Pads and abrasives

2 Pads and Abrasives During polishing, material is removed from the wafer surface by abrasive particles that are pressed by the polishing pad onto the surface. A pad with abrasives on its surface is shown [66] in Fig. 5.15. 

The removal rate depends on how many abrasive particles on the pad are pressed against the surface. The pad has been modeled [70-72] as a Langmuir 

FIGURE 5.15 Section of an ICIOOO pad, partially covered with abrasive particles, after CMP with a 12% slurry containing 200-nm abrasives [66]. 

FIGURE 5.16 Pad and slurry with abrasives. Abrasive particles attach to and release from the pad. 

The attachment rate Ton is proportional to the concentration of abrasives in the slurry [A] and to the number of available sites Ton = on [A] ns. The separation rate Toff depends on the number of occupied sites roff = At 

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The expression in Equation 5.4, and the model it is based on, uses two values to parameterize the interaction between pad and abrasive UoPad and Apad- It is reasonable to ask about [76] the effect of abrasive size on each of these. If there is little separation between sites, as appears to be the case shown in Fig. 5.15, then the site density oPad must decrease with increasing abrasive area, which is proportional to the square of the abrasive diameter d - If, however, the mechanical removal rate per particle increases with the particle diameter, then these effects would largely cancel each other. To understand the effect of abrasive diameter on Apad = koff/kon, it is helpful to think about how particle size affects the rates of attachment to and separation from the pad. The... 

Slurry Chemicals A large variety of materials (metals, alloys, insulators, semiconductors, etc.) are being polished. Each has a diffemt chemistry as far as chemical interactions with the slurry is concerned. Slurry chemicals affect primarily the chemical component, e.g., etch rate. However, chemical reactions modify the mechanical properties of the film, pad, and abrasive surfaces, which in turn affects the mechanical component. 

A prime example of where polyamide-imides enjoy prominence is in friction and wear applications. This means not only in a system where the polymer components are meant to operate with low-friction and low-wear characteristics, but also in new applications such as brake pads and abrasion wheels, where the operating conditions require low wear with high friction, a truly demanding scenario. Polyamide-imides can achieve this due to their high softening temperature and retention of strength, stiffness, and compression properties at elevated temperatures. 

In the dry pad condition, however, the friction force remained constant with the wafer velocity since there is no lubrication fihn under the wafer surface. This can be illustrated in terms of interaction with pad and abrasives (Figure 1.5). In the condition of high downforce or low wafer velocity, the wafer moves on the pad with thinner slurry film. This can cause increased interaction between the wafer surface and the abrasives supported by the polishing pad. In the condition of low downforce or high wafer velocity, wafCT behavior can be the opposite. Wafer can slide on the pad with thicker slurry film. This can result in less interaction between the wafer and the abrasives. 

Two types of approaches are available. In one, the fabric is padded with the cross-linker finish, dried, then sent to the garment cutter. The garments are then pressed and cured. In the second, the fabric is cured in fabric form, then fabricated into garments. It is then pressed and recured in hot-head presses. This double curing is particularly hard on the ceUulosic fiber in terms of strength and abrasion resistance. 

In 1996, Liu et al. [129] analyzed the wear mechanism based on the rolling kinematics of abrasive particles between the pad and wafer. They summarized that the kinetics of polishing are (1) material removal rate is dependent on the real contact area between the slurry particle and the wafer surface. The real contact area is related to the applied pressure, the curvature, and Young s modulus of the slurry... 

Xie, Y. and Bhushan, B., "Effects of Particle Size, Polishing Pad and Contact Pressure in Free Abrasive Polishing, Wear, Vol. 200,1996, pp. 281-295. 

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. 

Some work has been done to correlate oxide CMP performance with pad properties [46]. This work indicated that the specific gravity of the pads and the cross-linking densities affect polish performance. Other work has been done to correlate CMP performance with slurry composition [47]. This work suggests that the friction during polish is proportional to the removal rate when the abrasive content is greater than 10%, and inversely proportional to the removal rate when it is less than 10%. 

Chemical-mechanical planarization occurs when the surface of the wafer to be polished is forced against a polishing pad. Aqueous slurry that contains abrasive particles is placed on a polishing pad. The wafer is moved relative to the slurry-covered pad and the rate at which material is removed is often described by the heuristic equation called Preston s law ... 

As can be seen from the above discussions, the process parameters affect the tribology at the interface. In the following sections, the effect of various pad characteristics, slurry compositions, and abrasive particle characteristics on the tribology of CMP is elucidated. 

CMP is a wet process with DI water and/or polishing slurry present. During polishing, pad is under the attack of water, slurry chemicals, and abrasive particles at elevated temperature due to the friction force among wafer-particle-pad contact. This leads to changes in pad s physical and mechanical properties that influence polishing performance. 

Qualitatively, since the contact area increases linearly with applied pressure, the effective pressure is constant for a given pad. Soft, compliant pads have a larger contact area and lower effective pressure whereas hard, stiff pads have a smaller contact area and higher effective pressure. Thus soft pads push abrasive particles against the wafer over a larger area but with less force than hard pads do. 

Thus this model of the pad-abrasive interactions in CMP can be used to understand how the polishing rate depends on abrasive loading and abrasive size. 

On the chemistry side, the key knobs are pH, concentration of oxidizer, and abrasive content, given a relatively developed slurry formulation. Among these factors, pH has the most profound impact on removal rate as it determines how deep the oxidization process can penetrate into the tungsten film. A deeper penetration of the tungsten film allows higher removal rate provided that there is sufficient mechanical force to remove the softened film. The mechanical forces include downpressure, abrasive particles, and pad. In other words, at any given pH and oxidizer concentration, there is a removal rate limit. Within the limit, the removal rate follows the Preston equation. If the mechanical force is above this limit, scratch-type defects will be present. If the mechanical force is severely below what is needed to remove all the softened fill, corrosion-type defects will dominate the surface. 

Table 14.2 gives an overview of various pads and slurries, suitable for microfabrication. The compiled list is based on the information obtained from some consumable manufacturers as well as the experience accumulated at the author s lab as of Spring 2006. Besides the large suppliers like Cabot Microelectronics, Fujimi, and Rohm Haas, smaller companies are concentrating on custom-specific developments such as Kemesys in France as well as suppliers of abrasives or dispersions, for example, Degussa AG or H.C. Starck, both from Germany. Links to some of these suppliers are provided in the reference section [19]. 

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