Abstract:
The switch device comprises a pair of cavities, an elongate passage, a non-conductive fluid having a high electrical resistance, a conductive fluid having a high electrical conductivity and an electrical path. The passage is in fluid communication with the cavities and has a substantially elliptical cross-section over at least part of its length. The non-conductive fluid is disposed in each of the cavities. The conductive fluid is located in the passage. The electrical path is changeable between a connected state and a disconnected state by the non-conductive fluid separating the conductive fluid in the passage into non-contiguous conductive fluid portions.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    Published Japanese Patent Application No. S47-21645 discloses an example of a switch device for electrically switching solid electrodes by means of a conductive fluid. In this switch device, a conductive fluid composed of a liquid such as mercury is disposed movably inside a cylinder. The switch device is designed so that the conductive fluid is moved to one side by a pressure differential in a gas provided on both sides of the conductive fluid. When the conductive fluid moves, it touches electrodes that extend into the interior of the cylinder and forms an electrical connection between the electrodes. A drawback to this structure, however, is that the electrical connection characteristics of the switch device deteriorate as a result of the surfaces of the electrodes being modified over time by intermittent contact with the conductive fluid.  
           [0002]    Published Japanese Patent Application No. 2000-195389, assigned to the assignees of this disclosure and, for the United States, incorporated herein by reference, discloses a switch device structure that solves the above-mentioned problem. In this switch device structure, the electrical path is selectively changed from a connected state to a disconnected state by a conductive fluid such as mercury. However, the electrodes remain in constant contact with part of the conductive fluid, and the connected or disconnected state of the electrical path is determined by whether the conductive fluid exists as a single entity (connected state) or is separated into two or more conductive fluid portions (disconnected state). This eliminates the problem of poor connections that was a disadvantage of the switch device disclosed in published Japanese Patent Application No. S47-21645.  
           [0003]    In the switch device disclosed in published Japanese Patent Application No. 2000-195389, the material of the wall of the passage in which the conductive fluid is located has a low wettability with respect to the conductive fluid. Moreover, conventional manufacturing methods, such as anisotropically etching silicon, other types of dry etching, or a method such as applying a dry film, for forming the passage form the passage with a triangular, square, rectangular, trapezoidal or semicircular cross-sectional shape.  
           [0004]    [0004]FIG. 1 is a cross-sectional view of the passage of a typical prior art switch device. The passage  510  is formed in the silicon substrate by anisotropic etching. This design was proposed by J. Simon et al. in 6 J. MICROELECTRO-MECHANICAL SYS, (September 1997). The passage  510  has a triangular cross-sectional shape. The surface tension of the conductive fluid  520  causes the mercury to accumulate in the middle the passage, leaving gaps between the conductive fluid and the corners of the passage. Such gaps allow the non-conductive fluid to leak from the high-pressure side to the low-pressure side during operation of the switch device, which reduces the ability of the non-conductive fluid to move the conductive fluid. The effectiveness of the non-conductive fluid to move the conductive fluid can be increased by increasing the capacity of the device, such as a heater, that increases the pressure in the high-pressure side. However, when a heater is used as the pressure increasing device, increasing its capacity requires that the heater have a larger surface area or that it dissipate greater power. This not only increases the size of the switch device and increases the power consumption, but also lowers the degree of freedom in design.  
         SUMMARY OF THE INVENTION  
         [0005]    The invention solves the above problems, and provides a switch device that is more compact and uses less power than the conventional switch devices described above. The improvements are accomplished by reducing the leakage of the non-conductive fluid from the high-pressure side to the low-pressure side during operation of the switch device.  
           [0006]    The invention provides a switch device comprising a pair of cavities, an elongate passage, a non-conductive fluid having a high electrical resistance, a conductive fluid having a high electrical conductivity and an electrical path. The passage is in fluid communication with the cavities and has a substantially elliptical cross-section over at least part of its length. The non-conductive fluid is disposed in each of the cavities. The conductive fluid is located in the passage. The electrical path is changeable between a connected state and a disconnected state by the non-conductive fluid separating the conductive fluid in the passage into non-contiguous conductive fluid portions.  
           [0007]    The invention additionally provides a switch device comprising a pair of cavities, an elongate passage, a non-conductive fluid having a high electrical resistance, a conductive fluid having a high electrical conductivity and a wettable material. The passage is in fluid communication with the pair of cavities. The passage has a cross-sectional shape that, over at least a portion of the length of the passage, includes a corner. The non-conductive fluid is located in each of the pair of cavities. The conductive fluid is located in the passage in contact with the non-conductive fluid from the each of the cavities. The wettable material is wettable by the conductive fluid, is in contact with the conductive fluid and is located in at least part of the portion of the length of the passage where the cross-sectional shape includes the corner.  
           [0008]    Finally, the invention provides a method of making a switch device. In the method, a pair of plates, a non-conductive fluid having a high electrical resistance and a conductive fluid having a high electrical conductivity are provided. A pair of cavities and a passage that allows the pair of cavities to communicate are formed in at least one of the plates The passage has a cross-sectional shape that includes a corner over at least part of its length. The plates are mated. A portion of the non-conductive fluid is placed in each of the cavities. The conductive fluid is placed in the passage in contact with the portion of the non-conductive fluid in each of the cavities. A wettable film that is wettable by the conductive fluid is formed on at least one of the plates. The wettable film is located adjacent the corner of the cross-sectional shape and extends widthways and lengthways in the passage when the pair of plates is mated. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]    [0009]FIG. 1 is a cross sectional view of the channel of a conventional switch device.  
         [0010]    [0010]FIG. 2 is a plan view showing the structure of a first embodiment of a switch device according to the invention.  
         [0011]    [0011]FIG. 3 is a cross-sectional view along the line  3 - 3  in FIG. 2.  
         [0012]    [0012]FIG. 4 is a plan view showing the structure of a second embodiment of a switch device according to the invention.  
         [0013]    [0013]FIG. 5 is a cross-sectional view along the line  5 - 5  in FIG. 4.  
         [0014]    [0014]FIG. 6 is a plan view showing the structure of a third embodiment of a switch device according to the invention.  
         [0015]    [0015]FIG. 7 is a cross-sectional view along the line  7 - 7  in FIG. 6.  
         [0016]    [0016]FIG. 8 is a plan view showing the structure of a fourth embodiment of a switch device according to the invention.  
         [0017]    [0017]FIG. 9 is a cross-sectional view along the line  9 - 9  in FIG. 8. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0018]    Preferred embodiments of the switch device according to the invention will now be described in detail with reference to the Figures.  
         [0019]    [0019]FIGS. 2 and 3 show the structure of a first embodiment 1 of a switch device according to the invention. Three electrodes  31 ,  32 , and  33  are disposed along the length of the elongate passage  2  that is partially filled with a conductive fluid. The electrode  32  will be called the center electrode. The conductive fluid is shown separated into the three conductive fluid portions  11 ,  12 , and  13  that contact the electrodes  31 ,  32 , and  33 , respectively.  
         [0020]    The conductive fluid is preferably mercury. Gallium or another conductive material that is fluid at the operating temperature of the switch device may alternatively be used.  
         [0021]    Channels  41  and  42  extend from the cavities  51  and  52 , respectively, to the outlets  43  and  44 , respectively, laterally offset from one another along the length of the passage between the electrode  32  and the electrode  33 , and between the electrode  31  and the electrode  32 , respectively. The cavities  51  and  52  are filled with the non-conductive fluid  53  and  54 , respectively. Heaters  61  and  62  are located in the cavities  51  and  52 , respectively, for regulating the internal pressure of the non-conductive fluid in the cavities. The channels  41  and  42  transfer the non-conductive fluid from the cavities  51  and  52 , respectively, into the passage  2 .  
         [0022]    The switching operation of the switch device  1  is the same as of the switch device described in published Japanese Patent Application No. 2000-195389. For example, the conductive fluid portions  12  and  13  are initially joined together to form the conductive fluid  12 ,  13 , separated from the conductive fluid portion  11 . Thus, the conductive fluid  12 ,  13  electrically connects the electrode  32  to the electrode  33 , but the gap between the conductive fluid  12 ,  13  and the conductive fluid portion  11  electrically isolates the electrode  32  from the electrode  31 .  
         [0023]    The heater  61  generating heat causes the non-conductive fluid  53  in the cavity  51  to expand. The non-conductive fluid may be a gas, such as nitrogen, for example. The non-conductive fluid  53  passes through the channel  41  and enters the passage  2  through the outlet  43 . In the passage, the non-conductive fluid forms a gap in the conductive fluid  12 ,  13 . The gap separates the conductive fluid  12 ,  13  into the non-contiguous conductive fluid portions  12  and  13 . Separation of the conductive fluid  12 ,  13  into the conductive fluid portions  12  and  13  closes the gap between the conductive fluid portions  11  and  12 . The conductive fluid portions  11  and  12  unite to form the conductive fluid  11 ,  12 . The conductive fluid  11 ,  12  electrically connects the electrode  32  to the electrode  31 . The gap between the conductive fluid portion  13  and the conductive fluid  11 ,  12  electrically isolates the electrode  33  from the electrode  32 .  
         [0024]    The reverse operation occurs when the heater  62  generates heat. The non-conductive fluid  54  in the cavity  52  flows through the channel  42  into the passage  2  to form a gap in the conductive fluid  11 ,  12 . The gap electrically isolates the electrode  32  from the electrode  31 . Formation of the gap unites the conductive fluid portions  12  and  13  to form the conductive fluid portion  12 ,  13 . The conductive fluid  12 ,  13  electrically connects the electrodes  32  and  33 .  
         [0025]    The first embodiment of the invention provides an improvement in the cross-sectional shape of the passage  2  in the switch device just described to increase the operational efficiency and to reduce the size of the switch device.  
         [0026]    As shown in the cross-sectional view of FIG. 3, the passage  2  in this embodiment is composed of the grooves  73  and  74  formed in corresponding positions in the major surfaces of the first substrate  71  and the second substrate  72 , respectively. Joining the substrates together with their major surfaces in contact and the grooves  73  and  74  aligned with one another forms the passage  2 . Formed as described, the passage  2  has a substantially elliptical cross-sectional shape, as can be seen in FIG. 3. In this disclosure, unless otherwise stated, the term elliptical will be understood to encompass circular, the special case of the term elliptical in which the major and minor axes are of equal length. Similarly, the term semi-elliptical will be understood to encompass semicircular.  
         [0027]    The preferred material if the substrates  71  and  72  is glass. The grooves  73  and  74  have a substantially semi-elliptical cross-sectional shape and are about 0.1 to 0.2 mm wide and about 0.1 mm deep. The grooves are preferably formed in the substrates  71  and  72  by sandblasting with alumina particles, for instance. FIG. 3 also shows that, when the conductive fluid  12  is put into the passage  2  having an elliptical cross-sectional shape, the gap, if any, that exists between the conductive fluid  12  and the wall of the passage is very small.  
         [0028]    The conductive fluid  12  can be put into the passage  2  at the same time as the substrates  71  and  72  are joined together. Alternatively, the conductive fluid can be put in the groove formed in one of the substrates  71  and  72  before the substrates are joined. As a further alternative, the conductive fluid can be put into the passage  2  after the passage has been formed by joining the substrates  71  and  72  together.  
         [0029]    In a switch device having a passage  2  with an elliptical cross-sectional shape, and preferably formed by the method just described, the gap, if any, between the conductive fluid and the wall of the passage  2  is very small, as shown in the cross-sectional view of FIG. 3. Accordingly, the switch device  1  is subject to almost no pressure leakage or gas exchange past the conductive fluid  12 , and any increase in the pressure in each of the cavities  51  and  52  separates the conductive fluid into conductive fluid portions more efficiently. This allows the size of the heaters  61  and  62  to be reduced compared with a conventional switch device, or allows the heaters to be operated at lower power.  
         [0030]    In the embodiment just described, the number of component parts is reduced by forming the grooves  73  and  74  in both of the substrates  71  and  72  and by making the cross-sectional shapes of the portions  82  and  83  of the passage  2  and of the channels  41  and  42  similar to that shown in FIG. 3. However, according to the invention, only the portion  81  of the passage  2  that extends between the openings  43  and  44  of the channels  41  and  42 , respectively, must have a substantially elliptical cross-sectional shape and are preferably formed by forming grooves having a substantially semi-elliptical cross-sectional shape in both of the first and second substrates  71  and  72 . The portions  82  and  83  of the passage  2  and the channels  41  and  42  may alternatively have a semi-elliptical cross-sectional shape and may be formed by forming a groove in only one of the substrates  71  and  72 .  
         [0031]    [0031]FIGS. 4 and 5 illustrate a second embodiment  101  of a switch device according to the invention. The second embodiment of the switch device shown in FIGS. 4 and 5 is similar to the first embodiment of the switch device shown in FIGS. 2 and 3. Elements of the second embodiment having a similar function to elements of the first embodiment are indicated using the same reference numerals with  100  added and will not be described again.  
         [0032]    In the second embodiment  101 , the passage  102  has a semi-elliptical cross-sectional shape. The cross-sectional shape includes the corners  184  and  185  between the straight portion  186  and the semi-elliptical portion  187 . The wettable metal film  188  is located on a portion of the major surface of the substrate  172  that bounds part of the passage  102 .  
         [0033]    The preferred way of forming the passage  102  with a semi-elliptical cross-sectional shape is by forming the groove  175  having a semicircular or semi-elliptical cross-sectional shape in the first substrate  171  and joining the first substrate  171  to the first substrate  172  in which no groove is formed, as shown in FIG. 5.  
         [0034]    The wettable metal film  188  is located on part of the major surface of the substrate  172  in a region located at or near half-way between the openings  143  and  144  of the channels  141  and  142 . The wettable metal film extends lengthways along the length of the passage  102  towards both openings. The wettable metal film additionally extends widthways preferably at least as far as the corners  184  and  185  between the groove  175  and the substrate  172 . FIG. 5 shows the wettable metal film extending beyond this corner to ensure that the wettable metal film is present at the corners  184  and  185  notwithstanding alignment errors between the substrates  171  and  172 .  
         [0035]    The material of the wettable metal film  184  is a metal that is wetted by the conductive fluid  112 . Preferably, the wettable metal film is composed of thin films of chromium, nickel and gold. These films are deposited in order by vacuum deposition on the major surface of the substrate  172  for form the desired shape of the wettable metal film. Alternatively, the wettable metal film can include platinum, copper, tungsten, molybdenum, titanium, tantalum, iron, cobalt, palladium, or a combination of two or more of these metals. In the example shown, the wettable metal film also serves as the center electrode and is indicated as such by the reference numeral  132  in FIG. 5. However, this is not critical to the invention. The switch device may additionally include a center electrode separate from the wettable metal film.  
         [0036]    The preferred material of the substrates  171  and  172  is glass, and the groove  175  is preferably formed in the first substrate  171  by sandblasting with particles such as alumina.  
         [0037]    In a preferred embodiment, all three electrodes  131 ,  132  and  133  are formed simultaneously by the same thin film deposition process.  
         [0038]    In the second embodiment  101  of the switch device that includes the wettable metal film  188  located part-way along the length of the passage  102 , the gap, if any, between the conductive fluid  112  and the passage is very small, as shown in the cross-sectional view of FIG. 5. The small size of the gap is due to the effect of the semi-elliptical cross-sectional shape of the passage in the portion of the cross section of the passage having this cross-sectional shape, and the conductive fluid wetting the wettable metal film in the vicinity of the corners  184  and  185  between the semi-elliptical portion  187  and the straight portion  186  of the cross-sectional shape. Accordingly, the switch device  101  is subject to almost no pressure leakage or gas exchange past the conductive fluid, and any increase in the pressure in each of the cavities  151  and  152  moves or deforms the conductive fluid more efficiently. This allows either or both of the size and power dissipation of the heaters  161  and  162  to be reduced compared with a conventional switch device.  
         [0039]    An advantage of the second embodiment  101  over the first embodiment 1 is that there is no need to form a groove in both of the substrates. Additionally, whereas the efficiency of the first embodiment may be reduced if the alignment between the substrates  71  and  72  is not correct, the second embodiment provides some alignment tolerance by making the wettable metal film  188  located on the second substrate  172  wider than the width of the groove  175  formed on the first substrate  171 , as noted above.  
         [0040]    [0040]FIGS. 6 and 7 illustrate a third embodiment  201  of a switch device according to the invention. Elements of the third embodiment having a function similar to elements of the first embodiment 1 are indicated using the same reference numerals with  200  added and will not be described again. In the third embodiment, the wettable metal film  288  is located both on the major surface of the substrate  272  and in the groove  275  formed in the substrate  271 , and therefore substantially surrounds the passage  202 . The wettable metal film is located at or near half-way between the openings  243  and  244  of the channels  241  and  242 , respectively. The wettable metal film extends lengthways along the length of the passage  202  towards both openings. The wettable metal film extends widthways to surround the passage  202 .  
         [0041]    The third embodiment  201  of the switch device is made using a process similar to that described above for making the second embodiment  101 . However, after the groove  275  has been formed in the substrate  271 , metal films of chromium, nickel, and gold are deposited in order by masked vapor deposition into the groove  275  to form the wettable metal film portion  288   a . The wettable metal film portion  288   b  is also formed approximately in the middle of the major surface of the second substrate  272 . The wettable metal film portion  288   b  is also formed by vapor depositing and patterning thin films of chromium, nickel, and gold in that order.  
         [0042]    In the example shown, the wettable metal film  288  also serves as the center electrode and is indicated as such by the reference numeral  232  in FIG. 7. However, this is not critical to the invention, as noted above.  
         [0043]    The gap, if any, between the conductive fluid  212  and the passage  202  is very small, as shown in the cross-sectional view of FIG. 7. This is because the entire the region of the passage  202  that is surrounded by the wettable metal  288  is wetted by the conductive fluid  212 . Accordingly, the third embodiment of the switch device can be driven with lower power and more efficiently than the first and second embodiments.  
         [0044]    [0044]FIGS. 8 and 9 illustrate a fourth embodiment  301  of a switch device according to the invention. Elements of the fourth embodiment having a function similar to elements of the first embodiment 1 are indicated using the same reference numerals with  300  added and will not be described again. In the fourth embodiment, the passage  302  has a polygonal cross-sectional shape. In the example shown in FIG. 9, the passage  302  has a triangular cross-sectional shape as the most critical example of a polygonal shape.  
         [0045]    The preferred material of the first substrate  371  in the fourth embodiment is silicon. The silicon substrate is anisotropically etched using potassium hydroxide or another suitable etchant to form the groove  377  with a triangular cross section. As in the third embodiment, the wettable metal film  388  surrounds the passage  302  in a region centered on the mid-point between the outlets  343  and  344  of the channels  341  and  342 . The wettable metal film portion  388   a  is deposited in approximately half-way along the length of the groove  377  and the wettable metal film portion  388   b  is deposited approximately in the middle of the major surface of the second substrate  372 . The wettable metal film portions are formed by vapor depositing and patterning thin films of chromium, nickel, and gold in that order.  
         [0046]    Notwithstanding the triangular cross-sectional shape of the passage  302 , the gap, if any, between the conductive fluid  312  and the passage  302  is very small, as shown in the cross-sectional view of FIG. 9. This is because the entire region of the passage  302  that is surrounded by the wettable metal film is wetted by the conductive fluid  312 .  
         [0047]    Unlike the embodiments 1,  101  and  201  described above, the fourth embodiment  301  can be fabricated using anisotropic etching. Forming the groove  377  using anisotropic etching enables the dimensions of the groove to be controlled more accurately. This enables the groove to be made narrower and the entire switch device to be made smaller. Similar advantages are obtained when conventional dry etching is used instead of anisotropic wet etching. Furthermore, the wettable metal film  388  was made by masked vapor deposition in the example described. However, the wettable metal film can alternatively be made using a resist formation method involving plating, for example.  
         [0048]    The structure for minimizing the size of the gap between the conductive fluid and the inner walls of the passage was described above as being provided in the central region  81  of the passage  2  between the outlets  43  and  44  of the channels  41  and  42  connecting the passage to the cavities  51  and  52 . However, it is advantageous to provide this structure additionally in the outer regions  82  and  83  of the passage. The outer regions having such a structure latches the separated conductive fluid portions at specified locations when the conductive fluid is separated as shown in the Figures. This provides smoother and more reliable operation of the switch device.  
         [0049]    Accordingly, a method and apparatus have been provided for reducing the size, improving the efficiency, and reducing the power consumption of a miniature switch device in which a conductive fluid is used.  
         [0050]    Implementing the present invention yields a switch device that is higher in efficiency, smaller in size, and lower in cost than conventional switch devices. By minimizing or eliminating the gap between the conductive fluid and the passage, the increased internal pressure generated by the heater in one of the cavities will not leak into the other cavity, so the capability of the heater to separate the conductive fluid is maximized. Accordingly, the switch device can be produced with a smaller heater, or the heater can be driven at a lower power, among other advantages.  
         [0051]    One advantage of the present invention is that it provides a switch device that is more compact and uses less power. This is accomplished by reducing the leakage from the high-pressure side to the low-pressure side during operation of the switch device.  
         [0052]    According to the invention, a switch device that includes a small amount of a conductive fluid can be made smaller, its efficiency increased, and its power consumption reduced by defining the one or both of the cross-sectional shape and surface properties of the passage in which the conductive fluid is located as follows:  
         [0053]    (1) The cross-sectional shape of the passage is substantially elliptical;  
         [0054]    (2) The cross-sectional shape of the passage is substantially semi-elliptical and the cross-sectional shape includes a straight portion made from a wettable material that is wetted by the conductive fluid; and  
         [0055]    (3) The cross-sectional shape of the passage is polygonal and the inner wall of the passage is made of a wettable material that is wetted by the conductive fluid.  
         [0056]    The terms elliptical and semi-elliptical as used in this disclosure not only express pure mathematical shapes but also express shapes that approximate such mathematical shapes. Moreover, these shapes ignore fine irregularities that may exist in the surface of the inner wall of the passage. Additionally, there may be irregularities that are discontinuous in the lengthwise direction on the inner wall.  
         [0057]    When a drop of a conductive fluid, e.g., mercury, is put in a passage adjacent a non-conductive fluid, e.g., nitrogen gas, the conductive fluid will have a radius of curvature that is equal to or greater than the radius of curvature of the surface of the conductive fluid in contact with the non-conductive fluid. As a result, the gap will exist, but the gap will be no more than a few microns wide.  
         [0058]    The foregoing description of a preferred embodiment of the invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed, and modifications and variations are possible in light of the above teachings or may be acquired from practice of the invention. The embodiments were chosen and described in order to explain the principles of the invention and its practical application to enable one skilled in the art to utilize the invention in various embodiments and with various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined the claims appended hereto, and their equivalents.