Celesta E. White

Fabrication of microchannels using polycarbonates as sacrificial materials

The use of polycarbonates as thermally decomposable, sacrificial materials for the formation of microchannels is presented. Polycarbonates decompose in the temperature range of 200-300 °C. Two polycarbonates, polyethylene carbonate and polypropylene carbonate, have been used to fabricate microchannels in three different types of encapsulants: an inorganic glass (silicon dioxide), a thermoplastic polymer (Avatrel dielectric polymer) and a thermoset polymer (bisbenzoycyclobutene Cyclotene 3022-57). This paper presents the details of the fabrication process, a thermogravimetric analysis of the sacrificial materials, and the kinetic parameters for the decomposition process. The presence of oxygen or water was found to impact on the decomposition of the sacrificial material. This paper demonstrates the feasibility of forming buried air-cavities in a variety of encapsulants at a modest temperature, thus enabling the use of a wide range of dielectric materials with different thermal stabilities and properties.

Microsystems manufacturing via embossing of photodefinable thermally sacrificial materials

Substantial recent interest in microelectronics manufacturing has motivated significant work on non-traditional processes such as embossing-based lithography. This work has been generally limited to manufacturing conventional microelectronics, producing two dimensional patterned surfaces and structures. To date, little work has been done to produce microelectromechanical systems (MEMS), which can require production of complex three-dimensional and possibly free standing structures. This paper reports a novel method for manufacturing three-dimensional microstructures that can be freely standing and/or fully released. The method involves the use of thermally sacrificial polymers, i.e. materials that can be cleanly decomposed to gaseous products upon heating at elevated temperatures. Such sacrificial polymers can be directly embossed and subsequently overcoated with a variety of materials including other polymers, dielectrics, semiconductors, and metals. Following the deposition of the overcoat layer, further processing can be performed on the overcoat layer (e.g. selective etching or deposition of additional materials). Finally, the entire structure is heated to the decomposition temperature of the sacrificial polymer which results in the “dry” removal of the sacrificial layer, thus releasing the desired structures. The various sacrificial materials that have been investigated are polynorbornenes and polycarbonates, and the overlayer materials include polyimides, silicon oxide, and metals. This paper discusses the various properties of these sacrificial materials, the printing and processing conditions for these materials, and the use of this method for the fabrication of a MEMS based microfluidic system with free standing and suspended obstructions. This novel manufacturing technique meets the needs of MEMS manufacturing in that it can produce three dimensional and free standing microstructures. It permits the fabrication of devices and systems in only a few process steps that would otherwise be either substantially more complicated or impossible to achieve. This process of coating, embossing, and overcoating can also be repeated to build-up complex multi-layered structures.

Development of improved photosensitive polycarbonate systems for the fabrication of microfluidic devices

A technique was recently developed for the fabrication of microfluidic devices that involves using thermally sacrificial polymeric materials in conjunction with other conventional microelectronic processes. This method provides more versatility and choice for construction materials than other current techniques, and it enables the integration of higher levels of functionality into microfluidic systems (i.e., fully integrated multilevel fluidic systems with functional valves, pumping systems, other microelectromechanical system components, and microelectronic devices). This article describes recent results related to the development of photodefinable polycarbonates with improved properties for thermally sacrificial polymer applications. Results of the synthesis and characterization of a polycarbonate that can be directly patterned with the use of photoacid generators via acid-catalyzed thermolysis of polycarbonates are presented.

Photosensitive copolycarbonates for use as sacrificial materials in the fabrication of microfluidic and microelectromechanical devices

In the rapidly growing field of microfluidics, there is a tremendous need for alternative fabrication processes and for simple methods to integrate higher levels of functionality into microfluidic systems (i.e., fully-integrated, multi-level fluidic systems with functional valves, pumping systems, and other MEMS components). A fabrication technique recently developed at Georgia Tech involving thermally sacrificial polymeric materials allows for these innovations. In this method, which is completely compatible with traditional IC fabrication processes, thermally sacrificial polymers are coated onto a substrate and patterned into the shape of the desired channels and devices. These polymeric structures are then overcoated with a permanent structural material such as an inorganic glass or polymer. These steps can be repeated to produce complex, three-dimensional systems. Finally, the completed device structure is heated to the decomposition temperature of the sacrificial polymer which volatilizes to leave behind the desired open-channeled structures. These same materials and techniques can also be applied to the fabrication of a variety of microelectromechanical system components, including suspended membrane structures and cantilevers, that are integrated directly with IC devices on a common substrate. This process was first developed using functionalized polynorbornenes that decompose at temperatures near 425°C. In order to make this approach compatible with a wider range of substrates and structural materials, polymers with lower decomposition temperatures were desired, and polycarbonates were identified as a class of polymers with decomposition temperatures in the desired range (200-300°C). In addition, utilizing a polymer that can be patterned directly by conventional lithography greatly simplifies the fabrication process. By exploiting the acid-catalyzed thermolysis of polycarbonates, low decomposition temperature, photosensitive sacrificial polymers can be developed using mixtures of photoacid generators [PAG] and polycarbonates. Preliminary studies of several different polycarbonates, both photosensitive and non-photosensitive, have shown promising results, but optimization of these materials will be required to realize their full potential as sacrificial materials for use in microsystems manufacturing. The imaging characteristics of these polycarbonates vary greatly with the differing polymer thermal properties and polymer crystallinity, which are directly related to the polymer structure. A comparison of several new secondary and tertiary co-polycarbonates and their ability to maintain feature integrity during photolithography are presented.