#### Transcript Light Sources

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Xe Lamps
UV LEDs
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Photolithography
Examples of 5X Reticles:
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Photolithography
This is a square glass plate with a patterned
emulsion of metal film on one side. The mask
is aligned with the wafer, so that the pattern
can be transferred onto the wafer surface.
Each mask after the first one must be
aligned to the previous pattern.
Difraction Problem
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Phase-Shifting
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).
PhotolithographyDiffraction
• 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.
PhotolithographyDiffraction
PhotolithographyDiffraction
• 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)
PhotolithographyDiffraction
• In the latter expression k1 is
an experimental parameter
and depends on resist
properties and the
lithography system ( 0.60.8)
• 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,….
Photolithography-Diffraction
• The angle q in the figure 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 now, 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 diffracted orders).
• The 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 means that information
about the pattern is being lost.
PhotolithographyDiffraction
• 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.
Electron Beam
Lithography
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‫مطالب دیگر در موضوع لیتوگرافی‬
• Emitted when a fast
electron interacts with a
magnetic field. This causes
the electron to change
direction. As a result, the
electron will be
accelerated, causing it to