High temperature specimen stage and detector for an environmental scanning electron microscope

An environmental scanning electron microscope is provided which is capable of maintaining a specimen at a temperature up to approximately 1500.degree. C. In this environmental scanning electron microscope, a specimen chamber maintains the specimen enveloped in gas in registration with a final pressure limiting aperture of the objective lens assembly. The specimen chamber includes a specimen stage having a sample platform for supporting the specimen under examination at a first vertical height and a specimen heating assembly which includes a non-inductively wound heater coil which is positioned closely adjacent to the sample platform and extends to a second vertical height which is significantly above the first vertical height so that the top of the specimen is maintained at a high temperature. In this environmental scanning electron microscope, a heat shield assembly is positioned above the specimen heating assembly in the specimen chamber to avoid radiant heat loss and which has an adjustable bias voltage applied thereto to accelerate secondary electrons through its central opening to be collected by an electron detector. Moreover, in order to enhance image quality, the final pressure limiting aperture and the electron detector are biased at different voltages with the bias applied to the final pressure limiting aperture floated to provide for automatic compensation. A specimen stage moving assembly is also provided to move the specimen stage with respect to the heat shield assembly to enhance the field-of-view of the specimen.

FIELD OF THE INVENTION 
This invention relates to the field of environmental scanning electron 
microscopes ("ESEM"), and more particularly, to a hot temperature stage 
for an environmental scanning electron microscope which is capable of 
heating a specimen up to a temperature of approximately 1500.degree. C., 
and a detector which allows imaging of the specimen at high temperatures. 
BACKGROUND OF THE INVENTION 
As background, the advantages of a environmental scanning electron 
microscope over the standard scanning electron microscope ("SEM") lie in 
its ability to produce high-resolution electron images of moist or 
nonconductive specimens (e.g., biological materials, plastics, ceramics, 
fibers) which are extremely difficult to image in the usual vacuum 
environment of the SEM. The environmental scanning electron microscope 
allows the specimen to be maintained in its "natural" state, without 
subjecting it to the distortions caused by drying, freezing, or vacuum 
coating normally required for high-vacuum electron beam observation. Also, 
the relatively high gas pressure easily tolerated in the ESEM specimen 
chamber acts effectively to dissipate the surface charge that would 
normally build up on a nonconductive specimen, blocking high quality image 
acquisition. The ESEM also permits direct, real-time observation of liquid 
transport, chemical reaction, solution, hydration, crystallization, and 
other processes occurring at relatively high vapor pressures, far above 
those that can be permitted in the normal SEM specimen chamber. 
Typically, in an ESEM, the electron beam is emitted by an electron gun and 
passes through an electron optical column of an objective lens assembly 
having a final pressure limiting aperture at its lower end thereof. In the 
electron optical column, the electron beam passes through magnetic lenses 
which are used to focus the beam and direct the electron beam through the 
final pressure limiting aperture. 
The beam is subsequently directed into a specimen chamber through the final 
pressure limiting aperture wherein it impinges upon a specimen supported 
upon a specimen stage. The specimen stage is positioned for supporting the 
specimen approximately 1 to 10 mm below the final pressure limiting 
aperture so as to allow the beam of electrons to interact with the 
specimen. The specimen chamber is disposed below the optical vacuum column 
and is capable of maintaining the sample enveloped in gas, preferably 
nitrogen or water vapor, at a pressure of approximately between 10.sup.-2 
and 50 Torr in registration with the final pressure limiting aperture such 
that a surface of the specimen may be exposed to the charged particle beam 
emitted from the electron gun and directed through the final pressure 
limiting aperture. 
The typical specimen stage previously used in an environmental scanning 
electron microscope to image samples at high temperatures is illustrated 
in FIG. 1. In that prior specimen stage, the electron beam 1 strikes a 
surface of the specimen 2 which is supported in a recess 3 of a sample cup 
4 which is supported in a depression 5 of an insulating jacket 6. An 
annular heating assembly 7 is provided in the depression 5 between the 
insulating jacket 6 and the sample cup 4, which is best shown in FIG. 2. 
This heater assembly 7 is formed of an annular toroidal ceramic former 8 
having a heater wire 9 wound therearound such that the heater wire 9 is 
provided remote from the sample 2 and does not extend above the sample. In 
the prior specimen stage of FIG. 1, a heat shield 9a is positioned on top 
of the sample cup 4, the insulated jacket 6, and the heater assembly 7 
which has a central opening 9b such that the electron beam can pass 
therethrough and strike the specimen 2. 
For the reasons set forth above and below, the prior specimen stage for an 
environmental scanning electron microscope shown in FIGS. 1 and 2 has 
disadvantageously been unable to achieve sample temperatures of at least 
1500.degree. C. First, the heater wire 9 cannot be made of tungsten 
because it will oxidize in the water vapor of the environmental scanning 
electron microscope. Hence, platinum wire must be used, but platinum melts 
at 1700.degree. C. Second, the heater is provided too remote from the 
sample and is significantly hotter than the sample. Finally, heat is 
radiated and convected from the surface of the sample as is represented by 
arrow a in FIG. 1 so that the bottom of the sample is hotter than the top, 
but the user is observing the top of the sample and the top surface cannot 
then be maintained at temperatures of at least 1500.degree. C. due to this 
radiant heat loss. Hence, utilizing the toroidal heater of FIGS. 1 and 2 
in an environmental scanning electron microscope failed to achieve a 
sample temperature of 1500.degree. C. 
Objects of the Invention 
Therefore, it is an object of the present invention to provide a hot 
temperature specimen stage for an environmental scanning electron 
microscope which avoids the aforementioned deficiencies of the prior art. 
It is also an object of this invention to provide a hot temperature 
specimen stage for an environmental scanning electron microscope which 
achieves a sample temperature of at least approximately 1500.degree. C. 
It is a further object of this invention to provide a hot temperature 
specimen stage for an environmental scanning electron microscope wherein 
the sample cup is made of a high thermally conductive material to thereby 
minimize the temperature differential between the top and bottom of the 
sample cup. 
It is another object of this invention to provide a high temperature 
specimen stage for an environmental scanning electron microscope wherein 
the heater is wound as a coil so that the entirety of the wire is 
positioned close to the sample cup. 
It is yet another object of this invention to provide a high temperature 
specimen stage for an environmental scanning electron microscope wherein 
the heater assembly extends well above the sample so as to reduce the heat 
lost from the sample surface. 
It is still a further object of this invention to provide a high 
temperature stage for an environmental scanning electron microscope 
wherein the heater is in the form of a wound coil to minimize the magnetic 
field from the heating current which otherwise would cause deflection of 
the primary electron beam. 
Various other objects, advantages and features of the present invention 
will become readily apparent from the ensuing detailed description and the 
novel features will be particularly pointed out in the appended claims. 
SUMMARY OF THE INVENTION 
This invention relates to a hot temperature specimen stage for an 
environmental scanning electron microscope which is capable of achieving a 
sample temperature of up to at least approximately 1500.degree. C. In the 
environmental scanning electron microscope in which this hot temperature 
specimen stage is employed, an electron beam is generated by an electron 
gun which passes through an electron optical column until the electron 
beam is focused and scanned across the diameter of a final pressure 
limiting aperture provided at the lower end of the electron optical 
column. The final pressure limiting aperture separates the relatively high 
vacuum of the electron optical column from the relatively low vacuum of 
the specimen chamber. 
The specimen chamber is positioned below the electron optical column and is 
capable of maintaining the specimen enveloped in gas in registration with 
the final pressure limiting aperture such that a surface of the specimen 
may be exposed to the focused beam of electrons. A specimen stage is 
located within the specimen chamber and is positioned for supporting the 
specimen approximately 1 to 25 mm below the final pressure limiting 
aperture so as to allow the focused beam of electrons to interact with the 
specimen. In the specimen chamber, the specimen is maintained at a 
pressure between about 10.sup.-2 and 50 Torr, and preferably approximately 
10 Torr. 
In the high temperature specimen stage of the environmental scanning 
electron microscope of this invention, a sample platform is provided for 
supporting the specimen. The sample platform is in the form of a loose, 
disposable, sample cup made of a high thermally conductive material which 
is contained within a recess of an insulating jacket. The specimen is 
supported in a depression of the sample cup at a first vertical height. 
In this specimen stage, a specimen heater assembly heats the specimen and 
includes a non-inductively wound heater wire coil which is positioned 
closely adjacent to the sample cup and extends to a second vertical height 
of the specimen which is significantly above the first vertical height of 
the specimen so that the top surface of the specimen is maintained at a 
high temperature. 
In order to further avoid radiant heat loss, a heat shield assembly is 
provided in the specimen chamber above the specimen heating assembly. This 
heat shield assembly includes a central opening to permit the electron 
beam to pass therethrough and strike the sample. In addition, the heat 
shield assembly includes a plurality of thin ceramic insulating shields 
which act as heat reflectors and a plurality of perforated metal support 
plates integrally found therewith to avoid warping of the heat shield 
assembly at high temperatures. 
Further, the heat shield assembly is mounted separately from the specimen 
heater assembly which allows for a larger field of view. 
Accordingly, the design of this hot temperature specimen stage for an 
environmental scanning electron microscope achieves the following 
advantages over the previous specimen stage of FIG. 1: 
1. the sample cup is made of high thermally conductive material to minimize 
the temperature differential between the top and bottom of the sample cup; 
2. the heater assembly is formed as a non-inductively wound heater coil to 
minimize the magnetic field from the heating element which would otherwise 
cause a deflection of the primary electron beam which thereby allows the 
heater wire to be positioned close to the sample cup; and 
3. the heater coil extends significantly above the sample so that the heat 
lost from the top of sample is reduced. 
Further, an adjustable bias voltage of up to approximately 500V can be 
applied to the heat shield assembly to accelerate secondary electrons 
emanating from the surface of the specimen to pass through the central 
opening of the heat shield assembly to be collected by an electron 
detector assembly. In one embodiment, the electron detector assembly can 
be in the form of a thin ring electrode, and in order to enhance image 
quality, the final pressure limiting aperture can be biased at a different 
voltage than the bias applied to the thin ring electrode. In addition, the 
bias voltage applied to the final pressure limiting aperture can float to 
provide for automatic compensation. In order to suppress thermal 
electrons, a bias voltage of between approximately +50 and -50V can be 
applied to the sample cup. 
In this environmental scanning electron microscope, the heat shield 
assembly is aligned with respect to the final pressure limiting aperture 
and the electron detector and is separated from the specimen stage. Thus, 
the present invention provides a specimen stage moving assembly for 
laterally moving the specimen stage independently of the heat shield 
assembly to enhance the field-of-view of the specimen.

DETAILED DESCRIPTION OF CERTAIN PREFERRED EMBODIMENTS 
Referring now to FIGS. 3 and 4, the prior environmental scanning electron 
microscope of U.S. Pat. Nos. 5,362,964 and 5,412,211 is illustrated, the 
subject matter of which are incorporated by reference. In this 
environmental scanning electron microscope, a device for generating, 
amplifying and detecting secondary and backscattered electrons emanating 
from a surface of a sample being examined is provided. More specifically, 
a beam of electrons 12 is emitted through an electron optical column 10 
and the objective lens assembly 11 by an electron gun (not shown). The 
electron optical column 10 includes a final pressure limiting aperture 14 
at its lower end thereof. The final pressure limiting aperture 14 is 
formed within the lower end of an aperture carrier 15. This aperture 
carrier 15 is discussed in U.S. Pat. No. 4,823,006, the subject matter of 
which is incorporated by reference. This aperture carrier includes a 
second pressure limiting aperture 17 positioned above the final pressure 
limiting aperture 14 which communicates directly with the electron optical 
column 10. The electron beam passes through magnetic lenses 16 and 18 
which are used to control the intensity of the electron beam. Focusing 
means 20 locating within the objective lens assembly 11 adjacent to the 
vacuum column is capable of directing the beam of electrons through the 
final pressure limiting aperture 14. 
In the prior ESEM construction of FIG. 3, the beam is subsequently directed 
into a specimen chamber 22 through final pressure limiting aperture 14 
wherein it impinges upon a specimen 24 supported on a specimen stage. The 
specimen mount or stage 26 is located within the specimen chamber 22 and 
is positioned for supporting specimen 24 approximately 1 to 25 mm, and 
preferably 1 to 10 mm, below final pressure limiting aperture 14 so as to 
allow the beam of electrons to interact with the specimen. The specimen 
chamber is disposed below the electron optical column 10 and is capable of 
maintaining the sample 24 enveloped in gas, preferably nitrogen or water 
vapor, at a pressure of approximately between 10.sup.-2 to 50 Torr in 
registration with the final pressure limiting aperture such that a surface 
of the specimen may be exposed to the charged particle beam emitted from 
the electron gun and directed through the pressure limiting aperture 14. 
A preferred embodiment of a high temperature specimen stage 50 for an 
environmental scanning electron microscope of the present invention which 
achieves a sample temperature of at least approximately 1500.degree. C. is 
shown in FIG. 5. As will be explained in more detail below, this high 
temperature specimen stage uses the same principle as a standard furnace 
used to heat samples to high temperatures--the sample is surrounded by a 
furnace at the high temperature. 
As is shown in FIG. 5, in the high temperature specimen stage 50 of the 
present invention, the specimen 24 is supported on a sample platform 49. 
The sample platform includes a loose, disposable sample cup 52 which is 
contained within a recess 53 of an insulating jacket 54. The specimen 24 
is supported in a depression 51 formed in the top surface of the sample 
cup 52 at a first vertical height in the recess 53, designated by 
reference character A in FIG. 5. In the preferred embodiment, the sample 
cup 52 is made of a high thermally conductive material, preferably 
magnesium oxide. As shown in FIG. 5, the annular upper edge 55 of the 
insulating jacket 54 extends to a height substantially above the first 
vertical height A of the sample 24 supported in the sample cup 52. 
In the specimen stage 50, a specimen heating assembly 56 is supported in 
the recess 53 of the insulating jacket 54 which heats the specimen under 
examination. The specimen heater assembly 56 is positioned immediately 
adjacent to the sample cup 52 and extends to a height which is generally 
contiguous with the upper edge 55 of the insulating jacket 54. As shown in 
FIGS. 5 and 6, the specimen heater assembly 56 includes a non-inductively 
wound heater coil 58 which extends the entire length of the heater 
assembly 56 to a second vertical height in the recess 53, designated by 
reference character B in FIG. 5, which extends significantly above the 
first vertical height A of the sample. As a result of this design, the top 
surface of the specimen is maintained at a high temperature. 
The specimen heater assembly 56 further includes a ceramic coating 60 (see 
FIG. 5) covering the heater wire 58 to hold the shape of the heater wire 
58. In addition, in order to further insulate the sample platform, avoid 
further radiant heat loss and support the heater wire 58, a ceramic tube 
62 is positioned in the recess 53 of the insulating jacket 54 between the 
heater assembly 56 and the upstanding edge 57 of the insulating jacket 54. 
As is shown in FIG. 5, the sample cup 52, the heater assembly 56, and the 
ceramic tube 62 are supported within the recess 53 of the insulating 
jacket 54 by means of an insulated support stand 64 which is attached 
preferably by cement to the ceramic tube 62. Moreover, the ceramic coating 
60 of the heater assembly 56, the ceramic tube 62, the support stand 64, 
and the insulating jacket 54 are preferably made of aluminum oxide to 
thereby better direct heat to the sample cup 52. 
In order to avoid radiant heat loss upwardly from the specimen heater 
assembly 56, a heat shield assembly 68 is placed above the sample cup 52, 
heater assembly 56, ceramic tube 62 and the insulating jacket 54. As is 
shown in FIG. 5, the heat shield assembly 68 includes a central opening 74 
such that the electron beam 12 can pass therethrough and strike the 
specimen 24 supported on the sample cup 52. The heat shield assembly 68 
further includes a plurality of thin ceramic insulating shields, such as 
70a, 70b, and 70c, which act as heat reflectors, and a plurality of 
perforated metal support plates, such as 72a and 72b, integrally formed 
therewith to avoid warping of the heat shield assembly at high 
temperatures. In the preferred embodiment, the ceramic insulating shields 
70a-c are formed of alumina paper and the metal support plates 72a-b are 
formed of perforated stainless steel. The heat shield assembly 68 
including the thin ceramic insulating shields 70a-c and the perforated 
metal support plates 72a-b is supported in its proper position by means of 
an annular support ring 76. 
Therefore, the design of this hot temperature specimen stage for an ESEM 
achieves the following advantages over the prior ESEM specimen stage of 
FIGS. 1 and 2; namely: 
1. Since the sample cup 52 is made of a high thermally conductive material, 
the temperature differential between the top and bottom of the sample cup 
52 is minimized; 
2. Since the heater wire 58 is wound as a coil, the entirety of the heater 
wire is positioned close to the sample cup 52 in comparison to the prior 
torroidal heater assembly of FIGS. 1 and 2; 
3. Since the heater assembly 56 with the heater coil 58 extends 
significantly above the sample 24, the top of the sample 24 is irradiated 
by the heater coil 58 in order to maintain the sample at a high 
temperature. 
4. Since the heater wire 58 is wound non-inductively, the magnetic field 
from the heating current is minimized which otherwise would cause 
deflection of the primary beam. 
Accordingly, as a result of these advantageous features, the hot 
temperature specimen stage of the present invention can achieve a sample 
temperature of at least approximately 1500.degree. C. 
As aforementioned, the hot temperature specimen stage discussed above 
advantageously achieves an increased sample temperature and minimizes the 
temperature difference between the sample surface and the heater coil 56. 
However, it has been found that the foregoing hot temperature specimen 
stage creates an electrostatic shield around the specimen which causes 
inherent difficulties in extracting the electron signal through the 
central opening 74 of the heat shield assembly 68. 
In order to alleviate this inherent problem, as is schematically shown in 
FIG. 7, an adjustable bias voltage 80 is applied to the heat shield 
assembly 68 to thereby accelerate the secondary electrons emanating from 
the surface of the specimen 24 to pass through the central opening 74 of 
the heat shield assembly and be subsequently collected by final pressure 
limiting aperture detector 50. Preferably, the shield bias may be positive 
or negative depending upon the temperature and the bias applied to the 
detector 50 represented by reference numeral 84 in FIG. 7. Voltages up to 
500V may be required for suitable acceleration of the secondary electrons 
such that they are collected by the secondary electron detector 50. 
Referring again to FIG. 3, a ring detector 28 was provided in the specimen 
chamber of the environmental scanning electron microscope of U.S. Pat. 
Nos. 5,362,964 and 5,412,211 between the final pressure limiting aperture 
14 and the specimen 24. This ring electrode detector is disclosed as being 
preferably formed of a thin ring, made of metal, and having a wire 
thickness of approximately 50 to 1,000 microns. According to the '964 and 
'211 patents, the diameter of the ring detector 28 is slightly larger than 
the diameter of the final pressure limiting aperture 14 and is separated 
therefrom. 
When the primary beam 12 strikes the specimen 24, as in FIG. 3, secondary 
electrons 35 and backscattered electrons, such as 37, 38 and 39, are 
released from the sample. For purposes of illustration in the prior ESEM 
of FIG. 3, the '964 and '211 patents discussed that the bias voltage was 
to be applied to the ring electrode 28 of approximately +500V. The bullet 
detector 30 which forms the final pressure limiting aperture 14 is 
unbiased. In this configuration, high positive voltage on the ring 
electrode 28 causes the secondary electrons 35 emanating from the surface 
of the sample to be accelerated until they strike gas molecules of the 
gaseous environment in the specimen chamber 22. Multiple collisions with 
the gaseous environment cause other electrons to be released which are, as 
well, accelerated towards the ring electrode 28. Some examples of 
signal-gas interactions which are described in U.S. Pat. No. 4,992,662 to 
Danilatos and specifically incorporated herein are: gaseous scintillation, 
ionization, chemical combination, chemical disassociation, electron 
attachment, photo-ionization, X-ray reactions, rotational and vibrational 
collisions, collisions characterized by a particular energy loss, etc. 
According to the '964 and '211 patents, there will generally be many such 
collisions and eventually a cloud of hundreds or thousands of electrons 
will reach the ring electrode 28. The main objective, however, of the ring 
electrode 28 is to collect the electrons triggered by secondary electrons 
emanating from the specimen 24. 
However, as illustrated in FIG. 3, secondary electrons are also generated 
by gas collisions from other sources; namely: 
(a) collisions between the primary beam 12 and the gaseous environment of 
the specimen chamber, these secondary electrons being represented by 
reference numeral 43 in FIG. 3; 
(b) collisions between the backscattered electrons 37 that pass through the 
pressure limited aperture 14 and the gaseous environment of the specimen 
chamber 22, these secondary electrons being represented by reference 
numeral 45; 
(c) collisions between the backscattered electrons 38 which pass through 
the gaseous environment between the sample 24 and the remainder of the 
specimen chamber, these secondary electrons being represented by reference 
number 47; and 
(d) backscattered electrons 39 which strike the pressure limited aperture 
14, and generate secondary electrons which are referred to by reference 
numeral 49. 
All of the secondary electrons generated by these collisions are amplified 
by gas multiplication and the gaseous environment of the specimen chamber 
and add to the desired secondary electron signal. However, the secondary 
electrons that derive from the backscattered electrons, such as 43, 45, 47 
and 49, add an undesired backscattered component to the secondary electron 
image being received by the ring detector 28. Furthermore, the secondary 
electrons 43 created by collisions between the primary beam 12 and the 
gaseous environment of the specimen chamber cause an undesired background 
noise component. 
Thus, in order to enhance its signal capabilities, the environmental 
scanning electron microscope disclosed in U.S. Pat. Nos. 5,362,964 and 
5,412,211 incorporated an improved secondary electron detector which 
reduced the backscattered electron component of the signal, such as 
signals 43, 45, 47 and 49 present in the FIG. 3 example, and reduced the 
signal noise produced by the primary beam, such as signal 43. In the 
embodiment of the environmental scanning electron microscope of U.S. Pat. 
Nos. 5,362,964 and 5,412,211 shown in FIG. 4, the ring electrode 28 is 
biased at an electrical potential between approximately 200 and 2,000 
volts, and preferably 500 volts. Additionally, a pressure limiting 
aperture electrode 59 is formed integrally with the bullet detector 
defining the final pressure limiting aperture and is biased at an 
electrical potential between 200 and 2,000 volts and preferably 500 volts. 
In the ESEM of U.S. Pat. Nos. 5,362,964 and 5,412,211, the ring electrode 
28 and the pressure limiting aperture electrode 50 are preferably biased 
at the same electrical potential. 
As an example of the effect of the ESEM design of FIG. 4, if the ring 
electrode 28 and the final pressure limiting aperture electrode 50 are 
both biased at 500 volts, the desired secondary electrons 35 are 
accelerated and multiplied in the gaseous environment of the specimen 
chamber 22 to generate further secondary electrons 36 which are still 
collected by the ring electrode 28. However, in this configuration, most 
of the undesired secondary electrons are intercepted by the final pressure 
limiting aperture electrode 59. More specifically, the secondary electrons 
45 generated by collisions with the backscattered electrons 37 are 
attracted to the positive surface of the pressure limiting aperture 
electrode 59. Further, many of the secondary electrons 43 generated by 
collisions between the primary beam 12 and the gaseous environment of the 
specimen chamber 22 are also attracted to the pressure limiting aperture 
electrode 59. In addition, the secondary electrons 49 generated by 
collisions between a backscattered electron 39 and the pressure limiting 
aperture 50 will no longer be accelerated away from the pressure limiting 
aperture and no gas amplification occurs. Accordingly, most of the 
undesirable signal components are not collected by the ring electrode 28, 
and therefore, the image signal derived from the ring electrode 28 is a 
more pure secondary electron image having a lower noise level. 
A further object of this invention is to employ an improved gaseous 
electron detector in the ESEM utilizing the hot temperature specimen stage 
of the present invention similar to the gaseous electron detector shown in 
FIG. 20 of U.S. Pat. No. 5,412,211. As is shown in FIG. 8, an 
environmental scanning electron microscope employing printing circuit 
board technology is shown. In this ESEM, a printed circuit board 132 is 
positioned in the specimen chamber in a generally horizontal manner. The 
detector body 172 is mounted to the electron optical column and provides a 
path of the electron beam to pass therethrough. This detector body 172 is 
described in U.S. Pat. No. 5,412,211 as being similar to the aperture 
carrier described in U.S. Pat. No. 4,823,006 assigned to the common 
assignee of this application except that the detector head in FIG. 8 forms 
the lower portion of the detector body. As is shown in FIG. 8, this 
detector body 172 is threaded into the electron optical column 174. In 
this configuration, a signal collection ring electrode 136 extends 
downwardly from the printed circuit board 132 by means of support legs, 
such as 140a and 140b, and faces the specimen 24 under examination. As a 
result thereof, U.S. Pat. No. 5,412,211 describes that the primary beam 
passes through the final pressure limited aperture 144 and impinges upon 
the specimen 24. Secondary electrons emitted from the surface of the 
specimen are thus collected by the suitably biased signal ring electrode 
136. 
In order to minimize beam loss during the path of the electron beams 
through the gas between the detector and the sample, the electron beam 
path can be shortened by positioning the sample very close to the 
detector. In accordance therewith, an improved gaseous electron detector 
has been designed which creates a short electron beam path to the sample 
through the gaseous environment of the specimen chamber, but maintains a 
relatively long gas path to the detector. In the improved electron 
detector of FIG. 8, the final pressure limiting aperture 144 of the 
printed circuit board 132 angularly extends inwardly in an inverted 
conical arrangement through the collection ring electrode 136 of the 
printed circuit board 132. This reduces the length of the path of the 
electron beam through the chamber gas represented by the distance C in 
FIG. 8. However, the path from the sample 24 to the signal ring electrode 
136 represented by the distance D is still sufficiently long enough to 
obtain satisfactory detection performance. The incorporation of this 
improved electron detector in the environmental scanning electron 
microscope incorporating the hot temperature specimen stage of the present 
invention is best illustrated in FIG. 9. 
Referring now to FIG. 10, the image quality may be improved by 
incorporating the aforementioned features described in U.S. Pat. Nos. 
5,362,964 and 5,412,211, the disclosure of which is incorporated by 
reference. It has been found that the image quality can be further 
enhanced by biasing the final pressure limiting aperture 144 at a 
different voltage from the signal collection ring electrode 136. In 
practice, the final pressure limiting aperture 144 is isolated which 
allows the final pressure limiting aperture 144 to "float" to a stable 
voltage. "Floating" the bias voltage of the final pressure limiting 
aperture is desirable as it has been found to provide for automatic 
compensation. As a result thereof, superior image quality is obtained. The 
same performance and image quality can be achieved with a variable bias 
voltage, but in order to utilize a variable bias voltage, it has been 
found that another power supply and control of its voltage in the required 
way has to be provided. 
A further improvement of the hot temperature specimen stage for an 
environmental scanning electron microscope of the present invention is 
schematically shown in FIG. 11 wherein the bias voltage is applied to the 
thermally conductive sample cup 52. More specifically, at very high 
temperatures (above about 800.degree. C.), the surfaces of the sample 
start to emit a significant amount of thermal electrons. The shield 
biasing scheme described above (i.e., the heat shield assembly 68) 
extracts these thermal electrons as well as the required secondary 
electrons. However, at temperatures above about 1000.degree. C., these 
thermal electrons override the required secondary electrons. These thermal 
electrons can be suppressed by applying a bias voltage to the sample cup. 
This bias voltage applied to the sample cup 52 is represented by reference 
numeral 84 in FIG. 11. This biasing of the sample cup 52 also requires 
that the sample cup be electrically conductive. Bias voltages between +50 
and -50V have been found to be required to suitably suppress undesired 
thermal electrons. 
In the design of the high temperature specimen stage for an environmental 
scanning electron microscope of FIG. 5, the heat shield assembly 68 is 
formed integrally with the specimen stage. As is shown in FIGS. 12a and 
12b, the integrally formed stage/heat shield assembly/sample cup is moved 
laterally in unison in order to observe different parts of the sample. 
Hence, the observable area of the sample is limited by the size of the 
central opening 74 of the heat shield assembly 68. However, the central 
opening 74 of the heat shield assembly must be necessarily small (e.g., 2 
mm diameter) to prevent heat loss, and consequently, this relatively small 
opening limits the size of the sample that can be observed. 
In order to overcome this problem, another preferred embodiment of the high 
temperature specimen stage for an environmental scanning electron 
microscope of the present invention is shown in FIGS. 13a and 13b. As is 
shown in FIGS. 13a and 13b, the heat shield assembly 68 is fixed in 
relation to the final pressure limiting aperture 144 and the signal ring 
electrode 136. A small gap 90 is provided between the heat shield assembly 
68 and the specimen stage assembly. Thus, the specimen stage, and hence 
the sample 24, can be moved independently of the heat shield assembly 68. 
This allows a larger area of the specimen 24 to be observed, and 
accordingly, increases the field-of-view of the specimen. This separation 
of the heat shield assembly 68 from the specimen stage is further shown in 
FIG. 9. As is shown in FIG. 9, the specimen stage moving mechanism 210 
laterally moves the specimen stage 50 independently of the heat shield 
assembly 68 which stays aligned with the electron detector 136 and the 
final pressure limiting aperture 144. 
Accordingly, in accordance with the general objects of the present 
invention, a high temperature specimen stage for an environmental scanning 
electron microscope has been designed which achieves sample temperatures 
of up to approximately 1500.degree. C. In addition, the high temperature 
specimen stage for an ESEM of the present invention minimizes the 
temperature differential between the sample and the heater wire to 
maintain the top of the specimen as being at a high temperature. Moreover, 
the high temperature specimen stage of the present invention minimizes the 
magnetic field from the heating current which otherwise would cause 
deflection of the primary beam. 
As aforementioned, in this high temperature specimen stage, the bias 
voltage applied to the sample cup is combined with the bias voltage 
applied to the heat shield assembly to suppress thermal electrons while 
still collecting secondary electrons. In addition, a differential voltage 
can be applied between the secondary electron detection ring and the 
pressure limiting aperture to enhance imaging quality. Further, "floating" 
the bias of the final pressure limiting aperture can achieve automatic 
compensation. Moreover, in the high temperature specimen stage of the 
present invention, the heat shield assembly can be mounted separately from 
the specimen stage so that the specimen stage can be moved independently 
of the heat shield assembly to enhance the field-of-view of the specimen. 
Although the invention has been particularly shown and described with 
reference to certain preferred embodiments, it will be readily appreciated 
by those of ordinary skill in the art that various changes and 
modifications may be made therein without departing from the spirit and 
scope of the invention. It is intended that the appended claims be 
interpreted as including the foregoing as well as various other such 
changes and modifications.