I. Channeling studies of silicon interfaces. II. Diffusion barrier properties of titanium nitride

Citation

Cheung, Woontong Nathan (1980) I. Channeling studies of silicon interfaces. II. Diffusion barrier properties of titanium nitride. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/e47n-cg57. https://resolver.caltech.edu/CaltechETD:etd-10122006-090333

Abstract

NOTE: Text or symbols not renderable in plain ASCII are indicated by [...]. Abstract is included in .pdf document. Part I The channeling effect of MeV ions in crystalline materials has been applied to study the interfaces of metals, silicides and oxides with single-crystal Si substrates. The study was facilitated by the development of thin (1500-5000Å) Si crystals which enabled channeled ions to probe the interfacial region without first traversing the metal or silicide layers. These investigations revealed that a reaction occurs between the silicon and as-deposited metal layers. For the Ni-Si system, about three monolayers of Ni penetrate into Si and occupy interstitial sites. An interfacial layer is also found between silicides and Si. In contrast, an abrupt interface is found between SiO2 and Si. With conventional channeling techniques, the ion beam traverses first the overlayer and then the Si substrate. A Si peak is observed in the energy spectrum and the area of the peak can be converted to Si atoms/cm(2) from known Rutherford scattering cross-sections. The Si peak corresponds to the first few monolayers of crystalline Si (the "surface peak"), the non-registered Si at the interface and the amount of Si in the silicide or oxide. From recent theoretical and experimental studies on surfaces, the surface peak contribution can be accurately predicted. Using different overlayer thicknesses, the stoichiometry of silicides and oxides and the interfacial disordered Si can be determined. In the case of metal-Si interfaces, the interfacial reactivity can be monitored by measuring the amount of non-registered Si at the interface. However, the conventional channeling technique is applicable only when the multiple scattering of the incident beam by the overlayer is small (i.e. thin overlayers and low atomic number elements). Additional information on the nature of the interfacial disordered Si can be obtained by the thin-crystal channeling technique which involves the use of thin ([...]), self-supporting Si crystals. With the ions first traversing the Si thin crystal along a channeling direction, the channeled ions are steered away from the atomic rows. At the interfacial region, the channeled ions only interact with the interfacial Si atoms which are displaced [...] laterally from the atomic rows. There is no contribution from the first few crystalline Si layers to the measured Si peak; as there is in the case of conventional channeling. The combination of both conventional and thin-crystal channeling techniques provides further information about the interfacial disorder because of the different sensitivities of the two techniques to lateral displacements. Thin-crystal channeling has also been used to locate the relative position of foreign atoms with respect to the Si lattice near the interfacial region. This technique requires angular scan experiments along various axial and planar channeling directions. For metal-Si interfaces, channeling results show that an interfacial reaction is initiated at room temperature with as-deposited metal layers (Ni, Pd, Cr, V and Au); Ag is the only exception. With the Ni-Si system, the interfacial reaction can be greatly reduced by cooling the substrate to 170°K during Ni deposition and ion beam analysis. The temperature dependence of the interfacial reactivity emphasizes the kinetic nature of the metal-Si interfaces and the importance of interfacial studies at low temperatures for meaningful comparison with abrupt metal-semiconductor interface models. The amount of disordered interfacial Si is observed to be high for metals which are dominant diffusion species in silicide formation (e.g. Ni and Pd). A lattice location experiment on the Ni-Si interface shows that ~3 monolayers of Ni atoms are situated interstitially at the tetrahedral sites of crystalline Si. This observation supports the interstitial diffusion model which was proposed by Tu to explain the low-temperature formation of silicides. Interfaces between Si and Pd2Si, Ni2Si, NiSi or NiSi2 have also been investigated by thin-crystal channeling. The Ni2Si-Si interface shows ~4x10(16) atoms/cm(2) of excess Si above an "ideal" Ni2Si-Si interface. The other silicide-Si interfaces all show disordered interfacial Si of <8x10(15) atoms/cm(2). The amount of interfacial disorder correlates with the transformation temperature for the next stable silicide phase. For example, the Ni2Si phase (which exhibits the highest interfacial disorder) transforms to NiSi at ~350°C. In contrast, the other silicide phases, Pd2Si and NiSi, require a much higher transformation temperature (~700°C). The SiO2-Si interface has been studied by both conventional and thin-crystal channeling. The results show that thermally grown oxides on (110)Si are stoichiometric SiO2 at least down to a thickness of 4Å. By comparing the channeling data with predictions based on a various number of reconstructed Si layers, an abrupt SiO2-Si interface is deduced with 2 monolayers of the Si single crystal being reconstructed at the interface. Part II This part of the thesis is concerned with the deposition and evaluation of titanium nitride (TiN) layers as diffusion barriers in contact metallization on Si devices. The application of TiN to Si solar cell contacts (i.e., the TiN-Ti-Ag metallization scheme) has been demonstrated to withstand a 600°C, 10 min anneal without degrading the cell's performance. Titanium nitride films have been prepared by reactive sputtering of Ti in a nitrogen plasma. The nitride films are identified by TEM and X-ray diffraction to have the NaCl structure with a lattice parameter of 4.24±0.02Å. Electrical properties and atomic composition of the films have been studied as a function of sputtering RF power (500-1500 W) and nitrogen pressure (3-100 mT). Backscattering analysis shows that the films have a composition close to stoichiometric TiN but with a slight tendency for higher nitrogen content. Oxygen is the major contaminant in the nitride films. The oxygen content strongly depends on the sputtering parameters and a high oxygen content corresponds to high electrical resistivity of the TiN films. The lowest resistivity (~170[...]-cm) is obtained by sputtering with high RF power and low nitrogen pressure. The effectiveness of titanium nitride films as a diffusion barrier between various metals and single-crystal Si substrate or Ti on single-crystal Si substrate is investigated by backscattering spectrometry, SEM and EDAX. The temperature range of interest is from 400°C to 700°C. Various metals of high electrical conductivity such as Au, Ag, Al, Cu and Pd are used as the top metal layer. By interposing a thin layer of TiN ([...] 1000 Å) between the metal and substrate, the failure temperature (i.e., the temperature at which metal-substrate interdiffusion becomes significant) can be greatly increased. The failure temperature of the TiN layers as a diffusion barrier is related to the metal-silicon eutectic temperature. SEM studies show that the interdiffused layer is laterally non-uniform and is initiated at isolated spots across the sample's surface. It is believed that grain boundaries or pinholes in the TiN films are the weak links in the diffusion barrier properties of TiN. As an application for high-temperature diffusion barriers, the Ag-Ti-TiN metallization scheme has been tested on shallow-junction (~2000A) Si solar cells. The conventional Ti-Pd-Ag metallization scheme has been shown to fail after a 600°C, 10 min anneal which is required for a glass encapsulation process. With the TiN-Ti-Ag scheme, no degradation of cell performance can be observed after the 600°C, 10 min anneal if the TiN layer is [...]1500Å.

Thesis Files