Abstract:
A micro-electro-mechanical high frequency switch and method for manufacturing the high frequency switch, comprising the steps of: providing a substrate; forming a metal transmission line and a driving electrode on the substrate; forming a dielectric layer on said metal transmission line and said driving electrode; forming a micro-electro-mechanical switch; forming driving electrodes on and beneath the micro-electro-mechanical switch; such that the driving voltage of high frequency switch is reduced, the insertion loss is lowered, the isolation is high, and the functions of high frequency switch is improved.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    The present invention relates to a micro-electro-mechanical high frequency switch and method for manufacturing the same. Especially to those can be integrated with coplanar waveguide transmission line, integrated with micro-electro-mechanical package, increasing the actuating voltage, improving the insertion loss and isolation.  
         DESCRIPTION OF THE PRIOR ART  
         [0002]    As the development of information, high frequency techniques or components are applied to many purposes. Weather satellites, RF (high frequency) communication and weaponry systems, and wireless communication for civil use, etc., each needs various RF components. The RF switch is a basic and very important component of the high frequency components. The scope of applying this includes the Tx/Rx switch of the receiving end of an antenna, the switching of the multi-band or diversity antenna, the switching filter module or radio frequency module, and the multiplexing selectors of power/gain/loss/delay etc. There are various selections for the standards of the RF switch, according to the applied frequency band, the quantity of power, and the functions. In generally, except for being integratablized and being integrated in the MMIC (microwave monolithic IC) for portable products, and in principle of the loss tolerance of the whole multiplexing selector that does not over 2 dB, the main switching components should be low insertion loss, high isolation in the range of the using frequency band. The more prosperous the communication business is, the more complicated the corresponding functions of transceiver modules are, and the more important the switching components are. Therefore, the standards of the switching component are required more sternly. Presently, there are approximately two kinds of main products of the RF switch on the market: one is gallium arsenide metal-semiconductor field effect transistor (GaAs MESFET), another one is GaAs PIN diode. To operate the GaAs MESFET, by operating the gate voltage (negative voltage operating) to control the channel of both ends of the source and drain, thus it can be integrated with the integrated microwave IC (MMIC) to reduce the size and cost of the chip. However, since the breakdown voltage between the source and drain is being restricted, it is not suitable for operating with higher power. In order to achieve a good operation with high power, the way is connecting with several GaAs MESFET in series (M. Shifrin at.al, 1989), or, applying a capacitor between the drain and the source (Hitachi Ltd., 1999). But, the size of the chip is comparatively increased and, as the impedance mismatching and the inserting loss increased when several GaAs MESFET are series connected, the RF operation is restricted. The current-voltage properties of the PIN diode enable it to be a good RF switching component, and the properties of high breakdown voltage are satisfied with various power requirement. However, as applying the power source bias of the PIN diode, it is required the RF choke and capacitors that are selected appropriately. Therefore, since the module becomes more complicated so that it is not easy to make the PIN diode integrated with other MMIC, it can not be operated in linearity, and the cost of the component unit is pushed higher. Furthermore, there is also a tendency that the products are being designed power saving in the future. For example, a conventional phase-array antenna for radar communicating containing thousands of antenna elements, of which each antenna element requires several switches. Nevertheless, these switches bring on high cost, high power consuming, and their quality are not good enough. It is because there is insertion loss generated by the PIN diode while applying higher power and higher frequency. But comparing with the GaAs MESFET, the PIN diode has a low integrated extent and is high power consuming, and the FET has higher insertion loss. Therefore, as described above, with the applications requiring switching array and the applications for RF range such as X-band (centered at 10 GHz) and Ka-band (centered at 35 GHz), the micro-electro-mechanical switch can provide good properties of RF switches, and has the advantages of low power consuming, and high integrated process simultaneously.  
           [0003]    Presently, the disadvantages of the solid-state electronic type RF switch are that the insertion loss during “on” state is too large, and bad isolation occurred during “off” state. The micro-electro-mechanical switch can solve the above-mentioned disadvantages. The micro-electro-mechanical switch is not operated at the RF mode directly, and it enables itself to be adjusted or switched by its good micro-electro-mechanical actuators. The advantage of the RF components using micro-electro-mechanical techniques is, it is well isolated between the electro-mechanical and RF, that is, the signals between the RF networks would not be a short circuit or coupled to the actuating circuit. Another advantage is low power consuming, thus a linear switch is achieved because of low interference, and it can be integrated. It not only shows its excellent insertion loss and isolation and its excellent linear operation by using the micro-electro-mechanical techniques for manufacturing the switches. Moreover, the switch components with various frequencies, power and structures, which can be designed by adjustment of designing and the manufacturing process, can be collocated with modules with various standards. Wherein, the power consumed is so low while operating the static electrical enabled type switch at the on/off state, so it is very suitable for the relative components of portable, mobile products.  
           [0004]    Presently, the electrostatic actuated type RF micro-electro-mechanical switch includes series-connecting type switch and shunt-connecting type switch. In the scheme of micro-strip transmission line can the series-connecting type switch be used; while in the schemer of co-planar waveguide transmission line, both the series-connecting type switch and the shunt-connecting type switch can be applied. Presently, although the insertion loss of the series-connecting type at LF (low frequency) range is quite low, the insertion loss at RF range is quite high, and the isolation is quite well whenever at LF or RF ranges. The isolation of the shunt-connecting type switch at LF range is worse, the isolation at RF range is better, and the insertion loss is quite low whenever at LF or RF range. Therefore, in the conventional technology, if there is no improvement about the functions of the switch while applying it, the series-connecting type switch is more suitable for LF range operation, and the shunt-connecting type switch is more suitable for RF operation.  
           [0005]    [0005]FIGS. 1 a  through  1   c  are perspective views of the conventional electrostatic actuated type RF micro-electro-mechanical switch. As shown in FIG. 1 a,  the micro-electro-mechanical switch can be formed on the substrate  12 , wherein, it consists of a supporting portion  13 , a cantilever beam  14  connecting with supporting portion  13  used as a switching actuator, a driving electrode  15  certainly located under the cantilever beam  14  for providing the cantilever beam  14  with electrostatic actuating. According to the formula, the formula of the driving voltage of the switch is:  
               V   p     =         8        KG   3         27      ɛ                 A                 (   1   )                               
 
           [0006]    wherein K is the entirety spring constant of the cantilever beam  14 , G is the distance between the cantilever beam  14  and the driving electrode  15 , ε is the dielectric constant of the filler between the cantilever beam  14  and the driving electrode  15 , A is the area of the driving electrode  15 . It is known from the above formula that, in order to reduced the driving voltage of the switch, by lowering the entirety spring constant K, reducing the distance G between the cantilever beam  14  and the driving electrode  15 , increasing the area A of the driving electrode  15 , or increasing the dielectric constant ε of the filler between the cantilever beam  14  and the driving electrode  15 . However, in the conventional electrostatic actuated type RF micro-electro-mechanical switch, the distance G between the cantilever beam  14  and the driving electrode  15  should not be too short to avoid lowering the isolation of the RF micro-electro-mechanical switch at “off” state, and generally, it should be 1.5 μm to 2 μm. In this structure, the filler between the cantilever beam  14  and the driving electrode  15  is air or vacuum (dielectric constant ε˜1). Presently, changeable parameters are the area A of the driving electrode  15  and the entirety spring constant K of the cantilever beam  14 . According to the formula (2), the bending spring constant K of the cantilever beam  14  is:  
               K   B     =           3      EI       L   3          •                 I     =       BH   3     12               (   2   )                               
 
           [0007]    wherein, E is the Young&#39;s modulus of the cantilever beam  14 , I is moment of inertia of the cantilever beam  14 , B, H, and L is the width, depth, and length of the cantilever beam  14 . A lower elasticity constant K can be achieve by increasing the equivalent length, decreasing the depth and the width of the cantilever beam  14 , or using a material with lower Young&#39;s modulus E for the structure body of the cantilever beam  14 . But, the occupied area of the RF switch formed by the cantilever beam  14  with lower spring constant K is being larger, and the structure is quite unstable. Moreover, it may not recover during the restoring state, and thus lowering the switching speed. There is another disadvantage of the conventional electrostatic actuated type RF micro-electro-mechanical switch: the electrode  15  (force applied point) is located between the supporting point  13  and the switch contact point  16 , and it is a laborious but time saving lever. But, if the electrode  15  (force applied point) is moved out of the switch contact point  16 , it is a time wasting but labor saving lever. In the structure of the latter, while the cantilever beam  14  is contacting with the driving electrode  15 , the contact point  16  cannot contact with the contact point  17  closely, thus the properties of the RF switch is lowered.  
           [0008]    There is another disadvantages of the conventional electrostatic actuated type RF micro-electro-mechanical switch: while it is applied to the switch of the coplanar waveguide transmission line, the layout of the cantilever beam  14  and the driving electrode  15  are rather restricted. FIG. 1 b  shows the layout of the coplanar waveguide transmission RF switch. Referring to FIG. 1 b,  wherein  17   a,    17   c  are the ground plane of the coplanar waveguide transmission line, and  17   b  is the signal line of the coplanar waveguide transmission line. Since the ground plane of the coplanar waveguide transmission line  17   a  and  17   c  exist, they actuate the electrode  15  to be approached to the supporting point  13 , and if the force applied arm is fixed, the supporting point  13  must be moved outward, as a result, the whole area of the chip is increased. On the other hand, as shown in FIG. 1 c,  while the switch is switched down, the switch contacting points  16   a,    16   b,  and  16   c  are not at the same level, thus the switch contacting points  16   a,    16   b,  and  16   c  can not contact with the switch contacting points  17   a,    17   b  and  17   c  closely, and thus the isolation or the insertion loss is being worse.  
           [0009]    [0009]FIG. 2 shows another type of the conventional electrostatic actuated type RF micro-electro-mechanical switch. That is, shunt-connecting RF micro-electro-mechanical switch. As shown in FIG. 2, the micro-electro-mechanical switch can be formed on the substrate  22 , wherein it includes two side-supporting portions  23  applied as a switching actuator located certainly below the thin film  24 . There is a driving electrode  25  interposed under the transmission line for providing electrostatic force to the thin film  24 .  
           [0010]    As shown in FIG. 2, the advantage of the shunt-connecting RF micro-electro-mechanical switch is that the structure is more stable, thus it is suitable for the layout of coplanar waveguide transmission line, and the occupied volume is small. But the isolation of the shunt-connecting switch at LF range is quite bad, and the driving electrode  25  is formed before the transmission line  27   b  deposited, thus a non-continuous step is formed by the transmission line  27   b  formed afterward and the properties are affected. On the other hand, the driving voltage of this type of RF micro-electro-mechanical switch is quite high, for example, the shunt-connecting RF micro-electro-mechanical switch developed by the Raytheon Company (the thesis “Performance of low-loss RF MEMS capacitive switches” disclose by Goldsmith on August 1998 at IEEE Microwave and Guided Wave Letters, which the driving voltage is between 30˜50 volts.  
           [0011]    It is known from above, the RF micro-electro-mechanical switch can be classified by series-connecting switch and shunt-connecting switch. The series-connecting micro-electro-mechanical switch is suitable for LF fields, and the shunt-connecting micro-electro-mechanical switch is suitable for RF fields. However, the switching speed of the series-connecting micro-electro-mechanical switch is too low, and the driving voltage of the shunt-connecting micro-electro-mechanical switch is too high. There is required for considering choosing between the broadband, high speed, driving voltage and the circuit layout.  
         SUMMARY OF THE INVENTION  
         [0012]    The object of the present invention is to solve the above-mentioned disadvantages in the conventional techniques.  
           [0013]    Therefore, it is an object of the present invention to provide a micro-electro-mechanical RF switch combining with series-connecting type and shunt-connecting type and the method for manufacturing the same, which enables itself to be integrated with the layout of coplanar waveguide transmission lines easily, and enables the layout of the driving electrode to be optimized, thus, it enables the driving voltage of the RF switch to be lowered, and provides a micro-electro-mechanical RF switch with low insertion loss and high isolation, and, the properties of the RF switch is improved.  
           [0014]    Another object of the present invention is to provide a improved shunt-connecting micro-electro-mechanical RF switch and method for manufacturing the same, which enables the conventional shunt-connecting micro-electro-mechanical RF switch, of which the driving voltage to be lowered, and provides a micro-electro-mechanical RF switch with low insertion loss and high isolation, and, the properties of the RF switch is improved.  
           [0015]    Another object of the present invention is to provide a micro-electro-mechanical RF switch integrated with micro-electro-mechanical packaging and method for manufacturing the same without complicating the process, it enables the driving voltage to be lowered, and provides a micro-electro-mechanical RF switch with low insertion loss and high isolation, and, the properties of the RF switch is improved.  
           [0016]    Another object of the present invention is to provide another micro-electro-mechanical RF switch integrated with micro-electro-mechanical packaging and method for manufacturing the same without complicating the process, it enables the driving voltage to be lowered, and provides a micro-electro-mechanical RF switch with low insertion loss and high isolation, and, the properties of the RF switch is improved.  
           [0017]    In order to achieve the above-mentioned objects, according to the method of manufacturing the circuit board of the present invention, it includes the steps as below: forming a micro-electro-mechanical switch on the substrate, it comprises supporting portions; a cantilever beam, of which one end is connected with the supporting portion, used as a switch actuator; a driving electrode beneath the cantilever beam for providing static electricity to actuate the cantilever beam.  
           [0018]    In order to achieve the above-mentioned objects, according to the method of manufacturing the circuit board of the present invention, it further comprises the steps as below: the micro-electro-mechanical switch can be formed on the substrate, wherein including the supporting portion; a cantilever beam, of which one end is connected with the supporting portion, used as a switch actuator; a driving electrode beneath the cantilever beam for providing static electricity to enable the cantilever beam; in addition, an upper driving electrode located certainly on the switch for providing static electricity to enable the cantilever beam and lower the driving voltage thereof.  
           [0019]    In order to achieve the above-mentioned objects, according to the method of manufacturing the circuit board of the present invention, it further comprises the steps as below: the micro-electro-mechanical switch can be formed on the substrate, wherein including the supporting portion; a cantilever beam, of which one end is connected with the supporting portion, used as a switch actuator; a driving electrode beneath the cantilever beam for providing static electricity to enable the cantilever beam; in addition, an upper driving electrode located certainly on the switch for providing static electricity to enable the cantilever beam and lower the driving voltage thereof.  
           [0020]    The above objectives and advantages will become more apparent with explanation of the accompanying drawings. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0021]    [0021]FIGS. 1 a  through  1   c  are perspective views of the conventional series-connecting type and shunt-connecting type electrostatic actuated type RF micro-electro-mechanical switch.  
         [0022]    [0022]FIG. 2 is a perspective view of the conventional shunt-connecting type electrostatic actuated type RF micro-electro-mechanical switch.  
         [0023]    [0023]FIGS. 3 a  and  3   b  are perspective views of the electrostatic actuated type RF micro-electro-mechanical switch according to the first embodiment of the present invention.  
         [0024]    [0024]FIG. 4 is a perspective view of the electrostatic actuated type RF micro-electro-mechanical switch according to the second embodiment of the present invention.  
         [0025]    [0025]FIG. 5 is a perspective view of the electrostatic actuated type RF micro-electro-mechanical switch according to the third embodiment of the present invention.  
         [0026]    [0026]FIGS. 6 a  through  6   c  are views of the electrostatic actuated type RF micro-electro-mechanical switch, showing the relationship between the electrostatic repelling and attracting forces and the position of the switch. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0027]    The conventional series-connecting type and shunt-connecting electrostatic actuated type RF (high frequency) micro-electro-mechanical switch in FIGS. 1 a,    1   b,    1   c,  and  2  has described as above, it is not repeated here.  
         [0028]    [0028]FIGS. 3 a  and  3   b  are perspective views of the electrostatic actuated type RF micro-electro-mechanical switch according to the first embodiment of the present invention. The whole structure has the advantages combining with the advantages of the series-connection and shunt-connection, thus enables a more stable structure can be maintained, and achieves low driving voltage operation. As shown in FIG.  3   a,  the micro-electro-mechanical switch can be formed on a substrate  32 , wherein it comprises supporting portions  33 ; it also comprises a cantilever beam  34  used as a switching actuator, beneath the cantilever beam  34 , there is a driving electrode  35  for providing static electricity to actuate the cantilever beam  34 . In this structure, since the driving electrodes  35  is located at the two sides of the coplanar waveguide transmission line, in order to lower the driving voltage of the switch, the entire spring constant K of the cantilever beam  34  is not lowered and the distance G between the cantilever beam  34  and the driving electrodes  35  is not shorten, but the area A of the driving electrodes  35  is increased to lower the voltage for driving the switch.  
         [0029]    In addition, as shown in FIG. 3 b,  another structure of this embodiment can be stabilized by applying upper and lower bi-electrode on the driving electrode  35 . According to FIG. 3 b,  the micro-electro-mechanical switch can be formed on a substrate  32 , wherein it comprises supporting portions  33 , two fixed ends respectively across both ends of the coplanar waveguide transmission line and connecting with supporting portions  33 . It also comprises a cantilever beam  34  used as a switching actuator; there is a driving electrode  35   a  beneath the cantilever beam  34  for providing static electricity to enable the cantilever beam  34 ; there is a driving electrode  35   b  on the cantilever beam  34  for providing recovering static electricity to actuate the switch to be maintained stable.  
         [0030]    [0030]FIG. 4 is a perspective view of the electrostatic actuated type RF micro-electro-mechanical switch according to the second embodiment of the present invention. As shown in FIG. 4, it shows the embodiment, of which the structure is being proceeded with the micro-electro-mechanical packaging on the driving electrode  45 , an upper electrode  45   b  is provided as the upper electrode  45   a  in the first embodiment, thus the processing complication is reduced. As shown in FIG. 4, the micro-electro-mechanical switch can be formed on the substrate  42   a,  wherein it comprises supporting portions  43 , two fixed ends respectively across both ends of the coplanar waveguide transmission line and connecting with the supporting portions  43 . It also comprises a cantilever beam  44  used as a switching actuator; there is a driving electrode  45   a  beneath the cantilever beam  44  for providing static electricity to actuate the cantilever beam  44 . It also comprises a deep trough  42 ′ formed on another substrate  42   b;  an upper driving electrode  45   b  is formed by forming a metal layer on the deep trough  42 ′. Then, by the method of flip-chip or wafer level chip scale packaging, the substrate  42   b  including the deep trough  42 ′ and the upper driving electrode  45   b  is connected with the substrate  42   a  having the lower electrode  45   a,  the supporting portions  43 , and the cantilever beam  44 . The advantage of this embodiment is, not as the second embodiment that it is required for two sacrificial layers, that the processing complication is reduced greatly.  
         [0031]    [0031]FIG. 5 is a perspective view of the electrostatic actuated type RF micro-electro-mechanical switch according to the third embodiment of the present invention. The actuating principle of the micro-electro-mechanical RF switch is by combining mainly with the electrostatic repelling and attracting force.  
         [0032]    As shown in FIG. 5, it shows the embodiment, of which the structure is being proceeded with upper and lower bi-electrodes, wherein the micro-electro-mechanical packaging provides an upper electrode  55   b  used as the upper electrode  55   b  in the first embodiment for lowering the processing complication; the upper electrode have a structure of repelling electrode including the first metal layer  55   b - 1 , the first insulating layer  55   b - 2 , and the second metal layer  55   b - 3 .  
         [0033]    As shown in FIG. 5, the micro-electro-mechanical switch can be formed on the substrate  52   a,  wherein it comprises supporting portions  53 , two fixed ends respectively across both ends of the coplanar waveguide transmission line and connecting with the supporting portions  53 . It also comprises a cantilever beam  54  used as a switching actuator; there is a driving electrode  55   a  beneath the cantilever beam  44  for providing static electricity to actuate the cantilever beam  54 . It also comprises a deep trough  52 ′ formed on another substrate  52   b;  the first metal layer  55   b - 1 , the first insulating layer  55   b - 2 , and the second metal layer  55   b - 3  is formed by forming a metal layer on the deep trough  52 ′, thus the upper driving repelling electrode  55   b  is formed. Then, by the method of flip-chip or wafer level chip scale packaging, Then the substrate  52   b  including the deep trough  52 ′ and the upper driving electrode  55   b  is connected with the substrate  52   a  having the lower electrode  55   a,  the supporting portions  53 , and the cantilever beam  54 .  
         [0034]    The advantages of the above embodiment is: first, it is not as the second embodiment that two sacrificial layers are required, that the processing complication is reduced greatly; another, according to the analysis from FIG. 6 a,    6   b,    6   c  showing the action forces, by means of the upper and lower electrodes for combining the electrostatic repelling and attracting force as an actuating principle of the micro-electro-mechanical RF switch, the electrostatic force for driving the switch can be reduced greatly.  
         [0035]    As shown in FIG. 6 a,  the curve labeled as R indicates the relationship between the repelling force and the distance of the upper electrode; the curve labeled as A indicates the relationship between the repelling force and the distance of the lower electrode; the curve labeled as S indicates relationship between the combined force of electrostatic repelling and attracting force and the distance of the upper and lower electrodes.  
         [0036]    According to the curve of label A, a conventional electrostatic attracting force is provided for action force, since it is the farthest at the starting position A 1 , it is known from the formula F=Q 1 Q 2 /r 2  of electrostatic force that the action force between thereof is very little, so it requires greater driving voltage for driving the cantilever beam  54  to the switch contacting point  57 . According to the curve of label R, the acting force provided by electrostatic repelling force, since it is the nearest at the starting position R 1 , it is known from the formula F=Q 1 Q 2 /r 2  of electrostatic force that the action force between thereof is very great, but as the cantilever beam  54  is closed to the switch contacting point  57 , the electrostatic action force is gradually getting less, and so it is also required a great driving voltage to make the cantilever beam  54  to be contacted to the switch contacting point  57  completely.  
         [0037]    If the electrostatic repelling force is combined by the upper and lower electrodes, shown as the curve of label S, at the beginning when the cantilever beam  54  is closed to the switch contacting point  57 , the greater repelling force generated by the upper electrode can be used as the main force, and as the cantilever beam  54  is moved to the middle point between the upper electrode and the switch contacting point  57 , and the attracting force of the lower electrode is gradually getting greater, thus the electrostatic force of the whole curve S is much greater than a single electrostatic attracting or repelling force, therefore, the electrostatic force for driving the switch can be reduced greatly.  
         [0038]    As shown in FIG. 6 b,  the curve labeled as R indicates the relationship between the repelling force and the distance of the upper electrode; the curve labeled as A indicates the relationship between the repelling force and the distance of the lower electrode; the curve labeled as S indicates relationship between the combined force of electrostatic repelling and attracting force and the distance of the upper and lower electrodes.  
         [0039]    As the repelling force provided by the upper electrode is greater, the repelling force served as the main force is lasted to the position the cantilever beam  54  moved to that under the middle point between the upper electrode and the switch contacting point  57 , and the attracting force of the lower electrode becomes the main acting force gradually; similarly, the electrostatic force of the whole curve S is much greater than a single electrostatic attracting force or electrostatic repelling force, therefore, the electrostatic force for driving the switch can be reduced greatly:  
         [0040]    As shown in FIG. 6 c,  the curve labeled as R indicates the relationship between the repelling force and the distance of the upper electrode; the curve labeled as A indicates the relationship between the repelling force and the distance of the lower electrode; the curve labeled as S indicates relationship between the combined electrostatic repelling and attracting force and the distance of the upper and lower electrodes.  
         [0041]    As the repelling force provided by the upper electrode is much greater than the attracting force, it depended on the repelling force that the whole cantilever beam  54  is moved to the switch contacting point  57 , and compared with the conventional electrostatic attracting force, the electrostatic force for driving the switch can be reduced greatly.  
         [0042]    It is known from the above description that although the present invention has been described using specified embodiment, the examples are meant to be illustrative and not restrictive. It is clear that many other variations would be possible without departing from the basic approach, demonstrated in the present invention.