Unit 5: Applied Plasma & Vacuum Techniques (Lab: PHYDSC353P)
Table of Contents
1. Laboratory Objectives
This unit transitions from basic plasma observations to the industrial and research applications of plasma environments. You will learn to manage vacuum systems—a prerequisite for any plasma experiment—and study how plasma interacts with surfaces and external fields.
2. Measurement of Pumping Speed of a Vacuum Pump
A Vacuum Pump (Rotary or Diffusion) is characterized by its pumping speed (S), which is the volume of gas removed from the system per unit time.
Working Principle:
The speed is determined by observing the rate of pressure drop in a chamber of known volume (V).
Where P1 and P2 are the pressures at the beginning and end of time interval t. A plot of log(P) vs t yields a straight line from which S is calculated.
3. Verification of Paschen’s Law
Paschen’s Law defines the Breakdown Voltage (Vb)—the voltage required to start a discharge or "spark" between two electrodes in a gas—as a function of the product of gas pressure (p) and distance between electrodes (d).
Key Observation: There is a unique minimum voltage (Vmin) at a specific value of p · d. Below this value, it's hard to find an electron; above it, electrons lose too much energy in collisions to cause ionization.
4. Magnetic Confinement (Magnetic Mirror)
In a plasma, charged particles gyrate around magnetic field lines. A Magnetic Mirror is a configuration where the magnetic field is stronger at the ends of a region than at the center.
Physical Significance:
Particles with sufficient magnetic moment are reflected back toward the center of the region when they encounter the "stronger" field at the ends. This is a fundamental technique used in fusion research to keep high-temperature plasma away from reactor walls.
5. Principles of Plasma Etching and Deposition
Plasma is used extensively in the semiconductor industry to modify surfaces at the nanometer scale.
- Plasma Etching: Reactive species in the plasma (ions and radicals) react with the surface of a wafer to remove material in a highly directional (anisotropic) manner.
- Sputter Deposition: Ions from the plasma strike a "target" material, knocking atoms off that then land on a substrate to form a thin film.
Lab Exam Focus Corner
Frequently Asked Questions
- What is 'Outgassing'? The release of trapped gas molecules from the inner surfaces of a vacuum chamber. This often limits the ultimate pressure a pump can reach.
- Why does Paschen's curve have a minimum? Because at very low pressures (p · d is small), there are too few atoms to collide with; at high pressures, the mean free path is too short to gain sufficient energy for ionization.
Common Mistakes
- Leaking Valves: In pumping speed experiments, even a tiny leak in the "needle valve" or gaskets will make the pumping speed appear much lower than it actually is.
- High Voltage Safety: Paschen's law experiments often involve voltages over 1000V. Always ensure the "HT" (High Tension) supply is turned off before touching any electrodes.
Practical Tips
Tip: In vacuum experiments, always clean the gaskets with Isopropyl Alcohol (IPA) and apply a very thin layer of high-vacuum grease (like Apiezon) to ensure a hermetic seal.