Patent Application
20200402757
Image intensifiers typically include a photocathode layer and an electron multiplier. Generally, the photocathode layer converts incoming photons into electrons and the electron multiplier (transmission layer) generates multiple electrons from each single electron received from the photocathode layer. Often, the electron multiplier (gain layer) utilizes electron impact ionization as a gain mechanism to amplify electrons generated by the photocathode layer in response to incoming photons.
Unfortunately, image intensifiers that rely on electron impact ionization as a gain mechanism often experience performance degradation when electrons move along undesirable paths prior to or subsequent to an impact. For example, if electrons backscatter (reflect or bounce) off a surface of the electron multiplier, these backscattered electrons may create a halo effect that degrades image quality. Additionally or alternatively, some electrons, including backscattered electrons, may be lost between the photocathode layer and the electron multiplier (i.e., if the electrons are absorbed by a structure other than the electron multiplier). Still further, electrons may move laterally within the electron multiplier, thereby degrading the spatial fidelity of the electrons that are output by the electron multiplier. Thus, crosstalk between pixels or adjacent regions in the gain layer may cause performance degradation. To minimize performance degradation, the thickness of the gain layer is often minimized. However, a thin gain layer is susceptible to failure from mechanical breakage or other processes such as deflection from pressure differentials between cavities or from applied voltages between different layers.
Increasing the thickness of the gain layer to add support or adding other specialized features to the gain layer generally leads to signal degradation and may increase manufacturing complexity. Thus, there is a tradeoff between the thickness of the gain layer and the ability of the carriers to pass through the gain layer without signal loss or degradation of the carriers.
According to an embodiment, an electron multiplier for a Micro-Electro-Mechanical-Systems (MEMS) image intensifier includes a photocathode, a transmission or gain substrate layer, and a phosphor or sensing layer (also referred to as an anode). The photocathode layer comprises an input surface that receives electrons and an emission surface (opposite the input surface) that emits electrons into a vacuum or gap. The emitted electrons pass through the vacuum to an input surface of the gain substrate layer. The received electrons are amplified by the gain substrate layer, and are emitted into a vacuum at an emission surface (opposite the input surface) of the gain substrate layer. A first plurality of support ribs are present on the input surface of the gain substrate layer, and a second plurality of support ribs are present on the emission surface of the gain substrate layer, adding mechanical support to the thin gain substrate layer while minimizing signal degradation and loss.
In some aspects, the height of the first plurality of ribs and the height of the second plurality of ribs are about the same. In other aspects, the height of the first plurality of ribs is less than the height of the second plurality of ribs. In still other aspects, the height of the first plurality of ribs is greater than the height of the second plurality of ribs.
According to another embodiment, the height of the first plurality of ribs and the height of the second plurality of ribs are each set to provide a predetermined degree of mechanical rigidity to the gain substrate layer.
In other aspects, the spacing between the individual ribs of the first plurality of ribs are configured to allow electrons emitted from the photocathode to be received on the input surface of the gain substrate layer without substantial degradation from contacting the walls of the first plurality of ribs. The spacing is determined at least in part by the distance between the emission surface of the photocathode and the input surface of the gain substrate layer (gap distance). Similarly, the spacing between the individual ribs of the second plurality of ribs are configured to allow electrons to be emitted from the emission surface of the gain substrate layer without substantial degradation from contacting the walls of the second plurality of ribs. The radial spread of electrons may be determined by the following equation:
r=2*gap*√{square root over ((MTE/Vbias))} Eq. (1)
wherein the gap is the distance between surfaces (e.g., between the emission surface of the photocathode and the input surface of the gain substrate layer), MTE is the mean transverse energy of the emitted electrons, and Vbias is the bias between the two surfaces.
In general, the individual ribs of the first plurality of ribs are aligned with respective individual ribs from the second plurality of ribs. For example, each individual rib of the first plurality of ribs may be positioned such that a vertical axis passes through a rib of the first surface, through the gain substrate layer and through a corresponding rib of the second plurality of ribs.
In another aspect, blocking structures/regions can be used in a transmission mode image intensifier to maintain the spatial registration of carriers in the gain substrate layer. Ribs, on both the upper input surface and lower emission surface, may be used to increase the robustness of the thin silicon-based gain substrate layer.
Like reference numerals have been used to identify like elements throughout this disclosure.
A gain substrate layer for a MEMS image intensifier is presented herein. The gain substrate layer includes a first plurality of ribs on an upper input surface and a second plurality of ribs on a lower emission surface. In some aspects, each of the upper ribs is aligned with a corresponding lower rib.
The gain substrate layer presented herein includes p-type doped ribs on both the upper input surface and the lower emission surface. The ribs define a plurality of cells, or pixels, insofar as the term “doped” or variants thereof (e.g., referred to herein as doping, dopant, etc.) indicates that a dopant has been added to shift the Fermi levels within the silicon-based material, such that with p-type dopants, the ribs are shifted to predominantly positive charge carriers. Advantageously, since the ribs comprise a p-type doped material, the ribs prevent electron crosstalk between pixels. According to an embodiment, the ribs extend above an input surface of the gain substrate layer (e.g., electron multiplier) and below the output emission surface of the gain substrate layer. Further, halo effects created by backscattered electrons are minimized by the ribs, only on the input surface 312a.
By extending lateral spacers perpendicular to or substantially perpendicular to the input surface and emission surface of the gain substrate layer, height may be added to the thin gain substrate layer, as a grid type, post type, or other suitable structure, to provide mechanical support while maintaining signal integrity. Thus, an enhanced thin gain substrate layer (also referred to as an electron multiplier layer or a transmission layer), which is ideal for maintaining spatial registration of carriers (e.g., holes) in the electron multiplier layer, can be generated by modifying a thin gain substrate layer to include a first plurality of upper ribs and a second plurality of lower ribs in order to be mechanically robust over large areas while maintaining signal integrity.
Typically, ribs that extend above the input surface of the gain substrate layer cause at least some electrons to be lost between the cathode layer and the gain substrate layer (e.g., electrons that impact a surface of a rib may be lost). This effect may be minimized by reducing a first gap distance between the gain substrate layer and the photocathode layer, as well as by controlling the spacing of the ribs, or by varying the voltage according to Eq. (1).
The image intensifier provided herein is intended to be an example. The electron multiplier can be included in any MEMS image intensifier or light detection device now known or developed hereafter.
The layers are encapsulated in an appropriate vacuum housing 110, and may be connected to an external power supply comprising bias circuitry 150 to bias the physical layers of the device to suitable voltages. More specifically, the bias circuitry 150 may include one or more circuits to appropriately bias photocathode 301, gain substrate layer 302, and anode 340 of the MEMS image intensifier 300. The MEMS image intensifier is described in further detail below.
Various fabrication processes may be used to form the ribs. To form the first plurality of ribs 420a, a silicon-based layer may be generated on the input surface of the gain substrate layer with a height corresponding to the height of region 455a. Known fabrication processes involving etching may be used to form ribs, as posts, walls, etc. After etching, ribs are present in the active region 414, with region 455a intact.
To form the second plurality of ribs 420b, polysilicon-based layers may be built up on the emission layer of the gain substrate layer, to reach a height corresponding to the height of region 455b. Ribs are generated in the active region 414, with region 455b intact. Additional details regarding fabrication of the gain substrate layer are provided herein.
Region 415 represents a first gap distance between the tip of ribs 420a and photocathode 301. Region 416 represents a second gap distance between the tip of ribs 420b and phosphor/sensing layer 340.
The photocathode 301 includes an upper surface 405a and an output emission surface 405b. Photons 408, present in regions outside of the image intensifier, may contact the photocathode 301 to convert photons to electrons. When photons 408 impinge on the upper surface 405a of the photocathode 301, each impinging photon 408 has an associated probability of generating a free electron. Free electrons (e−) resulting from impinging photons 408 pass through photocathode 301 and are emitted from output emission surface 405b. In some aspects, the output emission surface 405b is activated to or configured to a negative electron affinity (NEA) state, using well-known techniques, to facilitate the flow of the free electrons (e−) from the output emission surface 405b of the photocathode into the first gap distance 401 of the vacuum.
In some aspects, photocathode 301 is a photocathode layer comprising semiconductor materials such as gallium arsenide (GaAs), which exhibits a photoemissive effect. It is noted that other type III-V materials may be used, including but not limited to GaP, GaInAsP, InAsP, InGaAs, etc. Alternatively, the photocathode may comprise a known bi-alkali material, which includes but is not limited to antimony-rubidium-cesium (Sb—Rb—Cs), antimony-potassium-cesium (Sb—K—Cs), sodium-potassium-antimony (Na—K—Sb), etc., which also has a photoemissive effect.
In this example, the photoemissive semiconductor material of the photocathode 301 absorbs photons at the upper surface 405a. The absorbed photons cause the carrier density of the semiconductor material to increase, thereby causing the photo-emissive semiconductor material to generate a photo-current of electrons (as shown by the arrows) passing through photocathode 301 for emission from the output emission surface 405b. In some aspects, photocathode 301 comprises blocking structures 410 (shown as triangles). The blocking structures are configured to focus the electrons (e.g., into streams of emitted electrons) as the electrons leave the photocathode 301 to travel through the first gap distance 401 to reach the gain substrate layer. Thus, the photocathode 301 convert photons 408 received from ambient light to electrons (e−), and releases these electrons towards the gain substrate layer 302 in streams that are spatially co-located with the open area between ribs 420a in the input surface 412a of the gain substrate layer 302.
The gain substrate layer 302, which may also be referred to as an electron multiplier layer or transmission layer, includes an input surface 412a and an emission surface 412b that is opposite the input surface 412a. Photocathode 301 is positioned above the input surface 412a, separated by first gap distance 401, such that the photocathode 301 emits electrons towards the input surface 412a.
Gain substrate layer 302 may include silicon and/or other semi-conductive materials such as, and without limitation, gallium arsenide (GaAs). In an embodiment, gain substrate layer includes silicon-based component(s) and is doped with a p-type dopant to generate a plurality of electrons 205 for each free electron 201 that impinges on an input surface 412a of the gain substrate layer.
An electric field (not shown) between the photocathode 301 and the gain substrate layer 302 accelerates the electrons emitted by the photocathode towards the gain substrate layer, causing the electrons to impact the input surface 412a of the gain substrate layer 302. The gain substrate layer 302 amplifies the received electrons forming electrons 205 and emits additional electrons (designated as Ge- and referred to as an electron interaction volume), via emission surface 412b, towards the phosphor/sensing layer 340. Thus, there is a gain of electrons through the gain substrate layer 302.
The gain substrate layer 302 provides a highly efficient gain, which allows the MEMS image intensifier 400 (more specifically, the phosphor/sensing layer 340 and screen/display 350) to output visible light that a user can view in a direct view system (e.g., a system where a user looks directly at screen/display 350 for an image) or in a digital system (e.g., a system where a user views a digital output from a camera focused on a phosphor screen).
The gain substrate layer 302, which may be formed using a wafer or other suitable substrate (e.g., a silicon-based substrate), comprises an active region 414 that is configured to receive and amplify electrons emitted by the photocathode 301. The active region 414 includes a lattice or grid of interior walls or posts, referred to as a first plurality of ribs 420a, positioned on the input surface 412a and formed, for example, using single crystal silicon processes such as etching (a subtractive process), or another suitable process.
Another lattice or grid of interior walls or posts, referred to as a second plurality of ribs 420b, positioned on the emission surface 412b and formed, for example, using polysilicon crystal or boron doped polysilicon processes (additive processes), or another suitable process. In some aspects, to create the second plurality of ribs 420b, doping profiles may be generated by trenching, and polysilicon deposition may be used to form layer(s). In some aspects, processes such as Lithography, Electroplating, and Molding (LIGA) or other processes may be used to build up polysilicon layers. In some aspects, a thick photoresist applied to emission surface 412b may be used to build up polysilicon layers to form the plurality of second ribs, also using an additive process.
Ribs 420a or 420b may be formed on the input surface 412a or emission surface 412b, respectfully, using any suitable fabrication technique that generates a lattice or grid of walls or posts. Blocking structures 430 may be formed along the emission surface 412b, in order to focus the flow of emitted electrons to the anode 340. Since the blocking structures are formed from a doped material (e.g., a p-doped material), the blocking structures prevent, or at least discourage, electrons from moving laterally within the gain substrate layer 302, because the doped material produces an electrostatic barrier (i.e. an electric field that repels negative charged carriers). The barriers or blocking structures 430 are described in further detail below (
The MEMS image intensifier 400 may be configured to optimize individual electron paths through the various layers of the device. For example, individual components of the blocking structures 410, ribs 420a, blocking structures 430, and ribs 420b may be aligned such that an electron is directed along a vertical axis through these various layers and gaps, to prevent signal degradation from electrons moving horizontally. Thus, an electron stream may originate in the photocathode layer, pass through the vacuum and in between ribs 420a to enter the gain substrate layer, and pass through the gain substrate layer between blocking structures 430, to exit between ribs 420b to reach the anode layer. Accordingly, the path may follow or track along a vertical axis to focus the emission of electrons in streams towards the phosphor/sensing layer 340. In other aspects, the first and second plurality of ribs 420a, 420b, and blocking structures 430 may be aligned in order to maximize signal gain. By aligning the ribs, and blocking structure 430, the flow of electrons is directed and signal gain is optimized through the gain substrate layer. In other aspects, the gain substrate layer may provide suitable electron multiplication without blocking structures 430; however, the first plurality of ribs 420a and the lower ribs 420b are aligned.
Thus, in some aspects, the first and second plurality of ribs 420a, 420b, are aligned to form a plurality of pixels, such that the regions between ribs correspond to a pixel. Areas or pixels on the upper input surface and lower emission surface are aligned as a result. Electrons entering the gain substrate layer are amplified during passage along a vertical axis, wherein the electrons enter a region corresponding to a first input pixel on the upper input surface and exit a region of a corresponding first output pixel on the lower emission surface (as shown by arrows in
In some aspects, ribs 420a are made of p-type doped material and are in contact with or extend, at least slightly, into the input surface 412a of the gain substrate layer 302. Similarly, ribs 420b are made of p-type doped material and are in contact with or extend, at least slightly, into the emission surface 412b of the gain substrate layer 302. The first plurality of ribs 420a is perpendicular or substantially perpendicular to the input surface 412a, and the second plurality of ribs 420b is perpendicular or substantially perpendicular to the emission surface 412b, respectfully.
The phosphor/sensing layer 340 is positioned below the emission surface 412b of the gain substrate layer 302, such that the phosphor/sensing layer 340 may receive electrons emitted from the emission surface 412b. A small amount of stray particles 460 (e.g., photons, ions, etc.) may undergo backscattering, leading to signal degradation and loss. The backscattered electrons may contact the second plurality of ribs 420b and/or emission surface 412b of the gain substrate layer 302. Stray particles 460 that impinge on emission surface 412b may convert to stray electrons and corresponding stray holes. Thereafter, the free electrons may be emitted from emission surface 412b to contact the phosphor layer (see also,
In an embodiment, anode 340 may include a phosphor screen to convert the amplified electrons (e−) 431 to photons. The input surface 425 of the phosphor/sensing layer 340 may be coated with a conducting material (not shown), such as chrome or some other suitable metal, to provide an electrical contact layer to receive the emitted electrons from emission surface 412b.
In another embodiment, anode 340 may comprise a conventional integrated circuit with a CMOS substrate and a plurality of collection wells, commonly used in image intensifier tubes. The electrons may be processed using signal processing equipment for CMOS sensors to produce an intensified image signal for display to the user.
In some aspects, the thickness of the gain substrate layer may be about 30-50 μm, and the thickness of the gain substrate layer including both sets of ribs (Ti) may be, without limitation, approximately 90-150 μm, wherein the first plurality of ribs 420a and the lower ribs 420b are included in the thickness. In some aspects, the thickness of the ribs may be about the same thickness as the gain substrate layer. Regions 415 and 416 correspond to vacuum or gap regions.
Gap distance (L1) is measured from the photocathode layer to the tip of ribs 420a and typically has a distance of about 100-500 μm. In some embodiments, the gap distance is about 254 μm. Gap distance (L2), the distance between the phosphor layer and the tip of the rib 420b, is in the range of about 250-385 μm. The height of ribs 420a does not need to be same as the height of ribs 420b. However, a rib from 420a is in vertical alignment with a rib from 420b, in order to minimize signal loss.
In an embodiment, the gain substrate layer may include first region 501 and second region 510, which are configured (e.g., via doping) to direct the flow of electrons 205 to emission areas 520 of emission surface 412b. Emission areas 520 may be activated to a NEA state to facilitate electron flow from emission surface 412b.
The first region 501, which is disposed adjacent to input surface 412a, may be doped to force electrons away from input surface 412a and into gain substrate layer 302, thus inhibiting recombination of electron-hole pairs at input surface 412a. Inhibiting recombination of electron-hole pairs at input surface 412a ensures that more electrons flow through gain substrate layer 302 to emission surface 412b, thereby increasing efficiency. The first region 501 may be doped with a p-type dopant such as boron or aluminum, and may be relatively heavily doped (e.g., about 1019 parts per cubic centimeter), approximately 100-300 nm deep. Other suitable dopants, concentrations, and dimensions for use with silicon semiconductors and other semiconductor materials, e.g., GaAs, will be readily apparent to those skilled in the art of semiconductor fabrication. In some aspects, the input surface 412a of the gain substrate layer 302 is coated with a conducting material (not shown), such as chrome, adjacent to the first region 501 to provide an electrical contact to the upper surface of the gain substrate layer 302.
The second region 510 may be a moderately p-doped material, and may also be referred to herein as a background region. The second region may also comprise blocking structures 430 that are relatively heavily doped (e.g., 1019 parts per cubic centimeter), e.g., with a p-type dopant such as boron or aluminum. The second region 510 (alone and/or in combination with first region 501), may be referred to as an electron multiplier region.
Blocking structures 430, disposed on the emission surface 412b, are doped (e.g., using P+ dopants) to repel electrons 205 to emission areas. Blocking structures 430 define blocking areas 214 of emission surface 412b, where electron flow into and out of gain substrate layer 302 is inhibited. Blocking areas 214 are regions at emission surface 412b that are effectively blocked by one or more of blocking structure 430, shield 220 and dielectric film 224. Blocking structures 430 may also help to maintain spatial fidelity, for example, by directing stray electrons (moving laterally) into the appropriate emission area 520, helping to ensure that electrons 205 enter and exit gain substrate layer 302 along a vertical trajectory and do not cross into adjacent emission areas. Blocking structures 430 may have a height H of approximately 24 μm. The emission areas 520 may have a diameter of about 1 μm, with a 6 μm pitch between blocking structures 430.
The blocking structures focus electrons to small areas, emission areas 520, for emission. Additionally, focusing allows the electrons to travel greater distances. By funneling the electrons, the frequency of electrons that contact sidewalls of ribs is greatly reduced. This facilitates flow of the electrons to the phosphor layer without or with minimal signal loss.
Gain substrate layer 302 may further include shields 220, which are doped to reduce and/or minimize effects of stray particles 460 (see,
In other aspects, gain substrate layer may further include a dielectric film 224 disposed on emission surface 412b, positioned atop the base of the blocking structures 430. A first surface of the dielectric film 224 may contact one end of ribs 420b and a second surface of the dielectric film may contact a surface of shield 220 at the emission surface 412b. In some aspects, dielectric film 224 may be optional.
Thus, shield 220 is disposed within a blocking structure 430, wherein the surface of shield 220 is disposed at the emission surface 412b and the dielectric film 224 is disposed atop the emission surface 412b. Ribs 420b are disposed atop dielectric film 224.
As mentioned previously, the distance between the photocathode and the tip of the ribs 420a may be reduced, allowing a lower voltage to be used. By decreasing this distance, a more compact and mechanically robust transmission mode image intensifier structure is provided allowing operation at lower voltages. Reduction of voltage and reduced rib spacings allow common wafer level fabrication processes to be used to make the image intensifier, and may lead to a reduction in the number of substrates.
Typically, the voltage will be set at a value suitable for extracting electrons from the photocathode and will provide enough energy to overcome the dead voltage of the gain layer to provide the desired gain. However, the voltage will be lower than a value which may lead to the generation of X-rays by the bombarding electrons. Thus, in some aspects, the upper limit is about 2000V and the lower limit is about 600V. The values may also depend upon the spacing between the tip of the ribs and the photocathode 301 (gap distance 401).
Gap 540 may be provided between first region 501 and blocking structures 430. Gap 540 may be sized or dimensioned such that the blocking structures do not interfere with the generation of electrons 205 at input surface 212a. This may provide the gain substrate layer with an effective electron multiplication area that equals or approaches 100% of an area of input surface 212a. Gap 540 may be, without limitation, approximately 1 μm to 49 μm or any other suitable distance in between. Other suitable dopants, concentrations, dimensions, and/or semiconductor materials, such as GaAs, may be used, as will be readily apparent to one skilled in the relevant art(s). The gain substrate layer 302 may generate several hundred electrons or more for each received electron. Accordingly, the number of electrons exiting the gain substrate layer 302 is significantly greater than the number of electrons that entered the gain substrate layer.
Still referring to
In an exemplary embodiment, the layers of dielectric film 224 are fabricated using conventional fabrication techniques that are readily apparent to those of skill in the art. In one exemplary embodiment, the first oxide layer 638 is approximately 100-300 nanometers thick, the metal layer 640 is approximately 100-300 nanometers thick, and the second oxide layer 644 is approximately 100-300 nanometers thick. In accordance with this embodiment, the total thickness of dielectric film 224 is approximately 300-900 nanometers.
The layers of the illustrated dielectric film 224 perform a variety of functions in the exemplary embodiment. The first oxide layer 638 prohibits the emission of electrons from the emission surface 412b of the semiconductor structure 600 in areas where it is deposited (the blocked area 648) thereby reducing any “dark current” by the ratio of area blocked by the first oxide layer 638 to the total area of the emission surface 412b. Dark current is the flow of electrons within a semiconductor structure produced by thermal variations, which creates random generation of electrons and holes, or noise, in the EBC.
In an exemplary embodiment, the metal layer 640 is biased to draw the increased number of electrons 205 toward it through the gain substrate layer. In the exemplary embodiment, the biasing is low, e.g., less than one volt, to prevent electrons from gaining enough energy to penetrate blocking structure 430 and to prevent damage to the gain substrate layer. In addition, the metal layer 640 acts as a blocking layer for light feedback in embodiments where a photoemitter or phosphor screen is used as a phosphor/sensing layer 340 (
The second oxide layer 644 is disposed on the metal layer 640 to inhibit the emission of electrons by the metal layer 640. Thus, noise attributable to the metal layer 640 is reduced. An individual rib of the plurality of ribs 420b is disposed on the second oxide layer 644. The width of the rib may be less than, equal to, or greater than the width of the dielectric film 224.
The illustrated emission areas 520 are geometric shapes (e.g., circles, squares, etc.) defined by the blocking structure 430. The emission areas 520 may be essentially any suitable geometric shape. In an exemplary embodiment, the blocking structure 430 extends for about 10-20 μm between emission areas 520 and the emission areas 520 are about 0.5-6.0 μm in diameter.
The individual EBCs form a matrix on semiconductor structure 700. Each of the EBCs, and their associated emission areas 520, correspond to regions of the input surface 412a such that the matrix of EBCs pixelate the electrons received at the input surface 412a of the gain substrate layer 302. The number of EBCs actually employed in an array may be many more or less depending on the size of the individual EBCs and the desired resolution of the image intensifier.
In the examples of
In the example of
In the examples of
At operation 902, an electron is received at a gain substrate layer having an input surface with a plurality of ribs, such as described herein in one or more examples.
At operation 904, for each received electron, a plurality of electrons is generated within the gain substrate layer, such as described in one or more examples herein.
At operation 906, the plurality of electrons is repelled from blocking regions of the gain substrate layer that are doped to repel electrons, towards emissions areas of an emission surface having a second plurality of ribs, wherein ribs of the first plurality of ribs are aligned with ribs of the second plurality of ribs, such as described in one or more examples herein.
Support ribs combined with shaping of the incoming electrons and carriers within the transmission layer (gain substrate layer) can provide carrier registration and mechanical support in a transmission device. This structure has several advantages, including providing thin transmission layers, which minimizes signal loss. Additionally, other benefits include increased mechanical strength of the transmission layer, reduced gap distance (L1) spacing, and lower voltage operation. Further, by limiting complexity at least in part through reduction of the number of substrates, many common fabrication processes may be used with these techniques. These techniques provide for improving rigidity of the thin transmission layer (gain substrate layer) without degrading image quality.
The techniques disclosed herein may be implemented with/as passive devices (i.e., with little or no active circuitry or additional electrical connections). Techniques disclosed herein are compatible with conventional high temperature semiconductor processes and wafer scale processing, including conventional CMOS and wafer bonding processes, and a variety of fabrication processes.
In some aspects, support ribs present on upper or/and lower surfaces may be made of any material. Support ribs may be free-standing or connected to an adjacent substrate. Support ribs may be in any suitable configuration, including a grid of posts, or parallel lines/walls. The emission surface is less than the size of the input surface, to minimize lost carriers. Support ribs may be any suitable shape, including round, square, oval, etc. The upper and lower ribs are aligned with blocking structures to minimize or eliminate signal loss for the area consumed by the support ribs.
Methods and systems are disclosed herein with the aid of functional building blocks illustrating functions, features, and relationships thereof. At least some of the boundaries of these functional building blocks have been arbitrarily defined herein for the convenience of the description. Alternate boundaries may be defined so long as the specified functions and relationships thereof are appropriately performed. While various embodiments are disclosed herein, it should be understood that they are presented as examples. The scope of the claims should not be limited by any of the example embodiments disclosed herein. While a particular embodiment has been shown and described in detail, adaptations and modifications will be apparent to one skilled in the art. Such adaptations and modifications of the embodiment may be made without departing from the scope thereof, as set forth in the following claims.
