Chemical etching
Encyclopedia
In industry
, etching, also known as chemical milling, is the process of using acid
s, base
s or other chemicals to dissolve unwanted materials such as metal
s, semiconductor
materials or glass. This process has been used on a wide variety of metals with depths of metal removal as large as 12mm (0.5 in). Selective attack by the chemical reagent on different areas of the workpiece surfaces is controlled by removable layers of material called masking or by partial immersion in the reagent. It has applications in the printed circuit board
and semiconductor
fabrication industries. It is also used in the aerospace industry to remove shallow layers of material from large aircraft components, missile skin panels, and extruded parts for airframes.
, where it was applied to the decoration of armour
. One such craftsman, Daniel Hopfer
(circa 1470-1536) of Augsburg
, Germany, is credited with being the first person to apply the method to printmaking
.
is used instead of maskant.
For steel
s
2% Nital is common etchant for plain carbon steels.
For copper
For silica
Etching is used widely to manufacture integrated circuit
s and Microelectromechanical systems
. In addition to the standard, liquid-based techniques, the semiconductor industry commonly uses plasma etching
.
Industry
Industry refers to the production of an economic good or service within an economy.-Industrial sectors:There are four key industrial economic sectors: the primary sector, largely raw material extraction industries such as mining and farming; the secondary sector, involving refining, construction,...
, etching, also known as chemical milling, is the process of using acid
Acid
An acid is a substance which reacts with a base. Commonly, acids can be identified as tasting sour, reacting with metals such as calcium, and bases like sodium carbonate. Aqueous acids have a pH of less than 7, where an acid of lower pH is typically stronger, and turn blue litmus paper red...
s, base
Base (chemistry)
For the term in genetics, see base A base in chemistry is a substance that can accept hydrogen ions or more generally, donate electron pairs. A soluble base is referred to as an alkali if it contains and releases hydroxide ions quantitatively...
s or other chemicals to dissolve unwanted materials such as metal
Metal
A metal , is an element, compound, or alloy that is a good conductor of both electricity and heat. Metals are usually malleable and shiny, that is they reflect most of incident light...
s, semiconductor
Semiconductor
A semiconductor is a material with electrical conductivity due to electron flow intermediate in magnitude between that of a conductor and an insulator. This means a conductivity roughly in the range of 103 to 10−8 siemens per centimeter...
materials or glass. This process has been used on a wide variety of metals with depths of metal removal as large as 12mm (0.5 in). Selective attack by the chemical reagent on different areas of the workpiece surfaces is controlled by removable layers of material called masking or by partial immersion in the reagent. It has applications in the printed circuit board
Printed circuit board
A printed circuit board, or PCB, is used to mechanically support and electrically connect electronic components using conductive pathways, tracks or signal traces etched from copper sheets laminated onto a non-conductive substrate. It is also referred to as printed wiring board or etched wiring...
and semiconductor
Semiconductor
A semiconductor is a material with electrical conductivity due to electron flow intermediate in magnitude between that of a conductor and an insulator. This means a conductivity roughly in the range of 103 to 10−8 siemens per centimeter...
fabrication industries. It is also used in the aerospace industry to remove shallow layers of material from large aircraft components, missile skin panels, and extruded parts for airframes.
History
The process is known to have been used by craftsmen in Europe in the middle agesMiddle Ages
The Middle Ages is a periodization of European history from the 5th century to the 15th century. The Middle Ages follows the fall of the Western Roman Empire in 476 and precedes the Early Modern Era. It is the middle period of a three-period division of Western history: Classic, Medieval and Modern...
, where it was applied to the decoration of armour
Armour
Armour or armor is protective covering used to prevent damage from being inflicted to an object, individual or a vehicle through use of direct contact weapons or projectiles, usually during combat, or from damage caused by a potentially dangerous environment or action...
. One such craftsman, Daniel Hopfer
Daniel Hopfer
Daniel Hopfer was a German artist who is widely believed to have been the first to use etching in printmaking, at the end of the fifteenth century...
(circa 1470-1536) of Augsburg
Augsburg
Augsburg is a city in the south-west of Bavaria, Germany. It is a university town and home of the Regierungsbezirk Schwaben and the Bezirk Schwaben. Augsburg is an urban district and home to the institutions of the Landkreis Augsburg. It is, as of 2008, the third-largest city in Bavaria with a...
, Germany, is credited with being the first person to apply the method to printmaking
Printmaking
Printmaking is the process of making artworks by printing, normally on paper. Printmaking normally covers only the process of creating prints with an element of originality, rather than just being a photographic reproduction of a painting. Except in the case of monotyping, the process is capable...
.
Wet etching
- Residual stresses in the part are removed prior to the machining process to prevent warping after the chemical milling process.
- To ensure good adhesion between the masking and part, the part undergoes a thorough degreasing and cleaning process. There needs to be good adhesion between the masking and the part to ensure uniform material removal during the milling process.
- The masking material is applied to the surfaces of the part to prevent etching from taking place on these surfaces. Some masking types include tapes or paints which are referred to as maskants. Other maskants which are commonly used as well are elastomers (rubber and neoprene) and plastics (polyvinyl chloride, polyethylene, and polystyrene).
- Masking which covers areas of desired etching can be removed by utilizing the scribe-and-peel technique.
- The exposed surfaces of the part are etched by the reagent. The machining process is affected by the temperature and the agitation of the reagent in the tank. A good removal rate requires careful consideration of these two parameters.
- After machining, the masking is removed and the parts are washed thoroughly to prevent further etching by residual reagent.
- The etched part can be further machined by other finishing operations.
Photochemical etching
This process is very similar to wet etching except that a photoresistPhotoresist
A photoresist is a light-sensitive material used in several industrial processes, such as photolithography and photoengraving to form a patterned coating on a surface.-Tone:Photoresists are classified into two groups: positive resists and negative resists....
is used instead of maskant.
Common etchants
For aluminiumAluminium
Aluminium or aluminum is a silvery white member of the boron group of chemical elements. It has the symbol Al, and its atomic number is 13. It is not soluble in water under normal circumstances....
For steel
Steel
Steel is an alloy that consists mostly of iron and has a carbon content between 0.2% and 2.1% by weight, depending on the grade. Carbon is the most common alloying material for iron, but various other alloying elements are used, such as manganese, chromium, vanadium, and tungsten...
s
- hydrochloricHydrochloric acidHydrochloric acid is a solution of hydrogen chloride in water, that is a highly corrosive, strong mineral acid with many industrial uses. It is found naturally in gastric acid....
and nitric acidNitric acidNitric acid , also known as aqua fortis and spirit of nitre, is a highly corrosive and toxic strong acid.Colorless when pure, older samples tend to acquire a yellow cast due to the accumulation of oxides of nitrogen. If the solution contains more than 86% nitric acid, it is referred to as fuming...
s - ferric chloride for stainless steels
- NitalNitalNital is a solution of alcohol and nitric acid commonly used for routine etching of metals. It is especially suitable for revealing the microstructure of carbon steels. The alcohol can be methanol, ethanol or methylated spirits....
(a mixture of nitric acidNitric acidNitric acid , also known as aqua fortis and spirit of nitre, is a highly corrosive and toxic strong acid.Colorless when pure, older samples tend to acquire a yellow cast due to the accumulation of oxides of nitrogen. If the solution contains more than 86% nitric acid, it is referred to as fuming...
and ethanolEthanolEthanol, also called ethyl alcohol, pure alcohol, grain alcohol, or drinking alcohol, is a volatile, flammable, colorless liquid. It is a psychoactive drug and one of the oldest recreational drugs. Best known as the type of alcohol found in alcoholic beverages, it is also used in thermometers, as a...
, methanolMethanolMethanol, also known as methyl alcohol, wood alcohol, wood naphtha or wood spirits, is a chemical with the formula CH3OH . It is the simplest alcohol, and is a light, volatile, colorless, flammable liquid with a distinctive odor very similar to, but slightly sweeter than, ethanol...
, or methylated spirits for mild steels.
2% Nital is common etchant for plain carbon steels.
For copper
Copper
Copper is a chemical element with the symbol Cu and atomic number 29. It is a ductile metal with very high thermal and electrical conductivity. Pure copper is soft and malleable; an exposed surface has a reddish-orange tarnish...
- cupric chloride
- ferric chloride
- ammonium persulfateAmmonium persulfateAmmonium persulfate 2S2O8 is a strong oxidizing agent. It is very soluble in water; the dissolution of the salt in water is endothermic. It is a radical initiator. It is used to etch copper on printed circuit boards as an alternative to ferric chloride solution...
- ammoniaAmmoniaAmmonia is a compound of nitrogen and hydrogen with the formula . It is a colourless gas with a characteristic pungent odour. Ammonia contributes significantly to the nutritional needs of terrestrial organisms by serving as a precursor to food and fertilizers. Ammonia, either directly or...
- 25-50 % nitric acidNitric acidNitric acid , also known as aqua fortis and spirit of nitre, is a highly corrosive and toxic strong acid.Colorless when pure, older samples tend to acquire a yellow cast due to the accumulation of oxides of nitrogen. If the solution contains more than 86% nitric acid, it is referred to as fuming...
. - hydrochloric acidHydrochloric acidHydrochloric acid is a solution of hydrogen chloride in water, that is a highly corrosive, strong mineral acid with many industrial uses. It is found naturally in gastric acid....
and hydrogen peroxideHydrogen peroxideHydrogen peroxide is the simplest peroxide and an oxidizer. Hydrogen peroxide is a clear liquid, slightly more viscous than water. In dilute solution, it appears colorless. With its oxidizing properties, hydrogen peroxide is often used as a bleach or cleaning agent...
For silica
- hydrofluoric acidHydrofluoric acidHydrofluoric acid is a solution of hydrogen fluoride in water. It is a valued source of fluorine and is the precursor to numerous pharmaceuticals such as fluoxetine and diverse materials such as PTFE ....
(HF) is a very efficient etchant for silicon dioxideSilicon dioxideThe chemical compound silicon dioxide, also known as silica , is an oxide of silicon with the chemical formula '. It has been known for its hardness since antiquity...
. It is however very dangerous if it comes into contact with the body.
Commonly etched metals and alloys
The following metals and alloys are commonly etched using ferric chloride:- Aluminum
- Brass
- Copper
- Beryllium-Copper
- Nickel
- Nickel Silver
- Phosphorus-Bronze
- Carbon Steel
- Stainless Steel - 300 Series, 400 series
- Spring Steel
- Zinc
- Monel
- PH15-7
- PH17-7
- Alloy 42
- Kovar
Etching in the semiconductor industry
Etching is used widely to manufacture integrated circuit
Integrated circuit
An integrated circuit or monolithic integrated circuit is an electronic circuit manufactured by the patterned diffusion of trace elements into the surface of a thin substrate of semiconductor material...
s and Microelectromechanical systems
Microelectromechanical systems
Microelectromechanical systems is the technology of very small mechanical devices driven by electricity; it merges at the nano-scale into nanoelectromechanical systems and nanotechnology...
. In addition to the standard, liquid-based techniques, the semiconductor industry commonly uses plasma etching
Plasma etching
Plasma etching is a form of plasma processing used to fabricate integrated circuits. It involves a high-speed stream of glow discharge of an appropriate gas mixture being shot at a sample. The plasma source, known as etch species, can be either charged or neutral...
.