Laboratory plasma etchers, also known as plasma cleaners or plasma ashers, are essential tools in modern scientific research and industrial applications, providing an efficient and effective method for cleaning, etching, and modifying surfaces at the microscopic level. These devices harness the power of plasma, the fourth state of matter, to achieve results unattainable through traditional cleaning methods. This article introduces the fundamentals of laboratory plasma cleaners, their operating principles, types, applications, and benefits.
Understanding Plasma and Its Uses
Plasma is often described as an ionized gas, containing a mixture of electrons, ions, and neutral particles. It’s created by adding energy (such as heat or electromagnetic fields) to a gas, which ionizes its molecules. In a laboratory setting, plasma cleaning leverages this high-energy state to remove contaminants and modify surface properties.
Operating Principle of Plasma Etcher/Cleaner
Laboratory plasma etchers/cleaners work by generating a plasma field within a confined space, usually a chamber. The process typically involves:
- Gas Introduction: A small amount of gas (like argon, oxygen, or nitrogen) is introduced into the vacuum chamber.
- Ionization: An electric field, created by radio-frequency (RF) power, ionizes the gas, forming plasma.
- Surface Interaction: The energetic particles in the plasma interact with the surface of the material placed inside the chamber, effectively cleaning or etching it.
Types of Plasma Cleaners/Etchers in the market
There are two major types of plasma cleaners/Etchers in the market: vacuum plasma system or atmospheric plasma jet. Here are the summary comparison of these two types of plasma equipment.
1. Vacuum plasma cleaner/etcher:
- Design: This type of plasma system operates in a vacuum chamber. The objects to be etched are placed inside the chamber, and the air is evacuated. Plasma is generated inside this controlled environment, typically using radio-frequency (RF) or microwave energy.
- Applications: Vacuum plasma etcher are widely used in microelectronics, semiconductor industries, and material sciences for cleaning and surface preparation.
- Pros: Offers a highly controlled environment, which is crucial for sensitive applications. Capable of handling complex cleaning tasks.
- Cons: More expensive and requires a vacuum setup, which can be cumbersome for larger objects.
2. Atmospheric Pressure Plasma Jet (APPJ):
- Design: APPJ devices generate plasma at atmospheric pressure and direct it through a nozzle towards the surface to be treated. They often use gases like argon or helium.
- Applications: Used in medical device manufacturing, textile industry, and for surface activation before bonding or printing.
- Pros: More versatile and easier to integrate into production lines. Suitable for treating specific areas on larger or irregularly shaped objects.
- Cons: Less control over the plasma environment compared to vacuum systems, which can affect consistency and uniformity.
Different choice of rf frequencies used in the plasma equipment
There are four major types of plasma generation power supplies: DC glow discharge power supply, kHz (kilohertz), MHz (megahertz), and GHz (gigahertz) power supplies. They primarily differ in their frequency ranges and the methods used to generate them. Each type has unique characteristics and applications.
- DC glow discharge: DC glow discharge is created by applying an electric field between two electrodes in a gas at low pressure. The electric field ionizes the gas, creating plasma. Plasma generated in DC glow discharge usually have very low density. It can be used for treatment of samples that only need very gentle surface activation. The cost of DC glow discharge plasma system is very low.
- KHz Range: Low-cost option of plasma generation, lower efficiency, less precise control. Many KHz plasma systems only offer low, medium or high power settings or a rough knob adjustment. Due to long period in each cycle, both ions and electrons can be accelerated to very high energy before the electric field reverse direction. High energy and long mean free path will cause them to hit the chamber wall or electrodes, and disappear before they have any chance to ionize or dissociate the neutral atoms or molecules. Therefore, KHz plasma system are usually orders of magnitude weaker than 13.56MHz plasma system at the same rf power setting.
- MHz Range: The most common frequency range for plasma generation, offering a good balance of control, efficiency, and uniformity. Ions are too slow to respond to the MHz rf electric field. The wavelength of 13.56MHz rf is 22.12 meter, which is significantly larger than the dimensions of the plasma chamber. Thus, 13.56MHz plasma system have very high efficiency and good uniformity.
- GHz Range: Microwave plasma system can achieve high plasma intensity at high microwave wattage. But the wavelength of the GHz microwave is usually on the same scale or shorter than the plasma chamber dimension. Standing waves will result in localized hot spot and cause very bad uniformity. Therefore, microwave plasma system usually works in remote/downstream mode. Plasma source is separated from the sample chamber.
How to operate a Vacuum Plasma System
Operating a vacuum plasma etcher/cleaner typically involves a series of steps to ensure safe and effective cleaning of materials using plasma technology. Here’s a generalized introduction:
- Preparation: Begin by preparing the items you intend to clean. Make sure they are compatible with plasma recipe intended to be used. If the samples are in powder form or have many loose or light parts, the pumping speed or venting speed should be limited to prevent the samples from being blown away by the strong air flow.
- Loading: Open the chamber of the plasma cleaner and carefully place the items inside. It’s crucial to arrange them so that the plasma can reach all surfaces you intend to clean.
- Sealing the Chamber: Check the top surface of the front door o-ring and make sure there is no dust particles on the o-ring. Close and securely seal the chamber. This step is vital to ensure a proper vacuum can be formed.
- Evacuation: Turn on the vacuum pump to evacuate air from the chamber. This process creates the low-pressure environment necessary for plasma formation.
- Gas Introduction: Introduce the process gas (commonly argon, oxygen, or a mixture) into the evacuated chamber. The type and amount of gas depend on the cleaning requirements.
- Plasma Generation: Activate the plasma generator. This typically involves applying a high-frequency electrical field to ionize the gas, creating plasma. The chemically reactive radicals or energetic ions interact with the surfaces, effectively cleaning them. In surface activation applications, plasma can also generate and deposit functional groups, such as hydroxyl functional groups (OH*), on the sample surface.
- Completion: After the cleaning cycle, which usually lasts a few minutes, deactivate the plasma generator and allow the chamber to return to atmospheric pressure. If CF4, SF6 type gas has been used to generate the plasma, its’ recommended to include a chamber flushing steps to totally remove the fluorine radicals in the chamber before opening the chamber for unloading.
- Unloading: Open the chamber and carefully remove the cleaned items.
For Tergeo series plasma systems, most of the operations have been fully automated except the sample loading and unloading part of the operation.
Impedance Matching Design
- Automatic: Automatically adjusts to optimize power transfer from the RF generator to the plasma, ensuring efficient plasma generation. Ideal for processes requiring consistent plasma conditions without manual intervention.
- Fixed: Set to a specific value and does not adjust during operation. Suitable for applications with stable and predictable plasma requirements. But the operation pressure range and gas species is very limited. Fixed impedance matching can be optimized for a certain condition. But the plasma density will suffer for all the other conditions.
- Manual: Allows the operator to manually tune the impedance. Offers flexibility but requires more expertise and attention during operation. It could results in inconsistent results.
Gas Delivery Methods
- Needle Valve: Basic, manual control of gas flow, offering rudimentary adjustment without precise measurement.
- Manual Rotameter: Allows visual monitoring and manual adjustment of gas flow, offering more accuracy than a needle valve.
- Mass Flow Controller (MFC): Provides precise, automated control over gas flow, essential for repeatable and consistent processes.
Plasma Chamber Materials
- Quartz: Offers high thermal stability and chemical inertness. Ideal for UV radiation processes.
- Pyrex Glass: A cost-effective option with good thermal resistance and moderate chemical resistance.
- Aluminum: Durable and easy to machine, but susceptible to chemical reactivity and oxidation.
- Manual Button Control: Basic and straightforward, using buttons to control the plasma cleaner. Suitable for simpler systems or less complex processes.
- Touch Screen Control with Recipes: Advanced interface allowing for programmable processes and recipe storage. Enhances repeatability and ease of use for complex procedures.
Process Control Techniques
- Pressure Sensor: Monitors chamber pressure, crucial for maintaining consistent plasma conditions.
- Gas Flow Sensor: Ensures precise control over gas introduction, important for process repeatability.
- Plasma Intensity Sensor: Monitors the intensity of the plasma, providing feedback for process optimization and control.
Applications of Laboratory Plasma Cleaners
- Surface Cleaning: Removal of organic contaminants from various substrates, essential in microscopy, semiconductor fabrication, and materials science.
- Surface Activation: Increasing the surface energy of materials to improve adhesion properties, crucial in medical device manufacturing and polymer bonding.
- Etching: Precisely removing material layers, used in microfabrication and MEMS (Micro-Electro-Mechanical Systems) production.
Benefits of Using Plasma Cleaners
- Effective Cleaning: Plasma cleaning is more thorough and efficient than traditional chemical or physical methods.
- Environmentally Friendly: Typically uses less hazardous chemicals compared to conventional cleaning methods.
- Versatility: Suitable for a wide range of materials, including metals, glass, ceramics, and polymers.
- Precision: Ideal for applications requiring high precision, such as in the electronics and nanotechnology fields.
Laboratory plasma cleaners/etchers represent a sophisticated and versatile tool in the arsenal of modern science and industry. By leveraging the unique properties of plasma, these devices offer unparalleled cleaning, etching, and surface modification capabilities. Whether for intricate semiconductor fabrication or routine laboratory cleaning, plasma cleaners provide an efficient, environmentally friendly, and precise solution to meet the evolving demands of scientific research and industrial applications.
- “Chapter 8 – Fundamentals and Applications of Plasma Cleaning”, Dinesh P.R. Thanu et al, Developments in Surface Contamination and Cleaning: Applications of Cleaning Techniques. 2019, Pages 289-353
- “Low-pressure plasma cleaning: a process for precision cleaning applications”, W Petasch a et al, Surface and Coatings Technology, Volume 97, Issues 1–3, December 1997, Pages 176-181
- “A comparative analysis on physical and chemical plasma cleaning effects on surfaces”, Wong Jun Hao et al, 2013 IEEE 15th Electronics Packaging Technology Conference,
- “Plasma cleaning and the removal of carbon from metal surfaces”, M.A. Baker et al, Thin Solid Films, Volume 69, Issue 3, 1 July 1980, Pages 359-368
- “Plasma Cleaning of Surfaces”, W. W. Balwanz, Surface Contamination. pp 255–269
- “Process control of RF plasma assisted surface cleaning”, H Steffen et al, Thin Solid Films, Volume 283, Issues 1–2, 1 September 1996, Pages 158-164