Abstract:
A liquid crystal micro display (lcmd) is manufactured by creating a hole in an lcmd surface, filling the lcmd with liquid crystal material through the hole, and then sealing the hole. The invention allows an lcmd to be tested before it is separated from other lcmds and packaged. As a result, the invention increases the yield and reduces the cost associated with lcmd manufacturing.

Description:
TECHNICAL FIELD OF THE INVENTION 
     The present invention relates to a method for testing and filling semiconductor based liquid crystal displays, also known as liquid crystal micro displays (“lcmds”). 
     BACKGROUND OF THE INVENTION 
     Lcmds are small liquid crystal displays that usually have a display area less than 1 square cm and a thickness of about 1 mm. They are primarily used as view finders in devices such as cameras but are also used as part of a larger display component wherein the image from the lcmd is projected or magnified. Each lcmd typically comprises hundreds of thousands of pixels but some can contain over a million. 
     Lcmd manufacturing is typically performed in a clean room environment wherein steps are taken to remove dust and other contaminating agents from the surrounding atmosphere. The degree to which a manufacturing environment is kept clean depends on factors such as the size and density of the integrated circuits contained in the lcmds, the desired quality of the lcmds, and the costs associated with maintaining different levels of cleanliness. Statistical models may be used to conduct a cost-benefit analysis for determining an ideal level of cleanliness for manufacturing lcmds of a certain type, size, and quality. 
     With reference to FIGS. 1 &amp; 2 (prior art), each batch of lcmds is typically made from two substrates. Usually, one substrate is a semiconductor layer, such as a silicon wafer  9 , containing many integrated circuits (“ICs”)  12 . Although, for illustration purposes, FIG. 1 shows that the silicon wafer  9  contains only nine ICs  12 , each silicon wafer  9  typically contains hundreds of ICs  12  arranged in rows and columns. Each IC  12  includes an array of pixels comprising IC electrodes  16  driven via corresponding switching elements  17 . The other substrate is typically a glass wafer  10  that has thereon one transparent electrode  15  per corresponding IC  12 . Each substrate is typically, but not necessarily, less than 1 mm thick; the thickness of each substrate may vary according to the manner in which the lcmds are to be used. 
     A sealant that forms lcmd wall  11  is applied to one of the substrates. Traditionally, the wall  11  does not completely surround each IC  12 —a small gap  13  remains through which the liquid crystal material flows to fill the lcmds. The silicon wafer  9  is then aligned and joined with the glass wafer  10  such that the transparent electrodes  15  are aligned with the corresponding ICs  12 . Spacers (not shown) are used to keep the substrates separated by a small distance which is typically on the order of a few micrometers. The spacers may, for example, be etched onto the silicon wafer. After the substrates are joined, lcmds  8  are formed, each containing an IC  12 . 
     Since the distance between the silicon wafer  9  and the glass wafer  10  is on the order of microns, viscosity limitations may make it impossible for liquid crystal material to reach many, if not most, of the inner lcmds  8  prior to their separation. Therefore, the lcmds  8  are filled with liquid crystal material via openings  13  after they are separated. 
     The lcmds  8  may be separated from each other by using, for example, a scribe and break process. In a scribe and break process, the semiconductor wafer  9  is scribed (typically with a specialized saw or laser) along scribe lines  14 , in order to weaken the locations where the separation is to take place. In addition, the glass wafer  10  is scribed using a cutting tool such as a laser or a specially designed saw. The wafers  9  &amp;  10  are typically then temporarily glued onto a flexible material that is then flexed in order to break up the wafers and separate the lcmds. The scribe and break process results in small debris of semiconductor and glass material that accumulate around lcmd openings  13 . 
     After the lcmds are separated, they are filled with liquid crystal material. The filling is preferably achieved in a vacuum unit in which the lcmds are immersed in liquid crystal material. After an lcmd  8  is filled, the hole  13  through which it is filled is then sealed with a glue or epoxy material. 
     The traditional filling process described above often results in a large number of defective lcmds because debris from the scribe and break process are frequently pulled into many of the lcmds  8  by the in-flowing liquid crystal material. The debris may cause an electronic malfunction or may distort an image by blocking or altering the path of the electromagnetic radiation controlled by the lcmd. The defects caused by the debris are not discovered until after the lcmds are packaged since the packaging provides the wiring through which the lcmd receives imaging and testing signals. 
     Packaging an lcmd involves mounting and wiring. The lcmd is mounted into/onto a packaging unit at a predetermined angle and location so that the image produced is properly focused and aligned. The lcmd is also wired to terminals on the packaging unit. These terminals will eventually be connected to and will receive imaging and other signals from a host device, such as, for example, a camera. The area surrounding the wiring connections is typically filled with a glue or epoxy material that stabilizes the connections and prevents the wires from touching each other. 
     After the lcmds are packaged, they are tested so that defective units may be detected and eliminated. The testing can be difficult and costly since each individual lcmd must be tested separately. The reason that the lcmds cannot be tested while they are still part of a substrate assembly is because the testing must take place after the liquid crystal filling process (which has traditionally needed to be performed after the separation of the lcmds  8  due to viscosity limitations). The packaging process is relatively expensive and may account for most of the cost of a finished lcmd. Packaging the lcmds prior to testing significantly increases the cost associated with defective units since such cost would also include the cost of packaging. 
     Based on the foregoing, there exists a need for a system and method of manufacturing and testing lcmds that result in a higher yield and lower costs. 
     SUMMARY OF THE INVENTION 
     A liquid crystal micro display (lcmd) is manufactured by creating a hole in an lcmd surface, filling the lcmd with liquid crystal material through the hole, and then sealing the hole. The invention allows an lcmd to be tested before it is separated from other lcmds and packaged. As a result, the invention increases the yield and reduces the cost associated with lcmd manufacturing. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 depicts an example top view of a prior art lcmd substrate assembly. 
     FIG. 2 depicts a cross sectional view of a prior art lcmd depicted in FIG.  1 . 
     FIG. 3 depicts a flow chart illustrating a method of manufacturing lcmds of the present invention. 
     FIG. 4 depicts an example top view of an lcmd substrate assembly used in the method illustrated in FIG.  3 . 
     FIG. 5 depicts a cross sectional view of an lcmd of FIG.  4 . 
     FIG. 6 depicts a step for testing an lcmd in accordance with an embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     With additional reference to FIGS. 4 and 5, FIG. 3 depicts a flow chart illustrating a method of manufacturing lcmds of the present invention. Lcmds are formed using two substrates. In one embodiment, the first substrate is a silicon wafer  21  (less than 1 mm thick) on which many (typically hundreds) of ICs are formed. Each IC  22  includes a large number (typically about half a million) of active pixels comprising electrodes  40  driven by corresponding switching elements  41 . The thickness of each substrate may vary according to the application for which the lcmds will be used. The second substrate is transparent and is typically a thin glass wafer  26  having the transparent electrodes  42  of a corresponding number of lcmds. The transparent electrodes  42  are made from a transparent conductive material such as indium-tin oxide. 
     Fill holes  33  are created in one of the wafers as indicated in step  3 A. The fill holes  33  are carefully positioned so as to provide access to the chamber  35  of each lcmd  30  without damaging the ICs  22  or the display area of the lcmd  30 . For example, a fill hole  33  may be positioned as illustrated in FIGS. 4 and 5. The fill holes  33  may be created in either the glass wafer  26  or the silicon wafer  21 . If the fill holes  33  are to be in the glass wafer  26 , then they may be created using a glass drilling tool such as a laser device or a rotary drill. However, the fill holes  33  are preferably created in the silicon wafer  21  using an anisotropic etch. The anisotropic etch creates a funnel-shaped fill hole  33  in the silicon wafer  21  such that the opening in the inner surface of the silicon wafer  21  is smaller than the opening in the outer surface, as illustrated in FIG.  5 . The anisotropic etch helps to more precisely place the fill holes  33  in the desired areas of the inner surface of the silicon wafer  21 . 
     After the fill holes  33  are created, wafers  21  and  26  are joined as indicated in step  3 B. This step typically involves applying a sealant material around each IC  22  and then joining the wafers to form lcmd units  30 . The lcmd units  30  are then filled with liquid crystal material via the fill holes  33 , as indicated in step  3 C. The filling is preferably achieved using a standard vacuum filling technique whereby lcmds are placed in a vacuum chamber (not shown) in which air pressure is subsequently reduced; the lcmd units are then lowered into a bath of liquid crystal material and the pressure in the vacuum chamber is reasserted such that the pressure difference between the lcmd chambers  35  and their surroundings forces the liquid crystal material into the lcmd chambers  35  through the fill holes  33 . Other filling methods may also be used, such as, for example, injecting or pouring the liquid crystal material into the lcmds through their respective fill holes  33 . These alternative filling methods may be facilitated by the creation of outlet holes in a substrate for allowing the air inside an lcmd chamber  35  to escape while the lcmd chamber  35  is being filled with liquid crystal material. After the lcmds are filled, the fill holes  33  (and any outlet holes) are sealed using a sealing object, such as a plug, or a sealing material such as glue, epoxy, or solder, as indicated in step  3 D. 
     The lcmds are then tested as indicated in step  3 E. Since the lcmds are still part of the same substrates and are still physically connected, they are easily handled during testing. Each row or column of lcmds may share the same testing signal(s) as illustrated in FIG.  4  and discussed in the related description below. Lcmds that appear to be defective are marked using, for example, an ink marker, so that they may be identified and disposed of at a later time. After the lcmds are tested, they are separated along scribe lines  36  (as indicated in step  3 F) using, for example, a scribe and break process as discussed above. By following the above described steps, the debris caused by the scribe and break process should not affect the quality or performance of the lcmds since they are filled and sealed before debris are generated. 
     It should be noted that in some implementations, steps  3 A- 3 F may occur out of the order illustrated in FIG.  3 . As a non-limiting example, step  3 B may occur before step  3 A. Furthermore, each one of steps  3 A- 3 F may comprise sub-steps. 
     FIG. 4 is a top view of an example lcmd substrate assembly formed by the silicon wafer  21  and the glass wafer  26  before being divided into individual lcmds. For illustration purposes only, the silicon wafer  21  is shown to contain only  9  ICs. Typically, however, such a silicon wafer would contain hundreds of ICs. Each IC, such as IC  22 , is surrounded by a sealant wall  23  and is resistively connected to other ICs and to a testing terminal, such as testing terminal  25 , located on the silicon wafer  21  and used for receiving a testing signal. A glass wafer  26  covers the ICs and is joined to the silicon wafer via the sealant walls  23  that surround the ICs. The glass wafer  26  is layered with typically one transparent electrode  42  (FIG. 5) per lcmd. Transparent electrodes  42  are made from a transparent material such as indium-tin oxide. Parallel paths, such as paths  28  and  29  are used to reduce the impact of open circuits during testing. The glass wafer  26  is placed over the silicon wafer in such a way as to not cover the testing terminals on the silicon wafer. 
     FIG. 5 is a cross sectional view of a simplified version an lcmd  30  of FIG.  4 . Lcmd  30  contains an lcmd chamber  35  that is filled with liquid crystal material through fill hole  33 . The filling is preferably performed in a vacuum chamber as discussed above. After the lcmd chamber  35  is filled, the fill hole  33  is sealed using a sealing object, such as a plug, or a sealing material such as glue, epoxy, or solder. The filling and testing processes of this invention are easier than the traditional filling and testing processes since lcmds do not have to be individually handled. Instead, lcmds  30  are filled and tested before they are separated. Furthermore, fewer defects are caused during the new filling process since no debris from the scribe and break process are pulled into the lcmds. 
     FIG. 6 illustrates the testing of an lcmd in accordance with an embodiment of the present invention. For illustration purposes only, very few pixel electrodes  40  and corresponding switching elements  41  are shown. However, each lcmd tested may contain hundreds of thousands or even millions of pixels. After the lcmds are filled and sealed, but before they are separated, an electric signal is sent to one or more ICs  22  through a testing terminal, such as testing terminal  25  (FIG.  4 ). The testing signal is routed through a connection  54  to a switching element  53  that is fabricated in or forms part of the IC  22 . Each connection, such as connections  54  and  56 , may be resistive and/or may incorporate a resistive element. The testing signal causes the switching element  53  to connect the pixel electrodes  40  to a grounding terminal  52  via respective switching elements  41 . The grounding terminal  52  may be located on the silicon wafer and may be grounded through a connection that is routed between scribe lines. 
     With all the pixel electrodes  40  grounded, a corresponding transparent electrode  42  (FIG. 5) on the glass wafer  26  may be driven with varying voltages to create an all “black”, an all “white”, and/or an intermediate gray display. Optical testing equipment such as, for example, a specialized camera, can then be used to evaluate the lcmd&#39;s performance in response to the testing signals. The optical testing equipment tests to see if the lcmd produces a non-uniform image. An lcmd image may be non-uniform for various reasons such as, for example, the presence of debris in the lcmd or incomplete liquid crystal filling. An lcmd that produces a non-uniform image can be marked using, for example, an ink marker, so that it can be disposed of after the lcmds are separated. The above described approach eliminates the difficulties associated with handling separate lcmds during testing as well as the unnecessary cost associated with packaging defective units. 
     It should be emphasized that the figures described above and attached hereto and the items shown therein are not necessarily drawn to scale or accurately proportioned, but rather, they represent simplified illustrations that help to clearly set forth the principles of the invention. Furthermore, the above-described embodiments of the present invention are merely possible examples of implementations setting forth a clear understanding of the principles of the invention. Many variations and modifications may be made to the above-described embodiments of the invention without departing substantially from the principles of the invention. All such modifications and variations are intended to be included herein within the scope of the disclosure and present invention and protected by the following claims.