THE USE OF HF SOLUTIONS WITH PVA BRUSH SCRUBBING Alkaline chemistries, like ammonium hydroxide are often used in brush scrubbing operations to remove particles. The use of a high pH chemistry helps by taking advantage of the zeta potentials to create repulsive charges in the particle, brush and wafer surfaces. For many brush scrubbing operations, ammonium hydroxide will increase performance, by preventing particle reattachment and extend brush lifetime by preventing brush loading. However, in some cases, alkaline chemistries may not be appropriate. Many trace metal contaminants are not soluble in a high pH solution and will tend to plate out onto the substrate surface. In some applications, a thin layer of oxide may need to be removed in order to deal with subsurface contamination1. Under specific conditions (high pH), ammonium hydroxide may roughen copper surfaces and therefore may not be suitable for copper post-CMP cleaning2. For these conditions, HF (hydrofluoric acid) may be a good choice. HF is somewhat unique in that its corrosive properties are not typically due to the presence of dissociated hydronium ions. Although HF does completely give up its proton, the fluoride ion remains bound to the hydronium ion by hydrogen bonding (see Fig. 1). As a result, very little free H3O is available. Figure 1. The behavior of HF in water The etching of silicon by HF is due to the highly polar nature of the H-F bond. The reaction is helped by the high dipole moment of the Si-O bond in the silicon oxide surface. The reaction proceeds by breaking the Si-O bond and forming the very strong Si-F bond and releasing H2O. HF then attacks the Si-Si bond, which has been polarized by the attached fluorine, leaving a hydrogen terminated surface3 (see Fig. 2). Figure 2. The etching and passivation of silicon by HF (3) Rippey PVA brush rollers are compatible with HF based cleaning solutions. HF cleaning chemistries can be dripped onto the brush during scrubbing or delivered through the brush providing even distribution and reduced brush loading4. The PVA polymer is not degraded by solutions of less than 5% HF. HF easily etches silicon oxide, whereas PVA is relatively resistant due to the weak acid nature of HF and the differences in the C-O and Si-O bond. The silicon-oxygen bond has a longer bond length (1.5Å compared to 1.1 for C-O) and a higher dipole moment. However, the PVA can harden in solutions between 5% and 10% HF. This is due to the replacement of water molecules attached to the polymer. In concentrations above 10% HF, PVA will start to weaken and swell. This is generally not a problem, since most cleaning solutions for post-CMP scrubbing applications are less than 1% HF. Compatibility testing for polymer materials is generally conducted by soaking the material in the solution of question for 24hr. The degree of swelling and any changes in the tensile strength are measured to determine compatibility. PVA is typically used in concentrations of less that 1% and well within the compatibility range of 0-5%. References 1. Ravkin, M.A., Hetherington, D.L., de Larios, J.M., Gardner, D.G., and Krusell, W.C., A New Chemical Mechanical Scrubbing Process using HF for Post-CMP Cleaning Applications, Technical Proceedings CMP-MIC Conference, San Jose (1996) 2. Zhao, E., Zhang, L., Li, H., Hymes, D., de Larios, J.M., Krusell, W.C., Copper CMP Cleaning using Brush Scrubbing, Technical Proceedings CMP-MIC Conference, San Jose (1998) 3. Higashi, G.S., and Chabal, Y.J., in: Handbook of Semiconductor Wafer Cleaning Technology (Werner Kern ed.), pp. 433-496, Noyes Publications, New Jersey (1993) 4. Hymes. D.J., Ravkin. M., Zhang. X., Krusell. W.C., Method and Apparatus for Cleaning of Semiconductor Substrates using Hydrofluoric Acid (HF), United States Patent #5,868,863, (1999) Return to Home Page Copyright 2011 ITW Rippey - All rights reserved				Return to Home Page