Mukesh Kulsreshath

Breakdown study of dc silicon micro-discharge devices

The influence of geometrical and operating parameters on the electrical characteristics of dc microcavity discharges provides insight into their controlling physics. We present here results of such a study on silicon-based microcavity discharge devices carried out in helium at pressure ranging from 100 to 1000 Torr. Different micro-reactor configurations were measured. The differences include isolated single cavities versus arrays of closely spaced cavities, various cavity geometries (un-etched as well as isotropically and anisotropically etched), various dimensions (100 or 150 µm cavity diameter and 0–150 µm depth). The electrode gap was kept constant in all cases at approximately 6 µm. The applied electric field reaches 5 × 107 V m−1 which results in current and power densities up to 2 A cm−2 and 200 kW cm−3, respectively. The number of microcavities and the microcavity depth are shown to be the most important geometrical parameters for predicting breakdown and operation of microcavity devices. The probability of initiatory electron generation which is volume dependent and the electric field strength which is depth dependent are, respectively, considered to be responsible. The cavity shape (isotropic/anisotropic) and diameter had no significant influence. The number of micro-discharges that could be ignited depends on the rate of voltage rise and pressure. Larger numbers ignite at lower frequency and pressure. In addition, the voltage polarity has the largest influence on the electrical characteristics of the micro-discharge of all parameters, which is due to both the asymmetric role of electrodes as electron emitter and the non-uniformity of the electric field resulting in different ionization efficiencies. The qualitative shape of all breakdown voltage versus pressure curves can be explained in terms of the distance over which the discharge breakdown effectively occurs as long as one understand that this distance can depend on pressure.

Ignition and extinction phenomena in helium micro hollow cathode discharges

Micro hollow cathode discharges (MHCD) were produced using 250 µm thick dielectric layer of alumina sandwiched between two nickel electrodes of 8 µm thickness. A through cavity at the center of the chip was formed by laser drilling technique. MHCD with a diameter of few hundreds of micrometers allowed us to generate direct current discharges in helium at up to atmospheric pressure. A slowly varying ramped voltage generator was used to study the ignition and the extinction periods of the microdischarges. The analysis was performed by using electrical characterisation of the V-I behaviour and the measurement of He*(3S1) metastable atoms density by tunable diode laser spectroscopy. At the ignition of the microdischarges, 2 µs long current peak as high as 24 mA was observed, sometimes followed by low amplitude damped oscillations. At helium pressure above 400 Torr, an oscillatory behaviour of the discharge current was observed just before the extinction of the microdischarges. The same type of instability in the extinction period at high pressure also appeared on the density of He*(3S1) metastable atoms, but delayed by a few µs relative to the current oscillations. Metastable atoms thus cannot be at the origin of the generation of the observed instabilities.

Width-dependent interaction of trench-like microdischarges arranged in sub-arrays on a single silicon-based chip

Multiple trench microdischarge reactor arrays based on a silicon (Si) platform were investigated as a concept to overcome production-based errors. The devices incorporated four sub-arrays of trench-like anisotropically etched cavities on a single 2 cm × 2 cm Si chip. Each sub-array consisted of an equal number of structures of equal length, depth and separation only varying in trench width. Experiments were performed in argon (Ar) close to atmospheric pressure at ac frequencies of the order of 10 kHz. The arrays were characterized by means of electrical measurements and by (phase-resolved) optical emission. It is shown that the whole device as well as the independent sub-arrays and the individual cavities behave in a similar way as the inverted-pyramid structures investigated beforehand. Under identical conditions, the voltage required for a first ignition increases with the width of the cavities. For maximum voltages high enough for operation of all sub-arrays, the individual arrays ignite within the excitation period in reverse order. For different excitation frequencies, self-pulsing and emission waves propagating independently across the sub-arrays were observed. Both phenomena show distinct characteristics for the individual sub-arrays being more prominent for the smaller cavities. The observations are interpreted within the frame of a simple physical picture, as a field and a surface-dependent process and the influence of long-living species such as metastables.

Ignition dynamics of dry-etched vertical cavity single-hole microdischarge reactors in ac regime operating in noble gases

Silicon-based multi-cavity microdischarge reactors allow the generation of large-area uniform glow discharges over a wide pressure range up to atmospheric pressure. These devices, fabricated using micro electro-mechanical system technology, have shown complex interactions between the individual cavities. To discriminate these interactions, devices with only one shallow vertical cavity are studied here. Operation characteristics are investigated using electrical and optical analysing techniques. The spatial and temporal dynamics of the discharge are investigated for positive and negative voltage polarity of the applied ac voltage ramp by phase-resolved imaging. Within each voltage half-period, emission from the single cavity shows repetitive pulsing features and distinct spatial distributions. In the positive half-period, ring-shaped structures develop, while the negative half-period is distinguished by a bell-shaped intensity distribution. Effects of pressure and operation frequency on the spatial and temporal intensity profiles are discussed.