CCD in the electron-in mode, coupled with a bi-alkali photocathode to produce UV photon conversion, provides the following desirable features: (1) high UV response of the bi-alkali photocathode; (2) excellent imaging quality of a CCD area array; and (3) high signal-to-noise ratio due to the EBS (electron bombarded silicon) gain of the CCD operating in a tube configuration. This paper describes the rationale and progress made in developing a CCD for use as an UV imager

Barton, J. B.

Chan, W. W.

Collins, D. R.

Rhines, W. C.

Roberts, C. G.

Sobieski, S.

English

NASA Technical Reports Server

DEVELOPMENT O F  A CCD FOR ULTRAVIOLET IMAGING 
USING A CCD PHOTOCATHODE COMBINATION* 
D. R. Collins, C. G. Roberts, W. W. Chan, W. C. Rhines, and J. B. Barton 
Texas Instruments Incorporated 
Dallas, Texas 
S .  Sobieski 
Goddard Space Flight Center 
Greenbelt, Maryland 
For  many space experiments, imaging and 
spectroscopy require excellent ultraviolet sensi- 
tivity. CCD imagers are promising sensors because 
of their low power, light weight, and high reliability. 
However, high responsivity at wavelengths below 0 .4  
micron is difficult to achieve with silicon CCDs even 
using thinned backside illumination, 
CCD in the electron-in mode, coupled with a bi-alkali 
photocathode to produce UV photon conversion, will 
provide the following desirable features: 
The use of a 
( 1 )  High UV response of the bi-alkali 
photo cathode 
Excellent imaging quality of a CCD 
area a r r ay  
High signal-to-noise ratio due to the EBS 
(electron bombarded silicon) gain of the 
CCD operating in a tube configuration 
(2) 
( 3 )  
This paper describes the rationale and progress 
made in  developing a CCD for use as an UV imager. 
>#This work is partially supported by the U. S. Army N V L  and NASA 
Goddard (DAAKOZ - 7 4 - C - 0 3 5 9 ). 
163 
https://ntrs.nasa.gov/search.jsp?R=19750020765 2020-03-22T19:54:10+00:00Z
The CCD area array geometry, and electrical and 
optical characteristics a re  described along with the 
technology used to fabricate the sensor. 
tages of using a thinned backside illuminated CCD 
in order to maintain MTF (modulation transfer func- 
tion) and gain for the electron-in mode of operation 
are explained. Experimental data obtained using a 
scanning electron microscope as the signal source is 
presented on gain vs. accelerating voltage and MTF 
degradation due to lateral carr ier  diffusion from the 
back surface. 
method of mounting the CCD within a tube structure 
to achieve backside illumination, cooling, multiple 
electrical connection to the CCD in the tube vacuum, 
and minimal cross  -contamination between the CCD 
and the photocathode. 
The advan- 
Detailed discussion is given on the 
I. INTRODUCTION 
Ultraviolet imaging and spectroscopy a r e  primary mission goals for a 
number of planned space experiments. 
violet from a platform above the absorbing terrestr ia l  atmosphere (0.3-micron 
cutoff) affords a number of advantages, including: (1) increased angular resolu- 
tion, ( 2 )  extended wavelength baseline, (3) accessibility of important spectral 
resonance transitions, and (4) data on extremely hot o r  nonthermal sources 
nearer to their peak. 
cal f i l ters and spectroscopy, coupled with spatial resolution perpendicular to 
the wavelength dispersion direction, will also provide important new informa - 
tion. 
particularly by observations of the stars in  hot-terminal stages of evolution, 
spatial characterization of the physical conditions within hot extended inter stel- 
lar clouds, and the delineation of the physics and structure of the central regions 
of normal and high -excitation galaxies. 
The capability for observing in  the ultra- 
The imaging of faint sources through band-limiting opti- 
Representative planned programs include the study of stellar evolution, 
Figure 1 shows the spectral responsivity of photon-in (direct view) thinned 
It may be seen that typical responsivity is unsatis- backside illuminated CCDs. 
factory below 0.4-micron wavelengths. But it is possible to fabricate devices 
164 
with near -ideal backside accumulated surfaces and obtain near -theoretical 
quantum efficiencies at 0.4 micron (absorption length 0. 13 micron in Si). 
ever, examination of spectral resonance transitions below 0.3 micron (50-A 
absorption length), such as the Lyman Q line at 0.1216 micron, is not feasible 
with a photon-in CCD. 
How- 
The purpose of this paper is to describe recent work in the development of 
an ultraviolet converter tube which will be particularly suited for space imaging 
o r  imaging spectroscopy. 
convert ultraviolet (and visible) photons into photoelectrons, which in  turn  are 
focused by an applied electrostatic field onto a thinned charge-coupled device in 
close proximity to the photocathode. 
CCD provides a target for the photoelectrons with an inherent gain depending 
upon the accelerating voltage applied. 
The tube uses a photocathode/window combination to 
In this electron bombardment mode, the 
II. PHOTOCATHODEITUBE SELECTION 
A number of different photoemissive materials/window combinations can 
be used to produce the photoelectrons. 
cie s* of several combinations involving semi-transparent coatings which a r e  
suitable for U V  applications. 
for the developmental tube since it provides sensitivity in the visible and UV, 
thereby simplifying testing and evaluation. 
based telescope is therefore possible. 
Figure 2 illustrates the quantum efficien- 
The Bi-alkali/MgFz combination has been selected 
Immediate application at a ground- 
Quartz has an optical cutoff at approximately 0.2 micron and hence is of 
limited use as window material. This 0.2-micron cutoff of quartz also prevents 
the use of conventional curved fiber optic windows used in electrostatic inverter 
tubes. Hence, for U V  applications, the flat window requires the use of either a 
proximity, magnetically focused, o r  pentode tube. From the standpoint of size, 
the proximity tube, has a distinct advantage since the photocathode and CCD a r e  
separated by only -0. 1 inch. However, the electric fields developed in such a 
tube structure may limit the accelerating potential that can be applied. 
*Ref. EMR Photoelectric, wall chart. 
165 
III. CCD TARGET FABRICATION 
The CCD imager chosen to be incorporated with a photocathode is a 160 X 
100 resolution element, n-channel, buried-channel, double-level A1 -AlzO3-Ad 
metallization, thinned, backside illuminated structure, This CCD imager is 
discussed in  detail in a companion paper (Ref. 1). 
sor is shown in Figure 3. 
impinging electrons, and thinning is necessary to maintain spatial resolution 
near the Nyquist limit. 
the CCD from the front (metallized) side, the presence of the necessary gate 
oxide layer would eventually result in the buildup of charge in the oxide layer 
and drastically affect operation of the CCD. 
A1 metallization surface-channel CCD was found to degrade within a few seconds 
when operated while being irradiated on the front (metallized) side in a SEM 
(scanning electron microscope, 15-kV acceleration voltage). The same device 
could be operated in the SEM for a period of days with no degradation, with the 
electrons impinging on the thinned back surface. 
A cross  section of the sen- 
The silicon membrane serves  as the target for 
Although in principle, the electrons could impinge upon 
The performance of a single-level 
IV. DESIRABLE PROPERTIES O F  A CCD MOUNTED WITHIN A TUBE 
The features desired o r  necessary for incorporating a CCD into a photo- 
cathode tube are listed below: 
Conventional semiconductor integrated circuit techniques for 
mounting the CCD. 
Parallel photocathode and CCD surfaces. 
Multiple electrical connections (-30) from the CCD to the tube 
exterior. 
Sturdy electrical connections (pins) at  the r ea r  rather than the 
side of the tube for ease in  electrical testing. 
Simple CCD header, which is part of the vacuum tube wall. 
Minimization of c ross  contamination between the CCD/ header 
and the photocathode. 
Header construction to facilitate cooling the CCD. 
Header construction to allow pressure equalization across the thin 
membrane, especially during vacuum evacuation of the tube. 
166 
( 9 )  Header construction to withstand the tube temperature bake cycles 
(-350 to 400°C) without harming the CCD. 
The ease with which the above properties can be achieved involves compli- 
cated tradeoffs in mounting and header, CCD, and tube fabrication. 
V. HEADER CONSTRUCTION 
Mounting a thinned backside illuminated CCD within a tube structure pre-  
sents additional problems over those encountered in mounting a frontside illumi- 
nated device. 
electrical connections to be made to the CCD. 
the CCD in such a fashion would be to flip-chip mount the CCD to a circular 
ceramic header with electrical feedthroughs. However, the concept of flip-chip 
mounting a CCD, much less a thinned CCD, would require a substantial CCD 
fabrication development effort. 
is mounted must have an opening in  it to expose the thinned silicon surface to the 
electron flux. Since the thin membrane will not withstand the pressure differen- 
tial of the vacuum, the CCD itself cannot be part of the vacuum wall. 
requires an additional vacuum wall element. 
making multiple external electrical connections requires the substrate to be part 
of the vacuum wall. 
"cover" over the CCD and pressure relief vents between the main tube volume 
and the cover cavity. 
The thinned surface must face the photocathode and yet allow 
An obvious method of mounting 
Hence the ceramic substrate on which the CCD 
This 
However, the desire for ease in 
This feature in  turn requires that the header have both a 
A proposed CCD header configuration is  shown in  Figure 4. The ceramic 
substrate is brazed to a Kovar reentrant flange. 
is formed in  two layers, providing a buried metallization path from the bond 
pads to the connector pins, which eliminates a potential vacuum leakage path. 
An alloy stage is brazed to the bottom of the ceramic substrate for mounting the 
thinned CCD. A seal flange is brazed to the top of the ceramic substrate. After 
the CCD has been alloyed to the alloy stage and bonded to the bond pads, a Kovar 
cover is heliarc welded to the seal flange, providing the outer vacuum wall. 
small hole through the ceramic substrate provides the pressure relief vent 
between the main part of the tube and the CCD cavity, 
sealed, the tube flange may be heliarced to the main tube body. 
The ceramic substrate itself 
A 
After the cover has been 
This header design has all of the desirable features listed above. The 
alloy stage provides a good thermal expansion match to the silicon CCD during 
167 
the prolonged 350-400" C tube fabrication process. 
contamination between the CCD and the photocathode is through the small vent 
holes. While the contamination of the CCD MOS structure by alkali vapors is  a 
real  concern, remote processing of the photocathode can reduce the probability 
of this occurrence. Should cross  Contamination prove to be a problem, it is 
possible to eliminate the vent holes and evacuate the two regions separately. 
Attaching a thermoelectric cooler to the Koval cover will permit easy cooling of 
the CCD. Accurate registration and planarity of the CCD to the photocathode a re  
assured by maintaining a flat surface on the alloy stage, and by proper alignment 
of the tube flange to the alloy stage. 
The only possibility for cross  
VI. SIMULATED TUBE OPERATION USING A SCANNING 
ELECTRON MICROSCOPE 
In order to obtain information on the electron-in mode of CCD operation 
prior to actual tube operation, a SEM was utilized. 
performed in this experiment, critical performance in the 8- to 20-kV accelerat- 
ing potential range was obtained. Figure 5 shows a plot of EBS (electron bom- 
barded silicon) gain vs. acceleration voltage for three different CCDs. For 
comparison, both theoretical and typical SIT (silicon intensified target) tube 
gains a re  presented. 
devices were tested with the electron beam at  a 45-deg angle of incidence to the 
target, increasing the effective "dead voltage. These devices were also fabri- 
cated with blue responses (0.4 micron) of less  than 1070, indicating insufficient 
back surface accumulation. 
response will provide lower dead voltages and higher gains. 
While no imaging could be 
The gains obtained a re  less  than ideal; however, the 
It is anticipated that sensors with near -ideal blue 
The effect of lateral car r ie r  diffusion on the MTF (modulation transfer 
function) for the electron-in mode of operation is similar to the results obtained 
with blue light. 
depletion depth is approximately 4 microns. 
brane is approximately 10-12 microns. 
microns in thickness, compared to the 23 -micron center -to-center separation 
of the resolution elements. 
diffusion is small. 
beam position, as a beam is traversed across adjacent resolution elements. 
For the resistivity of the p-type substrate used (8 52-cm), the 
The thickness of the thinned mem- 
Thus the undepleted silicon is only 6-8 
Thus the degradation in MTF due to lateral charge 
This may be seen in Figure 6 from a plot of output signal vs. 
168 
VII. CONCLUSIONS 
While the operation of a CCD in conjunction with a photocathode for UV 
imaging remains to be demonstrated, many problems concerned with this con- 
cept have been satisfactorily addressed. 
minated CCD has demonstrated gains of 3000 and excellent resolution. 
more, the difficult technology needed for mounting a thinned CCD within a tube 
appears feasible. Major questions still to be resolved involve operation and 
reliability of the photocathode and CCD once the tube is activated. 
The thinned electron-in backside illu- 
Further- 
While the major emphasis of this paper has been UV imaging, it is appro- 
priate to comment on the much broader consequences of this EBS CCD operation. 
The gains possible with a CCD/photocathode combination are not only useful for 
faint  UV imaging but are also extremely important for all LLLTV (low light level 
TV) applications. 
noise of the CCD (less than 100 electrons/pixel at room temperature), a simple 
CCD/ photocathode proximity tube should provide equivalent preformance to an 
ISIT (intensified SIT) tube. 
CCD at a sufficiently high level and reducing the noise such that G is approxi- 
mately ten times the rms noise equivalent number of electrons per packet, one 
should be able to sense individual photon events, since the signal charge per 
packet resulting from a single photoelectron will be approximately G. 
Due to the high gain preceding the CGD and the inherent low 
Furthermore, by maintaining the gain G of the EBS 
REFERENCE 
1. G. A. Antcliffe, L. J. Hornbeck, J. M. Younse, J. B. Barton, D. R. 
Collins, "Large A r e a  CCD Imagers for Spacecraft Applications, " Pro-  
ceedings of this Symposium. 
169 
9 160 x 100 BEST 
A 160 x 100 TYPICAL 
INCIDENT WAVELENGTH (MICRONS) 
Figure 1. Spectral responsivity of near -ideal and typical thinned backside 
illuminated CCD area imagers 
170 
60 
40 
I I I I I I 
SPECTRAL CHARACTER I ST ICs - 
20 
L 
u, w 
f 
10 
8 
6 
4 
2 
1 
100 
20 
0.1 0.2 0.3 0.4 0. 5 0.6 0.7 0.8 
L-. Visible .-_I 
Spect ru rn 
WAVELENGTH - pm 
Figure 2. Quantum efficiencies of different photocathode/window combinations 
as a function of wavelength (Transmission of the earth 's  atmosphere 
as a function of wavelength is also presented. ) 
171 
"2'3 
Metal 
Electrode 
U Incident Light U 
Figure 3. Cross section of a thinned CCD area imager 
(Membrane thickness is approximately 
10-12 microns. ) 
Tube Flange , 
33 Connector Pins 
/ 
Bond Pad Metallization 
\ Buried Metallization 
\ 
Ceramic Substrate 
Figure 4. CCD header configuration 
172 
E6S CCD GAIN 
z ACCELERATING VOLVAGE 
83 loo 
3 
v) 
10 
0 2 4 6 8 10 12 14 16 18 (KV) 
Figure 5. EBS gain of three CCDs VS. acceleration voltage. 
typical SIT data shown for comparison (Data taken with e-beam at 
45" incident angle to the thinned silicon surface. ) 
Theoretical and 
173 
CCD RESPONSE SPOT SCAN 
100 
90 
80 
70 
60 
50 
40 
30 
20 
10 
0 
I 
I 0 ~ 
1 Pixel 
-(O. 9 mils)- 
Spot Position 
I Channel Stops 
I Across Channel 
stops 
\ *  0
Figure 6. Plot of output signal vs. e-beam position relative to the pixel bound- 
a ry  for 'a  0. 9 X 0. 9 mil pixel (Data is presented for e-beam motion 
both parallel and perpendicular to the channel stops; e-beam accel- 
eration voltages of 8-10 kV were used with effective range of 
approximately 1 micron. Variation i n  data between the parallel and 
perpendicular data may be primarily due to inaccuracy i n  focusing 
of the e-beam on the silicon surface. ) 
174 


Development of a CCD for ultraviolet imaging using a CCD photocathode combination

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