GB2585647A

GB2585647A - Vapour cell

Info

Publication number: GB2585647A
Application number: GB1909799.7A
Authority: GB
Inventors: Dyer Terry
Original assignee: University of Strathclyde
Current assignee: University of Strathclyde
Priority date: 2019-07-08
Filing date: 2019-07-08
Publication date: 2021-01-20
Also published as: GB201909799D0; WO2021005051A1

Abstract

A vapour cell 2 comprises a substrate 4, a first layer 6 directly bonded to or integral with the substrate, and a chamber 8 at least partially formed in the first layer to hold a vapour or gas for exposure to a radiation. The substrate and the first layer may be formed from the same material. The cell is transparent to radiation R and may include a reflective element 20. A vapour may be formed from a precursor material 16, such as alkali metal, strontium, tellurium or iodine, which enters the cell via a channel 18. A second layer 12 may cover the cell; this layer may be indirectly bonded via a layer 11 of silicon. The cell may be a microelectromechanical (MEMS) vapour cell. The cell may be made from borosilicate glass. The invention reduces problems with stress and thermal expansion at the interface between the substrate and first layer.

Description

VAPOUR CELL

FIELD

The present disclosure relates to a vapour cell, such as a microelectromechanical systems (MEMS) vapour cell, and associated apparatus, system and method.

BACKGROUND

Vapour cells may find application in atomic clocks, atomic magnetometers and frequency references. In such applications, an atomic vapour is present in the vapour cell and interrogated using a laser. Vapour cells, such as for example MEMS vapour cells, typically comprise a glass-silicon-glass structure, having a vapour chamber defined by a silicon interposer between two glass sheets. An optical path length of the interrogating laser beam may be limited by the thickness of the silicon wafer used. In a common cell, the thickness of the silicon wafer may be about 2mm. In some applications, such as for example atomic magnetometry and frequency references, an increased optical path length for the interrogating laser beam may be desirable. However, silicon wafers having a thickness larger than 2mm may be difficult to source and/or expensive. Equipment that may be used for processing of such silicon wafers may not be compatible with the a wafer thickness larger than 2mm. Modifications to the equipment to accommodate a silicon wafer having a thickness larger than 2mm may be expensive. Additionally or alternatively, the formation of structures, such as for example holes or cavities, in such silicon wafers, for example using an etching method, may require increased process times and/or process costs and/or lead to increased equipment failure rates.

SUMMARY

In described examples, there is provided a vapour cell and associated apparatus, system and method. The vapour cell and associated apparatus, system and method may address one or more problems identified in the background section According to a first aspect of the present disclosure there is provided a vapour cell comprising a substrate, a first layer directly bonded to or integral with the substrate and a chamber at least partially formed in or defined by the first layer, the chamber being configured to hold or contain a vapour or gas, wherein the substrate and the first layer are formed from the same material.

By the substrate being directly bonded to the first layer, no intervening layer of a different material may be provided between the substrate and first layer. By providing a substrate and a first layer directly bonded or integral and formed from the same material, mechanical stresses between the substrate and the first layer (e.g. at an interface between the substrate and the first layer) may be reduced. Additionally or alternatively, the coefficient of thermal expansion of the substrate and the first layer may be the same (or substantially the same). This may lead to a strong chemical and/or mechanical bond between the substrate and the first layer.

The substrate and the first layer may be formed from a material that may be or comprise glass, such as borosilicate or aluminosilicate glass. This may allow for a range of thicknesses, such as for example a thickness larger than 2mm, of the substrate and/or the first layer to be used. For example, the substrate and/or first layer may be thicker than 2.5mm or 3mm or 4mm or more. The substrate and the first layer being formed from glass may result in reduced material and/or manufacturing costs of the vapour cell. Additionally or alternatively, the substrate and the first layer being formed from glass may result in an improved thermal stability of the vapour cell. This may be due to the thermal conductivity of glass being lower than the thermal conductivity of silicon and/or the specific heat capacity of glass being higher (e.g. about 20% higher) than the specific heat capacity of silicon.

The first layer may be fusion bonded to the substrate. The bond between the substrate and the first layer may be a glass directly to glass bond. By fusion bonding the first layer to the substrate, a number of anodic bonds, which may be used to form the vapour cell, may be reduced. The reduction of the number of anodic bonds may facilitate the fabrication or manufacture of the vapour cell, for example by allowing for an all wafer-level fabrication or manufacturing process.

The vapour cell may be configured to allow the chamber and/or the vapour contained therein to be exposed to radiation, e.g. from outwith or outside the vapour cell and provided into the chamber. The vapour cell may be configured to allow passage of the radiation through at least a part or all of the vapour cell. For example, the substrate and/or the first layer may be configured to allow the radiation to pass through at least a part or all of the vapour cell, e.g. the chamber. The substrate and/or the first layer may be transparent or optically transparent (e.g. substantially transparent or substantially optically transparent) to the radiation.

The first layer may comprise a thickness selected such that an optical path length of the radiation passing through at least part or all of the vapour cell is above 2mm or above 4mm. The first layer may comprise a thickness selected such that an optical path length of the radiation passing through at least a part or all of the vapour cell is in the range of about 0.5mm to 7mm.

The vapour cell may comprise a further chamber. The further chamber may be at least partially formed in the first layer. The further chamber may be configured to receive a precursor material. The precursor material may comprise an alkali metal or a compound thereof. The precursor material may comprise Caesium azide (CsN3), Rubidium azide (RbN3), Strontium (Sr), or Tellurium (Te2). The precursor material may be provided in the form of a chemical pill or piece. The chemical pill or piece may be formed from a compressed powder of the precursor material. Alternatively, the precursor material may comprise another material, such as for example a non-metal. The precursor material may comprise a halide such as Iodine (12) or the like. The further chamber may be configured to allow for activation of the precursor material. Activation of the precursor material may result in the formation of the vapour or gas.

The vapour cell may comprise a second layer. The second layer may be bonded, e.g. directly bonded, to the first layer. The second layer may be bonded to an opposite side of the first layer to the substrate. The second layer may be anodic bonded to the first layer. The second layer may be formed from silicon or a compound thereof. The bond between the second layer and the first layer may be a silicon to glass bond.

The vapour cell may comprise a third layer. The third layer may be bonded, e.g. directly bonded, to the second layer. The third layer may be anodic bonded to the second layer. The third layer may be formed from glass, such as borosilicate or aluminosilicate glass. The bond between the second layer and the third layer may be a silicon to glass bond. The third layer may be indirectly bonded to first layer, e.g. via a glass-silicon-glass bond. Alternatively, the third layer may be directly bonded to the first layer, e.g. by fusion bonding. The third layer may be configured to allow radiation to pass through at least part or all of the vapour cell. The third layer may be transparent or optically transparent (e.g. substantially transparent or substantially optically transparent) to the radiation.

The vapour cell may comprise a channel. The channel may be arranged to connect the further chamber to the chamber, e.g. so as to allow for diffusion or migration of the vapour or gas from the further chamber into the chamber. The channel may be arranged or formed in, and/or at least partly defined by, the first layer, the second layer and/or the third layer. The channel may be a partial height or full height channel, e.g. partial height or full height of the first layer.

The vapour cell may comprise a covering portion. The covering portion may comprise the second layer and/or the third layer. The covering portion may be arranged or arrangeable to enclose and/or seal the chamber and/or the further chamber.

The first layer may comprise at least one opening, e.g. a through opening, therein. At least one of the openings in the first layer may at least partly form the chamber and/or the further chamber. The first layer may define at least part or all of side walls of the chamber and/or the further chamber. The substrate may define at least one end wall of the chamber and/or the further chamber. The covering portion may define at least one other end wall of the chamber and/or the further chamber, which may be an end wall of the chamber that is opposite to the end wall formed by the substrate.

The vapour cell may comprise a first reflective element. The first reflective element may be arranged or arrangeable in the chamber, e.g. to reflect a portion or all of the radiation passing through at least a part or all of the vapour cell. By arranging the first reflective element in the chamber, an optical path of the radiation passing through at least a part or all of the vapour cell may be increased. This may allow for a signal to noise ratio of a measurement obtainable using the vapour cell to be increased.

Additionally or alternatively, a depolarisation rate of the atoms of the gas or vapour, which may be due to collisions between atoms or molecular species and one or more walls of the chamber, may be reduced.

The vapour cell may comprise a second reflective element. The second reflective element may be arranged or arrangeable in the chamber, e.g. to allow for reflection of a portion or all of the radiation passing through at least part of all of the vapour cell between the first and second reflective elements. The second reflective element may be arranged or arrangeable to receive and/or reflect at least part of the radiation reflected by the first reflective element. The second reflective element may be arranged or arrangeable opposite to the first reflective element. A reflective surface of the second reflective element may face in the opposite direction to that of the first reflective element. The second reflective element may be provided or locatable on a surface of the chamber opposite to a surface of the chamber upon which the first reflective element is provided or locatable. Both the first and second reflective elements may be configured to face into a chamber, e.g. from opposite sides of the chamber. The second reflective element may be arranged or arrangeable to reflect radiation back to the first reflective element. The first and/or second reflective element may be configured to reflect at least part of the radiation it receives out of the chamber, e.g. after one or more reflections of radiation between the first and/or second reflective elements. A lateral extension of the second reflective element may be smaller than a lateral extension of the first reflective element. By arranging the first and second reflective elements in the chamber, e.g. to allow for reflection of the radiation between the first and second reflective elements, an optical path of the radiation passing through at least a part or all of the vapour cell may be further increased.

One or more walls of the vapour cell may be configured to allow the radiation to pass through at least a part or all of the vapour cell in a lateral direction. The lateral direction may be or comprise a direction parallel (e.g. substantially parallel) to an interface between the substrate and the first layer and/or the first layer and the second layer. An optical path length of the radiation may be defined by a lateral extension of the vapour cell, e.g. an extension of the vapour cell in the lateral direction. The one or more walls may comprise one or more interior walls, such as for example one or more side walls of the chamber and/or the further chamber, and/or one or more exterior walls of the vapour cell. The one or more walls of the vapour cells may comprise at least two opposing walls, such as for example at least two opposing interior walls and/or at least two opposing exterior walls. The one or more exterior walls may be parallel (e.g. substantially parallel) to the one or more interior walls of the vapour cell. The one or more walls may comprise one or more curved surfaces. The one or more walls may be configured such that the radiation passes through the one or more walls at an angle, e.g. Brewster's angle. In other words, the one or more walls may be configured so as to extend at an angle, e.g. Brewster's angle, relative to a direction of propagation of the radiation. This may allow the radiation to pass through the vapour cell (or at least a part thereof) without or reduced losses. The configuration of the one or more walls may be achieved or formed by polishing and/or machining the one or more walls (or at least a part thereof).

According to a second aspect of the present disclosure there is provided a vapour cell comprising a substrate, a first layer fusion bonded to or integral with the substrate, and a chamber at least partially formed in or defined by the first layer, the chamber being configured to hold a vapour or gas for exposure to a radiation.

The vapour cell of the second aspect may comprise any of the individual features of the vapour cell of the first aspect separately to, or in combination with, any of the other features of the vapour cell of the first aspect. That is, the features of the vapour cell of the first aspect that could optionally apply to the vapour cell of the second aspect are not unnecessarily repeated for the sake of brevity and clarity.

According to a third aspect of the present disclosure there is provided a vapour cell comprising a substrate, a first layer bonded to or integral with the substrate, a chamber at least partially formed in or defined by the first layer, the chamber being configured to hold a vapour or gas for exposure to a radiation, the vapour cell being configured to allow passage of the radiation through at least a part or all of the vapour cell, and at least one reflective element, the reflective element being arranged or arrangeable in the chamber to reflect a portion or all of the radiation passing through the at least the part or all of the vapour cell. The first layer may be directly bonded to the substrate.

The vapour cell of the third aspect may comprise any of the features of the vapour cell of the first aspect and/or second aspect separately to, or in combination with, any of the other features of the vapour cell of the first and/or second aspect. That is, the features of the vapour cell of the first and/or second aspect that could optionally apply to the vapour cell of the third aspect are not unnecessarily repeated for the sake of brevity and clarity.

According to a fourth aspect of the present disclosure there is provided a vapour cell comprising a layer of a material and a chamber at least partially formed in or defined by the layer and closed at one end by the layer, the chamber being configured to hold or contain a vapour or gas.

The vapour cell may comprise a second layer, that may close the other end of the chamber. The second layer may comprise a glass layer. The second layer may be directly bonded to the layer of material, e.g. via fusion bonding. The second layer may be indirectly bonded to the layer of material, e.g. via a silicon intermediate, e.g. via anodic bonding.

The layer may be provided instead of the substrate and the first layer of the vapour cell of the first, second and/or third aspect. The layer may be provided in the form of a single sheet of material. The material may comprise glass, such as borosilicate or aluminosilicate glass. The chamber may comprise a blind hole or recess in the layer of material, which may then be sealed by the second layer.

The vapour cell of the fourth aspect may comprise any of the features of the vapour cell of the first aspect, second aspect and/or third aspect separately to, or in combination with, any of the other features of the vapour cell of the first, second and/or third aspect. That is, the features of the vapour cell of the first, second and/or third aspect that could optionally apply to the vapour cell of the fourth aspect are not unnecessarily repeated for the sake of brevity and clarity.

According to a fifth aspect of the present disclosure there is provided a method of manufacture of a vapour cell, the method comprising providing a substrate, bonding a first layer directly to the substrate, and forming at least part of a chamber in the first layer or providing the first layer with at least part of a chamber pre-formed therein, the chamber being configured to hold a vapour or gas for exposure to radiation, wherein the substrate and the first layer are formed from the same material.

The vapour cell may be a vapour cell of any of the first, second and/or third aspects.

The first layer may be fusion bonded to the substrate.

The method may comprise forming at least part of a further chamber in the first layer. The method may comprise arranging a precursor material in the further chamber. The chamber and/or the further chamber may be formed using a water jet cutting process. The water jet may comprise water, an abrasive material or a combination thereof. By using a water jet cutting process a smoothness of the walls of the chamber and/or further chamber may be improved or increased. Additionally or alternatively, the use of a water jet cutting process may facilitate the manufacture of a plurality of vapour cells on a substrate.

The method may comprise bonding a second layer to the first layer, e.g. directly to the first layer. The second layer may be anodic bonded to the first layer. The second layer may be or comprise a silicon layer.

The method may comprise forming a channel. The channel may be arranged to connect the further chamber to the chamber. The channel may be formed in the first layer and/or in the second layer.

The method may comprise bonding a third layer to the second layer, e.g. directly to the second layer. The third layer may be anodic bonded to the second layer.

The third layer may be or comprise a glass layer.

The method may comprise arranging a first reflective element in the chamber. The reflective element may be arranged or arrangeable in the chamber to reflect a portion or all of a radiation passing through at least a part or all of the vapour cell.

The method may comprise arranging a second reflective element in the chamber. The second reflective element may be arranged or arrangeable in the chamber, e.g. to allow for reflection of the portion or all of the radiation passing through at least a part or all of the vapour cell between the first and second reflective elements. The first and/or second reflective element may be arranged in the chamber, e.g. prior to bonding the second layer to the first layer and/or the third layer to the second layer.

The method may comprise activating the precursor material. Activating the precursor material may comprise irradiating the precursor material with an activation radiation, e.g. to heat the precursor material to above the activation temperature. Activation of the precursor material may result in the formation of the vapour or gas.

One or more or each of the substrate, the first layer, the second layer and the third layer may be formed from physically separate components. The method may comprise using wafer processing techniques comprising processing a plurality of wafers, e.g. by providing at least two glass wafers, the substrate being formed from one of the wafers and the first layer being formed from a different wafer. The second and/or third layer may be formed from respective wafers. The method may comprise cutting respective wafers, e.g. to respectively form a plurality of substrates, first layers, second layers and/or third layers that are usable to produce a plurality of the vapour cells. The method may comprise directly bonding one of the substrates formed from one of the wafers to one of the first layers formed from a different wafer. The use of wafer production techniques to form the vapour cells may lead to easier or more efficient production of the vapour cells. It may also be easier to form components such as the reflective elements, e.g. on the substrate or second layer or third layer. For example, a reflective coating could be applied to at least part of a surface of wafer, such as the wafer used to form the substrate and/or second layer or third layer. When cut, the reflective coating may form the first and/or second reflective element. The reflective coating may be formed using additive manufacturing and/or wafer processing techniques, e.g. sputter coating, chemical vapour deposition, spin coating, and/or the like.

According to a sixth aspect of the present disclosure there is provided a method of manufacture of a vapour cell, the method comprising providing a substrate, providing a first layer, fusion bonding the first layer to the substrate, and forming at least part of a chamber in the first layer, the chamber being configured to hold a vapour or gas for exposure to a radiation.

The method of the sixth aspect may comprise any of the features of the method of the fifth aspect.

According to a seventh aspect of the present disclosure there is provided a method of manufacture of a vapour cell, the method comprising providing a substrate, bonding a first layer to the substrate, forming at least part of a chamber in the first layer, the chamber being configured to hold a vapour or gas for exposure to a radiation, the vapour cell being configured to allow passage of the radiation through at least a part or all of the vapour cell, and arranging at least one reflective element in the chamber to reflect a portion or all of radiation passing through at least the part or all the vapour cell.

The method of the seventh aspect may comprise any of the features of the method of the fifth aspect and/or sixth aspect.

According to an eighth aspect of the present disclosure there is provided a method of manufacture of a vapour cell, the method comprising providing a layer of a material and forming at least part of a chamber at least partially in the layer, the chamber being configured to hold or contain a vapour or gas.

The step of forming the at least part of the chamber may comprise removing a portion of the layer, e.g. using a material removal process such as drilling or mechanical drilling or a water jet cutting process. The method may comprise forming the chamber by forming a blind hole or recess in the layer of material. The method may comprise closing the recess or chamber by a second layer. The method of the eighth aspect may comprise any of the features of the method of the fifth aspect, sixth aspect and/or seventh aspect.

According to a ninth aspect of the present disclosure there is provided an atomic clock comprising a vapour cell according to any of the first, second, third and/or fourth aspect.

According to a tenth aspect of the present disclosure there is provided an atomic magnetometer or magnetic sensor comprising a vapour cell according to any one of the first, second, third and/or fourth aspect.

According to a eleventh aspect of the present disclosure there is provided a frequency and/or time reference system comprising a vapour cell according to any one of the first, second, third, and/or fourth aspect.

It should be understood that the features defined above in accordance with any aspect, example or embodiment or below in relation to any specific embodiment described herein may be utilised, either alone or in combination with any other defined feature, in any other aspect, example or embodiment described herein. Furthermore, the present invention is intended to cover apparatus configured to perform any feature described herein in relation to a method and/or a method of using or producing or manufacturing any apparatus feature described herein.

BRIEF DESCRIPTION OF THE DRAWINGS

Various embodiments will now be described by way of example only and with reference to the following drawings, in which: Figures 1A to 10 schematically depict an exemplary vapour cells; Figures 2A to 2C schematically depict other exemplary vapour cells; Figures 3A to 3C schematically depict other exemplary vapour cells; and Figure 4 schematically depicts an exemplary process flow of a method of manufacture of the vapour cells of Figures 1A to 1C, 2A to 20 and/or Figures 3A to 30.

DETAILED DESCRIPTION OF THE DRAWINGS

Figure 1A shows an exemplary vapour cell 2. The vapour cell 2 may be provided in the form of a microelectromechanical systems (MEMS) vapour cell 2. The vapour cell 2 includes a substrate 4. The vapour cell 2 includes a first layer 6 directly bonded to the substrate 4. The vapour cell 2 comprises a chamber 8 at least partially formed in the first layer 6. The chamber 8 is configured to hold a vapour or gas 10 for exposure to a radiation R, in use. The substrate 4 and the first layer 6 are formed from the same material. The directly bonded substrate 4 and the first layer 6 are bonded without any intervening material that is different to the material from which the substrate 4 and the first layer 6 are formed.

By providing a substrate and a first layer formed from the same material and directly bonded together, mechanical stresses between the substrate and the first layer (e.g. at an interface between the substrate and the first layer) may be reduced. Additionally or alternatively, the coefficient of thermal expansion of the substrate and the first layer may be the same (or substantially the same). This may lead to a strong chemical and/or mechanical bond between the substrate and the first layer. Since the substrate 4 and the first layer 6 are formed from two physically separate entities that are bonded together, production processes may be simplified. For example, wafer processing techniques may be used to produce one or more or each of: the substrate 4, first layer 6, second layer 11 and/or third layer 12, wherein a plurality of one or more or each of these may be produced from different respective wafers and then bonded together as part of the production of the vapour cell 2.

In this example, the first layer 6 and/or the substrate 4 may be provided in the form of a sheet of material, which may be a wafer. The material the substrate 4 and the first layer 6 are formed from may comprise glass, such as for example borosilicate or aluminosilicate glass. This may allow for a range of thicknesses of the first layer 6, such as for example a thickness larger than 2mm, of the substrate and/or the first layer to be used. The provision of glass as the material the substrate and the first layer are formed from may result in reduced material and/or manufacturing costs of the vapour cell. Additionally or alternatively, having the substrate and the first layer formed from glass may result in an improved thermal stability of the vapour cell. This may be due to the thermal conductivity of glass being lower than the thermal conductivity of silicon and/or the specific heat capacity of glass being higher (e.g. about 20% higher) than the specific heat capacity of silicon.

The first layer 6 may be fusion bonded to the substrate 4. By fusion bonding the first layer 6 to the substrate 4, a number of anodic bonds, which may be used to form the vapour cell, may be reduced. The reduction of a number of anodic bonds may facilitate the fabrication or manufacture of the vapour cell, for example by allowing for an all wafer-level fabrication or manufacture process. Fusion bonding generally does not require the high voltage used in an anodic bonding process. By fusion bonding the first layer 6 to the substrate 4, defects that may arise from arcing and/or migration of sodium during anodic bonding may be reduced or eliminated.

A precursor material (not shown) may be arranged (or arrangeable) in the chamber 8. The precursor material may be activated, e.g. by heating the precursor material and/or irradiating the precursor material with an activation radiation, to produce the vapour or gas 10, as will be described below. The precursor material may comprise caesium azide (CsN3), for example. The precursor material may be provided in the form of a solid or fluid, e.g. in the form of a getter pill. Alternatively or additionally, the gas or vapour 10 may diffuse or migrate from another part of the vapour cell 2 into the chamber 8, as will be described below.

As described above, the chamber 8 is configured to hold the gas or vapour 10 for exposure to the radiation R. Expressed differently, the gas or vapour 10 may be irradiated by the radiation R, in use. The radiation R may be provided in the form of an interrogation radiation R. The radiation R may be produced by a radiation source, such as for example a laser source. The laser source may be provided in the form of a vertical cavity surface emitting laser (VCSEL). The vapour or gas 10 may comprise an alkali metal gas or vapour, such as for example Caesium (Cs), as will be described below. The radiation R may optically excite the gas or vapour 10. The frequency of the electronic transition of an alkali atom may, for example, be used as an absolute frequency reference for generating a clock signal. The interaction between the radiation R and the gas or vapour 10 may be referred to as optical interrogation. The radiation R may also be referred to as an interrogation radiation R. Optical interrogation at different frequencies within a frequency band may be used to identify a transition frequency, e.g. through absorption spectrum detection, to provide an absolute frequency reference for the clock.

The vapour cell 2 is configured to allow passage of the radiation R through the vapour cell 2. It will be appreciated that the terms "through the vapour cell" may encompass through at least a part or all of the vapour cell 2. For example, the substrate 4 and/or the first layer 6 may be configured to allow the radiation to pass through or traverse the vapour cell 2, e.g. the chamber 8. In this example, the substrate 4 and/or the first layer 6 are transparent or optically transparent (e.g. substantially transparent or substantially optically transparent) to the radiation R. The first layer 6 has a thickness T. The thickness T of the first layer 6 may be selected such that an optical path length of the radiation R passing through the vapour cell 2 (or simply the thickness T of the first layer itself) is above 2mm or above 4mm, for example. The thickness T of the first layer 6 may be selected such that the optical path length of the radiation R (or simply the thickness T of the first layer itself) is in the range of about 0.5mm to 7mm, for example. It will be appreciated that in some examples the thickness of the first layer may be selected such that the optical path of the radiation passing through the vapour cell (or simply the thickness T of the first layer itself) may be larger than 7mm. By selecting the thickness of the first layer such that an optical path length of the radiation passing through the chamber is above 2mm or above 4mm, the signal to noise ratio of measurements using the vapour cell that are dependent on properties of the vapour may be increased. Additionally or alternatively, a depolarisation rate of the atoms of the gas or vapour, which may be due to collisions between atoms or molecular species and one or more walls of the chamber, may be reduced.

Referring to Figure 1A, the vapour cell 2 comprises a second layer 12. The second layer 12 is indirectly bonded to the first layer 6. In this example, the second layer 12 is indirectly bonded to the first layer 6 via a third layer in the form of a layer of silicon 11. In this case, the second layer 12 is anodic bonded to the silicon layer 11 and the silicon layer 11 is anodic bonded to the first layer 6. The second layer 12 may be formed from glass, such as borosilicate glass. The borosilicate glass may be provided in the form of BOROFLOAT® 33 glass, but is not limited to this. The second layer 12 may be provided in the form of a sheet of material or cut from a wafer. The second layer 12 is arranged to enclose and/or seal the chamber B. Expressed differently, the vapour cell 2 comprises a covering portion 13, which comprises the second layer 12. The covering portion 13 is arranged to enclose and/or seal the chamber 8, e.g. to contain or hold the vapour or gas 10 in the chamber 8. In the exemplary vapour cell 2 shown in Figure 1A, the number of anodic bonds may be considered as being reduced from four to two anodic bonds, e.g. between the first layer 6 and the silicon layer 11 and between the silicon layer 11 and the second layer 12. The second layer 12 may be configured to allow the radiation R to pass through the vapour cell 2, e.g. the chamber B. For example, the second layer 12 is transparent or optically transparent (e.g. substantially transparent or substantially optically transparent) to the radiation R. In this example, the chamber 8 has one or more walls 8a, 8b, 8c, 8d, which enclose an interior of the chamber Be. The vapour cell 2 may be arranged such that the first layer 6 defines the sidewalls 8a, 8b of the chamber 8, the substrate 4 defines a bottom wall Sc and/or the second layer defines a top wall 8d. However, it will be appreciated that the vapour cell disclosed herein is not limited to such an arrangement. For example, in other examples, the wall of the chamber may be defined by the first layer or at least one wall of the chamber may be defined by a layer other than the substrate, the first layer and/or the second layer.

Figure 1B shows another example of a vapour cell 2. The exemplary vapour cell 2 shown in Figure 1B is similar to the vapour cell shown in Figure 1A. Any features described above in relation to the vapour cell shown in Figure 1A may also be part of or comprised in the vapour cell shown in Figure 1B.

In the example shown in Figure 1B, the second layer 12 is directly bonded to the first layer 6. The bond between the first layer 6 and the second 12 may be a glass directly to glass bond. In this case, the second layer 12 is fusion bonded to the first layer 6. By fusion bonding the second layer 12 directly to the first layer 6, the number of anodic bonds in the vapour cell may be reduced to zero.

One or more walls of the vapour cell 2 may be configured to allow the radiation R to pass through the vapour cell 2 in a lateral direction. The lateral direction may be, define or comprise a direction parallel (e.g. substantially parallel) to an interface between the substrate 4 and the first layer 6 and/or the first layer 6 and the second layer 12. An optical path length of the radiation R may be defined by the lateral extension of the vapour cell 2. The radiation R passing through the vapour cell 2 in the lateral direction is indicated by the dashed arrow In Figure 1B.

The one or more walls of the vapour cell 2 may be or comprise the side walls 8a, 8b of the chamber 8, which oppose each other. In addition or alternatively, the one or more walls may be or comprise one or more exterior walls of the vapour cell 2, such as for example at least two opposing exterior walls 9a, 9b. The at least two exterior walls 9a, 9h of the vapour cell 2 may be arranged to extend parallel (e.g. substantially parallel) to at least one or both side walls 8a, 8b of the chamber 8. The side walls 8a, 8b of the chamber 8 and/or the exterior walls 9a, 9b of the vapour cell 2 may comprise or define one or more curved surfaces. Additionally or alternatively, the side walls 8a, 8b of the chamber 8 and/or the exterior walls 9a, 9b of the vapour cell 2 may be configured such that the radiation R passes through at least one of the side walls 8a, 8b and/or the exterior walls 9a, 9b at an angle, e.g. Brewster's angle. In other words, at least one of the side walls 8a, 8b and/or the exterior walls 9a, 9b may be configured so as to extend at an angle, e.g. Brewster's angle, relative to a direction of propagation of the radiation R. This may allow the radiation to pass through the vapour cell 2 without or reduced losses. The configuration of at least one of the side walls 8a, 8b and/or the exterior walls 9a, 9b may be achieved by polishing and/or machining the one or more walls (or at least a part thereof). It will be appreciated that in other examples one or more walls of the vapour cell may be configured to allow the radiation to pass through the vapour cell in a direction perpendicular (e.g. substantially perpendicular) to the interface between the substrate and the first layer and/or the first layer and the second layer or at an angle relative to the interface between the substrate and the first layer and/or the first layer and the second layer.

Figure 10 shows another example of a vapour cell 2. The exemplary vapour cell 2 shown in Figure 10 is similar to the vapour cells shown in Figures 1A and/or 1B. Any features described above in relation to the vapour cell shown in Figures 1A and/or 1B may also be part of or comprised in the vapour cell shown in Figure 10.

The vapour cell 2 shown in Figure 10 comprises a layer 7 of a material. The layer 7 is provided in the form of a single sheet of material. The material comprises glass, such as borosilicate or aluminosilicate glass. In this example, the integral layer 7 is provided instead of the substrate 4 and the first layer 6 of the vapour cells 2 shown in Figures 1A and 1B. By providing the layer 7 instead of a substrate and a first layer, the formation or manufacture of the vapour cell may be facilitated. For example, the provision of the layer instead of a substrate and a first layer may reduce the number of bonds, e.g. fusion bonds, required to form the vapour cell and/or a number of process steps required to form the vapour cell. The chamber 8 is at least partially formed in or defined by the layer 7 and configured to hold or contain the vapour or gas 10, as described above. As will be described below, the chamber 8 may be formed as a blind hole or recess in the layer 7 using a material removal process, such as drilling or mechanical drilling or water jet cutting process, e.g. to remove a portion of the layer 7.

It will be appreciated that variations to the arrangement shown in Figures 1A to 10 are possible. For example, Figures 2A to 20 show other examples of a vapour cell 2. The exemplary vapour cells 2 shown in Figures 2A to 2C are similar to the vapour cell 2 shown in Figure 1. Any features described above in relation to the vapour cell shown in Figures 1A to 10 may also be part of or comprised in any of the vapour cells shown in Figures 2A to 20.

Referring to Figures 2A to 20, the vapour cell 2 comprises a further chamber 14. The further chamber 14 may be at least partially formed in the first layer 6. The further chamber 14 is arranged to be spaced from the chamber 8. In other words, a first portion 6a of the first layer 6 is arranged between the chamber 8 and the further chamber 14.

A precursor material 16 may be arranged (or arrangeable) in the further chamber 14. The precursor material 16 may comprise an alkali metal or a compound thereof. The precursor material 16 may comprise caesium azide (CsN3) or be in the form of a getter pill. The precursor material 16 may be provided or introduced in solid or liquid form. The further chamber 14 may be configured to allow for activation of the precursor material 16. For example, the precursor 16 material may be activated by heating the precursor material 16 to above an activation temperature. Activation of the precursor material 16 may result in the formation of the vapour or gas 10. For example, when caesium azide is used as the precursor material 16, the precursor material 16 may be heated to a temperature of about 450°C. This may allow the precursor material 16 to decompose and caesium and nitrogen gas may be produced. The precursor material 16 may be dissociated by irradiating the precursor material 16 with an activation radiation, such as for example ultraviolet (UV) radiation or laser radiation. However, it will be appreciated that in other example, the precursor material may be activated using another mechanism. Additionally or alternatively, a precursor material other than caesium azide may be used, such as for example Rubidium azide (RbN3), Strontium (Sr), or Tellurium (Te2). In some examples, the precursor material may comprise a non-metal material, such as Iodine (12).

The vapour cell 2 comprises a channel 18. The channel 18 is arranged to connect the further chamber 14 to the chamber 8, e.g. so as to allow for diffusion or migration of the vapour or gas 10 from the further chamber 14 into the chamber 8. In other words, subsequent to the activation of the precursor material 16, the produced gas or vapour 10 may diffuse or migrate from the further chamber 14 into the chamber 8. The channel 18 may be provided in the form of an overpass, e.g. over the first portion 6a of the first layer 6. In this example, the channel 18 is beneficially defined between the first layer 6 and the second layer 12, which may be simpler to produce. However, it will be appreciated that other arrangements of channel 18 could be used. The channel 18 may also be full height or only partial height of the first layer 6. The channel 18 may be arranged or formed in the first portion 6a of the first layer 6. The first portion 6a of the first layer 6 defines at least one sidewall 8a of the chamber 8. The other sidewall 8b of the chamber 8 is defined by a second portion 6b of the first layer 6. Referring to Figure 2B, the vapour cell 2 comprises a first reflective element 20. The first reflective element 20 is arranged in the chamber 8 to reflect a portion or all of the radiation R passing through the vapour cell 2. For example, in use, the radiation R may enter the chamber 8 via the substrate 4, for example at an angle a relative to the substrate 4, before being reflected by the first reflective element 20 and leaving the chamber 8 again via the substrate 4. The radiation R may be polarised. The angle a may be selected to correspond (e.g. substantially correspond) to Brewster's angle. By arranging the first reflective element in the chamber, the optical path of the radiation passing through the vapour cell may be increased. This may allow for the signal to noise ratio of measurements using the vapour in the vapour cell to be increased. Additionally or alternatively, a depolarisation rate of the atoms of the gas or vapour, which may be due to collisions between atoms or molecular species and one or more walls of the chamber, may be reduced.

Referring to Figure 20, the vapour cell 2 comprises a second reflective element 22. The second reflective element 22 may be arranged in the chamber 8 to allow for reflection of the radiation R between the first and second reflective elements 20, 22. The second reflective element 22 is arranged opposite to the first reflective element 20.

In this example, the first element 20 is arranged on or at the top wall 8d of the chamber 8, which may be defined by the second layer 12. The second reflective element 22 is arranged on or at the bottom wall Sc of the chamber 8, which may be defined by the substrate 4. A lateral extension of the second reflective element 22 is smaller than a lateral extension of the first reflective element 20.

For example, in use, the radiation R may enter the chamber 8 via substrate 4 at the angle a. The first reflective element 20 may be arranged to reflect the radiation R towards the second reflective element 22. The second reflective element 22 may be arranged to reflect the radiation R back towards the first reflective element 20. Subsequent to the reflection of the radiation R again by the first reflective element 20, the radiation R may leave the chamber 8 via the substrate 4. By arranging the first and second reflective elements in the chamber, the optical path of the radiation passing through the vapour cell may be further increased. This may allow for the signal to noise ratio of measurements using the vapour in the vapour cell to be increased. Additionally or alternatively, a depolarisation rate of the atoms of the gas or vapour, which may be due to collisions between atoms or molecular species and one or more walls of the chamber, may be reduced.

The first reflective element 20 and/or the second reflective element 22 may each be provided in the form of a mirror or passivated mirror or the like. The first reflective element 20 and/or the second reflective element 22 may each comprise a protective layer, which may be provided in the form of a dielectric layer.

It will be appreciated that the disclosed vapour cell is not limited to the above arrangement of first and second reflective elements. For example, a lateral extension of the first and second reflective elements may be the same (or substantially the same).

The first and second reflective elements may be arranged opposite to and/or offset relative to each other. The first and second reflective elements may be arranged so that the first reflective element reflects the radiation towards the second reflective element. The second reflective element may reflect the radiation towards the second layer. In other words, the radiation may leave the chamber via the second layer, subsequent to the reflection by the second reflective element. It will be further appreciated that in some examples the vapour cell may comprise only one of the first and second reflective elements. In other examples, the vapour cell may comprise further reflective element, e.g. in addition to the first and/or second reflective elements.

It will be appreciated that variations to the arrangement shown in Figures 2A to 2C are possible. For example, Figures 3A to 3C show other examples of a vapour cell 2. The exemplary vapour cells 2 shown in Figures 3A to 30 are similar to the vapour cells 2 shown in Figures 1A to 10 and 2A to 2C. Any features described above in relation to the vapour cells shown in Figures 1A to 10 and 2A to 20 may also be part of or comprised in any of the vapour cells shown in Figures 3A to 30. Figure 3A is a plan view of another example of a vapour cell 2. Figures 3B and 3B represent two possible alternatives and are sectional views of the vapour cell 2 along the line A-A' of Figure 3A. It will be appreciated that the features of the substrate, the second layer, the first and second reflective elements have been omitted from Figure 3A for clarity purposes.

In the exemplary vapour cells 2 shown in Figure 3A to 30 each vapour cell 2 comprises an alternative second layer 12a. The alternative second layer 12a is formed from silicon and is bonded, e.g. directly bonded, to the first layer 6 using anodic bonding. The alternative second layer 12a closes off the chamber 8, e.g. closes the top of the chamber 8.

Referring to Figures 3B and 30, the vapour cell 2 may comprise the first reflective element 20 and/or the second reflective element 22, as described above. In this example, the top wall 8d of the chamber 8 is defined by alternative second layer 12a. The first reflective element 20 is arranged on or at the top wall 8d. In this example, the alternative second layer 12a is part of or comprised in the covering portion 13. Figure 4 shows an exemplary process flow of a method of manufacture of a vapour cell 2.

The method comprises providing a substrate 4 (step 400). The method comprises bonding a first layer 6 directly to the substrate 4 (step 405). The first layer 6 may be fusion bonded to the substrate 6. As described above, the substrate 4 and the first layer 4 may are formed from the same material. The material the substrate 4 and the first layer 4 are formed from may comprise glass, such as for example borosilicate glass or aluminosilicate glass. The borosilicate glass may be provided in the form of BOROFLOATO 33 glass. It will be appreciated that in some examples a layer of material may be provided instead of the substrate 4 and the first layer 6. For example a single sheet of material may be provided instead of the substrate 4 and the first layer 6. This may facilitate the formation of the vapour cell 2 and/or reduce a number of bonds, e.g. fusion bonds, required to form the vapour cells. The material may comprise glass, such as for example borosilicate glass or aluminosilicate glass.

The method comprises forming at least part of a chamber 8 in the first layer 6 (step 410). As described above, the chamber 8 may be configured to hold a vapour or gas for exposure to a radiation. The method may further comprise forming at least part of a further chamber 14 in the first layer 6. The chamber 8 and/or the further chamber 14 may be at least partially formed in the first layer 6 using a material removal process, such as for example a water jet cutting process, the water jet comprising water, an abrasive material or a combination thereof. By using a water jet cutting process a smoothness of the walls of the chamber and/or further chamber may be improved or increased. Additionally or alternatively, the use of a water jet cutting process may facilitate the manufacture of a plurality of vapour cells on a substrate. It will be appreciated that in another example, material removal process may comprise etching, sand blasting, drilling or mechanical drilling or the like.

The step (410) of forming the chamber 8 and/or the further chamber 14 may comprise removing one or more portions of the first layer 6, e.g. using the material removal process and/or the other material process. One or more portions of the first layer 6 may remain. The remaining portions may comprise the first, second and/or third portions 6a, 6b, 6c of the first layer 6. The first, second and/or third portions 6a, 6b, 6c of the first layer 6 may define or provide one or more walls, such as for example the sidewalls 8a, 8b, of the chamber 8 and/or the further chamber 14.

In step 415, the method may comprise forming a part of the channel 18, e.g. using a material removal process, such as water jetting, etching, sand blasting or mechanical drilling or the like. Using a water jetting process to form the channel 18 may facilitate the formation of a plurality of vapour cells and/or provide accurate control of the dimensions of the channel 18. The part of the channel 18 may be formed by removing a portion of the first layer 6, e.g. the first portion 6a, using the material removal process. The channel 18 is arranged to connect the further chamber 14 to the chamber 8. The method may comprise arranging a precursor material 16 in the further chamber 14. It will be appreciated that in other examples the precursor material may be arranged in the chamber, as described above in relation to Figure 1.

The method comprises bonding a second layer 12, 12a to the first layer 6, e.g. directly or indirectly to the first layer 6.

Subsequent to step 410, the method may comprise indirectly bonding the second layer 12 to the first layer 6. This comprises providing or depositing the third layer 11 onto the first layer 6, wherein the third layer 11 is in the form of a silicon layer 11 or layer of other material that is different to the material used for the first and second layers 6, 12. In an example, the third layer 11 may be cut from a wafer, e.g. of silicon, in a shape that corresponds to the shape of the first layer 6 and directly bonded to the first layer 6 (to the one or more of the second and/or third portions 6b, 6c of the first layer 6), e.g. by anodic bonding. The first portion 6a may remain uncovered. This may allow for the formation of the channel 18, as will be described below. The method may comprise arranging the precursor material 16 in the further chamber 14. It will be appreciated that in other examples the precursor material may be arranged in the chamber, as described above.

The method may comprise bonding the second layer 12 to the third / silicon layer 11, e.g. directly to the third layer (step 435). The second layer 12 may be anodic bonded to the third layer 11. The second layer 12 may comprise the same material as the first layer 6 and/or the substrate 4. For example, the second layer 12 may be formed from glass, example borosilicate glass. The borosilicate glass may be provided in the form of BOROFLOAT® 33 glass. In this example, the second layer 12 forms or is comprised in the covering portion 13.

In this example, the channel 18 may be at least partly defined by the first layer 6, the second layer 12 and/or the third layer 11. The channel 18 may be formed by bonding the second layer 12 to the third layer 11. The channel 18 may be arranged or formed between the first layer 6, e.g. the first portion 6a, and the second layer 12. The channel 18 may be formed as an overpass, e.g. over the first remaining portion 6a of the first layer 6.

In step 440, the method may comprise arranging the first reflective element 20 and/or the second reflective element 22 in the chamber 8, as also described below in relation to step 425 of the method. The first and/or second reflective element 20, 22 may be arranged in the chamber 8, prior to bonding the third layer 11 to the second layer 12. In this example, the first reflective element 20 may be arranged on or at the top wall 8d of the chamber 8, which may be defined by the second layer 12. The second reflective element 22 may be arranged on the bottom wall 8c of the chamber 8, which may be defined by the substrate 4.

The method may comprise activating the precursor material 16 (step 445).

Activating the precursor material 16 may comprise irradiating the precursor material 16 with an activation radiation A, e.g. to heat the precursor material 16 to above the activation temperature, as described above. Activation of the precursor material 16 may result in the formation of the vapour or gas 10.

This process can be beneficially used to form the vapour cell 2 shown in Figures 2B and 2C but could be easily adapted to produce the other vapour cells 2 shown and described herein.

For example, in the production of the cells shown in Figures 3A to 3C (steps 415-425), the second layer 12a that closes the chamber 8 and forms at least part of a top surface of the chamber 8 may be formed from silicon or a compound thereof and anodic bonded to the first layer 6. It will be appreciated that in other examples, the second layer may be formed from glass, such as borosilicate glass or aluminosilicate glass, which may be fusion bonded to the first layer. The second layer 12a may be provided in the form of a sheet of material or cut from a wafer. The second layer 12a may be arranged to enclose and/or seal the chamber 8. In other words, the second layer 12a may form, be part of or comprised in a covering portion 13. The covering portion 13 is arranged to enclose and/or seal the chamber 8. The channel 18 may be formed by bonding the second layer 12 to the first layer 11. The channel 18 may be arranged or formed between the first layer 6, e.g. the first portion 6a, and the second layer 12.In step 425, the method may comprise arranging a first reflective element 20 in the chamber 8. The first reflective element 20 may be arranged in the chamber 8 to reflect a portion or all of the radiation passing through vapour cell 2. The method may comprise arranging a second reflective element 22 in the chamber 8. The second reflective element may be arranged in the chamber 8 to allow for reflection of a portion or all of the radiation passing through the vapour cell 2 between the first and second reflective elements 20, 22 such that the second reflective element 22 receives radiation reflected from the first reflective element 20. Beneficially, the second reflective element 22 can be configured to reflect radiation back to the first reflective element 20, which in turn is configured to reflect the radiation received from the second reflective element 22 out of the chamber via the glass substrate 8, e.g. after one, two or more passes of the radiation between the first and second reflective elements 20, 22. The second reflective element 22 may be arranged opposite to the first reflective element 20. A lateral extension of the second reflective element 22 may be smaller than a lateral extension of the first reflective element 20. It will be appreciated that the vapour cell disclosed herein is not limited to this arrangement of the first reflective element and/or the second reflective element. The first and/or second reflective element 20, 22 may be arranged in the chamber 8, prior to bonding the second layer 12a to the first layer 6 to seal the chamber 8. The first reflective element 20 may be arranged on or at the top wall 8d of the chamber 8, which may be defined by the second layer 12a. The second reflective element 22 may be arranged on the bottom wall 8c of the chamber 8, which may be defined by the substrate 4.

It will be appreciated that the order of the method steps may be different. One or more of the method steps may be used in isolation of each other or in a different combination. It will be appreciated that in some embodiment some of the method steps described above may be used in isolation or in combination with other method steps. The method may be used to manufacture any one of the vapour cells 2 shown in Figures 1A to 1C, 2A to 2C and 3A to 3C.

It will be appreciated that the method described above may be used to manufacture a plurality of vapour cells, e.g. using the same steps (400 to 425 and/or 400 to 445). The plurality of vapour cells may be formed on the same substrate. For example, about a 100 vapour cells may be formed on a substrate. Subsequent to the formation of the plurality of vapour cells, the vapour cells may be separated from one another e.g. using a cutting or dicing process, such as for a dicing saw or automatic dicing saw. The vapour cell 2 or the plurality of vapour cells may be formed at wafer level.

The vapour cell 2 may be part of or comprised in an atomic clock, an atomic magnetometer, magnetic sensor and/or a frequency and/or time reference system.

The applicant discloses in isolation each individual feature described herein and any combination of two or more such features, to the extent that such features or combinations are capable of being carried out based on the specification as a whole in the light of the common general knowledge of a person skilled in the art, irrespective of whether such features or combinations of features solve any problems disclosed herein, and without limitation to the scope of the claims. The applicant indicates that aspects of the invention may consist of any such individual feature or combination of features. In view of the foregoing description it will be evident to a person skilled in the art that various modifications may be made within the scope of the invention.

Claims

CLAIMS: 1. A vapour cell, such as a microelectromechanical systems (MEMS) vapour cell, comprising: a substrate; a first layer directly bonded to or integral with the substrate; and a chamber at least partially formed in the first layer, the chamber being configured to hold a vapour or gas; wherein the substrate and the first layer are formed from the same material.
2. The vapour cell of claim 1, wherein the material the substrate and the first layer are formed from comprises glass, such as borosilicate glass.
3. The vapour cell of claim 1 or 2, wherein the first layer is fusion bonded to the substrate.
4. The vapour cell of any preceding claim, wherein the vapour cell is configured to allow passage of radiation through at least a part or all of the vapour cell.
5. The vapour cell of claim 4, wherein the first layer comprises a thickness selected such that an optical path length of the radiation passing through at least part or all of the vapour cell is above 2mm or above 4mm or in the range of about 0.5mm to 7mm.
6. The vapour cell of any preceding claim, wherein the vapour cell comprises a further chamber at least partially formed in the first layer, the further chamber comprising or being configured to receive a precursor material for forming the vapour or gas and the vapour cell comprises a channel, the channel being arranged to connect the further chamber to the chamber so as to allow for diffusion of the vapour or gas from the further chamber into the chamber.
7. The vapour cell of claim 6, wherein the precursor material comprises an alkali metal or a compound thereof, such as Caesium azide (CsN3), or Sr, or Te2, or a nonmetal, such as 12.
8. The vapour cell of any preceding claim, wherein the vapour cell comprises a second layer, the second layer being bonded to an opposite side of the first layer to the substrate.
9. The vapour cell of claim 8, wherein the second layer is anodic bonded to the first layer.
10. The vapour cell of claim 8 or 9, wherein the second layer is formed from silicon or a compound thereof.
11. The vapour cell of claims 8 to 10, wherein the vapour cell comprises a third layer formed from glass, such as borosilicate glass, the third layer being bonded, e.g. anodic bonded, to the second layer.
12. The vapour cell of any one of claims 8 to 11, wherein the vapour cell comprises a covering portion, the covering portion comprising the second layer and/or the third layer, the covering portion being arranged to enclose and/or seal the chamber and/or the further chamber.
13. The vapour cell of any preceding claim, wherein the vapour cell comprises at least a first reflective element, the first reflective element being arranged in the chamber to reflect a portion or all of the radiation passing through at least a part or all of the vapour cell.
14. The vapour cell of claim 13, wherein the vapour cell comprises a second reflective element, the second reflective element being arranged in the chamber to receive and reflect at least a portion or all of the radiation reflected by the first receiving element.
15. The vapour cell of claim 14, wherein a lateral extension of the second reflective element is smaller than a lateral extension of the first reflective element
16. A vapour cell comprising: a substrate; a first layer fusion bonded to the substrate; and a chamber at least partially formed in the first layer, the chamber being configured to hold a vapour or gas for exposure to a radiation
17. A vapour cell comprising: a substrate; a first layer bonded to the substrate; a chamber at least partially formed in the first layer, the chamber being configured to hold a vapour or gas for exposure to a radiation; the vapour cell being configured to allow passage of the radiation through at least a part or all of the vapour cell; and at least one reflective element, the reflective element being arranged in the chamber to reflect a portion or all of the radiation passing through the at least the part or all of the vapour cell.
18. A method of manufacture of a vapour cell, the method comprising: providing a substrate; bonding a first layer directly to the substrate; and forming at least part of a chamber in the first layer or providing the substrate with at least part of a chamber pre-formed in the substrate, the chamber being configured to hold a vapour or gas for exposure to a radiation; wherein the substrate and the first layer are formed from the same material.
19. The method of claim 18, wherein the first layer is fusion bonded to the substrate.
20. The method of claim 18 or 19, wherein the method comprises forming at least part of a further chamber in the first layer and forming a channel, the channel being arranged to connect the further chamber to the chamber.
21. The method of claim 20, wherein the method comprises arranging a precursor material in the further chamber.
22. The method of any one of claims 18 to 21, wherein the chamber and/or the further chamber are formed using a water jet cutting process, the water jet comprising water, an abrasive material or a combination thereof.
23. The method of any one of claims 18 to 22, wherein the method comprises arranging a first reflective element in the chamber, the reflective element being arranged in the chamber to reflect a portion or all of a radiation passing through at least a part or all of the vapour cell, and optionally a second reflective element in the chamber, the second reflective element arranged in the chamber to allow for reflection of the portion or all of the radiation between the first and second reflective elements.
24. A method of manufacture of a vapour cell, the method comprising: providing a substrate; providing a first layer; fusion bonding the first layer to the substrate; and forming at least part of a chamber in the first layer, the chamber being configured to hold a vapour or gas for exposure to a radiation.
25. A method of manufacture of a vapour cell, the method comprising: providing a substrate; bonding a first layer directly to the substrate; forming at least part of a chamber in the first layer, the chamber being configured to hold a vapour or gas for exposure to a radiation; the vapour cell being configured to allow passage of the radiation through at least a part or all of the vapour cell; and arranging at least one reflective element in the chamber to reflect a portion or all of radiation passing through at least the part or all the vapour cell.
26. An atomic clock comprising a vapour cell according to any one of claims 1 to 17.
27. An atomic magnetometer or magnetic sensor comprising a vapour cell according to any one of claims 1 to 17.
28. A frequency and/or time reference system comprising a vapour cell according to any one of claims 1 to 17.