Wafer carriers importance

Preston s equation indicates a pressure dependency and if the pressure distribution across the surface of the wafer is not uniform, one expects a wafer-level removal rate dependency. Runnels et al, for example, report a model incorporating pressure dependencies to account for wafer scale nonuniformity [42]. The distribution of applied force across the surface of the wafer is highly dependent on the wafer carrier design, and significant innovation in head design to achieve either uniform or controllable pressure distributions is an important area of development. 

Unlike W plasma etch back process, the typical W CMP process usually removes the adhesion layer such as Ti/TiN or TiN during the primary polish. As a result, during the over polish step there is some oxide loss. Since the oxide deposition, planarization CMP (oxide CMP), and tungsten CMP steps are subsequent to each other, the oxide thickness profile could become worse further into the process flow. Therefore, the across-wafer non-uniformity of the oxide loss during W CMP process is one of the very important process parameters needs to be optimized. To determine the effect of the process and hardware parameters on the polish rate and the across-wafer uniformity, designed experiments were run and trends were determined using analysis of variance techniques. Table speed, wafer carrier speed, down force, back pressure, blocked hole pattern, and carrier types were examined for their effects on polish rate and across-wafer uniformity. The variable ranges encompassed by the experiments used in this study are summarized in Table I. 

As shown in Fig. 1, down-force or pressure is applied on the wafer through a wafer carrier. The wafer carrier plays an important role in polishing... 

An important step toward the understanding and theoretical description of microwave conductivity was made between 1989 and 1993, during the doctoral work of G. Schlichthorl, who used silicon wafers in contact with solutions containing different concentrations of ammonium fluoride.9 The analytical formula obtained for potential-dependent, photoin-duced microwave conductivity (PMC) could explain the experimental results. The still puzzling and controversial observation of dammed-up charge carriers in semiconductor surfaces motivated the collaboration with a researcher (L. Elstner) on silicon devices. A sophisticated computation program was used to calculate microwave conductivity from basic transport equations for a Schottky barrier. The experimental curves could be matched and it was confirmed for silicon interfaces that the analytically derived formulas for potential-dependent microwave conductivity were identical with the numerically derived nonsimplified functions within 10%.10... 

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]. 

Electroless metal deposition at trace levels in the solution is an important factor affecting silicon wafer cleaning. The deposition rate of most metals at trace levels depends mainly on the metal concentration and some may also depend on the interaction with other species as well. For copper the deposition rate at trace levels in HF solutions is different for n and p types. It depends on illumination for p-Si but not for n-Si. It is also different in HF and BHF solutions. In a HF solution the deposition process is controlled by both the supply of minority carriers and the kinetics of cathodic reactions. Thus, a high deposition rate occurs on p-Si only when both and illumination are present. In the BHF solution, the corrosion process is limited by the supply of electrons for p-Si whereas for n-Si it is limited by the dissolution of silicon because the reaction rate is indepaidmt of concentration and illumination. The amount of copper deposition does not correlate with the corrosion current density, which may be attributed to the chemical reactions associated with hydrogen reduction. More information on trace metal deposition can be found in Chapters 2 and 7. 

Next, how WID planarity can be improved is explained then, several methods for how to improve WIW global nonuniformity are introduced. According to Preston s law, the polishing rate is proportional to the load on the wafer and the relative speed between the wafer and the table. In order to improve WIW global nonuniformity, it is very important to control the profiles of the load on the wafer and the relative speed between the wafer and the table. The load on wafer is controlled by carrier design. The relative speed between the wafer and the table is controlled by changing the rotating speed of both the wafer and the table. 

Figure 16.2 also shows the simple stmcmre of Carrier (1). Carrier (1) incorporates two important functional components Retainer Ring (7), which is designed to control the rebound of Pad (5) and Backing Film (Membrane) (8), which controls the pressure applied to the back of Wafer (6). The details will be described later. 

The second main feature of Carrier B is Membrane (3). Membrane (3) is made of an elastic body-Uke rubber. As shown in Figure 16.9, Membrane (3) is divided into several areas. Through Shaft (8) and Holes (7), a different load can be applied to each divided area of Membrane (3). This means that the CMP removal rate of each divided area can be controlled. As mentioned earlier, deposition cannot be evenly formed on an in-coming wafer. By measuring the deposition distribution and controlling the load applied to each divided area during polishing, the CMP removal rate uniformity can be controlled. This is an important measure for CLC (closed loop control). 

In order to use /d-Ga20s wafers as substrate, it is very important to have a precise control of the electrical properties. At the beginning it was thought that the n-type conductivity of 0-G 2O3 originates on oxygen vacancies that singly or doubly ionized act as donors. Actually, the systematical change in carrier concentration... 

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