Ionenverteilungen beim Magnetronsputtern verschiedener Metalle in Argon/Sauerstoff

Transkript

1 Th. Welzel, K. Ellmer Institut für Solare Brennstoffe und Energiespeichermaterialien HahnMeitnerPlatz 1, D1419 Berlin Ionenverteilungen beim Magnetronsputtern verschiedener Metalle in Argon/Sauerstoff XVII. Erfahrungsaustausch "Oberflächentechnologie mit Plasma und Ionenstrahlprozessen Mühlleithen, März 21

2 Content: 1. Introduction 2. Experimental 3. Discharge Voltages 4. Mass Spectra 5. Positive Ion Distributions 6. Negative Ion Distributions 7. Summary

3 Introduction one of the main advantages of magnetron sputtering: assistance of the film formation by energetic particles neutrals from the sputtering process electrons from the plasma high energies due to electric fields ions (positive and negative) radicals compound formation by admixture of reactive gas component (O 2, N 2, CH 4, H 2 S,...) reactive mode... particularly high (negative) ion energies These ion energies may in some cases (e.g. microelectronics, photovoltaics) be too high and detrimental causing radiation damage in the films. Adjustment/suppression needs knowledge of ion formation mechanisms and energies.

4 Measurement of Ion Distributions schematic representation of the measurement setup V mass spectrometer: plasma process monitor PPM 422 (Inficon) in place of substrate holder 6 mm distance from target energy range:... ± 5 ev mass range:... 5 amu entrance orifice (Ø = 1 mm) at floating potential acceptance angle of few degrees: only perpendicular incidence sputter setup: circular planar magnetron ( = 5 mm), d.c. discharge, P = 5 W different target materials: Mg, Cu, In, W working gas: 1 % Ar (nonreactive) 5 % Ar + 5 % O 2 (reactive) working gas pressure:.5 Pa Pa

5 Target Voltages in Ar and Ar/O 2.9 Pa, 5 W const. power Depla et al., JAP 11 (27) 1331 const. current U met [V] U ox [V] U ox /U met γ met U ox /U met γ ox Mg 276 ± ± Cu 281 ± ± In 397 ± ± W 246 ± ± <.8 tungsten: according to U ox /U met γ ox /γ met much lower than for Cu (.87)

6 Mass Spectra with Cu Target + (E i = 1 ev) (E i = 17 ev) 1 6 Ar + Cu + + Ar Cu 2 + Cu Ar O + O 2 + Ar + Cu + Ar + 2 /CuO + + CuO 2 Cu 2 + CuAr + Cu 2 O + O O 2 O O 2 O 3 Cu CuO CuO 2 Cu 2 Cu 2 O Ar/O m/z

7 Positive Ion Distributions: Typical Spectra Ar Ar/O W + (184) Ar + (36) O + (16) general: peak at low energy (~ev pl ) highenergy tail (sputter distribution) additional Ar ev nonreactive (Ar): mainly M + and Ar + impurity signals of O + (MO x+ ) O + (16) W + (184) + WO 2 (216) reactive (Ar/O 2 ): mainly Ar +, O +, and O 2 + O + extended to very high energy weak M + (reduced sputter yield) different molecules M (y) O x W target, P = 5 W, p =.5 Pa, centre, 6 cm distance

8 Positive Ion Distributions: HighEnergetic Working Gas (Ar + ) Meas Pa.9 Pa 1.7 Pa broad maximum/extension at energies of several 1 ev to (some) 1 ev: dependent on target mass (not for Cu, Mg) dependent on gas phase collisions TRIM gas heating or projectile reflection # of Reflected Ar [a.u.] Energy of Reflected Ar [ev] TRIM simulations (Ar + (256 ev) W): similar distribution slightly higher energy (no collisions) only for heavy targets W target, P = 5 W, Ar, centre, 6 cm distance

9 Positive Ion Distributions: HighEnergetic Working Gas (O + ) Meas. TRIM # of Reflected O [a.u.] Pa.9 Pa 1.7 Pa ev T ~155 ev Energy of Reflected O [ev] 2 2 broad maximum/extension to energies far above 1 ev: observed for every target material MgO: low discharge voltage (~155 V) signal extents to full voltage equivalent TRIM simulations (O + (158 ev) MgO): energy of reflected O much too low In contrast to Ar + (reflection + postionisation), highenergetic O + is not due to reflected atoms instead to O. Mg target, P = 5 W, Ar/O 2, centre, 6 cm distance

10 Negative Ion Distributions: Typical Spectra Ar weak signals in nonreactive mode: Ar/O O (16) W (184) W (184) WO 3 (232) O (16) 5 metal ions, if E a > (W, In, Cu) weak impurities more signal(s) in reactive mode: O,O 2, M (y) O x highenergy (several 1 ev) peak lowenergy signal (< 1 ev) partially midenergy peaks general: lowenergy cutoff: E min =ev fl to enter PPM no detection of cold ions formed from working gas W target, P = 5 W,.5 Pa, centre, 6 cm distance

11 Negative Ion Distributions: HighEnergy Peak IDF of O for different target materials, normalised to the respective target voltage V T 1 4 O peak generally located at ev T negative O ions formed directly at the target surface accelerated by full sheath potential W d.c.,.5 Pa Ar/O 2 W, Cu In, Mg similar for all other negative ions Normalised Energy E i /ev T

12 Negative Ion Distributions: HighEnergy Tail f U surface binding energy g(e ) Gauß distribution around E mean (Mraz et al., JAP 1 (27) 2353) CuO E E E ( E) ~ g( E ) Emin (( E E ) + U ) CuO de O O 3 45 E mean = (338 ± 1) ev σ = (6.5 ± 1) ev Fitted values for U : O : 1.5 ev O 2 : 3. ev CuO :.75 ev CuO 2 :.2 ev

13 Negative Ion Distributions: MidEnergy Peaks (Cu) Count Rate [a.u.] CuO2 O2 Cu2O3 Cu2O2 CuO2 CuO CuO2 CuO3 x O O2 Cu CuO CuO2 CuO 2 CuO Cu O 2 O distinct peaks between ev T and ev fl : depend on target and detected ion fragments from molecular ions formed at the target (full energy) e.g.: O 2 (334 ev) O (167 ev) + O (167 ev) CuO 2 (334 ev) CuO (278 ev) +O(56 ev) possible formation paths: spontaneous decay electron collision + reattachment not: gasphase collisions scattering out of the small detection angle Cu target, P = 5 W,.5 Pa Ar/O 2 = 5/5, centre, 6 cm distance

14 Negative Ion Distributions: MidEnergy Peaks (W) W2O8 W2O3 W2O8 W2O2 W2O8 O2 W2O WO4 WO4 WO3 WO4 WO3 WO2 WO4 WO3 WO2 WO WO3 WO2 WO W O WO 3 WO 2 x WO x x W O parent molecules up to WO 4 and W 2 O x detected no WO or WO 2 at full energy of fragments of them not formed at the target dominant molecule WO 3 (E a > 2.5 ev, E a (W) =.8 ev, E a (O) = 1.5 ev) so far no correlation to neutral molecular abundance (signal depends on E a ) W target, P = 5 W,.5 Pa Ar/O 2 = 5/5, centre, 6 cm distance

15 Conclusions nonreactive sputtering: lowenergetic positive ions for target material ions with highenergy tail midenergetic reflected gas atoms (heavy targets) few negative ions (electronegative metals, impurities) reactive sputtering: variety of negative ions (incl. molecules) energy up to full target voltage equivalent negative ion fragmentation, some ions missing low sputter energy of molecules additional highenergetic positive ions (charge exchange)

16 Thank you for your attention!

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18 Magnetron Plasma Typical Potential Distribution (d.c.) positive ions from working gas (electron impact) reflexted atoms (electron impact) sputtered atoms (electron impact) Target Substrate negative Ionen aus working gas (electron attachment) emission from the target sputtered atoms (electron impact) various chargeexchange processes V Pl V fl ~ 5 V ~ 2 V ~ 1 V substrate: grounded (fixed) ~ 4 V V bias with negative bias potential (fixed) insulated (floating) V T Measurement with the PPM is performed against ground potential.

19 Positive Ion Distributions: Centre of Gravity for Metal Ions CoG [ev] ev.5 Pa.9 Pa 1.7 Pa heat of sublimation: 3.5 ev 2.5 ev ev W Cu In Mg Target Material W Pa.9 Pa 1.7 Pa 6 P= 5 W p =.5 Pa Ar/O 2 = 1/ centre, 7 cm distance 8 pressure increase: highenergy tail (sputter distribution) thermalised peak CoG: trend follows sublimation energy (better for heavy atoms less efficient scattering)

20 Negative Ion Distributions: High and LowEnergy Part 1 Ratio Low/High Energy O Parameter: P = 5 W p =.5 Pa Ar/O 2 = 5/5 centre, 7 cm distance highenergy O represents γ ox : 1 W Cu In Mg Target Material high ratio for WO x (low γ ox ), lowest for MgO x (high γ ox ) formation possibilities of lowenergetic O : (1) at the target sheath edge (2) from highenergetic O by several collisions and electron reattachment

21 Positive Ion Distributions (revisited): HighEnergetic O + O O Pa.9 Pa 1.7 Pa Pa.9 Pa 1.7 Pa ev T ~155 ev O spans whole range up to ev T charge exchange between target and substrate via electron impact: (a1) (a2) (b) O + e O + 2e O + e O + + 2e O + e O + + 3e Highenergetic O + results from O accelerated in the target sheath. Lowenergy peak is still due to ions formed by electronimpact from the cold working gas in the plasma bulk Mg target, P = 5 W, Ar/O 2, centre, 7 cm distance