1. Field
The technology described herein relates to micromechanical membranes and related structures and methods.
2. Related Art
Some microelectromechanical systems (MEMS) devices, including some MEMS oscillators, include a micromechanical resonating component or structure. The micromechanical resonating structure vibrates in response to electrical or mechanical excitation, with the vibration being used to generate an electrical signal. The resonating structure is typically on the order of several hundred microns or smaller.
Micromechanical resonating structures are typically formed of single crystal silicon because of perceived benefits of the material. Vibrating structures fabricated out of silicon exhibit low damping. In addition, silicon is readily available. Furthermore, numerous fabrication processes for working with silicon wafers have been established, and these processes can be used to precisely shape silicon to obtain a well controlled geometry for purposes of forming a silicon resonating structure.
According to one aspect of the technology, an apparatus is provided, comprising a first silicon membrane formed above a first cavity in a silicon substrate, and a second silicon membrane formed above a second cavity in the silicon substrate. At least one of the following conditions is met for the apparatus: (a) a thickness of the first silicon membrane differs from a thickness of the second silicon membrane; and (b) silicon oxide is formed on at least one of the first silicon membrane and the second silicon membrane, and a different configuration of silicon oxide is formed with respect to the first silicon membrane than with respect to the second silicon membrane.
According to another aspect of the technology, an apparatus is provided, comprising a first plurality of trenches formed in a first surface of a silicon substrate and arranged in a one-dimensional pattern. Each of the first plurality of trenches has a first opening area and a first depth, and the trenches of the first plurality of trenches are spaced by a first pitch. The apparatus further comprises a second plurality of trenches formed in the first surface of the silicon substrate and arranged in a one-dimensional pattern. Each of the second plurality of trenches has a second opening area and a second depth, and the trenches of the second plurality of trenches are spaced by a second pitch. At least one of the following conditions is met for the apparatus: (a) the first depth differs from the second depth; (b) the first opening area differs from the second opening area; and (c) the first pitch differs from the second pitch.
According to another aspect of the technology, an apparatus is provided, comprising a first plurality of trenches formed in a first surface of a silicon substrate and arranged in a one-dimensional pattern. At least some of the first plurality of trenches have a first opening area and a first depth, and the at least some of the first plurality of trenches are spaced by a first pitch. The apparatus further comprises a second plurality of trenches formed in the first surface of the silicon substrate and arranged in a one-dimensional pattern. At least some of the second plurality of trenches have a second opening area and a second depth, and the at least some of the second plurality of trenches are spaced by a second pitch. At least one of the following conditions is met for the apparatus: (a) the first depth differs from the second depth; (b) the first opening area differs from the second opening area; and (c) the first pitch differs from the second pitch.
According to another aspect of the technology, a method of forming a plurality of silicon membranes from a silicon substrate is provided. The method comprises forming a first plurality of trenches in a first surface of the silicon substrate arranged in a one-dimensional pattern. Each of the first plurality of trenches has a first opening area and a first depth, and the trenches of the first plurality of trenches are spaced by a first pitch. The method further comprises forming a second plurality of trenches in the first surface of the silicon substrate arranged in a one-dimensional pattern. Each of the second plurality of trenches has a second opening area and a second depth, and the trenches of the second plurality of trenches are spaced by a second pitch. At least one of the following conditions is met: (a) the first depth differs from the second depth; (b) the first opening area differs from the second opening area; and (c) the first pitch differs from the second pitch. The method further comprises annealing the silicon substrate.
According to another aspect of the technology, a method is provided, comprising forming a first silicon membrane from a silicon substrate by forming a first cavity in the silicon substrate, and forming a second silicon membrane from the silicon substrate by forming a second cavity in the silicon substrate. The method further comprises forming silicon oxide on at least a portion of at least one of the first silicon membrane and the second silicon membrane. A different silicon oxide configuration is formed with respect to the first silicon membrane than with respect to the second silicon membrane.
According to another aspect of the technology, an apparatus is provided comprising a silicon substrate having a one-dimensional trench pattern formed therein comprising a plurality of trenches arranged along one axis. The trench pattern is characterized by: (a) differing trench widths among multiple trenches of the pattern; and/or (b) differing periods between multiple trenches of the pattern; and/or (c) at least one trench of the pattern having a width that varies along a length of the trench.
According to another aspect of the technology, an apparatus is provided comprising a silicon substrate having a two-dimensional trench pattern formed therein. The two-dimensional trench pattern comprises a plurality of trenches arranged along two axes, wherein the trench pattern is characterized by at least one of the following conditions being met along at least one of the axes: (a) trench width is variable from trench to trench; and/or (b) trench period is variable from trench to trench.
According to another aspect of the technology, an apparatus is provided comprising a plurality of trenches formed in a substrate, wherein a trench width, pitch or shape varies among the plurality of trenches.
Various aspects and embodiments of the technology will be described with reference to the following figures. It should be appreciated that the figures are not necessarily drawn to scale. Items appearing in multiple figures are indicated by the same or similar reference number in all the figures in which they appear.
While the previously described perceived benefits of silicon account for its use in micromechanical resonating structures, silicon may also exhibit temperature dependent properties (such as a temperature dependent stiffness tensor) which are undesirable in some situations. Thus, temperature changes may induce temperature drift in devices utilizing silicon micromechanical resonating structures, such as temperature drift in oscillator signals generated by oscillators having silicon resonating structures. Temperature compensation of silicon resonating structures may be achieved by placement of compensating structures on the top and bottom of the silicon resonating structure. A non-limiting example of such a temperature compensation structure includes a layer of silicon oxide on both the top and bottom of the silicon resonating structure, as described in U.S. patent application Ser. No. 12/639,161, filed Dec. 16, 2009 under Attorney Docket No. G0766.70006US01, published as U.S. Patent Publication No. 2010/0182102 and entitled “Mechanical Resonating Structures Including A Temperature Compensation Structure,” which is hereby incorporated herein by reference in its entirety. The silicon oxide may react differently than the silicon to changes in temperature, for example exhibiting different stiffening behavior, which thus may compensate for temperature-induced variations in behavior (e.g., operating frequency or resonance frequency) of the silicon resonating structure.
Applicants have appreciated that silicon membranes suitable for forming micromechanical resonating structures may be formed using empty-space-in-silicon (ESS) principles, and furthermore that oxidation of such silicon membranes may then be performed to form temperature compensated structures. Thus, according to one aspect of the present invention, silicon membranes suitable for formation of micromechanical resonating structures are formed from a silicon substrate. The dimensions of the membranes (e.g., thickness and area) may be selected to facilitate subsequent formation of a mechanical resonating structure having desired vibratory characteristics. The silicon membranes may be formed using ESS principles, as will be further described below, and in some embodiments may be oxidized to form temperature-compensated structures.
Applicants have further appreciated that ESS principles may be used to form multiple silicon membranes on the same silicon substrate, which may be used to form distinct micromechanical resonating structures, for instance to be used in different MEMS devices. Moreover, Applicants have appreciated that it may be beneficial in some instances to form, on the same substrate, silicon membranes of different thicknesses and/or with different oxide configurations, for example to provide devices incorporating such structures with different mechanical properties (e.g., vibratory properties).
Thus, according to another aspect of the present invention, two or more silicon membranes are formed on the same silicon substrate and differ in one or more respects which may impact the vibratory characteristics of the membranes and thus the vibratory characteristics of resonating structures formed from the membranes. According to one such aspect, two or more of the silicon membranes may differ in their thicknesses, which therefore may result in the membranes exhibiting different vibratory characteristics. According to another such aspect, differing oxide configurations may be formed with respect to two or more of the silicon membranes. The oxide configurations may differ in terms of the presence or absence of oxide, the location of oxide, and/or the thickness of oxide.
According to another aspect of the present invention, multiple silicon membranes are formed on a silicon substrate using different trench patterns in conjunction with ESS principles. The trench patterns may differ in terms of the area of the openings of the trenches, the depths of the trenches, the aspect ratios of the trenches and/or the pitches of the trench patterns. Annealing of the silicon substrate after formation of the trenches may then result in silicon membranes of differing dimensions (e.g., different thicknesses), as a result of the differing trench patterns.
The aspects described above, as well as additional aspects, are described further below. These aspects may be used individually, all together, or in any combination of two or more, as the technology is not limited in this respect.
As mentioned, according to the present aspect, the membrane 114 may be suitable for formation of a mechanical resonating structure (e.g., by defining such a structure from the membrane, as will be described further below in connection with
According to one non-limiting embodiment, to provide suitable vibratory characteristics, the membrane thickness T may be between approximately 1 and 20 microns. According to another embodiment, T may be between approximately 1 and 10 microns (e.g., 2 microns, 5 microns, etc.). According to one embodiment, T may be less than approximately three wavelengths of a resonance frequency of interest of a mechanical resonating structure to be formed from the membrane. According to some embodiments, the thickness T is less than approximately two wavelengths of a resonance frequency of interest of a resonating structure to be formed from the membrane. In still other embodiments, the thickness T may be less than approximately one wavelength of a resonance frequency of interest (e.g., less than approximately one wavelength of a resonant Lamb wave supported by a mechanical resonating structure to be formed from the membrane). Thus, it should be appreciated that the thickness of the membrane may determine or depend on the types of waves to be supported by a resonating structure to be formed from the membrane. For example, a given thickness may limit the ability of the resonating structure to support Lamb waves, or certain modes of Lamb waves. Thus, the thickness may be chosen dependent on the types and/or modes of waves desired to be supported by a mechanical resonating structure to be formed from the membrane. According to any of those embodiments described above, the thickness T may be substantially uniform (as shown in
According to one embodiment, suitable vibratory characteristics of the membrane 114 may be provided by suitably selecting not only the thickness of the membrane, but also at least one other dimension (e.g., length or width) of the membrane. For instance, suitable selection of the ratio of the thickness (T) to the maximum dimension of L and W (i.e., the larger of L and W) may provide suitable vibratory characteristics of the membrane such that the membrane is suitable for formation of a mechanical resonating structure (e.g., a micromechanical resonating structure to be used in a MEMS oscillator). According to one non-limiting embodiment, the ratio of T to the larger of L and W is between 1:20 and 1:500 (e.g., 1:100, 1:200, 1:300, 1:400, etc.). According to an alternative embodiment, the ratio of T to the larger of L and W is between 1:20 and 1:100 (e.g., 1:20, 1:50, etc.). It should be appreciated that other ratios are also possible, and that those listed are provided for purposes of illustration and not limitation. It should also be appreciated that the rectangular shape of the membrane 114 illustrated in
In any of those embodiments described above, or any other embodiments described herein in which the membrane has a length (L) and width (W), L and W may have any suitable values. For example, one or both of L and W may be less than approximately 1000 microns, less than approximately 100 microns (e.g., 75 microns, 60 microns, 50 microns, 40 microns, or any other value within this range), between approximately 50 microns and 200 microns, between approximately 70 microns and 120 microns, between approximately 30 microns and 400 microns, or have any other suitable values. Also, L and W need not be the same, and may differ by any suitable amounts, as the various aspects described herein as relating to membranes having dimensions L and W are not limited in this respect. According to some embodiments, L and W may be selected such that the area A is between approximately 110% and 300% (e.g., approximately 120%, approximately 150%, approximately 230%, approximately 250%, etc.) of the area of a mechanical resonating structure to be formed from the membrane, or in other embodiments between approximately 110% and 200% of the area of a mechanical resonating structure to be formed from the membrane, as described below.
According to one aspect of the present invention, a membrane (e.g., a single crystal silicon membrane) formed on a substrate (e.g., a single crystal silicon substrate) and suitable for formation of a mechanical resonating structure (e.g., a micromechanical resonating structure) is oxidized to provide a temperature compensated structure of the type(s) previously described with respect to U.S. patent application Ser. No. 12/639,161 (i.e., including silicon sandwiched between two layers of silicon oxide). A non-limiting example is illustrated in
The illustrated apparatus 200 is similar to the apparatus 100 of
The access holes may be of any suitable number and positioning, as well as each having any suitable size and shape, to facilitate formation of a desired oxide configuration (e.g., oxidizing the cavity 112 and/or the bottom of the membrane 114).
To form the oxide illustrated in
As mentioned, the formation of the SiO2—Si—SiO2 multi-layer structure of apparatus 200 may provide temperature compensated functionality. Suitable selection of the ratio of the thickness of the silicon membrane to the total thickness of the silicon oxide layer(s) (e.g., the combined thickness of oxide layers on the top and bottom surfaces of the membrane) may provide for temperature compensation of a desired acoustic mode of vibration for a resonating structure formed from the membrane. For example, the ratio of the total thickness of the silicon oxide on the top and bottom surfaces of the membrane (when oxide is present on both the top and bottom surfaces of the membrane) to the silicon of the membrane may be between 1:0.1 and 1:10, between 1:0.5 and 1:3, between 1:0.75 and 1:1.25, or between 1:1 and 1:2, among other possible ratios. Thus, suitable values of the thickness of the oxide layer(s) may be determined from these ratios by reference to the suitable values of the thickness T of the membrane, described above.
Utilizing ESS principles with a subsequent oxidation step to form the oxidized structure illustrated in
As mentioned, membranes of the type described herein may be utilized to form a mechanical resonating structure that may serve as part or all of a MEMS device, such as a MEMS oscillator. A non-limiting example is illustrated in
The illustrated device 300 includes a micromechanical resonating structure 310 (reference number shown in
Formation of the micromechanical resonating structure 310 from a membrane, like that of
As illustrated in the cross-section of the device 300 shown in
As mentioned, various types and forms of mechanical resonating structures may be formed from suitable membranes (e.g., single crystal silicon membranes) according to the various aspects described herein, and
The mechanical resonating structure may have any shape, as the shape illustrated in
The mechanical resonating structures described herein may have any suitable dimensions, and in some embodiments may be micromechanical resonating structures. The mechanical resonating structure may have a thickness corresponding to the thickness of a membrane (plus any oxidation layers on the membrane) from which the mechanical resonating structure is defined, and thus may have any of the thicknesses previously described with respect to the thickness T.
According to some embodiments, the mechanical resonating structures described herein have a large dimension (e.g., the largest of length, width, diameter, circumference, etc. of the mechanical resonating structure) of less than approximately 1000 microns, less than approximately 100 microns, less than approximately 50 microns, or any other suitable value. It should be appreciated that other sizes are also possible. According to some embodiments, the devices described herein form part or all of a microelectromechanical system (MEMS).
The mechanical resonating structures may have any desired resonance frequencies and frequencies of operation, and may be configured to provide output signals of any desired frequencies. For example, the resonance frequencies and/or frequencies of operation of the mechanical resonating structures, and the frequencies of the output signals provided by the mechanical resonating structures, may be between 1 kHz and 10 GHz. In some embodiments, they may be in the upper MHz range (e.g., greater than 100 MHz), or at least 1 GHz (e.g., between 1 GHz and 10 GHz). In some embodiments, they may be at least 1 MHz (e.g., 13 MHz, 26 MHz) or, in some cases, at least 32 kHz. In some embodiments, they may be in the range of 30 to 35 kHz, 60 to 70 kHz, 10 MHz to 1 GHz, 1 GHz to 3 GHz, 3 GHz to 10 GHz, or any other suitable frequencies. Thus, it should be appreciated that the frequencies are not limiting, and that the membranes described herein may be designed to support such frequencies.
The mechanical resonating structures may be operated in various acoustic modes, including but not limited to Lamb waves, also referred to as plate waves including flexural modes, bulk acoustic waves, surface acoustic waves, extensional modes, translational modes and torsional modes. The selected mode may depend on a desired application of the mechanical resonating structure.
The mechanical resonating structure may be actuated and/or detected in any suitable manner, with the particular type of actuation and/or detection depending on the type of mechanical resonating structure, the desired operating characteristics (e.g., desired mode of operation, frequency of operation, etc.), or any other suitable criteria. For example, suitable actuation and/or detection techniques include, but are not limited to, piezoelectric techniques, electrostatic techniques, magnetic techniques, thermal techniques, piezoresistive techniques, any combination of those techniques listed, or any other suitable techniques. The various aspects of the technology described herein are not limited to the manner of actuation and/or detection.
According to some embodiments, the mechanical resonating structures described herein may be piezoelectric Lamb wave devices, such as piezoelectric Lamb wave resonators. Such Lamb wave devices may operate based on propagating acoustic waves, with the edges of the structure serving as reflectors for the waves. For such devices, the spacing between the edges of the resonating structure may define the resonance cavity, and resonance may be achieved when the cavity is an integer multiple of p, where p=λ/2, with λ being the acoustic wavelength of the Lamb wave of interest, understanding that the device may support more than one mode of Lamb waves. However, it should be appreciated that aspects of the technology described herein apply to other types of structures as well, and that Lamb wave structures are merely non-limiting examples.
As should be appreciated from the foregoing and from
As mentioned, Applicants have appreciated that in some instances it may be beneficial to form two or more membranes (e.g., single crystal silicon membranes) having different vibratory characteristics on the same substrate, such that the membranes may be incorporated into different devices (e.g., distinct oscillators) with different vibratory characteristics. Thus, according to another aspect, two or more silicon membranes may be formed on a silicon substrate, with the membranes differing in thickness. According to yet another aspect, two or more silicon membranes may be formed on a silicon substrate and differing oxide configurations may be formed with respect to the silicon membranes, such that differing mechanical characteristics may be provided.
As shown, at least two of the membranes (e.g., membrane 404a and 404d) may have differing thicknesses, and may furthermore have differing areas, although not all embodiments are limited in this respect. The differing thicknesses may result in the membranes exhibiting different vibratory characteristics, which may lead to differing behavior of mechanical resonating structures formed from the different membranes. Thus, the thickness of each membrane may be selected to provide desired vibratory characteristics, and the differences in thickness may therefore depend on the differences in desired vibratory characteristics. According to one embodiment, a thickness of one membrane may differ from a thickness of a second membrane by between approximately 1 micron and 20 microns (e.g., 2 microns, 5 microns, 10 microns, etc.). According to another embodiment, a thickness difference of two membranes may be between approximately 1 micron and 10 microns, and according to a further embodiment the difference may be between approximately 3 and 10 microns.
An apparatus including multiple silicon membranes of differing thicknesses, such as apparatus 400 of
As shown, the apparatus 500 in this non-limiting example includes a substrate 502 (e.g., a silicon substrate or any other type of substrate described herein) with four distinct trench patterns, 504a-504d, each of which is a one dimensional trench pattern (as will be seen and described further with respect to
In the non-limiting example of
In the non-limiting example of
As shown in
According to one embodiment, multiple one-dimensional trench patterns are formed in a substrate, with each being suitable to form a membrane. At least some trenches of a first pattern have a first opening area and a first depth. At least some of the trenches of the first pattern are spaced by a first pitch. At least some trenches of a second pattern have a second opening area and a second depth, and at least some of the trenches of the second pattern are spaced by a second pitch. According to one embodiment, at least one of the following conditions is met: (a) the first depth differs from the second depth; (b) the first opening area differs from the second opening area; and (c) the first pitch differs from the second pitch.
Thus, it should be appreciated that
As can be seen from
The trenches may be formed using various anisotropic dry etching techniques, including, but not limited to, deep reactive ion etching (DRIE), which is often used in combination with a cyclic passivation deposition (the combination being referred to as Bosch process or advanced silicon etch (ASE)). Alternatively, the trenches may also be formed by anisotropic wet etching techniques, including KOH, EDP and TMAH based etch chemistries as well as anodization based etch techniques. Depending on the parameters, i.e. the current density during the anodization process, the silicon might not be completely etched. It should be understood that in some cases the trenches will contain porous silicon residue.
As mentioned, the resulting apparatus (e.g., apparatus 500 of
As mentioned, Applicants have also appreciated that it may be beneficial in some instances to form multiple membranes on the same substrate with different oxide configurations, as the oxide configurations may impact the mechanical properties (e.g., the vibratory properties) of the structures and therefore different oxide configurations may result in structures with different vibratory characteristics. Thus, according to one aspect of the present invention, multiple membranes with different oxide configurations are formed on the same substrate. The oxide configurations may differ in terms of the presence or absence of oxide, the positioning/location of oxide, and/or the thickness of oxide, all of which may impact the mechanical properties of the structures. In addition, the membranes may differ in thickness. Three non-limiting examples are illustrated in
In addition, as illustrated, different oxide configurations may be formed with respect to the membranes. In the non-limiting example of
The apparatus 600b of
The apparatus 600c of
Subjecting the apparatus 600c to further oxidation may result in the formation of oxide within cavities 406b and 406d (but not within 406c) and therefore on the back surfaces of membranes 404b and 404d. It should be appreciated that such further oxidation (subsequent to formation of access holes 608 and 610) may result in different oxide thicknesses being formed on different portions of the apparatus. For example, since oxide 604 is already present on portions of the apparatus (e.g., within the cavity 406a and on the membrane 404b), further oxidation of the structure may deposit further oxide on those portions of the apparatus already having oxide. Thus, as an example, subjecting the apparatus 600c to oxidation may result in thicker oxide formed on the backside of the substrate 402, within cavity 406a, on resonating structure 606, and on the top surface of membrane 404b compared to any oxide formed within cavities 406b and 406d and on the top surfaces of membranes 404c and 404d. The oxide thicknesses may differ by between approximately 0.1 microns to 3 microns (e.g., by 0.5 microns, 1 micron, 1.5 microns, 2 microns, 2.5 microns, etc.), as a non-limiting example.
According to one embodiment, the apparatus 600c may be used to form multiple resonating structures. For example, as shown, the apparatus 600c includes resonating structure 606. Resonating structures may also be formed from membranes 404b and 404d, for example, such that three resonating structures with different oxide configurations and/or different thicknesses may be formed on the same substrate. These structures may then be used in distinct devices (e.g., in three different oscillators) exhibiting different operating characteristics.
It should be appreciated from
While some non-limiting examples of trench patterns suitable for forming membranes of the types described herein have been shown and described (e.g., see
The pattern 700b of
The pattern 700c of
The patterns of
The pattern 700e of
The pattern 700f of
The pattern 700g of
The pattern 700h of
The patterns in
The pattern 800a of
In general, it should be understood that any two or more of the trench pattern features illustrated in
As previously mentioned, use of the fabrication techniques described herein may offer benefits over SOI processing techniques, for example in the formation of stress free membranes with accurately controlled thicknesses (e.g., the thickness of the silicon layer may only be controlled to within approximately +/−0.5 microns using SOI techniques, compared to +/−0.02 microns using the techniques described herein). In addition, Applicants have appreciated that use of the techniques described herein may facilitate formation of through-silicon vias (TSVs), which may be more difficult to form if SOI techniques are used due to the insulating oxide layer associated with SOI wafers. For example, using the techniques described herein, the vias may be etched from the top-side of the substrate (e.g., from a top surface 116 of the substrate 110) and exposed by thinning the substrate from the backside after bonding to another wafer, also referred to as “blind vias.” Accordingly, in some embodiments, the TSVs may be smaller (e.g., only half the wafer thickness in some embodiments) than is attainable using SOI technology.
It should also be appreciated that the processing shown herein (e.g., the processing to form the apparatus described herein) may be performed on either the front side or back side of a substrate, or both. For example, it is possible to create cavities in the backside of the wafer at the same time as forming cavities in the front side of the wafer, and fabricate devices on the front and back. Alternatively, cavities (and corresponding membranes) may be formed only on a backside of a wafer and not on a front side. Also, it should be appreciated that the structures shown herein may be formed without the use of wafer bonding and without the use of SOI substrates, according to some embodiments.
The mechanical resonating structures described herein may be used as stand alone components, or may be incorporated into various types of larger devices. Thus, the various structures and methods described herein are not limited to being used in any particular environment or device. However, examples of devices which may incorporate one or more of the structures and/or methods described herein include, but are not limited to, tunable meters, mass sensors, gyroscopes, accelerometers, switches, filters, microphones, pressure sensors, magnetic field sensors and electromagnetic fuel sensors. According to some embodiments, the mechanical resonating structures described are integrated in a timing oscillator. Timing oscillators are used in devices including digital clocks, radios, computers, oscilloscopes, signal generators, and cell phones, for example to provide precise clock signals to facilitate synchronization of other processes, such as receiving, processing, and/or transmitting signals. In some embodiments, one or more of the devices described herein may form part or all of a MEMS.
Having thus described several aspects of at least one embodiment of the technology, it is to be appreciated that various alterations, modifications, and improvements will readily occur to those skilled in the art. Such alterations, modifications, and improvements are intended to be within the spirit and scope of the technology. Accordingly, the foregoing description and drawings provide non-limiting examples only.
In addition, while some references have been incorporated herein by reference, it should be appreciated that the present application controls to the extent the incorporated references are contrary to what is described herein.
This application claims the benefit under 35 U.S.C. §119(e) of U.S. Provisional Patent Application Ser. No. 61/347,169, filed on May 21, 2010 under Attorney Docket No. G0766.70020US00 and entitled “Micromechanical Membranes and Related Structures and Methods”, which application is hereby incorporated herein by reference in its entirety.
Number | Date | Country | |
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61347169 | May 2010 | US |