Microelectronic Engineering 19 (1992) 639-642 639 Elsevier
Smart imager for fast and inexpensive m a s s - s p e c t r o m e t e r back-ends
B. Dierickx a, D. Geeraert b and D. Nevejans c alMEC, Kapeldreef 75, B-3001 Leuven, Belgium
bprovinciale Industri61e Hogeschool Kortrijk, Graaf Karel De Goedelaan 5, B-8500 Kortrijk, Belgium
CBIRA, Ringlaan 3, B-1180 Ukkel, Belgium
A b s t r a c t
A new "smart" type of detector array is presented that is suited for application in the detector back-end of a mass spectrometer. The integration of digital position-encoding intelligence on chip allowed to increase the detection frequency in single-ion detection mode and to reduce significantly the number of peripheral circuits.
1. I N T R O D U C T I O N
M a s s - s p e c t r o m e t e r s are used in a e r o n o m y for m o n i t o r i n g minute concentrations of gases in the upper atmosphere. These spectrometers are contained in balloon payloads. Consequently they have to be lightweight, and operate autonomously for a few hours.
Actual m a g n e t i c m a s s - s p e c t r o m e t e r s are based on a c l a s s i c a l a c c e l e r a t o r - s e p a r a t o r section, a m i c r o - c h a n n e l plate, and a phos- phorescent screen for final electron to photon conversion. The light spot representing a detected ion, is read-out by a photo diode array.
2. SMART DETECTOR ARRAY
The present development uses the FUGA1013 "smart photo diode array".
Its 1024 photo diodes are operated as 256 pixels. Unlike linear CCD's or classical photo diode arrays, it does not yield a sequential video like signal, but it processes each frame ("scanned line") internally. The number of data output is therefore small; the device can operate at very high "image" frequencies. For the envisaged application, single ion
0167-9317/92/$05.00 © 1992 - Elsevier Science Publishers B.V. All rights reserved.
04u B. Dierickx et al. / Mass-spectrometer back-ends
d e t e c t i o n can be d o n e at speeds of m o r e than 100 kHz. T h e FUGA10[~ is a s u c c e s s o r o f the r e p o r t e d X Y W e v e n t d e t e c t o r chip [1-2].
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Fig. 1. Floor plan of the FUGA10[3. 1024 photo diodes are connected in groups of 4 to 256 integrating charge amplifiers and discriminators, yielding 0 (=dark) or 1 (=light). Two parallel priority ladders determine the leftmost and rightmost extent of the 'light spot'. The 4 diodes per amplifier, black-white inversion, and erosion are not of interest for the present application.
A f t e r each c l o c k c y c l e the FUGA10[3 yields two digital o u t p u t words: the p i x e l n u m b e r s of the l e f t m o s t resp. r i g h t m o s t t r a n s i t i o n f r o m d a r k to l i g h t in the pixel array. T h e d i s c r i m i n a t i o n b e t w e e n dark and light is d o n e i n s i d e t h e c h a r g e a m p l i f i e r s , and d e p e n d s on an e x t e r n a l l y a d j u s t a b l e t h r e s h o l d . T h e d e v i c e is p r o c e s s e d in a C M O S t e c h n o l o g y , operates on a 5 V supply, and interacts in a p u r e l y digital w a y with the o u t s i d e w o r l d .
3. F E A S I B I L I T Y S T U D Y
A set-up has been built to d e m o n s t r a t e the feasibility of a c o m p a c t , light and i n e x p e n s i v e m a s s - s p e c t r o m e t e r b a c k - e n d (fig. 2). T h e l i g h t spots e m e r g i n g f r o m the s p e c t r o m e t e r ' s s c r e e n are e m u l a t e d by a L . E . D . m o u n t e d in a m i c r o s c o p e objective. The L.E.D. is p u l s e d by an oscillator r u n n i n g a s y n c h r o n o u s l y w i t h t h e F U G A 1 0 [ ~ c l o c k . T h e n u m b e r of p h o t o n s in a p u l s e is c o n t r o l l e d by the e l e c t r i c a l p u l s e l e n g t h and the c u r r e n t a m p l i t u d e t h r o u g h the L.E.D.
A l t h o u g h the i n t e n s i t i e s of light pulses on the d e t e c t o r can vary within a
w i d e r a n g e , t h e i m p a c t m i d - p o i n t s m u s t be :very r e p r o d u c i b l e . T h e
B. Dierickx et al. / Mass-spectrometer back-ends 641
s e n s i t i v e a r e a o f the actual FUGA10[~ is 10240 ~tm by 450 ~tm. In a p r o d u c t i o n d e v i c e the length m u s t be i n c r e a s e d to 24 m m (1000 pixels on a 24 I~m pitch)
Fig. 2 Demonstration set-up for the back-end of a mass spectro- meter based on the FUGA1013.
A flat yellow (~. = 650 nm) L.E.D. is mounted in a micro- scope objective so as to project a small image of the light emitting surface on the FUGA1013 detector array. The shape of the spot is about Gaussian, with a 2c diameter of 80 I.t m, which is representative for a real spectrometer. Multiple spots can be realized with multiple independently driven L.E.D.s.
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4. E X P E R I M E N T A L R E S U L T S
The m a x i m u m d e t e c t i o n rate is 100 to 150 kHz. T h e f o l l o w i n g results are obtained at 25 kHz; the integration time of the c h a r g e amplifiers was 20 ~ts.
P o s i t i o n r e s o l u t i o n
T h e spot position is d e t e r m i n e d as the a v e r a g e of left and right e d g e position. A l t h o u g h the spot width m a y vary w i d e l y , one sees that the a v e r a g e p o s i t i o n is quite r e p r o d u c i b l e (Fig. 3). For low and m e d i u m i n t e n s i t i e s , the n u m b e r o f p o s i t i o n s that d e v i a t e f r o m the m e d i a n position is a few percents (Fig. 4); i.e. the spot position is r e p r o d u c i b l e within 20 I.tm (= 1/2 pixel).
L o w e r d e t e c t i o n t h r e s h o l d
F i g u r e 4 shows the detection results as a f u n c t i o n of the total n u m b e r of
photons in a pulse. T h e s e n u m b e r s were calibrated versus L.E.D. c u r r e n t
642 B, Dierickx et al. / Mass-spectrometer back-ends
a n d p u l s e d u r a t i o n . A l o w e r r e l i a b l e d e t e c t i o n t h r e s h o l d w a s d e t e r m i n e d at a b o u t 2 0 0 0 0 0 p h o t o n s p e r p u l s e .
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X-position [pixel number]
Fig.3 e x a m p l e o f t h e s t a t i s t i c a l distribution of the positions of a large number of detected spots, for a fixed spot position and fixed pulse conditions. X-axis: pixel number (in a row of 256 pixels of 40 ~m wide). Y-axis: the number of hits a c c u m u l a t e d in each p o s i t i o n during one second. The central positions are calculated as the
"average" of left and right
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