U.S. patent application number 09/325005 was filed with the patent office on 2003-07-17 for image sensor response enhancement using fluorescent phosphors.
Invention is credited to BOOTH, LAWRENCE A. JR..
Application Number | 20030133029 09/325005 |
Document ID | / |
Family ID | 23266043 |
Filed Date | 2003-07-17 |
United States Patent
Application |
20030133029 |
Kind Code |
A1 |
BOOTH, LAWRENCE A. JR. |
July 17, 2003 |
IMAGE SENSOR RESPONSE ENHANCEMENT USING FLUORESCENT PHOSPHORS
Abstract
A method and apparatus is described that is related to image
response enhancement of image sensors. In an image sensor, a
phosphor layer is placed between the incident photons and the image
sensor in which the phosphor layer converts incident photons from a
first wavelength to a second wavelength.
Inventors: |
BOOTH, LAWRENCE A. JR.;
(PHOENIX, AZ) |
Correspondence
Address: |
YONG S CHOI
BLAKELY SOKOLOFF TAYLOR & ZAFMAN LLP
12400 WILSHIRE BOULEVARD
7TH FLOOR
LOS ANGELES
CA
90025
|
Family ID: |
23266043 |
Appl. No.: |
09/325005 |
Filed: |
June 2, 1999 |
Current U.S.
Class: |
348/294 ;
257/E27.132; 257/E27.134; 348/164; 348/302; 348/E5.028;
348/E5.04 |
Current CPC
Class: |
H01L 27/14645 20130101;
H04N 5/374 20130101; H01L 27/14621 20130101; H01L 27/14609
20130101; H01L 27/14627 20130101 |
Class at
Publication: |
348/294 ;
348/164; 348/302 |
International
Class: |
H04N 003/14 |
Claims
What is claimed is:
1. An apparatus comprising: an image sensor; and a phosphor layer
adapted to said image sensor, wherein said phosphor layer converts
said incident photons in a first wavelength to photons in a second
wavelength.
2. An apparatus as in claim 1, wherein said image sensor having a
pixel array, each pixel adapted to convert incident photons into an
electrical signal.
3. An apparatus as in claim 1, wherein said first wavelength is in
a range of 400 nm to 700 nm and said second wavelength is in a
range of 900 nm to 1000 nm.
4. An apparatus as in claim 1, wherein said phosphor layer shifts
said incident photons in a first time domain to a second time
domain.
5. An apparatus as in claim 1, further comprising a color filter
array wherein said phosphor layer is positioned between said color
filter array and said pixel array.
6. An apparatus as in claim 1, further comprising an isotropic
emissions reflector, wherein said phosphor layer is positioned
between said isotropic emissions reflector and said pixel
array.
7. An apparatus as in claim 2, further comprising a plurality of
microlenses, wherein said phosphor layer is located between said
microlenses and said pixel array.
8. An apparatus as in claim 2, wherein said pixel array is a
passive pixel array.
9. An apparatus as in claim 2, wherein said pixel array is an
active pixel array.
10. A method comprising: providing an image sensor; and positioning
a phosphor layer corresponding to said image sensor, wherein said
phosphor layer converts said incident photons in a first wavelength
to photons in a second wavelength.
11. A method as in claim 10, wherein said phosphor layer is
absorbing photons of said first wavelength in a range of 400 nm to
700 nm; and re-radiating photons of said second wavelength in the
range of 900 nm to 1000 nm.
12. A method as in claim 10, wherein said phosphor layer is
absorbing photons is a first time domain; and re-radiating photons
in a second time domain.
13. A method as in claim 10, further comprising: positioning a
color filter array such that said phosphor layer is between said
color filter array and said image sensor.
14. A method as in claim 10, further comprising: positioning an
isotropic emissions reflector such that said phosphor layer is
between said isotropic emissions reflector and said image
sensor.
15. A method as in claim 10, further comprising: positioning a
plurality of microlenses such that said phosphor layer is between
said microlenses and said image sensor.
16. A system comprising: a memory device; an image sensor; a
phosphor layer corresponding to said image sensor, wherein said
phosphor layer converts said incident photons in a first wavelength
to photons in a second wavelength; and an imaging circuitry coupled
to said image sensor and said memory device, said imaging circuitry
configured to process electrical signals received from said image
sensor and further configured to store said processed electrical
signals in said memory device.
17. A system as in claim 16, wherein said image sensor having a
pixel array, each pixel adapted to convert incident photons into an
electrical signal.
18. A system as in claim 16, wherein said first wavelength is in
the range of 400 nm to 700 nm and said second wavelength is in the
range of 900 nm to 1000 nm.
19. A system as in claim 16, wherein said phosphor layer shifts
incident photons in a first time domain to a second time
domain.
20. A system as in claim 16, further comprising a color filter
array, wherein said phosphor layer is positioned between said color
filter array and said image sensor.
21. A system as in claim 16, further comprising an isotropic
emissions reflector, wherein said phosphor layer is positioned
between said isotropic emissions reflector and said image
sensor.
22. A system as in claim 17, further comprising a plurality of
microlenses, wherein said phosphor layer is positioned between said
microlenses and said pixel array.
23. A system as in claim 17, wherein said pixel array is a passive
pixel array.
24. A system as in claim 17 wherein said pixel array is an active
pixel array.
Description
FIELD OF THE INVENTION
[0001] The present invention relates to light response enhancement
of image sensors.
BACKGROUND INFORMATION
[0002] Due to its many advantages, Complementary Metal Oxide
Semiconductor (CMOS) image sensors have been of interest as
replacements for charged coupled devices (CCDs) in imaging
applications. A CMOS image sensor generally uses a single low power
supply and has a simpler system level design with high functional
integration when compared with a CCD image sensor. These factors
contribute to lowering system costs while providing for a potential
camera on a chip. Such features are highly desirable, for example,
in camcorders and digital cameras, where the devices may be reduced
to a size of a TV remote control and are highly portable.
Additionally, high resolution color images can be recorded for
hours on battery power because the CMOS image sensor has a low
power consumption.
[0003] The CMOS image sensor can be generally divided broadly into
two categories dependent on the type of pixel array used, the first
category being the passive pixel array and the second category
being the active pixel array. In the passive pixel array, each
pixel merely collects the charge generated by the photodiode and
transfers the collected charge to the imaging circuitry for image
processing. The active pixel array, on the other hand, includes an
amplification circuitry in each pixel to amplify the signal
represented by the charge generated by the photodiode before
transferring to the image circuitry for processing. The advantage
of the passive pixel array over the active pixel array is that each
pixel has minimal components allowing for a high fill factor which
in turn produces a high quantum efficiency. Fill factor generally
refers to the ratio of photo sensitive area to the pixel's overall
size. Quantum efficiency is a measure of light sensitivity and
refers to the ratio of photon generated electrons that a pixel
captures to the photon incident over the pixel area. However, one
of the disadvantages of the passive pixel array is that the charge
levels generated may be low and thereby insufficient to drive the
image circuitry to produce high quality images. In the active pixel
array, the pixel amplifies the signal represented by the charge and
is sufficiently able to drive the image circuitry. However, due to
several components being used for amplification, the fill factor is
generally low which in turn affects the quantum efficiency. The
active pixel array generally compensates for the low quantum
efficiency by using microlenses to focus the photons into the
sensitive area of the pixels that may otherwise strike the
insensitive area of the pixels. Microlenses, however, are expensive
and generally drives up the cost of manufacturing the active pixel
array sensor.
[0004] The CMOS image sensor technology is by no means a new
technology and both the CMOS image sensor and the CCD image sensor
were developed at about the same period. While there were many
advantages to using a CMOS image sensor over a CCD image sensor (as
described above), the CCD image sensor has prevailed over the CMOS
image sensor in imaging applications. One major reason is that the
CMOS image sensor has not been able to match the quality of the
image generated by the CCD, that is, light sensitivity has been one
issue in which the CCD image sensor has prevailed over the CMOS
image sensor. The CMOS image sensor, however, is now rapidly
gaining wide acceptance due to increased light sensitivity obtained
using various complicated and expensive enhancement technologies.
However, from a cost point of view, the low cost advantage of the
CMOS image sensor has severely eroded when compared with the cost
the CCD image device due to the enhancement technologies. It is
desired to boost the light sensitivity of a CMOS sensor so that
better quality images may be produced and where possible, maintain
the cost advantage of the CMOS sensor.
SUMMARY
[0005] A method and apparatus is described that is related to light
response enhancement of image sensors. A phosphor layer is placed
between the incident photons and the image sensor in which the
phosphor layer converts incident photons from a first wavelength to
a second wavelength.
[0006] Other features and advantages of the present invention will
be apparent from the accompanying drawings and detailed description
to be followed.
BRIEF DESCRIPTION OF THE DRAWINGS
[0007] The present invention is illustrated by way of examples and
not as limitations in the figures of the accompanying drawings in
which:
[0008] FIG. 1 is a schematic diagram of a pixel in a pixel array of
an exemplary CMOS image sensor;
[0009] FIG. 2 is a timing diagram of the pixel illustrated in FIG.
1;
[0010] FIG. 3 is a cross-sectional view of a pixel including a
color filter and microlens;
[0011] FIG. 4 is a diagram of a response spectrum of an exemplary
CMOS image sensor made of silicon;
[0012] FIG. 5 illustrates a CMOS image sensor comprising a color
filter array, a phosphor layer and a pixel array in accordance with
an embodiment of the invention;
[0013] FIG. 6 illustrates an excitation spectrum of a phosphor used
in one embodiment of the invention;
[0014] FIG. 7 illustrates an emission spectrum of the phosphor
above used in the above embodiment of the invention;
[0015] FIG. 8 illustrates a CMOS image sensor comprising a phosphor
layer and a pixel array in accordance with an embodiment of the
invention;
[0016] FIG. 9 illustrates a CMOS image sensor comprising an
isotropic emissions reflector, a phosphor layer and a pixel array
in accordance with an embodiment of the invention; and
[0017] FIG. 10 illustrates an imaging device using a phosphor layer
in accordance with an embodiment of the invention.
DETAILED DESCRIPTION
[0018] A method and an apparatus is described that enhances an
image response of an image sensor. In one embodiment, in an image
sensor having a pixel array, where each pixel is configured to
convert incident light into electrical charges, a phosphor layer is
placed between the pixel array and the incident light. The phosphor
layer absorbs the incident light and re-radiates the light at a
wavelength that corresponds to the optimal quantum efficiency of
the image sensor. To aid in the understanding of the invention, a
brief description is given below on the operation of a pixel in a
CMOS image sensor which should not be construed as a
limitation.
[0019] FIG. 1 illustrates a schematic diagram of a pixel of an
exemplary CMOS image sensor having an active pixel array. The
operation of the pixel will now be described. The transistor M4 is
used to precharge the photodiode D1 to reset power (herein VCCT).
Photons falling on the photodiode D1 generate electron-hole pairs
and the electrons are collected by a N type well, driving the diode
D1 to a lower voltage. This voltage is a function of the light
intensity and the time since precharge, commonly referred to as the
integration time. Sampling transistor M3 and storage capacitor C1
provide the "electronic shutter", that is, when the deassertion of
the SAMPLE signal is applied to transistor M3, the storage
capacitor C1 is isolated from the photodiode D1, capturing the
instant analog voltage across the photodiode D1. The transistor M2
is the access device and the transistor M1 comprises the top of a
source-follower. The load device ML is common to each bit line.
[0020] FIG. 2 illustrates a timing diagram that further aids in the
explanation of the operation of the pixel of FIG. 1. Initially,
RESET is asserted on the transistor M4, precharging the photodiode
D1 to approximately VCCT. SAMPLE is asserted simultaneously with
RESET allowing the storage capacitor C1 to precharge to the same
voltage level as the photodiode D1. Integration begins with the
deassertion of the RESET turning off transistor M4 and allowing the
photons falling on the photodiode D1 to dictate the voltage drop
across the photodiode D1. Since SAMPLE is still asserted, the
voltage drop of the storage capacitor C1 corresponds to the voltage
drop across the photodiode D1. With the deassertion of SAMPLE,
thereby, turning off transistor M3, the storage capacitor C1 is
isolated from the photodiode D1 capturing the instant voltage drop
across the photodiode D1. Readout is performed on a row by row
basis by asserting the Wordline WL which turns on transistor M2
allowing the voltage drop across the storage capacitor C1 to be
asserted across the load device ML and driving the Bitline BL on
each pixel in the row.
[0021] As described above, the CMOS image sensor captures images
based on photoelectric principles. Photons striking the light
sensitive area of the silicon causes electrons to be released
proportional to the light intensity. Stated differently, in one
method, the light sensitivity of the CMOS sensor can be determined
by the light intensity that is able to reach the photosensitive
area of the pixels. However, to distinguish color, according to one
method, various color filters are applied to the surface of the
pixel array which affects the light sensitivity of the CMOS sensor
as will be described further below. In one method, the color filter
array separates the colors of the incident light such that one
primary color (i.e., red, green or blue) falls on each pixel of the
pixel array. Thus, the position of each filter in the color filter
array determines the pattern to be captured by the pixels in the
pixel array. Each pixel captures the filtered incident light as
electrical charges that are converted into a voltage signal to be
processed by the imaging circuitry. Color filters are generally
applied to the surface of the pixel array using existing commercial
color filter array (CFA) materials. In this configuration, however,
the filter array generally blocks out roughly two-thirds of the
light intensity that is detectable by the sensor and thus, reduces
the light sensitivity of the sensor. To increase the light
sensitivity of the pixel array, according to one method,
microlenses are deposited on the surface of the sensor. The
microlenses allow photons to be focused into the photo sensitive
area of the pixel which would otherwise fall on the insensitive
area of the pixel. In this manner, the effective fill factor of the
sensor can be doubled or tripled, dependent on the efficiency of
the microlenses. FIG. 3 illustrates such configuration as described
above.
[0022] FIG. 4 shows a spectral response of an exemplary CMOS image
sensor made of silicon. The CMOS image sensor generally has an
optimal quantum response in the range of 900-1000 nanometers (nm).
Stated differently, the CMOS image sensor operates at optimal
quantum efficiency in the infra-red wavelengths. Where the CMOS
sensor is converting light in the visible spectrum into electrical
charges, the conversion is not being performed efficiently and
thus, CMOS sensor is not operating at optimal quantum efficiency.
Therefore, it is desirable to operate the CMOS sensor in the
optimal quantum efficiency spectrum.
[0023] FIG. 5 illustrates a CMOS image sensor 50 in accordance with
an embodiment of the invention. The CMOS sensor 50 comprises a
color filter array 51 and a pixel array 55. Between the color
filter array 51 and the pixel array 55 there is a phosphor layer 53
which absorbs the visible light passing through the color filter
array 51 and shifts the energy to a wavelength that corresponds to
the optimal quantum efficiency of the CMOS pixel array 55. The
principle is similar to that of a fluorescent tube in which
phosphors coated in the inner surface of the tube absorbs the
ultra-violet light generated by the fluorescent tube and
re-radiates as visible light. Similarly, within the image sensor
50, light that is passed through the color filter array 51 is
absorbed by the phosphor layer 53 and is re-radiated in the
infra-red spectrum where the pixel array 55 operates at optimal
quantum efficiency. Generally, the silicon of the photodiode in the
pixel array is more sensitive in the infrared spectrum than in the
visible light spectrum. The phenomena corresponds to the principles
of solid state physics.
[0024] FIGS. 6 and 7 illustrate a characteristic of an excitation
spectrum and an emission spectrum of a phosphor. Ideally, it is
desirable that the phosphor has an excitation spectrum in the range
of 400-700 nanometers. For the emission spectrum, it is desirable
to emit in the wavelength of 900-1000 nanometers. The excitation
and the emission spectrum shown in FIGS. 6 and 7 correspond to a
phosphor UMPKC60#5296 available from Phosphor Technology located in
Essex, England.
[0025] FIG. 8 illustrates a CMOS image sensor in accordance with
another embodiment of the invention. The CMOS sensor 80 comprises a
pixel array 83 and a phosphor layer 81. Photons that strike the
pixel array 83 is first absorbed by the phosphor layer 81 and
re-radiated in the infra-red wavelength where the image sensor 80
can operate at optimal quantum efficiency. In this configuration, a
color filter array is not used. The phosphor layer 81 comprises a
plurality of phosphor dots 82, each dot 82 corresponding to a pixel
85 in the pixel array 83. Each phosphor dot 82 absorbs photons of a
particular wavelength in the visible spectrum and re-radiates
photons of a wavelength in the infra-red spectrum. In this manner,
the phosphor layer 81 performs the color separation. Stated
differently, the position of each phosphor that absorbs a
particular wavelength (i.e., red, green or blue) in the phosphor
layer determines the RGB pattern to be captured by the pixels in
the pixel array. Because each phosphor dot absorbs only a
wavelength and emits in the infra-red wavelength, a color filter
array is no longer required.
[0026] One advantage of using a phosphor layer is that the time
domain may be adjusted such that the phosphor absorbs photons and
re-radiates at a longer period of time. This allows the pixels of
the pixel array to be exposed to a particular image for a longer
period of time. The sampling time of a CMOS sensor can be adjusted
to accommodate for the extended time, allowing for fuller
penetration of the incident photons into the surface of the pixel
array.
[0027] FIG. 9 illustrates a CMOS image sensor 90 in accordance with
an embodiment of the invention. In this embodiment, the image
sensor 90 comprises an isotropric emissions reflector 91, a
phosphor layer 93 and a pixel array 95. The role of the isotropic
emissions reflector is as follows: Generally, incident light
entering the CMOS image sensor has an infra-red wavelength
component. The infra-red wavelength component interferes with the
RGB pattern generated by the pixel array 95 in that the infra-red
wavelength photons converted by the phosphor layer 93 is
indistinguishable from the infra-red wavelength component of the
incident light. The isotropic emissions reflector filters the
infra-red wavelength component of the incident light by reflecting
it away from the phosphor layer 93. Another role of the isotropic
emissions reflector 91 is that the phosphor layer generally emits
the converted infra-red wavelength photons in both directions, that
is, towards the pixel array 95 and also towards the isotropic
emissions relfector 91. The isotropic emissions reflector 91
reflects the transmitted the infra-red wavelength photons back into
the phosphor layer 93. Isotropic emissions reflectors are
commercially available, for example, from OCLI located in Santa
Rosa, Calif.
[0028] FIG. 10 illustrates an embodiment of a imaging system 100 in
accordance with the present invention. The imaging system 100 may
be, for example, a camcorder or a digital camera or any system that
is related to digital processing. As shown in the figure, an image
sensor 101 including a phosphor layer in accordance with the
invention is coupled to an imaging circuitry 103 that is further
coupled to a memory device 105. The memory device 105 may be
semiconductor memories, magnetic disks or optical disks capable of
storing images. The operation of the imaging system 100 is as
follows: incident light containing an image falls on the image
sensor 101 designed in accordance with the present invention. The
image sensor 101 captures the image in a manner as described with
respect to FIGS. 1 and 2. The charges generated by the pixels are
sent to the imaging circuitry 103 for processing. The image
represented by the electrical signals is processed by the imaging
circuitry 103 and may be stored in the memory device 105 or
transmitted out of the imaging system.
[0029] In the foregoing specification, the invention has been
described with reference to specific embodiments thereof. It will,
however, be evident that various modifications and changes can be
made thereto without departing from the broader spirit and scope of
the invention as set forth in the appended claims. The
specification and drawings are accordingly, to be regarded in an
illustrative rather than a restrictive sense.
* * * * *