Silicon Wafer Cleaning
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The increase in etch and deposition steps, new materials, and new structures used in 2.5D and 3D packaging rely heavily on cleaning processes like photoresist strip and descum to ensure contamination free surfaces.
Prior to a wafer's entry into the fabrication process, its surface must be cleaned to remove any adhering particles and organic/inorganic impurities. Silicon native oxide also needs to be removed.
wafer cleaning procedures can make up 30% - 40% of the steps in the total manufacturing process.
Silicon wafer wet cleaning
Pirhana RCA SC1, SC2 DHF UPW (ultra pure water) Piranha (SPM) Sulfuric acid/hydrogen peroxide/DI water H2SO4/H2O2/H2O 3-4:1; 90°C SC-1 (APM) Ammonium hydroxide/hydrogen peroxide/DI water NH4OH/H2O2/H2O 1:4:20; 80°C SC-2 (HPM) Hydrochloric acid/hydrogen peroxide/DI water HCl/H2O2/H2O1:1:6; 85°C DHF Dilute hydrofluoric acid/DI water (will not remove copper) HF/H2O1:50 DIO3 Ozone in de-ionized water O3/H2O Optimized Mixtures DHF Dilute hydrofluoric acid/DI water HF/H2O 1:100 BHF Buffered hydrofluoric acid NH4F/HF/H2ORCA cleaning continues to be the primary FEOL pre-deposition cleaning in the industry today. RCA cleaning procedures are a combination of the different procedures described above. The process consists of consecutive SC-1 and SC-2 solutions, followed by treatment with a dilute HF solution or buffered oxide etch (BOE). The product is a clean, hydrogen-terminated silicon surface, ready to be used in the process flow.
Advanced Reticle Cleaning Traditional methods (RCA) for reticle cleaning degrade their optical properties and reticles can only be cleaned between 2 and 8 times before unacceptable degradation occurs due to surface roughening. Since ozone-based chemistries produce very little surface roughening, they are becoming preferred in reticle cleaning applications.
Ultrapure water is required for many process steps. Early stages of device fabrication require repeated steps for wafer cleaning, rinsing and surface conditioning. At many different stages in device manufacturing, it is used for surface cleaning, wet etch, solvent processing, and chemical mechanical planarization. Indeed, the latter unit process has become one of the largest consumers of UPW within the fab, requiring high volumes for slurry production and rinsing.
UPW is normally produced using reverse osmosis / deionised resin bed technologies; however, as device linewidths continue to shrink, the requirement for ever higher water purities in semiconductor applications is expected to increase beyond the capabilities of current production technologies. Indeed, modern semiconductor standards for ionic contaminants in UPW are so stringent that some analyses are beyond the detection limits of available analytical tools.
Silicon wafer dry cleaning
The most common application for dry substrate cleaning is photoresist stripping and "descum" steps. Photoresist stripping involves the removal of photoresist residues by reaction with atomic oxygen radicals or some other highly reactive species to produce a volatile product that can be pumped away by the vacuum system. Photoresist descum is a similar but milder process carried out after photoresist patterning and development to remove residual photoresist left in a developed area.
The increase in etch and deposition steps, new materials, and new structures used in 2.5D and 3D packaging have made cleaning processes like photoresist strip and descum increasingly important for high device yield. The varying levels of cleanliness requirements and the different materials employed in the manufacturing process have made the availability of multiple cleaning options in a product line increasingly important to high yield.
Surface activation, an important process tied to cleaning, prepares the surface for the next process step ensuring good quality adhesion resulting in high quality die. This can also be accomplished using plasma techniques. Acceptable cleaning processes for semiconductor applications must achieve the following challenging objectives:
MKS offers RF and microwave plasma alternatives to wet substrate surface cleaning. These alternatives are compatible with multiple process gases, ensuring the best clean based on material chemistries. Our RF and Microwave Plasma products are an economical and environmentally sustainable alternative to wet cleans, avoiding the use of acids and solvents that require special storage and disposal. The R*evolution® is an integrated remote plasma source that provides extremely clean reactive gas species for surface cleaning applications. It integrates a quartz vacuum chamber, an RF power supply and all necessary controls into a compact, self-contained unit easily installed on a tool's process chamber.
The AX7610 Downstream Plasma Source is a general duty microwave plasma source for use in remote plasma applications such as photoresist strip and passivation, surface modification, chamber cleaning and reactive gas generation. It can be configured with either a quartz tube for cleaning applications requiring atomic oxygen, hydrogen and nitrogen or a sapphire tube compatible with more corrosive reactive gas generation from species such as CF4 and NF3.
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Silicon Wafer Production
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