Layout in Deep Submicron...or, what are all these rules?

Ron Ho9/21/00

Introduction

• We’ve learned all of the “standard” DRs– EE271, taping out 0.8 to 0.25m chips

• There are also a number of newer DRs– For more advanced processes, fab steps– They may affect you in future tapeouts

• A brief intro to some of these rules– Not sure what processes use these...

Outline

• New metal density rules• Antenna rules and proper application• Phase-shift region coloring• Optical proximity correction• Odds and ends

resist

Metal Density

• Old rule: minimum metal density– For Al, metals were etched away

ILDmetal

High density Low density

This etching step takes a lot longer(“microloading”)

Solution: Add dummymetal structures hereto maintain minimummetal density

Metal Density

• New rule: max/min metal density– For Cu, metals are “poured” (damascene)– Review of dual-damascene

An amateur’sview of dualdamascene(“via-first”variation)

Ta barrier layerto prevent Cu fromdiffusing into Si

SiN layerfor etch stop

M2

M1

Metal Density

– Min rule: Ta barrier is hard to remove– Max rule: Cu metal is much softer than Ta

• “Selectivity” of Cu is 20x higher than for Ta

Low density:Mandate min. metal density

Barrier tough to remove

High density:Mandate max. density and width

Softness of Curesults in “dishing”

Metal Density

• Min density:• Around 30%/layer, in stepped windows• Windows are around 1mmx1mm square, steps of ~100m• “Dummification” metal structures required to add metal

• Max density• Around 70%/layer, again in stepped windows• Usually, max width + min spacing -> 90% density• Insert slots in lines or turn wide wires into parallel lines

• Rules checked in Dracula (not Magic)

• Reactive ion etch charges up metal lines– Charge can accumulate and zap a gate oxide– If a gate sees a long metal before a diffusion does

Antenna Rules

m4

m3

m2

m1gate

100

Safe: m3 is too short toaccumulate very much charge; won’t kill gate

gate

2000

Dangerous: lots of m3; willprobably accumulate lots ofcharge and then blow oxide

diff diff

diff

Antenna Rules

• Two solutions: bridging and node diodes– Bridging attaches a higher layer intermediary– Diode is a piece of diffusion to leak away charge

m4

m3

m2

m1

psub

gate

2000

Bridging keeps gate away from long metals until theydrain through the diffusion

gate ndiff

Node diodes are inactive duringchip operation (reverse-biased p/n);let charge leak away harmlessly

Antenna Rules

• Bridge or add node diodes if area ratio > limit– Most rulesets today use

• Sum(metal_area_not_tied_to_diff)/gate_area

– Examples from previous slides

• Be careful to account for etch rate!– Etching rates vary depending on geometries– May expose antennas of smaller or larger size– Note: this applies to Al, not Cu damascene

Antenna Rules

• In areas of lower metal density (microloading)– Slower etch can imply longer-lasting “islands”

Large island of metal lasts for a shorttime, but can be enough to gather a fatalcharge, especially if node X were alreadyclose to the ratio limit

Node “x”

Antenna Rules

• In regions of lots of narrowly space wires– Can get a slower etch effect from “e- shading”– Especially if resist aspect ratio is high

• Etching particles don’t enter trench as easily• Differential in etch rates, creating islands of metal

a b c d

Node “c,” e.g., has an etch-antenna that includes “a,” “b,” and “d”

Antenna Rules

• Antenna rules work incrementally– Disconnect all above M1; check non-diff nodes– Add M2; check non-diffusion-protected nodes; etc.

• Stefanos has such a flow for Magic (ext2ant)

• Ideally, antenna rules would include etching– Calculate ratios based on neighbors and etchrates– In practice, use a fudge factor on the allowed ratio

• Cu relaxes antenna rule (lower ratio)– Still must etch ILD, but no etchrate variability

Phase-Shifting Masks

• Lithography uses (partially) coherent light– Wavelength today is 248nm; changes slowly

Kahng et. al., 1999 DAC

Phase-Shifting Masks

• PSM enables higher resolution patterning– Exploiting constructive/destructive light

Kahng et. al., 1999 DAC

Phase-Shifting Masks

• PSM done typically on poly and contact– Most critical layers for narrow lines; PSM $$$

• Around any line, we need to flip phases• This is a 2-coloring problem

0o

0o

0o 180o

180o

180o 0o

0o

0o 180o

180o

180o

Phase conflicthere will create anunwanted line; need“trim mask” to kill it

Phase-Shifting Masks

• Two principal design rule effects– Orthogonal gates cannot be too close– Avoid interdigitating poly

• These should be checkable directly in Magic

0o

180o

180o

Orthogonal gates need to haveincreased spacing to allow roomfor the 0o section to the right ofthe vertical gate

Trying to duck the poly-poly ruleby interdigitating the fingers is bad

Optical Proximity Correction

• Also known as serifs and dog-ears– Layout is not WYSIWYG anymore

• Patterning through a reticle is tough– Holes in reticle act as low-pass filter

• Blurred edges• Squares in mask are blobby ovals in production

– We can predistort the image to compensate• Analogous to channel equalization

Optical Proximity Correction

• Standard fixes include:– Outside corner dog-ears– Inside corner cut-outs– Long line embellishments

Optical Proximity Correction

• Example

Schellenberg et. al., 1999 SPIE

Optical Proximity Correction

• This is a back-end flow, done after tape-out– Designers are unaware of OPC for the most part– Only real restriction: limit use of 45o routing

• 45o routes, with OPC, need more spacing to other wires• Only important to those of us who use Cadence tools...

– OPC does explode the database size• Imagine the size of a microprocessor database...

Odds and Ends

• Via spacing rules will change– Center-to-center instead of edge-to-edge

• Reflect the fact that vias are really more circular

– Most efficient packing is not a rectangular array• More like a checkerboard pattern

– Checkable in Magic (shrink, then edge-edge)

Odds and Ends

• EM rules are relaxed with Cu metal– Vias in Cu processes are also Cu (poured)

• Expect that via EM rules are also relaxed

– However, vias have some EM problems• Void formation at vias

– Defects in the hole formed during the dielectric etch

• Void formation along the Cu wire– Adhesion of the Cu to the barrier metal above isn’t great

– Voids will travel down the wire and get “stuck” in the vias