Diffusion barrier
Encyclopedia
A diffusion barrier is a thin layer (usually micrometres thick) of metal
usually placed between two other metals. It is done to act as a barrier to protect either one of the metals from corrupting the other.
Adhesion of a plated metal
layer to its substrate requires a physical interlocking, inter-diffusion
of the deposit or a chemical bond
ing between plate and substrate in order to work. The role of a diffusion barrier is to prevent or to retard the inter-diffusion of the two superposed metals. Therefore, to be effective, a good diffusion barrier requires inertness with respect to adjacent materials. To obtain good adhesion and a diffusion barrier simultaneously, the bonding between layers needs to come from a chemical reaction of limited range at both boundaries. Materials providing good adhesion are not necessarily good diffusion barriers and vice-versa. Consequently there are cases where two or more separate layers must be used to provide a proper interface between substrates.
and service life are critical parameters to select diffusion barrier materials. Many thin film
metal combinations have been evaluated for their adhesion and diffusion barrier properties. Aluminum provides good conductivity
, adhesion and reliability because of its oxygen
reactivity and the self-passivation
properties of its oxide. Copper
also easily reacts with oxygen but its oxides have poor adhesion properties. As for gold
its virtue relies in its inertness, and ease of application; its problem is its cost. Chromium
has excellent adhesion to many materials because of its reactivity. Its affinity for oxygen forms a thin stable oxide coat, a passivation layer which prevents further oxidation and provides inertness to corrosive environment. Nickel
, Nichrome
, tantalum
, hafnium
, niobium
, zirconium
, vanadium
, and tungsten
are a few of the metals combinations used to form diffusion barriers for specific applications. Conductive ceramic
s can be also used, such as tantalum nitride, indium oxide, copper silicide
, tungsten nitride
, and titanium nitride
.
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...
usually placed between two other metals. It is done to act as a barrier to protect either one of the metals from corrupting the other.
Adhesion of a plated metal
Electroplating
Electroplating is a plating process in which metal ions in a solution are moved by an electric field to coat an electrode. The process uses electrical current to reduce cations of a desired material from a solution and coat a conductive object with a thin layer of the material, such as a metal...
layer to its substrate requires a physical interlocking, inter-diffusion
Diffusion
Molecular diffusion, often called simply diffusion, is the thermal motion of all particles at temperatures above absolute zero. The rate of this movement is a function of temperature, viscosity of the fluid and the size of the particles...
of the deposit or a chemical bond
Chemical bond
A chemical bond is an attraction between atoms that allows the formation of chemical substances that contain two or more atoms. The bond is caused by the electromagnetic force attraction between opposite charges, either between electrons and nuclei, or as the result of a dipole attraction...
ing between plate and substrate in order to work. The role of a diffusion barrier is to prevent or to retard the inter-diffusion of the two superposed metals. Therefore, to be effective, a good diffusion barrier requires inertness with respect to adjacent materials. To obtain good adhesion and a diffusion barrier simultaneously, the bonding between layers needs to come from a chemical reaction of limited range at both boundaries. Materials providing good adhesion are not necessarily good diffusion barriers and vice-versa. Consequently there are cases where two or more separate layers must be used to provide a proper interface between substrates.
Selection
While the choice of diffusion barrier depends on the final function, anticipated operating temperatureOperating temperature
An operating temperature is the temperature at which an electrical or mechanical device operates. The device will operate effectively within a specified temperature range which varies based on the device function and application context, and ranges from the minimum operating temperature to the...
and service life are critical parameters to select diffusion barrier materials. Many thin film
Thin film
A thin film is a layer of material ranging from fractions of a nanometer to several micrometers in thickness. Electronic semiconductor devices and optical coatings are the main applications benefiting from thin film construction....
metal combinations have been evaluated for their adhesion and diffusion barrier properties. Aluminum provides good conductivity
Conductivity
Conductivity may refer to:*Electrical conductivity, a measure of a material's ability to conduct an electric current*Conductivity , also the specific conductance, is a measurement of the electrical conductance per unit distance in an electrolytic or aqueous solution*Ionic conductivity, a measure of...
, adhesion and reliability because of its oxygen
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
reactivity and the self-passivation
Passivation
Passivation is the process of making a material "passive", and thus less reactive with surrounding air, water, or other gases or liquids. The goal is to inhibit corrosion, whether for structural or cosmetic reasons. Passivation of metals is usually achieved by the deposition of a layer of oxide...
properties of its oxide. 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...
also easily reacts with oxygen but its oxides have poor adhesion properties. As for gold
Gold
Gold is a chemical element with the symbol Au and an atomic number of 79. Gold is a dense, soft, shiny, malleable and ductile metal. Pure gold has a bright yellow color and luster traditionally considered attractive, which it maintains without oxidizing in air or water. Chemically, gold is a...
its virtue relies in its inertness, and ease of application; its problem is its cost. Chromium
Chromium
Chromium is a chemical element which has the symbol Cr and atomic number 24. It is the first element in Group 6. It is a steely-gray, lustrous, hard metal that takes a high polish and has a high melting point. It is also odorless, tasteless, and malleable...
has excellent adhesion to many materials because of its reactivity. Its affinity for oxygen forms a thin stable oxide coat, a passivation layer which prevents further oxidation and provides inertness to corrosive environment. Nickel
Nickel
Nickel is a chemical element with the chemical symbol Ni and atomic number 28. It is a silvery-white lustrous metal with a slight golden tinge. Nickel belongs to the transition metals and is hard and ductile...
, Nichrome
Nichrome
Nichrome is a non-magnetic alloy of nickel, chromium, and often iron, usually used as a resistance wire. Patented in 1905, it is the oldest documented form of resistance heating alloy. A common alloy is 80% nickel and 20% chromium, by mass, but there are many others to accommodate various...
, tantalum
Tantalum
Tantalum is a chemical element with the symbol Ta and atomic number 73. Previously known as tantalium, the name comes from Tantalus, a character in Greek mythology. Tantalum is a rare, hard, blue-gray, lustrous transition metal that is highly corrosion resistant. It is part of the refractory...
, hafnium
Hafnium
Hafnium is a chemical element with the symbol Hf and atomic number 72. A lustrous, silvery gray, tetravalent transition metal, hafnium chemically resembles zirconium and is found in zirconium minerals. Its existence was predicted by Dmitri Mendeleev in 1869. Hafnium was the penultimate stable...
, niobium
Niobium
Niobium or columbium , is a chemical element with the symbol Nb and atomic number 41. It's a soft, grey, ductile transition metal, which is often found in the pyrochlore mineral, the main commercial source for niobium, and columbite...
, zirconium
Zirconium
Zirconium is a chemical element with the symbol Zr and atomic number 40. The name of zirconium is taken from the mineral zircon. Its atomic mass is 91.224. It is a lustrous, grey-white, strong transition metal that resembles titanium...
, vanadium
Vanadium
Vanadium is a chemical element with the symbol V and atomic number 23. It is a hard, silvery gray, ductile and malleable transition metal. The formation of an oxide layer stabilizes the metal against oxidation. The element is found only in chemically combined form in nature...
, and tungsten
Tungsten
Tungsten , also known as wolfram , is a chemical element with the chemical symbol W and atomic number 74.A hard, rare metal under standard conditions when uncombined, tungsten is found naturally on Earth only in chemical compounds. It was identified as a new element in 1781, and first isolated as...
are a few of the metals combinations used to form diffusion barriers for specific applications. Conductive ceramic
Ceramic
A ceramic is an inorganic, nonmetallic solid prepared by the action of heat and subsequent cooling. Ceramic materials may have a crystalline or partly crystalline structure, or may be amorphous...
s can be also used, such as tantalum nitride, indium oxide, copper silicide
Copper silicide
Copper silicide , also called pentacopper silicide, is a binary compound of silicon with copper. It is an intermetallic compound, meaning that it has properties intermediate between an ionic compound and an alloy. This a solid crystalline material is a silvery solid that is insoluble in water...
, tungsten nitride
Tungsten nitride
Tungsten nitride is an inorganic compound, a nitride of tungsten. It is a hard, solid, brown-colored ceramic material that is electrically conductive and decomposes in water....
, and titanium nitride
Titanium nitride
Titanium nitride is an extremely hard ceramic material, often used as a coating on titanium alloys, steel, carbide, and aluminium components to improve the substrate's surface properties....
.