1. Field of the Invention
The present invention relates to a deposition method for a transition-metal oxide containing dielectric, and furthermore to a capacitor or transistor structure with a transition-metal oxide based dielectric, and a memory device comprising the same.
2. Description of the Related Art
Although in principle applicable to arbitrary integrated semiconductor structures, the following invention and the underlying problems will be explained with respect to integrated DRAM memory circuits in silicon technology.
Memory cells of a DRAM device each comprise a capacitor for storing information encoded as electric charge retained in the capacitor. A reliable operation of the memory cells demands for a minimal capacitance of the capacitors and a sufficiently long retention time of the charge in the capacitors.
There is a major interest to further reduce the lateral dimensions of structures of a DRAM to a minimal feature size of 40 nm and below. Therefore, in order not to reduce the capacitance of the DRAM capacitors, it is desirable to compensate shrinking lateral dimensions of the capacitors by providing a dielectric layer with a high specific dielectric constant, or k-value. Simultaneously, care has to be taken not to increase leakage currents, which lead to a short retention time of the DRAM memory cell and are influenced by the band gap of the dielectric material, and in particular by the match between the band structure of the dielectric to the band structure of the capacitor electrodes.
For DRAM capacitors at a feature size of below 40 nm, zirconium oxide (ZrO2) and hafnium oxide (HfO2) are considered likely candidates for providing a base material of the capacitor dielectric. In the cubic or tetragonal crystallization phase, pure ZrO2 and HfO2 each reach a specific dielectric constant of k=35 to 40. The dielectric constant as well as the leakage current density of ZrO2 and HfO2 films can be influenced by adding one or more additional oxide materials as dopants to the dielectric film. However, in many cases the addition of a given dopant that increases the specific dielectric constant leads also to an increase of leakage currents.
It would therefore be advantageous if a deposition method for a zirconium or hafnium oxide based dielectric film could be provided that achieves to increase the specific dielectric constant above that of pure ZrO2 or HfO2, respectively, while maintaining a low leakage current density. It would further be advantageous if a deposition method could be provided that enables depositing the film at a precisely defined thickness, composition, and crystallization phase over a high-aspect ratio structure.
According to a first aspect of the invention, a deposition method for a transition-metal oxide containing dielectric comprises:
The transition metal used herein comprises at least one of zirconium and hafnium. The dopant used herein comprises at least one of barium, strontium, calcium, niobium, bismuth, magnesium, and cerium.
The method according to the invention uses two sets of precursors to deposit the transition metal oxide based material on the substrate. By the first set of precursors, a layer of transition metal containing material is deposited, while by the second set of precursors, a layer of dopant containing material is deposited. Each of the sets of precursors comprises water vapor, ozone, oxygen, or oxygen plasma as one of the precursors, which acts as oxidizing reactant with respect to the respective remaining precursor of each pair. The water vapor, ozone, oxygen, or oxygen plasma respectively sets free the transition metal of the first precursor and the dopant of the third precursor. A potential advantage of ozone is its higher cleaning effect, that is to say less residuals of the organic compounds of the first and third precursors remain in the dielectric film since ozone is capable of transforming organic parts of the first and third precursors into volatile gases. Water vapor, on the other hand, is potentially advantageous where clean separation of the organic parts of the precursors is desired without fragmenting the organic parts themselves.
By using in this way the technique known as Atomic Layer Deposition (ALD), the deposition method achieves a uniform distribution of both the transition-metal containing material and the dopant containing material across the surface of the substrate, even if the substrate is shaped in the form of a high-aspect-ratio structure, such as a structure comprising deep trenches for producing trench-type capacitors, or cylinder-or cup-type features for producing stacked-type capacitors.
As a result, the transition-metal containing material and the dopant containing material are deposited in defined quantities, each corresponding to a monolayer of one-molecule thickness or less, depending on the amount of sterical hindrance among the chosen precursor molecules, which limits coverage of the substrate surface by precursor molecules applied simultaneously. Since all atoms of the transition metal are placed in the immediate vicinity of a dopant atom in a highly controlled way, a temperature of the substrate, either during a separate annealing step or during the deposition process itself, can be chosen such that it induces rearrangement of neighboring atoms of the transition metal and dopant atoms together with oxygen atoms deposited in both monolayers in a common crystallization structure, in particular the perovskite structure, thus leading to the creation of a thin and precisely distributed film of high specific dielectric constant and low leakage current.
Preferred embodiments of the inventive deposition method are listed in the dependent claims 2 to 17.
A capacitor structure manufactured by the inventive method comprises a first and a second electrode of conducting material, with the dielectric film according to the invention disposed between both electrodes. The first and second electrodes each preferably are made of at least one of niobium nitride, titanium nitride, titanium silicon nitride, tantalum nitride, tantalum silicon nitride, tantalum carbide, carbon, tungsten, tungsten sulicide, ruthenium, ruthenium oxide, iridium, and iridium oxide. The dielectric film comprises zirconium or hafnium oxide and at least one of barium, strontium, calcium, niobium, bismuth, magnesium, and cerium. Preferably the dielectric film comprises a perovskite structure, which advantageously enables to provide both a high dielectric constant and a large bandgap, e.g. of 30-50 and 6 eV, respectively, in the case of SrZrO3. The complete film or only part of it may have this structure. The orientation of the structure may vary within the film.
According to an embodiment, the dielectric film comprises a dopant content of between 5 and 70 atomic percent of the dielectric film material excluding oxygen. Preferably, in order to favor forming of a perovskite crystal structure, the dielectric film comprises a dopant content of between 50 and 70 atomic percent of the dielectric film material excluding oxygen.
A semiconductor memory device may comprise a plurality of memory cells each comprising the inventive capacitor.
In the Figures:
a and 1b show schematic cross-sections of a substrate undergoing deposition of a dielectric film by a deposition method according to a first embodiment of the invention;
a shows a schematic cross-section of a substrate bearing a mixed dielectric film deposited by a method according to a second embodiment of the present invention;
b shows a schematic cross-section of a substrate bearing a nanolaminate dielectric film deposited by a method according to a third embodiment of the present invention; and
In the Figures, like numerals refer to the same or similar functionality throughout the several views.
A deposition method according to a first embodiment is illustrated by making reference to
As shown in
As shown in
As a result of carrying out the deposition method as described, a dielectric film 106 is deposited on the substrate 100, where the dielectric film 106 contains an approximately equal amount of zirconium oxide and strontium oxide. Since both of the zirconium oxide and the strontium oxide have been deposited in the form of stacked monolayers 102, 104, or in the form of at least one mixed monolayer as described above, by choosing the temperature of the substrate 100 during the deposition from a temperature range that is known to induce the formation of a given desired crystal structure comprising both zirconium and strontium along with oxygen, the dielectric film 106 is enabled to be formed in the desired crystallization structure. In particular, the mixed dielectric film 106 containing zirconium, strontium and oxygen can be provided in a crystallization structure such as the perovskite structure that is known to be associated with a desired set of properties including a high specific dielectric constant and large bandgap.
Optionally, a separate annealing step is performed after the deposition of the dielectric film, during which the substrate with the deposited dielectric film is heated to a defined temperature to induce crystallization in a desired crystallization structure. In this way, the duration of the annealing step and the choice of atmosphere in which to perform the annealing can be controlled in addition to the annealing temperature. Preferably, the annealing temperature lies between 200° C. and 1200° C., more preferably between 200° C. and 600° C. Suitable atmosphere gases include N2, O2, Ar, NH3, and N2O, with the annealing step lasting several seconds.
By choosing a suitable number of repetitions in which the deposition steps of
Since throughout the dielectric film 106 zirconium atoms and strontium atoms are distributed in close proximity to each other as a result of the alternating deposition of complete or fractional monolayers 102, 104, by choosing the temperature of the substrate 100 during a subsequent annealing step or during the deposition process itself from a range that leads to desired common crystallization structure of zirconium, strontium and oxygen such as the perovskite structure, the present embodiment enables depositing a dielectric film 106 of desired thickness d throughout which zirconium, strontium and oxygen are crystallized in the desired common structure. For example, in the described way a mixed dielectric film 106 of zirconium strontium oxide in the perovskite crystallization structure is enabled to be deposited at a desired thickness, thus providing a dielectric film 106 that provides a high dielectric constant with a high resistance against leakage currents across the dielectric film 106.
In this embodiment, however, the deposition steps of
By choosing the temperature of the substrate 100, either during the deposition process or preferably during a separate annealing step, from a range of temperatures that enables the formation of desired crystallization structures within the sublayers of zirconium oxide and strontium oxide, respectively, and/or the formation of desired mixed crystallization structures in the vicinity of the interfaces between the sublayers, a dielectric film 106 can be deposited at a desired thickness d that combines a high overall dielectric constant with a high overall resistivity against leakage currents, e.g. by providing the sublayers of one of the oxide materials in a crystallization structure with a known high dielectric constant interspersed with the sublayers of the respective other one of the oxide materials in a crystallization structure that is known to provide a particularly high band-gap, thus forming an effective barrier against leakage currents.
Furthermore, by choosing a particular sequence of monolayers containing either zirconium or dopant a mixed film with a desired concentration ratio such as 1:2, 2:3, 3:4 etc. may be deposited. For example by repeating the sequence Sr—Zr—Sr—Sr—Zr, where Sr stands for a deposition step for a strontium containing monolayer and Zr stands for a deposition step for a zirconium containing monolayer, a mixed dielectric film with a concentration ratio of 3:2 between strontium and zirconium may be deposited, corresponding to a dopant content of approximately 60% of the atoms of the dielectric film material excluding oxygen. Preferably, the ratio is chosen such that the dopant content is between 5 and 70 atomic percent of the dielectric film material excluding oxygen, most preferably between 50 and 70 atomic percent. The most preferred range enables to form an advantageous perovskite structure in which vacant zirconium atom positions allow the zirconium atoms to move within a rigid structure of dopant, e.g. strontium, and oxygen atoms. This structure is highly polarizable and thus leads to a particularly high specific dielectric constant.
The reference to zirconium in the above described embodiments is purely exemplary. In alternative embodiments, hafnium may be used instead of zirconium, or in conjunction with zirconium, as a transition metal, carrying out the deposition method essentially as described. Likewise, the use of strontium as a dopant in the above embodiments as described is purely exemplary. In alternative embodiments, barium, calcium, niobium, bismuth, magnesium, or cerium, as well as combinations of any of these, may be used instead of or in conjunction with strontium, as a dopant while carrying out the deposition method essentially as described.
In order to produce the capacitor structure shown, a trench 304 is formed into a substrate 300. The first electrode 100 is deposited on the surface of the trench 304 by a standard deposition technique. The dielectric 106 is applied directly on the first electrode 100 by one of the ALD processes taught along with the above embodiments. The second electrode 302 may be formed as polycrystalline silicon or a metallic electrode, preferably consisting of niobium nitride, titanium nitride, titanium silicon nitride, tantalum nitride, tantalum silicon nitride, tantalum carbide, carbon, tungsten, tungsten silicide, ruthenium, ruthenium oxide, iridium, or iridium oxide. These materials are electrical conductors well suited to function as electrodes of a capacitor. Their respective conduction bands are advantageously positioned such as to present a high resistivity of the interface of electrode and dielectric against leakage currents. Optionally, an interface layer of silicon nitride (not shown) is formed either between the first electrode 100 and the dielectric 106, or between the dielectric 106 and the second electrode, or both. Alternatively, for formation of a metal-insulator-silicon (MIS) instead of a metal-insulator-metal (MIM) structure, an interface layer of e.g. silicon nitride can be used between a silicon substrate and the dielectric 106, if a first electrode separate from the substrate is not used.
Although the present invention has been described with reference to preferred embodiments, it is not limited thereto, but can be modified in various manners which are obvious for persons skilled in the art. Thus, it is intended that the present invention is only limited by the scope of the claims attached herewith.
For example, the ALD processes as illustrated in