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Novel RF MEMS Switch and Packaging Concepts
KTH, Superseded Departments, Signals, Sensors and Systems.
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.
Series
Trita-ILA, ISSN 0281-2878 ; 0401
Keyword [en]
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
Identifiers
URN: urn:nbn:se:kth:diva-3817ISBN: 91-7283-831-0 (print)OAI: oai:DiVA.org:kth-3817DiVA: diva2:9672
Public defence
2004-09-10, 00:00
Note
QC 20100617Available from: 2004-08-26 Created: 2004-08-26 Last updated: 2010-06-18Bibliographically approved
List of papers
1. Selective wafer-level adhesive bonding with benzocyclobutene for fabrication of cavities
Open this publication in new window or tab >>Selective wafer-level adhesive bonding with benzocyclobutene for fabrication of cavities
2003 (English)In: Sensors and Actuators A-Physical, ISSN 0924-4247, E-ISSN 1873-3069, 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.

Keyword
adhesive bonding, BCB, selective bonding, helium leak tests, wafer-level encapsulation, temperature, dielectrics
National Category
Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:kth:diva-13412 (URN)10.1016/S0924-4247(03)00202-4 (DOI)000184795300009 ()
Note
QC 20100618Available from: 2010-06-18 Created: 2010-06-17 Last updated: 2017-12-12Bibliographically approved
2. Sealing of adhesive bonded devices on wafer level
Open this publication in new window or tab >>Sealing of adhesive bonded devices on wafer level
2004 (English)In: Sensors and Actuators A-Physical, ISSN 0924-4247, E-ISSN 1873-3069, Vol. 110, no 1-3, 407-412 p.Article in journal (Refereed) Published
Abstract [en]

In this paper. we present a low temperature wafer-level encapsulation technique to hermetically seal adhesive bonded microsystem structures by cladding the adhesive with an additional diffusion barrier. Two wafers containing cavities for MEMS devices were bonded together using benzocyclobutene (BCB). The devices were sealed by a combined dicing and self-aligning etching technique and by finally coating the structures with evaporated gold or PECVD silicon nitride. The sealing layer was inspected visually by SEM and helium leak tests were carried out. Devices sealed with silicon nitride and with known damage of the sealing layer showed a helium leak rate of about 7-14 times higher than the background level. Devices of the same size without damage in the sealing layer had a leak rate of only 1.5 times higher than the background level. Experiments with evaporated gold as cladding layer revealed leaking cracks in the film even up to a gold thickness of 5 mum. The sealing technique with silicon nitride shows a significant improvement of the hermeticity properties of adhesive bonded cavities, making this bonding technique suitable for applications with certain demands on gas-tightness.

Keyword
hermetic sealing, wafer-level encapsulation, adhesive bonding, benzocyclobutene, fabrication, cavities, mems
National Category
Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:kth:diva-13415 (URN)10.1016/j.sna.2003.06.003 (DOI)000188700700064 ()2-s2.0-1642566508 (Scopus ID)
Note
QC 20100617Available from: 2010-06-17 Created: 2010-06-17 Last updated: 2017-12-12Bibliographically approved
3. Low-voltage high-isolation DC-to-RF MEMS switch based on an S-shaped film actuator
Open this publication in new window or tab >>Low-voltage high-isolation DC-to-RF MEMS switch based on an S-shaped film actuator
2004 (English)In: IEEE Transactions on Electron Devices, ISSN 0018-9383, E-ISSN 1557-9646, Vol. 51, no 1, 149-155 p.Article in journal (Refereed) Published
Abstract [en]

This paper presents a new electrostatically actuated microelectromechanical series switch for switching dc to radio frequency (RF) signals. The device is based on a flexible S-shaped film moving between a top and a bottom electrode in touch-mode actuation. This concept, in contrast to most other microelectrocheinical systems (MEMS) switches, allows a design with a low actuation voltage independent of the off-state gap height. This makes larger nominal switching contact areas for lower insertion loss possible, by obtaining high isolation in the off-state. The actuation voltages of the first prototype switches are 12 V to open, and 15.8 V to close the metal contact. The RF isolation with a gap distance of 14.2 mum is better than -45 dB up to 2 GHz and -30 dB at 15 GHz despite a large nominal switching contact area of 3500 mum(2).

National Category
Engineering and Technology
Identifiers
urn:nbn:se:kth:diva-13416 (URN)10.1109/TED.2003.820655 (DOI)000187959600021 ()2-s2.0-0742286721 (Scopus ID)
Note
QC 20100617Available from: 2010-06-17 Created: 2010-06-17 Last updated: 2017-12-12Bibliographically approved
4. Design and fabrication aspects of an S-shaped film actuator based DC to RF MEMS switch
Open this publication in new window or tab >>Design and fabrication aspects of an S-shaped film actuator based DC to RF MEMS switch
2004 (English)In: Journal of microelectromechanical systems, ISSN 1057-7157, E-ISSN 1941-0158, Vol. 13, no 3, 421-428 p.Article in journal (Refereed) Published
Abstract [en]

This paper reports on design and fabrication aspects of a new microelectromechanical series switch for switching de and RF signals. The switch consists of a flexible S-shaped film with the switching contact, rolling between a top and a bottom electrode in electrostatic touch-mode actuation. This design allows a low actuation voltage independent of the contact distance in the off-state. With a large contact distance, large overlapping switching contact areas are possible by obtaining a high off-state isolation. The RF transmission line and the MEMS part of the switch are fabricated on separate wafers, allowing an implementation of the switch with different RF substrates. The final assembly is done on device level for the first prototypes, even though the design provides the possibility of an assembly by full wafer bonding, leading to a near-hermetic package integrated switch. The measured prototype actuation voltages are 12 V to open and 15.8 V to close the contacts, with a resistance of 275 mOmega of each contact at an estimated contact force of 102 muN. The measured RF isolation with a contact distance of 14.2 mum is better than -45 dB up to 2 GHz and -30 dB at 15 GHz, at a large nominal switching contact area of 3500 mum(2).

Keyword
film actuator, low-stress silicon nitride, MEMS switches, RF MEMS, touch-mode actuation
National Category
Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:kth:diva-13414 (URN)10.1109/JMEMS.2004.828723 (DOI)000221845700005 ()2-s2.0-3142685987 (Scopus ID)
Note
QC 20100617Available from: 2010-06-18 Created: 2010-06-17 Last updated: 2017-12-12Bibliographically approved
5. Low-cost glass-lid packaging by adhesive full-wafer bonding with two-step etched electrical feedthroughs
Open this publication in new window or tab >>Low-cost glass-lid packaging by adhesive full-wafer bonding with two-step etched electrical feedthroughs
(English)Manuscript (preprint) (Other academic)
National Category
Engineering and Technology
Identifiers
urn:nbn:se:kth:diva-13410 (URN)
Note
QC 20100617Available from: 2010-06-17 Created: 2010-06-17 Last updated: 2010-06-18Bibliographically approved
6. BCB contact printing for patterned adhesive full-wafer bonded 0-level packages
Open this publication in new window or tab >>BCB contact printing for patterned adhesive full-wafer bonded 0-level packages
2005 (English)In: Journal of microelectromechanical systems, ISSN 1057-7157, E-ISSN 1941-0158, Vol. 14, no 2, 419-425 p.Article in journal (Refereed) Published
Abstract [en]

Adhesive water bonding with a patterned polymer layer is increasingly attracting attention as cheap and simple 0-level packaging technology for microstructures, because the patterned polymer both fulfills the bonding function and determines the volumes between the two wafers housing the devices to be packaged. To be able to pattern a polymer, it has to be cross-linked to a certain degree which makes the material rigid and less adhesive for the bonding afterward. In this paper, a simple method is presented which combines the advantages of a patterned adhesive layer with the advantages of a liquid polymer phase before the bonding. The pattern in the adhesive layer is "inked" with viscous polymer by pressing the substrate toward an auxiliary wafer with a thin liquid polymer layer. Then, the substrate with the inked pattern is finally bonded to the top wafer. Benzocyclobuene (BCB) was used both for the patterned structures and as the "ink". Tensile bond strength tests were carried out on patterned adhesive bonded samples fabricated with and without this contact printing method. The bonding yield is significantly improved with the contact printing method, the fabrication procedure is more robust and the test results show that the bond strength is at least 2 times higher. An investigation of the samples' failure mechanisms revealed that the bond strength even exceeds the adhesion forces of the BCB to the substrate. Furthermore, the BCB contact printing method was successfully applied for 0-level glass-lid packaging done by full-wafer bonding with a patterned adhesive layer. Here, the encapsulating lids are separated after the bonding by dicing the top wafer independently of the bottom wafer.

National Category
Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:kth:diva-13413 (URN)10.1109/JMEMS.2004.839030 (DOI)000228308600025 ()2-s2.0-18844397024 (Scopus ID)
Note
QC 20100618Available from: 2010-06-18 Created: 2010-06-17 Last updated: 2017-12-12Bibliographically approved
7. A 2-bit reconfigurable meander slot antenna with RF-MEMS switches
Open this publication in new window or tab >>A 2-bit reconfigurable meander slot antenna with RF-MEMS switches
2005 (English)In: Antennas and Propagation Society International Symposium, 2005 IEEE: volym 2A, 2005, 396-399 p.Conference paper, Published paper (Refereed)
Abstract [en]

A novel antenna with two RF-MEMS switches placed symmetrically on an asymmetrical meander-shaped slot yielding four different operating frequencies is proposed. The final antenna was manufactured and the four different switching configurations were verified with dummy switches. The measured results were satisfying for all four frequencies with a return loss well above 10 dB. In addition, the actual switches were mounted on the antenna. This proved that the antenna properties are unaffected by the switches. Further use of additional switches on the same antenna is discussed in the paper.

Keyword
microswitches, multifrequency antennas, slot antennas, 2-bit meander slot antenna, RF-MEMS switches, asymmetrical meander-shaped slot, operating frequencies, reconfigurable meander slot antenna
National Category
Telecommunications
Identifiers
urn:nbn:se:kth:diva-13411 (URN)10.1109/APS.2005.1551826 (DOI)2-s2.0-33846882807 (Scopus ID)0-7803-8883-6 (ISBN)
Conference
2005 IEEE Antennas and Propagation Society International Symposium, 3-8 July 2005
Note

QC 20100617

Available from: 2010-06-17 Created: 2010-06-17 Last updated: 2012-09-20Bibliographically approved

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