Pad—wafer interface

By delivering slurry directly to the pad-wafer interface, process engineers have a great deal of latitude in controlling slurry distribution across the wafer during polish. In other words, they can design processes that do not suffer the limitations of pH or oxidizer concentration gradients across the wafer. Oxide and metal CMP processes are very different, so it is useful not only to be able to inject slurry directly to the wafer surface, but also to control where on the wafer the slurry is delivered. 

A polishing pad has a significant impact on the performance of the CMP process. It transports the slurry to the pad-wafer interface, impacts the polishing nonuniformity, and affects the global wafer and device planarity. Pads may consist of thin porous closed cell [28], open cell [29], or noncell [30] polyurethane material. The properties of polishing pad can be studied in detail... 

High porosity may also change the fluid dynamics at the pad—wafer interface caused by more slurry holding. New models considering pads with high porosity are desired to better understand and predict these dependencies. 

Lee et al. (2013) recently developed a semiempirical CMP model to include consumable properties of both slurry particles and pad asperities. The proposed model assumes plastic contact at the wafer—particle interface, elastic contact at the pad—particle interface, a particle size distribution, and a normally distributed pad asperity height. The distinguishing advantage of this model is that both particle size distribution and asperity height distribution have been included, as shown in Figure 6.4. 

Luo and Domfeld [110] introduced a fitting parameter H , a d5mamical" hardness value of the wafer surface to show the chemical effect and mechanical effect on the interface in their model. It reflects the influences of chemicals on the mechanical material removal. It is found that the nonlinear down pressure dependence of material removal rate is related to a probability density function of the abrasive size and the elastic deformation of the pad. 

WTW and RTR control of thickness are improved by the use of end-point detection systems and advanced process control. End-point detection, whether mechanical or optical, monitor the state of the wafer surface (film thickness, reflectivity, etc.) or of the entire polishing system (friction, slurry by products, etc.) in an attempt to predict when the desired amount of material has been removed (i.e., the end of process). End-point detection is most successful in processes where a change in the films on the wafer surface leads to an abrupt change in the optical or mechanical properties of the wafer surface. For example, copper CMP end point is easy to detect by optical means due to the large difference in reflectivity of the copper film compared to the barrier films. In contrast, end-point detection for small amounts of ILD removal is difficult due to the lack of change in the wafer surface or the wafer-pad interface. 

Viscosity Units of poise (=0.1 Pascal sec). Viscosity affects how easily the slurry flows. The more viscous a material, the more it resists flow. High slurry viscosity results in poor transport of reactants and products to and from the wafer surface. Viscosity also affects slurry transport across the wafer and lubrication of the wafer-pad interface. 

We have found that the entrainment of a slurry between a silicon surface and a polyurethane pad will cause the generation of subambient pressure at that interface. These pressures cause the silicon to be further impressed into the pad. We have measured these pressures and this paper reports on the pressure distribution maps over an area beneath a 100mm diameter silicon wafer. The pressures are generally not uniform. The leading 2/3 of the wafer has subambient pressures of the order of 50kPa and the trailing 1/3 of the wafer has positive pressures of approximately lOkPa. The reasons for the subambient pressures is related to the dynamics of the compression of pad asperities, the boundary effects of the silicon edge, the rebound of the asperities, and re-infiltration of the slurry. 

The friction of polymers or CMP processes can be attributed to two major sources in mixed modes deformation involving the dissipation of energy in quite a large volume around the local area of contact, and adhesion originating from the interface between the wafer and the pads (brush). The details of the deformation and adhesion will be discussed in Chapter 5, along with a clear schematic illustration. 

Previous studies have indicated that no hydrodynamic lubrication occurs during CMP.28 3la There is always a physical contact between the wafer and the polishing pad asperities. In the following section, we will see that there is enough evidence to prove interactions between a wafer and a pad. The boundary lubrication associated with tribochemical interactions plays a dominant role. In order to understand the mechanisms of boundary lubrication in CMP, the physical, electrochemical, and mechanical processes of interfaces must be considered. The mechanisms can be classified into the following categories based on the surface physical chemistry of materials involved during CMP. 

Flip-chip technology, as shown in Fig. 11.14, is similar to TAB technology in that successive metal layers are deposited on the wafer, ending up with solder-plated bumps over the device contacts. One possible configuration utilizes an alloy of nickel and aluminum as an interface to the aluminum bonding pads. A thin film of pure nickel is plated over the Ni/Al, followed by copper and solder. The copper is plated to a thickness of about 0.0005 in., and the solder is plated to about 0.003 in. The solder is then reflowed to form a hemispherical bump. The devices are then mounted to the substrate face down by reflow solder methods. During reflow, the face of the device is prevented from contacting the substrate metallization by the copper bump. This process is sometimes referred to as the controlled collapse process. 

---