Sputter coating typically occurs at pressures in the mTorr range, specifically from 0.5 mTorr to 100 mTorr. This pressure range is necessary to facilitate the sputtering process, where a target material is bombarded by ions from a plasma, usually argon, causing atoms from the target to be ejected and deposited onto a substrate.
Explanation:
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Base Pressure and Gas Introduction: Before the sputtering process begins, the vacuum chamber is evacuated to a base pressure, typically in the range of 10^-6 mbar or lower. This high vacuum environment ensures clean surfaces and minimal contamination from residual gas molecules. After achieving the base pressure, a sputtering gas, commonly argon, is introduced into the chamber. The gas flow can vary significantly, from a few sccm in research settings to several thousand sccm in production environments.
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Operating Pressure During Sputtering: The pressure during the sputtering process is controlled and maintained in the mTorr range, which is equivalent to 10^-3 to 10^-2 mbar. This pressure is crucial as it affects the mean free path of the gas molecules and the efficiency of the sputtering process. At these pressures, the mean free path is relatively short, around 5 centimeters, which influences the angle and energy with which the sputtered atoms reach the substrate.
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Influence of Pressure on Deposition: The high density of the process gas at these pressures leads to numerous collisions between the sputtered atoms and the gas molecules, causing the atoms to arrive at the substrate at random angles. This contrasts with thermal evaporation, where atoms typically approach the substrate at normal angles. The presence of the process gas near the substrate can also lead to gas absorption into the growing film, potentially causing microstructural defects.
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Electrical Conditions: During the sputtering process, a DC electrical current is applied to the target material, which acts as the cathode. This current, typically between -2 to -5 kV, helps in ionizing the argon gas and accelerating the ions towards the target. Simultaneously, a positive charge is applied to the substrate, which acts as the anode, attracting the sputtered atoms and facilitating their deposition.
In summary, the pressure during sputter coating is carefully controlled to be in the mTorr range, optimizing the sputtering process for efficient and effective deposition of materials onto substrates. This pressure control is essential for managing the interactions between the sputtered atoms and the process gas, ensuring the quality and properties of the deposited film.
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