Subtractive Processing & Pattern Transfer
Question
- Explain the difference between positive and negative photoresist. Which is generally preferred for high-resolution work and why?
Answer
In a positive resist, exposure breaks polymer chains (e.g. via photosolubilisation), making exposed regions more soluble in developer — exposed resist is removed. In a negative resist, exposure crosslinks the polymer, making exposed regions insoluble — unexposed resist is removed.
Positive resists are generally preferred for high-resolution lithography because crosslinking in negative resists causes swelling during development, limiting resolution. Positive resists dissolve cleanly with sharper edge definition.
Question
- Describe the lift-off process. What resist profile is required and why? State a rule of thumb for resist thickness relative to the deposited film.
Answer
Lift-off uses the resist as a stencil: pattern the resist, deposit material (typically by evaporation) over the entire surface, then dissolve the resist in solvent. The material on top of the resist lifts away, leaving patterned material only where the resist had openings.
An undercut (re-entrant) resist profile is essential — it creates a physical break between the film on the substrate and the film on the resist sidewall, ensuring clean separation. With a vertical or outward-sloping profile, the deposited film forms a continuous sheet and won’t lift off cleanly.
Rule of thumb: resist thickness should be at least 3× the deposited film thickness.
Question
- Compare wet etching and dry etching. Give at least four reasons why dry etching has largely replaced wet etching in modern silicon processing.
Answer
Wet etching is generally isotropic — it etches equally in all directions, causing mask undercut proportional to etch depth. This limits the minimum feature size achievable.
Dry etching advantages: (1) anisotropic (vertical) etch profiles are achievable, critical as feature sizes shrink and aspect ratios increase; (2) in-situ end-point detection is straightforward via optical emission spectroscopy or laser interferometry; (3) less chemical waste to dispose of; (4) higher yield — minor surface contamination that blocks wet etchants is removed by ion bombardment; (5) better profile control — sidewall angle can be tuned from vertical to sloped.
Question
- KOH is used as an anisotropic wet etchant for silicon. Explain the mechanism and describe the etch profiles obtained on (100) and (110) oriented wafers.
Answer
KOH etches Si along certain crystal planes at vastly different rates. The (111) planes are etched very slowly compared to (100) and (110) planes due to the higher density of bonds per unit area on (111) surfaces.
On a (100) wafer: the etch exposes (111) sidewalls at 54.74° to the surface, producing a V-groove or trapezoidal trench profile (inverted pyramid).
On a (110) wafer: the (111) planes are perpendicular to the surface, so the etch produces vertical sidewalls — useful for making deep, straight-walled trenches.
Question
- What is the difference between additive (lift-off/electroplating) and subtractive (etching) pattern transfer? When might you choose one over the other?
Answer
Additive: pattern the resist first, then deposit or plate material into the openings, then remove the resist. The deposited material remains as the pattern.
Subtractive: deposit a blanket film first, then pattern resist on top, then etch away unwanted material, then strip the resist.
Lift-off is preferred when the material is difficult to etch (e.g. noble metals like Au, Pt) or when dry etching would cause unacceptable damage. Subtractive etching is preferred in the Si industry for higher yield (>99.99%) and when vertical, well-controlled etch profiles are needed.