Abstract:
A surface acoustic wave (SAW) device comprising at least one heating element formed on the substrate; at least one temperature sensor having a first electric component on the substrate whose resistance varies with the temperature of the substrate and a second electric component whose resistance does not vary; and a temperature controller including an operational amplifier bonded in thermally conductive relationship to the substrate. The operational amplifier is responsive to the output of the temperature sensor to apply power to the heating element and thereby maintain the temperature of the substrate within a predetermined temperature range. The transducer, heating element, and first component are monolithically formed on the substrate, and only three electrical connections are on the substrate at voltage to off-SAW die points.

Description:
BACKGROUND 
       [0001]    The present invention relates to temperature compensated surface acoustic wave (SAW) devices. 
         [0002]    SAW devices utilize the localized propagation of acoustic waves on the surface of a planar piezoelectric substrate. SAW transduction between electrical signals and acoustic waves is accomplished by thin film metallic interdigital electrodes on the substrate surface. SAW propagation velocity is temperature sensitive, but SAW devices must often work over a wide temperature range, so devices may be mounted in a custom oven to maintain a fixed temperature above the maximum ambient temperature. 
         [0003]    An oven comprises a device holder, heater, temperature sensor, feedback temperature controller such as operational amplifier, thermal insulation, and electrical connections between the device and ambient. An oven contains (and is thus larger than) the ovenized device and consumes significant power. 
         [0004]    One example of an attempt to provide more efficient temperature compensation for a SAW device, is described in U.S. Publication 2008/0055022A1. The SAW substrate is contained within a vacuum housing which in turn is within a packaging, and a heater is located on the housing or the bottom of the SAW substrate, opposite the acoustic propagation surface. Although a distinct oven around the packaging is avoided, the heater is still remote from the propagation surface of the SAW substrate. 
         [0005]    Pending U.S. application Ser. No. 13/065,177 for “Monolithically Applied Heating Elements on SAW Substrate” discloses a “micro-oven” technique for heating and preferably temperature sensing only the localized surface where the surface acoustic waves actually exist. The heater and preferably associated temperature sensor are realized as thin film metallic meander resistor electrodes on the substrate propagation surface, which can be deposited monolithically with the transducers and other functional features from the same photomask and photolithographic manufacturing process. One embodiment is directed to a surface SAW device comprising a substrate having a working surface with an active zone capable of propagating an acoustic wave, at least one interdigital transducer on the working surface, and a heating element on the working surface, adjacent to at least the active zone, wherein the transducer and heating element have the same material composition. 
         [0006]    Although this monolithic heating system represents a major improvement over previous systems, a significant amount of heat generated by the system is not applied to the SAW surface due to connection losses. 
       SUMMARY 
       [0007]    The present invention is directed to a further improvement in the temperature control of SAW devices. 
         [0008]    Although operational amplifiers are often used as part of the temperature controller, we have discovered a novel way of configuring the inputs to a commercially available operational amplifier for simpler and more accurate control. 
         [0009]    The invention is preferably used in conjunction with heating and sensing elements realized as thin film metallic meander resistor electrodes on the substrate propagation surface. Ideally, the goal is to improve the micro-oven control circuitry by (1) minimizing the number wire bonds to off-SAW die points in order to reduce total thermal loss from the SAW die; (2) keeping all power dissipating components on the SAW-die so that they heat the SAW die and thereby minimize total micro-oven power; and (3) minimizing off-mask component count to reduce complexity and cost, and allow the control circuitry to be incorporated compactly entirely inside the SAW device package. 
         [0010]    The circuit is a basic servo loop which strives to zero the bridge voltage across the operational amplifier input terminals. Conventionally, the temperature varying sensor resistors are incorporated in the SAW mask, while the other three resistors in the bridge circuit are fixed external resistors. With the present invention, the functions of the resistors are switched. The three other resistors of the bridge circuit are now temperature variable, incorporated in the SAW mask, and built of the same thin metal film as the rest of the SAW device. The set point resistor is now an off-mask temperature invariant resistor, either bonded to the SAW substrate or mounted externally so that the set-point temperature may be controlled by the user. Preferably, the operational amplifier is also directly carried on the SAW substrate so that its dissipation energy further heats the substrate. 
         [0011]    With the preferred embodiment, only three wire bonds are required from the SAW substrate and amplifier to realize the micro-oven control circuit, even with an external set point resistor. These three wire bonds are for the source or bias voltage, the set point resistor, and reference voltage (e.g., ground). 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
         [0012]      FIG. 1  is a schematic of a conventional SAW band pass filter in a case with external heaters and temperature control; 
           [0013]      FIG. 2  is a schematic of the main operative portion of a SAW band pass filter with monolithic heating and sensing elements on the SAW substrate; 
           [0014]      FIG. 3  is a schematic of the main operative portion of a one-port SAW resonator with monolithic heating elements and monolithic temperature sensor; 
           [0015]      FIG. 4  is a circuit schematic for a controller and associated contacts according to the present invention, that can be used for a variety of SAW devices such as the resonator shown in  FIG. 3 ; 
           [0016]      FIG. 5  is a schematic of a one-port SAW resonator with bond pad configuration that reduces heat loss in accordance with the present invention; and 
           [0017]      FIG. 6  is a circuit schematic for an alternative controller, for the resonator of  FIG. 5 . 
       
    
    
     DETAILED DESCRIPTION 
       [0018]      FIG. 1  represents a conventional band pass SAW filter  10  comprising a piezo electric crystal substrate  12  encapsulated by casing  14  with intervening air gap  16 . The working surface  18  of the substrate is capable of transmitting acoustic surface waves, which are induced by input electric-to-mechanical transducer  20  and received by output mechanical-to-electric transducer  22 . The transducers  20 ,  22  are aligned with an axis of the crystalline structure of substrate  12 , such that the transducer waveforms travel along such axis on an active zone  24  of the working surface  18 . 
         [0019]    A source  26  of electrical input signal is delivered to a plurality of electrically conductive interdigital transducer fingers  28 , which by means of a piezo electric effect, generate an acoustic wave response on the active zone  24  according to the designed filter wavelength frequency selectivity. The filtered mechanical signal is picked up by the interdigital fingers  30  of the output transducer  22 , and delivered to load  32 . Generally, the wire leads of the source  26  and load  32  are connected to respective bus conductors  34 ,  36  at enlarged bond pads  38 ,  40 . The fingers  28 ,  30  buses  34 ,  36  and pads  38 ,  40  are typically formed on the working surface monolithically  18  by any of a variety of well-known lithographic processes. 
         [0020]    In  FIG. 1 , the oven is provided by a plurality of heater elements  42  on the outside surface of the casing  14 . The heat must pass through the air space  16  where temperature gradients in the casing are reduced such that the hot air in contact with the lateral surfaces of the crystal  12  is of substantially uniform temperature. A source of power  44  is connected to the heating elements  42 , and temperatures sensors  46  and associated controller  48  provide a control heater control signal  50  to the power source  44 . 
         [0021]      FIG. 2  shows one embodiment  100  a band pass filter of the type show in  FIG. 1 , with monolithic heating elements. The piezo electric crystal  102  has substantially the same working surface  104 , input transducer  106 , output transducer  108 , buses (represented at  110 ), interdigital fingers (represented at  112 ) and active zone  114  (represented by a dashed rectangle) as described with respect to  FIG. 1 . The most significant difference is that the heating elements  116  A, B are provided on the working surface  104 . In  FIG. 2  (likewise  FIG. 1 ) the working surface  104  is substantially rectilinear with opposite input and output ends  118 ,  120  and opposite sides  122 , 124 . The transducers  106 ,  108  are adjacent the input and output ends  118 ,  120 , with the active zone  114  situated between and including the transducers  106 ,  108 . The heating elements  116 A and  116 B can be situated along side margins  126 A and  126 B of the working surface  104 , between the active zone  114  and the sides  122 ,  124  of the working surface  104  of the substrate  102 . Importantly, the heating elements  116  are in a much more intimate relationship with the active zone  114  than is possible with conventional ovens. 
         [0022]    The temperature sensor  128  is likewise in a more intimate relationship with the active zone. In  FIG. 2 , the sensor  128  is on the side margin  126 B of the working surface, between the heating element  116 B and the side  124  of the working surface  104 , but as will be described below, a plurality of sensor are preferably situated between the heating elements and the active zone. 
         [0023]    The heating elements  116  are formed monolithically with at least the transducers  106 ,  108 . The term “monolithic” when used herein should be understood as in the field of semi-conductor technology, i.e., formed on a single crystal substrate. Multiple photolithographic steps can be used. The heaters, sensors, and resonator/filter pattern can be added to the substrate in a single photolithographic step (lowest cost). Multiple steps can be used if the required parameters (e.g., heater resistance) cannot be obtained in one step. This can still be considered monolithic. The invention can also be implemented with so-called “hybrid” features that are formed outside the substrate and then attached to the substrate. 
         [0024]      FIG. 3  is a schematic of the main operative portion of a one-port SAW resonator  200  with monolithic heater and temperature sensor. The substrate  202  has a central transducer  204  formed thereon, with alternating fingers, some of which are connected to bus  206 A and the others connected to bus  206 B. A first reflector grating  208 A is situated on one side of the transducer  204 , and another reflector grating  208 B is situated on the other side of the transducer  204 . A plurality of heaters  210 A, B, C, and D, are on the working surface between the gratings  208 A,  208 B and the edges of the substrate. A plurality of temperature sensors  212  A, B, C, and D are located on the working surface, respectively between each heater  210  A, B, C, and D, and the boundary (such as  208 ′) of each of the reflector gratings. At least one bond pad  214  is provided for the heaters, and at least one bond pad  216  is provided for the sensors. It should be appreciated that a bond pad can be shared, e.g., one sensor pad may be shared with one transducer bus. Conductive pads  218  between one or more heaters and conductive pads  220  between one or more sensors can be provided in a known manner. 
         [0025]    The transducer  204 , heaters  210 , and sensors  212  and preferably the respective transducer buses  206 , bond pads  214  for the heaters, and bond pads  216  for the sensors, are all monolithic with the substrate  202 . The location of the heaters  214  on the substrate close to the grating  208  provides a substantially uniform temperature at the active zone, and the location of the sensors  212  on the substrate  212  immediately adjacent to the grating  208  provides a more accurate measure of the temperature in the active zone. Furthermore, a plurality of sensors with an associated plurality of heaters, coupled to a control system that compares the outputs of four sensors, can be used to adjust the current differential to each heater for achieving uniformity in the temperature of the active zone. 
         [0026]    According to one embodiment of the present invention, a temperature controller for a band pass filter such as shown in  FIG. 2  or for a resonator such as shown in  FIG. 3 , can take the form of the schematic shown in  FIG. 4 , for improving the thermal efficiency of the micro-oven, by dissipating much of the heat associated with the resistors and controller amplifier, to the substrate rather than losing the heat through wire bonds to external components. The controller is carried on the SAW substrate. 
         [0027]    Resistors R 1 , R 2 , and R 3  may be considered as the temperature sensing elements, whereas resistor RS is a set point resistor which is preferably adjustable by the end user. The resistor R 1  and set point resistor RS form a first voltage divider and resistors R 2  and R 3  form a second voltage divider. Resistors R 1  and R 2  are connected at common node N 1  that is maintained with a bias voltage from an exterior source Vcc. The plus (+) terminal of the operational amplifier U 1  is connected to the common node N 2  of R 1  and RS. Similarly, the input to the minus (−) terminal of the operational amplifier is connected to the common node N 3  of R 2  and R 3 . Whenever a voltage difference is present between the plus (+) and minus (−) terminals of the operational amplifier, a net voltage commensurate with that difference is applied to the variable heating resistor RH. Heating resistor RH is arranged in a feedback loop from the output of the amplifier U 1  to the source voltage node Vcc. In a conventional manner, the amplifier is powered by an input connection to the voltage source node Vcc and is connected to ground. The resistor R 3  and the set point resistor RS are also connected to ground. It should be appreciated that R 1 , R 2 , and RH are on the SAW mask, and the operational amplifier U 1  is epoxy bonded to the SAW substrate. 
         [0028]    The operational effect of this circuit relies on the nominal values of all the resistors being equal, so the midpoint of each voltage divider should have an equivalent voltage. The midpoints of these voltage dividers are connected to the inverting and non-inverting inputs of the operational amplifier U 1 . However, R 1 , R 2  and R 3  are temperature variable and arranged on the substrate but RS is temperature invariable and/or arranged remote from the substrate. Since equivalent resistors R 2  and R 3  are positioned on the substrate, the midpoint of this voltage divider will not vary with the temperature of the substrate. The midpoint of the voltage divider R 1  and RS will vary with the temperature of the substrate, since R 1  is temperature variant and positioned on the substrate. As the temperature of the substrate changes, the inputs to the operational amplifier will change in a corresponding manner, causing feedback through RH to adjust the temperature of the substrate until the inputs to the operational amplifier balance again. RS is preferably adjustable, which permits the user to set the target temperature the controller will maintain on the substrate. 
         [0029]    In this embodiment, the three connections to the SAW substrate are VCC, ground, and RS (where it connects to R 1 , if RS is not positioned on the substrate). If one seeks to minimize connections to the substrate, only one ground connection should be used. Ground can be common to all those components, so only one ground connection to the chip is needed. It can be appreciated that from a hardware perspective, the only wire bonds that extend from the substrate are for the voltage source node Vcc, a common ground connection for resistor R 3  and the amplifier U 1 , and a ground for the set point resistor RS. The set point resistor RS can itself be off the substrate and adjustable by the user. 
         [0030]      FIGS. 5 and 6  show a one port SAW resonator and alternative temperature controller, labeled to show a direct correspondence of the electrical connections. The heater element  1  and temperature sensor  2  are deposited on the substrate surface. The bond pads for the heater elements are shown as  3   a  for the voltage and  3   b  for the return (heater control output line of the operational amplifier). The bond pads for the temperature sensor element (resistor component) are shown at  4   a  for the voltage tap and  4   b  for ground. The bond pads for the sensor voltage divider are shown at  5   a  for the voltage and  5   b  for ground. 
         [0031]    It can this be seen that only three electrical connections at voltage are needed on the substrate ( 3   a ,  4   a , and  5   a ) to off-SAW die points, for providing the function of temperature control. 
         [0032]    The controller circuits shown in  FIGS. 4 and 6  are equivalent. In  FIG. 4 , (i) the first, temperature dependent resistor R 1  is connected at its output end to input node N 1 ; (ii) the second, temperature invariant set point resistor RS is connected at its input end to the node N 1  and to ground; (iii) the third, temperature dependent resistor R 2  is connected at its output end to the other input node N 2 ; and (iv) the fourth, temperature dependent resistor R 3  is connected at its input end to the other node N 2  and ground. In  FIG. 6 , (i) the first, temperature dependent resistor R 1  is connected at its input end to input node N 1  and to ground; (ii) the second, temperature invariant set point resistor RS is connected at its output end to node N 1 ; (iii) the third, temperature dependent resistor R 2  is connected at its output end to the other input node N 2 ; and (iv) the fourth, temperature dependent resistor R 3  is connected at its input end to the other node N 2  and to ground.