Patent Application
20120025337
Transducers generally convert electrical signals to mechanical signals or vibrations, and/or mechanical signals or vibrations to electrical signals. Acoustic transducers, in particular, convert electrical signals to acoustic signals (sound waves) in a transmit mode (e.g., a speaker application), and/or convert received acoustic waves to electrical signals in a receive mode (e.g., a microphone application). Transducers, such as ultrasonic transducers, are provided in a wide variety of electronic applications, including filters. As the need to reduce the size of many components continues, the demand for reduced-size transducers continues to increase, as well. This has led to comparatively small transducers, which may be micromachined according to various technologies, such as micro-electromechanical systems (MEMS) technology.
Various types of MEMS transducers, such as piezoelectric ultrasonic transducers (PMUTs), include a resonator stack, having a layer of piezoelectric material between two conductive plates (electrodes), formed on a thin membrane. The membrane may be formed on a substrate over a cavity passing through the substrate. Typically, the substrate is formed of a material compatible with semiconductor processes, such as silicon (Si). The transducers may packaged by polishing the back side of the transducer substrate and mounting the polished transducer substrate directly onto a package substrate. For example, when the transducer is to be included in a lead frame package, the transducer substrate is typically mounted on a metal package substrate.
In conventional packaging, a coefficient of thermal expansion (CTE) of the transducer is significantly different from the CTE of the package in which it is mounted. Generally, CTE indicates the rate or proportion of change of a material or structure with respect to changes in temperature. The difference between the transducer and package CTEs results in varying responses to changes in temperature, both during packaging processes and during operation, which impose physical stress on the transducer. In other words, the source of parametric shifts in MEMS bending mode and/or thickness mode transducers due to die mounting and operating temperature variation, for example, is mismatch of thermal properties between the materials of the transducer and the package. The stress is most pronounced between the transducer substrate and the package substrate to which the transducer substrate is attached, due to the physical contact and significant CTE mismatch of the respective materials.
In a representative embodiment, a micro-electromechanical systems (MEMS) transducer device mounted to a package substrate includes an active transducer formed on a transducer substrate, and a stress mitigation structure between the transducer substrate and the package substrate. The stress mitigation structure reduces stress induced on the transducer substrate due to mismatched coefficients of thermal expansion (CTEs) of the transducer substrate and the package substrate, respectively.
In another representative embodiment, a method is provided for forming a packaged MEMS transducer device. The method includes forming a membrane on a top surface of a transducer substrate, the transducer substrate having a first CTE; forming a resonator stack on the membrane; forming a stress mitigation structure on a bottom surface of the transducer substrate; etching the stress mitigation structure and the transducer substrate to form a substrate cavity under the membrane; and attaching the stress mitigation structure to a package substrate having a second CTE different from the first CTE. The stress mitigation structure reduces stress induced on the transducer substrate due to the difference between the first and second CTEs.
In another representative embodiment, a packaged MEMS transducer device includes a transducer substrate having a first CTE, a membrane on the transducer substrate over a cavity formed through the transducer substrate, a resonator stack on the membrane, and a stress mitigation structure between the transducer substrate and a package substrate having a second CTE greater than the first CTE. The stress mitigation structure has a third CTE less than both the first CTE and the second CTE, the stress mitigation structure counter-acting physical responses of the package substrate to temperature fluctuations to reduce stress induced onto the transducer substrate.
The example embodiments are best understood from the following detailed description when read with the accompanying drawing figures. It is emphasized that the various features are not necessarily drawn to scale. In fact, the dimensions may be arbitrarily increased or decreased for clarity of discussion. Wherever applicable and practical, like reference numerals refer to like elements.
In the following detailed description, for purposes of explanation and not limitation, representative embodiments disclosing specific details are set forth in order to provide a thorough understanding of the present teachings. However, it will be apparent to one having ordinary skill in the art having had the benefit of the present disclosure that other embodiments according to the present teachings that depart from the specific details disclosed herein remain within the scope of the appended claims. Moreover, descriptions of well-known apparatuses and methods may be omitted so as to not obscure the description of the representative embodiments. Such methods and apparatuses are clearly within the scope of the present teachings.
Generally, it is understood that the drawings and the various elements depicted therein are not drawn to scale. Further, relative terms, such as “above,” “below,” “top,” “bottom,” “upper,” “lower,” “left,” “right,” “vertical” and “horizontal,” are used to describe the various elements' relationships to one another, as illustrated in the accompanying drawings. It is understood that these relative terms are intended to encompass different orientations of the device and/or elements in addition to the orientation depicted in the drawings. For example, if the device were inverted with respect to the view in the drawings, an element described as “above” another element, for example, would now be “below” that element. Likewise, if the device were rotated 90 degrees with respect to the view in the drawings, an element described as “vertical,” for example, would now be “horizontal.”
According to various embodiments, a transducer device, such as a MEMS ultrasonic transducer or a PMUT, includes an active transducer on a top surface of a transducer substrate and a stress mitigation structure on a bottom (opposite) surface of the transducer substrate. The active transducer includes a resonator stack, having a layer of piezoelectric material between two conductive plates (electrodes), formed on a membrane. The transducer device is mounted in a package, for example, by attaching the transducer device to a package substrate through the stress mitigation structure. The stress mitigation structure reduces or eliminates the effects of a CTE mismatch between the package substrate and the transducer substrate.
Referring to
In the depicted embodiment, the resonator stack 130 is shown as an annular resonator, where the cross-section is taken across the center. The annular resonator stack 130 may be substantially circular in shape, for example, although it may be formed in different shapes, such as ovals, squares, rectangles, or the like, without departing from the scope of the present teachings. Further, in various embodiments, the resonator stack 130 need not have annular shape, but may simply be a solid resonator stack on the substrate 110. The resonator stack 130 is substantially centered over the cavity 115, enabling mechanical movement of the membrane 120 and/or the resonator stack 130. Application of a time-dependent voltage to the resonator stack 130 causes a mechanical wave to be launched through the resonator stack 130 and the membrane 120. Generally, as the piezoelectric layer 135 of the resonator stack 130 and/or the membrane 120 oscillate in response to a mechanical perturbation (e.g., sound waves), forces generated by the perturbation induce stresses in the piezoelectric layer 135 resulting in generation of a voltage difference across the first and second electrodes 131, 132.
The transducer substrate 110 may be formed of various types of materials compatible with semiconductor processes, such as silicon (Si), gallium arsenide (GaAs), indium phosphide (InP), glass, sapphire, alumina, or the like, which is useful for integrating connections and electronics, thus reducing size and cost. The cavity 115 formed through the transducer substrate 110 may be substantially the same shape as the resonator stack 130, e.g., circular, although it may have any of a variety of sizes and shapes, such as oval, square, rectangular, or the like, without departing from the scope of the present teachings. The cavity 115 may be obtained by back side etching the bottom surface of the transducer substrate 110, which may include a dry etch process, such as a Bosch process, for example, although various alternative techniques may be incorporated. Formation of the transducer substrate 110 and the resonator stack 130 (on a membrane) is described, for example, by MARTIN et al. in U.S. patent application Ser. No. 12/495,443, which is hereby incorporated by reference.
In various embodiments, the transducer substrate 110 is formed of a different material than the package substrate 150. Accordingly, the transducer substrate 110 has a first CTE corresponding to the material from which it is formed, and the package substrate 150 has a second CTE corresponding to the material from which it is formed, where the first and second CTE are different from one another, resulting in a CTE mismatch. Typically, the second CTE is greater than the first CTE, indicating that the package substrate 150 expands at a greater rate in response to increases in temperature than the rate at which the transducer substrate 110 expands. Likewise, the greater second CTE results in the package substrate 150 contracting at a greater rate in response to decreases in temperature than the rate at which the transducer substrate 110 contracts. For example, when the transducer substrate 110 is formed of Si, the first CTE is about 2.5, and when the package substrate 150 is formed of Cu, the second CTE is about 17. Of course, the second CTE may be less than the first CTE in various configurations, although the same stress mitigation techniques described herein apply.
As discussed above, the difference (mismatch) between the first and second CTEs causes stress to be transferred to the transducer device 100, and more particularly to the transducer substrate 110, in response to temperature fluctuations, e.g., during die attach and other assembly processes and/or during operation the transducer device 100 after being mounted to the package substrate 150. The temperature fluctuations may result from changes in various conditions, including ambient environmental temperature, internally generated heat, or the like. Thus, according to various embodiments, the stress mitigation structure 140 is added to the bottom surface of the transducer substrate 110 prior to attaching the transducer device 100 to the package substrate 150. The stress mitigation structure 140 reduces the stress, e.g., induced on the transducer substrate 110 and/or other components of the transducer device 100, otherwise caused by the mismatch between the first and second CTEs. The various types and functionalities of the stress mitigation structure 140 are discussed below. Although
The membrane 120 may be formed of includes various materials compatible with semiconductor processes, such as boron silicate glass (BSG), silicon dioxide (SiO2), silicon nitride (SiN), polysilicon, aluminum nitride (AlN), or the like. The first and second electrodes 131, 132 may be formed of an electrically conductive material, such as molybdenum (Mo), tungsten (W), aluminum (Al), or the like. The first and second electrodes 131, 132 are electrically connected to external circuitry via contact pads (not shown), which may be formed of a conductive material, such as gold, gold-tin alloy or the like. The piezoelectric layer 135 may be formed of a thin film of piezoelectric material, such as AlN, zinc oxide (ZnO), lead zirconium titanate (PZT), or other piezoelectric film compatible with semiconductor processes. The thickness of the membrane 120 may range from about 0.05 μm to about 20 μm, thicknesses of the first and second electrodes 131, 132 may range from about 0.05 μm to about 10 μm, and the thickness of the piezoelectric layer 135 may range from about 0.1 μm to about 10 μm, for example, although the respective thicknesses may vary to provide unique benefits for any particular situation or to meet application specific design requirements of various implementations, as would be apparent to one skilled in the art.
In various embodiments, one or more passivation layers (not shown) may be formed on top and side surfaces of the resonator stack 130 and exposed portions of the membrane 120. The passivation layer(s) may be formed of various types of materials compatible with semiconductor processes, including BSG, SiO2, SiN, polysilicon, AlN, or the like, and may include multiple layers of the same or different materials. The total thickness of the passivation layer is generally sufficient to insulate the layers of the transducer device 100 from the environment, including protection from moisture, corrosives, contaminants, debris or the like, to which the transducer device 100 would otherwise be exposed. Passivation may have other design requirements to achieve desired frequency and sensitivity, as would be apparent to one of ordinary skill in the art.
Referring again to
More particularly, according to various embodiments, the stress mitigation structure 140 may be is formed of a CTE negating material having a third CTE lower than the first and second CTEs when the second CTE of the package substrate 150 is higher than the first CTE, and having a third CTE higher than the first and second CTEs when the second CTE is lower than the first CTE. The stress mitigation structure 140 thus reduces stress on the transducer device 100 by effectively counter-acting or “netting out” the physical responses of the package substrate 150 to temperature fluctuations before they induce stress onto the transducer substrate 110, thus negating the mismatch between the first and second CTEs, in whole or in part.
According to various alternative embodiments, when the stress mitigation structure 140 is formed of an isolating material that physically isolates the transducer substrate 110 from the package substrate 150, the stress mitigation structure 140 reduces stress on the transducer device 100 by blocking or otherwise absorbing the physical responses of the package substrate 150 to temperature fluctuations. Of course, as mentioned above, further embodiments may include a stress mitigation structure 140 formed of one or more materials that combine aspects of both CTE negating materials and isolating materials, described above, in various proportions.
Illustrative embodiments of the transducer device 100 with different materials as the stress mitigation structure 140 are discussed below. For purposes of simplifying the description, it is assumed that the transducer substrate 110 is formed of Si and the package substrate 150 is formed of Cu, for example, although the concepts apply with various alternative materials, taking into account the respective relative prosperities of these materials, as would be apparent to one of ordinary skill in the art.
According to an embodiment, the stress mitigation structure 140 is an oxide film, such as SiO2, grown or otherwise deposited on the bottom surface of the transducer substrate 110. SiO2, in particular, has a much lower CTE than Si, as well as a lower CTE than Cu, for example. Thus, the third CTE of the stress mitigation structure 140 formed of SiO2 is less than both the first CTE of the transducer substrate 110 formed of Si and the second CTE of the package substrate 150 formed of Cu. Accordingly, the stress mitigation structure 140 counteracts the stress induced on the transducer substrate 110 by the package substrate 150 in response to temperature fluctuations. The stress mitigation structure 140 may be a relatively thick oxide film, e.g., about 1 μm to about 20 μm, which may be applied using a chemical vapor deposition (CVD) process, for example. Otherwise, the oxide film may be applied using thermal oxidation.
According to another embodiment, the stress mitigation structure 140 may be a thin film of a non-etchable oxide, such as BSG, for example. That is, the chemical composition of the oxide is modified to make it non-etchable in release-etch chemistry. For example, MEMS transducer devices may utilize a sacrificial material, such as photoresist pattern or etch mask 245 discussed below with reference to
According to another embodiment, the stress mitigation structure 140 may be a thin film of a chemically optimized dielectric material, including chemically optimized dielectric glass. Chemically optimized refers to a dielectric material having a composition designed to meet compatibility with the wafer manufacturing process and that provides the desired CTE. Examples of a chemically optimized dielectric include various materials other than SiO2, such as SiN and AlN. The stress mitigation structure 140 formed of chemically optimized dielectric material may be deposited on the Si of the transducer substrate 110 using various deposition techniques, such as a CVD process, for example.
According to another embodiment, the stress mitigation structure 140 is a sheet of material or a separate wafer bonded to the bottom of the transducer substrate 110 using any of various lamination techniques. For example, the stress mitigation structure 140 may be formed of alumina, sapphire, doped Si02 (e.g., BSG or Pyrex®), or the like, bonded to the transducer substrate 110. In contrast to the three previously described embodiments which involve thin films, the lamination technique allows a much thicker interposer material between the transducer substrate 110 and the package substrate 150, resulting in greater mitigation of package induced stresses. Therefore, when the material of the stress mitigation structure 140 has a third CTE less than both the first CTE of the Si transducer substrate 110 and the second CTE of the Cu package substrate 150, as well as a substantial thickness, the lamination technique provides mitigation of package induced stresses both by negating the CTE mismatch between the transducer substrate 110 and the package substrate 150, and by physically isolating the transducer substrate 110 from the package substrate 150.
According to yet another embodiment, the stress mitigation structure 140 may be an organic compliant coating, such as benzocyclobutene (BCB) or SU-8 or other photoresist film, for example. The organic compliant film is an isolating material (as opposed to a CTE negating material), and thus the organic compliant film blocks, absorbs, or otherwise reduces the magnitude of the stress transferred into the transducer substrate 110. For example, the stress mitigation structure 140 may be formed of BCB having a thickness of about 0.5 μm to about 50 μm. This approach may be somewhat less robust than negating the mismatch between the first and second CTEs, discussed above, although it may suitable to applications with benign operating environments.
As stated above, the transducer device 100 may be an ultrasonic transducer fabricated using MEMS technology. When the transducer device 100 is a PMUT, for example, the translation is made through a piezoelectric material, e.g., by the piezoelectric layer 135. In various alternative embodiments, the transducer device 100 may be any type of micromachined transducer with a membrane having stress as a significant parameter, such as a capacitive micro-machined ultrasonic transducer (CMUT), in which case the translation is made through a capacitance variation. It is understood that other types and arrangements of transducers may be incorporated, without departing from the scope of the present teachings.
Referring to
Referring to
The resonator stack 130 may be an annular resonator including piezoelectric layer 135 formed between first and second electrodes 131, 132, and the resonator stack 160 may be an annular resonator including piezoelectric layer 165 formed between first and second electrodes 161, 162. The resonator stacks 130, 160 and may be formed by sequentially applying a layer of an electrically conductive material, such as Mo, W, Al, or the like, for first electrodes 131, 161, a piezoelectric thin film, such as AlN, ZnO, or PZT for piezoelectric layers 135, 165, and another layer of the electrically conductive material for second electrodes 132, 162. One or more passivation layers (not shown) may be included as well. The conductive layers may be respectively patterned, for example, using photolithography, although various alternative techniques may be incorporated, to provide the desired shapes of the first electrodes 131, 161 and second electrodes 161, 162.
Referring to
For convenience of illustration,
When the stress mitigation layer 140 is formed of SiO2 or other oxide film, for example, the SiO2 material may be grown on the bottom surface of the wafer substrate 210 using thermal oxidation. Alternatively, the SiO2 material may deposited using CVD, sputtering or thermal growth techniques, for example, which enables deposition of a thicker (e.g., about 1 μm to about 20 μm) layer of the SiO2 material. When the stress mitigation layer 140 is formed of some other type of thin film, such as non-etchable oxide (e.g., BSG) or chemically optimized dielectric (e.g., SiN or AlN), as discussed above, the material may deposited using CVD or sputtering techniques, for example.
When stress mitigation structure 140 is formed of an organic compliant coating (e.g., BCB or SU-8) or other isolating material, as discussed above, the stress mitigation structure 140 is relatively thicker. For example, when the stress mitigation structure 140 is formed of BCB, it is deposited to a thickness of about 0.5 μm to about 50 μm. The isolating material may be deposited using spin-on or sputtering techniques, for example.
Also, as discussed above, the stress mitigation structure 140 may be a sheet of material or separate wafer that is laminated or otherwise bonded to the bottom of the transducer substrate 110 using various lamination or bonding techniques. In this case, the stress mitigation structure 140 may be formed of alumina, sapphire, doped SiO2 (e.g., BSG or Pyrex®), or the like, for example. The sheet of material or separate wafer may include a CTE negating material having a third CTE that substantially or partially negates the difference between the first CTE of the wafer substrate 210 and a second CTE of the package substrate (not shown in
Referring to
As discussed above, for convenience of illustration,
The etch mask 245 may be formed by machining or by chemically etching a mask layer (not shown), previously applied to the stress mitigation structure 140, using photolithography, although various alternative techniques may be incorporated. However, in various embodiments where the stress mitigation structure 140 is a sheet of material or a separate wafer to be laminated to the wafer substrate 210, the stress mitigation structure 140 may itself be used as the etch mask in place of the etch mask 245. For example,
Referring to
As discussed above, the transducer devices 100, 101 may be singulated into corresponding dies (indicated by the vertical dashed line in
The transducer devices 100, 101, including the corresponding stress mitigation structures 140, may then be separately packaged, for example, in any of a variety of packaged devices, such as lead frames, dual-in-line packages, outline packages, ceramic packages (e.g., alumina), or the like. The packaging may be single chip packaging, e.g., including only one transducer device 100, 101, or multi-chip packaging, e.g., including both transducer devices 100, 101 or one transducer device 100, 101 with another chip. In various embodiments, the stress mitigation structure 140 is attached to the package substrate using various die attach adhesives, for example. The packaging may include addition of a lid to encapsulate the transducer devices 100, 101. Post processing may then be performed on the packages, including curing, wire bonding, etc.
Generally, examples of methods, materials and structures for fabricating a transducer device (e.g., without stress mitigation structures and associated fabrication techniques and processes) are described by RUBY et al. in U.S. Pat. Nos. 5,587,620, 5,873,153, 6,384,697 and 7,275,292; by BRADLEY et al. in U.S. Pat. No. 6,828,713; by FAZZIO et al. in U.S. Patent Application Pub. Nos. 2008/0122320 and 2008/0122317; by JAMNEALA et al. in U.S. Patent Application Pub. No. 2007/0205850; by RUBY et al. in U.S. Patent Application Pub. No. 2008/0258842; by FENG et al. in U.S. Patent Application Pub. No. 2006/0103492; and by MARTIN et al. in U.S. patent application Ser. No. 12/495,443; all of which are hereby incorporated by reference.
In alternative embodiments, the wafer substrate 210 may be formed with no cavities 117, 118. For example, using solidly mounted resonator (SMR) technology, the first and second resonator stacks 130, 160 may be formed over corresponding acoustic mirrors or Bragg Reflectors (not shown), having alternating layers of high and low acoustic impedance materials, formed in the wafer substrate 210. An acoustic reflector may be fabricated according to various techniques, an example of which is described by LARSON, III, et al. in U.S. Pat. No. 7,358,831, which is hereby incorporated by reference.
Notably, the teachings of the incorporated patents and patent applications are intended to be illustrative of methods, materials and structures useful to the present teachings, but in no way limiting to the present teachings.
According the various embodiments, a stress mitigation structure is able to mitigate differential materials thermal expansion stresses between a transducer device (e.g., a MEMS die) and a supporting package substrate, for example. The differential stresses to which the transducer device is subjected during package mounting processes required to permit the transducer device to be interconnected to end products/systems are reduced or eliminated. Likewise, differential stresses to which the transducer device is subjected during the functioning of such end product/systems in service, as surrounding temperatures shift to their high and low extremes, are reduced or eliminated.
The various components, materials, structures and parameters are included by way of illustration and example only and not in any limiting sense. In view of this disclosure, those skilled in the art can implement the present teachings in determining their own applications and needed components, materials, structures and equipment to implement these applications, while remaining within the scope of the appended claims.
