초록 ▼

The method for making single crystals, especially CaF2 crystals, includes tempering, in which the crystal is heated at <18 K/h to a temperature of 1000° C. to 1350° C. and held at this temperature for at least 65 hours with maximum temperature differences within the crystal of <0.2 K. Subseque

The method for making single crystals, especially CaF2 crystals, includes tempering, in which the crystal is heated at <18 K/h to a temperature of 1000° C. to 1350° C. and held at this temperature for at least 65 hours with maximum temperature differences within the crystal of <0.2 K. Subsequently the crystal is cooled with a cooling rate of at maximum 0.5 K/h above a limiting temperature between 900° C. to 600° C. and then further below this limiting temperature at maximum 3 K/h. The obtained CaF2 crystals have refractive index uniformity <0.025×10?6 (RMS) in a (111)-, (100)- and/or (110)-direction and a stress birefringence of less than 2.5 nm/cm (PV) and/or a stress birefringence of less than 1 nm/cm (RMS) in the (100)- or (110)-direction. In the (111)-direction the stress birefringence is <0.5 nm/cm (PV) and/or the stress birefringence is <0.15 nm/cm (RMS).

대표청구항 ▼

1. A method of making a highly uniform, low-stress single crystal, said method including tempering the crystal and said method comprising the steps of: a) heating the crystal at a temperature increase rate of less than 18 K/h to a temperature between 1000° C. and 1350° C. to temper the crystal; b) h

1. A method of making a highly uniform, low-stress single crystal, said method including tempering the crystal and said method comprising the steps of: a) heating the crystal at a temperature increase rate of less than 18 K/h to a temperature between 1000° C. and 1350° C. to temper the crystal; b) holding the crystal at said temperature reached during the heating of step a) for at least 65 hours, so that temperature differences up to a maximum value of 0.2 K are present within the crystal during the holding; c) subsequently cooling the crystal in a temperature range above a limiting temperature between 900° C. and 600° C. with a cooling rate that is at maximum equal to 0.5 K/h; and then d) cooling to temperatures below said temperature range with a cooling rate that is at maximum equal to 3 K/h. 2. The method as defined in claim 1, wherein said temperature increase rate during the heating is less than 10 K/h. 3. The method as defined in claim 1, wherein the tempering of the crystal occurs in an apparatus, in which a reducing atmosphere is present. 4. The method as defined in claim 1, wherein the tempering of the crystal occurs in the presence of at least one solid material selected from the group consisting of PbF2, ZnF2 and XeF2. 5. The method as defined in claim 1, wherein temperature differences up to a maximum value of 0.3 K are present within the crystal during the cooling of the crystal occurring above said limiting temperature between 900° C. and 600° C. 6. The method as defined in claim 1, wherein the crystal tempered in the tempering is in the form of a cylindrical body, independent from geometries of products formed later from the crystal. 7. The method as defined in claim 1, wherein the tempering occurs in an apparatus, and further comprising producing a vacuum of at least 10?4 mbar in the apparatus prior to the tempering in order to remove residual moisture and providing a pressure of 10 to 1050 mbar in the apparatus during the tempering. 8. The method as defined in claim 1, wherein the crystal is a calcium fluoride crystal. 9. The method as defined in claim 1, wherein the crystal has an optical quality that is sufficient for optical applications in directions different from a (111)-direction and said directions different from said (111)-direction include a (100)-direction and a (110)-direction. 10. A homogeneous calcium fluoride crystal with improved transmission obtained by a method as defined in claim 1 and having an RMS average value of refractive index uniformity (Δn)<0.025×10?6 in a (111)-direction, a (100)-direction and/or a (110) direction and having a PV value of the stress birefringence of less than 2.5 nm/cm and/or an RMS average value of the stress birefringence of less than 1 nm/cm in the (100)-direction and/or the (110)-direction and having a PV value of the stress birefringence of less than 0.5 nm/cm and/or an RMS average value of the stress birefringence of less than 0.15 nm/cm in the (111)-direction. 11. The homogenous calcium fluoride crystal as defined in claim 10, wherein said RMS average value of said refractive index uniformity (Δn) is <0.015×10?6 in said (111)-direction, said (100)-direction and/or said (110) direction; said PV value of the stress birefringence is less than 0.2 nm/cm and/or said RMS average value of the stress birefringence is less than 0.08 nm/cm in the (111)-direction and/or said PV value of the stress birefringence is less than 1 nm/cm and/or said RMS average value of the stress birefringence is less than 0.35 nm/cm in the (100)-direction and/or the (110)-direction. 12. A stepper, excimer laser, wafer, computer chip or integrated circuit containing a crystal as defined in claim 10 acting as an optical element. 13. An electronic unit or device containing a computer chip and/or an integrated circuit, wherein said computer chip and said integrated circuit each contain a crystal as defined in claim 10. 14. A stepper, excimer laser, wafer, computer chip or integrated circuit containing a crystal obtained by the method as defined in claim 1, which acts as an optical element with an optical axis or principle propagation direction that is not in a (111)-direction, wherein said crystal is a homogeneous calcium fluoride crystal having an RMS average value of refractive index uniformity (Δn)<0.025×10?6 in the (111)-direction, a (100)-direction and/or a (110)-direction and having a PV value of the stress birefringence of less than 2.5 nm/cm and/or an RMS average value of the stress birefringence of less than 1 nm/cm in the 100-direction and/or the (110)-direction and having a PV value of the stress birefringence of less than 0.5 nm/cm and/or an RMS average value of the stress birefringence of less than 0.15 nm/cm in the (111)-direction. 15. An electronic unit or device containing a computer chip and/or an integrated circuit, wherein said computer chip and said integrated circuit each contain a crystal obtained by the method as defined in claim 1, which acts as an optical element with an optical axis or principle propagation direction that is not in a (111)-direction, and wherein said crystal is a homogeneous calcium fluoride crystal having an RMS average value of refractive index uniformity (Δn) <0.025×10?6 in the (111)-direction, a (100)-direction and/or a (110)-direction and having a PV value of the stress birefringence of less than 2.5 nm/cm and/or an RMS average value of the stress birefringence of less than 1 nm/cm in a 100-direction and/or (110)-direction and having a PV value of the stress birefringence of less than 0.5 nm/cm and/or an RMS average value of the stress birefringence of less than 0.15 nm/cm in the (111)-direction.