Chromatic sensor-based- profilometer for the focusing mirror in the Scanning Helium Microscope D. Litwin *a , J. Galas a , S. Sitarek a , B. Surma b , B. Piatkowski b a Institute of Applied Optics, 18 Kamionkowska St, Warsaw, Poland b Institute of Electronics Materials Technology, Warsaw, Poland ABSTRACT The paper is aimed at describing measurement methodology for characterizing quality of a silicon wafer applied as a focusing element in the Scanning Helium Microscope and continues the struggle against phenomena decreasing accuracy. The focusing mirror being the heart of the system and the decisive factor, which defines the resolution of the microscope, indicates the importance of testing methods. The systems made specially for this purpose provide the ability to create maps of the surface shape, thickness and surface roughness of the wafers. The paper shows many multidisciplinary issues associated with the measurements procedures and concludes with discussion on accuracy limits. Keywords: profilometry, shape, thickness, flatness measurements, helium atom microscopy, confocal sensors 1. INTRODUCTION In recent years a tiny, 50 µm thick silicon wafer has become a crucial part of a new investigating instrument – a scanning helium atom microscope. In the microscope a helium atom beam is used as a probe and potentiality can be a useful tool in reference to surface and membrane science in biological and industrial environments 1,2,3,4 . Previously the Fresnel zone plate was used.to focus the beam 5,6,7 . However, due to some limitations of its usage and low intensity a new solution has emerged, which was the ultra thin silicon wafer that is deformed under a precise electric field 8-12 . In the microscope the specially shaped mirror focuses a helium atom beam onto a sample's surface. The mirror quality affects the diameter of the focused beam and consequently the microscope resolution. Thus, the mirror surface thickness and its shape must be controlled accurately. The Si membrane production process is a complex issue and it is very difficult to obtain membranes of uniform thickness, which makes the monitoring of the process very important. Flatness and thickness uniformity of the wafer must be measured in order to select the best plate to be used in the microscope. Nano-scale measurements of a Silicon wafer introduce a number of problems in mechanical and temperature stability of the measuring instrument. 2. OBJECT OF MEASUREMENT An orientation of the wafer during a measuring process and a mount are crucial because it has a very challenging diameter to thickness ratio. The Silicon wafer is 50 µm tick and its outer diameter is 50 mm. Intuitively one could think about horizontal position- e.g. the wafer would just sit on a flat glass surface. However in such case dust particles between the wafer and the glass surface could create a structure, which would modify the original shape. In addition it would be impossible to characterize the shape of the other side. Therefore the mount of the wafer is a complex problem itself. In our previous papers we demonstrated a holder that consisted of three pairs of balls arranged in a way that the wafer could be placed flexibly in the vertical position 15, 16 between them (Fig. 1.). The pairs of balls were put in two corresponding plates: the immovable and detachable both fastened vertically to the base bracket. In that way we avoided several drawbacks like: gravity sag, dust influence, mount influence on the original shape of the wafer. On the other hand it must be remembered that a thin wafer is a membrane that can vibrate. Fortunately, in case of thickness measurements a potential error caused by the vibration has no effect since the signals from both confocal heads compensate each other. *adlitwin@pro.onet.pl, +48 22 8133285, galasj@inos.pl 16th Polish-Slovak-Czech Optical Conference on Wave and Quantum Aspects of Contemporary Optics, edited by Agnieszka Popiolek-Masajada, Elzbieta Jankowska, Waclaw Urbanczyk, Proc. of SPIE Vol. 7141, 71411T · © 2008 SPIE · CCC code: 0277-786X/08/$18 · doi: 10.1117/12.822411 Proc. of SPIE Vol. 7141 71411T-1 2008 SPIE Digital Library -- Subscriber Archive Copy