Patent Publication Number: US-7719392-B2

Title: Ferroelectric varactors suitable for capacitive shunt switching

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application is a continuation-in-part of PCT Application US 2004/034266, filed Oct. 15, 2004, which claims the benefit of U.S. Provisional Application Ser. No. 60/512,631, filed Oct. 20, 2003, and is related to U.S. patent application Ser. No. 10/575,754, filed Apr. 13, 2006. 

   BACKGROUND OF THE INVENTION 
   The present invention relates to ferroelectric varactors, and in particular, to a ferroelectric varactor that is suitable for a capacitive shunt switch. 
   Electrically tunable microwave filters have many applications in microwave systems. These applications include local multipoint distribution service (LMDS), personal communication systems (PCS), frequency hopping radio, satellite communications, and radar systems. There are three main kinds of microwave tunable filters, mechanically, magnetically, and electrically tunable filters. Mechanically tunable filters are usually tuned manually or by using a motor. They suffer from slow tuning speed and large size. A typical magnetically tunable filter is the YIG (Yttrium-Iron-Garnet) filter, which is perhaps the most popular tunable microwave filter, because of its multioctave tuning range, and high selectivity. However, YIG filters have low tuning speed, complex structure, and complex control circuits, and are expensive. 
   One electronically tunable filter is the diode varactor-tuned filter, which has a high tuning speed, a simple structure, a simple control circuit, and low cost. Since the diode varactor is basically a semiconductor diode, diode varactor-tuned filters can be used in monolithic microwave integrated circuits (MMIC) or microwave integrated circuits. The performance of varactors is defined by the capacitance ratio, C max /C min , frequency range, and figure of merit, or Q factor at the specified frequency range. The Q factors for semiconductor varactors for frequencies up to 2 GHz are usually very good. However, at frequencies above 2 GHz, the Q factors of these varactors degrade rapidly. 
   Since the Q factor of semiconductor diode varactors is low at high frequencies (for example, &lt;20 at 20 GHz), the insertion loss of diode varactor-tuned filters is very high, especially at high frequencies (&gt;5 GHz). Another problem associated with diode varactor-tuned filters is their low power handling capability. Since diode varactors are nonlinear devices, larger signals generate harmonics and subharmonics. 
   Varactors that utilize a thin film ferroelectric ceramic as a voltage tunable element in combination with a superconducting element have been described. For example, U.S. Pat. No. 5,640,042 discloses a thin film ferroelectric varactor having a carrier substrate layer, a high temperature superconducting layer deposited on the substrate, a thin film dielectric deposited on the metallic layer, and a plurality of metallic conductive means disposed on the thin film dielectric, which are placed in electrical contact with RF transmission lines in tuning devices. Another tunable capacitor using a ferroelectric element in combination with a superconducting element is disclosed in U.S. Pat. No. 5,721,194. 
   With the advent of microelectromechanical system (MEMS) technology, attention has been focused on the development of MEMS devices for radio frequency (RF) applications. MEMS switches are one of the most prominent micromachined products that have attracted numerous research efforts in numerous years and have many potential applications such as impedance matching networks, filters, signal routing in RF system front-end and other high frequency reconfigurable circuits. MEMS switches provide many advantages over conventional electromechanical or solid-state counterparts in terms of low insertion loss, high isolation, low power consumption, high breakdown voltage, high linearity and high integration capability. The majority of today&#39;s MEMS switches employ electrostatic actuation and require a high actuation voltage, a major drawback of this type of switch. Recently, high relative dielectric constant Barium Strontium Titanium Oxide (BST) thin-films have been used in RF MEMS switches as a dielectric layer for reducing the actuation voltage requirements as well as improving isolation. Isolation can be improved more than 10 dB using ferroelectric thin-films of BST compared to dielectric materials such as Si 3 N 4 . 
   However, RF MEMS switches have several limitations such as, for example, relatively low speed, low power handling capability, required high actuation voltage, low reliability, low switching lifetime and high packaging cost. Although improvements are being made in these areas, challenges remain for commercial applications of RF MEMS switches. A ferroelectric varactor based capacitive shunt switch can over come most of the limitations of existing RF MEMS switches. 
   BRIEF SUMMARY OF THE INVENTION 
   It is against this background that the present invention is based on a coplanar waveguide (CPW) transmission line shunted by a ferroelectric varactor. The novelty in the implementation comes from the elimination any moving parts for switching and from the elimination of via connections. High resistivity silicon with a SiO 2  layer and a metallic layer deposited on top is used as the substrate. The substrate can be any low-loss microwave substrate such as, for example, Saphire, magnesium oxide, lanthanum aluminate, etc. A ferroelectric thin-film layer is deposited on a patterned bottom metal layer (metal1 layer) for the implementation of the varactor. A top metal electrode (metal2 layer) is deposited on the ferroelectric thin-film layer, and patterned to form a CPW transmission line, such that an overlapping area of the center conductor of the CPW in metal1 and the shorting line in metal2 layers defines the varactor area. By using the large area ground planes in the metal2 layer as well as the metal1 layer, a series connection of the ferroelectric varactor with the large capacitor defined by the ground planes on the top and bottom metal layers was created. The large capacitor acts as a short to ground, eliminating the need for any vias. The concept of switching ON and OFF state is based on the dielectric tunability of the BST thin-films. 
   Accordingly, it is a feature of the embodiments of the present invention to create a varactor shunt switch with improved isolation and insertion loss with reduced bias voltage. 
   It is another feature of the embodiments of the present invention to create a varactor shunt switch with lower bias voltage requirement, high switching speed, ease of fabrication and high switching lifetime. 
   It is yet another feature of the embodiments of the present invention to create a bandpass filter using varactor shunt switches. 
   It is still another feature of the embodiments of the present invention to create a tunable notch filter using varactor shunt switches. 
   Other features of the embodiments of the present invention will be apparent in light of the description of the invention embodied herein. 

   
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
     The following detailed description of specific embodiments of the present invention can be best understood when read in conjunction with the following drawings, where like structure is indicated with like reference numerals and in which: 
       FIG. 1  illustrates a cross-sectional view of the multiple layers of the capacitive shunt switch according to one embodiment of the present invention. 
       FIG. 2   a  is a pattern of the bottom metal electrode according to one embodiment of the present invention. 
       FIG. 2   b  is a pattern of the top metal electrode according to one embodiment of the present invention. 
       FIG. 2   c  is a top-view of a varactor according to one embodiment of the present invention. 
       FIG. 2   d  is a cross-sectional view of the varactor area according to one embodiment of the present invention. 
       FIG. 3  illustrates a top view of the capacitive shunt switch according to one embodiment of the present invention. 
       FIG. 4  represents the electric circuit model of the varactor shunt switch of  FIG. 3  according to one embodiment of the present invention. 
       FIG. 5  illustrates simulated isolation using different dielectric constants with the same varactor area according to one embodiment of the present invention. 
       FIG. 6  illustrates simulated insertion loss using different varactor areas with the same dielectric constant according to one embodiment of the present invention. 
       FIG. 7  illustrates simulated isolation and insertion loss of the varactor shunt switch for an optimized device according to one embodiment of the present invention. 
       FIG. 8  illustrates experimental measurements on the varactor shunt switch according to one embodiment of the present invention. 
       FIG. 9  illustrates experimental results versus the simulation results for the varactor shunt switch according to one embodiment of the present invention. 
       FIG. 10  illustrates a bandpass filter implementation using ferroelectric varactor shunt switches according to one embodiment of the present invention. 
       FIG. 11  illustrates simulated results of a bandpass filter implementation using ferroelectric varactor shunt switches according to one embodiment of the present invention. 
       FIG. 12  illustrates a ferroelectric varactor shunt switch bandpass filter MMIC response with tuning according to one embodiment of the present invention. 
       FIG. 13  illustrates a tunable notch filter implementation using ferroelectric varactor shunt switches according to one embodiment of the present invention. 
       FIG. 14  illustrates simulated results of a tunable notch filter implementation using ferroelectric varactor shunt switches according to one embodiment of the present invention. 
       FIG. 15  illustrates simulated results of a tunable notch filter implementation using ferroelectric varactor shunt switches with a set minimum depth according to one embodiment of the present invention. 
   

   DETAILED DESCRIPTION 
   In the following detailed description of the preferred embodiments, reference is made to the accompanying drawings that form a part hereof, and in which are shown by way of illustration, and not by way of limitation, specific preferred embodiments in which the invention may be practiced. It is to be understood that other embodiments may be utilized and that logical, mechanical and electrical changes may be made without departing from the spirit and scope of the present invention. 
   The concept of implementing shunt capacitance will be useful for a large number of MMICs such as, for example, tunable one-dimensional and two-dimensional electromagnetic bandgap (EBG) structures, tunable band-reject and bandpass filters, interference suppression systems, microwave switching applications, distributed phase shifters for microwave and millimeterwave frequencies. Furthermore, the present invention is also suitable for two-dimensional and three-dimensional EBG arrays. In addition, these switches could be used in analog and digital applications, such as, for example, interlayer coupling in multi-layered packages, isolation of specific subsystems with a larger system. This type of switch could also serve as a sensory element, since ferroelectric thin-films manifest piezo-electricity (useful for pressure sensors, accelerometers, etc.), pyroelectricity (for infra-red detectors), and electro-optic activity (voltage induced refractive index change for color filters, displays, optical switching, etc.). 
     FIG. 1  illustrates a cross-sectional view of the multiple layers of the varactor shunt switch. The varactor shunt switch is designed on CPW transmission line  10  with a multilayer substrate. A tunable ferroelectrical thin-film of BST  20  with a high dielectric constant (∈ r ≧100) can be used as a dielectrical layer and may have a thickness of about 100-400 nm on top of a platinum/gold layer  25  which may have a thickness of about 500 nm. A titanium adhesion layer  30  of about 20 nm is deposited between the platinum/gold layer  25  and the silicon oxide/high resistivity silicon substrate layer  35  and  40 . The silicon has resistivity of &gt;1 kΩ-cm and is typically about 6 kΩ-cm. The thickness of the silicon oxide layer  35  and the high resistivity silicon substrate  40  can be about 200 nm and about 20 mils respectively. 
   As a first step in the process, a patterned bottom electrode (metal1 layer) can be processed on the Si/SiO 2  substrate by electron-beam (e-beam) deposition (or sputtering) and lift-off photolithography technique.  FIG. 2   a  illustrates the pattern of the bottom metallic layer  25 . After the lift-off photolithography process for the platinum/gold layer  25 , the layer  25  may be covered by a 100-400 nm ferroelectric thin film  20  such as, for example, barium strontium titanate (BST), strontium titanate (STO) or any other non-linear tunable dielectric, using a pulsed laser ablation (PLD) process or by RF sputtering. Note that the ferroelectric thin-film can be used in the paraelectric state or in the ferroelectric state to optimize the switch performance based on the type of application. 
     FIG. 2   b  illustrates the pattern of the top metal electrode  15  that can be deposited on top of the ferroelectric thin film  20 . This top metal electrode  15  may be comprised of gold and includes the central signal strip  100  and the ground conductors  110  of the CPW. The top metal electrode  15  may be prepared by e-beam deposition (or sputtering) and lift off photolithography process. The ground conductors  50  in the bottom metallic layer  25  and top metal electrode  15  are effectively shorted, due to the large capacitance between these two layers, eliminating need for the via holes. 
   The top view of the finalized CPW is shown in  FIG. 2   c . In  FIG. 2   c , the varactor area  200  is defined by the overlap area between the top metal electrode  15  and the bottom metallic layer  25  as indicated by the dashed lines. The two ground conductors  50  of the bottom metallic layer  20  have exactly the same dimensions as the CPW ground lines  110  of the top metal electrode  15 . A shunt conductor  55  connects the two ground lines  50  in the metal1 layer  25 . The varactor area  200  is formed by the overlap of the shunt conductor  55  of the bottom metallic layer  25  and the central signal strip  100  of the top metal electrode  15  as illustrated by the dotted lines in  FIG. 3 . 
   When the capacitance of the varactor is very high (at 0V bias), the signal is coupled through the varactor and passes through the shunt conductor to the ground. The varactor capacitance is in series with the larger capacitance introduced by the overlapping of the ground conductors  50 ,  110  in the top metal electrode (metal2)  15  and the bottom metallic layer (metal1)  25 . The output is isolated from the input because of the signal being shunted to ground at 0V, resulting in the OFF state of the device. When a DC voltage is applied to the center conductor  100  of the CPW in the metal2 layer  15 , the dielectric constant of the ferroelectric thin-film  20  is reduced and results in a lower varactor capacitance. When the varactor capacitance becomes small, the majority of signal from the input will be passed on to the output, because of reduced coupling by the varactor, resulting in the ON state of the device. Large dielectric tunability results in high isolation and low insertion loss of the device. 
   In the cross section of the varactor, see  FIG. 2   d , the widths of the two overlapping ground lines  100  of the top metal electrode  15  and the ground lines  50  of the bottom metallic layer  25  are chosen such that a required value of capacitance is obtained based on the known relative permittivity (∈ r ) of the ferroelectric thin-film  20 . Tuning is obtained if a DC electric field is applied between the ground conductors  100  and the central signal strip  110  of the CPW (using CPW probes). The DC field changes the relative permittivity of ferroelectric thin-film  20 , and hence the capacitance of the varactor. 
   In one embodiment, the width of the center signal strip  110  of the CPW and the spacing between the center signal strip  110  and ground conductors  100  were chosen so that the characteristic impedance is close to about 50Ω and the line losses are minimized. The CPW line has the dimensions of Ground-Signal-Ground being 150 μm/50 μm/150 μm for DC-20 GHz on the high resistivity silicon substrate  35 . The spacing (S) between the center signal strip  110  and ground conductors  100  is taken as 50 μm and the geometric ration (k=W/(W+2S)) is equal to 0.333 of the CPW line. The device area is approximately 450 μm by 500 μm. The varactor area  200 , which is the overlap of the top metal electrode  15  and the bottom metallic layer  25  is approximately 75 μm 2 . 
   The simple circuit implementation as the present invention is compatible with Si MMIC technology, wherein the need for vias is eliminated in this two metal layer process. The switch is in the normally “OFF” state compared to MEMS capacitive shunt switches which are in the normally “ON” state. In addition, these switches are capable of switching at ˜30 ns switching speeds, where as the MEMS switches are slower (˜10 μs). Further, a lower bias voltage (&lt;10V) can be used compared to MEMS (40-50V) for switching. The varactor shunt switch can be designed for a bias voltage of less than 2 V. 
   The design trade between the isolation (OFF-state) and insertion (ON-state) loss depends on the varactor area  200  and the dielectric constant of the BST thin-films  20 . Large varactor area  200  and high dielectric constant are required to get the high isolation but it will increase the insertion loss. To keep the insertion loss at a minimum (&lt;1 dB), the optimized overlapping area  200  and dielectric constant are taken as 25 μm 2  and 1200 respectively. 
     FIG. 4  represents the electric circuit model of the varactor shunt switch of  FIG. 3 . 
   The electrical circuit model is obtained by shunting the varactor, with L  400  and Rs  410  being parasitic inductance and resistance respectively. The shunt resistance Rd  430  models the lossy (leakage conductance) nature of the varactor. The varactor capacitance  420  can be obtained by the standard parallel plate capacitance calculation, with the dielectric permittivity of the BST thin-film  20 , and the overlap area  200  of the center signal strip  110  and the shunt line  55 . The varactor capacitance is given by:
 
 Cv=∈   0 ·∈ rf   ·A/t   (1)
 
where ∈ 0  is the dielectric permittivity of free space, ∈ rf  is the relative dielectric constant of the ferroelectric thin-film  20  used, A is the area of the varactor, and t is the thickness of the ferroelectric thin-film  20 .
 
   The series resistance (Rs)  410  of the shunt conductor  55  in the bottom metal layer (metal1)  25 , where the signal is shunted to ground is calculated using Equation 2
 
 R=I /(σ wt )  (2)
 
where, σ is the conductivity of metal used in the top metal electrode  15 , w is the width of the conductor, l is the length of the line shunting to ground, and t is the thickness of the conductor.
 
   The inductance  400  (L) of the line is calculated using Equation (3)
 
 L =( Z   0 /(2 πf )sin(2 πl/λ   g )  (3)
 
where, Z 0  is the characteristic impedance of the CPW transmission line, f is the operating frequency, and λ g  is the guide-wavelength.
 
   The shunt resistance  430  (Rd) of the varactor can be calculated using Equation (4)
 
 Rd ( V )=1/(ω C ( V )tan δ)  (4)
 
where, C(V)  420  is the capacitance of the varactor and tan δ is the loss-tangent of the ferroelectric thin-film  20 .
 
   The performance (e.g., high isolation, low insertion loss, etc.) of the capacitive shunt switch depends on the dielectric tunability of the ferroelectric thin-film  20 . High capacitance value will increase the isolation in the OFF-state but it will also increase the insertion loss in the ON-state. The capacitance value can be increased by using a high dielectric constant of the ferroelectric thin-films  20  or large varactor area  200 . Increasing the dielectric constant of the ferroelectric thin-films  20  with same varactor area  200  does not change the isolation very much but the resonance frequency decreases due to the increased varactor capacitance, see  FIG. 5 .  FIG. 5  shows the isolation for the relative dielectric constants of 2000, 1500, 1200 and 1000 from left to right with a fixed varactor area of 5×5 μm 2 . 
   Further, insertion losses increase with increasing varactor area  200  as shown in  FIG. 6 .  FIG. 6  illustrates the insertion loss for a fixed dielectric constant of value 200 with the varactor areas of 15×15 μm 2 , 10×10 μm 2 , 10×5 μm 2 , and 5×5 μm 2  from left to right. 
   The simulated optimized dielectric constant of ferroelectric thin-films  20  is taken as 1200 for the OFF-state and 200 for the ON-state with a varactor area  200  of 5×5 μm 2 , or 25 μm 2 .  FIG. 7  illustrates the simulated isolation and insertion loss of the varactor shunt switch for the optimized device. The isolation of the device is better than 30 dB at 30 GHz and the insertion loss is below 1 dB below 30 GHz. 
   The varactor shunt switch was tested using a HP 8510 Vector Network Analyzer (VNA). A Line-Reflect-Reflect-Match (LRRM) calibration was performed over a wide frequency range (5 to 35 GHz). The sample was then probed using standard GSG probes. The dc bias was applied through the bias tee of the VNA.  FIG. 8  illustrates the experimental measurements performed on the varactor shunt switch for 0 V (i.e., the OFF state) and for 10 V dc bias (i.e., the ON state). In the measured device, the capacitance of the varactor at zero bias was about 0.85 pF and was reduced to about 0.25 pF for a bias voltage of 10 V, thereby, resulting in a dielectric tunability of more than 3:1. 
     FIG. 9  illustrates the experimental results obtained from the varactor shunt switch compared to the simulation results based on the electrical model developed for the device. The experimental results were obtained up to 35 GHz. Theoretical simulations performed on the same device indicates that the isolation (off-state S 21 ) improves to 30 dB near 41 GHz. A good agreement between the theoretical and experimental results over the frequency range of measurements can be seen as shown in  FIG. 9 . Therefore, the experimental data confirms the operation of the varactor shunt switch for microwave switching applications. 
   Table 1 demonstrates the comparison among solid-state switching devices, RF MEMS and the ferroelectric-based varactor shunt switch. The advantages of the varactor shunt switch include lower bias voltage requirement, high switching speed, ease of fabrication and high switching lifetime. 
   
     
       
         
             
             
             
             
           
             
               TABLE 1 
             
             
                 
             
             
               Device 
                 
                 
                 
             
             
               characteristics 
                 
               RF MEMS 
               Ferroelectric 
             
             
               and performance 
               Solid state 
               capacitive 
               varactor based 
             
             
               parameter 
               switches 
               shunt switches 
               shunt switch 
             
             
                 
             
           
          
             
               Type of switch 
               Normally OFF 
               Normally ON 
               Normally OFF 
             
             
                 
               or ON 
             
             
               Actuation voltage 
               Low (3-8 V) 
               High (40-50 V) 
               Low (&lt;10 V) 
             
             
               Switching speed 
               High (5-100 ns) 
               Low (~10 μs) 
               High (&lt;100 ns) 
             
             
               Isolation (dB) 
               &lt;20 db @ 
               Very high 
               High 
             
             
                 
               20 GHz 
               (&gt;40 dB @ 
               (&gt;20 dB @ 
             
             
                 
                 
               30 GHz) 
               30 GHz) 
             
             
               Insertion loss (dB) 
               &gt;1 db @ 30 GHz 
               Very low 
               Low 
             
             
                 
                 
               (&lt;1 db @ 
               (&lt;1.5 dB @ 
             
             
                 
                 
               30 GHz) 
               30 GHz) 
             
             
               Switching lifetime 
               High 
               Low 
               High 
             
             
               Packaging cost 
               Low 
               High 
               Low 
             
             
               Power handling 
               Poor (0.5-1 W) 
               Medium 
               Medium 
             
             
                 
                 
               (&lt;5 W) 
               (&lt;5 W) 
             
             
               Power consumption 
               Low (1-20 mW) 
               Almost zero 
               Almost zero 
             
             
               (OFF-state) 
             
             
               Breakdown voltage 
               Low 
               High 
               &lt;20 V 
             
             
               DC resistance 
               High (1-5 Ω) 
               Low (&lt;0.5 Ω) 
               Low (&lt;0.5 Ω) 
             
             
               Linearity 
               Low 
               High 
               High 
             
             
               IP3 
               Low (~+28 dBm) 
               High 
               Not available 
             
             
                 
                 
               (~+55 dBm) 
             
             
               Integration 
               Very good 
               Very good 
               Very good 
             
             
               capability 
             
             
                 
             
          
         
       
     
   
   Note that the ferroelectric varactor shunt switch performance predicted in the table are based on theoretical calculations. 
   In another embodiment, a frequency selective, or bandpass, filter can be implemented by connecting a sequence of varactor shunt switches, as shown in the circuit diagram in  FIG. 10 . The varactor shunt switch bandpass filter can be used as a replacement for a YIG type filter. As illustrated in  FIG. 10 , each of the varactor shunt switches  500  can be connected in series to create the bandpass filter. This is accomplished by cascading varactor shunt switches. The circuit diagram of the varactor shunt switch  500  used in the bandpass filter is similar to the circuit diagram illustrated in  FIG. 4 . The varactor capacitance (C(V)), shunt resistance (R d (V)), parasitic resistance (R s ) and inductance (L) can be chosen to select the frequency to be passed by the filter. As mentioned above, the capacitance of the shunt switch can be increased by increasing the dielectric constant of the ferroelectric thin film, changing the thickness of the ferroelectric thin film, or by creating a larger varactor area. 
   This resulting varactor shunt switch bandpass filter can be tunable by more than 500 MHz as illustrated in the simulated results graph in  FIG. 11 . In addition, by tuning selective capacitors, for example the capacitors in box  510  on  FIG. 10 , the bandpass filter can result in MMIC response as illustrated in  FIG. 12 . A varactor shunt switch bandpass filter is smaller (i.e., less bulky), cheaper to implement and uses less power than the typical YIG type filter. 
   In yet another embodiment, a tunable notch filter can also be implemented by using the varactor shunt switches as illustrated by the circuit diagram in  FIG. 13 . The circuit diagram of the varactor shunt switch  500  used in the notch filter is similar to the circuit diagram illustrated in  FIG. 4 . However, in this embodiment, the parasitic resistance and inductance are in parallel with an additional capacitor resulting in a resonant circuit. In this embodiment, the parasitic resistance can be about 1000Ω, the parasitic inductance can be about 0.66 nH, and the addition capacitor can be about 1 pF. When the notch is tunable by &gt;1 GHz, the simulation results are shown in  FIG. 14 . In  FIG. 15 , the simulation results are shown when the notch is tunable by &gt;1 GHz and the minimum depth is set to −50 dB. 
   It is noted that terms like “preferably,” “commonly,” and “typically” are not utilized herein to limit the scope of the claimed invention or to imply that certain features are critical, essential, or even important to the structure or function of the claimed invention. Rather, these terms are merely intended to highlight alternative or additional features that may or may not be utilized in a particular embodiment of the present invention. 
   Having described the invention in detail and by reference to specific embodiments thereof, it will be apparent that modifications and variations are possible without departing from the scope of the invention defined in the appended claims. More specifically, although some aspects of the present invention are identified herein as preferred or particularly advantageous, it is contemplated that the present invention is not necessarily limited to these preferred aspects of the invention.