Method for making a suspended membrane structure with buried electrode

Abstract

A microsystem and/or nanosystem type device is disclosed, comprising: a first substrate, or intermediate substrate, comprising a mobile part, a second substrate or support substrate, at least one lower electrode, and one dielectric layer (101) located between the first and second substrates, the dielectric layer being arranged between the lower electrode and the first substrate; the first substrate comprising through vias filled with conducting material in contact with said lower electrode.

Claims

1. At least one of a microsystem or a nanosystem device comprising: a first substrate, being an intermediate substrate, comprising a mobile part, a second substrate being a support substrate, at least one first electrode, being a lower electrode, formed of at least one conducting material, defined in a lower electrode layer, and one dielectric layer located between the first and second substrates, the dielectric layer being arranged in part or in whole between the lower electrode and the first substrate, a cavity disposed under the mobile part of the system, said lower electrode disposed under said cavity and having a top part facing at least part of the mobile part spaced apart from the mobile part, the first substrate comprising at least one through via filled with said at least one conducting material, said at least one through via and said lower electrode, both formed during a continuous process filling the at least one through via and forming the lower electrode, thereby both comprising identically the same said at least one conducting material throughout without interruption, and a bottom surface of the mobile part of the first substrate is exposed to the cavity.

2. The device according to claim 1, further comprising a second electrode, being an upper electrode, located on the first substrate, and in electrical contact with the at least one through via passing through the first substrate.

3. The device according to claim 2, the upper electrode being located on the intermediate substrate using conductors, or being supported by a third substrate.

4. The device according to claim 1, also comprising electrical contact zones between the lower electrode and the first substrate.

5. The device according to claim 1, the first substrate being made of a semiconducting material, for example silicon, or SiGe or SiC or SiGeC or GaAs or Ge or a semiconducting material in Group III-V, preferably doped, or a “silicon on insulator” (SOI) type substrate.

6. The device according to claim 1, the first substrate comprising several layers stacked on a substrate.

7. The device according to claim 1, further comprising a dielectric layer between the second substrate and the lower electrode layer.

8. The device according to claim 7, etched zones being defined in the lower electrode layer and in the dielectric layer located between the second substrate and the lower electrode layer.

9. The device according to claim 7, etched zones being defined in the lower electrode layer, said etched zones being filled with the material of said dielectric layer.

10. The device according to claim 1, wherein the cavity is at least as wide as the mobile part.

11. The device according to claim 1, wherein the cavity is wider than a width of the mobile part.

12. The device according to claim 1, wherein the first substrate comprising said at least one through via is filled with said at least one conducting material.

13. The device according to claim 1, wherein an extension member contacts specific locations of the first substrate to limit parasite capacitance.

14. The device according to claim 1, wherein an extension member of the lower electrode contacts specific locations of the first substrate.

15. The device according to claim 1, wherein an extension member contacts specific locations of the first substrate.

16. The device according to claim 15, wherein the extension member limits parasite capacitance.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 shows a NEMS/MEMS structure with 2 parallel capacitive electrodes, below and above the active layer,

(2) FIGS. 2A-2I show various steps in the production of a MEMS or NEMS type device, in which the layer used for assembly with the second substrate comprises cavities,

(3) FIGS. 3A-3D show a variant of steps in performing the method in FIGS. 2A-2I,

(4) FIGS. 4A-4B show a variant of the other steps in the production of a MEMS or NEMS type device, in which the layer used for assembly with the second substrate is continuous;

(5) FIG. 5 shows a simplified top view of the intermediate substrate, with a mobile structure;

(6) FIG. 6 shows an SOI substrate structure.

DETAILED PRESENTATION OF PARTICULAR EMBODIMENTS

(7) We will start by describing an example of a MEMS/NEMS structure shown in FIG. 1, which is a sectional view.

(8) We will refer to “substrate” 100, 200, 300 in the following, although we could also say a “layer”. Consequently, we will use either of these terms indifferently for these three elements.

(9) The structure in FIG. 1 may be made in 2 substrates 100, 200 that are subsequently called the first substrate and the second substrate respectively, superposed and assembled to each other preferably by direct bonding or molecular bonding.

(10) For a better understanding of the description, we will refer to a orthogonal coordinate system xyz as shown in FIG. 1, in other words with a Z axis perpendicular to the layers or substrates of the device, while the other two axes x and y are perpendicular to the Z axis and are perpendicular to each other.

(11) The structure in FIG. 1 comprises: a first substrate or active layer 100 in which the mobile or active part of the MEMS (or NEMS) is formed; a support substrate or second substrate 200; a set of lower electrodes 102 and a sacrificial layer 101 located between the first substrate and the second substrate, part of the sacrificial layer 101 being eliminated to form a cavity 101′ under the mobile part of the system; the bottom of this cavity is formed partly by the upper surface of an electrode 102 that faces at least part of the active zone of the MEMS; a set of upper electrodes 106 located at a certain distance from the active zone of the MEMS and that faces at least part of this active zone; interconnection means 111 that electrically connect the lower electrode layer 102 and the upper electrode layer 106 through the substrate 100.

(12) As can be seen in FIG. 1, the connections 111 are in the form of through pads, for example cylindrical section pads in the xy plane.

(13) In the example chosen, these connections are arranged outside the mobile parts of the MEMS in the xy plane on each side of the active part 120 of the MEMS, made in the substrate 100 in the example chosen. Their number may be variable, two are shown in the figure, one on each side of the active part 120, but this number may be arbitrary (or even equal to 1).

(14) Furthermore, one or several connections 111′ can also be seen between the set of lower electrodes 102 and the substrate 100, so that the active layer can be polarised at specific locations, for example to limit parasite capacitance phenomena.

(15) The material in the lower electrode 102 may be a semiconducting material, preferably doped, for example doped amorphous or poly-crystalline Si, or a metallic layer.

(16) Part of the first substrate that forms the active zone or mobile mass 120 is free to move, particularly along the Z axis. This displacement can be detected by lower electrodes 102 and upper electrodes 106.

(17) In the example described herein, a capacitive detection system is used. In the case of an RF component, for example, there will be a resistive membrane formed in a very slightly doped Si substrate.

(18) FIG. 5 shows a simplified top view of the substrate 100 with an example of a structure with a mobile mass 120 also comprising means of detecting movement of the mobile mass in the plane of the layer 120.

(19) The mobile mass may comprise one or several mobile combs, free to move relative to one or several fixed combs.

(20) The structure in this FIG. 5 shows a mobile mass 120 provided with inter-digitised electrostatic combs on its sides, some of which 500, 506 are fixed and others 504, 502 are mobile, a mobile comb 504, 506 being inter-digitised with a fixed comb 500, 502. A variation in the distance between a fixed comb and the mobile comb facing it will cause a movement of the mobile comb, which itself is due to a movement of the mobile mass 120 and therefore the device, causing a variation in the capacitance detected by means provided for this purpose.

(21) In general, the mobile part enables detection of movements imposed on the component.

(22) Furthermore, the mobile part may comprise one or several mobile or seismic masses.

(23) As can be seen also in FIG. 1, the level of upper electrodes 106 is not supported directly on the substrate 100, but it is connected to the pads 111 through pads 106′ that keep a given distance between the upper surface of the substrate 100 and the lower surface of the electrode layer 106.

(24) For example, the thickness of the substrate 100 may be between a few tens of μm and a few hundred μm, for example between about 10 μm and 100 μm or 500 μm.

(25) This substrate extends in the xy plane, the z axis being perpendicular to each of the substrates 100, 200. This is why the xy plane is also called the plane or principal plane of the device.

(26) The thickness of the device measured along the z axis may be very small compared with the lateral extensions of the device, in other words in comparison with the dimensions p and l of the device measured in the xy plane; p (measured along the x axis) may for example be between 100 μm and a few mm and l (measured along the y axis) may for example be of the order of a few hundred micrometers, for example between 100 μm and 10000 μm.

(27) The substrate 100 may be made from a semiconducting material, preferably monocrystalline, but also possibly polycrystalline. For example it may be made from silicon, or SiGe or SiC or SiGeC or GaAs or Ge or a semiconducting material in group III-V. The semiconducting material (and particularly silicon) may be doped, particularly in the above example to enable electrical conduction in the active layer.

(28) As a variant, this substrate 100 may be a “silicon on insulator” (SOI) type substrate comprising a substrate 10 made of a semiconducting material, a buried layer 11 of oxide and a thin semiconducting layer 12 for example made of monocrystalline silicon, as shown in FIG. 6. This type of substrate 100 may be used to create functions using for example the monocrystalline nature of the layer 12.

(29) An SOI substrate may also be used advantageously in which the layer 101 and the lower electrode 102 would be entirely or partly formed by the buried oxide layer 11 and the possibly doped thin silicon layer 12 respectively. The advantage of this embodiment is particularly that the layer 11 can be used for example as an etching stop layer.

(30) As a variant, this substrate may comprise several layers stacked on a semiconducting substrate, each layer for example being located in any of the materials mentioned above.

(31) The advantage is that additional interconnection levels may be made and/or functions can be added under the active layer.

(32) The substrate 200 may for example be made of Si or one of the other semiconducting materials mentioned above or a material transparent to light (for example glass or quartz, etc.) depending on the end purpose of the component. It may possibly be covered by a dielectric layer (not shown in the figure).

(33) In the remainder of this description, the lower part or side of the device refers to the part facing the free surface 200′ of the substrate 200, and the upper part or side of the device is the part facing the opposite side, namely the side of the electrodes 106.

(34) Another example of a MEMS/NEMS structure is shown in FIG. 3D, which is also a sectional view.

(35) The difference from the previous structure lies in the way in which the upper electrode layer 306 is connected to the substrate 100. The link between the upper electrode layer and the interconnections 111 that pass through the substrate 100 is made by conducting means 108, for example in the form of connection pads 108.

(36) The fabrication methods described below are only given as examples and may be applied to components other than the component with capacitive detection used for illustration.

(37) FIGS. 2A-2I show a first example of a method to make a MEMS type device with capacitive detection outside the plane.

(38) The starting point (FIG. 2A) is a substrate 100 that may be composed of one of the materials already mentioned above. This is the substrate 100 in which the active part of the MEMS will be made.

(39) A dielectric layer 101 (FIG. 2B) called the sacrificial layer is deposited or grown on this substrate 100, using a method for example such as thermal oxidation or by a CVD deposition, or a plasma enhanced CVD deposition (PECVD), or ALD (atomic layer deposition), or IBD (Ion Beam Deposition). This insulating layer 101 isolates the future lower electrode 102 from the first substrate 100. Therefore the function of this dielectric layer is essentially an electrical insulation function but it can also be used as a sacrificial layer.

(40) Photolithography and etching steps can then be done (FIG. 2C) to define cavities 111, 111′, 111″ in this first dielectric layer and for some of them, in the substrate 100. The cavities 111 can then be used to form contact means between the lower electrode and the upper electrode of the device, through the substrate 100. The cavities 111′ can be used to make alignment marks (111′) passing through the substrate 100, compatible with the alignment equipment. These marks are thus self-aligned with the lower electrode. The cavities 111″ are used to make contact zones between the substrate 100 and the lower electrode 102.

(41) These cavities 111, 111′ and 111″ can be etched in one or several steps to a depth greater than the thickness of the future MEMS component, typically of the order of 5 μm to 100 μm or 200 μm, for example, for a component said to have a high aspect ratio. In other words, the depth of the cavities 111, 111′ is greater than or equal to the thickness of the substrate 100 after thinning, as described below.

(42) The cavities are then filled with a conducting material 102 (FIG. 2D). The conducting layer 102 formed on the upper surface of the substrate 100 and that will be called the lower electrode or the lower electrode layer depending on the geometry of the electrodes made in this layer 102, is created during this step, thus enabling a very good quality contact between the conducting cavity and the lower electrode. The cavities 111′ used for alignment marks are filled at the same time as the interconnections.

(43) The method of depositing this conducting layer of the lower electrode will be chosen as a function of its nature: for example this deposition can be made by evaporation or by CVD or by plasma enhanced CVD, or by sputtering. Deposition of a layer of doped Si by LPCVD is attractive because it is compatible with most methods used in a semiconductor production line, which allows the designer of this type of MEMS a great deal of freedom in choosing technological steps for complete fabrication of the component.

(44) The cavities 111, 111′ may be isolated before filling with the conducting material. This isolation may be done by formation of a coating or a thin dielectric layer on the surfaces of each cavity 111, 111′, for example by thermal oxidation or by a deposition method such as CVD or plasma enhanced CVD (PECVD), ALD or IBD.

(45) Finally, the lower electrode 102 may be thinned, for example by physical or chemical etching or by mechanical-chemical polishing or by a combination of these methods.

(46) FIG. 2E shows the formation of a dielectric layer 104 above the lower electrode. This layer may be made by thermal oxidation or by a deposition such as a CVD deposition or plasma enhanced CVD (PECVD), or ALD or IBD, for example. This dielectric layer and the conducting layer 102 on which it is formed can then be etched to define the structure of the lower electrode as can be seen in FIG. 2E with the etched zones 105. The bottom of the etched zones is approximately equivalent to the surface of the layer 101.

(47) In the case of an SOI type substrate (FIG. 6), the surface layer 12 can form the membrane on which all or some of a micro or nano system is formed. The dielectric 11 of the SOI may then be used as a stop layer for etching the vias.

(48) An assembly can then be made by cementing this substrate 100 thus prepared with the second substrate 200, preferably in direct bonding (FIG. 2F). The surface of the dielectric layer 104 then comes into contact with the substrate 200, a layer of insulating or dielectric material being formed on this substrate 200; the assembly is then made with this dielectric layer.

(49) The first substrate 100, once assembled to the second substrate, is thinned (FIG. 2F) from its face 100′ opposite the interface between the two substrates 100, 200. This step thus defines the thickness of the MEMS/NEMS and exposes the interconnections 111 and exposes the alignment marks 111′.

(50) This thinning may be done alone or in combination, using one or several of the following techniques: chemical etching and/or dry etching and/or ionic implantation and separation of the substrate (for example see document EP 763849) and/or grinding, and/or mechanical-chemical polishing.

(51) After being thinned, the first substrate 100, is etched vertically (FIG. 2G) from its upper face opposite the interface between the two substrates 100, 200. This means that vertical etched zones 107 can be formed that pass through the substrate 100 and define the mobile parts of the active zone 120 of the device. The bottom of the etched zones corresponds approximately to the surface of the layer 101.

(52) This thus defines the active part of the MEMS.

(53) The etching used may be of the RIE type and preferably the DRIE type.

(54) The process can be stopped at this stage, in which case no upper electrode and no assembly with a substrate or cap 300 will be made (FIG. 3D), and therefore there will be no second electrode. If a lower electrode and an upper electrode enable detection of the movement of the mobile part outside the plane of the layers by capacitive variation, a single electrode may be sufficient for this detection although the use of a second electrode enables detection with better sensitivity.

(55) But on the contrary, it is also possible to continue the manufacturing method with the formation of an upper electrode.

(56) This can then be made above the active layer (FIG. 2H).

(57) This is why an oxide layer 105 is formed on the upper surface of the substrate 100; the material in this layer fills in or blocks off the cavities 107 left open during etching of the MEMS structure.

(58) Openings 105′ are made in the layer 105 above the top of the interconnections 111 in order to prepare the electrical connection between the upper electrode and these interconnections 111. A layer of conducting material 106 can then be deposited on the layer 105. The thickness of the layer 105 defines the distance between the upper part of the active part of the MEMS and the lower surface of the electrodes 106.

(59) This conducting layer 106 is then partially etched in order to form through zones 106″. The structure of the upper electrodes 106 is thus defined. The MEMS can also be released through the openings 106″ by elimination by wet, dry or vapour phase etching of the sacrificial layer 105. The electrodes 106 then remain in contact only with the interconnections 111 through the parts of the conducting material that were formed in the etched zones 105′ of the layer 105.

(60) The dielectric material present in the crossings 107 is also etched together with the portion of the layer 101 on which the active part of the MEMS is supported, thus forming a cavity 101′ between the active part and the lower layer of electrodes 102.

(61) The result obtained is then the structure shown in FIG. 2I, the active part of the MEMS being free between the two etched zones 101′ and 105″.

(62) FIGS. 3A to 3C show another example of the formation of the upper electrode 106a facing the active layer.

(63) Starting from the structure obtained during the step described above with reference to FIG. 2F, an oxide layer 105a is formed on the upper surface of the MEMS (FIG. 3A).

(64) Openings 105′a are formed in this layer 105a in order to prepare the electrical connection between the upper electrode and the interconnections 111. Therefore, openings 105′a are made above these connection pads 111.

(65) A deposition of conducting (or semiconducting) material, for example made of Ge, is then formed on the insulating layer and in openings 105′a, and then etched to define contact pads 108 that project above the free surface of the layer 105a (FIG. 3B). The height of these pads and the thickness of the layer 105a can define the distance that will separate the upper part of the substrate 100 and therefore the active zone of the MEMS, and the lower surface of the future electrode 106.

(66) The layer 105a and then the substrate 100 are then etched vertically (FIG. 3C) from the upper face, opposite the interface between the two substrates 100, 200. This etching step can form vertical etched zones 107 that pass through the substrate 100, but also the portion of the layer 101 located between the lower electrode 102 and the active zone of the MEMS. It leaves the connection means that were formed intact, and particularly the contact pads 108. Etching of the layer 101 leads to the formation of a cavity 101′ that exposes the active zone. The bottom of this cavity 101′ is approximately at the surface of the electrode layer 102. The active part of the MEMS is thus defined. Etching of the layer 100 may be of the RIE type, and preferably DRIE.

(67) The upper electrode may then be made above the active layer (FIG. 3D).

(68) The next step to achieve this is to assemble a substrate 300 provided with a conducting layer 301 on its surface that may have been previously etched and that will form the future upper electrode layer. This conducting layer is assembled with the free ends of the pads 108. The assembly is made by metallic cementing onto the contact pads.

(69) A cap 300 without an electrode 301 may also be advantageous in the case of a component operating with the single lower electrode.

(70) A variant of the steps described above in relation with FIGS. 2E-2F will now be described.

(71) The electrode layer 102 is etched in zones 102′ starting from the structure obtained at the end of the step described above with reference to FIG. 2D (FIG. 4A). The bottom of these etched zones is approximately the same as the upper surface of the layer 101. An insulating layer 104, for example an oxide layer, is formed on the upper surface of the layer 102. The material in this insulating layer 104 also fills the zones 102′ that were previously etched. The layer 104 may be formed by one of the techniques already mentioned above.

(72) The next step is to make an assembly by cementing this substrate 100 thus prepared with a second substrate 200, preferably by direct bonding (FIG. 4B). The surface of the dielectric layer 104 then comes into contact with the substrate 200; an insulating or dielectric layer may possibly be formed on this substrate 200, the assembly then being made with this dielectric layer.

(73) In this variant, the layer 104 remains continuous and is not etched. The electrode 102 is etched before this layer 104 is deposited, unlike the step in FIG. 2E in which the layer 104 is deposited first before making the etched zones 105, both in this layer and in the electrodes layer 102.

(74) Once assembled to the substrate 200, the substrate 100 may be etched along a vertical direction as explained above with reference to FIG. 2G. The next step is to perform the steps corresponding to FIGS. 2H-2I. Refer to the above description of these steps.