U.S. patent application number 10/233512 was filed with the patent office on 2003-01-02 for wafer with overlay test pattern.
Invention is credited to Bar-On, David, Mazor, Isaac, Yokhin, Boris.
Application Number | 20030002620 10/233512 |
Document ID | / |
Family ID | 24202382 |
Filed Date | 2003-01-02 |
United States Patent
Application |
20030002620 |
Kind Code |
A1 |
Mazor, Isaac ; et
al. |
January 2, 2003 |
WAFER WITH OVERLAY TEST PATTERN
Abstract
A method of X-ray analysis includes irradiating a spot on a
sample with X-rays along an X-ray beam axis. X-rays emitted from
the sample, responsive to irradiating the spot, are simultaneously
detected at a plurality of different azimuthal angles relative to
the beam axis. X-ray intensities detected at the different angles
in a common energy range are compared in order to determine a
property of the sample.
Inventors: |
Mazor, Isaac; (Haifa,
IL) ; Yokhin, Boris; (Nazareth Illit, IL) ;
Bar-On, David; (Givat Ella, IL) |
Correspondence
Address: |
Hoffman, Wasson & Gitler, P.C.
Suite 522
2361 Jefferson Davis Highway
Arlington
VA
22202
US
|
Family ID: |
24202382 |
Appl. No.: |
10/233512 |
Filed: |
September 4, 2002 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
10233512 |
Sep 4, 2002 |
|
|
|
09551715 |
Apr 18, 2000 |
|
|
|
6453002 |
|
|
|
|
Current U.S.
Class: |
378/49 |
Current CPC
Class: |
G01N 23/223 20130101;
G01N 2223/076 20130101 |
Class at
Publication: |
378/49 |
International
Class: |
G01N 023/223 |
Claims
1. A semiconductor wafer having at least an upper and a lower metal
layer deposited thereon, the layers comprising respective upper and
lower features defining a test pattern on the wafer, such that the
upper feature substantially shields the lower feature from
radiation when the upper and lower metal layers are properly
registered with one another, but does not shield at least a portion
of the lower feature when the layers are not properly
registered.
2. A wafer according to claim 1, wherein the lower feature
comprises a first metallic material, and wherein the upper feature
comprises a second metallic material, the first and second
materials having substantially different X-ray fluorescence
spectra.
Description
FIELD OF THE INVENTION
[0001] The present invention relates generally to X-ray
spectrometry, and specifically to methods and devices to detect and
analyze X-ray microfluorescence.
BACKGROUND OF THE INVENTION
[0002] X-ray microfluorescence analysis is a non-destructive
technique known in the art for determining the atomic composition
and thickness of thin films. Typically, a focused X-ray beam is
directed at a sample, and the X-ray fluorescence induced by the
interaction of the X-rays with the sample is detected by a detector
located near the sample. The composition and thickness of the
irradiated sample are determined from the intensity and energy of
the fluorescent X-ray photons.
[0003] In "Annular-Type Solid State Detector for a Scanning X-Ray
Analytical Microscope," Review of Scientific Instruments 66(9)
(September, 1995), pp. 4544-4546, which is incorporated herein by
reference, Shimomura and Nakazawa describe an annular germanium
detector located near an irradiated sample which transduces the
energy resulting from X-ray fluorescence into a single channel of
data.
[0004] U.S. Pat. No. 5,937,026, to Satoh, whose disclosure is
incorporated herein by reference, describes a microfluorescent
X-ray analyzer in which a capillary tube is used to deliver X-ray
excitation to a small region of a sample. The capillary passes
through a hole in the center of a flat plate solid-state X-ray
detector, which is used to detect fluorescent X-rays emitted by the
sample. The geometry of the capillary tube and the detector allows
fluorescent X-rays from a small excitation region to be detected
over a large solid angle.
[0005] U.S. Pat. No. 3,256,431, to Fraser, U.S. Pat. No. 3,581,087,
to Brinkerhoff and U.S. Pat. No. 5,778,039, to Hossain, whose
disclosures are incorporated herein by reference, describe systems
for detection and analysis of X-ray fluorescence using multiple
detectors. In all of these patents, a sample is excited by an X-ray
source, and the multiple detectors are used to detect the X-ray
fluorescence in different, respective energy domains. Typically,
the energy domains are chosen to correspond to emission bands of
different elements in the sample, so that comparative measurements
can be made of the relative concentrations of two elements, for
example.
[0006] U.S. Pat. No. 5,497,008, to Kumakhov, which is incorporated
herein by reference, describes analytic instruments using a
polycapillary X-ray optic, also known as a Kumakhov lens, for X-ray
fluorescence analysis or spectroscopy. The instruments described
use a single fluorescence detector.
SUMMARY OF THE INVENTION
[0007] It is an object of some aspects of the present invention to
provide improved apparatus and methods for X-ray microfluorescence
analysis.
[0008] It is a further object of some aspects of the present
invention to provide apparatus and methods for detection and
analysis of X-ray microfluorescence associated with very small
geometrical features of a sample.
[0009] It is yet a further object of some aspects of the present
invention to provide apparatus and methods for detection of faults
occurring in production of semiconductor devices.
[0010] In preferred embodiments of the present invention, an X-ray
microfluorescence analyzer comprises an X-ray source which
irradiates a small spot on a sample, and a plurality of individual
detectors arrayed around the spot, so as to capture X-ray photons
emitted from the sample responsive to the X-ray illumination.
Preferably, the detectors are arrayed in a generally symmetrical
pattern about the spot. A processing unit receives signals from the
detectors and processes them to compare the intensity of photon
emission captured by the different detectors, and thus to detect
variations in the intensity as a function of azimuth about the
irradiation beam. These variations are indicative of directional
inhomogeneity of the emission from the sample.
[0011] The detected azimuthal differences in the intensity of
emission in a selected energy range are preferably used to
determine properties of microscopic structures in the sample under
test. Alternatively or additionally, the differences are monitored
in order to accurately align the X-ray source and detectors with
such structures. The method of the present invention, wherein
multiple detectors are used simultaneously to measure emission in a
common energy range at different azimuths, is substantively
different from methods of X-ray fluorescence analysis known in the
art. Such methods, as described in the Background of the Invention,
are generally based on detection at only a single azimuth at any
given time. When multiple detectors are used, their purpose is to
measure emission in different, respective energy ranges, and
directional inhomogeneity of emission is not considered.
[0012] In some preferred embodiments of the present invention, the
analyzer is used to measure overlay errors between successive
layers, such as metallization layers, created on a semiconductor
wafer in the course of integrated circuit production. Preferably, a
test zone is created on the wafer, in which a pattern in a lower
layer, using a first element,is overlaid by a substantially
identical pattern in an upper layer, using a second, different
element. The first and second elements are typically metal
elements, although other types of X-ray detectable elements may
also be used. When the layers are in proper registration, the
pattern in the upper layer substantially shields the element in the
lower layer from X-rays and prevents X-ray photons from the first
element from reaching the detectors. When there is a registration
error, however, a portion of the pattern in the lower layer is
exposed to X-rays, so that photons from the first element can reach
the detectors. The processing unit analyzes the intensity and
direction of emission of these X-ray photons in order to determine
the degree and direction of misregistration between the upper and
lower layers.
[0013] In other preferred embodiments of the present invention, the
analyzer is used to determine the composition and thickness of
bumps formed on a surface of the sample. Such bumps typically
comprise metal bumps, which are formed on the upper surface of a
semiconductor wafer, for example, and are then used as contact
points between an integrated circuit made from the wafer and a
suitable chip carrier (in place of wire bonding) . The analyzer of
the present invention is used to measure the size and thickness of
these bumps, in order to verify that they will provide a suitable
connection to the chip carrier. To perform the measurement
accurately, however, it is necessary that the small spot that is
excited by the X-ray source be accurately aligned with one of the
bumps. Preferably, directional inhomogeneity of X-ray emission from
the bumps is measured so as to provide an indication of
misalignment between the spot and the bump, and thus to drive a
translation stage so that the spot and the bump are precisely
aligned. Alternatively or additionally, the processing unit
averages the signals from the different detectors to compensate for
any residual misalignment.
[0014] In still other preferred embodiments of the present
invention, the sample comprises a crystalline substance, such as
single-crystal silicon, which generates a diffraction pattern when
irradiated by the X-ray source. The diffraction pattern has
directional inhomogeneity, whose direction is determined by an
orientation angle of the substance. This diffraction pattern can
cause anomalies in measurement of X-ray fluorescence by the
analyzer. The processing unit detects the inhomogeneous diffraction
pattern by detecting differences in the signals that it receives
from the different detectors. Most preferably, the signal
differences are used to drive a rotation stage so as to align the
sample, relative to the detectors, in a manner that minimizes the
impact of the diffraction on the fluorescence measurement.
Alternatively, the signal differences may be used to determine the
crystal orientation.
[0015] There is therefore provided, in accordance with a preferred
embodiment of the present invention, a method of X-ray analysis,
including:
[0016] irradiating a spot on a sample with X-rays along an X-ray
beam axis;
[0017] simultaneously detecting X-rays emitted from the sample,
responsive to irradiating the spot, at a plurality of different
azimuthal angles relative to the beam axis; and
[0018] comparing intensities of the X-rays detected at the
different angles in a common energy range in order to determine a
property of the sample.
[0019] Preferably, irradiating the spot includes irradiating a spot
of microscopic size, and comparing the intensities includes
determining a geometrical property of a microscopic structure of
the sample. Further preferably, simultaneously detecting the X-rays
includes detecting X-ray emission using an array of detectors
positioned around the spot.
[0020] Preferably, comparing the intensities includes detecting an
inhomogeneity of the emitted X-rays as a function of azimuth. In a
preferred embodiment, detecting the X-rays includes detecting
X-rays diffracted from the sample, and detecting the inhomogeneity
includes determining an angle of diffraction of the X-rays from a
crystalline structure of the sample. Preferably, the method
includes introducing a relative rotation between the sample and an
array of detectors, responsive to the determined angle, so that the
X-rays are diffracted in a desired direction relative to the
detectors. Alternatively or additionally, detecting the X-rays
further includes detecting fluorescent X-rays emitted by the
sample, and comparing the intensities includes using the determined
angle to distinguish between the diffracted X-rays and the
fluorescent X-rays. Further alternatively or additionally,
irradiating the spot includes irradiating a generally symmetrical
feature of the sample, and detecting the inhomogeneity includes
detecting a misalignment of the spot with the feature.
[0021] In a preferred embodiment, detecting the X-rays includes
detecting X-rays emitted by a lower feature of the sample, at least
a portion of which is covered by an upper feature of the sample so
as to block irradiation of the covered portion of the lower
feature, and comparing the intensities includes assessing a
position of the upper feature relative to the lower feature.
[0022] There is also provided, in accordance with a preferred
embodiment of the present invention, a method for detecting
misregistration of upper and lower layers formed on the surface of
a sample, the layers including respective upper and lower features,
wherein the upper feature is designed to substantially cover the
lower feature, the method including:
[0023] irradiating an area of the sample including the upper and
lower features with X-rays;
[0024] detecting X-rays emitted by the sample in an energy range
that is characteristic of the lower feature; and
[0025] responsive to an intensity of the detected X-rays, assessing
an extent to which the lower feature is not covered by the upper
feature.
[0026] Preferably, detecting the X-rays emitted by the sample
includes detecting fluorescent X-rays emitted by a material that is
present in the lower feature. Further preferably, the material
present in the lower feature includes a first metallic material,
and the upper feature includes a second metallic material. Most
preferably, the sample includes a semiconductor wafer, and wherein
the upper and lower layers include upper and lower metal layers
formed on the wafer.
[0027] In a preferred embodiment, the method includes forming a
test pattern on the wafer, the test pattern including the upper and
lower features, which are designed so that when the upper and lower
metal layers are properly registered, the upper feature
substantially covers the lower feature. Preferably, detecting the
X-rays includes detecting X-rays emitted by the sample at a
plurality of different azimuthal angles relative to a beam axis of
the irradiating X-rays, and assessing the portion of the lower
feature that is not covered includes comparing the intensity of the
X-rays emitted at the different angles to determine a direction of
misregistration of the upper feature with the lower feature.
[0028] Preferably, assessing the extent to which the lower feature
is not covered by the upper feature includes determining a measure
of the area of the lower feature that is not covered by the upper
feature.
[0029] There is additionally provided, in accordance with a
preferred embodiment of the present invention, a method for X-ray
analysis of a generally symmetrical feature on the surface of a
sample, including:
[0030] irradiating the feature with a beam of X-rays having a beam
diameter at the surface of the sample that is substantially less
than a diameter of the feature;
[0031] detecting X-rays emitted from the sample, responsive to
irradiating the feature, at a plurality of different azimuthal
angles relative to an axis of the irradiating beam; and
[0032] analyzing a characteristic of the feature responsive to the
detected X-rays, using respective intensities of the X-rays
detected at the different angles to compensate for a misalignment
between the irradiating beam and the feature.
[0033] Preferably, detecting the emitted X-rays includes detecting
X-ray fluorescence due to an element of the feature, wherein
analyzing the characteristic using the respective intensities
includes summing the intensities of the X-rays detected at the
different angles.
[0034] In a preferred embodiment, the feature includes a metal bump
formed on the surface of a semiconductor wafer.
[0035] Preferably, analyzing the characteristic using the
respective intensities includes measuring a difference in the
respective intensities of the detected X-rays at opposing azimuths
relative to the axis of the irradiating beam. Most preferably,
using the respective intensities at the different angles includes
relatively shifting the beam and the sample responsive to the
measured difference in the intensities, so as to correct the
misalignment.
[0036] There is further provided, in accordance with a preferred
embodiment of the present invention, apparatus for X-ray
microanalysis, including:
[0037] an X-ray source, adapted to irradiate a spot on a sample
with X-rays along an X-ray beam axis;
[0038] a plurality of X-ray detectors, arrayed around the spot so
as to simultaneously receive X-rays emitted from the sample,
responsive to irradiation of the spot, at a plurality of different
azimuthal angles relative to the beam axis, and to generate
electrical signals responsive to the received X-rays; and
[0039] a processing unit, coupled to receive the electrical signals
from the detectors and, responsive to the signals, to compare
intensities of the X-rays received at the different angles in a
common energy range in order to determine a property of the
sample.
[0040] Preferably, the X-ray source includes X-ray optics,
configured to focus the X-ray beam to a spot of microscopic size on
the sample. Most preferably, the X-ray optics include a
polycapillary array.
[0041] There is moreover provided, in accordance with a preferred
embodiment of the present invention, apparatus for detecting
misregistration of upper and lower layers formed on the surface of
a sample, the layers including respective upper and lower features,
wherein the upper feature is designed to substantially cover the
lower feature, the apparatus including:
[0042] an X-ray source, adapted to irradiate an area of the sample
including the upper and lower features with X-rays;
[0043] a plurality of X-ray detectors, arrayed around the spot so
as to simultaneously receive X-rays emitted from the sample in an
energy range that is characteristic of the lower feature, and to
generate electrical signals responsive to the received X-rays;
and
[0044] a processing unit, coupled to receive the electrical signals
from the detectors and responsive to the signals, to assess an
extent to which the lower feature is not covered by the upper
feature.
[0045] There is furthermore provided, in accordance with a
preferred embodiment of the present invention, a semiconductor
wafer having at least an upper and a lower metal layer deposited
thereon, the layers including respective upper and lower features
defining a test pattern on the wafer, such that the upper feature
substantially shields the lower feature from radiation when the
upper and lower metal layers are properly registered with one
another, but does not shield at least a portion of the lower
feature when the layers are not properly registered.
[0046] Preferably, the lower feature includes a first metallic
material, and wherein the upper feature includes a second metallic
material, the first and second materials having substantially
different X-ray fluorescence spectra.
[0047] There is additionally provided, in accordance with a
preferred embodiment of the present invention, apparatus for X-ray
analysis of a generally symmetrical feature on the surface of a
sample, including:
[0048] an X-ray source, adapted to irradiate the feature with a
beam of X-rays having a beam diameter at the surface of the sample
that is substantially less than a diameter of the feature;
[0049] a plurality of X-ray detectors, arrayed around the spot so
as to simultaneously receive X-rays emitted from the sample,
responsive to irradiation of the spot, at a plurality of different
azimuthal angles relative to an axis of the irradiating beam, and
to generate electrical signals responsive to the received X-rays;
and
[0050] a processing unit, coupled to receive the electrical signals
from the detectors and, responsive to the signals, to analyze a
characteristic of the feature responsive to the detected X-rays,
using respective intensities of the X-rays detected at the
different angles to compensate for a misalignment between the
irradiating beam and the feature.
[0051] Preferably, the apparatus includes a translation stage,
coupled to be driven by the processing unit so as to shift the
sample relative to the detectors, so as to correct the
misalignment.
[0052] The present invention will be more fully understood from the
following detailed description of the preferred embodiments
thereof, taken together with the drawings in which:
BRIEF DESCRIPTION OF THE DRAWINGS
[0053] FIG. 1 is a schematic side view of an X-ray
microfluorescence analyzer, in accordance with a preferred
embodiment of the present invention;
[0054] FIG. 2 is a schematic top view of a detail of the analyzer
of FIG. 1, in accordance with a preferred embodiment of the present
invention;
[0055] FIG. 3 is a block diagram that schematically illustrates an
application of an X-ray microfluorescence analyzer in a
semiconductor production process, in accordance with a preferred
embodiment of the present invention;
[0056] FIG. 4 is a schematic, sectional illustration of a
metallization test pattern formed on a semiconductor wafer, in
accordance with a preferred embodiment of the present
invention;
[0057] FIG. 5 is a schematic top view of the pattern of FIG. 4;
and
[0058] FIG. 6 is a schematic, sectional view of a detail of the
analyzer of FIG. 1, applied to analyze bumps on the surface of a
sample, in accordance with a preferred embodiment of the present
invention.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
[0059] FIG. 1 is a schematic, pictorial illustration of a X-ray
microfluorescence analyzer 20, in accordance with a preferred
embodiment of the present invention. X-rays emitted by an X-ray
source 24 are collected by a focusing optic 26. X-ray source 24
preferably comprises an X-ray tube, such as an XTF 5011 tube
produced by Oxford Instruments, Inc., of Scotts Valley, Calif.
Optic 26 is preferably a monolithic polycapillary lens, such as
those produced by X-Ray Optical Systems, Inc., of Albany, N.Y. The
optic collects the X-rays and focuses them to a spot 28 on a sample
22. In the preferred embodiments described hereinbelow, the sample
comprises a silicon wafer, but analyzer 20 may similarly be applied
to samples of other types. Most preferably, spot 28 is
substantially circular with a diameter of the order of 50 m.
Alternatively, optic 26 comprises a monocapillary optic and/or
X-ray collimating pinholes, or any other suitable collimating means
known in the art.
[0060] Fluorescent X-rays, emitted by sample 22 in response to the
irradiating X-rays, are produced within spot 28, and are collected
by a plurality of detectors 30, 31, 33 and 35 arrayed around the
spot. (For clarity of illustration, only detectors 30 and 31 are
shown in FIG. 1. All of the detectors appear in FIG. 2.) The
arrangement and operation of the detectors are described in more
detail hereinbelow. Signals from the detectors are transferred to a
processing unit 32, which most preferably comprises a plurality of
pulse processors, with a separate input and processing channel for
each of the detectors. Each channel determines the spectral
intensity of emission captured by the respective detector,
preferably using methods of energy-dispersive processing, as are
known in the art. An output stage of processing unit 32 analyzes
sums and differences of the signals from the different detectors in
order to measure the spectral intensity of X-ray emission from the
sample and azimuthal variations in the intensity. These
measurements are used determine the local structure and composition
of sample 22 at spot 28, as described hereinbelow. The resultant
information is then output to a display 34.
[0061] Preferably sample 22 is scanned by a translation/rotation
stage 36, as is known in the art, under the control of processing
unit 32. Stage 36 is used to align spot 28 and the array of
detectors with features of interest in the sample and to allow
different areas of the sample to be tested. Alternatively the X-ray
tube, optics and detectors are together scanned over the
sample.
[0062] FIG. 2 is a schematic top view of spot 28 formed on sample
22 and of detectors 30, 31, 33 and 35 positioned around the spot,
in accordance with a preferred embodiment of the present invention.
Preferably, the detectors comprise four PIN diodes, such as type
S1223 PIN diodes produced by Hamamatsu Photonics, K. K., of
Hamamatsu City, Japan. The detectors are arranged symmetrically
about spot 28 and are preferably positioned as close as is
practically possible to the spot. Thus, in the present embodiment,
in which each of the detectors has an active collection area in the
form of a square of side 2.5 mm, the detectors are arranged in a
square having sides of approximately 9 mm and at a distance of
approximately 4 mm from the surface of sample 22. Preferably, as
shown in FIG. 1, the detectors are angled towards spot 28, in order
to increase the active area presented to the spot. It will be
appreciated that the number and type of detectors and their
dimensions and positions are described herein by way of example,
and other numbers, sizes, positions and types of detectors may
similarly be used. For example, another analyzer of this general
type is described in U.S. patent application Ser. No. 09/114,789,
which is assigned to the assignee of the present patent
application, and whose disclosure is incorporated herein by
reference.
[0063] During irradiation by source 24, spot 28 generates
fluorescent X-ray photons which are incident on the detectors,
wherein corresponding pulses are in turn generated in the detectors
and conveyed to processing unit 32. The processing unit analyzes
and counts the pulses from the plurality of detectors. Most
preferably, spot 28 and detectors 30, 31, 33 and 35 are maintained
substantially stationary in relation to sample 22 until sufficient
counts have been recorded by unit 32 in one or more spectral
regions of interest for a satisfactory measurement to be made.
Processing unit 32 then moves sample 22 using stage 36 to a new
spot to be analyzed.
[0064] When sample 22 comprises a crystalline substance, such as a
semiconductor wafer, X-rays incident at spot 28 are diffracted in a
characteristic diffraction pattern. This pattern is illustrated
schematically by a shaded pattern 38 on sample 22. Some of the
diffracted X-rays reach the detectors and can cause saturation of
the detectors or pulse pile-up in unit 32, or can otherwise confuse
measurements of X-ray fluorescence (XRF) by the detectors. Such
interference by diffracted radiation in XRF measurements is known
in the art. The accepted solution is to rotate the sample until, by
trial and error, the effect of diffracted radiation on the XRF
measurement is minimized.
[0065] FIG. 2 illustrates a superior solution to this problem that
is made possible by the present invention. Processing unit 32
measures the azimuthal inhomogeneity of the X-ray signals that are
received from spot 28, preferably by comparing a sum of the signals
from detectors 30 and 31 to a sum of the signals from detectors 33
and 35. In the orientation shown in FIG. 2, in which pattern 38
falls along a diagonal of the square defined by the detectors, the
two sums will be approximately identical. The interference of the
diffracted radiation with the XRF measurement will be minimized in
this orientation. Thus, to cancel the effect of the diffraction,
unit 32 preferably rotates stage 36 until the sums are equalized.
Alternatively or additionally, the initial difference between the
sum of the signals from detectors 30 and 31 and that from detectors
33 and 35 is computed by the processing unit and is then used to
calculate an optimal angle to which stage 36 should be rotated in
order to minimize the diffraction effect.
[0066] FIG. 3 is a block diagram that schematically illustrates a
system 40 for processing of semiconductor wafers using analyzer 20,
in accordance with a preferred embodiment of the present invention.
In this system, differential XRF analysis is used to verify that
microscopic features formed on the wafer have the proper shape,
composition and mutual registration. Analyzer 20 is preferably
integrated on-line with the processing of the wafer, so that as
successive layers are formed on the wafer, each new layer can be
tested and verified in turn. System 40 is shown here by way of
example, and integration of analyzer 20 into other types of
processing systems will be clear to those skilled in the art.
[0067] For each layer to be formed on the wafer, a
microlithographic stepper 42 forms a mask pattern on photoresist
that is deposited on the wafer surface, as is known in the art. The
photoresist is developed and etched by an etcher 44, and a metal
layer is filled into channels created by the etching, in a
metallization chamber 46. Alternatively, the order of the steps may
be reversed, so that the metal layer is applied to the entire wafer
surface, followed by patterning and etching of the metal. In either
case, analyzer 20 measures differential XRF signals generated by
the metal elements in the metal layers, as described hereinbelow.
Alternatively, XRF signals due to non-metal elements may be
detected and analyzed in like manner. When any deviation is
discovered in these differential measurements, the result is used
to adjust system 40 so as to correct the deviation on the next
wafer to be processed. For example, as illustrated in FIG. 3,
stepper 42 is adjusted to correct for misregistration of successive
metal layers detected by analyzer 20. After metallization, an
interlayer dielectric is deposited over the surface of the wafer in
a dielectric deposition chamber 48.
[0068] FIGS. 4 and 5 schematically illustrate a metallization
pattern 51 formed on wafer 22 for the purpose of testing the mutual
registration of successive metal layers, in accordance with a
preferred embodiment of the present invention. FIG. 4 is a
sectional view, while FIG. 5 is a top view of the wafer. The
pattern comprises a lower grid of pads 52 of a first metal, say
tungsten, which is overlaid by an upper grid of pads 54 of a second
metal, say aluminum or copper. The two grids are preferably formed
in the course of processing wafer 22 in system 40 and have a pitch
on the order of the pitch of functional circuit features that are
formed on the wafer. Preferably, the pattern is included in
lithographic masks that are written onto the wafer by stepper 42
during successive photolithography cycles. In this manner, pads 52
are deposited onto a lower dielectric level 50 as part of a lower
metal layer, after which pads 54 are deposited onto an upper
dielectric layer 56 as part of an upper metal layer.
[0069] The masks are designed so that when the upper and lower
metal layers are perfectly registered with one another, each of
pads 52 is fully covered by a respective one of pads 54. When there
is a misregistration of the metal layers, however, there will be an
offset of the upper and lower grids, as shown in the figures.
Similarly, if there is a discrepancy in the dimensions of the upper
and lower pads, due to inaccurate control of etching parameters,
for example, pads 54 may not cover the respective pads 52
completely. Thus, the mutual registration of pads 52 and 54 is
indicative of the registration of the functional metal layers
deposited on wafer 22.
[0070] Because metal pads 54 are relatively opaque to X-rays, the
portion of pads 52 that are directly below corresponding pads 54
receive relatively little irradiation from source 24. Furthermore,
most of the X-ray fluorescence emitted from this covered portion of
the lower pads will be absorbed by the corresponding upper pads.
Therefore, when the upper and lower metal layers are perfectly
registered, and pattern 51 is irradiated by source 24, the number
of fluorescent photons received by detectors 30, 31, 33 and 35 in
the characteristic emission bands of the first metal (from which
pads 52 are formed) will generally be small. Furthermore, the
signals received by each pair of opposing detectors (30 vs. 31, and
33 vs. 35) will be substantially symmetrical.
[0071] On the other hand, when there is an offset between the metal
layers, as shown in FIGS. 4 and 5, a portion of pads 52 is exposed
to irradiation and contributes photons to the flux received by the
detectors. The total magnitude of this flux gives a measure of the
extent of misregistration between the metal layers. A reference
flux level can be determined based on the detector signals measured
in a characteristic emission band of the second metal, from which
upper pads 54 are formed. Even small errors in registration of the
upper and lower pads, on the order of 1%, can be detected in this
manner, so that corrective action can be taken before the errors
become critical.
[0072] Furthermore, when the misregistration is relatively small,
the direction and magnitude of the misregistration offset can be
determined based on the difference between the fluorescence signals
received from opposing detectors in the emission band of the first
(lower) metal. Left-right misregistration (in the perspective of
FIG. 2) is thus given by the difference in signals between
detectors 30 and 31, while up-down misregistration is given by the
difference between detectors 33 and 35. The differences between the
signals are preferably normalized by a sum of the signals. Analyzer
20 thus provides feedback to stepper 42 (or to an operator of the
stepper) indicating not only that a registration error has
occurred, but also giving an estimate of the amount and direction
of adjustment that are needed in order to prevent the error from
occurring in subsequent wafers.
[0073] FIG. 6 is a schematic, sectional view of a detail of wafer
22 under test in analyzer 20, in accordance with another preferred
embodiment of the present invention. In this embodiment, metal
bumps 60 have been formed on an upper surface of the wafer. These
bumps, typically about 0.1 mm across, are used for soldering
microelectronic devices that are made from wafer 22 onto suitable
chip carriers, in place of wire bonding. Analyzer 20 is used to
ascertain that the composition and thickness of bumps 60 meet
design parameters.
[0074] When only a single X-ray detector is used to capture
fluorescent photons emitted from bumps 60, as in XRF microanalyzers
known in the art, the measurement of fluorescence intensity will be
prone to error due to inaccurate alignment of spot 28 with bump 60.
The error arises from variations that occur in XRF emission as a
function of the radiation angle relative to the surface of the
bump. This difficulty is overcome by differential measurement using
analyzer 20. Preferably, differences between the XRF signals
received from detectors 30, 31, 33 and 35 are measured in order to
determine how far off spot 28 is from the center of bump 60. When
the spot is perfectly centered, all of the detectors will give
substantially equal signals. The differences between the signals
are preferably provided as a negative feedback input to stage 36,
so as to drive the bump into alignment with the irradiation spot.
Alternatively or additionally, reliable XRF measurements can be
made even when the spot is not perfectly aligned on the bump, by
taking a sum or average of the signals received from the
detectors.
[0075] Although the preferred embodiments described herein deal
specifically with measurements made on certain specific features of
wafer 22, the principles of the present invention may be applied in
a straightforward manner to measurement of other microscopic
geometrical features on or near a surface of a semiconductor wafer.
Furthermore, these principles may similarly be applied to the
analysis of other types of microscopic features and structures, in
samples other than semiconductor wafers, and to the control of
production processes for making such features and structures. For
example, the principles described above with reference to FIG. 6
may be applied in other contexts in which X-ray emissions are
measured from a curved surface. The ratios of the signals received
by the opposing detectors are indicative of the slope of the
surface at the point of measurement. The location of the point of
measurement may be adjusted, based on the ratios, to find a point
of horizontal slope, as in the method of FIG. 6, or to find another
predefined slope.
[0076] It will thus be appreciated that the preferred embodiments
described above are cited by way of example, and that the present
invention is not limited to what has been particularly shown and
described hereinabove. Rather, the scope of the present invention
includes both combinations and subcombinations of the various
features described hereinabove, as well as variations and
modifications thereof which would occur to persons skilled in the
art upon reading the foregoing description and which are not
disclosed in the prior art.
* * * * *