Abstract:
Alignment films for use within a liquid crystal display (LCD) and methods for their manufacture are disclosed. Embodiments of the invention generally relate to process conditions that are selected to improve certain properties of the alignment films. The alignment films include a deposited layer of amorphous carbon, a hydrogenated amorphous carbon film or a hydrogenated diamond-like carbon (DLC) film.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application claims benefit of U.S. provisional patent application Ser. No. 60/624,618, filed Nov. 3, 2004, which is herein incorporated by reference. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     Embodiments of the invention generally relate to amorphous carbon films. More particularly, embodiments of the invention relate to hydrogenated diamond-like carbon (DLC) films for use as alignment films within a liquid crystal display (LCD).  
         [0004]     2. Description of the Related Art  
         [0005]     Active matrix liquid crystal displays (LCDs) are commonly used for applications such as computer and television monitors, cell phone displays, personal digital assistants (PDAs) and an increasing number of other devices. Generally, an active matrix LCD includes two glass plates having a layer of liquid crystal (LC) materials sandwiched therebetween. One of the glass plates typically includes a conductive film disposed thereon. The other glass plate typically includes an array of thin film transistors (TFTs) coupled to an electrical power source. Each TFT may be switched on or off to generate an electrical field between a TFT and the conductive film. The electrical field changes the orientation of the LC material to create a pattern on the LCD.  
         [0006]     The LC material employed in the LCD typically relies on alignment layers to orient the LC material in a predetermined direction. The conventional method of LC material alignment includes placing a thin film of the LC material on a mechanically rubbed polyimide film deposited on a suitable substrate of the LCD. Rubbing of the polyimide film creates micro-grooves that the LC material aligns itself with. However, problems with this rubbing method include tracks formed in the polyimide, non-uniformity of the micro-grooves and debris from the cloth used to rub the polyimide. These problems degrade the LCD, necessitate extra cleaning steps and otherwise limit development of the LCD.  
         [0007]     More recently, ion-beam treatment on diamond-like carbon (DLC) films used for the alignment of the LC material provides many advantages over conventional rubbed polyimide alignment films, such as, non-contact processing and alignment uniformity. Further, hydrogenated DLC films provide optical transparency suitable for use as the alignment film. Usually, plasma enhanced chemical vapor deposition (PECVD) followed by argon (Ar) ion beam irradiation forms the DLC films that are deposited for use as the alignment film. However, methods heretofore used have affected the desirability of the hydrogenated DLC for use as the alignment film within the LCD. Accordingly, it would be desirable to utilize process conditions that improve certain properties of the alignment film.  
         [0008]     Thus, there exists a need for changes in the manufacture of an LCD for deposition of a hydrogenated amorphous carbon film, such as a hydrogenated DLC film, to provide an improved alignment film within the LCD.  
       SUMMARY OF THE INVENTION  
       [0009]     Embodiments of the invention generally relate to process conditions that are selected to improve certain properties of an alignment film within a liquid crystal device (LCD). The alignment film includes a deposited layer of amorphous carbon, a hydrogenated amorphous carbon film, or a hydrogenated diamond-like carbon (DLC) film. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]     So that the manner in which the above recited features of the present invention can be understood in detail, a more particular description of the invention, briefly summarized above, may be had by reference to embodiments, some of which are illustrated in the appended drawings. It is to be noted, however, that the appended drawings illustrate only typical embodiments of this invention and are therefore not to be considered limiting of its scope, for the invention may admit to other equally effective embodiments.  
         [0011]      FIG. 1  is a sectional view of an exemplary liquid crystal display (LCD) having alignment films formed according to aspects of the invention.  
         [0012]      FIG. 2  is a schematic illustration of an apparatus that can be used for deposition of the alignment films.  
         [0013]      FIG. 3  is a graph of transmittance verses wavelength through a 200 angstrom thick film deposited in accordance with Example 1. 
     
    
     DETAILED DESCRIPTION  
       [0014]      FIG. 1  illustrates an exemplary liquid crystal display (LCD) having alignment films  30 ,  60  formed according to aspects of the invention. The LCD includes a lower substrate  10 , an upper substrate  90 , and a liquid crystal (LC) layer  50  interposed therebetween. The lower substrate  10  can be a glass substrate comprising a thin film transistor (TFT) array. A pixel electrode  20  is formed on the lower substrate  10 . A lower alignment film  30  is formed on the pixel electrode  20 . A color filter  80  is formed on the interior surface of the upper substrate  90 . A common electrode  70  is formed on the color filter  80 . An upper alignment film  60  is formed on the common electrode  70 . An upper polarizer  40  is disposed (or adhered) on the exterior surface of the upper substrate  90 . A lower polarizer  45  is disposed (or adhered) on the exterior surface of the lower substrate  10 . A backlight module  95  is disposed below the lower polarizer  45 .  
         [0015]     According to embodiments of the invention, the alignment films  30 ,  60  preferably have a high transmittance within a visible range of wavelengths (e.g., greater than 95% at 600 nanometers, more preferably, greater than 99% at 600 nanometers for thicknesses of 200 angstroms), a refractive index that is larger than 1.7, an intrinsic stress that is less than 2.0 giga Pascal, a percentage of sp3 bonds of greater than 70%, a resistivity that is greater than 10 11  Ohm-cm, a density of greater than 1.1 grams/cubic centimeter and a thickness variation of less than 10%. The alignment films  30 ,  60  can each be approximately 200 angstroms thick. However, other thickness (e.g., 50 to 500 angstroms) may be suitable for the alignment films  30 ,  60  disclosed. Additionally, as the intrinsic stress increases the alignment films  30 ,  60  can increasingly tend to peel away from the surfaces on which they are deposited.  
         [0016]      FIG. 2  is a schematic representation of a substrate processing system  103  that can be used to perform amorphous carbon layer deposition in accordance with the present invention. This apparatus typically comprises a process chamber  100 , a gas panel  130  and a control unit  110  along with other hardware components such as power supplies and vacuum pumps. Examples of the system  103  include AKT-1600 PECVD, AKT-4300 PECVD, AKT-5500 PECVD, AKT 10K PECVD, AKT 15K PECVD, AKT 25K PECVD and AKT 40K PECVD systems available from AKT America, Inc., Santa Clara, Calif.  
         [0017]     The process chamber  100  generally comprises a support pedestal  150  that holds a substrate  190  of the LCD. Depending on the specific process, the substrate  190  can be heated to some desired temperature prior to processing. An embedded heater element  170  heats the substrate support pedestal  150  and hence the substrate  190  via a bottom surface  192  of the substrate  190  supported above the pedestal  150 . For example, the pedestal  150  may be resistively heated by applying an electric current from an AC supply  106  to the heater element  170 . A temperature sensor  172 , such as a thermocouple, monitors the temperature of the pedestal  150 . The measured temperature is used in a feedback loop to control the AC supply  106  for the heating element  170  such that the substrate temperature can be maintained or controlled at a desired temperature which is suitable for the particular process application.  
         [0018]     A vacuum pump  102  evacuates the process chamber  100  and maintains the proper gas flows and pressure inside the chamber  100 . A showerhead  120 , through which process gases are introduced into the chamber  100 , is located above the substrate support pedestal  150 . The showerhead  120  connects to the gas panel  130 , which controls and supplies various gases used in different steps of the process sequence.  
         [0019]     The showerhead  120  and substrate support pedestal  150  also form a pair of spaced apart electrodes. When an electric field is generated between these electrodes, the process gases introduced into the chamber  100  are ignited into a plasma. Typically, the electric field is generated by connecting the substrate support pedestal  150  to a source of radio frequency (RF) power (not shown) through a matching network (not shown). Alternatively, the RF power source and matching network may be coupled to the showerhead  120 , or coupled to both the showerhead  120  and the substrate support pedestal  150 .  
         [0020]     Proper control and regulation of the gas flows through the gas panel  130  is performed by mass flow controllers (not shown) and the controller unit  110 , such as a computer. Illustratively, the control unit  110  comprises a central processing unit (CPU)  112 , support circuitry  114 , and memories containing associated control software  116 . This control unit  110  is responsible for automated control of the numerous steps required for substrate processing, such as substrate transport, gas flow control, temperature control, chamber evacuation, and so on. Bi-directional communications between the control unit  110  and the various components of the system  103  are handled through numerous signal cables collectively referred to as signal buses  118 .  
         [0021]     Plasma enhanced chemical vapor deposition (PECVD) techniques promote excitation and/or disassociation of the reactant gases by the application of the electric field to the reaction zone near the substrate surface, creating a plasma of reactive species. The reactivity of the species in the plasma reduces the energy required for a chemical reaction to take place, in effect lowering the required temperature for such PECVD processes.  
         [0022]     Amorphous carbon layer deposition to form the alignment films  30 ,  60  illustrated in  FIG. 1  is accomplished by plasma enhanced thermal decomposition of a hydrocarbon compound such as acetylene (C 2 H 2 ). For example, acetylene is introduced into the process chamber  100  under the control of gas panel  130 . The hydrocarbon compound is introduced into the process chamber as a gas with a regulated flow. The showerhead  120  allows process gases including the hydrocarbon compound from the gas panel  130  to be uniformly distributed and introduced into the process chamber  100 . When the process gas mixture exits the showerhead  120 , plasma enhanced thermal decomposition of the hydrocarbon compound occurs at a surface  191  of the heated substrate  190 , resulting in a deposition of an amorphous carbon layer on the substrate  190 .  
         [0023]     For some embodiments of the invention, the amorphous carbon layer is formed from a gas mixture of a hydrocarbon compound and an optional inert gas such as argon (Ar) or helium (He). The hydrocarbon compound has a general formula C x H y , where x has a range of between 1 and 4 and y has a range of between 2 and 10. For example, acetylene (C 2 H 2 ), methane (CH 4 ), propylene (C 3 H 6 ), propyne (C 3 H 4 ), propane (C 3 H 8 ), butane (C 4 H 10 ), butylene (C 4 H 8 ), or butadiene (C 4 H 6 ) as well as combinations thereof, may be used as the hydrocarbon compound. Similarly, a variety of gases such as hydrogen (H 2 ), nitrogen (N 2 ), ammonia (NH 3 ), or combinations thereof, among others, may be added to the gas mixture, if desired. An inert gas such as Ar, He and/or N 2  may be used to control the density and deposition rate of the amorphous carbon layer. The addition of H 2  and/or NH 3  can be used to control the hydrogen ratio of the amorphous carbon layer. Additionally, the ratio of the hydrogen to carbon in the hydrocarbon selected can be used to control the hydrogen ratio of the amorphous carbon layer.  
         [0024]     In general, the following deposition process parameters can be used to form the amorphous carbon layers that provide the alignment films  30 ,  60  of the LCD. For some embodiments, the process parameters range from a substrate temperature of about 100° C. to about 200° C., a chamber pressure of about 10 milli-torr to about 2000 milli-torr, a hydrocarbon gas (e.g., C 2 H 2 ) flow rate of about 80 standard cubic centimeters per minute (sccm) to about 320 sccm with a H 2  flow up to about 1280 sccm, an RF power of between about 125 watts to about 1000 watts, and a spacing of between about 700 mils to about 1300 mils between the shower head and the substrate. These processes were run on an AKT-1600 PECVD system having an internal volume of about 48 liters and being capable of processing substrates having an area up to 2,000 square centimeters. The substrate processed was 370 mm by 470 mm.  
         [0025]     Other deposition chambers are within the scope of the invention and the parameters listed above may vary according to the particular deposition chamber used to form the amorphous carbon layer. For example, other deposition chambers may have a larger or smaller volume, requiring gas flow rates that are larger or smaller than those recited for deposition chambers available from Applied Materials, Inc. In this respect, the deposition process parameters used to form the alignment films  30 ,  60  can include an RF power of about 0.05 watts per square centimeter (W/cm 2 ) to about 0.6 W/cm 2 . Additionally, gas density for C 2 H 2  can be between 1.5 cubic centimeters per liter (cc/L) and 7.0 cc/L. If present, gas density for H 2  can be between 3.0 cc/L and 27.0 cc/L and for Ar can be between 3.0 cc/L and 14.0 cc/L.  
         [0026]     The deposited amorphous carbon layer has an adjustable carbon:hydrogen ratio that ranges from about 10% hydrogen to about 60% hydrogen. For some embodiments, the layer is about 40% hydrogen to about 50% hydrogen. Controlling the hydrogen ratio of the amorphous carbon layer is desirable for tuning its optical properties. Specifically, as the hydrogen ratio decreases the optical properties of the deposited layer such as the index of refraction (n) and the absorption coefficient (k) increase. Due to the relative H 2  flow rate and the ratio of carbon:hydrogen present in the selected hydrocarbon, the carbon:hydrogen ratio present in the chamber can be 1:4, 8:1,16:1 or 1:1.  
         [0027]     Upper and lower adhesion layers  25 ,  65  may optionally be deposited on the respective electrodes  20 ,  70  prior to depositing the alignment films  30 ,  60 . Peeling of the alignment films  30 ,  60  from the electrodes  20 ,  70  can potentially occur due to insufficient adherence when directly applied to coatings, such as indium-tin-oxide (ITO), that form the electrodes  20 ,  70  on the substrates  10 ,  90 . Accordingly, the adhesion layers  25 ,  65  aid in preventing the alignment films  30 ,  60  from peeling away from the electrodes  20 ,  70  after being deposited.  
         [0028]     For some embodiments, the adhesion layers  25 ,  65  are formed from an amorphous silicon (a-Si), silicon nitride (SiN), silicon oxynitride (SiON), or silicon oxide (SiO). The adhesion layers  25 ,  65  can be thin, for example, less than about 20 angstroms and preferably about 5 angstroms. The thinness of the adhesion layers  25 ,  65  increases transmittance through the adhesion layers  25 ,  65  since the adhesion layers  25 ,  65  may not be transparent.  
         [0029]     In general, the following deposition process parameters can be used to form the adhesion layers  25 ,  65 . For some embodiments where the adhesion layers  25 ,  65  are a-Si, the process parameters include a substrate temperature of about 150° C., a chamber pressure of about 2100 milli-torr, a silane (SiH 4 ) gas flow rate of about 200 sccm with a H 2  flow up to about 1000 sccm, an RF power of about 200 watts, and a spacing of about 520 mils between the shower head and the substrate. These processes were run on an AKT-3500 PECVD system having an internal volume of about 124 liters and being configured to process a substrate that is 550 mm by 650 mm.  
         [0030]     According to aspects of the invention, the deposition process parameters used to form the adhesion layers  25 ,  65  include an RF power of about 0.02 W/cm 2  to about 0.2 W/cm 2 . Additionally, gas density is between 0.2 cc/L and 9.0 cc/L for SiH 4 , between 0.5 cc/L and 5.0 cc/L for ammonia (NH 3 ), between 2.5 cc/L and 25.0 cc/L for nitrogen (N 2 ), and/or between 3.0 cc/L and 45.0 cc/L for nitrous oxide (N 2 O) depending on the composition selected for the adhesion layers  25 ,  65 .  
       EXAMPLES  
       [0031]     Table 1 illustrates various examples of several embodiments of the invention that produced samples of amorphous carbon layers. These amorphous carbon layers are suitable for use within the LCD as the alignment films  30 ,  60 . While C 2 H 2  is used as the hydrocarbon, the process can be modified to obtain similar results. For example, CH 4  at 320 sccm and H 2  at 320 sccm provides an equivalent C:H ratio.  
                                                                                 TABLE 1                                               Pow-                       C 2 H 2     H 2     Ar   er   Pres.   Spacing   Temp           (sccm)   (sccm)   (sccm)   (W)   (mT)   (mils)   (° C.)                                    Example 1   160   640   0   500   300   1000   150       Example 2   320   0   0   200   100   700   100       Example 3   320   0   160   500   200   1000   150       Example 4   320   160   160   500   200   1000   150       Example 5 *   320   1280   0   800   300   1000   150                 * Example 5 was run on an AKT-3500 PECVD system having an internal volume of about 124 liters to process a substrate that was 550 mm by 650 mm while Examples 1-4 were run on the AKT-1600 PECVD system to process substrates that were 370 mm by 470 mm, thereby illustrating the ability to scale up.             
 
         [0032]     Table 2 shows some properties associated with films formed utilizing inputted parameters for the examples as defined in Table 1. Varying one or more of the following process parameters: the flow rates of C 2 H 2  and H 2 ; the RF power; the pressure in the chamber; the temperature of the substrate; and the spacing between the shower head and the substrate, produces an amorphous carbon film with unique combinations of parameters. These parameters include transparency, hydrogen content, sp3 bond percentage, intrinsic stress shown in Table 2 as giga Dyne per square centimeter, refractive index (RI), and resistivity. Accordingly, the appropriate process parameters can be selected to improve the amorphous carbon layer used for the alignment film.  
                                             TABLE 2                                       Stress           RI   (E9D/cm 2 )                                        Example 1   1.714   −7.0           Example 2   1.787   −8.4           Example 3   1.780   −4.9           Example 4   1.767   −5.8                      
 
         [0033]     In addition to the properties shown in Table 2, it has been determined by Rutherford Backward Scattering that a film created by Example 1 has a hydrogen percentage of 42% and a carbon percentage of 58%. Similarly, Example 5 has a hydrogen percentage of 42.5% and a carbon percentage of 57.5%. In addition, the density of Example 1 has been calculated to be 1.3477 g/cm 3 .  
         [0034]      FIG. 3  shows a graph of transmittance verses wavelength through a 200 angstrom thick film deposited in accordance with Example 1. Transmittance at 450 nm is 94.0% and is 98.4% at 600 nm. If desired, transmittance can be increased by reducing the thickness of the film.  
         [0035]     While the foregoing is directed to embodiments of the present invention, other and further embodiments of the invention may be devised without departing from the basic scope thereof, and the scope thereof is determined by the claims that follow.