Active solid-state devices (e.g. – transistors – solid-state diode – Voltage variable capacitance device
Reexamination Certificate
1995-03-22
2003-03-04
Dunn, Tom (Department: 1725)
Active solid-state devices (e.g., transistors, solid-state diode
Voltage variable capacitance device
C428S446000, C428S697000, C428S699000, C428S701000, C428S702000, C427S226000, C427S126300
Reexamination Certificate
active
06528863
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to a perovskite-containing composite material comprising a substrate, an intermediate layer of a first, titanium-containing perovskite and a covering layer of a second perovskite. The invention also relates to a method of manufacturing this perovskite-containing composite material, an electronic component comprising this composite material and a module having integrated passive components.
Such a perovskite-containing composite material is important for a large number of novel applications in microelectronics.
Ceramic perovskites are used in a wide range of applications in the semiconductor industry, because they offer interesting properties for special technical applications, such as for ferroelectric, dielectric, pyroelectric, piezoelectric or optoelectronic components.
For example, lead-zirconium-titanium-perovskites (PZT) are used, inter alia, for non-volatile, ferroelectric memories. Lanthanum-doped PZTs (PLZT) and complex titanium-containing perovskites, such as Pb(Mg, Nb)
x
Ti
1−x
O
3
are used as dielectric materials in capacitors, and PLZTs are also used as optoelectronic materials.
In combination with the known substrates, some perovskite composite materials have gained particular importance for the construction of modules comprising integrated passive components. To this end, passive components, such as capacitors, resistors and coils are integrated in electronic circuits by depositing said components on a silicon or aluminium-oxide substrate by means of thin-film techniques. Another important application of these composite materials is formed by capacitors for ferroelectric, non-volatile memories and random-access write-read memories.
In order to obtain perovskites having satisfactory properties (density, crystallinity), said perovskites must be sintered at high temperatures, however, as most of the substrates used in microelectronics cannot withstand said temperatures, jointly sintering the perovskites and said substrates is hampered.
Therefore, it is necessary to manufacture perovskite-containing composite materials which exhibit suitable properties when sintered at a temperature which does not adversely affect the substrate.
With regard to this, a method of manufacturing a thin film from a ferroelectric perovskite material is described, for example, in U.S. Pat. No. 5,198,269, the contents of which are hereby incorporated by reference; said method comprising the following steps:
a . providing a first substrate,
b. selecting a first sol-gel-perovskite precursor material, the crystallization of this first sol-gel-perovskite precursor material to the perovskite phase being insensitive to the first substrate and, after the heat treatment, the material being isostructural relative to the second ferroelectric, perovskite thin-film material,
c. depositing a first layer of the selected sol-gel-perovskite precursor material,
d. subjecting said first layer to a thermal treatment to form a first ferroelectric, perovskite thin-film material,
e. selecting a second sol-gel-perovskite precursor material, the crystallization of this second sol-gel-perovskite precursor material to the perovskite phase being sensitive to the substrate,
f. depositing a second layer of the selected sol-gel-perovskite starting material,
g. subjecting said second layer to a thermal treatment to form a second ferroelectric, perovskite thin-film material, the second layer of the second sol-gel-perovskite precursor material, after heat treatment, having better perovskite crystallinity when deposited on the first layer than if it would have been deposited directly on the substrate and heat treated.
The first sol-gel-perovskite starting material is selected to form, as a perovskite in step d., lead titanate (PbTiO
3
) or strontium titanate (SrTiO
3
).
The perovskite-containing composite materials manufactured in accordance with this method exhibit good crystallinity in the second perovskite layer. However, this second perovskite layer often comprises secondary crystalline phases (cf. U.S. Pat. No. 5,198,269 A, column 19, lines 2-5) which adversely affect the properties of the layer.
SUMMARY OF THE INVENTION
Therefore, it is an object of the invention to provide a perovskite-containing composite material which comprises a substrate, an intermediate layer of a first, titanium-containing perovskite and a covering layer of a second perovskite, and which exhibits improved properties.
According to the invention, this object is achieved by a perovskite-containing composite material in which both the first and the second perovskites are quaternary, or higher, complex perovskites.
In this composite material in accordance with the invention, the quaternary, titanium-containing perovskite of the intermediate layer acts as the nucleating layer for the formation of perovskite in the covering layer. Since this nucleating layer and the perovskites of the covering layer have a similar lattice constant and crystallographic space group, said nucleating layer enables single-phase perovskite to be formed in the covering layer during sintering and a reduction of the sintering temperature. Disturbing secondary phases, such as pyrochlore phases or the like, or amorphous phases do not occur in the covering layer. Such composite materials comprising a single-phase perovskite coating have an optimum texture and exhibit excellent characteristics, for example, for ferroelectric, non-volatile memories because they have a very high switched polarization. Such composite materials can also very suitably be used for capacitor applications because they have a remarkably high relative dielectric constant and hence a high surface capacitance.
A further advantage of the inventive composite material comprising a homogeneous, single-phase covering layer is that the covering layer can be structured so as to be clean-cut with sharp contours by means of known etch techniques. Composite materials having secondary phases in the perovskite-containing covering layer, however, form residues which are difficult to remove in these processes.
Within the scope of the invention it is preferred that both the first and the second perovskites contain titanium and that the titanium content of the first perovskite is higher than that of the second perovskite. The formation of the nucleating layer and the covering layer enables very good crystalline covering layers to be achieved.
Within the scope of the invention it is further preferred that the perovskite of the covering layer contains lead because, owing to the ferroelectric properties of the covering layer, such composite materials can very suitably be used, in particular, for a large number of microelectronic applications.
For several applications it is preferred that also the perovskite of the intermediate layer contains lead, so that its lattice symmetry corresponds to that of the perovskite of the coating.
Inventive composite materials whose perovskite of the covering layer is PbZr
x
Ti
1−x
O
3
, wherein 0.53≦x<1, can particularly suitably be used for non-volatile, ferroelectric memories and for piezoelectric applications.
Inventive composite materials whose perovskite of the covering layer is (Pb
1-1.5y
La
y
)Zr
x
Ti
1−x
O
3
, wherein 0.01≦y≦0.15 and 0.53≦×<1 or Pb(Mg, Nb)
x
Ti
1−x
O
3
, wherein 0.65≦x≦0.9 can very suitably be used to manufacture integrated capacitors and capacitors for dynamic, random-access write-read memories. Since the perovskite phase is free of pyrochlore, these capacitors have a high relative dielectric constant and a high surface capacitance.
In accordance with an advantageous embodiment of the invention, the perovskite of the intermediate layer is PbZr
x
Ti
1−x
O
3
, wherein 0<×≦0.35.
If this perovskite is used as the intermediate layer, crystallization of the perovskite in the covering layer starts at very low temperatures.
In accordance with the invention, the substrate preferably comprises a layer of silicon monocrystal. This combination of materials in
Brand Hans-Wolfgang
Klee Mareike K.
Larsen Poul
Bartlett Ernestine C.
Dunn Tom
Ildebrando Christina
Koninklijke Philips Electronics , N.V.
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