Atomic-force microscope makes silicon microlenses

The most commonly used method for microlens fabrication is resist reflow. In this approach, islands or columns of cylindrical resist are created by photo­lithography and are heated above the melting temperature, allowing the ­island to form a truncated-sphere or lens shape.

May 1st, 2005
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The most commonly used method for microlens fabrication is resist reflow. In this approach, islands or columns of cylindrical resist are created by photo­lithography and are heated above the melting temperature, allowing the ­island to form a truncated-sphere or lens shape. Refractive microlenses can then be fabricated by solidification of the reflowed resist while it is cooling down, along with optional anisotropic ­etching using the resist microlens as the etch mask. Unfortunately, the achievable shapes and focal lengths of refractive microlenses are limited by the resist-island properties and it is not possible to use a single photomask to fabricate unusually shaped surfaces required for diffractive or hybrid microlenses.

To overcome the drawbacks of the reflow technique, researchers in the Department of Physics and the Institute of Microelectromechanical Systems at National Tsing-Hua University (Hsinchu, Taiwan) have developed a new microlens-fabrication technique ­using ­atomic-force-microscope (AFM) local-anodic oxidation under ambient conditions.1

This AFM gray-scale oxidation technique allows refractive, diffractive, and hybrid microlens profiles to be created by means of programming the oxidation voltage between the AFM tip (cathode) and the substrate (anode) while scanning the tip across the surface, thus creating gray-scale (in terms of oxide height, which has an unlimited number of gray levels) surface oxide patterns for subsequent pattern transfer. Through the large difference between etch rates of AFM-oxidized and unoxidized surface regions in anisotropic dry-etching processes-such as reactive ion etching (RIE) or inductively coupled plasma etching-the scientists can create arbitrarily designed microlens structures. Compared with other direct-writing methods that use a focused laser beam, the AFM technique can be used to fabricate microlenses with a pixel size and pitch on the order of 10 nm-a 2× improvement over other methods.

Fabricating the microlenses

To demonstrate the technique, the scientists used a commercially available AFM system operated in contact mode in air at room temperature and about 60% humidity. The AFM probes had a force constant of 2.8 N/m and a resonance frequency of 70 kHz. Silicon (Si) was chosen as the microlens material because of its transparency in the IR, including the important 1.3- to 1.55-µm wavelength range for optical-fiber communications. Silicon substrates were cut from a silicon wafer covered by a thin native silicon dioxide (SiO2) layer.

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    An atomic-force-microscope (AFM) gray-scale ­patterning technique allows microlens arrays to be fabricated with superior focal lengths and low ­surface-roughness values of approximately 1 nm (left, top and bottom; scanning-electron micrograph). The process also can be used to fabricate unusual or ­arbitrarily shaped microlenses (top right; scanning-electron ­micrograph), difficult if not impossible to create using standard resist-reflow techniques. An infrared optical image of a 1 × 3 micro­lens array (bottom right) confirms the focusing function of AFM-­patterned silicon microlenses.

The process for producing a gray-scale oxide relief on the surface of the ­silicon substrate was controlled by a separate probe oxidation controller. When the AFM tip scanned to the first pixels of the scanning lines on the target area, the AFM controller synchronously sent a voltage pulse to trigger the probe oxidation controller. In response, the probe oxidation controller output a train of oxidation voltage pulses to the silicon substrate. A preprogrammed output-voltage matrix (512 × 512) was saved in the controller for the surface oxide patterning. Scanning speed of the AFM tip was typically 4 µm/s, corresponding to a patterning time of about 20 minutes for a 10 × 10-µm scanning area. The dry-etching process was performed with a RIE instrument; the experiment showed that the etched Si-microlens profile corresponded well to the oxidation-voltage profile with a linear relationship.

The researchers were able to fabricate microlenses with diameters of 2, 3, and 4 µm. Assuming a spherical geometry, the focal lengths of the three lenses were calculated to be 4, 9, and 16 µm, respectively. Considering that ­resist-­reflow techniques are limited to focal lengths of a few tens of microns, and that an AFM produces features with surface roughness below a few nanometers (compared to a few tens of nanometers for alternate techniques), the AFM technique represents a significant improvement in microlens fabrication.

In addition to the focal length and smoothness advantage, arbitrarily shaped structures can be fabricated by design. By using an oxidation-voltage function for a special oxide pattern, a continuous gray-scale structure can be built (see figure).

"The greatest strength of the AFM gray-scale oxidation technique is its ability to create high-resolution, arbitrarily designed microrelief structures, which can be used as masters for replication of micro-optical elements,¿ says Shangjr Gwo, professor of physics at the National Tsing-Hua University. " The throughput of this technique can be further improved by using parallel-probe arrays,¿
Gail Overton

1. C.-F. Chen et al., Optics Lett. 30(6) 652 (March 15, 2005).

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