# Lithography-Part 2

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```							       Lithography-Part 2

UCI, Winter 2011
Class 3
Content
   Photolithography limits
   Next Generation Lithography
(NGL)
–   EUV
–   X-Ray Lithography
–   E-beam
–   Ion-beam
   Other proposed lithography
scenarios of the future
Photolithography
 Three  ways to
improve resolution
Wmin (also R is used
in the text)
 We will derive this
expression and
analyze all the
different means of
reducing Wmin(also R)
Photolithography-NA
Photolithography-NA
Photolithography-Diffraction
   At smaller dimensions, diffraction effects
dominate
   If the aperture is on the order of l, the
light spreads out after passing through the
aperture. (The smaller the aperture, the
   If we want to image the aperture on an
image plane (resist), we can collect the
light using a lens and focus it on the image
plane.
   But the finite diameter of the lens means
some information is lost (higher spatial
frequency components).
Photolithography-Diffraction
   Image formed by a small circular
aperture (Airy disk) as an example
 Image by a point source forms a circle
with diameter 1.22lf/d *surrounded by
diffraction rings (airy pattern)
 Diffraction is usually described in
terms of two limiting cases
– Fresnel diffraction - near field.
– Fraunhofer diffraction - far field.
*If one considers diffraction through a
circular aperture, then the calculation
involves a Bessel function -- 1.220 is
approximately the first zero of the
Bessel function of the first kind, of
order one (i.e. J1), divided by π.
Photolithography-Diffraction
Photolithography-Diffraction
   Rayleigh suggested that a reasonable
criterion for resolution (R = distance
between A and B) is that the central
maximum of one point source lies at the
first minimum of the Airy pattern of the
other point (R = diameter of circle)
   The numerical aperture (NA) of a lens
represents the ability of the lens to collect
diffracted light and is given by NA = n sin
a in this expression n is the index of
refraction of the medium surrounding the
lens and a is the acceptance angle of the
lens ( n = 1 for air)
   When a is small, 2sin a =d/f
Photolithography-Diffraction
   In the latter expression k1 is an
experimental parameter and
depends on resist properties and
the lithography system ( 0.6-0.8)
   You may remember that, for a
plane wave incident on a grating
of period d, the angles q at which
the intensity maxima in the image
occur are given by: sin q = N l/d,
where N= 0,1,2,…. (Bragg’s
equation)---difference is slits vs
holes
Photolithography-Diffraction
   The angle q in the figure on the top
right is the maximum angle for which
diffracted light from the mask will be
collected for imaging by the lens.
   With sin q = N l/d only those values
of N for which the term on the right is
less than sin q are allowed. Thus, as
the period d gets smaller (l/d gets
larger), N gets smaller (i.e. lower
number of diffracted orders gets
through).
   The middle figure on the right shows
the spread of the diffracted orders for a
decrease in relative slit width (b).
   Because of this spreading effect, fewer
diffracted orders form the image. This
pattern is being lost.
Photolithography-Diffraction
   The figure on the right shows the effect of
including increasing numbers of diffracted orders
on the image of a slit of width w. You can think
of the aperture as truncating these diffracted
orders at some small number.
   The value of sin a for an optical system is the
numerical aperture, or NA. If the value of the
NA is small for a system, fewer orders will be
imaged, and the grating may not be resolved.
    It has been shown that the depth of focus, DOF,
or the range of focus for which a feature can be
resolved, is given by: DOF = ±k2 l/(NA)2
   The R and DOF equations sum up all of the
problems and the promise of optical lithography
using projection tools: The way to increase
resolution is to decrease the wavelength at which
the machine can operate, and to increase the
numerical aperture of the lens. However, both of
these options have the effect of decreasing the
depth of focus.
Photolithography-DOF
   The defocus tolerance (DOF)

   Much bigger issue in miniaturization
science than in ICs
A small aperture was used to ensure the foreground
stones were as sharp as the ones in the distance.

What you need here is a use a telephoto lens at its widest aperture.
Photolithography-DOF
Photolithography- MTF
   Another useful concept is the
modulation transfer function or
MTF
   MTF is the ratio between image
intensity modulation over the
object intensity modulation
   This parameter qualifies the
capability of an optical system
Photolithography- MTF

   Function describes
contrast as a function of
size of features on the
   Generally, MTF needs to
be > 0.5 for the resist to
resolve features
Photolithography- Coherence
   Only point sources are
completely coherent (light
waves impinging perpendicular
   In reality, light sources do have
a finite size resulting in partially
coherent light
   The definition for coherence is
S= NAc/NAo or also S =s/d (see
Figure)
   MTF depends on S : An S of
off
Photolithography- Coherence
Photolithography-OAI and Kohler
   “Off-axis illumination” also
allows some of the higher order
diffracted light to be captured
and hence can improve
resolution (by decreasing k1).
Photolithography-OAI and Kohler
   Kohler illumination systems
focus the light at the entrance
pupil of the objective lens. This
“captures” diffracted light
equally well from all positions
   This method improves the
resolution by bringing k1 down.
Photolithography- OPC
   Optical Proximity Correction
(OPC) can be used to
compensate somewhat for
diffraction effects.
   Sharp features are lost
because higher spatial
frequencies are lost due to
diffraction. These effects can
be calculated and can be
compensated for. This
improves the resolution by
decreasing k1.
   Extends resolution capability of
current optical lithography
   Takes advantage of the wave
nature of light
   PSM changes the phase of light
interference rather than
constructive interference
   Improves MTF of aerial image
on wafer. Making k1 smaller.
   A number of companies
now provide OPC and
phase shifting software
services.
which these make possible
allow sharper resist
images and/or smaller
feature sizes for a given
exposure system.
Photolithography-l
Photolithography-NA
   At the same time that exposure
wavelengths have been reduced,
improvements in lens design has
led to improvements in the NA of
exposure systems lens, see figure .
In the mid eighties an NA value of
approximately 0.4 was typical,
today 248nm exposure systems
are available with an NA greater
than 0.8. The physical limit to NA
for exposure systems using air as
a medium between the lens and
the wafer is 1, the practical limit
is somewhere around 0.9, with
recent reports suggesting that an
NA as high as 0.93 may be
possible for ArF systems in the
future .
Photolithography- k1
   The third element in the Rayleigh
equation is k1. k1 is a complex factor
of several variables in the
photolithography process such as
the quality of the photoresist and the
use of resolution enhancement
techniques such as phase shift
and optical proximity correction
(OPC). While exposure wavelengths
have been falling and NA rising, k1
has been falling as well, see figure .
The practical lower limit for k1 is
Photolithography-Immersion Litho
   From the discussion to this point, the resolution limit for 193nm exposure
systems may be calculated using the Rayleigh equation with, l = 193nm,
NA = 0.93 and k1 = 0.25 or

   From the above a highly optimized ArF exposure system has an absolute
maximum resolution of 52nm, sufficient for 65nm linewidths forecast in
2005, but not capable of meeting the 45nm linewidths forecast in 2007.
   The technical challenges with 157nm and shorter wavelength exposure
systems make any technique that can improve the resolution of the 193nm
exposure systems and delay the need to move to shorter wavelengths an
important development.
Photolithography-Immersion litho
   NA is determined by the acceptance angle of the lens and the index of refraction
of the medium surrounding the lens. The physical limit for an air based system is
clear, but what if a medium with a higher index of refraction is substituted for air?
Microscopy has for years used oil between the lens and the sample being viewed
for resolution enhancement and it is somewhat surprising that the semiconductor
industry has taken this long to seriously consider the merits of replacing air with
an alternative.
Photolithography-Immersion Litho

   The medium between the lens and the wafer being exposed needs to have an
index of refraction >1, have low optical absorption at 193nm, be compatible
with photoresist and the lens material, be uniform and non-contaminating.
Surprisingly, ultrapure water may meet all of these requirements. Water has an
index of refraction n = 1.47, absorption of <5% at working distances of up to
6mm, is compatible with photoresist and lens and in it’s ultrapure form is non-
contaminating.
Photolithography-Immersion Litho

Quiz: what does immersion litho do to DOF?
Next Generation Lithography (NGL)
Next Generation Lithography (NGL)
Next Generation Lithography : EUV

   Uses very short 13.4 nm light
   All reflective optics (at this
wavelength all materials
absorb!)
   Uses reduction optics (4 X)
   Step and scan printing
   Optical tricks seen before all
apply: off axis illumination
OPC
   Vacuum operation
   Laser plasma source
   Very expensive system
Next Generation Lithography : EUV
Next Generation Lithography: E-Beam
oDiffraction is not a limitation on resolution (l < 1 Å for 10-50 keV electrons)
oResolution depends on electron scattering and beam optics the size of the beam, can

reach ~ 5 nm
oTwo modes of operation:

oDirect writing with narrow beam

oElectron projection lithography using a mask :EPL

oIssues:

oThroughput of direct writing is very low : research tool or low pattern density

manufacturing
oProjection stepper (EPL) is in development stage only (primarily by Nikon).

oMask making is the biggest challenge for the projection method

oBack-scattering and second electron result in proximity effect –reduce

resolution with dense patterns there is also the proximity effect
oOperates in high vacuum (10
-6 –10-10 torr) –slow and expensive
Next Generation Lithography: E-
Beam
electron lithography is its low
lithography are:
throughput (approximately 5
(1) Generation of micron and
wafers / hour at less than 0.1 µ
submicron resist geometries
resolution). Therefore, electron
(2) Highly automated and
lithography is primarily used in
precisely controlled operation
(3) Greater depth of focus
and in situations that require
(4) Direct patterning without a
small number of custom
circuits.
Next Generation Lithography: E-Beam

   Electron scattering in resist and
substrate
   The scattered electrons also
expose the resist
   Interaction of e-and substrate +
– Elastic and in-elastic
scattering in the resist
– Back-scattering from
substrate and generation of
secondary e-
– 100 Å e-beam become 0.2
µm line
Next Generation Lithography: E-Beam
Next Generation Lithography: E-Beam
   Pattern directly written into
resist by scanning e-beam
   Device is just like an SEM with
– On-off capability
– Pixelation
– Accurate positioning
– E-beam blur
Next Generation Lithography: E-
Beam
   E-beam blur
Next Generation Lithography:E-Beam
   Thermionic emitters:
– Electrons “boiled” off the surface
by giving them thermal energy to
overcome the barrier (work
function)
– Current given by Richardson-
DushmanEquation
   Field Emitters:
– Takes advantage of the quantum
mechanical properties of electrons.
–Electrons tunnel out when the
surface barrier becomes very
narrow
– Current given by Fowler-
Nordheim equation
   Photo Emitters:
– Energy given to electrons by
incident photons
– Only photo-electrons generated
close to the surface are able to
escape
SCALPEL® (SCattering with Angular Limitation Projection
Electron-beam Lithography)

   EPL is e-beam with a mask for high-throughput
   The aspect of SCALPEL which differentiates it from
previous attempts at projection electron-beam
lithography is the mask. This consists of a low
atomic number membrane covered with a layer of a
high atomic number material: the pattern is
delineated in the latter. While the mask is almost
completely electron-transparent at the energies used
(100 keV), contrast is generated by utilizing the
difference in electron scattering characteristics
between the membrane and patterned materials. The
membrane scatters electrons weakly and to small
angles, while the pattern layer scatters them strongly
and to high angles.
   An aperture in the back-focal (pupil) plane of the
projection optics blocks the strongly scattered
electrons, forming a high contrast aerial image at the
wafer plane
SCALPEL® (SCattering with Angular Limitation Projection
Electron-beam Lithography)

   The functions of contrast generation
and energy absorption are thus
separated between the mask and the
aperture. This means that very little
of the incident energy is actually
thermal instabilities in the mask. It
should be noted that, although the
membrane scatters electrons weakly
compared to the scatterer, a
significant fraction of the electrons
passing through the membrane are
scattered sufficiently to be stopped
by the SCALPEL aperture.
SCALPEL® (SCattering with Angular Limitation Projection
Electron-beam Lithography)
Next Generation Lithography: x-
Rays
   X-ray lithography employs a shadow
printing method similar to optical
proximity printing. The x-ray
wavelength (4 to 50 Å) is much shorter
than that of UV light (2000 to 4000 Å).
Hence, diffraction effects are reduced
and higher resolution can be attained.
For instance, for an x-ray wavelength of
5 Å and a gap of 40 µ, R is equal to 0.2
µ.
   Became very important in MEMS:
LIGA
   Despite huge efforts seems abandoned       Grenoble Synchrotron
for NGL for now
Next Generation Lithography: x-
Rays
   Types of x-ray sources:
– Electron Impact X-ray
source
– Plasma heated X-ray source
» Laser heated
» E-beam heated
– Synchrotron X-ray source
Next Generation Lithography: x-Rays
   Mask: Needs a combination of
materials that are opaque
(heavy element, e.g. Au) and
transparent (low atomic mass
membrane, e.g. BN or S3N4) to
x-rays
   Diffraction is not an issue
viewgraph)
   Masks difficult to make due to
need to manage stress
   Dust less of a problem because
they are transparent to x-rays
Next Generation Lithography: x-Rays
   On account of the finite size of
the x-ray source and the finite
penumbral effect results which
edge of a feature.
is the lateral magnification error
gap and the non-vertical
incidence of the x-ray beam.
The projected images of the
mask are shifted laterally by an
amount d, called runout. This
runout       error    must      be
compensated for during the
Next Generation Lithography:IPL
   Ions scatter much less than                          Ion source

electrons so a higher                                    Ion beam

   Problems:
– Ion Beam source (e.g.
Gallium)
Electrostatic
(4:1 reduction)
– Beam forming
Reference
– Not as mature as EPL             plate

Step-and-scan
wafer stage

Vacuum chamber
Next Generation Lithography:IPL
   Ion lithography can achieve higher resolution than optical, x-ray, or electron
beam lithographic techniques because ions undergo no diffraction and scatter
much less than electrons. In addition, resists are more sensitive to ions than to
electrons. The Figure below depicts the computer trajectory of 50 H+ ions
implanted at 60 keV. As illustrated, the spread of the ion beam at a depth of
0.4 µ is only 0.1 µ. There is also the possibility of a resistless wafer process.
However, the most important application of ion lithography is the repair of
masks for optical or x-ray lithography, a task for which commercial systems
are available.
Next Generation Lithography:IPL

PMMA 1m
Au 100A
2 m Si3N4
Si wafer
develop

electroplating and etch off

backside etch
Future Lithography: Massive
Parallel Writing Arrays
   High-throughput direct-write
electron beam lithography.
electron affinity cathodes have
to improve throughput for
electron beam direct - write
applications.
   Massively parallel arrays of
atomic force microscopes (
AFMs ). Perhaps the ultimate
device in lithography might be
achieved by using amorphous
Si as a resist in conjunction
with a large array of AMFs.
Future Lithography:Microcontact
Printing
   Soft lithography:
– Replication of a “master-
pattern” using PDMS
(stamp)
– Inking the stamp with
molecules (thiols,
thioethers, alkoxysilanes,
chlorosilanes, etc.)
– Contact the stamp with the
substrate surface
– Monolayer formation at
regions of contact
Future Lithography: Nano-Imprint
Technology
   Nanoimprintlithography patterns a
resist by deforming the resist shape
through embossing (with a mold),
rather than by altering resist chemical
particle beams). After imprinting the
resist, an anisotropicetching is used to
remove the residue resist in the
compressed area to expose the
underneath substrate. 10nm diameter
holes and 40nm pitch in PMMA can
be achieved on Sior a metal substrate
and excellent uniformity over 1 square
inch.
Future Lithography: Nanoimprinting
Dip Pen Lithography
Scanning AFM Nanostencil
 Cantilever   tip with holes
Scanning AFM Nanostencil
Nanopatterning Methods Compared
Homework
1.   How does one derive a diameter of 1.22lf/d for the imaging of a small
aperture ?
2.   What does immersion lithography do to DOF?
3.   Demonstrate that F = (1+M)f=1/2NA

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