Microelectromechanical device with spacing layer

An interferometric modulating device is provided with a spacing layer positioned between the fixed reflector and the electrode. The spacing layer prevents shorting between the movable reflector and the electrode and provides a filtering cavity to improve color saturation.

BACKGROUND

Microelectromechanical systems (MEMS) include micro mechanical elements, actuators, and electronics. Micromechanical elements may be created using deposition, etching, 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 MEMS device is called an interferometric modulator. As used herein, the term interferometric modulator or interferometric light modulator refers to a device that selectively absorbs and/or reflects light using the principles of optical interference. In certain embodiments, an interferometric modulator may comprise a pair of conductive plates, one or both of which may be transparent and/or reflective in whole or part and capable of relative motion upon application of an appropriate electrical signal. In a particular embodiment, one plate may comprise a stationary layer deposited on a substrate and the other plate may comprise a metallic membrane separated from the stationary layer by an air gap. As described herein in more detail, the position of one plate in relation to another can change the optical interference of light incident on the interferometric modulator. Such devices have a wide range of applications, and it would be beneficial in the art to utilize and/or modify the characteristics of these types of devices so that their features can be exploited in improving existing products and creating new products that have not yet been developed.

SUMMARY

In certain embodiments, a device comprises an electrode, a fixed reflector, a movable reflector comprising an electrically conductive material, wherein an interferometric modulating cavity is defined between the movable reflector and the fixed reflector, the movable reflector being movable between at least a first position, a second position, and a third position. The device also comprises a spacing layer positioned between the fixed reflector and the electrode, the spacing layer providing a filtering cavity.

In certain embodiments, a device comprises an electrode, a fixed reflector, a movable reflector comprising an electrically conductive material, wherein an interferometric modulating cavity is defined between the movable reflector and the fixed reflector, the movable reflector being movable between at least a first position, a second position, and a third position. The device also comprises a spacing layer positioned between the fixed reflector and the electrode, the spacing laying being greater than or equal to 160 nm in thickness from a cross-sectional view of the device.

In certain embodiments, a device comprises an electrode, a first, fixed means for reflecting light, a second, movable means for reflecting light. The second reflecting means comprises an electrically conductive material, wherein an interferometric modulating cavity is defined between the first reflecting means and the second reflecting means. The second reflecting means is movable between at least a first position, a second position, and a third position. The device also comprises means for separating the first reflecting means and the electrode, the separating means providing a filtering cavity.

In certain embodiments, a method of making a device for modulating light comprises forming an electrode, forming a spacing layer, forming a fixed reflector, and forming a movable reflector comprising an electrically conductive material. An interferometric modulating cavity is defined between the movable reflector and the fixed reflector. The movable reflector is movable between at least a first position, a second position, and a third position. The spacing layer provides a filtering cavity.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The following detailed description is directed to certain specific embodiments of the invention. However, the invention can be embodied in a multitude of different ways. In this description, reference is made to the drawings wherein like parts are designated with like numerals throughout. As will be apparent from the following description, the embodiments may be implemented in any device that is configured to display an image, whether in motion (e.g., video) or stationary (e.g., still image), and whether textual or pictorial. More particularly, it is contemplated that the embodiments may be implemented in or associated with a variety of electronic devices such as, but not limited to, mobile telephones, wireless devices, personal data assistants (PDAs), hand-held or portable computers, GPS receivers/navigators, cameras, MP3 players, camcorders, game consoles, wrist watches, clocks, calculators, television monitors, flat panel displays, computer monitors, auto displays (e.g., odometer display, etc.), cockpit controls and/or displays, display of camera views (e.g., display of a rear view camera in a vehicle), electronic photographs, electronic billboards or signs, projectors, architectural structures, packaging, and aesthetic structures (e.g., display of images on a piece of jewelry). MEMS devices of similar structure to those described herein can also be used in non-display applications such as in electronic switching devices.

One embodiment comprises a display adopting tri-state interferometric modulating devices that have a high contrast ratio (CR) and a large gamut. In such a tri-state interferometric modulating device, a spacing layer provides better protection against shorting between the movable reflector and the electrodes. In one embodiment, the spacing layer defines a second interferometric modulating cavity that is found to provide saturated light over a range of light frequencies. As a result, the gamut of the display is enhanced.

One interferometric modulator display embodiment comprising an interferometric MEMS display element is illustrated inFIG. 1. In these devices, the pixels are in either a bright or dark state. In the bright (“on” or “open”) state, the display element reflects a large portion of incident visible light to a user. When in the dark (“off” or “closed”) state, the display element reflects little incident visible light to the user. Depending on the embodiment, the light reflectance properties of the “on” and “off” states may be reversed. MEMS pixels can be configured to reflect predominantly at selected colors, allowing for a color display in addition to black and white.

The depicted portion of the pixel array inFIG. 1includes two adjacent interferometric modulators12aand12b(collectively referred to as interferometric modulators12). In the interferometric modulator12aon the left, a movable reflective layer14ais illustrated in a relaxed position at a predetermined distance from an optical stack16a, which includes a partially reflective layer. In the interferometric modulator12bon the right, the movable reflective layer14bis illustrated in an actuated position adjacent to the optical stack16b.

The optical stacks16aand16b(collectively referred to as optical stack16), as referenced herein, typically comprise several fused layers, which can include an electrode layer, such as indium tin oxide (ITO), a partially reflective layer, such as chromium, and a transparent dielectric. The optical stack16is thus 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 substrate20. The partially reflective layer can be formed from a variety of materials that are partially reflective such as various metals, 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 embodiments, the layers of the optical stack16are patterned into parallel strips, and may form row electrodes in a display device as described further below. The movable reflective layers14a,14bmay be formed as a series of parallel strips of a deposited metal layer or layers (orthogonal to the row electrodes of16a,16b) deposited on top of posts18and an intervening sacrificial material deposited between the posts18. When the sacrificial material is etched away, the movable reflective layers14a,14bare separated from the optical stacks16a,16bby a defined gap19. A highly conductive and reflective material such as aluminum may be used for the reflective layers14, and these strips may form column electrodes in a display device.

With no applied voltage, the gap19remains between the movable reflective layer14aand optical stack16a, with the movable reflective layer14ain a mechanically relaxed state, as illustrated by the pixel12ainFIG. 1. However, when a potential difference is applied to a selected row and column, the capacitor formed at the intersection of the row and column electrodes at the corresponding pixel becomes charged, and electrostatic forces pull the electrodes together. If the voltage is high enough, the movable reflective layer14is deformed and is forced against the optical stack16. A dielectric layer (not illustrated in this Figure) within the optical stack16may prevent shorting and control the separation distance between layers14and16, as illustrated by pixel12bon the right inFIG. 1. The behavior is the same regardless of the polarity of the applied potential difference. In this way, row/column actuation that can control the reflective vs. non-reflective pixel states is analogous in many ways to that used in conventional LCD and other display technologies.

FIGS. 2 through 5Billustrate one exemplary process and system for using an array of interferometric modulators in a display application.

FIG. 2is a system block diagram illustrating one embodiment of an electronic device that may incorporate aspects of the invention. In the exemplary embodiment, the electronic device includes a processor21which may be any general purpose single- or multi-chip microprocessor such as an ARM, Pentium®, Pentium II®, Pentium III®, Pentium IV®, Pentium® Pro, an 8051, a MIPS®, a Power PC®, an ALPHA®, or any special purpose microprocessor such as a digital signal processor, microcontroller, or a programmable gate array. As is conventional in the art, the processor21may be configured to execute one or more software modules. In addition to executing an operating system, the processor may be configured to execute one or more software applications, including a web browser, a telephone application, an email program, or any other software application.

In typical applications, a display frame may be created by asserting the set of column electrodes in accordance with the desired set of actuated pixels in the first row. A row pulse is then applied to the row1electrode, actuating the pixels corresponding to the asserted column lines. The asserted set of column electrodes is then changed to correspond to the desired set of actuated pixels in the second row. A pulse is then applied to the row2electrode, actuating the appropriate pixels in row2in accordance with the asserted column electrodes. The row1pixels are unaffected by the row2pulse, and remain in the state they were set to during the row1pulse. This may be repeated for the entire series of rows in a sequential fashion to produce the frame. Generally, the frames are refreshed and/or updated with new display data by continually repeating this process at some desired number of frames per second. A wide variety of protocols for driving row and column electrodes of pixel arrays to produce display frames are also well known and may be used in conjunction with the present invention.

FIGS. 4,5A, and5B illustrate one possible actuation protocol for creating a display frame on the 3×3 array ofFIG. 2.FIG. 4illustrates a possible set of column and row voltage levels that may be used for pixels exhibiting the hysteresis curves ofFIG. 3. In theFIG. 4embodiment, actuating a pixel involves setting the appropriate column to −Vbias, and the appropriate row to +ΔV, which may correspond to −5 volts and +5 volts, respectively. Relaxing the pixel is accomplished by setting the appropriate column to +Vbias, and the appropriate row to the same +ΔV, producing a zero volt potential difference across the pixel. In those rows where the row voltage is held at zero volts, the pixels are stable in whatever state they were originally in, regardless of whether the column is at +Vbias, or −Vbias. As is also illustrated inFIG. 4, it will be appreciated that voltages of opposite polarity than those described above can be used, e.g., actuating a pixel can involve setting the appropriate column to +Vbias, and the appropriate row to −ΔV. In this embodiment, releasing the pixel is accomplished by setting the appropriate column to −Vbias, and the appropriate row to the same −ΔV, producing a zero volt potential difference across the pixel.

In theFIG. 5Aframe, pixels (1,1), (1,2), (2,2), (3,2) and (3,3) are actuated. To accomplish this, during a “line time” for row1, columns1and2are set to −5 volts, and column3is set to +5 volts. This does not change the state of any pixels, because all the pixels remain in the 3-7 volt stability window. Row1is then strobed with a pulse that goes from 0, up to 5 volts, and back to zero. This actuates the (1,1) and (1,2) pixels and relaxes the (1,3) pixel. No other pixels in the array are affected. To set row2as desired, column2is set to −5 volts, and columns1and3are set to +5 volts. The same strobe applied to row2will then actuate pixel (2,2) and relax pixels (2,1) and (2,3). Again, no other pixels of the array are affected. Row3is similarly set by setting columns2and3to −5 volts, and column1to +5 volts. The row3strobe sets the row3pixels as shown inFIG. 5A. After writing the frame, the row potentials are zero, and the column potentials can remain at either +5 or −5 volts, and the display is then stable in the arrangement ofFIG. 5A. It will be appreciated that the same procedure can be employed for arrays of dozens or hundreds of rows and columns. It will also be appreciated that the timing, sequence, and levels of voltages used to perform row and column actuation can be varied widely within the general principles outlined above, and the above example is exemplary only, and any actuation voltage method can be used with the systems and methods described herein.

FIGS. 6A and 6Bare system block diagrams illustrating an embodiment of a display device40. The display device40can be, for example, a cellular or mobile telephone. However, the same components of display device40or slight variations thereof are also illustrative of various types of display devices such as televisions and portable media players.

The display device40includes a housing41, a display30, an antenna43, a speaker45, an input device48, and a microphone46. The housing41is generally formed from any of a variety of manufacturing processes as are well known to those of skill in the art, including injection molding and vacuum forming. In addition, the housing41may be made from any of a variety of materials, including, but not limited to, plastic, metal, glass, rubber, and ceramic, or a combination thereof. In one embodiment, the housing41includes removable portions (not shown) that may be interchanged with other removable portions of different color, or containing different logos, pictures, or symbols.

The display30of exemplary display device40may be any of a variety of displays, including a bi-stable display, as described herein. In other embodiments, the display30includes a flat-panel display, such as plasma, EL, OLED, STN LCD, or TFT LCD as described above, or a non-flat-panel display, such as a CRT or other tube device, as is well known to those of skill in the art. However, for purposes of describing the present embodiment, the display30includes an interferometric modulator display, as described herein.

The components of one embodiment of exemplary display device40are schematically illustrated inFIG. 6B. The illustrated exemplary display device40includes a housing41and can include additional components at least partially enclosed therein. For example, in one embodiment, the exemplary display device40includes a network interface27that includes an antenna43, which is coupled to a transceiver47. The transceiver47is connected to a processor21, which is connected to conditioning hardware52. The conditioning hardware52may be configured to condition a signal (e.g., filter a signal). The conditioning hardware52is connected to a speaker45and a microphone46. The processor21is also connected to an input device48and a driver controller29. The driver controller29is coupled to a frame buffer28and to an array driver22, which in turn is coupled to a display array30. A power supply50provides power to all components as required by the particular exemplary display device40design.

The network interface27includes the antenna43and the transceiver47so that the exemplary display device40can communicate with one or more devices over a network. In one embodiment, the network interface27may also have some processing capabilities to relieve requirements of the processor21. The antenna43is any antenna known to those of skill in the art for transmitting and receiving signals. In one embodiment, the antenna transmits and receives RF signals according to the IEEE 802.11 standard, including IEEE 802.11(a), (b), or (g). In another embodiment, the antenna transmits and receives RF signals according to the BLUETOOTH standard. In the case of a cellular telephone, the antenna is designed to receive CDMA, GSM, AMPS, or other known signals that are used to communicate within a wireless cell phone network. The transceiver47pre-processes the signals received from the antenna43so that they may be received by and further manipulated by the processor21. The transceiver47also processes signals received from the processor21so that they may be transmitted from the exemplary display device40via the antenna43.

In an alternative embodiment, the transceiver47can be replaced by a receiver. In yet another alternative embodiment, network interface27can be replaced by an image source, which can store or generate image data to be sent to the processor21. For example, the image source can be a digital video disc (DVD) or a hard-disc drive that contains image data, or a software module that generates image data.

Processor21generally controls the overall operation of the exemplary display device40. The processor21receives data, such as compressed image data from the network interface27or an image source, and processes the data into raw image data or into a format that is readily processed into raw image data. The processor21then sends the processed data to the driver controller29or to frame buffer28for 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.

In one embodiment, the processor21includes a microcontroller, CPU, or logic unit to control operation of the exemplary display device40. Conditioning hardware52generally includes amplifiers and filters for transmitting signals to the speaker45, and for receiving signals from the microphone46. Conditioning hardware52may be discrete components within the exemplary display device40, or may be incorporated within the processor21or other components.

The driver controller29takes the raw image data generated by the processor21either directly from the processor21or from the frame buffer28and reformats the raw image data appropriately for high speed transmission to the array driver22. Specifically, the driver controller29reformats 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 array30. Then the driver controller29sends the formatted information to the array driver22. Although a driver controller29, such as a LCD controller, is often associated with the system processor21as a stand-alone Integrated Circuit (IC), such controllers may be implemented in many ways. They may be embedded in the processor21as hardware, embedded in the processor21as software, or fully integrated in hardware with the array driver22.

Typically, the array driver22receives the formatted information from the driver controller29and reformats the video data into a parallel set of waveforms that are applied many times per second to the hundreds and sometimes thousands of leads coming from the display's x-y matrix of pixels.

In one embodiment, the driver controller29, array driver22, and display array30are appropriate for any of the types of displays described herein. For example, in one embodiment, driver controller29is a conventional display controller or a bi-stable display controller (e.g., an interferometric modulator controller). In another embodiment, array driver22is a conventional driver or a bi-stable display driver (e.g., an interferometric modulator display). In one embodiment, a driver controller29is integrated with the array driver22. Such an embodiment is common in highly integrated systems such as cellular phones, watches, and other small area displays. In yet another embodiment, display array30is a typical display array or a bi-stable display array (e.g., a display including an array of interferometric modulators).

The input device48allows a user to control the operation of the exemplary display device40. In one embodiment, input device48includes a keypad, such as a QWERTY keyboard or a telephone keypad, a button, a switch, a touch-sensitive screen, or a pressure- or heat-sensitive membrane. In one embodiment, the microphone46is an input device for the exemplary display device40. When the microphone46is used to input data to the device, voice commands may be provided by a user for controlling operations of the exemplary display device40.

Power supply50can include a variety of energy storage devices as are well known in the art. For example, in one embodiment, power supply50is a rechargeable battery, such as a nickel-cadmium battery or a lithium ion battery. In another embodiment, power supply50is a renewable energy source, a capacitor, or a solar cell including a plastic solar cell, and solar-cell paint. In another embodiment, power supply50is configured to receive power from a wall outlet.

In some embodiments, control programmability resides, as described above, in a driver controller which can be located in several places in the electronic display system. In some embodiments, control programmability resides in the array driver22. Those of skill in the art will recognize that the above-described optimizations may be implemented in any number of hardware and/or software components and in various configurations.

The details of the structure of interferometric modulators that operate in accordance with the principles set forth above may vary widely. For example,FIGS. 7A-7Eillustrate five different embodiments of the movable reflective layer14and its supporting structures.FIG. 7Ais a cross section of the embodiment ofFIG. 1, where a strip of metal material14is deposited on orthogonally extending supports18. InFIG. 7B, the moveable reflective layer14is attached to supports at the corners only, on tethers32. InFIG. 7C, the moveable reflective layer14is suspended from a deformable layer34, which may comprise a flexible metal. The deformable layer34connects, directly or indirectly, to the substrate20around the perimeter of the deformable layer34. These connections are herein referred to as support posts. The embodiment illustrated inFIG. 7Dhas support post plugs42upon which the deformable layer34rests. The movable reflective layer14remains suspended over the gap, as inFIGS. 7A-7C, but the deformable layer34does not form the support posts by filling holes between the deformable layer34and the optical stack16. Rather, the support posts are formed of a planarization material, which is used to form support post plugs42. The embodiment illustrated inFIG. 7Eis based on the embodiment shown inFIG. 7D, but may also be adapted to work with any of the embodiments illustrated inFIGS. 7A-7C, as well as additional embodiments not shown. In the embodiment shown inFIG. 7E, an extra layer of metal or other conductive material has been used to form a bus structure44. This allows signal routing along the back of the interferometric modulators, eliminating a number of electrodes that may otherwise have had to be formed on the substrate20.

In embodiments such as those shown inFIG. 7, the interferometric modulators function as direct-view devices, in which images are viewed from the front side of the transparent substrate20, the side opposite to that upon which the modulator is arranged. In these embodiments, the reflective layer14optically shields the portions of the interferometric modulator on the side of the reflective layer opposite the substrate20, including the deformable layer34. This allows the shielded areas to be configured and operated upon without negatively affecting the image quality. Such shielding allows the bus structure44inFIG. 7E, which provides the ability to separate the optical properties of the modulator from the electromechanical properties of the modulator, such as addressing and the movements that result from that addressing. This separable modulator architecture allows the structural design and materials used for the electromechanical aspects and the optical aspects of the modulator to be selected and to function independently of each other. Moreover, the embodiments shown inFIGS. 7C-7Ehave additional benefits deriving from the decoupling of the optical properties of the reflective layer14from its mechanical properties, which are carried out by the deformable layer34. This allows the structural design and materials used for the reflective layer14to be optimized with respect to the optical properties, and the structural design and materials used for the deformable layer34to be optimized with respect to desired mechanical properties.

A common problem for all color displays, regardless of whether they are of the self-luminous type or the non-self-luminous type, is the synthesis of a full-color image from a limited set of primary colors. Several approaches to color synthesis have traditionally been employed for electronic displays. The most successful of these conform to the principles of additive color mixture and include optical superposition, spatial color synthesis, and temporal color synthesis.

Direct optical superposition of three primary color images is an effective and commonly used method in projection display systems, but is not readily amenable to most direct-view color display technologies. Spatial color synthesis has by far been the most successful method of color synthesis and remains the foundation of modern color display technology in devices like cathode ray tubes (CRT) and liquid crystal displays (LCD). Spatial color synthesis mixes sub-pixels of three or more primary colors (typically red (R), green (G) and blue (B)) in close proximity to generate a full spectrum.

An exemplary color display includes red, green, and blue display elements. Other colors are produced in such a display by varying the relative intensity of light produced by the red, green, and blue elements. Such mixtures of primary colors such as red, green, and blue are perceived by the human eye as other colors. The relative values of red, green, and blue in such a color system may be referred to as tristimulus values in reference to the stimulation of red, green, and blue light-sensitive portions of the human eye. The range of colors that can be produced by a particular display may be referred to as the color gamut of the display. In general, increasing the saturation of the primary colors increases the color gamut, or range of colors that can be produced by the display. While an exemplary color system based on red, green, and blue are disclosed herein, in other embodiments, the display may include modulators12(seeFIG. 8) having sets of colors that define other color systems in terms of sets of primary colors other than red, green, and blue.

In one embodiment of the display, each pixel includes one or more color modulators12, e.g., modulators configured to reflect red, green, and blue light, and one or more “white” modulators12configured to reflect white light. In such an embodiment, light from the red, green, and/or blue modulators12in their reflective states combines to output colored light. Light from the white modulators12can be used to output white light. Use of white in combination with color may increase the brightness or intensity of the pixels.

FIG. 8is a side cross-sectional view of an exemplary multi-state interferometric modulator12. The multi-state interferometric modulator12reflects light having particular spectral characteristics by positioning the movable reflective layer (or reflector)14to one of a plurality of selected positions81-85. As discussed above, a potential difference between a row and column electrode causes the movable reflective layer14to deflect. The exemplary modulator includes a conductive layer102of indium-tin-oxide (ITO) acting as a column electrode. In the exemplary modulator, the reflective layer14comprises an electronically conductive material which forms the row conductor.

In one embodiment, a dielectric layer104of a material such as silicon dioxide (SiO2) is positioned over a layer of molybdenum-chromium (MoCr) that forms a reflective surface of the optical stack (or the fixed reflector)16. As discussed above with reference toFIG. 1, the dielectric layer104prevents shorting and controls the separation distance between the movable reflector14and the fixed reflector16when the movable reflector14deflects. The optical cavity formed between the movable reflector14and the fixed reflector16thus includes the dielectric layer104. The relative sizes of items inFIG. 8have been selected for purposes of conveniently illustrating the modulator12. Thus, such distances are not to scale and are not intended to be representative of any particular embodiment of the modulator12.

As discussed above, the modulator12includes an interferometric modulating cavity formed between the movable reflector14and the fixed reflector16. The characteristic distance, or effective optical path length, L, of the optical cavity determines the resonant wavelengths, λ, of the optical cavity and thus of the interferometric modulator12. The resonant wavelength, λ, of the interferometric modulator12generally corresponds to the perceived color of light reflected by the modulator12. Mathematically, the distance L=½N λ, where N is an integer. A given resonant wavelength, λ, is thus reflected by interferometric modulators12having distances L of ½λ(N=1), λ(N=2), 3/2λ(N=3), etc. The integer N may be referred to as the order of interference of the reflected light. As used herein, the order of a modulator12also refers to the order N of light reflected by the modulator12when the movable reflector14is in at least one position. For example, a first order red interferometric modulator12may have a distance L of about 325 nm, corresponding to a wavelength λ of about 650 nm. Accordingly, a second order red interferometric modulator12may have a distance L of about 650 nm.

A list of examples of wavelength ranges for some common colors used in interferometric modulator displays are shown in the following table.

When the cavity19comprises a fluid having an index of refraction of approximately 1 (e.g., air), the effective optical path length, L, is substantially equal to the distance between the movable reflector14and the fixed reflector16. When the cavity19comprises a fluid having an index of refraction of greater than 1, the effective optical path length, L, may be different from the distance between the movable reflector14and the fixed reflector16.

In embodiments that include the dielectric layer104, which has an index of refraction greater than one, the interferometric modulating cavity is formed to have the desired optical path length by selecting the distance between the movable reflector14and fixed reflector16and by selecting the thickness and index of refraction of the dielectric layer104, or of any other layers between the movable reflector14and fixed reflector16.

In one embodiment, the movable reflector14may be deflected to one or more positions within a range of positions to output light of a corresponding range of colors. For example, the voltage potential difference between the row and column electrodes may be adjusted to deflect the movable reflector14to one of a plurality of selected positions in relation to the fixed reflector16.

Each of a particular group of positions81-85of the movable reflector14is denoted inFIG. 8by a line extending from the fixed reflector16to an arrow point indicating the positions81-85. Thus, the distances81-85are selected so as to account for the thickness and index of refraction of the dielectric layer104. When the movable reflector14is deflected to each of the positions81-85, each corresponding to a different distance L from the fixed reflector16, the modulator reflects light to a viewing position at the substrate20side with a different spectral response that corresponds to different colors of incident light being reflected by the modulator12.

Moreover, at position81, the movable reflector14is sufficiently close to the fixed reflector16, that the effects of interference are negligible and modulator12acts as a mirror that reflects substantially all colors of incident visible light substantially equally, e.g., as white light. The broadband mirror effect is caused because the small distance L is too small for optical resonance in the visible band. The reflective layer14thus merely acts as a reflective surface with respect to visible light.

At the position82, the distance L is such that the cavity operates interferometrically but reflects substantially no visible wavelengths of light because the resonant wavelength is outside the visible range.

As the distance L is increased further, a peak spectral response of the modulator12moves into visible wavelengths. Thus, when the movable reflector14is at position83, the modulator12reflects blue light. When the movable reflector14is at the position84, the modulator12reflects green light. When the movable reflector14is at the non-deflected position85, the modulator12reflects red light.

As noted above, having a separate state for outputting white light in a modulator140decouples the selection of the properties of the modulator controlling color saturation from the properties affecting the brightness of white output. The distance and other characteristics of the modulator12may thus be selected to provide a highly saturated color without affecting the white light produced in the first state. For example, in an exemplary color display, one or more of the red, green, and blue modulators12may be formed with optical path lengths L corresponding to a higher order of interference.

As discussed above, the dielectric layer104prevents shorting and controls the separation distance between the movable reflector14and the fixed reflector16when the movable reflector14deflects, but the thickness of the dielectric layer104should be sufficiently small so that the movable reflector14is sufficiently close to the fixed reflector16, for example, at position81, when modulator12reflects white light. However, the use of thin dielectric layer104alone does not provide adequate protection against shorting between the movable reflector14and the conductive layer102.

The modulator12may be formed using lithographic techniques known in the art, and such as described above with reference to the modulator12. For example, conductive layer102may be formed by depositing one or more layers of a transparent conductor such as ITO onto the substrate20. The substrate20may comprise any transparent material such as glass or plastic. The substrate20may have been subjected to prior preparation step(s), e.g., cleaning, to facilitate efficient formation of a subsequently formed layer. The conductive layers102are patterned into parallel strips, and may form columns of electrodes. The fixed reflector16may be formed by depositing one or more layers of MoCr onto the substantially transparent substrate20and/or the substrate20. The movable reflector14may be formed as a series of parallel strips of a deposited metal layer or layers (orthogonal to the column electrodes102) deposited on top of posts18and an intervening sacrificial material deposited between the posts18. Vias through one or more of the layers described above may be provided so that etchant gas, such as xenon difluoride, can reach the sacrificial layers. When the sacrificial material is etched away, the deformable metal layers are separated from the fixed layers by an air gap. A highly conductive and reflective material such as aluminum may be used for the deformable layers, and these strips may form row electrodes in a display device.

FIG. 9is a side cross-sectional view of an exemplary multi-state interferometric modulator12with a spacing layer103. In the multi-state interferometric modulator12, a spacing layer103of a material such as silicon dioxide (SiO2) is positioned between the fixed reflector16and the conductive layer102. The spacing layer103increases the distance between the movable reflector14and the conductive layer102and therefore decreases the capacitance between the movable reflector14and the conductive layer102and provides adequate protection against shorting between the movable reflector14and the conductive layer102.

At the same time, the spacing layer103defines a second interferometric modulating cavity between the fixed reflector16and the conductive layer102. The thickness and composition (index of refraction) of the spacing layer103can be adjusted to vary the effective optical path length of the second interferometric modulating cavity to produce a desired color filter. For example, the spacing layer103may be adjusted so that the second interferometric modulating cavity acts as red, green or blue band pass filters as desired, and therefore the multi-state interferometric modulator12reflects saturated red, green or blue light.

The reflectance of the conductive layer102and the fixed reflector16is low, but the coupled cavity effect exhibited by the first interferometric modulating cavity and the second interferometric modulating cavities are sufficient to increase the color saturation of reflected light.

Saturation refers to the narrowness of the range of wavelengths of light output. A highly saturated hue has a vivid, intense color, while a less saturated hue appears more muted and grey. For example, a laser, which produces a very narrow range of wavelengths, produces highly saturated light. Conversely, a typical incandescent light bulb produces white light that may have a desaturated red or blue color.

In one embodiment of the display, each pixel includes one or more tri-state modulators12which have the structure as shown inFIG. 9or similar structures. These tri-state modulators include at least a blue modulator, a green modulator, and a red modulator.FIG. 10is a schematic side cross-sectional view of a pixel including three tri-state interferometric modulators that have respective spacing layers. InFIG. 10, like parts are numbered similarly with respect to previous figures.

The blue modulator has three states. In the first state, the movable reflector14is at a first position, for example, position81, and the blue modulator substantially reflects white light. In the second state, the movable reflector14is at a second position, for example, position82, and the blue modulator substantially reflects no light. In the third state, the movable reflector14is at a third position, for example, position83, and the blue modulator substantially reflects blue light.

The green modulator has three states. In the first state, the movable reflector14is at a first position, for example, position81, and the green modulator substantially reflects white light. In the second state, the movable reflector14is at a second position, for example, position82, and the green modulator substantially reflects no light. In the third state, the movable reflector14is at a third position, for example, position84, and the green modulator substantially reflects green light.

The red modulator has three states. In the first state, the movable reflector14is at a first position, for example, position81, and the red modulator substantially reflects white light. In the second state, the movable reflector14is at a second position, for example, position82, and the red modulator substantially reflects no light. In the third state, the movable reflector14is at a third position, for example, position85, and the red modulator substantially reflects red light.

In such an embodiment, light from the red, green, and/or blue modulators12in their third states combines to output colored light. Light from the red, green, and/or blue modulators12in their first and second states can be used to output white or black light. Use of white in combination with color may increase the brightness or intensity of the pixels.

In order to increase the saturation of the light from the red, green and blue modulators, the thickness and index of refraction of the spacing layer103is selected respectively for the red, green and blue modulators such that the second interferometric modulating cavities in the red, green and blue modulators have corresponding desired optical path lengths.

As described above, the spacing layer103can be of any material and of any thickness. For example, if the spacing layer103is of material SiO2, the thickness of the spacing layer103for the red modulator is at least 140 nm. In one embodiment, when the thickness of the spacing layer103for the red modulator is 170 nm, the red light from the red modulator is highly saturated.

In one embodiment, if the spacing layer103is of material SiO2, the thickness of the spacing layer103for the blue modulator is at least 210 nm. When the thickness of the spacing layer103for the blue modulator is 230 nm, the blue light from the blue modulator is highly saturated.

In one embodiment, if the spacing layer103is of material SiO2, the thickness of the spacing layer103for the green modulator is at least 190 nm. When the thickness of the spacing layer103for the green modulator is 220 nm, the green light from the green modulator is highly saturated.

The production of an interferometric modulator device incorporating a spacing layer103between the conductive layer102and the fix reflector16requires only a few additional process steps compared to the production of an interferometric modulator device without the spacing layer103. In the example illustrated inFIG. 9, incorporation of the spacing layer103requires only an additional step of depositing the spacing layer103. The additional processing requirements can be further reduced or minimized if the spacing layer103comprises the same material as the dielectric layer104and/or if the conductive layer102is made of the same material as the fixed reflector16.

FIG. 11shows an example of the modeled reflectance spectra for two red tri-state modulators. Line111depicts the modeled spectral reflectance of a red tri-state modulator12having a 100 nm thick spacing layer103of SiO2. Line112depicts the modeled spectral reflectance of another red tri-state modulator12having a 170 nm thick spacing layer103of SiO2, to contrast with line111. As illustrated inFIG. 11, the red tri-state modulator12having a 170 nm thick spacing layer103of SiO2provides higher saturation over the red frequencies, i.e., between 625 nm to 740 nm, than the red tri-state modulator12having a 100 nm thick spacing layer103of SiO2.

FIG. 12shows an example of combined modeled reflectance spectra for red, green and blue tri-state modulators having different spacing layers. Line121depicts the modeled spectral reflectance of the blue tri-state modulator12having a 230 nm thick spacing layer103of SiO2. Line122depicts the modeled spectral reflectance of the green tri-state modulator12having a 220 nm thick spacing layer103of SiO2. Line123depicts the modeled spectral reflectance of the red tri-state modulator12having a 170 nm thick spacing layer103of SiO2. Line124depicts the modeled spectral reflectance when the red, green and blue tri-state modulators12are in the white state. Line125depicts the modeled spectral reflectance when the red, green and blue tri-state modulators12are in the black state.

FIG. 13shows the positions of the red, green, blue, and white colors perceived inFIG. 12in the CIE 1976 diagram. The color blue which is the perceived color of line121inFIG. 12is depicted as point131inFIG. 13. The color green which is the perceived color of line122inFIG. 12is depicted as point132inFIG. 13. The color red which is the perceived color of line123inFIG. 12is depicted as point133inFIG. 13. The color white which is the perceived color of line124inFIG. 12is depicted as point134inFIG. 13. As shown inFIG. 13, the points131,132and133are very close to the sRGB primary coordinates, and the point134is very close to D65. Here, sRGB is a standard RGB color space created cooperatively by HP™ and Microsoft™ for use on monitors, printers, and the Internet. D65is a standard white point of daylight, which is promulgated by the International Commission on Illumination (CIE) at the temperature of 6,500° K. D65 corresponds roughly to a midday sun in Western/Northern Europe.

FIG. 14shows an example of combined reflectance spectra for a display of a three-bit design. In this embodiment, the display has a plurality of red, green and blue tri-state modulators. Each pixel comprises a red subpixel, a green subpixel, and a blue subpixel. Each of the red, green and blue subpixels is represented by three bits. For each subpixel, one tri-state interferometric modulator is associated with the first bit. Two tri-state interferometric modulators are associated with the second bit. Four tri-state interferometric modulators are associated with the third bit. Therefore, seven tri-state interferometric modulators are associated with each subpixel, and twenty one tri-state interferometric modulators are associated with each pixel. This embodiment is only an example. Those skilled in the art may appreciate that numerous variations are possible.

Line141depicts the spectral reflectance of the blue tri-state modulators for the blue subpixel. Line142depicts the spectral reflectance of the green tri-state modulators for the green subpixel. Line143depicts the spectral reflectance of the red tri-state modulators for the red subpixel. Line144depicts the spectral reflectance when the red, green and blue tri-state modulators are in the white state. Line145depicts the spectral reflectance when the red, green and blue tri-state modulators are in the black state.

Experiment with the three-bit design display showed the effects as shown in the following table.

As shown above, the three-bit design display demonstrated a modeled gamut of 38% EBU (European Broadcast Union). The contract ration (CR) was modeled to be 18:1. The brightness (Y) was modeled to be 33%.

FIG. 15shows the positions of the red, green and blue colors perceived inFIG. 14in the CIE 1976 diagram. The color blue which is the perceived color of line141inFIG. 14is depicted as point151inFIG. 15. The color green which is the perceived color of line142inFIG. 14is depicted as point152inFIG. 15. The color red which is the perceived color of line143inFIG. 14is depicted as point153inFIG. 15. The color white which is the perceived color of line144inFIG. 14is depicted as point154inFIG. 15. As shown inFIG. 15, the points151,152and153are very close to the sRGB primary coordinates, and the point154is very close to D65.

As described above, in a tri-state interferometric modulator, a spacing layer may be provided between the fixed reflector and the electrodes. The spacing layer may provide adequate protection against shorting between the movable device and the electrodes. The spacing layer may also define a second interferometric modulating cavity so as to provide saturated light over a range of light frequencies. Thus, a display adopting such tri-state interferometric modulators may have a longer service life, a higher contrast ratio and a larger gamut.