Related fields include atomic layer deposition and metal-insulator-metal capacitors.
Traditional device scaling of logic devices based on silicon (Si) has encountered obstacles. Inherent material properties have placed limitations on further miniaturization, increases in processing speed, and other performance enhancements. For example, as gate conductor width decreases, gate dielectric thickness also needs to decrease to provide sufficient capacitance to control the transistor. Suppression of leakage current is critical to performance of a capacitor dielectric. Silicon oxide layers <2 nm thick are subject to unacceptably high leakage current due to tunneling effects.
Tunneling leakage decreases as a function of physical thickness. Therefore, there has been interest in gate dielectric materials that would exhibit the same capacitance as 1-2 nm thick silicon dioxide (SiO2) at a physical thickness too large for significant tunneling leakage (e.g., >=5 nm). Metal oxides with high dielectric constants (“high-k materials”) such as hafnium oxide (HfO2, k˜20) and zirconium oxide (ZrO2, k˜25) are, among others, being investigated as dielectric candidates to replace silicon oxide in such devices as back-end-of-line (BEOL) decoupling capacitors.
A quantity often measured for high-k dielectrics being substituted for silicon oxide is the effective oxide thickness (EOT=(kSiO2/k)t). In the equations, k=dielectric constant of the actual material, t=physical thickness of the actual material, and kSiO2=dielectric constant of SiO2˜3.9.
When a layer is thick enough to effectively suppress tunneling, other leakage mechanisms may still remain. For example, HfO2, ZrO2, and other high-k materials tend to crystallize after annealing, forming polycrystalline material that contains grain boundaries. The resulting grain boundaries provide paths for electrons to travel, thus increasing the leakage current.
Amorphous materials have no grain boundaries, and thus grain boundaries are eliminated as a leakage path.
Therefore, a need exists for a way to form a layer of high-k material that can be annealed without developing the leakage-current paths from grain boundaries in polycrystalline materials.
The following summary presents some concepts in a simplified form as an introduction to the detailed description that follows. It does not necessarily identify key or critical elements and is not intended to reflect a scope of invention.
A very thin layer (<15 Å) of a somewhat lower-k impurity material such as aluminum oxide (Al2O3, k˜8-9) is deposited by atomic layer deposition (ALD) under, over, or inside a thicker layer (˜30-100 Å) of a high-k host material such as hafnium oxide or zirconium oxide. When the high-k material crystallizes after annealing, the lattice is disrupted where the high-k material meets the impurity sub-layer(s). Potential paths for leakage current are similarly disrupted, particularly in the direction perpendicular to the impurity sub-layer(s) which is most critical to capacitor performance. Because the impurity sub-layers are so thin, the amount of impurity compared to the host material is too small to unacceptably reduce the dielectric constant of the composite layer.
Different embodiments may be selected to optimize different parameters. For instance, placing the impurity sub-layer inside the host material may produce very low leakage current, while depositing the impurity sub-layer first and depositing the host material layer directly on the impurity sub-layer may produce a very low EOT.
The leakage current can be reduced by sandwiching the Al2O3 inside the HfO2, while the EOT can be reduced by depositing the Al2O3 first and depositing the HfO2 directly on it. A post-metal anneal at 300-450 C for 1-120 min produced improved results when the process parameters were varied.
Some embodiments of a thin-film capacitor include a first conductive layer and a second conductive layer (operable as capacitor plates) with a composite dielectric layer between the conductive layers. The composite dielectric layer includes a host material with one or more impurity sub-layers. Each of the impurity sub-layers may be less than about 15 Å thick, or even <=3 Å thick. Near the impurity sub-layers, the lattice structure of the host material may be disrupted or distorted. The host material may be polycrystalline in the absence of lattice-disrupting impurities. The impurity sub-layer may also include some material that would be polycrystalline if no other materials disrupted its lattice structure.
In some embodiments, the dielectric constant k of the host material is greater than that of the impurity material. The dielectric constant k of the impurity may be less than 10. The dielectric constant k of the host material may be greater than 14. The dielectric constant k of the composite dielectric layer may be greater than 12, and may be greater than 80% of the dielectric constant k of the host material. The composite dielectric layer may be about 30-100 Å thick. The composite dielectric layer may have an EOT less than about 20 Å. The composite dielectric layer may have a leakage current density JG less than 0.1 μA/cm2 at 1.2V.
In some embodiments, the host material may include a metal oxide (e.g., HfO2 or ZrO2). In some embodiments, the impurity may include a metal oxide (e.g., Al2O3). The first or second conductive layer may include a metal nitride (e.g., TiN).
A thin-film capacitor may be made by forming a first conductive layer on a substrate, forming a host material layer above the first conductive layer, forming an impurity sub-layer less than about 15 Å thick in direct contact with the host material, forming a second conductive layer, and annealing the substrate. Annealing combines the host material and the impurity into a composite dielectric layer. The annealing temperature may be between about 300 and about 45° C., and its duration may be between about 1 and about 120 minutes. After annealing, the host material forms a polycrystalline lattice, which is disrupted in the vicinity of the impurity. The dielectric constant of the composite dielectric layer may be greater than about 12.
In some embodiments, the impurity sub-layer is formed by ALD. In some embodiments, the host material, or either or both of the conductive layers, are also formed by ALD. The impurity sub-layer may be formed before the host material, so that it is located between the first conductive layer and the host material. The impurity sub-layer may be formed after the host material, so that it is located between the host material and the second conductive layer. The impurity sub-layer may even be formed during the formation of the host material, so that it is located within the host material layer. In some embodiments, there may be more than one impurity sub-layer. The multiple impurity sub-layers may be embedded in the host material, above or below the host material, or a combination. The effective thickness of some impurity sub-layers may be less than 1 Å (averaged over a monolayer that is not fully contiguous).
The accompanying drawings may illustrate examples of concepts, embodiments, or results. They do not define or limit the scope of invention. They are not drawn to any absolute or relative scale. In some cases, identical or similar reference numbers may be used for identical or similar features in multiple drawings.
As used herein, a material is “crystalline” if it exhibits at least 30% crystallinity as measured by a technique such as x-ray diffraction (XRD). Terms of relative position such as “above”, “below”, “bottom”, “top”, “higher”, “lower”, “upper”, “over”, and “under” are used for convenience; unless clearly indicated otherwise, the scope of invention also includes embodiments with layers in the opposite order to that described. “On” indicates direct contact between described elements. “Above,” “over,” “below,” and “under” allow for intervening elements between the described elements. “About” means “within 10% of.”
In
A lattice structure or grain boundary forming in a layer (for instance, during annealing, which crystallizes many amorphous-as-deposited materials) can be disrupted near an impurity that does not fit into the lattice or continue the grain boundary. An amorphous material may be thought of as a limiting case with a lattice disrupted in so many places that it becomes undetectable. Where the lattice structure and particularly the grain boundaries are disrupted, so are the available paths for leakage current. An impurity sub-layer need not be amorphous to effectively disrupt the lattice or grain boundaries of the host material. It only needs to be structurally different enough to cause a discontinuity in the host material's leakage paths.
An impurity sub-layer may also affect the host material's dielectric constant. That effect depends wholly on the amount of the impurity compared to the host material. The impurity's effect on leakage current, however, depends on both the amount of the impurity and the nature of its dispersal through the host material. Therefore, a small amount of an impurity, strategically dispersed, may decrease leakage current significantly while only slightly affecting the layer's dielectric constant.
In a capacitor, leakage current travels between the two capacitor plates through the most conductive path(s) available. If the conductive paths can be blocked in one or more entire planes of the dielectric parallel to the plates, no highly conductive path between the plates remains. Current may then travel through material on both sides of the blockage plane, but it will not become leakage current if it cannot reach the opposite plate. Therefore, a small amount of impurity may be strategically dispersed to efficiently limit leakage if it is densely in concentrated in one or more thin planes parallel to the capacitor plates.
Because the impurity sub-layer 213A extends across the entire area of the capacitor in a plane between bottom conductive layer 102 and top conductive layer 104, leakage current is blocked by the lattice-structure disruption no matter where on the conductive layer it originates. This is a potential advantage over doping methods that scatter dopant throughout the bulk of the host material and may leave some leakage paths between the dopant loci. At the same time, impurity sub-layer 213 A may be very thin compared to host material 103, making the atomic percentage of the impurity in the composite dielectric very small. In some embodiments, more than 90% of the impurity in the composite dielectric may be confined to the impurity sub-layer. Because the impurity is only a small atomic percentage of the composite dielectric, the dielectric constant k of the composite dielectric is dominated by the dielectric constant of the host material 103. Thus, if host material 103 is a high-k material, the composite dielectric can also be high-k even if the impurity is medium-k or low-k.
In
In the following discussion, the word “substrate” will refer to underlying substrate 301 and any layers formed on it up to the present top surface. For example, in
In this example, first deposited material 323 is a component of a host material. In some embodiments, the host material may be a metal oxide. The metal oxide may be a high-k metal oxide such as hafnium oxide or zirconium oxide. The first deposited material 323 may be the metal component of the metal oxide, such as hafnium or zirconium. Precursor 324 may be a hafnium precursor such as HfCl4, tetrakis(dimethylamido)hafnium(IV), tetrakis(ethylmethylamido)hafnium(IV), bis(methyl-η5-cyclopentadienyl)dimethylhafnium, hafnium(IV) tert-butoxide, or bis(methyl-η5-cyclopentadienyl)methoxymethylhafnium. Precursor 324 may be a zirconium precursor such as ZrCl4, bis(methyl-η5-cyclopentadienyl)methoxymethylzirconium, tetrakis(dimethylamido)zirconium(IV), tetrakis(ethylmethylamido)zirconium(IV), or zirconium(IV) tert-butoxide. These options are intended as examples only and do not limit the scope of invention.
In
In this example, second deposited material 333 is a component of a host material. In some embodiments, the host material may be a metal oxide, perhaps a high-k metal oxide such as hafnium oxide or zirconium oxide. The second deposited material 323 may be the oxygen component of the metal oxide and may adsorb by oxidizing the metal. Precursor 324 may be an oxygen precursor such as water (H2O) or ozone (O3).
In
In this example, third deposited material 353 is a component of an impurity. In some embodiments, the impurity may be a metal oxide, perhaps a lower-k (compared to the host material) metal oxide such as aluminum oxide or lanthanum oxide. The third deposited material 353 may be the metal component of the metal oxide. Precursor 354 may be an aluminum precursor trimethylaluminum (TMA).
In
In this example, fourth deposited material 363 is a component of an impurity. In some embodiments, the impurity may be a lower-k (compared to the host material) metal oxide such as aluminum oxide. The fourth deposited material 363 may be the oxygen component of the lower-k metal oxide and may adsorb by oxidizing the metal. Precursor 364 may be an oxygen precursor such as water (H2O) or ozone (O3).
In
In
In some embodiments, the impurity sub-layer may be formed by ALD, but the host material layer or layers may be formed by some other process, such as a CVD, MOCVD, PVD, or epitaxy. Likewise, conductive layer layers may also either be formed by ALD or by any other suitable method.
At some point after the composite dielectric is formed 403, there is a heat treatment 410. Heat treatment 410 may be performed immediately after forming 403 the composite dielectric, or after forming 404 the top conductive layer, or after forming some intermediate layer, or it may be part of a subsequent process 499.
Forming 403 the composite dielectric includes forming at least one layer of host material (“host layer”) and at least one impurity sub-layer. Alternative examples include (a) initially forming 413 the impurity sub-layer and then forming 423 the host layer; (b) initially forming 433 the host layer and then forming 443 the impurity sub-layer; or (c) forming 453 a first part of the host layer, forming 463 the impurity sub-layer, and forming 473 the remaining part of the host layer. If multiple impurity sub-layers are desired, steps 453 and 463 may be iterated 483 multiple times before steps 473. Embodiments with multiple sub-layers may alternatively have the impurity sub-layers on the top, the bottom, or both. Any of the sub-layers or parts of the host layer may be 1 or fewer monolayers thick. Laminates of multiple layers may effectively form an alloy of the host material and the impurity material.
Experimental Results
In the graph, hollow data point markers are assigned to film stacks with a 3 Å impurity sub-layer, while solid data point markers are assigned to film stacks with the 5 Å impurity sub-layer. Diamond-shaped data point markers are assigned to stacks with an impurity sub-layer embedded in the center of the host layer. Square data point markers are assigned to stacks with the impurity sub-layer between the bottom electrode and the host material. Triangular data point markers are assigned to stacks with the impurity sub-layer between the host material and the top electrode.
Thus, hollow diamond data points 501 correspond to a 3 Å impurity sub-layer embedded between two 30 Å thicknesses of host material (30H+3A+30H). Solid diamond data points 502 correspond to a 5 Å impurity sub-layer embedded between two 30 Å thicknesses of host material (30H+5A+30H). Hollow square data points 503 correspond to a 3 Å impurity sub-layer between a bottom electrode and a 60 Å thickness of host material (3A+60H). Solid square data points 504 correspond to a 5 Å impurity sub-layer between a bottom electrode and a 60 Å thickness of host material (5A+60H). Hollow triangular data points 505 correspond to a 3 Å impurity sub-layer between the host material and the top electrode (60H+3A). Solid triangular data points 506 correspond to a 5 Å impurity sub-layer between the host material and the top electrode (60H+5A).
Encircled region 508 shows the stacks with a leakage current density less than 20 nA/cm2. The lowest measured leakage-current density came from (30H+5A+30H) stacks, although the (30H+3A+30H) and the (60H+5A) were also very low. The lowest measured EOT values were seen in the (3A+60H) group, but the leakage current density was higher. The lowest measured EOT values among the stacks with leakage current density less than 20 nA/cm2 were between 11 and 12 Å in this data-set.
Crystalline hafnium oxide has a peak 611 near 2θ=28.4° for monoclinic crystalline formations and a peak 612 near 2θ=30.4° for tetragonal and orthorhombic crystalline formations. Both peaks 611 and 612 are visible on curves 603 (3A+60H) and 604 (5A+60H), but are noticeably attenuated in curve 604 with the thicker impurity sub-layer. Thus the presence of the impurity sub-layer disrupts the host-material lattice and/or grain boundaries sufficiently to attenuate the host-material XRD peaks, compared to a host-material dielectric with no impurity. The monoclinic peak 611 is barely visible in curve 605 (60H+3A) and seems to be completely absent in curve 606 (60H+5A). The tetragonal/orthorhombic peak 612 is still visible in curves 605 and 606, but is weaker in curve 606 corresponding to the thicker impurity sub-layer. Neither of the two host-material peaks is resolvable above the approximately 20-30 counts of peak-to-peak background noise in curves 601 and 602 corresponding to the impurity sub-layer embedded in the center of the host material layer, suggesting that the central embedded impurity sub-layer caused the most disruption in the host material grain boundaries and/or lattice structure.
Although the foregoing examples have been described in some detail to aid understanding, the invention is not limited to the details in the description and drawings. The examples are illustrative, not restrictive. There are many alternative ways of implementing the invention. Various aspects or components of the described embodiments may be used singly or in any combination. The scope is limited only by the claims, which encompass numerous alternatives, modifications, and equivalents.