Abstract:
In accordance with the teachings described herein, low loss thin film capacitors and methods of manufacturing the same are provided. A low loss thin-film capacitor structure may include first and second electrodes and a polar dielectric between the first and second electrodes. The polar dielectric and the first and second electrodes collectively form a capacitor having an operational frequency band. The capacitor structure may also include one or more layers that affect the acoustic properties of the thin-film capacitor structure such that the capacitor absorbs RF energy at a frequency that is outside of the operational frequency band. A method of manufacturing a low loss thin-film capacitor may include the steps of fabricating a capacitor structure that includes a polar dielectric material, and modifying the acoustic properties of the capacitor structure such that the polar capacitor absorbs RF energy at a frequency that is outside of the operating frequency band of the capacitor structure.

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
       [0001]     This application claims priority from U.S. Provisional Application No. 60/670,805, titled “Systems and Methods for Improving the Loss of Thin Film Capacitors,” filed on Apr. 13, 2005, which is incorporated herein by reference in its entirety. 
     
    
     FIELD  
       [0002]     The technology described in this patent document relates generally to the field of thin film devices and fabrication. More particularly, the patent document describes a low loss thin film capacitor and methods of manufacturing the same.  
       BACKGROUND AND SUMMARY  
       [0003]     Ferroelectric and paraelectric capacitors have potential for use as decoupling or voltage-tunable capacitors (varactors) in RF systems. Some benefits of ferroelectric and paraelectric capacitors are small size, integration of different values and functions of capacitance, and low cost. Applications for ferroelectric and paraelectric capacitors may include tunable filters, voltage-controlled oscillators, tunable phase shifters, tunable matching networks, low-impedance power supplies, decoupling high-frequency signals at an IC bonding pad, or others. Integrated circuits including ferroelectric and paraelectric capacitors may, for example, be used in portable electronics for low-power wireless communication (e.g., cellular phones, pagers, PDAs, etc.), directional antenna systems, high clock-rate microphones, miniature DC to DC converters, or other devices.  
         [0004]     In accordance with the teachings described herein, low loss thin film capacitors and methods of manufacturing the same are provided. A low loss thin-film capacitor structure may include first and second electrodes and a polar dielectric between the first and second electrodes. The polar dielectric and the first and second electrodes collectively form a capacitor having an operational frequency band. The capacitor structure may also include one or more layers that affect the acoustic properties of the thin-film capacitor structure such that the capacitor absorbs RF energy at a frequency that is outside of the operational frequency band. A method of manufacturing a low loss thin-film capacitor may include the steps of fabricating a capacitor structure that includes a polar dielectric material, and modifying the acoustic properties of the capacitor structure such that the polar capacitor absorbs RF energy at a frequency that is outside of the operating frequency band of the capacitor structure. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0005]      FIGS. 1A and 1B  depict a typical thin-film capacitor integrated circuit.  
         [0006]      FIG. 2  is an example thin-film capacitor having a cavity that is fabricated in the substrate layer to create a void under the capacitor.  
         [0007]      FIG. 3  is an example thin-film capacitor having a cavity that is fabricated in the substrate and insulating layers to create a void under the capacitor  
         [0008]      FIG. 4  is another example thin-film capacitor having a cavity that is fabricated in the substrate layer to create a void under the capacitor.  
         [0009]      FIG. 5  is another example thin-film capacitor having a cavity that is fabricated in the substrate and insulating layers to create a void under the capacitor.  
         [0010]      FIG. 6  is an example thin-film capacitor that includes a multi-layer acoustic reflector or absorber.  
         [0011]      FIG. 7A  is an example thin-film capacitor in which the substrate layer is completely or partially removed to create a void under the capacitor.  
         [0012]      FIG. 7B  is an example thin-film capacitor in which the substrate layer is completely or partially removed and that includes a cavity fabricated in a carrier substrate above the capacitor.  
         [0013]      FIG. 8  is an example thin-film capacitor in which the substrate layer is completely or partially removed that includes a multi-layer acoustic reflector or absorber fabricated in the carrier substrate above the capacitor.  
         [0014]      FIG. 9  is an example thin-film capacitor in which a cavity is fabricated between the capacitor and the substrate layer.  
         [0015]      FIG. 10  is an example thin-film capacitor that uses a thin top electrode and a thin interconnect metallization to create a void above the capacitor.  
         [0016]      FIG. 11  is an example thin-film capacitor that includes a thin single-layer top electrode.  
         [0017]      FIG. 12  is a flow diagram illustrating an example method for fabricating a thin-film capacitor integrated circuit.  
         [0018]      FIG. 13  is a flow diagram illustrating another example method for fabricating a thin-film capacitor integrated circuit.  
         [0019]      FIG. 14  is a flow diagram illustrating a third example method for fabricating a thin-film capacitor integrated circuit.  
         [0020]      FIG. 15  is a flow diagram illustrating a fourth example method for fabricating a thin-film capacitor integrated circuit. 
     
    
     DETAILED DESCRIPTION  
       [0021]      FIGS. 1A and 1B  depict a typical thin-film capacitor integrated circuit.  FIG. 1A  depicts a cross-sectional diagram of the capacitor structure, and  FIG. 1B  depicts a top view showing the connections between the capacitor electrodes and an interconnect layer.  
         [0022]     With reference to  FIG. 1A , the capacitor structure includes two conducting electrodes  10  that are separated by a dielectric layer  12 . The conducting electrodes  10  may, for example, be fabricated using platinum or a platinum alloy. The dielectric layer  12  is fabricated using a polar dielectric material, such as barium strontium titanate (BST). The capacitor is fabricated on a substrate material  14  coated with an insulating layer  16  and an etch-resistant insulating layer  18 . The substrate  14  may, for example, be Si, Al 2 O 3 , sapphire, or some other type of insulting, semi-insulating or semiconducting material. The insulating layer  16  may be SiO 2 , and the etch-resistant insulating layer may be Si 3 N 4 , however other materials with similar functionality may also be used. Also illustrated in  FIG. 1A  are conducting interconnect layers  20  that may be used to electrically connect the capacitor electrodes to other circuitry, either within in the integrated circuit (IC) package or to external circuitry via bump pads  22 .  
         [0023]     A common problem associated with thin-film capacitors made with polar dielectric materials are high losses at specific frequencies, particularly in the 0.1 to 10 GHz range. These frequency-specific losses are fundamental to the properties of ferroelectric and paraelectric materials, and are directly related to the change of the dielectric constant with applied electric field.  
         [0024]      FIGS. 2-11  illustrate example low loss thin film capacitor structures that are fabricated from polar dielectric materials, while avoiding high, frequency-specific losses, particularly in the 0.1 to 10 GHz frequency range under an applied electric field in the range of about 0.1 megavolts/cm (MV/cm) to about 10 (MV/cm). This result is achieved by modifying the capacitor structure to change the acoustic reflections at the electrodes or other layers of the capacitor structure, such that the capacitor structure dielectric absorbs RF energy at a different frequency than the frequency required for the particular application. The cause of the RF energy absorption is electrostrictive resonance. When a ferroelectric material is voltage biased, for example to change the dielectric constant, the crystal lattice of the grains in the film are distorted. A varying electric field (from the RF signal) modulates this distortion, generating acoustic energy that is reinforced by the surrounding films in the structure. The example capacitor structures described herein are modified to increase the frequency at which the resonance is reinforced, resulting in capacitors having low RF energy absorption and high Q-values throughout the operational frequency band (e.g., throughout the 0.1 to 10 GHz frequency band under voltage biases between 0.1 MV/cm to 10 MV/cm.) These capacitors can be either single devices or integrated into a circuit on the substrate with other components such as other capacitors, resistors and/or inductors.  
         [0025]      FIG. 2  is an example thin-film capacitor structure having a cavity  30  that is fabricated in the substrate layer  14  to create a void  30  under the capacitor  10 ,  12 . The void  30  under the capacitor  10 ,  12  serves as an acoustic reflector, which modifies the electrostrictive resonance of the capacitor structure. In this manner, the electrostrictive resonance frequency of the capacitor structure may be modified to improve the loss in the operational frequency band (e.g., from 0.1 to 10 GHz.) The cavity  30  may be created by selectively removing the substrate  14 , for example by laser drilling, ultrasonic milling (e.g., for ceramic substrates), deep RIE, wet anisotropic etching (e.g., on &lt;110&gt; oriented silicon), or other fabrication techniques.  
         [0026]      FIG. 3  is an example thin-film capacitor structure having a cavity  40  that is fabricated both in the substrate  14  and insulating layers  16  to create a void under the capacitor. In this example the insulating layer  16  is etched after the substrate layer  14 . The insulating layer may, for example, be etched using a wet etch based on hydrofluoric acid, a dry RIE etch, or some other means. Removing the insulating layer  16  creates a thinner acoustic layer between the capacitor  10 ,  12  and the void  40 , further increasing the electrostrictive resonance frequency of the capacitor structure compared to the example of  FIG. 1 .  
         [0027]      FIG. 4  is another example thin-film capacitor structure having a cavity  50  that is fabricated in the substrate layer  14  to create a void  50  under the capacitor  10 ,  12 .  FIG. 5  is another example thin-film capacitor structure having a cavity  60 ,  62  that is fabricated in the substrate  14  and insulating layers  16  to create a void under the capacitor  10 ,  12 . The thin-film capacitors shown in  FIGS. 4 and 5  are similar to the examples of  FIGS. 2 and 3 , respectively, except that the substrate layer  14  is etched using an anisotropic backside wet etch. Anisotropic etching may, for example, be used in the case of a &lt;100&gt; silicon substrate.  
         [0028]      FIG. 6  is an example thin-film capacitor structure that includes a multi-layer acoustic reflector or absorber  70 . In this example the multi-layer acoustic reflector or absorber  70  is fabricated between the capacitor  10 ,  12  and the substrate layer  14 . The multi-layer reflector or absorber  70  includes two or more layers of alternating materials of different acoustic properties, which can be selected to either reflect or absorb the acoustic waves depending on the desired capacitor properties. The materials used to fabricate the layers of the acoustic reflector or absorber  70  and the thickness of the layers  70  may be selected to either reflect or absorb the acoustic wave at desired frequencies in order to modify the electrostrictive resonance of the capacitor structure in the operating frequency band (e.g., from 0.1 to 10 GHz.)  
         [0029]      FIG. 7A  is an example thin-film capacitor structure in which the substrate layer is completely or partially removed to create a void under the capacitor  10 ,  12 . In this example, the capacitor structure is bonded to a carrier substrate  80 , for example using flip-chip bonding methods. The carrier substrate  80  includes a substrate material  82 , and conducting layers  84  which provide connections to bonding pads  22  on the thin-film capacitor IC. In other examples, the carrier substrate may also include additional layers, including for example active and/or passive thin-film components. After the thin-film capacitor structure is bonded to the carrier substrate  80 , the substrate layer of the thin-film capacitor may be removed, leaving only the thin insulating layer  18  between the capacitor  10 ,  12  and an air void. In this manner, the air void under the capacitor  10 ,  12  serves as an acoustic reflector, which modify the electrostrictive resonance of the capacitor structure. Moreover, the carrier substrate  80  provides the necessary physical support to maintain the structural integrity of the thin-film capacitor structure after the substrate has been completely or partially removed. The thin-film capacitor substrate may, for example, be removed using either mechanical or chemical methods.  
         [0030]      FIG. 7B  is another example thin-film capacitor structure in which the substrate layer is completely or partially removed that includes a cavity fabricated in a carrier substrate  80  above the capacitor  10 ,  12 . This example is similar to the capacitor structure of  FIG. 7A , with the addition of the cavity  90  in the carrier substrate  80 . The cavity  90  above the capacitor  10 ,  12  forms an acoustic reflector, which further modifies the electrostrictive resonance of the capacitor structure.  
         [0031]      FIG. 8  is another example thin-film capacitor in which the substrate layer is completely or partially removed that includes a multi-layer acoustic reflector or absorber  100  fabricated in the carrier substrate  80  above the capacitor  10 ,  12 . The multi-layer reflector or absorber  100  includes two or more layers of alternating materials of different acoustic properties, which can be selected to either reflect or absorb the acoustic waves depending on the desired capacitor properties. The air void under the capacitor  10 ,  12 , which is created by completely or partially removing the substrate, serves as an acoustic reflector and combines with the multi-layer reflector or absorber  100  to modify the electrostrictive resonance of the capacitor  10 ,  12 . The materials used to fabricate the layers of the acoustic reflector or absorber  100  and the thickness of the layers  100  may be selected to either reflect or absorb the acoustic wave at desired frequencies in order to modify the electrostrictive resonance of the capacitor structure in the operating frequency band (e.g., from 0.1 to 10 GHz.)  
         [0032]      FIG. 9  is an example thin-film capacitor structure in which a cavity  110  is fabricated between the capacitor  10 ,  12  and the substrate layer  14 . The cavity  110  serves as an acoustic reflector, which raises the electrostrictive resonance frequency of the capacitor structure and may be used to modify the electrostrictive resonance of the capacitor structure in the operational frequency band (e.g., from 0.1 to 10 GHz.) The cavity  110  may be fabricated by etching a portion of the insulating layer  16  through front-side access holes  112 . The access holes  112  may, for example, be etched through the interlayer dielectric, lower electrode material and the underlying etch-resistant insulating layer to give access to the etchable insulating layer. The access holes  112  may then be lined with an etch-resistant insulating layer  18  to prevent damage to the capacitor  10 ,  12  and conducting layers  20 . The cavity  110  may be formed by wet etching the insulating layer  16  through the access holes  112 .  
         [0033]      FIG. 10  is an example thin-film capacitor structure that uses a thin top electrode  120  and a thin interconnect metallization  122  to create a void  124  above the capacitor  10 ,  12 ,  120 . The void  124  above the capacitor serves as an acoustic reflector, which modifies the electrostrictive resonance of the capacitor structure in the operational frequency band (e.g., from 0.1 to 10 GHz.) The void  124  is created by fabricating the top electrode  120  of the capacitor and the attached interconnect metallization  122  from thin conductive layers and by minimizing the amount of insulating material  126  between the top capacitor electrode  120  and the air medium  124 .  
         [0034]      FIG. 11  is an example thin-film capacitor structure that includes a thin single-layer top electrode  130 . In this example, the top electrode  130  of the capacitor includes an extended portion  132  that extends horizontally away from the capacitor in order to provide an electrical connection between the top electrode  130  and the interconnect metallization  136  The extended portion  132  of the top electrode  130  enables the interconnect metallization  136  to be horizontally offset from the top electrode  130 , thus minimizing the thickness of material between the top electrode  130  and the air medium above the capacitor. In this manner, the air medium above the capacitor  10 ,  12 ,  120  acts as an acoustic reflector, which modifies the electrostrictive resonance of the capacitor structure.  
         [0035]      FIG. 12  is a flow diagram illustrating an example method for fabricating a thin-film capacitor integrated circuit. At step  140 , a substrate layer is prepared with an etch-resistant layer on the surface to protect the capacitor and to act as an etch-stop for backside etching. The capacitor is fabricated on the substrate using conventional fabrication techniques at step  142 . At step  144 , the front side of the capacitor is protected with an etch-resistant layer, and another etch-resistant layer is deposited and patterned on the backside at step  146 . At step  148 , the substrate layer is completely or partially removed using wet or dry chemical etching or a combination of both. Either a timed etch or the etch-resistant layer may be used to terminate the chemical etch. Once the substrate is etched, the front and backside etch-resistant layers may be removed at step  150 , and any additional fabrication and/or packaging processing is performed at step  152 .  
         [0036]      FIG. 13  is a flow diagram illustrating another example method for fabricating a thin-film capacitor integrated circuit. At step  154 , a capacitor structure is fabricated on a substrate material using conventional fabrication techniques. Substrate material is then removed to a pre-determined depth at step  156 , stopping short of removing capacitor material. The substrate material may, for example, be removed using a laser, abrasive chemicals, ultrasonic milling and/or other mechanical or thermal means. Any additional fabrication and/or packaging processes may then be performed at step  158 .  
         [0037]      FIG. 14  is a flow diagram illustrating a third example method for fabricating a thin-film capacitor integrated circuit. At step  160 , the capacitor is fabricated on a substrate material using conventional fabrication techniques. An acceptor substrate is then fabricated at step  162  that includes one or more cavities or multi-layer acoustic reflector or absorber structures (e.g., the carrier substrate  80  in  FIGS. 7A, 7B  and  8 ). At step  164 , the acceptor substrate and the capacitor are aligned and bonded, for example using flip-chip bonding techniques. The original substrate is then completely or partially removed from the backside of the capacitor at step  166 . The substrate may, for example, be removed using mechanical or chemical methods. Any additional fabrication and/or packaging processing may then be performed at step  168 . The additional fabrication steps may include etching contact holes in the insulating layer on the backside and adding metal to reduce the series resistance of the capacitor.  
         [0038]      FIG. 15  is a flow diagram illustrating a fourth example method for fabricating a thin-film capacitor integrated circuit. At step  170  the capacitor is fabricated on a substrate material using conventional fabrication techniques. Capacitor devices are then singulated at step  172 . An acceptor substrate is fabricated at step  164  that includes one or more cavities or multi-layer acoustic reflector or absorber structures (e.g., the carrier substrate  80  in  FIGS. 7A, 7B  and  8 ). At step  176 , the acceptor substrate and the singulated capacitor are aligned and bonded, for example using flip-chip bonding techniques. Any additional fabrication and/or packaging processing may then be performed at step  178 . The additional fabrication steps may include etching contact holes in the insulating layer on the backside and adding metal to reduce the series resistance of the capacitor.  
         [0039]     This written description uses examples to disclose the invention, including the best mode, and also to enable a person skilled in the art to make and use the invention. The patentable scope of the invention may include other examples that occur to those skilled in the art.