Abstract:
The semiconductor inertial sensor is formed by a rotor element and a stator element electrostatically coupled together. The rotor element is formed by a suspended mass and by a plurality of mobile electrodes extending from the suspended mass. The stator element is formed by a plurality of fixed electrodes facing respective mobile electrodes. The suspended mass is supported by elastic suspension elements. The suspended mass has a first, larger, thickness, and the elastic suspension elements have a second thickness, smaller than the first thickness.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a MEMS-type high-sensitivity inertial sensor and to the manufacturing process thereof. 
   2. Description of the Related Art 
   As is known, techniques of micromachining of semiconductors are used also for manufacturing electromechanical microdevices (so-called micro-electro-mechanical-systems or MEMS), such as sensors and actuators of various types. In particular, the techniques of micromachining are advantageously used for manufacturing inertial sensors, utilized for example in the automotive sector or within apparatuses equipped with stand-by functions, for recovery of functionality starting from the stand-by condition upon detection of a movement. 
   Currently, inertial sensors are formed preferably by surface micromachining, wherein the mobile and fixed elements that form the sensor (rotor and stator and corresponding electrodes) are formed in a same structural layer, typically a semiconductor layer, of a mono-crystalline or polycrystalline type. 
   In this type of sensors, the thickness of the structural layer influences both the compliance of the structure to mechanical stresses (stiffness) and the mass. Any increase in the thickness of the structural layer brings about an increase in inertia (as a result of the increase in the mass), and consequently in the mechanical sensitivity of the sensor, i.e., the capacity for the rotor to modify its relative position when subjected to a stress, without any increase in the overall dimensions of the sensor. 
   However, the increase in the thickness determines an increase not only in the mass of the system but also in the stiffness of the springs, thus countering the improvement in the sensitivity of the sensor. 
   Not even other ways of increasing the mechanical sensitivity of the sensor are able to solve the problem. For example, by making springs that are more compliant, it is possible to increase the degree of movement of the rotor for a same applied stress; however, in this case the capacity to reject movements in other directions is reduced, and the sensor could yield false readings. 
   For a better understanding of the problem referred to above, in particular the one linked to the increase in thickness, reference may be made to  FIGS. 1-3 , corresponding to a known translational inertial sensor. 
   In  FIGS. 1 and 2 , an inertial sensor  1  comprises a body of semiconductor material, formed by a substrate  2  and by a structural layer  3 , overlying one another. The structural layer  3  forms a rotor  5  and a stator  6 , extending above the substrate  2  but set at a distance therefrom by an air gap  7 . The air gap  7  is obtained in a known way by removing a portion of a sacrificial layer, for example of silicon oxide, a remaining portion of which is designated by  8 . 
   The rotor  5  comprises a suspended mass  10 , here of a substantially parallelepipedal shape, and first and second mobile electrodes  11   a ,  11   b  extending from two opposite sides of the suspended mass  10  and arranged parallel to one another. The stator  6  comprises first and second fixed electrodes  12   a ,  12   b  extending parallel to one another and to the mobile electrodes  11   a ,  11   b . In particular, the first fixed electrodes  12   a  are comb-fingered to the first mobile electrodes  11   a , and the second fixed electrodes  12   b  are comb-fingered to the second mobile electrodes  11   b . The fixed electrodes  12   a ,  12   b  extend from a fixed supporting structure  13 , carried by the substrate  2 , only some parts whereof are visible in  FIGS. 1 and 2 . 
   As represented schematically in  FIG. 1 , the rotor  5  is supported by the fixed structure  13  via elastic elements or springs  15  that enable oscillation of the rotor  5  in the direction indicated by the arrow A. The anchorage portions of the rotor  5  are obviously electrically insulated from the stator  6  via junction insulations, dielectric insulations, or by trenches, in a per se known manner that is not illustrated in the drawings. 
   In use, the rotor  5  is biased at a sinusoidal a.c. voltage V 1 , as represented in  FIG. 1  by a voltage generator  16 , while the stator  6  is connected to a sensing circuit  20  comprising two operational amplifiers  21   a ,  21   b , each connected to a respective set of fixed electrodes  12   a ,  12   b.    
   In detail, the operational amplifiers  21   a ,  21   b  have an inverting input connected to the respective set of fixed electrodes  12   a ,  12   b , and a non-inverting input connected to ground. A feedback capacitor  23   a ,  23   b  is moreover connected between an output  24   a ,  24   b  and the inverting input of a respective operational amplifier  21   a ,  21   b . The resulting electrical diagram is illustrated in  FIG. 3 , which relates to both the operational amplifiers  21   a ,  21   b  and wherein consequently the elements represented have been identified without using the letters a and b. 
   As may be noted in particular from  FIG. 2 , the fixed electrodes  12  and the mobile electrodes  11  have a thickness t equal to the thickness of the structural layer  3  and a length l, and are arranged at a distance from the adjacent electrode of an opposite type (i.e., mobile or fixed) by a space g, which is variable and depends upon the instantaneous position of the rotor  5 . 
   In practice, as illustrated in the equivalent circuit of  FIG. 3 , the fixed electrodes  12  and the mobile electrodes  11  on each side of the suspended mass  5  form a variable capacitor  25  having a capacitance C 1  given by: 
                 C1   =         ɛ   0     ⁢   N   ⁢           ⁢     A   g       =       ɛ   0     ⁢   Nl   ⁢           ⁢     t   g                 (   1   )               
where ε 0  is the dielectric constant in a vacuum; N is the number of fixed electrodes  12  connected to each operational amplifier  21 ; l, t, g are the quantities indicated above; A represents the facing area, which here is approximately equal to l×t, since the length of facing between fixed electrodes  12  and mobile electrodes  11  can be considered, to a first approximation, equal to l.
 
   From Eq. (1) it is evident how the capacitance C 1  is directly proportional to the thickness t of the structural layer  3 . 
   In general, it is moreover possible to state that the mass M of the rotor  5  and hence substantially of the suspended mass  10  is given by the formula:
 
 M∝ρt   M   A=ρt   M   l   typ,rot   2   (2)
 
where ρ is the density of the material (silicon), t M  is the thickness of the suspended mass  10 , and l typ,rot  is the typical length (which is linked to the width of the suspended mass  10  and thus to the overall dimensions) of the sensor  1 .
 
   The stiffness k of a spring, instead, is given by: 
                 k   ∝       t   k       l     typ   ,   s     n               (   3   )               
where t k  is the thickness of the spring, l typ,s  is the typical length of the spring, and n is a coefficient linked to the type of sensor and is typically equal to 3 for planar sensors, whether of a linear type or of a rotational type.
 
   The sensitivity S of a sensor of this type is: 
                   S   ∝     M   k       =     ρ   ⁢           ⁢       t   M       t   k       ⁢       l       typ   .     ,   rot     2     ·     l     typ   ,   s     n                 (   4   )               
From Eq. (4) it may thus be noted that, in a typical micromachining process, in which the two thicknesses t M  and t k  are equal, the sensitivity S is invariant to the variation in thickness.
 
   Thus, currently, when it is desired to increase the sensitivity of the sensor, the design aims at increasing the occupation of area (i.e., l typ ) of the sensor either to increase the mass of the system or to reduce the stiffness of the elastic suspension springs. 
   Similar considerations apply to an inertial sensor of rotational type, the simplified structure whereof is shown in  FIG. 4  where, for reasons of clarity of illustration, the same reference numbers as those of  FIGS. 1 and 2  have been used. In detail,  FIG. 4  shows an inertial sensor  1 ′ having a rotor  5 , a stator  6 , a suspended mass  10 , mobile electrodes  11 , fixed electrodes  12 , springs  15 , and a fixed structure  13 . In a way not shown, the inertial sensor  1 ′ is connected to a sensing circuit similar to the sensing circuit  20  of  FIG. 1 , so that the inertial sensor  1 ′ has the equivalent circuit illustrated in  FIG. 3  and has an output voltage V 0  given by Eq. (4). 
   In this case, the inertial sensor  1 ′ has a moment of inertia J z  with respect to the axis Z of rotation equal to: 
                   J   Z     =       1   2     ⁢     MR   2               (   5   )               
where M is the mass (practically coinciding with that of the suspended mass  10 ), and R is the mean radius of the rotor  5 , substantially due to the radius of the suspended mass  10 .
 
   As may be noted, the moment of inertia is directly proportional to the mass, which is in turn directly proportional to the thickness. Since the mechanical sensitivity of the inertial sensor  1 ′ of rotational type is linked directly to the moment of inertia, the increase in the thickness of the structural layer accommodating both the rotor  5  and the stator  6  determines an increase in the mechanical sensitivity. However, also in this case this effect is nullified at the sensing circuit. As for the inertial sensor  1  of  FIGS. 1 and 2 , then, it is not possible to increase the sensitivity of the inertial sensor simply by increasing the thickness of the structural layer. 
   BRIEF SUMMARY OF THE INVENTION 
   According to an embodiment of the present invention, there are provided a semiconductor inertial sensor and the manufacturing process thereof. 
   In practice, according to one aspect of the invention, the thickness of the rotor and that of the spring (elastic suspension) are different from one another, even though they are provided in a same structural layer. In this way, it is possible to separate the influence of the thickness on the stiffness and on the mass of the inertial sensor. 

   
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
     For an understanding of the present invention, preferred embodiments thereof are now described, only as non-limiting examples, with reference to the attached drawings, wherein: 
       FIG. 1  is a schematic top view of a known inertial sensor, of translational type; 
       FIG. 2  is a partially broken perspective view of the inertial sensor of  FIG. 1 ; 
       FIG. 3  shows an equivalent circuit of the inertial sensor of  FIG. 1 ; 
       FIG. 4  is a schematic top view of a known inertial sensor of rotational type; 
       FIG. 5  is a partially ghost perspective view of an embodiment of an inertial sensor according to the invention, of translational type; 
       FIG. 6  is a cross-sectional view taken along section line VI-VI of  FIG. 5 ; 
       FIGS. 7-9  are cross-sectional views similar to that of  FIG. 6 , in successive manufacturing steps, according to a first embodiment of the process; and 
       FIGS. 10-15  are cross-sectional views taken along section line VI-VI of  FIG. 5 , in successive manufacturing steps, according to a second embodiment of the process. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   With reference to  FIG. 5 , an inertial sensor  30  comprises a body of semiconductor material formed by a substrate  31  and by a structural layer  32 , overlying one another. In the example illustrated, the structural layer  32  is formed by a stack of layers including a bottom semiconductor layer  33 , an intermediate dielectric layer  34 , and a top semiconductor layer  35 . As in the known inertial sensor  1 , a rotor  38  and a stator  39  are formed in the structural layer  32 ; the rotor  38  is moreover supported by elastic elements or springs  45 . 
   As may be noted, here the rotor  38  has a thickness greater than that of the stator  39  and of the springs  45 . In fact, the rotor  38  is formed by all the layers  33 - 35  of the structural layer  32 , while the stator  39  and the springs  45  are formed by just the bottom semiconductor layer  33 , as is evident particularly from the cross-section of  FIG. 6 . The rotor  38 , the stator  39 , and the springs  45  are also here arranged at a distance from the substrate  31  by an air gap  37 , which extends here between the substrate  31  and the structural layer  32 . 
   In a way similar to that of the known inertial sensor  1 , the rotor  38  comprises a suspended mass  40 , mobile electrodes  41  extending from the suspended mass and comb-fingered to fixed electrodes  42 . Also here the fixed electrodes  42  extend from a fixed structure  43  resting on the substrate  31  through a sacrificial layer  44 , a portion of which has been removed to provide the air gap  37 , as explained in greater detail hereinafter. A trench  46  extends between the stator  39  and the rotor  38 . 
   Furthermore, the springs  45  are shaped so as to enable oscillation of the rotor  38  in the direction indicated by the arrow A. Obviously, also here the regions of the fixed structure  43  electrically connected to the rotor  38  through the springs  45  are electrically insulated from the stator  39  with a junction insulation, with a dielectric, or by trenches, in a per se known manner, not illustrated in the drawings. 
   In practice, with the inertial sensor  30  illustrated in  FIGS. 5 and 6  it is possible to obtain a higher mass with the same occupation of area, maintaining the stiffness of the springs constant, thus increasing the sensitivity of the system, as emerges clearly from Eq. (4). Alternatively, the sensitivity can be increased, maintaining the mass of the system constant and reducing the stiffness of the springs. Likewise, it is possible to increase the mass of the system and the stiffness of the springs proportionally, obtaining a sensor which, given the same sensitivity, occupies a smaller area. 
   In fact, designating by t 1  the thickness of the bottom semiconductor layer  33  and designating by t 2  the thickness of the top semiconductor layer  35  and neglecting the thickness due to the intermediate dielectric layer  34 , the suspended mass  40  has a height substantially equal to t 1 +t 2 , since it is formed throughout the thickness of the structural layer  32 , while the fixed electrodes  42  and the springs  45  have a height equal to t 1  since they are formed just by the bottom semiconductor layer  33 . 
   Consequently, as compared to a traditional sensor having a structural layer with a thickness equal to the thickness t 1  of just the bottom semiconductor layer  33 , and given the same length, the springs  45  have equal stiffness k. 
   Instead, the inertial sensor  30  has a greater mass M, given the greater thickness of the suspended mass  40 , equal to t 1 +t 2 . Consequently, the inertial sensor  30  has greater sensitivity S. 
   A first embodiment of a manufacturing process for the structure of  FIGS. 5 and 6  is described hereinafter with reference to  FIGS. 7-9  corresponding to a cross-section taken through the area of the electrodes  41 ,  42 . 
   In detail (see  FIG. 7 ), a material wafer  50  comprising a substrate  31  is coated initially with a sacrificial oxide layer  44 . For example, the top surface of the substrate  31  can be oxidized thermally, or the sacrificial layer  44  can be deposited. Then, after deposition of a polysilicon germ layer, a first polysilicon growth is carried out, which leads to the formation of a bottom semiconductor layer  33 , of polycrystalline silicon, with a thickness, for example, of 15 μm. Then an intermediate dielectric layer  34  is formed, of material that resists etching of the silicon, for example silicon oxide deposited or grown thermally. Typically, the intermediate dielectric layer  34  can have a thickness of 1.6 μm. The intermediate dielectric layer  34  is then defined photolithografically, using a resist mask (not illustrated), so as to form first protective regions, designated again by  34 . In practice, the first protective regions do not cover the areas where the bottom semiconductor layer  33  is then to be etched, such as for example for forming the trench  46  between the rotor  38  and the stator  39 . Next, after deposition of a further polysilicon germ layer, a second polysilicon growth is performed, which leads to the formation of a top semiconductor layer  35  of polycrystalline silicon with a thickness of, for example, 15 μm. In this way, the structure of  FIG. 7  is obtained. 
   Next, a resist mask  51  is formed over the top semiconductor layer  35 . The resist mask  51  is defined to cover the areas of the top semiconductor layer  35  that are to be protected. In particular, the resist mask  51  covers the area where the rotor  38  must be defined (both of the suspended mass  40  and of the mobile electrodes  41 ; possibly openings in the resist mask  51  can be provided above the suspended mass  40 , where through holes are to be formed extending as far as the sacrificial layer  44  to enable its complete removal underneath the suspended mass  40 , in a per se known manner. In this case, also the intermediate dielectric layer  34  must have been previously removed in an aligned position to the openings of the resist mask  51 ). In this way, the structure of  FIG. 8  is obtained. 
   Then, using the resist mask  51 , a trench etch is performed, thereby removing the top semiconductor layer  35 , where it is uncovered; the etch stops at the intermediate dielectric layer  34 . Instead, where the portions of the intermediate dielectric layer  34  have been removed, the etch proceeds, removing the bottom semiconductor layer  33 . 
   In practice, as may be seen from  FIG. 9 , the top semiconductor layer  35  is removed above the fixed electrodes  42  and the springs  45 , but here the etch stops due to the presence of the intermediate dielectric layer  34 . Instead, where the first protective regions  34  are not present, the etch proceeds, and the exposed portions of the bottom semiconductor layer  33  are removed. The trench  46  is thus completed that separates the mobile electrodes  41  from the fixed electrodes  42  and, more in general, the rotor  38  from the stator  39 , as well as delimiting the springs  45  ( FIG. 5 ). The structure of  FIG. 9  is thus obtained, comprising a spring  45 , shown by a dashed line and schematically, which has a known shape (typically, a serpentine extending between the suspended mass  40  and the fixed structure  43 ), here formed by the bottom semiconductor layer  33  alone. 
   Finally, an etching step is performed, that causes removal of the sacrificial layer  44  both where it is uncovered and partially underneath the bottom semiconductor layer  33 . In practice, given the smaller thickness of the mobile electrodes  41  and of the fixed electrodes  42 , these are freed underneath; likewise the suspended mass  40  is freed, thanks to the openings (not illustrated) provided to this end. At the end of this process, the final structure illustrated in  FIG. 6  is obtained. 
   According to a different embodiment, illustrated in  FIGS. 10-15 , and wherein for sake of simplicity the same reference numbers employed previously are used, the starting point ( FIG. 10 ) is a wafer  60  formed by a stack, which includes a substrate  31 , a sacrificial layer  44 , a bottom semiconductor layer  33 , an intermediate dielectric layer  34 , a top semiconductor layer  35 , and a top dielectric layer  61 , for example of thermal oxide or deposited. The wafer  60  can be obtained in any known way, analogously to the description provided with reference to  FIG. 7  (formation of dielectric regions and polysilicon growth after deposition of nucleolus polycrystalline silicon) or using a multiple SOI wafer obtained by bonding monocrystalline silicon wafers. 
   Using a first resist mask  62 , first portions of the top dielectric layer  61  are removed ( FIG. 11 ). 
   Then, the first resist mask  62  is removed, and a second resist mask  63  is formed ( FIG. 12 ). The second resist mask  63  in part extends on the remaining portions of the top dielectric layer  61 , where the top semiconductor layer  35  is not to be removed (rotor and fixed structure  43 ), and in part extends directly on the top semiconductor layer  35 , i.e., regions where the top semiconductor layer  35  but not the bottom semiconductor layer  33  must be removed (typically the fixed electrodes  42  and the springs  45 ). 
   Using the second resist mask  63 , first the top dielectric layer  61  is etched (oxide etch, removing the exposed regions of the top dielectric layer  61 , to form second protective regions  61 ,  FIG. 13 ), then the top semiconductor layer  35  is etched (trench etch, removing the exposed portions of the top semiconductor layer  35 ). Finally, a further oxide etch is performed, removing the exposed portions of the intermediate dielectric layer  34 , to form third protective portions  34 . The structure of  FIG. 14  is thus obtained. 
   After removing the second resist mask  63 , a further trench etch is performed ( FIG. 15 ). In this step, the portions of the top semiconductor layer  35  not covered by the top dielectric layer  61  and no longer covered by the second resist mask  63  are removed; furthermore the exposed portions of the bottom semiconductor layer  33  are removed, where neither the second protective portions  61  nor the third protective portions  34  are present. In this way, the stator  39 , the rotor  38 , and the springs  45  are defined ( FIG. 5 ), and the top semiconductor layer  35  is removed above the fixed electrodes  42 . Thus the structure of  FIG. 15  is obtained. 
   At the end, as described with reference to the first embodiment of the manufacturing process, the sacrificial layer  44  is etched to free the mobile structures, to obtain the final structure of  FIG. 6 . 
   Finally, it is clear that numerous modifications and variations can be made to the inertial sensor and to the manufacturing process described and illustrated herein, all falling within the scope of the invention, as defined in the attached claims. 
   For example, in the first embodiment of the process, the initial structure, formed by the layers  31 ,  44 ,  33  of  FIG. 7 , can also be formed by bonding monocrystalline silicon wafers, and possibly the entire wafer  50  illustrated can be obtained using a dedicated SOI wafer. 
   With the polysilicon growth process described, when the bottom semiconductor layer  33  is monocrystalline and it is necessary to integrate electronic components in the same wafer, it is possible to remove portions of the intermediate dielectric layer  34  on the side of the sensor area so as to cause the top semiconductor layer  35  to grow in a monocrystalline form. 
   Finally, the same solution based onto a height difference, as described for the suspended mass and the elastic suspension elements, may be applied also to an inertial sensor of rotational type, such as the one illustrated in  FIG. 4 , where consequently the springs  15  are formed only in the bottom semiconductor layer  33 . 
   In addition, differently from the figures, also the stator  39  ( 6  in  FIG. 4 ) may be formed in the entire structural layer  32  and thus have a thickness t 1 +t 2 . 
   Various principles of the invention have been described with reference to inertial sensor type MEMS devices. Other embodiments of the invention include other types of MEMS devices including, for example, motors, adjustable mirrors, gyroscopes, etc. In particular, it will be recognized that by employing principles disclosed herein, MEMS devices can be designed in which the mass or thickness of a rotor element and the flexibility of a spring element can be selected independently, without sacrificing additional surface area of a substrate. 
   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. 
   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.