The CMP Process

References to these research and development efforts are given throughout this book. 

The pad — the key media enabling the transfer of mechanical forces to the surface being polished and 

The slurry — that provides both chemical and mechanical effects. 

Each of the above are extremely important and briefly discussed here. Most of the variables discussed in Chapter 3 could be categorized in one of the above groups. Temperature, pressure, relative velocity of the surface being polished with respect to pad (which is usually rotating), and pre- and post-CMP cleaning that may affect the final acceptance criteria for the polished surface are other parameters that play important roles. 

The slurry is the third important key player among the three listed above. Key slurry parameters that affect CMP are discussed in Chapter 3. Slurries provide both the chemical action through the solution chemistry and the mechanical action through the abrasives. High polishing rates, planarity, selectivity, uniformity, post-CMP ease of cleaning including environmental health and safety issues, shelf-life, and dispersion ability are the factors considered to optimize the slurry performance. For hard materials like W and Ta mechanical effects are more important. On the other hand for soft 

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Conditioning, an important technology in the CMP process, is to maintain the asperity structures on the pad surface, which force the abrasive particles against the wafer. A... 

The polishing pad, as another consumable material, also has a dominating effect in the CMP process, which is usually made of a matrix of cast polyurethane foam with filler material to control hardness of polyurethane impregnated felts. The pad carries the slurry on top of it, executes the polishing action, and transmits the normal and shear forces for polishing, thereby playing a very cnacial role in process optimization [44 6]. 

The CMP process is regarded as a combination of chemical effect, mechanical effect, and hydrodynamic effect [110-116]. Based on contact mechanics, hydrodynamics theories and abrasive wear mechanisms, a great deal of models on material removal mechanisms in CMP have been proposed [110,111,117-121]. Although there is still a lack of a model that is able to describe the entire available CMP process, during which erosion and abrasive wear are agreed to be two basic effects. 

In 1999, Luo and Domfeld [110] proposed that there are two typical contact modes in the CMP process, i.e., the hydro-dynamical contact mode and the solid-solid contact mode [110]. When the down pressure applied on the wafer surface is small and the relative velocity of the wafer is large, a thin fluid film with micro-scale thickness will be formed between the wafer and pad surface. The size of the abrasive particles is much smaller than the thickness of the slurry film, and therefore a lot of abrasive particles are inactive. Almost all material removals are due to three-body abrasion. When the down pressure applied on the wafer surface is large and the relative velocity of the wafer is small, the wafer and pad asperity contact each other and both two-body and three-body abrasion occurs, as is described as solid-solid contact mode in Fig. 44 [110]. In the two-body abrasion, the abrasive particles embedded in the pad asperities move to remove materials. Almost all effective material removals happen due to these abrasions. However, the abrasives not embedded in the pad are either inactive or act in three-body abrasion. Compared with the two-body abrasion happening in the wafer-pad contact area, the material removed by three-body abrasion is negligible. 

CMP processes for oxide planarization (ILD and STI) rely on slurry chemistry to hydrolyze and soften the Si02 surface. Mechanical abrasion then controls the actual material removal. Thus, the key process output control variables (i.e., removal rate and nonuniformity) are strong functions of the mechanical properties of the system, namely, the down force and the relative velocity between the pad and the wafer. Metal CMP processes such as copper CMP rely more on chemical oxidation and dissolution of the metal than mechanical abrasion to remove the metal overburden. Consequently, careful control of the chemistry of the CMP process is more important for these CMP processes than it is for oxide CMP. Thus, CMP tools and processes optimized for ILD may not be optimal for metal CMP and vice versa. 

These rules basically state that post-CMP cleaning must be done as part of the CMP process, and it must be done as quickly as possible after beginning the CMP process. Rule 7 is driving the design of cleaners comprised of... 

Water consumption by the CMP process is enormous. Estimates vary widely, but range up to a total of 30-40% of total fab consumption [10]. Whatever the value is, water consumption by CMP will increase for the foreseeable future as CMP becomes more widely implemented and as new wafer fabrication schemes make greater use of this process. 

As understanding of the CMP process improves, one can expect a great deal of work in all aspects of CMP modeling and simulation. These improvements are likely to extend over many length scales spanning wafer-level polish and uniformity concerns to die-level prediction to microscopic feature, chemical, and mechanical interactions. 

A number of models for wafer-scale nonuniformity have been published. While of significance for understanding the CMP process, they have limited relevance for this present chapter as, for the most part, pad effects are neglected. The reader is referred to the primary references listed here by focus area ... 

The expected impact of the material properties reviewed in Table II on the polishing mechanisms reviewed in the previous section is summarized in Table IV. As might be expected from previous discussion on structure vs properties, a high degree of interaction between properties and process effects is evident. Publicly available evidence to support materials properties effects on the CMP process is relatively limited. This is reviewed in Section IV. 

Summary of Expected Pad Material Property Effects on the CMP Process... 

A damaged layer between 1 to 10 nm thick according to the material and the polishing conditions is generated by the CMP process. This layer would seem to have to be removed as it presents poorly defined physical properties, for example, in terms of contamination, internal stress, insulating characteristics, and the like. Nevertheless the detrimental effects of this layer still have to be clearly demonstrated. 

As the charge of the particles is in equilibrium with the other species in solution, the zeta potential depends on the pH as well. Figure 13 shows the zeta potential of some materials concerned by the CMP process, where SiOj stands for both the substrate and the fumed glass slurries PVA is the material used for the scrubber brushes (poly vinyl alcohol) SijN. is used as a polish stop layer and AI2O3 and CeOj represent the alumina and ceria slurries, respectively. 

One of the metrology issues with the STI process is that the process utilizes three layers of different materials (1) thin thermal oxide (less than 200 A), (2) nitride (approximately 1500 A), and (3) the TEOS oxide above the active regions (see Fig. 7). Ideally, the CMP process polishes the TEOS oxide and stops at the nitride. In reality, after the polish, either a very thin residual TEOS oxide is present or the TEOS is completely gone and the nitride thickness is being measured. This poses some problems in the setup... 

Fig. 12. Illustration of the relationship between the delta and the final thicknesses across a wafer. Thickness variation is caused by the CMP process. The more thickness polished, the more thickness variation (higher standard deviation), and the higher the nonuniformity.

Figures 20a and 20b are SEM micrographs of dielectric cracks from a top view and from a cross-sectional view. The root cause was determined to be poor stress control in the TEOS film, not the CMP process. However, this problem may not have been as serious without CMP (i.e., CMP may exacerbate the problem). For CMP, the dielectric film must be highly compressive. If a film is tensile or not compressive enough, the overwhelming down force of CMP will impart significant shear stress, enough to break down the film. This is particularly true for the areas above the metal lines at ILD levels, where the surface has an abrupt step height in topography. A crack can start from the concave corner of the step and travel to the corner of a metal line, as shown in Fig. 20c.

A polishing pad has a significant impact on the performance of the CMP process. It transports the slurry to the pad-wafer interface, impacts the polishing nonuniformity, and affects the global wafer and device planarity. Pads may consist of thin porous closed cell [28], open cell [29], or noncell [30] polyurethane material. The properties of polishing pad can be studied in detail... 

What is tribometrology What are its applications during the CMP process In a fabrieation plant, one of the CMP stations encountered inconsistent material removal rate. Given that the slurry is fully functional, how would you use trihometrology to determine the source of inconsistency ... 

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