Patent Application
20160232858
This disclosure relates to microelectromechanical systems (MEMS) display devices and more particularly to annealed multilayer thin film stacks in movable reflective structures to improve creep resistance in MEMS display devices.
Electromechanical systems (EMS) include devices having electrical and mechanical elements, actuators, transducers, sensors, optical components such as mirrors and optical films, and electronics. EMS devices or elements can be manufactured at a variety of scales including, but not limited to, microscales and nanoscales. For example, microelectromechanical systems (MEMS) devices can include structures having sizes ranging from about a micron to hundreds of microns or more. Nanoelectromechanical systems (NEMS) devices can include structures having sizes smaller than a micron including, for example, sizes smaller than several hundred nanometers. Electromechanical elements may be created using deposition, etching, lithography, and/or other micromachining processes that etch away parts of substrates and/or deposited material layers, or that add layers to form electrical and electromechanical devices.
One type of EMS device is called an interferometric modulator (IMOD). The term IMOD or interferometric light modulator refers to a device that selectively absorbs and/or reflects light using the principles of optical interference. In some implementations, an IMOD display element may include a pair of conductive plates, one or both of which may be transparent and/or reflective, wholly or in part, and capable of relative motion upon application of an appropriate electrical signal. For example, one plate may include a stationary layer deposited over, on or supported by a substrate and the other plate may include a reflective membrane separated from the stationary layer by an air gap. The position of one plate in relation to another can change the optical interference of light incident on the IMOD display element. IMOD-based display devices have a wide range of applications, and are anticipated to be used in improving existing products and creating new products, especially those with display capabilities.
The systems, methods and devices of this disclosure each have several innovative aspects, no single one of which is solely responsible for the desirable attributes disclosed herein.
One innovative aspect of the subject matter described in this disclosure can be implemented in a MEMS display device including a substrate, a movable reflective structure over the substrate, where the movable reflective structure includes an annealed thin film stack, and one or more support structures over the substrate and connected to the movable reflective structure to support the movable reflective structure. The annealed thin film stack includes a first layer including an aluminum or aluminum alloy, a second layer including aluminum or aluminum alloy and over the first layer, and a third layer between the first layer and the second layer. The third layer is in contact with at least one of the first layer and the second layer, where the third layer includes a transition metal, the transition metal including at least one of: zirconium, scandium, ruthenium, titanium, tantalum, molybdenum, and chromium.
In some implementations, one or both of the first layer and the second layer is doped with about 0.1 atomic % to about 10 atomic % of the transition metal. In some implementations, a stress of the movable reflective structure is less than about 200 MPa. In some implementations, the third layer has a thickness of less than about 5 nm, and the first layer and the second layer each have a thickness of equal to or greater than about 20 nm. In some implementations, a reflectance of the movable reflective structure is greater than about 80%. In some implementations, each of the first layer and the second layer is doped to include between about 1 atomic % and about 20 atomic % of one or both of oxygen and nitrogen. In some implementations, the first layer and the second layer are substantially identical in composition and thickness.
Another innovative aspect of the subject matter described in this disclosure can be implemented in a method of manufacturing a MEMS display device. The method includes providing a substrate, forming a support structure over the substrate, forming a movable reflective structure over the substrate and connected to the support structure, and annealing the movable reflective structure. The movable reflective structure includes a first layer including aluminum or aluminum alloy, a second layer including aluminum or aluminum alloy and over the first layer, and a third layer between the first layer and the second layer. The third layer is in contact with at least one of the first layer and the second layer, where the third layer includes a transition metal, the transition metal including at least one of: zirconium, scandium, ruthenium, titanium, tantalum, molybdenum, and chromium.
In some implementations, forming the movable reflective structure includes depositing the first layer over the substrate, depositing the third layer on the first layer, and depositing the second layer on the third layer. In some implementations, depositing the first layer includes doping the first layer with one or both of oxygen and nitrogen, and depositing the second layer includes doping the second layer with one or both of oxygen and nitrogen. In some implementations, the method further includes depositing a fourth layer between the substrate and the first layer, where the fourth layer is substantially identical in thickness and composition with the third layer. In some implementations, the method further includes depositing a fifth layer between the substrate and the fourth layer, where the fifth layer is substantially identical in thickness and composition with the first layer and the second layer.
Details of one or more implementations of the subject matter described in this disclosure are set forth in the accompanying drawings and the description below. Although the examples provided in this disclosure are primarily described in terms of EMS and MEMS-based displays the concepts provided herein may apply to other types of displays such as liquid crystal displays (LCDs), organic light-emitting diode (“OLED”) displays, and field emission displays. Other features, aspects, and advantages will become apparent from the description, the drawings and the claims. Note that the relative dimensions of the following figures may not be drawn to scale.
Like reference numbers and designations in the various drawings indicate like elements.
The following description is directed to certain implementations for the purposes of describing the innovative aspects of this disclosure. However, a person having ordinary skill in the art will readily recognize that the teachings herein can be applied in a multitude of different ways. The described implementations may be implemented in any device, apparatus, or system that can be configured to display an image, whether in motion (such as video) or stationary (such as still images), and whether textual, graphical or pictorial. More particularly, it is contemplated that the described implementations may be included in or associated with a variety of electronic devices such as, but not limited to: mobile telephones, multimedia Internet enabled cellular telephones, mobile television receivers, wireless devices, smartphones, Bluetooth® devices, personal data assistants (PDAs), wireless electronic mail receivers, hand-held or portable computers, netbooks, notebooks, smartbooks, tablets, printers, copiers, scanners, facsimile devices, global positioning system (GPS) receivers/navigators, cameras, digital media players (such as MP3 players), camcorders, game consoles, wrist watches, clocks, calculators, television monitors, flat panel displays, electronic reading devices (e.g., e-readers), computer monitors, auto displays (including odometer and speedometer displays, etc.), cockpit controls and/or displays, camera view displays (such as the display of a rear view camera in a vehicle), electronic photographs, electronic billboards or signs, projectors, architectural structures, microwaves, refrigerators, stereo systems, cassette recorders or players, DVD players, CD players, VCRs, radios, portable memory chips, washers, dryers, washer/dryers, parking meters, packaging (such as in electromechanical systems (EMS) applications including microelectromechanical systems (MEMS) applications, as well as non-EMS applications), aesthetic structures (such as display of images on a piece of jewelry or clothing) and a variety of EMS devices. The teachings herein also can be used in non-display applications such as, but not limited to, electronic switching devices, radio frequency filters, sensors, accelerometers, gyroscopes, motion-sensing devices, magnetometers, inertial components for consumer electronics, parts of consumer electronics products, varactors, liquid crystal devices, electrophoretic devices, drive schemes, manufacturing processes and electronic test equipment. Thus, the teachings are not intended to be limited to the implementations depicted solely in the Figures, but instead have wide applicability as will be readily apparent to one having ordinary skill in the art.
Some implementations described herein relate to a MEMS display device having improved creep resistance and stress control. The MEMS display device can include a substrate, a movable reflective structure (e.g., mirror) over the substrate, and one or more support structures over the substrate and connected to the movable reflective structure to support the movable reflective structure. The movable reflective structure can include a multilayer thin film stack, where the multilayer thin film stack can include at least a thin transition metal layer sandwiched between two aluminum or aluminum alloy layers. The multilayer thin film stack can be annealed to permit the aluminum or aluminum alloy layers to be doped with the transition metal. The microstructures of the multilayer thin film stack can be altered or controlled through the annealing processes to control the stress in the movable reflective structure, improve the creep resistance of the MEMS display device, and minimize compromising the reflectance and electrical conductivity of the MEMS display device. In some implementations, the aluminum or aluminum alloy layers can be doped with oxygen and/or nitrogen to further enhance mechanical properties.
Particular implementations of the subject matter described in this disclosure can be implemented to realize one or more of the following potential advantages. The annealed multilayer thin film stack with a transition metal layer sandwiched between two aluminum or aluminum alloy layers provides greater resistance against creep. Accordingly, the MEMS display device can be more resistant to thermal cycles and mechanical stresses so that the lifetime of the MEMS display device can be increased. The annealed multilayer thin film stack can also increase the mechanical strength of the MEMS display device by reducing the stress in the movable reflective structure. The annealed multilayer thin film stack can also improve the optical characteristics of the MEMS display device by increasing the reflectance of the movable reflective structure. Furthermore, the incorporation of a transition metal layer between two aluminum or aluminum alloy layers can provide greater control over the mechanical, optical, and electrical properties of the movable reflective structure. In some implementations, such properties may be tuned by the thickness of the transition metal layer and/or the annealing conditions of the multilayer thin film stack.
An example of a suitable MEMS display device or apparatus, to which the described implementations may apply, is a reflective display device. Reflective display devices can incorporate interferometric modulator (IMOD) display elements that can be implemented to selectively absorb and/or reflect light incident thereon using principles of optical interference. IMOD display elements can include a partial optical absorber, a reflector that is movable with respect to the absorber, and an optical resonant cavity defined between the absorber and the reflector. In some implementations, the reflector can be moved to two or more different positions, which can change the size of the optical resonant cavity and thereby affect the reflectance of the IMOD. The reflectance spectra of IMOD display elements can create fairly broad spectral bands that can be shifted across the visible wavelengths to generate different colors. The position of the spectral band can be adjusted by changing the thickness of the optical resonant cavity. One way of changing the optical resonant cavity is by changing the position of the reflector with respect to the absorber.
The IMOD display device can include an array of IMOD display elements which may be arranged in rows and columns. Each display element in the array can include at least a pair of reflective and semi-reflective layers, such as a movable reflective layer (i.e., a movable layer, also referred to as a mechanical layer) and a fixed partially reflective layer (i.e., a stationary layer), positioned at a variable and controllable distance from each other to form an air gap (also referred to as an optical gap, cavity or optical resonant cavity). The movable reflective layer may be moved between at least two positions. For example, in a first position, i.e., a relaxed position, the movable reflective layer can be positioned at a distance from the fixed partially reflective layer. In a second position, i.e., an actuated position, the movable reflective layer can be positioned more closely to the partially reflective layer. Incident light that reflects from the two layers can interfere constructively and/or destructively depending on the position of the movable reflective layer and the wavelength(s) of the incident light, producing either an overall reflective or non-reflective state for each display element. In some implementations, the display element may be in a reflective state when unactuated, reflecting light within the visible spectrum, and may be in a dark state when actuated, absorbing and/or destructively interfering light within the visible range. In some other implementations, however, an IMOD display element may be in a dark state when unactuated, and in a reflective state when actuated. In some implementations, the introduction of an applied voltage can drive the display elements to change states. In some other implementations, an applied charge can drive the display elements to change states.
The depicted portion of the array in
In
The optical stack 16 can include a single layer or several layers. The layer(s) can include one or more of an electrode layer, a partially reflective and partially transmissive layer, and a transparent dielectric layer. In some implementations, the optical stack 16 is electrically conductive, partially transparent and partially reflective, and may be fabricated, for example, by depositing one or more of the above layers onto a transparent substrate 20. The electrode layer can be formed from a variety of materials, such as various metals, for example indium tin oxide (ITO). The partially reflective layer can be formed from a variety of materials that are partially reflective, such as various metals (e.g., chromium and/or molybdenum), semiconductors, and dielectrics. The partially reflective layer can be formed of one or more layers of materials, and each of the layers can be formed of a single material or a combination of materials. In some implementations, certain portions of the optical stack 16 can include a single semi-transparent thickness of metal or semiconductor which serves as both a partial optical absorber and electrical conductor, while different, electrically more conductive layers or portions (e.g., of the optical stack 16 or of other structures of the display element) can serve to bus signals between IMOD display elements. The optical stack 16 also can include one or more insulating or dielectric layers covering one or more conductive layers or an electrically conductive/partially absorptive layer.
In some implementations, at least some of the layer(s) of the optical stack 16 can be patterned into parallel strips, and may form row electrodes in a display device as described further below. As will be understood by one having ordinary skill in the art, the term “patterned” is used herein to refer to masking as well as etching processes. In some implementations, a highly conductive and reflective material, such as aluminum (Al), may be used for the movable reflective layer 14, and these strips may form column electrodes in a display device. The movable reflective layer 14 may be formed as a series of parallel strips of a deposited metal layer or layers (orthogonal to the row electrodes of the optical stack 16) to form columns deposited on top of supports, such as the illustrated posts 18, and an intervening sacrificial material located between the posts 18. When the sacrificial material is etched away, a defined gap 19, or optical cavity, can be formed between the movable reflective layer 14 and the optical stack 16. In some implementations, the spacing between posts 18 may be approximately 1-1000 μm, while the gap 19 may be approximately less than 10,000 Angstroms (Å).
In some implementations, each IMOD display element, whether in the actuated or relaxed state, can be considered as a capacitor formed by the fixed and moving reflective layers. When no voltage is applied, the movable reflective layer 14 remains in a mechanically relaxed state, as illustrated by the display element 12 on the left in
The processor 21 can be configured to communicate with an array driver 22. The array driver 22 can include a row driver circuit 24 and a column driver circuit 26 that provide signals to, for example a display array or panel 30. The cross section of the IMOD display device illustrated in
The details of the structure of IMOD displays and display elements may vary widely.
As illustrated in
In implementations such as those shown in
In
The process 80 continues at block 84 with the formation of a sacrificial layer 25 over the optical stack 16. Because the sacrificial layer 25 is later removed (see block 90) to form the cavity 19, the sacrificial layer 25 is not shown in the resulting IMOD display elements.
The process 80 continues at block 86 with the formation of a support structure such as a support post 18. The formation of the support post 18 may include patterning the sacrificial layer 25 to form a support structure aperture, then depositing a material (such as a polymer or an inorganic material, like silicon oxide) into the aperture to form the support post 18, using a deposition method such as PVD, PECVD, thermal CVD, or spin-coating. In some implementations, the support structure aperture formed in the sacrificial layer can extend through both the sacrificial layer 25 and the optical stack 16 to the underlying substrate 20, so that the lower end of the support post 18 contacts the substrate 20. Alternatively, as depicted in
The process 80 continues at block 88 with the formation of a movable reflective layer or membrane such as the movable reflective layer 14 illustrated in
The process 80 continues at block 90 with the formation of a cavity 19. The cavity 19 may be formed by exposing the sacrificial material 25 (deposited at block 84) to an etchant. For example, an etchable sacrificial material such as Mo or amorphous Si may be removed by dry chemical etching by exposing the sacrificial layer 25 to a gaseous or vaporous etchant, such as vapors derived from solid XeF2 for a period of time that is effective to remove the desired amount of material. The sacrificial material is typically selectively removed relative to the structures surrounding the cavity 19. Other etching methods, such as wet etching and/or plasma etching, also may be used. Since the sacrificial layer 25 is removed during block 90, the movable reflective layer 14 is typically movable after this stage. After removal of the sacrificial material 25, the resulting fully or partially fabricated IMOD display element may be referred to herein as a “released” IMOD.
Creep is the tendency of a solid material to move slowly or deform permanently under the influence of mechanical stresses. Creep is a time-dependent deformation mechanism that may or may not constitute a failure mode. MEMS devices may include one or more movable parts that may be subject to creep. MEMS devices can undergo deformation as a result of creep because of mechanical stresses and thermal energy. Such mechanical stresses may be below the yield strength of a material in the MEMS device, but over time and over elevated temperatures, the material will deform. Accordingly, mechanical actuation and thermal heating can cause atoms to diffuse around some of the layers of the MEMS device over time, eventually causing the MEMS device to fail. Creep resistance of the MEMS device can be increased by one or more of the following: reducing the operating temperature, reducing the applied stress levels, and changing the material.
In changing the composition of the material, the microstructure of the material may be controlled to limit the effects of creep. Deformation as a result of creep can be attributed to diffusional effects, such as the diffusion of atoms across grain boundaries. When the operating temperature increases, the thermal energy causes increased diffusion, thereby resulting in greater creep deformation. Solutions for limiting the mechanism of diffusional creep can include, for example, reducing the grain size of the material and making the material more mechanically robust.
Typically, some metallic layers and structures, especially those with low melting points or soft metal such as aluminum or gold, may be vulnerable to creep deformation due in part to its thermal and mechanical properties. Moreover, MEMS devices include movable parts that can undergo greater amounts of mechanical strain. Under elevated temperatures, the diffusion of atoms may increase, especially in regions exposed to more mechanical strain. Over time, this can lead to creep deformation that can result in device failure. Where movable parts include metallic layers and structures, these movable parts that are vulnerable to creep deformation may be strengthened to be more creep-resistant and more mechanically robust.
An example of a MEMS device can be a MEMS display device or MEMS display device element (e.g., pixel). In some implementations, the MEMS display device can include an IMOD, which is described with reference to
In some implementations, each of the one or more support structures 130 can include a tether or hinge 150 connected to a support post 180. The movable reflective structure 140 may be connected to each of the support posts 180 via tethers 150. In some implementations, the tethers 150 may be tangential to the movable reflective structure 140 and can reduce the residual stress in the MEMS display device 100. Other configurations for tethers 150, including straight, curved, or folded, are also possible. In some implementations, the tethers 150 may be made of metals, such as aluminum, aluminum-titanium, or aluminum-zirconium, or other materials such as amorphous or polycrystalline silicon, oxides, nitrides, and oxynitrides. In some implementations, the tethers 150 may include the same or substantially the same material as the movable reflective structure 140.
The substrate 200 can be made of any suitable substrate materials, including a substantially transparent material, such as glass or plastic. Substantial transparency as used herein may be defined as transmittance of visible light of about 70% or more, such as about 80% or more or about 90% or more. Glass substrates (sometimes referred to as glass plates or panels) may be or include a borosilicate glass, a soda lime glass, photoglass, quartz, Pyrex or other suitable glass material. A non-glass substrate can be used, such as a polycarbonate, acrylic, polyimide, polyethylene terephthalate (PET) or polyether ether ketone (PEEK) substrate. In some implementations, the substrate 200 can have dimensions of a few microns to hundreds of microns. For example, the substrate 200 can have a thickness between about 10 microns and about 1100 microns.
The stationary electrode 160 on the substrate 200 may be electrically conductive or include an electrically conductive layer. In some implementations, the stationary electrode 160 may be part of an optical stack. The optical stack can be at least partially absorbing of visible light and can include an optically absorbing material. An optically absorbing material can include, for example, a molybdenum-chromium compound having a thickness between about 20 Å and about 100 Å. The optical stack can include a plurality of sub-layers and can be configured similarly to the optical stack 16 in
The movable reflective structure 140 can be referred to as a movable electrode or a mirror. The movable reflective structure 140 can be electrically conductive or include an electrically conductive layer. The movable reflective structure 140 can include a plurality of layers (not shown), including but not limited to a reflective layer and an optical layer. The reflective layer can include a material that is configured to substantially reflect visible light, where the reflectance can be about 70% or more, such as about 80% or more or about 90% or more. In some implementations, the reflective layer can provide a mirror for interferometrically modulating light with the optically absorbing material in the stationary electrode 160.
In some implementations, the movable reflective structure 140 can include a reflective layer and a deformable layer, where the optical properties of the movable reflective structure 140 can be decoupled from its mechanical properties. The reflective layer can include a plurality of sub-layers, including but not limited to a reflective sub-layer, a dielectric sub-layer, and a metal sub-layer. The reflective sub-layer can have a thickness between about 100 Å and about 500 Å and can include aluminum or aluminum alloy. If a dielectric sub-layer is included in the reflective layer, the dielectric sub-layer can have a thickness between about 4000 Å and about 40000 Å to provide structural rigidity to the movable reflective structure 140. The dielectric sub-layer can include any suitable dielectric material such as nitrous oxide, silicon dioxide, silicon oxynitride, and silicon nitride. If a metal sub-layer is included in the reflective layer, the metal sub-layer can have a thickness between about 100 Å and about 1000 Å and can include aluminum, copper, aluminum-copper alloy, aluminum-titanium alloy, aluminum-zirconium alloy, or other electrically conductive material.
The movable reflective structure 140 may be configured to electrostatically actuate towards the stationary electrode 160 when a voltage is applied. The one or more support structures 130 may bend and cause the movable reflective structure 140 to deflect towards the stationary electrode 160. In some implementations, the movable reflective structure 140 can remain parallel or substantially parallel to the stationary electrode 160 during actuation. As the movable reflective structure 140 moves or otherwise deflects towards the stationary electrode 160, a gap distance between the movable reflective structure 140 and the stationary electrode 160 can influence the reflective properties of the MEMS display device 100. For example, different gap distances can reflect different wavelengths of light through the substrate 200, which gives the appearance of different colors.
The repeated movement of the movable reflective structure 140 and the one or more support structures 130 can cause the MEMS display device 100 to undergo several cycles of stress. The movable reflective structure 140 and the one or more support structures 130 can be made of metallic materials. In addition, during the operating lifetime of the MEMS display device 100, the MEMS display device 100 can be exposed to elevated temperatures. Thus, the MEMS display device 100 may be vulnerable to creep deformation and potential mechanical failure.
In some implementations, the one or more support structures 130 may be deformable. The one or more support structures 130 may permit the movable reflective structure 140 to actuate in the MEMS display device 100. Over time, the creep behavior in the movable reflective structure 140 can cause the one or more support structures 130 to become unable to restore to its original position after actuation. This can lead to poor image retention and device failure in the MEMS display device 100. For example, device failure can occur at connection points between the tethers 150 and the movable reflective structure 140 as a result of creep.
To increase the creep resistance of movable parts in the MEMS display device 100, the materials of the movable parts can be changed to provide greater creep resistance and mechanical strength. The movable reflective structure 140 can include aluminum or aluminum alloy. Examples of creep-resistant aluminum alloys can include but is not limited to aluminum-copper, aluminum-scandium, and aluminum-zirconium. The aluminum or aluminum alloy can serve as a reflective material for the movable reflective structure 140, where the reflectance of the movable reflective structure 140 can reflect greater than about 80% of visible light. The aluminum or aluminum alloy can serve as an electrically conductive material for the movable reflective structure 140, where the sheet resistance of the movable reflective structure 140 can be less than about 1, 10, or 100 ohms per square.
The aluminum or aluminum alloy may provide some resistance against creep, but stress control in the movable reflective layer 140 may be difficult with the aluminum or aluminum alloy. For example, when the movable reflective layer 140 in the MEMS display device 100 is subject to subsequent deposition and annealing, the movable reflective layer 140 with aluminum or aluminum alloy can end up with increased tensile stress. For example, the movable reflective layer 140 with aluminum or aluminum alloy can have a tensile stress of greater than about 500 MPa after annealing at 350° C.
The thin film stack 700 may also be referred to as an annealed multilayer thin film stack. The thin film stack 700 can have a total thickness of less than about 5 microns, or between about 50 nm and about 3 microns. The transition metal layer 720 can have a thickness of less than about 20% of the total thickness of the thin film stack 700, or less than about 10% of the total thickness of the thin film stack 700. For example, the reflective layers 710a, 710b can each have a thickness equal to or greater than about 20 nm, and the transition metal layer 720 can have a thickness of less than about 5 nm. The reflective layers 710a, 710b can each include aluminum or aluminum alloy, and the transition metal layer 720 can include at least one of: zirconium, scandium, ruthenium, titanium, tantalum, molybdenum, and chromium. For example, the transition metal can include zirconium.
The transition metal layer 720 upon doping into the reflective layers 710a, 710b, can improve creep resistance and reduce the stress of the reflective layers 710a, 710b while minimizing degradation of the optical and electrical properties of the reflective layers 710a, 710b. A movable reflective structure incorporating the thin film stack 700 can have a relatively low stress value, such as a stress value of less than about 200 MPa, or less than 100 MPa. Furthermore, the movable reflective structure incorporating the thin film stack 700 can have a sufficiently high reflectance of visible light, such as a reflectance of greater than about 80% or greater than about 90%. The reduced stress value and the sufficiently high reflectance value can be achieved even after annealing the thin film stack 700. In some implementations, the movable reflective structure incorporating the thin film stack 700 can have a relatively low sheet resistance value, such as less than about 1, 10 or 100 ohms per square.
Incorporating a transition metal layer 720 between two reflective layers 710a, 710b permits diffusion of transition metal atoms to dope the reflective layers 710a, 710b with the transition metal. Upon annealing, atoms of the transition metal may diffuse into the two reflective layers 710a, 710b from both sides of the transition metal layer 720. The thickness of the transition metal layer 720 and the annealing conditions can influence how the reflective layers 710a, 710b are doped. Each of the reflective layers 710a, 710b can be doped with about 0.1 atomic % to about 10 atomic % of the transition metal, or with about 0.5 atomic % to about 5 atomic % of the transition metal. The doped thin film stack 700 can provide greater creep resistance and stress control.
In some implementations, the thin film stack 700 can be doped to further include less than 20 atomic % of one or both of oxygen and nitrogen, or between about 1 atomic % and about 20 atomic % of one or both of oxygen and nitrogen. The thin film stack 700 can be doped with oxygen and/or nitrogen to further improve creep resistance and reduce stress. The aluminum or aluminum alloy may react with oxygen and/or nitrogen to form aluminum oxide, aluminum nitride, or aluminum oxynitride. When doping with one or both of oxygen and nitrogen, oxygen and nitrogen atoms can be provided at the grain boundaries of the reflective layers 710a, 710b to limit grain growth and limit the diffusion of atoms across the grain boundaries. The doping can occur by flowing oxygen and nitrogen gas during the aluminum or aluminum alloy deposition (e.g., sputter deposition). For example, oxygen gas can be flowed at less than about 5 standard cubic centimeters per minute (sccm), and nitrogen gas can be flowed at less than about 10 sccm while the sputtering gas such as argon or krypton can be ranged from 50 sccm to 100 sccm or 200 sccm or more.
The movable reflective structure incorporating the thin film stack 700 can be part of a “thin” film reflective structure or a “thick” film reflective structure. In implementations for a “thin” film reflective structure, the reflective layers 710a, 710b can be relatively thin, such as each having a thickness between about 10 nm and about 50 nm. However, the movable reflective structure in such implementations can include dielectric layers, where the dielectric layers can provide structural rigidity to the movable reflective structure and can each have a thickness between about 400 nm and about 4000 nm. In implementations for a “thick” film reflective structure, the reflective layers 710a, 710b can be relatively thick, such as each having a thickness between about 100 nm and about 1000 nm. The movable reflective structure in such implementations may or may not include dielectric layers to provide structural rigidity.
In
In
Different configurations of the thin film stack, different thicknesses of the transition metal layer, and different annealing conditions can influence the mechanical, optical, and electrical properties of the movable reflective structure, as shown in
Data in
In some implementations, doping the aluminum or aluminum alloy layers with oxygen and/or nitrogen can further improve the mechanical robustness of the movable reflective structure. Table 1 shows stress data for a movable reflective structure including an aluminum alloy mirror, where the stress values vary depending on the amount of nitrogen or oxygen gas introduced with the aluminum alloy. The stress values are taken before annealing and after annealing at about 350° C. for 3 hours. Oxygen and/or nitrogen gases are flowed in low amounts during the deposition of the aluminum alloy. For example, the oxygen and nitrogen gases can be introduced during sputtering of the aluminum alloy to form precipitates of aluminum oxide, aluminum nitride, or aluminum oxynitride. An inert carrier gas such as argon is also be flowed simultaneously.
The flow rates of nitrogen and oxygen gas can be adjusted to control the stress of the movable reflective structure having an aluminum alloy mirror. The flow rate of oxygen gas can be less than about 5 sccm and the flow rate of nitrogen gas can be less than about 10 sccm. In some implementations, the flow rate of oxygen gas can be less than about 4 sccm, and the flow rate of nitrogen gas can be less than about 5 sccm. As shown in Table 1, introduction of oxygen and nitrogen can reduce the stress of the movable reflective structure to be less than 100 MPa. Additionally, the sheet resistance of the movable reflective structure doped with oxygen and nitrogen is about 1.2 ohms per square for a movable reflective structure having a thickness of 40 nm. The movable reflective structure doped with oxygen and nitrogen can improve the stress of creep-resistant films while maintaining a relatively low sheet resistance.
At block 910, a substrate is provided. The substrate can include any suitable substrate material, such as glass or plastic. In some implementations, the substrate material can be substantially transparent to visible light. One or more display elements may be formed on the substrate for the MEMS display device. In some implementations, the one or more display elements can include an active matrix OLED, shutter-based light modulator, or IMOD. Each of the display elements can be part of a pixel in the display device. In some implementations, the process 900 can further include forming a stationary electrode or optical stack over the substrate, where the stationary electrode or optical stack can include an electrically conductive material.
At block 920, a support structure is formed over the substrate. In some implementations, the support structure can include a plurality of tethers or hinges symmetrically disposed around the edges of a movable reflective structure. The support structure can be formed on the substrate and configured to support a movable reflective structure. The support structure can be formed of a metal, such as aluminum or titanium, or other materials, such as oxides, nitrides, and oxynitrides. In some implementations, the support structure can include materials identical or substantially identical with the materials of the movable reflective structure. Thus, the support structure can include aluminum or aluminum alloy. The support structure may be bendable to permit the actuation of the movable reflective structure towards the substrate. In some implementations, the support structure can include a support post and a tether connected to the support post, where tether is connected to the movable reflective structure. The tether can be made of a flexible material while the support posts can be made of a relatively rigid material.
At block 930, a movable reflective structure is formed over the substrate and connected to the support structure, where the movable reflective structure includes a first layer including aluminum or aluminum alloy, a second layer including aluminum or aluminum alloy and over the first layer, and a third layer between the first layer and the second layer where the third layer is in contact with at least one of the first layer and the second layer. The third layer includes a transition metal, where the transition metal includes at least one of zirconium, scandium, ruthenium, titanium, tantalum, molybdenum, and chromium.
The movable reflective structure can constitute a movable electrode or mirror that is separated from the stationary electrode by a gap. The movable reflective structure can be configured to move across the gap towards the stationary electrode by electrostatic force. The movable reflective structure can be part of a pixel of the MEMS display device. Pixels of the MEMS display device can be arranged as an array to form a display. A gap distance between the movable reflective structure and the stationary electrode can reflect a certain wavelength of light to give the appearance of a particular color.
In some implementations, forming the movable reflective structure can include depositing the first layer over the substrate, depositing the third layer on the first layer, and depositing the second layer on the third layer. The movable reflective structure can form a thin film stack having a transition metal layer sandwiched between two aluminum or aluminum alloy layers. Each of the deposition steps may be subsequently followed by masking, patterning, etching, or planarization steps. Each of the layers can be deposited using deposition techniques known in the art, such as PVD, CVD, PECVD, ALD, and spin-coating. PVD processes can include pulsed laser deposition, sputter deposition, electron beam physical vapor deposition, and evaporative deposition. In some implementations, the movable reflective structure can further include an optical layer of titanium oxide, where the first layer is formed over the optical layer.
In some implementations, the first layer and the second layer of the movable reflective structure can be substantially identical in composition and thickness. The composition of the first layer and the second layer can include aluminum, such as pure aluminum, aluminum-scandium, aluminum-zirconium, or aluminum-copper. The thickness of the first layer and the second layer can each be equal to or greater than about 20 nm, such as between about 20 nm and about 400 nm. The thickness of the third layer can be less than about 5 nm. In some implementations, the thickness of the third layer can be about 10% or less than a total thickness of the movable reflective structure.
In some implementations, depositing the first layer can include doping the first layer with one or both of oxygen and nitrogen, and depositing the second layer can include doping the second layer with one or both of oxygen and nitrogen. Doping the first and second layers with oxygen and/or nitrogen can include flowing oxygen and/or nitrogen gas during reactive sputtering of aluminum or aluminum alloy. In some implementations, the flow rate of oxygen gas can be less than about 5 sccm and the flow rate of nitrogen gas can be less than about 10 sccm.
In some implementations, the process 900 further includes depositing a fourth layer between the substrate and the first layer, where the fourth layer is substantially identical in composition and thickness with the third layer. Therefore, another transition metal layer can be incorporated in the thin film stack of the movable reflective structure. In some implementations, the fourth layer can provide a more balanced structure in terms of stress.
In some implementations, the process 900 can further include depositing a fifth layer between the substrate and the fourth layer, where the fifth layer is substantially identical in thickness and composition with the first layer and the second layer. The fifth layer can include aluminum or aluminum alloy, and can have a thickness equal to or greater than about 20 nm. In some implementations, the fifth layer can provide a more balanced structure in terms of stress.
At block 940, the movable reflective structure is annealed. By annealing the movable reflective structure, the first layer and the second layer can be doped with the transition metal. Upon annealing, the transition metal can diffuse from both sides so that the first layer can be doped and the second layer can be doped with the transition metal. In some implementations, the first and the second layers can each be doped with about 0.1 atomic % to about 10 atomic % of the transition metal, or with about 0.5 atomic % to about 5 atomic % of the transition metal. In some implementations, the temperature during anneal can be between about 100° C. and about 600° C. It may be desirable to control the stress so that the stress of the movable reflective structure under as-deposited conditions and after annealing may be relatively similar. That way, the microstructure of the movable reflective structure can be thermally stable.
Such an annealed thin film stack in the movable reflective structure can reduce stress in a creep-resistant movable reflective structure, where the stress can be less than about 200 MPa or less than about 100 MPa. The annealed thin film stack can strengthen the mechanical robustness of the movable reflective structure without considerably degrading the optical and electrical properties of the movable reflective structure. For example, the sheet resistance of the movable reflective structure can be less than about 1 or less than about 10 ohms per square and the reflectance of the movable reflective structure can be greater than about 80%.
The display device 40 includes a housing 41, a display 30, an antenna 43, a speaker 45, an input device 48 and a microphone 46. The housing 41 can be formed from any of a variety of manufacturing processes, including injection molding, and vacuum forming. In addition, the housing 41 may be made from any of a variety of materials, including, but not limited to: plastic, metal, glass, rubber and ceramic, or a combination thereof. The housing 41 can include removable portions (not shown) that may be interchanged with other removable portions of different color, or containing different logos, pictures, or symbols.
The display 30 may be any of a variety of displays, including a bi-stable or analog display, as described herein. The display 30 also can be configured to include a flat-panel display, such as plasma, EL, OLED, STN LCD, or TFT LCD, or a non-flat-panel display, such as a CRT or other tube device. In addition, the display 30 can include an IMOD-based display, as described herein.
The components of the display device 40 are schematically illustrated in
The network interface 27 includes the antenna 43 and the transceiver 47 so that the display device 40 can communicate with one or more devices over a network. The network interface 27 also may have some processing capabilities to relieve, for example, data processing requirements of the processor 21. The antenna 43 can transmit and receive signals. In some implementations, the antenna 43 transmits and receives RF signals according to the IEEE 16.11 standard, including IEEE 16.11(a), (b), or (g), or the IEEE 802.11 standard, including IEEE 802.11a, b, g, n, and further implementations thereof. In some other implementations, the antenna 43 transmits and receives RF signals according to the Bluetooth® standard. In the case of a cellular telephone, the antenna 43 can be designed to receive code division multiple access (CDMA), frequency division multiple access (FDMA), time division multiple access (TDMA), Global System for Mobile communications (GSM), GSM/General Packet Radio Service (GPRS), Enhanced Data GSM Environment (EDGE), Terrestrial Trunked Radio (TETRA), Wideband-CDMA (W-CDMA), Evolution Data Optimized (EV-DO), 1×EV-DO, EV-DO Rev A, EV-DO Rev B, High Speed Packet Access (HSPA), High Speed Downlink Packet Access (HSDPA), High Speed Uplink Packet Access (HSUPA), Evolved High Speed Packet Access (HSPA+), Long Term Evolution (LTE), AMPS, or other known signals that are used to communicate within a wireless network, such as a system utilizing 3G, 4G or 5G technology. The transceiver 47 can pre-process the signals received from the antenna 43 so that they may be received by and further manipulated by the processor 21. The transceiver 47 also can process signals received from the processor 21 so that they may be transmitted from the display device 40 via the antenna 43.
In some implementations, the transceiver 47 can be replaced by a receiver. In addition, in some implementations, the network interface 27 can be replaced by an image source, which can store or generate image data to be sent to the processor 21. The processor 21 can control the overall operation of the display device 40. The processor 21 receives data, such as compressed image data from the network interface 27 or an image source, and processes the data into raw image data or into a format that can be readily processed into raw image data. The processor 21 can send the processed data to the driver controller 29 or to the frame buffer 28 for storage. Raw data typically refers to the information that identifies the image characteristics at each location within an image. For example, such image characteristics can include color, saturation and gray-scale level.
The processor 21 can include a microcontroller, CPU, or logic unit to control operation of the display device 40. The conditioning hardware 52 may include amplifiers and filters for transmitting signals to the speaker 45, and for receiving signals from the microphone 46. The conditioning hardware 52 may be discrete components within the display device 40, or may be incorporated within the processor 21 or other components.
The driver controller 29 can take the raw image data generated by the processor 21 either directly from the processor 21 or from the frame buffer 28 and can re-format the raw image data appropriately for high speed transmission to the array driver 22. In some implementations, the driver controller 29 can re-format the raw image data into a data flow having a raster-like format, such that it has a time order suitable for scanning across the display array 30. Then the driver controller 29 sends the formatted information to the array driver 22. Although a driver controller 29, such as an LCD controller, is often associated with the system processor 21 as a stand-alone Integrated Circuit (IC), such controllers may be implemented in many ways. For example, controllers may be embedded in the processor 21 as hardware, embedded in the processor 21 as software, or fully integrated in hardware with the array driver 22.
The array driver 22 can receive the formatted information from the driver controller 29 and can re-format the video data into a parallel set of waveforms that are applied many times per second to the hundreds, and sometimes thousands (or more), of leads coming from the display's x-y matrix of display elements.
In some implementations, the driver controller 29, the array driver 22, and the display array 30 are appropriate for any of the types of displays described herein. For example, the driver controller 29 can be a conventional display controller or an active matrix display controller (such as an IMOD display element controller). Additionally, the array driver 22 can be a conventional driver or an active matrix display driver (such as an IMOD display element driver). Moreover, the display array 30 can be a conventional display array or a MEMS display array (such as a display including an array of IMOD display elements). In some implementations, the driver controller 29 can be integrated with the array driver 22. Such an implementation can be useful in highly integrated systems, for example, mobile phones, portable-electronic devices, watches or small-area displays.
In some implementations, the input device 48 can be configured to allow, for example, a user to control the operation of the display device 40. The input device 48 can include a keypad, such as a QWERTY keyboard or a telephone keypad, a button, a switch, a rocker, a touch-sensitive screen, a touch-sensitive screen integrated with the display array 30, or a pressure- or heat-sensitive membrane. The microphone 46 can be configured as an input device for the display device 40. In some implementations, voice commands through the microphone 46 can be used for controlling operations of the display device 40.
The power supply 50 can include a variety of energy storage devices. For example, the power supply 50 can be a rechargeable battery, such as a nickel-cadmium battery or a lithium-ion battery. In implementations using a rechargeable battery, the rechargeable battery may be chargeable using power coming from, for example, a wall socket or a photovoltaic device or array. Alternatively, the rechargeable battery can be wirelessly chargeable. The power supply 50 also can be a renewable energy source, a capacitor, or a solar cell, including a plastic solar cell or solar-cell paint. The power supply 50 also can be configured to receive power from a wall outlet.
In some implementations, control programmability resides in the driver controller 29 which can be located in several places in the electronic display system. In some other implementations, control programmability resides in the array driver 22. The above-described optimization may be implemented in any number of hardware and/or software components and in various configurations.
As used herein, a phrase referring to “at least one of” a list of items refers to any combination of those items, including single members. As an example, “at least one of: a, b, or c” is intended to cover: a, b, c, a-b, a-c, b-c, and a-b-c.
The various illustrative logics, logical blocks, modules, circuits and algorithm steps described in connection with the implementations disclosed herein may be implemented as electronic hardware, computer software, or combinations of both. The interchangeability of hardware and software has been described generally, in terms of functionality, and illustrated in the various illustrative components, blocks, modules, circuits and steps described above. Whether such functionality is implemented in hardware or software depends upon the particular application and design constraints imposed on the overall system.
The hardware and data processing apparatus used to implement the various illustrative logics, logical blocks, modules and circuits described in connection with the aspects disclosed herein may be implemented or performed with a general purpose single- or multi-chip processor, a digital signal processor (DSP), an application specific integrated circuit (ASIC), a field programmable gate array (FPGA) or other programmable logic device, discrete gate or transistor logic, discrete hardware components, or any combination thereof designed to perform the functions described herein. A general purpose processor may be a microprocessor, or, any conventional processor, controller, microcontroller, or state machine. A processor also may be implemented as a combination of computing devices, such as a combination of a DSP and a microprocessor, a plurality of microprocessors, one or more microprocessors in conjunction with a DSP core, or any other such configuration. In some implementations, particular steps and methods may be performed by circuitry that is specific to a given function.
In one or more aspects, the functions described may be implemented in hardware, digital electronic circuitry, computer software, firmware, including the structures disclosed in this specification and their structural equivalents thereof, or in any combination thereof. Implementations of the subject matter described in this specification also can be implemented as one or more computer programs, i.e., one or more modules of computer program instructions, encoded on a computer storage media for execution by, or to control the operation of, data processing apparatus.
Various modifications to the implementations described in this disclosure may be readily apparent to those skilled in the art, and the generic principles defined herein may be applied to other implementations without departing from the spirit or scope of this disclosure. Thus, the claims are not intended to be limited to the implementations shown herein, but are to be accorded the widest scope consistent with this disclosure, the principles and the novel features disclosed herein. Additionally, a person having ordinary skill in the art will readily appreciate, the terms “upper” and “lower” are sometimes used for ease of describing the figures, and indicate relative positions corresponding to the orientation of the figure on a properly oriented page, and may not reflect the proper orientation of, e.g., an IMOD display element as implemented.
Certain features that are described in this specification in the context of separate implementations also can be implemented in combination in a single implementation. Conversely, various features that are described in the context of a single implementation also can be implemented in multiple implementations separately or in any suitable subcombination. Moreover, although features may be described above as acting in certain combinations and even initially claimed as such, one or more features from a claimed combination can in some cases be excised from the combination, and the claimed combination may be directed to a subcombination or variation of a subcombination.
Similarly, while operations are depicted in the drawings in a particular order, a person having ordinary skill in the art will readily recognize that such operations need not be performed in the particular order shown or in sequential order, or that all illustrated operations be performed, to achieve desirable results. Further, the drawings may schematically depict one more example processes in the form of a flow diagram. However, other operations that are not depicted can be incorporated in the example processes that are schematically illustrated. For example, one or more additional operations can be performed before, after, simultaneously, or between any of the illustrated operations. In certain circumstances, multitasking and parallel processing may be advantageous. Moreover, the separation of various system components in the implementations described above should not be understood as requiring such separation in all implementations, and it should be understood that the described program components and systems can generally be integrated together in a single software product or packaged into multiple software products. Additionally, other implementations are within the scope of the following claims. In some cases, the actions recited in the claims can be performed in a different order and still achieve desirable results.
