Cluster Ion Beams

Why Cluster Ion Beams?

As semiconductor devices scale, the energy of the ion implant also scales. Very low energy implant, and very advanced annealing technologies are required to produce Ultra Shallow Junctions, or “USJs”. Conventional implant systems cannot perform to expected levels at low energy due to fundamental physical limits in the extraction and transport of ion beams at low energy.
The low productivity of conventional systems at low energy is shown in Figure 2.

The use of cluster ion beams “unties the Gordian knot” and solves the productivity problem with low energy implant.

Cluster ions include many dopant atoms (N) per ion. In this way, the implanter operates at a higher energy (N times the process energy) which avoids the low energy extraction and transport limits. In addition, the dose rate is then N times the beam current. Using cluster ions, a conventional implanter can perform with N² the productivity compared to standard operation with single atoms of the dopant species.

There are further benefits to the implantation of cluster ions compared to conventional methods. In a standard implanter, productivity is enhanced by the use of decel, where the beam is extracted and transported at higher energy and then decelerated before impacting the wafer. This method always involves implanting with a fraction of the beam still at the higher energy or “energy contamination”. Such energy contamination makes the formation of USJs very difficult today, and impossible to meet future requirements. The use of cluster ion implantation eliminates the need for deceleration and therefore avoids energy contamination.

The SemEquip ion source has been designed specifically for the production and preservation of cluster ions. Our source produces cluster ions of all the species of interest to the semiconductor industry, including boron, arsenic and phosphorus. The critical need in the industry is for boron clusters, since the formation of P-type (Boron doped) USJs is the main bottleneck in semiconductor fabrication.

Benefit for USJ

The use of cluster ion beams directly addresses the low energy limitations of conventional ion implantation, in two major ways.  First, the use of a cluster of N atoms of the intended dopant requires that the implanter operate at NEo, where Eo is the energy per dopant atom.  This increase in energy avoids the low energy limits and allows a conventional ion implanter to operate in the energy range of it’s original design.  Secondly, there is only one electrical charge per cluster, rather than one charge per atom, so the charge density is much reduced, further improving the extraction and transport of these beams.  Putting these factors together, one can derive that an increase in dose rate of N² is possible for cluster ion beams, compared to conventional single atom implantation with the same effective energy.

Boron Cluster Ion Beams

The critical need is for P-type low energy implants, so clusters of boron are an exciting opportunity. SemEquip has pioneered the use of ClusterBoron^® (see products page) for the production of cluster ions containing 18 boron atoms. The use of this cluster ion allows the implanter to run with an extraction voltage of 10kV and still produce an implant process equivalent to 0.5keV monomer boron. The dose rate enhancement of using clusters makes the throughput of such a process many times higher than a conventional implant process for 0.5keV boron. In addition to the B18 cluster, the SemEquip ClusterIon^® source can produce many other clusters of boron, such as B₂, B₄, B₅, B₆, B₁₀, etc depending on the desired process.

Arsenic Tetramer Ion Beams

For N-type cluster implants, SemEquip uses a cluster of 4 arsenic atoms, which is called the arsenic tetramer (As₄, n=4).  Thus, the arsenic cluster process uses an energy four times the conventional arsenic process would use and has a charge density 4x lower than a conventional implant of the same dose rate.