JP3874521B2 - Method for forming ferroelectric memory electrode - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
This invention relates to a method for forming electrodes for use in a ferroelectric memory.
[0002]
[Prior art]
In recent years, the density of semiconductor memories has been increased, and recently, those using ferroelectric thin films as shown in the literature (Ceramics, Vol. 30 (1995), No. 6, pp 499-507). It attracts attention. For example, as such a semiconductor memory, there has been conventionally used an FET (Field Effect Transistor) element having an MFS (Metal-Ferroelectric-Semiconductor) structure. This element has a structure in which a ferroelectric thin film as an insulating film and an upper electrode as a gate electrode are sequentially stacked in a channel region of a normal FET. A memory cell using this element inverts the polarization of the ferroelectric thin film by applying a voltage between the gate electrode and the semiconductor substrate, that is, the ferroelectric thin film. The polarization induces electrons or holes in the channel region of the transistor to change the threshold voltage of the transistor. The stored information is identified by the magnitude of the drain current value at this time. The feature of this memory cell is that the cell size can be made smaller than one transistor and one capacitor (1Tr1Cp), and that information written when information is read can be read without being destroyed.
[0003]
However, in manufacturing this MFS-FET, it is very difficult to form a ferroelectric film having good characteristics on Si, which is a semiconductor, in consideration of lattice constant mismatch and thermal expansion coefficient matching. Further, when an oxide ferroelectric is directly formed on Si, an unnecessary film such as SiO ₂ is formed at the interface between the ferroelectric and Si. When such a film is formed, not only the operating voltage of the FET is increased, but there is a possibility that the polarization history characteristic of the ferroelectric film is deteriorated.
[0004]
Therefore, MFMIS-FET (Metal-Ferroelectric-Metal-Insulator-Semiconductor) with a structure in which a semiconductor substrate, gate oxide film, floating gate (lower electrode), ferroelectric, and control gate (upper electrode) are stacked in order from the bottom. Has been developed. In this FET, it is possible to select a metal electrode such as Pt, on which the ferroelectric is easily grown, as a base material for forming the ferroelectric. Furthermore, the layer structure below the ferroelectric layer can use the conventional FET structure and constituent materials as they are.
[0005]
[Problems to be solved by the invention]
However, even in the MFMIF-FET, when the polarization is reversed by repeatedly applying a voltage to the ferroelectric thin film, film fatigue occurs.
[0006]
For example, a Pt electrode normally used as the lower electrode is an aggregate of Pt columnar crystal particles. For this reason, there are grain boundaries of Pt particles in the Pt electrode, and lead (Pb) contained in the ferroelectric thin film provided on the Pt electrode is diffused along the grain boundary. Yes. As a result, oxygen deficiency occurs in the ferroelectric film and space charge is generated. It is considered that one of the causes of film fatigue is that the charge due to remnant polarization is canceled by this space charge.
[0007]
Therefore, as the lower electrode, an oxide electrode such as an iridium oxide (IrO ₂ ) electrode, for example, prevents diffusion of the constituent components of the ferroelectric substance because the oxide constituting the electrode is not a columnar crystal like Pt. It is considered possible.
[0008]
However, it is preferable to use platinum as the electrode in consideration of chemical stability and heat resistance.
[0009]
Bi-layered compounds are attracting attention as materials for ferroelectric thin films having fatigue resistance, and active research is being conducted particularly on SrBi ₂ Ta ₂ O ₉ -based substance groups.
[0010]
However, if a ferroelectric thin film made of this Bi layered compound is formed on the Pt electrode, Bi may diffuse along the Pt grain boundary and reach the semiconductor region located under the Pt electrode. This causes a problem that the characteristics of the semiconductor change and cannot be used as a device.
[0011]
Accordingly, there has been a demand for the appearance of an electrode containing platinum, which is preferable as an electrode material, and capable of suppressing diffusion of the constituent components of the ferroelectric and a method for manufacturing the electrode.
[0019]
[Means for Solving the Problems]
For this reason , the formation of the ferroelectric memory electrode preferably includes a step of forming the lower electrode by sputtering in an oxygen-containing atmosphere using a platinum target and a metal target other than platinum. Is good.
[0020]
Sputtering atoms jumping out of the target by performing sputtering in an oxygen-containing atmosphere using platinum and a metal that becomes a metal oxide that is oxidized and deposited on the grain boundaries of the platinum particles cause platinum and oxidized metal Including. The lower electrode formed in this manner is composed of platinum crystal particles and a metal oxide filling the grain boundaries between the particles. This electrode is an electrode having the characteristics as a platinum electrode as it is. Therefore, this lower electrode is excellent in chemical stability and heat resistance. Further, since the high crystallinity of platinum is maintained in this electrode, a ferroelectric film having high crystallinity can be formed by providing a ferroelectric film on this electrode. In addition, since the metal oxide is precipitated at the grain boundaries of the platinum particles, there is no possibility that the constituent components of the ferroelectric film diffuse along the grain boundaries of the platinum particles.
After the sputtering is completed and before the ferroelectric film is formed, the lower electrode is heated in an oxygen atmosphere.
As a result, the metal oxidation, which may be insufficient only by sputtering performed in an oxygen-containing atmosphere, can be sufficiently performed to such an extent that no impediment is caused in practice. Therefore, when the ferroelectric film is formed on the lower electrode, there is no possibility that oxygen deficiency occurs in the ferroelectric film even if heat treatment is performed. Further, diffusion of components constituting the ferroelectric from the ferroelectric film can be prevented by the metal oxide precipitated at the grain boundaries of the platinum particles.
[0021]
The area of the platinum target is preferably larger than the area of the metal target. The area of the metal target is made smaller than the area of the platinum target to the extent that the crystallinity of platinum in the electrode is not deteriorated. Moreover, this metal should just be the quantity which can fill the grain boundary of a platinum particle.
[0024]
Metals other than platinum include aluminum, yttrium, lanthanum, cerium, neodymium, gadolinium, terbium, dyspronium, ytterbium, titanium, zirconium, hafnium, vanadium, niobium, tantalum, chromium, molybdenum, tungsten, manganese It is preferably a metal selected from the group consisting of iron, cobalt, nickel, copper and zinc.
[0025]
These metals are easy to oxidize, chemically stable and easy to handle, and are suitable for sputtering.
[0026]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, an embodiment of the present invention will be described with reference to the drawings. Each figure is only schematically shown to the extent that the invention can be understood, and therefore the invention is not limited to the illustrated examples.
[0027]
The materials and numerical conditions used in this embodiment are only examples within the scope of the present invention, and therefore the invention is not limited to these conditions.
[0028]
<First Embodiment>
As a first embodiment of the present invention, a sample in which the ferroelectric memory electrode of the present invention is provided on a base will be described with reference to FIGS. 1, 2 and 3. FIG. FIG. 1 is a sectional view schematically showing the structure of a ferroelectric memory electrode according to the present invention. FIG. 2A is a plan view of a surface on which a sputtering gas collides with a target used when sputtering is performed in this embodiment. FIG. 2B is a configuration diagram schematically showing a main part of a sputtering apparatus using the target shown in FIG. FIG. 3 is a structural cross-sectional view of a sample used in this embodiment.
[0029]
First, a sample is formed. The base 10 of this sample is a base excellent in adhesion to the electrode and heat resistance. Here, the polysilicon layer 13 and the TiN barrier layer 15 are sequentially laminated on the silicon substrate 11 by a spin coater. These three layers (silicon substrate 11, polysilicon layer 13, and TiN barrier layer 15) are used as the base 10 (see FIG. 3).
[0030]
Next, a ferroelectric memory electrode 17 is formed on the base 10.
[0031]
In this embodiment, an electrode is formed on a base by sputtering in an oxygen-containing atmosphere using a platinum target and a metal target other than platinum.
[0032]
Here, aluminum (Al) is used as a metal target other than platinum. First, prepare a target. On a 6-inch platinum target 21 (about 1 inch is about 2.54 cm), 17 sheets of Al 23 having a thickness of 1 mm square and a thickness of 2 mm are arranged at equal intervals (FIG. 2A). )). The Al sheet 23 has a purity of 99.99% or more. The area of one Al sheet 23 is about 5% of the area of the platinum target 21. The amount of Al is sufficient as long as the aluminum oxide can fill the grain boundaries of the platinum crystal particles, so a small amount is sufficient. In order not to deteriorate the crystallinity of platinum, the area of one Al sheet 23 is preferably about 5% of platinum as described above. Further, the Al sheet 23 only needs to be placed on the platinum target 21. In this manner, a target in which the Al sheet 23 is arranged on the platinum target 21 is used.
[0033]
Next, sputtering is performed in an oxygen-containing atmosphere.
[0034]
The target 20 and the base 10 described above are aligned and installed in the sputtering apparatus 30. Sputtering is performed using an argon sputtering gas containing 10% by volume of oxygen. At this time, the gas pressure of the sputtering gas is 7 mTorr, the sputtering power is 500 kW, and the substrate temperature is 300.degree. After performing pre-sputtering for 2 minutes, the shutter 31 is opened and sputtering is performed for 3 minutes, whereby a platinum electrode 17 containing Al having a thickness of 200 nm is formed on the base 10 (FIG. 2B and FIG. 3).
[0035]
The cross-sectional structure of the electrode 17 obtained in this way is schematically shown in FIG. Such a structure can be confirmed by XPS (X-ray photoelectron spectroscopy) or SEM (scanning electron microscope).
[0036]
As a result, in the electrode 17 formed on the base 10, the platinum columnar crystal particles 41 form an aggregate without deterioration, and the crystal boundaries 41 of the crystal particles 41 are oxidized by the metal oxide 45. Aluminum is deposited (FIG. 1).
[0037]
Next, a ferroelectric thin film 19 is formed on the electrode 17.
[0038]
In this embodiment, for example, a coating film of SrBi ₂ Ta ₂ O ₉ is formed on the electrode 17 by a spin coater. Then, after drying this coating film and evaporating the solvent in the coating film, the coating film is temporarily fired to burn organic functional groups. Thereafter, main firing is performed to form a SrBi ₂ Ta ₂ O ₉ ferroelectric thin film 19 (FIG. 3).
[0039]
As a result, the lower electrode 17 of the ferroelectric film 19 is composed of the platinum columnar crystal particles 41 and the aluminum oxide 45 deposited at the grain boundaries 43 of the particles. From this, it is possible to prevent the constituent components such as Bi from diffusing along the grain boundaries 43 of the platinum particles 41 of the electrode 17. For this reason, deterioration of the device can be prevented. One cause of film fatigue of the ferroelectric film 19 can be solved. In addition, since the platinum crystallinity of the electrode 17 on which the ferroelectric film 19 is formed is maintained, the ferroelectric thin film 19 can also be formed with good crystallinity.
[0040]
Further, even when, for example, lead zirconate titanate, which is a material other than the Bi layered compound such as SrBi ₂ Ta ₂ O ₉ , is used as the material of the ferroelectric film 19, the film fatigue of the formed film 19 is improved. be able to.
[0041]
Moreover, as the metal to be the metal oxide 45 deposited on the grain boundaries 43 of the platinum particles 41, a material that is easily oxidized, chemically stable, inexpensive and easy to handle is preferable. Therefore, in addition to Al, a metal belonging to the 3 (IIIA) genus, particularly a metal selected from the metal group consisting of yttrium, lanthanum, cerium, neodymium, gadolinium, terbium, dyspronium and ytterbium, and 4 ( A metal belonging to the genus IVA), particularly selected from the metal group consisting of titanium, zirconium and hafnium, and a metal belonging to the genus 5 (VA), particularly among the metal group consisting of vanadium, niobium and tantalum. Selected from the group consisting of metals belonging to the genus 6 (VIA), particularly selected from the group consisting of chromium, molybdenum and tungsten, and manganese, iron, cobalt, nickel, copper, zinc it can.
[0042]
Even when these metals are used, the ferroelectric memory electrode can be formed in the same manner as described above.
[0043]
<Second Embodiment>
The sample of the second embodiment will be described. In this sample, an electrode for a ferroelectric memory is provided on a base, and these are manufactured almost in the same manner as in the first embodiment.
[0044]
Hereinafter, differences from the first embodiment will be described, and detailed description of points similar to those of the first embodiment will be omitted.
[0045]
First, as in the first embodiment, a polysilicon layer and a TiN barrier layer are sequentially stacked on a silicon substrate to form a base.
[0046]
Next, similarly to the first embodiment, Ar ions in Ar gas containing 10% by volume of oxygen are collided with a target in which an Al sheet is disposed on a platinum target. Thus, a film obtained by depositing sputtered atoms jumping out of the target on the base is used as an electrode. This electrode is composed of platinum columnar crystal particles and aluminum oxide precipitated at the grain boundaries.
[0047]
Here, in this embodiment, after the electrodes are formed, the structure including at least the base and the electrodes on the base is heat-treated in an oxygen atmosphere at a temperature of 500 ° C. for 30 minutes.
[0048]
As a result, in the electrode obtained by sputtering with a sputtering gas containing 10% oxygen, the risk of insufficient oxidation of aluminum can be avoided, and the aluminum deposited at the grain boundaries of the platinum crystal particles can be avoided. Oxidation can be performed sufficiently. Therefore, during the subsequent process of forming the ferroelectric film on the electrode, unoxidized aluminum exists in the electrode, and oxygen is extracted from the ferroelectric film to oxidize the unoxidized aluminum. , Oxygen deficiency can be prevented from occurring in the film. Therefore, the film fatigue of the ferroelectric film can be improved.
[0049]
As a result, the same effect as in the first embodiment can be obtained. Further, when the electrode formed in this embodiment is used as an electrode for a ferroelectric memory and a ferroelectric film is formed on this electrode, the film fatigue characteristics of the ferroelectric film can be further improved. Can be planned.
[0050]
In this embodiment as well, a metal belonging to the 3 (IIIA) group as a metal oxide to be deposited on the grain boundary of platinum particles of the electrode, particularly yttrium, lanthanum, cerium, neodymium, gadolinium, terbium, dyspronium. And a metal selected from the metal group consisting of ytterbium and a metal belonging to the 4 (IVA) genus, particularly a metal selected from the metal group consisting of titanium, zirconium and hafnium, and a 5 (VA) genus. Metals belonging to the group consisting of vanadium, niobium and tantalum, particularly metals selected from the group consisting of 6 (VIA) metals, especially consisting of chromium, molybdenum and tungsten Alternatively, any of manganese, iron, cobalt, nickel, copper, and zinc can be used.
[0051]
【The invention's effect】
As is clear from the above description, the ferroelectric memory electrode of the present invention comprises platinum crystal particles and metal oxides precipitated at the grain boundaries of the platinum crystal particles.
[0052]
For this reason, when this ferroelectric memory electrode is used as a lower electrode of a ferroelectric memory in which a lower electrode, a ferroelectric film and an upper electrode are sequentially provided on a base including a semiconductor substrate, a platinum electrode Therefore, this electrode is chemically stable and excellent in heat resistance. In addition, since the grain boundaries of the platinum crystal particles are filled with metal oxide, the ferroelectric components diffuse along the grain boundaries of the platinum crystals from the ferroelectric film provided on this electrode. Can be prevented. For this reason, generation | occurrence | production of the oxygen deficiency in a ferroelectric film accompanying the spreading | diffusion of lead can be suppressed. Therefore, improvement of fatigue of the ferroelectric film is expected. Further, since the diffusion of the ferroelectric thin film constituent elements that reach the semiconductor region from the ferroelectric film through the lower electrode can be suppressed, there is no possibility of changing the characteristics of the semiconductor. Therefore, deterioration of the device due to this can be prevented.
[Brief description of the drawings]
FIG. 1 is a schematic cross-sectional view showing the structure of a ferroelectric memory electrode according to a first embodiment.
2A is a schematic configuration diagram of a target used for sputtering, and FIG. 2B is a schematic diagram of a main part of a sputtering apparatus that performs sputtering.
FIG. 3 is a schematic cross-sectional view of a sample formed in the first embodiment.
[Explanation of symbols]
10: base 11: silicon substrate 13: polysilicon layer 15: TiN barrier layer 17: (for ferroelectric memory) electrode 19: ferroelectric (thin) film 20: target 21: platinum target 23: sheet-like Al, Al sheet 30: Sputtering device 31: Shutter
41: (platinum columnar) crystal particle 43: grain boundary 45: metal oxide (aluminum oxide)

Claims (4)

In forming the lower electrode of the ferroelectric memory comprising the lower electrode, the ferroelectric film, and the upper electrode sequentially on the base including the semiconductor substrate ,
Using a metal target other than platinum target and platinum, by sputtering in an oxygen-containing atmosphere, the platinum crystal grains, are constituted with a metal oxide which is deposited at the grain boundaries of platinum crystal grains the Forming a lower electrode;
An electrode for a ferroelectric memory, comprising: a step of heat-treating the lower electrode in an oxygen atmosphere after the sputtering is completed and before the ferroelectric film is formed. Forming method. The method for forming an electrode for a ferroelectric memory according to claim 1 ,
A method for forming an electrode for a ferroelectric memory, wherein an area of the platinum target is larger than an area of the metal target. The method for forming an electrode for a ferroelectric memory according to claim 1 ,
The metal constituting the metal target is aluminum, yttrium, lanthanum, cerium, neodymium, gadolinium, terbium, dyspronium, ytterbium, titanium, zirconium, hafnium, vanadium, niobium, tantalum, chromium, molybdenum, tungsten, manganese, iron, cobalt A method for forming an electrode for a ferroelectric memory, which is a metal selected from the group consisting of nickel, copper and zinc. The method for forming an electrode for a ferroelectric memory according to claim 1,
  The metal constituting the metal target is one of manganese and nickel
And a method of forming an electrode for a ferroelectric memory.

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