WO2019122924A1

WO2019122924A1 - Vacuum chamber, parts therefor and method for manufacturing the same

Info

Publication number: WO2019122924A1
Application number: PCT/GB2018/053750
Authority: WO
Original assignee: Teledyne E2V (Uk) Limited
Priority date: 2017-12-21
Filing date: 2018-12-21
Publication date: 2019-06-27
Also published as: GB201721557D0; GB2570441A; GB2570441B; EP3729205A1

Abstract

A vacuum chamber comprises a vacuum chamber wall (2) having an aperture (6) therethrough and an electromagnetic radiation transmissive window (4) positioned in the aperture. The window (4) has an external face (12) external to the chamber and an internal face (13) internal to the chamber, the external face (12) and the internal face (13) being substantially parallel and the external face having a larger area than the internal face. A peripheral surface (14) of the window is inclined relative to the normal to the plane of the internal and external faces, with a vacuum seal between the window and the wall at the inclined peripheral surface (14).

Description

VACUUM CHAMBER, PARTS THEREFOR AND METHOD FOR

MANUFACTURING THE SAME

FIELD OF THE INVENTION

This invention relates to vacuum chambers having a window, parts therefor and a method for manufacturing such vacuum chambers, and particularly but not exclusively to vacuum chambers included in cold atom or cold ion apparatus.

BACKGROUND

Cold atoms or ions are used to achieve quantum mechanical effects for use in cold atom clocks, gravimeters or other apparatus. The desired cold state is achieved by using a laser or lasers to reduce the kinetic energy of the atoms or ions in a low vacuum environment. Typically, the cold matter is then manipulated using lasers to produce an effect caused by the characteristic under investigation. Lasers may then be used to measure the effect produced. There are significant challenges in being able to meet both vacuum and optical requirements. Vacuum chambers are also used in non-quantum technology applications.

BRIEF SUMMARY

According to a first aspect of the invention, a vacuum chamber comprises: a vacuum chamber wall having an aperture therethrough and an electromagnetic radiation transmissive window positioned in the aperture, the window having an external face external to the chamber and an internal face internal to the chamber, the external face having a larger area than the internal face, a peripheral surface of the window being inclined relative to the normal to the plane of the internal and external faces, with a vacuum seal between the window and the wall at the inclined peripheral surface and the vacuum seal being extensive over less than or equal to two thirds of the dimension of the peripheral surface in a direction from the internal face to the external face. In one embodiment, the vacuum seal is extensive over one third or less of the dimension of the peripheral surface in a direction from the internal face to the external face. The inventors have appreciated that a vacuum seal over a shorter distance than over the entire thickness of the peripheral wall can still give the required vacuum performance that one might achieve with a longer vacuum seal but also is able to better withstand stresses due to thermal differentials because it is shorter.

The window configuration and vacuum within the vacuum chamber provide optical surfaces with good flatness. Flatness is particularly important for applications where light must travel several times through the window, where any deviations can lead to radiation losses and affect performance. The window flatness may be better than or equal to 40 nm over the central 90% of the window. The window may be

transmissive for radiation in at least one of: the visible spectrum, infra-red spectrum and ultra-violet spectrum.

In one embodiment, the external face and the internal face of the window are substantially parallel.

In one embodiment, the window is of circular cross section but other

configurations are possible. For example, it could be rectangular with rounded corners.

In one embodiment, the ratio of the thickness of the window to the diameter of the window is 1:5.

In one embodiment, the window has a thickness of less than lOmm. The window may have a thickness of less than 5 mm.

The peripheral surface of the window may be inclined at an angle of between 1 degree and 10 degrees relative to a normal to the internal face. However, the inclination may be at an angle greater or smaller than this range. In one embodiment, the angle of inclination is approximately 2 degrees.

The peripheral surface may be inclined at the same angle around its entire extent as this provides symmetry which can be desirable for applications where mechanical and thermal stresses are to be minimised where possible. In other applications which are less tightly constrained the angle of inclination is non-constant around the periphery.

In one embodiment, the window is brazed to the wall using filler material. In another embodiment, the window is sealed to the wall by a joint formed using a solid- liquid interdiffusion (SLID) process. In an SLID process, layers of metallic materials having different melting points are brought into contact. By diffusion, first in the liquid phase and then in the solid phase, one or more intermetallic compounds are formed with a re-melting temperature which is generally between the melting points of the initial materials. Bonds can thus be made at a low temperature and subsequently withstand higher temperature of use. In another embodiment, the window is sealed to the wall by a weld, which may be, for example, a laser weld.

The internal face of the window may be supported by a ledge or lip of the wall.

The chamber wall and the peripheral surface of the window may be substantially parallel to each other over the contact area between the window and the chamber wall. The contact area may have a length of between 0.5 mm and 5 mm in a direction from the internal face to the external face. In one embodiment, the contact area has a length of approximately 1 mm. The contact area between the window and the wall may be located in a region extensive over two thirds of the peripheral surface from the internal face towards the external face, and may be located in a region extensive over one third of the peripheral surface from the internal face towards the external face.

Various wall configurations may be used to support the window. For example, in one embodiment, a portion is flexible to mitigate stress due to differential thermal expansion.

The chamber wall may include a window frame within which the window is positioned and the window frame is joined to a portion of the chamber wall. This is particularly advantageous where several windows are located within a vacuum chamber as it allows the vacuum seal of each window/frame subassembly to be tested prior to it being joined to the main part of the chamber wall. The window frame or frames may comprise at least one of stainless steel, molybdenum, nickel copper alloy and mu-metal.

The chamber wall may comprise at least one of stainless steel, molybdenum, nickel copper alloy and mu-metal.

The vacuum chamber may be such that joint leakage via the vacuum seal is lower than or equal to 10 ¹⁰ mbar litres per second. The vacuum pressure within the chamber may be lower than or equal to 10 ⁸ mbar. These parameters have been found suitable for quantum technology applications such as magneto-optical traps.

According to a second aspect of the invention, a method of manufacturing a vacuum chamber includes the steps of:

a. positioning an electromagnetic radiation transmissive window in an aperture in a wall of the vacuum chamber, the window having an external face external to the chamber and an internal face internal to the chamber, the external face having a larger area than the internal face, a peripheral surface of the window being inclined relative to the normal to the plane of the internal and external faces;

b. making a vacuum seal between a peripheral surface of the window and the wall, the window having a deformation outwardly from the interior of the chamber after the seal is made and the vacuum seal being extensive over less than or equal to two thirds of the dimension of the peripheral surface from the internal face to the external face; and

c. then producing a vacuum within the chamber such that the sealed window is deformed in an inward direction to counteract at least partially the deformation of the window in an outwardly direction.

By using a method in accordance with the invention, outward deformation which may be produced by stresses in the subassembly when the window is sealed to the wall is at least somewhat compensated by the deformation inwardly when the vacuum is produced. This may provide a robust structure with good optical flatness, particularly if the deformation inwardly and deformation outwardly substantially match to provide net zero deformation. The peripheral surface of the window may be substantially parallel to the surface of the wall with which it is in contact.

In one method in accordance with the invention, the step is included of, during the vacuum sealing step, heating the window and applying force to the window in a direction inwards towards the interior of the vacuum chamber.

The window may be being heated to a temperature in the range 200 to 300 degrees Celsius.

The chamber wall including a window frame having the aperture therethrough, and the steps are included of making a vacuum seal between the window and the window frame, testing the vacuum seal prior to the window frame being joined to a portion of the chamber wall and joining the window frame to the portion with a vacuum seal.

According to a third aspect of the invention, a window assembly for a vacuum chamber comprises: a window frame having an aperture therethrough and an

electromagnetic radiation transmissive window positioned in the aperture, the window having an external face external to the chamber when included therein and an internal face internal to the chamber, the external face having a larger area than the internal face, a peripheral surface of the window being inclined relative to the normal to the plane of the internal and external faces, with a vacuum seal between the window and the window frame wall at the inclined peripheral surface and the vacuum seal being extensive over less than or equal to two thirds of the dimension of the peripheral surface from the internal face to the external face.

BRIEF DESCRIPTION OF THE DRAWINGS

Some embodiments of the present invention will now be described by way of example only, and with reference to the accompanying drawings, in which:

Figure 1 schematically illustrates in cross-sectional view a magneto-optical trap apparatus including a vacuum chamber in accordance with the invention;

Figure 2 schematically illustrates part of another vacuum chamber in accordance with the invention;

Figure 3 schematically illustrates part of a vacuum chamber in accordance with the invention;

Figure 4 schematically illustrates part of a vacuum chamber in accordance with the invention;

Figures 5a and 5b schematically illustrate plan and cross-sectional views respectively of a window and surrounding frame;

Figures 6a, 6b and 5c schematically illustrate plan, cross-sectional and perspective views respectively of a window and its surrounding frame; and

Figures 7a, 7b and 7c schematically illustrate steps included in a method in accordance with the invention.

DETAILED DESCRIPTION

With reference to Figure 1, in one embodiment, a magneto-optical trap (MOT) apparatus 1 includes a vacuum chamber 2 within which laser cooled atoms 3 are held during operation. First and second windows 4 and 5 of N-ZK7 glass (available from SCHOTT AG) are positioned in opposite sides of the vacuum chamber 2 in respective apertures 6 and 7 through the wall 8 of the vacuum chamber. The wall material is at least one of stainless steel, molybdenum, nickel copper alloy and mu-metal. The windows 4 and 5 are joined to the surrounding wall 8 by hermetic seals. Another suitable glass material is N-BK7 glass, also available from SCHOTT AG. Other glasses may be used instead, for example, fused silica glass.

The windows 4 and 5 are transmissive to electromagnetic radiation and allow laser light to be introduced into the chamber 2 during operation. Reflective surfaces 9 within the vacuum chamber 2 direct laser radiation to a region where cold atoms are held during operation.

The MOT 1 also includes an atomic source 10, which in this embodiment is a source of rubidium atoms, arranged to dispense rubidium atoms into the vacuum chamber. A pump tube 11 is extensive through the chamber wall 2 to give a connection to an external vacuum pump (not shown) to produce a vacuum within the vacuum chamber 2. The vacuum required is dependent on the purpose of the vacuum chamber. For example, for an MOT holding cold atoms, a suitable vacuum may be lower than or equal to 10 ⁸ mbar.

A single laser may be used and reflective surfaces arranged within and/or external to the vacuum chamber 2 such that the laser output light makes several passes through the chamber 2. In other embodiments, two or more lasers are used. In an initial operating mode, the laser or lasers are directed to reduce the kinetic energy of atoms in the central region of the vacuum chamber 2 and hence cool them. In subsequent operating modes, laser radiation is used to produce a quantum effect and then monitor the quantum effect.

The windows 4 and 5 are nominally identical and sealed in the same manner. The first window 4 is circular in plan view and has an external face 12 which is of larger surface area than its internal face 13 and is substantially parallel thereto. The peripheral surface 14 of the window 4 is inclined at an angle of about 2 degrees relative to the normal to the planes of the external face 12 and internal face 13. It should be noted that the taper is exaggerated in the drawings for the purposes of clarity. The ratio of the thickness of the window 4 to the diameter of the window 4 is between approximately 1:3 and 1:8 and in this embodiment is approximately 1:5. The thickness of the window 4 is less than lOmm.

The chamber wall 8 surrounding the window 4 includes a cylindrical portion 15 having an inner surface portion 16 which is inclined at substantially the same angle as the peripheral surface 14 of the window 4 such that the two surfaces are substantially parallel.

The window 4 is sealed to the surrounding inner surface portion 16 using a solid-liquid interdiffusion (SLID) process using gold and tin to form the bond. In another embodiment, the vacuum seal is made by brazing using filler material or by some other suitable process such as laser welding. The contact area between the window 4 and the inner surface portion 16 of the wall 8 is located in a region which is extensive over two thirds of the peripheral surface 14 from the internal face 13 towards the external face 12. The contact area has a length of approximately 1 mm in this embodiment, illustrated by arrow“a”.

Figure 2 illustrates part of another embodiment showing the configuration of a tapered window 17 similar to that shown in the embodiment of Figure 1 and a portion of a vacuum chamber wall 18 to which it is sealed. The stainless steel wall 18 includes a circular inwardly-projecting portion 19 of molybdenum. The circular portion 19 has an inclined surface 20 which is hermetically sealed to the periphery of the window 17.

Figure 3 illustrates part of another embodiment. In this case, an inwardly projecting ring 21 includes sections 2la, 2lb and 2lc at different angles and is of a thickness and material which allows movement with, for example, thermal changes, to reduce stresses that might otherwise occur during manufacturing and/or operating processes. A window 22 is sealed to a section 2la of the ring 21. A suitable material for ring 21 is molybdenum. The wall 23 of the vacuum chamber may be of stainless steel.

Figure 4 illustrates part of another embodiment showing the configuration of a tapered window 24 similar to that shown in the embodiment of Figure 1 and a portion of a vacuum chamber wall 25 to which it is sealed. An inwardly projecting ring 26 is angled downwardly as shown.

In another embodiment, as shown in Figure 5a and 5b, a tapered window 27 is hermetically sealed to a window frame 28. The peripheral surface 29 of the window 27 is inclined and is bonded to an inclined surface of the window frame 28 which is substantially parallel to the peripheral surface 29 to give a vacuum seal. After sealing, the join between the window 27 and frame 28 is tested. If satisfactory, the frame and window subassembly is then joined to the remainder of the vacuum chamber wall by welding or some other suitable technique. Figures 6a, 6b and 6c illustrate a window frame and a tapered window having a different configuration.

With reference to Figure 7a, a method in accordance with the invention includes positioning an electromagnetic radiation transmissive window 30 in an aperture in a wall 31 of a vacuum chamber 32, the window 30 having an external face 33 external to the chamber and an internal face 34 internal to the chamber 32. The external face has a larger area than the internal face. A peripheral surface 35 of the window 30 is inclined relative to the normal to the plane of the internal and external faces 33 and 34. A vacuum seal 36 is then made between the peripheral surface 35 of the window 30 and the wall 31. The vacuum seal is made using a solid-liquid interdiffusion (SLID) process with gold and tin to form the vacuum seal. During the vacuum sealing step, the window is heated to a temperature in the range 200 to 300 degrees Celsius. Force is applied to the window in a direction substantially normal to the internal face and external face of the window and in a downward direction.

When the sealing step is complete and the window 30 is sealed in position, the window 30 has a deformation in a direction outwardly from the interior of the chamber as shown by arrow“a”. The deformation occurs because stresses are produced as the wall 31 recovers from the heat used in sealing process and because of the configurations of the wall 31 and of the window 30.

A vacuum is then produced within the chamber as shown in Figure 7b, resulting in the sealed window 30 becoming deformed in an inward direction towards to interior of the chamber, as shown by arrow“b”.

With reference to Figure 7c, the amount of deformation in the inward and outward directions is such that they substantially cancel out one another to provide a window with good optical flatness. The window flatness may be better than or equal to 40 nm over the central 90% of the window.

The chamber wall may include a window frame having the aperture

therethrough. A vacuum seal is made between the window and the window frame. Then the vacuum seal is tested prior to the window frame being added to a vacuum chamber and the chamber placed under vacuum.

The present invention may be embodied in other specific forms without departing from its spirit or essential characteristics. The described embodiments are to be considered in all respects only as illustrative and not restrictive. The scope of the invention is, therefore, indicated by the appended claims rather than by the foregoing description. All changes that come within the meaning and range of equivalency of the claims are to be embraced within their scope.

Claims

1. A vacuum chamber comprising: a vacuum chamber wall having an aperture

therethrough and an electromagnetic radiation transmissive window positioned in the aperture, the window having an external face external to the chamber and an internal face internal to the chamber, the external face having a larger area than the internal face, a peripheral surface of the window being inclined relative to the normal to the plane of the internal and external faces, with a vacuum seal between the window and the wall at the inclined peripheral surface and the vacuum seal being extensive over less than or equal to two thirds of the dimension of the peripheral surface in a direction from the internal face to the external face.

2. The vacuum chamber as claimed in claim 1 wherein the window is of

substantially circular cross-section.

3. The vacuum chamber as claimed in claim 2 wherein the ratio of the thickness of the window to the diameter of the window is between approximately 1:3 and 1:8.

4. The vacuum chamber as claimed in claim 3 wherein the ratio of the thickness of the window to the diameter of the window is approximately 1:5.

5. The vacuum chamber as claimed in claim 1 wherein ratio of the thickness of the window to the longest dimension of the window though the planar geometric centre of the window is between approximately 1:3 and 1:8.

6. The vacuum chamber as claimed in any preceding claim wherein the window has a thickness of less than or equal to 10 mm.

7. The vacuum chamber as claimed in claim 6 wherein the window has a thickness of less than or equal to 5 mm.

8. The vacuum chamber as claimed in any preceding claim wherein the external face and the internal face are substantially parallel.

9. The vacuum chamber as claimed in any preceding claim wherein the peripheral surface of the window is inclined at an angle of between 1 degree and 10 degrees relative to a normal to the internal face.

10. The vacuum chamber as claimed in claim 9 wherein the angle of inclination is approximately 2 degrees.

11. The vacuum chamber as claimed in any preceding claim wherein the peripheral surface is inclined at the same angle around its entire extent.

12. The vacuum chamber as claimed in any preceding claim wherein the window is sealed to the wall by a brazed joint, by a soldered joint or by a weld.

13. The vacuum chamber as claimed in claim 12 wherein the window is sealed to the wall by a joint formed using a solid-liquid interdiffusion (SLID) process.

14. The vacuum chamber as claimed in claim 12 wherein the window is sealed to the wall by brazing using filler material.

15. The vacuum chamber as claimed in any preceding claim wherein the wall

includes a ledge which supports the internal face.

16. The vacuum chamber as claimed in any preceding claim wherein the chamber wall and the peripheral surface of the window are substantially parallel to each other over the area of the vacuum seal between the window and the chamber wall.

17. The vacuum chamber as claimed in claim 16 wherein the chamber wall

comprises a tubular part having a surface at which the window is joined to the wall.

18. The vacuum chamber as claimed in claim 16 wherein the chamber wall includes a member projecting from a tubular wall, the member having a surface substantially parallel to the inclined surface of the chamber wall at which the window is joined to the wall.

19. The vacuum chamber as claimed in claim 18 wherein the member is flexible to mitigate stress due to differential thermal expansion.

20. The vacuum chamber as claimed in claim 16, 17, 18 or 19 wherein the area of the vacuum seal has a length of between 0.5 mm and 5 mm in a direction from the internal face to the external face.

21. The vacuum chamber as claimed in claim 20 wherein the area of the vacuum seal has a length of approximately 1 mm.

22. The vacuum chamber as claimed in any preceding claim wherein the vacuum seal between the window and the wall is extensive over one third or less of the dimension of the peripheral surface in a direction from the internal face to the external face.

23. The vacuum chamber as claimed in any preceding claim and wherein the

chamber wall includes a window frame within which the window is positioned and the window frame is joined to a portion of the chamber wall.

24. The vacuum chamber as claimed in claim 23 wherein the window frame is joined to the portion by one of brazing and welding.

25. The vacuum chamber as claimed in claim 23 or 24 and including a plurality of window frames, at least one of the plurality including a window positioned in the chamber wall with a vacuum seal between the window and the wall, the window being transmissive to electromagnetic radiation, and a peripheral surface of the window being inclined such that an internal face of the window has a smaller surface area than an external face of the window, a peripheral surface of the window being inclined relative to the normal to the plane of the internal and external faces, with a vacuum seal between the window and the wall at the inclined peripheral surface and the vacuum seal being extensive over less than or equal to two thirds of the dimension of the peripheral surface from the internal face to the external face.

26. The vacuum chamber as claimed in claim 25 wherein the vacuum seal between the window and the wall is located in a region extensive over one third or less of the dimension of the peripheral surface in a direction from the internal face to the external face

27. The vacuum chamber as claimed in claim 24, 25 or 26 wherein the window frame or frames comprises at least one of stainless steel, molybdenum, nickel copper alloy and mu-metal.

28. The vacuum chamber as claimed in any preceding claim wherein the chamber wall comprises at least one of stainless steel, molybdenum, nickel copper alloy and mu-metal.

29. The vacuum chamber as claimed in any preceding claim wherein the window flatness is better than or equal to 40 nm over the central 90% of the window.

30. The vacuum chamber as claimed in any preceding claim wherein joint leakage via the vacuum seal is lower than or equal to 10 ¹⁰ mbar litres per second.

31. The vacuum chamber as claimed in any preceding claim wherein the vacuum pressure within the chamber is lower than or equal to 10 ⁸ mbar.

32. The vacuum chamber as claimed in any preceding claim wherein the window is transmissive for radiation in at least one of: the visible spectrum, infra-red spectrum and ultra-violet spectrum.

33. An ion or atom trapping arrangement including a vacuum chamber as claimed in any preceding claim.

34. A method of manufacturing a vacuum chamber as claimed in any preceding claim including the steps of: a. positioning an electromagnetic radiation transmissive window in an

aperture in a wall of the vacuum chamber, the window having an external face external to the chamber and an internal face internal to the chamber, the external face having a larger area than the internal face, a peripheral surface of the window being inclined relative to the normal to the plane of the internal and external faces;

c. then producing a vacuum within the chamber such that the sealed

window is deformed in an inward direction toward the interior of the chamber to counteract at least partially the deformation of the window in an outwardly direction.

35. The method as claimed in claim 34 wherein the seal between the window and the wall is located in a region extensive over one third or less of the dimension of the peripheral surface in a direction from the internal face to the external face.

36. The method as claimed in claim 34 or 35 wherein the amount of deformation in the outward direction and the inward direction is substantially the same.

37. The method as claimed in claim 34, 35 or 36 wherein the peripheral surface of the window is substantially parallel to the surface of the wall with which it is in contact.

38. The method as claimed in claim 34, 35, 36 or 37 and including, during the vacuum sealing step, heating the window and applying force to the window in a direction inwards towards the interior of the vacuum chamber.

39. The method as claimed in claim 38 and wherein the force is applied in a direction substantially normal to the internal face and external face of the window.

40. The method as claimed in claim 38 or 39 and the window being heated to a

temperature in the range 200 to 300 degrees Celsius.

41. The method as claimed in any of claims 34 to 40 and including making the

vacuum seal using a solid-liquid interdiffusion (SLID) process.

42. The method as claimed in claim 41 and the SLID process uses gold and tin to form the vacuum seal.

The method as claimed in any of claims 34 to 42 and the chamber wall including a window frame having the aperture therethrough, including the steps of making a vacuum seal between the window and the window frame, testing the vacuum seal prior to the window frame being joined to a portion of the chamber wall and joining the window frame to the portion with a vacuum seal.

44. A window assembly for a vacuum chamber comprising: a window frame having an aperture therethrough and an electromagnetic radiation transmissive window positioned in the aperture, the window having an external face external to the chamber when included therein and an internal face internal to the chamber, the external face having a larger area than the internal face, a peripheral surface of the window being inclined relative to the normal to the plane of the internal and external faces, with a vacuum seal between the window and the window frame wall at the inclined peripheral surface and the vacuum seal being extensive over less than or equal to two thirds of the dimension of the peripheral surface in a direction from the internal face to the external face.

45. A window assembly as claimed in claim 44 wherein the vacuum seal between the window and the wall is located in a region extensive over one third or less of the dimension of the peripheral surface in a direction from the internal face to the external face