U.S. patent application number 11/199189 was filed with the patent office on 2005-12-08 for solid image pickup apparatus.
This patent application is currently assigned to KABUSHIKI KAISHA TOSHIBA. Invention is credited to Monoi, Makoto.
Application Number | 20050270595 11/199189 |
Document ID | / |
Family ID | 18713878 |
Filed Date | 2005-12-08 |
United States Patent
Application |
20050270595 |
Kind Code |
A1 |
Monoi, Makoto |
December 8, 2005 |
Solid image pickup apparatus
Abstract
A solid image pickup apparatus which is not easily susceptible
to an influence of an emitted light caused by impact ionization.
The solid image pickup apparatus of the present invention includes
an output circuit for converting a signal charge outputted from a
photoelectric converter into an analog signal and outputting the
signal. The output circuit includes a charge-voltage converter for
converting the charge transferred from the photoelectric converter
into a voltage signal, a plurality of source follower circuits for
performing impedance conversion, and a reverse amplification
circuit. A gate length of a MOSFET constituting the final-stage
source follower circuit in the output circuit is longer than the
gate length of the MOSFET of another source follower circuit or the
like. Therefore, impact ionization does not occur in a position in
which the impact ionization most easily occurs, the emitted light
caused by the impact ionization does not enter the photoelectric
converter, and an image quality can be prevented from being
deteriorated.
Inventors: |
Monoi, Makoto; (Meguro-ku,
JP) |
Correspondence
Address: |
OBLON, SPIVAK, MCCLELLAND, MAIER & NEUSTADT, P.C.
1940 DUKE STREET
ALEXANDRIA
VA
22314
US
|
Assignee: |
KABUSHIKI KAISHA TOSHIBA
Tokyo
JP
|
Family ID: |
18713878 |
Appl. No.: |
11/199189 |
Filed: |
August 9, 2005 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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11199189 |
Aug 9, 2005 |
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09907945 |
Jul 19, 2001 |
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6954230 |
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Current U.S.
Class: |
358/513 ;
257/E27.153; 348/E3.022; 348/E5.079 |
Current CPC
Class: |
H04N 5/378 20130101;
H04N 5/357 20130101; H01L 27/14818 20130101; H01L 27/14825
20130101; H04N 5/37213 20130101 |
Class at
Publication: |
358/513 |
International
Class: |
H04N 001/46 |
Foreign Application Data
Date |
Code |
Application Number |
Jul 19, 2000 |
JP |
2000-219023 |
Claims
1-5. (canceled)
6. A solid image pickup apparatus comprising: a photoelectric
converter; and an output circuit, configured by connecting in
series a plurality of stages of circuit blocks of at least one of a
source follower circuit and an inverter, configured to output an
electric signal in accordance with a signal charge
photoelectrically converted by said photoelectric converter,
wherein at least one MOSFET among two MOSFET constituting the
circuit block at a final stage is selectively formed of a lightly
doped drain (LDD) structure.
7. The solid image pickup apparatus according to claim 6 wherein
only the MOSFET constituting a current source among two MOSFETs
constituting said circuit block at the final stage in said output
circuit is formed into the lightly doped drain (LDD) structure.
8. The solid image pickup apparatus according to claim 6, further
comprising a transfer section configured to successively transfer
the signal charge photoelectrically converted by said photoelectric
converter in a predetermined direction, wherein said output circuit
outputs the electric signal in accordance with the signal charge
transferred from said transfer section.
9. The solid image pickup apparatus according to claim 6, further
comprising: a plurality of voltage conversion circuits configured
to convert the charge accumulated in said photoelectric converter
into a voltage signal for each pixel; a plurality of MOS
transistors which are disposed for said plurality of voltage
conversion circuits, respectively, and which can supply the voltage
signal converted by said corresponding voltage conversion circuit
to said output circuit; and a shift register configured to control
an on/off state of said plurality of MOS transistors.
10-16. (canceled)
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application claims benefit of priority under 35USC Sec.
119 to Japanese Patent Application No. 2000-219023, filed on Jul.
19, 2000, the entire contents of which are incorporated by
reference herein.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The present invention relates to a solid image pickup
apparatus for transferring a signal charge photoelectrically
converted by a photoelectric converter such as a photodiode,
particularly to a technique for preventing impact ionization to
enhance an image quality.
[0004] 2. Related Background Art
[0005] Examples of a typical solid image pickup apparatus include a
CCD image sensor. The CCD image sensor is easily integrated, and
therefore broadly used in a scanner, digital copying machine, video
camera, and the like in which a large number of pixels are
required.
[0006] FIG. 7 is a plan view showing a schematic configuration of
the CCD image sensor. The CCD image sensor of FIG. 7 includes: a
photoelectric converter 11 in which a plurality of photosensitive
pixels 10 are linearly arranged, a CCD register 13 for transferring
a signal charge photoelectrically converted by the photoelectric
converter 11 and subsequently sent via a shift electrode 12 in a
direction of arrows in FIG. 7, and an output circuit 14 for
outputting an analog signal in accordance with the signal charge
moved to the end of the CCD register 13.
[0007] In recent years, with an increase of speed of an electronic
apparatus, there has been an increasing demand for an increase of
an output data rate of the CCD image sensor. Because of this, the
speed is raised by shortening a gate length of MOSFET in the output
circuit 14, enhancing a driving ability of MOSFET, or increasing a
current flowing through the circuit.
[0008] However, when the gate length of the MOSFET is shortened, an
electric field between source and drain is heightened, and an
electron/hole pair is generated by impact ionization. It is known
that the electron/hole pair is recombined to emit light as shown in
FIG. 8 (authored by Takeda, Nikkei MacGrow Hill Co., "Hot Carrier
Effect", p 37, 1987).
[0009] Additionally, in FIG. 8, reference numeral 30 denotes a gate
electrode, 31 denotes a source region, and 32 denotes a drain
region.
[0010] When the light reaches the photoelectric converter 11 of
FIG. 7, the signal charge attributed to the light is generated by
the photoelectric converter 11, and this deteriorates an image
quality.
SUMMARY OF THE INVENTION
[0011] According to the present invention, there is provided a
solid image pickup apparatus comprising:
[0012] a photoelectric converter; and
[0013] an output circuit, configured by connecting in series a
plurality of stages of circuit blocks of at least one of a source
follower circuit and an inverter, configured to output an electric
signal in accordance with a signal charge photoelectrically
converted by said photoelectric converter,
[0014] wherein a gate length of at least one MOSFET constituting
the circuit block at a final stage side in said output circuit is
set to be longer than the gate length of the MOSFET constituting
the circuit block at the previous stage.
[0015] Moreover, according to the present invention, there is
provided a solid image pickup apparatus comprising:
[0016] a photoelectric converter; and
[0017] an output circuit, configured by connecting in series a
plurality of stages of circuit blocks of at least one of a source
follower circuit and an inverter, configured to output an electric
signal in accordance with a signal charge photoelectrically
converted by said photoelectric converter,
[0018] wherein at least one MOSFET among two MOSFET constituting
the circuit block at a final stage is selectively formed of a
lightly doped drain (LDD) structure.
[0019] Furthermore, according to the present invention, there is
provided a solid image pickup apparatus comprising:
[0020] a photoelectric converter; and
[0021] an output circuit, configured by connecting in series a
plurality of stages of circuit blocks of at least one of a source
follower circuit and an inverter, configured to output an electric
signal in accordance with a signal charge photoelectrically
converted by said photoelectric converter,
[0022] wherein a semiconductor element configured to relax an
electric field is selectively inserted between two MOSFETs
constituting the source follower circuit at a final stage among
said plurality of stages of source follower circuits.
[0023] Additionally, according to the present invention, there is
provided a solid image pickup apparatus comprising:
[0024] a photoelectric converter;
[0025] an output circuit configured to output an electric signal in
accordance with a signal charge photoelectrically converted by said
photoelectric converter;
[0026] a shielding film formed above at least said photoelectric
converter; and
[0027] a shielding member disposed in at least one of a
semiconductor substrate between said photoelectric converter and
said output circuit and/or between said semiconductor substrate and
said shielding film.
[0028] According to the solid image pickup apparatus of the present
invention, the emitted light caused by impact ionization is not
easily incident upon the photoelectric converter, thereby improving
image quality.
BRIEF DESCRIPTION OF THE DRAWINGS
[0029] FIG. 1 is a circuit diagram showing a detailed configuration
of an output circuit of a solid image pickup apparatus according to
the present invention.
[0030] FIG. 2 is a plan view of the solid image pickup apparatus,
showing a position in which MOSFET having a long gate length is
formed.
[0031] FIG. 3 is an explanatory view of a third embodiment of the
solid image pickup apparatus according to the present
invention.
[0032] FIG. 4 is an explanatory view of a fourth embodiment of the
solid image pickup apparatus according to the present
invention.
[0033] FIG. 5 is a sectional view of a fifth embodiment of the
solid image pickup apparatus according to the present
invention.
[0034] FIG. 6 is a schematic plan view of a linear sensor using MOS
sensor.
[0035] FIG. 7 is a plan view showing a schematic configuration of a
CCD image sensor.
[0036] FIG. 8 is an explanatory view of an impact ion.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0037] A solid image pickup apparatus according to the present
invention will concretely be described hereinafter with reference
to the drawings.
First Embodiment
[0038] A first embodiment is structured such that an impact ion is
not easily generated at a rear stage in an output circuit 14. That
is, in the first embodiment, an electric field is prevented from
being high between source and drain of MOSFET in the output circuit
14, and therefore an electron/hole pair is inhibited from being
generated by impact ionization.
[0039] FIG. 1 is a circuit diagram showing a detailed configuration
of the output circuit 14 of the solid image pickup apparatus
according to the present invention, and FIG. 2 is a plan view of
the solid image pickup apparatus according to the present
invention. The output circuit 14 shown in FIG. 1 includes a
charge-voltage converter 1 for converting a charge transferred from
a photoelectric converter 11 of FIG. 2 into a voltage signal, a
plurality of stages of source follower circuits 2.sub.1 to 2.sub.n
for performing impedance conversion, and a reverse amplification
circuit 3 disposed between the stages of the source follower
circuits 2.sub.1 to 2.sub.n.
[0040] The charge-voltage converter 1 has a floating capacity 4 and
a reset gate 5. The reset gate 5, source follower circuit 2, and
reverse amplification circuit 3 are configured by combining
MOSFETS.
[0041] The source follower circuit 2.sub.1 and so on at the rear
stage in the output circuit 14 is structured such that much current
can be passed in order to enhance a driving ability. Concretely,
for example, a gate width is increased. However, when much current
flows through MOSFET, impact ionization easily occurs as described
above, and light emission by recombination easily occurs.
[0042] To solve the problem, in the first embodiment, gate lengths
L1 and L2 of MOSFETs Q1 and Q2 in the final-stage source follower
circuit 2.sub.1 in which the impact ionization most easily occurs
inside the output circuit 14 of FIG. 1 are set to be longer than
the gate length of MOSFET constituting another source follower
circuit or the reverse amplification circuit 3.
[0043] Concretely, the gate lengths of the MOSFETs Q1, Q2
constituting the final-stage source follower circuit 2.sub.1 are
set to such an extent that the impact ionization does not occur.
Therefore, the impact ionization does not easily occur, an emitted
light caused by the impact ionization does not easily enter the
photoelectric converter 11, and an image quality can be prevented
from being deteriorated.
Second Embodiment
[0044] In a second embodiment, some MOSFETs of a plurality of
MOSFETs constituting the source follower circuit 2 have a structure
in which the impact ionization does not easily occur.
[0045] The source follower circuit 2 has a current source
configured by the MOSFET Q2. The electric field is more easily
centralized and the impact ionization is more easily caused on a
drain side of the MOSFET Q2 than in any other portion.
[0046] To solve the problem, in the second embodiment, only the
MOSFET Q2 constituting the current source among the MOSFETs Q1 and
Q2 constituting the final-stage source follower circuit 2.sub.1 in
the output circuit 14 has a gate length longer than that of another
MOSFET in the output circuit 14.
[0047] Therefore, similarly as the first embodiment, the emitted
light caused by the impact ionization does not easily enter the
photoelectric converter 11. Moreover, the number of MOSFETs long in
gate length can be decreased as compared with the first embodiment.
Therefore, a decrease of operation speed can be minimized, and a
manufacturing process can be simplified.
[0048] Additionally, in the aforementioned first and second
embodiments, the gate length of the MOSFET constituting the
final-stage source follower circuit 2.sub.1 in the output circuit
14 is set to be longer than that of another MOSFET. However, the
MOSFET having a long gate length is not necessarily limited to the
source follower circuit 2.sub.1.
[0049] That is, as shown by an arrow y1 of FIG. 2, the gate lengths
of at least several of the respective MOSFETs constituting the
source follower circuit 2 at rear stage side in the output circuit
14 may be set to be longer than those of other MOSFETs.
Third Embodiment
[0050] In a third embodiment, MOSFET has a structure different from
that in the first and second embodiments, so that the impact
ionization does not easily occur.
[0051] FIG. 3 is an explanatory view of the third embodiment of the
solid image pickup apparatus according to the present invention,
and is a sectional view of the MOSFET constituting the source
follower circuit 2.sub.1 at final stage in the output circuit 14.
In the MOSFET of FIG. 3, a source region 31 and drain region 32 are
formed into a lightly doped drain (LDD) structure on both sides of
a gate electrode 30. Concretely, low-density regions 33, 34 are
disposed in the source region 31 and drain region 32, respectively.
Since the LDD structure is formed, concentration of the electric
field in the source region 31 and drain region 32 can be relaxed,
and the impact ionization does not easily occur.
[0052] Additionally, in FIG. 3, both the source region 31 and drain
region 32 are formed into the LDD structure, but either one region,
particularly only the drain region may be formed into the LDD
structure. In this case, deterioration of a property of MOSFET is
minimized.
[0053] Moreover, the MOSFET having the LDD structure is not
necessarily limited to the source follower circuit 2 at final
stage, but preferably at least several source follower circuits
including the initial-stage source follower circuit are not formed
into the LDD structure. In this case, a current to be passed
through the output circuit 14 can be increased, and the property of
the circuit is enhanced as compared with a case in which all
circuits are formed into the LDD structure.
[0054] Concretely, at least several MOSFETs at the final stage
shown by the arrow y1 of FIG. 2 are preferably formed into the LDD
structure.
Fourth Embodiment
[0055] In a fourth embodiment, a new MOSFET for relaxing the
electric field is added between two MOSFETs constituting the source
follower circuit 2.
[0056] FIG. 4 is an explanatory view of the fourth embodiment of
the solid image pickup apparatus according to the present
invention, and is a circuit diagram of the source follower circuit
2.sub.1 at final stage in the output circuit 14. The circuit of
FIG. 4 has a configuration in which a MOSFET Q3 for relaxing the
electric field is inserted between two MOSFETs Q1, Q2 constituting
the source follower circuit 2.sub.1. The MOSFET Q3 short-circuits a
drain terminal and gate terminal, and thereby serves as a
diode.
[0057] Since the MOSFET Q3 is disposed, the concentration of the
electric field on the drain side of the MOSFET Q2 constituting the
current source can be relaxed, and the impact ionization does not
occur.
[0058] Moreover, the source follower circuit in which the MOSFET Q3
is disposed is not necessarily limited to the source follower
circuit 2.sub.1 at final stage, but to simplify the structure, it
is preferable not to dispose the MOSFET Q3 in at least several
source follower circuits including the circuit at initial
stage.
[0059] Concretely, the transistor Q3 for relaxing the electric
field as shown in FIG. 4 may be inserted in at least several source
follower circuits 2 within an area at the final stage side shown by
the arrow y1 of FIG. 2.
Fifth Embodiment
[0060] In a fifth embodiment, a shielding member is disposed in an
incident route of the emitted light caused by the impact
ionization, so that the emitted light does not enter the
photoelectric converter.
[0061] FIG. 5 is a sectional view of the fifth embodiment of the
solid image pickup apparatus according to the present invention.
The emitted light caused by the impact ionization generated in the
output circuit 14 is passed through a route RT1 in a substrate, and
a route RT2 between the substrate and a shielding film 7, and is
incident upon the photoelectric converter 11.
[0062] In order to avoid the emitted light caused by the impact
ionization and passed through the routes RT1, RT2 of FIG. 5, in the
fifth embodiment, as shown in FIG. 5, shielding members 6 formed of
aluminum or another material are disposed along the proceeding
route of the emitted light caused by the impact ionization. Since
the shielding members 6 are disposed, passage of the emitted light
caused by the impact ionization can be interrupted, and the light
is not incident upon the photoelectric converter 11.
[0063] Additionally, a shape and number of the shielding members 6
are not especially limited. However, the shielding member 6 is
disposed at the rear stage in the output circuit 14 in which the
impact ionization easily occurs. Then, the emitted light can
efficiently be shielded by a small number of shielding members 6.
Concretely, the member is preferably disposed at the final stage in
the output circuit 14 shown by the arrow y1 of FIG. 2.
Sixth Embodiment
[0064] In the aforementioned first to fifth embodiments, examples
of the solid image pickup apparatus having the CCD register as a
transfer section have been described. However, solid image pickup
apparatuses having no CCD register, such as a MOS sensor, also have
a possibility that the image quality is deteriorated by generation
of an impact ion.
[0065] To solve the problem, in a sixth embodiment, the impact
ionization of the MOS sensor is prevented.
[0066] FIG. 6 is a schematic plan view of a linear sensor using the
MOS sensor. The MOS sensor of FIG. 6 has a photoelectric converter
11, source follower circuit 41, MOS switch 42, and shift register
43 for each pixel. A source electrode of each MOS switch 42 is
connected to an output line 44, and the output circuit 14 is
connected to the other end of the output line 44.
[0067] The charge accumulated in each photoelectric converter 11 is
converted to a voltage signal for each pixel in the source follower
circuit 41. As omitted from FIG. 6, a charge storage section
composed of a capacitor, and the like is disposed between the
source follower circuit 41 and the MOS switch 42.
[0068] The shift register 43 turns on the MOS switches 42 in order.
When each MOS switch 42 turns on, an output signal of the
corresponding source follower circuit 41 is inputted into the
output circuit 14 via the output line.
[0069] The output circuit 14 is constituted similarly as FIG. 1.
The gate length of at least one MOSFET constituting the source
follower circuit at final stage side of the output circuit 14 is
set to be longer than the gate length of the MOSFET constituting
the source follower circuit at previous stage. Therefore, similarly
as the first to fifth embodiments, the impact ion is not easily
generated, and the image quality is enhanced.
[0070] Moreover, in the linear sensor using the MOS sensor, among
the MOSFETs Q1, Q2 constituting the source follower circuit of the
output circuit 14 at final stage, the gate length of only the
MOSFET Q2 constituting the current source may be set to be longer
than the gate length of any other MOSFET in the output circuit
14.
[0071] Furthermore, in the linear sensor using the MOS sensor, the
source region 31 and drain region 32 of the MOSFET constituting the
final-stage source follower circuit 2.sub.1 in the output circuit
14 may be formed into the LDD structure.
[0072] Additionally, in the linear sensor using the MOS sensor, as
shown in FIG. 4, the MOSFET Q3 for relaxing the electric field may
be inserted between two MOSFETs Q1 and Q2 constituting the source
follower circuit 2.sub.1 at final stage in the output circuit
14.
[0073] Moreover, in the linear sensor using the MOS sensor, as
shown in FIG. 5, the shielding member may be disposed along the
proceeding route of the emitted light.
* * * * *