# Chemical milling

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Chemical milling or industrial etching is the subtractive manufacturing process of using baths of temperature-regulated etching chemicals to remove material to create an object with the desired shape. [1] [2] Other names for chemical etching include photo etching, chemical etching, photo chemical etching and photochemical machining. It is mostly used on metals, though other materials are increasingly important. It was developed from armor-decorating and printing etching processes developed during the Renaissance as alternatives to engraving on metal. The process essentially involves bathing the cutting areas in a corrosive chemical known as an etchant, which reacts with the material in the area to be cut and causes the solid material to be dissolved; inert substances known as maskants are used to protect specific areas of the material as resists. [2] [3]

## History

Organic chemicals such as lactic acid and citric acid have been used to etch metals and create products as early as 400 BCE, when vinegar was used to corrode lead and create the pigment ceruse, also known as white lead. [4] Most modern chemical milling methods involve alkaline etchants; these may have been used as early as the first century CE.

Armor etching, using strong mineral acids, was not developed until the fifteenth century. Etchants mixed from salt, charcoal, and vinegar were applied to plate armor that had been painted with a maskant of linseed-oil paint. The etchant would bite into the unprotected areas, causing the painted areas to be raised into relief. [4] Etching in this manner allowed armor to be decorated as if with precise engraving, but without the existence of raised burrs; it also prevented the necessity of the armor being softer than an engraving tool. [5] Late in the seventeenth century, etching became used to produce the graduations on measuring instruments; the thinness of lines that etching could produce allowed for the production of more precise and accurate instruments than were possible before. [6] Not long after, it became used to etch trajectory information plates for cannon and artillery operators; paper would rarely survive the rigors of combat, but an etched plate could be quite durable. Often such information (normally ranging marks) was etched onto equipment such as stiletto daggers or shovels.

In 1782, the discovery was made by John Senebier that certain resins lost their solubility to turpentine when exposed to light; that is, they hardened. This allowed the development of photochemical milling , where a liquid maskant is applied to the entire surface of a material, and the outline of the area to be masked created by exposing it to UV light. [7] Photo-chemical milling was extensively used in the development of photography methods, allowing light to create impressions on metal plates.

One of the earliest uses of chemical etching to mill commercial parts was in 1927, when the Swedish company Aktiebolaget Separator patented a method of producing edge filters by chemically milling the gaps in the filters. [8] Later, around the 1940s, it became widely used to machine thin samples of very hard metal; photo-etching from both sides was used to cut sheet metal, foil, and shim stock to create shims, recording heat frets, and other components. [9]

## Applications

Etching has applications in the printed circuit board and semiconductor fabrication industries. It is also used in the aerospace industry [10] to remove shallow layers of material from large aircraft components, missile skin panels, and extruded parts for airframes. Etching is used widely to manufacture integrated circuits and Microelectromechanical systems. [10] In addition to the standard, liquid-based techniques, the semiconductor industry commonly uses plasma etching.

## Process

Chemical milling is normally performed in a series of five steps: cleaning, masking, scribing, etching, and demasking. [2] Video of chemical milling process Learn more about the video

### Cleaning

Cleaning is the preparatory process of ensuring that the surface to be etched is free of contaminants which could negatively impact the quality of the finished part. [2] [11] An improperly cleaned surface could result in poor adhesion of the maskant, causing areas to be etched erroneously, or a non-uniform etch rate which could result in inaccurate final dimensions. The surface must be kept free from oils, grease, primer coatings, markings and other residue from the marking out process, scale (oxidation), and any other foreign contaminants. For most metals, this step can be performed by applying a solvent substance to the surface to be etched, washing away foreign contaminants. The material may also be immersed in alkaline cleaners or specialized de-oxidizing solutions. It is common practice in modern industrial chemical etching facilities that the workpiece never be directly handled after this process, as oils from human skin could easily contaminate the surface. [3]

Masking is the process of applying the maskant material to the surface to ensure that only desired areas are etched. [2] [3] Liquid maskants may be applied via dip-masking, in which the part is dipped into an open tank of maskant and then the maskant dried. Maskant may also be applied by flow coating: liquid maskant is flowed over the surface of the part. Certain conductive maskants may also be applied by electrostatic deposition, where electrical charges are applied to particles of maskant as it is sprayed onto the surface of the material. The charge causes the particles of maskant to adhere to the surface. [12]

The maskant to be used is determined primarily by the chemical used to etch the material, and the material itself. The maskant must adhere to the surface of the material, and it must also be chemically inert enough with regard to the etchant to protect the workpiece. [3] Most modern chemical milling processes use maskants with an adhesion around 350 g cm−1; if the adhesion is too strong, the scribing process may be too difficult to perform. If the adhesion is too low, the etching area may be imprecisely defined. Most industrial chemical milling facilities use maskants based upon neoprene elastomers or isobutylene-isoprene copolymers. [13]

### Scribing

Scribing is the removal of maskant on the areas to be etched. [2] For decorative applications, this is often done by hand through the use of a scribing knife, etching needle or similar tool; modern industrial applications may involve an operator scribing with the aid of a template or use computer numerical control to automate the process. For parts involving multiple stages of etching, complex templates using colour codes and similar devices may be used. [14]

### Etching

Etching is the immersion of the part into the chemical bath, and the action of the chemical on the part to be milled. [15] The time spent immersed in the chemical bath determines the depth of the resulting etch; this time is calculated via the formula:

${\displaystyle E={\frac {s}{t}}}$

where E is the rate of etching (usually abbreviated to etch rate), s is the depth of the cut required, and t is the total immersion time. [10] [15] Etch rate varies based on factors such as the concentration and composition of the etchant, the material to be etched, and temperature conditions. Due to its inconstant nature, etch rate is often determined experimentally immediately prior to the etching process. A small sample of the material to be cut, of the same material specification, heat-treatment condition, and approximately the same thickness is etched for a certain time; after this time, the depth of the etch is measured and used with the time to calculate the etch rate. [16] Aluminium is commonly etched at rates around 0.178 cm/h, and magnesium about 0.46 cm/h. [10]

Demasking is the process of clearing the part of etchant and maskant. [2] [17] Etchant is generally removed with a wash of clear, cold water. A de-oxidizing bath may also be required in the common case that the etching process left a film of oxide on the surface of the material. Various methods may be used to remove the maskant, the most common being hand removal using scraping tools. This is frequently time-consuming and laborious, and for large-scale processes may be automated. [18]

## Common etchants

For aluminium
For steels

2% Nital is common etchant for plain carbon steels.

For copper
For silica

## Notes

1. Harris 1976, p. xiii.
2. Çakir, O.; Yardimeden, A.; Özben, T. (August 2007). "Chemical machining" (PDF). Archives of Materials Science and Engineering. 28 (8): 499–502. Archived from the original (PDF) on 2015-04-12. Retrieved 13 February 2013.
3. Harris 1976, p. 32.
4. Harris 1976, p. 2.
5. Harris 1976, p. 6.
6. Harris 1976, p. 9.
7. Harris 1976, p. 10.
8. Harris 1976, p. 15.
9. Harris 1976, p. 17.
10. Fishlock, David (8 December 1960). "New Ways of Cutting Metal". New Scientist. 8 (212): 1535. Retrieved 13 February 2013.
11. Harris 1976, p. 31.
12. Harris 1976, p. 36.
13. Harris 1976, p. 33.
14. Harris 1976, pp. 37-44.
15. Harris 1976, p. 44.
16. Harris 1976, p. 45.
17. Harris 1976, p. 54.
18. Harris 1976, p. 56.

## Related Research Articles

Microelectromechanical systems (MEMS), also written as micro-electro-mechanical systems and the related micromechatronics and microsystems constitute the technology of microscopic devices, particularly those with moving parts. They merge at the nanoscale into nanoelectromechanical systems (NEMS) and nanotechnology. MEMS are also referred to as micromachines in Japan and microsystem technology (MST) in Europe.

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 material. As a method of printmaking, it is, along with engraving, the most important technique for old master prints, and remains in wide use today. In a number of modern variants such as microfabrication etching and photochemical milling it is a crucial technique in much modern technology, including circuit boards.

Printmaking is the process of creating artworks by printing, normally on paper. Printmaking normally covers only the process of creating prints that have an element of originality, rather than just being a photographic reproduction of a painting. Except in the case of monotyping, the process is capable of producing multiples of the same piece, which is called a print. Each print produced is considered an "original" work of art, and is correctly referred to as an "impression", not a "copy". Often impressions vary considerably, whether intentionally or not. The images on most prints are created for that purpose, perhaps with a preparatory study such as a drawing. A print that copies another work of art, especially a painting, is known as a "reproductive print".

Electroplating is a general name for processes that create a metal coating on a solid substrate through the reduction of cations of that metal by means of a direct electric current. The part to be coated acts as the cathode of an electrolytic cell; the electrolyte is a solution of a salt of the metal to be coated; and the anode is usually either a block of that metal, or of some inert conductive material. The current is provided by an external power supply.

A printed circuit board (PCB) mechanically supports and electrically connects electronic components using conductive tracks, pads and other features etched from one or more sheet layers of copper laminated onto and/or between sheet layers of a non-conductive substrate. Components are generally soldered onto the PCB to both electrically connect and mechanically fasten them to it. Printed circuit boards are used in nearly all electronic products and in some electrical products, such as passive switch boxes.

Isotropic etching is a method commonly used in semiconductors to remove material from a substrate via a chemical process using an etchant substance. The etchant may be in liquid-, gas- or plasma-phase, although liquid etchants such as buffered hydrofluoric acid (BHF) for silicon dioxide etching are more often used. Unlike anisotropic etching, isotropic etching does not etch in a single direction, but rather etches in multiple directions within the substrate. Any horizontal component of the etch direction may therefore result in undercutting of patterned areas, and significant changes to device characteristics. Isotropic etching may occur unavoidably, or it may be desirable for process reasons.

A corrosive substance is one that will damage or destroy other substances with which it comes into contact by means of a chemical reaction.

Gilding is a decorative technique for applying a very thin coating of gold to solid surfaces such as metal, wood, porcelain, or stone. A gilded object is also described as "gilt". Where metal is gilded, the metal below was traditionally silver in the West, to make silver-gilt objects, but gilt-bronze is commonly used in China, and also called ormolu if it is Western. Methods of gilding include hand application and gluing, typically of gold leaf, chemical gilding, and electroplating, the last also called gold plating. Parcel-gilt objects are only gilded over part of their surfaces. This may mean that all of the inside, and none of the outside, of a chalice or similar vessel is gilded, or that patterns or images are made up by using a combination of gilt and ungilted areas.

Dry etching refers to the removal of material, typically a masked pattern of semiconductor material, by exposing the material to a bombardment of ions that dislodge portions of the material from the exposed surface. A common type of dry etching is reactive-ion etching. Unlike with many of the wet chemical etchants used in wet etching, the dry etching process typically etches directionally or anisotropically.

Photoengraving is a process that uses a light-sensitive photoresist applied to the surface to be engraved to create a mask that shields some areas during a subsequent operation which etches, dissolves, or otherwise removes some or all of the material from the unshielded areas. Normally applied to metal, it can also be used on glass, plastic and other materials.

Also known as a "bonderizer" bonding agents are resin materials used to make a dental composite filling material adhere to both dentin and enamel.

Deep reactive-ion etching (DRIE) is a highly anisotropic etch process used to create deep penetration, steep-sided holes and trenches in wafers/substrates, typically with high aspect ratios. It was developed for microelectromechanical systems (MEMS), which require these features, but is also used to excavate trenches for high-density capacitors for DRAM and more recently for creating through silicon vias (TSVs) in advanced 3D wafer level packaging technology. In DRIE, the substrate is placed inside a reactor, and several gases are introduced. A plasma is struck in the gas mixture which breaks the gas molecules into ions. The ions accelerated towards, and react with the surface of the material being etched, forming another gaseous element. This is known as the chemical part of the reactive ion etching. There is also a physical part, if ions have enough energy, they can knock atoms out of the material to be etched without chemical reaction.

Etching is used in microfabrication to chemically remove layers from the surface of a wafer during manufacturing. Etching is a critically important process module, and every wafer undergoes many etching steps before it is complete.

Photochemical machining (PCM), also known as photochemical milling or photo etching, is a chemical milling process used to fabricate sheet metal components using a photoresist and etchants to corrosively machine away selected areas. This process emerged in the 1960s as an offshoot of the printed circuit board industry. Photo etching can produce highly complex parts with very fine detail accurately and economically.

Bathtub refinishing also known as bathtub resurfacing, bathtub reglazing or bathtub re-enameling is the process of refreshing the surface of a worn, damaged bathtub to a like-new condition. This process typically involves removing the drain(An important step that improves coating life-span but is not always possible) and repairing any damaged areas, including checking to see if the bathtub holds water after draining. Chips or cracks are repaired using Bondo or another type of polyester putty. After repairs are made the surface is prepped with an acid etching. Etching and wet sanding provide mechanical adhesion and clean the surface. Porcelain, enamel, and fiberglass tubs are non-porous and do not provide a good substrate for the new coating to attach to. Etching the surface provides a porous surface that will allow proper adhesion. Another possible method is to apply an adhesion-promoting bonding agent like silane to the surface before applying the coating. The two methods can be used in unison or independently. The greatest adhesion is generally achieved by using both methods together however, some newer refinishing processes claim they do not require the use of etching, by relying on silane alone. After preparing the surface, Prior to spraying any chemical coatings, most refinishers will protect themselves and their clients by completely masking the areas that can be affected by overspray and setting up a professional exhaust system rated to work with the type of coating system being applied. This is important for many reasons, the greatest being a possible fire or explosion hazard. Then the refinisher will use a NIOSH-rated fresh-air supplied breathing apparatus to and spray suite and gloves to protect themselves from the chemicals being used in the process. They also provide assistance with the quality of the spraying. By using at least a 1200-cfm exhaust unit, the refinisher can see better and may limit the overspray and settling on the surface. primer, is then applied, followed by a topcoat. Generally, a catalyzed two-component cross-link synthetic white coating is applied, but the coating does not have the durability or abrasive tolerance of the original glass-enamel coating of a factory-new bathtub. The coating should be in most cases between 5-8 mils in thickness when cured to provide the best, longterm results. This is typically achieved by spraying 2-coats of primer, and 3-coats of top-coat. A very experienced refinisher may be able to accomplish this with fewer coats depending on conditions.

Eco pickled surface (EPS) is a process applied to hot rolled sheet steel to remove all surface oxides and clean the steel surface. Steel which has undergone the EPS process acquires a high degree of resistance to subsequent development of surface oxide (rust), so long as it does not come into direct contact with moisture. EPS was developed by The Material Works, Ltd., which has filed several patent applications covering the process. It is primarily intended to be a replacement of the familiar acid pickling process wherein steel strip is immersed in solutions of hydrochloric and sulfuric acids to chemically remove oxides.

The Wright etch is a preferential etch for revealing defects in <100>- and <111>-oriented, p- and n-type silicon wafers used for making transistors, microprocessors, memories, and other components. Revealing, identifying, and remedying such defects is essential for progress along the path predicted by Moore's Law. It was developed by Margaret Wright Jenkins (1936-2018) in 1976 while working in research and development at Motorola Inc. in Phoenix, AZ. It was published in 1977. This etchant reveals clearly defined oxidation-induced stacking faults, dislocations, swirls and striations with minimum surface roughness or extraneous pitting. These defects are known causes of shorts and current leakage in finished semiconductor devices should they fall across isolated junctions. A relatively low etch rate at room temperature provides etch control. The long shelf life of this etchant allows the solution to be stored in large quantities.

Carbon tissue is a gelatin-based emulsion used as a photoresist in the chemical etching (photoengraving) of gravure cylinders for printing. This was introduced by British physicist and chemist Joseph Swan in 1864. It has been used in photographic reproduction since the early days of photography.

Polytetrafluoroethylene (PTFE), better known by its trade name Teflon, has many desirable properties which make it an attractive material for numerous industries. It has good chemical resistance, a low dielectric constant, low dielectric loss, and a low coefficient of friction, making it ideal for reactor linings, circuit boards, and kitchen utensils, to name a few applications. However, its nonstick properties make it challenging to bond to other materials or to itself.

Metal Assisted Chemical Etching is the process of wet chemical etching of semiconductors with the use of a metal catalyst, usually deposited on the surface of a semiconductor 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 and hydrofluoric acid. 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. This means that a pre-defined pattern of the metal on the surface can be directly transferred to a semiconductor substrate.

## References

• Harris, William T. (1976). Chemical Milling: The Technology of Cutting Materials by Etching. Oxford: Clarendon Press. ISBN   0 19 859115 2.