SiO 2 ETCH PROPERTY CONTROL USING PULSE POWER IN CAPACITIVELY COUPLED PLASMAS* Sang-Heon Song a) and Mark J. Kushner b) a) Department of Nuclear Engineering and Radiological Sciences University of Michigan, Ann Arbor, MI 48109, USA b) Department of Electrical Engineering and Computer Science University of Michigan, Ann Arbor, MI 48109, USA Nov AVS * Work supported by DOE Plasma Science Center and Semiconductor Research Corp.

AGENDA  Motivation for controlling f(  )  Description of the model  Typical Ar/CF 4 /O 2 pulsed plasma properties  Etch rate with variable blocking capacitor  Etch property with different PRF  Etch rate, profile, and selectivity  Concluding Remarks University of Michigan Institute for Plasma Science & Engr. SHS_MJK_AVS

CONTROL OF ELECTRON KINETICS – f(  )  Controlling the generation of reactive species for technological devices benefits from customizing the electron energy (velocity) distribution function. University of Michigan Institute for Plasma Science & Engr. e + CF 4 CF 3 + F + e k SHS_MJK_AVS

ETCH RATE vs. FLUX RATIOS University of Michigan Institute for Plasma Science & Engr. Ref: D. C. Gray, J. Butterbaugh, and H. H. Sawin, J. Vac. Sci. Technol. A 9, 779 (1991) Flux Ratio (F/Ar + )Flux Ratio (CF 2 /Ar + ) Etching Yield (Si/Ar + )  Large fluorine to ion flux ratio enhances etching yield of Si.  Large fluorocarbon to ion flux ratio reduces etching yield of Si. SHS_MJK_AVS

Ref: K. Ono, M. Tuda, H. Ootera, and T. Oomori, Pure and Appl. Chem. Vol 66 No 6, 1327 (1994)  Large chlorine radical to ion flux ratio produces an undercut in etch profile.  Etch profile result in ECR Cl 2 plasma after 200% over etch with different flux ratios p-Si University of Michigan Institute for Plasma Science & Engr. ETCH PROFILE vs. FLUX RATIOS  Flux Ratio (Cl / Ion) = 0.3  Flux Ratio (Cl / Ion) = 0.8 SHS_MJK_AVS

HYBRID PLASMA EQUIPMENT MODEL (HPEM)  Fluid Kinetics Module:  Heavy particle and electron continuity, momentum, energy  Poisson’s equation  Electron Monte Carlo Simulation:  Includes secondary electron transport  Captures anomalous electron heating  Includes electron-electron collisions E, N i, n e Fluid Kinetics Module Fluid equations (continuity, momentum, energy) Poisson’s equation T e, S b, S eb, k Electron Monte Carlo Simulation University of Michigan Institute for Plasma Science & Engr. SHS_MJK_AVS

MONTE CARLO FEATURE PROFILE MODEL (MCFPM)  The MCFPM resolves the surface topology on a 2D Cartesian mesh.  Each cell has a material identity. Gas phase species are represented by Monte Carlo pseuodoparticles.  Pseuodoparticles are launched with energies and angles sampled from the distributions obtained from the HPEM  Cells identities changed, removed, added for reactions, etching deposition. PCMCM Energy and angular distributions for ions and neutrals MCFPM Etch rates and profile University of Michigan Institute for Plasma Science & Engr.  Poisson’s equation solved for charging HPEM SHS_MJK_AVS

REACTOR GEOMETRY: 2 FREQUENCY CCP  2D, cylindrically symmetric  Ar/CF 4 /O 2 = 75/20/5, 40 mTorr, 200 sccm  Base conditions  Lower electrode: LF = 10 MHz, 500 W, CW  Upper electrode: HF = 40 MHz, 500 W, Pulsed University of Michigan Institute for Plasma Science & Engr. SHS_MJK_AVS

PULSE POWER Time  = 1/PRF Duty Cycle Power(t) P min P max University of Michigan Institute for Plasma Science & Engr.  Use of pulse power provides a means for controlling f(  ).  Pulsing enables ionization to exceed electron losses during a portion of the ON period – ionization only needs to equal electron losses averaged over the pulse period.  Pulse power for high frequency.  Duty-cycle = 25%, PRF = 50, 100, 200, 415, 625 kHz  Average Power = 500 W SHS_MJK_AVS

VARIABLE BLOCKING CAPACITOR  Due to the different area of two electrodes, a “dc” bias is produced on the blocking capacitor connected to the substrate electrode.  The temporal behavior of “dc” bias is dependent on the magnitude of the capacitance due to RC delay time. University of Michigan Institute for Plasma Science & Engr.  We investigated variable blocking capacitor of 10 nF, 1  F, and 100 F  100 F of blocking capacitor results in NO “dc” bias on the substrate. SHS_MJK_AVS

Typical Plasma Properties SHS_MJK_AVS

PULSED CCP: Electron Density & Temperature  Pulsing with a moderate PRF duty cycle produces nominal intra- cycles changes in [e] but does modulate T e.  Electron Density (x cm -3 )  Electron Temperature (eV) University of Michigan Institute for Plasma Science & Engr. MIN MAX  40 mTorr, Ar/CF 4 /O 2 =75/20/5  PRF = 100 kHz, Duty-cycle = 25%  HF = 40 MHz, pulsed 500 W  LF = 10 MHz, 250 V SHS_MJK_AVS ANIMATION SLIDE-GIF

PULSED CCP: ELECTRON SOURCES  The electrons have two groups: bulk low energy electrons and beam-like secondary electrons.  The bulk electron source is negative due to electron attachment and dissociative recombination.  The electron source by beam electrons compensates the electron losses and sustains the plasma.  by Bulk Electrons (x cm -3 s -1 )  by Secondary Electrons University of Michigan Institute for Plasma Science & Engr. MIN MAX  40 mTorr, Ar/CF 4 /O 2 =75/20/5  LF 250 V, HF 500 W SHS_MJK_AVS ANIMATION SLIDE-GIF

PULSED CCP: E-SOURCES and f(  ) University of Michigan Institute for Plasma Science & Engr.  40 mTorr, Ar/CF 4 /O 2 =75/20/5  PRF = 100 kHz, Duty-cycle = 25%  LF = 10 MHz, 250 V  HF = 40 MHz, pulsed 500 W  Rate coefficient of e-sources is modulated between electron source (electron impact ionization) and loss (attachment and recombination) during pulsed cycle. SHS_MJK_AVS ANIMATION SLIDE-GIF

SHS_MJK_AVS Etch Properties: Variable Blocking Capacitor

PULSED CCP: PLASMA POTENTIAL & dc BIAS  A small blocking capacitor allows the “dc” bias to follow the change during the pulse period.  Maximum ion energy gain = Plasma Potential – “dc” Bias University of Michigan Institute for Plasma Science & Engr.  PRF = 100 kHz, Duty-cycle = 25%  LF = 10 MHz, 250 V  HF = 40 MHz, pulsed 500 W  1  F  10 nF

ETCH PROFILE IN SiO 2 & IEAD: 1  F · With constant voltage, bias amplitude is constant but blocking capacitor determines “dc” bias.  Cycle Average IEAD  Etch Profile (600 sec) University of Michigan Institute for Plasma Science & Engr. Angle (degree) Width (  m) ANIMATION SLIDE-GIF  Pulsed HF 40 MHz 500 W  LF 10 MHz 250 V, Blocking Cap. = 1  F Energy (eV) Height (  m) SHS_MJK_AVS

ETCH PROFILE IN SiO 2 & IEAD: 10 nF · With smaller blocking capacitor, “dc” bias begins to follow the rf power and so produces a different IEAD. University of Michigan Institute for Plasma Science & Engr.  Pulsed HF 40 MHz 500 W  LF 10 MHz 250 V, Blocking Cap. = 1 nF SHS_MJK_AVS Angle (degree) Width (  m) ANIMATION SLIDE-GIF  Cycle Average IEAD  Etch Profile (600 sec) Energy (eV) Height (  m)

ETCH PROFILE IN SiO 2 & IEAD: NO dc BIAS · In absence of dc bias and for constant voltage, pulse power and is effect on f(  ) in large part determine etch properties. University of Michigan Institute for Plasma Science & Engr.  Pulsed HF 40 MHz 500 W  LF 10 MHz 250 V, Blocking Cap. = 100 F SHS_MJK_AVS Angle (degree) Width (  m) ANIMATION SLIDE-GIF  Cycle Average IEAD  Etch Profile (600 sec) Energy (eV) Height (  m)

POWER NORMALIZED ER: Blocking Capacitor · Power normalized etch rate is dependent not only on the pulse repetition frequency (PRF), but also the value of the blocking capacitor on the substrate at lower PRF. University of Michigan Institute for Plasma Science & Engr.  Pulsed HF 40 MHz 500 W  LF 10 MHz 250 V SHS_MJK_AVS CW kHz A B C A B C · F to Poly Flux ratio

· Electron source rate coefficient is modulated with f(  ) by pulse power. · Modulation is enhanced with smaller PRF. E-SOURCES and FLUX RATIO: PRF University of Michigan Institute for Plasma Science & Engr. SHS_MJK_AVS  Pulsed HF 40 MHz 500 W  LF 10 MHz 250 V  Blocking Cap. = 1  F · F to Poly Flux ratio

· Power normalized etch rate is large at 250 kHz with ion distribution extending to higher energies. University of Michigan Institute for Plasma Science & Engr.  Pulsed HF 40 MHz 500 W  LF 10 MHz 250 V  Without DC Bias on LF electrode ETCH RATE: POWER NORMALIZED CW kHz SHS_MJK_AVS  Cycle Average IEAD  Normalized Etch Rate Angle (degree) Energy (eV)

1  Pulsed HF 40 MHz 500 W  LF 10 MHz 250 V  Blocking Cap. = 1  F University of Michigan Institute for Plasma Science & Engr. SHS_MJK_AVS · EPD + Over Etch 50% ETCH PROFILE: CRITICAL DIMENSION 2 (1/A) (2/A) CW kHz  CD is compared at the middle and bottom of feature.  CW excitation produces bowing and an undercut profile.  Pulse plasma helps to prevent the bowing and under- cutting.  Smaller PRF has a tapered profile. A

ETCH SELECTIVITY: Between SiO 2 and Si  Pulsed HF 40 MHz 500 W  LF 10 MHz 250 V  Blocking Cap. = 1  F University of Michigan Institute for Plasma Science & Engr. SHS_MJK_AVS CW kHz  Silicon damage depth is compared in 2-D etch profile.  Pulsed operation helps to prevent the silicon damage.  Lower damage appears to be correlated with smaller F flux ratio at 250 kHz. · EPD + Over Etch 50%

CONCLUDING REMARKS  Extension of tail of f(  ) beyond that obtained with CW excitation produces a different mix of fluxes to substrate.  Etch rate can be controlled by pulsed operation with different pulse repetition frequencies.  Blocking capacitor is another variable to control ion energy distributions and etch rates. Smaller capacitance allows “dc” bias to follow the plasma potential in pulse period more rapidly.  Etch rate is enhanced by pulsed power operation in CCP.  Etch profile is improved with pulsed operation preventing undercut.  Etch selectivity of SiO 2 to Si is also improved with PRF of 250 kHz with a smaller fluorine flux ratio. University of Michigan Institute for Plasma Science & Engr. SHS_MJK_AVS