Peng Tian

Controlling VUV photon fluxes in low-pressure inductively coupled plasmas

Low-pressure (a few to hundreds of millitorrs) inductively coupled plasmas (ICPs), as typically used in microelectronics fabrication, often produce vacuum-ultraviolet (VUV) photon fluxes onto surfaces comparable to or exceeding the magnitude of ion fluxes. These VUV photon fluxes are desirable in applications such as sterilization of medical equipment but are unwanted in many materials fabrication processes due to damage to the devices by the high-energy photons. Under specific conditions, VUV fluxes may stimulate etching or synergistically combine with ion fluxes to modify polymeric materials. In this regard, it is desirable to control the magnitude of VUV fluxes or the ratio of VUV fluxes to those of other reactive species, such as ions, or to discretely control the VUV spectrum. In this paper, we discuss results from a computational investigation of VUV fluxes from low-pressure ICPs sustained in rare gas mixtures. The control of VUV fluxes through the use of pressure, pulsed power, and gas mixture is discussed. We found that the ratio, β, of VUV photon to ion fluxes onto surfaces generally increases with increasing pressure. When using pulsed plasmas, the instantaneous value of β can vary by a factor of 4 or more during the pulse cycle due to the VUV flux more closely following the pulsed power.

Controlling VUV photon fluxes in pulsed inductively coupled Ar/Cl2 plasmas and potential applications in plasma etching

UV/VUV photon fluxes in plasma materials processing have a variety of effects ranging from producing damage to stimulating synergistic reactions. Although in plasma etching processes, the rate and quality of the feature are typically controlled by the characteristics of the ion flux, to truly optimize these ion and photon driven processes, it is desirable to control the relative fluxes of ions and photons to the wafer. In prior works, it was determined that the ratio of VUV photon to ion fluxes to the substrate in low pressure inductively coupled plasmas (ICPs) sustained in rare gases can be controlled by combinations of pressure and pulse power, while the spectrum of these VUV photons can be tuned by adding additional rare gases to the plasma. In this work, VUV photon and ion fluxes are computationally investigated for Ar/Cl2 ICPs as used in etching of silicon. We found that while the overall ratio of VUV photon flux to ion flux are controlled by pressure and pulse power, by varying the fraction of Cl2 in the mixture, both the ratio of VUV to ion fluxes and the spectrum of VUV photons can be tuned. It was also found that the intensity of VUV emission from Cl(3p 44s) can be independently tuned by controlling wall surface conditions. With this ability to control ratios of ion to photon fluxes, photon stimulated processes, as observed in halogen etching of Si, can be tuned to optimize the shape of the etched features.

Properties of microplasmas excited by microwaves for VUV photon sources

Microplasma sources typically take advantage of pd (pressure × size) scaling by increasing pressure to operate at dimensions as small as tens of microns. In many applications, low pressure operation is desirable, which makes miniaturization difficult. In this paper, the characteristics of low pressure microplasma sources excited by microwave power are discussed based on results from experimental and computational studies. The intended application is production of VUV radiation for chemical analysis, and so emphasis in this study is on the production of resonant excited states of rare gases and radiation transport. The systems of interest operate at a few to 10 Torr in Ar and He/Ar mixtures with cavity dimensions of hundreds of microns to 1 mm. Power deposition is a few watts which produces fractional ionization of about 0.1%. We found that production of VUV radiation from argon microplasmas at 104.8 nm and 106.7 nm saturates as a function of power deposition due to a quasi-equilibrium that is established between the electron temperature (that is not terribly sensitive to power deposition) and the population of the Ar(4s) manifold.

Thomson scattering diagnostics and computational modeling of a low pressure microwave excited microplasma source

Summary form only given. Low-pressure microwave-excited microplasmas1 are promising sources of VUV photons for a variety of applications, including photoionization for mass spectrometry. A split-ring resonator microstrip architecture can be used to initiate and sustain these microplasmas using the extremely high electric field generated in the sub-millimeter gap between electrodes. The VUV flux, primarily the result of resonance radiation following electronic excitation of rare gas atoms, is a sensitive function of the distribution of electron energies. Direct measurement of the electron energy distribution (EED) could provide critical insight into the physics of the microplasma operation. We present Thomson scattering measurements of the EED in a split-ring resonator argon microplasma operating at 2.5 GHz and approximately 1 Torr. The diagnostic consists of a Q-switched Nd:YAG laser operating at 532 nm. Thomsonscattered light is collected with a high throughput (f/2) triple grating imaging spectrometer and an intensified CCD camera gated to the laser pulses. Stray and Rayleigh-scattered laser light, which can exceed the intensity of the Thomsonscattered light by factors of over 105, is filtered out using a mask placed between the first two gratings, which are operated in subtractive mode. Other available diagnostics include VUV flux measured using a vacuum UV monochromator. Plasma parameters are measured as a function of gas flow rate, absorbed microwave power, and spatial location both within the plasma cavity and in the downstream plume. Experimental results of the EED and VUV flux are compared with computational modeling using the Hybrid Plasma Equipment Model (HPEM).2 In this model, the EED and radiation transport are computed using Monte Carlo simulations, and neutral gas and plasma transport are addressed using fluid techniques. These experimental and computational modeling results provide a means for optimizing the VUV flux produced by the source.

Customizing arrays of microplasmas for controlling properties of electromagnetic waves

Summary form only given. Microplasma arrays are being investigated to manipulate the propagation of electromagnetic waves [1]. Such applications require control of plasma properties over large dynamic ranges across a large area, resulting in control of their absorption, dielectric and metamaterial properties. These microplasma arrays often operate at intermediate pressures of 10s to 100s of Torr, motivated by a tradeoff between obtaining fast response and having a high plasma density. pd scaling then implies plasma cavities of hundreds of microns, which are then replicated in arrays. Controlling cross-talk between microplasma units is a challenge since normally they are not physically isolated to reduce absorption or diffraction by structural components. Plasma properties of 1-D and 2-D microplasma arrays excited by pulsed dc-bipolar/unipolar waveforms were computationally investigated. Results will be discussed for investigations aimed at maximizing the time averaged electron density and dynamic range during pulses, and controlling cross-talk between microplasmas that are not physically isolated. Small arrays of microplasmas of hundreds of microns across (3-6 microplasma units) were investigated in rare gas mixtures operating in 10s-100s Torr pressure. The basic geometry is four microplasma cells operating in 60 Torr Ar generated with 300 V unipolar pulses of 100 ns duration. The electron density peaks up to 2 × 1014 cm-3 with the cathode fall region forming near the exposed cathode. Beam ionization by secondary electrons from the cathode is the major source of electrons with ionization by bulk electrons contributing approximately 30% of the total. Cross talk between discharges results from electrons drifting away from the peaks in plasma density towards maxima in plasma potential that occurs between the microplasma cells. In the base case, the influence of the crosstalk on plasma behavior is weak, which makes it possible to control different plasma cells separately even though they are not isolated physically. The predicted plasma properties of the arrays are used to evaluate the potential for controlling electromagnetic wave properties through simulation of microwave propagation through large arrays of such microplasmas.

UV emission and probe diagnostics and computational modeling of a low pressure microwave excited microplasma source

Summary form only given. Low-pressure microwave-excited microplasmas have a wide variety of potential applications, including their use as UV photoionization sources for mass spectrometry. Resonant microwave microstrip architectures can be used to initiate and sustain these microplasmas. When operated in a windowless configuration, the plasma plume exiting from an aperture in the plasma confinement structure contains a complex mixture of particle species (UV photons, groundstate neutrals, metastables, and plasma electrons and ions)1. Understanding the makeup of this plume is critical to optimize parameters for photoionization, or any other application where exposure to the plasma plume takes place. The microplasma source under investigation consists of a resonant microstrip pattern on alumina substrate, with an elongated 1.5 mm x 6.5 mm plasma confinement structure. 2.5 GHz microwave power is delivered at up to 5 W. Argon and helium/krypton mixtures are flowed through the confinement region at flow rates up to 10 sccm producing confinement region pressures near 1 Torr, with the plasma plume exiting into high vacuum through a 300μm-by-600μm aperture. The effect of confinement region and microstrip geometries, net absorbed microwave power, and source region pressures on the properties of the plasma in the source region and plume are investigated. Ultraviolet emission and Langmuir probe diagnostics are used to diagnose the plasma. The spatial distribution of the plasma density in the plume is measured for a range of microwave powers and flow rates. We also present computational modeling results of this microplasma using the Hybrid Plasma Equipment Model (HPEM)2 with comparisons to the experiments. The angular distribution of UV flux is observed, revealing a highly directional radiation pattern owing to an elongated source region. This flux magnitude and directionality is additionally a function of source region pressure, due to variation in the spatial distribution of the plasma and resonant UV photon absorption and quenching.

Plasma dynamics of microwave excited microplasmas in a sub-millimeter cavity

Summary form only given. Capacitively coupled microplasmas in dielectric cavities have a range of applications from VUV lighting sources for surface treatment to radical production. Due to the large surface-to-volume ratio of these devices, the wall mediated dynamics of plasma transport are important to the uniformity and confinement of the plasma. For example, there may be applications where a plume of ionized gas is desired from the microcavity - whereas other applications may require a confined plasma emitting only VUV photons.In this paper, we will discuss results from a computational investigation of the plasma dynamics in microwave excited micro plasma VUV lighting sources. A 2dimensional hydrodynamics model, the Hybrid Plasma Equipment Model, has been used in which radiation and electron energy transport are addressed using Monte Carlo techniques. The microdischarges have widths of:: 1 mm and lengths of :: 1 cm, operate at pressures of 1-20 Torr, with microwave power of 2-10s Watt at 2.5 GHz and a flow rate of several sccm. Gases are either pure rare gases or mixtures of rare gases. We found that the plasma operates in a mode that has both normal-glow and abnormal glow characteristics. Under usual operation in argon, plasmas are produced with a peak electron density of 1013 cm-3. The plasma may not fill the microdischarge cavity at low power. As the power is increased, the plasma expands to fill the cavity. In this regard, the plasma operates as a normal glow. The current density, however, increases with increasing power, and so in this regard, the plasma resembles an abnormal glow. The expansion of the plasma will eventually overfill the cavity, at which time a plasma plume is formed. These plasma dynamics are sensitive to gas mixture. The scaling of plasma confinement and VUV production as a function of aspect ratio, power and gas mixture will be discussed.

Properties of bipolar DC-pulsed microplasmas at intermediate pressures

Summary form only given. Microplasmas generated in spatially confined cavities have applications ranging from electrical switching and radical production to lighting. In these applications, there is often a tradeoff between obtaining a short response time of the plasma and maximizing plasma density, both of which optimize with higher pressure; and obtaining a uniform plasma, which optimizes with lower pressure. These scalings motivate operation at intermediate pressures, tens of Torr to 100 Torr, which by pd scaling corresponds to sizes of the micro-cavity of hundreds of microns. In many cases, the inner surfaces of the microplasma cavities are largely dielectric due to ease of fabrication or to maximize lifetime. These conditions then motivate use of some form of bipolar excitation.In this paper, we discuss results from a computational investigation of scaling of microplasmas excited by pulsed dc-bipolar waveforms with the goal of maximizing the time averaged electron density. The computational platform is the Hybrid Plasma Equipment Model, a 2-dimensional hydrodynamics model in which radiation transport, and electron and ion distributions are addressed using Monte Carlo techniques. We investigated plasmas of 10s-100s Torr excited by short DC bipolar pulses (a few ns) with pulse repetition periods ranging from tens to hundreds of ns using mixtures of rare gases. Cavity sizes are a few hundred microns. Quasi-steady state, time averaged electron densities in excess of 1015 cm-3 in Penning mixtures are predicted. Although ionization by bulk electrons is the major source, the uniformity of the plasma is sensitive to ionization due to sheath accelerated secondary electrons. The behavior of the plasma was asymmetric with respect to the polarity of the voltage pulses, with more ionization occurring on the anodic portion of the cycle, in large part due to the electrically floating dielectric boundaries.