The EarthCARE satellite mission objective is the observation of clouds and aerosols from low Earth orbit. The payload will include active remote sensing instruments being the W-band Cloud Profiling Radar (CPR) and the ATLID LIDAR. These are supported by the passive instruments Broadband Radiometer (BBR) and the Multispectral Imager (MSI) providing the radiometric and spatial context of the ground scene being probed. The MSI will form Earth images over a swath width of 150 km; it will image the Earth atmosphere in 7 spectral bands. The MSI instrument consists of two parts: the Visible, Near infrared and Short wave infrared (VNS) unit, and the Thermal InfraRed (TIR) unit. Subject of this paper is the VNS unit.
In the VNS optical unit, the ground scene is imaged in four spectral bands onto four linear detectors via separate optical channels. Driving requirements for the VNS instrument performance are the spectral sensitivity including out-of-band rejection, the MTF, co-registration and the inter-channel radiometric accuracy. The radiometric accuracy performance of the VNS is supported by in-orbit calibration, in which direct solar radiation is fed into the instrument via a set of quasi volume diffusers.
The compact optical concept with challenging stability requirements together with the strict thermal constraints have led to a sophisticated opto-mechanical design.
This paper, being the second of a sequence of two on the Multispectral Imager describes the VNS instrument concept chosen to fulfil the performance requirements within the resource and accommodation constraints.
Optical designers often insert or split lenses in existing designs. Here, we apply in the design of objectives for deep and extreme UV lithography an alternative method for adding new components that consists of constructing saddle points in the optical merit function landscape and obtaining new local minima from them. The design examples show that this remarkably simple method can be easily integrated with traditional design techniques. The new method has significantly improved our design productivity in all cases in which we have applied it so far. High-quality designs of lithographic objectives are obtained with this method.
The multidimensional merit function space of complex optical systems contains a large number of local minima. We illustrate a method to find new local minima by constructing saddle points, with examples of deep and extreme UV objectives. The central idea of the method is that, at certain positions in a system with N surfaces that is a local minimum, a thin meniscus lens or two mirror surfaces can be introduced to construct a system with N+2 surfaces that is a saddle point. When optimization rolls down on the two sides of the saddle point, two minima are obtained. Often one of these two minima can also be reached from several other saddle points constructed in the same way. With saddle-point construction we can obtain new design shapes from existing ones in a simple, efficient, and systematic manner that is suitable for complex designs such as those for lithographic objectives.
When extreme ultraviolet (EUV) mirror systems having several high-order aspheric surfaces are optimized, the configurations sometimes enter into highly unstable regions of the parameter space. Small changes of system parameters lead then to large changes in ray paths and optimization algorithms fail. A technique applicable for any rotationally symmetric optical system that keeps the configuration away from unstable regions during optimization is described. A finite-aberration quantity is computed for several rays, and its average change per surface is determined for all surfaces. For not too large values of these average changes, optimization remains stable. A design for EUV lithography is discussed.
Saddle-point construction (SPC) is a new method to insert lenses into an existing design. With SPC, by inserting and
extracting lenses new system shapes can be obtained very rapidly, and we believe that, if added to the optical designer's
arsenal, this new tool can significantly increase design productivity in certain situations. Despite the fact that the theory
behind SPC contains mathematical concepts that are still unfamiliar to many optical designers, the practical
implementation of the method is actually very easy and the method can be fully integrated with all other traditional
design tools. In this work we will illustrate the use of SPC with examples that are very simple and illustrate the essence
of the method. The method can be used essentially in the same way even for very complex systems with a large number
of variables, in situations where other methods for obtaining new system shapes do not work so well.
Optical designers often insert or split lenses in existing designs. Here, we present, with examples from Deep and Extreme UV lithography, an alternative method that consists of constructing saddle points and obtaining new local minima from them. The method is remarkable simple and can therefore be easily integrated with the traditional design techniques. It has significantly improved the productivity of the design process in all cases in which it has been applied so far.
The multidimensional merit function space of complex optical systems contains a large number of local minima that are connected via links that contain saddle points. In this work, we illustrate a method to construct such saddle points with examples of deep UV objectives and extreme UV mirror systems for lithography. The central idea of our method is that, at certain positions in a system with N surfaces that is a local minimum, a thin meniscus lens or two mirror surfaces can be introduced to construct a system with N+2 surfaces that is a saddle point. When the optimization goes down on the two sides of the saddle point, two minima are obtained. We show that often one of these two minima can be reached from several other saddle points constructed in the same way. The practical advantage of saddle-point construction is that we can produce new designs from the existing ones in a simple, efficient and systematic manner.
The merit function space of mirror systems for EUV lithography is studied. Local minima situated in a multidimensional merit function space are connected via links that contain saddle points and form a network. In this work we present the first networks for EUV lithographic objectives and discuss how these networks change when control parameters, such as aperture and field are varied and constraints are used to limit the variation domain of the variables. A good solution in a network obtained with a limited number of variables has been locally optimized with all variables to meet practical requirements.
When Extreme Ultraviolet mirror systems having several high-order aspheric surfaces are optimized, the configurations often enter into highly unstable regions of the parameter space. Small changes of system parameters lead then to large changes in ray paths, and therefore optimization algorithms crash because certain assumptions upon which they are based become invalid. We describe a technique that keeps the configuration away from the unstable regions. The central component of our technique is a finite-aberration quantity, the so-called quasi-invariant, which has been originally introduced by H. A. Buchdahl. The quasi-invariant is computed for several rays in the system, and its average change per surface is determined for all surfaces. Small values of these average changes indicate stability. The stabilization technique consists of two steps: First, we obtain a stable initial configuration for subsequent optimization by choosing the system parameters such that the quasi-invariant change per surface is minimal. Then, if the average changes per surfaces of the quasi-invariant remain small during optimization, the configuration is kept in the safe region of the parameter space. This technique is applicable for arbitrary rotationally symmetric optical systems. Examples from the design of aspheric mirror systems for EUV lithography will be given.