Copper Drift Research Articles

A 3D printed pure copper drift tube linac prototype.

Radio frequency cavities are among the most challenging and costly components of an accelerator facility. They are usually manufactured in individual parts, which are then joined by complex processes, e.g., several brazing steps. 3D printing has become an alternative to these conventional manufacturing methods due to higher cost efficiency, freedom in design, and recent achievement of high print quality for pure copper. A fully functional 3GHz drift tube linac (DTL) prototype was 3D printed in one piece, made from pure copper by selective laser melting (SLM). To achieve a higher surface quality, the DTL geometry was optimized for the SLM process. The DTL design is related to the design of the DTL part of the side-coupled DTL modules used in linac-based proton therapy facilities. The quality factor (8750) and the shunt impedance per unit length (102mΩm) of the printed prototype are already comparable to traditionally manufactured DTL structures and can be further enhanced by surface treatments.

Copper on the Northwest Coast, a biographical approach

This paper explores Indigenous use of copper metal to create objects of cultural importance on the Northwest Coast of North America, and the impacts of colonial contact on established cultural practices. Prior to contact (late 17th to early 19th century), native copper and occasional shipwreck drift copper was collected, traded, and used by Indigenous communities to fabricate meaningful objects such as Coppers. Following the introduction of foreign trade materials, copper continued to be used to create culturally significant artefacts but existing frameworks of interaction with the metal shifted to accommodate the new materials and social relations introduced by contact.This investigation highlights the utility of a biographical approach to gain an understanding of Indigenous artefacts and the relationships they engender. Specifically, a close material investigation of a Kwakwaka'wakw Copper fabricated from multiple pieces of metal is conducted. This copper alloy artefact results from a series of material choices, fabrication traditions, and usage that, when considered alongside archaeological and ethnographic data allow us insight in to social, economic, and political changes during the contact period.

Towards understanding intrinsic degradation and breakdown mechanisms in SiOCH low-k dielectrics

The degradation and breakdown mechanisms of a SiOCH low-k material with k = 2.3 (25% porosity) and thicknesses ranging from 90 nm to 20 nm were investigated. By combining the time dependent dielectric breakdown data at positive/negative bias stress with the thickness scaling results, dielectric failure is proven to be intrinsic and not influenced by copper drift or metal barrier deposition induced dielectric damage. It is shown that stress induced leakage current (SILC) can be used as a measure of dielectric degradation. Therefore, low field lifetimes can be safely estimated using SILC extrapolation. Based on our results, both the impact damage model and the power law model have a good accuracy for low field lifetime prediction. Recovery and anneal experiments are used to study the physical nature causing the observed negative flatband voltage shifts in our metal-insulator-semiconductor planar capacitor structures, where hydrogen induced unstable fast and slow donor type interface states are hypothesized to be the root cause of the observed shifts. We suggest that atomic hydrogen is released from the dielectric during electron injection and migrates to the interfacial region. Our model is further supported by an observed irreversible SILC change during the recovery and anneal studies. The degradation mechanism proposed in this work, supported by the low field lifetime data, provides a feasible explanation for intrinsic low-k dielectric failure.

Development of Laser and Photodefinable Toughened Benzocyclobutene Dielectric Materials for 3D-TSV Integration

3D IC integration based on TSV technology has been recognized as a key enabler for next generation of electronic devices with reduced size factor and improved performances. The adoption of 3D-TSV technology also requires the development of innovative interconnect solutions that reduces the size of signal routing and therefore imposes new demands on dielectric materials used to isolate the copper interconnects. Benzocyclobutene polymers (Dow's CYCLOTENE™ Advanced Electronic Resins) have been used to isolate copper interconnects in packaging applications for more than 20 years, due to a number of good attributes of the BCB polymer including low copper drift rate, low dielectric constant and low loss, low moisture absorption and proven reliability. However, the low fracture toughness and low elongation of BCB polymer has limited its use in stress buffer applications due to solder bump failure. Here we report the development of new laser and photodefinable toughened benzocyclobutene (BCB) dielectric materials that have following improved properties and benefits over commercial materials including: 1) Higher elongation to break at 25%, 2) Higher fracture toughness, 3) Improved lithographic performance, < 8μm minimal size feature, 4) Better stability, no change in Eo after 30 days at room temperature. The patterning and integration of these toughened benzocyclobutene materials and the processing conditions are also discussed. We believe this toughened BCB material will find wide applications as a stress buffer layer in 3-D IC.

Extensive analysis of resistance evolution due to electromigration induced degradation

Electromigration failure kinetic has been studied with resistance evolution versus time of interconnects during degradation. Tests were performed on dual damascene copper lines, issued from the 65 nm technology node, of various widths and lengths. All samples exhibit similar resistance evolution: an initial step, characterized by its height called Rstep, follows a linear kinetic characterized by its slope called Rslope. These two parameters were systematically extracted; Rstep is proportional to the critical volume of a void spanning the whole section of the line, and Rslope to the copper drift velocity. On one hand, the linewidth does not affect these two parameters. On the other hand, Rslope is highly dependent on the line length because of the Blech effect, while Rstep remains constant. Consequently, it was demonstrated that the classical linear function L/TTF=f(jL), where TTF is the time to failure, j is the current density, and L is the line length, used to study the Blech effect in interconnects could be substituted with the linear function RslopeL=f(jL), confirming that Rslope is a suitable parameter to study void growth kinetics. At 300 °C and 2 MA/cm2, a void growth velocity of 1.15×10−8 μm3/s was thus determined on long lines, and an activation energy of 0.95 eV was found. Finally based on the resistance analysis, an explanation is proposed concerning the larger spread observed on the TTF measured on short lines at low current density.

Study of copper drift during TDDB of intermetal dielectrics by using fully passivated MOS capacitors as test vehicle

A fully passivated Metal-Oxide-Semiconductor (MOS) capacitor [Tőkei Zs. Barrier integrity and reliability in copper low-k interconnects. ISTC 2005; 386–95] is used to study the intrinsic properties of a barrier between copper and dielectric in Back-End-Of-Line interconnects. Several barriers are studied and compared to each other. The test vehicle is also used to thoroughly investigate the role of thermal diffusion or field assisted ionic copper drift during Time-Dependent-Dielectric Breakdown (TDDB) by investigating the breakdown of dielectrics without a barrier at very low voltages. Comparing different barriers revealed that a “standard” PVD-based Ta barrier has a significantly better TDDB-performance compared to an 8 nm SiCN-barrier. For samples without barrier, it was found that long thermal anneals without the application of a stress voltage changes the distribution of failure times. Furthermore, the possibility of a bimodal distribution was argued when stressing these devices at a wide range of fields. A corollary is that, in presence of copper, both the E-model and the root- E-model do not apply for describing the experimental data obtained on these samples without barrier.

Copper ion drift in integrated circuits: Effect of boundary conditions on reliability and breakdown of low-k dielectrics

Copper drift in current interconnect structures is a reliability issue leading to premature low-k dielectric breakdown. Copper ions drift through insulating low-k dielectrics leading to an increase in the local electric field that, if high enough, could lead to device failure. We have solved the coupled continuity and Poisson’s equation using the boundary condition that the flux of copper ions equals zero [J(t,L)=0] at the cathode or dielectric-semiconductor interface. This condition closely mirrors the actual physics in interconnects and leads to a buildup or accumulation of copper ions at the dielectric-semiconductor interface. Significant differences are observed in the copper concentration and local electric field behavior near the interface when the condition of a blocking electrode [J(t,L)=0] compared to the concentration equals zero [C(t,L)=0] with implications for the reliability of the low-k dielectric and time to failure for the interconnect structure. The time to failure (TTF) is shown to depend on the boundary condition at the cathode Ce, the copper ion solubility in the dielectric, as well as the drift of copper ions through the dielectric. When intrinsic thermal effects are included in addition to the mass transport, a close fit is achieved with experimental TTF results of Hwang et al. [J. Appl. Phys. 101, 074501 (2007)] The inclusion of mass transport simulates the nonlinearity in TTF at low fields, a characteristic that has been observed experimentally.

Use of Resistance-Evolution Dynamics During Electromigration to Determine Activation Energy on Single Samples

In this paper, we present an analysis of resistance change events that are occurring during the degradation of interconnects. The events analyzed are the following: 1) a step jump in resistance and 2) a subsequent progressive linear increase of resistance. We first find that the height of the step is correlated with time to fail. We show that the occurrence of a step is likely due to the vertical growth of a void, and the variation of the resistance-step amplitude is due to the variation of the length of the void from sample to sample. We then find that resistance-increase rate of the progressive part is correlated with copper drift velocity. Thus, we show that it is possible to determine an activation energy on every sample, which is performing temperature change during the progressive part of the resistance increase.

CoWP as a Drift Barrier for Cu Ions Studied by Electric Measurements

Electrical characteristics of self-aligned cobalt–tungsten–phosphide (CoWP) as a copper (Cu) drift barrier for advanced Cu interconnects were investigated using blanket metal-insulator-metal (MIM) structures. The time-dependent dielectric breakdown measurements of the MIM structure under the bias-temperature stress tests demonstrated that the median time-to-failure (MTF) of chemical-vapor-deposited improves by more than 20 times by forming a 10 nm thick CoWP. The improved MTF is comparable to that measured with the reversed-bias stress in which no Cu-ion drift occurs. It is concluded that Cu-ion drift is suppressed by a thick CoWP barrier.

Copper drift in high-dielectric-constant tantalum oxide thin films under bias temperature stress

The use of high-dielectric-constant (high-κ) materials for embedded capacitors is becoming increasingly important. Tantalum oxide (Ta2O5) is a prominent candidate as a high-κ material for embedded capacitor use. Metal drift in Ta2O5 (κ∼25) was investigated by bias temperature stress and triangular voltage sweep testing techniques on metal/Ta2O5∕SiO2∕Si structures. At a temperature of 300°C and 0.75MV∕cm bias conditions, Al, Ta, and Ti do not diffuse in Ta2O5, but Cu clearly showed a drift. The Cu drift is attributed to the lack of a stable Cu oxide which can limit Cu ion generation and penetration.

Diffusion of Copper in Nanoporous Dielectric Films

Water adsorption is undesirable in microelectronics processing because it increases the dielectric constant of the insulator, may lead to the corrosion of metal lines, and can act as a source for the generation of copper ions that can lead to copper drift in a dielectric. We have shown by chemical modification of the surface that the degree of hydrophobicity is a function of the chemical nature of porous low-k dielectric films. Chemical surface modification of nanoporous silica films markedly reduces the moisture uptake and reveals the importance of chemically bound or adsorbed water species in the dielectric and how it triggers metal diffusion. That is, when the organic groups left by the surface modifier are removed by high-temperature sintering, the hydrophobicity of the dielectric film is markedly reduced, more copper ions are generated, and copper drift in the dielectric is increased significantly. We propose that water-related traps in the dielectric films have two effects on metal diffusion: (a) they ionize the metal to form a nonstoichiometric oxide, which acts as the source of metal ions for diffusion; and (b) water-related traps in the dielectric are generated by the action of the external electric field and once generated they create space-charged regions where the local electric field exceeds the external applied field by a substantial amount; this enhances metal charge injection. The relationship between moisture uptake and porosity in nanoporous dielectrics is presented and correlated to metal charge injection. A quantitative analysis by capacitance-voltage measurements is presented of Cu drift in dense and nanoporous low-k dielectric films that reveals the role of Cu ions in the degradation and breakdown of a dielectric. The mechanism for metal diffusion and charge injection, and its dependence on porosity, pore size, surface area, and surface chemistry of the dielectric, are discussed. A physically based mathematical model of diffusion through dense and nanoporous solids has been developed considering bulk and surface diffusion and different concentrations of water-related traps. This model is used to interpret some of the experimental results obtained and confirms that diffusion barriers markedly reduce the injection of copper into dielectrics.

Leakage mechanism in a porous organic low-k polymer

Leakage mechanism of a copper single damascene structure with a porous organic low-k polymer as the inter-metal dielectric between 1 and 4 mV/cm is investigated. Constant-rate voltage ramp is the main tool for investigation. Experiments were conducted between 25 and 250 °C. Passivated damascene structures with fully dense and porous copper diffusion barriers were investigated. If a fully dense barrier is used, the leakage mechanism is believed to be Frenkel–Poole emission. For a porous diffusion barrier above 175 °C, this mechanism is altered by copper drift diffusion.

Separation of copper ion-induced and intrinsic polymer instabilities in polyarylether using triangular voltage sweep

A low dielectric constant polyarylether (PAE) polymer was investigated for resistance to copper drift. Positive bias-temperature stressing (BTS) of metal/PAE/SiO2/Si capacitors led to large initial flat-band voltage shifts toward the left-hand side in the capacitance–voltage curve for all gate metals investigated (Pt, Al, and Cu). Upon triangular voltage sweep (TVS) testing, Pt/PAE/SiO2/Si capacitors exhibited no peak, whereas a single peak (indicative of mobile ions) was seen for Cu/PAE/SiO2/Si capacitors. The results indicate that an intrinsic polymer instability source, such as polarization (under bias), was the major cause for the BTS instability of the polymer. From TVS studies, Cu+ ion drift could be characterized in the temperature range of 150–250 °C, and mobility of Cu+ in PAE was extracted from Hillen’s analysis. A combination of BTS and TVS techniques is thus more useful for characterization of copper drift, and helps to resolve the intrinsic polymer effect from that of metal drift-related instability.

Ultralow copper drift in inductively coupled plasma silicon carbide dielectrics

This study investigates how copper penetration into inductively coupled plasma (ICP) trimethylsilane-based hydrogenated amorphous silicon carbide (a-SiCx:H) dielectric barriers depends on the films’ hydrogen concentrations. Depositing ICP a-SiCx:H films with low hydrogen concentrations (or few defect sites), copper drift rate in such films stressed at around 200 °C, 1 MV/cm, is as low as 105 ions/cm2 s, two to five times lower than those in plasma-enhanced chemical-vapor-deposited (PECVD) a-SiCx:H and nitride barriers. Accordingly, the time-to-breakdown of ICP a-SiCx:H subjected to bias-temperature stress at above 200 °C, 2.0–2.75 MV/cm, is almost one order of magnitude longer than that of PECVD a-SiCx:H.

Assessment of Reliability of cap layers used in Cu-Black DiamondTM Interconnects

AbstractBlack DiamondTM. (BD) is one of the primary candidates for use in copper-low k integration. Although BD is SiO2 based, it is vastly different from oxide in terms of dielectric strength and reliability. One of the main reliability concerns is the drift of copper ions under electric field to the surrounding dielectric layer and this is evaluated by voltage ramp (V-ramp) and time dependent dielectric breakdown (TDDB). Metal 1 and Metal 2 intralevel comb structures with different metal widths and spaces were chosen for dielectric breakdown studies. Breakdown field of individual test structures were obtained from V-ramp tests in the temperature range of 30 to 150°C. TDDB was performed in the field range 0.5 – 2 MV/cm. From the leakage between combs at the same level (either metal 1 or metal 2) Cu drift through SiC/BD or SiN/BD interface was characterized. It was found that Cu/barrier and barrier/low k interfaces functioned as easy paths for copper drift thereby shorting the lines. Cu/SiC was found to provide a better interface than Cu/SiN.

Copper drift in low dielectric constant insulator films caused by O 2+ primary ion beam

Copper depth profiling in low- k material methylsilsesquioxane (MSQ) was investigated. Cu was implanted into MSQ or thermally grown SiO 2 and depth profiling was carried out with O 2 + primary ions, and charge neutralization with electron flooding using a quadrupole SIMS. The obtained profile was deeper and broader than the theoretically predicted ones and Cu drifting during the depth profiling was implied. Though the primary ion energy was lowered to 1 keV, it was not enough to suppress the redistribution. Though MSQ showed shrinking by electron irradiation, this did not seem to be the major origin of the Cu redistribution.

Copper drift in methyl-doped silicon oxide film

Drift of copper (Cu) ions into methyl-doped silicon oxide film, a promising low-dielectric-constant (low-κ) material for inter-layer dielectrics applications in ultralarge scale integrated circuit (ULSI) interconnects, was investigated using bias temperature stressing (BTS)/capacitance–voltage (C–V), current–voltage (I–V) at elevated temperatures and time-dependent dielectrics breakdown (TDDB) tests. Standard and control metal–insulator–semiconductor capacitor test samples with “sandwich” dielectric layer structures were used in the tests. BTS in nitrogen (N2) ambient at an electric field magnitude up to 1.5 MV/cm and with temperatures between 175 and 275 °C was used to accelerate the Cu ion drift. By studying flat band voltage shift in C–V tests, leakage current magnitude in I–V tests and time to fail in TDDB tests, it is demonstrated that Cu ions readily drift into methyl-doped silicon oxide under electric fields at elevated temperatures. Results obtained using different techniques correlate well to each other. The activation energy of Cu drift is determined to be 0.76 eV in N2 ambient. The film is found to have poorer resistance to Cu drift than silicon oxide (SiO2) deposited using plasma-enhanced chemical vapor deposition (PECVD). Therefore, good Cu drift barrier layers are required for reliable ULSI interconnects using this low-κ material. A thin layer of PECVD SiC is proven to be a good Cu drift barrier layer.

Modeling Copper Diffusion in Silicon Oxide, Nitride, and Carbide

AbstractDensity functional theory was applied to simulate copper diffusion in silicon oxide, nitride, and carbide (SiOx, SiNx, SiCx). Because copper drift into oxide is significantly enhanced by negative bias, copper ions are the active diffusing species. Clusters and, in some cases supercells, modeling various ring configurations of the amorphous networks of silicon oxide, nitride, and carbide were employed. Interactions of both neutral copper and its cation, Cu+, with the network were explored. Calculations revealed a strong binding of Cu+ to SiOx, SiCx, and SiNx in contrast with neutral Cu. The Cu+ attraction to carbide clusters is significantly lower than to SiOx and SiNx, explaining the effective barrier properties of SiCx. The estimated lower bounds for activation energies for Cu+ hops between stable ring clusters of SiOx and SiNx are similar. This implies that the difference in Cu diffusion properties between oxides and nitrides is likely due to a higher percentage of large rings in amorphous oxides compared with nitrides. An approach to increasing the resistance of oxides to Cu+ diffusion is suggested.

The Simulation of Copper Drift in SiO2 during Bias Temperature Stress (BTS) Test

AbstractIn order to develop a reliable interconnect integration scheme by using Cu in ultra large scale integrated devices (ULSI), the evolutions of the concentration profile of copper ions in SiO2 was simulated under bias temperature stress (BTS) test. Diffusion equation was solved numerically in two electric field modes. One is constant electric field mode where copper drift was simulated with the assumption that electric field is constant within SiO2 film. In variable electric field mode, simulation was carried out considering the variation of electric field in SiO2 due to copper ions. The diffusion of copper ions in variable electric field mode is slower than that in constant electric field mode, because copper ions in SiO2 reduce electric field near the interface between Cu and SiO2. Flatband voltage shift ) (ΔVFB increases parabolically as BTS time increases in constant electric field mode. However, it has linear relation with BTS time in variable electric field mode, which is typically observed in experiments.

Use of a capacitance voltage technique to study copper drift diffusion in (porous) inorganic low- k materials

Cu + drift diffusion in two inorganic low- k materials is evaluated. The diffusion is investigated by measuring shifts in the flatband voltage of capacitance/voltage measurements on Cu gate capacitors after bias temperature stressing. The Cu + drift rate in SiO x C y (2.7≤ k≤3.1) is considerably lower than in PECVD oxide. This electrical method is not suited for a porous silicon resin ( k≈2.0) because of the occurrence of instabilities in the metal/dielectric. A modification of the measurement technique is required. The modification consists of measuring the Cu + ions, which have been diffused through the porous dielectric in an underlying thermal oxide layer. The Cu + drift mobility in the porous dielectric as measured by this method appears to be lower than in thermal oxide.