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Description  |
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FIELD OF THE INVENTION
The present invention relates to automated inspection of electronic
devices, in particular to automated inspection of electrical
interconnections formed on integrated circuits and the like.
BACKGROUND OF THE INVENTION
Recent advances in circuit manufacturing technology have spawned increases
in circuit complexity while simultaneously allowing for production of
smaller and smaller circuits. For example, with the advances in "bump" or
"flip chip" technology, electrical interconnections, which had previously
been on the order of 0.020 inches between centers, are approaching 0.001
inch (25 microns) between centers. In addition, as the manufacture of
circuit boards becomes ever more complex due to increasing lead count,
decreasing lead pitch, and the switch to double-sided construction, it is
important that the manufacturing engineer obtain real-time data about the
circuit manufacturing process.
Presently, the best circuit board production lines produce assemblies with
a solder joint defect rate of about 10-50 parts-per-million (PPM). Thus,
as the number of solder connections on a circuit board increases, it
becomes increasingly important to reduce joint solder-defect levels
through process control feedback, and circuit board inspection. For
example, for a circuit board design having 100,000 solder connections, and
a joint defect level of 10 PPM, less than 40% of the circuit boards
produced will be defect free.
The trend in manufacturing is toward more joints per board. There are
boards being designed with over 150,000 connections on a single small
board. In addition, advanced techniques are being used to increase the
complexity of interconnections on integrated circuit (IC) chips as well.
Therefore, additional means must be employed to further reduce the process
defect level. Even with further process improvements, statistical process
control, and closed loop feed-back, many products will still require an
inspection system to reduce the remaining defect levels to acceptable
rates for product shipment.
It has become apparent that, with increased circuit complexity and
decreased circuit size, visual inspection by subjective human inspectors
has become inadequate. Consequently, the circuit manufacturing industry
has sought to develop an automated circuit inspection system which is
capable of meeting the needs of present circuit board manufacturers.
Inspection systems in the past have met with limited success for through
hole technology boards and single-sided surface mount technology (SMT)
boards. The most successful of these has been automated transmission
X-ray. For example, U.S. Pat. No. 4,809,308 by Adams, et al., discloses an
automated transmission X-ray device for performing circuit board solder
quality inspections. It has been found, however, that automated
transmission X-ray exhibits additional problems with double-sided SMT
boards due to the overlapping interference of the images produced by the
top side components with the images produced by components on the bottom
side of the circuit board. That is, when an X-ray beam penetrates through
two separate connections on both sides of a circuit board, the image
formed on a detector is a composite image of both connections. This could
present serious problems when attempting to analyze each connection
individually. Because of the shortcomings exhibited by past automated
inspection systems, a new automated circuit inspection technology was
needed.
Recent developments in scanned-beam laminography (SBL) have provided
improved resolution and accuracy in the inspection of electronic devices,
particularly for high component density and double-sided circuit boards.
By laminographically scanning an electrical connection, a cross-sectional
image of the electrical connection can be produced which significantly
reduces the overlapping image interference exhibited in transmission X-ray
inspection systems. The introduction of SBL for the automated inspection
of circuit boards has provided a means for analyzing individual
connections on high density and double-sided circuit boards. Automated SBL
typically provides for increased image resolution without requiring the
complex mechanical operations that are typical of many automated
inspection systems. Thus, automated SBL has proven to be superior for
inspection of high density circuitry, and for applications which require
the inspection of multiple layers within an object. Such an automated
laminographic inspection system which produces cross-sectional images of
electrical connections on a circuit board is described in U.S. Pat. No.
4,926,452, issued May 15, 1990.
Until now, SBL automated circuit inspection systems have been suitable for
uses such as the inspection of solder connections on circuit boards.
However, with the advent of integrated circuit designs wherein the entire
circuitry that would normally be placed on a circuit board is instead
deposited onto a silicon substrate, higher resolution inspection systems
than had been previously contemplated have become a priority. With
chip-level connections in the sub-micron range (i.e., 1.0 microns and
smaller), the current technique of circuit inspection is not very
practical. Thus, a need exists for a high-resolution, automated circuit
inspection system which is capable of inspecting interconnections and
circuitry (e.g., within an integrated circuit) using a resolution
sufficient to analyze connections in the sub-micron range.
SUMMARY OF THE INVENTION
The present invention is directed to an apparatus and a method for the
inspection of micro-features of electrical components such as an
integrated circuit chip. The invention produces high resolution
cross-sectional images of the micro-features which are then analyzed by an
image analysis computer system. The cross-sectional images of the
inspected micro-features are analyzed to locate defects, and, if defects
are found, to determine the type of defects.
In particular, the present invention is a high resolution laminography
system which comprises a source of electrons which generates an electron
beam, a deflector for steering the electron beam, and a target wherein
electromagnetic energy is emitted from the target when the electron beam
impinges upon the target, which includes an imagable feature. The high
resolution laminography system further comprises a laminographic detector
for producing a laminograph of an object illuminated by the
electromagnetic energy emitted by the target, and an SEM detector for
producing an SEM micrograph of the target imagable feature in response to
illumination of the feature by the electron beam.
In one embodiment, the system as defined in Claim 1 further comprises an
image analysis system for analyzing characteristics of the SEM micrograph
image of the feature, and providing an output signal in response to the
analysis, and a feedback system which receives the output signal from the
image analysis system, processes the output signal and provides a control
signal to the electron beam deflector. In a particularly preferred
embodiment, the feedback system comprises a digital Look-Up-Table.
In a further embodiment, the imagable feature on the target comprises four
points, wherein two of the points lie on a line which is perpendicular to
a line defined by two others of the points.
In another embodiment, the target comprises a plurality of concentric
rings, or in an alternative embodiment, the target has a cylindrical
interior surface.
In a further embodiment, the SEM detector comprises a channeltron imager.
The present invention may also include an electron collector for
preventing electrons from striking the object, and a piezoelectric
translation stage for vertically positioning the object.
In still a further embodiment, the detector comprises a fluorescent screen,
an optical derotation device and a camera. In a preferred form of this
embodiment, the fluorescent screen comprises Gadolinium Oxysulfide.
The high resolution laminography system may also comprise a means for
supporting an object within a vacuum chamber, a source of electromagnetic
energy which illuminates the object, and a laminographic detector for
producing a laminograph of the object when illuminated by the
electromagnetic energy.
In one embodiment the detector is situated within the vacuum chamber, and
is supported by magnetic bearings.
In a further embodiment, the laminography system of the present invention
comprises a pair of differential vacuum pumps.
The method of producing high resolution laminographs in accordance with the
present invention comprises the steps of generating an electron beam,
steering the electron beam with a deflector, striking a target with the
electron beam, wherein electromagnetic energy is emitted from the target
when the electron beam impinges upon the target, the target having an
imagable feature, producing a laminograph of an object illuminated by the
electromagnetic energy using a laminographic detector, and producing an
SEM micrograph of the target imagable feature in response to illumination
of the feature by the electron beam.
The method of producing high resolution laminographs may also comprise the
steps of analyzing characteristics of the SEM micrograph image of the
feature, and providing an output signal in response to the analysis, and
providing a feedback system which receives the output signal from the
image analysis system, processes the output signal and provides a control
signal to the electron beam deflector.
In a further embodiment, the analyzing step comprises the step of,
determining the location of the feature image within the micrograph,
calculating the distance between the determined location of the feature
image and the center of the micrograph, and producing a voltage signal as
a function of the calculated distance appropriate to cause the feature
image to be centered within the micrograph.
The method of producing high resolution laminographs in accordance with the
present invention may also comprise the steps of supporting an object
within a vacuum chamber, illuminating the object using a source of
electromagnetic energy, and producing a laminograph of the object
illuminated by the electromagnetic energy using a laminographic detector.
In one embodiment, the method may also comprise the step of situating the
detector within the vacuum chamber.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a simplified schematic diagram which shows the main components of
a laminographic system and their approximate geometric relationship to one
another.
FIGS. 2a and 2b are simplified schematic diagrams which depict the
laminographic geometry used in the present invention, and illustrate the
effect of shifting the relative location of the path traced by the X-ray
source.
FIGS. 3a and 3b are simplified schematic diagrams which depict the
laminographic geometry used in the present invention, and illustrate the
effect of varying the radius of the path traced by the X-ray source.
FIGS. 4a-4e illustrate the manner in which images of features in different
planes within an object can be imaged using a laminography system.
FIG. 5 is a simplified schematic diagram of the high resolution
laminography system of the present invention.
FIG. 6a is a perspective view showing the general visual appearance of a
typical integrated circuit chip.
FIG. 6b is a side cross-sectional view showing the different trace layers
within an integrated circuit.
FIG. 6c is a plan view which shows the traces and electrical
interconnections typically found within the different layers of an
integrated circuit.
FIG. 7 is a schematic diagram illustrating the calibration procedure for
synchronizing the X-ray source and detector positions.
FIG. 8 is a schematic block diagram of the feedback control system used for
synchronization of the X-ray source and detector motions.
FIG. 9 shows an X-ray image of an exemplary fiducial trace pattern within
an integrated circuit.
FIGS. 10a and 10b are a flowchart which illustrate the method used to
calibrate the source with the detector.
FIG. 11 is a block diagram which shows the major elements of the computer
control and analysis system.
FIG. 12 is a flowchart that illustrates the method used to inspect the
different regions of a specimen under examination.
FIG. 13 is a diagram of the timing cycle for the coordinated motion of the
source and detector, and image acquisition by the camera and computer
system.
FIG. 14 is a simplified schematic diagram of an alternative embodiment of
the high resolution laminography system shown in FIG. 5.
FIGS. 15a and 15b, shows a cross-sectional view of an alternative target
formed as a hollow cylinder.
FIG. 16 shows a cross-sectional view of another embodiment of the target
having concentric rings.
FIG. 17 depicts a possible calibration pattern which may be embossed onto
the target.
FIG. 18 is a circuit block diagram which shows the basic functions
performed by a synchronization Look-Up-Table and drift compensation
circuitry used in accordance with the present invention.
FIGS. 19a and 19b depict a flowchart which details the overall method of
obtaining a high resolution laminographic image employed in accordance
with the present invention.
DETAILED DESCRIPTION OF THE INVENTION
As used throughout, the term "radiation" refers to electromagnetic
radiation, including but not limited to the X-ray, gamma and ultraviolet
portions of the electromagnetic radiation spectrum.
FIG. 1 shows a schematic representation of the laminographic geometry used
in the present invention. An object 10 under examination, for example, an
integrated circuit, is held in a stationary position with respect to a
source of X-rays 20 and an X-ray detector 30. Synchronous rotation of the
X-ray source 20 and detector 30 about an axis 40 causes an X-ray image of
a plane 60 within the object 10 to be formed on the detector 30. In the
embodiment shown in FIG. 1, the axis 40 is the common axis of rotation for
both the X-ray source 20 and the detector 30, however, it should be noted
that the rotation of the X-ray source 20 need not occur about the same
axis as the rotation of the detector 30. In practice it is sufficient that
the planes 62 and 64, defined by the rotation of the source 20 and
detector 30 respectively, are parallel to one another.
The image plane 60 is substantially parallel to the planes 62 and 64. As
the X-ray source 20 and the detector 30 rotate in synchronization, a
family of cones is defined around the circular path traced by the source
20 and the detector 30. Each cone has an apex defined by the X-ray source
20, and a base defined by the circular detector 30. The set of points
defined by the intersection of the entire family of cones around a
complete rotation of the source and detector constitutes the imaged
region, or field of view, of the focal plane 60. Thus, an in-focus,
cross-sectional X-ray image of the portion of the object 10 within the
field of view at the imaged region of the focal plane 60 is formed on the
detector 30 as the source and detector synchronously rotate about an
intersection point 70. Structures within the object 10 which lie outside
of plane 60 form a blurred X-ray image on the detector 30.
FIGS. 2a and 2b depict source and detector configurations that image
different regions, i.e., fields of view, within the same focal plane of an
object 80. In FIG. 2a, the source 20 is shown to rotate about a center
point in a circular path, A. In FIG. 2b, the source 20 is shown to rotate
in a second circular path, B, about another center point which is linearly
shifted from the center point of the path A. The object 80 shown in FIGS.
2a and 2b has test patterns in the shape of an arrow 81, and a cross 82
embedded within the object 80. In FIG. 2a, cones 90 and 92, defined by the
X-ray source 20 and detector 30 at two different locations along their
path of rotation, are shown to intersect in an image plane 93 at
substantially the same location as the arrow 81, so that as the source and
detector rotate in synchronization, an image of the arrow 81 is reinforced
on the detector 30. Thus, the configuration shown in FIG. 2a produces a
cross-sectional image of the arrow 81 on the detector 30. In FIG. 2b, a
different circular path that is horizontally displaced from the path
traced in FIG. 2a is followed by the X-ray source 20. In this case, cones
94 and 96, defined by the X-ray source 20 and detector 30 at two different
locations along their path of rotation, are shown to intersect in an image
plane 97 at substantially the same location as the cross 82. Thus, as the
source and detector rotate in synchronization, an image of the cross 82 is
reinforced on the detector 30, so that the configuration shown in FIG. 2a
produces a cross-sectional image of the cross 82 on the detector 30.
FIGS. 3a and 3b depict source and detector configurations that image
regions within different planes of an object 100. The object 100 also has
test patterns in the shape of the arrow 81, and the cross 82 embedded
within the object 100. In FIG. 3a, cones 102 and 104, defined by the X-ray
source 20 and detector 30 at two different locations along their path of
rotation, are shown to intersect in an image plane 105 at substantially
the same location as the arrow 81, so that as the source and detector
rotate in synchronization, an image of the arrow 81 is reinforced on the
detector 30 to produce a cross-sectional image of the arrow 81 on the
detector 30. In FIG. 3b, a different circular path, having a smaller
radius than the path shown in FIG. 3a, is followed by the X-ray source 20.
Thus, cones 108 and 110, defined by the X-ray source 20 and detector 30 at
two different locations along their path of rotation, are shown to
intersect in an image plane 106 at substantially the same location as the
cross 82, so that the configuration shown in FIG. 2a produces a
cross-sectional image of the cross 82 on the detector 30.
Thus, FIGS. 2a, 2b, 3a and 3b illustrate how different regions of a
specimen under inspection can be imaged onto the detector 30 by
manipulating the path traced by the X-ray source 20. For example, this may
be done by electrostatically deflecting an electron beam which produces
X-rays when it strikes various locations on a target. By deflecting an
electron beam, electrons may be caused to strike different regions of the
target in a desired pattern, thereby causing the X-ray source 20 to trace
a desired path on the target.
FIGS. 4a-4e show laminographs produced by the above described laminographic
technique. The object 10 shown in FIG. 4a has test patterns in the shape
of the arrow 81, the cross 82, and a circle 83 embedded within the object
10 in three different planes 60a, 60b and 60c, respectively.
FIG. 4b shows a typical laminograph of object 10 formed on detector 30 when
the point of intersection 70 lies in plane 60a of FIG. 4a. The image 120
of arrow 81 is in sharp focus, while the images of other features within
the object 10, such as the circle 83 and cross 82 form a blurred region
122 which does not greatly obscure the arrow image 120.
Similarly, when the point of intersection 70 lies in plane 60b, the image
130 of the circle 83 is in sharp focus as seen in FIG. 4d. The arrow 81
and cross 82 form a blurred region 132.
FIG. 4c shows a sharp image 140 formed of the cross 82 when the point of
intersection 70 lies in plane 60c. The arrow 81 and circle 83 form blurred
region 142.
For comparison, FIG. 4e shows an X-ray shadow image of object 10 formed by
conventional projection radiography techniques. This technique produces
sharp images 150, 152 and 154 of the arrow 81, circle 83 and cross 82,
respectively, which overlap one another. FIG. 4e vividly illustrates how
multiple characteristics contained within the object 10 may create
multiple overshadowing features in the X-ray image which obscure
individual features of the image.
FIG. 5 illustrates a schematic diagram of one embodiment of the high
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