U.S. patent application number 10/903985 was filed with the patent office on 2005-02-17 for specimen tip and tip holder assembly.
Invention is credited to Morrison, Robert.
Application Number | 20050035302 10/903985 |
Document ID | / |
Family ID | 27799691 |
Filed Date | 2005-02-17 |
United States Patent
Application |
20050035302 |
Kind Code |
A1 |
Morrison, Robert |
February 17, 2005 |
Specimen tip and tip holder assembly
Abstract
A specimen tip holder assembly for mounting a specimen tip in a
transmission electron microscope (TEM) is described. The specimen
tip holder assembly comprises a tip holder for supporting a
specimen tip. The tip holder is coupled to an elongate support for
movement in a direction substantially perpendicular to the axis of
the support. An actuator is mounted to the support for causing
motion of the tip holder relative to the support.
Inventors: |
Morrison, Robert;
(Wiltshire, GB) |
Correspondence
Address: |
FENWICK & WEST LLP
SILICON VALLEY CENTER
801 CALIFORNIA STREET
MOUNTAIN VIEW
CA
94041
US
|
Family ID: |
27799691 |
Appl. No.: |
10/903985 |
Filed: |
July 30, 2004 |
Current U.S.
Class: |
250/442.11 |
Current CPC
Class: |
H01J 37/20 20130101 |
Class at
Publication: |
250/442.11 |
International
Class: |
H01J 037/20 |
Foreign Application Data
Date |
Code |
Application Number |
Aug 1, 2003 |
GB |
0318134.4 |
Claims
We claim:
1. A specimen tip holder assembly for mounting a specimen tip in a
transmission electron microscope (TEM), the specimen tip holder
assembly comprising a tip holder for supporting a specimen tip, the
tip holder being coupled to an elongate support for movement in a
direction substantially perpendicular to the axis of the support,
and an actuator mounted to the support for causing motion of the
tip holder relative to the support.
2. A specimen tip holder assembly according to claim 1 wherein the
movement between the tip holder and the elongate support is
substantially rectilinear.
3. A specimen tip holder assembly according to claim 1 or claim 2
wherein the movement between the tip holder and the elongate
support is substantially parallel to an optic axis of the TEM.
4. A specimen tip holder assembly according to claim 1 wherein the
elongate support is provided with an elongate aperture or cavity
extending in the direction of movement, and the tip holder
comprises a protrusion which slidably engages with the elongate
aperture or cavity.
5. A specimen tip holder assembly according to claim 4 wherein the
elongate aperture or cavity extends in a direction substantially
parallel to an optic axis of the TEM.
6. A specimen tip holder assembly according to claim 1 wherein the
actuator is adapted to be operated from a location remote from the
tip holder.
7. A specimen tip holder assembly according to claim 1 wherein the
tip holder is adapted to removably support a specimen tip.
8. A specimen tip holder assembly according to claim 1 which
further comprises a specimen tip.
9. A specimen tip holder assembly according to claim 7 or claim 8
wherein the tip holder is provided with clamping means to removably
support the specimen tip.
10. A specimen tip holder assembly according to claim 7 or claim 8
wherein the tip holder comprises two members, coupled by a pivot
pin, and biassing means for urging the two members towards one
another, so as to clamp at least a portion of a specimen tip
between them.
11. A specimen tip holder assembly according to claim 1 wherein the
elongate support is a barrel, and the actuator extends within the
barrel.
12. A specimen tip holder assembly according to claim 1 wherein the
actuator comprises a rotatable camshaft extending along the
elongate support, arranged to engage a cam follower provided on a
slide forming part of or coupled to the tip holder, such that the
slide is caused to move in a direction substantially perpendicular
to the axis of the support as the camshaft is rotated.
13. A specimen tip holder assembly according to claim 12 wherein
the support is further provided with sealing means which prevent
the passage of gas between the camshaft and the elongate
support.
14. A specimen tip for supporting a specimen in a transmission
electron microscope, the tip being provided with a substantially
planar surface onto which a specimen is bonded, and a cut-out
located underneath the specimen through which electrons may
pass.
15. A specimen tip according to claim 14, wherein the specimen
comprises a sample.
16. A specimen tip according to claim 15 wherein the specimen
further comprises a mounting grid to which the sample is bonded, at
least the mounting grid being bonded to the planar surface.
17. A specimen tip holder assembly according to claim 1 wherein the
tip holder supports a specimen tip according to claim 14.
18. A transmission electron microscope having an evacuated cavity
into which a specimen tip holder assembly according to claim 1 or
claim 17 extends.
19. A transmission electron microscope according to claim 18
wherein the actuating means are operable from outside of the
evacuated cavity.
20. A method of mounting a specimen to a specimen tip for
supporting a specimen in a transmission electron microscope, the
method comprising the steps of: a) applying adhesive to at least
part of a planar surface on a specimen tip and/or to at least part
of a specimen; b) positioning the specimen on the specimen tip at a
location such that the passage of electrons through an at least
partially electron-transparent portion of the specimen is not
disrupted by the specimen tip; and c) bonding the positioned
specimen to the specimen tip by means of the adhesive.
21. A method according to claim 20, wherein the specimen comprises
a sample.
22. A method according to claim 21, wherein the specimen comprises
a mounting grid to which the sample is bonded.
23. A method according to claim 22, wherein the method further
comprises bonding the sample to the mounting grid prior to step
(a).
24. A method according to any of claims 20 to 23 further
comprising, prior to step (a) or after step (c), removing at least
some of the area of the specimen which overhangs the specimen
tip.
25. A method according to claim 20 wherein the adhesive is a high
vacuum compatible adhesive.
Description
[0001] This invention relates to specimen tips and tip holder
assemblies for supporting a sample to be analysed in a transmission
electron microscope. The invention further provides a method for
mounting a specimen to a specimen tip.
[0002] Transmission electron microscopy is a versatile tool widely
used across a range of research areas from biology to
physio-chemical subjects. The transmission electron microscope is
one of the most effective imaging devices available and can be used
to study not only the surface morphology but also the internal
structure and defects in diverse materials including metals,
semiconductors, proteins and polymers.
[0003] A schematic diagram illustrating the workings of a
transmission electron microscope (TEM) is shown in FIG. 1. In
essence, a TEM consists of an electron gun 1 which produces a
high-energy beam of electrons, and a series of electromagnetic
lenses 2 which control and collimate the beam along the optic axis
A-A onto a specimen under investigation. A thin sample 4
(.about.100 nm thick) is used so that electrons may be transmitted
through the material. The electrons are scattered by the sample and
form a diffraction pattern characteristic of that material. A
further series of lenses 5 can be used to form an image from the
electron beams that had been scattered as they passed through the
sample. The image or diffraction pattern may be viewed on a screen
6 coated with electron-fluorescent material, through a viewing
window 7, or converted to a picture shown on a monitor via camera
chamber 8. Alternatively, techniques for quantitative measurement
of the electron intensity across the image may be employed. The
whole TEM cavity is evacuated to a pressure of less than 10.sup.-2
Pa, to prevent the beam of electrons being disrupted by gas
particles.
[0004] In order to obtain the high resolution that is often
required in a TEM, the sample is generally positioned within the
electromagnetic lenses or "pole pieces" 2c and 5a. The gap inside a
high-resolution pole piece is small and this places a severe
restriction on its maximum size: typically the sample 4 is a disc,
3 mm in diameter and with a maximum height of approximately 1 mm
(FIG. 2). In order that electrons may be transmitted through the
sample, specialist sample preparation techniques are used to thin
the material to less than 100 nanometres. In most techniques,
material is removed from the centre of the sample disk using
electro-polishing methods or ion thinning until a hole 10 is made
in the centre of the disk. A thin, electron-transparent area 11
surrounds the hole.
[0005] The prepared sample 4 is placed in a specimen holder 3 which
extends through the wall of the TEM to hold the sample in the
required location. The end of the specimen holder in which the
sample is held is the specimen tip. This is generally an integral
part of the specimen holder 3. A variety of specimen tips are
available, examples of which are shown in FIGS. 3a, 3b and 3c. The
sample is generally held in a circular recess 12 in the specimen
tip, provided with a central aperture 13 intended to coincide with
the hole 10 and surrounding electron-transparent region 11 of the
sample 4. Often the tip is equipped with a thin mesh or grid which
provides the sample with further support. Finally, the sample is
held in place by a clip ring or similar mechanical fastening means.
Available holders include heating and cooling stages, electrical
stages which measure voltage and current in the specimen and
straining stages. Examples of such stages are described in U.S.
Pat. No. 5,225,683 which discloses various types of specimen tip
with conventional clamping and tilting means which can be
interchangeably mounted to a specimen holder.
[0006] Certain specialised techniques exist for mounting particular
types of sample. For example, GB-A-2121208 describes a technique
for freeze-drying and mounting cryosections. A pressure element is
used to locate the specimen and, due to the applied pressure, the
specimen becomes affixed to the mount during drying.
[0007] By combining a series of TEM images of a sample, it is
possible to create a three-dimensional model of the specimen. This
is termed "electron tomography" and is an important research tool
since many complex specimens cannot be adequately described by a
two-dimensional projection alone. For example, electron tomography
is often used to image small single particles, such as catalysts or
viruses.
[0008] In order to obtain the series of images required to form a
3-D model, the sample must be tilted through as large an angle as
possible. Conventional specimen holders as described above,
including those of U.S. Pat. No. 5,225,683, are bulky and, in the
confined space within a high-resolution pole piece, do not
generally allow the sample to be tilted by more than +/-40.degree..
It would be advantageous for the holder size to be reduced so that
the tilt range could be increased.
[0009] Furthermore, when tilting the sample it is important that
the area of interest on the sample does not move out of focus or
out of the viewing field. This is achieved by positioning the
sample such that the point of interest is at the same height as the
tilt axis. This is termed the "eucentric height". Conventionally
standard holders are designed such that a standard size and
thickness of sample will be approximately at the eucentric height
when placed in the TEM. The height of the specimen holder relative
to the TEM may be adjusted for focus in a goniometer which provides
movement of the holder in 3 directions (x, y, z) with respect to
the electron optics of the TEM. This is disclosed, for example, in
JP2001-068047. However, due to the variation in the thickness of
the sample, this technique does not guarantee that the sample is at
the eucentric point and it would be advantageous if the z-axis
height of the sample could be fine-tuned by the user.
[0010] U.S. Pat. No. 3,778,621 discloses a specimen tilting device
for an electron microscope. The device provides for tilting of a
mounted specimen about the X and Y axis, and lateral motion in the
X axis (parallel to the axis of the device). The specimen stage may
also be pivoted so as to move the specimen along an arcuate path in
the XY or YZ planes. However the device is of a complex and
delicate construction and does not provide the ability to fine-tune
the Z-axis position of the specimen without moving the specimen in
the XY plane also.
[0011] In accordance with a first aspect of the present invention,
a specimen tip holder assembly for mounting a specimen tip in a
transmission electron microscope comprises a tip holder for
supporting a specimen tip, the tip holder being coupled to an
elongate support for movement in a direction substantially
perpendicular to the axis of the support, and an actuator mounted
to the support for causing motion of the tip holder relative to the
support.
[0012] By forming the tip holder assembly from two parts and
providing actuating means, it is possible to displace the tip
holder relative to the support perpendicularly to the axis of the
support. This makes it possible to adjust the height of the
specimen in the TEM, allowing fine-tuning of the eucentric point.
In the description that follows, the term "specimen" ordinarily
comprises the sample to be investigated, although it may also
comprise an additional supporting object such as a mesh or grid, to
which the sample is mounted in use. The height adjustment is
incorporated into the specimen tip holder assembly and may be
manually controlled by the user. This in situ height adjustment
enables accurate and consistent setting of the specimen at the
eucentric height. This means that whatever the specimen thickness,
the height can be set to allow the same area of the specimen to
remain in view (even at high magnification) and generally in focus
whatever angle the holder is tilted to.
[0013] Preferably, the movement between the tip holder and the
elongate support is substantially rectilinear. The adjustment takes
place along a straight line in a single direction, avoiding any
displacement in the other axes. For example, if the movement is
substantially along the Z-axis (parallel to an optic axis of the
TEM), the specimen height may be adjusted without lateral movement
of the sample under examination. By having the Z-axis adjustment
integrated into the specimen tip holder assembly, it is possible to
set the microscope goniometer height to its optimum position for
eccentricity and still maintain the correct focus position for the
specimen. This also maximises the space available within the pole
piece gap for tilting the specimen using the microscope
goniometer.
[0014] Conveniently, the elongate support is provided with an
elongate aperture or cavity extending in the direction of movement
and the tip holder comprises a protrusion which slidably engages
with the elongate aperture or cavity. This ensures accurate linear
relative movement between the tip holder and elongate support. The
elongate aperture or cavity guides the motion of the tip holder.
Preferably, the elongate aperture or cavity extends parallel to an
optic axis of the TEM (in the Z-axis).
[0015] It would be possible to operate the actuator using an
internal mechanism. Preferably, however, the actuator is adapted to
be operated from a location remote from the tip holder.
[0016] A number of different arrangements for achieving this height
adjustment are conceivable, for example movement of the tip holder
could be powered by a motor, piezo-electric drive, thermal
expansion, lever or cantilever beam. Preferably, however, the
actuator comprises a rotatable camshaft extending along the
elongate support, arranged to engage a cam follower provided on a
slide forming part of or coupled to the tip holder, such that the
slide is caused to move in a direction substantially perpendicular
to the axis of the support as the camshaft is rotated. This
arrangement provides the operator with an easy and accurate way of
controlling the height of the specimen and does not contain any
complex parts which would require frequent maintenance.
Conveniently, the slide forms a protrusion on the tip holder which
slidably engages with an elongate aperture or cavity in the
elongate support.
[0017] Generally, the elongate support is a barrel and the actuator
extends within the barrel. This is a convenient arrangement,
providing support and protection for the actuating means and
resulting in a compact unit for fitting into a TEM.
[0018] Preferably, the barrel or other elongate support is further
provided with internal sealing means which prevent the passage of
gas between the camshaft and the barrel to maintain the vacuum in
the TEM cavity. Alternatively, sealing means may be provided
elsewhere in the tip holder assembly or left out of the design.
However, it is convenient to position sealing means in the barrel
where a good seal may be made between the camshaft and the barrel
itself. This prevents entry of gas into the TEM through the tip
holder assembly.
[0019] The tip holder may be integrally formed with a specimen tip.
However, it is preferable that the tip holder is adapted to
removably support a specimen tip. By making it possible to remove a
specimen tip from the tip holder, interchangeable specimen tips may
be used. Different styles of specimen tips may be chosen in
accordance with the sample to be investigated and the TEM
techniques to be employed. For example, many TEMs are equipped with
X-ray analysis apparatus and may be used to carry out techniques
such as energy dispersive spectroscopy (EDS). In such a case, it is
advantageous if the side of the specimen tip which faces the EDS
detector is cut back far enough not to be directly in the
detector's path. This means that the detector will only receive
signals from the sample material and mounting grid, should one be
employed.
[0020] Once the specimen is mounted onto a specimen tip, the
individual specimens can also be handled and stored as an assembly
with the interchangeable specimen tip. It is further preferable
that the specimen tip holder assembly further comprises a specimen
tip.
[0021] In order to removably support a specimen tip, the tip holder
could be provided with screw or clips with which to support the
specimen tip. However, the tip holder is preferably provided with
clamping means to removably support the specimen tip. There are
various ways in which the specimen tip may be clamped to the tip
holder, but preferably the tip holder comprises two members,
coupled by a pivot pin, and biassing means for urging the two
members towards one another, so as to clamp at least a portion of a
specimen tip between them. This straightforward arrangement is easy
to operate and will securely support the specimen tip in the
TEM.
[0022] According to a second aspect of the invention, a specimen
tip for supporting a specimen in a transmission electron microscope
is provided with a substantially planar surface onto which a
specimen is bonded, and a cut-out located underneath the specimen
through which electrons may pass. Effectively, the tip is shaped so
as to allow electrons to pass through the specimen without
obstruction by the specimen tip. By bonding the specimen onto the
specimen tip, it is possible to securely hold the specimen in place
without a need for complex mechanical clips or cages which add bulk
to the specimen holder. It is therefore possible to reduce the
dimensions of the specimen tip, thereby allowing the specimen to be
tilted through a greater angle in the confined space within the TEM
pole piece. This arrangement can achieve high tilt angles such as
+/-80.degree. of sample tilt in small, high resolution pole piece
gaps.
[0023] Robust samples may generally be mounted directly onto such a
specimen tip. However, more fragile samples may require further
support. The specimen in this case may therefore comprise a
mounting grid to which the sample is bonded. In use, the specimen
is then bonded to the planar surface of the specimen tip.
Effectively, the specimen (which may include a mounting grid and a
sample, or could comprise a sample alone) becomes an integral part
of the specimen tip.
[0024] Advantageously, the tip holder of the specimen tip holder
assembly supports a specimen tip as above described. The
combination of an assembly provided with in situ height adjustment
and reduced-size specimen tips provides a specimen holder
particularly well adapted for carrying out electron tomography in
an ultra high resolution TEM. The small specimen tips allow large
angles of sample tilt inside the TEM, and the movement of the tip
holder in a direction perpendicular to the axis of the barrel makes
it possible to accurately fine tune the height of the specimen so
that the point of interest is located at the eucentric height.
[0025] According to a further aspect of the invention, a
transmission electron microscope has an evacuated cavity into which
a specimen tip holder assembly according to the first aspect of the
invention extends. Preferably, the actuating means are operable
from outside of the evacuated cavity.
[0026] The invention also provides a method of mounting a specimen
to a specimen tip for supporting a specimen in a transmission
electron microscope, the method comprising the steps of:
[0027] a) applying adhesive to at least part of a planar surface on
a specimen tip and/or to at least part of a specimen;
[0028] b) positioning the specimen on the specimen tip at a
location such that the passage of electrons through an at least
partially electron-transparent portion of the specimen is not
disrupted by the specimen tip; and
[0029] c) bonding the positioned specimen to the specimen tip by
means of the adhesive.
[0030] This mounting procedure requires no mechanical fixing
components to be provided on the specimen tip. The size of the
specimen tip can therefore be reduced and the tilt angles
attainable inside the TEM increased. The specimen can also be
handled and stored attached to the specimen tip, which protects the
specimen and greatly increases its ease of handling. As previously
discussed, if the sample is sufficiently robust, the specimen may
comprise a sample alone. However, if the sample requires extra
support, the specimen may further comprise a mounting grid to which
the specimen is bonded. Therefore the method may further comprise
the step of bonding the sample to the mounting grid prior to step
(a).
[0031] At this stage the specimen is ready for use in the TEM.
However, the size of the assembly may be further reduced by
trimming the side portions of the sample and/or mounting grid. This
may be performed either before or after mounting the specimen to
the specimen tip. Therefore, the method may further comprise the
step of removing at least some of the area of the specimen which
overhangs or will overhang the specimen tip. Alternatively
physically smaller specimens may be used. Each of the above may
provide for improved angles of tilt for the specimen when mounted
to the tip.
[0032] After analysis in the TEM, if desired, the specimen can be
removed from the specimen tip by mechanical means or by soaking the
tip in a suitable solvent in which the bonding agent will
dissolve.
[0033] An example of a specimen holder assembly in accordance with
the present invention incorporating a specimen tip in accordance
with a second aspect of the present invention will now be described
and contrasted with known specimen holders with reference to the
accompanying drawings, in which:
[0034] FIG. 1 is a schematic diagram illustrating the workings of a
conventional transmission electron microscope;
[0035] FIG. 2a is a plan view of a typical specimen prepared for
analysis in a TEM;
[0036] FIG. 2b is a cross section along a line X of the specimen
shown in FIG. 2a;
[0037] FIGS. 3a, 3b and 3c illustrate the end portions of a variety
of conventional specimen holders;
[0038] FIG. 4 is a perspective view of an example of a specimen tip
holder assembly in accordance with the present invention,
supporting an example of a specimen tip in accordance with a second
aspect of the invention;
[0039] FIG. 5 is a plan view of the specimen tip holder assembly
and specimen tip shown in FIG. 4;
[0040] FIG. 6 is a side view of the specimen tip holder assembly
and specimen tip shown in FIGS. 4 and 5;
[0041] FIG. 7a is an enlarged portion of FIG. 5;
[0042] FIG. 7b is a cross section of a portion of the specimen tip
holder assembly and specimen tip illustrated in FIGS. 4 to 6, along
the line Y shown in FIG. 7a;
[0043] FIG. 8a is a perspective view of an example of a specimen
tip in accordance with a second aspect of the present invention,
with the specimen removed for clarity;
[0044] FIG. 8b is a perspective view of a second example of a
specimen tip in accordance with a second aspect of the invention,
with the specimen removed for clarity;
[0045] FIG. 9a is a perspective view of a specimen tip with a
specimen attached, and its sides trimmed; and
[0046] FIG. 9b is a perspective view of a specimen tip with a
mounting grid attached and its sides trimmed.
[0047] A specimen tip holder assembly for mounting a specimen tip
in a transmission electron microscope (TEM) is illustrated in FIGS.
4, 5 and 6. The assembly 20 consists of a barrel 21, long enough to
extend from the exterior of a TEM to the pole piece gap where the
sample is positioned in use, and a tip holder 22 coupled to one end
of the barrel 21. The tip holder 22 is adapted to support a
specimen tip 23. In the Figures, the specimen tip is shown as a
separate component, but it is envisaged that the specimen tip 23
could form an integral part of tip holder 22 if so desired. At its
other end, the barrel is equipped with a housing 24 designed to fit
against the side of a TEM, and a dial 25, discussed below. The
assembly 20 is provided with polymeric sealing rings 26 which are
fitted around the barrel in such a way that, when placed in a TEM,
the passage of gas between the barrel and the TEM is prevented.
This enables the TEM to maintain the high vacuum required for
successful operation.
[0048] The tip holder 22 and the barrel 21 are coupled together in
such a way that relative movement between the two components is
possible in a direction perpendicular to the axis of the barrel.
This arrangement is shown in FIG. 7b. Inside the barrel 21, there
is a chamber 35 located adjacent to the end of the barrel 21
supporting tip holder 22 and defined by the inner walls of barrel
21, slide retainer 37a and end plate 37b. The chamber or cavity 35
is typically elongate in the direction of movement. The end plate
37b is welded, or otherwise attached, to the inside wall of the
barrel 21, and is provided with a central aperture through which a
rear portion of the tip holder 22 passes. The aperture is
conveniently round but could be elongate in the direction of
movement. This has the added benefit of preventing any accidental
pivotal motion of the tip holder. The aperture is larger than the
portion of the tip holder passing therethrough, to allow relative
displacement of the tip holder. A slide 31 is disposed within
chamber 35 in such a way that it is able to move up and down along
a direction A. By re-orientating the slide and chamber arrangement
(and aperture if necessary), it would be possible to move the slide
back and forth in any direction perpendicular to the axis of the
barrel 21.
[0049] The tip holder 22 is fixed to the slide 31 by means of a
screw 34. This allows the tip holder to be easily removed for
maintenance, replacement or storage. Other fixing methods could
also be used, or alternatively the slide 31 could form an integral
part of tip holder 22. The slide 31 and the portion of the tip
holder 22 to which it is attached effectively form a protrusion on
the tip holder 22 which slidably engages the aperture or cavity in
the barrel 21.
[0050] A spring 33, or other means of imparting a biassing force to
the slide 31 is disposed between the slide 31 and the base of the
chamber 35. The slide 31 is provided with a cam follower 32 which
abuts a cam shaft 36. The spring 33 forces the cam follower 32
against the cam shaft 36.
[0051] Cam shaft 36 is rotatably mounted within barrel 21 and
extends from the slide 31 through the barrel 21 to the far end
where the cam shaft 36 is coupled with dial 25. To enable rotation
of camshaft 36, it is mounted in bearings. In this example, bushes
(not shown) are used, but roller or alternative types of bearings
may be employed. At its end 38, cam shaft 36 has an eccentric
diameter, defining a cam, which contacts cam follower 32 on slide
31. Internal sealing rings 39 are positioned between the cam shaft
36 and the barrel 21 to prevent gas entering the TEM through the
specimen holder.
[0052] On rotation of the cam shaft 36, slide 31 and tip holder 22
translate perpendicularly to the long axis of the barrel. The
specimen 4 essentially follows a substantially rectilinear path in
the Z-axis (substantially parallel to the optic axis A-A of the
TEM). This avoids any displacement in the other axes so that the
Z-height of the specimen 4 may be adjusted (and the beam focussed)
without lateral movement of the specimen. The spring 33 ensures
that the cam follower 32 and the cam (eccentric diameter) end 38 of
the cam shaft 36 stay in contact with each other throughout the
range of movement. The cam shaft may be rotated by turning dial 25
at the far end of the holder assembly 20. In this example, rotating
the dial clockwise raises the slide 31, tip holder 22 and the
specimen tip 23. Rotating the dial in a counter-clockwise direction
lowers the components. Of course, the assembly could be arranged so
that the directions are reversed. An operator can therefore adjust
the z-axis position of a specimen mounted in the TEM by rotating
the dial 25 at the rear of the holder assembly, outside of the
microscope. Thus, the eucentric height may be fine-tuned without
having to move the whole holder assembly 20 or having to re-focus
the electron beam.
[0053] To ensure that the mechanism does not exceed its lowest or
highest points of contact at minimum and maximum adjustment, there
is a slot, at an angle such as 180.degree., machined into the dial
face which mates with a pin (not shown) that protrudes from the
housing 24. This ensures that the eccentric diameter is at its
mid-point of contact at zero adjustment and that the cam shaft 36
cannot rotate through more than a certain angle, such as
+/-90.degree.. To aid the operators, and to improve the
reproducibility of the specimen height setting, there is also a set
of index marks 27 on the dial 25 and housing 24.
[0054] In one example, the total adjustment range is 760 microns
along the z-axis. The slide travel is therefore limited to +/-380
microns. The adjustment range can of course be increased or
decreased by varying the dimensions of the cam shaft 36 and the
slide 31.
[0055] In this example, the tip holder is arranged such that the
specimen tip 23 may be removed and replaced. The tip holder 22 is
equipped with a movable clamping arm 28, coupled to the main body
of the tip holder 22 via a pivot pin 29 and spring 30. The specimen
tip 23 is retained in the tip holder 22 by sliding it into a hole
in the end of the tip holder 22 and placing it between the clamping
arm 28 and the main part of the tip holder 22.
[0056] The pin 29 extends across the full width of the tip holder
22 and retains the clamping arm 28 in a slot in the tip holder 22,
permitting it to pivot. The spring 30 or alternative biassing means
puts pressure on one end of the clamping arm 28 so that equal
pressure is exerted on a flat surface machined onto the bottom of
the specimen tip 23. The specimen tip 23 is then constrained from
unwanted linear movement by the fit of the specimen tip 23 into the
hole in the tip holder 22, the rear face of specimen 23 contacting
a stop in the tip holder 22 and being held under spring pressure
from the clamping arm 28. The specimen tip 23 is constrained from
angular movement by the contacting flat surfaces of the specimen
tip 23 and the clamping arm 28. An operator may release the
clamping mechanism by applying slight pressure, for example with
the end of a pair of tweezers, to a circular recessed area 28a on
the spring end of the clamping arm 28. This pressure compresses the
spring 30 and pivots the clamping arm 28, releasing pressure on the
specimen tip 23.
[0057] Since the specimen tip 23 may be removed from the tip holder
22, it is possible to use interchangeable specimen tips. Different
styles of specimen tip may be selected for different types of
specimen material or different TEM techniques, as will be discussed
in detail below.
[0058] FIGS. 8a and 8b show two examples of specimen tips which may
be used in conjunction with the tip holder assembly described above
to support a specimen in a transmission electron microscope. Each
specimen tip 23a or 23b consists of a length of rigid material with
a thin planar surface 40 at one end and a flat edge 42 adjacent to
the other. The tip material may be selected according to its
intended application. For example, titanium would be adequate for
most applications but for X-ray microanalysis, beryllium is
preferable. The planar area may be provided with a cut-out such as
41a or 41b or may be otherwise shaped so as to allow passage of
electrons through the specimen 4 unimpeded by the tip 23. The
specimen tips 23a and 23b shown respectively in FIGS. 8a and 8b are
illustrated without the specimen attached so as to clearly show the
structure of the supporting tip itself.
[0059] A standard prepared TEM specimen 4, such as that shown in
FIG. 2, is positioned on the planar surface 40, so that the hole 10
and electron transparent region 11 of the specimen 4 are located
above cut-out 41a or 41b. The completed assembly is shown in FIG.
9a in which the edges of the specimen 4 have been trimmed for
reasons which will be discussed below. This step is however
optional. By carefully positioning the specimens 4 as shown, in use
the electron beam transmitted through the specimen will not be
intercepted by the specimen tip 23. The specimen 4 is bonded into
position on the specimen tip 23 by means of an adhesive or other
bonding agent.
[0060] The flat surface 42 at the rear end of the specimen tip can
be used to help mount the specimen tip 23 in a tip holder such as
that described above.
[0061] By bonding the specimen 4 to the specimen tip 23 rather than
using clips or other mechanical fixing means, the size of the
specimen tip may be significantly reduced. Indeed, the need for any
sort of specimen clamping or locating mechanism is eliminated thus
allowing a much thinner tip profile to fit between the narrow gap
of the TEM pole pieces. In the example shown, the specimen tip 23
is 1.5 mm in diameter. Unlike conventional specimen holders, it is
possible that the specimen tip 23 may be smaller than the specimen
4 mounted to it (typically 3 mm diameter), since no allowance need
be made for mechanical fixing apparatus. The reduced-sized specimen
tip makes it possible to significantly increase the specimen tilt
angles attainable in the TEM.
[0062] The specimen 4 in the form of a sample to be investigated,
may be mounted directly to a specimen tip 23 as shown in FIG. 9a.
However, the required specimen dimensions often result in a sample
being fragile and requiring extra support on a specimen tip.
Therefore a mounting grid 9 may be used (to which the sample is
bonded). This can then be bonded to the surface 40 of the tip 23.
FIG. 9b shows a mounting grid 9 and sample (not shown) bonded to a
specimen tip, trimmed as per FIG. 9a.
[0063] If desired, the size of the specimen tip assembly may be
further reduced by trimming the left and right edges of the
specimen sample or grid such that they are flush to the sides of
the tip as depicted in FIGS. 9a and 9b. This can be carried out
before or after the specimen is bonded to the specimen is bonded to
the specimen tip. Alternatively the sample could be prepared to a
reduced size (e.g. a 1.5 mm diameter disk) before attachment. Such
an arrangement can achieve +/-80% of sample tilt in small,
high-resolution pole piece gaps. The increased tilt range enables
TEM images of the specimen to be obtained over a very wide range of
angles and is of particular use when carrying out techniques such
as electron tomography.
[0064] Once bonded, the individual specimens can be handled and
stored as an assembly with the interchangeable specimen tip 23. By
bonding the specimen to the specimen tip 23, the tip 23 effectively
becomes the specimen and is much easier to handle, being much more
rigid than the specimen 4 alone. This protects delicate specimens
and also improves reproducibility of results obtained in the TEM
since it allows the specimen to be accurately repositioned each
time examination is carried out. If desired, the specimen can be
removed by mechanical means or by soaking the tip in a suitable
solvent for the bonding agent.
[0065] Various styles of specimen tip 23 may be used with the tip
holder assembly described above. In FIG. 8, a J-shaped tip 23a and
a C-shaped tip 23b are illustrated. Both tips are machined to
facilitate energy dispersive spectroscopy (EDS) analysis. The area
of the specimen tip which when in use would face the EDS detector
is removed so as not to be directly in the detector's path. The EDS
detector will then only receive signals from the sample and grid
materials and will not be contaminated with signals from the
specimen tip itself.
[0066] The C-shaped tip 23b should be compatible with most mounting
grids and specimens. The J-shaped tip 23a offers some more support
for thin mounting grids and fragile specimens. Other tip designs,
to suit specific types, shapes and sizes of specimen are envisaged,
including a full-circle aperture for use with particularly thin
specimens or in situations where EDS analysis is not to be carried
out. The interchangeable specimen tips 23 may be reusable or
disposable tailored to the specimen type in use.
* * * * *