Glass manufacturing – Processes of manufacturing fibers – filaments – or preforms – Process of manufacturing optical fibers – waveguides – or...
Reexamination Certificate
2001-12-21
2004-01-06
Derrington, James (Department: 1731)
Glass manufacturing
Processes of manufacturing fibers, filaments, or preforms
Process of manufacturing optical fibers, waveguides, or...
C065S414000, C065S421000, C065S017400, C065S137000
Reexamination Certificate
active
06672111
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to fused silica. More particularly, the invention relates to methods and apparatus used for adding metals such as aluminum to fused silica glass articles.
BACKGROUND OF THE INVENTION
As practiced commercially, fused silica optical members such as lenses, prisms, filters, photomasks, reflectors, etalon plates and windows are typically manufactured from bulk pieces of fused silica made in large production furnaces. Bulk pieces of fused silica manufactured in large production furnaces are known in the art as boules or ingots. Blanks are cut from boules or ingots, and finished optical members are manufactured from glass blanks, including manufacturing steps that may include, but are not limited to, cutting, polishing, and/or coating pieces of glass from a blank. These optical members are used in various apparatus employed in environments where they are exposed to high-power ultraviolet light having a wavelength of about 360 nm or less, for example, an excimer laser beam or some other high-power ultraviolet laser beam. The optical members are incorporated into a variety of instruments, including lithographic laser exposure equipment for producing highly integrated circuits, laser fabrication equipment, medical equipment, nuclear fusion equipment, or some other apparatus which uses a high-power ultraviolet laser beam.
In overview, boules are manufactured by reacting silicon-containing gas molecules in a flame to form silica soot particles. The soot particles are deposited on the hot surface of a rotating or oscillating body where they consolidate to the glassy solid state. In the art, glass making procedures of this type are known as vapor phase hydrolysis/oxidation processes, or simply as flame deposition processes. The term “boule” is used herein with the understanding that the term “boule” includes any silica-containing body formed by a flame deposition process.
Boules typically having diameters on the order of five feet (1.5 meters) and thicknesses on the order of 5-10 inches (13-25 cm) and larger can be routinely produced in large production furnaces. Multiple blanks are cut from such boules and used to make the various optical members referred to above. The principal optical axis of a lens element made from such a blank will also generally be parallel to the boule's axis of rotation in the furnace. For ease of reference, this direction will be referred to as the “axis 1” or “use axis”. Measurements made in a direction perpendicular to the axis
1
or use axis will be referred to as “off-axis” measurements.
As the energy and pulse rate of lasers increase, the optical members which are used in conjunction with such lasers are exposed to increased levels of laser radiation. Fused silica members have become widely used as the manufacturing material of choice for optical members in such laser-based optical systems due to their excellent optical properties and resistance to laser induced damage.
Laser technology has advanced into the short wavelength, high energy ultraviolet spectral region, the effect of which is an increase in the frequency (decrease in wavelength) of light produced by lasers. Of particular interest are short wavelength excimer lasers operating in the UV and deep UV (DUV) wavelength ranges, which includes lasers operating at about 193 nm and 248 nm wavelengths. Excimer laser systems are popular in microlithography applications, and the shortened wavelengths allow for increased line densities in the manufacturing of integrated circuits and microchips, which enables the manufacture of circuits having decreased feature sizes. A direct physical consequence of shorter wavelengths (higher frequencies) is higher photon energies in the beam due to the fact that each individual photon is of higher energy. In such excimer laser systems, fused silica optics are exposed to high energy photon irradiation levels for prolonged periods of time resulting in the degradation of the optical properties of the optical members.
It is known that laser-induced degradation adversely affects the performance of fused silica optical members by decreasing light transmission levels, altering the index of refraction, altering the density, and increasing absorption levels of the glass. Over the years, many methods have been suggested for improving the optical damage resistance of fused silica glass. It has been generally known that high purity fused silica prepared by such methods as flame hydrolysis, CVD-soot remelting process, plasma CVD process, electrical fusing of quartz crystal powder, and other methods, are susceptible to laser damage to various degrees.
One of the known methods for reducing absorption levels in the glass is to reduce total metal impurity levels of metals such as sodium, aluminum, and iron. For example, one known way of reducing metals impurities in the glass involves treating the refractory materials used in the fused silica production furnace with a halogen gas. Further details on this method are described in U.S. Pat. No. 6,174,509. Another known method of improving the transmission and durability of fused silica optical members is disclosed in U.S. Pat. No. 6,174,830, which discloses annealing silica glass members for 10 or more hours at 1000° C. so that the hydrogen content of the member is 5×10
18
molecules/cm
3
or less. While the method in U.S. Pat. No. 6,174,830 is advantageous in that it produces optical members having excellent properties, the annealing process takes a considerable amount of time and expense to produce such members.
Fused silica members can also exhibit transient absorption. As described in the article “Transient absorption in excimer-exposed silica,” by Charlene Smith, Nicholas Borrelli and Roger Araujo, Applied Optics, Vol. 39, No. 31, 5778-5784 (Nov. 1, 2000), the contents of which are incorporated herein by reference, transient absorption can take two forms. In one form, the transmittance of glass in the UV region recovers somewhat when the irradiation source is removed and redarkens quickly when reexposed to light. In the second form, absorption occurs upon the initial irradiation of the glass, and this absorption decreases with constant illumination of the optical member. This type of transient absorption will be referred to herein as an “absorption spike.” This absorption spike is problematic because optical elements contained in optical equipment such as stepper systems must be exposed to a sufficient number of pulses to work through the absorption spike and reduce to the absorption value to avoid the undesirable effects of absorption changes in an optical member when the optical member is placed in service. This exposure process requires optical equipment manufacturers to devote time and resources to work through the absorption spike to reduce absorption to an acceptable level.
The presence or the absence of the absorption spike has been traced to the level of molecular hydrogen dissolved in the glass. Typically, optical members containing high amounts of hydrogen, for example, a concentration of 10
19
molecules/cm
3
, do not have a measurable absorption spike. Accordingly, manufacturing of optical members with high amounts of molecular hydrogen is one way of reducing this absorption spike. Hydrogen can be introduced during the boule formation process or after formation of the boule.
Commonly assigned, copending United States Patent application, entitled “Fused Silica Containing Aluminum,” naming Dan Sempolinski as inventor, discloses that inclusion of low levels of aluminum (>50 ppb) in fused silica glass articles improves transmission, and the absorption spike is significantly reduced when compared to fused silica glass articles that do not contain additional aluminum. Low levels of aluminum, for example, as low as about 50 parts per billion and as high as about 900 parts per billion have been observed to improve the aforementioned properties in fused silica optical articles. Normally, impurity levels of aluminum in fused silica glass articles presently manufactured by the
Peters William P.
Sempolinski Daniel R.
Sproul Merrill F.
Wasilewski Michael H.
Corning Incorporated
Derrington James
Schaeberle Timothy M.
Suggs James V.
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