In the preceding chapter analytical solutions for AMOL type problems and the short- comings of scalar methods in comparison to vector methods were discussed. In this chapter vector modeling methods are introduced that will allow the solving of Maxwell’s equations and thus the characterisation of the propagation of light through the AMOL system. A review of possible modeling techniques is given, before reasoning for the choice of FEM. This is followed by an introduction to how the FEM method operates and how Maxwell’s equations can be represented in a suitable form, a simple example with an analytical solution is used to validate the technique. Finally the methods used to implement the AMOL system using the FEM are detailed, in particular the use of MATLAB/COMSOL and how two wavelengths and the photochromic layer were implemented.
4.2
MODELIG TECHIQUES
It is possible to analyze an AMOL type system of using vector based electromagnetic wave propagation with a number of modeling techniques; each of these techniques having positive and negative aspects and the choice of method is largely dependent on the particular problem being analysed and availability. Here four methods are briefly considered: finite difference time domain (FDTD) analysis, rigorous coupled wave analysis (RCWA), the PROLITH simulator and the finite element method (FEM).
4.2.1
FIITE DIFFERECE TIME DOMAI (FDTD)
FDTD was the first technique developed which worked by discretising differential equations through creating a grid in the geometric plane. It was first popularized in electromagnetics by a seminal paper by Yee [Yee 1966] and has since been continuously developed to adapt to a range of later developments. FDTD has several advantages as a numerical method [Taflove 2000]; it avoids the creation of a memory-intensive linear algebra equation, handles time changing materials easily and provides a systematic approach to problems. The Yee algorithm discretises the problem by looking at a unit cube in which each . field is surrounded by four / fields and vice versa, the propaga- tion of time is then recorded in a two step process in each time step, first the .-fields are calculated based on the /-fields, then the /-fields are updated based on the just calcu- lated . fields, thus providing a simple technique to model Maxwell's equations. It can be more difficult to effectively model dispersive materials in FDTD as it is not a frequency- based numerical method and the flexibility in choosing the mesh design is limited. However FDTD is a simple method to work with and has been widely and successfully used in electromagnetics and lithography modeling.
4.2.2
RIGOROUS COUPLED WAVE AALYSIS
Rigorous couple wave analysis (RCWA) has been used to analyse the interaction of light with various grating systems [Gaylord 1985, Moharam 1981] and multilayer structures [Lee 2004]. The method discretises the grating into a series of domains and then solves the wave equation from Maxwell’s laws in each domain, finally matching the fields at the boundaries to complete the analysis. This allows for diffraction effects and modu- lated conduction/absorption structures to be investigated through the summation of any diffractive orders created in the system. Although RCWA has been shown to be a valuable technique for explaining diffraction by gratings the nature of a two wavelength system with variable absorbance modulation such as AMOL would introduce additional complexity due to the changing absorbance with time and the absorbance grating height, as would introducing a multilayer stack.
4.2 MODELING TECHNIQUES
4.2.3
POSITIVE RESIST OPTICAL LITHOGRAPHY (PROLITH)
The positive resist optical lithography (PROLITH) simulator [KLATencor 2010] is a commercially available simulation for lithography, able to implement models of optical projection, contact and proximity lithographies as well as new features such as double patterning and some new lithography techniques such as EUV. The ability to model the complete process from exposure to feature formation (based on the Dill Equations (Eq 3.11) but fully developed to the latest developments, RCWA and FDTD techniques) has created a very strong tool for lithographic simulations, including stochastic effects. However the use of the two wavelength illumination and the introduction of an AML create new and novel problems that may not be able to be implemented easily in the commercial software package; modeling software designed for broader problems would allow more flexibility in the design of a model as well as enable greater understanding of the propagation of light in the layers, in particular regarding the evanescent fields. Also when new ideas are incorporated there is no guarantee that these will be suitable for the PROLITH system.
4.2.4
FIITE ELEMET METHOD (FEM)
FEM turns a system of differential equations into a set of algebraic equations through the recasting of the equation problem as an error minimisation followed by the division of the domain into a set of small elements with constant parameters. This allows the calculation of the solution, at a single frequency per simulation, and is suited to complex and/or changing domains as well as variation within domains. FEM is computationally intensive with regards to the required memory because the created algebraic elements must all be stored in a large matrix; it is also a mathematically complicated method and thus is not as easily developed as FDTD and is more difficult to run time-dependent simulations in FEM. However FEM has been well developed in recent years as is able to cope with a large array of problems and situations. FEM is particularly good when considering frequency specific materials and complex geometries and material inhomo- geneities [Davidson 2010] and has been successfully applied to many electromagnetic problems.
4.2.5
CHOICE OF METHOD
In this work FEM was chosen as the simulation tool to be used to develop a full vector model of the AMOL process due to the strengths of FEM in handling dispersive media and inhomogeneities like those present in the AMOL layer. FEM has also been demon- strated to be very effective in previous mask simulations [Burger 2005]. It was decided to be appropriate to initially consider only the steady state solution, assuming this could be achieved within a reasonable time period. It was felt that FEM could suitably handle the dual wavelength nature of the simulation through multiple, concurrent simulations and that the flexibility in material and geometry parameters would allow new ideas to be easily tested. In particular it was decided that a commercial FEM software, COMSOL multiphysics [COMSOL] be used as this offered the advantage of a confirmed and tested FEM package to work from as well as being able to interface to a MATLAB [Math- works] environment to allow more complex simulations to be run and for automated testing routines.
4.3
FIITE ELEMET METHOD MODELIG
FEM [Pepper 2006, Jin 2002, Henwood 1996] was pioneered in structural problems in Civil Engineering, particularly solid mechanics but has since been developed to be a strong tool in solving a broad range of differential equations (DEs) based problems. Although progress was initially limited by available computational power available and the large memory resources required growth in these technologies has allowed managea- ble access to problems in structural dynamics, thermodynamics and fluids as well as electromagnetics. FEM has become widely used in electromagnetic simulations [Webb 1995] and in various optical [Anderson 2009, Cummer 2006] and lithographic systems [Fikri 2003, Tejeda 1998] even given the computational power required for large do- mains. Here FEM is introduced, describing the background technique before creating an example problem in electromagnetics. Mention is also made of the errors involved and in the use of COMSOL Multiphysics, the commercial FEM software package used in this thesis, in tandem with Matlab.