Ultra Low-k Films Research Articles

Ultra Low-k Materials: Challenges of Scaling

High porosity and carbon doing are required to achieve low dielectric constant, k values for ultra low-k (ULK) materials based on silicon dioxide. Those characteristics create new challenges in the integration of ULK materials that are being developed for future interconnect scaling. Four of those challenges based on electrical characterizations of ULK films will be presented in this paper. First, a high concentration of porogen residue is typically present in ULK materials fabricated by Plasma Enhanced Chemical Vapor Deposition (PECVD). The high porogen residue concentration leads to a high leakage current density. Second, ULK materials based on Spin-on Glass (SOG) process demonstrate a low leakage current density. But early dielectric breakdown has been observed in SOG k=2.2 material. Third, PVD TaNTa barrier process can introduce significant damage to ULK materials. The damage level increases as the k value decreases or porosity increases. Four, direct deposition of ALD Ru barrier on ULK films results in a high leakage current density.

The TDDB Study of Post-CMP Cleaning Effect for L40 Direct Polished Porous Low K Dielectrics Cu Interconnect

In order to realize the high performance of RC delay, direct polished porous type ultra low-K film (ULK) is integrated into Cu dual-damascene interconnects for 45 nm and beyond. In the past, metal line seems not an important role in the world of TDDB. However, because of the minimized feature size and the relative fragile ULK film properties, it was found that the Cu roughness of polished surface also plays an important role to effect on the reliability such as TDDB from this generation. Post-cleaning of Cu CMP is a major process to manage the Cu surface roughness. In this paper, the correlation for post-cleaning time to the TDDB was discussed. The mechanism with two models was built up to explain this behavior as well.

Mechanistic study of ultralow k-compatible carbon dioxide in situ photoresist ashing processes. I. Process performance and influence on ULK material modification

In situ photoresist (PR) ashing processes are attractive because of the ease of process integration with plasma etching processes. The authors have examined the performance of carbon dioxide (CO2) as a source gas for in situ PR ashing processes compatible with ultralow k (ULK) materials and compared it with the results obtained using O2. They performed measurements of 193 nm PR ashing rates in a dual frequency capacitively coupled plasma reactor. The damage to porous ULK feature sidewalls was simulated by exposing blanket ULK films in a non-line-of-sight fashion in a small gap structure to the plasma-generated reactants. The pressure for the in situ ashing processes was varied from 10 to 100 mTorr, and the self-bias voltages ranged from floating potential to ∼−400 V. To increase line-of-sight etching of PR by inert ion bombardment, Ar/CO2 mixtures with up to 75% Ar were investigated. The ULK material modifications were analyzed by x-ray photoelectron spectroscopy and time-of-flight secondary ion mass spectroscopy (ToF-SIMS). Plasma-damage of the ULK material primarily is detected as the removal of carbon from the SiCOH ULK films. To compare the performance of different ashing processes for PR stripping from ULK material, the authors introduced an ashing efficiency (AE) parameter which is defined as the thickness of PR removed over the thickness of ULK simultaneously damaged, and can be considered a process figure of merit. AE with CO2 was about three times greater than AE with O2 for the same process conditions. When a 75% Ar/CO2 gas mixture was used and a −100 V substrate bias was applied during PR ashing, a PR ashing rate of 200 nm/min could be achieved for a 10 mTorr Ar/CO2 plasma. For this process, the measured AE was 230, more than 10× greater than AE achieved with O2 discharges using the same conditions. The authors found that ULK damage showed a direct dependence on the atomic oxygen densities of both CO2 and O2 discharges which was characterized by optical emission of discharges. The question whether in-diffusion of carbon species from CO2 discharges into ULK material was significant was also examined. For this the authors substituted C13O2 for C12O2 and performed ToF-SIMS analysis of the exposed ULK films. No significant amount of C13 from C13O2 plasmas was detected either on the surface or in the bulk of the C13O2 plasma-exposed ULK.

Mechanistic study of ultralow k-compatible carbon dioxide in situ photoresist ashing processes. II. Interaction with preceding fluorocarbon plasma ultralow k etching processes

Process interactions between fluorocarbon (FC) plasma ultralow k (ULK) dielectrics etching followed by carbon dioxide (CO2) in situ photoresist (PR) ashing on ULK damage have been studied in a dual frequency, capacitively coupled plasma reactor. Introduction of ULK trench/via sidewall damage was simulated employing blanket ULK films by exposing them in a non-line-of-sight fashion in a small gap structure to the plasma environment. ULK damage was quantified using the dilute hydrofluoric acid (0.5%/15 s) selective etching method. CO2 in situ ashing processes showed a chamber memory effect due to prior FC plasma etching, significantly increasing damage of pristine ULK films. For ULK plasma etching/PR ashing process sequences, ULK material surfaces were modified by FC plasma etching prior to the CO2 plasma exposure. X-ray photoelectron spectroscopy studies showed that the modifications consisted primarily of 1–2 nm FC coverage of the ULK. This FC deposit remained on the ULK surfaces during in situ CO2 processing and provided protection of the underlying ULK material. PR-patterned ULK structures were also processed employing the same processing conditions. The results obtained from the characterization of the resulting trench structures support the findings obtained with blanket films. CO2 in situ PR ashing processes performed at low pressure (10 mTorr) and enhanced by rf biasing provided a good combination of high PR stripping rate and low ULK damage introduction.

A 3 nm Self-Forming InOx Diffusion Barrier for Advanced Cu/Porous Low-k Interconnects

The copper diffusion barrier properties of a 3 nm self-forming InOx layer on a porous ultralow-k (p-ULK) film have been investigated. A 5 at. % In doped Cu film was directly deposited onto porous low-k films by co-sputtering, followed by annealing at various temperatures. Transmission electron microscopy (TEM) images showed that a 3 nm layer was self-formed at the interface between Cu–In and p-ULK films after annealing at 400 °C for 1 h. An EDS line scan on the region near this interface showed obvious accumulation of In at the interface. X-ray photoelectron spectroscopy (XPS) analyses indicated that the self-formed interfacial layer was InOx. The self-forming InOx layer prevented Cu agglomeration on the p-ULK film surface. The XPS atomic depth profiles showed that the self-formed InOx barrier was thermally stable against Cu diffusion to at least 500 °C for 5 h. The sheet resistance of the post 500 °C annealed Cu–In film was comparable to that of a pure Cu film. The Cu–In self-forming barrier approach may be a viable candidate for Cu/p-ULK interconnects.

Damage-Free Post-CMP Cleaning Solution for Low-<i>k</i> Fluorocarbon on Advanced Interconnects

As technology node progressing, ultra low-k film has been implemented to reduce RC delay in LSI circuit. A fluorocarbon (CFx) film is proposed as foreground ultra low-k film because of non-porous structure [1]. Although CFx film is expected to be stable for its structure advantage, damage-less process is anticipated to avoid dielectric constants change in subsequence process steps [2]. CMP and post CMP process are concerned to bring damage on devices, so the effect of post CMP cleaning solutions on CFx structure and electric property is evaluated.

Novel Carbon-Cage-Based Ultralow-$k$ Materials: Modeling and First Experiments

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> A new class of materials is presented that is supposed to be a potential candidate for isolating ultra low-k thin films between metal on-chip interconnects in future CMOS technology nodes. The ideal structure of the novel carbon-cage-based materials is described by a model that assumes an ordered network (mosaic structure) with fullerenes (C60) as the nodes and linker molecules along the edges of the mosaic cells. The interior of the network represents a nanopore of a 1-nm scale. According to the molecular design-based model, structures with simple cubic and diamond-like topology of the network are considered promising candidates. A dielectric constant (k value) of 1.7 and an elastic bulk modulus of about 20 GPa are predicted of ideal combinations of network topology and linker molecules. First experimental results, based on electron energy loss spectroscopy, X-ray absorption spectroscopy, nanoindentation, and atomic force microscopy are presented. A more controlled film fabrication process is needed to get more homogeneous thin films with characteristic material parameters as predicted by the model. </para>

The Preparation and Low-k Application of Asymmetric Functionalized Cyclodextrin Templated Nanoporous Silsesquioxane Films

Several kinds of Asymmetric Functionalized Cyclodextrin (AFCD), as a porogen, have been introduced for the first time to investigate their potential in the field of low-k applications. The novel AFCD templated films were compared in terms of their pore forming efficiency and the pore structure, such as pore size and pore size distribution. Of the AFCDs, btCD and tbsCD showed fairly good pore forming efficiencies. The pore diameter generating form of AFCD was larger than that of conventional CD (tCD) at a high porogen loading (> 30 wt%), which might have been due to the micelle-like assembled behavior of the AFCD porogen as a supramolecular surfactant. Ultra low-k films (k < 1.5) can be effectively made using the AFCD porogen. Mechanical properties, such as hardness and elastic modulus, decrease with increasing AFCD porogen loading.

Influence of HF catalyst on the microstructure properties of ultra low-k thin films prepared by sol-gel method

This paper reports that by using the hydrofluoric acid (HF) as the acid catalyst, F doped nanoporous low-k SiO2 thin films have been prepared by means of sol-gel method. The characterization of atomic force microscopy and Fourier transform infrared spectroscopy demonstrates that the HF catalyzed films are more hydrophobic. The N2 adsorption/desorption experiments show that the suited introduction of HF increases the porosity and decreases the pore size distribution (about 10 nm) in the films. The above results indicate that the hydrofluoric acid is the more suitable acid catalyst than the hydrochloric one for preparing nanoporous ultra low-k SiO2 thin films.

Evaluation of Barrier CMP Slurries and Characterization of ULK Material Properties Shifts Due to CMP

The integration of porous ultra low-k (ULK) films, with k = 2.2, into future technology nodes can add risk to yield and reliability. The porosity of the film makes it more susceptible to damage from CMP process. This work investigates the shifts in the dielectric constant (k) and refractive index (RI) that various barrier slurries have induced on ULK films. Post CMP treatment of these films seems to help in recovering the material property shifts caused by these barrier slurries. Chemical characterization of these films helped in identifying the reason for shifts that were induced in these films after exposure to CMP slurries.

The role of ultraviolet radiation during ultralow k films curing: Strengthening mechanisms and sacrificial porogen removal

This work proposes a fundamental understanding of structural transformation occurring during porogen extraction from as-deposited ultralow k (ULK) materials when exposed to ultraviolet (UV) radiation during thermal curing. Specific explanations are provided for as deposited films at high temperature (T&amp;gt;250 °C). This temperature range is sufficient to assess thin-film stability. Two distinguished regimes were identified in the curing process. During the first stage, the film shrinks strongly in similar proportion to SiCH3 break. Preferential impact of UV radiation on hydrocarbon porogen bonds leads also to a break of SiCH3 structures. In this work, 5 min of curing is enough to remove the porogen and create the max of porosity (33%). After the porogen removal step, the porous film shrinks under UV radiation leading to an increase of SiOSi bond concentration. A structural rearrangement of the bulk is initiated since the porogen is totally evacuated from the film. The increase of normalized infrared SiOSi peak during UV curing (related in literature to an improvement of mechanical properties) is mainly due to film densification (a few SiOSi bonds creation). The origin of shrinkage is explained by the breaking of SiCH3 which could turn into a SiH bond leading to a free volume reduction within the film. In this case, a correlation is found between shrinkage and elastic modulus. The resulting SiOSi structure densification drives, at first order, the improvement of the mechanical properties. UV radiation exposure shows no effect in silanol condensation and acts mainly on the porogen decomposition and bulk densification. Si-CH2-Si structure formation, pointed out in the literature as a possible reaction way to enhance film crosslinking, is identified in the nuclear magnetic resonance (NMR) broadband spectrum of the ULK film. However, resulted NMR and infrared peaks intensities suggest its formation as a minor. This study gives a full understanding of UV curing impact on ULK films (k≤2.5) where structure densification is identified as a main process to enhance the film strength in addition to the crosslinking.

Effects of Bias, Pressure and Temperature in Plasma Damage of Ultra Low-k Films

Effects of Bias, Pressure and Temperature in Plasma Damage of Ultra Low-k Films

Nonuniform structural degradation in porous organosilicate films exposed to plasma, etching, and ashing as characterized by x-ray porosimetry

X-ray porosimetry is used to characterize the porosity, the average film density, and the density of the wall material between the pores in ultralow-k films as a function of film thickness. These measurements are performed on films that have been plasma treated as well as plasma etched and an ashed to evaluate how these integration processes affect the pore characteristics of the interlayer dielectric. The damage, a decrease of porosity and an increase in the wall density, is strongly localized to the exposed surface of the film. The plasma etching and ashing tend to induce more surface damage than the plasma alone.

Comparison Between E-beam and Ultraviolet Curing to Perform Porous a-SiOC : H

Porous ultralow-k films are required by the microelectronics industry as interlayer dielectrics for technologies and below. These porous insulating films can be deposited by plasma-enhanced chemical vapor deposition using a porogen approach. It consists of the codeposition of a matrix precursor and a sacrificial organic porogen, and then on a post-treatment to remove the organic porogen phase and create porosity in the film. In this work, an e-beam assisted thermal curing was compared to an ultraviolet-assisted thermal curing. Basic film properties such as , film shrinkage, porosity, pore size, and pore size distribution were evaluated. NMR and Fourier transform infrared analyses were used to study the chemical modifications induced by the post-treatment. These analyses show that the post-treatment impact depends on the radiation used. Both treatments lead to a removal of terminal nonbridging bonds such as , , and and can contribute to a subsequent formation of crosslinks. Both treatments remove methyls from , but the e-beam induces a bond increase while the UV bulb used decreases the contribution. The cross-linking improvement induces an increase of Young’s modulus, the elastic properties being mainly correlated to the volumic bond concentration in the film.

Structural Transformation in PECVD Ultralow-k Material during Porogen Removal by UV Assisted Thermal Curing

ABSTRACTNext-generation microelectronic interconnects require the use of dielectrics with continuously lower permittivity (k) to overcome limitations induced by crosstalk parasitic signal delay. Using PECVD, Ultralow-k film (ULK, k ≤ 2.5) can be developed by creating pore inclusions within an organosilicate matrix through porogen approach. Both ULK deposition and subsequent curing process has to be adjusted in order to achieve optimized mechanical and electrical properties and maintain stability during integration. For this concern, the attention was recently focused on ultraviolet (UV) radiation to sustain the thermal curing. In the present work, a fundamental understanding of structural transformations occurring during porogen extraction from as-deposited ULK materials when exposed to thermal-assisted UV radiation is proposed. This thermal-assisted UV cure technique is very efficient in porogen removal since in a few minutes the desired porosity is reached. During the first stage of curing, the film shrinks strongly whereas the porosity is created. After porogen removal step, the porous film continues shrinking under UV radiation leading to an increase of SiOSi bonds concentration (film densification). The normalized FTIR SiOSi peak increase during UV curing (related in literature to an improvement of mechanical properties) is mainly due to the film densification, in addition to the SiOSi bridging bond creation. In this case, correlation is found between shrinkage and elastic modulus.

Study of plasma-induced damage of porous ultralow-k dielectric films during photoresist stripping

Plasma damage of low-k dielectrics during photoresist (PR) stripping in a dual-damascene process is a critical issue in the application of copper/low-k technology for ⩽45nm nodes to increase the signal processing speed of integrated circuit devices. In this article, a detailed and systematic work has been conducted to study the low-k damage on porous methyl silsesquioxane ultralow-k films using various PR strip chemistries and process conditions on a high density plasma reactor. The experimental results obtained from different test methodologies show that the low-k damage generated under fixed process conditions increases in the order of NH3&amp;lt;N2&amp;lt;H2∕N2&amp;lt;H2&amp;lt;O2. Among plasma control parameters, bias power has a very pronounced effect on low-k damage for reducing chemistries due to the acceleration of Si–C bond breaking by ion bombardment. Source power also affects the low-k damage significantly as it controls the ion density and flux to the wafer surface. The pressure effect is more complicated and shows different characteristics for oxidizing and reducing chemistries. The extent of low-k damage depends on the orientation of the wafer surface exposed to the plasma, leading to different sensitivity of the damage to the strip chemistry and process condition. Based on this work, an optimized chemistry and process regime are identified to effectively reduce low-k damage and achieve good strip process performance.

Dependence of CMP-induced delamination on number of low-k dielectric films stacked

Peeling during chemical mechanical polishing (CMP) is investigated with respect to the number of ultra low-k dielectric films in an interconnect stacking. It is shown that the addition of dielectric levels increases significantly the CMP-induced peeling. Stack fracture energies, measured by 4-point bending technique, are relatively less sensitive to the increase of level number, even if a degradation is observed. This leads to the conclusion that delamination during polishing depends highly on the elastic properties of the stack and that there is no simple correlation between stack adhesion and peeling during CMP. In this work, mechanical damages generated in dielectric stack during inter-level CMP were also investigated. It was shown that, if no peeling appears, inter-level CMP have no effect on stack reliability. This indicates that negligible “fatigue” effect, i.e. sub-critical crack growth, takes place during CMP.

Effect of Trimethylsilylation on the Film Stress of Mesoporous Silica Ultralow-k Film Stacks

The film stress of the ultralow-k mesoporous silica thin film and the α-SiC:H/ mesoporous silica film stack was studied. The as-calcined mesoporous silica exhibits a tensile film stress due to its volume contraction during bake and calcination. Trimethylsilylation of the mesoporous film results in a spring-back effect and thereby improves the mechanical properties of the porous film. Deposition of a plasma-assisted α-SiC:H layer on the mesoporous silica thin film can also relieve the tensile stress, and even made the film stack become compressively stressed. This is ascribed to the stress compensation and alkoxylation occurring during the α-SiC:H deposition.

Self-Assembled Monolayers as Cu Diffusion Barriers for Ultralow-k Dielectrics

Ultralow-k dielectrics are mainly porous and pose reliability issues as copper barrier integrity is compromised on porous surfaces. No work is reported on use of self-assembled monolayers (SAMs) as Cu diffusion barrier or to seal pores for ultralow-k dielectrics. We attached SAMs to ultralow-k film and confirmed by analytical techniques such as Auger and X-ray photoelectron spectroscopic analysis. We report reduction of leakage current and enhancement of dielectric breakdown voltage of the ultralow-k p-SiLK film after SAM attachment showing it to be a promising barrier material for prevention of Cu diffusion. H 2 /N 2 plasma treated p-SiLK film showed early breakdown and high leakage in spite of SAM attachment.

Porosimetry Measurements on Low Dielectric Constant—Thin Layers by Coupling Spectroscopic Ellipsometry and Solvent Adsorption-Desorption

Ellipsometric measurements are carried out on different commercial ultra low-K mesoporous thin films under several solvent vapor pressures. In a first step, both open porosity and skeleton refractive indices are estimated using linear and Lorentz-lorenz laws. In a second step, adsorption-desorption isotherms of selected solvents, giving refractive index as a function of the relative pressure, are acquired. Pore size distributions (PSD) are calculated. The effect of the nature of the adsorbate on the PSD is also discussed.