Optimizing Sputtering Technology: How to Enhance Effective Sputtered Area and Sputtering Rate

Optimizing Sputtering Technology: How to Enhance Effective Sputtered Area and Sputtering Rate

Effective Sputtered Area

The “effective sputtered area” on the sample refers to the region on the sample surface that is effectively covered by the sputtered particles, where a uniform film is deposited. This area depends on factors such as target diameter, target-to-substrate distance (TSD), sample size, and rotation mechanism. The smaller the TSD, the more concentrated the coverage area, and the higher the sputtering rate, but the uniformity might be affected.

Sputtering Rate

The sputtering rate is the thickness of the film deposited on the sample per unit time, typically expressed in nm/s or nm/min. It is directly proportional to the flux of sputtered particles reaching the sample. The sputtering rate is influenced by target power, TSD, pressure, and the sputtering mode (DC or Magnetron). In general, a smaller TSD results in a higher sputtering rate. Magnetron sputtering enhances ionization through a magnetic field, which increases sputtering efficiency, leading to a higher sputtering rate compared to traditional DC sputtering.

Geometrical Relationship and Coverage Area

Assuming the target diameter is 50mm, and the target-to-sample distance is H, the effective sputtered area depends on the diffusion angle of the sputtered particles. When the TSD is smaller (e.g., 40mm), the effective area is close to the target diameter. As the distance increases (e.g., 120mm), the coverage area expands, but the flux at the edges of the area decreases.

DC Sputtering vs. Magnetron Sputtering

• DC Sputtering: The sputtered particles have a wide diffusion angle, and the coverage area is limited, suitable for smaller samples.

• Magnetron Sputtering: Due to the magnetic field, which increases plasma density, the target material can sputter particles more efficiently, allowing a larger coverage area and maintaining a higher sputtering rate even at a larger distance.

  
  

Estimations for VPI Equipment

For a 50mm target diameter and a sample placed at different target-to-sample distances (e.g., 40mm, 70mm, 120mm), the following estimations can be made:

Factors Affecting the Sputtering Rate

The sputtering rate (R) is influenced by parameters such as target power (P), target-to-sample distance (TSD), pressure (p), and sputtering mode (DC or Magnetron). Increasing target power or decreasing TSD usually leads to a higher sputtering rate, but it may also lead to non-uniform thickness. Magnetron sputtering, by enhancing flux, increases the sputtering rate compared to traditional DC sputtering.

VPI Equipment Estimation

For VPI equipment, assuming a target diameter of 50mm and a sample-to-target distance of 50mm (5cm) and 100mm (10cm), the sputtering rate could be estimated as follows:

• At TSD = 50mm: The sputtering rate is higher due to minimal scattering.

• At TSD = 100mm: The rate decreases due to more scattering. The rate might drop to 50-70% of the rate at 50mm.

In the case of Magnetron sputtering, at a TSD of 50mm, the rate might be 10nm/min, but at 100mm, it could drop to 6-7nm/min. Traditional DC sputtering would show a further reduction in rate at the same distance.

Interaction of Sputtering Rate and Effective Area

In practical applications, there is a trade-off between sputtering rate and effective sputtered area:

• To increase coverage area (larger TSD or more sample placements), the sputtering rate may decrease.

• To increase the sputtering rate (shorter TSD or higher power), the coverage area might shrink, and the uniformity may decrease.

• Magnetron sputtering can alleviate the reduction in sputtering rate by enhancing flux, allowing for better balance between rate and coverage area.

Thus, for VPI equipment users, it is essential to adjust these parameters according to sample size, quantity, and required film thickness.

Selecting Target-to-Sample Distance (TSD)

• For smaller samples (e.g., optical fibers), a TSD of 40-60mm is recommended to maximize flux and rate.

• For larger samples, a TSD of 80-120mm can be used to increase effective coverage area, but sample rotation mechanisms should be employed to ensure uniformity.

Adjusting Power and Pressure

• In Magnetron sputtering, power can be increased to enhance flux and sputtering rate, but care must be taken to avoid overheating of the target.

• Pressure should be controlled within an optimal range to maintain stable plasma while preventing excessive scattering that reduces the sputtering rate.

• For low-vacuum applications, adjusting the TSD and power under high pressure or thicker gas layers can help compensate for reduced rate.

Sample Rotation and Uniformity

• Using sample rotation or tilting can improve film uniformity and reduce thickness variation at the edges.

• For applications requiring high uniformity (e.g., SPR coatings), shorter TSDs and Magnetron sputtering modes should be preferred.

By optimizing the target-to-sample distance, power, pressure, and other parameters, it is possible to effectively balance sputtering rate and effective sputtered area. For VPI equipment, adjusting these variables based on sample size, quantity, and deposition thickness will lead to more efficient and higher-quality thin film deposition.