Plasma etching is a form of

plasma processing Plasma processing is a plasma-based material processing technology that aims at modifying the chemical and physical properties of a surface. Plasma processing techniques include: *Plasma activation * Plasma ashing * Plasma cleaning * Plasma elec ...

used to fabricate

integrated circuit An integrated circuit (IC), also known as a microchip or simply chip, is a set of electronic circuits, consisting of various electronic components (such as transistors, resistors, and capacitors) and their interconnections. These components a ...

s. It involves a high-speed stream of glow discharge ( plasma) of an appropriate gas mixture being shot (in pulses) at a sample. The plasma source, known as etch species, can be either charged (

ion An ion () is an atom or molecule with a net electrical charge. The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by convent ...

s) or neutral (

atom Atoms are the basic particles of the chemical elements. An atom consists of a atomic nucleus, nucleus of protons and generally neutrons, surrounded by an electromagnetically bound swarm of electrons. The chemical elements are distinguished fr ...

s and

radicals Radical (from Latin: ', root) may refer to: Politics and ideology Politics *Classical radicalism, the Radical Movement that began in late 18th century Britain and spread to continental Europe and Latin America in the 19th century *Radical politics ...

). During the process, the plasma generates volatile etch products at room temperature from the

chemical reaction A chemical reaction is a process that leads to the chemistry, chemical transformation of one set of chemical substances to another. When chemical reactions occur, the atoms are rearranged and the reaction is accompanied by an Gibbs free energy, ...

s between the elements of the material etched and the reactive species generated by the plasma. Eventually the atoms of the shot element embed themselves at or just below the surface of the target, thus modifying the

physical properties A physical property is any property of a physical system that is measurable. The changes in the physical properties of a system can be used to describe its changes between momentary states. A quantifiable physical property is called ''physical ...

of the target.

Mechanisms

Plasma generation

A plasma is a high energetic condition in which a lot of processes can occur. These processes happen because of electrons and atoms. To form the plasma electrons have to be accelerated to gain energy. Highly energetic electrons transfer the energy to atoms by collisions. Three different processes can occur because of this collisions: *

Excitation Excitation, excite, exciting, or excitement may refer to: * Excitation (magnetic), provided with an electrical generator or alternator * ''Exite'', a series of racing video games published by Nintendo starting with ''Excitebike'' * Excite (web port ...

* Dissociation *

Ionization Ionization or ionisation is the process by which an atom or a molecule acquires a negative or positive Electric charge, charge by gaining or losing electrons, often in conjunction with other chemical changes. The resulting electrically charged at ...

Different species are present in the plasma such as electrons, ions,

, and neutral particles. Those species are interacting with each other constantly. Two processes occur during plasma etching: *generation of chemical species *interaction with the surrounding surfaces Without a plasma, all those processes would occur at a higher temperature. There are different ways to change the plasma chemistry and get different kinds of plasma etching or plasma depositions. One way to form a plasma is by using RF excitation by a power source of 13.56 MHz, a frequency allocated for this application in the

ISM bands The ISM radio bands are radio band, portions of the radio spectrum reserved internationally for ''industrial, scientific, and medical'' (ISM) purposes, excluding applications in telecommunications. Examples of applications for the use of radio fr ...

. The mode of operation of the plasma system will change if the operating pressure changes. Also, it is different for different structures of the reaction chamber. In the simple case, the electrode structure is symmetrical, and the sample is placed upon the grounded electrode.

Influences on the process

The key to develop successful complex etching processes is to find the appropriate gas etch chemistry that will form volatile products with the material to be etched as shown in Table 1. For some difficult materials (such as magnetic materials), the volatility can only be obtained when the wafer temperature is increased. The main factors that influence the plasma process: *Electron source *Pressure *Gas species *Vacuum

Surface interaction

The reaction of the products depend on the likelihood of dissimilar atoms, photons, or radicals reacting to form chemical compounds. The temperature of the surface also affects the reaction of products. Adsorption happens when a substance is able to gather and reach the surface in a condensed layer, ranging in thickness (usually a thin, oxidized layer.) Volatile products desorb in the plasma phase and help the plasma etching process as the material interacts with the sample's walls. If the products are not volatile, a thin film will form at the surface of the material. Different principles that affect a sample's ability for plasma etching: * Volatility *

Adsorption Adsorption is the adhesion of atoms, ions or molecules from a gas, liquid or dissolved solid to a surface. This process creates a film of the ''adsorbate'' on the surface of the ''adsorbent''. This process differs from absorption, in which a ...

Chemical Affinity In chemical physics and physical chemistry, chemical affinity is the electronic property by which dissimilar chemical species are capable of forming chemical compounds. Chemical affinity can also refer to the tendency of an atom or compound to com ...

*Ion-bombarding *

Sputtering In physics, sputtering is a phenomenon in which microscopic particles of a solid material are ejected from its surface, after the material is itself bombarded by energetic particles of a plasma or gas. It occurs naturally in outer space, and c ...

Plasma etching can change the surface contact angles, such as hydrophilic to hydrophobic, or vice versa. Argon plasma etching has reported to enhance contact angle from 52 deg to 68 deg, and, Oxygen plasma etching to reduce contact angle from 52 deg to 19 deg for CFRP composites for bone plate applications. Plasma etching has been reported to reduce the surface roughness from hundreds of nanometers to as much lower as 3 nm for metals.

Types

Pressure influences the plasma etching process. For plasma etching to happen, the chamber has to be under low pressure, less than 100 Pa. In order to generate low-pressure plasma, the gas has to be ionized. The ionization happens by a glow charge. Those excitations happen by an external source, which can deliver up to 30 kW and frequencies from 50 Hz (dc) over 5–10 Hz (pulsed dc) to radio and microwave frequency (MHz-GHz).

Microwave plasma etching

Microwave etching happens with an excitation sources in the microwave frequency, so between MHz and GHz. One example of plasma etching is shown here. Microwave plasma etching

Hydrogen plasma etching

One form to use gas as plasma etching is hydrogen plasma etching. Therefore, an experimental apparatus like this can be used:

Plasma etcher

A plasma etcher, or etching tool, is a tool used in the production of

semiconductor A semiconductor is a material with electrical conductivity between that of a conductor and an insulator. Its conductivity can be modified by adding impurities (" doping") to its crystal structure. When two regions with different doping level ...

devices. A plasma etcher produces a plasma from a process gas, typically

oxygen Oxygen is a chemical element; it has chemical symbol, symbol O and atomic number 8. It is a member of the chalcogen group (periodic table), group in the periodic table, a highly reactivity (chemistry), reactive nonmetal (chemistry), non ...

or a

fluorine Fluorine is a chemical element; it has Chemical symbol, symbol F and atomic number 9. It is the lightest halogen and exists at Standard temperature and pressure, standard conditions as pale yellow Diatomic molecule, diatomic gas. Fluorine is extre ...

-bearing gas, using a high frequency

electric field An electric field (sometimes called E-field) is a field (physics), physical field that surrounds electrically charged particles such as electrons. In classical electromagnetism, the electric field of a single charge (or group of charges) descri ...

, typically 13.56 MHz. A

silicon wafer In electronics, a wafer (also called a slice or substrate) is a thin slice of semiconductor, such as a crystalline silicon (c-Si, silicium), used for the fabrication of integrated circuits and, in photovoltaics, to manufacture solar cells. The ...

is placed in the plasma etcher, and the

air An atmosphere () is a layer of gases that envelop an astronomical object, held in place by the gravity of the object. A planet retains an atmosphere when the gravity is great and the temperature of the atmosphere is low. A stellar atmosph ...

is evacuated from the process chamber using a system of vacuum pumps. Then a process gas is introduced at low pressure, and is excited into a plasma through

dielectric breakdown In electronics, electrical breakdown or dielectric breakdown is a process that occurs when an electrically insulating material (a dielectric), subjected to a high enough voltage, suddenly becomes a conductor and current flows through it. All ...

Plasma confinement

Industrial plasma etchers often feature plasma confinement to enable repeatable etch rates and precise spatial distributions in plasmas. One method of confining plasmas is by using the properties of the

Debye sheath The Debye sheath (also electrostatic sheath) is a layer in a plasma which has a greater density of positive ions, and hence an overall excess positive charge, that balances an opposite negative charge on the surface of a material with which it is ...

, a near-surface layer in plasmas similar to the double layer in other fluids. For example, if the Debye sheath length on a slotted quartz part is at least half the width of the slot, the sheath will close off the slot and confine the plasma, while still permitting uncharged particles to pass through the slot.

Applications

Plasma etching is currently used to process semiconducting materials for their use in the fabrication of electronics. Small features can be etched into the surface of the semiconducting material in order to be more efficient or enhance certain properties when used in electronic devices. For example, plasma etching can be used to create deep trenches on the surface of silicon for uses in

microelectromechanical systems MEMS (micro-electromechanical systems) is the technology of microscopic devices incorporating both electronic and moving parts. MEMS are made up of components between 1 and 100 micrometres in size (i.e., 0.001 to 0.1 mm), and MEMS devices ...

. This application suggests that plasma etching also has the potential to play a major role in the production of microelectronics. Similarly, research is currently being done on how the process can be adjusted to the nanometer scale. Hydrogen plasma etching, in particular, has other interesting applications. When used in the process of etching semiconductors, hydrogen plasma etching has been shown to be effective in removing portions of native oxides found on the surface. Hydrogen plasma etching also tends to leave a clean and chemically balanced surface, which is ideal for a number of applications. Oxygen plasma etching can be used for anisotropic deep-etching of diamond nanostructures by application of high bias in inductively coupled plasma/reactive ion etching (ICP/RIE) reactor. On the other hand, the use of oxygen 0V bias plasmas can be used for isotropic surface termination of C-H terminated diamond surface.

Integrated circuits

Plasma can be used to grow a

silicon dioxide Silicon dioxide, also known as silica, is an oxide of silicon with the chemical formula , commonly found in nature as quartz. In many parts of the world, silica is the major constituent of sand. Silica is one of the most complex and abundan ...

film on a silicon wafer (using an oxygen plasma), or can be used to remove silicon dioxide by using a fluorine bearing gas. When used in conjunction with

photolithography Photolithography (also known as optical lithography) is a process used in the manufacturing of integrated circuits. It involves using light to transfer a pattern onto a substrate, typically a silicon wafer. The process begins with a photosensiti ...

, silicon dioxide can be selectively applied or removed to trace paths for circuits. For the formation of integrated circuits it is necessary to structure various layers. This can be done with a plasma etcher. Before etching, a

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 electronics industry. T ...

is deposited on the surface, illuminated through a mask, and developed. The dry etch is then performed so that structured etching is achieved. After the process, the remaining photoresist has to be removed. This is also done in a special plasma etcher, called an

asher Asher ( ''’Āšēr''), in the Book of Genesis, was the younger of the two sons of Jacob and Zilpah, and Jacob's eighth son overall. He was the founder of the Israelite Tribe of Asher. Name The text of the Torah states that the name אָ� ...

. Dry etching allows a reproducible, uniform etching of all materials used in silicon and

III-V semiconductor Semiconductor materials are nominally small band gap insulators. The defining property of a semiconductor material is that it can be compromised by doping it with impurities that alter its electronic properties in a controllable way. Because of ...

technology. By using inductively coupled plasma/reactive ion etching (ICP/RIE), even hardest materials like e.g. diamond can be nanostructured. Plasma etchers are also used for de-layering integrated circuits in

failure analysis Failure analysis is the process of collecting and analyzing data to determine the cause of a failure, often with the goal of determining corrective actions or liability. According to Bloch and Geitner, ”machinery failures reveal a reaction chain ...

Printed circuit boards

Plasma is used to etch printed circuit boards, including de-smear vias.

References

External links

*http://stage.iupac.org/publications/pac/pdf/1990/pdf/6209x1699.pdf{{Dead link, date=May 2020 , bot=InternetArchiveBot , fix-attempted=yes Plasma processing Semiconductor device fabrication