Semiconductor device manufacturing: process – Having magnetic or ferroelectric component
Reexamination Certificate
1998-06-05
2002-10-29
Pham, Long (Department: 2823)
Semiconductor device manufacturing: process
Having magnetic or ferroelectric component
C438S239000, C438S240000
Reexamination Certificate
active
06472229
ABSTRACT:
DETAILED EXPLANATION OF THE INVENTION
1. Industrial Application Field
This invention pertains to a method for manufacturing ferroelectric capacitors (especially ferroelectric capacitors with a lead zirconate titanate (PZT) film) and a method for manufacturing ferroelectric memory devices (especially nonvolatile semiconductor memories using the ferroelectric capacitor with a PZT film).
2. Prior Art
By forming a capacitor using a film made of PZT, a ferroelectric material, as the dielectric film, it is possible to manufacture a nonvolatile memory element having a simple configuration and using the remnant polarization characteristics of the capacitor, that is, a ferroelectric RAM (ferroelectric random access memory) known as a nonvolatile memory called FRAM.
In the conventional FRAM, however, the so-called “polarization fatigue” phenomenon, which will significantly deteriorate the polarization characteristics, tends to occur because the inversion (that is, the polarization inversion performed when data are written or read) is repeated during the operation of the PZT capacitor Cap. This phenomenon causes problems in developing practical devices. When the inversion is repeated about 1,000,000 times, the polarization intensity is reduced to about half of the original value or even less. Consequently, it has been difficult to develop devices with high reliability for repeated reading or writing operation.
Progress for Achieving this Invention
In Japanese Patent Application No. Hei 8[1996]-181358, the present inventors have already proposed a novel effective method (referred to as the previous invention hereinafter) that can alleviate the aforementioned polarization fatigue under two conditions. According to the previous invention, the first condition is to use and crystallize a PZT feed solution (used when forming a film by the sol-gel method) containing excess lead at a temperature higher than 650° C. The second condition is to use electrodes made of Ir, Ru, or other oxidizing metals.
The aforementioned first condition will be explained as follows. When amorphous PZT is deposited on a lower electrode made of Ir and is heated to a temperature higher than the crystallization temperature, crystallization is carried out in the thickness direction. A columnar grain structure is formed during the crystallization process, and the excess lead is pressed out to the surface to form a structural transition layer mainly composed of the Pb on the surface of the PZT layer.
In this case, the composition of a preferred PZT precursor solution is such that Pb=1.02-1.50 (with respect to Zr+Ti=1.0) (the ratio of Ti/Zr can be selected at will). If the Pb concentration is too low, it is difficult to form the aforementioned columnar structure (control of the PZT crystallization direction). On the other hand, if the Pb concentration is too high, the aforementioned structural transition layer becomes too thick to be removed by heat treatment.
When the sintering temperature in the heat treatment is too low (600° C.), it is easy to form the aforementioned structural transition layer mainly composed of Pb. On the other hand, the structural transition layer will disappear if the sintering temperature is controlled to 650° C. or higher. However, if the sintering temperature is too high, it is difficult to form the PZT crystal. Consequently, the sintering temperature should be maintained at 750° C. or lower.
As far as the sintering (annealing) temperature for PZT is concerned, for the PZT formed at 600° C., the remnant polarization intensity is almost reduced to 0 after 100,000,000 polarization inversions. On the other hand, the polarization characteristics of the capacitor formed at 650° C. are improved significantly. Also, the remnant polarization intensity of a sample formed at a temperature in the range of 650-700° C. is barely reduced after 100,000,000 inversions. The reason for the improvement in the fatigue characteristic property is that the structural transition layer made of the excess pB on the surface disappears at a temperature of 650° C. or higher.
In the following, the electrode material as the aforementioned second condition will be explained. Generally, Pt or another substance that cannot be oxidized is used for the electrodes of the PZT capacitor (in some cases, however, Au is used for the upper electrode alone). The previous invention has succeeded in alleviating the polarization fatigue by using metal Ir for both the lower and upper electrodes. For example, the polarization characteristics of acapacitor using Pt for its electrodes deteriorate significantly when the inversion is repeated more than 2×10
8
times. On the other hand, when Ir is used for the electrodes, deterioration in the polarization characteristics does not occur until the inversion is repeated 2×10
9
times.
As described above, the polarization fatigue characteristic property has a strong dependency on the material of the electrodes. Compared with other types of capacitors, the capacitor with both upper and lower electrodes made of Ir has a very stable remnant polarization intensity (Pr) during the polarization inversion. It is believed that the reason for the stable remnant polarization intensity is due to the oxidizability of the Ir metal.
Problems to be Solved by the Invention
While the previous invention has the aforementioned desirable features, there is still a problem remaining to be solved. That is, in a manufacturing process that satisfies the aforementioned conditions, it is difficult to keep the temperature low because the temperature (sintering temperature) for forming the PZT has to be kept at the level of 650° C. or higher.
When the temperature is high, the element region and wiring on the semiconductor substrate will be affected by the generated heat. For example, a change in the concentration of the impurities or damage of Al wiring might occur.
The purpose of this invention is to provide a method that can take advantage of the features of the previous invention and can also be used to manufacture capacitors free of polarization fatigue, even when the treatment is carried out at a low temperature.
Means to Solve the Problems
This invention provides a method for manufacturing a ferroelectric capacitor characterized by the fact that the process for forming the ferroelectric capacitor by sequentially laminating a ferroelectric film and a first electrode on a second electrode (for example, the process for forming the ferroelectric capacitor by sequentially laminating a lead zirconate titanate layer and an upper electrode made of iridium, iridium oxide, rubidium, rubidium oxide, platinum, or palladium on a lower electrode made of iridium, iridium oxide, rubidium, rubidium oxide, platinum, or palladium) has the following steps:
a step in which a ferroelectric material layer containing an excess amount of the specific constituent element of the aforementioned ferroelectric film is formed on the aforementioned first electrode (for example, a step in which an amorphous layer of lead zirconate titanate containing excess lead is formed on the aforementioned lower electrode);
a step in which the aforementioned ferroelectric material is crystallized by a heat treatment to form the aforementioned ferroelectric film (for example, a step in which the aforementioned amorphous layer is crystallized by a heat treatment to form a crystal layer of lead zirconate titanate);
a step in which the surface deposit mainly composed of the aforementioned specific constituent element and generated on the surface of the aforementioned ferroelectric film during the crystallization process is removed (for example, a step in which the surface deposit mainly made of the excess lead and generated on the surface of the aforementioned crystal layer during the crystallization process is removed by means of etching or dissolution); and
a step in which the aforementioned upper electrode is formed on the aforementioned crystal layer exposed after the aforementioned surface deposit is removed.
This invention als
Aoki Katsuhiro
Fukuda Yukio
Numata Ken
Brady W. James
Brairton Scott
Kempler William B.
Pham Long
Telecky , Jr. Frederick J.
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