Abstract:
A method of making a micro electromechanical switch or tunneling sensor. A cantilevered beam structure and a mating structure are defined on a first substrate or wafer; and at least one contact structure and a mating structure are defined on a second substrate or wafer, the mating structure on the second substrate or wafer being of a complementary shape to the mating structure on the first substrate or wafer. At least one of the mating structures includes a protrusion extending from a major surface of at least one of said substrates. A bonding layer, preferably a eutectic bonding layer, is provided on at least one of the mating structures. The mating structure of the first substrate is moved into a confronting relationship with the mating structure of the second substrate or wafer. Pressure is applied between the two substrates so as to cause a bond to occur between the two mating structures at the bonding or eutectic layer. Then the first substrate or wafer is removed to free the cantilevered beam structure for movement relative to the second substrate or wafer.

Description:
TECHNICAL FIELD  
         [0001]    The present invention relates to micro electromechanical (MEM) tunneling sensors and switches using dual wafers which are bonded together preferably eutectically.  
         RELATED APPLICATIONS  
         [0002]    This invention is related to other inventions which the subject of separate patent applications filed thereon. See: U.S. patent application Ser. No. ______ entitled “A Single Crystal, Dual Wafer, Tunneling Sensor or Switch with Silicon on Insulator Substrate and a Method of Making Same” (attorney docket 617965-3); U.S. patent application Ser. No. ______ entitled “A Single Crystal, Dual Wafer, Tunneling Sensor and a Method of Making Same” (attorney docket 617975-0); U.S. patent application Ser. No. ______ entitled “A Single Crystal, Dual Wafer, Gyroscope and a Method of Making Same” (attorney docket 618022-2); and U.S. patent application Ser. No. entitled “A Single Crystal, Tunneling and Capacitive, Three-Axes Sensor Using Eutectic Bonding and a Method of Making Same” (attorney docket 617808-9), all of which applications have the same filing date as this application, and all of which applications are hereby incorporated herein by reference.  
         BACKGROUND OF THE INVENTION  
         [0003]    The present invention provides a new process of fabricating a single crystal silicon MEM tunneling devices using low-cost bulk micromachining techniques while providing the advantages of surface micromachining. The prior art, in terms of surface micromachining, uses e-beam evaporated metal that is patterned on a silicon dioxide (SiO 2 ) layer to form the control, self-test, and tip electrodes of a tunneling MEM switch or sensor. A cantilevered beam is then formed over the electrodes using a sacrificial resist layer, a plating seed layer, a resist mold, and metal electroplating. Finally, the sacrificial layer is removed using a series of chemical etchants. The prior art for bulk micromachining has utilized either mechanical pins and/or epoxy for the assembly of multi-Si wafer stacks, a multi-Si wafer stack using metal-to-metal bonding and an active sandwiched membrane of silicon nitride and metal, or a dissolved wafer process on quartz substrates (Si-on-quartz) using anodic bonding. None of these bulk micromachining processes allow one to fabricate a single crystal Si cantilever (with no deposited layers over broad areas on the beam which can produce thermally mismatched expansion coefficients) above a set of tunneling electrodes on a Si substrate and also electrically connect the cantilever to pads located on the substrate and at the same time affording good structural stability. The fabrication techniques described herein provide these capabilities in addition to providing a low temperature process so that CMOS circuitry can be fabricated in the Si substrate before the MEMS switches and/or sensors are added. Finally, the use of single crystal Si for the cantilever provides for improved process reproductibility for controlling the stress and device geometry. A protrusion is formed on at least one of the substrates to provide better mechanical stability to the resulting switch or sensor.  
           [0004]    Tunneling switches and sensors may be used in various military, navigation, automotive, and space applications. Space applications include satellite stabilization in which MEM switch and sensor technology can significantly reduce the cost, power, and weight of the presently used gyro systems. Automotive air bag deployment, ride control, and anti-lock brake systems provide other applications for MEM switches and sensors. Military applications include high dynamic range accelerometers and low drift gyros.  
           [0005]    MEM switches and sensors are rather similar to each other. The differences between MEM switches and MEM sensors will be clear in the detailed disclosure of the invention.  
         BRIEF DESCRIPTION OF THE INVENTION  
         [0006]    Generally speaking, the present invention provides a method of making a micro electromechanical switch or sensor wherein a cantilevered beam structure and a mating structure are defined on a first substrate or wafer and at least one contact structure and a mating structure are defined on a second substrate or wafer. The mating structure on the second substrate or wafer is of a complementary shape to the mating structures on the first substrate or wafer. At least one of the two mating structures includes a silicon protrusion extending from the wafer on which the corresponding unit is fabricated. A bonding or eutectic layer is provided on at least one of the mating structures and the mating structure are moved into a confronting relationship with each other. Pressure is then applied between the two substrates and heat may also be applied so as-to cause a bond to occur between the two mating structures at the bonding or eutectic layer. Then the first substrate or wafer is removed to free the cantilevered beam structure for movement relative to the second substrate or wafer. The bonding or eutectic layer also provides a convenient electrical path to the cantilevered beam for making a circuit with the contact formed on the cantilevered beam.  
           [0007]    In another aspect, the present invention provides an assembly or assemblies for making a single crystal silicon MEM switch or sensor therefrom. A first substrate or wafer is provided upon which is defined a beam structure and a mating structure. A second substrate or wafer is provided upon which is defined at least one contact structure and a mating structure, the mating structure on the second substrate or wafer being of a complementary shape to the mating structure on the first substrate or wafer. At least one of the two mating structures includes a silicon protrusion extending from the wafer on which the corresponding unit is fabricated. A pressure and heat sensitive bonding layer is disposed on at least one of the mating structures for bonding the mating structure defined on the first substrate or wafer with the mating structure on the second substrate in response to the application of pressure and heat therebetween.  
       
    
    
     BRIEF DESCRIPTION OF THE FIGURES  
       [0008]    [0008]FIGS. 1A through 6A depict the fabrication of a first embodiment of the cantilever portion of a MEM sensor.  
         [0009]    [0009]FIGS. 1B through 6B correspond to FIGS.  1 A- 6 A, but show the cantilever portion, during its various stages of fabrication, in plan view:  
         [0010]    [0010]FIGS. 7A through 11A show, in cross section view, the fabrication of the base portion of the first embodiment tunneling sensor;  
         [0011]    [0011]FIGS. 7B through 11B correspond to FIGS.  7 A- 9 A but show the fabrication process for the base portion in plan view;  
         [0012]    [0012]FIGS. 12 and 13 show the cantilever portion and the base portion being aligned with each other and being bonded together preferably by eutectic bonding;  
         [0013]    [0013]FIGS. 14A and 15 show the completed MEM sensor according to the first embodiment in cross sectional view, FIG. 15 being enlarged compared to FIG. 14A;  
         [0014]    [0014]FIG. 14B shows the completed MEM sensor according to the first embodiment in plan view;  
         [0015]    [0015]FIGS. 16A through 21A depict, in cross sectional view, a modification applicable to the first embodiment of the cantilever portion of the MEM sensor;  
         [0016]    [0016]FIGS. 16B through 21B correspond to FIGS.  16 A- 21 A, but show the fabrication process for the modification in plan view;  
         [0017]    [0017]FIG. 22 depicts a side elevational section view of another embodiment of a MEM sensor, this embodiment having a preferably eutectic bond in a central region of its columnar support;  
         [0018]    [0018]FIG. 23 depicts a side elevational section view of yet another embodiment of a MEM sensor, this embodiment having a preferably eutectic bond adjacent the cantilevered beam  12 ;  
         [0019]    [0019]FIG. 24 depicts a side elevational section view of still another embodiment of a MEM sensor, this embodiment having a preferably eutectic bond in a central region of its columnar support as in the embodiment of FIG. 30, but also having a ribbon conductor on the cantilevered beam structure;  
         [0020]    [0020]FIG. 25 depicts a side elevational section view of another embodiment of a MEM sensor, t this embodiment having a preferably eutectic bond adjacent the cantilevered beam structure as in the case of the embodiment of FIG. 31, but also having a ribbon conductor on the cantilevered beam structure;  
         [0021]    [0021]FIG. 26 depicts a side elevational section view of still another embodiment of a MEM sensor, this embodiment having a preferably eutectic bond adjacent the cantilevered beam, but also utilizing a base structure having a silicon protrusion which forms part of the columnar support structure;  
         [0022]    [0022]FIG. 27 depicts a side elevational section view of yet another embodiment of a MEM sensor, this embodiment having a preferably eutectic bond adjacent the cantilevered beam and utilizing a base structure having a silicon protrusion which forms part of the columnar support structure as in the case of the embodiment of FIG. 26, but also utilizing a ribbon conductor on the cantilevered beam structure;  
         [0023]    [0023]FIG. 28 depicts a side elevational section view of another embodiment of a MEM sensor, this embodiment having a preferably eutectic bond in a central region of its columnar support, but also utilizing a base structure having a silicon protrusion which forms part of the columnar support structure;  
         [0024]    [0024]FIG. 29 depicts a side elevational section view of another embodiment of a MEM sensor, this embodiment having a preferably eutectic bond in a central region of its columnar support and a base structure having a silicon protrusion which forms part of the columnar support structure as in the embodiment of FIG. 28, but also utilizing a ribbon conductor on the cantilevered beam structure;  
         [0025]    [0025]FIG. 30 depicts a side, elevational section view of an embodiment of a MEM switch, this embodiment being similar to the sensor embodiment of FIG. 32, but being equipped with an additional pad which is used to apply electrostatic forces to the beam to close the switch;  
         [0026]    [0026]FIG. 31 depicts a side elevational section view of another embodiment of a MEM switch, this embodiment being similar to the switch embodiment of FIG. 38, but the preferably eutectic bond occurs adjacent the cantilevered beam as opposed in a central region of the columnar support;  
         [0027]    [0027]FIG. 32 depicts a side elevational section view of yet another embodiment of a MEM switch, this embodiment utilizing a base structure having a silicon protrusion which forms part of the columnar support structure for the cantilevered beam; and  
         [0028]    [0028]FIG. 33 depicts a side elevational section view of yet another embodiment of a MEM switch, this embodiment being similar to the switch embodiment of FIG. 32, but including an SiO 2  layer between the ribbon conductor and the Si of the cantilevered beam. 
     
    
     DETAILED DESCRIPTION  
       [0029]    Several embodiments of the invention will be described with respect to the aforementioned figures. The first embodiment will be described with reference to FIGS. 1A through 15. A second embodiment will be discussed with reference to FIGS. 16 through 21B. Further additional embodiments and modifications are described thereafter. Since some of the fabrication steps are the same for many of the embodiments, reference will often be made to earlier discussed embodiments to reduce repetition.  
         [0030]    The MEM devices shown in the accompanying figures are not drawn to scale, but rather are drawn to depict the relevant structures for those skilled in this art. Those skilled in this art realize that these devices, while mechanical in nature, are very small and are typically manufactured using generally the same type of technology used to produce semiconductor devices. Thus a thousand or more devices might well be manufactured at one time on a wafer. To gain an appreciation of the small scale of these devices, the reader may wish to turn to FIG. 15 which includes size information for a preferred embodiment of a MEM sensor utilizing the present invention. The figure numbers with the letter ‘A’ appended thereto are section views taken as indicated in the associated figure numbers with the letter ‘B’ appended thereto, but generally speaking only those structures which occur at the section are shown and not structures which are behind the section. For example, in FIG. 2A, the portion of the mask  14  which forms the upper arm of the letter E shaped structure seen in FIG. 2B does not appear in FIG. 2A since it is located spaced from the plane where the section is taken. The section views are thus drawn for ease of illustration.  
         [0031]    Turning to FIG. 1A and 1B, a starting wafer for the fabrication of the cantilever is depicted. The starting wafer includes a wafer of bulk n-type silicon (Si)  10  upon which is formed a thin layer of doped p-type silicon  12 . The silicon wafer  10  is preferably of a single crystalline structure having a &lt;100&gt; crystalline orientation. The p-type silicon layer  12  may be grown as an epitaxial layer on silicon wafer  10 . The layer  12  preferably has a thickness of in the range of 1 to 20 micrometers (μm), but can have a thickness anywhere in the range of 0.1 μm to 800 μm. Generally speaking, the longer the cantilevered beam is the thicker the beam is. Since layer  12  will eventually form the cantilevered beam, the thickness of layer  12  is selected to suit the length of the beam to be formed.  
         [0032]    Layer  12  may be doped with Boron such that its resistivity is reduced to less than 0.05 Ω-cm and is preferably doped to drop its resistivity to the range of 0.01 to 0.05 Ω-cm. The resistivity of the bulk silicon wafer or substrate  10  is preferably about 10 Ω-cm. Boron is a relatively small atom compared to silicon, and therefore including it as a dopant at the levels needed (10 20 ) in order to reduce the resistivity of the layer  12  tends to induce stress which is preferably compensated for by also doping, at a similar concentration level, a non-impurity atom having a larger atom size, such as germanium. Germanium is considered a non-impurity since it neither contributes nor removes any electron carriers in the resulting material.  
         [0033]    Layer  12  shown in FIGS. 1A and 1B is patterned using well known photolithographic techniques by forming a mask layer, patterned as shown at numeral  14 , preferably to assume the shape of a capital letter ‘E’ when viewed in plan view (see FIG. 2B). While the shape of the capital letter ‘E’ is preferred, other shapes can be used. In this embodiment, the outer peripheral portion of the E-shape will form a mating structure which will be used to join the cantilevered beam forming portion  2  of the sensor to its base portion  4  (see FIGS. 12 and 13).  
         [0034]    After the mask layer  14  has been patterned as shown in FIGS. 2A and 2B, the wafer is subjected to a plasma etch, for example, in order to etch through the thin layer of p-type doped silicon  12  and also to over etch into the silicon wafer  10  by a distance of approximately 500 Å.  
         [0035]    The mask  14  shown in FIGS. 2A and 2B is then removed and another photoresist layer  16  is applied which is patterned as shown in FIGS. 3A and 3B by providing two openings therein  16 - 1  and  16 - 2 . Opening  16 - 1  basically follows the outer perimeter of the ‘E’ shape of the underlying thin layer of p-type silicon  12  while opening  16 - 2  is disposed at or adjacent a tip of the interior leg of the ‘E’-shaped p-type silicon layer  12 .  
         [0036]    Layers of Ti/Pt/Au are next deposited over mask  16  and through openings  16 - 1  and  16 - 2  to form a post contact  18 - 1  and a tunnelling tip contact  18 - 2 . The Ti/Pt/Au layers preferably have a total thickness of about 2000 Å. The individual layers of Ti and Pt may have thicknesses in the ranges of 100-200 Å and 1000-2000 Å, respectively. After removal of the photoresist  16 , the wafer is subjected to a sintering step at approximately 520° C. to form an ohmic Ti—Si juncture between contacts  18 - 1  and  18 - 2  and the underlying layer  12 . As will be seen with reference to FIGS.  24 A- 28 B, the sintering step can be eliminated if a metal layer, for example, is used to connect contacts  18 - 1  and  18 - 2 .  
         [0037]    As another alternative, which does rely on the aforementioned sintering step occurring, post contact  18 - 1  may be formed by layers of Ti and Au (i.e without Pt), which would involve an additional masking step to eliminate the Pt layer from post contact  18 - 1 . However, in this alternative, the sintering would cause Si to migrate into the Au to form an Au/Si eutectic at the exposed portion of post contact  18 - 1  shown in FIGS. 4A and 4B. As a further alternative, the exposed portion of the post contact  18 - 1  shown in FIGS. 4A and 4B could simply be deposited as Au/Si eutectic, in which case the Pt layer in the post contact  18 - 1  could be optionally included. Post contact  18 - 1  may be eliminated if the subsequently described bonding between the cantilevered beam forming portion  2  and the base portion  4  occurs non-eutectically.  
         [0038]    As a result, the exposed portion of the post contact  18 - 1  shown in FIGS. 4A and 4B is formed, preferably either by Au or by Au/Si. When the cantilevered forming portion  2  and the base portion  4  are mated as shown and described with reference to FIGS. 12 and 13, one of the exposed mating surfaces is preferably a Au/Si eutectic while the other is preferably Au. Thus, exposed mating surfaces  18 - 1 ,  18 - 3  can preferably be either Au and Au/Si if the exposed mating surface on the base portion  4  is the other material, i.e., preferably either Au/Si or Au so that a layer of Au/Si confronts a layer of Au.  
         [0039]    After the structure shown by FIGS. 4A and 4B is arrived at, a layer of photoresist  20  is put down and patterned to have a single opening  20 - 2  therein as shown in FIGS. 5A and 5B. A layer of gold  26 , preferably having a thickness of 15,000 Å, is applied over the photoresist  20 ′ and the gold, as it deposits upon contact  18 - 2  through opening  20 - 2 , will assume a pyramidal-like or conical-like shape so as to form a pointed contact  26 - 2  due to the formation of an overhang at the opening  20 - 2  during the deposition of the gold layer  26 . After contact  26 - 2  is formed, the remaining photoresist  20 ′ is dissolved so that the cantilever beam structure then appears as shown in FIGS. 6A and 6B. The mating structure is provided by layer  18 - 1  in this embodiment. Those skilled in the art will appreciate that the size of the openings  16 - 1 ,  16 - 2  and  20 - 2  are not drawn to scale on the figures and that openings  16 - 2  and  20 - 2  would tend to be significantly smaller than would be opening  16 - 1 . As such, when a rather thick layer  26  of Au is deposited on the wafer, those skilled in the art will appreciate that there is some fill-in at the sides of a mask when layer  26  is deposited because of an increasing overhang which occurs at the edges of opening  20 - 2  as the deposition process proceeds. Since opening  20 - 2  is rather narrow to begin with, the Au deposited through opening  20 - 2 , which is shown at numeral  26 - 2 , assumes a pyramidal-like or conical-like shape. The thickness of the deposition of Au layer  26  is generally sufficiently thick to assure that layer  26  will close across the top of opening  20 - 2  during the deposition process and so that structure  26 - 2  assumes its pointed configuration.  
         [0040]    The layer of photoresist  20  is then removed so that then the cantilevered beam forming portion  2  of the sensor appears as depicted by FIGS. 6A and 6B.  
         [0041]    The fabrication of the base portion  4  of this embodiment of the MEM sensor will now be described with reference to FIGS. 7A through 11B. Turning to FIGS. 7A and 7B, a wafer  30  of silicon is shown upon which a layer of photoresist  50  has been deposited and patterned to assume preferably the outerperipheral shape of a capital letter ‘E’. The exposed silicon is then subjected to an etch, etching it back approximately 20,000 Å, to define a protruding portion  30 - 1  of wafer  30  under the patterned mask  50  of the photoresist. The photoresist mask  50  is then removed and wafer  30  is oxidized to form layers of oxide  52 ,  54  on its exposed surfaces. The oxide layers are each preferably about 1 μm thick. Of course, the end surfaces shown in FIG. 8A are not shown as being oxidized because it is assumed that the pattern shown in FIG. 8A (and the other figures) is only one of a number of repeating patterns occurring across an entire wafer  30 .  
         [0042]    Turning to FIGS. 9A and 9B, a layer of photoresist  56  is applied having an opening therein  56 - 1  which again assumes the outerperipheral shape of a capital letter ‘E’, as previously described. Then, a layer of Ti/Pt/Au  58 , preferably having a thickness of 2,000 Å, is deposited through opening  56 - 1  followed by the deposition of a layer  60  of an Au/Si eutectic preferably with a 1,000 Å thickness. Layers  58 - 1  of Ti/Pt/Au and  60 - 1  of the Au/Si eutectic are thus formed, which layers preferably follow the outerperipheral shape of a capital letter ‘E’, as can be clearly seen in FIG. 9B. Of course, if the post contact  18 - 1  (see FIG. 4A) is either formed of an Au/Si eutectic or has an Au/Si eutectic disposed thereon, then layers  60 ,  60 - 1  may be formed of simply Au or simply omitted due to the presence of Au at the exposed layer  58 - 1 .  
         [0043]    Photoresist layer  56  is then removed and a layer  62  of photoresist is applied and patterned to have (i) openings  62 - 2 ,  62 - 3  and  62 - 4 , as shown in FIG. 10A, (ii) openings for pads  40 - 1  through  40 - 4  and their associated ribbon conductors  42  and (iii) an opening for guard ring  44  and its pad, as depicted in FIG. 10B. For the ease of illustration, the opening for guard ring  44  is not shown in FIG. 10A. A layer  38  of Ti/Pt/Au is then deposited over the patterned photoresist layer  62  and through openings  62 - 2  through  62 - 4  therein forming contacts  38 - 3 ,  38 - 4  and  38 - 2  and the photoresist  62  is removed to thereby arrive at the structure shown in FIGS.  11 A and  11 B. Those contacts are interconnected with their associated pads  40 - 2  through  44 - 4  by the aforementioned ribbon conductors  42 , which contacts  40  and ribbon conductors  42  are preferably formed at the same time as contacts  38 - 3 ,  38 - 4  and  38 - 2  are formed. The outerperipheral layers  58 - 1  and  60 - 1  are also connected with pad  40 - 1  by an associated ribbon conductor  42 . The protrusion  30 - 1 , which preferably extends approximately 20,000 Å high above the adjacent portions of wafer  30 ′, and the relatively thin layers  58 - 1  and  60 - 1  form the mating structure for the base portion  4 .  
         [0044]    Turning to FIG. 12, the cantilevered beam forming is now bonded to base portion  4 . As is shown in FIG. 12, the two wafers  10  and  30  are brought into a confronting relationship so that their mating structure  18 - 1 ,  30 - 1 , 58 - 1  and  60 - 1  are in alignment so that layers  18 - 1  and  60 - 1  properly mate with each other. Pressure and heat (preferably by applying a force of 5,000 N at 400° C. between three inch wafers  2 ,  4  having 1000 sensors disposed thereon) are applied so that eutectic bonding occurs between layers  18 - 1  and  60 - 1  as shown in FIG. 13. Thereafter, silicon wafer  10  is dissolved so that the MEM sensor structure shown in FIG. 14 is obtained. The p-type silicon layer  12  includes a portion  12 - 2  which serves as the cantilevered beam and another portion which is attached to the base portion  4  through the underlying layers. The gold contact  26 - 2  is coupled to pad  40 - 1  by elements  18 - 2 ,  12 - 2 ,  12 - 1 ,  18 - 1 ,  60 - 1 ,  58 - 1  and its associated ribbon conductor  42 . If the bonding is done non-eutectically, then higher temperatures will be required.  
         [0045]    Protrusion  30 - 1  and layers  18 - 1 ,  60 - 1 , and  58 - 1  have preferably assumed the shape of the outerperpherial edge of a capital letter ‘E’ and therefore the moveable contact  26 - 2  of the MEM sensor is well protected by this physical shape. After performing the bonding, silicon layer  10  is dissolved away to arrive at the resulting MEM sensor shown in FIGS. 14A and 14B. The silicon can be dissolved with ethylenediamine pyrocatechol (EDP). This leaves only the Boron doped silicon cantilevered beam  12  with its associated contact  26 - 2  and its supporting or mating structure  18 - 1  bonded to the base structure  4 . Preferable dimensions for the MEM sensor are given on FIG. 15. The beam as preferably has a length of 200 to 300 μm (0.2 to 0.3 mm).  
         [0046]    Instead of using EDP as the etchant, plasma etching can be used if a thin layer of SiO 2  is used, for example, as an etch stop between layer  12  and substrate  10 .  
         [0047]    [0047]FIG. 15 is basically identical to FIG. 14, but shows the MEM sensor in somewhat more detail and the preferred dimensions of the MEM sensor are also shown on this figure.  
         [0048]    It will be recalled that in this embodiment, a layer of Ti/Pt/Au  18  was applied forming contacts  18 - 1  and  18 - 2  which were sintered in order to form an ohmic bond with Boron-doped cantilever  12 . It was noted that sintering could be avoided by providing a ribbon conductor between contacts  18 - 1  and  18 - 2 . Such a modification is now described in greater detail and is depicted starting with FIGS. 16A and 16B.  
         [0049]    According to this modification, the thin Si layer  12  formed on silicon wafer  10  may be (i) doped with Boron or (ii) may be either undoped or doped with other impurities and formed by methods other than epitaxial growth. If undoped (or doped with other impurities), then a thin etch stop layer  11  is formed between the thin Si layer  12  and the silicon wafer  10 . This configuration is called Silicon On Insulator (SOI) and the techniques for making an SOI structure are well known in the art and therefor are not described here in detail. The etch stop layer  11 , if used, is preferably a layer of SiO 2  having a thickness of about 1-2 μm and can then be made, for example, by the implantation of oxygen into the silicon wafer  10  through the exposed surface so as to form the etch stop layer  11  buried below the exposed surface of the silicon wafer  10  and thus also define, at the same time, the thin layer of silicon  12  adjacent the exposed surface. This etch stop layer  11  will be used to release the cantilevered beam from wafer  10 . If layer  12  is doped with Boron, it is doped to reduce the resistivity of the epitaxial layer  12  to less than 1 Ω-cm. At that level of Boron doping the epitaxial layer  12  can resist a subsequent EDP etch used to release the cantilevered beam from wafer  10  and thus an etch stop layer is not needed.  
         [0050]    Optionally, the silicon wafer  10  with the thin doped or undoped Si layer  12  formed thereon (as shown in FIGS. 16A and 16B) may be subjected to thermal oxidation to form a relatively thin layer of SiO 2  on the exposed surface of layer  12 . Layer  12  is preferably about 1.2 μm thick (but it can be thinner or thicker depending upon the application). The thickness of the optional SiO 2  layer is preferably on the order of 0.2 μm. To arrive at this point, both major surfaces may be oxidized and the oxide stripped from the bottom layer, if desired. The optional oxide layer may be used to provide an even better barrier against diffusion of Si from the beam into the Au of the tunneling tip to be formed at one end of the beam. This optional oxide layer may be used with any embodiment of the cantilevered beam, but is omitted from most of the figures for ease of illustration. It does appear, however, in FIGS. 25 and 27 and is identified there by element number  70 .  
         [0051]    Turning now to FIGS. 17A and 17B, a layer of photoresist  14  is then applied on layer  12  (or on the optional oxide layer  70 , if present) and patterned preferably to assume the same “E” letter shape as the layer photoresist  14  discussed with reference to FIGS. 2A and 2B. The structure shown in FIGS. 17A and 17B is then subjected to a plasma etch which etches through layers  11  and  12  into the silicon substrate  10  by approximately 500 Å. Then a layer of photoresist  16  is applied and patterned as shown by FIGS. 18A and 18B. The layer  16  of photoresist is patterned to assume basically the same arrangement and configuration as layer  16  discussed with respect to FIGS. 3A and 3B except that an additional opening  16 - 5  is included communicating between openings  61 - 1  and  16 - 2  to provide for the formation of a ribbon conductor  18 - 5  when a layer  18  of metals, preferably Ti/Pt/Au, is subsequently deposited on photoresist  16 . After depositing the layer  18 , the photoresist  16  is removed lifting off the portions of the layer  18  formed thereon, leaving portions  18 - 1 ,  18 - 2  and  18 - 5  of layer  18  on the underlying layer  12  as shown in FIGS. 19A and 19B, or on the optional oxide layer  70 , if present.  
         [0052]    After arriving at the structure shown in FIGS. 19A and 19B, a tunneling tip  26 - 2  is added by appropriate masking and deposition of layer  26  (see FIG. 5A) Au or a layer of Ti/Pt/Au, for example, thereby arriving at the structure shown by FIGS. 20A and 20B. If the silicon base  30  is formed with a protrusion  30 - 1  (see FIG. 8A, for example), then the MEM sensor can be completed as previously described with reference to FIGS. 12 and 13. After bonding the structure depicted by FIGS. 20A and 20B to the base structure  4  of FIGS. 11A and 11B and releasing the silicon wafer  10  from the cantilevered beam, the structure shown by FIGS. 21A and 21B is arrived at. The cantilevered beam  12  is preferably released by performing two plasma etches. The first etch dissolves wafer  10  and the second etch removes the etch stop layer  11 .  
         [0053]    The protrusion  30 - 1  can be omitted, if desired, in which case it is then replaced by making layer  58 - 1  and/or layer  60 - 1  of a relatively thick layer of metal, such as Ti/Pt/Au, with opposing layers of Au and Au/Si eutectic applied thereon to confront each other when the two portions are brought together and eutectically bonded as previously described. However, this often requires additional masking steps since the other metal layers normally formed at the same time as layers  58 - 1  amd/or  60 - 1  should remain thin. The use a protrusion  30 - 1  is preferred since the resulting structure is believed to be more stable and since it simplifies the formation of the various metal layers.  
         [0054]    Also instead of forming the protrusion from layer  30  of the base  4  portion, it could instead be formed from layer  10  of the cantilevered beam forming portion  2  or, as a further alternative, protrusions could be formed from both layers  10  and  30 . Preferably, however, the protrusion  30 - 1  is formed from the base portion  4 .  
         [0055]    [0055]FIG. 22 shows another embodiment of a MEM sensor. In this case the MEM sensor is shown in its completed form. With the information already presented herein, those skilled in the art will not find it difficult to modify the detailed description already given to produce this embodiment and still further embodiments, all of which will now be discussed.  
         [0056]    In the embodiment of FIG. 22, the preferable eutectic bond occurs closer to a center point in the supporting arm  80  between the Au and Au/Si layers and no protrusion is utilized in this embodiment. Otherwise this embodiment is similar to the embodiment described with reference to FIGS.  1 A- 15 . In the embodiment of FIG. 23, the preferable eutectic bond occurs between the Au and Au/Si layers which are arranged close to the cantilevered beam  12  as opposed to close to base  4 . In the case of the embodiments of FIGS. 22 and 23, the cantilevered beam  12  should have good conductivity so that it acts as a conduction path between contact  22 - 2  at the end of the beam  12  and contact  40 - 1  on the base  4 . Preferably the resistivity of the boron doped silicon cantilevered beam  12  is less than 0.05 Ω-cm. Due to the low resistivity of the beam  12 , EDP may be used to etch away substrate  10  (see FIGS. 10 and 11 and the related description). Preferably, however, a SOI wafer is used in the embodiments of FIGS. 22 and 23 and the SiO 2  layer  11  (FIGS.  16 A- 20 B) is used as an etch stop layer to protect the beam  12  when etching away substrate  10  and therefore layer  12  need not be doped with Boron (to protect against an EDP etch) but rather doped with other impurities to achieve a lower resistance.  
         [0057]    Comparing the embodiments of FIGS. 15, 21,  22  and  23 , the embodiments of FIGS. 15 and 21 are preferred since they only need a relatively thin metal mating layer and provide a more rigid Si post or protrusion  30 - 1  for better stability.  
         [0058]    The embodiments of FIGS. 24 and 25 are similar to the embodiments of FIGS. 22 and 23, but these two embodiments make use of the ribbon conductor  18 - 5  described with reference to FIGS. 16A through 21B. For these embodiments, if layer  12  is doped with Boron, the resistivity of the cantilevered beam  12  is preferably less than 1 Ω-cm. The ribbon conductor allows the use of higher resistivity silicon for the cantilevered beam  12 . If layer  12  is doped with Boron, then the cantilevered beam can be released from wafer  10  using EDP as the echtant. Preferably, an SIO construction is utilized with a SiO 2  stop layer  11  (See FIGS.  16 A- 21 B) utilized to protect the beam  12  while the substrate  10  is etched away.  
         [0059]    The embodiments of FIGS.  26 - 29  are similar to the embodiments of FIGS.  22 - 25 , respectively,but a substrate with a silicon protrusion  30 - 1  is utilized, as described with reference to the embodiments of FIGS.  1 A- 21 .  
         [0060]    Generally speaking, embodiments which utilize the a base substrate  30  with a silicon post or protrusion  30 - 1 , are believed to give the resulting sensors and switches better mechanical stability.  
         [0061]    The structure which has been described so far has been set up as a sensor. Those skilled in the art know not only how to utilize these structures as a sensor but also know how to modify these structures, when needed, to make them function as a switch. The sensor devices shown in the preceding figures are preferably used as accelerometers, although they can be used for other types of sensors (such as gyroscopes, magnetometers, etc.) or as switches, as a matter of design choice, and with appropriate modification when needed or desired.  
         [0062]    Four embodiments of a switch version of a MEM device in accordance with the present invention will now be described with reference to FIGS.  30 - 33 . In order to function as a switch, two metal pads  26 - 3  and  26 - 4  are deposited on the cantilevered beam structure  12  instead of a pointed contact  26 - 2 . In these embodiments the cantilevered beam  14  is preferably formed of undoped silicon. When the switch closes, the metal pad  26 - 4  bridges two contacts  38 - 5  and  38 - 6 , which are deposited at the same time that layer  38  is deposited on the base structure  4 . The ribbon conductor  18 - 5  described with reference to FIGS. 16A through 21B is utilized, due to the relatively high resistivity of undoped Si, to bring an electrical connection with metal pad  26 - 3  down to the base substrate  4 . The switch is closed by imparting an electrostatic force on the cantilevered beam  12  by applying a voltage between metal pads  38 - 3  and  26 - 3 . That voltage causes the metal pad  26 - 4  to make a circuit connecting contacts  38 - 5  and  38 - 6  when the metal pad  26 - 4  makes physical contact with those two contacts when the switch closes. Otherwise these embodiments are similar to the previously discussed embodiments. It should be noted, however, that since the cantilevered beam  12  is preferably formed of undoped silicon, the EDP etchant will not prove satisfactory. Instead the SiO 2  etch stop layer  11  described with reference to FIGS.  16 A- 21 B is preferably used to protect the beam  12  when etching away substrate  10 .  
         [0063]    In the embodiment of FIG. 32 the Au/Si eutectic layer is disposed next to the beam and in this embodiment the base structure  4  has a protrusion  30 - 1  which acts as a portion of the column  80  which supports the beam  12 . Of the switch embodiments, the embodiment of FIG. 32 is preferred for the same reason that sensors with a protrusion  30 - 1  in their base structures  4  are also preferred, namely, it is believed to give the resulting sensors and switches better mechanical stability.  
         [0064]    In FIG. 32 an SiO 2  layer  70  is shown disposed between beam  12  and layer  18 . Layer  18  preferably is formed of layers of Ti, Pt and Au. The Pt acts as a diffusion barrier to the Si to keep it from migrating into the Au contacts. If layer  18  does not provide adequate protection for whatever metal is used in making contacts, then the use of a diffusion barrier such a SiO 2  layer  70  would be appropriate.  
         [0065]    The structures shown in the drawings has been described in many instances with reference to a capital letter ‘E’. However, this shape is not particularly critical, but it is preferred since it provides good mechanical support for the cantilevered structure formed primarily by beam portion of layer  12 . Of course, the shape of the supporting structure or mating structure around cantilever beam  12  can be changed as a matter of design choice and it need not form the perimeter of the capital letter ‘E’, but can form any convenient shape, including circular, triangular or other shapes as desired.  
         [0066]    In the embodiment utilizing a ribbon conductor on the cantilevered beam  12 , the pads and contacts (e.g.  26 - 2  and  26 - 3 ) formed on the beam  12  are generally shown as being formed over the ribbon conductor  18 - 1 ,  18 - 2 ,  18 - 5 . The ribbon conductor on the beam can be routed in any convenient fashion and could butt against or otherwise make contact with the other metal elements formed on the cantilevered beam  12  in which case elements such as  26 - 2  and  26 - 3  could be formed directly on the beam  12  itself  
         [0067]    The contacts at the distal ends of the cantilevered beams are depicted and described as being conical or triangular. Those skilled in the art will appreciate that those contacts may have other configurations and may be flat in some embodiments.  
         [0068]    Throughout this description are references to Ti/Pt/Au layers. Those skilled in the art will appreciate that this nomenclature refers to a situation where the Ti/Pt/Au layer comprises individual layers of Ti, Pt and Au. The Ti layer promotes adhesion, while the Pt layer acts as a barrier to the diffusion of Si from adjacent layers into the Au. Other adhesion layers such as Cr and/or other diffusion barrier layers such as a Pd could also be used or could alternatively be used. It is desirable to keep Si from migrating into the Au, if the Au forms a contact, since if Si diffuses into an Au contact it will tend to form SiO 2  on the exposed surface and, since SiO 2  is a dielectric, it has deleterious effects on the ability of the Au contact to perform its intended function. As such, a diffusion barrier layer such as Pt and/or Pd is preferably employed between an Au contact and adjacent Si material. However, an embodiment is discussed wherein the diffusion barrier purposefully omitted to form an Au/Si eutectic.  
         [0069]    The nomenclature Au/Si or Au—Si refers a mixture of Au and Si. The Au and Si can be deposited as separate layers with the understanding that the Si will tend to migrate at elevated temperature into the Au to form an eutectic. However, for ease of manufacturing, the Au/Si eutectic is preferably deposited as a mixture except in those embodiments where the migration of Si into Au is specifically relied upon to form Au/Si.  
         [0070]    Many different embodiments of a MEM device have been described. Most are sensors and some are switches. Many more embodiments can certainly be envisioned by those skilled in the art based the technology disclosed herein. But in all cases the base structure  4  is united with the cantilevered beam forming structure  2  by applying pressure and preferably also heat, preferably to cause an eutectic bond to occur between the then exposed layers of the two structures  2  and  4 . The bonding may instead be done non-eutectically, but then higher temperatures must be used. Since it is usually desirable to reduce and/or eliminate high temperature fabrication processes, the bonding between the two structures  2  and  4  is preferably done eutectically and the eutectic bond preferably occurs between confronting layers of Si and Au/Si.  
         [0071]    Having described the invention with respect to certain preferred embodiments thereof, modification will now suggest itself to those skilled in the art. The invention is not to be limited to the foregoing description, except as required by the appended claims.