In situ spectroscopic ellipsometry as a versatile tool to study atomic layer deposition

Transcription

1 In situ spectroscopic ellipsometry as a versatile tool to study atomic layer deposition Erik Langereis Department of Applied Physics e.langereis@tue.nl Ellipsometry Workshop Enschede, 11th February 1 utline 1 / 3 Atomic layer deposition (ALD) - Basics of ALD - Materials deposited by ALD - Applications for ALD In situ spectroscopic ellipsometry as tool to monitor ALD Monitoring the film thickness - ALD growth & growth per cycle - Self-limiting chemistry - Initial film growth & substrate dependence - Nanolaminates Parametrizing the dielectric function - Metallic films: Electrical properties & electron scattering - Ultrahigh-k dielectrics Film composition & microstructure Thin film materials: Al3 Ta5 Er3 Ti TiN TaN Ta3N5 SrTi3 Pt Ru 1

2 Basics of ALD / 3 Atomic layer deposition (ALD) is a low temperature vapor-based deposition technique for ultrathin and conformal film growth with submonolayer growth control use saturative surface reactions to ensure self-limiting film growth Precursor A Precursor B Large variety in ALD materials by finding the appropriate combination of precursors Illustrative example: ALD of Al 3 / 3 Al(CH 3 ) 3 ALD cycle Al film thickness Number of ALD cycles H Submonolayer growth control Film thickness is ruled by the number of cycles chosen: Al ~1 Å growth per cycle Heil et al., J. Appl. Phys. 13, 133 ()

3 Illustrative example: ALD of Al / 3 Al(CH 3 ) 3 ALD cycle Mass spectrometry signal (A) Al(CH 3 ) 3 H H CH Al(CH 3 ) 3 H Al(CH 3 ) 3 H Al(CH 3 ) 3 H Time (s) H Surface chemistry rules ALD process Ligand exchange between Al(CH 3 ) 3 and H surface groups and H and CH 3 surface groups leads to CH formation Heil et al., J. Appl. Phys. 13, 133 () Illustrative example: ALD of Al 5 / 3 Al(CH 3 ) 3 ALD cycle Infrared absorbance x1-5 Al(CH 3 ) 3 chemisorption 9 cm cm -1 H CH x CH x stretching stretching deformation H exposure Wavenumber (cm -1 ) H Surface chemistry rules ALD process Surface alternately covered by H groups and CH 3 groups Langereis et al., Appl. Phys. Lett.. 9, 319 () 3

4 Illustrative example: ALD of Al / 3 Al(CH 3 ) 3 ALD cycle H Growth per cycle (Å) 1... Al(CH 3 ) 3 precursor H oxidant. 1 Precursor dose (ms) Saturative surface reactions Al(CH 3 ) 3 and H lead to self-limiting surface reactions that become independent of precursor flux van Hemmen et al., J. Electrochem. Soc. 15, G15 (7) Illustrative example: ALD of Al 7 / 3 Al(CH 3 ) 3 ALD cycle H Growth per Cycle (Å) CVD+ALD. ALD. Purge after Al(CH 3 ) 3 dose (s) Separated surface reactions Precursor and reactants have to be very well evacuated/separated from the reactor before pulsing the next precursor to avoid parasitic CVD

5 Plasma-assisted ALD / 3 Additional chemical reactivity Range of reactive species from a to provide additional reactivity to the ALD surface chemistry ALD cycle H N H -N xides and Metals Nitrides and Metals Key features of ALD 9 / 3 (1) Ultimate control of film growth and thickness digital thickness control () Relatively low substrate temperatures down to room temperature (3) Extremely high conformality/step coverage self-limiting surface reactions () Good uniformity on large substrates 3 mm and even bigger (5) Multilayer structures and nanolaminates easy to alternate between processes () Large set of materials and processes many different materials demonstrated 5

6 Key features ALD 1 / 3 Thin films synthesized by ALD 11 / H He Li Be B,N,* C N F Ne Na Mg,Te Al,N,* Si,N,* P * S Cl Ar K Ca,S,* Sc Ti,N,* V Cr,* Mn,S,Te Fe Co Ni Cu,S Zn,S,Te,Se Ga,N,* Ge As Se Br Kr Rb Sr,S,* Y,S Zr,N,* Nb,N Mo N Tc Ru Rh Pd Ag Cd S,Se,Te In,N,S,* Sn Sb Te I Xe Cs Ba S La*,S,* Hf,N,* Ta,N W,N,S,* Re s Ir Pt Au Hg Te Tl Pb S Bi Po At Rn Fr Ra Ac** Rf Db Sg Bh Hs Mt Ds Rg Lanthanoids* Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Actinoids** Th Pa U Np Pu Am Cm Bk Cf Es Fm Md No Lr Puurunen, J. Appl. Phys. 97, 1131 (5)

7 ALD applications: Semiconductor industry (= key driver) Metal-oxide-semiconductor field effect transistor (MSFET) (metal) gate 1 / 3 source channel gate oxide drain DRAM deep trench capacitors (aspect ratio up to 7) Thermally grown Si Deposited high-k oxide ALD applications: verview Transistor gates stacks Deep trench DRAM High density capacitors Hard disks Interconnect technology MEMS/Microsystems Corrosion protection Thin film encapsulation Li + -batteries Photonics Photovoltaics Displays Solid-state lighting ptical coatings Nanotechnology TU/e Beneq Philips + 13 / 3 Sandia/ U. Colorado Beneq - Philips 7

8 ALD applications: Ultrathin films conformal growth high material quality 1 / 3 Al /Ta 5 : x-ray mirror Szeghalmi et al., Appl. Phys. Lett. 9, (9) Ti /ZnS:Mn/Ti and Ti : inverse opals King et al., Adv. Mater. 17, 11 (5) Al : encapsulation of organic LED No film a-sin x :H film Langereis et al., Appl. Phys. Lett. 9, 1915 () Groner et al., Appl. Phys. Lett., 5197 () Garcia et al., Appl. Phys. Lett. 9, (). ALD Al film WVTR (g m - day -1 ) PEN Remote ALD nm Al PE-CVD 1 nm Si 3 N PE-CVD 1 nm Si PE-CVD 1 nm Al utline 15 / 3 Atomic layer deposition (ALD) - Basics of ALD - Materials deposited by ALD - Applications for ALD In situ spectroscopic ellipsometry as tool to monitor ALD Monitoring the film thickness - ALD growth & growth per cycle - Self-limiting chemistry - Initial film growth & substrate dependence - Nanolaminates Parametrizing the dielectric function - Metallic films: Electrical properties & electron scattering - Ultrahigh k dielectrics Film composition & microstructure Thin film materials: Al Ta 5 Er Ti TiN TaN Ta 3 N 5 SrTi Pt Ru

9 In situ spectroscopic ellipsometry during ALD 1 / 3 Measurement of the change in polarization of a light beam (at different wavelengths) upon reflection from a surface Film thickness and optical dielectric function extracted by model-based analysis J.A. Woollam Co. M-series: VIS + IR: ev VIS + UV: 1..5 ev ALD materials investigated Metal oxides Metal nitrides - Metals 17 / 3 Material Precursor gas Reducing/ xidizing agent Cycle time (s) Temperature (ºC) Al Al(CH 3 ) 3 H / 1 (5 3) Hf Hf[N(CH 3 )(C H 5 )] 11 9 (3 35) Er Er(thd) (15 3) Ti Ti[CH(CH 3 ) ] 3 (5 3) Ta 5 Ta[N(CH 3 ) ] (1 5) SrTi Star-Ti and Hyper-Sr > 5 (15-35) TiN TiCl H -N (1:1) (1 ) TaN x,x 1 Ta[N(CH 3 ) ] 5 H 37 5 (15 5) Ta 3 N 5 Ta[N(CH 3 ) ] 5 NH (15 5) Pt (CpCH 3 )Pt(CH 3 ) 3 / 17 3 (1-3) Ru CpRu(C) C H 5 / (3-) 9

10 In situ spectroscopic ellipsometry (SE): ptical modeling of metal oxides 1 / Al 1 1 Hf 1 Dielectric functions 1 1 ε 1 ε 1 ε ε Ta ε 1 Ti ε ε ε Photon energy (ev) In situ spectroscopic ellipsometry (SE): ptical modeling of metal nitrides 19 / 3 1 ε 1 TiN 5 1 ε 1 TaN 5 Dielectric functions ε ε ε Ta 3 N ε Photon energy (ev) 1

11 In situ spectroscopic ellipsometry (SE): verview optical model parameters / 3 See Topical Review: Langereis et al., J. Phys. D: Appl. Phys., 731 (9) utline 1 / 3 Atomic layer deposition (ALD) - Basics of ALD - Materials deposited by ALD - Applications for ALD In situ spectroscopic ellipsometry as tool to monitor ALD Monitoring the film thickness - ALD growth & growth per cycle - Self-limiting chemistry - Initial film growth & substrate dependence - Nanolaminates Parametrizing the dielectric function - Metallic films: Electrical properties & electron scattering - Ultrahigh k dielectrics Film composition & microstructure Thin film materials: Al Ta 5 Er Ti TiN TaN Ta 3 N 5 SrTi Pt Ru 11

12 Characterize ALD film growth by in situ SE / 3 Layer-by-layer film growth obtained by alternating ALD half-cycles - Linear growth with sub-monolayer control Metal oxide Al Ta 5 Ti Metal nitride Ta 3 N 5 TaN TiN Growth per cycle 1. Å (1 C). Å (5 C).5 Å ( C) Growth rate.5 Å (1 C). Å (1 C).37 Å ( C) Film thickness (nm) Al Ta 5 Ti Ta 3 N 5 TaN TiN Number of cycles Thickness increases linear with number of cycles slope yields growth per cycle (GPC) Characterize ALD film growth by in situ SE 3 / 3 Layer-by-layer film growth obtained by alternating ALD half-cycles - Linear growth with sub-monolayer control - Self-limiting chemistry essential for uniform and conformal growth Monitor film thickness while changing precursor/reactant dosing time Film thickness (nm) s s 15 s Number of cycles 3 s Growth rate (nm/cycle) single deposition run separate depositions Plasma exposure time (s) single deposition run yields saturation curves ex situ measurements not strictly necessary 1

13 Characterize ALD film growth by in situ SE / 3 Layer-by-layer film growth obtained by alternating ALD half-cycles - Linear growth with sub-monolayer control - Self-limiting chemistry essential for uniform and conformal growth - Establish temperature window for true ALD growth Plasma-assisted ALD using Metal oxide Precursor Al Al(CH 3 ) 3 Ta 5 Ta[N(CH ) 3 ] 5 Ti Ti[CH(CH 3 ) ] Growth per cycle (Å) Al Ta 5 Ti Substrate temperature ( C) Metal oxides can be deposited over a wide temperature range down to room temperature Characterize ALD film growth by in situ SE 5 / 3 Layer-by-layer film growth obtained by alternating ALD half-cycles - Linear growth with sub-monolayer control - Self-limiting chemistry essential for uniform and conformal growth - Establish temperature window for true ALD growth - Insight into nucleation relevant for nanometer thick films targeted Film thickness (nm) 1 TaN on HF-last Si TiN on Si (1 o C) Number of cycles Thickness (nm) 1 1 Plasma ALD Pt Thermal ALD Pt Number of cycles Knoops et al., Electrochem. Solid-State Lett. 1, G3 (9) Nucleation behavior can be investigated and film thickness can be controlled accurately 13

14 Characterize ALD with sub-monolayer sensitivity / 3 Er - Al nanolaminates Er-doped Al films Er Al Er Alternating Al and Er [Al precursor ] x [Er precursor ] y 3 Å of Al (3 cycles) Small precursor. Å of Er ( cycles) Bulky precursor Al[(CH 3 )] 3 utline 7 / 3 Atomic layer deposition (ALD) - Basics of ALD - Materials deposited by ALD - Applications for ALD In situ spectroscopic ellipsometry as tool to monitor ALD Monitoring the film thickness - ALD growth & growth per cycle - Self-limiting chemistry - Initial film growth & substrate dependence - Nanolaminates Parametrizing the dielectric function - Metallic films: Electrical properties & electron scattering - Ultrahigh k dielectrics Film composition & microstructure Thin film materials: Al Ta 5 Er Ti TiN TaN Ta 3 N 5 SrTi Pt Ru 1

15 ALD for high-density capacitors Specifications for DRAM and decoupling capacitors: High capacity density: > 5 nf/mm Dielectric thickness: ~1-5 nm High step coverage: > 95% for AR -3 Electrode thickness: 5-3 nm Electrode conductivity: < 3 µωcm Low leakage current (A/cm ) Specs: 1 1 V (DRAM) 1 3 V (Decap) Low thermal budget Specs: - C A C = εk d / 3 High-density trench capacitors Conformality requirements (too) demanding for CVD and PVD flux determines growth! Metal Insulator Metal ALD essential for conformal growth of - metal electrodes: TiN, TaN, Ru, Pt, - Ultrahigh k dielectrics: Al, Hf, SrTi Metallic films dielectric function 9 / 3 Metallic films show Drude absorption in NIR by conduction electrons Drude term scales with film resistivity Pt > TiN > TaN 1 ε Dielectric function ε Pt TiN TaN Photon energy (ev) ω n pu f jω j = nk = ε + ω iγ Dω j= 1 j + ω ω iγ ω j 15

16 Metallic films resistivity & electron MFP in TiN 3 / 3 Resistivity increases with Cl-content Resistivity (µωcm) in situ SE ex situ FPP Cl content (at.%) Resistivity: Γ ρ = ε 1 D ω pu Langereis et al., J. Appl. Phys. 1, 353 () Metallic films resistivity & electron MFP in TiN 31 / 3 Resistivity increases with Cl-content Resistivity (µωcm) in situ SE ex situ FPP Cl content (at.%) Especially electron mean-free-path is affected, electron density remains constant Electron MFP (nm) Deposition temperature ( C) Electron density (1 cm -3 ) Resistivity: Γ ρ = ε 1 D ω pu Electron mean free path: 1/ 3 3π ε ω MFP = ħ * ( m e) Γ / 3 pu D Electron density: N e * m ε = ω e pu Langereis et al., J. Appl. Phys. 1, 353 () 1

17 Finite size effects in ultrathin metallic films 3 / 3 Change in electrical properties with TiN film thickness Resistivity (µωcm) Film thickness (nm) Electron MFP (nm) Film thickness (nm) Electron density (1 cm -3 ) With decreasing film thickness: Increase in resistivity mainly attributed to decrease in electron MFP additional electron-sidewall scattering in ultrathin films Distinction of TaN x phases in dielectric function 33 / 3 Dielectric function ε 3 1 Plasma condition Composition 1 1) T d 3 s H TaN. ) T d 1 s H TaN.9 3) T d 5 s H -N (9:1) TaN 1.9 ) T d 1 s H -N (1:1) Ta 3 N.7 5) T d 1 s NH 3 Ta 3 N Photon energy (ev) Insulating Resistive Metallic Ta 3 N 5 Ta N 5 Ta 5 N Hexagonal TaN Ta N Cubic TaN Stampfl et al., Phys. Rev. B 71, 111 (5) Langereis et al., J. Appl. Phys. 1, 353 () 17

18 Ultrahigh k dielectrics: ALD of SrTi 3 / 3 Composition control to optimize film properties s ALD SrTi = x cycles ALD Ti + y cycles ALD Sr Star Ti: [Me 5 Cp]Ti(Me) 3 = [(CH 3 ) 5 C 5 ]Ti(CH 3 ) 3 Hyper Sr [ i Pr 3 Cp] Sr = [(C 3 H 7 ) 3 (C 5 H )] Sr Crystalline films yield ultrahigh k values s Post-deposition anneal required for crystallization ALD of SrTi - composition control 35 / 3 Cycle ratio Sr : Ti :1 1: :3 1:1 :1 3:1 1: [Sr]/[Ti] Ratio Refractive index (at 1.9 ev) Change in ST composition probed by dielectric function Transition in dielectric function with Sr-content in SrTi Low absorption related to amorphous structure of films 1

19 ALD of SrTi - crystallization 3 / 3 Post-deposition anneal of 3 nm thick SrTi films ([Sr]/[Ti] = 1.3) Film crystallization probed by in situ spectroscopic ellipsometry Changing microstructure probed by dielectric function for T RTA 5 C Planar capacitors: k > and leakage < 1-7 A/cm at 1 V Summary 37 / 3 Atomic layer deposition is an enabling technology for ultrathin film growth: sub-monolayer growth control with excellent conformality variety of materials at low deposition temperatures In situ spectroscopic ellipsometry during ALD provides many merits: By monitoring the film thickness as a function of the number of cycles the growth rate per cycle can be calculated during the ALD process ALD saturation curves can be obtained in a single deposition run Nucleation behavior can be studied on various substrates Sub-monolayer level surface changes can be probed ptical film properties can be obtained from the dispersion relation Electrical properties can be calculated from the Drude absorption in metallic films Insight into crystalline phase and composition of the films can be derived from the shape of the energy dispersion 19

20 Acknowledgments 3 / 3 PhD thesis Plasma & Materials Processing (PMP) Dr. Stephan Heil Hans van Hemmen Harm Knoops Adrie Mackus Jeroen Keijmel Menno Bouman Dr. Erwin Kessels Prof. Dr. Richard van de Sanden Dutch technology foundation Plasma-assisted assisted atomic layer deposition an in situ diagnostic study Erik Langereis Digital copy available at TU/e library (