Wet-Chemical Etching of Silicon
Revised: 2011-01-17 Source: www.microchemicals.eu/technical_information
Our Poster Crystalline Silicon
Crystallography, etch rates of Si (isotropic and anisotropic) and SiO2, Si-wafer production, summarized on a DIN A0 Poster ... Interested? Gladly we send you one or more posters for free (only valid for Europe)! Just send us a short e-mail: info@microchemicals.de
Since 2010, we supply our customers - beside photoresists, solvents, and etchants - also with c-Si wafers. Therefore, we are happy to provide you with technical support also in this field of microstructuring. Thank you for your interest!
Photoresists, developers, remover, adhesion promoters, etchants, and solvents ... Phone: +49 731 36080-409 www.microchemicals.eu e-Mail: sales@microchemicals.eu
MicroChemicals GmbH - Silicon Etching
Anisotropic Silicon Etching
Strong alkaline substances (pH > 12) such as aqueous KOH- or TMAH-solutions etch Si via Si + 4 OHSi(OH)4 + 4e-. Since the bonding energy of Si atoms is different for each crystal plane, and KOH/TMAH Si etching is not diffusion- but etch rate limited, Si etching is highly anisotropic: While the {100}and {110}-crystal planes are being etched, the stable {111} planes act as an etch stop: (111)-orientated Si-wafers are almost not attacked by the etch. (100)-orientated wafers form square-based pyramids with {111} surfaces. These pyramids are realised on c-Si solar cells for the purpose of reflection minimization. (110)-orientated wafers form perpendicular trenches with {111} side-walls, used as e. g. microchannels in micromechanics and microfluidics. The degree of anisotropy (= etch rate selectivity between different crystal planes), the etch rates, and the etching homogeneity depend on the etching temperature, atomic defects in the silicon crystal, intrinsic impurities of the Si crystal, impurities (metal ions) by the etchant, and the concentration of Si[100] atoms already etched. The doping concentration of the Si to be [111] etched also strongly impacts on the etching: During etching, Boron {111} doped Si forms borosilicate glass on the surface which acts as etch stop if the boron doping concentration exceeds 1019 cm-3. The following table lists etch rates of Si and the hard masks SixNy and [100] [010] SiO2, and etch selectivity between different [110] crystal planes as a function of the etchant.
Anisotropic etching {100} [100] {110} [110]
{111}
Photoresists, developers, remover, adhesion promoters, etchants, and solvents ... Phone: +49 731 36080-409 www.microchemicals.eu e-Mail: sales@microchemicals.eu -2-
MicroChemicals GmbH - Silicon Etching
Isotropic Etching of Silicon and SiO2
Etchant KOH (44%, 85C) TMAH (25%, 80C) EDP (115C) Etch rate ratio (100)/(111) (110)/(111) 300 37 20 600 68 10 Etch rate (absolute) (100) Si3N4 SiO2 1.4 m/min <1 /min 14 /min Advantages (+) Disadvantages (-)
(-) Metal ion containing (+) Strongly anisotropic (-) Weak anisotropy 0.3-1 <1 /min 2 /min m/min (+) Metal ion free (-) Weak anisotropy , toxic 1.25 m/min 1 /min 2 /min (+) Metal ion free, metallic hard masks possible
The following chemical reactions summarize the basic etch mechanism for isotropic silicon etching (steps 1-4), and SiO2 (only step 4) using a HF/HNO3 etching mixture: (1) NO2 formation (HNO2 always in traces in HNO3): HNO2 + HNO3 2 NO2 + H2O (2) Oxidation of silicon by NO2: 2 NO2 + Si Si2+ + 2 NO2(3) Formation of SiO2: Si2+ + 2 (OH)SiO2 + H2 (4) Etching of SiO2: SiO2 + 6 HF H2SiF6 + 2H2O In conclusion, HNO3 oxidises Si, and HF etches the SiO2 hereby formed. Fig. right-hand: High HF : HNO3 ratios promote rate-limited etching (strong temperature dependency of the etch rate) of Si via the oxidation (1) - (3), while low HF : HNO3 ratios promote diffusion-limited etch[HF] Increasing etch ing (lower temperature dependency of the etch rate) via rate temperastep (4). HNO3-free HF etches do not attack Si. ture dependThe SiO2 etch rate is determined by the HF-concentraency tion, since the oxidation (1) - (3) does not account. Compared to thermal oxide, deposited (e. g. CVD) IncreasSiO2 has a higher etch rate due to its porosity; wet ing selecoxide a slightly higher etch rate than dry oxide for tivity Si/ the same reason. SiO2 An accurate control of the etch rate requires a es as temperature control within 0.5C. Dilution with re nc acidic acid improves wetting of the hydrophobic ei t ra Si-surface and thus increases and homogenizes ch et the etch rate. Si Doped (n- and p-type) silicon as well as phos[H2O]+[CH3COOH] [HNO3] phorus-doped SiO2 etches faster than undoped Si or SiO2.
Our Silicon-Etch
Our Silicon-Etch AFN 549 for isotropic silicon etching has the composition HF : HNO3 : CH3COOH : H2O = 10.2 % : 39.5 % : 23.2 %: 27.1 % We supply this mixture in 2.5 L sales volumes in MR quality. Other grades/sales volumes available on request.
Photoresists, developers, remover, adhesion promoters, etchants, and solvents ... Phone: +49 731 36080-409 www.microchemicals.eu e-Mail: sales@microchemicals.eu -3-
MicroChemicals GmbH - Silicon Etching
Disclaimer of Warranty
All information, process guides, recipes etc. given in this brochure have been added to the best of our knowledge. However, we cannot issue any guarantee concerning the accuracy of the information. We assume no liability for any hazard for staff and equipment which might stem from the information given in this brochure. Generally speaking, it is in the responsibility of every staff member to inform herself/himself about the processes to be performed in the appropriate (technical) literature, in order to minimize any risk to man or machine.
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Photoresists, developers, remover, adhesion promoters, etchants, and solvents ... Phone: +49 731 36080-409 www.microchemicals.eu e-Mail: sales@microchemicals.eu -4-
