7 June 1996 Usage of an electrically controlled photomask for dry microlithography by femtosecond visible laser radiation
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Abstract
The femtosecond visible laser radiation ranked among the possible modem light sourses for the optical microlithography. For the intensive femtosecond laser radiation the multyphoton excitation of high molecular electron states prevails over the linear excitation. The excitation of these states effects the ionisation of the molecules (or of the molecular groops), by this means the thin film of the photoresist is turned to plasma in a time of hundreds femtoseconds. Ablation of the material is principally caused by this process. As the experiments and the estimations imply, the treated surface after ablation is not subjected to thermic or mechanical stresses. An estimated thickness of the heat affected zone is not more than 0.02 ptm. The shape of the ablated surface correlates with the profile of the laser radiation intensity to the power P (the value P is in the region 2 - 4). The treated surface after ablation is smooth. The typical depth of ablation (per one laser pulse) is about 0.2-0.4 tm .This depth depends only weakly on the intensity of laser radiation and on the chemical composition of the photoresist. By the same procedure it is possible to treat the photoresists, the semiconductors, and some optical materials. The high degree ionized plasma ejected from the ablated surface under action of laser radiation in principle does not pollute the optical system. In this manner the ablation by femtosecond laser pulses is convinient to use in microlithography. In this article it is suggested a manner of the forming of the profile of laser radiation with the use of an electrically controlled photomask. The required profile of the laser intensity is formed by the matrix of transmitting and reflecting cells of the photomask. The state ofevery cell is electrically controlled by the computer. The states of the set are variable with the frequency that is less or equal to 25 Hz. The suggested system of the optical microlithography: 1) does not need for the collection of constant photomasks. This collection completely is replaced by the collection of controlling computer programs 2) does not use the process of any gas or liquid development, because of the process of ablation is used in this system. 3) is adaptable to the process of fabrication of special-purposed custom-made microcircuites, hologramms and other optical components. It is necessary only to change the computer program. 4) in princible makes possible the realization of ablation with the linear resolution 0.8 ptm. KEYWORDS: femtosecond visible laser radiation, electrically controlled photomask, ablation, fabrication of custom-made components, dry microlithography.
© (1996) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.
Moishe S. Kitai, Moishe S. Kitai, } "Usage of an electrically controlled photomask for dry microlithography by femtosecond visible laser radiation", Proc. SPIE 2726, Optical Microlithography IX, (7 June 1996); doi: 10.1117/12.240971; https://doi.org/10.1117/12.240971
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