U.S. patent application number 10/849455 was filed with the patent office on 2005-11-24 for method of wafer height mapping.
Invention is credited to Liao, Chi-Hung, Liu, Louie, Sung, Yi-Chang, Turn, Li-Kong, Wang, Chun-Sheng.
Application Number | 20050259272 10/849455 |
Document ID | / |
Family ID | 35374849 |
Filed Date | 2005-11-24 |
United States Patent
Application |
20050259272 |
Kind Code |
A1 |
Wang, Chun-Sheng ; et
al. |
November 24, 2005 |
METHOD OF WAFER HEIGHT MAPPING
Abstract
An improved method of wafer height mapping using a wafer level
sensor eliminates or substantially minimizes the "spacing" in the
wafer height mapping data usually caused by having an exposure
field on a wafer whose width is less than the width of the
measurement spot array of the wafer level sensor and also not being
a multiple of the width of a single measurement spot. According to
the improved method, the measurement spot array is first translated
towards one edge of the exposure field and scanned. Then the
measurement spot array is translated towards the other edge of the
exposure field and scanned second time to map the area that was
missed during the first mapping scan.
Inventors: |
Wang, Chun-Sheng; (Jhubei
City, TW) ; Sung, Yi-Chang; (Jhubei City, TW)
; Liao, Chi-Hung; (Sanchong City, TW) ; Turn,
Li-Kong; (Taichung, TW) ; Liu, Louie; (Hsinchu
City, TW) |
Correspondence
Address: |
DUANE MORRIS LLP
IP DEPARTMENT (TSMC)
30 SOUTH 17TH STREET
PHILADELPHIA
PA
19103-4196
US
|
Family ID: |
35374849 |
Appl. No.: |
10/849455 |
Filed: |
May 19, 2004 |
Current U.S.
Class: |
356/601 |
Current CPC
Class: |
G03F 9/7088 20130101;
G03F 9/7034 20130101; G03F 9/7069 20130101; G01B 11/0608
20130101 |
Class at
Publication: |
356/601 |
International
Class: |
G01B 011/30 |
Claims
What is claimed is:
1. A method of wafer height mapping using a wafer level sensor,
comprising the steps of: determining edge positions of an exposure
field on a wafer, the exposure field having two edges defining a
width of the exposure field; aligning an array of measurement spots
to the exposure field, wherein the array of measurement spots is
centered within the exposure field; translating the array of
measurement spots in a first direction towards a first of the two
edges of the exposure field until the first edge is substantially
flush with a first closest measurement spot that is closest to the
first edge and the first measurement spot being within the exposure
field; scanning and mapping the exposure field, generating a first
mapped height data of the exposure field; translating the array of
measurement spots in a second direction towards a second of the two
edges of the exposure field until the second edge is substantially
flush with a second closest measurement spot that is closest to the
second edge and the second measurement spot being within the
exposure field; and scanning and mapping the exposure field a
second time, generating a second mapped height data for the
exposure field.
2. The method of claim 1, further comprising a step of merging the
first and second mapped height data for the exposure field.
3. The method of claim 1, wherein the step of translating the array
of measurement spots in the first direction is achieved by moving
the wafer relative to the array of measurement spots.
4. The method of claim 1, wherein the step of translating the array
of measurement spots in the first direction is achieved by moving
the array of measurement spots relative to the wafer.
5. The method of claim 1, wherein the step of translating the array
of measurement spots in the second direction is achieved by moving
the wafer relative to the array of measurement spots.
6. The method of claim 1, wherein the step of translating the array
of measurement spots in the second direction is achieved by moving
the array of measurement spots relative to the wafer.
7. A method of wafer height mapping using a wafer level sensor, an
exposure field having a width being defined on the wafer and the
wafer level sensor measuring height of the wafer's surface using
optical sensors based on moir interference by generating an array
of measurement spots, each of the measurement spots having an equal
width, the method comprising the steps of: determining whether the
width of the exposure field is a multiple of the width of one of
the measurement spots; determining edge positions of the exposure
field on the wafer, the exposure field having two edges defining
the width of the exposure field; aligning the array of measurement
spots to the exposure field, wherein the array of measurement spots
is centered within the exposure field; translating the array of
measurement spots in a first direction towards a first of the two
edges of the exposure field until the first edge is substantially
flush with a first closest measurement spot that is closest to the
first edge and the first measurement spot being within the exposure
field; scanning and mapping the exposure field, generating a first
mapped height data of the exposure field; translating the array of
measurement spots in a second direction towards a second of the two
edges of the exposure field until the second edge is substantially
flush with a second closest measurement spot that is closest to the
second edge and the second measurement spot being within the
exposure field; and scanning and mapping the exposure field a
second time, generating a second mapped height data for the
exposure field.
8. The method of claim 7, further comprising a step of merging the
first and second mapped height data for the exposure field.
9. The method of claim 7, wherein the step of translating the array
of measurement spots in the first direction is achieved by moving
the wafer relative to the array of measurement spots.
10. The method of claim 7, wherein the step of translating the
array of measurement spots in the first direction is achieved by
moving the array of measurement spots relative to the wafer.
11. The method of claim 7, wherein the step of translating the
array of measurement spots in the second direction is achieved by
moving the wafer relative to the array of measurement spots.
12. The method of claim 7, wherein the step of translating the
array of measurement spots in the second direction is achieved by
moving the array of measurement spots relative to the wafer.
Description
[0001] The present invention relates to wafer height detection and
level sensing, for use in lithographic equipment and more
particularly to an improved method of wafer height mapping.
[0002] In the manufacture of integrated circuits, lithographic
projection apparatus can be used to expose circuit patterns for
each layer of the integrated circuit onto an exposure area on a
semiconductor wafer substrate. Generally, the semiconductor wafer
substrate is coated with a layer of photosensitive material, such
as photoresist, during a typical integrated circuit manufacturing
process in order to expose and develop the photoresist with the
circuit patterns. The circuit patterns are imaged on to the
photoresist-coated semiconductor wafer through a mask or a
reticle.
[0003] Generally, a single semiconductor wafer contains an array of
adjacent dies (exposure area) which are successively irradiated
(exposed) via a reticle, one at a time. In one type of
photolithographic apparatus, each exposure area is irradiated by
exposing the entire reticle pattern on to the exposure area at
once. Such apparatus is commonly referred to as a wafer stepper. In
another type of photolithographic apparatus, step-and-scan
apparatus, each exposure area is irradiated by progressively
scanning the reticle pattern under the projection beam in a given
reference direction while synchronously scanning the wafer table
parallel or anti-parallel to this direction.
[0004] In both types of lithographic apparatus, it is important
that the reticle images are focused accurately on the exposure area
of the wafer surface. Although, wafers are polished to a very high
degree of flatness, a typical wafer surface will exhibit areas that
sufficiently deviate from perfect flatness that may affect the
focus accuracy of lithographic apparatus. To address this problem,
and to ensure that the reticle image is accurately focused on the
exposure area of the wafer surface, most lithographic apparatuses
are provided with a device called a wafer level sensor that
generates a map of wafer height to be used by the image projection
system of an exposure tool to focus the reticle image on to the
wafer's exposure areas. After a wafer is loaded on a wafer table,
the wafer table first positions the wafer under the level sensor
which maps the top surface of the wafer to gain topographical data
to aid in focusing of the reticle image during the image exposure
process.
[0005] The level sensor measures the height of the wafer surface,
relative to a physical reference surface of the wafer table at each
of the exposure area of the wafer and generates a map of the wafer
containing the surface height information for each of the exposure
area on the wafer. This is otherwise known as wafer leveling or
height mapping. Because the wafer height map contains height
information of the wafer surface measured relative to a physical
reference surface of the wafer table, the exposure tool can
calculate the vertical position of the exposure area on the wafer
from the physical reference surface of the wafer table, which is
always at a fixed predetermined position. The exposure tool then
adjusts the focal plane of the projection beam to coincide with the
surface of the exposure area.
[0006] The wafer height mapping scan (also called a measurement
scan) conducted by a wafer level sensor in these lithographic
apparatus will now be discussed to describe the shortcomings of
such system which will be addressed by the present invention. FIG.
1, shows an example of a pattern of exposure areas C of various
shapes and sizes arranged on a wafer 10. The different exposure
areas C represent different dies and are separated by scribe lanes
SL.
[0007] Wafer level sensors measure the height of a wafer surface
using optical sensors based on moir interference and the detailed
workings of the optics involved need not be discussed here. An
example of a lithographic apparatus utilizing such wafer level
sensor is the TwinScan.RTM. system available from ASML Netherlands
B.V. and a detailed description of the operation of the wafer level
sensor may be found in ASML's U.S. Pat. No. 6,674,510 to Jasper et
al., the disclosure of which is hereby incorporated by reference in
its entirety.
[0008] A typical wafer level sensor uses nine measurement spots to
measure the height of the wafer surface during its measurement
scan. The nine measurement spots are shown in FIG. 1 as a linear
array of nine optical spots 50 (interchangeably referred to herein
as measurement spots) arranged perpendicular to the scanning
direction. The array of measurement spots is of a size sufficient
to cover the width of the widest exposure area that can be exposed
in the apparatus. In this example, the linear array of nine spots
50 is sufficiently wide to cover the widest column of exposure
areas, the column A, on the wafer 10.
[0009] During the measurement scan, the wafer level sensor scans
the array of nine spots in a meander path 20 so that the center
spot 52 of the array 50 passes along the center line of each column
of exposure areas. Where the column of exposure areas (exposure
field) is the same width as the width of the array of the
measurement spots 50, as in the case of the column A, one
measurement scan will cover the full width of the exposure field
with the measurement spots. This is illustrated in FIG. 2 with each
of the nine measurement spots having a width of W and the spacing
between the measurement spots being represented by S. The width E
of the exposure field is the same as the width of the nine
measurement spots.
[0010] However, as illustrated in FIG. 3, when the wafer level
sensor is measuring the columns B and C of exposure areas on the
wafer 10, for example, the width of the exposure field in those
columns are narrower than the width of the array of measurement
spots 50. The result is that some of the measurement spots 50a fall
completely within the exposure field being scanned while other
measurement spots fall, either partially or completely, outside the
exposure field being scanned. Measurement spots 50b are partially
outside the exposure field and measurement spots 50c are completely
outside the exposure field. Typical wafer level sensors are
programmed to ignore the height data from these measurement spots
since they are not useable. This means that the areas 60 and 61 of
the exposure field do not get measured. This will result in a wafer
mapping data having areas of missing height data or "spacing"
between the scanned exposure fields. FIG. 4 is an illustration of a
wafer 70, where the mapped data, represented by the shaded area 72,
is overlaid on the grid of exposure fields 75 (i.e., the grid of
dies). The areas 77 between the mapped areas 72 represent the
"spacing" or the unmapped regions.
[0011] Since the width of the exposure fields vary depending on the
particular die designs, the width of the exposure fields can vary
from wafer to wafer and can vary within a wafer from column to
column, as shown in the wafer 10 of FIG. 1. Thus, one can see that
the problem described above does not exist even when the width E of
the exposure area is less than the total width of the nine
measurement spots in the array 50 as long as the width E of the
exposure area is substantially equal to a multiple of the width W
of a single measurement spot. FIG. 5 illustrates a case where the
width E of the exposure area is a multiple of the width W of a
single measurement spot. Specifically, E, in this example, is about
five times W. For the purpose of this discussion, the spacing S
between the measurement spots will be ignored since S is generally
substantially smaller than W. Because the width E of the exposure
field is about five times W, the center five measurement spots 50a
will measure the full width of the exposure field.
[0012] Thus, when the width of one or more columns of exposure
fields on a wafer is not substantially equal to a multiple of the
width of a measurement spot, the wafer mapping data will have
strips of unmeasured areas and will not properly represent the
whole wafer. If only one exposure field on a wafer presents this
condition, it may not cause much of a problem. However, if many or
all of the exposure fields on a wafer meet this condition,
substantial portion of the exposure fields of the wafer will not
get measured. This can result in focusing problems during the
reticle image exposure process.
[0013] Thus, there is a need for an improved wafer level sensor
where partial measurement scan of the exposure areas is minimized
or substantially eliminated.
SUMMARY
[0014] According to an aspect of the present invention, an improved
method of wafer height mapping to eliminate or substantially
minimize the "spacing" in the wafer height mapping data is
disclosed. According to an embodiment of the present invention, the
method of wafer height mapping comprises the wafer level sensor
first determining exposure area's edge positions. The level
sensor's array of measurement spots is aligned to the exposure
field. The level sensor's linear array of measurement spots is
translated or moved perpendicular to the scan direction so that one
of the two edges of the exposure area is substantially flush with
the edge of a first of the measurement spots. The column of the
exposure areas (hereinafter referred to as an "exposure field") is
scanned and mapped. Next, the array of measurement spots is
translated for the second time in the direction opposite the first
translation until the opposite edge of the exposure field is flush
with the nearest measurement spot. The exposure field is then
scanned and mapped for the second time, thus, generating a mapped
height data for the portion of the exposure field that was not
mapped during the first scan.
[0015] According to another embodiment of the present invention,
the method of wafer height mapping further comprises a step of
merging the first and second mapped height data for the exposure
field.
[0016] According to another embodiment of the present invention,
the method of wafer height mapping further comprises an initial
step of first determining whether the width of a given exposure
field is a multiple of the width of a single measurement spot. This
step allows the wafer level sensor to determine whether the
translation steps and double scanning of an exposure field is
necessary. If the width of a given exposure field is a multiple of
the width of the single measurement spot, the exposure field may be
mapped by a single scan of the exposure field. Thus, the wafer
level sensor will not perform the additional steps of translating
the measurement spot array and double scanning exposure fields
where the additional steps are not necessary.
[0017] The movement of translating the array of measurement spots
disclosed herein refers to a relative motion between the array of
measurement spots and the exposure field of the wafer. And this
relative motion may be achieved by either moving the wafer (i.e. by
moving the wafer stage) relative to the measurement spots (which is
produced by the projection and detection components of the wafer
level sensor) or moving the measurement spot array (i.e. moving the
projection and detection components of the wafer level sensor)
relative to the wafer (i.e. by moving the wafer stage).
BRIEF DESCRIPTION OF THE DRAWINGS
[0018] The invention will be better understood from the following
detailed description of exemplary embodiments thereof in
conjunction with the accompanying drawings in which:
[0019] FIG. 1 is a plan view illustration of a wafer showing the
wafer height mapping scan path conducted by a conventional wafer
level sensor;
[0020] FIG. 2 is a plan view illustration of a measurement spot
array scanning over an exposure field according to a conventional
wafer height mapping method, wherein the exposure field width is
same as the width of the measurement spot array;
[0021] FIG. 3 is a plan view illustration of a measurement spot
array scanning over an exposure field according to a conventional
wafer height mapping method, wherein the exposure field width is
less than the width of the measurement spot array and not being a
multiple of the width of a single measurement spot;
[0022] FIG. 4 is a plan view illustration of a wafer height map
data generated by a conventional wafer level sensor overlaid on a
wafer's exposure area grid;
[0023] FIG. 5 is a plan view illustration of a measurement spot
array scanning over an exposure field according to a conventional
wafer height mapping method, wherein the exposure field width is
less than the width of the measurement spot array but being a
multiple of the width of a single measurement spot;
[0024] FIG. 6 is a flow chart illustration of the steps involved in
the method of wafer height mapping according to an embodiment of
the present invention;
[0025] FIG. 7 is a plan view illustration of a measurement spot
array illustrating the first scan over an exposure field according
to an embodiment of the present invention, wherein the measurement
spot array has been translated in the first direction;
[0026] FIG. 8 is a plan view illustration of a measurement spot
array illustrating the second scan over an exposure field according
to an embodiment of the present invention, wherein the measurement
spot array has been translated in the second direction; and
[0027] FIG. 9 is a schematic illustration of a wafer level sensor
showing some of the major components.
[0028] Features shown in the above referenced drawings are not
intended to be drawn to scale, nor are they intended to be shown in
precise positional relationship. Like reference numbers are used to
illustrate like features in the various drawings.
DESCRIPTION
[0029] The method of wafer height mapping according to an
embodiment of the present invention eliminates or substantially
reduces the "spacing" in the wafer height mapping data observed in
a conventional wafer level sensor system when the width E of the
column of exposure areas is less than the width of the wafer level
sensor's measurement spot array and the width E is not a multiple
of the width W of a single measurement spot of the wafer level
sensor. A column of exposure areas on a wafer will be referred to
throughout this document as an "exposure field."
[0030] FIG. 6 is a flow chart 100 illustrating the steps of an
exemplary method according to an embodiment of the present
invention.
[0031] At step 101, before the wafer level sensor begins mapping a
wafer, the wafer level sensor's software determines the edge
positions of each of the exposure field. First, level sensor
circles the wafer edge and shine a light beam on the edge. The
reflection light signal will be received by a level sensor
receiver. After modeling, the wafer edge leveling data will be
collected. The details of how the software determines the edge
positions of each of the exposure field is well known in the art
and need not be discussed further.
[0032] At step 102, the array of measurement spots is aligned to an
exposure field (i.e., a column of exposure areas) on the wafer to
be scanned. In this initial alignment, the linear array of
measurement spots is centered on the exposure filed (i.e., the
center measurement spot of the nine spot array is aligned to the
center line of the exposure field) as in the conventional wafer
level sensor system.
[0033] At step 103, the wafer level sensor's array of measurement
spots is translated in a first direction perpendicular to the scan
direction so that one edge of the exposure field is substantially
flush with an edge of the nearest measurement spot and the
measurement spot being within the exposure field. This is
illustrated in FIG. 7. The center 80 of the array of measurement
spots is translated in a first direction D1 from the center line 85
of the exposure field so that the exposure field's edge E1 is
substantially flush with the edge of the nearest measurement spot
M2. The first direction D1 is perpendicular to the scan direction
SC. As can be seen, measurement spots M2 through M7 are fully
within the exposure field and measurement spots M1, M8 and M9 are
not. Of course, one should note that if the exposure field width E
was the same as the width of the array of measurement spots or a
multiple of the width W of a single measurement spot, there would
not be any need to translate the measurement spot array since the
edge E1 of the exposure field would be substantially flush with the
edge of the nearest measurement spot.
[0034] At step 104, the exposure field is scanned and mapped,
generating a first mapped height data of the exposure field. As
illustrated in FIG. 7, however, the first mapped height data for
this exposure field would be a partial representation of the
exposure field. The data from the measurement spot M8 substantially
covering the area 87 would be discarded because the measurement
spot is not wholly within the exposure area.
[0035] At step 105, the measurement spot array is translated in a
second direction, opposite the first translation direction, towards
the second edge of the exposure field, until the second edge of the
exposure field is substantially flush with a nearest measurement
spot, the measurement spot being within the exposure field. This is
illustrated in FIG. 8. The center of the array of measurement spots
is translated in a second direction D2 from the previous position
80a to a second position 80b so that the exposure field's second
edge E2 is substantially flush with the edge of the nearest
measurement spot M8. The second translation direction D2 is
opposite the first direction D1 and perpendicular to the scan
direction SC. As can be seen, after this second translation, the
measurement spots M3 through M8 are fully within the exposure field
and measurement spots M1, M2 and M9 are not. But more importantly,
the area 87, which was not mapped during the first scan is now
covered by the measurement spot M8 and will be mapped in subsequent
scanning.
[0036] The translation of the measurement spot array discussed
herein refers to a relative motion between the measurement spot
array and the exposure field (which is on the wafer). Thus, the
translation of the measurement spot array may be produced by either
moving the wafer stage (and thus the wafer) relative to the
measurement spot array (which is produced by the projection and
detection components of the wafer level sensor) or moving the
measurement spot array (i.e. moving the projection and detection
components of the wafer level sensor) relative to the wafer (i.e.
by moving the wafer stage).
[0037] At step 106, the exposure field is scanned and mapped for
the second time generating a second mapped height data for the
exposure field. The second mapped height data, however, includes
mapped height data for the portion of the exposure field that was
not mapped during the first scan. As illustrated in FIG. 8, the
mapped height data from this second scanning of this exposure field
includes mapped data for the area 87.
[0038] At step 107, the central processing unit (not shown) which
manages the operation of the wafer level sensor would merge the
mapping data from the two scan runs to generate a full height
mapping data for the particular exposure field with out any
"spacing." This data merging step conducted by the central
processing unit of a wafer level sensor is well known in the art
and further details need not be discussed.
[0039] The steps 101 through 107 may be repeated until all exposure
fields of the wafer are mapped.
[0040] According to another preferred embodiment of the present
invention, the method of wafer height mapping further comprises
determining, first, whether the particular exposure field can be
scanned once using the conventional method (i.e., with the
measurement spot array aligned to the center of the exposure field)
or the steps of translating the measurement spot array and double
scanning the exposure field is required. This determination can be
done by comparing the width of the particular exposure field to the
known width of a single measurement spot. As discussed in reference
to FIGS. 2, 3 and 5, if the width E of the exposure field is a
multiple of the width W of a single measurement spot, that exposure
field can be captured fully by the measurement spot array and the
scanned map data would contain a full height map data for that
exposure field. In other words, if the width E of the exposure
field is a multiple of the width W of a single measurement spot,
substantially all of the exposure field can be captured by the
measurement spot array without any of the measurement spots
partially falling in the exposure field. Thus, if the wafer level
sensor determines that the width E of a particular exposure field
meets this requirement, that exposure field only needs to be
scanned one time in a conventional manner (i.e., with the
measurement spot array aligned to the center line of the exposure
field). The over all wafer height mapping process will be more
efficient since the additional steps of translating and double
scanning exposure fields will only be conducted where
necessary.
[0041] The translation motion of the measurement spot array
discussed above may be enabled by utilizing any one of a variety of
known mechanisms. For example, conventional wafer level sensors are
already equipped with a set of stepper motors to manipulate the
position of the wafer stage. And the movement of the wafer required
to achieve the translation motion of the measurement spot array are
within the capabilities of most wafer level sensors without any
additional actuating devices may be achieved through the software
that controls the wafer stage movement in most conventional wafer
level sensors. However, if necessary or appropriate, additional
actuating devices such as stepper motors may be added to a
conventional wafer level sensor to move the necessary components in
order to translate the measurement spot array as desired. FIG. 9 is
a schematic illustration of a typical wafer level sensor system 200
showing the major components. The typical wafer level sensor 200
comprises primarily two sections, a projection component 210 and a
detection component 250. The projection component 210 comprises a
light source 220, a light guide 225, illumination optics 230, and
projection optics 235. The projection optics 235 includes a
projection grating diaphragm 237 for projecting the array of nine
measurement spots on to the wafer 10. The wafer 10 is carried on a
wafer stage 15. The detection component 250 comprises a detection
optics 260, a modulation optics 270, a photo elastic modulator 275,
and data acquisition module 280. The detection optics 260 includes
a detection grating diaphragm 265 for receiving the light signals
of the array of nine measurement spots reflected by the wafer 10.
One method of translating the measurement spot array is to add two
stepper motors, one motor to controllably move the projection
optics 235 and another motor to controllably move the detection
optics 260. Alternatively, the position of the wafer stage 15 may
be adjusted in relation to the measurement spot array to produce
the same effect as translating the measurement spot array itself.
Since in actuality, the scanning motion during the wafer height
mapping process is created by moving the wafer stage rather than
moving the wafer level sensor's optical components, this
alternative method of adjusting the position of the wafer stage to
create the effect of translating the measurement spot array
relative to the exposure fields of the wafer may be easier to
implement.
[0042] While the foregoing invention has been described with
reference to the above embodiments, various modifications and
changes can be made without departing from the spirit of the
invention. These various modifications are considered to be within
the scope of the invention defined by the appended claims.
* * * * *