Pad Scratching in Chemical-Mechanical Polishing: The Effects of Mechanical and Tribological Properties

Abstract

In chemical-mechanical polishing (CMP), even the soft pad asperities may, under certain conditions, generate scratches on the relatively hard surfaces being polished. In the present study, contact mechanics models of pad-induced scratching are formulated, and the effects of the hardness of the surface layers and of pad asperities as well as the interfacial friction are elucidated. Additionally, scratch-regime maps are proposed to provide criteria for scratching hard surface layers by the softer pad asperities. Furthermore, scratching indexes are introduced to predict the proportion of asperities in contact that are likely to scratch. The contact mechanics models of scratching have been validated by sliding experiments with two commercial CMP pads (Pad A and IC1000) and various thin-films (Al, Cu, SiO 2 , Si 3 N 4 , TiN and three low-k dielectrics) using deionized water as a "lubricant." Both the theoretical models and the experimental results show that the number of scratches increases as the scratching index exceeds 0.33. Al and Cu layers are found to be more susceptible to pad scratching due to their low hardness and high interfacial friction. The scratch-regime maps provide practical guidelines for mitigating pad scratching in CMP. Chemical-mechanical polishing (CMP), which employs both chemical and mechanical means to remove material from solid surfaces, is a planarization/polishing process. The CMP process is widely used in the manufacture of integrated circuits (IC), computer hard disks, optical glass, and micro-electromechanical systems (MEMS).1-5 Despite its universal usage, a persistent problem in CMP is the scratching of the surfaces being polished. As demands for metal interconnects and surface structures are becoming ever more stringent in integrated circuits, micro-and nano-scale scratching during CMP has lately emerged as a critical problem. 6-8The CMP process is carried out by rotating a wafer over a polishing pad under pressure, while chemical slurry containing hard, abrasive particles is provided at the wafer/pad interface. The basic mechanism of material removal in CMP is by "fine scratching" at the nano-scale by the abrasive particles.9-12 The nano-sized particles, 50-300 nm in diameter, "plow" the surface layer softened by chemical reactions. Such nanometer-scale scratches produce smooth, flat surfaces and thus are preferred.During the polishing process, however, the small abrasive particles may agglomerate due to inter-particle attraction, fluctuations in slurry delivery, and so on. Scratches generated by the agglomerated particles are termed surface defects, and the width of the scratches so generated is an order of magnitude greater than that of the scratches created by individual particles. The abnormally large, hard particles, which may cut the metal interconnects in IC chips and cause malfunctioning of the microelectronic devices, have generally been considered the primary sources of scratching.13-18 To minimize particle agglomeration, and thus mitigate particle-induced scratching, part...

Energy dissipation and constitutive modeling for a mechanistic description of pad scratching in chemical–mechanical planarization

“…The wafer is modeled as a linearly elastic material before yield. It has been shown that the polishing pad material plastically deforms and has time dependent mechanical behavior in CMP [3,[9][10][11]. The desired result of this analysis is to determine the elementary contact mechanics variables that influence scratch production.…”

“…Equation 9 provides the yield criterion for a relatively soft elastic sliding pad asperity over the harder wafer surface [3,14,15].…”

“…The real contact between a polishing pad and a wafer consists of many pad asperities in contact. A decrease in the deviation of asperity peaks on the pad surface will distribute the total load on the wafer more evenly, reducing the maximum total load for single asperity contacts [3]. Increasing the asperity radius of curvature increases the area of contact.…”

“…Relatively soft polishing pads can induce scratches on harder wafers that are large enough to greatly reduce manufacturing yields. Previous work investigating CMP pad scratching provides mechanical criteria based on contact mechanics which can predict scratching and presents methods to reduce damage [1][2][3]. Pad scratches are mitigated by decreasing relative hardness values of the pad and wafer materials, decreasing the coefficient of friction between the pad and wafer, and flattening the pad topography.…”

Scratching of Patterned Cu/Dielectric Surface Layers by Pad Asperities in CMP

“…Based on the theoretical and experimental work on monolithic layers, it has been found that scratching by pad asperities strongly depends on the pad-to-layer hardness ratio and the interfacial friction between the pad asperities and the surface layers [13]- [16].…”