The Effect of Sputtering Parameters on Thin Film Properties

The sputtering parameters in magnetron sputtering have a decisive impact on the properties of thin films. These parameters include sputtering gas pressure, sputtering power, target-to-substrate distance, substrate temperature, bias voltage, and sputtering gas types. By precisely controlling these parameters, the physical, chemical, and mechanical properties of the thin films can be optimized.

Sputtering Gas Pressure

Impact on Film Crystallinity: When the gas pressure is too high, the degree of gas ionization increases, but the sputtered atoms experience more collisions before reaching the substrate, resulting in significant energy loss. This limits their mobility upon reaching the substrate, leading to poorer crystallinity, and the film may exhibit an amorphous state or incomplete crystallization. Conversely, at too low a pressure, gas ionization is difficult, making it hard to achieve stable sputtering, leading to low deposition rates and inability to form continuous films. An optimal sputtering pressure ensures that the sputtered particles have enough energy to reach the substrate and crystallize well, resulting in good film quality.

Impact on Surface Roughness: At an appropriate sputtering pressure, sputtered atoms can uniformly deposit on the substrate, resulting in a smoother film surface. If the pressure is too high or too low, this uniformity is disrupted, leading to increased surface roughness. For example, at high pressures, the sputtered atoms arrive at the substrate unevenly after numerous collisions, increasing surface roughness.

Impact on Film Density: At lower pressures, the average free path of sputtered atoms is longer, and they arrive at the substrate with higher energy, which allows them to fill the pores in the film better and increase its density. In contrast, at high pressures, the energy loss of the sputtered atoms is significant, preventing effective pore filling and reducing film density.

Sputtering Power

Impact on Deposition Rate: Increasing the sputtering power enhances the energy of argon ions bombarding the target surface, increasing the sputtering yield and accelerating the deposition rate. However, if the power is too high, it can cause overheating of the target surface or even "poisoning" of the target, adversely affecting the stability of the deposition rate.

Impact on Film Structure: At low sputtering power, the energy of sputtered atoms reaching the substrate is low, resulting in weaker atomic mobility and smaller grain sizes, possibly forming polycrystalline or amorphous structures. At high sputtering power, the increased energy enhances atomic mobility and diffusion, promoting grain growth and crystallization, leading to larger grain sizes and better crystalline structures.

Impact on Film Stress: Variations in sputtering power can alter the growth rate and microstructure of the film, affecting the stress state within the film. Generally, films deposited at high sputtering power exhibit higher stress due to the rapid deposition process, which does not allow atoms sufficient time to adjust their positions, resulting in stress accumulation.

Target-to-Substrate Distance

Impact on Deposition Rate: If the target-to-substrate distance is too large, sputtered atoms encounter more collisions with gas molecules during flight, resulting in significant energy loss and a reduced number of atoms reaching the substrate, thus decreasing the deposition rate. Conversely, if the distance is too small, while energy loss is minimized, the concentrated distribution of sputtered atoms can affect the uniformity of the deposition rate.

Impact on Film Uniformity: An appropriate target-to-substrate distance allows sputtered atoms to be uniformly distributed on the substrate, forming a uniform film. If the distance is uneven or inappropriate, it can lead to variations in thickness and properties across different areas of the film, affecting its overall quality.

Substrate Temperature

Impact on Film Crystallinity: At lower substrate temperatures, the diffusion capability of sputtered atoms on the substrate surface is weak, preventing orderly arrangement and resulting in amorphous structures. As the substrate temperature increases, atomic diffusion improves, enhancing film crystallinity, increasing grain size, and leading to more complete crystallization.

Impact on Film Adhesion: Appropriately increasing the substrate temperature can enhance the adhesion between the film and the substrate. This is due to increased atomic diffusion and chemical reactions at the interface at high temperatures, forming a stronger bond. However, if the substrate temperature is too high, it may increase the difference in thermal expansion coefficients between the substrate and the film, generating thermal stress and reducing adhesion.

Bias Voltage (if applicable)

Impact on Film Quality: Applying an appropriate bias voltage to the substrate can give sputtered ions kinetic energy, accelerating their movement toward the substrate and bombarding it, which helps remove loosely bound atoms from the substrate surface and leaves tightly bound, defect-free film atoms, thereby improving film quality. Additionally, the bias voltage can attract argon ions to clean impurities from the substrate surface, further enhancing film quality.

Impact on Film Porosity: The bias voltage increases the energy of deposited ions, enhancing their mobility and allowing them to penetrate the substrate surface, reducing film porosity and increasing film density, thus improving film performance. However, if the bias voltage is too high, severe back-sputtering may occur, reducing the sputtering rate and creating defects within the film.

Sputtering Gas

Impact on Film Composition: The type and flow rate of sputtering gas can influence the reaction between target atoms and gas molecules during the sputtering process, thus altering the film composition. For instance, when sputtering metallic targets with reactive gases like oxygen, metal oxides may form in the film; using nitrogen may lead to metal nitrides.

Impact on Film Performance: The pressure and flow rate of sputtering gases also affect the energy and quantity of sputtered atoms, subsequently influencing the film's performance. For example, excessive gas flow can reduce the energy of sputtered atoms, impacting film crystallinity and density; insufficient gas flow may lead to instability in the sputtering process, affecting film uniformity.