Page 188 - Sami Franssila Introduction to Microfabrication

CMP: Chemical–Mechanical Polishing 167

consumption in the process (cf. etching in buffered HF).
At the end of CMP, a soft polishing step is often done: Direct Mixed Hydrodynamic
no slurry is used, just water. This step does not remove
solid material but is effective in washing away abrasive
particles and corrosive chemicals.
CMP tool input variables include the following: Friction

– platen rotation 10–100 rpm
– velocity 10–100 cm/s
– applied pressure (load) 10–50 kPa
– slurry supply rate 50–500 ml/min Log velocity
Figure 16.4 Stribeck diagram of CMP: three different
Pad type, compressibility, hardness and elastic modulus, lubrication modes
conditioning, pore size and ageing can be considered
variables too. Because there is a chemical component the slurry. Polish rate is very high. In the rolling
in CMP, temperature will have an effect on polish- contact mode (mixed lubrication mode), slurry particles
ing results. occasionally roll on the wafer surface. In the non-
CMP process factors resemble those encountered contact mode (hydrodynamic lubrication mode), slurry
in etching: particles are accelerated hydrodynamically and they
impart energy to the wafer surface, weakening the
– polish rate surface so that chemical attack can occur. Hydrodynamic
– selectivity lubrication takes place at high velocities at which the
– overpolish time load is borne by the fluid, and the system is well
– pattern density effects lubricated. Friction force between the pad and the wafer
– uniformity across wafer is very different in these modes and it is classified in a
– wafer-to-wafer repeatability. Stribeck diagram (Figure 16.4).
The penetration of the abrasive particles into the
Plasma etching and CMP resemble each other also substrate is very small indeed: this is the reason for
in the sense that both depend on interaction between smooth surfaces with no visible grooves or scratches.
chemical and physical processes: in etching, ion bom- Penetration depth is given by
bardment removes reaction products from surface; in
CMP, mechanical abrasion removes surface layers that R s = (3/4)d(P/2kE) 2/3 (16.1)
have been modified chemically, for instance, by oxida-
tive slurries. where d is the abrasive particle diameter (e.g., 100 nm),
Polish rate can be limited by transport of reactants, k is the filling factor of abrasive particles (for instance,
or by surface processes, just like etching. This can be 50%), P is the local pressure (not down force, which is
found out by varying the input variables: if the rate 10–50 kPa) and E is Young’s modulus of the surface
is unaffected by change in a variable, it cannot be being polished. Penetration depths are of the order of
the rate-controlling factor. Another similarity is pattern nanometres, which is similar to surface roughness after
dependency: small pattern density leads to higher rates. polishing, as would be expected. Increasing pressure will
Pattern size effect is, however, opposite: in CMP, lead to deeper penetration but also to higher removal
small patterns are polished faster, but, in etching, small rate. Sometimes, the abrasive particles agglomerate into
patterns will be etched slower than large ones. This will huge chunks, and this leads to much larger penetration
be discussed in Chapter 20. depths and will result in microscratches that are tens of
nanometres deep.
16.2 MECHANICS OF CMP
16.2.1 Preston model
There are three modes in polishing, depending on the
degree of contact between the pad and the wafer. In Polish rates have been measured experimentally by
the direct contact (boundary lubrication) mode, the pad Preston (in 1927) to obey the following equation:
makes contact with the wafer, resulting in high and
constant friction because there is no lubrication from R = H/ t = K p P ( s/ t) (16.2)