1. INTEGRATED CIRCUITS
Dr. Esam Yosry
Lec. #6

2. Lithography Introduction Lithography Photolithography
Photoresist Process
Types of Photoresist
Photomask
Clean Rooms
Lithography

3. Introduction (Chip Fabrication Cycle)

4. Introduction (Processes)
Oxidation
Diffusion
Ion Implantation
Deposition
Etching
Lithography
Deposition
Removal
Patterning
Modification of electrical properties
Introduction (Processes)

5. Lithography
Lithography is the process of transferring patterns drawn on a mask to a thin layer of radiation sensitive material (resist) covering the surface of the material to be etched (oxide or metal).
The main equipment is optical ultraviolet unit (UV wavelength 200 to 400 nm).
In this case the mask is called photomask (PM), the resist is called photoresist (PR) and the process is called photolithography.

6. Lithography
Patterning process consists of mask design, mask fabrication and wafer printing.

7. Lithography
In case of X-ray (wavelength 0.2 to 5 nm) the resist is called X-ray resist and the process is called X-ray lithography.
In electron-beam (EB) lithography (wavelength ≈ 0.1 nm) using EB resist.
The shorter the radiation wavelength the smaller the feature size that can be produced.
Since the EB radiates very small spots, no mask is needed and direct exposure of the EB resist is carried out (serial exposure of the resist or direct writing on the resist).

8. Lithography
When masks are used (photo- and X-ray- lithography), we irradiate the whole resist surface using parallel exposure.
EB lithography is used when the product volume is small because masks are very expensive to fabricate.
Masks are themselves fabricated by EB lithography for shaping chromium metallic layer deposited on quartz transparent supports.
Masks are only used to reproduce very large number of wafers exceeding

9. As processing equipment and technologies become more advanced, transistor size shrinks
In the 70’s, 8
Now, 0.02
This is due to advancement in lithography .
Each layer of dopant or material has its own layout (15 – 20 mask).
Lithography

10. Photolithography
Photolithography is the heart of integrated circuit processing. It is the method used to transfer the individual circuit design knowledge onto the silicon wafer.
Probably one half of the wafer fabrication costs go into obtaining proper photolithography.
The basic elements consist of:
An Align and Expose Tool
Masks containing design information
Photo-sensitive Resist

11. Basics of Photolithography
Silicon Wafer with Silicon Dioxide
SiO2
To be etched
Silicon Wafer

12. Basics of Photolithography
Apply photo resist
Photoresist
SiO2
Silicon Wafer

13. Basics of Photolithography
Apply photomask
Photomask
SiO2
Silicon Wafer

14. Basics of Photolithography
Apply Ultraviolet Light
Silicon Wafer
SiO2
Photomask
Decreasing feature size require the use of shorter λ

15. Basics of Photolithography
Develop photoresist
Silicon Wafer
SiO2

16. Basics of Photolithography
Etch Silicon Dioxide
Silicon Wafer
SiO2

17. Basics of Photolithography
Strip Photoresist
Silicon Wafer
SiO2

18. Photoresist Process
This flow chart shows the typical process used for VLSI Lithography.

19. Photoresist Process 1. Substrate Cleaning and Preparation
The surface is as clean as possible immediately after a high temp operation.
Acid cleaning may be necessary if wafers sit for a long time.

20. Photoresist Process 2. Wafer Coating
The goal of coating is to apply a uniform, defect free film of photoresist over The entire wafer. Spin Coating is used for VLSI
accelerate the wafer - fast
spin at constant speed to get uniform thickness
Before spin After spin
PR Wafer PR
Chuck

21. Photoresist Process 3. Pre-Bake (Soft-Bake) The Soft-Baked is used to:
Drive off solvent from resist. (reduced to ~5%)
Improve adhesion and anneal stresses in resist
Typical process Temp is °C
Ovens
Convection oven - Very stable over time. ~ 30 min
IR oven - Most popular min.
Hot plate - very effective for single wafers.

22. Photoresist Process 4. Expose
To produce the desired photochemical effects in the shortest time possible.
Time is important because:
Wafers are individually processed
Align machines are expensive
However, short times usually mean lower resolution.
Exposure to UV light chemically changes the resist
Silicon Wafer
SiO2
Photomask

23. Photoresist Process 5. Develop
Silicon Wafer
SiO2
Development removes the unwanted photoresist.
Positive Resist - Area exposed is removed
Negative Resist - Area not exposed is removed

24. Photoresist Process 6. Post-Bake 100-120°C 30 minutes
Hardens Resist Prior to Etch
Temperature is critical
Too high a Temp (>120°C) causes resist to flow
Extra high Temp (>180°C) causes resist to lift.

25. Photoresist Process 7. Inspection
The object of this inspection is to verify the photoresist is acceptable prior to etch.
A complete rework can now be performed if the photoresist is not acceptable.

26. Photoresist Process 8. Etch
Silicon Wafer
SiO2
Dry Plasma Etch is commonly used in commercial manufacturing lines.
We use Wet Etch with acids because of the cost and flexibility of the process.

27. Photoresist Process 9. Strip
It is a complete removal of the Photoresist.
Dry Plasma Strip in O2 is commonly used in commercial manufacturing lines.
We use Wet Chemical Strip in special solvents because of the cost and flexibility of the process.
Silicon Wafer
SiO2

28. Photoresist Process
PHOTORESIST AREA

29. Types of Photoresist

30. Photomask The pattern to be etched on the wafer surface is drawn on
the pohtomask
Photomasks are made from chromium
Many masks are needed in recent CMOS technologies. The # of masks depends on the process complexity

31. Clean Rooms
Photolithography must be carried out in a clean room otherwise dust particles causing fabrication errors.
The total number of dust particle are controlled with temp and humidity.
Clean rooms standards:
Class 100 < 100 pp ft3 (0.5µ)
Class 10 < 10 pp ft3 (0.5µ)
Class 1 < 1 pp ft3 (0.5µ)

32. Thanks
Many thanks to Prof. Hany Fikry and Prof Wael Fikry for their useful materials that help me to prepare this presentation.