Margaret B. Stern

Color separation by use of binary optics

We describe the preliminary design, fabrication, and demonstration of an array of micro-optics that is used to separate colors locally on the focal plane. The 64 x 64 array combines 100 microm x 100 microm, F/2 refractive microlenses with a 17-microm period grating. The microlenses concentrate the incoming radiation, while the grating disperses the radiation according to wavelength. The element, with a minimum feature size of 1 microm and total depth of 8 microm, is fabricated on a silicon wafer (for use in the 8-12-microm band) by means of a nonstandard binary-optics process.

Nanochannel fabrication for chemical sensors

In a novel chemical sensor, the chemical charge coupled device (CCD), electrostatic fields in nanocapillary channels smaller than a Debye length will be used to separate and concentrate ions in solution with a predicted detection limit of <1× 10−13 M. Conventional integrated circuit techniques are used to deposit thin dielectric and amorphous-Si films on a Si substrate and to lithographically define channel and reservoir structures. Hollow Si3N4 nanochannels with heights between 20 and 100 nm, widths between 0.5 and 20 μm, and lengths up to 5 mm have been fabricated by wet chemical etching of a sacrificial amorphous-Si layer in tetramethylammonium hydroxide. Initial modeling of a three-phase chemical CCD predicts the ability to select and concentrate ionic constituents by many orders of magnitude, according to their diffusion coefficients.

Preshaping photoresist for refractive microlens fabrication

Refractive microlenses are fabricated by the preshaped photoresist refractive optics by melting (P2ROM) process. A polar coordinate HeCd laser beam lithography system (laser writer) is used to directly expose a continuous spheroidal profile into photoresist to fabricate 200-μm-diam microlenses with speeds between f/1.5 and f/5. Interferometric surlace measurements, used to determine the optical quality of the preshaped microlenses, indicate reduced spherical aberration in slow lenses fabricated by the P2ROM method.

Dry etching for coherent refractive microlens arrays

Coherent arrays of refractive micro-optics are fabricated in the surface of silicon using a combination of lithographic and reactive-ion etching (RIE) techniques. The aspheric profile can be approximated in a stepwise manner by iterative steps of photolithography and RIE (binary optics technology), by direct etching of a preshaped polymer microlens etch mask into the substrate, or by analog etching of a lens profile directly into the substrate through a pinhole mask.

Line-edge roughness in sub-0.18-μm resist patterns

We have characterized line-edge roughness in single-layer, top-surface imaging, bilayer and trilayer resist schemes. The results indicate that in dry developed resists there is inherent line-edge roughness which results from the etch mask, resist (planarizing layer) erosion, and their dependence on plasma etch conditions. In top surface imaging the abruptness of the etch mask, i.e., the silylation contrast, and the silicon content in the silylated areas are the most significant contributors to line-edge roughness. Nevertheless, even in the case of a trilayer, where the SiO2 layer represents the near ideal mask, there is still resist sidewall roughness of the planarizing layer observed which is plasma induced and polymer dependent. The mechanism and magnitude of line-edge roughness are different for different resist schemes, and require specific optimization. Plasma etching of silicon, like O2 dry development, contributes to the final line-edge roughness of patterned features.

Laser-fabricated glass microlens arrays.

We report the fabrication of large, laser-formed refractive microlens arrays on doped borosilicate glass. Tuning the laser heat-source properties was found to give precise control over the melt process and the resulting lenslet geometries. Measurements of focal length, index, and surface shapes show excellent uniformity across these arrays.

Binary optics: A VLSI-based microoptics technology

Binary optics technology applies VLSI and ULSI processing techniques to the field of microoptics to enable the fabrication of unique wavefront engineering devices from x-ray to IR wavelengths. The nested character of these stepped diffractive surface relief structures, coupled with submicron linewidths and precise etch depths, puts stringent demands on process tolerances. We summarize our efforts to produce high-optical-quality diffractive and refractive microoptics with this technology. We also describe novel techniques developed to fabricate the deep structures needed to form high-aspect-ratio gratings and analog refractive lenslet arrays, including thick resist processing and deep anisotropic etching.

Fabricating binary optics: Process variables critical to optical efficiency

While conventional photolithographic and reactive ion etching techniques are employed in the manufacture of binary optical elements, the nested character of these diffractive surface relief structures, coupled with submicron linewidths and additive etch depths, puts stringent demands on process tolerances. To quantitatively assess the effects of process variables, we have utilized a mask set that incorporates ten different lenslet designs as single elements and as 10×10 arrays. All lenslets are 200 μm×200 μm with focal lengths ranging from 170 μm to 14 mm at 632.8 nm wavelength. Overlay registration accuracy is evaluated by optical microscopy of vernier‐style alignment marks, etch depths are determined by stylus profilometry, and linewidths are measured by optical and scanning electron microscopy. The optical efficiency is evaluated as a function of the focal length and compared to theoretical predictions. The eight‐phase‐level F/4 microlenses, measured at 96% of the expected value, exhibit the highest op...

Pattern transfer for diffractive and refractive microoptics

Pattern transfer technologies developed for very large and ultra-large scale integrated (VLSI and ULSI) circuit processing can be adapted to fabricate high fidelity microstructures for planar diffractive and refractive optical elements (DOEs and ROEs). Lithographic, dry-etching, and material deposition techniques used to pattern continuous and stepped microoptical surface relief structures in dielectric, metallic, and semiconductor materials are reviewed. Examples are given of multilevel DOEs, analog refractive lenslet arrays, and subwavelength structures.

Effect of refractive microlens array fabrication parameters on optical quality

Refractive microlens arrays with lens speed F/1 to F/5 are fabricated by the PROM (photoresist refractive optics by melting) technique. Optimal PROM fabrication parameters are determined from interferometric measurements of the optical quality. Elements in a hexagonal PROM microlens array, composed of 10,000 200 micrometers diameter F/2 lenses on 205 micrometers centers, exhibit less than one-tenth wave deviation from sphere. Four planes of these F/2 microlens arrays, each containing 10,000 lenslets have been assembled into an afocal imaging system.