Juha T. Rantala

Toward the development of miniaturized imaging systems for detection of pre-cancer

In this paper, we describe the progress toward the development of miniaturized imaging systems with applications in medical imaging, and specifically, detection of pre-cancer. The focus of the article is a miniature, optical-sectioning, fluorescence microscope. The miniature microscope is constructed from lithographically printed optics and assembled using a bulk micro-machined silicon microoptical table. Optical elements have been printed in a negative tone hybrid glass to a maximum depth of 59 /spl mu/m and an rms surface roughness between 10-45 nm, fulfilling the requirements of the miniature microscope. Test optical elements have been assembled using silicon-spring equipped mounting slots. The design of silicon springs is presented in this paper. Optical elements can be assembled within the tolerances of an NA=0.4 miniature microscope objective, confirming the concept of simple, zero-alignment assembly.

Realization of refractive microoptics through grayscale lithographic patterning of photosensitive hybrid glass.

Refractive microlenses with more than 50 microm sag are fabricated using grayscale lithography. Mechanical assembly features are made simultaneously alongside the microlenses to facilitate high precision assembly of miniature optical systems. The microlens elements are formed using lithographic patterning of photosensitive hybrid sol-gel glass requiring no etch transfer to the substrate material. Grayscale lithography enables the straightforward patterning of aspheric lenses and arbitrary surfaces within the material depth. Lessons learned in the design of a grayscale photomask are described. Characterization of the fabricated lens elements is reported including lens shape, surface quality, and image quality of a complete assembled imaging system.

Diffraction gratings in sol-gel films by direct contact printing using a UV-mercury lamp

Abstract We report on the fabrication of diffraction gratings in photosensitive sol–gel thin films by direct contact printing using a UV-mercury lamp. Titanium amplitude masks were used to replicate diffraction gratings into photosensitive sol–gel films by contact printing with an incoherent UV-light source. Gratings with 1-μm period were fabricated in sol–gel films. The diffraction efficiencies of each diffracted order were measured and compared to theoretical values. The demonstrated process of contact printing using a regular UV-light source in the optically compatible sol–gel material has potential for large-scale fabrication of submicrometer gratings at very low cost.

Binary-phase zone-plate arrays based on hybrid solgel glass

An organically modified silane zirconate-based solgel material is used for the fabrication of binary-phase zone-plate arrays. The synthesized hybrid solgel material has a negative tone under UV exposure and can be patterned by a UV-lithography process. The transmittance of the material is nearly 100%, and the refractive index is 1.52. Two different diffractive lens arrays with focal lengths of 5 and 42 cm have been fabricated. The average roughness of the zone surface is less than 20 nm. The diffraction efficiencies of the lens arrays are measured as a function of modulation depth and exposure dose. A diffraction efficiency of 30% is achieved.

Negative tone hybrid sol-gel material for electron-beam lithography

Abstract A novel high-resolution sol-gel material for electron-beam lithography and direct writing of micro-optical elements is reported. A negative tone hybrid sol-gel material was synthesized and used for fabrication of submicrometer grating structures by electron-beam lithography. Doses from 10 to 100 μC/cm 2 were able to crosslink the material and convert it into hybrid sol-gel glass. The sensitivity and the contrast of the material were found to be good with a linewidth of 0.5 μm.

Multilevel structures in sol-gel thin films with a single UV-exposure using a gray-scale mask

Multilevel structures in sol-gel thin films with a single step process are demonstrated. A negative-tone hybrid sol-gel material is UV-exposed through a gray-scale calibration mask. Test strips of different depths and a continuous test profile are demonstrated. A maximum depth difference exceeding 1.5 μm is obtained, which permits the fabrication of multilevel diffractive elements for use in visible light. This method of using sol-gel material with a gray-scale mask has the potential for low-cost fabrication of complex multilevel micro-optical structures.

Optical properties of spin-on deposited low temperature titanium oxide thin films

This letter presents a method to fabricate high quality, high refractive index titanium oxide thin films by applying liquid phase spin-on deposition combined with low temperature annealing. The synthesis of the liquid form titanium oxide material is carried out using a sol-gel synthesis technique. The material can be annealed at low temperature (150 C degrees ) to achieve relatively high refractive index of 1.94 at 632.8 nm wavelength, whereas annealing at 350 C degrees results in index of 2.03 at 632.8 nm. Film depositions are demonstrated on silicon substrates with 0.5% uniformity in thickness. Refractive indices and extinction coefficients are characterized over a broad wavelength range to demonstrate the optical performance of this novel aqueous phase spin-on deposited hybrid titanium oxide material.

Photolithographic processing of hybrid glasses for microoptics

Hybrid glass materials are used in the photolithographic fabrication of optical and optomechanical structures. Two different photolithographic hybrid glass processing methods are described. The first one is referred as photolithographic patterning and the second as direct photolithographic deforming of hybrid glass materials. No etch transfer of the photoimaged structures is needed. In the latter method even the chemical development step can be excluded from the fabrication. Fabrication of lens-arrays, gratings and other binary structures is presented. The synthesized hybrid glass materials feature minimum optical transmission of 97% at wavelengths ranging from 450 nm to 1600 nm and refractive index of, e.g., 1.53 at 632.8 nm. The photolithographic patterning resulted in structure heights in excess of 180 /spl mu/m with rms surface roughness values ranging from 10 to 45 nm. The direct photolithographic deforming resulted in structure heights in excess of 27 /spl mu/m with rms surface roughness values ranging from 1 and 15 nm.

Lithographic patterning of benzoylacetone modified SnO2 and SnO2:Sb thin films

Abstract The synthesis of directly UV-photopatternable pure and antimony-doped organo-tin materials is presented. UV-photopatternability has been achieved by using the synthesized benzoylacetone modified tin and antimony 2-isopropoxyethoxides. Photopatterned pure and antimony-doped organo-tin films are crystallized by thermal annealing in order to obtain conductive SnO2 and Sb:SnO2 thin films. The molar ratio between benzoylacetone and metal alkoxides has to be 2 in order to obtain crack-free, good-quality structures. The effects of UV-irradiation, increasing antimony doping level and benzoylacetone concentration on the electrical properties of the single-layered films are analyzed. The highest obtained conductivity was 20 S/cm. Benzoylacetone concentration and UV-irradiation has only a negligible effect on the film electrical conductivities.

UV light induced surface expansion phenomenon of hybrid glass thin films

Liquid-phase deposition of sol-gel method derived hybrid glass materials is utilized for fabrication of UV-light-sensitive thin films. The hybrid glass material undergoes a surface-relief deformation when exposed to UV light. The observed deformation phenomenon is in the form of a physical expansion of the exposed areas. The UV light induced surface expansion of the hybrid glass film was used to fabricate near-sinusoidal diffraction gratings with periods of 24 microm, 18 microm, 12 microm, and 9 microm. The maximum deformation when the material was patterned as a diffraction grating was 0.685 microm. The hybrid glass material features an index of refraction of 1.52 at 632.8 microm, rms surface roughness of 2.2 +/- 0.8 microm after processing, and extinction coefficients of 1.2 x 10-3 microm-1 and 0.47 x 10-3 mm-1 at wavelengths of 633 nm and 1550 nm, respectively.