DOE WEB SEMINAR, 2013.03.29 Electron energy distribution function of the plasma in the presence of both capacitive field and inductive field : from electron heating to plasma processing control 1 mm PR Si Plasma Electronics Laboratory Hyo-Chang Lee e-mail : flower4507@hanyang.ac.kr This work was performed with Dr. M.H. Lee, Prof. C.W. Chung Department of Electrical Engineering, Hanyang University, Republic of Korea
In this seminar 1. Motivation of this study 1-3 2. Experimental result Page Num. - Experimental set-up 4 - Observation of collisionless electron heating 5-8 - Effect of negative ions on electron heating 9-11 - Plasma processing control via modification of EEDF 12-16 3. Conclusions 17 4. Supplement 18 5. Discussions 19-22 - EEDF by additional capacitive power in strongly electronegative plasma
Log EEPF Correlation of Plasma parameters and Processing Applied E-field Collisional, Collisionless heating Ohmic heating where n m /w >> 1 E-field Wave External power e De eel en Collisionless heating where n m /w << 1 e T e n p Electron heating mechanism Wave-electron interaction Electron-neutral interaction EEDF e high Collision process e th e th Electron energy (ev) Plasma e low 0 Particle energy r Dissociation by electrons UV Ionization Ion electron Various collision processes Ionization, Excitation, Dissociation, etc Chemical reaction Ion acceleration Sheath Plasma processing Radicals UV Ion bombardment 1
Study on the electron heating mechanism : Theoretical and experimental studies for collisionless heating in single ICP or CCP. [1] H. A. Blevin et al., Phys. Fluids 13, 1259 (1970). [2] V. A. Godyak et al., Phys. Rev. Lett. 65, 996 (1990). [3] M. A. Lieberman, IEEE Trans. Plasma Sci. 16, 638 (1988). [4] M. M. Turner, Phys. Rev. Lett. 75, 1312 (1995). [5] M. M. Turner, Phys. Rev. Lett. 71, 1844 (1993). [6] V. Vahedi et al., J. Appl. Phys. 78, 1446 (1995). [7] I. D. Kaganovich et al., Appl. Phys. Lett. 69, 3818 (1996). [8] V. A. Godyak et al., Phys. Rev. Lett. 81, 369 (1998). [9] Y. M. Aliev et al., Phys. Plasmas 4, 2413 (1997) [10] I. D. Kaganovich, Phys. Rev. Lett. 82, 327 (1999). [11] A. V. Vasenkov et al., Phys. Rev. E 66, 066411 (2002). [12] C. W. Chung et al., Phys. Rev. Lett. 88, 095002 (2002). [13] O. V. Polomarov et al., Phys. Plasmas 12, 080704 (2005). [14] G.Y. Park et al., Phys. Rev. Lett. 98, 085003 (2007). [15] T. Mussenbrock et al., Phys. Rev. Lett. 101, 085004 (2008). [16] H. C. Lee et al., Appl. Phys. Lett. 96, 041503 (2010). Etc : Now, effect of frequency coupling on the collisionless heating. dual frequency CCP [17] H. C. Kim et al., Phys. Rev. Lett. 93, 085003 (2004). [18] M. M. Turner, Phys. Rev. Lett. 96, 205001 (2006). [19] Y. X. Liu et al., Phys. Rev. Lett. 107, 055002 (2011). CCP + ICP RF biased ICP? : Studies at electro-negative gases? 2
Log EEPF Log EEPF EEDF modification to control plasma processing e th: negative ion e th: ioniz, diss, exci, e th,1 e th,2 Electron energy (ev) Electron energy (ev) ex> F/CF x for SiO 2 etching Si etching To control the radical composition, control of EEDF & T e, not n e. 3
Experimental set-up Reactor Additional Inductive power Langmuir probe system Probe tip 1st 2nd 1st 2nd : RF Choke Vacuum seal DC blocking capacitor Reference ring probe Ceramic tube EEDF from Druyvesteyn method 2 2 die 2 e 2 2 Af e v e dv m n e emaz 0 f e e d e, 3 1 emaz e kteff e f d 2 0 e e e n e Plasma generation 1. Direct differentiation of the I-V characteristic curve. 2. AC superposition method. detailed description: Phys. Plasmas. 20 033504 (2013) 4
Pure capacitive discharge at Ar Capacitive (Bias) discharge - Driving frequency: 12.5 MHz - RF power: 80 W - Argon gas: 3 mtorr Bi-Maxwellian distribution Electro-positive plasma e high energy ambipolar potential well e low energy Stochastic heating 5
Additional inductive power to CCP at Ar Background plasma: fixed capacitive (Bias) power of 80 W Additional inductive power (0-80 W) Capacitive discharge + ICP inductive power Pure capacitive discharge Lee et al., Appl. Phys. Lett. 93 231503 (2008) 6
Additional inductive power to CCP at Ar T 1 : temperature of low energy electron group. T 2 : temperature of high energy electron group. n 1 : density of low energy electron group. n 0 : total electron density. 7
Additional inductive power to CCP at Ar (a) Pure ICP or CCP (b) Lee et al., Phys. Plasmas. 19 033514 (2012) T 1 Discharge Type ICP 80 W 1.46 ev Bi-Maxwellian (c) d 10 cm CCP ICP+CCP 0.70 ev 2.70 ev Bi-Maxwellian toward Maxwellian (d) n eff 2.0 10 7 s -1 n en ~ 10 6 s -1 Experimental evidence for collisionless heating of low energy electrons by inductive field, even in the E mode of ICP 8
Inductive field to Capacitive discharge at Ar/O 2 gas Plasma generation - Capacitive power: 120 W, Gas pressure: 10 mtorr Ar/O 2 =10:5 (sccm) O 2 =0:15 (sccm) Lee et al., J. Appl. Phys. 111 093301 (2012) 9
Inductive field to Capacitive discharge at Ar/O 2 gas Inductive power (W) 30 25 20 15 10 5 Evolution point of EEPF in the presence of CCP 15:0 10:5 5:10 0:15 Ar:O 2 flow rate (sccm) With O 2, smaller inductive power is needed for evolution of the EEPF 10
Possible low energy electron heating mechanism Electro-positive plasma e Electro-negative plasma e ambipolar potential well e high e high 0 e low r 0 e low 1 Pind npve A evambi n p = 9.3 10 4 8 cm -3, v e = 4.1 10 5 m/s (low energy group), A 2 Rl(R + l) = 0.26 m 2, and V ambi 0.31 V P ind 1.2 W Experiment: 2.5 W at O 2 plasma ambipolar potential well r High energy electrons large CCP power Low energy electrons small inductive power, they become high energy electrons large CCP power 11
Independent Control of Plasma Property using Extremely Low Inductive Power for Device Fabrication Process 25 Lowest electron-threshold-energy for radical generation (ev) 20 15 10 5 0 CF4 O2 CO2 SF6 SiF4 H2 N2 SiH4 CF 4 O 2 CO 2 SF 6 SiF 4 H 2 N 2 SiH 4 Gas type Ref. for the gases: Kimura et., J. Appl. Phys. 100 063303 (2006), 108 033305. (2010) 107 083308 (2010), Fiala et al., J. Appl. Phys. 86 152 (1999). Mao, M. et al., J. Phys. D: Appl. Phys. 44 435202 (2011). Janev, R. et al., Contrib. Plasma Phys. 43 401 (2003). 12
Control of EEDF Existing method Gas pressure RF power Low & high energy electrons EEDF Radicals, Driving frequency Gap length T e also, n e ion & electron energy flux etc Our method : Effectively low energy electrons EEDF Radicals, applying additional low inductive field to background plasma T e little change in n e Little change in ion & electron energy flux 13
To control EEDF Our method : applying additional low inductive field to background plasma (a) Inductive power for plasma control Matching network (b) Continuous-wave (CW) Inductive power EM wave penetration Electrode Tungsten substrate Main power for plasma generation Matching network 14
Plasma parameters & discharge characteristics Inductive power to CCP (a) Plasma density & effective electron temperature n e (cm -3 ) (b) Self-bias voltage CCP 120 W, O 2 30 mtorr 10 9 10 8-500 -510 2.7 2.4 2.1 1.8 1.5 1.2 0.9 0.6 0.3 1 10 100 Inductive power (W) T eff (ev) (c) OES data for O radicals Intensity (a.u.) 9000 Inductive power 8000 0W 10W 2W 20W 4W 30W 7000 6000 5000 4000 3000 777.4 Self-bias voltage (V) -520-530 -540-550 2000 1000 590.0 595.8 615.7 645.4 686.1 844.6-560 600 650 700 750 800 850-570 0 10 20 30 40 50 60 70 Inductive power (W) Wavelength (nm) 15
Log EEPF Photo-resistor ashing (a) External power (b) 1 mm EM Wave PR T e n p Electron heating mechanism Wave-electron interaction Electron-neutral interaction EEDF Si Collision process e th,1 e th,2 Electron energy (ev) Plasma Sample 4.0 ICP 0 W CCP 120 W ICP 4 W CCP 120 W ICP 10 W CCP 120 W ICP 20 W CCP 120 W 0.16 Chemical reaction Dissociation by electrons Ion acceleration + Ionization Ion Electron + Sheath Film Thickness (mm) 3.5 3.0 2.5 2.0 0.12 0.08 0.04 Ashing rate (mm/min) 0.00 wafer 1.5 Sample 0 4 10 20 Additional ICP power (W) for EEPF control controllable processing result with reduced wafer damage, which is usually caused by the ion energy flux to nano-materials or wafer. 16
Conclusions Collisionless heating of low energy electrons by extremely low inductive power to CCP. In an Ar/O 2 mixture capacitive discharge, the evolution of the EEDF is occurred at a smaller inductive power (a few W) and it is attributed to a combined effect of collisionless heating by capacitive and inductive electric fields at the presence of the negative ions This method, adding small inductive power to the background plasma, is applied to the plasma processing control. It is revealed that the ashing rate of the photo resistor is significantly enhanced due to the rich radicals produced by the controlled EEPF, even though the ion density/energy flux is not increased. plasma processing control. 17
10 11 CCP 80 W, Ar 3 mtorr 4 Log EEPF (ev -3/2 cm -3 ) Log EEPF (ev -3/2 cm -3 ) Supplement: Pulsed Inductive Power to CCP (a) 10 10 CW CCP 80 W, Ar 3 mtorr 2.4 2.0 n e (cm -3 ) 10 9 OFF PM ICP 20 W ON OFF 1.6 1.2 T eff (ev) 0.8 0 20 40 60 80 100 Time (ms) (b) n e (cm -3 ) 10 10 PM ICP 50 W ON 3 2 T eff (ev) 10 9 OFF OFF 1 0 0 20 40 60 80 100 Time (ms) 18
Discussion: for the result in SF 6 gas discharge with extremely high electronegativity GEC conference 2010. Oct. 6th Paris, France Plasma generation ICP: 13.56 MHz Matching network 12 cm 13 cm 8 cm Substrate Langmuir probe Bias: 12.5 MHz Matching network Additional capacitive field 19
Discussions: ICP + small capacitive power Gas pressure : 50 mtorr (d) Electro-positive plasma (a) e high energy ambipolar potential well e low energy (b) Sheath heating (stochastic & Ohmic) by Capacitive field (e) Electro-negative plasma significant reduction of ambipolar potential well e high energy (c) e low energy Sheath heating (stochastic & Ohmic) by Capacitive field 20
Discussions: ICP + small capacitive field At Ar/SF 6 of 50 mtorr at 100 W, as the capacitive power increases, - Ar/SF 6 = 9 T eff : slight increase with little change in EEPF. - Ar/SF 6 = 1 T eff : remarkable increase with the generations of high energy electrons on the EEPF. At Ar/SF 6 of 50%, as the capacitive power increases, - 3 mtorr at ICP power of 100 W T eff : slight increase with little change in EEPF. - 50 mtorr at ICP power of 300 W T eff : slight increase with little change in EEPF. Strong modification of EEDF and electron heating mechanism (related to the negative ions), when the capacitive field is applied. However, this strong electron heating by RF capacitive power in electronegative ICP has been not solved for theoretical or simulation approach. 21
Discussions: Plasma generation ICP: 13.56 MHz Matching netwok Related reference for electron heating by RF capacitive power in electro-positive ICP Substrate Bias: 12.5 MHz Matching netwok Langmuir probe Additional capacitive field Electro-positive plasma Gas pressure: Ar 2 mtorr, where l e > L Plasma generation: ICP power 60 W Additional RF capacitive power: 0-150 W Electro-negative plasma Gas pressure: 50 mtorr, where l e < L Plasma generation: ICP power 100 W Additional RF capacitive power: 0-15 W Bulk heating to Sheath heating Lee et al., Appl. Phys. Lett. 101 244104 (2012) Is there a behind electron heating mechanism in strongly electro-negative plasma? 22