Porous organosilicates useful for on-chip insulator applications can be prepared by templating the vitrification of low molecular weight silsesquioxanes (SSQs) using highly branched, thermally labile macromolecules which are subsequently removed in a thermal process to generate porosity. The process involves spin coating a mixture of the matrix material and the porogen (pore generator) followed by thermal curing to initiate vitrification and decomposition of the porogen. The morphology is fixed during the formation of the nanoscopic inorganicorganic hybrid and is maintained during foaming. This process generates controllable and stable morphologies where the void volume is determined by the porogen loading level. The porous materials are thermally robust and intrinsically hydrophobic without subsequent chemical treatment. Dielectric constants of < 2.2 are easily achieved for pore volumes of only 20%, and this porosity appears to be predominately closed cell in nature. These materials display a number of thermal mechanical and electric properties consistent with the requirements for on-chip insulator applications.

A porous polymer material, which is made of a two-phase composite and contains 35% porosity with a pore size less than 50Å, is found to have a dielectric constant of 1.8. It absorbs almost no water. The electrical properties, such as capacitance and leakage current, do not change with time and temperature.

Parylene-N films vapor deposited near liquid nitrogen temperature (77 K) undergo a unique ‘wave’ polymerization process in which a rapidly moving reaction front is apparent as the film changes from translucent to optically opaque. This moving reaction front produces a highly porous polymer film. The porosity of these films is approximately 80%. By capturing the wave process on video we have quantified the moving ‘wave’ velocity, which averages 11 cm/s. Timeaveraged deposition rates of the resulting opaque, porous films are more than 8 μm/min. This rate is more than two orders of magnitude greater than the measured deposition rates of nonporous films that are deposited at higher temperatures, at otherwise fixed conditions of monomer delivery rate and deposition chamber pressure.

Surface modified silica xerogel films of high porosity (60 - 90 %) and uniform thickness (0.4–2 μm) were fabricated at ambient pressure on silicon and silicon dioxide. The rheological properties that govern film uniformity were determined. A relation between the final dried film thickness and spin speed was developed. The porosity and thickness of the films could be controlled independently. The same porosity could be obtained over a wide range of aging time and temperature combinations. Fracture toughness was measured using the edge-lift-off technique. The best values were comparable to concrete. Surface modification was affected by treating the film with trimethylcholorosilane (TMCS) and other modifiers. Moisture adsorption was studied at 100% RH using a quartz crystal microbalance technique. Depending upon the degree and kind of surface treatment, films absorbed as much as 32% or as little as 2% of their weight in water. Dielectric constants (K), losses and breakdown strengths were comparable to values for calcined, bulk aerogels. Thin (≤ 500 Å) films of Copper (Cu) and Tantalum (Ta) were deposited on xerogel films and subjected to thermal annealing. No diffusion was observed within the limits of RBS. High-density plasma etching showed that the films etch an order of magnitude faster than conventional SiO2 films.

A novel spin on material derived from perhydropolysilazane that converts into ultra-low k inorganic films is described in this paper. The obtained films, cured at 400°C in N2 atmosphere, exhibit dielectric constants as low as 1.6 which do not change after holding the wafers in a clean-room mbient for 2 months. Cross-sectional SEM images of the cured films show the aggregation of small granules with diameters ranging from 5 to 30 nm. The films can be obtained by conventional SOG process: spin-coating, baking and curing, without any additional process such as hydrophobic treatment.

The average atomic compositions of the films are, Si/O/N/C = 40/55/5/0.5 (atomic %), by XPS analysis. These results indicate that the films have hydrogen silicon oxynitride structures. No evolution of H2O and NH3 was detected by TDS analysis in the temperature range of RT to 800°C. Hydrophobic Si-H and Si-H2 groups remaining in the film might prevent water absorption, resulting in the low dielectric constant.

The remainder of Si-H and Si-H2 constituents in the cured films is the result of selective oxidation reactions of perhydropolysilazane in the baking process with the use of a specific catalyst. The structures of the films are tailored by altering the amount of the catalyst. In this study, we also demonstrate the relationship between the effect of the catalyst and the film properties.

Phenyl and alkyl groups were introduced into silica glass in order to prevent the formation of a Si-O network and reduce its density and to reduce the dielectric constant of the glass. In this regard, the phenyl group was more effective than the alkyl group. The measured Si-O intensity of FTIR spectrum of the glass with introduced phenyl was only 14% of thermal SiO2 film, however, the thermally stable carbon component prevented further reduction in density after 400°C annealing.

Previously, the fabrication of air-gap structures for electrical interconnections was demonstrated using a sacrificial polymer encapsulated in conventional dielectric materials. The air-gaps were formed by thermally decomposing the sacrificial polymer and allowing the by-products to diffuse through the encapsulating dielectric. The diffusivity of the polymer decomposition products is adequate at elevated temperatures to allow the formation of air-gaps. This process was extended to form low dielectric constant, porous silica from commercially available methylsilsesquioxane (MSQ) by the addition of the sacrificial polymer to the MSQ. The porous MSQ film was thermally cured followed by decomposition of the NB at temperatures above 400°C. The dielectric constant of the MSQ was lowered from 2.7 to 2.3 by creating 70 nm pores in the MSQ. The voids created in the MSQ appeared to exhibit a closed-pore structure.

We studied the effect of pore diameter on the film properties of low-density porous material with special interest in the pore diameter range below 5nm. A novel low-density material, Interpenetrated SOG (IPS), was designed to realize such porous character. It is composed of a pyrolic template, which is an organic olygomer, and framework, made from SOG polymer and silica sol, forming an interpenetrated structure. Thermal decomposition of the organic component successfully resulted in films with pore diameter range below 5nm. It is found that the dielectric constant and the pore diameter depended on the density and the film strength was affected by the average pore diameter. Practically, films with the average pore size below 5nm possessed dielectric constants as low as 2.2–2.0 and sufficient strength.

In recent years there have been widespread efforts to identify low dielectric constant materials that can satisfy a number of diverse performance requirements necessary for successful integration into IC devices. This has led to extensive efforts to develop low k materials and the associated process integration. A particularly difficult challenge for material development has been to find the combination of low dielectric constant and good thermal and mechanical stability. In this paper recent characterization results for low k materials performed at the University of Texas will be reviewed, with an emphasis on the relationship of chemical structure to the aforementioned key material properties. For example, measurements showing the effect of film porosity on dielectric constant and thermal and mechanical properties is presented. This data, as well as that for other material types, demonstrates the tradeoffs between dielectric constant and thermomechanical properties that are often made during the course of material development.

Ellipsometric porosimetry allows measurement of the pore size distribution in a thin porous film deposited on top of any solid substrate. The most important concept of this technique is the use of in-situ ellipsometry to determine the amount of adsorbate adsorbed/condensed in the film. Changes in refractive index and film thickness are used for calculation of the amount of adsorbate. A room temperature porosimetry based on adsorption of vapours of some organic solvents has been developed. In this paper, a method of calculation of the pore size distribution and reliability of the ELP results is discussed and the validity of the Gurvitsch rule for organic adsorbates (toluene, heptane, CCl4) is examined. Porous silica films on top of Si wafers were used for this analysis.

A quantitative characterization of the thermal properties is required to assess the thermal performance of low dielectric constant materials. Recently we have developed a technique based on the 3-omega method for measuring the thermal conductivity of porous dielectric thin films. In this paper we present the results on the measurements of thermal conductivity of thin porous films using this method. A finite element method analysis is used to evaluate the approximations used in the measurement. Two porosity-weighted thermal resistor models are proposed to interpret the results. By studying the dependence of the thermal conductivity on porosity, we are able to discuss the scaling rule of thermal conductivity. Additionally, a steady state layered heater model is used for evaluating the significance of introducing porous ILDs into an interconnect structure.

The effect of a post plasma treatment on the dielectric properties and reliability of fluorine doped silicon oxide (SiOF) films deposited by electron cyclotron resonance chemical vapor deposition (ECRCVD) was studied. Also, the thermal stability of an electrodeposited Cu / sputtered Ta interconnect system with SiOF intermetal dielectrics was examined by annealing in a vacuum furnace. The stability of the dielectric constant of SiOF films was improved by O2 post plasma treatment. Surface modification by the plasma treatment was effective in prevention of water absorption. The Cu/Ta/SiOF/Si system was thermally stable at least up to 500°C for 3h. For the Cu/Ta/SiOF/Si multilayer structure, the plasma treatment seemed to play a big role in suppressing the interdiffusion between SiOF and metal interconnects. By C-V measurement, the electrical stability of the Cu/Ta/SiOF/Si multilayer structure was found to be stable up to 500°C for 2 h.

This paper reports degradation characteristics of low-k dielectric (FOX) in multi-level metal structures (comb-type capacitors) submitted to moisture stress. A large increase of leakage current (>105) and capacitance (up to ×3) is observed after moisture stress when only FOX is used as lateral dielectric, while moderate degradation takes place when an oxide liner is placed between FOX and metal lines. Enhanced moisture induced degradation is found on previously probed dices with respect to virgin devices. Systematic electrical measurements, combined with SEM analysis, are performed to find out the moisture diffusion path. When contact pads are damaged by previous probing (owing to the mechanical weakness of FOX in the pad stack), they provide a direct entry path for enhanced humidity intake. Humidity is also shown to enter through wafer border. Using a SiO2 liner combined with FOX improves considerably the resistance to moisture degradation.

The quest to improve the high performance in ULSI circuits, is driving the search for new materials with low dielectric constants (k=2.5–3.0) for the back end of the line (BEOL) interconnect structures. Novel SiCOH films comprising Si, C, O and H, have been deposited by a PECVD deposition technique. The films have been characterized as-deposited and after anneals of up to 8 hours at 400°C. The atomic composition of the films has been determined by RBS and FRES analysis and their optical properties have been determined by FTIR and index of refraction measurements. The mechanical properties have been determined by measurements of stress and of crack development velocity in water. Metal insulator silicon structures have been used to test the electrical properties of the SiCOH films. After an initial stabilization anneal, the SiCOH films are thermally stable up to 400°C have low tensile stresses (<50 MPa), an extremely low crack propagation velocity, and a hydrophobic behavior. According to the deposition conditions the films have dielectric constants in the range of 2.8 to 3.5. These film properties, combined with an easy-to-integrate deposition process indicate that the material has a strong potential as an interconnect dielectric.

The thermal and chemical stability of low k fluorinated amorphous carbon (a-C:F) material, deposited by a novel co-sputtering process using both polytetrafluoroethylene (PTFE) and graphite targets was investigated. Thin films of a-C:F with fluorine concentration of 2–55% were deposited, and carbon is observed by XPS in four distinct chemical states, C-C, C-F, C-F2, C-F 3. The relative intensity of C-Fx to C-C increased in intensity with increasing fluorine content and decreasing deposition temperature. Formation of tantalum fluoride was observed upon deposition of tantalum nitride, and the defluorination of the film could lead to reliability and delamination problems.

The adhesion of thin film polymers will be critical in the integration of low-κ materials into microelectronic processing. This study describes the adhesion of two promising low-κ polymers (polyimide and benzocyclobutene) to a silicon dioxide surface. Critical adhesion values were measured using interface fracture mechanics samples in a double cantilever beam geometry. The effect of subcritical (time-dependent) delamination was also evaluated for these systems. Subcritical debonding data are important in understanding the effect of environment and temperature on interface reliability. To that end, experiments were conducted over a range of humidities to elucidate the effect of moisture on interface delamination. The important effect of the acceleration of debond growth rates due to cyclic loading is also described. In addition, XPS studies are presented to characterize the debond path in these layered systems.

In this paper, we present studies on the integration process of CVD Cu with low-k fluorinated amorphous carbon (a-F:C) in single level and multilevel damascene structure. A thin layer of adhesion promoter material, SiC:H, was utilized to enhance the adhesion and mechanical properties of the damascene stacking layers. The SiC:H layer could also serve as a barrier to contain fluorine atoms from diffusion. The improved a-F:C damascene stacking layers are able to sustain the process of CMP, heat treatment, patterning and plasma etching. The fabrication of single and multi-level damascene structures is proved to be feasible. Some of the electrical performance data evaluated on the Cu/a-F:C damascene structure will be also presented in this paper.

Advanced on-chip interconnects using new materials and new integration architectures are necessary for current and future IC chips in order to meet the requirements in performance, reliability and manufacturing cost. Insulating materials with low dielectric constant (low-κ) and conductive materials with low-resistivity have drawn significant attention for their possible applications in IC interconnects. Dual damascene interconnect integration architectures not only offer process simplification and low cost, but also enable the use of low-resistive Cu for interconnect wiring. Use of low-κ materials in dual damascene architecture is challenging due to material and processing issues. In this paper, the evolution of advanced interconnects, materials and technology options, and some recent achievements in advanced interconnect systems of low-κ dielectric and dual damascene architectures for both Al and Cu metallization are reviewed and discussed.

The recent introduction of dual inlaid Cu and oxide based interconnects within sub-0.25μm CMOS technology has delivered higher performance and lower power devices. Further speed improvements and power reduction may be achieved by reducing the interconnect parasitic capacitance through integration of low-k interlevel dielectric (ILD) materials with Cu. This paper demonstrates successful multi-level dual inlaid Cu/low-k interconnects with ILD permittivities ranging from 2.0 to 2.5. Integration challenges specific to inorganic low-k and Cu based structures are discussed. Through advanced CMP process development, multi-level integration of porous oxide materials with moduli less than 0.5 GPa is demonstrated. Parametric data and isothermal annealing of these Cu/ low-k structures show results with yield comparable to Cu/oxide based interconnects.

The technologies utilizing Fluorinated Silicon Oxide (FSG, k=3.6) and Hydrogen Silsesquioxane (HSQ, k=3.0) have been established for 0.25-μm and 0.18-μm generation ULSIs. However, low-k materials for the next generation ULSIs, which have a dielectric constant of less than 3.0, have not become mature yet. In this paper, we review process integration issues in applying FSG and HSQ, and describe integration results and device performance using Fluorinated Amorphous Carbon (a-C:F, k=2.5) as one of the promising low-k materials for the next generation ULSIs.