Silica glass member, method for producing the same, and projection aligners using the same

A method for producing a silica glass member, comprising:

• a synthetic step of silica glass bulk comprising, in a synthetic furnace equipped with a burner having a plurality of tubes, ejecting a raw material and a combustion gas from the plurality of tubes of said burner to hydrolyze said raw material in an oxyhydrogen flame, and thereby synthesizing a silica glass bulk having a difference in the maximum and the minimum values of hydroxyl group concentration as measured in a plurality of points within a predetermined internal plane of the bulk of 50 ppm or lower;
• a step of cooling silica glass bulk, comprising cooling said silica glass bulk under a pressure range of from 0.01 to 0.15 MPa (abs) while controlling the cooling rate in the temperature region of from 1500 to 1800° C. to a range of from 5 to 10° C./min;
• a step of cutting silica glass bulk, comprising cutting out said silica glass bulk to obtain a silica glass member having the desired shape and size;
• and
• a step of heat treating silica glass member, comprising applying a heat treatment to said silica glass member to obtain a silica glass member in which a difference in the maximum and the minimum values of hydroxyl group concentration as measured in a plurality of points within a plane vertical to an optical axis whose center is the crossing point of its optical axis with the optical axis of the silica glass member is 50 ppm or lower, and in which the plurality of signed birefringence values obtained based on the birefringence values measured on several points within a plane vertical to an optical axis whose center is the crossing point of its optical axis with the optical axis of the silica glass member and the direction of the fast axis fall within a range of from −2.0 to +2.0 nm/cm.

A method for producing a silica glass member, comprising:

• a synthetic step of silica glass bulk comprising, in a synthetic furnace equipped with a burner having a plurality of tubes, ejecting a raw material and a combustion gas from the plurality of tubes of said burner to hydrolyze said raw material in an oxyhydrogen flame, and thereby synthesizing a silica glass bulk having a difference in the maximum and the minimum values of hydroxyl group concentration as measured in a plurality of points within a predetermined internal plane of the bulk of 50 ppm or lower;
• a step of cooling silica glass bulk, comprising, while holding said silica glass bulk inside said synthetic furnace, cooling said silica glass bulk based on the temperature difference between the silica glass bulk and the temperature of the external environment;
• a step of cutting silica glass bulk, comprising cutting out said silica glass bulk to obtain a silica glass member having the desired shape and size;
• a first heat treatment step of silica glass member, comprising elevating the temperature of said silica glass member to a predetermined temperature in the temperature range of from 1600 to 2300° C. in an inert gas atmosphere in a pressure range of from 0.01 to 0.15 MPa (abs);
• and
• a second heat treatment step of silica glass member, comprising, in an inert gas atmosphere maintained in a pressure range of from 0.01 to 0.15 MPa (abs), cooling said silica glass bulk while controlling the cooling rate in the temperature region of from 1500 to 1800° C. to a range of from 5 to 10° C./min to obtain a silica glass member in which a difference in the maximum and the minimum values of hydroxyl group concentration as measured in a plurality of points within a plane vertical to an optical axis whose center is the crossing point of its optical axis with the optical axis of the silica glass member is 50 ppm or lower, and in which the plurality of signed birefringence values obtained based on the birefringence values measured on several points within a plane vertical to an optical axis whose center is the crossing point of its optical axis with the optical axis of the silica glass member and the direction of the fast axis fall within a range of from −2.0 to +2.0 nm/cm.

A projection aligner comprising an exposure light source emitting a light having a wavelength of 250 nm or shorter as the exposure light, a reticle having formed thereon an original pattern image, an irradiation optical system which irradiates a radiation output from said exposure light source to said reticle, a projection optical system which projects a pattern image output from said reticle onto a photosensitive substrate, and an alignment system which aligns said reticle and said photosensitive substrate,

• in which at least a part of the silica glass member constituting said irradiation optical system, the silica glass member constituting said projection optical system, and said reticle, is made of a silica glass member for use with a light having a specific wavelength of 250 nm or shorter,
• in which the difference in the maximum and the minimum values of hydroxyl group concentration as measured in a plurality of points within a plane vertical to an optical axis whose center is the crossing point of its optical axis with the optical axiss of the silica glass member is 50 ppm or lower;
• in which the plurality of signed birefringence values obtained based on the birefringence values measured on several points within a plane vertical to an optical axis whose center is the crossing point of its optical axis with the optical axis of the silica glass member and the direction of the fast axis fall within a range of from −2.0 to +2.0 nm/cm,
• wherein said plurality of signed birefringence values of said silica glass member are expressed based on the formulae as follows:
• −2.0≦Bij≦2.0 nm/cm (1) Bij = ∑ k = 1 h ⁢ ⁢ A ijk h (2)
• where, in formula (1) and (2) above,
• i represents the number of said silica glass members L (i=1, 2, . . . m;
• 2≦m),
• j represents the number of concentrical virtual circles C present on an effective cross section vertical to the optical axis of said silica glass member L, each having a center on said optical axis and different from each other in radius from said optical axis (i=1, 2, . . . n;
• 1≦n),
• k represents the number of measuring points located on the circumference of said concentrical circles C (k=1, 2, . . . h;
• 1≦h),
• Aijk represents the signed birefringence value at the kth measuring point Pijk located on the circumference of a concentrical circle Cij of a silica glass member Li, and Bij represents the mean signed birefringence value corresponding to the arithmetic mean of the signed birefringence values of the measuring points located on the circumference of a concentrical circle Cij of a silica glass member Li.