2 OutlineIntroductionWet Chemical EtchingPlasma Etching
3 DefinitionRecall: Photolithography = process of transferring patterns (on masks) onto a thin layer of photoresistPhotoresist patterns must be transferred once more onto the underlying layers to produce circuit featuresPattern transfer accomplished by selectively etching unmasked portions of a layer
5 OutlineIntroductionWet Chemical EtchingPlasma Etching
6 UsesPrior to thermal oxidation or epitaxial growth, wafers are chemically cleaned to remove contaminationEspecially suitable for blanket etches (i.e., over the whole wafer surface) of polysilicon, oxide, nitride, metals, and III-V compounds.
7 Mechanism Reactants transported by diffusion to surface Reactions occur at surfaceProducts from surface removed by diffusion
8 Uniformity“Equality” of vertical etch rates at different sites on the wafer surfaceThis is actually non-uniformityAlternative definitions:s/m
9 Silicon EtchingMost 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 + 4NO2HF is used to dissolve the SiO2 layer:SiO2+ 6HF → H2SiF6 +2H2OWater can be used as a diluent for this etchant, but acetic acid is preferred.
10 Orientation-Dependent Etching Some etchants dissolve a certain crystal plane of Si faster than another planeFor Si, the (111) plane has more available bonds per unit area than the (110) and (100) planesTherefore, etch rate is slower for the (111) plane.
11 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].
12 SiO2 EtchingCommonly etched in a dilute solution of HF with or without NH4FAdding NH4F is called a buffered HF solution (BHF), also called buffered-oxide-etch (BOE)Reaction for SiO2 etching:SiO2+ 6HF → H2SiF6 +2H2OSiO2 can also be etched in vapor-phase HF.
13 OutlineIntroductionWet Chemical EtchingPlasma Etching
14 where RL = lateral etch rate, RV = vertical etch rate AnisotropyVertical features are desirable to increase circuit density.Quantitatively:where RL = lateral etch rate, RV = vertical etch rate
15 Plasma FundamentalsPlasma: ionized gas composed of equal numbers of positive and negative charges and a different number of unionized moleculesProduced when electric field is applied to a gas, causing gas to break down and become ionizedInitiated 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.
16 Plasma EtchingPlasma etching - chemical reaction combined with physical ion bombardmentOther names:ion millingsputter etchingreactive ion etchingreactive ion beam etchingFirst explored as a cheaper alternative to wet solvent resist stripping in 1960’s and 70’s
18 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.
19 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.
20 Laser InterferometryIntensity of laser light reflected off thin film surface oscillates.Period of the oscillation related to change in film thicknesswhere Dd = change in film thickness, l is the wavelength, and is the refractive index
21 Interferometry Example Typical signal from a silicide/polycrystalline Si gate etchDd for polysilicon = 80 nm (measured by using a He-Ne laser with l = nm)