Lecture 10 Wet Etching

Lecture 10 Wet Etching - • Can be very useful for making...

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Unformatted text preview: • Can be very useful for making complex shapes • Can be very surprising if not carefully thought out • Only certain “standard” shapes are routinely used – 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 $QLVRWURSLFHWFKLQJ – 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 ,VRWURSLFHWFKLQJ :HW(WFKLQJ &RQWUROV'RSLQJ(OHFWURFKHPLFDO)LOP4XDOLW\ 0DVN0DWHULDOV $SSOLFDWLRQV6LOLFRQ6LOLFRQ1LWULGH6LOLFRQ 'LR[LGH0HWDO 3URILOHV,VRWURSLFDQG$QLVRWURSLF .H\7HFKQRORJLHVRI:HW(WFKLQJ ± 2[LGL]HU ‡H[DPSOHV+2+12 ± $FLGRUEDVHWRGLVVROYHR[LGL]HGVXUIDFH ‡H[DPSOHV+621+2+ ± 'LOOXWHQWPHGLDWRWUDQVSRUWUHDFWDQWVDQGSURGXFWVWKURXJK ‡H[DPSOHV+2&+&22+ ‡.H\LQJUHGLHQWVLQDQ\ZHWHWFKDQW ± 7UDQVSRUWRIUHDFWDQWVWRWKHVXUIDFH ± 6XUIDFHUHDFWLRQ ± 7UDQVSRUWRISURGXFWVIURPWKHVXUIDFH ‡7KHHWFKLQJSURFHVVLQYROYHV (WFKLQJ&KHPLVWU\ Fundamentals of Micromachining Dr. Bruce K. Gale Wet Etching and Bulk Micromachining 6LOLFRQ (WFKLQJ 6LOLFRQ(WFKLQJ )LOP 6LOLFRQ(WFKLQJ 6XEVWUDWH 0DVN DQLVRWURSLF HWFKLQJ LVRWURSLF HWFKLQJ Cross-section 54.7° <100> Wafer 4 Top View R = k 0 [H 2 O ] [KOH ] e 1 4 − Ea kT Varies with Temperature and Concentration (see appendix C in Madou) Etch Rate (110) > (100) > (111) (100) > (110) > (111) w/ IPA KOH Etching 0DVNLQJ0DWHULDOV  3KRWRUHVLVW $FLGV2QO\   6L1  6L2 $QLVRWURSLF  .2+  ('3 (WK\OHQHGLDPLQH3\URFDWHFKRO  &V2+  1D2+  1++ +\GUD]LQH ,VRWURSLF  +)+12&+&22++  +)  +)1+) 6LOLFRQ(WFKLQJ 6LOLFRQ(WFKLQJ N 7KHUPDOILOPVEHVW N &9'ILOPVHWFKIDVWHU N 6SXWWHUHGILOPVSRRU 6L2 N &9'ILOPVEHVW N 6SXWWHUHGILOPVSRRU 6L1 0DVN/D\HUIRU.2+(WFKLQJ 0DVNV 6L1 LVEHVWYHU\VORZHWFKUDWH 6HOHFWLYLW\! 6L2 ZRUNVVHOHFWLYLW\|  0DVN'HVLJQ .2+(WFKHVH[SRVHGFRUQHUVTXLFNO\ 8VHVWDUSDWWHUQRUFUHDWHLQWHULRUFRUQHUVWRFUHDWH RXWHUFRUQHUV .2+(WFKLQJ 14 µm/hr 20 µm/hr 30 µm/hr = 0.5 µm/min 36 µm/hr 70°C 80°C 90°C 97°C R. B. Darling / EE-527 • Ethylene Diamine Pyrocatechol • Also known as Ethylene diamine - Pyrocatechol - Water (EPW) • EDP etching is readily masked by SiO , Si3N4, Au, Cr, Ag, 2 Cu, and Ta. But EDP can etch Al! • Anisotropy: (111):(100) ~ 1:35 • EDP is very corrosive, very carcinogenic, and never allowed near mainstream electronicmicrofabrication. • Typical etch rates for (100) silicon: EDP Etching of Silicon - 1 R. B. Darling / EE-527 • Presence of alkali metal (potassium, K) makes this completely incompatible withMOS or CMOS processing! • Comparatively safe and non-toxic. – Hot plate & stirrer. – Keep covered or use reflux condenser to keep propanol from evaporating. • Simple hardware: KOH Etching of Silicon - 2 250 g KOH 200 g normal propanol 800 g H 2O Use at 80°C with agitation • Anisotropy: (111):(110):(100) ~ 1:600:400 – ~1 µm/min for (100) Si planes; stops at p++ layers – ~14 Angstroms/hr for Si3N4 – ~20 Angstroms/min for SiO2 • Etch rates: – – – – • Typical and most used of the hydroxide etches. • A typical recipe is: KOH Etching of Silicon - 1 – Si + 2OH− + 4H2O → Si(OH)2++ + 2H2 + 4OH− • Overall redox reaction is: – Si(OH)2++ + 4OH− → SiO2(OH)22− + 2H2O R. B. Darling / EE-527 R. B. Darling / EE-527 • Silicate further reacts with hydroxyls to form a watersoluble complex: – 4H2O → 4OH− + 2H2 + 4h+ • Reduction of water: – Si + 2OH− + 4h+ → Si(OH)2++ • Oxidation of silicon by hydroxyls to form a silicate: – KOH, NaOH, CeOH, RbOH, NH 4OH, TMAH: (CH3)4NOH • Several hydroxides are useful: Hydroxide Etching of Silicon 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 – – – – – R. B. Darling / EE-527 Amine Gallate Etching of Silicon R. B. Darling / EE-527 20 sec. DI water rinse 120 sec. dip in 5% ascorbic acid (vitamin C) and H O 2 120 sec. rinse in DI water 60 sec. dip in hexane, C6H14 • Much safer than EDP • Typical recipe: – – – – • EDP etching can result in deposits of polymerizedSi(OH)4 on the etched surfaces and deposits of Al(OH)3 on Al pads. • Moser’ post EDP protocol to eliminate this: s EDP Etching of Silicon - 4 + 4H2O → Si(OH)6 2− + 2H 2 OH − catechol OH H2 C C H2 pyrazine N N NH2 R. B. Darling / EE-527 ethylene diamine H2N OH R. B. Darling / EE-527 – 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. • EDP has a faster etch rate on convex corners than other anisotropic etches: – 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. • Requires reflux condenser to keep volatile ingredients from evaporating. • Completely incompatible withMOS or CMOS processing! EDP Etching of Silicon - 3 – Si(OH)6 2− + 3C6H4(OH)2 → Si(C6H4O2)32− + 6H2O • Chelation of hydrous silica: – Si + 2OH− • Oxidation of Si and reduction of water: – NH2(CH2)2NH2 + H2O → NH2(CH2)2NH3 + + 1 L ethylene diamine, NH2-CH2-CH2-NH2 160 g pyrocatechol, C6H4(OH)2 6 g pyrazine, C4H4N2 133 mL H2O • Ionization of ethylene diamine: – – – – • Typical formulation: EDP Etching of Silicon - 2 – – – – – 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 CH3 N CH3 OH – – – – – R. B. Darling / EE-527 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 – Tetramethyl ammonium hydroxide: (CH )4NOH 3 – Tetraethyl ammonium hydroxide: (C H5)4NOH 2 R. B. Darling / EE-527 • 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 (NH OH) is one hydroxide which 4 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: • 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: R. B. Darling / EE-527 tetramethyl ammonium hydroxide (TMAH) H3C H3C Anisotropic Etch Stop Layers - 1 R. B. Darling / EE-527 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 = 52°C = 126°F in air. • Anisotropy: (111):(100) ~ 1:10 to 1:35 • Typical recipe: – No alkali metals {Li, Na, K, … }. – Used in positive photoresist developers which do not use choline. – Does not significantly etch SiO or Al! (Bond wire safe!) 2 • Tetra Methyl Ammonium Hydroxide • MOS/CMOS compatible: TMAH Etching of Silicon - 1 TMAH Etching of Silicon - 2 – – – – – – • Hydrazine is very dangerous! – 100 mL N2H4 – 100 mL H2O – ~2 µm/min at 100°C • Produces anisotropic etching of silicon, also. • Typical recipe: Hydrazine and Water Etching of Silicon -2.0 OCP: open-circuit potential -1.5 -0.5 +0.5 potential of Pt reference electrode 0.0 V, Volts R. B. Darling / EE-527 +1.0 (100) Si in 40% KOH at 60°C p-type Si n-type Si PP: passivation potential -1.0 I, mA/cm2 Electrochemical Etch Effects - 3 R. B. Darling / EE-527 • HF normally etches SiO2 and terminates on Si. • By biasing the Si positively, holes can be injected by an external circuit which will oxidize theSi 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 SiN4. 3 • 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 - 2 Si wafer V HF / H2O solution Si + 4h+ + 2OH→ Si(OH)22+ I R. B. Darling / EE-527 400 - 500 µm thick wafer R. B. Darling / EE-527 Pt reference electrode Electrochemical Etch Effects - 1 2-5 µm thick membrane heavily boron doped etch stop layer Anisotropic Etch Stop Layers - 2 ‡5HGR[UHDFWLRQVDUHWKRVHFRPSRVHGRIR[LGDWLRQRIRQHRU PRUHVSHFLHVDQGVLPXOWDQHRXVUHGXFWLRQRIRWKHUV ‡5HGXFWLRQLVWKHSURFHVVRIHOHFWURQJDLQRUGHFUHDVHLQ WKHR[LGDWLRQQXPEHU ‡2[LGDWLRQLVWKHSURFHVVRIHOHFWURQORVVRULQFUHDVHLQWKH R[LGDWLRQQXPEHU ‡7KHR[LGDWLRQQXPEHULVWKHQHWSRVLWLYHFKDUJHRQD VSHFLHV ± ,WLQYROYHVHOHFWURQWUDQVIHUSURFHVVHVDVSDUWRIWKHVXUIDFH UHDFWLRQV ‡(WFKLQJLVLQKHUHQWO\DQHOHFWURFKHPLFDOSURFHVV 6LOLFRQ(WFKLQJ5HGR[5HDFWLRQV R. B. Darling / EE-527 – 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. • Increasing the wafer bias above the OCP will increase the etch rate by supplying holes which will oxidize theSi. • Increasing the wafer bias further will reach thepassivation potential (PP) where SiO2 forms. 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This note was uploaded on 11/16/2011 for the course MSE 5960 taught by Professor Douglas during the Fall '04 term at University of Florida.

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