ULK Materials Research Articles

Etch characteristics of nanoscale ultra low-k dielectric using C3H2F6

Next generation semiconductor devices require ultra low dielectric constant (ULK) materials such as porous SiCOH on the back end of line structure for lower resistance and capacitance (RC) time delay, however, these ULK materials are easily damaged by the exposure to plasmas during the etching. In this study, etch characteristics of nanoscale TiN masked porous SiCOH such as etch rate, etch profile, surface damage, etc. and plasma characteristics by using C3H2F6 based gases have been investigated with a dual-frequency capacitively coupled plasma system (DF-CCP) and the results were compared with those by using conventional C4F8 based gases used for low-k dielectric etching. The results showed that, for the similar etch rates and etch profiles of porous SiCOH, lower sidewall damage was observed for the etching with the C3H2F6 compared to the C4F8. The analysis showed that it was related to less UV (less than 400 nm) emission and less fluorine radicals in the plasma for C3 H2F6 compared to C4F8, which leads to less fluorine diffusion to the sidewall surface of the etched porous SiCOH by the fluorine scavenging by hydrogen in C3H2F6.

Transparent Nacre‐like Composites Toughened through Mineral Bridges

AbstractBulk materials with remarkable mechanical properties have been developed by incorporating design principles of biological nacre into synthetic composites. However, this potential has not yet been fully leveraged for the fabrication of tough and strong materials that are also optically transparent. In this work, a manufacturing route that enables the formation of nacre‐like mineral bridges in a bioinspired composite consisting of glass platelets infiltrated with an index‐matching polymer matrix is developed. By varying the pressure applied during compaction of the glass platelets, composites with tunable levels of mineral bridges and platelet interconnectivity can be easily fabricated. The effect of platelet interconnectivity on the mechanical strength and fracture behavior of the bioinspired composites is investigated by performing state‐of‐the‐art fracture experiments combined with in situ electron microscopy. The results show that the formation of interconnections between platelets leads to bulk transparent materials with an unprecedented combination of strength and fracture toughness. This unusual set of properties can potentially fulfill currently unmet demands in electronic displays and related technologies.

Shear test on hard coated flip-chip bumps to measure back end of the line stack reliability

Introduction of ultra-low κ (ULK) material into advanced back end of line (BEOL) stacks has improved the electrical performance of the flip-chip microelectronic packages. However, the integration of the brittle and porous ULK materials presents several mechanical reliability challenges due to chip-package interaction effects. One of the primary failure modes in a organic flip-chip microelectronic package is delamination of interlayer dielectric (ILD) layers. Thermo-mechanical stresses that arise at the end of flip-chip assembly process can be high enough to cause fracture of the ILD layers in the vicinity of a solder bump. It has been observed from experiments that two bumps subjected to apparently same amount of thermo-mechanical stresses and same process history can exhibit difference in failure pattern. Current numerical models nor micro- scale interfacial fracture characterization experiments such as double cantilever beam (DCB) or four-point bend (FPB) tests are not capable of predicting such a failure. Such a behavior is possible only if the adhesion strength of the ILD layers are not constant but varies spatially from bump to bump. The difference in adhesion strength between adjacent bumps can be attributed to the difference in trace pattern layout or via distribution. In order to understand the failure at the bump level that is sensitive to capture differences in local trace patterns, microscale test techniques need to be developed.The focus of this work is to demonstrate a new reliable microscale test technique performed using nano-indenter to measure the mechanical reliability of BEOL stacks. A high-load spherical head is used to apply a known displacement on a solder bump coated with a ceramic. The force initial increases and suddenly drops indicating failure. Cross-sections of the tested bumps clearly indicate crack propagating through the BEOL stack while the coated bump remains intact. Since the forces subjected on the bump during the test is similar to the forces subjected on the bump during flip-chip assembly, the critical force can be used as a measure of fracture strength of the BEOL stack. The results from the test indicate that the critical force varies from bump to bump and the results are repeatable. Furthermore, finite-element model of the test is developed. The failure of the ILD layers is simulated using cohesive zone (CZ) elements and the mixed-mode traction-separation parameters required to characterize the CZ elements are extracted from macro-scale test techniques like DCB and FPB tests. The force-displacement (P-Δ) plot from the model mimics the experiment and predicts the average value of the critical force observed in experiment.

The stress analysis and parametric studies for the low-k layers of a chip in the flip-chip process

Using low-k/ultralow-k (LK/ULK) materials as the inter-layer dielectrics (ILD) and inter-metal dielectrics (IMD) in copper connections were implemented to meet the electrical performance requirements in the advanced chips. ULK materials are fragile and the mechanical failures in ULK materials are critical during chip packaging processes, such as the solder reflow in the flip-chip. A crack or delamination that initiates within the high thermo-mechanical stress regions can propagate into the active area of the chip in packaging, which involves the chip-packaging interaction (CPI) problem. In this study, we proposed a three-dimensional sub-modeling finite element approach considering an effective layer for the back end of line (BEOL) microstructures in the global model to improve the accuracy. The approach surmounted the difficulty of the large size difference between the chip and the ULK layers in computations. The stress analysis and parametric studies for a designed ULK chip with 40nm technical node under the flip-chip reflow was performed based on the present method. The effects of the selected parameters were ranked and the optimal combination of the factors was achieved.

In-situ study of the TDDB-induced damage mechanism in Cu/ultra-low-k interconnect structures

The time-dependent dielectric breakdown (TDDB) mechanism in Cu/ultra-low-k (ULK) on-chip interconnect structures has been raising serious reliability concerns for state-of-the-art microelectronic devices in recent years. An advanced experimental methodology, which combines transmission electron microscopy (TEM) imaging and an in-situ electrical test using a Hysitron PI95 TEM holder, was used to study TDDB-induced damage mechanisms and degradation kinetics in Cu/ULK interconnect structures. In flawless Cu/ULK interconnect structures with locally very thin Ta-based barriers, slow but massive Cu migration into the SiO2 layer was observed at the bottom corner of the M1 metal after the TaN/Ta barrier had been firstly ruptured by the Ta migration, driven by the high electrical field. More importantly, it was found that both the high electric field and the massive metal migration into the SiO2 can contribute to the final breakdown of the Cu/ULK interconnect structures. The final breakdown spot could be located either at the interface of the SiCN layer and the ULK material or in the SiO2 layer, which depends on the geometry, the kind of dielectric materials (particularly ULK) and the driving force (the high electric field). The results also highlight the importance of the quality of TaN/Ta barrier deposition process, especially at the bottom corner of the M1 metal, to ensure the functionality of the TaN/Ta barriers during the product lifetimes.

TiN Hard Mask Cleans with SC1 Solutions, for 64nm Pitch BEOL Patterning

Titanium Nitride metal hard mask was first introduced for BEOL patterning at 65 nm [1] and 45 nm nodes [2]. Indeed, in this “Trench First Hard Mask” (TFHM) backend architecture, the dual hard mask stack (SiO2 & TiN) allows a minimized exposure of ULK materials to damaging plasma chemistries, both for line/via etch sequence, and lithography reworks operations. This integration scheme was successfully used for a BEOL pitch down to 90 nm for the 28 nm node, however, for the 14 nm technology node, 64 nm BEOL minimum pitch is required for the first metal levels. Because it is unable to resolve features below 80 nm pitch in a single exposure, conventional 193 nm immersion lithography must be associated with dual patterning schemes, so called Lithography-Etch-Lithography-Etch (LELE) patterning [3] for line levels and self-aligned via (SAV) process [4] for via patterning. In both cases, 2 lithography/etch/clean sequences are necessary to obtain one desired pattern, and associated reworks also become more challenging since first pattern is exposed to resist removal processes (plasma + wet clean). The reference wet cleans that were developed for 65 to 28 nm TiN hardmask patterning, utilizes commonly used chemistry for BEOL post-etch cleans, i.e. diluted hydrofluoric acid (dHF) followed by deionized water Nanospray (DIWNS) on 300 mm single wafer tool.

Structural characterization of amorphous materials applied to low-k organosilicate materials

We present a methodology to create computational atomistic-level models of porous amorphous materials, in particular, an organosilicate structure for ultra-low dielectric constant (ULK) materials known as “SiCOH.” The method combines the ability to satisfy geometric and chemical constraints with subsequent molecular dynamics (MD) techniques as a way to capture the complexities of the porous and amorphous nature of these materials. The motivation for studying ULK materials arises from a desire to understand the origin of the material's weak mechanical properties. The first step towards understanding how these materials might behave under processing conditions that are intended to improve their mechanical properties is to develop a suitable computational model of the material and hence is the focus of this paper. We define the atomic-scale topology of ULK materials that have been produced by chemical vapor deposition-like experimental techniques. Specifically, we have developed a method of defining the initial atom configurations and interactions, as well as a method to rearrange these starting configurations into relaxed structures. The main advantage of our described approach is the ability of our structure generation method to maintain a random distribution of relevant structural motifs throughout the structure, without relying on large unit cells and periodic boundaries to approximate the behavior of this complex material. The minimization of the different models was accomplished using replica exchange molecular dynamics (REMD). Following the generation of the ‘equilibrium’ configurations that result from REMD for a ULK material of a pre-specified composition, we demonstrate that its structural properties, including bonding topology, porosity and pore size distribution are similar to experimentally used ULK materials.

Barrier Slurry Innovation for Advanced Node

As the development of technology node, ULK was used as isolation material in Cu damascene integration. ULK is a porous material with low mechanical strength and low adhesion. During CMP process, slurry additive and moisture will enter the hole of ULK material and result in the increasing of ULK K value, and also ULK film is easily damaged by mechanical force. To solve this problem, a capping layer is designed in Cu integration. In this paper, a novel barrier is developed to meet this integration need. Slurry chemistry/additives effect on RR selectivity and topo is evaluated in this paper.

Low-k films modification under EUV and VUV radiation

Modification of ultra-low-k films by extreme ultraviolet (EUV) and vacuum ultraviolet (VUV) emission with 13.5, 58.4, 106, 147 and 193 nm wavelengths and fluences up to 6 × 1018 photons cm−2 is studied experimentally and theoretically to reveal the damage mechanism and the most ‘damaging’ spectral region. Organosilicate glass (OSG) and organic low-k films with k-values of 1.8–2.5 and porosity of 24–51% are used in these experiments. The Si–CH3 bonds depletion is used as a criterion of VUV damage of OSG low-k films. It is shown that the low-k damage is described by two fundamental parameters: photoabsorption (PA) cross-section σPA and effective quantum yield φ of Si–CH3 photodissociation. The obtained σPA and φ values demonstrate that the effect of wavelength is defined by light absorption spectra, which in OSG materials is similar to fused silica. This is the reason why VUV light in the range of ∼58–106 nm having the highest PA cross-sections causes strong Si–CH3 depletion only in the top part of the films (∼50–100 nm). The deepest damage is observed after exposure to 147 nm VUV light since this emission is located at the edge of Si–O absorption, has the smallest PA cross-section and provides extensive Si–CH3 depletion over the whole film thickness. The effective quantum yield slowly increases with the increasing porosity but starts to grow quickly when the porosity exceeds the critical threshold located close to a porosity of ∼50%. The high degree of pore interconnectivity of these films allows easy movement of the detached methyl radicals. The obtained results have a fundamental character and can be used for prediction of ULK material damage under VUV light with different wavelengths.

Investigation of plasma etch damage to porous oxycarbosilane ultra low-k dielectric

There has been much interest recently in porous oxycarbosilane (POCS)-based materials as the ultra-low k dielectric (ULK) in back-end-of-line (BEOL) applications due to their superior mechanical properties compared to traditional organosilicate-based ULK materials at equivalent porosity and dielectric constant. While it is well known that plasma etching and strip processes can cause significant damage to ULK materials in general, little has been reported about the effect of plasma damage to POCS as the ULK material. We investigated the effect of changing the gas discharge chemistry and substrate bias in the dielectric trench etch and also the subsequent effect of the cap-open etch on plasma damage to POCS during BEOL integration. Large differences in surface roughness and damage behaviour were observed by changing the fluorocarbon depositing conditions. These damage behaviour trends will be discussed and potential rationalizations offered based on the formation of pits and craters at the etch front that lead to surface roughness and microtrenching.

Effects of VUV and EUV Radiation on Ultra Low-k Materials Damage

ABSTRACTLow-k dielectric films can be substantially damaged during plasma processing. High energy UV and VUV photons emitted by plasma play the key role in damaging the porous low-k films directly or indirectly by stimulating chemical reactions with radicals in plasma and plasma afterglow. The different ULK samples (k: 2.0-2.2, porosity: 30-50%, pore radius: 1-2 nm) were studied by exposing to five radiation sources at various wavelengths (VUV: 193 nm, 147 nm, 104-106 nm, 58.3 nm, and EUV: 13.5 nm). Time-spatial behavior of the ULK damage as a function of photons fluence was studied by FTIR spectroscopy and XRF analysis. It is shown that the degree of damage depends on wavelength of UV light. The major UV damage was observed at the wavelengths below 193 nm. The maximum damage corresponds to 147 nm while the degree of damage at 58.3 nm was much smaller. In the case of organosilicate (OSG) based ULK materials, the degree of damage, as a rule, increases with porosity. Organic low-k materials are damaged more than OSG at 193 nm, but at shorter wavelengths (147, 106, 58.3 and 13.5 nm) they are more stable than OSG. One-dimensional model for radiation absorption and dynamics of CH3 group destruction in ULK films was developed. The cross-sections of photons absorption and photo-stimulated Si-CH3 bond breaking in ULK films for 13.5 -147 nm wavelength range were derived from a combined experimental and modeling study. The obtained values allow to simulate the VUV/EUV induced modifications of low-k materials with different composition, to understand better the mechanisms of plasma damage and to generate ideas for controllable modifications of low-k materials.

Determination of Surface Energy Characteristics of Plasma Processed Ultra Low-K Dielectrics for Optimized Wetting in Wet Chemical Plasma Etch Residue Removal

The integration of porous ultra low dielectric constant materials (ULK) for isolation within the interconnect system of integrated circuits is a promising approach to reduce RC-delays and crosstalk due to shrinking feature sizes [1]. Actually the focus is on porous CVD-SiCOH materials, which consist of a Si-O-Si backbone and organic species (e.g. CH3) to lower polarizability and prevent moisture uptake to remarkably decrease the k-value [2]. The integration of porous low-k materials is very challenging, especially looking at patterning, resist stripping and etch residue removal, where commonly plasma processing has been applied. But plasma processing of ULK materials, especially using oxygen plasmas, is known to degrade electrical, optical and structural material properties by removing carbon from the film and densification of the surface near areas of the ULK [5]. Carbon depletion may also lead to the incorporation of-OH groups, which easily form silanols and therefore increase moisture absorption and k-values [2]. Besides the development of nondamaging plasma processes, wet cleaning is a promising alternative to avoid ULK damage while removing organic plasma etch residues. Additionally wet cleaning steps are always necessary to remove inorganic residues, which do not form volatile reaction products and can therefore not be removed by plasma processing.

Characterization of Post Etch Residues on Patterned Porous Low-k Dielectric Using Multiple Internal Reflection Infrared Spectroscopy

Plasma etching and ashing processes on intricate porous low-k dielectrics are known to introduce high levels of defects that may cause serious reliability issues. Post etch cleans to remove polymer residues could also further damage the porous low-k dielectrics. Compounding the difficulty of handling weak porous ULK materials, a lack of sensitive metrology to evaluate such damage is a serious hindrance to systematic development of plasma etching, restoration and cleaning processes. In this paper, a new metrology tool using Multiple Internal Reflection Infrared Spectroscopy (MIR-IR) was developed and tested using silicon wafer based IR waveguides. Our data demonstrates the capability of MIR-IR spectroscopy to monitor removal of thin polymeric post-plasma etch residues from porous low-k dielectrics and characterize plasma-induced dielectric damage.

Processing and moisture effects on TDDB for Cu/ULK BEOL structures

As scaling in Cu/BEOL structures causes critical dimensions to shrink, TDDB between Cu lines is increasingly becoming a reliability concern and increasingly limiting the choices of low-k and ULK materials which can be implemented [1] . Failure analysis has shown that the interface between the dielectric and the Cu capping layer is typically where dielectric breakdown occurs. This interface can be adversely affected by a number of processing steps and processing parameters including dicing, CMP, queue time between Cu CMP and Cu capping, and exposure to moisture [2] , [3] , [4] , [5] . In this study we examine the effect of these critical processing steps and parameters on TDDB. To focus on the effect of the processing steps and to minimize the effects of wafer to wafer and lot to lot variability, we have primarily used structures with Cu line spacings of 80–90 nm. For some IV experiments a spacing of 30–40 nm was used. Techniques for improving TDDB performance are also discussed.

Microwave characterization of porous SiOCH permittivity after integration dedicated to the 32 nm node

New porous ULK materials are now required for maintaining a constant RC factor as back-end dimensions shrink for each new technology node. Porous SiOCH ULK HF characterization, dedicated to the 32 nm node, has been performed, and its complex permittivity extracted up to 8 GHz. The impact of process integration on the porous SiOCH is highlighted, by an increase of its real permittivity and unwanted losses.

Characterization of ULK Material Properties Shifts Due to Liner CMP Slurries

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Evaluation of ellipsometric porosimetry for in‐line characterization of ultra low‐κ dielectrics

AbstractEllipsometric porosimetry (EP) has recently appeared to be a suitable non‐destructive technique for the characterization of porous ultra low‐κ (ULK) dielectrics. The analysis of ellipsometric spectra of a film during adsorption and desorption cycles of an adsorptive, allows the determination of its open pore fraction, pore size distribution, refractive index and thickness. Several issues are encountered when integrating low‐κ materials as inter‐metal dielectrics (IMD) in the CMOS architecture such as the deposition monitoring, the pore sealing efficiency and the process induced damages. In this study we focus on the capabilities of EP to characterize ULK materials and their integration processes. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Impact of Organic Acid and Gas Bubbling on Copper and Copper Oxide Etch-Rates in Diluted HF Solution

In the damascene integration scheme, copper is used as an interconnect material. In a damascene integration scheme, the etch treatment by a plasma process leads to the formation of polymer residues, copper sputtered on the dielectric side wall, copper oxidation at the bottom of the via and copper precipitates at the hard mask surface. In order to remove these impurities, a post-via etch cleaning is mandatory [1]. Several aqueous cleaning solutions constituted of diluted HF and an organic acid were tested. Diluted HF was mainly used to remove the polymer residues and the organic acid to clean the copper surface. These mixtures were already studied and compatible with several porous and dense ULK materials [1]. In this paper, the impact of organic acids and gas bubbling in diluted HF solution on copper and copper oxide-dissolution rate is presented. Firslty the copper oxide and the copper etch-rates and the roughness were obtained from the kinetics of copper dissolution determined by X-ray reflectometry characterization [3]. Secondly, speciation calculations were performed in order to explain the copper etch-rates differences as a function of the solutions and gas bubbling.

On the photoresist stripping and damage of ultralow k dielectric materials using remote H2- and D2-based discharges

Blanket films of ultralow dielectric constant (ULK) materials and 193nm photoresist films have been processed downstream from hydrogen and deuterium-based discharges produced using an inductively coupled plasma reactor. Photoresist ashing rates and ULK modifications have been determined as a function of process parameters. The explored ULK materials differed widely in porosity and carbon content. The effect of processing time, substrate temperature (200–300°C), and gas composition on the surface and bulk chemical composition of ULK materials was monitored and quantified by ex situ ellipsometry and time-of-flight secondary ion mass spectrometry (SIMS). The stripping rates of 193nm photoresist films were found to strongly depend on processing temperature and only weakly on the nature of the H2/additive gas mixture. The authors found that hydrogen (or deuterium) fully penetrates the high porosity ULK layer, whereas for low porosity materials, such penetration is limited to a 50nm near-surface region. SIMS measurements also reveal that H2 (D2) diffusion into carbon-rich ULK layers can cause substantial carbon depletion throughout the penetration region. ULK damage values increase with temperature and injection of gas additives such as argon, helium, and nitrogen to H2 or D2 process gases. For each ULK material, the amount of damage depends on the gas mixing ratio; in general, high percentages of nitrogen in H2∕N2 (or D2∕N2) mixtures cause the most damage. Overall, the results demonstrate that ULK ashing damage depends strongly on both ULK material properties and H2-based plasma process parameters. In addition, the authors observed in this work a kinetic isotope effect for stripping of 193nm photoresist films in H2∕D2∕N2-based discharges. For given ashing process conditions, the photoresist ashing rate decreases by a factor of 1.414 (or square root of 2) in D2 plasma compared to H2 plasma. This can be explained by the influence of the H or D mass on the chemical reaction rate through a change in the frequency of nuclear vibrations of the reacting atoms. The presence of the kinetic isotope effect for gas mixtures provides unambiguous evidence of the rate-limiting role of atomic hydrogen in the fundamental etching reaction. Simultaneously processed ULK materials showed minor film thickness changes (<10nm) in H2 or D2 discharges, and the ULK damage level does not reflect a kinetic isotope effect. Therefore the H∕D isotope effect can be used to separate H2∕D2 associated ashing and etching processes from other chemistries or mechanisms.

Interactions of photoresist stripping plasmas with nanoporous organo-silicate ultra low dielectric constant dielectrics

To evaluate potential solutions for reducing the damage to ultra low- k dielectrics during photoresist stripping in advanced interconnect technology, we have investigated the mechanisms of interactions between remote H 2, D 2 and N 2 discharges and porous organo-silicate materials. Extended sub-surface modifications take place in high carbon-content organo-silicates, whereas silica-rich dielectrics show negligible chemical damages during the same treatments. The nature of plasma/dielectric interactions depends primarily on the organic fraction of the ULK material. Methyl groups in silica-rich organo-silicates withstand the interaction with the plasma species. Conversely, large organic compounds in carbon-rich dielectrics experience cleavage reactions leading to volatile hydrocarbon formation and compositional changes. For conditions where stripping-induced damage is introduced, the effects scale with the substrate temperature in the range 200 °C–300 °C. The permeation of the ULK material by remote plasma species depends on its porosity.