1. Lithography - Chapter 5
LITHOGRAPHY
• Lithography is arguably the single most important technology in IC manufacturing.
• The SIA ITRS is driven by the desire to continue scaling device feature sizes.
Reduction of minimum size x0.7 
X-ray, e-beam, extreme UV, ion beam
• 0.7X in linear dimension every 3 years. Control of CD ~10% smallest size
• Placement accuracy ≈ 1/3 of feature size.
• ≈ 35% of wafer manufacturing costs for lithography.
• Note the ??? - single biggest uncertainty about the future of the roadmap.
SILICON VLSI TECHNOLOGY
Fundamentals, Practice and Modeling
By Plummer, Deal & Griffin
© 2000 by Prentice Hall
Upper Saddle River NJ

2. Historical Development and Basic Concepts
• Patterning process consists
of mask design, mask
fabrication and wafer
printing.
• It is convenient to divide the
wafer printing process into three
parts A: Light source, B. Wafer
exposure system, C. Resist.
• Aerial image is the pattern of
optical radiation striking the top
of the resist.
• Latent image is the 3D replica
produced by chemical processes
in the resist.

3. A. Light Sources
• Decreasing feature sizes require the use of shorter
• Traditionally Hg vapor lamps have been used which generate many spectral
lines from a high intensity plasma inside a glass lamp.
• (Electrons are excited to higher energy levels by collisions in the plasma.
Photons are emitted when the energy is released.)
• g line -
• i line (used for 0.5 µm, 0.35 µm)
• Brightest sources in deep UV are excimer lasers
(1)
• KrF (used for 0.25 µm, 0.18µm, 0.13 µm)
• ArF (used for 0.13µm, 0.09µm, )
• FF (used for ??)
• Issues include finding suitable resists and transparent optical components at
these wavelengths.

4. B. Wafer Exposure Systems
• Three types of
exposure systems
have been used.
5-25µm
• Contact printing is capable of high resolution but has unacceptable defect
densities.
• Proximity printing cannot easily print features below a few µm (except for
x-ray systems).
• Projection printing provides high resolution and low defect densities and
dominates today.
• Typical projection systems use reduction optics (2X - 5X), step and repeat or step
and scan mechanical systems, print ≈ 50 wafers/hour and cost $ M.

5. B1. Optics - Basics and Diffraction
• Ray tracing (assuming light travels in straight lines) works well as long as the
dimensions are large compared to .
• At smaller dimensions, diffraction effects dominate.
Small aperture
More spherical propagation
Aperture - fewer wavelets
Plane wave continues due to superposition
small
Free space
Spherical secondary wavelets - same
Huygens-Fresnel principle
• If the aperture is on the order of , the light spreads out after passing through
the aperture. (The smaller the aperture, the more it spreads out.)

6. Diffraction • If we want to image the aperture
A point source does not produce a point image:
• 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).
Lost information 
• A simple example
is the image formed
by a small circular
aperture (Airy disk).
• Note that a point image
is formed only if ,
or . 
Second rings
• Diffraction is usually described in terms of two limiting cases:
• Fresnel diffraction - near field.
• Fraunhofer diffraction - far field.

7. B2. Projection Systems (Fraunhofer Diffraction)
• These are the dominant systems in use today. Performance is usually described
in terms of: • resolution
• depth of focus
• field of view
• modulation transfer function
• alignment accuracy
• throughput
• Consider this basic optical projection
system.
• Rayleigh suggested that a reasonable
criterion for resolution is that the
central maximum of each point source
lie at the first minimum of the Airy
pattern.
• With this definition,
How to recognize A&B f
(2)
(3)
• The numerical aperture of the lens is by definition,
• NA represents the ability of the lens to collect diffracted light.

8. Parameters of Projection Systems
Resolution and Numerical Aperture
(4)
• k1 is an experimental parameter which depends on the lithography system and resist properties (≈ ).
• Obviously resolution can be increased by:
• decreasing k1
• decreasing
• increasing NA (bigger lenses)
• However, higher NA lenses also decrease the depth of focus.
Depth of focus DOF=  =/2(NA)2=k2/2(NA)2 NA   DOF  , but  R  Flat topology!
Ex. KrF, 248nm, NA=0.6, k1=0.75, k2=0.5, R=0.31m, DOF =0.34 m
• k2 is usually experimentally determined.
• Thus a 248nm (KrF) exposure system with a NA = 0.6 would have a resolution
of ≈ 0.3 µm (k1 = 0.75) and a DOF of ≈ ± 0.35 µm (k2 = 0.5).

9. Projection Immersion lithography9

10. Immersion lithography10

11. Hyper-NA Immersion Litho Tool
45-nm node (ASML) 11

12. Renesas process-technology roadmap.12

13. • Modulation Transfer Function (MTF) – a measure of the contrast that can be recognized by the resist.
-> 0.5 for feature recognition by PR
(6)
MTF depends on the feature size and on the spacial coherence of the light source
S = light source diameter/condenser diameter =s/d = NAcondenser optics/NAprojection optics
MTF applies to coherent light (steppers have partly coherent illumination).
Separated large structures 4-5x larger
Ideal mask aerial image
Structures closer: Due to partial overlapping of two airy discs
MTF=1
Resist has t o respond to the light intensity

14. Modulation Transfer Function
Plane wave
Modulation Transfer Function
Ideal point source
The same angle of arrival
• Finally, another basic concept is the
spatial coherence of the light source.
Spatially coherent source
• Practical light sources are not point
sources.
the light striking the mask will not be
plane waves.
Various angles S
not point
• The spatial coherence of the system is
defined as
(7)
or often as
(8)
• Typically, S ≈ 0.5 to 0.7 in modern systems.

15. Modulation Transfer Function
S=light source diameter/condenser diameter=s/d=NAcondenser optics/NAprojection optics
s≈ for s->0 optical intensity decreases
Diffraction effect  MTF
Less coherent light
Improvement for very small features
Lower contrast in the aerial image

16. B3. Contact and Proximity Systems (Fresnel Diffraction)
• Contact printing systems operate in the near field or Fresnel diffraction regime.
• There is always some gap g between the mask and resist.
small
Ringing effect  by incoherent light
Some resist exposure
W=feature size
Huygens is pronounced “Hoighens” - Dutch astronomer
Ringing due to constructive and destructive interference
• The aerial image can be constructed by imagining point sources within the
aperture, each radiating spherical waves (Huygens wavelets).
• Interference effects and diffraction result in “ringing” and spreading outside
the aperture.

17. Contact and Proximity Systems
(9)
• Fresnel diffraction applies when
• Within this range, the minimum resolvable feature size is
(10)
• Thus if g = 10 µm and an i-line light source is used, Wmin ≈ 2 µm.
• Summary of wafer printing systems:
The aerial image here can be calculated if <g<W2/ 
The minimum feature size that can be resolved here is Wmin(g)1/2
365 nm
10 m
W ≈ 2µm
For contact printing -> 0.1 µm (not used!)

18. C. Photoresists
hydrocarbon-based materials: they change their chemical properties upon illumination by photons
• Resists are organic polymers that are spun onto wafers and prebaked to produce
a film ≈ µm thick.
Negative(older –resolution limited by swelling)- more soluble when not exposed. Positive – more soluble when exposed. Important parameters:
Sensitivity – how much time is reqd. for changes [mJcm-2] (Ex. 100 mJcm-2 for g line and i-line resists, newer down to 20 mJcm-2 .
Resolution - exposure system rather not the resist limits the Resolution
Robustness to etching.
Photoresists for g-line and i-line: a hydrocarbon inactive resin, a (hydrocarbon) photoactive compound(PAC), and a solvent: PAC replaced by a Photo Acid Generator (PAG) to act as a chemical amplifier for DUV P.R.
G-line and I-line resists : Diazonaphtoquinones DNQ+resin.

19. g-Line and i-Line Resists
• Generally consist of 3 components:
• Inactive resin
• Photoactive compound (PAC)
• Solvent - used to adjust viscosity
• After spinning and baking resists ≈ 1:1 PAC and resin.
• Diazonaphthoquinone or DNQ resists are commonly used today for g-line and
i-line resists.
Two methyl groups &OH
• The base resin is novolac a long chain polymer consisting of hydrocarbon rings
with 2 methyl groups and 1 OH group attached. Novolac –inactive resin : would dissolve in a developer 15nm/s

20. • The PACs in DNQ resists are often diazoquinones.
Photoactive compound  in soluble developers it  solubility of PR 1-2nm/s
• The PACs in DNQ resists
are often diazoquinones.
The photoactive portion is
above the SO2.
• Diazoquinones are insoluble
in typical developers and reduce
the dissolution rate of unexposed
resists to ≈ nm sec-1.
Exposure
Easy dissolution in a basic developer NaOH/H2O , TMAN etc.
nm/sec
light
Stabilizes itself
• After exposure to light, the PAC
component in DNQ resists undergoes
a transformation (Wolff rearrangement)
into carboxylic acid which is soluble
in the developer (basic solution).
In water carboxylic

21. DUV Resists
• g-line and i-line resists have maximum quantum efficiencies < 1 and are
typically ≈ 0.3.
• Chemical amplification can improve this substantially.
• DUV resists all use this principle. A catalyst is used.
• Photo-acid generator (PAG) is converted to an acid by photon exposure. Later, in
a post exposure bake, the acid molecule reacts with a “blocking” molecule on a
polymer chain, making it soluble in developer AND REGENERATING THE
ACID MOLECULE
 catalytic action  sensitivity is enhanced.
These molecules make the resist insoluble in developer.
Acid labile group
Photo Acid Generator
T< 1 °C
120 °C  TEMPERATURE CONTROL
Acid changes the PR properties in baking – reacts with INSOL and converts it into soluble molecules in alkali developers.
Soluble in aqueous alkaline developer
Catalytic behavior of acids - reaction repeated (20-40mJcm-2)
Sensitivity (also easily affected by contaminations, dust etc.)
5x more than DNQ

22. Basic Properties of Resists
• Two basic parameters are used to describe resist properties, contrast and the
critical modulation transfer function or CMTF.
Contrast allows distinguishing light and dark areas on the mask. DUV resists have better contrast and better sensitivity because of chemical amplification.
• Contrast is defined as
(11)
Affected by processing conditions also
CONTRAST = SLOPE
• Typical g-line and i-line resists achieve values of and Df values of about
100 mJ cm-2. DUV resists have much higher values (5 - 10) and Df values of about
mJ cm-2. DUV: better contrast and better sensitivity.

23. Resists
• The aerial image and the resist contrast in combination, result in the quality of
the latent image produced. (Gray area is “partially exposed” area which
determines the resist edge sharpness.)
• By analogy to the MTF defined earlier for optical systems, the Critical Modulation Transfer Function (CMTF) for resists is defined as
(12)
• Typical CMTF values for g and i-line resists are about 0.4. Chemically amplified
DUV resists achieve CMTF values of (because of higher sensitivity).
• Lower values are better since in general CMTF < MTF is required for the resist
to resolve the aerial image.

24. Exposure Uniformity Degradation
• There are often a number of additional issues that arise in exposing resist.
Where z is the depth and  is the optical absorption. ( More photons absorbed at the surface)
Bleaching effect : Absorption of incoming photons decreases in already absorbed regions. It allows for uniformity improvements.
Non uniform resist thickness (hills and valleys).
Overexposure (bigger size)
Underexposure
Light intensity decreases
• Resist is applied as a liquid and hence "flows" to fill in the topography.
Resist thickness may vary across the wafer. This can lead to under or over
exposure in some regions and hence linewidth variations.

25. Standing Wave During Exposure
• Reflective surfaces below the resist
can set up reflections and standing
waves and degrade resolution.
• In some cases an antireflective coating
(ARC) can help to minimize these
effects. Baking the resist after
exposure, but before development
can also help.
Light reflected by the layer below PR .
Use Anti-Reflective coating to prevent SANDING WAVE.
Post-baking (before development) “scalloped edges” due to diffusion of PAC or PAG
(Photo courtesy of A. Vladar and P. Rissman, Hewlett Packard.)

26. Typical resist process: (Smaller dose for DUV resists) (Required for
(g and i-line resist parameters)

27. SubWavelength Lithography
(From Synopsis Website -
• Beginning in ≈ 1998, chip manufacturers began to manufacture chips with
feature sizes smaller than the wavelength of the light used to expose photoresist.
• This is possible because of the use of a variety of “tricks” - illumination system
optimization, optical pattern correction (OPC) and phase shift mask techniques.