High-K materials and metal gates for CMOS applications

Abstract

The scaling of complementary metal oxide semiconductor (CMOS) transistors has led to the silicon dioxide layer used as a gate dielectric becoming so thin that the gate leakage current becomes too large. This led to the replacement of SiO 2 by a physically thicker layer of a higher dielectric constant or 'high-K' oxide such as hafnium oxide. Intensive research was carried out to develop these oxides into high quality electronic materials. In addition, the incorporation of Ge in the CMOS transistor structure has been employed to enable higher carrier mobility and performance. This review covers both scientific and technological issues related to the high-K gate stack-the choice of oxides, their deposition, their structural and metallurgical behaviour, atomic diffusion, interface structure, their electronic structure, band offsets, electronic defects, charge trapping and conduction mechanisms, reliability, mobility degradation and oxygen scavenging to achieve the thinnest oxide thicknesses. The high K oxides were implemented in conjunction with a replacement of polcryslliane Si gate electrodes with metal gates. The strong metallurgical interactions between the gate electrodes and the HfO 2 which resulted an unstable gate threshold voltage resulted in the use of the lower temperature 'gate last' process flow, in addition to the standard 'gate first' approach. Work function control by metal gate electrodes and by oxide dipole layers is discussed. The problems associated with high K oxides on Ge channels are also discussed. Glossary ALD atomic layer deposition CB conduction band CBO conduction band offset CMOS complimentary metal oxide semiconductor CNL charge neutrality level CV capacitance voltage CVD chemical vapour deposition DB dangling bond DOS density of states EA electron affinity ECT equivalent capacitance thickness EOT equivalent oxide thickness ESR electron spin resosnance EWF effective work function FET field effect transistor GGA generalised gradient approximation LDA local density approximation MEIS medium energy ion scattering MIGS metal induced gap states MOCVD metla organic chemical vapour deposition MOS metal oxide semiconductor MOSFET metal oxide semiconductor field effect transistor MOx arbitrary metal oxide NMOS n-type MOS PMOS p-type MOS RCS remote Coulombic scattering RPS remote phonon scattering SBH Schottky barrier height TAT trap assisted tunnelling T inv inversion thickness Vt threshold voltage (of FET) Vfb flat band voltage VB valence band VBO valence band offset WF work function

Intrinsic electron traps in atomic-layer deposited HfO2 insulators

“…These results are in good agreement with those of other theoretical studies 29,30 and, importantly, do not depend significantly on the force-field used. The oxide bandgap width is close to 6 eV, to be compared to the 5.9 eV experimental values for a-HfO 2 31 or a-HfAlO x . 32 Twenty five oxygen vacancies were created in the fully relaxed a-HfO 2 structure by randomly removing O atoms in accordance with the distribution of the number of nearest neighbour Hf ions described above.…”

“…28), the generation of electron-hole pairs within the HfO 2 layer leads to the annihilation of trapped electrons. 22 Since the final charge state of the oxide stack after exposure to photons with h > 5.6 eV is indistinguishable from that of the as-prepared (neutral) oxide The inset illustrates the schematics of electron transitions during the EPDS experiment: photoionization of traps with energy E t (1), electron photoinjection from Si followed by trapping in HfO 2 (2), and electron-hole pair generation in HfO 2 (3). stack, we conclude that there is neither present a detectable density of hole traps nor donor-type gap states in the HfO 2 or Hf 0.8 Al 0.2 O x layers.…”

High-k perovskite gate oxide BaHfO3

“…1 The SiO 2 layer thickness required for gate dielectrics nowadays is so thin (∼1 nm) that the leakage current would exceed 1 A cm 2 , causing unacceptably high power dissipation. High-k dielectrics were introduced to solve this issue and binary oxides such as HfO 2 , Al 2 O 3 , and ZrO 2 were investigated.…”

“…To reduce the threading dislocations and enhance the electrical properties of the channel, an undoped BaSnO 3 buffer layer was grown on SrTiO 3 substrates before the channel layer deposition. The device exhibited a field effect mobility value of 52.7 cm 2 V 1 s 1 , a I on /I off ratio higher than 10 7 , and a subthreshold swing value of 0.80 V dec 1 . We compare the device performances with those of other field effect transistors based on BaSnO 3 …”

Intrinsic and defect related luminescence in double oxide films of Al–Hf–O system under soft X-ray and VUV excitation

“…Dielectrics with high permittivity (the so-called high-k dielectrics) such as hafnium oxide HfO 2 (κ= 12-40), zirconium oxide ZrO 2 (κ= 12-40), aluminum oxide Al 2 O 3 (κ= 10) and some others are replacing traditional dielectrics in silicon devices: silicon oxide SiO 2 (k=3.9) and silicon nitride Si 3 N 4 (κ= 3.9) [1][2][3]. Recently, among all high-k dielectrics hafniabased materials are considered as one of the most promising candidates for gate dielectrics in complementary metal-oxide-semiconductor (CMOS) technology, DRAM (dynamic random access memory) capacitors, and blocking insulators in Si-oxide-nitride-oxide-silicon (SONOS) -type flash memory cells [4,5].…”