JP2738761B2 - X-ray counter - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention relates to X-rays for soft X-rays necessary for an apparatus for analyzing the composition ratio of light elements or the concentration of light element impurities using fluorescent X-rays. It relates to a line counter.

[Conventional technology]

In a so-called light element having an element number smaller than that of Ca, the excitation efficiency of fluorescent X-rays is lower than that of a heavy element, and Auger electron emission has a higher probability. However, the use of Auger electrons whose escape depth is several nanometers makes it possible to analyze only the very surface of a substance. Therefore, in the analysis of bulk materials, X-ray fluorescence analysis is useful not only for heavy elements but also for light elements. Further, since the characteristic X-ray of the light element belongs to the axial X-ray region where the energy is 5 keV or less, the absorption by air in the atmosphere is large, so that it is necessary to measure in a vacuum. Therefore, for the measurement of fluorescent X-rays of light elements, a detector that is highly sensitive and compensates for the low excitation efficiency of fluorescent X-rays and that can withstand use in vacuum is indispensable. The high-sensitivity detector means a detector having a large-sized X-ray intake window capable of efficiently capturing omnidirectional fluorescent X-rays.

Incidentally, as a document describing this type of technology, M.
Funabashi, et.al .; “Compact fluorescence X-ray det
ector for surface EXAFS and X-ray standing wave m
easurements ", Rev. Sci. Instrum., Vol. 60, p. 2505- 2508,
1989 [Funabashi et al., "X-ray Fluorescence Detector for X-ray Absorption Edge Fine Structure and X-ray Standing Wave Measurement", Review of Scientific Instruments, Vol. 60, 2505-25
08, 1989].

[Problems to be solved by the invention]

At present, an X-ray semiconductor detector and a proportional counter using a PR gas or the like are used for measuring fluorescent X-rays of light elements. Since the X-ray semiconductor detector is a solid state device, there is no problem for use in a vacuum. However, it is not easy to increase the area of a semiconductor element required for achieving high sensitivity. As a semiconductor detector for soft X-rays, a Li-doped Si device (hereinafter, referred to as a Si (Li) device) is used, but the maximum area of a Si (Li) device that can be manufactured by current technology is 200 mm ² . . And 200mm ²
Soft X-rays of about 1 KeV or more can be measured with the Si (Li) element, and the maximum area of the Si (Li) element that can measure Kα X-rays of about 1 keV or less such as oxygen, nitrogen, and carbon is further limited to 80 mm ² Is done. When the element area is large, the in-plane uniformity of the element crystal cannot be ensured, so that the noise level becomes high, which hinders low energy X-ray measurement. Accordingly, there is a problem that it is difficult to increase the sensitivity required for compensating for the excitation efficiency of fluorescent X-rays with low light elements in the semiconductor detector for soft X-ray measurement. On the other hand, in a proportional counter using gas, it is easy to manufacture a counter having a large area window. However, for several tubes for measuring soft X-rays of about 1 keV or more, a Be film that can withstand the atmospheric pressure difference can be used as a window material.
In order to be able to measure even soft X-rays of 1 keV or less, the window material is limited to a polymer thin film that is vulnerable to a pressure difference. In the soft X-ray region, the energy of characteristic X-rays of each light element is close to each other, but the energy resolution of the proportional counter is about six times worse than that of the X-ray semiconductor detector.
Accordingly, the proportional counter has a problem in use in a vacuum since there is a possibility of gas leak, and there is a problem that the energy resolution is poor.

An object of the present invention is to solve the above-mentioned problems in the prior art, and to provide a counter for soft X-rays that can be used in ultra-high vacuum with high sensitivity and good resolution.

[Means for solving the problem]

In the present invention, in order to achieve the above object, the gas is excited by accelerating a photoelectron generated in the gas in proportion to the energy of the X-ray by passing the X-ray by a predetermined electric field and colliding with the gas. In the X-ray counter for measuring the X-rays by converting the fluorescence generated by the excitation into an electric signal with a photomultiplier tube, the gas cell into which the excitation gas is introduced and the photomultiplier tube are hermetically sealed. It is arranged inside the inner cylinder of the double cylinder having the structure, and the outer cylinder of the double cylinder is attached to the side wall of the vacuum chamber on the X-ray source side via a metal bellows so as to be adjustable in distance from the X-ray source, A polymer thin film is used for a window through which X-rays of the inner cylinder and the outer cylinder pass, and an airtight space sandwiched between the inner cylinder and the outer cylinder is evacuated to a vacuum.

[Action]

Conventional soft X-ray detectors, such as proportional counters and semiconductor detectors, increase the number of ion pairs generated by the photoelectric effect along the path through which X-rays pass by applying a high voltage in a gas or solid. On the other hand, the present invention converts the fluorescence generated by exciting the gas using photoelectrons generated in the gas in proportion to the energy of the X-ray by passing the X-ray and converting the generated fluorescence into visible light. The principle of measurement is a gas fluorescence proportional method that takes out an electric signal with an optoelectronic amplifier, and the high sensitivity is measured by a large area measurement window using a polymer thin film as a window material in order to adapt to soft X-ray measurement. In addition to compensating for the low excitation efficiency of fluorescent X-rays, the airtight space between the inner and outer cylinders is evacuated to a vacuum by making the counter tube an airtight double tube structure, thereby suppressing gas leakage from the measurement window. Vacuum Chan The point that enables the use of the inner and main features. Also, by using a metal bellows for a part of the outer casing of the double cylinder structure, the distance between the counter tube and the sample that emits fluorescent X-rays can be adjusted without breaking vacuum when connected to a vacuum chamber. By bringing the counter tube close to the sample, it becomes possible to capture most of the fluorescent X-rays generated in combination with the large-area measurement window. Therefore, the counting principle is different from that of the conventional proportional counter or semiconductor detector, and the housing structure of the counter is also different.

〔Example〕

Hereinafter, an embodiment of the present invention will be described with reference to FIG. FIG. 1 is a cross-sectional view showing a state in which an X-ray counter tube of the present invention is connected to a vacuum chamber (not shown) holding a sample therein, wherein 1 is an outer tube of the counter tube, and 2 is an outer diameter of 36 mm. , A window of an outer cylinder made of a Mylar membrane having a thickness of 0.6 micrometers, 3 is an inner cylinder of a counter tube, 4 is an inner cylinder window of a Mylar membrane having the same shape as the window of the outer cylinder, 5 is about 40 mmHg of Xe gas. A gas cell to be filled, 6 is a pipe for externally introducing Xe gas into the gas cell 5, and 7 and 9 are voltages applied from the outside by electric wirings 8 and 10, immediately after the window 4 of the inner cylinder and at a central position. Metal mesh provided, 11 is an organic wavelength conversion agent for converting the fluorescence emitted by Xe gas into visible light, 12 is a photomultiplier tube, 13 is a preamplifier, 14 is an electric signal from a preamplifier 13 Electrical wiring to take out the light, 15 is the photomultiplier tube 12
Wiring for supplying the high voltage required for
Is a cover of the counter tube integral with the inner tube 3 of the counter tube, 17 is an exhaust port for evacuating the space between the outer tube 1 and the inner tube 3, 18
Is a conflat flange for connection on the vacuum chamber side, 19 is a conflat flange on the counter tube side for connection to the flange 18, and 20 is a conflat flange for connection with the lid 16 integrally welded to the outer cylinder 1. , 21 has flanges 19, 20 at both ends
The metal bellows 23, 24 are welded to the outer circumference of the flanges 19, 20 at equal intervals at four places, and the support base of the rod-shaped bolt 22 for adjusting the amount of expansion and contraction of the metal bellows 21, 25, 26
Is a nut for fixing the rod-shaped bolt 22 to the support bases 23, 24.

In the above configuration, Xe
The fluorescent X-rays in the soft X-ray region incident on the gas cell 5 are photoelectrically absorbed by Xe to form a primary electron group.
When 170 eV and 5 keV are respectively applied to and 9, the primary electron group becomes 7
And Xe atoms are excited and emitted in the space between 9 and the organic wavelength conversion agent 11. This excitation light has a wavelength of about 17
It is ultraviolet light of 0 nanometer, and the total number of photons is proportional to the number of primary electrons, that is, the energy of the incident soft X-ray. Xe
After the excitation light is converted into visible light by the organic wavelength converter 11, the light enters the photomultiplier tube 12 and is converted into an electric signal.
The electric signal amplified by the preamplifier 13 is connected to the electric wiring 14.
Can be extracted from the soft X
The energy and photon number of the line can be measured. In contrast to the conventional gas-based proportional counter, which uses the gas amplifying effect of an avalanche, this counter uses the photoelectron amplifier to count the Xe fluorescence, so the energy resolution is lower than that of the proportional counter. Is about twice as good. If a sealed space between the inner cylinder and the outer cylinder is evacuated to vacuum by connecting a turbo molecular pump or the like to the exhaust port 17, even if leakage of Xe gas from the Xe gas cell 5 occurs, the degree of vacuum in the vacuum chamber is reduced. Since it has almost no effect, it can be used by connecting to an ultra-high vacuum chamber. Also, by adjusting the length of the rod-shaped bolt 22 between the support bases 23 and 24 with the nuts 25 and 26, the window of the counter tube can be brought close to the sample in the vacuum chamber.
Combined with the effect of a polymer window with a large area of 1000 square millimeters and low absorption, it is possible to measure fluorescent X-rays of weak light elements generated from the sample.

FIG. 2 shows the results obtained by measurement using the present invention, and 10 ^-8 To
FIG. 4 is a fluorescent X-ray spectrum when a ZnSe sample formed on a GaAs substrate held in a high vacuum chamber of the order of rr is irradiated with about 400 eV monochromatic soft X-ray. Part A where the spectrum is dense is the peak of carbon Kα fluorescent X-ray with energy of 277 eV, and the fluorescence from the trace carbon contaminated layer on the ZnSe surface which is not measured by the conventional proportional counter or semiconductor detector. X-ray has been detected. The half width of the carbon Kα fluorescent X-ray peak is about 100 eV, and the energy resolution is as good as 36%. As is apparent from the measurement results, the present invention has improved vacuum resistance, sensitivity to soft X-rays, and energy resolution as compared with the conventional proportional counter.

〔The invention's effect〕

As described above, when the present invention is used, fluorescent X-rays of light elements having low excitation efficiency can be measured with high resolution and high sensitivity in a vacuum chamber, so that an extremely thin oxide film or nitride film formed on the surface of a semiconductor substrate can be measured. There is an advantage that X-ray fluorescence from a film or X-ray fluorescence from a light element added in a trace amount in a semiconductor or the like can be measured with high sensitivity and high accuracy.

[Brief description of the drawings]

FIG. 1 is a cross-sectional view of one embodiment of the present invention, and FIG. 2 is an X-ray spectrum in a soft X-ray region measured by using the present invention. The intensity of the line (count). [Description of Signs] 1 ... Outer tube of counter tube, 2 ... Window of outer tube 3 ... Inner tube of counter tube, 4 ... Window of inner tube 5 ... Gas cell, 6 ... Piping for gas introduction 7, 9 Metal mesh 8, 10, 14, 15 Metal wiring 11 Organic wavelength conversion agent 12 Photomultiplier tube 13 Preamplifier 16 Counter tube lid 17 … Exhaust port 18, 19, 20… Conflat flange 21… Metal bellows, 22… Bolt bolt 23, 24… Bolt support 25, 26 …… Nut

Claims (1)

(57) [Claims] 1. A method in which a photoelectron generated in a gas in proportion to the energy of the X-ray due to passage of the X-ray is accelerated by a predetermined electric field and collides with the gas to excite the gas, and the fluorescence generated by the excitation is excited. X-ray counter for converting X-rays into electric signals with a photomultiplier tube and measuring X-rays, wherein the gas cell into which the excitation gas is introduced and the photomultiplier tube are formed in an airtight double cylinder inner cylinder. And the outer cylinder of this double cylinder is X
The distance between the X-ray source and the X-ray source is adjustable on the side wall of the vacuum chamber on the side of the X-ray source via metal bellows. An X-ray counter characterized in that an airtight space sandwiched between a cylinder and an outer cylinder is evacuated to a vacuum.