Transcript Nano-Electronics S. Mohajerzadeh University of Tehran

Nano-Electronics
S. Mohajerzadeh
University of Tehran
Lithography, nano-technology
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Lithography is transferring a desired pattern from a “mask” onto a
processed substrate.
Lithography remains essentially the same for micro and nano-electronics.
Standard Photolithography
Photolithography
apply resist
mask alignment/
exposure
develop
etching
resist removal
Lithography approaches
Contact
MFS = (d.)1/2 ~ 
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Proximity
MFS = ((d+g).)1/2 ~ 
Projection
MFS = 0.61 /NA ~ 
Contact mode, mask sits on the resist-coated sample, best resolution is
achieved.
“d” is the resist thickness and “g” is the gap between the sample and mask
(proximity mode).
In projection mode (most used for nano-lithography), numerical aperture of
the lens plays a crucial role.
Resolution limits
Masks
Reflective Optics
nH
nL
Reflective mask
Materials: Mo/Si
Layer thickness ~ 3nm
Absorber layer (TaN, Cr, W, etc)
Buffer layer (Ru, SiO2, etc)
Multilayer stack
Low thermal
expansion material
Nanometric levels
Various lithography sources
UV sources
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Mercury high pressure lamps, strong peaks at 436, 405 and 365nm
Nothing below 300nm,
Plasma torch, Extreme UVs, at 10nm
Transparency of the various glasses drops at lower wavelengths.
Quartz or fused silica can be used for deep UV illuminations
Extended UV lithography
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No transmission
lens
Reflection
condensing,
mirrors
Reticle,
reflecting metals
High resolution
Deep UV source
E-beam writers
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Electrons form a beam
to hit the surface on
the desired area.
Thermionic sources,
field emission sources,
(W or LaB6)
Brightness, W: 104
A/cm2, LaB6: 105, field
emission 107
Electron Trajectories
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Resolution is limited by the spot size, and the exposed area
Backscattered electrons can expose unwanted regions
Proximity effect, the shape of pattern affects the resolution
E-beam lithography
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Underetching results in the reduction of the pattern resolution.
Sharp vertical patterns are obtained by high energy electron beam
writing.
Possible parallel processing
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Scattering limited projection e-beam lithography (SCALPEL)
Use of scattering layer (Au, W) to stop the electrons in the undesired
regions,
E-beam writers
• Design rule: 100 to 70 nm
• Accelerating voltage: 50 kV
• Electron gun emitter: LaB6
single crystal
• Workpiece dimension: up to
178 mm square
• Field stitching accuracy: ± 9
nm
• Overlay accuracy: ± 12 nm
JBX-3030MV series
Electron Beam Lithography System
X-ray lithography
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X-ray beam has a wavelength of the order of 1Ǻ, suitable for high resolution
lithography.
Adsorption is a problem with the mask. Shadow masks or thin membranes
are used.
No lens is available for X-ray. Long pipes are used to form a coherent
beam.
AFM lithography
Mirkin group NWU
e.g. mercaptohexadecanoic acid
on atomically-flat gold surface
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The AFM tip is used to deliver liquid (resist) onto the surface of
sample.
Nano-metric resolution is achieved by this “true writing” approach.
Resolution improvement
Phase shift masks
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Phase shift leads to diffraction on the image side.
Phase shift is suitable only when two windows are
placed close together.
Dark field microscopy
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Illuminating the object at glancing incidence, to ensure
the main reflecting beam does not enter the microscope
Surface irregularities are highlighted and features as
small as 10nm high are detected.
Dust particles of the order of 100nm are observed.
Phase Objects
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A pure phase object has no contrast and we cannot see it.
A glass with a step on it will be seen as a flat surface.
Phase object changes the phase of the light and our eyes can
see the variations in the intensity and not the phase.
Zernike proposed the phase contrast microscopy and received
a Nobel prize for this invention.
Phase contrast scheme
The idea of phase retarding plate causes contrast on the
image plane.
Light passes the phase object and after passing through
the lens, the zero-order diffraction faces the phase
retarding plate and on the image plane we see contrast.
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Phase shift
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Instead of plate, a phase shifting ring is used.
Phase contrast microscopy
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Only first and second order diffracted beams are considered.
Other directions cannot reach the lens and are not
important.
The parallel diffraction beams forms new beams in diverging
directions.
On the transform (focal) plane, small dots are formed
corresponding to various diffraction beams.
Without a phase shift object, contrast is not formed and no
clear image is formed on the image plane.
A phase retarding/advancing plate yields a
constructive/destructive interference and hence a true image
is formed.
Simple explanation!
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If E0, E+1 and E-1 are amplitudes of zero, +1 and -1 diffracted beams
and assuming a plane wave nature for these light beams,
E(x,y,z,t)=Aexp(j(kxx + kyy + kzz –ωt) + Φ)
For zero order, kx=ky=0, kz=2π/λ and E0(x,y,0)=Aexp(j(Φ –ωt))
At z=0 plane, E+1(x)=ε exp(j(kxx–ωt)) and E-1(x)=ε exp(j(-kxx–ωt))
So, finally, Etot(x) = E0 + E+1 + E-1
and
I(x) = Etot Etot*
And I(x)= A2 + 2 ε2 + E0 E*+1 + E0E+1* + E0E-1* + E0E-1* + E+1E-1* +
E+1*E-1
Eventually!!, I(x)= A2 + 2 ε2 + 4A ε[cos Φ cos (kxx)] + 2 ε2 cos(2kxx)
By ignoring the last term, I(x)= A2 + 2 ε2 + 4A ε[cos Φ cos (kxx)]
I(x)= A2 + 2 ε2 + 4Aε2[cos Φ cos (kxx)],
Playing with (cos Φ) can cause a contrast image to occur on the
image plane. Say Φ=0, π,… in normal case, Φ =π/2 or ..
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Wollaston prism, made of a material with two
anisotropic charactersitics such as Calcite.
The light which is polarized, when traveling in the
direction parallel with the optical axis, has a speed
different with the perpendicular case, so phases are
different. (higher index of refraction in one direction)
Normaski prism produces two images of an object,
one for each polarization with a small relative
displacement.
For the case of DIC resolution is less than optical
resolution.
Differential interference contrast
Nano-imprint