Transcript Nanoscience & Contributions Of E-Beam Lithography Jan M. Yarrison-Rice Physics Dept. Miami University w/ Neil Smith & Scott Masturzo.

Nanoscience & Contributions
Of E-Beam Lithography
Jan M. Yarrison-Rice
Physics Dept.
Miami University
w/ Neil Smith & Scott Masturzo
Research Interests
• Surface Enhanced
Microscopies, e.g. SERS
• Single Molecule
Electronics
• Electrochemical Sensing
Exposure Schedule for Dimers
• Photonic Bandgap
(PBG) Structures
Introduction:
Use of Surface
Plasmons
for
Spectroscopy
Phys. Rev. Lett. 94, 057401 (2005)
High tech imager. A 100-micron-wide drop of glycerin
(dark ellipse) acts as a parabolic focusing mirror in a new
technique that can reveal details smaller than a wavelength of
light using an ordinary microscope. (The blue is scattered laser
light.)
A Microscope from Flatland
•
Biologists dream of a point-and-shoot camera that can reveal details smaller
than a wavelength of light in living cells.
•
The team imaged nanoscale holes, but they believe the technique could
ultimately take instant shots/movies of the biological nanoworld.
•
When light meets a metallic surface it generates waves called surface
plasmons, also known as two-dimensional (2D) light, which is made of
electromagnetic waves coupled with conducting electrons.
•
Igor Smolyaninov placed a microscopic sample onto a thin, metal-coated
glass surface, like a document on the surface of a photocopier, and deposit
a drop of glycerin on top of it.
•
They shine laser light through the glass, which creates surface plasmons in
the metal coating. The plasmons "sense" the sample by scattering off of it.
•
They can sense finer details than ordinary light because their wavelength is
only 70 nanometers, seven times shorter than that of the laser.
Plasmon Excitation in Metallic
Nanoparticles
George Schatz, NW University, J. Phys. Chem. B,107, 668-677 (2003).
a)
b)
a) Qualitative plasmon excitation for metallic spheres b) Near field electric
field resonance calculation in Ag trigonal prism with 100 nm sides for excitation
at 770 nm & 460 nm
Resonance Wavelength in Truncated Triangle
vs. Amount of Truncation
Surface Enhanced Spectroscopy
Surface Enhanced Microscopies
• Dimers – sharp edged
doublets
• Ag or Au - on glass for
optical access
• Size determined by
plasmon frequency of
nonlinear system
Challenges..
– Sharp corners
– Closely spaced
nanoparticles
100 nm square dimers separated by 50 nm
Dimer Requirements
• 50 to 200 nm feature
sizes
• Inter-feature spacing
as small as
50 nm
• Pattern on ITO glass,
silicon, or silicon
nitride/dioxide
2 micron squares exposed on silicon w/ 100 nm PMMA
Three Principle Uses of
E-beam
• Mask making for optical lithography in
integrated circuit industry
– Require resolution of 100-250 nm
• Direct write of proto-type integrated circuits
– GaAs integrated circuits & optical waveguides
• Research of scaling limits of circuits &
quantum effects at nano-dimensions
– Require resolution well below 100 nm
Basics: E-Beam Lithography
– Direct Writing Process
• Developed in the late 1960’s
• Based on the Scanning Electron Microscope (SEM).
• It is very slow; orders of magnitude slower than optical
lithography
• Is very expensive
– Beam size of 5 nm or smaller
– Line width control of 20 nm or better (used in
industry)
– Strengths/Weaknesses
• Has very good resolution
• Is very flexible, can be used in many materials, and
almost any pattern can be made this way.
Exposure and Processing
a)
Prepared Silicon
Wafer
b)
Exposed Resist
PMMA
Silicon Dioxide
Silicon
c)
Developed Resist
e)
Evaporated Metal
d)
f)
Etched Silicon
Dioxide
Completed Co-planar
Electrodes
Block Diagram of E-beam
Raith 150 E-Beam Machine
Raith 150 Laser for Stage
Raith 150 CCD Camera
Raith 150
Secondary
Electron Censor
E-beam Source
LaB6 emitter
Both use high V and Temp to
create e- emission
LaB6
Higher emission currents
Schottky
More intense
Small virtual source
Fewer focusing elements
Thermal Field
Emitter (Schottky)
Source Properties
source type
brightness
(A/cm2/sr)
source size
energy spread
(eV)
vacuum
requirement
(Torr)
tungsten
thermionic
~105
25 um
2-3
10-6
LaB6
~106
10 um
2-3
10-8
~108
20 nm
0.9
10-9
~109
5 nm
0.22
10-10
thermal (Schottky)
field emitter
cold
field emitter
Controls: E-Beam Column
Magnetic Lenses – Toroid of wrapped wire
coil in a soft iron box


The electrons (=0.008 nm at 25 keV) are focused with a magnetic
force.
Optical lenses have lower resolution (due to ), but electron lenses can
only focus and cannot remove abberations.
Magnetic Lenses

Force on electrons F = qv x B


Spiral nature impacts design of system


Creates a converging spiral motion of electrons
Deflection system is rotated physically wrt stage
coordinates
Final lens
Large bore, long working distance type
 Minimize deflection abberations
 Made of Ferrite rather than soft iron
 Minimize eddy currents

Control of E-beam
• Apertures & Electron Beam Deflection
– Spray
– Blanking
– Beam Limiting
N
N
N
Deflector
S
S
S
S
N
Stigmator
Sample Preparation: Resists
• Want high sensitivity
– Easier to absorb the energy of the electron
– Sensitivity increases with increased particle
size
BUT:
• Want high resolution
– More detail in pattern
– Better resolution requires smaller particle
So…….
Different Resists
• PMMA – polymethyl methacrylate
– Low sensitivity, but high resolution
– 10-4 C/cm2 at 30 keV for 4 nm pixel size
• PBS – polybutene solfone
– High sensitivity, but low resolution
– 10-6 C/cm2 at 30 keV for 40 nm pixel size
• HSQ – hydrogen silsesquioxane
– Low sensitivity & Negative Resist
– 5.5 x 10-4 C/cm2 at 30 keV
PMMA positive resists are based on special grades of polymethyl
methacrylate designed to provide high contrast, high resolution for ebeam, deep UV (220-250nm) and X-ray lithographic processes.
Standard products include 495,000 and 950,000 molecular weights (MW)
in a wide range of film thicknesses formulated in chlorobenzene, or the
safer solvent anisole.
http://www.microchem.com/products/pmma.htm
Sample Exposure: Writing on
•
•
•
•
•
•
•
the Resist
Prepare substrate – clean it
Spin 100-300 nm of PMMA onto a
substrate
Bake resist to relax molecular
stress from spin
Expose the design with the ebeam.
Develop PMMA and remove
scissioned resist
Evaporate metal onto wafer
Wash away the PMMA to leave the
desired design
PMMA
Substrate
Gold
How the exposure works
• PMMA is a polymer – a bowl full of
spaghetti for the non-chemists!
– The most common molecular weights are 950
and 495 K
• Exposure to e-beam will cause chain
scission
– New molecular weight is ~50K
• Solubility in developer solvent – MIBK and
ISA is function of molecular weight
Positive vs. Negative Resists
• Positive Resist
– Exposed resist is
developed, dissolves &
washes away
– What you write is what
will be ready for further
processing
• Negative Resist
– Exposed resist remains
– What you write is
blocked to further
processing
Charging on Sample
•Electrons require an exit path
•Some portion of the sample must have a metal coating or
be conducting
Checking Sample: Post-Exposure
Review of Sample – Dose
Matrices
Proximity Effect
Evidence of Proximity
Lithography Challenge
• Best practices to make small,
closely spaced features
–
–
–
–
Design of structure
Dosage choices
Aperture choice
Resist
• What design corrections to try
– Dosage schedules within feature for proximity
– Lines around area features to sharpen edges
– Dots and their use to sharpen corners
Proximity Effect Correction
• Design a larger gap than
required between two closely
spaced objects
Proximity Effect Correction
• Vary dosage in each feature
Another route to sharpness..
• Draw a thin line exposure around the
feature
• Add small dots on corners
Other Methods
• Ghost Pattern to even
out exposure
• Use lower beam
aperture -> lower current
Multilayer Resists
• A higher weight PMMA is
spun on top of a lower
weight PMMA
• E-beam writing and
developed
• Metal is evaporated onto
the PMMA
• The PMMA is removed
leaving evaporated metal
on substrate
High Weight PMMA
Low Weight PMMA
How Raith Works
• Write Fields
– 100 microns
• Stitch Field
– Alignment Marks
• Coordinate Transformation
• Laser stage control
– More precision for writing stitch fields
• Resolution of about 2 nm
Stitch
Fields
Shift to right
Write field is shifted
up slightly
Final Design Thoughts
• Proximity – design parameters & dose
matrices
• Write-field placement
• Alignment marks for multi-layer work
• Different apertures for different sized
features means:
– Different design layers
– Different dosages
– Different write speeds
Getting Started on Raith 150
• Three Coordinate Systems
• Alignment via contamination spots
used for:
Sharp Focus
Astigmatism
Electro-Chemical Sensors
• Interdigitated Arrays
– Long 100 to 500 nm
thick fingers w/ ~50
nm separation
– Large contact Pads
separated by mm
– Au or Ag on glass
Top: 500 nm digits, Bottom: 200 nm digits
Interdigitated Array #1
• 200 nm digits
• Separation 200 nm
• 495 PMMA A12 on Silicon
~100 nm thick
Challenges – Strong proximity effect
– Write field overlap
– Very different sized
structures combined
Interdigitated Array #2
• 150 nm digits
• Separated by 400
nm
• ITO on Glass
• 495 PMMA A12 to
100 nm thick
PBG Structure #1
Oxide cover layer
(75nm)
Nitride core
(250 nm)
• 2D arrays of etched
pores
Oxide buffer
(1.8 mm )
• Particular Structures
of Interest include:
Substrate
260
nm
22
260 nm
x
nm
y
5
450
nm
– De-multiplexer
– Polarization Switching
– Microcavity for
Sensing
Calculated Photonic Band
Gap
H (TE) Polarization
a =1.0
1
b=4.0804
f=0.28597
199 G vectors
50 k/direction
0.8
a/
0.6
0.4
0.428
0.391
Band Gap
0.2
0
-0.5
X
0

0.5
k
1
J
X
Schematic of 2D Lattice
Embedded in Planar
Wavguide
19 mm
Exiting HeNe Beam
Interaction
Region
3 Scan
A reas
14 mm
PBG Stripe
Bragg Ref lected
Beam
x
Input HeNe Beam
y
Near Field Optical Scans of
PBG Structure
• NSOM picture of E-field above/in lattice
Back-Reflected
E-Field
Modeling of Photonic Waveguide
Structure #2
•Triangular
Lattice -- 1.7 µm
guide
•1.55 µm input to
GaN structure
• 94%
transmission
PBG Cavities – Structure #3
Optical Add/Drop Multiplexers
w/ Scott Masturzo, ECECS, UC
• 1.55 mm light
introduced in
grating
• Propagates into
the PBG
waveguide silicon
on insulator (SOI)
structure
•Blue represents 228 nm Si capping layer
•Pink illustrates the 700 nm SiO2 cladding
•Burgundy arrows show the propagating infrared
light.
•Etching the Si creates the volumes of air shown
by the cylinders and long rectangular solids.
Multi-Step Design & Fabrication
• 30-40 nm layer of Mo
• Expose PMMA to air pores & waveguide
channel design
– Develop and Etch w/ CF4 & O2 (RIE)
• Remove PMMA
• Etch air pores completely w/ SF6 & O2
– Remove Mo with aquaregia solution
• Expose new PMMA layer to grating design
– Etch Si with CF4 & O2
Resulting Structures
• SOI PC channel waveguide w/
267.6 nm channel width
• Shallow grating of period 724 nm etched
into top Si layer of SOI.
Optical Characterization of
Structure
Magnetic Spin Structures
w/ Neil Smith, Mark Zimmerman and Mick Pechan
• Thin films of Permalloy (Ni:Fe/ 80:20) on Si
– 40 nm thick film
• Etch 100 nm air pores in a square lattice
300 nm sides, or a rectangular lattice 300 x
400 nm
– Using 100 nm PMMA and HNO3:HCL:H2O
(1:2:60)
• Study configurational anisotropy – pinning
of magnetic spin within plane
E-beam Results
• Dose Matrix
– Lower right to
upper left
Successful E-beam Exposure
10 mm aperture
Dot design of
30 nm diameter
23 pA beam current
100 x 100 micron
square requres
45 minutes
Still need work
on wet etching
Single Molecule Electronics
w/ Neil Smith, Archana Jaiswal, Thomas Scott, H. Zhou, & S. Zou
Single digit
electrodes
500 nm wide
35 nm gap
Attached to
contact pads
Dose and Design Matrices
Results w/ & w/out Gold
Averages Of Gaps
120.00
100.00
Width (nm)
80.00
60.00
40.00
20.00
0.00
40nm Dose
80
40nm Dose
100
40nm Dose
120
60nm Dose
80
60nm Dose
100
Average No Gold
60nm Dose
120
Average With Gold
80nm Dose
80
80nm Dose
100
80nm Dose
120
Trouble in E-beam City?
Success ?
Next processing step…
Not so easy…
Di-Metal Molecule
I-V Curve for Bulk Di-metal
Molecule
4.00E-10
Current (Amp.)
3.00E-10
2.00E-10
1.00E-10
0.00E+00
-3
-2
0
-1
-1.00E-10
-2.00E-10
Voltage (Volts)
1
2
3
Next Steps
• Thicker PMMA – 300 nm thick
• Different Design & Exposure Parameters
• Thicker layer of Gold
• Electrolysis to control gap – 3 nm
• Single molecule I-V curves