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CHEMICAL MECHANICAL POLISHING

CHEMICAL MECHANICAL POLISHING 17.9

solution of KOH, with pH more than 10, is typically used to lubricate the wafer during the buffing
process. The addition of a second platen adds another process step and therefore additional time to
polish wafers. To prevent dried slurry-related defects, one is still required to spin rinse-dry wafers
out of a two platen DIWO CMP tool.

17.4.2 Dry in-Dry out CMP Tool
As critical wafer dimensions decreased in size, so did allowable defect sizes and surface concentration
levels. DIWO CMP tools were no longer able to buff wafers and meet increasingly more stringent defect
criteria. Therefore, the DIDO CMP tool was developed. A DIDO CMP tools looks much like a DIWO
CMP tool; however, there is a dry wafer cassette placed on the “unload” or completed process station.
These tools also use a robot to load the dry wafers in to the completed cassette since water slides or tracks
can no longer be used. DIDO CMP tools also contain a wafer cleaning system that typically uses a spin
rinse or Marangoni drier. After buffing wafers with a second platen, wafers are then cued in a wet-stor-
age elevator or compartment for post-CMP cleaning and drying.
17.4.3 CMP Wafer Cleaners

CMP cleaning systems originally evolved separately from CMP tools. Their function is to chemi-
cally and mechanically remove defects and particles from wafers prior to spin rinse drying. Stand-
alone CMP cleaners for ILD and STI CMP typically use ammonia, dilute hydrofluoric acid, and
PVA-type brushes (Texwipe Inc., Rippey Inc.) to clean wafers. Hydrofluoric acid helps the cleaning
system to chemically dissolve particles, while ammonia reduces zeta potential (electrostatic-type
forces) that attracts particles to surfaces. More advanced cleaners use noncontact ultrasonic or mega-
sonic sound energy to remove particles. Megasonic cleaning systems use immersion baths or spray
jets backed with transducers to create high-frequency pressure waves in water. These pressure waves
can help break zeta potential and van der Waals attraction forces of particles attached to the wafer.
For very small particles, van der Waals attraction forces per unit area can be extremely high; there-
fore a lot of energy is required to remove small particles.

17.4.4 Internal CMP Cleaning Systems
CMP cleaners are now commonly found residing inside CMP tools. Increasing productivity require-
ments have made stand-alone cleaners obsolete except for process development. The trick to placing
a cleaning system into a CMP tool is floor and tool space. Even with strict space limitations, integrated
CMP cleaners nowadays are extremely effective and convenient. The productivity of cleaning mod-
ules is also improving as systems evolve. By using Marangoni or other solvent-type drying processes,
the need for extensive spin rinse drying of wafers is quickly going away. In addition, porous low-k
films are very delicate and can be damaged by the use of water-based clean chemistries. Therefore,
solvent-type CMP cleaning and drying systems will become more common in the future.

17.5 COMMON CMP PLATFORMS AND TOOLS

CMP systems have been in use for over 30 years. The first of these tools were based on the more tra-
ditional rotational CMP process. However, as time went on, process and productivity requirements
changed and the tools had to evolve. This section will discuss typical production of CMP tools and
architectures. In addition to tool configurations, this section will discuss some of the special features
of the different types of CMP tool sets.
17.5.1 Single Head Rotational Systems

Rotational tools for CMP are roughly based on lapping and polishing tools for glass, ceramic, or
metal components. These tools consist of a single polishing platen and a single polishing head known

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