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INTRODUCTION
This invention relates generally to the storing and retrieval for
subsequent reproduction of optical images and, more particularly, to
techniques for performing such operations utilizing solid state devices.
BACKGROUND OF THE INVENTION
The use of solid state devices has been suggested for storing and for
subsequent retrieval and reproduction of optical images as in a solid
state camera. One such proposed system utilizes a miniaturized electronic
camera and a magnetic disk or tape storage unit which can be packaged as a
hand-held or shoulder-supported apparatus. The electronic camera utilizes
a conventional charge-coupled device (CCD) imaging chip, the CCD thereon
containing an array of pixels and the chip being located in the focal
plane of the camera optics. The chip is exposed to the image and
photocarriers are produced in each pixel. For reading the image, special
electronic shifting circuits are used to shift the photocarriers on a
line-by-line, and subsequently a pixel-by-pixel, basis to an output
detector. The output detector signal is then stored on a video disk or
video tape, for example, and the camera is then ready for the next imaging
process. Because CCDs provide only very short term storage, the shifting
process must begin substantially immediately after the image has been
stored and must be completed within a relatively short time interval,
i.e., one which is much less than one second if the quality of the image
is to be preserved. If the shifting takes longer the quality of the image
stored in the CCD device gradually diminishes and the image ultimately
disappears.
Such an approach requires relatively elaborate electronic switching and
shifting circuitry and the attachment of a magnetic recording device to
the hand-held camera. According1y the device becomes relatively bulky and
heavy and inconvenient to carry. As mentioned above, because the image
that is stored in the CCD chip has a very short storage life, each image
must be read out and stored in the magnetic recording device within a very
short time after the chip has been exposed to the external image, in
practice almost immediately, and the next image cannot be obtained until
the overall switching, shifting and permanent storage recording process
for a previous image has been completed.
In contrast it would be preferable to provide for solid state imaging in
which images can be stored for much longer periods of time so that each
image need not be accessed immediately for processing but rather many
images can be stored in an array of solid state chips and then processed
at a later time. Moreover, it would be desirable to avoid the need for an
on-site magnetic storage device and for the elaborate electronic switching
and shifting circuitry within the camera for reading out the image which
has been formed and is to be stored. Currently there is no known solid
state camera which provides such advantages.
BRIEF SUMMARY OF THE INVENTION
This invention provides a solid state camera in which several images can be
stored for an extended period of time in an array of solid state devices
so that the processing of images need not be performed in the field at the
time when the image has originally been stored but can be processed at a
much later and more convenient time period when all of the images in an
array thereof can be processed together. Moreover, in accordance with the
invention the system utilizes a read-out mechanism which permits the
stored image to be read out optically so that complex electronic switching
and shifting circuitry conventionally required is eliminated. In a
particular embodiment, for example, the read-out process can be performed
by utilizing a scanning optical beam which utilizes known, reliable
electronic or mechanical scanning techniques.
Each of the solid state devices, or imaging chips, of the array thereof can
be suitably moved sequentially into the camera's focal plane so that the
image can be projected onto the focal plane for storage in the particular
chip which has been moved into position. In a preferred embodiment, when
all the chips of an array have been exposed to images, the substrate
containing the array of chips can be appropriately read out by a device
which is separate from the camera itself and which is capable of providing
output image information for any appropriate use, such as to produce a
print thereof or to store such output in the form of electrical signals on
a video recording medium which can be appropriately displayed on a TV
display system, for example. The imaging chips may be erased, either
singly or collectively, by the application of a voltage pulse at some
convenient time, e.g. just prior to exposure.
DESCRIPTION OF THE INVENTION
The invention can be described in more detail with the help of the
accompanying drawings wherein
FIG. 1 shows a plan view of an exemplary substrate containing an array of
appropriate solid state imaging chips;
FIG. 2 shows a plan view of an exemplary chip having a plurality of
appropriate solid state cells for forming each of the pixels of a complete
image;
FIG. 3 shows a diagrammatic view in cross-section of a pair of adjacent
solid state cells useful in explaining the image storing operation
therein;
FIG. 4,shows a diagrammatic view in cross-section of a pair of adjacent
solid state cells useful in explaining the optical readout operation
thereof; and
FIGS. 5 and 5A show diagrammatic views of an exemplary embodiment of an
overall camera apparatus and readout apparatus, respectively, of the
invention depicting the locations of significant portions thereof;
FIG. 5B shows a block diagram of an alternative readout apparatus using a
cathode ray tube;
FIG. 6 shows an exemplary electrical integrating circuit for providing an
output signal from an optically scanned imaging chip; and
FIGS. 7 and 7A show curves representing outputs from an exemplary imaging
cell read by known electrical readout techniques and read by the optical
readout technique of the invention, respectively.
In one exemplary embodiment of the invention, as can be seen in FIG. 1, a
substrate 10 carries a plurality of adjacent MNOS imaging chips 11 formed
therein, each of said chips being utilized for storing a separate image as
explained in more detail below. In a particular embodiment the substrate
10 may be circular in its plan view with adjacent chips 11 arranged near
the outer periphery thereof. The substrate can then be mechanically
rotated, for example, so that each chip can be sequentially placed in the
focal plane of an optical imaging system. Alternatively, imaging chips may
be arrayed in rows and columns on a rectangular substrate, for example,
and a bidirectional (X-Y) drive circuit can be used to place the chips in
the focal plane of an optical imaging system so as to more efficiently
utilize all portions of the substrate.
In accordance with a preferred embodiment of the invention, each of the
chips may be formed of metal-nitride-oxide-silicon (MNOS) cells which are
formed in a suitable array thereof on each chip. Such cells may be formed
in rows and columns on each chip, for example, each cell representing a
pixel of the overall image stored on the chip.
MNOS cells of the type proposed for use in the invention have been
described in U.S. Pat. No. 4,313,178, issued on Jan. 26, 1982 to E. Stern
et al. Briefly, as described in such patent, an MNOS cell is a solid state
device which is capable of providing long-term storage of analog signals.
Thus, input analog signals (e.g., optical images) can effectively be
permanently stored in the array of MNOS cells for subsequent readout and
processing. In such MNOS devices analog signals can be stored for time
periods up to several days and even a week or more in contrast to storage
time of about 100 milliseconds, or less, with CCD devices.
An exemplary chip, for example, is formed as an array of rows and columns
of MNOS cells 13 as shown in FIG. 2. In contrast with the use of arrays of
charge-coupled devices (CCDs) a minimum of two electrical leads are
required for operation of the MNOS cells. One electrical contact provides
an appropriate excitation voltage (V) while the other provides an
effective ground contact to the cells as can be seen more clearly in FIG.
3 discussed below. The voltage may be applied to each of the cells via an
optically transparent, electrically conductive electrode layer which may
be formed of a very thin layer of metal (e.g., chromium) or a layer of
electrically conductive polysilicon on the upper surface of the chip. The
transparent electrode layers of each of the chips of the substrate can be
appropriately interconnected to each other and thence to an electrical
contact terminal suitably placed on one surface of the substrate 10. Such
contact can be connected to a suitable voltage source (not shown in FIG. 2
but shown in FIG. 3, for example). A ground contact can also be formed on
the opposite surface of the chips and interconnected on the substrate in
contact with the lower surface of each of the MNOS cells and connected via
a suitable terminal to electrical ground, (not shown in FIG. 2 but shown
in FIG. 3, for example). To provide a practical idea of the chip
dimensions, for example, in the exemplary embodiment depicted in FIG. 3
the chips can be 1.0 cm. square, each of the solid state cells being about
10 micrometers square.
FIG. 3 shows in diagrammatic (and idealized) form a pair of exemplary
adjacent MNOS cells 13, the metal contact terminal 15 providing a voltage
V which is supplied to an electrode layer 14 associated with the cells.
The electrode layer is in turn in contact with a nitride layer 17 which is
above an oxide layer 18 which is above a silicon layer 19 (e.g. a p-type
silicon) to form an MNOS image storage cell, as discussed in detail in the
above-mentioned Stern et al. patent. Suitable P+ channel stop regions 19'
can be formed in the silicon layer between cells in order to isolate the
cells and to prevent migration of charges between cells as discussed in
the Stern et al. patent. The silicon layer is shown as p-type although
operation with n-type silicon is possible. While the device in the patent
utilizes a chromium-gold electrode 14, such electrode layer may also be in
the form of a transparent electrode layer of electrically conductive
polysilicon material, the thickness thereof being designed to pass a
selected primary color for use in storing color images. Color images may
also be formed by the use of a color filter mask which passes the primary
colors to appropriate groups of storage cells or by the use of separate
chips for each primary color. A suitable ground connection 20 is made to
the silicon layer.
The process for storing photocarriers so as to store a representation of an
image is described in detail in the Stern et al. patent. Briefly, the
positive potential (+V) applied to each electrode layer depletes the
region beneath the electrode of majority carriers. Input image light, as
from a scene, for example, is permitted to fall on the overall MNOS chip
array (FIG. 3) and photons absorbed in the depleted region of each cell
create pairs of photo-carriers. The holes are swept into the bulk material
and the electrons accumulate beneath the surface, causing the electric
field in the oxide layer to increase and consequently causing the
electrons to tunnel through the oxide layer into the trap sites shown
within the nitride layer near the oxide-nitride interface where they can
remain for a relatively long period of time. Accordingly, an image of the
scene is effectively stored on a substantially permanent basis in the
solid state dual dielectric structure. Such storage process is essentially
a solid state equivalent of a photographic film.
Once an image has been so stored, the image can be optically read out in
accordance with the invention by the use of an optical (light) beam (e.g.,
such as supplied by a flying spot scanner) as illustrated in FIG. 4, in
contrast with the electrical read-out technique disclosed in the Stern et
al. patent. The positive potential(+V) is applied as in the imaging
storage process. As indicated in the Stern et al. patent, and as
illustrated diagrammatically in the figures, the depth of the depleted
region is inversely proportional to the stored charge (i.e., the lower the
charge in the stored image the larger the depletion depth).
In accordance with the invention, it has been found that when the readout
light beam, e.g., light from the scanner, falls upon a particular cell,
photocarriers are created, as in the storage process, and the so generated
electrons accumulate at the surface and screen the silicon substrate from
the applied potential, thereby causing the depleted region to collapse.
Blue light is preferred for this purpose as it is absorbed at a shallower
depth and causes less discharge of adjacent cells. The intensity of the
readout beam is of little importance as long as it is of sufficient
intensity to completely discharge any cell, regardless of the stored
charge, within the allotted dwell time. Collapse of a depleted region
causes charge to flow into the base electrode in order to neutralize the
acceptors in the collapsed depleted region. Accordingly, a read-out signal
is now available at the output electrodes 21, the signal being inversely
proportional to the charge due to the image portion stored in each cell.
The output signal from each cell can be supplied to an integrator to
produce an output which is linearly related to the total charge (i.e., the
image intensity) in each cell. Suitable electrical integrator circuitry
therefor is shown in FIG. 6 and is of essentially a well-known design.
Such signal can then be utilized in a variety of ways in order to recreate
the stored image. For example, the signal can be stored on a magnetic disk
or tape and later supplied to a cathode ray tube or television imaging
tube via appropriate circuitry or it can be used to produce hard copy
images (prints).
After a time period on the order of 10 to 1000 milliseconds, dark currents
in the MNOS chip cause significant collapse of the depletion layers. At
this time the readout process may be halted temporarily, a negative pulse
used to discharge the accumulated dark currents, and then the positive
potential restored and readout resumed.
Such process and structure represents a novel imaging storage and readout
technique in that it utilizes optical addressing of the stored image via,
for example, an optical spot scanning beam in order to provide a charge
from the electrically stored information which has been stored on a long
term basis within each imaging cell. The electrical output which is
obtained from such optical addressing process is linearly related to the
stored input and can be used to provide a re-creation of the image in any
of several well-known ways. Because the long term storage elements are of
the type shown by the long-term MNOS image storage cells, for example,
rather than short-term CCD storage elements suggested in prior image
storage processes, this relatively slow optically-addressed readout method
may be used. Only two electrical connections are required for each cell,
the leads from each of the cells being appropriately interconnected as
shown in FIG. 3, for example, so that only two electrical leads need to be
utilized for the overall array of the chips. Furthermore, the patterns on
the chip surface are few and simple. Such structure can be contrasted with
a device using the CCDs which requires electrical readout techniques in
which a larger number of separate leads and more numerous and complex
patterns of metals, dielectrics, and doped regions are required for each
chip in order to implement the more rapid signal readout with the complex
switching and shifting operations utilized therein. The large number of
metal lines required in such latter devices prevents the use of closely
spaced CCD elements so that the density thereof, and the overall imaging
resolution, is relatively low. In the invention, however, because of the
need for only two electrical leads and simple patterns for each cell, the
cells can be placed much closer together, the resulting high cell density
on each chip providing an ultimate image which has much higher resolution.
It will be evident to those skilled in the art that the optically addressed
output technique described may suffer to some extent from a degraded
signal-to-noise ratio. Specifically, (1) dark current is collected from
the entire chip, and (2) the large output capacitance reduces the signal
energy with respect to thermal noise energy. If necessary for a given
implementation, a compromise may be struck between noise performance and
chip simplicity by dividing the chip area into rows, each of which is
addressed sequentially during readout. Row address may be achieved
electrically using field-effect-transistors (FETs) and associated decoding
circuitry or optically using optoelectronic switches and a second readout
beam. Pixel (or column) address is as described above.
As discussed above, MNOS devices can store images for relatively long
periods of time, e.g., for several days or more. Other devices which are
being developed can also be used. For example, floating gate devices as
described in the article, R. S. Withers et al., "Nonvolatile Analog
Memory: Floating-Gate Devices," Solid State Research Quarterly Technical
Summary, Aug. 1 to Oct. 31, 1982, Lincoln Laboratory, M.I.T.,
ESD-TR-82-105, Nov. 15, 1982, pp 75-77, can also provide long term storage
up to several weeks and even up to many months or more and can be
optically addressed for read-out purposes. Accordingly, any suitable
device for providing storage over an extended period of time which, as
used herein, can mean a period of time ranging from several seconds to
several months or more, can be used in the invention.
FIGS. 5 and 5A depict in diagrammatic form exemplary embodiments of an
arrangement of elements required in exemplary solid state camera and
readout devices, respectively, made in accordance with the invention. As
can be seen therein, a camera enclosure 25 has an appropriate shutter
mechanism 26 which supplies an image of an object via a suitable lens
system 27 to an MNOS imaging storage chip array 28. A suitable power
supply 29 supplies the desired voltage V. During the image storage
operation when the shutter is open an image of the object is stored in an
MNOS chip of the overall array thereof. The array of chips can be removed
from the camera enclosure following storage of all the images thereon and
placed in a separate read-out device which provides for the optical
addressing and read-out of the images stored therein.
During the read-out operation, for example, an appropriate optical read-out
illuminator 31 utilizing a light source and deflector to provide suitable
flying spot scanning operation, the structure of which would be well known
to those in the art, supplies a light beam 32 which is used to scan, i.e.
to optically address, each of the MNOS chips of the array 28 for readout
purposes. The output electrical signal 32 which results can then be
utilized in whatever manner is desired, e.g., for storage on a video disk
or tape, for supply to the image display circuitry of a cathode ray tube
or for use in providing a hard copy of the image. The MNOS chip may be
electrically erased, as described in the above Stern et al. patent, and
reused. In FIG. 5A the signal 32 can be suitably processed via signal
processor circuitry 33, in accordance with techniques well known to the
art, to format the output into a standard video output signal format, for
example. Appropriate and well-known electronic control circuitry 34 is
used to coordinate the operation of the light beam source and the MNOS
array, as would also be well known to the art.
The function of the optical scanning beam may be relatively inexpensively
achieved in a particular embodiment by using the camera optics itself to
project a spot from a cathode ray tube onto the MNOS chip to be read out.
If the screen 35 of a CRT 36 is placed in the focal plane of the MNOS
camera 25, as shown in FIG. 5B, and if a single bright spot is made, by
simple electronic means, to scan in a raster across the screen, a
corresponding spot will scan the MNOS chip. The resulting video output
signal 38 from the chip, along with appropriate timing information from
the CRT drive circuitry 39, may be supplied to an image storage, image
display, or other image reproduction device.
Thus a single optical read-out device can be used with an unlimited number
of separate image storage cameras. The read-out device, for example, may
be located at a store where the camera user takes the exposed substrate
and receives appropriate hard copy prints, for example. Alternatively,
each user may have his own separate optical read-out device which permits
him to read out the chips at his convenience to provide an optical display
thereof on a cathode ray tube (TV tube), for example, or to make his own
hard copy prints. In accordance with such embodiments the camera itself
can have a very compact structure and can be easy to carry and to use.
The use of an optical light beam for optically addressing each MNOS chip
has not been suggested by any previously proposed imaging systems.
However, it has been found that such optical addressing technique in
accordance with the invention provides accurate read out results
comparable to those obtained through the use of previously proposed more
complex electrical read-out techniques, as shown by FIGS. 7 and 7A. In
FIG. 7, a typical curve is shown demonstrating the relationship between
the voltage readout from an MNOS cell utilizing an electrical addressing
and readout process as described, for example, in the above Stern et al.
patent, as a function of the input image intensity thereto. Such a curve
can be obtained for testing purposes, for example, by simulating the image
intensity in an MNOS cell in accordance with the number of input optical
pulses of light which are applied thereto (i.e., the greater the number of
such pulses, the greater the simulated intensity). The output voltage,
which is proportional to the charge stored, is shown as a function of
different intensities (i.e., different numbers of pulses) applied to the
cell.
FIG. 7A shows the output as a function of simulated image intensity (i.e.
numbers of pulses) wherein the output represents the integrated current
output for the cell when utilizing optical addressing of an optical light
beam to read out the stored charge in accordance with the invention. Such
process provides a current output as discussed above which is then
integrated to produce a voltage output once the entire charge stored
therein has been discharged. As can be seen, the curve in FIG. 7A
essentially matches the curve in FIG. 7, the scaling of the current
integrated output being arbitrary depending on whatever scale factor is
used. Thus FIG. 7A shows that the use of an optical scanning beam to
address the MNOS cells in order to read out the stored information
provides as accurate a representation of the stored charge representing
the input image as does the more complex and cumbersome electrical
read-out process described in the previously issued Stern et al. patent.
While the particular embodiments of the invention described above represent
preferred embodiments thereof, modifications thereof may occur to those in
the art within the spirit and scope of the invention. For example,
although the camera and read-out devices are described as normally being
physically separate units, it may be desirable in some applications to
integrate them into a single physical unit in which they are either
inseparably constructed or in which they can be separated or joined, as
desired. Hence, the invention is not to be construed as limited to the
particular embodiments disclosed herein except as defined by the appended
claims.
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