Apparatus for irradiating a specimen by an electron beam

A specimen chamber is which a specimen to be irradiated by an electron beam is arranged is evacuated by an evacuation device. The evacuation device includes a nonvaporative bulk getter vacuum pump and an oil rotary pump. The oil rotary pump is used for evacuating the specimen chamber from the atmospheric pressure to 10.sup.-1 - 10.sup.-2 Torr, while the nonvaporative bulk getter vacuum pump is used for continuously evacuating the specimen chamber to 10.sup.-5 - 10.sup.-6 Torr. The steady vacuum of 10.sup.-5 - 10.sup.-6 Torr is maintained by only the nonvaporative bulk getter vacuum pump.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to apparatus for irradiating a specimen by an 
electron beam. More particularly, it relates to the apparatus suitable for 
use in an electron microscope, an electron probe X-ray microanalyzer, an 
Auger electron analyzing device and an electron-beam pattern drawing 
device. 
2. Description of the Prior Art 
In apparatus for irradiating a specimen by an electron beam, such as 
electron microscope, electron probe X-ray microanalyzer, Auger electron 
analyzing device and electron-beam pattern drawing device, it is important 
that the specimen is saved from staining. Although causes for the staining 
of the specimen are complicated, the main cause is deemed to lie in that 
hydrocarbon molecules adsorbed to the specimen form a layer which is 
stable both thermally and chemically, on the specimen in virtue of the 
irradiation by the electron beam. In order to prevent such stains of the 
specimen, attention has heretofore been turned to that a specimen chamber 
in which the specimen is accommodated is evacuated to the highest possible 
vacuum. From this viewpoint, a high vacuum pump such as oil diffusion pump 
and sputter ion pump is ordinarily used for maintaining the steady vacuum 
of the specimen chamber. These techniques are respectively disclosed in 
Japanese Pat. Publications Nos. 3224/74 and 48945/76. 
However, oil vapor flows backwards into the specimen chamber from the oil 
diffusion pump. In substance, accordingly, the use of the oil diffusion 
pump does not serve to prevent the staining of the specimen. The oil 
diffusion pump also has the problem that it takes as long a time as about 
20 - 30 minutes to steadily operate. When a trap for the oil vapor is 
interposed between the oil diffusion pump and the specimen chamber, the 
backward flow of the oil vapor into the specimen chamber is relieved, but 
the effect of preventing the backward flow is still unsatisfactory. In the 
case where the sputter ion pump is employed as the main evacuating pump, a 
higher degree of vacuum can be reached. On the other hand, however, it 
becomes a cause for the stains of the specimen that sputter substances 
produced within the pump adhere (deposit) onto the specimen. The adhesion 
of the sputter substances onto the specimen becomes more conspicuous as 
the sputter ion pump is brought nearer to the specimen for the purpose of 
keeping the vicinity of the specimen at a higher vacuum. Another problem 
is that ions and electrons produced within the sputter ion pump are 
captured as false signals by a detector for detecting information signals 
(of, for example, secondary electrons, reflected electrons, X-rays, 
cathode luminescence, Auger electrons, etc.) from the specimen. 
SUMMARY OF THE INVENTION 
An object of this invention is to provide apparatus for irradiating a 
specimen by an electron beam wherein the specimen is not stained under the 
state under which the specimen is put in a steady vacuum. 
Another object of this invention is to provide apparatus for irradiating a 
specimen by an electron beam wherein the staining of the specimen can be 
prevented by a very simple construction. 
Still another object of this invention is to provide apparatus for 
irradiating a specimen by an electron beam wherein a steady vacuum is 
attained in a short time. 
Regarding the prevention of stains of a specimen, the concept that the 
stains of the specimen are prevented by establishing the highest possible 
vacuum has been common. In contrast, this invention is based on the 
concept that, even when the degree of vacuum is not very high, the problem 
of the stains of the specimen will be solved if the vacuum atmosphere is 
clean. Premised on such a concept and in view of the causes for the stains 
of the specimen, the inventor noted to arrange the specimen within an 
inert gas atmosphere having an adequate pressure, and took notice of a 
nonvaporative bulk getter vacuum pump. Although this vacuum pump is 
generally available commercially, it is not favored when to be employed 
singly as a main evacuating pump for the apparatus of the specified type 
because the degree of arrival vacuum is low. The inventor's notice of the 
nonvaporative bulk getter vacuum pump was based on the thought that, since 
the pump exploits the principle of chemical adsorption, argon and helium 
existent in the vicinity of the specimen will not be discharged by the 
pump. Under this thought, the inventor carried out an experiment of 
comparisons between a case of employing the nonvaporative bulk getter 
vacuum pump and a case of employing the oil diffusion pump as the main 
evacuating pump for the specimen chamber in which the specimen was 
received. Then, it was recognized that although the latter is desirable 
merely from the viewpoint of high vacuum, the former is far more excellent 
than the latter in point of the prevention of the stains of the specimen. 
More specifically, it could be perceived that the specimen staining rate 
in the former is only, at most, about 1/10 of that in the latter, and that 
the low staining rate comes from the fact that argon in an amount 
appropriate for the prevention of the stains of the specimen remains in 
the specimen chamber without being emitted. 
According to this invention, means for generating an electron beam is 
disposed, and a specimen is irradiated by the particle beam. The specimen 
is arranged in a specimen chamber, which is kept in a steady vacuum state 
by only a nonvaporative bulk getter vacuum pump. The nonvaporative bulk 
getter vacuum pump is equipped with heating means for activation thereof. 
According to such expedients of this invention, the staining of the 
specimen is prevented as stated previously. Moreover, since an oil 
diffusion pump or a sputter ion pump having heretofore been employed as a 
main evacuating pump is merely replaced with the nonvaporative bulk getter 
vacuum pump, the construction of this invention is simpler even when 
compared with that of the prior art of low stain preventive effect. 
Further, the nonvaporative bulk getter vacuum pump has the advantage that 
it takes only several tens seconds - several minutes to establish the 
steady vacuum. As the heating means for activation is comprised, the 
nonvaporative bulk getter vacuum pump can be used over a long term by 
utilizing it. 
The other objects, advantages and features of this invention will become 
apparent from the following description taken with reference to the 
drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 1 shows a preferred embodiment of this invention. Referring to the 
figure, an electron gun portion 2 is driven by a high voltage supply 1. An 
electron beam 3 from the electron gun portion 2 is converged onto a 
specimen 8 by a converging lens 5 which is excited with a lens power 
supply 4 and by an objective lens 7 which is excited with a lens power 
supply 6. When the specimen 8 is irradiated by the converged electron beam 
3, information signals (secondary electrons, reflected electrons, absorbed 
electrons, X-rays, cathode luminescence, Auger electrons, etc.) peculiar 
to the specimen are obtained therefrom. For example, the secondary 
electrons among them are detected by a detector 9. The secondary electron 
signal thus detected passes through an amplifier 10 and further through a 
switch 11, and is introduced into a grid 12a of a cathode-ray tube 12 as a 
brilliance modulation signal. 
A scanning signal generator 13 generates vertical and horizontal scanning 
signals. These signals are impressed through an amplifier 14 on a 
deflector 15 which is arranged directly under the objective lens 7. Thus, 
the electron beam 3 is deflected in two dimensions. In turn, the specimen 
8 is two-dimensionally scanned by the converged electron beam 3. On the 
other hand, the vertical and horizontal scanning signals from the scanning 
signal generator 13 are also impressed on a deflector 12b of the 
cathode-ray tube 12 through an amplifier 16. Therefore, a screen 12d of 
the cathode-ray tube 12 is two-dimensionally scanned in synchronism with 
the scanning of the specimen 8 by cathode rays, i.e., an electron beam 12c 
of the cathode-ray tube 12. 
The Auger electrons emitted from the specimen 8 are led to an energy 
analyzer 17 and dispersed here in dependence on their energies, and the 
Auger electrons of specified energy are detected. The Auger electron 
signal thus detected is led to an amplifier 18. Further, it is introduced 
into the grid 12a of the cathode-ray tube 12 as a brilliance modulation 
signal by changing-over the switch 11. Through the change-over of the 
switch 11, accordingly, a secondary electron image and an Auger electron 
image of the scanned area of the specimen 8 can be selectively displayed 
on the screen 12d of the cathode-ray tube 12. 
Upon closure of a switch 19, ions are emitted from an ion source 21 by a 
high voltage supply 20. Using the ions, the specimen 8 can be etched. The 
ion etching can be employed in order to analyze a new surface of the 
specimen 8. The specimen 8 is mounted on a specimen stage 23 inside a 
specimen chamber 22. The specimen stage 23 can be finely adjusted by 
manipulating a knob 24 outside the specimen chamber 22. 
An auxiliary vacuum pump 27 and a main vacuum pump 28 are connected to the 
specimen chamber 22 through valves 25 and 26 respectively, and a vacuum 
gauge 29 is also connected thereto. The auxiliary vacuum pump 27 may be 
either an oil rotary pump or a sorption pump. The main vacuum pump 28 is a 
nonvaporative bulk getter vacuum pump. 
Now, the valve 25 is opened and the valve 26 is shut, and the auxiliary 
vacuum pump 27 is actuated. The specimen chamber 22 is thus evacuated 
preliminarily by the auxiliary vacuum pump 27. When the oil rotary pump is 
employed as the auxiliary vacuum pump 27, the specimen chamber 22 can be 
evacuated to the extent of 10.sup.-1 - 10.sup.-2 Torr. The sorption pump 
has an evacuating capability of about 10.sup.-4 - 10.sup.-5 Torr. However, 
when it is employed as the auxiliary vacuum pump 27, desirably it is 
stopped at the time when the specimen chamber 22 has reached 10.sup.-1 - 
10.sup.-2 Torr. When the specimen chamber 22 has reached 10.sup.-1 - 
10.sup.-2 Torr or so in this way, the valve 25 is shut and the valve 26 is 
opened, whereupon the specimen chamber 22 is evacuated by the 
nonvaporative bulk getter vacuum pump 28 being the main vacuum pump. The 
degree of arrival vacuum owing to this pump is in the order of 10.sup. -4 
- 10.sup.-5 Torr, and the period of time for attaining such degree of 
arrival vacuum is as short as several tens seconds - several minutes. When 
only the nonvaporative bulk getter vacuum pump is employed as the main 
vacuum pump 28, an inert gas such as argon and helium remains in the 
specimen chamber 22 without being discharged because this pump exploits 
the principle of chemical adsorption. Therefore, while the specimen 
chamber 22 is maintained at the steady vacuum state of approximately 
10.sup.-4 - 10.sup.-5 Torr, the staining of the specimen 8 is effectively 
prevented. An experiment has revealed that the specimen staining rate at 
the time when only the nonvaporative bulk getter vacuum pump is employed 
as the main vacuum pump 28 is less than about 1/10 of that at the time 
when an oil diffusion pump is employed. The evacuating capability of the 
nonvaporative bulk getter vacuum pump is gradually lost as the pump is 
used over a long term, but the rate of staining the specimen rather lowers 
until approximately 10.sup.-1 - 10.sup.-2 Torr is reached. However, as the 
degree of vacuum lowers, the scattering of electrons becomes prone to 
occur. In practice, therefore, it is desirable that the nonvaporative bulk 
getter vacuum pump is activated when its evacuating capability has lowered 
down to approximately 10.sup.-3 Torr. The activation is easily achieved by 
heating the pump to about 800.degree. - 900.degree. C in a pressure 
atmosphere of 10.sup.-1 - 10.sup.-2 Torr or so. 
According to the embodiment of FIG. 1, the stains of the specimen are 
effectively prevented, and the period of time for attaining the steady 
vacuum is very short. In addition, the activation of the nonvaporative 
bulk getter vacuum pump can be performed, and hence, the life thereof is 
rendered long. Further, the stains of the specimen are prevented without 
intentionally introducing the inert gas such as argon into the specimen 
chamber, and by exploiting the fact that since only the nonvaporative bulk 
getter vacuum pump is employed as the main vacuum pump, the inert gas such 
as argon which is not emitted thereby is left in the specimen chamber. It 
is therefore understood that the construction is very simple. 
Since the nonvaporative bulk getter vacuum pump 28 can evacuate the 
specimen chamber 22 from the atmospheric pressure to 10.sup.-4 - 10.sup.-5 
Torr, the auxiliary pump 27 is not always necessary. However, in case of 
using the nonvaporative bulk getter vacuum pump 28 in order to evacuate 
the specimen chamber from the atmospheric pressure, the number of the 
operations for activating the pump becomes larger than in the other case, 
so that also the life shortens. To the end of solving these problems, the 
use of the preliminary evacuating pump is desirable. 
FIG. 2 shows an embodiment of the nonvaporative bulk getter vacuum pump in 
FIG. 1. Referring to FIG. 2, a heater 31 and a nonvaporative bulk getter 
vacuum pump unit 32 which surrounds the heater 31 are arranged within a 
tube 30 which is to be connected to the specimen chamber 22 (refer to FIG. 
1). The tube 30 is made of a material emitting little gases, for example, 
stainless steel. The heater 31 consists of a heater wire and a ceramics 
bobbin round which the heater wire is wound. The nonvaporative bulk getter 
vacuum pump unit 32 consists of a base which is made of iron, stailess 
steel, nichrome or the like, and a nonvaporative bulk getter vacuum pump 
element which is formed on the surface of the base. It is in the shape of 
bellows as shown in the figure. The nonvaporative bulk getter vacuum pump 
element may be made of an alloy containing aluminum and zirconium. An end 
part of the tube 30 is formed with a flange 33, to which a lid 35 is 
attached through a metal gasket 34 made of gold, copper or the like. A 
central part of the lid 35 is formed as a hermetic seal 36. The heater 31 
is connected to a variable heating current source 38 through lead wires 37 
which penetrate the hermetic seal 36 in a vacuum-proof manner. The heater 
31 and the nonvaporative bulk getter vacuum pump unit 32 are supported by 
the lid 35 through supporters 39 and 40, respectively. 
When a current is caused to flow from the variable heating current supply 
38 through the lead wires 37 to the heater 31, the heater generates heat, 
whereby the nonvaporative bulk getter vacuum pump unit 32 is heated by 
radiation. When the temperature of the vacuum pump unit 32 is maintained 
at about 200.degree. - 400.degree. C by heating the pump unit in this 
manner, the chemical adsorptivity of the vacuum pump unit 32 increases. 
Thus, gas molecules floating in the surroundings of the vacuum pump unit 
32 are chemically adsorbed, with the result that the surroundings can be 
steadily maintained at the degree of vacuum of approximately 10.sup.-4 - 
10.sup.-5 Torr. Of course, on account of the chemical adsorption, the 
inert gas such as argon and helium remains as it is without being 
adsorbed. Since the nonvaporative bulk getter vacuum pump element exhibits 
a chemical adsorptivity even at the normal temperature (the capability, 
however, lowers to some extent), it is not always necessary to heat the 
element at all times. 
The nonvaporative bulk getter vacuum pump unit, especially the 
nonvaporative bulk getter vacuum pump element included therein loses the 
chemical adsorptivity gradually through its use over a long term. In such 
a case, while maintaining the interior of the tube 30 at approximately 
10.sup.-1 - 10.sup.-2 Torr with, e.g., an oil rotary pump, a large current 
is supplied from the variable heating current supply 38 to the heater 31, 
to heat the heater so that the nonvaporative bulk getter vacuum pump unit 
32 may become about 800.degree. - 900.degree. C. Thus, the gas molecules 
adsorbed on the surface of the nonvaporative bulk getter vacuum pump 
element are forced deep into the interior of the element, with the result 
that the activation of the nonvaporative bulk getter vacuum pump element 
is achieved in several minutes - several tens minutes. 
FIG. 3 shows another embodiment of the nonvaporative bulk getter vacuum 
pump in FIG. 1. Differences of this embodiment from the preceding 
embodiment in FIG. 2 are that a high-frequency coil 41 is arranged on the 
outer periphery of the tube 30, the nonvaporative bulk getter vacuum pump 
unit 32 being heated by bestowing a high frequency thereon from a radio 
frequency supply 42, and that the tube 30 is accordingly made of a 
refractory material exhibiting a low radio-frequency loss, for example, 
ceramics. 
According to the embodiment of FIG. 2, the heater is arranged directly in 
the tube 30, so that in activating the nonvaporative bulk getter vacuum 
pump element, the heater is oxidized or nitrided by a comparatively large 
quantity of oxygen or nitrogen in the atmosphere in which it is arranged. 
In extreme cases, the life of the heater ends due to several activations. 
Another problem is that, since the nonvaporative bulk getter vacuum pump 
element is heated by the radiant heat, the heating efficiency is inferior. 
In order to heat the element to about 800.degree. - 900.degree. C, the 
temperature of the heater must be raised to above 1,000.degree. C. 
According to the embodiment of FIG. 3, the life of the heater need not be 
feared, but there is the problem that the high-frequency power is 
required. By way of example, a high-frequency power of over 1 kW is 
necessary in order to heat the nonvaporative bulk getter vacuum pump 
element of 1 kg to about 800.degree. - 900.degree. C. 
FIG. 4 shows still another embodiment of the nonvaporative bulk getter 
vacuum pump in FIG. 1. Referring to FIG. 4, the heater 31 is made of a 
nichrome wire, and it is buried in a refractory material, such as alumina, 
magnesia and silica, 44 that is packed in a thermally-conductive bottomed 
tube 43 penetrating the lid 35. The refractory material has the function 
of shutting off the heater 31 from the open air substantially perfectly, 
and the function of transferring the heat of the heater 31 to the bottomed 
tube by conduction. The bottomed tube 43 is inserted in a 
thermally-conductive bottomed tube 45 in a manner to lie in close contact 
with the inner wall thereof. An opening part of the bottomed tube 45 is 
coupled with the central part of the lid 35. The nonvaporative bulk getter 
vacuum pump unit 32 lies in contact with the outer surface of the bottomed 
tube 45, and the lid 35 is welded to the open end of the tube 30. 
According to the embodiment of FIG. 4, the heat of the heater 31 is 
transferred to the nonvaporative bulk getter vacuum pump unit 32 through 
the refractory material 44, bottomed tube 43 and bottomed tube 45 
effectively by conduction. Therefore, the temperature of the heater 31 and 
that of the nonvaporative bulk getter vacuum pump unit 32 become 
substantially equal. This signifies that the heat transfer efficiency from 
the heater 31 to the nonvaporative bulk getter vacuum pump unit 32 is very 
good and that the activation of the nonvaporative bulk getter vacuum pump 
element can therefore be done by a low power consumption. Since the heater 
31 is shut off by the refractory material 44 so as not to come into 
contact with the atmosphere of the tube 30 or the open air, the oxidation 
or nitriding as in FIG. 2 lessens to the extreme, with the result that an 
extraordinarily long life of the heater 31 is achieved. It has been 
experimentally confirmed that the heater can endure the activating 
operations 1,000 times or more. 
FIG. 5 shows yet another embodiment of the nonvaporative bulk getter vacuum 
pump in FIG. 1. This embodiment differs from the preceding embodiment of 
FIG. 4 in that the bottomed tube 43 is omitted, and that the refractory 
material 44 is packed directly in the bottomed tube 45, the heater 31 
being buried therein. With the present embodiment, it is expected to 
obtain a heat transfer efficiency which is more excellent than in FIG. 4. 
FIG. 6 shows a further embodiment of the nonvaporative bulk getter vacuum 
pump in FIG. 1. In this embodiment, a plurality of ring-shaped 
nonvaporative bulk getter vacuum pump units 32 are arranged around the 
bottomed tube 45 so as to contact with the outer surface thereof. They are 
disposed through spacers 46 along the axis of the bottomed tube 45. Each 
of the nonvaporative bulk getter vacuum pump units 32 is made up of a 
nonvaporative bulk getter vacuum pump element 32a, and a container 32b 
which is filled up therewith. One side of the container 32b as extends 
along the axis of the bottomed tube 45 is open, and the other side is 
closed. The nonvaporative bulk getter vacuum pump element 32a is a porous 
compact, which is readily obtained by pulverizing aluminum and zirconium 
and sintering the powdery mixture. 
According to the present embodiment, the chemical adsorption area of the 
nonvaporative bulk getter vacuum pump elements and accordingly the total 
quantity of adsorption can be made much larger than in the foregoing 
embodiments of FIGS. 2 to 5. 
FIG. 7 shows another embodiment of the nonvaporative bulk getter vacuum 
pump in FIG. 1. A feature of the embodiment shown in FIG. 7 is that a part 
of the tube 30 close to a tube 47 is formed as a thin-walled portion 30a. 
According to this embodiment, the temperature rise of the flanges 30b is 
avoided even at the time of the activation of the nonvaporative bulk 
getter vacuum pump elements 32a, so that the tube 30 can be connected to 
the tube 47 through a rubber packing 48. 
FIG. 8 shows still another embodiment of the nonvaporative bulk getter 
vacuum pump in FIG. 1. In this embodiment, two of the nonvaporative bulk 
getter vacuum pump assemblies as shown in FIG. 6 or FIG. 7 are arranged in 
the tube 30, and the heaters 31 in the assemblies have the respective 
variable heating current supplies 38 connected thereto. 
With the present embodiment, it is expected that the evacuating rate and 
the total quantity of adsorption will be increased more. Of course, the 
number of the nonvaporative bulk getter vacuum pump assemblies may be 
increased as is necessary. 
FIGS. 9 and 10 show modifications of the nonvaporative bulk getter vacuum 
pump units illustrated in FIGS. 6 to 8. These modifications have features 
in the container 32b when compared with the preceding embodiments of FIGS. 
6 to 8. In FIG. 9, the container 32b is open at the outermost periphery. 
In FIG. 10, it has a partition wall in the middle, and it is open on both 
sides thereof. 
In the embodiments described above, various alterations and modifications 
can be made without departing from the substance of this invention. It is 
therefore to be understood that the scope of this invention ought to be 
defined on the basis of patent claims given below.