SCALING OF HOLLOW CATHODE MAGNETRONS FOR METAL DEPOSITION a) Gabriel Font b) Novellus Systems, Inc. San Jose, CA, 95134 USA and Mark J. Kushner Dept. of Electrical and Computer Engineering Urbana, IL, 61801, USA October 1999 a) Work supported by Novellus and SRC b) Present Address: University of Texas at San Antonio GEC99C01
AGENDA Introduction to Ionized Metal PVD and Hollow Cathode Magnetrons Description of Model Plasma properties of IMPVD of Cu using a HCM Comparison to Experiments Deposition and Target Erosion Concluding Remarks GEC99C15
COPPER INTERCONNECT WIRING The levels of interconnect wiring in microelectronics will increase to 8-9 over the next decade producing unacceptable signal propogation delays. Innovative remedies such as copper wiring are being implemented. Ref: IBM Microelectronics GECA99C12
IONIZED METAL PHYSICAL VAPOR DEPOSITION (IMPVD) In IMPVD, a second plasma source is used to ionize a large fraction of the the sputtered metal atoms prior to reaching the substrate. MAGNETS ANODE SHIELDS TARGET (Cathode) PLASMA ION NEUTRAL TARGET ATOMS INDUCTIVELY COUPLED COILS SECONDARY PLASMA + e + M > M+ + 2e WAFER Typical Conditions: CACEM98M03 10-30 mtorr Ar buffer 100s V bias on target 100s W - a few kw ICP 10s V bias on substrate SUBSTRATE BIAS UNIVERSITY OF ILLINOIS OPTICAL AND DISCHARGE PHYSICS
IMPVD DEPOSITION PROFILES METAL IONS In IMPVD, a large fraction of the atoms arriving at the substrate are ionized. METAL ATOMS Applying a bias to the substrate narrows the angular distribution of the metal ions. The anisotropic deposition flux enables deep vias and trenches to be uniformly filled. METAL SiO2 CACEM98M04 UNIVERSITY OF ILLINOIS OPTICAL AND DISCHARGE PHYSICS
HOLLOW CATHODE MAGNETRONS Hollow Cathode Magnetrons (HCM) are typically cylindrical devices with inverted metal cups and floating shields. Production tools have 300 mm substates. IRON PLATE 35 CATHODE MAGNETS Typical Operating Conditions: HEIGHT (cm) 30 25 20 15 10 5 IRON RING SHIELD (FLOATING) GROUND GAS INLET SUBSTRATE (FLOATING) SHIELD (GROUND) Pressure: 5-10 mtorr B-field: 100-300 G atcathode Voltage: 300-400 V Substrate: Floating or biased Buffer gas: Ar, 50-100 sccm Power: 2-5 kw 0 12 9 6 3 0 3 6 9 12 RADIUS (cm) PUMP GEC99C02
HOLLOW CATHODE MAGNETRONS-IMPVD HCM - IMPVD devices use magnetic confinement in the cathode for high ion densities and sputtering, and cusp B-fields to focus the plasma. ION CONFINEMENT AND SPUTTERING PLASMA FOCUSING DIFFUSION (SOME GUIDING) B-Field Vectors B-Field Magnitude GEC99C03
HYBRID PLASMA EQUIPMENT MODEL Es(r,θ,z,φ) v(r,θ,z) S(r,θ,z) PLASMA CHEMISTRY MONTE CARLO SIMULATION FLUXES ETCH PROFILE = 2-D ONLY = 2-D and 3-D I,V (coils) MAGNETO- STATICS CIRCUIT E ELECTRO- MAGNETICS FDTD MICROWAVE Bs(r,θ,z) E(r,θ,z,φ), B(r,θ,z,φ) ELECTRON MONTE CARLO SIMULATION ELECTRON BEAM ELECTRON ENERGY EQN./ BOLTZMANN NON- COLLISIONAL HEATING Es(r,θ,z,φ) N(r,θ,z) S(r,θ,z), Te(r,θ,z) µ(r,θ,z) FLUID- KINETICS SIMULATION HYDRO- DYNAMICS ENERGY EQUATIONS SHEATH LONG MEAN FREE PATH (SPUTTER) SIMPLE CIRCUIT Φ(r,θ,z,φ) V(rf), V(dc) Φ(r,θ,z) R(r,θ,z) Φ(r,θ,z) R(r,θ,z) EXTERNAL CIRCUIT MESO- SCALE SURFACE CHEMISTRY J(r,θ,z,φ) I(coils), σ(r,θ,z) VPEM: SENSORS, CONTROLLER, ACTUATORS GECA99C13
PHYSICS MODELS USED IN HCM SIMULATIONS Monte Carlo secondary electrons emitted from cathode Fluid bulk electrons (T e from energy equation with Boltzmann derived transport coefficients) Continuity, momentum and energy for all heavy species (multi-fluid, slip and temperature jump boundary conditions) Long-mean-free path transport for sputtered atoms with sputter-heating Implicit Poisson solution simultaneous with continuity and momentum for electrons Species in Model: e, Ar, Ar(4s), Ar +, Cu( 2 S), Cu( 2 D), Cu( 2 P), Cu + GECA99C14
DESCRIPTION OF SPUTTERING MODEL Energy of the emitted atoms (E) obeys the cascade distribution, an approximation to Thompson s law for Ei 100 s ev: F(E) = 2 1+ E 2 b E b E ΛE i ( E b + E), E ΛE 3 i 0, E > ΛE i where Λ = 4m i m T /( m i + m T ) 2 subscripts: b ~ binding, i ~ ion, T ~ target. ion flux fast neutral flux Target Al θ Al Ar The sampling of sputtered atom energy E from the cascade distribution gives wafer E = E b ΛE i RN E b + ΛE i 1 RN ( ) where RN is a random number in interval [0,1]. PEUG99M03 UNIVERSITY OF ILLINOIS OPTICAL AND DISCHARGE PHYSICS
HCM- Cu IMPVD: ELECTRON DENSITY, TEMPERATURE Electron densities in excess of 10 12 cm -3 are generated inside and in the throat of the cathode. Fractional ionizations are 10%. Electron temperatures peak at 4-5 ev in the cathode, and are a few ev downstream. E-Density (6.6 x 10 12 cm -3 ) E-Temperature (4.8 ev) Ar, 6 mtorr, 150 sccm, 325 V, 160 G GECA99C04
HCM- Cu IMPVD: ELECTRON SOURCES Secondary electron emission and beam ionization produce electron sources in the cathode. Ionization in the bulk plasma, however, is the major electron source. E-beam (3 x 10 17 cm -3 s -1 ) Bulk (5 x 10 18 cm -3 s -1 ) Ar, 6 mtorr, 150 sccm, 325 V, 160 G GECA99C05
HCM- Cu DENSITIES Neutral copper [Cu( 2 S) + Cu( 2 D)] undergoes rapid ionization and rarefaction in the throat of the cathode. The majority of neutral copper is in metastable Cu( 2 D). Fractional ionization of Cu in the bulk is 10's %. Cu (1.9 x 10 13 cm -3 ) GECA99C06 Cu + (1.5 x 10 12 cm -3 ) Ar, 6 mtorr, 150 sccm, 325 V, 160 G
HCM- GAS TEMPERATURE Average Gas Temperature (max = 1530 K) Gas heating occurs dominantly by symmetric charge exchange between Ar neutrals and ions, and produces significant rarefaction. Slip between neutrals and disparate rates of charge exchange produce differences in their maximum temperatures: Ar: 1546 K Cu: 900 K Ar, 6 mtorr, 150 sccm, 325 V, 160 G GECA99C07
ELECTRON DENSITY AND TEMPERATURE vs EXPERIMENT 3.5 EXPERIMENT MODEL ELECTRON TEMPERATURE (ev) 3.0 2.5 2.0 1.5 1.0 MODEL EXPERIMENT Electron Density 0.5 0 2 4 6 8 10 12 RADIUS (cm) Electron Temperature Langmuir probe measurements of electron density (2.8 cm above substrate) show densities and temperatures more highly peaked on axis. The model underpredicts the "jetting" of plasma from the throat of the cathode which likely results from long-mean-free path effects not captured by the ion model. Ar, 6 mtorr, 150 sccm, 325 V, 160 G GECA99C09
ELECTRON DENSITY, TEMPERATURE vs BIAS HCM devices have significantly "less steep" I-V characteristics compared to conventional magnetron discharges. For example, downstream plasma densities and power deposition scale nearly linearly with bias. Electron temperatures are weak functions of bias. ELECTRON DENSITY (10 11 cm -3 ) 25 20 15 10 5 EXPERIMENT MODEL 0 320 340 360 380 400 420 440 CATHODE BIAS (v) ELECTRON TEMPERATURE (ev) 3.2 2.8 2.4 2.0 1.6 1.2 0.8 0.4 EXPERIMENT MODEL 0.0 320 340 360 380 400 420 440 CATHODE BIAS (v) GECA99C18
HCM- ION AND CU FLUXES TO THE SUBSTRATE 10 FLUX (10 17 cm -2 s -1 ) 8 6 4 2 Ar + Cu (Direct) Cu + Cu( 2 D) Ion flux to the substrate is dominated by Ar+ Direct non-thermal Cu dominates the Cu fluxes to the substrate. The Cu flux is 25-30% ionized. 0 Cu( 2 S) Ar, 6 mtorr, 150 sccm, 325 V, 160 G GECA99C08
Cu DEPOSITION RATE EXPERIMENT ( x 1.33) MODEL The off-axis peak in direct sputter flux produces an offaxis peak in the deposition rate. Cu Deposition Rate (A/min) Ar, 6 mtorr, 150 sccm, 325 V, 160 G GECA99C10
TARGET EROSION CORNER DEPOSITION 0.3 0.2 0.1 0.0 EXPERIMENT MODEL 0 EROSION -0.1-0.2 L -0.3-0.4 0 5 10 15 POSITION ALONG TARGET (cm) HCM TARGET B-FIELD Magnetron trapping and the resulting large ion fluxes to the target occurs at the side walls. Net deposition of sputtered Cu occurs on the end walls while the side walls experience net erosion. Ar, 6 mtorr, 150 sccm, 325 V, 160 G GECA99C11
CONCLUDING REMARKS Hollow Cathode Magnetrons (HCM) are capable of producing metal deposition with plasma densities of 10 12 cm -3. The HCM operates somewhat as a remote plasma source with moderate electron temperatures near the substrate compared to the cathode interior. The configuration of the magnetic field is important in at least two respects: Jetting of plasma at throat of cathode Erosion profile inside the cathode Small HCMs having higher power densities experience significant rarefaction which ultimately limits their capability to produce high plasma densities and deposition rates. Large HCMs (10-20 cm diameter) which avoid these problems have been designed and built based on these scalings studies. GEC99C16