1. Lithography Instructor Abu Syed Md. Jannatul Islam1
Instructor
Abu Syed Md. Jannatul Islam
Lecturer, Dept. of EEE, KUET, BD
Department of Electrical and Electronic Engineering
Khulna University of Engineering & Technology
Khulna-9203

2. Very-high-purity, single-crystal silicon ingot
Basic Things 2
Electrical and mechanical properties of the wafer depend on the orientation of the crystalline structure, the impurity concentrations, and the type of impurities present.
The surface of the wafer is then polished to a mirror finish using chemical and mechanical polishing (CMP) techniques.
Crystal is then sawed (like a loaf of bread) to produce circular wafers that are 400μm to 600μm thick
Solid cylinder 10 cm to 30 cm in diameter and can be 1 to 2 m in length.
Very-high-purity, single-crystal silicon ingot

3. These variables are strictly controlled during crystal growth
Basic Things 3
When designating the relative doping concentrations in semiconductor material, it is common to use the + and – symbols.
If a large number of impurity atoms is added, the silicon will be heavily doped (e.g., concentration > ∼10^18 atoms/cm−3).
Depending on the types of impurity, either holes (in p-type silicon) or electrons (in n-type silicon) can be responsible for electrical conduction.
A specific amount of impurities known as doping allows the alteration of the electrical properties of the silicon, in particular its resistivity.
These variables are strictly controlled during crystal growth

4. Basic Things 4
The ability to control the type of impurities and the doping concentration in the silicon permits the formation of diodes, transistors, and resistors in integrated circuits.
Similarly, p+ and p− designations refer to the heavily doped and lightly doped p-type regions, respectively.
A heavily doped (low-resistivity) n-type silicon wafer is referred to as n+ material, while a lightly doped material (e.g., concentration < ∼1016 atoms/cm−3) is referred to as n−.

5. Why Lithography? 5
Simple layers of thin films do not make a Device.

6. Why Lithography? 6

7. History of Lithography7
It was invented in 1796 by German author and actor Alois Senefelder as a cheap method of publishing theatrical works.

8. “ writing a pattern on stone”
What is Lithography ? 8
Lithography comes from two Greek words, “lithos” which means stone and “graphein” which means write.
“ writing a pattern on stone”
Lithography is the transfer of geometric shapes on a mask to a smooth surface
It uses light or other forms of radiant energy to change the chemical properties of thin layers of films that have been coated on a substrate.
Typically 8-25 lithography steps and several hundred processing steps between exposure are required to fabricate a packed IC.

9. Lithography Does This! What is lithography ?9
Lithography is one of the 4 major processes in the top-down model
Lithography
Etching
Deposition
Doping
In order to perform the other 3 processes,
we must precisely define where to do them
Lithography Does This!

10. Interference lithography. Scanning Probe lithography
Types of Lithography 10
Photolithography
E-beam lithography
X-ray lithography.
Interference lithography.
Scanning Probe lithography

11. Photolithography Photolithography is the process11
Photolithography is the process
of transferring patterns of geometric shapes
on a mask
to a thin layer of photosensitive material (called photoresist)
covering the surface of a semiconductor wafer.
A light sensitive photoresist is spun onto the wafer forming a thin layer on the surface.
The resist is then selectively exposed by shining light through a mask which contains the pattern information for the particular being fabricated.
The resist is then developed which completes the pattern transfer from the mask to the wafer.

12. Photolithography is an optical means for transferring patterns12
Photolithography is an optical means for transferring patterns
onto a substrate
Overview of the Photolithography Process
Surface Preparation (Get rid of H2O, RCA clean, apply adhesion promoter
Deposit (Photoresist Coating by Spin Casting)
Soft Bake (90 – 120°C for 60 –120 sec to remove solvent from liquid photoresist
Photo Mask Alignment
Exposure (Pattern transfer)
Development (Remove soluble photoresist)
Hard Bake (100 – 180°C) to increase adhesion
Etching (Remove oxide)
Stripping (Photoresist removal)
Post Processing/Cleaning (Ashing)

13. Photolithography 13
Grow Oxide Layer

14. Photolithography 14
Add Photoresist

15. Photolithography 15
Photo-Mask

16. UV Exposed to Photomask to transfer pattern
Photolithography 16
UV Exposed to Photomask to transfer pattern

17. Photolithography 17
Remove Photoresist

18. Remove the oxide using Etching
Photolithography 18
Remove the oxide using Etching

19. Now remove the photo resist by ashing
Photolithography 19
Now remove the photo resist by ashing

20. ion implantation or diffusion
Photolithography 20
Diffuse new region by
ion implantation or diffusion

21. Photolithography 21

22. Photolithography 22

23. The exposed areas will become softened (for positive photoresist)
Photolithography 23
The surface patterns of the various integrated-circuit components can be defined repeatedly using photolithography
Here, a photographic plate with drawn patterns will be used to selectively expose the photoresist under a deep ultraviolet illumination (UV)
The exposed areas will become softened (for positive photoresist)
The exposed layer can then be removed using a chemical developer, causing the mask pattern to be duplicated on the wafer
Silicon dioxide, silicon nitride, polysilicon, and metal layers can be selectively removed using the appropriate etching methods
After the etching step(s), the photoresist is stripped away, leaving behind a permanent pattern of the photomask on the wafer surface

24. Photolithography 24

25. Photoresist Coating 25
The wafer surface is coated with a photosensitive layer called photoresist, using a spin-on technique

26. Photoresist Coating 26

27. Photoresist Composition27
The most commonly used positive resist consist of diazonaphtoquinone (DQ), which is the photoactive compound (PAC), and novolac (N), a matrix material called resin. Upon absorption of UV light, the PAC undergoes a structural transformation which is followed by reaction with water to form a base soluble carboxylic acid, which is readily soluble in basic developer (KOH, NAOH, TMAH etc.)

28. Types of Photoresist Positive Photoresist28
Positive Photoresist
Most commonly used in the IC industry.
Become soluble after exposure
Better resolution
Cheaper
Negative Photoresist
Becomes insoluble after exposure
When developed, the unexposed parts dissolved

29. Soft Bake 29
Used to evaporate the coating solvent and to densify the resist after spin coating.
Typical thermal cycles: °C for 20 min. in a convection oven, °C for 45 sec. on a hot plate
Commercially, microwave heating or IR lamps are also used in production lines.
Improves adhesion
Improves uniformity
Improves etch resistance
Improves line width control
Optimizes light absorbance characteristics of photoresist

30. What Is a Photomask? Material Used to make Photomasks:30
Photomasks are high precision plates containing microscopic images of electronic circuits.
Material Used to make Photomasks:
There are four types of material used to make photomasks;
quartz (the most commonly used and most expensive), LE, soda lime, and white crown.

31. Types of Photomask 31

32. Different Photomasks 32
15 – 20 different mask levels are typically required for a complete IC process

33. Defects in Photomask 33

34. Photomask Aligner 34

35. Light Sources Increasing Cost35
Source λ
Resolution
Hg lamp(g-line)
436 nm
400 nm
Hg lamp (i-line)
365 nm
350 nm
KrF
248 nm
150 nm
ArF
193 nm
80 nm F2
157 nm
Research
Increasing Cost
Difficulties lie in sources, and materials for optics and masks
Extreme UV, X-ray lithography reasearch topics

36. Wafer Exposure Systems36

37. Wafer Exposure Systems37

38. Contact Printing The mask is directly in contact with the wafer38
The mask is directly in contact with the wafer
Advantages
Simple
Low Cost
Disadvantages
Poor for small features
Mask damage may occur from contact
Defects from contaminants on mask or wafer due to contacting surfaces

39. Proximity Printing The mask is above the wafer surface Advantages39
The mask is above the wafer surface
Advantages
Mask damage is minimal
Good registration possible
Disadvantages
Poorer resolution due to distance from the surface
Diffraction errors

40. Projection Printing 40
An optical system focuses the light source and reduces the mask image for exposure on the surface
Advantages
Higher resolution
Lens system reduces diffraction error
Disadvantages
Errors due to focus of lens system may occur
Limiting factor in resolution can be due to optical system

41. Develop 41
Soluble areas of photoresist are dissolved by developer chemical
Visible patterns appear on wafer
windows
islands
vacuum chuck
spindle
developer
dispenser
to vacuum pump

42. Hard Bake Evaporate remaining photoresist Improve adhesion42
Evaporate remaining photoresist
Improve adhesion
Used to stabilize and harden the developed photoresist prior to processing steps
Eliminates the solvent burst effects in vacuum processing
Introduces some stress into the photoresist.
Needed for acid etching, e.g. BOE.

43. Etching Etch oxide with hydrofluoric acid (HF)43
Etch oxide with hydrofluoric acid (HF)
Only attacks oxide where resist has been exposed

44. Photoresist Removal (Stripping)44
Want to remove the photoresist and any of its residues.
– Positive photoresists:
• acetone
• trichloroethylene (TCE)
• phenol-based strippers
– Negative photoresists:
• methyl ethyl ketone (MEK), CH3COC2H5
• methyl isobutyl ketone (MIBK), CH3COC4H9
Plasma etching with O2 (Ashing) is also effective for removing organic polymer debris.

45. Performance Metrics 45
Resolution: minimum feature dimension that can be transferred with high fidelity to a resist film.
Registration: how accurately patterns on successive masks can be aligned (or overlaid) with respect to previously defined patterns.
Throughput: number of wafers that can be exposed/unit time for a given mask level.

46. Limitations of Optical Lithography46
Resolution becoming a challenge for deep-submicron IC process requirements
Complexity of mask production and mask inspection
High cost of masks

47. Electron Beam Lithography47
Involves direct exposure of the resist by a focused electron beam without a mask
Resolution as low as 10 – 25 nm

48. Electron Beam Lithography48
Electron gun generates beam of electrons
Condenser lenses focus the e-beam
Beam-blanking plates turn beam on and off

49. Electron Beam Lithography49
Advantages
Generation of submicron resist geometries
Highly automated and precisely controlled operation
Greater depth of focus than that available from optical lithography
Direct patterning on wafer without using a mask
Disadvantages
Low throughput
Expensive resists
Proximity effect: backscattering of electrons irradiates adjacent regions and limits minimum spacing between features

50. Extreme ultraviolet : EUV50
Next Generation Lithography
Vacuum operation
Laser plasma source
Very expensive system
Uses very short 13.4 nm light
Step and scan printing
All reflective optics (at this wavelength all materials absorb!)
Uses reduction optics (4X)
Optical tricks seen before all apply: off axis illumination (OAI), phase shift masks and OPC

51. Extreme ultraviolet : EUV51
Challenges:
EUV is strongly absorbed in all materials.
Lithography process must be performed in vacuum
Mask blank must also be multilayer coated to minimize its reflection.

52. X-ray Lithography Advantages: Low diffraction Shorter exposure time52
Advantages:
Low diffraction
Shorter exposure time
Scattering is minimum
X –rays pass through spots
1nm
Problems:
Masks are the most
Difficult and critical
Element of an XRL system
lacking of photoresist
1:1 printing
High energy x-ray destroy
conventional optics

53. X-ray Lithography 53