ECE/ChE 4752: Microelectronics Processing Laboratory Etching ECE/ChE 4752: Microelectronics Processing Laboratory Gary S. May February 19, 2004

Outline Introduction Wet Chemical Etching Plasma Etching

Definition Recall: Photolithography = process of transferring patterns (on masks) onto a thin layer of photoresist Photoresist patterns must be transferred once more onto the underlying layers to produce circuit features Pattern transfer accomplished by selectively etching unmasked portions of a layer

Etching Hierarchy

Outline Introduction Wet Chemical Etching Plasma Etching

Uses Prior to thermal oxidation or epitaxial growth, wafers are chemically cleaned to remove contamination Especially suitable for blanket etches (i.e., over the whole wafer surface) of polysilicon, oxide, nitride, metals, and III-V compounds.

Mechanism Reactants transported by diffusion to surface Reactions occur at surface Products from surface removed by diffusion

Uniformity “Equality” of vertical etch rates at different sites on the wafer surface This is actually non-uniformity Alternative definitions: s/m

Silicon Etching Most etchants are mixtures of HNO3 and HF in water or acetic acid (CH3COOH). HNO3 oxidizes silicon to form an SiO2 layer: Si + 4HNO3 → SiO2 + 2H2O + 4NO2 HF is used to dissolve the SiO2 layer: SiO2+ 6HF → H2SiF6 +2H2O Water can be used as a diluent for this etchant, but acetic acid is preferred.

Orientation-Dependent Etching Some etchants dissolve a certain crystal plane of Si faster than another plane For Si, the (111) plane has more available bonds per unit area than the (110) and (100) planes Therefore, etch rate is slower for the (111) plane.

KOH Etching KOH is an orientation-dependent etchant for Si. Solution with 19 wt % KOH in deionized water at 80 oC removes the (100) plane at a much higher rate than the (110) and (111) planes [ratio of etch rates for (100):(110):(111) planes = 100:16:1].

SiO2 Etching Commonly etched in a dilute solution of HF with or without NH4F Adding NH4F is called a buffered HF solution (BHF), also called buffered-oxide-etch (BOE) Reaction for SiO2 etching: SiO2+ 6HF → H2SiF6 +2H2O SiO2 can also be etched in vapor-phase HF.

Outline Introduction Wet Chemical Etching Plasma Etching

where RL = lateral etch rate, RV = vertical etch rate Anisotropy Vertical features are desirable to increase circuit density. Quantitatively: where RL = lateral etch rate, RV = vertical etch rate

Plasma Fundamentals Plasma: ionized gas composed of equal numbers of positive and negative charges and a different number of unionized molecules Produced when electric field is applied to a gas, causing gas to break down and become ionized Initiated by free electrons that gain kinetic energy from electric field, collide with gas molecules, and lose energy. Energy transferred causes the gas molecules to be ionized (i.e., to free electrons). Free electrons gain kinetic energy from the field, and the process continues.

Plasma Etching Plasma etching - chemical reaction combined with physical ion bombardment Other names: ion milling sputter etching reactive ion etching reactive ion beam etching First explored as a cheaper alternative to wet solvent resist stripping in 1960’s and 70’s

Etch Equipment

Etch Mechanism Etchant species generated in plasma. Reactant transported by diffusion to surface. Reactant adsorbed on the surface. Chemical reaction (along with ion bombardment) forms volatile compounds. Compounds desorbed from surface, diffused into the bulk gas, and pumped out by vacuum system.

End-Point Control Dry etching has less etch selectivity than wet. Plasma reactor must be equipped with a monitor that indicates when the etching process is to be terminated (“end point detection” system). Laser interferometry is used to determine the end point.

Laser Interferometry Intensity of laser light reflected off thin film surface oscillates. Period of the oscillation related to change in film thickness where Dd = change in film thickness, l is the wavelength, and is the refractive index

Interferometry Example Typical signal from a silicide/polycrystalline Si gate etch Dd for polysilicon = 80 nm (measured by using a He-Ne laser with l = 632.8 nm)