1. 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
b) Department of Electrical Engineering and Computer Science
University of Michigan Ann Arbor, Ann Arbor, MI 48109
55th AVS Symposium, October 2008
* Work supported by Semiconductor Research Corporation
JULINE_AVS08_01

2. 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

3. 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:
University of Michigan, Ann Arbor
Optical and Discharge Physics
JULINE_AVS08_03

4. 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:
University of Michigan, Ann Arbor
Optical and Discharge Physics
JULINE_AVS08_04

5. 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

6. MODELING PLASMA ETCHING: HIGH-k GATE STACKPR
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

7. 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

8. 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

9. 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

10. 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

11. 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

12. 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

13. 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

14. 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

15. University of Michigan, Ann Arbor Optical and Discharge Physics
Ar/BCl3 /Cl2 PLASMAS
Total ion density: x 1011 cm-3
Ion densities (cm-3): BCl x Cl x Cl x Ar x 1009
Neutral densities (cm-3): BCl x Cl x Cl x Ar 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

16. 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

17. 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:
University of Michigan, Ann Arbor
Optical and Discharge Physics
JULINE_AVS08_17

18. TRIMMING AND BARC ETCHING IN Ar/O2 PLASMAS
Ar/O2 = 5/95 used to simultaneously trim PR and etch BARC layer.
Species: Ar O O+ O O 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

19. 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

20. 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

21. HfO2 ETCH RATE AND BIAS VOLTAGEPR
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

22. 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

23. 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

24. 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

25. 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