Process principle
Laboratory scraper coaters use a precise gap control mechanism to apply photoresist to the surface of the substrate at a constant thickness. The key principle lies in the threaded or groove design of the scraper surface, which forms quantitative grooves during rotation, allowing for precise control of wet film thickness. For submicron-level preparation, the gap between the scraper and the substrate needs to be adjusted to the nanoscale, typically achieved by micron-scale spiral micrometers or piezoelectric actuators. The entire process should be completed in a clean environment to avoid uneven film thickness due to particle contamination.
Equipment composition
The device mainly consists of the following components:
High-precision scrapers: usually made of stainless steel or carbide surface, ground and processed until the surface roughness is less than 0.1 microns.
Substrate Table: Equipped with a vacuum adsorption system to ensure that the substrate remains flat and fixed during the coating process.
Drive system: The rotation speed of the scraper is controlled by servo motor, and the speed stability error is less than ±1%.
Gap adjustment unit: Using the differential thread principle, the adjustment accuracy can reach 0.1 microns.
Process flow
The submicron wet film preparation process includes the following steps:
1. Substrate Pretreatment: Plasma cleaning or solvent wiping is used to remove surface contaminants.
2. Photoresist application: The photoresist is applied to the front of the substrate in a linear fashion through a pipette.
3. Squeegee coating: Set the rotation speed and movement speed of the squeegee to spread the photoresist evenly. Reference formula for calculating the movement rate:
Film thickness = (scraper groove depth × solid content) / (coating speed × viscosity coefficient ×correction factor)
Among them, the groove depth is determined by the scraper specification, and the solids content is determined by the data provided by the photoresist supplier.
4. Pre-drying: Treat on a thermostatic plate at 80°C to 100°C for 60 seconds to remove part of the solvent.
Key parameters:
| Parameter Name | Typical values or ranges |
| The scraper groove is deep | 5 microns to 50 microns |
| Coating speed | 1 mm/s to 100 mm/s |
| Gap setting | 0.5 microns to 10 microns |
| photoresist solid content | 20% to 40% |
Influencing factors
Film thickness uniformity is affected by the degree of scraper wear (surface topography needs to be checked regularly), changes in photoresist viscosity (temperature fluctuations can lead to viscosity changes), and substrate surface energy (plasma treatment improves wettability). In addition, the coating speed and scraper pressure need to be adjusted in tandem to avoid streaks or bubble defects.
Quality control
After the preparation is completed, the following tests should be carried out: use white light interferometer to measure the film thickness distribution, and the deviation should be controlled within ±5%; use optical microscopy to check the defect density, which is required to be less than 1 per square centimeter. If the film thickness exceeds the specification, the scraper gap can be corrected or the dilution ratio can be adjusted.
Application examples
In the process of microelectronic pattern transfer, the thickness uniformity of the sub-micron photoresist wet film prepared by this method is statistically less than 3%. This process has been successfully used to fabricate test structures with a line width of 0.5 microns, and the quality of the pattern after subsequent etching meets the standard requirements.
References
1. Technical Guide to Laboratory Coating, Industry Standard Technical Report, 2023
2. Review of Submicron Thin Film Preparation Methods, Materials Processing Research, 2022
3. Manual of Lithography Process Parameter Optimization, Microfabrication Technology Series, 2021