U.S. patent application number 14/447855 was filed with the patent office on 2015-04-16 for light sensors having dielectric optical coating filters.
The applicant listed for this patent is Intersil Americas LLC. Invention is credited to Francois Hebert, Eric S. Lee, Michael I-Shan Sun.
Application Number | 20150102444 14/447855 |
Document ID | / |
Family ID | 49211009 |
Filed Date | 2015-04-16 |
United States Patent
Application |
20150102444 |
Kind Code |
A1 |
Lee; Eric S. ; et
al. |
April 16, 2015 |
LIGHT SENSORS HAVING DIELECTRIC OPTICAL COATING FILTERS
Abstract
Light sensors including dielectric optical coatings to shape
their spectral responses, and methods for fabricating such light
sensors in a manner that accelerates lift-off processes and
increases process margins, are described herein. In an embodiment,
a light sensor includes a photodetector sensor region formed in a
semiconductor substrate, a dielectric optical coating filter
covering the photodetector sensor region, and dummy dielectric
optical coating features beyond the photodetector sensor region,
wherein the dummy dielectric optical features include one or more
dummy corners, dummy islands and/or dummy rings. Alternatively, or
additionally, the dielectric optical coating filter includes
chamfered corners, which improves the thermal reliability of the
dielectric optical coating.
Inventors: |
Lee; Eric S.; (San
Francisco, CA) ; Sun; Michael I-Shan; (San Jose,
CA) ; Hebert; Francois; (San Mateo, CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Intersil Americas LLC |
Milpitas |
CA |
US |
|
|
Family ID: |
49211009 |
Appl. No.: |
14/447855 |
Filed: |
July 31, 2014 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
13530809 |
Jun 22, 2012 |
8836064 |
|
|
14447855 |
|
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|
61613283 |
Mar 20, 2012 |
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Current U.S.
Class: |
257/432 |
Current CPC
Class: |
H01L 31/103 20130101;
H01L 31/02162 20130101; H01L 31/18 20130101; H01L 21/0272 20130101;
H01L 31/101 20130101; H01L 31/02327 20130101 |
Class at
Publication: |
257/432 |
International
Class: |
H01L 31/0216 20060101
H01L031/0216 |
Claims
1. A light sensor, comprising: a photodetector sensor region formed
in a semiconductor substrate; a dielectric optical coating filter
covering the photodetector sensor region and covering a
circumferential region of the semiconductor substrate that
surrounds the photodetector sensor region; and dummy dielectric
optical coating features beyond the photodetector sensor region,
wherein the dummy dielectric optical coating features include one
or more dummy corners, dummy islands and/or dummy rings.
2. The light sensor of claim 1, wherein a portion of the dielectric
optical coating filter that extends beyond the photodetector sensor
region and covers the circumferential region includes a plurality
of dummy corners that are the dummy dielectric optical coating
features.
3. The light sensor of claim 1, wherein: the photodetector sensor
region has a photodetector footprint; the dielectric optical
coating filter has a filter footprint that is larger than and
encompasses the photodetector footprint; and the dummy dielectric
optical coating features comprise one or more dummy islands and/or
dummy rings of the dielectric optical coating that are located
beyond the filter footprint.
4. The light sensor of claim 1, wherein: the photodetector sensor
region has a photodetector footprint; the dielectric optical
coating filter has a filter footprint that is larger than and
encompasses the photodetector footprint; and the filter footprint
includes dummy corners that correspond to at least some of the
dummy dielectric optical coating features.
5. The light sensor of claim 1, wherein: the photodetector sensor
region includes a plurality of sides that define a periphery of the
photodetector sensor region; and the dummy dielectric optical
coating features comprise one or more dummy corners, dummy islands
and/or dummy rings of the dielectric optical coating adjacent to at
least one of the sides of the photodetector sensor region.
6. The light sensor of claim 1, wherein: the photodetector sensor
region includes a plurality of sides that define a periphery of the
photodetector sensor region; and the dummy dielectric optical
coating features comprise one or more dummy corners of the
dielectric optical coating adjacent each of the sides of the
photodetector sensor region.
7. The light sensor of claim 1, wherein the dielectric optical
coating filter has one or more chamfered corners.
8. The light sensor of claim 1, wherein the dielectric optical
coating filter includes: four chamfered corners; and four
peripheral sides connected by the four chamfered corners.
9. The light sensor of claim 7, wherein: the photodetector sensor
region has a photodetector footprint; the dielectric optical
coating filter has a filter footprint that is larger than and
encompasses the photodetector footprint; and the filter footprint
includes four peripheral sides connected by four chamfered
corners.
10. The light sensor of claim 1, wherein: the photodetector sensor
region has a photodetector footprint; the dielectric optical
coating filter has a filter footprint that is larger than and
encompasses the photodetector footprint; and the filter footprint
includes a plurality of dummy corners.
11. A light sensor, comprising: a photodetector sensor region
formed in a semiconductor substrate and having a rectangular
photodetector footprint; and a dielectric optical coating filter
covering the photodetector sensor region and having a filter
footprint that is larger than and encompasses the rectangular
photodetector footprint; wherein the rectangular photodetector
footprint includes four sides each of which is straight; and
wherein the filter footprint includes four sides at least one of
which include at least three dummy corners.
12. The light sensor of claim 11, wherein one or more of the dummy
corners comprises a 90 degree corner.
13. A light sensor, comprising: a photodetector sensor region
formed in a semiconductor substrate; and a dielectric optical
coating filter covering the photodetector sensor region and
covering a circumferential region of the semiconductor substrate
that surrounds the photodetector sensor region; and wherein the
dielectric optical coating filter includes four peripheral
sidewalls; and wherein at least one of the peripheral sidewalls of
the dielectric optical coating filter includes at least three dummy
corners.
14. A light sensor, comprising: a photodetector sensor region
formed in a semiconductor substrate, the photodetector sensor
region having a photodetector footprint; and a dielectric optical
coating filter covering the photodetector sensor region and
covering a circumferential region of the semiconductor substrate
that surrounds the photodetector sensor region, the dielectric
optical coating filter having a filter footprint that is larger
than and encompasses the photodetector footprint; wherein the
dielectric optical coating filter includes one or more chamfered
corners; wherein the filter footprint includes four peripheral
sides; and wherein one or more of the four peripheral sides of the
filter footprint include dummy corners.
15. The light sensor of claim 14, wherein: the one or more
chamfered corners comprise four of the chamfered corners; and the
dielectric optical coating filter comprises four peripheral sides
connected by the four chamfered corners.
16. The light sensor of claim 14, wherein: the four peripheral
sides of the filter footprint are connected by four chamfered
corners.
17. (canceled)
18. The light sensor of claim 2, wherein one or more of the dummy
corners comprises a 90 degree corner.
19. The light sensor of claim 10, wherein one or more of the dummy
corners comprises a 90 degree corner.
20. The light sensor of claim 13, wherein one or more of the dummy
corners comprises a 90 degree corner.
21. The light sensor of claim 14, wherein one or more of the dummy
corners comprises a 90 degree corner.
Description
PRIORITY CLAIM
[0001] This application is a Divisional of U.S. patent application
Ser. No. 13/530,809, filed Jun. 22, 2012, which claims priority
under 35 U.S.C. 119(e) to U.S. Provisional Patent Application No.
61/613,283, filed Mar. 20, 2012, both of which are incorporated
herein by reference. Priority is claimed to both of these
applications.
RELATED APPLICATION
[0002] This application is related to commonly assigned and
commonly invented U.S. patent application Ser. No. 13/530,675,
filed Jun. 22, 2012, which is entitled ENHANCED LIFT-OFF TECHNIQUES
FOR USE WHEN FABRICATING LIGHT SENSORS INCLUDING DIELECTRIC OPTICAL
COATING FILTERS (Attorney Docket No. ELAN-01280US1), which is
incorporated herein by reference.
BACKGROUND
[0003] FIG. 1 shows a cross section of an exemplary conventional
light sensor 102, which is essentially a single photodiode, also
referred to as a photodetector. The photodetector 102 includes an
N.sup.+ region 104, which is heavily doped, and a P.sup.- region
106 (which can be a P.sup.- epitaxial region), which is lightly
doped. All of the above is likely formed on a P.sup.+ or P.sup.++
substrate 108, which is heavily doped. It is noted that FIG. 1 and
the remaining FIGS. are not drawn to scale.
[0004] Still referring to FIG. 1, the N.sup.+ region 104 and
P.sup.- region 106 form a PN junction, and more specifically, a
N.sup.+/P.sup.- junction. This PN junction is reversed biased,
e.g., using a voltage source (not shown), which causes a depletion
region 110 around the PN junction. When light 112 is incident on
the photodetector 102 (and more specifically on the N.sup.+ region
104), electron-hole pairs are produced in and near the diode
depletion region 110. Electrons are immediately pulled toward
N.sup.+ region 104, while holes get pushed down toward P.sup.-
region 106. These electrons (also referred to as carriers) are
captured in N.sup.+ region 104 and produce a measurable
photocurrent, which can be detected, e.g., using a current detector
(not shown). This photocurrent is indicative of the intensity of
the light 112, thereby enabling the photodetector to be used as a
light sensor. The portion of the photodetector 102 that produces a
photocurrent in response to light incident on the photodetector can
be referred to as the photodetector sensor region, or simply as the
sensor region.
[0005] Photodetectors, such as but not limited to the exemplary
photodetector 102, can be used as ambient light sensors (ALSs),
e.g., for use as energy saving light sensors for displays, for
controlling backlighting in portable devices such as mobile phones
and laptop computers, and for various other types of light level
measurement and management. For more specific examples, ambient
light sensors can be used to reduce overall display-system power
consumption and to increase Liquid Crystal Display (LCD) lifespan
by detecting bright and dim ambient light conditions as a means of
controlling display and/or keypad backlighting. Without ambient
light sensors, LCD display backlighting control is typically done
manually whereby users will increase the intensity of the LCD as
the ambient environment becomes brighter. With the use of ambient
light sensors, users can adjust the LCD brightness to their
preference, and as the ambient environment changes, the display
brightness adjusts to make the display appear uniform at the same
perceived level; this results in battery life being extended, user
eye strain being reduced, and LCD lifespan being extended.
Similarly, without ambient light sensors, control of the keypad
backlight is very much dependent on the user and software. For
example, keypad backlight can be turned on for 10 seconds by a
trigger which can be triggered by pressing the keypad, or a timer.
With the use of ambient light sensors, keypad backlighting can be
turned on only when the ambient environment is dim, which will
result in longer battery life. In order to achieve better ambient
light sensing, ambient light sensors preferably have a spectral
response close to the human eye response and have excellent
infrared (IR) noise suppression. Such a spectral response is often
referred to as a "true human eye response" or a "photopic
response".
[0006] FIG. 2 shows an exemplary spectral response of a
photodetector (e.g., the photodetector 102) without any spectral
response shaping, e.g., using a filter covering the detector. FIG.
3 illustrates the spectral response of a typical human eye (also
known as the "true human eye response" or the "photopic response",
as mentioned above). As can be appreciated from FIGS. 2 and 3, a
potential problem with using a photodetector as an ambient light
sensor is that it detects both visible light and non-visible light,
such as infrared (IR) light, which starts at about 700 nm. By
contrast, notice from FIG. 3 that the human eye does not detect IR
light. Thus, the response of a photodetector can significantly
differ from the response of a human eye, especially when the light
is produced by an incandescent light, which produces large amounts
of IR light. This would provide for significantly less than optimal
adjustments if the photodetector were used as an ambient light
sensor, e.g., for adjusting backlighting, or the like. Accordingly,
various techniques have been attempted to provide light sensors
that have a spectral response closer to that of a human eye, so
that such light sensors can be used, e.g., for appropriately
adjusting the backlighting of displays, or the like. Some of these
techniques involve covering such light sensor with optical
filters.
[0007] Typically, organic based optical filters cannot be used to
provide a true human eye response, because organic based optical
filters do not sufficiently absorb and/or reflect infrared light.
Rather, non-organic filters, such as filters made of dielectric
optical coatings, are generally preferred because they provide
better performance. Such dielectric optical coatings, which are
made from stacks of various dielectric films, are conventionally
expensive to implement. This is in part because they are typically
patterned using a photoresist lift-off in a chemical solvent bath,
which is typically costly due to the relatively long residence time
(i.e., soak duration) in the photoresist solvent bath, and due to
the relatively narrow process margin. Alternatively, acoustic
cleaning can be used, which is also typically costly.
BRIEF DESCRIPTION OF THE DRAWINGS
[0008] FIG. 1 shows a cross section of an exemplary conventional
photodetector.
[0009] FIG. 2 shows an exemplary spectral response of a
photodetector, such as the photodetector of FIG. 1, without any
spectral response shaping.
[0010] FIG. 3 illustrates the spectral response of a typical human
eye.
[0011] FIG. 4 illustrates a conventional lift-off process with a
negative profile photoresist.
[0012] FIG. 5 illustrates a conventional lift-off process with a
positive profile photoresist.
[0013] FIG. 6 illustrates a soft bake process, according to an
embodiment of the present invention, which provides for a more
rapid lift-off process and an increased process margin.
[0014] FIG. 7 provides illustrations of the photoresist reflow that
occurs during the soft bake process.
[0015] FIG. 8 provides illustrations of the micro-cracks that occur
in the dielectric optical coating as a result of the soft bake
process.
[0016] FIG. 9 illustrates a thermal shock process, according to an
embodiment of the present invention, which provides for a more
rapid lift-off process and an increased process margin.
[0017] FIG. 10 illustrates an example of the temperature cycling
involved in the thermal shock process.
[0018] FIG. 11 illustrates a top down view of an exemplary light
sensor being manufactured, wherein the light sensor includes a
rectangular photodetector sensor region surrounded by photoresist
and covered by a dielectric optical coating.
[0019] FIGS. 12A, 12B, and 12C include perspective cross-sectional
views that correspond to a portion of the light sensor shown in
FIG. 11 during different steps of a fabrication process.
[0020] FIG. 13, which is used to describe an embodiment of the
present invention, illustrates a top down view of an exemplary
light sensor being manufactured, wherein the light sensor includes
a rectangular photodetector sensor region surrounded by photoresist
and covered by a dielectric optical coating, and wherein the
photoresist includes additional corners compared to FIG. 11.
[0021] FIGS. 14A, 14B, and 14C include perspective cross-sectional
views that correspond to a portion of the light sensor shown in
FIG. 13 during different steps of a fabrication process.
[0022] FIG. 15, which is used to describe an embodiment of the
present invention, illustrates a top down view of an exemplary
light sensor being manufactured, wherein the light sensor includes
a rectangular photodetector sensor region surrounded by photoresist
and covered by a dielectric optical coating, and wherein the
photoresist includes dummy islands that provide additional corners
compared to FIG. 11.
[0023] FIGS. 16A, 16B, and 16C include perspective cross-sectional
views that correspond to a portion of the light sensor shown in
FIG. 15 during different steps of a fabrication process.
[0024] FIG. 17 is used to describe an embodiment of the present
invention that combines the embodiment described with reference to
FIGS. 13 and 14, with the embodiment described with reference to
FIGS. 15 and 16.
[0025] FIG. 18, which is used to describe an embodiment of the
present invention, illustrates a top down view of an exemplary
light sensor being manufactured, wherein the light sensor includes
a rectangular photodetector sensor region surrounded by photoresist
and covered by a dielectric optical coating, and wherein the
photoresist includes a dummy ring that provides additional corners
compared to FIG. 11.
[0026] FIGS. 19A, 19B, and 19C include perspective cross-sectional
views that correspond to a portion of the light sensor shown in
FIG. 18 during different steps of a fabrication process of an
embodiment of the present invention.
[0027] FIG. 20 is a high level flow diagram used to summarize the
soft bake embodiments described with reference to FIGS. 6-8.
[0028] FIG. 21 is a high level flow diagram used to summarize the
temperature cycling embodiments described with reference to FIGS.
9-10.
[0029] FIG. 22 is a high level flow diagram used to summarize the
embodiments described with reference to FIGS. 11-19.
[0030] FIGS. 23 and 24 illustrates top down views of exemplary
light sensors being manufactured, wherein the light sensor includes
a rectangular photodetector sensor region surrounded by photoresist
and covered by a dielectric optical coating having chamfered
corners.
[0031] FIG. 25 is a high level flow diagram used to summarize the
embodiments described above with reference to FIGS. 23 and 25.
DETAILED DESCRIPTION
[0032] In the following detailed description, reference is made to
the accompanying drawings that form a part hereof, and in which is
shown by way of illustration specific illustrative embodiments. It
is to be understood that other embodiments may be utilized and that
mechanical and electrical changes may be made. The following
detailed description is, therefore, not to be taken in a limiting
sense. In the description that follows, like numerals or reference
designators will be used to refer to like parts or elements
throughout. In addition, the first digit of a reference number
identifies the drawing in which the reference number first
appears.
[0033] Certain embodiments of the present invention relate to
improving lift-off processes used to produce light sensors (e.g.,
ambient light sensors) with increased process margins. Accordingly,
before describing such embodiments of the present invention, it is
first useful to describe conventional lift-off processes, so that
the deficiencies being overcome by embodiments of the present
invention can be better understood. Accordingly, conventional
lift-off processes will first be described with reference to FIGS.
4 and 5. More specifically, FIG. 4, which includes FIGS. 4(a), (b),
(c) and (d), illustrates a conventional lift-off process that
utilizes a photoresist having a negative profile (which is also
known as a re-entrant profile). FIG. 5, which includes FIGS. 5(a),
(b), (c) and (d), illustrates a conventional lift-off process that
utilizes a photoresist having a positive profile (which is also
known as a tapered profile). The light sensors generally shown in
FIGS. 4 and 5, and other FIGS., can have a structure similar to the
photodetector 102 described with reference to FIG. 1, but are not
limited thereto, as would be clear to one of ordinary skill in the
art. For example, the light sensors can include P.sup.+/N.sup.-
junctions, or N.sup.+/P.sup.- junctions, or PIN, NPN, PNP or NIP
junctions, but are not limited thereto. Regardless of the exact
structure of the light sensor, the light sensor will includes a
photodetector sensor region that produces a photocurrent in
response to light incident on the light sensor.
[0034] Referring to FIGS. 4(a) and 5(a), a photodetector sensor
region 404, 504 formed in a semiconductor substrate 406, 506 is
covered with a photoresist layer 420, 520 (also simply referred to
as a photoresist). The photoresist 420, 520 is covered with a
photomask 430, 530 and is exposed to ultraviolet (UV) light. As the
term is used herein, UV light is also meant to also include deep
ultraviolet light (DUV). This causes exposure of a portion of the
photoresist to the UV light, while another portion of the
photoresist is unexposed to the UV light. Thereafter, a developer
is used to remove the photoresist that was unexposed to UV light,
without removing the photoresist that was exposed to UV light, as
shown in FIGS. 4(b) and 5(b). Depending upon the type of
photoresist used, the post-develop photoresist can have a negative
profile (also known as a re-entrant profile), as shown in FIG.
4(b), or a positive profile (also known as a tapered profile), as
shown in FIG. 5(b). Thereafter, a dielectric optical coating 440,
540 is deposited, to thereby form a dielectric optical filter over
the light sensor, as shown in FIGS. 4(c) and 5(c). The dielectric
optical coating, which can also be referred to as a dielectric
optical filter, is made from a stack of dielectric films. A
lift-off process using a chemical solvent is thereafter used to
remove the remaining photoresist 420, 520 and the dielectric
optical coating 440, 540 covering the photoresist, resulting in the
structures shown in FIGS. 4(d) and 5(d).
[0035] A problem with conventional lift-off techniques is that
following the dielectric optical coating deposition (illustrated in
FIGS. 4(c) and 5(c)), the lift-off process requires a very long
soak duration (on the order of multiple hours) due to the
photoresist not being fully exposed to the solvent. While the exact
soak duration (also know as lift-off duration) depends on the
thickness of the dielectric optical coating, the thickness of the
photoresist, the chemical solvent, and the type of lift-off
equipment used, tests have shown that soak durations of up to nine
hours may be necessary to perform the lift-off process. Such high
soak durations are necessary, at least in part due to the
dielectric optical coating being highly conformal, even where the
photoresist utilized has a negative profile. Various embodiments of
the present invention, which are described below, are used to
accelerate the lift-off process and increase the process margin for
photoresist removal following deposition of the dielectric optical
coating. By accelerating the lift-off process and increasing the
process margin, embodiments of the present invention provide for
more inexpensive light sensors that include dielectric optical
coatings to shape their spectral response.
[0036] Certain embodiments of the present invention described with
reference to FIG. 6, which includes FIGS. 6(a), (b), (c), (d), and
(e), involve the use of a short duration soft baking step (also
referred to as a soft bake process) that occurs after the
deposition of the dielectric optical coating, but before
performance of the lift-off step using a solvent.
[0037] Referring to FIG. 6(a), a photoresist 620 is deposited on a
photodetector sensor region 604 formed in a semiconductor substrate
606. Still referring to FIG. 6(a), a photomask 630 is used to
selectively expose a portion of the underlying structure to UV
light, causing exposure of a portion of the photoresist to the UV
light, while another portion of the photoresist is unexposed to the
UV light. As shown in FIG. 6(b), a developer is then used to remove
the photoresist 620 that was unexposed to UV light, without
removing the photoresist that was exposed to UV light. While the
profile of the remaining photoresist shown in FIG. 6(b) is a
vertical profile (which is also known as a neutral profile), the
photoresist profile can alternatively be negative or positive (as
shown in FIGS. 4(b) and 5(b), respectively).
[0038] Thereafter, the dielectric optical coating 640 is deposited,
to thereby form a dielectric optical filter over the light sensor,
as shown in FIG. 6(c). In accordance with an embodiment, the
photoresist has a softening temperature (T.sub.soft, which is also
known as the softening point) that is higher than the temperature
(T.sub.dep) at which the deposition of the dielectric optical
coating is performed. Stated another way, in accordance with an
embodiment, the optical coating deposition is performed at a
temperature (T.sub.dep) below the softening point (T.sub.soft) of
the photoresist, i.e., T.sub.dep<T.sub.soft. Most photoresists
have a softening point (T.sub.soft) in the range of 100-130 degrees
Celsius. At this point, the dielectric optical coating 640 covers
the photodetector sensor region 604, a circumferential area
surrounding the sensor region 604, and the portion of the
photoresist 620 that remained after the developer was used to
remove the photoresist that was unexposed to UV light.
[0039] In accordance with an embodiment, following the deposition
of the dielectric optical coating 640, a short duration soft bake
is performed at a temperature (T.sub.soft.sub.--.sub.bake) above
the softening point (T.sub.soft) of the photoresist, i.e.,
T.sub.soft.sub.--.sub.bake>T.sub.soft. While the duration of the
soft bake is dependent on the thickness and type of the
photoresist, it is estimated that the soft bake duration should be
about 0.5 to 1 minute per micron (.mu.m) thickness of the
photoresist. Since the typical photoresist thickness ranges from
about 3 to 6 .mu.m, the soft bake process would likely take about
1.5 minutes to about 6 minutes, but may take less or more time.
Preferably, the soft bake time does not exceed 15 minutes, and more
preferably does not exceed 5 minutes. The soft bake causes thermal
expansion of the dielectric optical coating and the photoresist,
and also causes photoresist reflow, which individually and
collectively cause tensile mechanical strain at the sidewall of the
dielectric optical coating, which in-turn causes micro-cracks to
form in the dielectric optical coating. The photoresist reflow is
generally shown at 650 in FIG. 6(d). The micro-cracks are generally
illustrated by the dashed crooked line 660 in FIG. 6(d). A lift-off
process using a solvent is thereafter performed to remove the
remaining photoresist and the dielectric optical coating covering
the photoresist, which results in the structure shown in FIG. 6(e).
The micro-cracks 660 provide additional avenues for the lift-off
solvent to reach the photoresist during the lift-off process,
thereby significantly reducing the soak duration required for
lift-off.
[0040] The use of the soft bake step should reduce the soak
duration (during the lift-off process) by at least a factor of two,
and potentially by a factor of ten or more. For example, it is
estimated that the lift-off process will take about 15 to 30
minutes following the soft bake step, which is significantly faster
than the up to nine hours it might take if the soft bake step was
not performed.
[0041] FIG. 7, which includes FIGS. 7(a), (b), (c), (d), (e) and
(f), illustrates in more detail how the photoresist reflow occurs
during the a short duration soft bake process. More specifically,
the photoresist in FIG. 7(a) is shown as having a vertical profile
(also known as a neutral profile), similar to the photoresist
profile shown in FIG. 6(b). As can be appreciated from FIGS. 7(b),
(c), (d) and (e), the photoresist profile begins to become rounded
due to reflow, resulting in the photoresist profile of FIG. 7(f) at
the end of the short duration soft bake process. In accordance with
specific embodiments, the short duration soft bake process occurs
for about 15 minutes or less, and preferably for about 5 minutes or
less, and even more preferably for about 2 minutes or less. In
specific embodiments, the short duration soft bake process occurs
for between about 30 and 60 seconds.
[0042] While it is preferred that the soft bake is performed at a
temperature (T.sub.soft.sub.--.sub.bake) above the softening point
temperature (T.sub.soft) of the photoresist, i.e.,
T.sub.soft.sub.--.sub.bake>T.sub.soft, micro-cracks would likely
also occur at temperatures just below the softening point
(T.sub.soft), e.g., within 10 degrees Celsius of the softening
point temperature. It is also noted that the soft bake process is
preferably not performed at a temperature that is much greater than
the softening point temperature (T.sub.soft), so at to prevent the
photoresist from liquefying.
[0043] While it is preferred that the soft bake is performed at a
temperature (T.sub.soft.sub.--.sub.bake) above the softening point
temperature (T.sub.soft) of the photoresist, i.e.,
T.sub.soft.sub.--.sub.bake>T.sub.soft, micro-cracks should also
occur at temperatures just below the softening point temperature
(T.sub.soft), e.g., within 10 degrees Celsius of the softening
point temperature. Accordingly, the short duration soft bake can be
more generally be performed within a predetermined range of the
softening point (T.sub.soft) of the photoresist to thereby form
and/or increase a number of micro-cracks in the dielectric optical
coating that is not covering the photodetector sensor region. In
accordance with an embodiment, the predetermined range of the
softening point temperature (T.sub.soft) of the photoresist is
+/-10 degrees Celsius of the softening point temperature
(T.sub.soft).
[0044] Referring now to FIG. 8, the arrows 802 shown therein point
to corners of the photoresist where micro-cracks 860 in the
dielectric optical coating originated due to the high mechanical
stresses caused by the soft bake process. The micro-cracks 860
illustrate the same type of micro-cracks 660 discussed above with
reference to FIG. 6.
[0045] Further embodiments of the present invention, which will be
described with reference to FIGS. 9 and 10, involve the use of
thermal shock (also referred to as temperature cycling) after the
deposition of the dielectric optical coating, but before
performance of the lift-off step using a solvent.
[0046] FIGS. 9(a), (b) and (c) of FIG. 9 are similar to FIGS. 6(a),
(b) and (c) of FIG. 6, and thus, need not be described again. More
specifically, in FIG. 9, the reference numerals 904, 906, 920, 930
and 940 represent substantially the same elements as the reference
numerals 604, 606, 620, 630 and 640 in FIG. 6, and thus these
elements need not be described again. Referring to FIG. 9(d) and
FIG. 10, after the deposition of the optical coating 940,
thermo-mechanical stresses are introduced by cycling between two
(or more) temperatures T1 and T2, where T1<<T2. In specific
embodiments, the temperature T2 is between about 50 and 150 degrees
Celsius greater than the temperature T1. More specifically, the
temperature T1 is within the range of about 0 to 50 degrees
Celsius, and the temperature T2 is within the range of about 100 to
150 degrees Celsius, in accordance with an embodiment. Preferably,
the temperature T2 is between about 90 and 120 degrees Celsius
greater than the temperature T1, the temperature T1 is within the
range of about 0 to 30 degrees Celsius, and the temperature T2 is
within the range of about 120 to 150 degrees Celsius. Alternative
temperatures may be used, depending on the photoresist
formulation.
[0047] In accordance with an embodiment, the temperature cycling is
performed within a dry oven with inert atmosphere. In accordance
with an embodiment, the transitions between T1 and T2, and between
T2 and T1, are relatively rapid, as shown in FIG. 10, to promote
stress and delamination of the dielectric optical coating 940 over
the photoresist 920, as well as photoresist reflow, all of which
contribute to micro-cracks 960. In accordance with an embodiment,
the temperature cycling is performed for about 60 to 120 seconds,
with the temperature remaining at each of the different temperature
(e.g., T1 and T2) for at least 10 seconds, before transitioning to
another one of the temperatures. In accordance with specific
embodiments, the temperature cycling process occurs for about 15
minutes or less, and preferably for about 5 minutes or less, and
even more preferably for about 2 minutes or less. In specific
embodiments, the temperature cycling process occurs for between
about 1 and 2 minutes. Other variations are possible, and within
the scope of an embodiment of the present invention.
[0048] Micro-cracks are caused by the temperature cycling, as a
result of thermal expansion and thermal contraction, and are
generally illustrated by the dashed crooked line 960 in FIG. 9(d).
The arrows 970 are used to illustrate delamination. Both the
dielectric optical coating 940 and the photoresist 920 will expand
and contract during the temperature cycling. However, photoresist
polymers have a much higher coefficient of expansion (typically at
least 10.times. higher) than dielectric optical coatings, which
advantageously contributes to the micro-cracking and delaminating
of the dielectric optical coating 960.
[0049] The use of the temperature cycling should reduce the soak
duration (during the lift-off process) by at least a factor of two,
and by potentially by a factor of ten or more. For example, it is
estimated that the lift-off process will take about 15 to 30
minutes following the temperature cycling step, which is
significantly faster than the up to nine hours it might take if the
temperature cycling step was not performed.
[0050] While transitioning between two temperatures (e.g., T1 and
T2) is sufficient, it is possible and within the scope of an
embodiment of the present invention that three or more different
temperatures (e.g., T1, T2 and T3) can be used to perform the
thermal shock.
[0051] In accordance with certain embodiments, the soft bake
process can be performed for a first period of time, and then the
temperature cycling process can be performed for a second period of
time, prior to using a chemical solvent bath to complete the
lift-off process. In accordance with other embodiments, the
temperature cycling process can be performed for a first period of
time, and then the soft-bake process can be performed for a second
period of time, prior to using a chemical solvent bath to complete
the lift-off process. In other words, the aforementioned soft-bake
and temperature cycling embodiments can both be used, e.g., one
after the other.
[0052] Referring now to FIG. 11, illustrated therein is a top down
view of an exemplary light sensor in the process of being
manufactured. More specifically, FIG. 11 corresponds to the point
in the fabrication process after which the following steps have
already been performed. A surface of a semiconductor substrate,
which includes a rectangular photodetector sensor region 1104, has
been covered with a photoresist. Photolithography has been used to
expose a portion of the photoresist while not exposing a portion of
the photoresist covering the photodetector sensor region 1104.
Additionally, a circumferential area 1110 that extends beyond the
sensor region 1104, having a width "d", was also not exposed. A
developer has been used to remove the portion of the photoresist
covering the photodetector sensor region 1104, as well as to remove
the portion of the photoresist covering the circumferential area
1110, leaving the portion of the photoresist 1120 that was exposed
during photolithography (which can be referred to as the exposed or
remaining photoresist 1120). Also, a dielectric optical coating has
been deposited over the sensor region 1104, over the
circumferential area 1110, and over the remaining photoresist 1120.
Accordingly, FIG. 11 essentially corresponds to a top down view of
what is shown in FIGS. 4(c) and 5(c) discussed above.
[0053] Still referring to FIG. 11, the sensor region 1104 is shown
as having a rectangular outer perimeter. The photoresist 1120
(which remained after the developer had been used to remove the
portion of the photoresist covering the photodetector sensor region
1104 and the circumferential area 1110) has a rectangular opening
that is similar in shape to the rectangular outer perimeter of the
sensor region 1104. This rectangular shaped opening in the
photoresist 1120 is achieved using a rectangular feature in the
photomask during the photolithography step that exposes a portion
of the photoresist while not exposing a portion of the photoresist
covering the sensor region 1104 and the circumferential area 1110.
The next step of the fabrication process would be to perform a
lift-off process using a solvent to remove the remaining
photoresist 1120 and the portion of the dielectric optical coating
covering the photoresist 1120, to thereby produces a structure
similar to what was shown in FIGS. 4(d) and 5(d). However, as
mentioned above, a problem with conventional lift-off techniques is
that following the dielectric optical coating deposition, the
lift-off process typically requires a very long soak duration (on
the order of multiple hours) due to the photoresist not being fully
exposed to the solvent (because the photoresist is covered by the
dielectric optical coating).
[0054] FIGS. 12A, 12B and 12C are perspective cross-sectional views
that correspond to a portion of the light sensor shown in FIG. 11,
during different steps of the fabrication process. FIG. 12A shows a
portion of a straight sidewall of the photoresist (over the sensor
substrate) following exposure to a developer. FIG. 12B shows the
same portion after deposition of the dielectric optical coating.
FIG. 12C illustrates how the dielectric optical coating remains and
the photoresist is removed as a result of the liftoff process.
[0055] The embodiments of the present invention described above
with reference to FIGS. 6-10 relied on a soft bake process and/or a
temperature cycling process to purposely form micro-cracks in (and
potentially cause delaminating of) the dielectric optical coating
covering the photoresist, which enable the lift-off process to be
performed much more quickly than if no (or less) micro-cracks were
formed. Additional embodiments of the present invention, which are
described below, use alternative techniques to purposely form
micro-cracks in the dielectric optical coating covering the
photoresist, and more specifically, to significantly increase the
number of micro-cracks. Thus, such additional embodiments can also
be used to significantly reduced the amount of time required for
the lift-off process.
[0056] The inventors have discovered that micro-cracks more readily
appear near sharp corners in the photoresist over which the
dielectric optical coating is deposited. In FIG. 11, note that the
rectangular opening in the photoresist 1120 includes only four
sharp corners. To increase the number of micro-cracks formed in the
dielectric optical coating covering the photoresist, one or more
dummy corners, dummy islands and/or dummy rings are included in the
photomask used during the photolithography step (to expose a
portion of the photoresist while not exposing a portion of the
photoresist covering the photodetector sensor region). Such added
corners, islands and rings are referred to as "dummy" features
because they are not actually needed to form an underlying feature
of the light sensor being manufactured, but rather, are included
for the sole purpose of increasing micro-crack formations as well
as the edge surface area, thereby improving lift-off efficiency and
consistency. More specifically, the additional corners increase
local strain, which results in micro-cracks that are similar to
those described above with reference to FIGS. 6(d) and 9(d). The
additional edge area and micro-cracks both accelerate the
photoresist removal. As will be appreciated from FIGS. 14A-C, 16A-C
and 19A-C, discussed below, features corresponding to the one or
more dummy corners, dummy islands and/or dummy rings will remains
following the photoresist removal step. However, because these
remaining features (which can be referred to as dummy features) are
outside the active photodetector sensor region, they do not
adversely affect operation of the final light sensor.
[0057] The various embodiments of the present invention, described
below with reference to FIGS. 13-19, involve the adding of dummy
corners, dummy islands and/or dummy rings (all of which add
additional corners) in the photomask, which result in additional
corners and edge surface area at the perimeters of the photoresist
and dielectric optical coating. Referring to FIG. 13, illustrated
therein is a top down view of photodetector sensor region 1304
having a rectangular perimeter, as was the case also in FIG. 11.
However, in contrast to FIG. 11, in FIG. 13 the perimeters of the
photoresist and dielectric optical coating perimeter have numerous
additional 90 degree corners 1350. These additional corners 1350,
which are examples of dummy corners, are achieved by adding dummy
corners in the photomask used to expose selective portions of the
photoresist to UV light. The added corners also increase the edge
surface areas (which can also be referred to as the perimeter
surface areas) of the photoresist and the dielectric optical
coating by about 40% compared to the edge surface areas in FIG. 11.
The dummy corners increase local strain, which results in an
increased number of micro-cracks that are similar to those
described above with reference to FIGS. 6(d), 8 and 9(d). The
additional edge surface areas and the additional micro-cracks both
accelerate the photoresist removal and increase the lift-off
process margin. In FIG. 13 the perimeters of the photoresist and
dielectric optical coating includes a total of forty-four corners,
forty of which can be considered dummy corners.
[0058] FIGS. 14A, 14B and 14C are perspective cross-sectional views
that correspond to a portion of the light sensor shown in FIG. 13,
during different steps of the fabrication process. FIG. 14A shows a
portion of a sidewall of the photoresist (over the sensor
substrate) following exposure to a developer, wherein the sidewall
includes four more 90 degree corners compared to FIG. 12A. FIG. 14B
shows the same portion after deposition of the dielectric optical
coating, similarly showing four more 90 degree corners compared to
FIG. 12B. FIG. 14C illustrates how the dielectric optical coating
remains and the photoresist is removed as a result of the liftoff
process. The portion of the dielectric optical coating that is
shown in FIG. 14C, but was not shown in FIG. 12C, can be considered
a dummy dielectric optical coating feature. However, because the
dummy dielectric optical coating feature is outside of the
photodetector sensor region, it will not adversely affect the
function of the resulting light sensor.
[0059] Referring now to FIG. 15, illustrated therein is a top down
view of a photodetector sensor region 1504 having a rectangular
perimeter, as was the case also in FIGS. 11 and 13. Also shown are
the circumferential area 1510 covered by the dielectric optical
coating (with no underlying photodetector sensor region and no
underlying photoresist), and the exposed photoresist 1520 covered
by the dielectric optical coating. In contrast to FIG. 11, in FIG.
15 the photoresist 1520 has several rectangular dummy islands 1560
(ten are shown) beyond the sensor region 1504. Each rectangular
dummy island 1560, which can also be referred to as a dummy island
opening in the photoresist, include four 90 degree corners 1550.
These additional corners 1550, which are also examples of dummy
corners, increase local strain, which results in an increased
number of micro-cracks that are similar to those described above
with reference to FIGS. 6(d), 8 and 9(d). The additional edge
surface area and the additional micro-cracks both accelerate the
photoresist removal and increase the lift-off process margin. In
FIG. 15, since each of the ten dummy islands 1560 adds four dummy
corners 1550, there are a total of forty dummy corners 1550.
[0060] FIGS. 16A, 16B and 16C are perspective cross-sectional views
that correspond to a portion of the light sensor shown in FIG. 15,
during different steps of the fabrication process. FIG. 15(a) shows
a portion of a sidewall of the photoresist (over the sensor
substrate) following exposure to a developer, wherein the
photoresist layer includes a rectangular opening or window having
four 90 degree corners. FIG. 16B shows the same portion after
deposition of the dielectric optical coating. FIG. 16C illustrates
how the dielectric optical coating remains and the photoresist is
removed as a result of the lift-off process. The portion of the
dielectric optical coating that is shown in FIG. 16C, but was not
shown in FIG. 12C, can be considered a dummy dielectric optical
coating feature. However, because the dummy dielectric optical
coating feature is outside of the photodetector sensor region, it
will not adversely affect the function of the resulting light
sensor.
[0061] FIG. 17 illustrate an embodiment which essentially combines
the embodiment described with reference to FIGS. 13 and 14 with the
embodiment described with reference to FIGS. 15 and 16. FIG. 17
shows a photodetector sensor region 1704 having a rectangular
perimeter, as was the case also in FIGS. 11, 13 and 15. Also shown
are the circumferential area 1710 covered by the dielectric optical
coating (with no underlying photodetector sensor region and no
underlying photoresist) and the exposed photoresist 1720 covered by
the dielectric optical coating. In FIG. 17, there are fourteen
dummy islands 1760, each of which provide four dummy corners 1750.
Also, there are an additional forty dummy corners 1750 not provided
by the dummy islands. Thus, in FIG. 17, there are a total of
ninety-six dummy corners 1750.
[0062] FIGS. 18 and 19A-C illustrate yet another variation on the
embodiments described above with reference FIGS. 13-17. FIG. 18
shows a photodetector sensor region 1804 having a rectangular
perimeter, as was the case also in FIGS. 11, 13, 15 and 17. Also
shown are the circumferential area 1810 covered by the dielectric
optical coating (with no underlying photodetector sensor region and
no underlying photoresist) and the exposed photoresist 1820 covered
by the dielectric optical coating. A dummy ring 1870 is provided,
which adds eight dummy corners 1850. Additionally, the dummy ring
1870 increases the edge surface areas of the photoresist and the
dielectric optical coating by about 100% compared to the edge
surface areas in FIG. 11. FIGS. 19A, 19B and 19C are perspective
cross-sectional views that correspond to the light sensor shown in
FIG. 18, during different steps of the fabrication process.
[0063] While the dummy corners shown in FIGS. 13-19 were shown as
being 90 degree corners, dummy corners of other angles are also
possible and within the scope of the present invention. Further, it
is noted that each of the dummy corners is preferably a sharp
corner, as opposed to a rounded corner, because sharp corners
result in more local stress and thereby will result in more
micro-cracks.
[0064] The embodiments described with reference to FIGS. 13-19 can
be combined with the embodiments described with reference to FIGS.
6-8 and/or the embodiments described with reference to FIGS.
8-9.
[0065] For illustrative purposes, exemplary additional details of a
dielectric optical coating (which can also be referred to as a
dielectric optical filter) are provided below. The dielectric
materials used to form a dielectric optical filter can include
silicon dioxide (SiO2), silicon hydride (SixHy), silicon nitride
(SixNy), silicon oxynitride (SixOzNy), tantalum oxide (TaxOy),
gallium arsenide (GaAs), gallium nitride (GaN), and the like.
Alternating layers in the optical filter may have a constant or
varying film thickness throughout the filter stack, in order to
achieve the desired optical response. By careful choice of the
exact composition, thickness, and number of these layers, it is
possible to tailor the reflectivity and transmissivity of the
optical filter to produce almost any desired spectral
characteristics. For example, the reflectivity can be increased to
greater than 99.99%, to produce a high-reflector (HR) coating. The
level of reflectivity can also be tuned to any particular value,
for instance to produce a mirror that reflects 90% and transmits
10% of the light that falls on it, over some range of wavelengths.
Such mirrors have often been used as beam splitters, and as output
couplers in lasers. Alternatively, the optical filter can be
designed such that the mirror reflects light only in a narrow band
of wavelengths, producing a reflective optical filter.
[0066] Generally, layers of high and low refractive index materials
are alternated one above the other. This periodic or alternating
structure significantly enhances the reflectivity of the surface in
the certain wavelength range called band-stop, which width is
determined by the ratio of the two used indices only (for
quarter-wave system), while the maximum reflectivity is increasing
nearly up to 100% with a number of layers in the stack. The
thicknesses of the layers are generally quarter-wave (then they
yield to the broadest high reflection band in comparison to the
non-quarter-wave systems composed from the same materials),
designed such that reflected beams constructively interfere with
one another to maximize reflection and minimize transmission. Using
the above described structures, high reflective coatings can
achieve very high (e.g., 99.9%) reflectivity over a broad
wavelength range (tens of nanometers in the visible spectrum
range), with a lower reflectivity over other wavelength ranges, to
thereby achieve a desired spectral response. By manipulating the
exact thickness and composition of the layers in the reflective
stack, the reflection characteristics can be tuned to a desired
spectral response, and may incorporate both high-reflective and
anti-reflective wavelength regions. The coating can be designed as
a long-pass or short-pass filter, a bandpass or notch filter, or a
mirror with a specific reflectivity.
[0067] In accordance with specific embodiments of the present
invention, an optical filter is used to shape the spectral response
of the underlying photo detector region to obtain a true human eye
spectral response, i.e., a response that is similar to that of a
typical human eye response. Alternative spectral responses are
possible, and within the scope of the present invention.
[0068] Various embodiments of the present invention will now be
summarized with reference to the high level flow diagrams of FIGS.
20-22.
[0069] FIG. 20 is used to summarize the short duration soft bake
embodiments described above with reference to FIGS. 6-8. Referring
to FIG. 20, at step 2002, a surface of a semiconductor substrate,
which includes a photodetector sensor region, is covered with a
photoresist having a softening point (T.sub.soft). At step 2004,
photolithography is used to expose a portion of the photoresist
while not exposing a portion of the photoresist covering the
photodetector sensor region. At step 2006, the portion of the
photoresist covering the photodetector sensor region is removed
using a developer. At step 2008, a dielectric optical coating is
deposited over the photodetector sensor region and over the
photoresist not covering the photodetector sensor region. At step
2010, a short duration soft bake at a temperature
(T.sub.soft.sub.--.sub.bake) is performed within a predetermined
range of the softening point (T.sub.soft) of the photoresist to
thereby form and/or increase a number of micro-cracks in the
dielectric optical coating not covering the photodetector sensor
region. At step 2012, the dielectric optical coating that is over
the photoresist not covering the photodetector sensor region is
lifted off, so that the resulting light sensor includes the
dielectric optical coating over the photodetector sensor region.
Additional details of the techniques summarized in FIG. 20 can be
appreciated from the above discussion of FIGS. 6-8.
[0070] FIG. 21 is used to summarize the temperature cycling
embodiments described above with reference to FIGS. 9-10. Referring
to FIG. 21, steps 2102, 2104, 2106 and 2108 of FIG. 21 are
identical to steps 2002, 2004, 2006 and 2008 described above with
reference to FIG. 20, and thus need not be described again. At step
2110, temperature cycling is performed by repetitively cycling back
and forth between at least two temperatures T1 and T2, to thereby
form and/or increase a number of micro-cracks in the dielectric
optical coating not covering the photodetector sensor region. In
specific embodiment, the temperature T2 is between about 50 and 120
degrees Celsius greater than the temperature T1. At step 2112, the
dielectric optical coating that is over the photoresist not
covering the photodetector sensor region is lifted off, so that the
resulting light sensor includes the dielectric optical coating over
the photodetector sensor region. Additional details of the
techniques summarized in FIG. 21 can be appreciated from the above
discussion of FIGS. 9-10.
[0071] FIG. 22 is used to summarize the embodiments described above
with reference to FIGS. 11-19. Referring to FIG. 22, at step 2202,
a surface of a semiconductor substrate, which includes a
photodetector sensor region, is covered with photoresist. At step
2204, photolithography is performed using a photomask to expose a
portion of the photoresist, while not exposing a portion of the
photoresist covering the photodetector sensor region. As was
described above with reference to FIGS. 13, 15, 17 and 18, the
photomask used at step 2204 includes one or more dummy corners,
dummy islands and/or dummy rings. At step 2206, the portion of the
photoresist covering the photodetector sensor region is removed
using a developer. At step 2208, a dielectric optical coating is
deposited over the photodetector sensor region and the portion of
the photoresist that remains following the removing at step 2206.
At step 2210, the dielectric optical coating that is over the
photoresist not covering the photodetector sensor region is
removed, so that the resulting light sensor includes the dielectric
optical coating over the photodetector sensor region. Additional
details of the techniques summarized in FIG. 22 can be appreciated
from the above discussion of FIGS. 11-19.
[0072] Packaged optical sensors are typically subject to
significant thermal stresses during accelerated reliability
testing, which can include, temperature cycling, moisture
sensitivity level (MSL) tests, highly accelerated stress tests
(HAST), etc. These stresses arise from the thermal expansion and
contraction of films underneath the dielectric optical coating on
the die substrate, as well as the thermal expansion and contraction
of the package encapsulation material (which is typically a
transparent epoxy). Such stresses can cause the dielectric optical
coating (that covers and extends beyond the photodetector sensor
region) to crack and/or delaminated, which reduces yield.
[0073] In accordance with specific embodiments of the present
invention, non-orthogonal (e.g., 45.degree., but not limited
thereto) chamfered corners are added to improve the thermal
reliability of the dielectric optical coating the covers the
photodetector sensor region. Such embodiments can be appreciated
from FIGS. 23 and 24, which are described below.
[0074] Referring first to FIG. 23, this figure is the substantially
the same as FIG. 15, except that chamfered corners 2380 are added
to the dielectric optical coating to improve the thermal
reliability of the dielectric optical coating over a photodetector
sensor region 2304. More specifically, FIG. 23 illustrates a top
down view of a photodetector sensor region 2304 having a
rectangular perimeter. Also shown are the circumferential area 2310
covered by the dielectric optical coating (with no underlying
photodetector sensor region and no underlying photoresist), and the
exposed photoresist 2320 covered by the dielectric optical coating.
As was the case in FIG. 15, the photoresist 2320 has rectangular
dummy islands 2360 beyond the sensor region 2304, which adds dummy
corners 2350. The chamfered corners 2380 can be achieved by
including chamfered corners in the photomask that is used during
the photolithography step that exposes a portion of the photoresist
while not exposing a portion of the photoresist covering the
photodetector sensor region 2304 and the circumferential area
2310.
[0075] Referring next to FIG. 24, this figure is substantially the
same as FIG. 18, excepts that chamfered corners 2480 are added to
the dielectric optical coating to improve the thermal reliability
of the dielectric optical coating over a photodetector sensor
region 2404. More specifically, FIG. 24 illustrates a top down view
of a photodetector sensor region 2404 having a rectangular
perimeter, a circumferential area 2410 covered by the dielectric
optical coating (with no underlying photodetector sensor region and
no underlying photoresist), exposed photoresist 2420 covered by the
dielectric optical coating, and a dummy ring 2470 that adds eight
dummy corners 2450.
[0076] FIGS. 23 and 24 illustrates how chamfered corners can be
included in the embodiments described above with reference to FIGS.
11-19 and 22. The chamfered corners can also be included in the
embodiments described above with reference to FIGS. 6-10 and 21. It
is also within the scope of an embodiment of the present invention
that the chamfered corners not be combined with the other
embodiments described herein.
[0077] FIG. 25 is used to summarize the embodiments described above
with reference to FIGS. 23 and 24. Referring to FIG. 25, at step
2502, a surface of a semiconductor substrate, which includes a
photodetector sensor region, is covered with photoresist. At step
2504, photolithography is performed using a photomask to expose a
portion of the photoresist, while not exposing a portion of the
photoresist covering the photodetector sensor region. As was
described above with reference to FIGS. 23 and 24, a portion of the
photomask used to not expose the portion of the photoresist
covering the photodetector sensor region includes chamfered
corners. At step 2506, the portion of the photoresist covering the
photodetector sensor region is removed using a developer. At step
2508, a dielectric optical coating is deposited over the
photodetector sensor region and the portion of the photoresist that
remains following the removing at step 2506. At step 2510, the
dielectric optical coating that is over the photoresist not
covering the photodetector sensor region is removed, so that the
resulting light sensor includes the dielectric optical coating over
the photodetector sensor region. Because of the chamfered corners
included in photomask used at step 2504, following the lifting off
at step 2510, a portion of the dielectric optical coating that
extends beyond the photodetector sensor region includes chamfered
corners. Additional details of the techniques summarized in FIG. 25
can be appreciated from the above discussion of FIGS. 23 and
24.
[0078] There exist both positive and negative types of
photoresists. When a positive photoresist is exposed to UV light
the chemical structure of the photoresist changes so that it
becomes more soluble in a developer. The exposed photoresist is
then washed away by the developer, leaving windows in the
photoresist where the photoresist was exposed to UV light.
Accordingly, when using a positive photoresist the photomask
includes an exact copy of the pattern which is to remain on the
wafer. Negative photoresists behave in the opposite manner. That
is, exposure to the UV light causes the negative photoresist to
become less soluble in a developer. Therefore, the negative
photoresist remains on the surface wherever it was exposed, and the
developer removes only the unexposed portions. Accordingly, a
photomask used with a negative photoresist includes the inverse (or
photographic "negative") of the pattern to be transferred.
[0079] In the embodiments described above, the photoresists (e.g.,
620, 920, 1120, 1230, 1520, 1720, 1820, 2320 and 2420) that were
described behaved as, and thus were, negative photoresists.
However, it is also within the scope of embodiments of the present
invention to use positive photoresists instead of negative
photoresists. Accordingly, steps 2004 and 2104 (in FIGS. 20 and 21,
respectively) can more generally involve defining a pattern in the
photoresist. Similarly, step 2204 (in FIG. 22) can more generally
involve performing photolithography using a photomask to define a
pattern in the photoresist that includes one or more dummy corners,
dummy islands and/or dummy rings. Further, step 2504 (in FIG. 25)
can more generally involve performing photolithography using a
photomask to define a pattern in the photoresist that includes
chamfered corners.
[0080] Embodiments of the present invention are also directed to
light sensors formed used the above described techniques, and
systems that include such sensors.
[0081] Although specific embodiments have been illustrated and
described herein, it will be appreciated by those of ordinary skill
in the art that any arrangement, which is calculated to achieve the
same purpose, may be substituted for the specific embodiments
shown. Therefore, it is manifestly intended that this invention be
limited only by the claims and the equivalents thereof.
* * * * *