The Polishing Pad

Polishing mode and the role of the fluid layer is poorly understood at this point. It is clear, however, that polishing mode and fluid layer thickness and continuity have important implications for polish rates and planarity. Thus, it is important that further studies in these areas be initiated. We shall return to the subject of fluid layers in Sections 4.5 and 5.2.2. 

One of the most important components of the CMP system is the polishing pad. Unfortunately, the pad is also one of the most poorly understood components. Polishing pad stracture and material properties are important in determining polish rate and planarization ability of a CMP process. However, the pad structure and properties are arrived at empirically for the most part Indeed, it is 

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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]. 

Orbital motion offers the capability of achieving high relative velocities without sacrificing tool footprint. This point is especially important as the semiconductor industry prepares to make the transition to 300-mm wafers. Several CMP tool concepts have been developed based on orbital motion. Some orbit the carrier while rotating the platen [13]. Others orbit the polishing pad while rotating the carrier [14]. Another design involves orbital (as well as arbitrary nonrotational) motion on a fixed polish pad [15]. 

OnTrak invented an intriguing pressure controller in which fluid pressure is used to fine-tune the pressure presented by the force apphed to the carrier against the polish pad [38-40]. Beneath a continuous polish pad is a... 

Second, friction during CMP generates heat that can affect the reaction kinetics as well as soften the polish pad. On most tools, heat transfer away... 

Any and all of these particles can become attached to the pad. As the surface of the pad accumulates more and more particles, the surface glazes and becomes smoother and less abrasive. Consequently, the removal rate declines. Also, as the pad becomes glazed it becomes smoother, which causes a decline in the ability of the polish pad to distribute slurry under the wafer. At present there is considerable debate as to whether CMP takes place in a contact regime or in a lubrication regime. It is likely that resolution of this issue will be necessary to establish what fundamental limits there are on slurry flow, as well as how these limits affect pad conditioning. 

In a typical modeling approach, the material removal rate is modeled as a function of easily controlled process parameters. The most basic model is one that predicts the bulk rate of material removal in a macroscopic fashion. An empirical observation by Preston is widely used, in which the rate of material thickness reduction is proportial to the product of (a) the relative velocity between the wafer and the polish pad and (b) the pressure on the surface of the wafer ... 

An important observation regarding the equations representing the deformation is that the shape of the deformation (with only vertical scaling factors) is independent of the material constants and only dependent on the area of force application. The deformation shape is therefore similar for different material properties. The material properties and applied force control only the actual amount of the deformation. This is ideal for our initial goal that was to determine the typical deformation profile of the polish pad. All that remains is the determination of the appropriate length scale to use to represent the deformation. 

Figure 17 shows the effective density using the elliptic filter with a characteristic length of 2.9 mm. The optimal length must be determined for each consumable set and process conditions since the planarization length is dependent not only on the polish pad type but also on the polish process conditions, notably the down force. 

Figure 5 shows a trend chart of polish nonuniformity. The data were collected from the daily qualification (three wafers per day) of a tool for a period of 1 month. The results indicate an unstable CMP process. As can be seen, the polish nonuniformity behavior was cyclic, gradually increasing and then suddenly dropping. These step-functional nonuniformity changes correspond closely to the polish pad changes. The reasons for these short-lived and run-away pads were probably the poor quality of the incoming pads after this batch of pads was used up and a new batch of pads was introduced, the nonuniformity problem dramatically decreased. 

An end effector, also called a pad dressor or a pad conditioner, is used to condition the polish pad to retrieve polish rate. If this is not done, the surface of a pad can become glazed and the pad austerity lost (see Fig. 9). The austerity of a pad is required in CMP otherwise, hydroplaning will occur, which means that contact between a pad and a wafer surface no longer exists. 

Fig. 10. Illustration of the polish pad surface profile created by the wafer carrier due to the polish (wear) rate being higher at the wafer edge than at the wafer center. An end effector can be used to compensate the profile if the duration vs position profile is set up correctly.

Scratches can also be caused by particles by another means. Since in CMP the slurry (particles included) stays on the polish pad, and the polish pad material is removed by the end effector, the end product of this complicated slurry-pad material reaction is a kind of substance with unknown characteristics and sizes. These substances can be the cause of the scratches. Figure 18 shows such substances collected by an end effector [16]. If the end effector is not properly cleaned, these particles can become hardened and cause severe scratches. To prevent this, a high-pressure water spray nozzle or a rotating brush can be used. 

Li W, Shin DW, Tomozawa M, Muraka SP. The effect of the polishing pad treatments on the chemical-mechanical polishing of Si02 films. Thin Solid Films 1995 270 601-606. 

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. 

During CMP, the temperature between the polishing pad and the wafer is elevated due to friction and chemical reactions [43,44]. The elevated temperature may have an impact on the surface adsorption behavior of the passivating agent. The passivating film may become thinner due to an... 

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