GB2589338A

GB2589338A - Vapour cells and methods for making vapour cells

Info

Publication number: GB2589338A
Application number: GB1917236.0A
Authority: GB
Inventors: Francis Dunne Barry; Alexander Kurt
Original assignee: INEX MICROTECHNOLOGY Ltd
Current assignee: INEX MICROTECHNOLOGY Ltd
Priority date: 2019-11-27
Filing date: 2019-11-27
Publication date: 2021-06-02
Also published as: GB201917236D0

Abstract

A vapour cell 102 and method of fabricating a vapour cell, having a cavity 104 for containing an atomic vapour, a glass window 120 allowing entry and exit of light 110 that stimulates the vapour, the cavity having at least one reflective surface 106, 108 that directs the light in the longitudinal direction 116 of the cavity. The cavity is made my etching a silicon substrate having a [110] crystal plane orientation. The cavity may have greater length than depth. There may be a pair of reflective surfaces to direct the light from entry to exit of the cavity. The cavity may contain a metal alkali vapour or alkali metal. The stimulating light may be laser light or ultraviolet light. The vapour cell may also have a heater 130 to control temperature or pressure of vapour in the cavity. The vapour cell may be used in an atomic clock or magnetometer. Etching may be performed by depositing a silicon dioxide mask on the silicon substrate.

Description

VAPOUR CELLS AND METHODS FOR MAKING VAPOUR CELLS Background [0001] Vapour cells are used in a variety of applications such as, for example, in producing atomic clocks, magnetometers, reference cells and the like. Manufacturing vapour cells of an appropriate size involves balancing optical or excitation path length with vapour cell dimensions. It is a challenge to increase the optical or excitation path length without dramatically increasing the distances between upper and lower windows through which the optical or other stimulus passes.

Brief description of the drawings

[0002] Example implementations will now be described by way of example only with reference to the accompanying drawings in which: [0003] figure 1 shows a vapour cell according to example implementations; [0004] figure 2 illustrates a vapour cell according to example implementations; [0005] figure 3 shows a vapour cell according to example implementations; and [0006] figure 4 depicts a flowchart for fabricating a vapour cell according to example implementations.

Detailed description of example implementations

[0007] Referring to figure 1, there is shown a view 100 of a vapour cell 102 according to an example implementation. The vapour cell 102 comprises a cavity 104 formed in a substrate 105. The substrate 105 in the example depicted can be a silicon substrate. The cavity 104 comprises first 106 and second 108 reflectors arranged to reflect light 110 from a light source 112 onto a detector 114. Light 110 from the light source 112 is reflected by the first reflector 106 to direct it onto the second reflector 108. The light 110 is further reflected by the second reflector 108 onto the detector 114. The detector 114 can be a photodetector. The light is an example of a stimulus, excitation stimulus or excitation energy. The light source can be at least one of an ultraviolet light source or a laser light source. The light source can be a continuous wave light source. The light source can be a broadband source or a narrowband source. At least one or both of the first 106 and second 108 reflectors can be coated with a material to influence the reflectivity of those reflectors 106 and 108. The material can comprise a ceramic.

[0008] It can be appreciated that the vapour cell has a longitudinal dimension 116 and a depth or transverse dimension 118. The light 110 is arranged to be reflected in the longitudinal direction of the longitudinal dimension 116, which is in contrast to known vapour cells that have a top window and a bottom window that allows light to pass through the vapour cells in the transverse direction with the light source and the detector being disposed on opposite sides of the vapour cell. By arranging for the light to travel in the longitudinal direction, the light is able to interact with, or otherwise stimulate, the gas vapour within the cavity 104 to a greater extent due to the path length in the longitudinal direction being greater than the path length in the transverse direction.

[0009] The cavity 104 of the vapour cell 102 is sealed using a pane of glass 120, which forms a window to allow the light 110 to enter and exit the cavity. The glass 120 is anodically bonded to the substrate 105. The glass 120 is an example of an optically transparent medium, or an example of a stimulus, excitation stimulus or excitation energy transparent medium.

[0010] At least one, or both, of the first 106 and second 108 reflectors can be formed from respective layers of reflective material deposited onto the substrate 105.

[0011] The light 110 from the light source 112 is arranged to contact the first reflector 106 at a predetermined angle of incidence 122. In the example implementation, the predetermined angle of incidence is 54.7 degrees. It will be appreciated that the angle between the incident light and the first reflector 106 is 90 degrees minus the angle of incidence. Therefore, in the example implementation shown, the angle between the incident light and the first reflector 106 is 35.3 degrees.

[0012] The light 110 is reflected by the reflector 106 at a predetermined angle of reflection 124, determined by the predetermined angle of incidence. In the example shown, the predetermined angle of reflection 124 is 54.7 degrees. The reflected light 110 is inclined relative to the first reflector 106 at a predetermined angle 125, given by 90 degrees less the predetermined angle of reflection 124. In the example implementation shown in figure 1, the reflected light 110 is inclined relative to the first reflector 106 at an angle of 35.3 degrees.

[0013] The light 110 reflected by the first reflector 106 travels in the longitudinal direction, where it is reflected by the second reflector 108. The second reflector 108 is inclined to reflect the light 110 to create a predetermined angle of incidence 126 onto the glass 120. In the example implementation depicted in figure 1, the predetermined angle of incidence 126 onto the glass 120 is 19.4 degrees. The light 110 is refracted by the glass through a predetermined angle of refraction 128. In the example implementation shown in figure 1, the predetermined angle of refraction 128 is 13.05 degrees. Having passed through the glass 120, the light 110 is refracted onto the detector 114.

[0014] The cavity 104 bears an alkali metal. The alkali metal is turned into an alkali metal vapour.

Example implementations can be realised in which the alkali metal is at least one of caesium or rubidium.

[0015] The substrate 105 can optionally be provided with a heater 130. The heater 130 is arranged to control or influence at least one, or both, of the temperature or pressure of the gas vapour within the cavity 104. The temperature or pressure control can be open loop or closed loop temperature or pressure control. The heater 130 can be formed via a resistive path between a pair of electrodes 132 and 134. The resistive path can take any form subject to providing heat to the cavity 104. The resistive path can be formed by depositing a material. Example implementations can be realised in which the resistive path is formed from a metal such as, for example, Platinum, Indium Tin Oxide, Tin Oxide or some other material.

[0016] Referring to figure 2, there is illustrated plan and perspective views 200 of the vapour cell 102. The cavity 104 can be formed in the substrate 105 by etching. The etching can be performed along the crystal plane <110>, which defines the shape of the cavity 104. The first 106 and second 108 reflectors are disposed in the corners of the cavity 104. The cavity 104 has a rhombus shape. The etchant can be, for example, Potassium Hydroxide (KOH) or Tetramethylammonium Hydroxide (TMAH).

[0017] The example implementation depicted in figure 2 provides a vapour cell, bearing the cavity 104, with a first, or an overall, predetermined lateral width 202. The overall, or maximum, lateral width 202 is 15 mm in the example shown. The cavity 104 has a respective lateral width 204. The respective lateral width 204 is less than the first predetermined lateral width 202. The respective lateral width 204 of the example shown is 11 mm. The substrate, once diced, containing the cavity 104, has a first, or overall, predetermined longitudinal length 206. In the example shown, the first, or overall, predetermined longitudinal length 206 is 22 mm. The cavity 104 has a respective longitudinal length 208. In the example depicted, the respective longitudinal length 208 is 15 mm. The respective longitudinal length 208 defines the separation of the first 106 and second 108 reflectors. The sides 210 to 216 of the cavity 104 have a predetermined length. In the example illustrated, the sides 210 to 216 have lengths of 13 mm. It can be appreciated that the cavity 104 has a predetermined depth 218. Example implementations can be realised in which the predetermined depth is in the range of 350 pm to 450 pm such as, for example, 400 pm.

[0018] Referring to figure 3, there is shown a view 300 of a vapour cell 302 according to an example implementation. The vapour cell 302 comprises a cavity 304 formed in a substrate 305. The substrate 305 in the example depicted can be a silicon substrate. The cavity 304 comprises first 306 and second 308 reflectors arranged to reflect light 310 from a light source 312 onto a detector 314. Light 310 from the light source 312 is reflected by the first reflector 306 to direct it onto the second reflector 308. The light 310 is further reflected by the second reflector 308 onto the detector 314. The detector 314 can be a photodetector. The light is an example of a stimulus or excitation stimulus. The light source can be at least one of an ultraviolet light source or a laser light source. The light source can be a continuous wave light source. The light source can be a broadband source or a narrowband source. At least one or both of the first 306 and second 308 reflectors can be coated with a material to influence the reflectivity of those reflectors 306 and 308. The material can comprise a ceramic.

[0019] It can be appreciated that the vapour cell has a longitudinal dimension 316 and a depth or transverse dimension 318. The light 310 is arranged to be reflected in the longitudinal direction of the longitudinal dimension 316, which is in contrast to known vapour cells that have a top window and a bottom window that allows light to pass through the vapour cells in the transverse direction with the light source and the detector being disposed on opposite sides of the vapour cell. By arranging for the light to travel in the longitudinal direction, the light is able to interact with, or otherwise stimulate, the gas vapour within the cavity 304 to a greater extent due to the path length in the longitudinal direction being greater than the path length in the transverse direction.

[0020] The cavity 304 of the vapour cell 302 is sealed using panes of glass 320A and 320B, which can form at least one window, or a pair of windows, to allow the light 310 to enter and exit the cavity. The panes of glass 320A and 320B are anodically bonded to the substrate 305. The panes of glass 320A and 320B are examples of an optically transparent medium, or examples of a stimulus, or excitation stimulus, transparent medium.

[0021] At least one, or both, of the first 306 and second 308 reflectors can be formed from respective layers of reflective material deposited onto the substrate 305.

[0022] The light 310 from the light source 312 is arranged to contact the first reflector 306 at a predetermined angle of incidence 322. In the example implementation, the predetermined angle of incidence is 54.7 degrees. It will be appreciated that the angle between the incident light and the first reflector 306 is 90 degrees minus the angle of incidence. Therefore, in the example implementation shown, the angle between the incident light and the first reflector 306 is 35.3 degrees.

[0023] The light 310 is reflected by the reflector 306 at a predetermined angle of reflection 324, determined by the predetermined angle of incidence. In the example shown, the predetermined angle of reflection 324 is 54.7 degrees. The reflected light 310 is inclined relative to the first reflector 306 at a predetermined angle 325, given by 90 degrees less the predetermined angle of reflection 324. In the example implementation shown in figure 3, the reflected light 310 is inclined relative to the first reflector 306 at an angle of 35.3 degrees.

[0024] The light 310 reflected by the first reflector 306 travels in the longitudinal direction, where it is reflected by the second reflector 308. The second reflector 308 is inclined to reflect the light 310 to create a predetermined angle of incidence 326 onto the glass 320A. In the example implementation depicted in figure 3, the predetermined angle of incidence 326 onto the glass 320A is 19.4 degrees. The light 310 is refracted by the glass 320A through a predetermined angle of refraction 328. In the example implementation shown in figure 3, the predetermined angle of refraction 328 is 13.05 degrees. Having passed through the glass 320A, the light 310 is refracted onto the detector 314.

[0025] The cavity 304 bears an alkali metal. The alkali metal is turned into an alkali metal vapour.

[0026] The substrate 305 can optionally be provided with a heater 330. The heater 330 is arranged to control or influence at least one, or both, of the temperature or pressure of the gas vapour within the cavity 304. The temperature or pressure control can be open loop or closed loop temperature or pressure control. The heater 330 can be formed via a resistive path between a pair of electrodes 332 and 334. The resistive path can take any form subject to providing heat to the cavity 304. The resistive path can be formed by depositing a material onto the glass 320B. Example implementations can be realised in which the resistive path is formed from a metal such as, for example, Platinum, Indium Tin Oxide, Tin Oxide or some other material.

[0027] The observations made above relating to figure 2 are equally applicable to figure 3.

[0028] Vapour cell fabrication [0029] Referring to figure 4, there is shown a view 400 of a flowchart 402 of processing steps for manufacturing a vapour cell according to an example implementation. At 404, a silicon wafer of predetermined dimensions and predetermined crystal orientation is coated on at least one side with silicon dioxide to a predetermined depth. Example implementations can be realised in which the predetermined crystal orientation is <110> and/or in which the predetermined dimensions comprise a predetermined length such as, for example, 150mm, and a predetermined depth, such as, for example, 650-675pm. The predetermined depth can be, for example, 5pm. The predetermined depth of silicon dioxide can be created using PECVD. Accordingly, an example implementation can be realised using a 150mm <110> orientation standard thickness (650-675pm) silicon wafer that is coated on at least one side with 5pm of plasma-enhanced chemical vapour deposition (PECVD) silicon dioxide. Depositing the silicon dioxide can take a predetermined period of time, such as, for example, 18mins.

[0030] At 406, the wafer is coated with a predetermined photoresist. The photoresist can be, for example, S1813 photoresist. The predetermined photoresist can have a predetermined depth such as, for example, 1.3pm.

[0031] At 408, the photoresist is pattemed using a mask that defines a path having a predetermined path length. Example implementations can be realised in which the path length is between 2mm-15mm. Example implementations can be realised in which the predetermined path length is 3mm.

[0032] The photoresist is developed at 410, and the silicon dioxide is etched down to the silicon wafer surface for a predetermined period of time. The etching can be a dry etch. The etch time can be of the order of 14 minutes.

[0033] The photoresist is removed at 412, and the wafer is cleaned at 414.

[0034] It will be appreciated that at least one or more of steps 404 to 414 taken jointly and severally, are arranged to deposit a mask that defines at least one channel or cavity intended to form the vapour cavity 104. When producing multiple vapour cells on a wafer, multiple masks for respective channels will be defined. Suitably, example implementations can be realised that deposit a mask that defines at least one cavity, or at least one channel, intended to form a cavity for bearing a vapour.

[0035] Next, at 416, the silicon wafer is etched to a predetermined depth using a predetermined etching technique. The predetermined depth can be, for example, 400pm. The predetermined technique can be, for example, Deep Reactive Ion Etching (DRIE). The etching can take a predetermined period of time such as, for example, 70 minutes. The etch uses the silicon dioxide as a hard mask. It will be appreciated that this etching is used to limit a subsequent etch. Example implementations can be realised in which the subsequent etch is 428 below, which is a wet etch.

[0036] At 418, the silicon dioxide is removed using a buffered oxide etch for a predetermined period of time. The predetermined period of time can be, for example, 1 hour.

[0037] The wafer is coated, at 420, to a predetermined depth with silicon nitride using a predetermined technique. The predetermined depth can be, for example, 400nm. The predetermined technique can be chemical vapour deposition such as, for example, Low Pressure Chemical Vapour Deposition of silicon nitride.

[0038] At 422, the wafer is coated to a predetermined depth with a predetermined photoresist according to a predetermined mask. Example implementations can be realised in which the predetermined depth is 1.3pm. Example implementations can be realised in which the predetermined photoresist is a 81813 photoresist. The mask is arranged to create the angled side walls during the etching process.

[0039] The photoresist is developed at 424 and the silicon nitride is etched to the surface of the silicon wafer at 426.

[0040] It will be appreciated that steps 416 to 426 are used to define preferential etching of the silicon in a bound or limited manner. Step 422 to 426, defines a mask that is used for the subsequent etching step of 428. The mask, such as, for example, the silicon nitride mask, inhibits or binds etching of the silicon in 428. Example implementations provide such a mask that limits or binds lateral etching of the silicon.

[0041] At 428, the wafer is etched until etching naturally stops. Example implementations can be realised in which the etching spans a predetermined period of time such as, for example, 14 hours. Example implementations can be realised in which the silicon is wet etched using Potassium Hydroxide or Tetra-Methyl Ammonium Hydroxide until it naturally stops.

[0042] The silicon nitride is removed at 430. Removing LPCVD silicon nitride can use hydrofluoric acid and hydrochloric acid in a predetermined ratio. The predetermined ratio can be 1:1. Removing the silicon nitride can take a predetermined period of time such as, for example, 2 hours.

[0043] Optionally, glass 120 or 320B can be anodically bonded, at 431, to the bottom surface of the silicon wafer to seal the bottom of the cavity, to help encapsulate the alkali metal in the cell.

[0044] At 432, an alkali metal is dispensed into the cavity created by etching.

[0045] Glass 120 or 320A is anodically bonded, at 434, to the top surface of the silicon wafer to encapsulate the alkali metal in the cell. It will be appreciated that a buffer gas can be added depending on the dispensing method. The heater 130 can be deposited in one or more than one location. For example, example implementations can be realised in which a heater 130 is disposed on at least one of the bottom on the silicon substrate, such as depicted in figure 1, on one or more, or all, of the panes of glass 120, 320A, or 320B. Depositing the heater 130 on the panes of glass 120, 320A or 320B can take place before the glass is bonded to the substrate. Alternatively, the heater 130 could be deposited on the glass 120, 320A or 320B after the glass has been bonded to the substrate.

[0046] The wafer is diced into individual chips at 436.

[0047] The cells are activated, at 438, using either a high-power ultraviolet light source or with a high-power laser depending on the dispense method. Example implementations can be realised in which at least one of the ultraviolet light source or laser can have an intensity of 28 mW/cm2.

[0048] Optionally, example implementations can be realised in which the foregoing methods comprise depositing a heater 130 that is arranged to control or influence at least one of the temperature or pressure of the gas vapour within the cavity 104.

[0049] Vapour cell use [0050] In use, light is arranged to be reflected on the cell cavity reflectors 106 and 108 to allow light to propagate longitudinally, that is, in a direction that is parallel to the glass cover. This ensures maximum light intensity travels through the cell such that it can be detected by the photodetector 114 mounted directly on the external glass surface 120 or 320A. Therefore, the laser 112, such as, for example, a Vertical Cavity Surface Emitting Laser, can be mounted and angled or directed relative to the cell cavity reflectors 106 and 108 to realise the foregoing. Mounting the laser 112 can use, for example, solder bump bonding where the solder ball diameters are varied across the surface of the VCSEL such that, when bonded to the external glass surface 120 of the vapour cell 102, the optical path is provided. It will be appreciated that the solder ball diameter can control the angle of incidence by varying the gap between the laser emitting surface and the glass surface. The solder balls will use surface tension to help align the laser to the glass 120 or 320A during the mounting process.

Claims

CLAIMS1 A vapour cell comprising a cavity for bearing a vapour; the cavity having an optically transparent window for at least one of ingress and egress of an optical stimulus, and at least one optically reflective surface for directing the optical stimulus in a longitudinal direction of a longitudinal axis of the cavity.
2. The vapour cell of claim 1, in which the cavity has a predetermined depth and a predetermined length; the predetermined length being greater than the predetermined depth.
3. The vapour cell of any preceding claim, in which the at least one optically reflective surface is arranged to direct an ingress of the optical stimulus in the longitudinal direction.
4 The vapour cell of claim 3, in which the longitudinal axis is oriented in the direction of the predetermined length and in which the ingress of the optical stimulus is at a predetermined orientation relative to a plane containing the longitudinal axis.
5. The vapour cell of any preceding claim in which the at least one optically reflective surface comprises a pair of surfaces oriented to support ingress and egress of the optical stimulus through the optically transparent window.
6. The vapour cell of any preceding claim, in which the cavity contains at least one of a metal alkali vapour or an alkali metal.
7. The vapour cell of any preceding claim, in which the optically transparent window supports both ingress and egress of the optical stimulus.
8. The vapour cell of any preceding claim, in which the optical stimulus comprises at least one of ultraviolet light or laser light.
9. The vapour cell of any preceding claim, comprising a heater arranged to control at least one of the temperature or pressure of a gas vapour within the cavity.
10. An atomic clock comprising a vapour cell of any preceding claim.
11. A magnetometer comprising a vapour cell of any preceding claim.
12. A method of fabricating a vapour cell comprising a cavity for bearing a vapour; the method cornprising etching (428) a substrate having a predetermined crystal plane orientation (Si(110)) to produce an elongate cavity having at least one reflective surface for directing an excitation energy in an elongate direction of the cavity.
13. The method of claim 12, comprising depositing at least one of an alkali metal or an alkali metal vapour in the cavity.
14. The method of either of claims 12 and 13, comprising forming (434) an excitation energy transparent window to seal the cavity; the excitation energy transparent window allowing the excitation energy to be directed to the at least one reflective surface.
15. The method of any of claims 12 to 14, comprising mounting at least one of an excitation energy source to direct the excitation energy onto the at least one reflective surface for reflection in the elongate direction.
16. The method of any preceding claim, in which said etching (428) comprises etching the substrate having the predetermined crystal plane orientation (Si(110)) to produce an elongate cavity having at least one further reflective surface for further reflecting the excitation energy in a cavity egress direction.
17. The method of claim 16, comprising mounting at least one excitation energy detector to receive the excitation energy directed in the cavity egress direction.
18. The method of any of claims 12 to 17, in which at least one or both of said etching comprises depositing silicon dioxide onto a wafer and creating a mask (404..414; 420..426) for etching the cavity.