Varactor design using area to perimeter ratio for improved tuning range

• US 20070069274A1
• Filed: 08/28/2006
• Published: 03/29/2007
• Est. Priority Date: 09/23/2005
• Status: Active Grant

• Current Assignee: Qorvo US, Inc. (Qorvo, Inc.)
• Original Assignee: CYCAD Group, LLC
• Inventors: York, Robert, Elsass, Christopher
• Granted Patent: US 7,728,377 B2
• Time in Patent Office: Days
• US Class Current: 257/312
• CPC Class Codes: H01G 7/06 having a dielectric selected for the variation of its permittivity with applied voltage, i.e. ferroelectric capacitors electrets H01G7/02
• CPC Class Codes: H01L 28/55 with a dielectric comprising a perovskite structure material
• CPC Class Codes: H01L 28/65 comprising a noble metal or a noble metal oxide, e.g. platinum (Pt), ruthenium (Ru), ruthenium dioxide (RuO2), iridium (Ir), iridium dioxide (IrO2)

Abstract

Parallel plate tunable varactors having a bulk capacitance contribution to a total capacitance increased compared to a fringing capacitance contribution are disclosed. The contribution of the bulk capacitance to the total capacitance of an exemplary BST varactor is increased by increasing the area/perimeter ratio of the active region, thereby improving the tunability and other properties of the varactor. In an exemplary embodiment, an active region of the varactor has a lateral shape with a perimeter that is less than a perimeter of an equivalent area square. In various exemplary embodiments, the shape of the active region may be substantially circular or substantially octagonal. Methods for fabricating and designing such varactors are also disclosed.

Claims

1. A parallel plate tunable varactor, the varactor comprising:

a first electrode;

a second electrode;

a thin film dielectric layer between the first electrode and the second electrode, wherein the first electrode, the second electrode, and the dielectric layer are integrated on a substrate, and a permittivity of the dielectric layer varies according to a DC bias applied between the first electrode and the second electrode; and

an active region defined by an overlap of the first electrode, the second electrode, and the dielectric layer, the active region having a lateral shape, wherein a perimeter of the lateral shape is less than a perimeter of an equivalent square having a same area as the lateral shape.

2. The varactor of claim 1, wherein the thin film dielectric layer comprises at least one of the materials selected from a group consisting of:

barium titanate (BT), strontium titanate (ST), and barium strontium titanate (BST).

3. The varactor of claim 1, wherein at least one of the electrodes comprises platinum.

4. The varactor of claim 1, wherein the perimeter of the lateral shape is at least 3.9% less than the perimeter of the equivalent square.

5. The varactor of claim 1, wherein the perimeter of the lateral shape is at least 8% less than the perimeter of the equivalent square.

6. The varactor of claim 1, wherein the lateral shape is a polygon with at least five sides.

7. The varactor of claim 6, wherein the lateral shape is an octagon.

8. The varactor of claim 6, wherein the lateral shape is a heptagon, a hexagon, or a pentagon.

9. The varactor of claim 1, wherein the lateral shape is a polygon with at least eight sides.

10. The varactor of claim 1, wherein the first electrode is a top electrode and the active region is defined by the top electrode.

11. The varactor of claim 1, wherein the fringe capacitance comprises at least 5% of a total capacitance of the varactor.

12. The varactor of claim 1, wherein the fringe capacitance comprises at least 10% of a total capacitance of the varactor.

13. The varactor of claim 1, wherein the fringe capacitance comprises at least 15% of a total capacitance of the varactor.

14. The varactor of claim 1, wherein the fringe capacitance comprises at least 25% a total capacitance of the varactor.

15. The varactor of claim 1, wherein each one of a plurality of corners defining the lateral shape are rounded.

16. A parallel plate tunable varactor, the varactor comprising:

a first electrode;

a second electrode;

a thin film dielectric layer between the first electrode and the second electrode, wherein the first electrode, the second electrode, and the dielectric layer are integrated on a substrate, and a permittivity of the dielectric layer varies according to a DC bias applied between the first electrode and the second electrode; and

an active region defined by an overlap of the first electrode, the second electrode, and the dielectric layer, the active region having a lateral shape that is substantially circular.

17. The varactor of claim 16, wherein the thin film dielectric layer comprises at least one of the materials selected from a group consisting of:

barium titanate (BT), strontium titanate (ST), and barium strontium titanate (BST).

18. The varactor of claim 16, wherein at least one of the electrodes comprises platinum.

19. The varactor of claim 16, wherein the first electrode is a top electrode and the active region is defined by the top electrode.

20. The varactor of claim 16, wherein a fringe capacitance of the varactor comprises at least 5% of a total capacitance of the varactor.

21. A parallel plate tunable varactor, the varactor comprising:

a first electrode;

a second electrode;

a thin film dielectric layer between the first electrode and the second electrode, wherein the first electrode, the second electrode, and the dielectric layer are integrated on a substrate, and a permittivity of the dielectric layer varies according to a DC bias applied between the first electrode and the second electrode; and

an active region defined by an overlap of the first electrode, the second electrode, and the dielectric layer, the active region having a lateral shape including an area and a perimeter, wherein the area is maximized compared to the perimeter.

22. The varactor of claim 21, wherein the thin film dielectric layer comprises at least one of the materials selected from a group consisting of:

barium titanate (.BT), strontium titanate (ST), and barium strontium titanate (BST).

23. The varactor of claim 21, wherein the lateral shape is circular.

24. An electronic device, the device comprising:

a substrate;

a plurality of tunable thin film varactors integrated on the substrate, wherein each varactor comprises an active region having a lateral shape with an area A and a perimeter P, and an A/P ratio for each varactor is greater than a pre-selected ratio determined by a process design rule.

25. The electronic device of claim 24, wherein each of the varactors include a thin film dielectric layer including at least one of the materials selected from a group consisting of:

barium titanate (BT), strontium titanate (ST), and barium strontium titanate (BST).

26. The electronic device of claim 24, wherein the perimeter P of the lateral shape is at least 8% less than a perimeter of an equivalent square of equal area A.

27. A method of designing a tunable thin film varactor, the method comprising:

determining a pre-selected area/perimeter ratio;

choosing a lateral shape for an active region, the lateral shape having a design area/perimeter ratio equal to or greater than the pre-selected area/perimeter ratio; and

designing a bottom electrode shape, a top electrode shape, and a dielectric layer shape defining the active region.

28. The method of claim 27, wherein determining a pre-selected area/perimeter ratio comprises identifying the area/perimeter ratio as a design rule for a specific process.

29. The method of claim 27, wherein choosing a lateral shape comprises choosing a lateral shape having a perimeter that is less than a perimeter of an equivalent square of equal area.