Lecture date: Monday, 2014.10.27 (lecture recording)
The purpose of etching is to transfer the pattern defined by lithography into the underlying layers.
Early etching techniques made use of liquid acids. Liquid acids can be highly selective—they etch some materials very strongly and other materials not at all—but they etch isotropically and eat into the sidewalls. For the smallest device features, this isotropic etching is unacceptable.
Plasma etching allows for anisotropic etching at the cost of some selectivity:
- The plasma is composed of charged species—electrons and ions
- Electrons also create highly reactive radical species
- Ions impact the surface at normal incidence (perpendicular to the surface)
e⁻ + CF₄ → CF₃ + F + e⁻ e⁻ + C₄F₈ → CF, C₂F
Ions are driven straight down into the surface; radicals float around in a thermal distribution
Keep in mind that the photomask will etch too! A fluorocarbon plasma can help protect the hydrocarbon-based mask with depositing elements (like C).
When an ion hits the surface, it deposits its energy within ~1nm within 10^-12 seconds. The ion current is quite low, so each impact is pretty much independent. The trick is to sustain a reasonable etch rate without overheating the wafer, and to transfer as much momentum as possible into a thin surface layer.
If there aren't reactions (e.g. with Ar+ ions), you still get sputtering. The downside to this is that sputter yields of various common materials are all about the same: sputtering is entirely nonselective!
Things etch engineers worry about:
- etch rate
- electrical damage (charging damage)
- radiation damage (X-rays, etc)
The variables that can be adjusted:
- gas composition
- gas flowrate and residence time
- gas pressure
- substrate (wafer) temperature
- electrical power (plasma power and bias power)
- electrical frequency
- magnetic field strength and orientation
→ all of these things can be varied with time (e.g. you can ramp the pressure up or down over the run)
A typical etch step takes a few minutes. How many wafers can I process per hour, on average, including all of the cleaning and maintenance steps?
A typical etch chemistry is composed of 4-6+ compounds: Ar, C₄F₈, N₂, H₂ O₂, He...
A technique called 'Design of Experiments' can be used to generate a response surface that describes how your variables of interest will vary with the parameters you can change.
Etching Si: CF4, CF4+O2; SF6, SF6+O2; NF3, Cl2 ,Br2, HCl, HBr
Etching SiO2, Si3N4: CF4, C2H6, C4H8; CHF3, C4F6 + O2, Ar, He
Etching Al: BCl3, Cl2, high temperature
Etching Cu: Cl2, high temperature
Etching Pt: very challenging!
"Etch Products" are pumped away or deposit on chamber walls.
Etch Chamber Configurations
Capacitively-Coupled Plasma (CCP) and Inductively-Coupled Plasma (ICP) are common configurations in industry.
Microwaves and magnetically-enhanced plasmas are commonly used in sputtering in an Electron Cyclotron Resonance (ECR) system.
Post plasma etch molecular dynamics simulation videos!