Abstract:
Provided are a distributed analog phase shifter and a method of manufacturing the same, which reduce a change in a characteristic impedance while changing a phase velocity with respect to an applied voltage. In the distributed analog phase shifter, a coplanar waveguide (CPW) is formed in a line form on a substrate. A plurality of ferroelectric capacitors is periodically loaded to the CPW. The ferroelectric capacitors include a ferroelectric film in a pattern form and defines the ferroelectric film affected by the applied voltage within an area of the ferroelectric capacitors. Accordingly, the change in the phase velocity with respect to the applied voltage is maintained without the change of the CPW characteristic and a return loss characteristic and a total insertion loss are improved since a total dielectric loss of the ferroelectric film is decreased.

Description:
CROSS REFERENCE TO RELATED APPLICATION  
       [0001]     This application claims the priority of Korean Patent Application No. 2003-56847, filed on Aug. 18, 2003, in the Korean Intellectual Property Office, the disclosure of which is hereby incorporated by reference in its entirety.  
       BACKGROUND OF THE INVENTION 1. Field of the Invention  
       [0002]     The present invention relates to a phase shifter for use in phased array antennas and a method of manufacturing the same, and more particularly, to a distributed analog phase shifter using an etched ferroelectric film and a method of manufacturing the same.  
         [0003]     2. Description of the Related Art  
         [0004]     In general, phase shifters are essential core parts of active phased array antennas that trace a satellite to make clear and perfect communications possible on a real-time basis even when the active phased array antennas move in a mobile communication system.  
         [0005]     Among such phase shifters, a ferroelectric distributed analog phase shifter controls a phase using a characteristic of a ferroelectric material having a dielectric constant that changes according to the strength of an applied electric field. Since the ferroelectric distributed analog phase shifter is small size and lightweight and has a quick-response characteristic, low dielectric loss, and high power processing capability, it is expected to substitute for currently widely used semiconductor devices.  
         [0006]     General ferroelectric phase shifters can be roughly classified into four types: a simple coplanar waveguide (CPW) type, a loaded line type, a type using a variable filter, and a reflective structure type in which variable capacitors are connected to terminals of a coupler.  
         [0007]     Conventional ferroelectric distributed analog phase shifters are generally combinations of the CPW type and the loaded line type.  
         [0008]     However, in the conventional ferroelectric distributed analog phase shifters, a ferroelectric film having a dielectric constant that changes with an applied voltage occupies the entire surface of a substrate. As a result, a characteristic impedance is significantly changed with a change in a phase velocity that is the most important factor of the ferroelectric distributed analog phase shifter. In other words, since the ferroelectric film having a dielectric constant that changes with an applied voltage is formed over the entire surface of a substrate, the dielectric constant of the ferroelectric film may significantly change, causing changes in not only the phase velocity but also a characteristic impedance. Such changes may have a negative influence upon characteristics of a circuit.  
         [0009]     Also, difficulty in extracting design parameters requires a number of repetitive processes. If a high voltage is applied to obtain a desired differential phase shift angle, a large difference in insertion loss occurs due to a change in the applied voltage.  
       SUMMARY OF THE INVENTION  
       [0010]     The present invention provides a distributed analog phase shifter comprising a substrate, a coplanar waveguide (CPW), and a plurality of ferroelectric capacitors. The coplanar waveguide is extended in a line form on the substrate, the plurality of ferroelectric capacitors is periodically loaded to the CPW, and ferroelectric materials of the ferroelectric capacitors are disposed in pattern forms.  
         [0011]     The CPW further comprises a ground plane disposed at both sides of a signal line on the substrate and substantially parallel to the signal line.  
         [0012]     Also, the plurality of ferroelectric capacitors comprises first electrodes, second electrodes, and ferroelectric materials. The first electrodes are branched from both sides of the coplanar waveguide at predetermined intervals. The second electrodes are extended from a ground line to correspond to the first electrodes. The ferroelectric materials are formed in pattern forms to overlap with the first electrodes and the second electrodes.  
         [0013]     The ferroelectric materials are formed of barium strontium titanate. The substrate is formed of one of high-resistance silicon, semi-insulting gallium arsenide, alumina, glass, sapphire, and magnesium oxide.  
         [0014]     According to another aspect of the present invention, there is provided a method of manufacturing a distributed analog phase shifter. The method comprises depositing a ferroelectric film on a substrate, etching the ferroelectric film to form ferroelectric patterns, depositing a metal layer on the substrate on which the ferroelectric patterns are formed, and forming a coplanar waveguide, first electrodes, a ground line, and second electrodes by etching the metal layer. The first electrodes and the second electrodes are formed such that portions of the first electrodes and second electrodes overlap with the ferroelectric pattern, respectively.  
         [0015]     The ferroelectric film is deposited using pulsed laser deposition. The ferroelectric film is etched using radio frequency ion milling. The metal layer is a deposition layer of gold/chrome. The metal layer is deposited using DC magnetron sputtering. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0016]     The above and other aspects and advantages of the present invention will become more apparent by describing in detail exemplary embodiments thereof with reference to the attached drawings in which:  
         [0017]      FIG. 1  is a plan view of a ferroelectric distributed analog phase shifter according to an embodiment of the present invention;  
         [0018]      FIG. 2  is a perspective view of the ferroelectric distributed analog phase shifter according to the embodiment of the present invention;  
         [0019]      FIGS. 3A through 3E  are sectional views of respective processes for explaining a method of manufacturing the ferroelectric distributed analog phase shifter according to the embodiment of the present invention;  
         [0020]      FIG. 4A  illustrates the ferroelectric distributed analog phase shifter according to the embodiment of the present invention;  
         [0021]      FIG. 4B  is an enlarged view of a portion A of  FIG. 4A ;  
         [0022]      FIG. 4C  is an enlarged view of a portion B of  FIG. 4B ;  
         [0023]      FIG. 5  is an equivalent circuit diagram of a distributed analog phase shifter according to the present invention;  
         [0024]      FIG. 6A  is a graph showing simulated return loss results with respect to frequencies for the distributed analog phase shifter according to the embodiment of the present invention;  
         [0025]      FIG. 6B  is a graph showing simulated insertion loss results with respect to frequencies for the distributed analog phase shifter according to the embodiment of the present invention;  
         [0026]      FIG. 6C  is a graph showing simulated differential phase shift results with respect to frequencies for the distributed analog phase shifter according to the embodiment of the present invention;  
         [0027]      FIG. 7A  is a graph showing measured return loss results with respect to frequencies for a distributed analog phase shifter manufactured according to the embodiment of the present invention;  
         [0028]      FIG. 7B  is a graph showing measured insertion loss results with respect to frequencies for the distributed analog phase shifter manufactured according to the embodiment of the present invention; and  
         [0029]      FIG. 7C  is a graph showing measured differential phase shift results with respect to frequencies for the distributed analog phase shifter manufactured according to the embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0030]     The present invention will now be described more fully with reference to the accompanying drawings, in which an exemplary embodiment of the invention is shown. The invention may, however, be embodied in many different forms and should not be construed as being limited to the embodiment set forth herein; rather, this embodiment is provided so that this disclosure will be thorough and complete, and will fully convey the concept of the invention to those skilled in the art. In the drawings, the thicknesses of layers and regions are exaggerated for clarity. Like reference numerals in the drawings denote like elements, and thus their description will be omitted.  
         [0031]     In the embodiment of the present invention, in order to minimize the change in a characteristic impedance while maintaining the change in the phase velocity of a ferroelectric distributed analog phase shifter with respect to an applied voltage, a ferroelectric film having a dielectric constant that changes with the applied voltage is formed in a pattern form on the minimum area on a substrate, i.e., only on the area for forming a capacitor. By reducing the area of the ferroelectric film in the ferroelectric distributed analog phase shifter, the change in a characteristic impedance can be reduced without affecting the change in a phase velocity with respect to a voltage change, thereby reducing return loss and insertion loss.  
         [0032]     Hereinafter, the ferroelectric distributed analog phase shifter will be described in detail with reference to  FIGS. 1 and 2 .  
         [0033]     As shown in  FIG. 1 , a ferroelectric distributed analog phase shifter  100  according to the embodiment of the present invention includes a CPW signal line  120  having an input unit and an output unit and ferroelectric capacitors that are periodically loaded to the CPW signal line  120 , e.g., an interdigitated capacitor (IDC)  150 .  
         [0034]     CPW denotes a transmission line having high impedance, i.e., a conductive line. As implied by the name of the IDC  150 , the IDC  150  is a ferroelectric capacitor interposed between the CPW signal line  120  and a CPW ground plane  130  and includes a ferroelectric pattern etched to a predetermined size, e.g., a barium strontium titanate (BST) pattern. Such a periodic structure supports slow wave propagation properties.  
         [0035]     More specifically, as shown in  FIG. 2 , the ferroelectric distributed analog phase shifter  100  includes a substrate  110 . It is preferable that the substrate  110  be formed of one of high-resistance silicon, semi-insulating gallium arsenide, alumina, glass, sapphire, and magnesium oxide. A CPW is disposed in a line form on a predetermined portion on the substrate  110 . A radio frequency (RF) input unit  120   a  and an RF output unit  120   b  are formed at two terminals of the CPW and the widths of the RF input unit  120   a  and the RF output unit  120   b  each may be larger than the width of the CPW signal line  120  by a predetermined width. The CPW signal line  120  includes a plurality of first electrodes  125  that are branched from both sides of the CPW signal line  120  at predetermined intervals.  
         [0036]     The CPW ground plane  130  is disposed at both sides of the CPW signal line  120  on the substrate  110 . The CPW ground plane  130  is apart from the CPW signal line  120  by a predetermined distance and is substantially parallel to the CPW signal line  120 . A plurality of second electrodes  135  are formed at predetermined intervals to correspond to the first electrodes  125 . The first electrodes  125  and the second electrodes  135  face each other while being apart by a small space.  
         [0037]     A ferroelectric pattern  140  etched to a predetermined size is disposed under the first electrodes  125  and the second electrodes  135 . The ferroelectric pattern  140  may be a BST film and constitutes the IDC  150  with the first electrodes  125  and the second electrodes  135 .  
         [0038]     The ferroelectric distributed analog phase shifter  100  can be formed as follows.  
         [0039]     First, as shown in  FIG. 3A , the ferroelectric film  140 , e.g., a BST film, is deposited on the substrate  110 . For example, the ferroelectric film  140  is deposited at a pressure of 150-300 mTorr and at a temperature of 700-800° C., using pulsed laser deposition (PLD). For example, the ferroelectric film  140  is deposited with a thickness of 0.35-0.45 μm.  
         [0040]     As shown in  FIG. 3B , a photoresist pattern  142  is formed on the ferroelectric film  140 , using a well-known photolithography process. It is preferable that the photoresist pattern  142  be formed in an area in which the IDC  150  is to be formed.  
         [0041]     As shown in  FIG. 3C , the ferroelectric film  140  is etched in the form of the photoresist pattern  142 , thus to form the ferroelectric pattern  140 . The ferroelectric film  140  may be etched using RF ion milling. Then the photoresist pattern  142  is removed using a well-known method.  
         [0042]     Thereafter, as shown in  FIG. 3D , a metal layer, e.g., a deposition layer of gold/chrome, is deposited with a thickness of approximately 1.5-2.5 μm using DC magnetron sputtering to form a CPW signal line and a CPW ground plane.  
         [0043]     As shown in  FIG. 3E , a portion of the metal layer is etched to form the CPW signal line  120 , the first electrodes  125 , the CPW ground plane  130 , and the second electrodes  135 . At this time, the first electrodes  125  and the second electrodes  135  are formed such that predetermined portions of the first electrodes  125  and second electrodes  135  overlap with the ferroelectric pattern  140 .  
         [0044]      FIG. 4A  illustrates the ferroelectric distributed analog phase shifter  100  according the embodiment of the present invention,  FIG. 4B  is an enlarged view of a portion A of  FIG. 4A , and  FIG. 4C  is an enlarged view of a portion B of  FIG. 4B .  
         [0045]     Referring to  FIGS. 4A through 4C , the CPW signal line  120  is disposed between the CPW ground plane  130  and the IDC  150  is disposed between the CPW signal line  120  and the CPW ground plane  130  at predetermined intervals. The IDC  150  overlaps with the first electrodes  125  branched from the CPW signal line  120 , the second electrodes  135  branched from the CPW ground plane  130 , the first electrodes  125 , and the second electrodes  135  and is formed of the ferroelectric pattern  140  having a pattern shape. The first electrodes  125  and the second electrodes  135  are apart by a predetermined distance and may have curved cross-sections such that they can be inserted into each other.  
         [0046]     The ferroelectric distributed analog phase shifter  100  can be expressed by an equivalent circuit as shown in  FIG. 5 .  
         [0047]     As shown in  FIG. 5 , the ferroelectric distributed analog phase shifter  100  includes inductors L CPW  connected in series at predetermined intervals and total capacitors C t  connected between the inductors L CPW  at predetermined intervals.  
         [0048]     The inductors L CPW  are generated by the CPW.  
         [0049]     Each of the total capacitor C t  includes a fixed capacitor C CPW  and a variable capacitor C IDC  (V) connected in parallel. The fixed capacitor C CPW  is generated between the CPW signal line  120  and the CPW ground plane  130  and does not depend on a voltage. The variable capacitor C IDC  (V) includes the first electrodes  125  of the CPW signal line  120 , the second electrodes  135  of the CPW ground plane  130 , and the ferroelectric pattern  140 . Since a dielectric constant of the ferroelectric pattern  140  changes with a voltage applied between the first electrodes  125  and the second electrodes  135 , a capacitance changes with the applied voltage.  
         [0050]     Therefore, the CPW to which the IDC  150  is periodically connected is regarded as a synthetic transmission line having a line capacitance increased by the capacitance of the IDC  150  per unit cell (cell  101  formed of one inductor and one total capacitor). As a result, characteristic impedance and phase velocity of a synthetic transmission line are changed by the applied voltage.  
         [0051]     The characteristic impedance and phase speed of the ferroelectric distributed analog phase shifter  100  are expressed as follows.  
                   Z     S   .   T   .   L       ⁡     (   V   )       =         L   CPW       (       C   CPW     +         C   IDC     ⁡     (   V   )       /     L   unit_cell         )           ⁢     
     ⁢           V     S   .   T   .   L       ⁡     (   V   )       =     1         L   CPW     ⁡     (       C   CPW     +         C   IDC     ⁡     (   V   )       /     L   unit_cell         )             ,             (   1   )             
        where Z S.T.L. (V) represents the characteristic impedance of the synthetic transmission line, V S.T.L. (V) represents the phase velocity of the integrated transmission line, L unit     —     cell  represents a distance between the IDC  150 , C CPW  and L CPW  represent values obtained by normalizing a line capacitance and an inductance of the CPW to which the IDC  150  is not connected, using the distance between the IDC  150 , i.e., L unit     —     cell , and C IDC (V) represents a capacitance of a voltage variable IDC connected to the CPW. It can be seen from Equation 1 that the characteristic impedance and phase velocity of the synthetic transmission line are a function of the applied voltage.        
 
         [0053]     Major design parameters of the distributed analog phase shifter, i.e., the characteristic impedance (Z S.T.L. (V)) of the CPW and the distance (L unit     —     cell ) between the IDC  150  are optimized such that they are impedance-matched when the capacitance (C IDC (V)) of the IDC  150  has the largest value, so as to reduce a change in insertion loss with respect to the change in the applied voltage considering that dielectric loss of a ferroelectric material is maximized when a voltage applied to the ferroelectric material is 0V, i.e., the capacitance of the IDC  150  that is a variable capacitor, is maximized.  
         [0054]     To meet the minimum insertion loss condition of the distributed analog phase shifter, in this embodiment, the characteristic impedance of the CPW is set to 70-80 Ω, and preferably, to 74 Ω (related contents are disclosed in IEEE trans MTT, 47, 1705(1999) by A. S. Nagra and R. A. York). Also, considering total size and metal loss of a circuit, the width of the CPW is be set to 40-60 μm, and preferably, to 50 μm. Thus, to meet a characteristic impedance of 74 Ω, a gap between the CPW signal line  120  and the CPW ground plane  130  may be set to 93 μm on a MgO substrate having a thickness of 0.5 mm.  
         [0055]     In general, a circuit having a periodic structure has a Bragg frequency and shows a desired characteristic only at frequencies less than the Bragg frequency. In this embodiment, the Bragg frequency is expressed as follows.  
                 f   bragg     =     1     π   ⁢           ⁢     L   unit_cell     ⁢         L   CPW     ⁡     (       C   CPW     +       C   IDC   MAX     /     L   unit_cell         )               ,           (   2   )             
        where  
       C   IDC   MAX       
 
 represents the maximum capacitance of the IDC  150 . To obtain a Bragg frequency sufficient to use the distributed analog phase shifter in an X band, the maximum capacitance of the IDC  150  using the ferroelectric film is set to 0.08 pF. By using this maximum capacitance, the Bragg frequency and the distance (L unit     —     cell ) between the IDC  150  have a relationship as follows.  
                 L   unit_cell     =     50     π   ⁢           ⁢     f   bragg     ⁢     L   CPW           ,           (   3   )             
       
 
         [0058]     Using Equation 3, the distance (L unit     —     cell ) between the IDC  150  may be set to about 0.65 mm. Thus, a parameter that is not set in the entire circuit is the structure of the IDC  150  having a maximum capacitance of 0.08 pF. Since the IDC  150  is connected to the CPW signal line  120  in parallel, the single IDC  150  should have a maximum capacitance of about 0.04 pF. Inventors of the present invention designed, manufactured, and tested IDCs having various structures to obtain the IDC having a maximum capacitance of about 0.04 pF. As a result, as shown in  FIGS. 1 and 2 , the IDC  150  is branched from both sides of the CPW signal line  120  in a finger form and the ferroelectric film exists in a pattern form. Also, it can be seen that a capacitance of 0.04 pF is obtained when the overlapping length of fingers disposed in the first electrodes  125  and the second electrodes  135 , the width of each of the fingers (the first electrodes  125  and the second electrodes  135 ), and the gap between the fingers are 5 μm, respectively.  
         [0059]      FIGS. 6A through 6C  are graphs showing HFSS simulation results to estimate characteristics of the distributed analog phase shifter  100  according to the embodiment of the present invention. In simulation, a required dielectric constant of the ferroelectric material is extracted from a physical structure of the IDC and a measurement result using a conformal mapping technique. As a result, simulation is performed in an environment that a dielectric constant ε R  is verified in a range of 1000-600 with respect to the applied voltage and loss tangent of the ferroelectric material is fixed to 0.1 regardless of the applied voltage in consideration of the worst state thin film.  
         [0060]      FIG. 6A  is a graph showing simulated return loss results with respect to frequencies for the distributed analog phase shifter  100  according to the embodiment of the present invention. It can be seen from  FIG. 6A  that return loss is maintained at about −25 dB or more at 10 GHz.  FIG. 6B  is a graph showing simulated insertion loss results with respect to frequencies for the distributed analog phase shifter  100  according to the embodiment of the present invention. It can be seen from  FIG. 6B  that insertion loss changes from −1.7 dB to −1.2 dB at 10 GHz.  FIG. 6C  is a graph showing simulated differential phase shift results with respect to frequencies for the distributed analog phase shifter  100  according to the embodiment of the present invention. It can be seen from  FIG. 6C  that differential phase shift is 36° at 10 GHz.  
         [0061]      FIGS. 7A through 7C  are graphs showing measured return loss, insertion loss, and differential phase shift when an applied voltage Bias ranges from 0V to 40V for a ferroelectric distributed analog phase shifter manufactured according to the embodiment of the present invention. An RF characteristic of the ferroelectric distributed analog phase shifter is measured using an HP8510C vector network analyzer and a GSG pico-probe.  
         [0062]     In practice, a device has a return loss of −17 dB or more over the entire bands as shown in  FIG. 7A , an insertion loss that changes from −1.1 dB to −0.7 dB at 10 GHz as shown in  FIG. 7B , and a differential phase shift of 24° at 10 GHz. At this time, differential phase shift can be increased by increasing the number of unit cell  101  of the ferroelectric distributed analog phase shifter.  
         [0063]     When comparing  FIGS. 6A through 6C  with  FIGS. 7A through 7C , it can be seen that simulated results are similar to return loss and differential phase shift of the actual device. In the case of insertion loss, simulated results are greater than insertion loss of the actual device. However, since simulation is performed in consideration of the worst-case scenario where loss tangent of the ferroelectric material is set to a relatively high value, i.e., 0.1, it is expected that there is no large difference in insertion loss in practice.  
         [0064]     The ferroelectric distributed analog phase shifter according to the present invention etches a ferroelectric film and uses the etched ferroelectric film as a pattern form, thereby achieving accuracy in variable RF device desig and reducing total insertion loss of the device.  
         [0065]     The art and spirit of the present invention are described in detail according to the embodiment of the present invention, however, should not be construed as being limited to the embodiment set forth herein. For example, in the above embodiment, an X-band distributed analog phase shifter is described as an example, but the present invention can be applied without restriction of a band.  
         [0066]     Also, partial etching of the ferroelectric film can be applied to any RF variable device using a ferroelectric film.  
         [0067]     As described above, according to the present invention, by controlling a distance (gap) between first electrodes and second electrodes of an IDC which is connected to a CPW, a change in the strength of an electric field applied to the ferroelectric film is maintained constantly and an operating applied voltage of the phase shifter using the ferroelectric film can be reduced without a change in a characteristic of a circuit.  
         [0068]     As described above, according to the present invention, a change in a characteristic impedance while maintaining the change in the phase velocity of the ferroelectric distributed analog phase shifter with respect to an applied voltage can be minimized and an operating applied voltage of the phase shifter using the ferroelectric film can be reduced without a change in a characteristic of a circuit.  
         [0069]     Also, by partially etching the ferroelectric film, accuracy in RF variable device design is improved, thereby improving a return loss characteristic of the device and reducing total insertion loss of the device.  
         [0070]     While the present invention has been particularly shown and described with reference to an exemplary embodiment thereof, it will be understood by those of ordinary skill in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims and their equivalents.