Vol. 34, No. 7 Journal of Semiconductors July 2013

Remote scavenging technology using a Ti/TiN capping layer interposedin a metal/high-k gate stack�

Ma Xueli(马雪丽)�, Han Kai(韩锴), and Wang Wenwu(王文武)

Institute of Microelectronics, Chinese Academy of Sciences, Beijing 100029, China

Abstract: High permittivity materials have been required to replace traditional SiO2 as the gate dielectric to extendMoore’s law. However, growth of a thin SiO2-like interfacial layer (IL) is almost unavoidable during the depositionor subsequent high temperature annealing. This limits the scaling benefits of incorporating high-k dielectrics intotransistors. In this work, a promising approach, in which an O-scavenging metal layer and a barrier layer preventingscavenged metal diffusing into the high-k gate dielectric are used to engineer the thickness of the IL, is reported.Using a Ti scavenging layer and TiN barrier layer on a HfO2 dielectric, the effective removal of the IL and almostno Ti diffusing into the HfO2 have been confirmed by high resolution transmission electron microscopy and X-rayphotoelectron spectroscopy.

Key words: scavenging technology; EOT; barrier layer; TEM; XPSDOI: 10.1088/1674-4926/34/7/076001 EEACC: 2550

1. Introduction

For 32 nm technology node and beyond, an equivalent ox-ide thickness (EOT) of < 1 nm in the MOSFET is requiredŒ1�.However, EOT scaling below 1 nm using simple “un-doped”high-k materials is challenging because a low-k interfaciallayer forms between the high-k dielectric and Si substrateduring high-k deposition and subsequent annealing processes,and the overall EOT increases. Several approaches to circum-vent this issue have been reportedŒ2�5�. Scavenging technol-ogy is one of these effective approaches. However, if scaveng-ing metal is deposited on a high-k gate dielectric directlyŒ6�8�,the high-k material will be modified due to the in-diffusion ofscavenging metal, resulting in adverse effects include the fol-lowing: (1) effective work function (EWF) change either bythe inherently low vacuum work function (WF) of scaveng-ing metals or by formation of fixed charges and/or interfacedipoles. (2) Excessive carrier mobility degradation and gateleakage current increase. In this work, remote O-scavengingof the SiO2-like interlayer is realized by using O-scavengingmetal Ti. In addition, TiN was employed as a diffusion barrierlayer to protect the high-k dielectric from degradation causedby in-diffusion of metal Ti due to its excellent thermal-stability.

2. Experiment

Figure 1 is a schematic diagram of this structure. TheTi/TiN stacks are inserted between the top metal and HfO2 tofulfill the O-scavenging and suppress Ti diffusion. HfO2 filmof �3.5 nm thickness was grown by PVD after preparing 1 nmthickness SiO2 on a p-type h100i Si substrate. Then thin TiNlayers were employed as barrier layers. Ti scavenging layerswere deposited on top of the TiN without breaking vacuum.An in-situ TiN film of �20 nm thickness was deposited as the

metal electrode. W of �70 nm thickness was used to cap thereactive TiN metal electrode to prevent its subsequent oxida-tion on exposure to air. Then each wafer was cleaved into twopieces, one was treated with rapid thermal annealing (RTA)process in N2 atmosphere at 600 ıC to promote the scavengingreaction further. Finally, a forming gas anneal (FGA) at 400 ıCfor 30min was performed on all samples. Control samples withTi but without thin TiN and without Ti and thin TiN were alsofabricated for reference. After the device fabrication process,the physical properties of all samples were analyzed by highresolution transmission electronmicroscopy (HRTEM), andX-ray photoelectron spectroscopy (XPS).

Fig. 1. Schematic diagram of the O-scavenging structure.

* Project supported by the Important National Science & Technology Specific Projects, China (No. 2009ZX02035) and the National NaturalScience Foundation of China (Nos. 61176091, 50932001).

† Corresponding author. Email: maxueli@ime.ac.cnReceived 9 November 2012, revised manuscript received 23 January 2013 © 2013 Chinese Institute of Electronics

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Fig. 2. HRTEM image of samples (a) with and (b) without Ti/TiN, before 600 ıC RTA.

Fig. 3. HRTEM image of samples with (a) and without (b) Ti/TiN, after 600 ıC RTA.

3. Results and discussion

3.1. Results

Figure 2 shows the HRTEM image of the as-deposited gatestacks with and without Ti/TiN layers. It can be seen that theSiO2-like interfacial layer (IL) becomes 1.84 nm from an ini-tial 1 nm thickness during the subsequent fabrication processshown in Fig. 2(b) and the IL thickness of the Ti-with sampleis �1.58 nm thinner �0.26 nm stemming from the fact thatTi is capable of scavenging oxygen from IL. Figure 3 showsthe HRTEM image of the gate stacks with and without Ti/TiNlayers, with both annealed at 600 ıC. It can be seen that theSiO2-like IL becomes 2.08 nm from an initial 1 nm thicknessduring the subsequent fabrication process shown in Fig. 3(b)and the IL thickness of the Ti-with sample is �1.26 nm thinner�0.82 nm stemming from the fact that Ti is capable of scav-enging oxygen from IL. The scavenging of SiO2-like IL is fur-ther confirmed by Si2p XPS analysis (Fig. 4), where the SiO2

related peak becomes smaller after the O-scavenging process.It also can be concluded that annealing can further promotethe scavenging and decrease the IL by comparing the results inFigs. 2(a) and 3(a). In addition, the effect of the barrier layerTiN on preventing metal Ti diffusing into high-k material isconfirmed by depth profile analysis using XPS (Figs. 5 and 6).Compared with the gate structure without a TiN barrier layerin Fig. 6, Ti penetration in the gate structure with a TiN layer

Fig. 4. XPS spectra of samples with andwithout a Ti/TiN bi-layer after600 ıC RTA. The open and closed circles represent samples withoutand with a Ti/TiN bi-layer, respectively.

in Fig. 5 is effectively suppressed.

3.2. Discussion

The reaction model is schematically illustrated in Fig. 7.The IL scavenging reaction proceeds in a remote way as theoxygen vacancies in the HfO2 layer act as mediators for Oatom diffusion across the HfO2 layer from IL and dissolving

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Fig. 5. Depth profile of sample with barrier layer TiN.

Fig. 6. Depth profile of sample without barrier layer TiN.

Fig. 7. Schematic of the remote scavenging reaction.

into the scavenging metal Ti layer. Experimental data suggestthat oxygen diffusion through HfO2 is quite rapid and this isnot a rate-limiting step for the scavenging processŒ9�. So a ther-modynamic driving force is needed for the IL decompositionand the incorporation of O atom released from IL into Ti over-

layer. And the latter is decided by the oxygen solubility in Ti.The overall reaction can be written as follows:

B2SiO2 C Ti !

B2Si C TiOB:

Ti has a large positive Gibbs free energy change and thereis an energy gain for forming TiOB rather than maintainingSiO2

Œ10�, which is the thermodynamic driving force.

4. Conclusion

In summary, HRTEM and XPS depth profiling were usedto study the remote oxygen scavenging by a Ti/TiN bi-layeron a HfO2/SiO2/Si structure. After Ti/TiN bi-layer depositionat room temperature, oxygen is released from the SiO2-like ILinto the Ti layer and the IL is reduced by �0.26 nm. Upon an-nealing at 600 ıC in N2 atmosphere, further reduction of theIL of about 0.82 nm occurs. Compared with direct-scavengingtechnology in which scavenging metal is deposited on high-kmaterial directly, remote-scavenging can not only realize O-scavenging due to a Ti layer but also protect a high-k dielectricfrom effectively being destroyed by into-diffusion of scaveng-ing metal Ti due to the TiN barrier layer.

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