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Ind. Eng. Chem. Res. 2005, 44, 1220-1225
Diffusion of Copper in Nanoporous Dielectric Films Oscar Rodriguez, Ravi Saxena, Woojin Cho, Joel L. Plawsky, and William N. Gill* Department of Chemical and Biological Engineering, Center for Integrated Electronics and Electronic Manufacturing, Rensselaer Polytechnic Institute, Troy, New York 12180
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) waterrelated 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. Introduction In an interconnect structure, the dielectric is in contact with other materials, such as a metal (copper), a diffusion barrier (for example, Ta or Ta nitride), or a hard mask (Si nitride). It is important to minimize the interactions between these different materials to ensure long-term device stability. However, some minimal interaction is necessary to provide bonding and good adhesion between layers. These interactions, such as interdiffusion or interfacial reaction, should be limited to a few nanometers beneath the interface in order to maintain the electrical integrity of devices. Porous materials have been proposed to replace SiO2 as the interlayer dielectric in future microelectronic devices. The dielectric constants of porous materials, such as silica xerogels, can be tailored by adjusting the porosity so that their use can be extended to more than one generation of devices. This is a distinct advantage for silica that in dense form, which together with aluminum, has been used for many years for interconnects. Copper, the present material of choice for IC metallization, is known to be a fast diffuser through both dense SiO2 and Si, but the extent of its drift through porous materials has not been determined and may indicate a need for an effective diffusion barrier if * Corresponding author. E-mail:
[email protected]. Phone: (518) 276-2880. Fax: (518) 276-4030.
silica-based low-k dielectrics are used. If a diffusion barrier is used, the concentration of copper ions in the dielectric will be smaller, and as the Poisson equation shows, to a first approximation the externally generated field will be approximately constant. Ion penetration from the metal under thermal or electrical stress can lead to premature failure of the dielectric. Although many papers have been published regarding the influence of Cu on the breakdown of the dielectric, the fundamental mechanism is still not clear and porous materials require more study. The issue of interdiffusion of metals and barriers with low-k dielectrics has received considerable attention with reports of diffusion of metals in silica-based materials occurring even at 100 °C.1 However, we have demonstrated that neither Cu nor Ta diffuse into a silica xerogel material via a thermally activated process in the absence of an electric field.2,3 The reports of pure thermal diffusion relied on Rutherford backscattering spectroscopy (RBS) detection, which has limited sensitivity for copper detection. We found that the agglomeration or dewetting of the metal or barrier material on the surface of the dielectric distorts the RBS spectrum, which may result in erroneous interpretation of the data as Cu+ diffusion.2 Microdefects, such as holes and hillocks, appear in the Cu film after a Cu/xerogel sample is annealed for 2 h at temperatures above 600 °C. These defects alone could be responsible for an erroneous appearance of pseudo-diffusion (low-energy Cu tail) in the RBS spectra
10.1021/ie049554r CCC: $30.25 © 2005 American Chemical Society Published on Web 10/08/2004
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of such a sample.3 However, no Cu oxidation or diffusion could be observed in the cases where the Cu/xerogel sample was coated with a 500 Å Si3N4 layer, which prevents oxidation, and then annealed (up to 646 °C). We can conclude that Cu does not diffuse into the xerogel at temperatures up to 646 °C and annealing times up to 2 h, based on the RBS analysis3 and in the absence of an electric field. Again, to obtain a more definitive result than is possible with RBS, one Cu/ xerogel and one Cu/SiO2 sample were annealed at 450 °C and examined with secondary ion mass spectroscopy (SIMS) before and after annealing. Typical detection limits of SIMS are in parts per billion, while those of RBS are in parts per million. It was found that, while significant amounts of Cu were present in SiO2 after annealing, practically no Cu could be detected in the xerogel.3 Leakage currents through xerogel and SiO2 capacitors were found to be on the order of 10-9-10-7 A/cm2, depending on the temperature. All of the I-t curves, for xerogel or SiO2 samples, show that an initial decrease in current with time occurs.1 This behavior is attributed to space charge accumulation that reduces the effective electric field across the dielectric and thus slows down the drift rate further.4 The current levels for xerogel capacitors are 1-2 orders of magnitude higher than those for PECVD SiO2 capacitor structures. This result suggests that the high internal surface area of the xerogel material promotes diffusion or at least charge migration along a surface path. Leakage currents from Cu/xerogel capacitors are unexpectedly about an order of magnitude lower than those from Al/xerogel or Au/xerogel capacitors.3 Mechanism of Cu+ Charge Injection and Diffusion In a metal-insulator-semiconductor (MIS) capacitor, the application of power sets up a cell that generates positive and negative charges from water species inherent in the silica-based dielectric. The OH- ions ionize copper to form a nonstoichiometric oxide, which acts as the source of Cu+ available for diffusion into the dielectric;5 they also contribute to the slowly moving anions in the dielectric. Metal must be ionized to respond to an electric field and singly ionized Cu ions are assumed to be injected since the product of diffusivity and solid solubility of Cu2+ in oxide is insignificant compared to that of Cu+ in oxide.4 Wieder and Czanderna6 showed that the formation of Cu oxides could be classified into five different temperature regimes. In the range of temperatures of 150-200 °C is a region of constant composition (CuO0.67).6 Metal or oxidizing species must diffuse through the metal oxide layer for the reaction to continue. Since Cu forms p-type oxides (e.g., Cu2O), the diffusion of metal ions through this metal-oxide layer toward dielectric occurs in preference to diffusion of larger OH- toward metal. The presence of negative and positive charges in the dielectric creates local space charged regions that distort the external electric field. As a consequence, the effective electric field near the metal/dielectric interface is lower than the external electric field which results in a decrease in the rate of injection of Cu ions. When the density of Cu ions in the dielectric reaches a critical value, a conduction path that links the cathode and anode may form, which triggers a breakdown. As opposed to Al, Cu
does not form apassivating oxide layer that could prevent further corrosion. Water in glasses can exist in a hydrogen-bonded form connected to a hydrophilic silanol, SiOH, group. Silanol groups tend to form on the surface of micro voids in the glass bulk.7-13 Significant loss of hydrogen-bonded species will occur only at high temperatures (>900 °C).14 Some species may react with the glass structure to form immobile hydroxyls12,14 as follows:
Si-O-Si + H2O ) 2SiOH In a water-related trap, an electron that is trapped initiates a chemical reaction. The SiOH centers capture an electron becoming negatively charged (SiO-) and release atomic hydrogen (H+). This chemical reaction consumes the water-related traps until no further trapping occurs.15 Hydrogen ions are known to possess substantially higher mobility than sodium, copper, or other metals16 and can move freely throughout the device, causing shifts in the characteristics of the MOS capacitor. The contamination of an oxide surface with organic compounds, like ethanol, induces an instability that consists primarily of the transport of protons (or a proton-associated species).15,17,18 The creation of positive oxide charge (holes or protons) by water molecules can also be related to19
H2O + Si+ f Si(OH)2 + H+ The Model The transport of copper ions occurs by both thermal diffusion and drift caused by the electric field. Thus, the copper flux, J, and the electric field, E, are given by
J ) µCuEC(t,x) - DCu
∂C(t,x) ∂V(t,x) ;E)∂x ∂x
(1)
where µCuE plays the same role as the convective velocity in flow systems. Then since the only source of Cu+ is the ionization of copper gate at the copperdielectric interface at x ) 0, and these ions are not recombined inside the boundaries of the dielectric, the diffusion equation is
∂2C(t,x) ∂C(t,x) ∂V(t,x) ∂ ) DCu + µCu C(t,x) 2 ∂t ∂x ∂x ∂x
(2)
and the Poisson equation is
∂2 V q ) (C(t,x) - n) ∂x2
(3)
where DCu and µCu are the diffusivity and mobility of copper, C(t,x), in the dielectric respectively; V, q, and are the voltage, elementary charge, and permittivity of the dielectric; n in the Poisson equation is a measure of the concentration of defects or negative charges in the dielectric that trigger and enhance the diffusion of copper ions. FEMLAB was used to solve eqs 2 and 3 by the finite element method. Equations 2 and 3 are nonlinear and must be solved simultaneously, and because the anions move slowly, as a first approximation, n is taken to be a constant.
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Now let us consider the current due to copper ion transport, which according to Greeuw and Hoenders20 is given by
I(t) )
q d L dt
∫0L xC(t,x) dx ) Lq ∫0L x
∂C(t,x) dx ∂t
(4)
Noting that eq 2 is
∂J(t,x) ∂C(t,x) )∂t ∂x
(2a)
And combining eqs 2 and 4, integrating by parts and assuming that the flux of copper ions at x ) L, the dielectric-semiconductor interface, is J(t,L) ) 0, gives the current as
I(t) )
∫0L J(t,x) dx ) qJm
q L
(5)
where Jm is the average flux of ions in the dielectric. Note that eq 5 includes both molecular diffusion and drift and applies exactly as long as J(t,L) ) 0, which is so for times less than L/µCuE, as is the case in Figure 5. Equation 5 indicates that I(t) is the average ionic flux of copper in the dielectric. Equation 5 was used to calculate the current in Figure 5 to compare with that measured experimentally. If we now insert eq 1 into eq 5 and neglect molecular diffusion, we obtain
I(t) )
∫0L µCuEC(t,x) dx
q L
(6)
Since µCuE has the dimensions of velocity, we see that, with DCu∂C(t,x)/∂x neglected, the current is the convection of the charge qCm where Cm is the average concentration of copper ions in the dielectric. At sufficiently small values of t, eq 6 may not hold as molecular diffusion may be very important. One can show easily that the molecular diffusion flux at x ) 0, xDCu/πtC(t,0), has a singularity as t f 0, causing it to be very large. This may contribute to the behavior of the current at small time and is not included in eq 6. Experimental System To study Cu drift through nanoporous silica, biastemperature stressing (BTS) measurements were done in metal-insulator-silicon (MIS) capacitors.4,16 Epitaxial p-type silicon wafers with an epi layer resistivity of 3-5 Ω-cm were used. The dielectric films used include nanoporous silica (xerogel), methyl-silsesquioxane (MSQ), and thermal SiO2. The MSQ films were included to determine if organic groups in the bulk of the film would inhibit copper charge injection. The nanoporous silica films were spin-coated on Si wafers using the two-solvent sol-gel method procedure described in our earlier publications; the surface modification step was done using 4 vol % solution of trimethyl chlorosilane (TMCS) in hexane.21,22,27 This modification adds organic groups to the surface and makes the films more hydrophobic. The thickness of the nanoporous silica films was ∼500 nm and the porosity ∼20%. Film thickness and porosity (calculated from the refractive index measurement) were determined by variable angle spectroscopic ellipsomety (VASE). The SiO2 films were grown at 900 °C using a wet-oxidation process. The thickness of SiO2 was ∼600 nm. Methyl-silsequioxane
(MSQ) is a spin-on polymer with stoichiometry (CH3SiO1.5)n and a ladder structure.23 Silsesquioxanes are obtained by hydrolytic condensation of trifunctional silanes, such as RSiC1,3, or RSi(OMe)3. Methyl-silsesquioxanes (MSQ) contain carbon and are prepared from methyltrialkoxysilanes. The structure after cure has a methyl group covalently bonded to each silicon atom. In dense form, these materials exhibit a dielectric constant of ∼2.7, good gap fill, and high thermal stability. The MSQ films used in this study were ∼40% porous and 500 nm thick. In all cases, copper dots, ∼500 nm thick, were deposited on top of the dielectric using e-beam deposition through a shadow mask to obtain circular dots of 0.5-1.0 mm in diameter. To complete the structure, the backside of the Si wafer was cleaned carefully with 10:1 buffered oxide etch to remove residual oxide in the surface; this cleaned surface was immediately metallized with ∼300 nm of Al. As a final step, the structure was annealed at 250 °C for 30 min in ∼3% H2 containing N2, to restore the quality of the SiO2/Si interface and to ensure an ohmic backside contact. High-frequency capacitance-voltage (CV) measurements were made on an HP4280A 1 MHz capacitance meter/CV plotter. A small ac signal of 10 mV rms was superposed on the applied dc bias. The maximum available voltage and temperature were 100 V and 300 °C, respectively. Usually, the BTS experiments on SiO2 and xerogels were performed at 200 °C and 1 and 1.5 MV/cm on MSQ. An MSI electronics light shield/hot chuck was used to hold the sample, which was vacuum held on the hot chuck and was in a N2 ambient throughout the stressing. A continuous shift in the CV profile in the negative direction after every BTS step is observed. This continuous shift is an indication of the presence of positive charges in the insulator injected from the metal (Cu) gate. As the BTS progresses, the amount of these mobile charges also increases. The Cu+ ion concentration can be obtained from the CV traces and used in eq 7, in which [Cu+] is the concentration of Cu ions in the dielectric per unit area (of the metal gate in the capacitor); Cdielectric is the capacitance of dielectric per unit area; q is the elementary charge; ∆Vfb is the voltage shift between the “before BTS” and the “after BTS” curves.
[Cu+] ) -
∆Vfb‚Cdielectric q
(7)
Results and Discussion Figure 1 shows the variation of [Cu+] concentration per unit area as a function of BTS time in SiO2, MSQ, and nanoporous silica. The surface modification step of the nanoporous silica films was done using 4 vol % solution of trimethylchlorosilane (TMCS) in hexane. Rogojevic et al.24 showed that TMCS markedly inhibits moisture adsorption up to xerogel porosities of 60%. The injection of copper in MSQ (tested at 1.5 MV/cm and 200 °C) and SiO2 (tested at 1 MV/cm, 200 °C) seems to follow a “saturation” scheme, where the initial injection and Cu+ drift rates into the dielectric are very high, but decay rapidly. The amount of copper ions accepted by MSQ is significantly lower than that by xerogel or SiO2, probably due to the organic content of the bulk of the
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Figure 1. Cu+ charges detected as a function of time. Cu/MSQ system tested at 1.5 MV/cm, 200 °C; MSQ was 40% porous and 400 nm thick. Cu/PECVD-SiO2 system tested at 1 MV/cm and 200 °C; PECVD-SiO2 was 500 nm thick. Cu/xerogel system tested at 1 MV/cm, 150 °C; xerogel was 18% porous and 585 nm thick and treated with TMCS.
Figure 2. Cu+ charges detected as a function of time. Cu/sintered xerogel tested at 0.25 MV/cm and 150 °C. Cu/PECVD-SiO2 system tested at 1 MV/cm and 200 °C; PECVD-SiO2 was 500 nm thick. Cu/xerogel and Cu/vapor-treated xerogel systems tested at 1 MV/ cm and 150 °C; xerogel was 18% porous and 585 nm thick and treated with TMCS.
MSQ. We speculate that at least 80% of the total amount of Cu+ ions injected in the dielectric occurs in the first 15 min. This rapid increase and saturation is consistent with the accumulation of Cu+ ions in the dielectric that changes the electric field and opposes further mobile ion injection. An increase in the inversion capacitance of the MIS capacitor after BTS implies that some Cu+ ions might have drifted through the dielectric and reached the dielectric-Si interface. In the conditions tested, the time in which the Cu+ ions reach the MSQ/Si interface is less than 15 min. Some of these Cu+ ions might become neutralized and generate electrically effective trap centers near the Si interface and in bulk Si substrate. These deep-level states generated by Cu near the dielectric-Si interface and in the Si substrate act as generation-recombination centers and tend to increase the generation-recombination of minority carriers and reduce their lifetime. At higher temperatures, the gradual increase in inversion capacitance becomes faster due to higher Cu ion drift rates and indicates that more Cu ions may have drifted through the dielectric and reached the dielectric-Si interface. The injection of Cu+ in surface-modified nanoporous silica (tested at 200 °C, 1 MV/cm) shows similar behavior to MSQ but accepts substantially more copper. In the conditions of the experiment, the concentration of Cu+ obtained from the CV curves using eq 7 is higher in nanoporous silica than in SiO2. The nanoporous xerogel can be viewed as an amorphous SiO2 (with some C and H), but with a higher defect density than dense SiO2 due to its highly branched microstructure. These defects (broken or dangling bonds, -OH groups) can cause more Cu+ penetration and higher leakage currents. Moisture absorption due perhaps to the difficulty in obtaining complete coverage to modify completely the high surface area also leads to higher currents. To separate the effect of TMCS on the surface chemistry from the effect of porosity on the leakage current and charge accumulation, we prepared nanoporous silica capacitors in which the porous film was annealed at 900 °C (sintered) for 2 h in air before metal deposition, and this removes the organic groups on the surface. By removing organic groups from the surface of the nanoporous silica, one reduces its hydrophobicity, and our experiments indicate that the porosity is practically unchanged. Hysteresis is observed in the CV curves obtained of some of these capacitors, even though the testing was done in a N2 purged ambient. This hysteresis effect is due to losely adsorbed moisture in
the surface of the dielectric and it is removed after an in situ annealing at 150 °C (no bias applied) of ∼5-10 min. The [Cu+] vs time profiles, shown in Figure 2, demonstrate the effect of sintering and confirm that the importance of surface chemistry is to reduce the initial drift rate of Cu+ and evidences saturation after less time. Cu diffuses readily into sintered nanoporous silica where the surface is hydrophilic because the groups introduced by surface treatment have been burned off. In general, the value of the dielectric constant depends on the microstructure of the nanoporous films; at low porosities (
