Abstract:
Most mechanical tests (compression testing, tensile testing, flexure testing, shear testing) of samples in the sub-mm size scale are performed under the observation with an optical microscope or a scanning electron microscope. However, the following problems exist with prior art force sensors as e.g they cannot be used for in-plane mechanical testing (a- and b-direction) of a sample; they cannot be used for vertical testing (c-direction) of a sample. In order to overcome the before mentioned drawbacks the invention comprises the following basic working principle: A force is applied to the probe ( 2 ) at the probe tip ( 1 ) of the sensor. The force is transmitted by the sensor probe ( 2 ) to the movable body ( 3 ) of the sensor. The movable body is elastically suspended by four folded flexures ( 4 ), which transduce the force into a deflection dx. This deflection is measured by an array of capacitor electrodes, called capacitive comb drive ( 6 ).

Description:
[0001]    The present invention relates to a capacitive micro force sensor according to claim  1 . 
       BACKGROUND 
       [0002]    Multiple methods for measuring force from the nanonewton (10 −9  N) to millinewton (10 −3  N) range exist such as atomic force microscopes, microscales, piezoresistive cantilevers and capacitive force sensors. These systems have been successfully used in different application fields including material science, microsystem development, nanotechnology, biological research, medical research, thin film characterization and quality control of miniaturized systems. Using these sensors in combination with a precise positioning device such as a micromanipulator, allows building compression and tensile testing systems for mechanical testing at the microscale. 
         [0003]    Capacitance is a measure of the electrical charge between two conductors separated by an air gap. A load applied to the sensor causes a deflection. As the conductors are moved closer to or farther from one another, the air gap changes, and so does the capacitance. The principle of capacitive micro force sensing is simple and effective and features an excellent sensitivity. MEMS fabrication technology allows the efficient fabrication of such sensors [1]. Due to the single-crystalline silicon structure of the sensor, the results are highly repeatable and the sensors are less likely to degrade over time. Capacitive MEMS force sensor designs are detailed in [1-3] and provided as a commercial product [4]. 
       Problems to be Solved 
       [0004]    Most mechanical tests (compression testing, tensile testing, flexure testing, shear testing) of samples in the sub-mm size scale are performed under the observation with an optical microscope or a scanning electron microscope as shown in  FIG. 4   a.  Capacitive MEMS sensors have shown their suitability for small scale metrology and their design as described in [1, 2]. However, the following problems exist with prior art force sensors  18 :
       a) Prior art sensors  18  cannot be used for in-plane mechanical testing (a- and b-direction) of the sample  14 . The reason is that when aligning the sensitive direction  22  horizontally (a-b plane), the prior art sensors  18  will touch the sample holder  13 , making the measurement impossible. The contact point  16  is shown in  FIG. 4   a.      b) Prior art sensors  18  cannot be used for vertical testing (c-direction) of the sample  14 . The reason is that when aligning the sensitive direction  22  vertically (c-direction), the prior art sensors  18  will touch the microscope lens  12 , making the measurement impossible. The contact point  16  is shown in  FIG. 4   a.      c) Prior art sensors may be used in combination with a long range stereomicroscope. However, the alignment of the prior art sensor  18  relative to the sample  14  is difficult, since the sensor will cover large parts of the field of view of the microscope. An observation of the sample  14  during the measurement is not possible. This covering of the line of sight  15  inhibits the observation of the sample  14  during testing. However, the visual information during the test is of great importance to observe physical processes such as crack propagation, viscoelasticity, structure failure or plastic deformation.   d) It is possible to perform measurements using existing force sensors  18  at an angle β without touching the microscope lens  12  or the sample holder  13  as shown in  FIG. 4   a ). However, the measurement data is difficult to interpret since the force components in a-, b- and c-direction cannot be measured directly and independently.       
 
       PRIOR ART 
       [0009]    Documents [1, 2] describe a comb drive based capacitive sensor design featuring a sensor probe which is overhanging the substrate on one side. The direction in which the prior art force sensor  18  is sensitive is parallel to the sensor probe as shown in  FIG. 4   a ). This design is suitable for the mechanical testing of a sample  14  under the microscope lens  12  at an angle β. However, measurements in horizontal (a-b-plane) or vertical direction (c-axis) are often times not possible due to the shape of the substrate and the sensor chip geometry. For some applications capacitive multi-axis force sensors according to [3,7] are used to have a higher flexibility and decompose force vectors into the a-, b- and c-axis components. However, multi-axis force sensors are difficult to calibrate at the sub-Millinewton range and therefore highly expensive. 
         [0010]    Prior art sensor designs described in [1]-[9] do not feature a probe ( 2 ) which is overhanging the substrate ( 7 ) on two sides ( 8 , 9 ), limiting the number of application for which the sensor can be used due to geometrical limitations. 
         [0011]    Simply mounting the prior art MEMS sensor chips rotated such that the sensor probe  2  is overhanging the substrate  7  one side does not solve the problem since the substrate  7  and the whole MEMS sensor chip are inside the line of sight  15  and are therefore blocking a large part of the microscope view. Also, this configuration is not suitable since the substrate touches the microscope lens  12  making the measurement impossible. 
     
    
     
       DETAILED DESCRIPTION 
         [0012]    For a more complete understanding of the present disclosure, and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which: 
           [0013]      FIG. 1  depicts a capacitive MEMS sensor in a schematic view; 
           [0014]      FIG. 2   a  depicts a depicts a prior art sensor by Sun Yu et al.; 
           [0015]      FIG. 2   b  depicts a capacitive MEMS sensor with some geometrical indications; 
           [0016]      FIG. 3  depicts the electrical and mechanical sensor buildup; 
           [0017]      FIG. 4   a  depicts the mechanical testing using prior art sensors; 
           [0018]      FIG. 4   b  depicts the mechanical testing using the sensor design according to the invention; 
           [0019]      FIG. 5   a  depicts a prior art sensor; 
           [0020]      FIG. 5   b  depicts an embodiment of the invention; 
           [0021]      FIG. 5   c  depicts an alternative embodiment of the invention. 
       
    
    
       [0022]      FIG. 1  shows a schematic view of the force sensor design.  FIG. 3  shows the electrical and mechanical buildup. The basic working principle is the following: A force is applied to the probe  2  at the probe tip  1  of the sensor. The force is transmitted by the sensor probe  2  to the movable body  3  of the sensor. The movable body  3  is elastically suspended by four folded flexures  4 , which transduce the force into a deflection dx. This deflection dx is measured by an array of capacitor electrodes, called capacitive comb drive  6 . A configuration of two comb drives  6  may be used for differential measurements. The restoring force Fr for the folded flexure design is given by 
         [0000]    
       
         
           
             
               
                 F 
                 r 
               
               = 
               
                 2 
                  
                 
                   
                     Etw 
                     3 
                   
                   
                     l 
                     s 
                     3 
                   
                 
                  
                 dx 
               
             
             , 
           
         
       
     
         [0000]    where l s  is the length of the sensor&#39;s flexures, E the Young&#39;s Modulus of silicon, t the thickness of the flexures, w the width of the flexures of the sensor and dx the deflection. The electrical capacitance C 1  and C 2  of the comb drive formed between the movable, common electrode  24  and the non-movable electrode  25  is given by 
         [0000]    
       
         
           
             
               
                 C 
                 1 
               
               = 
               
                 
                   C 
                   2 
                 
                 = 
                 
                   
                     n 
                      
                     
                         
                     
                      
                     ɛ 
                      
                     
                       
                         
                           l 
                           c 
                         
                          
                         t 
                       
                       
                         d 
                         1 
                       
                     
                   
                   + 
                   
                     n 
                      
                     
                         
                     
                      
                     ɛ 
                      
                     
                       
                         
                           l 
                           c 
                         
                          
                         t 
                       
                       
                         d 
                         2 
                       
                     
                   
                 
               
             
             , 
           
         
       
     
         [0000]    where n is the number of comb drive fingers, l c  the length of the comb drive fingers, d 1  the small gap of the capacitor electrodes and d 2  the spacing between a capacitor electrode pair. The deflection dx changes the capacitances to 
         [0000]    
       
         
           
             
               C 
               1 
             
             = 
             
               
                 ɛ 
                  
                 
                   
                     
                       l 
                       c 
                     
                      
                     t 
                   
                   
                     
                       d 
                       1 
                     
                     - 
                     dx 
                   
                 
               
               + 
               
                 ɛ 
                  
                 
                   
                     
                       l 
                       c 
                     
                      
                     t 
                   
                   
                     
                       d 
                       2 
                     
                     + 
                     dx 
                   
                 
               
             
           
         
       
       
         
           and 
         
       
       
         
           
             
               C 
               2 
             
             = 
             
               
                 ɛ 
                  
                 
                   
                     
                       l 
                       c 
                     
                      
                     t 
                   
                   
                     
                       d 
                       1 
                     
                     + 
                     dx 
                   
                 
               
               + 
               
                 ɛ 
                  
                 
                   
                     
                       
                         l 
                         c 
                       
                        
                       t 
                     
                     
                       
                         d 
                         2 
                       
                       - 
                       dx 
                     
                   
                   . 
                 
               
             
           
         
       
     
         [0023]    The change in capacitance is converted into an output voltage by the readout electronics. The readout electronics with the interface IC  11  is located directly in the sensor package  20 . Locating the interface IC as close as possible to the MEMS sensor chip  17  is important to minimize the amount of parasitic capacitance. Parasitic capacitance would reduce the quality of the output signal of the interface IC  11 . The range, sensitivity and resolution of the sensing system are easily varied by changing the length l s  of the flexures. 
         [0024]    The sensor package consists of the MEMS sensor chip  17 , the interface IC  11  and the substrate  7 . The MEMS fabrication process is detailed in the documents [1, 6, 7 and 9]. Silicon or silicon-on-insulator wafers are is used as a base material. However, fabricating the sensor chip  17  using a metal by electroplating or laser-cutting may also be possible. The fabrication process described in documents [6, 7] enables the sensor probe to be electrically insulated from the rest of the MEMS chip. Therefore, the probe may be used for simultaneous electrical measurements or for applying a current or electrical signal to the sample  14 . 
         [0025]    In the state of the art sensor design described in [1], the movable, common electrode  24  is located in the middle between the non-movable electrodes  25 , forming a differential capacitive comb drive as shown in  FIG. 2   a.  The MEMS sensor chip  17  according to a preferred embodiment of the invention features a different comb drive configuration as shown in  FIG. 2   b.  The non-movable electrodes  25  are moved to the same side next to each other also forming a differential comb drive. The new configuration is advantageous for reducing the MEMS sensor chip  17  size in y-direction and for wire-bonding the MEMS sensor chip  17  to the substrate  7  by reducing the length of the wires. 
         [0026]    After MEMS chip fabrication, the MEMS sensor chip  17  is mounted onto a substrate  7 , which is normally a printed circuit board  7  which also includes the interface IC and the rest of the readout electronics  11 . The sensor chip is located at the upper edge of the substrate  7  (substrate edge  8 ) and at the right or the left edge (substrate edge  9 ). The sensor is wire-bonded to the substrate. The sensor probe  2  is designed such that it&#39;s probe tip  1  is overhanging two sides  8 ,  9  of the substrate  7  by the distance p x  and p y  respectively as shown in  FIG. 2   b.  This is realized by introducing an angle α into the MEMS design or by choosing a bent probe shape. The values p x  and p y  can range from single micrometers of to several millimeters. The fact that the sensor probe is overhanging both substrate side  8  and substrate side  9 , allows the sensor to be used for a much higher number of applications. The sensor is used in combination with an optical microscope or a scanning electron microscope to observe the alignment of the sensor probe tip  1  and the sample  14 . 
         [0027]    In most cases the capacitive MEMS force sensors are used in combination with a precise micropositioning device such as a micromanipulator. The sensor can be mounted in different, adjustable orientations as shown in  FIG. 4   b.  The way the sensor is mounted on the micromanipulator depends on the desired sensing direction for a certain application. Some measurement tasks require the application of the force to the sample  14  in vertical direction (c-direction). Other applications require the application of the force to the sample  14  in horizontal direction (a-b-plane). 
         [0028]    For vertical measurements the sensitive direction  10  of the sensor is aligned with the c-axis. The substrate  7  may have a cutout  21  such that the sensor can also be used in the limited space available underneath the microscope lens  12  and above the sample holder  13 . For measurements in the a-b-plane the sensor is mounted at an angle β as shown  FIG. 4   b.  The angle β can be altered without changing the sensitive direction  10  which is a great advantage compared to prior art sensor designs since it simplifies the experimental setup, the application of the force in a-direction or b-direction and the interpretation of the measurement data. 
       Comparison to Existing Sensor Designs 
       [0029]      FIG. 5   a  . . .  5   c  compares the prior art sensor design  18  with the sensor proposed according to the invention. The sensitive direction of the prior art sensor  22  is parallel to the sensor probe as shown in  FIG. 5   a.  Also, the sensor probe is parallel to the long axis of the substrate. This is not the case in the new sensor  23  design shown in  FIG. 5   b  and  FIG. 5   c.  The sensitive axis is perpendicular to the long-axis of the sensor package  20  and the probe tip  1 . Also it can be seen that the sensor probe of the prior art sensor  18  is overhanging one side of the substrate only. In the new sensor  23  the probe is overhanging two sides  8 ,  9  of the substrate  7 . This is the upper substrate edge  8  and one of the adjacent substrate edges  9  of the upper substrate edge  8 . This adjacent edge of the substrate  9  can be on the right or left side of the substrate  7 . In the prior art sensor design the probe is straight, while the probe  2  of the new sensor  23  is tilted at an angle α as shown in  FIG. 2   b.  The sensitive direction  10  is perpendicular to the long axis of the substrate  7  (x-axis in  FIG. 5   a,    5   b,    5   c.  The prior art sensor features a sensitive direction parallel to the substrate edge  9  (left or right side), the new sensor features a sensitive direction parallel to substrate edge  8  (upper substrate edge). The MEMS sensor chip is located at the edge of two adjacent substrate sides  8 ,  9 . 
         [0030]    When looking at  FIG. 5   a  . . .  5   c,  it can be seen that for the prior art sensor  18  the sensitive direction of the prior art sensor  22  changes with the angle β. This means that the force vector applied to the sample  14  changes with the angle β as well. This is a disadvantage, since for a meaningful measurement the applied force should either be in a-direction, b-direction or c-direction.  FIG. 4   b  shows that the sensitive direction  10  (and therefore the applied force vector) does not change with the angle β when using the new sensor  23 . The mounting configuration in  FIG. 4   b  shows also that a small part of the field of view of the microscope is covered by the new sensor  23  only. 
       LIST OF USED REFERENCE NUMERALS AND ACRONYMS 
       [0000]    
       
           1  sensor probe tip 
           2  sensor probe 
           3  movable part of the sensor 
           4  flexures 
           5  non-movable part of the sensor 
           6  capacitive comb drive 
           7  substrate 
           8  edge of the substrate 
           9  edge of the substrate 
           10  sensitive direction 
           11  interface integrated circuit 
           12  microscope lens 
           13  sample holder 
           14  sample 
           15  line of sight 
           16  point of contact 
           17  MEMS sensor chip 
           18  prior art sensor 
           21  cutout in the substrate 
           22  sensitive direction of the prior art sensor 
           23  new force sensor 
         px distance from probe tip to substrate edge in x-direction 
         py distance from probe tip to substrate edge in y-direction 
         d 1  air gap between capacitor electrodes 
         d 2  air gap between capacitor electrode pairs 
         dx deflection of the movable part of the sensor 
         ls length of flexure 
         lc length of capacitor electrodes 
         t thickness of chip device layer, flexures and capacitor electrodes 
         w width of the flexure 
         E Young&#39;s Modulus 
         Fr restoring force 
         MEMS Microelectromechanical System 
         IC integrated circuit 
         α angle of sensor probe 
         β angle of substrate 
       
     
       LIST OF CITED DOCUMENTS 
     [1] Y. Sun, B. J. Nelson, “MEMS Capacitive Force Sensors for Cellular and Flight Biomechanics”, Biomedical Materials, Vol. 2, No. 1, 2007, pp. 16-22. 
     [2] WO 2010/112242 A1, 
       [0000]    
       
         
           
             &lt;&lt;PACKAGE AND INTERFACE OF A MICROFORCE SENSOR FOR SUB-MILLINEWTON ELECTROMECHANICAL MEASUREMENTS&gt;&gt; 
             Applicant: FemtoTools GmbH 
           
         
       
     
       [3] WO 2005/121812 A1 
       [0000]    
       
         
           
             &lt;&lt;MULTI-AXIS CAPACITIVE TRANSDUCER AND MANUFACTURING METHOD FOR PRODUCING IT&gt;&gt; 
             Applicant: ETH ZURICH
 
[4] FemtoTools AG, http://www.femtotools.com
 
           
         
       
     
       [5] WO 2007/147239 A1 
       [0000]    
       
         
           
             &lt;&lt;MEMS-BASED MICRO AND NANO GRIPPERS WITH TWO-AXIS FORCE SENSORS&gt;&gt; 
             Applicant: SUN, Yu; KIM, Keekyoung
 
[6] F. Beyeler, A. P. Neild, S. Oberti, D. J. Bell, Y. Sun, J. Dual, B. J. Nelson “Monolithically Fabricated Micro-Gripper with Integrated Force Sensor for Manipulating Micro-Objects and Biological Cells Aligned in an Ultrasonic Field” IEEE/ASME Journal of Microelectromechanical Systems, Vol. 16, No. 1, February 2007, pp. 7-15.
 
[7] F. Beyeler, S. Muntwyler, Z. Nagy, C. Graetzel, M. Moser, B. J. Nelson, “Design and calibration of a MEMS sensor for measuring force and torque acting on a magnetic microrobot” Journal of Micromechanics Microengineering, Vol. 18, 2008, pp 7.
 
           
         
       
     
       [8] US 2007/0251328 A1 
       [0000]    
       
         
           
             &lt;&lt;FORCE SENSOR PACKAGE AND METHOD OF FORMING THE SAME&gt;&gt; 
             Applicants: Thirumani A. Selvan; Raghu Sanjee.
 
[9] S. Muntwyler, B. E. Kratochvil, F. Beyeler, B. J. Nelson, “Monolithically Integrated Two-Axis Microtensile Tester for the Mechanical Characterization of Microscopic Samples”, IEEE/ASME Journal of Microelectromechanical Systems (JMEMS), Vol. 19, No. 5, October 2010, pp. 1223-1233