EP1346433A1

EP1346433A1 - Arrangement with adjustable capacitance and method for production thereof

Info

Publication number: EP1346433A1
Application number: EP01270921A
Authority: EP
Inventors: Roger WÖRDENWEBER; Roland Ott
Original assignee: Forschungszentrum Juelich GmbH
Current assignee: Forschungszentrum Juelich GmbH
Priority date: 2000-12-15
Filing date: 2001-12-07
Publication date: 2003-09-24
Also published as: DE10062614A1; WO2002049145A1

Abstract

The invention relates to an arrangement with adjustable capacitance, comprising an electrical conductor (10) and a ferro-electrical material (12) with a variable dielectric constant ε, whereby the capacitance may be adjusted by means of a change in the dielectric constant ε. The electrical conductor is embodied as an electrically conducting layer (10), supported on a substrate (14). The electrically conducting layer (10) comprises a recess (16) and the ferro-electric material (12) is arranged in the recess (16). The invention further relates to a method for the production of an arrangement with adjustable capacitance.

Description

Tunable capacity arrangement and method of making the same

Description:

The invention relates to an arrangement with tunable capacitance with an electrical conductor and a ferroelectric material having a changeable dielectric constant ε, the capacitance being tunable by changing the dielectric constant ε. The invention further relates to a method for producing an arrangement with a tunable capacitance with an electrical conductor and a ferroelectric material having a changeable dielectric constant ε, the capacitance being tunable by changing the dielectric constant ε.

The development of miniaturized high-frequency components (RF components) is of great interest for a wide variety of applications. For example, such components can be used ideally in the field of wireless communication technology, radar technology and comparable applications. For example, penetrable filters come as HF components and resonators, phase shifters and super-directional antennas in question.

For example, base stations for wireless communication are currently based on the principle of frequency division into several frequency ranges (channels) in multiplex mode. Instead of multiplexing, it is possible to use tunable filters, which are designed as miniaturized HF components. In this way, a more flexible division of the different frequency ranges can be ensured and interference signals, for example intermodulation products, can be masked out. Changes in the frequency ranges can also be made without replacing filters.

Different concepts exist for the production of tunable filters, which can be used for example in wireless communication base stations. Thin-film technology is ideal for miniaturizing such components. A known method with which a frequency tuning of resonators can be achieved is based on the use of ferroelectrics, the dielectric properties of which depend on the applied electric field. If a ferroelectric is therefore used as a dielectric in a capacitor, such a capacitor can be tuned by applying different electrical field strengths, as a result of which frequency adaptation can be achieved. However, the problem with arrangements of the prior art is that very high capacities have to be achieved when operating at high frequencies. As a result, larger areas of an arrangement must be provided with capacitive structures. To achieve this, for example, a meander solution is known.

Another disadvantage is that all suitable ferroelectrics are associated with high RF losses.

It should also be noted that the production of multilayer arrangements which consist of a ferroelectric and a low-loss ceramic high-temperature superconductor has hitherto usually led to damage to either the superconductor or the ferroelectric.

It should also be noted that the application of a DC voltage to change the field strength and correspondingly change the dielectric constant of the ferroelectric requires a special design of the components, since both contacts must be electrically isolated. This leads to serious design restrictions.

Furthermore, a sufficient variation of the dielectric constant is necessary for a sufficient frequency tuning. With the existing designs of the capacitance, very high voltages are usually necessary for this, which make an application impossible. The invention has for its object to eliminate the disadvantages mentioned and in particular to provide an arrangement with tunable capacity and with a layer structure,

(i) in which the layers do not damage one another, reducing HF losses in the ferroelectric,

(ii) with which small gap widths can be achieved to achieve high capacities, and

(iii) in the case of the small gap widths in combination with high fill factors, which describe the electrical field component in the electrodes, lead to realistic voltages for the capacitance matching of the components.

Furthermore, a method for producing such an arrangement is to be specified.

This object is achieved with the features of independent claims 1 and 15. Advantageous refinements are specified in the dependent claims.

The invention is based on the generic arrangement in that the electrical conductor is an electrically conductive layer that is supported by a substrate, that the electrically conductive layer has an interruption - also called a gap - and that only ferroelectric material is arranged in the gap of the capacitance. With such an arrangement

(i) reduces the amount of ferroelectric material, thereby limiting RF losses,

(ii) the fill factor of the electric field in the dielectric increases compared to conventional designs, and

(iii) minimizes the risk of arcing between the electrodes.

It is also advantageous that the arrangement of ferroelectric material in the interruption of the electrical layer prevents mutual degradation of the materials.

The electrically conductive layer is preferably arranged directly on the substrate. The electrically conductive layer is therefore in particular not arranged on a ferroelectric, so that the layers are not deteriorated during the production process.

It can also be advantageous for a buffer layer to be arranged between the electrically conductive layer and the substrate. Such a buffer layer can bring advantages with regard to special deposition processes as well as with regard to the structuring of the layers. In principle, however, the advantage of the invention is retained, namely that an opposite side degradation of the layers during the manufacturing process is avoided.

In a preferred embodiment, the ferroelectric material is SrTi0 ₃ . It is a ferroelectric which can be arranged according to the invention.

For the same reason it can be advantageous that the ferroelectric material is Ba ^ r ^ TiC ^. In general, almost all ferroelectrics can be used, whereby optimal conditions are characterized by high tunability and low losses.

It is particularly advantageous that the ferroelectric material has a greater layer thickness than the electrode and thus typically in the range of about 0.3 -

5 μm. With such layer thicknesses, dielectric constants can be realized which are in a suitable range for tuning the capacitor. Furthermore, it is then possible to deposit the electrodes using the liftoff method.

It is also advantageous that the ferroelectric material has the smallest possible but sufficient lateral

Has expansion in the range of about 0.5 - 10 microns. With such lateral expansions, which correspond to the interruption (the gap) in the electrically conductive layer, capacitance and frequency comparisons are possible, which are advantageous for many applications. The ferroelectric material preferably has a greater layer thickness than the electrically conductive layer. The electrically conductive material is thus reliably divided into two areas that are electrically insulated from one another, so that a capacitance is achieved.

The electrically conductive layer is preferably a high-temperature superconductor. High-temperature superconductors are being used more and more frequently due to very low RF losses in thin-film arrangements. The invention is of particular advantage in this connection, since the low losses of the high-temperature superconductor are now not canceled out by losses of a large amount of ferroelectric material.

However, it can also be advantageous for the electrically conductive layer to be a conventional electrical conductor. Depending on the application, the arrangement can also develop its advantageous properties in this way.

It is preferred that the dielectric constant ε can be changed by changing an electric field strength. The adjustment can thus be achieved by a physical quantity which can be handled in a simple manner.

It is advantageous that electrodes are arranged on the electrically conductive layer via electrodes to apply a voltage so that the electrical

Field strength and thus the dielectric constant ε can be changed.

It can be advantageous if electrically conductive material is arranged on the ferroelectric material. The electrically conductive material can thus be applied to the substrate and the structured ferroelectric in a one-step process.

In this context, it is also advantageous if a further electrode is arranged on the electrically conductive material arranged on the ferroelectric material. In this way, an electrical field can be generated in an advantageous manner, which is concentrated in the ferroelectric and can be used to adjust the capacitance.

The invention builds on methods of the prior art in that a ferroelectric layer is applied to a substrate structure, that a region is structured out of the ferroelectric layer and that an electrically conductive layer is applied to the substrate structure with an interruption. In connection with the method, it should be emphasized as particularly advantageous that there is no degradation of the ferroelectric layer and the electrically conductive layer, since both layers are applied to laterally separated areas of a substrate structure. The method is particularly advantageous if a uniform substrate is used as the substrate structure. This is a particularly simple solution, which leads to an equally simple arrangement.

However, it can also be in the sense of the production process and also in terms of the functionality of the arrangement that a substrate with a buffer layer arranged on the substrate is used as the substrate structure.

SrTi0 ₃ is preferably used as the ferroelectric material in the method. This material is suitable in terms of both application and structuring.

Likewise, Ba _j .Sri_ _j .TiO ₃ or other ferroelectrics can be used as the ferroelectric material.

It is particularly preferred that the ferroelectric material has a layer thickness in the range of about 0.3

5 μm is applied. Such a layer thickness is useful in view of the final thickness of the electrodes. Furthermore, it offers good conditions for further processing e.g. in the lift-off procedure.

For similar reasons, it is advantageous that a region is structured out of the ferroelectric material that has a lateral extent in the region of approximately 0.5 - 10 μm. This ensures a high capacity with simultaneous insulation between the electrodes.

The method is also advantageous in that the electrically conductive layer is applied in a smaller layer thickness than the ferroelectric layer. Thus, an “interruption” and thus a capacitor effect is provided “automatically” when the electrically conductive layer is applied by the ferroelectric layer.

The method is particularly advantageous in that electrically conductive material is arranged on the ferroelectric material and in that a third electrode is arranged on the electrically conductive material arranged on the ferroelectric material. Thus, three electrodes can be provided in one method step, which electrodes can be used to apply a voltage.

However, it can also be advantageous for electrically conductive material arranged on the ferroelectric material to be removed. An arrangement can thus be produced which has electrically conductive material only on the substrate structure.

It is advantageous that the structuring is carried out by lithography and dry etching. These are proven methods with which a ferroelectric can also be used The layer can be structured, with particularly precise structures with clearly defined edges being able to be produced, particularly in ion beam etching (IBE).

In this context, it is also advantageous that the interruption of the electrically conductive layer is generated either by etching or by lift-off methods.

Further improvements of the arrangement according to the invention can be achieved within the scope of the method according to the invention by post-treating an interface between the ferroelectric material and the electrically conductive material. Such an aftertreatment can be carried out, for example, with oxygen, especially since the interface is oriented normal to the substrate.

The invention is based on the surprising finding that RF losses can be greatly reduced due to the structure specified. This is due in particular to the fact that the ferroelectric, which is subject to high losses, is arranged only in the gap in the capacitance. Likewise, the possibility of a degradation of the layers becomes less likely since both the ferroelectric and the electrodes of the capacitance are deposited directly on the substrate or on the substrate provided with the buffer layer. This is particularly important with regard to a combination of a ferroelectric with a high-temperature superconductor. Overall, improved tunability, lower losses and higher puncture resistance are achieved. The invention will now be explained by way of example with reference to the accompanying drawings using preferred embodiments.

It shows:

Figure 1 schematically shows a circuit diagram of a receiving system for the mobile radio area;

FIG. 2 schematically shows a circuit diagram of a further reception system for the mobile radio area;

FIG. 3a shows a first embodiment of a high-frequency band stop resonator;

FIG. 3b shows a second embodiment of a high-frequency band stop resonator;

Figure 4 is a diagram for explaining the invention;

FIG. 5a shows a first layer structure for realizing a capacitance with a ferroelectric;

FIG. 5b shows a second layer structure for realizing a capacitance with a ferroelectric;

FIG. 6a shows a layer arrangement after a first method step;

FIG. 6b shows a layer arrangement after a second method step;

FIG. 6c shows a layer arrangement after a third method step;

Figure 7 is a diagram for explaining the advantages of the invention.

FIG. 1 schematically shows an arrangement in which high-frequency components (HF components) are used. It is a reception system that can be used, for example, in the mobile radio area. The arrangement consists of an antenna 120, which is connected to the input of a bandpass filter 122. The output signal of this bandpass filter is passed on to an amplifier 124, which is preferably designed as an LNA ("low noise a plifier"). The output signal of this amplifier 124 becomes a multiplexer 126 entered, with the multiplexer 126 providing several channels on the output side. Each of these channels has a mixer 128, a further band-pass filter 130 following the mixer, a further amplifier 132 following the band-pass filter 130, which in turn is preferably designed as an LNA, and an analog-to-digital converter (ADC), which is used by converted analog signals output to amplifiers 132.

The arrangement according to FIG. 1 works on the principle of frequency division into many frequency ranges (channels) in multiplex mode.

An alternative concept is shown in FIG. Again, an antenna 120 is provided as the first link in the reception chain. The output signal from this antenna is fed to a frequency-tunable bandpass filter 136. The frequency-tuned signal is now passed to an amplifier 124, preferably an LNA, and from there to a mixer 128. This mixer is followed, similar to each channel in the arrangement according to FIG. 1, by a bandpass filter 130, an amplifier 132 and an analog-digital converter 134.

Because the filter 136 can be tuned, the multiplexer 126 according to FIG. 1, which is disadvantageous in terms of hardware expenditure, is unnecessary; A variable frequency range can be covered by the tunable bandpass filter 136. Furthermore, the arrangement according to FIG. 2 the advantage that interference signals can be filtered out.

Different concepts exist for realizing tunable filters that can be used as component 136 in an arrangement according to FIG. 2, for example. If you want to manufacture such components in miniaturized form, thin-film technology lends itself, whereby a change in the dielectric properties of ferroelectrics is used to influence a capacitance, which in turn has an influence on the set frequency range.

FIG. 3a and FIG. 3b show two basic sketches of high-frequency band stop resonators. FIG. 3a shows a band stop resonator with a single capacitance. A microstrip line 138 transports an RF signal in the direction indicated by arrow 140 as an example. An LC resonator 142 is arranged in the area of the microstrip line 138 and provides a capacitance C and an inductance L. The frequency to which the arrangement according to FIG. 3a is tuned depends on the inductance and the capacitance of the resonators via the relationship

from. FIG. 3b shows an arrangement similar to that in FIG. 3a, the same reference numerals indicating comparable components. However, a band stop resonator with double capacity and additional feed lines 144 is provided here for the application of an electrical direct voltage. By applying direct voltages, for example, the dielectric constant ε of a ferroelectric can be influenced, which is introduced into the gap of the capacitance and thus has an influence on the capacitance and thus the frequency of the band stop resonator. Basically, efforts are made to tune to high frequencies, with mobile radio frequencies in the GHz range, so that it is necessary to set high capacities. On the other hand, it is generally not desirable to increase the inductances to increase the frequencies, since large inductances result in large areas and losses.

FIG. 4 shows the dependence of the dielectric constant ε of SrTi0 ₃ on the applied electric field and on the temperature.

The upper dashed line shows the course of the dielectric constant as a function of the temperature without an applied electric field. The dotted line shows the course of the dielectric constant as a function of temperature with an electric field of 10 kV / cm. The dash-dot line shows the course of the dielectric constant for an electric field of 30 kv / cm. The lower dash-dot-dot curve shows the behavior of the dielectric constant as a function of temperature in an electric field of 100 kv / cm. If such a dielectric constant, which is dependent on an applied voltage, is brought into the gap of the capacitance, the frequency to which the filter is tuned can be influenced by changing the electric field.

5a and 5b show two possibilities for realizing a capacitance with a ferroelectric.

A ferroelectric 112 is arranged on a substrate 114 in FIG. 5a. An electrically conductive layer 110, which has an interruption 116, is applied to the ferroelectric 112. This interruption 116 provides a capacitance which can be changed by changing the dielectric constant of the ferroelectric 112.

Another arrangement is shown in FIG. 5b. An electrically conductive layer 110 with an interruption is located on the substrate 114. A ferroelectric is now applied to the overall arrangement of substrate 114 and electrically conductive layer 110. Again, the capacitance which is realized by the elements of the electrically conductive layer 110 can be influenced by changing the dielectric constant of the ferroelectric 112. 6 shows method steps for producing an arrangement according to the invention.

FIG. 6a shows the result of a first method step in which a ferroelectric material 12 is applied as a layer to a substrate 14. This ferroelectric material can be, for example, SrTi0 ₃ or Ba _x Sr-_ _x Ti0 ₃ in a layer thickness of 0.3-5 μm.

FIG. 6 b shows the result of further processing, a small area having a lateral dimension of 0.5-10 μm being structured out of the ferroelectric layer 12 by means of lithography and dry etching (ion beam etching (IBE)). It is particularly important to ensure that a steep flank is realized in the remaining structure 12, which can be achieved, for example, by ion beam etching at 45 ° along the edges.

FIG. 6c shows the result of a further processing step in which a conductor 10, 18 was applied to the structure according to FIG. 6b. This conductor 10, 18 can tear off directly at the edges during application, so that a capacitance can be made available in this way. If direct tearing does not take place, the electrical insulation can be carried out by additional etching at an extremely flat angle on the ferroelectric 12. In the shown in Figure 6c Arrangement can be provided by the electrically conductive areas 10 and 18 two electrodes for supplying a DC voltage and thus for providing and changing a field strength acting in the area of the ferroelectric material 12.

The described manufacturing process provides a structure which has an electrical conductor 10 with an interruption. A capacity is made available by this interruption 16. Ferro-electrical material 12, which influences the capacitance, is located in the interruption 16. The conductive layer can consist of normal conductive material or high-temperature superconductor material.

FIG. 7 shows the tunability n = C (0) / C (U _dc ) as a function of the applied DC voltage U _dc . The flatter curves show the tunability of a conventional planar thin-film component made of SrTi0 ₃ and Cu, with the interruption in the electrically conductive layer being 6 μm. The two upper curves show the tunability of an inventive

Arrangement, with values for an interruption with 2 μm and for an interruption with 6 μm are shown. It can be seen that the structures according to the invention can be tuned over larger areas and with lower voltages. The shown in Figure 7 set measuring points were recorded at a temperature T = 70 K.

The features of the invention disclosed in the above description, in the drawings and in the claims can be essential for realizing the invention both individually and in any combination.

Claims

Expectations:

1. Arrangement with a tunable capacitance with an electrical conductor (10) and a ferroelectric material (12) having a changeable dielectric constant ε, the capacitance being tunable by changing the dielectric constant ε, characterized in that the electrical conductor has an electrically conductive layer (10 ) which is carried by a substrate (14), that the electrically conductive layer (10) has an interruption (16) and that ferroelectric material (12) is arranged in the interruption (16).

2. Arrangement according to claim 1, characterized in that the electrically conductive layer (10) is arranged directly on the substrate (14).

3. Arrangement according to claim 1, characterized in that a buffer layer is arranged between the electrical layer and the substrate.

4. Arrangement according to one of the preceding claims, characterized in that that the ferroelectric material (12) is SrTiθ ₃ .

5. Arrangement according to one of the preceding claims, characterized in that the ferroelectric material (12) Ba _j .Sri_-. OK ₃ .

6. Arrangement according to one of the preceding claims, characterized in that the ferroelectric material (12) has a layer thickness in the range of about 0.3 - 5 microns.

7. Arrangement according to one of the preceding claims, characterized in that the ferroelectric material (12) is a lateral

Has expansion in the range of about 0.5 - 10 microns.

8. Arrangement according to one of the preceding claims, characterized in that the ferroelectric material (12) has a greater layer thickness than the electrically conductive layer (10).

9. Arrangement according to one of the preceding claims, characterized in that the electrically conductive layer (10) is a high-temperature superconductor.

10. Arrangement according to one of claims 1 to 8, characterized in that the electrically conductive layer (10) is a conventional electrical conductor.

11. Arrangement according to one of the preceding claims, characterized in that the dielectric constant ε can be changed by changing an electric field strength.

12. Arrangement according to one of the preceding claims, characterized in that electrodes are arranged on the electrically conductive layer (10).

13. Arrangement according to one of the preceding claims, characterized in that on the ferroelectric material (12) electrically conductive material (18) is arranged.

14. Arrangement according to claim 13, characterized in that an electrode is arranged on the electrically conductive material (18) arranged on the ferroelectric material (12).

15. A method for producing an arrangement with a tunable capacitance with an electrical conductor (10) and a variable dielectric constant ε-containing ferroelectric material (12), the capacitance being adjustable by changing the dielectric constant ε, characterized in that a ferroelectric layer (12) is applied to a substrate structure (14) such that a region is structured out of the ferroelectric layer (12) and that an electrically conductive layer (10) is applied to the substrate structure (14) with an interruption.

16. The method according to claim 15, characterized in that a uniform substrate (14) is used as the substrate structure.

17. The method according to claim 15, characterized in that a substrate with a buffer layer arranged on the substrate is used as the substrate structure.

18. The method according to any one of claims 15 to 17, characterized in that SrTi0 _{3 is} used as the ferroelectric material (12).

19. The method according to any one of claims 15 to 18, characterized in that Ba _x Sri_ _x i0 _{3 is} used as the ferroelectric material (12).

20. The method according to any one of claims 15 to 19, characterized in that the ferroelectric material (12) in one

Layer thickness in the range of about 0.3 - 5 microns is applied.

21. The method according to any one of claims 15 to 20, characterized in that an area is structured out of the ferroelectric material (12), which has a lateral extent in the range of about 0.5 - 10 microns.

22. The method according to any one of claims 15 to 21, characterized in that the electrically conductive layer (10) is applied in a smaller layer thickness than the ferroelectric layer (12).

23. The method according to any one of claims 15 to 22, characterized in that electrically conductive material (18) is arranged on the ferroelectric material (12).

24. The method according to any one of claims 15 to 23, characterized in that on the ferroelectric material (12) arranged electrically conductive material (18) is removed.

25. The method according to any one of claims 15 to 24, characterized in that the structuring is carried out by lithography and dry etching.

26. The method according to any one of claims 15 to 25, characterized in that the interruption (16) of the electrically conductive layer (10) is produced by etching.

27. The method according to any one of claims 15 to 26, characterized in that an interface between the ferroelectric material (12) and the electrically conductive material (10) is post-treated.