Buried Silicide Layers Research Articles

Direct Bonding of Silicon to Platinum

We have studied the direct bonding of platinum to silicon for applications where an electrical conduction through the bonding interface is desired. Platinum has been deposited on silicon wafers by mean of physical vapor deposition. Levels of surface roughness compatible with direct bonding have been achieved for the deposited Pt thin films. The surface preparation of the Pt films was performed using a diluted HF or based chemistry. The sequence used allowed achieving the direct bonding of Pt to Si. The post bonding thermal treatments have been investigated. Such thermal treatments lead to the silicidation of the platinum layer. Adhesion between the wafers after the thermal treatments has been strengthened as measured by Maszara’s blade technique ( were achieved after a thermal treatment). The morphological evolution of the buried silicide layer has been followed for different thermal budget by scanning electron microscopy. The higher thermal budgets probed (i.e., , ) resulted in the agglomeration of the buried silicide layer, but without observing debonding of the two silicon wafers. The vertical electrical conduction has been measured for the final sample annealed at . Schottky type contacts have been obtained between the semiconductor and the silicide.

Study of silicon-on-insulator substrates incorporated with buried MoSi 2 layer

Silicon-on-Insulator (SOI) substrates incorporated with buried MoSi 2 were fabricated using room temperature plasma bonding technology and smart cut technology. The molybdenum disilicide phase formation and morphology were studied by means of four-point probe measurements, X-ray diffraction analysis, atomic force microscopy and transmission electron microscopy examination. It is found that the transition of high-resistance phase Mo 3Si to low-resistance phase h-MoSi 2 occurs at approximately 750 °C. The t-MoSi 2 phase emerges at approximately 900 °C. SOI substrate incorporated with buried silicide layer of complete t-MoSi 2 phase can be achieved by 900 °C annealing for 20 min.

Toward Silicon-Based Lasers for Terahertz Sources

Producing an electrically pumped silicon-based laser at terahertz frequencies is gaining increased attention these days. This paper reviews the recent advances in the search for a silicon-based terahertz laser. Topics covered include resonant tunneling in p-type Si/SiGe, terahertz intersubband electroluminescence from quantum cascade structures, intersubband lifetime measurements in Si/SiGe quantum wells, enhanced optical guiding using buried silicide layers, and the potential for exploiting common impurity dopants in silicon such as boron and phosphorus to realize a terahertz laser

Thermal stability of CoSi2 layers implemented in a silicon-on-insulator substrate

A silicon-on-metal-on-insulator substrate, consisting of a top Si layer, a buried CoSi2 layer and a buried SiO2 layer on a Si (1 0 0) substrate was formed using Co silicidation, wafer bonding and wafer splitting. It is shown that the buried silicide layers in this structure exhibit a much higher thermal stability than surface layers. Resistivity measurements and cross-sectional transmission electron microscopy investigations revealed that buried CoSi2 layers withstand furnace anneals at 1000 °C up to 2 h, while surface CoSi2 layers started to degrade after 10 min anneals at 1000 °C. The proposed substrate is most useful for BiCMOS applications.

Review of SiGe HBTs on SOI

This paper reviews progress in SiGe Heterojunction Bipolar Transistors (HBT) on Silicon-On-Insulator (SOI) technology. SiGe HBTs on SOI are attractive for mixed signal radio frequency (RF) applications and have been of increasing research interest due to their compatibility with SOI CMOS (Complementary Metal Oxide Semiconductor) technology. In bipolar technology, the use of SOI substrate eliminates parasitic substrate transistors and associated latch-up, and has the ability to reduce crosstalk, particularly when combined with buried groundplanes (GP). Various technological SOI bipolar concepts are reviewed with special emphasis on the state-of-the-art SOI SiGe HBT devices in vertical and lateral design. More in depth results are shown from a UK consortium advanced RF platform technology, which includes SOI SiGe HBTs. Bonded wafer technology was developed to allow incorporation of buried silicide layers both above and below the buried oxide. New electrical and noise characterisation results pointed to reduced 1/ f noise in these devices compared to bulk counterparts. The lower noise is purported to arise from strain relief of the device structure due to the elasticity of the buried oxide layer during the high temperature epitaxial layer growth. The novel concept of the silicide SOI (SSOI) SiGe HBT technology developed for targeting a reduction in collector resistance, as well as for suppressing the crosstalk, is outlined. The buried tungsten silicide layers were found to have negligible impact on junction leakage. Further to vertical SiGe HBTs on SOI, the challenges of fabricating a lateral SOI SiGe HBT structure are presented.

Fabrication and characterization of buried silicide layers on SOI substrates for BICMOS-applications

The successful fabrication of a silicon on metal on insulator (SOMI) substrate with a structured buried silicide layer for BICMOS applications is shown in this paper. The cobalt silicide is used as the buried silicide layer in the SOMI substrate because of its high thermal stability, low resistivity and easier fabrication process. Conventional cobalt salicide process was used to form CoSi 2 structures. The SOMI substrate was fabricated on the wafer level using wafer bonding, CMP and back grinding technologies. A SOMI substrate, consisting of a 300 nm thick top-Si, a buried thin CoSi 2 layer, a buried SiO 2 layer on a silicon substrate, was formed using an SOI substrate as the starting material. The buried silicide layer has a resistivity of 16.3 μΩ cm and shows a high thermal stability which is sufficient for device applications.

SiGe HBTs on Bonded SOI Incorporating Buried Silicide Layers

A technology is described for fabricating SiGe heterojunction bipolar transistors (HBTs) on wafer-bonded silicon-on-insulator (SOI) substrates that incorporate buried tungsten silicide layers for collector resistance reduction or buried groundplanes for crosstalk suppression. The physical structure of the devices is characterized using cross section transmission electron microscopy, and the electrical properties of the buried tungsten silicide layer are characterized using sheet resistance measurements as a function of bond temperature. Possible contamination issues associated with the buried tungsten silicide layer are investigated by measuring the collector/base reverse diode tics. A resistivity of 50 /spl mu//spl Omega/cm is obtained for the buried silicide layer for a bond anneal of 120 min at 1000/spl deg/C. Collector/base reverse diode tics show a voltage dependence of approximately V/sup 1/2/, indicating that the leakage current is due to Shockley-Read-Hall generation in the depletion region. Fitting of the current-voltage tics gives a generation lifetime of 90 ns, which is as expected for the collector doping of 7 /spl times/ 10/sup 17/ cm/sup -3/. These results indicate that the buried tungsten silicide layer does not have a serious impact on junction leakage.

Optical cavities for Si/SiGe tetrahertz quantum cascade emitters

The use of buried tungsten silicide layers for confinement of terahertz optical modes is described. Silicon-on-silicide substrates are prepared using a bond and etch-back technique, and the successful growth of extremely long (600 period) strain-balanced p-Si/SiGe quantum cascade heterostructures on these substrates is demonstrated. THz electroluminescence is observed from these structures at low temperature, when the structure is biased so as to obtain interwell (‘diagonal’) transitions between heavy and light hole subbands. The emission shows a strong polarisation dependence, indicating the efficacy of the silicide layer in confining long wavelength TM modes.

Role of ripening and defects in the formation of mesotaxial cobalt-disilicide layers

Previous studies of the formation of buried silicide layers through mesotaxy, i.e., the introduction of the metal component by ion implantation with subsequent annealing of the structure, have shown that high-quality films can be achieved, but at processing temperatures too high to be useful for integration with silicon device technology. Recently, we proposed a modified ripening model to describe the early stages of the formation of a buried film which intentionally excluded the role of defects. In the current study, the predictions of that model are tested by varying sample preparation conditions. We show that the shift of the silicide precipitate layer is correlated with the narrowing of the cobalt profile, thus requiring both processes to be described by the same mechanism. We also show in a TEM study that specific defect morphologies are correlated with specific properties of the forming buried film, requiring an extension of the previous model.

Role of buried ultra thin interlayer silicide on the growth of Ni film on Si(100) substrate

The presence of a buried, ultra-thin amorphous interlayer in the interface of room temperature deposited Ni film with a crystalline Si(100) substrate has been observed using cross sectional transmission electron microscopy (XTEM). The electron density of the interlayer silicide is found to be 2.02 e/A3 by specular X-ray reflectivity (XRR) measurements. X-ray diffraction (XRD) is used to investigate the growth of deposited Ni film on the buried ultra-thin silicide layer. The Ni film is found to be highly textured in an Ni(111) plane. The enthalpy of formation of the Ni/Si system is calculated using Miedema’s model to explain the role of amorphous interlayer silicide on the growth of textured Ni film. The local temperature of the interlayer silicide is calculated using enthalpy of formation and the average heat capacity of Ni and Si. The local temperature is around 1042 K if the interlayer compound is Ni3Si and the local temperature is 1389 K if the interlayer compound is Ni2Si. The surface mobility of the further deposited Ni atoms is enhanced due to the local temperature rise of the amorphous interlayer and produced highly textured Ni film.

Article

We present results on the formation of buried silicide layers at ion implantation doses in the range of 1-60% of the critical dose for formation of a uniform layer. We emphasize observations for the low-dose range of 1-5% where the precipitate density is quite dilute. The Co redistribution during post-implant annealing is measured using Rutherford backscattering techniques and secondary ion mass spectrometry. Experimental observations during post-implantation annealing at 1000°C involves (i) a contraction of the Co depth profile for all doses, (ii) shifting of the peak of the profile towards the bulk, and (iii) formation of a secondary Co peak near the surface. The secondary peak is only present in samples implanted to greater than 3% of the critical dose. The interpretation of the shift of the main peak and the occurrence of the secondary peak requires a model exceeding the standard ripening model used previously to describe mesotaxy. We suggest that more recent ripening-based concepts allow for a full description of these observations with a minimum of parameters, particularly not requiring interaction with the complex defect profiles formed initially during implantation. Essential for this model is a proper inclusion of precipitate-precipitate interactions and the role of diffusion screening.Key words: silicide, Co implantation, mesotaxy, precipitate, ripening, screening.

Surfactant-mediated growth of CoSi 2 on Si(100)

Thin co-deposited CoSi 2 films grown at 500–600°C on Si(100) are studied by scanning tunneling microscopy (STM). With a Si rich deposition we observe initially the formation of elongated three-dimensional CoSi 2 islands. The use of one preadsorbed atomic layer of As as a surfactant results in a drastic increase of the island density. This effect appears to be a consequence of a decreased rate of surface diffusion of Co and Si. At higher coverages the roughness of the CoSi 2 film is reduced considerably by the surfactant. The results are discussed with regard to the method of allotaxy which allows the fabrication of buried silicide layers. Here, the requirements for small precipitates and high growth temperature can possibly be met more efficiently using As as a surfactant.

TEM investigation of ion beam synthesized semiconducting FeSi2

Abstract Both as-implanted and annealed semiconducting FeSi 2 samples fabricated using ion beam synthesis technique were studied by transmission electron microscopy. A continuous buried silicide layer with larger precipitates and sharper interfaces formed during annealing at 900 °C for 18 h. Different orientation relationships between the silicide grains and the silicon substrate were found. The existence of a large variety of parallel lattice plane pairs with the available small mismatches between the two phases results in these preferred orientation relationships.

Silicon Surface Chemistry By IR Spectroscopy in the Mid- To Far-IR Region: H2O And Ethanol On Si(100)

ABSTRACTThe technique of external reflection infrared (IR) spectroscopy is used to study silicon surface chemistry. External reflection is enhanced by implanting a buried cobalt silicide layer in silicon to act as an infrared reflector. The preparation of clean well-ordered surfaces from the ion implanted substrates is demonstrated. The reactions of water and ethanol with Si(100) are investigated.

Electrical and structural properties of buried CoSi2 layers in Si(100) grown by molecular beam allotaxy

Buried, single-crystal CoSi2 layers in Si(100) were fabricated by molecular beam allotaxy, a new two-step method to fabricate buried epitaxial layers. At first CoSi2 precipitates embedded in Si(100) were grown in a molecular beam system. In a second step a continuous, buried silicide layer was formed by rapid thermal annealing. Buried layers with thicknesses ranging from 27 to 224 nm were fabricated and investigated by transmission electron microscopy, Rutherford backscattering, He ion channelling and various electrical methods. Electrical resistivity measurements between 4.2 and 300 K revealed a specific resistivity of 14 μΩ cm at room temperature and 1 μΩ cm at 4.2 K. The temperature dependence follows the Bloch-Grüneisen relation. The resistivity increases with decreasing layer thickness. Schottky diodes were fabricated and characterized using I-V and I-T methods. Excellent diodes were produced with barrier heights of 0.64 ± 0.03 eV and idealities of 1.08.

Precipitation kinetics in silicon during ion beam synthesis of buried silicide layers

Under appropriate conditions, high dose implantation of metal ions such as Co +, Ni +, Fe + or Cr + into heated Si substrates and subsequent annealing results in the formation of buried epitaxial silicide layers. In the present paper, the kinetics of the underlying transient processes is discussed. The formation of silicide precipitates during implantation parameters are deduced. The local diffusional nucleation model. From this, guidelines for a favorable choice of the implantation parameters are deduced. The local precipitate coarsening and the narrowing of the spatial silicide distribution is described by an inhomogenous Ostwald ripening process in which solute fluxes from both sides of the implanted peak are driven by solute gradients associated with the peaked distribution of the precipitate sizes. It is shown that the possibility to narrow the spatial silicide distribution by an annealing procedure is limited. This principal limitation explains the experimental findings that the formation of a continuous buried layer requires the implantation of a minimum peak concentration of metal ions

Growth parameters affecting the formation of buried CoSi2 by endotaxy of Co on Si(111)

At growth temperatures of ≊800 °C, Co deposited on Si(111) diffuses through a Si capping layer and exhibits oriented growth on buried CoSi2 seeds, a process referred to as endotaxy. This occurs preferentially to surface nucleation of CoSi2 under certain growth conditions. High-quality continuous single-crystal buried silicide layers have been obtained by endotaxy. This requires very low densities of attractive nucleation sites other than the buried seeds. For this reason, use of a silicon buffer layer and a low base pressure in the molecular-beam epitaxy system are found to be essential. Growth conditions required for high-quality layers and problems which result from growth outside these limits are discussed.

Evolution of buried compound layers formed by ion implantation

The coalescence of buried silicide layers formed by high implantation in silicon and high temperature annealing occurs via a precipitate coarsening mechanism that is different in (100) and (111) silicon. The results of an extensive study of these phenomena are summarized and compared with the formation of amorphous SiO 2 by implantation.

Optimum implantation conditions for ion beam synthesis of buried cobalt silicide layers in Si(100)

Ion beam synthesis of buried CoSi2 layers in Si(100) (Co+ energy=170 keV, dose=1.7×1017 ions cm−2) is studied as a function of implantation temperature (250→500 °C) and beam current density (1.6→3 μA cm−2). Conventional cross-section transmission electron microscopy and Rutherford backscattering spectrometry are used to correlate the experimental conditions with the amount of pinholes in the silicide layer and the flatness of the CoSi2/Si interfaces after annealing. Optimum implantation conditions yielding a pinhole-free buried silicide layer with flat interfaces are obtained.

Thin buried cobalt silicide layers in Si(100) by channeled implantations

Si(100) wafers have been implanted with 50 keV Co ions at elevated substrate temperatures (320 °C) in the dose range 7.8×1014–7.8×1016 at. cm−2. A comparison is made between channeled (along the Si 〈100〉 surface normal) and random (tilted by 7°) implantations. Co depth distributions are measured with secondary-ion mass spectrometry and compared to marlowe and trim simulations. Annealed samples are characterized by Rutherford backscattering spectrometry and transmission electron microscopy. Our data indicate that for channeled implantations the sputtering effect is strongly reduced as compared to random implantations. Also, the average penetration depth is increased by about 20%. As a consequence, annealing of our high-dose implanted samples yields either a discontinuous surface silicide layer (random case) or a pinhole-free buried silicide layer (channeled case).