Ultra Thin Sacrificial Diffusion Barriers - Control of Diffusion Across the Cu-SiO 2 Interface
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ABSTRACT In a copper-SiO 2 dielectric multilevel interconnection scheme a diffusion barrier is needed between the metal and the dielectric to prevent the diffusion of Cu into SiO 2 when subjected to thermal treatments and high electric bias. Ultra thin layers (50-100A) of Al and Ti are being investigated for use as the required diffusion barrier as well as an adhesion promoter between Cu and SiO 2 The results of our investigations, using C-V, resistance, and XPS measurements, will be presented and discussed. It will be shown that both Al and Ti reduce the SiO 2 surface leading to a metal, oxygen, and Si bonded layer which acts as diffusion barrier under applied electrical bias of 1.5 MV/cm at temperatures as high as 250-300°C. Upper layers of Al and Ti are consumed by reaction with Cu. Such reactions increase the electrical resistivity and corrosion resistance of Cu (especially with Al). The results and the applicability of such barriers in practical cases will be presented and discussed. INTRODUCTION In the effort to develop copper as an interconnect material for integrated circuits several issues must be addressed. One of these is copper's ability to diffuse through common dielectric and passivation materials, especially under applied bias'. There are many possiblt.materials that could act as a diffusion barrier, but many would greatly increase processing complexity, and also they can increase the metallization resistivity thus negating the advantage of using copper. Aluminum and titanium have some useful properties that make them promising candidates for this application. They are both highly reactive metals that form self passivating oxide layers in an oxidizing environment. This self passivation indicates that these oxide layers limit diffusion of reactants. Another important characteristic of these oxides is that they both have very high heats of formation, more so than that of SiO 22. This means that when deposited on SiO 2 they reduce it forming their own oxides. Any remaining metal will likely be dissolved into the thick copper layer 3. This paper investigates the effectiveness of aluminum and titanium, used in very thin layers on SiO 2 prior to copper deposition, as electrically effective but sacrificial diffusion barriers. It is shown that both are suitable for such applications, although titanium is known to substantially increase the resistivity of copper on alloying. PROCEDURE In order to analyze the diffusion barrier behavior of aluminum and titanium, we biastemperature (BT) stressed MOS capacitors using various metallization schemes. 1000 to 2000 A of dry thermal oxide was grown on three inch 100 p-type silicon wafers. On top of this was 319 Mat. Res. Soc. Symp. Proc. Vol. 318. @1994 Materials Research Society
sputtered 5000 A copper on 100 A aluminum or titanium. The metals were typically sputtered
in 5 millitorr argon at 2500 watts of power. Deposition rates were 400 A per minute for aluminum and titanium, and 800 A per minute for copper. Films of plain copper and also aluminum were deposited f
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