Thermal Silicidation Research Articles

Thermal Silicidation of Ni/SiGe and Characterization of Resulting Silicide Films Using Raman Spectroscopy and X-ray Diffraction

For advanced application specific devices, combinations of Si/Ge, Ge/Si, Si1-xGex/Si are frequently introduced in the device fabrication process. Epitaxy, condensation and annealing processes are commonly used for controlling the Ge content to a desired level. Strain and crystallinity can be affected by a small variation in composition or Ge content, which can result in device performance deterioration or failure. Thus, the composition, strain and crystallinity must be carefully monitored and controlled throughout the manufacturing process. The electrode formation process is also very critical for successful device fabrication. Any spikes and/or electrically active defects near junctions can be fatal to the device yield. Silicidation processes and resulting silicides must be investigated as much as possible using various non-invasive material characterization techniques.In this study, we report the thermal silicidation characteristics of Ni/Si1-xGex with various Ge content under different annealing temperatures in the range of 225oC ~ 400oC in N2 ambient. Thermal silicidation was performed in a stacked hotplate-based SAO-302LP system designed for 300mm wafers. For resulting silicide characterization, measurements of sheet resistance, Raman spectra, X-ray diffraction curves as well as scanning electron microscopy (SEM) observations were performed. We have studied Ge content and annealing temperature dependence on the resulting electrical and crystallographic silicide characteristics.Figure 1 shows 647nm excited Raman spectra and X-ray diffraction (XRD) curves from reference Si, and resulting silicide from Ni/Si0.75Ge0.25 before and after annealing under at 235oC, 310oC and 400oC.Raman spectra and sheet resistance measurements showed silicide formation and silicide layer thickening with increasing annealing temperature. XRD curves clearly indicated the change of crystalline phase of resulting silicide with annealing temperature increase. Various aspects of thermal silicidation and its characterization will be discussed at the conference. Figure 1

Thermal Silicidation of Ni/SiGe and Characterization of Resulting Nickel Germanosilicides

Thermal silicidation characteristics of Ni/Si1-xGex with various Ge content was studied under different annealing temperatures in the range of 225 °C ∼ 400 °C in 100% N2 ambient. TiN capped Ni/Si1-xGex/Si/SiO2/Si wafers with x values in the range of 0.15 and 0.30 in 0.05 intervals were used. Thermal silicide formation was performed in a stacked hotplate-based annealing system designed for industrial 300 mm wafer fabs. For silicide characterization, measurements of spectral reflectance, sheet resistance, Raman spectra as well as X-ray diffraction curves were performed to investigate changes of optical properties, electrical properties, crystallographic phases during silicide formation. Effects of Ge content and annealing temperature on the electrical and crystallographic properties of the resulting thermal silicides (nickel germanosilicides) were investigated. Reaction mechanisms were discussed based on the characterization results. Multiwavelingth micro-Raman spectroscopy was found to be very promising as a non-contact, in-line silicidation process monitoring technique. For production worthiness verification of the thermal silicidation process, temperature sensitivity curves of sheet resistance, its uniformity and detailed sheet resistance maps were investigated using 8 nm thick Ni films on 300 mm diameter, epitaxial Si0.8Ge0.2/p−-Si wafers without a capping layer. Manufacturability of Ni/Si0.80Ge0.20/Si wafers were also verified using a stacked hotplate-based, nearly isothermal, annealing system.

Thermal Silicidation of Ni/SiGe and Characterization of Resulting Silicide Films Using Raman Spectroscopy and X-ray Diffraction

Thermal silicidation characteristics of Ni/Si1-xGex with various Ge content was studied under different annealing temperatures in the range of 225oC ~ 400oC in N2 ambient. TiN capped Ni/Si1-xGex/Si/SiO2/Si wafers with x values in the range of 0.15 and 0.30 in 0.05 intervals were used. Thermal silicide formation was performed in a stacked hotplate-based annealing system designed for 300mm wafers. For silicide characterization, measurements of spectral reflectance, sheet resistance, Raman spectra as well as X-ray diffraction curves were performed to investigate optical properties, electrical properties, crystallographic phases and reaction mechanisms. Multiwavelingth micro-Raman spectroscopy was found to be very promising as a non-contact, in-line silicidation process monitoring technique. We have studied effects of Ge content and annealing temperature dependence on the electrical and crystallographic properties of the resulting thermal silicides (nickel germanosilicides).

Optimizing Adhesion of Laser Structured Plated Ni-Cu Contacts with Insights from Micro Characterization

An improved understanding of the interface situation between metal and laser-structured semiconductor for nickel-copper plated solar cell contacts is created by micro characterization using SEM and Raman spectroscopy. A special focus is set on laser ablation with high energy density and the resulting mechanical contact adhesion. This aims to increase the flexibility of the process towards variable cell and emitter designs. The analysis revealed two neglected aspects. First, on laser openings that were created with high laser energy input the thickness of the native oxide exceeds the typical dimension of 1nm. The oxide layer allows metal plating, but hinders the growth of nickel silicide and thus affects the mechanical properties of the contacts. As shown in this work the adhesion can be improved by a more thorough etching of the oxide before plating. Second, Raman spectroscopy revealed that the tension under the laser processed surfaces differs for varying energy inputs only after thermal silicidation was performed. Inadequately chosen laser parameters causes high tension in the laser treated silicon causing decreased adhesion. This work reveals the value of microscopic analysis for the optimization of laser processes for metallization.

Electrical and Mechanical Properties of Plated Ni/Cu Contacts for Si Solar Cells

Plated Ni/Cu/Ag contacts are an industrially feasible metallization approach for high efficiency c-Si solar cells with low surface doping concentrations (1018 cm-3 <ND < 1020 cm-3). The 2d-simulations of this work define the minimum requirements on the contact resistivity of metal contacts in a high efficiency solar cell design. The following experimental study of the contact resistivity of plated Ni/Cu/Ag contacts on lowly doped phosphorus emitter demonstrates low contact resistivities in the mΩcm2 regime, which enable solar cells with high fill factors. Furthermore, the paper analyzes the influence of the thermal silicidation process on pseudo-fill factor losses and on the mechanical contact adhesion. The contact adhesion is also studied with respect to the laser contact opening process. The results of this work demonstrate that the right choice of back-end processes enable plated Ni/Cu/Ag contacts with low contact resistivities in combination with high contact adhesions above 1 N/mm.

Electrical characterization of NiSi/Si interfaces formed by a single and a two-step rapid thermal silicidation

This paper investigates the electrical characteristics of NiSi Schottky barrier diodes (SBD). A single-step rapid thermal process (RTP) and a two-step RTP were employed to form the SBDs. The diode structures were designed so as to minimize edge leakage. The two-step silicidation process resulted in a significant reduction of the reverse leakage current density, suggesting an improvement of the interface characteristics. Temperature-dependent current–voltage measurements were used to analyse the interface characteristics. The two-step RTP process reduces the density of non-ideal micro-contacts with low barrier height, which are responsible for the reverse leakage current.

Anomalous scaling effect of tungsten/titanium nitride/titanium to silicon electrical contact resistance for subquarter micron microelectronic devices

This paper reports the anomalous scaling effect of tungsten/titanium nitride/titanium to (a) n+ and (b) p+ silicon electrical contact resistance in dynamic random access memory (DRAM) devices, upon post heat treatment following rapid thermal silicidation annealing. The electrical measurements on contacts of sizes ranging from 0.54 μm to 0.18 μm reveal that the increase in resistance becomes larger as the contact size decreases. Transmission electron microscopy (TEM) results show that the silicide film agglomeration proceeds more severely as the contact size decreases. To explain the size-dependent degradation of the contact resistance, numerical simulation of the shape evolution of the silicide film is performed. The results show that the poor film coverage, especially at the edge, accelerates the reduction rate in contact area.

Spectroscopic ellipsometry investigation of nickel silicide formation by rapid thermal process

Titanium and cobalt silicides are used widely in microelectronics fabrication. There are limitations for both silicides. With TiSi2, a linewidth dependence of sheet resistance for lines narrower than 0.35 μm has become dramatic. The transformation from the high-resistivity C49 phase to the low-resistivity C54 TiSi2 phase is nucleation limited. With CoSi2 there is much less linewidth dependence of the sheet resistance, but more Si is consumed to form the silicide. With the use of NiSi, these problems can be avoided as reported so far in literature. In this article we shall report an investigation of rapid thermal silicidation of nickel on single crystalline silicon wafers in the annealing range of 150–1150 °C. It has been found that there are five zones in the dependence of sheet resistance on silicidation temperatures as follows: below 175, 175–350, 350–650, 650–900, and above 900 °C. In order to extensively study the phase sequence for Ni/Si reactions and the kinetics of nickel silicide formation corresponding to the sheet resistance zones, the spectroscopic ellipsometry (SE) was used to characterize the thin silicide films. The sensitivity and usefulness of SE characterization for studying the intermetallic growth processes and the application of an appropriate algorithm for extracting the information from experimental data is demonstrated. The results obtained by this nondestructive SE technique are compared with measurements done by the well-established but destructive techniques, Rutherford backscattering, scanning electron microscopy, and energy dispersive x-ray spectrometry. A physical picture of nickel silicidation as a function of temperature is also presented.

Control and Impact of Processing Ambient During Rapid Thermal Silicidation

ABSTRACTThe introduction of rapid thermal processing for silicide formation has triggered a lot of research to temperature uniformity and reproducibility in RTP systems. In addition to the temperature, the ambient control is to be taken into account. Although gasses are specified to a low level of contaminants, the RTP step needs to be optimised for optimal contaminant reduction. Besides, the process wafer itself is a source of contamination.In this paper an overview will be given of the role of RTP ambient on the silicidation processes. The effect of the wafer on ambient purity will be highlighted. It will be shown that the use of a reactive capping layer during silicidation represents an adequate solution for both sources of contamination.

A characterization technique for the degradation characteristics of Ti/Si schottky-barrier diodes and ohmic contacts after thermal silicidation

The effects of thermal silicidation on both the nonideal current-voltage characteristics of the Schottky-barrier diodes and the degradation properties of the ohmic contacts have been self-consistently characterized by using a simple interfacial-layer theory. The abruptly changed nonideality of the measured I– V characteristics as TiSi 2 is formed can be interpreted as the variations of the interface properties, such as the thermal-equilibrium barrier height, the interfacial-layer capacitance, and the density distribution of the interface state. Moreover, different work functions for Ti and TiSi 2 and the change of the dominant type of the interface states and their density distributions are shown to be responsible for the fluctuations of the thermal-equilibrium barrier height after thermal silicidation. Furthermore, the interface parameters extracted from the fabricated Schottky-barrier diodes are used to calculate and compare the simultaneously-processed specific contact resistivities, and satisfactory agreement is obtained. Thus, the silicidation-related degradation characteristics of the ohmic contacts can be directly related to the variations of the interface properties at the Ti/Si interface.

Influence of impurities on ion beam induced TiSi 2 formation

For silicide formation, ion beam induced silicidation seems to be superior to thermal silicidation because the mixing destroys contaminants at the metal/Si interface resulting in reproducible, homogeneous silicide formation. However, it is not known if a high impurity content in the metal film has an influence on ion beam induced silicidation. Therefore, we studied the effect of oxygen and arsenic on TiSi 2 formation by ion beam mixing and subsequent rapid thermal annealing (RTA). For this purpose, Ti layers containing two different oxygen contents were prepared by sputtering. Then, mixing with As and Ge ions at different energies and doses and a subsequent two-step RTA treatment were performed. By RBS and cross-sectional TEM analysis, it could be detected that only Ge mixing with a maximum implanted concentration in the substrate and thermal reaction of pure Ti films resulted in lateral homogeneous TiSi 2 layers. All other mixing conditions led to inhomogenous, high-resistivity silicide layers consisting of large TiSi 2 grains embedded in TiSi. Therefore, it can be concluded that impurities which have a high affinity to Ti can strongly affect ion beam induced silicidation.

Low Resistance and Thermally Stable Ti-Silicided Shallow Junction Formed by Advanced 2-Step Rapid Thermal Processing and Its Application to Deep Submicron Contact

A low-resistivity thermally stable TiSi2 shallow junction applicable to a deep submicron contact has been developed. This was achieved through the use of an advanced 2-step rapid thermal silicidation called the “AAS” process and the use of an Al-based metal/TiN/Ti/TiSi2 contact technique. The low sheet resistance and high thermal stability of TiSi2 film on n+-Si reached the same levels as those on undoped Si, though the silicidation on n+-Si is more difficult than that on p+ or undoped Si. The film and contact resistivities were about 11.5 µΩ cm and 1∼2×10-8 Ω cm2, respectively. Ring oscillators utilizing these processes exhibited a 15% improvement in speed as compared with conventional cases [without self-aligned silicide (SALICIDE)].

A new oxidation-resistant CoSi 2 process for self-aligned silicidation (salicide) technology

This paper presents a new oxidation-resistant and self-aligned CoSi 2 process using amorphous-Si (a-Si)/Co bilayer metallization. It is shown that, even in an environment without introducing any flowing inert gas, the detrimental reaction between Co metal and ambient impurities, e.g. O 2 and H 2O, can be completely impeded by a covered thin a-Si layer during the thermal silicidation cycle. Moreover, despite a capped a-Si layer over Co film, the satisfactory self-aligned silicidation properties can be retained if the initial silicidation temperature is kept below 500°C in N 2 ambient. Furthermore, the dependences of both oxidation-resistant and self-aligned silicidation properties on the thicknesses of a-Si and Co films are studied in detail. Based on experimental analysis, the allowed process window for the a-Si film thickness can be determined and is shown to increase with increasing the Co film thickness. An empirical process rule is experimentally obtained to determine the optimal thickness relation between a-Si and Co films for the a-Si/Co bilayer process. Thus, a simple and practical self-aligned CoSi 2 process with extremely high immunity to ambient impurities is proposed for SALICIDE applications.

Enhancement of boron diffusion through gate oxides in metal-oxide-semiconductor devices under rapid thermal silicidation

It has recently been reported that there is anomalous enhanced diffusion of B through the gate oxide in metal-oxide-semiconductor (MOS) structures from B-implanted, p+-polycrystalline silicon gates upon annealing in the presence of H or F. This letter discusses the effects of TiSi2 formation on B penetration through the gate oxide in p+ polycrystalline silicon gate MOS devices. From secondary-ion mass spectrometry analyses, it is found that B penetration effect is enhanced by TiSi2 formation, for 950 and 1100 °C rapid thermal annealing, in spite of the fact that the F concentration in the gate oxide for samples with silicide is lower than that for samples without silicide. Furthermore, samples with a one-step TiSi2 formation process exhibit more serious B penetration effects than those with a two-step process. This indicates that the effect of silicide on B penetration is more complicated than simply acting as a sink for F. Pileup of B at the silicide/polycrystalline silicon interface, the generation of point defects such as Si vacancies and interstitials during silicide formation, and B-defect interactions must be taken into account to explain the results.

Simultaneous formation of contacts and diffusion barriers for VLSI by rapid thermal silicidation of TiW

The formation of (TixW1−x)Si2/(TixW1−x)N, by rapid thermal processing of TixW1−x on Si in an N2 ambient is investigated. An activation energy of 1.7 eV is obtained for silicide formation. A distinct snow-ploughing of As atoms is observed during silicide formation whereas the interfacial B concentration decreases with increasing silicide formation temperature. The diffusion barrier properties of the (TixW1−x)Si2/(TixWi1−x)N stack in contact with Al is investigated upon post-metal annealing. No interaction between the layers is found for temperatures as high as 475°C after 60 min. The improved thermal stability of the (TixW1−x)N layer in contact with Al is attributed to nitrogen blocking of the grain boundaries.

Study of the rapid thermal nitridation and silicidation of Ti using elastic recoil detection. I. Ti on Si

Study of the rapid thermal nitridation and silicidation of Ti using elastic recoil detection. I. Ti on Si

The effects of thermal silicidation of the current transport characteristics of Ti/〉111〈Si Schottky-barrier contacts

Using the interfacial-layer theory and assuming the Shockley-Read-Hall (SRH) statistics for the nonequilibrium occupancy of the interface states, the effects of Ti/〉111〈Si contact reactions on the nonideality of the current-voltage characteristics measured from the TiSi x (0 ⩽ x ⩽ 2)/ p( n)-Si Schottky-barrier diodes have been studied. From the analyses obtained by Auger Electron Spectroscopy (AES), X-ray diffraction, and I– V measurements, the abrupt change of thermal-equilibrium barrier height with respect to silicidation temperature is shown to be coincident with the drastic change of the measured ideality-factor in the forward I– V characteristics. This phenomenon has been justified to be correlated with the formation of stoichiometric TiSi 2 at the Ti/Si interface by thermal reactions. From the interface-state-apparent-spectrum (ISAS) measurements using both constant-temperature-Schottky-capacitance-spectroscopy (CTSCS) and I– V methods, we have shown that the transition of electrical characteristics should be attributed to the large variations of the ISAS when the TiSi 2 starts to form at the Ti/Si interface. Moreover, with the help of a simplified occupation function model for the interface states, the observed transition can be interpreted as the changes of thermal-equilibrium barrier height, real interface-state density, and effective minority-carrier to majority-carrier capture rate ratio.