Supplemental Information
This segment is used to supplement information for the IonCCD introduction and description
sections, as well as to reduce the number of figures in the manuscript.
Introduction
Photosensitive pixilated detectors are a product of two different technologies: charge-coupled devices
(CCDs) and complementary metal-oxide-semiconductors (CMOS). The first CCD and CMOS devices
were invented in the late 1960s at AT&T Bell Laboratories and Fairchild Semiconductor, respectively.
At the time, CMOS performance was limited by available lithographic technology, allowing CCDs to
dominate the market for the next 25 y. However, lithography and process control in CMOS fabrication
eventually reached levels that allowed CMOS sensor image quality to rival that of CCDs [1]. CCDs and
CMOS devices have a wide range of applications, from digital photography to astronomy (particularly
in photometry and optical and UV spectroscopy) [2–5]. Despite a tempting debate on which technology
is superior, they both seem to be more complementary than anything else, revolutionizing imaging
technology [1, 6].
A CCD consists of a silicon wafer patterned in the manner of integrated circuits into a
rectangular grid of voltage adjustable capacitors (“gates”) on which electrical charge can be stored, and
among which charge can be shifted around. They were originally intended to be used as computer
memory components, but have become much more important as imaging detectors of visible,
ultraviolet, and X-ray light. In astronomy, CCDs play a large role at these wavelengths. Their principal
advantages over older imaging detectors, such as photographic emulsions, are several; CCDs have
particularly high quantum efficiency (photons-to-signal conversion). CCD quantum efficiency at visible
wavelengths is typically in the range 0.5–0.9 compared to <0.1 for a good photographic emulsion.[7]
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This fact makes CCDs considerably more sensitive than photography; in other words, CCDs can detect
much fainter sources of light in a given amount of time than photographic emulsions. CCDs are quite
linear and the signal they produce in response to light is directly proportional to the number of photons
collected. The signal produced by CCDs is a rectangular array of voltages, each of which is proportional
to the number of photons collected in a certain element (pixel) of the image. Voltages are readily
measured and stored digitally for computer analysis. By contrast, photographs must be “digitized”
before they can be analyzed on computers and, usually, such digitization is carried out by something
like a CCD. Very sensitive imaging detectors similar to CCDs are currently in use in astronomy at
practically all wavelengths from far-infrared (ca. 200 μm) to X-ray (ca. 0.1 Å).
Description
Figure 1 shows the geometry of the detector to the naked eye and using atomic force microscopy. These
dimensions were selected mainly to use the IonCCD as a focal-plane array detector for a miniaturized
sector-field mass spectrometer of Mattauch-Herzog geometry (MH-MS). The basic operation of the
detector is illustrated in Figure 2 (see text for more details). The linearity study of the IonCCD is shown
in Figure 3. This study uses the same experimental setup used for Experiment II (see text) with 90o
incidence angle. In this case, the ion beam current was varied from 250 fA to 250 pA as measured by an
electrode plate replacing the IonCCD, and the IonCCD was operated with a 15-ms integration time. To
minimize the measurement errors, the beam current was also recorded on the mask plate for both runs
(IonCCD and electrode plate). The horizontal errors are due mostly to the electrometer readings; the
vertical errors are due mostly to the IonCCD noise measured as the sum of the pixel noise under the
peak. Two hundred consecutive frames were recorded for every ion-beam current value. The beam was
measured to be about 12 pixels wide at low beam current (see top insert) and does not broaden at high
beam current (see bottom insert). From the top insert one can see the limit of detection being defined by
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the noise floor, illustrated here by the pixel signal standard deviation used as error bars (±9 dN). The
signal-to-noise ratio values were calculated from the peak area and the sum of the pixel noise under the
peak. Figure 3 does not only show a 103 linear response, it also shows that the detection efficiency of
the IonCCD is well between the 63 ion/dN green line and the 100 ion/dN blue line (see the text for
detection efficiency values). The reader must keep in mind that no averaging was used to define the
dynamic range; when averaging is performed, the pixel noise drops easily by a factor of 10 extending
the linear dynamic range to 104. Furthermore, using different integration times can easily extend this
value to 105–6.
Figure 4 shows the beam profile at 90o and 45o incidence angles of a 1 keV Ar+ ion beam. Note
the broadening of the beam profile at 45o following the cosine rule. The discrepancy with the theoretical
value is within 10% and is due to the experimental deviation in the involved dimensions between both
configurations.
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Supplemental References
1.
Litwiller, D. CMOs versus CCD: Maturing technologies, maturing markets. Photonics Spectra
2005, 39(8), 54-+.
2.
Neely, A. W.; J. R. Janesick, A. CCD Antiblooming technique for Use in Photometry.
Publications of the Astronomical Society of the Pacific: 1993, 105(693), 1330–1333.
3.
Richards, D.W.; et al. Enhanced Detection of Normal Retinal Nerve-Fiber Striations Using a
Charge-Coupled-Device and Digital Filtering. Graefes Archive for Clinical and Experimental
Ophthalmology 1993, 231(10), 595–599.
4.
Kraft, R.P.; et al. Soft-X-ray Spectroscopy with Subelectron Readnoise Charge-Cupled Devices.
Nuclear Instruments and Methods in Physics Research Section a—Accelerators Spectrometers
Detectors and Associated Equipment 1995, 361(1/2) 372–383.
5.
Williams, G.; Janesick, J. Cameras with CCDs Capture New Markets. Laser Focus World 1996,
32(3), S5–S9.
6.
Litwiller, D. CCD versus CMOS: The Battle Cools off. Photonics Spectra 2002, 36(1), 102–103.
7.
Boyce, P. B. Low Light Level Detectors for Astronomy. Science 1977, 198(4313), 145–148.
4
Figure 1. (a) Photographic image of the IonCCD chip. (b) Atomic force microscope image shown as
3D view image taken in contact mode.
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Figure 2. Diagram illustrating the basics of the IonCCD detector operation (see text).
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Figure 3. IonCCD response to 1-keV ion beam. Plotted is the peak area of the profiled beam (shown in
both inserts) as a function of the beam current measured when the IonCCD is replaced with a plate
electrode. Note that the noise floor of the IonCCD is about 9 dN in the above insert (lowest signal data
point).
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Figure 4. Ion beam profile taken at 90o and 45o incidence angle.
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