Houda Graoui

Optimizing boron junctions through point defect and stress engineering using carbon and germanium co-implants

We report the fabrication of p+∕n junctions using Ge+, C+, and B+ co-implantation and a spike anneal. The best junction exhibits a depth of 26nm, vertical abruptness of 3nm∕decade, and sheet resistance of 520Ohm∕square. The junction location is defined by where the boron concentration drops to 1018cm−3. These junctions are close to the International Technology Roadmap specifications for the 65nm technology node and are achieved by careful engineering of amorphization, stresses, and point defects. Advanced simulation of boron diffusion is used to understand and optimize the process window. The simulations show that the optimum process completely suppresses the transient-enhanced diffusion of boron and the formation of boron-interstitial clusters. This increases the boron solubility to 20% above the equilibrium solid-state solubility.

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.

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.

B clustering in amorphous Si

The authors have investigated ultrashallow p+∕n-junction formation by solid-phase epitaxy, by using x-ray absorption near-edge spectroscopy (XANES) measurements at the B K edge. The authors demonstrate that B clustering occurs during the very early stages of annealing-induced Si recrystallization, i.e., when B is still in the 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. The XANES structure are assigned to B–B sp2 bonds that are present in B clusters with two or more B atoms. Boron clustering and diffusion are further investigated by means of concentration profile analysis of ad hoc amorphous on insulator structures that evidences a clear concentration threshold for clustering and a concentration dependent B diffusion.

Optimization of Fluorine Co-implantation for PMOS Source and Drain Extension Formation for 65nm Technology Node

The main challenges for PMOS ultra shallow junction formation remain the transient enhanced diffusion (TED) and the solid solubility limit of boron in silicon. It has been demonstrated that low energy boron implantation and spike annealing are key in meeting the 90 nm technology node ITRS requirements. To meet the 65 nm technology requirements many studies have used Fluorine co-implantation with boron and Si + or Ge + pre-amorphization (PAI) and spike annealing. The Fluorine has been shown to reduce TED but its energy needs to be well optimized with respect to the Boron implant energy. In this work we demonstrate that the fluorine dose need also to be carefully optimized otherwise a reverse effect can be observed. We will also show that the fluorine energy needs to be optimized with respect to boron energy and the final junction depends less on the Ge + preamorprhization energies between 2keV and 20 keV. Finally we will show that if an HF etch is implemented after Ge + PAI, improvements in both the junction depth

Nitric oxide rapid thermal nitridation for Flash memory applications

Rapid thermal annealing in nitric oxide (RTNO) has long been used for the formation of ultrathin silicon oxynitride gate dielectrics. Nitric oxide (NO) furnace anneals are used in the formation of floating gate Flash memory transistor tunnel oxides. Nitrogen is thus, incorporated to improve the oxide reliability during program/erase cycling endurance and data retention. We present here a study of rapid thermal annealing and oxide growth in nitric oxide using Applied Materials single-wafer rapid thermal process (RTP) that enables the RTNO anneal to operate at higher temperatures compared to furnace, thereby allowing two times greater incorporation of nitrogen at the silicon/silicon dioxide interface. At 1200°C, a greater than 11% peak interface nitrogen concentration as measured by secondary ion mass spectroscopy (SIMS) in a 75 Angstrom SiON film is achieved. Reliability testing using a floating gate flash memory capacitor with minority carrier source (implants) test vehicle shows that this increase in the peak interface nitrogen results in an improvement in the tunnel oxides program/erase cycling endurance and data retention. For future memory devices, for example 3D memory devices, the use of direct RTNO oxide growth for dielectric formations is possible. In this case, higher temperatures allow the growth of thicker oxides in pure NO at 1200°C, with greater nitrogen incorporation.

Ultrathin SiO 2 interface layer growth

A variety of processes based on radical oxidation (N2O/H2) and spike RTO are investigated in this study to grow ultrathin SiO2 layers. Their process space is mapped out to cover regimes of interest for gate-last or gate-first integration of high k dielectrics with metal gates. Applieds Centura RTP chamber is found to be readily compatible with the requirements associated with 22/20nm CMOS technology.

Nonthermal illumination effects on ultra-shallow junction formation

In this letter, we present direct and unambiguous experimental evidence for nonthermal illumination effects in boron or arsenic implanted silicon. Both, dopant diffusion and activation vary significantly with illumination. Depending on annealing temperature, diffusion is either enhanced or inhibited. The results have significant implications for modeling and formation of ultrashallow junctions.

Optimization of P+/N junction formation using solid phase epitaxy for the 100 nm technology node and beyond

P+/N diodes are fabricated using a low energy boron implant followed by solid phase epitaxial (SPE) growth at 600°C. The pre-amorphization step was done using Germanium at either 10 keV or 80 keV, both with a dose of 1 × 1015 ions/cm2. Next, boron was implanted at a range of energies from 0.5 keV to 5 keV. The sheet resistance (Rs) measurements and the secondary ion mass spectrometry (SIMS) analysis from the SPE based diodes showed very good results that meet the Junction depth and the sheet resistance requirements for the 100 nm and 70 nm technology nodes using a 10 keV Ge+ pre-amorphization and sub-keV boron implant. However, these diodes were leaky because of the end of range (EOR) defects positioned within their depletion regions. At higher boron energies (2-5 keV), the remaining EOR defects from the 10 keV germanium pre-amorphization step were positioned closer to the surface and farther from the depletion region. These diodes showed lower leakage current densities by two orders of magnitude and a breakdown voltage greater than -4 V. This highlights the strong relationship between the SPE diode characteristics and the remaining EOR position with regards to the depletion region.

Ultra-Shallow Junctions for the 65nm Node Based on Defect and Stress Engineering

The co-implantation of germanium, carbon, and boron with the optimum implant energies and doses makes it possible to create p + /n junctions with the sheet resistance of less than 600 Ohm/square and the slope of less than 3 nm/decade. The narrow process window is based on careful engineering of the amorphization, point defects, and stresses and includes standard 1050°C spike annealing. The germanium pre-amorphization suppresses the ion channeling for the subsequent boron implant. The tensile stress induced by the substitutional carbon atoms and the compressive stress induced by the substitutional germanium atoms slow down boron diffusion and help to make the junctions shallower. The stress gradient in the transition region from the strained carbon and germanium doped layers to the relaxed silicon underneath creates an uphill boron flux that makes the junction slope steeper. The optimum amount of carbon is placed in between the implanted boron and the implant damage, which is located below the amorphized layer. During the annealing, the carbon atoms capture silicon interstitials that are coming from the implant damage and form carbon-interstitial clusters. The analysis demonstrates that it is possible to capture over 95% of the interstitials this way before they have a chance to reach boron-doped layer. This completely suppresses the transient-enhanced boron diffusion (TED) and drastically reduces the amount of boron that is deactivated in boron-interstitial clusters (BICs). In fact, the point defect engineering with an optimized carbon profile allows to remove all non-equilibrium silicon interstitials that are generated by the following three sources: the implant damage below the amorphized layer, the rapid temperature ramp down, and the interstitials generated by boron at high concentrations (due to the effect known as boron-enhanced diffusion (BED)). The latter effect leads to significant increase of the apparent boron activation level beyond the well-characterized solid-state solubility level. We explain this effect as a reduction in formation of BICs due to the lack of interstitial supersaturation. In carbon-free silicon, high concentration boron is always accompanied by the non-equilibrium interstitials, coming from either the implant damage or the BICs even if boron is introduced into silicon by pre-deposition instead of the implantation. Extensive experiments and theoretical analysis based on simulation of the interaction of Ge, C, I, and B atoms, as well as the stress effects, point to the optimized process flow that improves the shape and parameters of the p + /n USJs.