Platen temperature control

As W CMP is a combination of chemical and mechanical actions, the temperature between the pad and the wafer will rise during polishing because of mechanical friction and chemical reactions. Without a precise control of platen temperature, the removal rate from wafer to wafer will not be consistent. For example, as shown in Fig. 9.12, the removal rates increase significantly and then reach a relative plateau for a typical slurry using a platen that is not temperature controlled. 

FIGURE 9.12 Removal rate profile for a tungsten process conducted on a platen without temperature control. 

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Press control is critical it is essential that the elastomeric compound reaches the required cure state to optimize product performance yet remains in the press the shortest time period to maximize productivity. To meet this objective, both compression and injection presses now use microprocessor controls, which enable variations in platen temperatures and compound cure characteristics to be accommodated without sacrificing product performance or productivity. 

Although typically not thought of as a platform for more than presenting a flat surface, platens are being exploited in several ways to enhance overall CMP performance. Included in these approaches are temperature control, slurry delivery routing, and pressure control. 

Temperature control is accomplished in one of three general ways. One method is by controlling the temperature of the platen, usually by means of an integral channel in the platen through which temperature-controlled heat transfer fluid flows. Second, the temperature of the slurry itself can be regulated prior to being dispensed onto the platen. Finally, a means of heating the backside of the wafer can be built into the carrier [42,43]. 

Good temperature control over aU areas of the tool is recommended and correctly placed cartridge heaters used in conjunction with thermocouples and efficient controllers give an ideal system. Mold heating can be by any of the conventional methods, but direct conduction from platens is probably the easiest and most versatile. Conventional methods of production of heat are by oil or electricity. 

Figure 4 shows schematically the STS Multiplex ICP etching system. The source plasma is generated by an inductively coupled coil supplied by a 1-3 kW 13.56 MHz RF generator. Another 13.56 MHz generator is used to power the platen electrode which allows independent control of the bias potential of the wafer relative to the source plasma. The wafer temperature is maintained at typically less than 80 C through temperature-controlled, pressurized helium supplied to the back of the wafer. Wafers may be either mechanically or electrostatically clamped to the platen electrode. Typical base pressures are in the 10 torr range. The system is equipped with an automatic pressure control valve which can operate in one of two modes. In fixed automatic pressure control (APC) mode, the APC position remains constant, and the chamber pressure is a function of gas flows and RF powers. In automatic APC mode, the APC valve adjusts to maintain the chamber pressure at a constant value. The tool has been developed... 

Earlier it was stated that press manufacturers have made substantial improvements to the temperature control of platens. This work has to be extended to ensure that the moulds become integrated into the system. Some thermal control systems are available which allow such control, but cost is cited against their use to eliminate cavity-to-cavity variation. It is not logical to expect any system to maintain close temperature control over an area of half a square metre from one pair of thermocouples Greater control will pay real dividends by allowing the technologist to optimise a cure time that is based on equal crosslink density rather than the cure time required for the coldest cavity. 

The particleboard copolymer resin was applied at an 8% rate based on oven dry weight as the binder for flakeboards. A commercial phenol formaldehyde resin, catalyzed with resorcinol resin and extra sodium hydroxide was similarly used as the control resin binder for flakeboard evaluation. The density was 48 pounds per cubic foot for oak flakeboards and 43 pounds per cubic foot for pine. The oak flakeboards were pressed at a platen temperature of 350°F, however, the temperature had to be dropped to 335 F for the pine flakeboards due to a deficiency of steam pressure at the time they were to be made. The press times were ranging from 60 seconds to 120 seconds (push button close to push button open). 

Accuracy and repeatability of temperature/time/velocity/pressure controls of injection unit, accuracy and repeatability of clamping force, flatness and parallelism of platens, even distribution of clamping on all tie rods, repeatability of controlling pressure and temperature of oil, oil temperature variation minimized, no oil contamination (by the time you see oil contamination damage to the hydraulic system could have already occurred), machine properly leveled. 

A fluid film process line is shown in Figure 2. It is a series of modules, each consisting of the pair of platens illustrated in the previous slide. Each module is 5 feet (1.524 meters) long and each can have controls to adjust the temperature and pressure of the entering air. Thus, knowing the profile of temperature and pressure needs of a specific foam chemical formulation, the line can be adjusted for optimum operation and minimum energy waste. 

Various tool architectures have addressed the problem in different ways. Fixed platen architectures often control the temperature of the platen through a recirculated heat exchange method (Figure 1.29), and sometimes they also control the temperature of the slurry being transported to the pad surface. 

As stated earlier, uniformity of seal depends on the perfect matching of two platens which under well-controlled conditions undoubtedly gives a better seal. The pattern of seal may be line, pyramid or cross-hatch, but the last of these is invariably preferred. In the case of the cylinder type, the seal is achieved at the circumferential contact between two rollers, one of which is heated. As the process dwell time is very short, a higher temperature generally has to be employed. 

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