EuCARD-2 Enhanced European Coordination for Accelerator Research & Development. Milestone Report

Transcription

1 CERN-ACC EuCARD-2 Enhanced European Coordination for Accelerator Research & Development Milestone Report Commissioning of the SAPI for Operation with Metal Photocathodes Noakes, T.C.Q. (STFC) et al 10 February 2014 The EuCARD-2 Enhanced European Coordination for Accelerator Research & Development project is co-funded by the partners and the European Commission under Capacities 7th Framework Programme, Grant Agreement This work is part of EuCARD-2 Work Package 12: Innovative Radio Frequency Technologies (RF). The electronic version of this EuCARD-2 Publication is available via the EuCARD-2 web site < or on the CERN Document Server at the following URL: < CERN-ACC

2 Grant Agreement No: EuCARD-2 European Coordination for Accelerator Research and Development Seventh Framework Programme, Capacities Specific Programme, Research Infrastructures, Combination of Collaborative Project and Coordination and Support Action MILESTONE REPORT COMMISSIONING OF THE SAPI FOR MILESTONE: MS73 Document identifier: Due date of milestone: End of Month 8 (November 2013) Report release date: 10/02/2014 Work package: Lead beneficiary: Document status: WP12: Innovative Radio Frequency (RF) Technologies STFC Final Abstract: A surface analysis / preparation installation has been constructed and commissioned at STFC Daresbury Laboratory for carrying out research into metal photocathode materials. The main analytical techniques provided are outlined and commissioning data presented for X-ray photoelectron spectroscopy, atomic force microscopy, Kelvin probe work function measurement and quantum efficiency measurement. This equipment is now ready to begin work on the study of a variety of different metal and metal alloy materials and surface preparation procedures to allow use as a photocathode in an RF photoinjector. Grant Agreement PUBLIC 1 / 14

3 Copyright notice: Copyright EuCARD-2 Consortium, For more information on EuCARD-2, its partners and contributors please see The European Coordination for Accelerator Research and Development (EuCARD-2) is a project co-funded by the European Commission in its 7th Framework Programme under the Grant Agreement no EuCARD-2 began in May 2013 and will run for 4 years. The information contained in this document reflects only the author s views and the Community is not liable for any use that may be made of the information contained therein. Delivery Slip Name Partner Date Authored by T.C.Q. Noakes, B.L. Militsyn, R. Valizadeh, K.J. Middleman, A.N. Hannah and L.B. Jones STFC 24/01/14 Reviewed by R. Nietubyc NCBJ 03/02/14 Approved by WP Coordinator P. McIntosh N. Baboi J. Plouin STFC DESY CEA 07/02/14 Approved by Project coordinator Maurizio Vretenar 07/02/14 Grant Agreement PUBLIC 2 / 14

4 TABLE OF CONTENTS 1. EXECUTIVE SUMMARY INTRODUCTION SURFACE ANALYSIS / PREPARATION INSTALLATION ELECTRON SPECTROSCOPY SCANNING PROBE MICROSCOPY OTHER CHARACTERISATION TECHNIQUES COMMISSIONING DATA X-RAY PHOTOELECTRON SPECTROSCOPY ATOMIC FORCE MICROSCOPY KELVIN PROBE AND WORK FUNCTION MEASUREMENTS CONCLUSIONS / FUTURE PLANS ANNEX: GLOSSARY Grant Agreement PUBLIC 3 / 14

5 1. EXECUTIVE SUMMARY Milestone MS73 concerns the construction and commissioning of a surface analysis / preparation installation for carrying out research into the preparation of metal photocathodes for use in normally conducting RF photoinjectors. A description of the constructed equipment and the various analytical techniques is provided. The primary analytical techniques are X-ray photoelectron spectroscopy for compositional and chemical characterisation and atomic force microscopy, which in this instance is used to assess the surface roughness of the photocathode materials. The data produced using these techniques can be compared with work function and quantum efficiency measurements for the same samples. Commissioning data for the XPS system is presented, taken from a clean silver sample. Good spectra can be obtained with a resolution of 1.1 ev which should be sufficient to provide surface compositional data which will help elucidate the effectiveness of various preparation techniques and the effect of controlled exposure to known contaminant species. AFM images are presented for a diffraction grating sub-micron calibration standard and a copper sample, typical of the photocathodes currently used in the VELA accelerator at STFC Daresbury Laboratory. The image from the calibration standard shows clear resolution of the 55 nm high ridges with a 278 nm period. This resolution should be adequate for the intended studies as suggested by the image of the copper sample. Kelvin probe work function measurements have been made on a separate vacuum system using a commercially available apparatus. These measurements have been compared with quantum efficiency measurements made with 265 nm LED source. These techniques willbe implemented on the surface analysis / preparation installation in the near future. The equipment is now ready to carry out a series of experiments on the use of alternate metal and metal alloy samples for use as photocathodes in RF photoinjectors. These studies will include the evaluation of various methods of preparing the surfaces and the effect of contaminants present in the demanding environment of the RF photoinjector on likely photocathode performance. 2. INTRODUCTION Work package 12 concerns research into innovative Radio Frequency (RF) technologies for accelerator applications, with task 12.5 relating to the development of photocathodes for RF photoinjectors and sub-task focussing on materials research into metal photocathodes used in normally conducting guns. The development of photocathodes able to provide the ultra-short, high-brightness bunches needed for modern electron accelerator based light sources is a key element in driving forward these technologies which will ultimately enable a wide range of scientific studies not only of structure and electronic properties of various experimental systems, but also of how these change on very short time scales. This report covers the development and commissioning of Surface Analysis/Preparation Installation (SAPI) for use in the study of metal photocathode materials. The newly commissioned instrument complements existing equipment at STFC Daresbury Laboratory and extends the surface analysis capability by adding surface topographic characterisation and work function measurements to high resolution electron spectroscopy and quantum efficiency Grant Agreement PUBLIC 4 / 14

6 measurements previously possible. The importance of surface characterisation in the field of photocathode research is becoming increasingly accepted, with similar capability being brought online in many other accelerator research laboratories worldwide as evidenced by presentations at several recent specialist workshops in this area (for example see The SAPI system constructed and commissioned at Daresbury is composed of a fast entry load lock and a series of ultra-high vacuum chambers which house the surface analysis instrumentation. The surface analytical techniques included in the apparatus are X-ray photoelectron spectroscopy (XPS), low-energy electron diffraction (LEED), atomic force microscopy (AFM) with the instrumentation also capable of Auger electron spectroscopy (AES), low-resolution scanning electron microscopy (SEM), scanning tunnelling microscopy (STM) and scanning tunnelling spectroscopy (STS). In addition, two other techniques, Kelvin probe (KP) work function measurements and quantum efficiency (QE) measurements, have also been commissioned on a separate instrument, but are likely to be implemented on this system in the near future. Construction April to July 2013 Commissioning August to November 2013 Available for research 30th November 2013 First experiments From January 2014 Proposed programme (up to M18) Preparation and analysis January to March 2014 of various polycrystalline metal cathodes Evaluation of single crystal metal photocathodes Fabrication and testing of thin film photocathodes April to June 2014 July to September 2014 Table 1. Key Dates for the Construction and Commissioning of SAPI and the Proposed Programme of Research. Grant Agreement PUBLIC 5 / 14

7 3. SURFACE ANALYSIS / PREPARATION INSTALLATION The SAPI instrument is housed in the dedicated vacuum science lab in the Cockcroft Institute on the Sci-Tech Daresbury campus, which includes STFC Daresbury Laboratory. The instrument is complementary to other existing capability within the laboratory for the characterisation of photocathode materials. The two main other systems that are used for studies of this kind are the ESCALABII high resolution XPS spectrometer and the transverse energy spread spectrometer (TESS), which can be used to measure both transverse and longitudinal energy distributions from photocathode materials. In addition, there is a dedicated photocathode preparation facility (PPF) for preparing semiconductor (primarily GaAs) photocathodes used in DC guns, which represents a separate strand of photocathode research within STFC. Fig. 1 The UHV Surface Analysis / Preparation Installation (SAPI). SAPI is built as an ultra-high vacuum system with three chambers housing the various surface analytical techniques. A picture of the system is shown in Figure 1. The pumping system for these chambers includes both a turbo-molecular pump and an ion pump since this has no moving parts an can be used simultaneously with the vibration sensitive scanning probe methods implemented. The ultimate vacuum which can be achieved after baking the system is in the low to high mbar region. Samples can be moved around the system using a combination of transfer arms and wobble sticks. There is a turbo pumped load lock for introducing samples into the system either from air or from a vacuum suitcase. The vacuum suitcase will allow samples to be moved between SAPI and the TESS system where further detailed measurement of the electron emission characteristics can be carried out. Design work on this suitcase is under way, but does not form part of the milestone reported here. The ability of the system to accept samples loaded from air means that a range of ex-situ treatments for metal photocathodes can be evaluated. For example, work has commenced on the assessment and optimisation of oxygen plasma treatments for producing clean copper Grant Agreement PUBLIC 6 / 14

8 surfaces suitable for use in the VELA normally conducting RF gun also housed at STFC Daresbury Laboratory. Along with ex-situ treatments there is also the capability to carry out some in-situ surface modification using equipment within the vacuum system. An ion gun is included for bombardment cleaning and there is also the capability to heat samples in-situ. Although not currently present, there are sufficient ports available to attach various deposition sources for sample dosing, the growth of thin films and the deposition of various coatings. Finally, a precision leak valve is fitted that will allow the controlled exposure of photocathode materials to gases typically found in the accelerator environment and thus allow degradation experiments to be carried out. As-loaded, cleaned and treated samples will be able to be characterised using the range of surface analytical techniques included on this apparatus. A high resolution electron energy analyser can be used in conjunction with either an X-ray or electron source to produce electron spectroscopic data as described in the following section. A scanning probe microscope (SPM) capable of being used in either AFM or STM modes is also included and described in the next sub-section. Finally, there is a section detailing the remaining characterisation techniques present on the system ELECTRON SPECTROSCOPY The SAPI instrument provides facilities for electron spectroscopic measurements, including both XPS and AES. These techniques may be particularly important for determining the effectiveness of surface cleaning procedures and the relative importance of various contaminant species on photocathode performance. The XPS technique relies on the use of a low bandwidth X-ray source, which when incident on the surface of a sample, causes photoionisation with the ejection of electrons from the core levels of the atoms. The energy of the ejected electrons is dependent on both the energy of the incoming photons and the binding energy of the core levels from which they come. Each element in the periodic table has a characteristic set of core levels and hence the energy spectrum is a direct reflection of the composition of the sample. The mean free path of the emitted electrons at typical emission energies is low and hence the technique is highly surface sensitive. Because the photoionisation cross-sections are well known the technique is highly quantitative. An additional feature of this technique is that the binding energies themselves are not rigidly fixed but can experience small shifts due to the chemical environment in which the target atom sits. The magnitude of these shifts have been extensively characterised and hence it is possible to determine from them the chemical state of the surface from the precise energy positions of the core level peaks. This chemical sensitivity is one of the key advantages of XPS over other similar analytical techniques. AES typically requires a similar experimental set-up to XPS but this time typically using an electron source in the kev energy range for the primary excitation. Again core level electrons are ejected and the resultant vacancy is quickly filled by and electron from the outer levels. The energy released by this de-excitation can either be emitted as an X-ray (forming the basis of electron probe micro-analysis) or as a third electron which is ejected with a characteristic energy determined by the energy levels of all three electrons in the process. Each element provides a characteristic pattern of Auger transitions allowing easy identification and thus compositional analysis. As with XPS the mean free path of the ejected electrons ensures a high degree of surface specificity and again cross-sections are well known allowing Grant Agreement PUBLIC 7 / 14

9 quantitative analysis of the data. Because the process involves three electrons, chemical shift data is much less useful and AES is rarely used in this way where XPS is preferred. However, the relative ease of focussing the electron beam used for excitation gives rise to better spatial resolution than XPS, which allows small spot size analysis and imaging variations of the technique to be used. High spatial resolution is the one of the key advantages of AES. SAPI system houses a Thermo Scientific ALPHA 110 high resolution hemispherical electron energy analyser with a multi-channeltron detector system. The ultimate resolution of this analyser is less than 5 mev and it has high sensitivity. The entrance lens and aperture system allows small area analysis to be carried out down to 100µm. The excitation source for XPS is a VG dual anode X-ray source allowing both Al Kα ( ev) and Mg Kα ( ev) to be produced. These sources provide line widths of 0.85 and 0.7 ev respectively. In addition, the system includes a VG LEG 200 scanning electron gun which has a spatial resolution of 0.2 µm. This electron source can be used for AES or in conjunction with a backscattered electron detector for imaging of the samples as a low resolution SEM SCANNING PROBE MICROSCOPY SAPI also has facilities for scanning probe microscopy. There are many variants of this technique of which the most common are STM and AFM. In STM an ultra-sharp tip with a potential applied is rastered across a sample and because of its sharpness and close proximity to the surface the high induced field gives rise to quantum mechanical tunneling and an observable current. Typically, the tip is scanned whilst maintaining a constant current and the variation in tip height required recorded as a topographical image. For AFM there are many different modes of operation, but most involve a tip mounted on a cantilever. In the most simple operational mode the tip is in contact with the surface and the movement of the cantilever is measured using an laser/optical system as the sample is scanned. In the more popular non-contact modes, the tip is vibrated so that it senses the attractive potential of the surface without touching and is thus much less damaging. STM has the advantage of extremely high spatial resolution which can image individual atoms under ideal conditions. AFM typically has lower spatial resolution but can be used with insulating samples such as oxides, since no current flows between the tip and the sample. For the proposed research on photocathode materials AFM will be preferred since the technique will be primarily used to determine the surface roughness for various samples and preparation procedures. Surface topography is thought to be a key element in defining the transverse energy spread of the emitted beams, with macroscopically rough samples giving much higher energy spread. STM may also be useful because of the ability to obtain spatially resolved electronic structural information via STS. In this mode the tip is fixed at a given point over the sample and the voltage scanned from positive to negative values whilst the tunnelling current is recorded. Interpretation of STS data is complicated because it involves a convolution of the empty and filled states of both the surface and the tip. Nonetheless it can be useful in seeing changes in electronic properties across the surface. The SAPI instrument includes an Omicron room temperature UHV STM/AFM instrument. The tips/cantilever arms are transferable so that it can be used in either STM or AFM mode without breaking vacuum. The instrument includes magnetic damping for vibration isolation, allowing to be used in non-ideal conditions typically present in a standard laboratory. The maximum scan range for both AFM and STM is 8µm x8µm x 3µm. The height resolution is Grant Agreement PUBLIC 8 / 14

10 less than 0.01 nm, with lateral resolution typically limited by the probe size/geometry. The STM/AFM is driven by a Nanotec Electronica Dulcinea control unit which is in turn run by WSxM control software. This software allows manipulation of the acquired images and post processing to generate both line scans and rms roughness values OTHER CHARACTERISATION TECHNIQUES The system also currently houses an Omicron rear view LEED optic. In LEED a beam of electrons in the ev range is incident on the surface of a sample, which if it is crystalline gives rise to diffracted spots characteristic of the geometric structure. Since the electrons are of low energy this technique only probes the outer most layers and hence provides information on the surface region. The LEED technique can provide detailed information on the precise positions of all atoms in this region, particularly by measuring and fitting by simulation the variation of spot intensity as a function of electron energy. However, since this is a complicated and computationally expensive procedure, the technique is more generally used simply as a measure of the surface periodicity and degree of ordering of the surface, which can be determined from a visual inspection of the diffraction image. In the context of photocathode research this technique will only be useful for work carried out on single crystal samples where a well-ordered surface structure is expected. Another experimental technique that it is hoped will be installed on this instrument in the near future is Kelvin Probe contact potential difference measurements of work function. This technique uses a well-defined probe which is vibrated in close proximity to the surface. If there is a difference in potential between the two, the varying capacitance causes the voltage to vary in a similar fashion. An externally applied opposing voltage is used to nullify this signal and its value is equal to the contact potential difference which, if the probe work function is well known, allows the sample work function to be calculated. The equipment purchased for these experiments is a KP Technology UHVKPm100, a commercial instrument with less than 3 mev resolution in the work function measurements, which can be bolted on to any UHV system with a spare DN40CF port. It is currently being commissioned on the ESCALABII instrument. Work function (along with the wavelength of the incident laser light) defines not only the quantum efficiency of the photocathode but also the energy spread of the emitted electrons. Monitoring the work function in concert with surface chemical changes will allow a better understanding of the various surface preparation procedures and contamination mechanisms to be gained. Quantum efficiency measurements are being made on the ESCALABII instrument using a Roithner LaserTechnik UVTOP260-HL-TO39 LED laser source of 265 nm wavelength, which has an integrated lens to ensure a small spot size at the sample. The source has a bandwidth of around 12 nm and intensity is calibrated using an Opto Diode Corp. AXUV100G large area UV sensor. Whilst this system is capable of making the quantum efficiency measurements required, work is underway to implement a higher powered, more stable, compact UV laser source (based on a Crylas FQSS266-Q4 module) which should have much better resolution (below 0.1 nm). The photocathode samples are electrically isolated to allow drain current measurements to be made using a Keithley 480 pico-ammeter. A similar, if not identical, system for measuring quantum efficiency will be put in place on the SAPI system. Since quantum efficiency is one of the key parameters in determining photocathode performance, the ability to make accurate measurements is essential to all the future work planned for this equipment. Grant Agreement PUBLIC 9 / 14

11 4. COMMISSIONING DATA 4.1. X-RAY PHOTOELECTRON SPECTROSCOPY Commissioning data for XPS has been taken using a well-defined pure silver sample mounted on a suitable backing plate for the SAPI sample transfer system. Whilst this sample was cleaned prior to insertion into the vacuum system, no further in-situ cleaning was carried out, so the surface would be expected to show signs of atmospheric contamination in the form of surface oxides and hydrocarbons. Signals from Ag, O and C are therefore to be expected. The commissioning data presented has been taken using the X-rays from the Mg anode of the gun, since this should provide the highest resolution for testing the analyser performance. Figure 2 shows a wide area scan of the XPS spectrum of the Ag sample Ag 3d 5/ Ag (MNN) Ag 3d 3/2 Counts 4000 Ag 3p 1/2 Ag 3p 3/2 O 1s 2000 C 1s Ag 4p Binding Energy (ev) Fig. 2 Wide area XPS spectrum from the Ag test sample. Grant Agreement PUBLIC 10 / 14

12 The scan shows clear peaks for Ag, O and C indicating as expected a relatively clean sample with modest atmospheric contamination. Figure 3 shows detailed scans of the Ag 3d, O 1s and C1s regions Counts Ag 3d 3/ Ag 3d 5/ Counts O 1s Counts C 1s Binding energy (ev) Binding energy (ev) Binding energy (ev) Fig. 3 Detailed XPS scans of the Ag, O and C peaks from the Ag test sample. From these detailed scans it is possible to measure the resolution of the system, which in the case of the Ag 3d peaks is approximately 1.1 ev. Whilst this resolution is above the minimum of 0.7 ev expected, it is possible that further optimisation of the analyser settings could improve this. However, the resolution seen here, whilst perhaps less than required for obtaining detailed chemical environment information, is certainly more than sufficient for simple compositional measurements, which will form the majority of those required in the photocathode studies envisaged ATOMIC FORCE MICROSCOPY The scanning probe microscope has been commissioned in non-contact AFM mode using both a standard sub-micron calibration sample and more realistic samples. Figure 4 shows the image obtained from the calibration sample. Fig. 4 AFM image of a calibration sample showing regularly spaced ridges. Grant Agreement PUBLIC 11 / 14

13 The image clearly shows the expected parallel ridges from this diffraction grating, which are around 55 nm high and have a periodicity of 278 nm. Figure 5 shows an AFM image taken from a polycrystalline copper sample loaded from air with no in-situ treatment. This material is likely to be similar to the copper photocathodes currently in use in the VELA electron injector at Daresbury. Fig. 5 AFM image of a polycrystalline copper sample. These images demonstrate the correct functioning of the AFM instrument and indicate lateral and vertical resolutions sufficient to determine the roughness of typical photocathode materials KELVIN PROBE AND WORK FUNCTION MEASUREMENTS Kelvin probe work function measurements have been made and compared with quantum efficiency measurements for a series of oxygen plasma treated copper samples. These samples have been prepared using different apparatus and in the case of one apparatus two separate samples were tested. These preliminary data are shown in Figure 6 and indicate that with increased annealing temperature the photocurrent rises, accompanied by a slight reduction in the work function. However, during the collection of these data it became evident that there were issues with the measurement of temperature in the vacuum system. It is possible that the reported values are much higher than the true temperatures of the samples. For this reason the heater stage and thermocouple assemblies in that instrument have now been replaced and further work to repeat these studies will be carried out in the near future. Grant Agreement PUBLIC 12 / 14

14 Work Function (ev) Photocurrent (na) Treatment 1 - Sample 1 Treatment 1 - Sample 2 Treatment 2 Treatment Annealing Temperature ( C) Fig. 6 Plots of work function and associated photocurrent for oxygen plasma treated copper samples. Nonetheless the data in Figure 5 do demonstrate that work function and quantum efficiency measurements can be routinely measured as required for the envisaged work in this area. 5. CONCLUSIONS / FUTURE PLANS The SAPI instrument for carrying out research into metal photocathode materials for RF photoinjectors has been constructed and commissioned. XPS and AFM data have been collected and sufficient resolution demonstrated for the proposed studies. On a separate instrument, work function and quantum efficiency measurements have been made; these techniques will be implemented on SAPI in the near future. The completion of this part of the project now allows a series of studies on metal photocathodes to be carried out. In the near future these will involve techniques for cleaning copper photocathodes and the evaluation of alternative metals. Going forward the work will be extended to the study of metal alloys, thin films and coatings on metal photocathodes. The ultimate goal is to produce photocathodes with higher quantum efficiency that are able to withstand the operating environment within a normally conducting RF photoinjector. Grant Agreement PUBLIC 13 / 14

15 ANNEX: GLOSSARY Acronym AES AFM KP LEED LED PPF QE RF SAPI SEM SPM STFC STM STS TESS UHV UV VELA XPS Definition Auger electron spectroscopy Atomic force microscopy Kelvin probe Low-energy electron diffraction Light emitting diode Photocathode preparation facility Quantum efficiency Radio frequency Surface analysis / preparation installation Scanning electron microscopy Scanning probe microscopy Science and Technology Facilities Council Scanning tunnelling microscopy Scanning tunnelling spectroscopy Transverse energy spread spectrometer Ultra-high vacuum Ultra-violet Versatile electron linear accelerator X-ray photoelectron spectroscopy Grant Agreement PUBLIC 14 / 14