Analysis of a 200/300mm vertical furnace with integrated metrology
04/01/2001
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SPECIAL SECTION: EUROPEAN TECHNOLOGY
Metrology Series - Part III
Tanja Claasen-Vujcic, Albert Hasper, ASMI nv., Bilthoven, The Netherlands
Michael Abraham, NanoPhotonics AG, Mainz, Germany
overview
Recently, very compact metrology units with excellent performance have become available, enabling integration of metrology equipment and processing equipment, such as furnaces. Factory simulations are used to show the relative performance of the furnace area in a fab with stand-alone metrology and with integrated metrology. Cycle time and cost/wafer can be reduced using integrated metrology, especially for 300mm fabs.
The constant demand to improve the overall equipment effectiveness (OEE) of semiconductor equipment has become especially imperative for 300mm production lines due to high wafer and equipment costs.
Optimizing individual equipment types does improve the OEE, but much more can be achieved with an integrated approach where interactions between different equipment types are also optimized.
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An important interaction exists between processing tools, such as furnaces, and metrology equipment. After the layers are deposited in furnaces, they are usually measured in a stand-alone metrology unit (e.g., an ellipsometer and a particle counter). After processing in the furnace ends, it usually takes more than half an hour to make metrology results available. If this feedback comes more quickly, the cycle time be can reduced and queue time issues eliminated. Also, critical interventions can be faster and equipment efficiency increased. The latter is particularly important for furnace recipes for which the next run is not started before the metrology results are available. In this case, integrated metrology is especially important for furnaces with the dual-boat feature, as in most 300mm furnaces, since it enables true use of that feature.
Such a challenging OEE improvement can be achieved if metrology units are integrated into furnaces. The process performance can then be evaluated directly after the processing. Recently, very compact metrology units with excellent price/performance ratios have become available. This has been the first condition for successful integration.
The relevance of integrated metrology units has been well recognized for processing equipment in general [1]. This article focuses on the furnace area in detail, basing the analysis on the A412, the 200/300mm vertical furnace from ASM with an on-board integrated metrology module.
Integration of processing and metrology
The A412 from ASM (see above and Fig. 1) is a four-boat, dual-reactor vertical batch furnace for processing of 200mm and/or 300mm wafers. The furnace can be equipped with a metrology unit comprising an ellipsometer (Ellipson from NanoPhotonics) with a notch aligner and a wafer ID reader from RECIF S.A.,
 Figure 1. The A412 from ASM: a 200/300mm four-boat dual-reactor vertical furnace with integrated metrology on board. |
Aussonne, France, and a separate metrology unit with a particle counter (Reflex 300 from NanoPhotonics) [2]. For the successful integration of metrology units into furnaces, these units must comply with a number of general specifications, which are listed in Table 1.
Important requirements concern footprint and throughput of the processing tool. Neither of them must be affected by the insertion of the integrated metrology. In the A412, the metrology units are inserted above the FOUP door openers (Fig. 2). In this way, the units are easily accessible by the wafer-handling robot, and the footprint of the system is preserved. Due to the dual-boat operation of the furnace, (i.e., parallelism of processing and loading/unloading), integrated metrology does not affect the throughput of the system. Finally, since there are two reactors in one furnace, they share the same metrology units, thus reducing the investment per reactor by 50%.
Ellipson is the only currently available ultra-compact ellipsometer on the market. Other solutions for film thickness measurements are based on standard UV-Vis reflectometry and cannot be used for measurements of film thickness below 50nm [3].
Table 2 depicts the roadmap for integration of the metrology equipment in the furnace. Currently, the described metrology units are successfully integrated. The next important milestone is development of compact metrology units for measurements on patterned wafers. This will result in a significant reduction of test wafer consumption and will thus have a major impact on cost of ownership. Currently, test wafers in 300mm fabs can contribute more than 30% to the overall cost of ownership of the furnace area.
Experimental results
The benefits of integrated metrology in 300mm fabs are to be proven in the coming years, when the first 300mm fabs start volume production. The performance of the Ellipson metrology unit has already been tested at various users' sites.
In this paper, we refer to measurements done at Infineon in Dresden. The Ellipson was coupled to the load port of a RECIF SPP 300 sorter. The set-up of the metrology box housing the Ellipson was developed by RECIF in collaboration with NanoPhotonics and Infineon [4]. Three types of measurements were done on thin oxide films. Reproducibility, long-term stability, and sensitivity of the unit to vibrations have been tested (see Fig. 3 on p. S8) [4].
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The latter is important, since the unit, when integrated in the processing tool, will be in the vicinity of the moving parts, such as a cassette-handling robot. The tests proved that Ellipson can be used in mechanically excited environments; the same mean value of 7.2nm was measured (across 30 measurements) in both static and excited situations, and 1s did not exceed 0.011nm. To determine long-term stability, over 4000 static measurements were done on a 1.120nm film. Stability proved to be excellent with 1s of 0.007nm and a range value of 0.05nm. This positive result was expected due to the compact, low mass design. For the same reason, sensitivity to vibrations is negligible.
Simulation objectives and tools
The real performance of a system can be analyzed well only in its production environment, in a real volume production fab. Such data for a 300mm fab are currently not available, and they cannot be expected before 2002. Therefore, for the purpose of evaluation of the potential benefits of integrated metrology, we have used logistic simulations. In these simulations, virtual machines or complete factories are "built" and their behavior analyzed under defined conditions [5].
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The ASM modeling and simulation group has developed a flexible modeling and simulation tool for this purpose. Models are built out of elements (processes) and interactions, which are defined in a very general sense. Examples of processes are processing tools, cassette robots, operators, and control units. Examples of interactions are material flow and information flow.
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Figure 3. Measurements done at Infineon SC 300 in Dresden demonstrate the repeatability of the thickness measurements on the NanoPhotonics Ellipson, with a) static results and b) results when the system was mechanically excited by vibrations with an amplitude of 300?m and a frequency of 25Hz [4].
The top levels of a fab model are shown in Fig. 4 on p. S10. The figure starts with a window showing a graphical representation of processors, the CAM host system, and the actual fab, as well as their interaction paths (small arrows).
When the fab processor is expanded, more details of the fab model are shown. The fab is considered to consist of three areas and a transport system between those areas. Material enters and leaves the fab in the first area (BayM). The furnace area is Bay1. All other operations in the fab, (lithography, ion implantation, etc.), take place in the third area (BayR).
The third window in Fig. 4 shows the furnace area, Bay1, in more detail. The furnace area consists of a bay stocker where the material enters and leaves the area, a collection of operators, the bay server (an extension of the CAM host), and a collection of equipment (cleaners, furnaces, and metrology). Equipment and operator activities in the furnace area are included in detail. Activities in other areas of the fab are included at a higher abstraction level.
Simulation input
Many cases can be analyzed. We will focus, however, on integrated and stand-alone metrology for both 200mm and 300mm production in a foundry type of fab with 5000 wafer starts/week, three different products, and 5% hot lots. Table 3 summarizes fab characteristics, as well as some cost assumptions. Notice that a certain $/wafer value is assigned to one hour of cycle time reduction [6].
 Figure 4. Top levels of the fab logistic model. The model has been built in the Smalltalk programming environment. |
The number of tubes needed has been determined with the help of logistic simulations. It is assumed that particle inspection is done on one product wafer and thickness measurement on three test wafers/batch of 100 wafers. It is also important to determine the scheduling rules used. We will analyze both modes — "wait for test results" and "do not wait for test results" — with respect to the activity before starting a new batch in the furnace.
Moreover, it is assumed that cassettes remain in the area stocker until results are available and all lots are returned to the stocker. Details of activities in other areas of the fab are included at a higher abstraction level.
Simulation results
The following cost-related items have been analyzed: cycle time in the furnace area, scrap, yield, throughput, usage of test wafers, and capital investment.
Table 4 summarizes the results, and the rest of this article
provides background for the numbers shown in the table.
Cycle time. By using integrated metrology, one expects shorter cycle time in the furnace area, due to the reduction/elimination of the following activities per process step:
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The arrows indicate that, depending on the number of measurements, the activities can be repeated.
Simulation results showed that the total cycle time in the furnace area for all three products was reduced from 183 hrs with stand-alone metrology to 171 hrs with integrated metrology. The reduction amounts to half a day (7%). This results in savings of $1.5 million/year for a 200mm fab and $3.0 million/year for a 300mm fab.
Scrap and defect yield. Quantification of scrap wafers and particularly of defect yield as influenced by integrated metrology is quite difficult and rather speculative. Therefore, we restrict ourselves to a summary of possible advantages and a modest estimation of savings.
The reduction of scrap gives integrated metrology potential advantages, including:
- reducing complete batches out of spec, due to a shorter feedback loop, making it possible to change the process setting on the fly; to use real-time process control; and to schedule maintenance better; and
- reducing errors and particles due to human factors.
The main reason for a better defect yield lies in real-time process control. In this calculation, we did not take defect yield into account. It may have a strong influence on the final result.
For scrap, we assumed that due to the advantages of integrated metrology listed above, an equivalent of two batches (0.004%) can be saved from loss per year. A 200mm wafer fab would save $140,000/year; a 300mm wafer fab would save $350,000/year.
Throughput. An important requirement for integrated metrology is that it should not decrease the throughput of the processing equipment in which it is inserted. The two modes — "wait for test results" and "do not wait for test results" — have a strong impact on the throughput.
In the case of "wait" mode, integrated metrology significantly improves the system's throughput, simply because the feedback loop has become shorter, and therefore the system idle time is shorter.
In the case of the "do not wait" mode, the throughput of the A412 is preserved due to the dual-boat approach. Each reactor in the A412 has two dedicated wafer boats. While one boat is in processing, the other boat can be prepared for the next run. This means that overhead activities can be performed in parallel with processing. The same is true for metrology steps, which can be seen as overhead activity. Therefore, integrated metrology does not influence the throughput of the A412, except in very short recipes where overhead activities can limit throughput.
For the "do not wait" mode, integrated metrology results in approximately 20% higher throughput compared to stand-alone metrology. For "wait" mode, throughput is preserved, unless the recipe is shorter than 80 min, in which case the throughput is decreased by 10%. This means that for the "do not wait" mode, integrated metrology results in fewer furnaces needed. Compared to the "do not wait" mode (where 37 furnaces are needed for the described fab), the "wait" mode simulations proved that 48 furnaces are needed in case of stand-alone metrology. With the integrated metrology, four fewer furnaces are needed (a total of 44 furnaces). This means about $4 million less capital investment for 300mm production.
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Test wafer consumption. For the consumption of test wafers, it is in principle irrelevant whether the metrology takes place in a stand-alone unit or in an integrated unit. Test wafer costs can thus be assumed to be equal for both approaches. However, contribution of test wafers to the overall costs has become particularly significant in 300mm production. Therefore, reduction of test wafer consumption is a must for both approaches. A shift towards measurements on production wafers has been recognized as inevitable.
Capital investment. The ASM A412 furnace is a dual reactor system. The two reactors share resources and thus also the same integrated metrology unit. This reduces the cost per reactor.
The estimated cost amounts to $125,000
eactor for a complete metrology unit comprising an ellipsometer, particle counter, wafer ID reader, and notch aligner.
For the fab used as an example in this paper, this means a $3.75 million investment if all 30 reactors (15 A412 systems) are equipped with complete integrated metrology units. We have assumed the worse case scenario, that is, no reduction of capital investment due to the replacement of the stand-alone metrology units. The most probable scenario is that only a fraction of the stand-alone metrology systems will be replaced, which would reduce the investment in the example to perhaps approximately $2M.
Conclusion
Ultra-compact integrated metrology tools for film thickness and particle measurements have been shown to perform well and thus to be ready for integration with processing tools. The main driving forces to invest in integrated metrology in the furnace area are:
- cycle time reduction of at least 7%;
- cost/wafer benefit in the furnace area of at least 6% for 300mm production;
- throughput increase for "wait for test results" mode of operation (when waiting for test results before starting a new batch), resulting in fewer furnaces being needed; and
- scrap wafer reduction.
Integrated metrology is particularly beneficial for 300mm production, due to high wafer and equipment costs. With modest calculations, we have shown that the return on investment in this case is less than 1.5 years. Once confidence is gained in integrated metrology tools, stand-alone tools can be partially replaced.
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The return of investment will then be less than a year. The next milestone in the development of integrated metrology tools is measurement on patterned wafers.
Acknowledgments
The authors acknowledge Rien van Driel of ASM for his valuable advice regarding factory simulations. We acknowledge Eckhard Marx for conducting the test measurements at Infineon Dresden, and RECIF SA, Aussonne, France, for their contribution to the set-up allowing these measurements.
References
- John Heaton, "The Urgent Need for Integrated Metrology," Solid State Technology, pp. 99-100, August 1999.
- Albert Hasper, "Thin Film Process Monitoring in 300mm Vertical Furnaces by Integrated Metrology," Proceedings of the AEC/APC Conference, March 2000.
- James M. Holden, Martin J. Seamons, "Characterizing a CVD-Integrated Metrology System," Semiconductor International, pp. 169-176, October 2000.
- Eckhard Marx, "Closer Control of 300mm Manufacturing Equipment by Integration of New Sensors for Wet Etch, Film Thickness, and CD Control," Proceedings of the AEC/APC Conference, September 2000.
- R. van Driel, B. Sluijk, "Analyzing the Furnace Area: How Cycle Time and Cost of Ownership are Influenced by Equipment Type and Scheduling Strategy," Solid State Technology, October 1997.
- VLSI Research Inc., Chip Insider, December 21, 1999.
Tanja Claasen-Vujcic received her PhD from the Delft University of Technology in the Netherlands and her MSc in electronics from the Eindhoven University of Technology. She has been with ASM since 1997, first as a simulation consultant and currently as a technical marketer. ASM, Rembrandtlaan 7-9, 3723 BG, Bilthoven, The Netherlands; ph 31 30 229 84 11, fax 30 229 38 23, email [email protected].
Albert Hasper received his PhD and MSc in electrical engineering at the University of Twente in the Netherlands. He is the worldwide product manager for vertical furnaces at ASM. Before that, he was active in the development of batch equipment for more than seven years.
Michael Abraham is the president and founder of NanoPhotonics AG, Mainz, Germany. Previously, he was active in the R&D of thin film and surface physics, optics, optical metrology, and microsystems technology at various R&D labs and universities.
Eckhard Marx earned his MSc degree in electrical engineering from the Technical University of Dresden. He is an APC project manager for sensors (process sensors, integrated metrology) at Infineon Technologies SC300 GmbH Dresden. He was formerly a manager for metrology. He joined Infineon five years ago.
The compact metrology unit comprising a wafer ID reader, notch aligner, and ellipsometer is inserted in the furnace just above the FOUP door openers. The single wafer robot transfers wafers from a boat or a cassette to the unit and back.