Solid-state imaging device

Details Solid-state imaging device Solid-state imaging device

2001-08-21

2004-08-31

Coleman, W. David (Department: 2823)

Semiconductor device manufacturing: process

Making device or circuit responsive to nonelectrical signal

Responsive to electromagnetic radiation

C438S065000, C438S073000, C438S075000

Reexamination Certificate

active

06784014

ABSTRACT:

RELATED APPLICATION DATA
The present invention claims priority to Japanese Application No. P2000-251241, filed Aug. 22, 2000, which application is incorporated herein by reference to the extent permitted by law.
BACKGROUND OF THE INVENTION
The present invention relates to a solid-state imaging device and a process for production thereof, the solid-state imaging device being characterized in that each pixel has a convex lens with an upwardly curved surface which is embedded in an inter-layer dielectric between a light-receiving portion and an on-chip lens.
The CCD solid-state imaging device should desirably have a reduced chip size and an increased number of pixels. Unfortunately, this object is not achieved simply by reducing the chip size while leaving the current pixel size as it is because such an attempt ends up with a reduction in the number of pixels and hence a reduction in resolution. Nor is the object achieved simply by increasing the number of pixels while leaving the current pixel size as it is because such an attempt ends up with an increase in chip size and hence an increase in production cost and a decrease in yields.
Therefore, for reduction in chip size and for increase in the number of pixels, it is essential to reduce the pixel size from the current one. If this object is achieved, it is possible to provide a small-sized CCD imaging device which keeps the current resolution unchanged or conversely to improve resolution while keeping the current chip size unchanged.
The problem with reduction in pixel size is that the amount of light incident on a unit pixel decreases and the light-receiving portion of a unit pixel becomes poor in sensitivity characteristics. Although the second difficulty can be overcome by improving the photoelectric conversion efficiency with a concomitant adverse effect of amplifying noise components, the result is a decrease in S/N ratio of image signals output from the CCD imaging device. In other words, it is not desirable to compensate for the loss of sensitivity characteristics resulting from reduction in pixel size only with improvement in photo-electric conversion efficiency, but it is desirable to improve as much as possible the condensing efficiency of each pixel, thereby preventing the decrease in S/N ratio.
An idea contrived from the above-mentioned standpoint is to form an on-chip lens (OCL) on the color filter formed on the light-receiving portion. This idea, however, is not practicable for a CCD imaging device having a pixel size smaller than 4×4 &mgr;m, because the efficiency of condensing light with an on-chip lens alone has almost approached the upper limit. A new technology to overpass the limit has been proposed in Japanese Patent Laid-open No. Hei 11-40787. This technology is concerned with an additional convex lens in filmy form of light-transmitting material which is formed in the layer between the on-chip lens and the light-receiving portion. This additional convex lens is designed to improve further the efficiency of condensing light. According to the disclosure, the convex lens is formed by the process which is explained below with reference to
FIGS. 6A
to
7
C. This process is referred to as “conventional process
1
”.
As shown in
FIG. 6A
, the process starts with fabrication of a silicon substrate
1
to form thereon the following components in the conventional way. A light-receiving portion
2
, charge transfer portions
3
−1
and
3
−2
, a gate portion (not shown) between the light-receiving portion
2
and the charge transfer portion
3
−1
, and a channel stopper (not shown) between the light-receiving portion
2
and the charge transfer portion
3
−2
. Transfer electrodes
5
are embedded in the insulating film
4
covering the charge transfer portions
3
−1
and
3
−2
. On the insulating film
4
is formed a shielding film
6
of high-melting metal which has an opening above the light-receiving portion
2
.
Then, a BPSG film
20
is formed on the shielding film
6
and the opening therein. The BPSG film
20
is planarized by reflowing as shown in FIG.
6
B. On the planarized film
20
is formed a light-transmitting film
21
a
from silicon nitride (P-SiN) or silicon oxide (P-SiO
2
) by plasma CVD as shown in FIG.
6
C.
The light-transmitting film
21
a
is coated with a resist. The resist film is patterned such that a region around the center of the light-receiving portion
2
remains. The patterned resist undergoes reflowing, so that it softens and forms a convex lens (resist pattern RP) having a prescribed curvature, as shown in FIG.
7
A.
Etching is performed under the condition that the resist and the light-transmitting film have almost the same selectivity. Etching removes the resist, while leaving the light-transmitting film in the form of convex lens
21
. The shape of the convex lens
21
conforms well to the shape of the resist pattern RP, as shown in FIG.
7
B.
Subsequently, the convex lens
21
is embedded in a planarizing film
9
. Finally, an on-chip color filter (OCCF)
10
and an on-chip lens (OCL)
11
are formed in the usual way, as shown in FIG.
7
C.
There is another process (“conventional process
2
”) in which the convex lens forming step in “conventional process
1
” is modified as explained below.
FIGS
8
A to
9
C are sectional views showing “conventional process
2
”.
As shown in
FIG. 8A
, “conventional process
2
” starts with fabrication of a silicon substrate
1
to form the following components thereon as in “conventional process
1
”. A light-receiving portion
2
, charge transfer portions
3
−1
and
3
−2
, an insulating film
4
, transfer electrodes
5
, and a shielding film
6
.
A PSG film or BPSG film
22
is formed on the shielding film
6
and the opening therein. The PSG film or BPSG film undergoes reflowing. According to “conventional process
2
”, this reflowing does not achieve complete planarizing but forms a depression
22
a
above the light-receiving portion
2
.
As shown in
FIG. 8B
, a light-transmitting film
23
a
of P-SiN or P-SiO
2
is formed on the PSG film or BPSG film
22
. On the light-transmitting film
23
a
is formed resist R which is subsequently planarized.
Etching is performed under the condition that the resist R and the light-transmitting film
23
a
have almost the same selectivity. Etching forms the light transmitting film
23
b
, with its surface planarized, as shown in FIG.
8
C.
As shown in
FIG. 9A
, a light transmitting film
23
c
is formed on the planarized surface. Then, a resist is applied to the light transmitting film
23
c
, and the resist film is patterned such that a region around the center of the light-receiving portion
2
remains. This patterning is following by reflowing. In this way there is obtained a resist pattern RP in the form of convex lens having a prescribed curvature.
Etching is performed again under the condition that the resist and the light-transmitting film have almost the same selectivity. Etching removes the resist, while leaving the light-transmitting film in the form of convex lens
23
. The shape of the convex lens
23
conforms well to the shape of the resist pattern RP, as shown in FIG.
9
B.
Finally, a planarizing film
9
is formed and an OCCF
10
and an OCL
11
are formed in the usual way, as shown in FIG.
9
C.
The CCD imaging element obtained by “conventional process
1
” and “conventional process
2
” functions in the following way as shown in FIG.
10
. The OCL
11
converges to some extent the incident ray (the vertical incident ray L
0
and the oblique incident ray L
1
with respect to the light-receiving plane). Another convex lens
21
(or
23
) under the OCL
11
further converges the converged incident light. The converged light reaches the light-receiving portion
2
. Thus, the convex lens
21
(or
23
) improves the efficiency of condensing incident rays. The convex lens
21
(or
23
) is particularly effective in condensing the oblique incident ray L
1
indicated by broken lines in FIG.
10
. This improves the sensitivity of each p

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