Low-energy plasma-enhanced chemical vapor deposition (LEPECVD) is a

plasma-enhanced chemical vapor deposition Plasma-enhanced chemical vapor deposition (PECVD) is a chemical vapor deposition process used to deposit thin films from a gas state (vapor) to a solid state on a substrate. Chemical reactions are involved in the process, which occur after crea ...

technique used for the

epitaxial Epitaxy (prefix ''epi-'' means "on top of”) is a type of crystal growth or material deposition in which new crystalline layers are formed with one or more well-defined orientations with respect to the crystalline seed layer. The deposited cry ...

deposition of thin

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 ...

(

silicon Silicon is a chemical element; it has symbol Si and atomic number 14. It is a hard, brittle crystalline solid with a blue-grey metallic lustre, and is a tetravalent metalloid (sometimes considered a non-metal) and semiconductor. It is a membe ...

germanium Germanium is a chemical element; it has Symbol (chemistry), symbol Ge and atomic number 32. It is lustrous, hard-brittle, grayish-white and similar in appearance to silicon. It is a metalloid or a nonmetal in the carbon group that is chemically ...

and SiGe alloys) films. A remote low energy, high density DC argon plasma is employed to efficiently decompose the gas phase

precursors Precursor or Precursors may refer to: *Precursor (religion), a forerunner, predecessor ** The Precursor, John the Baptist Science and technology * Precursor (bird), hypothesized genus of fossil birds that was composed of fossilized parts of unre ...

while leaving the epitaxial layer undamaged, resulting in high quality epilayers and high deposition rates (up to 10 nm/s).

Working principle

The

substrate Substrate may refer to: Physical layers *Substrate (biology), the natural environment in which an organism lives, or the surface or medium on which an organism grows or is attached ** Substrate (aquatic environment), the earthy material that exi ...

(typically a silicon

wafer A wafer is a crisp, often sweet, very thin, flat, light biscuit, often used to decorate ice cream, and also used as a garnish on some sweet dishes. They frequently have a waffle surface pattern but may also be patterned with insignia of the foo ...

) is inserted in the reactor chamber, where it is heated by a graphite resistive heater from the backside. An argon plasma is introduced into the chamber to ionize the precursors' molecules, generating highly reactive

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 ...

which result in the growth of an epilayer on the substrate. Moreover, the bombardment of Ar ions removes the hydrogen atoms

adsorbed 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 f ...

on the surface of the substrate while introducing no structural damage. The high reactivity of the radicals and the removal of hydrogen from the surface by ion bombardment prevent the typical problems of Si, Ge and SiGe alloys growth by thermal

chemical vapor deposition Chemical vapor deposition (CVD) is a vacuum deposition method used to produce high-quality, and high-performance, solid materials. The process is often used in the semiconductor industry to produce thin films. In typical CVD, the wafer (electro ...

(CVD), which are * dependence of the growth rate from the substrate temperature, due to the thermal energy needed for precursors decomposition and hydrogen

desorption Desorption is the physical process where Adsorption, adsorbed atoms or molecules are released from a surface into the surrounding vacuum or fluid. This occurs when a molecule gains enough energy to overcome the activation barrier and the binding e ...

from the substrate * high temperatures (>1000 °C for silicon) required to get a significant growth rate, which is strongly limited by the aforementioned effects * strong dependence of the deposition rate on the SiGe alloy composition, due to the large difference between the hydrogen desorption rate from Si and Ge surfaces. Thanks to this effects the growth rate in a LEPECVD reactor depends only on the plasma parameters and the gas fluxes, and it is possible to obtain

deposition at much lower temperatures compared to a standard CVD tool.

LEPECVD reactor

The LEPECVD reactor is divided in three main parts: * a loadlock, to load the substrates into the chamber without breaking the vacuum * the main chamber, which is kept in UHV at a base pressure of ~10

^

mbar * the plasma source, where the plasma is generated. The substrate is placed at the top of the chamber, facing down toward the plasma source. Heating is provided from the back side by

thermal radiation Thermal radiation is electromagnetic radiation emitted by the thermal motion of particles in matter. All matter with a temperature greater than absolute zero emits thermal radiation. The emission of energy arises from a combination of electro ...

from a resistive graphite heater incapsulated between two

boron nitride Boron nitride is a thermally and chemically resistant refractory compound of boron and nitrogen with the chemical formula B N. It exists in various crystalline forms that are isoelectronic to a similarly structured carbon lattice. The hexago ...

discs, which improve the temperature uniformity across the heater.

Thermocouple A thermocouple, also known as a "thermoelectrical thermometer", is an electrical device consisting of two dissimilar electrical conductors forming an electrical junction. A thermocouple produces a temperature-dependent voltage as a result of the ...

s are used to measure the temperature above the heater, which is then correlated to that of the substrate by a calibration done with an infrared

pyrometer A pyrometer, or radiation thermometer, is a type of remote sensing thermometer used to measure the temperature of distant objects. Various forms of pyrometers have historically existed. In the modern usage, it is a device that from a distance de ...

. Typical substrate temperatures for monocrystalline films are 400 °C to 760 °C, for germanium and silicon respectively. The potential of the wafer stage can be controlled by an external power supply, influencing the amount and the energy of radicals impinging on the surface, and is typically kept at 10-15 V with respect to the chamber walls. The process gases are introduced into the chamber through a gas dispersal ring placed below the wafer stage. The gases used in a LEPECVD reactor are

silane Silane (Silicane) is an inorganic compound with chemical formula . It is a colorless, pyrophoric gas with a sharp, repulsive, pungent smell, somewhat similar to that of acetic acid. Silane is of practical interest as a precursor to elemental ...

() and

germane Germane is the chemical compound with the formula Ge H4, and the germanium analogue of methane. It is the simplest germanium hydride and one of the most useful compounds of germanium. Like the related compounds silane and methane, germane is te ...

() for silicon and germanium deposition respectively, together with

diborane Diborane(6), commonly known as diborane, is the chemical compound with the formula . It is a highly toxic, colorless, and pyrophoric gas with a repulsively sweet odor. Given its simple formula, borane is a fundamental boron compound. It has att ...

() and

phosphine Phosphine (IUPAC name: phosphane) is a colorless, flammable, highly toxic compound with the chemical formula , classed as a pnictogen hydride. Pure phosphine is odorless, but technical grade samples have a highly unpleasant odor like rotting ...

() for p- and n-type doping.

Plasma source

The plasma source is the most critical component of a LEPECVD reactor, as the low energy, high density, plasma is the key difference from a typical

PECVD Plasma-enhanced chemical vapor deposition (PECVD) is a chemical vapor deposition process used to deposit thin films from a gas state (vapor) to a solid state on a substrate (materials science), substrate. Chemical reactions are involved in the pr ...

deposition system. The plasma is generated in a source which is attached to the bottom of the chamber.

Argon Argon is a chemical element; it has symbol Ar and atomic number 18. It is in group 18 of the periodic table and is a noble gas. Argon is the third most abundant gas in Earth's atmosphere, at 0.934% (9340 ppmv). It is more than twice as abu ...

is fed directly in the source, where

tantalum Tantalum is a chemical element; it has Symbol (chemistry), symbol Ta and atomic number 73. It is named after Tantalus, a figure in Greek mythology. Tantalum is a very hard, ductility, ductile, lustre (mineralogy), lustrous, blue-gray transition ...

filaments are heated to create an electron-rich environment by

thermionic emission Thermionic emission is the liberation of charged particles from a hot electrode whose thermal energy gives some particles enough kinetic energy to escape the material's surface. The particles, sometimes called ''thermions'' in early literature, a ...

. The plasma is then ignited by a DC discharge from the heated filaments to the grounded walls of the source. Thanks to the high electron density in the source the voltage required to obtain a discharge is around 20-30V, resulting in an ion energy of about 10-20 eV, while the discharge current is of the order of several tens of amperes, giving a high ion density. The DC discharge current can be tuned to control the ion density, thus changing the growth rate: in particular at a larger discharge current the ion density is higher, therefore increasing the rate.

Plasma confinement

The plasma enters the growth chamber through an

anode An anode usually is an electrode of a polarized electrical device through which conventional current enters the device. This contrasts with a cathode, which is usually an electrode of the device through which conventional current leaves the devic ...

electrically connected to the grounded chamber walls, which is used to focus and stabilize the discharge and the plasma. Further focusing is provided by a

magnetic field A magnetic field (sometimes called B-field) is a physical field that describes the magnetic influence on moving electric charges, electric currents, and magnetic materials. A moving charge in a magnetic field experiences a force perpendicular ...

directed along the chamber's axis, provided by external copper coils wrapped around the chamber. The current flowing through the coils (i.e. the intensity of the magnetic field) can be controlled to change the ion density at the substrate's surface, thus changing the growth rate. Additional coils ("wobblers") are placed around the chamber, with their axis perpendicular to the magnetic field, to continuously sweep the plasma over the substrate, improving the homogeneity of the deposited film.

Applications

Thanks to the possibility of changing the growth rate (through the plasma density or gas fluxes) independently from the substrate temperature, both thin films with sharp interfaces and a precision down to the nanometer scale at rates as low as 0.4 nm/s, as well as thick layers (up to 10 um or more) at rates as high as 10 nm/s, can be grown using the same reactor and in the same deposition process. This has been exploited to grow low-loss composition-graded waveguides for NIR and MIR and integrated nanostructures (i.e. quantum well stacks) for NIR optical amplitude modulation. The capability of LEPECVD to grow both very sharp quantum wells on thick buffers in the same deposition step has also been employed to realize high mobility strained Ge channels. Another promising application of the LEPECVD technique is the possibility of growing high aspect ratio, self-assembled silicon and germanium microcrystals on deeply patterned Si substrates. This solves many problems related to heteroepitaxy (i.e. thermal expansion coefficient and crystal lattice mismatch), leading to very high crystal quality, and is possible thanks to the high rates and low temperatures found in a LEPECVD reactor.

References

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External links

LEPECVD page on the website of L-NESS laboratory
of Politecnico di Milano, in Como, Italy. Chemical vapor deposition Plasma processing Semiconductor device fabrication Thin film deposition