Optical metrology to support the next generation of nanopositioning

可追溯的位移测量对于许多制造工艺和研究任务至关重要，可以追溯到SI的米定义，以便对测量结果进行任何有意义的比较。一系列技术能够进行具有亚纳米不确定性的可追溯测量;然而，所有技术最终都可以追溯到光波长，因此可以追溯到位移测量光学干涉法。在纳米尺度下，位移测量光学干涉仪的测量不确定性由非线性、测量相位（以及因此位移）中的误差主导，所述误差与照明光的波长的谐波是周期性的。商业干涉仪目前实现1 - 0.1nm的非线性，其中一些现有技术的干涉仪设计在理想情况下达到个位数皮米非线性。目前能够实现个位数皮米非线性的所有干涉仪设计在测量范围、成本或光学复杂性方面具有限制，并且在没有参照高度线性X射线干涉仪的仔细优化的情况下是否可以实现这种低非线性仍然不清楚。现有的干涉仪设计，能够皮米的非线性，因此不适合许多实际的位移测量任务，特别是表征和校准的下一代nanopositioningdevices.This项目将调查：改进相位正交薄膜涂层通过理想的涂层设计的建模和改进的控制涂层过程。将研究具有偏振无关相位响应的相位正交涂层的制造。将开发一种能够评估干涉仪光学系统内由于多次反射而产生的复杂非线性源的模型。该模型将允许调查高阶多次反射的影响，以及由于偏振泄漏引起的高阶影响。在目前的NPL干涉仪设计的非线性源将进行调查。这将涉及实际的测量，以及前面描述的建模方法的应用。将研究改进的干涉仪设计，消除先前确定的非线性源。最后，改进的干涉仪设计将应用于纳米定位平台的表征和校准。将采取的方法来回答这些问题（学生实际上将做什么）相位正交薄膜涂层用于NPL干涉仪中，以在干涉仪的正交输出之间引入90 °的相位变化。通过优化涂层设计和涂层工艺，干涉仪的信噪比将最大化，并且干涉仪之间的非线性的可变性最小化。薄膜涂层工艺也必须进行优化，以生产所需的极薄（2 - 20 nm）薄膜。为了改进干涉仪设计，必须理解现有设计中的非线性源。为此，需要一种建模技术，该技术可以处理吸收薄膜涂层和偏振光学器件，同时处理多次反射和多次通过偏振光学器件的高阶效应，而无需求助于有限的n阶计算。这样一个模型的发展允许调查的不完美的光学涂层和组件上的干涉仪的最终非线性的影响，在一种方式，将是不可能的与传统的琼斯演算方法。一旦确定了非线性的来源，该模型还可以用于研究消除来源的潜在方法。

Traceable displacement measurement is of vital importance to many manufacturing processes and research tasks, with traceability back to the SI definition of the metre required for any meaningful comparison of measurements to be made. A range of technologies are capable of traceable measurements with sub-nanometre uncertainties; however, all are ultimately traceable back to the wavelength of light and therefore to displacement measuring optical interferometry. At the nanoscale, the measurement uncertainty of displacement measuring optical interferometers is dominated by non-linearities, errors in the measured phase (and therefore displacement) that are periodic with harmonics of the wavelength of the illuminating light. Commercial interferometers currently achieve non-linearities of 1 - 0.1 nm, with some state of the art interferometer designs reaching single digit picometre non-linearities under ideal circumstances. All interferometer designs currently capable of achieving single digit picometre non-linearities have limitations, in measurement range, cost or optical complexity, and whether such low non-linearities can be achieved without careful optimisation referenced against a highly linear X-ray interferometer remains unclear. Existing interferometer designs that are capable of picometre non-linearities are therefore poorly suited to many practical displacement measurement tasks, in particular the characterisation and calibration of the next generation of nanopositioning devices.This project will investigate: Improved phase-quadrature thin film coatings through the modelling of ideal coating designs and improvement of control over the coating process. The fabrication of phase-quadrature coatings with a polarisation independent phase response will be investigated. A model capable of assessing the complex non-linearity sources that arise due to multiple reflections within interferometer optics will be developed. This model will allow investigation of the effects of higher order multiple reflections, and higher order effects due to polarisation leakage. Sources of non-linearity in current NPL interferometer designs will be investigated. This will involve both practical measurements, and the application of the previously described modelling approach. Improved interferometer designs will be researched, eliminating the non-linearity sources previously identified.Finally, improved interferometer designs will be applied to the characterisation and calibration of nanopositioning stages. The approach that will be taken to answer these questions (what the student will actually be doing) Phase-quadrature thin film coatings are used in NPL interferometers to introduce a phase change of 90 between the quadrature outputs of the interferometer. By optimising the coating design and coating process the signal to noise ratio of the interferometer will be maximised, and the variability of the non-linearities between interferometers minimised. The thin film coating process must also be optimised for the production of the very thin (2 - 20 nm) films required. In order to improve upon interferometer designs, the sources of non-linearity in existing designs must be understood. To do this, a modelling technique is required that can handle both absorbing thin film coatings, and polarising optics, whilst treating the higher order effects of multiple reflections and multiple passes through polarising optics without recourse to limited nth order calculations. The development of such a model permits the investigation of the effects of imperfect optical coatings and components on the final non-linearity of the interferometer in a way that would not be possible with a conventional Jones calculus approach. Once the sources of non-linearities are identified, the model can also be used to investigate potential ways to eliminate the source.