Chuankai Qiu

Refractive micro lens array made of dichromate gelatin with gray-tone photolithography

A novel method is proposed to fabricate refractive micro lens array using dichromate gelatin (DCG) and an enzyme. DCG has optical properties similar to glass but is sensitive to UV light. The enzyme solution is used to develop the optically exposed DCG instead of a conventional water-isopropanol developer, which greatly enhances the surface relief depth. The continuous surface relief structure is realized by a coded grey-tone mask. A computer program has been developed to code an arbitrary 3D surface relief shape into a grey-tone mask design. The DCG properties and the lithography process are described. The grey-tone coding principle is presented. Unlike the current processes where patterned photoresist relief structure has to be transferred to a substrate material, the fabricated DCG micro lens array can be directly used as an optical element. The new process opens a new route of micro lens fabrication, which is simple and low cost

Profile control technology for high-performance microlens array

A profile formation and control approach has been developed for manufacturing micro-optical elements with continuous profile and deep relief depth. Based on Dills exposure model, an effective expression for determining the exposure dose function is established by using a supposition of equivalent exposure threshold inside a resist layer. An analytical simplified formula is further deduced by taking absorbance as constant B, and the approximate condition is discussed. For evaluating the simplified formula, the profile error was calculated and analyzed by simulation. With the exposure dose function, the binary mask for manipulating the light distribution by means of a moving-mask lithographic method can be designed. Experimental results are given and show the comparative performance to the required profile and relief depth. A series of refractive microlens arrays with aspherical profiles, a wide range of numerical apertures (0.005 to 0.6), and high fill factors were accomplished in the lab and may be applied to many systems

Imaging simulation of maskless lithography using a DMD

A maskless lithography imaging simulation using digital micromirror device (DMD) was investigated. The DMD acts as a reflective spatial light modulator. The micro-mirrors on DMD can be instructed by the computer to tilt them ±12° off their normal position which produces a mask pattern, and then the mask pattern can be carried onto the surface of wafer by the imaging system. Because the imaging of the maskless lithography is a complex process, it is necessary to simulate and analyze its practical process. In this paper, we present a partial coherent imaging model of maskless lithography considering the practical projection characterization of DMD. With the model, it is convenient to simulate the lithography of arbitrarily shaped microstructure using DMD. Through calculation, the spatial image in maskless lithography process based on gray scale photolithography with DMD real-time masks was obtained.

Method and experimental study for diffractive/refractive micro-optical elements with continuous profile

A new method for microlens profile design was developed based on the analysis to the main parameters of microlens array, including micro profile formation, the numerical aperture ( NA ), the maximum sag depth for the refractive lens and the minimum zones width for the diffractive lens. With the relationships among the parameters, the microlens array in different profile can be determined effectively. The moving mask method[1] is used to realize the required profiles, an unique photolithography system have been built for implementing the mask moving exposure in both X and Y directions for the creation of microlens array. By modifying the binary moving mask, optimizing the photosensitive materials and the processing technique, the microlens profile error can be controlled in the range of 0.4µm~3µm depending on effective reliefdepth of the microlens. In our method, both diffractive and refractive microlens array with larger NA and higher fill factor can be fabricated for satisfying a plenty of purposes.

Method study for microlens array with continuous aspherical profile

In order to form the proper aspherical microlens array profile with larger NA on photosensitive materials, a method is developed based on the characteristics of resist and processing parameters during development, for designing the exposure distribution, an experience formula has been proposed in the paper. Using the moving mask method, the exposure energy distribution function related to the photolithographic mask function can be determined by the experience formula. The profile control procedure is formed especially for the deeper relief profiles, after the binary mask data are slightly modified, the micro-structure with aspherical lens profile can be fabricated on the selected thicker resist, the micro relief profile error can be controlled in a certain range. The micro-profile is farther transferred to fused silica by ICP etching system. By our method, the fast microlens array elements with good fidelity and reasonable roughness have been fabricated and applied to the laser diode collimating system.

Precompensation approach for improving the quality of laser direct writing patterns by a modified proximity function

By applying energy conservation to the effect of a laser beam on a resist, a modified proximity function is proposed. The measured data on energy absorption in a photoresist are fitted well with the modified function. By using the new model, an effective precompensation method can be used to correct proximity effects in laser direct writing. Experimental results have been obtained on a laser direct writing machine with line width of 0.6 ?m.

Microlens array and application systems

Performances of diffractive and refractive microlens array have been studied and compared. Besides the diffractive microlens array which can be fabricated by means of the microfabrication technologies, method for manufacturing continuous profile refractive microlens array with larger sag depth has been developed for solving the unique problems like diode laser alignment. Both diffractive and refractive microlens array with different numerical aperture and other parameters are fabricated according to the application requirements, experimental results are given.

Continuous micro-optics fabrication using halftone masks and proximity printing

It is reported in this paper a method to fabricate three dimensional continuous micro-optical structures using half tone masks and proximity printing. The frequency modulation coded half tone masks are employed to obtain the predetermined exposure distribution. Several components like Cylinder microlens arrays, phase correctors are realized in photoresists.

Micro-optical structure for wave aberration compensation of optical systems

A wave aberration compensation method for optical system and human eyes has been studied. The wave distortion of the human eye can be measured using a Shack-Hartmann sensor and described in an eight-order Zernike polynomial. By picking up the higher order’s terms of the wave aberration from the measured polynomial, characteristics of the higher order wave aberration were analyzed and concluded. The conjugate phase plates were designed based on the distribution of the single and the overall wave aberration of the eye. The thermal-filtered halftone mask technique is developed to manufacture such an abnormity micro-optical structure. The compensators for the single or the overall wave aberration were fabricated respectively. The relief profiles were evaluated using both three-dimensional profiler and interferometer. A primary experiment for compensating the higher-order wave aberration of an artificial eye was demonstrated.

Optical proximity correction for submicron lithography by laser direct writing

Generally, a laser direct writing lithography system can only produce feature sizes larger than its beam spot size. When the feature size is comparable to its spot size, corner rounding and line shortening appears. This is caused by optical proximity effect. The effect is mainly due to light intensity spread in a laser beam which causes the spread of photon energy in resist layer. A new pre-compensation method has been developed to correct the optical proximity effect. The method has been implemented in the ISI-2802 laser direct write system. Feature size down to 0.6 micrometers has been produced with the system which normally can only produce 1 micrometers lithography without proximity correction.