Bernd Dörband

Interferometric testing of optical surfaces at its current limit

Summary Today the production of microlithographic projection lenses drives interferometry to its very limits. Resolutions to a fraction of an Angstrom when measuring surface figure are necessary. Especially EUV lithography needs mirror surface qualities of approximately 0.1 nm RMS, with the consequence that metrology needs accuracies significantly below 1 Angstrom. Repeatability of a measurement is limited by the quality of the interferometric setup itself. Reproducibility is mainly influenced by handling, adjustment and thermal adoption to the testpiece, whereas accuracy is limited by the calibration process and procedure. Carl Zeiss uses a low coherent Fizeau interferometer with multi-fringe evaluation technique (DMI = D irect M easuring I nterferometry) to accumulate many phase maps at video rate. Cavities are kept short, air ventilation of the cavity during the measurements is applied. A weak aspheric surface for EUV application (so called ELT2-EUV asphere) was tested with a special interferometric setup using a single lens compensating system. The reproducibility of the measurement came out as 0.07 nm RMS (surface figure). The setup has been calibrated using a spherical calibration surface, whose accuracy was cross checked by two different absolute measurement procedures. We found that both measurements differ by an amount of only 0.15 nm RMS (surface figure). Taking into account the systematic errors of the interferometric setup, the accuracy of the asphere measurement can be estimated to 0.2 nm RMS (surface figure). Further improvements are necessary to drive accuracy to even lower limits and to make test procedures ready for production process.

Absolute measurement of non-comatic aspheric surface errors

An absolute test for rotationally symmetric aspheric surfaces was developed. All non-rotationally symmetric surface errors apart from those of comatic nature were determined interferometrically. For this purpose, the asphere was measured 12 times under different angles of rotation. The difference of a single interferogram and the average of all 12 interferograms gave the non-rotationally symmetric errors in an absolute manner apart from the comatic errors. With the purpose of noise reduction, the so-determined 12 interferograms of the non-rotationally symmetric errors were appropriately rotated and subsequently averaged. The rotationally symmetric errors were detected via stylus profilometry. Repeatabilities of the so-obtained aspheric surface errors are presented.

Handbook of Optical Systems: Volume 5: Metrology of Optical Components and Systems

The state-of-the-art full-colored handbook gives a comprehensive introduction to the principles and the practice of calculation, layout, and understanding of optical systems and lens design. Written by reputed industrial experts in the field, this text introduces the user to the basic properties of optical systems, aberration theory, classification and characterization of systems, advanced simulation models, measuring of system quality and manufacturing issues. In this Volume Volume 5 topics comprise the methods of measuring the properties of optical systems. The different fundamental techniques, such as propagation measurement and polarimetry, are introduced and discussed in detail and clarity. The presentation allows the reader, after having devised an optical system, to perform the measurements best suited to ascertain that the system fulfills the specific needs and requirements. The following chapters provide a survey on materials, coatings and surfaces of optical components, and combine this with a treatment of light and radiation. The book thus serves as a one-stop reference for metrology of optical systems.

Abbildungsfehler und optische Systeme

Optische Elemente haben die Aufgabe, Objekte moglichst fehlerfrei abzubilden. Dazu sollte jeder Punkt eines Objektes in einen scharfen Bildpunkt uberfuhrt werden. Leider gibt es drei Hauptgrunde, die eine exakt scharfe Abbildung verhindern und damit dem Hersteller von optischen Systemen das Leben schwer machen: Beugung an der Blende des abbildenden Systems Konstruktionsbedingte Abbildungsfehler Herstellungsfehler

Simulation and error budget for high precision interferometry

An accurate calculation of the error budget of individual interferometer components can only be based on a simulation of the total interferometric setup. We have shown that such a simulation can in fact make accurate predictions of the achievable measurement precision. Furthermore, a simulation allows calculating the maximal allowable drifts magnitudes. Using a simulation of the interferometric setup therefore allows for focusing on critical components and drifts on the one hand and for relaxing requirements for non-critical components and drifts on the other hand.