Down force

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

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

Data is taken from average values at 8 psi down force. Erosion data is wafer flat. Defect data is unclustered defects >0.3/rm. 

FIGURE 13.7 Removal rate and HDP-CVD oxide/Sl3N4 selectivity versus (down-force) X (table rpm) obtained on a rotary-type polisher using high-selectivity slurry. 

This results in a pressure difference of 0.87 PSl. Compared to a nominal down force of 7 PSl this can be neglected in a first order approximation. This leads to the 

Cheemalapati K, Li Y, Tang K, Bian G. Organic particles for copper CMP at low down force. Proceedings of 9th International CMP for ULSI Multilevel Interconnection Conference. 2004. P23-27. 

FIGURE 7.21 Removal rate versus table speed on 8" Cu blanket test wafers polished at 2 psi down force and 200 ml/min slurry flow rate on Stasbaugh n-Hance polisher. Square data points indicate the removal rate values with the organic particles, and diamond data points denote removal rate values for silica particles, both polished under identical formulation and abrasive concentration (from Ref. 110). 

Pad shaping is accomplished by adjusting the amount of time the pad conditioner spends at each radius [53]. Other control parameters process engineers may use include the down force exerted by the pad conditioner and the rotation speed of the conditioning disk. Typically, however, the 

The simplicity of early carrier designs was complicated by their use as part of the robot used to move wafers from the load cassette to the unload cassette. On a so-called gimbaled carrier, the down force was applied to a central point on a plate behind the wafer, and it was assumed that the applied force was transferred through the wafer backing plate to be distributed uniformly across the wafer. Lateral motion of the pad then caused a torque to be applied to the carrier. To compensate for this rotation, a gimbal was built into the carrier at the point where the down force was applied. 

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. 

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. 

Many processes involve criteria other than solids suspension, for example, crystallization, precipitation, and many types of leaching and chemical reactions. In crystallization, the shear rate around the impeller and other mixing variables can affect the rate of nucleation, and can affect the ultimate particle size. In some cases, the shear rate can be such that it can break down forces within the solid particle and can affect the ultimate particle size and shape. There are some very fragile precipitate crystals that are very much affected by the mixer variables. 

Double-sided brush scrubbing followed by a spin-rinse-dry step dominates the industry. An example of a double-sided brush scrubber is shown schematically in Fig. 16. The rollers keep the wafer positioned and rotating while the brushes dean debris from both the front and back sides of the wafer. The cleaning solution is delivered through the bristles themselves. Typically, the bristles are porous so that fresh solution can be delivered directly to the wafer and minimal amounts of debris accumulate on the brush. Increasing the down force increases the particle removal efficiency up 

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