Color image sensor with embedded microlens array

Details Color image sensor with embedded microlens array Color image sensor with embedded microlens array

1999-12-23

2001-04-24

Elms, Richard (Department: 2824)

Semiconductor device manufacturing: process

Making device or circuit responsive to nonelectrical signal

Responsive to electromagnetic radiation

C438S069000, C438S075000

Reexamination Certificate

active

06221687

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to solid state image sensors. More specifically, the present invention relates to a method for fabricating color image sensors and to a color image sensor fabricated by the method.
RELATED ART
Solid state color image sensors are used, for example, in video cameras, and are presently realized in a number of forms including charge-coupled devices (CCDs) and CMOS image sensors. These image sensors are based on a two dimensional array of pixels. Each pixel includes color filter located over a sensing element. An array of microlenses located over the color filter focuses light from an optical image through the color filter into the image sensing elements. Each image sensing element is capable of converting a portion of the optical image passed by the color filter into an electronic signal. The electronic signals from all of the image sensing elements are then used to regenerate the optical image on, for example, a video monitor.
FIG.
1
(A) is a cross-sectional view showing a portion of a conventional color image sensor
10
. Color image sensor
10
is formed on an n-type semiconductor substrate
11
having a p-well layer
15
. An array of photodiodes
20
and charge transfer regions
25
are formed in p-well layer
15
, and are covered by a silicon oxide or nitride film
30
. A polysilicon electrode
35
is located over charge transfer region
25
such that it is surrounded by film
30
. A photo-shielding metal layer
40
is formed over electrode
35
, and a surface protective coating
45
and a planarization layer
50
are formed over metal layer
40
. A color filter layer
60
is formed on planarization layer
50
, and an intermediate transparent film
70
is formed over color filter layer
60
. A microlens
80
for focusing light beams
85
is formed from silicon dioxide (SiO
2
) or a resin material on intermediate transparent film
70
. An air gap
90
is provided over microlens
80
, and a glass packaging substrate
95
is located over air gap
90
.
In operation, light beams
85
are focused by microlens
80
through color filter layer
60
such that they converge along the focal axis F of microlens
80
to strike photodiode
20
, wherein photoenergy from light beams
85
frees electrons in photodiode
20
. When a select voltage is applied to polysilicon electrode
35
, these freed electrons generate a current in charge transfer region
25
. A sensor circuit (not shown) of color image sensor
10
then determines the amount of light received by photodiode
20
by measuring the amount of current generated in charge transfer region
25
.
Conventional solid-state imaging device
10
is designed for light beams
85
whose incident angle is perpendicular to substrate
11
, as shown in FIG.
1
(A), before being focused by microlens
80
onto photodiode
20
. However, during actual operation of color image sensor
10
, light beams can strike microlens
80
at oblique incident angles. A consequence of these oblique light beams is shown in FIG.
1
(B). In particular, light beams
87
enter microlens
80
at an oblique angle, which directs light beams
87
away from focal axis F such that they converge at the edge of photodiode
20
. Because the photoenergy of light beams
87
is not fully transferred to photodiode
20
, color image sensor
10
is unable to generate an accurate image.
Another problem associated with conventional solid-state imaging device
10
is that non-standard packaging methods are required due to the formation of microlenses
80
over color filter layer
60
and intermediate transfer layer
70
. Standard packaging methods typically include securing a glass substrate to an IC device using a layer of cement (e.g., epoxy). This cement typically has an index of refraction that is the same as silicon-dioxide and other resins typically used to form microlens
80
and other layers of conventional solid-state imaging device
10
. Therefore, to facilitate proper focusing of the light beams, air gap
90
must be provided between glass packaging substrate
95
and microlens
80
. Because air gap
90
is used in place of cement, the packaging method used to produce conventional solid-state imaging device
10
is non-standard.
It would be possible to avoid the oblique light beam problem (discussed above) by moving microlens
80
closer to photodiode
20
, thereby shortening the distance traveled by the light beams between microlens
80
and photodiode
20
. This shortened distance would reduce the displacement of focused oblique light beams
87
(see FIG.
1
(B)) relative to the center of photodiode
20
, thereby transferring more photoenergy from these oblique light beams to photodiode
20
.
One possible method of moving microlens
80
closer to photodiode
20
would be to reduce the thickness of the various layers located below microlens
80
. A problem with this method is that the thicknesses of these underlying layers are not easily reduced. First, photo-shielding layer
40
is typically formed during the formation of aluminum wiring utilized to transmit signals to and from each pixel of conventional solid-state imaging device
10
. Therefore, the thickness of photo-shielding layer
40
is limited by the wiring specifications. Repositioning microlens
80
closer to photodiode
20
is further restricted by planarization layer
50
, which is required to provide a flat surface for forming color filter layer
60
and microlens
80
. Therefore, it is not possible to significantly reduce the distance between a surface-mounted microlens
80
and photodiode
20
in conventional solid-state imaging device
10
by reducing the thickness of the layers underlying microlens
80
.
Another possible method of moving microlens
80
closer to photodiode
20
would be to form microlens
80
under color filter layer
60
(i.e., between photodiode
20
and color filter layer
60
). This arrangement would also address the non-standard packaging problem because, with color filter layer
70
located above microlens
80
, it would be possible to use cement to secure glass packaging substrate
95
according to standard packaging methods. However, forming microlens
80
under color filter layer
60
is not practical because, as discussed above, the index of refraction of conventional microlens materials (i.e., resin) is the same as that of other materials typically used to produce conventional solid-state imaging device
10
. Therefore, because air gap
90
must be provided over conventional microlens
80
, it would be very difficult to produce conventional solid-state imaging device
10
with microlens
80
located under color filter layer
60
using conventional microlens materials.
What is needed is a method for fabricating a color image sensor that minimizes the distance between the microlens and photodiode, and minimizes the fabrication and production costs of the color image sensor.
SUMMARY
The present invention is directed to a method for producing a color CMOS image sensor in which the microlens structure is embedded (i.e., located between the photodiode array and the color filter layer), thereby avoiding the oblique light beam problem, discussed above, because each microlens is located closer to its associated photodiode than in conventional image sensor structures. In addition, because the color filter layer is located above the microlenses and sandwiched between two color transparent layers, conventional image sensor packaging techniques (i.e., applying cement to the upper color transparent layer, then applying a glass substrate) may be utilized to produce color CMOS image sensors.
In accordance with a first embodiment of the present invention, an image sensor is produced by depositing a dielectric (e.g., silicon-nitride) layer over an image sensing element (e.g., a photodiode), etching the dielectric layer to form a microlens, and then depositing a protective layer on the microlens, wherein the protective layer has an index of refraction that is different from that of the dielectric. When silicon-nitride is utilized as the dielectric,

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