The Semion Single Sensor is a Retarding Field Energy Analyser (RFEA). It is placed at any location inside a plasma reactor and measures the energy of ions hitting a surface using a single measurement sensor.
This retarding field analyser (RFA) is a fundamental plasma diagnostic to measure the ion energy distribution function and also measures the Ion flux, negative ions, temperature and bias voltage (Vdc).
The Semion system allows in situ process monitoring, confirmation of models, development of new processes and research applications in real time at the substrate location.
The Semion, sometimes called a retarding potential analyser, measures the ion energy and produces the ion energy distribution function (IEDF), ion flux, negative ions, temperature and Vdc at the substrate position inside a plasma reactor. The Semion retarding field energy analyser (RFEA) is widely used in many sectors across industry and research, such as plasma etching, coatings, plasma sputtering, PECVD and ion beam. The Semion Single Sensor is key plasma diagnostic instrument that can confirm models, aid new processes development and experiments that use plasma, plasma tool design and plasma research.
The Semion Single Sensor retarding field energy analyser consists of a replaceable button probe sensor mounted in a sensor holder which connects to a vacuum feed-through which is powered by a 19" rack mounted electronics unit. Users connect to the electronics unit by using a laptop or PC. The system comes with a number of holder options ranging from 50mm to 450mm which depends on the users application. In most cases users will select a 50mm sensor and physically move the sensor to different locations around the chamber. For other applications the user can select a sensor holder to match the size of their substrate or their electrode and in this case the sensor is located in the center of the holder plate.
The Semion retarding potential analyser allow users adjust in real time their plasma input parameters such as power levels, pressure, chemistry and frequency to find their optimum ion energy and ion flux for their application in real time. The system also takes useful measurements such as temperature, bias voltage (Vdc) and the energy and flux of both positive and negative ions.
While the Semion System is not used on the process side of plasma manufacturing is has been used to assist with chamber to chamber matching, fault detection, fingerprinting, RF plasma analysis, new processes development and preventative maintenance in an offline environment. In a nut shell, the Semion System helps eliminate the need for a trial and error approach thus saving time and money.
Time Averaged Measurements
Time averaged measurement is the most commonly used function which allows users to take a measurement such as an ion energy distribution averaged over a period of time.
Time Resolved Measurements
This allows the user to synchronise the ion energy distribution measurements with an external synchronisation signal. The user can then obtain detailed information on the ion energy distribution 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. *When sensor is mounted on a grounded electrode (custom hardware required).
Time Trend Measurements
This allows the user to monitor the variation of the ion energy distribution over a time period for a given process. This feature does not require external synchronisation and the timescales involved can be in range of seconds to hours.
The Semion retarding field energy analyser (RFEA), Electronics Unit is built to the highest quality with an eye catching design and built to withstand the most demanding of laboratory environments. The 19” Rack Mounted Unit contains the most advanced architecture designed to offer the most efficient measurements and analysis as close to real time as possible.
The Semion Software Suite offers the most advanced analysis of any plasma measurement instrument on the market today. Carefully crafted over the past ten years by some of the world’s leading scientists in Plasma Measurement. The analysis has been peer reviewed in multiple publications and cited in hundreds. The user interface is designed to be easy to use and clutter free reducing the need to spend time figuring out how to use the system.
The Semion retarding field energy analyser (RFEA), holder sits at the substrate position inside a plasma reactor. It can be placed at any position and even on a biased electrode. It can withstand temperatures up to 250°C without the need for cooling. The main purpose of the sensor holder is to hold the replaceable Button Probe sensors and to provide the customer with the option to match the holder size to that of their substrate. The holders are available in 100mm, 150mm, 200mm, 300mm, 450mm as standard and custom size holders are available on request.
Due to the harsh environments inside most plasma reactors we have designed the sensors to be replaceable. Depending on factors such as deposition rate and corrosive chemistries the sensors can have a life time of 10’s of hours to 100’s of hours inside a plasma reactor. With this in mind we have designed the sensor (Button Probe) to be disposable. Once the measurements start to drift you can simply replace the sensor and continue your measurements with very little down time.
Time in the lab is expensive and with this in mind we have designed our systems to allow users to be up and running is a very short period of time. For the beginner we have a “quick start” mode which allows users get the data they need as soon as they pump down their reactor and for the more advanced user we allow full access to the raw data.
| Ion Energy Range | 2000 eV - Vdc |
| Ion Current | 1 mA DC max |
| Ion Flux Ranges* | Low: 0.001 - 3 | Std: 0.01 - 50 | High: 0.1 - 700 (A/m²) |
| IEDF Resolution | ± 1 eV nominal |
*Choice dependent on plasma density
| Max RF Bias Voltage | 1 kV (peak-to-peak) |
| Max DC Bias Voltage | -1940 V |
| Bias Frequency Range (Time Averaged Measurements) | 100 kHz to 80 MHz |
| Bias Frequency Range (Time Resolved Measurements) | 0 Hz to 100 kHz |
| Time Resolution* | 5 µs |
*For pulsed plasmas with Semion mounted on a grounded electrode, custom hardware required.
| Number of Sensors | 1 |
| Probe Configuration | 4-grid |
| Button Probe Diameter | 33 mm |
| Holder Diameter | 50 mm, 70 mm, 100 mm, 150 mm, 200 mm, 300 mm, 450 mm and custom shapes |
| Holder Thickness | 5 mm |
| Max Operating Temperature | 200º C |
| Mounting | RFEA probe holder mounted on electrode |
| Probe Enclosure and Holder Material | Aluminium, anodized aluminium, stainless steel* and ceramic (Al2O3)* |
| RFEA Probe Cable Length | 650 mm standard (custom available) |
*on request
| Flange Type | CF40 (custom available) |
| Grid Voltage Range | -2 kV to +2 kV |
| Current Range | 100 pA to 2.4 mA |
| Connectivity | USB 2.0 |
| Operating System | Windows 2000 / XP / Vista / Windows 7 / Windows 8 |
| Pressure (Pascal) | <0.1 Pa to 40 Pa* |
| Pressure (Torr) | 0 to 300 mTorr* |
| Density Ranges (Ar at 3eV) | Low: 1.2 x 1012 to 7.4 x 1015 | Std: 2.0 x 1013 to 1.2 x 1017 | High: 2.7 x 1014 to 1.6 x 1018 (m-3) |
| Gas Reactivity | Inert to highly reactive |
*dependent on ion mean free path
| The Semion Single Sensor used in Plasma Etching applications |
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Ion flux and ion distribution function measurements in synchronously pulsed inductively coupled plasmasAbstractPulsed inductively coupled plasmas (ICPs) have recently attracted considerable interest in the field of integrated circuit fabrication. This is because pulsing provides more flexibility for tuning an etch process by bringing in new reactor control parameters: the pulsing frequency, the duty cycle of the pulses, and the phase between the ICP pulses and the bias pulses. Recent experiments have shown that the duty cycle has a strong influence on plasma chemistry. In this study, the authors look at the impact of the duty cycle on the time variations of the ion flux and on the time-averaged ion distribution function measured at the wafer surface in an ICP-processing chamber subject to pulse modulation of source excitation and bias at 1kHz, in several plasma chemistries. |
Semion and Langmuir Probe systems used to study the SiO2 sputter etch rate in RF-Biased ICP discharge.AbstractInductively coupled plasmas (ICPs) with radiofrequency (RF) biased pedestals are commonly used in the semiconductor industry for nanoscale etch and deposition processes. In this study, the authors investigate the sputter etch rate of SiO2 in this type of reactor as a function of ICP and RF-bias power. The key parameters measured were the ion energy distribution (IED), ion flux and sputter etch rate. |
| The Semion Single Sensor used in HiPIMS Plasma applications |
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Plasma diagnostics of low-pressure high-power impulse magnetron sputtering assisted by electron cyclotron wave resonance plasmaAbstractThe 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.
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Semion retarding field energy analyzer used to investigate reactive HiPIMS + MF sputtering of TiO₂ crystalline thin filmsAbstractHigh-power impulse magnetron sputtering (HiPIMS) systems have been previously studied with mid-frequency (MF) plasma excitation, utilizing the “off” period to enhance the deposition rate, decrease the working pressure, and improve HiPIMS plasma generation. Our latest application note looks at the time-resolved ion velocity distribution function (IVDF) in a high-power pulse plasma in three modes of excitation: pure HiPIMS, medium-frequency pulsed bipolar (MF 350 kHz) and hybrid pulsed HiPIMS + MF. |
Semion Retarding Field Energy Analyser (RFEA) used in a study to form Ti–Cu thin films with regard to controlling the copper releaseAbstractSerious complications in orthopaedic surgery include aseptic loosening and infection of artificial implants. A number of studies have looked at ways to reduce these complications, and copper has been found to be one of the most promising metal ions for deposition applications because of its lower toxicity and higher cytocompatibility. Various studies have shown that sufficient (about 5 mmol/l) copper release over at least several days is needed to inhibit and then kill all bacteria. This study aimed to prepare Ti–Cu film with strong initial antimicrobial and cytotoxic effect, followed by long-lasting but moderate copper release using HiPIMS-based systems. |
Semion used in a study of the effect of mid-frequency discharge assistance on dual-high power impulse magnetron sputteringAbstractThis study introduces a hybrid-dual-HiPIMS system based on the simultaneous combination of dual-HiPIMS and MF discharges. The main body of the study is the time-resolved diagnostics taken using a Semion System during the deposition of Ti–Cu films, revealing the degree of assistance made by the MF discharge. |
| The Semion Single Sensor used in Ion Beam Plasma applications |
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| Coming soon |
| The Semion Single Sensor used in PECVD applications |
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Ion energy distributions in bipolar pulsed DC discharges of methane measured at the biased cathodeAbstractIon processes at the growing surface of a depositing film are of huge importance in the achievement of special surface properties. The study of plasma processes at the sheath level complements the characterisation performed by other methods, and it provides a better assessment of the influence of deposition variables on material properties. Compact models of retarding field energy analysers (RFEA) are becoming popular instruments for measuring ion currents due to their rapid installation, high sensitivity to small currents and because no auxiliary pump is needed. |
| The Semion Single Sensor used in Space Plasma applications |
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| Coming soon |
| The Semion Single Sensor used in Plasma Sputtering applications |
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Ion energy distribution measurements in RF and pulsed DC plasma dischargesAbstractThe energy and flux of bombarding ions play a vital role in the etching and deposition of layers on the substrate surface. Asymmetric radio-frequency (RF) capacitively coupled plasma (CCP) reactors are commonly used for plasma etching. Monitoring of the flux and energy of ions arriving at the substrate in these reactors is essential for process optimization and the control of films’ microstructure. This study used a commercial retarding field energy analyser (RFEA), the Semion System from Impedans Ltd, to measure the ion energy distribution functions (IEDFs) of impacting ions at the powered electrode in a 13.56MHz driven, capacitively coupled, parallel plate discharge operated at low pressure.
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| Download the Semion Sensor System Theory of Operation in PDF format |
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 Theory of Operation: Semion System | Retarding Potential Analyser | Retarding Field Energy Analyser |
| Download: Application Note IEDF vs IVDF |
| Measuring Ion Velocity Distributions and Ion Energy Distributions using Retarding Field Energy Analyzers (RFEAs). |
Direct measurement of the ion energy distribution (IED) and total ion flux can be performed with the Semion System, using our advanced retarding field energy analyser (RFEA) technology. The RFEA is constructed from process compatible materials and the sensor’s miniature size allows it to be mounted on the substrate or any other surface inside the reactor. RFEAs have been used for decades to measure IEDs in plasma discharges with limited success. Most designs require mounting on a grounded surface to avoid complications with substrate biasing. Early designs were typically bulky and differential pumping was required for the device to operate even at the low pressures encountered in many plasma processes. The Semion System incorporates a miniature design to avoid the need for differential pumping. Operating pressures of up to 300mTorr can be achieved in Argon discharges. The Semion System also uses high impedance low-pass filters to allow the RFEA to float at the substrate bias potential. The system supports bias frequencies in the range 1kHz to 100MHz and bias potentials up to 3kV peak-to-peak maximum. High temperature cabling connects the RFEA to the external data acquisition unit through a vacuum feedthrough, which is mounted at the reactor wall and enables the sensor to operate to 250°C. Button probe sensing elements are easily replaceable, which is a convenient feature for the user especially when operating in processes using aggressive etching chemistry sets.
Figure 1: In-situ layout of the Semion Sensor System in a typical plasma chamber setup
Figure 2 shows a schematic of the Semion RFEA design. Ions enter the RFEA through an array of sampling apertures exposed to the plasma (only one aperture is depicted for simplicity). A grid G0, covers the internal side of the apertures and reduces the open area ‘seen’ by the plasma to a scale less the Debye length to prevent plasma entering the analyser. A second grid G1, in a plane parallel to G0, is biased with a negative potential relative to G0 to repel any electrons that may enter the device. A third grid G2, is biased with a positive potential sweep, creating a potential barrier for the positive ions. A fourth grid G3, creates a negative potential barrier that prevents electrons from escaping and ensures that they are also collected. A collector plate C, oriented in the same plane as the grids, collects the current of ions which cross the potential barrier set by G2. The data acquisition unit records the ion current at each potential applied to G2 and the graphical user interface displays the resultant current-voltage characteristic. The IED is also displayed - obtained by differentiation of the current-voltage characteristic. The potential configuration is depicted in figure 2.
Figure 2: Schematic of the Semion RFEA structure and grid potential configuration
The analyser (including G0, G1, G2, G3 and C), floats at the AC/RF component of the substrate bias potential. This is achieved by means of high impedance low-pass filters. These high impedance filters prevent disturbance of the applied bias signal and provide sufficient attenuation at the output to protect the measurement electronics. The RFEA chassis also floats at the DC bias component of the powered electrode potential. The required DC electric fields between adjacent grids are produced by setting the grid potentials relative to each other (not relative to ground). The Semion feedthrough interface provides a filtered connection to the RFEA chassis to enable a direct measurement of the acceptance angle of a sampling orifice is approximately 45º allowing detection of ions arriving at the surface within this angle. The calculated IED is the energy distribution of the ions perpendicular to the electrode surface.
A typical Semion system installation is shown in figure 3. The RFEA was mounted at the biased substrate holder in a plasma etch reactor. The plasma is sustained by the 2.45GHz source. The substrate holder is biased with 2MHz RF power to control the energy of the ions impacting the substrate. The working gas was pure Argon at a pressure of 10mTorr. IEDs were measured for a range of RF power levels applied to the substrate holder.
Figure 3: Semion Sensor Holder Installation
A typical current-voltage characteristic and IED are shown in figure 4. The RF power was set at 50W and the argon gas pressure was 10mTorr. The bi-modal saddle shaped IED structure associated with sinusoidal biasing is clearly visible.
Figure 4 (a) Current-voltage characteristic (dashed) and IED measured at 50W and 10mTorr
(b) Ion energy distribution varies as a function of RF power applied to the substrate holder while the pressure is maintained at 10mTorr throughout
D Gahan¹, S Daniels², C Hayden², D O’Sullivan¹ and M B Hopkins¹
1. Impedans Ltd, Unit 8 Woodford Court, Woodford Business Park, Santry, Dublin 17, Ireland
2. National Centre for Plasma Science and Technology, Dublin City University, Dublin 9, Ireland
Published 19 December 2011
A retarding field energy analyzer is used to characterize an asymmetric, 13.56MHz driven, capacitively coupled, parallel plate discharge operated at low pressure. The characterisation is carried out in argon discharges at 10 and 20mTorr where the sheaths are assumed to be collisionless. The analyzer is set in the powered electrode where the impacting ion and electron energy distributions are measured for a range of discharge powers. A circuit model of the discharge is used to infer important electrical parameters from the measured energy distributions, including electrode excitation voltages, plasma potential and sheath potentials. Analytical models of the ion energy distribution in a radio-frequency sheath are used to determine plasma parameters such as sheath width, ion transit time, electron temperature and ion flux. A radio-frequency compensated Langmuir probe is used for comparison with the retarding field analyzer measurements.
View online at stacks.iop.org/PSST/21/015002
U Czarnetzki¹, J Schulze¹², E Schüngel¹ and Z Donkó²
1 Institute for Plasma and Atomic Physics, Ruhr-University Bochum, 44780 Bochum, Germany
2 Research Institute for Solid State Physics and Optics of the Hungarian Academy of Sciences,Budapest, Hungary
Published 1 April 2011
We present an analytical model to describe capacitively coupled radio-frequency (CCRF) discharges and the electrical asymmetry effect (EAE) based on the non-linearity of the boundary sheaths. The model describes various discharge types, e.g. single and multi-frequency as well as geometrically symmetric and asymmetric discharges. It yields simple analytical expressions for important plasma parameters such as the dc self-bias, the uncompensated charge in both sheaths, the discharge current and the power dissipated to electrons. Based on the model results the EAE is understood. This effect allows control of the symmetry of CCRF discharges driven by multiple consecutive harmonics of a fundamental frequency electrically by tuning the individual phase shifts between the driving frequencies. This novel class of capacitive radio-frequency (RF) discharges has various advantages: (i) A variable dc self-bias can be generated as a function of the phase shifts between the driving frequencies. In this way, the symmetry of the sheaths in geometrically symmetric discharges can be broken and controlled for the first time. (ii) Almost ideal separate control of ion energy and flux at the electrodes can be realized in contrast to classical dual-frequency discharges driven by two substantially different frequencies. (iii) Non-linear self-excited plasma series resonance oscillations of the RF current can be switched on and off electrically even in geometrically symmetric discharges. Here, the basics of the EAE are introduced and its main applications are discussed based on experimental, simulation, and modelling results.
Online at stacks.iop.org/PSST/20/024010
H. B. Profijt, P. Kudlacek, M. C. M. van de Sanden, and W. M. M. Kessels
Published 25 February 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.
Online at Journal of the Electrochemical Society DOI: 10.1149/1.3552663
K Denieffe¹, C M O’Mahony¹, P D Maguire¹, D Gahan² and M B Hopkins²
1. N.I. Biomedical Engineering Centre, Nanotechnology Research Institute, University of Ulster,BT 37 0QB, Northern Ireland.
2. National Centre for Plasma Science and Technology, Dublin City University, Glasnevin, Dublin 9, Ireland.
Published 2 February 2011
Accurate measurement of ion current density and ion energy distributions (IEDs) is often critical for plasma processes in both industrial and research settings. Retarding field energy analysers (RFEAs) have been used to measure IEDs because they are considered accurate, relatively simple and cost effective. However, their usage for critical measurement of ion current density is less common due to difficulties in estimating the proportion of incident ion current reaching the current collector through the RFEA retarding grids. In this paper an RFEA has been calibrated to measure ion current density from an ion beam at pressures ranging from 0.5 to 50.0mTorr. A unique method is presented where the currents generated at each of the retarding grids and the RFEA upper face are measured separately, allowing the reduction in ion current to be monitored and accounted for at each stage of ion transit to the collector. From these I–V measurements a physical model is described. Subsequently, a mathematical description is extracted which includes parameters to account for grid transmissions, upper face secondary electron emission and collisionality. Pressure-dependent calibration factors can be calculated from least mean square best fits of the collector current to the model allowing quantitative measurement of ion current density.
View online at stacks.iop.org/JPhysD/44/075205
David Gahan, Borislav Dolinaj, Chanel Hayden, Michael B. Hopkins
Published 16 June 2009
A compact, floating retarding field energy analyzer for measurement of ion energy distributions impacting an electrode through a radio-frequency or pulsed bias sheath in a plasma discharge is presented. The analyzer is designed to sit on the electrode surface, in place of the substrate, and wide-band low pass filters allow it to float at the electrode potential. This avoids the need for modification of the electrode. The capabilities of the analyzer are demonstrated through ion energy distribution and electron energy distribution measurements at the electrode surface in an inductively coupled plasma reactor. For a sinusoidal radio-frequency driving signal applied to the electrode the analyzer is shown to resolve ions with different mass. When the radio-frequency power to the plasma pulsed the analyzer is used to resolve the ion energy distributions at different times in the pulse. The high energy tail of the electron energy distribution reaching the electrode surface is also measured. A comparison with a Langmuir probe shows exceptional agreement in the energy region where both devices overlap.
Online at Plasma Processes and Polymers DOI: 10.1002/ppap.200931607
C Hayden¹, D Gahan¹ and M B Hopkins¹²
1. National Centre for Plasma Science and Technology, Dublin City University, Glasnevin, Dublin 9,Ireland
2. Impedans Ltd, Invent Centre, Dublin City University, Glasnevin, Dublin 9, Ireland
Published 4 March 2009
Ion energy distributions are investigated in an inductively coupled radio-frequency discharge at low pressures. A Langmuir probe is used to characterize the discharge and a retarding field energy analyzer measures the ion flux and energy distributions impacting a remote rf driven electrode. Comparisons are made between capacitive and direct coupling of the rf bias potential. The effects of ICP power, rf bias voltage (0–75V amplitude), bias frequency (0.5–20 MHz) and discharge pressure (0.2–1.2 Pa) are presented. Results are shown for Ar, O2 and Ar–He discharges. A double layer was observed during source characterization measurements in an O2 discharge; however, the focus of this paper is on the behavior of ions through capacitively and directly coupled plasma sheaths.
Online at stacks.iop.org/PSST/18/025018
D Gahan¹, B Dolinaj² and M B Hopkins¹²
1. National Centre for Plasma Science and Technology, Dublin City University, Glasnevin, Dublin 9,Ireland
2. Impedans Ltd., Invent Centre, Dublin City University, Glasnevin, Dublin 9, Ireland
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 2–10mTorr. 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.
Online at stacks.iop.org/PSST/17/035026
D Gahan¹, B Dolinaj², and M B Hopkins¹
1. National Centre for Plasma Science and Technology, Dublin City University, Glasnevin, Dublin 9, Ireland
2. Impedans Ltd., Invent Centre, Dublin City University, Glasnevin, Dublin 9, Ireland
Published 10 March 2008
A retarding field energy analyzer designed to measure ion energy distributions impacting a radio-frequency biased electrode in a plasma discharge is examined. The analyzer is compact so that the need for differential pumping is avoided. The analyzer is designed to sit on the electrode surface, in place of the substrate, and the signal cables are fed out through the reactor side port. This prevents the need for modifications to the RF electrode—as is normally the case for analyzers built into such electrodes. The capabilities of the analyzer are demonstrated through experiments with various electrode bias conditions in an inductively coupled plasma reactor. The electrode is initially grounded and the measured distributions are validated with the Langmuir probe measurements of the plasma potential. Ion energy distributions are then given for various rf bias voltage levels, discharge pressures, rf bias frequencies—500 kHz to 30 MHz, and rf bias waveforms—sinusoidal, square, and dual frequency
View Online at scitation.aip.org/content/aip/journal/rsi/79/3/10.1063/1.2890100
Head of Engineering Logic Conductor Etch | Applied Materials, USA