1. Principles of Optical Lithography
Layout:
Motivation
IC manufacturing
Traditional method and
trends
Resolution enhancement
techniques
University of
Szeged
Principles of Optical Lithography, 2002 November 20.

2. „From Lithography to Optical Lithography”
lithography: Graphic Arts. any of several printing processes,
such as offset lithography, in which the image areas of a plate
are treated (photographically or directly by hand, as with a
lithographic crayon) to accept greasy inks and repel water, while
the nonimage area accept water and repel ink.
/Dictionary of Science and Technology/
A litográfia szó eredeti jelentése
“Object”
“Image”
printing process
Principles of Optical Lithography, 2002 November 20.

3. Optical lithography is the process used to transfer a
„From Lithography to Optical Lithography”
“Object”
“Image”
optical imaging process
Az optikai litográfia az IC gyártás egyik lépése amely során a maszkon lévő mintázatot egy lencserendszer segítségével egy fényérzékeny vékony rétegbe (resist) exponálják.
application: cameras, microscopes,
manufacturing of integrated circuits (IC)
Optical lithography is the process used to transfer a
pattern to a layer of an integrated circuit.
Principles of Optical Lithography, 2002 November 20.

4. Manufacturing of Integrated Circuits
Az IC gyartasanak lépései. Korkoros, azaz a képen látható gyártási folyamat tobbszor megismetlodik.
Chip fabrication occurs as a cycle of steps carried out as many as 20 times!
Principles of Optical Lithography, 2002 November 20.

5. Silicon crystal (ingot)
Manufacturing of Integrated Circuits 4
Si substrate
Light
Photoresist
SiO2
SiO2
Thermal oxidation 3
Si - substrate
Silicon crystal (ingot) 1
polished wafer 2
Silicon is the most abundant element on the Earth except for oxygen.
Silicon is a natural semiconductor, which means that it can be altered to be either an isolator or conductor.
Purified silicon is melted and then formed into cylindrical single crystals called ingots.
The ingots are sliced into wafers about mm (0.03 inch) thick.
In the step called polarization the wafers polished untill they have a mirror-smooth surface.
The diameter of the wafer is 8 inches (200 mm) and 12 inches (300 mm).
Bigger wafers mean that more chips can be made at one time, holding down the cost per chip.
Principles of Optical Lithography, 2002 November 20. 5 6 7 8 9

6. Photolithography - Equipment
Resist Dispenser
Spin Coater:
Used for the application of photoresist, primer, and developer.
Photoresist Thickness Control:
Photoresist Viscosity
Spin Speed
Temperature
Humidity
A subsztratra a resistet spin-coating eljarassal szokas felvinni.
Vacuum
Chuck
Hollow
Shaft
To House
Vacuum
Principles of Optical Lithography, 2002 November 20.

7. Photolithography - Equipment
Spin Speed vs. Thickness:
A resist vastagsaga a fordulatszammal allithato be.
Principles of Optical Lithography, 2002 November 20.

8. Positive vs. Negative Resist
Positive Resist:
Photo Mask
Resist
Silicon Substrate
Oxide
Pozitiv resist eseten a fennyel kolcsonhatasba lepo reszt tudjuk eltavolitani.
Light areas on the mask => photoresist is removed during development.
Long-chained molecules in resist are broken down to smaller chains by the UV light, which are easily dissolved by the developer solution.
Capable of smaller features, better resolution, but has poor adhesion and costs much more.
Principles of Optical Lithography, 2002 November 20.

9. Positive vs. Negative Resist
Photo Mask
Resist
Resist
Oxide
Oxide
Silicon Substrate
Dark areas on the mask => photoresist is removed during development.
Short-chain molecules in the resist are “cross-linked” by the UV light. Resultant longer chains are resistant to developer solution.
Better adhesion, but incapable of producing submicron features.
Only used (anymore) for certain specialty applications.
Principles of Optical Lithography, 2002 November 20.

10. Threshold of Photoresists
A photoresist kuszobszeruen viselkedik, azaz kell egy min. energia, hogy „valami törtenjen”
Principles of Optical Lithography, 2002 November 20.

11. Manufacturing of Integrated Circuits4
Si substrate
Light
Photoresist
SiO2
Silicon is the most abundant element on the Earth except for oxygen.
Silicon is a natural semiconductor, which means that it can be altered to be either an isolator or conductor.
Purified silicon is melted and then formed into cylindrical single crystals called ingots.
The ingots are sliced into wafers about mm (0.03 inch) thick.
In the step called polarization the wafers polished untill they have a mirror-smooth surface.
The diameter of the wafer is 8 inches (200 mm) and 12 inches (300 mm).
Bigger wafers mean that more chips can be made at one time, holding down the cost per chip.
Principles of Optical Lithography, 2002 November 20. 5 6 7 8 9

12. „From Optical Lithography to Projection Optical Lithography”
Contact Printing
Projection Printing
simplest design (M=1x, no optics)
easy to operate but requires mask cleaning
limited to ~1m features
contact printing: there is a physical contact between the mask and the resist, requires mask cleaning procedure and homogeneous illumination as large beam as the mask pattern
proximity printing: There is gap between the mask and the resist, therefore there is a resolution loss introduced by diffraction.
projection printing: It has 4 subcategories, the critical point is the field size, the bigger the field size the bigger the area we can image instead mechanical movement
step and scan: the whole reticle is devided into smaller part and these parts scanned sequantually.
Proximity Printing
simple design (M=1x, no optics)
easy to operate
limited to ~1  m features (diffraction)
some resolution loss due to gap
complex design (M=1x, no optics)
limited by field size, optical aberrations etc.
Principles of Optical Lithography, 2002 November 20.

13. Clean Room Environment
Principles of Optical Lithography, 2002 November 20.
Clean Room Environment
“2. line of defence”
“first line of defence”
Since we are in the sub micron range even very small dusts could cause series problems. Therefore the whole process is made in a clean environment called clean room. However, there is a second line of defence.

14. Early integrated circuit Salt-size transistors
Principles of Optical Lithography, 2002 November 20.
Transistor miniaturization
The first transistor
1948
Early integrated circuit
1973
Technological jump in 1958 when Kilby invented the integrated circuits.
The tendency is clear, to reduce the size of the electronic components
Salt-size transistors
1964
DRAM chip
1997

15. Transistor miniaturization
Principles of Optical Lithography, 2002 November 20.
Transistor miniaturization
Moore’s Law: the number of transistors quadruples every four years
Rock’s Law: the cost of a fab doubles every four years
Result 1: fewer advanced fabs and fewer fab operators, fabless/foundry partnerships
Result 2: Extend the lifetime of every generation, by means of super resolution techniques.
Moore law is empirical law.
New generation means for example when a new wavelength is introduced. New wavelength means new stepper, new materials, such as new resist. The introduction of a new generation is extremly expensive. Therefore, researches try to extend the lifetime of the “old” steppers by means of alternatives techniques, which are not require stepper modification. PST is a typical example.

16. Rayleigh resolution limit
Principles of Optical Lithography, 2002 November 20.
Rayleigh resolution limit
Egy homogen modon kivilagitott diffrakciolimitalt lencse fokuszaban az intenzitasprofil.

17. Rayleigh resolution limit
Principles of Optical Lithography, 2002 November 20.
Rayleigh resolution limit
Lateral Resolution
Critical Feature Size
Critical Dimension
Axial Resolution
Depth of Field
Depth of Focus
A feloldas az NA novelesevel, vagy a hullamhossz csokkentesevel erheto el. Ezek a hagyomanyos modszerek. De a k, rendszerre jellemzo parametert is lehet modositani.
How can we enhance the resolution?
Reducing the wavelength ()
Increasing the numerical aperture (NA)
Manipulating the k1 factor

18. Lithography Roadmap (reduction of )
Why excimer is the winner?
Principles of Optical Lithography, 2002 November 20.

19. Lithography Roadmap - Why Excimers
It is difficult to design and manufacture an
achromatic projection lens for excimer
lasers in deep UV due to the small difference
in chromatic dispersion between fused quartz
and fluorite, in addition to other material
related problems, such as UV-induced defect
formation.
A kromatikus hiba nagy hatrany az UV tartomanyban, mert akromatot nem lehet kesziteni.
 < 0.6 pm
Beam shaping
Principles of Optical Lithography, 2002 November 20.

20. Development of Projection Optics (increasing of NA)
Lithography represents the greatest
capital expense in an advanced fab
Strategies to stretch machine
capabilities and extend the lifetime
of every generation
Principles of Optical Lithography, 2002 November 20.

21. Development of Projection Optics (evolution of the stepper)
Principles of Optical Lithography, 2002 November 20.

22. Stepper
Principles of Optical Lithography, 2002 November 20.

23. Importance of DOF DOF is limited by thickness of the resist
surface smoothness (although the wafer is polished, aftert
several fabrication cycles the surface has a complex topology)
A DOF a struktúra miatt is nagy kell hogy legyen, a resist vastagsaga mellett ez egy eros limit.
Principles of Optical Lithography, 2002 November 20.

24. Chemical-Mechanical Polishing (CMP)
Minden egyes lepes utan polirozzuk le.
Principles of Optical Lithography, 2002 November 20.

25. Chemical-Mechanical Polishing (CMP)
Principles of Optical Lithography, 2002 November 20.

26. Importance of DOF The required minimum feature size reduces
from 0.24 micron to 0.04 micron
The required DOF is 0.5 micron in 2010,
/limited by the thickness of the resist/
Principles of Optical Lithography, 2002 November 20.

27. Images in lithography Designed Pattern Mask Aerial Image Real Image
Mask writing process
(e-beam or laser)
Mask
Stepper
(NA, M, , , etc.)
Aerial Image
Wafer reflection
Thin film effects
Real Image
Photochemistry
Diffusion
Latent Image
Development
Resist Image
Pattern transfer
Device Layer
Principles of Optical Lithography, 2002 November 20.

28. Aerial versus Resist image simulated by Solid-C
Principles of Optical Lithography, 2002 November 20.

29. Image formation based on Fourier Optics
m(x,y): electric field transmittance of the mask
P(fx,fy): transfer function of the optical system (different interpretations are possible)
Principles of Optical Lithography, 2002 November 20.

30. Image formation Manhattan Geometry
Principles of Optical Lithography, 2002 November 20.
Image formation Manhattan Geometry
Szigoru eloirasok a lehetseges, alkalmazhato alakzatokrol.

31. Image formation Number of diffracted orders
Principles of Optical Lithography, 2002 November 20.
Image formation
Number of diffracted orders
(Spatial Fourier components)
Image quality
Harom diffrakcios rendnek benne kell lennie az aperturaban. Kulonben nem oldhato fel a racs.
Conventional imaging requires
minimum 3 diffraction orders

32. Fourier series of a grating
Principles of Optical Lithography, 2002 November 20.
Fourier series of a grating
mask
1/2
k=0
k=1
k=2
k=40
f(x)=1/2
threshold of the photoresist
De a resist kuszobszeruen viselkedik, igy az elohivott kép lehet kontrasztos. 1 1 1
-3
-2 -  2 3

33. Resolution Enhancement Techniques
Input Parameters
Mirror
* Wavelength ()
Light Source
* Bandwidth
* Exposure
Filter
* Filter (annular, quadrupole, etc.)
* Spatial coherence ()
Condenser
Lens
Mask
* Binary or PS mask
(Manhattan geometry)
* Magnification (M)
* Numerical Aperture (NA)
* Resist system
* Thickness
* Absorp. Param. A, B
* Rate Constant C
* Refractive Index
* Development Model
* etc.
* Defocus
Pupil-Plane
Filter
* Pupil-plane filter/Aberrations
* Image model
Projection
Lens System
* Image flare
Wafer with
Photoresist
Principles of Optical Lithography, 2002 November 20.

34. Resolution Enhancement Techniques - OPC
Linewidth variation
The light intensity profile is not
significantly different for the isolated
1.0 micron line as for the 1.0 micron
line/1 micron spacing grating.
Isolated and grouped lines are expected
to print approximately the same with.
A feloldas erosen fugg a mintatol.
Significantly different light intensity
profiles depending upon whether the
lines are isolated or part of a grating
structure
The size of a line is dependent upon
its proximity to other geometries
Principles of Optical Lithography, 2002 November 20.

35. Resolution Enhancement Techniques - OPC
Silicon Image w/o OPC
Conventional (no OPC)
Valtoztassuk meg a maszkon a mintat ugy, hogy a vegso kep lehyen jo.
Original Layout
0.18 mm
OPC Layout
Silicon Image with OPC
Principles of Optical Lithography, 2002 November 20.

36. Resolution Enhancement Techniques - OPC
Problems:
Proximity effects
Nonlinearity
Line shortening
Corner rounding
Solution:
OPC (Optical proximity correction):
is the technique of predistorting mask patterns such
that the printed patterns are as close to the desired
shape as possible.
Principles of Optical Lithography, 2002 November 20.

37. Resolution Enhancement Techniques - OPC
OPC processes
1. Catastrophic OPC:
The goal is to guarantee presence of a pattern without feature-size control worries.
Precise dimension control is not critical. An example is to ensure a wire is continuous and that no open circuit will result.
2. One dimensional OPC (Linewidth variation minimization):
Vary the size of each feature on the photomask depending on its nominal dimension and environment /For example, dense and sparse lines are made smaller on the mask whereas lines of intermediate
periodicities are made larger/
3. Line shortening method:
4. Corner rounding:
biasing if there is room
Principles of Optical Lithography, 2002 November 20.

38. Resolution Enhancement Techniques - OPC
Uncorrected Test Pattern
Vertex Count=94
Super Aggressive Correction
Vertex Count=1106
Limited by the pixel size of the mask writing process!!!
Principles of Optical Lithography, 2002 November 20.

39. Resolution Enhancement Techniques /Off-Axis Illumination/0.
+1.
+2.
-1.
-2.
Photo-Mask
Projection Lens
Photo-Resist
Intensity in the
Pupil-plane
Conventional
Illumination
Off-Axis
TWO BEAM IMAGING!
Principles of Optical Lithography, 2002 November 20.

40. Resolution Enhancement Techniques Phase Shifting Techniques
Principles of Optical Lithography, 2002 November 20.

41. Resolution Enhancement Techniques Phase Shifting Techniques
Principles of Optical Lithography, 2002 November 20.

42. Resolution Enhancement Techniques Phase Shifting Techniques
Principles of Optical Lithography, 2002 November 20.

43. Resolution Enhancement Techniques Phase Shifting Techniques
Principles of Optical Lithography, 2002 November 20.

44. Resolution Enhancement Techniques Phase Shifting Techniques
A maszkon a fazistolo hiba nagyobb gondot okoz mint a krom hiba.
Principles of Optical Lithography, 2002 November 20.

45. Resolution Enhancement Techniques Phase Shifting Techniques
Phase-shifting masks are classified as “strong” and “weak”, according to their
ability to suppress the zero order diffraction component.
Principles of Optical Lithography, 2002 November 20.

46. Resolution Enhancement Techniques Phase Shifting Techniques
Phase Shifting Layer
Chrome
Alternating/Levenson PSM
Attenuated PSM
Phase Shifting Layer (T )
Dark Field Grating
(PS Layer)
Chromeless PSM
Outrigger PSM
Rim PSM
Interferometric PSM ?
Phase Shifting Method without
Phase Shifting Layer. a. b. c. d. e. f.
Principles of Optical Lithography, 2002 November 20.

47. Resolution Enhancement Techniques Phase Shifting Techniques
Four-beam imaging z
Front
Illumination
Reflective
Chrome
Layer
2·  x 
Fused
Silica
Substrate
Back
Illumination
Principles of Optical Lithography, 2002 November 20.

48. Resolution Enhancement Techniques Phase Shifting Techniques
Image Plane
Mask L3 L2 L1 1°
Optical Axis
CCD
0.7°
Piezo
Translator M1 M2
Attenuator
Ar+-ion laser
(=457.9) nm
Beam Splitter
Principles of Optical Lithography, 2002 November 20.

49. Resolution Enhancement Techniques Phase Shifting Techniques1
3.5
Intensity
X(m)
Calculated
Measured
CCD Image a b c d e f g h
R image
T image
Correct Phase
Wrong Phase
Principles of Optical Lithography, 2002 November 20.

50. What’s next for Optical Lithography
Principles of Optical Lithography, 2002 November 20.
What’s next for Optical Lithography

51. What’s next for Optical Lithography
Principles of Optical Lithography, 2002 November 20.
What’s next for Optical Lithography

52. Lithography Roadmap
Principles of Optical Lithography, 2002 November 20.

53. Development of Projection Optics (increasing of NA)
Catadioptric and immersion objectives
NANO Fórum 2006 December 13.