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

2. 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

3. 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)

4. 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

5. 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

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

7. Sacrificial LIGA ProcessNi
(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

8. 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)

9. 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

10. 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)

11. 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

12. 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

13. 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

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

15. 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

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

17. 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

18. 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 ㎛

19. 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

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

21. LIGA MEMS Technology Samples

22. LIGA MEMS Technology Samples
GEARS & ROTORS

23. 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

24. Recent Results for SLIGA

25. 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
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
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

26. 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

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

28. 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

29. 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

30. 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

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

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

33. 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`

34. 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

35. 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

36. 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

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

38. 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