Semion Single Sensor | Retarding Field Energy Analyser

Measuring Parameters

*Choice dependent on plasma density

Probe Bias Conditions

*For pulsed plasmas with Semion mounted on a grounded electrode, custom hardware required.

RFEA Probe

*on request

Feed-Through Assembly

Control Unit Electronics

Application Software

Operating Parameters

*dependent on ion mean free path

Theory of Operation: Semion System
Retarding Potential Analyser | Retarding Field Energy Analyser

Introduction

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

Theory of Operation

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.

Typical Results

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

Download the Semion Sensor System Theory of Operation in PDF format
Theory of Operation: Semion System | Retarding Potential Analyser | Retarding Field Energy Analyser

Characterization of an Asymmetric Parallel Plate Radio-Frequency Discharge Using a Retarding Field Energy Analyzer

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

Abstract

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

The Electrical Asymmetry Effect in Capacitively Coupled Radio-Frequency Discharges

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

Abstract

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

Ion and Photon Surface Interaction during Remote Plasma ALD of Metal Oxides

H. B. Profijt, P. Kudlacek, M. C. M. van de Sanden, and W. M. M. Kessels

Published 25 February 2011

Abstract

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

Retarding Field Energy Analyser Ion Current Calibration and Transmission

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

Abstract

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

Retarding Field Analyzer for Ion Energy Distribution Measurement Through a Radio-Frequency or Pulsed Biased Sheath

David Gahan, Borislav Dolinaj, Chanel Hayden, Michael B. Hopkins

Published 16 June 2009

Abstract

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

Ion Energy Distributions at a Capacitively and Directly Coupled Electrode Immersed in a Plasma Generated by a Remote Source

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

Abstract

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

Comparison of Plasma Parameters Determined with a Langmuir Probe and with a Retarding Field Energy Analyzer

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

Abstract

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

Retarding Field Analyzer for Ion Energy Distribution Measurements at a Radio-Frequency Biased Electrode

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

Abstract

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

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