Lecture 10 Thermal Oxidation, Part 1

9/7/2013

CHE323/CHE384 Chemical Processes for Micro- and Nanofabrication Silicon Dioxide

• Two Main purposes of SiO 2: – Electrical insulator Lecture 10 – Dopant diffusion barrier • Used in the process at these steps: Thermal Oxidation, part 1 – Device isolation (LOCOS or STI) – Gate Oxide – Poly gate spacer (for LDD) Chris A. Mack – Interlayer dielectric (ILD) for metal levels Adjunct Associate Professor – Sacrificial oxide • Deposition Methods: Reading : – Thermal growth Chapter 4, Fabrication Engineering at the Micro- and Nanoscale , 4 th edition, Campbell – Chemical Vapor Deposition (CVD)

Thermal Oxidation Oxidation Furnace

• Oxide is “grown” by supplying an oxygen Heating Elements source that reacts with the silicon wafer at Fused Silica Tube Input Gas high temperatures to form SiO 2 at the wafer Valve surface • Thermal oxides are valuable because they are: – Stable, intrinsically good insulators – Very clean/pure when thermally grown SiC Boat SiC – Have excellent Si/SiO 2 interface with low Water Cantilevered electrical defects (good for gate oxide) Paddle

Temperature Control Thermal Cycle

Dummy wafers at ends of boat

Temp Oxidation, O + HCl ±0.5 ºC temp. 1000ºC 2 Ramp Up, Ramp control required N + 5% O Down, N Temp “flat zone” 2 2 2 600 – 1200 ºC Push, N 2 + 5% O 2 Pull, N 2 range for furnace 750ºC

Position Time © Chris Mack, 2013 5 © Chris Mack, 2013 6

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Thermal Oxidation Cycle Electrical Defects

• Push – Low temp to reduce thermal stress (wafer warpage) –O included to prevent nitride formation mobile charge 2 Na + - • Dry Oxidation + K+ + –O2 + HCl – Slow, used for thin oxides or where high quality is required Fixed charge oxide trapped charge (incompletely oxidized Si) • Wet Oxidation (broken Si–0 bonds) interface trapped charge –O2 + H 2O – Fast, used for thick oxides (dangling Si bonds) + + + + + SiO 2 •N2 Anneal xxxxxxxxxxxxxxxxxxxxxxxx – Reduces surface states Silicon • Ramp down and Pull – Low Stress, low surface states, low fixed charge

Quality Control Lecture 10: What have we learned?

• Mobile Ions – introduced by contamination – Use good pre-clean • What is oxide used for in a CMOS process? – Careful backside handling • What are the advantages of thermal – Clean furnace components – Use HCl during oxidation (Cl reacts with mobile ions; results in oxidation? excess Cl in oxide film) • Interface States – fixed charge and interface trapped charge • Explain the basic workings of an oxidation – Avoid stress, which causes oxide-induced stacking faults furnace – Use N 2 or Ar during pull – Use HCl during oxidation • Why is HCl used in the oxidation process? • Thickness Control – Across wafer, wafer-to-wafer, and lot-to-lot • How does one insure good oxide thickness – Vertical furnaces have better temperature uniformity uniformity?