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1. 1 Please refer to Appendix Al for a more detailed description of the ErlEED 1000A unit Redudant since 2001 units are now supplied with wide range power supplies for a direct 110V or 220V connection Safety button since 2002 Push to operate ErLEED Optics and Power Supplies 13 5 3 ErLEED 3000D LEED mode As far as the LEED mode is concerned the ErLEED power supplies are more or less identical in their functionality Nevertheless they differ from each other significantly by their control possibilities As mentioned earlier the ErLEED 3000D unit can be controlled by either front panel input in a quasi analog manner or with the RFA PC software The mode of operation is indicated on the front display and can be selected in the System Menu or within the RFA PC software The LEED mode operation in this chapter is described with using the front panel input Please refer to ErLEED 3000D user manual The spectroscopy mode in chapter 5 4 is explained by using the RFA PC software Please refer to EeLEED RFA PC user manual The ErLEED optics should only be operated at a base pressure in the UHV chamber lower than 5 x 10 mbar if an LaB filament is used For Thoria coated Iridium filaments the pressure should be in the 10 mbar range e Before switching on Check the line voltage red commutator at the rear of the unit e Connect the mains plug e Connect the SHV cable to the screen voltage output at the rear panel of the unit and to th
2. the sample position is not correct the diffraction pattern is distorted In particular the unit cell appears slightly blown up if the sample is too close to the optics while it looks deflated if the sample distance is too large In addition the underground in the diffraction pattern is inhomogeneous when the suppressor voltage is varied in the region of small voltages Furthermore a broadening of the diffraction spots and a shift of the spot positions is observed The position and the intensity of the diffraction spots depend sensitively on the energy The sharpness of the spots is on one hand dependent on the voltages at the lens elements the Wehnelt cylinder and at the anode On the other hand it is a function of the quality of the sample surface i e of the cleanliness of the surface and the degree of the long range order ErLEED Optics and Power Supplies 5 2 ErLEED 1000A The ErLEED optics should only be operated at a base pressure in the UHV chamber lower than 5 x 10 mbar if a LaB filament is used For Thoria coated Iridium filaments the pressure should be in the 10 mbar range Before switching on Check the line voltage red commutator at the rear of the unit e Connect the mains plug e Check if the screen voltage is set to OFF e Connect the SHV cable to the High Voltage output at the rear of the unit and to the HV feedthrough for the screen voltage at the LEED flange of the optics e Connect the multipin cable t
3. time constant has however to be compensated for by a decrease of the scan velocity Finally a smoothing of the spectra by means of a computer will only in certain cases improve the quality of the measurement since important information is lost as a consequence of the smoothing procedure ErLEED Optics and Power Supplies 19 6 1 20 Chapter 6 Maintenance Maintenance Replacement of the filament The lifetime of the filaments used in the ErLEED optics depends crucially on the working conditions It is therefore recommended to carefully outgas the filaments after bakeout and to avoid working at base pressures above 5x10 mbar In particular LaBg cathodes react rather sensitively to any kind of contamination of their surface It might therefore be necessary to carefully reactivate the filament from time to time to reestablish stable emission characteristics e g after bakeout of the chamber or after exposure to surface contaminants like CO CmHn etc The filament of the ErLEED electron gun needs to be replaced when it is fused or when the emission characteristics deteriorate significantly due to excessive contamination of the cathode surface that cannot be removed by reactivation Spare filaments are usually available from stock at SPECS GmbH In the following the working steps required for the replacement of the filament will be explained in detail Important Choose a clean and dust free place in or outside of your laboratory fo
4. voltage Adjustment of the voltage at L1 3 offset Adjustment of the screen voltage Adjustment of the voltage at L2 gain Adjustment of the voltage at L2 offset Control LED for filament current Control LED for screen voltage Power switch Controlling devices rear panel EG qe c N O CI Se o a co Socket for mains plug Commutator 110V 220V line voltage Ventilator Multipin socket for the connection of the 12 pin cable leading to the LEED optics electron gun and grids SHV BNC socket for the fluorescent screen high voltage BNC socket of l0 monitoring output 0 10V voltage input for external control of electron energy Selector switch for internal external control of electron energy Safety button since 2002 Push to operate Redudant since 2001 units are now supplied with wide range power supplies for a direct 110V or 220V connection 24 ErLEED Optics and Power Supplies C W A L1 L2 L3 j e M ann O I Filament Usa is 5V Imax JA ELA Q L4 S G1 4 FLL lt f sf d ree SP oF Wehnelt Umax 150V Anode U max 1000V Imax 2MA L1 3 Umax 2000V Imax INA Suppressor Umax 350V L2 U max 1000V Imax IMA Energy Umax 1000V Screen Umax 7 5kV Tmax 0 75 mA Lmax 2MA Fig 1
5. when moving the mechanics Prior the first use the optics should be baked out The maximum bakeout temperature is 250 C also for versions with z retraction mechanism and or integrated shutter ErLEED Optics and Power Supplies 9 5 1 Chapter 5 Operation Operation General remarks In the following the main experimental parameters and their influence on the performance will be summarized for the LEED mode In the subsequent chapter the work conditions of the optics in LEED mode will be explained separately for ErLEED 1000A and for ErLEED 3000D units Finally the operation and optimisation of the ErLEED system in the RFA mode will be described in chapter 5 4 Influence of the gun elements The Wehnelt cylinder acts as an electrostatic aperture between the cathode and the anode It is on the same or on a negative potential with respect to the cathode and regulates the penetration of the anode potential into the direction of the cathode An increase of the Wehnelt voltage leads to a narrowing of the electron beam By that means the sharpness of the diffraction spots can be improved However if the voltage is increased above a certain value the spot intensities decrease and become completely suppressed finally The optimum Wehnelt voltage slightly depends on the energy This was taken into consideration in the concept of the ErLEED 3000D control unit where the Wehnelt voltage varies linearly as a function of the energy with adjustable ga
6. 1 Block diagram of the ErLEED 1000A control unit showing the voltages applied to the electron gun the fluorescent screen and the grids of the LEED optics A 4 grid optics is used as an example ErLEED Optics and Power Supplies 25 HAN ORE o 99 99 Voltage Cathode Current Anode Current Energy ErLEED 1000 A Wehnelt V Anode E e D L2 adi Off 7 Suppressor Cathode Anode Gain L1 3 Gain L2 Gain POWER Wehnelt Anode Offset L1 3 Offset L2 Offset Energy Front panel of the ErLEED 1000A control unit The function of the devices is described in the text Fig 1 2 Schematic view of the back panel of the ErLEED 1000A control unit The function of the devices is described in the text Fig 1 3 26 ErLEED Optics and Power Supplies
7. M8 screws Before mounting the optics to the UHV system a DN 100 150 CF copper gasket and the propper amount of M8 screws or a corresponding set of thread rods respectively should be placed at hand In the standard version the LEED flange has tapped holes A version with bored holes is available on request A UHV window is already mounted and UHV leak tested When mounting the optics it is of great importance to work under highly clean conditions Each dust particle which enters the region of the grids or the screen may lead to electric charging effects when operating the optics and may cause severe disturbance of the LEED pattern After removing the four M8 screws the optics can be taken out of the stainless steel housing The copper gasket which is used to protect the cutting edge has to be replaced by a new one If the optics is kept outside the vacuum for longer time it is recommended to cover the grids and screen in order to avoid the entering of dust particles The optics should however be mounted to the UHV chamber as soon as possible after removal of the housing In principle the optic can be mounted in any possible orientation However in case of an integrated shutter a 12 o clock mounting of the shutter UHV rotary motion drive is highly recommended It is not possible to mount the ErLEED 100 150 optics bottom up with an integrated shutter To ensure smooth moving be sure that the shuiter is closed and the driving shaft is not engaged
8. SP CS Surface Analysis and Computer Technology Optics and Power Supplies ErLEED User Manual 12 All rights reserved No part of this manual may be reproduced without the prior permission of SPECS GmbH User manual for the ErLEED optics and power supplies Version 1 2 of the 04 01 2002 SPECS order number for this manual 78000144 SPECS GmbH Voltastr 5 13355 Berlin Germany phone 49 30 467824 0 fax 49 30 4642083 http www specs de support specs de Chapter T Table of contents T Introductlioh ANAKAN NAA 1 2 Description of the optics 3 2 1 General remarks oxucexiexu imn n inxu inning m inn inrb unu ANaR apu eRRrUSErUS 3 2 2 Electron QUI LC 3 2 3 Grids and fluorescent screen 4 2 4 z retraction mechanism optional 4 2 5 Integrated shutter optional 4 3 Power supplies oae eee 7 4 Mounting of the ErLEED optics 9 5 SODCRAMON ecc m 10 5 1 General remarks ccce 10 5 2 EFEEED 10004 5 iio Deere eec ca eire ctu beret 13 5 3 ErLEED 3000D LEED mode 14 5 4 ErLEED 3000D RFA mode esee 15 6 Maintenance eeeeeeeee eene nenne nnn 20 6 1 Replacement of
9. average distances defect concentrations terrace distributions etc A 4 grid LEED optics can in addition be used as an electron energy analyzer or more precisely a retarding field analyzer RFA This offers the possibility of recording Auger electron spectra AES or energy electron loss spectra EELS with an energy resolution of a few eV The ErLEED system has been designed at the Institute of Solid State of the University of Erlangen N rnberg Germany It is fabricated under an exclusive license by SPECS GmbH Berlin Germany In order to guarantee the highest possible quality and performance of each system great importance is attached to the selection of the used materials and components as well as to the quality of the mechanical and electronical manufacturing ErLEED Optics and Power Supplies 1 The optics has been designed as a reverse view or back view optics respectively with a transparent fluorescent screen That means that it is possible to observe the diffraction pattern at the position of the LEED flange The miniaturized electron gun obstructs only a minor part of the LEED pattern The optics is offered in a 2 3 and 4 grid version In addition the mounting on a z retraction mechanism and the integration of a shutter is possible As power supplies or control units SPECS GmbH offers an analog and a digital version ErLEED 1000A and ErLEED 3000D respectively As an extension of the LEED system the PC based video data a
10. cquisition system AIDA PC for recording and processing of LEED data has been developed AES data may be comfortably recorded using the RFA PC software in combination with the ErLEED 3000D power supply with integrated lock in amplifier LIA All mentioned products are distributed exclusively by SPECS GmbH ErLEED Optics and Power Supplies 2 1 2 2 Chapter 2 Description of the optics Description of the optics General remarks The ErLEED optics is either mounted on a DN 100 CF or DN 150 CF flange Each flange carries the electrical feedthroughs for the operating voltages that is a 12 pin feedthrough DN 35CF for the electron gun and grid voltages and a HV feedthrough for the voltage supply of the fluorescent screen On the atmospheric side the LEED flange is equipped with a UHV DN 100 150 CF window Inside the vacuum the grids the fluorescent screen and the electron gun are mounted on a base ring see fig 2 1 All cables have been fixed to the mechanical setup in a way that the view on the LEED screen is not obstructed For the electrical connections crimp connectors have been used which facilitates service works Optional there are up to two DN 16 CF UHV rotary motion drives for a z retraction mechanism and or an integrated shutter Before the ErLEED optics is delivered it is completely checked in UHV During those tests the filament is carefully outgased and formed In a seperate test report supllied with the unit the optimum condit
11. e positioned in the centre of the screen The tolerance is only a few millimeters In addition the sample surface normal should be parallel to the optical axis of the LEED optics ErLEED Optics and Power Supplies 17 18 e Set the collector voltage to 500V e Activate the ramp generator and set the start and final energy of the scan to 950 eV and 1050 eV respectively e Set the scan time to 100 sec e Set the modulation amplitude to about 4 V peak to peak depending on the height of the signal e Select OUTPUT 1 of the oscillator to modulate the voltage at the suppressor grids Setting of the Lock In amplifier e Set the input frequency filter Bandpass Low or Highpass respectively to f 2100 Hz e Set the input sensitivity to about 1 meV depending on the height of the signal e Set the 1f reference frequency mode e Set the time constant to TC 0 3 sec or TC 1 sec In order to maximize the output signal of the LIA the phase shift between the measuring signal and the reference signal has to be adjusted In addition the deviation N E of the primary peak has to show a maximum on the low energy side and a minimum on the low energy side with the maximum and the minimum having approxi mately the same amplitude In the case of a strongly asymmetric primary peak the position of the sample should be checked Hint The fastest method to optimize the phase matching of the LIA is to select the manual mode of operation and t
12. e HV feedthrough for the screen voltage at the LEED flange of the optics e Connect the multipin cable to the grids e gun voltage output of the power supply and to the 12 pin feedthrough at the LEED flange e Check if the SENSE input of the power supply is short circuited either by a BNC end connector or by an external device for example the trip output of a Bayard Alpert ionization gauge e Switch on power An acoustical signal is heard a self test and the calibration of the unit is performed during about 10 sec e Set the voltages at the electron gun Wehnelt anode lens 1 3 lens 2 to the values in the test report using the corresponding potentiometers for gain and offset Refer to chapter 7 of the ErLEED 3000D user manual for a description of the operation of the control devices of the power supply The respective offset voltages are determined by extrapolation of the voltages in the test report to E 0eV The offset is adjusted with the electron energy set to zero In a second step the gain of the electron gun voltages is adjusted by setting the energy to 100eV or 300eV respectively and adjusting the voltages with gain control to the corresponding values given in the test report e Select the mode of the primary energy control of the power supply internal external remote RS232 see chapter 7 of the ErLEED 3000D user manual Set the energy to 100 eV e Set the suppressor voltage to maximum in order to maximize the energy windo
13. e signal at the output of the LIA An example for the second case are the elastically reflected primary electrons with an energy loss of 0 eV Switching from OUTPUT 1 to OUTPUT 2 therefore allows to distinguish between energy loss electrons and Auger electrons in the spectra ErLEED Optics and Power Supplies Concerning the setting of the LIA and the ErLEED 3000D control unit it is recommended to optimize the parameter adjustment using the strong primary peak of the elastically reflected electrons In a second step the spectrum of the much weaker Auger transitions can be recorded The procedure of the recording of the primary peak and the Auger spectra respectively will be described in the following Detection of the primary peak e Before switching on the power supply the matching unit has to be connected to the HV feedthrough on the LEED flange In addition to the signal matching circuit the matching unit contains a frequency filter and a preamplifier In order to provide the necessary supply voltage of 12 V the DIN cable coming from the matching unit has to be plugged into the socket labeled supply on the rear of the ErLEED 3000D unit e The signal output corresponding to the low voltage BNC socket on the matching unit has to be connected to the input of a Lock in amplifier LIA e The medium voltage BNC socket on the matching unit has to be connected to the collector voltage output at the rear of the power supply e The BNC sock
14. ection The output signal of the LIA is then the first derivative of the energy distribution i e N E When the 2f mode of the oscillator is set the modulation frequency is identical to the reference frequency In this case the energy distribution N E is obtained at the output of the LIA The frequency of the oscillator can be varied in discrete steps between 750 and 1250Hz please refer to chapter 7 5 7 of ErLEED RFA PC user manual It should however only be varied in special cases as such a variation makes a recalibration of the frequency filter in the matching unit necessary An electric charging of the screen is avoided by applying a positive voltage of about 500 V to it In order to separate this voltage from the measuring signal a matching unit has to be placed between the output of the screen and the input of the LIA In the matching unit used in connection with the ErLEED 3000D control unit an active frequency filter and a low noise signal preamplifier are integrated additionally When OUTPUT 1 is activated the voltage at the suppressor grids 2 and 3 is modulated If OUTPUT 2 is active the modulation is applied to the cathode potential As the energy of Auger electrons is independent of the excitation energy they can only be detected by a modulation of the suppressor voltage If instead the primary energy cathode potential is modulated only those electrons with an energy depending on the primary energy will contribute to th
15. emory of the ErLEED 3000D power supply The storing and recalling of parameter settings is explained in chapter 7 10 of the ErLEED 3000D user manual ErLEED 3000D RFA mode In the RFA mode the ErLEED 3000D power supply provides all necessary voltages to operate a LEED optics as a retarding field analyser RFA for electron energy loss spectroscopy EELS or Auger electron spectroscopy AES with a maximum adjustable primary electron energy of 3 keV For data recording an integrated Lock In amplifier and RFA PC software is used The software is described in a separate ErLEED RFA PC user manual All references in this chapter are based thereon although operation is possible with front panel input as well An external Lock In amplifier can be used optional with a diffrent version of RFA PC In order to obtain an energy resolution which is sufficient for spectroscopy experiments a highly homogenous retarding field is required Therefore a 4 grid LEED optics is necessary for this kind of experiments The ErLEED optics should only be operated at a base pressure in the UHV chamber lower than 5 x 10 mbar if an LaB filament is used For Thoria coated Iridium filaments the pressure should be in the 10 mbar range ErLEED Optics and Power Supplies 15 16 The signal at the fluorescent screen is N E dE where N E is the energy eU Supp distribution of the electrons emitted from the sample and Usupp the suppressor voltage at grids 2 and 3 of the opt
16. et labeled REF on the rear of the control unit provides the reference signal which corresponds to 1f or 2f of the oscillation frequency applied to the suppressor grids respectively for the Lock In detection It has to be connected to the reference input of the LIA which has to be set to 1f reference mode e n case of an integrated LIA data are recorded and displayed by RFA PC software In order to record a spectrum manually the output of the LIA has to be connected to the Y input of a XY recorder or to a computer respectively The energy axis corresponding to the X axis is driven by the U Mon output on the rear of the ErLEED 3000D e Switch on the power supply e Set the primary beam energy to 1000eV e Adjust the focus parameters according to the values given in the attached test report In the RFA mode of the power supply the voltages at lens 1 3 are set equal to the anode potential For focusing of the gun only the offset voltages are used as the primary energy of the electrons is generally kept constant during the experiments so that no broad range focusing is needed The screen high voltage is not active in the RFA mode Instead a collector voltage of up to 500V is applied to the screen in order to avoid electrical charging e Switch on the filament e Set the filament current to the working value given in the test report e Adjust the sample position In order to obtain the highest possible energy resolution the sample has to b
17. gain and offset While the quality of the focus at low energies is mainly determined by the offset the adjustment of the gain becomes important at higher energies 5300 eV In addition there is a strong interdependence of the voltages at lens 1 3 and lens 2 The general focusing procedure is to optimise all offset voltages of the e gun elements at low energies 50eV and then switching to high energies gt 3800eV to adjust the corresponding voltage gains leaving the offsets unchanged By repeating these steps several times a broad range focus is obtained iteratively Alternative focus conditions Due to the large number of adjustable voltages Wehnelt anode lens 1 3 and lens 2 there exists more than one optimum combination of the values Which one is chosen finally depends on the respective experimental requirements It might for example be necessary to have a good focus over a broad energy range the sharpest possible or the most intense spots etc The values stated in the ErLEED test report constitute a compromise between a broad range focus between about 50 and 500 eV and a homogeneous beam profile in this energy range In special cases there exists the necessity of a higher beam current or narrower spots in a smaller range of energies The standard parameter setup may serve as a good starting point to find a setting which is more appropriate for the given experimental conditions Filament The optimum filament current is a com
18. ge as well as the offset and gain of the lens and anode voltages are set by potentiometers on the front of the unit e 3 digital meters for current and voltage display The cathode potential corresponding to the electron energy is measured in reference to the internal ground The Wehnelt anode suppressor and lens voltages are measured and displayed relative to the cathode potential e Monitoring outputs for beam current 10 and electron energy e 0 10V voltage input for the external remote control of the electron energy e Currents and voltages supplied by the HV modules cathode 0 1000 V 2 mA lens 1 3 0 2000 V 1 mA lens 2 0 1000 V 1 mA anode 0 1000 V 2 mA fluorescent screen 0 7 5 kV 0 75 mA Wehnelt 0 150 V ErLEED Optics and Power Supplies 23 Controlling devices front panel mo Oy OO p CODO OW BE OD LCD 0 1 Adjustment of the voltage at L1 3 gain 12 13 14 15 16 17 18 Digital voltmeter 1 DVM 1 for the display of the energy in eV and the Wehnelt anode lens and suppressor voltages in V DVM for the display of the filament current in A DVMS for the display of the anode current in mA Selector switch for DVM 1 Adjustment of the energy cathode potential Adjustment of the filament heating voltage with ON OFF switch Adjustment of the Wehnelt voltage Adjustment of the gain of the anode voltage Adjustment of the offset of the anode voltage Adjustment of the gain of the suppressor
19. ics The energy distribution N E itself and its first derivation N E are obtained using Lock In technique For that purpose the cutoff voltage at the suppressor grids is modulated with a fixed frequency The corresponding intensity variation on the fluorescent screen serving as the electron detector is read out by a Lock In amplifier LIA Then the energy distribution N E is obtained from the oscillation amplitude of the 1w Fourier component of the signal The amplitude of the 2w component is giving the first derivation N E which corresponds to the standard representation of the spectra of Auger electrons excited by an electron beam In comparison to the differentiated spectra the undifferentiated spectra N E have the disadvantage that small peaks are often hidden by the strong background of secondary electrons The oscillator circuit providing the modulation voltage in the ErLEED 3000D power supply works at a preset quartz stabilized frequency of 1050 Hz 1f mode activated or 2100 Hz 2f mode activated respectively The maximum peak to peak amplitude is 12 V The oscillator signal is monitored at the f OUT output on the rear of the control unit The reference frequency at the f REF output is always twice the frequency of the oscillator in the 1f mode For standard operation the oscillator and the LIA are set to the 1f mode each and the voltage at the f REF output is us as the reference signal for the lock in det
20. in and offset The anode is always on a positive potential with respect to the cathode It accelerates the electrons emitted by the filament into the direction of the lens elements The optimum anode voltage is energy dependent too Therefore the ErLEED control units provide an adjustable gain and offset of the anode voltage in order to obtain a good focus over a broad energy range i The ErLEED 3000D unit can be operated using front panel input exlusively Via the RS232 interface and RFA PC software the unit can also be fully controlled by using a computer Both possibilities are described in detail in two separate user manuals ErLEED 3000D and ErLEED RFA PC respectively 10 ErLEED Optics and Power Supplies The lens elements 1 2 and 3 constitute an electrostatic single lens which is essentially responsible for the shaping of the electron beam In the diffraction experiments the electron source or more precisely the cross over point in front of the cathode is focused on the fluorescent screen where the sample acts as a mirror plane On the contrary the source is focused on the sample in the spectroscopy setup The lens elements 1 and 3 labelled lens 1 3 are on the same potential while the potential of lens 2 is independent of lens 1 3 In order to obtain a sharp focus over a broad range of energies the lens voltages have to vary as a function of the energy Therefore the energy voltage characteristics are fitted by a linear curve with
21. ions and emission currents are documented Electron gun The highly compact electron gun diameter 15 mm consists of the cathode the Wehnelt cylinder a double anode an electrostatic single lens elements L1 L2 L3 and the drift tube lens element L4 on internal ground potential see figure 1 1 Within the standard LEED operation the electron energy can be varied between 0 and 1000 eV With the ErLEED 3000D power supply energies up to 3000 eV can be set in for operation in RFA mode It is however recommended to limit the energy to about 2500 eV when the e gun is operated at high energies for a long period of time The operation of the e gun at high energy for a long time longer than 1 hour leads to an increase of the pressure in the cathode housing which results in a decrease of the cathode lifetime ErLEED Optics and Power Supplies 3 2 3 2 4 2 5 The construction of the e gun ensures that no light from the cathode leaves the housing into the direction of the LEED flange The standard cathodes are Thoria coated lridium hairpin filaments and optional Lanthanum hexaboride LaBg filaments with specially cut microfaces Generally the lifetime of the filaments depends sensitively on the pressure in the UHV chamber For LaBg filaments the pressure should not exceed 5 x 10 mbar Thoria coated Iridium filaments are less sensitive on the vacuum conditions They may be operated at pressures in the 10 mbar range Important The filament
22. mounted in the e gun is already outgased and formed by SPECS GmbH before the LEED optics is delivered Another forming of the filament at the customer side is not necessary The forming conditions after filament repalcement on side that is current and duration of the forming procedure are documented in the test report included in the delivery documents of the unit Grids and fluorescent screen The grids of the LEED optics are fabricated out of molybdenum They are gold coated to avoid potential changes as a result of work function differences The grids and the screen in the form of calottes with different diameters are arranged concentrically around a common center where the sample surface has to be positioned for the diffraction and spectroscopy experiments respectively The grid side of the screen is covered with ITO and coated with P43 cadmium free phosphor homogenous over the whole hemisphere For all electrical connections crimp connectors have been used z retraction mechanism optional In order to extend and withdraw the optics i e for evaporation processes the z retraction mechanism is used which is driven by a UHV rotary motion drive DN 16CF on the LEED flange The design of the cabling and isolation enables the operation of the LEED system in all positions and orientations The voltages should however be switched off when moving the optics Integrated shutter optional The integrated shutter can be used to protect the op
23. o maximize the amplitude of the primary peak in the differential spectrum with the energy set to the value corresponding to the peak maximum or minimum respectively Finally having optimized the phase of the LIA and the sample position the signal may eventually be further increased by a fineadjustment of the focus voltages please refer to chapter 7 5 4 of the ErLEED RFA PC user manual This further increase can be due to an increase of the beam current as well as to a sharper focus of the electron beam on the sample surface Detection of Auger electrons Once the optimisation of the signal for the primary peak is finished neither the sample position nor the settings of the phase and of the electron gun voltages should be changed afterwards However in order to record an AES spectrum in the N E mode a few parameters have to be readjusted e The start and the final value of the energy scan have to be set e The scan time has to be increased by a factor of 5 to 10 approximately e The input sensitivity of the LIA has to be increased by a factor of 100 to 1000 ErLEED Optics and Power Supplies e The time constant has to be increased by a factor of about 5 to 10 Hint In general the noise level in the resulting Auger spectra is more significant than in the spectrum of the primary peak It could in principle be reduced by choosing a higher time constant of the LIA In order to keep the energy resolution constant an increase of the
24. o the rear of the unit and to the 12 pin feedthrough at the LEED flange e Check if the potentiometer for the adjustment of the filament current is set to OFF e Switch on power The digital meters on the front of the unit display the actual currents and voltages e Set the voltages at the electron gun Wehnelt anode lens 1 3 lens 2 to the values in the test report using the corresponding potentiometers for gain and offset The respective offset voltages are determined by extrapolation of the voltages in the test report to E 0eV The offset is adjusted with the electron energy set to zero In a second step the gain of the electron gun voltages is adjusted by setting the energy to 100eV or 300eV respectively and adjusting the voltages to the corresponding values given in the test report e Set the suppressor voltage to maximum in order to maximize the energy window for the transmission of the electrons e Set the screen voltage to 6 kV e Set the energy to about 100 eV Switch on the filament heating current and set the current slowly within about 30 sec to the value which is given in the test report In any case avoid overheating of the filament Keep in mind that the emission anode current increases exponential with the filament current increasing linear The anode current should increase within a few seconds It decreases significantly when the Wehnelt voltage is increased e Adjust and optimize sample position
25. of the electrons corresponds to the potential difference between the cathode and ground The other voltages refer to the cathode potential The voltages at the electron lenses at the anode and at the Wehnelt cylinder only for ErLEED 3000D vary linearly as a function of the energy with adjustable gain and offset see fig 3 1 The detailed features and operation condition of the ErLEED 3000D unit is described in detail in separate user manuals coming with the unit ErLEED Optics and Power Supplies 7 voltage V voltage V 2000 1500 1000 500 Lens 0 200 offset 2 7V gain 0 03 Fig 3 1 1 3 offset 20V S gain 0 5 oO D offset 100V gain 1 5 Q 400 600 800 1000 0 200 400 600 800 1000 energy eV energy eV 1000 Wehnelt 800 600 oO g 400 g offset 350V 200 gain 0 5 0 400 600 800 1000 0 200 400 600 800 1000 energy eV energy eV Voltages at lens 1 3 lens 2 Wehnelt and anode as a function of the electron energy A corresponding set of focusing parameters is provided in the test report for each LEED optics In the case of the ErLEED 1000A control unit the Wehnelt voltage does not vary as a function of the energy ErLEED Optics and Power Supplies Chapter 4 Mounting of the ErLEED optics 4 Mounting of the ErLEED optics During transport the ErLEED optics is protected against shock and dust by a stainless steel housing which is fixed to the LEED flange with four
26. ontrolled by analog electronic circuits On the other hand the ErLEED 3000D power supply is driven digitally by an integrated microcomputer This offers a couple of interesting features concerning the operation of the unit such as for example the possibility of communication with a computer via a RS 232 interface storing and recalling of experimental parameters the permanent indication of all voltages etc Both units can also be used in connection with other commercial LEED optics Please contact SPECS GmbH for further information With the ErLEED 1000A the primary energy of the electron is variable from 0 1000 V while voltages up to 3000 V can be set in spectroscopy mode with the ErLEED 3000D Therefore the ErLEED 3000D unit can in addition be used as an RFA controller for the operation of the LEED optics as an Auger electron or electron energy loss spectrometer AES and EELS respectively Both units offer the possibility to control the electron energy by an external analog voltage Within both ErLEED power supplies the true beam current is measured by a fully floating measuring unit as potential drop across a 1 kQ resistance between external ground and internal zero Therefore a current of 1 A follows a voltage drop of 1mV In addition all significant voltages are permanently displayed on digital voltmeters DVM in the case of the ErLEED 1000A or on a large matrix LC display in the case of the ErLEED 3000D power supply The kinetic energy
27. promise between the largest possible emission which varies nearly exponentially as a function of the temperature of the filament a sharp focus and the smallest possible heating of the cathode environment Such a heating results in an increase of the pressure around the cathode which leads to sputtering of the active cathode surface and thus to a shortened lifetime of the cathode Due to a temporary deterioration of the vacuum conditions in the UHV chamber e g after bakeout gas deposition etc a decrease of the emission current is possible In this case the filament can be reactivated by applying the cathode forming current for some minutes The appropriate forming current is stated in the test report After the reactivating procedure the current has to be reset to its normal value to avoid a premature ageing of the filament ErLEED Optics and Power Supplies 11 12 Beam current measurement The beam current corresponds to the part of the electrons emitted by the filament which pass all apertures and lens elements and finally leave the electron gun It depends therefore on the emission current as well as on the energy and the focus voltages Having left the electron gun the electrons hit the sample and flow back to the control unit across common ground This current causes a voltage drop across a 1kQ resistance i e 190A causes a 1mV voltage drop which can be measured at the IO monitor socket at the rear of the control unit Fluorescent
28. r the exchange of the filament In addition we recommend to wear gloves when working on the optics e Remove the optics carefully from the UHV chamber Put a clean cover on top of the u metal or stainless steel shield to prevent the grids from being contaminated by dust particles e Loosen the three M2 screws around the magnetic shielding and carefully remove the shielding from the optics e Disconnect the connectors from the contact pins spot welded to the grids and screen respectively Note the number of the grid each cable is assigned to ErLEED Optics and Power Supplies e Loosen the 3 M4 nuts on the upper mounting ring carrying the assembly of grids and screen Take off the mounting ring and put the whole assembly under a clean dust free cover e Carefully remove the end cap from the Wehnelt cylinder If the end cap does not come off easily be careful not to damage the ceramic holder when exerting increased force on the electron gun In addition pay attention to the glass fiber isolation of the two cables for the cathode current supply Note the position and the way the cables are laid e Remove the cables from the filament contact pins e Pull the fixing cylinder out of the electron gun Note the position of the lateral groove in the cylinder with respect to the Wehnelt cylinder e The old filament can now be taken out of the electron gun by pulling equally on both contact pins Note the position and orientation respectivel
29. ring When the new filament is set to operation for the first time it is necessary to carefully outgas the electron gun Make sure that the pressure rise in your UHV chamber due to the heating of the filament does not exceed the range of 10 mbar In addition the filament needs to be activated or formed in order to obtain stable emission characteristics This applies particularly to LaBg filaments which usually require several hours of operation at elevated currents appr 2 0 Amps for activation If you have any further questions or in the case of doubt please do not hesitate to contact Specs for further instructions ErLEED Optics and Power Supplies 21 22 Chapter A Appendix ErLEED Optics and Power Supplies Chapter Al ertEep 1000A Main features Al ErLEED 1000A Main features and Specifications In the following the main features of the ErLEED 1000A control unit will be summarized Size 19 rack height 182 mm weight 10 kg e Fully floating unit measures true beam current e Electron energy variable between 0 1000eV e 5 high stability HV modules for independent supply and control of all voltages The lens voltages L1 3 and L2 and the anode voltage vary linearly as a function of the electron energy with individually adjustable gain and offset The suppressor voltage varies linearly without offset e The filament current the electron energy the Wehnelt the suppressor and the fluorescent screen volta
30. screen In order to make the electrons visible on the fluorescent screeen it is necessary to apply a positive high voltage between 5 and 7 kV to the screen In the case of the ErLEED 3000D control unit the difference between the cathode and the screen potential is kept constant i e the screen voltage decreases with increasing electron energy with the amplification becoming independent of the electron energy Suppressor Secondary electrons and electrons which are inelastically reflected by the sample with energy losses of some 10 eV can be kept away from the fluorescent screen by applying a negative voltage to the suppressor grids grid 1 for the 2 grid optics grid 2 for the 3 grid optics grid 2 and 3 for the 4 grid optics This results in a reduction of the background on the LEED screen as only elastically reflected electrons and electrons with small energy losses contribute to the diffraction pattern The suppressor voltage varies linearly as a function of the energy with zero offset At very small suppressor voltages narrow energy window a weak defocusing of the diffraction beams is observed This is due to a slightly inhomogenous potential distribution in the vicinity of the grids and deviations from an absolutely radial symmetric propagation of the electrons Sample position In order to generate a diffraction diagram on the LEED screen a well prepared single crystal surface has to be placed in the center of the grids and screen If
31. the filament 20 APPENDIX nanana 22 Al ErLEED 10004 Main features and SpecilicatlOns a reise eoe etu osea eaae eens acus 23 ErLEED Optics and Power Supplies Chapter 1 Introduction 1 Introduction Low Energy Electron Diffraction LEED is one of the oldest but also one of the most widely used methods in surface analysis Its main application is the structure determination of thin films and of clean and adsorbate covered crystal surfaces In addition LEED is used as a standard method for the characterization of the surface quality during the sample preparation prior to other UHV experiments Generally the structural information given by a LEED pattern results from the position and the intensity of the diffraction spots as well as from the spot profiles In particular the surface unit cell of the reciprocal lattice and the corresponding real space unit cell follow from the positions of the LEED spots The positions of the surface atoms are obtained from the so called I V curves which means that the spot intensities are measured as a function of the electron energy This method allows a determination of the structure parameters of the upper 4 or 5 layers with a precision of up to 0 01 Finally from the spot profiles the quality and degree of long range order at the surface can be deduced A measurement of the spot profile thus provides important statistical parameters of the surface such as
32. tics during extensive sample annealing evaporation or sputtering The voltages should be switched off when operating the shutter The shutter must be closed when using the z retraction mechanism Restricted orientation of the LEED optics applies Indium Tin Oxide layer ErLEED Optics and Power Supplies Viewport u metal shield 145 165 203 and 254 Other length on request 34 22 Rotary drive for z retraction 105 HV feedthrough optional DN150CF 8 flange 85 151 Fig 2 1 Schematic view of the ErLEED 150 optics and the LEED flange ErLEED Optics and Power Supplies pin no 2 grid optics 3 grid optics 4 grid optics pana Filament Cathode Wehnelt Filament Cathode Anode Lens Element 1 Lens Element 2 Lens Element 3 Lens Element 4 o o Oo oF A5 O NM grid 1 grid 1 mah e grid 1 grid 2 grid 2 1 grid 3 mak N grid 2 grid 3 grid 4 Fig 2 2 12 pin feedthrough layout on LEED flange viewed from atmospheric side for 2 3 and 4 grid optics ErLEED Optics and Power Supplies Chapter 3 Power supplies 3 Power supplies There are two power supplies available for ErLEED 100 150 optics ErLEED 1000A and ErLEED 3000D for analog and digital operation respectively In case of the ErLEED 1000A the high voltage modules are c
33. w for the transmission of the electrons e Switch on the screen high voltage and set it to 6 kV Le the energy and all volatges can be controlled with a knob on the front panel of the unit Redudant since 2001 units are now supplied with wide range power supplies for a direct 110V or 220V connection 14 ErLEED Optics and Power Supplies 5 4 Although the ErLEED 3000D power supply provides a maximum screen voltage of 10 kV it is sufficient in most cases to work at lower voltages Instead a maximum value of 7 kV is recommended For safety reasons the screen voltage and the filament heating are immediately switched off by the control unit if a spark over is generated when working at extreme voltages e Switch on the filament and set the filament current to the working value given in the test report The anode current should increase within a few seconds In any case avoid overheating of the filament Keep in mind that the emission anode current increases exponential while the filament current is increased linearly The filament current is set to zero when the filament voltage is switched off The value of the heating voltage is however stored In the case of a change of the mode of operation or of switching on off the control unit the filament heating voltage is automatically set to zero e Adjust and optimize sample position Once a suitable set of focus parameters is found we recommend to store it using the internal m
34. y of the contact pins of the filament with respect to the Wehnelt cylinder The ceramic base plate of the filament breaks easily if unequal force is exerted on the pins If this happens make sure that no fragments of the ceramics remain in the electron gun before mounting the new filament The filament base plate sits on a distance holder inside the Wehnelt cylinder ensuring the correct position of the filament tip with respect to the Wehnelt aperture The distance holder has four venting bores in its side These have to be orientated at 45 with respect to the corresponding bores in the surrounding Wehnelt cylinder such that during operation of the filament no light can leave the electron gun into the direction of the LEED flange e Carefully introduce the new filament into the Wehnelt cylinder maintaining the position and orientation of the contact pins of the old filament Make sure that the base plate sits evenly on the distance holder e Remount the fixing cylinder maintaining the previous orientation of the groove e Carefully connect the cables to the contact pins of the filament and lay the cables respecting the previous way of laying e Put the end cap onto the Wehnelt cylinder Pay attention to the isolation of the cables e Remount the grids screen assembly and reconnect the cables e Remount the u metal or stainless steel cylinder Make sure not to damage the isolation of the cables when the cylinder is slided over the mounting

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