Pattern transfer by etching orlift-off processes

Basic pattern transfer stepsPositive resist (~1mm thick)FilmsubstrateUV lightHard maskGlassopaque pattern

Exposuredevelopment- Exposed resist removed by developer - The resist pattern is the positive image of the pattern on the maskFilmetchingResist strippingUnexposed resist removed by resist striperResist protects the film areas

Pattern transfer by lift-off - Allows use of hard-to-etch materialslift-offetchingrounded topWing tips- Not for sub-micron features

Requirements and constraints on lift-offResist with slope or undercut profile: Must avoid lateral step coverage good edge definition depends on the lateral walls avoid tearing or peeling of the remaining metal easier to liftoff (solvent can reach the resist)Create a descontinuity of the deposited film provides an area through which a solvent can pass to remove the resista) bake resist at 110C before exposure(top surface becomes harder)b) Double resist: resist I/resist II

substrateresistsubstrateresist

Resist thickness > metal thickness Deposited film descontinuousFor thick metal leyers:

a) Increase the PR thicknessb) Multiple layers: Resist/oxide

Metal deposition temperature < ~250C (resist degradation)Some materials are deposited at high temperatures: - sputtered AlSiCu @400CBurn resist: ~100C(difficult to clean resist residues)Metal ductility determines success of liftoffDuctile materials (e.g. Au): poor edge definition or impossible to liftoffBritle materials: tend to fracture at the edge of pattern steps (film discontinuity)substrateresistsubstrateresist

Problems: fabrication of surface sensitivity devicesMagnetic tunnel junctionThe wafer surface was exposed to chemicals (vapor prime HMDS, resist, developer)The magnetic structure at the edges can be affected:Cannot deposit directly the device layers: resist1.5 mm(15 000)600

Pattern transfer by etchingIsotropic or anisotropic

selectivity

overetch

The resist and the substrate are also etchedIf the mask is etched too quickly: no control of the feature sizeTo cope with film thickness and etch rate non-uniformities

Pattern transfer by etchingDry etchingWet etching

Wet etchingSiO2 etching: SiO2 + 4 HF SiF4 + 2 H2OAl2O3 etching:

Anisotropic etching of Si: atomically smooth planes and atomically sharp edges, at low costSi etching:

Defect-selective wet etchingAl2O3 etching

Oxide pin-hole detectionEnhancement of local defects can be counted in an optical microscopeNucleation sites after wet etchingSi/NiFe 500/AlN 500

Dry etchingplasmaPlasma + acceleration

Mechanisms in a plasma etch process

Dry etching profiles

Avoid redeposition sample tilting