There are two particular difficulties for the immersive projection objective. First, a high NA causes increase of the light incidence angle on the component surface. NA1.35 can be used as an example the water refraction index is 1.437 corresponding to the objective system NA0.94. In such case, some components need to reduce the residual reflectance within 0-70°. The optical coating for immersive projection objective faces severer issues in terms of the large angle incidence than those for dry objective(NA0.75). In addition and from the view of film system design, film systems designed with materials such as MgF2, AlF3, LaF3, GdF3 and SiO2[18-19] prepared in traditional thermal evaporation and ion beam sputtering methods are insufficient to ensure the extremely low residual reflectance and S/P polarization split at a large angle incidence, when an ArF excimer light source with the operating wavelength of 193nm is used. Some common methods that eliminate the polarization[20-21] cannot be compatible with the requirement for large angle incidence and need to pursue better solutions.
对浸没式投影物镜来说, 存在两个特殊的难题:首先, 高NA导致了光线在元件表面入射角的增加, 以NA1.35为例, 水的折射率为1.437, 相当于物镜系统NA0.94, 这种情况下, 部分元件需要在0~70°范围内减少剩余反射, 相对干式物镜(NA0.75)而言, 浸没式投影物镜光学薄膜面临的大角度入射问题更加严峻。另外, 从膜系设计的角度考虑, 当工作波长为193 nm的ArF准分子光源时, 使用传统热蒸发和离子束溅射方法制备的MgF2、AlF3、LaF3、GdF3和SiO2[18-19]材料所设计的膜系很难保证在大角度入射时仍具有极低的剩余反射和S/P偏振分离, 另外一些常见消偏振方法[20-21]无法兼顾大角度入射需求, 需要寻求更好的解决办法。
From the view of film system design, the above broad angle antireflection(BAAR) film system can be realized with two ideas. On one hand, the best solution of a film system design depends on materials with high/low refraction indices known from the maximum value principle in mathematics. The larger refraction index difference results in better optical capabilities of a film system. The BAAR film system with more superior capabilities can be designed with importing film layers that have a lower refraction index. As shown in Fig. 1, lower the refractive index of the outermost material in the design, the smaller the residual reflectance and polarization separation when the film is incident at a large angle. On the other hand, a film system design within 0-70° has approached to the so-called "omni-directional antireflection film". An ideal solution uses the film system where the refraction index is shaded from the substrate to the air side, which has no interface and reflection and can effectively eliminate the polarization split[23-24]. However, an ideal film system with shaded refraction index is hard to be realized. The film system with shaded refraction index can be substituted approximately with the film system that has a gradient refraction index only according to the principle of Snell′s Law and following the rule that the film system interface with a smaller refraction index variation results in a larger initial incidence angle that begins to cause the polarization split. To realize both the above ideas, many possible attempts have been made in practice to prepare film layers with extremely low and adjustable refraction indices[25-26].
图 1 五层减反射膜系反射率与最外层材料折射率的关系。五层膜系，其中仅最外层材料折射率发生变化
Figure 1. Simulated relation between the refractive index of the top layer of an five-layer-antireflective coating and the reflectance. stack, five layers(only the refractive index of the top layer was changed)
从膜系设计的角度看, 可以通过两种思路实现上述宽角度减反(Broad Angle Anti-Reflection, BAAR)膜系:一方面, 由数学上的极大值原理可知, 一个膜系设计的最优解由高/低折射率材料所决定, 其折射率差值越大, 膜系的光学性能越好, 因此可以通过引入折射率更低的膜层设计出性能更加优异的BAAR膜系, 如图 1所示, 在设计中最外层材料折射率越低, 膜系大角度入射时的剩余反射率和偏振分离越小; 另一方面, 0~70°的膜系设计已接近所谓的"全向减反膜", 理想的解决方案是采用从基底到空气端折射率渐变的膜系, 这样的膜系无界面、无反射, 能够有效消除偏振分裂[23-24], 然而理想的折射率渐变膜系很难实现, 只能根据斯涅耳定律的原理, 遵循膜系界面折射率突变越小发生偏振分离的起始入射角越大的规律, 采用梯度折射率膜系近似替代折射率渐变膜系。为实现以上两种思路, 制备出超低折射率膜层和折射率可调的膜层, 人们在实践中尝试了多种可能[25-26]。
Among regular film-coated materials, MgF2 and cryolite have the lowest refraction index. MgF2 has the refraction index of approximately 1.44 at 193 nm. The cryolite has a lower refraction index, but it is not suitable for use in the objective due to its worse environmental adaptability. In addition, traditional PVD processes are not convenient to realize the refraction index adjustment. Thus only other methods can be pursued to realize an extremely low refraction index. It can be known from the equivalent medium approximation(EMA) model that a pore structure needs to be introduced into the film layer to reduce the film layer refraction index. Main methods to prepare film layers with an extremely low refraction index include template and sol-gel methods. In recent years, many technologies to prepare thin films with the template method use copolymer as the template where film layers with the refraction index of 1.11 in a visible range as well as a certain anti-friction capability can be prepared. But the high-temperature calcination at more than 450 ℃ shall be used for this method. It is insufficient to meet the requirement on film-coated component surface shape index for the lithographical objective(less than 1 nm for a single component). Compared with the template method, the sol-gel method has been widely used in recent years due to its simple reaction principle, relatively low preparation temperature(~200 ℃), and favorable surface hydrophobic modification[27-28].
在常规镀膜材料中, MgF2和冰晶石具有最低的折射率, 其中MgF2在193 nm处的折射率约为1.44, 冰晶石虽然具有更低的折射率, 但由于其环境适应性较差而不适合在物镜中使用。另外, 传统PVD工艺不便于实现折射率的调控, 因此超低折射率的实现只能寻求其他方法。由等效介质近似(EMA)模型可知, 为降低膜层折射率, 需要在膜层内引入孔隙结构, 主流的超低折射率膜层制备方法有模板法和溶胶-凝胶法。近年来模板法制备薄膜的技术多采用异量分子聚合物(copolymer)为模板, 可以制备出可见范围内折射率1.11的膜层, 并且具有一定的抗摩擦性能, 但该方法在工艺上需采用450 ℃以上的高温煅烧工艺, 很难保障光刻物镜对镀膜元件面形指标的要求(单个元件1 nm以下)。相对于模板法, 溶胶-凝胶法因其反应原理简单、制备温度(~200 ℃)相对较低、利于进行表面疏水修饰的工艺优势, 近年来被广泛采用[27-28]。
For optical coatings on the lithographical objective, the regular sol-gel method uses MgF2 as a base material to prepare materials with an extremely low refraction index. Different reaction paths can be adopted in realization. But the essential idea is basically consistent. The MgF2 sol material is obtained through reaction between the weak acidic salt or alcoholate that contains magnesium and the fluoric acid. Generally this material consists of self-organized nanometer particles that can form the MgF2 bubble structure after treatment with an autoclave or aging treatment. In addition, the catenulate tree structure is formed through hydrolytic polycondensation of TEOS. It wraps the bubble MgF2 particles to constitute an irregular tree structure. The sol sample obtained is used for thin film preparation through Czochralski method or the spin-coating method. Finally pore film layers are prepared to realize a refraction index lower than that of the lumpy MgF2. The refraction index of film layers obtained finally can be adjusted with parameters in the reaction. The lowest refraction index that can be realized with this method is slightly more than 1.1, but the mechanical strength of film layers is frequently low. To solve this problem, Ishizawa et al. use the viscous SiO2 solution for spinning on the MgF2 film layer prepared and heat to 100-200 ℃ to form the amorphous SiO2 between MgF2 particles, which makes mechanical strength of the film layer increase from approximately 25 MPa to about 135 MPa. In addition to the extremely low refraction index of the sol-gel film layer, its another advantage lies in a high resistance to laser damage that makes the film layer maintain integrity after exposure to the ArF laser irradiation at 5×107 pulses and the energy density of 600 mJ/cm2/pulse.
对于光刻物镜光学薄膜, 为制备出超低折射率材料, 常规的溶胶-凝胶方法均以MgF2为基础材料, 可以采用不同的反应路径, 但其本质思路基本一致:通过含镁的弱酸盐或醇盐与含氟酸反应获得MgF2溶胶原材料, 这种材料通常是自组织的纳米颗粒, 通过高压釜处理或老化处理形成MgF2水泡结构, 另外, 通过TEOS的水解缩聚反应, 形成链状的树形结构, 并将水泡状MgF2颗粒包裹其中, 成为无规则的树状结构。得到的溶胶样品通过提拉法或旋涂法制备薄膜, 最终制备出含孔隙的膜层, 实现了低于块状MgF2的折射率, 最终获得的膜层折射率可由反应过程中的各参数进行调控来获得。采用该方法可实现的最低折射率略大于1.1, 但通常膜层机械强度较低, 为解决这一问题, Ishizawa等人在制备完成的MgF2膜层上, 用SiO2粘合溶液甩胶并加温100~200 ℃, 使MgF2粒子之间形成了非晶SiO2, 将膜层的机械强度由~25 MPa提升至~135 MPa。溶胶-凝胶膜层除了具有超低折射率, 另一优势在于其具有较高的抗激光损伤能力, 在能量密度为600 mJ/cm2/pulse的条件下经历5×107脉冲的ArF激光辐照后, 膜层仍然保持完好。
As shown in Fig. 1, the MgF2 antireflection film prepared with the sol-gel method or traditional PVD film system with the sol-gel coated MgF2 film layer has good optical capacities at the vacuum ultraviolet waveband and deep ultraviolet waveband.
如图 1所示, 溶胶-凝胶方法所制备的MgF2减反膜, 或在传统PVD膜系上增镀溶胶-凝胶MgF2膜层, 在真空紫外与深紫外波段, 都有良好的光学性能。
When the two materials of high/low refractive index are co-evaporated, a specific refractive index film layer between the two materials can be realized by adjusting the ratio of the two materials. Realization can be made with two methods. One is the gaseous phase mixture method where two separate evaporation sources each evaporate one material and the required proportions are obtained by changing deposition rates of both materials. The other one is used to directly mix materials as per designated proportions in one evaporation source for evaporation, which is called the liquid phase mixture. The former is flexible for refraction index adjustment and is more applicable to cases where continuous adjustment of the refraction index is required. But its defects are also evident. Proportions of two materials are different in space distribution. Thus it is not suitable for the preparation of large diameter components. The latter can be used only to realize particular proportions of materials, but it can be used for large-caliber components with curved surface and is applicable more extensively.
当高/低折射率的两种材料共同蒸发时, 可以通过调控两种材料的配比, 实现介于两种材料之间的特定折射率膜层。具体实现可以采用两种方法:一种是气相混合, 即两个独立的蒸发源各自蒸发一种材料, 通过改变两种材料的沉积速率获得所需的配比; 另一种是直接按照指定配比将材料混合在同一蒸发源中再蒸发, 即所谓的液相混合。其中前一种方法对折射率的调整较为灵活, 更适用于需要连续调节折射率的情形, 但其缺陷也比较明显, 即两种材料的配比具有空间分布的差异性, 因此无法实现大口径元件的制备; 后一种方法只能实现特定材料配比, 但可用于具有曲面形状的大口径元件, 具有更广泛的适用性。
What needs to be pointed out specifically is that any refraction index between 1.20 and 1.44 can be realized with the aforesaid sol-gel method where the molar ratio between Si and Mg in two sols of MgF2 and SiO2 are controlled. The schematic diagram for mixture of two sols in different proportions and the corresponding refraction index are shown in Fig. 2. Recently, Xu Yao et al.[33-34] have prepared the antireflection film system with a gradient refraction index by controlling the molar ratio between mixed materials, which is potential to meet broad wavebands or the demand for broad angle antireflection.
图 2 两种溶胶(MgF2和SiO2)不同Si/Mg摩尔比混合实现可调折射率
Figure 2. Realization of the adjustable refraction index with mixture of two sols (MgF2 and SiO2) at different Si/Mg molar ratios
Design and realization approach of the BAAR film system have been presented in the above. Now indices of objectives NA0.75 and NA1.35 are used as an example to explain how to evaluate objective system indices. Impact of the objective on the incident light can be expressed with Jones Matrix. The pupil expressed with Jones Matrix is called Jones Pupil, components of which are not certain in physical significance. It is not easy to distinguish indices because of combination of different influence factors. Thus it is broken down into the form of physical pupil, components of which are independent from each other and certain in physical significance. They can be expressed with parameterization in the directional Zernike Polynomials. The objective′s Jones Pupil can be described with five pupil functions that have certain physical significances after simplification and decomposition:
上述内容介绍了BAAR膜系的设计及实现途径。现以NA0.75和NA1.35物镜的指标为例, 说明物镜系统级指标如何评估。物镜对入射光的影响可用琼斯矩阵表示, 用琼斯矩阵表示的光瞳称为琼斯光瞳。琼斯光瞳各个分量的物理意义不明确, 不同影响因素交织在一起, 不易进行具体指标的划分, 因此将其分解成物理光瞳的形式。物理光瞳的各分量相互独立且物理意义明确, 并可用方向泽尼克多项式进行参数化表征。通过简化与分解, 物镜的琼斯光瞳可以由5个具有明确物理意义的光瞳函数进行描述:
The diattenuation Jdia and the retardation Jret are two main influence factors of the polarization aberration, which correspond respectively to amplitude and phase splits. Generally it is required in the immersive projection objective with NA1.35 that the diattenuation is less than 0.5% and the retardation is less than 2 nm. It is also required that the apodization uniformity corresponding to each viewing field of the system is not less than 90% and the transmittance is not less than 60%.
式中, 二次衰减(diattenuation, Jdia)和延迟(retardation, Jret)为偏振像差的两个主要影响因素, 分别对应振幅分离和相位分离, 在NA1.35的浸没式投影物镜中, 一般要求其二次衰减 < 0.5%, 延迟 < 2 nm。另外要求系统各个视场对应的切趾均匀性(apodization uniformity, t)≥90%, 透过率≥60%。
Impact of the polarization aberration caused by film system on the imaging cannot be ignored at a certain extent of the objective numerical aperture. Shang Hongbo et al. use the objective system with NA0.75 for studies on the intensive lines imaging at 90 nm intervals and the imaging contrast. It is found through comparison between common and BAAR film systems in terms of diattenuation, retardation, apodization uniformity, transmittance and intensive line contrast at 90 nm intervals that the system retardation has been reduced significantly from 1.55 nm to 1.2 nm after use of the BAAR film system. In addition, the intensive line contrast at 90 nm intervals in the objective is increased from 0.08 to 0.89 by adjusting the objective system design simultaneously.
当物镜数值孔径达到一定程度时, 膜系所引起的偏振像差对成像的影响不容忽视。尚红波等采用NA0.75物镜系统对90 nm密集线条成像, 并对成像的对比度进行研究, 通过对比普通膜系和BAAR膜系对应的二次衰减、延迟、切趾均匀性、透过率和90 nm密集线条对比度等指标, 发现采用BAAR膜系后, 系统的延迟显著降低, 由1.55 nm降到1.2 nm。另外, 通过调整物镜系统设计, 使物镜90 nm密集线条对比度由0.08提升至0.89。
The film system has a bigger impact on the polarization aberration in an immersive objective with higher numerical aperture. System indices corresponding to different film systems such as regular film system, combined film system and combined BAAR film system that includes film layers with an extremely low refraction index are shown in Tab. 1.
表 1 不同方案对应的系统指标
Table 1. System indices corresponding to different schemes
Index requirement System index Regular film system Combined film system Combined BAAR film system that includes film layers with an extremely low refraction index Retardation/(RMS, nm) < 2.00 2.69 2.29 1.94 Diattenuation(RMS) < 0.005 0.009 0.015 0.010 Apodization uniformity > 0.90 0.79 0.89 0.94 Transmittance > 0.60 0.69 0.55 0.67
在数值孔径更高的浸没式物镜中, 膜系对偏振像差的影响更大。表 1是不同膜系对应的系统指标, 一种是常规膜系, 一种是组合膜系, 另一种是应用超低折射率膜层构成的组合BAAR膜系。
It can be seen from the table that retardation, apodization uniformity and transmittance of the film system have been increased significantly after use of film layers with the extremely low refraction index of 1.1 and the requirement for objective system indices can be met.
从表中可以看出, 膜系中应用折射率为1.1的超低折射率膜层后, 系统延迟、切趾均匀性和透过率均有较大提升, 能够满足物镜系统级指标要求。