# Patent

## Title
Use of metal native oxide to control stress gradient and bending moment of a released MEMS structure

## Abstract

In described examples, a MEMS device is formed by forming a sacrificial layer over a substrate and forming a first metal layer over the sacrificial layer. Subsequently, the first metal layer is exposed to an oxidizing ambient which oxidizes a surface layer of the first metal layer where exposed to the oxidizing ambient, to form a native oxide layer of the first metal layer. A second metal layer is subsequently formed over the native oxide layer of the first metal layer. The sacrificial layer is subsequently removed, forming a released metal structure.

## Background
CROSS-REFERENCE TO RELATED APPLICATIONS
This application is a divisional of U.S. patent application Ser. No. 14/467,521 filed Aug. 25, 2014, which is hereby incorporated by reference in its entirety.


BACKGROUND
This relates generally to microelectronic-mechanical systems (MEMS) devices, and more particularly to metal structures in MEMS devices.
A MEMS device may contain a released metal structure which is formed by depositing one or more metal layers over a sacrificial layer and subsequently removing the sacrificial layer. Attaining a desired shape of the released metal structure may be problematic, possibly due to internal stresses in the metal layers.
SUMMARY
In described examples, a MEMS device is formed by forming a sacrificial layer over a substrate and forming a first metal layer over the sacrificial layer. Subsequently, the first metal layer is exposed to an oxidizing ambient which oxidizes a surface layer of the first metal layer where exposed to the oxidizing ambient, to form a native oxide layer of the first metal layer. A second metal layer is subsequently formed over the native oxide layer of the first metal layer. The sacrificial layer is subsequently removed, forming a released metal structure.



BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1A through FIG. 1G are cross sections of an exemplary MEMS device containing a released metal structure, depicted in successive stages of fabrication.
FIG. 2A through FIG. 2F are cross sections of another exemplary MEMS device containing a released metal structure, depicted in successive stages of fabrication.
FIG. 3A through FIG. 3E are cross sections of a further exemplary MEMS device containing a released metal structure, depicted in successive stages of fabrication.



DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS
The figures are not necessarily drawn to scale, and they are provided merely to illustrate. Several aspects are described below with reference to example applications for illustration. Numerous specific details, relationships and methods are set forth to provide an understanding of the examples. One or more of the specific details may be unnecessary. In other instances, well-known structures or operations are not shown in detail. The disclosures are not limited by the illustrated ordering of acts or events, as some acts may occur in different orders and/or concurrently with other acts or events. Furthermore, not all illustrated acts or events are required to implement a methodology.
A MEMS device is formed by forming a sacrificial layer over a substrate and forming a first metal layer over the sacrificial layer. Subsequently, the first metal layer is exposed to an oxidizing ambient which oxidizes a surface layer of the first metal layer where exposed to the oxidizing ambient, to form a native oxide layer of the first metal layer. A second metal layer is subsequently formed over the native oxide layer of the first metal layer. The sacrificial layer is subsequently removed, forming a released metal structure in which a portion of the released metal structure is free of any direct contact with solid elements of the MEMS device.
The native oxide layer may be contiguous with a native oxide layer at a bottom surface of the released metal structure, or may be contiguous with a native oxide layer at a top surface of the released metal structure, or may be located between metal layers in the released metal structure. More than one native oxide layer may be formed in the released metal structure.
For the purposes of this disclosure, the term native oxide layer of a metal layer is understood to mean an oxide of the metal in the metal layer, formed by oxidation of metal atoms at a surface of the metal layer. It is further understood that the term native oxide layer precludes metal oxides formed by anodization, deposition of additional metal in an oxidizing ambient, ion implantation of oxidizing species, or immersion in a liquid oxidizing solution.
FIG. 1A through FIG. 1G are cross sections of an exemplary MEMS device containing a released metal structure, depicted in successive stages of fabrication. Referring to FIG. 1A, the MEMS device 100 is formed on a substrate 102. The substrate 102 may be, for example, a silicon wafer, a glass or sapphire panel, a ceramic substrate, a multilayer substrate, or other substrate suitable for forming the MEMS device 100. Furthermore, the substrate 102 may include elements of the MEMS device 100 formed prior to the instant stage of fabrication.
A sacrificial layer 104 is formed over the substrate 102. The sacrificial layer 104 may optionally be patterned so that a via hole 106 is formed through the sacrificial layer 104. The sacrificial layer 104 may include a photoactive polymer such as polyimide or photoresist, and the via hole 106 if present may be formed by a photolithographic process. Alternatively, the sacrificial layer 104 may include a non-photoactive polymer such as novolak resin or methyl methacrylate, and the via hole 106 if present may be formed by a mask and etch process. In a further version of the instant example, the sacrificial layer 104 may include inorganic dielectric material such as low-k dielectric material or porous material. Instances of the sacrificial layer 104 containing other materials are within the scope of the instant example.
A first metal layer 108 of the released metal structure is formed over the sacrificial layer 104. The first metal layer 108 may be, for instance, 10 nanometers to 60 nanometers of titanium aluminum alloy formed by physical vapor deposition (PVD), or may be 50 nanometers to 100 nanometers of sputtered aluminum with 0.1 to 3 percent silicon, copper and/or titanium, or may be 120 nanometers to 500 nanometers of titanium aluminum alloy formed by PVD. Instances of the first metal layer 108 containing other metals are within the scope of the instant example. The first metal layer 108 may extend conformally into the via hole 106 if present.
Referring to FIG. 1B, the first metal layer 108 is exposed to a first oxidizing ambient 110. The first oxidizing ambient 110 may be ordinary air at 20° C. to 25° C. and atmospheric pressure, so that an oxidizing species of the first oxidizing ambient 110 is predominantly oxygen molecules 112, as depicted schematically in FIG. 1B. Alternatively, the first oxidizing ambient 110 may be, for example, oxygen gas at less than atmospheric pressure, ozone or an oxygen containing plasma, so that the oxidizing species may include oxygen ions, ozone, or oxygen radicals. The first metal layer 108 may be exposed to the first oxidizing ambient 110 by removing the MEMS device 100 from a process tool in which the first metal layer 108 was formed, or the first oxidizing ambient 110 may be introduced into the process tool after the first metal layer 108 is formed.
Exposure to the first oxidizing ambient 110 forms a first native oxide layer 114 at a top surface of the first metal layer 108, the top surface being the surface of the first metal layer 108 exposed to the first oxidizing ambient 110, regardless of the orientation of the MEMS device 100 during exposure to the first oxidizing ambient 110. The first native oxide layer 114 may be, for example, 1 nanometer to 10 nanometers thick, depending on a composition of the first metal layer 108. In one version of the instant example, the first metal layer 108 is exposed to the first oxidizing ambient 110 for a sufficient time so that formation of the first native oxide layer 114 is substantially self-limiting, that is, a thickness of the first native oxide layer 114 remains substantially constant with prolonged exposure to the first oxidizing ambient 110.
Referring to FIG. 1C, a second metal layer 116 is formed over the first native oxide layer 114. In one version of the instant example, the second metal level 116 may have a same composition as the first metal level 108. In another version, the second metal level 116 may have a different composition from the first metal level 108. In the version in which the first metal level 108 is 10 nanometers to 60 nanometers of titanium aluminum alloy, the second metal layer 116 may be 30 nanometers to 80 nanometers of titanium aluminum alloy also formed by PVD. In the version in which the first metal level 108 is 50 nanometers to 100 nanometers of sputtered aluminum, the second metal layer 116 may be 100 nanometers to 150 nanometers of sputtered aluminum. In the version in which the first metal level 108 is 200 nanometers to 500 nanometers of titanium aluminum alloy, the second metal layer 116 may be 300 nanometers to 500 nanometers of titanium aluminum alloy also formed by PVD. Instances of the second metal layer 116 containing other metals are within the scope of the instant example. The second metal layer 116 may extend conformally into the via hole 106 if present.
Referring to FIG. 1D, the second metal layer 116 may optionally be exposed to a second oxidizing ambient 118. The second oxidizing ambient 118 may be, for example, an oxygen containing plasma, so that an oxidizing species includes oxygen molecules 112 and oxygen radicals 120, as depicted in FIG. 1D. Alternatively, the second oxidizing ambient 118 may be, for example, ordinary air, ozone or an oxygen containing plasma. The second metal layer 116 may be exposed to the second oxidizing ambient 118 by various methods, as described in respect to FIG. 1B.
Exposure to the second oxidizing ambient 118 forms a second native oxide layer 122 at a top surface of the second metal layer 116, the top surface being the surface of the second metal layer 116 exposed to the second oxidizing ambient 118, regardless of the orientation of the MEMS device 100 during exposure to the second oxidizing ambient 118. The second native oxide layer 122 may be, for example, 1 nanometer to 10 nanometers thick, depending on a composition of the second metal layer 116. In one version of the instant example, the second metal layer 116 is exposed to the second oxidizing ambient 118 for a sufficient time so that formation of the second native oxide layer 122 is substantially self-limiting.
Referring to FIG. 1E, a third metal layer 124 may optionally be formed over the second native oxide layer 122. In one version of the instant example, the third metal layer 124 may have a same composition as the first metal level 108 or the second metal level 116. In another version, the third metal layer 124 may have a different composition from the first metal level 108 and the second metal level 116. The third metal layer 124 may extend conformally into the via hole 106 if present.
Referring to FIG. 1F, a release process is performed on the MEMS device 100 which removes the sacrificial layer 104 of FIG. 1E. The release process may include, for example, exposing the MEMS device 100 to an ashing ambient 126 containing oxygen radicals 120 and/or ozone 128, using a downstream asher. The release process may possibly oxidize a bottom surface of the first metal layer 108 and/or a top surface of the third metal layer 124.
FIG. 1G depicts the MEMS device 100 after the release process is completed. The first metal layer 108, the first native oxide layer 114, the second metal layer 116, the second native oxide layer 122 if present, and the third metal layer 124 if present provide the released metal structure 130 of the MEMS device 100. A portion of the released metal structure 130 is free of any direct contact with solid elements of the MEMS device 100. In the instant example, the portion of the released metal structure 130 which is free of any contact with solid elements of the MEMS device 100 has a substantially flat shape; the substantially flat shape is provided by a balance of compressive stress in the first native oxide layer 114 and the second native oxide layer 122 if present, with a cumulative tensile stress in the first metal layer 108, the second metal layer 116 and the third metal layer 124 if present. The released metal structure 130 may be attached to the substrate 102 through the via hole 106 if present. If the via hole 106 is not present, the released metal structure 130 is attached to the substrate 102, possibly through other elements, not shown of the MEMS device 100.
Forming the native oxide layers 114 and 122 provides more process latitude compared to other methods of forming metal oxides, as thicknesses of the native oxide layers 114 and 122 are substantially self-limiting during exposure to oxidizing ambients. Also, stress levels of the native oxide layers 114 and 122 tend to be more reproducible compared to metal oxides formed by other methods. Thus, forming the released metal structure 130 to include the first native oxide layer 114 and the second native oxide layer 122 may advantageously reduce fabrication cost and complexity of the MEMS device 100.
FIG. 2A through FIG. 2F are cross sections of another exemplary MEMS device containing a released metal structure, depicted in successive stages of fabrication. Referring to FIG. 2A, the MEMS device 200 is formed on a substrate 202, for example, similar to the substrate 102 described in reference to FIG. 1A.
A sacrificial layer 204 is formed over the substrate 202. The sacrificial layer 204 may optionally be patterned so that via holes 206 are formed through the sacrificial layer 204. The sacrificial layer 204 may include materials and be patterned as described in reference to FIG. 1A.
A first metal layer 208 of the released metal structure is formed over the sacrificial layer 204. The first metal layer 208 may include aluminum, titanium, tungsten, and/or other metals. In the instant example, the first metal layer 208 is at least 20 nanometers thick. The first metal layer 208 may extend conformally into the via holes 206 if present.
Referring to FIG. 2B, the first metal layer 208 is exposed to a first oxidizing ambient 210. The first oxidizing ambient 210 may be for example, ordinary air, oxygen gas at less than atmospheric pressure, ozone or an oxygen containing plasma. An oxidizing species of the first oxidizing ambient 210 may be, for example, predominantly oxygen molecules 212, as depicted schematically in FIG. 2B, or may include oxygen ions, ozone, or oxygen radicals. As described in reference to FIG. 2B, the first metal layer 208 may be exposed to the first oxidizing ambient 210 by removing the MEMS device 200 from a process tool in which the first metal layer 208 was formed, or the first oxidizing ambient 210 may be introduced into the process tool after the first metal layer 208 is formed.
Exposure to the first oxidizing ambient 210 forms a first native oxide layer 214 at a top surface of the first metal layer 208, the top surface being the surface of the first metal layer 208 exposed to the first oxidizing ambient 210, regardless of the orientation of the MEMS device 200 during exposure to the first oxidizing ambient 210. The first native oxide layer 214 may be, for example, 1 nanometer to 10 nanometers thick, depending on a composition of the first metal layer 208. In one version of the instant example, the first metal layer 208 is exposed to the first oxidizing ambient 210 for a sufficient time so that formation of the first native oxide layer 214 is substantially self-limiting.
Referring to FIG. 2C, a second metal layer 216 is formed over the first native oxide layer 214. In the instant example, a thickness of the second metal layer 216 is such that substantially all of the second metal layer 216 will be converted to metal oxide during subsequent exposure to a second oxidizing ambient. The thickness of the second metal layer 216 may therefore be, for example, 1 nanometer to 5 nanometers, depending on the metals in the second metal layer 216. In one version of the instant example, the second metal level 216 may have a same composition as the first metal level 208. In another version, the second metal level 216 may have a different composition from the first metal level 208. The second metal layer 216 may extend conformally into the via holes 206 if present.
Referring to FIG. 2D, the second metal layer 216 is exposed to a second oxidizing ambient 218. The second oxidizing ambient 218 may include oxygen molecules 212 as depicted schematically in FIG. 2D, and/or may include other oxidizing species such as ozone and oxygen radicals. The second metal layer 216 may be exposed to the second oxidizing ambient 218 by various methods, as described in respect to FIG. 1B.
Exposure to the second oxidizing ambient 218 forms a second native oxide layer 222 which consumes substantially all of the second metal layer 216 of FIG. 2C. In the instant example, the second native oxide layer 222 contacts the first native oxide layer 214.
Referring to FIG. 2E, a release process is performed on the MEMS device 200 which removes the sacrificial layer 204 of FIG. 2D. The release process may include ozone molecules 228 and oxygen radicals 220 and may be performed as described in reference to FIG. 1F. The release process may possibly oxidize a bottom surface of the first metal layer 208.
FIG. 2F depicts the MEMS device 200 after the release process is completed. The first metal layer 208, the first native oxide layer 214 and the second native oxide layer 222 provide the released metal structure 230 of the MEMS device 200. A portion of the released metal structure 230 is free of any direct contact with solid elements of the MEMS device 200. The second native oxide layer 222 is free of direct contact with any metal layer in the portion of the released metal structure 230 which is free of any contact with solid elements of the MEMS device 200. In the instant example, the portion of the released metal structure 230 which is free of any contact with solid elements of the MEMS device 200 has a desired convex shape, which is provided by a greater magnitude of compressive stress in the first native oxide layer 214 and the second native oxide layer 222, than a stress in the first metal layer 208. The convex shape of the released metal structure 230 may provide an optical element or sensor element of the MEMS device 200. The released metal structure 230 may be attached to the substrate 202 through the via hole 206 if present. If the via hole 206 is not present, the released metal structure 230 is attached to the substrate 202, possibly through other elements, not shown of the MEMS device 200.
FIG. 3A through FIG. 3E are cross sections of a further exemplary MEMS device containing a released metal structure, depicted in successive stages of fabrication. Referring to FIG. 3A, the MEMS device 300 is formed on a substrate 302, for example, as described in reference to FIG. 1A. A sacrificial layer 304 is formed over the substrate 302. The sacrificial layer 304 may optionally be patterned so that a via hole 306 is formed through the sacrificial layer 304. The sacrificial layer 304 may include materials and be patterned as described in reference to FIG. 1A.
A first metal layer 308 of the released metal structure is formed over the sacrificial layer 304. The first metal layer 308 may include aluminum, titanium, tungsten, and/or other metals. In the instant example, a thickness of the first metal layer 308 is such that substantially all of the first metal layer 308 will be converted to metal oxide during subsequent exposure to a first oxidizing ambient and exposure to an oxidizing ambient of a release process which removes the sacrificial layer 304. The thickness of the first metal layer 308 may therefore be, for example, 2 nanometers to 10 nanometers, depending on the metals in the first metal layer 308. The first metal layer 308 may extend conformally into the via hole 306 if present.
Referring to FIG. 3B, the first metal layer 308 is exposed to a first oxidizing ambient 310. The first oxidizing ambient 310 may be for example, as described in reference to FIG. 1B and FIG. 2B. An oxidizing species of the first oxidizing ambient 310 may include, for example, oxygen molecules 312 and oxygen radicals 320, as depicted schematically in FIG. 3B, or may include oxygen ions or ozone. As described in reference to FIG. 1B, the first metal layer 308 may be exposed to the first oxidizing ambient 310 by removing the MEMS device 300 from a process tool in which the first metal layer 308 was formed, or the first oxidizing ambient 310 may be introduced into the process tool after the first metal layer 308 is formed.
Exposure to the first oxidizing ambient 310 forms a first native oxide layer 314 at a top surface of the first metal layer 308, the top surface being the surface of the first metal layer 308 exposed to the first oxidizing ambient 310, regardless of the orientation of the MEMS device 300 during exposure to the first oxidizing ambient 310. The first native oxide layer 314 may be, for example, 1 nanometer to 10 nanometers thick, depending on a composition of the first metal layer 308. In one version of the instant example, the first metal layer 308 is exposed to the first oxidizing ambient 310 for a sufficient time so that formation of the first native oxide layer 314 is substantially self-limiting.
Referring to FIG. 3C, a second metal layer 316 is formed over the first native oxide layer 314. In the instant example, the second metal layer 316 is at least 20 nanometers thick. In one version of the instant example, the second metal level 316 may have a same composition as the first metal level 308. In another version, the second metal level 316 may have a different composition from the first metal level 308. The second metal layer 316 may extend conformally into the via hole 306 if present.
Referring to FIG. 3D, a release process is performed on the MEMS device 300 which removes the sacrificial layer 304 of FIG. 3C. The release process may include oxygen molecules 312 and oxygen radicals 320 and may be performed as described in reference to FIG. 1F. In the instant example, the release process oxidizes the remaining first metal layer 308 of FIG. 3C to form a second native oxide layer 332 which contacts a bottom surface of the first native oxide layer 314. A portion of the first metal layer 308 may remain substantially unoxidized at a bottom of the via hole 306 if present. The release process may possibly oxidize a top surface of the second metal layer 316 to form a third native oxide layer 322.
FIG. 3E depicts the MEMS device 300 after the release process is completed. The first metal layer 308, the first native oxide layer 314, the second native oxide layer 332 the second metal layer 316 and the third native oxide layer 322 if present provide the released metal structure 330 of the MEMS device 300. A portion of the released metal structure 330 is free of any direct contact with solid elements of the MEMS device 300. The second native oxide layer 332 is free of direct contact with any metal layer in the portion of the released metal structure 330 which is free of any contact with solid elements of the MEMS device 300. In the instant example, the portion of the released metal structure 330 which is free of any contact with solid elements of the MEMS device 300 has a desired concave shape, which is provided by a greater magnitude of compressive stress in the first native oxide layer 314 and the second native oxide layer 332, than a stress in the second metal layer 316 and the third native oxide layer 322 if present. The concave shape of the released metal structure 330 may provide an optical element or sensor element of the MEMS device 300. The released metal structure 330 may be attached to the substrate 302 through the via hole 306 if present. If the via hole 306 is not present, the released metal structure 330 is attached to the substrate 302, possibly through other elements, not shown of the MEMS device 300.
Modifications are possible in the described embodiments, and other embodiments are possible, within the scope of the claims.

## Claims

1. A method of forming a MEMS device, the method comprising:
providing a substrate;
forming a sacrificial layer over the substrate;
forming a first metal layer over the sacrificial layer;
exposing the first metal layer to an oxidizing ambient so as to form a native oxide layer of the first metal layer at a top surface of the first metal layer;
forming a second metal layer on the native oxide layer; and
subsequently removing the sacrificial layer to form a released metal structure that includes the first metal layer, the native oxide layer and the second metal layer, such that the released metal structure is attached to the substrate, wherein at least first and second portions of the released metal structure are free of any direct contact with solid elements of the MEMS device, and wherein a third portion of the released metal structure has direct contact with the substrate in an area between the first and second portions.

2. The method of claim 1, wherein the native oxide layer is a first native oxide layer, wherein the oxidizing ambient is a first oxidizing ambient, and further comprising:
exposing the second metal layer to a second oxidizing ambient so as to form a second native oxide layer of the second metal layer at a top surface of the second metal layer; and
forming a third metal layer on the second native oxide layer, before removing the sacrificial layer.

3. The method of claim 1, wherein the native oxide layer is a first native oxide layer, wherein the oxidizing ambient is a first oxidizing ambient, and further comprising:
exposing the second metal layer to a second oxidizing ambient so as to form a second native oxide layer of the second metal layer that consumes substantially all of the second metal layer.

4. The method of claim 1, wherein the oxidizing ambient is ordinary air at 20° C. to 25° C. and atmospheric pressure.

5. The method of claim 1, wherein the oxidizing ambient is oxygen gas at less than atmospheric pressure introduced into a process tool in which the first metal layer is formed, after forming the first metal layer.

6. The method of claim 1, wherein forming the first metal layer includes forming a layer of metal selected from the group consisting of aluminum and titanium aluminum alloy.