chapter 5: lithography
DESCRIPTION
Chapter 5: Lithography. Introduction. The mechanism to print 2-D patterns to a thin film layer on the wafer surface. Masks are glass plates (soda lime or quartz glass) that contain the patterns. The patterns are first transferred from the mask to photoresist (PR), a light-sensitive polymer. - PowerPoint PPT PresentationTRANSCRIPT
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Chapter 5: Lithography
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Introduction
The mechanism to print 2-D patterns to a thin film layer on the wafer surface.
Masks are glass plates (soda lime or quartz glass) that contain the patterns.
The patterns are first transferred from the mask to photoresist (PR), a light-sensitive polymer.
After opening windows in the PR, the pattern is transferred to the thin film using etching techniques.
Complexity of a fabrication process is often measured by the number of photolithographic masks used in the process.
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Introduction
The concept is simple– Spin on a thin layer of light-sensitive photoresist– Selectively expose it to UV light
Causing chemical bonds to either form or break
– Develop to selectively remove the lighter weight PR The resist may be used as a mask for either etching or
for ion implantation Because of constraints of resolution, exposure field,
accuracy, throughput, and defect density, the implementation is not so simple– Very expensive– Very complex
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Introduction
• Steps in the mask fabrication process:
Designing 2D layout using CAD
tools
Transfer data to pattern generator
(mask maker)
Pattern generation on the mask plate coated with Cr&PR
Etching PR and then Cr
Inspection
Stripping PR
Glass plate with Cr
Only Glass
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Introduction
Mask Maker
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Introduction
• Steps in the photolithography
Clean wafer
deposit film (oxide, nitride, metal, …)
Coat with PR
Soft bake
Align masks
Expose Pattern
Develop PR
Hard Bake
Etch the deposited film
Remove PR
Typical for 1800 Series PR:Soft Bake: 110°C for 1min on a hotplateHard Bake: 110°C for 3min on a hotplate
PR1813 1.3µm @ 4krpm & 30secPR1827 2.7µm @ 4krpm & 30sec
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Introduction
Spinner
Hotplate
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Introduction
Mask Aligner
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Introduction
1
2
3,4
5,6
6
UV
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Introduction
7,8
9
10
Be aware that there are two different types of PR:
Positive PR: exposed areas will be developed
Negative PR: exposed areas will not be developed
Some common PRs:
1800 series (for thin) will be developed in MF 319
9200 series (for thick) will be developed in AZ 400
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Alignment Markers
Once a photolith process is done, the pattern developed is used to perform some additional process selectively on the wafer
– Etching trenches in Si or SiO2
– Making metalization runs– Implantation of dopants
Then the wafer will come back for another photolith step
Alignment markers are registration patterns that mate from one mask to another so that the multiple pattern sets match one another.
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Introduction
Positive resists provide better controllability for small features.
Positive resists are easier to work with and use less corrosive developers and chemicals.
Positive resists are the dominant type of photoresists today.
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Clear Field and Dark Field Masks
Most photolith engineers prefer clear field masks when possible– Easier to detect pattern on the wafer itself
as there is more clear glass in the mask
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Introduction
Demands placed on this process for– Resolution: smaller device structures– Exposure field: ever-increasing chip sizes– Placement accuracy: aligning with existing
layers– Throughput: manufacturing cost– Defects: yield and cost
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NTRS Lithography Requirements
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Introduction
The National Technology Roadmap for Semiconductors defines the future needs
Note especially– The driving force is the reduction of feature size– For every factor of two in reduction of area, there is
a reduction of 0.7 in the linear dimensions– The reduction is required every three years– The most commonly quoted feature size is not as
small as isolated MOS gate lines– Critical dimension (CD) control must improve (about
10% of minimum feature size)– Alignment accuracy must be about 1/3 of minimum
feature size– The printing area increases with time since we must
print one full die at a time
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Introduction
About 1/3 of the cost of a wafer cost (about $1000 for an 8-inch wafer) is associated with lithography; we have only a few hundred dollars per wafer to spend– Optical lithography is used down to
0.13 m (130nm) generations– For smaller dimensions, X-ray, direct e-
beam, or extreme UV (EUV) processes are used.
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Basic Concepts
We generally separate lithography into three parts– The energy source (photons or electrons)– The exposure system– The resist
The exposure tool, which includes the light source and the exposure system, creates the best image possible on the resist (resolution, exposure field, depth of focus, uniformity and lack of aberrations)– Optimization of the photoresist with the settings on
the exposure tool transfers the aerial image from the mask to the best thin film replica of the aerial image
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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)
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Light Source
A much smaller set of wavelengths used to expose the resist– to minimize optical distortion associated
with the lens optics.– to match the properties of the resist
Pick the wavelength that is heavily absorbed and causes changes in resist chemical properties
Two common monochromatic selections are the g-line at 436 nm and the i-line at 365 nm.
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UV Light Sources
To expose < 250nm wide lines, we need to use shorter wavelength light– 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 UV light
– These lasers must be continuously strobed (several hundred Hz) or pulsed to pump the excitation; can get several mJ of energy out
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Excimer Lasers
Low reliability due to etching of the electrodes and the optical windows by the energitic F ions
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E-beam Source
http://cmi.epfl.ch/metrology/img/LEO1550/LEOColumn.gif
Field Emission Gun (3), which provides the source of Field Emission Gun (3), which provides the source of the electron beam, is a W or LaFthe electron beam, is a W or LaF66 filament. filament.
Condenser Lens (7) are pairs of electromagnets that Condenser Lens (7) are pairs of electromagnets that are used to collimate the beam of electrons.are used to collimate the beam of electrons.
Beam Booster, composed of Anode (5), Vacuum Beam Booster, composed of Anode (5), Vacuum Tube (6), Apertures (8), Alignment Coils (9a, b, c), Tube (6), Apertures (8), Alignment Coils (9a, b, c), Stigmator (13), and Isolating Valve (15) is used to Stigmator (13), and Isolating Valve (15) is used to determine the energy of the electrons and to remove determine the energy of the electrons and to remove the electrons moving off-axis.the electrons moving off-axis.
Objective Lens (10,11) is another set of Objective Lens (10,11) is another set of electromagnets that focuses the electron beam onto electromagnets that focuses the electron beam onto the specimen (12), also containing the Deflecting the specimen (12), also containing the Deflecting System (14), which is another set of electromagnetics System (14), which is another set of electromagnetics that sweep the electrons across the field of view and that sweep the electrons across the field of view and off of the sample .off of the sample .
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X-Ray Source
High energy electrons collide with a metal. The transfer of energy results in the release of x-rays (short wavelength photons).
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Exposure System
There are three classes of exposure systems– Contact– Proximity– Projection
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Exposure System
Contact printing is the oldest and simplest The mask is put with the absorbing layer face
down 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
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Wafer Exposure Systems
Proximity printing solves the defect problem associated with contact printing– The mask and the wafer are kept about
5 – 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 b/c of the very short exposure wavelength (1-2 nm).
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Projection/Step and Repeat
For large-diameter wafers, it is impossible to achieve uniform exposure and to maintain alignment between mask levels across the complete wafer.– Masks are now called reticules
Projection printing is the dominant method today– They provide high resolution without the defect
problem– The mask 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 < 0.25 m– Their throughput is about 25 – 50 wafers/hour
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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
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Snell’s Law and Reflectivity
n1 sin(1) = n2 sin(2)
1 = T+R+A, where T is transmissionR is reflectionA is absorption
If 1 = /2, 2 = sin-1(n1/n2)
R = [(n1-n2)/(n1+n2)]2
http://scienceworld.wolfram.com/physics/SnellsLaw.html
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Refractive index of SiO2
http://www.ioffe.ru/SVA/NSM/nk/Oxides/Gif/sio2.gif
R = 3.5 in air
= 365nm
Transmission through two air-glass surfaces is
less than 93.1%.
http://www.mellesgriot.com/products/optics/images/fig5_12.gif
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Snell’s Law/Antireflective Coatings
when the layer thickness,t, is
t = (m+1)/4; m = 0,1,2…
R = 0 when n = (n1n2)1/2
tt
nn11 nn22
nn
http://en.wikipedia.org/wiki/File:Optical-coating-1.png
2
21
221
nnn
nnnR
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Young’s Single Slit Experiment
sin = /d
http://micro.magnet.fsu.edu/optics/lightandcolor/diffraction.htmlhttp://micro.magnet.fsu.edu/optics/lightandcolor/diffraction.html
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Amplitude of largest secondary lobe at point Q, Q, is given by:
Q = (A/r)f()d
where A is the amplitude of the incident wave, r is the distance between d and Q, and f() is a function of , an inclination factor introduced by Fresnel.
http://micro.magnet.fsu.edu/optics/lightandcolor/diffraction.htmlhttp://micro.magnet.fsu.edu/optics/lightandcolor/diffraction.html
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Young’s Double Slit Experiment
http://micro.magnet.fsu.edu/optics/lightandcolor/interference.htmlhttp://micro.magnet.fsu.edu/optics/lightandcolor/interference.html
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Diffraction of Light
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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 phasesFor unobstructed waves, we
propagate a plane waveFor light in the pin-hole, the ends
propagate a spherical wave.
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Diffraction of Light
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Basic Optics
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Basic Optics
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– If only part of the diffraction pattern is
collected and focused on the substrate, the image created is not identical to the one on the mask.
The light diffracted at higher angles contains information about the finer details of the structure and are lost
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Basic Optics
The image produced by this system is
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Basic Optics
The diameter of the central maximum is given by
Note that you get a point source only if d
light ofh wavelengtλ
length focal f
diameter lens focusing
22.1 maximum central ofDiameter
dd
f
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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.
In Fraunhofer diffraction, the image plane is far from the aperture, and there is a lens between the aperture and the image plane.
Fresnel diffraction applies to contact and proximity printing while Fraunhofer diffraction applies to projections systems
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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
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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.
– Lord Rayleigh suggested that the minimum resolution be defined by placing the maximum from the second point source at the minimum of the first point source.
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Fraunhofer Diffraction
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Fraunhofer Diffraction
With this definition, the resolution becomes
For air, n=1 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
light diffracted theof angle half maximum
lens andobject ebetween th material theof refraction ofindex
sin
61.0
sin2
22.122.1
n
nfn
f
d
fR
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Fraunhofer Diffraction
This is usually defined as the numerical aperture, or NA
Defined only for point sources as the point source Airy function was used to develop the equation
A more generalized equation replaces 0.61 by a constant k1 which lies between 0.6 and 0.8 for practical systems.
NAk
NAR
nNA
1
61.0
sin
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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
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Depth of Focus
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Depth of Focus
From this criterion, we have
For small
cos4/
22114/
22
2222
22sin
NAk
NADOF
NAf
d
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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
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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.
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Modulation Transfer Function
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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 MTF=1
As the feature size decreases, diffractions effects casue MTF to degrade
MINMAX
MINMAX
II
IIMTF
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Change in MTF versus Wavelength
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Contact and Proximity Systems
These systems operate in the Fresnel regime– If the mask and the resist are separated by some
small distance “g” and a plane wave is incident on the mask, light is diffracted at the aperture edges.
– As shown in next slide, there is 1. Small maximum at the edge from
constructive interference
2. Ringing caused by constructive and destructive interference
To minimize effects, multiple wavelengths of light may be used to expose PR
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Fresnel Diffraction
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Fresnel Diffraction
As g increases, the quality of the image decreases– The aerial image can be computed accurately when
where W is the feature size
– Within this regime, the minimum resolvable feature size is:
– Proximity aligner with a 10 m gap and an i-line source can resolve ~ 2 m features.
2Wg
gW min
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Resolution
A more exact solution for the theoretical resolution for proximity or contact aligners is given by:
Where is the wavelength of light used to exposure the pattern, g is the distance between the bottom of the mask and the top of the photoresist, z is the thickness of the photoresist (typically 0.8-1.2m).
22
3 zgR
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Fresnel Number
Fresnel diffraction when F ≥ 1 Fraunhofer diffraction when F << 1
g
WF
2
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Depth of Focus
http://www.research.ibm.com/journal/rd/411/holm1.gif
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Summary of the Three Systems
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Photoresists
Parameters that determine the usefulness of the resist include:– Sensitivity: a measure of how much light is
required to expose the resist - typically 100mJ/cm2
– Resolution where the effects of exposure, baking, developing should not degrade the quality of the image
– Chemical and physical properties: it must withstand chemical etching, mild temperature excursions, ion implantation
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Photoresists
Photoresists usually contain three components– Inactive resin (usually a hydrocarbon which
forms the base material)– Photoactive compound (PAC) – Solvent which is used to adjust the viscosity
The most common g- and i-line resists use– Diazonaphthoquinones (DNQ) as the PAC– Novolac as the resin– Propylene glycol monomethyl ether acetate
(PGMEA) as the solvent (this has replaced Cellosolve acetate, which is a toxic hazard)
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Basic Structure of Novolac
Novolac is a polymer containing hydrocarbon rings with 2 methyl groups and 1 OH group
The basic ring structure is repeated to form a long chain polymer
Novolac readily dissolves in developer at about 15 nm/s
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Diazoquinone
The photoactive part of the molecule is the part above the SO2
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Diazoquinone
The function of the PAC is to inhibit the dissolution of the resin in the developer– DNQ is essentially insoluble in developer
prior to exposure to light– When dissolved in the resin, DNQ reduce
the resist dissolution rate from ~ 15nm/s to 1-2 nm/s
When the resist is exposed to light, the diazoquinone molecule changes chemically and increases the dissolution rate to ~100nm/s.
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Properties and Characteristics of Resists
Two parameters are used to define the properties of photoresists– Contrast– Critical modulation transfer function (CMTF)
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Contrast
The ability of the photoresist to distinguish between various levels of light intensities.– It is experimentally determined by exposing
the resist to differing amounts of light, developed for a fixed time and measuring the thickness of resist remaining after developing.
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Photoresist Contrast
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Photoresist Contrast
For positive resists, material exposed to low light will not be attacked by the developer; material exposed to large doses will be completely removed
Intermediate doses will result in partial removal The contrast is the slope of this curve and is given by
Typical g- and i-line resists will achieve a contrast of = 2-3 and Qf values of 100 mJ/cm2
O
f
Q
Q10log
1
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Photoresist Contrast
The contrast is not a constant, but depends on process variables such as – development chemistry, – bake times, – temperatures before and after exposure,– wavelength of light, and – underlying structure
It is desirable to have as high a contrast as possible in order to produce the sharpest edges in the developed pattern
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Photoresist Contrast
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Modulation Transfer Function (MFT)
Defined in two points of the lithographic system.– MTF: Measure of the dark versus light intensities in
the aerial image produced by the projection system– CMTF: Measure of the exposed versus unexposed
regions in the high contract image focused on the PR
The CMTF is the minimum optical transfer function necessary to resolve a pattern in the resist– For g- and i-line resists, CMTF 0.4
110
110/1
/1
0
0resist
QQCMTF
f
f
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Effect of Resist Thickness
Resists usually do not have uniform thickness on the wafer– Edge bead: The build-up of resist along the
circumference of the wafer- There are edge bead removal systems
– Step coverage
Centrifugal ForceCentrifugal Force
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Effect of Resist Thickness
The resist can be underexposed where it is thicker and overexposed where it is thinner– This can lead to linewidth variations
Light intensity varies with depth below the surface due to absorption
where is the optical absorption coefficient– Thus, the resist near the surface is exposed first
A process called bleaching in which the exposed material becomes almost transparent (i.e., decreases after exposure)– Therefore, more light goes to deeper layers after
bleaching the near surface layer of PR
)exp()( 0 xIxI
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Photoresist Absorption
If the photoresist becomes transparent and if the underlying surface is reflective, reflected light from the wafer will expose the photoresist in areas we do not want it to.– This leads to the possibility of standing
waves (due to interference), with resultant waviness of the developed resist
We can solve this by putting an antireflective coating on the surface of the substrate before spinning the photoresist increases process complexity
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Standing Waves Due to Reflections
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Standing Waves Due to Reflections
http://www.lithoguru.com/scientist/lithobasics.htmlhttp://www.lithoguru.com/scientist/lithobasics.html
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(a) (b) (c)
Diffusion during a post-exposure bake (PEB) is often used to reduce standing waves.
Photoresist profile simulations as a function of the PEB diffusion length: (a) 20nm, (b) 40nm, and (c) 60nm.
http://www.lithoguru.com/scientist/lithobasics.htmlhttp://www.lithoguru.com/scientist/lithobasics.html
Removal of Standing Wave Pattern
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Mask Engineering
There are two ways to improve the quality of the image transferred to the photoresist– Optical Proximity Correction (OPC)– Phase Shift Masks (PSM)
We note that the lenses in projections systems are both finite and circular but most features on the mask are square.– The high frequency components of the pattern are
lost and the “squareness” of the corners of the pattern disappear.
– Can be taken into account by adjusting feature dimensions and shapes in the masks
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Mask Engineering
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Phase Shift Masks
In a projection system the amplitudes at the wafer add so that closely spaced lines interact; the intensity at the wafer is smeared– If we put a material of proper index of refraction on
part of the mask, we can retard some of the light and change its phase by 180 degree and the two portions of light interfere and cancel out.
The thickness of the PS layer is
n is the index of refraction of the phase shift material
12
nd
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Phase Shift Masks (PSM)
Intensity Intensity pattern is pattern is barely barely sufficient sufficient to resolve to resolve the two the two patterns.patterns.
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Scanning Projection Aligners
Projection aligners have been industry standard for about 20 years– It is easier to correct for aberrations in small
regions than in large Scan a small slit across the mask while the
wafer is simultaneously scanned– Scanning projection aligners must use 1:1
masksPattern on the mask is the same size as the one imaged on the wafer.
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Scanning Projection Printer
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Scanning Projection Systems
Cost effective and has high throughput– Linewidth control for smaller devices is
difficult– As chips became larger, it is more difficult
to produce good full wafer masks– With ULVI and WSI, this system could not
scale and was replaced by systems that exposed only a single die at a time
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Step-and-Repeat Projection Aligners
Exposed a limited portion of the wafer at a time– The image on the wafer is 4-5 times smaller
than the image on the mask or reticule.– Masks thus are much larger, and thus
repairable to some extent Steppers also allow better alignment because
they align on the exposure field rather than for the entire wafer– Wafer can be moved vertically to keep
image plane at some location as the PR
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Off-Axis Illumination
By changing the angle of incidence of the light on the mask, change the angle of the diffracted light– Although some of the diffracted light is lost
in this scheme, much of the higher order diffraction is captured
– As the resolution is decreased, it is harder to make these optics work
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Off-Axis Illumination
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Step and Scan
A hybrid has been developed called a “step-and-scan”, but is very complex and very expensive.
https://www.chiphistory.org/product_content/lm_asml_pas5500-400_step&scan_system_1990_intro.htm
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DNQ/Novolac Resist Process
The details of the process are more complex that described earlier
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DNQ/Novolac Resist Process
We first must consider adhesion– There can be one or more operations
depending on what is under the resist The wafer must be clean before resist is
applied It may need to be heated to a few hundred
degrees to drive off water Adhesion to Si is not as good as to metals and
silicon dioxide– Adhesion promoter, Hexamethyldisilane
(HMDS), may be needed
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DNQ/Novolac Resist Process
Dispensing the resist can be done either with a stationary or a slowly spinning wafer
The solvent evaporates rapidly after dispensing the resist and during the spin– Generally more uniform resist thicknesses
are obtained the faster the wafer is accelerated.
– The faster the final speed, the thinner the resist.
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DNQ/Novolac Resist Process
Exposure times and source intensity are reciprocal—one can reduce exposure times with more intense sources– Exposure time is increase by increasing the bake
temperature (due to decomposition of the PAC and thus decreased sensitivity)
Post-exposure bake is often done before development because the PAC can diffuse and this will eliminate the standing wave pattern
Post-development bake is done to remove standing wave pattern by flowing resist (90-100oC) or increase chemical/mechanical strength of resist (120-150oC) Long UV exposure can also be used to cross-link the
polymer chains in the remaining photoresist
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http://www.research.ibm.com/journal/rd/411/holm4.gif
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Measurement Methods
Measurement of– Mask Features and Defects– Resist Patterns– Etched Features– Alignment
Measure resist pattern after development– The aerial image is not generally
measurable Because of the complexity of the masks, the
inspection must be fully automated—manual observation under a microscope is not possible
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Mask Inspection System
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Measurement of Mask Features and Defects
Here, light is passed through the mask and collected by an image recognition system
Solid state detectors are used to collect the light The information is compared against the database of
the mask design or with an identical mask The inspection process is more difficult if the mask
contains OPC or is a PSM Often, defects found in this process can be corrected
– Lasers can burn off excess Cr or Fe oxide.– Adding absorber to clear areas is harder
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SEM Measurement
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State-of-the-Art
Capable of exposing down to ~ 10nm – E-beam lithography– X-ray lithography– Extreme UV lithography
E-beam and EUV are performed under vacuum– Throughput is very slow
New resist families are required– Most are very difficult to remove after use
Research needed on mask material for x-ray and EUV– Glass absorbs– Thickness of metal needed to block x-rays is very
thick (20-50m)