PROCEEDINGS ARTICLE | May 14, 2007
Proc. SPIE. 6607, Photomask and Next-Generation Lithography Mask Technology XIV
KEYWORDS: Signal to noise ratio, Dispersion, Quartz, Photons, Reflectivity, Optical testing, Scatterometry, Transmittance, Critical dimension metrology, Scatter measurement
For the first time, polarized broadband transmittance (T) plus reflectance (R) measurements, combined with
the Rigorous Coupled-Wave Analysis (RCWA) and the Forouhi-Bloomer dispersion equations for n and k,
were used to measure 2D trench dimensions. This is in contrast to traditional scatterometry, which is based
on reflectance-only measurements. T and R were measured from 190 to 1000 nm in one-nanometer intervals.
Inclusion of the transmittance measurements proved to be advantageous, because there is a greater sensitivity
of the T spectra to the sub-nanometer structural and/or material variations, which are difficult to detect with
R-only measurements. Furthermore, the intensity of T is much higher than the intensity of R, resulting in a
much improved signal-to-noise ratio, since intensity is proportional to number of photons reaching the
detector, which in turn is proportional to the signal. Thus, the higher the intensity, the higher the signal-to-noise,
and the better the repeatability and reproducibility of the results.
For the current study, 2D arrays of square and circular contact holes of various pitches were measured on an
After-Clean-Inspection (ACI) phase-shift mask, using a spectrophotometer-based instrument, capable of
collecting four continuous spectra during one measurement - two polarized reflectance spectra (Rs and Rp)
and two polarized transmittance spectra (Ts and Tp). The measured spectra were analyzed using the Forouhi-Bloomer dispersion equations, in conjunctions with RCWA algorithm, applied simultaneously to R and T polarized spectra. The method provided accurate and repeatable results for contact hole depths, critical dimensions film thicknesses and n and k spectra. High-resolution uniformity maps were obtained for all the parameters mentioned above.
