Microlens arrays are used in optical switches for telecommunications, in charge-coupled-device cameras to increase the amount of light reaching the pixels, and for other uses that potentially require them to be made in medium to large volumes. The ideal fabrication technique for a microlens array would result in accurate and tailorable lens shapes using relatively simple equipment. Real-life techniques have disadvantages: embossing can result in deformation-causing shrinkage, while ink-jet and photoresist-reflow techniques limit the choice of lens-element surface profiles. Replication may solve these problems, as demonstrated by an approach developed by scientists at Cornell University (Ithaca, NY), Clariant (Somerville, NJ), Lawrence Berkeley Laboratory (Berkeley, CA), and Lucent Technologies' Bell Labs (Murray Hill, NJ).
A stamp is first made that is a negative replica of a precisely made master; this is done by coating the master with an antistick film, placing a glass substrate 1 mm away from the master, filling the gap with a monomer that is heated to change it to an elastomer, and pulling the master away. To create a replicated array, the cavities in the stamp are filled with ultraviolet (UV)-curable epoxy and a lens-backing plate placed against the stamp. After UV curing, the stamp is removed, leaving a microlens array.
Important to the makeup of the epoxy is its 19%-weight fill of silica nanoparticles of 9 to 11 nm in diameter. Madanagopal Kunnavakkam, a senior research associate at Cornell, explains the advantages of adding the nanoparticles. "First, they reduce the organic material content of the lens," he explains. "This reduces moisture absorption and extends the life of the lens array. Moisture absorption can lead to swelling in organic materials that could lead to change in focal lengths and increased losses due to absorption of light by water, especially in the 1.3- to 1.5-µm communications band. Eventually the material would turn hazy if too much water is absorbed. Reducing the organic contents reduces all these effects." A second advantage, he says, is that the cured lens material exhibits improved mechanical properties, such as increased hardness and resistance to scratching.
"Third, the coefficient of thermal expansion of the cured epoxy is closer to that of the substrate and this reduces stresses induced by variations in temperature due to differential expansion," says Kunnavakkam. "We find that lenses made with pure epoxies often crack under thermal cycling, whereas the loaded epoxy lenses do not. This also has an added advantage of minimizing the focal-length shifts with temperature."
Profiles of the lenses taken with a scanning optical profilometer show that the central portions of the replicated lenses accurately duplicate the originals and have positional placement errors of less than 2 µm up to temperatures of 65°C. The researchers envision fabrication of gratings, waveguides, and other micro-optical components using this technique.
- M. V. Kunnavakkam et al., Appl. Phys. Lett. (Feb. 24, 2003).