Lecture 22 (CHE 323) Sputtering, part 1

Summary

In this lecture, Chris Mack introduces sputter deposition as a superior alternative to evaporation for depositing metals and dielectrics onto wafers. Sputtering offers better uniformity and film quality. The process involves using an inert gas like argon to create a plasma that sputters material from a target onto the wafer. Key factors discussed include pressure's impact on deposition rate and mean free path, achieving good step coverage through random deposition angles, and optimizing sputter yield for higher throughput. Magnetron sputtering, which uses a magnetic field to increase ion density and deposition rates, is highlighted as a popular technique.

Takeaways

• 🔬 Sputtering, or sputter deposition, is now the dominant technique for metal deposition, solving issues like uniformity and film quality seen in evaporation.
• ⚙️ A sputtering system consists of two parallel plates, where a voltage is applied. The cathode, made of the material to deposit, is bombarded by ionized argon gas, causing sputtering.
• 🌬️ Argon gas is commonly used in sputtering because it’s inert, ensuring the process remains physical rather than chemical.
• 🔋 The system uses either DC power (ideal for metals) or RF power (used for dielectrics) to ionize the argon gas and create plasma.
• 🧲 Pressure inside the sputtering chamber affects deposition rate and mean free path, influencing the uniformity and step coverage of the deposited material.
• 📏 Sputter yield is the number of target atoms ejected per argon ion. Optimizing ion energy is critical for maximizing this yield, typically between 1 and 2.
• 🚀 High ion energy can lead to ion implantation instead of sputtering, which is not ideal for deposition, requiring a balanced energy range for sputter yield.
• 🎯 Good step coverage is achieved by sputtering due to atoms arriving at various angles, improving upon the line-of-sight limitations of evaporation.
• 🔧 Magnetron sputtering enhances ion density by adding a magnetic field, increasing deposition rates without raising chamber pressure.
• 📚 Key concepts include the basic components of a sputtering system, comparison with evaporation, and understanding magnetron sputtering's role in improving deposition rates.

Q & A

What is the primary reason for sputtering being preferred over evaporation in metal deposition?

Sputtering is preferred over evaporation because it provides better control of film quality and uniformity. It also resolves many issues associated with evaporation, making it the dominant technique for depositing most metals and some dielectrics.

How does sputtering deposition work in general?

Sputtering works by applying a voltage across two parallel plates with a target material as the cathode. An inert gas, usually argon, is ionized to form a plasma. The ionized argon is accelerated towards the target, causing atoms to be knocked off (sputtered) and deposited onto the wafer.

Why is argon gas used in sputtering, and what is the advantage of it being chemically inert?

Argon is used because it is chemically inert, ensuring that the sputtering process remains physical rather than chemical. This makes it a part of the physical vapor deposition (PVD) category, allowing for a clean, controlled deposition of material without unwanted chemical reactions.

What is meant by 'step coverage' in the context of sputtering, and how does it differ from evaporation?

Step coverage refers to the ability of the deposition process to coat both the flat surfaces and sidewalls of features on a wafer. In sputtering, material arrives at a range of angles due to multiple collisions in the gas, improving step coverage compared to evaporation, which is more line-of-sight.

What role does pressure play in sputtering, and how does it impact deposition rates?

Pressure affects the deposition rate by controlling the number of argon ions available for sputtering. Higher pressure increases the number of collisions, but too much pressure can reduce the mean free path and result in excessive scattering, lowering efficiency. The optimal pressure balances ion generation and collision frequency.

What is sputter yield, and what factors influence it?

Sputter yield is the ratio of target atoms released per ion that hits the target. It is influenced by the ion energy—low energies result in no sputtering, while too high energies lead to ion implantation rather than sputtering. Optimal sputter yield occurs at a specific range of ion energies, typically in the tens of electron volts.

Why is magnetron sputtering used, and how does it enhance the deposition process?

Magnetron sputtering is used to increase the density of argon ions without raising the pressure. It introduces a magnetic field that causes electrons to spiral in the plasma, increasing the likelihood of collisions with argon atoms and enhancing the overall ion density and deposition rate.

How does ion energy impact sputtering efficiency?

Ion energy must be carefully controlled for effective sputtering. Too low of an energy prevents atoms from being knocked off the target, while too high of an energy leads to ion implantation rather than sputtering. The ideal energy is between these extremes, maximizing the sputter yield and deposition rate.

What are some limitations of sputtering compared to evaporation in terms of deposition angle and coverage?

Although sputtering improves step coverage compared to evaporation, it still has some shadowing effects. Sidewalls of features do not receive material from as wide a range of angles as flat surfaces, resulting in imperfect coverage in certain geometries.

What is the relationship between sputter yield and ion flux, and how does this affect throughput?

Throughput in sputtering is determined by both the sputter yield (number of atoms released per ion) and the ion flux (number of ions hitting the target). To maximize throughput, it is important to optimize both the sputter yield and the ion flux, which is often achieved by controlling pressure and using techniques like magnetron sputtering.