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The serial addition of sprite infrared detectors
002CU1891/88 53.00+ 0.00 PerolmonRarpk
Iqfmrd Phys.Vol.28, No. 4, pp. 271-278, 1988 Printedin GreatBritain
THE SERIAL ADDITION OF SPRITE INFRARED DETECTORS A. B. DEAN,’ P. N. J. DENNIS,’ C. T. ELLIOIT,’ D. H~BER? and J. T. M. WOTHERSPOON* ‘Royal Signals and Radar Establishment, Malvem, Worcestershire WR14 3PS, *Mullard Limited, Southarhpton. U.K.
U.K.and
(Received 11 March 1988) A&rat&-A new device structure for improving the sensitivity of SPRITE based thermal imaging systems by the serial addition of SPRITE filaments is described. By means of a number of readout electrades along a filament, a common bias supply can be used and the number of additional leads is minim&d. Results from 8 x 4 SPRITE structures operating in the 8-12 pm band at 80 K are presented.
INTRODUCTION
During the last few years major advances have been made in the development of new IR detectors for thermal imaging systems. The SPRITE detector, fabricated in Cadmium Mercury Telluride (CMT) which has been described previously(‘-3) has now been adopted for many high performance thermal imaging systems operating in both the medium and long wavelength bands. The device most commonly used in present systems consists of eight parallel filaments, each with one readout. The detector length is approximately 700 pm and the width 62.5 pm on a pitch of 75 pm. The readout contact is a bifurcated arm, which enables the array to be close packed and unstaggered. A particular feature of SPRITE detectors is the relative ease with which the thermal resolution can be increased by the addition of extra detectors on the focal plane. If these detectors are incorporated serially, in the scan direction, no change to the system optics or scanner is usually required and only minor modifications to the electronics. A technique is described in this paper which allows the serial addition to be carried out in a single filament, thereby minimising the number of leads and interconnects. A hybrid CMT silicon technology is described which has the potential for the future incorporation of additional focal plane signal processing in the substrate.
THE
CONCEPT
OF
SERIAL
ADDITION
Conceptually, serial addition of SPRITES could be performed in the same way as time-delayintegration for other types of detectors, as illustrated in Fig. l(a). For photoconductive elements this has the disadvantage that each device needs its own constant-current bias supply and the current must be supplied via leads into the cryogenic encapsulation, which can be a serious constraint for very large arrays. However, for the SPRITE detectors a considerably simpler solution is available as illustrated in Fig. l(b). A single filament of CMT is now used for each row of the 2-D array with only one bias supply. Pairs of readout contacts are equally spaced along the length with a separation greater than a minority carrier drift length. Consequently the noise at the different outputs is largely uncorrelated. By means of electronic time-delay-integration off the focal plane, an improvement in the signal-to-noise ratio, with respect to a single SPRITE will be obtained which is equal to the square-root of the number of readouts. Thus, there are two levels of integration one within the element and one performed conventionally. To bring out the readouts from the centre parts of a close-packed 2-D array, it has been necessary to develop a hybrid technology for the SPRITEs. Two such technologies have been investigated. In one a silicon substrate is prepared with diffused, high conductivity tracks to form the lead-outs from the array to bonding pads. Aluminium pads are also deposited on the other ends of the diffused tracks so that strap or loophole(*) connections may be made from the CMT to the tracks 271
A. B. DEANet al.
272
7
tMhEw
V&&y W3
(bl
Fig. 1. Schematic representation of the serial addition of (a) conventional devices, (b) SPRITE devices.
beneath. A CMT monolith is glued on top of the silicon tracks, isolation being ensured by an SiOz layer. The shape of the SPRITE filaments is defined by ion beam milling. The contacts from each readout to the correct diffused channel are fabricated by removing the glue and SiOz in the contact region and using a sputtered gold lead from the track, up and over the edge of the CMT filament, as shown schematically in Fig. 2. In the alternative process for the fabrication of substrates, trenches are chemically etched into a silicon substrate which is then oxidised for inter-track insulation. Aluminium tracks are then deposited in each trench by evaporation. The tops of the tracks are arranged to be coplanar with the surface of the substrate. The whole top surface of the substrate is coated with silicon nitride except for windows for readouts and wire bonding. This is to provide insulation and protection from mechanical damage. DESIGN
The aim of this device design was to replace an eight row SPRITE device in a system with an eight row by four serial array, which could improve the system sensitivity by a factor of 2. The original detector was 7OOpm long with 8 filaments on a 75 pm pitch and it was used in a
@TRAP
CONneT
li-;‘;.)_
CNT
Fig. 2. Schematic of an overlap contact used as a readout.
The serial addition of SPRITE IR detectors 4
273
l.O-
1 . % P
P d ii
.
0.s.
i
INTEQRATION
LENQTH turn)) -
Fig. 3. Signal integration along total length
serial/parallel system with f/2.7 optics and a scan velocity of 1.1 x 104cm s-‘. The fundamental concept of the design for this detector/encapsulation was that it should be able to be incorporated in the present system with the minimum of modification, It was with this constraint that the first device was designed. The usable sensitive area on the focal plane, as defined by the system optics, is approximately 2.25 mm square, thus the maximum length of each filament was nominally limited to 2250 pm and the width and pitch defined as for the previous detector. The bias field the device operates under is 30 V cm-‘, and the field of view f/2.7. In order to determine the optimum geometry of the device, the signal and its associated noise voltages at each readout must be determined. The signal is integated along the filament such that the carrier density at any position, y, is given by:
(1)
s=qT[l-exp(T)],
as shown in Fig. 3, where q is the quantum efficiency, cp is the photon flux and t is the thickness, I is the excess carrier drift length defined by: I= /LET,
(2)
where /r is the ambipolar mobility, E the bias field and T the minority carrier lifetime. If we consider a filament in which the separation of the readouts is x, and the length from the bias contact to the first readout is y, then: Sclj=rjT[l-exp-($)I, S(2)=q
:[I-exp-(?)I,
S(3l=q;[l
-exp-(Y+)],
S(4)=n$[l
-exp-(+)I,
The resulting signal after external time-delay and integration sintcgral =
i
n-l
S(n).
is then given by: (7)
Some of the optically generated excess carriers at readout n will survive recombination and will still exist at readout n + 1, giving correlated noise. In addition photo-generated carriers, produced
A. B. DEAN et al.
274
between readout n and readout n + 1 will give rise to uncorrelated noise. In order to determine the output noise voltage it is necessary to sum both the correlated and uncorrelated contributions for each of the readouts, summing linearly or in quadrature as appropriate. The noise at any single readout point along the filament is proportional to the square-root of the carrier density at that point. Thus the total noise at readout 1, V,,(l), is given by: V”,(1) = kS( l)‘i2,
(8)
where k is a constant. These noise carriers will then produce a correlated noise output at successive readouts, such that: = k
V,(l,j)
s(l)exp - VI”*,
(9)
where j = 14. The noise voltage at readout 2 can be calculated as follows. It contains a component, V,( 1,2), which is correlated to the noise at readout 1, and is given by equation (9) and also a value uncorrelated with readout 1, V,,(2,2) whose value is determined by the square-root of the difference between the total number of carriers, S(2), the number of carriers in the correlated component, i.e.:
. 112 1 12 exp _ (P 1 IR 1. l/2
V,(2,2) = k S(2) - S(l)exp
J’&, k)=
The noise at subsequent readouts which is correlated with V,(2,2) S(2) - S(l)exp - y
where k = 2-4. Similarly: V,(3,p)
S(3) - S(2)exp - 7
= [
exp_(k-2)x -,
is:
I
3)x
I
S(4) - S(3)exp - 7
V,(4,4) = [
(10)
(11)
(12)
(13)
These components can be represented as shown in Table I, where the horizontal components are fully correlated with each other and the vertical components are completely uncorrelated. V,,, V,, Vc and V,, are the linear sum of the rows and the total noise is determined by summing the components VA, Va, Vc and V, in quadrature: VTOU, = (Vi + v: + v; + v;)“?
(14)
The normalised signal-to-noise ratio can then be calculated from the values obtained in equations (7) and (14). This has then been plotted as a function of x in Fig. 4. For a filament length of 2268 pm the maximum separation is 560 pm, which gives a signal-tonoise ratio of -0.93. However, during the initial design of the detector there was some concern that an extra noise source could be generated in the bias contact, although in practice this was not Tabk I. Repmmtation
of the noise sourcu in 2-D SPRITE may Readout ttmber
Equation Equation Equation Equation
(9) (II) (12) (13)
I
2
3
4
UII) -
V(12) U22)
V(13) ~(23) V(33) -
K(M) ~(24) VW) VW)
-
Total comlated noir ---v* - VB - vc - vo 1 VT& equation (14)
‘rhe ti
0
addition
of SPRITE IR dnscton
roo MAD-OUT
800
460 OtmRAnon
hlml
*
Fig. 4. Normdimd S/N ratio U a function of madout acpamtion.
found to be the case. Consequently it was decided to make the first integration length, y, slightly greater than the subsequent readout separations. This also had the added bendit that the signal level S(l), as shown in Fig. 3 is increased and thus the amplifier requirements on this channel are less severe. The other constraint on the readout separations is to make them equivalent to an integral number of CCD clock pulses, as used in the external timc-deIay and integrate circuits. Hence a distance of 511 pm with an initial integration length of 647 pm was chosen, as iliustrated in Fig. 5. The resistance between the GMT taps and the input of the differential amplifiers is minim&d to reduce the Johnson noise. In the case of the silicon substrates highly.dopai tracks were used under the sensitive area, but evaporated aluminium films connected to the bonding pads around the substrate edge. The width of the tracks was cboscn to give identical resistances, thus the longer tracks to the centre tilamcnts are wider. The silicon used was 20 t2 cm p-type CZ Wacker (100) wafers. In the case of the substrates with meeased ahuninium tracks the maximum lead resistance was 2.5 0, compared with about 45 Q for the diffused track substrates. In both cases the substrates were designed on a 4.25 mm square carrier, which was the correct size to fit the encapsulation header. The method chosen to corincct the CMT 3Iament, with the appropriate diffused channel or embedded track was to use an overlap contact, and to enable this
ienoout Fig. 5. Layout for 2-D SPRlTk? may.
PICK-WCS
A. B. DEN+ et al.
Fig. 6. Photographs
to be ~~rnrn~at~ increasing the electric The CMT rnono~~s aligned with p~e~~n~ were used throughout readout contacts were shown in Fig. 6.
of 2-D SPRITE array.
the filament width was reduced in this region, This had the benefit of field in this zone, thus increasing the responsivity. were prepared by polishing and etching, then placed on the substrate and markers, and glued using an epoxy. Positive photor~~st piques the processing. The elements were contacted and divided and finally the deposited. An example of a device made by diffused track technology is
DEVICE
PERFORMANCE
The results reported below are for a device fabricated with recessed al~inium track technology. The CMT materiaf was prepared by Bridgman growth with a cut-off wavelength of - 11.0 #rn at 80 K and a carrier density of 2.3 x IO’”cmW3. A convenient method of assessing the unifo~ty of the filaments and the degree of integration is to scan a small spot of fight from a focussed He-Ne laser along the length. Two types of outputs are obtained; the one in Fig. 7(a) is the change in voltage across the length of the filament, measured with a very small bias field, This type of plot gives a measure of the filament uniformity. It is
QIBTllxcE
ALCWQ ZUMPLE &unl -)
tbf
00
Fig. 7. Spot scan mcawemcnts
Element number
3
4
f.?7E-+iS : 3 4
I.?4E+OS 1.88 E+ 03 1.998+03
1.34E+ll 1.19Et 11 1.14Ei it 1.14Ec 11
8.25E-09 9.ISE-(W I.O8E-f)8 1.09E-08
: 3 4
1.74 E + 03 IME+ 2.02 E + 03 l.%E.kOS
1.32Et 1.13Et 1.19E.e IAMES
11 11 II II
8.25 E - 09 9.tSE-09 I.%E-08 1.18E-06
t :
1.88E+03 I .47 E t 03 1.83EfOJ
1.38 E + I1 1.01 1.17E.t E+ 11 II
8.35E-09 9.158-09 9.41 E - 09
4
2.04E+05
1.06fz+ 11
1.21 E-08
:
1.80E+OS 1.88Ei.03 I.% E + OS 2.06E+03
1.34E.b 1.19Et 1.18ES 1.01 E+
II II It If
O&E-09 9.89 E - 09 1.03E-08 1.27E-OS
2.I3E+Of l&E+03 tI6E+Of ZISE+05 2.13E+03 2.16E+O3 2.13Et03
1.43E+ I.I8E+ I.t9E+ I.02Ei I.29E.k 1.24EC 1.ofJii+
I1 II II 11 II 11 II
9.30 E - 09 9.74E-09 1.14E-08 1.31 E-08 1.03E-08 l.OPE-08 1.24E-08
2.tOE+03 2.16EtO3 2.18E+05 2.02 E t 03
1.44Et 1.31 Et I.25 E + 9.28 E +
11 11 11 10
9.1SE-09 I.O3E-08 IBE-08 1.36E-08
1.83EfOf IME+ 2.12EtOS 2.18E-03
1.36ES 1.23E+ 1.21 Et 1.10E+
II 11 II II
8.55 E - 09 0.89 E - 09 IME-08 1.24E-OS
3 4 5
:
6
3 4 : 3 4
7
: 3 4
8
(a) along length of BIament, (b) across readouts.
Tabk 2. Performance of lypical SPRITE array Responsivity Readout W’O’OK~~~k~~, 1) Gmw?t number
t
2
2200
I ; 4
278
A. B. IhAn 81 al.
interesting to note the increased signal from the high reristanoe ngionr in the narrowed readouts. The sazond type of output shown in Fig. 7(b) is obtained at a field 30 V cm-’ and the readout is taken from each pair of readout ekctrodas. This plot shows the integration effect very clearly and also indicates that the degree of correlation between @acent readouts should be small. The performance of the device in terms of the usual detector parameters is summariscd in Table 2. The mean 500 K blackbody rcsponsivity of 31 outputs ir 1.97 x lo5 V W-’ and the mean 500 K blackbody detcctivity is 1.20 x 10” cm Hz’~ W-l as measured in f/2.7 field of view and a scan speed of 1.l x 10’cm s-‘. The effa$ive D* per row which results is 2.4 x 10” cm Hz’~ W-’ and the improved system sensitivity which can be achieved is reported by Braim.(s CONCLUSIONS A new CMT detector structure has been demonstrated which has the potential to improve the thermal sensitivity of SPRITE-ba&d imagcrs by a factor 2. To achieve this device multilayer technologies have been developed which allow close-packed, unstaggered 2-D arrays to be fabricated. The next step in this technology will be to include the t&delay-integration electronics on the focal plane in the silicon substrate thus reducing the dewar lcadout requirements even further and permitting the incorporation of even larger 2-D arrays in existing encapsulations. AcknowMgemrnr-Part of the work dcscribad wu rupported by DCVD. Crown copy-right 0 1988 REFERENCES I. C. T. Elliott, 2. C. T. Elliott, 3. A. Blrckbm, Prac. AIRDS 4. 1. Baker and 5. S. P. Bra&
E&crron. Lat. 17, 312 (1981). D. Day and D. J. Wilron, Itiored Phys. 22, 31 (1982). M. V. Btdunm, D. E. ChIton, W. A. E. Dunn. M. D. Janer, K. J. Oliver and J. T. M. Wotbcrspoon. Con/.. London (1981). R. A. Ballin@, SPIE Cqf. on infrared Techndogy ad Syams, Sao Diego (1984). to be published.
Iqfmrd Phys.Vol.28, No. 4, pp. 271-278, 1988 Printedin GreatBritain
THE SERIAL ADDITION OF SPRITE INFRARED DETECTORS A. B. DEAN,’ P. N. J. DENNIS,’ C. T. ELLIOIT,’ D. H~BER? and J. T. M. WOTHERSPOON* ‘Royal Signals and Radar Establishment, Malvem, Worcestershire WR14 3PS, *Mullard Limited, Southarhpton. U.K.
U.K.and
(Received 11 March 1988) A&rat&-A new device structure for improving the sensitivity of SPRITE based thermal imaging systems by the serial addition of SPRITE filaments is described. By means of a number of readout electrades along a filament, a common bias supply can be used and the number of additional leads is minim&d. Results from 8 x 4 SPRITE structures operating in the 8-12 pm band at 80 K are presented.
INTRODUCTION
During the last few years major advances have been made in the development of new IR detectors for thermal imaging systems. The SPRITE detector, fabricated in Cadmium Mercury Telluride (CMT) which has been described previously(‘-3) has now been adopted for many high performance thermal imaging systems operating in both the medium and long wavelength bands. The device most commonly used in present systems consists of eight parallel filaments, each with one readout. The detector length is approximately 700 pm and the width 62.5 pm on a pitch of 75 pm. The readout contact is a bifurcated arm, which enables the array to be close packed and unstaggered. A particular feature of SPRITE detectors is the relative ease with which the thermal resolution can be increased by the addition of extra detectors on the focal plane. If these detectors are incorporated serially, in the scan direction, no change to the system optics or scanner is usually required and only minor modifications to the electronics. A technique is described in this paper which allows the serial addition to be carried out in a single filament, thereby minimising the number of leads and interconnects. A hybrid CMT silicon technology is described which has the potential for the future incorporation of additional focal plane signal processing in the substrate.
THE
CONCEPT
OF
SERIAL
ADDITION
Conceptually, serial addition of SPRITES could be performed in the same way as time-delayintegration for other types of detectors, as illustrated in Fig. l(a). For photoconductive elements this has the disadvantage that each device needs its own constant-current bias supply and the current must be supplied via leads into the cryogenic encapsulation, which can be a serious constraint for very large arrays. However, for the SPRITE detectors a considerably simpler solution is available as illustrated in Fig. l(b). A single filament of CMT is now used for each row of the 2-D array with only one bias supply. Pairs of readout contacts are equally spaced along the length with a separation greater than a minority carrier drift length. Consequently the noise at the different outputs is largely uncorrelated. By means of electronic time-delay-integration off the focal plane, an improvement in the signal-to-noise ratio, with respect to a single SPRITE will be obtained which is equal to the square-root of the number of readouts. Thus, there are two levels of integration one within the element and one performed conventionally. To bring out the readouts from the centre parts of a close-packed 2-D array, it has been necessary to develop a hybrid technology for the SPRITEs. Two such technologies have been investigated. In one a silicon substrate is prepared with diffused, high conductivity tracks to form the lead-outs from the array to bonding pads. Aluminium pads are also deposited on the other ends of the diffused tracks so that strap or loophole(*) connections may be made from the CMT to the tracks 271
A. B. DEANet al.
272
7
tMhEw
V&&y W3
(bl
Fig. 1. Schematic representation of the serial addition of (a) conventional devices, (b) SPRITE devices.
beneath. A CMT monolith is glued on top of the silicon tracks, isolation being ensured by an SiOz layer. The shape of the SPRITE filaments is defined by ion beam milling. The contacts from each readout to the correct diffused channel are fabricated by removing the glue and SiOz in the contact region and using a sputtered gold lead from the track, up and over the edge of the CMT filament, as shown schematically in Fig. 2. In the alternative process for the fabrication of substrates, trenches are chemically etched into a silicon substrate which is then oxidised for inter-track insulation. Aluminium tracks are then deposited in each trench by evaporation. The tops of the tracks are arranged to be coplanar with the surface of the substrate. The whole top surface of the substrate is coated with silicon nitride except for windows for readouts and wire bonding. This is to provide insulation and protection from mechanical damage. DESIGN
The aim of this device design was to replace an eight row SPRITE device in a system with an eight row by four serial array, which could improve the system sensitivity by a factor of 2. The original detector was 7OOpm long with 8 filaments on a 75 pm pitch and it was used in a
@TRAP
CONneT
li-;‘;.)_
CNT
Fig. 2. Schematic of an overlap contact used as a readout.
The serial addition of SPRITE IR detectors 4
273
l.O-
1 . % P
P d ii
.
0.s.
i
INTEQRATION
LENQTH turn)) -
Fig. 3. Signal integration along total length
serial/parallel system with f/2.7 optics and a scan velocity of 1.1 x 104cm s-‘. The fundamental concept of the design for this detector/encapsulation was that it should be able to be incorporated in the present system with the minimum of modification, It was with this constraint that the first device was designed. The usable sensitive area on the focal plane, as defined by the system optics, is approximately 2.25 mm square, thus the maximum length of each filament was nominally limited to 2250 pm and the width and pitch defined as for the previous detector. The bias field the device operates under is 30 V cm-‘, and the field of view f/2.7. In order to determine the optimum geometry of the device, the signal and its associated noise voltages at each readout must be determined. The signal is integated along the filament such that the carrier density at any position, y, is given by:
(1)
s=qT[l-exp(T)],
as shown in Fig. 3, where q is the quantum efficiency, cp is the photon flux and t is the thickness, I is the excess carrier drift length defined by: I= /LET,
(2)
where /r is the ambipolar mobility, E the bias field and T the minority carrier lifetime. If we consider a filament in which the separation of the readouts is x, and the length from the bias contact to the first readout is y, then: Sclj=rjT[l-exp-($)I, S(2)=q
:[I-exp-(?)I,
S(3l=q;[l
-exp-(Y+)],
S(4)=n$[l
-exp-(+)I,
The resulting signal after external time-delay and integration sintcgral =
i
n-l
S(n).
is then given by: (7)
Some of the optically generated excess carriers at readout n will survive recombination and will still exist at readout n + 1, giving correlated noise. In addition photo-generated carriers, produced
A. B. DEAN et al.
274
between readout n and readout n + 1 will give rise to uncorrelated noise. In order to determine the output noise voltage it is necessary to sum both the correlated and uncorrelated contributions for each of the readouts, summing linearly or in quadrature as appropriate. The noise at any single readout point along the filament is proportional to the square-root of the carrier density at that point. Thus the total noise at readout 1, V,,(l), is given by: V”,(1) = kS( l)‘i2,
(8)
where k is a constant. These noise carriers will then produce a correlated noise output at successive readouts, such that: = k
V,(l,j)
s(l)exp - VI”*,
(9)
where j = 14. The noise voltage at readout 2 can be calculated as follows. It contains a component, V,( 1,2), which is correlated to the noise at readout 1, and is given by equation (9) and also a value uncorrelated with readout 1, V,,(2,2) whose value is determined by the square-root of the difference between the total number of carriers, S(2), the number of carriers in the correlated component, i.e.:
. 112 1 12 exp _ (P 1 IR 1. l/2
V,(2,2) = k S(2) - S(l)exp
J’&, k)=
The noise at subsequent readouts which is correlated with V,(2,2) S(2) - S(l)exp - y
where k = 2-4. Similarly: V,(3,p)
S(3) - S(2)exp - 7
= [
exp_(k-2)x -,
is:
I
3)x
I
S(4) - S(3)exp - 7
V,(4,4) = [
(10)
(11)
(12)
(13)
These components can be represented as shown in Table I, where the horizontal components are fully correlated with each other and the vertical components are completely uncorrelated. V,,, V,, Vc and V,, are the linear sum of the rows and the total noise is determined by summing the components VA, Va, Vc and V, in quadrature: VTOU, = (Vi + v: + v; + v;)“?
(14)
The normalised signal-to-noise ratio can then be calculated from the values obtained in equations (7) and (14). This has then been plotted as a function of x in Fig. 4. For a filament length of 2268 pm the maximum separation is 560 pm, which gives a signal-tonoise ratio of -0.93. However, during the initial design of the detector there was some concern that an extra noise source could be generated in the bias contact, although in practice this was not Tabk I. Repmmtation
of the noise sourcu in 2-D SPRITE may Readout ttmber
Equation Equation Equation Equation
(9) (II) (12) (13)
I
2
3
4
UII) -
V(12) U22)
V(13) ~(23) V(33) -
K(M) ~(24) VW) VW)
-
Total comlated noir ---v* - VB - vc - vo 1 VT& equation (14)
‘rhe ti
0
addition
of SPRITE IR dnscton
roo MAD-OUT
800
460 OtmRAnon
hlml
*
Fig. 4. Normdimd S/N ratio U a function of madout acpamtion.
found to be the case. Consequently it was decided to make the first integration length, y, slightly greater than the subsequent readout separations. This also had the added bendit that the signal level S(l), as shown in Fig. 3 is increased and thus the amplifier requirements on this channel are less severe. The other constraint on the readout separations is to make them equivalent to an integral number of CCD clock pulses, as used in the external timc-deIay and integrate circuits. Hence a distance of 511 pm with an initial integration length of 647 pm was chosen, as iliustrated in Fig. 5. The resistance between the GMT taps and the input of the differential amplifiers is minim&d to reduce the Johnson noise. In the case of the silicon substrates highly.dopai tracks were used under the sensitive area, but evaporated aluminium films connected to the bonding pads around the substrate edge. The width of the tracks was cboscn to give identical resistances, thus the longer tracks to the centre tilamcnts are wider. The silicon used was 20 t2 cm p-type CZ Wacker (100) wafers. In the case of the substrates with meeased ahuninium tracks the maximum lead resistance was 2.5 0, compared with about 45 Q for the diffused track substrates. In both cases the substrates were designed on a 4.25 mm square carrier, which was the correct size to fit the encapsulation header. The method chosen to corincct the CMT 3Iament, with the appropriate diffused channel or embedded track was to use an overlap contact, and to enable this
ienoout Fig. 5. Layout for 2-D SPRlTk? may.
PICK-WCS
A. B. DEN+ et al.
Fig. 6. Photographs
to be ~~rnrn~at~ increasing the electric The CMT rnono~~s aligned with p~e~~n~ were used throughout readout contacts were shown in Fig. 6.
of 2-D SPRITE array.
the filament width was reduced in this region, This had the benefit of field in this zone, thus increasing the responsivity. were prepared by polishing and etching, then placed on the substrate and markers, and glued using an epoxy. Positive photor~~st piques the processing. The elements were contacted and divided and finally the deposited. An example of a device made by diffused track technology is
DEVICE
PERFORMANCE
The results reported below are for a device fabricated with recessed al~inium track technology. The CMT materiaf was prepared by Bridgman growth with a cut-off wavelength of - 11.0 #rn at 80 K and a carrier density of 2.3 x IO’”cmW3. A convenient method of assessing the unifo~ty of the filaments and the degree of integration is to scan a small spot of fight from a focussed He-Ne laser along the length. Two types of outputs are obtained; the one in Fig. 7(a) is the change in voltage across the length of the filament, measured with a very small bias field, This type of plot gives a measure of the filament uniformity. It is
QIBTllxcE
ALCWQ ZUMPLE &unl -)
tbf
00
Fig. 7. Spot scan mcawemcnts
Element number
3
4
f.?7E-+iS : 3 4
I.?4E+OS 1.88 E+ 03 1.998+03
1.34E+ll 1.19Et 11 1.14Ei it 1.14Ec 11
8.25E-09 9.ISE-(W I.O8E-f)8 1.09E-08
: 3 4
1.74 E + 03 IME+ 2.02 E + 03 l.%E.kOS
1.32Et 1.13Et 1.19E.e IAMES
11 11 II II
8.25 E - 09 9.tSE-09 I.%E-08 1.18E-06
t :
1.88E+03 I .47 E t 03 1.83EfOJ
1.38 E + I1 1.01 1.17E.t E+ 11 II
8.35E-09 9.158-09 9.41 E - 09
4
2.04E+05
1.06fz+ 11
1.21 E-08
:
1.80E+OS 1.88Ei.03 I.% E + OS 2.06E+03
1.34E.b 1.19Et 1.18ES 1.01 E+
II II It If
O&E-09 9.89 E - 09 1.03E-08 1.27E-OS
2.I3E+Of l&E+03 tI6E+Of ZISE+05 2.13E+03 2.16E+O3 2.13Et03
1.43E+ I.I8E+ I.t9E+ I.02Ei I.29E.k 1.24EC 1.ofJii+
I1 II II 11 II 11 II
9.30 E - 09 9.74E-09 1.14E-08 1.31 E-08 1.03E-08 l.OPE-08 1.24E-08
2.tOE+03 2.16EtO3 2.18E+05 2.02 E t 03
1.44Et 1.31 Et I.25 E + 9.28 E +
11 11 11 10
9.1SE-09 I.O3E-08 IBE-08 1.36E-08
1.83EfOf IME+ 2.12EtOS 2.18E-03
1.36ES 1.23E+ 1.21 Et 1.10E+
II 11 II II
8.55 E - 09 0.89 E - 09 IME-08 1.24E-OS
3 4 5
:
6
3 4 : 3 4
7
: 3 4
8
(a) along length of BIament, (b) across readouts.
Tabk 2. Performance of lypical SPRITE array Responsivity Readout W’O’OK~~~k~~, 1) Gmw?t number
t
2
2200
I ; 4
278
A. B. IhAn 81 al.
interesting to note the increased signal from the high reristanoe ngionr in the narrowed readouts. The sazond type of output shown in Fig. 7(b) is obtained at a field 30 V cm-’ and the readout is taken from each pair of readout ekctrodas. This plot shows the integration effect very clearly and also indicates that the degree of correlation between @acent readouts should be small. The performance of the device in terms of the usual detector parameters is summariscd in Table 2. The mean 500 K blackbody rcsponsivity of 31 outputs ir 1.97 x lo5 V W-’ and the mean 500 K blackbody detcctivity is 1.20 x 10” cm Hz’~ W-l as measured in f/2.7 field of view and a scan speed of 1.l x 10’cm s-‘. The effa$ive D* per row which results is 2.4 x 10” cm Hz’~ W-’ and the improved system sensitivity which can be achieved is reported by Braim.(s CONCLUSIONS A new CMT detector structure has been demonstrated which has the potential to improve the thermal sensitivity of SPRITE-ba&d imagcrs by a factor 2. To achieve this device multilayer technologies have been developed which allow close-packed, unstaggered 2-D arrays to be fabricated. The next step in this technology will be to include the t&delay-integration electronics on the focal plane in the silicon substrate thus reducing the dewar lcadout requirements even further and permitting the incorporation of even larger 2-D arrays in existing encapsulations. AcknowMgemrnr-Part of the work dcscribad wu rupported by DCVD. Crown copy-right 0 1988 REFERENCES I. C. T. Elliott, 2. C. T. Elliott, 3. A. Blrckbm, Prac. AIRDS 4. 1. Baker and 5. S. P. Bra&
E&crron. Lat. 17, 312 (1981). D. Day and D. J. Wilron, Itiored Phys. 22, 31 (1982). M. V. Btdunm, D. E. ChIton, W. A. E. Dunn. M. D. Janer, K. J. Oliver and J. T. M. Wotbcrspoon. Con/.. London (1981). R. A. Ballin@, SPIE Cqf. on infrared Techndogy ad Syams, Sao Diego (1984). to be published.