Etching

Etching techniques include

Wet etch is used to remove contanimation.

Cleaning

There are patterned (after lithography) and unpatterned etches.

Etch chemicals are chosen by selectivity 6:1 etch vs 30:1

Chemicals etch isotropically resulting in undercutting of edges.

Dry etches produce sharp vertical sides.

Plasma etches are halogen based with free radicals. CF2 and CF3 mixed with Argon (Ar+) ions. Positive ions remove material physically.

Parameters to control in plasma etching:

Plasma etching is done in low-pressure vacuum, (10 torr with rough pumping, 0.1 millitorr final pumped pressure). Dielectric etch uses 100 millitorr.

Flourine-based gases are used (CHF3) and Ar for physical etch). The ratio of gasses is controlled by computer.

RF at 13.6 MHz. A matching network does impedance matching. A rotating magnetic field forces electrons in a spiral path.

In plasma glow region positive ions bombard surface for physical etch. Free flourine, eg. from SiF4 does a chemical etch. A polymer bonds to sidewall. Then Ar selectively attacks horizontal surfaces leaving steep side walls. Etching occurs at 10 C temperature. Carbon monoxide levels are monitored to signal the endpoint of the etch.

The corrosive gasses from the plasma etch go to a combustion system before venting to the atmosphere.

In a wet etch H2SO4 and H2O2 is sprayed on wafer to remove resist.

Oxide is removed by unpatterned etch. A nitride layer provides a patterned etch.

Phosphoric acides removes uneven surface - trench oxide.

Oxide damaged by ion implantation is removed and new oxide layer is regrown.

Control chambers keep chips at temperature between steps.

Poly etch uses 2-4 millitorr vacuum pressures and HBr and chlorine gas to etch polysilicon. Polysilicon etch is more chemical than mechanical. An optical spectrum analyser is used to detect the end of the polysilicon etch.

Planarization

Chemical-mechanical-physical (CMP) process using abrasive pads and mechanical motion. Pads are first reconditioned to remove caked slurry (slurry is silica particles blended with water - like wet sandpaper.) The slurry must be constantly monitored and stirred to keep silica particles from clumping. Rotating pads are vibrated to maintain pressure on wafer surface. After planarization, wafers are washed and brushed and then given a final rinse.

Contact holes

The high aspect ratio of contact holes makes this a critical etch. A barrier film is sputtered on, then a tungston layer. Metal planarization is used to remove and smooth excess material. Orbital polishing with oxidizers in a slurry form tungsten oxide which is then removed by silicon particles (?).

Metal etch

An Al-Cu alloy is deposited, then patterned with lithography, and a metal etch removes the excess metal. Turbo pump produces vacuum of 5-10 millitorr. 85 C temperature produced by heat lamps. Chlorine gasses and Ar introduces. BCl3 reacts with Al2Cl. The etching is almost totally chemical.

Ashing and sidewall passivation is then applied. Cl (which is corrosive) is removed with H2 to form gaseous HCl. The residue is cleanned with organic solvents.

The last layer is a nitide overglass.

Finally an atmospheric plasma etches the back side of the Silicon wafer to make it ultrathin.


Maintained by John Loomis, last updated 15 Dec 2003