Copper diffusion layer

Effects of current density(I) on the recovery of copper in the reactor can be seen in Fig. 5. As can be seen, the value of R increased gradually with increasing ciurent density, since the mass transfer rate of copper ion is proportional to the current density. Effects of amount of fluidized particles on the recovery of copper can be seen in Fig. 6. Note that the addition of a small amount of fluidized particles (W=1.0wt.%) to the reactor could increase the copper recovery up to 10 25%. It has been mderstood that the contacting of fluidized solid particles with the cathode plate could clean the siuface as well as decrease the diffusion layer of copper ion, which results in the increases of reaction rate and current efficiency, thus, the recovery of copper could be increased. 

O Sullivan describes the fundamental theory, mechanistic aspects and practical issues associated with autocatalytic electroless metal deposition processes. Current approaches for gaining fundamental understanding of this complex process are described, along with results for copper, nickel and various alloys. Emphasis is placed on microelectronic applications that include formation of structures that are smaller than the diffusion layer thickness which influences structure formation. 

The dynamics of upd reactions have also been examined by STM. The formation of the ordered copper/sulfate layer [354] and copper chloride layer [355] on Au(lll) was examined in a dilute solution of Cu where the reaction was under diffusion control so that growth proceeded on a time scale compatible with STM measurements [354]. In another study, the importance of step density on nucleation was examined and the voltammetric and chronoamperometric response for Cu upd on vicinal Au(lll) was shown to be a sensitive function of the crystal miscut, as... 

Drake [35] has measured the thickness of the diffusion layer during the electrodeposition of copper from an acidic-sulphate system. He obtained a value of approximately 200 pm for the silent system and values of 34 pm and 3.4 pm for ultrasonic frequencies of 1.2 MHz and 20 kHz respectively. The corresponding values of the limiting-current density were from 8 Am (silent) to 50 A m (1.2 MHz) to 500 A m (20 kHz), indicating a significant increase in the rate of deposition. 

In electronic applications, where it is common to deposit copper and/or copper alloy and tin in sequence, with a nickel diffusion barrier layer, 0.5 fim thick, between the layers present, no failure occurs. Without the nickel layers between bronze/-copper/tin layers themselves, for instance, intermetaUic brittle layer(s) and Kirkendall voids are formed, leading eventually to separation of the coated system and substrate. 

As a result of that reductive process, a deposit of copper metal (denoted in Eq. 2.2 by s for solid ) is formed on the carbon electrode surface. The prominent anodic peak recorded in the reverse scan corresponds to the oxidative dissolution of the deposit of copper metal previously formed. The reason for the very intense anodic peak current is that the copper deposit is dissolved in a very small time range (i.e., potential range) because, in the dissolution of the thin copper layer, practically no diffusion limitations are involved, whereas in the deposition process (i.e., the cathodic peak), the copper ions have to diffuse through the expanding diffusion layer from the solution to the electrode surface. These processes, labeled as stripping processes, are typical of electrochemically deposited metals such as cadmium, copper, lead, mercury, zinc, etc., and are used for trace analysis in solution [84]. Remarkably, the peak profile is rather symmetrical because no solution-like diffusive behavior is observed. 

The cyclic voltammetry behavior of the Cu(II) rotaxane, 4(5)2+ (Fig. 14.8b), is very different from that of 4, t l +. The potential sweep for the measurement was started at - 0.9 V, a potential at which no electron transfer should occur, regardless of the nature of the surrounding of the central Cu(II) center (penta- or tetracoordinate). Curve i shows two cathodic peaks a very small one, located at + 0.53 V, followed by an intense one at —0.13V. Only one anodic peak at 0.59 V appears during the reverse sweep. If a second scan ii follows immediately the first one i, the intensity of the cathodic peak at 0.53 V increases noticeably. The main cathodic peak at —0.15 V is characteristic of pentacoordinate Cu(II). Thus, in 4(5)2+ prepared from the free rotaxane by metalation with Cu(II) ions, the central metal is coordinated to the terdentate terpyridine of the wheel and to the bidentate dpp of the axle. On the other hand, the irreversibility of this peak means that the pentacoordinate Cu(I) species formed in the diffusion layer when sweeping cathodically is transformed very rapidly and in any case before the electrode potential becomes again more anodic than the potential of the pentacoordinate Cu2 + /Cu+ redox system. The irreversible character of the wave at —0.15 V and the appearance of an anodic peak at the value of + 0.53 V indicate that the transient species, formed by reduction of 4(5)2 +, has undergone a complete reorganization, which leads to a tetracoordinate copper rotaxane. The second scan ii, which follows immediately the first one i, confirms this assertion. 

The reforming reactor was built of copper powder, which could be sintered at temperatures between 500 and 700 °C, being low enough to avoid damage to the catalyst. In the same fabrication stage, the Cu/ZnO catalyst with a particle size between 300 and 500 pm was incorporated into the device. Copper and aluminum powder were used as inert materials for parts such as channels and diffusion layers. 

Consider the process of plating copper on a plane electrode. Near the electrode, copper ions are being discharged on the surface and their concentration decreases near the surface. At some point away from the electrode, the copper ion concentration reaches its bulk level, and we obtain a picture of the copper ion concentration distribution, shown in Fig. 6. The actual concentration profile resembles the curved line, but to simplify computations, we assume that the concentration profile is linear, as indicated by the dashed line. The distance from the electrode where the extrapolated initial slope meets the bulk concentration line is called the Nernst diffusion-layer thickness S. For order of magnitude estimates, S is approximately 0.05 cm in unstirred aqueous solution and 0.01 cm in lightly stirred solution. 

Simulation of the ESEM pattern for Cu2+-doped Mg-montmorillonite leads to a coordination number of six and a Cu2+-D distance of 0.29 nm [41]. X-ray diffraction shows that the smectite layers are about 1.04 nm apart when the ESR lineshape becomes isotropic with a single peak. This large basal-plane spacing and the ESEM data suggest a diffuse-layer Cu(H20)g+ species that tumbles sluggishly. Copper-doped Mg-hectorite whose layers are about 0.54 nm apart yields an ESR spectrum like those for beidellite and montmorillonite at low relative humidity, whereas with the layers 1.04 nm apart, the spectrum is again isotropic [39]. Figure 8 illustrates the three Cu2+ surface complexes that appear successively as a smectite... 

Experimental study indicated that the mass transport limiting species in copper-phosphoric acid solution is the so-called acceptor (water molecules), which diffuses into the diffusion layer and facilitates Cu removal [14]. In some cases, a salt film of metal ion complexes can form as an anodic layer to control the mass transport processes [15-17]. Mass transport limiting species can also be metal ions, which diffuse and migrate through anodic layers (ionconcentrated diffusion layer and/or salt film) into the bulk solution [17]. 

However, electropolishing is pattern sensitive and this may limit its applications. The passivation film or diffusion layer thickness is minimal above narrow features as the diffusion layer profile is unaffected by the copper surface profile. Therefore, the diffusion flux that corresponds to the removal rate is large for these features and planarization of these features is possible. For wide features, the diffusion layer profile follows the copper surface profile, the removal rate is low, and this leads to a conformal copper removal and inadequate planarization. 

EXAMPLE 14-1 Suppose that silver and copper are to be separated when both are present in 0.1 M solution. We take the formal potentials to be approximately equal to the standard potentials of 0.80 and 0.34 V. Assume the mass-transfer constant m to be 10 cm/s. For diffusion control, this corresponds to an effective diffusion-layer thickness 8 = Djm of the order of 10 cm, since D is usually about 10 dm s. For imstirred solutions, 8 is of the order of 0.04 cm it decreases rapidly with increasing stirring rate until convection control sets in, when it becomes approximately 10 cm. The constant A = AmjV = 20 x 10 /100 = 2 x 10 /s for a cell... 

Anyway, there are two effects of hydrogen evolution on copper electrodeposition leading to the formation of the honeycomb-like structures. The first effect is a stirring of the solution in the nearelectrode layer caused by a vigorous hydrogen evolution leading to the decrease of the diffusion layer thickness and the increase of the limiting diffusion current density.10 The second effect concerns... 

Figure 6.6. Fit of the diffuse layer model to copper adsorption by hydrous ferric oxide. The solid line represents the optimal ht for these data. The dashed line represents the fit corresponding to the best overall estimate of the Cu surface complexation constant obtained from 10 Cu adsorption edges. (From Dzombak and Morel. 1990.)...

This set of equations can be approximated with hand calculations or solved using a computer program such as the one described by Dzombak and Morel (1990). An example fit of the diffuse layer model is indicated in Figure 6.6 for copper adsorption to hydrous ferric oxide. 

The structure of the double layer and the specific surface adsorption can affect the reaction kinetics. In the absence of specific adsorption, copper ions position of the closest approach to the electrode surface is the Outer Helmholtz Plane (OHP). The potential at the OHP, potential drop through the diffuse layer and possibly because some ions are specifically adsorbed. These potential differences in the double layer, as known, can affect the electrode reaction kinetics [5]. 

Copper is going to replace aluminum as the material of choice for semiconductor interconnects due to its low electrical resistance and high electromigration resistance (1-4). An inlaid interconnect is used for copper metallization in which the insulating dielectric material is deposited first, trenches and vias are formed by patterning and selective dielectric etching, and then diffusion barrier and copper seed layer are deposited into the trenches and vias (5). 

All experiments were performed on 200mm wafers using Semitool s plating tool. Trenches with various geometries and aspect-ratios were patterned in silicon oxide coated wafers. Titanium Nitride (TiN) or Tantalum (Ta) diffusion barriers with nominal thickness of 300 A were deposited on the trenches by vacuum techniques such as PVD or CVD. Unless specified differently, a PVD copper adhesion layer with a nominal thickness of 200A was deposited on top of the barrier by PVD techniques. This thin PVD copper adhesion layer was electrochemically enhanced in Semitool s proprietary ECD seed plating solution prior to the full deposition from an acid copper sulfate bath. 

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