Phase-shifting masks (PSMs) have provided a breakthrough in the future semiconductor industry by further extending lithography to the submicrometer order. With the use of PSMs over the past several years, their requirements have changed with the developing semiconductor technology. This study investigated high-transmittance attenuated PSMs (HT-Att-PSMs) to satisfy the requirements of 20±5% transmittance and 180℃ phase shift at the exposure wavelength and less than 40% transmittance at the inspection wavelength. Regarding the wavelength, the inspection wavelength was targeted at 248nm for the ArF laser (exposure wavelength of 193nm) HT-Att-PSM, and 365nm for the KrF laser (exposure wavelength of 248nm) HT-Att-PSM.
With simulation, n(refractive index)-k(extinction coefficient)-d(thickness) charts were developed to show optimum ranges of the optical constant for the HT-Att-PSM. The simulation was performed using the matrix method to find the optimum range of the variables that yield a 180℃ phase shift and 20±5% transmittance at the exposure wavelength and less than 40% transmittance at the inspection wavelength. The simulation was verified by comparing calculated transmittance data with measured data.
Cr-based materials of chromium fluoride, chromium aluminum oxide, and chromium aluminum oxynitride were investigated to study the applicability to HT-Att-PSMs.
Chromium fluoride showed too high a transmittance slope with wavelength to be applicable as a HT-Att-PSM material. Also, the chromium fluoride films degraded in a humid condition. The degradation phenomena of chromium fluoride films in a 100% relative humid condition were investigated. Furthermore, a possible degradation mechanism consisting of multi-step reactions was proposed.
Chromium aluminum oxide was chosen as a new candidate for the HT-Att-PSM in this study. Using developed n-k-d charts and film properties, optimum film thickness and composition were determined for the HT-Att-PSM. Additionally, the optical band gap of chromium aluminum oxide films was studied as a basic property of optical material. However, chromium aluminum oxide was found to be difficult in dry etching. Therefore, new material or new etching method, which satisfied optical requirements for the HT-Att-PSM and provided easy dry etching, was needed.
For the solution material, chromium aluminum oxynitride was chosen, and it was also a new candidate for the HT-Att-PSM. The optical properties at the deep-UV region of chromium aluminum oxynitride films deposited with a reactive magnetron sputtering system were studied. Film properties were analyzed by varying the composition ratio of Al/(Al+Cr), flow rate ratio of $N_2/Ar$, and deposition power. Transmittance, the refractive index, the extinction coefficient, thickness, composition and etching characteristics were analyzed. This study also examined the relationship between film properties (optical constants, transmittance slope at the deep-UV region, composition, and deposition rate), and process variables (the composition ratio of Al/(Al+Cr), flow rate ratio of $N_2/Ar$, deposition power, and deposition pressure). With the studied film properties and developed n-k-d charts, optimum conditions of chromium aluminum oxynitride films for the HT-Att-PSM were established. Also, tests for dry etching of this material showed successful results. Therefore, chromium aluminum oxynitride is expected to be applied for the HT-Att-PSM at KrF laser and ArF laser.
Finally, on the basis of this study, new material was proposed for a future PSM, that is, the HT-Att-PSM at $F_2$ laser of 157nm.
PSMs are expected to be used more than 10 years and are required to be developed with varying requirements such as wavelength, transmittance, and illumination axis. The developed method in this study can be applied for the fore-processes developing new PSMs with some modifications according to the requirements. The sequences are summarized as development of n-k-d charts, studying the dependence of optical constants on process condition, and finding the optimum condition with the n-k-d charts and the studied dependence.
