8.1 Chapter 6. ic fabrication technology from 60s to date_part 1  (Page 9/16)

KINETICS OF GROWTH:

DRY OXIDATION

Si (solid) +O2 → SiO2(solid)

WET OXIDATION

Si(solid) +2H2O (steam) → SiO2 (solid) +2H2 ↑.

Dry Oxidation is much slower than Wet Oxidation as is evident from the Table 6.4 and Table 6.5.

Table (6.4) Oxide Growth (µm) versus temperature and time for dry oxidation for

<1 0 0>crystal orientation.

Table (6.5) Oxide Growth (µm) versus temperature and time for wet oxidation for

<1 0 0>crystal orientation.

This oxide layer is called thermal oxide and it uses a part of the silicon surface atoms . Hence 44% of SiO2 layer is below the silicon surface and 56% above the original silicon surface as shown in Figure 6.9.

Temperatures of 1000 ^o C and 1 hour thermal oxidation time are the typical oxidation parameters. Where thick layer (0.5 µm or 500nm thick) of SiO ₂ required one goes for WET OXIDATION and where thin layer (30 nm-100nm) accurately controlled is required there we go for DRY OXIDATION.

(1) Dry oxidation is slow but gives UNIFORM , RELATIVELY FREE FROM DEFECTS and ELECTRICALLY STABLE thin layer (30 nm to 100 nm) as required in MOSFET Fabrication . MOSFET requires insulating gate oxides(5 to 50 nm thick) and it requires 200 nm thick field oxide.

Initially because of instability and latch - on problems of MOSFET , high packing density which is afforded by MOS circuits was technically unfeasible. Eventually oxide was doped with phosphorous and P glass achieved . This anode P glass is impervious to Na ⁺ ions migrations and MOSFET was stabilized. This led to the development of VLSI and ULSI fabrication. (Andy Grove of Intel fame had an important role in developing the MOS Technology).

(2). Wet oxidation- This is carried out in steam ambient and as it is clear from the Table (6.4) it accelerates the growth rate of SiO ₂ but it is not as free from microscopic defects as is dry thermal oxides. Hence wet Thermal Oxides is primarily used as MASKS Application during diffusion and wet oxides are also used as Field Oxides.

In both types of thermal oxide, interface (Si/SiO ₂ ) trap densities are minimized.

The oxidation is carried out in oxidation furnace as shown in Figure (6.5) where loading of the wafers is done in Laminar Flow Hood. This minimizes contamination and dust particles on wafer surface.

The oxidation temperature is 900 ⁰ to 1200 ⁰ C and the typical gas flow rate is 1 cm/sec. The oxidation system uses microprocessors to regulate the gas flow sequence, to control the automatic insertion and removal of silicon wafers, to ramp the temperature up (i.e. to increase the furnace temperature linearly from a low to the oxidation temperature so that wafers will not wrap due to sudden temperature change), to maintain the oxidation temperature to within ±1 ⁰ C and ramp the temperature down when oxidation is completed.