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Run-to-Run Control of Photolithography Processes by W. Jarrett Campbell, Ph.D., KLA-Tencor Corporation
Run-to-run (R2R) control is rapidly becoming a key process control tool in the semiconductor industry. Due to the complexity and importance of the photolithography process, overlay and critical dimension are two common process parameters that are controlled via advanced process control. As the device fabrication process is extremely sensitive to key photolithography parameters, the benefits resulting from superior process control are significant.
Traditionally there have been two distinct approaches to process control. Statistical process control (SPC) is a technique in which the process output is monitored, usually ex situ, in order to detect an out of control process. SPC attempts to assign a causality relationship to an external disturbance. A process is considered out of control if output variance can be attributed to an assignable cause1. However, many times the machine has not reached an inoperable state. The operator simply compensates for the error by manipulation of a process input variable. SPC does not define the control action necessary to return a process to an in control state. This decision is left to the operator or control engineer. SPC has seen widespread acceptance in discrete parts manufacturing where processes generally have high repeatability and natural variability. The other approach to process control is APC. Sometimes referred to as engineering process control (EPC), APC uses measurements of important process variables to incorporate a feedback loop into the control strategy. The feedback loop uses a mathematical relationship to adjust process inputs based on the measure-ments in order to keep the product on target. APC accomplishes this by transferring variability in the output variable to an input control variable2.
Recently, a combination of SPC and APC has emerged to address processing issues in the semiconductor manufacturing industry. Known as run-to-run (R2R) control, this approach combines techniques from both SPC and APC in an attempt to reduce output variability. From an SPC standpoint, R2R control extends traditional process monitoring by monitoring control actions for abnormality. APC practitioners can view R2R control as a supervisory controller that manipulates the setpoints of underlying tool controllers. The ultimate goal of R2R control is that of batch control for a lot of wafers. By analyzing the results of previous batches, the R2R controller should be able manipulate the batch recipe in order to reduce output variability. The motivation for R2R control is a lack of in situ measurements of the product quality. Typically, in semiconductor manufacturing, the goal is to control qualities such as film thickness or electrical properties that are difficult, if not impossible to measure in realtime in the process environment. Most semiconductor products must be moved from the processing chamber to a metrology tool before an accurate measurement of the control variable value can be taken. Semiconductor processing tools generally have real-time controllers, typically PID loops, for controlled variables that can be measured in real-time. The variables are typically process inputs, such as chemical flow rates, or reactor states like temperature or pressure. The manufacturing engineer must specify a recipe that contains the setpoints of these inputs and states that will produce the proper output product. The job of the supervisory, Summer 2000
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