System Provides Non-Intrusive, & In-Situ Thin Film Characterization
Alexander E. Braun, Associate Editor -- Semiconductor International, 10/1/1998
T he use of specially treated fused silica fibers, making them capable of transmitting UV light without sustaining radiation damage, has enabled an analyzer capable of non-intrusive in-situ thin film characterization, with minimal chamber adjustment, whether it is an etch or deposition chamber or a load lock (Fig. 1).
The system, engineered by n&k Technology (Santa Clara, Calif.), uses reflective optics for its beam delivery system, which includes fiber coupling optics and imaging mirrors. It can measure the reflectance spectrum over the entire 190-900 nm range, with a good signal-to-noise ratio. A by-product of the system's optics is a long, 7 in. working distance between the optical probe and sample.
The incident beam's intensity can be adjusted and configured, or it can be blocked partially. This is useful for photolithography, where some DUV photoresists may be sensitive to visible light. The beam's intensity can be controlled, so that it will not affect the sample. Basic to the system, is that it enables light originating from the detector to be shined upon the sample in a controlled manner, having it reflect in an equally controlled way, to enable a maximum of raw data to be collected for analysis.
The analyzer uses two light sources, halogen tungsten (visible light) and deuterium (UV). The way it combines these into one beam as well as how it delivers the beam is key to in-chamber characterization.
The use of all reflective optics - the source and the detector - make for a smaller spot size of 50 mm, which opens a wider spectral range for analysis and makes it unnecessary to be close to the sample. The innovation of the system's optics is that the spot is achieved solely through reflective optics; no transmissive optics are involved. This reduces aberrations naturally induced by all optical systems, since there is no such thing as a perfect lens or perfect reflector. This can allow for a good signal-to-noise ratio in data being obtained through DUV as well as the rest of the spectrum.
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| Figure 1 |
Results are based on raw data obtained across the entire DUV to near IR spectral region. It then is analyzed using the n&k method, which produces thickness and n and k spectra from 190 to 900 nm for all types of thin film stacks. Since the analyzer does not require polarization phase shift measurements in its DUV measurements, it provides an additional metrology option and capability to the production line, which is easy to use and does not have to meet the constraints required by some other systems.
An application where this new system would be useful is in the deposit of silicon films, where it is unknown whether the film is amorphous or polycrystalline. Where various types of polycrystalline silicon are deposited, the interest is in determining whether resistivity changes in these films, depending on how they have been doped or processed. By correlating optical properties to electrical properties, it is possible to get an answer. Likewise, there are cases where a determination of the amount of nitrogen in a carbon film would be useful.
It is not possible to characterize thin films with raw reference data only. Raw reflectance data must be analyzed properly. Such data depends on film, substrate, surface roughness and thickness. Any changes in reflectance from one film to the next - even if of the same type - can be due to changes in any one of these factors.
This technology also can deal with transparent substrates and do
simultaneous reflectance and transmittance measurements from two sides
and through a given sample. That would be useful for applications with
media such as CDs. By measuring the reflectance from both sides and
transmittance through the sample, it would be possible to characterize,
fully and simultaneously, the various layers comprising these products. 
