This lecture, Part 1 of a two-part series, introduces sputter deposition as a superior and dominant technique for depositing metals and some dielectrics in micro and nanofabrication, addressing uniformity, control, and film quality issues inherent in older methods like evaporation. The presentation details the fundamental components of a parallel-plate sputtering system, emphasizing the use of an inert argon plasma to physically dislodge target material for deposition onto a wafer. Key parameters such as pressure, its impact on mean free path and resulting "good step coverage" (due to randomized particle trajectories), and sputter yield (atoms released per ion impact) are thoroughly explained, including the critical ion energy dependence. Finally, the lecture introduces magnetron sputtering, a method employing a magnetic field to increase plasma ion density and achieve higher deposition rates by forcing electrons into spiral paths, thus enhancing collision probability without increasing chamber pressure.
Summarizing Sputter Deposition: Principles and Applications
0:00 Sputter Deposition: A Superior Alternative: Sputter deposition is introduced as the dominant and improved technique for depositing most metals and some dielectrics in micro and nanofabrication, largely solving uniformity, control, and film quality issues seen in evaporation.
1:19 Basic Sputtering System Components: The simplest sputtering system consists of two parallel plates. The cathode is made of the material to be deposited (the target), and the wafer is placed on the anode. An inert gas (usually argon) is introduced to form a plasma.
3:12 The Sputtering Process (PVD): Argon gas is ionized to create a plasma. A negative voltage is applied to the cathode, accelerating the ionized argon towards the target. The physical impact ("sputtering") dislodges target material, which then randomly floats and deposits onto the wafer. This is classified as Physical Vapor Deposition (PVD).
4:03 Power Sources: DC power is preferred for metal deposition, while RF (radio frequency) power is beneficial for dielectrics.
5:06 Impact of Pressure on Deposition: Chamber pressure affects both the deposition rate (higher pressure means more argon ions, thus faster sputtering) and the mean free path of sputtered atoms.
5:53 Good Step Coverage: High pressure results in a short mean free path (around 0.1 Torr yields millimeter-scale mean free paths). This leads to many collisions, causing sputtered material to arrive at the wafer from random angles, which provides better "step coverage" compared to line-of-sight deposition methods like evaporation.
8:27 Sputter Yield Explained: Sputter yield is defined as the ratio of target atoms released per argon ion hitting the target. It is highly dependent on ion energy.
Threshold Energy: Below a few tens of electron volts (eV), the yield is zero.
Optimal Energy: There's an optimal ion energy that maximizes yield, typically aiming for 1 to 2 atoms per ion.
Ion Implantation vs. Sputtering: Too high energy (e.g., > few kilovolts) can lead to ion implantation instead of sputtering, causing the yield to drop to zero.
10:42 Increasing Throughput with Ion Flux: Deposition rate is a function of sputter yield and the flux (density) of ions hitting the target. Increasing pressure can increase ion flux but leads to more collisions and shorter mean free paths.
11:53 Magnetron Sputtering for Higher Rates: To increase ion density without increasing pressure, magnetron sputtering adds a magnetic field parallel to the wafer and perpendicular to the electric field. This forces electrons into spiral paths, significantly increasing their path length and thus their probability of colliding with argon atoms to create more ions. This results in a higher density plasma and faster deposition rates.
14:42 Key Learning Outcomes: After this lecture, viewers should be able to describe basic sputter system components, how good step coverage is achieved, define sputter yield, and explain magnetron sputtering.