Page 252 - Semiconductor Manufacturing Handbook

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CHEMICAL MECHANICAL POLISHING

CHEMICAL MECHANICAL POLISHING 17.7

• Regular pad replacement after wafer breakage is critical for defects
• Optimum subpad selection for best within wafer and within die uniformity and planarity (stacked
pads or two layers are suggested)
• Pad temperature control using an integrated, closed loop, cooling system
• Pad grooving (Rodel K-Groove), surface texturing, perforations, and/or slurry channeling to
improve mass transport
• Proper “break in” and contouring or flattening of the polishing pad prior to CMP
• Regular conditioning of the pad (in situ or ex situ) using a brush or diamond abrasive conditioner
• Periodic pad rinsing and cleaning, which may also include chemical washing of the pad
• Maintaining the pad wetness at all times will lower defects and improve process control
• Replacing the pad when it wears more than 50 percent of its original thickness is also suggested
for best results
• Storing pads dry and flat can also help improve polishing results
Diamond abrasive pad conditioners are typically used For ILD CMP. These conditioners consist
of diamond particles sintered, glued, or brazed to rigid or flexible surfaces. The purpose of the pad
conditioner is to periodically mechanically roughen the pad surface in between polishing runs. This
is called ex situ pad conditioning. Conditioning during the CMP process is also possible on some
platforms, and is known as in situ pad conditioning.
Companies such as 3M, Kinik, ATI, and TBW make diamond abrasive pad conditioners. When
the pad conditioner scrapes the pad, it opens up small closed bubbles or cells in the polyurethane pol-
ishing pad. These bubbles help aid fluid transport both to and from the wafer surface. In addition,
rigid diamond abrasive pad conditioners planarize defects or bumps on the pad surface, reducing
defects and improving polish uniformity. Channeling occurs as diamonds rake across the pad sur-
face, also improving fluid transport. The downforce one uses to condition the polishing pad will
directly influence bulk material rates during oxide CMP. Increasing pad conditioning downforce can
increase film removal rates. However, at higher conditioning loads, pad consumption and therefore
the process cost of ownership, will be higher. Inadequate pad conditioning will likely result in both
unstable polishing uniformity and film removal rate control. This is called polishing pad “glazing.”
One can trade off process stability for throughput with pad conditioning. Removal rates can also be
maintained by conditioning during the CMP process. This is called in situ pad conditioning. The
good news is that grid-based sintered diamond pad conditioners like those offered by 3M/Rodel pro-
*
vide both a long pad life and relatively high polishing rates. It is too bad that not all CMP tools can
be adapted to use these types of pad conditioners.
17.3.5 Endpoint Systems

Ensuring wafer-to-wafer process repeatability for CMP is very important to reduce over polishing.
It is common for more advanced CMP tools to use endpoint systems. There are a few types of com-
mon endpoint systems. Endpoint systems can be classified as either friction-based or optical metrol-
ogy systems. These two endpoint technologies will be discussed next.
Friction-based CMP endpoint systems are the most simple, and typically use platen motor drive
current to measure changes in polishing friction. When a wafer film planarizes, the polishing friction
slowly reduces and the wafer becomes smoother. In addition, when the film being polished clears,
polishing of the underlayer film commences. Since polishing friction is highly material dependent,
a strong surge or decrease in the platen drive motor can be observed. Friction-based endpoint sys-
tems work well for polishing larger features since they may not be sensitive enough to determine clear-
ing on smaller die features. They also assume that polishing uniformity is nearly perfect. If global

*
Dyer, T., and J. Schlueter, “Characterizing CMP Pad Conditioning Using Diamond Abrasives.” Micro Magazine, January 2002.

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