Patent Application
20060063392
The present invention relates generally to semiconductor processing, and more particularly to the application of nanoparticle technology in the formation of dielectric layers during semiconductor processing.
In the past, aluminum and its alloys have been the materials of choice for dual damascene interconnect metallurgies and other Back-End-Of-Line (BEOL) metallization structures utilized in semiconductor devices. These interconnects have typically been embedded in dielectric materials (e.g., SiO2) known as interlayer dielectrics (ILD). The delay in the BEOL structure, and hence its performance characteristics, are dictated by the product of the resistance of the interconnect and the capacitance provided by the ILD materials.
The physical limitations of aluminum interconnects, and in particular their relatively high resistivities, have prevented them from accommodating the need for increased circuit densities and speeds in semiconductor devices. Consequently, copper-based metallurgies have evolved as replacements for aluminum metallurgies in dual damascene interconnect metallurgies and other BEOL metallization structures. Copper-based interconnect metallurgies offer lower resistivities and comparatively lower susceptibility to electromigration failure compared to their aluminum counterparts. The use of copper-based metallurgies also allows capacitance to be exploited in the optimization of interconnect performance. To this end, ultra-low dielectric constant materials have begun supplanting SiO2 as the dielectric materials of choice in which metal lines are embedded.
The material properties of the ILD in dual damascene applications are essential for integration of low RC (Resistance/Capacitance) backend solutions. In particular, this dielectric material must be mechanically strong and possess a very low dielectric constant or k-value. In practice, this dielectric constant must be much lower than that of SiO2, which has a dielectric constant of about 4.1.
One of the challenges facing the commercial implementation of dual damascene processes is the lack of an optimal dielectric material for use in the ILD. In particular, most existing dielectric materials, such as silicon dioxide, porous hydrogen and carbon containing glasses (e.g., SiCOH), or dielectrics based on aromatic hydrocarbon thermosetting polymers, are either unsuitable for use in the ILD layer, are fraught with mechanical stability issues or exhibit k-value integrity issues during processing, or simply have too high of a k-value. Air gap ILDs possess potentially the best k-value, but come with significant patterning, processing and mechanical integrity challenges.
Other known low-k materials are unsuitable for use in dual damascene processes because they require that a protective layer be formed on the etched dielectric layer in order to protect the dielectric material during processing. For example, porous low-k materials require pore sealing for all of the processes used to etch them (i.e., for via formation and cleaning), as well as for the metallization processes used to form the metal interconnects. Potentially, each processing step (e.g., etching, post-etch treatment to remove etch residues, the preparation of exposed metal surfaces, and metal deposition) can degrade the BEOL dielectric constant. Silicon and oxygen-based low-k films whose low k-value is enabled by the presence of carbon are particularly prone to k-value degradation in this regard, since the only known processes that can adequately etch them for high performance and high density computing applications will themselves interact with the carbon and hydrogen in the films.
The use of boron-nitride (BN), and in particular, boron nitride nanotubes (BNNTs), as ILD materials has also been investigated. These materials have attractive hardness and can theoretically provide dielectric layers with very low K-values. However, BNNT compositions have a number of fundamental flaws that have prevented their successful use in BEOL applications. In particular, the k- values achieved with dielectric layers formed from these materials are found to be significantly higher than the values that are theoretically possible, and the mechanical properties of the resulting layers have been found to be inadequate. Moreover, BNNT layers are typically grown from catalyst layers. The extra steps involved in patterning (masking and etching) the catalyst at all the levels of metal and dielectric make this process prohibitively expensive.
There is thus a need in the art for a low-k dielectric material that is suitable for use in dual damascene processes and that exhibits good mechanical properties. There is also a need in the art for a dielectric material whose use in a damascene process does not require additional processing steps, such as those required to manage the impact of film porosity or to pattern catalyst layers. There is further a need in the art for dielectric materials whose k-values are resistant to change from processes that are commonly used in BEOL processes. These and other needs are met by the processes and materials disclosed herein and hereinafter described.
As used herein, the term “nanoparticle” is meant to encompass nanotubes, nanoballs, nanohorns, and other shapes assumed by materials on the nanoscale.
It has now been found that suspensions of nanoparticles, such as, for example, suspensions of boron-nitride nanotubes (BNNTs) in a liquid medium, can be used to make dielectric layers having low k-values, high structural integrity and excellent mechanical properties. These materials are also conducive to BEOL integration (that is, they are not adversely affected by BEOL etch, metallization and multilayer deposition processes). The use of purified suspensions of BNNTs to make dielectric layers through spin coating or through other evaporative processes is especially preferred, due to the particularly favorable combination of excellent hardness, exceptionally high Young's moduli, and reproducible dielectric constants of 2 or less that these materials and the associated methodology afford.
The nanoparticle ILD 12 will typically have a conductor 15 embedded in it. In advanced interconnect structures, the conductor 15 preferably comprises copper or its alloys, but in other applications the conductor may also comprise aluminum or other conductive materials. A diffusion barrier liner 14 may be disposed between nanoparticle ILD 12 and conductor 15. The diffusion barrier liner 14 typically comprises one or more layers of tantalum, titanium, tungsten, or nitrides of these metals. The diffusion barrier liner could also be a polymer, a so-called zero-thickness liner, a nanoparticle material or a bulk material deposited by chemical vapor deposition, such as BCN.
The top surface of conductor 15 is made coplanar with the top surface of the silicon nitride layer 13, usually through chemical-mechanical polishing (CMP). A cap layer 17, which may comprise a material such as SiCN, CoWP, or CoWB, is disposed on the conductor 15 and on the silicon nitride layer 13. The cap layer 17 acts as a diffusion barrier to prevent the diffusion of copper from the conductor 15 into the surrounding dielectric material. The cap layer may also act as an etch stop layer.
A first interconnect level is defined by the adhesion promoter layer 11, nanoparticle ILD 12, silicon nitride layer 13, diffusion barrier liner 14, conductor 15, and cap layer 17 in the interconnect structure shown in
The methodologies described herein for forming and defining the nanoparticle ILDs 12 and 19 in the structure depicted in
As depicted in
As depicted in
Next, as shown in
As shown in
Without wishing to be bound by theory, it is believed that methods for making nanoparticle interlayer dielectrics of the type illustrated in
It has now been found that, by creating purified suspensions or solutions of nanoparticles and then using these purified suspensions or solutions to construct the dielectric layer, as through evaporative processes such as spin coating or through other suitable processes, a dielectric layer which is free of catalyst contamination can be produced. The resulting layers have physical characteristics that are often markedly better than similar layers formed by a catalyst deposition method. In the case of BNNT, for example, the resulting layer is observed to have reproducible dielectric constants that are significantly lower than the k-values observed in dielectric layers formed by catalyst growth methodologies such as that depicted in
Another benefit of the approach described herein is that the resulting dielectric layer can then be etched to form an appropriately patterned dielectric layer. By contrast, catalyst-based methods require that the catalyst layer itself be patterned before the dielectric layer is grown upon it, an approach that results in extra processing steps and also increases the amount of catalyst contamination in the resulting system. This process is complicated by the fact that catalysts are typically difficult to etch. Moreover, many commonly used metal catalysts, such as Pt, form low volatility fluorides, chlorides and carbonyls. There is thus the added risk of particle contamination and cross-contamination with other processes. This is especially true with regard to the processes related to lithography since, in many fabrication lines, all of the wafers will have to pass through a single lithographic exposure/patterning tool.
One non-limiting embodiment of a methodology suitable for the formation of nanoparticle dielectric layers in accordance with the teachings herein is illustrated in
The generated particles are then purified and suspended 303 in a liquid medium. The purification process may be a single step process (for example, the step of forming the suspension may itself effectively purify the particles, especially if the liquid medium does not solvate or suspend the impurities), or it may be a multiple step process. The exact choice of purification technique may depend on such factors as the specific composition or chemistry of the nanoparticle material, the manner in which it was formed, and the types of impurities to be removed.
As a specific example of a purification technique that may be utilized prior to forming the suspension, the nanoparticles may be washed with a solvent that selectively dissolves or removes any metal catalyst impurities that may be present. The nanoparticles may also be purified by a suitable evaporative technique such as sublimation, or through recrystallization.
One particular technique that is useful in purifying some nanoparticle materials involves placing the material in a suitable liquid medium, such as methanol or another organic solvent, and keeping the material in suspension within the liquid medium for a prolonged period of time using sonication techniques. This technique may be supplemented, if necessary, by passing the suspension through a micro porous membrane.
Another technique that can be used to purify some nanoparticle materials involves refluxing the particles in a suitable solvent, such as H2O2, with subsequent rinsing in additional liquids or solvents (such as, for example, CS2 or methanol), followed by filtration. This later approach has been described in Tang et al., Science, Vol. 288, 492 (2000).
A number of materials may be used as the liquid medium in the methodologies described herein, with the exact choice of material depending in part on such factors as the chemical composition of the nanoparticles and their average particle size. Preferably, the liquid medium is an aqueous medium, which may be, for example, pure distilled water or an aqueous mixture of one or more other materials. A variety of other media may also be used, including, but not limited to, alcohols (including, for example, methanol, ethanol, and isopropyl alcohol), peroxides (including both hydrogen peroxide and organic peroxides), polyols, polyvinylpyrrolidone, polar or nonpolar organic solvents (such as, for example, methylene chloride and CS2), and the like. Various mixtures of the foregoing may also be used as the liquid medium. Formation of the suspension may be facilitated with sonication, heating, reflux, or other techniques as are known to the art.
After the purified suspension is formed, it may be used to generate a dielectric layer 305 on a substrate through a number of suitable techniques, including both evaporative and non-evaporative processes. Preferably, the dielectric layer is formed through a spin coating process. As indicated in
After the dielectric layer is initially formed, it may be patterned as necessary to form the final device. One specific, non-limiting embodiment of this process is illustrated in
With reference to
With reference to
As shown in
Referring to
Referring now to
As shown in
As shown in
The trench and vias are then backfilled with copper 535 or another suitable conductor, as shown in
The physical properties of particular types of nanoparticles are an important consideration in their selection for use in the processes described herein, including the ILD formation processes described above. In particular, the polarizability of these materials is an important consideration, since it can have a significant impact on the ability of the nanoparticles to form a stable suspension. Polarizability of BN nanotubes has been reported in the literature. See, for example, J. Kongsted, A. Osted, L. Jensen, P. Astrand, K. Mikkelsen, “Frequency Dependent Polarizability of BN Nanotubes: A Theoretical Study”, J. Phys. Chem., B, 105 10243 (2001), which is incorporated here by reference in its entirety. As explained therein, the polarizability of BN nanotubes, an example of which is depicted herein in
For a canonical nanotube with 100 atoms and a 9 angstrom by 9 angstrom unit cell (that is, a 9 angstrom length and diameter), the increment in number of atoms per unit length is
Nincrease
where l is in angstroms, and the increment in number of atoms per unit of radial distance is
Nincreas
where d is the tube diameter in angstroms. For reference, the conversion between angstroms and a.u. is
1 a.u.˜0.5 angstroms (EQUATION 3)
Hence, the canonical tube has a polarizability of approximately 1000 (a.u.3). The polarizability (units of volume) as a function of length and width is therefore
α=25d+22.5l (EQUATION 4)
where the units are in a.u.
The unit cell of a nonotube depends on the nanotube type, which defines the nanotube volume. Comparing the unit cell volume with the polarization “volume” allows one to calculate the dielectric constant using the Clausius Mosotti Equation:
(the high frequency terms have been dropped, since they are typically negligible). The term α/V can be calculated for BN nanotubes using the data from the Kongsted reference cited above. Upon doing so, it is found that
where l is the tube length (not the unit length).
It is to be noted that this calculation assumes the worst case scenario, that is, it assumes that the unit cell length is approximately 9 angstroms, which is the case for zig-zag and armchair nanotubes. However, the unit cell length is much longer for chiral nanotubes.
Consider a high k-value (20) tube and a low k-value (2) tube. EQUATION 5 then dictates that 60/V˜0.2 for a dielectric constant of 20, and that α/V˜0.08 for a dielectric constant of 2. The tube “design” curve for dielectric constant 2 tubes is then:
The design curve for dielectric constant 20 tubes is then
The relationship between l and d for k=2 tubes is depicted in
A number of techniques may be employed to adjust the dimensions of nanotubes and other nanoparticles after their formation. These techniques can be used, for example, to achieve nanoparticles having a desired distribution of lengths and diameters as described above. Thus, for example, in some instances, prolonged sonication of nanoparticles in a suitable medium has been found to reduce the dimensions of the particles. As a specific example of this approach, sonication of carbon nanotubes in a mixture of concentrated sulfuric and nitric acids has been observed to systematically shorten the length distribution of the carbon nanotubes as a function of exposure time to the acid. This methodology is described more fully in Jie Liu et al., “Fullerene Pipes”, Science, Vol. 280, pp. 1253-1256 (May 22, 1998).
Another technique that may be useful in some applications for reducing the dimensions of nanoparticles utilizes diamond particles as an abrasive material for cutting the nanoparticles. This technique is described more fully in L. Stepanek et al., “Nano-Mechanical Cutting and Opening of Single Wall Carbon Nanotubes”, Chem. Phys. Lett., 2000, 331, 125-131. In this technique, diamond particles are used to effectively grind the nanoparticles in order to cut them into shortened particles.
In a further technique, a soft cutting agent, such as a cyclodextrin, may be used to reduce the dimensions of the nanoparticles. This approach is particularly suitable where the smaller particles have greater solubility in the cutting agent than the larger particles, because the reduction in dimensions of the nanoparticles is often observed to slow or stop as solvation occurs. In some applications, this technique may be used in combination with a physical grinding technique, such as the technique of Stepanek et al.
In some embodiments of the methodologies and devices disclosed herein, the dielectric layer may consist of multiple layers of nanoparticles. For nanoparticles that adhere to one another well, these multilayer structures will typically have the dielectric constant of bulk BN over the adhering volume, but may have mechanical properties that differ from a monolithic layer of the material having the same dimensions.
Various means may be used to produce nanoparticles for use in the methodologies and devices described herein. Such means include, but are not limited to, laser ablation, DC discharge, plasma enhanced chemical vapor deposition (PECVD), CND, atom beam deposition, and milling and post annealing growth methods.
In the case of BN nanotubes, these materials may be deposited from milled BN grown after annealing (annealing is typically conducted at about 1200° C.). BNNT can be grown in this manner free of contaminants or metal catalyst particles. BNNTs may be generated by ablation of bulk BN, and may then be collected and purified. BNNTs may also be readily suspended in water and subsequently deposited and grown into BNNT membranes through evaporation. This approach has been used to form well defined membranes from BNNTs. Thus, for example, 0.2 mg/ml suspensions have been used to generate a membrane consisting of 0.5 micron length tubes.
In the process steps described herein which require the etching of BNNTs, various methodologies and materials may be used for this purpose. Preferably, such etching is achieved through the use of chlorine plasma etching, though particular applications may dictate different etching chemistries. For example, the use of sufficiently polymerizing fluorocarbon plasmas, driven with appropriate ion energy to preserve anisotropy, may be beneficial in some applications because such plasmas are self-sealing. By contrast, chlorine plasmas tend to be characterized by etching mediated by a passivated surface with a relatively thin “chlorinated layer” and, as such, may not self seal. However, this typically does not lead to radial etching, as neutral driven etching (side-walls) is slow. Even though, in the case of Cl2 plasmas, the surface may not seal as when a polymer layer is deposited from a CxFy based plasma, ion bombardment may collapse any open structures, thereby resulting in sealing.
Material penetration post-etch processes can be dealt with, in some instances, by coating the nanoparticles with silica, since silica has been demonstrated to adhere well to certain types of nanoparticles. The existence of poor anisotropy in the etching process can be dealt with through a Bosch-like etch process in which isotropic processes can be used to yield highly anisotropic etch features. Alternatively, if trench and via walls are sufficiently sloped, an argon plasma may be used to morphologically damage the surface and collapse the open tubes.
Applying a Bosch etch process to the process integration around a BNNT interconnect could be used to address issues beyond profile control, such as pore sealing. For example, halogen etch steps could be alternated with argon etch steps designed to close pores as the halogen etch progresses. Alternatively, it may be desirable to round the corners of a trench opening to enhance subsequent barrier coverage. It may be undesirable to do this after etching is completed and the trench and vias in an interconnect structure have opened to another metal layer through the etch stop layer. The process of etching the corners or vias to form facets may sputter the bottom of the BNNT via or trench structure depositing sealing BN material on the sidewalls, further enhancing sidewall sealing. In a more traditional Bosch etch process, a halogen based etch process may be alternated with a plasma process that is substantially a deposition process such as a process that would deposit BCN, paralyne or polymer generated by a c-C4F8, C4F6 or c-C5F8 plasma. The action of etching during the non-depositing etch steps would remove deposit from the bottom of the BNNT interconnect structure. Varying degrees of sidewall deposition during the deposition step could enable novel interconnect structures such as a wine-glass profile or other profiles desired for interconnect reliability imperatives.
The dielectric layers made in accordance with the teachings herein exhibit a number of very desirable properties. For example, dielectric films based on BNNTs are insulating with a band gap that is ˜5 eV. Moreover, and in contrast to carbon nanotubes (CNTs), the electronic properties of BNNTs are typically independent of nanotube size (radius and length), and are also typically independent of whether the tubes are multi-walled or not, although they can be significantly affected by radial defects akin to “pimples”.
The nanoparticles employed in the methodologies and devices described herein can be made to occur in many configurations. Thus, for example, BNNTs may be produced in the form of long ropes or filaments of up to 40 microns in length (2-10 nm in diameter), to lengths in the sub-micron region. Zig-zag nanotubes and open and closed-end nanotubes may also be produced. Other nanoparticles can be made to form balls, horns and other such shapes.
Dielectric layers based on BNNTs can be produced in accordance with the teachings herein which have very desirable mechanical properties. Thus, for example, dielectric layers can be produced in accordance with the teachings herein that have Young's moduli within the range of about 1.1 to about 1.3 TPa. BNNTs are also more found to be more resistant to strain induced defects than their CNT counterparts, and exhibit greater mechanical strength than BN films that are not based on nanoparticles. Without wishing to be bound by theory, it is believed that the stiffness of BNNTs may make up for the buckling tendency of the pure BN film, while offering mechanical advantages from the perspective of materials processing (i.e., planarization with CMP).
Various dielectric layers may be made in accordance with the teachings herein. These layers may be formed in a single deposition process, or may be built up through multiple depositions, using the same or different suspensions, to achieve multilayer structures. For example, deposition of BNNT may be alternated with deposition of diverse materials, such as silicon nitride. Also, layers of BNNT having differing thicknesses may be formed by the stepwise deposition of BNNT using suspensions of differing concentrations, or by using different amounts of suspensions having the same concentration. Multilayered structures may also be formed in accordance with the teachings herein wherein different layers comprise different nanoparticles. Thus, for example, multilayered structures may be formed which consist of alternating BNNT and CNT layers.
While the nanoparticle layers described herein, especially those based on BNNT, are particularly useful in a damascene process, it is to be understood that the methodology and dielectric layers described herein are more broadly useful, and hence have use in many applications requiring high strength and/or low-k layers.
Thus, for example, the methodologies disclosed herein may be suitably adapted to produce layers and structures in other applications and devices. One such application is in the fabrication of microelectromechanical structures (MEMS), where nanoparticle-based materials may be used, for example, to produce walls in trench perimeter schemes, and/or may be used to produce anchor portions of MEMS structures. As a specific example, BNNTs may be used to construct a strong, low-k dielectric anchor in a MEMS device.
As shown in
As shown in
After deposition of the nanoparticle layer and patterning of roof 567, an outline mask (not shown) is used in conjunction with a suitable etching technique such as deep reaction ion etching to create a second trench 571 outside of the perimeter of the first trench, as shown in
As shown in
It will also be appreciated that, if desired, the anchor portion of the device can be made entirely out of the nanoparticle material. In this case, rather than creating a perimeter trench 559 as shown in
The methodologies disclosed herein have been described primarily as they relate to the formation of dielectric layers and structures. It will be appreciated, however, that the purification techniques disclosed herein are generally applicable to the formation of high-purity layers and structures. The actual dielectric properties will depend, among other things, on the choice of nanoparticles employed in the material. Thus, for example, the principles described herein may also be used to generate high-purity layers or structures that are conducting or semi-conducting. This may be accomplished, for example, through the use of carbon nanotubes.
The compositions and methodologies described herein may also be applied to the formation of tiling or other structures for the control of CMP.
This problem may be overcome by placing polishing dummy features, comprising BNNTs or other nanoparticle compositions, in open locations having widths, for example, in a range of 5-10 microns. One such location lies between memory array 642 and 644. However, in accordance with one particular, non-limiting embodiment of the tiling methodology disclosed herein, polishing dummy features 682 are inserted into the mask, preferably at the same feature level as the conductors 672. The resulting pattern is seen in a semiconductor device in
Typically, the polishing dummy features 682 are at least approximately 0.3 mm away from the nearest conductor 672 (active feature) within the area having the highest active feature density, and often will be at least 1 mm away. In this specific embodiment, the polishing dummy features 682 closest to the active features within the area having the highest active feature density lie in a range of approximately 3-5 mm away from each other (polishing dummy feature-to-active feature spacing). The minimum gaps and spacing for polishing dummy features may be determined at least in part from the polishing characterization.
The polishing dummies may be formed using the various deposition and etching techniques described herein for nanoparticle materials. After the conductors 672 and polishing dummy features 682 are formed over the substrate 670, the insulating layer 674 is formed by depositing one or more insulating films over the conductors 672 and polishing dummy features 682. The average elevation of the upper surface of insulating layer 674 for the left-hand portion of
After depositing the layer 674, it is then polished to achieve a substantially planar surface 692 as shown in
The methodologies disclosed herein may be used in conjunction with various molding and stamping processes to produce a variety of structures that incorporate nanoparticle compositions. One example of such a process is depicted in
With reference to
Referring now to
Referring now to
A number of variations are possible in the methodology depicted in
It will also be appreciated that the structure depicted in
A variety of structures are achievable through processes of the type depicted in
Moreover, depending in part on the dimensions of the surface features of the mold, it is possible to produce a variety of structures in which the nanoparticles are oriented within the surface features of the mold. Orientation of the nanoparticles can be useful in producing structures having improved mechanical and/or electrical properties, depending on the choice of nanoparticle composition.
The methodologies described herein may also be applied to the formation of layers or components in various devices and heterostructures. For example, these methods may be used to define one or more layers of a nanoparticle composition in a heterojunction bipolar transistor or other such device, or to create spacers, based on BNNT or other nanoparticle materials, around gate structures.
The above description of the invention is illustrative, and is not intended to be limiting. It will thus be appreciated that various additions, substitutions and modifications may be made to the above described embodiments without departing from the scope of the present invention. Accordingly, the scope of the present invention should be construed solely in reference to the appended claims.
