Wet Etching - UWEE
EE-527: MicroFabricationWet EtchingR. B. Darling / EE-527 / Winter 2013
Outline General features of wet chemical etchingIsotropic Si etchingAnisotropic Si etchingAnisotropic GaAs etchingIsotropic etching of SiO2, Al, and CrSelective etching and etch stopsSpecial etching techniques–––––Electrically biased etchingContact and via etchesPad etchesDefect delineation etchesEtching of probe tipsR. B. Darling / EE-527 / Winter 2013
Etch Uniformity and Roughness The uniformity of an etch gives a bound on how level a surface it willproduce after starting from an initially flat surface.– Uniformity is a long-scale measure of surface height variation. The roughness of an etch gives a bound on how flat a surface it willproduce after starting from an initially flat surface.– Roughness is a short-scale measure of surface height variation. Measures of uniformity are usually derived from measurements of etchdepth: (R rate; D depth)Rmax RminDmax DminUNI 100% 100%Rmax RminDmax Dmin Measures of roughness are usually given as an RMS height variation:RUF H RMSR. B. Darling / EE-527 / Winter 2013
Etch Selectivity The selectivity of an etch is the ratio of the etch rate for the material itis desired to remove versus the etch rate for some other material that itshould not remove:Rdesired material to etchSEL Rundesired material to etch Example: For 10:1 BOE etching a Si wafer surface that contains SiO2,aluminum metalization, and Si3N4 spacers:– 10:1 BOE SEL for SiO2 / aluminum 15:1– 10:1 BOE SEL for SiO2 / Si3N4 100:1– 10:1 BOE SEL for SiO2 / Si substrate 10,000 : 1 Selectivity is usually dependent upon etch formulation, concentration,temperature, and mixing level. So it can be tuned.R. B. Darling / EE-527 / Winter 2013
Etch Anisotropy Isotropic etching– Same etch rate in all directions– Lateral etch rate is about the same as vertical etch rate– Etch rate does not depend upon the orientation of the mask edge Anisotropic etching– Etch rate depends upon orientation to crystalline planes– Lateral etch rate can be much larger or smaller than vertical etchrate, depending upon orientation of mask edge to crystalline axes– Orientation of mask edge and the details of the mask patterndetermine the final etched shape Can be very useful for making complex shapes Can be very surprising if not carefully thought out Only certain “standard” shapes are routinely usedR. B. Darling / EE-527 / Winter 2013
Etching Chemistry The etching process involves:– Transport of reactants to the surface– Surface reaction– Transport of products from the surface Key ingredients in any wet etchant:– Oxidizer examples: H2O2, HNO3– Acid or base to dissolve the oxidized surface examples: H2SO4, NH4OH– Dilutent media to transport reactants and products through examples: H2O, CH3COOHR. B. Darling / EE-527 / Winter 2013
Redox Reactions Etching is inherently an electrochemical process:– It involves electron transfer processes as part of the surfacereactions. The oxidation number is the net positive charge on aspecies. Oxidation is the process of electron loss, or increase in theoxidation number. Reduction is the process of electron gain, or decrease in theoxidation number. Redox reactions are those composed of oxidation of one ormore species and simultaneous reduction of others.R. B. Darling / EE-527 / Winter 2013
HNA Etching of Silicon - 1 Hydrofluoric acid Nitric acid Acetic acid Produces nearly isotropic etching of Si Overall reaction is:– Si HNO3 6HF H2SiF6 HNO2 H2O H2– Etching occurs via a redox reaction followed by dissolution of theoxide by an acid (HF) that acts as a complexing agent.– Points on the Si surface randomly become oxidation or reductionsites. These act like localized electrochemical cells, sustainingcorrosion currents of 100 A/cm2 (relatively large).– Each point on the surface becomes both an anode and cathode siteover time. If the time spent on each is the same, the etching willbe uniform; otherwise selective etching will occur.R. B. Darling / EE-527 / Winter 2013
HNA Etching of Silicon - 2 Silicon is promoted to a higher oxidation state at an anodicsite which supplies positive charge in the form of holes:– Si0 2h Si2 NO2 from the nitric acid is simultaneously reduced at acathode site which produces free holes:– 2NO2 2NO2 2h The Si2 combines with OH to form SiO2:– Si2 2OH Si(OH)2 SiO2 H2O The SiO2 is then dissolved by HF to form a water solublecomplex of H2SiF6:– SiO2 6HF H2SiF6 2H2OR. B. Darling / EE-527 / Winter 2013
HNA Etching of Silicon - 3 Nitric acid has a complex behavior:– Normal dissociation in water (deprotonation): HNO3 NO3 H – Autocatalytic cycle for production of holes and HNO2: HNO2 HNO3 N2O4 H2O N2O4 2NO2 2NO2 2h 2NO2 2H 2HNO2– NO2 is effectively the oxidizer of Si Its reduction supplies holes for the oxidation of the Si.– HNO2 is regenerated by the reaction (autocatalytic)– Oxidizing power of the etch is set by the amount of undissociatedHNO3.R. B. Darling / EE-527 / Winter 2013
HNA Etching of Silicon - 4 Role of acetic acid (CH3COOH):– Acetic acid is frequently substituted for water as the dilutent.– Acetic acid has a lower dielectric constant than water 6.15 for CH3COOH versus 81 for H2O This produces less dissociation of the HNO3 and yields a higheroxidation power for the etch.– Acetic acid is less polar than water and can help in achievingproper wetting of slightly hydrophobic Si wafers.R. B. Darling / EE-527 / Winter 2013
HNA Etching of Silicon - 5Silicon Anodic SiteEtchant SolutionSi2 2OH Si(OH)2 SiO2 H2OSi0 2h Si2 SiO2 6HF H2SiF6 2H2OHNO2 HNO3 N2O4 H2ON2O4 2NO22NO2 2NO2 2h 2NO2 2H 2HNO2HNO3 NO3 H Silicon Cathodic SiteR. B. Darling / EE-527 / Winter 2013
HNA Etching of Silicon - 60100HF (49%)25751505035 m/min253 m/min2 m/min1 m/min275HNO3 (70%)10001007550250CH3COOH (99%)R. B. Darling / EE-527 / Winter 2013
HNA Etching of Silicon - 7 Region 1 – For high HF concentrations, contours are parallel to the lines ofconstant HNO3; therefore the etch rate is controlled by HNO3 inthis region.– Leaves little residual oxide; limited by oxidation process.Region 2– For high HNO3 concentrations, contours are parallel to the lines ofconstant HF; therefore the etch rate is controlled by HF in thisregion.– Leaves a residual 30-50 Angstroms of SiO2; self-passivating;limited by oxide dissolution; area for polishing. Region 3– Initially not very sensitive to the amount of H2O, then etch ratefalls of sharply for 1:1 HF:HNO3 ratios.R. B. Darling / EE-527 / Winter 2013
Isoetch ContoursEXAMPLE:HF:HNO3:CH3COOH3:2:5 ratio by volume0100HF (49%)2025755050255 m/min303 m/min2 m/min1 m/min75HNO3 (70%)10001007550250CH3COOH (99%)R. B. Darling / EE-527 / Winter 2013
Silicon Crystal StructureSibacDiamond structure:a b c 5.43095 A at 300 K 90 R. B. Darling / EE-527 / Winter 2013
Anisotropic Etching of Silicon - 1 Differing hybridized (sp3) orbital orientation on differentcrystal planes causes drastic differences in etch rate. Typically, etch rates are: (110) (100) (111). The (111) family of crystallographic planes are normallythe “stop” planes for anisotropic etching. There are 8 (111) planes along the x y z unit vectors. Intersections of these planes with planar bottoms producethe standard anisotropic etching structures for (100) Siwafers:– V-grooves– pyramidal pits– pyramidal cavitiesR. B. Darling / EE-527 / Winter 2013
Anisotropic Etching of Silicon - 2(100) surface orientation(111)54.74Silicon(110) surface orientation(111)SiliconR. B. Darling / EE-527 / Winter 2013
Anisotropic Etching of Silicon - 3Symmetrical, anisotropically etched pyramidal pit:[110][100]SiO2 mask[111]Arbitrary mask features will become anisotropically etched to the [110]aligned rectangle which contains them:R. B. Darling / EE-527 / Winter 2013
Anisotropic Etching of Silicon - 4Progressiveanisotropicetching of acantileverR. B. Darling / EE-527 / Winter 2013
Hydroxide Etching of Silicon Several hydroxides are useful:– KOH, NaOH, CeOH, RbOH, NH4OH, TMAH: (CH3)4NOH Oxidation of silicon by hydroxyls to form a silicate:– Si 2OH 4h Si(OH)2 Reduction of water:– 4H2O 4OH 2H2 4h Silicate further reacts with hydroxyls to form a watersoluble complex:– Si(OH)2 4OH SiO2(OH)22 2H2O Overall redox reaction is:– Si 2OH 4H2O Si(OH)2 2H2 4OH R. B. Darling / EE-527 / Winter 2013
KOH Etching of Silicon - 1 Typical and most used of the hydroxide etches. A typical recipe is:––––250 g KOH200 g normal propanol (isopropanol has too low of a flash point)800 g H2OUse at 80 C with agitation Etch rates:– 1 m/min for (100) Si planes; stops at p layers– 14 Angstroms/hr for Si3N4– 20 Angstroms/min for SiO2 Anisotropy: (110):(100):(111) 600:400:1 Masking films: SiO2, Si3N4, but not photoresist.R. B. Darling / EE-527 / Winter 2013
KOH Etching of Silicon - 2 Simple hardware:– Hot plate & stirrer.– Keep covered or use reflux condenser to keep propanol fromevaporating. Presence of alkali metal (potassium, K) makes thiscompletely incompatible with MOS or CMOS processing! Comparatively safe and non-toxic, aside from the high pHof the KOH solution.– It is still a very strong alkali solution which can cause burns!R. B. Darling / EE-527 / Winter 2013
EDP Etching of Silicon - 1 Ethylene Diamine Pyrocatechol Also known as Ethylene diamine - Pyrocatechol - Water(EPW) EDP etching is readily masked by SiO2, Si3N4, Au, Cr, Ag,Cu, and Ta. But EDP can etch Al! Anisotropy: (100):(111) 35:1 EDP is very corrosive, very carcinogenic, and neverallowed near mainstream electronic microfabrication. Typical etch rates for (100) silicon:70 C14 m/hr80 C20 m/hr90 C30 m/hr 0.5 m/min97 C36 m/hrR. B. Darling / EE-527 / Winter 2013
EDP Etching of Silicon - 2 Typical formulation:––––OH1 L ethylene diamine, NH2-CH2-CH2-NH2160 g pyrocatechol, C6H4(OH)26 g pyrazine, C4H4N2133 mL H2ONcatecholpyrazine Ionization of ethylene diamine:– NH2(CH2)2NH2 H2O NH2(CH2)2NH3 NOHOH Oxidation of Si and reduction of water:H2CH2NCH2NH2ethylene diamine– Si 2OH 4H2O Si(OH)6 2 2H2 Chelation of hydrous silica:– Si(OH)6 2 3C6H4(OH)2 Si(C6H4O2)32 6H2OR. B. Darling / EE-527 / Winter 2013
EDP Etching of Silicon - 3 Requires reflux condenser to keep volatile ingredients fromevaporating. Completely incompatible with MOS or CMOS processing!– It must be used in a fume collecting bench by itself.– It will rust any metal in the nearby vicinity.– It leaves brown stains on surfaces that are difficult to remove. EDP has a faster etch rate on convex corners than otheranisotropic etches:– It is generally preferred for undercutting cantilevers.– It tends to leave a smoother finish than other etches, since fasteretching of convex corners and protruding edges produces apolishing action.R. B. Darling / EE-527 / Winter 2013
EDP Etching of Silicon - 4 EDP etching can result in deposits of polymerized Si(OH)4on the etched surfaces and deposits of Al(OH)3 on Al pads. Moser’s post EDP protocol is used to eliminate this:––––20 sec. DI water rinse120 sec. dip in 5% ascorbic acid (vitamin C) and H2O120 sec. rinse in DI water60 sec. dip in hexane, C6H14R. B. Darling / EE-527 / Winter 2013
Amine Gallate Etching of Silicon Much safer than EDP Typical recipe:–––––100 g gallic acid305 mL ethanolamine140 mL H2O1.3 g pyrazine0.26 mL FC-129 surfactant Anisotropy: (100):(111) 50:1 to 100:1 Etch rate: 1.7 m/min at 118 CR. B. Darling / EE-527 / Winter 2013
TMAH Etching of Silicon - 1 Tetra Methyl Ammonium Hydroxide MOS/CMOS compatible:– No alkali metals {Li, Na, K, }.– TMAH is used in many positive photoresist developers.– Does not significantly etch SiO2 or Al! (Bond wire safe!) Anisotropy: (100):(111) 10:1 to 35:1 Typical recipe:–––––250 mL TMAH (25% from Aldrich)375 mL H2O22 g Si dust dissolved into solutionUse at 90 CGives about 1 m/min etch rateH3CH3CCH3NOHCH3tetramethyl ammonium hydroxide(TMAH)R. B. Darling / EE-527 / Winter 2013
TMAH Etching of Silicon - 2 Hydroxide etches are generally safe and predictable, butthey usually involve an alkali metal which makes themincompatible with MOS or CMOS processing. Ammonium hydroxide (NH4OH) is one hydroxide whichis free of alkali metal, but it is really ammonia which isdissolved into water. Heating to 90 C for etching willrapidly evaporate the ammonia from solution. Ballasting the ammonium hydroxide with a less volatileorganic solves the problem:– Tetramethyl ammonium hydroxide: (CH3)4NOH (TMAH)– Tetraethyl ammonium hydroxide: (C2H5)4NOH (TEAH) TMAH has recently been found to be more toxic thanpreviously thought – it appears to be a neurotoxin.R. B. Darling / EE-527 / Winter 2013
Hydrazine and Water Etching of Silicon Produces anisotropic etching of silicon, also. Typical recipe:– 100 mL N2H4 (hydrazine)– 100 mL H2O– 2 m/min at 100 C Hydrazine is very dangerous!––––––A very powerful reducing agent (used for rocket fuel)Flammable liquidTLV 1 ppm by skin contactHypergolic: N2H4 2H2O2 N2 4H2O (explosively)Pyrophoric: N2H4 O2 N2 2H2O (explosively)Flash point 52 C 126 F in air.R. B. Darling / EE-527 / Winter 2013
Anisotropic Etch Stop Layers - 1 Controlling the absolute depth of an etch is often difficult,particularly if the etch is going most of the way through awafer. Etch stop layers can be used to drastically slow the etchrate, providing a stopping point of high absolute accuracy. Boron doping is most commonly used for silicon etching. Requirements for specific etches:–––––HNA etch actually speeds up for heavier dopingKOH etch rate reduces by 20 for boron doping 1020 cm-3NaOH etch rate reduces by 10 for boron doping 3 1020 cm-3EDP etch rate reduces by 50 for boron doping 7 1019 cm-3TMAH etch rate reduces by 10 for boron doping 1020 cm-3R. B. Darling / EE-527 / Winter 2013
Anisotropic Etch Stop Layers - 2heavily boron doped etch stop layer2-5 m thick membrane400 - 500 mthick waferEtch stop layers are commonly used to create membranes and cantilevers for MEMS devices.R. B. Darling / EE-527 / Winter 2013
Electrochemical Etch Effects - 1IVSi 4h 2OH Si(OH)22 Si waferPt reference electrodeHF / H2O solutionThe supply of holes (h ) to oxidize Si is a common element of all Sietches. Because of this, Si etches can be electrically controlled.R. B. Darling / EE-527 / Winter 2013
Electrochemical Etch Effects - 2 HF normally etches SiO2 and terminates on Si. By biasing the Si positively, holes can be injected by anexternal circuit which will oxidize the Si and formhydroxides which the HF can then dissolve. This produces an excellent polishing etch that can be verywell masked by LPCVD films of Si3N4. If the etching is performed in very concentrated HF (48%HF, 98% EtOH), then the Si does not fully oxidize whenetched, and porous silicon is formed, which appearsbrownish. Porous silicon has some unusual electroluminescentproperties: It will glow bright orange under electroninjection.R. B. Darling / EE-527 / Winter 2013
Electrochemical Etch Effects - 3I, mA/cm2n-type SiOCP:open-circuitpotential-2.0(100) Si in 40% KOH at 60 Cp-type Si-1.5-1.0PP:passivationpotential-0.50.0 0.5 1.0V, Voltspotential of Ptreference electrodeR. B. Darling / EE-527 / Winter 2013
Electrochemical Etch Effects - 4 The open circuit potential (OCP) is the difference betweenthe half-cell potentials of the Si wafer and the Pt electrode.– This is usually about 1.50 V, varying a bit with temperature andsolution concentration. Increasing the wafer bias above the OCP will increase theetch rate by supplying holes which will oxidize the Si. Increasing the wafer bias further will reach the passivationpotential (PP) where SiO2 forms.– This passivates the surface and terminates the etch. The SiO2creates an insulating film which drastically reduces the currentflow.– The HF / H2O solution does not exhibit a PP, since the SiO2 isconstantly and rapidly dissolved by the HF.R. B. Darling / EE-527 / Winter 2013
GaAs Crystal StructureGabAsacZinc blende structure:a b c 5.6533 A at 300 K 90 R. B. Darling / EE-527 / Winter 2013
Anisotropic Wet Etching of GaAs - 1 Like silicon, GaAs crystalizes in a face-centered cubic (FCC)lattice, but it has a basis of 1 Ga 1 As atom, whereas siliconhas a basis of 2 Si atoms. A cubic crystal lattice has 8 equivalent (111) planes, but thediffering basis of GaAs creates 4 (111) planes that areterminated by Ga atoms and 4 (111) planes that are terminatedby As atoms. The (111)As planes are highly reactive, while the (111)Gaplanes are fairly inert. This gives rise to a different type of wet etching anisotropy: Typical etch rates: (110) (111)As (100) (111)Ga This is also typical of most other compound semiconductors.R. B. Darling / EE-527 / Winter 2013
Anisotropic Wet Etching of GaAs - 2 Depending upon the crystal orientation relative to the mask edge,anisotropic etching of GaAs can produce ramped edges, or undercut(dovetailed) edges:z-axissquare mask featurefor etch pitx-axispoints outof pageundercutat mask edgey-axisnormal to (011) planeramped at mask edgeR. B. Darling / EE-527 / Winter 2013
Wet Etching of GaAs There are many effective etch compositions; a sampling:H2SO4 : H2O2 : H2OBr : CH3OHHCl : H2O2 : H2OH3PO4 : H2O2 : H2ONH4OH : H2O2 : H2O Any of these etches must be tuned to achieve the properramp and dovetail characteristics that are required.R. B. Darling / EE-527 / Winter 2013
Isoetch Contours for H2SO4 : H2O2 : H2O0100H2O2 (30%)257550503 m/min252 m/min1 m/min0.5 m/min75H2SO4 (98%)10001007550250H2OR. B. Darling / EE-527 / Winter 2013
Wet Etching of SiO2 Almost always requires HF in some form:– HF : H2O– HF : NH4F (Buffered Oxide Etch BOE)– HF : HCl Etch rate is highly dependent upon how the SiO2 was created:– Thermal oxidation creates the most dense and electronically suitableoxide for MOSFETs with generally the slowest etch rate.– LPCVD deposited oxides are generally less dense, have more electronicdefects, and etch quicker than thermal oxides.– Sputtered oxides are generally less dense still, have even moreelectronic defects, and etch still faster than the LPCVD oxides.– Special glass insulating layers have different etch rates still: Low Temperature Oxide (LTO) Phospho-Silicate Glass (PSG)R. B. Darling / EE-527 / Winter 2013
Buffered Oxide Etch (BOE) Normal etching of SiO2 will deplete the F ion concentration,leading to an etch rate which changes over time. This can be fixed by buffering the HF with another source ofthe F ion: NH4F. Buffering with NH4F also slows the etch rate and results inmore polishing of the Si surface (atomically flatter). Reactions:– Etching: SiO2 6HF H2SiF6 2H2O– Buffering: NH4F NH3 HF Many commercial compositions exist:– 5:1, 6:1, 7:1, 10:1, 20:1, 30:1, 50:1, and 100:1.– Ratios are NH4F (40% in H2O) to HF (49% in H2O)R. B. Darling / EE-527 / Winter 2013
Concentrations of HF and NH4F in BOE Increasing the NH4F concentration reduces the HF concentration,because the F ion is becoming more sequestered by the ammonium.Equilibrium is set by the buffering reaction: NH4F NH3 HFExample: 10:1 BOE: 36.2% NH4F and 4.7% HF at 20 C.10:1 BOEGraph from Honeywell “improved BOE” studyR. B. Darling / EE-527 / Winter 2013
BOE Etching of Thermal (Native Grown) SiO2 Note that in the literature, 10:1 BOE is not the same as 10:1 HF!– 10:1 BOE means 10 NH4F (40%) to 1 HF (49%)– 10:1 HF means 10 H2O to 1 HF (49%) Some typical etch rates at 20 C:Etch SolutionEtch Rate – thermal SiO26:1 BOE90 nm/min 1.50 nm/sec10:1 BOE53 nm/min 0.88 nm/sec20:1 BOE30 nm/min 0.50 nm/sec10:1 HF28 nm/min 0.47 nm/sec50:1 HF
Etching is inherently an electrochemical process: – It involves electron transfer processes as part of the surface reactions. The oxidation number is the net positive charge on a species. Oxidation is the process of electron loss, or increase in the
Etching is a process of removing material from the substrate’s surface. In general, there are two categories that etching can be divided into dry etching, and wet etching. The focus of this section will be solely on dry etching. Wet etching, a process where the substrate is submerged
etching is usually faster than the rates for many dry etching processes and can easily be changed by varying temperature or the concentration of active species. Wet Etch Synonyms: chemical etching, liquid etching Definition: Wet etching is a material removal process that uses liquid chemicals or etchants to remove materials from a wafer.
Review: GaAs etching overview; wet and dry etching; Ref. (Ashby, C.I.H., 1990a) Review: InP wet chemical etching; with (1) defect or damage revealing etchant table, (2) polishing etchant table, and (3) pattern etchant table; Ref. (Adachi, S., 1990b) Review: wet and dry chemical etching of GaAs; classifies wet etchants as non-electrolyte (those with
Plasma Etching Page 2 OUTLINE Introduction Plasma Etching Metrics – Isotropic, Anisotropic, Selectivity, Aspect Ratio, Etch Bias Plasma and Wet Etch Summary The Plasma State - Plasma composition, DC & RF Plasma Plasma Etching Processes - The principle of plasma etching, Etching Si and SiO2 with CF4
Al etching start and thus to diff erent etching depths or times (Fig. 118). The formation of hydrogen in the etching reaction is also problematic for a homogeneous etching result. The constantly produced H 2 bubbles stick to the surface and block the etching process through a sup
After etching, line width and the length only for the tail along the circuit was measured to obtain etching factor. Etching factor was defined as shown in Figure 3. From the etching test results, etching factor improved when matte side surface roughness decreased. Flatter foil seemed to be better to create narrower traces.
Wet Etching vs Dry Etching In wet etchants, the etch reactants come form a liquid source In dry etchants, the etch reactants come form a gas or vapor phase source and are typically ionized-Atoms or ions from the gas are the reactive species that etch the exposed film Selectivity : In general, dry etching has less selectivity than wet .
Dry plasma etching has become the dominant patterning technique for the group-III nitrides, due to the shortcom-ings in wet chemical etching. Plasma etching proceeds by either physical sputtering, chemical reaction, or a combination of the two often referred to as ion-assisted plasma etching, Physical sputtering is dominated by the
