초록 ▼

The method of making a single crystal, especially a CaF2 single crystal, 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.

The method of making a single crystal, especially a CaF2 single crystal, 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)-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 ▼

We claim: 1. A method of making a highly uniform, low-stress calcium fluoride single crystal, said method including tempering the calcium fluoride single crystal and said method comprising the steps of: a) heating the calcium fluoride single crystal at a temperature increase rate of less than 18 K/

We claim: 1. A method of making a highly uniform, low-stress calcium fluoride single crystal, said method including tempering the calcium fluoride single crystal and said method comprising the steps of: a) heating the calcium fluoride single crystal at a temperature increase rate of less than 18 K/h to a temperature between 1050° C. and 1350° C. to temper the crystal; b) holding the calcium fluoride single 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, static radial temperature gradients within the crystal are less than 0.013 K/cm, and static axial temperature gradients are less than 0.07 K/cm; c) subsequently cooling the calcium fluoride single 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 the calcium fluoride single crystal 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 calcium fluoride single 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 calcium fluoride single 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 calcium fluoride single 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 calcium fluoride single 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 calcium fluoride single 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. 9. The method as defined in claim 1, wherein the calcium fluoride crystal has an RMS average value of refractive index uniformity (Δn) less than 0.025×10-6 in a (100)-direction or a (110)-direction and a PV value of stress birefringence of less than 2.5 nm/cm and an RMS average value of the stress birefringence of less than 1 nm/cm in the (100)-direction or the (110)-direction. 10. The method as defined in claim 9, wherein the calcium fluoride crystal has a PV value of the stress birefringence of less than 0.5 nm/cm and an RMS average value of the stress birefringence of less than 0.15 nm/cm in the (111)-direction. 11. The method as defined in claim 10, wherein said PV value of the stress birefringence is less than 0.2 nm/cm and said RMS average value of the stress birefringence is less than 0.08 nm/cm in the (111)-direction.