What is glow discharge optical emission spectrometer?
Glow discharge spectroscopy
is a new type of surface analysis technique that enables the direct analysis of solid samples. Over the years, the glow depth profiling technique has been widely used in the field of material surface analysis, and people are increasingly valuing this technique. Glow discharge optical emission spectrometer
offers unique and irreplaceable advantages in profiling the surface and depth of materials. Initially, the glow discharge spectrometer originated in the steel industry and was mainly used for the determination of galvanized steel sheets and passivation films on steel surfaces.
What is glow discharge?
The physical phenomenon of a gas being broken down under the action of an electric field and conducting electricity is usually called a gas discharge. There are two forms of gas discharge, glow discharge and arc discharge. Glow discharges
are subdivided into normal glow discharges and abnormal glow discharges. They are the basic parts of the magnetron sputtering coating process to produce plasma.Glow discharge
(or abnormal glow discharge) can be formed by DC or pulsed DC target power through the gas discharge, but also with AC (rectangular wave bipolar pulse IF power, sine wave IF and RF) target power through the vacuum chamber gas discharge. In the gas discharge, what kind of working gas, high or low air pressure, the size of the current density, the distribution, and high or low electric and magnetic field strength, different materials, shape and location characteristics of the electrode, and other factors will not only affect the process and nature of the discharge but also affect the nature and color of the radiation light when the discharge.
A. DC glow discharge
a. When DC voltage is added between the cathode and anode, the remaining electrons and ions in the working gas in the chamber make directional movements under the action of the electric field, so the current increases from zero.
b. When the voltage between the electrodes is large enough, all charged ions can reach their respective electrodes, when the current reaches a certain maximum.
c. When the discharge voltage between the electrodes is greater than a certain critical value (ignition glow voltage), the discharge current will suddenly and rapidly rise, and the voltage between the cathode and the anode will drop steeply and remain at a lower stable value. The working gas is breakdown, and ionization, and produces plasma and self-sustaining glow discharge, which is the basic process of "Townsend discharge", also known as small current normal glow discharge.
d. The cathode of the magnetron target is connected to the negative terminal of the target power supply, and the anode is connected to the positive terminal of the target power supply. The normal sputtering must be in the gas discharge volt-ampere characteristic curve in the "abnormal glow discharge region" operation. Its characteristic is that with the adjustment of the power output of the magnetron target working voltage increases, the sputtering current should also be synchronized with a slow rise.
B. Pulsed DC glow discharge
The gas discharge of a single pulse of a pulsed or sinusoidal half-wave IF target power supply should be consistent with the variation pattern of the abnormal glow discharge section of the DC gas discharge volt-ampere characteristic curve and the preceding section. It can be regarded as the gas discharge volt-ampere characteristics in a single pulse of the discharge reproduction. The pulsed DC target power supply starts glow sputtering during the pulse and naturally extinguishes the glow between pulses (difficult to distinguish with the naked eye due to the high frequency).
After the sputtering target glow discharge, if the power supply output pulse repetition frequency is high enough and the conductive ions in the vacuum cavity have not been completely neutralized, it will lead to the second (later) repetition pulse of the re-glow voltage and the sputtering target working voltage close or the same. If the sputtering target glow discharge after the power supply output pulse repetition frequency is very low or arc extinguishing time is too long and the conductive ions in the vacuum cavity body has been largely neutralized, it will lead to the second (later) repeat pulse re-glow voltage to a higher value, and ignition glow when the high voltage is close to or the same.
Working principle of glow discharge optical emission spectrometer
Glow discharge optical emission spectrometer
is an emission spectrometer. It works by using a light source to put the sample element in an excited state, and when the outer electrons of the sample element return from the high-energy state to the low-energy state, the characteristic spectrum is emitted. The characteristic spectra of the elements in the sample are analyzed by the emission of the element. Glow discharge lamp
is the light source of the glow discharge instrument, and there are two electrodes in an RF glow discharge light source. The diameter of a 4mm tubular copper electrode is one of them, which is the anode, and the anode is grounded. And the sample as the cathode, making an RF potential maintained, this is the potential generated by high frequency (RF) power induction. The low-pressure argon gas is charged into the glow discharge lamp, and a small number of argon ions are spontaneously generated in the lamp, which passes through the anode-cathode gap under the action of RF potential, making the high-speed oscillation generated. The accelerated argon ions and argon atoms collide, making more argon ions and electrons generated, making the plasma formation, which is also known as the glow discharge
. The high-speed argon ions in the plasma reach the surface of the sample (cathode) so that the material on the surface of the sample is sputtered out uniformly and diffused into the glow-discharge plasma. The diffusion is dissociated and atomized in the plasma, which is excited and causes the emission of the characteristic spectra of the sample components.
The light from the sample is focused and spectroscopic using an optical system and reaches the high dynamic range detector HDD, which receives the light signal. The optical signal is converted into an electrical signal and sent to a computer for processing using an electronic control system. The computer is equipped with special software, which compares the standard curve preset in the software with the light intensity signals of various elements so that the concentration of each element can be measured.
Glow discharge vs plasma cleaning
A. What is plasma?
a. Plasma in vacuum magnetron sputtering coating technology is generally formed by the discharge of a working gas under the action of an electric field. The atoms that make up the molecules gain enough kinetic energy to start separating from each other, the outer electrons of the atoms become free from the nucleus, and the atoms that lose electrons become positive ions. This process is called ionization. A plasma is an ionized gas, a collection of ions, electrons, high-energy atoms, etc. In this aggregate, positive ions and electrons always appear in pairs, in approximately equal numbers, and the whole is quasi-electrically neutral. It is an ionized state consisting of charged particles and is called the fourth state of matter - the plasma state.
b. A voltage or electric field is applied to a gas discharge to form a plasma, accompanied by the movement of conducting ions, particles, electrons, etc. At this point, they flow through the plasma with an electric current, which is the conductivity of the plasma.
c. During the vapor deposition process, the working gas and the target metal atoms are ionized by high-energy electron impact into a plasma consisting of electrons, gas ions and metal ions, and other conductive particles.
B. Glow discharge cleaning
The device used to generate plasma in a plasma cleaning machine is set up in a sealed container with two electrodes to form an electric field, with a vacuum pump to achieve a certain degree of vacuum.
As gases become thinner and thinner, the molecular spacing and the distance of free motion of molecules or ions also become longer. By the action of an electric field, they collide and form a plasma. These ions are highly reactive and their energy is sufficient to break almost all chemical bonds and cause chemical reactions on any exposed surface. Different gases of the plasma have different chemical properties. For example, the plasma of oxygen has high oxidation properties and can oxidize the photoresist reaction to generate gas, thus achieving the cleaning effect. The plasma of corrosive gas has good anisotropy so that it can meet the needs of etching. The use of plasma treatment emits a glow, so it is called glow discharge treatment.
During the glow discharge, electrons and positive ions move toward the anode and cathode respectively under the action of the electric field at both poles of the discharge and accumulate near the poles to form a space charge area. Because the drift speed of positive ions is much smaller than that of electrons, the charge density of the positive ion space charge area is much larger than that of the electron space charge area, making the entire interpolar voltage almost entirely concentrated in a narrow region near the cathode. This is a significant feature of the glow discharge. And in the normal glow discharge, the voltage between the two poles does not change with the current.
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