Aspect Ratio Dependent Twisting and Mask Effects During

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

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 Si. O 2 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 MINGMEI_AVS 08_AGENDA University of Michigan Institute for Plasma Science and Engineering

CHALLENGES IN HARC ETCHING Mask Erosion Ref: Oxford Instruments Bowing Ref: JJAP, 46, p 7873 (2007) Ref: ULVAC Technologies Twisting · 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 MINGMEI_AVS 08_01 Institute for Plasma Science and Engineering

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. MINGMEI_AVS 08_02 University of Michigan Institute for Plasma Science and Engineering

MONTE CARLO FEATURE PROFILE MODEL · Monte Carlo techniques address plasma surface interactions and evolution of surface profiles. Ions, HEE, radicals and neutrals · Electric potential is solved using Successive Over Relaxation (SOR) method. Charged particles - + + + Mask Si. O 2 Polymer + + + Si -6 MINGMEI_AVS 08_03 0 151 University of Michigan Institute for Plasma Science and Engineering

SURFACE REACTION MECHANISM · Etching of Si. O 2 is dominantly through a formation of a fluorocarbon complex. · Si. O 2(s) + Cx. Fy+(g) Si. O 2*(s) + Cx. Fy#(g) · Si. O 2*(s) + Cx. Fy(g) Si. O 2 Cx. Fy(s) · Si. O 2 Cx. Fy (s) + Cx. Fy+(g) Si. Fy(g) + CO 2 (g) + Cx. Fy#(g) · Further deposition by Cx. Fy(g) produces thicker polymer layers. · Sputtering of photo resist and redeposition. MINGMEI_AVS 08_04 · PR(s) + Cx. Fy+(g) PR(g) + Cx. Fy#(g) · PR(g) + Si. O 2 Cx. Fy(s) + PR(s) University of Michigan Institute for Plasma Science and Engineering

FLUOROCARBON ETCHING OF SIO 2 · 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/C 4 F 8/O 2 = 80/15/5, 300 sccm, 40 m. Torr, RF 1 k. W at 10 MHz, DC 200 W/-250 V. MINGMEI_AVS 08_05 University of Michigan Institute for Plasma Science and Engineering

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 m. Torr, LF: 10 MHz, 300 W, 440 V/dc=-250 V · DC: 200 W, -470 V MINGMEI_AVS 08_06 ANIMATION SLIDE-GIF University of Michigan Institute for Plasma Science and Engineering

HIGH ENERGY ELECTRON (HEE) FLUXES · HEE fluxes increase with increasing RF bias power due to increase in plasma density. · 40 m. Torr, RF 10 MHz, DC 200 W/-250 V, Ar/C 4 F 8/O 2 = 80/15/5, 300 sccm · HEE flux increases with increasing DC voltage. · HEE is naturally generated by RF oscillation (when VDC=0 V). · 40 m. Torr, RF 4 k. W/1. 5 k. V at 10 MHz, Ar/C 4 F 8/O 2 = 80/15/5, 300 sccm MINGMEI_AVS 08_07 University of Michigan Institute for Plasma Science and Engineering

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 m. Torr, Ar/C 4 F 8/O 2 = 80/15/5, 300 sccm, RF 10 MHz, DC 200 W/-250 V. MINGMEI_AVS 08_08 University of Michigan Institute for Plasma Science and Engineering

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 m. Torr, Ar/C 4 F 8/O 2 = 80/15/5, 300 sccm, RF 10 MHz, DC 200 W/250 V. MINGMEI_AVS 08_09 University of Michigan Institute for Plasma Science and Engineering

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 m. Torr, Ar/C 4 F 8/O 2 = 80/15/5, 300 sccm, RF 1 k. W at 10 MHz, DC 200 W. Different random seeds Without HEE Different random seeds With HEE Aspect Ratio = 1: 25 MINGMEI_AVS 08_10

HEE EFFECTS on TWISTING: COARSE MESH · Coarse mesh (~5 nm) with photo resist erosion on the top. Without HEE · 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 (75 nm) to the plasma and statistican nature of fluxes. · 40 m. Torr, Ar/C 4 F 8/O 2 = 80/15/5, 300 sccm, RF 5 k. W at 10 MHz. Aspect Ratio = 1: 20 With HEE MINGMEI_AVS 08_11 University of Michigan Institute for Plasma Science and Engineering

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 e. V), low DC voltage causes broader angular distribution and lower particle density. · 40 m. Torr, Ar/C 4 F 8/O 2 = 80/15/5, sccm, RF 1. 5 k. V at 10 MHz. MINGMEI_AVS 08_12 300 University of Michigan Institute for Plasma Science and Engineering

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 m. Torr, Ar/C 4 F 8/O 2 = 80/15/5, 300 sccm, RF 1. 5 k. V at 10 MHz. MINGMEI_AVS 08_13 Aspect Ratio = 1: 20 University of Michigan Institute for Plasma Science and Engineering

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 m. Torr, Ar/C 4 F 8/O 2 = 80/15/5, 300 sccm, RF 5 k. W at 10 MHz. Aspect Ratio = 1: 30 MINGMEI_AVS 08_14 ANIMATION SLIDE-GIF University of Michigan Institute for Plasma Science and Engineering

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 m. Torr, Ar/C 4 F 8/O 2 = 80/15/5, 300 sccm, RF 5 k. W at 10 MHz. Aspect Ratio = 1: 30 MINGMEI_AVS 08_14 University of Michigan Institute for Plasma Science and Engineering

MASK MATERIAL EFFECTS · Hard mask is not etched or sputtered easily. · PR has an etching selectivity of ~10 over Si. O 2. · Bowing occurs at the middle height of trench with the hard mask. · Bowing occurs right under the PR layer. · 40 m. Torr, Ar/C 4 F 8/O 2 = 80/15/5, 300 sccm, RF 5 k. W at 10 MHz. (AR=30) MINGMEI_AVS 08_15 (AR=30) (AR=40) University of Michigan Institute for Plasma Science and Engineering

BOWING MECHANISM Ions & HEE · 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 m. Torr, Ar/C 4 F 8/O 2 = 80/15/5, 300 sccm, RF 5 k. W at 10 MHz. MINGMEI_AVS 08_16 E-Field University of Michigan Institute for Plasma Science and Engineering

PROPOSED METHODS OF BOWING ELIMINATION · Many methods have been proposed to address bowing. · Deposit a protective layer onto PR. · Sputtering protective layer away at later stage of etching. PR HM · 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 Si. O 2 etching. MINGMEI_AVS 08_17 University of Michigan Institute for Plasma Science and Engineering

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. MINGMEI_AVS 08_18 University of Michigan Institute for Plasma Science and Engineering