Pad Surface Shape

If we knew the mean pad surface shape then Equations 6.41 and 6.42 would provide the local load balanced solid contact polishing pressure. However, the pad mean surface shape is itself influenced by the local contact pressure. If we knew the pressure distribution, then the displacement field of the pad under the wafer at equilibrium could be found by solving the momentum equations [25] ... 

After solving the above elasticity problem, the mean pad surface shape ( under loading is then the z coordinate M3 of the displacement at the top of the pad. 

N Number of sweeping cycles. makes the pad surface shape... 

The smaller the conditioner diameter, the flatter the pad surface shape. 

Z.C. Li, E. Baisie, X.H. Zhang, Diamond disc pad conditioning in chemical mechanical planarization (CMP) a surface element method to predict pad surface shape. Precis. Eng. 36 (2) (2012) 356-363. 

Plotted with the rate is pad surface temperature (x s in Fig. 1), as measured for similar process conditions. The stark similarity of the shape of the curves adduces a strong hint as to the nature of the removal rate. Since the process conditions are not changing during this ramp, a direct causal temperature/rate relationship can clearly be inferred. 

The purpose of the conditioning disk is to present the conditioning abrasive to the pad surface in a way that will deliver the necessary pad conditioning effects. Currently, all three of the required effects are treated as a single effect and handled through physical abrasion of the pad surface with a diamond-studded end-effecor (called a disk). The use of diamonds is necessary because of the extreme hardness of the abrasives used in the slurry. The presence of such abrasives on the pad would rapidly wear down materials softer than diamond. Several major suppliers of diamond conditioner disks are available, each one with its own combination of disk properties. A variety of physical disk formats is available, from solid disks to open patterns (such as ribbed or honeycomb shapes). Likewise a variety of ways to bind the diamonds to the disk exist, and they impact disk lifetimes and possibly wafer scratch damage. There is also a choice of diamond types, which affects the shape of the abrasive as it protrudes from the disk surface. 

The slurry mix or mixtures that satisfy the desired consistency is used to determine its setting characteristic. A mix passing the consistency test is poured onto a 152 by 152 mm asphalt felt pad and shaped to 6 mm thickness using a 6 mm template. At the end of 15 min, at 25° 1°C and 50% 5% relative humidity, a white paper towel or tissue is lightly pressed or blotted on the slurry surface. If no brown stain is transferred to the paper, the slurry is considered set. If a brown stain does appear, repeat the blot procedure at 15 min intervals. After 3 h of blotting, 30 min (or longer) blot intervals would be suitable. 

A feature topography on the wafer surface is considered as a rigid body whose cross-sectional shape is described by a height function/(x, y) in xy coordinates. The pad is assumed to be a massive (semi-inhnite) elastic body with a flat surface. The relation between the displacement of the pad surface at each point w(x, y) and the contact pressure p(x, y) can be obtained from... 

Figure 4.20.A shows a more recent cell reported by Cobben et al. It consists of three Perspex blocks, of which two (A) are identical and the third (B) different. Part A is a Perspex block (1) furnished with two pairs of resilient hooks (3) for electrical contact. With the aid of a spring, the hooks press at the surface of the sensor contact pads (4), the back side of which rests on the Perspex siuface, so the sensor gate is positioned in the centre of the block, which is marked by an engraved cross as in the above-described wall-jet cell. Part B is a prismatic Perspex block (2) (85 x 24 x 10 mm ) into which a Z-shaped flow channel of 0.5 mm diameter is drilled. Each of the wedges of the Z reaches the outside of the block. The Z-shaped flow-cell thus built has a zero dead volume. As a result, the solution volume held between the two CHEMFETs is very small (3 pL). The cell is sealed by gently pushing block A to B with a lever. The inherent plasticity of the PVC membrane ensures water-tight closure of the cell. The closeness between the two electrodes enables differential measurements with no interference from the liquid junction potential. The differential signal provided by a potassium-selective and a sodium-selective CHEMFET exhibits a Nemstian behaviour and is selective towards potassium in the presence of a (fixed) excess concentration of sodium. The combined use of a highly lead-selective CHEMFET and a potassium-selective CHEMFET in this type of cell also provides excellent results.

Figure J shows an example of the top surface features of an MCP designed for electrooptical-signal-processing applications (33). The MCP has 18 chip attach pads surrounded by dumbbell-shaped pads for wire bonding and repair. The top surface also contains off-package I/Os along two sides, wide power distribution lines, and sites for decoupling capacitors. In this design, the package size of 2.25 by 2.25 in. (5.7 by 5.7 cm) was determined by the top-layer features rather than by the maximum interconnection density.

The wafer contour determines the area of contact between the wafer and the pad along with the abrasives. Thus, the amount of surface asperity interaction and the particle-wafer interaction also depends on the wafer contour. The fluid film that is in contact with the wafer surface is also dependent on the wafer contour. Thus, the pressure experienced by the wafer at different applied pressures and velocities changes with the shape of the surface. Scarfo et al. [20] conducted polishing tests on wafer samples with concave, convex, and intermediate surface contours and noted that the shape of the wafer affects the coefficient of friction. 

Pad Hardness Hardness, measured in relative units based on the type and mode of the indentation, is generally a measure of the ability of the pad to maintain its shape, and thus it is linked to elastic and viscoelastic properties. Harder pads are expected to provide better planarity. Softer pads, on the other hand, may provide better (defect-free) surfaces. Whereas it is difficult to measure the hardness of the pads, the viscoelastic behaviors can be measured to provide the necessary information. 

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