Ultrafast Polymerization Inhibition by Stimulated Emission Depletion for Three-dimensional Nanolithography
Ultrafast Polymerization Inhibition by Stimulated Emission Depletion for Three-dimensional Nanolithography
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DOI：
10.1002/adma.201103758
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发表时间：
2012-03-08
影响因子：
29.4
通讯作者：
Wegener, Martin
中科院分区：
文献类型：
--
作者：
Fischer, Joachim;Wegener, Martin
最近的三维直接激光写入（DLW）[1 - 5]光学光刻实验受到受激发射耗尽（STED）光学显微镜的启发。[6-9]使用由季戊四醇四丙烯酸酯中的光引发剂7-二乙基氨基-3-噻吩甲酰香豆素与定制的光焦点组合的新型光致抗蚀剂，衍射屏障确实可以在横向和轴向方向上被打破，使这种形式的光刻真正达到纳米尺度。[10]然而，潜在的消耗机制一直不明确。在本通讯中，我们进行光刻实验的时间延迟的激发和耗尽脉冲的可变中心波长。这些数据揭示了具有不同光谱特征的慢分量和快分量。快分量表现出约1 ns的时间常数，并在频谱上遵循预期的增益谱。因此，它可以坚定地被指定为受激发射。慢分量与受激发射不同，在10 ns~1 μ s范围内。这些总体数据允许系统地优化下一代STED-DLW光学光刻的条件。激光直写（DLW）光刻可以被看作是平面（2D）电子束光刻的三维（3D）对应物。[1-5]它通常允许在单个处理步骤中制造几乎任意复杂的3D结构。通常，光致抗蚀剂中的光引发剂分子通过紧密聚焦的近红外激光束经由双光子吸收而被激发，它们产生自由基，并且仅在聚焦体积内引发聚合反应。聚合体积元素（体素）是通常通过扫描样本或焦点创建的更复杂结构的构建块。然而，由于典型的最小横向（轴向）特征尺寸为100 nm（250 nm），DLW还不是真正的纳米技术。利用收缩[11 - 12]可以减小特征尺寸，但通常不适用。然而，线宽和阿贝意义上的分辨率（即光栅的最小周期）不能混淆。[10]受衍射极限的限制，实际上无法获得低于210 nm（510 nm）的横向（轴向）周期。[10]今天，荧光显微镜的相应限制可以通过几种方式克服，由SW Hell和他的方法称为受激发射耗尽（STED）显微镜开创。[6-9]通过耗尽焦斑外围分子的第一激发单重态，STED物理上减少了有效激发体积，其程度被认为是根本上有限的。正如Hell在2000年提出的，[7]这种较小的激发体积不仅可以用于荧光显微镜，而且还可以将光化学反应限制在纳米级。显然，这将是非常可取的，以提高分辨率的DLW向电子束或深紫外光刻，同时完全保持其3D能力。最近，第一个实验对3D光学光刻超过衍射极限已经公布。[13 - 15]将STED的概念转化为光刻，意味着用第一激光（激发激光）引发聚合反应，并用第二激光（耗尽激光）快速、可逆和局部停止（或抑制）它。然后可以使用具有零强度点的空间成形的耗尽焦点（例如，甜甜圈焦点）来将有效反应体积朝向零收缩，理想地，朝向远低于衍射极限的空间尺度收缩。阻止或抑制DLW中的聚合反应不仅可以通过STED来实现，而且可以通过不同的方式来实现，包括光催化聚合。
Recent experiments on three-dimensional direct-laser-writing (DLW)[1–5] optical lithography have been inspired by stimulatedemission-depletion (STED) optical microscopy.[6–9] Using a novel photoresist composed of the photoinitiator 7-diethylamino-3-thenoylcoumarin in pentaerythritol tetraacrylate combined with tailored foci of light, the diffraction barrier could indeed be broken in the lateral as well as in the axial direction, bringing this form of lithography truly to the nanometer scale.[10] However, the underlying depletion mechanism has been ambiguous. In this Communication, we perform lithography experiments with time-delayed excitation and depletion pulses of variable center wavelength. These data reveal a slow and a fast component with distinct spectral signatures. The fast component exhibits a time constant of about 1 ns and spectrally follows the anticipated gain spectrum. It can thus firmly be assigned to stimulated emission. The slow component is distinct from stimulated emission and lies in the range from 10 ns to 1 μs. These overall data allow for systematically optimizing the conditions in next-generation STED-DLW optical lithography. Direct-laser-writing (DLW) optical lithography can be viewed as the three-dimensional (3D) counterpart of planar (2D) electron-beam lithography.[1–5] It routinely allows for the fabrication of nearly arbitrarily complex 3D structures in a single processing step. Typically, photoinitiator molecules in a photoresist are excited via two-photon absorption by a tightly focused near-infrared laser beam, they generate radicals, and initiate a polymerization reaction only within the focal volume. The polymerized volume element (voxel) is the building block for more complex structures that are usually created by scanning either sample or focus. However, with typical minimum lateral (axial) feature sizes of 100 nm (250 nm), DLW is not yet a true nano-technology. Exploiting shrinkage [11–12] can reduce feature sizes but is not generally applicable. However, linewidth and resolution in the sense of Abbe (ie, minimum period of a grating) must not be confused.[10] Governed by the diffraction limit, lateral (axial) periods below 210 nm (510 nm) were inaccessible in practice.[10]Today, the corresponding limitation in fluorescence microscopy can be overcome in several ways, pioneered by SW Hell and his approach called stimulated-emission-depletion (STED) microscopy.[6–9] By depleting the first excited singlet state of the molecules in the periphery of the focal spot, STED physically reduces the effective excitation volume, the extent of which was believed to be fundamentally limited. As proposed by Hell in 2000,[7] this smaller excitation volume cannot only be used for fluorescence microscopy but, eg, also to confine photochemical reactions to the nanoscale. Obviously, it would be highly desirable to improve the resolution of DLW towards that of electron-beam or deep-UV lithography while fully maintaining its 3D capability. Recently, first experiments towards 3D optical lithography beyond the diffraction limit have been published.[13–15] Translated to lithography the idea of STED means to initiate the polymerization reaction with a first laser (the excitation laser) and to quickly, reversibly, and locally stop (or inhibit) it with a second laser (the depletion laser). A spatially shaped depletion focus with points of zero intensity (like, eg, a donut focus) can then be used to contract the effective reaction volume towards the zeros, ideally to spatial scales way below the diffraction limit. Stopping or inhibiting the polymerization reaction in DLW cannot only be accomplished by STED but has been realized in different ways including photo …