![Mace wiki](https://upload.wikimedia.org/wikipedia/commons/d/d0/Mace_wiki.png)
Metal Assisted Chemical Etching (also known as MACE) is the process of wet chemical
etching
Etching is traditionally the process of using strong acid or mordant to cut into the unprotected parts of a metal surface to create a design in intaglio (incised) in the metal. In modern manufacturing, other chemicals may be used on other types ...
of
semiconductors
A semiconductor is a material which has an electrical resistivity and conductivity, electrical conductivity value falling between that of a electrical conductor, conductor, such as copper, and an insulator (electricity), insulator, such as glas ...
(mainly
silicon
Silicon is a chemical element with the symbol Si and atomic number 14. It is a hard, brittle crystalline solid with a blue-grey metallic luster, and is a tetravalent metalloid and semiconductor. It is a member of group 14 in the periodic tab ...
) with the use of a
metal
A metal (from Greek μέταλλον ''métallon'', "mine, quarry, metal") is a material that, when freshly prepared, polished, or fractured, shows a lustrous appearance, and conducts electricity and heat relatively well. Metals are typicall ...
catalyst
Catalysis () is the process of increasing the rate of a chemical reaction by adding a substance known as a catalyst (). Catalysts are not consumed in the reaction and remain unchanged after it. If the reaction is rapid and the catalyst recyc ...
, usually deposited on the surface of a
semiconductor
A semiconductor is a material which has an electrical resistivity and conductivity, electrical conductivity value falling between that of a electrical conductor, conductor, such as copper, and an insulator (electricity), insulator, such as glas ...
in the form of a thin film or nanoparticles. The semiconductor, covered with the metal is then immersed in an etching solution containing and
oxidizing agent
An oxidizing agent (also known as an oxidant, oxidizer, electron recipient, or electron acceptor) is a substance in a redox chemical reaction that gains or "Electron acceptor, accepts"/"receives" an electron from a (called the , , or ). In ot ...
and
hydrofluoric acid
Hydrofluoric acid is a Solution (chemistry), solution of hydrogen fluoride (HF) in water. Solutions of HF are colourless, acidic and highly Corrosive substance, corrosive. It is used to make most fluorine-containing compounds; examples include th ...
. The metal on the surface catalyzes the reduction of the oxidizing agent and therefore in turn also the dissolution of silicon. In the majority of the conducted research this phenomenon of increased dissolution rate is also spatially confined, such that it is increased in close proximity to a metal particle at the surface. Eventually this leads to the formation of straight pores that are etched into the semiconductor (see figure to the right).
[Huang, Z., Geyer, N., Werner, P., De Boor, J., & Gösele, U. (2011). Metal‐assisted chemical etching of silicon: a review: in memory of Prof. Ulrich Gösele. Advanced materials, 23(2), 285-308.] This means that a pre-defined pattern of the metal on the surface can be directly transferred to a semiconductor substrate.
History of development
The metal assisted chemical etching of semiconductors is a relatively new technology in semiconductor engineering and therefore it is not yet a process that is used in industry. The first attempts of MACE consisted of a silicon wafer that was partially covered with aluminum and then immersed in an etching solution.
[Dimova-Malinovska, D., et al. "Preparation of thin porous silicon layers by stain etching." Thin Solid Films 297.1-2 (1997): 9-12.] This material combination lead to an increased etching rate compared to bare silicon. Often this very first attempt is also called galvanic etching instead of metal assisted chemical etching.
Further research in this direction showed that a not dense film of a noble metal that is deposited on the surface of a silicon wafer can also locally increase the etching rate. In particular, it was observed that noble metal particles sink down into the material when the sample is immersed in an etching solution containing an oxidizing agent and
hydrofluoric acid
Hydrofluoric acid is a Solution (chemistry), solution of hydrogen fluoride (HF) in water. Solutions of HF are colourless, acidic and highly Corrosive substance, corrosive. It is used to make most fluorine-containing compounds; examples include th ...
(see image in the introduction).
[Li, Xiuling, and P. W. Bohn. "Metal-assisted chemical etching in HF/H 2 O 2 produces porous silicon." Applied Physics Letters 77.16 (2000): 2572-2574.] This method is now commonly called the metal assisted chemical etching of silicon.
Besides silicon, other semiconductors were also successfully etched with metal assisted chemical etching, such as
silicon carbide
Silicon carbide (SiC), also known as carborundum (), is a hard chemical compound containing silicon and carbon. A semiconductor, it occurs in nature as the extremely rare mineral moissanite, but has been mass-produced as a powder and crystal sin ...
[Leitgeb, Markus, et al. "Metal assisted photochemical etching of 4H silicon carbide." Journal of Physics D: Applied Physics 50.43 (2017): 435301.] or
gallium nitride
Gallium nitride () is a binary III/ V direct bandgap semiconductor commonly used in blue light-emitting diodes since the 1990s. The compound is a very hard material that has a Wurtzite crystal structure. Its wide band gap of 3.4 eV affords it ...
[Dıaz, Diego J., et al. "Morphology evolution and luminescence properties of porous GaN generated via Pt-assisted electroless etching of hydride vapor phase epitaxy GaN on sapphire." Journal of applied physics 94.12 (2003): 7526-7534.] but the main portion of research is dedicated to MACE of silicon.
Also it could be shown that both
noble metal
A noble metal is ordinarily regarded as a metallic chemical element that is generally resistant to corrosion and is usually found in nature in its raw form. Gold, platinum, and the other platinum group metals (ruthenium, rhodium, palladium, o ...
s such as
gold
Gold is a chemical element with the symbol Au (from la, aurum) and atomic number 79. This makes it one of the higher atomic number elements that occur naturally. It is a bright, slightly orange-yellow, dense, soft, malleable, and ductile met ...
,
platinum
Platinum is a chemical element with the symbol Pt and atomic number 78. It is a dense, malleable, ductile, highly unreactive, precious, silverish-white transition metal. Its name originates from Spanish , a diminutive of "silver".
Platinu ...
,
palladium
Palladium is a chemical element with the symbol Pd and atomic number 46. It is a rare and lustrous silvery-white metal discovered in 1803 by the English chemist William Hyde Wollaston. He named it after the asteroid Pallas, which was itself na ...
, and
silver
Silver is a chemical element with the Symbol (chemistry), symbol Ag (from the Latin ', derived from the Proto-Indo-European wikt:Reconstruction:Proto-Indo-European/h₂erǵ-, ''h₂erǵ'': "shiny" or "white") and atomic number 47. A soft, whi ...
, and
base metal
A base metal is a common and inexpensive metal, as opposed to a precious metal such as gold or silver. In numismatics, coins often derived their value from the precious metal content; however, base metals have also been used in coins in the past ...
s such as
iron
Iron () is a chemical element with symbol Fe (from la, ferrum) and atomic number 26. It is a metal that belongs to the first transition series and group 8 of the periodic table. It is, by mass, the most common element on Earth, right in f ...
,
nickel
Nickel is a chemical element with symbol Ni and atomic number 28. It is a silvery-white lustrous metal with a slight golden tinge. Nickel is a hard and ductile transition metal. Pure nickel is chemically reactive but large pieces are slow to ...
,
[Volovlikova, Olga V., et al. "Influence of Etching Regimes on the Reflectance of Black Silicon Films Formed by Ni-Assisted Chemical Etching." Key Engineering Materials. Vol. 806. Trans Tech Publications Ltd, 2019.][Azeredo, B. P., et al. "Silicon nanowires with controlled sidewall profile and roughness fabricated by thin-film dewetting and metal-assisted chemical etching." Nanotechnology 24.22 (2013): 225305.] copper
Copper is a chemical element with the symbol Cu (from la, cuprum) and atomic number 29. It is a soft, malleable, and ductile metal with very high thermal and electrical conductivity. A freshly exposed surface of pure copper has a pinkis ...
, and
aluminium
Aluminium (aluminum in American and Canadian English) is a chemical element with the symbol Al and atomic number 13. Aluminium has a density lower than those of other common metals, at approximately one third that of steel. I ...
[Shahnawaz Uddin, Md. Roslan Hashim and Mohd Zamir Pakhuruddin "Aluminium-assisted chemical etching for fabrication of black silicon" Materials Chemistry and Physics. (2021); 124469; ISSN 0254-0584; https://doi.org/10.1016/j.matchemphys.2021.124469.] can act as a
catalyst
Catalysis () is the process of increasing the rate of a chemical reaction by adding a substance known as a catalyst (). Catalysts are not consumed in the reaction and remain unchanged after it. If the reaction is rapid and the catalyst recyc ...
in the process.
Theory
![Mace mechanism](https://upload.wikimedia.org/wikipedia/commons/4/4c/Mace_mechanism.png)
Some elements of MACE are commonly accepted in the scientific community, while others are still under debate.
[Huang, Z., Geyer, N., Werner, P., De Boor, J., & Gösele, U. (2011). Metal‐assisted chemical etching of silicon: a review: in memory of Prof. Ulrich Gösele. Advanced materials, 23(2), 285-308.] There is mutual agreement that the reduction of the oxidizing agent is catalyzed by the noble metal particle (see figure to the left). This means that the metal particle has a surplus of positive charge which is eventually transferred to the silicon substrate. Each of the positive charges in the substrate can be identified as a hole (h
+) in the
valence band
In solid-state physics, the valence band and conduction band are the bands closest to the Fermi level, and thus determine the electrical conductivity of the solid. In nonmetals, the valence band is the highest range of electron energies in w ...
of the substrate, or in more chemical terms it may be interpreted as a weakened Si-Si bond due to the removal of an electron.
The weakened bonds can be attacked by a
nucleophilic species such as HF or H
2O, which in turn leads to the dissolution of the silicon substrate in close proximity to the noble metal particle. From a thermodynamic point of view, the MACE process is possible because the
redox potential
Redox potential (also known as oxidation / reduction potential, ''ORP'', ''pe'', ''E_'', or E_) is a measure of the tendency of a chemical species to acquire electrons from or lose electrons to an electrode and thereby be reduced or oxidised respe ...
of the redox couple corresponding to the used oxidizing agents (
hydrogen peroxide
Hydrogen peroxide is a chemical compound with the formula . In its pure form, it is a very pale blue liquid that is slightly more viscous than water. It is used as an oxidizer, bleaching agent, and antiseptic, usually as a dilute solution (3% ...
or
potassium permanganate
Potassium permanganate is an inorganic compound with the chemical formula KMnO4. It is a purplish-black crystalline salt, that dissolves in water as K+ and , an intensely pink to purple solution.
Potassium permanganate is widely used in the c ...
) are below the valence band edge at the electrochemical energy scale. Equivalently, one could say that the
electrochemical potential
In electrochemistry, the electrochemical potential (ECP), ', is a thermodynamic measure of chemical potential that does not omit the energy contribution of electrostatics. Electrochemical potential is expressed in the unit of J/ mol.
Introductio ...
of the
electron
The electron ( or ) is a subatomic particle with a negative one elementary electric charge. Electrons belong to the first generation of the lepton particle family,
and are generally thought to be elementary particles because they have no kn ...
in the etching solution (due to the presence of oxidizing agent) is lower than the electrochemical potential of the electron in the substrate, hence electrons are removed from the silicon. In the end, this accumulation of positive charge leads to the dissolution of the substrate by hydrofluoric acid.
[Huang, Z., Geyer, N., Werner, P., De Boor, J., & Gösele, U. (2011). Metal‐assisted chemical etching of silicon: a review: in memory of Prof. Ulrich Gösele. Advanced materials, 23(2), 285-308.]
MACE consists of multiple individual reactions. At the metal particle, the oxidizing agent is reduced. In the case of hydrogen peroxide this can be written down as follows:
:
H2O2 + 2H+ -> 2H2O + 2h+
The created holes (h
+) are then consumed during the dissolution of silicon. There are several possible reactions via which the dissolution can take place, but here just one example is given:
:
Si + 6HF + 4h+ -> SiF6^- + 6H+
Besides this generally accepted mechanism of MACE there are still some not finally clarified aspects. The model proposed above requires contact of the metal particle with the silicon substrate which is somehow conflicting with the etching solution being underneath the particle. This can be explained with a dissolution and redeposition of metal during MACE. In particular it is proposed, that some metal ions from the particle are dissolved and eventually are re-deposited at the silicon surface with a redox reaction. In this case the metal particle (or even larger noble metal thin films) could partially maintain contact to the substrate while also etching could partially take place underneath the metal.
[Geyer, Nadine, et al. "Model for the mass transport during metal-assisted chemical etching with contiguous metal films as catalysts." The journal of physical chemistry C 116.24 (2012): 13446-13451.]
It is also observed that in the vicinity of straight pores as shown in the introduction also a micro-porous region between the pores is formed. Generally this is attributed to holes that diffuse away from the particle and hence contribute to etching at more distant locations.
[Li, Xiuling. "Metal assisted chemical etching for high aspect ratio nanostructures: A review of characteristics and applications in photovoltaics." Current Opinion in Solid State and Materials Science 16.2 (2012): 71-81.] This behavior is dependent on the doping type of substrate as well as on the type of noble metal particle. Therefore it is proposed that the formation of such a porous region beneath the straight pores depends on the type of barrier that is formed at the metal/silicon interface. In the case of a upward
band bending
In solid-state physics, band bending refers to the process in which the electronic band structure in a material curves up or down near a junction or interface. It does not involve any physical (spatial) bending. When the electrochemical potential ...
the electric field in the
depletion layer
In semiconductor physics, the depletion region, also called depletion layer, depletion zone, junction region, space charge region or space charge layer, is an insulating region within a conductive, doped semiconductor material where the mobile ...
would point towards the metal. Therefore holes cannot diffuse further into the substrate and thus no formation of a micro-porous region is observed. In the case of downward band-bending holes could escape into the bulk of the silicon substrate and eventually lead to etching there.
[Lai, Ruby A., et al. "Schottky barrier catalysis mechanism in metal-assisted chemical etching of silicon." ACS applied materials & interfaces 8.14 (2016): 8875-8879.]
Experimental procedure of MACE
![Mace particles](https://upload.wikimedia.org/wikipedia/commons/c/c8/Mace_particles.png)
As already stated above MACE requires metal particles or a thin metal thin film on top of a silicon substrate. This can be achieved with several methods such as
sputter deposition
Sputter deposition is a physical vapor deposition (PVD) method of thin film deposition by the phenomenon of sputtering. This involves ejecting material from a "target" that is a source onto a "substrate" such as a silicon wafer. Resputtering is re ...
or
thermal evaporation
A thermal column (or thermal) is a rising mass of buoyant air, a convective current in the atmosphere, that transfers heat energy vertically. Thermals are created by the uneven heating of Earth's surface from solar radiation, and are an example ...
.
[Kabalan, Amal. "A Comparative Study on the Effects of Passivation Methods on the Carrier Lifetime of RIE and MACE Silicon Micropillars." Applied Sciences 9.9 (2019): 1804.] A method to obtain particles from a continuous thin film is thermal
dewetting
In fluid mechanics, dewetting is one of the processes that can occur at a solid–liquid, solid–solid or liquid–liquid interface. Generally, dewetting describes the process of retraction of a fluid from a non-wettable surface it was forced t ...
.
[Backes, A., et al. "Influence of metallic catalyst and doping level on the metal assisted chemical etching of silicon." Scripta Materialia 114 (2016): 27-30.]
These deposition methods can be combined with
lithography
Lithography () is a planographic method of printing originally based on the immiscibility of oil and water. The printing is from a stone (lithographic limestone) or a metal plate with a smooth surface. It was invented in 1796 by the German a ...
[Chang, Shih‐Wei, et al. "Densely packed arrays of ultra‐high‐aspect‐ratio silicon nanowires fabricated using block‐copolymer lithography and metal‐assisted etching." Advanced functional materials 19.15 (2009): 2495-2500.] such that only desired regions are covered with metal. Since MACE is an anisotropic etching method (etching takes place not in all spatial directions) a pre-defined metal pattern can be directly transferred into the silicon substrate.
Another method of depositing metal particles or thin films is
electroless plating
Electroless plating, also known as chemical plating or autocatalytic plating, is a class of industrial chemical processes that create metal coatings on various materials by autocatalytic chemical reduction of metal cations in a liquid bath. This c ...
of noble metals on the surface of silicon. Since the redox potentials of the redox couples of noble metals are below the valence band edge of silicon, noble metal ions can (like described in the theory section) inject holes (or extract electrons) from the substrate while they are reduced. In the end metallic particles or films are obtained at the surface.
[Smith, Zachary R., Rosemary L. Smith, and Scott D. Collins. "Mechanism of nanowire formation in metal assisted chemical etching." Electrochimica Acta 92 (2013): 139-147.]
Finally, after the deposition of the metal on the surface of silicon, the sample is immersed in an etching solution containing hydrofluoric acid and oxidizing agent. Etching will take place as long as the oxidizing agent and the acid are consumed or until the sample is removed from the etching solution.
Applications of MACE
![Etch anisotropy](https://upload.wikimedia.org/wikipedia/commons/7/72/Etch_anisotropy.png)
The reason why MACE is heavily researched is that it allows completely anisotropic etching of silicon substrates which is not possible with other wet chemical etching methods (see figure to the right). Usually the silicon substrate is covered with a protective layer such as
photoresist
A photoresist (also known simply as a resist) is a light-sensitive material used in several processes, such as photolithography and photoengraving, to form a patterned coating on a surface. This process is crucial in the electronic industry.
T ...
before it is immersed in an etching solution. The etching solution usually has no preferred direction of attacking the substrate, therefore isotropic etching takes place. In semiconductor engineering, however it is often required that the sidewalls of the etched trenches are steep. This is usually realized with methods that operate in the gas-phase such as
reactive ion etching
Reactive-ion etching (RIE) is an etching technology used in microfabrication. RIE is a type of dry etching which has different characteristics than wet etching. RIE uses chemically reactive plasma to remove material deposited on wafers. The pl ...
. These methods require expensive equipment compared to simple wet etching. MACE, in principle allows the fabrication of steep trenches but is still cheap compared to gas-phase etching methods.
Porous silicon
Metal assisted chemical etching allows for the production of
porous silicon
Porous silicon (abbreviated as "PS" or "pSi") is a form of the chemical element silicon that has introduced nanopores in its microstructure, rendering a large surface to volume ratio in the order of 500 m2/cm3.
History
Porous silicon was disco ...
with
photoluminescence
Photoluminescence (abbreviated as PL) is light emission from any form of matter after the absorption of photons (electromagnetic radiation). It is one of many forms of luminescence (light emission) and is initiated by photoexcitation (i.e. photon ...
.
[Li, Xiuling, and P. W. Bohn. "Metal-assisted chemical etching in HF/H 2 O 2 produces porous silicon." Applied Physics Letters 77.16 (2000): 2572-2574.]
Black silicon
Black silicon
Black silicon is a semiconductor material, a surface modification of silicon with very low reflectivity and correspondingly high absorption of visible (and infrared) light.
The modification was discovered in the 1980s as an unwanted side effect ...
is silicon with a modified surface and is a type of
porous silicon
Porous silicon (abbreviated as "PS" or "pSi") is a form of the chemical element silicon that has introduced nanopores in its microstructure, rendering a large surface to volume ratio in the order of 500 m2/cm3.
History
Porous silicon was disco ...
. There are several works on obtaining
black silicon
Black silicon is a semiconductor material, a surface modification of silicon with very low reflectivity and correspondingly high absorption of visible (and infrared) light.
The modification was discovered in the 1980s as an unwanted side effect ...
using MACE technology. The main application of black silicon is
solar energy
Solar energy is radiant light and heat from the Sun that is harnessed using a range of technologies such as solar power to generate electricity, solar thermal energy (including solar water heating), and solar architecture. It is an essenti ...
.
References
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Etching
Chemistry
Research
Semiconductors
Engineering