Selective wafer-level adhesive bonding with benzocyclobutene for fabrication of cavities

In this work we describe an adhesive wafer-level bonding technique in which the adhesive material is structured prior to bonding. This technique can be used to create encapsulated cavities of different heights and sizes for surface micromachined devices directly in the bonding layer. Benzocyclobutene (BCB) was used as the adhesive bonding material. The structuring of the BCB was done either by dry etching or by using photosensitive BCB. The process parameters needed to achieve a high bond quality while retaining the shapes of the structures in the intermediate bonding layer have been investigated extensively. Both dry-etch and photosensitive BCB were found to be suitable for selective adhesive bonding. The dry-etch BCB must be soft-baked to a polymerisation degree of 50-60% to both withstand the patterning procedure and to be sticky enough for the following bonding. Soft-baking is not necessary for the photosensitive BCB. For both types of BCB, good bond results have been achieved with a bonding pressure of 2-3 bar and full curing of the BCB at 250 degreesC for I h. Furthermore, helium leak tests have been performed to investigate the suitability of selective adhesive bonding for applications with demands on quasi-hermetic seals. Cavities created with this bonding techniques showed a leak rate between 1.4 x 10(-8) and 4.8 x 10(-8) kg m(2) s(-3) (1.4 x 10(-7) and 4.8 x 10(-7) mbar l s(-1)), which is 3-10 times higher than the limit of MIL-STD 883E. Therefore, this encapsulation technique does not provide sufficient gas-tightness to fulfill the requirements of hermetic electronic encapsulations. (C) 2003 Elsevier B.V. All rights reserved.