Microfabricated structures and processes for manufacturing same

Abstract

Various techniques for the fabrication of highly accurate master molds with precisely defined microstructures for use in plastic replication using injection molding, hot embossing, or casting techniques are disclosed herein. Three different fabrication processes used for master mold fabrication are disclosed wherein one of the processes is a combination of the other two processes. In an embodiment of the first process, a two-step electroplating approach is used wherein one of the metals forms the microstructures and the second metal is used as a sacrificial support layer. Following electroplating, the exact height of the microstructures is defined using a chemical mechanical polishing process. In an embodiment of the second process, a modified electroforming process is used for master mold fabrication. The specific modifications include the use of Nickel-Iron (80:20) as a structural component of the master mold, and the use of a higher saccharin concentration in the electroplating bath to reduce tensile stress during plating and electroforming on the top as well as sides of the dummy substrate to prevent peel off of the electroform. The electroforming process is also well suited towards the fabrication of microstructures with non-rectangular cross sectional profiles. Also disclosed is an embodiment of a simple fabrication process using direct deposition of a curable liquid molding material combined with the electroforming process. Finally, an embodiment of a third fabrication process combines the meritorious features of the first two approaches and is used to fabricate a master mold using a combination of the two-step electroplating plus chemical mechanical polishing approach and the electroforming approach to fabricate highly accurate master molds with precisely defined microstructures. The microstructures are an integral part of the master mold and hence the master mold is more robust and well suited for high volume production of plastic MEMS devices through replication techniques such as injection molding.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS / INCORPORATION BY REFERENCE [0001] This application claims priority to provisional U.S. Patent Applications Ser. Nos. 60 / 506,641; 60 / 506,226; 60 / 506,321; 60 / 506,424; and 60 / 506,635 all filed on Sep. 26, 2003, and all of which are incorporated herein by reference in their entirety.GOVERNMENT SUPPORT [0002] The U.S. Government has a paid-up license in this invention and the right in limited circumstances to require the patent owner to license others on reasonable terms as provided for by the terms of grant no. AF F30602-00-1-0569 awarded by the Defense Advanced Research Projects Agency (DARPA).[0003] This patent application is being filed concurrently with U.S. Patent Applications having attorney docket numbers 200057.00008, 200057.00009, 200057.00010, and 200057.00011, which are incorporated herein by reference in their entirety. TECHNICAL FIELD [0004] Embodiments of the present invention generally relate to the fabrication of ultra-high precisi...

AI Technical Summary

Benefits of technology

[0018] Disclosed herein is an embodiment of a two-step electroplating technique, wherein the height of the microstructures on the master mold is controlled precisely using a polishing step after electroplating of the microstructures. The structural integrity of the microstructures is preserved by the use of a second sacrificial support metal which is also electroplated, following the first electroplating step. The second sacrificial metal is selectively etched out after the polishing step. The polishing step ensures that all the microstructures are of uniform height (across the entire substrate) and furthermore each microstructure is exhibits uniform height along its cross-sectional profile. The polishing step also ensures that the surface roughness of the microstructures is minimized to yield high-quality replicated features.

[0020] Certain embodiments of the present invention provide an elegant solution to creating a negative image of an array of Piano-Convex microlenses on a master mold and subsequently replicating the Plano-Convex lens array structure on the plastic substrate.

[0022] Certain embodiments of the present invention are concerned with developing a master mold for plastic replication, wherein the master mold is a discrete component, which can be easily assembled into the injection mold block for plastic replication.

Problems solved by technology

However, specifically for microfluidic and more specifically for BioMEMS related applications, Silicon and Glass were observed to have many limitations in terms of the surface exposed to the fluids and / or biological samples.

One specific problem is non-specific adsorption of proteins commonly encountered in biological samples.

With the high intrinsic cost of these substrates, the serial processing further adds to the cost of the device.

However, Silicon master-mold cannot be easily used with injection molding equipment due to processing complexities and the relatively fragile nature of the Silicon substrate.

Hot embossing is an inherently slow-process and cannot match the short-cycle times of injection molding.

However, the milling process is limited in terms of achievable feature size and aspect ratio (the ratio of height to width for a microstructure).

One of the difficulties in this method is the fabrication of structures that do not have a rectangular cross-section.

Another major drawback in UV-LIGA based master mold fabrication is non-uniform electroplating thickness in photoresist mold patterns having different dimensions (specifically different widths).

This may give rise to problems where the microstructure needs to be very accurately defined e.g. in microfluidic applications.

Furthermore, another problem of the current-crowding effect is the non-uniform cross-sectional profile achieved after electroplating, wherein typically (along the cross-section) the center of the microstructure is plated to a lower height as compared to the edges of the microstructure.

Though, this approach allows for uniform plating, it is fairly complex to set it up and may not be suitable for plating large areas.

One of the primary issues in electroforming is stress control.

If the stress is not controlled properly, the electroformed master mold is severely distorted making it useless for plastic replication.

This issue has limited the use of electroforming techniques for injection molding applications.

Specifically, if a master mold feature is wider at the top than the bottom, it is not possible to injection mold (or emboss) a copy of it since the mold pattern will be “stuck” into the plastic substrate.

Such profiles are very difficult to create (except by using complex gray-scale lithography techniques as explained earlier) using UV-LIGA techniques.

Finally, an issue of concern in master molds created using either UV-LIGA or electroforming techniques, is the surface roughness of the microstructures.

For most BioMEMS applications, increased surface roughness leads to poor performance; specifically in the case of Capillary Electrophoresis (CE) chips, wherein poor surface quality can render the chip unusable for separation applications.

Obviously, for Optical MEMS applications, even slight surface imperfections can lead to deviation in the optical path characteristic ergo, poor device performance.

Images