Plastic and nonmetallic article shaping or treating: processes – With measuring – testing – or inspecting
Reexamination Certificate
2001-10-08
2004-08-24
Fiorilla, Christopher A. (Department: 1731)
Plastic and nonmetallic article shaping or treating: processes
With measuring, testing, or inspecting
C264S656000, C419S010000, C419S023000, C419S036000
Reexamination Certificate
active
06780353
ABSTRACT:
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
Not Applicable
REFERENCE TO A MICROFICHE APPLENDIX
Not Applicable
BACKGROUND—FIELD OF INVENTION
The present invention relates to the fabrication of micromolds and micromold components. More specifically, the present invention relates to the miniaturization of parts beyond the capability limits of current manufacturing technologies.
BACKGROUND—DESCRIPTION OF PRIOR ART
The universal trend, in the electronics, aerospace, medical and other fields of industry, toward smaller, more complex, lighter, more tightly toleranced, and more highly integrated products to be used in more aggressive, corrosive, hotter or constrained environments, creates a situation which has pushed current manufacturing technologies to their limits of capability.
Hence new manufacturing techniques to mass-produce parts with design features in the micrometer or even nanometer range are urgently needed and, indeed, will become a vital condition for progress and economic survival.
On engineering drawings, linear dimensions are usually toleranced by applying a widely accepted tolerance grading system such as the International Tolerance (IT) grades as outlined in ANSI B4.2. IT grades range from IT01 to IT16 and above, each grade corresponding to a range of tolerances depending on the basic size to which the tolerance applies. For instance the IT4 grade on a basic dimension of 5 mm is 0.004 mm, meaning that the part dimension has to be held within 5.002 mm and 4.998 mm. An IT12 grade on the same basic size is 0.120 mm, meaning that the 5 mm dimension now has to be held within plus or minus 0.120 mm. IT grades for mass-produced items usually range from IT12 through IT16.
Each type of machining process has its particular range of IT grades it can achieve. For instance lapping and honing usually cover the IT4 through IT5 grades whilst milling, drilling and punching operations generally cannot do better than an IT10 grade.
Developments in modem machine tools have been mainly driven by efforts to increase productivity and reduce reliance on the human operator. In addition to being very costly capital items, these machines are often designed and programmed for specific tasks—like robotic welders in car assembly operations—and are difficult to reprogram for different tasks than those for which they were conceived. They also usually are not suitable for mass-production of parts with design features at or below the millimeter. For instance, even using today's most advanced micromachining technologies, the drilling of precise holes of say 0.050 mm in diameter or less to a depth of several mm, would be an extremely difficult if not altogether impossible challenge.
Practically, tolerance capabilities vary widely from workshop to workshop, and indeed from machine tool operator to machine tool operator, and it is a common occurrence to find machining shops unable to make full use of the capabilities of modern machine tools for lack of experienced operators. In summary, only the best precision machining shops, like those found in the watch industry, are able to consistently maintain machining tolerances below 0.05 mm but this is usually only on small production batches and not for mass-produced items.
A number of new techniques, collectively grouped under the term ‘micromolding’ is currently generating great interest due to their potential to overcome the limitations of conventional machining. Clearly, the challenge of making micromolded parts is thus shifted to that of making micromolds.
One technique of fabricating micromolds uses a laser to machine patterned relief microstructures onto a polymer substrate. The machined polymer substrate is then electroplated and the metal inverse used as a micromold. The limitations in design are those of laser machining, i.e. limited definition, poor surface finish at the micrometer level, shallow depth of penetration, high capital investment, high operating cost, etc.
Another way in which the prior art has attempted to make micromolds is by first making oversized versions of these items by powder injection molding technology. After extraction of the organic binder, the green parts are sintered to their final density while undergoing volume shrinkage. An example of such efforts is disclosed in Wiech, Jr., U.S. Pat. No. 5,234,655 where, through a series of iterative cycles, micromolds are achieved. Each cycle consists of making a first green part in a first mold cavity. After sintering, during which the green part shrinks about 20% linearly, the product is used as a mold core in a second mold cavity and a second green part is produced and sintered in turn, upon which, it too, shrinks about 20% linearly to become a miniature of the first mold cavity. The cycle can, in principle, be repeated to produce smaller and smaller molds, each new mold being reduced in size with reference to the previous mold by an amount corresponding to the total shrinkage resulting from the inevitable double sintering step during each cycle. The inadequacy of this empirical method in achieving a precise overall shrinkage in the minimum number of cavity-core cycles, will be readily apparent to those skilled in the art, who will also understand the need to have a molding material that has a precisely predetermined and constant shrinkage factor engineered into it.
Another prior art avenue for making micromolds is by resorting to the manufacturing techniques used in the fabrication of integrated circuits (ICs) where thin material layers are iteratively deposited or selectively etched away on or from the surface of silicon wafers, making use of prior art techniques such as photolithography, molecular beam epitaxy (MBE), low pressure chemical vapor deposition (LPCVD), sputtering, reactive ion etch (RIE) processes, chemical mechanical polishing (CMP), etc.
In recent years these IC fabrication processes have been applied to fabricate mircomolds for so-called Micro-Electro-Mechanical Systems or MEMS, the millimeter to micrometer-scaled devices utilized in a growing number of commercial applications such as micromotors, actuators, sensors, heat exchangers, filters, microvalves and pumps, medical instruments, biomedical implants, etc.
The most basic method used to fabricate MEMS micromolds is to successively deposit and etch thin films of sacrificial polycrystalline silicon dioxide or and structural polycrystalline silicon layers, each about 2-4 &mgr;m thick. With each deposition/etch cycle protrusions are created. Other processes use silicon carbide as the structural material. The use of thin films, about 2-4 &mgr;m thick, has been cited as the major limitation to the development of MEMS micromolds. Also the deposition/etch cycles restrict design complexity.
For example, in Ghosh et al, U.S. Pat. No. 5,735,985 a first micromold is first created in a silicon wafer by dry etching technology. Next a soft stamp is produced by pouring silicone rubber over the etched surface and, after curing, the silicone replica is in turn used as a micromold.
Another problem stems from the fact that IC fabrication processes used to make MEMS micromolds are not universally accessible. Hence, many companies who would like to explore the potential of MEMS technology have limited options for getting devices prototyped or manufactured.
Yet another problem of the prior art is that advanced simulation and modeling tools for MEMS micromold design are still lacking, resulting in a relatively inaccurate prediction of dimensional accuracy. As a result, the MEMS design process is usually performed in a trial-and-error fashion requiring several iterations before the specification requirements are satisfied. This non-ideal design methodology combined with the length of time and high cost associated with MEMS prototyping results in a very inefficient and ineffective scenario for commercial product development. Frequently, the quality of MEMS micromolds fabricated at academic or commercial facilities is low. Part of the problem is that the technology is so new.
In conclusion, the prior art has n
Billiet Romain L.
Nguyen Hanh T.
Fiorilla Christopher A.
Foley & Lardner
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