Abstract:
A family of glasses from the SiO 2 —Al 2 O 3 —P 2 O 5  ternary system exhibiting high strain point, transparency, and low coefficient of thermal expansion. The glasses have the following composition, expressed in mol percent and calculated from the glass batch on an oxide basis: 55-80 SiO 2 , 12-30 Al 2 O 3 , and 2-15 P 2 O 5 .

Description:
RELATED APPLICATION 
     This application claims the benefit of priority from Provisional U.S. Patent Application No. 60/533,765, filed, Dec. 30, 2003, the content of which is incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     The invention relates to Al 2 O 3 —P 2 O 5 —SiO 2  glasses that are characterized by a high strain point, a low coefficient of thermal expansion and relatively low density. 
     BACKGROUND OF THE INVENTION 
     The materials of the present invention are most importantly substrate candidates for electronic devices. Several processes in the manufacture of electronic devices such as liquid crystal displays (LCDs), solar cells, electronics, microelectronics etc. include steps that are performed at extremely high temperatures. For example, active matrix LCDs 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 oh the glass substrate. By contrast, a-Si requires discrete driver chips that must be attached to the display periphery utilizing integrated circuit packaging techniques. The most efficient poly-Si processing methods operate at temperatures of at least 800° C., such processes enable formation of poly-Si films having extremely high electron mobility (for rapid switching) and excellent TFT uniformity across large areas. This fabrication process typically consists of successive deposition and patterning of thin films using elevated temperature processes which result in the substrate being heated to temperatures in the range of 800° C. Fused silica has a sufficiently high strain point of 990-1000° C., but its thermal expansion coefficient (C.T.E.) is significantly lower than that of silicon (C.T.E. 5×10 −7 /° C. v. C.T.E. 37×10 −7 /° C.). 
     For other electronic devices, common, processing steps also require high temperature substrates to withstand processing. Most high level electronic fabrication requires annealing of the gate oxide and dopant activation. These processes occur at temperatures in excess of 800° C. 
     Even in the case of single crystal silicon (x-Si) fabrication techniques that employ a thin layer of single crystal silicon bonded to a substrate, high temperature substrates are required. Single, crystal silicon allows for even greater electron mobility than that achieved with poly-Si. The bonding step often requires high temperatures as well as the gate oxide and dopant activation steps previously described. 
     A need exists, then, for a glass that (1) has a high strain point (&gt;800° C.), (2) does not require costly heat treatments after fabrication, (3) has a CTE match close to that of silicon, and (4) can be melted in a conventional melting unit. In addition, the glass will preferably be transparent to visible radiation and be chemically durable. These several qualities are needed in glasses for production of such varied products as flat panel displays, photovoltaic cells, photomasks, optomagnetic disks and tubing and fiber applications that require stability at high temperatures. 
     Flat panel displays employ sheet glass that necessarily is transparent at visible wavelengths as well as into the ultra violet. It is also necessary that the glass sheet be adapted to production of a silicon layer on the glass surface. Initially, the silicon layer applied was amorphous silicon (a-Si). Fabrication of such devices required temperatures no greater than 350° C. Suitable glasses were readily available for use under these conditions. 
     The evolution from a-Si to poly-Si and x-Si as a coating material has presented a major challenge to use of a glass substrate. Poly-Si and x-Si coatings require much higher processing temperatures, in the range of 600-1000° C. 
     A primary purpose of the present invention is to provide a glass that has properties suited to production of a poly-Si or x-Si coating on its surface. 
     Another purpose is to produce a glass having a sufficiently high strain point to permit processing at 800-900° C. 
     A further purpose is to provide a glass that can be melted by conventional procedures, and that can provide a substrate for application of a high quality, poly-Si or x-Si film. 
     A still further purpose is to provide an electronic device, in particular, a flat panel display, and having a high-quality, poly-Si or x-Si, thin film on its surface. 
     Another purpose is to provide a novel glass family consisting essentially of Al 2 O 3 —P 2 O 5 —SiO 2 , and optionally containing selected oxides such as alkali, alkaline earth, transition metal oxides, as well as oxides of the lanthanide series. 
     SUMMARY OF THE INVENTION 
     The invention resides in part in a family of ternary Aluminophosphosilicate glasses having a Al 2 O 3 /P 2 O 5  molar ratio of greater than 1, a strain point in excess of 800° C., a melting temperature of 1650° C. or less, a water-white clarity, and a coefficient of thermal expansion of 5-40×10 −7 /° C. 
     The invention further resides in an electronic device having a poly-silicon film on a transparent, glass substrate, the substrate being an Alumino-phosphosilicate glass having a Al 2 O 3 /P 2 O ratio of greater than 1, a strain point in excess of 800° C., and a coefficient of thermal expansion of 5-40×10 −7 /° C. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
         FIG. 1  demonstrates a viscosity curve for a preferred composition of the present invention. 
     
    
    
     DESCRIPTION OF THE INVENTION 
     Broadly stated, the present glasses have compositions falling within the following ranges, expressed in mole % as calculated from the glass batch on an oxide basis: 
     
       
         
               
               
               
             
           
               
                   
                   
               
             
             
               
                   
                 SiO 2   
                 55-80% 
               
               
                   
                 Al 2 O 3   
                 12-30% 
               
               
                   
                 P 2 O 5   
                  2-15% 
               
               
                   
                 RO 
                  0-15% 
               
               
                   
                   
               
             
          
         
       
     
     Any number of fluxes (modifying oxides) may be added to the batch in order to impart 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. Modifying oxides may be selected from alkali metals, alkaline earth metals, transition metals as well as oxides of the lanthamide 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 , ZnO, CdO, PbO, Bi 2 O 3 , Gd 2 O 3 , Lu 2 O 3  and/or B 2 O 3 . Therefore, for purposes of this invention, R shall be Mg, Ca, Y, Sr, Zr, Hf, As, Sn, Li, La, Ge, Ga, Sb or any other element that fits the definition of appropriate modifiers above. 
     These glasses have the following characteristic properties: 
     
       
         
               
               
               
             
           
               
                   
                   
               
             
             
               
                   
                 Strain Point 
                 &gt;800° C. 
               
               
                   
                 CTE 
                 &gt;8 × 10 −7 /° C. 
               
               
                   
                 Melting 
                 &lt;1650° C. 
               
               
                   
                 Density 
                 &gt;2.2 g/cm 2   
               
               
                   
                   
               
             
          
         
       
     
     A preferred embodiment has compositions within the above identified ranges, again in mol % on an oxide basis, as calculated from the glass batch: 
     
       
         
               
               
               
             
           
               
                   
                   
               
             
             
               
                   
                 SiO 2   
                 60-70% 
               
               
                   
                 Al 2 O 3   
                 15-25% 
               
               
                   
                 P 2 O 5   
                  5-10% 
               
               
                   
                 RO 
                 &lt;7% 
               
               
                   
                   
               
             
          
         
       
     
     TABLES I and II, below, set forth several compositions, in mol % on an oxide basis, illustrative of compositional ranges of the invention. The actual batch ingredients may comprise any materials, either oxides or other compounds, which when melted together with the other batch components, will be converted into the desired oxide in the proper proportions. 
     The batch ingredients were compounded, tumble mixed together thoroughly to aid in producing a homogeneous melt, and charged into platinum crucibles. After placing lids thereon, the crucibles were moved into furnaces operating at temperatures of 1600-1650° C. The crucibles were then removed after approximately 16 hours and the melt so-formed cast into a steel mold. The glass patty was then removed from the mold and placed into an annealing furnace at a temperature at the annealing point of the glass (approximately 900° C. The glasses were then removed from the annealing furnace and allowed to cool. 
     Tables I and II also list measurements of several physical and chemical properties determined on the glasses in accordance with techniques conventional in the glass art. Thus, the linear coefficient of thermal expansion (CTE) over the temperature range 0-300° C. is expressed in terms of ×10 −7 /° C. and determined by dilotometry; the softening point is expressed in ° C. as determined by parallel plate viscometry; and strain point and annealing temperature are expressed in ° C. as determined by beam bending viscometry. The durability in HCl was determined by measuring the weight loss (mg/cm2) after immersion in a bath of aqueous 5% by weight HCl at 95° C. for 24 hours. The liquidus temperature was measured utilizing a standard gradient furnace test. The melting temperature is the temperature at which the glass melt demonstrates a viscosity of 300 poises. 
     As can be observed from the Tables, not all tests were performed for all compositions. 
     
       
         
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
             
           
               
                 TABLE I 
               
               
                   
               
               
                 Series 
                 891 
                 891 
                 891 
                 891 
                 891 
                 891 
                 891 
                 891 
               
               
                 Code 
                 HIM 
                 HIN 
                 HIP 
                 HIQ 
                 HIS 
                 HIT 
                 HJB 
                 HOE 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 SiO 2   
                 71 
                 71 
                 70 
                 70 
                 69 
                 69 
                 68 
                 70 
               
               
                 Al 2 O 3   
                 21 
                 22 
                 22 
                 23 
                 22 
                 23 
                 24 
                 18 
               
               
                 P 2 O 5   
                 8 
                 7 
                 8 
                 7 
                 9 
                 8 
                 8 
                 7 
               
               
                 Y 2 O 3   
               
               
                 ZrO 2   
                   
                   
                   
                   
                   
                   
                   
                 5 
               
               
                 Softening Pt. 
                   
                   
                 1197 
                 1178 
                 1203 
                 1185 
               
               
                 (° C.) 
               
               
                 Annelaing 
                 911 
                 922 
                 911 
                 917 
                   
                 927 
                 920 
                 912 
               
               
                 Temp. (° C.) 
               
               
                 Strain Pt. 
                   
                 865 
                   
                 864 
                   
                 860 
                   
                 857 
               
               
                 (° C.) 
               
               
                 Melting 
               
               
                 (° C.) 
               
               
                 CTE RT−300   
                   
                   
                 9.9 
                 10.1 
                 8.6 
                 10.1 
                   
                 12.5 
               
               
                 (× 10 −7 /° C.) 
               
               
                 Density 
                 2.325 
                 2.347 
                 2.337 
                 2.353 
                 2.326 
                 2.35 
               
               
                 (g/cm 2 ) 
               
               
                 UV Cutoff λ 
                   
                   
                 332 
                 330 
                 337 
                 333 
               
               
                 (nm) 
               
               
                 Durability 
                   
                   
                 0.49 
                   
                   
                   
                 0.5 
               
               
                 (mg/cm 2 ) 
               
               
                 Refractive 
                 1.486 
                 1.491 
                 1.487 
                 1.492 
                 1.485 
                 1.491 
               
               
                 Index 
               
               
                   
               
             
          
         
       
     
     
       
         
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
             
           
               
                 TABLE II 
               
               
                   
               
               
                 Series 
                 891 
                 891 
                 891 
                 891 
                 891 
                 891 
                 891 
                 891 
               
               
                 Code 
                 HTS 
                 HTB 
                 HTP 
                 HVP 
                 HTU 
                 HZC 
                 HXA 
                 IAL 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 SiO 2   
                 65.6 
                 70 
                 65.6 
                 62.1 
                 65.6 
                 68 
                 65 
                 65 
               
               
                 Al 2 O 3   
                 21.8 
                 18 
                 21.9 
                 20.7 
                 21.8 
                 19 
                 22 
                 22 
               
               
                 P 2 O 5   
                 7.6 
                 7 
                 7.6 
                 7.2 
                 7.6 
                 8 
                 8 
                 8 
               
               
                 Y 2 O 3   
                 2.5 
                   
                   
                   
                   
                 2.5 
                 2.5 
                 2.5 
               
               
                 ZrO 2   
                 2.5 
                 3.5 
                   
                   
                 2.5 
                 2.5 
                 2.5 
               
               
                 MgO 
                   
                 1.5 
               
               
                 BaO 
                   
                   
                 5 
                 10 
               
               
                 La 2 O 3   
                   
                   
                   
                   
                 2.5 
                   
                   
                 2.5 
               
               
                 Softening Pt. 
                 1139 
                 1181 
                 1160 
                   
                 1111 
                   
                 1135 
               
               
                 (° C.) 
               
               
                 Anneal 
                 870 
                   
                 885 
                   
                 879 
                 900 
                 886 
               
               
                 Temp (° C.) 
               
               
                 Strain Pt. 
                 846 
                   
                 824 
                   
                 829 
                 851 
                 837 
               
               
                 (° C.) 
               
               
                 Melting 
                 1650 
               
               
                 (° C.) 
               
               
                 CTE RT−300   
                 18.2 
                 14.2 
                 20.6 
                 34.3 
                 19.6 
                 18.4 
                 16.8 
                 16.8 
               
               
                 (× 10 −7 /° C.) 
               
               
                 Liquidus 
                   
                   
                   
                   
                   
                 1450 
                 1525 
                 1460 
               
               
                 Temp (° C.) 
               
               
                   
               
             
          
         
       
     
     The strain points are somewhat lower than fused silica. However, they are substantially higher than available, conventionally melted glasses, and are quite adequate for their intended purpose. It should be noted that the glass transition temperatures for the ternary oxides are all greater than 900° C., which in turn indicates strain points all in excess of 850° C. 
     For several compositions, the liquidus temperature is below 1500° C. A preferred composition has a liquidus temperature of 1450° C. (891 HZC). 
       FIG. 1  is a curve demonstrating the viscosity of a preferred composition (891 HTS) over a wide range of temperatures. 
     As can be further appreciated, the low thermal expansion characteristics of these glasses make them excellent candidates for technological situations that require refractoriness and thermal shock resistance. 
     In situations where the glass of the present invention is to be utilized as a substrate supporting a thin layer of silicon it may be necessary to coat the surface with a barrier layer that will protect the silicon from becoming contaminated by the glass. Such barrier layers are common and known to those of skill in the art. Examples of suitable barrier layers include silica as well as silicon nitride. 
     In other applications, it may be advantageous to color the glass. Addition of transition metal oxides are known to add color to the glass. For example small amounts of cobalt are known to impart a blue or gray color to the glass. 
     While the invention has been described with respect to a limited number of embodiments, those skilled in the art, having benefit of this disclosure, will appreciate that other embodiments can be devised which do not depart from the scope of the invention disclosed herein. The scope of the invention is defined by the attached claims.