Ultra-shallow Junction Formation Research Articles

The impact of nitrogen co-implantation on boron ultra-shallow junction formation and underlying physical understanding

In this paper, we show that boron transient enhanced diffusion can be reduced to different extents by varying the distribution of nitrogen atoms in the junction. This is attributed to the relative location of nitrogen atoms with respect to boron profile and end-of-range defect band, affecting the interactions between dopants and defects upon annealing. In addition, variations in boron dopant activation and deactivation are also observed. Similar to fluorine co-implantation, it is proposed that nitrogen atoms react with vacancy point defects to form nitrogen–vacancy clusters that will trap the interstitials emitted from end-of-range defects. However, we report that the interstitial sink efficiency of nitrogen atoms is not as good as the co-implanted carbon atoms, which is noticed from the dopant deactivation curves. In terms of extended defect evolution, the results clearly indicate that end-of-range defects can be stabilized by choosing the optimized co-implant condition of N. Finally, a deeper physical understanding on the diffusion/clustering pathways of nitrogen co-implantation is achieved to supplement and explain the boron diffusion and deactivation behaviors.

Advanced activation trends for boron and arsenic by combinations of single, multiple flash anneals and spike rapid thermal annealing

Millisecond annealing as an equipment technology provides temperature profiles which favour dopant activation but nearly eliminate dopant diffusion to form extremely shallow, highly electrically activated junctions. For the 45-nm technology node and beyond, precisely controlled gate under-diffusion is required for optimum device performance. Therefore, on boron and arsenic beamline-implanted wafers, various annealing schemes were investigated for the formation of ultra-shallow and custom-shaped junctions. The main scheme consisted of flash annealing with peak temperatures ranging from 1250 to 1300 °C, combined with spike rapid thermal annealing with peak temperatures in the range from 900 to 1000 °C to achieve a desired junction depth. As alternative, to reduce the sheet resistance of pMOS and nMOS source–drain extensions, combinations of two or three flash anneals in succession were tested. Finally, the standard flash anneal condition of a 750 °C intermediate temperature followed by the flash anneal was changed to a high intermediate temperature of 950 °C followed by the flash anneal up to 1300 °C. The results of all these annealing schemes were analysed by four-point probe measurement. Selected samples were analysed by Hall-effect measurements for peak activation, and by secondary ion mass spectrometry for profile shape as well as diffusion effects. Transmission electron microscopy was used to study residual defects. Selected boron and arsenic dopant profiles were also compared to predictive simulation results which address the diffusion and activation at extrinsic concentrations.

High thermal stability of vacancy clusters formed in MeV Si-self-ion-implanted Si

We have shown that considerable vacancy defects, introduced by MeV Si self-ion implantation, can survive a 900°C∕5min annealing for gate formation. By analyzing the trap-limited Si interstitial diffusion, we have characterized these vacancy clusters. Furthermore, we show that the remaining vacancies are sufficient to reduce B diffusion. The study suggests that MeV ion implantation, a promising approach for ultrashallow junction formation in metal-oxide-semiconductor device fabrication, can be inserted before gate formation (involving high temperature annealing) to avoid irradiation damage on gate structures.

Physically based kinetic Monte Carlo modeling of arsenic-interstitial interaction and arsenic uphill diffusion during ultrashallow junction formation

A kinetic arsenic-interstitial interaction model has been developed to study and predict arsenic transient enhanced diffusion (TED) and deactivation behavior during ultrashallow junction (USJ) formation. This model is based on density functional theory and has been verified by previous experiments in which the significant role of interstitial mechanism in arsenic TED was revealed. The mechanism of enhanced and retarded arsenic diffusion in different point defect environments is investigated by utilizing this model in kinetic Monte Carlo simulation. The arsenic-interstitial pair, with low binding energy and low migration energy, is shown to be the major contributor to arsenic TED in silicon interstitial-rich situations. In addition, by using this model, we demonstrate the transient existence of arsenic-interstitial clusters (AsnIm) during postimplant annealing and propose their possible role in deactivation for short time annealings such as laser annealing and spike annealing. Moreover, we have developed a novel surface-trap based kinetic Monte Carlo model to simulate arsenic uphill diffusion in proximity of the Si∕SiO2 interface. The simulation results show that the activation behavior of the uphill portion of arsenic has considerable impact on the junction sheet resistance. The activation behavior of this arsenic is expected to become more important when USJ depth is scaled down further.

Understanding of Boron Junction Stability in Preamorphized Silicon after Optimized Flash Annealing

Recent research in ultrashallow junction formation has been greatly focused on the development of various advanced annealing techniques. Flash annealing has become one of the most likely candidates to achieve the stringent junction requirements for the forthcoming generation of complementary metal oxide semiconductor devices. In this paper, we present an extensive study on the stability of highly active and ultrashallow junction in preamorphized silicon formed by optimized flash annealing. Our results demonstrate a strong improvement in junction stability by using the multiple-pulse and pre-spike rapid thermal anneal flash annealing schemes. The deactivation of the flash-annealed junction is clearly shown to be correlated to the different levels of self-interstitial supersaturation, resulting from the release of excess silicon interstitials from the end-of-range defects. We show that optimized multipulse flash annealing could minimize the interaction between point defects and dopant atoms, enabling improvement in junction properties.

Extended 'Five-Stream' Model for Diffusion of Implanted Dopants in Silicon during Ultra-Shallow Junction Formation in VLSI Circuits

Ion implantation of different dopants (donors and acceptors) into crystalline silicon with subsequent thermal annealing is used for the formation of ultra-shallow p-n junctions in VLSI technology. The experimentally observed phenomenon of transient enhanced diffusion (TED) during annealing hinders further downscaling of advanced VLSI circuits. However, modern mathematical models of dopant diffusion, which are based on the so-called “five-stream” approach, and software packages such as SUPREM4 encounter difficulties in describing TED. In this work, an extended five-stream model for diffusion in silicon is developed, which takes into account all the possible charge states of point defects (vacancies and silicon self-interstitials) and diffusing pairs “dopant atom–vacancy” and “dopant atom–silicon self-interstitial”. The model includes diffusion and drift of differently charged point defects and pairs in the internal electric field and the kinetics of interaction between unlike species. The expressions for diffusion fluxes and sink/source terms that appear in the non-linear, non-steady-state reaction-diffusion equations are derived for both donor and acceptor dopants accounting for multiple charge states of the diffusing species.

Realization of ultrashallow junctions by plasma immersion ion implantation and laser annealing

To achieve the requirements of the 45nm ITRS technology node and beyond, beamline implantation has reached its limit in terms of low energies. Plasma immersion ion implantation (PIII) is thus an alternative doping technique for the formation of ultrashallow junctions for source/drain extension in silicon devices. In this study, the authors present some results obtained on the PIII prototype called PULSION® designed by the IBS French company. In previous works [F. Torregrosa et al., Proceedings of the 14th International Conference on Ion Implant Technology, 2004 (unpublished); Proceedings of the 16th International Conference on Ion Implant Technology, 2006 (unpublished), p. 6], it has been shown that this machine offers the possibility to reach ultralow energy implantations and then to obtain implantation depths of only a few nanometers. One of the main issues is then to highly activate these junctions with a limited diffusion of dopants. Wafers have been implanted by PULSION® with acceleration voltages from 1to2kV at saturation dose, with or without preamorphization implantation (PAI). Then, they have been annealed by a XeCl excimer laser with a wavelength of 193nm, with energy densities from 275to600mJ∕cm2 and several shots. Electrical and physicochemical characterizations such as secondary-ion-mass spectrometry, four-point probe, and optical noncontact measurements were then performed. In this article, the authors investigate the effects of PAI, implantation parameters, and laser fluence on the junction specifications (depth, sheet resistance, and leakage current).

Very shallow boron junctions in Si by implantation and SOD diffusion obtained by RTP

Because of their very large integration capabilities and continuous scaling, the CMOS devices are the basic element in the current-integrated circuits. Their scaling up to sub-micrometric scale presents advantages like diminution of power consumption, faster devices and a larger level of integration. But the physics limitations begin to be important at these dimensions, anomalous effects like hot electrons, leakage currents and punch through, among others, appear. These effects can be reduced if, at the source/drain region, shallow junctions are obtained with junction depth (xj) less than 200nm. To achieve this goal, new junction fabrication methods, which include pre-amorphization [S.D. Kim, C.M. Park, J.C.S. Woo, Formation and control of box-shaped ultra-shallow junction using laser annealing and pre-amorphization implantation, Solid State Electron. 49 (2005) 131–135] are required. Other alternative techniques that do not require ion implantation [T. Uchino, P. Ashburn, Y. Kiyota, T. Shiba, A CMOS-compatible rapid vapor-phase doping process for CMOS scaling, IEEE Trans. Electron Devices 51(1) (2004) 14–19.], in order to prevent surface crystal damage and as a consequence the inhibition of boron interstitial clusters and {311} defects [R.T. Crosby, K.S. Jones, M.E. Law, L. Radic, Dislocation loops in silicon–germanium alloys: the source of interstitials, Appl. Phys. Lett. 87 (192111) (2005) 1–3.], which are the trigger of the “transient enhanced diffusion” (TED) process are used. In this essay, it is shown that rapid thermal process, allow the fabrication of very shallow junctions with a xj less than 300nm by using with high energies and high doses of boron/BF2 ions implantation. By this way the slow dissolution of the dislocation loops, present at the end of range (EOR) of the implanted boron, allow this process. These obtained junctions are compared with those prepared by using the spin on doping (SOD) technique. The diffusion profiles obtained by both processes and their electrical properties are measured and compared for their application as S–D regions in a current CMOS process.

Vacancy engineering for ultra-shallow junction formation

Vacancy engineering has been proven to be a viable alternative to pre-amorphisation and solid phase epitaxy for creating ultra-shallow junctions. This paper investigates the effect of implantation order – i.e. whether the dopant implant is preceded or followed by the vacancy generating co-implant – in terms of Rs and diffusion. It is found that for optimal efficiency the co-implant should be performed first. Ultra-shallow boron layers created using this sequence can result in highly stable, ultra-shallow boron layers which do not deactivate significantly during annealing at 700°C for times ∼15min.

Characteristics of pulsed plasma doping sources for ultrashallow junction formation

Plasma doping of semiconductors is being investigated for low energy ion implantation to form ultrashallow junctions. In plasma doping, ions are extracted from a quasicontinuous plasma using a pulsed bias on the substrate. Plasma-based implantation techniques have the potential for higher throughput than those attainable with conventional accelerator beamlines due to the higher current densities possible with plasma sources. In this work, results from a computational investigation of plasma sources for doping of semiconductors will be discussed. An inductively coupled plasma (ICP) was used to generate ions at pressures of a few to tens of millitorr. A pulsed bias up to −20kV having lengths of tens of microseconds was applied to the substrate to accelerate the ions. Results are presented for Ar∕NF3 gas mixtures which serve as surrogates for the Ar∕BF3 mixtures that would provide boron doping. The consequences of bias voltage waveform, ICP power, operating pressure, and aspect ratio of the reactor on discharge characteristics and ion energy and angular distributions (IEADs) to the substrate will be discussed. The shape of the bias waveform has important consequences on the IEADs not only because of the transit times of the ions but also due to the instabilities that may be launched into the plasma. The aspect ratio of the reactor influences the angular uniformity of the IEADs, particularly when using large biases.

Heat Transport Analysis for Flash Lamp Annealing

The silicon wafer temperature change during flash lamp annealing is theoretically evaluated. A calculation model is developed on the basis of a finite difference method taking into account various heat transport phenomena, such as heat radiation from a lamp to a silicon surface, reflection at the silicon surface, heat radiation from a hot silicon surface, light absorption in silicon, and thermal conduction. The effect of the light absorption in a silicon wafer on the temperature profile is negligible at the position of ultrashallow junction formation. The largest temperature slope is shown to be formed by flash lamp annealing because of the agreement of the temperature slope obtained by taking into account heat absorption in silicon with that obtained by assuming heat absorption only at the surface, without accounting for the light absorption in silicon. This study further concludes that the surface reflectivity strongly affects the silicon temperature.

Perspective on state-of-the-art ultrashallow junction formation

Perspective on state-of-the-art ultrashallow junction formation

Modeling Ultra Shallow Junctions Formed by Phosphorus-Carbon and Boron-Carbon Co-implantation

AbstractA new carbon-interstitial clustering model has been developed. The model has been implemented into the process simulator Sentaurus Process. Model parameters have been calibrated using fundamental marker layer experiments. B diffusion retardation in the C doped layer as well as Sb diffusion enhancement in the region close to a layer with high C concentration are successfully simulated. The calibrated model has been applied to simulations of ultra-shallow junction formation by high dose P-C and B-C co-implantation. It is assumed that, in regions which are amorphized by ion implantation and recrystallized by solid phase epitaxy, C is in the substitutional state right after the recrystallization. In contrast, in non-amorphized regions, C is assumed to be in clusters at the beginning of thermal annealing. A good agreement between simulation and experimental results has been achieved. The dependence of dopant diffusion on implanted C dose and spike annealing temperature has been reproduced.

Experimental evidence of B clustering in amorphous Si during ultrashallow junction formation

The authors have investigated ultrashallow p+∕n-junction formation by solid-phase epitaxy, by using x-ray absorption near-edge spectroscopy measurements on the B K edge. A clear fingerprint of B–B clusters is detected in the spectra. The authors demonstrate that B clustering occurs during the very early stages of annealing-induced Si recrystallization, i.e., when B is still in an amorphous matrix. After complete regrowth the local structure around B remains the same as in the amorphous phase, implying that B clusters are transferred to the crystalline structure.

Millisecond Anneal and Short-Channel Effect Control in Si CMOS Transistor Performance

In this letter, the effects of the millisecond anneal in conjunction with conventional spike anneal on the p-n junction formation in CMOS devices are studied. The results reveal that the millisecond and spike annealing sequence plays an important role in the implanted boron p+/n junction formation. On blanket Si wafers, the millisecond anneal followed by the spike anneal increases implanted boron solid solubility in crystalline silicon by ~18% compared to that obtained using reversed annealing sequence under the same annealing conditions. This result substantially alters the short-channel effect behaviors in the fabricated CMOS devices, resulting in opposite threshold-voltage behaviors in PMOS and NMOS devices when using boron as NMOS halo implant. The results also provide useful insights into ultrashallow-junction formation and short-channel effect control when scaling CMOS technology

Laser annealing of plasma implanted boron for ultra-shallow junctions in Silicon

This work combines plasma doping implantation (PLAD) with laser annealing using excimer laser, for the formation of ultra-shallow junctions. For that purpose, high dose BF3 was implanted in n-type silicon wafers using PLAD. The as implanted material was investigated by high resolution TEM, measured by SIMS and simulated by Monte-Carlo codes. Subsequently, the samples were annealed using a KrF laser source at 248nm with a pulse duration 20ns and different fluence values. Laser annealing completely recrystallizes the amorphous layer as monitored by TEM measurements, fully activates the dopants achieving low sheet resistance values as shown by Van der Pauw measurements and results in box-shaped dopant profiles with movement less than 10nm at the junction depth as measured using SIMS.

Physical insight into ultra-shallow junction formation through atomistic modeling

We use atomistic simulations to gain physical understanding of relevant dopant–defect interactions involved in junction formation. The analysis of the energetics of B–Si interstitial clusters (BICs) indicates that a high Si interstitial supersaturation is necessary to nucleate BICs in crystalline Si, but only a Si interstitial supersaturation slightly larger than that set by BICs is enough to cause the stabilization and growth of preexisting BICs. We have analyzed the mechanisms associated to B uphill diffusion and deactivation in preamorphized Si upon subsequent annealing. Both phenomena occur simultaneously and they are the result of the trapping of B atoms by preexisting B clusters in the high concentration region.

Effect of Pulse Duration on Formation of Ultrashallow Junction by Excimer Laser Annealing

An ultrashallow junction (USJ) was formed by KrF excimer laser annealing. The effect of laser pulse duration on USJ formation was discussed through the evaluations of junction depth, sheet resistance and crystal defects. The possibility of reducing crystal defects by long-pulse laser annealing was discussed. A simple one-dimensional thermal diffusion model was used to analyze the results considering the solidification velocity after melting. The calculated results for the relationship between melt depth and laser energy density qualitatively agreed well with the experimental results. The results also showed that longer pulse duration made solidification velocity lower. Lower solidification velocity is the key to reducing the residual defects.

CVD Delta-Doped Boron Surface Layers for Ultra-Shallow Junction Formation

A new doping technique is presented that uses a pure boron atmospheric/low-pressure chemical vapor deposition (AP/LPCVD) in a commercially available epitaxial reactor to form less than 2-nm-thick δ-doped boron-silicide (BxSi) layers on the silicon surface. For long exposure B segregates at the surface to form a very slow growing amorphous layer of pure B (α-B). The electrical properties of the as-deposited α- B/BxSi stack have been studied by fabricating and measuring diodes where the B depositions are formed directly in the diode contact windows for different exposure times. It is demonstrated that the presented doping technique can be used to form high-quality δ-doped p+n junctions. Moreover, the formed α-B/BxSi layer is an attractive source of high-concentration B dopants for thermal annealing processes that does not induce any transient-enhanced diffusion (TED) effects.

Influence of surface adsorption in improving ultrashallow junction formation

The continual downscaling of silicon devices for integrated circuits requires the formation of transistor (p-n) junctions that are progressively shallower yet incorporate increasing levels of electrically active dopant. In the case of implanted arsenic, the authors show that both goals can be accomplished simultaneously and controllably through the adsorption of small amounts of atomic nitrogen on the Si(100) surface.