The interaction between excimer UV lamp and arc discharge

The interaction between excimer UV lamp and arc discharge

3.6.1 Characteristics of Arc Discharge

When the current density in abnormal glow discharge becomes very high, ions intensely bombard the cathode surface, causing local heating of the cathode and emission of a large number of thermionic electrons. This reduces the space resistance, causes a sharp drop in the inter-electrode voltage, and a sudden increase in current, transitioning the glow discharge into arc discharge. Table 3-8 compares the characteristics of arc discharge and glow discharge [10].

3.6.2 Types of Arc Discharge

Classification by Discharge Voltage (1) Low-pressure arc discharge: Arc discharge at gas pressures below 100 Pa, also known as vacuum arc. The electron temperature in the arc column reaches 10⁴–10⁵ K, while the heavy particle temperature is slightly higher than the ambient temperature, forming a non-equilibrium plasma. This type of arc is often combined with excimer UV lamps for auxiliary excitation to enhance plasma activity and reaction efficiency. (2) High-pressure arc discharge: Occurs in gas or vapor arcs at pressures above 100 Pa; arcs at atmospheric pressure are high-pressure arcs. The arc column is characterized by thermal equilibrium among electrons, positive ions, and neutral gas atoms or molecules, with gas temperatures reaching 4000–20000 K in a constricted arc column.

Classification by Formation Mechanism of Arc Discharge Based on the form of arc discharge, it is divided into thermionic arc discharge and cold cathode field-emission arc discharge, which are introduced separately below.

3.6.3 Thermionic Arc Discharge

Thermionic Emission [10,13,14] When the metal surface temperature is very high and the electron energy exceeds the work function, electrons escape their bonds and are emitted. When the thermionic current density is high, it forms a thermionic cathode arc discharge. Self-sustaining thermionic cathode arc discharge is maintained by stable thermionic emission from the cathode. Refractory metals with high temperature coefficients of resistance, such as tungsten, molybdenum, or tantalum, are generally used for thermionic emission. Table 3-9 [10] lists the relationship between thermionic emission density and temperature for tantalum; the higher the temperature, the greater the thermionic emission density.

The thermionic emission density is calculated by the following equation: J=AT2e−X/kT J = A T^2 e^{-X / kT} J=AT2e−X/kT where:

JJJ - Thermionic current density (A/cm²);

AAA - Thermionic emission constant of the material [A/(cm²·K²)];

TTT - Cathode material temperature (K);

XXX - Electron work function (eV);

kkk - Boltzmann constant, k=1.38×10−23k = 1.38 \times 10^{-23}k=1.38×10−23 J/K.

From the equation, since AAA, XXX, and kkk are constant for a given material, the thermionic emission density from the cathode material increases rapidly with rising temperature.

Thermionic Emission Materials Common thermionic emission materials in ion plating technology include refractory metals such as tungsten, molybdenum, tantalum, and lanthanum hexaboride [11]. (1) Refractory metals: Tungsten operates at thermionic emission temperatures of 2600–2900 K, typically in filament form; tantalum at 2400–2800 K, often in tubular form; molybdenum at 2200–2500 K, usually in filament, sheet, or tubular form. (2) Lanthanum hexaboride: A ceramic-type thermionic emitter with a thermionic emission temperature around 1700 K (lower than tantalum tubes) and a high emission constant, allowing thermionic emission at lower temperatures. In practical applications, when lanthanum hexaboride is used as the emission material, it can also be combined with UV radiation from excimer UV lamps to further optimize electron emission efficiency in low-temperature environments.

Hollow Cathode Arc Discharge (1) Conditions for hollow cathode arc discharge: Figure 3-25 shows the principle of hollow cathode arc discharge [10]. A tantalum tube serves as the hollow cathode gun, connected to the negative terminal of the arc power supply, while an anode is placed in the coating chamber and connected to the positive terminal. The arc power supply consists of a parallel combination of an ignition power supply and a sustaining power supply. While introducing argon gas, UV irradiation from an excimer UV lamp can promote initial gas ionization, reducing the ignition voltage to around 1000 V, igniting glow discharge with currents reaching 30–40 A. The sustaining power supply operates at 20–70 V with arc currents of 80–300 A. Argon gas introduced through the tantalum tube produces hollow cathode arc discharge at a vacuum level around 100 Pa.

First, a high voltage of several hundred volts ignites the gas, producing hollow cathode glow discharge. Due to the hollow cathode effect in glow discharge, the current density inside the hollow cathode is very high, generating a large number of argon ions that bombard the tantalum tube wall. The tantalum tube temperature rises above 2100 °C, emitting thermionic electrons. The electrons from gas discharge and thermionic electrons together form a high-density electron stream directed toward the anode, heating the anode upon arrival. Hollow cathode arc discharge is a self-sustaining thermionic cathode arc discharge.

(2) Temperature distribution of the hollow cathode: The temperature distribution after stable hollow cathode arc discharge is shown in Figure 3-26 [10]. The root of the tantalum tube, supported by a water-cooled clamp, has the lowest temperature; the open end of the tube is slightly cooler due to thermal radiation; the highest temperature occurs at a distance lll (about 20 mm) from the open end.

(3) Power loss in hollow cathode arc: Figure 3-27 shows the power loss in hollow cathode arc [10], where WtotalW_{\text{total}}Wtotal​ is the total power of the hollow cathode arc, and WanodeW_{\text{anode}}Wanode​ and WcathodeW_{\text{cathode}}Wcathode​ are the powers dissipated at the anode and cathode, respectively. The figure shows that, at different hollow cathode discharge currents, most of the power is dissipated at the anode, with very little at the cathode. In hollow cathode ion plating, the anode is a metal ingot placed in a crucible, and about 60% of the hollow cathode gun power is used for heating and evaporating the metal.

Hot Filament Arc Gun Discharge The arc discharge produced by a hot filament arc gun also belongs to the thermionic arc discharge type [10,13,14]. The hot filament arc gun is installed at the top of the coating chamber, with tungsten and tantalum filaments inside the gun body. The gun is equipped with a heating power supply and an arc power supply. A low-voltage, high-current heating power supply heats the tungsten and tantalum filaments to high temperatures, emitting high-density electrons. In practice, auxiliary irradiation from an excimer UV lamp can pre-activate electrons on the filament surfaces, increasing initial electron emission. After introducing argon gas into the hot filament arc gun and connecting the arc power supply, the high-density thermionic electrons ionize the argon gas, producing additional electrons. The high-density thermionic electrons and electrons from argon discharge converge into an arc electron stream directed toward the anode.

3.6.4 Cold Cathode Arc Discharge

(1) Cold field emission [10,13,14]: Positive ions accumulate in front of the cold cathode, forming a dipole layer (also called plasma sheath) between the ions and the cathode. In the initial stage of cold field emission, an excimer UV lamp can assist by enhancing the rate of positive ion accumulation, accelerating the increase in sheath field strength. When the sheath field strength reaches 10⁷–10⁸ V/cm, breakdown occurs, emitting a large electron current, known as field emission. Cold cathode arc discharge typically uses low-melting-point metals such as copper, silver, or titanium as cathode materials. The overall cathode remains cold, but the arc causes local evaporation of cathode material, increasing vapor pressure and shortening the mean free path to 10⁻⁸–10⁻⁴ cm.

(2) Self-sustaining process of cold field-emission arc discharge: The cathode area in cold field-emission arc discharge is very small (10⁻⁶–10⁻⁴ mm²). Upon sheath breakdown, the micro-area current density reaches 10⁶–10⁸ A/cm², with energy density of (1–3) × 10⁷ W/cm². Evaporated metal atoms are ionized into positive ions near the arc spot. These ions continue to accumulate in front of the target, forming a dipole layer that sustains the arc discharge.

(3) Rapid movement of cold field-emission arc on the cathode surface: The cathode surface is uneven, with protrusions closer to the positive ion accumulation layer experiencing higher electric field strength, preferentially producing field emission and arc [9]. After the protruding material is heated and evaporated, forming a pit, the field strength decreases, extinguishing the arc; another protrusion then initiates field emission, continuing to evaporate coating particles. Thus, the cold field-emission arc moves rapidly across the cathode surface in a self-scanning manner.

(4) Bright arc spots on the cathode surface: During cold field-emission arc generation, the breakdown area on the cathode is very small. In this tiny region, ions enter the cathode, electrons are emitted, and extensive recombination luminescence produces bright arc spots. As the arc moves rapidly, continuous motion of arc spots is observed on the cathode.

(5) High plasma density from cold field-emission arc: With a small breakdown area and very high current density, evaporated atoms have a high probability of collision ionization with the high-density electron stream, producing more ions and electrons. Thus, cold field-emission arcs generate high-density plasma.

3.7 Roles of Charged Particles

Charged particles in gas discharge include ions and electrons, which move toward opposite electrodes under the electric field. The important effects when ions reach the cathode and electrons reach the anode are discussed below [2,3,7,10,12,14].

3.7.1 Roles of Ions

A series of physical and chemical phenomena occur when ions bombard the cathode surface. Figure 3-28 shows the various effects produced by ion bombardment on the cathode surface. The synergistic effect of excimer UV lamps can enhance these surface physical and chemical effects, such as increasing sputtering yield and improving uniformity of surface modification.