CMP (Chemical Mechanical Polishing)

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

mel bartels march 2001