Regulating Solid State Diffusion in Semiconductor Processing

Transcription

1 Regulating Solid State Diffusion in Semiconductor Processing Edmund G. Seebauer Department of Chemical Engineering University of Illinois at Urbana-Champaign Support: NSF, DOE, Chartered Semiconductor 1

2 MOSFET Schematic Source Gate Drain Poly-Si Interconnect Glass Channel SiO 2 Heavily-doped Si Silicon substrate Silicide 2

3 International Technology Roadmap for Semiconductors! Feature size decreases 30% per 3 yrs! Cost/function decreases 50% per 3 yrs 3

4 Transistor Scaling and Diffusion! Surface/volume ratios increase! Surface/interface diffusion can dominate transport! Nonthermal means needed to enhance surface diffusion 4

5 Application: Hemispherical Grained Silicon for DRAMs! Rough surface increases capacitance/area! Formed by heating amorphous Si! Crystallization begins at free surface! Grains mushroom from surface by surface diffusion 5

6 UIUC-grown HSG 100µ µm Top view 100µ µm Cross-section 6

7 ! Smooths surfaces of! Al, Cu for metallization! Glasses for passivation Application: Reflow! Submicron length scales surface diffusion dominates! Simulators model effects accurately Cu on 0.35µm trench 375 C 450 C 7

8 Application: Sputter Deposition Ballistic flux shadowing Diffusional smoothing Process simulators can predict growth behavior Simulation by GROFILMS Actual Ti film 8

9 Surface Second Harmonic Generation! Interface polarization Laser P ~ χ 1 E + χ 2 :EE + PMT! χ 1 yields everyday reflection: adsorption insensitive ν Filter 2ν! χ 2 yields SHG: adsorption sensitive! For centrosymmetric bulk, χ 2 0 only at interface 9

10 Second Harmonic Microscopy! Spatially resolved SHG, D from profile spreading! Image before and after heating! Magnification ~ 20x, diffraction limited Laser Array 10

11 Experimental Set-up! Signal averaging at 10 Hz takes ~ 5 min! Imaging does not induce profile spreading J. Opt. Soc. Am. B 10 (1993)

12 Calibration Curve: Ge on Si(111) 6! Developed by Auger spectroscopy! Linear up to ~0.7 ML SH Signal (arb units) θ 12

13 Second Harmonic Images: Ge on Si(111) Unprocessed profiles After image processing Step Diffused 0.6 Step Diffused SH Signal (arb units) θ Ge Pixel Number Position (µm) 13

14 Arrhenius Plot: Ge on Si(111) 10-8 T ( o C) E a = 2.48 ev! Activation energy, prefactor both enormous! Independent of surface concentration D (cm 2 /s) D o = 6 x 10 2 cm 2 /s /T (K -1 ) Phys. Rev. B 55 (1997)

15 Surface Diffusion Physics on Si at High T " Composite surface diffusivity D M depends on: " Mobility of mobile species " Number of mobile species " E total = E hop + E pair formation ev " D o,total = (10-3 cm 2 /s) exp( S pair formation /kt)

16 Illumination-Influenced Surface Diffusion: Experimental Results! In, Ge, Sb on Si(111) Phys. Rev. Lett. 81 (1998) 1259 Phys. Rev. B 61 (2000) T ( o C) , p, n! Measured by second harmonic microscopy D (cm 2 /s) 10-9 Thermal! D increases for n-type, decreases for p-type material Illuminated /T (K -1 ) 16

17 Surface Diffusion Physics on Si at High T " Composite surface diffusivity D M depends on: " Number of vacancies, mobile atoms " Numbers depend on charge state " For all adsorbates (at 2 W/cm 2 ): " E diff changes by 0.3 ev " Prefactor changes by ~ 100 " Invariance with adsorbate suggests underlying commonality: Ionization of surface vacancies 17

18 Quantum Calculations Motivation Charge states of Si surface vacancies unknown, difficult to measure experimentally Quantum calculations can predict vacancy energy levels and formation energies Implementation Ab-initio pseudopotential calculation using DFT Supercell approach, periodic boundary conditions CASTEP software (Accelrys Inc.) Si(100)-(2 1) surface: mono- and divacancy 18

19 Formation Energies for Si Monovacancy " Four stable charge states (+2,0,-1,-2) " Comparable results for other vacancy types Formation Energy (ev) Upper Monovacancy 0 K Fermi Energy (ev) Si surface supports a variety of charged vacancies, consistent with picture for optically driven diffusion 19

20 Analogies between Surface, Bulk Diffusion Bulk Surface Hopping Interstitial motion Adatom motion diffusion Vacancy motion Vacancy motion Kick-in/kick-out Exchange Overall mass Clustering Islanding transport Vacancy-interstitial Vacancy-adatom formation formation Point defect ionization Point defect ionization 20

21 Bulk Diffusion Physics in Si at High T " Rate of total mass transport depends on: " Mobility to mobile species " Number of mobile species " Numbers and mobilities may depend on charge state photoexcitation 21

22 Ion Implantation Ultrashallow Junction Formation " Implantation leaves lattice damage " Dopant must be activated electrically Rapid Thermal Annealing " High-powered lamps " Rapid heating/cooling " Defects promote unwanted diffusion 22

23 Measurement of Bulk Diffusion " Isotopically-depleted layer grown epitaxially via LPCVD " 30 Si step profile created " Doping level uniform (eliminates drift effects) 28 Si (enriched) 30 Si (depleted) 28 Si (natural) 30 Si (natural) 30 Si 23

24 Non-thermal Illumination Effects: p-type! 850ºC, 4 hr, cm -3 p-type, 0.05 W/cm 2 30 Si (cm -3 ) 1e+22 1e+21 1e+20 1e+19 1e+18 hv dark as-grown D(hν) = cm 2 /s D(dark) = cm 2 /s 1e x (nm) Diffusion inhibition by a factor of

25 Non-thermal Illumination Effects in Bulk Si! 900ºC, 30 min, cm -3 n-type, 2 W/cm 2 30 Si (cm -3 ) 1e+22 1e+21 1e+20 1e+19 1e+18 1e+17 hv dark as-grown x (nm) D(hν) = cm 2 /s D(dark) = cm 2 /s Diffusion enhancement by a factor of 90 Results like surface diffusion, impt in rapid thermal processing 25

26 Application for Nonthermally-Driven Diffusion: Nanoparticle Growth on a-si Motivated by Hemispherical Grained Si growth for DRAMs Grow amorphous Si by chemical vapor deposition Anneal (645 C) Grain growth rate may be nonthermally influenced 75 sec 90 sec 105 sec 26

27 Directed Self-Assembly Using Amorphous Semiconductors Expose a-si with subcritical nuclei to hν or e-beam just below crystallization temp Ostwald ripening Dots, walls nm 100µm 27

28 Applications of Self-Assembly Method Particle arrays, walls, narrow pores for:! Nonvolatile memory at high density, low power! Sensors! Photonic band gap materials! Flat panel displays! Solar cells! On-chip nanopore devices for probing electrical activity of biological molecules 28

29 Diffusion at Solid Interfaces: Copper in Interconnect Structures! Diffusion of Cu at interfaces remains uninvestigated! Barrier layers! Etch stops! Interlayer dielectrics! Could be major means of transport during processing, use! Problems with line-to-line shorting, Si deep levels We seek to make first measurements of Cu diffusion at these interfaces 29

30 Problems! Inaccessibility of solid-solid interfaces! Electron, ion spectroscopies no good! Small number of diffusing atoms (< 1 monolayer)! Need for probe with interface specificity! Raman, IR, ellipsometry no good We employ optical second harmonic microscopy 30

31 Second Harmonic Microscopy at Solid Interfaces! Implementation similar to free surfaces! Create profile ex situ! Image through transparent overlayer Laser Array Solid 1 Solid 2 31

32 Measurement Concept! Position Cu source near one end of interface! Heat structure! Freeze concentration profile and image! Structure below made at Chartered Semiconductor Si 3 N 4! 5 nm Ta separates Cu from ILD SiO 2 Si 3 N 4 Cu 32

33 Preliminary Imaging Results! Profiles are very flat! Diffusion through Ta barrier rate limiting! Diffusion at upper or lower interface? 700 C 33

34 Low-Energy Ion-Surface Interactions! In beam-assisted deposition, plasma etching, effects of substrate T usually ignored! In much early work, E ion >> kt! Repulsive potentials govern! Now E ion closer to kt, leading to possible E-T interactions! Attractive potentials more important! Question: What are mechanisms for these interactions?! Now nearly virgin territory 34

35 Ge/Si(111) Surface Diffusion: Physical- Chemical Interactions T ( o C) Two temperature regimes: Low T only prefactor increases High T prefactor and E diff decrease D (cm 2 /s) D thermal D ion, 35 ev D ion, 54 ev D ion, 65 ev Phys. Rev. Lett., 82 (1999) /T (K -1 ) 35

36 Calculation Method: MD Simulation! Si or Ge (111)-(1 1)! Ne +, Ar +, Xe + < 65 ev! Stillinger-Weber/ Universal pot.! Monitor frequency (per ion) of 4 possible events: 36

37 Energy Effects, Constant T Determine E Thres where f 0.01 Fits use f = A (E 1/2 E Thres 1/2 ) Per Impact Probabilities 0.4 Sputtering Adatom formation Knockin Bulk vacancy Ar-Si, 1100K Thresholds E (ev) 37

38 Temperature Effects, Constant E Threshold temperature exists! Occurs for all events, such as sputtering, knock-in Fits use f = A(T T Thres ) Per impact probability 0.2 Ar-Si sputtering 65eV T (K) 38

39 At Threshold, T and Ion Energy are Coupled Direct trade-off between E and T E Total = σkt + E Thres (new result!) ~100eV ~10-65eV ~10-65eV Large amplification factor σ ~ Ar-Si sputtering 60 E Thres (ev) Ge Si T (K) 39

40 Physics: Ion Mass Variation Look for standard momentum matching effects Expect big variation with mass, but observe little E Total (ev) Si Sputtering σ (Ne) (Ar) (Xe) Atomic Number

41 Chemistry: Substrate Variation! Look for bond strength effects σ (slope) invariant E Thres (ev) Ge Si E Total (intercept) scales with cohesive energy, melting temp T (K) 41

42 Experimental Energy Effects Determine E Thres where D increases, using SHM Fits use: D ion = A (E 1/2 E Thres 1/2 ) + D thermal D (10-10 cm 2 /s) Ar-Si(111) 900K Threshold E (ev) 42

43 E-T Coupling Strength Matches Simulation E Total σ 24 Sim. 70 to Expt E Thres (ev) Ar-Si T (K) 43

44 Unimportance of Ion Mass Matches Simulation 240 Observe almost no variations! E Total (ev) Low T ( K) σ (Ne) (Ar) (Xe) Atomic Number 44

45 Apparent Mechanism! Low E! Long-range interaction! Corrugation flat! Nonlocal momentum transfer! High E! Short-range interaction! Corrugation rough! Local momentum transfer! Intermediate E! Medium-range interaction! Corrugation asperities only if target atom ~0.3 A off site 45

46 Summary! New, nonthermal ways (hν, ions) to transport semiconductor atoms have come to light! Numerous applications in microelectronic processing (RTP, dry etching, ion-assisted deposition) and nanotechnology! Solid interfaces SHM imaging demonstrated 46

47 Acknowledgements to:! Postdocs! Harry Ho Yeung Chan! Graduate Students! Kapil Dev! Rod Ditchfield! Mike Jung! Diana Llera-Hurlburt! Shih Hwee Tey! Mike Wang 47