Direct Energy Influx Measurements. in Low Pressure Plasma Processes

Transcription

1 Direct Energy Influx Measurements in Low Pressure Plasma Processes A.L. Thomann, GREMI Orléans R. Dussart, N. Semmar, J. Mathias, T. Lecas L. Bedra, P.A. Cormier, V. Dolique

2 Outline I. Introduction: Why energy flux measurements in low pressure plasmas? Different kinds of sensors II. Presentation of the energy flux diagnostic: Calibration procedure developped at GREMI Sensitivity of the sensor III. Measurements in low pressure plasmas: IV. Conclusion ICP Ar plasma: charged particle and neutral contributions Magnetron sputter deposition IR radiation emitted by the heated target Reactive sputtering Other reactive processes Dusty plasmas Silicon etching

3 I. Why measuring the energy influx at the substrate? Deposisted energy determines the efficiency of the surface modification process and drives the characteristics of the treated material The energy released at the surface could give some insight into the involved mechanisms

4 I. Why measuring the energy influx at the substrate? Case of plasma sputtered thin films: thermal energy Transition: kinetic energy thermal energy thermal energy thermal energy Schematic structure: Zone: Characteristics: substrate substrate substrate substrate substrate Zone Ia Zone Ib Extended structure zone model from S. Mahieu et al: TSF 515 (2006) 1229 Zone Ic -no preferential orientation Zone T -preferential orientation: fastest growth direction Zone II -preferential orientation: lowest surface energy Structure zone model from Anders A.(Thin Solid Films 518 (2010) ) for energetic deposition

5 I. How to estimate the energy fluxes? Energy balances with data from the (gas phase - plasma) characterization rough, a posteriori estimation Energetic contributions: - condensing atoms - particle bombardment (ions, electrons, reflected neutrals etc.) - chemical reactions (with reactive plasma species) - radiations (plasma, IR from heated devices etc.) Direct energy measurements during plasma processes: an old issue! probes for radiative transfer (radiometer) (R. Gardon 1953, K. Ellmer 1999 etc.) calorimetric probes reccording of the thermogram, (D.J. Ball 1972, J.A. Thornton 1978, R. Wendt 1997, H. Kersten since 1994 etc.) active probes compensation heated probes thermopile Seebeck effect

6 I. Calorimetric probes H. Kersten et al (Vacuum 63 (2001) 385); P.A. Cormier et al (J. Phys D: Appl. Phys. 43 (2010) ) M. Cada et al (J. Appl. Phys. 102 (2007) ) R. Wendt et al (J. Appl. Phys. 82(5) (1997) 2115)

7 I. Calorimetric probes P in = C s dt S, heat dt dt S, cool dt Heating Cooling Calibration: knowledge of the heat capacity Cheap, sturdy, simple probes but: reccording of the thermogram lasts at least 2 min the sensitivity is poor (small contributions may not be detected) difficult to discriminate various kinds of energetic contributions delicate calibration

8 II. The thermopile 1600 thermocouple junctions per cm² (Nichrome (1)/Constantan(2)). Each thermocouple produces a voltage directly proportional to the energy influx J. The total voltage V HFM is the sum of these voltages. Active junction Metal 1 Metal 2 Reference junction Heat flux T + T T M 1 M 2 T T+ T V J Seebeck effect: Because of the temperature gradient induced by the energy flux (J), the thermopile delivered a voltage Thin film technology : fast time response (µs) sensitivity (0.1mW/cm2)

9 II. The Heat Flux Microsensor (HFM) A commercial heat flux micro-sensor (Vattel@) is used: Thermopile (Seebeck effect) + Pt resistance Zynolite coating Thermopile (response time ms) Pt100 Heat Flux Meter active area 6 mm Connexions 6 mm Detection of radiation, convection and conduction heat fluxes

10 II. The Heat flux diagnostic The HFM is inserted into a water cooled holder HFM rod Calibration is carried out in vacuum, following a NIST protocol and using IR radiation from a black body (Stefan law) HFM active surface Φ ( 4 4 a ε T a ε ) 12 = σ T2 Home made Black body Thermal shields

11 II. Experimental conditions of measurement Example of calibration curve The sensor is cooled : to avoid losses by IR back emission to detect low intensity contributions En influx (mw/cm2) HFM voltage (µs) To work on a true surface a copper plate is pasted on the HFM: this has no effect on the calibration

12 II. Comparison thermopile/calorimetric probe Measurements in front of a broad beam ion source (Christian Allbrecht University, Kiel, Germany) In a wide range of energy flux density In front of an large anisotropic beam Good agreement Calorimetric probe HFM signals P.A. Cormier et al (J. Phys. D: Appl. Phys. 43 (2010) )

13 II. sensitivity of the diagnostic ICP argon plasma (100W, 0.66 Pa) Gas flow on: gas conduction (ICP argon plasma on : 25 mw/cm 2 ) contribution 0.8 mw/cm 2 J cond 2k = π m Ae ap T g ( T T ) g s For a=0.86, T g =300K, T s =278K, p=0.66pa: J cond = 0.9 mw/cm 2 Offset due tu radiative transfer between the reactor at room temperature and the cooled HFM L. Bedra et al (J. Phys. D: Appl. Phys. 43 (2010) )

14 II. sensitivity of the diagnostic Sputtered Al thin film submitted to O 2 flow N Energy released by oxydation: 20 mw/cm 2 With O2 = P O2 2πmkT 2N g O2 J ox = Hox N Av H ox = kJ/mol, C C sticking sticking J ox (10-45)mW/cm 2 Formation of a passivation layer Energy flux density (mw/cm2) O 2 on O 2 off O 2 on Time (s)

15 II. Separation of various contributions Measurements in front of magnetron cathodes (DCMS)

16 II. Separation of various contributions Measurements in front of magnetron cathodes (DCMS) (mosaic target, 9cm away, 10-3 mb Ar) Energy flux density (mw/cm2) Thermal contribution 250W 200W 150W 100W 50W Collisional transfers Time (s) 300W Sharp increase : collisional tranfers (condensation, ion - e- effect etc.) Slow increase : IR radiation from a heated device (the target)

17 III. Measurements in an ICP Ar plasma Contribution of charged particles / neutrals R. Dussart et al (Proceedings of ISPC 2007) Langmuir probe measurements: estimation of Ne, KTe, Vp, Vf Adsorption measurements: estimations of [Ar*] and Tg φ cond ch. P i B ( 2kT + e( V V E ) J = 0,6 n u ) + = 2k πm B Ar ap T g e (T g T P w ) S φ * = ξ rec * N m 8kT π m g Ar E m

18 III. Measurements in an ICP Ar plasma Contribution of charged particles / neutrals Gas conduction Ar* contribution

19 III. Measurements in an ICP Ar plasma Contribution of charged particles / neutrals Good correlation between calculated and measured energy flux values The difference could be due to uncertainties on Langmuir probe measurements Gas conduction = 10% of the total energy flux Contribution of metastables is negligible

20 III. Measurements in an ICP Ar plasma Contribution of sputtered particles 0,5 Pa, 400W, 10cm 1 Pa, -200V, Pt L. Bedra et al (J. Phys. D: Appl. Phys. 43 (2010) ) In this experimental configuration, energetic contribution of sputtered atoms is well separated from Ar RF plasma one

21 III. Measurements in an ICP Ar plasma Contribution of sputtered particles 0,5 Pa, 400W, 10cm Energy deposit ed by atom (ev) E Pt = 10,8 ev Target bias volt age (V) R. Dussart et al., Microsensors, Igor Minin (Ed.), ISBN: , InTech, (2011) Very good agreement!!! Ability to quantify and study a minor energetic contribution E ϕ / τ = deposition F Ar + measured on the target E = E cd + E c Determined from SRIM and Thompson formula sputtering yield f(e Ar+ ) E Pt = = 10. 2eV

22 III. Measurements in an ICP Ar plasma Effect of the energetic particle 0,5 Pa, 400W Plasma contribution Sputtered atom contribution Energy flux density (mw.cm -2 ) Target/substrate distance (cm) Energy flux density (mw/cm2) Target/substrate distance (cm) Deposition performed at 3 cm and 18 cm away from the metal target different energetic conditions

23 III. Measurements in an ICP Ar plasma Effect of the energetic specy 0,5 Pa, 400W, 18cm, 30 0,5 Pa, 400W, 3cm, The densest film is obtained when sputtered atoms carry le most energy, whereas the global energy transfer in the lowest! Echantillon Pt atom number (RBS) (at/cm2) Thickness calculated from RBS (nm) Thickness measured on SEM images (nm) Plasma contribution (mw/cm2) Sputtered atom contribution 30min, 18cm min30s, cm

24 III. Magnetron sputter deposition Co-sputtering of mosaic targets for the synsthesis of AlCoCuCrFeNi alloy (11cm away, 0.43 Pa) Fe Co Ni Cr Cu Al DePhy reactor with the three targets lightened on Energy influx (mw/cm²) T1 T2 T W 140 T T2 80 T2 T3 60 T1 T Time (s)

25 III. Magnetron sputter deposition Evidencing of 4 families of deposits with different morphology, microsctructure and chemical composition Group A (BCC) Group B (FCC) Chemical composition is varied close to the equimolarity Group C (a) Group D (BCC) Stabilization of a given solid solution can be driven by: - chemistry or - deposited energy

26 III. Magnetron sputter deposition Influx (mw/cm2) A C A D A B B B B A A B B C C Total power (W) D C A A No correlation between the energy deposited and the structure: Chemical composition is the driving force

27 III. Magnetron sputter deposition IR light emission from the target Ti target, 0,66 Pa, 400W, pdc (50 khz 1,2µs off 95% duty cycle) IR emitted by the target surface is clearly visible IR contribution is increased with B magnetic flied when the sputtering plasma is confined close to the target P..A Cormier at al., JAP 113, (2013)

28 III. Magnetron sputter deposition IR light emission from the target Un-balanced magnetic field Balanced magnetic field

29 III. Magnetron sputter deposition IR light emission from the target Ti target, 0,66 Pa, 400W, B magnetic field IR emission in enhanced in pulsed sputtering processes it could overcome the plasma contribution!!

30 III. Magnetron sputter deposition IR light emission from the target Ti target, 0,66 Pa, DC, B magnetic field What would be the effect of this IR contribution on the thin film growth????

31 III. Magnetron sputter deposition IR light emission from the target: indirect information on the target surface state Ti target, 0,66 Pa, DC, B magnetic field At equilibirum the target temperature can be evaluated from the IR part ϕ rad = f Met Cu σ ( 4 4 ε T ε T ) Met Met Cu Cu Percentage Target Plasma IR Power of IR surface contribution contribution τ (s) (W) emission temperature (mw/cm2) (mw/cm2) ( C) % % % % 9 577

32 III. Magnetron sputter deposition Reactive sputtering: well known transition between metal and oxide regime, and hysteresis phenomenon Al target, 0,4 Pa, DC, 400W Al I( 396 nm)/ari(750 nm) evolution with respect to O 2 percentage Evolution of the cathode during sputtering of Aluminum target with Ar/O 2 plasma

33 III. Magnetron sputter deposition Reactive mode Al target, 0,4 Pa, DC, 400W Energy flux is different in metal and oxide modes High energy is released at the transition between both modes

34 III. Magnetron sputter deposition Reactive mode Al target, 0,4 Pa, DC, 400W Φ 45 mw/cm2: could be the oxidation of the depositing Al thin film Φ 800 mw/cm2: different process, may be due to O- effect

35 III. Other reactive processes Silicon etching Cryogenic process in SF6 plasma Energy flux density (mw.cm -2 ) SF 6 plasma on Si SF 6 plasma on SiO 2 Ar plasma on SiO Time (s) RF Antenna Si substrats are pasted on the sensor Aror SF 6 Gas HFM Plasma source Diffusion chamber Pumps Confinement coil Huge energy released in SF6 plasma: clear evidence of the etching reaction between Si and F

36 III. Other reactive processes Silicon etching Cryogenic process in SF6 plasma Energy flux density (mw.cm -2 ) (a) Ar 800 W SF W 1000 W 800 W 600 W Ar 400 W 1200 W 200 W Time (s) (b) Energy flux density (mw.cm -2 ) SF 6 plasma on Si Ar plasma on SiO 2 SF 6 plasma on SiO 2 Argon plasma on Si Plasma source power (W) H r = ΦM ρ S i v s i g S S 2 1 Estimated value: ± 400 kj/mol Value given in literature: kj/mol

37 III. Other reactive processes Dusty plasmas: synthesis of µc-si, chemical annealing H effect on the synthesis of µc-si LPICM plasma chamber, silane plasma Particles are expected to be formed at 1 Torr and for P 20W

38 III. Other reactive processes Dusty plasmas: synthesis of µc-si, chemical annealing At 1 Torr and for P 20W decrease of the signal several after plasma ignition: formation of primary Si clusters 1 Pa Kinetic of the desrease changes with the power, this was expected since the steps of growth evolves At 2 Torr the kinetic of powder formation is not the same 2 Pa

39 III. Other reactive processes Dusty plasmas: synthesis of µc-si, chemical annealing 2 torr SiH4/H2 plasma: discharge regime of particle growth by SiHx group sticking H2 plasma: chemical annealing Various processes are evidenced by studying the energy fluxes Elementary mechanisms can be showed

40 IV. Main conclusions Heat flux diagnostic Direct energy influx measurements were performed with a thermopile (included in an homemade diagnostic) in various low pressure plasma processes With this diagnostic: very small contributions (less than 1mW/cm2) can be detected discrimination between collisional transfers and radiative contribution from heated device is possible change in process regime can be evidenced this tool is very usefull for the control of low pressure plasma processes

41 IV. Main conclusions Physics of the plasma/surface interaction Evidencing of: deposition parameters playing the major role on a characteristic of the film surface reaction at the substrate (molecule recombination, oxydation etc.) contribution due to interaction with energetic species Coupling with thin film and gas phase analysis - better understanding of the involved mechanisms (Determination of the role played by the energetic vector ) - estimation of intrinsic data (enthalpy, sticking coefficient etc.)

42 S. Konstantinidis, A. Balhami ChIPS, Université de Mons, 20 place du parc, 7000 Mons, Belgique H. Kersten Institute of Experimental and Applied Physics, University of Kiel, Allemagne P. Roca i Cabarrocas, S.N. Abolmasov LPICM CNRS-Ecole Polytechnique, Palaiseau Cedex, France