Electron Beam Nano Lithography

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Electron Beam Nano Lithography
Lesley Raven Anglin, Ayesha Nzeribe, and Dr.
Weilie Zhou
Any intelligent fool can make things bigger,
more complex, and more violent. It takes a touch
of genius and a lot of courage to move in the
opposite direction. -- Albert Einstein
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Abstract
The procedure of Nano Lithography is by no means
an easy one. From preparing the silicon sample
wafer to performing final lift off, each step
requires a high degree of precision and timing.
Sample preparation demands the utmost cleanliness
because the smallest piece of dust can have
catastrophic consequences. Designing and
configuring the pattern must be completed exactly
to guidelines, or the pattern may be distorted.
Lifting off the excess metal plating may have to
be repeated several times until all scraps are
removed. This delicate process paves the way to
the future in twenty-five-nanometer lines.
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Introduction
Electron Beam Nano Lithography is an advanced
tool in the world of nanotechnology. It has been
used in the design and manufacture of microchips,
nanosensors, and nano magnetic devices. Its
versatility with different developing methods and
materials make it a integral part of future
research. This revolutionary process will
continue to be at the forefront of nanotechnology
for years to come.
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Sample Wafer Preparation
950PMMA (Polymethyl Methacrylate) C Resist 3
in Chlorobenzene, which is a positive photo
resist with an intrinsic resolution of
less than10 nanometers A 76.2 ? 0.5 mm diameter
silicon wafer is cleaned (left) and then 2 mL of
PMMA is put in the center and spun at 4000 rpm
using a Cookson Electronics Spin Coater Model
P6700, which results in an end polymer thickness
of 150 nm to 200 nm
www.microchem.com
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Pattern Design and Configuration
Patterns are designed using the Design CAD
software (left) which is also integrated into
Nanometer Pattern Generating System (NPGS). NPGS
is the program that directs the FESEM to expose
the resist and write the pattern
The NPGS Run File Editor (right) controls the
parameters of the pattern, such as,
magnification, line dosage (nC/cm), line spacing
(nm), and distance between patterns (µm). Note
the different colors in Design CAD correspond to
different line dosages
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Callibrating the Microscope
By using a gold standard (below), the calibration
of the LEO 1530 Field Emission Scanning Electron
Microscope (FESEM) becomes a matter of routine.
Astigmatism, focus, and working distance to the
sample are checked before writing to optimize the
lithography.
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Writing the Pattern
Processing the Run File sends a recalibration
command to the PCI516 board, which is the
interface between NPGS and the FESEM NPGS
controls stage movement, magnification and
movement of the electron beam The electron beam
follows the designed pattern exposing the PMMA.
This breaks the bonds of the polymer
Different Batch Programs can be incorporated into
the program, such as, displaying reminders on the
screen before pattern writing begins, or sending
email reminders when the pattern is completed
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Developing and Sputtering
Sample is developed in a 31 solution
of Isopropyl Alcohol (IPA) to Methyl
Isobutyl Ketone (MIBK) for 75 seconds and
dried with Argon gas Developing dissolves the
broken bonds of PMMA which leaves a stencil of
the resist on the wafer Sample is sputtered
with an 10 nm of gold according to the
thickness monitor on the above pictured Cressingto
n 308R Specialty Coating System
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Preliminary Viewing
The pictured wheel is the desired standard. The
color change is due to charging of the area,
which is a good sign for successful lift off. The
widening at the top is due to the resist shifting
while viewing. Previewing is skipped when ones
skills are honed.
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Common Errors
Astigmatism
Out of Focus
Over Exposure
Stigmation causes pattern distortion in
perpendicular directions. Note the 12 oclock and
6 oclock directions appear normal, but the 3
oclock and 9 oclock directions are underexposed
Being out of focus causes the lines to be larger
than normal and blooming around the
intersections. Note that the only points that are
properly exposed are the intersections of lines
The PMMA is changed from a positive to a negative
resist in the center, this is clear sign
of over exposure. This recurring error later
indicated a problem with the beam blanker
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Lift Off
The stencil is removed by dissolving the PMMA
in acetone for several minutes. This removes
the excess metal and leaves the exposed portion
on the wafer, pictured left.
Sometimes not all of the stencil is removed and
parts start to peel away from the wafer, pictured
right. This is easily solved by soaking
the sample in acetone again.
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Dots
Width 140 nm
Height 125 nm
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Magnetic Rings
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Zig Zag Lines
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Whats Next?
Magnetic ? Nanodevice and Sensor
Fabrication
Photonic ? Devices Fabrication
Lithography ? Induced Self-Assembly
Bioelectronics ?
MEMS ? Fabrication
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Referenes and Acknowledgments
References www.jcnabity.com - NPGS
Homepage www.microchem.com - PMMA
manufacturer Funding National Science
Foundation (DMR-0243977) Special Thanks To Al
Lysse and Dr. Joe Nabity