The volt-ampere characteristic curve of 172 nm excimer light gas discharge
3.3.7 Luminescence Phenomena in Gas Discharge
Modern Ion Plating Technology
(Continuation of Luminescence Phenomena in Gas Discharge)
Excitation Luminescence
Resonantly excited atoms will transition back to stable energy levels within 10⁻⁸ s, releasing the energy absorbed during the excitation transition. This energy is emitted in the form of photons, producing excitation luminescence. In certain low-pressure discharges, such as those involving xenon, this process can also generate vacuum ultraviolet radiation, including 172 nm excimer light.
Recombination Luminescence
In the discharge space, when ions encounter electrons, they recombine to form stable atoms. The energy originally absorbed is released in the form of photons during the recombination process, producing recombination luminescence.
Due to differences in atomic structure and the energy of electron energy levels, the frequencies of excitation luminescence and recombination luminescence vary, leading to different luminescence colors. During ion plating, processes such as the recombination of electrons and ions, three-body recombination of metal ions, electrons, and reaction gases, and the transition of resonantly excited atoms back to stable states continuously occur, generating luminescence. The more intense the ionization, the stronger the light intensity. Different elements have different electron energy levels, so their luminescence colors differ: argon emits a sky-blue light, and titanium nitride films deposited with nitrogen 通入 exhibit a cherry-red color. From the above reaction processes, it can be seen that in the discharge space of ion plating, high-energy electrons continuously collide inelastically with gas, producing high-energy excited atoms and ions. These high-energy excited atoms and ions then undergo a series of complex collisions with low-energy gas atoms and metal atoms, resulting in ions and atoms of various energy levels in the discharge space. The discharge space is filled with a large number of active particles, whose energy is much higher than that of the introduced gas atoms and evaporated metal atoms. All high-energy particles in the plating space originate from high-energy electrons. Therefore, the more electrons there are in the plating space, the more high-energy particles are present during plating, the higher the total energy of the film particles, and the more favorable it is for chemical reactions to form high-quality films. Modern ion plating technologies strive to obtain more electrons to increase the plasma density in the plating space, thereby enhancing the total energy of film particles.
3.4 Gas Discharge
3.4.1 Process of Gas Discharge
Avalanche Discharge
Evacuate the container; when the vacuum degree is between 1∼10 Pa, turn on the power supply between the cathode and anode. At this point, the original electrons in the vacuum chamber accelerate under the action of the electric field. When the electron energy reaches a certain value, they collide with neutral gas molecules and ionize them. Ions accelerate toward the cathode, and electrons accelerate toward the anode; during this process, more collision ionization occurs, and the number of charged particles increases rapidly, forming an electron avalanche process (also called avalanche discharge), as shown in Figure 3-5 [3]. This discharge is a non-self-sustained discharge process; if the original ionization source is removed, the discharge stops immediately.
From the Paschen curve, it can be seen that there is a minimum cathode voltage required to initiate glow discharge. Practice shows that once gas discharge starts, the discharge voltage drops sharply, indicating that the selected ignition voltage is not the minimum ignition voltage. Generally, after the gas is ignited, the discharge voltage automatically decreases to the voltage that maintains self-sustained discharge. For example, the ignition voltage of argon discharge with iron-based materials is approximately 265 V.
3.4.2 Volt-Ampere Characteristic Curve of Gas Discharge
Measurement of the Volt-Ampere Characteristic Curve
The voltage-current variation curve between the two electrodes of gas discharge is called the volt-ampere characteristic curve. Figure 3-7 shows the device for measuring the volt-ampere characteristic curve of gas discharge [10], where Eₐ is a DC power supply and Rₐ is a variable resistor. After turning on the power supply Eₐ, measure the voltage and current between the electrodes, and plot the voltage-current variation curve during gas discharge, i.e., the volt-ampere characteristic curve [3−12].
Characteristics of the Volt-Ampere Characteristic Curve
Figure 3-8 shows the volt-ampere characteristic curve of gas discharge [3]:
Non-Self-Sustained Dark Discharge Region (Ⅰ)
In segment AB of Figure 3-8, a very weak current (10⁻¹² A) appears as the voltage gradually increases from 0. This current is generated by space ionization caused by residual electrons in the space. In this region, although the discharge current increases with the inter-electrode voltage, the current is weak and no luminescence is visible; this is called non-self-sustained dark discharge, which belongs to the avalanche-like Townsend discharge process.
Self-Sustained Dark Discharge Region (Ⅱ)
Starting from point B, the self-sustained discharge stage begins, producing avalanche-like Townsend discharge. The voltage at point B (Vᵇ, also called breakdown voltage) is generally higher than the minimum ignition voltage Uₘᵢₙ under the experimental conditions (p,d). Weak luminescence appears in segment BC, which is called the self-sustained dark discharge region.
Transition Region (Corona Discharge Region Ⅲ, Early Normal Glow Discharge Region Ⅳ)
If the resistance in the circuit is not large, the discharge quickly transitions to point E through the transition region: the voltage drops sharply to Vₙ, the current increases abruptly, and strong glow appears on the cathode, entering the normal glow discharge region.
Normal Glow Discharge Region (Ⅴ)
From the volt-ampere discharge characteristic curve, Vᵇ is the ignition voltage, and Vₙ is the voltage that maintains stable normal glow discharge (i.e., Uₘᵢₙ in the Paschen curve). After transitioning to normal glow discharge from point E, only part of the cathode surface emits light (called the cathode spot). Reducing the current-limiting resistor increases the glow area. The inter-electrode current increases with the glow area, but the voltage of normal glow discharge remains constant-this is a characteristic of normal glow discharge.
(Annotations for Figure 3-8: Volt-Ampere Characteristic Curve of Gas Discharge Ⅰ-Non-Self-Sustained Dark Discharge Region Ⅱ-Self-Sustained Dark Discharge Region Ⅲ-Corona Discharge Region Ⅳ-Early Normal Glow Discharge Region Ⅴ-Normal Glow Discharge Region Ⅵ-Abnormal Glow Discharge Region Ⅶ-Transition Region from Glow to Arc Discharge Ⅷ-Arc Discharge Region Vᵇ-Discharge Ignition Voltage Vₙ-Normal Glow Discharge Voltage Vᵍ-Arc Discharge Voltage)
Abnormal Glow Discharge Region (Ⅵ)
When the cathode spot covers the entire cathode surface (i.e., reaching point F), the inter-electrode current increases as the inter-electrode voltage increases-this is the abnormal glow discharge stage. Segment FG exhibits the characteristics of abnormal glow discharge, with a current density of the mA/cm² level. Near point G, the cathode current density is very high; the local cathode is heated to a strong thermionic emission state, the space resistance decreases, the voltage drops sharply, the current increases abruptly, and it easily transitions to arc discharge.
Transition Region from Glow to Arc Discharge (Ⅶ)
Starting from point G, the inter-electrode voltage drops sharply from several hundred volts to tens of volts, and the current density jumps from the mA/cm² level to the 100 A/cm² level. Point G is the arc ignition voltage, and segment GH is the transition region from abnormal glow discharge to arc discharge. This phenomenon is easy to achieve if the power supply allows.
Arc Discharge Region (Ⅷ)
Starting from point H, it enters a stable region with a voltage of tens of volts and a current density above 100 A/cm². In specialized applications, similar low-pressure environments can produce intense ultraviolet emissions, such as 172 nm excimer light from excited xenon dimers.
