Patent Publication Number: US-8541860-B2

Title: Antireflective coating

Description:
This application is a divisional of application Ser. No. 11/314,772, filed Dec. 21, 2005. 
    
    
     TECHNICAL FIELD 
     The present invention relates generally to a device and method useful in an image display system, and more particularly to a device and method for an antireflective coating to improve image quality in an image display system. 
     BACKGROUND 
     Image display systems that employ image projection techniques, such as spatial light modulators (SLMs) using arrays of digital micromirror devices (DMDs), deformable mirrors, and liquid crystal on silicon, can suffer from a degraded contrast ratio (a ratio of brightest white to darkest black producible) due to reflections of a light used to project the images from parts of the arrays not intended to reflect light. Light can scatter from support structures, apertures, vias, and so forth from the array of light modulators. The scattered light can effectively reduce the darkness of the darkest black, therefore reducing the contrast ratio. For example, in an SLM using DMD technology, light can scatter from electrode structures, micromirror support structures, mirror vias, and so on. 
     One technique to reduce reflection is to place an antireflective coating on metal structures in the array of light modulators. Since the metal structures can be a significant source of unintended reflection, the use of the antireflective coating can significantly improve the contrast ratio. For example, uncoated aluminum can have a reflectivity as high as 92 percent while coated aluminum can have a reflectivity of approximately two to three percent. 
     One disadvantage of the prior art is that the application of the antireflective coating on the metal structures does not address the unintended reflection from other portions of the array of light modulators, such as the interlayer dielectrics, the substrate material, metal structures buried under dielectric layers, and so forth. 
     A second disadvantage of the prior art is that by not addressing the light reflecting off the non-metallic structures, it is possible to have an undesired change in the color point by using different layer thicknesses. Changes in the thicknesses of the interlayer dielectric layers can have an especially pronounced effect upon the color point. Changing the color point can result in an undesired colorcast to the images being displayed. 
     SUMMARY OF THE INVENTION 
     These and other problems are generally solved or circumvented, and technical advantages are generally achieved, by preferred embodiments of the present invention which provide a device and method for an antireflective coating to improve image quality in an image display system. 
     In accordance with a preferred embodiment of the present invention, a semiconductor device with an antireflective coating is provided. The semiconductor device includes a first high refractive index layer overlying a reflective surface of an integrated circuit and a first low refractive index layer overlying the first high refractive index layer. The semiconductor device also includes a second high refractive index layer overlying the first low refractive index layer and a second low refractive index layer overlying the second high refractive index layer. 
     In accordance with another preferred embodiment of the present invention, a method for forming an antireflective coating on a semiconductor device is provided. The method includes forming a first high refractive index layer on a reflective surface and forming a first low refractive index layer on the first high refractive index layer. The method also includes forming a second high refractive index layer on the first low refractive index layer and forming a second low refractive index layer on the second high refractive index layer. 
     In accordance with another preferred embodiment of the present invention, an integrated circuit is provided. The integrated circuit includes a first layer overlying integrated structures of the integrated circuit, the first layer comprising a first material with a high refractive index and a second layer overlying the first layer, the second layer comprises a second material with a low refractive index. The integrated circuit also includes a third layer overlying the second layer, the third layer comprises a third material with a high refractive index and a fourth layer overlying the third layer, the fourth layer comprises a fourth material with a low refractive index. 
     An advantage of a preferred embodiment of the present invention is that it also reduces unintended reflections from the non-metallic portion of the array of light modulators, as well as reflections from structures that are buried underneath the dielectric layers. The result is a display system with a better contrast ratio, and hence, better quality images. 
     A further advantage of a preferred embodiment of the present invention is that the color point can be stabilized so that there is substantially no undesired colorcast to the images being displayed, even if the manufacturing process introduces wide variations in the thicknesses of the interlayer dielectric layers. 
     The foregoing has outlined rather broadly the features and technical advantages of the present invention in order that the detailed description of the invention that follows may be better understood. Additional features and advantages of the invention will be described hereinafter which form the subject of the claims of the invention. It should be appreciated by those skilled in the art that the conception and specific embodiments disclosed may be readily utilized as a basis for modifying or designing other structures or processes for carrying out the same purposes of the present invention. It should also be realized by those skilled in the art that such equivalent constructions do not depart from the spirit and scope of the invention as set forth in the appended claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a more complete understanding of the present invention, and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which: 
         FIGS. 1   a  and  1   b  are diagrams of an SLM system and a detailed view of a cross-section of a light modulator array; 
         FIGS. 2   a  through  2   c  are diagrams of an antireflective coating for structures in an integrated circuit, according to a preferred embodiment of the present invention; 
         FIGS. 3   a  and  3   b  are diagrams of antireflective coatings, according to a preferred embodiment of the present invention; 
         FIGS. 4   a  through  4   c  are diagrams of sequences of events in the fabrication of an integrated circuit with an antireflective coating, according to a preferred embodiment of the present invention; and 
         FIGS. 5   a  and  5   b  are diagrams of reflectance vs. light wavelength for a light modulator array with and without the antireflective coating, according to a preferred embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS 
     The making and using of the presently preferred embodiments are discussed in detail below. It should be appreciated, however, that the present invention provides many applicable inventive concepts that can be embodied in a wide variety of specific contexts. The specific embodiments discussed are merely illustrative of specific ways to make and use the invention, and do not limit the scope of the invention. 
     The present invention will be described with respect to preferred embodiments in a specific context, namely an SLM system using digital micromirror devices (DMDs) as light modulators, wherein the SLM system can make use of any number of light component colors. The invention may also be applied, however, to other SLM systems, such as those using deformable mirror or liquid crystal on silicon light modulators, as well as other applications where there is a need to reduce unintended reflections. Examples of these applications include light modulators used in non-display applications, including displacement monitors, accelerometers, photolithography, and so forth, as well as micro-electromechanical (MEM) devices used, for example, in laser pointers. 
     With reference now to  FIG. 1   a , there is shown a diagram illustrating an SLM system  100 . The SLM system  100  features a light source  105  that provides light to a light modulator array  110 . The light modulator array  110  contains a plurality of light modulators (not shown) that manipulate the light to display an image on a display plane  115 . The light modulators in the light modulator array  110  can make use of a wide variety of light modulator technologies, such as positional micromirror, deformable mirror, liquid crystal on silicon, and so forth. For discussion purposes, the focus will be placed upon positional micromirrors used in a digital micromirror device (DMD). However, this should not be construed as being limiting to the spirit and scope of the present invention. 
     Depending upon a positional micromirror&#39;s position, light from the light source  105  can either reflect onto the display plane  115  or to some other portion of the SLM system  100 . The light reflecting onto the display plane  115  forms the image being displayed and the light reflecting to another portion of the SLM system  100  is not visible on the display plane  115 . Therefore, the state of picture elements can be displayed. If a picture element (or a group of picture elements) is to be on, then a positional micromirror(s) associated with the picture element will reflect light from the light source  105  onto the display plane  115  and the portion of the display plane  115  will be lit. On the other hand, if the picture element is to be off, then the positional micromirror will reflect light from the light source  105  to a different portion of the SLM system  100  and the portion of the display plane  115  will be dark. 
     However, light from the light source  105  can reflect from structures on the light modulator array  110  other than the light modulators themselves. This unintended reflection can have a negative effect on the SLM system&#39;s contrast ratio, thereby reducing overall image quality. For example, it is possible for light to reflect from apertures, vias, support structures, electrode structures, micromirror supports, and so forth on and in the light modulator array  110 . 
     With reference now to  FIG. 1   b , there is shown a diagram illustrating a detailed view of a portion of the light modulator array  110 , wherein positional micromirrors are used as light modulators. The diagram illustrates a portion of the light modulator  110  with three positional micromirrors, such as positional micromirror  150 . The diagram also illustrates other structures making up the light modulator array  110 , such as a positional micromirror structure  160 , electrode structure  165 , and so forth. Additionally, there can be structures such as vias  170 , metal conductors  175 , and so on buried beneath or in a modulating layer  180 . It is possible for light from the light source  105  to travel through gaps, such as gap  185 , present between adjacent positional micromirrors and strike these structures (as well as a top surface of the modulating layer  180 ) and scatter. The light from the unintended reflections can strike the display plane  115 , degrading the contrast ratio of the SLM system. 
     As discussed previously, a prior art technique makes use of an antireflective coating applied to metal structures that are external to the modulating layer  180 . A similar term, antireflective layer, can be used interchangeably with antireflective coating without affecting the spirit of the present invention. The antireflective coating can reduce the amount of unintended reflection. However, the application of the antireflective coating on the external metal structures does not address reflections from structures buried under the modulating layer  180  or undesirable scatter from the top surface of the modulating layer  180 . Furthermore, a problem with color point variation as a function of varying layer thicknesses of an interlayer dielectric used to provide passivation to the modulating layer  180 , which can lead to undesirable colorcast being present in images displayed by the SLM system, is not addressed in the use of the antireflective coating on the external metal structures. According to a preferred embodiment of the present invention, it is possible to apply an antireflective coating underneath the top surface of the modulating layer  180  so that reflections from structures buried under the modulating layer  180  can be reduced as well as providing a stable color point for widely varying interlayer dielectric thicknesses. 
     With reference now to  FIGS. 2   a  through  2   c , there are shown diagrams illustrating an antireflective coating that can be applied to a reflective surface to reduce reflectivity and an exemplary antireflective coating applied on a metallic surface, according to a preferred embodiment of the present invention. The diagram shown in  FIG. 2   a  illustrates reflective surface  205 , such as a metal structure, and an antireflective layer  210  that can be applied on top of the reflective surface  205 . Alternatively, rather than applying the antireflective layer  210  on individual structures, the antireflective layer  210  can be applied on top of an entire layer. For example, if a modulating layer, such as the modulating layer  180  ( FIG. 1   b ), has metal layers one, two, and three (with metal layer three being a topmost metal layer), then the antireflective layer  210  can be applied on top of the modulating layer after metal structures are formed in the metal layer three. The antireflective layer  210  can then cover the entire modulating layer and can reduce reflections from all structures in and beneath the modulating layer. 
     According to a preferred embodiment of the present invention, the antireflective layer  210  comprises four layers of material with differing refractive indices, with alternating layers of high refractive index material and low refractive index material. More than four layers can be used, however, the effectiveness of the antireflective layer  210  may not increase significantly with additional layers. Preferably, a first layer  215  of the antireflective layer  210  has a high refractive index. For example, the first layer  215  can be formed using TiN (titanium nitride) or silicon rich nitride (SiRN). A second layer  220  of the antireflective layer  210  should have a low refractive index and can be formed from a dielectric metal oxide, such as silicon dioxide (SiO 2 ), aluminum oxide (Al 2 O 3 ), or silicon oxynitride (SiON). A third layer  225  of the antireflective layer  210  should have a high refractive index and can be formed using the same material as was used to form the first layer  215 . Alternatively, a different material may be used in the third layer  225 . A fourth layer  230  of the antireflective layer  210  should have a low refractive index and can be formed using the same material as was used to form the second layer  220 . Alternatively, a different material may be used in the fourth layer  230 . Depending upon the material used for the fourth layer  230 , a passivation layer may be applied to provide a measure of protection for the antireflective layer  210  and underlying structures. For example, if silicon dioxide is used to form the fourth layer  230 , it may not be necessary to add a passivation layer. 
     It should be understood to those skilled in the art of semiconductor manufacturing that the stoichiometry of these layers may not be exact. Thus in addition to the nominal compositional elements, amounts of other elements may be present. For example, in the case of titanium nitride, commonly denoted TiN, the stoichiometric, crystalline compound is Ti 3 N 4 . However, a titanium nitride thin film is better represented as Ti x N y O z , where x, y, and z are integer values and y&gt;x&gt;z. This is due to the deposition method for the TiN layer, which typically involves reactive sputtering of Ti metal in a nitrogen-containing plasma. 
     The layers in the antireflective layer  210  effectively function as a light trap to prevent light that passes through the antireflective layer  210  and reflects from the reflective surface  205  from passing back through the antireflective layer  210 . The thicknesses of the layers in the antireflective layer  210  can be determined after considering factors such as: wavelength of the light being reflected, desired color point, amount of permissible reflection, and so forth. 
     The diagram shown in  FIG. 2   b  illustrates an exemplary antireflective coating formed from four layers of materials with differing refractive indices, wherein the antireflective coating is formed on top of the reflective surface  205  made of aluminum. The first layer  215  of the antireflective coating can be made using TiN (with a typical refractive index of around 2.5), the second layer  220  of the antireflective coating can be made using SiO 2  (with a typical refractive index of around 1.4) or SiON (with a typical refractive index of around 1.6), the third layer  225  of the antireflective coating can be made using TiN, and the fourth layer  230  of the antireflective coating can be made using SiO 2 . Although shown in  FIG. 2   b  as being made from TiN, the high refractive index layers, the first layer  215  and the third layer  225 , can also be made from semiconducting metals, a metal oxide, or a metal nitride with a refractive index of at least 1.7. The thicknesses of the low refractive index layers (the second layer  220  and the fourth layer  230 ) can be referred to as thicknesses L 1  and L 2  and can range from 3 nm (nanometers) to 10,000 nm, with a preferred thickness range being 10 nm to 2,500 nm. While a preferred low refractive index material is SiO 2  or SiON, a wide variety of materials with a refractive index between that of air and TiN can be used, for example, insulating metal oxides and metal halides with a refractive index of less than 1.7 can be used. Additionally, materials such as aluminum (Al); titanium (Ti); tantalum (Ta); tungsten (W); silicon (Si); chromium (Cr); alloys of Al, Ti, Ta, W, Si, Cr; silicon dioxide (SiO 2 ); silicon oxynitride (SiON); titanium nitride (TiN), titanium aluminum nitride (TiAlN), and titanium dioxide (TiO 2 ), with TiN, TiAlN, and TiO2 being exemplary members of compounds describable as semiconductive binary and ternary compounds, can also be used. The two high refractive index layers (the first layer  215  and the third layer  225 ) can have thicknesses (referred to as thicknesses H 1  and H 2 ) chosen to minimize the intensities of undesired reflections from the completed structure. Layer thicknesses for H 1  and H 2  range from 5 nm to 1000 nm, and preferably from 5 nm to 250 nm. The diagram shown in  FIG. 2   c  illustrates thickness ranges for the layers of the antireflective coating. 
     The dual high refractive index layers (the first layer  215  and the third layer  225 ), shown in  FIG. 2   b  as being made from TiN, are arranged in what is commonly referred to as a split layer geometry. Each of the two high refractive index layers has a low refractive index antireflective coating. For example, the first layer  215  has the second layer  220  as its antireflective coating and the third layer  225  has the fourth layer  230  as its antireflective coating. The split layer geometry can be recognized by those of ordinary skill in the art of the present invention as being an optical trap. 
     An exemplary preferred embodiment of the present invention includes a first high refractive index layer with a thickness of approximately 20 nm of TiN, a first low refractive index layer with a thickness of approximately 50 nm of SiO 2 , a second high refractive index layer with a thickness of approximately 20 nm of TiN, and a second low refractive index layer formed from two layers of low refractive index material, a first layer with a thickness of approximately 50 nm of SiON and a second layer with a thickness of approximately 900 to 1100 nm of SiO 2 . A second exemplary preferred embodiment of the present invention includes a first high refractive index layer with a thickness of approximately 50 nm of TiN, a first low refractive index layer with a thickness of approximately 800 to 1100 nm of SiO 2 , a second high refractive index layer with a thickness of approximately 75 nm of TiN, and a second low refractive index layer with a thickness of approximately 67 nm of SiON. 
     According to a preferred embodiment of the present invention, a difference in the refractive index of the material used in the high refractive index layer and the refractive index of the material used in the low refractive index layer for light in the visible spectrum should be at least 0.25. Furthermore, the material used in the high refractive index layer can be a semiconducting metal, a metal oxide, or a metal nitride with a refractive index of at least 1.7 for light in the visible spectrum. Additionally, the material used in the low refractive index layer can be made from one or more insulating metal oxides or metal halides with a refractive index of at most 1.7 for light in the visible spectrum. 
     With reference now to  FIGS. 3   a  and  3   b , there are shown diagrams illustrating alternate layer arrangements for the antireflective coating, according to a preferred embodiment of the present invention. Each of the two low refractive index layers (the second layer  220  and the fourth layer  230  of  FIG. 2   a ) of the antireflective layer can comprise multiple layers of low refractive index materials, such as combinations of layers of SiO 2  and SiON. The diagram shown in  FIG. 3   a  illustrates an antireflective coating on an aluminum layer  305  comprising two TiN layers  315  and  325  (high refractive index layers) alternating with two combination low refractive index layers  320  and  330  made up of two or more layers of SiO 2  and SiON. According to a preferred embodiment of the present invention, the combination low refractive index layers  320  and  330  can have similar thickness ranges (5 nm to 10000 nm with a preferred thickness range of 10 nm to 2500 nm) as the single material low refractive index layers  220  and  230  ( FIGS. 2   a  and  2   b ), with the number of alternating layers and their respective thicknesses being based upon factors such as desired color point, reflectivity, and so forth. In addition to SiO 2  and SiON, other low refractive index materials can be used. 
     With reference now to  FIG. 3   b , in addition to the embodiment discussed above, wherein the two low refractive layers  320  and  330  are made from alternating layers of low refractive index materials, it is also a preferred embodiment that a first low refractive index layer, such as a first low refractive index layer  340  of two low refractive index layers be made from alternating layers of low refractive index materials, while a second low refractive index layer, such as second low refractive index layer  335 , be made from a single low refractive index material or vice versa. As shown in  FIG. 3   b , the first low refractive index layer  340  is formed with two layers made from two low refractive index materials, a first layer  345  made from SiON and a second layer  350  made from SiO 2 . Although shown as being formed with two alternating layers of low refractive index material, more than two alternating layers can be used. Furthermore, it is possible to form the low refractive index layer  335  from multiple layers of low refractive index material and the low refractive index layer  340  from a single low refractive index material. 
     With reference now to  FIGS. 4   a  through  4   c , there are shown diagrams illustrating sequences of events in the manufacture of an integrated circuit, such as a light modulator array, with an antireflective coating for structures in a substrate of the integrated circuit, according to a preferred embodiment of the present invention. A sequence of events  400  in the manufacture of an integrated circuit, such as one containing a light modulator array, is shown in  FIG. 4   a . The diagram shown in  FIG. 4   a  illustrates the sequence of events  400  in the manufacture of a typical integrated circuit and a sequence of events in the manufacture of an actual integrated circuit can vary for different integrated circuits. 
     Initially, the fabrication of the integrated circuit can begin with the formation of structures in a substrate of the integrated circuit (block  402 ). The structures formed in the substrate can include transistors, conductors, metal layers, vias, and so forth. Once the structures in the substrate of the integrated circuit have been formed, an antireflective coating can be formed (block  404 ). The antireflective coating can either be formed on the structures individually or it can be formed on a top surface of the substrate as a whole and cover all structures in the substrate as well as any portion of the substrate not containing a structure. After the antireflective coating has been formed on the substrate, a passivation coating can be applied (block  406 ). The passivation coating can be used to protect the substrate, the structures on and in the substrate, the antireflective coating, and so forth. Depending upon the antireflective coating, the passivation coating may not be needed. For example, if a top layer of the antireflective coating is formed from SiO 2  and has adequate thickness, then the passivation coating may not be required, since the passivation coating is typically made from SiO 2 . 
     After the application of the passivation coating or the formation of the antireflective layer (if the passivation coating is not needed), external structures for the integrated circuit can be formed (block  408 ). For example, if the integrated circuit is a light modulator array making use of positional micromirrors, then support structures for the positional micromirrors, as well as the positional micromirrors, can be formed. Following the formation of external structures, any additional operations needed to complete the integrated circuit can be completed (block  410 ). The additional operations can include bonding wire pads together, forming electrodes, and so on. The integrated circuit is now complete and can undergo singulation, testing, labeling, packaging, and so forth. 
     The diagram shown in  FIG. 4   b  illustrates a detailed sequence of events  420  in the formation of the antireflective coating on the substrate of the integrated circuit. The sequence of events  420  can be illustrative of the formation of an antireflective coating (block  404 ,  FIG. 4   a ). The formation of the antireflective coating can begin with the formation of a first high refractive index layer (block  422 ). According to a preferred embodiment of the present invention, TiN can be used to create the first high refractive index layer. The first high refractive index layer can have a thickness, H 1 , as specified and illustrated previously. After forming the first high refractive index layer (block  422 ), a first low refractive index layer can be formed (block  424 ). The first low refractive index layer can serve as an antireflective coating for the first high refractive index layer. Preferred materials for use in the formation of the first low refractive index layer can be SiO 2  and SiON. However, materials with a refractive index ranging from that of air to that of TiN can be used. The first low refractive index layer can have a thickness, L 1 , wherein the thickness can range from 5 nm to 10,000 nm, with preferred thicknesses ranging from 10 nm to 2,500 nm. 
     A second high refractive index layer can now be formed on top of the first low refractive index layer (block  426 ). Again, TiN can be a preferred material for the creation of the second high refractive index layer. The second high refractive index layer can have a thickness, H 2 , in an application where the antireflective coating has four layers. Finally, a second low refractive index layer can be formed (block  428 ). Preferred materials for use in the formation of the first low refractive index layer can be SiO 2  and SiON. However, materials with a refractive index ranging from that of air to that of TiN can be used. The first low refractive index layer can have a thickness, L 1 , wherein the thickness can range from 5 nm to 10,000 nm, with preferred thicknesses ranging from 10 nm to 2,500 nm. 
     If the antireflective coating is to have more than four layers, the formation of the high refractive index layers and the low refractive index layers can continue, with a final layer of the antireflective coating being a low refractive index layer. 
     The diagram shown in  FIG. 4   c  illustrates a detailed sequence of events  440  in the formation of a combination low refractive index layer of the antireflective coating. Each of the low refractive index layers in the antireflective coating (the first low refractive index layer and the second low refractive index layer, for example) can be formed from multiple layers of low refractive index materials. For example, the first low refractive index layer can be formed from alternating layers of SiO 2  and SiON. To form a multilayer low refractive index layer, a first layer can be formed with a thickness, CL 1  (block  442 ), and then a second layer can be formed with a thickness, CL 2  (block  444 ). Any additional layers can also be added to the low refractive index layer until a desired number of layers is achieved (block  446 ). Although the discussion illustrates the formation of the low refractive index layer from alternating layers of two low refractive index materials, more than two low refractive index layers can be used. The thicknesses of the alternating layers of low refractive index materials can be dependant upon factors such as desired color point, desired reflectivity, and so forth. 
     With reference now to  FIGS. 5   a  and  5   b , there are diagrams illustrating reflectance vs. light wavelength for exemplary light modulator arrays with and without the antireflective coating, according to a preferred embodiment of the present invention.  FIG. 5   a  illustrates reflectance (in percentage) vs. light wavelength for a light modulator array without the antireflective coating. As shown in  FIG. 5   a , the reflectance can range from a low of about 20% to a high of about 45% for visible light spectrum (from 400 nm to 750 nm).  FIG. 5   b  illustrates reflectance (in percentage) vs. light wavelength for a light modulator array with the antireflective coating. As shown in  FIG. 5   b , the reflectance ranges from a low of about 10% to a high of 22% for the visible light spectrum. The antireflective coating significantly reduces the reflectance of the light modulator array. 
     Although the present invention and its advantages have been described in detail, it should be understood that various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the invention as defined by the appended claims. 
     Moreover, the scope of the present application is not intended to be limited to the particular embodiments of the process, machine, manufacture, composition of matter, means, methods and steps described in the specification. As one of ordinary skill in the art will readily appreciate from the disclosure of the present invention, processes, machines, manufacture, compositions of matter, means, methods, or steps, presently existing or later to be developed, that perform substantially the same function or achieve substantially the same result as the corresponding embodiments described herein may be utilized according to the present invention. Accordingly, the appended claims are intended to include within their scope such processes, machines, manufacture, compositions of matter, means, methods, or steps.