Deep Submicron CMOS Scaling 1990s Replace Locos with Shallow Trench Isolation

# Deep Submicron CMOS Scaling (1990s): Step 13 — Replace LOCOS With Shallow Trench Isolation

## 1. Why LOCOS Ran Out of Room

The `mosfet_scaling_1970s` series' step 7 established LOCOS (local oxidation of silicon) as the standard field-isolation technique separating neighboring transistors, growing a thick field oxide through a nitride mask to block unwanted channel formation between devices. LOCOS's oxidation is inherently lateral as well as vertical — oxidant diffuses sideways under the edge of the nitride mask during the thermal growth, producing a tapered oxide encroachment known as the "bird's beak" that consumes active-area silicon beyond the nominally masked boundary. At 1980s geometries, the bird's beak's lateral extent was a tolerable area tax; at deep submicron 1990s geometries, where devices and their required isolation spacing were both shrinking toward LOCOS's fixed-ish bird's-beak dimension, the encroachment started consuming an unacceptably large fraction of the shrinking cell area, and the active-area edge rounding it produced also complicated controlling channel width precisely at these dimensions.

## 2. The Shallow Trench Isolation Alternative

Shallow trench isolation (STI) replaces thermal field-oxide growth with a purely geometric approach: etch a trench of the desired isolation width directly into the silicon between active areas, deposit a dielectric (commonly CVD oxide) to fill the trench, and planarize the surface back to flat — the chemical-mechanical polishing capability from Step 12 doing exactly that planarization job here as well. Because the trench's width and depth are set by lithography and etch, not by a lateral thermal diffusion process, STI has essentially no bird's-beak-equivalent encroachment: the isolation boundary is where the trench was etched, full stop, which lets isolation width shrink in step with everything else at a given node rather than being bottlenecked by a geometry LOCOS could not shrink as fast.

## 3. What STI Trades Away

STI's vertical sidewalls, unlike LOCOS's gradually tapered field-oxide edge, create sharper corners at the top of the active-area/trench boundary, which can locally concentrate electric field at the gate edge directly above that corner and create a parasitic leakage or threshold-shift effect at the very edge of the transistor if the corner is left too sharp. 1990s STI process development spent considerable effort on corner rounding techniques — controlled etch profiles and oxidation steps specifically aimed at softening this corner — to recover the clean isolation edge behavior LOCOS's gradual taper gave for free, in exchange for the area efficiency STI's lack of a bird's beak provided.

## 4. Why STI Became the Standard Despite the New Corner Problem

The area savings STI offered at deep submicron pitches were large enough, and the corner-rounding fix tractable enough with sufficient process development, that STI displaced LOCOS as the mainstream isolation technique across 1990s deep submicron logic processes, even though LOCOS's simpler, purely-thermal process had no equivalent corner-engineering burden. This is a recurring pattern in this series: nearly every deep submicron fix (LDD grading, salicide, STI) trades a new, manageable second-order problem for the removal of a first-order area or reliability constraint that scaling had turned from tolerable into limiting.

Isolation Area Tax: LOCOS Bird's Beak vs. STI Trench isolation area consumed per device against process generation, LOCOS vs. STI LOCOS — bird's-beak encroachment becomes a growing fraction of cell area STI — trench width shrinks with the node, no lateral encroachment penalty the point where LOCOS's fixed-ish bird's beak becomes unacceptable at shrinking pitch process generation advances left to right, cell pitch shrinks STI's isolation boundary is set by etch, not by lateral thermal diffusion the area saved funds STI's own new corner-rounding development burden
LOCOS's Taper vs. STI's Sharp Corner cross-section comparison of the isolation edge each technique produces LOCOS FIELD OXIDE gradually tapered bird's-beak edge, soft transition, costs lateral area STI TRENCH sharp vertical sidewall, no lateral encroachment, but a sharper top corner CORNER ROUNDING FIX controlled etch/oxidation softens the sharp corner's field concentration STI trades a solved area problem for a new, but manageable, corner-engineering problem

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