With greater complexity in ever thinner films used in manufacturing advanced CMOS transistors, such as the gate dielectric stacks formed by atomic layer deposition and graded diffusion, there is vital need for additional metrology capability in the fab. Spectroscopic ellipsometers (SE) have been used for decades to provide thickness and composition information for thin dielectric films, and expanding the wavelength of the spectrum inherently provides more information.
Most broadband SEs have been limited by optics hardware to resolving as low as 190nm wavelengths, but a lot of interesting information regarding dielectrics can be found down below 190nm. In particular, for complex siliconOxide-siliconNitride-siliconOxide (ONO) dielectric stacks (see figure, below), insight down to 150nm allows for thickness and composition information to be extracted from a single measurement. With this in mind, KLA-Tencor modified its broadband SE tool to push the wavelength from a 190nm lower-wavelength limit down to 150nm.
The 8500 is the first tool in a newly named Aleris product family for KLA-Tencor, and in coming months “there will be other tools in the family to address other modules in the customer fab,” said Namrata Vora, product marketing manager in the company’s films and scatterometry division. The company claims >25 tools in the Aleris family are already in the field, with the 8500 reportedly in production for gate-length control at 65nm, and for multi-layer interconnect measurement.
The tool uses patented reflective optics for focusing, ensuring the same focal points for different wavelengths, explained Vora. “This lets us use a much smaller spot size, allowing us to measure product wafers.” Compared to the prior generation tool, the tightened optics allow for the minimum spot or box size to be reduced by ~20%. Low spectral distortions minimize noise-levels, which helps with tool matching for manufacturing.
“To prevent absorption of the wavelength at 150nm, we have to purge the optics with an inert gas but no vacuum is needed,” reminded Vora. This latter comment perhaps refers to startup OEM Metrosol’s vacuum ultraviolet (VUV) SE tool (released in July), which requires vacuum pumps and chambers but does allow for further insight down to 120nm.
The 8500 is more than just an extended range SE, however, since it has many additional handling and inspecting capabilities tuned to the unique requirements of manufacturing ICs at advanced device nodes. For example, laser desorption removes airborne molecular contamination from up to eight hours of queue-time exposure; to ensure accurate measurements.
Stress measurement on the 8500 is done using laser rastering across the wafer and sub-millimeter spatial resolution at the detector to measure at little as 10μm of bow due to thin films. The measurement technique is much faster than the former z-axis focus. — E.K.