Electrically tunable device and a method relating thereto

• US 20010054748A1
• Filed: 06/20/2001
• Published: 12/27/2001
• Est. Priority Date: 06/20/2000
• Status: Active Grant

• Current Assignee: Optis Cellular Technology LLC (Brevet Capital)
• Original Assignee: Telefonaktiebolaget LM Ericsson
• Inventors: Petrov, Peter, Gevorgian, Spartak, Wikborg, Erland, Ivanov, Zdravko
• Granted Patent: US 6,563,153 B2
• Time in Patent Office: Days
• US Class Current: 257/595
• CPC Class Codes: H01L 29/93 Variable capacitance diodes, e.g. varactors

Abstract

The present invention relates to a thin film ferroelectric varactor device (10) comprising a substrate layer (1), a ferroelectric layer structure (12) and an electrode structure (9₁,9₂). The ferroelectric layer structure (12) comprises a number of ferroelectric layers (2,4) and a number of intermediate buffer layers (3) arranged in an alternating manner. At least a first (2) and a second (4) layer of said ferroelectric layers have different Curie temperatures, i.e. the dielectric constant of the first ferroelectric layer (12) has a maximum at a temperature which is different from the temperature at which the dielectric constant of the second ferroelectric layer (4) has a maximum.

Claims

1. A thin film ferroelectric varactor device (10;

20;

30;

40;

50;

60) comprising a substrate layer (1;

1A;

1B;

1C;

1D;

1E;

1F), a ferroelectric layer structure (12;

12A;

12B;

12C;

12D;

12E) and an electrode structure (9₁, 9₂;

9A₁, 9A₂;

. . ;

9E₁, 9E₂) characterized in that the ferroelectric layer structure (12;

12A;

12B;

12C;

12D;

12E) comprises a number of ferroelectric layers (2,4;

2A,4A;

2B,4B,6B;

2C,4C, 6C) and a number of intermediate buffer layers (3;

3A,5A;

3B,5B,7B;

3C,5C,7C) arranged in an alternating manner and in that at least a first and a second of said ferroelectric layers, have different Curie temperatures, i.e. the dielectric constant of the first ferroelectric layer has a maximum at a temperature which is different from the temperature at which the dielectric constant of the second ferroelectric layer has a maximum.

2. A varactor device according to

claim 1

, characterized in that at least said first and second ferroelectric layers (2,4;

2A,4A;

2B,4B,6B;

2C,4C, 6C) are chemically isolated from each other by an intermediate buffer layer (3;

3A,5A;

3B,5B,7B;

3C,5C,7C) and in that the chemical elemental composition of the ferroelectric layers (2,4;

2A,4A;

2B,4B,6B;

2C,4C,6C) differ in that the content of at least one element is different.

3. A varactor device according to

claim 1

or

2

, characterized in that at least some of the substrate layer (1;

1A;

1B;

1C;

1D;

1E;

1F), the layers of the ferroelectric layer structure (12;

. . .;

12E) and the electrode structure (9₁, 9₂ ;

. . .;

9E₁, 9E₂) have matched crystal structures.

4. A varactor device according to

claim 3

, characterized in that all layers have lattice matched crystal structures.

5. A varactor device according to any one of claims 1-4, characterized in that the layers of the ferroelectric layer structure (12;

. . .;

12E) comprise single crystalline (epitaxial) films.

6. A varactor device according to any of the preceding claims, characterized in that the ferroelectric layers comprise ceramic materials.

7. A varactor device according to

claim 6

, characterized in that the ceramic materials) comprise perovskite oxides or solid solutions thereof ABCO₃, wherein A is anyone of e.g. Ba, Na etc., B is anyone of e.g. Sr, K, etc., C is anyone of Ti, Nb, etc.

8. A varactor device according to

claim 7

, characterized in that each ferroelectric layer (2,4;

. . . ;

2C,4C,6C) (i) has an elemental composition of Ba_xiSr_1-xiTiO₃ respectively, wherein the content xi is different for at least two ferroelectric layers to produce different Curie temperatures for said at least two ferroelectric layers.

9. A varactor device according to

claim 7

, characterized in that the ferroelectric layers (2,4;

. . .;

2C,4C,6C) comprise Na_xiK_1-xi NbO₃ respectively, wherein the content xi is differ least two feroelectric layers to produce different Curie temperatures.

10. A varactor device according to

claim 8

or

9

, characterized in that for at least two, or at least said first and said second, ferroelectric layer i, i+1,0<

xi<

1;

0<

x(i+1)<

1.

11. A varactor device according to any one of the preceding claims, characterized in that the intermediate buffer layers (3;

3A,5A;

3B,5B,7B;

3C,5C,7C) comprise dielectric films.

12. A varactor device according to

claim 11

, characterized in that at least one of the dielectric film layers has an elemental composition of MgO, LaA10₃, CeO₂ or a material with similar properties.

13. A varactor device according to

claim 11

or

12

, characterized in that at least one of the dielectric film layers has an elemental composition of WO₃ or a material with similar properties.

14. A varactor device according to any one of the preceding claims, characterized in that at least one of the intermediate buffer layers (5C) comprises a multilayer structure comprising a number of sublayers (5C₁, 5C₂, 5C₃) wherein at least one sublayer has an elemental composition of MgO or similar and at least one sublayer has an elemental composition of WO₃.

15. A varactor device according to any one of the preceding claims, characterized in that the substrate layer (1;

. . .;

1E) has an elemental composition of MgO, LaA10₃ or a material with similar properties and in that the electrode structure comprises longitudinal arranged electrodes (9₁,9₂ ;

. . . ;

9E₁,9E₂) e.g. of gold defining a gap between them.

16. A varactor device according to any one of the preceding claims, characterized in that the ferroelectric layers have a thickness substantially <

1 μ

m, and in that the intermediate buffer layers have a thickness substantially <

100 nm.

17. A varactor device according to any one of the preceding claims, characterized in the ferroelectric layer structure comprises a ferroelectric nanostructure with ultra-thin ferroelectric layers having a thickness approximately <

100 nm.

18. A varactor device according to any one of the preceding claims, characterized in that adjacent to the electrode structure a buffer layer (8₁,8₂;

. . . 8D₁,8D₂) (e.g. of thin metallic Mg-films) is provided.

19. A varactor device according to any one of the preceding claims, characterized in the ferroelectric layer structure (12B;

12C) comprises three or more ferroelectric layers.

20. A varactor device according to any one of the preceding claims, characterized in that the temperature dependence of the varactor is given by the selection of Curie temperatures/values on xi/1-xi of an element of the composition of the respective ferroelectric layers.

26. Use of a number of a varactor device(s) according to any one of the preceding claims in tunable microwave (RF) circuits, such as tunable resonators, filters, phase shifters, delay lines, mixers, harmonic generators or similar.

21. A thin film ferroelectric varactor device (10;

20;

30;

40;

50;

60) comprising a substrate layer (1;

1A;

1B;

1C;

1D;

1E;

), a ferroelectric layer structure and an electrode structure (9₁,9₂ ;

. . . ;

9E₁,9E₂), characterized in that the ferroelectric layer structure (12;

. . . ;

12E) (comprises a number of ferroelectric layers (2,4;

2A,4A;

2B,4B,6B;

2C,4C,6C) and a number of intermediate buffer layers (3;

3A,5A;

3B,5B,7B;

3C,5C,7C) which preferably are dielectric, that the ferroelectric layers and the dielectric layers are arranged in an alternating manner such that ferroelectric layers between which an intermediate buffer layer is provided are chemically separated from each other and in that they have a different elemental composition such that the temperature dependence of the dielectric constants of the respective layers is different.

22. A thin film ferroelectric varactor device according to

claim 21

, characterized in that the ferroelectric layers comprise ceramic materials such as perovskite oxides or solid solutions of the type A_xiB_1-xiCO₃ wherein xi is different for at least subsequent ferroelectric layers between which an intermediate buffer layer is arranged.

23. A varactor device according to

claim 22

, characterized in that for each ferroelectric layer, i;

i=1 , . . ., N;

N being the number of ferroelectric layers, 0<

xi<

1.

24. A varactor device according to

claim 23

, characterized in that the ferroelectric layers i, i=1 , . . . N;

N being the number of layers, comprise Ba_xi Sr_1-xi TiO₃ or Na_xi K_1-xiNbO₃.

25. A varactor device according to

claim 23

or

24

, characterized in that all layers of the varactor device have matching crystal structures and in that the Curie temperatures of the respective layers are different, wherein the Curie temperatures are controlled by the value on xi, such that the temperature dependence of the capacitance of the varactor can be controlled.

27. A method of producing a thin film ferroelectric varactor device comprising an substrate layer structure, a ferroelectric layer structure and an electrode structure, characterized in that it comprises the steps of:

providing a ferroelectric structure on the substrate layer structure, including the steps of;

providing an intermediate, preferably dielectric, buffer layer between each of a number of ferroelectric layers, for at least two ferroelectric layers (i;

i+1), selecting different contents (xi;

x(i+1) of a first element of the respective elemental compositions of the layers (i;

l+1), selecting the contents (xi;

x(i+1)) such that the dielectric constants at the different layers will have different Curie temperatures such that the desired operational temperature range of the varactor will be between the Curie temperatures of the two ferroelectric layers;

disposing the electrode structure on the ferroelectric layer structure.

28. A method of operating a tunable thin film ferroelectric varactor device produced according to the method of

claim 27

using a vacuum deposition method, preferably laser ablation, to deposit a number of thin films to provide a crystal matched structure, said films comprising a substrate layer, a ferroelectric layer structure and an electrode structure, characterized in that it comprises the steps of;

operating the varactor device in the temperature interval between the Curie temperatures of the at least two ferroelectric layers;

applying the desired DC-tuning voltage.