Multilayer Film Stacks Research Articles

Residual stresses and mechanical properties of a ZnO pyroelectric sensor

The multilayer composite film stack is a common feature of the microsensors based on thin film technology. In this paper, we propose an analytical model to investigate the Young’s moduli, hardness, and residual stresses of the constitutive film layers of a multilayer film stack. A multilayer film stresses model is derived to evaluate the residual stress distributions in the constitutive film layers of a ZnO pyroelectric sensor. A good agreement among the multilayer film stresses model, the grain morphologies characterized by scanning electron microscopy, the mechanical properties testing by nanoindentation, and the voltage responsivity measurement of the ZnO pyroelectric sensor is found in this paper. The proposed multilayer film stresses model can therefore be used to quantitatively analyze the film stresses and proceed to the optimization of thin film deposition process.

Reducing imaging defects in high-resolution photolithography

Photoresist defects can occur in large, seemingly noncritical features in high-resolution photolithography although it is capable of resolving features of much smaller dimensions. Both optical illumination and wafer topography may contribute to the formation of resist defects in large features. This letter examines optical simulations to uncover the specific nature of these resist defects and proposes local correction methods to improve the photolithographic process. For example, adding a back-etched phase feature in the center of a large isolated trench feature reduces the illumination-responsible resist scumming effect, and optimizing the multilayered film stack can eliminate the resist defect when printing a large contact feature on the structured wafer substrate.

Analyses of interface adhesion between porous SiOCH low-k film and SiCN layers by nanoindentation and nanoscratch tests

In this study, an Si/PEOX/SiCN/SiOCH/SiCN multilayered film stack has been prepared by chemical vapor deposition. The bonding configurations between porous SiOCH film and SiCN etch stop layers have been analyzed by X-ray photoelectron spectroscopy, and the interface adhesion has been investigated by nanoindentation and nanoscratch tests. Elements of Si, C, N, and O constructed an interlayer region of about 10 nm with mixing bonds of Si to C, Si to N and Si to O at the interface between the porous SiOCH film and SiCN layers. During nanoindentation and nanoscratch tests, interface delamination occurred between SiOCH and SiCN layers due to the accumulation of sufficient shear stresses around the indented regions. The interface adhesion energy was accordingly measured as 1.68 J/m 2 by nanoindentation test with an applied load of 30 mN. With higher applied loads, the measured interface adhesion energy decreased. By the nanoscratch test, the interface adhesion energy was measured as about 0.91 J/m 2, lower than that obtained by nanoindentation test due to the mode mixity effect.

Effect of van der Waals forces on molecularly thin lubricant in the magnetic storage head-disk interface

The disjoining pressure of lubricant in the magnetic storage device head-disk interface (HDI) was obtained using Lifshitz theory [Sov. Phys. JETP 2, 73 (1956)] and multilayer film stack model. For this purpose the normal-air-gap-centered calculation of the disjoining pressure was replaced by the lubricant-layer-centered system. In the air-centered system the attractive force between the head and media was found to increase as the lubricant layer was thinned. By switching to the lubricant-centered system the disjoining pressure in a thin film of lubricant in the HDI was found to be negative at small head-to-media separation. By equating the free lubricant disjoining pressure with the lubricant in the head-disk interface, a thinning of the lubricant in the interface due to the presence of the head was calculated. A detailed thermodynamic analysis of the disjoining pressure of the lubricant in the interface revealed that the calculated negative disjoining pressure was due to the dominance of the air-gap-centered interaction energy for small air gaps. The impact on the lubricant film in the operating hard disk drive is discussed, and the possible role of negative disjoining pressure on the lubricant accumulation rate on the slider is highlighted.

Effect of media overcoat on van der Waals interaction at the head–disk interface

A multilayer Lifshitz calculation of van der Waals interactions at the magnetic storage head–disk interface is presented. The calculation of the Hamaker function for model multilayer film stacks that incorporate different media overcoat materials is obtained. These functions allow the comparison of interfacial forces when the media is coated with diamondlike carbon, with and without the inclusion of hydrogen and annealing, and SiNx overcoat films. A significant difference, ∼30%, in the magnitude of the interfacial force with the different overcoats is calculated.

Mechanical Characterization of Multilayer Thin Film Stacks Containing Porous Silica Using Nanoindentation and the Finite Element Method

AbstractNovel metal/dielectric material combinations are becoming increasingly important for reducing the resistance-capacitance (RC) interconnection delay within integrated circuits (ICs) as the device dimensions shrink to the sub-micron scale. Copper (Cu) is the material of choice for metal interconnects and SiO2 (with a dielectric constant k = ∼ 3.9) has been used as an interlevel dielectric material in the industry. To meet the demands of the international road map for semiconductors, materials with a significantly lower dielectric constant are needed. In this study, the effects of porosity and layer thicknesses on the mechanical properties of a multilayer thin film (Cu, Ta and SiO2)-substrate (Si) system are examined using nanoindentation and finite element (FE) simulations. A micromechanics model is first developed to predict the stress-strain relation of the porous silica based on the homogenization method for composite materials. An FE model is then generated and validated to perform a parametric study on nanoindentation of the Cu/Ta/SiO2/Si system aiming to predict the mechanical properties of the multilayer film stack.

AFM of ion-etched cross-sections: a method for analysing the morphology of dense hard coatings

An ion-milling-based sample preparation is described, which reveals the morphology of nanocrystalline hard coatings. The method has been successfully used for columnar TiAlN and nanocomposite TiAlSiN single layers, as well as for TiAlN/TiAlSiN multilayer films. Atomic force microscopy ( AFM) images of cross-sectioned samples prepared with our method give relevant insight about the packing of the nanograins and therefore about the growth kinetics. While the observations are also a simple alternative to transmission electron microscopy (TEM), they give essentially complementary information on density of grain (column) boundaries and the interlamellar growth in multilayer stacks. Because the sample preparation technique is composition-sensitive, it allows identification of the layer type in multilayer film stacks. This has been used for nanoindentation work on selected features like interlamellar boundaries, substrate/coating interfaces, grains with a particular composition and layers with thickness higher than 20 nm.

Enhancement of adhesion strength of Cu layer on single and multi-layer dielectric film stack in Cu/low k multi-level interconnects

An understanding of adhesion strength at Cu/barrier interface and Cu layer properties at different metal level is necessary to develop a robust planarization (CMP) process without peeling and delamination. This paper quantitatively analyzes the relation between stress and adhesion strength of Cu and its underlying layers of Cu interconnection. The properties of Cu layer on single and dual SiOC dielectric stacks with two different seed layer thickness were systematically studied. Effects of single and dual dielectric stack on stress, adhesion strength and growth orientation of Cu was explored in detail. Strongest adhesion strength between Cu and its underlying layers was found on single dielectric stack of 500 Å Cu seed on Ta barrier. It was reduced significantly after 6 kÅ of Electroplated (ECP) Cu layer was deposited on the stacks of dual dielectric scheme. The adhesion strength/interfacial bonding of dielectrics with Cu was deteriorated by 20% as a result of high stress ECP Cu layer without annealing. In this study, it was found that the properties of conductive Cu layer were strongly influenced by number of dielectric stacks. The results of this study further strengthens our claim that low stress films of dielectric stacks are necessary and essential for enhancing adhesion strength and interfacial bonding between Cu/dielectric layer. Moreover, annealing enhanced the growth of (1 1 1) Cu seed which in turns improved ECP Cu deposition. Better film morphology, improved uniformity of (1 1 1) Cu seed is essential for stress free Cu layer growth.

Tunable and Switchable Multiple-Cavity Thin Film Filters

A family of thin film interference filters is described that incorporates amorphous silicon layers for wide thermo-optic tunability and the potential for multiple cavity designs. Plasma enhanced chemical vapor deposition (PECVD) of hydrogenated amorphous silicon (a-Si:H) onto fused silica or crystalline silicon wafer substrates produces films with high index (n=3.71), low loss at 1500 nm (k<4/spl times/10/sup -6/), and thermo-optic index coefficients approximately ten times larger than those of the dielectric materials typically used in wavelength division multiplexing (WDM) filters. Pairing amorphous silicon with low index (n=1.77, k=1/spl times/10/sup -6/) silicon nitride companion layers followed by post-process annealing leads to multilayer film stacks which may be cycled up to 400 C without delamination or failure, resulting in thermal index modulations as large as 4% and enabling a wide variety of dynamic thin film device designs. In this paper we survey the range of device applications and show experimental demonstrations for two quite different classes of functionality. One class of filters is tunable in wavelength in the conventional sense. Single and dual-cavity narrowband filters are described with temperature coefficients of center wavelength 85-172 pm//spl deg/C in the 1550 nm WDM band and tuning ranges as large as 60 nm, an order of magnitude larger thermal tunability than for conventional thin film narrowband filters. A second class of filters is fixed in wavelength but switchable in transmission, based on hybrid structures which combine tunable semiconductor films and cavities with static dielectric films and cavities in an integrated coating design. To demonstrate the principle of a switchable add/drop filter, we fabricated a five-cavity, 117 layer, 200 GHz filter by combining a single cavity of a-Si:H/a-Si/sub x/N/sub y/ films deposited by PECVD with four cavities of dielectrics Ta/sub 2/O/sub 5//SiO/sub 2/ deposited by ion assisted e-beam evaporation. By varying the temperature, this device can be switched thermo-optically between transmission and reflection states at a fixed channel 1548.3 nm with a contrast ratio 18.4 dB. Stable devices can be obtained even with large internal temperature changes in microscopic volumes provided that layer-to-layer and layer-to-substrate adhesion is robust, film stresses are well controlled through coefficients of thermal expansion matching, and devices are annealed at or above maximum operational temperatures. Hitless filters can be obtained by structuring thermo-optic filters in two independently heated portions. Thermo-optically actuated thin film semiconductor devices are manufacturable and testable on a wafer scale and may be packaged by methods adapted from those for conventional thin film filters.

A Thermodynamic Approach in Tuning Phase Stability in Nanocomposite Multilayers

ABSTRACTChanges in the crystallographic phase stability of individual layers in a multilayered thin film stack are expected to have a significant influence upon the functional properties of the structure. The ability to predict and tune these phase stability states is of relevant importance in order to maximize the functional properties of the multilayer. A classical thermodynamic methodology, based upon competitive volumetric and interfacial free energies, has been used in the prediction and subsequent confirmation of the hcp to bcc phase stability in a Ti/Nb multilayer. An outcome of this model is a new type of phase stability diagram that can be used to predict the hcp Ti and bcc Ti phase stability as a function of length scale and volume fraction. The Ti layers were subsequently alloyed with a bcc-stabilizing element. The alloyed sputtered deposited Ti layers were able to stabilize the bcc Ti phase to a larger layer thickness as compared to the unalloyed Ti/Nb multilayers. The percentage of alloying element added to the Ti layer in controlling the critical transition thickness between the two phase states had good agreement with the predictions proposed by the thermodynamic model.

Fabrication and analyses of negative-birefringent thin film compensator with characteristic matrix

We fabricated a multilayer negative-birefringent film as a phase compensator to minimize light leakage from twisted nematic liquid crystal displays in oblique incidence. The multilayer film stack is composed of alternating thin layers of silicon dioxide and titanium dioxide, which is magnetron sputtered on the glass substrates. The, experiment agrees with the characteristic matrix calculation. Since each layer is not 'thin' enough compared with visible spectra, the multilayer thin stack induces interference phenomena, and consequently reduces the transmittance. In order to eliminate the interference, we optimized the multilayer thin film to achieve high transmittance in the region of visible spectra.

In situ spectral ellipsometry for real-time thickness measurement: Etching multilayer stacks

A polarization modulated spectroscopic ellipsometer is used in situ to measure the thicknesses of films in real time during semiconductor plasma etch processing. Utilizing the speed of phase modulation multichannel detection, and digital signal processing techniques, this ellipsometer is capable of acquiring spectral data in less than 75 ms. Efficient algorithms were developed for determining layer thicknesses from the measured spectra of multilayer film stacks in real time with a typical solution time of a few seconds. The measured thicknesses and etch rates are used to anticipate interfaces in multiple layer stacks and control process end points. The ability of the spectral ellipsometer to measure multilayer stacks during etching is demonstrated by the etching of a stack consisting of silicon nitride, polycrystalline silicon, and silicon dioxide. Such stacks are commonly used as masks for field oxidation for electrical isolation in memory device fabrication. An isotropic plasma etch is used to remove this film in a single-wafer process environment.

Phase Transformations in Co/Nb and Co/Zr Multilayer Film Stacks.

AbstractIon-beam mixing and isothermal annealing techniques were applied to induce phase transformations in Co/Nb and Co/Zr multilayer film stacks. Transmission electron microscopy(TEM) and x-ray diffraction(XRD) were used to characterize the microstructure of the films. Interfacial morphology and chemistry of the films were examined by Rutherford backscattering spectrometry(RBS) and cross-section transmission electron microscopy(XTEM). The formation of amorphous phases was found in both systems by either technique. A comparison of the phase transformation mechanisms induced by ion-beam mixing and isothermal annealing is given. The thermodynamic and kinetic factors controlling the phase formation and stability are discussed. For the isothermal annealing, the final stable configuration is predicted by equilibrium phase diagrams.