Semiconductor device manufacturing: process – Having magnetic or ferroelectric component
Reexamination Certificate
2003-01-09
2004-09-14
Niebling, John F. (Department: 2812)
Semiconductor device manufacturing: process
Having magnetic or ferroelectric component
C438S295000
Reexamination Certificate
active
06790677
ABSTRACT:
CROSS-REFERENCE TO RELATED APPLICATION
The present application is based on Japanese priority application No.2002-056146 filed on Mar. 1, 2002, the entire contents of which are hereby incorporated by reference.
BACKGROUND OF THE INVENTION
The present invention generally relates to semiconductor devices and more particularly to the method of forming a ferroelectric film and fabrication process of a semiconductor device having a ferroelectric film.
A ferroelectric random access memory (FRAM) is a semiconductor device that has a ferroelectric capacitor. A ferroelectric capacitor is a capacitor having a ferroelectric film sandwiched by a pair of capacitor electrodes, and a ferroelectric random access memory stores information in the ferroelectric capacitor insulation film in the form of spontaneous polarization. Thus, an ferroelectric random access memory functions as a non-volatile memory device.
In view of the fact that reversal of spontaneous polarization can be caused in a ferroelectric capacitor insulation film by mere application of a reversing electric field thereto without injection of a current, a ferroelectric random access memory can be written with information (reversal of spontaneous polarization) with minimum electric power. Further, such reversal of the spontaneous polarization can be conducted with very high speed.
Typically, a ferroelectric capacitor for use in a ferroelectric random access memory uses a layer of a perovskite ferroelectric material such as PZT (Pb(Zr,Ti)O
3
), SBT(SrBi
2
Ta
2
O
9
), and the like, for the ferroelectric capacitor insulation film, and the ferroelectric capacitor insulation film is formed on a lower electrode formed of a material such as Pt (platinum). The lower electrode, in turn, is connected to a conductive region of a transistor by way of a conductive plug such as W(tungsten) provided in an interlayer insulation film.
In order that the ferroelectric film undergoes the desired reversal of polarization with minimum drive voltage, it is necessary that the crystal grains in the ferroelectric film are aligned in a predetermined crystal orientation. Typically, a crystal orientation of <111> or <001> is used in the case of a PZT film, wherein the crystal orientation of <001> is most preferable in view of the maximum electric performance of the film.
On the other hand, there is a possibility in such a conventional construction that the tungsten plug or platinum lower electrode undergoes oxidation at the time of formation of the ferroelectric capacitor insulation film because of the oxidizing ambient that is used at the time of formation of the ferroelectric film.
In view of the problems noted above, the inventor of the present invention has proposed the use of Ir(iridium) for the material of the electrode of the ferroelectric capacitor used in a ferroelectric random access memory. In the case of using iridium, the problem of increase of resistance at the lower capacitor electrode caused by oxidation of the lower capacitor electrode is minimized because of the fact that iridium oxide.
When using iridium for the lower electrode film, it is preferable to form the ferroelectric film by way of an MOCVD process in view of the controllability of the ferroelectric composition and in view of feasibility of mass production.
At the time of forming a ferroelectric film by way of an MOCVD process, it is necessary to cause a decomposition of metal organic source materials of the ferroelectric film in a reactor such that deposition of a ferroelectric film takes place on the iridium substrate, wherein it should be noted that such a decomposition process requires oxygen.
On the other hand, oxygen thus introduced into a reactor, while causing the necessary decomposition of the metal organic source materials of the ferroelectric film, causes also an unwanted oxidizing reaction on the surface of the iridium film, on which the deposition of the ferroelectric film takes place. As a result of such an oxidizing reaction, the surface of the iridium lower electrode is converted to iridium oxide (IrO
2
).
When the surface of the iridium lower electrode is completely oxidized and there is formed a film of iridium oxide covering the surface of the Ir electrode, the crystal grains of the ferroelectric film thus grown on the iridium oxide film take various crystal orientations, and the desired <001> orientation is no longer obtained. Thereby, problems such as increase of drive voltage at the time of writing or erasing of information are caused associated with the degradation of the electric performance of the ferroelectric film, which in turn is caused by the uncontrolled alignment of orientation of the crystal grains in the ferroelectric film.
Further, it has been difficult to control the progress of the oxidization reaction of the iridium lower electrode and it has been difficult to control the timing of ferroelectric film formation such that the formation of the ferroelectric film is started before there is caused substantial oxidation in the iridium lower electrode.
SUMMARY OF THE INVENTION
Accordingly, it is a general object of the present invention to provide a novel and useful method of forming a ferroelectric film and fabrication process of a semiconductor device having a ferroelectric film wherein the foregoing problems are eliminated.
Another object of the present invention is to provide a method of forming a ferroelectric film while controlling the atmosphere of the ferroelectric film formation at a critical point of oxidation-reduction reaction.
Another object of the present invention is to provide a method of forming a ferroelectric film, comprising the steps of:
forming a layer by a material that takes a metal state in a reducing ambient and an oxide state in an oxidizing ambient; and
depositing a ferroelectric film on a surface of said layer by supplying gaseous sources of said ferroelectric film and an oxygen gas and causing a decomposition of said gaseous sources at said surface of said layer,
said step of depositing said ferroelectric film being started with a preparation step in which the state of said surface of said layer is controlled substantially to a critical point in which the state of said layer changes from said metal state to said oxide state and from said oxide state to said metal state.
Another object of the present invention is to provide a method of fabricating a semiconductor device, comprising the steps of:
forming an active element on a substrate;
forming a lower electrode over said substrate in electrical connection with said active element, said step of forming said lower electrode being conducted by using a material that takes a metal state in a reducing ambient and an oxide state in an oxidizing ambient for said lower electrode;
depositing a ferroelectric film on a surface of said lower electrode by supplying gaseous sources of said ferroelectric film and an oxygen gas and by causing a decomposition of said gaseous sources at said surface of said lower electrode; and
depositing an upper electrode on said ferroelectric film,
said step of depositing said ferroelectric film being started with a preparation step in which the state of said surface of said lower electrode is controlled substantially to a critical point in which a state of said lower electrode changes from said metal state to said oxide state and from said oxide state to said metal state.
According to the present invention, the surface of the layer is only partially covered by an oxide film such that the crystal structure of the material forming the layer is exposed at the time of starting the deposition of the ferroelectric film as a result of the preparation step. Alternatively, the surface of the layer is covered with an extremely thin oxide film such as the one having the thickness of several atomic layers of oxygen or less, at the time of starting the deposition of the ferroelectric film by a CVD process. As a result, the ferroelectric film deposited on the layer inherits the crystal orientation of the layer in the metallic state even when t
Kennedy Jennifer M.
Niebling John F.
Westerman Hattori Daniels & Adrian LLP
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