A novel MEMS actuation technique has been developed for scanned beam display and imaging applications that allows driving a two-axes scanning mirror to wide angles at high frequency. This actuation technique delivers sufficient torque to allow non-resonant operation as low as DC in the slow-scan axis while at the same time allowing one-atmosphere operation even at fast-scan axis frequencies great enough to support SXGA resolutions. Several display and imaging products have been developed employing this new MEMS actuation technique. Exceptionally good displays can be made by scanning laser beams much the same way a CRT scans electron beams. The display applications can be as diverse as an automotive head up display, where the laser beams are scanned onto the inside of the car’s windshield to be reflected into the driver’s eyes, and a head-worn display where the light beams are scanned directly over the viewer’s vision. For high performance displays the design challenges for a MEMS scanner are great. The scanner represents the system’s limiting aperture so it must be of sufficient size; it must remain flat to fractions of a wavelength so as to not distort the beam’s wave front; it must scan fast enough to handle the many millions of pixels written every second; and it must scan in two axes over significant angles in order to “paint” a wide angle, two-dimensional image. Using the new actuation method described, several MEMS scanner designs have been fabricated which meet the requirements of a variety of display and imaging applications.
Since lasers have the most saturated colors, laser display can express the natural color excellently. Laser scanning display has merits of simple structure and high optical efficiency. We designed a new scanning mirror which has a circular mirror plate with an elliptical outer frame and is electrostatically driven by vertical combs arranged at the outer frame. This eye-type mirror showed a larger deflection angle compared to the rectangular and the elliptical mirrors. To increase the driving force twice, stationary comb electrodes are arranged at the upper and lower sides of the moving comb fingers, together. The diameter of the mirror plate is 1.0 mm, and the lengths of the major and minor axes of the outer frame are 2.5 mm and 1.0 mm, respectively. Using this scanning mirror, we obtained an optical scanning angle of 32 degrees when driven by the ac control voltage of the resonant frequency in the range of 22.1 ~ 24.5 kHz with the 100 V dc bias voltages. We demonstrated the full color XGA-resolution video image with the size over 30 inches using an eye-type scanning mirror. The successful development of compact laser TV will open a new area of home application of the laser light.
This contribution deals with design, fabrication and test of a micromachined resonant scanner usable for horizontal deflection of the laser beam in a projection display. The electrostatically driven plate is separated from the mirror in order to reduce air damping and electrostatic non linearity. The device consists of a circularly shaped mirror which is suspended by torsion beams in the center of an elastically suspended driving plate. A resonator with two rotational degrees of freedom is arranged in this way. The rotation axes of mirror and driving plate are the same. A suitable design of the properties of the two degrees of freedom resonator leads to a significant amplification of the oscillation of the mirror compared to the oscillation of the driving plate. The first resonant mode is a rotation of both plates with nearly the same magnitude at a frequency of approx. 5 kHz. The second mode with paraphase deflection at 24 kHz shows a deflection amplification by a ratio of 53 and is used for scanning operation. A supporting part made of glass carries two electrodes in the region of the driving plate and has a micro sandblasted hole beneath the mirror. Bulk micromachining KOH wet etching of the electrode gap size on the back side of the driving plate, reactive ion etching for contour shaping of the mirror, of the driving plate and of the torsion beams and anodic bonding have been used for fabrication of the mechanical structure. The mirror is evaporated by an aluminum layer. Applying a voltage of 380V results in a mechanical deflection of ± 5.5 degrees at 24 kHz at atmosphere pressure. The device shows very small dynamic warp (<100nm) of the mirror plate even though the relatively large size of 2.2 mm diameter because of the thickness of 280 µm. The measured mechanical Q-factor is 5100.
This paper addresses different highly reflective optical coatings on micro scanning mirrors (MSM) for applications in the NIR-spectral region to enable new applications like laser marking and material treatment at high optical power density. In the case of MSM with an unprotected Al coating, the absorption limits the maximal power density because of induced heating. The damage threshold for unprotected Al coatings was investigated. In addition highly reflective enhanced metallic and dielectric multilayer coatings for the NIR have been developed and characterized. These coatings resolve the problems of unprotected aluminum coatings related to NIR absorption and the resulting limitation of applicable laser power density. The coatings ensure a high reflectance even in corrosive environments. Enhanced metallic broadband reflectors reach a reflectivity of 98.7% at 1064 nm whereas narrow-band dielectric multilayer coatings reach a reflectivity of 99.7% at 1064 nm.
This paper presents the properties of the Sigma7300 which is a commercial DUV laser pattern generator based on spatial light modulator (SLM) technology designed to meet the requirements of the 65-nm technology node and below. The introduction of spatial light modulators provides a possibility for optical mask writers to combine high resolution and accuracy with short write time making it possible to write most of the high end mask layers in a cost effective way. The Sigma7300 mask writer is developed by Micronic Laser Systems whereas the SLM, which is a combined MEMS and CMOS component with individually controllable movable micromirrors, is developed by the Fraunhofer-IPMS institute in Dresden. The SLM allows parallel writing of one million pixels with a frame rate of up to 2 kHz. The technology offers resolution enhancement advantages from stepper technology not available in other mask patterning tools.
The semiconductor industry has been driven by significant improvements in optical-lithographic capability. As feature sizes on the wafer shrink faster than the wavelength of the exposing illumination, increasingly complex and expensive steps such as immersion, resolution-enhancement techniques, and optical-proximity correction (OPC) are required. Traditionally, high costs have been amortized over large volumes of chips, and by progressive technological maturity. Optical lithography using MEMs-based spatial-light modulators provides an alternative means of lithography. Significantly lower costs-of-ownership coupled with throughputs acceptable for mask manufacturing, mask prototyping, and low-volume-chip manufacturing are the enabling attributes of such techniques. At MIT, we have pursued a unique version of this technology, which we call Zone-Plate-Array Lithography (ZPAL). In ZPAL, an array of high-numerical-aperture diffractive lenses (for example, zone plates) is used to create an array of tightly focused spots on the surface of a photoresist-coated substrate. Light directed to each zone plate is modulated in intensity by one pixel on an upstream spatial-light modulator. The substrate is scanned, and patterns of arbitrary geometry are written in a “dot-matrix” fashion. In this paper, we describe results from our proof-of-concept ZPAL system and its future potential. Lithography using distributed, tightly focused spots presents a different set of advantages and challenges compared to traditional optical-projection lithography. We discuss some of these issues and how they bear on practical system designs.
The Fraunhofer IPMS and Micronic Laser Systems AB have developed a technology for microlithography using spatial light modulation (SLM). This technology uses an array of micromirrors as a programmable mask, which allows parallel writing of 1 million pixels with a frame rate of up to 2 kHz. The SLM is fabricated at the IPMS using its high-voltage CMOS process. The mirrors are fabricated by surface micromachining using a polymer as sacrificial layer. The mirrors are operated in an analog mode to allow sub-pixel placement of pattern features. This paper describes the function of the SLM with an emphasis on the stability of the mirror deflection and a method to improve it which has been implemented.
High reflecting low-stress optical coatings for the next-generation of micro mechanical mirrors have been developed. The optimized metal systems are applicable from VUV and DUV down to the UV and VIS spectral region and can be integrated in the technology of MOEMS, such as spatial light modulators (SLM) and micro scanning mirrors. This optimized metal designs enable to reconcile high optical performances with adequate mechanical properties and convenient CMOS compatibility. Currently, micro-mirror arrays with enhanced highly reflective coatings for DUV (λ = 193 nm) and VUV (λ = 157 nm) exist as prototypes.
Optical coherence tomography (OCT) is an emerging imaging technique that can provide high-resolution cross-sectional images of biological tissues. OCT has been used to detect various cancers including those in gastrointestinal tracts, bladder, and respiratory pathways. For in vivo imaging in visceral organs, small size and fast speed are essential, which can be achieved by using MEMS (Microelectromechanical systems) technology. In this paper, design and experimental results of a miniature endoscopic OCT imaging probe based on unique single-crystal silicon (SCS) MEMS micromirrors are reported. Several generations of one-dimensional (1D) micromirrors with a size of 1 mm by 1 mm have been fabricated. The resonant frequencies and radii of curvature of the micromirrors are about 0.5 kHz and 0.25 m, respectively. The packaged MEMS-OCT probe is 5 mm in diameter. About 15-μm axial resolutions, 20-μm transverse resolutions and 5-frames/s image rates are obtained.
This paper provides an overview of several years of research in the use of polyimide MEMS actuators for medical imaging applications, including high frequency ultrasound and optical coherence tomography (OCT). These scanning devices are microfabricated out of polyimide substrates using conventional integrated circuit technology. The material properties of the polyimide allow very large scan angles to be realized and also allow the resonant frequencies of the structures to be in the appropriate ranges for real-time imaging. The primary application of these probes is endoscopic and catheter-based imaging procedures. The microfabrication enables the creation of very small devices essential for compact imaging probes. In addition, they can be fabricated in bulk, reducing their cost and potentially making them disposable to reduce the cost of patient care and minimize the potential for patient cross-contamination. Several different scanning geometries and actuators have been investigated for imaging applications, including both forward-viewing and side-scanning configurations. Probes that utilize both electrostatic polyimide actuators and piezoelectric bimorphs to mechanically scan the ultrasound or OCT imaging beams will be presented. These probes have been developed for both use in both ultrasound and OCT imaging systems. Medical applications of these probes include the early detection of cancerous and precancerous conditions in the bladder and other mucosal tissues. These imaging probes will allow the physician to visualize the subsurface microstructure of the tissues and detect abnormalities not visible through the use of conventional endoscopic imaging techniques. Prototype devices have been used to image geometric wire phantoms, in vitro porcine tissue, and in vivo subjects. The progress made over the last several years in the development of these polyimide scanning probes will be presented.
Intravascular ultrasound (IVUS) imaging has become an essential imaging modality for the effective diagnosis and treatment of cardiovascular diseases during the past decade enabled by innovative applications of piezoelectric transducer technology. The limitations in the manufacture and performance of the same piezoelectric transducers have also impeded the improvement of IVUS for emerging clinically important applications such as forward viewing arrays for guiding interventions and high resolution imaging of arterial structure such as vulnerable plaque and fibrous cap, and also implementation of techniques such as harmonic imaging of the tissue and of the contrast agents. Capacitive micromachined ultrasonic transducer (CMUT) technology shows great potential for transforming IVUS not only to satisfy these clinical needs but also to open up possibilities for low-cost imaging devices integrated to therapeutic tools. We have developed manufacturing processes with a maximum process temperature of 250°C to build CMUTs on the same silicon chip with integrated electronics. Using these processes we fabricated CMUT arrays suitable for forward viewing IVUS in the 10-20MHz range. We characterized these array elements in terms of pulse-echo response, radiation pattern measurements and demonstrated its volumetric imaging capabilities on various imaging targets.
Minimally invasive medical therapy can reduce both healthcare costs and patient suffering. The development of submillimeter scale instruments falls in a gap of manufacturing technologies between traditional machining and microfabrication techniques. To address this need we have developed a fabrication technique based upon laser machining of tubular structures combined with shaped-memory alloy actuators to create compliant devices for minimally invasive interventions. The initial application of this approach has been to develop a forward viewing intravascular ultrasound scanner for use in guiding intravascular interventions in situations where traditional angiography and intravascular ultrasound are unable to provide adequate guidance. The ultrasound device is less than 1.5 mm in diameter and provides imaging at 20 frames per second. Imaging currently is performed with a 20 MHz 800 micron diameter transducer producing axial resolutions of approximately 150 microns. Device optimization has resulted in peak strains of less than 1% within the compliant structure resulting in device life greater than 200,000 cycles providing usable times greater than twice the anticipated procedure length. The design concepts embodied in this initial implementation will serve as a platform for a variety of self actuated minimally invasive tools.
We have developed a unique miniature confocal optical scanning microscope as an endoscopic application and successfully obtained a real time image. The confocal microscope can observe the longitudinal direction of a scanning head including the electrostatic 2-D MEMS scanner and an aspherical objective lens. Waterproof packaging of the scanning head is accomplished. The MEMS scanner and the objective lens in the head are assembled precisely and compactly. The MEMS scanner has a gimbal structure and the mirror flatness and Q factor are controlled by a stress-controlled silicon nitride. The scan angle of 12deg., the mirror flatness of 80nm(PV) and the mirror reflectance of over 85% are achieved. The diameter and length of the scanning head are 3.3mm and 8mm, respectively and it can insert into a channel of the endoscope. The lateral resolution of 0.5um, the depth resolution of 2.9um, the field of view of 100*75um and over 20/s of frame rate needed for the in-vivo inspection was achieved.
In this paper we present scanning micromirrors, actuated by self-aligned, bidirectional, vertical electrostatic combdrives, for dual-axes confocal microscopy. The fabrication process, which is based on Deep Reactive Ion Etching (DRIE) of Silicon-on-insulator (SOI) wafers with two silicon device layers, requires only three lithography steps for one-dimensional scanners, while an additional two lithography steps must be performed to create two-dimensional scanners. Only front side processing is required and the two oxide layers of the double SOI wafers provide efficient and reliable etch stops. These features combined with the fact that the combs are self aligned, enable high-speed, high-resolution microscanners with stable and reliable operation as required for endoscopic implementations of confocal microscopes.
Bimaterial microcantilevers arranged into focal plane arrays (FPAs) can function as uncooled IR imaging devices. In order to analyze the performance of such devices and compare various FPAs, it is essential to have an in-depth understanding of their operation, figures of merit, and fundamental limitations. We give an overview of figures of merit that are applicable to both cooled and uncooled IR detectors. Specific focus of this chapter is a performance analysis for microcantilever IR detectors with an optical readout. We discuss responsivity of microcantilever IR detectors and analyze the different sources (and mechanisms) of noise present in them. A model SiNx microcantilever device with an Al layer in the bimaterial region was fabricated and its performance as an IR detector was analyzed.
A micrograting interferometer has been fabricated to use in measuring the static and dynamic performance of MEMS devices. These measurements aid in qualifying the functionality of fabricated MEMS devices, as well as improving fabrication techniques. The metrology system uses a phase sensitive diffraction grating for interferometric axial resolution and a microfabricated lens for improved lateral resolution. In addition, active control is applied to the system to reduce the impact of mechanical vibrations and insure a high degree of measurement sensitivity. The control scheme is demonstrated successfully in the scanning of MEMS devices in the experiment. A deformable grating, which controls measurement sensitivity, has been fabricated and integrated with optoelectronics in small volume. Experiments with the integrated package demonstrate that the measurement sensitivity can be adjusted by actuating the deformable grating. This integrated single device illustrates that the deformable grating sensor can be expanded to form arrays for parallel measurement of MEMS device.
This paper reports the design, fabrication and opto-mechanical characterization of a deformable mirror to correct spherical aberration in future optical data storage standard (Blu-Ray Disc). The integrated mirror is realized in standard semiconductor technology to produce very low cost devices. The device is based on the electrostatic actuation of a 10 μm thick silicon membrane (4 mm diameter) obtained from a 4' SOI wafer glued with polymer paste over concentric or hexagonal electrodes obtained from another 4' silicon wafer. Optical wavefront measurements compared with theoretical calculations demonstrate that an applied voltage of only 40 V on the three concentric electrodes allow to perfectly correct aberrations. Moreover we showed that the shape of the optical deformation induced by the mirror can precisely be controlled by the design of electrodes and applied voltages. A Peak to Valley optical deformation up to 4 μm can be achieved with an applied voltage of only 70 V. Finally dynamic measurement showed that the device is able to work at a frequency of 100 Hz that is higher that needed for foreseen application.
A polymer-based dynamic microlens system that can provide variable focal length and field-of-view (FOV) is fabricated and tested for its optical imaging characteristics. A flexible polydimethylsiloxane (PDMS) polymer membrane is used to form the lens surface. Two such membranes are combined with a spacer in between to form the fluidic lens chamber. The entire assembly is actuated by fluidic pressure using an external syringe pump to form either a double convex (DCX) or double concave (DCV) lens. The relationship between the focal length (f) and FOV of this dynamic lens as a function of the volume of the fluid pumped into or out of the lens chamber is investigated. The focal length of the single dynamic lens system can be tuned over the range of 75.9 to 3.1 mm and -75.9 to -3.3 mm, respectively, for the DCX and DCV lens configurations. The FOV that could be achieved using this dynamic lens system as DCX and DCV lenses is in the range of 0.12 to 61 degrees and 7 to 69 degrees, respectively. The smallest f-number (f/#) of 0.61, which corresponds to a numerical aperture of 0.64, could be achieved for a single dynamic lens system. An integrated two or three variable focal length DCV microlens system to provide wide FOV has also been fabricated and tested. The effective focal length of the integrated dynamic microlens system with two and three DCV lenses can be tuned in the range of -37.9 to -2.1 mm and -25.3 to -1.8 mm, respectively. The FOV achieved using the integrated two and three variable focal length DCV microlens systems were in the range of 8 to 76.7 degrees and 11.5 to 90.4 degrees, respectively.
We present a means for forming images using micromirror arrays. Using an array of 2D tilt mirrors it is possible to create an image
whose resolution is much higher than the number of mirrors. We
present several types of simulations, including images produced by
graphical ray tracing for linear MEMS array of containing 1 and 2
mirrors, and general NxN configurations.