Author: dpl

CAFMaD
Centre for Advanced Functional Materials and Devices
Cebn

Snapshot Electron Spectroscopy using a Linear Array Electron Counting Detector
DP Langstaff, DA Evans, RE Cross, S Cooil, M Gunn
Aberystwyth University, Aberystwyth, SY23 3BZ, United Kingdom

Introduction
An electron counting detector array, developed at Aberystwyth University, has been applied to electron spectroscopy [1,2].
By using the detector array to take snapshots of the photoemission spectrum during materials processing, it is possible to gain new insights into the physics, chemistry and electronic processes as they take place.
The MCD-1536 Detector Array
The MCD-1536 is a readout device for microchannel plates.
A pair of microchannel plates mounted above a custom Application Specific Integrated Circuit (ASIC) provide the initial gain of the detector. The charge cloud emerging from the microchannel plate stack is collected on the ASIC where it is amplified and converted to a digital signal. Each pixel on the array is furnished with a 16-bit counter which counts the electrons arriving on the plate above it.
The pixel electronics incorporate protection circuitry to prevent damage from microchannel plate excursions.
The ASIC is mounted on a ceramic substrate with printed connections.
The microchannel plate stack is mounted above the ASIC in a holder formed from photoetched stainless steel and ceramic plates.
The detector module is mounted at the focal plane of a VG CLAM4 electron energy analyser.
Detector Specifications
1536 pixels in linear array
Pixel size 25µm x 5mm
Total Active Area: 38.4mm × 5mm
16-bit counter per pixel
≈660,000 transistors
Fabricated on AMI semiconductor 0.5µm digital cMOS process
Microchannel Plate Specifications
Hamamatsu (based on F2370-01)
12mm channel diameter
15mm channel pitch
0.48mm channel length
40:1 l/d ratio
Maximum Count Rate Data
The detector operates in an event counting mode, and, as such, offers highly linear operation up to high count rates.
The range of linear operation for the detector is assessed by taking the ratio of 2 photoemission spectra at different x-ray powers.
Here a maximum count rate in excess of 4kHz/pixel (equivalent to 3MHz across the array) is demonstrated.
Detector Control Electronics
In order to maintain the electrostatic potentials within the electron analyser, the front face of the microchannel plate on the detector assembly needs to be maintained at the same potential as the analyser retarding voltage. The microchannel plate accelerating voltage means that the rear of the microchannel plate and hence the detector ASIC needs to float at up to 3kV from ground.
The solution adopted is to float the detector control electronics and communicate via a high speed optically isolated TCP/IP interface.
The control electronics are based on the National Instruments CompactRIO-9004 embedded controller. This comprises a processor running at 195MHz interfaced to I/O modules via a 3 million cell FPGA.
Time critical parts of the controller, such as the integration timer and ASIC readout functionality are implemented directly on the FPGA.
The control electronics can read out a frame of data from the detector in approx 1400µs, or somewhat under 1µs/pixel. These performance figures are compatible with industrially relevant materials processing techniques.
Data is made available on the embedded controller and can be collected on the host PC for further analysis and storage.
Real-Time Photoelectron Spectroscopy
Photoelectron spectroscopy (PES) is a technique yielding detailed information about chemical composition, chemical state and electronic structure at the surface of a material.
Conventional PES sweeps the photoelectron spectrum through the analyser and is usually a relatively slow, off-line technique. By employing an advanced detector, it is possible to take snapshots of regions of interest in the spectrum while materials processing takes place.
UHV Experimental System Ap-REES
An independent UHV system has been commissioned for real-time electron spectroscopy. The single chamber system utilizes a VG CLAM electron analyser fitted with the array detector.
X-ray source: 300W Twin anode (Al kα and Mg kα)
Sample stage heating & cooling: 77K to 1100K
Access for HR Raman Spectrometer superhead
Deposition systems: Electrospray, K-cells (Organic & metal).
Heating samples allows for desorption studies as well as sample cleaning. The ability to cool samples allows in-situ luminescence studies on electrospray deposited films such as PEO/CuPc blends. The temperature range also allows real time studies of catalytic processes.
The Ap-REES real-time electron spectroscopy system
References
Langstaff D P and Chase T 2007 “A Multichannel Detector Array with 768 Pixels Developed for Electron Spectroscopy” Nucl. Inst. Meth. A 573 169-71
Langstaff D P, Evans D A, Roberts O R and Zhu X 2009 “Progress on the Aberystwyth electron counting array” Nucl. Inst. Meth. A 604 133-137
Evans D A, Roberts O R, Vearey-Roberts A R, Langstaff D P, Twitchen D J and Schwitters M 2007 “Direct observation of Schottky to ohmic transition in Al-diamond contacts using realtime photoelectron spectroscopy” Appl. Phys. Lett. 91 132114
Evans D A, Roberts O R, Williams G T, Vearey-Roberts A R, Bain F, Evans S, Langstaff D P, Twitchen D J 2009 “Diamond-metal contacts: interface barriers and real-time characterization” J. Phys. C. 21 (in press)
Acknowledgements
This work has been supported by the Engineering and Physical Sciences Research Council (EPSRC), the Welsh Assembly Government Academia for Business (A4B) programme, Element Six Ltd. and the Centre for Advanced Functional Materials and Devices (CAFMaD). Rachel Cross is supported on a Knowledge Economy Skills Scholarship (KESS)

Fibre Optic Interface

Level Shifting Buffers

Compact RIO controller

Link to Detector

MCP Supply

Isolated PSU

9th International Conference on Position Sensitive Detectors 12th—16th September 2011, Aberystwyth, UK