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What is claimed is:
1. A method of controlling a lithographic tool operating on a surface of a
workpiece, the method comprising steps of:
observing the surface of said workpiece; and
wherein said observing comprises:
viewing adjacent sites at a first location on said surface with separate
beams of radiation to obtain data of said surface by differential phase
shift induced on said beams by reflection from said surface; and
converting said differential phase shift to amplitude data to produce an
image point of said first location;
said method further comprising steps of
repeating said observing step at additional locations spaced from said
first location to obtain further image points, all of said image points
constituting detected image of points on said surface;
providing a reference image pattern; and
correlating detected image with said reference image by use of an edge
correlation sum.
2. A method of controlling a lithographic tool having an exposure head and
a microscope for processing a wafer having a plurality of fields
containing product areas, said wafer being covered with a layer of
photoresist, the method providing for a correction of exposure dosage of
the photoresist, the method comprising steps of:
exposing a number of said fields, said exposing producing a latent image in
said photoresist for each of said number of fields;
moving said wafer to a microscope operative to obtain image data of said
photoresist by differential phase shift;
measuring amplitude obtained from the differential phase shift across
latent images for each of said number of fields, said step of exposing
producing a change of thickness in said photoresist;
obtaining previously measured photoresist thickness data for all of said
fields;
calculating exposure correction;
moving the wafer back to the exposure head; and
exposing said photoresist with a further dosage of radiation to adjust the
thickness of the photoresist.
3. a method of controlling a lithographic tool having a microscope and an
exposure head for processing a wafer, the wafer having a coating of
photoresist and comprising a plurality of fields each of which includes a
product area, the method providing for a correction of focus of the
exposure head, the method comprising steps of:
exposing a number of the fields with stepped focus;
moving said wafer to said microscope, said microscope being operative to
produce a detected image of a latent image by differential phase shift;
measuring points of the detected image across the latent image for each cf
said number of fields;
calculating a best focus position;
returning the wafer to said exposure head;
adjusting said focus; and
exposing the photoresist by said exposure head with a new value of focus.
4. A method of controlling a lithographic tool having a microscope and an
exposure head for processing a wafer, the wafer having a coating of
photoresist and comprising a plurality of fields each of which includes a
product area, the method providing for a measuring of line width in a
latent image in said photoresist, the latent image in the photoresist
being produced by exposure of the photoresist, the method comprising the
steps of:
exposing a number of said fields with step exposure and focus, the exposure
producing a latent image in the photoresist;
moving the wafer to said microscope;
measuring a distance between each position of image points lying on each
side of a line in said latent image to obtain a measure of the line width;
calculating a best focus and an exposure combination by comparison of the
line width with a line-width set point;
returning the wafer to said exposure head;
setting a new focus and amounts of exposure at said exposure head; and
exposing the photoresist with a new combination of focus and quantity of
exposure.
5. A method of controlling a lithographic tool having a microscope and an
exposure head for processing a wafer, the wafer having a coating of
photoresist and comprising a plurality of fields each of which includes a
product area, exposure of the photoresist via a reticle in said exposure
head producing a latent image with verification marks in the photoresist,
the method providing for an alignment of the wafer with the exposure head,
the me&:hod comprising steps of:
exposing a number of said fields, one field at a time;
moving the wafer to said microscope, said microscope being operative to
produce the detected image of the latent image by differential phase
shift;
locating an overlay verification aid;
measuring average overlay values of offset in each of two dimensions for
each of said number of fields;
correcting coordinates of said tool for a field position;
returning the wafer to said exposure head; and
exposing said photoresist with new overlay values.
6. A method of evaluating a bake cycle by use of a lithographic tool having
a microscope and an exposure head for processing a wafer, the wafer having
a coating of photoresist and being subjected to a bake cycle, exposure of
the photoresist via a reticle in said exposure head producing a latent
image with verification marks in the photoresist, said microscope being
operative to produce a detected image of the latent image by differential
phase shift, the method comprising:
prebaking the wafer;
exposing the entire wafer by said exposure head;
moving said wafer to a bake station;
placing said wafer on a heated vacuum chuck;
employing said microscope to measure the amplitude of points of said
detected image;
evaluating dimensions of said verification marks in an overlay;
determining whether the amplitude of points of the detected image is
sufficient to indicate adequate baking;
quenching the wafer by moving the wafer to a cooled platen; and
developing the photoresist.
7. A method of employing a lithographic tool to obtain latent image
enhanced global alignment by use of verification marks and tool
registration marks, the tool having a microscope and an exposure head for
processing a wafer, the wafer having verification marks thereon and being
coated with photoresist, exposure of the photoresist by a reticle in said
exposure head producing a latent image with verification marks in the
photoresist, said microscope being operative to produce a detected image
of a latent image in the photoresist by differential phase shift, the
method comprising steps of:
exposing registration and verification marks at a number of sites on the
wafer;
employing the lithographic tool to locate the registration marks;
moving the wafer under said microscope;
locating and measuring overlay verification marks for each of said number
of sites;
correcting locations of said registration marks by fitting a polynomial,
the deriving translation, rotation, and magnification terms of the
polynomial to determine corrective positioning of the entire wafer;
moving the wafer under said exposure tool;
using corrected fit to predict all site positions; and
exposing all sites of the wafer.
8. A method of controlling a lithographic tool having a microscope and an
exposure head for processing a wafer, the wafer having a coating
photoresist and comprising a plurality of dies, there being primary
verification marks and tool registration marks located on the surface of
said wafer, the method comprising steps of:
exposing said photoresist via a reticle in said exposure head to produce a
latent image with secondary verification marks in the coating of
photoresist;
observing a surface of said coating by use of said microscope, said
microscope being operative to produce a detected image of the latent image
by differential phase shift;
observing a surface of said coating by use of said microscope, said step of
observing including a viewing of adjacent sites at a first location on
said coating surface with separate beams of radiation to obtain data of
said coating surface by differential phase shift induced on said beams by
reflection from said coating surface, said step of observing also
including a converting of said differential phase shift to amplitude data
to produce an image point of said first location in said detected image;
repeating said observing step at additional locations on said coating
surface spaced from said first location to obtain further image points in
said detected image, all of said image points constituting said detected
image;
wherein said observing step further comprises a step of illuminating said
coating with radiation to which said coating is transparent to provide
reflections from said coating surface and from an interface between said
coating and said wafer, the reflections providing images of said primary
and said secondary verification marks in said detected image; and
wherein said method further comprises:
correlating said detected image with reference images of verification marks
to obtain a centroid of each of said verification marks; and
measuring the distance between the centroids to determine alignment of the
wafer. |
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Claims |
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Description |
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BACKGROUND OF THE INVENTION
This invention relates to the construction of semiconductor devices and
other layered devices by photolithography including steppers for
displacing a semiconductor wafer relative to a mask or a reticle and, more
particularly, to the use of latent images of alignment marks formed in a
photoresist layer for increased precision in the locating of a wafer
relative to a mask.
Numerous forms of semiconductor devices including integrated circuits, by
way of example, are constructed by building up layers of material on a
substrate, etching away portions of the material, followed by steps of
deposition of other substances which may include dopants. The locations of
the various materials are precisely controlled to insure an accurate
formation of the various semiconductor structures. Typically, layers of
photoresist are deposited and exposed by photolithographic apparatus
including masks to delineate the forms, in two dimensions, of the desired
structural elements. In the case of construction of numerous chips from a
single wafer, a mask used in the construction of any one chip is also used
for constructing the other chips. This is accomplished by stepping the
wafer under an optical system including the mask for exposure of the
individual wafer regions, or dies, of the respective chips. At each die,
photoresist is exposed, typically with ultraviolet light by the optical
system to form a latent image of the mask in a layer of photoresist. The
wafer is then removed from the stepper, and developer is applied to the
photoresist to develop the image. Etches selective to the development of
exposed photoresist remove portions of the photoresist to prepare the
wafer for the next stage of material deposition or other types of etching
processes. At a later stage in the manufacture the wafer is returned to
the stepper for exposure of the dies to another mask.
It is noted that the foregoing use of photolithographic apparatus in the
printing of successive circuit patterns on the surface of a wafer can also
be employed in the manufacture of reticles or masks is in the manufacture
of semiconductor devices. Generally speaking, the use of the term mask is
understood to include the term reticle or E-beam or maskless direct write
control pattern for the purposes of the present invention.
To ensure accurate alignment of the wafer relative to the optical system,
it is the practice in the initial stage of manufacture to produce tool
registration aids on the wafer substrate in the marginal region
surrounding each die, the registration aid being outside the product area
of a die. A tool registration aid may be formed as a depression in the
substrate surface, for example, wherein the aid is formed by an etch. A
primary verification aid or mark may also be formed on the substrate.
During subsequent steps in the manufacturing process, secondary
verification marks located in the marginal region of a mask are projected
onto a layer of photoresist and developed to allow alignment of the
primary and the secondary verification marks.
The foregoing procedure presents a problem in that the verification marks
imprinted in the photoresist must be observed in a manner which does not
detract from the use of the photoresist in the process of imaging the
details of an integrated circuit or other subject matter of the
semiconductor device. Under present practice in which the verification
marks imprinted on the photoresist are not usable until after the
photoresist has been developed, the wafer is removed from the stepper,
processed with a developing liquid, and then examined under a microscope
to determine the degree of alignment between verification marks and
registration marks. This wafer is known as a send ahead wafer, and must be
reprocessed in the event that the alignment is poor. The stepper is then
adjusted to correct the alignment, after which subsequent wafers can be
processed by the stepper. The current practice is particularly
inconvenient where relative few wafers are to be processed, and wherein
frequent reticle changes may be required by a need to place several
product patterns on a single wafer. The send-ahead wafer is a randomly
selected product substrate from a batch. Alternatively, a monitor
substrate which does not have a product being developed thereon may be
employed to check alignment. In either case, the additional manufacturing
steps represent a significant inconvenience.
SUMMARY OF THE INVENTION
The aforementioned problem is overcome and other advantages are provided by
a manufacturing process employing a wafer stepper and photolithographic
apparatus wherein, in accordance with the invention, a latent image of
verification marks imprinted on a layer of photoresist is employed for
alignment with registration marks previously produced on the substrate of
a wafer. The invention is advantageous in that the latent image can be
employed without the need for development of the exposed photoresist,
thereby to preserve the latent image of subject matter for a product area
on a die of the wafer. Therefore, alignment can be accomplished without
removal of the wafer from the stepper. If inadequate alignment is found,
the alignment can be corrected without removal of the wafer from the
stepper with the result that only the first die, or a first plurality of
dies, which dies have been exposed for the alignment process, have been
lost while all the other dies on the wafer can now be exposed under
conditions of correct alignment.
In accordance with a further embodiment of the invention, the latent image
is viewed by an optical system employing illumination of the latent image
with a photon energy lower than that required for exposure of the
photoresist. Thereby, no alteration of the latent image occurs during the
illumination. For example, where exposure of the photoresist to produce
the latent image is accomplished by use of light in the deep ultraviolet
region of the spectrum, the viewing of the latent image is accomplished
with illumination with lower-frequency light in the near ultraviolet
region of the spectrum.
In accordance with a further feature of the invention, examination of the
latent image is accomplished with a microscope and further reticle
elements operating in accordance with differential phase contrast,
preferably Nomarski differential interference contrast. This produces a
received beam of light in which changing depth and/or changing index of
refraction of the photoresist layer produces an elliptically polarized
wave which, upon being viewed through a polarization analyzer, produces
variation in amplitude at the plane of a detector. The detector may be
constructed as an array of charge-coupled devices (CCD) or a videcon which
is scanned to extract data of a two-dimensional image at the plane of the
detector. Variation in intensity across the detected image is stored as a
set of pixels each represented as a multibit digital signal. The stored
signal has at least several shades of gray to enable accurate observation
of the blatant image.
An interesting feature of the optical processing by differential
interference contrast (DIC) is the fact that the detected image shows a
light region where the depth of the photoresist is decreasing and a dark
region where the depth of the photoresist is increasing. Between these two
regions, the image shows a medium value of gray. Exposure of the
photoresist material to the ultraviolet radiation induces a slight
decrease in thickness of the photoresist, and a change in the index of
refraction. For example, a line having a width of two microns would appear
in the detected image as a length of dark fringe spaced apart from a
length of light fringe, the two fringes being spaced apart on centers by
the foregoing width of two microns. The width of a fringe is a measure of
the width of a transition region between the normal depth and refractive
index, and the reduced thickness and altered refractive index.
In accordance with features of the invention, the foregoing characteristics
of the DIC detected image permit convolution of the detected image and
alignment with a reference image of verification marks to reduce the
effect of noise on the determination of the precise location of the
detected verification mark. Such noise results from, by way of example,
deposits of aluminum and facets of polysilicon which may be present in the
wafer. A comparison of the centroid of a detected secondary verification
mark with the centroid of a primary verification mark produces a
positional error signal which is used in connection with an error signal
of a servomechanism of the stepper to refine the accuracy of the stepper.
Similarly, the DIC detected image can be employed for enhanced global
alignment in situations where verification marks for several dies are
examined and averaged together to produce a position correction signal.
Also, in accordance with the invention, the strength of the detected image,
specifically the intensity of a light or dark fringe, varies in accordance
with the sharpness of focus of the exposing radiation. Therefore,
examination of the detected image can be used to adjust the optical system
for best focus. Furthermore, in view of the fact that the thickness and/or
the refractive index of the exposed photoresist is dependent on the amount
of exposure, namely the intensity and duration of the exposure, and is
also dependent on the amount of heating in those photoresist materials
which must be baked after exposure, examination of the detected image can
also be used as a measure of exposure and a measure of baking.
It is to be noted that the theory of the invention for the measurement of
parameters of the photoresist based on a viewing of a latent image in the
photoresist is applicable to a variety of manufacturing situations in
which a layer of photoresist is employed. The construction of multilayered
semiconductor devices represents a common occurrence of a situation
employing photoresist in the construction process. However, other
important manufacturing situations, by way of example, relate to the
configuration of ferroelectric materials in miniaturized recording heads,
and the construction of masks and reticles used for constructing various
devices. Accordingly, the methodology of the invention has applicability
to diverse manufacturing processes, and is not limited to the construction
of semiconductor devices. Nor is the invention limited to optical exposure
tools but rather, encompasses E-beam, ion and X-ray exposure tools.
BRIEF DESCRIPTION OF THE DRAWING
The aforementioned aspects and other features of the invention are
explained in the following description, taken in connection with the
accompanying drawing wherein:
FIG. 1 shows a graphical representation of a typical patterned substrate
with both plan and cross-sectional views of overlay patterns and latent
image patterns, FIG. 1 being divided into four sections identified as
FIGS. 1A-1D wherein FIG. 1A is a stylized view of a wafer, FIG. 1B is an
enlarged view of one die of the wafer, FIG. 1C is an enlarged view of a
square box-shaped primary verification mark surrounded by a similarly
shaped secondary verification mark of a latent image, and FIG. 1D shows an
L-shaped primary verification mark surrounded by an L-shaped secondary
verification mark;
FIG. 2 is a diagrammatic view of optical equipment employed in the practice
of the invention, the optical equipment including a modified Nomarski
differential interference contrast (NDIC) microscope with autofocus and
electro-optical phase modulator, the latter producing rotation of an
optical connector;
FIG. 3 is a diagrammatic view of a measurement subsystem of the invention
integrated with a standard step and repeat microlithographic exposure
tool;
FIG. 4 shows an imaging detector output resulting from response of the NDIC
latent image for the measurement of exposure dose, tool focus, and direct
latent image line width measurement, FIG. 4 being presented in four
sections as FIGS. 4A-4D, and wherein FIG. 4A shows variation in
photoresist thickness with exposure, FIG. 4B shows a differential image
for the geometry of FIG. 4A, FIG. 4C is a graph of image amplitude as a
function of exposure dosage, and FIG. 4D is a graph of image amplitude as
a function of focus;
FIG. 5 is a flow diagram for real-time in-situ exposure monitoring and
control;
FIG. 6 is a flow diagram for real-time in-situ focus monitoring and
control;
FIG. 6A is a flow diagram for direct measurement of latent image
line-width;
FIG. 7 is a flow diagram for real-time in-situ overlay monitoring and
control;
FIG. 8 is a diagrammatic view of a wire-frame, or reference pattern, used
as a standard for convolution in an overlay position-determining
algorithm, the view showing also representative results of a convolution
calculation;
FIG. 9 is a stylized representation of successive steps employed in the
formation of layers of material for the construction of a semiconductor
device, such construction being advantageously implemented by the
invention;
FIG. 10 is a diagrammatic view of a photoresist post-exposed bake thermal
developer station;
FIG. 11 is a graphical representation of an NDIC image ratio as a function
of bake time for a thermal developer station;
FIG. 12 is a graphical representation of tool grid and registration aid
locations for blatant image enhanced global alignment; and
FIG. 13 is a flow diagram for latent image enhanced global alignment.
DETAILED DESCRIPTION
FIGS. 1A-1D describe alignment marks employed in the practice of the
invention for aligning a wafer 20 with a reticle or mask in
photolithographic apparatus to be described hereinafter. FIG. 1A discloses
a set of six dies 22 by way of example, it being understood that, in
practice, hundreds of dies may be present in a wafer. Global alignment
marks 24 are provided for initial alignment of the wafer 20 upon placing
the wafer within a stepper of the photolithographic apparatus.
In FIG. 1B, in stylized view, a portion of the wafer 20 of FIG. 1A has been
enlarged to show a die 22 and a marginal region 26 surrounding the die 22.
Two primary verification marks 28 and two tool registration marks 28B are
arranged about the die 22, within the marginal region 26, and formed
directly within a surface of the wafer 20. In the practice of the
invention, during a manufacturing step wherein a photoresist layer is
disposed upon the wafer 20, secondary verification marks provided by a
reticle or mask are projected onto the layer of photoresist. Two such
secondary verification marks 30 are shown in FIG. 1B. The verification
marks 30 may have a desired configuration, such as a square-shaped picture
frame as shown in FIG. 1B, or an L-shaped configuration as shown by an
alternative primary verification mark 28A and an alternative secondary
verification mark 30A in FIG. 1D. The verification mark 30 or 30A is
substantially larger than and encloses the verification mark 28 or 28A so
that, upon an initial alignment of the wafer 20 with the stepper, provided
by the optical system of the stepper as will be described hereinafter, the
primary verification mark 28 or 28A falls within the secondary
verification mark 30 or 30A. While the secondary verification mark 30 or
30A is shown to be larger than the primary verification mark 28 or 28A, it
is to be understood that the relative sizes may be reversed such that the
secondary verification mark would be made smaller than the primary
verification mark (not shown) with the latter enclosing the former.
FIG. 1C shows further details in the conduction of the registration and the
verification marks 28 and 30. Therein, the corner portion of the picture
frame in each of the marks has been deleted leaving four straight segments
of marking arranged in the square configuration. The four mark segments in
each of the verification marks 28 and 30 are sufficiently long so as to
provide overlap between the corresponding mark segments of the
verification marks 30 and 28 even upon displacement of the centroids of
the marks relative to each other, as might occur upon initial alignment of
the wafer with the stepper. An offset between the centroid positions, as
measured in an X coordinate, of an X-Y coordinate system, is also shown in
FIG. 1C.
As will be disclosed hereinafter, upon a viewing of the verification marks
28 and 30 by a scanning of a detected image along a coordinate axis, for
example the X axis, edge lines of a mark segment are noted by a change of
intensity in the detected image, this being a characteristic of the
Nomarski differential interference contrast response of the microscope
system employed in viewing the marks. Such an NDIC response is also
indicated in FIG. 1C for the situation in which a scanning of the detected
image is accomplished along a scan line parallel to the X axis and
intersecting a portion of the verification mark 30 which is parallel to
the Y axis.
FIG. 2 shows an optical system 32 employed by the invention for
accomplishing a fine measurement of offset between primary and secondary
verification marks, which fine measurement is to be employed by stepper
positioning apparatus to accomplish a more accurate locating of the wafer
within the stepper. The optical system 32 includes a source 34 of light,
typically near ultraviolet, a condenser lens 36 for collimating rays of
light from the source 34, and a rotator 38 of the electric vector of
collimated light incident upon the rotator 38 from the lens 36. The
rotator 38 comprises an electrooptic liquid crystal phase shifter which
introduces phase shifts differentially to orthogonal components of the
electric vector of the light incident upon the rotator 38 from the lens
36. The amount of differential phase shift is established in response to a
voltage applied to the rotator 38 from a voltage controller 40. The output
beam of light from the rotator 38 is indicated by line 42.
The optical system 32 further comprises a vidicon 44, a polarization
analyzer 46, a half-silvered mirror 48 comprising a glass plate 50 with a
layer 52 of silver disposed on a surface of the plate 50, a Nomarski
modified Wollaston prism 54 comprising two prismatic elements 56 and 58
which meet along an interface 60, and a microscope objective 62. Also
shown in FIG. 2 are components of a stepper 64, namely a stage 66 (shown
in simplified form) for supporting the wafer 20 in front of the objective
62, and a servo unit 68 for positioning the stage 66 to align the wafer 20
in accordance with command signals developed by use of the optical system
32.
The vidicon 44 is provided by way of example as a detector of an image of
subject matter of the wafer 20, it being understood that another form of
area detector, such as a charge coupled device, may be employed instead of
the vidicon 44. The vidicon 44 constitutes a part of an imager 70, the
imager 70 further comprising scan control unit 72 and a digitizer 74 which
are connected to the vidicon 44, and a memory 76 such as a random access
memory connected to an output terminal of the digitizer 74. The scan
control unit 72 directs the vidicon 44 to perform a raster scan of an
image incident upon the vidicon 44 so as to enable the vidicon 44 to
output data of the image as a sequence of scan frames each of which
provides image data in the form of a sequence of scan lines. Data
outputted by the vidicon 44 is sampled by the digitizer 74 and converted
to a sequence of multibit digital words, for example, 8-bit words, which
are applied to the memory 76 to be stored therein. An image/signal
processor 78 which includes a computer 80 and timing circuitry 82 outputs
timing signals for synchronizing operation of the scan control unit 72 and
the digitizer 74. In addition, the processor 78 addresses the memory 76
for obtaining data from the memory 76. Furthermore, the processor 78
applies a signal to the voltage controller 40 for designating an amount of
phase shift to provide a | | |
