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Langmuir Spatial Probe

Langmuir Spatial Probe

Plasma Measurements On The Move

The Langmuir Spatial Probe uses an automated linear drive to scan across a plasma and take measurements of plasma parameters at different locations. The spatial profiling of the plasma can help solve issues around uniformity.

The Langmuir Spatial Probe measures the spatial profile of the plasma like: plasma density, floating potential, plasma potential, ion current density, and the electron energy distribution function. Our system uses the most up to date probe theory available, drawing on Orbital Motion Limited and as the pressure regimes change, moving on to Allen Boyd Reynolds to account for collisions.

The Langmuir Probe equipped with a linear drive system helps to characterise the plasma parameters at different locations inside a plasma reactor.



Features

Automated Spatial Scanning | Single & Double Probe Combined |
Advanced RF Compensation | High Speed Acqusition


Applications

Fundamental Research | Process Development | Equipment Design | Model Validation




image of linear drive is for representative purposes only

Overview

The Langmuir Spatial Probe is the key instrument used to measure the internal parameters of plasma while using an automated linear drive to scan across the bulk of the plasma. key parameters measured are electron density, Ion density, electron temperature, plasma potential, floating potential and the electron energy distribution function (EEDF). The Langmuir Spatial Probe provides plasma characterisation measurements in DC, RF, microwave, continuous and pulsed plasma. The Langmuir Spatial Probe analyses ion and electron trajectories to obtain accurate measurements of the plasma parameters in a wide range of plasma applications. The Langmuir Spatial Probe is the fastest and most reliable Langmuir probe in the world (time resolution 12.5ns). In addition to speed and reliability. The Langmuir Probe provides the most advanced and trusted, fully automated data analysis in real time.

The Impedans Langmuir Probe system comes complete with interchangeable single and double probe tips (at no extra charge) which can be used with the same electronics unit and also comes complete with a fully automated linear drive system. This allows users conduct experiments across different reactors and allows plasma characterisation measurements in reactors which have a poor ground return.

The Langmuir Probe is used to establish plasma process repeatability and uniformity. It helps in the understanding plasma changes and the impact on surface treatment. The Langmuir Spatial Probe is an essential plasma process diagnostic to understand the correlation between plasma inputs and the plasma state.

Plasma Parameters Measured

  • Floating Potential Uniformity
  • Plasma Potential Uniformity
  • Plasma Density Uniformity
  • Ion Current Density Uniformity
  • Electron Energy Distribution Function (EEDF)

Measurement Functionality

Time Averaged Measurements
This provides an average over time of the plasma parameters measured by the Langmuir Spatial Probe in the bulk of the plasma.

Time Resolved Measurements
This allows the user to synchronise the plasma parameters measured by the Langmuir Spatial Probe with an external synchronisation signal. The user can then obtain detailed information on the plasma parameters as a function of time or phase through the synchronisation pulse period. Typically the pulse period would be on a timescale of milliseconds to microseconds.

Time Trend Measurements
This allows the user to obtain information on the variation of the plasma parameters as time progresses through a particular process. This feature does not require external synchronisation and the timescales involved can be in range of seconds to hours.

Further Product Information

Langmuir Probe Automated Electronics and Software

Electronics UnitThe user-friendly electronics and software takes accurate and reliable measurements with a speed not seen on any other commercial Langmuir Probe. Using an intelligent pre-scan feature, the optimal plasma parameter measurements are performed easily and repeatedly.

Langmuir Probe Tip Cleaning

An automated probe tip cleaning feature is provided as standard to facilitate the cleaning of the probe tip. This is especially useful in deposition plasma. Both automated and manual cleaning procedures are supported by the Single Langmuir Probe systems application software.

Langmuir Probe Replaceable Head

Langmuir Single and Double Probe Heads are easily replaced with the “Easy-Fit” probe tip holder design. Probe tip material is tungsten as standard, with molybdenum, platinum and invar available. Custom probe lengths, diameters, materials, and shapes can be supplied on request and easily updated in the software analysis.

Langmuir Probe Shaft

Probe shaft material is ceramic as standard but custom materials are available on request. Custom probe lengths, diameters and shapes can be supplied on request and easily updated in the software analysis.

Time-Resolved Measurements

A high-speed mode is available to support high resolution time-resolved measurements with a time-step resolution of 12.5ns for pulsed and low frequency applications. Trigger frequencies up to 1MHz are supported, and a built-in programmable delay allows gating of the probe measurements.

Time Averaged Measurements

In applications where high speed resolution is not required averaging the measurement over a number of data points can be used to significantly reduce the noise.

External Trigger

Integrated in the Langmuir Probe System is an external trigger TTL compatible 10Hz to 1MHz

DC Compensation

A novel pre-scan feature eliminates the need for a DC compensation electrode. Voltage shifts in the IV characteristic are monitored at various probe current drains. The plasma to ground resistance is determined and the appropriate correction is applied.

RF Compensation

The Langmuir probe shaft is constructed from metal, coated with a fine layer of ceramic to maximize the coupling of the probe tip to the plasma and eliminate the need for a compensation electrode.

Measuring Parameters

Floating Potential -145V to 145V
Plasma Potential -100V to 145V
Plasma Density 10⁶ to 3x10¹³cm⁻³
Ion Current Density 1µA/cm² to 300mA/cm²
Electron Temperature 0.1 to 15eV
Electron Energy Distribution Function 0 to 100eV

Langmuir Probe Specifications

Plasma Power Source DC, RF, Microwave, Continuous, Pulsed Plasma
RF Plasma Broadband Probe 2MHz to 100MHz
Probe Length 300mm to 1400mm (Custom Available)
Probe Diameter 10mm (Custom Available)
Probe Tip Length 10mm (Custom Available)
Probe Tip Diameter 0.4mm (Custom Available)
Probe Tip Material W, Ta, Ni, Pt. (Custom Available)
Probe Customisation 90°, 45° Bend (Custom Available)
Maximum Operating Temperature 230°C (Custom up to 1200°C)

Linear Drive

Step Resolution 0.025mm
Control Mechanism Automated through software
Drive Length 300mm, 450mm, 600mm or Custom

Electronics Control Unit

Probe Voltage Scan Range -150V to +150V
Current Range 15nA to 150mA or 1.5µA to 1A for high current densities
Communication USB 2.0
Sampling Rate 80 MSPS (V,I)
Data Acquisition Resolution 4.5mV, 4.5nA
Time Resolved Step Resolution 12.5nS
External Trigger TTL Compatible 10Hz to 1MHz

Application Software

Operating System Windows 2000 / XP / Vista / Windows 7 / Windows 8

Operating Parameters

Pressure (Pascal) 0 to 1,000Pa
Pressure (Torr) | Single Probe 0 to 10Torr
Pressure (Torr) | Double Probe 0 to 760Torr
Gas Temperature 20º to 1000º
Density 10⁴cm⁻³ to 10¹⁴cm⁻³
Gas Reactivity Inert to Highly Reactive
Power Frequency DC (0kHz) • pDC (0 to 350kHz) • MF (0 to 1MHz) • RF (1MHz to 100MHz) • Microwave (1GHz to 3 GHz)

The Langmuir Spatial Probe used in Dusty Plasma applications

Langmuir Probe used in an RF Dusty Plasma to measure the dust charge and density

Abstract

It is reported that 50 per cent of the failure in semiconductor production is due to dust particle contamination in plasma processes such as plasma etching. This study aimed to develop a method for measuring the density and charges of dust particles in a capacitive coupled cylinder discharge chamber in mixtures of gases SiH4 / C2H4 / Ar. A Langmuir probe was employed to gain such parameters as electron density and ion density of the dusty plasma.

LP02: Langmuir Probe used in an RF Dusty Plasma to measure the dust charge and density


The Langmuir Spatial Probe used in Plasma Etching applications

End-point detection of polymer etching using Langmuir Probes

Abstract

Determining accurately the end point of a plasma-etching process is extremely important for integrated circuit fabrication, as overetching can result in the removal of part of the film, or substrate lying under the film to be etched, and/or in extra undercut of the film. End-point detection is traditionally performed using several different techniques including the four most common techniques: measurement of the DC self-bias voltage, mass spectrometry, emission spectrometry, and interferometry.

LP03: End-point detection of polymer etching using Langmuir Probes

Langmuir Probe used in experimental and numerical investigations of the phase-shift effect in capacitively coupled discharges

Abstract

In this study, the authors have used a Langmuir Probe to measure the electron density and the electron energy distribution function in phase-shift controlled capacitively coupled plasmas. To verify the experimental results, a 2D fluid model was also used to calculate the electron densities at various phase differences.

LP10: Langmuir Probe used in experimental and numerical investigations of the phase-shift effect in capacitively coupled plasma discharges


The Langmuir Spatial Probe used in HiPIMS Plasma applications

Plasma diagnostics of low-pressure high-power impulse magnetron sputtering assisted by electron cyclotron wave resonance plasma

Abstract

The study focused on the plasma measurement of parameters to explore the assistance of the electron cyclotron wave resonance (ECWR) on the evolution of HiPIMS discharge, where it has several benefits.

SE04/LP08: Plasma diagnostics of low pressure HiPIMS assisted by ECWR plasma

Langmuir Single Probe used in determining the temporal evolution of negative ion density in the afterglow of reactive HiPIMS of titanium in an argon/oxygen gas mixture

Abstract

This study used a Langmuir Single Probe to determine the temporal evolution of the oxygen negative ion and electron densities during the offtime of a reactive HiPIMS discharge operating in argon–oxygen gas mixtures. The aim of the study was to add to the knowledge base information about oxygen negative ion dynamics that will help many plasma processing methods.

LP09: Langmuir Single Probe used in determining the temporal evolution of negative ion density in the afterglow of reactive HiPIMS of titanium in an argon/oxygen gas mixture

The Langmuir Spatial Probe used in PECVD applications

A study of plasma parameters in a BAI 730 M triode ion plating system by means of a Langmuir probe and plasma mass and energy spectroscopy

Abstract

Plasma-assisted processes are widely used in various areas of modern manufacturing, including physical vapour deposited (PVD) hard coatings, and there is an ongoing need for characterisation of plasma parameters. Beside the conventional Langmuir probe technique, plasma spectroscopy is being more and more widely used. This paper compares the results obtained by the Langmuir plasma probe with the results of energy and mass resolved plasma spectroscopy during three modes of operation (heating, etching, deposition) in a commercial triode ion plating system (Blazers BAI 730 M) used to deposit hard coatings like TiN, CrN and Ti(C,N).

LP06: A study of plasma parameters in a BAI 730 M triode ion plating system by means of a Langmuir probe and plasma mass and energy spectroscopy


The Langmuir Spatial Probe used in Space Plasma applications

Langmuir probe used in a lunar dust application to measure the electron density and energy distribution

Abstract

The single Langmuir probe is ideal for measuring and understanding the nature of the photoelectron plasma environment above a VUV illuminated surface. It is this photoelectron plasma region that provides the local environment for dust particles in a lunar environment.

LP01: Langmuir probe used in a lunar dust application to measure the electron density and energy distribution

Hall Effect Thruster plasma plume characterization with probe measurements and self-similar fluid models

Abstract

Hall effect thrusters (HET) are currently recognized as a good propulsion means for long missions and moves that require large velocity increments. The plasma plume of a HET exhibits a relatively large divergence angle of about 45°, and investigating this plume and its expansion into space is vital for understanding these devices and assessing the mechanical and electrical interactions of the exhaust plasma plume with the spacecraft itself and the surrounding environment.

LP04: Hall Effect Thruster plasma plume characterization with probe measurements and self-similar fluid models


The Langmuir Spatial Probe used in Plasma Sputtering applications

Observation of two-temperature electrons in a sputtering magnetron plasma

Abstract

Understanding electron transport in sputtering magnetrons is essential for the understanding of the operation of these devices, which are used for sputter etching and thin-film deposition. Several recent experiments have used Langmuir probes to investigate the electron component of a magnetron plasma and mechanisms of electron transport. For instance, Rossnagel and Kaufman reported that the electron temperature and density decrease with distance from the cathode, and noted the presence of a high-temperature electron tail under certain conditions, such as in a He discharge.

LP05: Observation of two-temperature electrons in a sputtering magnetron plasma

Time-resolved plasma characterisation of modulated pulsed power magnetron sputtering using a Langmuir probe

Abstract

With the knowledge that no investigation of certain key parameters of the plasma during Modulated Pulse Power Magnetron Sputtering (MPPMS) has yet been published, in this study, the authors employed a Langmuir probe to measure temporally resolved electron density, ion density, electron temperature, floating potential and plasma potential during the process.

LP07: Time-resolved plasma characterisation of modulated pulsed power magnetron sputtering using a Langmuir probe

Introduction

The concept of the Langmuir probe was developed almost a century ago and is named after its inventor Irving Langmuir. The Langmuir probe was the first diagnostic tool used for studying plasmas in detail and it is still widely used today. Langmuir probes, in principle, provide a simple and relatively inexpensive diagnostic for measuring the plasma parameters. However, there are a number of issues in the design and interpretation of Langmuir probe characteristics which have led in the past to a wide disparity in measured parameters obtained under similar conditions. Part of this difficulty results from an imprecise knowledge of the RF discharge parameters, voltage, current and deposited power, but this has been partially resolved by the availability of VI probes to make accurate measurements of the discharge RF parameters.

In-situ layout of the Single Langmuir Probe in a typical plasma chamber setup

Figure 1: In-situ layout of the Single Langmuir Probe in a typical plasma chamber setup

Theory of Operation

The Langmuir is a conducting wire placed inside the plasma with a variable bias, V applied. The current I, is measured as a function of V. This is called the I(V) characteristic and it has three regions; the electron collection region, the electron retardation region and the ion collection region.

Simple Langmuir Probe Schematic

Figure 2: Simple Langmuir probe schematic

Modern Langmuir probe systems are quite complex diagnostic systems with many features to prevent the pitfalls of probe diagnostics; some of which are less than obvious to the non- expert. Even with these precautions the probe system will be limited to a range of plasma conditions where accurate results are obtainable.

Current Voltage I(V) characteristic of a single Langmuir probe

Figure 3: The Current Voltage I(V) characteristic of a single Langmuir probe

A probe system can provide the following parameters: floating potential, Vf, which is the point where ion collection current equals electron retarding current. This is the potential at which an isolated object will float when placed in the plasma. The plasma potential, Vp, is the potential of the space the plasma occupies. It is normally positive with respect to the surrounding chamber to prevent excessive loss of electrons. Below Vp the electrons are retarded by the probe potential and the electron current falls off at a rate that depends on the electron average energy. Above Vp we enter the electron collection region. It is possible to calculate the electron density, Ne from the magnitude of the current at the plasma potential. The ion density, Ni is determined from the ion current in the ion collection region. A difficulty here is that the ion current needs to be extrapolated back to the plasma potential, Vp, to make accurate measurement of Ni.[1] This requires a theory to understand ion orbits around the probe and the effect of collisions in the sheath. The electron temperature, kTe, is determined from the rate at which the electron current falls in the electron retarding region. Here again, the best accuracy is achieved when the ion current is extrapolated and removed from the total current. Finally the electron energy distribution function (EEDF), can be obtained from the second derivative of the I(V) characteristic with zero energy at the plasma potential. The EEDF requires an accurate removal of the ion current and an accurate determination of the Plasma potential, Vp. It is possible to integrate the EEDF to get the electron density and the average electron energy and these measurement are independent and can be used as checks on the values of Ne and KTe derived directly from the I(V) characteristics.

The Langmuir probe can obtain data as a function of position with a resolution of a few mm and in time with resolution in the range of ns. The order of presentation of the parameters above can loosely be regarded as in order of difficulty, with the floating potential being the easiest to obtain and the EEDF being the most complex.

A suitable benchmark of a high quality EEDF can be obtained from the two-temperature structure in the case of the low-pressure RF discharge in Ramsauer-type gas such as argon[2] and the EEDF of a molecular gas which has a characteristic hole in gases such as nitrogen[3] due to the large in-elastic cross-section associated with vibrational excitation. These features are well characterised but difficult for a probe system to resolve and can act as a bench-mark on the quality of a probe system. A major objective of a commercial Langmuir probe system design is that it can be operated by an inexperienced operator and give reliable results.

Plasma can be formed either by a direct current (DC), a radio frequency (RF) current or microwave MW source. In the case of an RF plasma the plasma potential will tend to follow the RF bias by a amount that depends on both the amplitude of the RF but more importantly the configuration of the plasma; with a symmetrical parallel plate configuration having the highest magnitude of fluctuations of Vp and the inductive coupled plasma often having the lowest fluctuations in Vp. The removal of RF fluctuation in Vp relative to ground is normally achieved by means of the Passive Probe method first developed by Gagne and Cantin[4]. The probe is forced to float at the RF potential by ensuring that the probe-plasma impedance, Zp, is much less than the probe-ground impedance, Zs. The probe circuit is then a potential divider with the RF potential appearing across Zs and the probe floats relative to the plasma potential. When Zs > > Zp the characteristic is similar to a DC characteristic with the RF voltage fluctuations removed.

Schematic of Passive RF Compensation

Figure 4: Schematic of passive RF compensation

The Impedans Langmuir probe uses a high impedance RF filter to maximise Zs and the probe shaft is metallic with a thin ceramic layer to capacitively couple the probe to the plasma and minimise the Impedance between the probe tip and the plasma. The plasma to ground impedance is normally low at RF. This shunt capacitance of the ceramic coated probe holder dominates the plasma-probe impedance but has no effect on the direct current collected to the probe, the large area of the holder ensures that Zp is as small as possible. The probe holder diameter is minimised to prevent the probe structure from depleting the plasma. The blocking impedance is Zs > 100 kΩ over a broad range of frequencies.

Passive probe compensation works well at frequencies above 1MHz, where the plasma parameters are not modulated by the RF. At lower frequencies, < 1MHz, the plasma parameters change depending on the phase of the applied voltage waveform. Passive compensation is no longer valid below 1MHz and we need to move to time resolved measurement of the I(V) characteristic. This is achieved by using a sync signal coherent with the power source and an Advanced Boxcar Mode developed by Impedans.

Figure 5: Applied theories for electron and ion collection

The video above demonstrates the probe collecting ion and electron current, depending on the polarity of the potential sweep. At very low pressures the ion current to the probe is limited by orbital motion due to the ions angular momentum. As pressure increases collisions in the sheath reduce the effect of orbital motion. The probe calculates the number of collisions and applies the correct theory.

References:

[1] M.B. Hopkins J. Res. Natl. Inst. Stand. Technol. Vol. 100, No. 4, p. 415
[2] M. M. Turner, R. A. Doyle, and M. B. Hopkins, Measured and simulated electron energy distribution functions in a low-pressure radio frequency discharge in argon, Appl. Phy. Letts. 62(25), 3247 3249 (1993).
[3] M. M. Turner and M. B. Hopkins, Anomalous sheath heating in a low pressure RF discharge in nitrogen, Phys. Rev. Lett. 69, 3511 (1992).
[4] R. R. J. Gagne and A. Cantin, J. Appl. 43, 2639 2647 (1972).

 

Download the Langmuir Probe Theory of Operation in PDF format
Theory of Operation: Langmuir Probe

Electron flow properties in the far-field plume of a Hall thruster

K Dannenmayer and Stéphane Mazouffre

Published 29 April 2013

Abstract

The plasma properties were investigated in the far-field plume of a 1.5 kW class Hall thruster using a single, cylindrical Langmuir probe. The plasma potential, the electron temperature and the electron density were measured at 191 positions, providing a detailed map of the plume pattern. This map shows that the plasma plume of a Hall thruster is an expanding jet that is symmetric about the thruster axis. The large data set was also used for a detailed analysis of the electron flow properties. This analysis reveals that the plasma plume of a Hall thruster is an isentropic expansion. In addition, the momentum conservation equation shows there is a polytropic relationship between the plasma potential and the electron density with a γ smaller than that for an atomic gas due to ionization.

Online at Plasma Sources Sci. Technol. 22 035004


Plasma parameters and electron energy distribution functions in a magnetically focused plasma

C. M. Samuell, B. D. Blackwell, J. Howard and C. S. Corr

Published 12 March 2013

Abstract

Spatially resolved measurements of ion density, electron temperature, floating potential, and the electron energy distribution function (EEDF) are presented for a magnetically focused plasma. The measurements identify a central plasma column displaying Maxwellian EEDFs at an electron temperature of about 5 eV indicating the presence of a significant fraction of electrons in the inelastic energy range (energies above 15 eV). It is observed that the EEDF remains Maxwellian along the axis of the discharge with an increase in density, at constant electron temperature, observed in the region of highest magnetic field strength. Both electron density and temperature decrease at the plasma radial edge. Electron temperature isotherms measured in the downstream region are found to coincide with the magnetic field lines.

Online at Phys. Plasmas 20, 034502 (2013)


Plasma drift in a low-pressure magnetized radio frequency discharge

D Gerst, S Cuynet, M Cirisan and S Mazouffre

Published 25 January 2013

Abstract

A bright strip-like structure is observed in a low-pressure capacitively coupled radio frequency discharge with a magnetic field perpendicular to the plasma flow. The structure forms for a broad set of operating conditions in different gases. Measurements indicate that the strip acts as a path for the electrons to cross the magnetic field, which makes the discharge inhomogeneous and non-symmetrical. The strip intensity is strongly reduced when switching from a capacitive to an inductive discharge. A theoretical analysis shows that the strip results from a drift of the magnetized electrons perpendicular to the magnetic and electric fields, which is intercepted by the dielectric walls of the discharge tube. In capacitive mode, the drift is mostly governed by the electric field whereas the pressure dominates in inductive mode.

Online at D Gerst et al 2013 Plasma Sources Sci. Technol. 22 015024 doi:10.1088/0963-0252/22/1/015024


Time-Resolved Measurements of Plasma Properties Using Electrostatic Probes in the Cross-Field Discharge of a Hall Effect Thruster

K. Dannenmayer, P. Kudrna, M. Tichý, S. Mazouffre

Published 9 Jan 2013

Abstract

Time-resolved measurements of the plasma parameters are performed in the plume of a cross-field discharge. The plasma potential is measured with a cylindrical Langmuir probe and an emissive probe. The electron temperature and density are measured with a cylindrical Langmuir probe. The cross-field discharge is maintained in a harmonic oscillation regime to guarantee reproducible conditions for all measurements

Online at DOI: 10.1002/ctpp.201310011


Plasma characterization of the superconducting proton linear accelerator plasma generator using a 2MHz compensated Langmuir probe

C. Schmitzer, M. Kronberger, J. Lettry, J. Sanchez-Arias and H. Störi

Published 16 February 2012

Abstract

The CERN study for a superconducting proton Linac (SPL) investigates the design of a pulsed 5 GeV Linac operating at 50 Hz. As a first step towards a future SPL H− volume ion source, a plasma generator capable of operating at Linac4 or nominal SPL settings has been developed and operated at a dedicated test stand. The hydrogen plasma is heated by an inductively coupled RF discharge e− and ions are confined by a magnetic multipole cusp field similar to the currently commissioned Linac4 H−ion source. Time-resolved measurements of the plasma potential, temperature, and electron energy distribution function obtained by means of a RF compensated Langmuir probe along the axis of the plasma generator are presented. The influence of the main tuning parameters, such as RF power and frequency and the timing scheme is discussed with the aim to correlate them to optimum H− ion beam parameters measured on an ion source test stand. The effects of hydrogen injection settings which allow operation at 50 Hz repetition rate are discussed.

Online at Rev. Sci. Instrum. 83, 02A715 (2012)

Langmuir Probe Tip Removal and Replacement

Operational video showing Impedans unique and easy method for switching between single and double Langmuir Probe tips.


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