Microstructure of Buried Thin Etch Stop Films Formed by Nitrogen Implantation into Silicon
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MICROSTRUCTURE OF BURIED THIN ETCH STOP FILMS FORMED BY NITROGEN IMPLANTATION INTO SILICON A. ROMANO-RODRIGUEZ', A. EL-HASSANI" 3 , A. PEREZ-RODRIGUEZ', J. SAMITIER', J.R. MORANTE', J. ESTEVE2 AND M.C. ACERO 2 'LCMM, Departament de Ffsica Aplicada i Electrbnica, Universitat de Barcelona, Diagonal 645-647, E-08028 Barcelona, Spain 2 3Centro Nacional de Microelectr6nica CNM-CSIC, Campus UAB, E-08193 Bellaterra, Spain permanent address: Dept. of Physics, University Moulay Ismail, PB 4010, Zaitoun, Meknes, Morroco ABSTRACT The microstructure of buried layers obtained by medium to high dose nitrogen ion implantation in silicon for etch-stop applications is investigated as a function of the implantation conditions (dose and temperature). Samples are analyzed by TEM, SIMS, FTIR and XPS measurements. The correlation between the data from the different techniques allows to characterize the different layers in the structure, determining the phases induced during the process, the crystalline nature of the layers and the presence of structural defects. The obtained data show the gettering of nitrogen after annealing in both a buried layer around the implantation peak and in the surface region. The nitrogen concentration in these regions and the formation and nature of silicon nitride precipitates show strong dependences with the implantation dose and temperature. INTRODUCTION Micromachining is the most widely employed technique for the obtention of silicon sensors. This consists in the selective attack of specific regions of the wafers using anisotropic etchants. To stop the etch at a specific depth, so-called "etch-stop" layers are required, which show a high resistance to the etch process compared to the one silicon presents. The most suited method is to selectively dope with boron the area where the etch has to be stopped, as highly boron doped layers (hole concentrations above 7x10'9 cm- 3) are weakly attacked by the usual etchants, KOH, EDP, ... [1]. However for integrated sensors, where the control electronics of the sensors has to be implemented on the same wafer as the sensor, the boron doped layers do not survive the high-temperature treatments required for CMOS processing, because boron strongly diffuses at high temperature, making this approach unsuitable. Other possibility is electrochemical silicon etching [2], although it has the drawback of the need to contact samples for their biasing. It is well known that silicon oxide and silicon nitride are very weakly etched by the above mentioned solutions. Based on these facts SOI substrates have been used for the fabrication of sensors [3]. However the SOI wafers are quite expensive and a cheaper approach should be studied. Furthermore silicon nitride is more efficient as etch-stop than silicon oxide. This explains why some attempts have been made to form etch-stop layers by nitrogen implantation into silicon [4]. In this work medium to high dose nitrogen is implanted into silicon, followed by high Mat. Res. Soc. Symp. Proc. Vol. 308. ©1993 Materials Research Society
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