Abstract:
A frequency standard has a cell formed in a cavity of a substrate. The cell contains a metal alkali vapor. The substrate has an optical path that intersects the cell. A light source is supported by the substrate and supplies light through the first optical path to the cell, and a light detector is supported by the substrate and receives light through the second optical path from the cell. The sealed vapor-filled cell is surrounded by a vacuum cavity enclosure. Bridges between the cell and the substrate may be used to thermally isolate the cell in the cavity and allow closed loop temperature control of the cell.

Description:
TECHNICAL FIELD OF THE INVENTION  
         [0001]    The present invention relates to a frequency standard that can be used for such devices as atomic clocks.  
         BACKGROUND OF THE INVENTION  
         [0002]    In frequency standards that rely on alkali metal source atoms, such as atoms of cesium 133 or rubidium 85 or 87, a modulatable light source, such as a laser light source, is used to optically pump the source atoms contained in a cell of the frequency standard. A sealed, optically transparent cell contains the source atoms and any buffer gases, and the RF modulated light from the light source is directed through suitable optics into the cell. When the source atoms within the cell absorb light of a particular wavelength that is modulated at a particular modulation frequency, they emit a light signal whose intensity has a sharply defined peak at this wavelength. This light signal is detected as an output of the frequency standard.  
           [0003]    This detected light may then be used to control the frequency of the light source emission so that the intensity of the light output from the source atoms is maintained at this peak. Because the peak intensity is very sharply defined, the modulation frequency can then be used to very accurately drive a clock.  
           [0004]    Present atomic frequency standards have sizes averaging in the vicinity of 3 inches by 3 inches by 6 inches. Efforts have been made to reduce this size particularly for applications in the fields of telecommunications, satellite navigation transmitters and receivers, and the like.  
           [0005]    Once such effort has been directed to a design involving a frame element on which an optical physics package and an electronic control and detection package are mounted. The optical physics package includes a solid state laser source, a linear polarizer, a circular polarizer, a sealed and windowed metallic cell containing the source and buffer gas atoms, and a photodetector. The electronic control and detection package cooperates with the physics package to control and modulate the laser source and to detect the light output. This package is reported to have a size of 1.5 inches by 1.5 inches by 2.5 inches.  
           [0006]    The present invention achieves even smaller dimensions by employing MicroElectroMechanical Systems (MEMS) technology in fabricating both the optics and the detection components on the same substrate. The size of the MEMS frequency standard according to the present invention may be on the order of 1.5 mm deep by 1.5 mm high by 2.0 mm long.  
         SUMMARY OF THE INVENTION  
         [0007]    In accordance with one aspect of the present invention, a frequency standard comprises a two-layer substrate made by bonding together first and second substrates, a cell, first and second optical paths, a light source, and a light detector. The cell is formed in a cavity of the substrate, and the cell contains a vapor of metal alkali atoms. The first and second optical paths are formed inside the substrate so as to intersect the cell. The light source is supported by the substrate and supplies light through the first optical path to the cell. The light detector is supported by the substrate and receives light through the second optical path from the cell.  
           [0008]    In accordance with another aspect of the present invention, a frequency standard comprises a substrate, a cell, at least first and second bridges, an optical path, a light source, and a light detector. The cell is formed in a cavity of the substrate, and the cell contains metal alkali atoms. The at least first and second bridges suspend the cell within the cavity. The optical path is provided through the substrate, and the optical path intersects the cell. The light source is supported by the substrate and supplies light through the optical path to the cell. The light detector is supported by the substrate and receives light from the cell through the optical path.  
           [0009]    In accordance with yet another aspect of the present invention, a method comprises the following: etching a substrate to form an etched volume; forming a transparent oxide wall on the substrate in the etched volume; etching the substrate so as to form a cavity around the wall, so as to form an optical path in the substrate intersecting the wall, and so that the wall is mechanically attached to the substrate by bridges; placing a metal alkali within the wall; providing the substrate with a light source arranged to supply light through the first optical path to the wall; providing the substrate with a light detector arranged to receive light through the second optical path from the wall; and, engaging the substrate with a top cap. The top cap is hermetically sealed to the substrate to proved a vacuum enclosure for the alkali metal vapor cell suspended by bridges. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]    These and other features and advantages will become more apparent from a detailed consideration of the invention when taken in conjunction with the drawings in which:  
         [0011]    [0011]FIG. 1 is a top view of an atomic frequency standard in accordance with the present invention;  
         [0012]    [0012]FIG. 2 is a cross-section side view of the atomic frequency standard shown in FIG. 1; and,  
         [0013]    [0013]FIG. 3 is an isometric view of the atomic frequency standard shown in FIG. 1. 
     
    
     DETAILED DESCRIPTION  
       [0014]    A frequency standard  10  is shown in FIGS. 1, 2, and  3  and includes a lower substrate  12 , an upper substrate  40 , and a top cap  50 . Substrates  12  and  40 , for example, may be silicon substrates. A light source  14  is provided in the substrate  12  at one side thereof. The light source  14 , for example, may be a vertical cavity surface emitting laser (VCSEL) and can be separately fabricated and inserted into a corresponding well in the substrate  12 . Alternatively, the light source  14  can be directly fabricated into the substrate  12  using known integration techniques. Similarly, a light detector  16  is provided in the substrate  12  at another side thereof. The light detector  16 , for example, may be a photodiode detector and can be separately fabricated and inserted into a corresponding well in the substrate  12 . Alternatively, the substrate  12  can be made from a semiconducting material, and the light detector  16  can be directly fabricated into the substrate  12  using known integration techniques.  
         [0015]    During etching of the lower substrate  12 , v-shaped containment grooves  18  and  20  are selectively formed to receive first and second optical processors  22  and  24 , respectively. Similarly, grooves are etched in upper substrate  40 . The first optical processor  22 , for example, may include a lens and prism and a quarter wave plate circular polarizer. The lens and prism and a quarter wave plate circular polarizer may be on the side of the cell closer to the light source  14 , and the quarter wave plate circular polarizer may be on the other side nearer the detector  16 . The second optical processor  24 , for example, may include a lens.  
         [0016]    Respective hermetic seals are provided between the substrate  12 , the first and second optical processors  22  and  24 , and the upper substrate  40 . For example, the first and second optical processors  22  and  24  may be soldered into the v-shaped containment grooves  18  and  20  so as to form part of the hermetic seals. The hermetic seals allow the cavity  26  to be evacuated so that the alkali metal vapor cell  28  is thermally isolated from the substrates  12  and  40 . The transparent top cap  50  is bonded hermetically to the two-layer substrate to provide the vacuum enclosure for the vapor cell  28 .  
         [0017]    During etching of the substrate  12 , a portion of a chamber  26  is formed in the substrate  12 . A transparent oxide, such as silicon dioxide, is grown or deposited on the upper substrate  40  forming this portion of the chamber  28 . Etching of the substrate  40  is continued so that the top and sides of the cell  28  are formed. The bottom of the cell  28  is suspended from the substrate  12  by bridges  30 . The bridges  30 , which may be thermal insulating bridges, provide thermal isolation between the cell  28  and the substrates  12  and  40 . Deep Reactive Ion Etching (DRIE) can be used for the etching described above. The cell  28  is formed by bonding together the two substrates  40  and  12  with a hermetic seal such as Pb—Sn reflow solder.  
         [0018]    The cell  28  has a first cell portion  32  and a second cell portion  34 . An alkali metal such as rubidium is deposited in the second cell portion  34  and the alkali metal is capped with a passivation layer such as an aluminum layer. The first and second cell portions  32  and  34  are coupled by a small slit or tunnel. In one embodiment of the present invention, the cap and/or walls of the adjoining layer may be made transparent so that the alkali metal can be heated by a laser so as vaporize the alkali metal. The vapor pressure of the metal alkali is sufficient to cause the vaporized metal alkali to fill the first cell portion  32  to a saturation vapor pressure at the desired temperature, such as 85° C.  
         [0019]    In another embodiment of the present invention, the frequency standard  10 , when fabricated at least sufficiently for the cell  28  to be sealed, may be placed in an oven and heated to a temperature that causes the metal alkali in the second cell portion  34  to vaporize and that causes the resulting alkali metal vapor to fill the first cell portion  32 .  
         [0020]    In still another embodiment, a heater  36  in contact with the cell  28  may be energized to heat the metal alkali in the second cell portion  34  sufficiently to cause the metal alkali in the second cell portion  34  to vaporize and to cause the resulting alkali metal vapor to fill the first cell portion  32 . It is desirable to closely control the temperature of the cell with low levels of heater power.  
         [0021]    The upper substrate  40  is etched to form a chamber  42  above the cell  28 , to provide surfaces on which mirrors  44  and  46  may be formed, and to provide v-shaped containment grooves to receive the first and second optical processors  22  and  24 . Accordingly, the v-shaped containment grooves formed in the upper substrate  40  align with the v-shaped containment grooves  18  and  20  formed in the substrate  12  in order to contain the first and second optical processors  22  and  24 . The mirror  44  directs light from the light source  14  to the first optical processor  22  and through the alkali metal cell  28 , and the mirror  46  directs light from the second optical processor  24  to the light detector  16 .  
         [0022]    The upper substrate  40  is attached to the substrate  12  so that the chamber  42  aligns with the chamber  26  and so that the mirrors  44  and  46  have the relative positions shown in FIG. 2. Standard wafer bonding techniques may be used to attach the upper substrate  40  to the lower substrate  12 . The upper substrate  40 , for example, may be a semiconductor wafer and/or a silicon wafer. The mirrors  44  and  46 , for example, may be fabricated by etching silicon with KOH, a well known anisotropic etch for silicon, producing mirror surfaces.  
         [0023]    The cavity formed by the chambers  26  and  42  may be evacuated to form a vacuum around the cell  28 . For example, the transparent sealing wafer  50  may be attached to the substrate  12  within a vacuum thereby creating and preserving a vacuum within the cavity  26 . Because of the vacuum within this cavity, the thermal path from the cavity to the outside world has a very low thermal conductance. The low thermal conductance makes it possible to keep the temperature of the cavity stable with very little applied power.  
         [0024]    Bond pads  52  may be formed on the substrate  12  in order to electrically drive the light source  14 , the light detector  16 , the heater  36 , and/or any electronic connections needed in the frequency standard  10  to external devices.  
         [0025]    The first and second optical processors  22  and  24  may employ diffractive optic components. Such components can be made much smaller than their respective refractive counterparts, and are therefore more compatible with a MEMS process. Diffractive optics can be used to redirect, collimate, linearly polarize, and/or circularly polarize the light going into and exiting from the cavity formed by the chambers  26  and  42 .  
         [0026]    Although it has been conventionally thought in the past that linear polarizers were a required element of frequency standards of the type described herein, a separate linear polarizer is unnecessary if a well polarized VCSEL is used as the light source  14 .  
         [0027]    The remaining electronics for a device, such as an atomic clock, using the frequency standard  10  may be integrated in either the substrate  12  or the upper substrate  40  or elsewhere.  
         [0028]    Accordingly, the MEMS approach described herein for the frequency standard  10  results in the frequency standard  10  having a small size, low mass, and low power requirements. Also, MEMS fabrication offers other advantages such as high volume, low cost batch production and rapid commercialization. Moreover, the design described above has relatively large flat surfaces for solder reflow seals between wafers  12  and  40 , and between wafers  40  and  50 .  
         [0029]    Certain modifications of the present invention have been described above. Other modifications will occur to those practicing in the art of the present invention. For example, the bridges  30  may be silicon nitride.  
         [0030]    Also, the bridges  30  may be formed wholly or partially as springs in order to protect the bridges  30  from mechanical shocks. For example, the bridges  30  may be formed into zig-zag patterns that gives slightly when the substrate  12  and the upper substrate  40  are bonded together. By enabling the structure to flex, the process of bonding the substrate  12  and the upper substrate  40  together is made easier and more tolerant of processing imperfections.  
         [0031]    Accordingly, the description of the present invention is to be construed as illustrative only and is for the purpose of teaching those skilled in the art the best mode of carrying out the invention. The details may be varied substantially without departing from the spirit of the invention, and the exclusive use of all modifications which are within the scope of the appended claims is reserved.