Wafer cleaning simply means- to get rid of particles and contamination. The objective of the wafer cleaning process is the removal of chemical and particle impurities without altering or damaging the wafer surface or substrate. The surface of the wafer must be maintained not affected so that roughness, corrosion or pitting negates the results of the wafer cleaning process.
The yield of a silicon wafer is inversely related to the defect density (cleanliness and particle count) from the processing done on the wafer. One way to reduce defect density and increase yields is to use efficient wafer cleaning processes that remove particle contaminants efficiently. With smaller semiconductor devices and geometries the removal of smaller particles from silicon wafers has become more critical. Small particles can be difficult to remove since strong electrostatic forces exist between the particles and the wafer substrate.
Fabrication Process Steps
These are the steps mainly involved in fabrication process
- Wafer Surface Preparation and Cleaning
- Photo resist Deposition
- Photo resist soft baking
- Lithography- Mask Alignment and PR Exposure
- PR Development
- PR Hard Baking
- Development Inspection
- PR Removal/Stripping
- Final Inspection Device Testing
This blog is focused on wafer cleaning process that is being used widely in order to prepare the substrate.
The chemistry used in standard wafer cleaning has remained largely unchanged during the last 30 years. It is based on the use of the RCA clean process that utilizes acidic hydrogen peroxide and ammonium hydroxide solutions. While this is the primary method still used by industry, what has changed recently is its implementation along with new optimized cleaning technology that includes ozone cleaning and mega sonic cleaning systems.
There might be different types of impurities that might be associated with the substrates and for different types of substrates and different types of impurities we used different types of cleaning method and those might be classified as follows.
|Wafer Cleaning Step||Purpose|
|Pre-Diffusion Clean||Creates a surface that is free from metallic, particulate and organic contaminants. In certain cases native oxide or chemical oxides need to be removed.|
|Metallic Ion Removal Clean||Eliminate metallic ions which can have detrimental effect on device operation.|
|Particle Removal Clean||Particle removal from surface using chemical or mechanical scrubbing using Megasonic cleaning.|
|Post Etch Clean||Remove photo resist and polymers left after etching process. Remove photo resist and solid residue including “etch polymer”.|
|Film Removal Clean||Silicon nitride etching/strip, Oxide etching/strip, Silicon etching and metal etching/strip.|
The RCA clean is a standard set of wafer cleaning steps which need to be performed before high-temperature processing steps (oxidation, diffusion, CVD) of silicon wafers in semiconductor manufacturing. Werner Kern developed the basic procedure in 1965 while working for RCA, the Radio Corporation of America. It involves the following chemical processes performed in sequence:
- Removal of the organic contaminants (organic clean + particle clean).
- Removal of thin oxide layer (oxide strip, optional).
- Removal of ionic contamination (ionic clean).
The RCA cleaning process can be carried out in two steps called SC1 and SC2.
SC1- Organic Clean + Particle Clean
5 parts of DI water + 1 part of ammonia hydroxide + 1 part of aqueous H2O2 , this mixture is prepared and then we keep the substrate in the prepared solution and heat the solution at 75 or 80 °C for 10 minutes.
SC2- Ionic Clean
5 parts of DI water + 1 part of Hydrochloric Acid + 1 part of aqueous H2O2 , this mixture is prepared and then we keep the substrate in the prepared solution and heat the solution at 75 or 80 °C for 10 minutes.
Provided the RCA clean is performed with high-purity chemicals and clean glassware, it results in a very clean wafer surface while the wafer is still submersed in water. The rinsing and drying steps must be performed correctly (e.g., with flowing water) since the surface can be easily re-contaminated by organics and particulates floating on the surface of water.
Piranha Etch Clean
Piranha etch is a popular process for cleaning silicon wafers, but it must be tightly controlled to be effective. Piranha etch clean, also known as Piranha solution, is used to remove large amounts of organic residues from wafer substrates. It effectively removes photoresist and other hard to remove organic materials. The typical mixture ratio is 3 parts of sulphuric acid and 1 part of 30% hydrogen peroxide. Other protocols use higher sulphuric acid ratios of 4 to 1 or as high as 7 to 1. Since the mixture is a strong oxidizing agent, it not only removes most organic matter, it will also hydroxylase most surfaces and makes them hydrophilic (water-compatible). The mixture is highly corrosive and must be prepared and handled with special care. The piranha etch process can be done in heated quartz tanks where both temperature and chemical concentration affect the etch rate.
Pre Diffusion Cleaning
Pre-diffusion cleans are critical steps during the wafer manufacturing process. They are done several times during the manufacturing sequence and require a significant amount of equipment capacity. They are done as the final step before the silicon wafer goes through a diffusion furnace operation. Particle and metallic contamination must be minimized and etch uniformity must be optimized while high throughput in maintained.
Depending on the nature of the contaminant, a variety of aggressive chemicals, including those from the cleaning methods mentioned above, may be used to achieve clean surfaces. Pre-diffusion cleaning is a critical process because particles or contaminants on the wafer surface are likely to be driven into the wafer as well, causing unpredictable electrical properties that result in defective or low-quality semiconductor output.
Megasonic cleaning is a type of acoustic cleaning, related to ultrasonic cleaning. It is a gentler cleaning mechanism, less likely to cause damage, and is used in wafer, medical implant, and industrial part cleaning. Similar to ultrasonic cleaning, megasonics utilizes a transducer that usually sits atop a piezoelectric substrate. The transducer creates an acoustic field at a much higher frequency (typically 0.8–2 MHz) compared to ultrasonic cleaning (20-200 kHz). As a result, the cavitation that occurs is gentler and on a much smaller scale. Megasonics are currently used mainly in the electronics industry for preparation of silicon wafers. These methods include mechanical and wet chemical baths in combination with the use of high frequency megasonic cleaning equipment.
UV Ozone Cleaning
The UV Ozone Cleaner can clean a wide range of substrates, such as quartz, silicon, gold, nickel, aluminium, gallium arsenide, alumina, and glass slides. The UV Ozone Cleaning process is a photo-sensitized oxidation process in which the contaminant molecules of photoresists, resins, human skin oils, cleaning solvent residues, silicone oils, and flux are excited and/or dissociated by the absorption of short wavelength UV radiation. Atomic oxygen is simultaneously generated when molecular oxygen is dissociated by 185nm and ozone by 254nm ultraviolet wavelengths. The 254nm UV radiation is absorbed by most hydrocarbons and also by ozone. The products of this excitation of contaminant molecules react with atomic oxygen to form simpler, volatile molecules which desorbs from the surface. Therefore, when both UV wavelengths are present atomic oxygen is continuously generated, and ozone is continually formed and destroyed.