Optimization of Wafer-Back Pressure Profile in Chemical Mechanical Planarization

Abstract

In chemical mechanical planarization, a rotating wafer is pressed facedown against a rotating pad, while a slurry is dragged into the pad–wafer interface to assist in planarizing the wafer surface. Due to stress concentration, the interfacial contact stress near the wafer edge generally is much higher than that near the wafer center, resulting in a spatially nonuniform material removal rate and hence an imperfect planarity of the wafer surface. Here, integrating theories of fluid film lubrication and two-dimensional contact mechanics, we calculate the interfacial contact stress and slurry pressure distributions. In particular, the possibility of using a multizone (i.e., piecewise constant) wafer-back pressure profile to improve the contact stress uniformity is examined by studying a particular case with realistic parameter settings. The numerical results indicate that using a two-zone wafer-back pressure profile with optimized zonal sizes and pressures can increase the “usable” wafer surface area (within which the average contact stress nonuniformity is below 0.1%) by as much as 12%. Using an optimized three-zone wafer-back pressure profile, however, does not increase the usable wafer surface area much further.

Effects of Wafer Carrier Design on Contact Stress Uniformity in CMP

“…Of course, the pad has a finite thickness in reality, and so using the above result for a 2D half-space (of infinite extent) may seem questionable. However, as shown in one previous work [6], with properly increased pad stiffness, the numerical results of the present simplified model would agree well with that of full finite element computations for axisymmetric pads of finite thickness.…”

“…MPa, here we shall take p E = 400 MPa. It has been shown by Yang et al [6] that, for the half-space of infinite extent (the pad in the present model) to be effectively as rigid as a finite-sized pad, such an artificially increased elastic modulus is necessary.…”

“…One of the most important factors causing NU is the phenomenon of stress concentration on the wafer edge. We integrate our previous models focusing exclusively upon the presence of the retaining ring [5] or upon the use of a multi-zone wafer-back pressure profile [6] to reduce NU. Technically, the retaining ring and the wafer each has to satisfy an individual force balance.…”