
- Advanced Photonics
- Vol. 6, Issue 3, 030502 (2024)
Abstract
Joseph Braat (professor emeritus TU Delft and former research fellow at Philips Research) received the 2019 Holst Memorial Lecture Award for his important contributions in to diffraction-limited optical imaging and scanning. Photo courtesy of TU Eindhoven.
1 Introduction: Early Career and Influences
2 Contributions to Optical Disc Systems
Figure 2.An example of the 0.38 NA lithography projection lens designed by Joseph Braat published in “Quality of microlithographic projection lenses,”
3 Early Stages of Lithography
(Braat, cont'd.) In the optics research group at Philips, at the beginning of the 1960s, there was a mask-making lab to enable the production of optical patterns (initially binary black–white) on substrates for integrated circuit boards with individual electrical components. When the first integrated circuit was made in the USA (Fairchild, 1959), this mask laboratory focused on producing patterns for integrated planar transistors. At the beginning of the 1970s, the research people who started optical data storage also devoted time to this mask lab and to the new lithographic technology.
The initial lithography technology was so-called “contact lithography.” The only optical principle involved was Fresnel diffraction, which determines the gradual blurring of the mask features as a function of the distance between the mask and chip surface in quasi-contact. Contact lithography with unit magnification reached its limits at roughly 2-micron feature size in the optical domain. The maximum lateral size of a chip was of the order of 10 cm. Smaller details were feasible by a shift to shorter wavelengths, such as X-ray wavelengths.
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A step forward was projection lithography, using the same masks and the same 1:1 magnification. At Philips, a system with 0.20 numerical aperture (NA) was built, but the optics were inevitably complicated and bulky. The way out was reduction lithography combined with the stepping principle to increase both resolution and field size (step-and-repeat). The first reduction projection lens at Philips, with a track length of 60 cm, was produced by the French company Cerco, a specialist in astronomical imaging and space optics. As requested by Philips, this projection lens used both the g-line and the h-line of a mercury high-pressure lamp to smear out the standing wave patterns in the exposed photo-resist. Cerco was able to deliver some well-engineered prototypes but later struggled with the production of larger quantities. Moreover, the as-designed field quality was rather low at the edges of the field.
In the mid-1970s, a new projection lens was designed with 0.30 NA and a square field of view. Such a design should have more relaxed tolerances as compared to the previous design. The first prototype was unfortunately not OK. It also turned out that at several locations within the projection lens, very tight tolerances had to be respected during manufacturing. These alarmingly tight tolerances were discovered in an analysis of the fabricated lens using my own design program. Unfortunately, Philips could not obtain a satisfactory number of projection lenses for its first series of step-and-repeat wafer steppers because of the insufficient quality of lens fabrication at Cerco.
For various reasons, Philips had the intention to sell its lithography activity and invited potential buyers, including a two-week in-depth technical visit of optical and mechanical professionals from Perkin-Elmer to Philips Science and Industry (1979). They were dissatisfied with both the projection lens system (dispersion issues and the field quality) and the mechanics (wafer stage transport using oil hydraulics). Finally, in 1984, Philips’s lithography division became a joint venture with ASM (a Dutch clean-room equipment company managed by Arthur del Prado) and the new company got the name ASM-Lithography (ASML).
Figure 3.Joseph Braat in front of the Optics Research Group at TU Delft, where he worked as a professor from 1988 to 2008 (photo courtesy of Roland Horsten).
4 Transition to Academia
Figure 4.Cover of the book
5 Leadership in Optical Societies
6 Career Reflections
Image Infomation Is Not Enable7 Book Authorship (Imaging Optics, published by Cambridge University Press)
(Braat, cont'd.) During the year 2006, I had a five-month sabbatical leave at the University of Rome, in the laboratories of Concita Sibilia and Mario Bertolotti, with the task to write a chapter on the “Assessment of optical systems by means of point-spread functions” (published in the book series Progress in Optics, edited by Emil Wolf). It was a very gratifying experience, a longer period of work devoted entirely to scientific research. It strengthened an earlier idea of mine: the writing of a book on optical imaging for novice or mature researchers in optics. Together with a close colleague, Peter Török from Imperial College London, we had previously made a list of contents for such a book. It should unite optical subjects, starting from basic electromagnetics, going to geometrical optics and optical design, to diffraction optics, and then to various subjects in optical imaging. The list got updated, and we started writing the book. For me, it coincided with my retirement from university. It allowed me to work on the book subjects in a quiet room of the university while simultaneously staying in close contact with my former colleagues. I soon discovered that book writing is time-consuming. I typically wrote a hundred (densely) printed pages a year. The progress of both authors was not comparable for the simple reason that Peter had a full-time job and a research group to run; I had the advantage of being free of any management obligations. The original 50-50 distribution of work was not possible in practice, and this meant that the book writing took more time for me than the initially projected four to five years. The almost 1000-page book Imaging Optics (Fig. 4) was published in May 2019 by Cambridge University Press and was well received. In my view, it now serves as one of the reference works that the professionals, engineers, and scientists in optics want to have close at hand. The book has also proven to be useful as a general introduction to optics for younger generations during their training period.
Yifeng Shao is a postdoc researcher from the Optics Research Group at the Delft University of Technology. During 2013–2018, he studied in the Optics Research Group at Delft University of Technology as a PhD candidate. His PhD project involves the study of optical system design methodology and computational imaging algorithms. Currently, his research focuses on optical metrology applications for the semiconductor industry, including aberration retrieval and image restoration for the scanning electron microscope (SEM) and EUV lensless diffractive imaging using ptychography.
References

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