Improvement of Post CMP Cleaning Efficiency through Surface Energy Optimization and Use of Viscoelastic Fluids
Authors: Ozbek Sebnem , Walker Travis , Cashen Ryan Kristian , Basim G. Bahar 
Published: February 2020

The chemical and mechanical actions of the CMP process to achieve planarization and material removal also affect the defectivity and post cleaning requirements spontaneously. The wafer surface is cleaned from the excessive chemicals and the slurry particles from the post CMP operations through utilization of the fundamentals of colloids and surface chemistry. The problem of attachment of the slurry particles on the wafer surface is becoming a more severe yield issue, as the microelectronic device dimensions keep decreasing continuously. Furthermore, in integrated circuit fabrication, over 50% of the yield losses occur because of the insufficient cleaning of the wafers [1].

To address the challenges of the post CMP cleaning, this study focuses on the correlation of the surface energy of the wafer post CMP to the particle count remaining on the wafer surface for the oxide surfaces as a model approach. To study the effects of surface nature of the substrate, measurements of surface roughness, contact angle, surface energy, and surface charge of the glass slides were measured and compared to the particle count on the wafers post CMP cleaning treatment. To study the particle retention on the surface, particle removal experiments were conducted in a rinsing flow cell and a siphoning flow cell by implementing impinging jet (rinsing) flow and siphoning flow, respectively. Consequently, better post CMP cleaning formulations were suggested for the oxide-based CMP applications with optimal surface energy conditions fulfilled.

The surface free energy and the work of adhesion values were observed to correlate with the surface roughness of the substrates such that, as the surface roughness values increased, the surface free energy and work of adhesion values also increased. As the surface free energy increases, it becomes more difficult to remove particles from the surface without damaging the surface quality. Therefore, water becomes insufficient for removal of particles that remained on the surface. Therefore, a new method of post CMP cleaning is proposed that utilizes viscoelastic fluids to increase the removal of surface particles by taking advantage of the additional elastic stresses that are induced on the particles [2]. After coating the substrate surface with slurry particles, polymeric solutions with 0.7 wt% PAM and with 0.5 wt% PEO were used as the viscoelastic fluid. The optimum amount of polymeric solution was determined to be 40. The CMP-treated glass slides were fully coated and tested for the percent removal of particles. After conducting the siphoning flow experiment, multiple images were taken at 30 FPS (frames per second), and these images were analyzed to conduct particle removal analyses. The use of viscoelastic fluids was observed to increase the removal of particles due to the drag force from the elastic stresses that were applied on the particles by the viscoelastic fluids.

References:

Zhang, A.A. Busnaina, M.A. Fury, S.Q. Wang. The removal of deformed submicron particles from silicon wafers by spin rinse and megasonics. Journal of Electronic Materials, 29:199–204, 2000.

W. Walker, T.T. Hsu, S. Fitzgibbon, D.S.L. Mui, J. Zhu, A. Mendiratta, C.W. Frank, G.G. Fuller. Enhanced particle removal using viscoelatic fluids. Journal of Rheology, 58(1): 63–68, 2013.