Aspect Ratio Dependent Twisting and Mask Effects During Plasma Etching of SiO2 in Fluorocarbon Gas Mixture* Mingmei Wang1 and Mark J. Kushner2 1Iowa State University, Ames, IA 50011 USA mmwang@iastate.edu 2University of Michigan, Ann Arbor, MI 48109 USA mjkush@umich.edu http://uigelz.eecs.umich.edu 55th AVS, October 2008, Boston, MA *Work supported by the SRC, Micron Inc. and Tokyo Electron Ltd.

University of Michigan Institute for Plasma Science AGENDA Issues in high aspect ratio contact (HARC) etching. Approaches and Methodologies Electric field buildup due to charge deposition. Feature twisting; trench to trench variation when etching at critical dimension (CD). High energy electron (HEE) effects on feature twisting in SiO2 etching over Si. Varied mesh resolution due to computing limitation. Photo resist sputtering and redeposition. Twisting and bowing during etch in features patterned with photo resist (PR) and hard mask (HM). Concluding Remarks University of Michigan Institute for Plasma Science and Engineering MINGMEI_AVS08_AGENDA

CHALLENGES IN HARC ETCHING Ref: Oxford Instruments Mask Erosion Bowing Twisting Ref: ULVAC Technologies Ref: JJAP, 46, p7873 (2007) Etched features for advanced micro-electronic devices have aspect ratios (AR) approaching 100. Twisting, bowing and consequences of mask erosion challenge maintaining CD. In this poster, results from a computational investigation of these processes are presented. University of Michigan Institute for Plasma Science and Engineering MINGMEI_AVS08_01

HYBRID PLASMA EQUIPMENT MODEL (HPEM) Electromagnetics Module: Antenna generated electric and magnetic fields. Electron Energy Transport Module: Beam and bulk generated sources and transport coefficients. Fluid Kinetics Module: Electron and Heavy Particle Transport. Plasma Chemistry Monte Carlo Module: Ion, Higher Energy Electron (HEE) and Neutral Energy and Angular Distributions. Fluxes for feature profile model. University of Michigan Institute for Plasma Science and Engineering MINGMEI_AVS08_02

MONTE CARLO FEATURE PROFILE MODEL Monte Carlo techniques address plasma surface interactions and evolution of surface profiles. Electric potential is solved using Successive Over Relaxation (SOR) method. Ions, HEE, radicals and neutrals Charged particles + - Mask SiO2 Polymer Si University of Michigan Institute for Plasma Science and Engineering -6 151 MINGMEI_AVS08_03

SURFACE REACTION MECHANISM Etching of SiO2 is dominantly through a formation of a fluorocarbon complex. SiO2(s) + CxFy+(g)  SiO2*(s) + CxFy#(g) SiO2*(s) + CxFy(g)  SiO2CxFy(s) SiO2CxFy (s) + CxFy+(g)  SiFy(g) + CO2 (g) + CxFy#(g) Further deposition by CxFy(g) produces thicker polymer layers. Sputtering of photo resist and redeposition. PR(s) + CxFy+(g)  PR(g) + CxFy#(g) PR(g) + SiO2CxFy(s)  SiO2CxFy(s) + PR(s) University of Michigan Institute for Plasma Science and Engineering MINGMEI_AVS08_04

FLUOROCARBON ETCHING OF SIO2 DC augmented single frequency capacitively coupled plasma (CCP) reactor. DC: Top electrode RF: Substrate Plasma tends to be edge peaked due to electric field enhancement. Plasma densities in excess of 1011 cm-3. Ar/C4F8/O2 = 80/15/5, 300 sccm, 40 mTorr, RF 1 kW at 10 MHz, DC 200 W/-250 V. University of Michigan Institute for Plasma Science and Engineering MINGMEI_AVS08_05

University of Michigan Institute for Plasma Science 10 MHz LOWER, DC UPPER: PLASMA POTENTIAL LF electrode passes rf current. DC electrode passes combination of rf and dc current with small modulation of sheath potential. Ar, 40 mTorr, LF: 10 MHz, 300 W, 440V/dc=-250V DC: 200 W, -470 V University of Michigan Institute for Plasma Science and Engineering ANIMATION SLIDE-GIF MINGMEI_AVS08_06

HIGH ENERGY ELECTRON (HEE) FLUXES HEE fluxes increase with increasing RF bias power due to increase in plasma density. 40 mTorr, RF 10 MHz, DC 200 W/-250 V, Ar/C4F8/O2 = 80/15/5, 300 sccm HEE flux increases with increasing DC voltage. HEE is naturally generated by RF oscillation (when VDC=0 V). 40 mTorr, RF 4 kW/1.5 kV at 10 MHz, Ar/C4F8/O2 = 80/15/5, 300 sccm University of Michigan Institute for Plasma Science and Engineering MINGMEI_AVS08_07

ION ENERGY ANGULAR DISTRIBUTIONS (IEADs) IEADs for sum of all ions. Peak in ion energy increases with increasing rf bias power while IEAD narrows. Higher energy ions increase maximum positive charging of feature. 40 mTorr, Ar/C4F8/O2 = 80/15/5, 300 sccm, RF 10 MHz, DC 200 W/-250 V. University of Michigan Institute for Plasma Science and Engineering MINGMEI_AVS08_08

HEE ENERGY ANGULAR DISTRIBUTIONS HEE energy increases with increasing rf bias power. Narrower angular distribution (-2０~ 2０) than for ions. Peak at maximum energy with long tails. 40 mTorr, Ar/C4F8/O2 = 80/15/5, 300 sccm, RF 10 MHz, DC 200 W/-250 V. University of Michigan Institute for Plasma Science and Engineering MINGMEI_AVS08_09

HEE EFFECTS ON TWISTING: FINE MESH Atomic scale mesh size (~3 Å). Ions hitting the surface deposit charge. Electrons may scatter. Statistical composition of fluxes into small features produces occasional twisting. Twisting occurs randomly without considering HEE (3/20). HEE neutralizes charge effectively deep into the trench. 40 mTorr, Ar/C4F8/O2 = 80/15/5, 300 sccm, RF 1 kW at 10 MHz, DC 200 W. Without HEE With HEE Different random seeds Aspect Ratio = 1:25 MINGMEI_AVS08_10

HEE EFFECTS on TWISTING: COARSE MESH Without HEE With HEE Coarse mesh (~5 nm) with photo resist erosion on the top. Bowing occurs at later stage of etching due to reflection from sloped profile of eroded PR. HEE fluxes improve feature profiles. Trench to trench differences due to small opening (75nm) to the plasma and statistican nature of fluxes. 40 mTorr, Ar/C4F8/O2 = 80/15/5, 300 sccm, RF 5 kW at 10 MHz. Aspect Ratio = 1:20 University of Michigan Institute for Plasma Science and Engineering MINGMEI_AVS08_11

HEE ENERGY ANGULAR DISTRIBUTIONS HEE energy increases with increasing DC voltage. Narrower angular distribution is obtained at high voltage with longer tails. At low energy region (<500 eV), low DC voltage causes broader angular distribution and lower particle density. 40 mTorr, Ar/C4F8/O2 = 80/15/5, 300 sccm, RF 1.5 kV at 10 MHz. University of Michigan Institute for Plasma Science and Engineering MINGMEI_AVS08_12

TWISTING ELIMINATION: DC VOLTAGE Different random seeds Two group of profiles are selected from 21 cases with different random seed number generators. HEE neutralizes positive charge deep into the trench. Higher HEE energy and flux produce better profiles and higher etch rates: VDC=0 V, twisting probability=7/21. VDC=500 V, twisting probability=5/21. VDC=750 V, twisting probability=3/21. 40 mTorr, Ar/C4F8/O2 = 80/15/5, 300 sccm, RF 1.5 kV at 10 MHz. University of Michigan Institute for Plasma Science and Engineering Aspect Ratio = 1:20 MINGMEI_AVS08_13

PHOTO RESIST SPUTTERING and PROFILE BOWING Time sequence of feature etching. Photo resist is eroded during process broadening view-angle to plasma. Bowing occurs at later stage of etching as view-angle and slope of PR increases. 40 mTorr, Ar/C4F8/O2 = 80/15/5, 300 sccm, RF 5 kW at 10 MHz. Aspect Ratio = 1:30 University of Michigan Institute for Plasma Science and Engineering ANIMATION SLIDE-GIF MINGMEI_AVS08_14

PHOTO RESIST SPUTTERING and PROFILE BOWING Time sequence of feature etching. Photo resist is eroded during process broadening view-angle to plasma. Bowing occurs at later stage of etching as view-angle and slope of PR increases. 40 mTorr, Ar/C4F8/O2 = 80/15/5, 300 sccm, RF 5 kW at 10 MHz. Aspect Ratio = 1:30 University of Michigan Institute for Plasma Science and Engineering MINGMEI_AVS08_14

University of Michigan Institute for Plasma Science MASK MATERIAL EFFECTS Hard mask is not etched or sputtered easily. PR has an etching selectivity of ~10 over SiO2. Bowing occurs at the middle height of trench with the hard mask. Bowing occurs right under the PR layer. 40 mTorr, Ar/C4F8/O2 = 80/15/5, 300 sccm, RF 5 kW at 10 MHz. (AR=30) (AR=30) University of Michigan Institute for Plasma Science and Engineering (AR=40) MINGMEI_AVS08_15

University of Michigan Institute for Plasma Science E-Field BOWING MECHANISM With hard mask, as etch depth increases, ions with a small incident angle hit the side wall. Statistical deposition of charge produces deflection of narrow angle ions. With photo resist etching, ions hitting PR surface reflect to the side wall of trench. 40 mTorr, Ar/C4F8/O2 = 80/15/5, 300 sccm, RF 5 kW at 10 MHz. Ions & HEE University of Michigan Institute for Plasma Science and Engineering MINGMEI_AVS08_16

PROPOSED METHODS OF BOWING ELIMINATION Many methods have been proposed to address bowing. PR HM Deposit a protective layer onto PR. Sputtering protective layer away at later stage of etching. Multiple layers of mask materials (upper PR, lower hard mask). Increase HEE flux and energy to further neutralize positive charge on trench bottom and side walls. Control ion energy as the etch proceeds to utilize selectivity difference between PR and SiO2 etching. University of Michigan Institute for Plasma Science and Engineering MINGMEI_AVS08_17

University of Michigan Institute for Plasma Science CONCLUDING REMARKS HEE effects on eliminating twisting in HARC etching have been computationally investigated in fluorocarbon plasmas. Statistical nature of ion fluxes into small features produce lateral electric fields which deflect ions. HEE neutralizes positive charge deep into the trench to eliminate ion trajectory change and accelerate etching. Photo resist sputtering leads to bowing at top of feature profile. Bowing occurs at middle of feature in HARC (AR~40) etching. University of Michigan Institute for Plasma Science and Engineering MINGMEI_AVS08_18