Platen polishers

Fig. 7. Defect and micro-scratch counts in an extended run over 3 days with on oxide wafers using AEP ring and a 2 platen polish process.

The first CMP tools, based on rotational platen silicon wafer polishing tools, had low throughput values (10 to 18 wfr/hr). Since the implementation of these first-generation tools for polishing, CMP tool design has... 

The removal rate on first-generation polishers can be improved by simply increasing the rotation rate. However, nonuniformity can suffer if the platen run-out is not controlled. Platen run-out is a measure of the wobble a platen undergoes during rotation and must be kept to a minimum to ensure good performance. 

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]. 

The platens on which wafers are polished have also evolved over time. Traditional polishing, as is done on first- and second-generation CMP tools, is done on a hard platen. The reason for a hard platen, of course, is to present as close to absolutely flat a surface as possible against which the wafer is pressed. Ideally, platens rotate perfectly. In practice, however, there is a small amount of run-out, or wobble, which limits the ability of the tool to polish films uniformly, especially at high rotation speeds. 

One goal of a bulk delivery system is to provide constant pressure to a CMP polisher, so that the peristaltic metering pump in the polisher can provide constant slurry flow to the polisher platen. While many schemes have been devised to minimize the fluctuations in pressure, there are many... 

On-site filtration of CMP slurry is one of the remedial approaches used to reverse the effects of changes to slurry particles between manufacture and delivery to the polisher platen. Figure 11 shows the locations that can be considered for slurry filtration. The candidate locations include... 

The disadvantages of pad A, as compared to pad B, are twofold (1) the grooving imposing a pad-life limit, because once the grooves are worn out, the pad can no longer polish and (2) the difficulty in putting the whole pad successfully on a platen without entrapping bubbles to accomplish this, more skilled technicians are required. Perforated pads do not have these problems. 

Experiment no. Down force (psi) Platen speed (rpm) Carrier Polishing rate (A/min) Nonuniformity (%) ... 

Typically, there are three principal parameters in a CMP recipe, namely, the down force, the platen rotation speed, and the carrier rotation speed. If we put the results of the DOE study from Table I into contour plots, it is easier to see the influences of each factor on the polish nonuniformity. Figures 6-8 show the relationships between recipe parameters and the polish rate and nonuniformity. We see, from Fig. 6, that the polish rate and nonuniformity can be simultaneously improved by using a larger down force. However, if the down force is set too high, scratches on a wafer may result. Decreasing the platen speed and the carrier speed decreases nonuniformity, as we can see from Figs. 6 and 7. However, decrease of the platen speed also decreases the polish rate, as shown in Fig. 6. 

Fig. 6. Contour plot of the CMP down force vs the platen speed. The output responses are polish nonuniformity and rate.

In a CMP process, a rotating polymer-based pad is pressed against the polished metal/oxide layer surface of a wafer while slurry or a combination of slurry and other chemicals is introduced into the polish platen. In a post-CMP cleaning process, residual abrasive particles and residues of polished layers are removed via flushing of the post-CMP solution and gentle mechanical... 

Typically, both the wafer carrier and platen are rotated in the same direction. A downforce is applied while the wafer carrier and platen are rotated on their own axes coc and co, respectively. The polishing slurry is dispensed from a tube located at the center of the pad, and as the platen rotates the slurry is transported between the wafer and the pad [3-6]. In addition to the rotary platform, an orbital design has also been implemented (Fig. 3.3) [2]. The operating principle for the orbital design is similar to the rotary platform, except that the polishing head and table are in orbital motion to each other. In addition, the slurry is usually delivered through the pad. 

As W CMP is a combination of chemical and mechanical actions, the temperature between the pad and the wafer will rise during polishing because of mechanical friction and chemical reactions. Without a precise control of platen temperature, the removal rate from wafer to wafer will not be consistent. For example, as shown in Fig. 9.12, the removal rates increase significantly and then reach a relative plateau for a typical slurry using a platen that is not temperature controlled. 

Is it possible to conduct barrier polishing on a separate platen using dedicated slurry, similar to copper CMP If yes, explain the process and both the positive and negative effects. 

ECMP process is typically non-Prestonian therefore, the control knobs of conventional CMP, such as downpressure and platen speeds, cannot be directly used to modify the polishing profile. The control knobs of ECMP are mainly the charges applied to each zone. Different voltages are used for simultaneous completion of the applied charge by each zone. 

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