In the modern society, the demand for chips is very large, and chip lithography is the most important, complex and expensive process in the chip manufacturing process. Its precision determines the manufacturing process of the chip and the performance of the device. So, a deep understanding of chip lithography process plays a vital role in chip manufacturing.
What is chip lithography?
Photolithography is an important step in the fabrication of integrated circuits. Chip lithography is to apply a thin layer of photoresist on the surface of the silicon wafer, and leave the pattern of the mask plate on the photoresist after light and development, and then engrave the pattern on the mask plate onto the IC substrate to be processed, including silicon wafer, other metal layers, dielectric layers, etc.
General requirements for chip lithography
- High resolution graphics
- High photoresist sensitivity
- High precision of interlayer alignment
- Low defect density
Composition of photoresist
- Polymer solvent
1. High resolution
- The thinner the photoresist, the higher the resolution
- The thinner the photoresist, the lower the resistance to etching and ion implantation
2. High etch resistance (requires thick film)
3. Good adhesion
4. Strong injection shielding ability and less pinholes (requires thick film)
5. Wide Process window – Adapt to process changes
According to different properties, photoresists can be divided into positive and negative resists.
In the early stage of process development, negative resist has always been dominant in the chip lithography process. With the emergence of VLSI IC and 2-5 micron pattern size, negative resist can no longer meet the requirements. Positive glue appeared later, but the disadvantage of positive glue is poor bonding ability.
- The most critical step in IC manufacturing
- The most expensive device in the IC wafer
- Most Challenging Technology
- Determining the minimum feature size
What are 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
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.
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)
- The developer dissolves part of the photoresist
- Positive photodevelopers usually use weakly alkaline solvents
- 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)
- 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.
In short, chip lithography is an extremely complex process, which includes many auxiliary processes in addition to these main processes. Therefore, in order to ensure the quality of chips, when producing semiconductor chips or other components, chip lithography usually needs to be repeated many times for a substrate.
Chip lithography technology is an important part of chip technology . In addition to mastering the technical process of chip lithography, the manufacturing technology equipment of the chip industry chain such as lithography machines is indispensable . For example, chip architecture and EDA tools in design ; lithography, etching, etching and other equipment in manufacturing; materials also include a large number of chemical reagents, inert gases, masks, etc.
Chip lithography is the most important and complex process in chip manufacturing. Chip lithography needs to go through many steps, and each step cannot be ignored.
Basic steps of lithography:
Pre-baking and pre-treatment
Bake after coating
Alignment and Exposure
Post exposure bake
Post development bake
ASML(Advanced Semiconductor Material Lithography), one of the world's largest semiconductor equipment manufacturers, provides lithography machines and related services for semiconductor manufacturers. TWINSCAN series is the world's highest precision, highest production efficiency, and most widely used high-end lithography models. The vast majority of semiconductor manufacturers in the world purchase TWINSCAN models from ASML, such as Intel, Samsung, TSMC, and SMIC.