micromachining 기술 ii (non-silicon based) 최범규 (choi, bumkyoo) 서강대학교...
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Micromachining 기술 II
(Non-Silicon Based)
최범규 (Choi, Bumkyoo)
서강대학교 기계공학과
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Introduction• MEMS Technology
– Micro Electro Mechanical Systems– IC fabrication processing is the basis– Silicon bulk micromachining & wafer to wafer
bonding– Surface micromachining
• Planar processing with lateral etching– High Aspect Ratio MEMS
• Basic LIGA process• Sacrificial LIGA process• LIGA-like process
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High Aspect Ratio MEMS• LIGA Process
– Basic LIGA process (Dr. Ehrfeld)– Sacrificial LIGA process
• Prof. Guckel in the University of Wisconsin
• LIGA-like process– Georgia Institute of Technology (Prof. Allen)– Tohoku University (Prof. Esashi)– Technical University of Berlin (Dr. Reichl)
• SCREAM (Single Crystal Reactive Etching & Metallization)– Cornell University (Prof. McDonald)
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Basic LIGA Process
X-rayMask
Resist
Base plate
PMMAstructure
ElectroplatedMetal
Metalstructure
X-rays
(a) Irradiation
(b) Development
(d) Removal of the PMMA
(c) Electroplating
Lithographie
(Lithography)
Galvanoformung
(Electroplating)
Abformung
(Plastic Molding)
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Sacrificial LIGA Process
substrate
substrate
substrate
PMMA
Sacrificial layer
Plating base(Ti/Ni)
(1) Pattern sacrificial layer
(2) Sputter plating base
(3) Cast and anneal PMMA
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substrate
Ni
Sacrificial LIGA Process
(4) Align X-ray mask and expose PMMA by synchrotron radiation
(5) Developing PMMA andelectroplate Ni
substrate
PMMA
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Sacrificial LIGA Process
substrate
Ni (6) Remove PMMA and platingbase to clear access to the sacrificiallayer
(7) Etch sacrificial layer to undercutand free Ni structure
Ni Ni
cavitysubstrate
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X-ray Lithography
• Two Major Applications– Submicron VLSI
• Spun-on PR layers in 1 micron range
• Modest x-ray flux densities
– Micromechanics• Thick PR in ten to hundred micron range
(Deep x-ray lithography: DXRL)
• High intensity (Synchrotron source)
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Main Issues on DXRL
• Synchrotron with a beam line and fixturing for the mask and substrate
• The thick photoresist process– Coating
• multi-spinning, casting and in situ polymerization• PMMA film
– Developing in a long immersion time• The selectivity of the developer must nearly be infinity• Swelling and distortions must be avoided
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Main Issues on DXRL• X-ray photons are short wave length particles
– No diffraction effects (Limit device dimensions to 2-3 wavelengths of the radiation) for mask dimensions above 0.1 µm
– No standing wave problems (limit exposures of thick PR by optical means)
• A suitable mask– Mask blank not absorbing any photons
• A low atomic no. membrane in a micron range of thickness
– Absorber
• A high atomic number material (gold or tungsten)
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Main Issues on DXRL
• Absorber– The desired contrast ratio determines the thickness for a
given mask blank
– For very thick photoresist, thicknesses of several microns are required
– Normally electroplated
– Bath compatibility w/ the photoresist system, built-in strain and deposit uniformity are difficulties
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Substrate for the LIGA
• A suitable plating base must be supported– Sputtered Cr/Ni base gives good results for basic LIGA
– Ti/Ni is preferred when the base must be removed locally in order to uncover a sacrificial layer (not intermixing)
– Sputtered Ti and Cr are adhesive metal films
• Non-interference with the plating bath• Silicon, quartz, sapphire, glass, plastic and metal
are satisfactory
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Electroplating
• A nickel sulfamate plating system– The bath was operated at 50 C at a pH of 4.3– A plating current density of 50 mA/cm2
– Pulse platingPulse the current at the frequency of a few Hz
– Directed flow platingA laminar stream of solution is directed against the
plated surface
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Preliminary Results for SLIGA
• Close-up of nickel gear - Inside diameter: 55 ㎛ - Tickness of gear: 50 ㎛
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Preliminary Results for SLIGA
• Gears with keyway - The slot in the inner diameter of the center gear allows for the insertion of a key in order to lock the gear to a shaft
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Preliminary Results for SLIGA
• Stator configuration of a magnetic four-pole motor - Electroplated nickel of height 100 ㎛
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Preliminary Results for SLIGA
• Loaded magnetic micromoter - Was operated with several gears to several thousand rpm at 40 gauss or so - Frictional losses are quite low
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Preliminary Results for SLIGA
• Assembled large motion Structure - 5 components - The band width: 4 ㎛ Stretched to an estimated strain level of 0.1% - The nickel height: 100 ㎛
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Conceptual Differential Transducer with Double Sided Overload Protection
Electroplated NickelOverpressure Stop
Polysilicon Diaphragm
Si Wafer
Si Wafer
Si Overpressure Stop Si Wafer
Lower Gap
Upper Gap
• Basic device is surface micromachined polysilicon pressure sensor
• Diaphragm displacement limited by substrate and bridge
• Nickel stop produced by SLIGA process
• Readout can be piezo resistive and capacitive
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Nickel Overpressure StopsWith a thickness of 100µm and a gap of 0.8µm
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LIGA MEMSLIGA MEMS Technology Samples
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LIGA MEMSLIGA MEMS Technology Samples
GEARS & ROTORS
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Electrostatic Relay
The center shuttle and cantilever beams are free from the substrate, while the square pads around the periphery are fixed to the substrate
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Recent Results for SLIGASLIGA
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참고 문헌• H. Guckel, T. Christenson, K. Skrobis, D. Denton, B. Choi, E.
Lovell, J. Lee, S. Bajikar, T. Chapman, "Deep X-ray and UV Lithographies for Micromechanics, "Technical Digest, Solid-State Sensor and Actuator Workshop, June 1990, pp. 118-122
• H. Guckel, K. Skrobis, T. Christenson, J. Klein, S. Han, B. Choi, E. Lovell, "Fabrication of Assembled Micromechanical Components Via Deep X-ray Lithography," 4th IEEE MEMS Workshop, IEEE Pub. 91 CH2957-9, Jan. 1991, pp. 74-79
• H. Guckel, K. Skrobis, T. Christenson, J. Klein, S. Han, B. Choi, E. Lovell, T. Chapman, "On the Application of Deep X-ray Lithography with Sacrificial Layers to Sensor and Actuator Construction," Transducers '91, IEEE, June 1991
• H. Guckel, D. Burns, C. Rutigliano, E. Lovell, B. Choi, "Diagnostic Microstructures for the Measurement of Intrinsic Strain in Thin Films," Journal of Micromechanics and Microengineering, Vol. 2, No. 2, 1992
• 최범규 , "LIGA 공정과 응용 ," 물리학과 첨단기술 , Vol. 3, No. 3, Sept. 1994, pp. 35-38
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LIGA like Process (GIT)• Analogous to the LIGA process
except that polyimide is used as the electroplating mold
• A plating base is deposited on the substrate
• Photosensitive polyimide is spun on top of the seed layer and soft baked
• It is imaged into the desired pattern
• Electroplating and polyimide stripping are performed
PolyimideMold
ElectroplatedMetal
Metalstructure
PolyimideSeed layer
UV LightOpticalMask
Substrate
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LIGA like Process (GIT)
• Detailed SEM of a copper gear illustrating the extremely sharp sidewall profiles– 40µm in width– 45µm in height
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LIGA like Process (GIT)
• A gear/pin structure using the combination of this process and postassembly techniques
• The gear and pin height are 50µm and the gear/pin gap is less than 2 µm
• The gear is free to spin around the pin
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LIGA like Process (Tohoku Univ.)
• Polyimide is patterned by O2 RIE with a proper mask material
• Polyimide is used as a plating mold
• Electroplating and polyimide removal is performed
(a) 02 Reactive Ion Etching
(b) Removal of a mask material
(c) Electroplating
(d) Removal of the polyimide
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LIGA like Process (TUB)
• Almost same as LIGA except exposure source (UV lithography)
• Sputter a thin film plating base• Coat thick photoresist layers (15 to 80 µm)• Expose PR by UV light and develop it• Electroplate the metal• Remove PR and plating base
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LIGA like Process (TUB)
• SEM of a photoresist space/line pattern– layer thickness : 80 µm
– pitch : 50 µm
– aspect ratio : 3.2
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LIGA like Process (TUB)
• SEM of the cross section of planar coil tracks
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Submicron HAR Structure
• SCREAM (Single Crystal Reactive Etching and Metallization)– SC-GaAs is an important material for high-speed VLSI
circuit, monolithic microwave IC and optical laser-based communication systems
– The process includes CAIBE and RIE to produce suspended and movable structures with up to a 25:1 aspect ratio of vertical depth (10 µm ) to lateral width (400nm)
– Silicon nitride is used as an etch mask, structural stiffener and electrical insulator`
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Submicron HAR Structure
• The starting material is Si doped SC-GaAs
• A 350nm layer of PECVD nitride I is deposited
• Photoresist is applied over the nitride I layer, and is patterned
• SC-GaAs is removed by CAIBE etching
• A 300 nm layer of PECVD nitride II and 250nm layer of aluminium are deposited
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Submicron HAR Structure
• A 3.6µm thick photoresist layer is spun on the AL
• The PVS (Predominantly Vertical sidewall) electrode pattern is created in the PR using photolithography
• The electrode pattern in the PR is transferred to the Al using Cl2/BCl3-RIE process which clears the Al on the top, sidewalls, and the bottom
• The nitride II layer is etched back with a CHF3/O2-RIE
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Submicron HAR Structure
• The SC-GaAs mechanical structures are released from the SC-GaAs substrate by etching laterally, underneath the SC-GaAs
lines using a BCl3-RIE
• The photoresist is stripped by an O2 plasma etch after the SC-GaAs undercut etch
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SCREAM Process (Cornell)
• SEM micrograph showing SC-GaAs circular ring and angled straight-line features after the CAIBE
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Conclusion
• IC processing is basically planar and the fabrication of HAR structures makes 3D world possible
• The fabrication process depends upon ways of treating a thick PR to make a molder
• Electroplating is essential except SCREAM
• The quality and the aspect ratio are best in LIGA process
• The access to the synchrotron radiation would not be easy and fabrication cost would relatively high
• There is no best solution for every case and thus we could choose a proper method for each special case