Materials for attenuated phase mask application at 193 nm

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Title: Materials for attenuated phase mask application at 193 nm
Author: Butt, Shahid
Abstract: The electronic revolution is driven by the circuits and devices fabricated on silicon. Since the invention of the integrated circuit some thirty years ago the circuit complexity has increased manifold. A large portion of this increase can be attributed to the ability to print smaller features. The process of printing these small features is termed lithography. The ability to develop new lithographic techniques will determine to a large extent whether the semiconductor industry can maintain its rapid growth rate. Currently lithographic techniques that utilize 193 nm radiation are being developed to replace the 248 nm based lithographic processes. The trend towards smaller features has meant development of optics and materials that can be used at shorter wavelengths. It has also meant the implementation of optical enhancement techniques, to extend the useful lifetime of the available technology. Phase shift technology is currently being developed to extend the useful lifetime of 193 nm based systems. Of the various types of phase shift masks, the attenuated phase shift type seems attractive because they are relatively easy to manufacture. However, at shorter wavelengths the number of materials that can be used as single layer shifters is few. The situation is further aggravated by the additional requirements placed on mask material, such as minimal transmission at alignment wavelength, low resistivity and the ability to be dry etched. The fore-mentioned requirements preclude the use of stoichiometric elements and compounds as materials suitable for a single layer attenuated phase shift mask. However, it may be possible to combine a variety of materials to obtain the desired optical and physical properties. By carefully choosing the absorbing and the non-absorbing components of the material mix, the optical and the physical properties can be engineered to meet the specifications. In case of thin films, the material properties can be varied by adjusting the deposition conditions. For this work reactive sputtering was used to deposit the films. By adjusting the partial pressure, power and time, and through the use of mosaic targets the material properties were investigated. Furthermore, films were also deposited in a layered structure that allowed for a new set of film properties. Several materials were investigated for this work. By following a simple process of elimination the material was first evaluated for its optical properties followed by an evaluation of its physical properties. Films that were investigated were composed of molybdenum silicon oxide, under-stoichiometric aluminum nitride, under-stoichiometric silicon nitride and layered films of tantalum and silicon nitride. The primary selection factor for these films were there optical properties around 193 nm. Following this the films were evaluated for there properties in the visible regime. Finally the films were evaluated for there physical properties, such as stability and volatility of the etched compounds. Only under-stoichiometric aluminum nitride met all the requirements for attenuated phase shift mask material.
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Date: 1997-08

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