System Measures Amorphous Layer Depth Non-Destructively
Alexander E. Braun, Senior Editor -- Semiconductor International, 3/1/2001
 |
Technologies used for this are a four-point probe, spreading resistance profiling, and secondary ion mass spectroscopy (SIMS). The probe measures sheet resistance but not junction depth, so a profiling technique, usually SIMS, is used. SIMS measurements, however, have low throughput and are destructive, so the process tends to be used in the analytical lab and is not effective for measuring uniformity — critical with 300 mm wafers.
About a year ago, Boxer Cross Inc. (Menlo Park, Calif.) introduced its BX-10 platform, a high-throughput in-line, non-contact measurement system that works on patterned wafers and is capable of measuring junction depth and uniformity on an in-line basis. It enables the fab to implant, anneal and measure — all in-line and non-destructively — and provides post-anneal characterization of shallow junctions. It now provides implanted dose measurement and measurement of the pre-amorphizing implant (PAI) — the amorphous layer depth. The PAI is used before doping in the shallow junction process. If an attempt is made to do a shallow implant directly into the silicon, crystal channeling may result — ions go deeper and scatter more. If the surface is amorphized and implanting is done into an amorphous layer, channeling is eliminated.
There have been two methods used to characterize PAI depth. The most precise is cross-sectional transmission electron microscopy (TEM), but it is destructive and time-consuming. The other, variable-angle spectroscopic ellipsometry, requires deep understanding of the optical properties of the interface between the amorphous layer and the substrate, and can result in a process-dependent and inaccurate measurement.
The Boxer Cross system provides a process-independent direct measurement of amorphous layer depth with TEM-precision results, and can be used with silicon and germanium implants over a wide range of doses and energies. It follows the shallow-junction process, measuring amorphous layer depth, allowing the anneal to take place and measuring post-anneal junction depth.
|
|
The platform uses a two-laser measurement system (Figure). The first creates an excess carrier distribution, using photons to generate electron hole pairs in the substrate. Because of the junction's built-in field, the carriers line themselves at the interface's edge. Because a material's refractive index is a function of carrier concentration, a refraction index gradient is built at the interface. The second laser is used on the carrier distribution and on reflections obtained from the front surface and from the carriers that build up at the interface. This results in an interference signal originating from the front-surface reflection and the reflection at the depth of the interface's edge. Because the signal has a built-in reference — the front surface — it is a true measure of depth. The measurement is done in material with a high refractive index. Although the laser's wavelength is 1 µm in air, because silicon and amorphous silicon have a refractive index of about 4, the wavelength is only 0.25 µm once the laser hits the materials, making the measurements like ultraviolet measurements. Studies done on CVD layers demonstrate it is possible to get better than 1 Å depth resolution for junction-depth measurements. Similar results using amorphous layers have been demonstrated, showing depth resolutions that exceed those of other techniques such as SIMS or TEM.
For additional information on inspection, measurement and test, go to www.semiconductor.net/imt
