Physics 358 Nanolithography

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Physics 358 Nanolithography
E-beam lithography The scan interface and
electron-beam resists
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E-beam lithography
Schematic of the ElectronScribe interface
Image acquisition
beam blanker
SEM
Data Acquisition Board
external scan enable
x-y scan coils
16 bit D/A converters
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E-beam lithography
Factors determining resolution Sensitivity of
the resist (i.e., dosage, ?C/cm2) Resolution of
the D/A board (16 bits 65536 points) Speed of
the board (minimum dwell time, 5-10 ?s) Minimum
current (5-10 pA)
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E-beam lithography
Factors determining resolution Example PMMA
sensitivity 300 ?C/cm2 Current 10
pA Minimum dwell time 10 ?s Therefore, area
per pixel is (10 pA x 10 ?s)/300 ?C in
cm2 Working this out gives 3 x 10-12 cm2 or 3 x
10-4 ?m2 Step size is square root of this 1.7
x 10-2 ?m2 or 17 nm For comparison, resolution
of board is 16 bits Field of view 120 ?m, step
size is 1.8 nm.
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E-beam lithography
Factors determining resolution However, writing
time is also important! Suppose we want to
expose a 100 ?m x 100 ?m square at 10 pA with all
the other parameters the same Area per pixel is
3 x 10-4 ?m2 , total area is 104 ?m2 Total
number of pixels is 3 x 107 Total time is 3 x
107 x 10 ?s or 300 seconds 5 minutes
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E-beam lithography
Solution is to write large area structures at
higher currents (and therefore shorter
times) Step size is larger, but do not need
resolution for large structures Smaller
structures written at smaller probe currents and
therefore higher resolution Solution is to write
structures on different layers and change probe
currents and perhaps magnification
between layers Problem image shifts with probe
current/mag change !! Solution Design enough
overlap to take care of image shift
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E-beam lithography
Electron beam resists Recording and transfer
media for e-beam lithography. Polymers
dissolved in a liquid solvent. Two types of
e-beam resists positive- positive resists
develop away exposed regions negative -the
developed region remains behind after
development. Positive e-beam resists-Main-chain
scission when exposed to e-beam Negative
e-beam resists -Formation of interchain
linkages Positive e-beam resists PMMA (Poly
methyl methacrylate), EBR-9 (another acrylate
based resist), PBS (Poly butene-1-sulphone), ZEP
(a copolymer of a -chloromethacrylate and a
-methylstyrene). Negative tone e-beam resists
COP ( an epoxy copolymer of glycidyl
methacrylate and ethyl acrylate) and Shipley SAL
(has 3 components, a base polymer, an acid
generator, and a crosslinking agent).
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E-beam lithography
Electron beam resists
depends on molecular weight
Note can also cross-link PMMA at high dosage
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E-beam lithography
Electron beam resists Thickness of PMMA depends
on spin speed
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E-beam lithography
Substrate electron-beam interactions
Higher voltages penetrate deeper-less exposure of
resist
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E-beam lithography
Substrate electron-beam interactions
Higher voltages penetrate deeper-less exposure of
resist, finer resolution Eliminating substrate
leads to minimum linewidths (can do this by using
free standing windows, e.g., silicon nitride
windows)
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E-beam lithography
Proximity effect
Area around each point exposed is also exposed by
backscattered/secondary electrons. This is
called the proximity effect,. which limits the
resolution One needs to take into account
proximity effect in designing samples, e.g.,
large areas exposed next to small features will
broaden the small feature size. Conversely,
smaller structures require greater dosage than
larger structures.
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E-beam lithography
Multilayer resists Recall problem of
liftoff Solution is to use a bilayer Lower
layer has higher sensitivity, develops
faster Can use lower molecular weight PMMA, LOR
(lift-off resist) or other controlled resists
like PMGI Can also use trilayers, with central
layer being a metal.