Studies show that
1. polishing is not a function of hardness meaning that polishing is
not a function of wear (if polishing a function of hardness, then harder
glass should polish slower, but it does not, therefore polishing is not
a function of wearing down the glass)
2. polishing is not a function of softness meaning that polishing is not
a function of flow (if polishing a function of softness, then softer
glass should flow more and polish faster, but it does not, therefore
polishing is not a function of glass flow or softness)
3. polishing is a function of chemical durability meaning that chemical
reactions influence polishing
4. chemical leaching (H2O or dilute acid) increases polishing rate, and,
polish rate substantially lower in oil and dry polishing than in water
polishing, therefore chemical effect softens the glass to make abrasion
easier
Conclusion: polishing is a chemical mechanical process
How exactly does this occur?
1. role of water in oxide polishing
a. water enters the glass and softens it
1. amount of water entry depends on
pressure and velocity of polishing tool
2. water enters by breaking the Si-O
bonds (which most glass is composed of), giving Si-OH (Si-O bonds fully
hydrated Si(OH)4), which is highly soluable in water)
3. water entry into the glass is
accelerated by compressive stress imposed into the surface by the
abrasive particles, and soluability increases as a result of compressive
stress and hydrostatic pressure
a. material removed
(dissolution) is highest just in front of the moving abrasive particle
and lowest (condensation) behind the abrasive particle, net removal only
occurs when some of the dissolved Si(OH)4 is removed from the vicinity
of the surface by variety of mechanisms including turbulent motion of
the slurry, absorption onto the abrasive particle, precipitation and
formation of colloidal SiO2 which is swept away
2. chemical reaction between abrasive and oxide surface
a. highly accelerated polish rates, for instance, ceria
abrasive exhibits a chemical tooth property which accelerates the polish
rate, estimated 5x10^8 more efficient than silica, with a resulting
polish rate 43 times greater for ceria than for silica abrasives
b. 5 reaction steps important in determining the rate of
mass transport during polishing
1. water moves into the glass surface
2. water reacts with the surface leading
to dissolution of the glass surface under the influence of the load
3. some dissolution products absorb onto
the abrasive particles and are moved away from the surface
4. some dissolution products redeposit
back onto the surface
5. surface dissolution occurs between
particle impacts
c. chemical tooth property
1. ceria and zirconia accelerate removal
of SiO2 by chemically reacting and bonding with the SiO2 surface.
a. This occurs because
the free energy of formation of CeO2 and ZrO2 is less than that of SiO2.
b. Therefore ceria and
zirconia abrasives are able to reduce the SiO2 and bond with the
surface.
c. bonding between
abrasive and surface increases the shearing force of the abrasive
particle, increasing probability of removal of material within the
indentation volume
d. in addition, because
abraded material remains bonded to the abrasive, the probability that
the abraded material will be removed form the vicinity of the surface
increases
e. consequently, ceria
and zirconia abrasives yield greater removal rates than abrasives such
as diamond which do not exhibit the chemical tooth property
f. water also has a
role in chemical tooth property
3. polish rate results
a. polish rate varies linearly with pressure and
velocity
b. slurry abrasive size and concentration
1. not affected by particle size
2. affected by fill factor
a. at high particle
concentrations, smaller particles increases number of particles in
contact
b. at low
concentrations, small number of particles in contact and slower
polishing rates
ref Chemical Mechanical Planarization of Microelectronic Materials
John Wiley and Sons Inc, Wiley-Interscience
publication
by Joseph M Steigerwald, Shyam P Murarka, Ronald J Gutmann
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