Francesca Iacopi

Low-k dielectric materials

Abstract Performance improvements in microelectronic integrated circuits (ICs) over the past few decades have, for the most part, been achieved by increasing transistor speed, reducing transistor size, and packing more transistors onto a single chip. Smaller transistors work faster, so ICs have become faster and more complex. An emerging factor that may disrupt this trend is the slowing speed of signal propagation within the chip. Signal delays, caused by the interconnection wiring, increase with each generation of scaling and may soon limit the overall performance of the integrated system.

Short-ranged structural rearrangement and enhancement of mechanical properties of organosilicate glasses induced by ultraviolet radiation

The short-ranged bonding structure of organosilicate glasses can vary to a great extent and is directly linked to the mechanical properties of the thin film material. The combined action of ultraviolet (UV) radiation and thermal activation is shown to generate a pronounced rearrangement in the bonding structure of thin organosilicate glass films involving no significant compositional change or film densification. Nuclear magnetic resonance spectroscopy indicates loss of –OH groups and an increase of the degree of cross-linking of the organosilicate matrix for UV-treated films. Fourier transform infrared spectroscopy shows a pronounced enhancement of the Si–O–Si network bond structure, indicating the formation of more energetically stable silica bonds. Investigation with x-ray reflectivity and ellipsometric porosimetry indicated only minor film densification. As a consequence, the mechanical properties of microporous organosilicate dielectric films are substantially enhanced while preserving the organosili...

Plasma-enhanced chemical vapour deposition growth of Si nanowires with low melting point metal catalysts: an effective alternative to Au-mediated growth

Au nanoparticles are efficient catalysts for the vapour?solid?liquid (VLS) growth of semiconductor nanowires, but Au poses fundamental reliability concerns for applications in Si semiconductor technology. In this work we show that the choice of catalysts for Si nanowire growth can be broadened when the need for catalytic precursor dissociation is eliminated through the use of plasma enhancement. However, in this regime the incubation time for the activation of VLS growth must be minimized to avoid burying the catalyst particles underneath an amorphous Si layer. We show that the combined use of plasma enhancement and the use of a catalyst such as In, already in a liquid form at the growth temperature, is a powerful method for obtaining Si nanowire growth with high yield. Si nanowires grown by this method are monocrystalline and generally oriented in the direction.

SCINTILLATING ARRAY GAMMA CAMERA FOR CLINICAL USE

Abstract Dedicated gamma cameras for specific clinical application are representing a new trend in Nuclear Medicine. They are based on Position Sensitive Photo Multiplier Tubes (PSPMT). The main intrinsic limitation of large area PSPMT (5″ diameter) is the photocathode glass window. Coupling to a planar scintillation crystal strongly affects the useful active area and the intrinsic spatial resolution. To overcome this limitation at University of Rome “La Sapienza” was developed the first 5″ diameter gamma camera consisting of a Hamamatsu R3292 PSPMT coupled to 50 × 50 YAP: Ce scintillating array. The array pixel size is 2 × 2 mm2 and the overall dimension of multi-crystal is 10 × 10 × 1 cm3. Resistive chains were used to calculate the centroid. The scintillating array produces a focused light spot minimising the spread introduced by PSPMT glass window. The intrinsic spatial resolution varied between 2 and 2.7 mm. The position linearity and useful active area resulted in good agreement with intrinsic one obtained by light spot irradiation. The real limitation was the poor energy resolution of an individual crystal (40%) and the poor uniformity response of PSPMT (within ±15%). A correction matrix was then carried out by which a 57% of total energy resolution was obtained for the whole matrix. The camera is currently operating as Single Photon Emission Mammography (SPEM) and it is producing breast functional images for malignant tumour detection using the same geometry as standard X-ray mammography.

Optical Property Changes in Low-k Films upon Ultraviolet-Assisted Curing

Ultraviolet-assisted curing (UV curing) has been applied recently to enhance the mechanical properties of low- k films. Knowledge about which UV energies are most effective is still limited and the consequences of applying the UV-curing process to integrated stacks in on-chip interconnects are unknown. To clarify these open questions, we investigated the optical properties of a SiCOH low- k layer by purged UV spectroscopic ellipsometry in the energy region 2-9 eV. The complex refractive index of the low- k film shows an absorption edge with a superimposed absorption band at 6.4 eV that vanishes upon UV-assisted curing. A comparison with Fourier transform infrared transmission demonstrates that the absorption at 6.4 eV must be attributed to the organic porogens, which also influence the absorption edge. Further analysis reveals the redshift of the absorption edge of silica-based low- k films with the presence of carbon species. The measured optical properties permit simulation of the standing wave pattern of light within the films in differently configured stacks.

Factors affecting an efficient sealing of porous low-k dielectrics by physical vapor deposition Ta(N) thin films

The deposition of homogeneous thin films on porous substrates has been investigated. The thin film deposition of Ta(N) by physical vapor deposition on porous films with different average pore sizes and material compositions has been studied. The continuity of Ta(N) films on top of porous low-k dielectrics is evaluated by means of ellipsometric porosimetry combined with sheet resistance and atomic force microscopy measurements. Interface reactions are analyzed by x-ray photoelectron spectroscopy profiling. It has been observed that the minimal Ta(N) thickness required to obtain a continuous metal layer on top of the porous film depends, on the one hand, on the porosity and pore size and, on the other hand, on the chemical interaction of the thin film with the porous substrate. The sealing of pores is favored by the presence of carbon in the dielectric matrix. This is explained through a mechanism of local enhancement of the degree of crosslinking in the dielectric matrix, catalyzed by Ta.

Orientation-dependent stress relaxation in hetero-epitaxial 3C-SiC films

Residual stresses in epitaxial 3C-SiC films on silicon, for chosen growth conditions, appear determined by their growth orientation. Stress evaluation locally with Raman spectroscopy, and across a 150 mm wafer with curvature measurements, indicate that thin films can be grown on Si(100) with residual tensile stresses as low as 150 MPa. However, films on Si(111) retain a considerably higher stress, around 900 MPa, with only minor decrease versus film thickness. Stacking faults are indeed geometrically a less efficient relief mechanism for the biaxial strain of SiC films grown on Si(111) with 〈111〉 orientation. Residual stresses can be tuned by the epitaxial process temperatures.

Post patterning meso porosity creation: a potential solution for pore sealing

The creation of meso porosity in single damascene structures after patterning has been investigated to facilitate the sealing of the sidewalls by iPVD barriers. The dielectric stack consists of developmental porous SILK (v7) resin (SiLK is a trademark of The Dow Chemical Company) and a chemical vapor deposited hard mask. Porous SILK (v7) resin was selected since the temperature of vitrification of the material is lower than the temperature of porogen burn out. Creation of meso porosity after patterning results in smooth trench sidewalls, leading to an improved iPVD barrier integrity, as opposed to the conventional process sequence, which gives rise to large, exposed pores at the sidewall.

Compressive stress relaxation through buckling of a low-k polymer-thin cap layer system

The thermomechanical stability of a system composed of a metallic cap layer on top of a low-k thermosetting polymer film is investigated. It is observed that, when metal layers with high compressive stresses are used, a stress relaxation takes place during thermal anneal at temperatures above 300 °C through buckling of the two-layer system (wrinkling on rigid base). When designing low-k films for interconnects, this should be considered through a careful analysis of structural stability. The onset of this instability is allowed by the high compliance of the polymeric film, due to its transition from elastic to viscoelastic behavior through creep phenomena. This mechanism is more pronounced when a polymer film with 20% subtractive porosity is used.

Properties of porous HSQ-based films capped by plasma enhanced chemical vapor deposition dielectric layers

This article presents a study on Dow Corning® XLK™, an inorganic porous material with about 50% porosity and a dielectric constant of 2.0. It focuses on matters linked to sealing the porous film by depositing a plasma enhanced chemical vapor deposition (PECVD) dielectric cap layer. The study shows that the material can be modified during cap deposition due to the fast diffusion of reactants and radicals through the porous network, and acquire totally new properties which can be either beneficial or detrimental, depending on the chosen process. In particular, it is found that cap deposition processes on XLK in an oxidizing ambient, as used for SiO2 deposition, should be avoided. On the other hand, a beneficial modification of the dielectric film has been observed after SiC:H capping. It is also shown that there exists a critical thickness of capping material below which the cap layer reveals the presence of pinholes. The critical thickness value for a PECVD SiC:H cap layer on top of an XLK film is around 2...