Title: Method for making a color image sensor with recessed contact apertures prior to thinning
Abstract: The invention relates to method for making a color image sensor. The method comprises:
- the formation, on the front face of a semiconductive wafer (10), of a series of active zones (ZA) comprising image detection circuits and each corresponding to a respective image sensor, each active zone being surrounded by input/output pads (22),
- the transfer of the wafer by its front face against the front face of a temporary supporting substrate (20),
- the elimination of the major part of the thickness of the silicon wafer, leaving a fine silicon layer (30) on the substrate, this fine silicon layer comprising the image detection circuits.
Furthermore:
- firstly, layers of color filters (18) are deposited and then etched on the semiconductive layer thus thinned,
- secondly, prior to the transfer of the semiconductive wafer to the substrate, on the front face of the wafer, metallized apertures (25) are formed extending to a greater depth than the elements of the image detection circuits formed on the surface of the wafer, and the step of elimination of the major part of the thickness of the semiconductive wafer includes the baring, from the rear, of the metallization (22) of the metallized apertures.
Patent Number: 6,960,483 Issued on 11/01/2005 to Pourquier
| Inventors:
|
Pourquier; Eric (Voreppe, FR)
|
| Assignee:
|
Atmel Grenoble S.A. (Saint Egreve, FR)
|
| Appl. No.:
|
485743 |
| Filed:
|
August 30, 2002 |
| PCT Filed:
|
August 30, 2002
|
| PCT NO:
|
PCT/FR02/02979
|
| 371 Date:
|
February 4, 2004
|
| 102(e) Date:
|
February 4, 2004
|
| PCT PUB.NO.:
|
WO03/01966 |
| PCT PUB. Date:
|
March 6, 2003 |
Foreign Application Priority Data
| Current U.S. Class: |
438/22 |
| Intern'l Class: |
H01L 021/00 |
| Field of Search: |
438/20,22
257/98
|
References Cited [Referenced By]
U.S. Patent Documents
| 5144747 | Sep., 1992 | Eichelberger.
| |
| 5250843 | Oct., 1993 | Eichelberger.
| |
| 6646289 | Nov., 2003 | Badehi.
| |
| Foreign Patent Documents |
| 1 094 511 | Apr., 2001 | EP.
| |
| 99 40624 | Aug., 1999 | WO.
| |
Other References
Patent Abstract of Japan, vol. 007, No. 040 (E-159), Feb. 17, 1983 & JP 57 192052 A.
|
Primary Examiner: Coleman; W. David
Attorney, Agent or Firm: Lowe Hauptman & Berner, LLP
Claims
1. A method for making an image sensor, comprising the steps of:
forming, on the front face of a semiconductive wafer, of a series of active zones
comprising image detection circuits and each corresponding to a respective image
sensor, each active zone being surrounded by input/output pads;
transferring of the wafer by its front face against the front face of a temporary
supporting substrate;
eliminating of the major part of the thickness of the silicon wafer, leaving
a fine silicon layer on the substrate, this fine silicon layer comprising the image
detection circuits,
depositing, layers of color filters and then etching on the semiconductive layer
thus thinned,
prior to the transfer of the semiconductive wafer to the substrate, on the front
face of the wafer, metallized apertures are formed extending to a greater depth
than the elements of the image detection circuits formed on the surface of the
wafer, eliminating the major part of the thickness of the semiconductive wafer
includes the baring, from the rear, of the metallization of the metallized apertures,
dicing, the substrate into individual sensors after the deposition and the etching
of the color filters.
2. The method according to claim 1, wherein the remaining thickness of the thinned
semiconductive layer is about 3 to 20 micrometers.
3. The method according to claim 1, wherein a sheet of transparent material is
placed on the thinned semiconductive layer covered with color filters.
4. The method according to claim 2, wherein a sheet of transparent material is
placed on the thinned semiconductive layer covered with color filters.
Description
The invention relates to electronic image sensors, and especially to very small-sized
sensors with dimensions that enable the making of miniature cameras such as those
that are to be incorporated into a portable telephone.
Apart from great compactness, the image sensor should have high sensitivity
under weak light and excellent colorimetrical performance.
Furthermore, the entire camera needs to be made by the most economical
methods possible so as not to make the apparatus prohibitively costly.
To achieve this result, it is sought, firstly, to make the image sensor and the
electronic processing circuits if possible on a same silicon substrate and, secondly,
as far as possible, to carry out the deposition of the different layers, the etching
operations, the heat-processing operations etc. collectively on a silicon wafer
comprising many identical sensors, and then dice the wafer into individual sensors.
However, the methods hitherto proposed for making color image sensors and
the structures of these sensors are not entirely satisfactory from this viewpoint.
The methods of manufacture are not industrially efficient; they remain far too
costly and their efficiency is far too low for large-scale manufacturing applications,
or else the performance of the image sensor is not high enough.
The present invention proposes a method of manufacture and a corresponding image
sensor that minimizes the costs of manufacture while presenting excellent quality
and especially compactness, high sensitivity and high colorimetrical performance.
To this end, the invention propose a method for making an image sensor, comprising:
- the formation, on the front face of a semiconductive wafer, of a series
of active zones comprising image detection circuits and each corresponding to a
respective image sensor, each active zone being surrounded by input/output pads,
- the transfer of the wafer by its front face against the front face of
a temporary supporting substrate,
- the elimination of the major part of the thickness of the semiconductive
wafer, leaving a very fine semiconductive layer comprising the image detection
circuits on the substrate,
- this method being characterized in that:
- firstly, layers of color filters are deposited and then etched on
the semiconductive layer thus thinned,
- secondly, prior to the transfer of the semiconductive wafer to the
substrate, on the front face of the wafer, metallized apertures are formed extending
to a greater depth than the elements of the image detection circuits formed on
the surface of the wafer, and the elimination of the major part of the thickness
of the semiconductive wafer includes the baring, from the rear, of the metallization
of the metallized apertures,
- and finally, the substrate is diced into individual sensors after
the deposition and the etching of the color filters.
Preferably, the active zones comprise a matrix of photosensitive elements
as well as control circuits of the matrix and associated image-processing circuits
receiving signals coming from the photosensitive elements of the active area. The
circuits thus associated with the matrix are preferably masked against light by
a layer of aluminum, only the matrix being exposed to light.
The transfer of the semiconductive wafer to the temporary substrate can be done
by gluing, classic soldering, anodic bonding or by simple molecular adhesion (i.e.
through the very great force of contact between two surfaces having great planeity).
The thinning of the semiconductive wafer after transfer to the substrate and
before the deposition of the filters can be done in many different ways: thinning
by lapping, chemical thinning, a combination of both types of thinning (firstly
mechanical thinning and then chemical finishing or else mechanical machining in
the presence of chemicals). The thinning can also be done by a preliminary embrittlement
of the wafer at the desired dicing level, in particular by in-depth hydrogen implantation
in the desired dicing plane. In this case, the hydrogen implantation is done at
a shallow depth in the semiconductive wafer before the transfer of the wafer to
the substrate. The thinning is then done by heat processing which dissociates the
wafer at the level of the implanted dicing plane, leaving a thin semiconductive
layer in contact with the substrate.
The very great thinning of the wafer reduces its thickness from several hundreds
of micrometers before transfer to the substrate to 3 to 20 micrometers after transfer
to the substrate. Thinning is a major factor in the quality of the sensors since
it enhances colorimetrical performance and sensitivity. With non-thinned sensors,
illuminated by the side in which there are formed the numerous insulating and conductive
layers that serve to define the image detection circuits, the light that has crossed
a color filter is scattered on photosensitive dots corresponding to different colors,
thus impairing colorimetrical performance. Furthermore, the sensitivity of a thinned
sensor is improved because the photons reach a wider silicon region than in the
case of the non-thinned sensors, since they are not stopped by the metal layers
which are opaque and take up a large part of the surface area corresponding to
each photosensitive dot.
It will be understood that the thinning, however, complicates the problems of
manufacture because, after thinning, the silicon loses its rigidity and becomes
very brittle, and that, furthermore, there arises the problem of connecting the
image detection circuits with the exterior. The solution of the invention mitigates
this difficulty and enables the making of the image sensors with great efficiency.
The connection pads of the sensors thus made are in the front of the substrate,
on the side where the thinned silicon is located, the light being received from
the same side for the formation of an image.
The substrate and the silicon layer are in close contact and the active circuit
elements of the wafer are therefore well protected on this side.
Finally, on the thinned silicon layer, covered with color filters, it is
possible to place either a transparent sheet or a passivation layer or again microlenses
facing each sensor. These operations are preferably carried out on the substrate
in wafer form, before it is diced into individual sensors.
For example, the thickness of the substrate is about 500 micrometers for a substrate
with a diameter of 15 to 20 cm. The thickness of the silicon wafer is 500 to 1000
micrometers before thinning (with a diameter of 15 to 30 centimeters), and then
3 to 20 micrometers after thinning.
Planarization layers, made of polyimide for example, may be deposited
on the silicon wafer before transfer to the intermediate substrate.
Other features and advantages of the invention shall appear from the following
detailed description, made with reference to the appended drawings, of which:
FIG. 1 shows the structure of an image sensor made on a silicon wafer before
the positioning of color filters;
FIG. 2 shows the formation on this wafer of apertures filled with a metallization layer;
FIG. 3 shows the operation of transfer of the silicon wafer by its front face
to a supporting substrate;
FIG. 4 shows the supporting substrate with the silicon wafer after thinning
of the wafer;
FIG. 5 shows the substrate, bearing a thinned silicon layer on which a mosaic
of color filters has been deposited.
FIG. 1 shows the general structure of a silicon wafer on which classic techniques
have been used to make the image detection circuits of a multiplicity of image sensors.
The silicon wafer
10 has a thickness of several hundreds of micrometers,
for a diameter of 150 to 300 millimeters.
The image detection circuits (the matrix of photosensitive dots, transistors
and interconnections) are fabricated on one face of the silicon wafer, which may
be called the front face and is the upper face in FIG.
1. Fabrication implies,
firstly, various operations of diffusion and implantation in the silicon, from
the upper face of the wafer, to form especially photosensitive zones
12,
and, secondly, successive operations for the deposition and etching of conductive
layers
14 and insulating layers
16 forming a stack on top of the
photosensitive zones
12. The insulating and conductive layers form part
of the image detection circuits and enable the collection of electrical charges
generated in the photosensitive zones by an image projected on the sensor.
If the sensor were to be made by means of a classic technology, then a mosaic
of color filters would be deposited on the surface of the wafer. According to the
invention, they are not deposited at this stage and preliminary operations are performed.
For each individual image sensor formed on the silicon wafer, input/output pads
surround an active surface comprising both a matrix of photosensitive zones and
associated electronic circuits.
These input output pads (
22 in FIG. 2) are connected to the conductive
layers
14 and, in the present invention, are formed as follows: apertures
25 are hollowed out in the stack of insulating layers
16 as well
as in the depth of the silicon substrate. The depth of the apertures
25
is greater than the depth of all the image detection circuits (photosensitive zones,
interconnections, etc.) formed in the silicon wafer. This depth corresponds more
or less to the silicon thickness that will remain after the subsequent thinning
of the silicon and will contain these image detection circuits.
The thinning of the silicon, which will be done by the rear of the wafer, will
in principle reach exactly the bottom of the apertures
25. However, it will
be seen that the depth of the apertures
25 may be slightly different from
the desired thickness for the thinned silicon. Typically, the depth of the apertures
25 is about 5 to 20 microns inside the silicon of the wafer, namely about
15 to 30 micrometers below the surface of the stack of conductors and insulating
layers
14,
16.
In the apertures
25, there is preferably formed firstly an insulating
layer
26. Then a metal layer is deposited and etched. This metal layer will form
the connection pads
22. These connection pads come into contact with one
of the conductive layers
14. If, for this purpose, it is necessary to make
apertures for the local baring of a layer
14, the insulating layers
16
which cover the layer
14 are hollowed out locally before the deposition
of the metal layer in the aperture
25.
No color filters are deposited at this stage but the wafer is transferred by
its
front face to a temporary substrate
20 (FIG.
3). The substrate is
a wafer having the same diameter as the wafer
10 and a similar thickness
to ensure the rigidity of the structure while it is being made. It may furthermore
be constituted by another silicon wafer. The transfer can be done after the deposition
of a planarization layer serving to fill the relief features created on the front
face of the silicon wafer by the operations of deposition and etching of the stack
of conductive and insulating layers. This planarization layer does not need to
be transparent
FIG. 3 represents the structure on a smaller scale than that of FIG. 1 in order
to show the entire individual sensor comprising an active zone ZA and connection
pads
22 around the active zone ZA.
The transfer of the silicon wafer to the supporting wafer
20 can be done
by several means. The simplest means could be quite simply that of holding the
wafer by molecular adhesion, since the great planeity of the surfaces in contact
generates very high contact forces. Gluing is also possible. It is also possible
to carry out a soldering by means of the connection pads
22 and corresponding
pads formed beforehand in the substrate
20. In this case, it can furthermore
be envisaged that the substrate
20 will comprise auxiliary active or passive
circuit elements, that are connected to these pads and are therefore capable of
being directly connected to the image sensor.
After the silicon wafer has been transferred by the front face to the supporting
wafer, the major part of the thickness of the silicon wafer
10 is eliminated
so as to leave only a thickness of about 8 to 30 micrometers, including the thickness
of the stack of layers. What remains of the silicon wafer is no more than a superimposition
of a few micrometers (for example about ten micrometers) for the stack of layers
14,
16 and about three to 20 micrometers for the remaining silicon
thickness, including the photosensitive areas
12. The remaining thickness
is that of the layer
30 of FIG. 3 containing the photosensitive zones
12
of FIG.
1.
The thinning reveals the metallization of the connection pads
22 so that
they become electrically accessible by the rear face of the silicon wafer (the
front face being the upper face in FIG. 1, pointing upwards and covered with the
substrate
20 in FIGS.
4 and
5).
If the thinning goes slightly beyond the deepest part of the metallization, it
must allow a part of this metallization to remain in order to make access possible.
If the thinning is slightly short (by a few micrometers) of the deepest part of
metallization, it is possible to envisage the subsequent opening of access apertures
through the rear face of the thinned silicon, these apertures baring the metallizations
22.
The thinning operation can be done by mechanical machining (lapping) terminated
by chemical machining, or by chemical machining only, or by mechanical machining
in the presence of chemicals or again by a particular method of separation necessitating
a preliminary implantation of an embrittling impurity in the plane that will demarcate
the thinned silicon layer.
In the case of this separation by implantation of impurities, the implantation
must be done before the transfer of the silicon wafer to the supporting wafer.
Indeed, the implantation is done by the front face of the silicon wafer, throughout
the surface of the wafer and at a depth that will define the dicing plane. The
preliminary implantation is preferably hydrogen implantation. It can be done at
various stages of the making of the wafer, but the separation of the thickness
of the wafer along the implanted dicing plane can be done only when the silicon
wafer has been attached to the supporting wafer.
The upper surface of the thinned silicon layer
30 can be processed (fine
lapping, chemical cleaning, mechanical/chemical polishing, etc.) in order to eliminate
the surface defects, leading to a multiple-sensor wafer whose general structure
is that of FIG.
2.
A mosaic of color filters
18 is then deposited on the surface of the layer
30 (FIG.
4). However, one or more additional layers can be deposited
before the deposition of the color filters, especially passivation layers, anti-reflection
layers and other layers, electrical activation layers etc.
A glass film, or an individual lens for the image sensor, or a matrix of microlenses
having the same spacing pitch as the color filters
18 may be deposited on
the rear face (the face to be illuminated) of the structure after the deposition
and etching of the color filters.
The connection pads that have been bared by the thinning operation may be used
for a "wire-bonding" type connection (the wires
54 being soldered to the
pads) or a "flip-chip" type of connection (the chip being placed upside down with
the connection pads against the corresponding pads of the printed circuit board
with intermediate conductive bosses
56). In this case, the sensor is illuminated
through the top of the printed circuit board and the board must have an aperture
facing the photosensitive matrix.
In these different embodiments, the structure formed on the substrate
40
may be tested on the wafer by means of the connection pads. The test may be performed
in the presence of light, image patterns, etc.
The structure is diced into individual sensors for packaging only at the end
of this fabrication process.
*
