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Illumination & Displays

Building better head-mounted displays using microfabrication

It may be possible to use microfabrication techniques to build smaller and lighter head-mounted displays than are available today, while retaining the same resolution and field of view.
12 May 2006, SPIE Newsroom. DOI: 10.1117/2.1200604.0217

Head-mounted displays (HMD) are used in virtual and augmented reality,1,2 medical imaging,3 and head-up displays.4 Most use cathode-ray tubes or liquid-crystal displays to generate the image, and the perceived image quality is a function of the display, the working environment, and the viewer's eyes.

Now miniature scanner technologies are being used in HMDs. For example, the virtual retinal display developed at the University of Washington Human Interface Technology Lab in the early 1990s was originally driven by acousto-optic scanners. A similar system was later produced by Microvision using scanners built with micro-electromechanical systems (MEMS) technologies. The display uses mirrors that can be deflected in two axes to raster-scan a laser or LED light source, forming a virtual image at the viewer's retina.5 This approach has many size limitations. For instance, using mirrors of less than a few millimeters in diameter is impractical, because mirror scanners and grating deflectors must be larger than the diameter of the source beam to avoid clipping it or adding diffraction. There are also issues with reducing the diameter of CRTs or LCDs, since this reduces the number of pixels they can support, hence reducing its resolution and/or field of view (FOV). Despite these issues, shrinking the display system is still desirable: it enables the production of more-comfortable HMDs, and their wider use.

We have developed a 2D micro-image display device using MEMS technology that may overcome these size-reduction limits while matching the resolution and FOV of mirror-based displays. This optical scanner includes a microfabricated, polymer-based cantilever waveguide that is electromechanically deflected by a 2D piezoelectric actuator6,7 (see Figure 1). The direction in which the light beam is emitted from the tip of the cantilever waveguide is controlled by displacing it in two orthogonal directions. The waveforms for the XY actuators and to modulate the LED light are created using a field programmable gate array (FPGA).

We built the cantilever waveguide using an SU-8 epoxy-based negative-tone photoresist (MicroChem, MA). The resist was chosen for its ability to produce good vertical-side-wall profiles and to control dimensions over the entire structural height.7 We started with a silicon wafer with one polished side, on which we deposited a thin layer of silicon dioxide to buffer the coupling of the light beam into the waveguide. The cantilever waveguide was then patterned on this oxide layer. A second SU-8 film was put on the wafer's back to define a window beneath the cantilever waveguide. The final step was to release the structure using a reactive-ion etching process. SEM images of the cantilever, tapered section and fiber input groove show fairly straight sidewalls and a relatively smooth surface (Figure 1).

Figure 1. a) System view of polymeric waveguide for a 2D display system. b) SEM micrograph of the microfabricated waveguide structure, with a 75μm-wide and 100μm-high cantilever beam.

The cantilever waveguide is driven by two thin-strip bimorph piezoelectric transducers acting vertically and horizontally. The FPGA was configured to generate a horizontal scanning frequency of 22Hz with a vertical scanning frequency of 4466Hz, allowing for 406 vertical lines at 44 frames per second. Our prototype was built using a fairly-coarse MEMS process, limiting its resolution to 20 × 10 pixels. With a finer MEMS process, we hope to achieve resolutions of 320 × 200 pixels or more.

The image to be displayed is sent as a bitmap to the Matlab image-manipulation program, which converts it into a code that can be used to program the FPGA. We have successfully demonstrated simple patterns using this system, as shown in Figure 2: a mirror-image of an L shape; and the author's last name in Chinese. These were captured using a digital microscope at 60× magnification.

Figure 2. a) A mirror-imaged L shape pattern and b) Wang in Chinese are displayed by the optical scanner.

We have demonstrated that we can use a microfabricated cantilever-waveguide scanner to build a microdisplay system. In future, we will reduce the tip cross-section to overcome size limitations while maintaining resolution and FOV. We will also work toward integrating components, including actuators and light sources, with a controller to create a system-on-chip for display devices. This will make the display system more cost effective.

Wei-Chih Wang
Microtechnology Lab/Department of Mechanical Engineering, University of Washington
Seattle, WA
Electronic Engineering Department, Southern Taiwan University of Technology
Tainan, Taiwan
Dr. Wang's principal research is in the area of optical MEMS, fiber-optic sensors, and biomedical instrumentation. His recent work has been devoted to developing novel polymer micro-sensors and actuators for biomedical application. In addition, Dr. Wang has served as a program committee and session chair for the SPIE Nondestructive Evaluation for Health Monitoring and Diagnostics conference since 2003.