For several years the industry has recognized the need of developing low k dielectric material and high conductivity metal for high performance interconnect. Low k dielectric will impact both power and delay favorably, while higher conductivity metal will reduce delay time. In order to be useful, new low k dielectric materials must be carefully characterized for their electrical, chemical, thermal and mechanical properties. In addition, their impact on process integration, fabrication cost and device reliability must also be considered. Since the gestation period for introducing a new material is very long, a set of standard testing methodologies are required to speed up the development process. This review will discuss various material options and the progress of material development and characterization methodologies. Example results will be provided for assessing these parameters.

The mechanical, thermal, electrical and moisture absorption properties of rod-like, fluorinated polyimides and copolyimides developed in our laboratory are briefly reviewed. Further characterization work on the most promising candidates for interlayer dielectric applications is reported. This includes in- and out-of-plane refractive index measurements to estimate the isotropy of the dielectric constant, isothermal TGA measurements to assess thermal stability at processing temperatures and dynamic-mechanical analysis to ascertain the glass transition temperature and the mechanical modulus at temperatures in excess of the glass transition

Low dielectric constant interlayer dielectric ( ILD) materials are required for the advanced silicon integrated electronics such as those in the ULSI era[3, 10]. We have investigated several such materials. In this paper the results of our investigations of the materials and electrical properties, processing ( to form ILD ), and applicability of a DuPont fluorinated polyimide are described and discussed. Weight loss, FTIR, and ellipsometric measurements have been carried out. The DuPont fluorinated polyimide thin film was observed to be thermally stable up to 450°C, which is monitored by using dynamic TGA with a ramping rate of 10°C/min or 5°C/min in N2 Ambient. Also MIPOS capacitor characterization, effect of temperature and moisture on these properties have been determined. The dielectric constant was observed to be as low as 2.5 and the refractive index is around 1.63, both being stable up to 450°C. However, the DuPont Fluorinated polyimide exhibited a flat band voltage shift on C-V curve after 400°C annealing in vacuum environments for 1 hr. Compatibility with copper as the interconnecting metal has been determined and discussed[8]. It is concluded that this polymer is a possible candidate for ILD application.

Several novel processes have been developed in our lab to synthesize and deposit polymers that have extremely low dielectric constants (below 2.5) as thin films. These reactive intermediates are condensed onto wafers in a vacuum chamber and no chemical waste is produced. Furthermore, cold traps can be used to recover uncondensed monomers through the pumping system. Because of the extremely low dielectric constant, very low moisture uptake and high temperature stability of these vapor depositable materials, these environmentally friendly polymers are very attractive for future electronic applications as interlayer dielectrics. Examples are Parylene-F(fluorinated), poly(naphthalene) (PNT-N), poly(fluorinated naphthalene) (PNT-F), and Teflon AF 1600 (amorphous). These films were deposited at substrate temperatures between −20 and 350 °C and at deposition pressure was 10−4−2.5 Torr. The film thicknesses were between 0.1–10 μ. and dielectric constants were between 1.9–2.5. The thermal stabilities of these films were between 360–590 °C. Among them, poly(fluorinated naphthalene) dissociates at about 590 °C. Poly(naphthalene) and poly(fluorinated naphthalene) are obtained as films by vaporizing monomers in vacuo and transporting the vapor to a hot surface.

Poly(indanes) can be prepared by the photoinduced cationic polymerization of a variety of diisopropenyl aromatic monomers. Typically, polymerization proceeds rapidly under UV irradiation catalyzed by diaryliodonium salt photoinitiators to give hard, transparent films. Investigations have shown that the polymerization proceeds mainly by a dimerization followed by a ringclosure process to yield indane structures along the polymer backbone. Film-forming mixtures containing either pure monomer or mixtures of monomers together with a poly(indane) prepolymer can be spin coated onto silicon wafers. Measurements made on the photopolymerized coatings give low dielectric constants. These coatings also display excellent thermal stability.

Among the more promising approaches to minimizing capacitance in the multilevel interconnect of integrated circuits containing sub-half micron metal spacings is the development of organic polymers which exhibit high performance in key attributes such as thermal stability, low dielectric constant, and low moisture absorption coupled with high outgassing rates of what little moisture may be present. The use of such polymers as the intermetal dielectric can reduce power consumption and cross talk, while increasing signal propagation speed. While polyimides are the most extensively characterized polymer thin film dielectrics, and are in many cases suitable for the intermetal dielectrics in multichip modules, their tendency to absorb significant quantities of moisture, coupled with relatively slow outgas characteristics (presumably due to hydrogen bonding between water molecules and the carbonyls of the polyimide) constitute significant impediments to throughput in the fabrication of IC interconnects.

The search for alternative polymers which incorporate the “good” characteristics of polyimides while exhibiting improvements in electrical, moisture, and processing characteristics led us to the development of nominally 1 μm spin-on films derived from a family of noncarbonyl containing aromatic polyethers. Fluorinated poly(arylethers) based on decafluorobiphenyl exhibit thermal stability comparable to polyimides, from ten to forty times lower moisture absorption, dielectric constants in the mid-two's, and good retention of storage modulus above their glass transition temperatures. The precursor spin-on solutions, formulated in low toxicity organic solvents, exhibit excellent shelf life, and can be prepared with extremely low levels of metallic contamination. This paper describes the synthesis and both solution and film properties of this newly developed class of highly processible thermally stable polymers, first reported by Mercer, et. al. [1]. The characteristics of the polymers when spin-coated on silicon wafers is emphasized. Thermal and thermomechanical properties of nominally 10-25 μm free standing films are also described.

A new diamine monomer, N,N'-bis(4-aminophenyl)-2,5-(diisopropoxycarbony)benzene-l,4-dicarboxamide was synthesized. Using the diamine, several alternating copolyimide precursors were successfully synthesized by its condensation polymerization with dianhydrides. The structure and properties (mechanical properties and thermal expansivity) of alternating copolyimides imidized thermally were characterized, and compared with those of the corresponding randomcopolyimides prepared from the copoly(amic acid)s. The alternating copolyimides exhibited relatively higher chain order and in-plane orientation in films than the random copolyimides. The higher chain order and in-plane orientation led to the higher tensile modulus and lower thermal expansivity in the alternating copolyimides. Such effect of comonomer sequence was found to be highly pronounced in copolyimides consisting of comonomers which have a large difference in the chain rigidity. It was demonstrated in this study that the formation of alternating copolyimides is a suitable route to improve the performance of properties.

Foamed polyimides have been developed in order to obtain thin film dielectric layers with very low dielectric constants for use in microelectronic devices. In these systems the pore sizes are in the nanometer range, thus, the term “nanofoam”. The polyimide foams are prepared from block copolymers consisting of thermally stable and thermally labile blocks, the latter being the dispersed phase. Foam formation is effected by thermolysis of the thermally labile block leaving pores the size and shape corresponding to the initial copolymer morphology. Nanofoams prepared from a number of polyimides as matrix materials, were investigated as well as a number of thermally labile polymers. The foams were characterized by a variety of experiments including, TEM, SAXS, WAXD, DMTA, density measurements, refractive index measurements and dielectric constant measurements. Thin film foams, with high thermal stability and dielectric constants approaching 2.0, can be prepared using the copolymer/nanofoam approach.

The dielectric properties of the heat-resistant heterocyclic polymers are governed by their chemical structure and the absence of unwanted dipoles. A polyphenylquinoxaline is taken as an example of the effect of the dipole symmetry. In the particular case of polyimides, it is obvious that the dehydrocyclization of the intermediate polyamic acid has to be completed to produce the lowest achievable permittivity. This is illustrated by the polycondensation in solution of the polyimides based on 4,4'-(9H-fluoren-9-ylidene)-bisbenzeneamine. In addition, the spatial conformation of the macromolecular chain can also play an important role. The chemistry of a polyimide prepared from 4,4'-(1,3-benzenedicarbonyl) bis(1,2-benzenedicarboxylic acid dianhydride) allows to discuss this topic. However, a dielectric constant of 2.7 seems to be the ultimate value achievable for dense films prepared with the most performing polymers. The introduction of homogeneously distributed micro-voids allows the preparation of dielectric films with a permittivity lower than 2.0. The modification of a polymer with tertiobutyl carbonate (t.BOC) groups is a promising approach because it offers a large processability window.

We have investigated plasma fluorination of hydrocarbon polymer films spincoated onto silicon wafers. Fluorinated hydrocarbon polymers gave low dielectric constants of 2.0 – 2.4 which was close to that of PTFE. However, the thermal stability of these fluorinated polymers inferior to that of the non-fluorinated polymers. We believe that this is due to a decrease in the molecular weight resulting from C-C bond cleavage. We think that such a side reaction will be controlled by removing residual oxygen in a fluorination chamber. The molecular structures of the fluorinated polymers were analyzed by X-ray photo emission spectroscopy. Finally, we proposed direct patterning of interlayer dielectrics using plasma fluorination.

Material technology for future microelectronics will require advances in all facets of materials and processing. Low dielectric constant resins that exhibit facile processing and good thermal and mechanical behavior represent one area of needed research and development. The dielectric constant must be lower than that of amorphous silicon dioxide and possess the right properties for integration with future metallurgies such as copper. Several thermoset resins that were predicted to possess the necessary characteristics have been synthesized and studied. These include a copolymer of 1,3,5-tris(2-allyloxy-hexafluoro-2-propyl) benzene with polymethylhydrosiloxane and several cyanate ester resins. Thermal gravimetric analysis indicated significant degradation between 300 and 400 degrees C depending upon the resin. Dielectric constants were measured up to 40 GHz and ranged from 2.25 to 2.75. Compatibility with copper multilevel processing was addressed. The processability of the dielectric resins was investigated to address the integration issues associated with the fabrication process.

Low dielectric constant organic materials are ideal for use as interconnect dielectrics forintegrated circuits. As compared to silicon dioxide, organic dielectrics with K <3.84 reducepower dissipation, crosstilk and RC delays in interconnects. Curing is essential afterdeposition of these materials to initiate polymerization reactions and form films of desirableelectrical properties. For high performance and reliability, low thermal budget processing isa necessity. Rapid isothermal processing (RIP) based on the use of dual spectral sources isa potential technique to lower the thermal budget. In this paper, we demonstrate the role ofphotoeffects in the curing of polyimide films (K∼2.6) using a rapid isothermal processor witha dual spectral source (Tungsten Halogen and vacuum ultra violet (VUV) lamps) as a sourceof optical and thermal energy. Lamp configurations that allowed a greater availability of ultraviolet and vacuum ultra violet photons on the film to initiate physical and chemical processesallowed a lower curing temperature to achieve the same level of immidization. Furthermore, these samples also gave the lowest leakage current and film stress. Therefore, the rapidheating and cooling features of rapid isothermal processing in conjunction with lowerprocessing temperature through the use of high energy photons to enhance surface reactionsgive superior film properties

Low dielectric constant polymers offer many advantages in circuit performance, such as power dissipation, crosstalk and RC delay, when used as inter-layer dielectrics (ILDs). Silicon dioxide, a material commonly used as an ILD has a dielectric constant of 4.0. Organic polymers that have dielectric constant values ranging from 2.0 to 3.0 offer attractive alternatives to SiO2. However, it has been a great challenge to find organic polymers with thermal stability up to 450 °C. We have characterized thermal properties of polymers using thermal desorption analysis. isothermal TGA and FTIR to identify weak functional groups. In addition, we have measured the hardnesses and moduli of these polymers and found that the values are much lower than those of SiO2.Stress distributions in the interconnect system were analyzed using finite element modeling in order to understand potential reliability problems.

Multilevel interconnects for high performance ULSI need low dielectric constant materials for inter-metal layer dielectric, or ILD, to reduce signal propagation delay, power consumption and cross talk noises. We have studied the physical, dielectric and processing properties of a wide variety of promising low dielectric polymeric materials for ILD applications. This paper presents capacitance values measured over a wide range of temperature, and a summary of the measured dielectric properties. The anisotropy 1,2 of dielectric properties were determined experimentally, the vertical (out-of-plane) dielectric properties were determined by MIM (Metal-Insulator-Metal) measurements, and the horizontal (in-plane) dielectric properties were determined by intra-line measurements of sub-half micron serpentine and comb test structures.

In recent years, polyimide film has become a commonly used insulator for microelectronic devices including advanced semiconductor chip and high density packaging. It has the advantages of low dielectric constant, thermal stability and low processing cost. In general, it is important to understand both the thermal and mechanical properties of a microelectronic material. Thermal and mechanical properties play very important roles in terms of determining the device structural integrity. Mismatch of material properties could cause feasibility and reliability problems. These properties are also essential for mechanical stress and thermal stress modeling studies. Biphenyldianhydride-Phenylenediamine (BPDA-PDA) and Pyromellitic Dianhydride-Oxydianiline (PMDA-ODA) are two commonly used polyimide films in electronic industry. Their thermal expansion coefficients and mechanical properties were studied and reported here.

The glass transition temperature, Tg, of polystyrene (PS) thin films on silicon wafers with different surface preparations was determined in order to explore the effect of interfacial interaction energy on local chain dynamics. To enhance interface behavior, the film thickness included in this work was as low as 50 Å. PS with monodisperse molecular weights was chosen as a model polymer. Two different silicon surfaces were prepared: sulfuric acid-clean and hydrogen passivated. Tg of thin films was determined by monitoring the film thickness as a function of temperature. Tgwas identified as the temperature where the thermal expansion coefficient underwent an abrupt change. X-ray reflectivity was used to determine film thickness with a precision of a few angstroms. Tg of PS was found to depend strongly on the initial film thickness, as well as the substrate surface.

A review of the most common methods of permittivity measurements on thin films, printed-wiring and circuit boards, and substrates is presented. Transmission-line techniques, coaxial apertures, open resonators, surface-wave modes, and dielectric resonators methods are examined. The frequency range of applicability and typical uncertainties associated with the methods are summarized.

A practical, low-cost research system for the fluorescent optical imaging of BCB (DVS-bisBCB) thin films has been used for the first time to evaluate coating quality on spin coated silicon wafers. In general, DVS-bisBCB thin films are easily produced defect free with a high degree of planarization. Various features of the coating are enhanced visually by the fluorescence making detection, digital storage, and quantification easier. Examples of features found in defective coatings made intentionally by a process to generate several common thin film defects are variations in film thickness, foreign particles, pinholes, and residual polymer in vias. The fluorescent bands of the normally transparent resin are easily excited in the near UV with a mercury lamp, causing a semi-opaque visible emission which could be observed by conventional imaging hardware. The BCB fluorescent quantum yield, or efficiency, is similar to fluorescent dyes so that only a small amount of BCB need be present in a substrate to allow optical inspection.

This strong fluorescent property of DVS-bisBCB polymer, not possessed by many polyimide resins, could reduce labor costs of manual inspections and improve multichip module (MCM) processing yields. This digital imaging technique has potential for further development as a cost-effective automated optical inspection (AOI) method.

Low dielectric constant insulators offer the potential of improved interconnection delay and conductor packing density in advanced ICs, both with current metallization schemes and with future technologies such as copper. While polymer materials are very promising in such applications, significant issues must be addressed before oxide-based materials are replaced in mainstream applications. This invited paper reviews the directions of our program, which has emphasized the use of vapor deposited polymers compatible with uniform deposition over large diameter wafers and copper metallization. Therefore, emphasis is placed on polymer material characteristics compatible with inlaid metal (ie. Dual Damascene) patterning.

As device geometries and operating voltage continue to scale while functional density increases, it is imperative to reduce the RC time delay. The replacement of Si0 2 as an intermetal dielectric with an insulator of lower dielectric constant is a particularly attractive solution since it provides immediate performance improvement through reduction in capacitance. An embedded polymer integration scheme improves the interconnect performance through line-to-line capacitance reduction by using polymer only between tightly spaced lines. The gapfill polymeric materials do not degrade the electromigration performance of standard multilayered TiN/Al/TiN interconnects. Embedded polymers alleviate many of the integration and reliability problems associated with polymer integration, and can be easily adopted into a standard production process.