Patent Publication Number: US-6222508-B1

Title: VLSI visual display

Description:
This is a continuation of application Ser. No. 08/922,268, filed Sep. 2, 1997, which is a continuation of application Ser. No. 08/519,269, filed Aug. 25, 1995 now U.S. Pat. No. 5,867,134. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to visual displays and more particularly to visual displays having miniature displays that are compatible with very large scale integrated (“VLSI”) circuit technology. 
     BACKGROUND OF THE INVENTION 
     Current commercially available electronic displays are dominated by cathode ray tubes (“CRTs”) and liquid crystal displays (“LCDs”). LCDs offer advantages in power, size, and safety, while CRTs are well understood and inexpensive to manufacture. Recent advancements in LCD technology have led to the development of large area, high resolution LCDs, initially using a passive matrix scanning technique (i.e., pixels in the display are directly controlled by the address lines), and more recently using active matrix addressing techniques (i.e., the pixels in the display are controlled by addressing transistors associated with each pixel). 
     Compared with passive matrix LCDs, active matrix LCDs produce higher quality images but require greater capital investments in manufacturing. To manufacture active matrix LCDs, photolithographic masks of the same size as the LCDs and processing equipment suitable for handling the same size substrate are required. In addition, active matrix displays are generally built on glass substrates using either amorphous or polycrystalline thin film transistors (“TFTs”) which typically exhibit poor electrical characteristics and low yield. Further, because thin film transistors typically operate at a relatively low speed, the speed of the display is thus limited. Moreover, due to the low device density and low yield associated with thin fin transistors, the resolution of such LCDs is limited. 
     Besides the above-described TFT LCDS, there has been a great deal of interest in developing silicon backplane LCDs. In contrast to TFT LCDs which use thin film transistor circuits fabricated on a glass substrate as a backplane, silicon backplane LCDs use integrated circuits fabricated on single crystalline silicon substrates as backplanes. Due to the advantages of silicon integrated circuits over TFT circuits, silicon backplane LCDs operate at a higher speed and offer higher resolution and higher yield. 
     Kaneko, E., “Liquid Crystal TV Displays: Principles and Applications Of Liquid Crystal Displays,” KTK Scientific Publishers, 1987, describes a MOS transistor switch matrix addressed liquid crystal TV display. The TV display comprises a MOS transistor-addressed pixel array formed on a single crystalline silicon substrate; the array is integrated with its driving circuit as a hybrid and is connected to the driving circuits by wire bonding. This TV display suffers several significant drawbacks. First, because the yield for the array decreases drastically with increased array size, the size of the display is limited to about one inch by one inch, which is generally too small to be used as a TV or computer display. Second, the driver circuits are not monolithically integrated with the display array, which makes it expensive to fabricate and which also limits the resolution of the display since it is very difficult to interconnect the driver circuits with an array having a large number of pixels using wire bonding. Consequently, the resolution of the described TV display is limited for the purposes described herein. For example, one of the described TV displays has only 240×240 pixels formed on an area of 3.6 cm×4.8 cm. 
     Jared et al., “Electrical Addressed Spatial Light Modulator That Uses A dynamic Memory”, Optical Letters, (16:22), pp. 1785-1787 (Nov. 15, 1991), describes an electrically addressed spatial light modulator that consists of a silicon VLSI backplane chip and a ferroelectric-liquid-crystal (“FLC”) cell. The VLSI chip includes a 64×64 array of pixels located at the center of the chip; it is fabricated using a 2 micron, n-well, complementary metal-oxide-semiconductor (“CMOS”) process. The 64×64 pixel array appears to be monolithically formed with its electronic addressing circuits on the VLSI chip. However, this type of modulator would not be useful for image displays because the array area is too small. 
     Despite the clear advantages of VLSI backplane LCDs over TFT LCDs in speed and resolution, they are not viewable with the naked eye and are not used commercially for image displays. 
     Accordingly, it is an object of the present invention to provide a visual display that is small in size but has a high resolution and can be seen by the naked eye; 
     it is another object of the present invention to provide a visual display that is compatible with VLSI technology, operates at a high speed and has low power consumption; 
     it is a further object of the present invention to provide a visual display with reduced image aliasing; and 
     it is still a further object of the present invention to provide a visual display with reduced optical distortion. 
     SUMMARY OF THE INVENTION 
     The present invention provides a visual display which comprises a miniature display compatible with VLSI technology to display thereon images having desired quality but are generally too small to be viewed by the naked eye and an optical system disposed in front of the miniature display for enlarging such images to be visible at the desired image quality. 
     The miniature display includes an array of display elements of sufficient resolution and a VLSI circuit monolithically integrated with the array for driving the display elements. The VLSI circuit includes input terminals for providing electrical signals thereto to generate the images. The VLSI circuit may further include a signal processing circuit or a microprocessor for processing signals applied through the input terminals to affect the displayed image. 
     In a preferred embodiment, the visual display includes a miniature display that is a liquid crystal display and the visual display further includes a light source for providing illumination to the liquid crystal display. The liquid crystal display comprises a transparent cover positioned above the array of display elements, a transparent electrode deposited on the surface of the cover facing the array, and liquid crystal material such as dynamic scattering mode liquid crystal material disposed between the array and the transparent electrode. 
     In another preferred embodiment, a visual display with reduced image aliasing is provided. The visual display includes a miniature display having an array of display elements with the positions of the display elements be displaced by small random distances from the positions of elements in a corresponding symmetrical array. Preferably, the layout of the array is made with the use of a software silicon compiler program to determine the positions and the displacement of the display elements. 
     In another preferred embodiment, a visual display including a miniature display having an array of super-pixels, each comprising a central display element surrounded by a plurality of ancillary display elements. In the array, the central display elements are directly driven by a VLSI circuit and the ancillary display element are driven in response to electrical signals from neighboring central display elements. Image aliasing in the display is reduced. 
     In another preferred embodiment, a visual display including a miniature display made to compensate for optical distortion of the optical system used to view the images displayed by the miniature display. The optical distortion of the optical system is characterized by an optical distortion function. The miniature display includes an array of display elements which are positioned and scaled in accordance with an inverse function of such optical distortion function. Preferably, the array is made with the use of a silicon software compiler program to determine the positions of and to scale the display elements. 
     The present invention also provides a virtual image display which utilizes the visual display of the present. The virtual image display comprises an eye glass frame, at least one lens having a reflective surface mounted therein, and a visual display of the present invention. The visual display is positioned such that images displayed thereon are reflected by the lens into an eye of a person wearing the frame. 
     Preferably, the reflective surface of the lens is curved in accordance with a three-dimensional elliptical surface, and the visual display and the eye of the viewer are positioned at two focal points of the elliptical surface respectively such that the images displayed by the visual display are reflected by the reflective surface right into the eye of the person wearing the eye glass frame. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other features, objects, and advantages of the present invention will become more apparent from the following detailed description in conjunction with the appended drawings in which: 
     FIG. 1 depicts a reflective-mode visual display of the present invention; 
     FIG. 2 illustrates a transmissive-mode visual display of the present invention; 
     FIGS. 3A and 3B illustrate a virtual image display utilizing the visual displays of the present invention; 
     FIG. 4 depicts the physical arrangement of a LCD of the present invention; 
     FIGS. 5A-C show the schematic circuit diagram of a VLSI backplane, the circuit diagram and the layout of a single pixel in the VLSI backplane, respectively; 
     FIGS. 6A and 6B illustrate the operation of a single pixel cell having dynamic scattering liquid crystal material; 
     FIG. 7 illustrates an alternative embodiment of the VLSI backplane; 
     FIG. 8 illustrates a preferred embodiment of the visual display having an array of randomly displaced pixels for reducing image aliasing; 
     FIG. 9 depicts another preferred embodiment of the visual display having an array of super-pixels; 
     FIG. 10A illustrates the optical distortion of a magnifier having short focal length; 
     FIG. 10B shows a computer-displayed drawing depicting the inverse distortion function for a LEEP-type optical system; 
     FIG. 10C shows a top view of a fabricated pixel array designed to compensate for optical distortion. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to FIG. 1, a visual display  100  of the present invention comprises a reflective-mode miniature display  105  producing images on its front surface  110 , a light source  115  for illuminating front surface  110 , and an optical system  120  for magnifying the images displayed by the miniature display so that the images are visible to the naked eye. 
     Alternatively, a visual display of the present invention (not shown) comprises a miniature display compatible with VLSI technology and includes an array of display elements of sufficient resolution to provide a desired image quality, a VLSI circuit for driving the display elements to form images on the array that are generally too small to be viewed by the naked eye, and an optical system disposed in front of the miniature display for recording such images such that they can be displayed or viewed by the naked eye. The VLSI circuit is monolithically integrated with the array and it includes input terminals for providing electrical signals thereto to generate such images. 
     Miniature display  105  includes a reflective-mode liquid crystal display  104  and is compatible with VLSI technology. As defined hereinafter, a display is compatible with VLSI technology if it includes an array of display elements for display images thereon and a circuit for driving the array to display the images and if the array and its driving circuit are monolithically formed on a single crystalline semiconductor substrate. Preferably, a display compatible with VLSI technology has a high resolution. 
     Referring to FIG. 2, in an alternative embodiment, a visual display  200  comprises a transmissive-mode miniature display  205  compatible with VLSI technology, a light source  215  for providing a backside illumination, and an optical system  220  for magnifying images displayed on the front side of the miniature display. In a preferred embodiment, the transmissive-mode miniature display is a transmissive-mode liquid crystal display. 
     FIGS. 3A and 3B illustrate an eye-glass type virtual image display which utilizes the visual displays described above. The virtual image display comprises an eye glass frame  300 , a pair of lenses  305  and  310  mounted on the frame, and two visual displays  301  of the type described above. Lenses  305  and  310  have reflective surfaces  304  and  309 , respectively, for reflecting images generated by the visual displays to the eyes of the person wearing the frame. The reflected images are seen by the person as if they were displayed on a distant virtual screen  330 . 
     Preferably, each of the reflective surfaces is curved in accordance with a three-dimensional elliptical surface. Moreover, the visual display is positioned and the frame designed such that the visual display and an eye of a viewer wearing the frame are approximately located at the two focal points respectively of such elliptical surface; this ensures that the image displayed on the visual displays are reflected directly by the elliptical surface into the eyes of the viewer. 
     The reflective surfaces may be formed to cover the entire lenses or only portions of the lenses for a heads-up display. To provide electrical signals to the liquid crystal displays, the virtual image display may further include a video cassette recorder, a video compact disc player, or a computer (not shown) electrically connected to the liquid crystal displays. 
     It should be apparent to one skilled in the art that, although FIGS. 3A and 3B depict a virtual display of the present invention with reflective mode visual displays, transmissive-mode visual displays may also be used in place of the reflective-mode visual displays, which is also within the scope of the present invention. 
     In the preferred embodiment, the miniature display is a liquid crystal display compatible with VLSI technology. The liquid crystal display in accordance with the present invention is described below in conjunction with FIGS. 4-7. 
     FIG. 4 illustrates the physically arrangement of a preferred reflective-mode liquid crystal display. For clarity, the components of the LCD are purposely drawn to be vertically spaced apart. As shown, an LCD  400  of the present invention includes a VLSI backplane  410  and a glass cover  430  having a transparent electrode  425  such as a layer of indium-tin-oxide deposited on the surface of the cover facing the backplane. Disposed between the backplane and the glass cover are a polyamide spacer ring  415 , liquid crystal material  435  and a bonding pad  420  used to join the cover glass and the spacer. The VLSI backplane herein refers to an array of display elements and a circuit for driving the array monolithically formed on a single crystalline substrate. 
     FIG. 5A depicts a schematic circuit diagram of a preferred VLSI backplane  500  which includes an array  510  consisting of rows and columns of display elements  515 . Each display element is a minimum controllable display unit and is referred hereinafter as a pixel. To obtain desired resolution, the array contains a large number of pixels formed on a small area. For example, the array may include 2,000 by 2,000 (4 mega) pixels formed on one square centimeter area with each pixel having a dimension of 5 by 5 microns. As it is known to one of skill in the art, this kind of resolution is in general only obtainable by the use of VLSI technology. 
     In the VLSI backplane, a row scanning bus driver  520 , connected to the rows of display elements through a row bus  521 , operates to sequentially scan rows of the pixels so that each frame of the images is formed by scanning the entire array line by line. The row scanning bus driver has a clock terminal  501  for receiving an external clock signal and a terminal  502  for receiving a row control signal. 
     The backplane also includes a video signal buffer  525  connected to the columns of pixels via a column bus  524  and a video signal driver  526  coupled to the video signal buffer. Video signal driver  526  receives a clock pulse signal from terminal  503  and serial video data from terminal  504 ; it then sequentially shifts the serial video data to form a row of video data to be written into a row of the pixels. The row of data is subsequently provided to the video signal buffer. Upon receiving a synchronized signal at a terminal  505 , the video signal buffer provides the row of data to column bus  524  while a row of the pixels is turned on by the row scanning bus driver. Consequently, the row of video data is stored into the row of pixels and a line of image is thus formed. The process is then repeated line by line until a frame of image is formed. Preferably, the row scanning bus driver, the video signal buffer and the video signal driver are all CMOS circuits formed on a single crystalline silicon substrate. 
     FIGS. 5B and 5C illustrate a schematic circuit diagram and the layout of an individual pixel  515 . As shown, individual pixel  515  includes a reflective pixel electrode  511  to be physically in contact with the liquid crystal material, a MOS storage capacitor  512  for storing a video signal charge sufficient to drive the liquid crystal material between successive frame periods, and a MOS field effect transistor  513  operating as a switch to the pixel. The pixel is selected by applying an row scanning signal to the gate terminal of MOS transistor  513 . When the pixel is selected and a video signal is provided to a source terminal  516  from the column bus  524 , MOS transistor  513  is turned on and the video signal is provided from the source terminal to the drain terminal of the MOS transistor and to the pixel electrode and the capacitor. 
     The principle of operation for a single pixel is now described in conjunction with FIGS. 6A and 6B. Referring to FIG. 6A, in each pixel, dynamic scattering (“DS”) mode liquid crystal material  600  is disposed between a reflective pixel electrode  605  and a transparent ground electrode  610  formed on the cover glass. Before the liquid crystal material is filled in vacuum into the space between the reflective pixel electrode and the ground electrode, the surfaces of both electrodes are treated with a coupling agent such as alkoxysilane so that, when there is no electrical signal applied to the electrodes, molecules of the DS liquid crystal material are aligned perpendicularly to the electrodes. At this state, the DS liquid crystal material is transparent to light and an incident illumination will traverse the DS liquid crystal material and be reflected by the reflective display electrode. As a result, the pixel appears to have a silver color. 
     Referring to FIG. 6B, when a video signal is applied between the electrodes, the perpendicular alignment of the DS liquid crystal molecules is destroyed and the incident illumination is scattered. At this state, the pixel appears gray and the degree of grayness depends on the amplitude of the applied video signal. 
     When the entire array operates, gates of successive rows of the MOSFETs are turned on sequentially from top to bottoms by pulses from the row scanning bus drivers and appropriate video signals are simultaneously applied to the source terminals of the MOS transistors from the video signal buffer. The video signals charge the storage capacitors associated with the MOS transistors to levels related to the video signals. The charges stored in the capacitors are then retained for a frame period, leaking off slowly through the liquid crystal layer. As the capacitors are discharged through the DS liquid crystal layer, the layer produces a gray shade or an image that corresponds to the video signals applied to the capacitors during addressing. 
     For the transmissive-mode visual display shown in FIG. 2, a transmissive-mode LCD having a VLSI backplane is used. The VLSI backplane includes a thin substrate transparent to a backside illumination and thin layers of doped silicon or poly-silicon which is transparent to light as pixel display electrodes. Images are generated by illuminating the backside of the VLSI backplane and by providing video signals to pixel display electrodes to modulate the local transmissivity of the liquid crystal material to generate images. 
     The transmissive-mode VLSI backplane may be formed by first fabricating the array of pixels and its driving circuit on an opaque silicon substrate and subsequently thinning the substrate until it becomes transparent. Another way to fabricate the transparent. backplane is first forming a thin layer of single crystalline epitaxial silicon on a silicon dioxide substrate, fabricating the array and its driving circuit on the silicon layer, and then removing the silicon dioxide substrate. 
     FIG. 7 illustrates the schematic circuit diagram of a VLSI backplane for another embodiment of the visual display, wherein like elements are similarly designated as those in FIG.  5 A. As depicted, a VLSI backplane  500  includes inter alia a microprocessor  550  monolithically formed on the same substrate with a pixel array  510 , a row scanning bus driver  520 , a video signal driver  526  and a video signal buffer  525 . 
     Illustratively, microprocessor  550  is interconnected to row bus driver  520  via a bus  555 , to video signal driver  526  via a bus  560 , and to video signal buffer  525  via a bus  561 . The microprocessor receives input video signals and other signals through terminals  501 - 505  and processes the video signals before providing the processed signals to the row scanning bus driver, the video signal driver and the video signal buffer. Thus real-time image signal processing may be performed by the microprocessor. As it will be apparent to one of skill in the art, a signal processing circuit may be used in replace of or in combination with the microprocessor circuit. 
     In an alternative embodiment (not shown), a VLSI backplane including a matrix of microprocessors is provided. Each of the microprocessors is located in the vicinity of and associated with one or more pixels. The microprocessors are used to perform local-algorithm imaging processing tasks such as resolution enhancement by interpolation or contrast enhancement and are monolithically integrated with the pixel array and the driver circuits. 
     In another preferred embodiment, the visual display of the present invention incudes a miniature display having a VLSI backplane designed for reducing image aliasing. The backplane includes a pixel array with the positions of the pixels displaced by small random distances from the positions of pixels in a corresponding symmetrical array. This perturbation of pixel positions subtly breaks the symmetry and regularity of the pixel array and as a result greatly reduces image aliasing. 
     FIG. 8 illustrates the array of displaced pixels. In the drawing, pixels  800  represents a corresponding symmetrical array and pixels  805  are the result of displacing pixels  800  by small random distances. Preferably, the array is designed with the use of a software silicon compiler program and the displacement of pixel position is implemented by the use of such silicon compiler program. In designing the array with displaced pixels, factors such as the maximum allowable displacement for each pixel without noticeable image distortion and the minimum acceptable distance between the pixels should be considered. Those factors are important in preventing image distortion and pixel overlapping. 
     As compared with other available methods for reducing image aliasing which generally requires additional circuitry for real-time signal processing, the random displacement of pixel positions uses no additional circuitry and requires no real-time signal processing. Moreover, it can be accomplished by modifying existing silicon circuit design tools without substantially additional cost. It should be apparent to those skilled in the art that the present method for reducing aliasing by random displacement of pixel positions is applicable to any types of displays using a pixel array and it is not limited to LCDs. 
     In another preferred embodiment, the visual display of the present invention includes a miniature display having an array of super-pixels for reducing image aliasing. As depicted in FIG. 9, each super-pixel comprises a central display element surrounded by a number of ancillary display elements and a switch MOSFET. For example, a super-pixel  900  includes a central display element  905  surrounded by ancillary display elements  906 ,  907 ,  908 ,  909 ,  910 , and  911 . Neighboring super-pixels share the ancillary display elements interposed between them. For example, super-pixels  900  and  915  share ancillary display elements  911 . 
     In each super-pixel, only the central display element is directly driven by a driver circuit. For example, central display element  905  is driven by the driver circuit via MOSFET  912  through a row address line  913  and a column address line  914 ; its surrounding ancillary elements are not directly driven by the driver circuit through the address lines. 
     Each ancillary display element, however, is driven by a signal from an NAND gate responsive to two video signals from its neighboring central display elements. For example, ancillary element  911  is driven by a signal at the output of an NAND gate  917  which are responsive to video signals from neighboring central display elements  905  and  916 . 
     Obviously, other types of circuits may also be used in addition to or place of the NAND gates. For example, instead of being responsive to the video signals of neighboring central display elements, the ancillary elements may response to signals other than the video signals of the two neighboring central display elements, which is also within the scope of the present invention. 
     In yet another preferred embodiment, the visual display of the present invention incudes a miniature display having a pixel array made to compensate for non-linear optical distortion of an optical system used to view the images display on the pixel array. Although there are currently available methods for compensating optical distortion in an artificially-generated “virtual-reality display” by, for example, pre-processing image data in real-time to pre-warp the image data before it is “distorted” by a lens and providing the processed data to a display, those methods generally require extensive hardware for real-time image processing and their utilities are thus severely limited. 
     The optical distortion of an optical system is characterized by an optical distortion function. To compensate for the optical distortion, the positions and shapes of the pixels in the array are determined in accordance with an inverse function of the optical distortion function. As a result, the effect of the optical distortion on the images that are finally seen through the optical system is substantially compensated. No real-time signal processing or additional circuitry is required. 
     FIG. 10A illustrates what a regular square grid structure would look like if it is seen through a typical short-focal-length optical magnifier. The drawing shows that the image seen through the central portion of the magnifier is not distorted but at the periphery of the magnifier there is substantial distortion. It is seen that the local magnification M (r i ) of the magnifier decreases with an axial radius r i  and each point on the image moves radially inward toward the center of the magnifier. This type of distortion, typically called negative or barrel distortion, is typical for a magnifier having a short focal length. 
     The optical distortion of an optical magnifier is characterized by an optical distortion function which can be determined by a ray tracing experiment. Generally the distortion function for a short focal length magnifier is radially symmetrical and monotonically increasing with the axial radius. After the distortion function of the magnifier is determined, the inverse function of the distortion function may be derived by, for example, a third order polynomial fit to the distortion function. The resultant inverse function for such magnifier may be expressed as 
     
       
           D   −1 ( r   i )=( r   i   +kr   i   3 )  
       
     
     where D −1 (r i ) is the approximate inverse function of the distortion function, r i  is the axial radius of a point on the magnifier and k is a constant of approximately 0.32. FIG. 10B shows a computer-displayed drawing depicting the inverse distortion function for a LEEP-type optical system. 
     To compensate for the distortion introduced by an optical system used to view a pixel display, the inverse function of the distortion function is used to position and scale the individual pixels of the display. Since the pixel array is very regular, albeit of peculiar and specific form, a silicon compiler may be written to implement a scaleable architecture, where a pixel array can be “auto-instanced and routed” with the specification of only a few parameters such as DISTORTION_FUNCTION, CHROMATIC_ABERRATION, ARRAY_WIDTH, and PACKING_DENSITY. 
     FIG. 10C shows the top-view of a fabricated pixel array designed in accordance of the present invention to compensate for typical short-focal-length magnifier distortion. The layout of this array was generated with a silicon compiler program which may be written to even compensate for the varying warped pixel capacitance by adjusting the size of the buried storage capacitor for each pixel. The compiler program used is written in C computer language and it provides complete pixel array layout in the CADENCE design tool language called SKILL. 
     Alternative and additional description of the present invention is contained in the attached Appendix which is intended to be a part of specification of this patent application. 
     As will be apparent to those skilled in the art, numerous modifications to the present invention may be made within the scope of the present invention, which is not intended to be limited except in accordance with the following claims.