Introduction to Plasma Cleaner, Plasma Etcher and Plasma Asher

Laboratory plasma etchers and plasma asher, often referred to as plasma cleaners when used for surface treatment, have become indispensable tools across scientific research and industrial sectors. These devices offer an efficient and precise method to clean, etch, and modify surfaces down to the microscopic level, harnessing the unique properties of plasma – the fourth state of matter. Traditional cleaning techniques simply cannot match the capabilities that plasma etchers/cleaners provide in precision control at nanometer scale. This article delves into the fundamentals underpinning these powerful tools, exploring their operating mechanisms, the various types available, their diverse applications, and the key benefits they offer.

Tergeo plasma cleaner, plasma etcher
Laboratory plasma cleaner
Tergeo-Pro Plasma Etching Equipment
Tabletop plasma equipment

Understanding Plasma and Its Uses

To grasp how plasma etchers/cleaners work, we must first understand the nature of plasma itself. Essentially an ionized state of gas, plasma contains a mixture of electrons, ions, and neutral particles. It is generated by introducing energy sources like electromagnetic fields to a gas, which causes the gas molecules to become ionized. In laboratory applications, it is this energetic state of ionized particles that allows plasma to effectively remove surface contaminants and modify the properties of various materials.

Operating Principle of Plasma Etcher/Cleaner

Laboratory plasma cleaners work by generating a plasma field within a confined space, usually a vacuum chamber or an enclosure. The process typically involves:

  1. Gas Introduction: A small amount of gas (like argon, oxygen, or nitrogen) is introduced into the vacuum chamber for vacuum plasma system. For atmospheric pressure plasma system, much higher flow of gas is required.
  2. Ionization: An electric field, created by radio-frequency (RF) power, ionizes the gas, generating plasma.
  3. Surface Interaction: The energetic particles in the plasma interact with the surface of the material placed inside the chamber, effectively cleaning or etching it.

The controlled environment of the vacuum chamber in a vacuum plasma etcher/cleaner allows precise control of the plasma conditions and more uniform plasma distribution to achieve the desired surface modification or cleaning effect on the target material.

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:

  1. Design: This type of plasma cleaner operates in a vacuum chamber. The samples to be cleaned 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
  2. Applications: Vacuum plasma etcher are widely used in microelectronics, semiconductor industries, and material sciences for cleaning and surface preparation.
  3. Pros: Vacuum plasma systems offer a highly controlled environment, which is crucial for sensitive applications. Capable of handling complex cleaning tasks. The gas flow, vacuum level in a vacuum plasma system can be precisely controlled. Vacuum chamber offers a contained environment. Unlike the atmospheric plasma jet, the plasma can be generated over the whole volume of the vacuum chamber, which allow it to process the expoed sample surface uniformly without scanning. 
  4. Cons: Vacuum plasma system are generally more expensive and requires a vacuum setup. Due to the size of the vacuum chambr, it can’t handle large samples. 

2. Atmospheric Pressure Plasma Jet (APPJ):

  1. 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, or mixture of other gasses.
  2. Applications: Used in medical device manufacturing, textile industry, and for surface activation before bonding or printing.
  3. Pros: More versatile and easier to integrate into production lines. Suitable for treating specific spot areas on larger or irregularly shaped objects. APPJ is well suited to carry out spot treatment on large samples.
  4. Cons: Less control over the plasma environment compared to vacuum systems, which can affect consistency and uniformity. Spot beam has intrinsically bad uniformity. To cover large areas, multiple heads or scanning could be required. The gas flow rate required in atmospheric plasma jet is many orders of magnitude higher than the vacuum plasma system. The operation cost of the speciality gas could be significant.

Different choice of rf frequencies used in the plasma equipment

The choice of radio frequency (RF) used in plasma equipment plays a crucial role in determining the characteristics and performance of the generated plasma. Among the key factors influencing plasma systems’ design, the type of RF power supply employed is often more significant than the absolute wattage rating. A 13.56 MHz plasma system operating at a mere 15 watts can generate a much stronger plasma than a 40 kHz system operating at 400 watts.

Plasma generation power supplies can be broadly categorized into four main types: DC glow discharge, kilohertz (kHz) range, megahertz (MHz) range, and gigahertz (GHz) range. Each type has distinct frequency ranges and generation methods, resulting in 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 cleaner typically involves a series of steps to ensure safe and effective cleaning of materials using plasma technology. Here’s a generalized introduction to the operation of a vacuum plasma system:

  1. 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.
  2. 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. In some situations, you may have to clean the sample multiple times to make sure all the important surface are expoed to the plasma.
  3. Sealing the Chamber: CCheck 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. Dust particles or sharp debris on the surface of o-ring can cause vacuum leak or even damage the surface of the o-ring.
  4. Evacuation: Turn on the vacuum pump to evacuate the gas from the chamber. This process creates the low-pressure environment necessary for plasma ignition.
  5. 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. In automatic plasma systems, user can simply specify the gas flow rate in the recipes for each channels.
  6. Plasma Etching and Cleaning: Activate the rf power supply and ignite the plasma. 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.
  7. 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.
  8. Unloading: Vent the chamber with clean air, nitrogen or even argon to atmospheric pressure, then open the chamber door 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.

The operation of the fully automatic Tergeo series plasma cleaner through the touchscreen user interface

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 wall offers high thermal stability and chemical inertness. In system with quartz chamber wall, the high voltage rf electrodes can be placed outside the plasma volume, which increases the usable volume and improves the plasma uniformity.
  • Pyrex Glass: A cost-effective option with good thermal resistance and moderate chemical resistance.
  • Aluminum: Durable and easy to machine. Electrodes must be placed inside the plasma volume, which reduces the usable volume of the plasma. Plasma density will be significantly higher in regions close to the rf electordes and results in wrose uniformity. High voltage electrodes in plasma attracts high energy ions and cause more ion sputtering of the metal electrodes. The sputtered metal can redeposit on the samples surface and cause metal contamination issues for applications where trace amount of metal contamition is a concern. 

User Interfaces

  • 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. More suitable for production runs to achieve repeatable and consistent results. Automatic system is also much easier to operate and requires less training.

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

  1. Surface Cleaning: Removal of organic contaminants from various substrates, essential in microscopy, semiconductor fabrication, and materials science.
  2. Surface Activation: Increasing the surface energy of materials to improve adhesion properties, crucial in medical device manufacturing and polymer bonding.
  3. Etching: Precisely removing material layers, used in microfabrication and MEMS (Micro-Electro-Mechanical Systems) production.

Benefits of Using Plasma Cleaners

  1. Effective Cleaning: Plasma cleaning is more thorough and efficient than traditional chemical or physical methods.
  2. Environmentally Friendly: Typically uses less hazardous chemicals compared to conventional cleaning methods.
  3. Versatility: Suitable for a wide range of materials, including metals, glass, ceramics, and polymers.
  4. Precision: Ideal for applications requiring high precision, such as in the electronics and nanotechnology fields.

Conclusion

Laboratory plasma etchers (plasma cleaners) are sophisticated and versatile tools for many modern science and industrial applicaitons. By leveraging the unique properties of plasma, these devices offer unparalleled precision control in cleaning, etching, and surface modification capabilities at microscale. 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.

Reference

  1. Chapter 8 – Fundamentals and Applications of Plasma Cleaning”, Dinesh P.R. Thanu, Development in Surface Contamination and Cleaning: Applications of Cleaning Techniques. 2019, Pages 289-353
  2. 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
  3. A comparative analysis on physical and chemical plasma cleaning effects on surfaces”, Wong Jun Hao et al, 2013 IEEE 15th Electronics Packaging Technology Conference,
  4. 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
  5. Plasma Cleaning of Surfaces”, W. W. Balwanz, Surface Contamination. pp 255–269
  6. “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

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