Design of FG/CS-LPP and material removal uniformity experiment on ZrO₂ ceramic

Abstract

A novel processing technology based on the functionally graded and composite structured lapping and polishing plate (FG/CS-LPP) is proposed for effectively improving the material removal uniformity. The FG/CS-LPP has the characteristics of required quasi-continuous distribution of Young's modulus in the radial direction and composite structure in the longitudinal direction. Moreover, the certain variation of Young's modulus does not tend to influence the material removal uniformity and two-step processing technique makes the shrinkage deformation of the composites reduce to 0.03 mm. Based on the Preston equation, the FG/CS-LPP can realize smooth inverse proportional stress distribution and make processing parameters decoupling of K (Preston coefficient) and P (contact stress). Through numerical simulation analysis and experimental verification, the designed FG/CS-LPP successfully achieves material removal uniformity on ZrO₂ ceramic.

Introduction

Hard and brittle materials are widely used in the modern information industry, for example, optical glass is applied to the camera lens and LCD screen [1], microcrystalline zirconium ceramic is applied to smartphone rear screen [2], and silicon wafer is applied to electronic chip manufacturing [3]. With the advancement of Moore's Law in the information industry, surface finishing of hard and brittle materials has become an urgent need. Chemical mechanical polishing (CMP) is currently the most important processing technology, which can provide global or local planarization of processed materials. Since the technology was proposed by Walsh and Herzog in 1965, it has made great progress in decades of application [4]. However, this technology still has certain deficiency in the aspect of material removal uniformity.

The Preston equation (MRR = KPV) reveals the material removal mechanism and it is widely adopted by researchers [5]. The material removal rate (MRR) is proportional to the contact stress (P) and relative velocity (V) of the workpiece. The Preston coefficient K mainly depends on the effective action rate of abrasive particles. Campione et al. [6] reported a method to evaluate the stress distribution generated in polishing large-sized ceramic slabs and assess how it changes when modifying the parameters of the laws of motion. Liao et al. [7] established a contact model of the lap and conditioner to obtain the pressure distribution at the interface and proposed a novel creep model for the viscoelastic behavior of pitch lap. The method of changing contact stress was commonly used to adjust the material removal characteristic, as seen in other relevant studies [8], [9], [10], [11]. Meanwhile, the trajectory and distribution of abrasive particles have a significant effect on the material removal uniformity. Wang et al. [12] conducted relevant experiments to maximize the material removal rate and minimize the within wafer non-uniformity simultaneously for the multi-zone CMP of a 12-inch wafer. Fang et al. [13] designed a textured fixed-abrasive pad based on the bee colony theory and kinematic analysis. The various geometric patterns of the abrasive pad change the global distribution of fixed abrasives and effectively polishes the surface of sapphire glass in different regions to achieve the purpose of flattening. Enomoto et al. [14] introduced a spiral-structured pad with fixed-abrasive layers and abrasive holding layers to increase the quality and efficiency of fixed-abrasive tools. The finishing experiments revealed a much higher finishing efficiency and good surface quality compared to those obtained in conventional polishing with ceria slurry.

Under the guidance of the Preston equation, many researchers realized the importance of controllable contact stress of the workpiece and motion state of the abrasive particles. Also, the functionally graded materials (FGMs) have many applications [15], [16], [17], [18]. In our previous work [19], a particle-reinforced polymer-matrix FGMs was prepared, whose structure and properties change in the radial direction quasi-continuously, and the abrasive particles are exposed to the rubber surface in a semi-consolidated manner to provide a stable rubbing action on the workpiece. Further, in this paper, a novel processing technology based on the functionally graded and composite structured lapping and polishing plate (FG/CS-LPP) is proposed in the hope that it can effectively improve the material removal uniformity. The FG/CS-LPP has the characteristics of required quasi-continuous distribution of Young's modulus in the radial direction and composite structure in the longitudinal direction. Thus, it has the advantages of smooth inverse proportional stress distribution in the radial direction for making the product of P and V constant and processing parameters decoupling of K and P with uniform effective action rate of abrasive particles.