The emerging need for integrated metrology standards
04/01/2002
 Authors (L to R) are Tanja Claasen-Vujcic, Michael Abraham, and Marjorie Sijtsma; Albert Hasper's photo was not available. |
Recent developments in semiconductor manufacturing show a shift from off-line to in-line or integrated inspection methods, such as integrated film thickness and particle measurements. This shift is a result of constant efforts to improve overall equipment effectiveness of semiconductor equipment. The aim of integrated metrology is to enable inspection of processed wafers directly after processing. Doing this results in cycle-time reduction, cost-of-ownership improvements, improvements in queue-time issues, and throughput increases. The latter is especially important for so-called gated processes, where a second run is only started when the results of the first run are known and approved.
Moreover, for tools with dual-boat operation, integrated metrology is a must for gated processes. Otherwise, one cannot use the capability of continuous processing enabled by the dual-boat operation. This kind of operation is common for 300mm furnaces: While one boat is in the processing sequence, the other can already be unloaded and loaded with new wafers. This enables continuous processing and 100% reactor utilization.
Implementation of integrated metrology into fabs is not without its challenges. Overall, it requires a different approach compared to common practice, where metrology tools are managed by a specialized department. Often there is even a physical separator between processing tools and metrology tools in the fab.
With integrated metrology, metrology tools become part of the process equipment. At least part of the task, along with some of the knowledge and responsibilities of the metrology department, must be transferred to production. This includes not only operational issues, but also validation and maintenance of the tools.
The integration of metrology into process tools requires that the process equipment provider, the manufacturer of integrated metrology, and the chip manufacturer work closely together. Since standards for the fab integration of metrology are not yet available, such cooperation has to lead to a pragmatic implementation of an integrated metrology infrastructure incorporating new procedures for automated tool calibration, tool matching, and tool self-testing (see figure).
Calibration. Just like a stand-alone metrology tool, integrated metrology tools need to be calibrated according to a schedule. Since the integrated tool is not available for a standard, operator-based calibration procedure, the calibration has to be automated and the calibration procedure has to be integrated into fab logistics.
Therefore, from time to time, a set of calibration wafers must be provided to the integrated metrology tools.
Tool matching. All integrated metrology tools used in the furnace area of a fab, for example, have to be matched to show the same result on the same wafer. An appropriate strategy might be to match the ensemble of the integrated tools to a reference stand-alone tool. This procedure has to be automated and needs a number of tool-matching wafers to be sent through the ensemble of the metrology tools starting with the stand-alone reference tool.
Sensor health (self-test). Since integrated metrology is responsible for go, no-go decisions in a running process, the status of the integrated metrology unit has to be recorded and communicated to the host. This test should not have an impact on throughput and should preferably be carried out in the background at tool level (i.e., not at the host level). The host will be notified only in the event of a failure.
Automatic process control. Measurement data are communicated to the host via the process tool. The host computer can either decide to acknowledge or to inhibit the start-up of the next load, or even to change the process parameters for the next batch, using statistical process control and advanced process control algorithms. In our work with ASMI furnaces, these decisions can also be made at the furnace.
Integrated metrology will only be successful when these issues, and others that will eventually be identified, are addressed in standard ways.
Marjorie Sijtsma is project manager of integrated metrology at ASMI N.V., Jan van Eycklaan 10, NL-3723 BC Bilthoven, The Netherlands, e-mail [email protected]. The co-authors of this article are Michael Abraham from NanoPhotonics AG and Tanja Claasen-Vujcic and Albert Hasper from ASMI N.V.