Tungsten film stress

The stress of CVD-W films can vary, depending on the deposition conditions [Joshi et al.51, Clark et al.52, Blumenthal et al.148, Sivaram et al.149), by one order of magnitude (ie. from 3xl09 to 13xl09 dyne/cm2) and is mostly tensile. Experience has shown that for a plug process the stress is seldom a problem since the majority of the film is removed during the etch back process. Loss of adhesion is usually not observed in the blanket-plug process. When the interfaces between the different films are clean the adhesion will be formed by chemical bonds (1-2 eV) instead of (weak) physical forces (ca. 0.2 eV).To remove a film with an adhesion of 1 Ev per 

For an interconnect application the situation will be different than for a plug application. Two problems can occur  

---

The determination of tungsten film stress can be done in several ways. A convenient method is to determine the bow of the wafer and use the equation of Stoney ... 

Figure 5.1 Bow of a wafer on which a one micron thick tungsten film is deposited with a tensile stress of lOxlO9 dyne/cm2. A 6" wafer can have a bow of approximately 50 /Zm.

Tungsten peel Tungsten does not adhere to quartz and even thin films tend to peel leading to unacceptable particle counts. Although this could potentially be solved by first depositing an adhesion layer like polysilicon, the high stress values of thick tungsten films will soon cause... 

Figure 12.17 Stress versus working gas pressure for 500-nm-thick tungsten films sputtered in argon and xenon. (Reprinted from Mamor etal., 1995, with permission from Elsevier.)...

If a single metal film is deposited on an oxide, the sheet resistance measurement results can by easily interpreted and converted to the thickness. In practice, however, this is not usually the case. For example, in W CVD, the tungsten is not directly deposited on oxide due to high residual stress and unreliable adhesion. A titanium (Ti) layer must be first deposited as a glue layer. In addition, to prevent the fluorine in the CVD-precursor WFg from directly reacting with Ti (a strong catalytic reaction will occur), a barrier layer of titanium nitride (TiN) must be deposited on top of the Ti. As a result, we have a trilayer film of W on TiN on Ti on oxide, as shown schematically in Fig. 21. This poses some problems in accuracy in the four-point probe measurements. Based on the resistivities in Table VI, the... 

Also adhesion between the tip and sample can cause deformation of the sample. Several theories have been developed to include the effect of adhesive forces. In the JKR theory adhesion forces outside the contact area are neglected and elastic stresses at the contact line are infinite [80]. Even under zero load, the adhesion force results in a finite contact radius a=(9jtR2y/2 E)1/3 as obtained from Eq. 7 for F=0. For example, for a tip radius R=10 nm, E=lGPa, typical surface energy for polymers y=25 mN/m, and typical SFM load F=1 nN, the contact radius will be about a=9.5 nm and 8=9 nm, while under zero load the contact radius and the deformation become a=4.5 nm and 8=2 nm, respectively. The experiment shows that under zero load the contact radius for a 10 nm tungsten tip and an organic film in air is 2.4 nm [240]. The contact radius caused only by adhesion is almost five times larger than the Hertzian diameter calculated above. It means, that even at very small forces the surface deformation as well as the lateral resolution is determined by adhesion between the tip and sample. 

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. 

Hoffman, D. W. and Thornton, J. A. (1977), The compressive stress transition in aluminum, vanadium, zirconium, niobium and tungsten methyl films sputtered at low working pressure. Thin Solid Films 45, 387-396. 

Limiting the film thickness is generally the most easily accomplished approach. As a rule of thumb, the thickness of a high modulus material such as chromium or tungsten should be limited to less than 500 A to avoid excessive residual stress. If the film thickness is to exceed that value, some technique for stress monitoring and control should be developed. 

---