Microelectromechanical structure and method of forming microelectromechanical structure

Abstract

To fabricate a microelectromechanical (MEMS) arm, a release structure is disposed on a basic structure having an anchor structure. A lower protective dielectric layer is deposited on the release structure. The arm structure is formed on the lower protective dielectric layer and the anchor structure. The release structure is removed by etching with a fluorine-based etchant to form a MEMS arm fixed to the anchor structure and including the arm structure and the lower protective dielectric layer. The anchor structure may be a first electrode, and the basic structure further includes a second electrode and a spacer inserted between the first and second electrodes. The release structure is disposed on the second electrode and the spacer, and etching further removes the spacer. The result is a capacitive MEMS structure with a cantilever arm capacitively coupled to the second electrode.

Description

MEMS arm for MEMS none The following content is related to semiconductor devices and their manufacturing technology, microelectromechanical (MEMS) technology, MEMS actuator and sensor devices and their manufacturing, MEMS comb drives and their manufacturing, and related technologies. none The following disclosure provides many different embodiments, or examples, for implementing the different features of the provided subject matter. Specific examples of components and configurations are described below to simplify the disclosure. Of course, these are merely examples and are not intended to be limiting. For example, in the following description, the formation of a first feature above or on a second feature may include an embodiment in which the first feature and the second feature are formed in direct contact, and may also include an embodiment in which an additional feature may be formed between the first feature and the second feature so that the first feature and the second feature are not in direct contact. In addition, the disclosure may repeat reference numbers and / or letters in various examples. This repetition is for the purpose of simplicity and clarity and does not, in itself, indicate the relationship between the various embodiments and / or configurations discussed. Furthermore, for ease of description, spatially relative terms, such as "below," "beneath," "lower," "above," "upper," and the like, may be used herein to describe the relationship of one element or feature to another element or feature(s) illustrated in the figures. Spatially relative terms are intended to encompass different orientations of the device in use or operation in addition to the orientation depicted in the figures. The device may be otherwise oriented (rotated 90 degrees or at other orientations), and the spatially relative descriptors used herein should be interpreted accordingly. Microelectromechanical (MEMS) devices typically include a MEMS arm structure that can move in response to external stimuli or can be actuated to move in response to electrical input. For example, a MEMS arm in the form of a cantilever arm made of metal or other conductive material can be configured with its anchored end connected to a first electrode and its free end positioned proximate to a second electrode. In this design, the first and second electrodes form a capacitive drive. The capacitance between the second electrode and the cantilever arm will change as the cantilever arm moves toward or away from the second electrode. Such capacitive MEMS cantilever arms can be used as sensors, where the input to be measured (i.e., sensed) modifies the position of the cantilever arm relative to the second electrode. For example, the input can be acceleration, which moves the cantilever arm. In other types of MEMS-based sensors, acoustic waves or magnetic signals, for example, can be the input being measured. For example, if the cantilever arm has magnetic material at its free end, the arm can move in response to an external magnetic field. Such a capacitive MEMS cantilever arm can alternatively be used as a MEMS actuator. In this case, a voltage or other electrical input between the first electrode and the second electrode generates an electrostatic force on the cantilever arm, thereby generating motion. In some designs, the cantilevered arm can be a connector, with a component (such as a comb) attached to the free end of the cantilevered connector arm. Such a comb can provide stronger electrostatic interactions. In some methods for forming a MEMS arm, a sacrificial layer (also known as a release layer) is formed, followed by the formation of the arm structure (e.g., a patterned metal layer) intended to become the cantilever arm. For example, the sacrificial layer may be made of silicon, while the arm structure is a patterned metal layer. A fluorine-based etch process then removes the release layer. Advantageously, fluorine-based etchants have a higher selectivity for etching silicon than metals, so the fluorine-based etch removes the release layer without removing the arm structure. With reference to Figures 1 to 5, a process for manufacturing a MEMS arm including a cantilever arm is described. Figure 5 illustrates the final structure including the first electrode 10 and the second electrode 12 and the cantilever arm 40, while Figures 1 to 4 show cross-sectional views of successive steps in its manufacture. Figure 1 illustrates a basic structure on which the cantilever arm 40 is to be formed. The exemplary basic structure includes a capacitive drive of the first electrode 10 and the second electrode 12. The first electrode 10 and the second electrode 12 are suitably formed of a conductive material. In some non-limiting illustrative embodiments, the first electrode 10 comprises a silicon material, such as, for example, silicon, polysilicon, amorphous silicon, or a combination thereof, although other materials besides silicon are also contemplated. The first electrode 10 is covered with a first electrode protection dielectric layer 14; similarly, in some non-limiting illustrative embodiments, the second electrode 12 comprises a silicon material, such as, for example, silicon, polysilicon, amorphous silicon, or combinations thereof, although materials other than silicon are also contemplated. The second electrode 12 is covered with a second electrode protection dielectric layer 16. The first electrode protection dielectric layer 14 and the second electrode protection dielectric layer 16 comprise a dielectric material that is not removed by a fluorine-based etchant intended for removing the release layer. For example, as a non-limiting illustrative example, the first electrode protection dielectric layer 14 and the second electrode protection dielectric layer 16 may comprise silicon oxide (e.g., stoichiometric silicon dioxide, SiO 2, or non-ideal ratio of silicon oxide), silicon nitride (for example, ideal ratio of Si 3N 4 or non-stoichiometric silicon nitride), silicon carbide, silicon oxynitride carbide, or glass such as undoped silica glass (USG), fluorosilicate glass (FSG), borophosphosilicate glass (BPSG), or various combinations thereof, although other materials are also contemplated. As non-limiting illustrative examples, the thickness of the first electrode protection dielectric layer 14 and the second electrode protection dielectric layer 16 can be in the range of 0.1 to 0.3 microns, although thicknesses outside this range are also contemplated. In other embodiments, the first electrode 10 and the second electrode 12 may comprise a type of conductive material that is not strongly etched by the fluorine-based etchant intended for removing the release layer. For example, the first electrode 10 and the second electrode 12 may comprise a metal such as aluminum (Al) or copper (Cu), a metal alloy such as AlCu (optionally further including silicon, i.e., AlSiCu), a metal nitride such as titanium nitride (TiN) or tantalum nitride (TaN), or combinations thereof, although other materials are also contemplated. In these embodiments, the first electrode protection dielectric layer 14 and the second electrode protection dielectric layer 16 may optionally be omitted. As further shown in FIG. 1 , in addition to the capacitive drive including the first electrode 10 and the second electrode 12, the basic structure further includes a spacer 18 interposed between the first electrode 10 and the second electrode 12. The illustrative spacer 18 is intended to be removed by a fluorine-based etchant that will be used to remove the release layer. Therefore, the spacer 18 should be made of a material that is strongly etched by the fluorine-based etchant, such as a silicon material (e.g., silicon, polysilicon, amorphous silicon, or a combination thereof, although other materials are also contemplated). In other embodiments, it is contemplated that the spacer between the first electrode 10 and the second electrode 12 is not removed by the fluorine-based etchant. In such alternative embodiments, the spacer should comprise an electrically insulating material so that the first electrode 10 and the second electrode 12 in the final capacitive MEMS structure are electrically isolated from each other. The basic structure of FIG. 1 can be fabricated in a variety of ways. In one embodiment, a lithographically patterned etch is performed on a substrate or layer of silicon material, which is intended to form first electrode 10 and second electrode 12, to form trenches in the substrate or layer. Layers of dielectric material are then deposited, including first electrode protection dielectric layer 14 and second electrode protection dielectric layer 16, to cover first electrode 10 and second electrode 12 with respective first electrode protection dielectric layer 14 and second electrode protection dielectric layer 16. A lithographically controlled deposition is then performed to fill the trenches with silicon material, thereby forming spacers 18. In the illustrative example, first electrode 10 is intended to serve as an anchor for a cantilever arm. For this purpose, first electrode 10 is elevated above second electrode 12, as shown in FIG. 1, thereby providing a step between first electrode 10 and second electrode 12. This can be accomplished in various ways, such as by lithographically patterned deposition of additional silicon material to raise the height of the first electrode 10 (before forming the first electrode protection dielectric layer 14), or by lithographically patterned etching of silicon material to lower the height of the second electrode 12 (before forming the second electrode protection dielectric layer 16). Referring now to FIG. 2 , a release structure 20 is disposed over the base structure. The release structure 20 is suitably formed from a silicon material that is etched by a fluorine-based etchant to be used in a subsequent release step. For example, the release structure 20 may comprise a silicon material such as silicon, polysilicon, amorphous silicon, or a combination thereof, although materials other than silicon are also contemplated. In the illustrative example, the release structure 20 is formed from two distinct first and second layers 22, 24 of different silicon materials. For example, in one embodiment, the first layer 22 comprises a silicon material that forms a smooth interface with the underlying first and second electrodes 10, 12, formed as polysilicon using a deposition technique that provides small grains; while the second layer 24 comprises a silicon material that can be deposited at a faster deposition rate, such as polysilicon deposited with a larger average grain size. This is merely one non-limiting illustrative example. In one non-limiting illustrative embodiment, the total thickness of the release structure 20 is approximately 1-2 microns, although greater or lesser thicknesses are contemplated. The thickness of the release structure 20 will determine the unbiased spacing between the second electrode 12 and the cantilever arm (which in turn will affect the unbiased capacitance), and therefore the thickness of the release structure 20 is selected to provide a design baseline unbiased spacing / capacitance. As shown in FIG. 2 , due to the height difference between the first electrode 10 and the second electrode 12 , the illustrative release structure 20 includes a step located near the location of the spacer 18 . Referring now to FIG. 3 , a cantilever arm to be formed has one end anchored to an anchor. In the illustrative example, the anchor is the first electrode 10 . Thus, as shown in FIG. 3 , an opening 26 is formed in at least a portion of a release structure 20 disposed above the first electrode 10 , for example, by etching through a photolithographic pattern of the release structure 20 . In one suitable method, the opening 26 is etched using a fluorine-based etchant that etches the silicon material of the release structure 20 but not the first electrode protection dielectric layer 14 , such that the first electrode protection dielectric layer 14 serves as an etch stop for etching the opening 26 . The size of the opening 26 should be large enough to provide adequate anchoring of the cantilever arm, thereby ensuring structural reliability under the expected forces that may be applied to the cantilever arm during use of the MEMS structure. In the non-limiting illustrative example, the opening 26 has a circular area with a diameter of approximately 2-3 microns, although larger or smaller opening sizes, as well as openings having non-circular areas, are contemplated. Referring now to FIG. 4 , a lower protective dielectric layer 28 is deposited at least over the release structure 20 and, in the exemplary embodiment, also within the opening 26 (see FIG. 3 ). This is followed by the formation of an arm structure 30 disposed over the first and second electrodes 10, 12, and over the spacer 18 therebetween. The arm structure 30 (along with the lower protective dielectric layer 28) is intended to form a cantilever arm 40 (see FIG. 5 ) after the release step, wherein the cantilever arm 40 has one end anchored to an anchor (in the example of FIG. 5 , the anchor is the first electrode 10). For the illustrative capacitive MEMS structure fabricated in the illustrative examples of FIG. 1 through FIG. 5 , the cantilever arm 40 is to be capacitively coupled to the second electrode 12. Therefore, the arm structure 30 (along with the underlying lower protective dielectric layer 28) is also disposed over the second electrode 12, as shown in FIG. 4 . The purpose of the lower protective dielectric layer 28 is to protect the lower surface of the arm structure 30 during the subsequent fluorine-based etch, as will be described with reference to FIG. 5 . In some non-limiting illustrative embodiments, the lower protective dielectric layer 28 comprises silicon oxide (eg, stoichiometric silicon dioxide, SiO 2, or non-ideal ratio of silicon oxide), silicon nitride (for example, ideal ratio of Si 3N 4 or non-ideally matched silicon nitride), silicon carbide, silicon oxynitride carbide, or glass such as USG, FSG, BPSG, or various combinations thereof. These materials are highly resistant to fluorine etching and thus provide protection for the lower surface of the arm structure 30 during etching. The thickness of the lower protective dielectric layer 28 is selected to provide adequate protection for the lower side of the arm structure 30 during the subsequent fluorine etching, which in turn depends on various factors, such as the type of material comprising the lower protective dielectric layer 28 and the nature of the fluorine etching, such as the etching tool used, the etchant concentration, and the etching time. In some non-limiting illustrative examples, the lower protective dielectric layer 28 can have a thickness between 0.2 microns and 1 micron, although thicknesses outside this range are also contemplated. The arm structure 30 is suitably formed as a layer disposed on the release structure 20 (with the underlying protective dielectric layer 28 interposed therebetween, as shown in FIG. 4 ). The deposited arm structure 30 also covers the interior of the opening 26 to provide a predetermined anchoring of the cantilever arm 40 to the first electrode 10 as an anchor structure (see FIG. 5 ). In some non-limiting illustrative embodiments, the arm structure 30 comprises a metal (e.g., copper), a metal alloy (e.g., AlCu), AlSiCu, a metal nitride (e.g., TiN or TaN), or a combination thereof, although other materials are also contemplated. The material and thickness of the arm structure 30 are selected to provide a resulting cantilever arm with selected design-based properties, such as strength, flexibility, etc. In some non-limiting illustrative examples, the arm structure 30 may have a thickness between 0.2 microns and 1 micron, although thicknesses outside this range are also contemplated. The arm structure 30 (together with the underlying lower protective dielectric layer 28 ) conforms to the upper surface of the underlying release structure 20 , and thus, in the non-limiting illustrative example, the arm structure 30 has a bend 32 as shown in FIG. 4 . Referring to FIG. 5 , the release structure 20 is removed by etching using a fluorine-based etchant (schematically indicated by fluorine atoms 42 in FIG. 5 ). This results in the release of the unanchored end of the arm structure 30 to form a cantilever arm 40. The cantilever arm 40 is also known as a microelectromechanical (MEMS) arm. In this embodiment, the cantilever arm 40 is fixed at one end to the first electrode 10 as an anchor structure and has an opposite end that is free to move in response to an electrical input between the first electrode 10 and the second electrode 12, or in response to another type of stimulus such as an acoustic wave, acceleration, etc. As shown in FIG. 5 , the cantilever arm 40 includes the arm structure 30 and an underlying protective dielectric layer 28. Etching can be performed using various etching tools and etching processes, such as reactive ion etching (RIE), inductively coupled plasma etching (ICP), remote plasma etching, etc., as some non-limiting illustrative examples. The fluorine-based etchant includes fluorine or a fluorine compound as an active etching species. For example, a fluorine-based etchant may include CF 4 Gas and / or SF 6 gases, as two non-limiting illustrative examples. Fluorine is a highly reactive atom, and thus the active fluorine or fluorine compound of the fluorine-based etchant effectively etches the release structure 20 (see FIG. 4 ), which, as previously described, is formed of a silicon material such as silicon, polycrystalline silicon, amorphous silicon, or a combination thereof. As can also be seen by comparing FIG. 4 and FIG. 5 , the spacer 18 (see FIG. 4 ) is removed by the fluorine-based etchant. To this end, the spacer 18 is made of a material that is etched by the fluorine-based etchant, such as a silicon material (e.g., silicon, polycrystalline silicon, amorphous silicon, or a combination thereof, although materials other than silicon are also contemplated). The removal of the spacer 18 results in the first electrode 10 and the second electrode 12 being spaced apart from each other by the width of the (now removed) spacer 18. It should be understood that in some embodiments, although the first electrode 10 and the second electrode 12 are suitably made of a silicon material, they are protected from etching by a fluorine-based etchant by respective first electrode protection dielectric layers 14 and second electrode protection dielectric layers 16. The first and second electrode protection dielectric layers 14 and 16 are made of a suitable resistive material, such as silicon oxide, silicon nitride, silicon carbide, silicon oxynitride carbide, glass, or various combinations thereof. As previously described, the arm structure 30 comprises a metal or metal alloy, a metal nitride, or a combination thereof. Fluorine-based etchants generally have a higher selectivity for etching silicon materials than metals, metal alloys, and / or metal nitrides. In other words, fluorine-based etchants generally etch silicon materials very quickly, while fluorine-based etchants may not etch metals, metal alloys, and / or metal nitrides, or may etch them at a much slower rate than the etching rate of silicon materials. Therefore, the fluorine-based etchant used in the release step selectively etches and removes the release structure 20 of the silicon material while leaving the arm structure 30 behind, thereby releasing the arm structure 30 (except for the location where it is anchored to the first electrode 10) to form the cantilever arm 40. Therefore, due to the high selectivity of the fluorine-based etch, the benefits provided by the lower protective dielectric layer 28 are not significant. However, it is recognized herein that fluorine-based etchants can attack the metal, metal alloy, or metal nitride material of the arm structure 30, albeit at a much slower rate than the etching rate of the silicon material of the release structure 20 (and the silicon material of the spacer 18). For example, in the reference MEMS arm structure studied herein, scanning electron microscopy (SEM) of the AlCu surface after etching with a fluorine-based etchant revealed that the surface included protrusions or other non-planar features. Without being limited to any particular theory of operation, it is believed that these protrusions are caused by fluorine attacking the AlCu surface during etching. Based on the recognition that fluorine-based etching can produce protrusions or other non-planar features on the AlCu surface, the present invention discloses the inclusion of a lower protective dielectric layer 28 to protect the lower surface of the arm structure 30 during the fluorine-based etching. As previously described, in some non-limiting illustrative embodiments, the lower protective dielectric layer 28 comprises silicon oxide, silicon nitride, silicon carbide, silicon oxynitride carbide, glass, or various combinations thereof. These materials are impermeable to, or at least highly resistant to, etching by fluorine-based etchants. Therefore, the lower protective dielectric layer 28 disposed on the lower surface of the arm structure 30 protects the lower surface from attack by the fluorine-based etchant. Without the lower protective dielectric layer 28, the fluorine etch would directly interact with the lower surface of the arm structure 30, which could result in the formation of protrusions or other non-planar features on the arm structure 30. Protecting the arm structure 30, and in particular its lower surface, from fluorine attack by the lower protective dielectric layer 28 is beneficial because fluorine-based etchant attack on the lower surface of the arm structure 30 can change the spacing between the lower surface of the cantilever arm 40 and the upper surface of the second electrode 12. This spacing, represented as spacing d in FIG. 5 , is small and is approximately equal to the thickness of the relief structure 20 (as shown in FIG. 4 , before removal by the fluorine-based etch). The spacing d can vary from the thickness of the relief structure 20 due to various factors, such as sagging due to gravity and / or electrostatic attraction or repulsion caused by electrostatic charge accumulation on the second electrode 12 and / or the cantilever arm 40, which can attract the cantilever arm 40 toward the second electrode 12 or repel the cantilever arm 40 away from the second electrode 12, depending on the polarity of the electrostatic charge. As previously described, in some non-limiting illustrative embodiments, the release structure 20 has a thickness of approximately 1-2 microns, although greater or lesser thicknesses are contemplated, and thus, in such embodiments, the spacing d is also approximately 1-2 microns, although greater or lesser spacing values ​​are contemplated. The distance d has a strong influence on the operation of the capacitive MEMS device. The voltage difference between the first electrode 10 and the second electrode 12 It is manifested as a voltage difference between the cantilever arm 40 and the second electrode 12 (Any voltage drop along the cantilever arm 40 is neglected.) The electric field present in the gap of spacing d between the cantilever arm 40 and the second electrode 12 is The value of Therefore, The value of the spacing d is given, and This inverse relationship between the spacing d and d means that a small change in the spacing d due to fluorine attack on the lower surface of the cantilever arm can significantly change the strength of the electric field existing between the cantilever arm 40 and the second electrode 12. . Additionally, it was observed in the SEM that fluorine attack on the lower surface of the cantilever arm was spatially non-uniform, resulting in surface roughness and / or protrusions. Without being limited to any particular theory of operation, it is believed that this constitutes AlCu surface damage due to attack by the fluorine-based etchant. Such roughness and / or protrusions can locally modify the electrostatic potential difference between the cantilever arm 40 and the second electrode 12, resulting in localized higher electric field regions at the protrusions, which in turn can lead to performance degradation and / or failure mechanisms such as crown discharges at such protrusions. These problems are alleviated by including a lower protective dielectric layer 28 as disclosed herein, such that the cantilever arm 40 includes the arm structure 30 and the lower protective dielectric layer 28. Referring to FIG6 and FIG7 , the illustrative capacitive MEMS device of FIG5 can be used in sensor applications ( FIG6 ) or as an actuator ( FIG7 ). In the sensor device of FIG6 , the sensor readout circuit 44 measures the capacitance (or a related electrical parameter, such as voltage or current) between the first electrode 10 and the second electrode 12 . If the cantilever arm 40 moves in response to a stimulus, this will be detected as a change in the capacitance (or related electrical parameter) measured by the sensor readout circuit 44 . The stimulus that causes the cantilever arm 40 to move can be, for example, acceleration (in which case the sensor device of FIG6 is suitably an accelerometer), or an acoustic wave. In the actuator device of FIG. 7 , the actuator driver circuit 46 applies a voltage difference (or other drive signal, such as current) between the first electrode 10 and the second electrode 12, thereby generating an electrostatic force (e.g., repulsion and / or attraction) between the second electrode 12 and the cantilever arm 40, resulting in actuated motion of the cantilever away from the second electrode 12 (repulsion) and / or toward the second electrode 12 (attraction). Applying an alternating voltage or current (i.e., an AC signal) via the actuator driver circuit 46 can induce oscillatory motion of the cantilever arm 40. The resulting actuated motion can drive motion of a component connected to the cantilever arm 40, generate acoustic waves, or perform another useful power function. In these non-limiting illustrative applications, the sensor readout circuit 44 or the actuator driver circuit 46 are suitably implemented as electronic circuits including transistors, MOSFETs, etc. Referring to FIG8 , a cross-sectional view of another illustrative MEMS arm embodiment is shown. The MEMS arm of FIG8 is similar to the MEMS arm of FIG5 and includes a first electrode 10 and a second electrode 12, a first electrode protection dielectric layer 14 and a second electrode protection dielectric layer 16, and a cantilever arm 50 including an arm structure 30 and a lower protection dielectric layer 28. In the embodiment of FIG8 , the arm structure 30 includes at least two layers 30 of different materials. 1 and 30 2, each layer comprising a metal, a metal alloy, a metal nitride, or a combination thereof. In the embodiment of FIG. 8, the cantilever arm 50 includes a layer 30 1 and 30 2 stacked arm structure 30 and the lower protective dielectric layer 28. The fabrication of the MEMS arm of FIG8 follows the sequence previously described with reference to FIG1 to FIG5, except that two layers 30 are deposited consecutively at the stage depicted in FIG4. 1 and 30 2. In one contemplated application of the embodiment of FIG. 8 , two layers 30 1 and 30 2 can form a bimetallic thermometer. In such an embodiment, the two layers 30 1 and 30 2 are made of different metals with different coefficients of thermal expansion. Therefore, when the temperature of the arm structure 30 changes, one metal expands (or contracts) faster than the other, causing the cantilevered arm 50 to bend toward or away from the second electrode 12, thereby changing the capacitance. This change in capacitance (or another related electrical parameter) can be measured by the sensor readout circuit 44 of FIG. 6, thereby providing temperature sensing functionality. Referring to FIG. 9 , a cross-sectional view of another illustrative MEMS arm embodiment is shown. The MEMS arm of FIG. 9 is similar to the MEMS arm of FIG. 5 and includes first and second electrodes 10 and 12, first and second electrode protection dielectric layers 14 and 16, and a cantilever arm 60 including an arm structure 30 and a lower protection dielectric layer 28. However, in the embodiment of FIG. 9 , the lower protection dielectric layer 28 includes a dielectric layer stack including a first lower protection dielectric layer 28. 1 and the second lower protective dielectric layer 28 2. The fabrication of the MEMS arm of FIG. 9 follows the sequence previously described with reference to FIG. 1 to FIG. 5 , except that, at the stage depicted in FIG. 4 , the deposition sequence includes depositing a first lower protective dielectric layer 28 1. Then deposit the second lower protective dielectric layer 28 2. Then, the arm structure 30 is formed by depositing or otherwise forming the first lower protective dielectric layer 28. 1 and the second lower protective dielectric layer 28 2 is a suitable different fluorine resistant etchant material. For example, the first lower protective dielectric layer 28 1 may include silicon oxide (e.g., stoichiometric silicon dioxide, SiO 2, or non-ideal ratio of silicon oxide), silicon nitride (for example, ideal ratio of Si 3N 4 or non-ideal ratio of silicon nitride), silicon carbide, silicon oxynitride carbide, or glass such as USG, FSG, BPSG, or various combinations thereof; a second lower protective dielectric layer 28 2 may include silicon oxide (e.g., stoichiometric silicon dioxide, SiO 2, or non-ideal ratio of silicon oxide), silicon nitride (for example, ideal ratio of Si 3N 4 or non-ideally matched silicon nitride), silicon carbide, silicon oxynitride carbide, or glass such as USG, FSG, BPSG, or various combinations thereof; wherein the first lower protective dielectric layer 28 1 and the second lower protective dielectric layer 28 2 are different materials. These are also only some non-limiting illustrative examples. In the embodiment of FIG. 9 , the cantilever arm 60 thus comprises an arm structure 30 and a lower protective dielectric layer 28 , wherein the latter comprises a first lower protective dielectric layer 28 1 and the second lower protective dielectric layer 28 2 dielectric layer stack. Although FIG. 9 shows that the lower protection dielectric layer 28 is composed of two first and second lower protection dielectric layers 28 1 and 28 2, but it will be appreciated that this can be extended to three, four, or more constituent dielectric layers. Referring to FIG. 10 , a cross-sectional view of another illustrative MEMS arm embodiment is shown. The MEMS arm of FIG. 10 is formed by forming a lower protective dielectric layer 28 to include a first lower protective dielectric layer 28 . 1 and the second lower protective dielectric layer 28 2 dielectric layer stack (as shown in FIG. 9 ), and also the arm structure 30 is formed into two layers 30 made of different materials. 1 and 30 2 stack, and effectively combines the embodiments of FIG. 8 and FIG. 9, thereby forming a first lower protective dielectric layer 28 1 and the second lower protective dielectric layer 28 2 (as described with reference to FIG. 9 ), and two layers 30 of different materials 1 and 30 2 (as described with reference to FIG. 8 ) of the cantilevered arm 70 . The illustrative examples of Figures 1 through 10 are MEMS arms, wherein the MEMS arm is a cantilever arm 40 (or 50, or 60, or 70) anchored at one end to a first electrode 10, and its unanchored (i.e., free) end positioned adjacent to a second electrode 12 (e.g., with a spacing d as shown in Figures 5 through 7). This forms a capacitive MEMS device, in which the capacitance between the cantilever arm and the second electrode 12 can be utilized to perform sensing functionality (e.g., Figure 6), or can operate as a capacitive actuator (e.g., Figure 7). Although not shown, the free end of the cantilever arm 40 can be connected to a component, such as a comb, so that movement of the cantilever arm 40 causes movement of the comb (or other component) attached to the free end of the cantilever arm 40. It should be understood that other types of MEMS arms can be similarly fabricated. As another example, instead of a cantilever configuration, the MEMS arm can be fixed to (i.e., suspended from) anchors at both ends of the arm, and the middle portion of the MEMS arm can flex in response to electrical input, acoustic waves, or other types of stimuli (while both ends remain anchored). In another example, the MEMS arm can be a spring fixed at both ends. These are merely further non-limiting examples. In any such embodiments, the MEMS arm comprises a cantilever or suspended arm of a metal, metal alloy, metal nitride, or a combination thereof, which is released by removing a release layer using an etchant employing a fluorine-based etchant. For example, the methods disclosed herein are useful when fabricating a MEMS spring structure comprising a suspended arm of a metal, metal alloy, metal nitride, or a combination thereof. Referring to FIG. 11 , a cross-sectional view of a MEMS device including a drive comb 100 is shown. Drive comb 100 includes a comb 102 secured to a cantilever arm, which includes cantilever arm 40 including arm structure 30 and an underlying protective dielectric layer 28. Comb 102 may comprise, for example, polysilicon (as shown in FIG. 11 ), or silicon, amorphous silicon, or a combination thereof, and may be covered with a protective dielectric layer, such as an oxide, which is not removed by etching using a fluorine-based etchant, as further shown in FIG. Comb drives can be used, for example, in MEMS-based accelerometers, where acceleration is detected as a change in the capacitance of the comb drive structure. When used as a sensor, an external condition, such as an acoustic wave (or more generally, a pressure wave), light, a magnetic signal, or the like, is converted by the comb drive into an electrical signal, such as a voltage or current. In contrast, when used as an actuator, an applied electrical signal (such as a voltage or current) drives the movement of the comb, which can be used as a transducer to produce sound or other types of signals. Thus, the sensing or actuation operation is similar to that described with reference to FIG. 6 or FIG. 7 , respectively, but the comb 102 is fixed to the cantilever arm 40 to provide a larger area for electrostatic interaction. The exemplary MEMS device of FIG. 11 further includes springs 104, an intermediate frame 106, metal springs 108, and an outer frame 110. All of these components are fabricated in a first (e.g., upper) wafer 112, which is bonded to a second (e.g., lower) wafer 114 having a cavity 116 formed therein. Legend 118 indicates the materials used in the MEMS device of FIG. 11 . Referring to FIG12 through FIG18 , cross-sectional views of intermediate structures of a fabrication process for fabricating the illustrative MEMS device of FIG11 are shown. Note that the same reference numerals are used in the cross-sectional views of successive fabrication steps in FIG12 through FIG18 , and only the numerals associated with the step shown in a given cross-sectional view are discussed with reference to that cross-sectional view. FIG12 through FIG18 also each include a legend 118 , and the same hatching is used to indicate the various materials labeled in legend 118 . FIG12 shows a cross-sectional view of the device fabrication at a stage where the combs 102 have been formed in the silicon material of the upper wafer 112 and the lower wafer 114 has been processed to form the cavities 116 and bonded to the upper wafer 112. Also visible in FIG12 is the release structure 20 formed from two different layers 22 and 24 of different silicon materials, as previously described with reference to FIG2 . Figure 13 shows a cross-sectional view after deposition of the lower protective dielectric layer 28. As shown in Figure 13, the lower protective dielectric layer 28 is deposited as a conformal blanket layer that covers the entire upper surface. Figure 14 shows a cross-sectional view after photoresist 120 has been applied for etching of the underlying protective dielectric layer 28. The photoresist is deposited as a blanket layer of photoresist and then patterned, as shown in Figure 14, leaving patterned photoresist 120 covering only the areas corresponding to the drive combs 100. FIG15 shows a cross-sectional view after etching of the lower protective dielectric layer 28 and stripping of the photoresist 120. The etching removes the blanket lower protective dielectric layer 28 except where it was protected by the patterned photoresist 120 shown in FIG14; thus, after etching, the remaining lower protective dielectric layer 28 only covers the area of ​​the drive comb 100, as shown in FIG15. FIG16 shows a cross-sectional view of the arm structure 30 after deposition and lithographic patterning and etching of the AlCu material. As shown in FIG16 , the deposited and patterned AlCu layer is disposed in the region of the drive comb 100, as well as in the region of the spring 104 and the intermediate frame 106. Therefore, the spring 104 is made of the same material as the arm structure 30 (e.g., AlCu). FIG17 shows a cross-sectional view after a passivation oxide layer 122 is deposited over the arm structure 30. FIG17 also shows a deposited and patterned protective oxide 123 disposed over the spring 104, the intermediate frame 106, the outer frame 110, and portions of the area destined to become the metal spring 108. The areas of the metal spring 108 not covered with the protective oxide 123 serve as metal pads. Figure 18 shows a cross-sectional view after the lithographically controlled silicon etch step. Figure 18 illustrates some remaining photoresist 124 (eg, at least 4 microns thick in some embodiments). Returning to FIG. 11 , a cross-sectional view is shown after the arm 30 has been released using a fluorine-based etchant. The lithographically controlled etch of FIG. 18 forms an opening for the fluorine-based etchant to enter, which is used to release the arm structure 30 by removing the silicon material release structure 20 (which, in the illustrated example, includes two layers 22 and 24 of different silicon materials). Because the fluorine-based etchant removes silicon, the surfaces of the silicon regions remaining after the fluorine-based etch are appropriately protected during the etch by being covered with a protective oxide. For example, the cavity 116 and other exposed surfaces of the lower wafer 114 are covered with the protective oxide, as shown in FIG. 11 . In the following, some further embodiments are described. In a non-limiting illustrative embodiment, a method for fabricating a microelectromechanical (MEMS) structure is disclosed. The method includes forming a release structure disposed on a base structure, wherein the base structure includes an anchor structure; depositing a lower protective dielectric layer on at least the release structure; forming an arm structure disposed on the lower protective dielectric layer and on the anchor structure; and removing the release structure by etching with a fluorine-based etchant to form a MEMS arm secured to the anchor structure, the MEMS arm including the arm structure and the lower protective dielectric layer. In a non-limiting illustrative embodiment, a method of fabricating a MEMS structure is disclosed. The method includes providing a capacitive actuator including a first electrode and a second electrode; forming a release structure including a silicon material disposed on the second electrode; depositing a lower protective dielectric layer on at least the release structure; forming an arm structure disposed on the lower protective dielectric layer and on the first electrode; and removing the release structure by etching with a fluorine-based etchant to form a cantilever arm secured to the first electrode and capacitively coupled to the second electrode, the cantilever arm including the arm structure and the lower protective dielectric layer. In a non-limiting illustrative embodiment, a MEMS structure includes: a first electrode; a second electrode; and a cantilever arm fixed to the first electrode and above the second electrode with a gap therebetween. The cantilever arm includes an arm structure and a lower protective dielectric layer disposed on an underside of the arm structure. The foregoing summarizes the features of several embodiments so that those skilled in the art can better understand the aspects of the present disclosure. Those skilled in the art will appreciate that they can readily use this disclosure as a basis for designing or modifying other processes and structures for implementing the same purposes and / or achieving the same advantages of the embodiments introduced herein. Those skilled in the art will also recognize that such equivalent constructions do not depart from the spirit and scope of the present disclosure, and that various changes, substitutions, and replacements may be made herein for such equivalent constructions without departing from the spirit and scope of the present disclosure. 10: First electrode 12: Second electrode 14: First electrode protection dielectric layer 16: Second electrode protection dielectric layer 18: Spacer 20: Release structure 22: Layer / first layer 24: Layer / second layer 26: Opening 28: Lower protection dielectric layer 28 1: First lower protective dielectric layer 28 2: Second lower protective dielectric layer 30: Arm structure 30 1,30 2: Layer 32: Bend 40, 50, 60, 70: Cantilever arm 42: Fluorine atom 44: Sensor readout circuit 46: Actuator drive circuit 100: Drive comb 102: Comb 104: Spring 106: Middle frame 108: Metal spring 110: Outer frame 112: First wafer / upper wafer 114: Second wafer / lower wafer 116: Cavity 118: Legend 120: Photoresist 122: Passivation oxide layer 123: Protective oxide 124: Photoresist d: Spacing The aspects of the present disclosure are best understood from the following detailed description when read in conjunction with the accompanying figures. It should be noted that, in accordance with standard practices in the industry, various features are not drawn to scale. In fact, the dimensions of various features may be arbitrarily increased or decreased for clarity of discussion. Figures 1, 2, 3, and 4 schematically illustrate cross-sectional views of successive intermediate structures of a manufacturing process for making a MEMS arm. Figure 5 schematically illustrates a cross-sectional view of a MEMS arm formed by etching the intermediate structure of Figure 4 using a fluorine-based etchant. Figure 6 schematically illustrates a capacitive MEMS sensor including the MEMS arm of Figure 5. Figure 7 schematically illustrates a capacitive MEMS actuator including the MEMS arm of Figure 5. Figures 8, 9, and 10 schematically illustrate cross-sectional views of additional illustrative MEMS arm embodiments. Figure 11 schematically illustrates a cross-sectional view of a MEMS device including a drive comb appropriately secured with an arm structure such as that shown in Figures 5 and 8 to 10. 12 , 13 , 14 , 15 , 16 , 17 , and 18 schematically illustrate cross-sectional views of intermediate structures of a fabrication process for fabricating the illustrative MEMS device of FIG. 11 . Domestic deposit information (please note in the order of deposit organization, date, and number) None Foreign deposit information (please note in the order of deposit country, organization, date, and number) None 10: First electrode 12: Second electrode 14: First electrode protection dielectric layer 16: Second electrode protection dielectric layer 28: Lower protective dielectric layer 30: Arm structure 40: Cantilever arm 42: Fluorine atom d: spacing

Claims

1. A method of manufacturing a microelectromechanical structure (MEMS), the method comprising the steps of: forming a release structure disposed on a base structure, wherein the base structure includes an anchor structure, the anchor structure including a first electrode and a first electrode protection dielectric layer disposed on the first electrode; depositing a lower protective dielectric layer on at least the release structure, wherein the lower protective dielectric layer contacts the first electrode protection dielectric layer; forming an arm structure disposed on the lower protective dielectric layer and the anchor structure; and removing the release structure by etching with a fluorine-based etchant to form a MEMS arm fixed to the anchor structure, the MEMS arm including the arm structure and the lower protective dielectric layer.

2. The method as described in claim 1, wherein: The basic structure further includes a second electrode and a spacer inserted between the first electrode and the second electrode, the release structure being disposed on the second electrode and the spacer; the spacer is further removed by etching with the fluorine-based etchant; and the microelectromechanical structure manufactured by the method includes a capacitive microelectromechanical structure, wherein the microelectromechanical arm is capacitively coupled to the second electrode.

3. The method as described in claim 2, wherein: The first electrode comprises silicon, polycrystalline silicon, amorphous silicon, or a combination thereof, and the first electrode protective dielectric layer is not removed by etching with the fluorine-based etchant; the second electrode comprises silicon, polycrystalline silicon, amorphous silicon, or a combination thereof, and is covered with a second electrode protective dielectric layer, which is not removed by etching with the fluorine-based etchant; and the spacer comprises silicon, polycrystalline silicon, amorphous silicon, or a combination thereof.

4. The method as described in claim 2 further comprises the following steps: forming a comb-like structure fixed to the arm structure, the comb-like structure comprising silicon, polycrystalline silicon, amorphous silicon, or a combination thereof, the comb-like structure being covered with a comb-like structure protective dielectric layer, the comb-like structure protective dielectric layer not being removed by etching with the fluorine-based etchant; wherein the capacitive microelectromechanical structure comprises a comb-like drive.

5. A method of manufacturing a microelectromechanical structure (MEMS), the method comprising the steps of: providing a capacitive drive including a first electrode and a second electrode, wherein the capacitive drive includes a first electrode protection dielectric layer disposed on the first electrode; forming a release structure including a silicon material disposed on the second electrode; depositing a lower protective dielectric layer at least on the release structure, wherein the lower protective dielectric layer contacts the first electrode protection dielectric layer; forming an arm structure disposed on the lower protective dielectric layer and on the first electrode; and removing the release structure by etching with a fluorine-based etchant to form a cantilever arm, the cantilever arm being fixed to the first electrode and capacitively coupled to the second electrode, the cantilever arm including the arm structure and the lower protective dielectric layer.

6. The method as described in claim 5, wherein the materials of the lower protective dielectric layer and the first electrode protective dielectric layer are both oxides.

7. The method as described in claim 5, wherein the arm structure is formed of two layers of different materials.

8. A microelectromechanical structure (MEMS) comprising: a first electrode; a first electrode protective dielectric layer disposed on the first electrode; a second electrode; and a cantilever arm fixed to the first electrode and above the second electrode, the first electrode and the second electrode having a gap between them, the cantilever arm including an arm structure and a lower protective dielectric layer disposed below the arm structure, wherein the lower protective dielectric layer contacts the first electrode protective dielectric layer.

9. The microelectromechanical structure as claimed in claim 8, further comprising: a comb-like structure fixed to the cantilever arm, wherein the microelectromechanical structure includes a capacitive microelectromechanical structure, the capacitive microelectromechanical structure including a comb-like drive.

10. The microelectromechanical structure as described in claim 8, further comprising: at least one spring comprising the same material as the arm structure.

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