The importance of radio frequency discharge for 172nm&222nm excimer lamps

The importance of radio frequency discharge for 172nm&222nm excimer lamps

3.5.5 Radio Frequency Discharge

Low-pressure glow discharges include DC glow discharge, medium-to-high frequency discharge, radio frequency (RF) discharge, and microwave discharge. The latter two forms are widely used in plasma-enhanced chemical vapor deposition (PECVD) and plasma polymerization. Table 3-7 lists the frequencies of several power sources [6,8,13-15]. This section mainly introduces radio frequency discharge.

Table 3-7 Frequencies of Several Power Sources

Characteristics of RF Discharge In a vacuum vessel, two electrodes are placed and connected to a high-frequency power source (frequency of 13.56 MHz). When the power is turned on, high-frequency discharge occurs, known as radio-frequency glow discharge (RF discharge) [6,8,13-15]. The polarity of the two electrodes alternates rapidly, causing electrons to oscillate back and forth between the plates, increasing the probability of collision ionization with gas atoms. Due to their large mass, ions can be considered stationary at high frequencies, and recombination on the electrodes is lower than in DC discharge, allowing electrodes to be placed outside the vessel. The ignition/maintenance voltage for discharge is lower than in DC discharge, and the plasma density can reach ~10¹¹–10¹² /cm³.

Phenomena in High-Frequency Discharge High-frequency discharge differs from low-frequency discharge in the characteristics of the two-electrode discharge:

Low-frequency discharge: When low-frequency AC is applied to the electrodes, the glow alternates between the poles, with each half-cycle resembling DC glow discharge.

High-frequency discharge: There is no alternating glow between the electrodes; the luminescence intensity/color in each region is stable, the plasma column is centered, and the discharge near the two electrodes is completely symmetric (because when the electric field direction changes, the space charge/plasma region does not have time to redistribute or deionize). (Figure 3-16: Distribution of light between electrodes in low-frequency and high-frequency discharge: a) Low-frequency discharge; b) High-frequency discharge)

Electrode Configurations in RF Discharge RF discharge electrodes can be placed inside or outside the discharge tube. Internal electrodes are commonly flat-plate type, while external ones often use induction coils.

(1) Internal Electrodes Figure 3-17 shows a flat-plate electrode RF discharge device [10]: The electrode connected to the RF power source is equipped with a shielding cover; the grounded electrode (worktable) holds the workpiece and is grounded together with the discharge chamber. In the high-frequency electric field, electrons oscillate multiple times, increasing collision probability and achieving plasma density of ~10¹¹–10¹² /cm³. The discharge pressure is ~1–100 Pa, enabling large-area uniform thin films, applied in semiconductors, optoelectronic devices, etc. This device can also be combined with 172 nm excimer light and 222 nm UV light for auxiliary treatment to improve the surface properties and structural uniformity of thin films. (Figure 3-17: Flat-plate internal electrode RF discharge device, including electrodes, RF voltage, workpiece, plasma, thermocouple, gas nozzle, vacuum pump, SiH₄, NH₃, pressure gauge)

(2) External Electrodes (Electrodeless Discharge) External electrode devices feature an induction coil outside the reaction chamber [6,8,10,13-15]. Figure 3-18 shows an electrodeless RF discharge device [10]: It relies on a high-frequency magnetic field to generate an induced electric field that excites the plasma. With no electrodes inside the device, pure plasma can be obtained, beneficial for preparing high-purity thin films. In the high-frequency electric field, electron velocity greatly exceeds ion velocity, allowing stable discharge maintenance even at low pressure (~1–10 Pa) due to high collision probability.

Impedance Matching for RF Power Supply [6,8] To maximize high-frequency power transmission, a matching circuit (matching coupling circuit) is required between the load and the power source: Capacitive coupling for flat-plate electrodes; inductive coupling for high-frequency induction electrodeless devices.

Applications of RF Discharge

Sputtering of Insulating Films When sputtering insulating films with DC power, the insulating film blocks ions, causing charge accumulation and breakdown. RF power alternates electrode polarity, neutralizing electrode charges and ensuring normal discharge. Additionally, introducing 172 nm excimer light and 222 nm UV light during sputtering can further optimize the density and adhesion of insulating films, reducing defects.

Self-Bias Generated by High-Frequency Electrodes [6,8] In capacitively coupled flat-plate electrode devices, electron mobility greatly exceeds that of ions, so the high-frequency electrode remains negative for most of the cycle, forming a self-bias (500–1000 V), similar to the cathode drop in DC glow discharge, maintaining stable discharge.

Assisting Discharge at Atmospheric Pressure Plays an important role in atmospheric-pressure glow discharge and dielectric barrier discharge.

3.5.6 Microwave Discharge

Microwave discharge converts microwave energy into internal energy to excite/ionize gas and produce plasma, with a common frequency of 2.45 GHz. Figure 3-19 shows a schematic of a microwave discharge device [6]. Ionization mechanism: After electron collision, the electric field reverses direction just in time, causing continuous increase in electron velocity/energy. Plasma density reaches ~10¹⁵ /cm³, with power concentration possible without electrodes. Microwaves are transmitted via waveguide or coaxial cable and coupled resonantly with the discharge cavity (coupling forms seen in microwave PECVD). (Figure 3-19: Microwave discharge device, including microwave generator (2.45 GHz, 1.25 kW), cooling water, three-stub tuner, isolator, power meter, discharge tube, cavity resonator, etc.)

3.5.7 Glow Discharge at Atmospheric Pressure

Low-pressure discharge requires vacuum systems, which are costly, so atmospheric-pressure glow discharge technologies have been developed. Main forms include corona discharge, dielectric barrier discharge (DBD), and atmospheric-pressure glow discharge (requiring high breakdown voltage or RF/microwave power).

Corona Discharge A form of discharge at atmospheric pressure [6,8,16], divided into positive and negative corona: Device (Figure 3-20a [16]): Cathode is filamentary (wire electrode), anode is plate or cylindrical, with significant curvature difference. Parameters: Discharge voltage ~10–100 kV, pressure ~10⁵ Pa, current density ~10⁻⁶–10⁻⁴ A/cm². Characteristics: Small discharge range, low/uneven energy, with flickering and ozone smell; mostly limited to laboratories. (Figure 3-20: Negative corona discharge: a) Device; b) Photo)

Dielectric Barrier Discharge (DBD) Electrodes are covered with insulating dielectric, and high-voltage AC produces discharge [6,8,16-18], also known as DBD-CVD (atmospheric-pressure plasma-enhanced chemical vapor deposition): Structure (Figure 3-21 [16]): Flat-plate or tubular structures, with dielectrics such as glass or bakelite. Parameters: Pressure ~10⁵ Pa, frequency 50 Hz–1 MHz, amplitude up to 100 kV. Applications: Tubular for chemical reactors; flat-plate USED for modification/grafting of polymers/metals. During treatment, combining with 172 nm excimer light and 222 nm UV light enables more efficient control of surface chemical groups, enhancing modification effects. Discharge characteristics: At high voltage, filamentous micro-discharges occur (Figure 3-22 [16]), cylindrical (radius 0.1–0.3 mm), lasting 10–100 ns, current density 0.1–1 kA/cm². Electrode spacing ~3 mm; uniform gap ensures stable discharge.

Significance of Atmospheric-Pressure Coating (Figure 3-23 [19]) Low-pressure treatment (Figure 3-23a): Requires large vacuum system. Atmospheric-pressure DBD treatment (Figure 3-23b): No vacuum system needed, more suitable for roll-to-roll processing of textiles, etc. (Figure 3-21: Common dielectric barrier discharge structures; Figure 3-22: Filamentous discharge photo; Figure 3-23: Comparison of chemical fiber product treatment devices: a) Vacuum system; b) Atmospheric-pressure DBD)