Glossary

/Glossary
Glossary 2017-01-25T11:09:05+00:00

Glossary

Beam pulsing 2017-01-12T10:24:49+00:00

Electron sources from STAIB produce an electron beam that can be pulsed to produce sharp power pulses up to the microsecond range. Applications include sample annealing, heat treatment, evaporation, heat shocks, and luminescense studies.

Beam pulsing is available on all focused electron sources offered by STAIB Instruments. Several standard pulsing options are available to cover a wide range of frequencies. Customization is also available to meet the needs of specific experiments, such as additional ranges of pulse widths.

Fast pulsing systems
Pulsing to 100 Hz can be provided as an option to the electron source power supply. For pulsing beyond this frequency, in addition to the electron source and power supply, a pulsing unit plus a special high voltage cable, that provides an input from the pulse unit (a pulse control signal), are needed. The pulsing unit has an internal clock or allows an external TTL pulse signal to give it a reference signal. Several standard pulsing units are available to cover a large range of frequencies. Other ranges of pulse widths are available on request.

Pulsing unit FP-1 FP-10 FP-100
Minimum pulse length 3 μs 10 μs 100 μs
Maximum pulse length 50 μs 200 μs 20 ms

Please contact our office for specific details.

Reflection High Energy Electron Diffraction (RHEED) 2017-02-01T11:15:07+00:00

is a powerful technique for studying the structures of crystal samples in a wide variety of research and process chambers.
 

Slider01rheedpatternElectrons can be diffracted by a surface in a similar way as visible light, except that the associated wavelength is much smaller. Low energy electrons have a wavelength that is in the order of atomic spacing and can be diffracted by lattice atoms. High energy electrons have a wavelength much smaller than the lattice spacing and diffraction conditions can be reached simply by setting the incident beam at a very small incidence angle to the surface. The diffracted electrons build a diagram consisting of a superposition of spots at Bragg angles for the bulk material, streaks from the surface diffraction, and Kikuchi lines from channeling effects.

staib-instruments-EH30-pic02.jpgThe ability to monitor variations of the diagram during deposition is most valuable. RHEED delivers real time detailed information about the crystal structure of the bulk and the smoothness and crystalline quality of the surface layer with a sub-monolayer resolution. When used in a deposition device, the diffraction diagram instantly displays the surface modifications. The intensity of diffraction spots or streaks is used to monitor the deposition at a layer-by-layer sensitivity (technique named RHEED oscillations).

RHEED is the premium choice technique for growth monitoring since multiple different parameters can be captured simultaneousley in situ, and in real time.

In order to produce sharp diffraction spots, the high energy primary RHEED beam (typically above 10 keV) must be of small size, and highly collimated. Electron guns for RHEED fulfill these requirements using suitable electron optical lenses, which allow full electronic control of the beam parameters. The electron gun is mounted at grazing incidence angle towards the sample surface. The diffracted electrons are imaged on a fluorescent screen mounted on the opposite side of the sample. The screen can be protected with a shutter when not in use. The diffraction diagram displayed is digitalized using a CCD camera.

The video feed can be analyzed in a suitable RHEED acquisition software. Here, the area of interest can be set on the RHEED diagram to follow the signal intensity variations, which can be instantly converted into growth rate and layer thickness.

The standard setup requires an electron gun suitable for RHEED with an energy in the range of 10 to 60 keV. The electron gun is mounted onto the vacuum chamber to achieve a glazing incidence angle to the surface. The angle to the surface should be adjustable in the range of 1 to 3 degrees. Previously, this has been done using precise mechanical orienters. Today state-of-the-art uses a very precise electronic beam rocking control (STAIB Beam Rocking and STAIB Advanced Beam Steering™). The distance between gun and sample can be quite large to allow full clearance around the sample. The multilayer fluorescent screen includes a special phosphorous deposited on lead glass (to block X-rays generated in the chamber). It is designed to enhance contrast by blocking light and low energy electrons from inside the chamber. A shutter can be used to protect the screen and extend its lifetime.
The acquisition and digitalization of the diffraction diagrams is performed by a low noise CCD camera with selectable sensitivity and speed. Changes of the crystal structure, the number of layers deposited, and the deposition rate can be measured during the process simultaneousley.
  • Beam rocking capability to electronically fine adjust the incidence angle.
  • A high beam energy option for operation of RHEED in large vessels, or in order to work with poorly diffracting samples (high energy allows more bulk diffraction) and samples with poor and complex crystal structures (such as textures, nanofibers). High energy guns of 100 keV are also used for gas diffraction.
  • Addition of pumping stage (differential pumping) to the electron gun allows it to work under poor vacuum conditions (TorrRHEEDTM), a common requirement for many deposition systems, such as PLD, IBD, MOMBE and more.
  • Blanking the electron beam (beam blanking) allows fast control of the beam (switching on/off) that can be used for rotating samples and to drastically reduce the electron impact effects on electron sensitive surfaces (organic materials).
Magnetic shielding 2017-01-12T10:24:49+00:00

STAIB Instruments electron sources can be equipped with magnetic shielding. The shielding may be necessary because a magnetic field influences the electron beam. A static or DC magnetic field will shift the beam and an alternating field will wobble the beam. This can cause fluctuations in beam position, size and current.

The magnetic shielding is made of a highly ferromagnetic material. For this reason, it can be used to shield low frequency magnetic fields from the surrounding environment.

Auger Electron Spectroscopy (AES) 2017-01-12T10:24:49+00:00

Principle

The sample is bombarded with an electron beam of sufficient energy to create core level ionizations, leaving a hole in a specific electronic shell. Recombination occurs by filling the hole with an electron from an outer energy level. The released energy is carried outside either by an X-ray photon or another electron from the atom, the Auger electron. Like for X-ray emission, the energy of the Auger electron is characteristic of the atomic element and does not depend on the energy or kind of excitation used.

Technique

The sample (gas or solid) is bombarded by a focused electron beam of 3 to 10 keV energy. The energy of the emitted electrons is analyzed by a highly sensitive energy analyzer, in which the filter energy (so called Pass Energy) is scanned over the Auger lines and the signal is detected using an electron multiplier. The measured energy distribution is digitalized and displayed by a computer program. Data are saved in various formats for processing and quantification.

Set up

The minimal configuration is an electron gun of 3-10 keV energy and an energy analyzer. The best design is to have the electron gun built into the analyzer. Modern analyzers have a selectable energy resolution and large working distance (offering good sample clearance). The STAIB ESA-family is optimized for AES and provides the most advanced compact analyzer design on the market.

Useful options

  • It is important to analyze clean, contamination free samples. The most practical and universal cleaning method is the use of an ion beam (of Argon or other heavy gases) to sputter away the surface layers (which are mostly contamination).
  • Ion sources can also be used for Ion back-Scattering Spectroscopy (ISS), and AES depth profiling.
  • Excitation for AES can be operated with any electron source, provided it delivers a very low noise, stable beam in the correct energy range. When combined with a micro-focus source (delivering a small spot of high spatial resolution), very small sample areas can be analyzed, down to about 100 nm. Scanning Electron Microscopy (SEM) and Scanning Auger Microscopy (SAM) utilize a beam that is scanned over the sample, to map the surface elements.
  • Computer control of the analyzer system allows users to pre-set series of measurements (i.e. for over night).
  • Special systems have been designed to perform AES measurements at higher vacuum pressures, i.e. see AugerProbeTM.
  • Very compact AES analyzers, with small diameter and large working distance, can be used in systems where the space available is very tight (like MBE chambers).
Electron Energy Loss Spectroscopy (EELS) and REELS 2017-03-16T09:53:20+00:00

This method refers to the measurement of specific energy losses suffered by a Primary Electron (PE) traveling through a surface or inside a solid target. All interactions belong to elementary excitation in solids that theoretically describe quantified interactions of the PE with the solid bulk and surface. Each kind of interaction has its own energy meaning that the PE will lose a characteristic amount of energy for each excitation which is the basic feature of EELS.

The most common interactions are:

  • ionization of a core atomic level (followed by X-ray or Auger emission)
  • the excitation or ionization of outer energy levels leading to intra- and inter-band transitions
  • the plasmon excitations of the electron gas of the valence / conduction band.

Experimentally, plasmon excitation are normally dominant.

There are two different ways to measure energy losses, and their results are very different:

  • In Transmission, where the PE crosses a thin sample as found in Transmission Electron Microscopy (TEM). This technique is referred to as transmission Electron Energy Loss Spectroscopy (comonly undestood as EELS).
  • In Reflection, where the PE impact a thick sample and the energy loss spectra is measured from electron back scattered from the surface. This technique is referred to as Reflection Electron Energy Loss Spectroscopy (REELS).

staib-instrumets-EELS-REELSThe following figure shows two possible configurations for transmission and reflection. The incidence angle is normal to the surface in transmission, but it can have any orientation in reflection. An interesting geometry is using a high energy PE beam at grazing angle to the surface (RHEED geometry) and a RHEA analyzer.

Although both techniques generate similar excitations, they differ by the fact that in transmission by choosing proper sample thicknesses the background of electrons scattered inside the target is kept very low as compared to energy loss intensities. In contrast, all electrons in reflection mode must be scattered by a large angle. Therefor a larger background of scatted electrons is added to the energy loss signals. Fortunately, this background is negligible for energies near the PE energy. Thus, Characteristic Energy Losses (CEL) like plasmon and band transition can be well measured in reflection mode whereas the larger loss energies corresponding to deeper core level excitation will be superposed to a large background of scattered electrons.

The dominant CEL structures measured in reflection are Plasmons, low energy ionization levels and band transitions. The corresponding loss energies do not depend on the atomic number Z but are governed by the outer shell structure of the solid, thus making CEL very sensitive to the chemical environment of surface layers.

For REELS, energy analyzers with a very good energy resolution are required. The analyzers should reach this resolution especially at higher PE energies in the keV range, and, additionnally work in constant energy resolution mode (providing constant resolution at all energy). Standard CMA are therefore not well suited for this kind of operation.
In contrast, all STAIB Instrument energy analyzer are powerful REELS analyzers.

Standby operation 2016-10-26T09:01:00+00:00

The standby operation reduces the filament heating current and is used during longer operation pauses.

Advanced beam steering™ for RHEED 2016-11-29T10:54:26+00:00

STAIB Instruments has designed a patented system of beam steering that electronically controls the beam position and beam incidence angle.

With the patented STAIB TorrRHEED™, the incident angel can be changed with a unique double deflection system. The beam can easily be manipulated on the substrate without changing the mechanical alignment. This gives the operator the desired degrees of freedom for easy and simple alignment of the RHEED setup. RHEED diffraction conditions can be quickly and reproducibly fine tuned at any time, without the cumbersome mechanical manipulation approach used by other manufacturers.

X-ray Photelectron Spectroscopy (XPS) 2017-01-12T10:24:49+00:00

Principle

The sample is illuminated with X-ray radiation of sufficient energy to create core level ionizations of the atoms. The photon energy is transferred to the electron, with some leaving the sample as photoelectrons. The kinetic energy of the emitted photoelectron is given by the difference between photon energy and binding energy of the electron in the sample. Unlike for X-ray or Auger emission, the energy of the photoelectron depends on the X-ray energy used. As X-ray radiation is almost monochromatic, the energy of the measured line can be simply converted into binding energies.

Technique

An X-ray source producing quasi-monochromatic radiation (using Al or Mg anodes) is used to generate photoelectrons of different energies. The energy of the emitted electrons is analyzed by a highly sensitive, high energy resolution energy analyzer. The filter energy is scanned over the photoelectron lines and the signal is detected using an electron multiplier. The measured energy distribution is digitalized and displayed by a computer program. Data are saved in various formats for processing and quantification.

Set up

The minimum configuration requires an X-ray source and a high resolution energy analyzer suitable for XPS. The X-ray source is equipped with two anodes, Al and Mg, delivering different photon energies so that the photoelectron lines can be differentiated from the Auger lines (at fixed kinetic energy). Modern analyzers have selectable energy resolution and large working distance (good sample clearance). Having the electron source built into the analyzer provides ease of use and alignment, with full AES capability on a single flange. The STAIB DESA- family is optimized for XPS and AES.

Useful options

  • It is important to analyze clean, contamination free samples. The most practical and universal cleaning method is the use of an ion beam (of Argon or other heavy gases) to sputter away the surface layers (which are mostly contamination). The same ion source can be used for Ion back-Scattering Spectroscopy (ISS).
  • Computer control of the analyzer system allows users to pre-set series of measurements (i.e. for over night).
  • When combined with a micro-focus source, full Auger capabilities become available, including AES, SEM and SAM.
  • A special low energy electron source can be used for charge compensation on isolating samples.
Beam blanking 2016-11-07T09:53:38+00:00

The unique design of STAIB’s electron sources has several advantages over the classic triode gun configuration.

Beam blanking is one such advantage. The beam can be switched on and off electrically by controlling electron extraction from the filament. This method avoids the generation of stray electrons.

The basic beam blanking option (standard on all STAIB RHEED systems), allows the beam to be turned on and off manually from the remote control box.

Additionally a full beam blanking option is available. Here the electron beam can be turned on and off using a TTL type input through a BNC connector.

The full beam blanking option can be used to keep the electron load on the sample very low. Electron bombardment of sensitive surfaces can be kept to a minimum. The blanking signal can also be triggered synchronously with external signals to improve the use of the electron source in the presence of stray magnetic fields by compensating their effect. The external trigger can also be utilized to synchronize with rotating samples, which is important for in situ monitoring in some deposition techniques.

Beam rocking 2016-11-07T09:54:27+00:00

For RHEED analysis, the electron beam must impinge onto the surface at grazing incidence.

The unique STAIB feature, beam rocking, allows precise adjustment and variation of the incidence angle using electronic controls, without moving the sample. Electron sources from STAIB Instruments are uniquely designed to allow this precise electronic control of the beam position using sophisticated electron beam deflection optics.

When equipped with the beam rocking option, specially designed optics shift the electron path off axis in the gun and refocus it onto the sample, maintaining the spot position. In this way, the incidence angle can be precisely controlled electronically without either modifications of the geometry of the gun or motion of the sample. The electron gun can be mounted on the vacuum chamber without using a bellows for mechanical adjustment. The beam rocking option is very helpful in critical cases where the sample position cannot be adjusted easily during the growth. In these cases, beam rocking allows precise variation of the incidence angle.

Beam scanning 2017-02-02T08:33:54+00:00

STAIB Instruments offers Scanning Electron Microscopy (SEM) products designed around our electron sources.

For beam diameters below ca. 10 microns, bright and detailed images of the sample are obtained in SEM mode. A number of specific electron guns covering a wide range of beam diameters and energies are available.

The elements of the system are:

  • Electron gun and power supply
    Any of our microfocus sources may be used. Other STAIB electron sources may be used if higher lateral resolution is not needed.
  • Scanning amplifier
  • Current amplifier
  • Secondary electron detector
  • Data acquisition system
Complete service kit 2016-11-07T10:22:03+00:00

The kit contains one spare filament and tools for standard maintenance.

Advanced beam steering™ for electron sources 2016-11-29T10:53:51+00:00

STAIB Instruments has designed a patented system of beam steering that electronically controls the beam position and deflection angle.

With the patented STAIB Advanced beam steering™, the beam angle can be changed with a unique double deflection system. The beam can easily be manipulated on the sample without changing a mechanical alignment. This gives the operator the desired degrees of freedom for easy and simple alignment of the electron beam setup. Even double beam rocking can easily be performed.

Computer control 2017-01-12T10:24:49+00:00

The computer control option for STAIB electron guns consists of two parts.

1: Power supply computer control plug
The electron source power supply is equipped with an optional computer connection plug for use with analog control signals. The electron source system can either be operated using the manual control dials, or by using the optional computer control. The control voltages can either be provided by 3rd party software through a suitable DAC interface, or by the STAIB computer control module.

2: STAIB computer control module
The STAIB computer control option includes the cables and microprocessor interface to connect to the power supply computer control plug. The STAIB software sends the input signals to the power supply to control the system functions. Full parameter sets can be stored, modified, and reused. The advanced version of the STAIB computer control module can retrieve and copy the adjusted values from the manual control dials, providing the user with previously unavailable ease of use.

The option runs under the WindowsTM XP, WindowsTM 7, WindowsTM 10 operating systems, using a USB connection.

RHEED computer control includes a microprocessor interface module and a software package, and makes use of the RHEED very precise and repeatable. User’s parameters can be saved and retrieved, the beam can be turned on and off using the beam blanking option, and the incidence angle can be adjusted using the optional beam rocking feature of the gun.

Diagnostic box 2016-11-07T10:23:35+00:00

The major functions of the power supply can be easily tested using the diagnostic box connected to the diagnostic plug on the rear of the power supply.

Differential pumping for RHEED 2017-01-12T10:24:49+00:00

In many growth environments, the vacuum conditions are not good enough to operate an electron gun.

RHEED systems from STAIB Instruments are available with a differential pumping option for pressures greater than 10-5 mbar. By utilizing a turbo pump (>40 l/s) attached to a special flange on the electron gun, the system continues to work well up to a few 10-3 mbar. Efficient pumping of the filament section of the source offers unbeatable versatility. For higher pressures, the double differential pumping option (TorrRHEED™) is available, which still allows the complete electronic control of the beam.

The differential pumping option is available on all RHEED systems from STAIB Instruments in the range of 15 eV to 60 keV.

Electronic beam control 2016-11-07T10:20:42+00:00

lectron sources from STAIB Instruments are uniquely designed to allow precise electronic control of the beam position and current. Sophisticated electron beam deflection optics accurately control the beam position onto the
sample.

Features achieved by this precise control are:

  • Full compensation of the earth’s magnetic field for all beam energies.
  • Energy compensated deflection of the beam. During analyses, the beam energy can be
    changed without readjusting the focus or beam position.
  • The deflection optics allow the capability of electronic beam rocking. The electron path is shifted
    off axis in the gun and refocused onto the sample, maintaining the spot position. In this way,
    the incidence angle can be controlled electronically without either modifications of the geometry of
    the gun or motion of the sample. The electron gun can be mounted on the vacuum chamber
    without using a bellows for mechanical adjustment.

The electronic beam control option is very helpful in even more critical cases where the sample position cannot be adjusted easily during the growth. In this case, beam rocking allows a precise variation of the incidence angle.

Microprocessor controlled beam current regulation 2016-11-07T09:57:36+00:00

The Beam Current Controller (BCC) is a microcontroller based unit used to regulate the electron beam current of the electron gun, as used in RHEED or AUGER techniques.

Regulation of the electron beam current to a fixed, precise, user-adjusted value is important when the electron gun is working in a higher pressure range and when using reactive gases in the vacuum chamber. Once adjusted, the BCC will regulate the beam current to the assigned value, even after gun parameters like beam energy or focus are modified. Operating values can be saved and will be kept after restarting the BCC module, ensuring identical experimental conditions.

Parallel beam shift for RHEED 2016-11-29T10:56:25+00:00

The advanced beam steering is also used in the parallel beam shift option, which combines beam rocking (in X) and the parallel shifting of the beam (in Y). The RHEED beam can thus be used to analyze different positions on the substrate at the same RHEED angle (incidence and azimuth) (see RHEED for IBAD and RHEED for combinatorial deposition).

RHEED power supply and remote control 2016-11-07T09:52:31+00:00

The power supply, up to 60 keV, is compact and designed with state-of-the-art electronic components.

Gun settings, energy, filament current, and beam intensity, are displayed using digital readouts. The power supply is remote controlled by a compact control box to adjust the beam intensity, focus, and position. The system is equipped with a diagnostic capability, which allows a quick and complete test of the RHEED system electronics.

µ-metal shielding 2016-11-29T11:01:48+00:00

Mounted on an adaptor flange that is designed for the user’s chamber, the shield mounts between the
chamber and gun flanges. It consists of two µ metal tubes, one fixed and the other a telescopic design
to be adjusted on site, without precise measurement in advance. It can be adapted to fit the user’s
chamber. The shield should extend beyond the cryoshroud for best protection, especially from the effects
of charging of the cryoshroud. An additional advantage is that the shield protects the electron gun from
deposition and it can be easily cleaned.

Differential pumping for electron sources 2017-01-12T10:24:49+00:00

In many challanging environments, the vacuum conditions are not good enough to operate an electron gun.

Electron sources from STAIB Instruments are available with a differential pumping option for pressures greater than 10-5 mbar. By utilizing a turbo pump (>40 l/s) attached to a special flange on the electron gun, the system continues to work well up to a few 10-3 mbar. Efficient pumping of the filament section of the source offers unbeatable versatility. For higher pressures, the double differential pumping option (e.g. TorrRHEED™) is available, which still allows the complete electronic control of the beam.

Parallel beam shift for electron sources 2016-11-29T10:56:45+00:00

The Advanced beam steering™ is also used in the parallel beam shift option, which combines beam rocking (in X) and the parallel shifting of the beam (in Y). The electron beam can thus be used to analyze different positions on the sample at the same angle (incidence and azimuth).