Semiconductor device manufacturing: process – Making device or circuit responsive to nonelectrical signal – Responsive to electromagnetic radiation
Reexamination Certificate
2000-04-12
2002-07-09
Chaudhari, Chandra (Department: 2813)
Semiconductor device manufacturing: process
Making device or circuit responsive to nonelectrical signal
Responsive to electromagnetic radiation
C438S072000
Reexamination Certificate
active
06417022
ABSTRACT:
FIELD OF THE INVENTION
The present invention generally relates to a method for fabricating micro-lens for color filters and devices fabricated and more particularly, relates to a method for fabricating long focal length micro-lens for color filters and devices fabricated by such method.
BACKGROUND OF THE INVENTION
Color fillers have been used in image sensors for CCD or CMOS applications for producing color images. An essential part of the color filters is the multiplicity of micro-lenses that is formed on top of the color filters in order to focus a light beam. Micro-lenses can be fabricated by a variety of different methods. one of the more frequently used methods is by a standard photolithographic technique followed by reflowing photoresist strips that are formed in cylinders or squares into hemispheres. The technique is carried out by exposing the photoresist to process temperatures in excess of the glass transition temperature of the micro-lens material. At a temperature higher than the glass transition temperature, the cohesive force in the material that forms the solid patterns causes the surface area to be minimized forming a hemisphere.
A typical process for forming a multiplicity of micro-lenses for color filters is shown in
FIGS. 1A and 1B
. Referring now to
FIG. 1A
, wherein a process flow chart
10
for fabricating a micro-lens is shown. The process flow-chart
10
corresponds to the structure
20
shown in
FIG. 1B
for a CMOS image sensor including a multiplicity of micro-lenses
12
and a multiplicity of color filters
22
. As shown in
FIG. 1B
, a photodiode
24
is first formed in the surface of a substrate
26
, which also includes a series of metal conductors
28
covered by a layer of passivation
30
that forms an irregular upper surface
32
. The irregular upper surface
32
is then planarized by depositing a layer of dielectric material
34
over the passivation layer
30
. As shown in
FIG. 1A
, after the planarization step
14
has been performed, a subsequent step
16
is carried out to form a color filter layer
16
providing red, green and blue color elements. As a final step, a micro-lens spacer
36
is applied by step
18
following which a micro-lens
12
is formed by step
38
.
A more detailed view of the conventional color filter
20
is shown in
FIGS. 2A
,
2
B,
3
A and
3
B.
FIG. 2A
illustrates an enlarged, cross-sectional view of the image sensor
20
before the micro-lens reflow process, while
FIG. 2B
illustrates an enlarged, cross-sectional view of the image sensor
20
after the micro-lens reflow process. By using the conventional technology, the micro-lens material layer (not shown) is first spin coated on the surface of the spacer layer
36
. The thinnest possible thickness for the micro-lens material layer formed by using the presently available spin coating technology is between about 1.3 &mgr;m and about 1.4 &mgr;m. This is achieved at a spinning speed of about 4500 RPM. By using the conventional technology, the focal length of the micro-lens fabricated is about 5~6 &mgr;m for a 6 &mgr;m diameter micro-lens. The focal length achieved is not sufficient for the new generation color filters, i.e. for 0.35 &mgr;m technology CMOS image sensors which require a focal length of at least 7 &mgr;m and preferably 10 &mgr;m. The longer focal length is necessary in order for a light beam to focus on the photodiode
24
that is formed in the substrate
26
. A multiple number of micro-lenses is shown in a plane view of
FIG. 3B
while an enlarged, cross-sectional view of a single micro-lens formed by the conventional method with a large diameter is shown in FIG.
3
B.
The conventional CMOS image sensor shown in
FIG. 2B
has only one metal layer
28
formed in a passivation layer
30
and a planarization layer
34
. In modern semiconductor devices used as image sensors, a multiple number of metal layers, for instance
3
metal conductor layers are utilized in a 0.35 &mgr;m technology CMOS image sensor. The small diameter of the micro-lens
12
, which is contributed by the large thickness of the micro-lens material spin-coated (
FIG. 2A
) cannot produce a micro-lens that has the necessary focal length of at least 7 &mgr;m and preferably at least 10 &mgr;m. In order to reduce the thickness of the micro-lens material, as shown in
FIG. 2A
, a significantly higher rotational speed must be utilized in a spin coating process, i.e. a speed higher than 4500 RPM for producing a thickness of 1.4 &mgr;m micro-lens material. At such high rotational speed, various other processing problems can be caused which include a possible loss of wafer from the wafer platform due to a break in vacuum used in holding the wafer. It is therefore impossible to vary the processing parameters of a spin coating process in order to provide a thinner layer of the micro-lens material, or to produce a multiplicity of micro-lenses each having a focal length longer than 7 &mgr;m.
It is therefore an object of the present invention to produce a long focal length micro-lens in an image sensor application that does not have the drawbacks or shortcomings of conventional methods.
It is another object of the present invention to provide a method for making a long focal length micro-lens for use in color filters that have a focal length of at least 7 &mgr;m.
It is a further object of the present invention to provide a method for making a long focal length micro-lens for colored filters that is suitable for fabricating 0.35 &mgr;m technology CMOS image sensors.
It is another further object of the present invention to provide a method for making a long focal length micro-lens for colored filters that can be used for CMOS image sensors that have at least two metal conductor layers.
It is still another object of the present invention to provide a method for making a long focal length micro-lens for color filters that is suitable for producing a micro-lens material layer of less than 1 &mgr;m thick.
It is yet another object of the present invention to provide a method for making a long focal length micro-lens for color filters suitable for use in CMOS image sensors wherein a micro-lens material is first coated to a thickness of less than 1 &mgr;m, and then patterned and reflowed into micro-lenses having a focal length larger than 7 &mgr;m for 6 &mgr;m diameter micro-lenses.
It is still another further object of the present invention to provide a color filter that has a multiplicity of long focal length micro-lenses built thereon wherein each of the multiplicity of micro-lenses has a focal length of at least 10 &mgr;m for 6 &mgr;m diameter micro-lenses.
It is yet another further object of the present invention to provide a color filter that has a multiplicity of long focal length micro lenses built thereon for a CMOS image sensor application that contains at least two metal conductor layers with metal conductors embedded in at least two insulating material layers.
SUMMARY OF THE INVENTION
In accordance with the present invention, a method for making long focal length micro lenses for color filters and a device made by such method are provided.
In a preferred embodiment, a method for making long focal length micro-lenses for color filters can be carried out by the operating steps of first providing a semiconductor substrate that has at least one photodiode formed in a top surface; forming at least two metal conductor layers with metal conductors embedded in at least two insulating layers; forming a photoresist layer on top of the at least two metal conductor layers, the photoresist layer further includes a color agent therein for forming color filters of at least one of red, green and blue colors; depositing a layer of micro-lens material on top of the photoresist layer; patterning the layer of micro-lens material into at least four discrete regions for each micro-lens with pre-set spacing therein between; and reflowing the at least four discrete regions for the micro-lens and forming the micro-lens.
The method for making a long focal length micro-lens for color filters may further include
Chang Bi-Cheng
Hsiao Yu-Kung
Lu Kuo-Liang
Pan Sheng-Liang
Chaudhari Chandra
Taiwan Semiconductor Manufacturing Co. Ltd.
Tung & Associates
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