Haze Aberration Detection using Weir PW

1
Haze Aberration Detection using Weir PW

• Haze aberration has been shown to result in
  side-lobe formation next to the immediate edge.
• Side-lobe formation results not from the
  mathematical Gibbs Phenomenon description but
  rather from the physical introduction of
  Spherical Aberration and other perturbations into
  the optical train
• The reticle is an intimate part of the optical
  system of the scanner.
• Haze introduces scatter and aberrations that
• Result in image perturbations that reach way
  beyond the immediate deposition
• Influence adjacent features
• Result in perturbations even when they do not
  touch the feature.
• First degrade the Bossung Response of the reticle
  image and eventually the dose required to create
  an on-size image.

2
Reticle Haze
Haze formation on feature edges does directly
influence the edge, however the effects are more
far-reaching.
Critical Reticle Haze Directly influences this
features definition through side-lobe ringing
artifacts (Gibbs Phenomenon) that distort the
square wave of the chrome or PSM edge
Haze
3
Feature Edge Effects

• Three graphic instances of reticle-face haze
  formation are shown in this illustration.
• Feature Edge formations
• Most likely to form since they represent they
  form on high-energy initiation seed sites where
  edge scatter and depositions will first adhere to
  the mask. Previous presentation illustrated the
  influence based on the Gibbs effect that results
  in square-wave degradation by image ringing
  resulting in edge-intensity over/under shoot and
  in side-lobe generation
• Clear area Haze
• Not addressed as a factor
• On-Feature Haze formation
• Not addressed as a factor

On Obscuration Feature Haze Formation Influence
on Feature profile?
Critical Reticle Haze Directly influences this
features definition through side-lobe ringing
artifacts (Gibbs Phenomenon) that distort the
square wave of the chrome or PSM edge
Haze Influence on Feature Response?
4
Seed sites for Haze

• Haze deposition first forms on high-energy areas
  known as seed-sites
• Seed sites are not singularities forming at one
  or two isolated points
• Haze initiation will form across an extended
  area of the mask surface
• Formation is a function of the interaction of the
  feature loading
• localized optical wavefront characteristics
  (lens edge verses center)
• Wavelength of illumination
• Local surface contaminants on the mask
• From manufacture
• Cleaning
• Also tend to located on high-energy feature edges
• Areas where edges are undercut or chrome is
  thinned from etch.
• Areas of unequal etch or PSM feature thickness
  that result in non-optimum wave extinction during
  phase shifting.

5
Knife-edge optical effects
To see the effects of haze formation at a feature
edge consider the opticians knife-edge. The thin
chrome obscuration acts as a knife edge
discontinuity. Knife edge analyses have been used
for years in optics development because they
allow the aberrations of the lens to be
accurately measured. The Profile at the edge of
the knife edge is NOT a pure Dirac step function
as assumed in the Gibbs Phenomenon. It is a
complex Intensity gradient that incorporates the
basic Gibbs Phenomenon artifact plus a stronger
variance caused by optics- limited distortions,
scatter and localized changes in the effective
Numeric Aperture caused by the finite edge. This
results in an intensity profile that behaves like
a Gibbs function but is actually stronger in
intensity than described in the original
presentation. The Chrome is not a true knife edge
in that its thickness is actually many
wavelengths across. The thickness therefore
directly compounds profile changes by
polarization and coherence perturbations.
Intensity Profile
Chrome
6
Scatter effects
The chrome feature image is further complicated
in that it is supported by a quartz substrate.
The wavefront at the feature edge therefore
encounters a change in the index of refraction
(Quartz-to-Air) at the same time that it
encounters the chrome feature obscuration. The
index change at the edge results in scatter and
this in turn reduces the edge resolution. The
effect also interacts with the image wavefront to
induce localized aberrations. ATT, in 1982, was
issued a series of patents for glass photomask
coverplates to protect the chrome photomask
elements. The coverplate interface to the mask
incorporated an index matching fluid to prevent
this scatter and reflection interference. This
patent also noted the improvement in image
resolution and depth-of-focus that resulted
because the chrome was now encapsulated in a
continuous index of refraction and scatter was
eliminated.
scatter
Intensity Profile
Air
Wavefront
7
Open-area haze formation

• Haze does not form randomly. It needs a
  high-energy seed-site.
• Seed-sites therefore start areas containing
• localized damaged from repair
• Undercut
• Impurities or localized stress in the quartz
  substrate
• The resulting wavefront will be a convolution of
  the intensity profile across the hazed area PLUS
  the chrome edge profiles from nearby features as
  far as 2 microns away PLUS the scatter added by
  the chrome edge, haze edge and internal haze
  phase boundaries from acrylic crystalline
  transitions.
• The translucent haze area also behaves as a
  micro-lens and introduced refractive aberrations
  that further interfere with the wavefront.
• Summary Isolated haze introduces wavefront
  distortions and aberrations that influence nearby
  features.

Haze profile
Chrome edge profile
8
Chrome-obscured haze
Chrome is not a complete blocker of the
wavefront. Its complex index of refraction
results in a portion of the electromagnetic wave
that penetrates and thin film and interacts with
the overall image formation. In short, chrome is
translucent even at deep-UV illumination. The
wavefront intensity and phase immediately above
the chrome surface is not zero. A haze element
will react with the chrome causing thinning,
cracking and other localized physical
reactions. Scatter from other parts of the
imaging layer will interact with this wavefront
and also be gathered by the lenticular behavior
of the translucent haze. This results in
localized aberrations of near edge images not
directly involved with the haze seed. Summary On
chrome haze has a smaller but still finite
perturbing influence on the wavefront that
introduces aberrations and scatter.
9
Effects of Haze Seed Formation Summary

• Haze does not form on isolated singularities
• Haze formation is a high-energy area effect.
• Haze does not have to be intimately associated
  with a feature edge to influence image formation
• Early-formation isolated segments act as
  micro-lens elements
• On-feature surface haze influences overall
  scatter and dark-image formation.
• The image of the photomask is converted to a
  frequency spectrum at the entrance pupil of the
  lens. Scatter and aberrations from haze change
  this spectrum and also change the influence of
  the inherent lens aberrations on the image that
  results in large-area image degradation
• All lenses retain finite coma and spherical
  aberration as balanced aberrations tuned to the
  ideal photomask image.

10
Consider Thirty years of process windows
Collapsed line
BCD

• J. Bossung, SPIE 1977 vol. 100
• This is a well established technique

Next few slides are from TEA Systems Class
Lithograph Control and Characterization
11
Review Bossung curve analysis

• A Isofocal Dose
• Dose at which feature size is independent of
  Focus
• B Locus of Best Focus
• Best Focus is located at the maxima or minima
  of each dose curve
• The greater the curvature, the greater will be
  the aberrations of the system
• UCL/LCL
• Upper and Lower Control Limits for the process
• EL
• Exposure Latitude or the dose range over which
  the feature size lies between the UCL and LCL
• DoF
• Depth of Focus or the focus range over which the
  feature size lies between the UCL and LCL

Zernike Analyses are a quantitative method of
lens aberration analysis. Bossung curve
characteristics can show the presence and effect
of aberrations. More strongly than dose
reduction, HAZE CAUSES ABERRATIONS
12
Typical Bossung Focus analysis for center-site
Field Layout with measured-site shown in red.
J. W. Bossung, SPIE (1977) Vol. 100
13
Sites from field center 4 corners
Plotted sites in red

• An aberration free lens would result in exactly
  duplicated feature response.
• The scatter and aberrations caused by localized
  haze result in this phenomenon.
• More haze more scatter Image perturbations

14
Ideal focus/aberration response of features
Feature Size

• Aberration free features result in
• Linear feature size response to dose (blue line)
• An unchanging Best Focus response (flat) of the
  features for changes in dose as shown here for
  the near-zero change in focus with dose

15
Aberration influence on Feature response
Cd vs size curves
Feature Size
Dense Best Focus
Isolated Best Focus

• In this example of Contact response and proximity
  we see
• Un-influenced (flat) Best Focus response for
  widely separated contacts (blue black lines)
• Aberrated Best Focus response (red line) for
  small, dense vias
• The onset of Haze introduces aberrations that can
  be seen much sooner than simple dose change from
  neutral density obscuration effects.

16
Side Lobe
Cause Aberration and scatter NOT dose-change

• In experimental SEMs, side lobes are seen inside
  a line (left) and outside a trench (right )
• Figure shows experimental SEMs of side lobes for
  a line and trench for 8 attPSM. Because the
  Gibb's phenomenon takes place on either side of
  the discontinuity, the side lobes can be seen
  inside the line and outside the trench.

17
Side Lobe
Notice the perturbation of CD and feature
profiles as a result of the side-lobes resulting
from induced aberrations
18
Influence of Spherical Aberration

• Calculated with 3rd 5th Orders

i-line rim PSM 0.35 um contact hole SEM
(overexposed)
10 attenuation PSM, 0.35 um hole NA0.5, DUV,
s0.3
From TEA Systems Class Lithograph Control and
Characterization
19
Side Lobe Formation

• The intensity of a side lobe increases with
  higher transmission.
• However the stronger effect is the aberration of
  the wavefront emanating from the local hazed area
• Wavefront will influence both the immediate
  feature AND other nearby features through the
  introduction of spherical aberrations into the
  image.

20
Conclusions 1

• Previously Shown
• Resist erosion is inevitable, however, when using
  att PSM with higher transmissions.
• The constructive interference among the secondary
  maxima of nearest neighbors increases the
  intensity of side lobes.
• The worst situation is when the secondary maxima
  of four neighboring contact holes interact at
  their diagonal interaction and produce maximum
  intensity regions.
• Now Recognize
• The overexposures shown previously do not
  illustrate the effect of the haze on overall
  profile and process response.
• Process aberrations will extend well beyond the
  hazed area

21
Conclusions 2

• Weir PW
• Aberrations are quickly discovered through the
  Weir PW analysis of perturbation and feature
  response uniformity across the full-wavefront
  process window.
• The influence of the haze-induced aberration
  directly upon the process-robustness of the
  reticle feature can be directly measured using
  our techniques.
• This technique will discover the onset of haze
  formation very much earlier than reflectance or
  transmission intensity monitors
• The following slides illustrate the Weir PW tools
  for detection, identification and location of the
  influence of Haze Formation.

22
Process-Window Derived Feature and DoF
Focus Uniformity
Focus Response Analysis
Depth of Focus Uniformity
The Feature Derived Best Focus is next
calculated for every site on the field. This
focus-contour is not the exact focus-wavefront of
the lens but it is the response surface
experienced by the measured feature and rapidly
degrades with the onset of Haze. Similarly the
features Depth-of-Focus (DoF) can be visualized
for every point on the exposure.
23
Isofocal Analysis
Isofocal deviant curves
Aberration level at each dose
Dose Response Analysis

• Isofocal Analysis All-sites on field, BCD
  features Vertical (black) and Horizontal (red)
• Calculate IsoFocal dose for each feature and site
• Classic IsoFocal point is found when the 2nd
  derivative of the process window 0
• Bossung IsoFocal deviant the magnitude of the
  3rd Bossung curve coefficient
• Isofocal point is at the minimum for the curve
• 2) Lower curves Level of aberrations for the
  dose plotted as magnitude of 2nd Bossung curve
  coefficient
• 3) IsoFocal performance is highly sensitive to
  both lens and feature design and so responds to
  Haze

24
Exposure Latitude _at_ Best Focus
Exposure Latitude at Best Focus
BCDv
Dose Analysis Derivatives
BCDh
Contour Plot
Vector Format Plot
The Exposure Latitude Percentage uniformity can
now be plotted for the field at Best Focus
providing an improved characterization for resist
setup. Any point modeled in the field will see
the influence of nearby haze incidence and EL
will degrade as haze increases
25
Dose Uniformity _at_ BF for 80 nm
BCD Horizontal
Dose Analysis Derivatives
BCD Vertical
Focus errors removed and having calculated
the Feature v Dose response for each site, we
can now calculate the optimum dose needed to
obtain the feature target value at each site.
These contours still contain reticle
non-uniformity, lens aberration and
scan-perturbations. Haze onset will quickly
degrade this reticle and scanner specific
signature
26
At Best Focus/dose for 80 nm
BCDh
Focus Error at 22 mj/cm2
BCDv
27
Conclusions 3

• Weir PW
• The Weir PW techniques directly measure the
  process degradation of the reticle to feature
  profiles, loss of Exposure Latitude and
  Depth-of-Focus reduction through modeling of the
  response across the entire image wavefront.
• Weir PW answers the need for rapid haze detection
  and the avoidance of process yield loss long
  before the effects are noticed through the
  demands of the haze for increase exposure-dose.
• When first detected, the user can monitor the
  gradual degradation in full-field process window
  and conveniently schedule reticle cleaning or
  process correction