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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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R2R controller is to adjust these recipes to reduce variability in the output product.

R2R control is further necessitated by the non-stationary nature of most semiconductor processes. While SPC is designed for stationary processes where output variations are independent, R2R control is able to compensate for drifting processes where output variations are correlated. The variation correlation is typically caused by changes in the processing environment. For example, in a deposition process, the reactor walls may become fouled by deposition as many products are processed.

This slow drift in the reactor chamber state requires small changes to the batch recipe in order to ensure that the product outputs remain on target. Eventually, the reactor chamber will be cleaned to remove the wall deposition, causing a step disturbance in the process.

Just as the R2R controller compensates for the drifting process, it will also compensate for the step disturbance to return the process to target after an environment change.

Many manufacturers have concentrated their efforts in R2R control on the photolithography process. Because lithographic processes are perhaps the most critical device fabrication steps, R2R control has the potential to significantly impact the quality and maufacturability of semiconductor devices. Using advanced APC software such as KLA-Tencor’s Catalyst, several large semiconductor manufacturers have applied R2R control to their manufacturing processes in order to minimize variations in both critical dimension (CD) and overlay registration.

Overlay control

One type of R2R control often employed in device fabrication is overlay control. The purpose of overlay R2R control is to minimize the errors in registration between subsequent masking layers. There are many types of overlay errors that may occur during manufacturing. Some of these errors include translation, rotation, magnification, and shear. Examples of these overlay errors on a wafer-scale are shown in Figure 1.

A typical means of controlling overlay errors is to setup a feedback loop between the overlay metrology tool and the masking tool via an APC software system. The APC system continually monitors overlay errors at each masking operation to detect slow drifts or sudden shifts. When a disturbance in overlay is detected by the APC system, the software automatically updates the stage and reticle offset parameters on the masking tool 66

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Grid Translation

Grid Magnification

Grid Rotation

Grid Shear/Orthogonally

Figure 1. Examples of overlay errors.

in order to eliminate the overlay errors. Figure 2 illustrates a typical feedback system for overlay control.

When overlay R2R control is implemented, many manufacturing benefits result. Semiconductor manuacturers have reported increased Cpk, reduced rework, reduced send-ahead wafers, and decreased engineering time devoted to stepper matching. Advanced Micro Devices’ Fab 25 has reported that their implementation of overlay R2R control has decreased overlay-specific photolithography rework by over 50 percent and three sigma translation errors were reduced by greater than 20 percent. In addition, AMD has been able to eliminate test-wafer and send-ahead qual procedures for overlay calibration because these procedures are now handled exclusively by the APC software.3

Figure 2. Overlay feedback system.

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CD control

Another key process parameter in photolithography is CD. Just as overlay can be controlled using a feedback system, CD variations can be minimized using R2R control. However, CD control is not as easy as using a feedback system between the stepper and the CD metrology tool. This is because there is an etch bias, shown in Figure 3, that results during the post-photolithography etching process.

Etch Bias FI CD DI CD

Previous Layers Figure 3. CD bias induced by etch process.

Instead, a combined feedforward-feedback control system must be built around the etch process to ensure that the final inspection (FI) CD is at the appropriate process target.

First, the CD is measured after the development inspection (DI). This value is used in a feedforward manner to allow customization of the etch process recipe on a lot-by-lot basis. In other words if variability in the DICD value for a lot is measured, it can be directly compensated for by manipulation of that lot’s etch recipe.

In addition to feedforward control, feedback control is performed by monitoring the FICD resulting from the etch process. The APC system can detect drifts or shifts in etch bias caused by disturbances to the etch chamber.

The feedback system can then change the etch recipe appropriately to eliminate any systematic disturbances in the etch bias. Typically, the feedforward and feedback information is combined using a mathematical model of the etch process to determine an appropriate etch time for each lot.

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One difficulty of this CD control approach is that drifts in the upstream photolithography process can be compensated for after the fact, but cannot be corrected directly at their source. Imagine a scenario where stepper drift has caused the DICD values after photolithography to drift so far that even the feedforward control system cannot properly compensate for incoming DICD variation. An example would be a case where the etch time required is outside the allowed process window. In order to prevent such difficulties, a second feedback loop, often called a cascade loop, can be implemented between the etcher and photolithography tools.

The purpose of the cascade loop is to ensure that etch times remain centered in the allowable process window.

This is done by manipulating the DICD target of the photolithography process. For example, if the etch tools have drifted such that long etch times are required to achieve the desired FICD target, the photolithography recipe can be adjusted in order to target a new DICD value that will not require as much etch to achieve the same DICD target. This feedback system is unique from those previously discussed because the monitored output of the control loop is actually the recipe settings used in the etch process. The manipulated variable in this control loop is the process target in the photolithography process.

Once a cascade loop is in place to set the DICD targets, it may also be desirable to add a third feedback control loop around the photolithography process to ensure variations in DICD are minimized. This third control loop is a simple feedback loop between the CD metrology tool and the stepper. Although the feedforward controller at the etch process can compensate for variations in DICD, the etch controller will perform better if the variations in incoming DICD are localized to a small operating region. This allows more precise modeling of the etch process and results in better control of FICD. Although it has been shown that several recipe settings including post-exposure bake time and develop time can affect changes in CD4, the most popular recipe setting used to control DICD is the exposure dose. Dose is often chosen because the photolithography process tends to have a strong, linear relationship between changes in exposure dose and changes in DICD. Once these three control loops are put into place, a comprehensive control system is now available to minimize variations in CD across the patterning process.

Figure 4 represents a schematic of such a control system.

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Summary

Figure 4: Comprehensive CD Control Strategy

Because CDs are very closely tied to semiconductor device performance, it is easy to imagine that the superior process control achieved through implementation of R2R control can have significant impact on the manufacturing process. IC manufacturers have validated that implementation of R2R control of CD can result in tremendous financial and manufacturing benefits. In particular, Advanced Micro Devices has reported that R2R control of CD has lead to a greater than 8 percent increase in overall device speed. This boost in performance allowed AMD to realize approximately $40 million in increased revenue per year. On the manufacturing side, AMD also reported that photolithography rework for CD variation was reduced by over 90 percent and that one sigma variation in FICD was reduced by 45 percent5.

Once the R2R control systems are developed, they must be implemented in software and integrated into the manufacturing facilities. This integration effort is the single largest roadblock preventing rapid deployment of R2R control solutions throughout the semiconductor industry. Advanced APC software, like KLA-Tencor’s Catalyst*, eases the integration effort by providing a software framework in which APC applications can be developed and implemented into semiconductor manufacturing systems.

The benefits of applying R2R control are significant.

One semiconductor manufacturer’s experience with R2R control is summarized in Table 1.

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CD

Overlay

Rework reduced 90%

Rework reduced 50%

Std. dev. reduced 45%

Std. dev. reduced 20%

Speed increased 8%

Eliminated test quals

Revenue increased $40 million per year

Eliminated need for manual tool matching

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* Note:

Catalyst is the result of a three year, ten million dollar NIST-sponsored joint research project between KLA-Tencor’s Control Solutions division, Advanced Micro Devices, and Honeywell. The research project established SEMATECH and SEMI standards for APC software. Catalyst is the first commercial APC software to be based on these standards and it is SEMATECH CIM Framework compliant.

References

Integrating APC into the fab

Table 1: Results from R2R Control Production Implementations

R2R control is rapidly becoming a key process control tool in the semiconductor industry. Because of the complexity and importance of the photolithography process, overlay and CD are two common process parameters that are controlled via R2R control. Advanced Process Control (APC) software, such as KLA-Tencor’s Catalyst, provides a means of integrating R2R control solutions into today’s device fabrication facilities. By using such software, many of the top semiconductor manufacturers have been able to reduce the effort, cost, and time required in deploying APC in their production environments.

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1. Douglas C. Montgomery. Introduction to Statistical Quality Control. John Wiley & Sons, 2nd edition, 1991.

2. Douglas C. Montgomery, J. Bert Keats, George C. Runger, and William S. Messina. Integrating Statistical Pprocess Control and Engineering Process Control. Journal of Quality Technology, 26(2), April 1994.

3. Christopher A. Bode. Run-to-Run Control of Photolithography Overlay. Proceedings of SEMATECH AEC/APC Symposium XI. October 1999.

4. Thomas F. Edgar, Stephanie W. Butler, W. Jarrett Campbell, Carlos Pfeiffer, Chris Bode, Sung Bo Hwang, and K.S. Balakrishnan. Automatic Control in Microelectronics Manufacturing: Practices, Challenges, and Possibilities.

Automatica. Accepted for Publication.

5. Anthony J. Toprac and W. Jarrett Campbell. Run-to-Run Control Using the APC Framework. Proceedings of SEMATECH AEC/APC Symposium X, October 1998.

6. Terry Caudell. APC: An Enabling Technology in the Subquarter Micron Era. Proceedings of AEC/APC Workshop Europe. March 2000.