Due to their high catalytic activity in solutions as well as their usefulness in preparation of dispersed metal catalysts, clusters, also known as colloids, had been a very important aspect in science since its discovery in 18th century. A cluster is an atom or molecule ensemble whose size is in between a molecule and bulk solid. In IC manufacturing, clusters of electrically charged ions are used to form high quality films of dielectrics, metals, and other wafer layers at low substrate temperature. By ionizing and energizing the gas atoms that are condensed into clusters, this process, known as cluster ion beam, induces interactions between the cluster ions and the target atoms. The simultaneous interaction of the ions and the target atoms produces non-linear sputter and film formation.
Some important features of cluster ion beam are-
- Formation of macro aggregates or cluster. It has been considered that it is difficult, if possible, to generate clusters of solid material at room temperature. But with cluster ion beam, large clusters of few hundreds to thousands of atoms are formed through pure evaporation expansion process.
- Accelerating the atoms with high potential energy helps cluster ion beam in film deposition at low temperature.
The clusters are formed from the abrupt decrease in temperature of the gas vapor. One common method is by using an evaporation cell, where the evaporant is heated up until its vapor expands [adiabatically] to more than 100 to 10-3 or 10-7 mbar. The adiabatic expansion will suddenly cool down the vapor, thus, forming clusters of atoms. The clusters are ionized by electron bombardment and accelerated by high potentials. Both the neutral clusters, at 0.1 eV, and ionized clusters, at few eV higher than the neutral, are diffused to the substrate and get deposited as a film of pure and excellent adhesion.
There are different techniques that can be employed to generate clusters depending on instances of the requirement such as the need for free clusters, or the deposited ones. Size, material type, and boiling/melting temperature also influence the type of method for creating the clusters. Six of the common cluster formation methods are:
1. Supersonic Expansion
In supersonic expansion, a stream of gas is expanded into a vacuum from a nozzle. Clusters are formed if there is an adequate collision before the vapor reaches the complex part of the beam and its adiabatic cooling ceases. More likely, the generated clusters will have up to a few hundred of atoms, low translational temperature and speed distribution, but high vibrational temperature, which means there is a greater chance for the atoms from the escaping clusters to evaporate. To support cluster production, the expansion of the beam is constrained in a conical nozzle. This will not only increase the number of collisions, but will also produce larger clusters and improve the center-line flux.
Supersonic expansion is best suited for metals with low boiling points because of the difficulty in vaporizing such materials through heat. Clusters produced from metal expansion in an oven contain only a small number of atoms. Thus, applications requiring clusters from metals prefer supersonic expansion wherein a heated metal vapor at a partial pressure of 10 to 100 mbar is mixed with an inert gas (e.g. argon) at a stagnation pressure of several atmospheres. During the adiabatic expansion, the temperature of the inert gas will lower rapidly, assisting metal vapor cooling through collisions.
2. Inert Gas Condensation
This technique is similar with supersonic expansion in a way that both use nozzle. But for the case of inert gas condensation, the clusters are formed prior the expansion in the high vacuum region. A flowing stream of cold inert gas cools down the metal vapor, which over saturates the vapor and condenses it into clusters. The clusters then exits through the nozzle to the high vacuum region where adiabatic expansion occurs. The cold inert gas creates low internal temperature clusters, which, some scientists say, drives the negligibility of re-evaporation. The size of clusters formed from inert gas condensation usually ranges from dimers to approximately 105 atoms, with a controlled rate of growth. On Sattler’s prototype source in 1980, clusters with a few hundreds of atoms of antimony, bismuth, and lead are produced at a detected count rate of 10 per second. To date, the count rate of clusters from this technique has improved to 1010 up to 1011 per second.
One weak point of supersonic expansion is that when the large clusters cannot keep up with the velocity of the carrier gas to maintain a kinetic energy in the beam to less than 1 eV. Inert gas condensation is often used on metal clusters whose evaporation temperature is higher than the attainable point of supersonic expansion.
3. Laser Vaporization
Laser vaporization generates clusters by vaporizing the material in a pulse-laser and entailing the vapor to a pulsed flow of cold inert gas, in the same manner as in the inert gas condensation. However, this technique employs a greater stagnation pressure than in the inert gas condensation, which intensifies adiabatic cooling during the expansion in the high vacuum area. Laser vaporization can produce clusters that have a few hundred atoms of almost any metal, even the refractory metals.
4. Electrical Arc Discharge
Also known as PACIS (Pulsed Arc Cluster Ion Source), this technique is a derivative of laser vaporization method. But instead of indirectly vaporizing the material with a pulsed-laser, an electrical arc is used to directly vaporize it. This produces clusters containing about fifty atoms intensified to up to a few ångstrom. Ten percent of these clusters have already been ionized by the electrical arc, thus, eliminating a separate stage for cluster ionization.
5. Ion Bombardment
By bombarding the material with heavy high-energy ions, ion bombardment or sputtering technique ‘knocks-off’ clusters of atoms from the material. This produces hot clusters of positive, negative, and neutral charges, which eventually cool down as they travel down the target. However, due to the high-energy spread of the ‘knocked-off’ ions, usually up to 30 eV, the cluster ions are having a trouble to smoothly land on the target. Hence, this technique is best suited for small-sized clusters because the intensity of distribution falls away with cluster size.
6. Liquid Metal Ion Source
Liquid metal ion source (LMIS) are often used to form multiply charged clusters of metals with low melting point. A capillary filled with a liquid metal is charged with an electric field, which thrusts the liquid metal to the Taylor cone. The field at the tip of the cone is usually very strong that it may ripped-off the cluster ions. There is also a possibility for Coulomb explosion to happen since the multiply charged clusters are mutual. Similar to ion bombardment, clusters generated by liquid metal ion source also experiences high kinetic energy spread, making it difficult for the ions to attain soft-landing on the target.
The determination of the appropriate cluster formation method for a specific project relies on some criteria such as the parameters listed in Table 1. This summarizes the relative performance, based on the best case, of each technique.
|Cluster Formation Technique||Cluster Size||Beam Energy Spread||Cluster Internal Temperature||Cluster Material||Cluster Charge||Beam Intensity||Cost|
|Supersonic expansion||<100||lowvaries with size||high||alkali metals||neutrals||high||rational|
|Inert gas condensation||2 to >105||low<0.1eV||low||metals with melting points up to Ag/Fe||neutrals||high0.6 to 40 nA||rational|
|Laser vaporization||<200||low||low||any metal||neutrals||high|
|Electrical arc discharge||<50||low||low||any metal||neutrals and ions||high2Å per pulse||rational|
|Ion bombardment||<10 to 20||20 to 30eV||high||any metal||neutrals and ions||10 nA||rational|
|Liquid metal ion source||<100||10 to 50 eV or100 to 200 eV||high||metals with melting points up to Au||Ions||20 nA at 2m||rational|
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