Optoelectronic Image Detectors in Chemistry, An Overview - ACS

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Optoelectronic Image Detectors in Chemistry, An Overview

Downloaded by 110.14.44.225 on May 9, 2018 | https://pubs.acs.org Publication Date: June 21, 1979 | doi: 10.1021/bk-1979-0102.ch001

YAIR TALMI EG&G Princeton Applied Research Corporation, Princeton, NJ 08540

Spectrometric information can be obtained either by scanning across the spectral region of interest or by simultaneously monitoring this region in its entirety. Two instrumentation approaches to simultaneous spectrometric detection, that have evolved in the last two decades are multiplex and multichannel techniques. The former, utilizes transform methods based upon either Hadamand or Fourier mathematics. This approach has been rigorously studied both theoretically and experimentally and has been proven commercially feasible. However, its simultaneous (multiplex) advantage has been realized only for the IR region, where the spectrometric system is typically detector-noise limited. The multiplex advantage has been only partially realized with VUV to near IR-, electron- and ion-spectrometers, where the overall performance of the detector system is rarely limited by the detector noise itself. In fact, in a few cases, where "dense" spectra were studied, a "multiplex-disadvantage" has been actually observed (1,2). The alternative approach is "parallel-detection"; an array of detectors is placed across the focal plane of a polychromator and the dispersed radiation is simultaneously measured. By far, the most widely used and commonly available parallel detector is the photographic emulsion. It has a fixed and accurate geometric registration that allows for reliable wavelength calibration. Its operation is methodically and practically simple, its cost is affordable and its physical dimensions, and therefore spectral resolution, practically unlimited, depending only on the design of the polychromator. Unfortunately, the sensitivity (quantum efficiency) of these detectors is poor, their transfer characteristics non-linear (gamma ^ 1 ), their accuracy and precision are marginal and require tedious and time consuming calibration procedures and worse yet, the validity and usefulness of the spectral data gathered can be verified only after the "platedevelopment" process, thus, often resulting in a loss of invaluable, and at times, irreproducible experimental data. Because of its wide acceptance as a VUV-near IR spectrometric detector, the PMT has emerged as the natural electro-optical parallel detector. This has been accomplished by placing an array of 0-8412-0504-3/79/47-102-003$05.75/0 © 1979 American Chemical Society Talmi; Multichannel Image Detectors ACS Symposium Series; American Chemical Society: Washington, DC, 1979.


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MULTICHANNEL IMAGE DETECTORS

mini-PMTs i n predetermined p o s i t i o n s (corresponding to s p e c t r a l regions of i n t e r e s t ) across the f o c a l plane of a polychromator, each w i t h i t s a s s o c i a t e d readout e l e c t r o n i c s . Such d e t e c t i o n systems, d e s p i t e t h e i r clumsiness have been proven very u s e f u l i n v a r i o u s r o u t i n e spectrometric s t u d i e s , where the same few s p e c t r a l f e a t u r e s (10-4P) are repeatedly i n t e r r o g a t e d , where absolute ( s p e c t r o r a d i o m e t r i e ) s p e c t r a l data i s not r e q u i r e d and where the most c r i t i c a l and d e s i r a b l e performance f e a t u r e s are; a very wide dynamic range, high s e n s i t i v i t y and low s t r a y l i g h t , r a t h e r than optimal f l e x i b i l i t y i n the i n t e r a c t i o n between the s p e c t r o s c o p i s t and h i s experiment. A t y p i c a l example of t h i s design approach i s the " d i r e c t - r e a d e r " ; a r a t h e r expensive, and dedicated, " f a c tory-tuned" multichannel spectrometer t h a t i s capable of monitori n g up to f o r t y s p e c t r a l l i n e s simultaneously and i s used almost e x c l u s i v e l y i n conjunction with a r c s , sparks and plasmas f o r r o u t i n e elemental a n a l y s i s . In the l a s t decade, as the s t a t e - o f - t h e - a r t TV cameras have adequately matured, t h e i r use as spectrometric p a r a l l e l d e t e c t o r s has been demonstrated and g r a d u a l l y gained acceptance among s p e c t r o s c o p i s t s . A l l TV-detectors, and more g e n e r a l l y o p t o e l e c t r o n i c image d e t e c t o r s (OID) i n c l u d i n g image-orthicon and i s o c o n , s i l i c o n , lead-oxide and KC1 v i d i c o n s and a v a r i e t y of s o l i d - s t a t e imagers, such as self-scanned photodiode a r r a y s , charge-coupled and c h a r g e - i n j e c t i o n d e v i c e s , are by t h e i r very nature, m u l t i channel p a r a l l e l photon d e t e c t o r s which can a c c u r a t e l y transform o p t i c a l images i n t o t h e i r corresponding e l e c t r o n i c images. A l l OIDs comprise three b a s i c components; a transducer to convert photon images to t h e i r e l e c t r i c a l analogs, a device f o r s t o r i n g these " l a t e n t " e l e c t r i c a l images and a readout (video) mechanism to r e c o n s t r u c t the s t o r e d images and transmit them ( i n real-time) to a d i s p l a y monitor or v i a the use of an A/D converter to s t o r e them i n a d i g i t a l memory f o r f u r t h e r data p r o c e s s i n g and manipulation. H i s t o r i c a l l y , the trend i n the TV-detector i n d u s t r y has been toward s i m p l i f i c a t i o n v i a c o n s o l i d a t i o n o f these components. Thus, i n f i r s t generation imagers, e.g., charge-coupled d e v i c e s , a l l three components i n c l u d i n g the video p r e a m p l i f i e r i t s e l f are combined on a s i n g l e m o n o l i t h i c s i l i c o n c r y s t a l wafer Q ) . Although most experts p r e d i c t t h a t t h i r d generation s e l f scanned s o l i d s t a t e imagers w i l l e v e n t u a l l y become the OIDs of c h o i c e , a t the present time, t h e i r compromised performance, low manufacture y i e l d and t h e r e f o r e , l i m i t e d commercial a v a i l a b i l i t y , g r e a t l y l i m i t t h e i r use as spectrometric d e t e c t o r s . An exception to t h a t are self-scanned photodiode a r r a y s . Because v i d i c o n imagers and p a r t i c u l a r l y the s i l i c o n v i d i c o n w i t h i t s v a r i o u s i m a g e - i n t e n s i f i e d d e r i v a t i v e s are much more readi l y a v a i l a b l e and t h e i r behavior and performance are b e t t e r cont r o l l e d (even i f not always f u l l y understood), a b r i e f d e s c r i p t i o n of t h e i r p r i n c i p l e s of o p e r a t i o n may be necessary i f t h e i r spect r o m e t r i c performance (and t h a t of other imagers as well) i s to be p r o p e r l y and i n t e l l i g e n t l y i n t e r p r e t e d . The h e a r t of the SV i s a s i n g l e m o n o l i t h i c s i l i c o n c r y s t a l

Talmi; Multichannel Image Detectors ACS Symposium Series; American Chemical Society: Washington, DC, 1979.


1.

TALMI

Overview of Optoelectronic Applications

5

wafer with a m i c r o s c o p i c a r r a y of a few m i l l i o n diode j u n c t i o n s grown on i t (Figure 1 ) . A l l diodes have a common cathode and i s o l a t e d anodes s e l e c t i v e l y addressed by a scanning (readout) e l e c t r o n beam. The diodes f u n c t i o n as photodiodes, g e n e r a t i n g the p r o d u c t i o n and storage o f e l e c t r o n - h o l e p a i r s upon i n c i d e n c e o f UV t o near-IR photons. A c o n t i n u o u s l y scanning e l e c t r o n beam recharges a l l photodiodes t o an equal and p r e s e t r e v e r s e d ^ b i a s p o t e n t i a l . Exposure o f the t a r g e t t o photons or e l e c t r o n s causes p r o d u c t i o n of elec-*t r o n - h o l e p a i r s t h a t combine t o d e p l e t e the s u r f a c e charge. When the beam scans again, t h i s time a p a r t i a l l y d e p l e t e d r e g i o n , a r e c h a r g i n g c u r r e n t flows. T h i s c u r r e n t i s p r o p o r t i o n a l t o the depleted charge and l i k e w i s e to the d e n s i t y o f the e l e c t r o n - h o l e pairs. I t t h e r e f o r e , i s p r o p o r t i o n a l a l s o t o the number o f photons (or photoelectrons) i n c i d e n t on each diode. The imaging (and s p e c t r a l ) r e s o l u t i o n o f the SV i s l i m i t e d by the diameter o f the scanning e l e c t r o n beam, t y p i c a l l y 25jum. However, any number of d i o d e s can be grouped together (by a com^ puter-pre-programmed addressing m a n i p u l a t i o n o f the readout beam) and t h e i r combined s i g n a l s t o r e d i n the same memory c e l l . Thus, a t r a d e - o f f between r e s o l u t i o n and signal^to^-noise r a t i o (SNR) can be accomplished, i n a manner analogous to t h a t achieved by v a r y i n g the s l i t width o f a spectrometer. Because the readout n o i s e o f s i l i c o n v i d i c o n s (SV) i s approximately 2000 e l e c t r o n rms, the s m a l l e s t s i g n a l s t h a t can be detected ( i n each r e s o l u t i o n element) are a few thousand photons i n m a g n i t u d e ; t y p i c a l quantum e f f i c i e n c y (QE) of s i l i c o n i s 10% t o 80% i n the UV t o near-IR r e g i o n . To d e t e c t s i n g l e p h o t o e l e c t r o n s , an image i n t e n s i f i c a t i o n s e c t i o n i s added t o the s i l i c o n v i d i c o n . The r e s u l t a n t imager, a s i l i c o n i n t e n s i f i e d t a r g e t (SIT) v i d i c o n , has a photocathode (transducer) t h a t converts the photon image i n t o a corresponding p h o t o e l e c t r o n image. The generated e l e c t r o n image i s a c c e l e r a t e d ( t y p i c a l l y 7 ~ 9 kv) and focused onto the s i l i c o n t a r g e t . Since the number o f e l e c t r o n - h o l e charge p a i r & produced on the t a r g e t i s p r o p o r t i o n a l t o the p o t e n t i a l o f the i n c i d e n t e l e c t r o n s (approximately one charge p a i r per 3.6 ev) an i n t e r n a l g a i n of approximately 1500 i s t y p i c a l l y achieved, i . e . , t h i s i s the number o f e l e c t r o n s produced from each p h o t o e l e c t r o n emitted from the photocathode (the e l e c t r o n energy i s p a r t i a l l y absorbed by the s i l i c o n oxide o v e r c o a t ) . With t h i s g a i n , the s i g n a l i s s u f f i c i e n t l y enhanced (compared t o the readout noise) t o a l l o w the d e t e c t i o n of v e r y weak s i g n a l s . F u r t h e r g a i n , i f necessary, can be obtained by adding another image i n t e n s i f i c a t i o n stage;the i n t e n s i f i e d s i l i c o n i n t e n s i f i e d t a r g e t (ISIT) d e t e c t o r . In f a c t , the ISIT i s capable of d e t e c t i n g a s i n g l e p h o t o e l e c t r o n per r e s o l u t i o n element (500 channels i n the l i n e a r mode and a few thousands of r e s o l u t i o n elements i n the two-dimensional or random^access mode o f o p e r a t i o n ) . OIDs are now a v a i l a b l e which can be used as p a r a l l e l l i g h t d e t e c t o r s with up t o 2000 channels i n the l i n e a r mode o f o p e r a t i o n and many tens o f thousands i n the two-dimensional mode of



Figure I.

Diode structure and principles of operation of the silicon vidicon detector



Medium to high light level illumination

Of

(SNR) single channel detector

(SNR) TV detector

Submicrosecond to picosecond temporal range

Ultrarapid light phenomena

Ultralow light level illumination Preamplifier shot-noise limitation, e.g., astronomical and some luminescence measurements

Sampling or analysis-time limitations, e.g., insufficient amounts of samples for a destructive spectrometric analysis

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Measurements impossible to perform with a singlechannel, scanning spectrometer

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Uninterrupted long exposure of the target to the signal (signal buildup on target) followed by a single readout-integration

TSaN;

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Single readout or continuous read-and-store-in-memory accumulation

Fa

Fa

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F a (A/)'*; N .= number of individual spectral resolution elements simultaneously monitored by the TV detector

SNR advantage (F)* or time saving advantage (TS) Single readout or continuous read-and-store-in-memory accumulation

Advantages off multichannel (parallel) TV detectors

Photon shot-noise or preamplifier shot-noise limitations, or source-flicker-noise limitations but utilizing "source compensation" techniques

Low light illumination

Applications

Table 1


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9 i I


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M U L T I C H A N N E L I M A G E DETECTORS

operation. The r e l a t i v e m e r i t s of OIDs ( p a r a l l e l d e t e c t o r s ) , compared t o s i n g l e - c h a n n e l s p e c t r o m e t r i c d e t e c t o r s , are d e r i v e d from the s i m u l t a n e i t y ( m u l t i p l e x advantage) a t which whole s p e c t r a l r e g i o n s are detected. This f e a t u r e r e s u l t s i n a s i g n i f i c a n t improvement i n the measured SNR o r • i n a p r o p o r t i o n a l r e d u c t i o n i n the observ a t i o n time r e q u i r e d f o r the measurement. A summary of these advantages i s g i v e n i n Table I . The t h i r d mode of d e t e c t o r opera t i o n d e s c r i b e d i n t h i s Table, the " i n t e g r a t i o n " mode, i s very unique t o OIDs. At low temperatures, e.g., -50 C, the d e t e c t o r can be exposed t o u l t r a - l o w - l i g h t l e v e l s i g n a l s f o r long p e r i o d s of time (with the readout beam turned o f f ) u n t i l the " l a t e n t " e l e c t r i c a l , on-target-"developed", s i g n a l i s s u f f i c i e n t l y h i g h t o be r e a d - o f f the storage device (target) w i t h an adequate S/N r a t i o . In t h i s mode of d e t e c t i o n , subphoton-per-second l i g h t s i g n a l s can be measured q u a n t i t i v e l y , F i g . 2 . Moreover, because e n t i r e s p e c t r a are simultaneously monitored, any source f l u c t u a t i o n can be readi l y compensated f o r (vide i n f r a ) . Because of t h e i r accurate geometric r e g i s t r a t i o n , i . e . , s p e c t r a l - t o - s p a t i a l (channel p o s i t i o n ) t r a n s f o r m a t i o n , t h e i r i n t e g r a t i o n and storage f e a t u r e s , i . e . , OIDs are energy r a t h e r than power d e t e c t o r s , and because of t h e i r two-dimensional charact e r i s t i c s (area a r r a y ) , OIDs are p a r t i c u l a r l y s u i t e d f o r numerous computer-data h a n d l i n g and data p r o c e s s i n g techniques which can g r e a t l y f a c i l i t a t e the i n t e r p r e t a t i o n of raw s p e c t r o m e t r i c data. These techniques have been p r e v i o u s l y d i s c u s s e d elsewhere (4.) and w i l l be only b r i e f l y summarized here. Because the EG&G PARC o p t i c a l - m u l t i c h a n n e l a n a l y z e r (OMA) has been s p e c i f i c a l l y designed t o operate w i t h OIDs, i t w i l l be f r e quently r e f e r r e d t o i n the f o l l o w i n g d i s c u s s i o n , where v a r i o u s computer manipulations are d e s c r i b e d and assessed. Background and blank s u b t r a c t i o n : Since the OMA i s a curve (spectrum) manipulator, i t e a s i l y lends i t s e l f t o s u b t r a c t i o n of a background dark-charge (and "pattern") spectrum and/or a blank spectrum from each acquired a n a l y t e spectrum. The r e s u l t a n t anal y t e spectrum. The r e s u l t a n t a n a l y t e spectrum i s thus f r e e of any d e t e c t o r or blank e.g., s o l v e n t , d i s t o r t i o n s / F i g . 3. Channel-to-channel s p e c t r a l response c o r r e c t i o n : Normalizat i o n f a c t o r s necessary t o c o r r e c t f o r channel-to-channel s p e c t r a l response v a r i a t i o n s and v a r i a t i o n s i n the s p e c t r a l t r a n s f e r e f f i of the o p t i c a l system, e.g., g r a t i n g e f f i c i e n c y , can be s t o r e d i n memory and thus p r o v i d e the means f o r an automatic spectrum c o r r e c t i o n , F i g . 4. S p e c t r a l S t r i p p i n g : Mathematical c u r v e - f i t t i n g manipulations (5), r e q u i r i n g h i g h l y accurate geometric (wavelength) r e g i s t r a t i o n , can be performed i n order t o q u a n t i t a t i v e l y deconvolute mixtures of known i n d i v i d u a l components. Such techniques, s u c c e s s f u l l y implemented w i t h the OMA, can s i g n i f i c a n t l y reduce the number and


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10 msec) o f the t a r g e t . To o b t a i n the complete time and s p e c t r a l i n f o r m a t i o n cont a i n e d i n a s i n g l e event, e.g., f l a s h p h o t o l y s i s , the r o t a t i n g o p t i c a l s l i t , F i g . 9,10 , i s used. A wheel with s i x t y h o r i z o n t a l s l i t s , e q u a l l y spaced (10mm apart) i s p l a c e d i n f r o n t o f the entrance s l i t (10mm height) o f a spectrometer. R o t a t i o n o f the wheel causes one s l i t t o enter the spectrometer f i e l d e x a c t l y as the p r e v i o u s one leaves i t . Each p o s i t i o n o f the r o t a t i n g slit (time a x i s ) corresponds t o an OMA t r a c k a t the f o c a l plane o f the spectrometer. The OMA i s e x t e r n a l l y t r i g g e r e d by the event moni t o r e d , and data a c q u i s i t i o n , along the t a r g e t t r a c k s b e g i n s . When the experiment i s ocmpleted, a s h i f t - p r o g r a m i s u t i l i z e d t o p l a c e the i n i t i a l t r a c k (time zero) a t the top o f the data s e t . The temporal r e s o l u t i o n (time elapsed between two consecutive t r a c k s ) i s v a r i a b l e and i s p r a c t i c a l l y determined by the width o f the choppers' s l i t s ( and the corresponding number o f t r a c k s on the t a r g e t ) and by i t s v e l o c i t y . The performance o f the system i s l i m i t e d t o events whose p e r s i s t a n c e i s s h o r t e r than l / 6 0 t h o f a wheel r e v o l u t i o n - t i m e . Longer events w i l l cause a double exposure. However, with synchronous g a t i n g o f the d e t e c t o r , double exposure can be prevented (15). A t the present time, the temporal r e s o l u t i o n range p r o v i d e d by the r o t a t i n g s l i t i s approximately 8 jus t o 3 ms, however, use o f other chopping d e v i c e s , e.g., r o t a t i n g mirr o r s , should s i g n i f i c a n t l y extend i t . The time r e s o l v e d s p e c t r a of a xenon f l a s h (lamp) i s shown i n F i g . 11 . T h i s system can be a p p l i e d t o medium speed s p e c t r o m e t r i c k i n e t i c s t u d i e s i n c l u d i n g , stop-flow, T-jump, and f l a s h p h o t o l y s i s . 3. Microseconds t o nanoseconds: As p r e v i o u s l y mentioned, gated i n t e n s i f i e d v i d i c o n s can p r o v i d e temporal r e s o l u t i o n s as low as 40 nsec. New i n t e n s i f i e r s , e.g., microchannel p l a t e s , may extend t h i s range t o 1-5 nsec o r l e s s . However, by f a r , the most u s e f u l t o o l f o r ns t o ps spectroscopy i s the s t r e a k camera, i . e . , an u l t r a - r a p i d t e m p o r a l - t o - s p a t i a l e l e c t r o n i c image sweeper (16). Streak cameras convert an o p t i c a l s i g n a l , e.g., a spectrum, i n t o


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SPECTROMETER INPUT SLIT

Figure 9.

The experimental setup for studies of transient optical phenomena using a rotating optical slit

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Optoelectronic Image Detectors in Chemistry, An Overview - ACS

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