Reaction Mechanism and Profile Evolution for HfO2 High-k Gate-stack Etching: Integrated Reactor and Feature Scale Modeling* Juline Shoeba) and Mark J. Kushnerb) a) Department of Electrical and Computer Engineering Iowa State University, Ames, IA 50011 jshoeb@eecs.umich.edu b) Department of Electrical Engineering and Computer Science University of Michigan Ann Arbor, Ann Arbor, MI 48109 mjkush@eecs.umich.edu http://uigelz.eecs.umich.edu 55th AVS Symposium, October 2008 * Work supported by Semiconductor Research Corporation JULINE_AVS08_01

University of Michigan, Ann Arbor Optical and Discharge Physics AGENDA High-k Dielectrics Modeling Platforms HfO2 Gate-Stack Modeling Challenge Goals and Premises for Etch Mechanism Etching Mechanism HfO2 & SiO2 Etching with Si Selectivity TiN Etching Photo-resist Trimming and BARC Etching HfO2 Etch Rate and Bias Voltage Selectivity: Calibration and Collaboration Conclusion University of Michigan, Ann Arbor Optical and Discharge Physics JULINE_AVS08_02

HIGH-k DIELECTRICS IN GATE STACK As the gate length shrinks in the technology node, the gate oxide leakage current increases with decreasing SiO2 thickness. The use of SiO2 in gate stacks is limited by already approximately one monolayer . High-k dielectrics such as HfO2, offer a thicker oxide without degrading gate capacitance. URL:http://www.research.ibm.com/journal/rd/433/campbell.html University of Michigan, Ann Arbor Optical and Discharge Physics JULINE_AVS08_03

HIGH-k/METAL GATE TRANSISTORS High-k / poly- Si gate MOSFETs have challenges: Degraded channel mobility. Higher threshold voltage. High-k / Metal gates: Reduced mobility degradation addressed by SiO2 layer underlying HfO2. Offers lower dielectric leakage. High-k dielectrics are typically more compatible with non-silicon substrates (e.g., Ge). URL: http://www.research.ibm.com/journal/rd/433/campbell.html University of Michigan, Ann Arbor Optical and Discharge Physics JULINE_AVS08_04

HfO2 GATE STACK MODELING High-k metal oxides are being used as SiO2 replacements to minimize gate leakage. HfO2 has promising properties and can be integrated into current process streams. For process integration and speed, desirable to simultaneously etch entire gate stack…Success with Ar/BCl3/Cl2 plasmas. Challenge: Modeling is required to speed process development and optimization. There exists no fundamental database for process. Develop mechanism based on experience and data from literature. University of Michigan, Ann Arbor Optical and Discharge Physics JULINE_AVS08_05

MODELING PLASMA ETCHING: HIGH-k GATE STACK PR Coils Energy and angular distributions for ions and neutrals BARC Plasma TiN Metal HfO2 SiO2 Substrate Wafer Si Hybrid Plasma Equipment Model (HPEM) Plasma Chemistry Monte Carlo Module (PCMCM) Monte Carlo Feature Profile Model (MCFPM) University of Michigan, Ann Arbor Optical and Discharge Physics JULINE_AVS08_06

HYBRID PLASMA EQUIPMENT MODEL (HPEM) EEM (Electromagnetics Module) EETM (Electron Transport Module) FKM (Fluid Kinetics Module) EETM FKM PCMCM Electron energy equations are solved S Te μ Continuity, momentum, energy equations; and Poisson’s equation are solved Energy and angular distributions for ions and neutrals EΦ,B S EMM Maxwell equations are solved N ES E SCM Calculates energy dependent reaction probabilities PCMCM (Plasma Chemistry Monte Carlo Module) SCM (Surface Chemistry Module) University of Michigan, Ann Arbor Optical and Discharge Physics JULINE_AVS08_07

MONTE CARLO FEATURE PROFILE MODEL (MCFPM) The MCFPM resolves the surface topology on a 2D Cartesian mesh to predict etch profiles. Each cell in the mesh has a material identity (For this work cells as 1x1 nm). Gas phase species are represented by Monte Carlo pseuodoparticles. Pseuodoparticles are launched towards the wafer with energies and angles sampled from the distributions obtained from the PCMCM. HPEM PCMCM Energy and angular distributions for ions and neutrals MCFPM Provides etch rate And predicts etch profile University of Michigan, Ann Arbor Optical and Discharge Physics JULINE_AVS08_08

GOALS AND PREMISES FOR ETCH MECHANISM Optical and Discharge Physics Goal: High selectivity of HfO2 over Si at low ion energies to minimize damage. Premise 1: HfO2 etching is a multistep process requiring Hf-O bond breaking and separate removal of Hf and O. Premise 2: Si etch rate slowed by BClx polymer. BClx first produces Si-B bonding to assist deposition of polymer. Premise 3: TiN etching is analogous to other metals with volatile products (e.g., Al). Use experiments from literature to build mechanism. Material Etch Rate (A0 /min) Threshold (eV) HfO2 90 25 Si 100 29 TiN 400 _ Material Bond Bond (eV) HfO2 Hf-O 8.3 SiO2 Si-O Si Si-Si 3.4 Ref: L.Sha and J. P. Chang, J. Vac. Sci. Technol. A, v. 22, 88 (2004) Iowa State University Optical and Discharge Physics JULINE_AVS08_09

INITIAL GATE STACK FOR SIMULATION 58 nm wide PR with photolithography limited rounded top. Will trim PR to 32 nm PR height : 260 nm HfO2 : 20 nm thick SiO2 : 10 nm thick Si substrate PR BARC TiN HfO2 SiO2 Si University of Michigan, Ann Arbor Optical and Discharge Physics JULINE_AVS08_10

ETCHING MECHANISM IN Ar/BCl3/Cl2 PLASMA HfO2 Etching Bond Breaking M+(g) + HfO2(s)  HfO(s) + O(s) + M(g) M+(g) + HfO(s)  Hf(s) + O(s) + M(g) Adsorption Cl(g) + Hf(s)  HfCl(s) BClx(g) + O(s)  BClxO(s) Etching M+(g) + HFClx (s)  HfClx(g) + M(g) M+(g) + BClxO (s)  ByOClx(g) + M(g) Selectivity with respect to Si results from deposition of BClx polymer on Si BClx(g) + Si(s)  SiBClx(s) BClx(g) + SiBClx(s)  SiBClx(s) + Poly-BClx(s) University of Michigan, Ann Arbor Optical and Discharge Physics JULINE_AVS08_11

ETCHING MECHANISM IN Ar/BCl3/Cl2 PLASMA SiO2 Etching Bond Breaking M+ (g) + SiO2(s)  SiO(s) + O(s) + M(g) M+(g) + SiO2Cl(s)  SiOCl(s) + O(s) + M(g) M+(g) + SiO(s)  SiO*(s) + M(g) M+(g) + SiOClx(s)  SiOClx*(s) + M(g) Adsorption Cl(g) + SiO(s)  SiOCl(s) Cl(g) + SiOCl(s)  SiOCl2(s) Etching M+(g) + SiOClx *(s)  SiClx(g) + O(s) M+(g) + SiO*(s)  SiClx(g) + O(s) University of Michigan, Ann Arbor Optical and Discharge Physics JULINE_AVS08_12

ETCHING MECHANISM IN Ar/BCl3/Cl2 PLASMA TiN Etching TiN etching mechanism has three steps like other metal nitride etching, e.g. AlN. Bond Breaking M+(g) + TiN(s)  Ti(s) + N(g) + M(g) Adsorption Cl(g) + Ti(s)  TiCl(s) Cl(g) + TiCl (s)  TiCl2(s) Cl(g) + TiCl2(s)  TiCl3(s) Etching M+(g) + TiClx(s)  TiClx(g) + M(g) University of Michigan, Ann Arbor Optical and Discharge Physics JULINE_AVS08_13

University of Michigan, Ann Arbor Optical and Discharge Physics Ar/BCl3 /Cl2 PLASMAS Ar/BCl3/Cl2 = 5/40/55 used for etching of HfO2 with selectivity to Si. Species: Ar+ BCl2+ BCl+ Cl+ Cl2+ Ar BCl2 BCl Cl Cl2 Cl* Ar* BCl2+ and Cl fluxes largely determine HfO2 etch rate. To ensure high selectivity average ion energy was around 30 eV. Conditions: Ar/BCl3/Cl2 = 5/40/55, 5 mTorr, 300 W ICP University of Michigan, Ann Arbor Optical and Discharge Physics JULINE_AVS08_14

University of Michigan, Ann Arbor Optical and Discharge Physics Ar/BCl3 /Cl2 PLASMAS Total ion density: 3.2 x 1011 cm-3 Ion densities (cm-3): BCl2+ 1.56 x 1011 Cl+ 1.0 x 1011 Cl2+ 1.0 x 1010 Ar+ 1.0 x 1009 Neutral densities (cm-3): BCl2 1.30 x 1013 Cl 1.0 x 1014 Cl2 6.5 x 1013 Ar 6.0 x 1012 Conditions: Ar/BCl3/Cl2 = 5/40/55, 5 mTorr, 300 W ICP University of Michigan, Ann Arbor Optical and Discharge Physics JULINE_AVS08_15

University of Michigan, Ann Arbor Optical and Discharge Physics Ar/BCl3 /Cl2 PLASMAS BCl2+ is dominant ion in bond breaking of HfO2 . Cl is dominant neutral which is adsorbed by the Hf atoms after Hf-O bonds are broken. BCl2 is needed to form Si-B bonds and polymers. BCl2 is adsorbed by the O atoms of broken Hf-O bonds. Conditions: Ar/BCl3/Cl2 = 5/40/55, 5 mTorr, 300 W ICP University of Michigan, Ann Arbor Optical and Discharge Physics JULINE_AVS08_16

PHOTO-RESIST TRIMMING Photo-resist trimming is used to produce < 50 nm features due to the limitation of 193 nm lithography. Roughness in PR can be a major issue below 50 nm. Trimming by plasma etching smoothens bump and lessens roughness. URL:http://www.semiconductor.net/article/CA239573.html University of Michigan, Ann Arbor Optical and Discharge Physics JULINE_AVS08_17

TRIMMING AND BARC ETCHING IN Ar/O2 PLASMAS Ar/O2 = 5/95 used to simultaneously trim PR and etch BARC layer. Species: Ar+ O2+ O+ O O2 Ar* O2* O* BARC etch rate should be 1.5 times higher than PR to retain CD after trim and BARC etching. Conditions: Ar/O2 = 5/95, 5 m Torr, 300 W ICP University of Michigan, Ann Arbor Optical and Discharge Physics JULINE_AVS08_18

PHOTO-RESIST TRIMMING AND BARC ETCHING IN Ar/O2 PLASMA Sputtering X+(g) + PR(s)  PR(g) + X(g) X+(g) + BARC(s)  BARC(g) + X(g) Etching O+ (g) + PR(s)  COH(g) O+ (g) + BARC(s)  COH(g) Bond Breaking X+(g) + TiN(s)  Ti (s) + N(g) + X(g) Adsorption O(g) + Ti(s)  TiO(s) Oxide Etching M+(g) + TiO2(s)  TiO2(g) Cl(g) + TiO(s)  TiOCl(s M+(g) + TiOClx(s)  TiOClx(g) + M(g) University of Michigan, Ann Arbor Optical and Discharge Physics JULINE_AVS08_19

PHOTO-RESIST TRIMMING PR was trimmed and BARC was etched in Ar/O2 plasmas. Starting with a 80 nm wide PR, the process shrank down the width to 32 nm. Entire BARC layer should be etched to avoid undesired masking. PR BARC TiN HfO2 SiO2 Si Trimming Employing Ar/O2 plasmas Etching Employing Ar/BCl3/ Cl2 Plasmas Initial Gate-stack University of Michigan, Ann Arbor Optical and Discharge Physics JULINE_AVS08_20

HfO2 ETCH RATE AND BIAS VOLTAGE PR BARC TiN HfO2 The etch rate of HfO2 increases with bias voltage, thereby enhancing polymer sputtering and causing lower selectivity. SiO2 Si 30V Bias 60V Bias 100V Bias Ar/BCl3/Cl2 = 5/40/55, 5 mTorr, 300 W ICP University of Michigan, Ann Arbor Optical and Discharge Physics JULINE_AVS08_21

SELECTIVITY: CALIBRATION Complex reaction mechanism requires calibration. Example: Selectivity of HfO2 over Si requires layer of Poly-BClx, a competition between deposition and sputtering. BClx(g) + Si(s)  SiBClx(s) BClx(g) + SiBClx(s)  SiBClx(s) + P-BClx(s) M+ (g) + P-BClx(s)  M(g) + BClx(g) In the absence of fundamental data, reaction mechanisms must be derived through sensitivity studies. University of Michigan, Ann Arbor Optical and Discharge Physics JULINE_AVS08_22

SELECTIVITY: Si-B BOND AND POLYMER FORMATION TiN HfO2 SiO2 PR PR Si BARC Si-B Bond formation p= 0.4 Polymer formation p= 0.5 BARC TiN TiN TiN HfO2 HfO2 HfO2 SiO2 SiO2 SiO2 Si Si Si Si-B Bond formation p= 0.00 Si-B Bond formation p= 0.05 Polymer formation p= 0.05 Polymer formation p= 0.00 Ar/BCl3/Cl2 = 5/40/55, 5 mTorr, 300 W ICP JULINE_AVS08_23

SELECTIVITY: POLYMER SPUTTERING Incidence of high energy ions can sputter of polymer which exposes the Si. M+ (g) + POLY (s) POLY* (s) + M (g) M+ (g) + POLY* (s) BCl2 (g) + M (g) Selectivity reduces inversely with polymer sputtering probability. TiN HfO2 SiO2 PR Si  Polymer Sputtering p= 0.10  BARC TiN TiN HfO2 HfO2 SiO2 SiO2 Si Si Polymer Sputtering p= 0.8 Polymer Sputtering p= 1.0 Ar/BCl3/Cl2 = 5/40/55, 5 mTorr, 300 W ICP University of Michigan, Ann Arbor Optical and Discharge Physics JULINE_AVS08_24

University of Michigan, Ann Arbor Optical and Discharge Physics CONCLUDING REMARKS HfO2 Gate-Stack modeling is very challenging as reaction mechanism has little experimental database. Etch rates for HfO2,SiO2 and TiN agreed with literature values. Undercutting of the gate-stack was not significant. Both Si-B bond formation and polymer deposition are essential to ensure high selectivity. At low bias, the etching of Si essentially stopped due to the formation of polymer ensuring no damage to the Si substrate. University of Michigan, Ann Arbor Optical and Discharge Physics JULINE_AVS08_25