Chip lithography process

Basic steps of lithography:

1. Wafer Clean

• Remove contaminants
• Remove particles
• Reduce pinholes and other defects
• Improve photoresist adhesion

Basic steps: chemical cleaning- rinsing -drying.

2. Chip lithography process – pre-bake

• Drain and dry
• Remove moisture from the surface of the silicon wafer to improve the adhesion of the photoresist to the surface
• Usually at 100°C
• Simultaneously with pre-processing

3. Chip lithography process – pre-treatment

• Prevent the resist from detaching from the silicon wafer surface during development
• Usually combined with pre-bake
• The silicon wafer should be cooled before coating

4. Wafer cooling

• Silicon wafer needs to be cooled before coating
• Silicon wafers are cooled on a cooling plate
• Temperature affects the viscosity of the photoresist – affects the thickness of the photoresist

5. Uniform glue

• Silicon wafer is adsorbed on the vacuum chuck
• Liquid photoresist drops in the center of the silicon wafer
• The chuck rotates, and the photoresist spreads under the action of centrifugal force
• High-speed rotation, the photoresist evenly covers the surface of the silicon wafer
• First rotate at low speed ~500rpm
• Then rise to ~3000-7000rpm

6. Bake after coating

• Most of the solvent in the photoresist is evaporated.
• Solvents help get thin photoresist films but absorb light and affect adhesion
• Post-exposure bake time and temperature depend on process conditions
• Overbaking: polymerization, reduced photosensitivity
• Insufficient postbaking: affects adhesion and exposure

7. Wafer cooling

• Need to cool down to ambient temperature
• Silicon wafers are cooled on a cooling plate
• Thermal expansion rate of silicon: 2.5X10-6/°
• For an 8-inch silicon wafer, a change of 1°C causes a diameter difference of 0.5 microns

8.  Alignment

Chip lithography alignment technology is an important step before exposure and is one of the three core technologies of chip lithography. Generally, the alignment accuracy is required to be 1/7—1/10 of the thinnest line width.

With the improvement of lithographic resolution, the requirement for alignment accuracy is getting higher and higher. For example, for a line width of 45am, the alignment accuracy is required to be around 5am.

9. Exposure

Exposure methods include:
a. Contact Printing: The mask plate is directly in contact with the photoresist layer.
b. Proximity Printing: The mask plate is slightly separated from the photoresist layer, which is about 10-50 μm.
d. Stepper exposure (Stepper)
c. Projection Printing. A lens is used between the mask and the photoresist to focus light to achieve exposure.

Two parameters in exposure need to be considered:
1. Exposure energy (Energy)
2. Focal length (Focus)

10. Development

• The developer dissolves part of the photoresist
• Positive photodevelopers usually use weakly alkaline solvents
• Tetramethylhydrogen
• Transfer the pattern on the mask to the photoresist
• Three basic steps: developing, cleaning, and drying

Now there are two development methods: one is wet development, which is widely used in IC and micro-processing, and the other is dry development.

A. Batch Development – consuming a lot developer; but low development uniformity

B. Continuous Spray Development/Auto-rotation Development.

C. Puddle Development

Common problems in development:

• Incomplete Development.
• Under Development
• Over Development

11. Post development bake

• Evaporate the solvent in the photoresist
• Improved etch resistance and ion implantation resistance
• Improves adhesion between photoresists and silicon wafer surfaces
• Polymerizes and stabilizes photoresist
• Photoresist flows to fill pinholes

12. Graphic inspection

• Unqualified silicon wafers will be reworked with photoresist removed
  – Patterning of photoresist is temporary
  – Patterns after etching and implantation are permanent
• Lithography is reworkable
• Cannot be reworked after etch and implant
• Optical microscope
• Scanning Electron Microscope (SEM)

13. Check

• Alignment accuracy
• Critical Dimension (CD)
• Surface defects such as scratches, pinholes, stains, contaminants, etc.

14. Etching or ion implantation

Etching is the process of selectively removing specific parts of a deposited layer by chemical means. Etching is generally divided into electron beam etching and lithography. Chip lithography requires a high degree of cleanliness for materials to ensure the flatness of materials. Electron beam etching, slow and expensive equipment.

15. Photoresist removal

The main purpose of photoresist is to protect the part of the substrate under the photoresist. After the chemical or mechanical treatment process is completed, it needs to be removed. The methods of degumming mainly include wet degumming and dry degumming.