Radiation imagery chemistry: process – composition – or product th – Imaging affecting physical property of radiation sensitive... – Making named article
Reexamination Certificate
2001-08-06
2004-12-21
Huff, Mark F. (Department: 1756)
Radiation imagery chemistry: process, composition, or product th
Imaging affecting physical property of radiation sensitive...
Making named article
C430S311000, C430S320000, C700S119000, C700S120000, C700S121000, C264S401000
Reexamination Certificate
active
06833234
ABSTRACT:
BACKGROUND OF THE INVENTION
The technical field of the invention is photolithography and, in particular, stereolithographic patterning of materials.
Stereolithographic patterning enables rapid prototyping of complicated three-dimensional structures. Parts are built up in a layer by layer manner typically from 3D computer representations with no tooling or mounting changes during the build operation. Until recently, most techniques concentrated on fabricating macroscopic components (i.e. >1 cm
3
) with limited resolutions (i.e. >1 mil, or 25 micrometers). Recently, there has been growing interest in merging microelectronics with mechanical structures to create miniaturized microelectromechanical systems (MEMS) such as waveguide structures, microflulidic systems, and sensors. MEMS can offer cost savings, in terms of size reduction and/or increased functionality, especially when arrays of devices are implemented. Lithographic patterning is well established on the micro domain; however, structures are generally limited to extrusions of two-dimensional patterns because of the planar nature of lithography. Stereolithographic patterning on the micro-domain offers a greater optimization of structural elements and increased flexibility in package design.
Stereolithographic techniques utilizing photosensitive polymers have primarily suffered from either (1) poor layer thickness control and limited lateral resolutions or (2) significant process complexity because of the need for intermediate plating and development steps to be performed on a layer by layer basis. The previous methods reported utilize: (1) solution polymerization, (2) a combination of photoresist patterning with plating techniques, or (3) photoresist laminates.
In solution polymerization, a focused laser beam is scanned in a vector-based fashion over a solution of low molecular weight acrylic or epoxy resin. Laser-induced radicals promote polymer—polymer linkage (crosslinking) building up larger chained molecules, which then precipitate out of solution. The process was principally designed for the fabrication of macroscale parts and does not readily scale to the finer feature sizes and layer thicknesses achieved using integrated circuit (IC)-based resists and coating techniques. In solution polymerization, the coating technique is suited for macroscopic parts where individual layers are on the order of 10-50 micrometer thick. The fabrication process involves repeatedly lowering an elevated platform to a given depth per cycle permitting fresh material to overcoat the previously defined layer. The viscous nature of the resins used in solution polymerization require excessive leveling times to coat layers less than several micrometers. Even with plates or wipers to assist in leveling each layer, the coating technique cannot match the precision and uniformity achieved with the coating techniques employed in integrated circuit processing. The ultimate lateral resolutions of the photosensitive polymers used in solution polymerization are also inherently poorer than the photosensitive polymers used in integrated circuit processing. Solution polymerization-based systems rely on chain propagation of reactive radicals to initiate further polymerization; the photogenerated radicals, in the liquid state, readily diffuse. Lateral resolution is, as a result, limited. In addition, support structures are necessary to pattern reentrant structures (regions with no physical supports underneath) which are difficult to remove in the micro-domain.
In a second method, photoresist patterning and electroplating steps used in integrated circuit processing are combined to fabricate metallic structures. This process requires performing development, plating, and planarization steps for each and every layer, thereby adding significant process complexity and reducing throughputs. The photoresist/electroplating combination has only been viable for the creation of structures with a minimal number of layers. The introduction of the additional processing steps is not prohibitive in such circumstances.
Structures have also been fabricated using laminates of photosensitive polymers. Adhesives have been used to attach multiple layers of Dupont Riston®, a photopolymer material used to pattern printed circuit boards. The thickness of such laminates so fabricated have been on the order of 20 micrometers thick or greater; and it has been unclear whether the use of thinner layers is possible in view of manufacturing and handling constraints. Three-dimensional parts have been fabricated using a combination of repetitive exposure and plating techniques. Laminates of polymethyl methacrylate (PMMA) have also been employed having similar thicknesses. Such layers are attached using solvent bonding by melting a surface skin between the layers. In thin layers, this causes pronounced distortions in the previously exposed portions or, otherwise, adversely affects the development process.
None of the above methods can take direct use of advancements in high resolution photoresists and coating techniques already developed for the patterning of integrated circuits without the extra complexity of intermediate plating and development steps. The methods summarized above have been, in part, designed around a universal constraint that is posed because of solvent intermixing between layers of photoimageable resists. Solvent intermixing between the layers washes out previously defined features, alters the dissolution characteristics of the photoresist, and may possibly also cause thickness non-uniformities during coating.
Methods have been developed in semiconductor processing for layering multiple layers of photosensitive material. However, these techniques have been limited to bilayer or trilayer schemes and the resultant composite structures are not readily extendible to stereolithographic patterning that typically require a multitude of layers. Known methods for placing multiple layers of photosensitive polymers utilize: (1) two separate layers with immiscible solvents or an immiscible barrier layer between miscible layers (e.g. top-surface antireflection coatings, contrast enhancement layers, and lift-off based processes), (2) crosslinking to render an underlayer insoluble to the solvent of the next layer (e.g. bottom-layer anti-reflection coatings or in bi- or trilayer schemes where one layer serves as a planarization layer over reflective topographies), or (3) a sufficiently weak solvent in the applied layer to limit penetration into an underlayer to fabricate undercut profiles. All these methods present limitations in stereolithographic patterning.
All these methods present limitations in stereolithographic patterning and suffer from one or more of the following problems: Application of one layer destroys the photoimaging properties of another layer or the photoimaging properties are purposely destroyed through crosslinking to prevent solvent penetration, different developers are required for each layer complicating the development process or more typically, one layer is completely soluble in the developer of the other layer leading to delamination, or the composite structure is only compatible with a single exposure.
In the first case, different materials are applied for each layer. The top layer is purposely designed to be completely soluble in the underlayer's developer, or a different developer is required for each layer. In multiple layers of the composite structure, delamination would occur or different developers would be cycled on each layer complicating the development process. In the second case, the photoimaging properties of the resist are destroyed by crosslinking. Rather, a pattern imaged from the top surface is transferred through it in a second development stage, typically using a plasma based process. Again, each layer of the structure produced from this composite structure requires cycling between different development steps and significantly complicates the development process in multiple layers of this composite structure. Lastly, solvent weake
Bloomstein Theodore M.
Kunz Roderick R.
Palmacci Stephen T.
Cronin Kevin M.
Engellenner Thomas J.
Huff Mark F.
Massachusetts Institute of Technology
Nutter & McClennen & Fish LLP
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