Chemical Mechanical Polishing: Threshold Pressure and Mechanism

Abstract

The material polishing or removal mechanism has been studied for chemical mechanical polishing (CMP) as used for integrated circuit fabrication.There can exist a threshold pressure in the CMP process due to the tribological interaction among abrasive particles, the polished wafer surface, and the polishing pad. The CMP material removed is negligible if the applied pressure to the wafer is less than the threshold value. A new polishing rate or removal rate equation is derived and it explains the anomalous pressure dependence of polishing rate, which was observed in a variety of dielectric and metal CMP processes. The existence of a threshold pressure in CMP processes has significant implications to CMP consumable development and CMP process optimization.Chemical mechanical polishing (CMP) has found broad applications in fabrication of integrated circuits (IC). It is a vital technology which enables shallow trench isolation on an Si substrate, global planarization of interlevel dielectrics, and inlaid (damascene) metal wiring for on-chip multilevel interconnects. Since there are a large number of variables involved in a CMP process, it is unlikely that an optimized process will be realized empirically. A thorough understanding of the fundamental polishing mechanism is required in CMP consumable (pad and slurry) development and CMP process optimization for advanced IC fabrication.Preston's equation 1 has been widely used as the fundamental polishing rate equation in CMP consumable characterization, CMP process optimization, and modeling of CMP pattern density dependence. In Preston's equation, the polishing rate or removal rate (RR) of the polished material is described aswhere P is the pressure applied to the polished wafer, V is the relative velocity between the polishing pad and the polished wafer surface, and K p is Preston's coefficient.Recently, it was shown that the linear dependence of removal rate on pressure (RR ∝ P) in Preston's equation is only applicable to polishing with a pad whose hardness is similar to or harder than that of the abrasive particles and the polished surface. 2,3 In the CMP process for IC fabrication, the polishing pads are typically made of polyurethane polymers, which are much softer than the abrasive particles and the wafer surface. For these soft pads, the CMP removal rate has a sublinear dependence on the pressurewhere K(V) is a function of the relative velocity V and other CMP parameters.In this paper, a microscopic analysis of the interaction among abrasive particles, the polished wafer surface, and polishing pad is presented to elucidate the polishing mechanism in a CMP process. It is shown that there can exist a threshold pressure in CMP processes for IC fabrication because of the tribological interaction between the abrasive particles, the polished wafer surface, and the polishing pad.Polishing Mechanism In a CMP process for IC fabrication, a wafer is pressed onto a polishing pad by a wafer carrier while slurry, a solution containing abrasive particles and reactive chemicals,...

Particle Technology in Chemical Mechanical Planarization

“…This model 16) proposes that at low pressures the friction at the substrate-abrasive interface is higher, causing the abrasive particles to roll against the wafer substrate; this leads to negligible material removal. As the pressure increases, the friction at the pad-abrasive interface increases and beyond a threshold pressure the abrasives start being dragged by the pad across the substrate causing a sliding motion of the particles leading to appreciable material removal.…”

Modeling

“…6.1). We note that various analyses of three-body pad-waferslurry particle contact mechanics and adhesion [3][4][5][6][7][8] lead to other dependencies of the removal rate on pressure and velocity than the one we use here. We choose (6.11) because of its historical relationship with glass polishing.…”

Mechanical model of nanoparticles for material removal in chemical mechanical polishing process

“…They also found that MRR = K(PU) 1/2 . Shi and Zhao [9,10] demonstrated that MRR = K(P 2/3 − Pth 2/3 )U for critical pressure when the pad was soft. These models are called phenomenological models.…”

“…Compared with the phenomenological models [6][7][8][9][10][11][12], contact theories and models are more effective in revealing the material removal mechanism in the CMP process. However, only a few studies focused on the removal models of erosive wear, which may be a significant form of material removal [25].…”