Atomic Layer Deposition Barrier Layers on Large Silver-coated Mirrors

尽管天文观测中使用了所有令人眼花缭乱的技术，而天文学应用又反过来推动了这项技术的发展，但仍然很难制造出在所有光学波长都高度反射、又足够耐用的金属反射镜涂层。传统的铝涂层是耐用的，但在所有波长都不是高度反射的。银具有更好的全波长反射率，但除非用巧妙的方法将其沉积在玻璃镜坯上，否则会在短时间内形成针孔并剥落。这种方法的发展是本研究的目标，也是一个对许多不同类型的未来仪器具有深远应用的目标。提高耐久性的技术途径的实质是原子层沉积(ALD)。ALD是一种相对较新的、利用自限表面反应的顺序化学气相过程。ALD涂层具有几个理想的性能，特别是良好的薄膜结构、共形覆盖率和良好的厚度均匀性和可控性。目前，ALD已成为硅片行业中广泛使用的成熟工艺。然而，到目前为止，ALD仅限于相对较小的基片尺寸，并且由于其相对较慢的速度，很少用于光学领域。该项目将在通过传统物理气相沉积(PVD)沉积的银的基础上放置ALD阻挡层，以获得比单独使用PVD更耐用的薄膜。建议的工艺是这样一种混合工艺，大部分涂层使用PVD工艺，而阻挡层则添加了ALD系统。ALD沉积速度的提高也计划在合理的时间内覆盖更大的衬底。该项目具有巨大的更广泛的影响，因为它能够极大地提高各种天文仪器的实际性能。此外，学生将参与其中，并强烈承诺用简单的语言解释研究及其结果。该项目的资金由美国国家科学基金会天文科学部通过其先进技术和仪器计划提供。

Even with all the dazzling technology that has been brought to bear on astronomical observations, and the furthering of that technology in turn that has been inspired by astronomical applications, it is still difficult to make a metal mirror coating that is highly reflective at all optical wavelengths yet durable enough to last in field use. Traditional aluminum coatings are durable, but not highly reflective at all wavelengths. Silver has better all-wavelength reflectance, but will develop pin-holes and flake away in a short time unless clever methods are used to deposit it on a glass mirror blank. The development of such methods is the goal of this investigation, and a goal with far-reaching applications to many different kinds of future instruments.The essence of the technical approach to improved durability, high-reflectance silver coatings, is Atomic Layer Deposition (ALD). ALD is a relatively new, sequential chemical vapor process taking advantage of self-limiting surface reactions. ALD coatings have several desirable properties, notably excellent film structure, conformal coverage, and excellent thickness uniformity and control. ALD is now a mature process used widely in the silicon wafer industry. To date, however, ALD has been limited to relatively small substrate sizes and is rarely used for optics, owing to its relatively slow speed. This project will place ALD barrier layers on top of silver deposited by conventional physical vapor deposition (PVD) to get films more durable than PVD alone. The process proposed is a thus hybrid process, with most of the coating done with PVD processes and the barrier layers added with the ALD system. Improvements in ALD deposition speed are also planned to allow coating of larger substrates in reasonable times.This project has immense broader impacts in that it enables dramatically improved practical performance of a vast range of astronomical instrumentation. In addition, students will be involved, and there is a strong commitment to plain-language explication of the research and its results.Funding for this project is being provided by NSF's Division of Astronomical Sciences through its Advanced Technologies and Instrumentation program.