Kojima, ‘New micro stereo lithography for freely moved 3D micro structure – super IH process with submicron resolution’, in Proceedings of IEEE: MEMS’98, IEEE, Piscataway, NJ, USA, pp..
Trang 1of insulator patterned on a flat Cu disk In selective
electroplating, pressure is applied between the Cu anode
with the mask and the Ni substrate (cathode)
Blanket deposition is also based on the electroplating
technique, but without a mask Basically, the
blanket-deposited material (e.g Ni) is different from the selective
plated one (Cu), so that one of them acts as the sacrificial
material and could be removed later The planarization
is done by lapping the surplus materials to achieve a
precise layer thickness and flatness before deposition
of the subsequent layer By repeating the above steps,
a metallic 3-D microstructure can be formed
(Figure 11.20)
The EFAB process is in its development stage The
resolution obtained is around 25 mm and the smearing
caused by lapping and ‘misregistration’ also affects the
fabrication precision Moreover, the fabrication speed is
a concern since too many time-consuming electroplatingsteps are involved, although a throughput of two planar-ized 5 mm layers per hour or about 50 layers per day wasanticipated [55]
11.4.5.5 Localized electrochemical deposition
A localized electrochemical deposition apparatus isschematically shown in Figure 11.21 [53] The tip of asharply pointed electrode is placed in a plating solutionand brought near the surface where deposition is tooccur A potential is applied between the tip and thesubstrate The electric field generated for electrodepo-sition is then confined to the area beneath the tip, asshown in Figure 11.21(a)
Structural material
Sacrificial (support) material (g)
Figure 11.20 The EFAB process: (a) electroplating through an instant mask; (b) instant-mask removal; (c) blanket deposition of the structural material; (d) planarization by polishing; (e) repetition of electroplating, blanket deposition and planarization until the final structure is formed; (f) remove of the sacrificial materials; (g) cross-sectional view of one layer consisting of structural material and sacrificial materials [55] A Cohen, G Zhang, F Tseng, U Frodis, F Mansfeld, P Will, EFAB: rapid, low-cost desktop micromachining of high aspect ratio true 3-D MEMS, Proc IEEE MEMS’ 99, ß 1999 IEEE
Polymeric MEMS Fabrication Techniques 299
Trang 2In principle, truly 3-D microstructures can be formed
by using localized electrochemical deposition, provided
it is ‘electrically continuous’ with the substrate The
spatial resolution of this process is determined by the
size of the microelectrode Another important parameter
that needs to be considered in this process is the
electro-deposition rate The electro-deposition rate in this case can be
6 mm/s – two orders of magnitude greater than those of
conventional electroplating [53] The shape and
geome-try of the microelectrode used for localized
electroche-mical deposition is critical for the deposition profile
11.4.6 Metal–polymer microstructuresComposite metal/polymer microstructures are becomingvery popular for MEMS A process developed in cabrera
et al [70] allows build layer-by-layer the 3-D object so
as to obtain conductive and non-conductive partstogether, instead of manufacturing them separately andassembling afterwards, for example, to build the cylind-rical object described in Figure 11.22, which consists of ametallic element (‘Part 1’) freely rotating inside a poly-mer housing (‘Part 2’) The major steps involved in thefabrication include the following:
Electroplating of copper to make Part 1
‘Local’ laser silver plating on the polymer to get theconductive base for the following
11.5.1 Architecture combination by MSLArchitecture combination is a technology for buildingcomplicated structures by mechanically connecting two
or more architectures made by different micromachiningprocesses This approach can enable fabrication of a systemconsisting of LIGA linkages driven by a Si micromotor
Fine
electrode
Deposit Mandrel
Plating solution (a)
Micro stepping
motor controller
Figure 11.21 Localized electrochemical deposition for 3-D
micro-fabrication: (a) concept; (b) apparatus [53] Madden, J.D.;
Hunter, I.W., ‘‘Three-dimensional microfabrication by localized
electrochemical deposition,’’ Journal of Microelectromechanical
Systems, Volume 5, Issue 1, ß 1996 IEEE
Metal(Part 1)
(Part 2)
AirPolymer
Figure 11.22 Complex 3-D metal–polymer part [70].
300 Smart Material Systems and MEMS
Trang 3and housed in a polymer structure (Figure 11.2)
Photo-forming (its use here is the same as in MSL) is developed
for this because of its relatively high resolution and 3-D
fabrication capability (Figure 11.23(c)) [71]
Since in this approach the components fabricated with
different processes are joined together during the
photo-forming process, their proper alignment is critical to
achieve a successful architecture combination
11.5.2 MSL integrated with thick-film lithography
Many micromechanical components have been
fabri-cated using planar processes, such as thin-film and
bulk-silicon micromachining and high-aspect-ratio
micromachining (e.g LIGA, deep RIE and thick-resist
lithography), which have high fabrication resolutions,
but do not allow true 3-D fabrications On the other
hand, MSL allows the building of 3-D complex
micro-structures, but with limited resolution and the problemsassociated with the manipulation and assembling ofthe obtained polymer structures An approach of com-bining MSL and thick-resist lithography may provide aunique technique to build 3-D microstructures withmore functions [72]
11.5.3 AMANDA processAMANDA is a process which combines surface micro-machining, micromolding and diaphragm transfer tofabricate micro-parts from polymers A flexible dia-phragm with other functional or structural materials isdeposited and patterned on a silicon substrate using asurface micromachining process The molding process isthen used to build the housing for the fabricated dia-phragm and is then transferred from the silicon substrate
to the polymeric housing Hence, the AMANDA process
Elevator driver
Elevator
Window Laser oscillator
Resin container
Pin hole
Beam shutter
Condenser Head driver
Finish the first layer
Pull up the elevator for thickness of one layer
Repaet these operations
to make the object shape
Figure 11.23 (a) 3-D micro-fabrication by the combined process; (b) schematic of a photoforming system; (b) process flow for photoforming [71] T Takagi, and N Nakajima, Architecture combination by micro photoforming process, Proc IEEE MEMS 94,
ß 1994 IEEE
Polymeric MEMS Fabrication Techniques 301
Trang 4allows low-cost production of reliable micro devices by
batch fabrication
As an example for the AMANDA process, the
fabri-cation process for a pressure transducer is shown in
Figure 11.24 A silicon wafer is covered with 60 nm of
gold by PVD and then with 1.5 mm of polyimide by
spin-coating The polyimide is patterned by photolithography
and an additional 100 nm gold is evaporated on top of the
polyimide layer The second layer of gold is patterned
to form strain gauges A second polyimide disk with a
thickness of 30 mm is built on these strain gauges by
spin-coating and photolithography
The housing of AMANDA devices are produced by
molding Typically, several housings can be fabricated in
a batch Injection molding is generally used for the
molding in AMANDA in order to save time [73] The
housing can be molded from thermoplastic materials
such as polysulfone, PMMA, PA, PC, PVDF or PEEK
[73] Mold inserts are fabricated by milling and drilling
with an CNC machine, LIGA, deep RIE, etc
The diaphragm is then transferred into the housing An
adhesive is injected into the cavities inside the housings
In the example shown in Figure 11.24, the housings are
‘adhesively’ bonded to the polyimide on the wafer The
polyimide outside the housing is cut and the housing,
together with the polyimide diaphragm, is then separated
from the wafer The polyimide can be peeled off from the
wafer because adhesion of the first gold layer to silicon is
low Usually, the diaphragm is encapsulated by a second
shell, which is molded and bonded similarly to the first shell
The dimensional accuracy of the microstructuresfabricated by the AMANDA process depends on thelithography, precision of the mold insert and moldingprocess and alignment and temperature control duringbonding of the molded part and diaphragm The lateralaccuracy of the pattern on the diaphragm can be very highbecause it is fabricated by photolithography Transfer ofthe diaphragm to the polymer housing causes an overallshrinkage due to thermal expansion of the housing and theheating for bonding The precision of the mold insert forhousing fabrication can be very high if the LIGA process
is used The precision of molding can be of severalmicrons but can be improved with injection molding orhot-embossing molding Disadvantages of this process are
in the alignment and control of shrinkage which affects thedimensional accuracy of the AMANDA process [73]
REFERENCES
1 B Zhu and V.K Varadan, ‘Integrated MOSFET based hydrophone device for underwater applications’, Proceed- ings of SPIE, 4700, 101–110 (2002).
2 C.W Hull, ‘Apparatus for production of three-dimensional objects by stereolithography’, US Patent 4 575 330 (1984).
3 J.C Andre´, A Le Me´haute´ and O de Witte, ‘Dispositif pour re´alisar un mode`le de pie`ce industrielle’, French Patent,
8 411 241 (1984).
4 H Kodama, ‘Automatic method for fabricating a dimensional plastic model with photo-hardening polymer’, Review of Scientific Instruments, 52, 1770–1773 (1981).
three-Electric contact
Fluidic contacts Molded housings (b)
(d) (c)
(a)
Polyimide Gold Silicon waferPolyimide disk Strain gauges
Adhesive Figure 11.24 Major flow of AMANDA process; (a) a diaphragm is fabricated by silicon surface micromachining; (b) housings are fabricated by molding or mechanical machining; (c) a diaphragm is transferred from the silicon substrate to the housing; (d) diced chips with electric and fluidic contacts [73] Reprinted from Sensors and Actuators A, 70, W.K.Schomburg, R Ahrens, W Bacher,
C Goll, S Meinzer, A Quinte, AMANDA—low-cost production of microfluidic devices, pp 153–158, Copyright 1998, with permission from Elsevier
302 Smart Material Systems and MEMS
Trang 55 P.F Jacobs, Rapid Prototyping and Manufacturing:
Funda-mentals of Stereolithography, Society of Manufacturing
Engineers, Dearborn, MI, USA (1992).
6 D Kochan, Solid Freeform Manufacturing, Elsevier,
Amsterdam, The Netherlands (1993).
7 K Ikuta, and K Hirowatari, ‘Real three dimensional
microfabrication using stereo lithography and metal
mold-ing’, in Proceedings of IEEE: MEMS’93, IEEE, Piscataway,
NJ, USA, pp 42–47 (1993).
8 T Katagi and N Nakajima, ‘Photoforming applied to fine
machining’, in Proceedings of IEEE:MEMS’93 IEEE,
Pis-cataway, NJ, USA, pp 173–178 (1993).
9 X Zhang, X.N Jiang and C Sun, ‘Micro-stereolithography
of polymeric and ceramic microstructures’, Sensors and
Actuators: Physical, A77, 149–156 (1999).
10 K Ikuta, T Ogata, M Tsubio and S Kojima, ‘Development
of mass productive micro stereo lithography (Mass-IH
pro-cess)’, in Proceedings of IEEE: MEMS’96, IEEE, Piscataway,
NJ, USA, pp 301–305 (1996).
11 P.F Jacobs, Stereolithography and Other RP&M Technologies:
From Rapid Prototyping to Rapid Tooling, American Society of
Mechanical Engineers, New York, NY, USA (1996).
12 S Zissi, A Bertsch, J.Y Jezequel, S Corbel, J.C Andre and
D.J Lougnot, ‘Stereolithography and microtechnologies’,
Microsystem Technologies, 2, 97–102 (1996).
13 A Bertsch, S Zissi, J.Y Jezequel, S Corbel and J.C Andre,
‘Microstereolithography using a liquid crystal display as
dynamic mask-generator’, Microsystem Technologies, 3,
42–47 (1997).
14 T Nakamoto and K Yamaguchi, ‘Consideration on the
producing of high aspect ratio micro parts using UV
sensitive photopolymer’, in Proceedings of the Seventh
International Symposium on Micro Machine and Human
Science, IEEE, New York, USA, pp 53–58 (1996).
15 S Monneret, V Loubere and S Corbel,
‘Microstereolitho-graphy using a dynamic mask generator and a non-coherent
visible light source’, Proceedings of SPIE, 3680, 553–561
(1999).
16 L Beluze, A Bertsch and P Renaud,
‘Microstereolithogra-phy: a new process to build complex 3D objects’,
Proceed-ings of SPIE, 3680, 808–817 (1999).
17 T Katagi and N Nakajima, ‘Photoforming applied to fine
machining’, in Proceedings of IEEE: MEMS’93, IEEE,
Piscataway, NJ, USA, pp 173–178 (1993).
18 K Ikuta, S Maruo and S Kojima, ‘New micro stereo
lithography for freely moved 3D micro structure – super IH
process with submicron resolution’, in Proceedings of IEEE:
MEMS’98, IEEE, Piscataway, NJ, USA, pp 290–295 (1998).
19 B.P Wayne, Principles and Applications of Photochemistry,
Oxford University Press, Oxford, UK (1988).
20 S Maruo and S Kawata, ‘Two-photon-absorbed
near-infrared photopolymerization for three-dimensional
micro-fabrication, Journal of Microelectromechanical Systems, 7,
411–415 (1998).
21 K Suzumori, A Koga and R Haneda, ‘Microfabrication of
integrated FMAs using stereo lithography’, in Proceedings
of IEEE: MEMS’94, IEEE, Piscataway, NJ, USA, pp 136–
141 (1994).
22 L Weber, W Ehrfeld, H Freimuth, M Lacher, H Lehr and
B Pech, ‘Micro molding – a powerful tool for the large scale production of precise microstructures’, Proceedings of SPIE,
2879, 156–167 (1996).
23 T Hanemann, R Ruprecht and J.H HanBelt, ‘Micromolding and photopolymerization’, Advanced Materials, 9, 927–929 (1997).
24 L Lin, C.-J Chiu, W Bache and M Heckele, rication using silicon mold insert and hot embossing’, in MHS’96, Proceedings of the Seventh International Sympo- sium Micro Machine and Human Science, IEEE, Piscataway,
‘Microfab-NJ, USA, pp 67–71 (1996).
25 J Akedo, M Ichiki, K Kikuchi and R Maeda, ‘Fabrication
of three-dimensional micro structure composed of different materials using excimer laser ablation and jet molding’, in Proceedings of the IEEE: The Tenth Annual International Workshop on Micro electro Mechanical Systems, IEEE, Piscataway, USA, pp 135–140 (1997).
26 Y Xia and G.M Whitesides, ‘Soft lithography’, Angewandte Chemie; International Edition, 37, 350–375, (1998).
27 X.-M Zhao, Y Xia and G.M Whitesides, ‘Fabrication of three-dimensional micro-structures: microtransfer molding’, Advanced Materials, 8, 837–840 (1996).
28 E Kim, Y Xia and G.M Whitesides, ‘Polymer ture formed by moulding in capillaries’, Nature (London),
microstruc-376, 581–584 (1995).
29 E Kim, Y Xia, X.-M Zhao and G.M Whitesides, assisted microcontact molding: a convenient method for fabricating three-dimensional structures on surfaces of polymers’, Advanced Materials, 9, 651–654 (1997).
‘Solvent-30 Y Hirata, H Okuyama, S Ogino, T Numazawa and H Takada, ‘Piezoelectric composites for micro-ultrasonic transducers realized with deep-etch X-ray lithography’, in Proceedings of IEEE: MEMS’95, IEEE, Piscataway, NJ, USA, pp 191–196 (1995).
31 S.N Wang, J.-F Li, R Toda, R Watanabe, K Minami and
M Esashi, ‘Novel processing of high aspect ratio 1–3 structures of high density PZT’, in Proceedings of IEEE: MEMS’98, IEEE, Piscataway, NJ, USA, pp 223–228 (1998).
32 W Bacher, W Menz and J Mohr, ‘The LIGA technique and its potential for microsystems — a survey’, IEEE Transac- tions: Industrial Electronics, 42, 431–441 (1995).
33 J.Elders, H.V Jansen, M Elwenspoek and W Ehrfeld,
‘DEEMO: a new technology for the fabrication of structures’, in Proceedings of IEEE: MEMS’95, IEEE, Piscataway, NJ, USA, pp 238–243 (1995).
micro-34 H Freimuth, V Hessel, H Koelle, M Lacher, W Ehrfeld, T Vaahs and M Brueck, ‘Formation of complex ceramic miniaturized structures by pyrolysis of poly(vinylsilazane), Journal of the American Ceramics Society, 79, 1457–1465 (1996).
35 V Piotter, T Benzler, T Hanemann, H Wollmer, R Ruprecht and J Hausselt, ‘Innovative molding technologiesPolymeric MEMS Fabrication Techniques 303
Trang 6for the fabrication of components for microsystems,
Pro-ceedings of SPIE, 3680, 456–463 (1999).
36 Website: [http://potomac-laser.com/applications_micromold.
htm].
37 H Becker and U Heim, ‘Silicon as tool material for polymer
hot embossing’, in Proceedings of IEEE: MEMS’99, IEEE,
Piscataway, NJ, USA, pp 228–231 (1999).
38 O Kemmann, C Schaumburg and L Webber, ‘Micro
moulding behavior of engineering plastics’, Proceedings
of SPIE, 3680, 464–471, (1999).
39 L Weber, W Ehrfeld, M Begemann, U Berg and F Michel,
‘Fabrication of plastic microparts on wafer level’,
Proceed-ings of SPIE, 3874, 44–52 (1999).
40 W.S Beh, I.T Kim, D Qin, Y Xia and G.M Whitesides,
‘Formation of patterned microstructures of conducting
polymers by soft lithography and applications in
microelec-tronic device fabrication’, Advanced Materials, 11, 1038–
1041 (1999).
41 J.M English and M.G Allen, ‘Wireless micromachined
ceramic pressure sensors’, in Proceedings of IEEE:
MEMS’99, IEEE, Piscataway, NJ, USA, pp 511–516 (1999).
42 A.H Epstein, S.D Senturia, G Ananthasuresh, A Ayon, k.
Breuer, K.-S Chen, F Ehrich, G Gauba, R Ghodssi, C.
Groshenry, S Jacobson, J Lang, C.-C Mehra, J Mur
Miranda, S Nagle, D Orr, E Piekos, M Schmidt, G.
Shirley, S Spearing, C Tan, Y.-S Tzeng and I Waitz,
‘Power MEMS and ‘Power MEMS and microengines’, in
Proceedings of Transducers’97: International Conference
on Solid State Sensors and Actuators, Vol 2(2), IEEE,
Piscataway, NJ, USA, pp 753–756 (1997).
43 H.H Bau, S.G.K Ananthasuresh, J J Santiago-Aviles, J.
Zhong, M Kim, M Yi, P Espinoza-Vallejos and L.
Sola-Laguna, ‘Ceramic tape-based meso systems
tech-nology’, in Proceedings of the ASME International
Mechanical Engineering Congress and Exposition on
Micro-Electro-Mechanical Systems (MEMS), ASME, New York,
NY, USA, pp 491–498 (1998).
44 D.L Polla and L.F Francis, ‘Ferroelectric thin films in
microelectromechanical systems applications’, MRS
Bulle-tin, 59–65 (July 1996).
45 V.K Varadan, V.V Varadan and S Motojima,
‘Three-dimensional polymeric and ceramic MEMS and their
appli-cations’, Proceedings of SPIE, 2722, 156–164 (1996).
46 B.C Mutsuddy and R.G Ford, Ceramic Injection Molding,
Chapman & Hall, London, UK (1995).
47 R.M German, and A Bose, Injection Molding of Metals and
Ceramics, Metal Powder Industries Federation, Princeton,
NJ, USA (1997).
48 R Roy, D Agrawal, J Cheng and S Gedevanishvili, ‘Full
sintering of powdered-metal bodies in a microwave field’,
Nature (London), 399, 668–670 (1999).
49 W Bartusch, P Mehringer and G.A Muller, ‘Microwave
sintering – from the laboratory to industrial scale’,
Kera-mische Zeitschrift, 50, 810–817 (1998).
50 X.N Jiang, C Sun, X Zhang, B Xu and T.H Ye,
‘Micro-stereolithography of lead zirconate titanate thick film on
silicon substrate’, Sensors and Actuators: Physical, 87A, 72–77 (2000).
51 X Zhang, X.N Jiang and C Sun, ‘Micro-stereolithography for MEMS, in Proceedings of the ASME International Mechanical Engineering Congress and Exposition on Micro-Electro-Mechanical Systems (MEMS), ASME New York, NY USA, pp 3–9 (1998).
52 M.L Griffith, and J.W Halloran, ‘Stereolithography of ceramics’, in Proceedings of the 27th International SAMPE Technical Conference, SAMPE, Covina, CA, USA, pp 970–979 (1995).
53 T Benzler, V Piotter, T Hanemann, K Mueller, P Norajitra,
R Ruprecht and J Hausselt, ‘Innovations in molding technologies for microfabrication’, Proceedings of SPIE,
3874, 53–60 (1999).
54 J.D Madden and I.W Hunter, ‘Three-dimensional fabrication by localized electrochemical deposition’, Jour- nal of Microelectromechanical Systems, 5, 24–32 (1996).
micro-55 A Cohen, G Zhang, F Tseng, U Frodis, F Mansfeld and P Will, ‘EFAB: rapid, low-cost desktop micromachining of high aspect ratio true 3-D MEMS’, in Proceedings of IEEE: MEMS’99, IEEE, Piscataway, NJ, USA, pp 244–251 (1999).
56 C.S Taylor, P Cherkas, H Hampton, J.J Frantzen, B.O Shah, W.B Tiffany, L Nanis, P Booker, A Salahieh and R Hansen, ‘A spatial forming – a three dimensional printing process’, in Proceedings of IEEE: MEMS’94, IEEE, Piscat- away, NJ, USA, pp 203–208 (1994).
57 Y.P Kathuria, ‘Rapid prototyping: an innovative technique for microfabrication of metallic parts’, in Proceedings of the Seventh International Sympossium on Micro Machine and Human Science, IEEE, Piscataway, NJ, USA, pp 59–65 (1996).
58 J.B Mohler and H.J Sedusky, Electroplating for the lurgist, Engineer and Chemist, Chemical Publishing Co., Inc., New York (1951).
Metal-59 W Blum and G.B Hogaboom, Principles of Electroplating and Electroforming, McGraw-Hill, New York, NY, USA (1949).
60 L.T Romankiw, ‘A path from electroplating through graphic masks in electronics to LIGA in MEMS’, Electro- chimica Acta, 42, 2985–3005 (1997).
litho-61 R.J von Gutfeld and K.G Sheppard, ‘Electrochemical microfabrication by laser-enhanced photothermal pro- cesses’, IBM Journal of Research and Development, 42, 639–653 (1998).
62 E.W Becker, W Ehrfeld, P Hagmann, A Maner and D Muenchmeyer, ‘Fabrication of microstructures with high aspect ratios and great structural heights by synchrotron radiation lithography, galvanoforming and plastic moulding (LIGA process)’, Microelectronic Engineering, 4, 35–56 (1986).
63 A.B Frazier and M.G Allen, ‘Metallic microstructures cated using photosensitive polyimide electroplating molds’, Journal of Microelectromechanical Systems, 2, 87–94 (1993).
fabri-64 J.-M Quemper, S Nicolas, J.P Gilles, J.P Grandchamp, A Bosseboeuf, T Bourouina and E Dufour-Gergam, ‘Permal-
304 Smart Material Systems and MEMS
Trang 7loy electroplating through photoresist molds’, Sensors and
Actuators: Physical, A74, 1–4, (1999).
65 B Loechel, A Maciossek, H.J Quenzer and B Wagner,
‘Ultraviolet depth lithography and galvanoforming for
micromachining’, Journal of Electrochemical Society, 143,
237–244 (1996).
66 Y Konaka and M.G Allen, ‘Single- and multi-layer
elec-troplated microaccelerometers’, in Proceedings of IEEE:
MEMS’96, IEEE, Piscataway, NJ, USA, pp 168–173 (1996).
67 J.-B Yoon, B.K Kim, C.H Han, E Yoon and C.K Kim,
‘Surface micromachined solenoid on-Si and on-glass
induc-tors for RF applications’, IEEE Electron Device Letters, 20,
487–489 (1999).
68 C.H Ahn and M.G Allen, ‘Micromachined planar inductors
on silicon wafers for MEMS applications’, IEEE
Transac-tions : Industrial Electronics, 45, 866–876 (1998).
69 Q Lin, K.G Sheppard, K.G M Datta and L.T Romankiw,
‘Laser-enhanced electrodeposition of lead–tin solder’,
Journal of the Electrochemical Society, 139, L62–L63 (1992).
70 M Cabrera, A Bertsch, J Chassaing, J.Y von Jezequel and
J C Andre, ‘Microphotofabrication of very small objects: pushing the limits of stereolithography’, Molecular Crystals and Liquid Crystals, 315, 223–234 (1998).
71 T Takagi and N Nakajima, ‘Architecture combination by micro photoforming process’, in Proceedings of IEEE: MEMS’94, IEEE, Piscataway, NJ, USA, pp 211–216 (1994).
72 A Bertsch, H Lorenz and P Renaud, ‘Combining stereolithography and thick resist UV lithography for 3D microfabrication’, in Proceedings of IEEE: MEMS’98, IEEE, Piscataway, NJ, USA, pp 18–23 (1998).
micro-73 W.K Schomburg, R Ahrens, W Bacher, C Goll, S Meinzer and A Quinte, ‘AMANDA – low-cost production of micro- fluidic devices’, Sensors and Actuators, A70, 153–158 (1998).
Polymeric MEMS Fabrication Techniques 305
Trang 8The integration of an MEMS sensor with electronics has
several advantages when dealing with small signals The
function of electronics is to make sure that the MEMS
components operate correctly The state-of-the-art in
MEMS is combination with ICs, utilizing advanced
packaging techniques to create a system-on-a-package
(SOP) or a system-on-a-chip (SIP) [1] However, in such
cases it is important that the process used for MEMS
fabrication does not adversely affect the electronics
MEMS devices can be fabricated as pre- or
post-sing modules, which are integrated by standard
proces-sing steps The choice of integration depends on the
application and different aspects of its implementation
technology Various approaches for their integration with
microelectronics are considered in this section
In general, three possibilities exist for monolithic
integration of CMOS and MEMS: (a) CMOS first, (b)
MEMS in the middle, and (c) MEMS first [2,3] In
addition, a hybrid approach, known as a multichip
module is also used often for such integration Each of
these methods has its own advantages and disadvantages
A comparison is listed in Table 12.1 It may be recalled
that a number of materials, such as ceramics, are used in
the fabrication of various MEMS, unlike in CMOS
Annealing of polysilicon or sintering of most ceramics
generally require higher processing temperatures, often
exceeding that allowed in CMOS For example, at
temperatures in excess of about 800C, aluminum
metal-lizations may diffuse and cause performance
degrada-tion Hence, if ceramic processing at a higher
temperature is involved, it may be preferable to fabricate
the MEMS first In contrast, if the MEMS involvesdelicate structures, several common CMOS processes,such as ‘lift off’, may degrade the MEMS performance.Hence, the choice of process sequence is highly depen-dent on the particular MEMS structure at hand
12.1.1 CMOS first process
In this approach, first developed at UC Berkeley, thetemperature limitation due to aluminum is eliminated byusing tungsten as the conducting layer [4] In thisprocess, known as ‘modular integration of CMOS withmicrostructures’ (MICSs), CMOS circuits are first fabri-cated using conventional processes, and polysiliconmicrostructures are then fabricated on the top afterpassivating with SiN and using a phosphosilicate glass(PSG) sacrificial layer Rapid thermal annealing (RTA) ofpolysilicon in nitrogen at 1000C for ‘stress relief’ doesnot affect the CMOS performance A cross-sectionalview of the device is shown in Figure 12.1 In analternate approach, MEMS fabrication is limited tobelow 400C so that these steps do not adversely affectthe CMOS fabricated first Some examples of successfulmicrosystems fabricated by this approach as listed inTable 12.2
12.1.2 MEMS first process
In the method, MEMS structures are first fabricated onthe silicon wafer [12,13] The primary advantage is thathigher processing temperature can be used to achievebetter process optimization In this process, developed atthe Sandia National Laboratories, shallow trenches arefirst anisotropically etched on the wafer and the MEMS is
Smart Material Systems and MEMS: Design and Development Methodologies V K Varadan, K J Vinoy and S Gopalakrishnan
# 2006 John Wiley & Sons, Ltd ISBN: 0-470-09361-7
Trang 9built within these trenches [14] Silicon nitride and
sacrificial oxide may be deposited within these trenches
for the MEMS structures A polysilicon layer on top of
these layers helps establish contacts with the subsequent
CMOS processing Chemical–mechanical planarization
(CMP) and high-temperature annealing are done to
optimize this polysilicon layer The sacrificial oxide
covering the MEMS structure is removed after
fabrica-tion of the CMOS device A photoresist is used as a
protective layer during the release process A
cross-sectional view of a typical device fabricated with this
process in shown in Figure 12.2 Some examples of
successful microsystems fabricated by this approach are
listed in Table 12.3
12.1.3 Intermediate processThe simplest form of an integrated MEMS device is wherethe existing layers for fabricating the IC are used for themechanical components in MEMS [17–19] Standardmicroelectronics processes require a number of layers ontop of the wafer, such as oxide, polysilicon, metal andnitride Utilizing these layers in an MEMS requires only afew additional steps of masking and etching, as explained
in Figure 12.3 Some examples of successful tems fabricated by this approach are listed in Table 12.4
microsys-12.1.4 Multichip moduleThe incompatibilities in the fabrication processes ofMEMS and ICs have made their monolithic integrationdifficult Multichip module (MCM) packaging provides
an efficient solution to integrate MEMS with tronic circuits as it supports a variety of die types in acommon substrate without the need for resorting tosignificant changes in the fabrication process of eithercomponent Several sensors, actuators or a combinationcan be combined in a single chip using the MCMtechnique [22] Using this approach, both surface- andbulk-micromachined components may be integrated withthe electronics When using this approach, separateprocedures are required for releasing and assemblingthe MEMS structures without degrading the package orother dies in the module
microelec-Several variants of this approach exist: high-densityinterconnect (HDI), chip-on-flex (COF) and micro-modulesystem (MMSs) MCM-D These are compared in
Tungsten
metallization
Gatepoly TiN/TiSi2
Poly-polycapacitor PSG
NitridepassivationPoly 1Poly 2
308 Smart Material Systems and MEMS
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Trang 10Table 12.5 In standard HDI process the dies are embedded
in cavities milled on the base substrate and then a thin-film
interconnecting layer is deposited on top of the
compo-nents Holes for the interconnecting vias are made by laser
ablation using a 350 nm argon ion laser Physical access tothe MEMS die is provided by an additional laser ablationstep Figure 12.4(a) shows a typical HDI process flow,compared with an augmented HDI process for MEMS
Table 12.2 Examples of the CMOS first approach for the fabrication of microsystems [5]
accelerometer increase temperature limit of
CMOS; by MICS process
micro-mirror micro-mirrors integrated over a
static random access memory
Delphi Automotive
Systems
University of Acceleration MEMS parts built by additive [8]
Bremen/Infineon switch electroplating technology
thermal for electrical isolation andimager mechanical supportStanford Biosensor with A hybrid glass/PDMS/silicon [10]
University disposable chamber in a cell cartridge
cartridges that includes fluidic interchanges,
physiological sensors andenvironmental regulation
Systrems acceleration polysilicon sensor wafer
sensor with a CMOS substrate
Ped
TEOF Field Oxide
PE nitride Pad
Figure 12.2 Cross-sectional view of a typical device fabricated with an MEMS – first fabrication process developed at the Sandia National Laboratories [14] J.H Smith, S Montague, J.J Snieowski, J.R Murray, and P.J McWhorter, ‘‘Embedded micromechanical devices for monolithic integration of MEMS with CMOS,’’ IEDM’95 Tech Digest, # 1995 IEEE
Integration and Packaging of Smart Microsystems 309
Trang 11packaging (Figure 12.4(b)) by an additional laser-ablation
step to allow physical access to the MEMS die The
windows in the dielectric overlay above the MEMS device
are selectively etched used laser ablation COF is a
lower-cost variant of HDI in which a molded plastic substrate
replaces the ceramic
In the MCM-D approach, the interconnected layers are
deposited on the substrate and the dies are mounted
above these Interconnection between the dies and the
packaging is done by wire bonding Most of the common
wet-etching techniques are not suitable for bulk
micro-machining of structures while following this approach
Hence, isotropic dry etching using XeF2can be used for
selectively etching silicon Wet etching using HF can,
however, be used for releasing the surface-micromachined
structure parts of this chip after shielding the bulk
micro-machined parts by a positive photoresist
The main disadvantage of the MCM approach is thepossibility for signal degradation due to parasitic effectsbetween the components and the apparent added packag-ing expenses
12.2 MEMS PACKAGING
Packaging is the science of establishing interconnectionsbetween the various subsystems and providing an appro-priate operating environment for the electromechanicalcircuits to process the gathered information The disci-pline of microelectromechanical systems (MEMS) wasdeveloped so closely with silicon processing that most
of the early packaging technologies for MEMS weredirectly adapted from microelectronics However, incontrast to the case of microelectronics, most MEMSdevices need a physical access to the outside world,
Table 12.3 Examples of the MEMS first approach for the fabrication of microsystems [5]
MEMS (iMEMS) process
Electronics device SiO2/SiN/SiO2, is first
Laboratory,Zurich/ deposited for etching trenches
electrical shieldingMicrosystems Pressure sensor Micromachined parts are added [16]
Technology and angular to the CMOS fabricated parts by
Laboratory, MIT rate sensor wafer fusion bonding
doped silicon p + doped silicon
Siesmic mass
Suspension Anchor
Figure 12.3 Integration of surface micromachining with CMOS [17] Hierold, C, Hildebrandt, A, Naher, U, Scheiter, T, Mensching,
B, Steger, M, Tielert, R ‘‘A pure CMOS surface micromachined integrated accelerometer,’’ The 9th Annual Intl Workshop on Micro Electro Mechanical Systems, 1996, MEMS ’96, Proceedings IEEE, 11–15 Feb, # 1996 IEEE
310 Smart Material Systems and MEMS
Trang 12either to mechanically react with an external parameter
or to sense a physical variable In state-of-the art
micro-electronics, the device normally accesses the outside
world via electrical connections alone and the systems
are totally sealed and isolated Therefore, unlike
electro-nic packaging, where a standard package can be used for
a variety of applications, MEMS packages tend to be
customized
Challenges in the design of packaging depend on the
overall complexity of the ultimate application of the
device However, there are no sharp boundaries between
these classes The size of the package, choice of its shape
and material, alignment of the device, mounting for the
isolation of shock and vibration and sealing are some of
the many concerns in MEMS packaging Considerations
in packaging may be different, depending on whether the
device is used as an MOEMS, RF MEMS or simpler
sensors or actuators Furthermore, special considerations,
such as biocompatibility, may have to be examined when
designing the packaging of a system Many important
lessons that have been learned throughout years of
experience in the microelectronics industry could beadapted to the packaging of MEMS devices
In MEMS, mechanical structures and electrical ponents are combined to form a functional system Whilepackaging, these electrical and mechanical componentsare interconnected and the electrical inputs are interfacedwith external circuits MEMS components can be extre-mely fragile and must be protected from mechanicaldamage and hostile environments This section presentsthe fundamentals of microelectronic packaging adaptedfor MEMS technology
com-12.2.1 Objectives in packagingThe objective of packaging is to integrate all components
of a system such that cost, mass and complexity areminimized The MEMS package should protect thedevice, while at the same time letting it perform itsintended functions with less attenuation of signal in agiven environment [23,24] Packaging is an expensiveprocess since it seeks to protect relatively fragile struc-tures integrated into the device For a standard integratedcircuit, the packaging process may take up to 95 % of thetotal manufacturing cost Issues in MEMS packaging aremore difficult to solve due to stringent requirements inprocessing and handling and the diversity and fragilenature of the microstructures
MEMS packages provide a mechanical support, anelectrical interface to the other system components andprotection from the environment In addition, packagesshould also provide an interface between the system andthe physical world Many of the MEMS sensors oftenrequire an interface between the sensing media and thesensing area For example, a pressure-sensor packagingrequires incorporation of a pressure port to transmit fluidpressure to the sensor This makes the major differencebetween the standard semiconductor device packages andthe MEMS packages
12.2.1.1 Mechanical supportOnce the MEMS devices are wire-bonded and otherelectrical connections are made, the assembly must beprotected by covering the base or by encapsulating theassembly in plastic or ceramic materials and the electricalconnections are usually made through its walls If thepackaging creates excessive stress in the sensing structure,
it can cause a change in device performance Managingpackage-induced stress in the device becomes importantfor MEMS package design With most MEMS beingmechanical systems, protection and isolation of such
Table 12.4 Examples of the intermediate approach
for the fabrication of microsystems [5]
Table 12.5 A comparison of various MCM
technologies [22] Butler J.T., Bright V.M., Chu P.B
and Saia R.J., Adapting Multichip module foundries
for MEMS packaging, Proc of IEEE International
Conf on Multichip modules and High density
Packaging, # 1998 IEEE
Dielectric material Polyimide Kapton
metallization
Die-interconnection Wire Direct
Maximum operating 100–400 >1 GHz
Requirement None Additional laser
to MEMS dies
Integration and Packaging of Smart Microsystems 311
Trang 13devices from thermal and mechanical shock, vibration,
acceleration and other physical damages during their
operation is critical to their performance The mechanical
stress affecting a system depends on the application For
example, the device package for a military aircraft is
different from those used in communication satellites
The coefficient of thermal expansion of the package
material should be equal to that of silicon for reliability
because the thermal cycle may cause cracking or
delami-nation if they are unmatched
12.2.1.2 Electrical interface
The connection between the MEMS and the signal lines
is usually made with wire bonds or flip-chip die
attach-ments and multilayer interconnections Wire bonds and
other electrical connections to the device should be made
with care taken to protect the device from scratches and
other physical damages DC and RF signals to the
MEMS systems are given through these connections
and interfaces In addition, these packages should be
able to distribute signals to all components within the
package Examples of the external interfaces required
when packaging variuos types of devices are shown inTable 12.6
12.2.1.3 Protection from environmentMany of the MEMS devices and sensors are designed tomeasure variables from the surrounding environment.MEMS packages must protect the micromachined partsfrom the environment and at the same time it mustprovide interconnections to electrical signals, as well asaccess to and interaction with the external environment.The hermetic packaging generally useful in microelec-tronic devices is not suitable in such MEMS devices.These devices might be integrated with the circuits ormounted on a circuit board Special attention in packa-ging can protect a micromachined device from aggres-sive surroundings and mechanical damage Elements thatcause corrosion or physical damage to the metal lines aswell as other components, such as moisture, remain aconcern for many MEMS devices Moisture may beintroduced into the package during fabrication and beforesealing can damage the materials For example, alumi-num lines can corrode quickly in the presence of moist-ure Junctions of dissimilar metals can also corrode in thepresence of moisture
Hermetic MEMS packages provide good barriers toliquids and gases In hermetic packages, the electricalinterconnections through a package must confirm her-metic sealing Wire bonding is the popular technique toelectrically connect the die to the package Bonding ofgold wires is easier than bonding aluminum wires Theuse of wire bonding has serious limitations in MEMSpackaging due to the application of ultrasonic energy at
Mill substrate and attach die Bond pads
Die
CMOS
Apply dielectric layer and laser drill vias Dielectric
Sputter metallization and apply next dielectric layer Metal
Laser-ablated windows for MEMS access (a)
(b)
Figure 12.4 (a) HDI process; (b) MEMS access in the HDI
process [22] Butler J.T., Bright V.M., Chu P.B and Saia R.J.,
Adapting Multichip module foundries for MEMS packaging,
Proc of IEEE International Conf on Multichip modules and High
density Packaging, # 1998 IEEE
Table 12.6 External interfaces required whenpackaging various types of devices
Device Electrical Non-electrical
interface interface
OutputMEMS sensors Output Fluid channels
(gas/liquid)Physical contact(pressure/temperature)None (navigational)MEMS actuators Control Fluid channels
(micro pump)RF-MEMS Control RF cables/connectorsMOEMS Control Optical fibers/couplers
312 Smart Material Systems and MEMS
Trang 14a frequency between 50 to 100 kHz as these frequencies
may stimulate oscillations by the microstructures Since
most microstructures have resonant frequencies in the
same range, the chance of structural failure during the
wire bonding is high [25]
In most spaceborne applications, parts are hermetically
sealed due to the perceived increase in reliability and to
minimize the outgassing When epoxies or cyanate esters
are used to attach the die, they outgas while curing
Outgassing is a concern for many devices since these
particles could be deposited on the components, hence
degrading their performance This leads to ‘stiction’ and
corrosion of the device Die-attachment materials with a
low Young’s modulus allow the chip to move during the
ultrasonic wire bonding, so resulting in low bond
strength
12.2.1.4 Thermal considerations
MEMS devices used for present-day applications do not
have a high-power-dissipation requirement The thermal
dissipation from MEMS devices is not a serious problem
since the temperature of the MEMS devices usually does
not increase substantially during operation However, as
the integration of MEMS with other high-power devices,
such as amplifiers, in a single package increases, the need
for heat dissipation arises to ensure proper operations of
these devices Thus, thermal management is an important
consideration in package design
Heat-transfer analysis and thermal management
beco-me more complex by packaging different functional
components into a tight space This miniaturization
also raises issues such as coupling between the system
configurations and the overall heat dissipation to the
environment The configuration of the system shell
becomes important for heat dissipation from the system
to the environment [26,27] Heat spreading in the thin
space is one of the most important modes of heat transfer
in compact electronic equipment and microsystems As
the system shrinks, the space available for installation of
a fan or pump inside the system shell disappears and the
generated heat has to be dissipated through the shell to
the surrounding environment In general, the primary
motives in heat-transfer design are to diffuse heat as
rapidly as possible and to maximize the heat dissipation
from the system shell to the environment
12.2.2 Special issues in MEMS packaging
Although it follows a similar path as microelectronics
packaging, the design of MEMS packages does need to
address several unique challenges Some of these, as well
as their typical solutions, are described in the followingparagraphs
12.2.2.1 Release of structuresDuring the fabrication of MEMS polysilicon structures
by surface micromachining techniques, these are tected against damage or contamination by silicon diox-ide layers In order to release these polysilicon structures,the oxide layers should be etched out, often by HFsolution The issue here is the timing of this releaseetch, vis-a`-vis the packaging If this is done before thestart of packaging, it may weaken the structure, but ifdone during or after packaging, there is scope for con-tamination and incompatibility issues Another asso-ciated risk is stiction – a phenomenon by whichmicrostructures tend to stick to one another after release.This is caused by capillary action of the droplets of therinse solutions used after etching and may be reduced byincorporating ‘dimples’ into the structures Other solu-tions, such as freeze drying and critical CO2drying, arealso useful to reduce stiction after release To furtherreduce the possibilities of stiction during the lifetime ofthe device, non-stick dielectric films may be insertedduring the fabrication process
pro-12.2.2.2 Die separation
‘Dicing’ is a common process used in microelectronicsfabrication for separating mass-produced devices Thecurrent standard die-separation method adopted for silicon
is to cut the wafer by using a diamond-impregnated blade.The blade and the wafer are ‘flooded’ with high-puritywater while the blade spins at 45 000 rpm This creates noproblem for standard ICs because the surface is essentiallysealed to the effects of water and silicon dust However, if
a released MEMS device is exposed to water and debris,the structures may break off or get clogged and themoisture may adversely affect their performance Efforts
to protect these surfaces with photoresist and other ings have provided only limited success Another possi-bility is to delay the release of the structures to until afterthe dicing An alternate process called wafer cleaving,used in III–IV semiconductor lasers, may also be useful inMEMS die separation [3]
coat-12.2.2.3 Die handlingDuring automated processes, vacuum pick-up heads arecommonly used in handling the die in microelectronics.Integration and Packaging of Smart Microsystems 313
Trang 15As these may not be used for MEMS devices, due to the
presence of delicate structures, additional clamp
attach-ments are required to handle the MEMS die, possibly by
their edges However, the requirement for this special
equipment may be eliminated by wafer level
encapsula-tion In this approach, a capping wafer is used during
dicing, such that each MEMS chip has a protective chip
attached to it These wafers are bonded by using direct
binding or anodic bonding However, the additional
process steps required may cause an increase in the
cost of the device
12.2.2.4 Interfacial stress
Thermal annealing is required for MEMS structures
fabricated with polysilicon There are several other
processes during packaging of the device (such as the
use of hard solders for die attachments, package lid
sealing, etc.) that may introduce additional thermal
stress The application of high temperatures for these
purposes on a complex structure, such as MEMS
invol-ving several materials with varying coefficients of
ther-mal expansion (CTEs) may result in device deformation,
misalignment of parts, change in the resonant frequencies
of the structures and ‘buckle’ in long beam elements
Lower-moduli die-attach materials may solve these
pro-blems to a limited extent but may introduces additional
complications, such as ‘creep over time’ [3] They may
also allow the chip to move during wire bonding, so
resulting in low bond strength
12.2.2.5 Control of outgassing
Many die-attachment materials outgas during their
cur-ing These vapors and moisture may deposit on structures
and cause stiction or corrosion and may result in
degra-dation of performance The solution may include using
low-outgassing materials and/or the removal of
outgas-sing vapors during the die-attachment curing process
12.2.3 Types of MEMS packages
Although MEMS represent a relatively new topic, the
methods of packaging of very small mechanical devices
are not new For example, the aerospace and watch
industries have been performing this task for a very
long time However, MEMS applications usually require
specialized package designs, depending on the
applica-tion and optimizaapplica-tion procedures In general, the possible
group of packages for MEMS can be categorized into
metal, ceramic, plastic and multilayer packages
12.2.3.1 Metal packagesMetal packages are often used in MMIC and hybridcircuits due to their thermal dissipation and electromag-netic shielding effectiveness In addition, these packagesare sufficiently rugged, especially for larger devices.Hence, these are also often preferred for MEMS applica-tions Materials like CuW (10/90), SilverTM (Ni–Fealloy), CuMo (15/85) and CuW (15/85) are good thermalconductors and have higher coefficients of thermalexpansion (CTEs) than silicon
A ‘baking’ step is performed before the final assembly
in order to remove trapped gas and moisture, thusreducing the possibility of corrosion Au–Sn solders arepreferred since these are especially suited when joiningdissimilar materials An alternate method is to use weld-ing by localized heating methods, such as by the use oflasers The primary limitation of these packages is thepresence of the glass or ceramic ‘feedthroughs’, as thesemay be brittle if not handled properly
12.2.3.2 Ceramic packagesCeramic packaging is one of the most common typesused in the microelectronics industry, due to its featuressuch as low mass, low cost and easy mass production.The ceramic packages can be made hermetic, adapted tomultilayer designs and be easily integrated for the signal-
‘feedthrough’ lines Multilayer packages reduce the sizeand cost of integration of multiple MEMS into a singlepackage The electrical performances of the packagescan be tailored by incorporating multilayer ceramics and
‘interconnect’ lines
Co-fired multilayered ceramic packages are structed from individual ‘green’ pieces of thin films.Metal lines are deposited in each film by thick-filmprocessing, such as screen printing, and via holes forthe interconnections to be drilled The unfired layers arethen stacked and aligned and laminated together by firing
con-at high tempercon-atures MEMS and the necessary nents are then attached using epoxy or solders and wirebonds are made
compo-There are several problems associated with the mic packaging The ‘green state’ shrinks during the firingprocess and the amount of shrinkage depends on thenumber of via holes (and hence may be different in eachlayer.) The ceramic-to-metal adhesion is not as strong asthe ceramic-to-ceramic adhesion The processing tem-peratures of ceramics limit the choice of metal lines asthe metal may react with ceramics at high temperatures
cera-In such cases, metals used are W and Mo are employedbut if low-temperature ‘co-fired’ ceramics (LTCCs) are
314 Smart Material Systems and MEMS