Abstract:
A system and method in accordance with an embodiment reduces the cross-axis sensitivity of a gyroscope. This is achieved by building a gyroscope using a mechanical transducer that comprises a spring system that is less sensitive to fabrication imperfection and optimized to minimize the response to the rotations other than the intended input rotation axis. The longitudinal axes of the first and second flexible elements are parallel to each other and parallel to the first direction

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
       [0001]    Under 35 U.S.C. 120, this application is a Divisional Application and claims priority to U.S. patent application Ser. No. 13/361,261, filed on Jan. 20, 2012, entitled “MEMS DEVICE WITH IMPROVED SPRING SYSTEM,” which claims priority to U.S. provisional application Ser. No. 61/553,031 filed on Oct. 28, 2011, entitled “MEMS GYROSCOPE WITH IMPROVED SPRING SYSTEM”, all of which are incorporated herein by reference in their entireties. 
     
    
     FIELD OF THE INVENTION  
       [0002]    The present invention relates generally to MEMS devices and more particularly to springs utilized in such devices. 
       BACKGROUND OF THE INVENTION  
       [0003]    A gyroscope is a sensor that measures angular velocity about a sensitive axis. An ideal yaw gyroscope is sensitive to angular velocity about the Z-axis, which is normal to a plane of the sensor. Ideal pitch and roll gyroscopes are sensitive to angular velocities about the X-axis and the Y-axis which are orthogonal in the plane of the sensor. Ideally, all three gyroscope sensitive axes are mutually orthogonal. 
         [0004]    Fabrication imperfections can cause the sensitive axis to deviate from the ideal input axis. For example a yaw gyroscope, which responds to angular velocity about the Z-axis, can also respond to angular velocity about the X-axis and/or the Y-axis. 
         [0005]    Cross-axis sensitivity is a measure of the gyroscope sensitivity to angular velocity about an axis orthogonal to the intended input axes. Cross-axis sensitivity causes errors in a system using a gyroscope because the gyroscope responds to angular velocity about an axis other than the intended axis. For example, if the yaw gyroscope has cross-axis sensitivity, it would be responsive to the angular velocity around the X-axis and/or the Y-axis. Hence, the output of the yaw gyroscope would show a response as if there is a Z axis angular velocity although the actual angular velocity is around the X-axis and/or the Y-axis. Correcting the errors caused by cross-axis sensitivity requires calibration of each gyroscope, which is costly. 
         [0006]    MEMS gyroscopes are typically fabricated from silicon. The silicon layer is etched using deep reactive ion etching (DRIE). The gyroscopes are formed using batch fabrication, which means several thousand gyroscopes are formed in the single etch step. Gyroscopes using conventional springs are more responsive to the cross-axis sensitivity because conventional springs couple in-plane motion to out-of-plane motion due to fabrication errors. The challenge is to produce a high accuracy gyroscope with high yield and small size to maintain low cost. The present invention addresses such a need. 
       SUMMARY OF THE INVENTION  
       [0007]    A system and method in accordance with an embodiment reduces the cross-axis sensitivity of a MEMS force sensor such as a gyroscope. This is achieved by building a gyroscope using a mechanical transducer that comprises a spring system that is less sensitive to fabrication imperfection and optimized to minimize the response to the rotations other than the intended input rotation axis. A key feature of the present invention is a spring system that is less sensitive to the fabrication imperfections caused by the non-idealities in the deep reactive etching process which is used to manufacture high aspect ratio micro gyroscopes and other force sensors. 
         [0008]    The spring system minimizes coupling of the in-plane motion to out-of plane motion, which is due to the non-ideal (non vertical) cross section of the springs caused by fabrication imperfections. In-plane to out-of plane coupling is the main cause of cross axis sensitivity, in which a gyroscope for example responds to angular velocity about axes other than the intended sensitive axis. 
         [0009]    Gyroscopes using conventional springs are more responsive to the cross-axis sensitivity because conventional springs couple in-plane motion to out-of-plane motion due to fabrication errors. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         [0010]      FIG. 1  illustrates an embodiment of a conventional micro machined yaw gyroscope. 
           [0011]      FIG. 2A  illustrates the cross-section of a cantilever beam with an ideal cross-section. 
           [0012]      FIG. 2B  illustrates the cross-section of a cantilever beam with a non-ideal or non-vertical cross-section. 
           [0013]      FIGS. 3A and 3B  illustrate a common spring system. 
           [0014]      FIGS. 4A and 4B  illustrate a spring system in accordance with an embodiment. 
           [0015]      FIG. 5  illustrates a second embodiment of a spring system. 
           [0016]      FIG. 6  illustrates a third embodiment of the spring system. 
           [0017]      FIG. 7  illustrates a yaw gyroscope in accordance with an embodiment. 
           [0018]      FIG. 8  illustrates a second embodiment of a yaw gyroscope in accordance with an embodiment. 
       
    
    
     DETAILED DESCRIPTION  
       [0019]    The present invention relates generally to MEMS devices and more particularly to springs utilized in such devices. The following description is presented to enable one of ordinary skill in the art to make and use the invention and is provided in the context of a patent application and its requirements. Various modifications to the preferred embodiment and the generic principles and features described herein will be readily apparent to those skilled in the art. Thus, the present invention is not intended to be limited to the embodiment shown but is to be accorded the widest scope consistent with the principles and features described herein. 
         [0020]      FIG. 1  illustrates an embodiment of a conventional micro machined yaw gyroscope  10 . The yaw gyroscope  10  is comprised of a drive frame  11 , a proof mass  12 , drive springs  13   a - 13   d,  sense springs  14   a  and  14   b,  anchors  15   a  and  15   b,  transducers  16   a  and  16   b,  and electrostatic drive combs  17   a - 17   d.  The yaw gyroscope  10  is suspended over and parallel to a substrate  18 . The drive frame  11  is supported by the drive springs  13   a - 13   d  each of which extends from the support anchors  15   a  and  15   b  attached to the substrate  18 . The gyroscope  10  includes a proof mass  12  which is attached to the drive frame  11  by the sense springs  14   a  and  14   b.    
         [0021]    The drive frame  11  and the proof mass  12  are driven into oscillation in an X-direction in a plane by the drive comb structures  17   a - d  which are coupled to a alternating voltage source (not shown) and generate alternating electrostatic forces in the plane. The proof mass  12  is typically oscillated at a frequency of between 10 kHz to 40 kHz. In an embodiment, the frequency is greater than 20 kHz. Rotating the yaw gyroscope  10  with an angular velocity can impart a Coriolis force to the oscillating proof mass  12 . The Coriolis force is proportional to the angular velocity and depends on the orientation of the axis of the angular velocity with respect to the oscillation direction of the proof mass. The Coriolis force, the angular velocity, and the oscillation direction of the proof mass are mutually orthogonal. 
         [0022]    In the yaw gyroscope  10 , angular velocity about the Z-axis imparts a Coriolis force in the Y-direction on the proof mass  12  oscillating in the X-direction. The Coriolis force in the Y-direction imparted to the proof mass  12  is sensed by measuring the motion of the proof mass  12  in the Y-direction by the use of the transducers  16   a  and  16   b.  The transducers  16   a  and  16   b  may be electrodes that form capacitances to the proof-mass  12 , wherein the capacitances change as a result of the proof-mass motion. 
         [0023]    In the yaw gyroscope  10 , angular velocity about the Y-axis imparts a Coriolis force in the Z-direction on the proof mass  12  oscillating in the X-direction. In an ideal yaw gyroscope  10 , the motion generated by the Coriolis force in the Z-direction imparted on the proof mass  12  is usually insignificant compared to the motion generated by the Coriolis force in the Y-direction because the out-of-plane stiffness of the sense springs is usually much bigger than the in-plane stiffness of the sense springs  14   a  and  14   b.  The difference between the in-plane and out-of plane stiffness is achieved by increasing the thickness (H) to width (W) ratio (aspect ratio) of the sense springs  14   a  and  14   b.  In other words, the springs  14   a  and  14   b  etched with high aspect ratio can provide the needed difference between the in-plane and out-of plane stiffness to minimize the motion of the proof mass  12  in the Z direction. 
         [0024]    In bulk micromachining, high aspect ratio structures can be generated with the use of deep reactive etching process (DRIE). DRIE can provide a thickness to width ratio greater than 20:1 for single crystal silicon structures. However, although DRIE is a good process to manufacture high aspect ratio structures, it includes some non-idealities. One of the most important drawbacks of the DRIE is the non-vertical or tilted cross sections generated during the etching process. The tilt angle of the sidewall of the cross section is generally called the showerhead angle. Non-vertical cross sections occur due to the non-uniform distribution of the etchants among the wafer during the DRIE process. 
         [0025]    The effect of the showerhead angle on the flexure elements in the microstructures can be explained by cantilever beams.  FIG. 2A  illustrates the cross-section of a cantilever beam with an ideal cross-section  20 .  FIG. 2B  illustrates the cross-section of a cantilever beam  20 ′ with a non-ideal or non-vertical cross-section. 
         [0026]    If a force is applied in a direction to the cantilever beam  20  with the ideal cross section, the beam  20  purely deflects in the direction of the force. However, due to the non-idealities of the micro machining process the cross section of the cantilever beam  20 ′ can be non-vertical as shown in  FIG. 2B . 
         [0027]    A cantilever beam  20  with non-ideal cross section has different characteristics than the beam with the ideal cross section. If a force (Fi) is applied to the cantilever beam  20 , as shown in  FIG. 2A , the beam  20  will deflect purely in the X direction. However, if a force applied to the non-ideal beam  20 ′, as shown in  FIG. 2B , the beam will tend to deflect in the i 1  direction because the compliant axis of the beam  20 ′ is not parallel to the X-Y plane. In response to a force in the X-direction, the deflection of the beam  20 ′ will be both in the X-direction and the Z-direction. Similarly if a force is applied in the Z-direction to the beam  20 ′, the beam  20 ′ will deflect both in X- and Z-directions. 
         [0028]    In micro mechanisms various types of beams or spring systems have been used to provide compliance to the mass that they are attached. One of the common spring systems that have been used in the microstructures is shown in  FIG. 3A . The spring system  25  that is shown in  FIG. 3A  is comprised of two flexible elements  26   a  and  26   b  and one rigid element  27 . In this spring system  25 , the deflection of the flexible elements  26   a  and  26   b  is minimized by the added rigid element. When a force  28  is applied to the spring system  25 , the flexible elements  26   a  and  26   b  deflect and the rigid element rotates around the Z-axis as shown in  FIG. 3B . This type of spring system  25  ideally deflects in the direction of the force applied as the cantilever beams given previously. However, due to the non-idealities in the etching process the cross section of the flexible elements  26   a  and  26   b  of the spring system  25  can be non-vertical as shown in  FIG. 2B . Hence, the displacement of the spring system  25  will also deviate from the ideal condition. For example, if an in-plane force is applied, in the Y-direction, to this spring system  25  with non-ideal flexible element cross section, the resulting deflection of the spring system  25  will be both in the Y-direction and the Z-direction. 
         [0029]    Microstructures are generally built to be used as sensors like accelerometers, gyroscopes, compass etc. The basic principle behind the microstructures is usually based on sensing the externally applied forces. In such microstructures, externally applied force is converted to deflection and the deflection is sensed by various types of transducers. Consequently, ideal force input to deflection output is crucial in order to build a sensor with good performance. In other words, it is desired to have a resulting deflection solely in the direction of the force applied. If the externally applied force generates a deflection in the unintended direction of sensing, the sensor can have a degraded response, can give erroneous results, or can have cross-axis sensitivity. 
         [0030]    Specifically, deviation of the intended motion direction from the applied force direction may cause problems in the conventional gyroscopes. One of the main problems is named as cross-axis sensitivity. Cross-axis sensitivity is a measure of the undesired response of a sensor other than the intended axis of measurement for angular velocity sensors and accelerometers. It is the erroneous measurement of the sensor as a result of the imperfections of the sensing transducer. 
         [0031]    An ideal micromachined gyroscope, which does not have any cross-axis sensitivity, will only respond to the intended input rotation axis. If a micromachined gyroscope does not accurately reflect the intended input rotation axis, measurements of the gyroscope will be erroneous. 
         [0032]    If the non-idealities like non-vertical cross-section, shown in  FIG. 2A , is present on the drive springs  13   a - 13   d  or the sense springs  14   a  and  14   b  of a conventional yaw gyroscope  10 , shown in  FIG. 1 , the drive frame  11  or proof-mass  12  will move both in the intended direction and partially in an unintended direction. There will be two different effects on the operation of the gyroscope depending on which of the springs have a non-vertical cross-section. 
         [0033]    If the drive springs  13   a - 13   d  have a non-vertical cross section, they will tend to deflect both in X-direction and Z-direction during the motion of the drive frame  11  which is actuated by the comb structures  17   a - 17   d.  The effect of the non-vertical cross-section is that the proof-mass  12  oscillates both in the X-direction and partially in the Z-direction instead of oscillating only in the X-direction as with the ideal cross-section of the drive springs  13   a - d.    
         [0034]    When the proof mass  12  oscillates in the X-direction, angular velocity about the Z-axis, causes a Coriolis force in the Y-direction. The Coriolis force in the Y-direction causes the proof-mass  12  to move in the Y-direction which is measured by the transducers  16   a  and  16   b.  However, the proof mass  12  oscillates not only in X-direction but also in the Z-direction due to the non-vertical cross-section of the drive springs  13   a - d.  Thus, angular velocity about the X-axis will also cause a Coriolis force in the Y-direction. Hence, the proof-mass  12  will move in the Y-direction in response to angular velocity about the Z-axis and the X-axis, causing the yaw gyroscope  10  to have cross-axis sensitivity. 
         [0035]    If the sense springs  14   a  and  14   b  have a non-vertical cross section, they will tend to deflect both in the Y-direction and the Z-direction due to a Coriolis force acting on the proof-mass  12 . 
         [0036]    When the proof mass  12  oscillates in the X-direction, angular velocity about the Y-axis, causes a Coriolis force in the Z-direction. In the ideal case, the Coriolis force in the Z-direction causes the proof-mass  12  to move only in the Z-direction but not in the Y-direction, so the transducers  16   a  and  16   b  do not respond to angular velocity about the Y-axis. However, due to non-vertical cross-section of the sense springs  14   a  and  14   b,  the Z-directed Coriolis force causes the proof-mass  12  to move both in the Z-direction and the Y-direction. The proof-mass  12  motion in the Y-direction will be detected by the transducers  16   a  and  16   b.  Hence the yaw gyroscope  10  will have cross-axis sensitivity because it responds to angular velocity about the Z-axis and the Y-axis. 
         [0037]    Pitch gyroscopes and roll gyroscopes sense angular velocity about an axes in the plane of the gyroscope. Pitch or roll gyroscopes may comprise a proof-mass oscillating in the plane and a transducer that senses out-of-plane motion of the proof-mass resulting from Coriolis forces in the Z-direction. Similar to yaw gyroscopes, non-vertical cross-section may also cause cross-axis sensitivity in pitch or roll gyroscopes due to the coupling of in-plane to out-of plane motion. 
         [0038]    The forces applied to the cantilever beam  20  of  FIG. 2 , the spring system  25  in  FIG. 3A , and to the drive springs  13   a - 13   d  and sense springs  14   a  and  14   b  of the conventional gyroscope  10  are transverse to the longitudinal axis of the flexible elements. Since the force applied is transverse to the longitudinal-axis, the deflections of the flexible elements are due to bending. If the flexible element has a non-vertical cross-section, the bending will cause deflections not only in the direction of the force applied but also in a direction orthogonal to the intended axis of motion which causes undesired motion of the structures coupled to the spring system. 
         [0039]      FIG. 4A  illustrates a spring system  30  in accordance with an embodiment. The spring system  30  comprises a first flexible element  32   a  coupled to a rigid element  34 . The rigid element  34  in turn is coupled to a second flexible  32   b  element. The first flexible element  32   a  is anchored, but in other embodiments may be connected to other mechanical structures. The longitudinal axes of the first and second flexible elements  32   a  and  32   b  are parallel to each other and parallel to the intended direction of motion. The midpoint  35  of the first flexible element and the midpoint  36  of the second flexible element are aligned along the axis  37  perpendicular to the intended direction of displacement. When a force  38  acts along the axial direction, or longitudinal-axis, of the flexible elements  32   a  and  32   b,  spring system deflects in the intended direction of displacement The force  38  causes both flexible elements  32   a  and  32   b  to bend in the plane and the rigid element  34  acts as a lever arm to increase the deflection as shown in  FIG. 4B . 
         [0040]    If a force is applied to the spring system  30 , the flexible elements  32   a  and  32   b  will bend in-plane and the spring system  30  will deflect in the Y-direction as shown in  FIG. 4B . If the flexible elements  32   a  and  32   b  have a non-vertical cross-section, the flexible elements  32   a  and  32   b  will bend in-plane and bend out-of-plane. The out-of-plane bending will cause the rigid element  34  to rotate about the X-axis. If both flexible elements  32   a  and  32   b  have the same cross-section, for example due to the both elements  32   a  and  32   b  having the same angle of the etching, the Z-axis displacement at the end of the spring system  30  is significantly reduced compared to the conventional spring systems with non-vertical cross-sections. The out-of-plane displacement is approximately proportional to the distance between the midpoint of the flexible elements  32   a  and  32   b  in the Y-direction. Thus the spring system  30  deflects in the direction of the in-plane force with substantially no deflection out-of-plane, which can be used to reduce cross-axis sensitivity. 
         [0041]    Two alternate embodiments of the low cross-axis spring system are shown in  FIG. 5  and  FIG. 6 .  FIG. 5  illustrates a second embodiment of the spring system. This embodiment has all the same features of the first embodiment, but the shape of the rigid element  34 ′ is different and the flexible elements  32   a ′ and  32   b ′ connect to the same side of the rigid element. This embodiment operates in a similar manner to the first embodiment and similarly minimizes cross-axis coupling. 
         [0042]      FIG. 6  illustrates a third embodiment of a spring system  50 . The spring system  50  comprises first, second, and third flexible elements  52   a - 52   c  coupled to a rigid element  54 . The second flexible element  52   b  is anchored, but in other embodiments may also be connected to other mechanical structures. Similar to the first embodiment, the longitudinal axes of the first, second and third flexible elements  52   a - 52   c  are parallel to each other and parallel to the intended direction of motion. The midpoints of all three flexible elements  52   a - 52   c  should be substantially aligned along an axis perpendicular to the direction of motion  58 . Opposing forces are applied along the axial direction, or longitudinal-axis, of the first and third flexible elements  52   a  and  52   c.  The forces cause all flexible elements to bend in the plane. The rigid element  54  causes the endpoints of the first and third flexible elements  52   a  and  52   c  to move in opposite directions. This spring system  50  provides for two points to move in opposite directions with low cross-axis coupling. 
         [0043]    The spring systems introduced in  FIGS. 4A ,  4 B,  5  and  6  are advantageous in sensors, such as accelerometers, gyroscopes, magnetometers, force sensors, etc., that comprise a mechanical element that moves in response to a force and a transducer for measuring the deflection of the mechanical element. The cross-axis sensitivity of such sensors is reduced by using the low-cross axis spring system. 
         [0044]      FIG. 7  illustrates a yaw gyroscope  100  in accordance with an embodiment. The yaw gyroscope  100  comprises a guided mass system  190  which includes the spring systems  110   a  and  110   b,  drive mass  140  and proof mass  150  on a substrate  101 . Spring systems  110   a  and  110   b  comprise the flexible elements  103   a - b  and  105   a - b,  and the rigid lever arms  104   a  and  104   b.  The gyroscope  100  is supported by the flexible elements  105   a  and  105   b  each of which extends from a support anchor  102  attached to a substrate  101 . Spring systems  110   a  and  110   b  are coupled to the drive mass  140  by the flexible elements  103   a  and  103   b.  The proof-mass  150  is flexibly connected to the drive mass  140  via sense spring systems  180   a - 180   d.  Sense spring systems  180   a - 180   d  are stiff in the X-direction such that when the guided mass system  190  is driven, the proof mass  150  also translates with the drive mass  140  in the X-direction. 
         [0045]    The guided mass system  190  is driven into oscillation by the electrostatic drive comb structures  160   a  and  160   b  which are coupled to an alternating voltage source. Similar comb structures (not shown) may be capacitive sensors that are transducers for measuring the motion of the drive mass  140 . When the guided mass system is oscillating, the drive mass  140  and the proof-mass  150  oscillate in the X-direction. Angular velocity about a Z-axis will cause a Coriolis force to act on the proof mass  150  in the Y-direction resulting in motion of the proof mass  150  in the Y-direction. A transducer  170  is used to sense the motion of the proof mass  150  in the Y-direction which provides a measure of the angular velocity about the Z-input axis. The transducer  170  may be an electrode that forms a capacitance to the proof-mass  150 , wherein the capacitance changes as a result of the proof-mass motion. 
         [0046]    Imperfections in the fabrication process can cause the flexible elements  103   a  and  103   b,    105   a  and  105   b  to have non vertical cross sections in the gyroscope  100  configuration shown in  FIG. 7 . Even if the flexible elements  103   a  and  103   b,    105   a  and  105   b  have a non-vertical cross section, the drive mass  140  and the proof-mass  150  will move in the X-direction and the out-of plane motion is minimized due to spring system  110   a  and  110   b.  Consequently, the non-ideal response of the yaw gyroscope  100  to the angular velocity about the X-axis, is reduced compared to gyroscopes using conventional springs. The yaw gyroscope  100  will not respond to the angular velocity about the X axis and will not have cross-axis sensitivity 
         [0047]    Since the yaw gyroscope  100  given in  FIG. 7  is oscillated along the X-axis during the drive motion, angular velocity about the Y-axis, causes a Coriolis force in the Z-direction on the proof mass  150 . Even if the flexible elements of the spring systems  180   a - 180   d  have a non-vertical cross section, the Coriolis force in the Z-direction causes the proof-mass to move only in the Z-direction but not in the Y-direction since the spring system  180   a - 180   d  prevents the coupling of forces in the Z-direction to motion in the Y-direction. In this manner, the yaw gyroscope  100  will not respond to the angular rate about the Y axis and will not have cross-axis sensitivity. 
         [0048]      FIG. 8  illustrates a second embodiment of a yaw gyroscope comprising a guided mass system  200  in accordance with the present invention. The guided mass system  200  comprises a symmetric guided mass system  201  which includes spring systems  210   a  and  210   b,  drive masses  140   a  and  140   b,  and proof masses  150   a  and  150   b.  Spring systems  210   a  and  210   b  comprise the drive flexures  105   a  and  105   b,    103   a - 103   d  and the rigid lever arms  204   a  and  204   b.  Spring systems  210   a  and  210   b  are coupled to the drive masses  140   a  and  140   b  by the springs  103   a,    103   c  and  103   b,    103   d,  respectively. The proof-masses  150   a  and  150   b  are flexibly connected to the drive masses  140   a  and  140   b  via sense springs  180   a - 180   d  and  180   e - 180   h,  respectively. When the guided mass system  200  is driven into oscillation by the AC voltage coupled to the comb drive structures  160   a - 160   d,  the drive masses  140   a  and  140   b  and the proof masses  150   a  and  150   b  translate anti-phase in the X-direction. 
         [0049]    Angular velocity about the Z-input axis will cause Coriolis forces to act on the proof masses  150   a  and  150   b  resulting in motion of the proof masses  150   a  and  150   b  anti-phase along the Y-direction. The amplitude of the motion of the proof masses along the Y-direction is proportional to the angular velocity. Transducers  170   a  and  170   b  are used to sense the motion of the respective proof masses  150   a  and  150   b  along the Y-direction. 
         [0050]    Similar to the previous embodiment, spring systems  210   a  and  210   b  reduce the cross-axis coupling between X-direction and Z-direction deflections. Hence even if the flexible elements have a non-vertical cross-section, when the guided mass system is driven into oscillation, drive masses  140   a  and  140   b  and the proof-masses  150   a  and  150   b  will oscillate only in the X-direction and unlike the conventional gyroscope the masses will not oscillate in the Z-direction. Consequently, the non-ideal response of the yaw gyroscope to the angular velocity about the X-axis, is reduced compared to gyroscopes using conventional springs. 
         [0051]    Similar to the previous embodiment, spring systems  180   a - 180   h  prevent Z-directed Coriolis forces from causing Y-direction motion of proof-masses  150   a  and  150   b.  Hence, the yaw gyroscope will not respond to the angular rate about the Y axis and will not have cross-axis sensitivity. 
         [0052]    Although the present invention has been described in accordance with the embodiments shown, one of ordinary skill in the art will readily recognize that there could be variations to the embodiments and those variations would be within the spirit and scope of the present invention. Accordingly, many modifications may be made by one of ordinary skill in the art without departing from the spirit and scope of the appended claims.