Cylindrical microdischarge cavities 200–400 μm in diameter and 0.5–5 mm in depth have been fabricated in silicon and operated at room temperature with neon or nitrogen at specific power loadings beyond 10 kW/cm3. The discharges are azimuthally uniform and stable operation at N2 and Ne pressures exceeding 1 atm and ∼600 Torr, respectively, has been realized for 400 μm diameter devices. Spectroscopic measurements on neon discharges demonstrate that the device behaves as a hollow cathode discharge for pressures >50 Torr. As evidenced by emission from Ne and Ne+ (2P,2F) states as well as N2 (C→B) fluorescence (316–492 nm), these discharge devices are intense sources of ultraviolet and visible radiation and are suitable for fabrication as arrays.

Microdischarge devices fabricated in silicon

The transformations of the electron energy distribution (EEDF), their concentration, and plasma space potential along the discharge gap between the hollow rectangular cathode and the mesh anode were experimentally studied. The discharge was 3 cm long, with a cross-section of 10 cm2. A new version of measurements with several single probes with the formation of current-voltage characteristics (IVC) in the probe circuit was proposed with the simultaneous application of voltages in the form of a combination of periodic and noise signals. The proportions of the signals were varied for different sections of the current-voltage characteristics. Measurements along the central axis of the discharge were taken. The dynamic range of the EEDF was not less than 4 orders of magnitude at the electron concentrations of 2–13 × 1010 cm−3, which exceeds the best known achievements. Measurements for discharge in helium at reduced pressures of 1–1.2 mbar and currents of 150–400 mA showed that the EEDFs differ from Maxwell ones, with an excess of fast electrons in the region of 10–20 eV at medium energies 4–6 eV. The fraction of fast electrons decreased in regions closer to the anode, which is associated with the nonlocality of the mechanism of the spectrum formation of free electrons. EEDFs transformations led to the space dependence of electron drift velocities on the plasma area. The dependence of the voltage drop across the cathode on the gas pressure and discharge current was noted.

Spatial Variations of Plasma Parameters in a Hollow Cathode Discharge

Dynamics of hollow cathode discharge using fluid models with and without metastable atom involvement in helium

Large-area capacitive discharges driven at frequencies higher than the usual industrial frequency of 13.56 MHz have attracted recent interest for materials etching and thin film deposition on large-area substrates. Standing wave and skin effects can be important limitations for plasma processing uniformity, which cannot be described by conventional electrostatic theory. An electromagnetic theory is developed for a discharge having two plates of radius R and separation 2l, which accounts for the propagation of surface and evanescent waves from the discharge edge into the centre and the role of capacitive and inductive fields in driving the power absorption. Examples of discharge fields are given having substantial standing wave and/or skin effects. The conditions for a uniform discharge without significant standing wave and skin effects are found to be, respectively, λ0>>2.6(l/s)1/2R and δ>>0.45(dR)1/2, where λ0 is the free space wavelength, s is the sheath width, δ = c/ωp is the collisionless skin depth, with c the speed of light and ωp the plasma frequency, and d = l-s is the plasma half-width. Taking the equality for these conditions for a discharge radius of 50 cm, plate separation of 4 cm, and sheath width of 2 mm, there is a substantial skin effect for plasma densities 1010 cm-3, and there is a substantial standing wave effect for frequencies f70 MHz.

Standing Wave and Skin Effects in Large Area, High Frequency Capacitive Discharges

In this paper, the electron density in radio frequency (RF) hollow cathode discharge (HCD) is investigated in experiments and in simulation based on two-dimensional (2D) fluid model. The role of hole diameter on plasma density in RF HCD have been explored at various gas pressures. It is found that the optimal hole size for maximum plasma density decreases along with the increase of gas pressure, which is confirmed by simulated results. The simulations reveal that the high plasma density is owing to the hollow cathode effect in the hollow cathode. It is obtained that the optimal hole diameter in RF HCD is approximately sum of twice thickness of plasma sheath and triple the electron-neutral mean free path.

Shao-Wei Xu

Papers

Experimental and Theoretical Optimization of Radio Frequency Hollow Cathode Discharge