MOSFET Scaling 1970s Develop Locos Field Isolation

# MOSFET Scaling (1970s): Step 7 — Developing LOCOS Field Isolation

Recessed, selectively grown oxide replaces simple thick-field-oxide isolation. As transistor density rose, the thick field oxide separating neighboring devices had to shrink in lateral extent without losing its job of preventing parasitic conduction between adjacent transistors. LOCal Oxidation of Silicon (LOCOS), developed at Philips and widely adopted across the industry through the 1970s, uses a patterned silicon nitride mask to grow thick field oxide only where isolation is needed while protecting the active transistor areas, producing a more compact and more controllable isolation structure than earlier blanket-oxide approaches.

## 1. The Nitride Mask-and-Grow Sequence

A thin pad oxide is grown everywhere, then silicon nitride (which blocks oxidant diffusion) is deposited and patterned to cover only the areas that will become active transistors. Thermal oxidation then proceeds only in the exposed field regions, growing thick oxide there while the nitride-protected active areas keep only their thin pad oxide. Stripping the nitride afterward leaves a wafer with thick field oxide already in place around each transistor footprint, self-aligned to the active-area pattern by construction.

## 2. Real Diagram: LOCOS Cross-Section — Masked Oxidation Sequence

LOCOS Process Cross-Section Sequence nitride protects active area; oxide grows thick only in exposed field regions 1. Pad Oxide + Nitride Deposit thin oxide everywhere, nitride blocks oxidant 2. Pattern Nitride nitride remains only over intended active areas 3. Field Oxidation thick oxide grows only in exposed (field) regions 4. Strip Nitride active areas keep thin oxide; field is now isolated bird's-beak lateral encroachment at the mask edge is the main LOCOS design-rule cost

## 3. The Bird's Beak Problem

LOCOS has an intrinsic artifact: oxidant diffuses laterally under the edge of the nitride mask during growth, producing a tapered oxide encroachment into the nominally active area called a 'bird's beak' for its cross-sectional profile. Each generation's bird's beak consumed active-area width that the mask designer had intended to keep, which meant isolation spacing design rules had to budget for encroachment — a smaller but structurally analogous problem to the alignment-tolerance budgeting covered in step 5, now moved to the isolation boundary instead of the channel boundary.

## 4. Why LOCOS Still Won Despite the Bird's Beak

Compared to its predecessors, LOCOS still delivered a large net density win because it eliminated the need for a separately masked, independently positioned thick-oxide region with its own alignment margin; the bird's beak consumed a predictable, shrinking fraction of the available isolation spacing rather than an independent tolerance budget. LOCOS, in refined forms, remained the standard isolation technique for roughly two more decades after this series' 1970s window, which is a reasonable measure of how well the basic masked-oxidation idea scaled.

## 5. Companion Diagram: Stage Sequence and Trend Summary

MOSFET Scaling (1970s): Step 7 stage sequence and schematic outcome trend for this step The Nitride Mask-an… 1 Real Diagram: LOCOS… 2 The Bird's Beak Pro… 3 Why LOCOS Still Won… 4 sequence progression outcome trend (schematic) companion diagram: stage sequence and schematic trend for this step

## Developing LOCOS Field Isolation's Place in the Process Lineage

This is step 7 of 20 in the MOSFET Scaling (1970s) series, tracing planar NMOS scaling from early 1970s field pressure through the fixed-voltage-scaling break that opens the adjacent HMOS (1978) series.

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