Shallow Junction Research Articles

Improved vertical MOSFET performance using an epitaxial channel and a stacked silicon-insulator structure

A vertical MOSFET (VMOST) incorporating an epitaxial channel and a drain junction in a stacked silicon-insulator structure is presented. In this device structure, an oxide layer near the drain junction edge (referred to as a junction stop) acts as a dopant diffusion barrier and consequently a shallow drain junction is formed to suppress short channel effects. To investigate the scalability of this device, a simulation study in the sub-100 nm regime calibrated to measured results on the fabricated devices is carried out. The use of an epitaxial channel delivers 50% higher drive current due to the higher mobility of the retrograde channel and the junction stop structure delivers improvements of threshold voltage roll-off and drain-induced barrier lowering compared with a conventional VMOST.

A study on the performance of metal-oxide-semiconductor-field-effect-transistors with asymmetric junction doping structure

Diode currents of MOSFET were studied and characterized in detail for the ion implanted pn junction of short channel MOSFETs with shallow drain junction doping structure. The diode current in MOSFET junctions was analyzed on the point of view of the gate-induced-drain leakage (GIDL) current. We could found the GIDL current is generated by the band-to-band tunneling (BTBT) of electrons through the reverse biased channel-to-drain junction and had good agreement with BTBT equation. The effect of the lateral electric field on the GIDL current according to the body bias voltage is characterized and discussed. We measured the electrical doping profiling of MOSFETs with a short gate length, ultra thin oxide thickness and asymmetric doped drain structure and checked the profile had good agreement with simulation result. An accurate effective mobility of an asymmetric source–drain junction transistor was successfully extracted by using the split C–V technique.

Gettering Behavior of Transition Metals in Low Energy, High Dose Ion Implanted Silicon

Metallic contamination on silicon surfaces has a detrimental impact on ULSI device performance and yield. Surface metal impurities degrade gate oxide integrity, while metal impurities dissolved in silicon provide recombination centers, resulting in junction leakage. Surface metal impurities penetrate silicon by the collision with dopant during ion implantation and are also diffused in silicon by subsequent annealing [. If metal impurities are present around junctions, they can cause junction leakage. In order to avoid the junction leakage, metal impurities must be away from junction. It was reported that iron can be gettered in the region of dopants implanted at high energy [. On the other hand, little work has been reported on the behavior of metal impurities in shallow junction. In this study, the gettering behavior of various transition metals in low-energy and high-dose ion-implanted silicon has been demonstrated.

Optimization of Amorphous Si/Crystalline Si Heterojunction Solar Cells by BF2 Ion Implantation

In this study we evaluated two approaches to improving the efficiency of amorphous Si/crystalline Si (a-Si/c-Si) heterojunction solar cells by BF2 ion implantation. First, emitter layer formation was compared for the cases of B and BF2 ion implantation when using the same 7° tilt angle. Second, emitter layer formation was compared between a 7° tilt angle and a 60° tilt angle when using BF2 ion implantation. The experimental results reveal that the fluorine in BF2 passivates the defects at the a-Si and a-Si/c-Si interface, and ion implantation at a high 60° tilt angle forms a shallow solar cell junction. The emitter layer formed by BF2 ion implantation with a 60° tilt angle in an a-Si/c-Si heterojunction solar cell achieves the highest short circuit current density (JSC) of 36.85 mA/cm2 with a conversion efficiency (η) of 14.41%.

Optimization of Amorphous Si/Crystalline Si Heterojunction Solar Cells by BF2Ion Implantation

In this study we evaluated two approaches to improving the efficiency of amorphous Si/crystalline Si (a-Si/c-Si) heterojunction solar cells by BF2 ion implantation. First, emitter layer formation was compared for the cases of B and BF2 ion implantation when using the same 7° tilt angle. Second, emitter layer formation was compared between a 7° tilt angle and a 60° tilt angle when using BF2 ion implantation. The experimental results reveal that the fluorine in BF2 passivates the defects at the a-Si and a-Si/c-Si interface, and ion implantation at a high 60° tilt angle forms a shallow solar cell junction. The emitter layer formed by BF2 ion implantation with a 60° tilt angle in an a-Si/c-Si heterojunction solar cell achieves the highest short circuit current density (JSC) of 36.85 mA/cm2 with a conversion efficiency (η) of 14.41%.

Physical understanding of cryogenic implant benefits for electrical junction stability

We investigate the effect of cryogenic temperature implants on electrical junction stability for ultra shallow junction applications for sub-32 nm technology nodes and beyond. A comprehensive study was conducted to gain physical understanding of the impact of cryogenic temperature implants on dopant-defect interactions. Carborane (C2B10H12) molecule, a potential alternative to monomer boron was implanted in carbon preamorphized silicon substrates at cryogenic implant temperatures. Results indicate implants at cryogenic temperatures increase dopant activation with reduced diffusion, resulting in lower sheet resistance for a lower junction depth. Further, this study emphasizes the benefits of co-implants performed at cryogenic temperatures as alternative to traditional preamorphizing implants.

CMOS Leakage and Power Reduction in Transistors and Circuits: Process and Layout Considerations

Power reduction in CMOS platforms is essential for any application technology. This is a direct result of both lateral scaling—smaller features at higher density, and vertical scaling—shallower junctions and thinner layers. For achieving this power reduction, solutions based on process-device and process-integration improvements, on careful layout modification as well as on circuit design are in use. However, the drawbacks of these solutions, in terms of greater manufacturing complexity (and higher cost) and speed degradation, call for “optimized” solutions. This paper reviews the issues associated with transistor scaling and related solutions for leakage and power reduction in terms of topological design rules and layout optimization for digital and analog transistors. For standard cells and SRAMs cells, leakage aware layout optimization techniques considering transistor configuration, stressors, line-edge-roughness and more are presented. Finally, different techniques for leakage and power reduction at the circuit level are discussed.

Nickel Silicide Formation Using Excimer Laser Annealing

In this work, we report on a self-aligned nickel silicide formation technique based on excimer laser annealing (ELA). We evaluate this process for the front contact formation of industrial PERC type solar cells on random pyramid textured Si surfaces where damage to surface texture, emitter passivation, or to the shallow junction should be avoided or minimized. PERC type solar cells obtained by POCl3 diffusion were processed on large area (12.5x12.5cm2) CZ-Si. Self-aligned litho-free Ni/Cu contacts defined by ps-laser ablation of the SiO2/SiNx anti-reflective coating (ARC) and subsequent ELA of the Ni layer were compared to conventional Ag screen printed contacts.The novel ELA process results in an absolute gain in Jsc of 0.8mA/cm2 as well as a drop of 0.3Ω.cm2 in series resistance (Rs) compared to SP Ag contacts due to reduced shading and resistance losses. This leads to 0.5% absolute increase in efficiency from 19.3% to 19.7% since other characteristics (Voc, pFF) could be maintained to the same level. In this work, the best performing cell with the ELA process reached an outstanding 20.0% energy conversion efficiency with Jsc=39.3mA/cm2, Voc=649.8mV, and FF=78.3%.

The breakdown mechanism of a high-side pLDMOS based on a thin-layer silicon-on-insulator structure

A high-side thin-layer silicon-on-insulator (SOI) pLDMOS is proposed, adopting field implant (FI) and multiple field plate (MFP) technologies. The breakdown mechanisms of back gate (BG) turn-on, surface channel punch-through, and vertical and lateral avalanche breakdown are investigated by setting up analytical models, simulating related parameters and verifying experimentally. The device structure is optimized based on the above research. The shallow junction achieved through FI technology attenuates the BG effect, the optimized channel length eliminates the surface channel punch-through, the advised thickness of the buried oxide dispels the vertical avalanche breakdown, and the MFP technology avoids premature lateral avalanche breakdown by modulating the electric field distribution. Finally, for the first time, a 300 V high-side pLDMOS is experimentally realized on a 1.5 μm thick thin-layer SOI.

Advanced alignment technique for precise printing over selective emitter in c‐Si solar cells

ABSTRACTSignificant improvements in c‐Si solar cells performance can be achieved acting on the emitter or on the contact formation side. At the moment, an efficiency gain around 0.5–0.6%abs [1] has been demonstrated for selective emitter (SE) cells, mainly caused by the improvement of the blue response of the shallow emitter. Nevertheless, in case of sub‐optimal alignment of the printed metal with the deep emitter, the shallow junction depth leads to a significantly narrower firing process window, consequently causing high contact resistance and poor shunting behavior. Moreover, after Anti Reflective Coating (ARC) the contrast between differently doped areas is poor and often prevents the screen printer vision system from correctly recognizing the pattern for metal printing alignment. Thus, we developed a new equipment, high precision selective emitter, for precise alignment over SE able to guarantee recognition of every SE pattern and consequent printing repeatability within +/‐30 µm without rejections or misalignments. In this work, results from experimental investigations and mass production are reported and discussed, including also comparison with standard alignment methods and further feasible applications. New equipment for precise alignment over SE able to guarantee recognition of every SE pattern and consequent printing repeatability within +/‐30μm without rejections or misalignments has been developed (examples of SE patterns are reported in the following pictures). Test performed at production sites confirm that an efficiency gain of 0.1% and a significant reduction of rejection can result from the increased robustness of the presented method. Finally, also a correlation between sheet resistance of heavily doped area and image contrast has been demonstrated. Copyright © 2011 John Wiley & Sons, Ltd.

ChemInform Abstract: Nucleation and Growth of Epitaxial Silicide in Silicon Nanowires

AbstractReview: 139 refs.

Low-Complexity Full-Melt Laser-Anneal Process for Fabrication of Low-Leakage Implanted Ultrashallow Junctions

Good-quality ultrashallow n + p junctions are formed using 5-keV amorphizing As+ implantations followed by a single-shot excimer laser anneal for dopant activation. By using an implant that is self-aligned to the contact windows etched in an oxide isolation layer, straightforward processing of the diodes is achieved with postimplantation processing temperatures kept below 400°C. A possible source of junction leakage at the perimeter caused by dip-etch enlargement of the contact window, also confirmed by transmission electron microscopy (TEM) analysis, is identified, and diode performance is improved by increasing the junction/contact window overlap. The optimum performance in terms of low leakage, shallow junctions, and low resistivity is achieved for 30° tilted implants and by applying a thin laser-reflective aluminum layer. This work isolates the minimum requirements for achieving low-leakage diode characteristics.

Formation of shallow junctions in gallium and phosphorus compensated silicon for cell performance improvement

Solar cells have been fabricated from gallium and phosphorus compensated Czochralski silicon wafer. It is found that these solar cells have shallower n+p junctions, compared to those based on conventional silicon. The cell efficiencies under 1 sun illumination are on average 0.2% higher than those of conventional solar cells. This improvement is associated with the better spectral response in the short-wavelength range, due to the formation of shallower junctions.

Numerical Analysis for Radiation Resistant InGaP Solar Cell

Numerical analyses are carried out to optimize radiation resistant of InGaP solar cell under the effect of low energy protons. The radiation degradation parameters of InGaP used to investigate the effect of cell configuration, base thickness, junction depth, base carrier concentration, and AlInP window layer. Numerical analyses for both cells structure n–p and p–n shows that, the n–p structure is more radiation resistant in a solar cell with shallow junction (0.05 µm), thin base thickness (0.4 µm) and low base carrier concentration (1 ×1016 cm-3), while p–n cell structure is relatively radiation resistant in deep junction solar cell (0.1 µm) with thin base thickness and low base carrier concentration. The formation of AlInP window layer increases the radiation resistant of p–n cell structure, while no significant change in the radiation resistant of n–p cell structure was observed due to the formation of AlInP.

Improvement of Heavy Dopant Doped Ni-Silicide Using Ytterbium Interlayer for Nano-Scale MOSFETs with an Ultra Shallow Junction

In this paper, a novel Ni silicide with Yb interlayer (Yb/Ni/TiN) on a boron cluster (B18H22) implanted source/drain junction is proposed for the first time, and its thermal stability characteristics are analyzed in depth. The proposed Ni-silicide exhibits a wider RTP temperature window for uniform sheet resistance, surface roughness and better thermal stability than the conventional structure (Ni/TiN). In addition, the cross-sectional profile of the proposed Ni-silicide showed less agglomeration despite the high temperature post-silicidation annealing, and it can be said that the proposed structure was little dependence on the temperature post-silicidation annealing. The improvement of Ni silicide properties is analyzed and found to be due to the formation of the rare earth metal--NiSi (YbNi2Si2), whose peaks were confirmed by XRD. The junction leakage current of the p + -n junction with Yb/Ni/TiN and B18H22 implantation is smaller than that with Ni/TiN by almost one order of magnitude as well as improving the thermal stability of ultra shallow junction.

Reconstruction of the last marine connectivity of the Zagros basin, based on fossil evidences of the clastic-dominated Bakhtiari Formation

In this study, new paleontology and lithology foundations on the Bakhtiari Formation were obtained from Shalamzar area in Zagros basin. A depositional mollasic cycle was divided into Bk1, Bk2 and Bk3 units from the base to the top of the Formation. Bk2, with limestone- and mudstone-dominated deposits, was found to alternate with conglomerates and form a marine succession between the two limestone clast-supported conglomeratic continental units. A diverse fossil association has been described for the first time and consists of foraminifera (Oolina spp., Oolina hexagona, Textularia spp., Spiroloculina spp., Peneroplis spp., Elphidium spp., Planulina sp., Amphistegina spp., Lagena spp. Globigerina spp., Globigerinoides spp., Uvigerina spp., Triloculina spp., Quinqueloculina spp., Spiroloculina spp. Nodosariid., Orbitina spp., Borelis melo, Bolivina spp. and Miogypsina spp.), a gastropod community of Strombus spp., Potamides spp., Turitella spp., Cyprea cf. persica and Pleurotomaria sp. and a higher energy colony of scleractinian corals comprising Porites branching, Porites-encrusting massive Favites spp., Favia spp., Tarbellastrea spp. and Acropora spp. The early–middle Miocene marine level of the Bakhtiari Formation, confirms a nearshore accommodation through a middle shelf and an open coastal lagoon (inner shelf) within a fluvial-dominated delta or open estuary during the last presence of a marine trough in the Zagros basin. This saline to hypersaline shallow marine regime was probably dominated in tropical to subtropical conditions. Palaeobiogeography analysis reflects a last seaway faunal exchange between the area and early–middle Miocene basins of Pakistan, India, Australia, Turkey, Greece, Romania Algeria, Egypt, Italy, France, Portugal and Austrian/Hungarian border, which reconstructs a shallow junction between Zagros basin and Mediterranean Tethys and Indo-pacific Ocean during Burdigalian–Langhian (Badenian) before the terminal Tethyan event in late middle Miocene and Tortonian.

Precise Control of Spike Anneal Process for Advanced CMOS

Aggressive scaling of planar bulk CMOS device has resulted in the need for ultra-shallow junctions (USJ) formation. This represents some special concern are not only integration solution, but also the individual process precise control to meet the evolutional requirements. In the USJ particular, as junction thickness (depth) decreases, the series resistance of the junction increases. Therefore, the dopants concentration must to be increased to improve the resistance. Dilemma, the diffusion is an important issue in shallow junction technology. The spike anneal process was widely adopted at advanced CMOS fabrication to meet the device requirements. In this paper, how to precisely control the processes of spike anneal is discussed. The two major interacted process factors, peak-temperature and residence-time must be separately tunable, not only to meet the requirements of device parameter, but also to achieve an operational precise controlling for process setup, optimization and maintenance.

Abnormal Dopant Distribution in $\hbox{POCl}_{3}$-Diffused $\hbox{N}^{+}$ Emitter of Textured Silicon Solar Cells

We investigated 2-D dopant distribution in a -diffused emitter formed on textured Si solar cells using transmission electron microscopy (TEM) combined with selective chemical etching. TEM and simulation results demonstrate that the convex and concave regions of a pyramid in the textured Si surface show deeper and shallower junctions, respectively. By considering a strong dependence of phosphorus (P) diffusion on the Si interstitials, the abnormal profile of emitter in the textured Si surface could be attributed to the inhomogeneous distribution of Si interstitials caused by the geometry of the pyramid texture.

Electronic Structures of Donor-Acceptor-Donor Trimer Codopants in Silicon

The total energies of donor-acceptor-donor D2A trimer codopants (D = As and Sb, A = B, Al, Ga, In and Tl) in Si and their electronic geometrically stable structures were studied using ab initio calculations in order to propose new dopants for the formation of ultra shallow junctions with high carrier concentrations in the source/drain regions. The results of the calculations indicated that the trimer codopants were formed in Si and were stable. The trimer codopants are also able to activate the inactive complexes As2V and Sb2V by codoping acceptor atoms that occupy vacant sites. In particular, As2Al, As2Ga, Sb2B and Sb2Ga resulted in both shallower donor levels and higher solid solubility compared to traditional single donor atoms such as As and Sb.

Application of positron annihilation technique to front and backend processes for modern LSI devices

Developments of the semiconductor industry depend on ability to decrease the size of integrated circuits. Significant efforts to introduce new materials and their fabrication processes have been made in order to achieve such trends. However, one faces difficulties in the control of materials properties caused by atomic scale disorder. In the present paper, we used monoenergetic positrons to characterize ultra shallow junction formed on Si substrates and materials buried in LSI circuit structures. It was shown that the positron annihilation technique is a powerful characterization tool for modern Si-technology related materials.