Is tungsten carbide target easy to sputter? Compare different materials and explore the optimization of sputtering process

1. How does the sputtering efficiency of tungsten carbide target compare with other targets?

Tungsten carbide (WC), as a high-quality cemented carbide material, has been widely recognized in industrial applications because of its extremely high hardness, good wear resistance and corrosion resistance. In the field of sputtering technology, tungsten carbide targets are widely used to prepare films with high hardness and wear resistance. However, the sputtering efficiency of tungsten carbide is relatively low, which directly affects its application in thin film deposition. In order to understand the sputtering efficiency of tungsten carbide targets, we must first understand the basic definition and influencing factors of sputtering efficiency, and then compare it with other common targets to comprehensively evaluate the advantages and disadvantages of tungsten carbide in sputtering applications.

1.1 Basic definition of sputtering efficiency

Sputtering efficiency is usually measured by the sputtering yield, which is the number of atoms that can be sputtered from the surface of the target when each incident ion bombards it. Sputtering yield can be expressed as:

Y= Number of sputtered atoms Number of incident ions Y= \frac{\text{Number of sputtered atoms >{\text{Number of incident ions >Y= Number of incident ions Number of atoms sputtered

Factors that affect the sputtering yield include the energy of incident ions, the physical and chemical properties of the target (such as surface binding energy, melting point, density, etc.), and the sputtering environment (such as the type and pressure of the working gas, sputtering power, etc.). For tungsten carbide targets, its high melting point and high density characteristics make the sputtering process require higher energy, which usually results in a lower sputtering yield than some metal targets.

1.2 Influence of physical and chemical properties of tungsten carbide on sputtering efficiency

Tungsten carbide's high hardness and high melting point are the main reasons why it is favored in many industrial applications, but these properties also have a significant impact on the material removal efficiency during sputtering.

1. High hardness: tungsten carbide Vickers hardness is usually above 1600 HV, much higher than most metal materials. This means that the binding force between the surface atoms is very strong, and the incident ions require a higher energy to effectively knock the surface atoms out of the target. This strong interatomic binding force is one of the main reasons for the low sputtering efficiency of tungsten carbide targets.
2. High melting point: The melting point of tungsten carbide is about 2870°C, which is very high in metals and metal compounds. The high melting point means that the target is not easy to melt or soften during sputtering, which further increases the difficulty of atom removal during sputtering.
3. High density: The density of tungsten carbide is about 15.63 g/cm³, much higher than common metals such as aluminum (2.70 g/cm³) or titanium (4.51 g/cm³). The high density increases the mass of the target, and due to the dense arrangement of atoms, the incident ions need to overcome greater resistance to initiate effective sputtering. This is also another reason for the low sputtering yield of tungsten carbide targets.

1.3 Interaction between incident ion energy and tungsten carbide target during sputtering

During sputtering, the incident ions are usually high-energy ions (such as argon ions), which collide on the surface of the target and transfer energy to the target atoms. For tungsten carbide targets, due to its high atomic binding energy, only incident ions with high enough energy can effectively break the binding force between atoms, resulting in the sputtering of atoms.

However, too high an energy of incident ions can also have negative effects, such as:

Damage effect: Excessive ion energy may lead to structural damage on the surface of the target, and even cause structural changes on the surface of the target surface, which may affect the quality and uniformity of sputtering products.

Secondary effects: High-energy ions may trigger the emission of secondary electrons, which may interfere with the stability of the plasma and thus affect the overall efficiency of the sputtering process.

Therefore, in actual operation, the energy of incident ions needs to be optimized to minimize the adverse effects while maximizing the sputtering yield.

1.4 Comparison of sputtering efficiency of tungsten carbide targets with other common targets

In order to fully understand the sputtering efficiency of tungsten carbide targets, we need to compare it with several common metal targets (such as copper, aluminum, titanium). These metal targets are widely used in thin film deposition, and their sputtering efficiency has been thoroughly studied and validated in the industry.

Copper (Cu) target: Copper has a melting point of 1085°C and a density of 8.96 g/cm³, which is a low-melting point, high-density metal. During the sputtering process, the sputtering yield of the copper target is usually higher, mainly because the interatomic binding force of copper is weak, and the incident ion can easily remove the copper atom from the surface of the target. Experimental data show that the sputtering yield of copper can reach 2.0-3.0 atoms/ions, which is several times that of tungsten carbide.

Aluminum (Al) target: Aluminum has a melting point of 660°C and a density of 2.70 g/cm³, which is a light metal. During the sputtering process, the low density and melting point of aluminum make it easier for the surface atoms to be sputtered by incident ions. The sputtering yield of aluminum is usually between 1.0-2.0 atoms/ions, which, although lower than copper, is still significantly higher than tungsten carbide.

Titanium (Ti) target: Titanium has a melting point of 1668°C and a density of 4.51 g/cm³. As a transition metal, titanium shows moderate sputtering yield during the sputtering process. The atom binding energy of titanium is high, but due to its strong chemical activity, the titanium atoms formed during sputtering are easy to deposit on the substrate, and the sputtering yield is usually between 0.8-1.5 atoms/ions, which is close to tungsten carbide, but still slightly higher than tungsten carbide.

2. Sputtering characteristics of tungsten carbide targets

The sputtering characteristics of tungsten carbide targets play an important role in materials science and thin film deposition technology. The complexity of the sputtering process is due to the interaction of multiple physical and chemical factors, and a thorough understanding of these factors is essential to optimize process parameters and improve film quality. This part will be from the definition and measurement of sputtering yield, the specific sputtering yield analysis of tungsten carbide target, as well as the key factors affecting the sputtering process of three aspects of the in-depth analysis of the sputtering behavior of tungsten carbide target and its performance in practical applications.

2A. Definition and measurement of sputtering yield

2A.1 Definition of sputtering yield

Sputter Yield (Y) is a core parameter used to measure the efficiency of the sputtering process and is defined as the number of atoms per unit of incident ions that result in sputtering from the surface of the target. The expression of sputtering yield is usually expressed as:

Y=Nsputtered atomsNincident ionsY = \frac{N_{\text{sputtered atoms>}{N_{\text{incident ions>}Y=Nincident ionsN sputtered atoms

Where, Nsputtered atomsN_{\text{sputtered atoms>Nsputtered atoms are the total number of atoms sputtered from the surface of the target. Nincident ionsN_{\text{incident ions>Nincident ions is the total number of incident ions bombarded by the target.

The sputtering yield is not only related to the physical and chemical properties of the target, but also affected by the energy of incident ions, incident Angle, surface state of the target and other factors. For example, high-energy incident ions can usually break the atomic bonds on the surface of the target, thereby increasing the sputtering yield; The variation of incidence Angle may lead to nonlinear variation of sputtering yield.

2A.2 Calculation method of sputtering yield

The calculation of sputtering yield can be carried out through the combination of experimental measurement and theoretical simulation. In theory, methods such as Molecular Dynamics (MD) simulation and Monte Carlo (MC) simulation can be used to predict the sputtering yield. These methods take into account the interaction between incident ions and target atoms, energy transfer and multiple scattering effects, and can provide more accurate prediction of sputtering yield.

In the experiment, the measurement of sputtering yield is mainly achieved through the following methods:

1. Gravimetric method: By measuring the weight change of the target before and after sputtering, combined with the number of incident ions to calculate the sputtering yield. The accuracy of this method is high, but the sensitivity of the equipment is high, and it is suitable for the target with fast sputtering speed.
2. Film thickness method: Deposit a film on the substrate, and calculate the sputtering yield indirectly by measuring the growth rate of the film thickness. Commonly used thickness measurement techniques include ellipsometry measurement and X-ray reflection measurement. This method is suitable for the calculation of sputtering yield in the process of high precision film preparation.
3. Surface analysis technology: such as X-ray photoelectron spectroscopy (XPS) and secondary ion mass spectrometry (SIMS), these technologies determine the sputtering yield by analyzing the changes in the concentration of elements on the surface of the target, especially for the sputtering process of multi-layer targets or composite materials.

The combination of theoretical simulations and experimental measurements can provide a more comprehensive understanding of the sputtering yield. For example, theoretical simulations can predict sputtering yields under different conditions, while experimental measurements can verify these predictions, thus guiding the optimization of the actual process.

2B. Sputtering yield of tungsten carbide

2B.1 Sputtering yield analysis of tungsten carbide target

The sputtering yield of tungsten carbide (WC) target is usually low, which is closely related to its physical and chemical properties. WC has a high melting point (2870°C), high hardness (about 1600 HV) and high density (15.63 g/cm³), which makes WC exhibit high atomic binding energy and low atomic mobility during sputtering, thereby reducing its sputtering yield.

Under typical sputtering conditions, the sputtering yield of tungsten carbide targets is usually between 0.2-0.5 atoms/ions. This value is much lower than some common metal targets, but it is close to the sputtering yield of other high-hardness materials (such as titanium nitride, TiN).

The sputtering yield of tungsten carbide is also significantly affected by the process parameters. For example, under DC magnetron sputtering (DCMS) conditions, with the increase of sputtering power, the sputtering yield of WC increases, but its rise is limited, mainly because too high power may lead to an increase in the surface temperature of the target, which in turn leads to unstable sputtering behavior.

2B.2 Comparison of sputtering yield with other targets

Comparing tungsten carbide targets with other common targets helps to understand its uniqueness in sputtering process.

Copper (Cu) : The sputtering yield of copper is about 2.0-3.0 atoms/ions, which is significantly higher than tungsten carbide. The low melting point and high mobility of copper make it easy to remove atoms during sputtering.

Aluminum (Al) : The sputtering yield of aluminum is 1.0-2.0 atoms/ions. Although aluminum is a light metal, its atomic mass is low, but because of its low melting point and low binding energy, aluminum target in the sputtering process shows a high yield.

Titanium (Ti) : The sputtering yield of titanium is 0.8-1.5 atoms/ion, and the high binding energy and moderate melting point of titanium make its sputtering yield slightly lower than aluminum and copper, but still higher than tungsten carbide.

This comparison shows that tungsten carbide requires a higher energy input in the sputtering process, but it also provides a higher film quality, especially in applications that require extremely high hardness and wear resistance.

2C. Factors affecting tungsten carbide sputtering

2C.1 Effect of target temperature on sputtering efficiency

Target temperature is one of the important parameters that affect the sputtering process. Increasing the temperature of the target usually reduces the binding force between the atoms on the surface of the target, making it easier for the atoms to be sputtered out. However, for tungsten carbide targets, due to its high melting point, the influence of temperature is more complex.

Low temperature sputtering: At lower temperatures, the sputtering yield of tungsten carbide targets is usually lower because the target surface atoms have higher binding energy and are difficult to remove.

High temperature sputtering: With the increase of temperature, the sputtering yield may increase, but it should be noted that too high temperature may lead to thermal stress or micro-cracks on the surface of the target, which will affect the uniformity and quality of the sputtered film. Therefore, in the sputtering process of tungsten carbide target, it is necessary to find a balance point on temperature control, which can not only improve the sputtering efficiency, but also ensure the quality of the film.