CAN PLASMA MODELING BE A PREDICTIVE TOOL IN PROCESS DEVELOPMENT?

1. CAN PLASMA MODELING BE A PREDICTIVE TOOL IN PROCESS DEVELOPMENT? ETCHING OF VERY HIGH ASPECT RATIO FEATURES AND GATE STACKS Mingmei Wang, Juline Shoeb, Yang Yang and Mark J. Kushner University of Michigan Ann Arbor, MI 48109 USA mjkush@umich.eduhttp://uigelz.eecs.umich.edu October 2008 AVS08_MJK

2. University of Michigan Institute for Plasma Science and Engineering CONGRATULATIONS • Congratulations to Prof. Herb Sawin on his retirement. • With apologies to www.despair.com AVS08_MJK

3. University of Michigan Institute for Plasma Science and Engineering AGENDA • Past, present and future expectations for process modeling. • How do we get from here to there? • We model equipment well, why not processes? • Extremely high aspect ratio features • HfO2 gate-stack etching • Concluding Remarks • Work supported by Semiconductor Research Corp., Applied Materials Inc., Tokyo Electron Ltd. and Intevac. AVS08_MJK

4. University of Michigan Institute for Plasma Science and Engineering 1991 NRC REPORT: “PLASMA PROCESSING OF MATERIALS” • In 1991, the US National Research Council assessed the status of plasma processing of materials. • Findings: • “Currently, computer-based modeling and plasma simulation are inadequate for developing plasma reactors.” • “There is no fundamental obstacle to improved modeling…nor to the eventual creation of computer aided design tools for designing plasma reactors.” • Ref: “Plasma Processing of Materials”, NRC, 1991. AVS08_MJK

5. University of Michigan Institute for Plasma Science and Engineering 1991 NRC REPORT: WHAT IS NECESSARY TO ACHIEVE GOALS • To achieve the modeling goals set out in the 1991 NRC Report: • A reliable and extensive plasma data base against which the accuracy of simulations of plasmas can be compared. • A reliable and extensive input data base for calculating plasma generation, transport and surface interaction. • Efficient algorithms and supercomputers for simulating magnetized plasmas in 3 dimensions. • Ref: “Plasma Processing of Materials”, NRC, 1991. AVS08_MJK

6. University of Michigan Institute for Plasma Science and Engineering 2007 NRC REPORT: PLASMA SCIENCE: ADVANCING KNOWLEDGE IN THE NATIONAL INTEREST • NRC Decadal Study on Plasma Science (http://www.nap.edu/catalog.php?record_id=11960) • Low Temperature Plasmas: • Extreme challenges face modeling and the allied sciences to develop comprehensive and validated theories, computer models and databases that place predictive capabilities in the hands of technologists. “This represents the highest level of challenge and the highest potential return…to both quantify and advance our understanding of low temperature plasmas, and to leverage that understanding by speeding the develop of society benefiting technologies.” Ref: Adapted from “Plasma Science: Advancing Knowledge in the National Interest”, NRC, 2007. AVS08_MJK

7. University of Michigan Institute for Plasma Science and Engineering DOE/OFFICE OF SCIENCE WORKSHOP • Low Temperature Plasma Science: Not only the Fourth State of Matter but All of Them (September 2008) • Priorities in Modeling and simulation • 1 - Expand plasma capabilities to combine theory, simulation, and reacting flow equations to model closely-coupled, stochastic processes. • 4 - Develop multi-scale methods describing interactions of plasmas with nanoscale features such as nano-particles and nano-textured surfaces. AVS08_MJK

8. University of Michigan Institute for Plasma Science and Engineering 1991 to 2008…..Have we accomplished anything? AVS08_MJK

9. University of Michigan Institute for Plasma Science and Engineering EQUIPMENT AND PROCESS MODELING • The ability to craft structures and functionality on surfaces ultimately depends on the ability to control of charged and neutral species (and specify to produce a feature). • Can we achieve this degree of sophistication? AVS08_MJK

10. University of Michigan Institute for Plasma Science and Engineering MODELS FOR DESIGNING PLASMA EQUIPMENT • We do well in modeling plasma equipment • Vary power, pressure, geometry, gas mixture in the model…We can predict the change in fluxes. • Parametric variations better represented than absolute values. • Subtleties of equipment (e.g., materials, gaps) can be captured. • Instabilities and long term transients are problematic. • High frequency and non-standard excitation are challenging but doable. • In principle there is a path forward for most of these issues. • 3D CCP simulation showing plasma penetration through confinement rings (Ref: Kenney, Rauf, Collins, 2008) AVS08_MJK

11. University of Michigan Institute for Plasma Science and Engineering MODELS FOR DESIGNING PROCESSES • We do less well in modeling processes. • Vary power, pressure, geometry, gas mixture in the model… Quantitative, first principles predictions of profiles (or better yet, specify the reactor conditions to produce a profile) are lacking. • In principle, this is a “chemistry issue”. If we knew the reaction mechanism, we could represent the process well. • Subtleties abound (e.g., mixed layers, reflection from non-planar local topology) that are not known or that are not captured. • Subtleties may not be resolvable from first principles (e.g., MD) on large enough scale lengths for process design without massively parallel computing approaching weapons design. • Even then fundamentals parameters are not known….. • Is there a path forward? Less clear…. AVS08_MJK

12. University of Michigan Institute for Plasma Science and Engineering WHY IS IT DIFFICULT?: TIMESCALE AND CHEMISTRY • Technological plasmas have vastly different timescales that must be addressed in models. • Integrating timestep (numerical stability): t • Dynamic timescale (resolve phenomena): T • Plasma transport: • Dielectric relaxation t = /  1 ps – 10 ns • T = ns - ms • Surface chemistry: • t = s, T = 10 s • Will not be resolved “conventionally.” AVS08_MJK

13. University of Michigan Institute for Plasma Science and Engineering WHY IS IT DIFFICULT?: TIMESCALE AND CHEMISTRY • Computationally representing the gas phase and surface chemistries (but primarily surface) are now as large an intellectual challenge as addressing timescales. • PR morphology weakly dependent on temperature with only ion bombardment. • Add UV…hugely dependent. AVS08_MJK

14. University of Michigan Institute for Plasma Science and Engineering WHAT IS THE PATH TO INTEGRATED DESIGN • Tool design: Advance model infrastructure on par with weapons design. • Process design: Advance plasma chemistry knowledge base on par with combustion and catalysis. • Most direct: Understand linkages. AVS08_MJK

15. University of Michigan Institute for Plasma Science and Engineering ONE-PATH UP THE MIDDLE: HYBRID MODELING • Hybrid models resolve multi-physics over multi-scales. • Compartmentalize physical processes into modules having minimum of overlap. • Establish interdependencies • Time slice on physics times-scales. AVS08_MJK

16. TOOL DRIVEN PROCESS DESIGN: University of Michigan Institute for Plasma Science and Engineering TWISTING IN EXTREMELY HIGH ASPECT RATIO FEATURES: AVS08_MJK

17. University of Michigan Institute for Plasma Science and Engineering HIGH ASPECT RATIO CONTACT (HARC) ETCHING • Processes for HARC etching with aspect ratios > 50-100 are being developed for capacitors and through wafer vias. • Twisting, bowing and curvature of features is randomly observed. • Extremely high selectivity for mask emphasizes controlling PR erosion profiles. AVS08_MJK

18. University of Michigan Institute for Plasma Science and Engineering CHARGING IN HARC • Features are so small that random fluctuations of fluxes of radicals, ions and electrons into holes produces variations. • Closely related to charging • High energy ions penetrate deep into feature. • Electrons charge top of feature. • Charging of features produce internal E-fields that affect trajectories. • Randomness of charging leads to erratic paths. • Scaling law: Flux/cm2 = constant is prohibitive as feature sizes shrink AVS08_MJK

19. 0 -6 151 University of Michigan Institute for Plasma Science and Engineering MONTE CARLO FEATURE PROFILE MODEL • Psuedoparticles representing neutrals, ions and electrons are directed towards surface. • IEADs from HPEM - All ions neutralize striking surface, depositing charge. Reflect as neutrals. • EEADs of high energy secondary electrons from HPEM – Electrons scatter from surface f(,). • Maxwellian fluxes for bulk electrons with Lambertian distribution. • Charge neutral fluxes averaged over many rf cycle but randomly not-neutral. • Electric potential is solved using Successive Over Relaxation (SOR) method. • The charge of pseudo-particles is adjusted to account for finite sized particles. Potential AVS08_MJK

20. University of Michigan Institute for Plasma Science and Engineering SiO2 / Si HARC ETCH: EFFECTS OF CHARGING • Etch rate higher with increasing power. • Without charging: • Generally straight profiles. • With charging: • Ion trajectories perturbed. • Overcome with voltage. • Some evidence of randomness due to small contact area. • 10 mTorr, Ar/C4F8/O2 = 80/15/5, 10/ 40 MHz, 500 W. • Without Charging • With Charging AVS08_MJK

21. University of Michigan Institute for Plasma Science and Engineering SiO2/Si HARC ETCH: RANDOMNESS OF CHARGING? • 6 trenches receiving “same” fluxes. • Stochastic nature of fluxes produces random twisting. • Similar behavior observed experimentally. • Effect is amplified by finite size of particles and mesh. • 10 mTorr, Ar/C4F8/O2 = 80/15/5, 300 sccm, LF 4 kW, HF 500 W. AVS08_MJK

22. University of Michigan Institute for Plasma Science and Engineering MELD TOOL AND PROCESS DESIGN DC-AUGMENTED RF • Single (or dual) frequency CCP…with external, negative dc bias on opposing electrode. • DC ion current produces dc e-beam current incident onto wafer. • dc e-beam, mono-energetic and narrow in angle, penetrates deep into feature to neutralize excess positive charge. AVS08_MJK

23. 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 mTorr, LF: 10 MHz, 300 W, 440V/dc=-250V • DC: 200 W, -470 V ANIMATION SLIDE-GIF AVS08_MJK

24. University of Michigan Institute for Plasma Science and Engineering 10 MHz LOWER, DC UPPER: [e], ION ENERGY DISTRIBUTIONS • Ion energy distribution to wafer is many degrees, 150 eV in width. • Electron energy distributions onto wafer is narrower in angle and broader in energy. • E-beam reflects instantaneous potential difference between electrodes. • Ar, 40 mTorr, LF: 10 MHz, 300 W, 440V/dc=-250V; DC: 200 W, -470 V AVS08_MJK

25. University of Michigan Institute for Plasma Science and Engineering HEE EFFECTS on TWISTING: • E-beam current neutralizes sufficient charge to prevent major twisting. • Difference in etch depth results from randomness of fluxes. • Bowing occurs at later stages due to reflection from sloped profile of eroded PR. • 40 mTorr, Ar/C4F8/O2 = 80/15/5, 300 sccm, RF 5 kW at 10 MHz, DC 200 W. Without HEE Aspect Ratio = 1:20 With HEE

26. University of Michigan Institute for Plasma Science and Engineering ENERGY DISTRIBUTIONS vs DC VOLTAGE • Electrons • Ions • RF voltage constant while varying DC voltage. • Slightly IEAD due to increase in plasma density with VDC. • At low energy, low VDC broadens EEAD. • 40 mTorr, Ar/C4F8/O2 = 80/15/5, 300 sccm, RF 1.5 kV at 10 MHz. AVS08_MJK

27. University of Michigan Institute for Plasma Science and Engineering TWISTING ELIMINATION: DC VOLTAGE • 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. Different random seeds Aspect Ratio = 1:20 MINGMEI_AVS08_12

28. PROCESS DRIVEN TOOL DESIGN: University of Michigan Institute for Plasma Science and Engineering HfO2 GATE STACK ETCHING AVS08_MJK

29. University of Michigan Institute for Plasma Science and Engineering HfO2 GATE STACK MODELING • High-k metal oxides are being used as SiO2 replacements to minimize gate leakage. • For process integration and speed, desirable to simultaneously etch entire gate stack…Success with Ar/BCl3/Cl2 plasmas. • Challenge: • Modeling is needed to speed process development and optimization. • No fundamental database in the open literature for plasma-surface interactions. • Must develop mechanism based on experience, sparse data from literature… • Perform sensitivity studies to calibrate. AVS08_MJK

30. University of Michigan Institute for Plasma Science and Engineering 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) AVS08_MJK

31. University of Michigan Institute for Plasma Science and Engineering PR TRIMMING AND GATE STACK ETCHING • PR trimming (80 to 32 nm) and BARC removal in Ar/O2 plasma. • Features are small enough that random fluxes and micro-masking are important. • High selectivity to Si required. PR BARC TiN HfO2 SiO2 Si Initial Feature TrimmingAr/O2 plasma Stack Etching Ar/BCl3/ Cl2 Plasma ANIMATION SLIDE-GIF AVS08_MJK

32. University of Michigan Institute for Plasma Science and Engineering HfO2 ETCH RATE AND BIAS VOLTAGE • The etch rate of HfO2 increases with bias voltage, thereby increasing polymer sputtering and causing lower selectivity. PR BARC TiN HfO2 SiO2 Si 30V Bias 60V Bias 100V Bias ANIMATION SLIDE-GIF • Ar/BCl3/Cl2 = 5/40/55, 5 mTorr, 300 W ICP AVS08_MJK

33. University of Michigan Institute for Plasma Science and Engineering SELECTIVITY: CALIBRATION • Without fundamental data, reaction mechanism requires calibration. • Selectivity of HfO2 over Si depends on a layer of Poly-BClx maintained by a competition between bonding-deposition and sputtering. • Bonding: • BClx(g) + Si(s)  SiBClx(s) Polymer Deposition: BClx(g) + SiBClx(s)  SiBClx(s) + P-BClx(s) Sputtering: M+(g) + P-BClx(s)  M(g) + BClx(g) Sputtering • Calibration is obtained through sensitivity studies. AVS08_MJK

34. University of Michigan Institute for Plasma Science and Engineering WHAT IS NEEDED FOR PREDICTABILITY? • Become self-aware of the proper algorithms and resolution for the local physics. • Transparently generate data and reaction mechanisms. • Integration of spatial and time scales, and physical processes, by improving computational techniques and harnessing high performance computing.. • Adopting standards for exchange of algorithms, modules and data. • Dedicated experimental collaborative efforts – Take a lesson from combustion sciences. • Extreme challenges face modeling…to place predictive capabilities in the hands of technologists ...highest level of challenge and the highest potential AVS08_MJK