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Original Publication Date: 2003-Sep-18
Included in the Prior Art Database: 2003-Sep-18
Document File: 2 page(s) / 92K

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Methyl formate is shown to be a selective stripping solvent for the dissolution of thick PMMA films in the presence of several varieties of cycloaliphatic epoxy-based polymers. The method described avoids the swelling and damage to epoxies caused by other solvents and is especially useful for MEMS structures developed using X-ray lithography. Methyl formate is also a good solvent for PMMA indirectly crosslinked by high-energy radiation.

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  Disclosed is a method of dissolving thick films of PMMA (poly(methyl metacrylate)) without damage to epoxy polymers present in the same structure.

PMMA is an important polymer for both structural and lithography uses. In particular, in the manufacturing of microelectromechanical systems (MEMS) by the LIGA (Lithographie, Galvanoformung, Abformung) process, PMMA is typically used as X-ray resist in the form of thick (100-400 ˝m) films. After exposure, development and plating through the resist mask, the resist has to be stripped. Stripping of the thick resist is a lengthy process; for example, it takes 2-3 hours in acetone at room temperature. Furthermore, in multilayer structures it is common practice to protect the metal layers against corrosion by backfilling the structure with a polymer-based encapsulant. The solvents that are typically used for PMMA stripping tend to damage exposed areas of epoxy polymers used as dielectric encapsulants, in particular cycloaliphatic epoxy resins.

The process described below was tested on a specific type of microelectromechanical structure, i.e., an integrated variable reluctance magnetic minimotor. Its structure comprises three metallic layers separated by dielectric (for further details see [1]-[3]). The dielectric most conveniently used is a silica-filled epoxy resin, for reasons of thermal-expansion matching. The epoxy polymer can be of the reworkable type, where the cycloaliphatic groups carrying the epoxy functions are connected by acid-sensitive ketal or acetal groups [4]. Alternatively, the polymer can be of the more common, non-reworkable type, e.g., Union Carbide ERL-4421 (3,4-epoxycyclohexylmethyl 3,4-epoxycylohexanecarboxylate) where similar cycloaliphatic-epoxy entities are connected by ester groups.

In the process of building the structure, PMMA is used as the X-ray resist; it needs to be stripped away after each level of metallization is completed. The epoxy encapsulant can become exposed to PMMA-stripping solvent in at least two ways. One is when "windows" are left open in the seed layer for one level of metallization, in order to expose lithography alignment marks in the previous level. Another is when accidental cracks develop in the metal seed layer, partially exposing the dielectric underneath. The epoxy dielectric then swells, cracks and delaminates.

It is important therefore to find a PMMA stripping solvent that swells and dissolves PMMA faster than it swells the epoxy dielectric. Common solvents used for this purpose - acetone, 2-butanone, methylene chloride - are unsatisfactory in this regard. They swell the dielectric too fast, so it is irreversibly damaged before all PMMA can be removed. Ethyl acetate is also not satisfactory, being both slow and not selective enough. By c...