Abstract:
A substrate for flat panel display glasses comprising a glass the P 2 O 5 —SiO 2 —Al 2 O 3  ternary system which yields stable glasses exhibiting high strain point temperatures, resistance to devitrification, good chemical durability, excellent dielectric properties, coefficients of thermal expansion that can be tailored to match that of silicon, and having liquidus viscosities that enable forming by conventional methods. The glass comprises the following composition as calculated in weight percent on an oxide basis: P 2 O 5  33-75%, SiO 2  2-52%, Al 2 O 3  8-35%.

Description:
RELATED APPLICATION  
       [0001]     This application claims the benefit of priority from Provisional U.S. Patent Application No. 60/533,784, filed, Dec. 31, 2003, the content of which is incorporated herein by reference. 
     
    
     FIELD OF THE INVENTION  
       [0002]     The invention relates to Al 2 O 3 —P 2 O 5 —SiO 2  glasses that are characterized by a high strain point, a coefficient of thermal expansion that can be matched to silicon, and exhibiting high viscosities at their liquidus temperatures, rendering them especially suitable for use as substrates in flat panel display devices.  
       BACKGROUND OF THE INVENTION  
       [0003]     Liquid crystal displays (LCDs) are passive displays which depend upon external sources of light for illumination. Most commonly, LCDs are manufactured in an active matrix addressed format in which an array of diodes, metal-insulator-metal (MIM) devices, or thin film transistors (TFTs) supplies an electronic switch to each pixel. Two sheets of glass form the structure of the display. The separation of the sheets is the critical gap dimension of 5-10 um that contains the liquid crystal material. In order to maintain uniformity of the gap dimension, extremely precise flatness of the glass sheet is required.  
         [0004]     Active matrix liquid crystal displays (AMLCDs) employ an active device such as a diode or thin film transistor at each pixel thereby enabling high contrast and high response speed. Although many display devices currently utilize amorphous silicon (a-Si), the processing of which may be accomplished at temperatures under 450° C., polycrystalline-silicon (poly-Si) processing is preferred. Poly-Si has a much higher drive current and electron mobility thereby increasing the response time of the pixels. Further, it is possible, using poly-Si processing, to build the display drive circuitry directly on the glass substrate. By contrast, a-Si requires discrete driver chips that must be attached to the display periphery utilizing integrated circuit packaging techniques. Poly-Si processing methods operate at higher temperatures than those employed with a-Si TFTs. Such processes enable formation of poly-Si films having extremely high electron mobility (for rapid switching) and excellent TFT uniformity across large areas. The actual temperature required is mandated by the particular process utilized in fabricating the TFTs. Those TFTs with deposited gate dielectrics require 600-650° C., while those with thermal oxides require about 800° C. Both a-Si and poly-Si processes demand precise alignment of successive photolithographic patterns, thereby necessitating that the thermal shrinkage of the substrate be kept low.  
         [0005]     The temperature requirements have mandated the use of glasses exhibiting high strain points in order to avoid thermal deformation at temperatures above 600° C.  
         [0006]     It is generally accepted that four properties are deemed mandatory for a glass to exhibit in order to fully satisfy the needs of a substrate for LCDs: 
        First, the glass must be essentially free of intentionally added alkali metal oxide to avoid the possibility that alkali metal from the substrate can migrate into the transistor matrix;     Second, the glass substrate must be sufficiently chemically durable to withstand the reagents used in the TFT deposition process;     Third, the expansion mismatch between the glass and the silicon present in the TFT array must be maintained at a relatively low level even as processing temperatures for the substrates increase; and,     Fourth, the glass must be capable of being produced in high quality thin sheet form at low cost; that is, it must not require extensive grinding and polishing to secure the necessary surface finish.        
 
         [0011]     The last requirement is a particularly difficult one to achieve inasmuch as it demands a sheet glass production process capable of producing essentially finished glass sheet, such as the overflow downdraw sheet manufacturing process described in U.S. Pat. No. 3,682,609. That process requires a glass exhibiting a very high viscosity at the liquidus temperature plus long term stability, e.g. periods of 30 days, against devitrification at melting and forming temperatures.  
         [0012]     Most glasses to date that fulfill the requirements set forth above are based on eutectic compositions in the alkaline earth boroaluminosilicate systems. The present invention explores a compositional area whose benefits for use as a substrate for display devices will be made evident.  
       SUMMARY OF THE INVENTION  
       [0013]     The present invention describes a glass-forming area in the P 2 O 5 —SiO 2 —Al 2 O 3  system which yields stable glasses with high use temperatures, resistance to devitrification, good chemical durability, excellent dielectric properties, coefficients of thermal expansion that can be tailored to match that of silicon, and having liquidus viscosities that enable forming by conventional methods. The glass comprises the following composition as calculated in weight percent on an oxide basis: P 2 O 5  33-75%, SiO 2  2-52%, Al 2 O 3  8-35%, with the stipulation that the P/Al atomic ratio should lie between 1.3 and 4.0. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]      FIG. 1  describes interpolated liquidus phase relations (dark solid lines) in the SiO 2 —Al 2 O 3 —P 2 O 5  system from binary data along the SiO 2 —P 2 O 5 , SiO 2 —Al 2 O 3 —Al 2 O 3 —P 2 O 5  and AlPO 4 —SiO 2  joins as given in Phase Diagrams for Ceramists (American Ceramic Society). Cation per cent is used because Si 2 O 4  and AlPO 4  are isostructural. The glass-forming area we claim is outlined in a polygon; the dots indicate good glasses.  
         [0015]      FIG. 2  shows absorbance curves of Cr—, V—, Fe—, and Mn-doped glasses (base composition=YVF from Table 1).  
         [0016]      FIG. 3  is a plot of viscosity over a range of temperatures for an exemplary composition.  
         [0017]      FIG. 4  is a plot demonstrating strain points and annealing points for glasses over a range of SiO 2  concentrations.  
         [0018]      FIG. 5  is a dielectric constant curve plotting dielectric constant v. temperature for a representative glass composition.  
         [0019]      FIG. 6  is a plot of viscosity over a range of temperatures comparing an exemplary composition of the present invention and a commercially available glass made utilizing a downdraw process. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0020]     Very few ternary glass-forming systems have not already been thoroughly evaluated. Moreover, few compositional systems that contain no alkali or alkaline earth cations are known to provide stable, non-devitrifying glasses of excellent quality.  
         [0021]     A novel glass-forming area has been discovered in the basic system P 2 O 5 —SiO 2 —Al 2 O 3 . While attempting to synthesize the crystalline compound Si 2 AlO(PO 4 ) 3  through melting, an excellent glass was formed, to the surprise of the inventors. The glass-forming area was expanded along the join Al(PO 3 ) 3 —SiO 2  and on the Al 2 O 3 -rich (peraluminous) side of this join by further melt exploration (see  FIG. 1 ). Glasses poorer in Al 2 O 3  from this join (subaluminous) were deemed problematic from the standpoint of chemical durability.  
         [0022]     An important feature of these glasses is their thermal stability, specifically their resistance to phase separation, devitrification, and deformation at temperatures of 800° C. or even 900° C. The coefficient of thermal expansion ranges from roughly 25 to 55×10-7/° C. and can be tailored to match that of silicon. No alkali or alkaline earth cations are present in these glasses, allowing excellent dielectric behavior. In addition, low liquidus values are expected along cotectic boundaries between SiO 2 , AlPO 4 , and Al(PO 3 ) 3  phases in the preferred composition areas. The peraluminous glasses described herein have shown excellent resistance to boiling water.  
         [0023]     These properties were recognized as being potentially and particularly important for relatively refractory, low-density substrates for silicon or other substrates where the absence of alkali and related glass modifiers is demanded (such as low dielectric substrates). There is a need for readily-meltable glasses which resist deformation and devitrification at high temperatures, have no mobile cations, and possess a useful range of thermal expansion coefficients.  
         [0024]     The advantages of these glasses involve their superior deformation resistance over conventional glasses. They can be heated to 800° C. or even 900° C. without noticeable deformation. Annealing points of many compositions exceed 750° C. Expansion of the glasses range from 25 to 55×10-7/° C. and can be tailored to match that of silicon. The glasses contain no alkali or alkaline earth metals and can be made from relatively inexpensive batch materials. Many of these glasses possess low liquidus temperatures and are compatible with a wide range of forming processes.  
         [0025]     The ternary glass composition area-defined herein in weight % is as follows: 
        SiO 2  2-52     Al 2 O 3  8-35     P 2 O 5  33-75, where 1.3&lt;P/Al&lt;4.0.        
 
         [0029]     It has also been found that boric oxide, B 2 O 3 , additions are effective in stiffening glasses that otherwise might deform above 800° C. In addition, B 2 O 3  is expected to lower the liquidus temperature and further stabilize the glasses against devitrification. The level of B 2 O 3  added to the ternary compositions is typically up to 10% by weight. With this component added, the broader quaternary glass compositions are as follows: 
        SiO 2  2-52     Al 2 O 3  8-35     P 2 O 5  30-75     B 2 O 3  0-10, where 1.3&lt;P/Al&lt;4.0.        
 
         [0034]     It is believed that B 2 O 3  additions help to immobilize P 2 O 5  in the glass structure as BPO 4  units, thereby increasing stiffness and improving chemical durability. Other oxides such as ZrO 2  can be added in amounts up to 6% where appropriate. These glasses can also be doped with transition element or rare earth cations to yield potentially useful optical properties.  
         [0035]     As noted, glasses in the composition region have excellent dielectric properties with dielectric constants as low as 4.5 (and usually flat with respect to temperature and KHz) and resistivities at 250° C. of 10 14 -10 16.5 . The dielectric constant curves for a representative glass (908 ZCP) are shown in  FIG. 5 .  
         [0036]     The glasses of the present invention were melted in platinum crucibles at temperatures of 1600-1650° C. (although lower temperatures could be used for many of these glasses), cast into patties, and annealed at 750° C. Typical P 2 O 5  loss is about 1%. Descriptions of the glasses as melted and as subsequently heat-treated are described in Table 1. Physical properties obtained to date are also listed in the table.  
         [0037]      FIG. 2  shows the absorbance curves for a typical aluminosilicophosphate glass (908 YVF in Table 1) doped with various ions. The glass appears to provide a mildly reducing environment, with the iron occurring predominantly as Fe 2+  and chromium as Cr 3+  (and no Cr 6+ , as evidenced by the UV edge.) The 650 nm absorption of Cr 3+  also is red-shifted, suggesting a weaker crystal field than obtained in conventional soda lime or aluminosilicate glasses.  
         [0038]      FIG. 3  shows a viscosity curve for exemplary composition 908ZCA. As one of skill in the art can appreciate, the composition has a very high viscosity at the liquidus, in excess of 1,000,000 poises. This makes it an excellent candidate for downdraw manufacturing processes (e.g. the fusion or slot draw process). The preferred manufacturing process for the glasses of the present invention is via a downdraw sheet manufacturing process (e.g. the fusion or slot draw process) in which glass sheets are formed while traveling in a downward direction. In the fusion or overflow downdraw forming process, molten glass flows into a trough, then overflows and runs down both sides of a pipe, fusing together at what is called the root, (when the pipe ends and the two overflow portions of glass rejoin) and is drawn downward until cool. The overflow downdraw sheet manufacturing process is described for example in U.S. Pat. No. 3,338,696 and U.S. Pat. No. 3,682,609.  
         [0039]      FIG. 6  shows a viscosity curve for another exemplary composition (908 ZAU). The viscosity at the liquidus for this composition is greater than 10,000,000 poise. These low liquidus glasses can be melted as low as 1400-1450° C., significantly minimizing phosphorous volatility during melting. For comparative purposes, a viscosity curve for a commercially produced glass, manufactured by the fusion process, (Corning Incorporated Code 1737) is included in  FIG. 6 .  
                                                         TABLE 1                       Glass Compositions and Properties                                wt % (batched)                                           908   ZCA   YVF   YVY   YWQ   YWW   YXA   ZAU   ZCD   ZCK               SiO 2     21.4   31.3   30.1   38.8   47.5   39.8   27.4   41.5   39.8       Al 2 O 3     13.1   13.3   12.8   16.9   10.7   17.3   13.1   16.4   30.4       P 2 O 5     54.7   55.4   53.3   44.3   39.3   40.3   54.7   12.1   49.9       B 2 O 3     4.8   0   0   0   2.5   2.6   4.8   0   0       ZrO 2     0   0   3.8   0   0   0   0   0   0       Glass appearance       clear   clear   Clear   clear   clear, seedy   clear   clear   clear       Heat treatments       900° C.-4 h       some slump       some haze   some haze       clear   some haze   sl haze                       no slump   no slump       sl. slump       sl slump       850° C.-4 h       clear                   clear   clear   clear       Liquidus T ° C.   875   1110           &gt;1250       Refractive index       1.499                           1.485       Density g/cm 3     2.486   2.485                   2.486       2.374       CTE 25-500° C.   48   47       32           53       37       (×10 −7 /° C.)       Anneal point (° C.)   729   728       713       Strain point (° C.)   681   677       660       2 h boiling water   good   good       good           good       irid.               wt % (batched)       908   ZCP   ZCS   ZCX   ZDA   ZEY   ZFA   ZFC   ZFV   ZFW               SiO 2     39.5   31.2   25.8   24.4   16.9   19.7   16.1   12.4   13.3       Al 2 O 3     15.6   19.5   21.1   17.6   20.8   18.3   19.8   22.4   24.1       P 2 O 5     40.1   44.5   53.1   53.2   62.3   57.2   59.3   58.2   62.6       B 2 O 3     4.8   4.8   0   4.8   0   4.8   4.8   7   0       Glass   Clear   clear   clear   clear   clear   clear   clear       appearance       Heat treatments       900° C.-4 h   Clear   clear   some haze   Clear   clear   clear   clear           no slump   no slump   slump   sl. slump   slump   some slump   some slump       850° C.-4 h           clear.   Clear                   no slump   sl slump       Liquidus T ° C.       Refractive index           1.491       Density g/cm 3     2.313           2.413       CTE 25-500° C.       (×10 −7 /° C.)       Anneal point       (° C.)       Strain point (° C.)       2 h boiling water   rough surf               very   rough surf   excellent                           good                    
         [0040]     An additional series of melting experiments were conducted at 1600-1650° C. for 4 hours on 500-1000 gm batches of the appropriate mixtures of SiO 2 , Al(PO 3 ) 3  and Al 2 O 3  contained within Pt crucibles. For compositions on the Al(PO 3 ) 3  —SiO 2  join, it was observed that clear glasses can be formed with SiO 2  contents ranging from 7 to nearly 30 weight %. In addition, clear glasses be formed over a similar range of SiO 2  concentrations, but with higher content up to about 35% and, thus, expected superior chemical durability. Physical property data including anneal point (T a ), strain point (T str ) and thermal expansion coefficient (CTE) for these glasses is provided in the following Table 2:  
                                                                     TABLE 2                                               CTE                       T a     T str     25-500° C.       Code   % Al 2 O 3     % P 2 O 5     % SiO 2     (° C.)   (° C.)   (×10 −7 /° C.)                                891 HHM   17.9   74.7   7.4   757   709   59       891 HHN   17.4   72.4   10.2   756   714   55       891 HHO   16.8   70   13.2   755   707   56       891 HHP   16.2   67.5   16.3   758   718   56       891 HHQ   15.5   64.8   19.7   743   694   56       891 HHR   14.8   61.9   23.3   741   697   55       891 HHS   14.1   58.8   27.1   731   683   59       891 HLX   18.6   64.9   16.5   725   679   55       891 HLY   21.2   62.2   16.6   701   656   51       891 HLZ   23.7   59.5   16.8   680   631   46       891 HMA   20.4   66.2   13.4   721   678   51       891 HMC   25.4   61   13.6   —   —   —       891 HOQ   26   66.3   7.7   —   —   —       891 HOT   25.1   70   4.9   —   —   —       891 HOW   26.6   71   2.4   —   —   —                  
 
         [0041]     The data given above and displayed in  FIG. 4  indicate that the highest strain points are achieved for glasses with (1) Al 2 O 3 /P 2 O 5  ratios of ⅓ (i.e. lying on the on the Al(PO 3 ) 3 —SiO 2  join and (2) SiO 2  contents of 7-18%, with glass 891 HHP being a preferred composition. The trend towards lower strain points at SiO 2  levels greater than 18% may be a reflection of the existence of a binary eutectic involving SiO 2  and Al(PO 3 ) 3  at some higher SiO 2  concentration. Similarly, the downward trend in strain point with increasing Al 2 O 3  content at constant SiO 2  levels may be due to the existence of a thermal valley between the liquidus surfaces of Al(PO 3 ) 3  and AlPO 4 .  
         [0042]     As noted, any number of fluxes (modifying oxides) may be added to the batch in order to impart these and other desired characteristics. While these fluxes typically lower the strain point of the native glass, they are often necessary for any or all of the following purposes: raise the CTE, lower the liquidus temperature, obtain a preferred strain point for compaction, absorption at specific wavelengths, ease the melting, modify density or modify durability. The effects that certain oxides have on the physical and chemical characteristics of glass are generally known. Fluxes may be added in amounts up to 15%, or as limited by solubility. Fluxes are preferably added in amounts less than 10% in total. The glass compositions therefore are identified as: 
        SiO 2  2-52     Al 2 O 3  8-35     P 2 O 5  30-75     RO 0-15        
 
         [0047]     Modifying oxides may be selected from alkali metals, alkaline earth metals, transition metals as well as oxides of the lanthanide series. Specific examples include Y 2 O 3 , ZrO 2 , HfO 2 , MgO, CaO, SrO, BaO, As 2 O 3,  SnO 2 , Li 2 O La 2 O 3 GeO 2 , Ga 2 O 3 , Sb 2 O 3 , Na 2 O, K 2 O, Rb 2 O, Cs 2 O, BeO, Sc 2 O 3 , TiO 2 , Nb 2 O 5 , Ta 2 O 5 , ZnO, CdO, PbO, Bi 2 O 3 , Gd 2 O 3 , Lu 2 O 3  and/or B 2 O 3.  As demonstrated, several examples of representative glasses were melted containing various fluxes. Therefore, for purposes of this invention, R shall be Mg, Ca, Y, Sr, Zr, Hf, As, Sn, Li, La, Ge, Ga, Sb, Ba, Sb, Ti, Ta, Zn, or any other element that fits the definition of the appropriate modifiers above.  
                                                                                                     TABLE 3                       wt %   ZHR   ZIR   ZIT   ZIU   ZIV   ZIW   ZIX   ZIY   ZKH   ZJB                                SiO 2     23.2   22.3   22.8   22.0   22.9   22.4   22.6   22.1   21.3   25.9       Al 2 O 3     15.1   14.5   14.9   14.4   14.9   14.6   14.7   14.4   13.9   12.4       P 2 O 5     56.9   54.6   55.8   53.9   56.1   54.9   55.3   54.1   52.2   51.6       B 2 O 3     4.8   4.6   4.7   4.5   4.7   4.6   4.6   4.5   4.4   4.5       ZrO 2         4.0                           3.8       CaO           1.9       La 2 O 3                 5.2       MgO                   1.3       SrO                       3.4       ZnO                           2.7           4.8       SnO 2                                 4.9       Y 2 O 3                                     8.7       Sb 2 O 5                                         0.9       Liq T ° C.   990                           ˜1240       Density   2.470       CTE   51   47                   50   51       Anneal   736   738                   705   724   727       Strain   686   685                   654   675   670                  
 
         [0048]     As can readily be appreciated by the disclosed experimental data, the disclosed glass compositions present excellent candidates for display applications. They have strain points that are slightly higher than the alkaline earth boroaluminosilicate glasses presently in commercial use. They also provide the benefit of having lower viscosities at 1600° C., allowing them to be self-fining. It is believed that the partial volatization of the P 2 O 5  (in amounts approximating 1%) aid in refining of the glass. As a consequence, arsenic or other common fining agents will likely not be required during the manufacturing process.