Langmuir Probe Method Research Articles

Diagnostics of chlorine inductively coupled plasmas. Measurement of electron temperatures and electron energy distribution functions

The goal of the study that begins with this paper is the creation of a base set of parameters (densities and temperatures or energy distributions of all charged and neutral species) in a chlorine transformer-coupled plasma, measured with the same commercial plasma source over an extensive range of pressure and power. Electron temperatures Te and electron energy distribution functions (EEDFs) are reported as a function of pressure (1–20 mTorr) and power (10–1000 W) during slow etching of SiO2-covered Si wafers. Te values are obtained both by trace rare gases optical emission spectroscopy and Langmuir probe methods. EEDFs are obtained with the Langmuir probe. A zero-dimensional (global) model with revised rate coefficients is used to compute Te from both minimum (pressure, total plasma density, gas temperature, and wall recombination coefficient) and maximum (all experimentally measured parameters necessary to calculate Te) sets of input parameters.

Global model and scaling laws for inductively coupled oxygen discharge plasmas

For inductively coupled oxygen rf discharge plasmas, a global model which consists of the energy and particle balance equations is formulated. The energy balance equation includes energy losses for electron-neutral elastic collision, excitation, ionization, dissociation, pair production, and charged particle wall loss. Particle balance equations are written for all species of interest. For a specified discharge length and diameter, absorbed power, pressure, electron temperature-dependent reaction rate coefficients and surface recombination constants, we solve these equations to determine the densities of all species and the electron temperature. We measure these parameters by the Langmuir probe method. According to the prevailing particle loss mechanism, the parameter space can be divided into a volume recombination-loss-dominated region and an ion-flux-loss-dominated region. Based on the global model equations, the scaling laws of plasma variables with the control parameters for the ion-flux-loss-dominated region are estimated and compared with the experimental results.

Observations of the anode boundary layer in free-burning argon arcs

In order to make clear the physical grounds of the potential drop in front of the anode, namely, the anode fall in atmospheric free-burning argon arcs, the results of experimental measurements of the laser-scattering method and Langmuir-probe method are presented. The experimental results show that the anode boundary layer at low arc currents such as 50 A remarkably deviates from local thermodynamic equilibrium (LTE), whereas the boundary layer at higher arc currents such as 150 A preserves a similar state to the LTE. The Langmuir-probe measurements also show that the anode fall for 50 A is positive, whereas that for 150 A is negative. From these results, an assumption regarding the physical state of the anode boundary layer in the free-burning argon arcs is presented synthetically and it is also concluded that the sign and magnitude of the anode fall in the arcs relate vary closely to the thermal state of the anode boundary layer and that the thermal state should be influenced strongly by the arc current density, namely, the electron number density.

Plasma conditions for as-grown low temperature poly-Si formation on SiO2 substrate by sputtering and plasma enhanced chemical vapor deposition processes

We investigated the deposition condition of the plasma processing for the formation of as-grown poly-Si thin film on the SiO2 substrate with low temperature (300 °C) processing. We intended to compensate the thermal energy loss due to the low temperature processing with the low kinetic energy ion bombardment assist. We adopted both plasma enhanced chemical vapor deposition (PECVD) (SiH4/Ar/H2) and sputtering (Si-target/Ar/H2) processes. Plasma parameters were measured by the advanced Langmuir probe method tuned for 182.5 MHz rf plasma. Crystallinity of the film was estimated mainly by x-ray diffraction. Crystallization occurred by in situ hydrogenation during the deposition. In our experiment, ion bombardment energy was approximately 23 eV. We define the normalized ion (ni) flux number and investigated the correlation between the ni and the film crystallinity. The result is as follows, the crystal size enlarges with the increasing ni up to ∼20 but it diminishes as the ni increases over ∼20. Under the same plasma condition, the PECVD processed Si films have better crystallinity than the sputtering processed ones. The film thickness and crystal size are 1000 and 219 Å, respectively, at the optimum value of the ni in the PECVD process. Atomic hydrogen is thought to be very effective for the crystallization of the film and, as for the ion assist effect, there exists an optimum value of ni for a certain ion bombardment energy.

A Novel and Convenient Method for Monitoring Processing Plasma: The Insulated Pulse Probe Method

The “insulated pulse probe method” is presented as a useful and convenient method suitable for monitoring plasmas for processing. This method is essentially stable against the change of the probe surface conditions and is free from the fatal problems particular to the conventional Langmuir probe method in reactive plasmas. In this method, the probe surface is originally covered with an insulating layer, and the time response of the probe current is measured on application of a square voltage pulse to the probe. The theoretical analysis on the response of the probe current is presented, and the electron temperature measured by this method in Ar plasma shows good agreement with that measured by the conventional Langmuir probe method. In addition, it is proposed that a silicon wafer under processing is usable as the probe to observe the plasma just in front of the wafer.

The ion current density and spectroscopic study in a straight magnetic filtering arc deposition system

The changes of plasma states during cathodic arc discharges of Ti and TiAl under various magnetic fields of straight magnetic solenoid were analyzed by Langmuir probe method and optical emission spectroscopy (OES). The ion emission intensity of Ti II for Ti arc discharge was enhanced by a factor of 2. with increasing magnetic field up to 150 Gauss compared to that under no magnetic fields. For discharging TiAl with ambient N 2 under 150 Gauss, however, the emission of Al exhibited dominant neutral emission of Al I only with slight increase of ionized Al II emission while Ti II emissions were increased at a similar ratio as measured in Ti discharge. The enhancement of electron density induced by a straight solenoid magnetic field promoted the effective ionization of metal vapors, therefore producing a high fraction of ionized vapor flux including macroparticles, which contributed to the increase of deposition rate and sputtering of macroparticles seated on the film surface by energetic metal ion flux. The changes of surface morphology and film formation behaviors of TiN and (TiAl)N arc was evaluated with a variation of external magnetic field in conjunction with plasma conditions obtained from Langmuir probe, OES and XRD analyses. The corresponding hardness and friction coefficient variations are also illustrated.

An ECR sheet plasma with a slot antenna and permanent magnets

A plasma source is developed to create broad thin films. Instead of generating large plasmas, substrates will be scrolled in uniform sheet plasmas to extend the process area. The plasmas are generated by electron cyclotron resonance; microwaves are radiated from a rectangular waveguide with a slot on its E-plane. This slot antenna is put between two permanent magnets. Their poles face each other so that a cusp field is formed in front of the slot. Plasmas are generated with gas; plasma parameters are measured by the Langmuir probe method and visible-range spectroscopy. The density uniformity is 9% within 20 cm length when a short plunger terminates the slot antenna. Spectra of bands and oxygen atomic lines are observed.

Preparation of Rutile TiO 2 Films by RF Magnetron Sputtering

TiO2 films were prepared by rf (13.56 MHz) magnetron sputtering using a mixture of Ar and O2 gases. At a total pressure of 2 mTorr, 100% rutile TiO2 films were successfully obtained on non heated substrates with rf power of 200 W, while 100% anatase TiO2 films were deposited at a pressure of 20 mTorr. Spatial profiles of both emission of excited species and plasma parameters were measured by optical emission spectroscopy (OES) and the Langmuir probe method. At a pressure of 2 mTorr, it was found that high-energy electrons are generated at a certain radial position near the cathode surface where the transverse magnetic field is maximum, and the strong localization of plasma was observed. It was proven that the energetic species impinging on the growing film are responsible for the formation of the rutile phase even if the substrate is at room temperature.

Low-temperature in situ cleaning of silicon (100) surface by electron cyclotron resonance hydrogen plasma

Low-temperature, defect-free, in situ cleaning of silicon prior to homoepitaxy is successfully developed by an electron cyclotron resonance hydrogen plasma treatment in an ultrahigh vacuum chamber. The plasma potential distribution was measured by a Langmuir probe method to understand the effect of the substrate dc bias during hydrogen plasma cleaning. It changes from downhill to uphill distribution as the dc bias changes from a negative to a positive value, which leads to a decrease in the ion number density arriving at the substrate and results in the complete suppression of the defect formation in the Si substrate. In situ hydrogen plasma cleaned Si wafer always resulted in higher quality epilayers than ones cleaned only by so-called hydrogen passivation after the HF dip. We found that there is a critical dose of the hydrogen ions during in situ plasma cleaning beyond which crystalline defects are observed in the Si substrate, subsequently leading to the poor crystallinity of the epilayers. The dose of the hydrogen ions during plasma cleaning can be effectively controlled by the substrate dc bias, the microwave power, the magnet current, and the cleaning time.

ITO Thin Films Prepared by Magnetron Sputtering Method Using ITO Target. Effects of Plasma Conditions and Substrate Temperature on ITO Film Properties.

ITO (indium tin oxide) thin films have been prepared by the magnetron sputtering method. The effects of plasma conditions on ITO thin-film properties have been investigated by means of the Langmuir probe method. Furthermore, the effects of substrate temperature have been investigated. The following results were obtained : (1) Polycrystal ITO thin films were prepared at 40°C substrate temperature. (2) High electron temperature (Te) is necessary for ITO thin-film crystallization. (3) The magnetic field plays an important role in generating high-Te plasma.

Experimental investigation of particulate formation in He-SiH4 modulated RF discharges

Formation processes of particulates in radiofrequency power-modulated SiH4 discharges have been studied using two kinds of laser light scattering, a modified Langmuir probe and absorption methods. The results suggest that particulates are formed by three phases of nucleation, rapid growth and growth saturation. In the nucleation phase, the formation may be caused by short-lifetime radicals such as SiH2, SiH or Si. In the rapid growth phase, SiHn+ (n=0-3) ions and/or SiH3 radicals seem to make an important contribution to particulate growth, if only direct influx of the particles onto a particulate is taken into account. In the growth saturation phase, the saturation may be explained by taking into account the decrease in enhancement factor of the ion-collecting areas of particulates and the increase in their loss due to the reduction of ambipolar field in the periphery of the discharge column. Power modulation in SiH4 radiofrequency discharges is found to be practically very effective for decreasing the sizes of particulates.

Study on Growth Processes of Subnanometer Particles in Early Phase of Silane RF Discharge

Growth processes of particles formed in an early phase of silane RF plasmas are investigated using the modified Langmuir probe, spectroscopic and photodetachment methods. From the probe measurements, the particles are estimated to be on the order of subnanometer size, and they are found to nucleate and grow principally around the plasma/sheath boundary near the RF electrode, where optical emission intensity of SiH radicals is high. These spatial profiles are different from those of positive and negative ion densities during discharging periods. These results suggest that many short-lifetime neutral radicals are necessary for the nucleation and subsequent growth of a particle. The photodetachment experiments and scanning electron microscope (SEM) observation also show that the particles are in the size ranges below about 200 atoms and about 1 nm. Spatial profile measurements of probe currents and negatively charged particle density obtained by the photodetachment method confirm that the mass of the particles around the plasma/sheath boundary near the RF electrode is large.

Time-Resolved Measurements of RF Plasmas Using Electrostatic Probe Having Compensation Circuits for Space Potential Fluctuation

The time-resolved Langmuir probe method using a laser-heated emissive probe to compensate space potential fluctuation is modified to improve the limit of amplitude for compensation and reliability of current detection in order to experimentally study RF plasma under various conditions. Time-resolved electron temperatures and electron densities are measured at various input powers and pressures of Ar and He in 13.56 MHz RF asymmetric discharge. Not only the electron temperature but also the electron density change was synchronized with excitation RF. The period of variation is not the same as second harmonics, but is a fundamental period of 13.56 MHz. The electron temperature and the self-bias depend on the excitation RF power. Such evidence may indicate that plasma parameters are closely related with the electron acceleration in the electrode sheath and the plasma caused by the self-bias field.

Novel in situ method to detect subnanometer-sized particles in plasmas and its application to particles in helium-diluted silane radio frequency plasmas

A new Langmuir probe method is proposed to reveal processes of particulate nucleation and its subsequent early growth in silane radio frequency (rf) plasmas. In the method, subnanometer-sized particles grown during the rf-discharge-on phase are detected as negative ions in the subsequent afterglow phase in which temperature and density of electrons have decayed. Preliminary experiments applying the method to helium-diluted silane rf plasmas have the following features of particle growth: subnanometer-sized particles are mainly generated around the plasma/sheath boundary near the rf electrode; they grow with increase in the rf-discharge-on period, Ton, for Ton≲a gas residence time of the discharge.

Formation processes of particulates in helium-diluted silane RF plasmas

Particulate formation processes in SiH/sub 4/ RF plasmas are studied using two laser light scattering (LLS) methods and a newly developed Langmuir probe method together with a discharge-modulation method. From the experiments, the following results are obtained: particulates tend to appear around the plasma/sheath boundary of the RF electrode in their early growing phase and, in the subsequent rapid growing phase, grow there at a rate of a few tens nm/s high compared to the film deposition rate; after the rapid growing phase, the growth rate decreases considerably; size and density of particulates amount to 60/spl sim/180 nm and 10/sup 8//spl sim/10/sup 8/ cm/sup /spl minus/3/; spatial distributions of subnanometer-size particles detected by the probe method are similar to those of particulates of above a few tens nm in size observed by the LLS methods. These results suggest that short lifetime neutral radicals primarily contribute to particulate nucleation and subsequent early growth; the particulate growth rate during the rapid growing phase can be explained by taking into account the contribution of positive-ion and/or neutral radical fluxes to the particulate surface. Furthermore, the considerable decrease in the growth rate after the rapid growing phase may suggest that the positive ions mainly contribute to the rapid growth. >

Plasma Oscillation Method for Measurements of Absolute Electron Density in Plasma

A new and simple method for precise measurements of electron density in a plasma with use of plasma oscillations is described. A weak electron beam is injected from a hot filament and excites electron waves oscillating at the plasma frequency ?p, whose sharp spectrum, obtained using a detector, gives the electron density. This plasma oscillation method is free of many difficulties often encountered with a Langmuir probe, such as thin film deposition, plasma potential fluctuation, and magnetic fields. The measurement technique is theoretically analyzed and experimentally demonstrated in the density measurements of an inductively coupled rf plasma, together with a comparison with the Langmuir probe method.

Optical and Langmuir Probe Investigations of Excitation Temperatures in a Low-Pressure Glow Discharge with Variations in Discharge Gas Identity

In an effort to more fully understand the excitation and ionization mechanisms present in a low-pressure (0.3-3 Torr), low-current (0-5 mA), diode geometry glow discharge, preliminary studies of electron characteristics with variations in discharge pressure and fill gas identity were undertaken. For comparison, excitation temperatures were measured by both optical and Langmuir probe methods. Neither method showed electron excitation in the plasma to vary substantially with changes in discharge pressure or changes in discharge gas composition, including quenching with methane.

直流グロー放電の形態観察と放電特性に与える金属蒸発粒子の影響

The purpose of the present report is to comprehend the property in the D.C. glow discharge under the conditions which is used for the actual plasma process. We observed the glow discharge morphologies, using the Langmuir probe method. We examined the influence of the metal (Al, Ag and Sn) vapor on the glow discharge phenomena.It was found that the morphologies of the glow discharge was classified into four types. Evaporation of Al into glow discharge increased the discharge current but decreased the discharge voltage. On the other hand, the introduction of Ag-vapor or Sn-vapor decreased the discharge current and increased the discharge voltage. These results suggest that there are the different states for metal vapor in glow discharge. It is concluded that these phenomena are mainly caused by the value of the ionization energy.

ITO Thin Films Prepared by Magnetron Sputtering Method Using ITO Target. Effects of Plasma Conditions and Substrate Temperature on ITO Film Properties.

ITO (indium tin oxide) thin films have been prepared by the magnetron sputtering method. The effects of plasma conditions on ITO thin-film properties have been investigated by means of the Langmuir probe method. Furthermore, the effects of substrate temperature have been investigated. The following results were obtained : ( 1 ) Polycrystal ITO thin films were prepared at 40 °C substrate temperature. ( 2 ) High electron temperature (Te) is necessary for ITO thin-film crystallization. Moreover, the magnetic field plays an important role in generating high-Te plasma.

Plasma density dependence of the oxidation rate of Si by in situ during process rapid ellipsometry

Oxidation of silicon in a radio frequency discharge oxygen plasma was studied using in situ during process rapid ellipsometry (1 s resolution and interval). Plasma characteristics were also determined by the Langmuir probe method and emission spectroscopy. From the measurement of in situ during process rapid ellipsometry, oxidized film thickness of silicon increased steeply right after starting rf discharge and slowly after ≊3000 s. It was found that thickness change rate measured by in situ during process rapid ellipsometry immediately after starting the rf discharge was strongly proportional to O2+ ions density.