Abstract:
The present disclosure relates to a method of adjusting the resonance frequency of a vibrating element, comprising measuring the resonance frequency of the vibrating element, determining, using abacuses and as a function of the resonance frequency measured, a dimension and a position of at least one area of modified thickness to be formed on the vibrating element so that the resonance frequency thereof corresponds to a setpoint frequency, and forming on the vibrating element, an area of modified thickness of the determined dimension and position.

Description:
BACKGROUND 
       [0001]    1. Technical Field 
         [0002]    The present invention relates to micro-machined vibrating elements located in some electromechanical components such as resonators, motion sensors (accelerometers, rate gyros, and the like) and vibration sensors. The present invention applies in particular to the manufacture of such components. 
         [0003]    2. Description of the Related Art 
         [0004]    Nowadays, electromechanical components with vibrating element(s) are particularly provided on numerous portable devices, such as mobile phones, and other mobile communication terminals. It is therefore wished to miniaturize these components. In addition, electromechanical resonators tend to replace quartz resonators in electronic circuits, in particular due to the high frequencies they can reach, their lower electrical consumption and their ability to be integrated with other circuits on a semiconductor substrate. 
         [0005]    Electromechanical components with a vibrating element generally comprise an electrode for detecting actuating movement of the vibrating element. The vibrating element has a specific resonance frequency linked to its dimensions, to the way it is supported, and to the elastic properties of the material of which it is made and to the way it is actuated. The resonance frequency Fr of a vibrating element may be determined by the following equation: 
         [0000]    
       
         
           
             
               
                 
                   Fr 
                   = 
                   
                     
                       1 
                       
                         2 
                          
                         π 
                       
                     
                      
                     
                       
                         k 
                         m 
                       
                     
                   
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
           
         
       
     
         [0000]    where m is the mass of the vibrating element and k its stiffness. 
         [0006]    According to the application, the vibrating element may have various shapes such as that of a beam or a rectangular plate maintained by one or more ends, or a disc maintained by one to four or more points distributed on its periphery. They can vibrate according to a flexion or volume mode. Such vibrating elements may be made by depositing onto a support, for example a silicon wafer, a layer of a sacrificial material, then a layer of the material desired for the vibrating element going beyond the sacrificial layer. The layer in the material constituting the vibrating element is then cut according to the outlines thereof, for example by photo-etching, keeping links between the vibrating element and the support, and the sacrificial layer is removed for example by plasma etching. The vibrating element is thus only maintained by its links to the support, and is therefore free to vibrate. Such vibrating elements may thus be collectively manufactured on silicon wafers. 
         [0007]      FIGS. 1A ,  1 B and  1 C schematically show an example of resonator.  FIG. 1A  is a partial, schematic top view of the resonator.  FIGS. 1B and 1C  are cross-sectional views along planes B-B and C-C, respectively. The resonator comprises a vibrating element  1 , for example in the shape of a bar of rectangular shape and cross-section. The element  1  is fixed by arms  2  to anchor areas  3 . The arms  2  are arranged along a center line  5  of the element  1 . The element  1  is surrounded by an empty area  6  and electrodes  4  symmetrically arranged in relation to the line  5 . 
         [0008]    In  FIGS. 1B and 1C , the element  1 , the arms  2  and the anchors  3  are formed in a thin layer, for example in single-crystal or poly-crystal silicon, on a support  10 , for example consisting of a silicon wafer. An empty area  6 ′ located between the support  10  and the element  1  results from the partial removal of a sacrificial layer  7 . 
         [0009]    During operation, the element  1 , at least partially made of a conductive material, is subjected to a first potential and the electrodes  4  are subjected to a second potential. The difference between the first potential and the second potential creates an electrostatic force tending to cause a distortion of the element  1 . The element  1  then begins to oscillate at its resonance frequency around the line  5  in the case of a vibration in the plane of  FIG. 1A . The distortion of the element  1  causes a capacitance variation between the element  1  and the electrodes  4 , this variation may be detected on one of the two electrodes  4 . 
         [0010]    Due in particular to the effects of miniaturization and manufacturing process, differences of resonance frequency may be observed between vibrating elements assumed to be identical, collectively formed on a same silicon wafer. Thus, differences in resonance frequencies of +/−1000 parts per million (ppm) (10 −4 %) n relation to a target resonance frequency, may be observed between the vibrating elements formed on a same wafer. These differences may be observed in particular between vibrating elements formed near the center of a wafer and those formed near the edge of the wafer. These differences may result from variations of parameters of photo-etching processes, which introduce differences of widths, lengths, and/or thicknesses of the vibrating elements. 
         [0011]    It may be desirable to limit these resonance frequency variations to some ppm, even to values lower than the ppm. Indeed, the applications relating to mobile telephony transmissions, such as Global systems of Mobile Communications (GSM) have a resonance frequency precision around the ppm. Global positioning system (GPS) receivers must have a resonance frequency precision lower than the ppm. Other applications (Ethernet, Bluetooth) require precisions around some dozens of ppm. 
         [0012]    It has already been suggested to measure the resonance frequency of vibrating elements collectively formed on a wafer before individualization, so as to discard the elements having a difference with a rated resonance frequency higher than a threshold. However, if the required precision is high, this solution tends to discard a significant part of the resonators formed on a wafer. 
       BRIEF SUMMARY 
       [0013]    Embodiments relate to a method of adjusting the resonance frequency of a vibrating element, comprising: measuring a resonance frequency of the vibrating element, determining using abacuses, and as a function of the resonance frequency measured, dimensions and a position of at least one area of modified thickness, to be formed on the vibrating element so that the resonance frequency of the vibrating element corresponds to a setpoint frequency, and forming on the vibrating element, an area of modified thickness of the determined dimensions and position. 
         [0014]    According to an embodiment, each area of modified thickness is made by forming a patch on the vibrating element. 
         [0015]    According to an embodiment, forming each patch on the vibrating element is performed by adding a layer of a chosen material, having a determined thickness, and by removing the layer added outside areas where a patch is to be formed. 
         [0016]    According to an embodiment, forming the patchs on the wafer is performed by removing from the vibrating element a layer outside areas where a patch is to be formed. 
         [0017]    According to an embodiment, the resonance frequency of the vibrating element is increased by forming a patch on the vibrating element on an area where the stress to which the vibrating element in resonance is subjected is maximum. 
         [0018]    According to an embodiment, the resonance frequency of the vibrating element is decreased by forming a patch on the vibrating element on an area where the stress to which the vibrating element in resonance is subjected is minimum. 
         [0019]    According to an embodiment, the abacuses are obtained by forming areas of modified thicknesses of different dimensions and/or thicknesses on an area of the vibrating element where the stress to which the vibrating element in resonance is subjected is minimum, and by measuring the resonance frequency of the vibrating element, or with analytical or finite element models. 
         [0020]    Embodiments also relate to a method of manufacturing electronic components collectively formed on a wafer comprising: forming on the wafer vibrating elements, measuring the resonance frequency of each of the vibrating elements on the wafer, and steps of implementing the previously defined method. 
         [0021]    According to an embodiment, the measurements of resonance frequency are taken once for a manufacturing line of electronic components, forming patchs on the wafer being made using a photoetching mask defined from the dimensions and positions of determined patchs. 
         [0022]    According to an embodiment, the patchs are formed on the wafer before the vibrating elements. 
         [0023]    Embodiments also relate to an electronic component comprising a vibrating element, having a modified resonance frequency implementing the method as previously defined. 
         [0024]    According to an embodiment, each patch is made in a material chosen in the group comprising silicon, silicon oxide, silicon nitride, chromium and gold. 
         [0025]    According to an embodiment, the vibrating element has the shape of a beam or a plate maintained between two ends, or a disc maintained at the periphery thereof by four points. 
         [0026]    According to an embodiment, each patch is made on a face of the vibrating element facing a support on which the component is formed. 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         [0027]    Example embodiments of the invention will be described below in relation with, but not limited to the appended figures wherein: 
           [0028]      FIG. 1A  previously described schematically shows in top view a device comprising a vibrating element according to prior art, 
           [0029]      FIGS. 1B and 1C  previously described are longitudinal and transversal cross-section views of the device of  FIG. 1A , 
           [0030]      FIG. 2A  schematically shows in side view a vibrating element in the shape of a beam, susceptible of vibrating in a direction where it has its smaller dimension, 
           [0031]      FIG. 2B  shows a curve of the stress to which is subjected the vibrating element of  FIG. 2A , 
           [0032]      FIG. 3A  shows in top view a vibrating element, according to another embodiment, 
           [0033]      FIG. 3B  shows in side view the vibrating element of  FIG. 3A , 
           [0034]      FIG. 4A  shows in top view a vibrating element, according to another embodiment, 
           [0035]      FIG. 4B  shows in side view the vibrating element of  FIG. 4A , 
           [0036]      FIG. 5A  shows in top view a vibrating element, according to one embodiment, 
           [0037]      FIG. 5B  shows in side view the vibrating element of  FIG. 5A , 
           [0038]      FIG. 6A  shows in top view a vibrating element, according to one embodiment, 
           [0039]      FIG. 6B  shows in side view the vibrating element of  FIG. 6A , 
           [0040]      FIG. 7  shows variation curves of the resonance frequency of a vibrating element as a function of the position and radius of a perforation made in the vibrating element, 
           [0041]      FIG. 8  shows variation curves of the resonance frequency of a vibrating element as a function of the width and thickness of a patch made on the vibrating element, 
           [0042]      FIG. 9  shows variation curves of the resonance frequency of a vibrating element as a function of the width of a patch made on the vibrating element, for various materials constituting the patch, 
           [0043]      FIGS. 10A ,  10 B show variation curves of the resonance frequency of vibrating elements as a function of the width of a patch made on the vibrating element, 
           [0044]      FIGS. 11A ,  11 B show in top view and cross-section view a vibrating element in the shape of a disc, 
           [0045]      FIG. 11C  shows curves of the stress to which is subjected the vibrating element of  FIG. 11A , 
           [0046]      FIGS. 12A ,  12 B show in top views variations of the vibrating element of  FIGS. 11A-11C , according to various embodiments, 
           [0047]      FIG. 13A  shows in top view a vibrating element, according to one embodiment, 
           [0048]      FIG. 13B  shows in side view the vibrating element of  FIG. 13A . 
       
    
    
     DETAILED DESCRIPTION 
       [0049]    US Patent Publication No. 2010/0019869 offers to form, in the vibrating element, columns in a material having a Young modulus temperature coefficient opposite to that of the material in which the vibrating element is formed. This solution does not allow process corners to be corrected on a same wafer. 
         [0050]      FIG. 2A  shows a micro-machined vibrating element  11  in the shape of a beam, maintained along two opposite edges by anchor areas  12 ,  13  formed on a support SB. The element  11  may vibrate in a direction perpendicular to the plane thereof.  FIG. 2B  shows a curve C 1  of the stress to which is subjected the element  11  when it is subjected to a vibration at its resonance frequency. The stress curve C 1  has maximum values M 1 , M 2  near fixed points (anchor areas  12 ,  13 ) of the element  11 , followed by minimum values m 1 , m 2  when going toward the center of the element  11 . The minimum values m 1 , m 2  are separated by a central area I 1  corresponding to an average stress value. 
         [0051]    It has been observed that by arranging a patch on one or more regions of a face of the vibrating element, chosen according to the stress to which these regions are subjected, it is possible to increase or decrease the resonance frequency of the vibrating element. Thus, if a region subjected to a minimum stress is covered by a patch, the mass and stiffness of the vibrating element increase. As stiffness is modified in a low stress region, the modification has little effect on the resonance frequency. Therefore the result of the equation (1) is that the resonance frequency of the vibrating element decreases. On the other hand, if a region of the vibrating element subjected to high stress is covered by a patch, the resulting increase of stiffness of the vibrating element in these regions is going to affect the resonance frequency more that the resulting mass increase. According to the equation (1), the resonance frequency of the vibrating element is therefore going to increase. The resonance frequency variations thus obtained also depend on the thickness of each patch added and on the material of which it is made. Thus, the bigger the thickness of each patch is, the more significant the resonance frequency correction. 
         [0052]      FIGS. 3A to 6B  show the vibrating element  11  comprising one or two patchs arranged on the upper face thereof, according to various embodiments. In  FIGS. 3A ,  3 B, the patchs P 1 , P 2  are arranged near the anchor areas  12 ,  13 . Thus, according to the curve C 1 , the patchs P 1 , P 2  cover regions subjected to maximum stress (M 1 , M 2 ). The result is that the resonance frequency of the element  11  is increased in relation to a same vibrating element not comprising the patchs P 1 , P 2 . 
         [0053]    In  FIGS. 4A ,  4 B, a patch P 3  is arranged near the center of the element  11 , i.e., on a region subjected to an average stress value corresponding to the region I 1  of the curve C 1 . The result is that the resonance frequency of the element  11  is increased in relation to a same vibrating element not comprising the patch P 3 . 
         [0054]    In  FIGS. 5A ,  5 B, the patchs P 4 , P 5  are arranged near regions subjected to minimum stress (areas m 1 , m 2  of the curve C 1 ). The resonance frequency of the element  11  is then decreased in relation to a same vibrating element not comprising the patchs P 1 , P 2 . This phenomenon is due to an increase of the mass of the element  11  without modifying its stiffness coefficient k in the stressed regions. 
         [0055]    In  FIGS. 6A ,  6 B, a layer P 6  covers all the upper face of the vibrating element  11 . According to the curve C 1 , the vibrating element  11  has a stressed surface larger than its not stressed surface. The result is that the layer P 6  will tend to increase the resonance frequency of the element  11 . 
         [0056]    It may therefore be chosen to increase or decrease the resonance frequency of the element  11  using, for example, a same material. 
         [0057]    The patchs P 1 , P 2 , P 3 , P 4 , P 5 , P 6  may extend across the width of the element  11 , as shown in  FIGS. 3A ,  4 A,  5 A,  6 A, or only on a part of the width, for example centered on a longitudinal center line of the element  11 . The patchs may be made of various materials such as Si, Cr, SiO 2 , Si 3 N 4 , Au, or any other suitable material, which may be chosen according to their densities and mechanical properties, in particular their Young moduli. 
         [0058]      FIG. 7  shows variation curves of the resonance frequency FR of a vibrating element in the shape of a beam, such as that shown in  FIG. 2A , maintained along two opposite edges. The resonance frequency of the vibrating element is modified by forming a perforation in the vibrating element according to an axis perpendicular to the plane thereof. The curves of  FIG. 7  are obtained by varying the position HP of the perforation along the vibrating element from a fixed point, for different radius values HR of the perforation. The numerical values shown in  FIG. 7  correspond to a vibrating element made of silicon 50 μm long, 20 μm wide and 1.4 μm thick. In these conditions, the resonance frequency of the vibrating element is of 4.95122 MHz. When the position of the perforation varies along the vibrating element, the resonance frequency has minimum values m 3 , m 4  when the perforation is near the fixed edges of the vibrating element, maximum values M 3 , M 4  near the regions m 1 , m 2  ( FIGS. 2A ,  2 B) and an intermediate local minimum value  12  between the values M 3 , M 4  and corresponding to the region I 1 . The maximum values M 3 , M 4  are slightly higher (around 1,000 ppm or 0.1%) than the resonance frequency of the vibrating element without perforation. The result is that the resonance frequency of the vibrating element may be increased or decreased according to the region of the vibrating element where the perforation is made. The curves of  FIG. 7  are inverted (the values M 3 , M 4  become minimum values, and the values m 3 , m 4  and  12  become maximum values) if, instead of forming a perforation, matter is added by forming a patch on the vibrating element. 
         [0059]    The differences in resonance frequency, in relation to a rated frequency, exhibited by vibrating elements formed on a same silicon wafer are usually within a range from approximately −1,000 to approximately +1,000 ppm, and depend on the shape and dimensions of the vibrating element. Such differences appear to be totally compatible with the correction possibilities offered by arranging one or more patchs in determined regions of the vibrating element. Thus,  FIG. 8  shows variation curves of the resonance frequency FR as a function of the width W, for various thicknesses T of a patch (P 4  or P 5 ) arranged on one of the regions m 1 , m 2  of a vibrating element ( FIGS. 5A ,  5 B) having features similar to those subject of the curves of  FIG. 7 . The curves of  FIG. 8  show that the resonance frequency of the vibrating element may be decreased using one or two patchs arranged on the regions m 1 , m 2  of the vibrating element and having a width lower than around 6 μm, and increased if these same patchs have a width higher than 6 μm. 
         [0060]      FIG. 9  shows variation curves of the resonance frequency FR as a function of the width W of a patch, when this patch has a thickness of 20 nm and is formed on one of the regions m 1 , m 2 , in various materials such as silicon (Si), silicon oxide (SiO 2 ), silicon nitride (Si 3 N 4 ) and chromium (Cr). The curves of  FIG. 9  show that if a significant correction (around 2,000 ppm) for decreasing the resonance frequency is necessary, chromium may be used as the material constituting the patchs. 
         [0061]      FIGS. 10A ,  10 B show variation curves of the resonance frequency FR of vibrating elements, as a function of the width W of a patch 30 nm thick (chosen for example according to  FIG. 8 ), arranged on the region m 1  and/or m 2  of the vibrating element. The curves of  FIGS. 10A and 10B  may be those of two vibrating elements formed on a same semiconductor wafer. In  FIG. 10A , the vibrating element has a resonance frequency of around 4.9492 MHz (at W=0) instead of a setpoint resonance frequency FC of 4.95122 MHz. The frequency correction to be made is therefore of around +2 kHz, i.e., around +400 ppm. According to the curve of  FIG. 10A , the frequency FC may be reached with a patch of 6.902 μm. 
         [0062]    In  FIG. 10B , the vibrating element has a resonance frequency of around 4.9522 MHz instead of the setpoint resonance frequency FC at 4.95122 MHz. The frequency correction to be made is therefore of around −1 kHz, i.e., around −200 ppm. According to the curve of  FIG. 10B , the setpoint frequency may be reached with a patch of 0.883 μm. The resolution of the resonance frequency correction of a vibrating element may thus reach 5 ppm with the current photolithography techniques. 
         [0063]      FIGS. 11A ,  11 B show a vibrating element  21  in the shape of a disc linked to a support fixed by four arms  24  which may for example be uniformly distributed at the periphery of the disc.  FIG. 11B  is a section view along an axis X or Y. The arms are linked to anchor areas  22 . Electrodes  23  are formed facing the periphery of the disc between the arms  24  and the anchor areas  22 .  FIG. 11C  shows curves C 2 , C 3  of the stress of the vibrating element  21  subjected to a vibration according to a volume mode called “elliptic” or “wine glass”, at the resonance frequency of the element  21 . The curve C 2  corresponds to the stress to which is subjected the element  21  along the axis X, and the curve C 3 , to the stress to which is subjected the element  21  along the axis Y, during an oscillation. During a following oscillation, the stress to which is subjected the element  21  along the axes X and Y is inverted between the curves C 2 , C 3 . The curves C 2 , C 3  have minimum values m 1 ′, m 1 ″, m 2 ′, m 2 ″ along the edges of the element  21  and a maximum value M 1 ′ near the center of the element  21 . 
         [0064]    The resonance frequency of such a vibrating element in the shape of a disc may also be adjusted to match a setpoint frequency using patchs arranged on the regions m 1 ′, m 1 ″, m 2 ′, m 2 ″ or the region M 1 ′. Thus  FIGS. 12A ,  12 B show vibrating elements which resonance frequency is adjusted using patchs P 11 , P 12 . In  FIG. 12A , the vibrating element  21  comprises four patchs P 11  formed along the edge of the vibrating element, away from the arms  24 . In the example of  FIG. 12A , each patch P 11  has the shape of a spindle, which has a width W 1  along the radius of the disc  21 . The width W 1  of each patch P 11  may be adjusted according to the frequency correction to be obtained. Admittedly, to adjust the frequency correction, it is also possible to adjust the length of the patch. The frequency correction obtained by the patchs P 11  may be negative if their width W 1  is small, or positive when W 1  is greater than a particular value. 
         [0065]    In  FIG. 12B , the resonance frequency of the vibrating element  21  is increased using a patch P 12  formed at the center of the disc  21 . The frequency correction may be adjusted by modifying the radius W 2  of the patch P 12  (or its thickness). 
         [0066]    The possibilities of correction of the resonance frequency of vibrating elements, such as previously described, are taken advantage of in a method for collectively manufacturing components comprising a vibrating element, such as by implementing photolithography processes on a wafer of semiconductor material. It has been observed that on the semiconductor wafer the distribution of differences between a rated resonance frequency and the resonance frequency of the vibrating elements made on the wafer are substantially invariant from one batch to another. Measuring these differences to determine their distribution on the wafer may therefore be performed once. This distribution is then used to determine the positions and dimensions (lengths, widths, thicknesses, and the like) of patchs to be formed on each of various vibrating elements of the wafer so as to reduce this frequency difference to an acceptable value, for example lower than 5 ppm. These patch positions and dimensions are then used to form a photo-etching mask allowing patchs to be formed at the dimensions and positions wanted on each of the vibrating elements in which resonance frequency is to be corrected. 
         [0067]    In one embodiment, the patchs may be formed by depositing a layer of the material chosen on the whole surface of the wafer after forming the vibrating elements, then etching the layer deposited by a photolithography process so as to leave only the patchs at wanted dimensions and locations of the wafer. The layer forming the patchs may be deposited using various known means. In some embodiments, the thickness of the layer deposited in which the patchs are formed is precisely controlled. Thus, the layer forming the patchs may be made by atomic layer deposition (ALD), physical vapor deposition (PVD), epitaxy if the layer forming the patchs is made of silicon, oxidation if the layer forming the patchs is made of silicon oxide, and the like. 
         [0068]    The positions and dimensions of the patchs to be formed according to the resonance frequency difference to be corrected may be determined using an abacus, such as that of  FIG. 8 , which are constituted from series of measurements taken on the vibrating element on which patchs of various dimensions are formed. The positions and dimensions of the patchs to be formed may also be determined with analytical or Finite element models. 
         [0069]    It is to be noted that the patchs may be formed below the layer forming the vibrating elements, i.e., by depositing a first layer onto the sacrificial layer, etching the first layer to form the patchs, then by depositing the second layer on the patch and etching the second layer to form the vibrating elements. In  FIGS. 13A ,  13 B, patchs P 4 ′, P 5 ′ are arranged below the vibrating element  11 . 
         [0070]    If the patchs are made of the same material as that forming the vibrating elements, they may also be made by increasing the layer forming the vibrating elements by the thickness of the patchs, and removing a superficial layer having the thickness of the patchs at the locations without patchs. 
         [0071]    It will be clear to those skilled in the art that the present disclosure is susceptible to various other embodiments and applications. In particular, the disclosure is not limited to the vibrating elements previously described, but may apply to all the vibrating elements susceptible of being integrated on a wafer in a material such as semi-conductor, ceramics, glass or quartz. Furthermore, the disclosure is not limited to electrostatically actuated and capacitive motion detection vibrating elements. The disclosure may apply to actuated micro-machined vibrating elements, or to vibrating elements whose movements are detected by other means, such as by piezoelectric, piezoresistive or magnetic effect. 
         [0072]    Admittedly, the resonance frequency of the vibrating elements may be modified simply by forming in or on the vibrating element one or more areas of modified thicknesses, being understood that an area of modified thickness may also be a perforation going through the vibrating element. 
         [0073]    The various embodiments described above can be combined to provide further embodiments. These and other changes can be made to the embodiments in light of the above-detailed description. In general, in the following claims, the terms used should not be construed to limit the claims to the specific embodiments disclosed in the specification and the claims, but should be construed to include all possible embodiments along with the full scope of equivalents to which such claims are entitled. Accordingly, the claims are not limited by the disclosure.