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Farshid Karbassian

Farshid Karbassian. Oxidation. Oxide Layer Applications Types of Oxidation Dry Oxidation Wet Oxidation Modeling C-V Measurement. Outline. Oxide Layer Applications. Nitride stress buffer. Defect removal. Gate dielectric. Oxide Applications: Native Oxide.

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Farshid Karbassian

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  1. Farshid Karbassian Oxidation

  2. Oxide Layer Applications Types of Oxidation Dry Oxidation Wet Oxidation Modeling C-V Measurement Outline

  3. Oxide Layer Applications Nitride stress buffer Defect removal Gate dielectric

  4. Oxide Applications: Native Oxide Purpose: This oxide is a contaminant and generally undesirable. Sometimes used in memory storage or film passivation. Silicon dioxide (oxide) p+ Silicon substrate Comments: Growth of native oxide layer at room temperature takes 3-4 hours up to about 12 Å.

  5. Oxide Applications: Gate Oxide Gate oxide Gate Source Drain Transistor site p+ Silicon substrate Purpose: Serves as a dielectric between the gate and source-drain parts of MOS transistor. Comments: Common gate oxide film thickness range from about 30 Å to 50 Å. Dry oxidation is the preferred method.

  6. Oxide Applications: Field Oxide Field oxide Transistor site p+ Silicon substrate Purpose: Serves as an isolation barrier between individual transistors to isolate them from each other. Comments: Field oxide thickness ranges from 2,500 Å to 15,000 Å. Wet oxidation is the preferred method.

  7. Oxide Applications: Barrier Oxide Purpose: Protect active devices and silicon from follow-on processing. Barrier oxide Metal Diffused resistors p+ Silicon substrate Comments: Deposition to several hundred Angstroms thickness.

  8. Oxide Applications: Pad Oxide Nitride Pad oxide Passivation Layer Bonding pad metal Purpose: Provides stress reduction for Si3N4 ILD-5 M-4 ILD-4 M-3 Comments: Very thin layer of oxide is deposited.

  9. Oxide Applications: Implant Screen Oxide Ion implantation Screen oxide p+ Silicon substrate High damage to upper Si surface + more channeling Low damage to upper Si surface + less channeling Purpose: Sometimes referred to as “sacrificial oxide”, screen oxide, is used to reduce implant channeling and damage. Assists creation of shallow junctions. Comments: Thermally grown

  10. Oxide Applications: Insulating Layer between Metals Purpose: Serves as protective layer between metal lines. Interlayer oxide Passivation layer Bonding pad metal ILD-5 M-4 ILD-4 M-3 Comments: Deposition

  11. 1. Nitride deposition 2. Nitride mask & etch 3. Local oxidation of silicon Nitride Silicon SiO2 growth Silicon SiO2 Pad oxide (initial oxide) SiO2 Silicon Silicon Nitride 4. Nitride strip Silicon Silicon Cross section of LOCOS field oxide (Actual growth of oxide is omnidirectional) LOCOS Process

  12. Silicon oxynitride Bird’s beak region Nitride oxidation mask Selective oxidation Silicon dioxide Pad oxide Silicon substrate Selective Oxidation and Bird’s Beak Effect

  13. 3. Sidewall oxidation and trench fill 1. Nitride deposition 2. Trench mask and etch Oxide over nitride Nitride Silicon Silicon Silicon Silicon Pad oxide (initial oxide) 5. Nitride strip 4. Oxide planarization (CMP) Trench filled with deposited oxide Oxide Sidewall liner Silicon Silicon Cross section of shallow trench isolation (STI) STI Isolation

  14. Crystallization of silicon dioxide is very undesirable, since it is not uniform and crystal boundaries provide easy paths for impurities and moisture. Pre-oxidation Cleaning Therefore, pre-oxidation wafer cleaning is performed to eliminate crystallization.

  15. Pre-oxidation cleaning is performed to remove particles, organic and inorganic contaminants, native oxide and surface defects. Pre-oxidation Cleaning

  16. RCA Standard Cleaning I (SC-1) NH4OH:H2O2:H2O 1:1:5 – 1:2:7 (70-80OC) DI water RCA Standard Cleaning II (SC-2) HCl:H2O2:H2O 1:1:6 – 1:2:8 (70-80OC) DI water RCA Cleaning RCA: Radio Corporation of America

  17. When wafers are submerged in RCA I solution, particles and organic contaminants oxidize, and their byproducts are either gaseous (e.g. CO), or soluble in the solution (e.g. H2O). In RCA II, H2O2 oxidizes the inorganic contaminants and HCl reacts with the oxides to form soluble chlorides, which allows desorption of contaminants from the wafer surface. RCA Cleaning

  18. Native oxide on Si is of poor quality and needs to be stripped, especially for the gate oxide which requires the highest quality. This is performed either in HF:H2O solution or in HF vapor etcher. After native oxide stripping, some F atoms bind with Si atoms and form Si-F bonds on the silicon surface. HF etching

  19. Thermal Oxidation Process Flow Chart • Oxidation Furnace • O2, H2 , N2 , Cl • Flow rate • Exhaust • Temperature • Temperature profile • Time • Wet Clean • Chemicals • % solution • Temperature • Time • Inspection • Film thickness • Uniformity • Particles • Defects

  20. Depending on the quality and thickness which is required for the oxide layer, wet or dry oxidation may be used. Former is faster, but latter is cleaner and makes a better interface. Thermal Oxidation

  21. Dry Oxidation • In dry oxidation, pure oxygen gas (5s at least) is used. At high temp. O2 molecules diffuse across an existing oxide layer to reach the Si/SiO2 • interface.

  22. Dry Oxidation

  23. Oxygen supplied to reaction surface O, O2 Oxygen-oxide interface SiO2 Oxide-silicon interface Si Diffusion of Oxygen Through Oxide Layer TEM image of Si/SiO2

  24. Horizontal Diffusion Furnace

  25. Vertical Diffusion Furnace

  26. Horizontal and Vertical Furnace

  27. Vertical Furnace Process Tube Thermocouple measurements System controller Temperature controller Profile TCs Control TCs Overtemperature TCs Heater 1 Heater 2 Heater 3 TC

  28. Si/SiO2 Interface Silicon Interface State Charge (Positive) Oxygen Dangling Bond SiO2 Si-SiO2 Interface Si

  29. 0.55t t 0.45t After oxidation Before oxidation Consumption of Silicon during Oxidation

  30. At high temp. H2O dissociates and form hydroxide, HO, which can diffuses in the SiO2 layer faster than O2 . A wet oxidation system may have a boiler or a bubbler or maybe it is a pyrogenic steam system, which is more common. Wet Oxidation

  31. Wet Oxidation System Pyrogenic Steam System

  32. Wet Oxidation System Bubbler System

  33. Dry Oxidation Vs. Wet Oxidation Dry oxidation Wet oxidation

  34. Deal/Grove (Kinetic) Model Assumptions: Temperature: 700 - 1300 oC Pressure: 0.2 - 1.0 atm SiO2 thickness: 0.03 - 2 μm x2 + Ax = B(t + τ) ; τ = time for initial oxide thickness d0 A = 2D/κ B = 2DC0 /C1 x = [Bt + 0.25 A2 + d02 + A d0]0.5 – A/2

  35. Color chart Oxide Measurement

  36. C-V Measurement Oxide Measurement

  37. Any questions?

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