Vertex Single Sensor | Ion Energy Distribution

Vertex Single Sensor

Seeing Your Ions From a Different Perspective

The Vertex Single Sensor measures the ion energy distribution as a function of aspect ratio hitting a surface inside a plasma reactor.

This unique plasma characterisation instrument can also measure the ion energy, ion flux and bias voltage.

The angle of ions hitting the bottom of a feature can be determined from the ion energy distribution as a function of aspect ratio which allows users monitor the quality of the etch profile at a given location in the plasma reactor.



Features

Small Footprint | Substrate Bias Compatible | High RF Bias Resistant |
Easily Installed | High Temperature Resistant


Applications

High Aspect Ratio Processing | Model Validation | Fundamental Research |
Process Development | Equipment Design




Overview

The Vertex Single Sensor measures the ion energy distribution as a function of aspect ratio, ion energy, ion flux, negative ions and Vdc at a surface inside a plasma. The Vertex Single Sensor is increasingly used in many applications in industry and research, such as plasma etching, ion beams, and plasma sputtering for plasma characterisation and plasma interaction analysis. The Vertex Single Sensor saves the user valuable time in confirming models, developing new processes and in experiments that use plasma, plasma tool design and plasma research.

The Vertex Single Sensor is housed in a 19" rack mounted electronics unit, vacuum feed-through, and a sensor holder located anywhere inside a plasma or beam chamber. The location can even be a RF or DC powered electrode and can use a replaceable Button Probe sensor. The electronics unit connects to a laptop or a PC and uses the Vertex intelligent software suite. The Vertex Single Sensor has a number of holder options ranging from 50mm to 450mm and will depend on the users application. Users can select a 50mm sensor and physically move the sensor to measure in different chamber locations. In some applications the user can select a sensor holder to match the size of their substrate holder or their electrode and in this case the sensor is located at a specific location such as the center of the holder plate.

The Vertex Single Sensor allows users to change their plasma input parameters or beam source location in real time to find the optimum ion aspect ratio for a specific total ion energy and ion flux in their application. The system also takes useful measurements such as ion energy, temperature, voltage direct current and the energy and flux of negative ions. While the Vertex System is not used on the process side of plasma manufacturing it has been used to measure side wall pattern in etch, beam divergence, elastic scattering to assist with chamber to chamber matching, fault detection, researching new processes, in an offline environment. For the first time, by directly measuring ion angle the Vertex System helps eliminate the need for a trial and error approach saving time and money.

Plasma Parameters Measured

  • Ion Energy as a Function of Aspect ratio (1 Location)
  • Ion Energy (1 Location)
  • Ion Flux (1 Location)
  • Negative Ion (1 Location)*
  • Bias Voltage Vdc (Average)

Measurement Functionality

Time Averaged Measurements
This provides an average over time of the ion angular and energy distribution arriving at the substrate position

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 angular and 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. *Note: in standard energy mode only, for pulsed plasmas, with Vertex mounted on a grounded or floating electrode.

Time Trend Measurements
This allows the user to obtain information on the variation of the ion energy distribution as a function of aspect ratio 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

Vertex Electronics Unit

Electronics UnitThe Vertex 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.

Software Suite

The Vertex 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.

Sensor Holder

The Vertex sensor 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 electrode or substrate. The holders are available in 100 mm, 150 mm, 200mm, 300mm, 450mm as standard and custom size holders are available on request.Holder Options

Replaceable Button Probe

Button ProbesDue 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 from 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.

Installation

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.

Measuring Parameters

Aspect Ratio Range 0.5 to 20
Aspect Ratio Resolution 0.5
Ion Energy Range 2000eV - Vdc
Ion Current 1mA DC max
Ion Flux Range 0.01 - 50 (A/m²)
IEDF Resolution ± 1eV nominal

Probe Bias Conditions

Max RF Bias Voltage 1kV pk-to-pk
Max DC Bias Voltage -1940V
Bias Frequency Range (Time Averaged Measurements) 100kHz to 80MHz
Bias Frequency Range (Time Resolved Measurements) 0Hz to 100kHz
Time Resolution 100 µs

*for pulsed plasmas with Vertex mounted on a grounded or floating electrode

RFEA Probe

Number of Sensors 1
Probe Configuration 4-grid
Button Probe Diameter 33mm
Holder Diameter 50mm, 100mm, 150mm, 200mm, 300mm and custom shapes
Holder Thickness 5mm
Max Operating Temperature 200ºC
Mounting RFEA Probe holder mounted on electrode
Probe Enclosure and Holder Material Aluminium, anodized aluminium, stainless steel* and ceramic (Al2O3)*
RFEA Probe Cable Length 650mm standard (custom available)

*on request

Feed-Through Assembly

Flange Type CF40 (Custom Available)

Control Unit Electronics

Grid Voltage Range -2kV to +2kV
Current Range 100pA to 2.4mA
Connectivity USB 2.0

Application Software

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

Operating Parameters

Pressure (Pascal) 0 to 40Pa*
Pressure (Torr) 0 to 300mTorr*
Density (for Ar at 3eV) 1012 to 1018m-3
Gas Reactivity Inert to highly reactive

*dependent on ion mean free path

The Vertex Single Sensor used in Dusty Plasma applications
Coming soon
The Vertex Single Sensor used in Plasma Etching applications
Coming soon
The Vertex Single Sensor used in HiPIMS Plasma applications
Coming soon
The Vertex Single Sensor used in Ion Beam Plasma applications
Coming soon
The Vertex Single Sensor used in PECVD applications
Coming soon
The Vertex Single Sensor used in Space Plasma applications
Coming soon
The Vertex Single Sensor used in Plasma Sputtering applications
Coming soon

Overview

The incoming ions have an energy component in the X direction perpendicular to the sampling aperture EI and an energy component in the Y direction EII parallel to the sampling aperture

Incoming Ion Angle

Variable Aspect Ratio

The aspect ratio determines if the EII energy component is such that the ion gets collected at the aperture electrodes or passes through the apertures for detection at the collector of the RFEA.

Incoming Ion Angle

Variable Aspect Ratio Using a Variable Bias

The electric field applied in the EI direction determines if the EII energy component is such that the ion gets collected at the aperture electrodes or passes through the apertures for detection at the collector of the RFEA.

Incoming Ion Angle

Aperture Between Grid 2 and Grid 3

Incoming Ion Angle

Theory

The RFEA is designed to have electric fields in the X direction only, it is a planar system with all grids parallel to each other. The discriminator potential at G2 is used to separate ions with different energy and acts only on the EI component of the incoming ion. The EII component of the ion energy is unaffected by the electric fields inside the sensor. At any location inside the sensor the EII component of the ion energy is identical to the EII energy component of the ion as it entered the sensor through the sampling aperture.

By varying the potential difference between G2 and G3, the acceptance angle for which ions can enter the sensor is varied.

AR selection aperture

The aperture between grid 2 and grid 3 is designed to have a specific geometrical AR. Figure 3 shows a schematic representation of the aperture used to measure IEDs at the bottom of features with different ARs. The physical geometrical aperture is fixed with AR (l/w) of 0.5. The effective geometrical AR for positively charged ions can be varied by applying a potential difference (ΔV) between grid 2, which covers the entrance plane of the aperture, and grid 3, which covers the exit plane of the aperture.

Grid 2 is biased to discriminate ions based on their energy. In other words, grid 2 is used to select ions with a specific energy. As a result, the selected ions have close to zero energy when they cross grid 2 i.e. the process of selecting the ions of a particular energy involves the application of a retarding field that reduces the ion energy to almost 0 eV. In order to change the effective geometrical AR, we need to determine the ion trajectory through the aperture in terms of velocity (ν) rather than energy. The kinetic energy of the ion is equal to ½mν². Since the AR is effectively a ratio between ν⊥ and ν∥ then the ion mass (m) can be eliminated and

v∥ = √E∥

s⊥ = √(e∆V)

where the energy E has units of eV. The ion’s position (s) within the aperture (relative to the entrance plane) is given by Newton’s second law of motion, assuming the ion’s initial velocity in the perpendicular direction is zero

s∥ = v∥t

s⊥ = ½a⊥t²

The time taken for an ion to reach the aperture exit plane, in terms of its perpendicular acceleration, is √(2l/a⊥) . The perpendicular velocity at the exit plane v⊥=a⊥ t=√e∆V and therefore the perpendicular acceleration is e∆V/2l. An expression for the perpendicular position can be written as

s⊥ = (e∆V/4l)t²

The position of the ion in the parallel direction, in terms of its parallel energy, is √(E∥)t. The perpendicular position can now be written in terms of the parallel position

s⊥² = (e∆V/4lE∥)s∥²

Solving for the ion’s parallel position with respect to the exit plane gives

s∥² = 4E∥l²/e∆V

Using basic geometric considerations, the AR is v⊥/v∥ or √(E⊥/E∥). In the same reference frame, any ion with s∥>1 will hit the side wall of the aperture. Solving for s∥=1 and substituting E⊥/AR² for E∥ gives the equivalent geometrical AR for the ion energy selected by grid 2 and the potential difference applied between grid 2 and grid 3

AR = 2l/w√E⊥/e∆V

The software user interface (UI) allows the user to select an AR (or series of ARs) and the algorithm automatically selects ions with specified energy and applies the appropriate ΔV to keep AR constant for the duration of a specified AR scan.

Ion energy distribution as a function of aspect ratio

This graph shows the ion flux and IEDs measured for different ARs, using the technique outlined above. These measurements were taken at the powered electrode in a capacitively coupled parallel plate reactor. The driving frequency was 13.56 MHz, the working gas was oxygen and the pressure was 0.5 Pa. The drop in ion flux and the change in shape of the IED as the AR is increased is seen as expected. This implementation of the variable AR method is more convenient than the method described by Cunge et al.

Ion angle distribution measurement with a planar retarding field analyzer

Shailesh Sharma¹, David Gahan², Paul Scullin², Stephen Daniels¹ and M. B. Hopkins²

1. Dublin City University, Glasnevin, Dublin 9, Ireland
2. Impedans Limited, Chase House, City Junction Business Park, Northern Cross, Dublin 17, D17 AK63, Ireland

Published 02 November 2015

Abstract

A new technique is presented to measure the angular distribution of plasma ions bombarding the substrate surface with a planar retarding field analyzer. By varying the effective aspect ratio of the analyzer’s aperture, ions with different angular spread that are allowed through the device for detection are controlled. The analytical theory developed to define the ion current as a function of incident ion angle, ion energy, aperture geometry, and aspect ratio is shown. The method used to vary the effective aspect ratio of the aperture is also discussed. The mathematical theory is derived and the numerical solution discussed. Ion energy distributions, as a function of ion angle, with resolution as low as 3° can be measured.

View online at scitation.aip.org/content/aip/journal/rsi/86/11/10.1063/1.4934808


Ion angular distribution as a function of ion energy

Sweep Average 16 (Quick Scan)
Sweep Average 128 (Detail Scan)


Ion energy distribution function

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