The RF power for plasma typically operates at a high frequency of 13.56 MHz. This frequency is used to dissociate reactant gases and generate plasma, which has a significant impact on film stress. Additionally, a secondary RF power at a lower frequency, typically less than 500 kHz, can be used to enhance ion bombardment and modify film stress, as well as improve step coverage in film deposition. The choice of RF power frequency can influence the chemistry and ion bombardment during deposition, allowing for adjustments to achieve desired film properties.
Key Points Explained:
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Primary RF Power Frequency:
- Frequency and Function: The primary RF power typically operates at a high frequency of 13.56 MHz. This frequency is chosen because it is widely available for industrial use and is effective in dissociating reactant gases to generate plasma.
- Impact on Film Stress: The use of high-frequency RF power has a strong effect on film stress. This is due to the ionization and dissociation of gases, which can influence the structural integrity and stress levels within the deposited films.
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Secondary (Bias) RF Power:
- Frequency and Purpose: Secondary RF power operates at a lower frequency, typically less than 500 kHz. This lower frequency is used to cause more ion bombardment of the sample surface.
- Role in Film Deposition: By providing additional ion bombardment, the secondary RF power allows for better control over film stress and can improve the step coverage of film deposition into trench features. This is particularly useful for achieving uniform and dense films.
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Adjusting RF Power for Desired Outcomes:
- Frequency Adjustment: The frequency of RF power can be adjusted to influence the chemistry and ion bombardment during the deposition process. This flexibility allows for the modification of film properties to meet specific requirements.
- Dual-Frequency Reactor: Using a mixture of low- and high-frequency signals in a dual-frequency reactor can further enhance control over the deposition process. This approach enables the optimization of both plasma density and film characteristics.
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Impact of RF Power on Chamber Pressure:
- Lower Pressure for RF Systems: In RF systems, the plasma can be maintained at a much lower pressure, often less than 15 mTorr, compared to higher pressures required for DC sputtering. This lower pressure reduces collisions between target material particles and gas ions, facilitating a more direct pathway for particles to reach the substrate.
- Advantages for Insulating Materials: The combination of lower pressure and the use of radio waves instead of direct current makes RF sputtering ideal for target materials with insulating qualities.
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Uniformity and Film Quality:
- High-Frequency Benefits: Depositing films at high frequencies (13.56 MHz) results in better uniformity compared to low frequencies. This is because high frequencies equalize the electric field strength across the substrate, reducing differences in deposition rates between the edge and center of the plate.
- Trade-offs: While high frequencies produce denser films, they can also cause greater damage to the substrate. Therefore, careful selection and adjustment of RF power frequencies are crucial for balancing film quality and substrate integrity.
By understanding and manipulating the RF power settings, lab equipment purchasers can optimize the plasma generation process to achieve desired film properties, ensuring high-quality and consistent results in various applications.
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