field oxide
# Field Oxide (FOX)
Field oxide provides isolation outside active device regions in an appropriate process architecture.
## Connect the name to its role
Document whether this film is a target, mask, liner, electrode or temporary layer at the step being discussed. The same material can have different roles in different modules.
## Integration context
LOCOS field oxide is grown locally using an oxidation mask and exhibits a characteristic lateral transition. STI instead uses etched silicon trenches and isolation fill. FOX may also appear as local mask terminology, so a layer name alone should not be used to infer that the process uses LOCOS.
## What to verify
Confirm the isolation architecture and active-area boundary before choosing a strip or opening step.
Before selecting removal conditions, identify all exposed neighboring materials and the amount of loss each can tolerate. Verify the result using the measurements appropriate to the structure, then check that the surface is suitable for the next deposition, contact or clean step.
## Related terminology
field oxide, FOX. Short abbreviations are local process vocabulary and can have other meanings; use the full module name when searching.
## Source and scope
[Thermal oxide growth and deposited films](https://www.nanofab.ualberta.ca/capabilities/thin-film-deposition/). The diagram is an original teaching schematic. Layer choices and process families describe an example rather than a universal manufacturing recipe.
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## Additional existing reference
Field oxide is a thick silicon dioxide layer (typically 200-600nm) grown or deposited in non-active areas of the semiconductor wafer to provide electrical isolation between adjacent transistors, preventing parasitic conduction pathways that would cause unintended device interaction. Historical LOCOS process: Local Oxidation of Silicon was the primary field oxide formation technique through the 0.25μm technology node—(1) grow pad oxide (~10nm) on silicon, (2) deposit silicon nitride mask (~100nm), (3) pattern nitride to expose isolation regions, (4) thermally oxidize exposed silicon at 1000-1100°C in wet O₂ to grow thick field oxide (the nitride mask prevents oxidation in active device areas), (5) strip nitride and pad oxide. LOCOS creates a tapered oxide edge called a "bird's beak" where oxide grows laterally under the nitride mask—this encroachment consumes active area and limited LOCOS scalability to ~0.25μm. Modern STI replacement: Shallow Trench Isolation replaced LOCOS below 0.25μm—trenches are etched into silicon and filled with deposited oxide (HDP or HARP oxide), then planarized by CMP. STI eliminates the bird's beak, provides perfectly vertical isolation boundaries, and enables much denser transistor packing. However, the concept of field oxide as the isolation dielectric remains unchanged—STI fill oxide serves the same electrical isolation function as LOCOS field oxide. Field oxide thickness must be sufficient to keep the parasitic field transistor threshold voltage well above supply voltage (typically 2-3× Vdd)—the thick oxide under interconnect routing and between devices ensures no conduction path forms. At advanced nodes, STI oxide quality, stress, and interface properties affect adjacent transistor performance through stress coupling and charge trapping.