Field-emission transmission electron microscope and operation method thereof

An object of the present invention is to realize a field-emission transmission electron microscope which is able to cope with both observation of an electron-microscopic image of a high brightness and microanalysis. A low aberration condenser lens 4 is disposed at the farthest position from a specimen 7, and a short focal length lens 5 is disposed at the midpoint between the specimen 7 and the condenser lens 4. In the case of an observation of an electron-microscopic image, the condenser lens unit is operated for enlargement in which the condenser lens 4 and the condenser lens 5 are driven in an interlocking motion. When the size of a beam spot on a specimen is to be reduced, a condenser lens 6 disposed close to the specimen between the condenser lens 5 and the specimen 7 is driven to make the condenser lens unit be operated for reduction. The coexistence of a small illuminating angle and the illumination of a specimen with a fine beam spot is realized, which makes it possible for a field-emission transmission electron microscope to have both functions, being able to observe a bright electron microscopic image and to perform an element analysis.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a field-emission transmission electron 
microscope. To be more precise, it relates to a field-emission 
transmission electron microscope comprising an electrostatic lens for 
extracting electrons from a field-emission cathode and for accelerating 
the electrons with a predetermined accelerating voltage. Additionally, a 
condenser lens for focusing the accelerated electron beam and illuminating 
a specimen with the focused electron beam is provided. The invention 
relates, in particular, to the constitution and the operation method of 
the condenser lens portion. 
2. Description of the Related Art 
In the case of a conventional transmission electron microscope, for 
example, a thermionic electron source in which thermionic electrons are 
emitted from tungsten W or LaB.sub.6 being heated, or a field-emission 
electron source in which electrons are extracted from a hairpin cathode 
having a small radius of curvature with a strong electric field applied to 
it, is used as an electron source for generating an electron beam. 
In the case of an electron microscope using a thermionic electron source, 
an electron source diameter of the thermionic electron source is as large 
as 2 to 10 .mu.m, and in an observation with high magnification, in order 
to make an area on a specimen to be illuminated by an electron beam be 
less than 1 .mu.m, the magnification of the illumination system shall be 
less than 1. An example of such a case is described in a Japanese Patent 
laid open under Provisional Publication No. 126951/80. 
In the case of an electron microscope using a field-emission electron 
source, the illumination system is constituted with an electrostatic lens 
and a single condenser lens. An example of such a case is described in 
Japanese Patent Publication No. 117534/85. 
In recent years, there has been a demand for a transmission electron 
microscope having functions both of observation of high resolution imaging 
and of microanalysis. In a conventional transmission electron microscope 
using a thermionic electron source, the magnification of an illumination 
system becomes less than 1 in both cases. Because the size of a thermionic 
electron source is as large as 2 to 10 .mu.m, reduction of the source size 
is necessary to prevent a decrease in the brightness of an image in the 
case of an observation with high magnification, and reduction of the 
source size to a large extent is needed in the case of microanalysis. 
However, because of the insufficiency in the brightness of an electron 
source, it is almost impossible to make the diameter of an electron beam 
on a specimen less than 10 nm in order to obtain a necessary probe current 
for an analysis. 
On the other hand, in the case of a field-emission transmission electron 
microscope mounted with a field-emission electron source which has higher 
brightness and a smaller source size than the case of a thermionic 
electron source, there is a possibility of having a function of analyzing 
a small area and a function of an observation of high resolution imaging. 
In order to realize these two functions, observation of high resolution 
imaging and microanalysis, it is necessary to make the magnification of an 
illumination system less than 1 in the case of microanalysis, and to make 
the magnification of the illumination system more than 1 in the case of 
observation of high resolution imaging. In other words, in the case of a 
field-emission electron source, the electron source size is as small as 
about 10 nm; however, in order to perform a microanalysis of a small area 
on a specimen in reducing an electron beam diameter, for example, down to 
less than 1 nm on the specimen, it is necessary to make the magnification 
of the illumination system be less than 1/10. On the other hand, it is 
necessary to make an illuminating angle on a specimen small to obtain a 
high resolution electron microscopic image. 
As shown in FIG. 2, let us assume that an electron beam extracted at an 
extracting voltage of V1 from a field-emission electron source 1 is 
accelerated to an accelerating voltage V0 by an electrostatic lens 2 and 
is focused on a specimen 7 by a condenser lens 8; let a symbol .alpha. 
indicate an exit half angle of an electron source to be limited by an 
aperture 9 and let a symbol .beta. indicate an illuminating angle on a 
specimen, then the magnification of the illumination system M is expressed 
as, according to the Helmholtz equation, 
EQU M=.alpha./.beta. (V1/V0).sup.0.5. 
Let a symbol .omega. indicate the angular current density (emission current 
per unit solid angle) of the field-emission cathode 1, then a beam current 
I to be limited by the aperture 9 is expressed as I=.pi..alpha..sup.2 
.omega.. 
When tungsten W of bearing [310] is used as the field-emission cathode 1, 
the maximum angular current density .omega. is about 50 .mu.A/sr. In order 
to obtain a bright enlarged image on a fluorescent screen, a beam current 
of the order of I=4nA is necessary, and the symbol .alpha. to satisfy the 
condition is calculated to be .alpha.=5 mrad. An extracting voltage is 
usually in the range of 4 kV to 6 kV. Assuming that an extracting voltage 
V1=6 kV, accelerating voltage V0=200 kV, and .alpha.=5 mrad, the 
magnification M of the illumination system is 
EQU M=0.86/.beta. (where the unit of .beta. is mrad). 
Therefore, in order to obtain an illuminating angle .beta. of less than 0.5 
mrad being necessary for a high resolution observation, the magnification 
M has to be made as large as possible. 
In a conventional example, however, a condenser lens 8 is a single 
condenser lens, so that it is impossible to change magnification by a 
large extent from reduction to enlargement using the condenser lens 8. 
Because of this, when a lens is disposed in a position capable of 
obtaining an electron beam of less than 1 nm (suitable for microanalysis) 
in an observation with high magnification, sufficiently large 
magnification cannot be obtained; thereby, an illuminating angle of less 
than 0.5 mrad cannot be obtained, which makes it impossible to perform a 
high resolution observation. In contrast with this, when a lens is 
disposed in a position to be able to obtain a large magnification, 
microanalysis of a minute portion of less than 1 nm becomes impossible. 
Therefore, in the case of an example of a conventional field-emission 
transmission electron microscope, it is impossible to deal with both 
cases, high resolution observation and microanalysis, with a single 
condenser lens. 
SUMMARY OF THE INVENTION 
An object of the present invention is to realize a field-emission 
transmission electron microscope which is able to deal with both cases, 
observation of an electron microscopic image having high brightness and 
microanalysis. 
In order to achieve the above-mentioned object, the present invention 
offers a field-emission transmission electron microscope comprising: an 
electrostatic lens which extracts electrons from a field-emission cathode 
and accelerates the electrons to a desired accelerating voltage; an 
illuminating means for illuminating a specimen with a focused electron 
beam which is obtained by focusing the accelerated electrons with a 
condenser lens unit being composed of a plurality of lenses; and a driving 
means for selectively combining the above-mentioned lenses for making the 
magnification of the lens unit to be used for enlargement and also for 
reduction. 
A preferable constitution of the above-mentioned plurality of lenses is 
shown in the following: the lenses form a compound condenser lens of at 
least three stages comprising a first condenser lens disposed farthest 
from a specimen, a second condenser lens having a short focal length 
disposed about at the midpoint between the specimen and the first 
condenser lens, and a third condenser lens of a low aberration constant 
disposed between the second condenser lens and the specimen. 
In the operation of the above-mentioned field-emission transmission 
electron microscope, in the case of an observation of an electron 
microscopic image of the above-mentioned specimen, the first and the 
second condenser lenses are driven simultaneously in an interlocking 
motion to make the magnification of the compound condenser lens be more 
than 1. In a case where X-ray analysis, etc. are performed in reducing the 
size of an electron beam spot on the specimen down to a fine spot, the 
third condenser lens is driven to make the magnification of the compound 
condenser lens unit be less than 1. 
In the case of a thin equivalent magnetic lens which is generally used as a 
condenser lens, let a symbol D indicate an average value of apertures 
provided on an upper side magnetic pole and a lower side magnetic pole, 
and let a symbol S indicate the distance between the upper side magnetic 
pole and the lower side magnetic pole, then a focal length f and a 
spherical aberration constant Cs can be approximately expressed as 
EQU f/(S+D)=25/Ex.sup.2 ( 1) 
where lens excitation Ex=(NI)/(.PHI..sub.0).sup.0.5, 
Cs=5.0 f.sub.3 /(S+D).sup.2 ( 2) 
where .PHI..sub.0 is an accelerating voltage for an electron beam (unit: 
volt), and NI is the ampere-turns of a lens coil. As seen from equation 
(1), a lens of a shorter focal length can be obtained for a smaller value 
of (S +D), when the lens excitation Ex is a constant, and from equation 
(2), a lens of a smaller spherical aberration constant can be obtained for 
a larger value of (S+D), when the focal length is a constant. 
As described in the above, in the case of a field-emission transmission 
electron microscope according to the present invention, a condenser lens 
unit has a 3 stage constitution: a low aberration condenser lens is 
disposed at a farthest position from a specimen, and a short focal length 
condenser lens is disposed at the midpoint between the above-mentioned low 
aberration condenser lens and the specimen. Further, in a case of an 
observation of an electron microscopic image, the above-mentioned low 
aberration condenser lens and short focal length condenser lens are 
operated in an interlocking motion, and the magnification of the condenser 
lens unit is operated for enlargement. When an electron beam is to be 
reduced down to a fine spot on the specimen, another low aberration lens 
disposed close to the specimen between the above-mentioned short focal 
length lens and the specimen is driven to make the magnification of the 
condenser lens unit be operated for reduction. Owing to the 
above-mentioned constitution, it is made possible to realize a 
field-emission transmission electron microscope which is able to deal with 
both observation of an electron microscopic image of high brightness and 
element analysis.

DESCRIPTION OF PREFERRED EMBODIMENTS 
An embodiment according to the present invention will be explained in 
detail without reference to the accompanying drawings. 
FIG. 1 is a drawing showing the constitution of an essential part of a 
first embodiment of a field-emission transmission electron microscope 
according to the present invention. An electron beam extracted from a 
field-emission electron source 1 is accelerated to an accelerating voltage 
V0 and illuminates a specimen 7 through a condenser lens unit. The 
condenser lens unit is constituted with, starting from the farthest 
position from the specimen 7, a first condenser lens 4, a second condenser 
lens 5, and a third condenser lens 6. These 3 lenses, 4, 5 and 6, are 
driven by a driving means 10 being selectively combined. 
FIG. 3 is a lay diagram for explaining the operation principle of the 
above-mentioned embodiment. At first, a case where a condenser lens unit 
is used for enlargement will be explained. As shown in FIG. 3, let symbols 
a.sub.1, b.sub.1 and f.sub.1 indicate the distance between the object 
surface of the lens 4 and the lens, the distance between the image surface 
of the lens 4 and the lens, respectively; the focal length of the lens 4, 
and let symbols a.sub.2, b.sub.2 and f.sub.2 indicate the distance between 
the object surface of the lens 5 and the lens, the distance between the 
image surface of the lens 5 and the lens, and the focal length of the lens 
5, respectively and let a symbol L1 indicate the distance between the lens 
4 and the specimen. Then the magnification Mc of an illumination system, 
under the condition that an electron beam can be focused on the specimen 7 
with the first condenser lens 4, farthest from the specimen, and the 
second condenser lens 5, second farthest from the specimen, can be 
expressed as 
EQU Mc=(.sub.1 /a.sub.1).times.(b.sub.2 /a.sub.2) =(1/a.sub.1) 
{(-1/f.sub.2)b.sub.2.sup.2 +(L1/f.sub.2)b.sub.2 -L.sub.1)}(3). 
When equation (3) is partial-differentiated with respect to b.sub.2, 
EQU .alpha.Mc/.alpha.b.sub.2 =(1/a.sub.1 f.sub.2) (L1-2b.sub.2). 
Therefore, when b.sub.2 =2/L1, the magnification Mc of an illumination 
system becomes a maximum value, (L1/a.sub.1 f.sub.2) 
.times.(L1/4-f.sub.2). In other words, if the focal length of the lens 5 
is under the same condition, a maximum magnification can be obtained when 
the lens 5 is disposed at the midpoint between the lens 4 and the 
specimen. A larger magnification can be obtained when the lens 4 is 
disposed farther from the specimen and the lens 5 is used at a shorter 
focal length. The value of lens excitation Ex has an upper limit decided 
by magnetic saturation of the magnetic path of a lens, so that to obtain a 
focal length that is as short as possible, it is advantageous to use a 
lens having a smaller value of (S+D), which makes the focal length shorter 
for the same lens excitation Ex. The lens aberration of the lens 4 is 
enlarged by the lens 5, so that it is desirable to use a lens having as 
low an aberration constant as possible as the lens 4. To be concrete, less 
than several tens of mm is desirable. 
Next, the case where the size of an electron beam spot on a specimen is 
reduced will be explained. When the spot size is significantly reduced in 
the last stage lens, the lens aberration in the preceding stages is also 
much reduced; therefore, the only lens aberration of note is that of the 
last stage lens. If an illumination system is composed of only the first 
and the second condenser lenses, 4 and 5, the last stage lens is the 
second lens 5. In this case however, when the second condenser lens 5 is a 
small lens having a short focal length, as seen from equation (2), since 
spherical aberration becomes larger in the case of a smaller lens, a fine 
spot on a specimen cannot be obtained. 
Therefore, in order to obtain a fine spot, it is necessary to provide 
another low aberration lens, in addition to the second condenser lens, 
having a short focal length. When the condenser lens is disposed as close 
as possible to a specimen, the magnification of the condenser unit becomes 
small and the focal length becomes also small; thereby lens aberration is 
made small. If a third condenser lens 6 is disposed between the second 
condenser lens 5 and the specimen 7 at a position as close as possible to 
the specimen, and the third condenser lens 6 is a low aberration lens 
having a large value of (S+D), then a minute spot with little lens 
aberration can be obtained on the specimen. 
Next, the explanation of the embodiment shown in FIG. 1 will be given. 
When a distance L1, the distance between the condenser lens 4 and the 
specimen 7, is longer, larger magnification can be obtained; however, 
considering the height of a device, L1 chosen to be 300 mm in this case. 
The condenser lens 5 is disposed at the midpoint of the distance L1, and 
then the distance L2 between the condenser lens 5 and the specimen 7 is 
150 mm. The condenser lens 6 is disposed close to the specimen 7; the 
distance L3 between the condenser lens 6 and the specimen 7 is 50 mm in 
this case. Let symbols S1, S2 and S3 indicate the distances between the 
upper side magnet poles and the lower side magnet poles of the condenser 
lenses, 4, 5 and 6, and let symbols D1, D2 and D3 indicate the average 
aperture diameters of upper side magnet poles and the lower side magnet 
poles. In order to make the focal length of the condenser lens 5 as short 
as possible, the value of (S2+D2) has to be made small; in this case, 
(S2+D2) is made to be 10 mm. In order to make the lens aberration of the 
condenser lens 4 and the condenser lens 5 small, they are made large in 
size. In this case, they are made to be S1+D1=S3+D3=50 mm. 
An electron beam extracted from the field-emission electron source 1 is 
subjected to a lens action of the electrostatic lens 2. Since the 
electrostatic lens 2 has a large lens aberration constant, in general, it 
is used in a way to make the influence of the lens action weak; it is used 
under the condition that a virtual image is focused behind the electron 
source 1 at a distance in the range of 100 mm to 2000 mm. The distance 
between the condenser lens 4 and the electrostatic lens 2 has to be long 
enough to have a space to accelerate an electron beam up to a desired 
accelerating voltage; usually a distance of more than 500 mm is considered 
to be necessary for the above-mentioned distance. In this case, the 
following calculations are performed assuming that the distance a1 between 
the condenser lens 4 and the focusing position of the electrostatic lens 2 
is 1000 mm. 
When a compound lens is used for enlargement, condenser lenses 4 and 5 are 
driven simultaneously. Assuming that the maximum value of the lens 
excitation Ex of the condenser lens 5 is 10, from equation (1), the 
minimum focal length of the condenser lens 5 is obtained as f=2.5 mm; the 
magnification Mc, when the condenser lenses 4 and 5 are used for 
enlargement, can be obtained from equation (3) as Mc=8850 a1=8.85. 
When the position of the condenser lens 5 is shifted by 10% from the 
midpoint between the condenser lens 4 and the specimen 7, that is, when 
L2=0.4 and L1=120 mm, and when L2=0.6 and L1=180 mm, the values of Mc, in 
respective cases, are found to be Mc=8.46 and Mc =8.52; the magnification 
is decreased by about 5%, but in an actual mounting, when it is not 
possible to mount the condenser lens 5 at the midpoint, it can be disposed 
at a position in the range of L2=0.5 (1.+-.0.1). 
When it is desirable to shift the position of the condenser lens 5 further, 
it can cope with further reduction of the size of the condenser lens 5. 
When the size of the condenser lens 5, (S2+D2), is made to be 8 mm, the 
minimum focal length f of the condenser lens 5 can be obtained from 
equation (1) as f=2 mm; even when L1=300 mm and L2=220 mm, the 
magnification of the condenser lens unit Mc can be obtained as Mc=8580 a1 
=8.58; thus, the deterioration in the magnification caused by the shift in 
the position of the condenser lens 5 from the midpoint between the 
specimen 7 and the condenser lens 4 can be handled with further reduction 
of the size of the condenser lens 5. 
In order to make a beam spot formed on the specimen 7 as small as possible, 
the condenser lens 6 is driven. When the condenser lens 6 is driven alone, 
the magnification Mc becomes as Mc=b.sub.2 /(a1+250)=0.04, and it is made 
possible to reduce the size of the electron source 1 to a fine spot on the 
surface of the specimen 7. 
When the reduction ratio at the condenser lens 6 is made large and the 
accelerating voltage is high, the lens aberration d on the specimen 7 is 
approximately decided by the spherical aberration and the diffraction 
aberration of the condenser lens 6. Let the symbol .beta. indicate an 
illuminating angle on the specimen 7, let a symbol Cs indicate a spherical 
aberration constant of the condenser lens 6, and let a symbol .lambda. 
indicate the wavelength of an electron beam. Then, the spherical lens 
aberration and diffraction lens aberration, ds and d.lambda. are obtained 
as ds=0.5 Cs.beta..sup.3, and d.lambda.=0.61 .lambda./.beta., 
respectively. The lens aberration d is determined by calculating the 
square root of the sum of the square of ds and the square of d.lambda., 
and the minimum value dmim is obtained as 
EQU dmin=0.77 Cs.sup.1/4.lambda.3/4. (4) 
When the above value is applied to equation (2), Cs is obtained as Cs=250 
mm. For example, when the value of .lambda.=0.0025 nm at the accelerating 
voltage of 200 kV is applied to equation (4), dmin is obtained as 
dmin=1.08 nm; thus, when a low aberration lens is used as the condenser 
lens 6, the value of lens aberration becomes very small. 
FIGS. 4A, 4B, 4C and 4D are lay diagrams showing the operation of the above 
described embodiment. In FIGS. 4A, 4B, 4C and 4D, the lenses to be driven 
are expressed with thick lines and the lens not to be driven is expressed 
with a thin line. The condenser lens unit is composed of three lenses, and 
when the lens unit is used for enlargement, the condenser lens 4 and the 
condenser lens 5 are driven in an interlocking motion as shown in FIG. 4A. 
When the lens unit is used for reduction, the condenser lens 6 is driven 
independently as shown in FIG. 4B. In the case of a reducing use, as shown 
in FIGS. 4C and 4D, besides the condenser lens 6, the condenser lens 4, or 
both condenser lens 4 and condenser lens 5, can be driven simultaneously. 
Even in a case where an equivalent objective forward magnetic lens is used 
in the last stage of an illumination system, the above-mentioned functions 
can be achieved with the constitution of the condenser lens unit as 
described in the above without any modification.