Semiconductor device manufacturing: process – Coating of substrate containing semiconductor region or of... – Multiple layers
Reexamination Certificate
2002-08-01
2004-10-26
Smith, Matthew (Department: 2825)
Semiconductor device manufacturing: process
Coating of substrate containing semiconductor region or of...
Multiple layers
C438S763000
Reexamination Certificate
active
06809040
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method for manufacturing laminated dielectrics used as materials for electronic components such as piezoelectric actuators and the like and, more particularly, to a method for manufacturing laminated dielectrics consisting of a laminated body of oxide dielectric layers such as PZT etc. and inner electrode layers containing a base metal as a main constituent.
2. Description of the Related Art
Laminated dielectrics utilized in electronic components such as piezoelectric actuators, and the like, are formed generally by alternately laminating green sheets of oxide dielectric material such as PZT (lead zirconate titanate) and metal layers to be used as inner electrodes, and by integrally sintering the laminated body as one unit. Conventionally, noble metals such as Pd, Pt, Pd—Ag, etc. are widely used as materials for the inner electrodes. However, noble metals are very expensive and increase the manufacturing cost.
Thus, in recent years, attention has been increasingly paid to base metals (CU, N
1
, etc.) as the material for the inner electrodes in laminated dielectrics. Although, besides being inexpensive, base metals have other advantages such as good migration resistance, there are also problems associated with the fact that base metals tend to be easily oxidized in an oxidizing atmosphere at high temperatures. Therefore, in sintering a laminated body that employs a base metal as inner electrodes, it is necessary to bake the laminated body in a neutral or reducing atmosphere in order to avoid deterioration of the function of electrodes.
As the neutral or reducing atmosphere, a mixed gas of N
2
, H
2
, CO, CO
2
, H
2
O, etc. has been typically used. In order to facilitate the adjustment of the oxygen concentration in the atmosphere, Japanese Patent Publication (Kokai) No. 05-335177, for example, discloses a method in which a mixed gas having CO
2
or CO gas of high purity as a main component and containing H
2
and O
2
gases optionally adjustable in concentration, is employed. With this method, the concentration of oxygen can be quickly adjusted to a target value by varying the supply of H
2
and O
2
gases.
When, however, compounds containing PbO are used in the composition as in oxide dielectrics such as PET, if H
2
is contained in the atmosphere gas during sintering, reduction of PbO takes place at temperatures of 300° C. and higher, and the resulting Pb may diffuse, leading to the problem of fluctuations in the composition. Further, when the oxide dielectric contains alkaline earth metals in the composition, if H
2
O is contained in the atmosphere, the alkaline earth metals may react with H
2
O and lead to expansion of the laminated body (room temperature ~400° C.), allowing only small-sized laminated dielectrics to be fabricated.
SUMMARY OF THE INVENTION
It is an object of the present invention to resolve the above described problems and to provide a method, for manufacturing large-sized laminated dielectrics of high quality, which permits adjustment of the atmosphere during sintering process to be accomplished easily, and which can avoid the problems such as fluctuations of the composition of oxide dielectrics and expansion due to reaction with the atmosphere gas.
According to a first aspect of the invention, a method for manufacturing laminated dielectrics is provided wherein a laminated body consisting of oxide dielectric layers and inner electrode layers containing base metals as the main component is sintered in a reducing atmosphere characterized in that the above mentioned sintering step is performed in a mixed gas having CO
2
gas as the main constituent and optional amount of CO gas and O
2
gas, and that the oxygen partial pressure in the atmosphere is adjusted within a predetermined range by adjusting the amount of the above mentioned CO gas and O
2
gas.
According to the present invention, sintering of laminated dielectrics is performed in a mixed gas having CO
2
gas as a main constituent, and further containing CO gas and O
2
gas of arbitrarily adjustable concentrations in order to control the oxygen partial pressure in the atmosphere gas at the target oxygen partial pressure. Thus, if the atmosphere in the oven deviates from this target value, amount of supply of CO gas or O
2
gas can be increased or decreased so as to quickly adjust the oxygen partial pressure to within a predetermined range. Problems with the conventional method in which H
2
gas and H
2
O are contained in the atmosphere, such as the fluctuation of the composition of the dielectrics or the expansion of the dielectrics, do not arise with the method of the present invention.
The atmosphere in the oven is maintained in an equilibrium of CO
2
gas, CO gas and O
2
gas determined by the equation CO
2
+CO⇄2CO+(1/2)O
2
. By adjusting the amount of CO gas and O
2
gas utilizing this equilibrium system, the oxygen partial pressure in the atmosphere can be easily and stably controlled, and the variation and fluctuation of the atmosphere can be suppressed. Therefore, the atmosphere in the oven can be easily adjusted during a sintering process, and large-sized laminated dielectrics having good appearance and high quality can be obtained.
According to a second aspect of the invention, during the above mentioned sintering step, it is desirable to maintain the ratio of gas components in the mixed gas supplied to the oven preferably in the range of CO
2
:CO:O
2
=5000:500~0:20~0, and to maintain the oxygen partial pressure in the atmosphere gas within a proper range so that melting of the base metal electrodes and diffusion of the electrode into the composition can be suppressed.
More specifically, according to a third aspect of the invention, during the above mentioned sintering step, it is desirable to adjust the oxygen partial pressure of the atmosphere gas in the range, depending upon the sintering temperature, such that the reduction of the oxide composing the above mentioned oxide dielectric layers does not occur and the electrical conductivity of the base metal composing the above mentioned inner electrode layers is not impaired. Since the range of oxygen partial pressure in which oxidative diffusion of the base metal electrodes can be suppressed and fluctuation of the composition of the dielectric component can be avoided varies with the sintering temperature, the atmosphere should be adjusted to achieve the optimum oxygen partial pressure depending upon the sintering temperature in order to obtain the laminated dielectrics of better quality and higher reliability.
According to a fourth aspect of the invention, preferably, a reduction processing step in which the above mentioned laminated body is subjected to heat treatment in a reducing atmosphere is provided for performing reduction processing on the inner electrode layers prior to the above mentioned sintering step. During this reduction processing step, the oxygen partial pressure of the atmosphere gas (oxygen partial pressure outside the oven) is preferably maintained in the range of 1×10
−14
~1×10
−25
atm so that metallizing process can be accomplished while suppressing the reduction of the oxide dielectrics and avoiding melting of the base metal electrode and diffusion of the electrode into the composition.
According to a fifth aspect of the invention, processings such as sintering etc. can be advantageously performed by disposing aerating boards, formed of materials having no reactivity with the above mentioned laminated body, in generally symmetrical positions on the upper and lower sides, or in a position either on the upper or lower side of the laminated body, so as to be in contact therewith. The above mentioned aerating board can be formed of materials having substantially no reactivity with the laminated body and having aerating pores, for example, oxides of alkaline earth metals such as MgO.
According to a sixth aspect of the invention, a spacer layer consisting of a honeycomb body, a porous stru
Kihara Noriaki
Nagaya Toshiatsu
Shindo Hitoshi
Sumiya Atsuhiro
Suzuki Yasunori
Malsawma Lex H.
Nixon & Vanderhye PC
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