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
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
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
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
mel bartels march 2001