Structured Abrasive CMP: Length Scales, Subpads, and Planarization
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3M Advanced Materials Technology Center Building 201-1W-28, 3M Center, St. Paul, MN 55144-1000, [email protected] ABSTRACT Chemical-Mechanical Planarization with structured abrasive uses a subpad to manage the pressure variations due to loading over a range of length scales. The effect of subpad construction on pressure responses related to those scales is illustrated. A minimum length scale for the effect of the subpad is established via contact mechanics. Differences between one- and two-layer subpads are shown. Uniform compression, point loading, and edge exclusion are considered briefly. A model of the subpad as a plate on an elastic foundation is applied to the problem of die doming. The roles of process pressure, die size, and subpad construction are illustrated. Planarization at the intra-die, die, and wafer scales are related to the subpad construction. INTRODUCTION Chemical-Mechanical Planarization involves often-conflicting requirements at various length scales--e.g. uniform removal at the wafer scale, but non-uniform removal of high areas to achieve planarization at the feature scale. In conjunction with machine process controls, the management of pressure by the consumables is one key to balancing these requirements. 3M is developing abrasive systems for CMP in which the abrasive particles are incorporated into resin-based structures of precise dimensions on the surface of polymeric webs [ 1-3]. In order to manage pressure, the structured abrasive web can be laminated to a multilayer subpad. A typical construction is shown in Figure 1. The subpad is made up of the layers between the machine platen and the structured abrasive. Because the thin fixed abrasive layer constituting the process surface is independent of the subpad, there is great flexibility in subpad design. The primary mechanical elements in the pad can be changed while leaving the abrasive layer-and thus the details of the material removal process-unchanged, apart from the pressure distribution. The objective of this study is to identify the controlling features of structured abrasive system performance relative to the length scales of concern. The focus is on the role of the Structured Abrasive on Backing AD18
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Figure 1. Schematic of typical construction of pad for slurry-free CMP showing subpad 51
Mat. Res. Soc. Symp. Proc. Vol. 566 ©2000 Materials Research Society
subpad. The aim is to use ideas from applied mechanics to illustrate the relative effects of the subpad construction for the intra-die, die, and wafer scales. Such a physics-based understanding of the interaction of subpad constructions and length scales can inform the choice of subpad construction for experiments to optimize the CMP process. THEORY Load Transfer into the Subpad There is a minimum length scale for loading of the subpad. Consider a point asperity contacting the surface of the abrasive layer. As the related stress transfers through the abrasive layer, it spreads out, so
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