A TUNNELING SENSOR OR SWITCH AND A METHOD OF MAKING SAME

Abstract: A method of making a micro electro-mechanical 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: A Tunneling Sensor or Switch and a Method of Making Same Technical Field The present disclosure relates to micro electro-mechanical (MEM) tunneling sensors and switches using dual wafers which are bonded together preferably eutectically.. The present diclosure also relates to a single crystal, dual wafer, tunneling sensor or switch with substrate protrusion and a method of making same. Background of the Invention 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. 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. 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 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. 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 Figures 1 A through 6A depict the fabrication of a first embodiment of the cantilever portion of a MEM sensor. Figures IB through 6B correspond to Figures 1 A - 6 A, but show the cantilever portion, during its various stages of fabrication, in plan view: Figures 7 A through 11 A show, in cross section view, the fabrication of the base portion of the first embodiment tunneling sensor; Figures 7B through 1 IB correspond to Figures 7A - 9 A but show the fabrication process for the base portion in plan view; Figures 12 and 13 show the cantilever portion and the base portion being aligned with each other and being bonded together preferably by eutectic bonding; Figures 14A and 15 show the completed MEM sensor according to the first embodiment in cross sectional view, Figure 15 being enlarged compared to Figure 14A; Figure 14B shows the completed MEM sensor according to the first embodiment in plan view; Figures 16A through 21 A depict, in cross sectional view, a modification applicable to the first embodiment of the cantilever portion of the MEM sensor; Figures 16B through 21 B correspond to Figures 16A - 21 A, but show the fabrication process for the modification in plan view; Figure 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; Figure 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; Figure 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 Figure 30, but also having a ribbon conductor on the cantilevered beam structure; Figure 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 Figure 31, but also having a ribbon conductor on the cantilevered beam structure; Figure 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; Figure 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 Figure 26, but also utilizing a ribbon conductor on the cantilevered beam structure; Figure 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; Figure 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 Figure 28, but also utilizing a ribbon conductor on the cantilevered beam structure; Figure 30 depicts a side elevational section view of an embodiment of a MEM switch, this embodiment being similar to the sensor embodiment of Figure 32, but being equipped with an additional pad which is used to apply electrostatic forces to the beam to close the switch; Figure 31 depicts a side elevational section view of another embodiment of a MEM switch, this embodiment being similar to the switch embodiment of Figure 38, but the preferably eutectic bond occurs adjacent the cantilevered beam as opposed in a central region of the columnar support; Figure 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 Figure 33 depicts a side elevational section view of yet another embodiment of a MEM switch, this embodiment being similar to the switch embodiment of Figure 32, but including an SiO 2 layer between the ribbon conductor and the Si of the cantilevered beam. Detailed Description Several embodiments of the invention will be described with respect to the aforementioned figures. The first embodiment will be described with reference to Figures 1 A through 15. A second embodiment will be discussed with reference to Figures 16 through 21 B. 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. 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 Figure 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 Figure 2A, the portion of the mask 14 which forms the upper arm of the letter E shaped structure seen in Figure 2B does not appear in Figure 2 A since it is located spaced from the plane where the section is taken. The section views are thus drawn for ease of illustration. Turning to Figure 1 A and IB, 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 <100> 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. 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. Layer 12 shown in Figures -1 A and IB 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 Ε' when viewed in plan view (see Figure 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 Figures 12 and 13). After the mask layer 14 has been patterned as shown in Figures 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 1 by a distance of approximately 500 A. The mask 14 shown in Figures 2 A and 2B is then removed and another photoresist layer 16 is applied which is patterned as shown in Figures 3 A and 3B by providing two openings therein 16-1 and 16-2. Opening 16-1 basically follows the outer perimeter of the Ε' 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 Ε'-shaped p-type silicon layer 12. 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 A. The individual layers of Ti and Pt may have thicknesses in the ranges of 100-200 A and 1000-2000 A, 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 Figures 24A-28B, the sintering step can be eliminated if a metal layer, for example, is used to connect contacts 18-1 and 18-2. 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 Figures 4 A and 4B. As a further alternative, the exposed portion of the post contact 18-1 shown in Figures 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. As a result, the exposed portion of the post contact 18-1 shown in Figures 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 Figures 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. After the structure shown by Figures 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 Figures 5A and 5B. A layer of gold 26, preferably having a thickness of 15,000 A, 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 Figures 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. The layer of photoresist 20 is then removed so that then the cantilevered beam forming portion 2 of the sensor appears as depicted by Figures 6A and 6B. The fabrication of the base portion 4 of this embodiment of the MEM sensor will now be described with reference to Figures 7 A through 1 IB. Turning to Figures 7 A 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 A, 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 Figure 8A are not shown as being oxidized because it is assumed that the pattern shown in Figure 8A (and the other figures) is only one of a number of repeating patterns occurring across an entire wafer 30. Turning to Figures 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 Ε\ as previously described. Then, a layer of Ti/Pt/Au 58, preferably having a thickness of 2,000 A, is deposited through opening 56-1 followed by the deposition of a layer 60 of an Au/Si eutectic preferably with a 1,000 A 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 Figure 9B. Of course, if the post contact 18-1 (see Figure 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. 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 Figure 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 Figure 10B. For the ease of illustration, the opening for guard ring 44 is not shown in Figure 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 Figures 11 A and 1 IB. 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 A 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. Turning to Figure 12, the cantilevered beam forming is now bonded to base portion 4. As is shown in Figure 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 Figure 13. Thereafter, silicon wafer 10 is dissolved so that the MEM sensor structure shown in Figure 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. 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 Ε' 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 Figures 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 Figure 15. The beam as preferably has a length of 200 to 300 μm (0.2 to 0.3 mm). 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. Figure 15 is basically identical to Figure 14, but shows the MEM sensor in somewhat more detail and the preferred dimensions of the MEM sensor are also shown on this figure. 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 Figures 16A and 16B. 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. Optionally, the silicon wafer 10 with the thin doped or undoped Si layer 12 formed thereon (as shown in Figures 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 Figures 25 and 27 and is identified there by element number 70. Turning now to Figures 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 Figures 2A and 2B. The structure shown in Figures 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 A. Then a layer of photoresist 16 is applied and patterned as shown by Figures 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 Figures 3 A 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 Figures 19A and 19B, or on the optional oxide layer 70, if present. After arriving at the structure shown in Figures 19A and 19B, a tunneling tip 26-2 is added by appropriate masking and deposition of layer 26 (see Figure 5A) Au or a layer of Ti/Pt/Au, for example, thereby arriving at the structure shown by Figures 20A and 20B. If the silicon base 30 is formed with a protrusion 30-1 (see Figure 8 A, for example), then the MEM sensor can be completed as previously described with reference to Figures 12 and 13. After bonding the structure depicted by Figures 20A and 20B to the base structure 4 of Figures 11 A and 1 IB and releasing the silicon wafer 10 from the cantilevered beam, the structure shown by Figures 21 A 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. 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. 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. Figure 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. In the embodiment of Figure 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 Figures 1 A - 15. In the embodiment of Figure 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 Figures 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 Figures 10 and 11 and the related description). Preferably, however, a SOI wafer is used in the embodiments of Figures 22 and 23 and the SiO 2 layer 11 (Figures 16A - 20B) 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.. Comparing the embodiments of Figures 15, 21, 22 and 23, the embodiments of Figures 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. The embodiments of Figures 24 and 25 are similar to the embodiments of Figures 22 and 23, but these two embodiments make use of the ribbon conductor 18-5 described with reference to Figures 16A through 2 IB. 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 SiO2 stop layer 11 (See Figures 16A - 2 IB) utilized to protect the beam 12 while the substrate 10 is etched away. The embodiments of Figures 26 - 29 are similar to the embodiments of Figures 25 — 25-, respectively ,but a substrate with a silicon protrusion 30-1 is utilized, as described with reference to the embodiments of Figures 1A - 21. 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. 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. Four embodiments of a switch version of a MEM device in accordance with the present invention will now be described with reference to Figures 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 Figures 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 Figures 16A - 21B is preferably used to protect the beam 12 when etching away substrate 10. In the embodiment of Figure 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 Figure 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. In Figure 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. 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. 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. 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. 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. 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. 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. 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.

Claims: What is claimed is: 1. A method of making a MEM switch or tunneling sensor comprising the steps of: (a) defining a cantilevered beam structure and a mating structure on a first substrate or wafer; (b) forming at least one contact structure and a mating structure on a second substrate or wafer, at least one of the two mating structures including a protrusion extending from the substrate or wafer from which the at least one protrusion is defined; (c) positioning the mating structure of the first substrate or wafer into a confronting relationship with the mating structure of the second substrate or wafer; (d) bonding a layer associated with said mating structure on the first substrate or wafer with a layer associated with the mating structure on the second substrate or wafer, 2. The method of claim 1 wherein the mating structure on the second substrate or wafer being of a complementary shape to the mating structure on the first substrate or wafer, the method further comprising the step of removing at least a portion of the first substrate or wafer to release the cantilevered beam structure. 3. A method of making a MEM switch or tunneling sensor as claimed in claim 1 wherein the first and second substrate or wafer are formed of silicon. 4. A method of making a MEM switch or tunneling sensor as claimed in claim 3 wherein the silicon forming the second substrate or wafer is of a single crystalline structure. 5. A method of making a MEM switch or tunneling sensor as claimed in claim 4 wherein the crystalline structure of the silicon is <100>. 6. A method of making a MEM switch or tunneling sensor as claimed in claim 5 wherein the silicon is n-type. 7. A method of making a MEM switch or tunneling sensor as claimed in claim 3 wherein the silicon forming the first substrate or wafer is of a single crystalline structure. 8. A method of making a MEM switch or tunneling sensor as claimed in claim 7 wherein the crystalline structure of the silicon in the first substrate or wafer is <100>. 9. A method of making a MEM switch or tunneling sensor as claimed in claim 8 wherein the silicon of the first substrate or wafer is n-type. 10. A method of making a MEM switch or tunneling sensor as claimed in claim 1 wherein heat is applied together with pressure between the two substrates so as to cause an eutectic bond to occur between the two mating structures. 11. A method of making a MEM switch or tunneling sensor as claimed in claim 1 wherein the cantilevered beam structure is formed by: (a) forming an epitaxial layer of silicon on said first substrate or wafer, said epitaxial layer being doped; (b) masking and etching the epitaxial layer of silicon to define a beam structure disposed on said first substrate or wafer; and (c) wherein the cantilevered beam structure is released by removing the first substrate or wafer by etching. 12. A method of making a MEM switch or tunneling sensor as claimed in claim 11 wherein a contact is formed on an end of said beam structure by depositing a metal through a small opening in a temporary mask layer, the small opening being sufficiently small that the metal being deposited tends to overhang the small opening increasingly as the deposition of the metal proceeds whereby the contact being deposited through the small opening assumes an elongate shape of decreasing cross section as the deposition proceeds. 13. A method of making a MEM switch or tunneling sensor as claimed in claim 11 wherein a contact is formed on an end of said beam structure by depositing a metal through an opening in a temporary mask layer, the metal deposited through the opening forming a planar contact at the end of said beam structure. 14. A method of making a MEM switch or tunneling sensor as claimed in claim 11 wherein etching accomplished by ethylenediamine pyrocatechol as an etchant. 15. A method of making a MEM switch or tunneling sensor as claimed in claim 14 wherein the epitaxial layer is doped with boron at a sufficient concentration to reduce the resistivity of the epitaxial layer to less than 0.05 Ω-cm. 16. A method of making a MEM switch or tunneling sensor as claimed in claim 15 wherein a layer of metal, is selectively deposited on said epitaxial layer and sintered at an elevated temperature to form first and second ohmic contacts on said epitaxial layer, said second ohmic contact being disposed near a distal end of the beam structure and the first ohmic contact forming the mating structure on the first substrate or wafer. 17. A method of making a MEM switch or tunneling sensor as claimed in claim 16 wherein the layer of metal is formed of individual layers of Ti, Pt and Au. 18. A method of making a MEM switch or tunneling sensor as claimed in claim 1 wherein the cantilevered beam structure is formed by: (a) forming an etch stop layer on said first substrate or wafer (b) providing a thin layer of silicon on said etch stop layer; (c) masking and etching the thin layer of silicon to define a beam structure disposed on said first substrate or wafer; and (d) wherein the cantilevered beam structure is released by (i) removing the first substrate or wafer by a first etch, the etch stop layer being resistant to said first etch, and (ii) removing the etch stop layer by a second etch. 19. A method of making a MEM switch or tunneling sensor as claimed in claim 18 wherein the layer of silicon is undoped and wherein (i) a contact is formed on said beam stracture spaced from the mating structure and (ii) conductive material is formed on said beam structure between said contact and said mating structure. 20. A method of making a MEM switch or tunneling sensor as claimed in claim 18 wherein the layer of silicon is doped in order to render it conductive. 21. A method of making a MEM switch or tunneling sensor as claimed in claim 1 further including forming contacts, preferably of Ti/Pt/Au, on said second substrate or wafer, at least one of said contacts on the second substrate or wafer defining, in combination with the protrusion, the mating structure on the second substrate or wafer. 22. A method of making a MEM switch or tunneling sensor as claimed in claim 21 wherein an eutectic layer is provided by a layer of Au-Si eutectic formed on the Ti/Pt/Au contact on said second substrate or wafer and/or by a layer of Au-Si eutectic formed on an ohmic contact on the first substrate or wafer. 23. A method of making a MEM switch or tunneling sensor as claimed in claim 1 wherein the bonding occurs eutectically and wherein silicon for the eutectic bond is provided by the silicon substrate of the second substrate or wafer at the mating structure. 24. A method of making a MEM switch or tunneling sensor as claimed in claim 1 wherein a layer of metal, preferably formed of individual layers of Ti, Pt and Au, is selectively deposited on said cantilevered beam structure to form a first ohmic contact and a second layer of metal without a diffusion barrier, preferably Ti/Au, is selectively deposited on said cantilevered beam structure to form a second ohmic contact, both metal layers being sintered at an elevated temperature, said first ohmic contact being disposed near a distal end of the beam structure and the second ohmic contact forming the mating structure on the first substrate or wafer. 25. A method of making a MEM switch or tunneling sensor as claimed in claim 1 wherein a , layer of metal, preferably formed of individual layers of Ti, Pt and Au, is selectively deposited on said cantilevered beam structure and is sintered at an elevated temperature to define a first ohmic contact, said first ohmic contact being disposed near a distal end of the beam structure and wherein mating of the first and second substrates or wafers occurs between a metal layer, preferably Au, deposited on the second substrate or wafer and the cantilevered beam structure defined on the first substrate or wafer. 26. A method of making a MEM switch or tunneling sensor as claimed in claim 1 wherein a patterned metal layer, preferably formed of Ti Pt/Au, is selectively deposited on said cantilevered beam structure to form first and second interconnected contacts on said cantilevered beam structure, the contacts being interconnected by an elongate conductive ribbon layer, preferably of Ti/Pt/Au, said second interconnected contact being disposed near a distal end of the beam structure, the elongate layer being disposed longitudinally on the beam structure and preferably substantially narrower than the cantilevered beam structure, said first interconnected contact forming the mating structure on the first substrate or wafer. 27. A method of making a MEM switch or tunneling sensor as claimed in claim 26 further including forming a patterned metal layer, preferably formed of Ti Pt/Au, supported by said protrusion which protrudes from a major surface of said second substrate or wafer, said patterned metal layer on the second substrate or wafer defining, in combination with the protruding portion, the mating structure on the second substrate or wafer. 28. A method of making a MEM switch or tunneling sensor as claimed in claim 27 further including forming an Au/Si eutectic layer of at least one of said metal layers. 29. A method of making a MEM switch or tunneling sensor as claimed in claim 28 wherein the bonding occurs eutectically between the eutectic layer and an adjacent a layer of Au. 30. A method of making a MEM switch or tunneling sensor as claimed in claim 27 wherein the bonding occurs eutectically and wherein silicon for the eutectic bond is provided by the silicon substrate of the second substrate or wafer at the mating structure. 31. A method of making a MEM switch or tunneling sensor as claimed in claim 1 wherein the cantilevered beam structure is formed by: (a) forming a etch stop layer on said first substrate or wafer, (b) forming an thin layer of silicon on said etch stop layer; (c) masking and etching the thin layer of silicon to define a beam structure disposed adjacent said first substrate or wafer; (d) removing the first substrate or wafer by use of an etchant to which said etch stop layer is resistant; and (e) removing said etch stop layer using an etchant to which said cantilevered beam structure is resistant. 32. A method of making a MEM switch or tunneling sensor as claimed in claim 31 wherein a layer of Ti Pt/Au is selectively deposited on said thin layer to form first and second metal contacts on said thin layer, said second contact being disposed near a distal end of said beam structure and said first contact forming the mating structure on the first substrate or wafer. 33. A MEM switch or tunneling sensor assembly comprising: (a) a beam structure and a mating structure defined on a first substrate or wafer; (b) at least one contact structure and a mating structure defined on a second substrate or wafer; (c) at least one of the two mating structures including at least one protrusion extending from the substrate or wafer from which the at least one protrusion is defined; and (d) a bonding layer disposed on at least one of said mating structures for bonding the mating structure defined on the first substrate or wafer to the mating structure on the second substrate or wafer, the mating structures being joined one to another at said bonding layer. 34. A MEM switch or tunneling sensor assembly as claimed in claim 33 wherein said assembly is for making a MEM switch or tunneling sensor therefrom, the bonding layer being pressure/heat sensitive, the mating structure on the second substrate or wafer being of a complementary shape to the mating structure on the first substrate or wafer. 35. A MEM switch or tunneling sensor assembly as claimed in claims 33 or 34 wherein the first and second substrate or wafer are formed of silicon. 36. A MEM switch or tunneling sensor assembly as claimed in claim 35 wherein the silicon forming the first and second substrate or wafer are of a single crystalline structure. 37. A MEM switch or tunneling sensor assembly as claimed in claim 36 wherein the crystalline structure of the silicon is <100>. 38. A MEM switch or tunneling sensor assembly as claimed in claim 37 wherein the silicon is n-type. 39. A MEM switch or tunneling sensor assembly as claimed in claim 34 wherein the cantilevered beam structure is formed from a thin layer of silicon on said first substrate or wafer, said thin layer being doped with a dopant. 40. A MEM switch or tunneling sensor assembly as claimed in claim 33 wherein the cantilevered beam structure is formed from an epitaxial layer of silicon on said first substrate or wafer, said epitaxial layer being doped with a dopant. 41. A MEM switch or tunneling sensor assembly as claimed in claim 39 wherein a pointed contact is disposed on an end of said beam structure. ' 42. A MEM switch or tunneling sensor assembly as claimed in claim 39 wherein the thin layer is doped with Boron at a sufficient concentration to reduce the resistivity of the thin layer to less than 1 Ω-cm, the thin layer being an epitaxial layer. 43. A MEM switch or tunneling sensor assembly as claimed in claim 39 wherein the thin layer is doped with Boron at a sufficient concentration to reduce the resistivity of the thin layer to less than 0.05 Ω-cm, the thin layer being an epitaxial layer. 44. A MEM switch or tunneling sensor assembly as claimed in claims 40 or 43 further including first and second contacts on said epitaxial layer, said second contact being disposed near a distal end of the beam structure and said first contact forming at least a portion of the mating structure on the first substrate or wafer. 45. A MEM switch or tunneling sensor assembly as claimed in claim 44 wherein said first and second contacts are a first and second ohmic contact. 46. A MEM switch or tunneling sensor assembly as claimed in claim 44 wherein said first and second contacts are preferably formed of Ti/Pt/Au. 47. A MEM switch or tunneling sensor assembly as claimed in claim 44 further including metal contacts, preferably formed of Ti/Pt/A, disposed on said second substrate or wafer, at least one of said contacts on the second substrate or wafer in combination with the protrusion defining the mating structure on the second substrate or wafer. 48. A MEM switch or tunneling sensor assembly as claimed in claim 45 wherein a layer of metal is disposed on the first and second ohmic contacts, a first portion of the layer of metal being disposed on said first ohmic contact and forming at least a portion of the mating structure on the first substrate or wafer and a second portion of the layer of metal forming a pointed contact on said second ohmic contact. 49. A MEM switch or tunneling sensor assembly as claimed in claim 48 further including metal contacts disposed on said second substrate or wafer, at least one of said contacts on the second substrate or wafer cooperating with a protruding portion formed from the second substrate to define at least a portion of the mating structure on the second substrate or wafer. 50. A MEM switch or tunneling sensor assembly as claimed in claim 47 wherein the bonding layer is provided by a layer of Au-Si eutectic disposed on the at least one metal contact on said second substrate and/or by a layer of Au-Si eutectic disposed on the first contact formed on the first substrate or wafer. 51. A MEM switch or tunneling sensor assembly as claimed in claim 49 wherein the bonding layer is provided by a layer of Au-Si eutectic disposed on the metal contact on said second substrate or wafer and/or by a layer of Au-Si eutectic disposed on the first portion of the layer of metal on the first substrate or wafer. 52. A MEM switch or tunneling sensor assembly as claimed in claim 50 wherein the silicon for the eutectic bond is provided by the silicon substrate of the first or second substrates or wafers of the mating structures. 53. A MEM switch or tunneling sensor assembly as claimed in claim 49 wherein the bond layer is an eutectic bonding layer and wherein silicon for the eutectic bond is provided by the silicon substrate of the first or second substrates or wafers at the mating structure. 54. A MEM switch or tunneling sensor assembly as claimed in claim 39 further including first and second contacts on said epitaxial layer, said second contact being disposed near a distal end of the beam structure and said first contact forming at least a portion of the mating structure on the first substrate or wafer. 55. A MEM switch or tunneling sensor assembly as claimed in claim 33 wherein an at least one protruding portion protrudes from a major surface of said second substrate or wafer and wherein said assembly further includes metal contacts on said second substrate or wafer, at least one of said contacts on the second substrate or wafer defining, in combination with the protruding portion associated with the second substrate, the mating structure on the second substrate or wafer. 56. A MEM switch or tunneling sensor assembly as claimed in claim 55 wherein the bonding layer is provided by a layer of Au-Si eutectic disposed on a metal contact on said second substrate or wafer and/or by a layer of Au-Si eutectic disposed on the first contact. 57. A MEM switch or tunneling sensor assembly as claimed in claim 33 wherein first and second interconnected metal contacts are disposed on said beam structure, the contacts being interconnected by an elongate ribbon layer of metal, said second interconnected contact being disposed near a distal end of the beam structure, the elongate ribbon layer being disposed longitudinally on the beam structure and said first interconnected contact forming at least a portion of the mating structure on the first substrate or wafer. 58. A MEM switch or tunneling sensor assembly as claimed in claim 57 wherein the at least one protruding portion protrudes from a major surface of said second substrate or wafer and wherein metal contacts are disposed on said second substrate or wafer, at least one of said contacts on the second substrate or wafer defining, in combination with the protruding portion, at least a portion of the mating structure on the second substrate or wafer. 59. A MEM switch or tunneling sensor assembly as claimed in claim 58 wherein the bonding layer is provided by a layer of Au-Si eutectic disposed on the metal contact on said second substrate or wafer and/or by a layer of Au-Si eutectic disposed on first interconnected contact.

Inventors: CHANG DAVID T, KUBENA RANDALL L, CHANG, DAVID, T., KUBENA, RANDALL, L.
Applicants: CHANG DAVID T, HRL LAB LLC, KUBENA RANDALL L, CHANG, DAVID, T., HRL LABORATORIES, LLC, KUBENA, RANDALL, L.
Application year: 2001
Main classification: H01L21
IPC codes: B81B3, B81C1, B81C3, G01C19, G01P9, H01H1, H01H59