Apparatus for quantitative secondary ion mass spectrometry

An apparatus for quantitative secondary ion mass spectrometry comprising a sealed chamber for storing a sample containing a light impurity element which is to be analyzed, secondary ion generating means for bombarding a primary ion beam onto the sample so as to allow the sample to emit a secondary ion of the light element, and quantitative analyzing means for detecting the secondary ion so as to quantitatively analyze the light element contained in the sample. First evacuating means evacuates said sealed chamber to an ultrahigh vacuum during quantitative analysis. First cryopanel means is arranged to surround the sample, and first cooling means keeps said first cryopanel means at a cryogenic temperature during quantitative analysis so that said first cryopanel means adsorbs a gas present in said sealed chamber.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to an apparatus for quantitative secondary 
ion mass spectrometry and, more particularly, to such an apparatus 
suitable for use in quantitatively analyzing trace amounts of light 
elements contained in a material. 
2. Description of the Prior Art 
An apparatus for quantitatively analyzing secondary ions using a mass 
spectrometer is known wherein ion (primary ion) beams such as argon, 
cesium or the like bombard a material, e.g., a metal material or a 
semiconductor material placed in a sealed chamber kept under vacuum so as 
to allow it to emit elements contained in the material as ions (secondary 
ions) and to quantitatively analyze the emitted secondary ions (see R. E. 
Honig: Advances in Mass Spectrometry, edited by A. R. West, Applied 
Science Publishers, Barking, 1974, p.337). In an apparatus for 
quantitative secondary ion mass spectrometry of this type, when trace 
amounts of a light element contained in a sample such as hydrogen, carbon, 
nitrogen, oxygen, silicon, sulfur and the like are analyzed, the interior 
of the sealed chamber, particularly, a surrounding portion of the sample 
to be analyzed placed in the sealed chamber must be kept at an ultrahigh 
vacuum so as to reduce background light elements which are contained in 
the residual gases in the vacuum. 
However, in a conventional apparatus, the overall sealed chamber is 
evacuated simply by an ion pump or a cryopump. For this reason, the 
interior of the sealed chamber cannot fully be evacuated, and residual 
gases undesirably remain therein. In addition, contaminating gases are 
released from the surface of the sample, and contaminating gases can also 
be introduced into the sealed chamber from a primary ion source. A 
pressure in the sealed chamber, particularly at the surrounding portion of 
the sample, is increased by these gas components, and light elements as 
major components of these gases cannot be prevented from sticking in the 
surface of the sample. FIG. 1 shows a conventional apparatus provided with 
a cryopump. The apparatus includes a sealed chamber 1 which is provided 
with the cryopump 7 at its one end. The cryopump 7 is constituted by a 
refrigerator 7a, a cryopanel 7b, an outer cryopanel (or shield panel) 7c 
and a baffle 7d. The sealed chamber 1 holds therein a sample holder 4 on 
which a sample S is to be placed, an electrostatic lens 5 and a deflection 
electrode 6. A pipe is connected to the chamber 1 and provides a 
passageway for a primary ion beam 3 generated from a primary ion source 
(not shown). Only the cryopump cannot fully evacuate the chamber 1. 
Further, even if an ion pump 8 is provided for the chamber 1, a 
surrounding portion of the sample S cannot be fully evacuated. When the 
light elements are analyzed in such a state, a detection limit of a target 
light element is limited by the residual gas pressure near the sample, and 
light elements contained in a sample in trace amounts cannot be analyzed. 
For example, FIG. 2 shows an analytical result of the oxygen detection 
limit when the present inventors quantitatively analyzed an impurity 
oxygen contained in a gallium arsenide (GaAs) crystal using the 
conventional apparatus. As is apparent from FIG. 2, in the conventional 
apparatus, the detection limit of oxygen is on the order of 10.sup.16 
/cm.sup.3. Since an impurity oxygen concentration in the high purity GaAs 
crystal is on the order of 10.sup.15 /cm.sup.3 or less, such a low 
impurity concentration cannot be analyzed by the conventional apparatus. 
SUMMARY OF THE INVENTION 
It is therefore an object of the present invention to provide an apparatus 
for quantitative secondary ion mass spectrometry in which background light 
elements which are present in a sealed chamber, particularly near a 
sample, can be evacuated so that trace amounts of light impurity elements 
can be precisely analyzed. 
In order to achieve the above object, there is provided an apparatus for 
quantitative secondary ion mass spectrometry comprising: a sealed chamber 
for storing a sample containing a light impurity element which is to be 
analyzed, means for bombarding a primary ion beam onto the sample so as to 
allow the sample to emit a secondary ion beam of the light element, 
quantitative analyzing means for detecting the secondary ion beam so as to 
quantitatively analyze the light element contained in the sample, 
evacuating means for evacuating the sealed chamber to an ultrahigh vacuum 
during quantitative analysis, cryopanel means arranged to surround the 
sample, and cooling means for keeping the cryopanel means at a cryogenic 
temperature during quantitative analysis so that the cryopanel means 
adsorbs gases present in the sealed chamber. 
Primary ion beams are normally irradiated from a primary ion source into 
the sealed chamber through a sealed path. 
In an aspect of the present invention, in order to prevent undesirable 
gases from being introduced into the sealed chamber from the primary ion 
source, at least one evacuation means is provided for a primary ion beam 
path so as to keep the path at an ultrahigh vacuum. 
In a second aspect of the present invention, in order to evacuate the 
sealed chamber, at least two pumps are provided, and the sample is placed 
in a common evacuation region of the two pumps.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The present invention will be described hereinafter with reference to the 
accompanying drawings. Note that in the drawings, the same reference 
numerals denote the same parts. 
FIG. 3 shows an example of an apparatus for quantitative secondary ion mass 
spectrometry of the present invention. The apparatus 10 comprises a 
cylindrical chamber body 11 made of a material (e.g., stainless steel) 
which can withstand a high baking temperature of, e.g., 250.degree. C. or 
more. A flange portion 11b at an open end portion 11a of the chamber body 
11 is connected to an ultrahigh vacuum evacuation pump, e.g., an ion pump 
12. An ion pump evacuates the interior of the chamber body 11 to an 
ultrahigh vacuum in the following manner. Electrons generated upon 
application of a high voltage and helically moving in a high-intensity 
magnetic field ionize gas molecules so as to cause them to be attracted to 
a cathode or to propell them by electrodes into an auxiliary pump or ion 
trap. A pump of this type is well known to those skilled in the art. 
A hollow cylinder 13 is provided at a closed end portion 11c of the chamber 
body 11 opposite to the end portion 11a. The cylinder 13 communicates with 
the interior of the chamber body 11 and has a diameter smaller than that 
of the chamber body 11. A refrigeration unit, e.g., a helium refrigeration 
unit 14 for cooling the wall of a cryopanel 19 (described later) to a 
cryogenic temperature is connected to a flange portion 13a of the cylinder 
13. The unit 14 includes a thermally conductive rod 20. In operation, the 
cryopanel 19 which is cooled to a cryogenic temperature of, e.g., 30 K or 
less by the refrigeration unit 14 serves as a cryopump, so that gases are 
adsorbed by the surface of the cryopanel 19 so as to evacuate the interior 
thereof to an ultrahigh vacuum at high pumping speed. The unit 14 is 
preferably made of a material which can withstand a high baking 
temperature of, e.g., 250.degree. C. or more. 
The chamber body 11 is sealed by the ion pump 12 and the refrigeration unit 
14, and thus provides a sealed chamber. 
A cylindrical elongated pipe 15 of stainless steel is vertically provided 
on a circumferential surface of the chamber body 11. The pipe 15 
communicates with the interior of the chamber body 11. An open end portion 
of the pipe 15 is connected to a primary ion source PIS, e.g., an ion 
source of argon, cesium or the like for generating primary ion beams PIB. 
The primary ions from the ion source PIS bombard a sample S (to be 
described later) through an electrostatic lens 16 and a deflection 
electrode 17 provided in the chamber body 11 via the pipe 15. 
The sample S to be analyzed for light elements is placed on a sample holder 
18 provided in the chamber body 11. The cryopanel 19 is in the form of a 
hollow cylinder with a bottom and is provided so as to surround the sample 
S. A closed end portion 19a of the cryopanel 19 is thermally connected to 
the refrigeration unit 14 through a cold head 21a of a thermally 
conductive rod 21 which is connected to a cold head 20a of the 
refrigeration unit 14. On the other hand, an open end portion 19b of the 
cryopanel 19 opposes the ion pump 12, so an evacuation conductance of the 
pump 12 is not degraded. When the cryopanel is so arranged, the sample S 
within the cryopanel 19 is positioned within a common evacuation region of 
the ion pump 12 and the cryopump so that a pressure around the sample S 
can be further decreased. 
The cryopanel 19 cooled to 30 K or less, preferably to 20 K or less by the 
refrigeration unit 14 effectively adsorbs gas molecules introduced into 
the chamber body 11 from the ion source PIS together with primary ion 
beams PIB and gas molecules released from the surface of the sample S so 
as to keep the portion surrounding the sample S under the ultrahigh 
vacuum. 
As shown in FIG. 5, the cryopanel 19 is provided with an inlet port 41 for 
the primary ion beams PIB, a detection port 42 for detecting the secondary 
ions emitted from the sample sputtered by the primary ion beams PIB, a 
sample S insertion port 43, a notch 44 for the sample holder 18, and an 
observation window opening 45. 
A hollow cylindrical shield panel 22 with a bottom is provided between the 
chamber body 11 and the cryopanel 19. The panel 22 has a diameter slightly 
smaller than that of the chamber body 11. A flange portion 23a of a hollow 
cylinder 23 is provided at a closed end portion of the panel 22 so as to 
communicate with the interior of the chamber body 11. The shield panel 22 
is connected to the cold head 20a at the flange portion 23a. The shield 
panel 22 is cooled by the refrigeration unit 14 to an intermediate 
temperature (e.g., 77 K) between a room temperature and a temperature to 
which the cryopanel 19 is cooled. If the panel 22 is not provided, the 
object of this invention can still be achieved. However, if provided, heat 
from the chamber body 11 usually placed at room temperature is prevented 
from directly radiating to the cryopanel 19, thus reducing a heat load on 
the cryopanel 19. Therefore, the refrigeration unit 14 can be of a 
low-power type. 
Referring to FIG. 4, a detector (mass spectrometer) 32 (known per se) for 
detecting secondary ions SI and converting them to electron pulses is 
provided in the chamber body 11 so as to oppose the secondary ion 
detection port 42 of the cryopanel 19 through a hollow cylinder 31 which 
is formed on the circumferential surface of the chamber body 11 so as to 
extend in a direction perpendicular to the pipe 15. The detector 32 is 
connected to a system 37 to count and process the pulses and display the 
results outside the chamber body 11. 
An annular pipe 33 which has one closed end is provided on the 
circumferential surface of the chamber body 11 opposite to the cylinder 
31. A portion near a closed end portion 33a of the pipe 33 is expanded so 
as to constitute a preevacuation chamber 34. A metal gate valve 36 in the 
pipe 33 is provided between the chamber 34 and the chamber body 11. 
Referring again to FIG. 3, the pipe 15 defining the path of the primary ion 
beams PIB has an enlarged-diameter portion 15a connected to the ion source 
PIS. The portion 15a is connected to a vacuum pump 23 such as a cryopump, 
ion pump or turbo molecular pump (preferably a turbo molecular pump) so as 
to exhaust the gases from the ion source PIS. 
The pipe 15 has another enlarged-diameter portion 15b relatively near the 
chamber body 11. A disk-shaped cryopanel 24 having a small central through 
hole 24a for transmitting the primary ion beams PIB is coaxially provided 
in the portion 15b. The cryopanel 24 is connected to a refrigeration unit 
25 similar to the bakable refrigeration unit 14 through thermally 
conductive rods 26 and 27. A hollow annular shield panel 28 is arranged so 
as to surround the cryopanel 24. The shield panel 28 is connected to the 
rod 27 of the refrigeration unit 25. The shield panel 28 functions the 
same as the shield panel 22. The central portion of the cryopanel 24 is 
not covered by the shield panel 28 and is exposed to the gas flow from the 
primary ion source PIS. 
A metal gate valve 29 is provided between the portions 15a and 15b of the 
pipe 15. 
In order to analyze light elements contained in the sample S using the 
above apparatus 10, the gate valves 29 and 36 are opened, and the vaccum 
pumps 23 and 35 are driven with the ion pump 12 and the refrigeration unit 
14 deactivated. Thus, the overall apparatus is baked at a temperature of 
250.degree. C. or more, preferably, while the pressure in the apparatus is 
reduced to 10.sup.-8 to 10.sup.-9 Torr. Since the refrigerators 14 and 25 
are of a bakable type and gate valves 29 and 36 are made of a 
heat-resistant metal as described above, baking can be performed without 
any problem. With this baking process, water content and gas components 
attached to or absorbed in the inner wall of the apparatus can be released 
and are exhausted by the vacuum pumps 23 and 35. After baking, the gate 
valves 29 and 36 are closed, the ion pump 12 and the refrigeration unit 14 
are driven so as to evacuate the interior of the chamber body 11 to an 
ultrahigh vacuum, and the refrigeration unit 25 is driven to cool the 
cryopanel 24 to a cryogenic temperature. Thereafter, the sample S is 
introduced into the preevacuation chamber 34 and the chamber 34 is 
evacuated by the vacuum pump 35. Then, the gate valve is opened, the 
sample S is placed at a predetermined position on the sample holder 18, 
and the gate valve 36 is closed. 
After the above operations, the gate valve 29 is opened, and the primary 
ion beams PIB irradiate the sample S from the primary ion source PIS 
through the through hole 24a of the cryopanel 24, the electrostatic lens 
16, the deflection electrode 17 and the through hole 41 of the cryopanel 
19. The secondary ions SI emitted from the sample S by bombarding of the 
primary ion beams PIB are subjected to analysis by the mass spectrometer, 
and the light elements contained therein are thus analyzed. During 
analysis, if the ion source PIS is evacuated only by the vacuum pump 23, 
contaminating gas components generated from the ion source PIS cannot be 
prevented from flowing into the chamber body 11. In other words, since the 
pressure of the ion source PIS is 1.times.10.sup.-6 Torr, gas flow in the 
pipe 15 is turned into molecule flow, and gas molecules freely move in the 
vacuum. In this state, even when the evacuation is performed by the vacuum 
pump 23, only gas molecules which are introduced through the port of the 
vacuum pump 23 are evacuated. However, when the cryopanel 24 is further 
provided downstream to the vacuum pump 23, contaminating gas molecules 
which are not evacuated by the vacuum pump 23 can be trapped by the 
cryopanel 24. Therefore, only few contaminating gas molecules can be 
introduced into the chamber body 11. The cryopanel 19 is provided so as to 
surround the sample S placed in the chamber body 11. For this reason, if 
contaminating gases are introduced from the ion source PIS or if gases are 
released from the surface of the sample S, since the cryopanel 19 
effectively traps these gases, the portion surrounding the sample S can be 
effectively evacuated. Therefore, light elements in the chamber body 
vacuum which adversely influence analysis can be substantially eliminated. 
When the apparatus of the present invention is used, light elements, such 
as hydrogen, carbon, nitrogen, oxygen, silicon, sulfur, and the like, 
contained in trace amounts in a material, e.g., a metallic material and a 
semiconductor material can be precisely quantitatively analyzed. 
In the embodiment shown in FIGS. 3 and 4, the ion pump and the cryopump are 
preferably arranged opposite to each other; they can also be arranged 
perpendicularly to each other. 
In the embodiment shown in FIGS. 3 and 4, a two-stage differential 
evacuation system at the primary ion source side comprises the vacuum pump 
23 and the disk-shaped cryopanel 24 cooled by the refrigeration unit 25. 
The two-stage evacuation system, particularly the cryopanel, is not 
limited thereto. FIG. 6 shows another two-stage evacuation system. In this 
evacuation system, a cryopanel 51 provided near the vacuum pump 23 
comprises a disk plate 51a and a hollow cylinder 51b extending through a 
central portion of the plate 51a. Primary ion beams PIB pass through the 
cylinder 51b. The inner diameter of the cylinder 51b is preferably 
decreased to that which does not prevent the primary ion beams from 
passing therethrough. A shield panel 28 is provided so as to surround the 
disk plate 51a of the cryopanel 51. The disk plate 51a of the cryopanel 51 
is thermally connected to the refrigeration unit 25. Gas molecules 
introduced into the housing 11 passing through the cryopanel 51 are 
limited to those passing through the narrow cylinder 51b. Other gas 
molecules are trapped by the surface of the cryopanel 51 or the shield 
panel 28, or reflected by a buffer plate 52 and evacuated by the vacuum 
pump 23. When the cryopanel 51 is used, the amount of gas molecules from 
the ion source PIS is reduced to 1/10 to 1/20 that when the apparatus 
shown in FIGS. 3 and 4 is used. The buffer plate 52 can be used in the 
apparatus shown in FIGS. 3 and 4. 
FIG. 7 shows still another example of the two-stage evacuation system. In 
this evacuation system, a cryopanel means comprises two disk plates 61 and 
62 each having a through hole formed in its central portion and spaced 
from each other. These disk plates are supported by a support plate 63 
which is connected to a rod 26 of a refrigeration unit 25. The cryopanel 
means has the same effect as that of the cryopanel 51 shown in FIG. 5. 
In the apparatus described above, the cryopanel can be formed of aluminum, 
oxygen free copper, nickel plated oxygen-free copper and the like. 
However, as shown in FIG. 8, a substrate 71 made of an aluminum-based 
metallic material, i.e., aluminum or aluminum alloy which is subjected to 
anodic oxidation so as to form porous anodic oxide films 72 (enlarged in 
FIG. 7) on two surfaces thereof is preferably used. Each anodic oxide film 
72 has a number of pores 73 which extend substantially vertically to the 
surface of the substrate 71. Diameters and density of the pores 73 depend 
upon a temperature and a concentration of an electrolytic solution and a 
current density. Normally, each pore has a diameter of about 10 nm to 30 
nm, and a density thereof is about 10.sup.10 to 10.sup.11 /cm.sup.2. For 
example, when anodic oxidation is performed at a voltage of 15 V, a 
temperature of 20.degree. C., and a current density of 1 A/dm.sup.2 using 
the sulfate electrolytic solution of a 15% concentration, the anodic oxide 
film of a pore diameter of 16 nm and a density of 8.times.10.sup.10 
/cm.sup.2 is obtained. In this manner, since the anodic oxide, film is 
porous, a surface area thereof is considerably larger than that of a flat 
panel. 
In the cryopanel, when an arrival rate of gases from the vacuum to the 
surface thereof is constant, a surface covering ratio of trapped gas 
molecules decreases proportionally to a reciprocal number of its surface 
area. Therefore, when the cryopanel is formed of an aluminum-based 
metallic material on which a porous oxide film is formed by anodic 
oxidation, an increase in an amount of gases trapped due to an increase in 
surface area can be, e.g, 400 times the amount trapped by the cryopanel 
having the flat surface. The cryopanel having the anodic oxide film can 
trap even helium, hydrogen, and neon. In addition, since the anodic oxide 
film has good mechanical strength and thermal stability, even if it is 
greatly vibrated or subjected to a repeated heat cycle, it will not peel. 
The thickness of the anodic oxide film can be 10 to several tens of 
micrometers. However, the film is preferably set to be thick enough to 
increase an effective trapping area. 
FIG. 9 shows another example of the cryopanel having the anodic oxide film. 
The cryopanel is formed in such a manner that alumimum-based metallic 
materials 82 are cladded on two surfaces of an oxygen free copper 
substrate 81 and are subjected to anodic oxidation in the same manner as 
described above so as to form porous anodic oxide films 72. Since copper 
has a better thermal conductivity than aluminum, this cryopanel is more 
easily cooled. 
Note that the shield panels 22 and 28 can be formed of the same material as 
that of the cryopanel. 
Table 1 below shows vacuum pressures in a chamber and detection limits of 
various light elements contained in GaAs of the conventional apparatus 
having no cryopanels 19 and 24 and the apparatuses I and II of the present 
invention. The apparatus I is the same as the conventional one except that 
a cryopanel is provided to surround the sample as shown in FIGS. 3 and 4. 
The apparatus II is as shown in FIGS. 3 and 4. Note that the cryopanel was 
cooled to a temperature of 20 K, and the vacuum pressure mentioned in 
Table 1 is a pressure within the overall chamber. 
TABLE 1 
______________________________________ 
Apparatus Apparatus 
Conven- I of II of 
tional present present 
Item apparatus invention invention 
______________________________________ 
Vacuum pressure 
.sup. 2 .times. 10.sup.-9 
.sup. 7 .times. 10.sup.-10 
.sup. 1 .times. 10.sup.-11 
(Torr) or less 
Vacuum pressure 
.sup. 3 .times. 10.sup.-9 
.sup. 9 .times. 10.sup.-10 
.sup. 5 .times. 10.sup.-11 
during analysis 
(Torr) 
Detection 
C 2 .times. 10.sup.16 
5 .times. 10.sup.15 
2 .times. 10.sup.14 
limits of 
O 6 .times. 10.sup.16 
7 .times. 10.sup.15 
4 .times. 10.sup.14 
light el- 
Si 5 .times. 10.sup.14 
1 .times. 10.sup.14 
2 .times. 10.sup.13 
ements in 
S 2 .times. 10.sup.15 
5 .times. 10.sup.14 
1 .times. 10.sup.13 
GaAs 
(atoms/cm.sup.3) 
______________________________________ 
Table 1 shows that the detection limits of the apparatus of the invention 
are improved due to decrease in the pressure around the sample and that 
the pressure in the overall chamber is also decreased with the apparatus 
of the invention. An effect with respect to the detection limits of light 
elements is considerable with oxygen (O) and carbon (C). In addition, as 
is apparent from Table 1, the detection limits with respect to silicon 
(Si) having the same mass number as that of carbon oxide (CO) and sulfur 
(S) having the same mass number as that of oxygen molecule (O.sub.2) can 
be lowered to 1/5 or 1/20 level. The cooling temperature of the cryopanel 
19 is most effective when it is as low as possible, preferably, 20 K or 
lower. Even when the cooling temperature is 30 K or slightly higher, the 
detection limits could be improved. 
As described above, in the apparatus for quantitative secondary ion mass 
spectrometry according to the present invention, since light element 
components of residual gases in the vacuum surrounding a sample to be 
analyzed can be effectively evacuated by a cryopanel, background light 
elements preset during analysis can be greatly reduced. Therefore, when 
the apparatus of the present invention is used particularly for analysis 
of a high purity semiconductor material, quantitative analysis of trace 
amounts of light elements in the semiconductor material and evaluation of 
concentration distribution therein can be performed. This allows high 
quality of a semiconductor crystal and a semiconductor integrated circuit.