A 512 x 320 element charge coupled imager has been fabricated and tested under cooled slow scan conditions to evaluate the device's performance under a variety of long integration and slow scan readout conditions. Operation in a low blooming mode of operation enables the sensor to be exposed with portions of the image heavily overexposed, without those areas spreading into adjacent picture areas. Device design, layout, and operating mode are described, and presents experimental results, including displayed images taken under cooled slow scan operation, are reported

Giovachino, D. L.

Rodgers, R. L., III

English

NASA Technical Reports Server

COOLED SLOW-SCAN PERFORMANCE 
O F  A 512 X 320 ELEMENT CHARGE-COUPLED IMAGER* 
R. L. Rodgers III and D. L, Giovachino 
RCA Corporation 
Electronic Components 
Lancaster, Pennsylvania 
A 512 X 320 element charge-coupled imager 
has been fabricated and tested under cooled slow- 
scan conditions to evaluate the device's performance 
under a variety of long integration and slow-scan 
readout conditions. Operation in a low-blooming 
mode of operation enables the sensor to be exposed 
with portions of the image heavily overexposed, 
without those a reas  spreading into adjacent picture 
areas. 
layout, and operating mode, and presents experi- 
mental results, including displayed images taken 
under cooled slow - scan operation. 
This paper describes the device design, 
A 512 X 320 element imager based on charge-coupled device (CCD) 
technology has been fabricated for use in a variety of television applications, 
including those applications requiring slow-scan operation. 
cell count has been described previously (Ref. 1). 
layer doped polysilicon gate structure (Ref. 2). 
by doping Nt regions in a P-layer of polysilicon on top of the channel oxide. 
This results in low leakage between gates, 
stable sealed channel structure results. 
most wavelengths, and there are no opaque areas to cause aliasing. 
spectral response range extends from 420 nM to 11 00 nM (Ref. 3). 
The choice of this 
This device utilizes a single- 
The individual gates are formed 
Since there a r e  no exposed gaps, a 
The polysilicon layer is transparent to 
The useful 
*This work was performed for the Jet Propulsion Laboratory, California 
Institute of Technology, sponsored by the National Aemnautics and Space 
Administration, under Contract No. NAS 7-1 00. 
116 
https://ntrs.nasa.gov/search.jsp?R=19750020761 2020-03-22T19:53:54+00:00Z
The device is organized into a three-phase vertical frame transfer system 
(Ref .  3). N-surface channel operation is utilized. An electrical bias charge 
(fat zero) is inserted into the horizontal register to improve horizontal resolu- 
tion. An optical bias light is used to introduce bias charge into the body of the 
image area, 
Conventional television requirements involve 'observation of objects in 
motion in real  time in a similar fashion to motion picture filming. 
in relatively short exposure times to avoid image motion smearing and flicker 
in the display. 
a t  a fast readout rate to keep up with the picture rate (i. e., 30 frames/sec and 
a 6-MHz data rate). In many scientific imaging applications, the requirement 
to record, transmit, o r  process video at  high data rates presents a problem. 
In those applications where a single frame of video is sufficient, slow-scan 
readout may be used following an exposure to reduce the data rate, 
exposure time may be lengthened to expose faint images, as is done with film, 
Both of these techniques place severe requirements on the dark charge genera- 
tion rate and signal handling capability of the sensor, 
have experimentally exhibited smaller signal handling capability and higher dark 
current generation rates than surface-channel devices. 
choice of a surface-channel structure for the 512 X 320 element sensor being 
described. 
exposure times to reduce the thermal generation of dark current. 
is packaged in a hermetic, edge-contacted, 24-connection ceramic dual in-line 
package, 
focused onto the sensor. 
chip and facilitates easy cooling. 
This results 
A large number of picture elements must be read out each frame 
Also the 
Buried-channel devices 
This has led to the 
Sensor cooling is required for very slow readout rates and/or long 
The device 
The package contains an optical glass window to allow the image to be 
The packaging has a low thermal impedance to the 
There is no residual image remaining after the picture has been read out. 
This means that no special prepare o r  erase cycles a r e  required for  proper 
slow-scan operation. All that i s  required is proper setting of the exposure. 
The sensor is normally operated in a run-stop-run mode of operation for slow 
scan. 
continuously reading out the dark signal until i t  i s  desired to make an exposure. 
The vertical clocks a r e  stopped during the time the shutter is opento expose 
The sensor is set  up to operate a t  the desired readout rate and is left 
the image. After the shutter is closed, the slow-scan readout at  the preset 
117 
rate begins. 
out any dark o r  light charge buildup in this register. 
The horizontal clocks a r e  kept running during the exposure to read 
Many scenes a r e  low contrast and do not present small area overload 
problems to the CCD sensor. 
areas  (e. g., star fields). 
problem with CCD imagers. 
sensor operating in i ts  low-blooming mode of operation (Refs. 3, 4). A l l  of the 
small circular images in the scene a r e  0.25 mm in diameter on the sensor. 
The one on the right center is exposed a t  saturation and represents the original 
image size. 
four images a r e  at  a lOOX overload. 
less  than a factor of two in diameter with this lOOX overload. 
enlargement is known to be from the lens itself. 
Other scenes may contain strongly overloaded 
Containment of charge during overloads can be a 
Figure 1 shows the performance of the 512 X 320 
The image a t  the upper left is a t  a 1OX overload. The remaining 
It can be seen that the image size grows 
Part  of the 
Figure 2 shows what would have happened i f  the low-blooming mode of 
operation had not been used. 
because i t  is confined by the channel stop diffusions in the horizontal direction. 
The low-blooming mode of operation is accomplished by accumulating the sub- 
strate under the two-phase gates next to the phase gate collecting charge during 
picture exposure. 
sensing site, preventing charge spreading. 
biased into depletion during the normal readout transfers. 
devices cannot use this method of charge confinement during exposure since the 
surface cannot be accumulated (Ref. 2). 
a surface-channel structure instead of a buried-channel structure for  this 
512 X 320 sensor. 
The blooming takes place in the vertical direction 
This electronically extends the channel stops around each 
The sensor's phase gates a r e  then 
Buried-channel 
This was another reason for choosing 
The pictures shown in Figures 1-6 were all taken with a 10-second 
picture frame readout time. 
pictures. 
except Figure 2, in which blooming was allowed to occur to demonstrate the 
effectiveness of blooming control. Figures 3 and 4 show live imaging of the 
authors. 
with a 2-second exposure. Figures 1, 2, and 5 were taken with a 30-second 
exposure, followed by the 10-second readout. 
different s tar  fields taken from a book. 
The sensor was cooled to -6°C for all of the 
All of the pictures were made in the low-blooming mode of operation 
Figure 3 was taken with a 1/4-second exposure. Figure 4 was taken 
Figure 5 is a picture of two 
The white line in the middle is the 
118 
division between the two pages of the book. 
It was exposed for 67 seconds, followed by a readout of 10  seconds. 
equivalent illumination on the sensor in the highlights is approximately 
5 X - 1 X fc of 2856-K illumination. The sensor w a s  measured to 
have a dark current density of 2 nA/crn2 at 24°C. 
limited resolution in all parts of the picture (i, e. # .  512 X 320 cell resolution). 
However, the Tektronix 604 display used for the pictures cannot resolve 
individual ras ter  lines. 
structure. 
pattern is present in the display even when the video is disconnected and varies 
with scan rate. 
display unit, 
Figure 6 is a picture of the earth, 
The 
The sensor exhibited cell- 
All of the photographs exhibit a horizontal line 
This is not generated by the 512 X320 imager., This background 
It is believed to be due to 60-cycle modulation in the 604 
Figure 7 shows a portion of one video line of information a t  the output of 
the imager (l6-kHz data rate, corresponding to 10-second readout). 
is shown with white hegative. The output floating diffusion is reset a t  the end of 
each clock period so that the next cell output will  be present for most of the next 
clock phase. 
further video processing or  filtering was  necessary to make the displayed photos. 
The reset  pulses that are present merely blank off the beam for a small part of 
an element time and a r e  not resolved in the displayed picture. 
The video 
This presents the data in a pseudo sample and hold output. No 
In summary, i t  has been shown that a 512 X 320 element CCD imager may 
be operated a t  scan rates much slower than standard television (i. e. , 10-second 
frame time compared to 33-msec frame time) and integrated for much longer 
times (i. e., 67 seconds compared to 16 msec)  by slowing down the scanning and 
using a modest amount of cooling. 
These results do not represent the limiting performance possible from 
such a sensor even a t  this temperature a s  regards frame time and integration 
time. 
rate at  the lowest operating temperature. 
The ultimate performance will be limited by the dark current generation 
REFERENCES 
1. R, L. Rodgers, 111, "Charge-Coupled Imager for  525-Line Television, 
IEEE Intercon Proceedings, New York, N. Y., March 1974. 
2. R. L, Rodgers, 111, "512 X 320 Element Silicon Imaging Device, IEEE 
ISSCC Proceedings, Philadelphia, Pa., February 1975. 
119 
3. RCA SID51232 Data Sheet, January 1975. 
4. W. F. Kosonocky, J. E. Carnes, M. G. Kovac, P. Levine, F. V. 
Shallcross, and R. L. Rodgers, "Control of Blooming in 
Imagers, RCA Review, Vol. 35, No. 1 ,  March 1974. 
ar ge-Coupled 
120 
M tz 
d 
0 
.rl L)
122 
123 
Figure 7. Expanded video output (white negative, l6-kHz data rate, 10-sec 
readout) 
124 


Cooled slow-scan performance of a 512 by 320 element charge-coupled imager

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