Ray Duffy

Diffusion, activation, and regrowth behavior of high dose P implants in Ge

Time evolution of the chemical profile, electrical activity, and regrowth of P implanted in Ge at a concentration above the maximum equilibrium solubility is investigated at 500°C rapid thermal annealing temperature. During the first anneal, a second, epitaxial regrowth of a part of the amorphous layer leads to P trapping in substitutional sites at a level of about 4×1020atoms∕cm3. However, nonsubstitutional P atoms frozen in the crystal at high concentration during recrystallization form large, inactive precipitates of peculiar circular shape. Simultaneously, long annealing time leads to continuing, extensive P out- and indiffusion affecting both the P chemical profile and junction sheet resistance.

Solid phase epitaxy versus random nucleation and growth in sub-20nm wide fin field-effect transistors

The authors investigate the implications of amorphizing ion implants on the crystalline integrity of sub-20nm wide fin field-effect transistors (FinFETs). Recrystallization of thin body silicon is not as straightforward as that of bulk silicon because the regrowth direction may be parallel to the silicon surface rather than terminating at it. In sub-20nm wide FinFETs surface proximity suppresses crystal regrowth and promotes the formation of twin boundary defects in the implanted regions. In the case of a 50nm amorphization depth, random nucleation and growth leads to polycrystalline silicon formation in the top ∼25nm of the fin, despite being only ∼25nm from the crystalline silicon seed.

Highly manufacturable FinFETs with sub-10nm fin width and high aspect ratio fabricated with immersion lithography

We investigate scalability, performance and variability of high aspect ratio trigate FinFETs fabricated with 193 nm immersion lithography and conventional dry etch. FinFETs with fin widths down to 5nm are achieved with record aspect ratios of 13. Excellent nMOS and pMOS performance is demonstrated for narrow fins and short gates. Further improvement in nMOS performance can be achieved by eliminating access resistance that is currently attributed to poor re-crystallization of implantation damage in narrow fins. Fully-depleted FinFETs show strongly improved short channel effect (SCE) control when the fin width is scaled, even without halo-implants. Nearly ideal DIBL and sub-threshold slope (SS) are achieved down to 30nm gate length. Low leakage devices are realized by combining a fully depleted channel, HfSiO high-k dielectric, mid-gap TiN metal electrodes, and aggressive fin width scaling. Symmetrical threshold voltages (±0.35 V) are achieved. It is demonstrated that selective epitaxial growth on source and drain regions is essential to limit parasitic resistance in narrow fin devices. Parametric spread is dominated by gate length variations in short devices but within-die fin width variations are still evident for long devices.

Boron uphill diffusion during ultrashallow junction formation

The recently observed phenomenon of boron uphill diffusion during low-temperature annealing of ultrashallow ion-implanted junctions in silicon has been investigated. It is shown that the effect is enhanced by preamorphization, and that an increase in the depth of the preamorphized layer reduces uphill diffusion in the high-concentration portion of boron profile, while increasing transient enhanced diffusion in the tail. The data demonstrate that the magnitude of the uphill diffusion effect is determined by the proximity of boron and implant damage to the silicon surface.

Suppression of phosphorus diffusion by carbon co-implantation

The impact of Si interstitial (Sii) flux suppression on the formation of P junctions by rapid thermal annealing (RTA) is demonstrated. Here we investigate the role of amorphization coupled with C co-implantation on P diffusion and its activation. From experiments on C co-implants in a-Si versus c-Si, we conclude that only a small fraction of C interacts with Si interstitials (Sii). We have demonstrated that optimization of implants followed by spike RTA yields extensions suitable for gate lengths of 30nm, with vertical depth Xj=20nm (taken at 5×1018at.∕cm3), abruptness of 3nm/decade, and Rs=326Ω/◻.

Boron diffusion in amorphous silicon and the role of fluorine

We demonstrate that boron diffuses at high concentrations during low-temperature thermal annealing in amorphous silicon pre-amorphized by germanium ion implantation. For a typical boron ultrashallow junction doping profile, concentrations as high as 2×1020 cm−3 appear to be highly mobile at 500 and 600 °C in the amorphous silicon region before recrystallization. In crystalline silicon at the same temperatures the mobile boron concentration is at least two orders of magnitude lower. We also show that boron diffusivity in the amorphous region is similar with and without fluorine. The role of fluorine is not to enhance boron diffusivity, but to dramatically slow down the recrystallization rate, allowing the boron profile to be mobile up to the concentration of 2×1020 cm−3 for a longer time.

Groups III and V impurity solubilities in silicon due to laser, flash, and solid-phase-epitaxial-regrowth anneals

In this work the authors studied impurity solubilities of groups III and V elements in silicon resulting from laser anneal, flash anneal, and solid-phase-epitaxial regrowth. Rutherford backscattering channeling analysis was used to determine substitutional impurity depth profiles generated from the difference between the random and aligned spectra. Despite the large difference in peak temperatures and times, the anneals produce similar results with maximum solubilities beating the maximum equilibrium values by one to two orders of magnitude depending on the impurity. The correlation between the metastable solubility and the equilibrium distribution coefficient allows a prediction of values for other impurities not extracted experimentally.

Leakage optimization of ultra-shallow junctions formed by solid phase epitaxial regrowth

Ultra-shallow p+ junctions formed by solid phase epitaxial regrowth (SPER) have promise for sub-65 nm CMOS technologies. Due to above-equilibrium solid solubilities and minimal diffusion, such junctions can far outperform spike-annealed junctions in terms of resistance, abruptness, and depth. However, the low-temperature annealing does not dissolve the end of range defects creating concerns for junction leakage in the device. In this work, we show how SPER junctions can be optimized to meet the ITRS junction profile and low-power leakage requirements of the 45 nm CMOS node [International Technology Roadmap for Semiconductors (Semiconductor Industry Association, San Jose, CA, 2001)]. Diode leakage is shown to decrease with Ge amorphization depth and B dose and energy. Leakage is shown to increase dramatically with the background doping level. Increasing the regrowth, or post-annealing, thermal budget improves leakage and can be optimized to avoid deactivation. The inclusion of a preanneal does not affect t...

Atomistic analysis of the evolution of boron activation during annealing in crystalline and preamorphized silicon

We use kinetic nonlattice Monte Carlo atomistic simulations to investigate the physical mechanisms for boron cluster formation and dissolution in complementary metal-oxide semiconductor (MOS) processing, and the role of Si interstitials in the different processes. For this purpose, B implants in crystalline Si as well as B implants in preamorphized Si are analyzed. For subamorphizing B implants, a high concentration of Si interstitials overlaps with the B profile and this causes a very quick B deactivation for both low- and high-dose B implants. For B implants in preamorphized silicon, B is activated during the regrowth of the amorphous layer if the B concentration is lower than 1020cm−3 and remains active upon annealing. However, if B concentrations higher than 1020cm−3 are present, as occurs in the formation of extensions in p-channel MOS transistors, B atoms are not completely activated during the regrowth. Moreover, the injection of Si interstitials from the end-of-range defects leads to additional B ...

Doping fin field-effect transistor sidewalls: Impurity dose retention in silicon due to high angle incident ion implants and the impact on device performance

The three dimensional (3D) nature of a fin field-effect transistor (FinFET) structure creates new challenges for an impurity doped region formation. For the triple gate FinFET, both top and side surfaces require high levels of dopant incorporation to minimize access resistance. In this work, we investigate the use of conventional ion implantation for the introduction of impurities in this 3D silicon structure. Specifically, we evaluate sidewall impurity dose retention at various angles of incidence. The retention of dose is determined by (i) trigonometry of the implant angle in the 3D fin system, (ii) backscattering, and (iii) material properties of the target surface. Dose retention is most sensitive to the implant angle. For a fixed implant projected range, lighter ions are more likely to be ejected from the target. Thus, heavier ions are better for dose retention. The influence of sidewall dose retention on the electrical performance of fully depleted FinFETs was investigated by means of 3D device simu...