Porous Ultra Low-k Research Articles

Surface-Localized Sealing of Porous Ultralow-k Dielectric Films with Ultrathin (

Semiconductor integrated circuit chip industries have been striving to introduce porous ultralow-k (ULK) dielectrics into the multilevel interconnection process in order to improve their chip operation speed by reducing capacitance along the signal path. To date, however, highly porous ULK dielectrics (porosity >40%, dielectric constant (k) <2.4) have not been successfully adopted in real devices because the porous nature causes many serious problems, including noncontinuous barrier deposition, penetration of the barrier metal, and reliability issues. Here, a method that allows porous ULK dielectrics to be successfully used with a multilevel interconnection scheme is presented. The surface of the porous ULK dielectric film (k = 2.0, porosity ∼47%) could be completely sealed by a thin (<2 nm) polymer deposited by a multistep initiated chemical vapor deposition (iCVD) process. Using the iCVD process, a thin pore-sealing layer was localized only to the surface of the porous ULK dielectric film, which could minimize the increase of k; the final effective k was less than 2.2, and the penetration of metal barrier precursors into the dielectric film was completely blocked. The pore-sealed ULK dielectric film also exhibited excellent long-term reliability comparable to a dense low-k dielectric film.

A novel tri-layer nanoindentation method to measure the mechanical properties of a porous brittle ultra-low-k dielectric thin film

The mechanical properties of two porous ultra-low-k (ULK) dielectric thin films (porous carbon-doped silicon dioxide (pSiCOH); and octamethylcyclotetrasiloxane (OMCTS)) were measured by nanoindentation. Since a direct contact of a nanoindentation tip with a thin and brittle ULK film will likely induce damage to the film and thus alters its mechanical response, the mechanical properties measured from direct nanoindentation on an ULK film do not necessarily represent the behavior of an ULK thin film in service condition. To circumvent this difficulty, a novel tri-layer configuration was used for sample preparation. In this method, a thin layer of tetraethyl orthosilicate (TEOS) silica was deposited on an ULK thin film to protect it from direct contact with a nanoindenter tip. A nanoindenter tip was in contact with TEOS coating on the layered film in nanoindentation, and the collected data was subsequently analyzed by finite element analysis. The Ogden hyperfoam model was used as the constitutive law for the ULK film in simulations, and found to be appropriate. Results indicate that the linear elastic model is only applicable for very small strains. At large strains the ULK films show nonlinear behavior due to densification of the porous ULK material under compaction. The moduli for the pSiCOH and OMCTS dielectric films were determined as 2.6 GPa and 5.2 GPa, respectively.

Improvement of Rough Interface Between Barrier/Seed Layer and Porous Ultralow k Film for 28nm Technological Node and Beyond

With critical dimension of device scaled down to 28 nm technological node and beyond, porous ultralow k (PULK) film as an insulator is used in Cu interconnects to further reduce resistance–capacitance (RC) delay. The integration of the PULK film faces more severe challenges due to the presence of porosity. Plasma treatments and wet cleaning processes have been considered to be critical steps to impact the PULK film properties. In this paper, the effects of the various plasma treatments and wet cleaning processes on the PULK films were investigated. Results indicated that NH3, N2, and CHF3 plasma treatments resulted in the formation of a thin and densified layer on the surface of the PULK films, which prevented TaN/Ta barrier diffusing inside the pores to avoid forming rough interface. In addition, specific EKC520 product + dilute hydrofluoric acid (DHF) were used to clean the PULK film treated by the CHF3 plasma, and the rough interface was observed after the barrier deposition. Moreover, the specific EKC520 product was used to clean the PULK film treated by the N2 plasma, and there was smooth interface after the barrier deposition. However, the DHF chemical could remove the thin and densified layer from the surface of the PULK film to cause the re-exposure of pores. During barrier deposition, the TaN barrier diffused inside pores resulting in the formation of the rough interface.

Exploration of Chemical Bonding Transformation Mapping to Assist Low-k Dielectric Nanostructure Fabrication

In leading edge integrated circuit manufacture, reduction of RC time delay by incorporation of porous ultra-low k (ULK) interlayer dielectrics (ILD) into Cu interconnect nanostructure continues to pose major integration challenges. The main challenge is porosity induced to lower the dielectric constant of the ILD has the undesirable side effect of compromising its mechanical and chemical stability. As a result, these materials are more susceptible to the damages caused by RIE plasma etching processes. Besides the challenge of handling weak porous ULK materials, a lack of sensitive metrology to guide systematic development of plasma etching, restoration and cleaning processes is the major stumbling block. To assist in the overall integration effort, we have been actively exploring Multiple Internal Reflection Infrared Spectroscopy (MIR-IR) and associated IR spectroscopic techniques as a sensitive (sub-5nm) characterization tool to assist the fabrication process of ULK nanostructure with targeted critical dimension. In this paper, we will report new findings obtained on chemical bonding modification across fluorocarbon etch residues and low-k dielectric interface after manufacture relevant processing treatments including plasma etching, ashing, UV curing and post-etch cleaning. MIR-IR metrology showed useful spectroscopic details of chemical bonding transformations taking place during plasma etch and strip processing on both blanket and dielectric trench structures. The observed increase of Si-OH and C=O and the decrease of Si-CH3 IR absorption bands correlated well with increasing plasma induced damage, K increases verified by MIS structure, especially for more aggressive stripping process. With functional group-specific chemical derivatization, chemical bonding structure of 1-2 nm post-etch residue on porous low-k trench structure was established. UV-induced modification of fluorocarbon polymer residue to improve its subsequent wet cleaning removal was studied by FTIR, XPS and SEM. The overall goal is to apply new insights on chemical bonding transformation mapping to achieve cleans-friendly plasma etches on creating ULK dielectric nanostructures with minimal dielectric damages.

Preparation of porous ultra low k films using different sacrificial porogen precursors for 28 nM technological node

In order to reduce resistance capacitance (RC) delay and crosstalk between adjacent interconnect lines, the porous ultra low k (PULK) film with dielectric constant value ranging from 2.4 to 2.6 was introduced to be used as an insulator of the Cu integration in back end of the line (BEOL) for 28nm technological node. In this paper, the PULK films were deposited by plasma enhanced chemical vapor deposition (PECVD) using two different sacrificial organic porogen precursors of bicycloheptadiene (BCHD) and alpha-terpinene (ATRP), and then the above co-deposited films were expose to UV radialization for removing sacrificial organic porogens to create pores. The effects of the different sacrificial organic porogen on the PULK film properties such as the dielectric constant, pore and porosity, k shift, mechanic strength, and RC were studied in detail. Results indicated that the hardness and modulus of the PULK film for the BCHD and ATRP precursors were 0.7 and 5.1GPa and 1.0 and 7.1Gpa, and their pore radiuses and porosities were 1.18nm and 25% and 0.95nm and 23%. The dielectric constant value of the PULK film for the BCHD and ATRP precursors were measured as 2.48 and 2.59 with Hg probe. The adhesion test indicated that the interfacial strength of the PULK film for the ATRP precursor was higher than that of the PULK film for the BCHD precursor. By the RC examination, it was seen that RC curve of the PULK film for the ATRP precursor was located below the curve of the PULK film for the BCHD precursor.

Development of a High-Intensity UV Exposure Apparatus under a High-Pressure CO&lt;sub&gt;2&lt;/sub&gt; Gas Atmosphere to Manufacture Large-Area Porous Ultralow-k Polyimide Substrates for Flexible Print Circuits

Development of a High-Intensity UV Exposure Apparatus under a High-Pressure CO&lt;sub&gt;2&lt;/sub&gt; Gas Atmosphere to Manufacture Large-Area Porous Ultralow-k Polyimide Substrates for Flexible Print Circuits

Adhesion analysis for on-chip interconnect structures by beam bending techniques with optical crack length determination

Crack propagation in BEoL (back-end of line) interconnect structures is a concern in terms of chip reliability. Due to the application of porous ultra low-k (ULK) materials, a careful trade-off is necessary between reducing signal delays and also decreasing mechanical properties of the interconnect structures. Therefore, fast and reliable measurement techniques for characterizing mechanical properties as film adhesion and fracture toughness are needed. In the present investigation, a camera assisted double-cantilever beam (DCB) technique has been used to obtain locally resolved adhesion measurements within the die. Two chip designs, produced in leading-edge technology nodes with two different ULK materials, were tested. The Gc values measured within the die show a correlation to density and arrangement of the Cu structures. ULK containing metallization levels with lowest k values have been identified as preferred crack paths. The measurements are compared with results of the four-point bending (4PB) test not having the local resolution of DCB but typically used for unpatterned film stacks and possessing a mode I/II load mixity. The DCB tests yield reduced Gc values compared to the 4PB tests, which can be understood as result of the different loading modes of both techniques.

Pore Sealing of Porous Ultralow-k Dielectrics by Self-Assembled Monolayers Combined with Atomic Layer Deposition

Due to its excellent step coverage characteristics, atomic layer depositions (ALD) deposit their material not only on top of underlying surfaces but also into the pores of porous low-k films. In this study, the film characteristics caused by pre-treatments of the low-k material, followed by deposition of self assembled monolayers (SAMs) from an 11-cyanoundecyltrichlorosilane precursor and finally by an HfO2 Atomic Layer Deposition were determined. By adequate optimization of the surface treatment, the SAM deposition and the ALD it was possible to obtain complete pore sealing after ALD, without penetration of the Hf into de porous low-k film.

New generation of reactive pre-clean prior to barrier–seed deposition to preserve ULK integrity

Advanced interconnects development requires introduction of porous ultra low k (ULK) dielectrics enhancing product performance through parasitic capacitance reduction. ULK layers are characterized by higher carbon content and lower mechanical properties due to porosity increase. Consequently, ULK dielectrics are more prone to be damaged during plasma treatment in interconnects build [1]. Pre-clean processes prior to barrier/seed deposition are a potential area of concern for ULK integrity. Reactive pre-clean processes based on He/H2 plasma are investigated in this paper through two types of hardware. The new pre-clean chamber is an upgrade of conventional pre-clean hardware and combines optimized H2 plasma and active wafer temperature control to ensure efficient copper oxide reduction [3] and via cleaning. The aim of the paper is to evaluate performance of the new generation of reactive pre-clean compared to conventional hardware. To overcome ULK dielectric integration challenge, pre-clean processes need to ensure efficient copper oxide reduction and via bottom cleaning with minimum impact on ULK material properties. The ''new reactive pre-clean'' addresses these requirements for k=2.5 ULK materials. The new reactive pre-clean demonstrates a robust process window over a wide range of temperature and pressure settings for all characterized aspects. The ''new reactive pre-clean'' technology is seen to be a promising pre-clean candidate for next generation of porous ULK dielectric material ([email protected]?2.5) integration for 28nm node and beyond.

Influence of thermal cycles on the silylation process for recovering k-value and chemical structure of plasma damaged ultra-low-k materials

The integration of porous ultra low-k (ULK) materials within the interconnect system of integrated circuits is one of the most promising approaches to reduce RC-delay or crosstalk. The application of plasma processes for patterning, cleaning and photoresist removal is known to degrade the electrical parameters of the ULK by carbon depletion, densification and introduction of silanol species. In this study we investigate the influence of different thermally assisted immersion processes on the effectiveness of a silylation based k-restore process of plasma damaged porous low-k dielectrics. The results show that network recovery effects like methyl incorporation are different at the surface and in the depth of the damaged area addicted to precursor temperature and curing conditions. The most promising process sequence was shown to consist of an immersion at temperatures of 100-150^oC with subsequent UV anneal depending on the precursor used for recovery process.

Development of Porosimetry Techniques for the Characterization of Plasma-Treated Porous Ultra Low-k Materials

For the sub-32 nm node, porous SiCOH dielectrics (p-SiCOH) are integrated using dual damascene patterning by etching trenches and vias into the porous material. One challenge is to control the process conditions to minimize the plasma-induced damage of p-SiCOH materials at the bottom and at the sidewall of the trenches. Ellipsometric Porosimetry has been adapted to characterize the plasma-treated materials for both horizontal and vertical surfaces. Since surface modifications can cause adsorption and desorption delays and hydrophobicity loss, porosimetry measurements operated with multi-solvent and kinetic protocols are required. Quantitative measurements of vertically patterned materials are demonstrated using periodic structures of porous material and a Scatterometric Porosimetry analysis. Results show a good sensitivity of the measurement to the different process conditions but also highlight a different impact of the plasma processes on patterned materials compared with blanket films.

Development of Porosimetry Techniques for the Characterization of Plasma-Treated Porous Ultra Low-K Materials

not Available.

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.

Effect of TaN Stoichiometry on Barrier Oxidation and Defect Density in 32nm Cu/Ultra-Low K Interconnects

AbstractThe scaling of BEOL interconnect technology in ULSI circuitry requires the integration of Cu wiring with ultra-low K (ULK) dielectrics. We present the results of a study of the interaction between different-stoichiometry Ta(N)/Cu barrier processes and porous ULK dielectrics (k=2.4) at 32nm groundrules Auger and diffraction analysis of blanket wafers was used to benchmark two different stoichiometries of TaN barrier deposited using commercially-available ionized PVD sources. Comparison TEM and EDX/EELS images indicates that barrier oxidation is occurring in the low nitrogen-content Ta(N) barrier, which is absent at the higher stoichiometry. These differences are further manifested in defect-density analysis of patterned wafers comparing the two processes. These results illustrate the critical importance the TaN barrier properties play in enabling the integration of Cu/ULK interconnects at 32nm at beyond.

Surfactants as an Additive to Wet Cleaning Solutions for Plasma Etch Residue Removal: Compatibility to a Porous CVD-SiCOH Ultra Low-k Dielectric Material

With decreasing feature sizes and integration of porous low-k dielectrics within the interconnect system of integrated circuits, plasma etch residue removal becomes challenging. Plasma cleaning processes are known to degrade low-k material's properties and wet cleaning becomes a promising alternative. Using water based cleaning solutions turns out to be critical as they may not be able to wet low-energy residue surfaces or to penetrate into small features. Lowering the surface tension of the solutions will be essential and the application of surfactants is a promising approach. We investigated the static and dynamic surface tension of different surfactant solutions in DIW and their compatibility to a porous CVD-SiCOH low-k dielectric. The choice of the rinsing solution applied after surfactant treatment turned out to be essential for removing residual surfactant species. Optical, electrical and structural analysis showed that in contrast to DIW an IPA-rinse is able to remove remaining surfactant species.

Surfactants as an Additive to Wet Cleaning Solutions for Plasma Etch Residue Removal: Compatibility to a Porous CVD-SiCOH Ultra Low-k Dielectric Material

not Available.

Thermal and Electrical Properties of PVD Ru(P) Film as Cu Diffusion Barrier

Thermal and electrical properties of physical vapor deposition (PVD) Ru(P) film deposited on porous ultra low-k (p-ULK) material as Cu diffusion barrier were studied. The phosphorous concentration can be tuned by adjusting Ar to PH 3 ratio of the sputtering gases. The leakage current depends on phosphorous concentration. Higher phosphorous content in Ru film has lower leakage current. No obvious phosphorous content dependence was observed when the amorphous Ru(P) film crystallized. The X-ray diffraction (XRD) graphs and energy dispersive spectrometer’s (EDS) atomic depth profiles show that the Ru(P) film deposited on p-ULK can effectively block Cu diffusion when the sample is subjected to 800 °C 5 min annealing. The phosphorous doped Ru film improves diffusion barrier properties and leakage current performance. The improved Ru(P) barrier capable of direct Cu plating could be a potential candidate for advanced metallization.

Process Dependence on Defectivity Count on Copper and Dielectric Surfaces in Post-Copper CMP Cleaning

At critical dimensions of 65nm and lower, the tolerances for yield impacting “killer” particle defects become ever tighter and this is especially true for Cu PCMP cleaning. This is a critical step in reducing wafer defectivity and metal contamination directly impacting yield and reliability. A successful candidate must exhibit a number of additional attributes not least of which are compatibility to Cu while selectively removing copper oxides (CuOx), and be fully compatible with porous ULK and barrier materials. Conventionally PCMP cleans typically require two different acidic or alkaline solutions containing a surfactant to drive particle removal and surface metal preparation; however this is not without its limitations e.g. crack propagation, water absorption, low-k wetting, water marks and galvanic corrosion. It would be advantageous if Cu PCMP could be performed with a single neutral pH cleaning solution. The paper will present results of work conducted on a neutral, single step Cu Post CMP (PCMP) cleaning solution, EKC PCMP5510™, and will discuss the influences of different process parameters in optimising defect removal from Cu and Ultra Low-K (ULK) dielectric surfaces (K=2.5) thus highlighting the fundamental differences in cleaning either surface.

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.