ELSEVIER Diamond and Related Materials 5 (1996) 840-844 E’DAMOND RELATED MATERIALS Surface smoothing of diamond membranes by reactive ion etching process C. Vivensang a+, L. Ferlazzo-Manin b, M.F. Ravet b, G. Turban a, F. Rousseaux b, A. Gicquel ’ a Laboratoire des Plasmas et des Couches Minces, UMRllO, Institut des Materiaux de Names, 2 rue de la Houssiniere, 44072 Nantes Cedex 03, France b Laboratoire de Microstructures et de Microelectronique, CNRS, 196 avenue Henri Ravera, 92225 Bagneux, Cedex, France ’ Laboratoire d’lngenierie des Mattriaux et des Hautes Pressions, CNRS, Universite Paris-Nord, avenue Jean-Baptiste Clement, 93430 Villetaneuse, France Abstract This paper describes a procedure based on reactive ion etching developed to reduce the surface roughness of CVD diamond thin films. The technique involves etching a bilayer made up of diamond and a planarizing SiOZ layer in an SF6-O2 plasma mixture. Etching conditions have been determined which yield equal rates for both diamond and the SiOZ coverlayer and favour the removal of diamond peaks. Smoothed surfaces have been characterized by SEM, AFM and XPS: the results exhibit a significant decrease in roughness. This novel technique offers promising prospects for polishing thin diamond membranes for X-ray lithography applications without the removal of significant amounts of diamond material. The optical transmittance of diamond membranes, mainly affected by light scattering due to surface roughness, is significantly improved using this method of planarization. Keywords: Diamond; Planarization; Membrane; Reactive ion etching 1. Introduction Although diamond appears to be the best candidate as a membrane material for the next generation of X-ray lithography masks, the roughness of as-deposited CVD diamond films may be a limitation. The surface morphol- ogy must be compatible with the high resolution pat- terns, and the optical transmittance required for the alignment procedures is mainly affected by light scatter- ing due to surface roughness. Moreover, it has been established that diamond films with high crystallinity and high purity offer the best properties regarding mechanical strength and radiation durability, but at the sacrifice of surface smoothness. For these reasons, the polishing of diamond films to improve the surface rough- ness is of major interest in the fabrication of fully suitable X-ray masks. The film roughness may be reduced during diamond growth by increasing the nuclei density, promoting secondary nucleation [1,2] and texturing the film by control of crystalline orientation [ 31. Post-deposition treatments (ion beam etching, laser planing and mecha- nochemical polishing based on the diffusion of metals etc.) [4-6] are well adapted for thick films, but are not suitable for thin films due to the large amount of material removed. In addition, mechanical techniques may induce 0925-9635/96/$15.00 0 1996 Elsevier Science S.A. All rights reserved SSDZ 0925-9635(95)00368-l damage and a loss of fracture strength for membrane purposes. Therefore we have developed a method of planarization using reactive ion etching processes suit- able for diamond films thinner than 10 urn. The tech- nique involves creating a bilayer made up of the diamond film to be smoothed and a coverlayer. The bilayer is then etched such that the overlayer screens and protects the grain boundaries whilst the diamond peaks are etched away. The coverlayer must fill correctly the diamond interstices. In this paper, we describe the effects of direct etching on diamond films and report the planarization of SiO,/diamond bilayers. In addition, a study of two SiOZ deposits used as coverlayers (plasma- enhanced (PE) CVD SiOZ and rapid thermal (RT) CVD SiOJ is presented. 2. Experimental details The polycrystalline diamond films to be smoothed were deposited on Si wafers in a bell jar microwave- enhanced plasma reactor (MWCVD) as described else- where [ 71. The deposition process was optimized for the production of diamond membranes [ 1,2,8]. The diamond films (Fig. l(a)) are typically 2 urn thick and