Integrated Metrology Moves to Real-Time, Better Accuracy
Alexander E. Braun, Senior Editor -- Semiconductor International, 12/1/1999
Over the last year, the progress of integrated metrology has picked up speed, driven by the need to make faster and more accurate measurements while reducing wafer travel. Particularly with copper and the complexities this brings -- new films, low-k dielectrics, etc. -- it has become obvious that improvements in metrology systems invariably result in better process control, and that integration offers a king's highway to it.
An example of this is the NanoSpec 9000, which Nanometrics (Sunnyvale, Calif.) will introduce at SEMICON Japan. This film measurement system can integrate three reflectometry techniques: visible, 400-800 nm; DUV, 190-400 nm; and FTIR, 2.5-25 µm. It is capable of simultaneously determining thickness, refractive index, extinction coefficient and dielectric dopant concentration. The tool is a response to the need to monitor dielectric dopant concentrations of materials, particularly in the case of the new films and low-k.
The 9000 follows the so-called 'supertool' modular architecture, in that it is composed of modules taken from stand-alone products; in this case the FTIR, which has been integrated to monitor dopant concentrations. This design philosophy is becoming widespread in the industry.
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Fig.
1 The integration of three reflectometry techniques-- visible, DUV
and FTIR provides the capability to simultaneously determine thickness,
refractive index, extinction coefficient and dielectric dopant
concentration for materials like boron, phosphorous, carbon and fluorine.
(Source: Nanometrics Inc.) |
Before, it was necessary to take the wafer to a stand-alone film thickness measurement system, measure data points to get a film thickness average, then move it to the stand-alone manual FTIR system, input the average film thickness number and extract the FTIR data. Three tools were involved: the process tool, film measurement system and FTIR. Instead of only a few minutes, it could take more than two hours for the data to get to the process engineer. Besides turnaround time, another advantage of integrated metrology systems is that they are activated by the process tool, requiring no operator interaction. Stand-alone FTIRs often needed trained engineering personnel.
Historically, dopant concentration measurement has been a tough nut to crack, particularly due to the difficulty of doing it in real time, which would provide both better process control and higher process tool efficiency. The 9000 uses a library of algorithms (dispersion and PSL-type models) to determine thickness, refractive index, extinction coefficient and dielectric dopant concentration. Data can be stored in a database, or downloaded to the process tool and fab host computer.
The capability to measure thickness and dopant concentrations in real time, before the wafer leaves the CVD process tool, gives engineers immediate feedback, enabling closer process monitoring and tighter specs. In film thickness and optical constant mode, the system measures both product and monitor wafers. For determining dopant concentrations, thickness and optical constants on monitor wafers, the system is marginally slower; but there is considerable advantage for the fab to be able to acquire all these film parameters simultaneously and in real time.
Definitely, integrated supertools appear to be the shape
of things to come in the world of metrology.
