Ashok Kumar Sinha

Effect of texture and grain structure on electromigration in Al-0.5%Cu thin films

Abstract The electromigration resistance of Al-0.5%Cu meander lines was found to increase with increasing grain size s and degree of {111} preferred orientation and with decreasing spread σ in the grain size distribution. This dependence on microstructure can be expressed in terms of the empirical quantity ( s σ 2 ) log ( I 111 I 200 ) 3 which correlates well with the electromigration lifetime of films obtained by different deposition techniques.

Thermal stresses and cracking resistance of dielectric films (SiN, Si3N4, and SiO2) on Si substrates

In device‐manufacturing technology, it is important to understand why dielectric films crack. With this objective in mind, we have constructed an apparatus for measurement of thermal stresses in thin films (25–500 °C), obtained results on various reactively plasma deposited (RPD) Si‐N and CVD SiO2 films, and developed a model which quantifies the cracking resistance of different types of RPD Si‐N films. Measurements were made of the coefficient of thermal expansion α (T), which increases on going from SiO2→Si3N4→SiN→Si and the intrinsic stress, which is compressive for RPD Si3N4, nearly zero for thermal SiO2 and tensile for RPD SiN and CVD SiO2. The cracking resistance of Si‐N film at a given temperature is functionally related to its density, intrinsic stress, thermal mismatch with Si, and the deposition temperature.

Linewidth dependence of electromigration in evaporated Al‐0.5%Cu

The linewidth dependence of electromigration damage has been evaluated for 2.5‐cm‐long, 1–4‐μm‐wide, e‐gun‐evaporated Al‐0.5%Cu conductions. It is observed that the ≲1‐μm lines are much longer lived than the ≳2‐μm lines, reversing the trend at wider widths. These results are rationalized on the basis of the ’’bamboo’’‐type grain structure of narrow lines in contrast to the much more heterogeneous structure of the wider meanders.

Microstructure, growth, resistivity, and stresses in thin tungsten films deposited by rf sputtering

The growth process and the microstructure of very thin W films (80–500 A) deposited by rf sputtering on SiO2 and Si substrates have been observed by transmission electron microscopy (TEM). The resistivity and stress in these films have been related to the film microstructure, composition, and to the deposition conditions (substrate bias and rf deposition power). Thin W films deposited on silicon dioxide substrates under zero or positive bias have been found to grow in two distinct growth stages. Stage I corresponds to the formation of a thin continuous film (80–100 A thick) of β‐W. The β‐W phase has the A‐15 crystal structure and has been identified as a faulted W3W compound. A small grain size (50–100 A) is characteristic of the β‐W film. Stage II corresponds to the transformation of the β‐W film into a pure α‐W film with the bcc crystal structure. This thermally activated phase transformation takes place in the temperature range 100–200 °C. It is characterized by the growth of α‐W nuclei until complete ...

Elastic stiffness and thermal expansion coefficients of various refractory silicides and silicon nitride films

Abstract The elestic stiffness parameter E f (1−ν f ) and the thermal expansion coefficient αf were obtained for four different silicides (TiSi2, TaSi2, MoSi2 and WSi2) and for two different nitrides (chemically vapor-deposited Nitrox Si3N4 and r.f. plasma SiN) from stress-temperature measurements on identical films deposited on two different substrate materials. The values determined for αf and E f (1−ν f ) were quite similar for all silicides and averaged 15 ppm °C−1 and 1.1 × 1012 dyncm−2 respectively. The thermal mismatch of these silicides is such that, once safely formed, the silicide film should be able to withstand high temperature processing steps without cracking. For the nitrides the α values were essentially the same (approximately 1.5 ppm°C-1), although the larger value of E f (1−ν f ) chemically vapor-deposited Si3N4 film (3.7 × 1012 as opposed to 1.1 × 1012 dyn cm-2) indicates that it is somewhat stiffer than the SiN film.

The temperature dependence of stresses in aluminum films on oxidized silicon substrates

Abstract In situ measurements were carried out of stress at the AlSiO2 interface at various temperatures (25–500 °C) and for various film thicknesses (0.2–1.6 microm). These data are complemented with microstructural studies by transmission electron microscopy. For the aluminum films studied, the intrinsic structural component was very small (less than 2 × 108 dyn cm−2). On heating, thermal mismatch led to a compressive stress, with dσ/dT ≈ −2 × 107 dyn cm−2 °C; these films yielded at ‖σ‖ ; ⩽ 5 × 108 dyn cm−2, primarily through diffusion creep and grain growth. On cooling from about 450–500 °C, thermal mismatch led to a tensile stress which was limited mainly by dislocation slip. The final room temperature value after thermal cycling ranged from 0.5 × 109 dyn cm−2 for a 1.5 microm film to 8 × 109 dyn cm−2 for a 0.2 microm film; these values are believed to represent the critical stress for the generation of dislocation loops within the grains. The grain size of cold-deposited aluminum films ranged from about 0.2 microm for films 0.45 microm thick to about 2 microm for films 1.5 microm thick. Thermal cycling led to an order-of-magnitude increase in the grain size together with the formation of dislocation networks within the grains.

Refractory silicides of titanium and tantalum for low-resistivity gates and interconnects

A study of the refractory-gate metallization schemes had been undertaken to provide a low-resistivity metallization for LSI and VLSI. In this paper, we describe an overview of the efforts made in this direction and present two different metallization schemes which lead to a resistivity of <=20 and 40 /spl mu//spl Omega/spl dot/cm at the gate level. These schemes involve formation of titanium and tantalum silicides on polysilicon gates, respectively. The recommended structure ia a metal or a cosputtered alloy/polysilicon/gate oxide/substrate which, when sintered, gives the desired structure silicide/polysilicon/gate oxide substrate. By the use of 1000-/spl aring/ Ti or Ta, the sheet resistance of nearly 1 or 2 Omega//spl square/, respectively, can be routinely obtained. The silicides are mechanically strong and can be dry etched using radial-flow or barrel-type plasma reactors. The Ta silicide structure is found to be very stable throughout standard processing and can be retrofitted in the present processing sequence. Ti silicide structures are similarly stable except for the reactivity of the silicide with HF-containing reagents. The Ti silicide metallization scheme can therefore be employed in processing with changes incorporated to avoid HF-silicide contact.

Electrical properties of Si‐N films deposited on silicon from reactive plasma

Because reactive‐plasma‐deposited Si‐N films might offer certain advantages in silicon integrated‐circuit technology, we have evaluated their electrical properties and compared them with those of CVD Si3N4. Various films with compositions in the range 0.75?Si/N?1.9 were studied using C‐V and I‐V measurements, the latter at temperatures up to 200 °C. The dominant conduction mode appears to be Frenkel‐Poole emission; this is consistent with the observed linear I‐vs‐V1/2 relationship and magnitudes of various parameters. The resistivities (range 104–1021 Ω cm at 2×106 V/cm) and dielectric strengths (range 0.8×106 to 8×106 V/cm) are found to depend upon the film composition and on an as yet undefined structural parameter (for Si/N?0.75). The interface between Si‐N and Si is associated with a high density of surface charge (≳1012 cm−2) and a large trapping instability.

Oxidation of tantalum disilicide on polycrystalline silicon

Oxidation characteristics of the tantalum disilicide films have been investigated in the temperature range of 900°–1050 °C in dry oxygen and steam ambients. The silicide does not oxidize in dry oxygen and oxidizes in steam at a rate lower than that of doped polycrystalline silicon films as long as there is a polycrystalline silicon layer between the silicide and the gate oxide. Under these circumstances, the silicide retains its electrical and mechanical characteristics. The oxide on the silicide has an etch rate (in buffered hydrofluoric acid) similar to that of thermal SiO2 on silicon. Electrical characteristics of the oxide appear to be similar to those of the wet oxide on polycrystalline silicon. In the absence of polycrystalline silicon, between the silicide and the gate oxide, oxidation leads to a loss in the conductivity of the silicide and eventually to a mechanical instability of the film. An oxidation mechanism, which assumes silicon diffusion by substitution through the silicide, has been proposed.

MOS Compatibility of high-conductivity TaSi 2 /n + poly-Si gates

The MOS-VLSI parameters and process compatibility of a high-conductivity refractory silicide gate with a sheet resistance of ∼ 2 Ω/□ have been evaluated. The gate metallization typically consisted of 2.5 kÅ TaSi2/2.5 kÅ poly-Si, which was sintered prior to patterning with a CF4/O2plasma etch. Measurements were made to determine the metal work function, oxide fixed charge, surface-states density, dielectric strength, oxide defect density, lifetime, current leakage, and the flat-band voltage stability with respect to mobile charge contamination, slow trapping, and hot-electron trapping. On IGFETs (500-Å SiO2, As-implanted source/ drain), VTand β measurements were made as a function of the back-gate bias and the channel length as small as 2 µm. The MOS and IGFET parameters are nearly ideal and correspond to those expected of n+poly-Si gates. Static and dynamic bias-temperature aging stability of the VFBis excellent. These characteristics are preserved through subsequent standard VLSI process steps. However, certain process and structure limitations do exist and these have been defined.