For over two years, intensive efforts at SEMATECH and elsewhere have focused on identifying low dielectric constant (low ε) materials which possess all of the required properties and processing characteristics needed for integration into standard IC fabrication lines. To date, no material candidate has been shown to satisfy this impressive list of requirements. For some candidates, drawbacks related to material properties such as poor thermal stability or electrical performance have been identified; in other cases, problems in process integration, for example difficulties in patterning have stalled progress.

In this paper, most of the current leading candidates for the low ε IC IMC application are identified and discussed. An attempt is made to correlate structure/property relationships in these materials with their relative attributes and deficiencies as they relate to the IMD application. Key differences in chemistry and property/processing characteristics are contrasted for low c silicon-oxygen polymers and for purely organic polymers. Novel dielectrics such as porous organic and inorganic thin films are also discussed in terms of their properties and associated process integration challenges. Since the needs for global planarization and low c IMD are occurring within roughly the same generation of minimum feature size (˜ 0.25 μm), the chemical mechanical polishing (CMP) of low dielectric constant thin films and/or of SiO2 layers deposited above them is briefly discussed. Both subtractive metalization and damascene processes are included, and the required low dielectric constant film properties and processing characteristics are contrasted for each process. Finally, the author's views on future trends in low dielectric constant materials development are presented, with an emphasis on identifying the types of chemical structures which may prove viable for this most demanding of all polymer film applications.

We have compared anisotropy of spin cast and vapor deposited polyimide (VDP) films, using internal reflection infrared spectroscopy. The films were deposited directly on the internal reflection element. We find that spin cast films are more anisotropic than their VDP counterparts, with the polyimide chains tending to align parallel to the substrate. Both films are found to contain more and less ordered regions. Within the ordered regions, the plane of the phenyl ring tends to align parallel to the substrate.

Parylene VIPTM AF-4 dielectric is a potential low εR candidate for ULSI manufacture. The search for new IMD materials with low dielectric constant (k ≤ 2.5) to enable sub 0.18 micron technologies is focusing on new polymers, deposited by either spinning or CVD methods. Two classes of requirements have to be satisfied for a material to be successful, i.e., used in volume device manufacturing. First, a set of physical characteristics have to be met, among the most important are thermal stability above 400 °C, mechanical stability, and good adhesion to a variety of substrates. Then, a second set of more stringent requirements have to be met related to device integration. For example, electrical performance in a device and dry etching for via formation. We report results on the evaluation of Parylene AF-4, deposited by vapor-deposition polymerization of tetrafluoro-p-xylylene. We present data on deposition characteristics, film composition and purity, thermal stability as well as preliminary electrical data.

A novel polymer dielectric derived from tris-perfluorocyclobutene(PFCB) monomer has been investigated as a thin film dielectric. The dielectric constant of this material has been measured at 2.35. PFCB is applied from a hydrocarbon solvated solution using conventional spin coating equipment onto the substrate and produces highly uniform films after curing at 300 °C.

This paper describes the processing and properties of films derived from PFCB, and examines the interactions between the polymer and metals at the interfaces. In particular, the interfaces formed by the deposition of Cr or Co onto PFCB are unchanged after 30 minutes at 390 °C. The interface formed by the application of the polymer onto Ta is similarly unaffected by high temperatures. The interfaces formed by the deposition of Ta or Ti onto the polymer surface did not remain intact, with substantial quantities of the metals permeating the polymer after high temperature exposures. This behavior is discussed relative to the thermo-physical properties of the metal fluorides.

Fluorocarbon polymers exhibit excellent dielectric properties; however, their limited thermal and mechanical stability render them unsuitable for most interlevel dielectric applications. In this work we explore the plasma assisted deposition of fluorocarbon films from hexafluoropropylene (C3F6) and hydrogen in order to understand how the mechanical and thermal properties might be improved. Plasma activation and ion bombardment promote the formation of a crosslinked, amorphous film structure. Several process parameters were examined including temperature and rf driving frequency. The results demonstrate that crosslinked fluorocarbon polymers with enhanced mechanical strength can be produced from C3F6 in a plasma environment. Most of the films studied here had dielectric constants in the range of 2 to 2.5. Thermal stability (to 400 °C), however, was not achieved.

Transparent, nanophase-separated, inorganic-organic hybrid polymers with dielectric constants below 3.0 have been prepared from reactively functionalized poly(amic ester) derivatives and substituted, oligomeric silsesquioxanes. These hybrid materials are stable to 400 °C and above and form tough, crack-free films. Induced cracking and crack propagation studies performed with the application of external stress suggest a maximum critical film thickness of at least 2.0 μm under severe stress conditions. These hybrid materials appear to be significantly toughened by the chemical incorporation of the polyimides relative to organically modified silicates and spin-on-glasses without significantly effecting other important polymer properties of the silicates.

Synthesis and characterization of three classes of poly(arylene ether oxazole)s and poly(arylene ether thiazole)s with trifluoromethyl substituents in various positions is described. Based on these examples, the influence of the chemical structure of the polymers on properties such as glass transition temperature, solubility, and thermal stability will be discussed. Structural parameters studied are the number and nature of the heteroaromatic rings, the number and position of the trifluoromethyl groups, and the structure (flexibility, polarity) of various linking groups.

The thermal curing (imidization) of the polyimide film has been investigated using DSC and FTIR spectroscopy. The rate and the degree of imidization of the film was found to be dependent on curing temperature, time and thickness. The degree of imidization of the film rapidly increased between 200°C and 350°C and then remained constant for curing above 350°C, indicating that the imidization reaction is complete at 350°C. Also, it was found that imidization proceeds in two stages under isothermal condition, which was characterized by an initial fast imidization step that changes into a second, slower imidization process. The slower imidization rate at the second stage is discussed and explained. Higher degree of imidization was obtained with thicker films because of the higher retention of the solvent in the thicker film. Depolymerization process during thermal curing has also been studied with FTIR. The highest degree of the depolymerization is reached with thermal curing between 225 and 275°C.

The dielectric and mechanical properties of low permittivity dielectric/metal system consisting of fluoropolymers and copper were studied. The types of polymers in this study include sputtered Teflon, plasma-deposited fluoropolymers and the soluble Teflon AF1600. All these fluoropolymers have dielectric constants below 2.0 and dissipation factors below 0.001. Copper was either evaporated or sputtered. Adhesion of copper to fluoropolymers was studied using scratch and peel tests. Different thermal treatment techniques were applied to enhance adhesion, including preannealing in vacuum or in atmosphere, as well as postannealing in vacuum. The results show generally poor adhesion for evaporated copper, with substantial improvement for sputtered copper; however thermal treatments enhance adhesion in some cases. FTIR, surface tension and XPS experiments were carried out in an effort to understand these effects. Our findings will be discussed in terms of the various effects contributing to copper/fluoropolymer adhesion.

Polytetrafluoroethylene (PTFE) has been studied as a low dielectric constant material for ULSI. A novel nanoparticle dispersion of PTFE was developed that permits the spin-coat deposition of PTFE with a thickness range of 0.2 to 1.5 μm. These PTFE nanoemulsions are aqueous emulsions containing sub-50 nm size PTFE particles and surfactant that are thermodynamically stable, optically clear, and have low viscosity and surface tension. The films cast from this nanoemulsion are uniform in thickness with a standard deviation of < 2%. From FTIR spectra, significant amounts of C-F bonds (1153 cm−1 and 1211 cm−1) are detected in the films. The index of refraction from ellipsometry measurement is about 1.35 and the dielectric constant measured from high frequency C-V curves is about 1.85. The dielectric strength is about 170 V/ μm. TGA data indicates a weight loss rate of less than 0.25%/hr. at 425 °C. The moisture absorption is less than 0.01%. After sintering, the films are extremely resistant to chemical attack by sulfuric acid, buffered HF, and positive photoresist developer. The etch rate in an oxygen plasma at 30 W is around 200 nm/min. Stud pull tests indicate good adhesion to SiO2, Al, and Cu. Results of thermal, dielectric, chemical, and adhesion tests indicate that these PTFE films have potential for use as an integrated circuit dielectric material.

Considerable progress has been made in development of thin films of nanoporous silica (also known as aerogels or low density xerogels) for ILD and IMD applications. Advantages of these materials include high thermal stability, small pore size, and similarity to conventional deposition processes, precursors and final material (silica). We have previously reported success in synthesizing low density, low dielectric constant (K<2) thin films using ambient pressure processing. However, processing of those films was complicated due to large number of process steps and difficulties in independently controlling both film thickness and film porosity.

Nanoglass has now developed a new process which considerably reduces the number of process steps and allows independent control of both film thickness and porosity. The dielectric constant of the films can be tailored between 1.3 and 2.5. These films have improved mechanical properties due to controlled pore size and narrow pore size distribution and also because of higher density. The trade-offs between density, mechanical strength and dielectric constant for these types of porous solids will be elucidated. The known properties of the film and the process flow for deposition and post-deposition curing and the role of the relative rates of reaction, gelation, aging, and drying will be presented.

The continued scaling of device feature size demands the use of low permittivity intermetal dielectric materials. Porous silica xerogel films have low dielectric permittivity through the incorporation of micropores into the SiO2 network. A feasible xerogel process has been developed. Crack-free and uniform silica xerogel films up to two microns in thickness with targeted porosity were readily coated. Xerogel materials completely filled 0.3 μm wide gaps with a 2:1 aspect ratio. MOSCAP measurements revealed a low permittivity and high dielectric breakdown strength. The dielectric breakdown strength is expected to be higher than that of ambient air because the average pore size of in the xerogel film is much smaller than the mean free path of the ambient air. Surface treated xerogel films were found to be hydrophobic as indicated by the absence of adsorbed moisture peaks in FTIR spectra. Xerogel films maintained their porosity after deposition of dense capping layers and a subsequent process under 700 atm Ar pressure at 400 °C. Test structures containing xerogel were successfully planarization with CMP and went through a tungsten plug deposition process without delamination nor collapsing. These results reflect the reasonable mechanical strength of xerogel films.

Low dielectric-constant mesoporous silica films were prepared by condensation of a silicate network around surfactant micellar structures. Adherent, porous films 0.5–1.0 μm in thickness, and containing an ordered assemblage of ≈2 nm diameter pores were synthesized by spin-coating water/ethanol-based solutions containing a silica precursor and surfactant template. In this paper, film deposition conditions are described, and film thickness, porosity, refractive index and dielectric constant measured by ellipsometry are presented. Using a coating solution containing tetraethyl orthosilicate (TEOS) and a cationic cetyltrimethylammonium chloride (CTAC) surfactant template, the film porosity and dielectric properties were controlled over a wide range by adjusting the CTAC/TEOS molar ratio. With the CTAC/TEOS ratio between 0.1 and 0.15, the pores were highly ordered in hexagonal arrays after heat treatment at 550 °C. With a CTAC/TEOS ratio of 0.21, films with a pore volume of ≈64% (≈36 vol% silica) could be synthesized. The measured index of refraction for these highly porous films at 500 nm wavelength was 1.16, indicating that these films are potentially useful as low K interlayer dielectrics.

Si-O-F alloys have static dielectric constants (εs) significantly lower than SiO2. Infrared absorption spectra provide the basis modeling the molecular structure of these alloys. Contributions of electronic and vibrational transitions to εs are discussed in terms of an empirical chemical bonding model. Ab initio calculations are then used to identify inductive effects of Si- F bonds on the properties of Si-O-Si groups that are back-bonded to the Si atom of the Si-F group. These calculations provide a theoretical framework for understanding how relatively low concentrations of F atoms produce the significant decreases in εs reported for Si-O-F alloys.

The reliability of SiOF films for intermetal dielectrics in multilevel interconnections of ULSIs is investigated. SiOF films were deposited by ECRCVD using H-free source gases, i.e., SiF4 and O2. The effect of post plasma treatment on the moisture absorption and dielectric properties of SiOF films were carried out in terms of air exposure time. The reliability test of Cu / TiN / SiOF / Si specimen was carried out in terms of temperatures by RTA in N2 ambient. After O2 plasma treatment, no appreciable peak directly related to moisture absorption was detected. The C-V characteristics of the O2 plasma treated SiOF film showed that the film remained to hold the sound dielectric properties even after boiling treatment. The Cu / TiN / SiOF / Si system was found to be reliable up to 600 °C.

A characterization of Plasma-Enhanced CVD Fluorinated Silicate Glass (FSG) is presented. This study investigates the behavior of FSG film in the Chemical Mechanical Polishing (CMP) environment and compares those characteristics to undoped TEOS (UTEOS), Thermal Oxide and Si-Rich oxide capped FSG films. The removal rate, refractive index (RI), surface roughness, contact angle, water content by FTIR and thermal desorption spectroscopy (TDS) were studied.

The FSG films are polished ˜ 10% faster than the undoped PECVD oxide films. Their composition was slightly changed after CMP as can be seen by the minor increase in the RI. A layer of Si-Rich oxide (SRO) was found to have a stabilizing effect on the FSG film during CMP and post CMP cleaning operations, and thus can be used in the intermetal dielectric schemes that require low dielectric constant FSG layers.

SiO2 films have been prepared from tetraisocyanatesilane (TICS, Si(NCO)4) by using a conventional capacitively coupled RF(13.56MHz) plasma-enhanced CVD method. The films show poor water resistivity, which originates in inclusion of NCO in the films. This has been improved by mixing O2 to the source gas. SiO:F films have been prepared by mixing SiF4. The film with F concentration of 6at.% has shown dielectric constant of 3.3, resistivity of 3.6×1015Ωcm at 1MV/cm and break-down electric field of 7.7MV/cm. Infrared spectrum of the film have contained no noticeable OH peak even after exposing to the air for 2 weeks.

One solution to reduce the time constant of ultra large scale integrated circuit (ULSI) is the use of a low dielectric constant intermetal film like fluorinated silicon oxide (SiOF). We could obtain SiOF films with low dielectric constant as low as 2.6 and good step coverage by adding CF4 to SiH4 and N2O in plasma-enhanced chemical vapor deposition (PECVD) process. To investigate the dielectric constants due to each polarization and the reason for the decrease in the dielectric constant, we used capacitance-voltage (C-V) and ellipsometry measurements, and Kramers-Kronig transformation. The decrease in dielectric constant could not be completely explained by the reduction in ionic and electronic polarization. We could detect silanol groups, Si-OH in the films and their decrease with increasing CF4 flow rate. It is suggested that the main polarization component to decrease dielectric constant is such as orientational polarization. The step coverage of film was improved by adding CF4. It is suggested that the reduction in the sticking probability of films forming species due to the change in surface state improved the step coverage.

The thermal stability of fluorinated SiO2 films (SiOF) was found to be dependent on F content and the type of substrate upon which the film was deposited. SiOF films with a range of F concentrations were deposited using an electron cyclotron resonance (ECR) plasma upon Si, Al/Si, TiN/Al/Si, and Al/SiO2/Si substrates. Following deposition, the films were deliberately hydrated and/or annealed and their stability assessed. Hydration was found to only affect the high F content films. Capacitance changes with annealing in the high F content films were found to occur beginning at 200°C. These changes, which were independent of substrate type, likely occurred due to desorption of H2O in the films. After annealing of the high F content films up to 400°C, a reduction in F content was found for SiOF films on some substrates. Significant reductions were found for SiOF films on Al/Si substrates, while little or no change was found for films on TiN/Al/Si, Al/SiO2/Si, or Si substrates. Local chemical analysis of those films which showed F reduction indicated that the F profile was approximately uniform throughout the layer and did not pile-up at the interface. The substrate-dependent thermal instability exhibited by these films suggests the chemical nature or qualities of the substrate may play a role in the F reduction reaction.

A variety of diamondlike carbon (DLC) materials were investigated for their potential applications as low-k dielectrics for the back end of the line (BEOL) interconnect structures in ULSI circuits. Hydrogenated DLC and fluorine containing DLC (FDLC) were studied as a low-k interlevel and intralevel dielectrics (ILD), while silicon containing DLC (SiDLC) was studied as a potential low-k etch stop material between adjacent DLC based ILD layers, which can be patterned by oxygen-based plasma etching

It was found that the dielectric constant (k) of the DLC films can be varied between >3.3 and 2.7 by changing the deposition conditions. The thermal stability of these DLC films was found to be correlated to the values of the dielectric constant, decreasing with decreasing k. While DLC films having dielectric constants k>3.3 appeared to be stable to anneals of 4 hours at 400 °C in He, a film having a dielectric constant of 2.7 was not, losing more than half of its thickness upon exposure to the same anneal. The stresses in the DLC films were found to decrease with decreasing dielectric constant, from 700 MPa to about 250 MPa. FDLC films characterized by a dielectric constant of about 2.8 were found to have similar thermal stability as DLC films with k >3.3. The thermally stable FDLC films have internal stresses <300 MPa and are thus promising candidates as a low-k ILD.

For the range of Si contents examined (0-9% C replacement by Si), SiDLC films with a Si content of around 5% appear to provide an effective etch-stop for oxygen RIE of DLC or FDLC films, while retaining desirable electrical characteristics. These films showed a steady state DLC/SiDLC etch rate ratio of about 17, and a dielectric constant only about 30% higher than the 3.3 of DLC.