8b - Etching Chemistry • The etching process involves:...

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Unformatted text preview: 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 oxidized surface • examples: H2SO4, NH4OH – Dillutent media to transport reactants and products through • examples: H2O, CH3COOH .H\7HFKQRORJLHVRI:HW(WFKLQJ 3URILOHV,VRWURSLFDQG$QLVRWURSLF $SSOLFDWLRQV6LOLFRQ6LOLFRQ1LWULGH6LOLFRQ 'LR[LGH0HWDO &RQWUROV'RSLQJ(OHFWURFKHPLFDO)LOP4XDOLW\ 0DVN0DWHULDOV 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 etch rate, depending upon orientation of mask edge to crystalline axes – Orientation of mask edge and the details of the mask pattern determine 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 used 6XEVWUDWH )LOP 0DVN LVRWURSLF HWFKLQJ DQLVRWURSLF HWFKLQJ 6LOLFRQ(WFKLQJ ,VRWURSLF  +)+12&+&22++  +)  +)1+) $QLVRWURSLF  .2+  ('3 (WK\OHQHGLDPLQH3\URFDWHFKRO  &V2+  1D2+  1++ +\GUD]LQH 0DVNLQJ0DWHULDOV  3KRWRUHVLVW $FLGV2QO\   6L1  6L2 Crystallographic etching • recall crystal lattice is face centered cubic (FCC), with two atom basis [at (0,0,0) and (1/4, 1/4, 1/4) ] • two “interpenetrating” FCC lattices a (100) (110) (111) (111) planes • • (111) planes etch the slowest, tend to be cleavage planes 54.74° (111) wrt (100) (110 ) ed ge pla atomic (111) ne (1 1 1) a tom ic p l ane lane (100) p • edge of “pit” lines up with (110) (110) edge Anisotropic Etching of Silicon - 2 (111) 54.74 (100) surface orientation Silicon (110) surface orientation (111) Silicon Anisotropic Etching of Silicon [ 110] [100] [111] SiO 2 m ask Masking • assume bulk crystalline (100) silicon substrate combined with anisotropic etch – results in pyramidal shape • bounding (111) planes can be reached using a variety of mask shapes – square mask opening, (100) wafer orientation, side of square is aligned to the (110) flat – what happens if you use a circular mask opening? • undercutting of the mask occurs until the (111) planes are reached • still forms a pyramidal pit! Other mask openings • in general mask is undercut until (111) planes are reached – bars undercut until bounding planes are reached • “cross”-shaped mask opening will also undercut to form pyramidal pit Other shapes? – star – Longhorn • “obtuse corner” cross – different bending stress / properties KOH Etching Etch Rate (110) > (100) > (111) (100) > (110) > (111) w/ IPA Varies with Temperature and Concentration (see appendix C in Madou) R = k 0 [H 2 O ] [KOH ] e 4 <100> Wafer 54.7° Cross-section Top View 1 4 − Ea kT .2+(WFKLQJ 0DVNV 6L1 LVEHVWYHU\VORZHWFKUDWH 6HOHFWLYLW\! 6L2 ZRUNVVHOHFWLYLW\|  0DVN'HVLJQ .2+(WFKHVH[SRVHGFRUQHUVTXLFNO\ 8VHVWDUSDWWHUQRUFUHDWHLQWHULRUFRUQHUVWRFUHDWH RXWHUFRUQHUV 0DVN/D\HUIRU.2+(WFKLQJ N &9'ILOPVEHVW N 6SXWWHUHGILOPVSRRU 6L1 6L2 N 7KHUPDOILOPVEHVW N &9'ILOPVHWFKIDVWHU N 6SXWWHUHGILOPVSRRU 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− KOH Etching of Silicon - 1 • Typical and most used of the hydroxide etches. • A typical recipe is: – – – – 250 g KOH 200 g normal propanol 800 g H2O Use at 80°C with agitation • Etch rates: – ~1 µm/min for (100) Si planes; stops at p++ layers – ~14 Angstroms/hr for Si3N4 – ~20 Angstroms/min for SiO2 • Anisotropy: (111):(110):(100) ~ 1:600:400 KOH Etching of Silicon - 2 • Simple hardware: – Hot plate & stirrer. – Keep covered or use reflux condenser to keep propanol from evaporating. • Presence of alkali metal (potassium, K) makes this completely incompatible with MOS or CMOS processing! • Comparatively safe and non-toxic. 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: (111):(100) ~ 1:35 • EDP is very corrosive, very carcinogenic, and never allowed near mainstream electronic microfabrication. • Typical etch rates for (100) silicon: 70°C 80°C 90°C 97°C 14 µm/hr 20 µm/hr 30 µm/hr = 0.5 µm/min 36 µm/hr EDP Etching of Silicon - 2 • Typical formulation: – – – – 1 L ethylene diamine, NH2-CH2-CH2-NH2 160 g pyrocatechol, C6H4(OH)2 6 g pyrazine, C4H4N2 133 mL H2O OH OH N N catechol pyrazine • Ionization of ethylene diamine: – NH2(CH2)2NH2 + H2O → NH2(CH2)2NH3+ + OH− H2N H2 C C H2 NH2 • Oxidation of Si and reduction of water: – Si + 2OH− + 4H2O → Si(OH)6 2− + 2H2 ethylene diamine • Chelation of hydrous silica: – Si(OH)6 2− + 3C6H4(OH)2 → Si(C6H4O2)32− + 6H2O EDP Etching of Silicon - 3 • Requires reflux condenser to keep volatile ingredients from evaporating. • 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 other anisotropic etches: – It is generally preferred for undercutting cantilevers. – It tends to leave a smoother finish than other etches, since faster etching of convex corners produces a polishing action. Amine Gallate Etching of Silicon • Much safer than EDP • Typical recipe: – – – – – 100 g gallic acid 305 mL ethanolamine 140 mL H2O 1.3 g pyrazine 0.26 mL FC-129 surfactant • Anisotropy: (111):(100): 1:50 to 1:100 • Etch rate: ~1.7 µm/min at 118°C TMAH Etching of Silicon - 1 • Tetra Methyl Ammonium Hydroxide • MOS/CMOS compatible: – No alkali metals {Li, Na, K, …}. – Used in positive photoresist developers which do not use choline. – Does not significantly etch SiO2 or Al! (Bond wire safe!) • Anisotropy: (111):(100) ~ 1:10 to 1:35 • Typical recipe: – – – – – 250 mL TMAH (25% from Aldrich) 375 mL H2O 22 g Si dust dissolved into solution Use at 90°C Gives about 1 µm/min etch rate H3C H3C CH3 N CH3 OH tetramethyl ammonium hydroxide (TMAH) TMAH Etching of Silicon - 2 • Hydroxide etches are generally safe and predictable, but they usually involve an alkali metal which makes them incompatible with MOS or CMOS processing. • Ammonium hydroxide (NH4OH) is one hydroxide which is free of alkali metal, but it is really ammonia which is dissolved into water. Heating to 90°C for etching will rapidly evaporate the ammonia from solution. • Ballasting the ammonium hydroxide with a less volatile organic solves the problem: – Tetramethyl ammonium hydroxide: (CH3)4NOH – Tetraethyl ammonium hydroxide: (C2H5)4NOH Hydrazine and Water Etching of Silicon • Produces anisotropic etching of silicon, also. • Typical recipe: – 100 mL N2H4 – 100 mL H2O – ~2 µm/min at 100°C • Hydrazine is very dangerous! – – – – – – A very powerful reducing agent (used for rocket fuel) Flammable liquid TLV = 1 ppm by skin contact Hypergolic: N2H4 + 2H2O2 → N2 + 4H2O (explosively) Pyrophoric: N2H4 + O2 → N2 + 2H2O (explosively) Flash point = 52C = 126°F in air. Various issues • safety hazards – EDP, hydrazine potentially quite hazardous – ammonia released from TMAH at elevated temperatures • hydrogen bubbles – all the reactions tend to produce H2 • bubble formation can locally “mask” etch leading to rough surfaces • bubbles trapped inside sacrificial regions can stop etch or cause breakage 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 a wafer. • Etch stop layers can be used to drastically slow the etch rate, 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 doping KOH etch rate reduces by 20× for boron doping > 1020 cm-3 NaOH etch rate reduces by 10× for boron doping > 3 × 1020 cm-3 EDP etch rate reduces by 50× for boron doping > 7 × 1019 cm-3 TMAH etch rate reduces by 10× for boron doping > 1020 cm-3 Anisotropic Etch Stop Layers - 2 heavily boron doped etch stop layer 2-5 µm thick membrane 400 - 500 µm thick wafer Electrochemical Etch Effects - 1 I V Si + 4h+ + 2OH→ Si(OH)22+ Si wafer Pt reference electrode HF / H2O solution Electrochemical Etch Effects - 2 • HF normally etches SiO2 and terminates on Si. • By biasing the Si positively, holes can be injected by an external circuit which will oxidize the Si and form hydroxides which the HF can then dissolve. • This produces an excellent polishing etch that can be very well 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 when etched, and porous silicon is formed, which appears brownish. Electrochemical Etch Effects - 4 • Increasing the wafer bias above the OCP will increase the etch rate by supplying holes which will oxidize the Si. • Increasing the wafer bias further will reach the passivation potential (PP) where SiO2 forms. – This passivates the surface and terminates the etch. – The HF / H2O solution does not exhibit a PP, since the SiO2 is dissolved by the HF. HNA Etching of Silicon • 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 the oxide by an acid (HF) that acts as a complexing agent. – Points on the Si surface randomly become oxidation or reduction sites. These act like localized electrochemical cells, sustaining corrosion currents of ~100 A/cm2 (relatively large). – Each point on the surface becomes both an anode and cathode site over time. If the time spent on each is the same, the etching will be uniform; otherwise selective etching will occur. HNA Etching of Silicon • 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 higher oxidation power for the etch. – Acetic acid is less polar than water and can help in achieving proper wetting of slightly hydrophobic Si wafers. HNA Etching of Silicon - 6 0 100 HF (49%) 25 75 1 50 50 3 500 µm/min 25 100 µm/min 10 µm/min 0 100 H2 O 75 50 25 0 50 µm/min 75 HNO3 (70%) 100 2 HNA Etching of Silicon • Region 1 – For high HF concentrations, contours are parallel to the lines of constant HNO3; therefore the etch rate is controlled by HNO3 in this region. – Leaves little residual oxide; limited by oxidation process. Region 2 – For high HNO3 concentrations, contours are parallel to the lines of constant HF; therefore the etch rate is controlled by HF in this region. – 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 rate falls off sharply for 1:1 HF:HNO ratios. 3 Isoetch Contours EXAMPLE: HF:HNO3:H2O 3:2:5 ratio by volume HF (49%) 75 0 100 20 25 50 50 25 30 500 µm/min 100 µm/min 10 µm/min 50 µm/min 50 25 75 HNO3 (70%) 100 0 0 100 H2 O R. B. 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This note was uploaded on 09/02/2010 for the course MEEN 5050 taught by Professor Himanshuj.sant during the Spring '10 term at University of Utah.

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