Micromachining 기술 II (Non-Silicon Based) 서강대학교 기계공학과 최범규 (Choi, Bumkyoo)

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

High Aspect Ratio MEMS LIGA Process LIGA-like 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)

Basic LIGA Process Lithographie (Lithography) Galvanoformung X-rays X-ray Mask Lithographie (Lithography) Galvanoformung (Electroplating) Abformung (Plastic Molding) Resist Base plate (a) Irradiation PMMA structure (b) Development Electroplated Metal (c) Electroplating Metal structure (d) Removal of the PMMA

Sacrificial LIGA Process Sacrificial layer (1) Pattern sacrificial layer substrate Plating base(Ti/Ni) substrate (2) Sputter plating base PMMA (3) Cast and anneal PMMA substrate

Sacrificial LIGA Process (4) Align X-ray mask and expose PMMA by synchrotron radiation PMMA substrate Ni (5) Developing PMMA and electroplate Ni substrate

Sacrificial LIGA Process Ni (6) Remove PMMA and plating base to clear access to the sacrificial layer substrate Ni cavity substrate (7) Etch sacrificial layer to undercut and free Ni structure

X-ray Lithography Two Major Applications Submicron VLSI Micromechanics 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)

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

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)

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

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

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 plating Pulse the current at the frequency of a few Hz Directed flow plating A laminar stream of solution is directed against the plated surface

Preliminary Results for SLIGA Close-up of nickel gear - Inside diameter: 55 ㎛ - Tickness of gear: 50 ㎛

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

Preliminary Results for SLIGA Stator configuration of a magnetic four-pole motor - Electroplated nickel of height 100 ㎛

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

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 ㎛

Conceptual Differential Transducer with Double Sided Overload Protection 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

Nickel Overpressure Stops With a thickness of 100µm and a gap of 0.8µm

LIGA MEMS Technology Samples

LIGA MEMS Technology Samples GEARS & ROTORS

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

Recent Results for SLIGA

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

LIGA like Process (GIT) Polyimide Seed layer UV Light Optical Mask Substrate 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 Polyimide Mold Electroplated Metal Metal structure

LIGA like Process (GIT) Detailed SEM of a copper gear illustrating the extremely sharp sidewall profiles 40µm in width 45µm in height

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

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

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

LIGA like Process (TUB) SEM of a photoresist space/line pattern layer thickness : 80 µm pitch : 50 µm aspect ratio : 3.2

LIGA like Process (TUB) SEM of the cross section of planar coil tracks

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`

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

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

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

SCREAM Process (Cornell) SEM micrograph showing SC-GaAs circular ring and angled straight-line features after the CAIBE

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