TUNGSTEN AS INTERCONNECT MATERIAL

In this chapter we will focus on contact and via fill using the blanket tungsten approach. In chapter 5 we will discuss another application of blanket tungsten, namely, that of tungsten as the interconnect material. 

Tungsten. In ULSI type of circuits there is a trend to more often use tungsten as the interconnect material. See also chapter V. 

There are at least four points of concern when using tungsten as the interconnect material. These are stress, roughness, resistivity, and etchability of the film. In the sections below we will discuss each of these problem areas. 

Oxidation Behavior Tungsten is susceptible to oxidation by oxygen at temperatures above 300°C. The reaction is not self-limiting and is enhanced because its forms W03 which is volatile at higher temperatures. Therefore, when tungsten is used as an interconnect material, precaution is needed to prevent oxidation especially during subsequent dielectric depositions. 

The interconnect section of the IC has similarly seen continuous innovation in feature scaling as well as in materials. Pure aluminum single-layer interconnect was replaced by Cu-doped aluminum multilayer interconnect with tungsten vias, followed by the widespread implementation of damascene Cu metallization in the 1990s. A useful figure of merit for the speed of the interconnect is 1/(RC), where R is the resistance of the interconnect line and C is the capacitance associated with the interconnect. The introduction of Cu metallization improved interconnect speed, since the lower resistivity of copper... 

Interconnect. Three-dimensional structures require interconnections between the various levels. This is achieved by small, high aspect-ratio holes that provide electrical contact. These holes include the contact fills which connect the semiconductor silicon area of the device to the first-level metal, and the via holes which connect the first level metal to the second and subsequent metal levels (see Fig. 13.1). The interconnect presents a major fabrication challenge since these high-aspect holes, which may be as small as 0.25 im across, must be completely filled with a diffusion barrier material (such as CVD titanium nitride) and a conductor metal such as CVD tungsten. The ability to fill the interconnects is a major factor in selecting a thin-film deposition process. 

In reflectometry, the light passes through the films to be measured. Beneath the transparent films, there must be an opaque substrate through which light does not pass. The substrate characteristics must be modeled correctly to calculate the thicknesses of the films above. In silicon processing, theoretically, any of the commonly used metal materials, such as the titanium nitride (TiN), aluminum (Al), and tungsten (W), can be used as substrates. However, in reality, whereas a PMD oxide can be measured on the polysilicon material used in poly interconnections, an ILD oxide can not be measured directly on TiN, because the TiN layer used is too thin to be opaque. TiN is semitransparent if its thickness is less than 1000 A. A thin... 

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