Thin-film capacitors and process for manufacturing the same

Abstract: A thin-film capacitor according to the invention comprises a substrate (1), a lower electrode (3), a dielectric (4), and an upper electrode (5) in the structure stacked in sequence from bottom to top. The lower electrode directly on which the dielectric layer is formed is made from a member selected from the group consisting of ruthenium, ruthenium oxide, ruthenium silicide, rhenium, rhenium oxide, rhenium silicide, osmium, osmium oxide, osmium silicide, rhodium, rhodium oxide, rhodium silicide, iridium, iridium oxide and iridium silicide.

Description: This invention relates to thin-film capacitors and, more particularly, to those serving as circuit components of ICs and LSIs. This invention further relates to a process of manufacturing these thin-film capacitors. With developing integrated-circuit technique, electronic circuits become more and more miniaturized, and in turn the miniaturization of capacitors essential as circuit components for various electronic circuits has become a more important problem. Active components such as transistors are being miniaturized rapidly whereas miniaturization of thin-film capacitors comprising a dielectric thin-film layer, which are formed to operate together with active components on the same substrate, fall so behind that it is becoming a major inhibitory factor against making in more LSI or VLSI form. This is because of hitherto using, as material for dielectric thin-films, only materials having up to 10 at most in dielectric constant, such as SiO₂ and Si₃N₄, and therefore it is required for miniaturization of thin-film capacitors to develop dielectric thin-films having great dielectric constants. Of oxides falling under ferroelectric represented by chemical formula ABO₃, perovskite oxides such as BaTiO₃, SrTiO₃ and PbZrO₃, ilmenite oxides such as LiNBO₃, or Bi₄Ti₃O₁₂ are available not only as themselves alone but also as solid solutions thereof, in form of single crystal or ceramic. These oxides are known to have dielectric constants ranging from 100 to as much as 10000, and widely used for ceramic capacitors. Formation of thin-films from these materials is markedly effective for miniaturization of thin-film capacitors, and approaches to this have been carried out for a considerably long time. Particularly, examples resulting in relatively good characteristics are stated in a paper (Processing of the IEEE Vol. 59(10), 1440-1447) as follows: thin-films of BaTiO₃ formed by sputtering and then heat treatment exhibited dielectric constants ranging from 16 (when formed at room temperature) and 1900 (when formed by heat treatment at 1200°C). For the formation of the conventional dielectric thin-films as of BaTiO₃ mentioned above so as to have high dielectric constants, high temperature treatment is required. These, therefore, can not be formed on the surface of the lower electrode, which is a structural element of the thin-film capacitor, unless the lower electrode is made from a noble metal having a high melting point, such as platinum or palladium. If we try to form them from general electrode material such as aluminium, nichrome or copper, then they might be subjected at high temperatures to evaporation and interaction with the dielectric film, resulting in a marked drop in the dielectric constant of the dielectric film. Even if made from a high melting point noble metal such as platinum or palladium, the formation of dielectric thin-films at 300°C or higher temperatures may bring the surface of the electrode into rough texture due to recrystallization. The dielectric film layer formed on such an electrode has ununiform thickness, and therefore when voltage is applied, the smaller thickness is affected by the stronger electric field. This presents a problem with the dielectric breakdown characteristic. Electrode material widely used for LSIs or VLSIs at present is a polycrystalline silicon or a low-resistance silicon layer formed by addition of an impurity at a high doping level to a part on a silicon substrate. These electrodes are collectively called "silicon electrodes" hereinafter. Silicon electrodes for which fine processing technique has already been established are extensively used, and hence only if a good thin-film of high dielectric constant can be formed on a silicon electrode, this could be applied to fabrication of IC capacitors. In the prior art, however, for example, a paper of SrTiO₃ (IBM Journal of Research and Development, November 1969, pp. 686-695) states on pages 687-688 that when a thin-film of high dielectric constant material is formed on a silicon electrode, an about 100 Å-thick layer equivalent to silicon dioxide (SiO₂) inevitably appears along the interface between them. Owing to its low dielectric constant, this interfacial layer contributes to a remarkable drop of the effective dielectric constant of the high dielectric constant film, and thus to make substantially invalid the advantage of using high dielectric material. There is another similar report of BaTiO₃ (Journal of Vacuum Science and Technology Vol. 16(2), 315-318, refer to page 316). It therefore is an object of the invention to provide thin-film capacitors made from high dielectric constant material typically such as BaTiO₃ or SrTiO₃, thereby having high capacity densities and good insulating characteristic, and suitable as component of silicon ICs. Another object of the invention is to provide a process of manufacturing the thin-film capacitors according to the invention. According to one aspect of the present invention, there is obtained a thin-film capacitor comprising a first electrode layer consisting essentially of a member selected from the group consisting of ruthenium, ruthenium oxide, ruthenium silicide, rhenium, rhenium oxide, rhenium silicide, osmium, osmium oxide, osmium silicide, rhodium, rhodium oxide, rhodium silicide, iridium, iridium oxide and iridium silicide, a dielectric layer consisting essentially of an oxide ferroelectric material formed on the first electrode layer, and a second electrode layer formed on the dielectric layer. According to another aspect of the present invention, there is provided a process of manufacturing thin-film capacitors comprising the steps of forming a dielectric layer consisting essentially of an oxide ferroelectric material on a first electrode layer, the first electrode layer consisting essentially of a member selected from the group consisting of ruthenium, ruthenium oxide, ruthenium silicide, rhenium, rhenium oxide, rhenium silicide, osmium, osmium oxide, osmium silicide, rhodium, rhodium oxide, rhodium silicide, iridium, iridium oxide and iridium silicide, and forming a second electrode layer on the dielectric layer. Suitable oxide ferroelectrics for use in the present invention are perovskite oxides, ilumenite oxides, and Bi₄Ti₃O₁₂. Preferred examples of peroskite oxides are BaTiO₃, SrTiO₃, PbTiO₃, PbZrO₃ and solid solutions of them. Preferred examples of ilmenite oxides are LiNbO₃, LiTiO₃ and solid solutions of these. As the second electrode, aluminium is preferably used. Besides a third electrode layer made of polycrystalline silicon may be provided under the first electrode layer. An advantage of the present invention resides in providing thin-film capacitors having a high dielectric constant and a good insulating characteristic by making the first electrode layer of the thin-film capacitor from ruthenium, etc. never subjected to surface-roughnening by high temperature treatment. Another advantage of the present invention is to permit a thin-film capacitor having a uniform high dielectric constant independent of the thickness of the dielectric layer to be fabricated on a silicon substrate in virtue of that unlike conventional silicon electrodes, no silicon oxide layer of low dielectric constant is formed along the interface with the dielectric layer. The above and further objects, features and advantages of the present invention will become more apparent from the following detailed description, provided purely by way of example, taken in conjunction with the accompanying drawings, wherein Fig. 1 is a sectional view of a thin-film capacitor illustrating the first and second embodiments (Examples 1 and 2) of the present invention; Figs. 2(a) and 2(b) and Figs. 3(a) and 3(b) are histograms of dielectric strength from the results of Examples 1 and 2, respectively; Fig. 4 is a sectional view of another thin-film capacitor illustrating the third and fourth embodiments (Examples 3 and 4) of the present invention; Fig. 5 is a sectional view of further thin-film capacitor illustrating the fifth embodiment (Example 5) of the present invention; and Fig. 6 is a graph of the sixth embodiments (Example 6) according to the present invention which shows the relationship between dielectric constant and layer thickness. Example 1 Referring to Fig. 1 which illustrates the structure of the thin-film capacitor formed in the first embodiment of the present invention, a silicon substrate 1, an insulating silicon oxide layer 2, a lower electrode 3, a dielectric layer 4 of BaTiO₃, and an upper electrode 5 of aluminium layer are stacked in sequence from bottom to top. In the first processing step, silicon oxide layer 2 of 1 µm thickness was formed on the surface of a single crystal of silicon by a wet oxidation technique. The thermal oxidation was carried out at 1100°C in an atmosphere wherein the flow rate ratio of oxygen gas to hydrogen gas was kept 1 to 1. Lower electrode layer 3 of 0.5 µm thick was formed by a DC magnetron sputtering technique using a target of sintered Ru or RuSi₂ under the conditions: atmosphere of Ar gas or a mixed gas of Ar and O₂, 4 x 10⁻³ Torr, and substrate temperature 100°C. The BaTiO₃ layer 4 of 0.5 µm thick was constructed at a substrate temperature of 600°C at 1 x 10⁻² Torr in an Ar-O₂ mixture gas, using powder target of stoichiometric composition, by a high-frequency magnetron sputtering technique. Upper Al-electrode 3 of 0.5 µm thick was formed by a DC sputtering technique. This capacitor has an effective area amounting to 3 x 5 mm². Next will be described the differences in characteristics of the BaTiO₃ layer between, as lower electrode 3, a layer of palladium, a high melting point metal, and layers formed according to the present invention. Fig. 2(a) shows a histogram of dielectric strength of the BaTiO₃ layer using a layer formed according to the present invention as the lower electrode, and Fig. 2(b) gives that using Pd layer having a thickness of 0.5 µm. Herein "dielectric strength" is defined as "electric field strength when current reaching 1 x 10⁻⁴ A/cm²". It demonstrates this Example has a great insulating characteristic: as much as approximately 3 times in dielectric breakdown strength, and no dispersion in distribution. After removing a part of the BaTiO₃ layer by etching, the surface roughness of lower electrode 3 was measured with a needle-type surface roughness tester with the results: average values (Ras) were 50 Å for ruthenium, or the like, layer and 380 Å for Pd layer. Thus ruthenium, or the like, layer proved to be better. Besides the surface or roughness of lower electrode 3 was about 30 Å, respectively, as measured each before forming the BaTiO₃. These suggest that the difference in insulating characteristic between both may be attributed to surface roughness of lower electrode 3 induced by the high temperature treatment for forming the BaTiO₃ layer. Layers of ruthenium, ruthenium oxide, ruthenium silicide and stacked structures consisting of these gave similar effects as lower electrode 3. Example 2 Again referring to Fig. 1, a silicon substrate 1, an insulating silicon oxide layer 2, a lower electrode 3, a dielectric layer 4 of BaTiO₃, and an upper electrode 5 of aluminium layer are stacked in sequence from bottom to top. In the first processing step, silicon oxide layer 2 of 1 µm thick was formed on the surface of a single crystal of silicon by the wet oxidation technique. The terminal oxidation was carried out at 1100°C in an atmosphere wherein the flow rate ratio of oxygen gas to hydrogen gas was kept 1 to 1. Lower electrode layer 3 of 0.5 µm thick was formed by the DC magnetron sputtering technique using a target of sintered Re or ReSi₂ under the conditions: atmosphere of Ar gas or a mixed gas of Ar and O₂, 4 x 10⁻³ Torr, and substrate temperature 100°C. The BaTiO₃ layer 4 of 0.5 µm thick was constructed at a substrate temperature of 600°C at 1 x 10⁻² Torr in an Ar-O₂ mixture gas, using powder target of stoichiometric composition, by the high-frequency magnetron sputtering technique. Upper Al-electrode 3 of 0.5 µm thick was formed by the DC sputtering technique. This capacitor has an effective area amounting to 3 x 5 mm². Next will be described the differences in characteristics of the BaTiO₃ layer between, as lower electrode 3, a layer of palladium, a high melting point metal, and layers formed according to the present invention. Fig. 3(a) shows a histogram of dielectric strength of the BaTiO₃ layer using a layer formed according to the present invention as the lower electrode, and Fig. 3(b) gives that using Pd layer having a thickness of 0.5 µm. Herein "dielectric strength" is defined as "electric field strength when current reaching 1 x 10⁻⁴ A/cm²". It demonstrates this Example has a great insulating characteristic: as much as approximately 3 or more times in dielectric breakdown strength, and no dispersion in distribution. After removing a part of the BaTiO₃ layer by etching, the surface roughness of lower electrode 3 was measured with a needle-type surface roughness tester with the results: average values (Ras) were 50 Å for rhenium, or the like, layer and 380 Å for Pd layer. Thus rhenium, or the like, layer proved to be better. Besides the surface or roughness of lower electrode 3 was about 30 Å, respectively, as measured each before forming the BaTiO₃. These suggest that the difference in insulating characteristic between both may be attributed to surface roughness of lower electrode 3 induced by the high temperature treatment for forming the BaTiO₃ layer. Layers of rhenium, rhenium oxide, rhenium silicide and stacked structures consisting of these gave similar effects as lower electrode 3. Moreover in place of rhenium, rhenium oxide and rhenium silicide can be used osmium, rhodium, iridium and silicides or oxides of these as material of the lower electrode layer with good results in dielectric strength of the BaTiO₃ layer as much as 3 times or more than the conventional Pd lower electrode layer. Table 1 summarizes various materials and respective dielectric strengths of the BaTiO₃ layer formed on the lower electrode made from each material. Example 3 Referring to Fig. 4 which illustrates the structure of the thin-film capacitor to be formed, a single-crystal silicon substrate 6, an insulating silicon oxide layer 7, a lower electrode 8 consisting of polycrystalline silicon layer and a layer 9 as of ruthenium, a dielectric layer 10 of BaTiO₃, and an upper electrode 11 of aluminium layer are stacked in sequence from bottom to top. The polycrystalline silicon layer 8 of 0.3 µm was constructed at 300°C by a plasma CVD technique. Into this polycrystalline silicon layer 8, arsenic ions were injected at a dose of 2 x 10¹⁶ cm⁻² and an acceleration voltage of 70 kV, and heat-treated at 900°C for 20 minutes to obtain sheet resistance of about 100 Ω/□. The other layers were formed in the same way as described in Example 1. In this case, the polycrystalline silicon layer 8 serves for the improvement of the adhesion characteristic between the silicon oxide insulating layer 7 and the lower electrode. The formation of the layer 9 as of ruthenium on the polycrystalline silicon layer allowed to fabricate a thin-film capacitor having an insulating characteristic as good as in Example 1. Example 4 Again referring to Fig. 4 which illustrates the structure of the thin-film capacitor to be formed, a single-crystal silicon substrate 6, an insulating silicon oxide layer 7, a lower electrode 8 consisting of polycrystalline silicon layer and a layer 9 as of rhenium, a dielectric layer 10 of BaTiO₃, and an upper electrode 11 of aluminium layer are stacked in sequence from bottom to top. The polycrystalline silicon layer 8 of 0.3 µm was constructed at 300°C by plasma CVD technique. Into this polycrystalline silicon layer 8, arsenic ions were injected at a dose of 2 x 10¹⁶ cm⁻² and an acceleration voltage of 70 kV, and heat-treated at 900°C for 20 minutes to obtain sheet resistance of about 100 Ω/□. The other layers were formed in the same way as described in Example 2. In this case, the polycrystalline silicon layer 8 serves for the improvement of the adhesion characteristic between the silicon oxide insulating layer 7 and the lower electrode. The formation of the layer 9 as of ruthenium on the polycrystalline silicon layer allowed to fabricate a thin-film capacitor having an insulating characteristic as good as in Example 2. Example 5 Referring to Fig. 5, the structure of the thin-film capacitor to be formed comprises a single crystal silicon substrate 12 with thereon a low resistance region 13 formed by doping phosphorus at a low level, and a silicon oxide layer 14 as an insulating interlayer formed on the surface of the substrate and provided with two contact holes of which one is filled as with ruthenium to make a part of a lower electrode layer 15 and the other with aluminium layer 16 to form a terminal connecting to the lower electrode 15 through the low resistance region 13. The lower electrode layer 15 may be formed to fill the contact hole and overlap the silicon oxide layer 14. The structure further comprises a BaTiO₃ layer 17 on the lower electrode layer, and an upper Al electrode 18 on the BaTiO₃ layer 17. In this Example, for permitting the connection through the low resistance region 13, the lower electrode layer is formed on a single crystal silicon, and the thus-constructed thin-film capacitor has proved to have a dielectric characteristic as good is in Example 1. Example 6 In the following will be described the difference between the dielectric constant of the BaTiO₃ layer with the polycrystalline silicon layer as the lower electrode, and that with a layer according to the invention. At present polycrystalline silicon film is generally used as electrode layers on silicon LSIs. Fig. 6 shows graphs representing the relationship of the BaTiO₃ layer between dielectric constant and thickness, plotting the results when the lower electrode is a ruthenium silicide layer according to the present invention and a polycrystalline silicon layer, respectively. It reveals that the dielectric constant of the BiTiO₃ layer is about 240 constant against the thickness thereof, that is, thickness-independent with the layer according to the present invention whereas with the polycrystalline silicon layer, the dielectric constant thereof is thickness-dependent and becomes smaller as the thickness decreases. This suggests, as described earlier, that a silicon oxide layer having a low dielectric constant may be formed along the interface between the BaTiO₃ layer and the polycrystalline silicon, bringing about a reduction of the effective dielectric constant of the BaTiO₃. As in Examples 3 and 4, the lower electrode may be formed into double-layer structure consisting of a layer as of ruthenium or rhtnium and another layer of polycrystalline silicon thereunder. In this structure, the polycrystalline silicon layer produces effect to improve the adhesion of the layer as of ruthenium or rhenium to the single crystal silicon substrate and to the silicon oxide region. Also taking into consideration that a planarization technique for filling contact holes with polycrystalline silicon film has been established, the advantage to using it as part of the lower electrode is great. Finally formation of the lower electrode as a layer consisting of at least one selected from the group consisting of ruthenium, rhenium, osmium, rhodium, iridium, and silicides and oxides of these, as in this Example, affords the advantage of fabricating a thin-film capacitor having a high fixed dielectric constant independently of the thickness of the dielectric layer on a silicon substrate.

Claims: A thin-film capacitor comprising: a first electrode layer consisting essentially of a member selected from the group consisting of ruthenium, ruthenium oxide, ruthenium silicide, rhenium, rhenium oxide, rhenium silicide, osmium, osmium oxide, osmium silicide, rhodium, rhodium oxide, rhodium silicide, iridium, iridium oxide and iridium silicide; a dielectric layer consisting essentially of an oxide ferroelectric material formed on said first electrode layer; and a second electrode layer formed on said dielectric layer. A thin-film capacitor as recited in claim 1, further comprising a third electrode layer formed under said first electrode layer and consisting essentially of polycrystalline silicon. A thin-film capacitor as recited in claim 1, wherein said oxide ferroelectric is a member selected from the group consisting of perovskite type oxides, ilmenite oxides and Bi₄Ti₃O₁₂. A thin-film capacitor as recited in claim 1, wherein said oxide ferroelectric is a member selected from the group consisting of BaTiO₃, SrTiO₃, PbTiO₃, PbZrO₃ and solid solutions thereof. A thin-film capacitor as recited in claim 2, wherein said oxide ferroelectric is a member selected from the group consisting of perovskite type oxides, ilmenite oxides and Bi₄Ti₃O₁₂. A thin-film capacitor as recited in claim 2, wherein said oxide ferroelectric is a member selected from the group consisting of BaTiO₃, SrTiO₃, PbTiO₃, PbZrO₃ and solid solutions thereof. A process of manufacturing thin-film capacitors comprising the steps of: forming a dielectric layer consisting essentially of an oxide ferroelectric on a first electrode layer, said first electrode layer consisting essentially of a member selected from the group consisting of ruthenium, ruthenium oxide, ruthenium silicide, rhenium, rhenium oxide, rhenium silicide, osmium, osmium oxide, osmium silicide, rhodium, rhodium oxide, rhodium silicide, iridium, iridium oxide and iridium silicide; and forming a second electrode layer on said dielectric layer. A process of manufacturing thin-film capacitors as recited in claim 7, wherein said oxide ferroelectric is a member selected from the group consisting of perovskite oxides, ilumenite oxides, and Bi₄Ti₃O₁₂. A process of fabricating thin-film capacitor as recited in claim 7, wherein said oxide ferroelectric is a member selected from the group consisting of BaTiO₃, SrTiO₃, PbTiO₃, PbZrO₃ and solid solutions thereof.

Inventors: MATSUBARA SHOGO, MIYASAKA YOICHI, MATSUBARA, SHOGO, MIYASAKA, YOICHI
Applicants: NIPPON ELECTRIC CO, NEC CORPORATION
Application year: 1990
Main classification: H01L29
IPC codes: H01L21, H01L27