Optics has been a technology for a long time. Often, just when it looked like the technology was beginning to diminish in impact, a new technology related to optics was born and completely rejuvenated the field. This caused significant innovation in relatively short periods of time. For example, the development of the laser in the early 1960s gave rise to laser printers, CD disk drives, optical recording, fiber optic communication etc. and has produced an enormous increase in practical devices as well as economic value. We are now faced with another opportunity. This opportunity is the ability to fabricate small optical "benches" or structures using the same material that has enabled the computer revolution, viz. silicon. While this field is relatively new, what might we expect from this new capability? The opportunity for displays, sensors, communication, etc. are beginning to open up and present new business and innovation possibilities. Optics has always benefited from system enabling technologies and Micro-Electro-Mechanical-Systems is a major new development. In this paper I will look at where we might go with Optics and MEMS or MOEMS. Examples of display concepts, etc. will be discussed that illustrate the great potential of these combined capabilities in just one area. It is likely that we will not be limited by the technology but by our imagination in its use.
For the past six years, Digital Light Processing technology from Texas Instruments has made significant inroads in the projection display market. With products enabling the world’s smallest data and video projectors, HDTVs, and digital cinema, DLP technology is extremely powerful and flexible. At the heart of these display solutions is Texas Instruments Digital Micromirror Device (DMD), a semiconductor-based “light switch” array of thousands of individually addressable, tiltable, mirror-pixels. With success of the DMD as a spatial light modulator for projector applications, dozens of new applications are now being enabled by general-use DMD products that are recently available to developers. The same light switching speed and “on-off” (contrast) ratio that have resulted in superior projector performance, along with the capability of operation outside the visible spectrum, make the DMD very attractive for many applications, including volumetric display, holographic data storage, lithography, scientific instrumentation, and medical imaging. This paper presents an overview of past and future DMD performance in the context of new DMD applications, cites several examples of emerging products, and describes the DMD components and tools now available to developers.
For typically small volume production of MEMS, MOEMS, fine feature PCB, high density chip packaging and display panels, especially for lab tests, low cost and the capability to change the original design easily and quickly are very important for customers and researchers. BALL Semiconductor Inc.'s Maskless Lithography Systems (MLS) feature the Digital Mirror Device (DMD) as the pattern generator to replace photo-masks. This can remove masks from UV lithography, and dramatically reduce the running cost and save time for lab tests and small volume production. At Ball Semiconductor Inc, 1.5μm line/space, 10μm line/space, and 20μm line/space Maskless Lithography Systems were developed.
In our MLS, an 848×600 microlens and spatial filter array (MLSFA) was used to focus the light and to filter the noise. In order to produce smaller line-space than 16μm the MLSFA was used to get smaller UV light pad (compared with the SVGA DMD’s micro-mirror: 17μm×17μm) and to filter the noise produced from the DMD, optical lens system, and micro lens array. This MLSFA is one of the key devices for our Maskless Lithography System, and determines the resolution and quality of maskless lithography.
A novel design and fabrication process of a single-package MLSFA for our Maskless Lithography System will be introduced. To avoid problems produced by misalignment between a two-piece spatial filter and microlens array, MEMS processing is used to integrate the microlens array with the spatial filter array. In this paper, the self-alignment method used to fabricate exactly matched MLSFA will be presented.
An innovative High Resolution Maskless Lithography System (Hi-Res MLS) was designed using Texas Instruments’ SVGA DMD, which employs additional micro-optics with a combination of low and high NA projection lens systems. A low power mercury-arc lamp filtered for G-line (λ = 435.8 nm) was used as the light source. Exposure experiments were performed using Ball patented Point Array scrolling or scanning method and proprietary data conversion, extraction and transfer software algorithms. In each scan, the field-width (W) was approximately 8.47 mm with the field-length (longitudinal field) only limited by memory capacity. DMD frame rates of up to 5 kHz (kframe/s) were achievable, which were synchronized to the stage motion. In this experiment, TSMR-8970XB10 photo-resist, diluted to 3.8 cP with PR thinner was employed. The photo-resist was spin-coated on a glass substrate to 1.0-μm thickness with 0.1-μm uniformity. A 0.4-μm step-size was used and 27000 DLP frames were extracted and transferred to the DMD driver. Results indicated consistent 1.8 μm L/S resolved across the entire field-width of 8.47 mm. At certain locales, 1.5-μm L/S was also resolved. The potential of this maskless lithography system is substantial. Even at the current level of performance, the system is sufficient for applications in MEMS, MOEMS, photomasking, high resolution LCD, high density PCB, etc. Higher productivity is predicted by lens system designed for H-line (λ = 405 nm), by using Ball’s violet diode laser systems, and the development of real-time driver.
The visual displays of contemporary military flight simulators lack adequate definition to represent scenes in basic fast-jet fighter tasks. For example, air-to-air and air-to-ground targets are not projected with sufficient contrast and resolution for a pilot to perceive aspect, aspect rate and object detail at real world slant ranges. Simulator display geometries require the development of ultra-high resolution projectors with greater than 20 megapixel resolution at 60 Hz frame rate. A new micromirror device has been developed to address this requirement; it is able to modulate light intensity in an analog fashion with switching times shorter than 5 μs. When combined with a scanner, a microlaser and Schlieren optics, a linear array of these flexible micromirrors can display images composed of thousands of lines at a frame rate of 60 Hz. The approach selected for light modulation and the micromirror fabrication process flow are reviewed. Static and dynamic performances of these electrostatic MOEMS are described. Preliminary results following the integration of the described modulator into a projector prototype are reported. Developments toward a fully addressable 2000 × 1 flexible micromirror array are presented. The specifications and design of the CMOS circuit required to control this micromirror array are described. Packaging issues related to these large arrays are discussed.
This paper presents an overview of the design issues and tradeoffs that affect the key elements of a retinal scanned display including optics, light source module, and biaxial MEMS scanner. A prototype miniature RSD display system incorporating these design concepts is described along with its measured performance.
Dual capacitively driven MEMS mirrors, when driven with a sinusoidally varying voltage, will experience a nonlinear torque. Nonlinearities arise from the nonlinear relationship between voltage and torque and the fact that the capacitor gap is a function of the angular displacement of the mirror. At high maximum scan angles, the dynamics of the mirror exhibit behavior similar to that of a nonlinear harmonic oscillator with a softening spring. This behavior is characterized by amplitude instability relative to frequency and results in the need to implement additional control algorithms to achieve stable operation.
In this work we present experimental results demonstrating the effects of the nonlinearities and the implications of these nonlinearities for high fidelity image rendering using these scanning mirrors. We then present an analysis of the nonlinearities in the system showing significant nonlinearities up to the 10th order. We present numerical results consistent with the observed behavior of the MEMS scanning mirror. Next, we present an analysis showing that the nonlinearities can be significantly reduced applying a non-sinusoidal voltage signal. Finally, we discuss control issues, solutions and implications for high fidelity image rendering using these scanning mirrors.
The Fraunhofer Institute of Microelectronic Circuits and Systems, Dresden, has been developing resonant Micro Scanning Mirrors for several years. They are designed for large deflection angles at low driving voltages in resonant operation. A couple of changes and optimizations in the layout of the 2-scanner that have improved performance and reliability are presented and discussed.
Different variants have been fabricated by bulk-micromachining in bonded silicon-on-insulator-substrates and have been characterized. Mirror plate and gimbal can be rectangular or elliptical, now. New comb structures of the driving electrodes allow optimization of either capacitance, damping or electromechanical stability. Complex insulation structures reduce parasitic capacitance and increase reliability and mechanical stability.
Various design variants were fabricated and characterized. Low-frequency devices with characteristic frequencies under 500 Hz reached scan ranges over 45° (±11.2 degrees mechanically) at voltages below 20V. The high-frequency devices with 8.4 kHz / 1 kHz reached 17.5° / 35° at 40 V / 30 V, respectively.
A scanning two-axis tilt mirror has been modeled, fabricated and tested. The tilt mirror device is fabricated from single crystal silicon using bulk micromachining technology. The mirror is octagonal and is suspended by outer torsion hinges, a gimbal, and inner torsion hinges. Response to a driving voltage is investigated, along with frequency response. Finite element modeling was performed and the results compared with experimental data, with good agreement. Using automated and semi-automated placement equipment, linear arrays of the tilt mirrors have been produced.
A MEMS electromagnetic optical scanner for horizontal scanning in a commercial laser scanning microscope has been developed. Major specifications include mirror size: 4.5 × 3.3 mm2, resonant frequency: 4 kHz, changeable scan angle: 2.1 - 16 °, mirror flatness: <244 nm, and scan angle stability: <0.1 %. Initial development started with prototyping a scanner with polyimide hinge, but the stiffness and the Q-factor of polyimide hinge were found insufficient to realize the required resonant frequency and scan angle. On the other hand, a scanner with single crystal silicon hinge has been successfully developed. The electromagnetic scanner has a copper-electroplated driving coil and an improved magnetic circuit to reduce power consumption. A scanner controller using the output signal from an integrated sensing coil was also developed, and sufficient scan angle stability was obtained. The scanner has survived the life test of over 140 billion cycles. It has successfully satisfied all the specifications including not only the fundamentals such as resonant frequency and maximum scan angle but also the ones for commercial products such as scanning stability and durability. It has been commercialized as a part of our product OLS1100 (remodeled as OLS1200 as of Aug. 2002).
A high-frequency resonant horizontal scanner and a linearly driven vertical scanner at display frame rates can create a 2-D raster for video display. The combined motion of the two scanners forms a sinusoidal raster in the vertical direction where the raster line spacing is uniform only at the center and becomes progressively nonuniform towards the left and right edges of the display screen. Nonuniformities degrade the image quality and can be corrected by the addition of a third scanner to the system. Last year we reported the requirements and some of the early results in our MEMS-based raster correction scanner development effort. Since then, a lot of progress was made and the scanner was successfully incorporated into an SXGA resolution helmet-mounted display system. In this paper we report the results of thick copper coil development, new coil and magnet design for electromagnetic actuator, thermal flatness testing, new mounting design, and finally the performance measurements for the HMD system with a raster correction scanner.
We discuss magnetic actuation for Microvision’s bi-axial scanners for retinal scanning displays. Compared to the common side-magnet and moving-coil approach, we have designed, assembled and tested a novel magnet configuration, with magnets above and below the moving coil. This design reduces the magnet sizes significantly without sacrificing performance, and opens further improvement paths as well.
We present an ASIC for synchronized excitation of electrostatically driven resonant micro scanning mirrors which have been developed and fabricated at the Fraunhofer IMS for several years. The mirror oscillation is excited with a rectangular driving signal and operated close to the characteristic frequency or higher. Deflection amplitude is maximum if the driving voltage is switched off precisely at the cross-over of the oscillation. We have developed and fabricated a mixed signal ASIC that starts the oscillation and runs the mirror with high efficiency by detecting the oscillation cross-over. For that, the ASIC senses the varying capacitance of the actuator’s comb-drive electrodes and converts it with a clocked charge amplifier into a voltage. The amplified capacitance signal is processed by a synchronization module which detects the minimum of the capacitance signal corresponding to the cross-over of the oscillation. An integrated charge pump provides the driving voltage. The ASIC has been fabricated at the facilities of the IMS with a 1.2 μm CMOS process. Tests with a demonstrator PCB have shown that the synchronization works highly efficient with a value of appr. 96 %. A mechanical deflection angle of ± 13° was achieved for a micro-mirror with a characteristic frequency of 250 Hz at a driving voltage of 13.5 V, only.
Iridigm has produced the first color MOEMS based reflective display. The display relies on a family of MOEM devices that are called Interferometric Modulators or iMoDs. With dimensions of 240 × 160 pixels at 100 dpi, it exhibits contrast and brightness that is comparable or superior to existing LCD based solutions. The particular combination of characteristics within these devices is what provides for the improved performance, and facilitates the manufacture of such displays as well. These characteristics, their impact on display performance as well as the overall display system will be described and discussed.
A diffraction-based interferometric optical detection method for micromachined acoustic sensors can provide better sensitivity as compared to conventional capacitance detection schemes especially at low frequency range. The optical detection method, complete with optoelectronics readout, can be integrated with a capacitive micromachined acoustic transducer. The method is utilized on a 19×19 capacitive micromachined ultrasonic transducer (cMUT) array to demonstrate ultrasonic imaging of wire targets at 750kHz in air. A silicon photodiode (PD) array is also designed and fabricated in a standard 1.3μm CMOS technology, and through-wafer etching of holes for optical interconnect is performed on the same silicon platform. Further improvement of displacement sensitivity in a resonant-cavity-enhanced (RCE) acoustic sensor is theoretically analyzed including the loss effect in the mirror, and the theoretical results are experimentally verified by measurements on devices with a thin metallic bottom mirror made of silver.
Interrogation and analysis of hazardous agents in a hostile environment is difficult using currently available visible spectroscopic instrumentation due to both the size and cost of existing devices. One proposed method to decrease the size and cost of current visible spectrometers uses Micro-Opto-Electro Mechanical Systems (MOEMS) and die-level photo-diodes instead of static bulk gratings and linear detector arrays. We propose a grating based electrostatic comb driven visible micro-spectrometer that allows spectroscopy to be performed in the harshest of environments for a cost that, up until now, has been restricted. Our proposed device is approximately 50 mm2 in size, which makes it portable enough to gather spectroscopic data discreetly and remotely. The use of MOEMS and precision micro-machined optics allows this device a level of accuracy and mechanical ruggedness that conventional bulk grating spectrometers lack. Commercial-off-the-shelf (COTS) optical components are combined with state-of-the-art (SOA) photolithography techniques that reduce production costs while increasing manufacturability. The novel design of this spectroscopic device allows it to be utilized in a wide range of applications, from collecting data on an unmanned ground vehicle or acting as a passive sensor that remotely evaluates the introduction or reduction of certain reagents. Principles of assembly, operation and testing will be presented in this paper.
In this paper, we present a system-level simulation and analysis of a diffractive optical MEM Grating Light Valve. The simulations are performed in a system-level multi-domain CAD framework developed at the University of Pittsburgh. Including the electrical, mechanical, and optical domains, this framework allows the user to design micro-optical systems by examining performance measures of the entire system. In this paper, we provide a brief background of the models that are used for signal and device simulation, and use these results for the simulation and analysis of the promising GLV device for applications in a projection system.
The Fraunhofer IMS in Dresden is developing and fabricating spatial light modulators (SLMs) for micro lithography with DUV radiation. The accuracy of analog modulation is very important for the resulting accuracy of the generated features. On the other hand, fabrication tolerances create variations for example in spring constant, zero voltage deflection, and reflectivity. The slightly different response curves of the individual pixels therefore require an individual calibration. The parameters of these are stored in a look-up table so that the proper addressing voltage for the required optical response can be selected. As the deflection angle as well as the size of the SLM pixels are quite small, a direct measurement of the pixel response is not straightforward. An optical system similar to the one in the lithography machine has been set up, where the SLM is operating as a phase grating and the image is generated by a spatial filter. The pixel deflection can be calculated from the aerial image for isolated deflected pixels. The background pixels, that are not calibrated yet, contribute some error to this calculation. However, this error is not very large. Simulations regarding the accuracy of this measurement are discussed, and experimental results are shown.
The Fraunhofer Institute for Microelectronic Circuits and Systems (FhG-IMS) has developed spatial light modulators (SLM), which are used in a pattern generator for DUV laser mask writing developed by Micronic Laser Systems. They consist of micromirror arrays and allow massive parallel writing in UV mask writers. The chip discussed here consists of 2048 × 512 individually addressable mirrors and can be run at a frame rate of 1 to 2 kHz. For this application it is necessary that the SLMs can be operated under DUV light without changing their performance. This paper discusses a failure mechanism of the SLMs when operated in DUV light and countermeasures to eliminate this effect.
We present novel approaches to the fabrication of spatial light modulators based on thin viscoelastic layers. These layers are formed against two chips: the bottom one carries an interdigitated electrode structure and the top one is a sacrificial chip coated with a metal layer or a stack of materials. By etching away the top chip with bulk silicon techniques, a directly coated and planarized elastic layer results with very high optical quality. The surface is deformed in a sinusoidal shape under electrostatic load when alternating potentials are applied on the underlying electrodes. With this effect, solid-state alternatives for Eidophor projectors can be fabricated. The top chip can contain either a 125nm gold layer or a 50nm nitride and 80nm aluminum layer. After curing, the chip is encapsulated in a flexible elastomer based etch holder and placed in a 33wt% KOH solution at 85°C. This etches away the silicon of the top chip and stops on either the nitride or gold. The surface has a 100% optical fill factor over the active region and can scale easily to various resolutions and spectral ranges. Measurements of the surface has shown local initial deformations below 0.10 λ. Experiments done with devices with 50-100μm electrode size and 5μm spacer distance have shown significant far-field scattering under application of 300V potential difference between the electrodes. Further development will include optimizations of the modulation efficiency. Applications can be found in high performance projection displays, optical lithography and optical communication networks.
Elliptical-boundary deformable mirrors have been developed for focus control of an optical beam incident at forty-five degrees with respect to the surface normal. The mirrors are silicon nitride membranes 1.4×1 mm in size, designed to accommodate a 1 mm diameter beam. Two electrostatic actuation zones provide control over spherical aberration. Focal lengths ranging from infinity to 36 mm have been achieved, and the mirror surface figure has been characterized to quantify aberration. Residual aberrations have been observed to be less than λ/5 (peak to peak) measured at λ = 660 nm.