GB2310318A - Ferroelectric device - Google Patents

Description

2310318 PERROELECTRIC COMPONENT AND METHOD OF MANUFACTURE

The present invention relates to a ferroelectric component, for example, a pyrodetector. A pyrodetector consists of an active sensor layer of a pyroelectric material provided on both sides with an electrode. Perovskite from the family of lead titanates or organic pyroelectrics such as polyvinylidene fluoride (PVDF), for example, are used as pyroelectric materials. Lasting polarization of the sensor layer can be achieved in a strong electrical field or appears automatically on epitaxial growth of the pyroelectric.

The active sensor layer of a pyrodetector reacts to absorption of infrared radiation, and to the temperature increase caused thereby, by way of a buildup of an electric voltage, which can be read off at the electrode. To obtain a maximum measuring signal for a given radiation, the pyroelectric properties of the sensor layer and especially the pyroelectric coefficient must be optimized. This can be achieved with an oriented or even monocrystalline pyroelectric layer.

A quick and easy response is achieved if the pyrodetector has only a slight thermal capacity. This is normally effected by reducing the layer thicknesses of the pyroelectric and back-etching of the substrate to a membrane on which the detector element is built up. A further increase in sensitivity can be achieved if a read-out and amplifier circuit for evaluating the measuring signal can be connected to the detector element without a large wiring outlay or, even better, if the read-out and amplifier is integrated together with the detector element in a single component.

A further problem exists in the manufacture of integrated pyrodetector arrays, in which a plurality of detector elements is integrated on a component in order thereby to generate a heat image of an IR source. A pyrodetector array of this kind must in addition be optimized to as high a density of individual detector elements component as possible, to obtain a heat image with a better resolution.

A pyrodetector array with a c-axis-oriented active sensor layer is known from an article by R. Takayama et al, 11Pyroelectric Infrared Array Sensors Made Of cAxis-oriented La-Modified PbT'03 Thin Films", Sensors and Actuators, A21 to A23, pp 508 to 512 (1990). The sensor layer described there consists of lanthanummodified lead titanate thin films (PLT), which are grown orientedly over a magnesium oxide monocrystal as a substrate. In a later process stage, the PLT layer is exposed by back- etching of the magnesium oxide substrate and coated from below with an electrode.

From DE-A 43 23 821 an integrated pyrodetector element is known in which first buffer layers and above these oriented electrode and active sensor layers are deposited over a [1001-silicon substrate. The requisite readout and evaluation circuits can be integrated directly into the substrate underneath the buffer layers.

A disadvantage of the said pyrodetectors with oriented ferroelectric or pyroelectric layers is the requirement that exclusively monocrystalline or at least oriented layers can be used in manufacture. A thin membrane layer of amorphous silicon oxide and/or nitride which is advantageous for the components themselves is therefore not suitable for the known pyrodetectors with oriented ferroelectric layers.

The present invention therefore seeks to provide an improved ferroelectric component.

According to the invention, there is provided a ferroelectric component with: an amorphous membrane layer; at least one orientedly grown intermediate layer; a first orientedly grown electrode layer; an orientedly grown ferroelectric layer; and a second electrode layer. Further configurations of the invention and a method of manufacturing the component follow from the remaining claims.

The invention utilizes the discovery that certain materials can be deposited under defined conditions in thin layers such that their crystallographic layer planes are oriented parallel to the surface of the substrate. A material of this kind can then be used as an intermediate layer over an amorphous base layer and facilitates the assembly above it of a ferroelectric component with an oriented first electrode layer and an oriented ferroelectric layer. A ferroelectric component is thus obtained which combines for the first time a mechanically sound, electrically insulating and effectively freely etchable amorphous membrane layer with the good pyroelectric properties of an oriented ferroelectric layer. If the component is also built up over a crystalline substrate consisting of silicon, for example, the component according to the invention can be designed as a highly integrated pyrodetector array. The high pyroelectric coefficient of the oriented ferroelectric layer results in an adequate measuring signal even in the case of a small detector area. The amorphous membrane permits specific back-etching as far as the membrane, which can then serve as an etching barrier. owing to this, the individual detector elements can be separated from one another electrically and thermally by etching spaces in the pyroelectric layer, so that cross-talk between adjacent elements is substantially reduced. On the other side, the substrate can be etched back or removed completely underneath the pyrodetector element to reduce the thermal capacity of the individual detector element and is also reduce the thermal bridge and thus the cross-talk.

Any substrate can be used for an individual component, for example ceramic, a semiconductor or glass. However, if several ferroelectric components are arranged on an integrated component, for example in a pyrodetector array, a semiconductor substrate is preferably used. Readout and evaluation circuits can then be integrated in the substrate. Highly integrated components can thus be manufactured simply and with little wiring outlay, which components also facilitate quick measurement and thus a high measuring frequency.

The amorphous base layer consists for example of silicon oxide and/or silicon nitride. A preferred membrane consists of a triple layer comprising a silicon nitride layer, a silicon oxide layer and a further silicon nitride layer. With the optimum layer thickness ratios, different tensile and compressive stresses in this triple layer can be counterbalanced such that no overall stress of any kind results for the membrane as a whole or for the triple layer.

The intermediate layer consists of a material which has either a layerlike crystal structure or which can be grown orientedly under suitable deposition conditions. Such conditions may be set in particular in plasma processes, in which the plasma deposition is "in equilibrium" with the corresponding plasma etching process, which is brought about by bombarding the growing layer with particles of the plasma. If the deposition conditions are set so that the plasma deposition process just predominates, then preferably such crystals grow which exhibit the lowest etching rate compared with the preferably anisotropic plasma etching. An example of such a material is stabilized zirconium oxide, which grows under the said conditions preferably in such a way that the [0021 crystal plane is oriented parallel to the substrate surface. On an is oriented zirconium oxide layer of this kind further oriented layers including an oriented first electrode layer and the oriented ferroelectric layer can then be grown. The mixed oxides containing copper which are also known as high-temperature superconductors, for example yttrium barium copper oxide (YBaCuO), can be named here as an example of a material with a layerlike crystal structure.

An electrode layer can now be grown orientedly directly on this intermediate layer. However, it is also possible first to deposit orientedly a further matching layer on the intermediate layer to achieve better lattice matching of the intermediate layer and first electrode layer. Orientedly growing layers of the kind which have the [2001 crystal structure favourable for the ferroelectric layer are particularly suitable for such matching layers.

A [2001-oriented magnesium oxide layer or the yttrium barium copper oxide already mentioned, for example, are suitable as a matching layer over an intermediate layer of orientedly grown zirconium oxide (YSZ).

The first electrode layer is applied directly to the intermediate layer or a matching layer if this is present. It is possible to apply platinum for example in an advantageous [2001-orientation, which in the ferroelectric component according to the invention is therefore particularly well-suited to the first electrode layer. It is also possible to use electrically conductive perovskite from the class of cobaltates.

An oriented ferroelectric layer can be grown over the oriented first electrode layer. Suitable materials for the ferroelectric layer are selected from the family of lead-containing perovskite materials, especially the titanates or zirconate titanates. This i ferroelectric layer grows on the oriented electrode layer likewise with [2001-orientation, the crystallographic c-axis being aligned vertically to the surface of the substrate.

The method of manufacturing the component according to the invention is explained in greater detail with reference to two practical examples and the related two figures.

For a better understanding of the present nvention, and to show how it may be brought into effect, reference will now be made, by way of example, to the accompanying drawings, which are diagrammatic cross-sections through the layer construction of two practical examples of the invention.

A first practical example will now be explained with reference to Figure 1.

On a substrate, here a [1001-oriented silicon wafer S, an amorphous membrane layer MS is produced. In the practical example a triple layer is applied for this purpose with the layer sequence silicon nitride/silicon oxide/silicon nitride. The membrane layer as a whole (triple layer) is deposited in a thickness of approx. 1 gm using current thin-film processes. The layer thicknesses of the individual layers of the triple layer are optimized here such that overall a membrane layer MS free of tensile and compressive stresses results.

An intermediate layer W1 is now deposited orientedly over this. (Fully) stabilized zirconium oxide YSZ serves as the chosen material. The intermediate layer W1 is deposited using known thinfilm processes, the deposition conditions being set such that selective growth of [0021oriented crystallites is achieved. To do this, RF sputtering for example can be used, the substrate temperature being maintained at 400 to 6500C and preferably at approx. 6000C. A polycrystalline target is used and worked in an atmosphere of 90 percent argon and 10 percent oxygen. A similar process is known for example from an article by A. K. Stamper et al in J. Appl. Phys. 70 (4), 15 August 1991, Pages 2046 to 2051. The intermediate layer ZS1 is deposited in a thickness which provides sufficient structural information for the following oriented deposition processes. For this, approx. 50 nm is sufficient.

A further process for the selective deposition of a [2001-oriented YSZ layer is known from an article by N. Sonnenberg et al in J. Appl. Phys. 74(2), 15 July 1993, Pages 1027 to 1034. Here an ion-beam-aided vapour deposition process (IBAD) is used on a glass substrate at a temperature of approx. 6001C.

Above the intermediate layer W1, a further orientedly growing and latticematched matching layer AS1 is now deposited. In the practical example, a [2001-oriented magnesium oxide layer of a thickness of approx. 50 nm is applied for this purpose.

The first electrode layer E1 is now deposited over the matching layer AS1. Platinum, which can be deposited by vapour deposition or sputtering in [2001orientation, is selected as a suitable electrode material for this. It is deposited in a thickness of 50 nm maximum. Over the oriented electrode layer E1 the ferroelectric layer FS1 now grows orientedly under suitable manufacturing conditions in such a way that the c-axis is vertical to the surface of the substrate. This corresponds to an [0011orientation. Pure or suitably doped lead titanate is used as the material.

For a pyroelectric component, the ferroelectric layer FS1 is deposited in a thickness of approx. 1 gm. Finally, a second electrode layer E'l is produced, for which no orientation and thus no special deposition conditions are necessary.

is A second practical example will now be explained with reference to Figure 2.

As in the first practical example, an amorphous membrane layer MS in the form of the said triple layer is applied over a silicon substrate S. Stabilized zirconium oxide (YSZ) is again used as the intermediate layer ZS2. In this practical example, the intermediate layer ZS2 can be deposited orientedly if the abovementioned conditions are observed. However, it is also possible to deposit the intermediate layer ZS2 without orientation and use this just as a buffer layer for the further matching layer AS2. Yttrium barium copper oxide of a thickness of approx. 50 nm can be grown as the matching layer AS2. Here the layer-like crystal structure is oriented such that the layer planes are oriented parallel to the surface of the substrate. This corresponds to a [0011-orientation.

Of the many other suitable materials, layer-like materials which have a B'203 layer structure, such as Bi.T3012, for example, can also be named here.

A first electrode layer E2, a ferroelectric layer FS2 and a second electrode layer E'2 are produced by analogy with the first practical example as further layers to complete the component. Here too a ferroelectric layer is obtained with the desired preferred orientation [0011, which has a high pyroelectric coefficient, for example. The component produced can therefore be used especially as a pyrodetector.

In a modification of the first practical example, the first electrode layer E1 is applied directly above the oriented intermediate layer ZS1 without a further matching layer (not shown in the figure). In a further variation of the arrangement, the first electrode layer E1 is replaced by an electrically conductive lanthanum/strontium-cobaltate layer. A highly oriented ferroelectric layer with [0011-orientation can be deposited over this also.

The ferroelectric components according to the invention can be used as already mentioned as pyrodetectors. However, it is also possible to utilize the piezoelectric effect of ferroelectric layers for a piezoelectric component. A further application utilizes the hysteresis effect of the polarization of the ferroelectric layer accompanying the orientation. In this way the ferroelectric component can also be operated as a capacitor, which is suitable in turn for the permanent storage of data. A store of this kind is suitable for writing, since the direction of polarization can be reversed by applying an opposing voltage. In contrast to a DRAM store, a storage cell of this kind retains the information written in as long as no counter-voltage sufficient to effect reverse polarity is applied. The ferroelectric component according to the invention is also particularly suitable for the manufacture of an integrated component, in which a number of several individual elements are manufactured, structured and wired up integrated on a single substrate. The component according to the invention makes structuring easier, as the amorphous membrane can serve as an etching stop layer. The oriented layers produced on the membrane are more easily accessible to anisotropic etching processes than correspondingly unoriented layers. The component can be assembled on a silicon substrate, it also being possible to integrate current semiconductor circuits in the substrate in addition to the ferroelectric component.

Claims (13)

1. Ferroelectric component with: an amorphous membrane layer; at least one orientedly grown intermediate layer; a first orientedly grown electrode layer; an orientedly grown ferroelectric layer; and a second electrode layer.

2. Component according to claim 1, in which the ferroelectric layer comprises a perovskite material from the family of lead titanates.

3. Component according to claim 1 or 2, in which the material of the orientedly grown electrode layer comprises platinum.

4. Component according to any one of claims 1 to 3, in which the intermediate layer is selected from stabilized zirconium oxide, yttrium barium copper oxide or B'J'3012.

5. Component according to any one of claims 1 to 4, in which provided between the intermediate layer and the first electrode layer is a matching layer.

6. Component according to claim 5, in which provided above an intermediate layer of stabilized zirconium oxide is a matching layer of magnesium oxide MgO or of yttrium barium copper oxide.

7. Method of manufacturing a ferroelectric component with oriented ferroelectric layer comprising steps in which: an amorphous membrane layer is produced on a substrate, an intermediate layer with a crystallographic layer structure is deposited on the amorphous membrane layer such that its layer plane is oriented on growth parallel to the substrate surface; and further oriented layers are deposited above the orientedly grown intermediate layer, which layers comprise at least a first electrode layer chosen from platinum and electrically conductive lanthanum/strontium-cobaltates, and a ferroelectric layer.

8. Method according to claim 7, in which yttrium-stabilized zirconium oxide YSZ with [2001 orientation is deposited alternatively as an intermediate layer.

9. Method according to claim 8, in which the YSZ intermediate layer is produced by RF sputtering, the substrate temperature being maintained at 400 to 6500C.

10. Method according to claim 8, in which the YSZ intermediate layer is produced by ion-beam-aided vapour deposition, the substrate temperature being maintained at 500 to 6500C.

11. Method according to any one of claims 7 to 10, in which at least one further matching layer selected from <200> oriented magnesium oxide and YBaCuo is deposited orientedly above the intermediate layer.

12. A ferroelectric component substantially as herein described, with reference to the accompanying drawings.

13. A method of manufacturing of a ferroelectr component substantially as herein described, with reference to the accompanying drawings.