The present invention relates to a method for manufacturing a semiconductor device provided with electrodes or wiring that are formed sandwiching an adhesion layer on a dielectric film.
In semiconductor memories, the reduction in size of memory cells due to the miniaturization of the design is progressing. For example, DRAM (Dynamic Random Access Memory) memory cells, which are one type of semiconductor memories, are made of a memory cell transistor and a capacitor for charging electron. In DRAMs, even though the memory cells have been reduced in size and the area of the capacitor projected onto the substrate (referred to as xe2x80x9ccapacitor areaxe2x80x9d in the following) has become small, it is not possible to reduce the capacitance of the capacitor, in order to reduce the power consumption and in order to prevent soft errors. The capacitance of the capacitor is generally proportional to the relative dielectric constant of the dielectric material used for the dielectric film (capacitance dielectric film) constituting the capacitor as well as to the capacitor area, and inversely proportional to the film thickness of the capacitance dielectric film. However, if the film thickness of the capacitance dielectric film is made small in order to increase the capacitance of the capacitor, then the leak current in the capacitor increases. As a result, it becomes necessary to shorten the refresh cycle of the memory cells, so that the power consumption increases. This means that there is a limit to how thin the capacitance dielectric film can be made.
In order to address this problem, in recent years, the use of dielectric materials with high relative dielectric constant (high-k material) for the capacitance dielectric film has been researched as a way of increasing the capacitance of the capacitor. High-k materials that have been researched in detail include for example metal oxides such as aluminum oxide or tantalum pentoxide (composition formula: Ta2O5), barium strontium titanium oxide (composition formula: (Ba(1-x)Srx)TiO3; referred to as xe2x80x9cBSTxe2x80x9d in the following), lead zirconium titanium oxide (referred to as xe2x80x9cPZTxe2x80x9d in the following), and strontium bismuth tantalum oxide (referred to as xe2x80x9cSBTxe2x80x9d in the following), which have a perovskite crystal structure.
When the dielectric film is formed using such a high-k material, then, in general, chemical reactions are utilized often, and the formation of the dielectric film is performed in an oxidizing atmosphere, so that if silicon, which has been used conventionally, is used as the electrode material, the silicon tends to be oxidize. That is to say, a silicon oxide film with a low relative dielectric constant is formed, so that it becomes difficult to increase the capacitance of the capacitor. Consequently, a precious metal or a refractory metal or the like is used for the electrodes of capacitors using such a high-k material for the capacitance dielectric film. Furthermore, a precious metal or a refractory metal or the like is used also for the electrodes of capacitors using a ferroelectric material instead of a high-k material for the capacitance dielectric film.
More specifically, when the high-k material tantalum pentoxide is used for the capacitance dielectric film, then ruthenium (symbol of element: Ru), tungsten (symbol of element: W), molybdenum (symbol of element: Mo) or the like is used for the electrodes. Furthermore, when BST is used for the capacitance dielectric film, then Ru, ruthenium dioxide (composition formula: RuO2), platinum (symbol of element: Pt), iridium (symbol of element: Ir) or the like is used for the electrodes. Moreover, when a ferroelectric material such as SBT or PZT is used for the capacitance dielectric film, then Pt, Ir, iridium dioxide (composition formula: IrO2) or the like is used for the electrodes.
FIG. 3 shows the cross-sectional structure of a conventional capacitor using BST for the capacitance dielectric film.
As shown in FIG. 3, an interlayer dielectric film 52 is formed on a semiconductor substrate 51, on which a memory cell transistor (not shown in the figure) is formed. In the interlayer dielectric film 52, a plug 53 for connection with this memory cell transistor is formed. On the interlayer dielectric film 52 including the top of the plug 53, an adhesion layer 54 is formed, and a lower electrode 55 is formed on that adhesion layer 54. The adhesion layer 54 is made of titanium (symbol of element: Ti) or tantalum (symbol of element: Ta) or of an oxide or a nitride of these metals. On the lower electrode 55, a capacitance dielectric film 56 is formed so as to cover the top surface and the lateral surfaces of the lower electrode 55. On the capacitance dielectric film 56, an upper electrode 57 is formed. The three-layered structure of the lower electrode 55, the capacitance dielectric film 56 and the upper electrode 57 constitutes the capacitor. The lower electrode 55 and the upper electrode 57 are made of Pt. The capacitance dielectric film 56 is made of a BST film with a thickness of about 25 nm.
Here, the adhesion of the lower electrode 55 to the dielectric film is weak, so that if the lower electrode 55 is formed directly on the interlayer dielectric film 52, then there is the possibility that the lower electrode 55 separates from the interlayer dielectric film 52. To prevent this, the adhesion layer 54 made of a metal such as Ti or Ta or an oxide (e.g. TiOx, TaOx) or a nitride (e.g. TiNx, TaNx) of these metals is arranged between the lower electrode 55 and the interlayer dielectric film 52, thus improving the adhesion of the lower electrode 55 to the underlying dielectric film. It should be noted that recently, oxides and nitrides of, for example, titanium aluminum, tantalum silicon or tantalum aluminum have been used as the material for the adhesion layer 54.
However, such an adhesion layer oxidizes much easier than the electrodes made of a refractory metal or precious metal. Furthermore, depending on the thickness and the formation method of the adhesion layer, the metal atoms constituting the adhesion layer (referred to as xe2x80x9cadhesion layer metalxe2x80x9d in the following) may be diffused throughout the lower electrode and be deposited on the surface of the lower electrode. When in this situation a high-k film such as a BST film is formed as the capacitance dielectric film, then it is ordinarily formed in an oxidizing atmosphere of about 300 to 700xc2x0 C., so that the adhesion layer metal that has been deposited on the surface of the lower electrode is oxidized. As a result, a volume expansion occurs due to the oxidized layer formed on the surface of the lower electrode, so that an excess force is exerted on the capacitor portion or film separation occurs.
On the other hand, a method of suppressing the diffusion of the adhesion layer metal throughout the electrode and the deposition on the electrode surface is conceivable in which a sufficiently oxidized adhesion layer is formed first. However, the following problems occur if in the course of forming, for example, a titanium oxide (TiOx) film as the adhesion layer a Ti film is annealed in an oxidizing atmosphere to form the TiOx film, or the TiOx film is deposited while letting Ti react with oxygen in a vapor phase, or the TiOx film is deposited using a reactive sputtering process, by admixing oxygen when sputtering Ti.
In the case of annealing a Ti film in an oxidizing atmosphere, a temperature of at least 500xc2x0 C. is necessary, so that impurities included in the source and drain regions of the transistor already formed on the substrate are diffused again, so that the desired transistor properties cannot be obtained.
In the case of depositing the TiOx film while letting Ti react with oxygen, or in the case of depositing the TiOx film using a reactive sputtering process, the Ti may not be sufficiently oxidized, so that as mentioned above, Ti atoms are diffused throughout the Pt film (i.e. the Pt electrode) serving as the lower electrode, and deposited on the Pt electrode surface. Therefore, when a BST film, a Ta2O5 film or a PZT film with high relative dielectric constant are formed as the capacitance dielectric film on the Pt electrode, the Ti atoms deposited on the Pt electrode surface are oxidized, thereby forming a Ti oxide film with low relative dielectric constant on the Pt electrode surface. As a result, the formation of this Ti oxide film causes an excessive force to be exerted between the lower electrode and the capacitance dielectric film, and film separation occurs. Furthermore, because the relative dielectric constant of the Ti oxide film is low, it is in the end not possible to attain a capacitor with a large capacitance.
Furthermore, if the Ti atoms are deposited on the Pt electrode surface, the compositional balance between, for example, Ba, Sr and Ti in a BST film formed as the capacitance dielectric film on the Pt electrode is destroyed due to the influence of the Ti atoms deposited on the Pt electrode surface, so that the desired properties of the BST film cannot be attained. Furthermore, if Ti atoms are deposited on the Pt electrode surface, and if a ferroelectric material having a perovskite structure, such as PZT or SBT, is used for the capacitance dielectric film, then, as a result of the Ti atoms entering the perovskite structure, it may not be possible to attain the desired film properties required by a capacitance dielectric film using a ferroelectric material as described above.
In the stacked capacitor structure of the conventional example shown in FIG. 3, there are no particular problems if the film thickness of the adhesion layer is thick at 10 nm or more. On the other hand, the cup-type capacitor structure shown in FIG. 4A requires to making the lower electrode as well as the adhesion layer thinner.
FIG. 4A is a cross-sectional drawing of conventional cup-type capacitors, and
FIG. 4B is a graph showing the aspect ratio of the cups (recesses) in which the upper electrode is formed as a function of the thickness of the lower electrode (including the thickness of the adhesion layer).
As shown in FIG. 4A, an insulating layer 60 is provided with a recess 60a for capacitor formation, and a plug 61 is buried below the recess 60a in the dielectric film 60. A lower electrode 62 is formed on the bottom of the recess 60a, including the top surface of the plug 61, and the walls of the recess 60a, sandwiching an adhesion layer (not shown in the figure), thus yielding a recess 60b. Furthermore, a capacitance dielectric film 63 is formed on the dielectric film 60 including the recess 60b, yielding a recess 60c. On the capacitance dielectric film 63, an upper electrode 64 is formed, yielding a recess 60d. That is to say, the cup-type capacitor is constituted by a three-layered structure of the lower electrode 62, the capacitance dielectric film 63 and the upper electrode 64. Here, the lower electrode 62 and the upper electrode 64 are made of Pt films. Furthermore, the capacitance dielectric film 63 is made of a BST film of 25 nm thickness.
It should be noted that in FIG. 4A, the adhesion layer is drawn unitarily with the lower electrode 62. Furthermore, in FIG. 4A, xcex1 indicates the thickness of the lower electrode 62 including the thickness of the adhesion layer, xcex2 indicates the separation width between memory cells, and 2F indicates the array pitch between the cup-type capacitors.
As shown in FIG. 4B, as the thickness xcex1 of the lower electrode (including the adhesion layer) becomes large, the aspect ratio of the recesses (cups) 60c in which the upper electrode 64 is formed becomes extremely large, so that the formation of the upper electrode 64 becomes impossible in practice. As further shown in FIG. 4B, as the array pitch 2F (in FIG. 4B, 2F is indicated by its half value F) of the cup-type capacitor becomes small, the aspect ratio for the same value of xcex1 becomes large.
Consequently, in cup-type capacitors, if the thickness of the adhesion layer is not reduced together with the thickness of the lower electrode in order to miniaturize the memory cell, then it is difficult to bury the upper electrode inside the cup.
It should be noted that the results shown in FIG. 4B have been obtained by keeping the separation width xcex2 constant at 50 nm, and varying the half value F of the array pitch 2F over 0.10 xcexcm, 0.13 xcexcm and 0.15 xcexcm. Here, the half value F corresponds to a design rule for the transistor, and in current DRAMs, this design rule F for the transistor is such that memory cells are designed to have an area of 2Fxc3x974F=8F2. That is to say, the pitch of the capacitors is 2F on the shorter side and 4F on the longer side. Consequently, 4A shows the cross-sectional structure along the shorter side of a cup-type capacitor.
It is an object of the present invention to present a method for manufacturing a semiconductor device in which a sufficient adhesion can be maintained between a dielectric film and an electrode or wiring, in which a decrease of the dielectric constant when forming a capacitance element can be prevented, and in which the formation of an electrode can be performed easily when forming a capacitance element such as a cup-type capacitor having an electrode that is buried in a recess.
In order to achieve these objects, a first method for manufacturing a semiconductor device includes the steps of forming a metal layer on a dielectric film, forming an adhesion layer by subjecting the metal layer to an oxidation process using a liquid having oxidizing power, and forming an electrode or wiring on the adhesion layer.
With this first method for manufacturing a semiconductor device, first a metal layer is formed on a dielectric film, and then the metal layer is subjected to an oxidation process using a liquid acting as an oxidizing agent, so that the metal layer can be sufficiently oxidized, and an electrode or wiring can be formed on the thusly formed adhesion layer. Thus, the metal atoms in the adhesion layer do not diffuse throughout the electrode or wiring and are not deposited on the surface of the electrode or wiring, so that no oxide film will be formed on the surface of the electrode or wiring in the steps after the electrode or wiring has been formed. As a result, film separation caused by volume expansion of the oxide can be prevented, so that it is possible to form an adhesion layer with favorable adhesion. Consequently, a semiconductor device can be manufactured, in which a sufficient adhesion between the electrode or wiring and the dielectric film can be maintained.
Furthermore, when a capacitance element has been formed with this first method for manufacturing a semiconductor device, the metal atoms in the adhesion layer below the lower electrode are not deposited on the surface of the lower electrode, so that an oxide film with low relative dielectric constant is not formed on the surface of the lower electrode when forming a capacitance dielectric film made of a material with high dielectric constant on the lower electrode. Consequently, a decrease in the dielectric constant of the capacitance element, that is, a decrease of the capacitance can be prevented.
In the first method for manufacturing a semiconductor device, it is preferable that the metal layer is made of titanium.
In that case, an adhesion layer with favorable adhesion made of titanium oxide can be reliably formed by sufficiently oxidizing the titanium metal layer.
In the first method for manufacturing a semiconductor device, it is preferable that the thickness of the metal layer is at least 1 nm and at most 10 nm.
In that case, in addition to the previously described effects, a sufficient adhesion can be maintained while forming the adhesion layer by sufficiently oxidizing the metal layer, because the thickness of the metal layer has been set to at least 1 nm and at most 10 nm. Furthermore, since the thickness of the metal layer that is turned into the adhesion layer is at least 1 nm and at most 10 nm thin, if the first method for manufacturing a semiconductor device is applied to the formation of cup-type capacitors, an increase in the aspect ratio of the cups (recesses) in which the upper electrodes are buried can be prevented. Consequently, the upper electrodes can be formed easily.
In the first method for manufacturing a semiconductor device, it is preferable that the liquid having oxidizing power is water, hydrogen peroxide water or ozone water.
In that case, an adhesion layer with favorable adhesion can be reliably formed by sufficiently oxidizing the metal layer.
A second method for manufacturing a semiconductor device includes the steps of forming a metal layer on a dielectric film, forming an adhesion layer by subjecting the metal layer to an oxidation process using a liquid having oxidizing power, forming a first electrode on the adhesion layer, forming a capacitance dielectric film on the first electrode, and forming a second electrode on the capacitance dielectric film.
With this second method for manufacturing a semiconductor device, first a metal layer is formed on a dielectric film, and then the metal layer is subjected to an oxidation process using a liquid acting as an oxidizing agent, so that the metal layer can be sufficiently oxidized, and a first electrode (lower electrode of the capacitance element) can be formed on the thusly formed adhesion layer. Thus, the metal atoms in the adhesion layer do not diffuse throughout the lower electrode and are not deposited on the surface of the lower electrode, so that no oxide film will be formed on the surface of the lower electrode when forming a capacitance dielectric film made of a material with high relative dielectric constant on the lower electrode. As a result, film separation caused by volume expansion of the oxide can be prevented, so that it is possible to form an adhesion layer with favorable adhesion. Furthermore, since no oxide film with low relative dielectric constant is formed on the lower electrode surface, a decrease in the dielectric constant of the capacitance element, that is, a decrease in the capacitance can be prevented. Consequently, a semiconductor device can be manufactured that has a capacitance element, in which a sufficient adhesion between the lower electrode and the underlying dielectric film can be maintained, and in which a sufficient capacitance can be ensured.
In the second method for manufacturing a semiconductor device, it is preferable that the metal layer is made of titanium.
In that case, an adhesion layer with favorable adhesion made of titanium oxide can be reliably formed by sufficiently oxidizing the titanium metal layer.
In the second method for manufacturing a semiconductor device, it is preferable that the thickness of the metal layer is at least 1 nm and at most 10 nm.
In that case, in addition to the previously described effects, a sufficient adhesion can be maintained while forming the adhesion layer by sufficiently oxidizing the metal layer, because the thickness of the metal layer has been set to at least 1 nm and at most 10 nm. Furthermore, since the thickness of the metal layer that is turned into the adhesion layer is at least 1 nm and at most 10 nm thin, if the second method for manufacturing a semiconductor device is applied to the formation of cup-type capacitors, an increase in the aspect ratio of the cups (recesses) in which the upper electrodes are buried can be prevented. Consequently, the upper electrodes can be formed easily to the bottom of the cups, and the yield of the capacitors can be improved.
In the second method for manufacturing a semiconductor device, it is preferable that the liquid having oxidizing power is water, hydrogen peroxide water or ozone water.
In that case, an adhesion layer with favorable adhesion can be reliably formed by sufficiently oxidizing the metal layer.