Cleaning and liquid contact with solids – Processes – Including application of electrical radiant or wave energy...
Reexamination Certificate
1999-06-29
2003-03-04
Gulakowski, Randy (Department: 1746)
Cleaning and liquid contact with solids
Processes
Including application of electrical radiant or wave energy...
C134S001000, C134S002000, C134S003000, C134S026000, C134S028000, C134S029000, C134S902000
Reexamination Certificate
active
06526995
ABSTRACT:
BACKGROUND OF THE INVENTION
After semiconductors are manufactured, they are subjected to chemical-mechanical polishing (CMP). The most common way to clean silicon wafers after chemical mechanical polishing is with a brush cleaning tool. The problem with such as system is that the abrasiveness of the brush often damages the wafer. Alternatively, there are some systems where spray tools are employed to clean wafer surfaces. These methods do not have optimal cleaning ability. Another technique is to use megasonics combined with immersion into other solvents. This technique, and the chemicals used is described in detail below.
Megasonics
Megasonics is a cleaning method for contamination-sensitive products. Integrated circuit, hard drive, raw silicon, mask, flat panel display, and other devices affected by contamination are cleaned using megasonics to help meet stringent cleanliness requirements. The megasonic method is commonly employed to clean semiconductor wafers themselves. A megasonic cleaning system uses high frequency (700-1,000 KHz) acoustic energy pressure waves produced in a liquid. The liquid is contained in a cleaning tank. Because the pressure waves are effective for removing particles from silicon wafers without causing damage to the wafers themselves, it is an excellent nondestructive cleaning technique. Megasonics maybe used to effectively removing 0.15-micron particles from silicon wafers and other products, without damage.
Megasonics cleaning uses the piezoelectric effect (the generation of electric polarization in certain dielectric crystals as a result of the application of mechanical stress) to enable removal of submicron particles from substrates. A ceramic piezoelectric crystal is excited by high-frequency AC voltage, causing it to vibrate. This vibration generates an acoustic wave that is transmitted to and through the cleaning fluid, producing controlled cavitation. Cavitation is the formation of gas bubbles within the liquid created by the application of mechanical force. Because the cavitation is formed by acoustic waves, it is said that megasonic employs “acoustic cavitation.” Acoustic cavitation is caused by the pressure variations in sound waves moving through the cleaning fluid. As the acoustic wave passes across the surface of the wafer, particles are removed. Acoustic cavitation provides sufficient energy to overcome particle adhesion forces and cause particles to be removed. Controlled megasonics cavitation becomes acoustic streaming, which pushes the particles away so they do not reattach to the material being cleaned.
It is useful to contrast megasonic cleaning with ultrasonic cleaning to fully understand the megasonic process. The fundamental difference between ultrasonic cleaning and megasonics cleaning lies in the frequency that is used to generate the acoustic waves. Ultrasonic cleaning uses frequencies between 20-350 kH. These frequencies produce random cavitation. Megasonics cleaning uses higher frequencies at 700-1000 kHz. This produces controlled cavitation.
An important distinction between the two methods is that the higher megasonic frequencies do not cause the violent cavitation effects found with ultrasonic frequencies. This significantly reduces or eliminates cavitation erosion and the likelihood of surface damage to the product being cleaned. Parts that would be damaged by ultrasonic frequencies or cavitation effects can often be cleaned without damage in a megasonic bath using the same solution. With ultrasonics, cavitation occurs throughout the tank, and all sides of submerged parts are cleaned. With megasonics, only the side of the part that is facing the piezo electric device is cleaned.
Chemicals Used in Megasonics
Megasonics cleaning may be used with a variety of chemistries. Although megasonics is used primarily for particle removal, it may also be used to increase the efficiency of chemical cleaning with surfactants or detergents. Removal of other contaminants depends on the solutions in the tank.
Deionized water (DI) water is commonly used in megasonic application. An “ion” is an atom or group of atoms that is not electrically neutral, but instead carries a positive or negative electric charge. As the names suggest, deionized water is water without such ions present. Deionized Water of ultrahigh purity (very low in contaminants) is of high electrical resistance which makes it useful for cleaning various semiconductor substrates.
Wafers are often rinsed in pure DI water after being immersed in a chemical bath both to remove the chemicals, and to further clean the wafer. As relates to megasonics, the chemicals chosen are substances (aqueous or organic) designed to clean the semiconductor wafer (or other assembly) by dissolving the contaminants present on its surface. The rinse of the DI water removes the residue of chemical that still remains on the wafer from previous immersions. DI water has a pH of 7 meaning that it neither an alkaline nor acidic but rather is neutral.
DI water may also be used to dilute the concentration of a chemical in the megasonic bath (or other immersion baths). It is common to mix various chemicals in different concentrations in DI water. One chemical commonly used is ammonia hydroxide. Another is hydrogen peroxide. Hydrogen peroxide is a strong oxidizing chemical used in particle removal chemistries. SC-1, a solution commonly used in megasonics, is a mixture of ammonia hydroxide, hydrogen peroxide and water. Hydrofluoric acid (HF) is also commonly used. Hydrofluoric acid is a corrosive acid used in etching and cleaning. It is commonly used for the removal of metals or thin oxides of glass. Another common corrosive acid used in particle removal chemistries is hydrochloric acid (HCL). Hydrochloric acid is commonly found in SC-2 or HPM solution which is a solution used for metallic removal.
Surfactants may also be used in megasonics. A surfactant is a surface active solution, and is used to lower the surface tension of a liquid. Surfactants reduce interfacial tension between two liquids or a liquid and a solid. Because of this property, surfactants reduce the ability for particles to adhere to the wafer. Detergents are made up principally from surfactants. One surfactant commonly used to clean wafers is known as NWC-601.
SUMMARY OF THE INVENTION
It is generally known that exposure time, the length of time the wafer is subjected to the megasonic cleaning process, is an exceptionally important variable affecting megasonics cleaning. Indeed, it is well-known that as exposure time increases, particle redeposition decreases. The inherent problem with using increased exposure times is that cycle time (the time needed to clean each batch of wafers) correspondingly increases, thereby adding additional manufacturing cost and production time. Today, typical exposure times are 10 to 20 minutes. The exposure times of 7 minutes or less are not currently employed for post-chemical mechanical polish or back-grinding operations, and are commonly believed to be insufficient to reach desired levels of cleaning efficiency. What is not known is that using two or more megasonic cycles of shorter duration, as opposed to a single cycle of longer duration, increases cleaning efficiency (the ratio of particles on a material before the cleaning process to the particles remaining after cleaning) without significantly effecting the overall cycle time.
The term “cycle” has a specific meaning in the semiconductor fabrication industry. By “chemical cycle” I mean exposing the wafer to the same, or approximately the same, chemicals in close temporal proximity to achieve a desired result. One aspect of my invention is performing one “chemical cycle” as two separate megasonic chemical “sub-cycles.” Preferably, the very chemical bath of the first chemical megasonic sub-cycle is used again for the second chemical megasonic sub-cycle, and there is a rinse in between. Where the megasonic chemical bath is not a sub-cycle, it is simply labeled a “chemical megasonic cycle” instead of a “chemical megasonic sub-cycle.” Thus, I have used the t
Fogg and Associates LLC
Gulakowski Randy
Intersil America's Inc.
Smetana J
LandOfFree
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