S. Whelan

Characterization by medium energy ion scattering of damage and dopant profiles produced by ultrashallow B and As implants into Si at different temperatures

High depth resolution medium energy ion scattering (MEIS) has been used to examine the influence of dynamic defect annealing on the damage formed in silicon substrates irradiated with ultralow energy ions (1 keV B+, 2.5 keV As+). Samples were implanted to doses ranging from 3×1014 to 2×1016 cm−2 at sample temperatures −150/−120, 25, and 300 °C. For all doses examined, B implantation at 25 and 300 °C produced a near-surface disordered layer 3–4 nm thick. For doses above 1×1015 cm−2, a second, deeper damaged layer was resolved at a depth greater than the peak of the projected range (Rp) of the implanted ions. For irradiations at −150 °C, MEIS and transmission electron microscope studies indicated the formation of a continuous amorphous layer, extending from the deeper damage region to the surface. However, epitaxial regrowth of this layer was not complete after a 30 s anneal at 600 °C, being arrested near Rp by clusters containing B. The dependence of B transient enhanced diffusion on the implant temperatur...

The dependence of arsenic transient enhanced diffusion on the silicon substrate temperature during ultralow energy implantation

The redistribution of As during high-temperature annealing has been investigated as a function of the Si(100) substrate temperature (−120 °C, +25 °C, and +300 °C) during 2.5 keV implantation (to 1.5×1015atoms/cm2). Each implant produced a damaged near-surface region, the extent of which varied with implant temperature. Samples implanted at each temperature were annealed in a nitrogen ambient with a few percent oxygen for 10 s at 550, 925, and 975 °C. The changes in implant damage and dopant distributions both prior to and following annealing were investigated using medium energy ion scattering and secondary ion mass spectrometry. Transient enhanced diffusion (TED) of the dopant was observed for all implant temperatures after 925 °C annealing with the 25 °C implant showing the deepest diffusion. Between 925 and 975 °C annealing, the As diffusion rate in the 300 °C exceeded that of the 25 °C implant. Significantly, the −120 °C implant displayed less TED of As compared to the higher temperature implants foll...

The production and use of ultralow energy ion beams

An ion accelerator, purpose built to produce beams at energies down to 10 eV with current densities in the 10–100 μA cm−2 range, is described. Fitted with dual ion source assemblies, the machine enables ultralow energy ion implantation and the growth of films and multilayers to be carried out under highly controlled conditions. The accelerator delivers ion beams into an ultrahigh vacuum chamber, containing a temperature controlled target stage (range −120 to +1350 °C), where they are used to study the fundamental physics relating to the interaction of ultralow energy ions with surfaces. This knowledge underlies a wide range of ion-beam and plasma-based technologies and, to illustrate its importance, results are presented from investigations designed to determine the optimum conditions for the growth of diamond-like and aluminum films by ion-beam deposition and the formation of ultrashallow junctions in semiconductors by 2.5 keV As+ implantation. The later investigation shows how transient arsenic diffusio...

Dopant behaviour and damage annealing in silicon implanted with 1 keV arsenic

The level of activation in ultra-shallow As doped Si as a function of the anneal condition has been investigated with spreading resistance profiling (SRP), four point probe (FPP) and Van der Pauw (VDP) methods. Double alignment medium energy ion scattering (MEIS) and low energy secondary ion mass spectrometry (SIMS) have been used to assess the damage annealing and dopant behaviour in the near surface regions. An inactive dopant solid solution was formed in Si following re-growth of the amorphous layer. When annealing in an oxidising ambient, although a high fraction of the implanted dose remains trapped in the oxide layer, a higher level of electrical activation is observed than compared to the non-oxidising anneal. Evidence of dopant out diffusion is observed during high temperature annealing in a non-oxidising gas ambient. The processes that occur during the anneal in the near surface regions of the sample have been discussed and related to the level of dopant activation achieved.

Implant temperature dependence of transient-enhanced diffusion in silicon (100) implanted with low-energy arsenic ions

The diffusion of arsenic implanted into silicon at low ion energies (2.5 keV) has been studied with medium-energy ion scattering, secondary ion mass spectrometry and four-point probe measurements. The dopant redistribution together with the corresponding damage recovery and electrical activation produced by high-temperature (550–975°C) rapid thermal anneals has been investigated for a range of substrate temperatures (+25, +300 and −120°C) during implant. Initial results show an implant temperature dependence of the damage structure and arsenic lattice position prior to anneal. Solid-phase epitaxial regrowth was observed following 550°C, 10 s anneals for all implant temperatures and resulted in approximately 60% of the implanted arsenic moving to substitutional positions. Annealing at 875°C resulted in similar arsenic redistribution for all implant temperatures. Following annealing at 925°C, transient-enhanced diffusion was observed in all samples with more rapid diffusion in the +25°C samples than either the −120 or +300°C implants, which had similar dopant profiles. In the 975°C anneal range, similar rates of implant redistribution were observed for the +300 and +25°C implants, while diffusion in the −120°C sample was reduced. These observations are discussed qualitatively in terms of the nature and density of the complex defects existing in the as-implanted samples.

Damage and Dopant Profiles Produced by Ultra-Shallow Boron And Arsenic Ion Implants into Silicon at Different Temperatures Characterised by Medium Energy Ion Scattering.

Medium energy ion scattering (MEIS), operated at sub-nm depth resolution in the double alignment configuration, has been used to examine implant and damage depth profiles formed in Si(100) substrates irradiated with 2.5 keV As+ and 1 keV B+ ions. Samples were implanted at temperatures varying between 150°C, and 300°C to doses ranging from 3X1014 to 2X1016 cm-2. For the As implants the MEIS studies demonstrate the occurrence of effects such as a dopant accommodation linked to the growth in depth of the damage layer, dopant clustering, as well as damage and dopant movement upon annealing. Following epitaxial regrowth at 600°C, approximately half of the As was observed to be in substitutional sites, consistent with the reported formation of AsnV complexes (n≤4), while the remainder became segregated and became trapped within a narrow, 1.1 nm wide layer at the Si/oxide interface MEIS measurements of the B implants indicate the formation of two distinct damage regions each with a different dependence on implant dose, the importance of dynamic annealing for implants at room temperature and above, and a competing point defect trapping effect at the Si/oxide interface. B+ implantation at low temperature resulted in the formation of an amorphous layer due to the drastic reduction of dynamic annealing processes. Notably different dopant distributions were measured by SIMS in the samples implanted with As at different temperatures following rapid thermal annealing (RTA) up to 1100°C in an oxidising environment. Implant temperature dependent interactions between defects and dopants are reflected in the transient enhanced diffusion (TED) behaviour of As.

High depth resolution characterization of the damage and annealing behaviour of ultra shallow As implants in Si

The relationship between damage formation/annealing and As profile redistribution has been studied using low energy As implants into Si at 2.5 keV at doses between 3 × 1013 cm-2 and 2 × 1015 cm-2 at room temperature. Samples were annealed at temperatures between 600 and 1050°C. High depth resolution medium energy ion scattering (MEIS) and secondary ion mass spectrometry (SIMS) were used to characterise the damage build up and As profiles as a function of implant dose and anneal temperature. MEIS studies showed that damage does not accumulate according to the energy deposition function but proceeds from the surface inwards. This is ascribed to the accumulation of collision cascade produced interstitials that are attracted to and settle at initially the oxide/Si interface and later to the advancing amorphous/crystalline interface. Dopant depth profiles agreed well with TRIM calculations for doses ≥ 4 × 1014 cm-2. However, for lower doses the dopant was observed to have a profile nearer to the surface, due to trapping in the narrow surface damaged layer, in which it is more easily accommodated. Following epitaxial regrowth at 700°C, MEIS showed that -50 % of the As has moved into substitutional sites, consistent with activation and/or the formation of inactive AsnV clusters (n ≤ 4), while the remainder had segregated to and become trapped in a ≤1 nm wide layer, clearly located on the Si side of the oxide/Si interface. Very low energy SIMS analysis at normal incidence is able to resolve these ultra shallow peaks, including the As pileup following epitaxial regrowth. They also confirmed that As retention was complete during dose build up and annealing.

The influence of substrate temperature and the anneal ambient on the redistribution of arsenic implanted into silicon at ultra-low energy

The effects of implant temperature and anneal gas environment on the redistribution of arsenic implanted into silicon at 2.5 keV to high concentrations has been studied. The as-implanted damage structures were observed to be strongly dependent on implant temperature. The transient enhanced diffusion observed during 10 second anneals was found to be highly dependent on the presence of oxygen. When annealing was carried out in pure nitrogen, significant loss of dopant occurred and the TED was not dependent on implant temperature. In an environment containing oxygen, all the dopant was retained and more extensive, implant temperature dependent redistribution was observed.