Abstract:
A film bulk acoustic resonator (FBAR) comprises a piezoelectric film sandwiched between a top electrode and a bottom electrode. A temperature sensor is provided to sense a temperature to determine a temperature induced frequency drift for the FBAR. A voltage controller operatively connected to the temperature sensor supplies a direct current (DC) bias voltage to the FBAR to induce an opposite voltage induced frequency drift to compensate for the temperature induced frequency drift.

Description:
FIELD OF THE INVENTION  
       [0001]     Embodiments of the present invention relate to film bulk acoustic resonators (FBARs) and, more particularly to such devices stabilized against temperature drift.  
       BACKGROUND INFORMATION  
       [0002]     Film bulk acoustic resonator (FBAR) technology may be used as a basis for forming many of the frequency components in modern wireless systems. For example, FBAR technology may be used to form filter devices, oscillators, resonators, and a host of other frequency related components. FBAR may have advantages compared to other resonator technologies, such as Surface Acoustic Wave (SAW) and traditional crystal oscillator technologies. In particular, unlike crystals oscillators, FBAR devices may be integrated on a chip and typically have better power handling characteristics than SAW devices.  
         [0003]     The descriptive name given to the technology, FBAR, may be useful to describe its general principals. In short, “Film” refers to a thin piezoelectric film such as Aluminum Nitride (AIN) sandwiched between two electrodes. Piezoelectric films have the property of mechanically vibrating in the presence of an electric field as well as producing an electric field if mechanically vibrated. “Bulk” refers to the body or thickness of the sandwich. When an alternating voltage is applied across the electrodes the film begins to vibrate. “Acoustic” refers to this mechanical vibration that resonates within the “bulk” (as opposed to just the surface in a SAW device) of the device.  
         [0004]     The frequency characteristics of FBAR devices tend to be influenced by temperature which may be undesirable for wireless communication applications. For example, for cell phone applications, the operation temperature specification may be between −35 and +85° C. Such extreme temperature variations may be encountered for example in a closed automobile where a cell phone may be kept. Because of temperature induced frequency drift, pass band windows are typically designed appreciably larger than they otherwise would be and transition bands sharper. Such design constraints tend to degrade insertion loss and demand more stringent processing requirements leading to reduced production yield. These constraints may be illustrated in a current FBAR filter design where there is only a 12 MHz (mega-Hertz) frequency variation budget governed by communication standards and material properties. A temperature variation from −35 to +85° C. may induce a frequency drift in the FBAR filter that consumes about 6 MHz, thus leaving only 6 MHz for processing variations.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0005]      FIG. 1  is a cross-sectional view of a film bulk acoustic resonator (FBAR);  
         [0006]      FIG. 2  is a schematic of an electrical circuit of the film bulk acoustic resonator (FBAR) shown in  FIG. 1 ;  
         [0007]      FIG. 3  is a graph illustrating the temperature induced frequency drift for an FBAR;  
         [0008]      FIG. 4  is a graph illustrating the DC bias voltage induced frequency drift for an FBAR;  
         [0009]      FIG. 5  is an example FBAR oscillator circuit including a bias voltage source for compensating for temperature induced frequency drift;  
         [0010]      FIG. 6  is an example FBAR filter circuit including a bias voltage source for compensating for temperature induced frequency drift; and  
         [0011]      FIG. 7  is an example physical lay-out for the FBAR filter circuit shown in  FIG. 6 .  
     
    
     DETAILED DESCRIPTION  
       [0012]     An FBAR device  10  is schematically shown in  FIG. 1 . The FBAR device  10  may be formed on the horizontal plane of a substrate  12 , such as silicon and may include an SiO 2  layer  13 . A first layer of metal  14  is placed on the substrate  12 , and then a piezoelectric layer  16  is placed onto the metal layer  14 . The piezoelectric layer  16  may be Zinc Oxide (ZnO), Aluminum Nitride (AIN), Lead Zirconate Titanate (PZT), or any other piezoelectric material. A second layer of metal  18  is placed over the piezoelectric layer  14 . The first metal layer  14  serves as a first electrode  14  and the second metal layer  18  serves as a second electrode  18 . The first electrode  14 , the piezoelectric layer  16 , and the second electrode  18  form a stack  20 . As shown, the stack may be, for example, around 1.8 μm thick. A portion of the substrate  12  behind or beneath the stack  20  may be removed using back side bulk silicon etching to form an opening  22 . The back side bulk silicon etching may be done using deep trench reactive ion etching or using a crystallographic-orientation-dependent etch, such as Potassium Hydroxide (KOH), Tetra-Methyl Ammonium Hydroxide (TMAH), and Ethylene-Diamene Pyrocatechol (EDP).  
         [0013]     The resulting structure is a horizontally positioned piezoelectric layer  16  sandwiched between the first electrode  14  and the second electrode  16  positioned above the opening  22  in the substrate  12 . In short, the FBAR  10  comprises a membrane device suspended over an opening  22  in a horizontal substrate  12 .  
         [0014]      FIG. 2  illustrates the schematic of an electrical circuit  30  which includes a film bulk acoustic resonator  10 . The electrical circuit  30  includes a source of radio frequency “RF” voltage  32 . The source of RF voltage  32  is attached to the first electrode  14  via electrical path  34  and attached to the second electrode  18  by the second electrical path  36 . The entire stack  20  can freely resonate in the Z direction  31  when an RF voltage  32  at resonant frequency is applied. The resonant frequency is determined by the thickness of the membrane or the thickness of the piezoelectric layer  16  which is designated by the letter “d” or dimension “d” in  FIG. 2 . The resonant frequency is determined by the following formula: 
 f 0 V/2d, where  f 0 =the resonant frequency,  V=acoustic velocity of piezoelectric layer, and  d=the thickness of the piezoelectric layer.  
         [0015]     It should be noted that the structure described in  FIGS. 1 and 2  can be used either as a resonator or as a filter. To form an FBAR, piezoelectric films  16 , such as ZnO, PZT and AIN, may be used as the active materials. The material properties of these films, such as the longitudinal piezoelectric coefficient and acoustic loss coefficient, are parameters for the resonator&#39;s performance. Performance factors include Q-factors, insertion loss, and the electrical/mechanical coupling. To manufacture an FBAR the piezoelectric film  16  may be deposited on a metal electrode  14  using for example reactive sputtering. The resulting films are polycrystalline with a c-axis texture orientation. In other words, the c-axis is perpendicular to the substrate.  
         [0016]     As previously noted, the frequency of the FBAR device  10  drifts with temperature. This is undesirable for most wireless applications since stable frequency characteristics over the range in which the device is expected to operate is preferred.  FIG. 3  illustrates the drift phenomena. For a center frequency of about 1587 MHz at 50° C. the frequency of the FBAR device may drift up to 1589 MHz if the temperature drops to 0° C. and may drift down to 1586 MHz if the temperature rises to 100° C. The drift appears fairly linear over a given temperature range. While this drift may not be large, it may nevertheless be troubling for designers since modern wireless devices operate within tight frequency ranges. For an AIN based FBAR, the temperature coefficient of frequency (TCF), a, is about −25 ppm (parts per million) per degree Celsius.  
         [0017]     According to embodiments of the invention, a direct current (DC) bias voltage may be applied across the FBAR device to compensate for temperature induced frequency drifts since the frequency of the FBAR may also be affected by a strong electric field in the piezoelectric film. For an AIN based FBAR at ˜1.6 GHz, the measured voltage coefficient of frequency (VCF), β, is ˜−9 ppm/Volt. It is inversely proportional to the AIN thickness (proportional to electric field strength), and consequently proportional to resonance frequency for a given bias voltage.  
         [0018]      FIG. 4  illustrates the effects of a DC bias voltage to an FBAR device. It is noted that the voltage induced frequency drift between the DC voltage range of −100 to 100 Volts is approximately linear. In this example, for a center frequency of 1587.7 MHz, linear function may be expressed as y=−0.0144x+1587.7. Thus, according to embodiments of the invention, an applied DC bias voltage may be used to provide a voltage induced frequency drift in the opposite direction to compensate for the temperature induced frequency drift.  
         [0019]      FIG. 5  shows a simple oscillator circuit using an FBAR  50 . Oscillator circuits may be used in wireless devices such as cell phones  51 . The oscillator may comprise an amplifier  52  having a first input  54  connected to ground and a second input  56  connected to a feedback loop  58  comprising a capacitor  60  connected to the output terminal  62  and a shunt capacitor  64  connected between the output terminal  62  and ground. A coupling capacitor  66  may connect the FBAR  50  to the feedback loop  58 . A temperature sensor  60 , such as a thermistor, may be placed in proximity to the FBAR  50  to detect the temperature influencing the FBAR  50 . A controller  62  determines the DC bias voltage suitable to compensate to any temperature induced frequency drift of the FBAR  50 . Thereafter, the appropriate DC Bias voltage may be applied to the FBAR  50 . A high impedance RF choke or resistor  64  may be employed between the FBAR  50  and voltage source controller  62  to prevent shorting at high frequencies. The DC bias voltage may be calculated as:  
       V   =       α   ⁡     (     T   -     T   o       )       β           
 Where, V=DC bias Voltage; 
        α=Temperature Coefficient of Frequency (TCF) for a given piezoelectric film;     β=Voltage Coefficient of Frequency (VCF) for a given piezoelectric film; and     T−T o =a detected shift in temperature.          
         [0023]      FIG. 6  shows FBAR devices used to form a filter such as may also be found in a wireless device. The particular filter shown is a ladder filter comprising a plurality of FBAR devices  70  connected in series between an input  72  and an output  74  and a plurality of FBAR devices  80  connected in parallel between the input  72  and output  74 . Coupling capacitors  82  may be used between the parallel connected FBAR devices  80  and ground. As previously discussed, a temperature sensor  60  may be used to monitor the temperature influencing the FBAR devices  70  and  80  on a real time basis. A controller  62  may use the temperature data from the sensor  60  to calculate the DC bias voltage suitable to compensate for temperature induced frequency drift.  
         [0024]     The ladder filter of  FIG. 6  may be configured such that the piezoelectric polarization direction of all FBAR devices is the same. That is, nodes  84  indicated by an open circle are connected to the positive terminal  86  of the controller  62  and those nodes  88  indicated by a solid circle are connected to the negative terminal  90  of the controller  62  such that the DC electric field is applied in the same direction for all FBAR devices  70  and  80 . The DC voltage may reverse polarity as the temperature changes to compensate for frequency drifts in either direction from a center frequency. Each node,  84  and  88 , may be connected to the DC controller  62  via a high impedance radio frequency (RF) choke or resistor  64 .  
         [0025]      FIG. 7  shows an example physical layout for the ladder filter discussed with reference to  FIG. 6  with like items from previously described figures labeled with like reference numerals. In particular, a plurality of serially connected FBAR devices  70  and parallel connected FBAR devices  80  are connected between an input  72  and an output  74 . Each of the FBAR devices may comprise a bottom metal electrode  14 , a piezoelectric film  16 , and a top metal electrode  18 . When depositing the piezoelectric film  16 , the piezoelectric polarization direction of all resonators ( 70  and  80 ) is oriented either from bottom to top or from top to bottom, depending on the particular material. In this fashion, the top electrode  18  of an FBAR is connected to the top electrode of an adjacent FBAR. Similarly, the bottom electrode  14  of an FBAR is connected to the bottom electrode of an adjacent FBAR. While the layout may vary, the top electrodes  18  for each FBAR should be consistently connected to V+86 and bottom electrodes  14  connected to V−90 in order to shift the frequency of all FBAR devices ( 70  and  80 ) in the same direction for an applied bias voltage. The connection lines  92  and  94  may be made of low resistivity metals, such as Al, Au, Pt, Cu, Mo, or W. The high impedance radio frequency (RF) choke or resistor  64  may comprise impedance lines may be made from high resistivity materials, such as poly silicon, TiN.  
         [0026]     The above description of illustrated embodiments of the invention, including what is described in the Abstract, is not intended to be exhaustive or to limit the invention to the precise forms disclosed. While specific embodiments of, and examples for, the invention are described herein for illustrative purposes, various equivalent modifications are possible within the scope of the invention, as those skilled in the relevant art will recognize.  
         [0027]     These modifications can be made to the invention in light of the above detailed description. The terms used in the following claims should not be construed to limit the invention to the specific embodiments disclosed in the specification and the claims. Rather, the scope of the invention is to be determined entirely by the following claims, which are to be construed in accordance with established doctrines of claim interpretation.