UItramicroscopy 42-44 (1992) 1376-1380 / / l ~ g l / / ~ ~ North-Holland Topography of sputtered Si(100) surfaces studied by STM in air G. Gottschalk, Th. Fries, C. Becker and K. Wandelt lnstitut fiir Physikalische Chemie, Unil,ersith't Bonn, Wegelerstrasse 12, 5300 Bonn 1, Germany Received 12 August 1991 Investigations of the influence of Ar ion bombardment on the topography of Si(100) surfaces are presented. The dependence of surface roughness on the angle of incidence and ion dose has been studied. The samples,which were cut from commercially available wafers, have been prepared in a UHV system by argon ion etching at 3 keV primary energy. They have been exposed to air after preparation and have also been investigated by the STM in air. This ensured that the silicon samples were able to develop a native oxide layer just as in technological applications. A series of samples sputtered at normal incidence with different ion doses shows a maximum in surface roughness of 0.75 nm correlated to an ion dose of 240× 1015 ions/cm 2. A series, dependent on the angle of incidence, taken at an ion dose of 360× 1015 ions/cm 2 shows a maximum surface roughness of 1.2 nm at a sputtering angle of 60 ° off surface normal. The results are in good agreement with theoretical calculations. I. Introduction The modern semiconductor industry is still re- lying on silicon as the prime material for manu- facturing semiconductor devices. With increasing integration density, that is, smaller structure di- mensions, the surface topography of the commer- cially available wafers plays an increasingly im- portant role for the manufacturing process. As the structural dimensions are getting into the nanometer regime, the scanning tunneling micro- scope (STM) proves to be the adequate tool for investigations of the semiconductor surfaces [1,2]. Even at this scale it would be useful to conduct the investigations under conditions that are com- parable to the conditions used during the manu- facturing process, that is, under ambient pres- sure. Since microstructuring of semiconductor sur- faces is usually done by ion etching we focus our interest on the topographical changes produced by ion bombardment. One can easily imagine that knowledge of the microscopic structure of the surface after ion beam processing is of great technological interest. One of the problems en- countered in imaging silicon surfaces by STM is the native oxide layer of some nanometers thick- ness which evolves even after only short exposure to air. A widely spread opinion is that this oxide layer hinders imaging of those samples by STM. As our investigations show, this problem can be overcome using the right tunneling parameters. The gap voltage should be about -2.5 V sam- ple bias, as is customary for semiconductor sur- faces. The tunneling current should be in the range of 0.1 nA-1 nA. We have found that the most important parameter is the scanning speed. Our images were taken at a speed of 10 s per frame. For slower scanning speeds the repro- ducibility of the topography was lost. Since at slower scanning speeds the local charge transfer from the sample to the tip is increased, we be- lieve that this can be attributed to charging of the sample surface due to the limited conductivity of the sample surface. Using the parameters described above, stable imaging of oxidized silicon surfaces can easily be accomplished. This enables us to investigate ion- 0304-3991/92/$05.00 © 1992 - Elsevier Science Publishers B.V. All rights reserved