GB2175739A

GB2175739A - X-ray generator

Info

Publication number: GB2175739A
Application number: GB08608765A
Authority: GB
Inventors: Xiang-Rong Yu
Original assignee: BEIJING CENTER FOR PHYSICAL AN
Current assignee: BEIJING CENTER FOR PHYSICAL AN
Priority date: 1985-04-10
Filing date: 1986-04-10
Publication date: 1986-12-03
Also published as: CN85102523B; CN85102523A; GB8608765D0

Abstract

An X-ray generator with horizontal arrangement for X-ray photoelectron spectrometer includes a four target adjustable, interchangeable, discrete and small cubic target head 10, an interchangeable window frame 9, a double filaments assembly 8, a cryo-reservoir 1, a heat transmitter 6, an ion cleaning chamber 14, and a transporting rod 11. The target head 10 and the window material (21) of the window frame 9 can be cleaned in the ion cleaning chamber 14 and be interchanged outside the vacuum system through the operation of the transporting rod 11 and ultra high vacuum valve 12, without deterioration of the ultra high vacuum in the sample analysis chamber 7. <IMAGE>

Description

SPECIFICATION X-ray generator This invention relates to X-ray generators, in particular for an X-ray photoelectron spectrometer. Based on the same concept, however, it is also applicable to lithographic, medical and other analytical equipment using X-ray generators.

An X-ray generator is one of the vital parts in an X-ray photoelectron spectrometer, and its performance is closely related to the quantity and quality of information obtained by Xray photoelectron spectroscopy (XPS) experiments.

Hitherto, X-ray generators used for XPS instrumentation have been of three main types, namely single-anode (for example, K. Siegbahn et al; EXCA: Atomic, Molecular and Solid State Structure Studied by Means of Electron Spectroscopy, ova Acta Regiae Soc. Sci; Upsaliensis, Ser. IV, Vol; 20(1967) ), dual-anode (for example, K.Yates, A. Barrie and F.J.

Street, J. Phys E6, 130 (1973) ), and quadranode (for example, Q.C.Herd, UK Patent Application GB 2133208A) X-ray generators.

At present, the dual-anode X-ray generator (Fig. 1) is available on most commercial X-ray photoelectron spectrometers. A later quadr-anode X-ray generator is also now available. Fig.

2 is a schematic drawing of the quadr-anode X-ray generator. It has a multi-facetted target (T1-4) and two filaments (F 1 and 2) respectively disposed in lower and upper positions for emitting electrons to bombard a selected face of the anode. The target is rotatable to bring any selected anode face opposite a selected filament by means of a remote control indexing mechanism. The target has four facets (T1,2,3,4) each of different anode material, e.g. Mg, Al, AG, Ti, deposited on a copper base, with a centrally located electron trap (g) to avoid cross-over interference.

It is an object of this invention to provide an improved X-ray generator.

The present invention consists in an X-ray generator assembly having a discrete and interchangeable target head.

Preferably, the generator assembly has an interchangeable window frame supporting a window positioned in use between the target head and the sample to be irradiated.

Suitably, the generator assembly has a cryoreservoir and a heat transmitter connected between the cryo-reservoir and the target the dissipate heat from the target.

Preferably, the assembly has a transporting rod adapted to engage the target head and the window frame to enable the target head or window frame to be withdrawn from the assembly.

Preferably, the generator assembly includes an ion cleaning chamber connected in series with a sample analysis chamber containing the target head so that the target head can be withdrawn into the cleaning chamber, the cleaning chamber being equipped with an etching ion gun to clean surface contamination from anodes on the target head.

The invention will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 illustrates a known dual-anode Xray generator, Figure 2 illustrates a known multi-facetted anode for an X-ray generator, Figure 3 is a diagrammatic illustration of an X-ray generator in accordance with the present invention, Figure 4 is a cross-section through the target head and window frame of the X-ray generator, Figure 5 is a cross-section through the target head, Figure 6 is a section on line VI-Vl of Fig. 5, Figure 7 is an end view of the window frame, Figure 8 is a plan view, partly in section, of the window frame, and Figure 9 is a detail view of a transporting rod of the X-ray generator.

As shown in Fig. 3, the X-ray generator assembly consists of a number of components arranged horizontally. The assembly includes a cryo-reservoir 1 to contain a cryogen, for example liquid nitrogen (at about 77 K). The reservoir is made of double metal cylinders isolated from one another and matched with the ultra-high vacuum system. An isolated thermocouple (not shown) is fitted in the reservoir as a coolant sensor to monitor the level of liquid nitrogen and to interlock with the high voltage power supply for the X-ray generator. A heat transmitter 6 has one end welded into the lower part of the inner cylinder of the reservoir 1.The heat transmitter extends through a ceramic spacer 4 into the sample analysis chamber 7 and its other end, which is square in cross-section, fits intimately into a cavity in the target head 10, so that heat is dissipated from the target head through the heat transmitter. The portion of the heat transmitter 6 engaging the reservoir 1 is of stainless steel, and is connected by a spring sheet to a copper portion which engages the target head 10.

The target head 10 is in the form of a hollow tetragonal prism, having four target anodes 10', 10'2, 10'3, 10'4, which may be of different materials. The target head 10 is supported in the sample analysis chamber by the heat transmitter 6 and the transporting rod 11, described below.

The positive high electrical potential (for example 1-20 KV) applied to the target head is supplied through a high-voltage terminal assembly 2, for example with a maximum 28 KV rating. Filament current is supplied through a terminal assembly 3 with three conductors, for example each of maximum 15A rating.

The filament assembly 8 consists of a ceramic base 25, two filaments 24a and 24b and two shield electrodes 23 which block the direct line of sight between the filaments and the target head. Each electrode 23 has a suitable slot for passage of electrons emitted by the filament, and acts to focus electrons onto the target, suitable potential being applied to the electrodes, normally the same potiential as the filaments, which are near earth potential.

The filaments 24a and 24b can be selectively or simultaneously heated to emit electrons.

The interchangeable window frame 9 has a cylindrical body which is carried by a support sleeve 5 and which is located by a pin on the sleeve 5 engaging in a slot 28 in the window frame. The window frame can be positioned on or taken off the sleeve 5 by means of the transporting rod 11 through an adaptor 29.

Projecting from the body of the window frame is a curved plate 27, having a suitable slot, on which the selected window material can be mounted and held in place by a slotted sheet 22 fixed to the plate 27.

The copper support sleeve 5 is fixed in the sample analysis chamber and serves to support the filament assembly 8 and the window frame 9.

The transporting rod 11 extends into the sample analysis chamber 7 and has at its end a screw thread 30 which can engage with an internal screw thread at one end 10" of the target head, to enable the target head to be moved on and off the heat transmitter 6. The screw thread 30 also enables the adaptor 29 to be fixed to the end of the transporting rod 11, so that the window frame 9 can be removed by causing the adaptor to engage the body of the window frame. The transporting rod 11 enables the target head to be moved into an ion cleaning chamber 14 and enables the target head or the window frame to be withdrawn from the vacuum system through a vacuum valve or a sealing gland 16 so that the target head or window frame (and hence the window material) can be interchanged.

The transporting rod 11 can be operated manually or by stepping-motors (not shown).

The ultra-high-vacuum sample analysis chamber 7 accommodates the X-ray generator, an electron lens 20, samples 26, a pumping port and various accessories for XPS experiments.

A UHV valve 12, manually or electro-pneumatically operated, enables the transporting rod 11 to enter the sample analysis chamber.

The valve is interlocked with the high voltage power supply for the X-ray generator, so that while the valve is opened, to allow the transporting rod to operate in the sample analysis chamber, the high voltage supply is overridden and cannot be energised.

The ion cleaning chamber 14 is connected to the sample analysis chamber through the valve 12, and has pumping port 15 connected for example to a turbo-molecular pump. The chamber 14 contains a simple etching ion gun 13 to clean surface contamination from the anodes of the target head.

To enable the target head or window frame to be withdrawn, an insertion lock 17 provided with a differential pumping line 18 is connected to the sealing gland 16.

To assist the positioning of the target head and window frame, the outer end of the transporting rod 11 moves against a positioning scale 19.

The described embodiment has the following desirable capabilities: 1. The target is small and discrete, and can be easily adjsuted and interchanged without the deterioration of ultra-high vacuum in the sample analysis chamber, so that one can select in principle unlimited characteristic X-ray energies (for example, from higher energy Ti, Ag, Au, Zr to medium energy Si, Al, Mg, Na and to lower energy anode materials F, Zr and Y etc.) to excite photoelectrons and Auger electrons from different levels of an atom and different depths beneath the surface of a solid.

2. The surface of the anodes on the target can be cleaned in the vacuum system to eliminate X-ray "ghost" interference lines coming from contamination and oxidation on the surface of anodes, to get quality results of quantitative analysis, and to enable some reactive anode materials for example, Y etc, to be used.

3. The window material can be easily interchanged and cleaned without the deterioration of UHV in the sample analysis chamber, so one can reasonably match the selected window material with the anode in use to get maximum and constant output of the characteristic X-ray radiation.

4. There is increased anode loss power to modify the ratio of signal to noise and to shorten the analysis period. It is particularly meaningful to have suitable intensity from a monochromatic X-ray source. The difficulty in generating high output of primary X-rays, arising mainly from the traditional approach of heat dissipation of the target using water coolant, is overcome.

5. The routine operation of an X-ray generator runs without cooling water to avoid some considerable problems for instance of water consumption or noise generated by water cooling equipment, corrosion and blockage of the pipes and valves in a water cooling system.

6. There is no cross-over interference at all between the anodes on a multifacetted target head so simplifying the interpretation of an XPS spectrum.

Claims

1. An X-ray generator assembly having a discrete and interchangeable target head.

2. An X-ray generator assembly according to Claim 1 having an interchangeable window frame supporting a window positioned in use beweeen the target head and a sample to be irradiated.

3. An X-ray generator assembly according to Claim 1 or Claim 2 having a cryo-reservoir and a heat transmitter extending between the cryo-reservoir and the target head to act as a heat dissipator for the X-ray target.

4. An X-ray generator assembly according to Claim 1 and having a transporting rod adapted to engage the target head to enable its position to be adjusted or the head to be withdrawn from the assembly.

5. An X-ray generator assembly according to Claim 2 and having a transporting rod adapted to engage the target head and the window frame to enable the target head or the window frame to be withdrawn from the assembly.

6. An X-ray generator assembly according to any preceding claim having an ion cleaning chamber in series with sample analysis chamber and equipped with an etching ion gun to clean surface contamination from the anode on the target head, and means for moving the target head from the sample analysis chamber to the ion cleaning chamber.

7. An X-ray generator assembly as claimed in Claim 6, in which the means for moving the target head comprise a transporting rod adapted to extend through the ion cleaning chamber into the sample analysis chamber and to releasably engage the target head.

8. An X-ray generator assembly substantially as herein described with reference to Figs. 3 to 9 of the accompanying drawings.

9. An X-ray monochromator having an Xray generator assembly in accordance with any of the preceding claims.

10. An X-ray photoelectron spectrometer having an unmonochromatized and/or monochromatized X-ray generator assembly in accordance with any one of the preceding claims.