WO2005063633A1

WO2005063633A1 - Method for fabricating glass panels

Info

Publication number: WO2005063633A1
Application number: PCT/US2003/037981
Authority: WO
Inventors: Glen Cook; Martin Goller; Randy Rhoads; Robert Thomas
Original assignee: Corning Incorporated
Priority date: 2003-11-28
Filing date: 2003-11-28
Publication date: 2005-07-14
Also published as: CN1878731A; KR20060107829A; KR101093914B1; AU2003293146A1; JP2007521218A

Abstract

A method of forming glass with substantially reduced inclusions and cord includes reducing a shear force while maintaining a predetermined stirring efficiency in a stirring operation by properly selecting a stirrer diameter, a stirrer speed and a coupling distance. A stirring apparatus (100) includes a stirring vessel (101), which receives the fluid glass (105) at an input (107). This glass is then stirred by rotation of a shaft (104) having a stirrer (103) to improve the homogeneity of the glass material (106) that is delivered at the output (108). The coupling distance is the distance between the edges of the stirrer blades and the interior walls (102) of the stirring vessel. The glass may be used to form glass sheets for use in flat panel displays such as liquid crystal displays for use in computer monitors, personal digital assistants, and cellular telephones. The glass sheets may have thin film transistor elements formed thereon for use in forming pixels on the display.

Description

Method for Fabricating Glass Panels

Background

[0010] Display devices are used in a variety of applications. For example, glass substrates are in used active matrix liquid crystal displays (AMLCD) and thin film transistor liquid crystal displays (TFT-LCD) for notebook computers, flat panel desktop monitors, LCD televisions, and internet and communication devices, to name only a few.

[0020] In many LCD-based display devices, certain electronic components are often fabricated on the glass substrate of the display device. Often, the transistors are thin-film transistors (TFT), and are complementary metal oxide semiconductor

(CMOS) devices. In these applications, it is beneficial to form the semiconductor structure directly on the glass material of the display.

[0030] Thus, many LCD displays often comprise a glass substrate with the transistors formed over the glass substrate, and beneath a layer of LC material.

The transistors are arranged in a patterned array, and are driven by peripheral circuitry to provide switching on desired voltages to orient the molecules of the LC material in the desired manner. The transistors thus create the picture elements

(pixels) of the display.

[0040] With many of these display devices, the requirements of homogeneity of the glass material have significantly increased. To this end, the glass substrates used in these display devices are beneficially, if not necessarily, free of inclusions and surface abnormalities. These requirements place certain demands on the manufacturing process.

[0050] Four particular defect types are beneficially mitigated in order to provide glass substrates useful in the display devices referenced above. To wit, cord, erosion- sourced platinum, condensation-sourced inclusions and precipitation- sourced inclusions are usefully reduced or substantially eliminated.

[0060] Cord is a chemical inhomogeneity in the glass, which is manifested as a visible defect in display glass from corrugations on the surface of the glass sheet where the length of the corrugation is parallel to the draw direction. These corrugations, which are on both surfaces of the sheet, create a lens effect that can produce alternating longitudinal light and dark bands spaced a few millimeters apart when light is passed through the sheet. These corrugations may also create color or streak patterns in the final display.

[0070] Erosion-sourced platinum inclusions are often generated by a shear stress imposed on the stir chamber wall and stirrer blades. These small inclusions can create a small surface discontinuity in the glass surface. The small surface discontinuities have a potential to create performance defects in the TFT and color filter side of the display. These defects can have a significant impact on the color filter side of the TFT cell because the color filter is fabricated in a whole surface process rather than a discrete area process as in the TFT side of the cell. [0080] Condensation-sourced inclusions are often generated from volatilization of precious metals or chemical components of the glass at the free surface at the top of the stir chamber condensing on colder surfaces and then subsequently falling in to the molten glass and creating a solid or gaseous defect. The solid defects create problems in the product as described above. The gaseous defects can perturb the surface but more likely become an optical defect in a pixel of the finished product.

[0090] Precipitation-sourced platinum inclusions are often generated in the manufacturing system down stream of the stirrer chamber where the glass flow is substantially laminar. Platinum (Pt) or other elements from the manufacturing process diffuse into the flowing glass at the interface of the glass and the vessel. The amount of the element (Pt or Rhodium (Rh)) present in the glass is a function of the temperature and time the glass is in intimate contact with the vessel, and solubility and diffusivity of the element in the glass. Down stream precipitation of these elements is interdependent on the concentration of the element, the thermodynamics and kinetics of the elements crystallization, and the time- temperature history prescribed by the manufacturing process. [00100] Accordingly, what is needed is a method and apparatus for fabricating glass panels that addresses at least the issues presented above. For example, reductions in the shear stress are needed to meet present and future platinum micro-inclusion requirements for LCD display glass substrates.

Summary

[00110] In accordance with an example embodiment, a method of forming glass with substantially reduced inclusions and cord includes reducing a shear force while maintaining a predetermined stirring efficiency in a stirring operation by properly selecting a stirrer diameter, a stirrer speed and a coupling distance.

Brief Description of the Drawings

[00120] The invention is best understood from the following detailed description when read with the accompanying drawing figures. It is emphasized that the various features are not necessarily drawn to scale. In fact, the dimensions may be arbitrarily increased or decreased for clarity of discussion.

[00130] Fig. 1 is a cross-sectional view of a stirring chamber and stirrer in accordance with an example embodiment.

[00140] Fig. 2 is a table showing conditions of constant stirring efficiency in accordance with an example embodiment.

[00150] Fig. 3 is a graph of required stirring efficiency as a function of speed in accordance with an example embodiment.

[00160] Fig. 4 is a table showing conditions of constant stirring efficiency in accordance with an example embodiment.

[00170] Fig. 5 is a graphical representation of product loss versus period in accordance with an example embodiment.

[00180] Fig. 6 is graphical representation of shear stress versus stirring efficiency in accordance with an example embodiment.

[00190] Fig. 7 is a shear stress on the inner walls of the stirring container versus temperature in accordance with an example embodiment.

[00200] Figs. 8a and 8b are images of dye in glass material with improved mixing of the glass in accordance with an example embodiment and poor mixing of the glass per known methods, respectively.

[00210] Fig. 9 is a graphical representation of the intensity versus pixel distance in accordance with an example embodiment.

[00220] Fig. 10 is a graphical representation of the stirrer efficiency index, versus shear rate distance in accordance with an example embodiment.

Detailed Description

[00230] In the following detailed description, for purposes of explanation and not limitation, example embodiments disclosing specific details are set forth in order to provide a thorough understanding of the present invention. However, it will be apparent to one having ordinary skill in the art having had the benefit of the present disclosure, that the present invention may be practiced in other embodiments that depart from the specific details disclosed herein. Moreover, descriptions of well- known devices, methods and materials may be omitted so as to not obscure the description of the present invention.

[00240] Briefly, by way of example embodiments described herein, a stirring device (stirrer) geometry used in the fabrication of glass sheets for use in various display products is described. The geometry of the stirrer is described relative to stirrer efficiency and shear stress on platinum surfaces, and an extension of this technology to reduce precipitation sourced inclusions. Furthermore, example embodiments at the same time reducing the shear stress created inclusions referenced previously reduce cord in the glass display created by the discrete glass material of differing viscosity. The latter is addressed by increasing the rate of output from the stirring chamber, which reduces the period of these chemical inhomogeneity-creating corrugations in the final product. In example embodiments the distance is increased between the stirrer blades and the stir chamber wall (referred to as the coupling distance "C"), while the circular velocity of the stirrer is increased. This reduces the incidence of platinum inclusions as well as cord in the resultant product.

[00250] Fig. 1 shows a stirring apparatus 100 in accordance with an example embodiment. The stirring apparatus 100 includes a stirring vessel 101 , which receives the fluid glass 105 at an input 107. This glass is then stirred by a stirrer 103 to improve the homogeneity of the glass material 106 that is delivered at the output 108. The glass material 105 thus flows in a downward direction and the radial motion of the stirrer 103 about its shaft 104 is responsible for reducing if not substantially eliminating the inhomogeneities in the glass materials in examples embodiment.

[00260] As will become clearer as the present description continues, increasing the shear stress on the glass material 105 will decrease the inhomogeneous nature of the glass material 106. However, the greater the shear stress, the greater is the tendency of material from the stirrer 103 and stirring chamber 101 to be forcefully removed from these elements and thus introduced into the glass material 106. Ultimately, if this occurs, defects in the material will manifest in surface non- uniformities and optical aberrations in the glass panels. Accordingly, example embodiments address these phenomena so that both cord and inclusions are dissipated to an acceptable level for use in present and future glass applications where these must be minimized. In accordance with example embodiments described herein, an optimum coupling distance is realized, and a rate of rotation of the stirrer 103 that both effects the desired stirring to reduce inhomogeneities and the desired rate of discharge of the glass material 106, so that the distance between any remaining inhomogeneous or discrete portions of the glass material is small enough that cord is substantially eliminated in the resultant glass sheet, and inclusions are substantially eliminated as well.

[00270] In accordance with illustrative embodiments, Pt inclusions having a diameter of approximately 50μm or greater are substantially eliminated. Of course, the threshold size of the Pt inclusions is beneficially reduced to less than 50μm according to example embodiments. Cord is measured by a gauge having an output that is intensity contrast between the light and dark bands of cord. Under this type of measure, the method of fabricating glass in accordance with example embodiments results in a contrast measure that is less than approximately 0.67%. [00280] As referenced previously, the incidence of cord in the resultant glass sheet is particularly problematic. To this end, it is believed that cord results at least from refractory corrosion products and glass volatilization products. For example, the inhomogeneous glass (e.g., glass 105) entering the stirring system may have an elevated alumina concentration, or silica concentration, or both. From physical model studies it is clear that when the stirring process receives a continuous input stream of inhomogeneous glass, it generates an output stream containing discrete elements of the inhomogeneity albeit at significantly reduced concentration levels. The individual width and the pitch of the discrete elements are inversely proportional to stirrer speed. To wit, the increased stirrer speed can reduce the inhomogeneity through the resultant shear stress imparted on the discrete glass. [00290] Furthermore, it is believed that the visibility of the surface corrugations depends on their amplitude and pitch. As described in further detail below, as the pitch between the corrugations decreases due to faster speed of the output of the glass material from the stirring chamber 101 (e.g., glass material 106) the optical effect of the corrugations becomes less easy to see (even if their amplitude does not change) so that the target stirring efficiency actually decreases. This is a reflection of the influence of stirrer speed on visibility of the cor . This property makes it possible to increase the coupling distance to achieve further reductions in shear stress.

[00300] Quantitatively, the effectiveness of the stirring, E, can be approximated as: BD²NVτ

[00310] E = (1 ) Q^lμ

[00320] where k is a constant of proportionality, B is the number of blades on the stirrer, D is the blade diameter, N is the stirrer speed, τ is the shear stress, V is the volume of stir chamber, Q is the flow rate of glass in the chamber, and μ is the viscosity of the glass.

[00330] Eqn. (1 ) shows that if E is to be maintained while τ is reduced, D and V must be increased. According to example embodiments, a larger stirrer diameter and larger stirred volume (i.e., increased size of the stir chamber 101) along with reduction in viscosity effectively reduces the formation of micro-Pt inclusions by reducing shear stress about 80% on the platinum stirring parts (stir chamber and stirrer). According to example embodiments, further reduction in shear stress is effected by examining the effect of the coupling "C" between stirrer and stir chamber on shear stress.

[00340] The shear stress can be represented: _mn„„. dv V... -V ._mll πDN-0 μπDN ...

[00350] τ = μ— = μ- ≡lL = _μ = ^7T~ (²)

[00360] where v is the velocity of the glass material and C is the coupling distance between blades of the stirrer 103 and interior wall 102 of the stir chamber

101.

[00370] Substituting Eqn. (2) into Eqn. (1 ) gives:

[00390] where k' = πk.

[00400] If E, B, V and Q are kept constant, the effects of varying D, N and C can be evaluated from the following equation:

^■■ Consent (4)

[00410] The goal is to find a condition where the left side of Eqn. (4) is kept constant for a minimum shear stress. Using normalized numbers, the results of Eqn. (4) can be seen in the table of Fig. 2.

[00420] The area in the stir chamber where the stirrer applies the greatest shear stress to the glass is the region of closest approach between the blades of the stirrer 103 and the interior walls 102 of the stirring container. Stated differently, both the shear rate (the velocity gradient of the fluid between two surfaces) and the shear stress are at their highest in the coupling area next to the stir chamber wall. This area is often referred to in the art as the coupling area. By increasing the distance between the edges of the blades of the stirrer 103 and the interior walls 102, the coupling distance "C", the volume of highly sheared glass increases proportionally to the lower sheared volume between the outside diameter of the shaft 104 and the tip of the blades ("the blade area"). In an example embodiment, it is beneficial that the coupling distance is between approximately 5/8" and approximately 7/8" in a stir chamber having a diameter of 9.875". Moreover, the ratios of high shear volume to low shear volume for these two coupling distances in according to an example embodiment (in a 9.875" stir chamber diameter) are 0.315 and 0.396, respectively. At a coupling distance larger than approximately 0.75", the stirrer efficiency as measured in the physical model drops off substantially. [00430] It is noted that Eqn. 3 above, based on physical modeling of stirrers for a number of glass applications, predicts that stirring efficiency (E) is directly proportional to stirrer speed (N) and inversely proportional to the square root of coupling (C ). Experience with glass stirring applications other than LCD sheet shows that scale-up of a stirring process can be done successfully if the stirring efficiency (E) is the same for both cases. To wit, Eqn. 3 predicts that if coupling distance ( C ) is doubled, for example, the speed ( N ) would need to be increased by 40% to compensate so that E would remain the same. [00440] However, Fig. 3 is a graphical representation of the stirrer efficiency necessary to make acceptable cord quality in LCD sheet glass as a function of stirring speed and coupling. Curve 301 shows the stirring efficiency required to realize acceptable cord in the glass sheets. For LCD glass, the required stirring efficiency is not the same for all cases that meet the cord requirement. The required stirring efficiency actually decreases as the coupling and the speed are increased (for the same stir chamber diameter). Measurements show that the spacing of the surface corrugations becomes smaller as the speed is increased, making the cord more difficult to see. Even though the speed necessary to achieve acceptable cord would be expected to increase as coupling is increased, the necessary speed does not increase as much as expected from Eqn. 3. Thus, shear stress applied to the stirrer and the stir chamber platinum parts can be decreased as coupling distance is increased up to a certain value of C. According to example embodiments, when C is increased beyond that value, illustratively above values in the range of approximately 0.75" and approximately 1 .0", stirring will become less effective, requiring even greater stirring efficiency and shear stress to meet the cord requirement. As such, per an example embodiment, there is an optimum coupling at which shear stress on the platinum parts is minimized. In accordance with an illustrative embodiment, this optimum coupling is in the range of approximately 0.5" to approximately 0.75" for fabricating glass panels for LCD applications.

[00450] The curve 301 fitted through the three operating points above can be approximated by the power function r _rge, = 1366.6N-°-⁷¹¹⁴ (5)

[00460] From this empirical example, the left side of Eqn. (4) is not a constant since the target E is not a constant. Substituting Eqn. (5) into Eqn. (4) takes this into account and results in:

[00470] From Eqn. (6) the table of Fig. 2 can be revised to render the table of Fig. 4. From the table of Fig. 4, it is evident that that a desirable reduction in shear stress can be achieved while still making acceptable cord quality by increasing the coupling distance. As can be appreciated, the shear stress increases as coupling is increased in order to meet the cord requirements. In accordance with an example embodiment there is an optimum coupling where shear stress on the platinum parts is minimized. In an example embodiment, the optimum coupling that produces the minimum shear stress for acceptable cord quality is in the range of approximately 0.3125" to approximately 0.75" range for the stirring chamber and stirrer described herein. [00480] As can also be appreciated the referenced limit of the coupling distance in the range of approximately 0.3125" to approximately 0.75" is also the coupling distance beyond which eqns. 5 and 6 do not hold. To this end, eqns. 5 and 6 are based on experience with actual stirrers with coupling distance. Physical modeling further suggests that stirring performance begins to fall off between C=0.75" and C=1.25". It is further noted that, in an example embodiment in which the stir chamber has a 9.875" diameter a coupling distance of O.75" is near the optimum. [00490] An objective of reducing shear stress on platinum surfaces is to reduce platinum erosion inclusions in the drawn sheet glass that result in the rejection of the product. Indirect evidence that this objective has been achieved by increasing the coupling distance is shown in Figure 5. To this end, Fig. 5 shows product loss, in percent, is shown for glass from a known process, where each period is a period of manufacture. The first five sets of data, periods 1 through 5, show product loss for glass made with a 9.25" stirrer in a 9.875" stir chamber. The last three periods, periods 6 through 8, are example embodiments using a stirrer with an 8.375" diameter, and a stir chamber with a 9.875" inner diameter (C = 0.75"). As can be readily determined, the finishing loss is reduced by approximately 40% or more by the implementation of the coupling parameters referenced above in connection with an example embodiment.

[00500] It is noted that roughly three-quarters of the reduction in shear stress of an example embodiment is accomplished by lowering the viscosity by about two- thirds and about one-quarter by enlargement of the stirring system. The reduction in viscosity may be effected by increasing the temperature of the glass material in the stirrer. In one example embodiment this is realized by increasing the glass temperature by approximately 80 degree °C; namely from 1400 °C, at which the glass viscosity is about 3000 poise (gram-cm^"1-sec^"1) to 1480°C at which the glass viscosity is about 1000 poise. This equates to the referenced reduction in viscosity of two-thirds.

[00510] It is noted that this substantial increase in temperature may decrease the strength of the precious metal parts of the stirring chamber and the stirrer, and increase the volatility of the platinum parts due to oxidation. To this end, the continuous oxidation of platinum alloy parts and condensation of the platinum oxide on colder surfaces creates a source of platinum particles that can fall into the glass and result in defects. A temperature reduction is necessary to reduce this phenomenon. In addition, a temperature reduction in the stir chamber reduces the temperature of the glass entering the stir chamber, which will reduce the opportunity to dissolve platinum from the stir chamber and the stirrer into the glass. This will ultimately reduce the potential for precipitation of platinum defects from solution.

[00520] In accordance with illustrative embodiments, the temperature reduction can be achieved without an increase in shear stress by increasing the coupling distance to drop shear stress and then decreasing temperature to bring shear stress back to its former level. The effect of coupling on shear stress was shown in the table of Fig. 4, and can be seen graphically in Fig. 6, which shows the shear stress versus stirring efficiency in accordance with example embodiments. [00530] Curve 601 represents the relationship between shear stress and stirring efficiency for a known, relatively high viscosity stirrer. (It is noted that all shear stresses shown on this plot are shown at 1O00 poise for comparison.) The line 602 connecting curve 601 with the curve 603 is a line of constant coupling, C. Operating lines for other stirrers whose diameter ranged between 5.25" and 9.25" with coupling of 0.3125" would fall on this line, so this shows the effect of increasing the stirrer diameter on shear stress. Following this line to the larger stirrer system (9.875" diameter), from smaller (5.875" diameter) conditions, results in a reduction of shear stress from about 0.38 psi to 0.27 psi at 1000 poise. This is a significant reduction of shear to on the order of approximately 30%. [00540] The connecting line 604 between line 603 (C=0.3125") and line 605 (C=.75") represents the operating line for required stirring efficiency as coupling changes from approximately 0.3125" to approximately 0.75". This shows that in an example embodiment shear stress can be further reduced by about 50% (to about 0.14 psi) by increasing the coupling distance. Based on the definition of stirring efficiency (E) from eqn. 1 , an illustrative range for the stirring efficiency to realize acceptable cord and inclusions is approximately 175 to approximately 250 over the range of coupling of 0.3125" to 0.75". For example, a 9.875" ID stir chamber, two different stirrer diameters, 9.25" (0.3125" coupling) and 8.375" (0.75" coupling), are implemented in example embodiments. The required stirring efficiency (E) for the 9.25" diameter stirrer is approximately 250. The required stirring efficiency for the 8.375" diameter stirrer is approximately 175. For the same 1000 poise viscosity, the shear stress for the 9.25" diameter stirrer was 0.270 psi. The shear stress for the smaller 8.375" diameter stirrer was 0.136 psi. Using these parameters, the stirrers of the example embodiments, result in cord that is acceptable for many applications including LCD displays. [00550] It is noted that the benefit from the coupling increase can also be utilized to reduce temperature if the shear stress of 0.27 psi is acceptable for platinum erosion inclusions at lower system temperatures. Fig. 7 is a graphical representation of the shear stress on the inner walls of the stirring container versus temperature. This graph shows the operating characteristics of a stirrer (C = 0.75") in accordance with an example embodiment. The point 701 on the graph shows the proven operating condition for a stirrer of another example embodiment, particularly operating at 1480 °C and 16 RPM. Following the line of constant stirring efficiency 702 (E = 180) to the shear stress 703 (0.27 psi ) for another stirrer-will allow a temperature reduction from 1480 °C to about 1430°C without increasing the shear stress above what has been successfully achieved with the other stirrer. This may be desirable if more inclusions are coming from a solution/precipitation reaction than from physical erosion. It may also be possible if the erosion resistance of the platinum parts increases faster with lower temperatures than the impact of increased shear stress on the platinum parts.

Example [00560] In accordance with example embodiments, a series of oil modeling experiments were conducted to determine the coupling distance that delivered the optimal stirring efficiency as a function of shear stress between the stir blades and the stir chamber wall. Stirrers were constructed with blades sized to deliver coupling distances of 5/16", ¹/₂", 9/16", 5/8", 11/16", ³λ" and 7/8". The stirrers were tested over a range of rotational speeds typically from 9 rpm to 25 rpm. This yields a range of shear rates between the blades and the interior wall from approximately 5 s^"1 to 20 s^"1. For purposes of illustration shear rates in excess of approximately 15 s^" at 1000 poise viscosity cause unacceptable Pt erosion, and thus inclusion in the resultant products. [00570] Stirrer effectiveness was measured by analyses of the stirrers' ability to disperse an injected dye, both spatially and temporally. A transparent plastic chamber of approximating geometrically the stirrer chambers in production was used. The glass was simulated by polyisobutylene oil at a viscosity, density and flow rate to provide kinematic similitude with production glass in the hot stir chamber. A relatively small volume of dyed oil (of viscosity approximately 25 times that of the clear matrix oil) was injected into the stir chamber and allowed to flow into the stir chamber and through the rotating blades, and then exit the chamber, [part of sentence deleted] The model includes the use of known equipment, including a square transparent pipe to facilitate undistorted observation and imaging of the mixed oil stream.

[00580] A linescan image (not shown) of the exiting dyed (magenta-red) oil was obtained. A digital video shows the distribution of the dye in the volume of stirred glass as a function of position and time at a reference plane. The images are green filtered and processed to maximize contrast. The roughly parabolic shape of the dye front is characteristic of the fully developed pipe flow.

[00590] The spatial homogeneity of the oil is assessed from the intensity contrast of the image. To wit, low contrast indicates good mixing and high contrast is poor. This is shown in Figs. 8a and 8b, respectively.

[00600] The above measures of homogeneity can be quantified through numerical interrogation of the bitmapped intensity data, which is shown in Fig. 9, which is a graph of the intensity versus pixel distance. Contrast is taken as a function of spatial variation of intensity in the image, and dispersion as a function of the characteristic log normal peak and decay of the pulse of dye. The dispersion and homogeneity numbers are combined to give a total index of the stirrer effectiveness (SEI), and scales between approximately 0 and approximately 4000 for stirrers examined in this study, with higher numbers indicating superior stirring. Fig. 10 shows is a graphical representation of the stirrer effectiveness index versus the shear rate. This measure is entirely different from the stirrer efficiency, E, but is believed to behave in the same way as E. Generally, E is the ratio of the interfacial surface area between the inhomogeneity in the glass and the parent glass itself after stirring to before stirring.

[00610] For example, the interfacial area for a golf ball in a pond is the surface of the ball in contact with the water, i.e. the outside surface of the ball. However, if the ball is cut into little pieces and thrown back into the water, the interfacial surface area between the ball and the water would be the sum of the surface area of all of the particles. This would be much greater than the surface area of the original ball. Thus, the ratio of the interfacial area after cutting up the ball to before cutting up the ball is a large number and representative of how finely the ball was chopped up. The basis for the preceding equations is a theoretical derivation of the ratio of interfacial surface areas before and after stirring. This is an interesting concept but not readily measurable. In the example embodiment of Fig. 7 a different measure of the effectiveness of stirring is used. It is based on watching the discrete elements (nested parabolic lines) of tracer leaving the stir chamber and measuring the intensity and spacing of the lines.

[00620] As seen in Fig. 10, stirring effectiveness increases with increasing the coupling distance from 5/16" to approximately 5/8" and then falls off again at greater blade-wall spacing. In particular, the stirring effectiveness index increases with the coupling distance of curves 1001 , 1002, 1003, and decreases with coupling distances of curves 1004, 1005 and 1006. This indicates an optimal balance between the coupling distance, blade surface area and rotational speed exists for this class of stirrer, and that optimization of these parameters in production can give reduced cord loading and reduced platinum inclusions from the stirrer erosion.

[00630] It is desirable for peak SEI in Figure 10 to be as high as possible and occur at the lowest possible shear rate or shear stress. To some extent, however, sometimes a trade-off or compromise must be made. For example, curve 1005 (coupling of 0.75") has the second-highest SEI peak of the curves of the example embodiments of Fig.10. However, this peak occurs at a relatively high shear rate. An even higher SEI peak is obtained at a lower shear rate (or shear stress) in curve 1003, which is the SEI versus shear rate for a stirrer with coupling of 5/8". Curve 1004 has a peak that is lower than the peaks of curves 1003 and 1005, and a shear rate that is between the shear rates at the peak of curves 1003 and 1005. As such, it may be useful to provide models such as described in connection with Fig. 10 to determine which coupling and thus which elements provide the optimal SEI and minimal shear rate.

[00640] The example embodiments having been described in detail in connection through a discussion of example embodiments, it is clear that modifications of the invention will be apparent to one having ordinary skill in the art having had the benefit of the present disclosure. Such modifications and variations are included in the scope of the appended claims.

Claims

Claims:

1. A method of forming glass with substantially reduced inclusions and cord, the method comprising: providing a shear rate of a predetermined level, while maintaining a predetermined stirring efficiency in a stirring operation by properly selecting a stirrer diameter, a stirrer speed and a coupling distance.

2. A method as recited in claim 1 , wherein the shear rate is less than approximately 15 s^"1.

3. A method as recited in claim 1 , wherein the stirring efficiency is in a range of approximately 175 to approximately 250.

4. A method as recited in claim 1 , wherein the coupling distance the coupling distance is between approximately 5/8" and approximately 7/8" and a stir chamber has a diameter of 9.875".

5. A method as recited in claim 4, wherein a ratios of a high shear volume to a low shear volume for the coupling distance of 5/8" is approximately 0.315 and for the coupling distance of 7/8" is approximately 0.396.

6. A method as recited in claim 1 , wherein the coupling distance is less than or equal to approximately 0.75".

7. A method as recited in claim 3, further comprising selectively modifying the stirrer speed, or the coupling distance, or both to produce a stirring efficiency in the range.

8. A method as recited in claim 1 , wherein the cord has an intensity contrast of less than approximately 0.67%.

9. A method as recited in claim 1 , wherein the inclusions are less than approximately 50 μm in diameter.

10. A method as recited in claim 1, wherein the inclusions are Pt-sourced inclusions, inclusions.

11 . A method of fabricating glass sheets for use in displays, the method comprising: providing a shear rate of a predetermined level, while maintaining a predetermined stirring efficiency in a stirring operation by properly selecting a stirrer diameter, a stirrer speed and a coupling distance.

12. A method as recited in claim 11 , wherein the displays are one or more of liquid crystal displays, portable telephone displays and personal digital assistants (PDAs).

13. A method as recited in claim 10, further comprising, after the fabricating, forming thin film transistors on a surface of the glass sheet for use in picture elements of the displays.

14. A method as recited in claim 11 , wherein the shear rate is less than approximately 15 s^"1.

15. A method as recited in claim 11 , wherein the stirring efficiency is in a range of approximately 175 to approximately 250.

16. A method as recited as recited in claim 11 , wherein the coupling distance the coupling distance is between approximately 5/8" and approximately 7/8" and a stir chamber has a diameter of 9.875".

17. A method as recited in claim 16, wherein a ratios of a high shear volume to a low shear volume for the coupling distance of 5/8" is approximately 0.315 and for the coupling distance of 7/8" is approximately 0.396.

18. A method as recited in claim 1 , wherein the coupling distance is less than or equal to approximately 0.75".

19. A method as recited in claim 3, further comprising selectively modifying the stirrer speed, or the coupling distance, or both to produce a stirring efficiency in the range.

20. A method as recited in claim 1, wherein the cord has an intensity contrast of less than approximately 0.67%.

21. A method as recited in claim 1 , wherein the inclusions are less than approximately 50 μm in diameter.

22. A method as recited in claim 11, wherein the inclusions are Pt-sourced inclusions, inclusions.