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Selective wafer-level adhesive bonding with benzocyclobutene for fabrication of cavities
KTH, Superseded Departments, Signals, Sensors and Systems.
KTH, Superseded Departments, Signals, Sensors and Systems.ORCID iD: 0000-0002-0525-8647
KTH, Superseded Departments, Signals, Sensors and Systems.ORCID iD: 0000-0001-9552-4234
2003 (English)In: Sensors and Actuators A-Physical, ISSN 0924-4247, Vol. 105, no 3, 297-304 p.Article in journal (Refereed) Published
Abstract [en]

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.

Place, publisher, year, edition, pages
2003. Vol. 105, no 3, 297-304 p.
Keyword [en]
adhesive bonding, BCB, selective bonding, helium leak tests, wafer-level encapsulation, temperature, dielectrics
National Category
Electrical Engineering, Electronic Engineering, Information Engineering
URN: urn:nbn:se:kth:diva-13412DOI: 10.1016/S0924-4247(03)00202-4ISI: 000184795300009OAI: diva2:325303
QC 20100618Available from: 2010-06-18 Created: 2010-06-17 Last updated: 2011-12-12Bibliographically approved
In thesis
1. Novel RF MEMS Switch and Packaging Concepts
Open this publication in new window or tab >>Novel RF MEMS Switch and Packaging Concepts
2004 (English)Doctoral thesis, comprehensive summary (Other scientific)
Abstract [en]

Radio-frequency microelectromechanical systems (RF~MEMS) are highly miniaturized devices intended to switch, modulate, filter or tune electrical signals from DC to microwave frequencies. The micromachining techniques used to fabricate these components are based on the standard clean-room manufacturing processes for high-volume integrated semiconductor circuits. RF~MEMS switches are characterized by their high isolation, low insertion loss, large bandwidth and by their unparalleled signal linearity. They are relatively simple to control, are very small and have almost zero power consumption. Despite these benefits, RF~MEMS switches are not yet seen in commercial products because of reliability issues, limits in signal power handling and questions in packaging and integration. Also, the actuation voltages are typically too high for electronics applications and require additional drive circuitry.

This thesis presents a novel MEMS switch concept based on an S-shaped film actuator, which consists of a thin and flexible membrane rolling between a top and a bottom electrode. The special design makes it possible to have high RF isolation due to the large contact distance in the off-state, while maintaining low operation voltages due to the zipper-like movement of the electrostatic dual-actuator. The switch comprises two separately fabricated parts which allows simple integration even with RF circuits incompatible with certain MEMS fabrication processes. The two parts are assembled by chip or wafer bonding which results in an encapsulated, ready-to-dice package. The thesis discusses the concept of the switch and reports on the successful fabrication and evaluation of prototype devices.

Furthermore, this thesis presents research results in wafer-level packaging of (RF) MEMS devices by full-wafer bonding with an adhesive intermediate layer, which is structured before bonding to create defined cavities for housing MEMS devices. This technique has the advantage of simple, robust and low temperature fabrication, and is highly tolerant to surface non-uniformities and particles in the bonding interface. It allows cavities with a height of up to many tens of micrometers to be created directly in the bonding interface. In contrast to conventional wafer-level packaging methods with individual chip-capping, the encapsulation is done using a single wafer-bonding step. The thesis investigates the process parameters for patterned adhesive wafer bonding with benzocyclobutene, describes the fabrication of glass lid packages based on this technique, and introduces a method to create through-wafer electrical interconnections in glass substrates by a two-step etch technique, involving powder-blasting and chemical etching. Also, it discusses a technique of improving the hermetic properties of adhesive bonded structures by additional passivation layers. Finally, it presents a method to substantially improve the bond strength of patterned adhesive bonding by using the solid/liquid phase combination of a patterned polymer layer with a contact-printed thin adhesive film.

Place, publisher, year, edition, pages
Stockholm: KTH, 2004. xii, 142 p.
Trita-ILA, ISSN 0281-2878 ; 0401
0-level packaging, adhesive bonding, BCB, benzocyclobutene, bond strength, contact printing, film actuator, glass lid encapsulation, glass lid packaging, helium leak test, hermetic packaging, hermeticity, high isolation switch, low stress silicon nitride, low volt
National Category
Engineering and Technology
urn:nbn:se:kth:diva-3817 (URN)91-7283-831-0 (ISBN)
Public defence
2004-09-10, 00:00
QC 20100617Available from: 2004-08-26 Created: 2004-08-26 Last updated: 2010-06-18Bibliographically approved

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