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Novel process control strategies for 300 mm semiconductor production
Microelectronic Engineering 45 (1999) 247–256
Novel process control strategies for 300 mm semiconductor production a, a a a,b Lothar Pfitzner *, Richard Oechsner , Claus Schneider , Heiner Ryssel , c c Manfred Riemer , Mario von Podewils b
a Fraunhofer-IIS-B, Schottkystrasse 10, D-91058 Erlangen, Germany ¨ ¨ Erlangen-Nurnberg ¨ , Cauerstrasse 6, D-91058 Erlangen, Lehrstuhl f ur Elektronische Bauelemente der Universitat Germany c Thesys, Haarbergstrasse 61, D-99097 Erfurt, Germany
Abstract Semiconductor industry is one of the fastest growing businesses. Integrated circuit production process is becoming more and more challenging as the increase in complexity and in wafer size continues. Semiconductor manufacturing comprises hundreds of mostly complex processing steps, almost each of them has to be controlled within a narrow process window. Measurement steps are more or less frequently used in the production processing. Quite often it is required to take those wafers that have to be tested or measured out of the production lots, which affects logistics and, even worse, adds costs for such monitor wafers. It is of utmost importance to check new strategies in monitoring for the enhancement of yield at any possible level. Three practical examples for integrated metrology, X-ray-induced photoelectron spectroscopy and ellipsometry integrated into a cluster tool and ellipsometry integrated into a vertical furnace have been demonstrated and will be described. Considerations of cost savings by integrated metrology are made concerning increase in equipment availability, yield improvement and reduction in cleanroom space. 1999 Elsevier Science B.V. All rights reserved.
1. Introduction Semiconductor processing is driven by a steady increase in complexity of the microelectronic chips allowing higher number of gates and functions. This is achieved by a decrease of minimum dimensions and a multitude of additional number of processing steps. In contrast to these requirements, the production costs have to be reduced. Important contributions to such cost reductions arise from the use of larger wafers and from lower contamination and defect levels, combined with efforts towards better equipment utilization. Above features typically require more monitoring and control steps. However, these non-productive steps are a significant cost factor. These additional steps also lower the cycle time, they require space in the cleanroom, and they require a high number of monitoring and test wafer. All this results in the *Corresponding author. Tel.: 1 49-9131-761-110; fax: 1 49-9131-761-112. 0167-9317 / 99 / $ – see front matter PII: S0167-9317( 99 )00161-6
1999 Elsevier Science B.V. All rights reserved.
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L. Pfitzner et al. / Microelectronic Engineering 45 (1999) 247 – 256
need of novel concepts and developments concerning less expensive advanced process control and monitoring strategies.
2. Integrated metrology Monitoring has to be used for quality control in semiconductor production. Monitoring steps are performed to continuously control equipment state, process state, and wafer state. Monitoring is important during the ramp-up of a new fab, a new process, or a shrink version but also necessary for control of continuous operation. In any case, monitoring of ramp-up and production is very costly and time-consuming. Therefore, new monitoring strategies must aim especially at a reduction in costs. This can be done by integrated metrology which is able to shorten the ramp-up periods as well as to reduce the amount of monitoring wafers. In this context, integrated metrology includes in situ metrology as well as integrated in-line metrology. In the case of in situ metrology, sensors are integrated into processing equipment and can be used for real-time data acquisition during the process and direct closed-loop process control. Integrated in-line metrology means that the sensor is directly attached to the processing equipment and offers immediate response to measurements and, optionally, feedback control. In Fig. 1, three different scenarios for a fab ramp-up are shown: all curves start at a low yield assigned to the status when the equipment is inside the specifications. Curve one is the conventional learning curve. Curve two displays a faster ramp-up phase caused by a reduced cycle time due to the use of new monitoring techniques. A further increase in ramp-up can be achieved by minimizing the number of prototype runs. Especially prototypes are endangered to suffer from misprocessing. The integration of metrology may assist in such prototyping.
Fig. 1. Ramp-up time reduction for different strategies including integrated metrology.
L. Pfitzner et al. / Microelectronic Engineering 45 (1999) 247 – 256
249
In the following, three examples for integrated metrology are presented. Cluster tools offer possibilities to integrate metrology controlling the wafer state developed for in-line quality control as well as for implementation of feedback or feed forward control. In the first example described, X-ray-induced photoelectron spectroscopy (XPS) surface analysis was developed and integrated as an attached module [1]. Film thickness measurement by ellipsometry was integrated as a second example where integration was done in the form of in-line metrology. These monitoring tools allow to characterize cluster tools including cleaning steps by using XPS and the control of layer deposition
Fig. 2. Flexible MESC / CTMC compliant metrology module with XPS measurement system.
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L. Pfitzner et al. / Microelectronic Engineering 45 (1999) 247 – 256
steps based on ellipsometry. The third example for integrated metrology consists of an in situ layer thickness sensor in a vertical furnace used for sensing layer growth and run-to-run control [2,3]. The XPS analysis was implemented in a separate proprietary module attached to the cluster platform due to throughput considerations and due to the need of an ultra-high vacuum [4]. The X-ray source and the detection system based on a cylindrical mirror analyzer are integrated as a single mechanical unit which allows a single-flange mounting (Fig. 2). UHV-compatible, high-precision xy-mapping and z-leveling for focusing are provided. The module uses its own module controller. A VME bus computer with a real-time, multi-tasking operation system OS-9 was employed. The software was developed by using process automation language (PAL) in a CONTROLVision environment (Brooks, Canada). The module controller supervises the states of the module components (XPS system, vacuum system and wafer mapping stage) and communicates with the cluster controller via a TCP/ IP network (CTMC interface). The XPS system is fully controlled by the module controller. This includes the management of measurement recipes, the initialization of power supplies, the starting of measurement sequences as well as the calculation and the graphical representation of measurement results. For software and hardware integration of the attached module, the SEMI MESC / CTMC [5] standards were applied. Fig. 3 shows a comparison of measurements on the uncleaned wafer surface consisting of native oxide, the VPC-cleaned wafer surface and the regrown wafer surface. Different bonding states can be distinguished, thus allowing cleaning efficiency and surface passivation to be optimized and controlled [6]. A different approach was chosen for the integrated film thickness measurement. A commercial single-wavelength ellipsometer was mounted onto the transport chamber (Fig. 4). Since standard vacuum flanges were employed, a compact design was required. A proprietary optical light-guiding system was used to realize an arrangement of the sensor heads perpendicular to the platform. Film thickness measurements on the wafer, which reside on the robot end effector, can be performed routinely during the handling sequence with accuracy and repeatability close to the performance in stand-alone equipment (Fig. 5). The third example for integrated metrology is an in situ layer thickness sensor in a vertical furnace
Fig. 3. XPS spectra for control of cleaning process in a gate stack cluster tool.
L. Pfitzner et al. / Microelectronic Engineering 45 (1999) 247 – 256
251
Fig. 4. Ellipsometer arrangement in the cluster platform.
used for sensing layer growth and run-to-run control [2,3]. Spectroscopic ellipsometry was chosen because of its high accuracy and versatility. The mechanical integration of the spectroscopic ellipsometer was performed only with minor modifications of the mechanical furnace geometry (Fig.
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L. Pfitzner et al. / Microelectronic Engineering 45 (1999) 247 – 256
Fig. 5. Repeatability of layer thickness measurements using an ellipsometer integrated in a cluster tool. Between every measurement the wafer was placed into the load lock.
6). The two ellipsometer parts, the polarizer and the analyzer, were placed on the base plate of the furnace. Together with the base plate and the wafer carrier they form a mechanical unit which moves vertically with the boat loader. The ellipsometer light beam is guided by quartz glass prisms into the furnace tube and directed onto the wafer. The prisms are operated in total internal reflection mode. The introduced additional phase shift can be determined and subtracted from the measured phase shift. A real-time controller and a run-to-run controller integrated into the equipment controller utilize the layer thickness data measured in the in situ and in the post process mode for corrections during the process run and for the following run. Fig. 7 shows an example of a thermal dry oxidation process at 9008C which was terminated at an oxide thickness of 25 nm by end-point monitoring with the in situ spectroscopic ellipsometer. The oxide growth could not be stopped immediately by switching from oxygen to nitrogen flow. The oxide growth continued for an additional 0.35 nm. This post process
Fig. 6. Integrated ellipsometer in a vertical furnance.
L. Pfitzner et al. / Microelectronic Engineering 45 (1999) 247 – 256
253
Fig. 7. Twenty-five nm thermal oxidation process at 9008C terminated by end-point-detection monitoring.
oxidation could easily be compensated. A precalculation by process simulation turned out to be the most flexible way, allowing an easy implementation into the computer for the process recipes.
3. Cost and yield considerations The total productivity of a wafer fab depends on many factors. Equipment efficiency including throughput, availability, process performance, contamination, and defect generation is an important part of the cost-effectiveness of semiconductor production. Here, not only processing equipment, but also the metrology equipment is contributing and has to be taken into account [7]. Optimized work in progress (WIP) is another important cost factor. Additional benefits result from the reduction of cleanroom space, the reduction of wafer transport and handling operations, and the decrease in cycle times. For all the above-mentioned sources for enhancement of productivity, integrated metrology is of high importance. The benefit of integrated sensors will in many cases exceed the additional costs already within a few months. Figs. 8–10 show estimations of typical cost savings by increase in equipment availability, yield improvement and reduction in cleanroom space. The shaded areas indicate the fields and the volume of possible cost reductions. A distinction was made with respect to estimations coming from mass production and from ASIC manufacture. In addition, mini-environments as a different approach for clean wafer environment are taken into consideration separately.
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Fig. 8. Improvement of equipment availability by integrated metrology.
Fig. 9. Yield improvement by integrated metrology.
L. Pfitzner et al. / Microelectronic Engineering 45 (1999) 247 – 256
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Fig. 10. Reduction of cleanroom space by integrated metrology.
4. Conclusions Novel process control strategies will be based on increased application of integrated metrology. Such integrated metrology using in situ metrology or in-line metrology, whether based on existing equipment components or attached sensors / modules will be of high importance to reduce monitoring wafer, cleanroom space, learning turns. Especially, it will be of high importance to increase equipment availability and throughput. Reduction in costs and increase of productivity will be the most important results.
References ¨ [1] I. Kasko, R. Ochsner, C. Schneider, L. Pfitzner, H. Ryssel, in: I. Olefjord, L. Nyborg, D. Briggs (Eds.), in ECASIA 1997 Proceedings, A Novel XPS System for Integration into Advanced Semiconductor Equipment for In-Line Process Control, Vol. 1089, Wiley, UK, 1997. [2] R. Berger, C. Schneider, W. Lehnert, L. Pfitzner, H. Ryssel, in: SPIE-Proceedings on Process, Equipment, and Materials Control in Integrated Circuits Manufacturing II, Austin, Advanced Process Control System for Vertical Furnace, Vol. 2876, 1996, pp. 16–26. [3] IVPS Final Report, Project No. 01M2925 B, Erlangen, 1996.
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L. Pfitzner et al. / Microelectronic Engineering 45 (1999) 247 – 256
[4] L. Pfitzner, C. Schneider, H. Ryssel, C. Werkhoven, L. Deutschmann, O. Spindler, in: Proceedings of ISSM 96, Tokyo, The Challenge of Multi-Component, Multi-Vendor Cluster Tools, 1996, pp. 54–57. [5] SEMI Book of Standards, Mountain View, 1996. ¨ [6] B. Froeschle, F. Glowacki, A.J. Bauer, I. Kasko, R. Ochsner, C. Schneider, in: T.J. Riley, J.C. Gelpey, F. Roozeboom, S. Saito (Eds.), Mater. Res. Symp. Proc., Rapid Thermal and Integrated Processing VI, Cleaning Process Optimization in a Gate Oxide Cluster Tool Using an In-Line XPS Module, Vol. 470, 1997. [7] C. Schneider, L. Pfitzner, H. Ryssel, in: Proceedings of ISSM 97, San Francisco, Modular Metrology Tools for Productivity Enhancement in Wafer Fabs, 1997, pp. B21–B24.
Novel process control strategies for 300 mm semiconductor production a, a a a,b Lothar Pfitzner *, Richard Oechsner , Claus Schneider , Heiner Ryssel , c c Manfred Riemer , Mario von Podewils b
a Fraunhofer-IIS-B, Schottkystrasse 10, D-91058 Erlangen, Germany ¨ ¨ Erlangen-Nurnberg ¨ , Cauerstrasse 6, D-91058 Erlangen, Lehrstuhl f ur Elektronische Bauelemente der Universitat Germany c Thesys, Haarbergstrasse 61, D-99097 Erfurt, Germany
Abstract Semiconductor industry is one of the fastest growing businesses. Integrated circuit production process is becoming more and more challenging as the increase in complexity and in wafer size continues. Semiconductor manufacturing comprises hundreds of mostly complex processing steps, almost each of them has to be controlled within a narrow process window. Measurement steps are more or less frequently used in the production processing. Quite often it is required to take those wafers that have to be tested or measured out of the production lots, which affects logistics and, even worse, adds costs for such monitor wafers. It is of utmost importance to check new strategies in monitoring for the enhancement of yield at any possible level. Three practical examples for integrated metrology, X-ray-induced photoelectron spectroscopy and ellipsometry integrated into a cluster tool and ellipsometry integrated into a vertical furnace have been demonstrated and will be described. Considerations of cost savings by integrated metrology are made concerning increase in equipment availability, yield improvement and reduction in cleanroom space. 1999 Elsevier Science B.V. All rights reserved.
1. Introduction Semiconductor processing is driven by a steady increase in complexity of the microelectronic chips allowing higher number of gates and functions. This is achieved by a decrease of minimum dimensions and a multitude of additional number of processing steps. In contrast to these requirements, the production costs have to be reduced. Important contributions to such cost reductions arise from the use of larger wafers and from lower contamination and defect levels, combined with efforts towards better equipment utilization. Above features typically require more monitoring and control steps. However, these non-productive steps are a significant cost factor. These additional steps also lower the cycle time, they require space in the cleanroom, and they require a high number of monitoring and test wafer. All this results in the *Corresponding author. Tel.: 1 49-9131-761-110; fax: 1 49-9131-761-112. 0167-9317 / 99 / $ – see front matter PII: S0167-9317( 99 )00161-6
1999 Elsevier Science B.V. All rights reserved.
248
L. Pfitzner et al. / Microelectronic Engineering 45 (1999) 247 – 256
need of novel concepts and developments concerning less expensive advanced process control and monitoring strategies.
2. Integrated metrology Monitoring has to be used for quality control in semiconductor production. Monitoring steps are performed to continuously control equipment state, process state, and wafer state. Monitoring is important during the ramp-up of a new fab, a new process, or a shrink version but also necessary for control of continuous operation. In any case, monitoring of ramp-up and production is very costly and time-consuming. Therefore, new monitoring strategies must aim especially at a reduction in costs. This can be done by integrated metrology which is able to shorten the ramp-up periods as well as to reduce the amount of monitoring wafers. In this context, integrated metrology includes in situ metrology as well as integrated in-line metrology. In the case of in situ metrology, sensors are integrated into processing equipment and can be used for real-time data acquisition during the process and direct closed-loop process control. Integrated in-line metrology means that the sensor is directly attached to the processing equipment and offers immediate response to measurements and, optionally, feedback control. In Fig. 1, three different scenarios for a fab ramp-up are shown: all curves start at a low yield assigned to the status when the equipment is inside the specifications. Curve one is the conventional learning curve. Curve two displays a faster ramp-up phase caused by a reduced cycle time due to the use of new monitoring techniques. A further increase in ramp-up can be achieved by minimizing the number of prototype runs. Especially prototypes are endangered to suffer from misprocessing. The integration of metrology may assist in such prototyping.
Fig. 1. Ramp-up time reduction for different strategies including integrated metrology.
L. Pfitzner et al. / Microelectronic Engineering 45 (1999) 247 – 256
249
In the following, three examples for integrated metrology are presented. Cluster tools offer possibilities to integrate metrology controlling the wafer state developed for in-line quality control as well as for implementation of feedback or feed forward control. In the first example described, X-ray-induced photoelectron spectroscopy (XPS) surface analysis was developed and integrated as an attached module [1]. Film thickness measurement by ellipsometry was integrated as a second example where integration was done in the form of in-line metrology. These monitoring tools allow to characterize cluster tools including cleaning steps by using XPS and the control of layer deposition
Fig. 2. Flexible MESC / CTMC compliant metrology module with XPS measurement system.
250
L. Pfitzner et al. / Microelectronic Engineering 45 (1999) 247 – 256
steps based on ellipsometry. The third example for integrated metrology consists of an in situ layer thickness sensor in a vertical furnace used for sensing layer growth and run-to-run control [2,3]. The XPS analysis was implemented in a separate proprietary module attached to the cluster platform due to throughput considerations and due to the need of an ultra-high vacuum [4]. The X-ray source and the detection system based on a cylindrical mirror analyzer are integrated as a single mechanical unit which allows a single-flange mounting (Fig. 2). UHV-compatible, high-precision xy-mapping and z-leveling for focusing are provided. The module uses its own module controller. A VME bus computer with a real-time, multi-tasking operation system OS-9 was employed. The software was developed by using process automation language (PAL) in a CONTROLVision environment (Brooks, Canada). The module controller supervises the states of the module components (XPS system, vacuum system and wafer mapping stage) and communicates with the cluster controller via a TCP/ IP network (CTMC interface). The XPS system is fully controlled by the module controller. This includes the management of measurement recipes, the initialization of power supplies, the starting of measurement sequences as well as the calculation and the graphical representation of measurement results. For software and hardware integration of the attached module, the SEMI MESC / CTMC [5] standards were applied. Fig. 3 shows a comparison of measurements on the uncleaned wafer surface consisting of native oxide, the VPC-cleaned wafer surface and the regrown wafer surface. Different bonding states can be distinguished, thus allowing cleaning efficiency and surface passivation to be optimized and controlled [6]. A different approach was chosen for the integrated film thickness measurement. A commercial single-wavelength ellipsometer was mounted onto the transport chamber (Fig. 4). Since standard vacuum flanges were employed, a compact design was required. A proprietary optical light-guiding system was used to realize an arrangement of the sensor heads perpendicular to the platform. Film thickness measurements on the wafer, which reside on the robot end effector, can be performed routinely during the handling sequence with accuracy and repeatability close to the performance in stand-alone equipment (Fig. 5). The third example for integrated metrology is an in situ layer thickness sensor in a vertical furnace
Fig. 3. XPS spectra for control of cleaning process in a gate stack cluster tool.
L. Pfitzner et al. / Microelectronic Engineering 45 (1999) 247 – 256
251
Fig. 4. Ellipsometer arrangement in the cluster platform.
used for sensing layer growth and run-to-run control [2,3]. Spectroscopic ellipsometry was chosen because of its high accuracy and versatility. The mechanical integration of the spectroscopic ellipsometer was performed only with minor modifications of the mechanical furnace geometry (Fig.
252
L. Pfitzner et al. / Microelectronic Engineering 45 (1999) 247 – 256
Fig. 5. Repeatability of layer thickness measurements using an ellipsometer integrated in a cluster tool. Between every measurement the wafer was placed into the load lock.
6). The two ellipsometer parts, the polarizer and the analyzer, were placed on the base plate of the furnace. Together with the base plate and the wafer carrier they form a mechanical unit which moves vertically with the boat loader. The ellipsometer light beam is guided by quartz glass prisms into the furnace tube and directed onto the wafer. The prisms are operated in total internal reflection mode. The introduced additional phase shift can be determined and subtracted from the measured phase shift. A real-time controller and a run-to-run controller integrated into the equipment controller utilize the layer thickness data measured in the in situ and in the post process mode for corrections during the process run and for the following run. Fig. 7 shows an example of a thermal dry oxidation process at 9008C which was terminated at an oxide thickness of 25 nm by end-point monitoring with the in situ spectroscopic ellipsometer. The oxide growth could not be stopped immediately by switching from oxygen to nitrogen flow. The oxide growth continued for an additional 0.35 nm. This post process
Fig. 6. Integrated ellipsometer in a vertical furnance.
L. Pfitzner et al. / Microelectronic Engineering 45 (1999) 247 – 256
253
Fig. 7. Twenty-five nm thermal oxidation process at 9008C terminated by end-point-detection monitoring.
oxidation could easily be compensated. A precalculation by process simulation turned out to be the most flexible way, allowing an easy implementation into the computer for the process recipes.
3. Cost and yield considerations The total productivity of a wafer fab depends on many factors. Equipment efficiency including throughput, availability, process performance, contamination, and defect generation is an important part of the cost-effectiveness of semiconductor production. Here, not only processing equipment, but also the metrology equipment is contributing and has to be taken into account [7]. Optimized work in progress (WIP) is another important cost factor. Additional benefits result from the reduction of cleanroom space, the reduction of wafer transport and handling operations, and the decrease in cycle times. For all the above-mentioned sources for enhancement of productivity, integrated metrology is of high importance. The benefit of integrated sensors will in many cases exceed the additional costs already within a few months. Figs. 8–10 show estimations of typical cost savings by increase in equipment availability, yield improvement and reduction in cleanroom space. The shaded areas indicate the fields and the volume of possible cost reductions. A distinction was made with respect to estimations coming from mass production and from ASIC manufacture. In addition, mini-environments as a different approach for clean wafer environment are taken into consideration separately.
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Fig. 8. Improvement of equipment availability by integrated metrology.
Fig. 9. Yield improvement by integrated metrology.
L. Pfitzner et al. / Microelectronic Engineering 45 (1999) 247 – 256
255
Fig. 10. Reduction of cleanroom space by integrated metrology.
4. Conclusions Novel process control strategies will be based on increased application of integrated metrology. Such integrated metrology using in situ metrology or in-line metrology, whether based on existing equipment components or attached sensors / modules will be of high importance to reduce monitoring wafer, cleanroom space, learning turns. Especially, it will be of high importance to increase equipment availability and throughput. Reduction in costs and increase of productivity will be the most important results.
References ¨ [1] I. Kasko, R. Ochsner, C. Schneider, L. Pfitzner, H. Ryssel, in: I. Olefjord, L. Nyborg, D. Briggs (Eds.), in ECASIA 1997 Proceedings, A Novel XPS System for Integration into Advanced Semiconductor Equipment for In-Line Process Control, Vol. 1089, Wiley, UK, 1997. [2] R. Berger, C. Schneider, W. Lehnert, L. Pfitzner, H. Ryssel, in: SPIE-Proceedings on Process, Equipment, and Materials Control in Integrated Circuits Manufacturing II, Austin, Advanced Process Control System for Vertical Furnace, Vol. 2876, 1996, pp. 16–26. [3] IVPS Final Report, Project No. 01M2925 B, Erlangen, 1996.
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L. Pfitzner et al. / Microelectronic Engineering 45 (1999) 247 – 256
[4] L. Pfitzner, C. Schneider, H. Ryssel, C. Werkhoven, L. Deutschmann, O. Spindler, in: Proceedings of ISSM 96, Tokyo, The Challenge of Multi-Component, Multi-Vendor Cluster Tools, 1996, pp. 54–57. [5] SEMI Book of Standards, Mountain View, 1996. ¨ [6] B. Froeschle, F. Glowacki, A.J. Bauer, I. Kasko, R. Ochsner, C. Schneider, in: T.J. Riley, J.C. Gelpey, F. Roozeboom, S. Saito (Eds.), Mater. Res. Symp. Proc., Rapid Thermal and Integrated Processing VI, Cleaning Process Optimization in a Gate Oxide Cluster Tool Using an In-Line XPS Module, Vol. 470, 1997. [7] C. Schneider, L. Pfitzner, H. Ryssel, in: Proceedings of ISSM 97, San Francisco, Modular Metrology Tools for Productivity Enhancement in Wafer Fabs, 1997, pp. B21–B24.