Some organic and inorganic substances are required to participate in semiconductor manufacturing. In addition, since the process is always carried out in a clean room with human participation, semiconductor wafers are inevitably contaminated by various impurities.
According to the source and nature of the contaminants, they can be roughly divided into four categories: particles, organic matter, metal ions and oxides.
1. Particles:
Particles are mainly some polymers, photoresists and etching impurities.
Such contaminants usually rely on intermolecular forces to adsorb on the surface of the wafer, affecting the formation of geometric figures and electrical parameters of the device photolithography process.
Such contaminants are mainly removed by gradually reducing their contact area with the surface of the wafer through physical or chemical methods.
2. Organic matter:
The sources of organic impurities are relatively wide, such as human skin oil, bacteria, machine oil, vacuum grease, photoresist, cleaning solvents, etc.
Such contaminants usually form an organic film on the surface of the wafer to prevent the cleaning liquid from reaching the surface of the wafer, resulting in incomplete cleaning of the wafer surface.
The removal of such contaminants is often carried out in the first step of the cleaning process, mainly using chemical methods such as sulfuric acid and hydrogen peroxide.
3. Metal ions:
Common metal impurities include iron, copper, aluminum, chromium, cast iron, titanium, sodium, potassium, lithium, etc. The main sources are various utensils, pipes, chemical reagents, and metal pollution generated when metal interconnections are formed during processing.
This type of impurity is often removed by chemical methods through the formation of metal ion complexes.
4. Oxide:
When semiconductor wafers are exposed to an environment containing oxygen and water, a natural oxide layer will form on the surface. This oxide film will hinder many processes in semiconductor manufacturing and also contain certain metal impurities. Under certain conditions, they will form electrical defects.
The removal of this oxide film is often completed by soaking in dilute hydrofluoric acid.
General cleaning sequence
Impurities adsorbed on the surface of semiconductor wafers can be divided into three types: molecular, ionic and atomic.
Among them, the adsorption force between molecular impurities and the surface of the wafer is weak, and this type of impurity particles is relatively easy to remove. They are mostly oily impurities with hydrophobic characteristics, which can provide masking for ionic and atomic impurities that contaminate the surface of semiconductor wafers, which is not conducive to the removal of these two types of impurities. Therefore, when chemically cleaning semiconductor wafers, molecular impurities should be removed first.
Therefore, the general procedure of semiconductor wafer cleaning process is:
De-molecularization-deionization-de-atomization-deionized water rinsing.
In addition, in order to remove the natural oxide layer on the surface of the wafer, a dilute amino acid soaking step needs to be added. Therefore, the idea of cleaning is to first remove organic contamination on the surface; then dissolve the oxide layer; finally remove particles and metal contamination, and passivate the surface at the same time.
Common cleaning methods
Chemical methods are often used for cleaning semiconductor wafers.
Chemical cleaning refers to the process of using various chemical reagents and organic solvents to react or dissolve impurities and oil stains on the surface of the wafer to desorb impurities, and then rinse with a large amount of high-purity hot and cold deionized water to obtain a clean surface.
Chemical cleaning can be divided into wet chemical cleaning and dry chemical cleaning, among which wet chemical cleaning is still dominant.
Wet chemical cleaning
1. Wet chemical cleaning:
Wet chemical cleaning mainly includes solution immersion, mechanical scrubbing, ultrasonic cleaning, megasonic cleaning, rotary spraying, etc.
2. Solution immersion:
Solution immersion is a method of removing surface contamination by immersing the wafer in a chemical solution. It is the most commonly used method in wet chemical cleaning. Different solutions can be used to remove different types of contaminants on the surface of the wafer.
Usually, this method cannot completely remove impurities on the surface of the wafer, so physical measures such as heating, ultrasound, and stirring are often used while immersing.
3. Mechanical scrubbing:
Mechanical scrubbing is often used to remove particles or organic residues on the surface of the wafer. It can generally be divided into two methods: manual scrubbing and scrubbing by a wiper.
Manual scrubbing is the simplest scrubbing method. A stainless steel brush is used to hold a ball soaked in anhydrous ethanol or other organic solvents and gently rub the surface of the wafer in the same direction to remove wax film, dust, residual glue or other solid particles. This method is easy to cause scratches and serious pollution.
The wiper uses mechanical rotation to rub the surface of the wafer with a soft wool brush or a mixed brush. This method greatly reduces the scratches on the wafer. The high-pressure wiper will not scratch the wafer due to the lack of mechanical friction, and can remove the contamination in the groove.
4. Ultrasonic cleaning:
Ultrasonic cleaning is a cleaning method widely used in the semiconductor industry. Its advantages are good cleaning effect, simple operation, and can also clean complex devices and containers.
This cleaning method is under the action of strong ultrasonic waves (the commonly used ultrasonic frequency is 20s40kHz), and sparse and dense parts will be generated inside the liquid medium. The sparse part will produce a nearly vacuum cavity bubble. When the cavity bubble disappears, a strong local pressure will be generated near it, breaking the chemical bonds in the molecules to dissolve the impurities on the wafer surface. Ultrasonic cleaning is most effective for removing insoluble or insoluble flux residues.
5. Megasonic cleaning:
Megasonic cleaning not only has the advantages of ultrasonic cleaning, but also overcomes its shortcomings.
Megasonic cleaning is a method of cleaning wafers by combining the high-energy (850kHz) frequency vibration effect with the chemical reaction of chemical cleaning agents. During cleaning, the solution molecules are accelerated by the megasonic wave (the maximum instantaneous speed can reach 30cmVs), and the high-speed fluid wave continuously impacts the surface of the wafer, so that the pollutants and fine particles attached to the surface of the wafer are forcibly removed and enter the cleaning solution. Adding acidic surfactants to the cleaning solution, on the one hand, can achieve the purpose of removing particles and organic matter on the polishing surface through the adsorption of surfactants; on the other hand, through the integration of surfactants and acidic environment, it can achieve the purpose of removing metal contamination on the surface of the polishing sheet. This method can simultaneously play the role of mechanical wiping and chemical cleaning.
At present, the megasonic cleaning method has become an effective method for cleaning polishing sheets.
6. Rotary spray method:
The rotary spray method is a method that uses mechanical methods to rotate the wafer at a high speed, and continuously sprays liquid (high-purity deionized water or other cleaning liquid) on the surface of the wafer during the rotation process to remove impurities on the surface of the wafer.
This method uses the contamination on the surface of the wafer to dissolve in the sprayed liquid (or chemically react with it to dissolve), and uses the centrifugal effect of high-speed rotation to make the liquid containing impurities separate from the surface of the wafer in time.
The rotary spray method has the advantages of chemical cleaning, fluid mechanics cleaning, and high-pressure scrubbing. At the same time, this method can also be combined with the drying process. After a period of deionized water spray cleaning, the water spray is stopped and a spray gas is used. At the same time, the rotation speed can be increased to increase the centrifugal force to quickly dehydrate the surface of the wafer.
7.Dry chemical cleaning
Dry cleaning refers to cleaning technology that does not use solutions.
The dry cleaning technologies currently used include: plasma cleaning technology, gas phase cleaning technology, beam cleaning technology, etc.
The advantages of dry cleaning are simple process and no environmental pollution, but the cost is high and the scope of use is not large for the time being.
1. Plasma cleaning technology:
Plasma cleaning is often used in the photoresist removal process. A small amount of oxygen is introduced into the plasma reaction system. Under the action of a strong electric field, the oxygen generates plasma, which quickly oxidizes the photoresist into a volatile gas state and is extracted.
This cleaning technology has the advantages of easy operation, high efficiency, clean surface, no scratches, and is conducive to ensuring product quality in the degumming process. Moreover, it does not use acids, alkalis and organic solvents, and there are no problems such as waste disposal and environmental pollution. Therefore, it is increasingly valued by people. However, it cannot remove carbon and other non-volatile metal or metal oxide impurities.
2. Gas phase cleaning technology:
Gas phase cleaning refers to a cleaning method that uses the gas phase equivalent of the corresponding substance in the liquid process to interact with the contaminated substance on the surface of the wafer to achieve the purpose of removing impurities.
For example, in the CMOS process, the wafer cleaning uses the interaction between gas phase HF and water vapor to remove oxides. Usually, the HF process containing water must be accompanied by a particle removal process, while the use of gas phase HF cleaning technology does not require a subsequent particle removal process.
The most important advantages compared to the aqueous HF process are much smaller HF chemical consumption and higher cleaning efficiency.
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Post time: Aug-13-2024