Novel submicrometer MOS devices by self-aligned nitridation of silicide

A new MOS technology is developed for submicrometer MOS devices. In this new technology, TiSi2is formed on the source and drain diffused layers by self-aligned silicidation to reduce the sheet resistance, and TiN is formed in the contact holes by self-aligned nitridation of TiSi2. This TiN can be used as an effective barrier metal between Al and Si. TiSi2is prepared by a two-step annealing method to prevent a reaction between Ti and the field oxide. PSG cap annealing after TiSi2formation provides excellent p-n junction characteristics and relatively low silicide sheet resistance of 4 ω/□ even after annealing at 950°C for 30 min. TiN is formed by direct thermal nitridation of TiSi2in N2ambient at a temperature higher than 900°C after contact hole formation. The formation of TiN is confirmed by AES, ESCA, and X-ray diffraction analysis. The TiN formed by direct thermal nitridation is found to prevent Al diffusion into the Si substrate even for post-metallization annealing at 500°C for 1 h. The characteristics of devices fabricated by this new technology also are determined.     

Correlation of microstructure with electrical behavior of Ti/GaN schottky contacts

We correlate structural and electrical characteristics of as-deposited and low-temperature annealed Ti contacts on GaN. Temperature dependent currentvoltage measurements are used to determine the effective barrier heights of the respective contacts, while high-resolution transmission electron microscopy is utilized for structural characterization. As-deposited Ti contacts are slightly rectifying with an effective barrier height of ∼200 meV. After annealing at 230°C, the barrier height increases to values of ∼450 meV. A similar behavior of Schottky contacts with more strongly rectifying diodes upon low-temperature annealing is observed for Zr metal contacts on GaN. As-deposited Ti already forms a thin TiN layer at the GaN interface. After annealing at 230°C, the average thickness and the distribution of TiN grains remain practically unchanged, but the interface with GaN roughens. We correlate the observed barrier height changes with interface roughness and phase formation and we discuss the results in terms of interface damage and the Schottky-Mott theory.     

Titanium nitride films with high oxygen concentration

Titanium nitride (TiN) films with 10–15% oxygen incorporation were prepared by direct-current reactive sputtering in a mixture of argon and nitrogen with an electronicgrade titanium target. These TiN films were found to exhibit a tensile stress and a grain size of about 200Å. No degradation in barrier properties was detected by Rutherford backscattering spectroscopy, Auger profiling and x-ray diffraction even after a 30 min 560° C argon anneal. These TiN films formed Schottky barrier diodes on n(100)-Si with nearly ideal barrier properties and a barrier height of 0.55 eV. The conduction mechanism in the TiN/p(100)-Si diodes was found to be quasi-ohmic. A work function of 4.5 eV was obtained for these TiN films.     

Characterization of TiN/TiSi2 bilayer for application to ULSI

The properties of TiN/TiSi₂ bilayer formed by rapid thermal annealing (RTA) in an NH₃ ambient after the titanium film is deposited on the silicon substrate is investigated. It is found that the formation of TiN/TiSi₂ bilayer depends on the RTA temperature and a competitive reaction for the TiN/TiSi₂ bilayer occurs at 600°C. Both the TiN and TiSi₂ layers represent titanium-rich films at 600°C anneal. The TiN layer has a stable structure at 700°C anneal while the TiSi₂ layer has C₄₉ and C₅₄ phase. Both the TiN and TiSi₂ layers have stable structures and stoichiometries at 800°C anneal. When the TiN/TiSi₂ bilayer is formed, the redistribution of boron atoms within the TiSi₂ layer gets active as the anneal temperature is increased. According to secondary ion mass spectroscopy analysis, boron atoms pile up within the TiN layer and at the TiSi₂−Si interface. The electrical properties for n⁺ and p⁺ contacts are investigated. The n⁺ contact resistance increases slightly with increasing annealing temperature but the p⁺ contact resistance decreases. The leakage current indicates degradation of the contact at high annealing temperature for both n⁺ and p⁺ junctions.     

The effects of process conditions and substrate on copper MOCVD using liquid injection of (hfac)Cu(vtmos)

Copper MOCVD (metalorganic chemical vapor deposition) using liquid injection for effective delivery of the (hfac)Cu(vtmos) [1,1,1,5,5,5-hexafluoro-2,4-pentadionato(vinyltrimethoxysilane) copper(I)] precursor has been performed to clarify growth behavior of copper films onto TiN, <100> Si, and Si₃N₄ substrates. Especially, we have studied the influences of process conditions and the substrate on growth rates, impurities, microstructures, and electrical characteristics of copper films. As the reactor pressure was increased, the growth rate was governed by a pick-up rate of (hfac)Cu(vtmos) in the vaporizer. The apparent activation energy for copper growth over the surface-reaction controlled regime from 155°C to 225°C was in the range 12.7–32.5 kcal/mol depending upon the substrate type. It revealed that H₂ addition at 225°C substrate temperature brought about a maximum increase of about 25% in the growth rate compared to pure Ar as the carrier gas. At moderate deposition temperatures, the degree of a <111> preferred orientation for the deposit was higher on the sequence of <Cu/Si<Cu/TiN<Cu/Si₃N₄. The relative impurity content within the deposit was in the range 1.1 to 1.8 at.%. The electrical resistivity for the Cu films on TiN illustrated three regions of the variation according to the substrate temperature, so the deposit at 165°C had the optimum resistivity value. However, the coarsened microstructures of Cu on TiN prepared above 275°C gave rise to higher electrical resistivities compared to those on Si and Si₃N₄ substrates.     

Thermal Stability of Nitride-Based Diffusion Barriers for Ohmic Contacts to <Emphasis Type="Italic">n</Emphasis>-GaN

The use of TaN, TiN, and ZrN diffusion barriers for Ti/Al-based contacts on n-GaN (n ∼ 3 × 10¹⁷ cm⁻³) is reported. The annealing temperature (600–1,000°C) dependence of the Ohmic contact characteristics using a Ti/Al/X/Ti/Au metallization scheme, where X is TaN, TiN, or ZrN, deposited by sputtering was investigated by contact resistance measurements and Auger electron spectroscopy (AES). The as-deposited contacts were rectifying and transitioned to Ohmic behavior for annealing at ≥600°C. A minimum specific contact resistivity of ∼6 × 10⁻⁵ Ω-cm⁻² was obtained after annealing over a broad range of temperatures (600–900°C for 60 s), comparable to that achieved using a conventional Ti/Al/Pt/Au scheme on the same samples. The contact morphology became considerably rougher at the high end of the annealing range. The long-term reliability of the contacts at 350°C was examined; each contact structure showed an increase in contact resistance by a factor of three to four over 24 days at 350°C in air. AES profiling showed that the aging had little effect on the contact structure of the nitride stacks.     

Electromigration mechanisms in aluminum lines

Special test structures were used for investigating electromigration mechanisms. Large-grained Al lines of different lengths and widths were interconnected by varying TiN auxiliary layers. Test current densities lay between 4 × 10⁵ A/cm² and 6 × 10⁵ A/cm² at 200°C. Considering electromigration threshold, grain boundary electromigration was eliminated and interface electromigration appeared, affecting the conductive Al/TiN interface. Interface electromigration clearly contributes to the mass flow of Al lines, and thus can be detrimental for the reliability of metallization. The interface diffusion activation energy is comparable to the grain boundary activation energy. Contrary to a conductive interface, the technical Al surface does not contribute to mass flow. The elimination of interface effects finally brings out homogeneous bulk electromigration. The drift velocity was directly measured after a stress period of 8300 hours at 200°C. For a current density of 4 × 10⁵ A/cm² bulk drift velocity was 7 × 10^?12 cm/s, while grain boundary electromigration surpassed this value by a factor of 300. Electromigration threshold was ascertained for grain boundary as well as for interface and bulk diffusion.     

A transmission electron microscopy study of microstructure evolution with increasing anneal temperature in Ti/Al ohmic contacts to n-GaN

Al(200 nm)/Ti(20 nm)/n-GaN contacts have been studied using transmission electron microscopy (TEM) and the resulting microstructures correlated with the observed variation in specific contact resistance (ρ_c). A minimum ρ_c value of 7×10⁻⁷ Ωcm² was obtained after annealing at 550°C for 1 min in argon. Bulk metal and interfacial phases have been characterized, and explanations for the observed electrical behavior are proposed. A transition from TiN to AlN at the interface occurs between 650°C and 700°C.     

Formation of TiN/TiSi2/p+-Si/n-Si by rapid thermal annealing (RTA) silicon implanted with boron through titanium

A low temperature method of fabricating conductive (3.5 Ω/ sq.) p⁺/n junction diodes possessing excellentI-Vcharacteristics with reverse-bias leakage less than -3 nA.cm^-2at -5 V is described. Single crystal n-type 〈100〉 Si is implanted with 60 keV¹¹B+through 0.028-µm thick sputtered Ti film. Rapid thermal annealing (RTA) in an N2ambient simultaneously forms a 0.36-µm deep p⁺/n junction and a 0.063-µm thick bilayer of TiN and TiSi2with a resistivity of 22 µΩ.cm. The electrical properties of these diodes are not degraded by annealing for 30 min at 500°C, suggesting that the outer layer of TiN is an effective diffusion barrier between TiSi2and Al.     

Enhanced thermal stability of a sputtered titanium-nitride film as a diffusion barrier for capacitor-bottom electrodes

The effect of a thin RuO_x layer formed on the Ru/TiN/doped poly-Si/Si stack structure was compared with that on the RuO_x/TiN/doped poly-Si/Si stack structure over the post-deposition annealing temperature ranges of 450–600°C. The Ru/TiN/poly-Si/Si contact system exhibited linear behavior at forward bias with a small increase in the total resistance up to 600°C. The RuO_x/TiN/poly-Si/Si contact system exhibited nonlinear characteristics under forward bias at 450°C, which is attributed to no formation of a thin RuO_x layer at the RuO_x surface and porous-amorphous microstructure. In the former case, the addition of oxygen at the surface layer of the Ru film by pre-annealing leads to the formation of a thin RuO_x layer and chemically strong Ru-O bonds. This results from the retardation of oxygen diffusion caused by the discontinuity of diffusion paths. In particular, the RuO_x layer in a nonstoichiometric state is changed to the RuO₂-crystalline phase in a stoichiometric state after post-deposition annealing; this phase can act as an oxygen-capture layer. Therefore, it appears that the electrical properties of the Ru/TiN/poly-Si/Si contact system are better than those of the RuO_x/TiN/poly-Si/Si contact system.     

Effects of post-annealing by the rapid thermal process on the characteristics of MOCVD-Cu/TiN/Si structures

Effects of rapid thermal annealing on the characteristics of Cu films deposited from the (hfac)Cu(VTMS) precursor and on the barrier properties of TiN layers were studied. By the post-annealing, the electrical characteristics of Cu/TiN and the microstructures of Cu films were significantly changed. The properties of Cu films were more sensitive to the annealing temperature than the annealing time. Sheet resistances were decreased in 400–450°C ranges, and abrupt increases were observed above 750°C. It was also found that the copper films showed pronounced grain growth with the (111) preferred orientation. The grain growth and condensation of copper were observed below 500°C without formation of the CuO and Cu₂O phase resulting in surface degradation. Above 500°C, the oxide compound of copper was partially formed on the surface and the inter-reaction on the Cu-TiN interface was started. The inter-reaction of Cu-Ti and Cu-Si interface vigorously occurred and the surface roughness was continuously deteriorated above 650°C. It revealed that the optimum annealing conditions for MOCVD-Cu/PVD-TiN structures to enhance the electrical characteristics without degradation of TiN barriers were in the range of 400°C.     

High Quality Carbon Nanotubes on Conductive Substrates Grown at Low Temperatures

For carbon nanotubes (CNTs) to be exploited in electronic applications, the growth of high quality material on conductive substrates at low temperatures (<450 °C) is required. CNT quality is known to be strongly degraded when growth is conducted on metallic surfaces, particularly at low temperatures using conventional chemical vapor deposition (CVD). Here, the production of high quality vertically‐aligned CNTs at low substrate temperatures (350–440 °C) on conductive TiN thin film using photo‐thermal CVD is demonstrated by confining the energy required for growth to just the catalyst using an array of optical lamps and by optimizing the thickness of the TiN under‐layer. The thickness of the TiN plays a crucial role in determining various properties including diameter, material quality, number of shells, and metallicity. The highest structural quality with a visible Raman D‐ to G‐band intensity ratio as low as 0.13 is achieved for 100 nm TiN thickness grown at 420 °C; a record low value for low temperature CVD grown CNTs. Electrical measurements of high density CNT arrays show the resistivity to be 1.25 × 10^‐2 Ω cm representing some of the lowest values reported. Finally, broader aspects of using this approach as a scalable technology for carbon nanomaterial production are also discussed.     

Investigation of Ti/Al and TiN/Al thin films as the stable ohmic contact for p-type 4H-SiC

Interfacial reactions, surface morphology, and current-voltage (I-V) characteristics of Ti/Al/4H-SiC and TiN/Al/4H-SiC were studied before and after high-temperature annealing. It was observed that surface smoothness of the samples was not significantly affected by the heat treatment at up to 900°C, in contrast to the case of Al/SiC. Transmission electron microscopy (TEM) observation of the Ti(TiN)/Al/SiC interface showed that Al layer reacted with the SiC substrate at 900°C and formed an Al-Si-(Ti)-C compound at the metal/SiC interface, which is similar to the case of the Al/SiC interface. The I-V measurement showed reasonable ohmic properties for the Ti/Al films, indicating that the films can be used to stabilize the Al/SiC contact by protecting the Al layer from the potential oxidation and evaporation problem, while maintaining proper contact properties.     

Novel Diffusion‐Barrier Materials Against Oxygen for High‐Density Dynamic Random Access Memory Capacitors

A new design concept for diffusion barriers in high‐density memory capacitors is suggested, and both RuTiN (RTN) and RuTiO (RTO) films are proposed as sacrificial oxygen diffusion barriers. The newly developed RTN and RTO barriers show a much lower sheet resistance than various other barriers, including binary and ternary nitrides (reported by others), up to 800 °C, without a large increase in the resistance. For both the Pt/RTN/TiSi_x/n⁺⁺poly‐plug/n⁺ channel layer/Si and the Pt/RTO/RTN/TiSi_x/n⁺⁺poly‐plug/n⁺ channel layer/Si contact structures, contact resistance—the most important electrical parameter for the diffusion barrier in the bottom electrode structure of capacitors—was found to be as low as 5 kohm, even after annealing up to 750 °C. When the RTN film was inserted as a glue layer between the bottom Pt electrode layer and the TiN barrier film in the chemical vapor deposited (Ba,Sr)TiO₃ (CVD–BST) simple stack‐type structure, the RTN glue layer was observed to be thermally stable to temperatures 150 °C higher than that to which the TiN glue layer is stable. Moreover, the capacitance of the physical vapor deposited (PVD)–BST simple stack‐type structure adopted TiN glue layer initially degraded after annealing at 500 °C, and, thereafter, completely failed. In the case of the RTN and RTO/RTN glue layers, however, the capacitance continuously increased up to 550 °C. Thus, the new RTN and RTO films, which act as diffusion barriers to oxygen, are very promising materials for achieving high‐density capacitors.     

Ohmic contacts to n-type GaN

Ohmic contacts to n-GaN using Ag, Au, TiN, Au/Ti, Au/Mo/Ti, and Au/Si/Ti have been studied. The Fermi level of GaN appears to be unpinned, and metals and compounds with work functions less than the electron affinity resulted in ohmic contacts. Reactively sputter deposited TiN was ohmic as deposited. However, Au/Ti, Au/Mo/Ti, and Au/Si/Ti required heat treatments to form ohmic contacts, with the best being an RTA at 900°C. Ag and Au were shown to diffuse across the GaN surface at T>500°C; therefore, they are unstable, poor ohmic contact metallizations as single metals. The other contact schemes were thermally stable up to 500°C for times of 30 min.     

Epitaxial TiN based contacts for silicon devices

We have grown high quality epitaxial TiN/Si(100) and Cu/TiN/Si(100) heterostructures by pulsed laser deposition. The epitaxial TiN films have the same low (15 μΩ-cm) resistivity as TiSi₂ (C-54) phase with excellent diffusion barrier properties. In addition, Schottky barrier height of TiN was close to that of TiSi₂ (0.6-0.7 eV). Auger and Raman spectroscopy revealed that the films were stoichiometric TiN and free from oxygen impurities. The x-ray diffraction and transmission electron microscope (TEM) results showed that the TiN films deposited at 600°C were single crystal in nature with epitaxial relationship TiN|| Si. The Rutherford baskscattering channeling yield for TiN films was found to be in the range of 10–13%. The epitaxy of Cu on TiN was found to be cube-on-cube, i.e., Cull<100>TiN||Si. The Cu/TiN and TiN/Si interfaces were found to be quite sharp without any indication of interfacial reaction. The growth mechanism of copper on TiN was found to be three-dimensional. We discuss domain matching epitaxy as a mechanism of growth in these large lattice mismatch systems, where three lattice constants of Si(5.43?) match with four of TiN(4.24?) and seven units of Cu(3.62?) match with six of the TiN. Thus, for next generation of device complementary metal oxide semiconductor structures, Cu/TiN/Si(100) contacts hold considerable promise, particularly since Cu is a low resistivity metal (1.6 μΩ-cm) and is considerably more resistant to electromigration than Al. The implications of these results in the fabrication of advanced microelectronic devices are discussed.     

Use of a TiN barrier to improve GaAs FET ohmic contact reliability

We have used a reactively sputtered TiN diffusion barrier to prevent interpenetration of a (Ni)GeAuPt ohmic contact layer and Ti/Pt/Au overlay on GaAs devices baked at 250-300°C in air. Planar GaAs MESFET's and TLM patterns were fabricated and iteratively tested and baked. Devices without TiN showed severe degradation in morphology and dc and RF performance. Devices with TiN remained essentially unchanged.     

Improving the Reliability of Si Die Attachment with Zn-Sn-Based High-Temperature Pb-Free Solder Using a TiN Diffusion Barrier

The thermal fatigue reliability of Si die-attached joints with Zn-30wt.%Sn, high-temperature, Pb-free solder was investigated, focusing on the interfacial microstructure and joining strength of a Cu/solder/Cu joint during thermal cycling. A sound die attachment on an aluminum nitride (AlN) direct-bonded copper (DBC) substrate was achieved by forming Cu-Zn intermetallic compound (IMC) layers at the interface with the Cu of the substrate. During the thermal cycling test performed between −40°C and 125°C, thermal fatigue cracks were induced by the growth of Cu-Zn IMCs at the interface with the Cu. A␣thin titanium nitride (TiN) film was applied to suppress the formation of Cu-Zn IMCs. Adequate joint formation was accomplished by using an Au/TiN-coated DBC substrate, and only the TiN layer was observed at both interfaces. In conjunction with the TiN diffusion barrier, the Si die-attached joint created with Zn-30wt.%Sn solder exhibited a stable interfacial microstructure during thermal cycling. No microstructural changes, such as IMC formation, grain growth or formation of fatigue cracks, were observed, and the joining strength was maintained even after 2000 cycles.     

Silicide resistors for integrated circuits

Thin-film resistors are useful in monolithic integrated circuits whenever high sheet resistance (ρs> 1 kΩ/sq) or radiation hardness are required. Silicide resistive films (MoSi2, CrSi2, and Si-Cr) deposited by dc sputtering have been shown to be compatible with monolithic circuit production end require no protective overlayer. Si-Cr films 200-300 Å thick have ρs, and temperature coefficients of resistance (TCR) ranging from about 1 kΩ/sq and +150 ppm/°C (CrSi2) to 20 kΩ/sq and -1400 ppm/°C (17 at % Cr). MoSi2is best suited for resistor applications requiring 100-200Ω/sq. MoSi2films are about 700 Å thick at 200 Ω/sq, compared to < 100 Å for 200-Ω/sq Ni-Cr, and their TCR is -125 ppm/°C. Typical stability for unprotected silicide resistors in TO-5 packages at 200°C, no load, is < ±3 percent during the first 200 h and < ± 0.5 percent during the next 2000 h. The films are stable during short term exposure to high temperatures as encountered during monolithic or hybrid circuit ceramic package sealing.     

The effects of deposition temperature on the interfacial properties of SiH4 reduced blanket tungsten on TiN glue layer

Low pressure chemical vapor deposition tungsten films were deposited at various temperatures, using a WF₆−SiH₄−H₂ gas mixture. The impurity distribution at the W/TiN interface was investigated by Auger electron spectroscopy depth profiling. Some fluorine accumulation at the interface is observed when the tungsten is deposited below 300°C. However, above 300°C, no accumulation of fluorine could be observed. A result obtained from thermodynamic calculations using SOLGASMIX-PV suggests that this phenomenon is closely associated with the highly oxidized surface layer of TiN at the initial stage of deposition. The reaction of the gas mixture with the TiN surface layer seems to enhance the fluorine accumulation, which lowers the adherence of the interface and increases the contact resistance.