Heidi Hockel

Additive lithography for fabrication of diffractive optics

An innovative fabrication technique is introduced that is based on multiple-exposure techniques for micro-optics fabrication. This method utilizes various exposure times and combinations of binary and analog photo masks to sculpture complex photoresist profiles. It also demonstrates the fabrication of analog structures from the multilevel structures thus formed by using resist reflow.

Analog micro-optics fabrication by use of a two-dimensional binary phase-grating mask.

The utilization of phase-mask technology for the fabrication of an analog micro-optics profile with a thick photoresist was investigated. A two-dimensional phase-grating mask with pi phase depth can produce a desired analog variation of exposure intensity, which allows one to vary the thickness of an analog photoresist after its exposure by a photolithographic stepper and development of the photoresist. A two-dimensional phase-grating mask of square pixels was simulated, designed, and fabricated. The fabrication of analog micro-optics in a thick SPR-220 photoresist by use of this phase mask was also demonstrated.

Development of a two-dimensional phase-grating mask for fabrication of an analog-resist profile

Fabrication of a thick analog profile with photoresist is a difficult task in photolithography. We demonstrate that a binary phase-grating photomask with an appropriate period and duty cycles is capable of manipulating the exposure illumination in an analog fashion and can be used for fabrication of the desired analog micro-optics profiles on the surface of a thick photoresist. By choosing the proper period and variation of duty cycle of the phase-grating mask, one can create the desired analog intensity of exposure illumination for an optical stepper. This allows the formation of a wide range of analog micro-optics profiles with an SPR 220-7 photoresist. The numerical convolution of the diffraction efficiency curve and resist exposure characteristics is used to predict the final resist profile and also to design the appropriate duty-cycle distribution for the binary phase grating. As a demonstration of this technology, we fabricated a variety of micro-optical elements, such as a positive lens, ring lens, prism, and vortex of approximately 100-200 microm diameter, by using a phase-grating mask fabricated in a poly(methyl methacrylate) electron-beam resist.

Analog micro-optics fabrication by use of a binary phase grating mask

In this paper, we present a new photo-mask technology capable of forming a continuous relief micro-optic profile on thick photo-resist. This technique eliminates many of the drawbacks of gray-scale and half-tone masking technology. An optical stepper is used to fabricate binary phase gratings of pi phase depth on a transparent quartz reticle. When the phase reticle is used in the stepper an analog intensity profile is created on the wafer. The period is constrained allowing for control of the 0th order in the stepper. The duty cycle of the phase gratings can be varied in such a way to provide the proper analog intensity profile for a wide range of micro-optics on the photo-resist. The design, analysis, and fabrication procedures of this technique will be discussed.

Design and fabrication of trihedral corner-cube arrays using analog exposure based on phase masks

Trihedral corner cube arrays are efficient retro-reflectors. They are integral parts in numerous imaging and sensing applications. However, the fabrication of these trihedral arrays can prove to be both difficult and cost prohibitive. Using a phase-only mask, we have fabricated an array of analog reflectors which can then be tiled using a photolithographic stepper. The elements are designed using a fixed period and varying fill factor to create the analog slope of each side wall. The overall depth of the array can be controlled by both the exposure and etching processes to ultimately create the desired effect. After etching, a single coating of metal finishes the process, and the elements can then be diced out and integrated into each specific application. The etched arrays may alternatively be used as a mold to create high volumes of the desired element. The design and fabrication parameters for trihedral corner cube arrays will be discussed in detail. The advantages and limitations will then be discussed.

Additive lithography for refractive micro-optics

An innovative fabrication technique is introduced that is based on multiple exposure techniques for micro-optics fabrication. This approach is compatible with conventional lithography systems used in Integrated Circuit manufacturing and can be applied to thick and thin photoresists. The additive concept is centered on the idea of using multiple exposures to remove the desired amount of resist without resorting to multiple etching steps. This presentation will explain how the additive technique, used with thin and thick resists, will revolutionize our capability to efficiently form refractive lenses and micro-optics for optical beam shaping and transforming. The quality and reproducibility of these elements will also be discussed.

Improved fabrication accuracy of Bragg gratings

Bragg gratings have been used relatively extensively in recent years due to their highly dispersive and wavelength selective nature. Typically used as a reflective structure, the gratings reflect specific wavelengths at specific locations along the structure based on the grating periodicity to spatially shape an incident pulse of light according to its spectral components. Usually the purpose is to either compress or stretch the pulse. Unfortunately, fabrication tolerances severely limit the amount of chirp per unit of waveguide length that can be placed on a Bragg grating. For some applications, a few nanometers of chirp over a meter or more of waveguide would be ideal, yet placement accuracy of individual features is usually far less than is needed for such a task. We propose an alternative fabrication method which would provide a long grating with substantially increased placement accuracy. Instead of fashioning the grating in the typical linear manner, a waveguide is fabricated in a spiral shape. This has been done for delay lines and amplifier structures in the past. However, we propose to incorporate a radial grating underneath it. This provides us an additional degree of freedom, since the period of the grating changes very linearly with its radius, and a waveguide can be accurately positioned on top of it so as to gradually spiral inwards (or outwards) and change radius (and, hence, grating period) very slowly along its length. We present fabrication results, optical comparisons between similar linear and spiral structures, and preliminary theoretical modeling of the structures.

Refractive micro-optics fabrication with a 1-D binary phase grating mask applicable to MOEMS processing

We present a new photomask technology capable of forming a continuous rotationally symmetric microstructure in thick photoresist. This technique eliminates many of the drawbacks of grayscale and half-tone masking technology. A binary phase grating of pi phase depth on a transparent quartz mask plate is fabricated in PMMA resist using an e-beam direct writing technique. When the phase mask is used in the stepper, an analog intensity profile is created on the wafer. The period is constrained, allowing for control of the zero-order in the stepper. The duty cycle of the phase gratings can be varied in such a way to provide the proper analog intensity profile for a wide range of micro-optics on the photoresist. The design, analysis, and fabrication procedures of this technique are discussed. This processing technique can be applied to many MOEMS devices that require refractive elements for optical processing. The method greatly simplifies the device process, reducing the cost and improving the device yield.

Presculpting of photoresists using additive lithography

In this paper we present the fabrication of refractive micro optical elements by additive sculpting of the photoresist using binary amplitude masking techniques. We also present the fabrication of micro optical elements by pre sculpting the photoresist before reflow. This enables the use of fewer masking patterns while allowing us to obtain smooth profiles on the resist. The resist can be pre sculpted into any shape by using a set of binary patterns thus allowing us to fabricate refractive beam shaping elements.

Optical vortex coronagraphs

Starlight transmitted through an optical vortex was observed to form a dark core in the image of the star despite atmospheric turbulence. This affords opportunities to use optical vortices in the hunt for extrasolar planets and zodiacal dust. Observations using a 60-inch f/16 telescope will be reported.