Surface Selective Atomic Layer Deposition of Hafnium Oxide for Copper Diffusion Barrier Application Using Tetrakis(diethylamino)Hafnium and Ethanol

Atomic layer deposition (ALD) is a thin film deposition technique, in which substrate surface is exposed to reactants in cyclic manner separated by discrete purging steps. Thus one atomic layer of material is deposited per ALD cycle. This cycle is repeated until desired thickness is achieved. The bottom up nature of this deposition process facilitates excellent conformality, uniformity and thickness and composition tunability.

Patterning of ALD films for various device applications is commonly performed by removing the specific part of the film or by selectively depositing films on specific areas of the surface. In the former method, patterning is done with the use of extra processing steps such as etching, lithography etc. The latter method involves the use of masking layers to prevent ALD on specific areas.^{1, 2} Maskless ALD, i.e., surface selective atomic layer deposition of thin films has recently garnered considerable interest due to its simplicity. This method of patterning ALD films is unique in that no surface mask is required. Materials nucleate on different surfaces to different extents. This difference in nucleation time is used to achieve selective atomic layer deposition (SALD). In our earlier studies on SALD of HfO₂ and TiO₂, we used water as the oxygen source.^{3, 4}

In this study, ALD of HfO₂ was carried out using tetrakis(diethylamino)hafnium and a primary alcohol (ethanol) in a custom built ALD reactor (Fig. 1). Unlike common ALD processes, here we used ethanol as the oxygen source. Ethanol reacts with the hydroxyl (-OH) groups on silicon surface and forms alkoxides that serve as a starting surface for the ALD film growth. Thus the film growth is obtained on the Si surface. Figure 2 shows the thickness tunability of the HfO₂ ALD process on Si surface using ethanol as the oxygen source. HfO₂ was found to grow at a rate of 0.05 nm/cycle, without any growth inhibition on the silicon surface. Linear dependence indicates self-limited growth of HfO₂ thin film, typical to the ALD process. On the other hand, significant growth delay was observed on copper surface. The use of ethanol reduces surface copper oxide into metallic copper and reduces the number of potential ALD nucleation sites. Thus selective deposition of HfO₂ was achieved on silicon surface over copper surface. Such selectivity depends on many ALD processing parameters and substrate pretreatment conditions. In this report, the effect of these parameters on the extent of selectivity will be discussed in detail along with copper diffusion barrier characteristics of such selectively deposited ultrathin (1-2 nm-thick) HfO₂ layers. The growth rate on silicon side was measured using spectral ellipsometry. X-ray photoelectron spectroscopy was used to probe the presence of HfO_­2 on copper surface. Results of this study can be used to selectively deposit ultra-thin layers of HfO₂ on silicon surface over copper surface for copper diffusion barrier applications.  

 This work was supported in part by the National Science Foundation (CBET 1067424 and EEC 1062943). We also thank Dr. Gregory Jursich for many helpful discussions.

References

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3. Q. Tao, G. Jursich and C. Takoudis, Appl. Phys. Lett., 96, 192105 (2010).

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