Abstract:
A process for the fabrication of an inertial sensor with failure threshold includes the step of forming, on top of a substrate of a semiconductor wafer, a sample element embedded in a sacrificial region, the sample element configured to break under a preselected strain. The process further includes forming, on top of the sacrificial region, a body connected to the sample element and etching the sacrificial region so as to free the body and the sample element. The process may also include forming, on the substrate, additional sample elements connected to the body.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to a process for the fabrication of an inertial sensor with failure threshold.  
           [0003]    2. Description of the Related Art  
           [0004]    As is known, modern techniques of micromachining of semiconductors can be advantageously exploited for making various extremely sensitive and precise sensors, having further small overall dimensions. The so-called MEMS sensors (or micro-electro-mechanical-system sensors), are sensors that can be integrated in a semiconductor chip and are suitable for detecting various quantities. In particular, both linear and rotational MEMS accelerometers with capacitive unbalancing are known. In brief, these accelerometers are normally provided with a fixed body and of a mobile mass, both of which are conductive and are capacitively coupled together. In addition, the capacitance present between the fixed body and the mobile mass may vary, and its value depends upon the relative position of the mobile mass with respect to the fixed body. When the accelerometer is subjected to a stress, the mobile mass is displaced with respect to the fixed body and causes a variation in the coupling capacitance, which is detected by a special sensing circuit.  
           [0005]    As mentioned previously, MEMS accelerometers are extremely sensitive and precise; however, they are not suitable for being used in many applications, mainly because they are complex to make and their cost is very high. On the one hand, in fact, the processes of fabrication involve the execution of numerous non-standard steps and/or the use of non-standard substrates (for example, SOI substrates); on the other hand, it is normally necessary to provide feedback sensing circuits based upon differential charge amplifiers, the design of which frequently involves some difficulties.  
           [0006]    In addition, in many cases the precision of capacitive MEMS sensors is not required and, indeed, it is not even necessary to have an instantaneous measurement of the value of acceleration. On the contrary, it is frequently just necessary to verify whether a device incorporating the accelerometer has undergone accelerations higher than a pre-set threshold, normally on account of impact. For example, the majority of electronic devices commonly used, such as cell phones, are protected by a warranty, which, however, is no longer valid if any malfunctioning is due not to defects of fabrication but to an impact consequent on the device being dropped onto an unyielding surface or in any case on a use that is not in conformance with the instructions. Unless visible damage is found, such as marks on the casing or breaking of some parts, it is practically impossible to demonstrate that the device has suffered damage that invalidates the warranty. On the other hand, portable devices, such as cell phones, exactly, are particularly exposed to being dropped and consequently to getting broken, precisely on account of how they are used.  
           [0007]    Events of the above type could be easily detected by an inertial sensor, which is able to record accelerations higher than a pre-set threshold. However, the use of MEMS accelerometers of a capacitive type in these cases would evidently lead to excessive costs. It would thus be desirable to have available sensors that can be made using techniques of micromachining of semiconductors, consequently having overall dimensions comparable to those of capacitive MEMS sensors, but simpler as regards both the structure of the sensor and the sensing circuit. In addition, also the processes of fabrication should be, as a whole, simple and inexpensive.  
         BRIEF SUMMARY OF THE INVENTION  
         [0008]    The purpose of the present invention is to provide a process for the fabrication of an inertial sensor with failure threshold, which will enable the problems described above to be overcome.  
           [0009]    According to an embodiment of the present invention, a process is provided for the fabrication of an inertial sensor with failure threshold, including the step of forming at least one sample element embedded in a sacrificial region on top of a substrate of a semiconductor wafer, the sample element being configured to fracture under a preselected force. The process further includes forming, on top of the sacrificial region, a body connected to the sample element, and etching the sacrificial region, so as to free the body and the sample element.  
           [0010]    The process may include the step of making a weakened region of the sample element. The weakened region may be made by forming a narrowed region or notches in the sample element.  
           [0011]    The process may include forming a plurality of sample elements, each configured to fracture under the preselected force.  
           [0012]    According to an alternative embodiment of the invention, a method for manufacturing an inertial sensor is provided, comprising forming, on a semiconductor substrate, a sample element configured to break under a preselected strain, the sample element having a first end coupled to the substrate, and forming, above the semiconductor substrate, a semiconductor material body coupled to a second end of the sample element. The method may include forming a weakened region on the sample element, with the sample element configured to break at the weakened region under the preselected strain.  
           [0013]    According to this embodiment, the sample element may have a T shape, the first end being the cross-bar portion of the T and being coupled to the substrate at extreme ends thereof, the second end being the upright portion of the T.  
           [0014]    The method may also include forming an additional sample element having a first end coupled to the substrate, a second end coupled to the semiconductor material body, and configured to break under the preselected strain.  
           [0015]    According to another embodiment of the invention, A method of measuring movement of a device is provided, including providing a circuit in the device configured to permanently change the conductive state of a conductive path in the event the device is subjected to an acceleration exceeding a preselected level, applying a potential at first and second ends of the conductive path, and detecting a change in the conductive state of the conductive path.  
           [0016]    The method may further include breaking a semiconductor structure through which the conductive path passes in the event the device is subjected to the acceleration. This step may be performed by moving a first semiconductor body relative to a second semiconductor body in response to inertial forces resulting from the acceleration, the semiconductor structure being coupled at a first end thereof to the first body and at a second end to the second body, the movement of the first body causing a flexion of the structure, resulting in the breaking thereof.  
           [0017]    The device may be a cell phone, and the preselected level may be selected to correspond to an acceleration caused by a drop of the device to an unyielding surface from a preselected height. The preselected level may also be selected to be equal to or less than an acceleration sufficient to damage the device. 
       
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING(S)  
       [0018]    For a better understanding of the invention, some embodiments thereof are now described, purely by way of non-limiting examples and with reference to the attached drawings, in which:  
         [0019]    [0019]FIGS. 1 and 2 are cross-sectional views through a semiconductor wafer in successive steps of fabrication in a first embodiment of the process according to the present invention;  
         [0020]    [0020]FIG. 3 is a top plan view of the wafer of FIG. 2;  
         [0021]    [0021]FIG. 4 illustrates an enlarged detail of FIG. 3;  
         [0022]    [0022]FIG. 5 is a cross-sectional view of the wafer of FIG. 3 in a subsequent fabrication step;  
         [0023]    [0023]FIG. 6 is a top plan view of the wafer of FIG. 5;  
         [0024]    [0024]FIGS. 7 and 8 are cross-sectional views of the wafer of FIG. 6 in a subsequent fabrication step, taken along the planes of trace VII-VII and VIII-VIII, respectively, of FIG. 6;  
         [0025]    [0025]FIG. 9 is a top plan view of the wafer of FIG. 7, in a subsequent fabrication step, in which an inertial sensor is obtained;  
         [0026]    [0026]FIGS. 10 and 11 are cross-sectional views of the wafer of FIG. 9, taken along the planes of trace X-X and XI-XI, respectively, of FIG. 9;  
         [0027]    [0027]FIGS. 12 and 13 are cross-sectional views through a composite wafer and a die, respectively, obtained starting from the wafer of FIG. 9;  
         [0028]    [0028]FIG. 14 is a schematic view of the top three quarters of a device incorporating the die of FIG. 13;  
         [0029]    [0029]FIG. 15 is a schematic illustration of an inertial sensor of the type illustrated in FIGS.  9 - 13  in an operative configuration;  
         [0030]    [0030]FIG. 16 is a detail of an inertial sensor obtained according to a variant of the first embodiment of the present process;  
         [0031]    [0031]FIG. 17 is a top plan view of an inertial sensor obtained according to a further variant of the first embodiment of the present process;  
         [0032]    [0032]FIG. 18 is a cross-sectional view of the sensor of FIG. 17;  
         [0033]    [0033]FIG. 19 is a top plan view of an inertial sensor obtained according to a second embodiment of the present invention;  
         [0034]    [0034]FIG. 20 illustrates an enlarged detail of FIG. 19;  
         [0035]    [0035]FIG. 21 is a top plan view of an inertial sensor obtained according to a third embodiment of the present invention;  
         [0036]    [0036]FIG. 22 illustrates an enlarged detail of FIG. 21;  
         [0037]    [0037]FIG. 23 is a schematic illustration of two inertial sensors of the type illustrated in FIG. 21 in an operative configuration;  
         [0038]    [0038]FIG. 24 is a cross-sectional view through a semiconductor wafer in an initial fabrication step of a process according to a fourth embodiment of the present invention;  
         [0039]    [0039]FIG. 25 is a top plan view of the wafer of FIG. 24;  
         [0040]    [0040]FIG. 26 illustrates the wafer of FIG. 24 in a subsequent fabrication step;  
         [0041]    [0041]FIG. 27 is a top plan view of the wafer of FIG. 26 in a subsequent fabrication step, in which an inertial sensor is obtained;  
         [0042]    [0042]FIG. 28 is a cross-sectional view through the wafer of FIG. 27, taken according to the plane of trace XXVI-XXVI of FIG. 27;  
         [0043]    [0043]FIG. 29 is a plan view of a detail of an inertial sensor obtained according to a fifth embodiment of the present invention  
         [0044]    [0044]FIG. 30 is a side view of the detail of FIG. 29; and  
         [0045]    [0045]FIG. 31 is a side view of the detail of FIG. 29, obtained according to a variant of the fifth embodiment of the present invention. 
     
    
     DESCRIPTION OF THE INVENTION  
       [0046]    With reference to FIGS.  1 - 13 , a wafer  1  of semiconductor material, for example monocrystalline silicon, comprises a substrate  2 , on which a thin pad oxide layer  3 , for example 2.5 μm thick, is thermally grown. A conductive layer  5  of polysilicon, having for example a thickness of between 400 and 800 nm and a dopant concentration of 10 19  atoms/cm 3 , is then deposited on the pad oxide layer  3  and is defined by means of a photolithographic process. Two T-shaped samples  6  are thus obtained, having respective feet  6   a , aligned with respect to one another and extending towards one another, and respective arms  6   b  parallel to one another (FIGS.  2 - 4 ). The feet  6   a  and the arms  6   b  of each sample  6  are set in directions identified by a first axis X and, respectively, by a second axis Y, which are mutually orthogonal (a third axis Z, orthogonal to the first axis X and the second axis Y, is illustrated in FIG. 2). In addition, at respective ends of the arms  6   b  of both the samples  6  anchoring pads  8  are made, of a substantially rectangular shape and having a width greater than the arms  6   b . As illustrated in FIG. 4, each of the samples  6  has a first weakened region  9  and a second weakened region  10 . In particular, in both of the samples  6 , the first weakened region  9  and the second weakened region  10  are made as narrowed portions of the foot  6   a  and, respectively, of one of the arms  6   b . In addition the weakened regions  9 ,  10  are defined by notches  11  with a circular or polygonal profile, made in an area of joining  6   c  between the foot  6   a  and the arms  6   b  and traversing the sample  6  in a direction parallel to the third axis Z. The thickness of the conductive layer  5  of polysilicon, the dimensions of the feet  6   a  and of the arms  6   b  of the samples  6 , and the conformation of the weakened regions  9 ,  10  determine the mechanical resistance to failure of the samples  6  themselves. In particular, acting on the shape and on the dimensions of the notches  11  defining the first weakened region  9  and the second weakened region  10 , it is possible to obtain pre-set failure thresholds of the samples  6  along the first, second and third axes X, Y and Z. Preferably, all the mechanical failure thresholds are basically the same.  
         [0047]    Next, a sacrificial layer  12  of silicon dioxide is deposited so as to coat the pad oxide layer  3  and the samples  6 . In practice, the pad oxide layer  3  and the sacrificial layer  12  form a single sacrificial region in which the samples  6  are embedded. The sacrificial layer  12  is then defined by means of a photolithographic process comprising two masking steps. During a first step, first openings  14  are made in the sacrificial layer  12 , exposing respective ends of the feet  6   a  of the samples  6 , as illustrated in FIG. 5. In a second step of the photolithographic process (FIG. 6), both the sacrificial layer  12  and the pad oxide layer  3  are selectively etched, so as to make second openings  15 , exposing portions of the substrate  2 .  
         [0048]    Subsequently, a conductive epitaxial layer  16  is grown on the wafer  1 , the said layer having a thickness, for example, of 15 μm and a dopant concentration of 10 18  atoms/cm 3 . In detail, the epitaxial layer  16  coats the sacrificial layer  12  entirely and extends in depth through the first and the second openings  14 ,  15  until the samples  6  and the substrate  2 , respectively, are reached (FIG. 7 and  8 ).  
         [0049]    The epitaxial layer  16  is then selectively etched, preferably by reactive-ion etching (RIE), and the sacrificial layer  12  and the pad oxide layer  3  are removed. In greater detail, during the step of etching of the epitaxial layer  16 , the following are formed: a mobile mass  18 ; anchorages  19 , provided on the portions of the substrate  2  previously exposed by the second openings  15 ; a plurality of springs  20 , connecting the mobile mass  18  to the anchorages  19 ; and a ring-shaped supporting structure  21 , which surrounds the mobile mass  18 , the samples  6 , the springs  20 , and the corresponding anchorages  19  (see FIG. 9, in which the sacrificial layer  12  and the pad oxide layer  3  have already been removed).  
         [0050]    The mobile mass  18  is connected to the substrate  2  by the springs  20 , which are in turn constrained to the anchorages  19  (FIG. 11). The springs  20 , which are per se known, are shaped so as to enable oscillations of the mobile mass  18  with respect to the substrate  2  along each of the three axes X, Y, Z, at the same time, however, preventing rotations. The mobile mass  18  is moreover constrained to the substrate  2  through the samples  6 . In greater detail, the mobile mass  18  has, in a median portion, a pair of anchoring blocks  22 , projecting outwards in opposite directions along the second axis Y. The anchoring blocks  22  are connected to the end of the foot  6   a  of a respective one of the samples  6 , as illustrated in FIG. 10. In turn, the samples  6  are anchored to the substrate  2  through the anchoring pads  8 . By controlling the duration of etching of the sacrificial layer  12  and of the pad oxide layer  3 , the silicon dioxide is in fact removed only partially underneath the anchoring pads  8 , which are wider than the feet  6   a  and the arms  6   b  of the samples  6 ; thus, residual portions  3 ′ of the pad oxide layer  3 , which are not etched, fix the anchoring pads  8  to the substrate  2 , serving as bonding elements.  
         [0051]    The sacrificial layer  12  and the remaining portions of the pad oxide layer  3  are, instead, completely removed and, hence, the mobile mass  18  and the samples  6  are freed. In practice, the mobile mass  18  is suspended at a distance on the substrate  2  and can oscillate about a resting position, in accordance with the degrees of freedom allowed by the springs  20  (in particular, it can translate along the axes X, Y and Z). Also the samples  6  are elastic elements, which connect the mobile mass  18  to the substrate  2  in a way similar to the springs  20 . In particular, the samples are shaped so as to be subjected to a stress when the mobile mass  18  is outside a relative resting position with respect to the substrate  2 . The samples  6  are, however, very thin and have preferential failure points in areas corresponding to the weakened regions  9 ,  10 . For this reason, their mechanical resistance to failure is much lower than that of the springs  20 , and they undergo failure in a controlled way when they are subjected to a stress of pre-set intensity.  
         [0052]    In practice, at this stage of the process, the mobile mass  18 , the substrate  2 , the springs  20  with the anchorages  19 , and the samples  6  form an inertial sensor  24 , the operation of which will be described in detail hereinafter.  
         [0053]    An encapsulation structure  25  for the inertial sensor  24  is then applied on top of the wafer  1 , forming a composite wafer  26  (FIG. 12). In particular, the encapsulation structure  25  is an additional semiconductor wafer, in which a recess  27  has previously been opened, in a region that is to be laid on top of the mobile mass  18 . The encapsulation structure  25  is coupled to the ring-shaped supporting structure  21  by the interposition of a layer of soldering  29 . Next, the compound wafer  26  is cut into a plurality of dice  30 , each die comprising an inertial sensor  24  and a respective protective cap  31 , formed by the fractioning of the encapsulation structure  25  (FIG. 13).  
         [0054]    The die  30  is finally mounted on a device  32 , for example a cell phone. Preferably, the device  32  is provided with a casing  33 , inside which the die  30  is fixed, as illustrated in FIG. 14. In addition (FIG. 15), the inertial sensor  24  is connected to terminals of a testing circuit  35 , which measures the value of electrical resistance between said terminals. In greater detail, the anchoring pads  8  of the arms  6   b , in which the second weakened regions  10  are formed, are connected each to a respective terminal of the testing circuit  35 .  
         [0055]    In normal conditions, i.e., when the inertial sensor  24  is intact, the samples  6  and the mobile mass  18  form a conductive path that enables passage of current between any given pair of anchoring pads  8 . In practice, the testing circuit  35  detects low values of electrical resistance between the anchoring pads  8 . During normal use, the device  32  undergoes modest stresses, which cause slight oscillations of the mobile mass  18  about the resting position, without jeopardizing the integrity of the inertial sensor  24 .  
         [0056]    When the device  32  suffers a shock, the mobile mass  18  of the inertial sensor  24  undergoes a sharp acceleration and subjects the samples  6  and the springs  20  to a force. According to the intensity of the stress transmitted to the inertial sensor  24 , said force can exceed one of the thresholds of mechanical failure of the samples  6 , which consequently break. In particular, failure occurs at one of the weakened regions  9 ,  10 , which have minimum strength. In either case, the conductive path between the two anchoring pads  8  connected to the testing circuit  35  is interrupted, and hence the testing circuit detects a high value of electrical resistance between its own terminals, thus enabling recognition of the occurrence of events that are liable to damage the device  32 .  
         [0057]    According to a variant of the embodiment described, shown in FIG. 16, T-shaped samples  37  are provided, which present a single weakened region  38 . In particular, the weakened region  38  is a narrowed portion defined by a pair of notches  39 , which are oblique with respect to a foot  37   a  and arms  37   b  of the samples  37 .  
         [0058]    According to a further variant, illustrated in FIGS. 17 and 18, the two T-shaped samples  6  are located in a gap  36  between the substrate  2  and the mobile mass  18  and have the end of the respective feet  6   a  in mutual contact. In addition, both of the samples  6  are fixed to a single anchoring block  22 ′ set centrally with respect to the mobile mass  18  itself.  
         [0059]    The process according to the invention has the following advantages. In the first place, for fabrication of the inertial sensor  24 , processing steps that are standard in the microelectronics industry are employed. In particular, the following steps are carried out: steps of deposition of both insulating and conductive layers of material; photolithographic processes; a step of epitaxial growth; and standard steps of etching of the epitaxial silicon and of the insulating layers. Advantageously, a single step of thermal oxidation is carried out, and consequently the wafer  1  is subjected to modest stresses during the fabrication process. The yield of the process is therefore high. In addition, the inertial sensor  24  is obtained starting from a standard, low-cost substrate.  
         [0060]    The process described consequently enables inertial sensors with failure threshold to be produced at a very low cost. Such sensors are particularly suitable for use where it is necessary to record the occurrence of stresses that are harmful for a device in which they are incorporated and in which it is superfluous to provide precise measurements of accelerations. For example, they can be advantageously used for verifying the validity of the warranty in the case of widely used electronic devices, such as, for example, cell phones.  
         [0061]    In addition, the inertial sensors provided with the present method have contained overall dimensions. In inertial sensors, in fact, large dimensions are generally due to the mobile mass, which must ensure the necessary precision and sensitivity. In this case, instead, it is sufficient that, in the event of a predetermined acceleration, the mobile mass will cause breaking of the weakened regions of the samples, which have low strength. It is consequently evident that also the mobile mass can have contained overall dimensions.  
         [0062]    The use of a single anchoring point between the samples and the mobile mass, as illustrated in the second variant of FIGS. 17 and 18, has a further advantage as compared to the ones already pointed out, because it enables more effective relaxation of the stresses due to expansion of the materials. In particular, it may happen that the polysilicon parts which are even only partially embedded in the silicon dioxide (samples and portions of the epitaxial layer) will be subjected to a compressive force, since both the polysilicon, and the oxide tend to a expand in opposite directions during the fabrication process. When the oxide is removed, the action of compression on the polysilicon is eliminated, and the polysilicon can thus expand. Clearly, the largest expansion, in absolute terms, is that of the mobile mass, since it has the largest size. The use of a single anchoring point, instead of two anchorages set at a distance apart enables more effective relaxation of the stresses due to said expansion, since the mobile mass can expand freely, without modifying the load state of the samples.  
         [0063]    The inertial sensors obtained using the process described are more advantageous because they respond in a substantially isotropic way to the mechanical stress. In practice, therefore, just one inertial sensor is sufficient to detect forces acting in any direction.  
         [0064]    A second embodiment of the invention is illustrated in FIGS. 19 and 20, where parts that are the same as the ones already illustrated are designated by the same reference numbers. According to said embodiment, an inertial sensor  40  is made, having L-shaped samples  41 . As in the previous case, the samples  41  are obtained by shaping a conductive polysilicon layer deposited on top of a pad oxide layer (not illustrated herein), which has in turn been grown on the substrate  42  of a semiconductor wafer  43 . Using processing steps similar to the ones already described, the mobile mass  18 , the anchorages  19  and the springs  20  are subsequently obtained.  
         [0065]    In detail, the samples  41  have first ends connected to respective anchoring blocks  22  of the mobile mass  18 , and second ends terminating with respective anchoring pads  41  fixed to the substrate  2 , as explained previously. In addition, notches  42  made at respective vertices  43  of the samples  41  define weakened regions  44  of the samples  40 .  
         [0066]    [0066]FIGS. 21 and 22 illustrate a third embodiment of the invention, according to which an inertial sensor  50  is obtained, made on a substrate  54  and provided with substantially rectilinear samples  51  that extend parallel to the first axis X. In this case, during the RIE etching step, in addition to the mobile mass  18 , two anchorages  52  and two springs  53  of a known type are provided, which connect the mobile mass  18  to the anchorages  52  and are shaped so as to prevent substantially the rotation of the mobile mass  18  itself about the first axis X.  
         [0067]    The samples  51  have first ends soldered to respective anchoring blocks  22  of the mobile mass  18  and second ends terminating with anchoring pads  55 , made as described previously. In addition, pairs of transverse opposed notches  57  define respective weakened regions  58  along the samples  51  (FIG. 22).  
         [0068]    Alternatively, the weakened regions may be absent.  
         [0069]    The inertial sensor  50  responds preferentially to stresses oriented according to a plane orthogonal to the samples  51 , i.e., the plane defined by the second axis Y and by the third axis Z. In this case, to detect stresses in a substantially isotropic way, it is possible to use two sensors  50  connected in series between the terminals of a testing circuit  59  and rotated through 90° with respect to one another, as illustrated in FIG. 23.  
         [0070]    With reference to FIGS.  24 - 28 , according to a fourth embodiment of the invention, a pad oxide layer  62  is grown on a semiconductor wafer  60  having a substrate  61 . Next, a conductive layer  63  of polycrystalline silicon (here indicated by a dashed line) is deposited on the pad oxide layer  61  and is defined to form a sample  64 , which is substantially rectilinear and extends parallel to the first axis X (FIG. 25). The sample  64  has an anchoring pad  65  at one of its ends and has a weakened region  66  defined by a pair of notches  67  in a central position.  
         [0071]    A sacrificial layer  69  of silicon dioxide is deposited so as to coat the entire wafer  60  and is then selectively removed to form an opening  68  at one end of the sample  64  opposite to the anchoring pad  65 .  
         [0072]    An epitaxial layer  70  is then grown (FIG. 26), which is etched so as to form a mobile mass  71 , anchorages  72 , springs  73 , and a supporting ring (not illustrated for reasons of convenience). The epitaxial layer extends into the opening  68  to form a connection region  88  between the sample  64  and the mobile mass  71 .The sacrificial layer  69  and the pad oxide layer  62  are removed, except for a residual portion  62 ′ of the pad oxide layer  62  underlying the anchoring pad  65  (FIGS. 27 and 28). The mobile mass  71  and the sample  64  are thus freed. More precisely, the mobile mass  71 , which has, at its center, a through opening  74  on top of the sample  64 , is constrained to the substrate  61  through the anchorages  72  and the springs  73 , which are shaped so as to prevent any translation along or rotation about the first axis X. In addition, the sample  65  has opposite ends, one connected to the substrate  2  through the anchoring pad  65 , and the other to the mobile mass  71  at connection region  88 , and is placed in a gap  76  comprised between the mobile mass  71  and the substrate  61 .  
         [0073]    In this way, an inertial sensor  80  is obtained, which is then encapsulated through steps similar to the ones described with reference to FIGS. 12 and 13.  
         [0074]    Also in this case, the use of a single anchoring point between the sample and the mobile mass advantageously enables effective relaxation of the stresses due to expansion of the mobile mass.  
         [0075]    According to one variant (not illustrated), the sample is T-shaped, like the ones illustrated in FIG. 9.  
         [0076]    [0076]FIG. 29 illustrates a detail of a sample  81 , for example a rectilinear one, of an inertial sensor obtained using a fifth embodiment of the process according to the invention. In particular, the sample  81  has a weakened region defined by a transverse groove  82  extending between opposite sides  83  of the sample  81 .  
         [0077]    The groove  82  is obtained by means of masked etching of controlled duration of the sample  81  (FIG. 30).  
         [0078]    Alternatively (FIG. 31), a first layer  85  of polysilicon is deposited and defined. Then, a stop layer  86  of silicon dioxide and a second layer  87  of polysilicon are formed. Finally, a groove  82 ′ is dug by etching the second layer  87  of polysilicon as far as the stop layer  86 .  
         [0079]    Finally, it is evident that modifications and variations may be made to the process described herein, without thereby departing from the scope of the present invention. In particular, the weakened regions can be defined by using side notches in the samples together with grooves extending between the side notches. In addition, the weakened regions could be defined by through openings that traverse the samples, instead of by side notches.  
         [0080]    All of the above U.S. patents, U.S. patent application publications, U.S. patent applications, foreign patents, foreign patent applications and non-patent publications referred to in this specification and/or listed in the Application Data Sheet, are incorporated herein by reference, in their entirety.  
         [0081]    From the foregoing it will be appreciated that, although specific embodiments of the invention have been described herein for purposes of illustration, various modifications may be made without deviating from the spirit and scope of the invention. Accordingly, the invention is not limited except as by the appended claims.