Focused Ion Beam Nanofabrication

Transcription

1 Focused Ion Beam / Focused Electron Beam NT II Focused Ion Beam Nanofabrication Nanotechnology for Engineers : J. Brugger (LMIS-1) & P. Hoffmann (IOA)

2 Nova 600 NANOLAB (FEI) Dual-Beam Instrument Dual Beam: FIB and FEB in one instrument 2

3 Milling FIB a Jack of all Trade Doping Imaging Deposition Lithography 3

4 Table of Contents Introduction Ion Source Ion Optics Ion-Solid Interaction Milling Imaging Applications 4

5 Introduction Field emission reported the first time by R W Wood in 1897 (electrons) Theory based on quantum mechanical tunnelling (Fowler and Nordheim 1928) Field Ion Microscope (FIM) introduced in the 50 s. For the first time atomic resolution has been achieved. (Müller 1951) Field ionisation based FIB were first developed in early 70 s. 5

6 Introduction Principle Surface modification I + I + e- e N 0 - e - Surface modification due to Interaction of impinging ions with the surface Elastic interaction displacement, sputtering, defects, ionimplantation Sample holder Sample A Inelastic interaction secondary e -, secondary ions, X-ray, photons γ Moving the beam Surface patterning 6

7 Instrumentation Ion source (GFIS, LMIS) Suppressor: Improves the distribution of extracted ions Extractor: High tension used for ion extraction Spray aperture: First refinement First lens: Parallelise the beam Upper octopole: Stigmator Variable aperture: Defines current Blanking deflector and aperture: Beam blanking Lower octopole: Raster scanning Second lens: Beam focusing MCP (Multichannel plat): Collecting secondary electrons used for imaging Reyntjens S: J. Micromech. and Microeng. 11 (2001)

8 Ion Source a) Gas Field Ionisation Source (GFIS) atoms (molecules) are trapped by polarizations forces Trapped atoms hop on the surface until they are ionised Ionisation: tunneling process with probability D: D α e -c(i- Φ) V I : Ionisation potential Φ : Work function of emitter V : El. Potential c : constant Ions are ejected from the surface 8

9 Ion Source a) Gas Field Ionisation Source (GFIS) Cooling the tip higher residence time τ r leads higher ionisation rate Ions: H +, He +, Ne +, etc -1 a) low current di = 1 μ A sr dω n dω = sinϑ dϑ dϕ L = 1 a) largest reported value (J. Orloff: High Resolution Focused Ion Beams, Kluwer Academic, 2003) 9

10 Ion Source b) Liquid Metal Field Ionization Source (LMIS) High electrical fields at the apex of a rod leads to detachment of Ions Liquid metal film is drawn into conical shape of the rod (W or Rh) Wide variety of ion species including Al, As, Au, B, Be, Cs, Cu, Ga, Ge, Fe, In, Li, Pb, Si, Sn, U, and Zn Reservoir U Solid substrate (W) Capillary flow Taylor cone Ga + source from FEI Counter electrode 10

11 Ion Source b) Liquid Metal Field Ionization Source (LMIS) γ Surface force F S = 2, γ : surface tension r inward force 2 ε0 E q Coulomb force F C, E = outward force πε r 0 Maximum charge may be placed on the surface Rayleigh limit: F C Liquid droplet q = 8π εγr Rh 0 ε 0 = C 2 /J m dielectric constant 3 r F S charges Formation of Taylor Cone 11

12 Ion Source b) Liquid Metal Field Ionization Source (LMIS) Properties of metals used in LMIS Properties Low melting point Low volatility at melting point Low surface free energy Low solubility in substrate Reason Minimise reaction between liquid and substrate Conserves supply of metal; promotes long source life Promotes flow of liquid and wetting of substrate Dissolution of substrate alters the alloy composition 12

13 Ion Source b) Liquid Metal Field Ionization Source (LMIS) Melting point T m [K] Boiling point T B [K] Vapor pressure p at T m [Torr] T at which p= 10-6 mbar [K] Bi < Ga < In < Sn < Au As < Orloff J, M. Utlaut, L. Swanson: High Resolution Ion Beams, Kluwer Academic (2003) 13

14 Ion Source LMIS or GFIS di μa Current dω sr Cryogenic operation Resolution [nm] Lifetime [h] LMIS 20 no 5 a) 1500 GFIS 1 Yes 50 b) unlimited Current and operation near ambient temperature are in favour for using LMIS Melting temperature T m = 310 K and low vapour pressure favour Ga source for LMIS a) Orloff J, M. Utlaut, L. Swanson: High Resolution Ion Beams, Kluwer Academic (2003) b) Escovitz W., T. Fox and R. Levi-Setti: Scanning Transmission Ion Microscopy with a Field Ionisation Sourc, Proc. Nat. Acxad. Sci. USA 72 (1975)

15 Ion Optics Introduction Intensity: Brightness β: di, Current per steradian dω 2 di β =, current per steradian per unit area per volt d Ω da V β s β t Brightness is conserved over the system and independent of magnification: β s 2 2 di di = = = βt d Ω da V d Ω da V s s t t x s x t α s α t ion source lens target source Typical values for β ~ 10 A cm -2 sr -1 15

16 Ion Optics Electrostatic lens Charged particles are accelerated in electrical field E qe i a =, a E! mi a( A) > a( B) A Ion r and v( A) < v( B) l l r B Net acceleration towards the center V V ~ 0.5 V A (V A : Acceleration Voltage) 16

17 Ion Optics Beam properties Current I follows Gaussian distribution r - σ 2 beam centre r = 0 Diameter of the beam is defined: (FWHM : full width half maximum) I 0 I(r, σ ) = e σ 2π d b 2 b I(, ) 0 σ : standard deviation I 0 : total current r : radial coordinate, d σ 2 1 = I 2 Total current I 0 I ( σ) = I(r, σ) dr 0 0 Typical currents and beam diameters I 0 [pa] d b [nm]

18 Ion Optics Aberrations Astigmatism: Spherical aberration Chromatic aberration: Not all particles have exactly the same energy Space charge effects: more important for ions than for electrons 18

19 Ion-Solid interaction sputtering implantation damage electron emission thermal energy Courtesy John Melngailis 19

20 Ion-Solid interaction sputtering Example Cross section of a tip deposited by FEB 20

21 Ion-Solid interaction Sputtering Physical sputtering: removal of material by elastic collisions between ions and target atoms Sputtering occurs at energies E > hundred ev Typical ion-energy E: E > 5keV Sputtering occurs via collision cascades Most ejected atoms origin from the top few atomic layers 21

22 Ion-Solid interaction Sputtering Rates R s N e R s = = Ni ejected atoms = incoming ions Courtesy John Melngailis 22

23 Ion-Solid interaction Volume per Dose V D V = V D I t V: Volume I: Current t: Time 23

24 Ion-Solid interaction Sputtering Yield Sputtering yield depends on incident angle φ φ Higher probability of collision cascades near the surface at higher φ Sputtering yield has maximum for φ = 75 24

25 Redeposition Sputtering yield can not be used to determine material removal Redeposition needs to be considered for precise structuring Scan speed redeposition sample 25

26 Gas-Assisted Etching Ga + gas inlet gas Enhanced milling rate Redeposition is reduced due to volatile reaction products Typical gases: Cl 2, I 2, H 2 O, XeF 2 Etch enhancement: gas gas Sample Si Al W SiO 2 Cl None none XeF none

27 Gas-Assisted Etching Model Yield of chemical etching is linear to the surface coverage atoms N(t) N(t) Yield Y = = s, : surface coverage, s: maximum yield ion N N 0 0 N N N N = Fg 1- - msj(t) - N N τ 0 0 des adsorption reaction desorption Solution for uniform beam: Replenish: N(t) = N e R Fg - t N 0 F: gas flow g: sticking coefficient J: ion flux τ des : desorption constant m: number of molecules participating in reaction N D : density of adsorbed molecules at the beginning of dwell period N R : density of adsorbed molecules at the end of dwell period Deplete: N(t) = N e + N 1 - e Fg + Jms gf + Jms gf + Jms - t - t N Fg 0 N0 D 0 27

28 Gas-Assisted Etching Model Removal by physical sputtering AS and chemical etching AR removed atoms Y = = ions AR + AS Jt D AR = Js N t = t D 0 t = 0 N(t) dt AS depends on the ion energy and how the the energy from ion impact is dissipated in the presence of a reactive precurser 28

29 Gas-Assisted Etching Interdigitated electrodes milled without gas-assisted etching Interdigitated electrodes milled using gas-assisted etching 29

30 Imaging Ions and secondary electrons may be used for imaging Interaction of ions with solids leads to generation of secondary electrons Potential emission (Auger neutralization) e - φ w Kinetic emission Inelastic collisions may result in excitation or ionisation of atoms E F E ion E ion > 2φ w a) a) Bajales N. et al.: Surface Science 579, L97-L102 (2005) 30

31 Imaging Yield of secondary electrons depends on material Material depending contrast Yield decreases with atomic number Z Low penetration depth z p (10 nm < z p < 100 nm at 30kV) 31

32 Imaging FIB and Electron Microscopy - a Comparison Resolution: FIB and SEM are comparable; FIBs: up to 5nm, SEMs: up to 3nm Sample handling: Both FIB and SEM comparable Voltage contrast imaging: FIB performs better than low-voltage SEM (low intrinsic depth of ions) Material analysis: SEM allows EDX, FIB doesn't (excication energy!). FIB would allow micro- SIMS (some systems are installed) 32

33 Applications TEM-lamellas and Lift-out 15um Lift-out TEM grid, 3mm diameter 33

34 Applications cross-section SIM image of Co tip deposited using FEB SEM image of Co tip deposited using FEB 34

35 Applications Absolute pressure sensor Sealing Deposition process Reference pressure p = 10-6 mbar Finished encapsulation deposition Reyntjens, S. and Puers, R.: A review of focused ion beam applications in microsystem technology. J Micromech. Microeng. 11 (2001)

36 Applications Optical Filter Array of 20x20 coaxial structures Zoom of sub-wavelength coaxial structure Cross-section Pt deposition Ti layer Au SiO 2 36

37 Applications Chip Modification Insertion of electrical connection: 1) Removal of isolating layer (milling) 2) Pt deposition (FIB deposition) 37