Abstract:
A pedestal structure and its fabrication method stress release assembly of micromechanical sensors, in particular acceleration sensor, angular rate sensors, inclination sensors or angular acceleration. At least one silicon seismic mass is used as sensing element. The at least one silicon seismic mass is joined to the silicon frame via at least one assembly pedestal, the surface of which is bonded to a covering wafer, either glass or silicon.

Description:
BACKGROUND AND SUMMARY 
     The invention relates to a micromechanical device, such as an acceleration sensor, angular rate sensor, inclination sensor, or angular acceleration sensor, in which a seismic mass is used as sensing element. 
     Many devices having micromechanical structures are currently known. One problem associated with many of such structures is that manufacture of the devices introduces internal stresses in the structure and participating in the components which measure the parameter to which the sensor is sensitive. Generation of such stresses causes problems. It often results in the sensor having an offset or varying unpredictably with temperature or over the sensing range of the sensor. This results in each sensor requiring individual testing and appropriate means, either via mechanical or electrical compensation, to be provided in order for the sensor to operate accurately and consistently. It will be appreciated that this can cause a considerable increase in sensor cost, as well as reducing reliability. 
     Many attempts have been made to overcome the problem associated with induced stress. Most of the approaches are, however, dependent upon employing very specific materials, either in the device components or in the encasing packaging of the device, meaning that are inflexible and cannot be broadly applied to different device types. Many have an additional problem they in requiring extremely complex and costly manufacturing steps which again increase cost and which can be time consuming and result in many rejected devices. 
     According to the present invention there is provided A micromechanical device comprising: 
     a pedestal member connected, in use, to a support wall and bonded, in use, to an encasing member; and 
     wherein the pedestal member has a rim formed around at least a portion of its outer periphery, the rim extending away from the encasing member and supporting at least one sensing component of the device; and 
     wherein the pedestal member is elongate, with its longer dimension extending in a direction substantially perpendicular to that of the support wall to which it connects. 
     The device may comprise the support wall which may be arranged such that it surrounds both the pedestal member and the component. 
     The pedestal member may be bonded to the encasing member in a discontinuous manner. 
     The component may be connected to the pedestal member by one or more planar flexible hinges. 
     The micromechanical device may be an acceleration sensor, an angular rate sensor, an inclination sensor, or an angular acceleration sensor. 
     A gap between the component and the encasing member may be provided and may be formed by an etched recess in the encasing member. 
     Electrical contacts may be provided with the component or suspension member by the provision of direct electrical contacts located on the edge of the pedestal and on the contact surface of the encasing member. Alternatively, electrical conductors may be provided by implantation of impurities or by sputter deposition of film onto the pedestal structure. 
     Electrical crossings may be provided perpendicular to the direction of elongation of the pedestal member in order to further reduce stresses in the overall device structure. The device may be formed from silicon. 
     A method of manufacturing the device is also provided. Within this invention, at least one silicon seismic mass may be joined to a silicon support wall frame via the pedestal structure, the surface of which is bonded to the encasing member which is, either glass or silicon. 
     This pedestal structure and its method of assembly according to the invention has the advantage that the coupling between the sensing element of the sensor and the frame of the sensor is minimised by using the pedestal member, the bearing surfaces of which are small compared to the surface area from which the device is formed. This reduces the assembly-related strains, stress and associated temperature-induced variations of the overall device, thus simplifying the evaluation electronics of the device. 
     Other provisions of the structure and its method of assembly according to the invention are also advantageous. 
     The pedestal member can be fabricated easily, being produced in the same process that structures the micromechanical components of the device, such as a sensing seismic elements and their suspension systems. This structuring process is especially advantageous because well known and established micromechanical structuring processes, such as wet and dry anisotropic silicon etching, can be used for this purpose. 
     A particular advantage of the pedestal member and its assembly method according to the invention is that the geometry and the manner of its structuring can be selected in accordance with the function of the sensing element and its fabrication sequence. 
     A special advantage of this invention is that any bonding between the covering wafers and the micromachined silicon wafer, which caries the component, the pedestal member and the support wall, takes place at the wafer level, resulting in economical, easy to handle batch processing. According to the invention, a multitude of ready-structured devices, which have not yet been cut in individual devices, can be bonded simultaneously to the encasing member, then separated, for instance by sawing. 
     The bonding technique that forms any sealed cavity and anchors the pedestal member to the encasing member is to be chosen depending on the material of the encasing member. If glass is used for covering, then an anodic bonding technique is suitable; if silicon is used, then silicon-to-silicon bonding techniques are advantageously suited. For other materials, soldering bonding techniques can be successfully employed. The atmosphere composition and its pressure can be freely selected and preserved within any sealed cavity by the anodic bonding technique, which makes this technique particularly attractive. 
     The device and its method of assembly according to the invention allows the optional implementation in its structure of (i) press-contacts, a method of passing electrical conductive paths between the wafers; (ii) buried crossings, a method of passing electrical conductive paths through the bulk of pedestal; (iii) direct crossings, a method of passing electrical conductive paths across the pedestal; and (iv) surface conductors along the pedestal. 
     The invention particularly enables the realisation of a compact sensor as no other stress-releasing structures or mounting techniques, internal or external, being required. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The pedestal structure and its method of assembly according to the present invention will now be described, with reference to the accompanying drawings, in which: 
     FIG. 1 is a section through a sensor device according to the invention having its sensing components joined to its frame by means of a pedestal member; 
     FIG. 2 is a top view of a sensing component attached to the frame by means of a pedestal member structure; 
     FIGS. 3 a  and  3   b  are cross-sectional views taken along lines A—A and B—B respectively in FIG. 2; 
     FIG. 4 is a cross-sectional view taken along A—A in 
     FIG. 2, during wafer-based processing of a sensor device recording to the invention; 
     FIG. 5 is a longitudinal cross-sectional view of a pedestal member structure and the mechanical loads generated by the packaging and/or temperature variations, in an example of the invention; 
     FIGS. 6 a ,  6   b ,  6   c  and  6   d  are transverse cross-sectional views of a pedestal member showing various options for structuring. 
     FIGS. 7 a ,  7   b ,  7   c  and  7   d  are cross-sectional views taken along line A—A in FIG. 2, in alternative examples; 
     FIGS. 8 a ,  8   b ,  8   c  and  8   d  are transverse cross-sectional views of the pedestal member structure showing various options of structuring, in alternative examples; and 
     FIG. 9 is a longitudinal cross-sectional view of the pedestal member taken along line A—A in FIG. 2, in alternative examples of SOI wafer-based processing. 
    
    
     DETAILED DESCRIPTION 
     FIG. 1 shows a cross-sectional view of an inertial sensor device  10  in accordance with the present invention. The inertial sensor  10  is realised by bonding a structured silicon wafer  30  between two encasing member  20  and  21 , which can be structured or not, and which can be glass or silicon, depending on the sensor function and operating principle. FIG. 2 shows an exemplary top view of an inertial sensor according to the present invention. 
     The substrate wafer  30  contains at least one silicon seismic mass  32  which acts as the sensor&#39;s primary sensing component, attached to a fixed silicon frame  31  by means of at least one flexible suspension system  33  and a rigid pedestal member  40 . The suspension systems are indicated in the figures as double clamped straight beams, but they may have any arbitrary planar shape. 
     With this configuration, an impressed mechanical signal causes the sensing components  32  to undergo limited displacement(s) with respect to the fixed silicon support wall and frame  31  thus proportionally modifying either the stress level in at least one suspension system  33  or the separation gap  50  between at least one sensing element  32  and the encasing member  20 . Using a transducing principle, for instance a piezoresistive effect or capacitance changes, the sensor  10  provides an electrical output signal proportional to the impressed mechanical input. The Assembly-related strains and stress and the associated temperature-induced variations negatively affect the proportionality between the electrical output signal and the impressed mechanical input, however. 
     The assembly-related strains and stress and the associated temperature-induced variations transmitted to the sensing component are reduced by using the principles of the invention. 
     In the substrate wafer  30  at least one pedestal member  40  is structured by a combination of anisotropic wet and dry etching steps, as follows by way of example in the preferred fabrication process. 
     A. Into the boron doped substrate wafer  30 , the lateral geometry of at least one pedestal member  40 , of at least one seismic mass  32  and of the fixed silicon frame  31  is defined by at least one implantation of donor impurities, such as phosphorous. Different doses may be used for the pedestal member, seismic masses and silicon support wall and fixed frame dependent upon the specific sensor&#39;s application and design. At least one drive-in process is used to diffuse the donor impurities deep into the silicon wafer until the desired thickness of these structures is achieved. The shape of these structures is defined by the location of the p-n junctions, deep junctions, formed between the n-type-implanted silicon and the p-type substrate. These drive-in steps generate the rounded features illustrated throughout the cross-sectional views shown in FIGS. 1 to  3  and  5  to  8 . 
     B. An n-type epitaxial layer is grown on top of the substrate wafer, with a thickness that depends on the sensor&#39;s specific application and design. This epitaxial layer is separated from the p-type substrate by a shallower p-n junction, said shallower junction, apart from several other purposes in the sensor&#39;s functionality, provides the entire front surface of the substrate wafer  30  with a continuous n-type material that will be used as an etch stop layer during a subsequent wet etching step. The epitaxial layer also gives the opportunity of structuring the pedestal member  40  and the seismic masses  32  on several layers, as well as providing the material of the suspension system  33 . 
     C. A recess  50 , providing separation gap, is etched into the epitaxial layer in order to provide a separation between any movable structures, such as the seismic masses  32  and the suspension systems  33 , and the top encasing member  20 , thus allowing limited motion of these structures. 
     D. An anisotropic wet etching step, in combination with an electrochemical etch-stop technique is performed from the back side of the substrate wafer. This is used to selectively remove the p-type material while the n-type material remains unetched. Depending on the crystal orientation of the silicon substrate wafer, on the etching window and its orientation with respect to the silicon crystal and on the anisotropic properties of the etching solution, the lateral walls of the fixed silicon frame  31  are pyramidal in shape. The formed structures are defined by the shape of the deep and shallow junctions, resulting in thicker regions (members) if etch stop occurs on the deep junctions and thinner regions (membranes) if etch stop occurs on the shallow junctions. 
     E. The final shaping of at least one pedestal member  40 , at least one seismic mass  32  component, at least one suspension system  33  and the fixed silicon frame  31  is achieved by dry, reactive ion or plasma etching (or combination thereof) through the said membranes. FIG. 2 shows the structured substrate wafer  30  after this processing step. 
     F. A suitable bonding technique is used to join and permanently bond the structured substrate wafer  30  with two encasing members  20  and  21 , which may also be structured to form a sealed cavity  34 . During this fabrication step, the top surface  41  of the pedestal member structure  40  is firmly bonded to the top encasing member  20 , completing the assembly process of the sensor element  10  and all its components. If glass wafers are used for encasement, then an anodic bonding technique is suitable; if silicon wafers are used, then silicon-to-silicon bonding techniques are advantageously suited. 
     In FIG. 2, the pedestal member structure  40  is located inside the dashed rectangle and the cross-hatched areas represent the surface of the structured substrate wafer that is firmly bonded on the top encasing member  20 . 
     Apart from the above mentioned fabrication steps, several other conventional steps such as photolithography, selective growth and etch back of thermal oxides, deposition and patterning of thin metallic films can be used within a generally inexpensive mass production process flow. 
     The preferred fabrication process described above, including the method of assembly, allows a simultaneous structuring and fabrication of all of the device elements and relies on well known and established micromachining processes. 
     The examples described hereinafter are based only on two levels of structuring: the thicker regions defined by implantation and drive-in and the thinner regions (structured membranes) patterned in the epitaxial layer. However, in alternative fabrication processes, the members may be realised by several implantations and diffusion processes and the epitaxial layer may be replaced as well by at least one implanted and diffused layer, resulting in several levels of structuring with various thickness for the pedestals members  40 , the seismic masses  32 , the suspension systems  33  and the fixed silicon frame  31 . 
     In alternative fabrication processes the starting material of the substrate wafer  30  can be of n-type. In this case acceptor impurities are implanted and driven-in to form the pedestal members  40 , the seismic masses  32  and the fixed silicon frame  31 . Consequently, the epitaxial layer, if used, is also of a p-type silicon and the wet anisotropic etching of silicon is to be combined with an etch stop technique that allows the selective removal of the n-type material of the substrate while not etching the p-type silicon. The photovoltaic etch stop technique or the high boron doping etch stop are advantageous in this particular context. 
     In alternative fabrication processes the starting substrate wafer  30  can be an SOI wafer of any kind, that is it consists of a single crystal silicon top layer  74  separated from the bulk silicon by a buried, very thin layer of insulating oxide  75 . In this case the pedestal member structure is formed by etch stop on the buried oxide layer followed by a deep reactive etching for lateral definition. The separation gap  50  between the movable silicon structures, such as the seismic masses  32  and the suspension systems  33 , and the top encasing member  20 , is still employed in order to allow a limited motion of these structures. FIG. 4 shows a cross sectional view of the intertial sensor  10  after the last processing step in the SOI wafer-based alternative processing. 
     With a suitable design for etching window, wet isotropic etching of a silicon in combination with a suitable etch stop technique can be used to remove all the excess substrate material. 
     FIG. 2 shows an example device, which is a sensor and its preferred geometry according to the present invention. FIGS. 3 a  and  3   b  show cross-sectional views along lines A—A and B—B of FIG.  2 . 
     The pedestal member structure  40  has an elongate shape consisting of a longer dimension (pedestal length) and a smaller dimension (pedestal width) where the pedestal length and width are mutually a perpendicular. The pedestal structure comprises a thick portion  44  and associated bonded surface  41  or bonded top area. A thinner rim  43  (pedestal rim) and any number of transversal shallow recesses  42  (direct crossing). The thick portion  44  is formed by selective removal of the substrate material with an etch stop on said deep p-n junctions. The pedestal rim  43  is formed by selective removal of the substrate material with an etch stop on shallow p-n junctions. The direct crossings  42  are produced by shallow wet or dry etching of either the said pedestal member structure  40  or the encasing member  20 , or both. 
     One end of the pedestal member structure is attached along its width to a wall of fixed silicon frame  31  while the other end, a pedestal tip, is free as indicated in FIG. 3 a.    
     In order to minimise the longitudinal strains and stress(es) that may originate in the fixed frame, the suspension systems  33  that flexibly connect the seismic masses  32  to the pedestal member  40  are attached as far as possible from the edges of the wall of fixed silicon frame  31 , that is close to the pedestal tip, in attachments of said hinge  45 . The attachments of hinges  45  are indicated in FIG. 3 a  with dotted line, indicating the fact that they are not crossed by the sectioning line. 
     Except for the attachment to the fixed silicon frame, the bonded top area  41  and the attachments of hinges  45 , all the remaining pedestal surfaces are free and, therefore, no strains or stress can develop along them. 
     In order to reduce further on the transmitted strain and stress, the suspension system can be attached directly to the pedestal rim  43  and as far as possible from the thick portion  44  and bonded top area  41 , resulting an overall T-shaped pedestal, as indicated in FIG.  2 . FIG. 3 b  shows a transversal cross-section through the inertial sensor  10 , showing the pedestal member structure  40  and indicating the recommended location of the attachments of hinges  45 . 
     The length of the pedestal member and the width of the pedestal rim in the regions of the attachments of hinges are preferably as large as possible, their size being limited only by the available space within the sensor die. At the same time, the bonded top area  41  should be minimised in order to reduce the stress developed due to possible material mismatch between the substrate wafer  30  and the encasing member  20 . However, in order to achieve a rigid and strong bonding, the bonded top area  41  must not be reduced below certain limits that depend mainly on the chosen bonding technique. 
     In the preferred embodiment, as indicated in FIG. 5, only longitudinal mechanical loads can generate strain and stress in the attachments of hinges  45  and through the suspension system  33  further on into the device component  32 . However, owing to the pedestal geometry and method of assembly according to the present invention, the transmitted loads are several orders of magnitude smaller than the loads exerted on the silicon fixed frame  31 . 
     The mechanical strains and stress through the pedestal member structure  40  decrease from the wall of the silicon fixed frame  31  towards the pedestal tip and from the bonded top area  41  towards the sides and underneath free surfaces of the pedestal member. With the suspension systems attached as indicated in FIGS. 2,  3   a  and  3   b , the transmitted loads to the suspension systems  33  are torques which generate a negligible rotation around the attachments of hinges  45 . If the suspension systems  33  are attached exactly at the pedestal tip, along the pedestal length, the transmitted load  90  is a bending momentum. 
     The direct crossing  42  provide an advantageous way of reducing the bonded top area  41  between the pedestal member  40  and the encasing member  20 , thus minimising the transmitted load  90 , while preserving a required width which allows a strong and rigid bonding. 
     In FIGS. 6 a  to  6   d  several designs for the device are shown in transversal cross-sectional view and in analogy with the examples shown in FIGS. 1,  2 ,  3   a ,  3   b  and  5 . 
     FIG. 6 a  shows a transversal cross-sectional view through a basic example pedestal member structure  40 , wherein the separation gap  50  is achieved in two distinctive steps: a primary or recess  51  is structured by etching the silicon, either wet or dry, along the pedestal rim  43 , and a secondary (optional) recess  52  is structured by etching the silicon, either wet or dry above the suspension systems  33  and/or seismic masses  32 . The dashed line in FIG. 6 a  represents the surface of the substrate wafer  30  if the optional recess  52  is omitted. 
     Generally, the pedestal member structure  40  is electrically conductive, therefore electrically non-conductive layers, such as thermal oxides or regions, such as p-n junctions, must be employed whenever several electrically conductive paths are to be isolated from each other and/or from the pedestal bulk material. 
     FIGS. 6 b ,  6   c  and  6   d  show further optional features that can be implemented along with the basic pedestal member structure, depicted in FIG. 6 a.    
     The present invention allows the implementation of direct electrical contacts between a patterned metal film  61  located on the encasing member  20  and a patterned metal film  62  located on the substrate wafer  30 , by pressing and squeezing them into each other during the wafer bonding step, resulting in an electrical contact (press-contact)  60 . The press-contacts  60  allow the direct transfer of electric signals from conductive paths located on the encasing member  20  to conductive paths located on the substrate wafer  30 , a feature of importance in the design of micromachined sensors. At least one of the two metallic films  61  and  62  employed within a press contact  60  should be a soft metal that deforms and flows easily under pressure. In addition, the total area of each of the said press-contacts  60  should preferably remain small in order to allow for lateral flow of the soft metal film and the total thickness of the two metallic films  61  and  62  employed within a press-contact should be slightly larger than the said film recess  51 , but not excessively larger in order not to hinder the bonding process between the pedestal member  40  and the encasing member  20  occurring nearby. 
     FIG. 6 b  shows an example of press-contacts  60  in accordance with the present invention. 
     The present invention also allows the implementation of buried conductive paths (buried crossings) transversally through the thick portion  44 . The buried crossings are fabricated by a patterned implantation of a p-type impurity inside the thick portion  44  and a subsequent drive-in to the desired depth, such that a p-n junction forms between the implanted area and the underlying n-type thick portion  44 , followed by the epitaxial growth which buries the p-type conductor (buried conductor)  70 . Additionally, p-type impurities are implanted in at least two isolated islands and subsequently driven-in, resulting in surface conductors  71 , where surface conductors  71  are driven-in long enough to contact the buried conductor  70 , such that a transversal, continuous and buried electrical path between at least two press-contacts  60  is formed. 
     FIG. 6 b  shows an example of buried crossings in accordance with the present invention. 
     Surface conductors  71  can be used to carry isolated electrical signals along the pedestal rim  43 , where the separation gap  50  or recess rim  51  is used to prevent the bonding between the areas in which the surface conductors  71  are located and the encasing member  20  and where an-oxide layer  72  is used to passivate the p-n junctions that separate the said surface conductors  71  from the n-type material of the pedestal member  40 . 
     Metal conductors  62  can be used to carry isolated electrical signals along pedestal rim  43 , where an oxide layer  72  is used to electrically isolate the metal conductors  62  from the pedestal member  40  and where the separation gap  50  of recess rim  51  is used to prevent the bonding between the metal conductor  62  and the covering wafer  20 . 
     The present invention allows the implementation above and across the thick portion  44  of direct conductive paths within the said direct crossings  42 , wherein a patterned metal film  61  located on the encasing member  20  is the conductive element which is electrically isolated from the thick portion  44  by the separation gap  50  or recess rim  51 . 
     FIG. 6 c  shows surface conductors and metal conductors either along the pedestal rim  43  or across the thick portion  44  in accordance with the present invention. 
     The lateral walls of the thick portion  44 , adjacent to the said direct crossings  42 , are shown with dotted lines, indicating the fact that they are not crossed by the sectioning line. 
     The present invention allows the implementation above and across the thick portion  44  of direct conductive paths within the said crossings  42 , wherein a patterned metal film  62  located on the thick portion  44  is the conductive element which is electrically isolated from the thick portion  44  by means of an oxide layer  72  and from the encasing member  20  by the separation gap  50  or recess rim  51 . 
     The present invention allows the implementation above and across the pedestal member  40  of direct conductive paths within the direct crossings  42 , wherein a patterned surface conductor  71  located within the pedestal member  40  is the conductive element which is electrically isolated from the pedestal member  40  by means of a p-n junction and has its surface passivated by an oxide layer  72 . This in turn is separated from the encasing member  20  by the separation gap  50  or recess rim  51 . 
     FIG. 6 d  shows examples of the surface conductors and metal conductors across the pedestal member  40  in accordance with the present invention. The lateral walls of the pedestal member  40 , adjacent to the said direct crossings  42 , are shown with dotted lines, indicating the fact that they are not crossed by the sectioning line. 
     Side views of the said direct crossings  42 , with a direct conductive path realised as patterned metal film  61  or without are shown in FIGS. 3a and 5. 
     In alternative fabrication processes the pedestal material can be of a p-type. Consequently, the epitaxial layer, if used, is also of a p-type silicon. In this case donor impurities are implanted and driven-in to form the said buried conductors  70  and said surface conductors  71 . 
     FIGS. 7 a  to  7   d ,  8   a  and  8   d  to  9  show examples of the invention realised in alternative fabrication processes. 
     In alternative fabrication processes the separation gap  50  can be realised either totally or partially in the top encasing member  20 . 
     In alternative fabrication processes the pedestal member can have a structured thickness, achieved by using several and different implantation and drive-in steps, depending on the specific device application and design and on the attachment location of the suspension systems. 
     FIGS. 7 a  and  7   b  show cross-sectional views along lines A—A and B—B of FIG. 2, presenting an alternative pedestal member structure  40  in an alternative fabrication process in which the separation gap  50  is totally realised in the encasing member  20 . 
     FIG. 7 c  shows a cross-sectional view along line A—A of FIG. 2, presenting an alternative example of the pedestal member structure in an alternative fabrication process in which the pedestal member  40  has a structured thickness and the separation gap  50  is totally realized within the substrate wafer  30 . 
     FIG. 7 d  shows a cross-sectional view along line A—A of FIG. 2, presenting an alternative example of the pedestal member structure in the alternative fabrication process in which the pedestal member  40  has a structured thickness and the separation gap  50  is totally realised in the top covering wafer  20 . 
     FIG. 8 a  shows a cross-sectional view along line B—B of FIG. 2, presenting alternative examples of the pedestal member structure in the alternative fabrication process in which the separation gap  50  is partially realised in the top encasing member  20  and partially realised in the substrate wafer  30 , wherein the separation gap  50  consists of the rim recess  51  realised in the top encasing member  20  and at least one optional recess realised either within the substrate wafer  30 , denoted by reference designation  52 , or within the encasing member  20 , denoted by reference designation  53 . 
     FIG. 8 b  presents examples of press-contacts  60  and buried crossings in the pedestal member structure of FIG. 8 a.    
     FIGS. 8 c  and  8   d  present examples of electrical conductors along the pedestal rim  43  and/or across the pedestal member  40  and the direct crossings  42 , in the pedestal structure of FIG. 8 a . The lateral walls of the pedestal member  40 , adjacent to the direct crossings  42  are shown with dotted lines, indicating the fact that they are not crossed by the sectioning line. 
     FIG. 9 shows a cross-sectional view along line A—A of FIG. 2, presenting an alternative embodiment of the pedestal member structure in an alternative fabrication process in which the substrate wafer  30  is of an SOI type, that is it consists of a single crystal silicon top layer  74  separated from the bulk silicon by a buried, very thin layer of insulating oxide  75 . The separation gap  50  is depicted as being realised totally in the encasing member  20 , in comparison with the example shown in FIG. 4 where the separation gap  50  is realised totally within the top silicon layer  74 .