Abstract:
An embodiment of the present invention discloses a Liquid Crystal on Silicon (LCOS) display unit, in which a Metal-Insulator-Metal (MIM) capacitor consisting of a micromirror layer, a insulation layer and a light shielding layer is formed by grounding the light shielding layer on a pixel switch circuit layer. Therefore the pixel switch circuit and the capacitor are in vertical distribution, that is, the switch circuit and the capacitor both have an allowable design area of the size of one pixel. Another embodiment of the present invention provides a method for forming a Liquid Crystal on Silicon (LCOS) display unit.

Description:
This application claims the priority of Chinese Patent Application No. 200610119059.8, filed Dec. 4, 2006, the entire disclosure of which is incorporated herein by reference. 
     FIELD OF THE PRESENT INVENTION 
     The present invention relates to semiconductors, and more particularly, to a Liquid Crystal on Silicon (LCOS) display unit and a method for forming the same. 
     BACKGROUND OF THE PRESENT INVENTION 
     In recent years, many new technologies related to Liquid Crystal Displays (LCD) have come forth, and among others, Liquid Crystal on Silicon (LCOS) is a hot technology. LCOS relates to a new reflective micro LCD projection technology. To form an LCOS structure, transistors are grown on a silicon substrate; a drive panel (also referred to as Complementary Metal-Oxide-Semiconductor-LCD (CMOS-LCD)) is fabricated using a semiconductor process; the transistors are flattened by polishing technology and plated with aluminum to act as micro mirrors; thus a Complementary Metal-Oxide-Semiconductor (CMOS) substrate is formed; then the CMOS substrate is jointed with an upper glass substrate having transparent electrodes, and liquid crystal is injected into the structure. An encapsulation test is then performed. 
     Compared with conventional LCD and Digital Light Processing (DLP) technologies, LCOS has the following technical advantages: a) high light utilization efficiency: LCOS is similar to LCD technology, and mainly different from LCD in that LCOS is a reflective imaging system, such that the light utilization efficiency may reach 40% or more which is equivalent to the light utilization efficiency of DLP, while the light utilization efficiency of transmissive LCD only reaches about 3%; b) small volume: LCOS may integrate periphery circuits such as driver ICs to a CMOS substrate completely, so as to reduce the number of periphery ICs and the encapsulation cost, and decrease the whole volume; c) high resolution: since the transistor and the driver circuits of an LCOS are both fabricated in a silicon substrate and located under the reflective surface, they don&#39;t occupy surface area, and only pixel gaps occupy opening area, while Thin Film Transistors (TFTs) and wires of a transmissive LCD both occupy the opening area, so that both the resolution and the opening ratio of LCOS are higher than those of transmissive LCD; d) more mature manufacturing technology: the manufacturing of LCOS may be divided into Front of Line (FL) semiconductor CMOS manufacturing and End of Line (EL) liquid crystal panel jointing and encapsulating. There have been mature designing, simulating, fabricating and testing technologies for the FL semiconductor CMOS manufacturing, and now the product yield has reached above 90% with a very low cost; as for the EL liquid crystal panel jointing and encapsulating, although the yield is only 30% at now, since the manufacturing of liquid crystal panel has been developed rather maturely, the yield may be increased more rapidly than that of digital micromirror device (DMD) in theory. As a result, LCOS has more chances to be the mainstream technology than DLP. Therefore, LCOS technology has a bigger potential in application markets of digital camera, digital video camera, projector, monitor, large size TV and mobile telephone. 
     In LCOS technology, each pixel switch circuit consists of a Metal-Oxide-Semiconductor Field-Effect Transistor (MOSFET) and a capacitor. In conventional processes, the capacitor occupies half of the whole pixel area. As the circuit area decreases, the capacitor area also decreases. This will increase refresh rate in practical use. In order to increase capacitance, U.S. Pat. No. 6,437,839 discloses an LCOS pixel with multiple capacitors, the structure of which is shown in  FIG. 1 , and formed by the following steps: a diffusion region is formed on a substrate  40  as a top electrode of a first capacitor; an oxide layer is formed on the diffusion region as a dielectric layer  22   b  of the first capacitor; a first polysilicon layer  26  is formed on the dielectric layer  22   b  as a common electrode of two capacitors; a second oxide layer is formed on the first polysilicon layer  26  as a dielectric layer  24   b  of a second capacitor; a second polysilicon layer is formed on the second oxide layer as a top electrode  24   a  of the second capacitor; an insulation layer  42  is formed on the above structure; and an interconnection structure and an micromirror layer is formed on the above structure. As shown in  FIG. 1 , the capacitance is increased in the improved structure, but the processes are also increased, resulting in increased processing cost. 
     At present, each micromirror on the substrate surface serves as a display pixel, and each display pixel has a switch circuit. A pixel switch (MOSFET) and a capacitor must be designed in the area of a pixel. In order to lower the display refresh rate, the capacitance needs to be as large as possible. However, as restricted by the pixel area, if the capacitor occupies the pixel area in too high proportion, the performance of the switch circuit may be affected inevitably. For example, since the design area of the MOSFET is decreased, the insulation performance of the MOSFET may be affected and current leakage may occur. 
     SUMMARY OF THE PRESENT INVENTION 
     The present invention aims to solve the following problems: as higher and higher resolution is required for a display, the area of each pixel is becoming smaller and smaller; as restricted by the pixel area, it is difficult to obtain a pixel switch circuit with high performance and hold a high capacitor voltage level at the same time. 
     In an aspect of the present invention, there is provided a Liquid Crystal on Silicon (LCOS) display unit, which includes: a silicon substrate; a pixel switch circuit layer on the silicon substrate, the pixel switch circuit layer including a Metal-Oxide-Semiconductor Field Effect Transistor (MOSFET); a light shielding layer on the pixel switch circuit layer; an insulation layer on the light shielding layer; a micromirror layer on the insulation layer, the micromirror layer, the insulation layer and the light shielding layer constituting a capacitor, and the micromirror layer being electrically connected with a source of the MOSFET, wherein the light shielding layer is grounded. 
     Optionally, a mirror connecting pad is formed in the light shielding layer to be isolated from the light shielding layer; and the mirror connecting pad is electrically connected with the source of the MOSFET. 
     Optionally, an opening is formed in the insulation layer; and the opening is filled with a conductive material which electrically connects the micromirror layer to the mirror connecting pad. 
     Optionally, the light shielding layer is electrically connected with a ground pad via a through hole of the pixel switch circuit layer; and the ground pad is grounded. 
     Optionally, the insulation layer has a thickness ranging from 100 Å to 1000 Å. 
     Optionally, the insulation layer is made of silicon oxide, silicon nitride, silicon oxynitride, hafnium oxide, aluminum oxide, zirconium oxide or a combination thereof. 
     Optionally, the micromirror layer is made of aluminum. 
     Optionally the light shielding layer is made of metallic titanium, titanium nitride, AlCu alloy, titanium nitride or a combination thereof. 
     In another aspect of the present invention, there is provided a method for forming a Liquid Crystal on Silicon (LCOS) display unit, which includes: forming a pixel switch circuit layer on a silicon substrate, the pixel switch circuit layer including a Metal-Oxide-Semiconductor Field Effect Transistor (MOSFET); forming a light shielding layer on the pixel switch circuit layer; forming an insulation layer on the light shielding layer; forming a micromirror layer on the insulation layer; the micromirror layer, the insulation layer and the light shielding layer constituting a capacitor, and the micromirror layer being electrically connected with a source of the MOSFET, wherein the light shielding layer is grounded. 
     Optionally, the method further includes forming in the light shielding layer a mirror connecting pad to be isolated from the light shielding layer, wherein the mirror connecting pad is electrically connected with the source of the MOSFET. 
     Optionally, the method further includes forming an opening in the insulation layer and filling the opening with a conductive material, wherein the conductive material electrically connects the micromirror layer to the mirror connecting pad. 
     Optionally, the light shielding layer is electrically connected to a ground pad via a through hole in the pixel switch circuit layer; and the ground pad is grounded. 
     Optionally, the insulation layer has a thickness ranging from 100 Å to 1000 Å. 
     Optionally, the insulation layer is made of silicon oxide, silicon nitride, silicon oxynitride, hafnium oxide, aluminum oxide, zirconium oxide or a combination thereof. 
     Optionally, the micromirror layer is made of aluminum. 
     Optionally, the light shielding layer is made of metallic titanium, titanium nitride, AlCu alloy, titanium nitride or a combination thereof. 
     Compared with the prior art, the present invention has the following advantages: a Metal-Insulator-Metal (MIM) capacitor consisting of a micromirror layer, a insulation layer and a light shielding layer is formed by grounding the light shielding layer on a pixel switch circuit layer. Therefore the pixel switch circuit and the capacitor are in vertical distribution, that is, the switch circuit and the capacitor both have an allowable design area of the size of one pixel. Thus, the present invention makes full use of the area of the whole pixel, increases the capacitance and decreases the refresh rate of the LCOS display unit. 
     Additionally, the present invention also increases the design area of a switch circuit, and switch circuits with high performance may be designed according to different requirements, so that design flexibility of switch circuits may be increased. 
     The present invention also simplifies the procedure for manufacturing capacitors, thus decreasing the overall chip manufacturing cost. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram illustrating the structure of a capacitor of a LCOS display unit in prior art; 
         FIGS. 2A to 2L  are schematic diagrams illustrating a method for forming an LCOS display unit according to an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     The above objects and advantages of the present invention will become more apparent with reference to the following description of the preferred embodiments given in conjunction with the accompanying drawings. 
     A method for forming a Liquid Crystal on Silicon (LCOS) display unit is now described, which includes: forming a pixel switch circuit layer on a silicon substrate, the pixel switch circuit layer including a Metal-Oxide-Semiconductor Field-Effect Transistor (MOSFET); forming a light shielding layer on the pixel switch circuit layer; forming an insulation layer on the light shielding layer; forming a micromirror layer on the insulation layer; wherein the micromirror layer, the insulation layer and the light shielding layer constitute a capacitor, the micromirror layer is electrically connected with a source of the MOSFET, and the light shielding layer is grounded. 
       FIGS. 2A to 2J  illustrating a method for forming an LCOS display unit according to an embodiment of the present invention. The embodiments of the present invention will be described hereunder in detail in conjunction with the drawings. 
     Referring to  FIG. 2A , first, a pixel switch circuit layer  22  including a Metal-Oxide-Semiconductor Field-Effect Transistor (MOSFET) is formed on a silicon substrate  21 . The technology for forming the pixel switch circuit layer  22  is known to those skilled in the art. In a preferred embodiment of the present invention, the pixel switch circuit layer  22  includes an interlayer insulation layer  201 , and ground pads  202 , a signal pad  203 , a connection pad  204  which are inserted in the interlayer insulation layer  201 , as well as connection through holes. The ground pads  202  are connected to ground, the signal pad  203  is adapted to apply a voltage to the MOSFET of a drive circuit, the signal pad  203  is connected with a drain of the MOSFET of the pixel switch circuit layer  22  via a through hole, and a source of the MOSFET is electrically connected with the connection pad  204  via a through hole. 
     Then, a first conductive layer  205  is formed on the interlayer insulation layer  202 . The first conductive layer  205  is made of single or multiple layers of conductive materials. In a preferred embodiment of the present invention, the first conductive layer  205  is a multi-layer structure consisting successively of metallic titanium, titanium nitride, AlCu alloy, titanium nitride and metallic titanium. Preferably, the first conductive layer  205  has a thickness ranging from 1000 Å to 6000 Å. 
     Referring to  FIG. 2B , an island-like mirror connecting pad  206  and a light shielding layer  205   a  are formed in the first conductive layer  205  by the existing photolithography and etching technologies. The purpose for forming the light shielding layer  205   a  is to prevent the light from leaking onto the circuit devices in the silicon substrate. In case that the light leaks onto the circuit devices, it will affect the performance and life of the circuit, and thus a specific layer of metal is required to shield light. Gaps  206   a  between the mirror connecting pad  206  and the light shielding layer  205   a  insulatively isolate the mirror connecting pad  206  from the light shielding layer  205   a . The mirror connecting pad  206  is electrically connected with the source of the MOSFET of the pixel switch circuit layer via the connection pad  204  and a through hole. The light shielding layer  205   a  is electrically connected with ground pads  202  via through holes in the pixel switch circuit layer  22 . The ground pads  202  are grounded. 
     Referring to  FIG. 2C , a silicon oxide layer  207  is form on the light shielding layer  205   a  and the mirror connecting pad  206  as well as in the gaps  206   a  by a high-density plasma chemical vapor deposition (CVD). Since the gaps  206   a  are filled, grooves are generated on the surface of the silicon oxide layer  207  toward the gaps  206   a . The silicon oxide layer  207  formed has a thickness ranging from 200 nm to 1000 nm. 
     Referring to  FIG. 2D , an organic Bottom Anti-Reflecting Layer (BARC)  208  is formed on the surface of the silicon oxide layer  207 . The organic bottom anti-reflecting layer  208  has a good flowability and fills up the grooves on the surface of the oxide layer  207  completely. The purpose for forming the organic bottom anti-reflecting layer  208  is to fill the grooves on the surface of the silicon oxide layer  207  so as to protect the part of the silicon oxide towards the grooves and keep the surface flat after etching. 
     Referring to  FIG. 2E , the organic bottom anti-reflecting layer  208  is removed. After the removing, the residual organic bottom anti-reflecting layer  208   a  fills up the grooves on the surface of the silicon oxide layer  207 . The method for partially removing the organic bottom anti-reflecting layer  208  is well known to those skilled in the art. In a preferred embodiment of the present invention, O 2  plasma is employed to etch the organic bottom anti-reflecting layer  208 , such that part of the organic bottom anti-reflecting layer  208  reacts with the O 2  plasma and generates gases such as CO 2  and H 2 O, which escape from the surface of the silicon substrate. 
     Referring to  FIG. 2F , the silicon oxide layer  207  is removed. There is a residual silicon oxide layer  207   a  on the surfaces of the light shielding layer  205   a  and the mirror connecting pad  206  as well as in the gaps  206   a . The technology for removing the silicon oxide layer  207  is well known to those skilled in the art. In a preferred embodiment of the present invention, dry etching is employed to etch the silicon oxide layer  207 . After the silicon oxide layer  207  is removed, due to the inherent defect of the dry etching, bumps may be formed on the surface of the residual silicon oxide layer  207   a  at the gaps  206   a . These bumps will cause unevenness of the finally generated micromirror which is covered with liquid crystal. If the surface of the micromirror is uneven, the liquid crystal orientation and the electric field applied to the liquid crystal will be affected, so that the display performance will be affected. 
     Referring to  FIG. 2G , the surface of the residual silicon oxide layer  207   a  is planarized. During the planarization process, the residual silicon oxide layer  207   a  on the light shielding layer  205   a  is removed until the light shielding layer  205   a  is exposed. Thereafter, a smooth surface consisting of the light shielding layer  205   a , the mirror connecting pad  206 , and the silicon oxide  207   b  filled in the gaps  206   a  is formed. The technology for planarizing the surface of the residual silicon oxide layer  207   a  is well known to those skilled in the art. In a preferred embodiment of the present invention, the surfaces of the residual silicon oxide layer  207   a  and the light shielding layer  205   a  are polished simultaneously by a Chemical-Mechanical Polishing (CMP) device. Finally, a smooth, flat surface is formed. 
     Referring to  FIG. 2H , an insulation layer  209  is formed on the surfaces of the light shielding layer  205   a , the mirror connecting pad  206  and the silicon oxide  207   b  filled in the gaps  206   a . The insulation layer  209  may be made of silicon oxide, silicon nitride, silicon oxynitride or a combination thereof, and the insulation layer  209  may also be made of high-k dielectrics such as hafnium oxide, aluminum oxide, zirconium oxide and so on. In a preferred embodiment of the present invention, a three-layer ONO structure made of silicon oxide, silicon nitride and silicon oxide is used to serve as an insulation layer  209 , and the insulation layer  209  has a thickness ranging from 100 Å to 1000 Å. The method for forming the insulation layer  209  is well known to those skilled in the art. 
     Referring to  FIG. 2I , an opening  210  is formed on the insulation layer  209 . The opening  210  exposes the mirror connecting pad  206 . The method for forming the opening  210  is well known to those skilled in the art. 
     Referring to  FIG. 2J , a metal layer  211  is formed on the insulation layer  209   a  and in the opening  210 . The metallic material filled in the opening electrically connects the metal layer  211  with the mirror connecting pad  206 . A metallic material with a high reflectivity which may be aluminum, silver or an alloy thereof is employed to form the metal layer  211 . In a preferred embodiment of the present invention, the metal layer  211  is formed by using metallic aluminum. The metal layer  211  has a thickness ranging from 1000 Å to 6000 Å. Due to the existence of the opening, after the metal layer  211  is formed, a groove is formed on the surface of the metal layer  211  toward the opening  210 , and the depth range of the groove is 100 Å to 200 Å. 
     Referring to  FIG. 2K , the metal layer  211  is planarized to form a micromirror layer  211   a . The technology for planarizing the metal layer  211  is well known to those skilled in the art. In a preferred embodiment of the present invention, a Chemical-Mechanical Polishing (CMP) device is employed to planarize the surface of the metal layer  211 , finally forming a smooth, flat micromirror layer  211   a . The micromirror layer  211   a  has a thickness ranging from 800 Å to 5800 Å. The micromirror layer  211   a  is electrically connected with the mirror connecting pad  206  via the metal filled in the opening  210 , so that the micromirror layer  211   a  is connected to the source of the MOSFET of the pixel switch circuit layer  22 . 
     In an embodiment of the present invention, a structure of an LCOS display unit obtained based on the implementing of the above process is shown in  FIG. 2K . The structure includes: a pixel switch circuit layer  22  formed on a silicon substrate  21 , the pixel switch circuit layer  22  including a Metal-Oxide-Semiconductor field-effect transistor (MOSFET); a light shielding layer  205   a  on the pixel switch circuit layer  22 ; an insulation layer  209   a  on the light shielding layer  205   a ; a micromirror layer  211   a  on the insulation layer  209   a . The micromirror layer  211   a , the insulation layer  209   a  and the light shielding layer  205   a  constitute a capacitor, in which the micromirror layer  211   a  is electrically connected with a source of the MOSFET, and the light shielding layer  205   a  is grounded. 
     An island-like mirror connecting pad  206  insulated from the light shielding layer  205   a  is formed in the center of the light shielding layer  205   a . The light shielding layer  205   a  is electrically connected with ground pads  202  in the pixel switch circuit layer  22 , and the ground pads  202  are grounded; the mirror connecting pad  206  is electrically connected with a source of the MOSFET via a connection pad  204  formed in the pixel switch circuit layer  22 ; an opening  210  is formed in the insulation layer  209   a , and the micromirror layer  211   a  is electrically connected with the mirror connecting pad  206  via a metallic material filled in the opening  210  which is the same as that of the micromirror layer  211   a . The insulation layer  209   a  has a thickness ranging from 100 Å to 1000 Å. 
     As shown in  FIG. 2K , the micromirror layer  211   a , the insulation layer  209   a  and the light shielding layer  205   a  constitute a metal-insulator-metal (MIM) capacitor. The upper electrode (mirror  211   a ) of the capacitor is electrically connected with the source of the MOSFET, and the lower electrode (light shielding layer  205   a ) of the capacitor is electrically connected with the ground pads  202 . Thus, the MOSFET and the capacitor constitute a dynamic random access memory (DRAM). When a voltage is applied on a gate of the MOSFET of the pixel switch circuit layer  22 , the MOSFET is turned on, and since the upper electrode of the capacitor is electrically connected with the source of the MOSFET, the capacitor is charged via the voltage applied on the signal pad  203 . The capacitor area is the area of the whole pixel, moreover the insulation layer  209   a  is thin thus the capacitance of the capacitor is increased, such that the refresh rate is decreased. The insulation layer  209   a  is thin, so the formed thickness of the opening  210  is small, and the contact resistance between the micromirror layer  211   a  and the mirror connecting pad  206  is decreased. At last, since the insulation layer  209   a  is thin, the probability of a light entering onto the silicon devices through diffuse reflection is reduce, and accordingly the light shielding is enhanced. 
     Referring to  FIG. 2L , a liquid crystal layer  23  is on the micromirror layer  211   a ; and an upper glass substrate layer  24  is on the liquid crystal layer  23 , polarized light comes from the top of the upper glass substrate layer  24 . 
     Additionally, according to the embodiments of the present invention, the design area of a switch circuit is also increased, and switch circuits with high performance may be designed according to different requirements, so that design flexibility of switch circuits may be increased. F or example, a switch with a N-type Metal-Oxide-Semiconductor (NMOS) and a P-type Metal-Oxide-Semiconductor (PMOS) in parallel may be designed to improve the display gamma; or a NMOS may be designed to increase the width of the device so as to improve display reaction speed. 
     The present invention is described above in, but not limited to, the preferred embodiments. It is noted that those skilled in the art may make modifications and variations, without departing from the basic principle of the present invention; any of those modifications and variations shall fall into the protected scope of the present invention defined by the following claims.