The era of silicon Ultra-Large-Scale-Integration (ULSI) has spurred an everincreasing level of functional integration on-chip, driving a need for greater circuit density and higher performance. While traditional transistor scaling has thus far met this challenge, interconnect scaling has become the performance-limiting factor for new designs. Both interconnect resistance and capacitance play key roles in overall performance, but modeling simulations have highlighted the importance of reducing parasitic capacitance to manage crosstalk, power dissipation and RC delay. New dielectric materials with lower permittivity (k) are needed to meet this challenge. This paper summarizes the process integration and reliability issues associated with the use of novel low k materials in multilevel interconnects.

Valuable information on the structure and formation of metal-polymer interfaces originates from radiotracer measurements of metal diffusion at the interface, structural investigations by means of transmission electron microscopy, and computer simulations on the interplay of atomic metal diffusion and aggregation. Moreover, X-ray photoemission spectroscopy has largely contributed to our present understanding of the interfacial chemistry and the early stages of interface formation. While reactive metals always form relatively sharp interfaces with polymers, metals of lower reactivity diffuse into polymers at elevated temperatures and have a very strong tendency to agglomerate. The extent of diffusion appears to be determined by the initial stage of the deposition process. Here sticking coefficients recently measured for metals on virgin polymer surfaces deviate markedly from unity. Diffusion into the polymer increases strongly at low deposition rates. No significant diffusion is expected from a continuous metal film unless metal ions are formed at the interface. Metal ions are highly mobile and do not aggregate due to electrostatic repulsion. The model emerging from these observations allows us to predict the salient features of interface formation between metals and polymers in general and particularly with respect to the new low-k polymers.

Low-K silica films were grown from the liquid phase using methyl-triethoxy-silane. Physical and chemical properties were investigated together with electronic properties. The film had dense methyl groups and showed insulating characteristics even under as-grown conditions. The dielectric constant, low-field resistivity and breakdown field-strength were 3.6, 1013 Ωcm and IMV/cm, respectively, for the as-grown film. They were improved to 2.6, more than 1015 Ωcm and more than 3MV/cm, respectively, after 300°C vacuum annealing

Variations in the electrical and mechanical properties of silsesquioxane spin-on glass thin films are examined as a function of curing time and temperature. Particular attention is paid to the trade-off between producing low dielectric constant films, suitable for advanced microelectronic interconnection structures, and mechanically stable films, able to withstand semiconductor wafer fabrication processes. Two critical aspects of the mechanical stability of spin-on glasses are shown to be: the positive thermal expansion mismatch with silicon–leading to tensile film stresses; and reactivity with water–leading to susceptibility to stress-corrosion cracking.

Structures of polysilsesquioxanes {(R- SiO1.5)n with R=H, CH3, C6H5} in spin-on thin films are investigated in relation to their dielectric properties. IR spectroscopy in combination with results from quantum chemistry calculations shows that the initial hydrido-silsesquioxane films (cured to 250 °C) exhibit more symmetric ring structures than those for methyl-silsesquioxanes. Moreover, IR spectra indicate that increasing the cure temperature above a critical value, which varies with the Si-R moiety, causes extensive three-dimensional cross-linking in silsesquioxanes via breakage of the Si-R bond and formation of networks of O-Si-O structures with a lower ring symmetry than the initial materials. Dielectric properties do not appear to vary with the structural symmetry about the O-Si-O moiety nor with the R substituents, but rather depend on the extent of three-dimensional cross-linking as seen by the loss of Si-R absorbance. Highly cross-linked silsesquioxanes show a higher dielectric value and no variation with temperature from −100 °C to 150°C range, whereas a lower value and a negative temperature dependence are seen for dielectric constants of silsesquioxane samples with little loss of Si-R absorbance.

The versatility of e-beam processing provides the capability of tailoring the properties of spin-on polymers. Using different e-beam processing schemes, one can obtain a variety of films such as a homogeneous film with modified properties from the original, a film with variable properties as a function of depth, or a film having two distinct layers. In this paper, we demonstrated how to utilize this advantage of e-beam process to facilitate integration of spin-on polymers in the back-end-of-line manufacturing.

To meet the requirements of low-K and low-stress intermetal dielectric (IMD) for future ULSI devices, a novel temperature-difference liquid-phase deposition (TD-LPD) method is proposed. The deposition solution of supersaturated silicic acid with high concentration of fluorine can be achieved by raising deposition temperature larger than 15 °C from dissolution temperature (0 °C). Because fluorine atoms can easily be incorporated with the technique, TDLPD fluorine-doped SiO2 (FSG) exhibits low-K (∼3.4) and low-stress (∼40MPa) property. In this paper, to study the interaction between TD-LPD FSG and moisture, the FSG is annealed and moisture stressed repeatedly as in a real process. Since K is sensitive to moisture absorption, and the stress is sensitive to the dehydration reaction between Si-OH's, the both are monitored as indices. A feasible mechanism is proposed to explain the variation in K/stress during annealing and boiling cycles.

Plasma ion implantation (PII) doping technique has been utilized to prepare a new lowdielectric constant (low k) material SiO(F,C). Fluorine and carbon were implanted into SiO2 films by CF4 PII using an ICP plasma reactor. The effective dielectric constant of the films was significantly reduced after PH doping. An analysis of a double layer model indicated that a high quality dielectric layer with a dielectric constant down to 2.8 can be achieved by an optimized PII process. Contrasting to other conventional low-k material techniques, PII process also improved bulk resistivity and electrical field breakdown strength. The improvement possibly resulted from adding carbon into the films. The etching effect of CF4 PII could be beneficial to planarization and gap filling of dielectric interlayer.

Thin films of nanoporous poly(methylsilsesquioxane) have been prepared from organic/inorganic polymer hybrids. Low molecular weight 6-arm star poly(caprolactone)s are incorporated into partially pre-condensed methylsilsesquioxane resin. Upon heating, the glass resin cross-links around the organic polymer templates. Thermal decomposition of the orgainc polymer produces nanoporous thin films which show tunable dielectric constants as low as 1.9 and exhibit properties suitable for use in interconnect devices. Experimental data including porosity, morphology, and dielectric constants are presented.

Pulsed plasma enhanced chemical vapor deposition films have been grown from C2H2F4, CH2F2, and CHCLF2. C-Is x-ray photoelectron spectroscopy (XPS) indicates a prevalence of C-CF species in the films from C2H2F4 and CH2F2, whereas CF2 species dominate the films from CHC1F2. The CFx species distributions for the films are largely controlled by the competition between CF2-producing and HF elimination reactions in the pulsed plasmas. Dominance by HF elimination produces films with high C-CF and CF concentrations (e.g., CH2F2), whereas dominance by CF2-producing reactions leads to films with higher CF2 concentrations (e.g., CHCIF2). The % CF3 in the film is lowest for the precursor having the lowest F:H ratio, CH2F2. Little or no hydrogen was detected in the deposited films. Thermal degradation of films from C2H2F4 and CH2F2, as probed by solid-state nuclear magnetic resonance (NMR) spectroscopy, shows a loss through CF3 detachment and HF elimination. Pulsed plasma films from all three precursors gave dielectric constants of 2.4, with loss tangents on the order of 10−2. Dielectric measurements of pulsed plasma films from hexafluoropropylene oxide (HFPO) gave a dielectric constant of 2.0 ± 0.1 with a loss tangent of 0.009.

We investigated phase behaviors of polyimide blends such as polyethermide(PEI)/polystyrene(PS), PEI/polyamideimide(PAI), PEI/polyethyleneoxide(PEO), and PAI/PEO systems. Our sample systems exhibited lower critical solution temperature(LCST) phase behaviors. In the PEI/PS system, phase transition occurred near or above the glass transition temperature(Tg) of PS and the critical temperature of the system increased with decreasing molecular weight of PS. For PEI/PAI and PEI/PEO systems, the critical temperature of PEI/PAI system is higher than that of PEI/PEO system. Phase transition temperatures of PAI/PEO systems appeared near or below the melting point of PEO.

The utility of a variable duty cycle, pulsed plasma polymerization technique to produce low dielectric constant materials (k < 2.3) is described. The molecular compositions (and thus the dielectric constants) of the plasma polymers are controllable via changes in the plasma duty cycles employed during synthesis, all other reaction variables being held constant. In the present study, this compositional controllability under pulsed conditions is illustrated with two fluoroaromatic monomers. The dielectric constants of the films decrease as the plasma duty cycles employed during polymerization are decreased. Although the as deposited films exhibit relatively poor thermal stability, it was discovered that post-plasma annealing of the films, particularly at 400 °C under N2, provides dramatic improvements in the thermal stability of these materials. Most importantly, this enhanced thermal stability is achieved with relatively minor changes in the dielectric properties of these materials. In fact, synthesis of high thermal stability films having k < 2.0 is demonstrated in this work using the perfluoroaromatic monomer perfluoroallyl benzene.

Xerogel films of high porosity were fabricated using an ambient pressure technique. The same porosity can be obtained with different microstructures by varying the aging time of the films. The dielectric constant of these films as a function of porosity at 1 MHz follows correlations originally developed for bulk aerogels. Diffusion of copper is orders of magnitude faster in these xerogels than in the corresponding thermal oxide. An activation energy of 0.9 eV was estimated based on a convective diffusion model.

As integrated circuit sizes decrease below 0.25 microns, device performance will no longer improve at the same rate as for past generations because of RC interconnect delay which becomes significant as compared to the intrinsic gate delay. Parallel approaches to address this are to use a lower resistance metal (i.e., copper instead of aluminum) and to use a dielectric material with a dielectric constant significantly below that of dense silica (∼4). Recently, considerable progress has been made in development of thin films of nanoporous silica for these applications. Advantages include high thermal stability, small pore size, similarity to conventional spin-on deposition processes and spin-on glass precursors and final material (silica). The dielectric constant of nanoporous silica can be tailored between ∼1 and 3 which allows its’ implementation at multiple technology nodes in integrated circuit manufacture.

Recent development efforts have been focused on; 1) simpler and more reproducible deposition processes, 2) a more complete understanding of processing-property relationships for this material, 3) scale-up of manufacturing to yield a range of precursor products with stability for at least six months and very high purity, and 4) working with customers to integrate this material into both aluminum/gapfill and copper/damascene process flows. This paper targets several specific issues related to nanoporous silica use including water adsorption, pore size distribution control, processing at commercially viable throughputs, and obtaining thickness and dielectric uniformity across 200 mm wafers and wafer to wafer.

Highly porous (15% dense) silica thin films with k < 2.2 have been produced using a colloidal sol-gel process. This process uses relatively inexpensive, commercially available colloidal silica and potassium silicate. These films were deposited by spin coating. The parameters for spin coating (spin speed, gelation time, gel composition, etc.) have been optimized and coherent films with thicknesses ranging from 0.7–2.0 μm can be reproducibly fabricated. Preliminary work shows that the porosity of these films ranges from a few rn to < 1 μm. The dielectric properties were examined for frequencies up to 1 MHz. Compatibility of the films with IC processing was also investigated.

Glass Capillary Arrays (GCA) are low density columnar monolithic structures made of soda-lime glass. This structure, in which 76% of volume between the columns, the channels, is void, also has a greatly reduced dielectric constant in comparison with bulk glass. We have measured the index of refraction and absorption of samples of GCA's in the X-band, 8 × 109 Hz to 1.2×1010 Hz, for various orientations of the channels with respect to the polarization. For channels perpendicular to the polarization direction we have measured an index of refraction of 1.15. In comparison the index of refraction of (bulk) soda-lime glass is 2.6. We also examined the absorption in the far-infrared (FIR) frequency range between 6×1011 Hz and 6×1012 Hz. In this frequency range we obtain a k2 dependence due to losses in the glass matrix at higher frequencies. The results of the X-band and FIR results are interpreted in terms of an effective medium theory of the real and imaginary part of the dielectric constant of the composite.

Ion beam analysis techniques have become very useful for characterization of low k materials. Studies on several ion beam analysis techniques will be discussed. Rutherford Backscattering Spectrometry (RBS) provides a very powerful analytical technique for the thickness and porosity measurements on porous Si0 2 films. Nuclear Reaction Analysis (NRA) techniques for hydrogen and fluorine profiling are very useful to characterize fluorinated polymer and fluorinated oxide films. Examples of low k materials including Si02:F, Parylene-AF and Teflon-AF will be discussed. Fluorine diffusion in to metals and various interface effects between metal and low k materials will be presented.

Currently, the IC industry is researching the integration of a variety of materials to meet the low dielectric constant requirement for improved back-end of line (BEOL) interconnect performance. One critical dimension for successful ntegration of these new materials is maintaining mechanical integrity through multilayer processes. This includes both cohesive and adhesive fracture resistance. The latter adds additional complexity in that adhesive toughness is a function of the adherend materials and the processes used to join them. Hence, many good dielectric materials may be rematurely eliminated from further research not because of inherently poor adhesion but because of the necessity to optimize processing strategies.

In this paper, we use the modified Edge Liftoff Test (m-ELT) to quantify the mechanical adhesion of multilayer blanket coatings. A specific example is used to demonstrate the utility of combining the m-ELT with surface analysis to optimize the reliability of low-K dielectric resins for use in ULSI applications. The system studied consists of a Cyclotene™ 5021(BCB) low-K material integrated with CVD aluminum for single level, damascene structures. The effects of liner layer metallurgy and surface plasma treatments are measured. Surface analysis is done on the failed parts to understand the location of the failure. In this way recommendations for process optimization can be made.

A great interest exists to reduce power consumption, cross-talk, and RC-delay in ULSI devices by replacing SiO2(k=3.9−4.3) with a polymeric material (k<3.0). Unlike SiO2, polymeric thin films exhibit a complex morphology which varies with the polymer's thermal history, deposition temperature, and film thickness. Since the morphology of the polymer thin film ultimately affects its properties such as its dielectric constant, methods need to be developed to understand the morphological changes in polymer thin films. Further, the polymer thin films must exhibit a high thermal stability due to the relatively high back-end-of-line (BEOL) processing temperature. The polymers which exhibit high thermal stability often contain a mainchain benzene ring. Since benzene has a high anisotropic molecular polarizability, optical birefringence can be used to monitor the polymer chain conformation as a function of the polymer's thermal history, thickness or deposition conditions. Poly(p-xylylene) and poly(tetraflouro-p-xylylene) are shown to have a large negative birefringence, increasing until polymer decomposition. Poly(p-xylylene) becomes increasingly more negatively birefringent after a crystallographic phase change at 220°C. A high negative birefringence results in a large in-plane capacitance, negatively impacting the polymers potential benefit. owever, poly(chlorop-xylylene) and poly(dichloro-p-xylylene) exhibit positive birefringence, which increases until their crystalline melting points at ∼290°C and ∼380°C and thereafter decreases due to film disruption. Conclusions will be drawn based on this positive birefringence for the future molecular design of CVD polymers to decrease their in-plane dielectric constant. Urther poly(chloro-p-xylylene) and poly(p-xylylene) are investigated as a function of deposition temperature and film thickness.

Dielectric anisotropy was studied using two fluorinated polymers(fluorinated polyirnide and fluorinated poly(aryl ether)), which are promising candidate as interlayer dielectrics(LLD). From the measurement of the optical anisotropy, the fluorinated polyimide has larger birefringence than fluorinated poly(aryl ether) because the fluorinated polyimide is a rigid rod-like polymer, while the fluorinated poly(arly ether) has an ether linkage to allow the flexibility. The large optical birefringence in the fluorinated polyimide may indicate the large dielectric anisotropy. The dielectric anisotropy of more flexible fluorinated poly(arly ether) was very similar to the expected anisotropy from optical birefringence, but the fluorinated polyimide with rigid rod-like chains has much smaller dielectric anisotropy than expected in the spin-coated film. From the FTLR spectra of the rigid rod-like polymer, the olecular orientation of the rod-like polymer confined in the submicron gaps is shown to be different from that of the blanket film spin-coated on the substrate. This change of the molecular orientation in the confined submicron gap is responsible for the change of the dielectric anisotropy.