The Langmuir Probe™ is one of the most common and widely used plasma diagnostics and plasma characterisation instruments to measure plasma parameters in the bulk of the plasma.
The Langmuir Probe™ measures plasma parameters such as floating potential, plasma potential, plasma density, ion current density, electron energy distribution function and electron temperature.
The Langmuir Probe™ is by far the best commercial Langmuir Probe on the market, with it's ultra fast repeatable measurements and the inclusion of both a Single and Double Langmuir Probe in each system is evidence of Impedans commitment to its customers.
The Langmuir Probe™ is a precision plasma measurement instrument used in a wide range of plasma laboratory applications. The Langmuir Probe™ is the key plasma diagnostic used by scientists interested in plasma characterisation to measure the internal parameters of the bulk of the plasma. Among the key parameters measured are electron density, ion density, electron temperature, plasma potential, floating potential and the electron energy distribution function (EEDF). The Langmuir Probe™ provides plasma parameter measurement in DC, RF, microwave, continuous and pulsed plasma.
The Langmuir Probe™ has the most advanced technology on the market and analyses ion and electron trajectories to obtain accurate measurements of the real plasma parameters in a wide range of plasma applications. The Langmuir 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 both a single Langmuir Probe and a Double Langmuir Probe (at no extra charge) which can be used with the same electronics unit. This allows users conduct experiments across different reactors and allows measurements in reactors which have a poor ground return.
The Langmuir Probe™ is used to establish plasma process repeatability. It helps the user to understand plasma changes and the impact on surface treatment. The Langmuir Probe™ is an essential plasma process diagnostic to understand the correlation between plasma inputs and the plasma state. The Langmuir Probe™ reduces process and tool development time, as well as the time to market for new plasma products. Pulsed plasmas are used to tailor the electron or ion energy and The Langmuir Probe™ is an integral part of pulsed process development.
Time Averaged Measurements
This provides an average over time of the plasma parameters measured by the Langmuir Probe in the bulk of the plasma.
Time Resolved Measurements
This allows the user to synchronise the plasma parameters measured by the Langmuir 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.
The 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.
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 Single Probe TipProbe tips 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.
Probe shafts are easily replaced with the “Screw-Fit” probe shaft design. Probe tip 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.
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.
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.
Integrated in the Langmuir Probe System is an external trigger TTL compatible 10Hz to 1MHz
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.
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.
|Floating Potential||-145V to 145V|
|Plasma Potential||-100V to 145V|
|Plasma Density||10⁶ to 3x10¹³cm⁻³|
|Ion Current Density||1uA/cm² to 300mA/cm²|
|Electron Temperature||0.1 to 15 eV|
|Electron Energy Distribution Function||0 to 100eV|
|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||6.5mm (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)|
|Probe Voltage Scan Range||-150V to +150V|
|Current Range||15nA to 1A|
|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 1 MHz|
|Operating System||Windows 2000 / XP / Vista / Windows 7 / Windows 8|
|The Langmuir Probe used in Dusty Plasma applications|
|The Langmuir Probe used in Plasma Etching applications|
|The Langmuir Probe used in HiPIMS Plasma applications|
|The Langmuir Probe used in PECVD applications|
|The Langmuir Probe used in Space Plasma applications|
|The Langmuir Probe used in Plasma Sputtering applications|
The concept of the Langmuir probe was developed almost a century ago and 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.
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.
Figure 1, Schematic of the Single Langmuir Probe.
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.
Figure 2, 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. 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 [1,2] and the EEDF of a molecular gas which has a characteristic hole in gases such as nitrogen  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 . 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.
Figure 3, 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 k? 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,
 M.B. Hopkins J. Res. Natl. Inst. Stand. Technol. Vol. 100, No. 4, p. 415
 V. A. Godyak and R. B. Piejak, Abnormally low electron energy and heating-mode transition in a low-pressure argon rf discharge at 13.56 MHz, Phys. Rev. Lett. 65(8), 996 999 (1990).
 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).
 M. M. Turner and M. B. Hopkins, Anomalous sheath heating in a low pressure rf discharge in nitrogen, Phys. Rev. Lett. 69, 3511 (1992).
 R. R. J. Gagne and A. Cantin, J. Appl. 43, 2639 2647 (1972).
Vitezslav Stranaka, Corresponding author contact information, E-mail the corresponding author, Ann-Pierra Herrendorfa, Harm Wulffa, Steffen Drachea, Martin Cadab, Zdenek Hubickab, Milan Tichyc, Rainer Hipplera
Published 15 May 2013
The effect of energetic ion bombardment on TiO2 crystallographic phase formation was studied. Films were deposited using high-power impulse magnetron sputtering (HiPIMS) assisted by an electron cyclotron wave resonance (ECWR) plasma. The ECWR assistance allows a significant reduction of pressure down to 0.075 Pa during reactive HiPIMS deposition and subsequently enables control of the energy of the deposited species over a wide range. Films deposited at high ion energies and deposition rates form rutile with (101) a preferred orientation. With decreasing ion energy and deposition rates, rutile is formed with random crystallite orientation, and finally at low ion energies the anatase phase occurs. It is supposed that particles gain high energy during the HiPIMS pulse while the ECWR discharge is mostly responsible for substrate heating due to dissipated power. However, the energetic contribution of the ECWR discharge is not sufficient for annealing and phase transformation.
Steffen Drachea, Vitezslav Stranaka, Ann-Pierra Herrendorfa, Martin Cadab, Zdenek Hubickab, Milan Tichyc, Rainer Hipplera
Published April 2013
The paper focuses on time-resolved diagnostics of unipolar hybrid-dual-High Power Impulse Magnetron Sputtering (hybrid-dual-HiPIMS) discharges. The newly developed sputtering system is based on a combination of dual-HiPIMS with a mid-frequency (MF) discharge. The most important feature of hybrid-dual-HiPIMS systems is the MF pre-ionization which causes/allows: (i) a significant reduction of working pressure by more than one order of magnitude, and (ii) faster ignition and development of HiPIMS pulses. Parameters such as mean electron energy, electron density and electron energy probability function (EEPF) were obtained from time-resolved Langmuir probe diagnostics to demonstrate the aforementioned effects. Calorimetric probe diagnostics were used for determination of the total power density flux. Power flux contributions of particular species, e.g. ions, electrons and neutrals, were estimated as well.
Boyd D Blackwell, Juan Francisco Caneses, Cameron M Samuell, John Wach, John Howard and Cormac Corr
Published 4 October 2012
The Magnetized Plasma Interaction Experiment (MAGPIE) is a versatile helicon source plasma device operating in a magnetic hill configuration designed to support a broad range of research activity and is the first stage of the Materials Diagnostic Facility at the Australian National University. Various material targets can be introduced to study a range of plasma–material interaction phenomena.
Initially, with up to 2.1 kW of RF at 13.56 MHz, argon (10¹⁸–10¹⁹ m⁻³) and hydrogen (up to 10¹⁹ m⁻³ at 20 kW) plasma with electron temperature ~3–5 eV was produced in magnetic fields up to ~0.19 T. For high mirror ratio we observe the formation of a bright blue core in argon above a threshold RF power of 0.8 kW. Magnetic probe measurements show a clear m = +1 wave field, with wavelength smaller than or comparable to the antenna length above and below this threshold, respectively. Spectroscopic studies indicate ion temperatures
K Dannenmayer, P Kudrna, M Tichý and S Mazouffre
Published 13 September 2012
Time-resolved measurements using electrostatic probes are performed in the far-field plume of a low-power permanent magnet Hall effect thruster. These measurements are necessary in order to account for the non-stationary behavior of the discharge. The plasma potential is measured by means of a cylindrical Langmuir and a sufficiently heated emissive probe, the electron temperature and density are measured with a cylindrical Langmuir probe. The thruster is maintained in a periodic quasi-harmonic oscillation regime by applying a sinusoidal modulation to a floating electrode in the vicinity of the cathode in order to guarantee repeatable conditions for all measurements. The modulation depth of the discharge current does not exceed approximately 10%. In order to achieve synchronism, the frequency of the modulation has to be close to the natural frequency of the observed phenomena. It is different depending on whether the discharge current or the plasma potential is selected as a reference. The measurements show that the fluctuations of the electron density follow the discharge current fluctuations. The time evolution of the plasma potential and the electron temperature is similar. The time-averaged properties of the discharge remain almost uninfluenced by the modulation. Measurements of the plasma potential with the two different probes are in good agreement. The observed phenomena are similar for Xe and Kr used as propellant gases.
Francis F Chen
Published 6 September 2012
Helicon discharges have extensively been studied for over 25 years both because of their intriguing physics and because of their utility in producing high plasma densities for industrial applications. Almost all measurements so far have been made away from the antenna region in the plasma ejected into a chamber where there may be a strong magnetic field (B-field) but where the radiofrequency (RF) field is much weaker than under the antenna. Inside the source region, the RF field distorts the current–voltage (I–V) characteristic of the probe unless it is specially designed with strong RF compensation. For this purpose, a thin probe was designed and used to show the effect of inadequate compensation on electron temperature (Te) measurements. The subtraction of ion current from the I–V curve is essential; and, surprisingly, Langmuir's orbital motion limited theory for ion current can be used well beyond its intended regime.
Online at Plasma Sources Sci. Technol. 21 055013
L. Chang, M. J. Hole, J. F. Caneses, G. Chen, B. D. Blackwell and C. S. Corr
Published 31 August 2012
A radio frequency field solver based on Maxwell's equations and a cold plasma dielectric tensor is employed to describe wave phenomena observed in a cylindrical non-uniform helicon discharge. The experiment is carried out on a recently built linear plasma-material interaction machine: The magnetized plasma interaction experiment [Blackwell et al., Plasma Sources Sci. Technol. (submitted)], in which both plasma density and static magnetic field are functions of axial position. The field strength increases by a factor of 15 from source to target plate, and the plasma density and electron temperature are radially non-uniform. With an enhancement factor of 9.5 to the electron-ion Coulomb collision frequency, a 12% reduction in the antenna radius, and the same other conditions as employed in the experiment, the solver produces axial and radial profiles of wave amplitude and phase that are consistent with measurements. A numerical study on the effects of axial gradient in plasma density and static magnetic field on wave propagations is performed, revealing that the helicon wave has weaker attenuation away from the antenna in a focused field compared to a uniform field. This may be consistent with observations of increased ionization efficiency and plasma production in a non-uniform field. We find that the relationship between plasma density,static magnetic field strength, and axial wavelength agrees well with a simple theory developed previously. A numerical scan of the enhancement factor to the electron-ion Coulomb collision frequency from 1 to 15 shows that the wave amplitude is lowered and the power deposited into the core plasma decreases as the enhancement factor increases, possibly due to the stronger edge heating for higher collision frequencies.
Online at Phys. Plasmas 19, 083511 (2012)
K Dannenmayer, P Kudrna, M Tichý and S Mazouffre
Published 29 November 2011
The far-field plume of a 1.5 kW Hall effect thruster is mapped with a Langmuir probe and an emissive probe. Time-averaged measurements of the plasma potential, the electron temperature and the electron number density are performed for different operating conditions of the thruster. The influence of the discharge voltage, the cathode mass flow rate as well as the magnetic field strength is investigated. The plasma potential decreases from 30 V at 300 mm on the thruster axis to 5 V at 660 mm and at 60°, the electron temperature decreases from 5 to 1.5 eV. The electron number density drops from 3.5 × 10¹⁶ to 1 × 10¹⁵ m⁻³ in the far-field plume. The values of the plasma potential and electron temperature measured with the Langmuir probe and the emissive probe are in good agreement.
Lara Popelier, Ane Aanesland and Pascal Chabert
Published 15 July 2011
Electronegative plasmas are plasmas containing a significant fraction of negative ions, when magnetized they are very often segregated: the core is electropositive or weakly electronegative whereas a highly electronegative plasma forms at the periphery. At strong magnetic fields this segregation can lead to the formation of ion–ion plasmas almost free of electrons close to the walls or extraction surfaces and allows access to both positive and negative ions. The PEGASES thruster aims at alternately extracting and accelerating positive and negative ions from the ion–ion plasma region to provide thrust by both types of ions. The acceleration schemes depend on the possible control of the potential in an ion–ion plasma relative to the acceleration grids. In this paper continuous extraction and acceleration of positive ions from the PEGASES thruster is investigated by a retarding field energy analyser. It is shown from the measured ion energy distribution functions that the continuous acceleration potential can be controlled by biasing bare electrodes in contact with the region of the plasma with high electron density (i.e. the weakly electronegative plasma core). A grounded grid placed in the ion–ion region allows consequently the acceleration of positive ions, where the ion velocity is controlled by the bias applied to the electrodes in the plasma core. In contrast, when the grid in the ion–ion region is biased, positive ion beams are not detected downstream of the grid. The results indicate that biasing a grid positively in the ion–ion region may result in an electronegative space-charge sheath in front of the grid, which traps the positive ions inside the thruster.
H. B. Profijt, P. Kudlacek, M. C. M. van de Sanden and W. M. M. Kessels
Published February 25, 2011
The influence of ions and photons during remote plasma atomic layer deposition (ALD) of metal oxide thin films was investigated for different O2 gas pressures and plasma powers. The ions have kinetic energies of ≤35 eV and fluxes of ∼1012–1014 cm−2 s−1 toward the substrate surface: low enough to prevent substantial ion-induced film damage, but sufficiently large to potentially stimulate the ALD surface reactions. It is further demonstrated that 9.5 eV vacuum ultraviolet photons, present in the plasma, can degrade the electrical performance of electronic structures with ALD synthesized metal oxide films.
Harald B. Profijt, Pavel Kudlacek, M. C. M. Van de Sanden and W. M. M. Kessels
The influence of oxygen ions and photons during remote plasma atomic layer deposition (ALD) of metal oxide thin films was investigated for pressures of 0.5-3 Pa and plasma powers of 100-500 W. Ions have kinetic energies of ~15-35 eV and fluxes of ~10^13-10^14 cm^-2 s^-1 towards the substrate surface. The ion energy is low enough to prevent substantial ion-induced film damage, however the total energy flux provided to the substrate is sufficiently large to potentially stimulate the ALD surface reactions, e.g., through ligand desorption and adatom migration. Furthermore, it is demonstrated that the presence of VUV photons with energies of ~9.5 eV can deteriorate the electrical performance of electronic structures with ALD synthesized metal oxide films.
D Gahan, B Dolinaj and M B Hopkins
Published 31 July 2008
A comparison is made between plasma parameters measured with a retarding field energy analyzer (RFEA), mounted at a grounded electrode in an inductive discharge, and a Langmuir probe located in bulk plasma close to the analyzer. Good agreement between measured plasma parameters is obtained for argon gas pressure in the range 2–10 mTorr. Parameters compared include time averaged plasma potential, the tail of the electron energy distribution function (EEDF), the electron temperature and the ion flux. This highlights the versatility of the RFEA for determining plasma parameters adjacent to the surface where probe measurements are not easily made. Combination of the probe and energy analyzer has enabled the measurement of the EEDF to a higher energy than otherwise possible.