Uniform Material Removal

Chemical etching of the silicon wafer in mixtures of HF and HNOj is used to remove surface work damage and stress from the slicing operation as well as to provide 

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Multilayer or stacked pads are commonly used in the CMP processes for better polishing performance such as uniform material removal and good planarization across the wafer surface. A multilayer or stacked pad usually consists of a stiff, hard top layer and a soft, flexible subpad, and possibly some medium layer(s), as shown in Fig. 5.3. 

Microscopic images, Figure 1.36, of weathered samples illustrate the material-dependent influence of acid precipitation. The acidic weathered sample surfaces clearly exhibit the locally limited, additional effect caused by droplets of simulated acid dew. They include spots due to structural differences on PA 6, etching pits on PA12, or bizarre surface defects on the acid loaded ABS sample that indicate non-uniform material removal from the styrene and butadiene phases [92]. 

The most common filter pack material is quartz (silica). Quartz is relatively inert, readily available, and workable therefore, it is preferred to replace formation materials removed from the borehole. The grain size of the filter material (i.e., sand or gravel) should be chosen based on the characteristics of the formation to be monitored and the slot size of the screen. Sand and gravel are available in various uniform sizes to accommodate different monitoring environments. 

Guo, Y., Tang, J., Dornfield, D. (1998). A finite element model for wafer material removal rate and non-uniformity in chemical mechanical polishing process. Proc. 3rd Int. CMP for ULSI Multilevel Interconnect. Conf, Santa Clara, pp. 113-118. 

The CMP output parameters include removal rate, planarization efficiency, surface finish, material removal rate selectivity, wafer-to-wafer uniformity, within-wafer uniformity, dishing and erosion, and defect levels [15]. 

One of the key performance metrics for a CMP process is the polish rate, including the uniformity of the polish rate across the wafer and from wafer to wafer. In the previous sections and in Chapter 4, material removal is discussed in terms of fundamental principles. This section discusses the polish rate in terms of process variables and emphasizes what tools the process engineer has to improve polish rate and polish rate control. 

The CMP process is composed of a chemical effect from nanosize ceramic particles and a physical effect from the pressed pad. Pads and slnrries are the consumables of a CMP process. The polishing pads consist of polynrethane. Generally two types of pads (hard and soft types) are simultaneously used in the CMP process. A hard pad gives better local (within die) planarity, bnt a soft pad gives better uniformity of material removal across the entire wafer. A hard pad is mounted onto a softer pad to form a stacked pad. Figure 15.1 shows a stacked CMP pad the hard pad is the Rodel 1C-1000 and the soft pad is the Rodel Snba-I V. ... 

The third element of the interaction, is the pad as shown in Fig. 1. The purpose of the pad is to provide uniform pressure to the asperities in a narrow range in the direction normal to the wafer. In this way, the material removal occurs so that the poUshed material surface evolves toward a planar one. There are a number of additional properties of the pad that are important and have been empirically optimized. These properties include the pad hardness, its abiUty to be conditioned, its density, its moduU, as well as several other physical and chemical properties. 

The characteristics of CMP are material removal rate (MRR), thickness uniformity, and surface quality, and they are directly related to device characteristics and productivity. These characteristics are determined by each factor as per Figure 1.3. 

The corrosion-like electrochemical process of material removal refers to spatially uniform general corrosion of the metal surface. However, the wet CMP environment can also support certain other types of undesirable electrochemical corrosions, such as localized pitting, and bimetallic/galvanic decomposition that contribute to surface defects. The considerations for mitigating these defects constitute a major aspect of slurry (additive) selection, which in turn can be facilitated by the use of electrochemical techniques. 

Note that this means that the material removed from the plane of contact is uniformly distributed over the surface of the sphere, rather than accumulating at the neck. For small values of 0, i.e., for small neck radius, cos 0 = 1 - 0 /2, so that Eq. (8.25) becomes... 

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