MEMS frequency standard for devices such as atomic clock

[0001] The present invention relates to a frequency standard that can be used for such devices as atomic clocks.

[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.

[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.

[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 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.