Negative Photoresist Lithography Process
Lithography is one of the most important process steps in semiconductor manufacturing. The main function is to copy the graphics on the mask board to the silicon wafer, so as to prepare for the next etching or ion implantation process. The cost of lithography is about 1/3 of the whole silicon wafer manufacturing process, and the time consumption is about 40~60% of the whole silicon wafer manufacturing process.
Detailed Steps
Substrate Clean
After different processes, the surface of the substrate has been seriously contaminated, common are: organic impurities contaminated, Particle contamination, metal ion contamination and so on. Cleaning the substrate with ethanol (Product No.: E130059) or trichloroethylene (Product No.: T119718) as solvent can effectively remove the stain. After washing and rinsing the substrate, the substrate is baked for 20 minutes at 120-200°C. Finally, there are no organic impurities and surface particles left on the substrate.
Primer Vapor
Uniform height and consistent coating thickness can be achieved using a rotary coating system. The substrate should be coated with photoresist before spin coating begins. Do not apply photoresist when spinning, as it may cause uneven distribution. Square or rectangular substrates are best applied at low RPM (50-1000 RPM). An undiluted resist at 75 RPM produced a coating of about 2.5μM, with thicker edges and corners of the substrate. The thickness of the resist depends on the spin rate, acceleration, and viscosity of the resist. Circular substrates are best coated at high RPM (2000-5000 RPM). Substrate rotation at a speed of > 5000 rpm has little effect on the thickness of the baffle. The dependence of thickness on spin rate decreases with the decrease of viscosity. For a more uniform, thinner film and to avoid thickening the edges of the substrate, acceleration to peak rotational speed of 0.1 seconds is optimal. Using a spin coating system, film thicknesses range from 0.3 to 2μM. To avoid etching penetration and pinhole formation, thick films (1-2μM) are usually preferred. However, thick films can lead to a decrease in resolution. For better results, apply two thin coats.
Pre-bake
In order to achieve a strong adhesion between the resist and the substrate, all remaining solvents and volatile components need to be removed by evaporation. Failure to thoroughly bake the resist will affect the crosslinking necessary for the resist process. But attention should also be paid to avoid excessive baking, otherwise it will lead to fogging or decomposition of the resist. Pre-bake at 82°C for 20 minutes is generally recommended.
Photoresist Coating
Any light source with near ultraviolet radiation can be used to expose photoresist. Large area light sources should only be used for thick lines (50 μM or larger). For thin line patterns, a light source that diffuses less must be used. Recommended fine stripe light sources are carbon, high-pressure mercury vapor, or xenon flash. Proper exposure requires about 100 MW/cm2. The intensity of the light. Thickness and processing variables can affect exposure. As long as the light source produces 10 MW/cm2. The minimum amount of irradiation on the substrate surface, 1-10 seconds exposure time can be. Exposure energy should not vary by more than 10% of the optimal value, otherwise fine line sharpness and reproducibility will be lost. The diffraction effect of photoresist can cause cross-linking under the mask, resulting in line widening up to 2.5μM.
Development
Spray the developer onto the coated substrate for 10-60 seconds, then rinse several times with isopropyl alcohol (Product No.: I292350). Then dry the surface with nitrogen or pure compressed air.
Hard Bake
Drying of the photoresist is required to remove residual solvents and volatile components and to help enhance the chemical stability and adhesion of the photoresist. The recommended post-baking time is 10-20 minutes and the temperature is 120°C, never exceeding 148°C.
Etching
For most etching steps, parts of the top layer of the wafer are protected by a "shield " that cannot be etched, allowing selective removal of specific parts of the layer. In some cases, the material is photo-resistive, similar to the principle used in lithography. In other cases, etching masks need to be resistant to certain chemicals, and silicon nitride can be used to create such "masks."
Photoresist Removal
The main function of the photoresist is to protect the substrate section under the photoresist during chemical or mechanical processing of the entire area. So when the above process is over, the photoresist should be removed, this step is referred to as de-gluing. There are two common ways to remove glue: wet and dry. Wet de-gluing can be subdivided into organic solvent de-gluing (using organic solvent to remove photoresist) and inorganic solvent (using some inorganic solvent to oxidize the carbon element in organic matter such as photoresist into carbon dioxide, and then remove it). Dry gluing is the use of plasma to remove the photoresist.
Precautions
1. Pay attention to avoid dust and impurity pollution;
2. Relative humidity should be controlled between 30-50%;
3. Appropriate lighting, preferably gold fluorescent, yellow incandescent, or white fluorescent with yellow or orange filters;4. Maintain proper ventilation.