ALD of Scandium Oxide from Tris(N,N'-Diisopropylacetamidinato

ALD of Scandium Oxide from Tris(N,N’-diisopropylacetamidinato)Scandium and Water

Philippe P. de Rouffignac, Roy G. Gordon Dept. of Chemistry and Chemical Biology, Harvard University, Cambridge, MA [email protected] (617) 495-4017

Scandium oxide is considered to be a high-k dielectric with good leakage properties. In combination with rare earth elements like dysprosium, gadolinium, and lanthanum, scandium produces ternary metal scandates that exhibit promising characteristics. These oxides have dielectric constants of ~22, have large bandgaps and high conduction band offsets with respect to silicon, remain amorphous to high temperatures, and are thermodynamically stable. Using ALD to deposit films for advanced MOSFET and DRAM applications is becoming more prevalent, and as such new ALD precursors and deposition methods are needed to have access to many different materials like the rare-earth scandates. In the first step towards synthesizing scandates it is necessary to have a fully developed scandium oxide ALD process.

Scandium oxide thin films were deposited using ALD from a new scandium i precursor Sc( Pr2amd)3 and water. The precursor has sufficiently high volatility (0.04 torr at 100°C) and reacts with water to produce Sc2O3 with a minimal amount of carbon or nitrogen incorporation in the thin films. The growth rate of the film saturates at 0.7 Å/cycle and the ALD temperature window extends from 225°C to 300°C. The thickness is linear with the number of cycles and has an inhibition of 8-10 cycles on HF last silicon. The composition of the films was determined by Rutherford backscattering (RBS), and the crystallinity was determined by low-angle x-ray diffraction (XRD). The films were amorphous as-deposited, but were polycrystalline upon annealing at 600°C. Other characterization methods used include atomic force microscopy, spectroscopic ellipsometry, and cross-sectional high resolution transmission electron microscopy. Electrical measurements were made using either sputtered Pt contacts or in-situ ALD WN to form MIS and MIM capacitor structures. Capacitance-voltage and current-voltage curves were measured for films with various thicknesses. The dielectric constant was determined to be 13 with a leakage current density of less than 1 x 10-8 A/cm2 for a film with an EOT of 2.0nm. ALD of Scandium Oxide from tris(NN’N,N -diisopropylacetamidinato) scandium and Water

Philippe de Rouffignac [email protected] Andrew Yousef Gordon Group Harvard University Kevin Kim Department of Chemistry and Chemical Biology Roy Gordon Applications and Characteristics of Sc2O3 Scandi um Ox ide Thin Films: • Polycrystalline above 400ºC • Stable in direct contact with silicon • Relatively high dielectric constant 14-17 • Refractive index of 1.8-1.9 • Optically transparent in the visible

Applications: •multilayer antireflection and protective coatings •light-emitting diodes •thin film interference polarizers •laser optical coatings

ALD Sc2O3 can be used for ternary rare-earth metal oxides such as GdScO3, DyScO3, and LaScO3

Gordon Group Excellent properties for use as a gate dielectric or Harvard University Department of Chemistry as the insulator in MIM capacitors. and Chemical Biology Precursor Characteristics

Crystal structure if we get it in time…

N N N N LaSc • Precursor: scandium diisopropylacetamidinate N N Sc(amd)3 • Sublimation: 50mtorr at 125C

Gordon Group • Stability in air > 5min5 min Harvard University Department of Chemistry • Yield of purified precursor = 71% and Chemical Biology • Residue from TG = 1.2% Sc(amd)3 and H2O Saturation Curves

LtiitLow reactivity

0.7 % of Sc(amd)3 ends up in film

Growth Temperature of 290ºC HF last (100) Silicon substrates (At dep. Temp. 290oC, dep. Pres. 0.3 torr)

Gordon Group Sc(()amd)3 N2 ppgurge H2O N2 ppgurge Harvard University o Department of Chemistry [150 C] [R.T.] and Chemical Biology Decomposition is 3% at 360ºC

No decomposition below 330ºC

RtiittfReactivity to surface i ncreases with increasing deposition temperature

Linear growth rate of 0.28 Å/cycle

No significant inhibition on HF-last Si

Gordon Group Harvard University Department of Chemistry and Chemical Biology Stoichiometry from RBS 0.5 1.0 1.5 1 2 Energy (MeV) 0.4 0.5 0.6 0.7 6 RBS simulation gives 10 5 stoichiometry of 4 Sc2O3.1 8 3 carbon <1%

2 Normalized Yield Yield dd 6 1

0 200 250 300 350 400 Channel rmalize oo 4 NN

2 Film T= 30nm 0 0 200 400 600 800 1000 Gordon Group Harvard University Ch a nnel Department of Chemistry and Chemical Biology No elements other than Sc and O detected in film. Surface and film morphology

1.5nm As-Deposited 290°C Cubic polycrystalline RMS = 2.3Å (222) (400)

U.) RTA 1m in 10000C

450°C H2/N2 0 anneal(4l (4m in ) Intensity (A. RTA 4min 450 C RMS = 3.3Å As-Deposited 2900C

15 20 25 30 35 40 45 50 2I

1000°C N2 anneal (1min) Crystallization increases RMS = 3.6Å with anneal temperature

Roughness increases with Vertical scale of anneal temperature Gordon Group AFM images is 1.5nm Harvard University Department of Chemistry Roughness < 4 Å and Chemical Biology after 1000°C vacuum anneal Capillary Tube Penetration: ATA Test tf for ALD ALDB Beh av ior

1130ȝm penetration of Sc2O3 film in a 20 ȝm diameter fused silica tube

Gordon Group Harvard University Yields an aspect ratio of up to 57 Department of Chemistry and Chemical Biology Electrical Properties of a thick Sc2O3 Film

• Bubbler temp. = 152°C • 290°C Deppposition Temp • Thickness = 29nm • Avg. EOT (annealed) = 6. 67nm • Avg. K = 16.9 • Avg. Breakdown Field (annealed) = 3. 50 MV/cm

Gordon Group Harvard University Department of Chemistry and Chemical Biology Electrical Properties of an ultrathin Sc2O3 film

•EOT = 1.80 nm •J = 7.63 x 10-6 A/cm2 at 1V •Small frequency dispersion •Ideal CV shape •Low dissipation and no hysteresis after anneal •Vfb has a positive shift of 300mV from ideal

RTA 5min at Gordon Group 450ºC Pt/5.5 nm Sc2O3/Si (100) n-type Harvard University Department of Chemistry and Chemical Biology EOT vs. Thickness As-Deposited at 290C and Annealed in forming gas at 450C

7 As-Deposited As-Deposited Y= 0.23X + 0.43 6 R= 0.9985 K =169= 16.9 ± 030.3 IL = 0.43 ± .05 nm 5 )

mm 4 Annealed 3 K = 18.7 ± 0.5 EOT (n 2 Annealed IL = 0690.69 ± .08 nm Y= 0.208X + 0.69 1 R= 0.997 Annealing increases dielectric 0 constant and increases 0 5 10 15 20 25 30 extrapolated interfacial layer. Thickness (nm)

Gordon Group Harvard University Die lec tr ic Cons tan t (from s lope ) Department of Chemistry Intercept provides interfacial oxide thickness and Chemical Biology J vs. EOT

for annealed Sc2O3 10

0.1 • Leakage is lower

0.01 than equivalent SiO2 films

) 1E-3 2

1E-4 • Not as low as many other high-K (A/cm 1E- 5

J oxides 1E-6 Sc O 1E-7 2 3 SiO 2 1E-8 1.0 1.5 2.0 2.5 3.0 3.5 Gordon Group EOT (nm) Harvard University Department of Chemistry and Chemical Biology Summary

Pure, self-limited Sc2O3.1 thin films were grown from a new precursor i Sc( Pr2-Me-amd)3

Electrical properties are acceptable, but not ideal.

Low growth rate.

Large ALD temperature window <275º-350º with no decomposition.

NihibiiNo inhibition on HF-last s ilicon sur faces.

Use of ALD Sc2O3 to deposit more complex ternary oxides and for specia lty coa tings app lica tions s hou ld be feas ible w ith this type o f precursor.

Gordon Group Harvard University Department of Chemistry and Chemical Biology Acknowledgements

PfProfessor R oy G Gdordon

Andrew Yousef Kevin Kim Damon Farmer for HRTEM

National Science Foundation

Gordon Group Harvard University Department of Chemistry and Chemical Biology 2.REG 3 x 1 0 Sp ectrum Index [vb1 , 0]

16 egionegion 2 2 RRRR

Name Pos. Ar ea %Area 14 Region 2 532.36 18388.9 56.996 Region 2 534.25 13854.3 43.004

12 SS

CP 10

540 538 536 534 5322.REG 530 528 526 524 522 3 Binding Energy (eV) x 1 0 24 Sp ectrum Index [vb1 , 0] Name Pos. Ar ea %Area

22 2 2 Region Region Region 2 532.72 27874.3 61.824 Region 2 534.46 17189.2 38.176

16 SS 14 CP

8 Gordon Group Harvard6 University Department of Chemistry 540 538 536 534 532 530 528 526 524 522 and Chemical Biology Binding Energy (eV)