EP3112315B1

EP3112315B1 - Method for filling csac absorption cells with high-purity alkali metal

Info

Publication number: EP3112315B1
Application number: EP14884030.9A
Authority: EP
Inventors: Jian Zhu; Erwin HELL
Original assignee: CETC 55 Research Institute
Current assignee: CETC 55 Research Institute
Priority date: 2014-02-27
Filing date: 2014-09-02
Publication date: 2021-07-14
Anticipated expiration: 2034-09-02
Also published as: EP3112315A1; KR101824789B1; EP3112315A8; CN103864007A; EP3112315A4; KR20160013122A; CN103864007B; WO2015127577A1

Description

FIELD OF THE INVENTION

The present invention relates to a method for filling wafer-based chip-scale atomic clock absorption cells with a high-purity alkali metal.

BACKGROUND OF THE INVENTION

All our daily life, scientific research, navigation, surveying and mapping and other work can't be separated from time. Time measurement relates to two quantities-epochs and time intervals. Any natural phenomena with periodical changes can be used to measure time. As timing instruments have also constantly developed with humans' progress over 3,500 years, people's demand for timing accuracy is becoming higher and higher. As for timing instruments, people developed sundials which take advantage of the periodic change law of earth rotation to identify time changes, then sand clocks, astronomical clock towers, mechanical pendulum clocks, quartz clocks, atomic clocks and optical clocks. It is clear that all of them use the natural phenomena with periodic changes to measure time.
As one of the most accurate timing instruments at present, the theory of the atomic clocks was first put forward in the 1930s and then the atomic clocks were produced. Gradually, more and more atomic clocks have been used in national defense and scientific research. In recent years, miniature chip-scale atomic clocks produced by using MEMS (Micro-Electro-Mechanical Systems) technology have begun to develop. The development of clocks will make breakthroughs in receivers' clock performance, be more widely applied to timing frequency standards of all kinds and have a revolutionary social impact.
An atomic clock is an instrument which realizes accurate time measurement by using atomic transition radiation frequency between hyperfine energy levels in the atomic ground state. The miniature atomic clock based on the CPT (Coherent Population Trapping) phenomenon is an inexorable development trend of atomic clock miniaturization. And the miniaturization of its core chip alkalis metal vapor cavity plays a critical role in the miniaturization of the atomic clock.
At present, the technical methods for filling micro absorption cells with an alkali metal can mainly be divided into two categories. One category is to directly inject a pure alkali metal (such as rubidium and cesium) into the absorption cell. This category needs an sophisticated large vacuum equipment and a strict vacuum environment. A trace of residual oxygen in the cavity may lead to the oxidization of the alkali metal and then reduce the service life of the atomic clocks. The second category is to directly inject alkali metal compounds into the absorption cell cavity to produce the corresponding alkali metal through chemical reactions. This category needs to strictly control the amount of the compounds which are injected into the absorption cell while the residual impurities of the reactions may remain in the absorption cell and then affect the atomic clock performance. In addition, the two categories share the same disadvantage: absorption cells need to be filled one by one so it is difficult to realize production on a large scale.

SUMMARY OF THE INVENTION

The present invention puts forward a method for filling wafer-based chip-scale atomic clock absorption cells with a high-purity alkali metal. The present invention aims at overcoming the problem of the prior art that alkali metals are extremely prone to oxidization during the production of atomic clock absorption cells. Through the method of wafer level filling and partial reactions,
An atomic clock is an instrument which realizes accurate time measurement by using atomic transition radiation frequency between hyperfine energy levels in the atomic ground state. The miniature atomic clock based on the CPT (Coherent Population Trapping) phenomenon is an inexorable development trend of atomic clock miniaturization. And the miniaturization of its core chip alkalis metal vapor cavity plays a critical role in the miniaturization of the atomic clock.
JP 2013 125907 A discloses a method of manufacturing a gas cell of an atomic oscillator. The gas cell consists of a first substrate made of glass, a second substrate made of silicon, and a third substrate. In one embodiment of JP 2013125907 A , the method includes: forming a connecting groove, a first through hole, and a second through hole on the silicon substrate, wherein the first through hole and the second through hole penetrate the silicon substrate, and are connected to each other through the connecting groove; bonding the first substrate to one surface of the second substrate, placing an alkali metal generating agent in the second through hole, and bonding the third substrate to the other surface of the second substrate; irradiating the alkali metal generating agent by using a laser light to generate alkali metal gas, and filling the first through hole with the alkali metal gas; deforming the first substrate in the region where the connecting groove is formed and to eliminate the space between the first through hole and the second through hole, and enclosing the alkali metal gas in the first through hole. In addition, US 2012/243088 A1 recites a gas cell manufacturing method. The method includes: arranging solid substances at a plurality of holes each of which is provided on each of a plurality of cells; accommodating gas in inner spaces of the cells through an air flow path connected to the holes; and sealing the spaces by melting the solid substances to close the holes. In addition, US 7 893 780 B2 discloses an alkali beam cell system including a reversible alkali beam cell. The reversible alkali beam cell includes: a first chamber configured as a reservoir chamber to evaporate an alkali metal during a first time period and as a detection chamber to collect the evaporated alkali metal during a second time period; a second chamber configured as the detection chamber during the first time period and as the reservoir chamber during the second time period; and an aperture interconnecting the first and the second chambers.
At present, the technical methods for filling micro absorption cells with an alkali metal can mainly be divided into two categories. One category is to directly inject a pure alkali metal (such as rubidium and cesium) into the absorption cell. This category needs a sophisticated large vacuum equipment and a strict vacuum environment. A trace of residual oxygen in the cavity may lead to the oxidization of the alkali metal and then reduce the service life of the atomic clocks. The second category is to directly inject alkali metal compounds into the absorption cell cavity to produce the corresponding alkali metal through chemical reactions. This category needs to strictly control the amount of the compounds which are injected into the absorption cell while the residual impurities of the reactions may remain in the absorption cell and then affect the atomic clock performance. In addition, the two categories share the same disadvantage: absorption cells need to be filled one by one so it is difficult to realize production on a large scale.

SUMMARY OF THE INVENTION

The present invention puts forward a method for filling wafer-based chip-scale atomic clock absorption cells with a high-purity alkali metal. The present invention aims at overcoming the problem of the prior art that alkali metals are extremely prone to oxidization during the production of atomic clock absorption cells. Through the method of wafer level filling and partial reactions, the present invention realizes the filling of all absorption cells on the wafer at the same time to meet the demand for large-scale production of atomic clocks. The present invention is characterized by low cost and high efficiency.
Accordingly, the present invention defines a method according to claim 1. A preferred embodiment of the present invention is defined in claim 2.
The invention has the advantages of easy processes and rapid, low-cost, large-scale production of atomic clock alkali metal absorption cells.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1a is a schematic representation of using the MEMS ICP etching technique to form micro grooves 102, alkali metal placement cavity grooves 103 and absorption cell cavity grooves 104 on the double-side polished silicon wafer 101.
FIG. 1b is a schematic representation of using the three-layer wafer level anodic bonding technique to form prefabricated temporary micro flow channels 108, absorption cell cavities 104 and alkali metal placement cavities 103.
FIG. 1c is a schematic representation of making the alkali metal compound decompose by adjusting the temperature of the alkalis metal placement cavities 103 to produce alkali metal gas which passes through the prefabricated temporary micro flow channel and cooling part of the absorption cell cavity to make the alkali metal gas congeal in the absorption cell cavities.
FIG. 1d is a schematic representation of sealing all the alkali metal absorption cell cavities.
FIG. 2 is a schematic representation of a single alkali metal absorption cell of an atomic clock.

In the drawings, the following reference numbers are used: 101. double-side polished silicon wafers; 102. shallow micro grooves; 103. alkali metal compound placement cavities; 104. alkali metal absorption cell cavities; 105. sheet glass A; 106. alkali metal compounds; 107. sheet glass B; 108. temporary micro flow channels; 109. alkali metal vapor; 110. equipment for partial cooling; 111. high-purity solid alkali metal; 201. high-purity alkali metal; 202. silicon alkali metal absorption cell cavities; 203. upper-layer glass; and 204. lower-layer glass.

DETAILED DESCRIPTION OF THE EMBODIMENTS

A method for filling wafer-based chip-scale atomic clock absorption cells with a high-purity alkali metal comprising at least one of rubidium and cesium, wherein the method comprises:

(1) using an MEMS ICP etching technique to carve a shallow microgroove (102) on the first side of a polished double-sided silicon wafer, to carve an array of absorption cell cavities within the scope of said microgroove, to carve a placement cavity within said scope of said microgroove at the center of said polished double-sided silicon wafer, wherein said absorption cell cavities and said placement cavity have a through hole structure running through both sides of said polished double sided silicon wafer;
(2) bonding the second side of the polished double-sided silicon wafer with a first sheet glass through a a silicon-glass wafer level anodic bonding process, putting a precomputed amount of an alkali metal compound into the placement cavity and bonding the first side of the polished double-sided silicon wafer with a second sheet glass through a glass-silicon wafer level anodic bonding process, so that the shallow microgroove and said second sheet glass define a temporary flow channel for an alkali metal vapour;
(3) controlling the intensity of a decomposition reaction of the alkali metal compound by separately adjusting the temperature of the placement cavity to decompose the alkali metal compound and to produce a needed amount of the alkali metal and vaporizing the alkali metal to produce said alkali metal vapour;
(4) diffusing the alkali metal vapour through the temporary flow channel, and cooling the absorption cavities to condense the alkali metal vapour in the absorption cavities; and
(5) re-bonding the first side of the polished double-sided silicon wafer with the first glass sheet with glass-silicon wafer level anodic bonding while applying an electrostatic force to make said first glass sheet bending under the influence of said force, in such a way that the temporary flow channel is eliminated so that the absorption cells are sealed at the same time.

The glass-silicon-glass three-layer wafer level anodic bonding technique which is used in the present invention is carried out in two steps: the first step is to form temporary micro flow channels for alkali metal vapor; and the second step is to reuse the anodic bonding technique to realize the sealing of the alkali metal compound.
Make the alkali metal compound in the placement cavities react chemically by separately adjusting the temperature of the placement cavities to produce high-purity alkali metal through decomposition. The intensity of decomposition reactions can be controlled by adjusting the temperature of the alkali metal placement cavities.
The flow channel of the alkali metal vapor is that the alkali metal vapor diffuses into the absorption cell through silicon-glass temporary micro flow channel and congeals in the absorption cell through partial cooling.
Make the silicon wafer with the temporary micro flow channels re-bond with the sheet glass. During the re-bonding process, increase pressure or voltage to make the sheet glass bend under the influence of electrostatic force to eliminate prefabricated temporary micro channels and realize the sealing of all alkali metal absorption cell cavities.

EMBODIMENTS

Carve a shallow micro groove 102 which is 1-2 µm in depth and 80-90 mm in diameter on one side of a 4-cun double-side polished silicon wafer 101, and an alkali metal compound placement cavity 103 which is 20 mm in diameter at the center of the double-side polished silicon wafer 101. Carve an array of alkali metal absorption cell cavities 104 in square which is 2 mm in length within the scope of the shallow micro groove 102 on the double-side polished silicon wafer 101. The alkali metal compound placement cavity 103 and the alkali metal absorption cell cavity 104 both have a through-hole structure, running through the double-side polished silicon wafer 101, as shown in FIG. 1a.
Bond the side of the double-side polished silicon wafer 101 without the shallow micro groove 102 with a sheet glass A 105 through the silicon-glass wafer level anodic bonding. Put a well-computed amount of alkali metal compound 106 in the alkali metal compound cavity 103, and bond the side of the double-side polished silicon wafer 101 with the shallow micro groove 102 with a sheet glass B 107 through the silicon-glass wafer level anodic bonding. The shallow micro groove 102 and the sheet glass B 107 constitute the micro flow channel of the alkali metal vapor together to form a small vacuum environment, as shown in FIG. 1b.
Separately adjust the temperature of the alkali metal compound placement cavity 103 by a reaction device to make the alkali metal compound 106 decompose to separate out alkali metal vapor 109. The alkali metal vapor 109 diffuses in the whole cavity including the alkali metal absorption cell cavity 104 through the temporary micro flow channel 108. An equipment for partial cooling 110 can adjust the temperature of the double-side polished silicon wafer 101 excluding the alkali metal compound placement cavity 103. The alkali metal vapor 109 congeals at the bottom of the alkali metal absorption cell cavity 104 to become a high-purity solid alkali metal 111 used to fill the alkali metal absorption cell cavity 104, as shown in FIG. 1c.
Re-bond the double-side polished silicon wafer 101 with the sheet glass B 107 through the silicon-glass wafer level anodic bonding. During the bonding process, increase pressure (1800 mbar-2000 mbar) or voltage (-800 V- -1000 V) to make the sheet glass B 107 bend under the influence of electrostatic force to eliminate the prefabricated temporary micro flow channel 108 and realize the sealing of all the alkali metal absorption cell cavities 104, as shown in FIG. 1d.
As shown in FIGS. 1a to 1d, the atomic clock alkali metal absorption cells made by the wafer level technique can be cut into individual atomic clock alkali metal absorption cells by slicing the wafer. As shown in FIG. 2, a high-purity alkali metal 201 is placed in a silicon alkali metal absorption cell cavity 202 which is sealed by an upper-layer glass 203 and a lower glass 204.

Claims

A method for filling wafer-based chip-scale atomic clock absorption cells with a high-purity alkali metal (201) comprising at least one of rubidium and cesium, wherein the method comprises:
(1) using an MEMS ICP etching technique to carve a shallow microgroove (102) on the first side of a polished double-sided silicon wafer (101),
to carve an array of absorption cell cavities (104) within the scope of said microgroove (102),
to carve a placement cavity (103) within said scope of said microgroove (102) at the center of said polished double-sided silicon wafer (101),
wherein said absorption cell cavities (104) and said placement cavity (103) have a through hole structure running through both sides of said polished double-sided silicon wafer (101);

(2) bonding the second side of the polished double-sided silicon wafer (101) with a first sheet glass (105) through a glass-silicon wafer level anodic bonding process,
putting a precomputed amount of an alkali metal compound (106) into the placement cavity (103),
and bonding the first side of the polished double-sided silicon wafer (101) with a second sheet glass (107) through a glass-silicon wafer level anodic bonding process, so that the shallow microgroove (102) and said second sheet glass (107) define a temporary flow channel (108) for an alkali metal vapour (109);

(3) controlling the intensity of a decomposition reaction of the alkali metal compound (106) by separately adjusting the temperature of the placement cavity to decompose the alkali metal compound (106) and to produce a needed amount of the alkali metal (201),
and vaporizing the alkali metal to produce said alkali metal vapour (109);

(4) diffusing the alkali metal vapour (109) through the temporary flow channel (108),
and cooling the absorption cavities (104) to condense the alkali metal vapour (109) in the absorption cavities (104); and

(5) re-bonding the first side of the polished double-sided silicon wafer (101) with the first glass sheet (107) with glass-silicon wafer level anodic bonding while applying an electrostatic force to make said first glass sheet (107) bending under the influence of said force, in such a way that the temporary flow channel (108) is eliminated so that the absorption cells (104) are sealed at the same time.
The method of claim 1, characterized in that in (5), a pressure or a voltage is applied on the first sheet glass (107) to bend the first sheet glass under the influence of said electrostatic force and to eliminate the temporary flow channel (108).