U.S. patent application number 15/076556 was filed with the patent office on 2016-07-14 for integrated devices with photoemissive structures.
The applicant listed for this patent is David Lewis Adler. Invention is credited to David Lewis Adler.
Application Number | 20160203938 15/076556 |
Document ID | / |
Family ID | 50025475 |
Filed Date | 2016-07-14 |
United States Patent
Application |
20160203938 |
Kind Code |
A1 |
Adler; David Lewis |
July 14, 2016 |
INTEGRATED DEVICES WITH PHOTOEMISSIVE STRUCTURES
Abstract
An apparatus is disclosed for the examination and inspection of
integrated devices such as integrated circuits. X-rays are
transmitted through the integrated device, and are incident on a
photoemissive structure that absorbs x-rays and emits electrons.
The electrons emitted by the photoemissive structure are shaped by
an electron optical system to form a magnified image of the emitted
electrons on a detector. This magnified image is then recorded and
processed. For some embodiments of the invention, the photoemissive
structure is deposited directly onto the integrated device. In some
embodiments, the incidence angle of the x-rays is varied to allow
internal three-dimensional structures of the integrated device to
be determined. In other embodiments, the recorded image is compared
with a reference data to enable inspection for manufacturing
quality control.
Inventors: |
Adler; David Lewis; (San
Jose, CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Adler; David Lewis |
San Jose |
CA |
US |
|
|
Family ID: |
50025475 |
Appl. No.: |
15/076556 |
Filed: |
March 21, 2016 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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13507895 |
Aug 3, 2012 |
9291578 |
|
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15076556 |
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Current U.S.
Class: |
257/48 ;
438/22 |
Current CPC
Class: |
G21K 1/00 20130101; G01N
23/044 20180201; G21K 7/00 20130101; H01J 1/78 20130101; G01N
2223/611 20130101; H01L 27/0203 20130101; G01N 23/04 20130101; H01L
23/49827 20130101; G01N 2223/1006 20130101 |
International
Class: |
H01J 1/78 20060101
H01J001/78; H01L 27/02 20060101 H01L027/02; H01L 23/498 20060101
H01L023/498; G21K 7/00 20060101 G21K007/00 |
Claims
1. An integrated device, on which a photoemissive structure has
been manufactured.
2. The device of claim 1, in which the photoemissive structure
comprises material containing gold.
3. The device of claim 2, in which the photoemissive structure
comprises a layer of gold having a thickness between 5 nm and 100
nm.
4. The device of claim 1, in which the photoemissive structure
comprises material containing cesium iodide (CsI).
5. The device of claim 4, in which the material containing cesium
iodide (CsI) comprises a layer of cesium iodide having a thickness
between 3 nm and 200 nm.
6. The device of claim 1, in which a planarization layer is
manufactured between the integrated device and the photoemissive
structure.
7. The device of claim 1, in which the integrated device is an
integrated circuit.
8. The device of claim 1, in which the integrated device is a
silicon interposer with through-silicon-vias.
9. An object comprising an integrated circuit, additionally
comprising: a photoemissive structure that has been deposited onto
the object comprising the integrated circuit, in which the
photoemissive structure comprises: a photoemissive layer and an
emissive coating.
10. The object of claim 9, in which the thickness of the
photoemissive layer is 100 nm or less; and the thickness of the
emissive coating is 100 nm or less.
11. The object of claim 9, in which the photoemissive layer is a
layer comprising gold, and the emissive coating comprises a layer
of cesium iodide (CsI) deposited onto the photoemissive layer.
12. The object of claim 9, in which the object additionally
comprises a planarization layer placed between the integrated
device and the photoemissive structure
13. The object of claim 9, in which the integrated circuit has been
fabricated on a silicon substrate, and the silicon substrate has
been thinned to have a thickness of 50 microns or less.
14. The object of claim 13, in which the photoemissive structure is
deposited onto the object after the silicon substrate has been
thinned.
15. A method of preparing an object comprising an integrated
circuit for examination in an x-ray photoemission microscope,
comprising: depositing a photoemissive structure onto the object
comprising the integrated circuit, in which the photoemissive
structure comprises: a photoemissive layer and an emissive
coating.
16. The method of claim 15, in which the thickness of the
photoemissive layer is 100 nm or less; and the thickness of the
emissive coating is 100 nm or less.
17. The method of claim 15, in which the deposition of the
photoemissive structure comprises: depositing a layer of material
comprising gold to serve as the photoemissive layer; and depositing
a layer of material comprising cesium iodide (CsI) onto the layer
of material comprising gold to serve as the emissive coating.
18. The method of claim 15, additionally comprising the deposition
of a planarization layer placed between the integrated device and
the photoemissive structure.
19. The method of claim 16, in which the integrated circuit has
been fabricated on a silicon substrate, and in which said method
additionally comprises the step of: thinning the silicon substrate
to have a thickness of 50 microns or less.
20. The method of claim 19, in which the photoemissive structure is
deposited onto the object after the silicon substrate has been
thinned.
Description
RELATED INVENTIONS
[0001] The present application is a Divisional of U.S. patent
application Ser. No. 13/507,895, filed Aug. 3, 2012 and entitled
X-RAY PHOTOEMISSION MICROSCOPE FOR INTEGRATED DEVICES, which is
herein incorporated by reference in its entirety.
FIELD OF THE INVENTION
[0002] This invention relates to the examination of integrated
devices, such as integrated circuits, by transmitting x-rays
through the device and magnifying the resulting images; and in
particular, to the use of a hybrid system which converts the
transmitted x-rays to electron-beams, which are then magnified
using electron optics for the resolution of physical structures
much smaller than 100 nm in size. The particular embodiments
disclosed here allow for the observation of the device at multiple
angles to determine the two-dimensional and three-dimensional
structures within the device without physically damaging the
device, and, when paired with a reference image or a reference
database, can also be used as an inspection system for devices of
unknown quality.
BACKGROUND OF THE INVENTION
[0003] The initial discovery of x-rays by Rontgen in 1895 [W. C.
Rontgen, "Eine Neue Art von Strahlen" (Wurzburg: Verlag und Druck
der Stahel'schen K. Hof- und Universitats-Buch- und Kunsthandlung,
Wurzburg, Germany, 1896); "On a New Kind of Rays," Nature, Vol. 53,
pp. 274-276 (Jan. 23 1896)] was in the form of shadowgraphs, in
which the contrast of x-ray transmission for biological samples
(e.g. bones vs. tissue) allowed internal structures to be revealed
without damaging the samples themselves. However, because of their
short wavelength (10 to 0.01 nm, corresponding to energies in the
range of 100-100,000 eV), and the absence of materials for which
the refractive index for x-rays differs significantly from 1, there
are no easy equivalents to refractive or reflective optical
elements so commonly used in optical system design. So, even now,
the most common use of x-rays is still as a simple shadowgraph,
observing the structure of bones and teeth in the offices of
doctors and dentists.
[0004] Early x-ray "microscopy," developed more than 50 years after
the initial discovery of x-rays, simply consisted of elaborate
shadowgraph apparatus, in which the diverging x-rays cast a shadow
larger than the object [S. P. Newberry and S. E. Summers, U.S. Pat.
No. 2,814,729]. With the advent of computer data collection, it
became possible to gather more information from the specimen,
changing the relative positions and illumination angles of the
x-ray source and specimen in a systematic way. Using multiple
transmission measurements taken at multiple angles around the
specimen, images can be synthesized by computer that represent a
2-dimensional or 3-dimensional model of the specimen [G. N.
Hounsfield, U.S. Pat. No. 3,778,614]. The "slices" of interior
bodies so revealed are amazing to look at, revealing a great deal
about the internal structures without invasive surgery. However, as
far as the physics of the x-ray interaction with the specimen,
these tomographic reconstructions represent nothing more than an
elaborate map of x-ray absorption--a sophisticated shadowgraph.
[0005] Over time, other imaging tools for x-ray optical systems
were invented. Apparatus using grazing incidence reflection off of
surfaces provided cone reflectors [C. G. Wang, U.S. Pat. No.
4,317,036] and capillary collimators [F. Kumasaka et al., U.S. Pat.
No. 5,276,724] to allow a diverging x-ray beam to be manipulated
into a collimated beam or to concentrate x-rays onto a
specimen.
[0006] With the development of high-resolution patterning with
electron-beam lithography in the 1970's, Fresnel zone plates, which
use diffractive properties to effectively focus an electromagnetic
wave, could now be manufactured at the small dimensions suitable
for use with short x-ray wavelengths. [J. Kirz, "Phase zone plates
for x rays and the extreme uv", Journal of the Optical Society of
America, Vol. 64(3), pp. 301-309 (March 1974)]. Zone plates can be
used both to shape and focus the illuminating optics and also to
collect and focus the transmitted x-rays onto a detector [G.
Schmahl and D. Rudolph, "X-Ray Microscopy" pp. 192-202, (Springer
Verlag, Berlin, 1984); and U.S. Pat. No. 4,870,674]. Variations
using phase-contrast rings [G. Schmal [sic] and D. Rudolph, U.S.
Pat. No. 5,550,887] have been developed, and are now commonly used
in contemporary x-ray microscopes.
[0007] Unfortunately, what a zone plate microscope design may have
in resolution may not be matched in imaging speed. The diffractive
properties of the zone plate are tuned to a specific wavelength,
meaning that most of the energy in a broad-band x-ray source is
discarded. Synchrotron sources may increase brightness for a
particular wavelength, but are not suitable for portable systems
and, at the selected wavelength, the best diffraction efficiency
that can be achieved is still under 35%.
[0008] Because of this, the microscopy of specimens requiring high
speed and high resolution use electron microscopy instead, either
as scanning electron microscopes (SEMs) or transmission electron
microscopes (TEMs). Being charged particles, electrons can be
easily controlled and focused using electric and magnetic fields,
and the science and technology of electron optics is a
well-developed and established field. [L. Reimer, "Electron
Optics", Section 2 of Ch. 2 of "Scanning Electron Microscopy:
Physics of Image Formation and Microanalysis, 2.sup.nd Edition",
(Springer Verlag, Heidelberg, 1998)].
[0009] Electron beams require that the sample and the beam path
must all be in a vacuum. Since any sample would lose all its water
in the desiccating environment of a vacuum chamber, this does not
represent a way of observing most biological samples in their
"natural" condition. Also, depending on their energy, electrons
tend to be absorbed with the first few nanometers of a sample,
making them extremely useful for the observation of surfaces, but
not so useful for the observation of internal structures. Samples
must be thinned to be less than 100 nm thick, and often only a few
tens of nm thick, before they can be used in a TEM.
[0010] In an attempt to combine the penetrating power of x-rays
with the control and resolution possible with electron-beams, a
hybrid of x-ray microscopy and photoemissive electron microscopy,
or PEEM, has been developed [O. H. Griffith and W. Engel,
"Historical perspective and current trends in emission microscopy,
mirror electron microscopy and low-energy electron microscopy,"
Ultramicroscopy, Vol. 36, p. 1 (1991)]. Although PEEM is usually a
technique in which a surface is excited from the front and
photoelectrons also emitted from the same front surface, a
photocathode mounted on a sufficiently thin membrane can allow
excitation from the back side through a membrane [H. Hirose, U.S.
Pat. No. 5,045,696].
[0011] FIG. 1 illustrates a prior art hybrid x-ray/PEEM system as
disclosed by F. Cerrina and T. B. Lucatorto on Drawing Sheet 2 of
U.S. Pat. No. 6,002,740. In this system, described as being a
system to inspect masks for x-ray lithography, the mask 22 is
placed between a source of x-rays 30 and converter 18 comprising a
photo-emitting cathode 16 mounted on a membrane 19. When the
converter 18 is illuminated through the membrane 19 by x-rays, it
emits electrons 32 whose intensity is "directly proportional to the
local intensity of the x-rays impinging thereon."
[0012] The electrons 32 emitted from the converter 18 are then
highly magnified by a set of electron optics in the electron
microscope 17. The electron microscope 17 forms an image of the
mask pattern that may be fed to the computer system 20 for analysis
and display.
[0013] The Cerrina disclosure describes a hybrid x-ray/PEEM
inspection system for x-ray lithography masks, in which the system
emulates an x-ray lithography system. [H. Smith and M.
Schattenberg, "X-ray lithography from 500 to 30 nm," IBM Journal of
Research and Development, Vol. 37(3), p. 319 (1993)]. The
configuration described requires placing the photoemitting cathode
relative to the mask in the same position that a photoresist-coated
wafer would be placed in an x-ray lithography system, allowing the
image to mimic what the mask would print. In such a lithography
system, both the mask and the wafer are placed in a vacuum in close
proximity for proximity printing, with a distance of less than 25
microns separating them to minimize distortions, [A. D. Dubner et
al., "Diffraction effects in x-ray proximity printing," Journal of
Vacuum Science and Technology B, Vol. 10(5), pp. 2234-2242 (1992)]
but not in direct contact to avoid damaging the mask or wafer.
[0014] Such hybrid systems were proposed but never applied to x-ray
lithographic mask inspection because x-ray lithography did not
achieve any widespread commercial adoption. Such systems have been
built and demonstrated for various biological and mineral samples.
[R. N. Watts et al., "A transmission x-ray microscope based on
secondary-electron imaging," Review of Scientific Instruments, Vol.
68, p 3464 (1997); G. De Stasio et al., "Soft-x-ray transmission
photoelectron spectromicroscopy with the MEPHISTO system," Review
of Scientific Instruments, Vol. 69, p. 3106 (1998), and "MEPHISTO
spectromicroscope reaches 20 nm lateral resolution," Review of
Scientific Instruments, Vol. 70, p. 1740 (1999); Y. Hwu et al.,
"Using photoelectron microscopy with hard x-rays," Surface Science,
Vol. 480, pp. 188-195 (2001)]. However, many biological structures
are well observed by variations of conventional optical and x-ray
tomographic tools, making the complexity of these hybrid systems
unnecessary for many biological applications.
[0015] But, for one particular class of specimens, variations on
this hybrid technique may be perfectly suited, and are the subject
of the invention disclosed here.
[0016] One problem that has recently emerged is the need to examine
products containing integrated devices, such as integrated circuits
(ICs), to verify that the devices have been manufactured as
specified. This is especially important when the security and
integrity of the devices may be an issue, in which is it necessary
to insure that additional circuitry (e.g. RF antennas to relay
signals from unauthorized sources) have not been inserted during
the manufacturing process. When all circuit structures are encased
within a single package, verification of the actual contents of the
circuit is difficult.
[0017] Current examination techniques for these circuit packages
require destructive testing, taking the circuit package and
removing material layer by layer, photographing and analyzing the
circuit patterns of each layer as they are exposed with either an
optical microscope, or with an electron microscope for smaller
structures. This can be very tedious and time consuming. With the
components of the most modern ICs quickly approaching 20 nm in
size, and potentially becoming as small as 5 nm in future
generations, there is a real need for an imaging technique which
has the resolution to identify these small features and also the
speed to observe multiple layers of devices and interconnects over
a 1 cm by 1 cm area in a manageable amount of time.
[0018] An approach using the transmissive power of x-rays to
examine the internal contents of a circuit will not require the
destruction of the circuit itself, and has the potential to provide
both the resolution needed and the speed required.
[0019] Systems using an x-ray microscope for the inspection of
integrated circuits have been disclosed by the Xradia Corporation
[W. Yun and Y. Wang, U.S. Pat. No. 7,119,953; Y. Wang et al., U.S.
Pat. No. 7,394,890; M. Bajura et al., U.S. Pat. No. 8,139,846;
www.xradia.com]. FIG. 2 illustrates a prior art x-ray microscope
system as disclosed on Drawing Sheet 2 of U.S. Pat. No. 7,119,953.
In such a system, x-rays from a source 1110 are collected by a
condenser 1120, which relays x-rays from the source 1110 to the
test object 1010. This condenser 1120 is described in some
embodiments as a capillary condenser with a suitably configured
reflecting surface, while in others as a zone plate. The converging
beam from the condenser 1120 irradiates the test object 1010, and
the radiation emerging from the test object 1010 is scattered and
diffracted out of the path of the direct radiation beam. An
objective 1118 is therefore used to form an image of the object,
collecting the scattered x-rays. This objective 1118 is described
as being possibly a zone plate lens, a Wolter optic, or a Fresnel
optic. In some embodiments, an additional phase plate 1116, often
in the form of a ring around the center axis of the system, is
included to enhance contrast. Both the phase plate 1116 and the
objective 1118 are described as being attached to a
"high-transmissive substrate" 1140 to form a composite optic 1138.
The image of the test object 1010 is formed on a detector 1125,
which is described as possibly comprising in some embodiments a
charged coupled device (CCD), and in some embodiments comprising a
scintillator, and in others being a film-based detector.
[0020] X-ray systems with Fresnel zone plate (FZP) optics such as
this prior art Xradia system can be effective for the
non-destructive examination of integrated circuits, but the
limitations of the zone plate optics [J. Kirz and D. Attwood, "Zone
Plates", Sec. 4.4 of the "X-ray Data Booklet"
(xdb.lbl.gov/Section4/Sec_4-4.html)] reduce the wavelength range
over which x-rays can be effectively collected, and increase the
time to collect data for a complete IC. The system is therefore
very slow and inefficient for collecting large volumes of data on
multiple layers of an IC.
[0021] There is therefore a need for a system that can combine the
penetrating power of x-rays with the easy control possible in
electron imaging, and in particular for the application to the
microscopy of sub-100 nm structures in integrated circuits to allow
rapid, non-destructive testing and inspection of those integrated
circuits.
BRIEF SUMMARY OF THE INVENTION
[0022] The invention disclosed with this application is an
apparatus for the examination and inspection of an integrated
device such as an integrated circuit. In this invention, a
photoemissive structure placed in a vacuum chamber converts
incident x-rays, which have been transmitted through the integrated
device, into the emission of electrons, and the electrons emitted
by the converter are shaped by an electron optical system to form a
magnified image of the emitted electrons on a detector.
[0023] In preferred embodiments of the invention, the x-ray
intensity pattern incident on the photoemissive structure will have
a profile representing the attenuation of x-rays in the integrated
device under examination, and the materials of the photoemissive
structure will be selected so that the number of electrons emitted
are proportional to the intensity of the incident x-rays.
[0024] The magnified image produced by the detector can then be
recorded and processed. In some embodiments, the image is compared
with a corresponding image of a device known to be correct. In
another embodiment, the image is compared to a database
representation of the structures in the circuit.
[0025] In yet another embodiment, the integrated device under
examination is mounted on a stage and the incident x-rays are moved
through a series of angles and positions relative to the integrated
device, and a set of corresponding transmission images recorded.
These images can then be assembled using computed laminography
algorithms with a digital computer to create a 2-D or 3-D
representation of the specimen. This synthesized representation can
then be compared to a reference image or database, allowing the
embodiment to be used as an inspection system.
[0026] In some embodiments of the invention, the integrated device
under examination is mounted outside the vacuum chamber containing
the photoemissive structure and the electron optics.
[0027] In other embodiments of the invention, the photoemissive
structure is coated directly onto the window of the vacuum system
that contains the electron optics, reducing the distance between
the specimen and the photoemissive structure.
[0028] In other embodiments of the invention, the specimen to be
examined is directly coated with the photoemissive layer, and
mounted within the vacuum system containing the electron optics,
and is illuminated by x-rays through a suitably transparent window
in the wall of the vacuum chamber.
BRIEF DESCRIPTION OF THE DRAWINGS
[0029] FIG. 1 illustrates a prior art hybrid x-ray/PEEM system as
disclosed in U.S. Pat. No. 6,002,740.
[0030] FIG. 2 illustrates a prior art x-ray microscope system from
the Xradia Corporation as disclosed in U.S. Pat. No. 7,119,953.
[0031] FIG. 3 illustrates a cross-section view of a microscope
system according to one embodiment of the present invention, in
which the integrated device is mounted outside the vacuum chamber
and the photoemissive structure is mounted within the vacuum
chamber.
[0032] FIG. 4 illustrates a detailed cross-section view of the
integrated device and photoemissive structure for the embodiment
illustrated in FIG. 3.
[0033] FIG. 5 illustrates a typical copper x-ray absorption
spectrum.
[0034] FIG. 6 illustrates a detailed cross-section view of the
electron image converter and imaging system for the embodiment
illustrated in FIG. 3.
[0035] FIG. 7 illustrates a schematic of the control systems to be
used for the embodiment illustrated in FIG. 3.
[0036] FIG. 8 illustrates a cross-section view of a microscope
system according to a second embodiment of the invention, in which
the integrated device is outside the vacuum chamber and the
photoemissive structure is coated onto the window of the vacuum
chamber.
[0037] FIG. 9 illustrates a detailed cross-section view of the
integrated device and photoemissive structure for the embodiment
illustrated in FIG. 8.
[0038] FIG. 10 illustrates a cross-section view of a microscope
system according to a third embodiment of the invention, in which
the integrated device is inside the vacuum chamber and the
photoemissive structure is coated onto the integrated device.
[0039] FIG. 11 illustrates a detailed cross-section view of the
integrated device and photoemissive structure for the embodiment
illustrated in FIG. 10.
[0040] FIG. 12 illustrates a detailed cross-section view of the
integrated device and photoemissive structure for a variation of
the embodiment illustrated in FIG. 10 in which the angle of
incidence is variable.
[0041] FIG. 13 illustrates a detailed cross-section view of the
integrated device and photoemissive structure for a variation of
the embodiment illustrated in FIG. 10.
[0042] FIG. 14 illustrates a detailed cross-section view of the
integrated device and photoemissive structure for a fourth
embodiment of the invention.
[0043] Note: The elements in the drawings illustrate the elements
of the invention and their general relationships, but should not be
interpreted as scale drawings. For example, in FIG. 3, the entrance
window 125 in the vacuum chamber 120 may be only a few mm in
diameter or smaller, while the entire vacuum chamber 120 may be as
long as a meter, but these elements have not been shown at these
relative dimensions here. Likewise, in FIG. 4, in an integrated
device 160, the silicon substrate 162 may be 500 microns thick,
whereas the layer comprising integrated structures 164 may be only
10-20 microns thick. The illustrations do show the general
relationships sufficiently so that one skilled in the art would be
able to reproduce the invention accordingly.
[0044] Note: The cross-section views have been selected to
represent elements in a plane in which the x-rays and emitted
electrons are traveling. Some of the elements also presented in the
cross-section views, and in particular the stage controls 132 and
332 inside the vacuum chamber 120 as well as the external stage
controls 232 would typically be, at least in part, below or above
the plane of the illustrated cross-section, especially for the
region through which the x-rays or emitted electrons are traveling,
so that these mechanical elements will not block the x-rays.
However, the illustrations and descriptions in the specification
present the general relationships sufficiently so that one skilled
in the art would be able to reproduce the invention
accordingly.
DETAILED DESCRIPTIONS OF EMBODIMENTS OF THE INVENTION
First Embodiment of the Invention
[0045] One embodiment of the apparatus according to the invention
for the examination of an integrated device 160 is illustrated in
FIG. 3 through FIG. 7. FIG. 3 illustrates a cross section view of
the overall system. FIG. 4 illustrates a cross-section in detail of
the integrated device 160 and photoemissive structure 170. FIG. 5
illustrates the x-ray energy absorption for copper, used in many
integrated devices. FIG. 6 illustrates a cross section in detail of
the image converter 180 and the imaging system 190. FIG. 7
illustrates a schematic of the various elements connected to the
system controller 110.
[0046] Turning first to FIG. 3, the apparatus comprises a source of
x-rays 100 and also comprises beam-shaping optics 104 which provide
a beam of x-rays 111, directed onto the integrated device 160, with
a predetermined energy (i.e. wavelength) spectrum and a
predetermined angular distribution. The energy spectrum can be a
broad emission spectrum, or filtered to have a specific set of
wavelengths, or can comprise some other combination of wavelengths.
Likewise, the angular distribution can be a diverging beam, a
converging beam, or a collimated beam.
[0047] The source of x-rays 100 can be any x-ray source, including
a synchrotron, a fixed target x-ray tube, a rotating anode source,
a laser plasma source, or other sources that will be well known to
those skilled in the art. The source of x-rays 100 can be operated
to continuously emit x-rays, or be operated in a pulsed mode. The
beam shaping optics 104 can comprise any of a number of x-ray beam
shaping tools, including capillary collimators, grazing incidence
reflecting cones, and zone plates. However, a beam of x-rays for
this system would generally have a numerical aperture of
approximately nine milliradians (9 mrad).
[0048] The system further comprises a vacuum chamber 120 and a
means of establishing a vacuum 121 within the vacuum chamber 120,
such as a vacuum pump. The means of establishing a vacuum 121 may
use a valve 122 for vacuum control, which in some embodiments of
the invention can allow the vacuum chamber to be detached from the
means of creating a vacuum 121 once the vacuum is established.
Additionally, the wall of the vacuum chamber 120 may comprise
junctions 123 to preserve vacuum at any mechanical or electrical
access points in the system, and vacuum seals 124 at other access
points such as windows.
[0049] In this embodiment of the invention, on the side of the
system exposed to the beam of x-rays 111, the integrated device 160
is mounted outside the vacuum chamber 120, using an external stage
230. The external stage 230 may be attached to the vacuum chamber
120 or, in some embodiments, may be independently supported and not
in physical contact with the vacuum chamber. In some embodiments of
the invention, this external stage 230 will be fixed in place, and
in others the external stage 230 may have external stage controls
232 to adjust position and orientation. Adjustments enabled for the
external stage 230 through the controls 232 may include motion in
the x-y plane perpendicular to the axis of propagation of the
x-rays; it may also include rotation about the x- or y-axis, and it
may also include translation in the z-axis along the axis of
propagation of the x-rays, and may also include rotation around the
z-axis. It may also include rotation and/or translation about any
axis or axes.
[0050] Adjustment of the external stage controls 232 can be
governed by an external stage controller 234, which is generally a
system of electronics also outside the vacuum chamber 120. In some
embodiments, this external stage controller 234 can in turn be
controlled by the overall system controller 110, which can
designate an organized scan of positions and orientation angles of
the external stage 230 to facilitate the examination of the entire
integrated device 160 at multiple exposure angles. These controls
may make it possible for a relatively small beam of x-rays to be
used to examine the entire area of an integrated device 160 much
larger than the diameter of the x-ray beam 111.
[0051] After passing through the integrated device 160, the
intensity of the beam of x-rays 111 will be modified by the
absorption or scattering of x-rays within the integrated device
160. The modified x-ray beam 211 enters the vacuum chamber 120
through an entrance window 125, made from a material selected to be
relatively transparent to x-rays, such as beryllium or diamond. It
is desired that this be a uniform material, so that the intensity
profile of the internal modified beam of x-rays 311 (i.e. inside
the vacuum chamber 120) is proportional and nearly identical to
that of the external modified beam of x-rays 211.
[0052] Once inside the vacuum chamber 120, the internal modified
beam of x-rays 311 will encounter a photoemissive structure 170.
Some x-rays will be absorbed by the photoemissive structure 170,
while the remaining unabsorbed beam of x-rays 511 exits the
photoemissive structure 170 and proceeds on into the vacuum chamber
120. In some embodiments of the invention, the unabsorbed beam of
x-rays 511 will eventually be absorbed by a beam dump 108 elsewhere
in the vacuum system.
[0053] The absorption of x-rays in this structure stimulates the
emission of electrons 179 from the photoemissive structure 170.
However, since the elements of the photoemissive structure 170 are
often thin films, in some embodiments of the invention a support
structure 270 may also be used provide additional mechanical
support for the photoemissive structure 170. Care should be taken
in the selection of materials for the support structure 270 so that
significant distortions to the internal modified beam of x-rays 311
are not introduced.
[0054] The support structure 270 and photoemissive structure 170
can be supported and adjusted using a stage 130 within the vacuum
chamber 120. This stage 130 may be attached to the vacuum chamber
120, as illustrated in FIG. 3. In some embodiments of the
invention, this stage 130 will be fixed in place, and in others the
stage 130 may have stage controls 132 to adjust position and
orientation. Adjustments enabled for the stage 130 may include
motion in the x-y plane perpendicular to the axis of propagation of
the x-rays; it may also include rotation about the x- or y-axis,
and it may also include translation in the z-axis along the axis of
propagation of the x-rays, and may also include rotation around the
z-axis.
[0055] Adjustment of the stage controls 132 can be governed by a
stage controller 134, which is generally a system of electronics
also outside the vacuum chamber 120. In some embodiments, this
stage controller 134 can in turn be controlled by the overall
system controller 110, which can designate an organized scan of
positions and angles of the stage 130 to improve the signal from
the emitted electrons 179.
[0056] FIG. 4 shows a detailed cross-section view of this
embodiment of the integrated device 160 and photoemissive structure
170 according to the invention in more detail. A typical integrated
device 160, such as an integrated circuit, will comprise a
substrate 162 of a material, such as silicon, on which a layer
comprising integrated structures 164 has been fabricated. The
devices will often be in the form of a plurality of structures,
often fabricated in several planar processing steps that form
devices and the structures interconnecting them. The illustration
of FIG. 4 shows a cross-section of a representation of a layer
comprising integrated structures 164 which contains many structures
fabricated using 4 planar layers, which include dielectric material
165, shown as grey, and numerous metal interconnect structures 166,
shown as black. For simplicity, only a few structures have been
illustrated in FIG. 4, but for many contemporary integrated
circuits, the metal interconnect structures are often manufactured
using copper and the number of metal interconnect structures number
in the billions. Typical overall x- and y-dimensions for an
integrated device 160 may be 1 cm.times.1 cm, while the dimensions
of the interconnect structures 166 are often smaller than 100 nm,
and can be as small as 20 nm in a contemporary integrated
circuit.
[0057] For some embodiments of the invention, especially when used
to observe copper interconnect structures, the energy of the beam
of x-rays 111 can be selected so that a significant portion of the
x-rays have energy greater than the energy of the copper K-band
absorption edge. FIG. 5 illustrates a graph showing a typical plot
of the copper x-ray absorption near edge structure (XANES)
absorption spectrum. When the spectrum of x-ray energy for the beam
of x-rays 111 is chosen to have a significant fraction above the
copper K-band absorption at 8.98 eV, the copper structures will
strongly absorb many of the x-rays, while the other dielectric
structures will transmit the x-rays. This will result in the
varying degrees of x-ray transmission through the integrated device
and therefore improved contrast. The external modified beam of
x-rays 211 will therefore have a strongly varying intensity
profile, representing copper interconnect structures.
[0058] Returning to FIG. 4, The photoemissive structure 170 may
comprise a layer 172 of photoemissive material that generates a
cascade of electrons 177 when irradiated with x-rays. This layer
172 can be fabricated using a material such as gold, but other
photoemissive materials will be known to those skilled in the art
of electron-material interactions. It is desired that the
composition of the material of this layer 172 be relatively
uniform, so that the cascade of electrons 177, comprising primary
electrons generated by the x-rays and also secondary electrons
generated in turn by the primary electrons, has an electron density
that is in proportion to the local flux of x-rays passing through
the material.
[0059] The thickness of the layer 172 must also be selected with
care, as a layer 172 that is too thin may not generate a strong
cascade of electrons 177, while a layer 172 that is too thick may
generate a cascade of electrons 177 whose number is no longer in
proportion to the flux of incident x-rays. In one embodiment of the
invention, layer 172 is fabricated using gold having a thickness of
approximately 50 nm.
[0060] Once the cascade of electrons 177 is generated, some of
these electrons exit the layer 172 of photoemissive material.
However, in some cases, the material used to fabricate the layer
172 of photoemissive material may be selected to have the
capability of generating a large number of electrons from a few
absorbed x-rays, but which may also have a large work function.
This may reduce the emission of some of the generated electrons,
which can lead to a reduction in the overall signal strength.
[0061] Therefore, in some embodiments of the invention, it is
desired that the photoemissive structure 170 additionally comprise
an emissive coating 174. The material used to fabricate this
emissive coating 174 can be chosen to have a low work function, so
that it is more likely that the cascade of electrons 177 initiated
by the x-ray exposure in the layer 172 of photoemissive material
will result in a large number of emitted electrons 179. The
material used for the emissive coating 174 can also be chosen so
that the electrons of the cascade of electrons 177 that are
transferred into the emissive coating 174 can generate additional
secondary electrons, forming an amplified electron cascade 178.
Materials such as cesium iodide (CsI), which has a low work
function and good electron generation properties, may be used for
the emissive coating 174 in some embodiments of the invention. In
one embodiment of the invention, the emissive coating 174 is a CsI
layer with a thickness of 100 nm. In another embodiment of the
invention, the emissive coating 174 is a CsI layer with a thickness
of 5 nm.
[0062] Returning to FIG. 3, in some embodiments of the invention,
there will be a voltage applied to the photoemissive structure
using electrical contact 176 and electrical lead 142. The relative
voltage compared to the voltage applied to the subsequent electron
optics, such as cathode lens 152, will be set such that the emitted
electrons 179 are accelerated away from the surface of the
photoemissive structure 170 towards the electron optics.
[0063] It will be known to those skilled in the art that other
architectures for the photoemissive structure can be designed
comprising additional layers, and in which voltage differences
between the layers of the photoemissive structure 170 are
established as well, to accelerate the electrons between the layers
of the photoemissive structure 170.
[0064] Some embodiments of the invention will have a voltage
controller 140 that uses electrical lead 142, connecting to the
photoemissive structure 170, and cathode lens electrical lead 144,
connecting to the cathode lens 152, to set the relative voltage of
the photoemissive structure 170 and the cathode lens 152. If the
voltage provided through the lead 142 to the electrical contact 176
is significantly more negative than the voltage provided through
the lead 144 to the cathode lens, then the emitted electrons 179
will be accelerated away from the photoemissive structure 170 and
into the electron optical system. In some embodiments of the
invention, a voltage difference of twenty kilovolts (20 kV) will be
established between electrical lead 142 and cathode lens electrical
lead 144. In another embodiment of the invention, a voltage
difference of fifty kilovolts (50 kV) will be established.
[0065] A typical electron optical system comprises a combination of
electron optics, such as the cathode lens 152, apertures 154, beam
steering optical elements 156 and transfer and projection lenses
158. The electron optics can be positioned inside the vacuum
chamber 120, such as when the electron optical design uses
electrostatic lenses, or be positioned outside the vacuum chamber
120, such as when magnetic lenses are used, as illustrated in FIG.
3. The optics can also be configured to be adjustable for imaging
properties such as astigmatism and other aberration corrections,
and in particular to adjust the position and orientation of the
cathode lens 152 relative to the photoemissive structure 170. These
can be predetermined adjustments, or may be adjusted by signals
from the system controller 110 in response to feedback about the
imaging performance of the system. In some embodiments of the
invention, the stage controls 132 may be used on the stage 130
holding the photoemissive structure 170 to adjust the position and
orientation of the photoemissive structure 170 relative to the
electron optics and the optical axis of the electron optics.
[0066] In some embodiments of the invention, the electron optics
will comprise beam steering optical elements 156 so that the
emitted electrons 179 are no longer co-linear with the unabsorbed
beam of x-rays 511. This allows the unabsorbed beam of x-rays 511
to fall into a beam dump 108, where it is absorbed and therefore
prevented from leaving the vacuum chamber, reducing the risk of
inadvertent radiation exposure.
[0067] In some embodiments of the invention, the various electron
optical elements form a magnified electron image 159 of the emitted
electrons 179 in the final image plane of the electron optical
system. In some embodiments, the magnified electron image 159 will
be 150 times larger than the pattern of emitted electrons 179. In
other embodiments, the magnified electron image 159 will be 1,500
times larger than the pattern of emitted electrons 179.
[0068] The electron optical system will typically be designed such
that the image plane is within the vacuum chamber 120. An image
converter 180 is placed at this image plane that emits photons 198
when excited by energetic electrons of the magnified electron image
159. These photons 198 are generally visible photons (i.e. with a
wavelength between 400-700 nm), although in some embodiments the
emitted photons 198 may be infrared or ultraviolet photons. Some of
the emitted photons 198 from the image converter 180 exit the
vacuum chamber 120 through exit window 127, and are collected by
the imaging system 190. In some embodiments, this imaging system
190 comprises a video system or a CCD array to create electronic
signals corresponding to the emitted photons 198.
[0069] FIG. 6 illustrates a detailed cross-section view of the
structure of the image converter 180 and imaging system 190 for
this embodiment of the invention. In this embodiment, the image
converter 180 comprises a scintillator 184. Such scintillators are
common in electron microscopy, and many variations containing a
variety of phosphors that emit photons when stimulated with
energetic electrons will be known to those skilled in the art. The
scintillator 184 can comprise a phosphorescent material, such as
zinc sulfide (ZnS) doped with manganese (Mn) or other elements, a
structure comprising a crystal material such as Yttrium Aluminum
Garnet (YAG), or compositions comprising various rare earth
elements. The fabrication of the image converter 180 should result
in a uniform structure of material in the scintillator 184, so that
the intensity of the emitted photons 198 is in proportion to the
number of incident electrons absorbed from the magnified electron
image 159.
[0070] In some embodiments, the image converter 180 may comprise
additional layers, such as a conducting layer 186 on the side of
the image converter 180 on which the electrons of the magnified
electron image 159 are incident. In some embodiments of the
invention, this conducting layer 186 can be attached electrically
using an electrical lead 182 to set the image converter 180 to a
specific voltage. In some embodiments of the invention, the
specific voltage on the electrical lead 182 will be set to zero
volts, and the lead 182 therefore provides a path to ground for the
absorbed electrons of the magnified electron image 159. In some
embodiments, the voltage may be set by voltage controller 140. In
some embodiments, the conducting layer 186 is fabricated using a
material that reflects photons such as a metallic thin film. In
some embodiments, the conducting layer 186 will be approximately 50
nm thick, and fabricated using a material comprising aluminum. This
provides an additional benefit of taking any photons from the
scintilator 184 emitted in the direction of the incoming electrons
and reflecting them back towards the exit window 127, where they
add to the intensity of the emitted photons 198.
[0071] Outside the vacuum chamber, an imaging system 190 can be
used to produce an image 199 of the emitted photons 198. In some
embodiments, this imaging system 190 comprises a lens system 192,
an image sensor 194, and image processing electronics 196 that can
be used to convert the image 199 of the emitted photons 198 into
electronic signals. In some embodiments, the lens system 192 forms
a magnified image of the emitted photons 198. In some embodiments,
this magnification is by a factor of 100. In some embodiments, the
image sensor 194 will be a charge-coupled device (CCD) array. In
some embodiments, the signals will be a represent the image using
video formats. In other embodiments, these signals will be a
collection of still images.
[0072] If the materials of the photoemissive structure 170 and the
image converter 180 are well selected and uniformly fabricated, and
the adjustments of the electron optics 152, 156 and 158 are made to
minimize aberrations and distortions, the final electronic signals
from the imaging system 190 will represent a magnified image of the
x-ray transmission of the corresponding portion of the integrated
device 160.
[0073] FIG. 7 illustrates in more detail a schematic of the control
systems to be used for some embodiments of the invention. The
electronic images generated by the imaging system 190 can be
transmitted to the system controller 110. This controller 110 can
comprise a means for governing the source of x-rays 100, such as
adjusting the x-ray intensity, pulsing the source, or making
adjustments to the beam shaping optics 104 to collimate or
concentrate the beam.
[0074] This controller 110 can further comprise a means for
electronic input and output 114.
[0075] This controller 110 can further comprise an electronic
processor 116. This processor 116 may be programmed to manage the
external stage controller 234 that drives the external stage
controls 232 that adjust the position and orientation for the
external stage 230 supporting the integrated device 160. This
processor 116 may also be programmed to manage the stage controller
134 that drives the stage controls 132 that adjust the position and
orientation for the stage 130 supporting the photoemissive
structure 170. This processor 116 may be programmed to manage the
voltage controller 140 that adjusts the relative voltage of the
photoemissive structure 170 and the cathode lens 152, and may also
control the voltage setting for the electrical lead 182 for the
scintillator 184. This processor 116 may also be programmed to
adjust the settings and aberration controls of the cathode lens
152.
[0076] In some embodiments of the invention, the controller 110
will also comprise electronic data storage 118, which can be used
to record the position and orientation set for the external stage
230, the stage 130, and the control voltages for the photoemissive
structure 170 and image converter 180, as well as the corresponding
images collected by the imaging system 190.
[0077] In some embodiments, the information and signals
representing images recorded in the electronic data storage 118 can
be combined to synthesize a two-dimensional (2-D) or
three-dimensional (3-D) representation of the integrated device 160
or portions thereof.
[0078] In some embodiments of the invention, these synthesized 2-D
or 3-D representations can be compared with a stored representation
of an integrated device known to be correctly manufactured, or a
database representation of the design rules or the layout of the
device as designed, and the resulting comparison used to evaluate
the attributes of the integrated device 160 being examined. Such a
system can be used as an inspection system for manufacturing
quality control.
Second Embodiment of the Invention
[0079] FIG. 8 and FIG. 9 illustrate another embodiment of an
apparatus according to the invention. FIG. 8 illustrates a cross
section view of the overall system, and FIG. 9 illustrates a
cross-section in detail of the integrated device 160 and
photoemissive structure 170.
[0080] As in the previously described embodiment of the invention,
an x-ray source 100 produces a beam of x-rays 111 which are
partially absorbed by the integrated device 160 under examination,
forming a modified beams of x-rays 211.
[0081] As in the previously described embodiment of the invention,
the integrated device 160 is mounted outside the vacuum chamber
120, using an external stage 230. In some embodiments of the
invention, this external stage 230 will be fixed in place, and in
others the external stage 230 may have external stage controls 232
to adjust position and orientation. Adjustment of the external
stage controls 232 can be governed by an external stage controller
234, which is generally a system of electronics also outside the
vacuum chamber 120. In some embodiments, this stage controller 234
can in turn be controlled by the overall system controller 110,
which can designate an organized scan of positions and angles of
the external stage 230 to facilitate the examination of the
integrated device 160.
[0082] After passing through the integrated device 160, the
modified beam of x-rays 211 enters the vacuum chamber 120. However,
in this embodiment of the invention, the photoemissive structure
170 has been fabricated directly onto the support window 225 of the
vacuum chamber 120. The support window 225 may be similar in design
and fabrication to the window 125 described in the previous
embodiment, and will also be made using a material transparent to
x-rays, such as beryllium or diamond, but may also need to be of a
different thickness or composition to serve as both a window for
the vacuum chamber 120 and also as a mechanical support for the
photoemissive structure 170.
[0083] This configuration has some advantages, in that the need for
stage 130, stage controls 132 and stage controller 134 inside the
vacuum chamber are eliminated, along with the corresponding
feedthrough junctions 123. Also, the need to select two materials,
the window 125 and the support structure 270, for mechanical and
x-ray transmission properties is simplified to the selection of a
single material for support window 225.
[0084] In some embodiments of the invention, a vacuum chamber may
be designed in which the position and orientation of the window can
also be adjusted relative to the electron optics and the optical
axis of the electron optics. However, since windows for vacuum
systems are typically fixed in place, in the embodiment as
illustrated here, the photoemissive structure 170 also becomes
fixed in position and orientation. Any relative changes in position
or orientation angle between the integrated device 160 and the
photoemissive structure 170 would then need to be controlled
through the position and orientation of the external stage 230 for
the integrated device 160.
[0085] Likewise, because the support window 225 will function as a
seal for the vacuum chamber 120 and therefore be near or in contact
with the walls of the vacuum chamber 120, care must be taken in
setting the voltage for electrical lead 142 relative to the
electron optics so that electrical shorting through to vacuum
chamber 120 does not occur.
[0086] As in the previously described embodiment of the invention,
after transmission through the photoemissive structure 170, the
unabsorbed beam of x-rays 511 can proceed in the vacuum chamber 120
to a beam dump 108 where it is absorbed.
[0087] As in the previously described embodiment of the invention,
the emitted electrons 179 are directed by a set of electron optics
to form a magnified image 159 at an image converter 180. As in the
previous embodiment of the invention, the photons 198 emitted by
the image converter 180 leave the vacuum chamber through exit
window 127 and are converted to electronic signals in imaging
system 190. These are transmitted to a controller 110, which can
record these images using electronic data storage 118.
[0088] As in the previously described embodiment of the invention,
the information and signals representing images recorded in the
electronic data storage 118 can be combined to synthesize a
two-dimensional (2-D) or three-dimensional (3-D) representation of
the integrated device 160 or portions thereof.
[0089] As in the previously described embodiment of the invention,
these synthesized 2-D or 3-D representations can be compared with a
stored representation of an integrated device known to be correctly
manufactured, or a database representation of the design rules or
the layout of the device as designed, and the resulting comparison
used to evaluate the attributes of the integrated device 160 being
examined. Such a system can be used as an inspection system for
manufacturing quality control.
Third Embodiment of the Invention
[0090] FIG. 10 through FIG. 13 illustrate another embodiment of an
apparatus according to the invention. FIG. 10 illustrates a
cross-section view of the overall system, and FIG. 11 illustrates a
cross-section in detail of the integrated device 160 and
photoemissive structure 170. FIG. 12 and FIG. 13 each illustrate a
cross-section in detail of the integrated device 160 and
photoemissive structure 170 for two different variations of the
embodiment.
[0091] As in the previously described embodiments of the invention,
an x-ray source 100 produces a beam of x-rays 111. In this
embodiment, however, the beam of x-rays directly enters the vacuum
chamber 120 through the entrance window 125 without passing through
the integrated device 160, becoming the interior unmodified beam of
x-rays 411. The modifications that make the interior unmodified
beam of x-rays 411 different from the incident beam of x-rays 111
are only due to absorption and scattering from the window 125.
[0092] In this embodiment, the photoemissive structure 170 is
deposited directly onto the integrated device 160 to be examined.
Both the integrated device 160 and the attached photoemissive
structure 170 are entirely contained within the vacuum chamber 120.
Therefore, the need for external stage 230, external stage controls
232, and external stage controller 234 are eliminated. However,
stage 330 within the vacuum chamber 120 is now used to hold both
the integrated device 160 and the photoemissive structure 170, and
to adjust their positions and orientation angle relative to the
interior beam of x-rays 411 as well. The design of the stage 330
and the stage controls 332 may be very similar to the stage 130 and
stage controls 132 in the previous embodiments. However, the
additional thickness and support requirements for holding both the
integrated device and the photoemissive structure may require some
variation in design.
[0093] In some embodiments, both the position and the orientation
of the integrated device 160 may be adjustable using stage controls
332 for the stage 330, making it possible for a relatively small
beam of x-rays to be used to examine the entire area of an
integrated device 160 much larger than the diameter of the slightly
modified x-ray beam 411. Adjustment of the stage controls 332 can
be governed by a stage controller 334, which is generally a system
of electronics outside the vacuum chamber 120. In some embodiments,
this stage controller 334 can in turn be controlled by an overall
system controller 110, which can designate an organized scan of
positions and angles of the stage 130 to facilitate the examination
of the integrated device 160. In some embodiments of the invention,
the stage controls 332 may be used on the stage 330 holding the
integrated device 160 and photoemissive structure 170 to adjust the
position and orientation of the photoemissive structure 170
relative to the electron optics and the optical axis of the
electron optics.
[0094] As in the previously described embodiments of the invention,
the photoemissive structure 170 may comprise a layer 172 of
photoemissive material and an emissive coating 174. However, in
this embodiment the need for an additional support structure 270
for the photoemissive structure 270 is eliminated, since the
structure 170 has been deposited directly on the integrated device
160.
[0095] FIG. 12 illustrates a variation of this embodiment of the
invention. In the previously illustrated embodiments, the angle of
incidence of the beam of x-rays 111 and therefore also modified
x-ray beam 411 is perpendicular (i.e. has an angle of incidence at
or near 90.degree.) to the surface of the integrated device 160. In
this variation of the embodiment, motion of the x-ray source 100 or
adjustment of the beam-shaping optics 104 can provide x-rays
incident on the integrated device 160 at some value .theta. which
is not 90.degree., and in fact in some embodiments may be
adjustable to provide a multiplicity of angles of illumination,
either simultaneously or in a programmed time sequence. The angled
beam of x-rays 611 will have an intensity pattern that is different
from the normal incidence case of FIG. 11, and therefore the
trajectory and intensity pattern of the cascade of electrons 777
and the amplified electron cascade 778 will be different from the
trajectory and intensity pattern of the cascade of electrons 177
and the amplified electron cascade 178 in the normal incidence case
of FIG. 11.
[0096] However, once the electrons 179 are emitted from the surface
of the photoemissive structure 170, they are accelerated towards
the cathode lens 152, and a magnified image is formed by the image
converter 180 and imaging system 190, as in the previous
embodiments of the invention.
[0097] Such an embodiment can be used in conjunction with various
image processing algorithms such as those for computed
laminography, also known as digital tomosynthesis, synthetic
laminography, or computerized synthetic cross sectional imaging, in
which images from multiple angles are collected to and processed to
provide a 3-dimensional representation of the layers of the
integrated device 160. In some cases, a simple parallax computation
from two images at different angles may be enough to infer 3-D
structural information. In other cases in which the basic structure
(i.e. layer thicknesses and approximate feature sizes) are known,
collecting images for a few multiple angles near perpendicular may
provide enough information to infer 3-D detailed structural
information.
[0098] The advantage of computed laminography algorithms over more
commonly used computed tomography (CT) algorithms is that
transmission information from a wide range of angles around the
sample need not be collected. When the integrated device 160 is in
a vacuum chamber requiring a window 125 for x-ray transmission, and
the alignment with the electron optics can be delicate, the wide
range of motion required by many tomography algorithms can require
a system that is mechanically complex. When only a few angles and
views are required, the integrated device 160 and photoemissive
structure 170 can remain aligned with the electron optics, and even
at times immobile, and only the angle of incidence of the beam of
x-rays 611 need be changed.
[0099] Although we describe the integrated device 160 and
photoemissive structure 170 as being in "direct contact" in this
embodiment, it will be known to those skilled in the art that there
may be some configurations in which it may be best to deposit
additional layers of buffer material between the integrated device
160 and the photoemissive structure 170, to provide a more
physically flat, chemically neutral, or electrically insulating
surface. Such a planarization layer 370 is illustrated in FIG. 13.
This may be especially important if particular voltage settings are
desired for the electrical lead 142, since the lead may be also in
contact or close proximity to the integrated device 160 and the
stage 130.
[0100] As in the previous embodiments, some x-rays from the
unmodified beam of x-rays 411 are absorbed or scattered in the
integrated device, and then stimulate the emission of electrons 179
from the photoemissive structure 170.
[0101] As in the previously described embodiments of the invention,
after transmission through the photoemissive structure 170, the
unabsorbed interior beam of x-rays 511 can proceed in the vacuum
chamber 120 to a beam dump 108 where it is absorbed.
[0102] As in the previously described embodiments of the invention,
the emitted electrons 179 are directed by a set of electron optics
and form a magnified image 159 at an image converter 180. As in the
previous embodiment of the invention, the photons 198 emitted by
the image converter 180 leave the vacuum chamber through the exit
window 127 and are converted to electronic signals in imaging
system 190. These are transmitted to a controller 110, which can
record these images using electronic data storage 118.
[0103] As in the previously described embodiments of the invention,
the information and signals representing images stored in the
electronic data storage 118 can be combined to synthesize a
two-dimensional (2-D) or three-dimensional (3-D) representation of
the integrated device 160 or portions thereof.
[0104] As in the previously described embodiments of the invention,
these synthesized 2-D or 3-D representations can be compared with a
stored representation of an integrated device known to be correctly
manufactured, or a database representation of the design rules or
the layout of the device as designed, and the resulting comparison
used to evaluate the attributes of the integrated device 160 being
examined. Such a system can be used as an inspection system for
manufacturing quality control.
Other Embodiments of the Invention
[0105] FIG. 14 illustrates a cross-section in detail of the
integrated device 160 and photoemissive structure 170 for another
embodiment of an apparatus according to the invention.
[0106] In this embodiment, both the integrated device 160 and the
photoemissive structure 170 are within the vacuum chamber 120, but
the integrated device has a connected stage 430 with connected
stage controls 432 that are attached to the stage 130 and stage
controls 132 for the photoemissive structure 170. The connected
stage 430 may be designed to allow the easy and rapid insertion of
integrated devices 160, and to adjust their position and
orientation not only with respect to the interior unmodified beam
of x-rays 411, but also relative to the photoemissive structure
170. In some embodiments, the connected stage 430 will be designed
to allow the integrated device 160 to be moved to be in very close
proximity to the photoemissive structure 170. In some embodiments,
photoemissive structure also has an independent support 470, which
may be similar in design and material composition to the support
structure 270 described in the previous embodiments.
[0107] The advantage to such a configuration is that the device 160
and the structure 170 can be placed in relatively close proximity,
minimizing the distortion from propagation and scattering that can
occur with propagation, without actually being in mechanical and
electrical contact. In some embodiments, the position and
orientation angle of the integrated device 160 can be adjusted
independently.
[0108] The disadvantage to such a configuration is that both the
device 160 and the structure 170 must now have either independent
mounting systems within the vacuum chamber 120, or a well designed
single stage for mounting that will allow the integrated device 160
to be inserted in close proximity to the photoemissive structure
170 and its independent support 470, and also allow its removal,
without damaging or misaligning the photoemissive structure 170.
The design of such mounting systems can be costly, especially when
required to be used entirely within a vacuum chamber.
[0109] There are other design concerns for the joined stage 130,
stage control 132, and connected stage 430 and its stage controls
432. If the photoemissive structure is to remain in a stable
position relative to the electron optics, which is better for
distortion control, then the design of the connected stage 430
holding the integrated device must be offset with enough distance
from the photoemissive structure 170 so that changes in the
relative orientation angle and position of the integrated device
160 relative to the beam of x-rays 411 could be made without the
device coming in contact with the structure 170, disrupting the
alignment with the electron optics.
[0110] In an alternative embodiment, the integrated device 160
could be mounted in close proximity to the photoemissive structure
170 and its independent support 470 and the motions of the stage
130 and the connected stage 430 rotated together if various
orientation angles for the integrated device relative to the
interior unmodified beam of x-rays 411 is desired. This may reduce
the potential distortions caused by greater distance between device
160 and structure 170, but may increase the distortions caused by
potential misalignments between the photoemissive structure 170 and
the electron optics. Also, unless the photoemissive structure 170
were the same dimensions as the integrated device, a translation of
the device 160 in x-y coordinates relative to the structure 170 may
be required, so that the entire device can eventually be observed.
To increase signal strength, a high x-ray flux is desired, and
spreading the x-ray beam to cover the entire integrated device will
reduce flux considerably.
Performance: Speed and Resolution
[0111] Given the descriptions above, the time to collect an image
from a 1 cm.times.1 cm integrated device can be estimated, and
compared to prior art Fresnel zone plate (FZP) systems such as
those previously described.
[0112] The relative imaging throughput of this system can be
estimated using three factors: [0113] 1. Flux of x-ray illumination
[0114] 2. Contrast in the specimen under examination [0115] 3.
Detection efficiency.
[0116] As noted above, the spectrum of the x-ray source used with
the disclosed invention can be a broadband source, and in
particular one in which a significant fraction of the x-rays have
higher energy than the copper K-absorption edge. The source
brightness can be as high as 5.times.10.sup.10 x-ray
photons/mm.sup.2 srad, while the brightness in the FZP system is at
least a factor of 10 smaller, since only the characteristic 8 keV
copper K.alpha. fluorescence photons are used.
[0117] Also, the numerical aperture (NA) of the system disclosed
here can be approximately 9 mrad, while the NA of a FZP system is
typically 3 mrad. The reduction in angle by a factor of 3 leads to
a reduction in the amount of x-ray photons that can be collected
and used to illuminate the specimen, reducing incident flux by a
factor of 9.
[0118] These two differences alone lead to an increase in
throughput by a factor of at least 90 due to the increased incident
x-ray flux for the invention disclosed here.
[0119] The second factor affecting imaging speed (throughput) is
image contrast. The imaging contrast (signal) depends on sample
materials and x-ray energy (wavelength). As mentioned above, a FZP
typically system uses 8 keV copper K.alpha. fluorescence as its
x-ray source, to which copper interconnects in an integrated device
are mostly transparent (as was illustrated in FIG. 5). By using
broadband light, with a significant portion with energy greater
than the copper K-edge absorption at 8.9 keV, the contrast can
increase by a factor of 15 or more. For an IC with a 50 micron
thick silicon substrate, this corresponds to a 7.times. increase in
signal contrast for a 1 micron thick copper line. The corresponding
increase in throughput is a factor of 7.sup.2 or 49.times..
[0120] The overall detector quantum efficiency (DQE) for the for
the system according to the invention (factoring in the conversion
from x-rays to an electron cascade in the photoemissive structure
170, the conversion of electrons to photons by the image converter
180, and then to electronic signals in the imaging system 190) is
similar to that of existing FZP systems, about 2%. Therefore, the
overall improvement in imaging speed is found by multiplying the
increase in throughputs due to increased x-ray flux (90.times.) and
image contrast (49.times.). This leads to an overall improvement in
throughput for the disclosed system over the prior art FZP of
90.times.49=4,410.
[0121] Prior art FZP systems have been designed to inspect details
of an integrated device, but not for high-resolution examination of
entire ICs. To form a complete image of a 1 cm.times.1 cm IC using
such a prior art system would take 200,000 hours (22.8 years).
However, using the improvements of a system according to the
invention disclosed here, data collection could occur 4,410 times
faster, and a complete image could be collected in 45.3 hours--less
than 2 days.
[0122] Of course, speed is not the only metric for such a
microscope or inspection system. For integrated devices with 20 nm
features, a resolution of 20 nm is desired. The resolution of a
system according to the invention is partly determined by the
diffusion of the cascade of electrons 177 in the photoemissive
structure 170. This diffusion reduces the localization of the
electron excitation, and causes a blur in the image.
[0123] This diffusion depends on the geometry and material
composition of the photoemissive structure 170. A thicker structure
will increase diffusion, creating more blur. Previous studies of
x-ray-electron hybrid imaging systems [L. A. Bakaleynikov, E. Yu.
Flegontova, and E. Zolotoyabko, "Combined X-ray-electron Imaging
Techniques: Limitations on Lateral Resolution," Journal of Electron
Spectroscopy and Related Phenomena, Vol. 151, pp. 97-104 (2005)]
indicate that excitation of a photoemissive thin film of gold by an
x-ray point-source can produce electron emission with a resolution
as small as 20 nm. The electron optics can be designed such that
they faithfully maintain this resolution without further
degradation.
Further Extensions and Limitations.
[0124] Although this disclosure presents an apparatus for the
microscopic examination of integrated devices, and in particular
copper integrated circuit structures, it will be recognized that
the term "integrated devices" as used here can represent any
manufactured object with small (e.g. micro- or nano-scale)
features, such as silicon interposers with thru-silicon-vias
(TSVs), packages containing multiple integrated circuits (3D-IC
structures), especially those with TSVs to connect them vertically,
MEMS and NEMS devices such as micro-actuators and micro-sensors, RF
antenna structures, integrated optical devices, multi-function IC
packages for cellular phones, photomasks, metamaterials, magnetic
storage devices, and others that will be known to those skilled in
the art.
[0125] It will also be recognized that the apparatus disclosed here
can be used for the examination of objects other than manufactured
integrated structures. Such objects can include mineral formations
or biological tissue samples, especially biological tissue samples
that may have been metallized for enhanced contrast. As long as the
wavelength range for the beam of x-rays is selected such that there
is measurable contrast in the absorption or scattering of x-rays
from the internal structures of the object under investigation, a
system as disclosed here can be used to investigate these internal
structures as well.
[0126] It will also be recognized that this microscope can be used
as a component of an inspection system, in which the
representations of the 2-D and 3-D structures are compared with
stored reference data. These data can be either a reference image
or set of images from a similar device known to have been properly
manufactured (a "Golden Image"), or data from a similar region in
the integrated device previously that has been measured, or from a
database representing the integrated device design rules or
geometric structures as designed.
[0127] With this application, several embodiments of the invention,
including the best modes for various circumstances, have been
disclosed. It will be recognized that, while specific embodiments
may be presented, elements discussed in detail only for some
embodiments may also be applied to others. For example, the image
collection discussed in detail only for the first embodiment can be
applied to other embodiments as well. Likewise, the angular
variation of the x-rays described in detail in the third embodiment
may find application in other configurations.
[0128] While specific materials, designs, configurations and
fabrication steps have been set forth to describe this invention
and the preferred embodiments, such descriptions are not intended
to be limiting. Modifications and changes may be apparent to those
skilled in the art, and it is intended that this invention be
limited only by the scope of the appended claims.
* * * * *
References