Multi-Domain Production Ellipsometry Characterizes BARCs

Alexander E. Braun, Associate Editor -- Semiconductor International, 11/1/1998

When, in photolithography, the mask pattern is transferred to the photoresist using light from the stepper, wafer topology imperfections may cause variations in photoresist thickness. This can lead to multiple reflections within the photoresist, producing constructive or destructive interference, possibly affecting the wafer's resulting pattern resolution. Over time, several methods (including various organic antireflective coating and chemically amplified 'dyed' photoresists) have been developed to decrease thin-film interference effects and reflective notching to improve pattern resolution.

Sub-0.25 µm linewidth applications use DUV photolithography. This requires more stringent control of critical dimension variations in pattern resolution. Inorganic bottom antireflection coatings (BARCs) have become essential in reducing reflections for DUV applications, while interfering least with the process flow. Two important features make Si-rich nitrides ­ SiN_x, SiO_xN_y ­ good candidates for BARCs. They conform to the underlying substrate with good thickness uniformity, and by altering composition the films' optical properties can be tuned. This tunability of optical properties allows for a thinner, more absorbing film with performance optimized at the exposure wavelength.

Fig. 1. The SpectraLaser optical system uses four-wavelength simultaneous multi-angle ellipsometry with UV reflectometry to enable measurements of t, n and k of SiNx at 248 nm. (Source: Rudolph Technologies)

Accurately tuning a film's optical properties requires tight control of the process parameters. BARC parameters measured in production applications include reflectance at the exposure wavelength, film thickness and optical constants. Reflectance measurements can identify the source of the problem if the film's reflectance is out of tolerance. BARC characterization, then, is an integral part of the metrology control, which was not the case with previously used organic materials.

Spectral reflectometry (traditionally used in photolithography applications) cannot reliably measure thickness and optical constants simultaneously. Ellipsometry, although well suited for the simultaneous measurements of the thickness and optical contraints, is hampered in DUV by a more complex optical system resulting in lower light collection efficiency, compared to reflectometry.

A method developed by Rudolph Technologies (Flanders, N.J.), which combines spectral reflectometry and ellipsometry, enables reliable measurements of t, n and k of SiN_x at 248 nm. Used in partnership, DUV spectral reflectometry and multiple-domain visible ellipsometry appear to create a powerful technique well-suited for simultaneously measuring thickness and characterizing inorganic BARCs' optical constants in production applications.

The method, which is indirect in nature (Fig. 1), uses a segregated BARC layer model and Effective Medium Approximation (EMA) to represent the optical properties of the SiN_x material. EMA's limitations of the interpretation of highly non-stoichiometric SiN_x are circumvented by correcting the estimated value of the material's absorption at the exposure wavelength by using a reflectance measurement. Results were found consistent with those obtained using research-grade spectroscopic ellipsometry. The latter measurements were interpreted using the EMA and a Lorentz-Tauc oscillator (LTO) approximation. The composition of the SiN_x measured using the EMA model correlates well with the deposition parameters and the results of auger electron spectroscopy made of the material.