Thermal Oxidation and Growth of Insulators (Chapter 3 - Jaeger 3) Key advantage of Si: Oxidation of Si into SiO (glass) Major factor in making Silicon the main semiconductor Grown at high temperature in pure O (dry) or steam (wet) Glass is chemically inert Glass makes hard, dielectric layer Note: density changes with oxide type: dry is denser than wet Dry thermal oxide.7 g/cc Wet thermal oxide.18 g/cc
Glass Use in Semiconductors Looking at MosFet shows some of Glass applications :LOCOS (LOCal Oxidation of Silicon)
Uses of Oxide Films in IC s Both furnace grown oxides and Chemical Deposited (CVD) oxides CVD is where chemical reactions create the oxide Furnace oxide is denser greater hardness, chemical resistance. Also has better electrical characteristics, Higher resistivity, dielectric constant and breakdown voltage
Growth of Oxide Films Done at high temperature (in a furnace) in oxidizing gas Thickness control and density determine process Dry oxidation (denser oxides: gate oxide) Wet Oxidation (lower density: Thick masking, Field oxides)
Growth of Oxide Films Furnace growth (charts) of SiO depends on wet/dry and crystal orientation. The calculated oxide thickness with time for different oxidation time at different temperature and different wafer orientations is shown in figures below
(a) According to Fig., it would take.8 to grow 0. µm oxide in dry oxygen at 1100 o C. (b) The total oxide thickness at the end of the oxidation would be 0.5 µm which would require 1.5 to grow if there was no oxide on the surface to begin with. However, the wafer thinks it has already been in the furnace 0.4. Thus the additional time needed to grow the 0.3 µm oxide is 1.5-0.4 = 1.1.
(a) From Table3.1, B = 7.7x10 1.3 µ m exp kt µ m For T = 173 K, B = 0.036 ( 0.05µ m) 0.05µ m ( 0.µ m) τ = µ m 0.036 + µ m 0.169 B A and B A = 0.174 = 3.71x10 µ m = 0.169 t 6.00 µ m exp kt = µ m 0.036 X 0.µ m + µ m 0.169 i = 5nm 0.174 =.70 (b)[part c in example]: From Table 3.1, B = 3.86x10 t = ( 0.µ m) µ m 0.314 ( 0.5µ m) µ m 0.314 0.78 µ m exp kt µ m For T = 173 K, B = 0.314 τ = 0.µ m + µ m 0.74 0.5µ m + µ m 0.74 = and 0.397 B A 0.397 = 9.70x10 B A = 1.07 note: 7 µ m = 0.74.05 µ m exp kt X i = 0.µ m X i = 0
Good Oxide Growth Processes Start with removing wafers from storage Clean wafers Load wafers into boat in Laminar Flow Hood Boat pushed slowly into furnace After time/temperature/gas cycle wafers removed Most important factor: Prevention of wafer contamination
Handling Wafers Wafers kept in a cassette (up to 5 wafers) Old handling: wafer tweezers could damage wafers, carry dirt Modern: Vacuum tweezers, pencils or wands clean, less damage but more likely to drop
Standard RCA (Radio Corporation of America ) Cleaning Process Standard clean of wafers before any hot process Not possible after any metal level depositions Process different for initial pre-oxidation, and post oxidation Chemicals & RCA Clean Always use Deionized (DI) Water Use Electronic Grade Chemicals 7% NH 4 OH (Ammonium Hydroxide) 30% unstabilized H O (Hydrogen Peroxide) 49% HF (Hydrofluroic Acid) 30% HCl (Hydrochloric Acid)
Cascade Rinse Rinse in DI water to remove contaminates Cascade 3 level final rinse Place wafers in lower level (twice used water) Move to nd level (once used water) Top level final clean Spin Dry Spin wafer Spin up wafer to 1000 rpm to remove water some water left on the back of wafer Blow with high speed N to remove water on the back
Wafers Loading Wafers loaded into quartz boats Wafer loading depends on style of boat Most common is Craddle type (used here) Wafers perpendicular to gas flow get high density Slotted has wafers parallel to gas flow Flat/slab has wafers lying flat usually done for CVD process
Oxidation Furnace Furnace is at elevate temperature Wafers loaded slowly into furnace to prevent thermal shock Manual loading for oxide growth load at lower T than oxidation Production fabrications use autoloaders that move wafers in very slowly Then can load furnace at oxidation temp Problem for research labs auto loader is as big as furnace
Furnace Temperature Programs Furnaces under computer control Load wafers below 800 o C Ramp up temperature to oxidation level rate limited by power of furnace Temp. flat during oxidation Cool down ramp, limited by cool rate Problem takes long time to ramp up and down Hence production Fabrications load more slowly at oxidation temp
3 Zone Oxidation Furnace Furnace have 3 heat zones (usually used for production fabrications) Each heat zone separately controlled heating coils Temperature of outer zones adjusted to keep central zone flat T.
Gas Flow in Oxidation Furnace Nitrogen atmosphere during ramp up and ramp down Little oxide growth them, keeps system clean During oxidation add Oxygen or steam Oxygen in Gas form Steam produced by Bubbler DI water in heater near 100 o C Nitrogen bubbled tough bubbler Nitrogen gas flows into a hot water bath to generate wet nitrogen bubbles and carry vapor into the process tube
Measuring Oxide thickness with Color Chart Rough measure of wafer by color appearance Optical interference in oxide selects colors Derived by Riezman and Van Gelder (1967), Accurate to ±.5 nm Works on non-absorbing films.
Ellipsometry Film Measurement Non destructive optical measurement of transparent films Uses change of state of light polarization when reflected at angle from film Use lasers as light source Complicated calculations now done automatically Refractive index of silicon dioxide is typically 1.45 Alternative: profile mechanical measurement of thickness after etching Fig. Concept of ellipsometry Fig. Automatic ellipsometer measuring system