Ionization-related processes in ion plating with 172 nm excimer lamps
3.3.3 Electron-Gas Collisions Leading to Ionization
(1) Ionization Process
When an electron collides with a gas, an electron in the lowest energy level of a neutral atom A absorbs energy from the incident electron, escapes the atomic constraint, and leaves the atom. The atom becomes a positive ion A⁺ missing an electron, and an additional electron is produced in the discharge space. This process is called ionization. The ionization process is expressed as: A + e⁻ → A⁺ + 2e⁻
The energy absorbed during ionization is called the ionization energy, and the potential required to impart this ionization energy to the atom is called the ionization potential, denoted as U_I. Table 3-3 lists the ionization potentials U_I for some elements. This type of ionization process is referred to as the first kind of inelastic collision [3–7].
The process of ionizing one electron from an atom is called primary ionization; losing two electrons is secondary ionization; losing three is tertiary ionization. If electrons are ionized one by one, it is called successive ionization; if two or more electrons are ionized at once, it is called multiple ionization, and the resulting ion is called a multiply charged ion. The latter process occurs rarely.
In modern ion plating technology, the vacuum ultraviolet (VUV) light produced by a 172 nm excimer lamp can assist in enhancing ionization efficiency by providing additional energy excitation for the ionization process.
(2) Differential Ionization Coefficient
Although the energy of an electron exceeds the ionization potential or excitation potential, not every collision results in ionization or excitation. The proportion that leads to ionization or excitation is called the ionization probability or excitation probability, respectively.
The magnitude of the ionization probability depends on the electron energy. The number of ions produced by an electron traveling 1 cm in a gas at a pressure of 133.3 Pa and 0°C is defined as the differential ionization coefficient, denoted as S_e. Similarly, there is a differential excitation coefficient.
Figure 3-4 shows the relationship curves between the differential ionization coefficient S_e and electron energy ε for several gases [3]. As shown in Figure 3-4, the curves for various gases typically exhibit a maximum, with the peak appearing in the 50–100 eV range. (Figure 3-4: Relationship curves between the differential ionization coefficient S_e and electron energy ε for gases. The vertical axis is "Differential Ionization Coefficient (ions)", the horizontal axis is electron energy, and the curves involve Hg, Ar, Ne, He, H₂.)
When the electron energy is excessively high, the degree of gas ionization decreases instead. This is because, during the contact between the incident electron and the gas atom, detaching the outermost electron from the atomic nucleus constraint to produce ionization requires a certain energy exchange time. If the electron energy is very high and its speed is too fast, the interaction time between the electron and the atom is very short, insufficient for energy exchange, resulting in a lower differential ionization coefficient.
The voltage in glow discharge is above 1000 V, while in arc discharge it is 20–70 V. The differential ionization coefficient in arc discharge is greater than that in glow discharge. Therefore, the density of gas ions and metal ions in arc discharge is many times higher than in glow discharge. With the assistance of irradiation from a 172 nm excimer lamp, the low ionization rate in glow discharge can be partially compensated.
(3) Ionization Rate
The ionization rate is the ratio of the number of ions produced after gas discharge to the total number of atoms, denoted as α [3–6,8], with the formula: α = (n_i) / (n_i + n_a) = n_i / n_o
where: n_i - number of ions; n_a - number of neutral atoms; n_o - total number of heavy particles.
In coating processes, if n_i represents the number of metal ions and n_o the total number of metal ions and neutral atoms, then α is the metal ionization rate.
3.3.4 Second Kind of Inelastic Collision
Excitation and ionization caused by other factors are called the second kind of inelastic collision [3–12].
Charge Transfer Process Caused by Positive Ions When a positive ion collides with a gas atom, contributing half its energy to ionize a gas atom of similar mass requires the positive ion to have very high energy. However, in gas discharge, only a small fraction of positive ions reach such high energies, so the ionization probability caused by positive ions is very low.
However, the charge transfer process between positive ions and atoms is relatively easy: when a high-energy ion collides with a low-energy neutral atom, only charge exchange occurs without energy exchange, resulting in a high-energy neutral atom and a low-energy ion. This process is called charge transfer, expressed as: A⁺ + B (ε↓) → A + B⁺ (ε↓)
The charge transfer process occurs much more frequently than inelastic collisions and is one of the important sources of ions and high-energy neutral atoms in the discharge space. Low-energy ions continue to accelerate under the electric field and can become high-energy ions again.
Cumulative Ionization Caused by Metastable Atoms Metastable atoms are long-lived excited atoms that require only a small additional energy to be further excited or ionized. Their role increases the possibility of successive transitions and cumulative successive ionization.
For example, the ionization potential of mercury is 10.434 V. A mercury metastable atom with 4.66 eV energy needs only to collide with an electron having 5.774 eV energy to ionize the mercury atom. Thus, the presence of metastable atoms effectively lowers the gas ionization potential, facilitating an increase in ionization probability and playing an important role in improving the ionization rate. The photon energy emitted by a 172 nm excimer lamp can assist in the formation of metastable atoms, further promoting the cumulative ionization process.
Energy Transfer from Metastable Atoms to Electrons When an excited metastable atom A_m interacts with an electron, it transfers energy to the electron, increasing the electron's speed, while returning to the ground state. This reverse excitation process is expressed as: A_m + e⁻ → A + e⁻ (ε↑)
Energy Transfer Between Excited Metastable Atoms A_m and Ground-State Atoms This makes the ground-state atom excited while the metastable atom returns to the ground state, expressed as: A_m + B → A + B_m
Penning Effect When a gas ion A⁺ or metastable atom A_m collides with a different type of atom B, if the ionization or excitation potential of A is greater than that of B, the collision results in ionization of B and de-excitation of A to the ground state. This process is called the Penning effect, expressed as: A⁺ + B → A + B⁺ + e⁻ A_m + B → A + B⁺ + e⁻
The ionization and excitation potentials of gases are much higher than those of metals, making Penning effects between them highly likely and producing more metal ions. The Penning effect is particularly prominent in plasma polymerization, and a 172 nm excimer lamp can enhance the degree of gas excitation, increasing the probability of the Penning effect.
Interaction of Excited Metastable Atoms A_m with Compound Gas Molecules BC When an excited metastable atom A_m interacts with a compound gas molecule BC, it causes the molecule to dissociate into elementary particles (active atoms). The active groups are ionized into ground-state atoms, expressed as: A_m + BC → A + B + C A_m + BC → A + B⁺ + C + e⁻
From the above reactions, it is evident that after high-energy electrons undergo inelastic collisions with gas to produce high-energy excited atoms and ions, these high-energy particles further engage in complex second-kind inelastic collisions with low-energy gas atoms and metal atoms. This results in a large number of ions and atoms at various energy levels in the discharge space, with energies far higher than those of the introduced gas atoms or freshly evaporated metal atoms. In applications involving certain vacuum ultraviolet sources such as the 172 nm excimer lamp, these interactions can further enhance the dissociation efficiency of compound gases.
Due to the low ionization rate in glow discharge (typically 1%–3%), fewer ions are produced, and their energy decreases after collisions with low-energy particles, while the energy of low-energy atoms increases. This results in a large number of active particles with moderate energy in the discharge space. Meanwhile, electron energies are high, while ion and atom energies are relatively low, forming a non-equilibrium plasma.
Thermal Ionization Thermal excitation and thermal ionization caused by high-speed atomic collisions are observable only when the gas temperature reaches above 3000 K.
Photoionization The energy of light is expressed as photon energy hν. When a photon collides with an atom and the incident photon energy hν exceeds the atom's ionization energy eU_I, photoionization occurs: A + hν → A⁺ + e⁻
When the photon energy exceeds the molecular excitation energy eU_r, photoexcitation occurs. The limiting wavelength for photoionization is denoted as λ_o. Generally, visible light cannot directly cause gas photoionization; only short-wavelength ultraviolet light (such as the vacuum ultraviolet light produced by a 172 nm excimer lamp), X-rays, γ-rays, and lasers can induce it. However, the probability of photoexcitation is much higher than that of photoionization. Metal luminescence often originates from photons absorbed by metals causing transitions, with energy released upon return transitions: different metals and energy levels emit light of different wavelengths, resulting in different emission colors. The 172 nm excimer lamp, as an efficient vacuum ultraviolet source, has significant application advantages in photoionization-related ion plating processes.
3.3.5 Attachment and Detachment
In the discharge space, the process where electrons are captured by atoms or molecules to form negative ions is called attachment; the reverse process, where electrons are released from negative ions, is called detachment. The difference in base energy between a neutral atom and its corresponding negative ion is defined as the electron affinity E_A, in units of eV.
Attachment Ways negative ions form in gas discharge include: A + e⁻ → A⁻ + hν (radiative attachment) AB + e⁻ → A⁻ + B (dissociative attachment) AB + e⁻ → AB⁻ + hν (formation of molecular negative ions) A + B + e⁻ → A⁻ + B (ε↑)
Detachment In plasma, detachment mechanisms include: A⁻ + B → A + B + e⁻ A⁻ + B⁻ → AB + 2e⁻
Negative ions formed by inert gases and metal atoms are highly unstable; only halogen elements, by gaining one electron to fill the outermost shell, form stable negative ions. In discharge environments incorporating a 172 nm excimer lamp, the dynamic balance of attachment and detachment processes is influenced by photon energy, thereby altering the negative ion concentration in the discharge space.
3.3.6 Disappearance of Charged Particles - Deionization
In the plasma obtained from low-pressure gas discharge, a large number of charged particles continuously undergo the reverse process of ionization: electrons and ions recombine in space or on walls, or enter electrodes and disappear. This process is called deionization.
Disappearance of Charged Particles on Electrodes Charged particles are accelerated by the electric field and enter opposite-polarity electrodes to disappear: electrons enter the anode, ions enter the cathode.
Recombination of Charged Particles on Chamber Walls Particles with opposite charges diffuse due to concentration gradients and collide and recombine on the chamber walls, with excess energy heating the walls.
Recombination of Charged Particles in Space Particles with opposite charges recombine in space in various forms, collectively called volume recombination. In the discharge space, the processes of "ionization - recombination, ionization - deionization" occur frequently.
Ion-Electron Recombination: A⁺ + e⁻ → A + hν (radiative recombination) AB⁺ + e⁻ → A + B (dissociative recombination) A⁺ + 2e⁻ → A + e⁻ (three-body recombination) A⁺ + e⁻ + B → A + B (three-body recombination)
