Slurry particle

Global planeness and large scale scratches are usually evaluated by HDI instruments as shown in Fig. 3(a) [8], which is a surface reflectance analyzer to measure flatness, waviness, roughness of a surface, and observe scratches (Fig. 3(h)), pits (Fig. 3(c)), particles (Fig. 3(d)) on a global surface. These surface defects can also be observed by SEM, TEM, and AFM. Shapes of slurry particles can be observed by SEM and TEM, and their movement in liquid by the fluorometry technique as shown in Chapter2. 

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... 

To measure an individual particle surface interaction and its material removal effects. Because of the complexity of the polishing system, it is highly desirable to characterize the physical and chemical behavior of individual interactions while other components are fixed. AFM technology can be provided to explore slurry particle interactions with different surfaces in different liquid ambient. 

To test the transport rules of slurry particles in the flow field during CMP. An understanding of the slurry transport and particles motion beneath the wafer plays an important role in revealing the interaction process... 

Slurry particle agglomeration can take place in the slurry in which abrasive particles and colloids coalesce to form extended particles. 

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... 

Nagahara et al, The Effect of Slurry Particle Size on Defect Levels for a BPSG CMP Process, Proceedings of 1996 VMIC Conference (June 1996), p. 443. 

Chemical mechanical polishing appears to consist of two cooperating physical mechanisms [9]. First, chemical interaction of the slurry with material at the surface of the wafer weakens the surface to be polished. Second, the weakened surface is mechanically removed by a combination of slurry particles, polish pad asperities, and hydrodynamic effects. The extent... 

Filtration can reduce the particles adhered to the wafer, but only slightly. Filtration will, however, reduce scratch defects. Slurry particles can be more effectively cleaned if more elaborate post-CMP clean chemistry is used [8]. 

Slurry Particles Surface roughness Plug recess Fe content... 

Fig. 9. AFM image of post CMP surface showing nanoscratches produced by slurry particle...

Change in slurry particle Change in rate (carrier related) pad texture (grooves, perforations, texture) Slurry flow slurry nonuniformity ... 

Slurry particle-reaction product buildup Rate defectivity... 

Figure 1 shows the surface configuration after STI CMP. Both the nitride stop layer and the isolation oxide contaminated with Si02 slurries, particles, and metals must be cleaned. 

To prevent large slurry particles from scratching wafers, filters can be used on the slurry lines. The drawbacks of filters in the slurry lines, however, are that (1) the slurry flow rate may decrease as the filter is being clogged and (2) the CMP polish rate can decrease as the filter approaches its end of life. 

A choice of a filter with the correct membrane pore size can prevent the shift in polish rate. The pore size chosen must be significantly larger than the slurry particle sizes. For example, if the mean of the slurry particle size is 1000 A, the pore size should be a few microns, so that no normal sized particles will be filtered out. Frequent filter changes can prevent filter clogging. 

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. 

Purge-and-trap collection is well adapted to biological samples such as blood or urine that are soluble in water (Pellizzari et al. 1985a Peoples et al. 1979), and is readily adapted from techniques that have been developed for the analysis of carbon tetrachloride in water and wastewater. For water- insoluble materials, the purge-trap approach is complicated by uncertainty of partitioning the analyte between sample slurry particles and water. 

Slurries have been used to introduce soil samples into an ETV with ICP-MS detection [332] as well as directly into an ICP-MS using a Babington-type nebulizer [333]. Although slurry sample introduction eliminates the problems associated with sample dissolution, care is required to ensure that the slurry particles are small enough to be completely vaporized in the ICP. Agglomeration of particles in the slurry before introduction to the nebulizer must be prevented in order to maintain constant transport efficiency into the ICP... 

Constant-rate filtration is employed sometimes when an improperly used centrifugal pump may break down the slurry particles. In fact, however, centrifugal pumps are most often chosen for filtration operations. The following example shows the relevant calculations. 

Some products will not grind easily and may have strong crystals up to 1 mm In size or at least significant numbers beyond 150 microns. Slurry particles as large as this will cause very low production rates in the small media mill or necessitate repeated passages through the mill. 

An example of the steady-state rheometric test at room temperature for various slurries of similar chemical compositions is shown in Fig. 2.11. Slurry viscosity decreases as shear rate increases hence, the tested slurry is a non-Newtonian liquid and cannot be characterized by measuring viscosity at a single shear rate. Decrease in the slurry viscosity can also be ascribed to the deagglomeration of the slurry particles caused by shearing. A steady-state rheometric test distinguishes the slurries with only minor differences in their compositions, as shown in Fig. 2.11. One area of further opportunity in terms of... 

Typical industrial plastic selection criteria have focused on pin-on-disk tests (involving plastic sliding over steel) and sand slurry abrasion tests. The CMP environment, however, is very different from these typical industrial tests. In CMP, the retaining ring plastic is subjected to a plastic-to-plastic adhesive force component involving the polyurethane pad, chemical attack from the chemicals in the slurry, as well as an abrasive component associated with slurry particles. 

Wang C, Sherman P, Chandra A. Modeling and analysis of pad surface topography and slurry particle size distribution effects on material removal rate (MRR) in chemical mechanical planarization. Int J Manuf Technol Manag 2005 7(5/6) 504-529. 

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