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Original Publication Date: 2003-Sep-18
Included in the Prior Art Database: 2003-Sep-18
Document File: 3 page(s) / 94K

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The use of a silica-filled epoxy precursor is shown to make possible the fabrication of stable MEMS structures in which epoxy dielectric layers separate metallization layers. The epoxy precursor undergoes slow polymerization and stress-reducing annealing at temperatures below the polymer?s final glass transition. A TCE as low as 29 ppm/oC in the cured filled epoxy prevents delamination driven by thermal mismatch in subsequent processing

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  Disclosed is a method of generating the polymer dielectric layers separating metallization layers in MEMS (microelectromechanical systems) structures, in such a way that TCE-driven stresses between materials and internal stresses in the polymer are minimized.

In the thick structures that are formed in MEMS and especially in high aspect ratio plated MEMS (also known as "Hi MEMS"), where thickness can reach 0.3 mm or more, proper matching of thermal coefficients of expansion (TCE) is important when thermal processing is involved, even to modest temperatures as low as 100 oC. Metals and alloys are an integral part of MEMS structures, and in many instances so are polymers. The former have relatively low TCE values, e.g. 20 ppm/oC, while the latter may exhibit TCE's approaching 250-300 ppm/oC; this is true of many epoxy polymers, which are preferred for encapsulation purposes due to their precursors' low viscosities and to the absence of solvents in their formulations. Combining materials with such vastly different TCE values in a structure leads, upon thermal treatment, to interface delamination which cannot be readily overcome, even by heating, and especially cooling, at impractically low rates.

An obvious way to reduce the extent of TCE mismatch is to incorporate a low-TCE filler in the polymer. However the filler and its concentration have to be chosen judiciously, in order to obtain the maximum TCE benefit while suffering a minimal penalty in other properties such as viscosity. In the present case, a cycloaliphatic diepoxide formulation (see for example [1]), containing a silica filler was employed in the fabrication of a multilayer, magnetic minimotor that involved five lithographic mask levels (for structure and fabrication details see [2]-[4] ). The silica-filled epoxy used here had a TCE value of 29 ppm/oC, which is relatively close to that of Cu (ca. 19 ppm/oC).


After removal of the plating seed layer from the minimotor wafer following electrodeposition, the wafer was placed in a template on a hot plate at 45 oC. The template was derived from a Rodel polishing template and was glued to a 5" Si wafer. The top of the template extended about 30 micron above the top of the Cu conductors or Permalloy magnetic features. An acetal diepoxide containing 50-57 wt% fused silica filler (closely sized to 10 microns particle size) was applied to the wafer using a syringe-type applicator, first to the centers of the motor stators so as to enable wicking of the epoxy into the conductor region and to prevent void formation in the latter. The epoxy was leveled using a blade that traveled over the wafer supported on the template surface.

Curing was carried out under a cover in an oven, first at 80 oC for 60 min, then at 115 oC for 5 hours in a nitrogen atmosphere. The temperature was changed at a rate of 1 oC/min. Careful leveling of the oven shelf was...