Alessandra Satta

Materials aspects, electrical performance, and scalability of Ni silicide towards sub-0.13 μm technologies

Ni-silicide phase formation with and without a Ti capping layer was studied by sheet resistance, x-ray diffraction and transmission electron microscopy. Ni monosilicide is found to be the stable phase in a temperature range from 400 to 600 °C. At lower temperatures the Ni2Si phase is found to be present. For temperatures higher than 700 °C NiSi is converted into NiSi2. Pyramidal NiSi2 precipitates were found to grow epitaxially along the Si〈111〉 planes for annealing temperatures as low as 310 °C. The epitaxial NiSi2 grains were found to disappear when the annealing temperature is increased. Stress buildup during Ni silicidation was measured in situ and could be correlated to the formation of the different Ni-silicide phases. The stress induced by Ni-monosilicide formation compares favorably to the stress induced by Co disilicide and Ti disilicide. The average silicon consumption required to obtain a certain sheet resistance was found to be 35% lower for Ni monosilicide compared for Co disilicide. It was f...

Heavy ion implantation in Ge: Dramatic radiation induced morphology in Ge

High dose ion implantation of heavy elements in Ge induces a rough surface and profile distortions when measured with secondary ion mass spectrometry. In the case of Sb large subsurface holes are also induced by the implantation. The formation of these subsurface structures starts abruptly at a dose between 5∙1014 and 1015at∕cm2. The addition of a SiO2 capping layer on top of Ge prevents the formation of the surface roughness, but has limited impact on the void formation. These voids originate from vacancy clustering during the implant process. Anneal studies show that it is impossible to remove these structures by annealing, limiting the usefulness of high dose Sb implants in Ge for junction formation. In the case of As implantation a similar surface roughness is seen but no void formation. Adding a cap layer removes the surface roughness in this case and improves the secondary ion mass spectroscopy profiles.

Germanium: The Past and Possibly a Future Material for Microelectronics

In 1947, the first transistors were fabricated in Bell Labs using bulk germanium as the semiconducting material. For this work its inventors, John Bardeen and Walter Brattain shared the 1956 Nobel Prize in Physics, along with William Shockley. About a dozen years later, the integrated circuit was independently invented by Jack Kilby, who used Ge substrates, and by Robert Noyce, who used silicon, and for which Kilby received the 2000 Nobel Prize in Physics (Noyce had passed on in 1990). Germanium was the predominant material for solid state devices through the 1950s and early 1960s, but its use was largely replaced with silicon during the 1960s. There are a number of reasons for this shift, but the ready formation of a high quality thermal oxide (SiO2) for silicon as compared to the water soluble oxides for Ge (GeO, GeO2) and the difficulty this poses for device performance and integration is a major reason.

Active dopant characterization methodology for germanium

In order to reach the ITRS goals for future complementary metal-oxide semiconductor technologies there is a growing interest in using germanium as an alternative substrate material in view of its higher mobility. Different species and thermal budgets are presently being investigated in order to determine the most likely candidates for the required junction formation. A key issue is the accurate determination of the achievable electrical activation, i.e., the reliable measurement of the sheet resistance and electrical depth profile. In order to be applicable to Ge-based junctions, standard techniques such as the spreading resistance probe and scanning spreading resistance microscopy (SSRM) need to be reevaluated in terms of their performance and operational conditions. First, the significantly different behavior of germanium calibration curves (versus silicon) will be discussed. Next, the shape and characteristics of the probe imprints (Ge is softer than Si) and the differences in raw data behavior will be...

Electronic structure calculations of vacancy parameters in transition metals: Impact on the bcc self-diffusion anomaly

The group dependence, i.e., the variation with the number of d valence electrons, of vacancy parameters in transition metals with the body-centered cubic (bcc) structure is investigated via a combination of electronic structure calculation techniques. A semiempirical tight-binding d-band approach is proposed that shows that the position of the Fermi level with respect to the pseudogap governs the sharp variations along a transition metal series of (1) the formation energy, (2) the relaxation energy, (3) the migration energy, and (4) the electronic contribution to the formation and migration entropies. These predicted trends are confirmed by first-principles calculations in 5d bcc metals (bcc-Hf, Ta, and W) including structural relaxations within plane-wave pseudopotential computations performed on supercells containing up to 54 sites. The agreement with available experimental data is very conclusive. A fast version of the full potential linear muffin-tin orbital method is then used to show the weak influence of the method within density functional theory in the local density approximation and to generalize these results--without relaxation--to the 3d and 4d series. This data base allows us to test the validity for vacancy studies of spd tight-binding models proposed in the literature.

Defect Removal, Dopant Diffusion and Activation Issues in Ion-Implanted Shallow Junctions Fabricated in Crystalline Germanium Substrates

The formation of shallow junctions in germanium substrates, compatible with deep submicron CMOS processing is discussed with respect to dopant diffusion and activation and damage removal. Examples will be discussed for B and Ga and for P and As, as typical p- and n-type dopants, respectively. While 1 to 60 s Rapid Thermal Annealing at temperatures in the range 400-650oC have been utilized, in most cases, no residual extended defects have been observed by RBS and TEM. It is shown that 100% activation of B can be achieved in combination with a Ge pre-amorphisation implant. Full activation of a P-implant can also be obtained for low-dose implantations, corresponding with immobile profiles. On the other hand, for a dose above the threshold for amorphisation, a concentration-enhanced diffusion of P occurs, while a lower percentage of activation is observed. At the same time, dose loss by P out-diffusion occurs, which can be limited by employing a SiO2 cap layer.

Microstructure evolution from the atomic scale up

Abstract Microstructure evolution under external forces results from the complicate interplay of competing events originating at the atomic scale. The movement and interaction of, e.g., dislocations, grain boundaries, microcracks, occurs via many elementary atomic-scale events, which can be conveniently grouped into “geometric” and “topological”: the former can modify only the size and shape of the microstructure elements, while the latter may alter their number and connectivity (e.g. turning a bunch of dislocations into a grain boundary). We present and discuss the results of atomic-level simulations of both isolated and interacting defects. Then we describe a mesoscopic simulation framework based on a variational formulation of the dissipated work rate; such a model allows to correlate the elementary, atomic-scale events into a microstructure evolution model of great richness and complexity.

Depth resolved study of impurity sites in low energy ion implanted As in Si

An extended x-ray absorption fine structure investigation in depth-resolved mode allows us to identify the different sites of the arsenic along its concentration profile in shallow junctions, obtained by low energy arsenic implantation of silicon. In the deeper part of the sample, arsenic mainly occupies substitutional sites and vacancy–arsenic complexes are evidenced, whereas in the region close to the surface a mixed phase of arsenic aggregates and arsenic impurities is present. First principles calculations supporting the observations are presented.

Advanced carrier depth profiling on Si and Ge with micro four-point probe

In order to reach the ITRS goals for future complementary metal-oxide semiconductor technologies, there is a growing need for the accurate extraction of ultrashallow electrically active dopant (carrier) profiles. In this work, it will be illustrated that this need can be met by the micro four-point probe (M4PP) tool. M4PP sheet resistance measurements taken along beveled Si and Ge blanket shallow structures will be investigated. From the differential sheet resistance changes, the underlying carrier profile can be reconstructed without the need to rely on a complicated contact modeling, i.e., M4PP carrier profiling is an absolute carrier depth profiling technique. On Si, it is found that the more sensitive a structure is to carrier spilling along the bevel, the better the M4PP system performs relative to conventional spreading resistance probe (SRP) due to its much lower probe pressure in combination with its sensitivity to what happens around the probes (and not underneath them). Also for Ge, the same iss...

Electrical Performance of Ge Devices

The chapter reviews the electrical performance of basic structures such as MOS capacitors and pn diodes, which gives an insight in the performance parameters and a better understanding of the basic mechanisms involved. A good control of the performance of these building blocks is essential for optimizing the transistor performance. The chapter explains some basic measurement techniques. In this chapter, the two major building blocks of the Ge-based MOSFET (metal-oxide-semiconductor field-effect transistor) are reviewed––namely, the p-n junction and the Ge/insulator/metal gate stack. The chapter also outlines the basic principles of operation of these devices.