Conformity - November 2008 - (Page 38) With the publication of E78, tool manufacturers and end users have a common document from which to work when developing their electrostatic management programs. E78 promotes a partnership between tool manufacturers and end users to create products, processes, and cleanroom environments that moderate the effects of static charge. In this partnership, equipment manufacturers design in static control to meet the specified sensitivity levels. Awareness of the problem on both sides is the best way to determine an acceptable level of static charge that will not affect yield or throughput. However, each tool platform is unique in its processes, materials used, and automation employed, and a thorough ESD/ESA characterization should be done to comply with E78 guidelines. SEMI E78-0706 In summary, E78 was developed to minimize the negative impact on productivity caused by static charge in the semiconductor manufacturing environment. It provides methods of measurement and recommendations for maximum levels of allowable charge on wafers, wafer containers, parts of the input/output ports in mini-environments, internal walls and surfaces, etc., in order to eliminate ESD electrical damage to product as well as particle attraction issues. It also provides methods of measurement to ensure immunity to ESD-related equipment failures (ESD stress testing). However, E78 does not address the various inner processes that take place in automated wafer processing equipment. For example, chamber operation evaluations are not required as part of an E78 static risk evaluation of the semiconductor tool. Wafer charging and discharging mechanisms often exist inside these semiconductor tool wafer-processing chambers. The remainder of this article reviews a typical chamber design, along with some of the charging and discharging mechanisms that were detected, documented, analyzed and subsequently eliminated as part of a thorough ESD/ESA chamber evaluation. We feel that such evaluations are important to accomplish along with standard E78 evaluations and static control implementations in the equipment front-end modules (EFEM) areas that are commonly addressed. Chamber Evaluation In our wafer chamber evaluations, we have frequently observed and determined that any process involving a wet chemical or DI water application has a strong propensity to result in a significant charging event. Therefore, it is essential to remove the charge during and immediately after such a fluid dispensing operations in order to eliminate potential damage to devices on the wafer. In this case study, the chamber was completely enclosed during normal operations – similar in structure to a basic chamber design shown in Figure 1. The charging characteristics of the wafer could be measured by both direct contact and non-contact techniques. Real-time voltage monitoring was performed with an oscilloscope. Once the monitoring equipment was in place, the operation was started and the chamber “baseline” measurement was recorded (see Figure 2). The voltage on the wafer inside the chamber was monitored continuously in real-time in order to determine all potential risks with the operations. As can be followed on the scope trace in Figure 2, the points of interest corresponding to major transitions during the process recipe are reviewed below: 1. Fixture movement charges wafer: The voltage on the wafer spikes to +2Kv due to the up/down movement of the wafer. Fluid discharges wafer: A rapid discharge of the wafer occurs when the first fluid application begins. Such a rapid discharge is often responsible for charged device model (CMD) ESD damage to the devices on the wafer. Static sparks can be visually witnessed on occasion with such a discharge if the voltage levels are high enough. Some have even caused “pit mark” type damage to the wafer surface. However, even without an ESD pitting event, severe damage can occur to the device. Spinning fixture recharges wafer: As the chuck begins spinning at a relatively low RPM, the wafer begins to charge negatively (to –1Kv approximately). Fluid discharges wafer: The (negatively) charged wafer now dangerously discharges at the beginning of the second fluid application. CDM ESD damage is likely at this point in the process recipe. Spinning fluid recharges wafer: The wafer again significantly charges when the high-speed spin process begins and fluid is radially slung from the wafer. The voltage on the wafer at this point reaches +6Kv, which is typical in many high-speed wafer-drying operations. 2. 3. 4. 5. Figure 1: Typical wafer chamber design 38 Conformity november 2008
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