MEMS Microlens Has Adjustable Focal Length

Breck Hitz

MEMS photonic devices have proved useful in many applications, including telecom, displays and imaging. But developing an efficient and reliable microlens, one whose focal length can be easily adjusted, is a goal that has eluded engineers. Recently, scientists at National Tsing Hua University in Hsinchu, Taiwan, proposed and demonstrated a tiny lens that may come closer to achieving that end than have previous designs.

Figure 1. A false-color image shows the temperature variation across a microlens whose focal length is changed by heating the silicon ring that surrounds it.

There are several types of variable-focus microlenses, each with drawbacks. Liquid lenses can be adjusted by applying an electric field, but they are subject to evaporation. The problem can be avoided by enclosing the liquid in a thin, flexible skin, but the lens is then subject to distortion from the weight of the liquid. Another technique involves a liquid crystal lens whose focal length can be adjusted by tuning the crystal’s refractive index with an applied electric field. The problem here is caused by fringing fields, which often introduce unacceptable aberrations.

Figure 2. A top view (left) and side view (right) show the polymer lens surrounded by a silicon conducting ring. When a current is applied through the heaters, thermal expansion causes the lens, whose edges are constrained by the silicon ring, to bulge in the middle. Images reprinted with permission of IEEE Photonics Technology Letters.

The new method takes a different tack. It surrounds a tiny polymer lens with a single-crystal silicon ring, and heats the whole microscopic assembly with a pair of tiny heaters (Figures 1 and 2). As the materials expand, the edge of the polymer lens is constrained by the ring, and the curvature of its surface changes. Experimentally, the scientists observed focal length changes between 1088 and 1852 μm as they applied heat.

Figure 3. The profile of the polymer lens changes as different currents are applied to the heaters.

They fabricated the lens in Figure 1 by starting with a silicon-on-glass wafer and applying a photoresist. The components were etched from the silicon layer, and the photoresist was removed. The researchers deposited the polymer into the silicon conduction ring. Surface tension across the top of the polymer formed a near-perfect lens. Finally, they cured the polymer at 150 °C for 15 minutes.

To characterize the microlens, they applied three tests. They interferometrically measured the deformation of the lens surface as a function of the current applied to the two heaters (Figure 3) and measured the temperature distribution across the whole device with a thermal infrared microscope as well as the focal length with a CCD camera (Figure 4).

Figure 4. By positioning the CCD camera at the focal plane, the scientists measured a focal length that is adjustable between 1018 and 1852 μm for the tunable microlens.

They saw that the focal length varied linearly with applied heat between 1018 and 1852 μm.

IEEE Photonics Technology Letters, Nov. 1, 2006, pp. 2191-2193.