in Situ Studies of Nitridation of Cu/Ti Thin Films Using Environmental Cell in Transmission Electron Microscopy
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ABSTRACT Co-deposited Cu/Ti thin films were heated at various temperatures in an ammonia ambient in an environmental cell fitted in to the column of transmission electron microscope (TEM). The reaction dynamics was observed in situ and recorded on video using a TV camera with 1/30 sec. time resolution. The nitridation of titanium accompanied by nucleation and growth of copper particles started at 370'C. Ti 2N formed at lower temperatures while TiN was formed above 400'C. The nucleation of crystals occurred simultaneously (within a minute) throughout the film indicating no effect of electron beam on reaction process. The growth rate of copper particles was observed to vary slightly from one particle to another indicating varying growth rate for different facets.
INTRODUCTION: Transmission electron microscopy (TEM) is one of the most popular ex-situ techniques used to characterize the micro structure of synthesis products which can be related to their properties. In order to understand the reaction mechanism and kinetics involved, a number of samples are obtained under varying thermodynamic and kinetic conditions and are compared at various stages of reaction. It is a tedious process and has some limitations as the sample may change during the process of its removal from the reaction chamber to the microscope. It is very easy to miss the formation of intermediate phases, diffusion conditions etc. We have taken an in-situ approach to study mechanism and kinetics of reactions by TEM using an environmental cell (E-cell) fitted in the microscope column which allows us to control the pressures from a few milli torr to 20 torr while the heating stage allows us to heat the samples up to 1000)C (depending upon the gas pressure). The dynamic observation is the most useful tool not only to pin point the reaction temperatures but also to record the reaction rates. Copper has attracted more and more attention in recent year as an alternate to aluminum in multilevel metallization of very large scale integrated (VSLI) circuits in sub micron level devices. It has lower resistance for electric current and higher resistance to electron migration [1]. But just like Al, Cu reacts with Si at low temperatures to form copper silicide (Cu 3Si). Moreover, arsenic, often used as a dopant, migrates through the metal silicide layer and is lost to the ambient [2,3]. A layer of TiN or CrN is often used to control Cu/Si inter diffusion as well as arsenic migration without degrading the electronic properties significantly. This barrier layer can be formed on Si/Si0 2 surfaces (a) by direct sputtering of the nitride layer, (b) by chemical vapor deposition (CVD) using metal precursor and NH 3, and (c) by nitriding 251 Mat. Res. Soc. Symp. Proc. Vol. 317. ©1994 Materials Research Society
the metal film using either N2 or NH 3. The nitridation of metal layer deposited on Si/Si0 2 wafers is most commonly used in industrial processing because of better quality and low costs. The Cu/Si, Cu/Ti, Ti/N reactions have been studied ex-situ for bulk material
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