Endoscopes are widely used for diagnosis in human internal organs, but conventional endoscopes can only collect surface information. Thus, biopsy is often needed. Newly-developed endoscopes that employ optical coherence and scanning micromirrors can be used to replace risky, painful and time-consuming biopsy, for bio-imaging applications such as early cancer detection in internal organs, and artery imaging for diagnostic and intraoperative procedures. For imaging internal organs, small endoscope probes are desirable. Illustration:1 depicts schematics of a conventional micromirror and an endoscope. An endoscope’s ability to probe obscure parts of the body is limited by its size. We propose micromirrors that allow greater miniaturization of endoscopes compared to previous designs. Other features include improved optical alignment, power consumption and robustness. Scanning is achieved by novel curved bimorph microactuators.

Micromirrors belong to a broader class of devices known as MEMS (Microelectromechanical Systems). MEMS fabrication typically involves micromachining on semiconductor wafer. MEMS have mechanical components that move or deform during device operation. The large scan range of thermal bimorph micromirrors makes them suitable for bio-imaging applications. Similar to bimetal strips, thermal bimorphs bend due to temperature change. Temperature change is typically achieved by an embedded resistive heater. Bimorphs employed for micromirrors are all straight, which wastes significant space given that optical beams are typically circular.

We propose micromirrors actuated by curved bimorphs. Illustration:2 shows the schematic of a mirror actuated by a semicircular bimorph along with scanning electron microscope images of the novel micromirrors. The bimorph consists of aluminum and tungsten, both 0.6 microns-thick. The mirror-plate consists of 20 microns-thick silicon coated with aluminum. The advantages of these novel designs over previous designs actuated by straight bimorphs are as follows:

• The unique bending and twisting deformation of curved bimorphs is utilized to achieve low mirror center-shift during scanning, which improves optical alignment.
• Two-dimensional scanning can be achieved using a single electrical signal (Illustration:3). Previous designs required up to four signal lines, which hampered miniaturization.
• Only a single actuator beam is used, resulting in a compact layout.
• Twisting deformation of curved actuators ensures high resonant frequencies.
• Improved robustness is achieved because no brittle materials are employed.
• Power consumption is low as only a single actuator is used. The device shown in Illustration:2-2(c) can scan 60 degrees at 11 mW power input.
• High thermal diffusivity of aluminum and tungsten ensures fast thermal response.

The novel micromirrors will be employed in miniature endoscopes for early cancer detection, diagnosis of diseased arteries, early detection of tooth decay etc. Potential economic and societal benefits are as follows:

• Diagnosis by optical methods is potentially cheaper, less invasive and less traumatic than traditional biopsy procedures. This invention is a step towards comprehensive point-of-care testing.
• Minimally-invasive procedures improve geriatric care as the elderly may be too frail to undergo conventional procedures.
• Early detection of diseases significantly reduces treatment cost and effort. This translates into billions of dollars in savings. Furthermore, early detection will extend the economically productive part of our life.