Basic Concepts

1
Basic Concepts

• Antireflective coating is used to prevent
  reflections from the chrome coming back into the
  resist
• Occasionally AR coatings are deposited on wafers
  also
• Develop the resist and etch to remove the metal
• We get good dimensional control because the Cr is
  very thin (80nm)
• It is critical that the areas beneath where the
  Cr is removed be highly transparent at the
  wavelength of the light used in the wafer
  exposure system.
• Masks (reticules) for steppers (step and repeat
  systems) are 4x to 5x larger than what is printed
• Relaxes minimal feature requirements on mask
• Masks for steppers print usually only one or two
  die at a time any defect in the mask gets
  reproduced for every die!

2
Reflectivity

• At the interface of two bulk layers

http//www.mellesgriot.com/products/optics/images/
fig5_12.gif
3
Antireflectivity Coatings
             

• For l/4 thick films
• Ideal index of refraction for antireflective
  coating is v(nairnglass)

4
Basic Concepts

• We generally separate lithography into three
  parts
• The light source
• The exposure system
• The resist
• The exposure tool creates the best image possible
  on the resist (resolution, exposure field, depth
  of focus, uniformity and lack of aberrations)
• The photoresist transfers the aerial image from
  the mask to the best thin film replica of the
  aerial image (geometric accuracy, exposure speed,
  resist resistance to subsequent processing)

5
Light Source

• Historically, light sources have been arc lamps
  containing Hg vapor

• A typical emission spectra from a Hg-Xe lamp
• Low in DUV (200-300nm) but strong in the UV
  region (300-450nm)

6
Light Source

• To minimize problems in the lens optics, the lamp
  output must be filtered to select on of the
  spectral components.
• Two common monochromatic selections are the
  g-line at 436 nm and the i-line at 365 nm.
• The i-line stepper now dominates the 0.35 ?m
  market

7
Light Sources

• For 0.18 and 0.13, we use two excimer lasers (KrF
  at 248 nm and ArF at 193 nm)
• These lasers contain atoms that do not normally
  bond, but if they are excited the compounds will
  form when the excited molecule returns to the
  ground state, it emits
• These lasers must be continuously strobed
  (several hundred Hz) or pulsed to pump the
  excitation
• Can get several mJ of energy out
• Technical problems have been resolved for KrF and
  these are used for 0.25 and 0.18 ?m
• ArF is likely for 0.18 and 0.10 ?m technical
  problems remain

8
Exposure System

• There are three classes of exposure systems
• Contact
• Proximity
• Projection

9
Exposure System

• Contact printing is the oldest and simplest
• The mask is put down with the Cr in contact with
  the wafer
• This method
• Can give good resolution
• Machines are inexpensive
• Cannot be used for high-volume due to damage
  caused by the contact
• Still used in research and prototyping situations

10
Wafer Exposure Systems

• Proximity printing solves the defect problem
  associated with contact printing
• The mask and the wafer are kept about5 25 ?m
  apart
• This separation degrades the resolution
• Cannot print with features below a few microns
• The resolution improves as wavelength decrease.
  This is a good system for X-ray lithography
  because of the very short exposure wavelength
  (1-2 nm).

11
Wafer Exposure Systems

• For large-diameter wafers, it is impossible to
  achieve uniform exposure and to maintain
  alignment between mask levels across the complete
  wafer.
• Projection printing is the dominant method today
• They provide high resolution without the defect
  problem
• The mask (reticule) is separated from the wafer
  and an optical system is used to image the mask
  on the wafer.
• The resolution is limited by diffraction effects
• The optical system reduces the mask image by 4X
  to 5X
• Only a small portion of the wafer is printed
  during each exposure
• Steppers are capable of lt 0.25 ?m
• Their throughput is about 25 50 wafers/hour

12
Optics Basics

• We need a very brief review of optics
• If the dimensions of objects are large compared
  to the wavelength of light, we can treat light as
  particles traveling in straight lines and we can
  model by ray tracing
• When light passes through the mask, the
  dimensions of objects are of the order of the
  dimensions of the mask
• We must treat light as a wave

13
Optics Basics

• Diffraction occurs because light does not travel
  in straight lines
• Pass a light through a pin-hole we see that the
  image is larger than the hole
• This cannot be explained by ray tracing

14
Diffraction of Light
15
Diffraction of Light

• The Huygens-Fresnel principle states that every
  unobstructed point of a wavefront at a given time
  acts as a point source of a secondary spherical
  wavelet at the same frequency
• The amplitude of the optical field is the sum of
  the magnitudes and phases
• For unobstructed waves, we propagate a plane wave
• For light in the pin-hole, the ends propagate a
  spherical wave.

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Diffraction of Light
17
Youngs Single Slit Experiment
sinq l/d
http//micro.magnet.fsu.edu/optics/lightandcolor/d
iffraction.html
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Amplitude of largest secondary lobe at point Q,
eQ, is given by eQ a(A/r)f(c)d where A is
the amplitude of the incident wave, r is the
distance between d and Q, and f(c) is a function
of c, an inclination factor introduced by
Fresnel.
http//micro.magnet.fsu.edu/optics/lightandcolor/d
iffraction.html
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Youngs Double Slit Experiment
http//micro.magnet.fsu.edu/optics/lightandcolor/i
nterference.html
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Basic Optics

• This diffraction bends the light
• Information about the shape of the pin hole is
  contained in all of the light we must collect
  all of the light to fully reconstruct the pattern
• The following diagram shows how the system works
• Note that the focusing lens only collects part of
  the diffraction pattern
• The light diffracted at higher angles contains
  information about the finer details of the
  structure and are lost

21
Basic Optics
22
Basic Optics

• The image produced by this system is

23
Basic Optics

• The diameter of the central maximum is given
  by
• Note that you get a point source only if d ? ?

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Basic Optics

• There are two types of diffraction
• Fresnel, or near field diffraction
• Fraunhofer, or far field diffraction
• In Fresnel diffraction, the image plane is near
  the aperture and light travels directly from the
  aperture to the image plane (see Figure 5-4)
• In Fraunhofer diffraction, the image plane is far
  from the aperture, and there is a lens between
  the aperture and the image plane (see Figure 5-6)
• Fresnel diffraction applies to contact and
  proximity printing while Fraunhofer diffraction
  applies to projections systems
• There are powerful simulations systems for both
  cases

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Fraunhofer Diffraction

• We define the performance of the system in terms
  of
• Resolution
• Depth of focus
• Field of view
• Modulation Transfer Function (MTF)
• Alignment accuracy
• throughput

26
Fraunhofer Diffraction

• Imagine two sources close together that we are
  trying to image (two features on a mask)
• How close can these be together and we can still
  resolve the two points?
• The two points will each produce an Airy disk
  (5-7)
• Lord Rayleigh suggest that we define the
  resolution by placing the maximum from the second
  point source at the minimum of the first point
  source

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Fraunhofer Diffraction
28
Fraunhofer Diffraction

• With this definition, the resolution
  becomes
• For air, n1
• ? is defined by the size of the lens, or by an
  aperture and is a measure of the ability of the
  lens to gather light

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Fraunhofer Diffraction

• This is usually defined as the numerical
  aperture, or NA
• This really is defined only for point sources, as
  we used the point source Airy function to develop
  the equation
• We can generalize by replacing the 0.61 by a
  constant k1 which lies between 0.6 and 0.8 for
  practical systems

30
Fraunhofer Diffraction

• From this result, we see that we get better
  resolution (smaller R) with shorter wavelengths
  of light and lenses of higher numerical aperture
• We now consider the depth of focus over which
  focus is maintained.
• We define ? as the on-axis path length difference
  from that of a ray at the limit of the aperture.
  These two lengths must not exceed ?/4 to meet the
  Rayleigh criterion

31
Depth of Focus
32
Depth of Focus

• From this criterion, we have
• For small ?

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Fraunhofer Diffraction

• From this we note that the depth of focus
  decreases sharply with both decreasing wavelength
  and increasing NA.
• The Modulation Transfer Function (MTF) is another
  important concept
• This applies only to strictly coherent light, and
  is thus not really applicable to modern steppers,
  but the idea is useful

34
Fraunhofer Diffraction

• Because of the finite aperture, diffraction
  effects and other non-idealities of the optical
  system, the image at the image plane does not
  have sharp boundaries, as desired
• If the two features in the image are widely
  separated, we can have sharp patterns as shown
• If the features are close together, we will get
  images that are smeared out.

35
Modulation Transfer Function
36
Fraunhofer Diffraction

• The measure of the quality of the aerial image is
  given by
• The MTF is really a measure of the contrast in
  the aerial image
• The optical system needs to produce MTFs of 0.5
  or more for a resist to properly resolve the
  features
• The MTF depends on the feature size in the image
  for large features MTF1
• As the feature size decreases, diffractions
  effects casue MTF to degrade

37
Change in MTF versus Wavelength
38
Contrast and Proximity Systems

• These systems operate in the near field or
  Fresnel regime
• Assume the mask and the resist are separated by
  some small distance g
• Assume a plane wave is incident on the mask
• Because of diffraction, light is bent away for
  the aperture edges
• The effect is shown in the next slide
• Note the small maximum at the edge this results
  from constructive interference
• Also note the ringing
• As a result, we often use multiple wavelengths

39
Fresnel Diffraction
40
Fresnel Diffraction

• As g increases, the quality of the image
  decreases because diffraction effects become more
  important
• The aerial image can generally be computed
  accurately whenwhere W is the feature size
• Within this regime, the minimum resolvable
  feature size is

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Depth of Focus
http//www.research.ibm.com/journal/rd/411/holm1.g
if
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Summary of the Three Systems