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New Approaches in Wet Chemical Etching for Defect Delineation in Silicon-on-Insulator Substrates

New Approaches in Wet Chemical Etching for Defect Delineation in Silicon-on-Insulator Substrates. 216 th ECS Meeting, Vienna E1 – Analytical Techniques for Semiconductor Materials and Process Characterization VI October 05, 2009

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New Approaches in Wet Chemical Etching for Defect Delineation in Silicon-on-Insulator Substrates

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  1. New Approaches in Wet Chemical Etching for Defect Delineation in Silicon-on-Insulator Substrates 216th ECS Meeting, Vienna E1 – Analytical Techniques for Semiconductor Materials and Process Characterization VI October 05, 2009 J. Mählißa, A. Abbadieb, F. Brunierb and B. O. Kolbesena aGoethe University, Institute of Inorganic and Analytical Chemistry, Frankfurt, Germany bR&D Department SOITEC S.A., Parc technologique des Fontaines, Bernin, France email: maehliss@chemie.uni-frankfurt.de

  2. Outline ________________________________________________________ •Introduction • Useoforganicoxidizing agents - 1,4-benzoquinone (p-BZQ) - 2,3,5,6-tetrachloro-1,4-benzoquinone (p-CA) • Useof tetrafluoroboric acid (asdissolvingagent) • Activationenergies – indicators for preferential etching • Summary and outlook • Acknowledgements

  3. Introduction ________________________________________________________ • wetchemical etching iswidelyusedtocharacterizecrystaldefects in substrates and monitor their densities • atpresentdilute Secco (K2Cr2O7/HF) etching solutionsareusedasstandard on thinfilmslikesilicon-on-insulator (SOI):  sensitive enoughtodiscriminatebetween theenergylevel in theperfectlattice and localenergylevelsaround a crystal defect ( “strainfield”)  “preferential etching” createsetchpits whichcanbeeasilycountedunder an microscope 3

  4. Introduction ________________________________________________________  BUT: Secco contains toxic and carcinogenic Cr(VI)  need for alternative Cr(VI)-free preferential etching solutions  most logical approach: - dilute Secco (0.04 M) recipe as starting point: 0.6 g K2Cr2O7 in 100 ml H2O : 50 ml HF (50 %) - Cr(VI) replaced by other inorganic oxidizing agents (among them e.g. Se(VI)  similar electron configuration of their valence shells)  did not match the Secco result - Cr(VI) replaced by organic oxidizing agents  matches the Secco result

  5. Use of organic oxidizing agents ________________________________________________________ • 1,4-benzoquinone (p-BZQ) • E0 = 699.8 mV • widelyused in organicchemistry •  redox equilibriumis well understood

  6. Use of organic oxidizing agents ________________________________________________________ • startingpoint Secco recipe: •  as p-BZQ isonlyslightly soluble in H2O an organic solvent hastobeused • instead •  precondition: organic solvent hastobemisciblewithwater ( aqueous HF) •  severalorganicsolventstested •  etch rate dependent on solvent • used and amountofdissolved • p-BZQ

  7. Use of organic oxidizing agents ________________________________________________________  from experimental resultsacetone was chosen recommendedcomposition 2 g p-BZQ dissolved in 100 ml acetone  “p-BZQ etching solution” + 50 ml HF (50 %)  very smooth and homogeneoussurface after etching  ER = 3 nm/min at 23 ˚C  suitable for thinfilmslike SOI  etchremoval vs. etching time is linear  easy calculationofremoval 7

  8. SEM AFM µm µm µm µm Use of organic oxidizing agents ________________________________________________________  delineatesdefectsof all types •SOI: - process-induceddefects (oxidation-inducedstackingfaults, OiSFs) - grown-in defects (vacancyagglomerates, oxygenprecipitates) p-BZQ etching solution Secco (0.04 M)

  9. Use of organic oxidizing agents ________________________________________________________  goodcorrelationtothe Secco defectdensities:  for SOI substrateswithstandard (samples 1 and 2) veryhigh (sample 3) low (sample 4) OiSF densities 9

  10. Use of organic oxidizing agents ________________________________________________________ • oxidation/dissolutionmechanismof Si • 1. oxidationof Si by p-BZQ – generation of a positivelycharged Si • 2. fluorinationof H-terminated Si by HF – developmentof H2 • 3. break upof Si-Si back bonds – formationof SiF4

  11. Use of organic oxidizing agents ________________________________________________________ drawbacks: •p-BZQ istoxic • formationof a charge-transfercomplex (quinhydrone)  precipitation on the wall ofthebeaker after some time  checkingof BZQ-derivatives with sterical groups e.g. 2,5-di-tert-butyl-p-BZQ

  12. Use of organic oxidizing agents ________________________________________________________ • 2,3,5,6-tetrachloro-1,4-benzoquinone (p-chloranil) • E0 = 712.0 mV •  noformationof a quinhydrone complex •  p-chloranil (p-CA) is not toxic: Xi, N •  p-CA isnot soluble in H2O •  amountof p-CA as well asaqueouscontent ( HF) hadtobereduced • recommendedcomposition • 0.25 g p-CA dissolved in 100 ml acetonitrile • + 10 ml HF (50 %)

  13. Use of organic oxidizing agents ________________________________________________________  very uniform etchremoval  delineatesdefectsclearly p-CA in acetonitrile + HF Secco (0.04 M)

  14. Use of organic oxidizing agents ________________________________________________________  higheretchrateswith p-CA (upto 20 nm/min at 23 ˚C) thanwith p-BZQ  also suitable for application on thick SOI (> 100 nm)  uniform etchremoval on thick SOI  defectsaredelineatedclearly recommendedcomposition 0.5 g p-CA dissolved in 100 ml 1,4-dioxane + 20 ml HF (50 %) dark “dot” with halo dark “dots” without halo (dissolution of BOX below etch pit by subsequent HF-dip produced halo)

  15. 5 µm Use of organic oxidizing agents ________________________________________________________ • • defect delineation process two steps: 1. defect etching solution proceeds preferentially at the defect site  small etch pit is produced which reaches the buried oxide (BOX) 2. dip in HF dissolves the BOX below the etch pit  halo is produced which increases visibility SOI defect BOX preferential etching HF-dip BOX dissolution bulk Si

  16. Use of organic oxidizing agents ________________________________________________________ • tetrafluoroboric acid •  used in theorganicchemistryas“fluorinatingagent” •  replacementof HF whichishighlytoxic: T+ (requiresspecialequipment • and caution in handling) •  in aqueous solution HBF4ispresentas BF4- and H3O+ •  HBF4isonlyslightly sensitive tohydrolysis: • HBF4 + H2O HBF3(OH) + HF •  classified “C” (corrosive) and not “T+” •  HBF4dissolved in diethylether ( nofree HF) has same etch rate •  HBF4isthedissolvingagent! K1 = 2.3·10-3 at 25 ˚C K2 << K1

  17. Use of organic oxidizing agents ________________________________________________________ recommendedcomposition 0.3 g p-CA dissolved in 100 ml acetonitrile  “p-CA etching solution” + 25 ml HBF4 (50 %)  first non-toxic etching solution!!  ER = 2 nm/min at 40 ˚C  veryhomogeneousetch removal  delineatesdefectsclearly

  18. Use of organic oxidizing agents ________________________________________________________  HBF4-dip after preferential etching OiSFs after 1h HBF4-dip at 40 ˚C OiSFs after 2h HBF4-dip at 40 ˚C OiSFs after 3h HBF4-dip at 40 ˚C  completereplacementof HF ispossible! (but: long HBF4-dip needed)

  19. Use of organic oxidizing agents ________________________________________________________ •  dissolution rates of SiO2: • HF (50 %) at 20 ˚C = 2700 nm/min HBF4 (50 %) at 20 ˚C = 2.5 nm/min • HBF4 (50 %) at 40 ˚C = 7.5 nm/min • dissolution mechanism of SiO2 by HBF4 •  direct fluorination of Si (no free F- or HF present or necessary) 19

  20. Use of organic oxidizing agents ________________________________________________________  verygoodcorrelationtothe Secco on SOI sampleswith different defect densities  similar preferential etching characteristics

  21. Activation energies – indicators for preferential etching ________________________________________________________ • preferential etching characteristics may bedependent on: • - Ea  indicatingpolishing or preferential etching •  Ea for a diffusion-controlledprocessislessthanthat for a reaction-controlled • one •  only in a reaction-controlledprocess will thestrainfieldaround a crystal • defectaffectthelocalreaction rate and thustheetch rate • etching of Si: polishing < 15 kJ/mol < preferential • - DEa indicatingthecharacteristicof preferential etching P P 21

  22. Activation energies – indicators for preferential etching ________________________________________________________ •  calculation of Ea from the Arrhenius Equation: • with k representing the reaction rate which can be measured by the etch rate •  values for Ea are in the same range •  confirming that the p-CA etching solution is a preferential etching solution as • well P  Ea = 34.1±1.2 kJ/mol  Ea = 32.5±1.1 kJ/mol P P P

  23. indenter tip Activation energies – indicators for preferential etching ________________________________________________________ • measurementofD Ea(= Ea- Ea) •  calculationofEabymeasuringthedepthoftheetchpitsproducedat • artificiallycreateddislocations •  for a reliable measurement: depths of etch pits had to be within a certain window •  AFM-tip must reach the bottom of pit P D D b) a) making of defined rapid thermal annealing creation of artificial dislocations indenter mark dislocation half-loops indenter mark epi Si etch pit preferential etching development of etch pits c) d) measurement of depth of etch pits by AFM AFM-tip

  24. Activation energies – indicators for preferential etching ________________________________________________________ • AFM line scan •  for a reliable measurement: • the distance from the indenter mark had to • be standardized •  depth of pits decrease with increasing • distance due to decreasing inclination • angles removal perfect lattice: 115 nm depth of etch pits: 72 nm  Ea = 32.9±2.2 kJ/mol  D Ea = -1.2 kJ/mol (= -3.5 %) • D Eaare in the same range •  may indicate similiar preferential • etching characteristics •  confirmed by same defect densities in • SOI substrates obtained after etching Secco (0.04 M): D p-CA etching solution:  D Ea = -1.4 kJ/mol (= -4.3 %) 24

  25. Summary and Outlook ________________________________________________________ • organic oxidizing agents open a new door to the field of preferential etching • p-BZQ and p-CA both delineate defects of different types in SOI • very good correlation to the Secco defect densities •  organic oxidizing agents proved to be a suitable alternative for toxic and • carcinogenic chromate/dichromate (Secco) • HBF4 as non-toxic fluorinating agent is able to replace HF (T+) •  for the first time a completely non-toxic etching solution is presented • as a candidate for Secco replacement on thin films • first promising results were obtained on sSOI • D Ea might be important parameter for comparison • of preferential etching characteristics

  26. Acknowledgements ________________________________________________________ This work was supported by SOITEC S.A., Parc technologique des Fontaines, Bernin, France. Thanks to: - Oleg Kononchuk - Béryl Blondeau - Muriel Chaupin - Romana Hakim - Doris Ceglarek (SEM and AFM measurements) - Yvonne Filbrandt-Rozario Thank you for your attention!

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