c05 - 1 virginia tech chapter 5: lithography. c05 - 2 virginia tech introduction the mechanism to...
TRANSCRIPT
![Page 1: C05 - 1 Virginia Tech Chapter 5: Lithography. C05 - 2 Virginia Tech Introduction The mechanism to print 2-D patterns to a thin film layer on the wafer](https://reader036.vdocuments.site/reader036/viewer/2022070417/56649e0a5503460f94af2432/html5/thumbnails/1.jpg)
C05 - 1Virginia Tech
Chapter 5: Lithography
![Page 2: C05 - 1 Virginia Tech Chapter 5: Lithography. C05 - 2 Virginia Tech Introduction The mechanism to print 2-D patterns to a thin film layer on the wafer](https://reader036.vdocuments.site/reader036/viewer/2022070417/56649e0a5503460f94af2432/html5/thumbnails/2.jpg)
C05 - 2Virginia Tech
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.
![Page 3: C05 - 1 Virginia Tech Chapter 5: Lithography. C05 - 2 Virginia Tech Introduction The mechanism to print 2-D patterns to a thin film layer on the wafer](https://reader036.vdocuments.site/reader036/viewer/2022070417/56649e0a5503460f94af2432/html5/thumbnails/3.jpg)
C05 - 3Virginia Tech
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
![Page 4: C05 - 1 Virginia Tech Chapter 5: Lithography. C05 - 2 Virginia Tech Introduction The mechanism to print 2-D patterns to a thin film layer on the wafer](https://reader036.vdocuments.site/reader036/viewer/2022070417/56649e0a5503460f94af2432/html5/thumbnails/4.jpg)
C05 - 4Virginia Tech
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
![Page 5: C05 - 1 Virginia Tech Chapter 5: Lithography. C05 - 2 Virginia Tech Introduction The mechanism to print 2-D patterns to a thin film layer on the wafer](https://reader036.vdocuments.site/reader036/viewer/2022070417/56649e0a5503460f94af2432/html5/thumbnails/5.jpg)
C05 - 5Virginia Tech
Introduction
Mask Maker
![Page 6: C05 - 1 Virginia Tech Chapter 5: Lithography. C05 - 2 Virginia Tech Introduction The mechanism to print 2-D patterns to a thin film layer on the wafer](https://reader036.vdocuments.site/reader036/viewer/2022070417/56649e0a5503460f94af2432/html5/thumbnails/6.jpg)
C05 - 6Virginia Tech
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
![Page 7: C05 - 1 Virginia Tech Chapter 5: Lithography. C05 - 2 Virginia Tech Introduction The mechanism to print 2-D patterns to a thin film layer on the wafer](https://reader036.vdocuments.site/reader036/viewer/2022070417/56649e0a5503460f94af2432/html5/thumbnails/7.jpg)
C05 - 7Virginia Tech
Introduction
Spinner
Hotplate
![Page 8: C05 - 1 Virginia Tech Chapter 5: Lithography. C05 - 2 Virginia Tech Introduction The mechanism to print 2-D patterns to a thin film layer on the wafer](https://reader036.vdocuments.site/reader036/viewer/2022070417/56649e0a5503460f94af2432/html5/thumbnails/8.jpg)
C05 - 8Virginia Tech
Introduction
Mask Aligner
![Page 9: C05 - 1 Virginia Tech Chapter 5: Lithography. C05 - 2 Virginia Tech Introduction The mechanism to print 2-D patterns to a thin film layer on the wafer](https://reader036.vdocuments.site/reader036/viewer/2022070417/56649e0a5503460f94af2432/html5/thumbnails/9.jpg)
C05 - 9Virginia Tech
Introduction
1
2
3,4
5,6
6
UV
![Page 10: C05 - 1 Virginia Tech Chapter 5: Lithography. C05 - 2 Virginia Tech Introduction The mechanism to print 2-D patterns to a thin film layer on the wafer](https://reader036.vdocuments.site/reader036/viewer/2022070417/56649e0a5503460f94af2432/html5/thumbnails/10.jpg)
C05 - 10Virginia Tech
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
![Page 11: C05 - 1 Virginia Tech Chapter 5: Lithography. C05 - 2 Virginia Tech Introduction The mechanism to print 2-D patterns to a thin film layer on the wafer](https://reader036.vdocuments.site/reader036/viewer/2022070417/56649e0a5503460f94af2432/html5/thumbnails/11.jpg)
C05 - 11Virginia Tech
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.
![Page 12: C05 - 1 Virginia Tech Chapter 5: Lithography. C05 - 2 Virginia Tech Introduction The mechanism to print 2-D patterns to a thin film layer on the wafer](https://reader036.vdocuments.site/reader036/viewer/2022070417/56649e0a5503460f94af2432/html5/thumbnails/12.jpg)
C05 - 12Virginia Tech
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.
![Page 13: C05 - 1 Virginia Tech Chapter 5: Lithography. C05 - 2 Virginia Tech Introduction The mechanism to print 2-D patterns to a thin film layer on the wafer](https://reader036.vdocuments.site/reader036/viewer/2022070417/56649e0a5503460f94af2432/html5/thumbnails/13.jpg)
C05 - 13Virginia Tech
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
![Page 14: C05 - 1 Virginia Tech Chapter 5: Lithography. C05 - 2 Virginia Tech Introduction The mechanism to print 2-D patterns to a thin film layer on the wafer](https://reader036.vdocuments.site/reader036/viewer/2022070417/56649e0a5503460f94af2432/html5/thumbnails/14.jpg)
C05 - 14Virginia Tech
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
![Page 15: C05 - 1 Virginia Tech Chapter 5: Lithography. C05 - 2 Virginia Tech Introduction The mechanism to print 2-D patterns to a thin film layer on the wafer](https://reader036.vdocuments.site/reader036/viewer/2022070417/56649e0a5503460f94af2432/html5/thumbnails/15.jpg)
C05 - 15Virginia Tech
NTRS Lithography Requirements
![Page 16: C05 - 1 Virginia Tech Chapter 5: Lithography. C05 - 2 Virginia Tech Introduction The mechanism to print 2-D patterns to a thin film layer on the wafer](https://reader036.vdocuments.site/reader036/viewer/2022070417/56649e0a5503460f94af2432/html5/thumbnails/16.jpg)
C05 - 16Virginia Tech
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
![Page 17: C05 - 1 Virginia Tech Chapter 5: Lithography. C05 - 2 Virginia Tech Introduction The mechanism to print 2-D patterns to a thin film layer on the wafer](https://reader036.vdocuments.site/reader036/viewer/2022070417/56649e0a5503460f94af2432/html5/thumbnails/17.jpg)
C05 - 17Virginia Tech
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.
![Page 18: C05 - 1 Virginia Tech Chapter 5: Lithography. C05 - 2 Virginia Tech Introduction The mechanism to print 2-D patterns to a thin film layer on the wafer](https://reader036.vdocuments.site/reader036/viewer/2022070417/56649e0a5503460f94af2432/html5/thumbnails/18.jpg)
C05 - 18Virginia Tech
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
![Page 19: C05 - 1 Virginia Tech Chapter 5: Lithography. C05 - 2 Virginia Tech Introduction The mechanism to print 2-D patterns to a thin film layer on the wafer](https://reader036.vdocuments.site/reader036/viewer/2022070417/56649e0a5503460f94af2432/html5/thumbnails/19.jpg)
C05 - 19Virginia Tech
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)
![Page 20: C05 - 1 Virginia Tech Chapter 5: Lithography. C05 - 2 Virginia Tech Introduction The mechanism to print 2-D patterns to a thin film layer on the wafer](https://reader036.vdocuments.site/reader036/viewer/2022070417/56649e0a5503460f94af2432/html5/thumbnails/20.jpg)
C05 - 20Virginia Tech
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.
![Page 21: C05 - 1 Virginia Tech Chapter 5: Lithography. C05 - 2 Virginia Tech Introduction The mechanism to print 2-D patterns to a thin film layer on the wafer](https://reader036.vdocuments.site/reader036/viewer/2022070417/56649e0a5503460f94af2432/html5/thumbnails/21.jpg)
C05 - 21Virginia Tech
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
![Page 22: C05 - 1 Virginia Tech Chapter 5: Lithography. C05 - 2 Virginia Tech Introduction The mechanism to print 2-D patterns to a thin film layer on the wafer](https://reader036.vdocuments.site/reader036/viewer/2022070417/56649e0a5503460f94af2432/html5/thumbnails/22.jpg)
C05 - 22Virginia Tech
Excimer Lasers
Low reliability due to etching of the electrodes and the optical windows by the energitic F ions
![Page 23: C05 - 1 Virginia Tech Chapter 5: Lithography. C05 - 2 Virginia Tech Introduction The mechanism to print 2-D patterns to a thin film layer on the wafer](https://reader036.vdocuments.site/reader036/viewer/2022070417/56649e0a5503460f94af2432/html5/thumbnails/23.jpg)
C05 - 23Virginia Tech
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 .
![Page 24: C05 - 1 Virginia Tech Chapter 5: Lithography. C05 - 2 Virginia Tech Introduction The mechanism to print 2-D patterns to a thin film layer on the wafer](https://reader036.vdocuments.site/reader036/viewer/2022070417/56649e0a5503460f94af2432/html5/thumbnails/24.jpg)
C05 - 24Virginia Tech
X-Ray Source
High energy electrons collide with a metal. The transfer of energy results in the release of x-rays (short wavelength photons).
![Page 25: C05 - 1 Virginia Tech Chapter 5: Lithography. C05 - 2 Virginia Tech Introduction The mechanism to print 2-D patterns to a thin film layer on the wafer](https://reader036.vdocuments.site/reader036/viewer/2022070417/56649e0a5503460f94af2432/html5/thumbnails/25.jpg)
C05 - 25Virginia Tech
Exposure System
There are three classes of exposure systems– Contact– Proximity– Projection
![Page 26: C05 - 1 Virginia Tech Chapter 5: Lithography. C05 - 2 Virginia Tech Introduction The mechanism to print 2-D patterns to a thin film layer on the wafer](https://reader036.vdocuments.site/reader036/viewer/2022070417/56649e0a5503460f94af2432/html5/thumbnails/26.jpg)
C05 - 26Virginia Tech
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
![Page 27: C05 - 1 Virginia Tech Chapter 5: Lithography. C05 - 2 Virginia Tech Introduction The mechanism to print 2-D patterns to a thin film layer on the wafer](https://reader036.vdocuments.site/reader036/viewer/2022070417/56649e0a5503460f94af2432/html5/thumbnails/27.jpg)
C05 - 27Virginia Tech
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).
![Page 28: C05 - 1 Virginia Tech Chapter 5: Lithography. C05 - 2 Virginia Tech Introduction The mechanism to print 2-D patterns to a thin film layer on the wafer](https://reader036.vdocuments.site/reader036/viewer/2022070417/56649e0a5503460f94af2432/html5/thumbnails/28.jpg)
C05 - 28Virginia Tech
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
![Page 29: C05 - 1 Virginia Tech Chapter 5: Lithography. C05 - 2 Virginia Tech Introduction The mechanism to print 2-D patterns to a thin film layer on the wafer](https://reader036.vdocuments.site/reader036/viewer/2022070417/56649e0a5503460f94af2432/html5/thumbnails/29.jpg)
C05 - 29Virginia Tech
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
![Page 30: C05 - 1 Virginia Tech Chapter 5: Lithography. C05 - 2 Virginia Tech Introduction The mechanism to print 2-D patterns to a thin film layer on the wafer](https://reader036.vdocuments.site/reader036/viewer/2022070417/56649e0a5503460f94af2432/html5/thumbnails/30.jpg)
C05 - 30Virginia Tech
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
![Page 31: C05 - 1 Virginia Tech Chapter 5: Lithography. C05 - 2 Virginia Tech Introduction The mechanism to print 2-D patterns to a thin film layer on the wafer](https://reader036.vdocuments.site/reader036/viewer/2022070417/56649e0a5503460f94af2432/html5/thumbnails/31.jpg)
C05 - 31Virginia Tech
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
![Page 32: C05 - 1 Virginia Tech Chapter 5: Lithography. C05 - 2 Virginia Tech Introduction The mechanism to print 2-D patterns to a thin film layer on the wafer](https://reader036.vdocuments.site/reader036/viewer/2022070417/56649e0a5503460f94af2432/html5/thumbnails/32.jpg)
C05 - 32Virginia Tech
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
![Page 33: C05 - 1 Virginia Tech Chapter 5: Lithography. C05 - 2 Virginia Tech Introduction The mechanism to print 2-D patterns to a thin film layer on the wafer](https://reader036.vdocuments.site/reader036/viewer/2022070417/56649e0a5503460f94af2432/html5/thumbnails/33.jpg)
C05 - 33Virginia Tech
Young’s Single Slit Experiment
sin = /d
http://micro.magnet.fsu.edu/optics/lightandcolor/diffraction.htmlhttp://micro.magnet.fsu.edu/optics/lightandcolor/diffraction.html
![Page 34: C05 - 1 Virginia Tech Chapter 5: Lithography. C05 - 2 Virginia Tech Introduction The mechanism to print 2-D patterns to a thin film layer on the wafer](https://reader036.vdocuments.site/reader036/viewer/2022070417/56649e0a5503460f94af2432/html5/thumbnails/34.jpg)
C05 - 34Virginia Tech
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
![Page 35: C05 - 1 Virginia Tech Chapter 5: Lithography. C05 - 2 Virginia Tech Introduction The mechanism to print 2-D patterns to a thin film layer on the wafer](https://reader036.vdocuments.site/reader036/viewer/2022070417/56649e0a5503460f94af2432/html5/thumbnails/35.jpg)
C05 - 35Virginia Tech
Young’s Double Slit Experiment
http://micro.magnet.fsu.edu/optics/lightandcolor/interference.htmlhttp://micro.magnet.fsu.edu/optics/lightandcolor/interference.html
![Page 36: C05 - 1 Virginia Tech Chapter 5: Lithography. C05 - 2 Virginia Tech Introduction The mechanism to print 2-D patterns to a thin film layer on the wafer](https://reader036.vdocuments.site/reader036/viewer/2022070417/56649e0a5503460f94af2432/html5/thumbnails/36.jpg)
C05 - 36Virginia Tech
Diffraction of Light
![Page 37: C05 - 1 Virginia Tech Chapter 5: Lithography. C05 - 2 Virginia Tech Introduction The mechanism to print 2-D patterns to a thin film layer on the wafer](https://reader036.vdocuments.site/reader036/viewer/2022070417/56649e0a5503460f94af2432/html5/thumbnails/37.jpg)
C05 - 37Virginia Tech
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.
![Page 38: C05 - 1 Virginia Tech Chapter 5: Lithography. C05 - 2 Virginia Tech Introduction The mechanism to print 2-D patterns to a thin film layer on the wafer](https://reader036.vdocuments.site/reader036/viewer/2022070417/56649e0a5503460f94af2432/html5/thumbnails/38.jpg)
C05 - 38Virginia Tech
Diffraction of Light
![Page 39: C05 - 1 Virginia Tech Chapter 5: Lithography. C05 - 2 Virginia Tech Introduction The mechanism to print 2-D patterns to a thin film layer on the wafer](https://reader036.vdocuments.site/reader036/viewer/2022070417/56649e0a5503460f94af2432/html5/thumbnails/39.jpg)
C05 - 39Virginia Tech
Basic Optics
![Page 40: C05 - 1 Virginia Tech Chapter 5: Lithography. C05 - 2 Virginia Tech Introduction The mechanism to print 2-D patterns to a thin film layer on the wafer](https://reader036.vdocuments.site/reader036/viewer/2022070417/56649e0a5503460f94af2432/html5/thumbnails/40.jpg)
C05 - 40Virginia Tech
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
![Page 41: C05 - 1 Virginia Tech Chapter 5: Lithography. C05 - 2 Virginia Tech Introduction The mechanism to print 2-D patterns to a thin film layer on the wafer](https://reader036.vdocuments.site/reader036/viewer/2022070417/56649e0a5503460f94af2432/html5/thumbnails/41.jpg)
C05 - 41Virginia Tech
Basic Optics
The image produced by this system is
![Page 42: C05 - 1 Virginia Tech Chapter 5: Lithography. C05 - 2 Virginia Tech Introduction The mechanism to print 2-D patterns to a thin film layer on the wafer](https://reader036.vdocuments.site/reader036/viewer/2022070417/56649e0a5503460f94af2432/html5/thumbnails/42.jpg)
C05 - 42Virginia Tech
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
![Page 43: C05 - 1 Virginia Tech Chapter 5: Lithography. C05 - 2 Virginia Tech Introduction The mechanism to print 2-D patterns to a thin film layer on the wafer](https://reader036.vdocuments.site/reader036/viewer/2022070417/56649e0a5503460f94af2432/html5/thumbnails/43.jpg)
C05 - 43Virginia Tech
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
![Page 44: C05 - 1 Virginia Tech Chapter 5: Lithography. C05 - 2 Virginia Tech Introduction The mechanism to print 2-D patterns to a thin film layer on the wafer](https://reader036.vdocuments.site/reader036/viewer/2022070417/56649e0a5503460f94af2432/html5/thumbnails/44.jpg)
C05 - 44Virginia Tech
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
![Page 45: C05 - 1 Virginia Tech Chapter 5: Lithography. C05 - 2 Virginia Tech Introduction The mechanism to print 2-D patterns to a thin film layer on the wafer](https://reader036.vdocuments.site/reader036/viewer/2022070417/56649e0a5503460f94af2432/html5/thumbnails/45.jpg)
C05 - 45Virginia Tech
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.
![Page 46: C05 - 1 Virginia Tech Chapter 5: Lithography. C05 - 2 Virginia Tech Introduction The mechanism to print 2-D patterns to a thin film layer on the wafer](https://reader036.vdocuments.site/reader036/viewer/2022070417/56649e0a5503460f94af2432/html5/thumbnails/46.jpg)
C05 - 46Virginia Tech
Fraunhofer Diffraction
![Page 47: C05 - 1 Virginia Tech Chapter 5: Lithography. C05 - 2 Virginia Tech Introduction The mechanism to print 2-D patterns to a thin film layer on the wafer](https://reader036.vdocuments.site/reader036/viewer/2022070417/56649e0a5503460f94af2432/html5/thumbnails/47.jpg)
C05 - 47Virginia Tech
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
![Page 48: C05 - 1 Virginia Tech Chapter 5: Lithography. C05 - 2 Virginia Tech Introduction The mechanism to print 2-D patterns to a thin film layer on the wafer](https://reader036.vdocuments.site/reader036/viewer/2022070417/56649e0a5503460f94af2432/html5/thumbnails/48.jpg)
C05 - 48Virginia Tech
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
![Page 49: C05 - 1 Virginia Tech Chapter 5: Lithography. C05 - 2 Virginia Tech Introduction The mechanism to print 2-D patterns to a thin film layer on the wafer](https://reader036.vdocuments.site/reader036/viewer/2022070417/56649e0a5503460f94af2432/html5/thumbnails/49.jpg)
C05 - 49Virginia Tech
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
![Page 50: C05 - 1 Virginia Tech Chapter 5: Lithography. C05 - 2 Virginia Tech Introduction The mechanism to print 2-D patterns to a thin film layer on the wafer](https://reader036.vdocuments.site/reader036/viewer/2022070417/56649e0a5503460f94af2432/html5/thumbnails/50.jpg)
C05 - 50Virginia Tech
Depth of Focus
![Page 51: C05 - 1 Virginia Tech Chapter 5: Lithography. C05 - 2 Virginia Tech Introduction The mechanism to print 2-D patterns to a thin film layer on the wafer](https://reader036.vdocuments.site/reader036/viewer/2022070417/56649e0a5503460f94af2432/html5/thumbnails/51.jpg)
C05 - 51Virginia Tech
Depth of Focus
From this criterion, we have
For small
cos4/
22114/
22
2222
22sin
NAk
NADOF
NAf
d
![Page 52: C05 - 1 Virginia Tech Chapter 5: Lithography. C05 - 2 Virginia Tech Introduction The mechanism to print 2-D patterns to a thin film layer on the wafer](https://reader036.vdocuments.site/reader036/viewer/2022070417/56649e0a5503460f94af2432/html5/thumbnails/52.jpg)
C05 - 52Virginia Tech
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
![Page 53: C05 - 1 Virginia Tech Chapter 5: Lithography. C05 - 2 Virginia Tech Introduction The mechanism to print 2-D patterns to a thin film layer on the wafer](https://reader036.vdocuments.site/reader036/viewer/2022070417/56649e0a5503460f94af2432/html5/thumbnails/53.jpg)
C05 - 53Virginia Tech
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.
![Page 54: C05 - 1 Virginia Tech Chapter 5: Lithography. C05 - 2 Virginia Tech Introduction The mechanism to print 2-D patterns to a thin film layer on the wafer](https://reader036.vdocuments.site/reader036/viewer/2022070417/56649e0a5503460f94af2432/html5/thumbnails/54.jpg)
C05 - 54Virginia Tech
Modulation Transfer Function
![Page 55: C05 - 1 Virginia Tech Chapter 5: Lithography. C05 - 2 Virginia Tech Introduction The mechanism to print 2-D patterns to a thin film layer on the wafer](https://reader036.vdocuments.site/reader036/viewer/2022070417/56649e0a5503460f94af2432/html5/thumbnails/55.jpg)
C05 - 55Virginia Tech
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
![Page 56: C05 - 1 Virginia Tech Chapter 5: Lithography. C05 - 2 Virginia Tech Introduction The mechanism to print 2-D patterns to a thin film layer on the wafer](https://reader036.vdocuments.site/reader036/viewer/2022070417/56649e0a5503460f94af2432/html5/thumbnails/56.jpg)
C05 - 56Virginia Tech
Change in MTF versus Wavelength
![Page 57: C05 - 1 Virginia Tech Chapter 5: Lithography. C05 - 2 Virginia Tech Introduction The mechanism to print 2-D patterns to a thin film layer on the wafer](https://reader036.vdocuments.site/reader036/viewer/2022070417/56649e0a5503460f94af2432/html5/thumbnails/57.jpg)
C05 - 57Virginia Tech
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
![Page 58: C05 - 1 Virginia Tech Chapter 5: Lithography. C05 - 2 Virginia Tech Introduction The mechanism to print 2-D patterns to a thin film layer on the wafer](https://reader036.vdocuments.site/reader036/viewer/2022070417/56649e0a5503460f94af2432/html5/thumbnails/58.jpg)
C05 - 58Virginia Tech
Fresnel Diffraction
![Page 59: C05 - 1 Virginia Tech Chapter 5: Lithography. C05 - 2 Virginia Tech Introduction The mechanism to print 2-D patterns to a thin film layer on the wafer](https://reader036.vdocuments.site/reader036/viewer/2022070417/56649e0a5503460f94af2432/html5/thumbnails/59.jpg)
C05 - 59Virginia Tech
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
![Page 60: C05 - 1 Virginia Tech Chapter 5: Lithography. C05 - 2 Virginia Tech Introduction The mechanism to print 2-D patterns to a thin film layer on the wafer](https://reader036.vdocuments.site/reader036/viewer/2022070417/56649e0a5503460f94af2432/html5/thumbnails/60.jpg)
C05 - 60Virginia Tech
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
![Page 61: C05 - 1 Virginia Tech Chapter 5: Lithography. C05 - 2 Virginia Tech Introduction The mechanism to print 2-D patterns to a thin film layer on the wafer](https://reader036.vdocuments.site/reader036/viewer/2022070417/56649e0a5503460f94af2432/html5/thumbnails/61.jpg)
C05 - 61Virginia Tech
Fresnel Number
Fresnel diffraction when F ≥ 1 Fraunhofer diffraction when F << 1
g
WF
2
![Page 62: C05 - 1 Virginia Tech Chapter 5: Lithography. C05 - 2 Virginia Tech Introduction The mechanism to print 2-D patterns to a thin film layer on the wafer](https://reader036.vdocuments.site/reader036/viewer/2022070417/56649e0a5503460f94af2432/html5/thumbnails/62.jpg)
C05 - 62Virginia Tech
Depth of Focus
http://www.research.ibm.com/journal/rd/411/holm1.gif
![Page 63: C05 - 1 Virginia Tech Chapter 5: Lithography. C05 - 2 Virginia Tech Introduction The mechanism to print 2-D patterns to a thin film layer on the wafer](https://reader036.vdocuments.site/reader036/viewer/2022070417/56649e0a5503460f94af2432/html5/thumbnails/63.jpg)
C05 - 63Virginia Tech
Summary of the Three Systems
![Page 64: C05 - 1 Virginia Tech Chapter 5: Lithography. C05 - 2 Virginia Tech Introduction The mechanism to print 2-D patterns to a thin film layer on the wafer](https://reader036.vdocuments.site/reader036/viewer/2022070417/56649e0a5503460f94af2432/html5/thumbnails/64.jpg)
C05 - 64Virginia Tech
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
![Page 65: C05 - 1 Virginia Tech Chapter 5: Lithography. C05 - 2 Virginia Tech Introduction The mechanism to print 2-D patterns to a thin film layer on the wafer](https://reader036.vdocuments.site/reader036/viewer/2022070417/56649e0a5503460f94af2432/html5/thumbnails/65.jpg)
C05 - 65Virginia Tech
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)
![Page 66: C05 - 1 Virginia Tech Chapter 5: Lithography. C05 - 2 Virginia Tech Introduction The mechanism to print 2-D patterns to a thin film layer on the wafer](https://reader036.vdocuments.site/reader036/viewer/2022070417/56649e0a5503460f94af2432/html5/thumbnails/66.jpg)
C05 - 66Virginia Tech
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
![Page 67: C05 - 1 Virginia Tech Chapter 5: Lithography. C05 - 2 Virginia Tech Introduction The mechanism to print 2-D patterns to a thin film layer on the wafer](https://reader036.vdocuments.site/reader036/viewer/2022070417/56649e0a5503460f94af2432/html5/thumbnails/67.jpg)
C05 - 67Virginia Tech
Diazoquinone
The photoactive part of the molecule is the part above the SO2
![Page 68: C05 - 1 Virginia Tech Chapter 5: Lithography. C05 - 2 Virginia Tech Introduction The mechanism to print 2-D patterns to a thin film layer on the wafer](https://reader036.vdocuments.site/reader036/viewer/2022070417/56649e0a5503460f94af2432/html5/thumbnails/68.jpg)
C05 - 68Virginia Tech
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.
![Page 69: C05 - 1 Virginia Tech Chapter 5: Lithography. C05 - 2 Virginia Tech Introduction The mechanism to print 2-D patterns to a thin film layer on the wafer](https://reader036.vdocuments.site/reader036/viewer/2022070417/56649e0a5503460f94af2432/html5/thumbnails/69.jpg)
C05 - 69Virginia Tech
Properties and Characteristics of Resists
Two parameters are used to define the properties of photoresists– Contrast– Critical modulation transfer function (CMTF)
![Page 70: C05 - 1 Virginia Tech Chapter 5: Lithography. C05 - 2 Virginia Tech Introduction The mechanism to print 2-D patterns to a thin film layer on the wafer](https://reader036.vdocuments.site/reader036/viewer/2022070417/56649e0a5503460f94af2432/html5/thumbnails/70.jpg)
C05 - 70Virginia Tech
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.
![Page 71: C05 - 1 Virginia Tech Chapter 5: Lithography. C05 - 2 Virginia Tech Introduction The mechanism to print 2-D patterns to a thin film layer on the wafer](https://reader036.vdocuments.site/reader036/viewer/2022070417/56649e0a5503460f94af2432/html5/thumbnails/71.jpg)
C05 - 71Virginia Tech
Photoresist Contrast
![Page 72: C05 - 1 Virginia Tech Chapter 5: Lithography. C05 - 2 Virginia Tech Introduction The mechanism to print 2-D patterns to a thin film layer on the wafer](https://reader036.vdocuments.site/reader036/viewer/2022070417/56649e0a5503460f94af2432/html5/thumbnails/72.jpg)
C05 - 72Virginia Tech
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
![Page 73: C05 - 1 Virginia Tech Chapter 5: Lithography. C05 - 2 Virginia Tech Introduction The mechanism to print 2-D patterns to a thin film layer on the wafer](https://reader036.vdocuments.site/reader036/viewer/2022070417/56649e0a5503460f94af2432/html5/thumbnails/73.jpg)
C05 - 73Virginia Tech
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
![Page 74: C05 - 1 Virginia Tech Chapter 5: Lithography. C05 - 2 Virginia Tech Introduction The mechanism to print 2-D patterns to a thin film layer on the wafer](https://reader036.vdocuments.site/reader036/viewer/2022070417/56649e0a5503460f94af2432/html5/thumbnails/74.jpg)
C05 - 74Virginia Tech
Photoresist Contrast
![Page 75: C05 - 1 Virginia Tech Chapter 5: Lithography. C05 - 2 Virginia Tech Introduction The mechanism to print 2-D patterns to a thin film layer on the wafer](https://reader036.vdocuments.site/reader036/viewer/2022070417/56649e0a5503460f94af2432/html5/thumbnails/75.jpg)
C05 - 75Virginia Tech
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
![Page 76: C05 - 1 Virginia Tech Chapter 5: Lithography. C05 - 2 Virginia Tech Introduction The mechanism to print 2-D patterns to a thin film layer on the wafer](https://reader036.vdocuments.site/reader036/viewer/2022070417/56649e0a5503460f94af2432/html5/thumbnails/76.jpg)
C05 - 76Virginia Tech
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
![Page 77: C05 - 1 Virginia Tech Chapter 5: Lithography. C05 - 2 Virginia Tech Introduction The mechanism to print 2-D patterns to a thin film layer on the wafer](https://reader036.vdocuments.site/reader036/viewer/2022070417/56649e0a5503460f94af2432/html5/thumbnails/77.jpg)
C05 - 77Virginia Tech
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
![Page 78: C05 - 1 Virginia Tech Chapter 5: Lithography. C05 - 2 Virginia Tech Introduction The mechanism to print 2-D patterns to a thin film layer on the wafer](https://reader036.vdocuments.site/reader036/viewer/2022070417/56649e0a5503460f94af2432/html5/thumbnails/78.jpg)
C05 - 78Virginia Tech
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
![Page 79: C05 - 1 Virginia Tech Chapter 5: Lithography. C05 - 2 Virginia Tech Introduction The mechanism to print 2-D patterns to a thin film layer on the wafer](https://reader036.vdocuments.site/reader036/viewer/2022070417/56649e0a5503460f94af2432/html5/thumbnails/79.jpg)
C05 - 79Virginia Tech
Standing Waves Due to Reflections
![Page 80: C05 - 1 Virginia Tech Chapter 5: Lithography. C05 - 2 Virginia Tech Introduction The mechanism to print 2-D patterns to a thin film layer on the wafer](https://reader036.vdocuments.site/reader036/viewer/2022070417/56649e0a5503460f94af2432/html5/thumbnails/80.jpg)
C05 - 80Virginia Tech
Standing Waves Due to Reflections
http://www.lithoguru.com/scientist/lithobasics.htmlhttp://www.lithoguru.com/scientist/lithobasics.html
![Page 81: C05 - 1 Virginia Tech Chapter 5: Lithography. C05 - 2 Virginia Tech Introduction The mechanism to print 2-D patterns to a thin film layer on the wafer](https://reader036.vdocuments.site/reader036/viewer/2022070417/56649e0a5503460f94af2432/html5/thumbnails/81.jpg)
C05 - 81Virginia Tech
(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
![Page 82: C05 - 1 Virginia Tech Chapter 5: Lithography. C05 - 2 Virginia Tech Introduction The mechanism to print 2-D patterns to a thin film layer on the wafer](https://reader036.vdocuments.site/reader036/viewer/2022070417/56649e0a5503460f94af2432/html5/thumbnails/82.jpg)
C05 - 82Virginia Tech
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
![Page 83: C05 - 1 Virginia Tech Chapter 5: Lithography. C05 - 2 Virginia Tech Introduction The mechanism to print 2-D patterns to a thin film layer on the wafer](https://reader036.vdocuments.site/reader036/viewer/2022070417/56649e0a5503460f94af2432/html5/thumbnails/83.jpg)
C05 - 83Virginia Tech
Mask Engineering
![Page 84: C05 - 1 Virginia Tech Chapter 5: Lithography. C05 - 2 Virginia Tech Introduction The mechanism to print 2-D patterns to a thin film layer on the wafer](https://reader036.vdocuments.site/reader036/viewer/2022070417/56649e0a5503460f94af2432/html5/thumbnails/84.jpg)
C05 - 84Virginia Tech
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
![Page 85: C05 - 1 Virginia Tech Chapter 5: Lithography. C05 - 2 Virginia Tech Introduction The mechanism to print 2-D patterns to a thin film layer on the wafer](https://reader036.vdocuments.site/reader036/viewer/2022070417/56649e0a5503460f94af2432/html5/thumbnails/85.jpg)
C05 - 85Virginia Tech
Phase Shift Masks (PSM)
Intensity Intensity pattern is pattern is barely barely sufficient sufficient to resolve to resolve the two the two patterns.patterns.
![Page 86: C05 - 1 Virginia Tech Chapter 5: Lithography. C05 - 2 Virginia Tech Introduction The mechanism to print 2-D patterns to a thin film layer on the wafer](https://reader036.vdocuments.site/reader036/viewer/2022070417/56649e0a5503460f94af2432/html5/thumbnails/86.jpg)
C05 - 86Virginia Tech
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.
![Page 87: C05 - 1 Virginia Tech Chapter 5: Lithography. C05 - 2 Virginia Tech Introduction The mechanism to print 2-D patterns to a thin film layer on the wafer](https://reader036.vdocuments.site/reader036/viewer/2022070417/56649e0a5503460f94af2432/html5/thumbnails/87.jpg)
C05 - 87Virginia Tech
Scanning Projection Printer
![Page 88: C05 - 1 Virginia Tech Chapter 5: Lithography. C05 - 2 Virginia Tech Introduction The mechanism to print 2-D patterns to a thin film layer on the wafer](https://reader036.vdocuments.site/reader036/viewer/2022070417/56649e0a5503460f94af2432/html5/thumbnails/88.jpg)
C05 - 88Virginia Tech
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
![Page 89: C05 - 1 Virginia Tech Chapter 5: Lithography. C05 - 2 Virginia Tech Introduction The mechanism to print 2-D patterns to a thin film layer on the wafer](https://reader036.vdocuments.site/reader036/viewer/2022070417/56649e0a5503460f94af2432/html5/thumbnails/89.jpg)
C05 - 89Virginia Tech
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
![Page 90: C05 - 1 Virginia Tech Chapter 5: Lithography. C05 - 2 Virginia Tech Introduction The mechanism to print 2-D patterns to a thin film layer on the wafer](https://reader036.vdocuments.site/reader036/viewer/2022070417/56649e0a5503460f94af2432/html5/thumbnails/90.jpg)
C05 - 90Virginia Tech
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
![Page 91: C05 - 1 Virginia Tech Chapter 5: Lithography. C05 - 2 Virginia Tech Introduction The mechanism to print 2-D patterns to a thin film layer on the wafer](https://reader036.vdocuments.site/reader036/viewer/2022070417/56649e0a5503460f94af2432/html5/thumbnails/91.jpg)
C05 - 91Virginia Tech
Off-Axis Illumination
![Page 92: C05 - 1 Virginia Tech Chapter 5: Lithography. C05 - 2 Virginia Tech Introduction The mechanism to print 2-D patterns to a thin film layer on the wafer](https://reader036.vdocuments.site/reader036/viewer/2022070417/56649e0a5503460f94af2432/html5/thumbnails/92.jpg)
C05 - 92Virginia Tech
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
![Page 93: C05 - 1 Virginia Tech Chapter 5: Lithography. C05 - 2 Virginia Tech Introduction The mechanism to print 2-D patterns to a thin film layer on the wafer](https://reader036.vdocuments.site/reader036/viewer/2022070417/56649e0a5503460f94af2432/html5/thumbnails/93.jpg)
C05 - 93Virginia Tech
DNQ/Novolac Resist Process
The details of the process are more complex that described earlier
![Page 94: C05 - 1 Virginia Tech Chapter 5: Lithography. C05 - 2 Virginia Tech Introduction The mechanism to print 2-D patterns to a thin film layer on the wafer](https://reader036.vdocuments.site/reader036/viewer/2022070417/56649e0a5503460f94af2432/html5/thumbnails/94.jpg)
C05 - 94Virginia Tech
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
![Page 95: C05 - 1 Virginia Tech Chapter 5: Lithography. C05 - 2 Virginia Tech Introduction The mechanism to print 2-D patterns to a thin film layer on the wafer](https://reader036.vdocuments.site/reader036/viewer/2022070417/56649e0a5503460f94af2432/html5/thumbnails/95.jpg)
C05 - 95Virginia Tech
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.
![Page 96: C05 - 1 Virginia Tech Chapter 5: Lithography. C05 - 2 Virginia Tech Introduction The mechanism to print 2-D patterns to a thin film layer on the wafer](https://reader036.vdocuments.site/reader036/viewer/2022070417/56649e0a5503460f94af2432/html5/thumbnails/96.jpg)
C05 - 96Virginia Tech
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
![Page 97: C05 - 1 Virginia Tech Chapter 5: Lithography. C05 - 2 Virginia Tech Introduction The mechanism to print 2-D patterns to a thin film layer on the wafer](https://reader036.vdocuments.site/reader036/viewer/2022070417/56649e0a5503460f94af2432/html5/thumbnails/97.jpg)
C05 - 97Virginia Tech
http://www.research.ibm.com/journal/rd/411/holm4.gif
![Page 98: C05 - 1 Virginia Tech Chapter 5: Lithography. C05 - 2 Virginia Tech Introduction The mechanism to print 2-D patterns to a thin film layer on the wafer](https://reader036.vdocuments.site/reader036/viewer/2022070417/56649e0a5503460f94af2432/html5/thumbnails/98.jpg)
C05 - 98Virginia Tech
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
![Page 99: C05 - 1 Virginia Tech Chapter 5: Lithography. C05 - 2 Virginia Tech Introduction The mechanism to print 2-D patterns to a thin film layer on the wafer](https://reader036.vdocuments.site/reader036/viewer/2022070417/56649e0a5503460f94af2432/html5/thumbnails/99.jpg)
C05 - 99Virginia Tech
Mask Inspection System
![Page 100: C05 - 1 Virginia Tech Chapter 5: Lithography. C05 - 2 Virginia Tech Introduction The mechanism to print 2-D patterns to a thin film layer on the wafer](https://reader036.vdocuments.site/reader036/viewer/2022070417/56649e0a5503460f94af2432/html5/thumbnails/100.jpg)
C05 - 100Virginia Tech
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
![Page 101: C05 - 1 Virginia Tech Chapter 5: Lithography. C05 - 2 Virginia Tech Introduction The mechanism to print 2-D patterns to a thin film layer on the wafer](https://reader036.vdocuments.site/reader036/viewer/2022070417/56649e0a5503460f94af2432/html5/thumbnails/101.jpg)
C05 - 101Virginia Tech
SEM Measurement
![Page 102: C05 - 1 Virginia Tech Chapter 5: Lithography. C05 - 2 Virginia Tech Introduction The mechanism to print 2-D patterns to a thin film layer on the wafer](https://reader036.vdocuments.site/reader036/viewer/2022070417/56649e0a5503460f94af2432/html5/thumbnails/102.jpg)
C05 - 102Virginia Tech
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)