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Parallel recording for an electron spectrometer on a Scanning Transmission Electron Microscope
122
Ultramicroscopy 28 (1989) 122-125 North-Holland, Amsterdam
PARALLEL R E C O R D I N G F O R A N E L E C T R O N S P E C T R O M E T E R ON A SCANNING TRANSMISSION ELECTRON MICROSCOPE S.D. B E R G E R * and D. M c M U L L A N Cavendish Laboratory, Madingley Road, Cambridge CB3 0HE, UK Received at Editorial Office 3 November 1988; presented at Workshop March 1988
A parallel recording system is being built for the electron-energy-lossspectrometer of a VG Microscopes HB501 scanning transmission electron microscope (STEM). A two-dimensional charge-coupled device (CCD) lens-coupled to an ytrrium aluminium garnet (YAG) scintillator is the electron detector. The operating mode for the CCD is described and the expected performance compared with that of a detector using a self-scanned photodiode array. Provision is being made for the bright-field and energy-lossimages to be produced.
I. Introduction Several parallel recording systems for electronenergy-loss spectrometry (EELS) using silicon detectors have been the subject of recent papers [1-5]. These systems include a single-crystal yttrium aluminium garnet (YAG) scintillator fibreoptically coupled to a self-scanned diode linear array [1,2], a self-scanned diode linear array used in the EBIC m o d e [3], and a Y A G scintillator lens-coupled to a charge-coupled detector (CCD) [4,5]. The latter combination is being used for the system that is being built for the V G Microscopes HB501 STEM at the Cavendish Laboratory.
2. CCD parallel detector for a S T E M F r o m the start it was decided that a two-dimensional C C D should be used as the detector because of the very low noise read-out that can be achieved, thus making lens coupling to a Y A G scintillator feasible. There are a number of advantages in using a two-dimensional detector apart from the obvious one of facilitating the setting-up
* Now at: AT&T Bell Laboratories, Murray Hill, New Jersey 07974, USA.
of the electron optics: in particular it is possible to store m a n y spectra on the chip before reading out, and, most important, the effects of afterglow of the Y A G scintillator can be avoided (see section
3). The detector is a camera, manufactured by Wright Instruments [6], which uses an English Electric P8600 C C D sensor. This C C D has an imaging area 8.5 × 6.4 m m 2 (385 columns of 288 pixels 20 # m square), and when cooled ( - 150 o C) the read-out noise is less than 10 electrons RMS. Calculations, confirmed b y tests, show that with f / 2 lens coupling only about 3 electrons are generated in the C C D per high-energy electron on the Y A G , but even so, because of the low read-out noise, the D Q E is reasonable at low count rates; and because of the low temperature the dark signal is negligible for an exposure as long as 1 h. The D Q E is given by
~
l
=[
~
E+n2'
where E is the n u m b e r of incident high-energy electrons and n is the read-out noise referred to the scintillator, i.e. the C C D read-out noise divided by the number of C C D charges produced by a single electron. In the present example, the minimum signal for a D Q E > 0.5 is only 10 high-
0304-3991/89/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
123
S.D. Berger, D. McMullan / Parallel recording for electron spectrometer on S T E M Table 1 Comparison of self-scanned diode array [2] and CCD detectors
Self-scanned array + YAG CCD+YAG
Minimum count per channel a) (electrons) 200 10
Maximumcount per channel b) (electrons) 4.5x10 s 1 Xl0 s
Dynamic range 2x103 I xl04
Integration time at 10 electrons/s rate c) (s) 20 1
Number of channels (resolved) 512 385
a) For DQE = 0.5; per integration period. b) Per integration period. c) For DQE-0.5
energy electrons (30 C C D charges), so that the integration time for a 10-electrons s -1 per channel signal ( D Q E = 0.5) is 1 s. The C C D saturation charge per pixel is 3 × 105 and the dynamic range 104. These calculations are summarised in table 1 and compared with the self-scanned diode array used by Egerton and Crozier [2] (250 diode charges per 100 keV electron on the YAG, read-out noise estimated to be 3500 electrons RMS, and 512 resolved channels). The point spread function of the scintillations in the Y A G (30 btm with 100 keV electrons, measured by Strauss et al. [4] and confirmed b y ourselves), is wider than the pixel dimension of the C C D (20 btm) so that optical demagnification is necessary if the full resolution of the C C D is to be realised; 2 x demagnification should be suitable. The dispersion of the spectrometer (about 2 /~m/eV) is being increased b y three quadrupole lenses [1] to match the resolution of the detector. The P8600 is a frame transfer device [7] with an image area and a storage area which is masked (see fig. 1). The spectrum will be focussed on the image area with the dispersion parallel to the rows, and will cover a width of, say, 50 C C D pixels (i.e. at right angles to the dispersion); this width can be controlled by adjustment of the quadrupoles. At the end of an exposure period the charges will be shifted down rapidly (in about 250 /~s) and binned into the first row of pixels in the storage area; the binned charges will then be shifted down one row so that the first row is ready to receive another spectrum from the imaging area. It will thus be possible to store up to 290 spectra before a read-out is necessary, i.e. when the first spectra to be recorded has been shifted
down to the read-out register at the lower edge of the CCD. The read-out will be done by clocking the readout register to shift the line of charges serially to the output amplifier; this will take about 13 ms per line so that read-out of all 290 spectra will be completed in about 5 s. The shifting of the charges on the chip does not introduce any additional noise and can be done using the standard camera software. A similar operating m o d e has been proposed by Strauss et al. [4], but they focus the spectrum 385 COLUMNS
I0 10
s~ S~ s¢
IMAGE AREA 288 ROWS
STORAGE AREA 290 ROWS
OU"
R¢3 Re 2 Re I Fig. 1. Schematic diagram of surface of English Electric P8600 CCD showing the imaging and storage areas [7].
124
S.D. Berger, D. McMullan / Parallel recording for electron spectrometer on S T E M
down to a width of only 5 pixels, defined by a mask, and binning takes place at the read-out register. Masking may lead to errors as it does with photodiode arrays where either the diode length defines the unmasked area or a separate mask is used. In both cases, curvature of the spectrum can cause difficulties and accurate positioning is needed because the sensitivity of a diode varies over its surface, important when the width of the spectrum is less than the mask aperture, and electrons are lost at the edge of the beam if this overfills the aperture. Recording the unmasked full width with a two-dimensional CCD is to be preferred since the actual area illuminated can be displayed and the alignment checked to ensure that the same pixels are used as were during calibration. There are 385 columns in the P8600, so that there will be this number of channels per spectrum; however, the length of the spectrum can be increased by changing the potential of the spectrometer drift tube between exposures and recording up to say five 385-channel sections cyclically. An optical shutter will time the exposures and it will thus be possible to accumulate approximately equal charges in the low- and high-loss sections of the spectrum. The minimum exposure allowed by the shutter is about 10 msec and this may be too long for recording the zero-loss peak without saturation of the CCD; provision is therefore being made for automatically introducing a neutral density filter into the optical path. Alternatively, the coupling lens could be stopped down while recording this part of the spectrum. The Wright CCD camera will fiat-field, drift correct, and add the sections to give a spectrum of nearly 2000 channels, each with a maximum count of about 5 × 106 electrons.
3. Scintillator afterglow Although a YAG scintillator has a fast primary luminescent decay (~-- 80 ns) [8], there are slower components of the decay which can cause errors if relatively weak spectra are recorded immediately after the zero-loss peak. These errors can be avoided if different areas of the YAG, and of
course of the CCD, are used for the high and low intensity sections of the spectrum by shifting with deflection coils before or following the quadrupoles. The zero-loss and plasmon sections will be placed at the top of the CCD (i.e. furthest from the storage area) and the lower intensity sections progressively further down. The former will be shifted to the storage area by 288 clock pulses, but for the lower-intensity ones fewer clock pulses will be used (sufficient for the charge pattern to be binned) and any spurious image due to the afterglow of the zero-loss peak will not reach the storage area but will be lost at the top edge of the image area by reverse docking after the wanted spectrum has been binned.
4. Bright field detector In the standard HB501 STEM the energy-loss spectrometer detector (a photomultiplier) is used with the slit wide open for bright-field imaging and with a narrow slit to produce an energy-loss image. The CCD detector does not have a fast enough response for bright field detection and therefore the photomultiplier will be retained and the beam from the spectrometer deflected by coils to a separate YAP scintillator optically coupled to the photomultiplier [9]. If this scintillator is masked so that only a slit-like area is exposed, say 100 #m wide, then energy-loss images can be recorded, the effective width of the slit being controlled by varying the magnification of the quadrupoles.
5. Conclusions A CCD coupled to a YAG scintillator would appear to be the most suitable type of detector for the parallel recording of EELS spectra. Apart from the advantages the CCD shares with other solid-state photon detectors such as self-scanned photodiode linear arrays (high responsive quantum efficiency, stable sensitivity and geometry, robustness, and small size), it can also have an extremely low read-out noise which leads to high DQE at low light levels, making lens coupling to the YAG an attractive proposition. CCDs are
S.D. Berger, D. McMullan / Parallel recording for electron spectrometer on S T E M
generally two-dimensional detectors, and this again offers several advantages over self-scanned linear arrays: adjustment of the spectrometer and quadrupole lenses is facilitated; any curvature in the spectrum is unimportant and extreme accuracy in positioning is unnecessary because the sensitive area of the detector is not defined by a mask; spectra can be stored on the chip; and there is the possibility of using different areas of the YAG for high- and low-intensity parts of the spectrum so that errors caused by afterglow of the scintillator can be avoided.
References [1] O.L. Krivanek, C.C. Ahn and R.B. Keeney, Ultramicroscopy 22 (1987) 103.
125
[2] R.F. Egcrton and P.A. Crozicr, J. Microscopy 148 (1987) 157. [3] A.J. Bourdillon and W.M. Stobbs, in: Electron Microscopy 1986 (Proc. 11th Intern. Congr. on Electron Microscopy, Kyoto, 1986), Vol. 1, Eds. T. Imura, S. Maruse and T. Suzuki (Japan. Soc. of Electron Microscopy, Tokyo, 1986). 523. [4] M.G. Strauss, I. Naday, I.S. Sherman and N.J. Zaluzec, Ultramicroscopy 22 (1987) 117. [5] D. MeMullan, in: Proc. 45th Annual EMSA Meeting, Baltimore, MD, 1987, Ed. G.W. Bailey (San Francisco Press, San Francisco, 1987) p. 526. [6] Wright Instruments Ltd., 4 Chalkw¢ll Park Road, Enfield, Middlesex EN1 2AJ, UK. [7] English Electric, CCD Technical Notes 1-9 (1983). [8] R. Autrata, P. Scheuzr, J. Kvapil and Jo. Kvapil, in: Electron Microscopy 1984 (Proc. 8th European Congr. on Electron Microscopy, Budapest, 1984), Eds. A. Csanfidy, P. RShlieh and D. Szabo (Budapest, 1984) p. 617. [9] D. MeMuUan and S.D. Bergcr, Ultramicroscopy 25 (1988) 349.
Ultramicroscopy 28 (1989) 122-125 North-Holland, Amsterdam
PARALLEL R E C O R D I N G F O R A N E L E C T R O N S P E C T R O M E T E R ON A SCANNING TRANSMISSION ELECTRON MICROSCOPE S.D. B E R G E R * and D. M c M U L L A N Cavendish Laboratory, Madingley Road, Cambridge CB3 0HE, UK Received at Editorial Office 3 November 1988; presented at Workshop March 1988
A parallel recording system is being built for the electron-energy-lossspectrometer of a VG Microscopes HB501 scanning transmission electron microscope (STEM). A two-dimensional charge-coupled device (CCD) lens-coupled to an ytrrium aluminium garnet (YAG) scintillator is the electron detector. The operating mode for the CCD is described and the expected performance compared with that of a detector using a self-scanned photodiode array. Provision is being made for the bright-field and energy-lossimages to be produced.
I. Introduction Several parallel recording systems for electronenergy-loss spectrometry (EELS) using silicon detectors have been the subject of recent papers [1-5]. These systems include a single-crystal yttrium aluminium garnet (YAG) scintillator fibreoptically coupled to a self-scanned diode linear array [1,2], a self-scanned diode linear array used in the EBIC m o d e [3], and a Y A G scintillator lens-coupled to a charge-coupled detector (CCD) [4,5]. The latter combination is being used for the system that is being built for the V G Microscopes HB501 STEM at the Cavendish Laboratory.
2. CCD parallel detector for a S T E M F r o m the start it was decided that a two-dimensional C C D should be used as the detector because of the very low noise read-out that can be achieved, thus making lens coupling to a Y A G scintillator feasible. There are a number of advantages in using a two-dimensional detector apart from the obvious one of facilitating the setting-up
* Now at: AT&T Bell Laboratories, Murray Hill, New Jersey 07974, USA.
of the electron optics: in particular it is possible to store m a n y spectra on the chip before reading out, and, most important, the effects of afterglow of the Y A G scintillator can be avoided (see section
3). The detector is a camera, manufactured by Wright Instruments [6], which uses an English Electric P8600 C C D sensor. This C C D has an imaging area 8.5 × 6.4 m m 2 (385 columns of 288 pixels 20 # m square), and when cooled ( - 150 o C) the read-out noise is less than 10 electrons RMS. Calculations, confirmed b y tests, show that with f / 2 lens coupling only about 3 electrons are generated in the C C D per high-energy electron on the Y A G , but even so, because of the low read-out noise, the D Q E is reasonable at low count rates; and because of the low temperature the dark signal is negligible for an exposure as long as 1 h. The D Q E is given by
~
l
=[
~
E+n2'
where E is the n u m b e r of incident high-energy electrons and n is the read-out noise referred to the scintillator, i.e. the C C D read-out noise divided by the number of C C D charges produced by a single electron. In the present example, the minimum signal for a D Q E > 0.5 is only 10 high-
0304-3991/89/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
123
S.D. Berger, D. McMullan / Parallel recording for electron spectrometer on S T E M Table 1 Comparison of self-scanned diode array [2] and CCD detectors
Self-scanned array + YAG CCD+YAG
Minimum count per channel a) (electrons) 200 10
Maximumcount per channel b) (electrons) 4.5x10 s 1 Xl0 s
Dynamic range 2x103 I xl04
Integration time at 10 electrons/s rate c) (s) 20 1
Number of channels (resolved) 512 385
a) For DQE = 0.5; per integration period. b) Per integration period. c) For DQE-0.5
energy electrons (30 C C D charges), so that the integration time for a 10-electrons s -1 per channel signal ( D Q E = 0.5) is 1 s. The C C D saturation charge per pixel is 3 × 105 and the dynamic range 104. These calculations are summarised in table 1 and compared with the self-scanned diode array used by Egerton and Crozier [2] (250 diode charges per 100 keV electron on the YAG, read-out noise estimated to be 3500 electrons RMS, and 512 resolved channels). The point spread function of the scintillations in the Y A G (30 btm with 100 keV electrons, measured by Strauss et al. [4] and confirmed b y ourselves), is wider than the pixel dimension of the C C D (20 btm) so that optical demagnification is necessary if the full resolution of the C C D is to be realised; 2 x demagnification should be suitable. The dispersion of the spectrometer (about 2 /~m/eV) is being increased b y three quadrupole lenses [1] to match the resolution of the detector. The P8600 is a frame transfer device [7] with an image area and a storage area which is masked (see fig. 1). The spectrum will be focussed on the image area with the dispersion parallel to the rows, and will cover a width of, say, 50 C C D pixels (i.e. at right angles to the dispersion); this width can be controlled by adjustment of the quadrupoles. At the end of an exposure period the charges will be shifted down rapidly (in about 250 /~s) and binned into the first row of pixels in the storage area; the binned charges will then be shifted down one row so that the first row is ready to receive another spectrum from the imaging area. It will thus be possible to store up to 290 spectra before a read-out is necessary, i.e. when the first spectra to be recorded has been shifted
down to the read-out register at the lower edge of the CCD. The read-out will be done by clocking the readout register to shift the line of charges serially to the output amplifier; this will take about 13 ms per line so that read-out of all 290 spectra will be completed in about 5 s. The shifting of the charges on the chip does not introduce any additional noise and can be done using the standard camera software. A similar operating m o d e has been proposed by Strauss et al. [4], but they focus the spectrum 385 COLUMNS
I0 10
s~ S~ s¢
IMAGE AREA 288 ROWS
STORAGE AREA 290 ROWS
OU"
R¢3 Re 2 Re I Fig. 1. Schematic diagram of surface of English Electric P8600 CCD showing the imaging and storage areas [7].
124
S.D. Berger, D. McMullan / Parallel recording for electron spectrometer on S T E M
down to a width of only 5 pixels, defined by a mask, and binning takes place at the read-out register. Masking may lead to errors as it does with photodiode arrays where either the diode length defines the unmasked area or a separate mask is used. In both cases, curvature of the spectrum can cause difficulties and accurate positioning is needed because the sensitivity of a diode varies over its surface, important when the width of the spectrum is less than the mask aperture, and electrons are lost at the edge of the beam if this overfills the aperture. Recording the unmasked full width with a two-dimensional CCD is to be preferred since the actual area illuminated can be displayed and the alignment checked to ensure that the same pixels are used as were during calibration. There are 385 columns in the P8600, so that there will be this number of channels per spectrum; however, the length of the spectrum can be increased by changing the potential of the spectrometer drift tube between exposures and recording up to say five 385-channel sections cyclically. An optical shutter will time the exposures and it will thus be possible to accumulate approximately equal charges in the low- and high-loss sections of the spectrum. The minimum exposure allowed by the shutter is about 10 msec and this may be too long for recording the zero-loss peak without saturation of the CCD; provision is therefore being made for automatically introducing a neutral density filter into the optical path. Alternatively, the coupling lens could be stopped down while recording this part of the spectrum. The Wright CCD camera will fiat-field, drift correct, and add the sections to give a spectrum of nearly 2000 channels, each with a maximum count of about 5 × 106 electrons.
3. Scintillator afterglow Although a YAG scintillator has a fast primary luminescent decay (~-- 80 ns) [8], there are slower components of the decay which can cause errors if relatively weak spectra are recorded immediately after the zero-loss peak. These errors can be avoided if different areas of the YAG, and of
course of the CCD, are used for the high and low intensity sections of the spectrum by shifting with deflection coils before or following the quadrupoles. The zero-loss and plasmon sections will be placed at the top of the CCD (i.e. furthest from the storage area) and the lower intensity sections progressively further down. The former will be shifted to the storage area by 288 clock pulses, but for the lower-intensity ones fewer clock pulses will be used (sufficient for the charge pattern to be binned) and any spurious image due to the afterglow of the zero-loss peak will not reach the storage area but will be lost at the top edge of the image area by reverse docking after the wanted spectrum has been binned.
4. Bright field detector In the standard HB501 STEM the energy-loss spectrometer detector (a photomultiplier) is used with the slit wide open for bright-field imaging and with a narrow slit to produce an energy-loss image. The CCD detector does not have a fast enough response for bright field detection and therefore the photomultiplier will be retained and the beam from the spectrometer deflected by coils to a separate YAP scintillator optically coupled to the photomultiplier [9]. If this scintillator is masked so that only a slit-like area is exposed, say 100 #m wide, then energy-loss images can be recorded, the effective width of the slit being controlled by varying the magnification of the quadrupoles.
5. Conclusions A CCD coupled to a YAG scintillator would appear to be the most suitable type of detector for the parallel recording of EELS spectra. Apart from the advantages the CCD shares with other solid-state photon detectors such as self-scanned photodiode linear arrays (high responsive quantum efficiency, stable sensitivity and geometry, robustness, and small size), it can also have an extremely low read-out noise which leads to high DQE at low light levels, making lens coupling to the YAG an attractive proposition. CCDs are
S.D. Berger, D. McMullan / Parallel recording for electron spectrometer on S T E M
generally two-dimensional detectors, and this again offers several advantages over self-scanned linear arrays: adjustment of the spectrometer and quadrupole lenses is facilitated; any curvature in the spectrum is unimportant and extreme accuracy in positioning is unnecessary because the sensitive area of the detector is not defined by a mask; spectra can be stored on the chip; and there is the possibility of using different areas of the YAG for high- and low-intensity parts of the spectrum so that errors caused by afterglow of the scintillator can be avoided.
References [1] O.L. Krivanek, C.C. Ahn and R.B. Keeney, Ultramicroscopy 22 (1987) 103.
125
[2] R.F. Egcrton and P.A. Crozicr, J. Microscopy 148 (1987) 157. [3] A.J. Bourdillon and W.M. Stobbs, in: Electron Microscopy 1986 (Proc. 11th Intern. Congr. on Electron Microscopy, Kyoto, 1986), Vol. 1, Eds. T. Imura, S. Maruse and T. Suzuki (Japan. Soc. of Electron Microscopy, Tokyo, 1986). 523. [4] M.G. Strauss, I. Naday, I.S. Sherman and N.J. Zaluzec, Ultramicroscopy 22 (1987) 117. [5] D. MeMullan, in: Proc. 45th Annual EMSA Meeting, Baltimore, MD, 1987, Ed. G.W. Bailey (San Francisco Press, San Francisco, 1987) p. 526. [6] Wright Instruments Ltd., 4 Chalkw¢ll Park Road, Enfield, Middlesex EN1 2AJ, UK. [7] English Electric, CCD Technical Notes 1-9 (1983). [8] R. Autrata, P. Scheuzr, J. Kvapil and Jo. Kvapil, in: Electron Microscopy 1984 (Proc. 8th European Congr. on Electron Microscopy, Budapest, 1984), Eds. A. Csanfidy, P. RShlieh and D. Szabo (Budapest, 1984) p. 617. [9] D. MeMuUan and S.D. Bergcr, Ultramicroscopy 25 (1988) 349.