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Traditionally there have been two distinctapproaches to process control. Statisticalprocess control (SPC) is a technique in whichthe process output is monitored, usually exsitu, in order to detect an out of controlprocess. SPC attempts to assign a causalityrelationship to an external disturbance. Aprocess is considered out of control if outputvariance can be attributed to an assignablecause1. However, many times the machinehas not reached an inoperable state. Theoperator simply compensates for the errorby manipulation of a process input variable.SPC does not define the control action nec-essary to return a process to an in controlstate. This decision is left to the operator orcontrol engineer. SPC has seen widespreadacceptance in discrete parts manufacturingwhere processes generally have highrepeatability and natural variability.

The other approach to process control isAPC. Sometimes referred to as engineeringprocess control (EPC), APC uses measure-ments of important process variables toincorporate a feedback loop into the controlstrategy. The feedback loop uses a mathe-matical relationship to adjust process inputsbased on the measure-ments in order to keepthe product on target. APC accomplishesthis by transferring variability in the outputvariable to an input control variable2.

LithographyS P E C I A L F O C U S

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 parametersthat are controlled via advanced process control. As the device fabrication process is extremely sensitive to key photolithographyparameters, the benefits resulting from superior process control are significant.

Recently, a combination of SPC and APC has emergedto address processing issues in the semiconductor man-ufacturing industry. Known as run-to-run (R2R) con-trol, this approach combines techniques from both SPCand APC in an attempt to reduce output variability.From an SPC standpoint, R2R control extends tradi-tional process monitoring by monitoring controlactions for abnormality. APC practitioners can viewR2R control as a supervisory controller that manipu-lates the setpoints of underlying tool controllers. Theultimate goal of R2R control is that of batch controlfor a lot of wafers. By analyzing the results of previousbatches, the R2R controller should be able manipulatethe batch recipe in order to reduce output variability.

The motivation for R2R control is a lack of in situmeasurements of the product quality. Typically, insemiconductor manufacturing, the goal is to controlqualities such as film thickness or electrical propertiesthat are difficult, if not impossible to measure in real-time in the process environment. Most semiconductorproducts must be moved from the processing chamberto a metrology tool before an accurate measurement ofthe control variable value can be taken. Semiconductorprocessing tools generally have real-time controllers,typically PID loops, for controlled variables that can bemeasured in real-time. The variables are typicallyprocess inputs, such as chemical flow rates, or reactorstates like temperature or pressure. The manufacturingengineer must specify a recipe that contains the set-points of these inputs and states that will produce theproper output product. The job of the supervisory,

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R2R controller is to adjust these recipes to reduce vari-ability in the output product.

R2R control is further necessitated by the non-station-ary nature of most semiconductor processes. While SPCis designed for stationary processes where output varia-tions are independent, R2R control is able to compen-sate for drifting processes where output variations arecorrelated. The variation correlation is typically causedby changes in the processing environment. For example,in a deposition process, the reactor walls may becomefouled by deposition as many products are processed.This slow drift in the reactor chamber state requiressmall changes to the batch recipe in order to ensure thatthe product outputs remain on target. Eventually, thereactor chamber will be cleaned to remove the walldeposition, causing a step disturbance in the process.Just as the R2R controller compensates for the driftingprocess, it will also compensate for the step disturbance toreturn the process to target after an environment change.

Many manufacturers have concentrated their efforts inR2R control on the photolithography process. Becauselithographic processes are perhaps the most criticaldevice fabrication steps, R2R control has the potentialto significantly impact the quality and maufacturabili-ty of semiconductor devices. Using advanced APC soft-ware such as KLA-Tencor’s Catalyst, several large semi-conductor manufacturers have applied R2R control totheir manufacturing processes in order to minimizevariations in both critical dimension (CD) and overlayregistration.

Overlay controlOne type of R2R control often employed in device fab-rication is overlay control. The purpose of overlay R2Rcontrol is to minimize the errors in registrationbetween subsequent masking layers. There are manytypes of overlay errors that may occur during manufac-turing. Some of these errors include translation, rota-tion, magnification, and shear. Examples of these over-lay errors on a wafer-scale are shown in Figure 1.

A typical means of controlling overlay errors is to setupa feedback loop between the overlay metrology tooland the masking tool via an APC software system. TheAPC system continually monitors overlay errors at eachmasking operation to detect slow drifts or suddenshifts. When a disturbance in overlay is detected by theAPC system, the software automatically updates thestage and reticle offset parameters on the masking tool

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

When overlay R2R control is implemented, manymanufacturing benefits result. Semiconductor manu-acturers have reported increased Cpk, reduced rework,reduced send-ahead wafers, and decreased engineeringtime devoted to stepper matching. Advanced MicroDevices’ Fab 25 has reported that their implementationof overlay R2R control has decreased overlay-specificphotolithography rework by over 50 percent and threesigma translation errors were reduced by greater than20 percent. In addition, AMD has been able to elimi-nate test-wafer and send-ahead qual procedures foroverlay calibration because these procedures are nowhandled exclusively by the APC software.3

Grid Translation Grid Magnification

Grid Rotation Grid Shear/Orthogonally

Figure 1. Examples of overlay errors.

Figure 2. Overlay feedback system.

S P E C I A L F O C U S

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CD controlAnother key process parameter in photolithography isCD. Just as overlay can be controlled using a feedbacksystem, CD variations can be minimized using R2Rcontrol. However, CD control is not as easy as using afeedback system between the stepper and the CDmetrology tool. This is because there is an etch bias,shown in Figure 3, that results during the post-pho-tolithography etching process.

Instead, a combined feedforward-feedback control sys-tem must be built around the etch process to ensurethat the final inspection (FI) CD is at the appropriateprocess target.

First, the CD is measured after the developmentinspection (DI). This value is used in a feedforwardmanner to allow customization of the etch processrecipe on a lot-by-lot basis. In other words if variabilityin the DICD value for a lot is measured, it can bedirectly compensated for by manipulation of that lot’setch recipe.

In addition to feedforward control, feedback control isperformed by monitoring the FICD resulting from theetch process. The APC system can detect drifts or shiftsin etch bias caused by disturbances to the etch chamber.The feedback system can then change the etch recipeappropriately to eliminate any systematic disturbancesin the etch bias. Typically, the feedforward and feedbackinformation is combined using a mathematical modelof the etch process to determine an appropriate etchtime for each lot.

One difficulty of this CD control approach is thatdrifts in the upstream photolithography process can becompensated for after the fact, but cannot be correcteddirectly at their source. Imagine a scenario where stepperdrift has caused the DICD values after photolithographyto drift so far that even the feedforward control systemcannot properly compensate for incoming DICD varia-tion. An example would be a case where the etch timerequired is outside the allowed process window. Inorder to prevent such difficulties, a second feedbackloop, often called a cascade loop, can be implementedbetween the etcher and photolithography tools.

The purpose of the cascade loop is to ensure that etchtimes remain centered in the allowable process window.This is done by manipulating the DICD target of thephotolithography process. For example, if the etch toolshave drifted such that long etch times are required toachieve the desired FICD target, the photolithographyrecipe can be adjusted in order to target a new DICDvalue that will not require as much etch to achieve thesame DICD target. This feedback system is uniquefrom those previously discussed because the monitoredoutput of the control loop is actually the recipe settingsused in the etch process. The manipulated variable inthis control loop is the process target in the photolitho-graphy process.

Once a cascade loop is in place to set the DICD targets,it may also be desirable to add a third feedback controlloop around the photolithography process to ensurevariations in DICD are minimized. This third controlloop is a simple feedback loop between the CD metrol-ogy tool and the stepper. Although the feedforwardcontroller at the etch process can compensate for varia-tions in DICD, the etch controller will perform betterif the variations in incoming DICD are localized to asmall operating region. This allows more precise mod-eling of the etch process and results in better control ofFICD. Although it has been shown that several recipesettings including post-exposure bake time and devel-op time can affect changes in CD4, the most popularrecipe setting used to control DICD is the exposuredose. Dose is often chosen because the photolithogra-phy process tends to have a strong, linear relationshipbetween changes in exposure dose and changes inDICD.

Once these three control loops are put into place, acomprehensive control system is now available to mini-mize variations in CD across the patterning process.Figure 4 represents a schematic of such a control system.

Previous Layers

DI CD

FI CD

Etch Bias

S P E C I A L F O C U S

Figure 3. CD bias induced by etch process.

SummaryR2R control is rapidly becoming a key process controltool in the semiconductor industry. Because of thecomplexity and importance of the photolithographyprocess, overlay and CD are two common process para-meters that are controlled via R2R control. AdvancedProcess Control (APC) software, such as KLA-Tencor’sCatalyst, provides a means of integrating R2R controlsolutions into today’s device fabrication facilities. Byusing such software, many of the top semiconductormanufacturers have been able to reduce the effort, cost,and time required in deploying APC in their produc-tion environments.

References1. Douglas C. Montgomery. Introduction to Statistical Qual-

ity Control. John Wiley & Sons, 2nd edition, 1991.2. Douglas C. Montgomery, J. Bert Keats, George C. Runger,

and William S. Messina. Integrating Statistical PprocessControl and Engineering Process Control. Journal of Qual-ity Technology, 26(2), April 1994.

3. Christopher A. Bode. Run-to-Run Control of PhotolithographyOverlay. Proceedings of SEMATECH AEC/APC Sympo-sium XI. October 1999.

4. Thomas F. Edgar, Stephanie W. Butler, W. Jarrett Camp-bell, Carlos Pfeiffer, Chris Bode, Sung Bo Hwang, andK.S. Balakrishnan. Automatic Control in Microelectron-ics Manufacturing: Practices, Challenges, and Possibilities.Automatica. Accepted for Publication.

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

6. Terry Caudell. APC: An Enabling Technology in the Sub-quarter Micron Era. Proceedings of AEC/APC WorkshopEurope. March 2000.

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Because CDs are very closely tied to semiconductordevice performance, it is easy to imagine that the supe-rior process control achieved through implementationof R2R control can have significant impact on themanufacturing process. IC manufacturers have validatedthat implementation of R2R control of CD can resultin tremendous financial and manufacturing benefits. Inparticular, Advanced Micro Devices has reported thatR2R control of CD has lead to a greater than 8 percentincrease in overall device speed. This boost in perfor-mance allowed AMD to realize approximately $40 millionin increased revenue per year. On the manufacturing side,AMD also reported that photolithography rework forCD variation was reduced by over 90 percent and thatone sigma variation in FICD was reduced by 45 percent5.

Integrating APC into the fabOnce the R2R control systems are developed, theymust be implemented in software and integrated intothe manufacturing facilities. This integration effort isthe single largest roadblock preventing rapid deploy-ment of R2R control solutions throughout the semi-conductor industry. Advanced APC software, likeKLA-Tencor’s Catalyst*, eases the integration effort byproviding a software framework in which APC applica-tions can be developed and implemented into semicon-ductor manufacturing systems.

The benefits of applying R2R control are significant.One semiconductor manufacturer’s experience withR2R control is summarized in Table 1.

S P E C I A L F O C U S

Table 1: Results from R2R Control Production Implementations6.

CD Overlay

Rework reduced 90% Rework reduced 50%

Std. dev. reduced 45% Std. dev. reduced 20%

Speed increased 8% Eliminated test quals

Revenue increased Eliminated need for$40 million per year manual tool matching

Figure 4: Comprehensive CD Control Strategy

* Note: Catalyst is the result of a three year, ten milliondollar NIST-sponsored joint research project betweenKLA-Tencor’s Control Solutions division, AdvancedMicro Devices, and Honeywell. The research projectestablished SEMATECH and SEMI standards for APCsoftware. Catalyst is the first commercial APC softwareto be based on these standards and it is SEMATECHCIM Framework compliant.