Sputtering. Ion-Solid Interactions

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Melina Ward
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1 ssistant Professor Department of Microelectronic Engineering Rochester Institute of Technology 82 Lomb Memorial Drive Rochester, NY Tel (716) Fax (716) Page 1 Ion-olid Interactions ticking probability versus ion energy Thermal energies -- ~<0.02 ev unity sticking coefficient (physi and chemisorption) ev sticking probability decreases reaching a minimum at ~ 20eV ev the sticking coefficient increases and approaches 1 again. Page 2

EDU Page 1 Ion-olid Interactions ticking probability versus ion energy Thermal energies -- ~<0.

2 Ion-olid Impact Initially when the ion is a few ngstroms away there are electron exchange processes occuring (on the time scale of s). Ions capture electrons from the solid (IP of the ion > work function of the solid) thus the ionic species become neutralized s the distance further decreases: the gas atom (ie the ion) and the solid atom form a quasi-molecular species. (atomic orbitals begin to overlap and form an unstable species) Further decreases in the distance: electron-electron repulsion and the Pauli exclusion principle start to dominate which results in separation and collisional re-ionization of the neutrals (IMPCT). Reflection: as θ approaches 0 and M2 >> M1 Page 3 putter Yield number of ejected (sputtered) target species/incident particle Is a function of: M1 and M2 inding Energy Incident ngle Nuclear stopping power Incident Particle Energy Page 4

3 Regimes ingle knock on (low energy) Linear Cascade (medium energy) Thermal pike (high energy) Page 5 Linear Cascade Model Incident ion produces a cascade of atom displacements igmund Theory ΛF ( E) where : Λ is a materials constant which reflects the binding energy (Us), the range of atoms, and the number of ejected F d D accounts for the energy deposition in the sample surface and is a function of particle angle, and target parameters recoiled atoms that get displaced the ion type, energy, incident Page 6

the binding energy (Us), the range of atoms, and the number of ejected F d D accounts for the energy deposition in the sample

4 3α 4M1M 2 4π ( M + M where : α is a function of E is incident ion igmund Theory 1 incident angle 0.1 > α > 1.4 but often has a value of M1is the Mass of the ion M2 is the mass of the target 2 2 E ) 2 U M(target)/M(ion) and energy Us is the binding energy of the target ions Good for low energy (<1keV) Page 7 inding Energy Us typcially assumed to be the heat of sublimation (2-5eV) For, typical threshold energies are ~ 4xUs ee sputtering yield plots and note the different regimes Threshold, ~ linear, and the implantation Page 8

4 M1is the Mass of the ion M2 is the mass of the target 2 2 E ) 2 U M(target)/M(ion) and energy Us is the binding energy of the target

5 igmund Theory High Energy 0.42α where : ( E) / U n de 1 n( E) is the nuclear stopping power dz N where : N atomic volume (atoms/volume) Z1Z2q M1s' n ( E) n 4πa M + M where : Z is the atomic number a is the effective radius over which the nuclear charge is screened by the been tabulated) electrons during the collision (~ ngstroms) s' ( E) is the reduced nuclear cross - section (which have n Page 9 ψ ' ψ ' Target n n +n C n /n C n /n C' C' teady tate Target composition lloys C (1 n C (1 n C ' C' C C g g / n) / n) and Initial Flux Ratio ψ n C g ψ ngc ψ atom flux sputter yield C concentration n g number of gas atoms impinging on target Modified urface Concentration C' C (1 n / n) g C' C (1 n / n) ψ ψ C C Page 10 g teady tate Flux

effective radius over which the nuclear charge is screened by the been tabulated) 2 1 2 electrons during the collision (~ 0.1-0.

6 Miscellaneous Energy Distribution (sputtered species have energies ~ 2-7eV (> Evaporation) better adhesion and increased film density ngular distribution cosine law ( θ ) 1 Incident ion angle ~ up to ~ 70 (0) cos( θ ) degrees Page 11 ystems ee Overhead slide on sputtering system Page 12

distribution cosine law ( θ ) 1 Incident ion angle ~ up to ~ 70 (0)

7 Pressure effects DC < ~ 10mTorr electron mean free path too large and not enough ions strike the target for efficient secondary electron generation Increasing pressure increases ionization and the plasma current s pressure increases plasma current increases s pressure increases ion scattering increases ~100 mtorr optimum Page 13 DC cm P < x > G& d th s gρ(1 γ e ) E where Pd discharge power density(w/cm2) < xth > is the mean distance the sputtered atoms travel before they become thermalized g is the cathode - anode gap distance ρ is the atomic density γe is the Townsend secondary electron coef. E is the average sputtering energy Page 14

optimum Page 13 DC cm P < x > G& d th s gρ(1 γ e ) E where Pd discharge power density(w/cm2) < xth > is the mean distance the sputtered atoms travel before they

8 Triode dd a thermionic or cold cathode source to inject electrons into plasma to increase the plasma density Target -V node V ubstrate Holder Disadvantage is plasma non-uniformity over the target Page 15 ias Impose a small (50-300V) bias on the substrate to create a flux of low energy ions Improved film adhesion Improved step coverage Increased film density Decreased resistivity in metals Change in hardness and residual stress Improved Optical reflectivity Improved dielectric strength Page 16

the substrate to create a flux of low energy ions Improved film adhesion Improved step coverage Increased film density

9 ias Entrapped Gas content ~ Vb 2 Preferentially sputter physisorbed and weakly bound chemisorbed species so purer material is generated Page 17 C (rf) dvantages can sputter all materials Disadvantages more complicated (ie requires an impedance matching network to deliver maximum power to the target) Electrode size effects for an C plasma both electrodes should sputter how do we maximized the voltage drop at the target cathode? Page 18

(ie requires an impedance matching network to deliver maximum power to the target) Electrode size effects for

10 V V rf G C C where G rf C is the capacitance ( ε/d) ds is the sheath thickness ds ( rf ) V electrode sheath ( ) ds G V 4 ds ( rf ) ds ( G) C ssuming the same current density at each Vrf VG G rf G rf rf G 3 4 (G) (rf) V(ac) Tie the substrate holder and the rest of the chamber together as the grounded electrode to maximize (G) Page 19 Reactive Many oxides, nitride, carbide, sulfides can be sputtered from ceramic targets however the binding energy (Us) is typically large so the sputter yield is low low throughput! olution Reactive sputtering sputter from metallic cation target and flow a reactive gas containing the relevant anion Page 20

maximize (G) Page 19 Reactive Many oxides, nitride, carbide, sulfides can be sputtered from ceramic targets however the binding energy (Us) is typically large

11 Reactive ssumptions: Elemental target has a sputter yeild m Target sputtering is due only to inert working gas Compounds sputtered from the target with a sputter yield c deposit as molecules uniform ion current density (j) flows over the target area (t) The collecting substrate surface area is (s) The fraction of the target area covered by the compound is θt The fraction of target un-reacted metal is 1-θt The fraction of the substrate area covered by the compound is θs The fraction of substrate un-reacted metal is 1-θs The flux of reactive gas flux (φr) is proportional to the partial pressure throuh 1/4 nv(avg) Reactive gas molecules do not stick to compound but stick to metal target with a sticking coefficient of αt Page 21 Target teady tate Compound Film Formation Rate Φ α 1 θ ) a ( j / q) θ r t( t t t t c where q is the electron charge and a is the number of compound molecules formed by one reactive gas molecule Reactive ( j / q) where : t Total Target Erosion Rate [ c θ t + m (1 t ] t R ( j / q) θ ) ubstrate Mass alance [ θ (1 θ )] + Φ α (1 θ ) b ( j / q) [ (1 θ ) ] c t t s b is the number of metal atoms in the compound terms on the left reflect the two contributions to compound formation rate on the substrate;1. due to sputter deposition of from the target onto the metal fraction of r the substrate and 2. is due to reaction of this metal with the reactive gas the right hand term is the metal sputtered from the target Onceθ t is calculated from aboveθ s can be determined s s s m t t θ / b the compound from s Page 22

$metal is 1-θt The fraction of the substrate area covered by the compound is θs The fraction of substrate un-reacted metal is 1-θs The flux of reactive gas flux (φr) is proportional to the partial$

12 Reactive Gas Kinetics Q Qt + Qs + Qp Where Q is the total gas flow Qt Φ r α t (1-θ t ) t Qs Φ r α s (1-θ s ) s nd QpP r where is the system pumping speed Cf figure 5-6 in the book Page 23 Rf Magnetron ystem ystem ase Pressure ~5x10-9 Torr Load-lock 3 flexible 2 ources ubstate heat (~ 800 o C), and ias pplications Combinatorial Materials Deposition Multi-layers Metals, emiconductors, Insulators Page 24

ystem ystem ase Pressure ~5x10-9 Torr Load-lock 3 flexible 2 ources ubstate heat (~ 800 o C), and

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Vacuum Evaporation Recap

Sputtering Vacuum Evaporation Recap Use high temperatures at high vacuum to evaporate (eject) atoms or molecules off a material surface. Use ballistic flow to transport them to a substrate and deposit.