Transcript Electroplating

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
– 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
X-rays
X-ray
Mask
Lithographie
Resist
Base plate
(a) Irradiation
(Lithography)
Galvanoformung
PMMA
structure
Electroplated
Metal
(b) Development
(Electroplating)
Abformung
(c) Electroplating
Metal
structure
(d) Removal of the PMMA
(Plastic Molding)
Sacrificial LIGA Process
Sacrificial layer
substrate
(1) Pattern sacrificial layer
Plating base(Ti/Ni)
substrate
(2) Sputter plating base
PMMA
(3) Cast and anneal PMMA
substrate
Sacrificial LIGA Process
PMMA
substrate
Ni
substrate
(4) Align X-ray mask and
expose PMMA by synchrotron
radiation
(5) Developing PMMA and
electroplate Ni
Sacrificial LIGA Process
Ni
substrate
Ni
substrate
Ni
cavity
(6) Remove PMMA and plating
base to clear access to the sacrificial
layer
(7) Etch sacrificial layer to undercut
and free Ni structure
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)
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
Electroplated Nickel
Overpressure Stop
Upper Gap
Polysilicon Diaphragm
Lower Gap
Si Wafer
Si Overpressure Stop
Si Wafer
Si Wafer
• 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)
UV Light
Optical
Mask
Polyimide
Seed layer
Substrate
Polyimide
Mold
Electroplated
Metal
Metal
structure
• 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
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.)
(a) 02 Reactive Ion Etching
(b) Removal of a mask material
(c) Electroplating
(d) Removal of the polyimide
• Polyimide is
patterned by O2 RIE
with a proper mask
material
• Polyimide is used
as a plating mold
• Electroplating and
polyimide removal
is performed
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 laserbased 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