大双折射深紫外光学晶体偏硼酸锂的生长及偏振器件研制

Polarizer made by deep-ultraviolet birefringent crystal plays important roles in the polarized illumination optical system for ArF lithography. It is customary to use MgF2 crystal due to its high transparency at 193 nm. However, the relative small birefringence of MgF2 leads to a small splitting angle. If want to increase the splitting angle, one has to increase the length of the polarizer which impedes the compactness of the device. Therefore, it is urgent to develop new deep-ultraviolet optical material with large birefringence. According to the study of the structure-properties relationship of borates, we found that the ∞[BO2] chains formed by corner sharing BO3 units would facilitate the generation of large birefringence. We successfully selected the lithium metaborate LiBO2 from the structural database. It has been shown that the crystal possesses a wide transparency window (164−3480nm) and large birefringence (0.135@589.5nm), which is the known borate that possesses the shortest ultraviolet cutoff edge and largest birefringence. It is congruent melting, therefore, large crystals can be grown from melt by Czochralski method. This project will focus on exploration of the temperature-field and growth technology for LiBO2 crystal, and obtain large crystals with sizes larger than ϕ30 mm. A polarization splitting prism used at 193 nm will be designed and processed, and the extinction ratio, transmittance and laser damage threshold of the prism will be comprehensively studied. This work will build theoretical and experimental foundation for the application of the crystal as a deep-ultraviolet polarizer.

利用深紫外双折射晶体制作的偏振器件在激光光刻的偏振照明系统中有重要应用，目前ArF光刻机通常使用的是在193nm透过率高的MgF2晶体，但由于其双折射率小，导致分束角小，增大分束角则需增加棱镜长度，不利于装置紧凑化，因此亟待开发双折射率大的深紫外光学材料。对硼酸盐结构-性能研究发现，BO3基团共顶点连接成链且平行排列有利于产生大双折射率，在结构数据库中筛选出偏硼酸锂(LiBO2)晶体。研究发现其透光波段宽(164−3480nm)，深紫外双折射率大(0.135@589.5nm)，是已知硼酸盐中紫外截止边最短同时双折射率最大的材料；晶体为同成分熔融，可采用提拉法长大晶体。本项目在此基础上，研究提拉法生长LiBO2晶体的温场和工艺，获得尺寸大于ϕ30 mm 的高质量晶体，设计加工用于193nm的偏光棱镜，系统研究消光比、透射比、激光阈值等性能，为该晶体作为深紫外偏光器件的应用奠定理论和实验基础。

本项目成功研发了一种新型深紫外双折射晶体——偏硼酸锂。突破了深紫外双折晶体偏硼酸锂的生长技术，采用提拉法生长出尺寸达到Ø55 × 34 mm3的高质量晶体；探索了偏硼酸锂晶体的加工工艺，加工了相应的晶体器件并测试了透过率、激光损伤阈值、硬度、折射率等性能，结果表明偏硼酸锂晶体是已知的硼酸盐深紫外双折射晶体中紫外截止边最短(164 nm)、同时双折射率最大(≥ 0.168 @266 nm)的材料。鉴于偏硼酸锂优异的深紫外双折射性能，提出了一种基于偏硼酸锂晶体的Glan偏振器的设计方案。通过结构-性能关系研究提出高平面度的∞[BO2]链是设计深紫外双折射晶体材料的优势基元，为设计开发新型深紫外双折射晶体提供了思路。上述结果表明偏硼酸锂晶体是具有重要应用前景的深紫外双折射材料。.以第一作者或者通讯作者在Light Sci. Appl.、Nat. Commun.等期刊发表论文11篇，影响因子均大于5.0。项目期内授权新型紫外、深紫外双折射晶体新材料的制备、生长、用途等方面的美国发明专利1件，授权中国发明专利7件。获得2019年新疆青年科技奖。培养博士研究生3名、硕士研究生3名。.