© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.pss-rapid.com pss Phys. Status Solidi RRL 3, No. 6 , 205– 207 (2009) / DOI 10.1002/pssr.200903128 Ultra-thin nanocrystalline diamond films (<100 nm) with high electrical resistivity Mathieu Lions *, 1, 2 , Samuel Saada 1 , Jean-Paul Mazellier 2 , François Andrieu 2 , Olivier Faynot 2 , and Philippe Bergonzo 1 1 CEA, LIST, Diamond Sensor Laboratory, 91191 Gif-sur-Yvette, France 2 CEA, LETI, MINATEC, 38054 Grenoble, France Received 7 May 2009, revised 22 May 2009, accepted 26 May 2009 Published online 4 June 2009 PACS 68.55.A –, 73.61.Cw, 77.55.+ f, 81.05.Uw, 81.07.Bc, 81.15.Gh * Corresponding author: e-mail mathieu.lions@cea.fr, Phone: + 331 69 08 89 59 © 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 1 Introduction Diamond exhibits at room tempera- ture very high resistivities although it has one of the high- est thermal conductivities. This has made diamond a very good candidate to be used as a sub-micron layer of great interest as a thermal heat spreader in SOD (Silicon-On- Diamond) applications [1, 2]. Today, current SOD stand- ards require dielectric layer thicknesses of 150 nm or below, while the diamond initial layers during growth are often known to be composed of small nanocrystals with poor electrical and thermal quality [3]. To match the prop- erties required, diamond synthesis has to be significantly improved at the early stages of growth for films to attain namely a high electrical resistivity of 10 13 Ω cm or above necessary for its use as a buried dielectric layer. 2 Experimental set-up The reactor used for the syn- thesis of diamond is a home made MPCVD reactor equip- ped with a 2.45 GHz – 2 kW SAIREM microwave genera- tor. The base pressure of the reactor is 10 –9 hPa and high purity research grade gases were used: N55 (> 99.9995%) for methane and N90 (> 99.99999990%) for hydrogen (a catalytic purifier is present on the gas line). The substrate holder is insulated from the wall of the CVD reactor to apply a voltage during the BEN step. We use 2 inch silicon (100) substrates (300 µm thick) on a rotating substrate hol- der to improve the uniformity of the process (below 2% on 2 inch) [4]. The MPCVD process is all computer control- led to improve the reproducibility of the initial steps of growth. The nucleation density was calculated using FEG- SEM imaging (HITACHI S-4500) coupled with image analysis. Thickness and roughness were characterized us- ing a UV–Vis Spectroscopic Ellipsometer (Horiba Jobin- Yvon, UVISEL) at a fixed angle of 70°. Gold 450 µm 2 square contacts were evaporated using an electron beam evaporator on the diamond film. Electrical measurements were carried out with a Keithley 6517 to measure the cur- Thick diamond films are known to exhibit remarkably high electrical resistivity and thermal conductivity. However, on thin films, difficulties are often observed to achieve such per- formances. In this study, the synthesis of ultra-thin diamond films was optimized towards the possibility to maintain high dielectric performances on layers compatible with today re- quirements for Silicon-On-Diamond technology, and namely aiming at films with thicknesses equal or below 150 nm. The nucleation of diamond nanocrystals is crucial to obtain films with thickness lower than 100 nm. A Bias Enhanced Nuclea- tion step (BEN) was improved to achieve nucleation densities above 10 11 cm –2 although the process was also tuned to limit the size of the nanocrystals during this step. The control of the carbonization of the silicon substrate is also essential to reach such a density with a high reproducibility. The BEN is followed by a growth step with optimized conditions. The films were characterized by SEM and Spectroscopic Ellip- sometry. Electrical conductivity measurements were con- ducted on thin diamond films and values obtained on layers below 100 nm were as high as 5 × 10 13 Ω cm; a value signifi- cantly higher than the state of the art for such thin films.