Megasonic cleaning

In this section, a case study is presented in which polished thermal oxide wafers were cleaned using megasonic cleaning and SCI chemistry. The effects of sonic power, temperature, and oxide etching on cleaning efficiency are examined. 

Another factor that does not depend on power and is shared by DI water and SCI cleaning is the acoustic boundary layer thickness, which is a function of frequency and viscosity only. The boundary layer decreases from thousands of micrometers to a fraction of a micrometer when megasonics is applied at any power. The acoustic boundary layer thickness is inversely proportional to the frequency and directly proportional to the viscosity of the fluid. For example, a flow with a velocity of 4 m/s (maximum streaming velocity for the considered equipment) has a hydrodynamic boundary layer thickness of 1500 pm at the center of the wafer. By contrast, the acoustic boundary layer thickness on a wafer in a 850-kHz megasonic cleaning tank is 0.61 pm. The effect of a very... 

POST-CMP MEGASONIC CLEANING USING DILUTE SCI SOLUTION... 

An example of post-CMP clean process sequence design for defect reduction is the hybrid clean process as illustrated in Figure 17.25. In this approach, an acidic chemical is used in the roller bmsh steps in sequence to dissolve the metal oxide PR/FM while an alkaline clean chemical is plumbed to the megasonic tank without power to clean off the remaining PR/FM and passivate the Cu surface. In other words, the megasonic clean station is adopted only as a rinse tank with an alkaline chemical solution. The use of acidic clean chemical solution in the bmshes to dissolve metal oxide is the key to the reduction of PR/FM and the ehmination of circular ring defects. The application of a basic chemical rinse step provides further reduction in surface defects and passivation of the Cu surface to prevent the formation of HM and DE defects. 

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