novel dielectrics for advanced semiconductor devices

1. novel dielectrics for advanced semiconductor devices Cristiano Krug and Gerry Lucovsky Department of Physics North Carolina State University

2. outline band edge states - nanocrystalline HfO2 and ZrO2theory and experiment inherent limitations engineering solutions band edge states - non-crystalline Zr and Hf silicate alloys theory and experiment inherent limitations engineering solutions novel device structure experimental result proposed device structures research plan

3. band edge states nanocrystalline HfO2 and ZrO2 theory and experiment inherent limitations engineering solutions

4. theory -- crystal field and Jahn-Teller term-splittings model calculation - ZrO2 band edge d-states T2g J-T - orthorhombic monoclinic C-F cubic Eg two issues can XAS detect mixtures of tetragonal and monoclinic nano-crystallites? and can mixtures account for range of defect state energies in electrical measurements ?

5. Eg (2) T2g (3) Eg (2) T2g (4+) nano-crystallite grains - different for different processing Stefan Zollner’s results at Freescale - XRD and SE multiple features in T2g region are indicative of mixture of monoclinic and tetragonal by XRD as-deposited MO-RPECVD films by IR are monoclinic similar result for ZrO2

6. theory -- crystal field and Jahn-Teller term-splittings model calculation - ZrO2 band edge d-states T2g J-T - orthorhombic monoclinic C-F cubic Eg can XAS detect mixtures of tetragonal and monoclinic nano-crystallites? YES model predicts at least 4 features in T2g band observed for reactive evaporation, but not for MO-RPECVD

7. p-bonded d*-states/defects at conduction band edge in absorption constant (e2) andconductivity (PC) onset of strong optical absorption - lowest Eg state - optical band gap optical band gap model calculations indicate band edge defect state is associated with a Jahn-Teller distortion at internal grain boundary and is intrinsic to nano-crystalline thin films

8. tunneling but not F-P x'port trapping/Frenkel-Poole transport localized band edge J-T d*-statesinherent asymmetry in transport and trapping including BTI’s trap depth 0.5-0.8 eV, same state PC and band edge abs. Z . Yu et al., APL 80, 1975 (2002), in Chap 3.4 - High-K dielectrics, M. Houssa (ed), IOP, 2004.

9. T2g T2g J-T - orthorhombic monoclinic Eg Eg crystal field and Jahn-Teller term-splittings model calculation using Zr and O atomic states can mixtures account for range of defect state energies in electrical measurements ? YES C-F cubic 3x energy scale ~ 0.5 - 0.8 eV

10. engineering solutions NEC solution limit applied bias so that injection into band edge defect states is not possible modify band tail states by alloying with divalent (MgO) or trivalent oxides (Y2O3) e.g. Y2O3 in cubic zirconia introduces vacancies random distribution gives cubic structure and eliminates J-T term splittings, but evidence for absorption associated with excitations to/from midgap state issue: is this state electrically active ? study has just been undertaken

11. VUV spectroscopic ellipsometry and UV-VIS transmission term-spitting removed - but new absorption band at ~4.1 eV ZrO2-9.5%Y2O3 cubic zirconia 4.1 eV sub-band-gap absorption -O vacancies Jahn-Teller term-split d-states of nc-ZrO2not in Y-Zr-O, but edge broadened

12. outline band edge states non-crystalline Zr and Hf silicate alloys theory and experiment inherent limitations engineering solutions

13. IR results - GB Rayner - PhD thesis, NCSU Si-O-1 group shoulder ~ 950 cm-1 grows with increasing x in as-films deposited changes continuously with annealing in inert ambient, Ar SiO2 features at 1068, 810 and 450 cm-1 sharpen up with increasing Tann chemical phase separation “non-crystalline” by XRD, but, x=0.23 nano-crystalline by TEM and EXAFS

14. comparison of extended x-ray absorption fine structure and x-ray diffraction crystallite size difference for x ~ 0.25 and x ~ 0.5 from HRTEM x~0.25, ~3 nm x~0.5, ~10 nm

15. chemical phase separation (CPS) in Zr silicate and ZrSiON alloys after 900°C annealing dE~0.5 eV n-c CPS doubly degenerate Eg feature in non-crystalline Zr silicate alloys independent of alloy composition after 900°C anneal, chemical phase separation and crystallization Eg narrowed/shifted 0.5 eV in Zr silicate, asymmetric in ZrSiON

16. statistical/mean field disruption of SiO2 network 1:1 representation of silicate alloys i) metal ions, Na1+, Ca2+, Y3+, Zr4+, etc.. disrupt network converting bridging Si-O-Si to terminal Si-O1- group ii) number of terminal groups valence of metal ion, 1 for Na, 2 for Ca, 3 for Y and 4 for Zr iii) connectedness of network defined by shared corners, Cs between SiO4/2 units iv) Cs = 4 perfect 3 D network, Cs= 1,0 completely disrupted mixture of SinOm molecular ions and metal ions xo cs cs = 0 rate of network disruption increases with valence of metal ion when normalized on a per/atom basis for x > xo for Cs = 0, silicate is “inverted” and SinOm are minority species

17. pseudo-ternary (SiO2)1-x-y(Si3N4)y(ZrO2)x alloysremote plasma enhanced chemical vapor deposition (SiO2)0.4(Si3N4)0.25(ZrO2)0.35 as-deposited amorphous alloy – significant Si oxynitride bonding after anneal at 1000°C – chemical phase separation into SiO2, nanocrystalline ZrO2 with N-bonding

18. pseudo-ternary (SiO2)1-x-y(Si3N4)y(ZrO2)x alloysremote plasma enhanced chemical vapor deposition (SiO2)0.3(Si3N4)0.4(ZrO2)0.3 as-deposited amorphous alloy – significant Si oxynitride bonding after anneal at 1000°C – no chemical phase separation and self-organization encapsulating ZrSiO4 bonding groups viable engineering solution, k~9-10, EOT to 0.7-0.8 nm

19. Ge – direct deposition of SiO2 with & without pre-oxidation, 0.5-0.6 nm same as RPAO step for GaN ~Vfb -Qf Dit 0.4 W-cm n-type - Al pre-oxidation of Ge leads to an increase in Dit, but a decrease in negative fixed charge – next step interface nitridation! novel device structures (one example) experimental results for Ge-SiO2 no preoxidation C-V is as good as the best discussed by Saraswat of Stanford Univ. at Workshop on Future Electronics 2005 two approaches i) 15 oxidation followed by plasma nitridation ii) grow 3-5 atomic layers of pseudo-morphic Si on Ge and oxidize surgically to prevent Ge-O bond formation use on-line AES this worked in mid-late 80's, but was not followed-up

20. research plans device testing - ZrO2-Y2O3 and atomically engineered ZrSiON alloys nitrided Ge interfaces - two approaches nano-scale vertical p-n junctions (~25 nm diameter!) a precursor to vertical MOS devices (SRC patent application in process)