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With the rapid development of today's technology, plasma technology has been widely used in many professional fields, and has become more and more important.
At present, the cleaning industry has higher and higher requirements for cleaning. In some occasions, conventional cleaning can not meet the requirements. Plasma cleaning is ideal for solving these precision cleaning requirements, and it is in line with the current environmental protection situation.
1 Plasma Cleaning 1.1 Plasma Cleaning Mechanism Plasma is an ionized gas having substantially equal densities of positive ions and electrons. It consists of ions, electrons, free radicals, photons, and neutral particles. It is the fourth state of matter.
Plasma cleaning relies on various cleaning methods for substances in the "plasma state", which is the most thorough stripping cleaning of all cleaning methods.
In terms of reaction mechanism, plasma cleaning generally includes the following processes: an inorganic gas is excited to a plasma state; a gas phase substance is adsorbed on a solid surface; an adsorbed group reacts with a solid surface molecule to form a product molecule; and the product molecule resolves to form a gas phase The reaction residue leaves the surface.
The biggest feature of plasma cleaning is that it can be processed regardless of the object to be processed and the type of substrate, and the cleaning of the whole and partial and complex structures can be realized. Plasma cleaning is conducive to environmental protection. Only trace gases are used in the work, and no pollutants are discharged. The plasma cleaning process is low in cost, easy to use, can handle various materials, and has good uniformity and repeatability. Plasma cleaning is a type of dry cleaning, and the main advantages compared to wet cleaning are shown in Table 1.
Table 1 Plasma cleaning and wet chemical cleaning compared to plasma cleaning Wet chemical cleaning process is easy to control time and chemical solvent has a greater impact on the process. One wash, almost no residue may need further removal and treatment or need more Step cleaning reaction by-product is gas, which can be discharged by vacuum pumping. A large amount of waste is required to be further processed. The gas required for the reaction is mostly non-toxic. Most solvents and acids are quite toxic. 2 Plasma cleaning is classified into plasma treatment processes, including chemical reactions and Physical reaction two cleaning processes.
1.21 Chemical Process In a chemical plasma process, a free radical molecule chemically reacts with an element on the surface of the object to be cleaned. The products after these reactions are very small, volatile molecules which can be withdrawn by a vacuum pump. In organic cleaning applications, the main by-products generally include water, carbon monoxide and carbon dioxide.
Plasma cleaning based on chemical reaction, fast cleaning, good selectivity, and most effective in removing organic pollutants. The disadvantage is that it will oxidize on the surface. A typical chemical plasma cleaning uses an oxygen plasma.
1.22 Physical Process In the physical process, the physical cleaning process of atomic and ion bombardment with high energy and high speed requires high energy and low pressure. Atoms and ions are brought to maximum velocity before bombarding the surface of the object to be cleaned.
Because of the need to accelerate the plasma, high energy is required so that the speed of atoms and ions in the plasma can be higher. Low pressure is required to increase the average distance between atoms before collision between atoms. This distance refers to the mean free path. The longer this path, the higher the probability of bombarding the ions on the surface of the object to be cleaned.
Plasma cleaning based on physical reactions does not chemically react itself, and does not leave any oxides when cleaning the surface of the sample. The disadvantage is that it will damage the surface and have a large thermal effect. A typical physical plasma cleaning is argon plasma cleaning.
1.3 Factors Affecting Plasma Cleaning 1.31 Process Gas and Related Parameters Argon Gas: Physical bombardment is the mechanism of argon gas cleaning. Argon is the most effective physical plasma cleaning gas because of its large atomic size. The surface of the sample can be bombarded with great force. Positive argon ions will be attracted to the negative electrode plate. The impact force is sufficient to remove any dirt on the surface. These gaseous contaminants are then discharged through a vacuum pump.
Oxygen: The plasma reacts with compounds on the surface of the sample in a chemical process. For example, organic contaminants can effectively remove oxygen plasma from contaminants with oxygen plasma to produce carbon dioxide, carbon monoxide and water. In general, chemical reactions are better at removing organic pollutants.
Hydrogen: Hydrogen can be used to remove metal surface oxides.
It is often used in combination with argon to increase the removal rate. There is a general concern about the flammability of hydrogen, and the amount of hydrogen used is very small. The bigger worry is the storage of hydrogen. We can use a hydrogen generator to generate hydrogen from water. Thereby removing the potential harm.
CF4/SF6: Fluorinated gases are widely used in the semiconductor industry and in the PWB (Printed Circuit Board) industry. There is only one application in IC packaging. These gases are used in the PADS process, by which oxides are converted to oxyfluorides, allowing flowless soldering.
Pressure: The pressure in the reaction chamber is a function of the process gas flow rate, the chamber leakage rate, and the pumping speed. The choice of process gas determines the plasma cleaning mechanism (physical, chemical or physical chemistry).
The lower pressure of the physical process. Physical plasma cleaning requires that excited particles collide with the substrate surface. If the process pressure is high, the excited particles will collide with other particles many times before reaching the cleaning surface, losing energy. However, if the pressure is reduced too much, there will not be enough active reactive components to clean the substrate in a reasonable amount of time.
The chemical process relies on the chemical reaction of the gas plasma with the surface of the substrate, so the greater the number of reactive particles, the greater the cleaning power and the higher the pressure. Higher pressures allow the chemical process to have a faster cleaning rate.
Power: Increasing plasma power increases cleaning speed and cleaning strength by increasing the ion density and ion energy within the plasma. However, if the power is increased too much, it may be harmful to the substrate and will be ineffective for the result.
Time: Generally speaking, the best cleaning effect is achieved in the shortest process time, which is the balance of process time and power, pressure and gas type. The power setting is mainly balanced with the processing time. Increasing the power can reduce the processing time, however, it will increase the temperature inside the chamber. For many processing processes, higher temperatures are equivalent to catalysts and can result in uneven or excessive etching. Therefore, in order to optimize the process, it is necessary to consider these two parameters comprehensively.
1.32 Plasma Cleaning Mode Plasma cleaning equipment is designed with three types of electrode holders used as the anode, cathode and suspension poles of the system. Depending on the application, moving and adjusting the electrode holder can produce two modes of plasma, direct plasma mode and downstream plasma mode.
The direct plasma mode is placed between the anode and the cathode. In this configuration, all positive and negative ions will flow through the two poles without isolation, which is the most aggressive mode. The cleaning sample can be placed on the anode or on the cathode and placed on the anode for the most intense cleaning.
The downstream plasma mode is the mounting mode of the anode, cathode, and floating pole. In this configuration, only positive ions will reach the floating pole and negative ions will be trapped by the positive electrode. This configuration produces the weakest plasma for cleaning some of the electrical components, MOS components, and the like.
1.33 Effects of Load on Plasma Cleaning Sometimes a plasma that works well on one or two components does not work in a fully loaded warehouse. This is the effect of the load on him for two main reasons: many materials are Degassing under vacuum, gas is generated from the surface of the material, replacing the reaction gas to hinder the process. If this happens, the pump's pumping time will be extended and the set process pressure will be significantly higher than when the tank is empty.
Material additions and water vapor will cause degassing. For example, the degassing of a sponge is stronger than the degassing of a solid material of the same size.
All materials are made up of their respective degassing capabilities. This is a serious problem because of chemical reactions in plasma cleaning. If there is a chemical equilibrium point, at this point, the chemical reaction or cleaning will slow down or stop. To solve this problem, we should try to change the gas flow rate and the pumping capacity of the pump.
Another problem affecting the load is related to the number of shelves or the conductivity of the material. As the shelf increases, the energy of the RF is distributed over a larger area, resulting in a lower density of the plasma, which is often overcome by increasing processing time, increasing power or gas pressure.
2 Low-temperature plasma cleaning equipment Low-pressure plasma below several hundred Pa is often in a non-thermal equilibrium state. At this time, electrons hardly lose energy during collision with ions or neutral particles. We call such plasma low temperature. plasma.
Plasma cleaning equipment for my production. Its functional components include: reaction chamber, mechanical frame, gas supply system, vacuum system, RF power supply and control system.
21 Equipment Introduction 21.1 The reaction chamber adopts a flat structure, which is favorable for uniform plasma distribution.
21.2 The choice of RF power plasma generator and its frequency selection are the two most important parameters to ensure plasma quality and process flexibility. For cleaning applications, the frequency of the RF generator is standard and it determines if the gas is excited to a plasma state. 1356MHz is the most commonly used frequency. Also associated with the RF generator is a matching network. If the impedance load cannot be accurately adjusted, the RF generator will be damaged due to the feedback of the wave. The adjustment of the matching is necessary in obtaining a good cleaning effect. A suitable matching system and a high quality RF generator will automatically adjust the load impedance even when the cleaning conditions and operation are changed.
This guarantees optimum plasma density and repeatability.
21.3 Vacuum System The pressure in the plasma system is affected by several factors, the most important of which is the vacuum pump. In any given vacuum system, the maximum vacuum is limited by the capabilities of the vacuum pump. Typically, the pressure is between 10 and 100 Pa21.4. In order to ensure maximum repeatability during the plasma process, some means for controlling the gas flow, especially for the reaction gas, is required. The typical situation is: the flow rate is 0~200SCCM. The 21.5 control system uses the PLC with touch screen man-machine interface to control the parameters of the plasma power supply and the various input and output devices. The advantage is that the human-machine interface is friendly, flexible, stable and reliable. Reduce human error. At the same time, it is convenient to continuously improve the performance of the equipment through software during the continuous process experiment. The recipe mode is used for process management, which facilitates the input and management of various parameters. At the same time, the parameters of different content can be graded to meet the different requirements of equipment operators, process engineers and equipment engineers for human-computer interaction. And the protection of multiple software and hardware chains is designed in the equipment to ensure the safety of operators and equipment.
22 equipment features 56MHz RF power supply, with automatic impedance matching device, can provide stable processing technology; PLC control with human-machine interface, safety interlock, reliable control, easy to operate; recipe mode process parameter management, process Flexible; all features are built-in and compact; the flexible shelving structure provides both direct plasma and co-current plasma modes for the process, and the flexibility to adjust the distance to place different cleaning objects.
3 Conclusion Plasma cleaning equipment as a precision dry cleaning equipment, suitable for cleaning of hybrid integrated circuits, monolithic integrated circuit packages and ceramic substrates; used in semiconductors, thick film circuits, component packaging, silicon wafer etching, Precision cleaning in industries such as vacuum electronics, connectors and relays removes organic and oxide layers such as grease and oil on metal surfaces.
It can also be applied to the activation of surfaces such as plastics, rubber, metals and ceramics, as well as life science experiments.
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