Patent Publication Number: US-2005128385-A1

Title: Pixel structure for a liquid crystal on silicon display

Description:
FIELD OF THE INVENTION  
      The present invention relates generally to a liquid crystal on silicon (LCoS) display, and more particularly, to a pixel structure of an LCoS display.  
     BACKGROUND OF THE INVENTION  
      LCoS is the critical technology for next generation of reflective LC projector and rear projection television (TV), and has the most advantages of dramatically reducing the manufacture cost of display panel while achieving high resolution. The distinction between LCoS and thin film transistor (TFT) liquid crystal display (LCD) is that both of top and bottom substrates of TFT-LCD are glass plates, but only top substrate of LCoS is glass plate. The bottom substrate of LCoS is silicon semiconductor, and thus LCoS is a technology combining LCD with semiconductor CMOS process.  
       FIG. 1  shows a pixel structure  10  of a conventional LCoS, which comprises a pixel electrode  114 , an insulator  112  on the pixel electrode  114 , three planar reflectors  110  on the insulator  112 , a layer of liquid crystal  104  above the reflectors  110  and the insulator  112 , and a glass plate  102  above the layer of liquid crystal  104 . The incident light  116  is vertically incident into the glass plate  102  and is vertically reflected out of the glass plate  102  by the reflectors  110 . Due to the optical paths of the incident light  116  and the reflective light  118  are identical or similar, this conventional structure needs optical device such as splitter to separate the incident light  116  and reflective light  118 , resulting in reduced brightness and contrast.  
      Therefore, it is desired a pixel structure for an LCoS which separates the optical paths of the incident light and the reflective light so as to enhance the light throughput and contrast.  
     SUMMARY OF THE INVENTION  
      Accordingly, one object of the present invention is to provide a pixel structure of an LCoS that diffracts or refracts an oblique incident light at specific angles out of the glass plate in the LCoS display.  
      Another object of the present invention is reflecting an oblique incident light at specific angles out of the glass plate in the LCoS display by diffraction or refraction by reflectors with reflective surface in different slopes.  
      Yet another object of the present invention is reflecting an oblique incident light at specific angles out of the glass plate in the LCoS display by diffraction or refraction by gratings with length close to or shorter than the wavelength of the incident light.  
      Still another object of the present invention is reflecting an oblique incident light at specific angles out of the glass plate in the LCoS display by diffraction or refraction by reflectors coated with multilayer coatings of different refractive indexes.  
      In a pixel structure for an LCoS display, according to the present invention, an insulator is formed on a pixel electrode by chemically mechanical polishing (CMP), several reflectors on the insulator, a passivation formed on the reflectors and insulator, a transparent conductor on the passivation, a layer of LC above the conductor, and a glass plate above the layer of liquid crystal.  
      In one embodiment, the reflector includes one or more oblique metal plates or high reflective multilayer coatings to reflect the oblique incident light to produce the reflective light at specific angles by diffraction or refraction out of the glass plate. In another embodiment, the reflector includes optical gratings or multilevel diffractive reflector to reflect the oblique incident light. In still another embodiment, the reflector includes a planar reflective surface with one or more coatings thereon to reflect the oblique incident light. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      The above and other objects, features and advantages of the present invention will become apparent to those skilled in the art upon consideration of the following description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings, in which:  
       FIG. 1  shows a pixel structure of a conventional LCoS;  
       FIG. 2  shows the simplified cross-sectional view of an embodiment pixel structure for an LCoS according to the present invention;  
       FIG. 3  shows a variation of the pixel structure shown in  FIG. 2 ;  
       FIG. 4  shows a further variation of the pixel structure shown in  FIG. 2 ;  
       FIG. 5  shows a variation of the pixel structure shown in  FIG. 4 ;  
       FIG. 6  shows a further variation of the pixel structure shown in  FIG. 4 ;  
       FIG. 7  shows the simplified cross-sectional view of another embodiment pixel structure for an LCoS according to the present invention;  
       FIG. 8  is an enlarged view of the optical grating in  FIG. 7 ;  
       FIG. 9  shows a variation of the pixel structure shown in  FIG. 7 ;  
       FIG. 10  shows a further variation of the pixel structure shown in  FIG. 7 ;  
       FIG. 11  shows a variation of the pixel structure shown in  FIG. 10 ;  
       FIG. 12  shows the relation between the incident angle and the period of the optical grating;  
       FIG. 13  shows the simplified cross-sectional view of yet another embodiment pixel structure for an LCoS according to the present invention;  
       FIG. 14  shows a variation of the pixel structure shown in  FIG. 13 ; and  
       FIG. 15  shows a variation of the pixel structure shown in  FIG. 14 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
       FIG. 2  shows the simplified cross-sectional view of an embodiment pixel structure for an LCoS according to the present invention. A pixel structure comprises a pixel electrode  214 , an insulator  212  formed on the pixel electrode  214  by CMP, several reflectors  210  on the insulator  212  to reflect an oblique incident light  216 , a passivation  208  formed on the reflectors  210  and insulator  212  by CMP, a conductor  206  on the passivation  208 , a layer of liquid crystal  204  above the conductor  206 , and a glass plate  202  above the layer of liquid crystal  204 . The conductor  206  is directly connected to the pixel electrode  214 . The angles Φ between each of the reflectors  210  and the insulator  212  are the same, and the lengths L and heights h of the reflectors  210  are also the same. The reflector  210  includes a high reflective metal such as Al, Ag or their alloy. Alternatively, the reflector  210  may be formed with multilayer coatings of high reflectivity. As shown in  FIG. 2 , the incident light  216  is incident into the glass plate  202  with an incident angle θ i , and after refracted by the glass plate  202 , the light  218  becomes at an angle θ i ′. The refractive light  218  reaches the reflector  210  through the layer of LC  204 , the conductor  206  and the passivation  208 , and reflected by the reflector  210  to produce the reflective light  220  at an angle θ o ′. The reflective light  220  passes through the glass plate  202  and has a final output angle θ o . The output angle θ o  is in the range of 0 to 65 degrees, the incident angle θ i ′ within the pixel  20  is in the range of 10 to 80 degrees, and the reflected angle θ o ′ within the pixel  20  is in the range of 0 to 45 degrees. On the other hand, each oblique reflector  210  has a height h of 0.05 to 5 μm and a length L of 0.05 to 15 μm, and the incident angle Φ is in the range of 0.5 to 45 degrees. When the length L of the reflector  210  is larger than the wavelength λ of the incident light  218 , for example with the ratio of L/λ larger than 20, the reflection caused by the reflector  210  will not appears obvious diffraction. While the length L of the reflector  210  is smaller than or close to the wavelength λ of the incident light  218 , for example with the ratio of L/λ between 0 and 20, the reflection caused by the reflector  210  will have obvious diffraction to enhance the light throughput and contrast. In this embodiment, due to the incident angles Φ to each reflector  210  and insulator  212  all the same, the panel can only reflect the incident light at one color or one specific wavelength, and thus three panels are used to separately modulate the reflective brightness of red, green and blue lights. In addition, the height h or the length L of the reflectors  210  can be arranged in an order or in a regular distribution.  
       FIG. 3  shows a variation of the pixel structure shown in  FIG. 2 , where a pixel structure  20   a  is similar to the pixel structure  20  of  FIG. 2  in that they both have a pixel electrode  214 , an insulator  212 , several reflectors, a passivation  208 , a conductor  206 , a layer of LC  204 , and a glass plate  202 . However, the reflectors of the pixel  20   a  are divided into three groups  210   a   1 ,  210   a   2  and  210   a   3  with an oblique angles Φ a1 , Φ a2  and Φ a3  between each of them and the insulator  212 , and the lengths L a1 , L a2  and L a3  and the heights h a1 , h a2  and h a3  of them are also different. Moreover, the number of the reflectors in each group may be also different, i.e. at different densities of distributions. As a result, this embodiment can reflect three color lights by the varied reflectors. Likewise, if the ratios L a1 /λ a1 , L a2 /λ a2  and L a3 /λ a3  of the lengths L a1 , L a2  and L a3  of the reflectors to the wavelengths λ a1 , λ a2  and λ a3  of the incident lights are all larger than 20, the diffraction effect will be nonobvious. However, the refraction and reflection effects can be used for reflecting light at specific angles to enhance the light throughput and contrast. In contrast, if the ratios L a1 /λ a1 , L a2 /λ a2  and L a3 /λ a3  lie in the range of 0 to 20, the diffraction effect will be obvious for the light reflection and thus to enhance the light throughput and contrast. Moreover, the lengths L a1 , L a2  and L a3  and the heights h a1 , h a2  and h a3  of the reflectors  210   a   1 ,  210   a   2  and  210   a   3  arranged in an order or in a regular distribution.  
       FIG. 4  shows a further variation of the pixel structure shown in  FIG. 2 , where a pixel structure  20   b  is similar to the pixel structure  20  of  FIG. 2  in that they both have a pixel electrode  214 , an insulator  212 , a passivation  208 , a conductor  206 , a layer of LC  204 , and a glass plate  202 . However, the reflectors of the pixel  20   b  include only three oblique reflectors  210   b  each having a same length L b  and a same height h b  and a same oblique angle Φ b  to the insulator  212 , thereby one panel of this embodiment only reflects one color light. Again, when the length L b  of the reflector  210   b  is larger than the wavelength λ of the incident light  218 , i.e., the ratio L b /λ is larger than 20, no obvious diffraction appears to the reflective light  220 , while the refraction and reflection effects can be used for reflecting light at specific angles to enhance the light throughput and contrast. If the length L b  of the reflector  210   b  is smaller or near to the wavelength λ of the incident light  218 , i.e., the ratio L b /λ between 0 and 20, obvious diffraction appears to the reflective light  220  and thus to enhance the light efficiency and contrast.  
       FIG. 5  shows a variation of the pixel structure shown in  FIG. 4  with the difference that the conductor  206  in  FIG. 4  is connected to the pixel electrode  214  through the conductive reflector  210   c , while the conductor  206  in  FIG. 5  is directly connected to the pixel electrode  214 .  
       FIG. 6  shows a further variation of the pixel structure shown in  FIG. 4 , where the included angles Φ R , Φ G  and Φ B  of the reflectors  210 R,  210 G and  210 B to the insulator  212  are all different to each other, and the lengths L R , L G  and L B  and height h R , h G  and h B  of the reflectors  210 R,  210 G and  210 B are also different to each other. Therefore, one panel can reflect three color lights in this embodiment. As shown, a red incident light  222  with an incident angle θ iR  produces a refractive light  224  with an angle θ iR ′ after refracted by the glass plate  202 . The refractive light  224  traverses through the LC  204 , the conductor  206  and the passivation  208  to the reflector  210 R, and reflected by the reflector  210 R to produce the reflective light  226  at an angle θ oR ′, which is further refracted to an angle θ oR  out of the glass plate  202 . Similarly, the green incident light  228  and the blue incident light  234  become the refractive lights  230  and  236  after refracted by the glass plate  202 , and further become the reflective lights  232  and  238  after reflected by the reflectors  210 G and  210 B, which are further refracted out of the glass plate  202  at specific angles θ oG  and θ oB . The angles θ oR , θ oG  and θ oB  all lie in the range of 0 to 45 degrees. The reflectors  201 R,  210 G and  210 B each can only reflect the red, green or blue lights individually, and imposes no effect to the other two color lights. For example, when the green incident lights  2281  and  2284  become the refractive lights  2282  and  2285  after refracted, and further become the reflective lights  2283  and  2286  after reflected by the reflectors  210 R and  210 B, the reflective lights  2283  and  2286  are finally refracted by the glass plate  202  at alternative angles θ oGR  and θ oGB , thereby inducing no effect for the angles θ oGR  and θ oGB  different from the proper θ oG .  
       FIG. 7  shows the simplified cross-sectional view of another embodiment pixel structure for an LCoS according to the present invention. Likewise, a pixel structure  30  comprises a pixel electrode  214 , an insulator  212 , several reflectors  310 , a passivation  208 , a conductor  206 , a layer of LC  204 , and a glass plate  202 . However, optical gratings  310  are used for the reflectors herewith. The incident light  216  produces a refractive light  218  after refracted by the glass plate  202 , and the ratio L′/λ of the length L′ of each optical grating  310  and the wavelength λ of the incident light lies in the range of 0 to 20 to thereby reflect the refractive light  218  with obvious diffraction. Then the reflective light  220  is refracted out of the glass plate  202  at a specific angle with enhanced light efficiency and contrast. In this embodiment, each period a of the gratings  310  has the same value, and the pixel structure  30  can only reflect one color light for one panel. Moreover, the lengths L′ of each optical grating  310  are distributed equally or regularly.  
       FIG. 8  is an enlarged view of the optical grating  310  in  FIG. 7 , which includes a series of strip metals arranged regularly or periodically on the insulator  212 . Particularly, the lengths of the strip metals  3102 ,  3104 ,  3106 ,  3107 ,  3108  and  3109  are L′ 1 , L′ 2 , L′ 3 , L′ 4 , L′ 5  and L′ 6 , respectively, and the gaps between each two adjacent strip metals are w 1 , w 2 , w 3 , w 4  and w 5 , respectively, where both the lengths L′ 1 , L′ 2 , L′ 3 , L′ 4 , L′ 5  and L′ 6  and the gaps w 1 , w 2 , w 3 , w 4  and w 5  decrease gradually in an order. As a result, the lengths, gaps and direction of arrangement will affect the angle and direction of reflective light.  
      For illustration, the parameters and effects observed on the pixel structure  30  of  FIG. 7  when the incident light  216  has a wavelength of 500 nm and an output angle θ o  is 0 degree are listed in Table 1. The relation between the incident angle θ i  and period a of the grating  310  is  
                           TABLE 1                                   Incident Angle θ i     Period a (um)                                                    10   3.16729           15   2.12503           20   1.60809           25   1.30141           30   1.1           35   0.95889           40   0.85565           45   0.77781                      
 
 According to Table 1, the period a determines the incident light angles, and when the period a is smaller the incident light angle is larger. 
 
       FIG. 9  shows a variation of the pixel structure shown in  FIG. 7 . The pixel structure  30   a  hereof is noted that the optical gratings are divided into three groups  310   a   1 ,  310   a   2  and  310   a   3 , with different periods a 1 , a 2  and a 3  and lengths L′ a1 , L′ a2  and L′ a3  thereof, and the number of the optical gratings in the respective group are also different, i.e., different densities of distributions, thereby three color lights can be reflected by one panel of this embodiment.  
       FIG. 10  shows a further variation of the pixel structure shown in  FIG. 7 . Particularly, each optical grating  310   b  hereof includes a plurality of metals in stack on the insulator  212 . Similarly, the ratio L′ b /λ of the length L′ b  of the optical grating  310   b  to the wavelength λ of the incident light  216  lies in the range of 0 to 20, and thus the diffraction effect is produced and much more than that in  FIG. 7 . In this embodiment, each period a b  has the same value, and one panel can therefore reflect only one color light. Moreover, the length of each optical grating  310   b  is selected regularly or periodically. The optical grating  310   b  in this embodiment can be also formed with one layer of metal and multilayer coatings thereon, or a multilayer coating of high reflectivity, in which each coating has a different refractive index.  
      For illustration, the parameters and effects observed on the pixel structure  30   b  of  FIG. 10  when the incident light  216  has a wavelength of 550 nm and an output angle θ o  is 15 or 30 degrees are listed in Table 2. Modulating the period a b  and the incident angle θ i , the first and second order diffractive ratio of the incident light  216  is  
                               TABLE 2                       Period a b     Reflective Angle θ o     Height   1R   2R                                                    0.6   15   0.4   0.874129           0.7   15   0.4   0.92764       0.8   15   0.4   0.92043       0.9   15   0.4   0.858215       0.6   30   0.4   0.96468       0.7   30   0.4   0.94933       0.8   30   0.4   0.882393       0.9   30   0.4   0.865683       1   30   0.4   0.853313       0.6   30   0.5   0.89832       1   30   0.7       0.868452       1   30   0.8       0.91208                  
 
 In Table 2, 1R and 2R denote the diffractive ratios for the first and second order to the incident light  218 . The better range of diffraction effect in Table  2  can be determined by  
               y   =     0.8   +     5.1   ×     ⅇ     -     (       x   -   5.5     7.6     )               ,   and           [     EQ   ⁢     -     ⁢   1     ]                 y   =     0.1   +     4.6   ×     ⅇ     -     (       x   -   0.4     27     )               ,           [     EQ   ⁢     -     ⁢   2     ]             
 
 where, y is the incident angle θ i , and x is the period a b .  FIG. 12  shows the curves  32  and  34  for the equations EQ-1 and EQ-2, respectively, and the better range for diffraction effect is among that between the curves  32  and  34 . 
 
       FIG. 11  shows a variation of the pixel structure shown in  FIG. 10 . The pixel structure  30   b  hereof is noted in that the multilevel diffractive reflectors are divided into three groups  310   c   1 ,  310   c   2  and  310   c   3 , with different number and length of multilayer in stack and the periods a′ c   1 , a′ c   2  and a′ c   3  thereof. In this embodiment, one panel can reflect three color lights.  
       FIG. 13  shows the simplified cross-sectional view of yet another embodiment pixel structure for an LCoS according to the present invention. The pixel structure  40  hereof is similar to the foregoing embodiments, except that three planar reflectors  410  are used and a microprism  402  (or air) is buried in the passivation  208  and above the planar reflectors  410 . Each microprism  402  has an angle Φ′ (or a slope), a length L″ and a height h″. The refractive index of the passivation  208  is n 1 , and that of the microprism  402  is n 2 , where n 1  is not equal to n 2 , and n 1 -n 2  is larger or equal to 0.02. After the incident light  216  refracted by the glass plate  202 , the refractive light  218  arrives the microprism  402  through the layer of LC  204 , the conductor  206  and the passivation  208 . The refractive light  218 ′ perpendicular to the planar reflector  410  is produced after the refractive light  218  is refracted by the microprism  402 , with an angle θ o ′ after reflected by the reflector  410  and refracted once again by the microprism  402 , and finally refracted out of the glass plate  202  with the output angle θ o . If the ratio L″/λ of the length L″ of the microprism  402  and the wavelength λ of the incident light  216  is larger than 20, the diffraction will not appear. In this case the refraction and reflection effects can be used to reflect the light to specific angles to enhance the light efficiency and contrast. If the ratio L″/λ lies in the range of 0 to 20, obvious diffraction will appear and enhance the light efficiency and contrast. Moreover, since the angle Φ′ (or slope), length L″ and height h″ of the microprisms  402  in each and other reflectors are all the same, the panel reflects only one color light in this embodiment.  
       FIG. 14  shows a variation of the pixel structure shown in  FIG. 13 . The pixel structure  40   a  in  FIG. 14  includes several microprism  402   a  buried in each planar passivation  208 . If the ratio L″/λ of the length L″ of the microprism  402   a  and the wavelength λ of the incident light  216  is larger than 20, diffraction effect will not appear but refraction effect will. If the ratio L″/λ is in the range of 0 to 20, diffraction effect will appear and can be used to enhance the light efficiency and contrast. Moreover, since the angle Φ′ a  (or slope), length L″ a  and height h″ a  of each reflector is same, the pixel structure  40   a  reflects one color light.  
       FIG. 15  shows a variation of the pixel structure shown in  FIG. 13 . For the reflectors hereof, the lengths L″ b1 , L″ b2  and L″ b3 , the heights h″ b1 , h″ b2  and h″ b3 , and angles Φ′ b1 , Φ′ b2  and Φ′ b3  of the microprisms  402   b   1 ,  402   b   2  and  402   b   3  are all different, and the number (or density) of the microprisms  402   b   1 ,  402   b   2  and  402   b   3  are also different. Therefore, the pixel structure  40   b  in this embodiment can reflect three color lights at the same time. If the ratios L″ b1 /λ, L″ b2 /λ and L″ b3 /λ of the lengths L″ b1 , L″ b2  and L″ b3  of the microprisms  402   b   1 ,  402   b   2  and  402   b   3  in contact with the reflector  410  to the wavelength λ of the incident light  216  are larger than 20, diffraction effect will not appear but refraction and reflection effect can be used to reflect the light at specific angles. If the ratios L″ b1 /λ, L″ b2 /λ and L″ b3 /λ lie in the range of 0 to 20, diffraction will appear and can be used to enhance the light efficiency and contrast. Moreover, the microprisms  402   b   1 ,  402   b   2  and  402   b   3  on each reflector can be arranged regularly or periodically, and the distribution of microprisms  402   b   1 ,  402   b   2  and  402   b   3  on each and other reflectors can be different.  
      While the present invention has been described in conjunction with preferred embodiments thereof, it is evident that many alternatives, modifications and variations will be apparent to those skilled in the art. Accordingly, it is intended to embrace all such alternatives, modifications and variations that fall within the spirit and scope thereof as set forth in the appended claims.