Radiation imagery chemistry: process – composition – or product th – Imaging affecting physical property of radiation sensitive... – Forming nonplanar surface
Reexamination Certificate
2003-01-15
2004-08-31
McPherson, John A. (Department: 1756)
Radiation imagery chemistry: process, composition, or product th
Imaging affecting physical property of radiation sensitive...
Forming nonplanar surface
C430S323000, C430S945000, C430S330000, C428S410000, C428S913000, C501S011000, C501S053000, C216S080000
Reexamination Certificate
active
06783920
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to the field of microfabrication of glass ceramic materials. More particularly, the present invention relates to the manufacture of structure within photoactivable glasses and ceramics using variable laser exposures.
BACKGROUND OF THE INVENTION
Glass is a highly versatile and functional-material that can be designed with specific properties, such as high compressive strength and durability, corrosion resistance and chemical inertness, low thermal and electrical conductivity, negligible porosity and biocompatibility. Consequently, glass has received significant attention for potential applications in a variety of scientific and industrial fields, including aerospace engineering, photonics, optoelectronics, biology and biochemistry, and microelectromechanical systems (MEMS) design. These applications require integrated and functional structures that range in scale from the microscale of less than 100 &mgr;m, to the mesoscale of 100 &mgr;m to 10 mm, and to the macroscale of greater than 10 mm. For example, integrated photonic and biofluidic devices have feature dimensions varying from submicrons to the hundreds of microns. In the intermediate mesoscale domain, arrays of glass microfluidic channels and precision volume containers, such as microtiter plates, are often utilized to facilitate the automation of chemical and biochemical analysis.
The ability to fabricate intricate microstructures in glass materials is essential to meet the increasing demands of future microtechnology development. These advanced design requirements will involve the fabrication of high and low aspect ratio structures with variegated heights on a common substrate. Microscale and mesoscale structure fabrication in glass materials have typically been achieved using optical lithographic patterning and chemical etching, mechanical micromilling and direct thermal ablation with ultrafast lasers. Although lithography and mechanical cutting methods permit the formation of two dimensional patterns, these approaches do not allow the fabrication of true three dimensional structures with high and low aspect ratios.
In principle, conventional material removal methods can be applied to the cofabrication of two-dimensional microstructures with different aspect ratios on a shared substrate. However, the processing steps are numerous and costly for these approaches to be practical and economical. For microelectromechanical systems applications requiring concurrent and proximal high aspect ratio and low aspect ratio features, the fabrication solution generally involves the use of an elaborate masking sequence. The sequential masking steps are intended to protect physically the low aspect ratio structures during the long duration etching time needed to form the high aspect ratio structures. An alternative approach concerns the introduction of dopants and impurities into the substrate that selectively alter the local etching rate and permit the concurrent formation of high and low aspect ratio features. Another alternative is to generate the high and low aspect ratio structures on separate substrates. Following preparation, the substrates can be joined or packaged together to merge the variegated aspect ratio structures. Unfortunately, these alternative solutions are time consuming, difficult and expensive to implement. Laser ablation or micromilling methods can fabricate continuously variable aspect ratio structures in glass or ceramic materials. However, the fabricated structures suffer from residual thermal induced effects, such as structural stress, cracks and optical defects.
A prior method demonstrated in the related application teaches fabricating embedded structures in glass and ceramic materials that are photostructurable glass ceramic material, commonly called pyrocerams or photocerams. One photostructurable material has been commercially available under the tradename Foturan™. This photostructurable material is a preferred photoceramic material, but other glass ceramic materials are also suitable for photostructuring of embedded structures using laser exposure. A predetermined laser energy dose and wavelength are applied to the Foturan photoceramic material. The laser is a pulsed ultraviolet laser. The laser provides a pulsed laser beam using a lens defining a beam waist at a focal depth that is moved during exposure relative to the exposed material. The choice of the UV wavelength is critical. The wavelength is preferably at the very edge of the spectral region where photoceram transitions from being strongly absorbing to weakly absorbing. In the weakly absorbing spectral region, the wavelength of the laser is outside the strong absorption band of the photoceramic material. Hence, the absorption of radiation is very small so that the process is photon inefficient but enables the controlled focused exposure of any volume including an embedded focal volume defining an embedded three-dimensional structure. The focused beam illuminates the material with the intensity peaking in the focal volume. The number of pulses and pulse fluence controls the amount of the exposure dose so that the exposure outside the focal depth, that is, outside the depth of the optical field in a collateral volume, is insufficient for conversion of the material to the soluble crystalline phase. Within the depth of focus region, that is, the focal volume, the combined effect of the focused laser beam fluence in Joules/cm
2
, and the dose in terms of the number of laser pulses is beyond a critical dose that is required for conversion to the crystalline phase. The focused pulsed ultraviolet laser and a computer-controlled sample positioning stage and shutter provides motion controls for moving the material relative to the focus laser beam during selective exposure of the focal volume. True three-dimensional patterns can be formed by moving the sample using an XYZ positioning stage in XYZ directions. Motion and shutter operations are both computer controlled. For example, laterally moving the workpiece in the XY plane can create an embedded tunnel within the material, while moving vertically for adding via openings to the end sections of a tunnel so as to undercut the structure above leaving an anchored but suspended structure. The result is an embedded microstructure or an exposed pixel defined in the focal volume.
The method can be used to create one or more stacked embedded structures. There is no critical exposure above and below depth of focus. The material is only critically exposed in the focal volume region where the administered laser dose is above a critical dose. Repeated exposures at different depth of focus enable the formation of stacked embedded structures. Precise structural definition is realized for creating one or more embedded structures because collateral volume regions above and below the focal volume at the depth of focus do not accumulate the critical exposure dose. The critical dose is based on sufficient per pulse fluence that is the energy per unit area in a single pulse and the number of pulses. For a given laser pulse width and wavelength, the per pulse fluence is proportional to irradiance. The exposure process is a nonlinear optical process, that is, the critical dose is a nonlinear function of the per pulse laser fluence. That is, the critical dose required for conversion to the crystalline phase is both a function of the per pulse fluence and the number of applied pulses. The dose dependence is nonlinear in per pulse fluence and is cumulative. The most intense portion of the focused pulsed laser light is sufficient to deliver the critical dose over a predetermined number of pulses. Exploiting the nonlinear aspect of the exposure process allows for creation of stacked structures at any desired focal depth. The wavelength is in the weak absorption region so that a critical dose is not delivered to the collateral volume where the pulsed laser light is not focused and not as intense as in laser focal volume where the laser light provides a critical dose. Hence, the pulsed
Helvajian Henry
Livingston Frank E.
Chacko-Davis Daborah
McPherson John A.
Reid Derrick Michael
The Aerospace Corporation
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