Active solid-state devices (e.g. – transistors – solid-state diode – Responsive to non-electrical signal
Reexamination Certificate
2003-01-22
2004-08-10
Nelms, David (Department: 2818)
Active solid-state devices (e.g., transistors, solid-state diode
Responsive to non-electrical signal
C257S053000, C257S108000, C257S225000, C257S252000, C438S048000, C438S049000, C438S051000, C438S054000, C438S570000
Reexamination Certificate
active
06774444
ABSTRACT:
This application claims priority to Japanese Patent Application Number JP2002-015078 filed Jan. 24, 2002, which is incorporated herein by reference.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a semiconductor device with a charge-storing unit, to a solid-state imaging device such as a charge-coupled device (CCD) image sensor or a complementary metal-oxide semiconductor (CMOS) image sensor, and to a method for making the semiconductor device or the solid-state imaging device.
2. Description of the Related Art
FIG. 4
is a cross-sectional view showing an example layer structure around a pixel unit of a conventional CCD solid-state imaging device.
The solid-state imaging device includes an epitaxial layer
11
formed on an N-type silicon substrate
10
. The epitaxial layer
11
includes a photosensor unit (photodiode)
20
that functions as an imaging pixel and a CCD vertical transfer unit
30
. A dotted line &agr; indicates the border between the epitaxial layer
11
and the substrate
10
.
The vertical transfer unit
30
is formed as a strip extending in a direction perpendicular to the plane of paper in
FIG. 4. A
plurality of vertical transfer units
30
is aligned at a regular interval.
Each photosensor unit
20
is formed as a dot that constitutes a two-dimensional matrix. The photosensor units
20
generate signal charges by photoelectric conversion of light incident on the substrate surface and store the generated signal charges.
Referring again to
FIG. 4
, a transfer electrode
40
is disposed in a region directly above the vertical transfer unit
30
and at the top of the epitaxial layer
11
. The transfer electrode
40
is separated from the vertical transfer unit
30
by an insulating film (not shown) therebetween. Each of the transfer electrodes
40
is formed as a strip extending in a direction parallel to the plane of paper of FIG.
4
. The transfer electrodes
40
are sequentially aligned in a direction perpendicular to the plane of the paper of FIG.
4
.
A vertical-transfer clock pulse is applied to the transfer electrode
40
so as to sequentially transfer the signal charges read out from the photosensor unit
20
to the vertical transfer unit
30
. The transferred signal charges are output to a horizontal transfer unit (not shown).
The horizontal transfer unit that received the signal charges from the vertical transfer unit
30
transfers the signal charges in the horizontal direction, i.e., the direction parallel to the plane of the paper of
FIG. 4
, so as to output the signal charges to a charge-detection amplifier (not shown). The charge-detection amplifier converts the signal charges to voltage signal or current signal and outputs the converted signals.
A light-shielding film
50
is formed to cover each transfer electrode
40
. The transfer electrode
40
is separated from the light-shielding film
50
by an insulating film (not shown) therebetween.
An opening
52
is formed between the light-shielding films
50
to expose the light-receiving surface of the photosensor unit
20
. While light enters the photosensor unit
20
via the opening
52
, the light-shielding film
50
inhibits light from entering the sections other than the photosensor unit
20
.
An on-chip-lens (OCL)
60
for condensing incident light is formed above the light-shielding film
50
.
An embedded transfer channel
32
and a second P-type well region (2PW)
34
surrounding the transfer channel
32
are formed in the epitaxial layer
11
. The transfer channel
32
and the second P-type well region (2PW)
34
form a transfer path for the signal charges and thus function as the vertical transfer unit
30
.
An overflow barrier (OFB) layer that allows a vertical overflow structure is formed under the photosensor unit
20
. The OFB layer is a potential barrier prepared by forming a first P-type well region (1PW, shaded region in
FIG. 4
)
70
in the silicon substrate
10
.
The photosensor unit
20
consists of an upper layer, which is a P-type impurity region, and a lower layer, which is an N-type impurity region. The region around the interface between the N-type impurity region and the OFB layer is a depletion region. The charges generated in and around the depletion region flow into the photosensor unit
20
via the depletion region.
Accordingly, in order to improve the sensitivity of the photosensor unit
20
, the depletion region must be extended over a wider range. In order to extend the depletion region, the position of the first P-type well region (1PW)
70
in the substrate must deep.
Conventionally, in order to form the first P-type well region
70
at a deep position, boron ions for forming P-type well region
70
are first implanted in the silicon substrate
10
, and the epitaxial layer
11
of N-type is then formed on the implanted silicon substrate
10
.
To be more specific, referring to
FIG. 5A
, a resist mask with openings is placed on the N-type silicon substrate
10
(first substrate), and ions of a P-type impurity are implanted into the N-type silicon substrate
10
to form the first P-type well region
70
. As shown in
FIG. 5B
, a lightly doped N-type epitaxial layer (second substrate) is then formed, and, subsequently, a transfer channel, a photosensor, and the like are formed in the epitaxial layer.
However, growing of the N-type epitaxial layer after ion plantation has the following problems.
First, a high temperature of, for example, approximately 1,100° C. or more is necessary to form the epitaxial layer
11
. Because of the high temperature, boron ions for forming the first P-type well region
70
implanted into the N-type silicon substrate
10
diffuse externally. As a result, as shown in
FIG. 5C
, the distribution of the boron concentration in the first P-type well region
70
becomes more spread compared with the boron distribution concentration immediately after the implantation.
Since both the substrate
10
and the epitaxial layer
11
are N-type, the difference in concentration between the substrate and the epitaxial layer contributes as a p-type impurity. However, the amount of the difference is easily changed according to the growth conditions of the epitaxial layer.
For example, when the process time is long, a significantly large amount of boron ions are driven out by external diffusion. Moreover, the n-type impurity is also driven out due to external diffusion. External diffusion of impurities become significant and the amount of impurities becomes highly unpredictable as the temperature is increased.
Moreover, external diffusion continues even during formation of the epitaxial layer. Since impurity ions also diffuse into the growing epitaxial layer, the amount of the impurity ions becomes further unstable.
In order to form the epitaxial layer, SiH
4
, dichlorosilane, trichlorosilane, or the like that contains phosphorus or arsenic as the N-type impurity is used. The type of impurity for the epitaxial layer is selected depending on the partial pressure of the N-type impurity in the gas, the growth rate, and the process time. The boron ions that have been driven out by external diffusion may be incorporated into the epitaxial layer, thereby degrading the controllability of the amount of the impurity.
Accordingly, the impurity concentration of the first P-type well region
70
and that of the epitaxial layer
11
are difficult to control.
The instability in impurity concentration in these regions results in instability in formation of the depletion region. Moreover, it also results in varying of the voltages applied to the substrate during accumulation of saturating signal charges and in varying of high voltages applied to the substrate for flushing the charges in the substrate direction, i.e., voltages for operating electronic shutters.
Furthermore, because the impurity concentration in the photosensor is widely distributed, the size of the depletion region change accordingly. Since incident light reaches a different depth depending on the wavelength, there is a problem in that the sensitivity varies depen
Depke Robert J.
Holland & Knight LLP
Huynh Andy
Nelms David
Sony Corporation
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