Active solid-state devices (e.g. – transistors – solid-state diode – Non-single crystal – or recrystallized – semiconductor... – Field effect device in non-single crystal – or...
Reexamination Certificate
2000-11-10
2002-07-02
Sherry, Michael J. (Department: 2829)
Active solid-state devices (e.g., transistors, solid-state diode
Non-single crystal, or recrystallized, semiconductor...
Field effect device in non-single crystal, or...
C349S044000, C349S111000
Reexamination Certificate
active
06414337
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to liquid crystal display devices, such as a microdisplays and more particularly to an apparatus and methods for generating an accurate, visually appealing black frame around the image being displayed on either a display screen or for a near to the eye or binocular application microdisplays.
BACKGROUND OF THE INVENTION
Microdisplays are a type of liquid crystal display (LCD). LCDs are commonly used in portable televisions, portable computers, and cellular phones to display information to the user. LCDs act in effect as a light valve, i.e., they allow transmission of light in one state and block the transmission of light in a second state. When used as a high resolution information display, as in the application of the present invention, the LCDs are typically arranged in a dot-matrix configuration with independently addressable pixels. Each individual pixel is controlled to selectively modulate light from a backlight (transmission mode), from a reflector (reflective mode), or from a combination of the two (transflective mode). The matrix of pixels is laid out on a semiconductor substrate or die, which is produced from a semiconductor wafer or other suitable substrate such as a silicon wafer. Multiple die can be generated from a single wafer or substrate.
Typically, the image generating portion of the microdisplay, is constructed by bonding a piece of glass or transparent material G to an electrical circuit fabricated on a semiconductor or other suitable substrate chip S, as illustrated in FIG.
1
. The glass or transparent material G is held to the silicon S by a perimeter seal PS, which is generally formed of an epoxy or other similar sealant-type material. Liquid crystal LC fills the region between the glass or transparent material G and semiconductor substrate or silicon S, and inside the perimeter seal PS, as also shown in FIG.
1
. The region inside of the perimeter seal is generally considered to be available as the viewing or active area of the display, i.e., this is the area where the pixels and liquid crystal are located.
A drawback of this construction is that when an image is displayed upon a screen or viewed directly, from the microdisplay; the perimeter seal PS, and any space between the inside of the perimeter seal PS and outer perimeter of pixels, are visible. In a projected or viewed image, this region of the display is not visually appealing. Another drawback of this construction is that stray light bounces off of the perimeter seal PS surfaces and surfaces adjacent to the perimeter seal PS. This can lead to a reduction in the contrast ratio of the display, which is undesirable.
In one solution to the above described problems in the prior art, a black frame BF is placed on the top surface of the glass G. In one embodiment of this solution, the frame is formed by cutting a piece of metal coated black into the desired pattern. The metal frame is then adhered to the top surface of the glass G. Alternatively, the frame is formed by depositing a black ink on the top surface of the glass G in the desired pattern. The black color of the coating or ink absorbs stray light SL, and thus prevents the image of the perimeter seal PS from being visible, as shown in FIG.
2
. Any other light L that is directed onto a pixel P will be reflected back. Thus, as long as the opening in the frame is made so that the perimeter seal PS (and any other aspects of the image that are not desired to be projected) is protected from incident light, the protected image will be masked.
There are, however, a number of drawbacks to the Black Frame and Black Ink solutions. One is the parallax of incoming light. Another is Snell's Law. Parallax refers to the phenomenon where lightwaves which enter the glass are not perpendicular to the surfaces of the glass. These light rays strike the interior and exterior faces of the glass and the pixels on the Fabricated Semiconductor at an angle and are reflected back out of the glass at a complementary angle.
With the phenomenon described by Snell's Law, light waves are bent as they travel through glass to air and glass to LC fluid boundaries, in each direction, into and out of the glass.
Because light rays must travel through the thickness of the glass G before striking the pixel array, the impacts of parallax and Snell's Law must be accounted for.
FIG. 3
illustrates these impacts as it travels through the display.
The primary aspect of the parallax problem results from the fact that there is some finite distance, D between the edge of the black frame BF and the first illuminated pixel P due to the distance the light must travel through the glass G. See FIG.
3
. By calculating this distance, it is possible to develop an opening size in the black frame BF that will allow the appropriate region of the display to be illuminated, so that the perimeter seal PS is covered, but the pixels are left open. However, if the angle of incidence of the outermost light rays varies, which it often does because of the incident angle and f# of each different lens system, the region of the display that is illuminated changes.
Two different prior art solutions were developed to deal with this latter problem. One involves creating a custom black frame opening and location for each different optical lens system. A drawback of this solution is that it requires customization of what is an otherwise standard product, which increases product cost and complexity. Another solution to this latter problem, involves increasing the number of pixels on the silicon. In this arrangement, “unused” pixels adjacent to the perimeter seal or black frame are driven black. A drawback of this solution is that it reduces the number of displays that can be produced from each wafer because each silicon die is increased in size. Another drawback of this latter solution is that “black” pixels BP are not truly black. This is because of the contrast ratio of the display. Thus, with this latter solution, a gray region GR is projected between the black frame BF and the projected pixels PP. This effect is illustrated in FIG.
4
.
The major disadvantage of the solution employing a black frame or printing black ink on the top surface of the glass is that the tolerances in the size of the opening of the black frame. The positioning of the black frame poses additional problems of requiring 20 or more additional pixels on each side of the display to compensate for these inaccuracies. These additional pixels not only decrease the yield from each wafer, but they also create the gray ring to be displayed in the projected or observed images described above, depending upon the microdisplay application.
Yet another disadvantage of this solution is that because the black frame is outside of the focal plane of the optics system (the focal plane is at the surface of the silicon die), the edges are slightly blurry.
Another problem with using black frames is thermal management. The black frame absorbs energy, and thus becomes hot. Removing the energy, particularly in high energy lighting projection systems, using microdisplays, without causing it to damage the liquid crystal, is problematic.
Still another drawback of using black frames, applied on the top surface of the glass, is that manufacturing such devices is expensive. Each frame has to be added to the microdisplay one at a time. Because alignment tolerances are critical, this step thus requires a lot of time and specialized equipment.
Another aspect of microdisplay devices relevant to the present invention relates to the connection of what is known as the VCOM or ITO signal. This is a voltage that exists on a thin layer of transparent metal, ITO (Indium Tin Oxide), which is coated on the inside surface of the glass G. This voltage sets up the potential difference across the liquid crystal. It provides the reference voltage for the pixel voltage on the silicon S. It is important that the connections to the ITO be of low resistance so as to provide low power consumption and high contra
Day Kevin
Slater Andrew R.
Baker & Botts LLP
Saricar Asok Kumar
Sherry Michael J.
Three-Five Systems, Inc.
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