^TM (DLP) technology. DLP technology is based on the Digital Micromirror Device^TM (DMD) microchip, a MEMS technology that is a semiconductor digital light switch that precisely controls a light source for projection display and hardcopy applications. DLP technology provides a unique business opportunity because of the timely convergence of market needs and technology advances. The world is rapidly moving to an all- digital communications and entertainment infrastructure. In the near future, most of the technologies necessary for this infrastrucutre will be available at the right performance and price levels. This will make commercially viable an all-digital chain (capture, compression, transmission, reception decompression, hearing, and viewing). Unfortunately, the digital images received today must be translated into analog signals for viewing on today's televisions. Digital video is the final link in the all-digital infrastructure and DLP technoogy provides that link. DLP technology is an enabler for digital, high-resolution, color projection displays that have high contrast, are bright, seamless, and have the accuracy of color and grayscale that can be achieved only by digital control. This paper contains an introduction to DMD and DLP technology, including the historical context from which to view their developemnt. The architecture, projection operation, and fabrication are presented. Finally, the paper includes an update about current DMD business opportunities in projection displays and hardcopy.

The rapid expansion in number and scope of research projects in the general area of micromachining technology makes the field especially interesting. Advances, particularly from universities and research institutes, suggest that increased commercial development is likely in a number of new fields. The nature of the fabrication technologies used to make these prototype parts, however, lead to difficulties in quickly reaping commercial benefits from these technolgoical advancements. This is in contrast to the nature of commercial development in the integrated circuit industry, where standardized processes result in rapid development of systems and products which use new designs.