1. Lithography
Layout, Mask, photo

2. Index Quick Review Layout Basics Mask Basics Lithography Key Steps
Details
Optics
Production
1-Dec-18

3. Layout, Mask & Photo Layout Mask Photo (Lithography)
Convert electrical design to physical design
2D ‘picture’ of the final chip (quantitative)
Mask
Creates a physical layout (similar to photo negative)
Can be used to general many chips (similar to obtaining many copies from a negative)
Photo (Lithography)
Process of obtaining a ‘picture’ from a ‘negative’
(ie generating shapes on wafer from a mask)
1-Dec-18

4. Chip manufacturing: Snap shot
Electrical Chip Design R C
Physical “Layout” Design
Blue Print-
Photo
“negative”
Creating the chip
Review
Testing
“Print”
Quality Control
1-Dec-18

5. Layout: Basics Electrical circuit Physical circuit N P+ Oxide
Diode
Resistor
Electrical circuit
Physical
circuit N P+
Oxide
Substrate (p silicon)
Oxide N P+
Substrate (p silicon)
Oxide
1-Dec-18

6. Layout: Basics Layout Example
Each layer is made one at a time. Hence each layer has a ‘photo negative’ or mask
Therefore, each layer must be specified separately in the ‘layout’ file. It is a computer file (softcopy)
M1 layout
Example
A sample part of a chip
Layout provides the top view (2D)
The ‘depth’ is controlled by the deposition process (and not by the layout)
1-Dec-18

7. Layout: Basics Z1 Z2 Layout will provide information on x and y
Substrate (p silicon)
Oxide N P+ Y1 Y3 Y2 Y4 X1 X2 X3 X4 Z1 Z2
Layout will provide information on x and y
Process flow sheet will provide information on z
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8. Layout: Basics Example
A sample part of a chip M1
Via 12 M2
The file has different layers (numbered from 1 to...)
A table which correlates the layers to numbers is also provided
1-Dec-18

9. Layout: Basics
Via and contacts are shown as squares (or in rare cases rectangles) in the layout. Others are in rectangular geometry
Cheater rules for SRAM
However, the mask will have circular (or in rare cases elliptical) holes
Completely filling rectangular features with high aspect ratio (with metal) is difficult
Aspect ratio: Depth/width (or diameter)
width
Low aspect ratio
High aspect ratio
Dia
height
1-Dec-18

10. Layout: Basics Hierarchy vs Flat file Levels, Layers
GDS (stream format), GDSII, MEBES, CIF
Review
Softcopy (quantitative)
of what is ideally required
Top view (2D) only
Hierarchy
1-Dec-18

11. Mask: Basics
The layout file is given as ‘input’ the mask making machine
E-beam writer (electron beam)
Scans the “blank” mask
Mask may be a plate of glass, coated with chromium. On top of chromium, a photosensitive coating is applied
A mask is often referred to as ‘chrome’
E-Beam, ‘sensitizes’ the areas
Develop the resist
Remove chromium on the ‘exposed’ areas
1-Dec-18

12. Mask: Basics Relatively slow process Mask has to be very very clean.
beam goes to each ‘pixel’ which has to be written ‘on’
switches “on” and then “off”
moves to the next pixel which has to be written on
Mask has to be very very clean.
Any particle on the mask will cause incorrect ‘feature’ in all the chips
==> chips will to fail
A mask is then coated with a clear film, to protect it.
Eg. If a particle falls on the mask, then you can remove the clear film and put a new film. (Easy to ‘repair)
1-Dec-18

13. Mask: Basics A mask may be 100 mm x 100 mm (for example)
The size is decided by the size of the lens in the lithographic tool
A mask is usually 4 to 5 times larger than the actual feature. The features are ‘reduced/ zoomed out’ during the lithographic process
Correspondingly, a mask is referred to as ‘4x mask’ or ‘5x mask’
Example: If you want a 100 nm wide line on the wafer
layout file will show 100 nm wide line on the screen
mask making machine will enlarge everything by 4
Actually, layout file will be converted to another file which will enlarge it by 4 and then the mask making machine will use it.
The process is sometimes called MDP (Mask Data Preparation)
Mask will have 400 nm wide opening and
Lens in the litho process will ‘shrink’ the image to 100 nm.
1-Dec-18

14. Mask: Basics A mask may be 100 mm x 100 mm (for example)
So, one ‘print’ will be about 25 mm by 25 mm, on the wafer.
A chip may be only 5 mm by 4 mm
So, one mask will have perhaps 20 chips, if the chip is small
The gaps between the ‘real chips’ are used for
alignment marks (TBD)
test structures (TBD)
1-Dec-18

15. Mask: Basics If a mask gets dirty in a particular location,
with in a ‘field’, a die will always fail
may also happen due to lens issues
Pizza masks, to verify design changes
1-Dec-18

16. Mask: Basics A mask may be dark-field or light-field
Depends on the photo resist that will be used in the process
If photo resist dissolves in the exposed regions, it is positive photo resist; else negative
+ve resist;
dark field mask
-ve resist;
light field mask
Desired structure
Mask Review
1-Dec-18

17. Lithography: Overview
Shape Definition
Key steps
Details of steps
Optics: Relevant information
Production: Relevant information
Review of Lithography
1-Dec-18

18. Lithography: Basic steps
Coat the wafer with light sensitive material (photo-resist)
Place the mask over the wafer and align with the wafer. Adjust the focus of the camera
Expose the wafer to light (for a controlled time period)
Exposed photoresist will have changed chemically
Wash the wafer+photoresist in a solvent (Developing the resist)
Removes only exposed photo resist
Etch the ‘open’ areas
Remove all the remaining resist
Animation
1-Dec-18

19. Lithography: Schematic
Note: Some schematics will show mask between lens and wafer. In reality, at least one lens will come between the mask and the wafer

20. Lithography: Basic Steps
Coat the wafer with light sensitive material (photo-resist)
Mask over the wafer, align, focus
Exposure for a precise amount of time
Move (Step) to the next ‘Field’
Stepper
Actually, the wafer moves, not the litho system
Repeat the above procedure
1-Dec-18

21. Lithography: Basic Steps
After the whole wafer is exposed....
Repeat for other wafers in the lot
Crude schematic below
Develop the lot
Etch the open areas
Remove the remaining photo resist and the etch products
Note: Etch product forms a thin film or ‘veil’. Removal of the veil is called ‘de-veil’
1-Dec-18

22. Lithography: Details Photo Resist Coating
Mostly positive resists used now
More resistance to pinhole formation
Easier removal (stripping)
More expensive
Harder to develop (solubility differential is not high)
-ve resist; most area open
+ve resist; most area blocked
1-Dec-18

23. Lithography: Details Photo Resist Coating Prepare the wafer
Clean the wafers (to remove particles)
Dry ( C bake)
Primer (HMDS- Hexa Methyl Di Silazane)
Immersion (liquid)
Spin coat (liquid) and high speed drying
Vapor prime
Photo resist coating (spin coating)
1 um thick (± 1% variation)
1-Dec-18

24. Lithography: Details Photo Resist Coating Dynamic coating
Low rpm (500) dispense
High rpm (5000) thinning (form uniform film)
Final resist thickness depends on rpm and time
and viscosity, surface tension, drying characteristic of solvent...
Moving arm to enhance uniformity
1-Dec-18

25. Photo Resist properties
Lithography: Details
Photo Resist properties
Contains photosensitive materials, sensitivity enhancers (called sensitizers), solvent
Resolution
Contrast
Response to a given wavelength (spectral response)
Solubility differential between unexposed vs exposed regions
Etch/ Implant resistance
1-Dec-18

26. Lithography: Details Pre Expose Bake Soft Bake:
To drive out the solvent
Resist is still ‘soft’, but not liquid like
Temperature:
Baking options:
Hot Plate (manual, moving)
Convection Oven (may form crusts)
Infra-red (wafer surface heated)
Microwave (volume heating)
Vacuum baking (radiation)
1-Dec-18

27. Wafer (with resist coating)
Lithography: Details
Exposure
Litho Tool (current): Projection aligners
Stepper vs Scanner
Historical: Contact, proximity aligners
Proximity:
Mask almost in contact with wafer
Mask may become dirty over time, but not as bad as contact
1x mask, minor optical distortions
Contact:
Mask in contact with wafer
Mask may become dirty over time
1x mask, no optical issues
Wafer (with resist coating)
MASK
1-Dec-18

28. Lithography: Details Exposure
Litho Tool (current): Projection aligners
Stepper vs Scanner
Stepper: What we saw before
Scanner: Actually Step & Scan
Need smaller lens for the same sized mask
Or with same sized lens, can handle larger mask
Stepper
Step & Scan
Lens
Lens
MASK
1-Dec-18

29. Schematic
1-Dec-18

30. Lithography: Details Exposure Light Sources Hg Lamp: Filter
G,H, or I-line (365 nm)
Excimer Lasers (DUV)
ArF193 nm , RF 248 nm etc
EUV, X-Ray, E Beam (in R&D)
EUV: Reflective optics G I H
400 nm
Intensity
600 nm
1-Dec-18

31. Lithography: Details Develop Post Exposure Bake
Similar to pre-exposure bake
Help minimize standing wave effects
Developing
Immersion
Spray develop
Plasma (dry) develop
Rinse
Hard bake: 200 C, 30 mins (for example)
To protect from etch/implantation
Resist flow
Etch
1-Dec-18