MEMS atomic clock air chamber and preparation method thereof

Abstract

The invention provides an MEMS atomic clock air chamber and a preparation method thereof. The method comprises the following steps: selecting a silicon wafer, and etching the front surface of the silicon wafer to form a silicon groove structure; carrying out anodic bonding on the front surface of the silicon wafer etched with the silicon groove and a glass wafer; forming top glass and a glass optical window on the silicon wafer by adopting a glass backflow process, and flattening the upper surface of the top glass; photoetching and etching the back surface of the silicon wafer, etching a channel structure, and forming an optical cavity and an alkali metal placing groove; alkali metal is put in, anodic bonding is carried out on the face, where the channel structure is etched and the optical cavity is formed, of the glass wafer to package the optical cavity, and the alkali metal is activated. According to the invention, the novel on-chip MEMS atomic clock air chamber with the optical cavity with multiple reflection capability is constructed, so that the length of an optical path in the atomic clock air chamber can be obviously increased, and the interaction strength between light and alkali metal atoms is enhanced.

Description

Technical Field

[0001] This disclosure relates to the field of atomic clock technology, and in particular to a microelectromechanical system (MEMS) atomic clock gas chamber and its preparation method. Background Technology

[0002] Atomic clocks, which utilize atomic energy level transitions to generate highly stable and precise frequency reference signals, are currently the most accurate time measurement tools in the world, playing a crucial role in navigation, positioning, synchronous communication, and autonomous driving. Traditional atomic clocks are large and consume a lot of power, making it difficult to meet the growing demands for portability and miniaturization. Chip-scale atomic clocks, while maintaining high precision, can achieve significant reductions in size and power consumption, and have become an important direction for the future development of atomic clocks.

[0003] Chip-scale atomic clock technology based on the principle of coherent population trapping (CPT) is relatively mature. It achieves frequency stability through coherent coupling between the hyperfine energy level transitions of atoms in an alkali metal chamber and the external microwave frequency. Therefore, the atomic clock chamber is considered the core physical component of a chip-scale atomic clock, and its performance and size directly affect the size, power consumption, and integration density of the atomic clock.

[0004] Currently, due to the overall size of chip-scale atomic clocks, it is very difficult to further increase the cavity length of atomic clock gas cells, which cannot meet the ever-increasing demand for optical path length. Summary of the Invention

[0005] In view of this, the main objective of this disclosure is to provide a MEMS atomic clock gas cell and its preparation method, so as to increase the optical path length in the atomic clock gas cell, enhance the interaction strength between light and alkali metal atoms, and meet the growing demand for optical path length in the atomic clock gas cell.

[0006] To achieve the above objectives, this disclosure provides a method for fabricating a MEMS atomic clock gas chamber, the method comprising:

[0007] Select a silicon wafer and etch a silicon groove structure on the front side of the silicon wafer;

[0008] The front side of the silicon wafer with etched silicon grooves is anodicly bonded to the glass wafer;

[0009] A glass reflow process is used to form a top glass and a glass optical window on a silicon wafer, and the upper surface of the top glass is planarized.

[0010] Photolithography and etching are performed on the back side of the silicon wafer to etch channel structures, forming optical cavities and alkali metal placement grooves; and

[0011] An alkali metal is placed in the cavity, and the side of the optical cavity formed by etching out the channel structure is anodicly bonded to the glass wafer to encapsulate the optical cavity, and the alkali metal is activated.

[0012] In the above scheme, in the step of selecting a silicon wafer and etching a silicon groove structure on the front side of the silicon wafer, the silicon wafer is selected as a single-crystal silicon wafer with double-surface polishing and crystal orientation. <100> A silicon wafer with a resistivity ≤0.001Ω·cm and a thickness of not less than 500μm is used. Then, using photoresist and SiO2 as a mask, silicon grooves are etched on the front side of the silicon wafer using photolithography and deep reactive ion etching processes, with an etching depth of 200μm~400μm.

[0013] In the above scheme, in the step of anodic bonding between the front side of the silicon wafer with etched silicon grooves and the glass wafer, the glass wafer is selected as BF33 high borosilicate glass.

[0014] In the above scheme, the step of forming the top glass and glass optical window on the silicon wafer using the glass reflow process involves hot reflowing the anoly bonded sample in a high-temperature tube furnace. The glass wafer is softened and melted at high temperature, and finally reflowed into the silicon groove to fill the silicon groove, forming the top glass and glass optical window after reflow. The reflow temperature is selected as 750℃~900℃.

[0015] In the above scheme, the planarization step of the top glass surface is performed by chemical mechanical polishing.

[0016] In the above scheme, in the step of photolithography and etching the back side of the silicon wafer to etch out the channel structure and form the optical cavity and alkali metal placement groove, photoresist and SiO2 are used as masks, and photolithography and deep reactive ion etching processes are used to photolithography and etch out the channel structure to form the optical cavity and alkali metal placement groove.

[0017] In the above scheme, the alkali metal is selected as Rb or Cs.

[0018] In the above scheme, in the steps of placing alkali metal, anodicly bonding the side of the optical cavity formed by etching the channel structure to the glass wafer to encapsulate the optical cavity, and activating the alkali metal, the glass wafer is selected as BF33 high borosilicate glass, and the alkali metal is activated by laser heating.

[0019] To achieve the above objectives, this disclosure also provides a MEMS atomic clock chamber, fabricated using the method described above. The MEMS atomic clock chamber includes an optical cavity, an optical window, and a reflector, wherein:

[0020] The optical cavity is a sealed cavity formed by bonding a silicon wafer with a cavity structure to an upper glass wafer and a lower glass wafer, and light propagates within the sealed cavity.

[0021] The optical window is formed by the glass wafer being reflowed into a vertical silicon groove etched on the silicon wafer, and light enters and exits the optical cavity through the optical window;

[0022] The reflector is an optical cavity formed by etching a silicon wafer, perpendicular to the silicon sidewall.

[0023] In the above scheme, the optical cavity is provided with an alkali metal placement groove, which is formed at the same time as the optical cavity is formed by photolithography and etching on the back side of the silicon wafer.

[0024] As can be seen from the above technical solution, the MEMS atomic clock chamber and its fabrication method provided in this disclosure, by constructing a novel on-chip MEMS atomic clock chamber with an optical cavity capable of multiple reflections, can significantly increase the optical path length in the atomic clock chamber, thereby enhancing the interaction strength between light and alkali metal atoms. This meets the increasing demand for longer optical path lengths in atomic clock chambers and has at least the following beneficial effects compared with existing technologies:

[0025] 1. Adjustable optical path length: The MEMS atomic clock gas cell and its fabrication method proposed in this disclosure construct a novel on-chip MEMS atomic clock gas cell. This MEMS atomic clock gas cell has an optical cavity with multiple reflection capabilities. The optical path is on-chip propagation, and the optical path length is adjustable according to requirements, which significantly improves the path length of the interaction between light and alkali metal atoms.

[0026] 2. Simplified fabrication process: The MEMS atomic clock gas cell and its fabrication method proposed in this disclosure use the optical cavity formed by etching a silicon wafer and its perpendicular silicon sidewall as a reflector, which greatly simplifies the fabrication process of the 45° reflector and allows for the simultaneous and free placement of multiple 45° reflectors. This enables the adjustment of the optical path length according to actual needs, thus meeting the growing demand for optical path length in atomic clock gas cells.

[0027] 3. Good process compatibility: The MEMS atomic clock gas cell and its preparation method proposed in this disclosure are compatible with micro-nano manufacturing processes and can be easily integrated with micro light sources and detectors on-chip, effectively reducing the overall size of the atomic clock. Attached Figure Description

[0028] The foregoing contents, as well as other objects, features, and advantages of this disclosure, will become clearer from the following description of embodiments with reference to the accompanying drawings, in which:

[0029] Figure 1 This is a flowchart of a method for preparing a MEMS atomic clock gas cell according to an embodiment of the present disclosure;

[0030] Figure 2 for Figure 1 The diagram shows a silicon groove structure etched on the front side of a silicon wafer during the fabrication process of a MEMS atomic clock gas cell.

[0031] Figure 2a For corresponding Figure 2 Top view of the silicon groove structure;

[0032] Figure 3 for Figure 1 A schematic diagram illustrating the anodic bonding of the front side of a silicon wafer with etched silicon grooves to a glass wafer in the fabrication process of a MEMS atomic clock gas cell.

[0033] Figure 4 for Figure 1 A schematic diagram illustrating the glass reflow process used in the fabrication of the MEMS atomic clock gas cell to form the top glass and glass optical window on a silicon wafer.

[0034] Figure 5 for Figure 1 A schematic diagram illustrating the planarization of the top glass surface after glass reflow during the fabrication process of the MEMS atomic clock gas cell.

[0035] Figure 6 for Figure 1 A schematic diagram of the process of fabricating the gas chamber of a MEMS atomic clock, in which photolithography and etching are performed on the back of a silicon wafer to etch out the channel structure, forming an optical cavity and an alkali metal placement groove;

[0036] Figure 6a For corresponding Figure 6 A bottom view forming the optical cavity and alkali metal placement tank;

[0037] Figure 7 for Figure 1 A schematic diagram illustrating the process of fabricating a MEMS atomic clock gas chamber by etching a channel structure to form an optical cavity and then anodicly bonding the optical cavity to a glass wafer and activating the alkali metal.

[0038] Figure 7a For along Figure 7 A bottom view of the section at the centerline aa'.

[0039] [Explanation of Labels in the Attached Image]

[0040] 100 is a silicon wafer, 101 is a silicon trench, 102 is a silicon support structure, and 103 is a sidewall mirror.

[0041] 200 is the glass wafer, 201 is the top glass, and 202 is the glass optical window.

[0042] 301 is the optical cavity, 302 is the alkali metal placement tank, and 400 is the alkali metal. Detailed Implementation

[0043] The embodiments of the present disclosure will now be described with reference to the accompanying drawings. However, it should be understood that these descriptions are exemplary only and are not intended to limit the scope of the disclosure. In the following detailed description, numerous specific details are set forth to provide a thorough understanding of the embodiments of the present disclosure for ease of explanation. However, it will be apparent that one or more embodiments may be practiced without these specific details. Furthermore, descriptions of well-known structures and techniques are omitted in the following description to avoid unnecessarily obscuring the concepts of the present disclosure.

[0044] The terminology used herein is for the purpose of describing particular embodiments only and is not intended to limit this disclosure. The terms “comprising,” “including,” etc., as used herein indicate the presence of the stated features, steps, operations, and / or components, but do not exclude the presence or addition of one or more other features, steps, operations, or components.

[0045] All terms used herein (including technical and scientific terms) have the meanings commonly understood by those skilled in the art, unless otherwise defined. It should be noted that the terms used herein are to be interpreted in a manner consistent with the context of this specification, and not in an idealized or overly rigid way.

[0046] Furthermore, the technical features involved in the different embodiments of this application described below can be combined with each other as long as they do not conflict with each other.

[0047] Currently, the atomic clock cells used in chip-scale atomic clocks are mainly manufactured using microelectromechanical systems (MEMS) technology. Their structure is typically a three-layer structure composed of glass-silicon-glass stacks. Alkali metals and mixed buffer gases are stored within through-cavities formed after etching the silicon wafer. Light propagates along the glass-silicon-glass direction and interacts with the alkali metals and mixed buffer gases. The optical path length depends on the thickness of the silicon wafer. Thicker silicon wafers are typically used to increase the path length for the interaction between light and alkali metal atoms; however, the thickness of the silicon wafer is often no more than 2 millimeters, thus the number of atoms is constrained by the cell volume, limiting the signal-to-noise ratio improvement of the CPT signal. Reflective atomic gas cells integrate mirrors into the cell. The cell typically contains two mirrors. Light is reflected by the first mirror and propagates horizontally, then reflected by the second mirror and incident on the detector. This increases the cavity length of the atomic gas cell, improving the path length for the interaction between light and alkali metal atoms. The ideal mirror angle is 45°, however, typically... <100> The surface reflection angle of an oriented silicon wafer after anisotropic etching is 54.74°. Previous studies have reported a method using a diffraction grating to adjust the beam direction, achieving horizontal light propagation. However, a certain distance must be maintained between the diffraction grating and the mirror to avoid the influence of other diffraction modes, thus increasing the thickness. Other studies have explored depositing a metal reflective layer on a glass slide and then mechanically cutting it to fabricate a 45° Bragg mirror. The mirror is then integrated into an atomic gas cell via local anodic bonding. However, this method requires customized development, is complex, costly, and difficult to mass-produce.

[0048] To increase the optical path length in the atomic clock chamber, enhance the interaction strength between light and alkali metal atoms, and meet the growing demand for longer optical path lengths in the atomic clock chamber, this disclosure provides a MEMS atomic clock chamber and its fabrication method.

[0049] like Figure 1 As shown, Figure 1 The present invention provides a flowchart of a method for fabricating a MEMS atomic clock gas cell according to an embodiment of the present disclosure, the method comprising the following steps:

[0050] Step 1: Select a silicon wafer and etch a silicon groove structure on the front side of the silicon wafer;

[0051] According to the embodiments of this disclosure, the process corresponding to this step is as follows: Figure 2 and Figure 2a As shown, Figure 2 for Figure 1 The diagram shows a silicon groove structure etched on the front side of a silicon wafer during the fabrication process of a MEMS atomic clock gas cell. Figure 2a For corresponding Figure 2 A top view of the silicon groove structure.

[0052] In this step, the silicon wafer 100 can be selected from single-crystal silicon double-surface polishing and crystal orientation. <100> A silicon wafer with a resistivity ≤0.001Ω·cm and a thickness of not less than 500μm is used. Using photoresist, SiO2, etc. as a mask, a silicon groove 101 is etched on the front side of the silicon wafer 100 using photolithography and deep reactive ion etching processes, with an etching depth of 200μm~400μm.

[0053] Step 2: Anodicly bond the front side of the silicon wafer with etched silicon grooves to the glass wafer;

[0054] According to the embodiments of this disclosure, the process corresponding to this step is as follows: Figure 3 As shown, Figure 3 for Figure 1 This is a schematic diagram illustrating the anodic bonding of the front side of a silicon wafer with etched silicon grooves to a glass wafer in the fabrication process of a MEMS atomic clock gas chamber.

[0055] In this step, the front side of the silicon wafer 100 with the etched silicon groove 101 is anoly bonded to the glass wafer 200, which can be BF33 high borosilicate glass.

[0056] Step 3: The top glass and glass optical window are formed on the silicon wafer using a glass reflow process, and the upper surface of the top glass is planarized.

[0057] According to the embodiments of this disclosure, the process corresponding to this step is as follows: Figure 4 and Figure 5 As shown, Figure 4 for Figure 1 This diagram illustrates the process of forming the top glass and glass optical window on a silicon wafer using a glass reflow process in the fabrication of the gas cell for a MEMS atomic clock. Figure 5 for Figure 1 This is a schematic diagram illustrating the planarization of the top glass surface after glass reflow during the fabrication process of the MEMS atomic clock gas chamber.

[0058] In this step, the anoly bonded sample is subjected to thermal reflow in a high-temperature tube furnace. The glass wafer 200 is softened and melted at high temperature, and finally reflowed into the silicon recess 101 to fill it, forming the top glass 201 and the glass optical window 202 after reflow. The reflow temperature can be selected from 750℃ to 900℃. The upper surface of the top glass 201 is planarized, which can be achieved by chemical mechanical polishing.

[0059] Step 4: Perform photolithography and etching on the back side of the silicon wafer to etch out the channel structure, forming the optical cavity and alkali metal placement groove;

[0060] According to the embodiments of this disclosure, the process corresponding to this step is as follows: Figure 6 and Figure 6a As shown, Figure 6 for Figure 1 This diagram illustrates the process of photolithography and etching on the back of a silicon wafer to create channel structures, forming optical cavities and alkali metal placement grooves in the fabrication of the MEMS atomic clock gas chamber. Figure 6a For corresponding Figure 6 A bottom view showing the optical cavity and alkali metal placement tank.

[0061] In this step, photoresist, SiO2, etc., are used as masks, and photolithography and deep reactive ion etching processes are employed to perform photolithography and etching on the back side of the silicon wafer 100 to etch out the channel structure, forming the optical cavity 301 and the alkali metal placement groove 302. The alkali metal used can be Rb or Cs.

[0062] Step 5: Place the alkali metal, and anodicly bond the side of the etched channel structure to form the optical cavity to the glass wafer to encapsulate the optical cavity, and activate the alkali metal.

[0063] According to the embodiments of this disclosure, the process corresponding to this step is as follows: Figure 7 and Figure 7a As shown, Figure 7 for Figure 1 The schematic diagram illustrates the process of fabricating a MEMS atomic clock gas chamber by etching a channel structure to form an optical cavity, anodicly bonding one side of the optical cavity to a glass wafer, and activating the alkali metal. Figure 7a For along Figure 7 A bottom view of the section at the centerline aa'.

[0064] In this step, the side of the optical cavity 301 formed by etching the channel structure is anoly bonded to the glass wafer 200. The glass wafer 200 can be BF33 high borosilicate glass, and the alkali metal is activated by laser heating.

[0065] As can be seen from the above embodiments, the method for fabricating the MEMS atomic clock gas cell provided in this disclosure constructs a novel on-chip MEMS atomic clock gas cell. This MEMS atomic clock gas cell has an optical cavity with multiple reflection capabilities, which can significantly increase the optical path length in the atomic clock gas cell, thereby enhancing the interaction strength between light and alkali metal atoms and meeting the growing demand for optical path length in the atomic clock gas cell.

[0066] based on Figures 1 to 7 The method for fabricating a MEMS atomic clock gas cell shown in the disclosure also provides a MEMS atomic clock gas cell, which includes an optical cavity, an optical window, and a reflector, wherein:

[0067] The optical cavity is a sealed cavity formed by bonding a silicon wafer with a cavity structure to an upper glass wafer and a lower glass wafer, and light propagates within the sealed cavity.

[0068] The optical window is formed by the glass wafer being reflowed into a vertical silicon groove etched on the silicon wafer, and light enters and exits the optical cavity through the optical window;

[0069] The reflector is an optical cavity formed by etching a silicon wafer, perpendicular to the silicon sidewall.

[0070] According to an embodiment of this disclosure, the optical cavity is provided with an alkali metal placement groove, which is formed simultaneously with photolithography and etching of the back side of a silicon wafer to form the optical cavity.

[0071] As can be seen from the above embodiments, the MEMS atomic clock gas cell and its preparation method proposed in this disclosure construct a novel on-chip MEMS atomic clock gas cell with an optical cavity capable of multiple reflections. The optical path is propagated within the chip, and the optical path length is adjustable according to requirements, which significantly improves the path length of the interaction between light and alkali metal atoms.

[0072] The MEMS atomic clock gas cell and its fabrication method proposed in this disclosure use the vertical silicon sidewall of the optical cavity formed by etching a silicon wafer as a reflector, which greatly simplifies the fabrication process of the 45° reflector and allows for the simultaneous and free placement of multiple 45° reflectors. This enables the adjustment of the optical path length according to actual needs, thus meeting the growing demand for optical path length in the atomic clock gas cell.

[0073] The MEMS atomic clock gas chamber and its fabrication method proposed in this disclosure are compatible with micro-nano manufacturing processes and can be easily integrated with micro light sources and detectors on-chip, effectively reducing the overall size of the atomic clock.

[0074] Those skilled in the art will understand that the features described in the various embodiments of this disclosure can be combined and / or combined in various ways, even if such combinations or combinations are not explicitly described in this disclosure. In particular, the features described in the various embodiments of this disclosure can be combined and / or combined in various ways without departing from the spirit and teachings of this disclosure. All such combinations and / or combinations fall within the scope of this disclosure.

[0075] The embodiments of this disclosure have been described above. However, these embodiments are for illustrative purposes only and are not intended to limit the scope of this disclosure. Although various embodiments have been described above, this does not mean that the measures in the various embodiments cannot be used advantageously in combination. Various substitutions and modifications can be made by those skilled in the art without departing from the scope of this disclosure, and all such substitutions and modifications should fall within the scope of this disclosure.

Claims

1. A method for fabricating a MEMS atomic clock gas cell, characterized in that, The method includes: Select a silicon wafer and etch a silicon groove structure on the front side of the silicon wafer; The front side of the silicon wafer with etched silicon grooves is anodicly bonded to the glass wafer; A glass reflow process is used to form a top glass and a glass optical window on a silicon wafer, and the upper surface of the top glass is planarized. Photolithography and etching are performed on the back side of the silicon wafer to etch channel structures, forming optical cavities and alkali metal placement grooves; and An alkali metal is placed in the cavity, and the side of the optical cavity formed by etching out the channel structure is anodicly bonded to the glass wafer to encapsulate the optical cavity, and the alkali metal is activated.

2. The method for fabricating a MEMS atomic clock gas cell according to claim 1, characterized in that, In the step of selecting a silicon wafer and etching a silicon groove structure on the front side of the silicon wafer, the silicon wafer is selected as a single-crystal silicon wafer with double-surface polishing and crystal orientation. <100> A silicon wafer with a resistivity ≤0.001Ω·cm and a thickness of not less than 500μm is used. Then, using photoresist and SiO2 as a mask, silicon grooves are etched on the front side of the silicon wafer using photolithography and deep reactive ion etching processes, with an etching depth of 200μm~400μm.

3. The method for fabricating a MEMS atomic clock gas cell according to claim 1, characterized in that, In the step of anodic bonding of the front side of the silicon wafer with etched silicon grooves to the glass wafer, the glass wafer is selected as BF33 high borosilicate glass.

4. The method for fabricating a MEMS atomic clock gas cell according to claim 1, characterized in that, In the step of forming the top glass and glass optical window on the silicon wafer using the glass reflow process, the anoly bonded sample is thermally reflowed in a high-temperature tube furnace. The glass wafer is softened and melted at high temperature, and finally reflowed into the silicon groove to fill the silicon groove, forming the reflowed top glass and glass optical window. The reflow temperature is selected as 750℃~900℃.

5. The method for fabricating a MEMS atomic clock gas cell according to claim 1, characterized in that, In the step of planarizing the upper surface of the top glass, the planarization is performed using a chemical mechanical polishing method.

6. The method for fabricating a MEMS atomic clock gas cell according to claim 1, characterized in that, In the step of photolithography and etching the back side of the silicon wafer to etch out the channel structure and form the optical cavity and alkali metal placement groove, photoresist and SiO2 are used as masks, and photolithography and deep reactive ion etching processes are used to photolithography and etch out the channel structure to form the optical cavity and alkali metal placement groove.

7. The method for fabricating a MEMS atomic clock gas cell according to claim 6, characterized in that, The alkali metal is selected as Rb or Cs.

8. The method for fabricating a MEMS atomic clock gas cell according to claim 1, characterized in that, In the steps of placing alkali metal, anodicly bonding the side of the etched channel structure to form the optical cavity with the glass wafer to encapsulate the optical cavity, and activating the alkali metal, the glass wafer is selected as BF33 high borosilicate glass, and the alkali metal is activated by laser heating.

9. A MEMS atomic clock gas chamber, prepared by any one of claims 1 to 8, characterized in that, The MEMS atomic clock gas chamber includes an optical cavity, an optical window, and a reflector, wherein: The optical cavity is a sealed cavity formed by bonding a silicon wafer with a cavity structure to an upper glass wafer and a lower glass wafer, and light propagates within the sealed cavity. The optical window is formed by the glass wafer being reflowed into a vertical silicon groove etched on the silicon wafer, and light enters and exits the optical cavity through the optical window; The reflector is an optical cavity formed by etching a silicon wafer, perpendicular to the silicon sidewall.

10. The MEMS atomic clock gas chamber according to claim 9, characterized in that, The optical cavity is provided with an alkali metal placement groove, which is formed simultaneously with photolithography and etching of the back side of the silicon wafer to form the optical cavity.

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