Radiation imagery chemistry: process – composition – or product th – Imaging affecting physical property of radiation sensitive... – Forming nonplanar surface
Reexamination Certificate
1999-08-31
2001-06-05
Chea, Thorl (Department: 1752)
Radiation imagery chemistry: process, composition, or product th
Imaging affecting physical property of radiation sensitive...
Forming nonplanar surface
C430S323000, C205S070000, C216S002000, C216S033000, C216S039000, C264S219000, C264S318000
Reexamination Certificate
active
06242163
ABSTRACT:
FIELD OF THE INVENTION
This invention relates generally to micromachining methods. More particularly, it relates to a method for fabricating complex millimeter and sub-millimeter parts out of ceramic and metallic materials using layered molds.
BACKGROUND ART
A variety of methods are available for fabricating complex microscopic and mesoscopic parts out of ceramics and metals, in which feature or part sizes are below 1 millimeter. These sizes are not accessible by conventional machining techniques, including milling and turning. Micromachining techniques can be divided into a few main groups: thin film techniques, micro-stereolithography (&mgr;-SLA), photo-chemical etching, and LIGA.
Thin film techniques include etching, sputtering, and chemical vapor deposition. Material is deposited onto a silicon substrate, which is etched away after the part is formed. Complex multi-material assemblies can be produced with high accuracy. However, only certain materials can be deposited in sufficient quality with these techniques. Feature height is limited to about 10 &mgr;m, above which internal stresses built up during thin-film deposition cause delamination. In addition, microscopic parts formed by this technique cannot be combined with conventionally machined parts to form an assembly. A related technique involves electroplating into patterned silicon substrates. Generally, a thin seed layer is deposited onto the wafer as an electroplating base before electroplating. This technique also has height limitations, and complex structures with overhangs cannot be electroplated.
SLA is widely used for manufacturing macroscopic prototypes out of polymeric materials, and has been adapted for micropart fabrication, as described in T. Nakamoto et al., “Manufacturing of three-dimensional micro-parts by UV laser induced polymerisation,” Journal of Micromechanics and Microengineering, Vol. 7, pp. 89-92 (1997). Parts are made by scanning a UV laser over a photo-curable resin. In regions where the laser penetrates the liquid surface, the resin solidifies and forms the part. SLA achieves complex three-dimensional structures, but accuracy is limited by the spot size of the laser and its penetration depth. It is also difficult to remove the uncured resin from microscopic cavities.
In photo-chemical etching, a focused laser beam removes material by photo-chemical ablation. This process has been used to shape silicon with an argon-ion laser in a chlorine atmosphere, and is described in T. M. Bloomstein and D. J. Ehrlich, “Laser deposition and etching of three-dimensional microstructures,” IEEE, Transducers '91, pp. 507-511. Laser ablation allows the creation of real three-dimensional structures. Its main disadvantage is the slow speed of the process. Since surfaces must be scanned in a serial fashion, photo-chemical etching cannot compete with the highly parallel lithographic processes, especially when a large number of devices are manufactured or if large volumes of material must be removed.
LIGA (Lithographie, Galvanik, Abformung) involves the use of a bright X-ray beam to irradiate polymers such as polymethyl methacrylate (PMMA). The irradiated PMMA degenerates into shorter polymer chains that are soluble in certain solvents. By covering some regions Qf the polymer with an X-ray mask, microstructured polymer parts can be manufactured. LIGA can produce polymer microstructures with very high precision and high aspect ratios. By electroplating into the polymer mictrostructures, metal parts can be formed. However, standard LIGA cannot be used to produce complex, multi-layered parts.
A method for producing complex structures using PMMA sheets patterned by LIGA has been disclosed in U.S. Pat. Nos. 5,378,583 and 5,496,668, issued to Guckel et al. Multiple layers can be stacked to form complex microstructures of up to 1 mm in height. This method has some significant drawbacks, however. Both X-ray masks and access to the synchrotron light source needed for X-ray generation are extremely expensive. In fact, while the LIGA method is, in theory, capable of highly parallel manufacturing of numerous identical or different parts, incorporating an X-ray source into mass production is unfeasible. Furthermore, part material that can be cast or plated into polymer microstructures is quite limited. The mold must be able to withstand the filling process without heat- or pressure-induced deformation, and the part material must be unaffected by the method (usually chemical) used to remove the polymer. The thin, flexible polymer is also difficult to handle and align, with both the X-ray mask and subsequent layers. Polymer layers also cannot be combined with macroscopic layers to form macroscopic parts incorporating microscopic features.
There is still a need for a process for accurately fabricating complex, three-dimensional microstructures that can be used for mass production.
OBJECTS AND ADVANTAGES
Accordingly, it is a primary object of the present invention to provide a highly accurate method for fabricating complex, three-dimensional microscopic and mesoscopic parts. The parts can include overhangs and curved surfaces.
It is a further object of the invention to provide a method for fabricating parts up to 1 mm high.
It is an additional object of the invention to provide a method in which the finished part can easily be removed from its mold, without retaining any unwanted material.
It is another object of the present invention to provide a method that can produce large quantities of parts in parallel, in an economically viable fashion.
It is an additional object to provide a method for fabricating parts from a wide range of ceramic and metallic materials.
It is a further object to provide a method that uses a mold that is easy to form, handle, and align, and is made of rigid, strong material.
Finally, it is an object of the invention to provide a technique for fabricating macroscopic parts that that have regions or features of microscopic or mesoscopic dimensions.
SUMMARY
These objects and advantages are attained by Micro-Mold Shape Deposition Manufacturing, a method for shaping microscopic parts and assemblies out of ceramic and metallic materials using layered silicon molds. The molds are made using standard silicon-processing technologies, and then filled with the desired material. The mold is removed to obtain the final part.
In one embodiment, a method of fabricating a precursor to a part, “surface patterns are etched in a plurality of silicon wafer layers, ” at least one of which contains a through-etched region. The wafers are stacked and bonded together to create a mold, which is then filled with a gelcasting material, which may be a ceramic, metallic, or other gelcasting slurry. The gelcasting material is then solidified to produce the precursor. The mold may also contain patterned wax layers, which have pattern features wider than 1 mm, so that the resulting precursor has both microscopic and macroscopic regions. The method may also include a first additional step of removing the precursor from the mold, preferably by chemically etching the silicon mold, and a second additional step of sintering the precursor to form the part.
This embodiment also includes the use of a mold containing only one layer. The layer is made by creating a surface pattern in a mold material; the surface pattern includes a feature of width between 1 &mgr;m and 1 mm. Preferably, the mold material is a silicon wafer, and the pattern is created by wet or dry etching; features may be up to 10 mm wide. The mold is then used as described above to create a precursor and a micropart.
Also provided is a method for fabricating parts made of a material that does not require sintering to form the finished part. A layered mold is formed as described above. In this case, the layered mold is filled with part material to produce the part. The mold can either be created from the wafer layers and then filled, or each wafer layer can be filled after it is stacked and bonded to the previously filled wafer layer. The mold may also be attached to a support
Cheng Yih-Lin
Cooper Alexander
Leitgeb Rudolf
Prinz Friedrich
Stampfl Jurgen
Board of Trustees of the Leland Stanford Junior University
Chea Thorl
Lumen Intellectual Property Services Inc.
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