DE102020204570A1 - Method for filling a steam cell with inert gas and filling device - Google Patents

Abstract

The invention relates to a method for filling a steam cell which has a silicon wafer (1) with at least one recess for forming a steam cell cavity (5) which is filled with at least one alkali metal and closed with a glass wafer (3), with noble gas, an opening (30) in the glass wafer (3) being created by melting it using a laser (110), noble gas being introduced into the vapor cell cavity (5) in addition to the alkali metal located there, a glass ball (122), which is set up to completely seal the opening (30) in the glass wafer (3), is guided onto the opening (30), and the glass ball (122) is melted by means of the laser (110) in such a way that the glass ball ( 122) fuses with the glass wafer (3).

Description

The present invention relates to a method for filling a steam cell, which has a silicon wafer with at least one recess for forming a steam cell cavity, which is filled with at least one alkali metal and closed with a glass wafer, with noble gas and a filling device for carrying out a such procedure.

State of the art

One can use rotation rate sensors or gyroscopes based on MEMS to determine a change in a rotational orientation in space. These are inexpensive and small. Its deviation is around 1 ° / hour and its accuracy enables autonomous cars, for example, to stay in lane for around 40 seconds if all other driver assistance systems fail. For example, they can serve as a backup for radar positioning, video assistance positioning and GPS positioning.

Laser gyroscopes, which can be used for aircraft navigation, are much more precise. They are based on the optical Sagnac effect and their deviation is only approx. 0.0035 ° / hour. However, they are relatively large and expensive and therefore hardly suitable for use in vehicles.

An alternative option is to use NMR gyroscopes ("Nuclear Magnetic Resonance"). These evaluate nuclear magnetic resonance signals from atomic nuclei with a non-vanishing magnetic moment. These can be produced in miniature versions and show a deviation of approx. 0.02 ° / hour. This makes them up to 50 times more accurate than MEMS gyroscopes.

In order to provide nuclear magnetic resonance-based gyroscopes, glass is usually used to realize a vapor cell filled with rubidium (Rb) and xenon (Xe). However, the scalability of the manufacturing costs is poor in the transition to large quantities. Silicon is much more suitable as a base material. Since silicon is not transparent in the optical spectral range, a heterogeneously structured system consisting of silicon and glass wafers, which can be hermetically tightly connected to one another using wafer bonding processes such as anodic bonding, is recommended. In this way, by bonding a glass wafer to a previously structured silicon wafer, an optically accessible, hermetically sealed cavity can be created that forms the vapor cell.

In the case of nuclear magnetic resonance-based gyroscopes, an additional challenge is that a defined mixture of isotopically pure Xe-129 and Xe-131 must be brought into the steam cell at a defined pressure in addition to the rubidium. Isotopically pure Xe is very expensive. The isotopically pure Xe mixture can be introduced between the glass and silicon wafers during the final bonding process by filling the entire volume of the bonder, which has a volume of a few liters, with the xenon mixture at a pressure of around 250 mbar. However, this is significantly too expensive for an inexpensive production of nuclear magnetic resonance-based gyroscopes. There is therefore a need for an inexpensive method for filling the vapor cell with noble gas, in particular xenon.

The pamphlet RU 2578890 C as well as the pamphlet CN 106 219 481 A describe methods of placing alkali metals in a hermetically sealed volume.

Disclosure of the invention

According to the invention, a method for filling a steam cell with noble gas and a filling device with the features of the independent claims are proposed. Advantageous refinements are the subject matter of the subclaims and the description below.

The invention makes use of the measure of filling a silicon wafer with at least one recess for forming a vapor cell cavity in advance with at least one alkali metal, admixtures of gases such as nitrogen being conceivable. This vapor cell cavity is preferably sealed in advance with a diffusion barrier for the alkali metal and closed with a glass wafer, in particular hermetically shielded by bonding. This glass wafer is then melted at a local point by means of a laser, so that an opening is created. In addition to the alkali metal located there, noble gas is introduced or filled into the vapor cell cavity through this opening. The noble gas can in particular be xenon, helium, krypton or neon or a special isotope mixture of the gases, for example Xe-129 and Xe-131. A noble gas mixture with a specially adjusted isotope mixture of, for example, xenon with at least one other gas, such as, for example, helium, neon or krypton, is also conceivable. Filling with other gases is also conceivable. In particular, nitrogen is advantageous as a further filling gas for NMR gyroscope vapor cells. This serves as a buffer gas and ensures that the alkali metal atoms and the noble gas atoms do not lose their spin polarization due to wall collisions. Thereafter, a glass ball, which is set up, completely opens the opening in the glass wafer to seal, led to the opening. The glass ball is melted by means of the laser in such a way that the glass ball fuses with the glass wafer, expediently in a hermetically sealed manner.

These process steps make it possible to dispense with an excess volume of noble gases. The method according to the invention makes it possible to use at most twice the required fill quantity of noble gas. This is particularly economical and inexpensive.

In an advantageous embodiment, before an opening is created, a filling nozzle is approached to a surface of the glass wafer and is tightly connected to it, with inert gas being introduced through the filling nozzle. The opening is then expediently produced within the surface area enclosed by the filling nozzle. This is advantageous because in this way gas losses can be further reduced.

Before the opening is produced, a volume enclosed by the filling nozzle and the surface of the glass wafer is preferably emptied using a vacuum pump. In this way, unwanted gas admixtures can be reduced to the desired gas composition.

Expediently, after the opening has been created, the volume enclosed by the filling nozzle and surface of the glass wafer is further emptied with the vacuum pump, whereby additions to the alkali metal are pumped out of the steam cell cavity, but the non-gaseous alkali metal remains in the steam cell cavity. In this way, it is not necessary to provide a pure alkali metal directly in advance, but admixtures of gases from the ambient air, such as nitrogen, for example, which can then be pumped out at this point.

In particular, the opening is produced in a region which is located closer to an edge of the glass wafer than to a center of the glass wafer. In other words, the opening in the glass wafer is preferably produced in an area which is closer to a wall of the steam cell cavity than to the center of the steam cell cavity. This results in the advantage that with respect to the cavity walls through the glass wafer approximately or precisely can be radiated centrally into the cavity without the light beam being impaired by the optical properties of the glass wafer which have changed as a result of the fusion of the sealing ball. This is advantageous because in this way a central area for optical irradiation of the vapor cell cavity, in particular with regard to its optical properties, remains unchanged or unprocessed.

In a particularly advantageous embodiment, the noble gas is a defined xenon isotope mixture, in particular composed of Xe-131 and Xe-129. This is advantageous because an external magnetic field can be determined in this way due to the different gyromagnetic ratio.

The alkali metal rubidium is preferred. This is particularly advantageous because it is gaseous at around 110 to 120 ° C and rubidium electron spins couple well with xenon nuclear spins.

The individual steps of the method are expediently carried out by means of a filling device. This is advantageous because a filling device that is optimized for the steps of the method improves the scalability for large quantities and can reduce costs.

According to a further aspect of the invention, a filling device is proposed which has a filling nozzle which is set up to be approached to a surface of a glass wafer which seals a vapor cell cavity and to be sealed or tightly connected to it; a vacuum pump which is connected to the filler neck by means of at least one first valve and is set up to empty the filler neck; a gas cylinder which has noble gas at a defined pressure and is connected to the filler neck by means of at least one second valve and is set up to fill noble gas into the filler neck; a laser which is configured to create an opening in the hermetic shielding of the glass wafer by melting the glass; a glass ball container which is connected to a guide by means of at least one third valve and is set up to guide a glass ball onto the opening for sealing the opening. The filling device is expediently set up to carry out a method according to the first aspect of the invention.

This filling device is suitable for carrying out the method according to the invention very efficiently.

In particular, the at least one first valve and / or the at least one second valve and / or the at least one third valve is designed as a solenoid or solenoid valve. Such valves can be controlled particularly easily.

The at least one second valve is preferably designed as a 3-way valve, with a first connection of the 3-way valve leading to the gas cylinder, with a second connection of the 3-way valve leading to a connection of the at least one first valve, a third connection of the 3-way valve leads to the vacuum pump, so that the Vacuum pump can drain not only the filler neck, but also a passage from the 3-way valve to at least one first valve. This is a particularly advantageous structure that enables very efficient filling without unnecessary amounts of ambient gas.

Further advantages and embodiments of the invention emerge from the description and the accompanying drawing.

The invention is shown schematically in the drawings using an exemplary embodiment and is described below with reference to the drawings.

Figure list

• 1 shows a silicon wafer with two recesses for the formation of a vapor cell cavity each, which is filled with solid alkali metal and nitrogen and is sealed with a diffusion barrier for the alkali metal and is hermetically shielded with a glass wafer by bonding, before a preferred embodiment is carried out method according to the invention;
• 2 until 10 show the silicon wafer off 1 with a preferred embodiment of a filling device according to the invention during various steps of a preferred embodiment of a method according to the invention.

Embodiments of the invention

In 1 a silicon wafer is shown and denoted by 1. This silicon wafer 1 has two recesses 5 , 6th on. These recesses 5 , 6th form vapor cell cavities and are filled with solid rubidium as alkali metal and nitrogen. They are also provided with a diffusion barrier 4th sealed and with a first 2 and second glass wafer 3 hermetically shielded by bonding.

This arrangement forms the starting material for a preferred embodiment of a method according to the invention.

This wafer stack is produced, for example, by anodic bonding of a Borofloat glass wafer to a silicon wafer, in which recesses in the form of through holes, for example approx. 2 mm in diameter, have previously been made, for example by deep reactive ion etching. Following this first anodic bonding process, the composite of glass and silicon wafers is provided with a thin diffusion barrier for rubidium made of, for example, Al ₂ O ₃ (aluminum oxide) in a thickness of approximately 8 to 12 nm, in particular approximately 10 nm, for example through an atomic layer deposition process. Then rubidium in the form of, for example, rubidium nitrate (RbN ₃ ) dissolved in water or ethanol is introduced into the cavities of the wafer assembly, for example by dispensing or by an inkjet process. After the solution has dried, a second glass wafer is made 3 , which is coated on one side with a diffusion barrier for Rb, is hermetically sealed onto the Rb-filled composite of glass and silicon wafers using an anodic bonding process. In the last step, the rubidium nitrate in the vapor cell cavities is converted into elemental (solid) rubidium (Rb) and nitrogen (N ₂ ) by thermal treatment or by UV radiation.

In the following, a preferred embodiment of the invention is based on the 2 until 10 described in which a preferred embodiment of a filling device according to the invention 2 is shown in different stages of the filling process.

Filling device 2 has a filler neck 170 with a seal in order to approach it to the surface of the second glass wafer and to seal it with this. It has a trapezoidal cross-section here. Furthermore, an opening is provided on the side that can be approached to the second glass wafer.

The filling device 100 also has a vacuum pump 160 on that here by means of a first valve 130 and a second valve 140 with the filler neck 170 is connected and is set up for this purpose, the filler neck 170 to drain.

The filling device also includes a gas cylinder 150 , which has a defined mixture of nitrogen and a defined noble gas (mixture) at a defined pressure and is connected to the filling nozzle by means of the first and second valve and is set up for this purpose, into the filling nozzle 170 fill in the gas. The gas mixture can contain, for example, 200 to 300 mbar, in particular 250 mbar N2, 20 to 30 mbar, in particular 25 mbar Xe-129 and 20 to 30 mbar, in particular 25 mbar Xe-131.

It's a laser 110 provided together with a focusing lens 111 is set up an opening in the hermetic shield of the second glass wafer 3 by melting the glass. The exit opening of the laser 110 here protrudes into the filler neck, purely as an example 170 inside and the focusing lens 111 is arranged inside the filler neck.

A glass ball container 120 is by means of a third valve 121 connected to a guide and set up for this purpose, a glass ball 122 on one through to guide the opening generated by the laser to seal the opening.

The first valve 130 is here with a valve control 131 connected and designed as a solenoid valve. The second valve 140 is designed here as a 3-way valve. A first connection of the second valve 140 leads to the gas cylinder 150 , a second connection of the second valve 140 leads to a connection of the first valve 130 and a third connection of the second valve 140 leads to the vacuum pump 160 so that the vacuum pump not only has the filler neck 170 , but can also drain a passage from the second valve to the first valve.

First is the filler neck 170 to a surface of the second glass wafer 3 approached and sealed with this, so that there is an opening of the filler neck 170 over a vapor cell cavity 5 is located. This situation is in 2 shown.

The first valve 130 is opened and at the second valve 140 the second and third ports are opened. This creates a passage from the filler neck 170 through the first valve 130 and the second valve 140 to the vacuum pump 160 generated. One of the filler neck 170 and the surface of the second glass wafer 3 enclosed volume is with the vacuum pump 160 emptied. After that, the hermetic shielding of the second glass wafer 3 by melting using the laser 110 by a focused laser beam 112 an opening 30th generated. This situation is in 3 shown. The first 130 and second valve 140 stay open for a while until the nitrogen is also out of the vapor cell cavity 5 is mostly pumped out. However, the solid rubidium remains in the vapor cell cavity 5 .

The first valve 130 and the second valve 140 are closed, causing the filler neck 170 and in the steam cell cavity 5 a vacuum remains. This is in 4th shown.

The first connection and the second connection of the second valve 140 are opened. Furthermore, the first valve 130 opened. In this way there is a passage from the gas cylinder 150 to the filler neck 170 free, so that the nitrogen-xenon-isotope mixture in the filler neck 170 and into the steam cell cavity 5 flows. This situation is in 5 shown.

The first valve 130 and the second valve 140 are closed again. In the filler neck 170 and the nitrogen-xenon isotope mixture is now in the vapor cell cavity. This is in 6th shown.

After that the third valve 121 in the glass container 120 opened. A glass ball 122 comes out of the glass container 120 on the opening 30th led to the sealing of the opening. This situation is in 7th shown.

The third valve 121 is closed again and the glass ball 122 is on the opening 30th . This situation is in 8th shown.

By means of the laser 110 is made by a focused laser beam 112 the glass ball 122 melted. This is in 9 shown.

The glass ball melts when it melts 122 Hermetically sealed with the second glass wafer 3 . This is in 10 shown.

The solid rubidium can be converted into the gaseous state for use in particular by heating.

QUOTES INCLUDED IN THE DESCRIPTION

This list of the documents listed by the applicant was generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Patent literature cited

• RU 2578890 C [0007]
• CN 106219481 A [0007]

Claims (14)

A method for filling a steam cell, which has a silicon wafer (1) with at least one recess for forming a steam cell cavity (5) which is filled with at least one alkali metal and sealed with a glass wafer (3), with noble gas, comprising the steps: - Generating an opening (30) in the glass wafer (3) by melting by means of a laser (110); - Introducing noble gas into the vapor cell cavity (5) in addition to the alkali metal located there; - Guiding a glass ball (122), which is designed to completely seal the opening (30) in the glass wafer (3), onto the opening (30); - Melting the glass ball (122) by means of the laser (110) in such a way that the glass ball (122) fuses with the glass wafer (3). Procedure according to Claim 1 , a filling nozzle (170) being approached to a surface of the glass wafer (3) and tightly connected to it before an opening (30) is created, the introduction of noble gas into the vapor cell cavity (5) through the filling nozzle (170) he follows. Procedure according to Claim 2 , a volume enclosed by the filling nozzle (170) and the surface of the glass wafer (3) being emptied with a vacuum pump (160) before an opening (30) is produced. Procedure according to Claim 3 , wherein after the opening (30) has been created, the volume enclosed by the filling nozzle (170) and the surface of the glass wafer (3) is further emptied with the vacuum pump (160), whereby residual gases are pumped out of the vapor cell cavity (5), the alkali metal however, remains as a solid in the vapor cell cavity (5). Method according to one of the preceding claims, wherein the opening (30) is produced in a region which is located closer to a wall of the steam cell cavity (5) than to the center of the steam cell cavity (5). Method according to one of the preceding claims, wherein the noble gas is a defined mixture of Xe-131 and Xe-129. A method according to any preceding claim, wherein the alkali metal is rubidium. Method according to one of the preceding claims, wherein, in addition to introducing noble gas into the steam cell cavity (5), further gases, which preferably comprise nitrogen, are introduced into the steam cell cavity (5). Method according to one of the preceding claims, wherein the walls of the vapor cell cavity (5) are provided with a diffusion barrier (4) for the alkali metal before the method is carried out. Method according to one of the preceding claims, wherein the steps of the method are carried out by means of a filling device (100). Filling device (100), which has the following: - A filling nozzle (170) which is set up to be approached to a surface of a glass wafer (3), which seals a vapor cell cavity (5), and to be sealed with this; - A vacuum pump (160) which is connected to the filling nozzle (170) by means of at least one first valve (130) and is set up to empty the filling nozzle (170); - A gas cylinder (150) which has noble gas at a defined pressure and is connected to the filling nozzle (170) by means of at least one second valve (140) and is set up to fill the noble gas into the filling nozzle (170); - A laser (110) which is set up to produce an opening (30) in the hermetic shielding of the glass wafer (3) by melting the glass; - A glass ball container (120) which is connected to a guide by means of at least one third valve (121) and is set up to guide a glass ball (122) onto the opening for sealing the opening (30). Filling device according to Claim 11 , wherein the at least one first and / or second and / or third valve (130, 140, 121) is designed as a solenoid valve. Filling device according to Claim 11 or 12th , the at least one second valve (140) being designed as a 3-way valve, a first connection of the 3-way valve leading to the gas cylinder (150), a second connection of the 3-way valve leading to a connection of the at least one first valve (130), a third connection of the 3-way valve leading to the vacuum pump (160), so that the vacuum pump (160) not only has the filling nozzle (170), but also a passage from the 3rd -Way valve to at least one first valve (130) can empty. Filling device (100) according to one of the Claims 11 until 13th , which is set up to use a method according to one of the Claims 1 until 10 perform.

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