WO2004077672A1

WO2004077672A1 - Microwave switching method and device

Info

Publication number: WO2004077672A1
Application number: PCT/SE2004/000232
Authority: WO
Inventors: Daniel RÖNNOW; Eugen Veszelei; Jan Isidorsson; Staffan Bruce
Original assignee: Racomna Research Ab
Priority date: 2003-02-27
Filing date: 2004-02-19
Publication date: 2004-09-10
Also published as: SE0300536L; SE0300536D0; SE527452C2

Abstract

A method is disclosed for obtaining a device for control and switching of microwave or radio frequency signals on a substrate material generally adapted for microwave or RF operation. Typically the substrate may be a plate of quarts or similar material, on which an area of a trivalent ‘rare earth’ metal or an alloy thereof is deposited. This area is subjected to hydrogen and will act as a metal or as a semiconductor depending on the amount of hydrogen present. The area of the trivalent metal then acts as a switch for a microwave or radio frequency signal. An arrangement with hydrogen is provided for switching the trivalent metal area between its states of dihydride conducting metal state and a trihydride semiconducting state. A device for control and switching of Megaherz and Gigahertz radio signals is formed on a substrate for instance provided with strip-lines or corresponding surfaces of gold connecting a trivalent area acting as the switching or controlling element. The opposite surface of the substrate is typically having a deposited layer of a highly conducting metal forming a ground plane. The device will constitute a switched antenna, filter, stub, capacitance, resistor and similar microwave or radio frequency component, respectively.

Description

Microwave switching method and device

TECHNICAL FIELD

The present invention relates to a method for obtaining a device for switching and controlling the signal propagation properties operating in the radio frequency and microwave frequency region.

BACKGROUND

Radio frequency integrated circuits (RFIC) incorporate transmission lines, capacitors, inductors, and resistors, as well as active components, such as transistors and diodes. Generally radio frequency integrated circuits have a layer of metallization for transmission lines on a dielectric substrate. Components such as resistors, capacitors, transistors, and diodes are mounted on the substrate.

In RFIC:s there are often a number of tuning or trimming stubs, that can be manually trimmed in order to provide for variations in the signal properties of the circuit. Trimming is made by changing the geometry of the structures in the metallic layer, or by using active components.

In monolithic microwave integrated circuits (MMIC) also in addition to transmission lines on the substrate, the active and passive circuit elements are grown on the substrate. The substrate contains a semiconductor layer and there are several layers of metallic, dielectric, and resistive films.

Some passive components are realised in RFIC:s by certain metal patterns, i.e. certain patterns or geometries in the conducting layer. Thus, for instance, capacitors, inductors, filters, etc can be realised. Structures that are made in the metallic layer cannot be changed. Hence, it is not possible to use them for trimming.

Metal hydrides go from a metallic state to a semiconducting or insulating state with increasing hydrogen content. The direct current (DC) resisitivity increases and optical properties go from being reflective metallic to transparent semiconducting. Thin layers of these metals can be reversibly switched between a metallic and a semiconducting or insulating state. For most metal hydrides the hydrogan rich state is best referred to a semiconducting and accordingly such a terminiology will be used in the following.

Yttrium is one metal that can be used. When yttrium is exposed to hydrogen it changes to yttrium dihydride, which is a metal with lower resistance than yttrium. Further exposure to hydrogen gives yttrium trihydride, which is a wide bandgap semiconductor. Yttrium dihydride has a resistivity of Sx lO⁷ Ωm and yttrium trihydride has a resistivity of 7x 1 CH Ωm [1]. The optical properties change from highly reflective for yttrium dihydride, which is typical for a metal, to transparent, typical for a wide bandgap semiconductor. The switching between yttrium dihydride and trihydride is reversible, and this property is utilised in optical switches.

Several metallic materials can form metal hydrides and have been found to have switchable optical and electrical properties: 1) Trivalent metals like yttrium (Y), scandium (Sc), lanthanum (La) or alloys of these.

2) The 'rare earth' metals with atomic numbers 58 - 71 (Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu) [2].

3) Mg, and alloys of Mg with 'rare earth' metals [3], [4], [12]. 4) Multilayer stacks, with a layer thickness of 1 - 2 nm, of a trivalent metal and magnesium (Mg) [4]. 5) Alloys of Mg and a transition metal [5].

Below we discuss yttrium, but any metal could be used, if it is not explicitly stated otherwise. Table 1 illustrates change in resistivity for some switchable metal hydrides between their metallic and semiconducting states. Also given are the layer thicknesses and the cap layers, with a catalytically active metal. D stands for deuterium.

Table 1

* Values are compensated for the resistivity of the cap layer

The switching of the metal hydrides is due to the exchange of hydrogen. If yttrium is exposed to molecular hydrogen at room temperature and atmospheric pressure, yttrium hydride is formed. First yttrium dihydride is formed being metallic and with further hydration yttrium trihydride is formed being a semiconductor. If the yttrium trihydride is not exposed to hydrogen, but for instance to air, the hydrogen concentration will decrease: yttrium trihydride will change back to yttrium dihydride. This process will be faster if the sample is heated.

The molecular hydrogen has to be dissociated to atomic hydrogen. A cap layer of a catalytically active metal can be used on top of the metal film to promote the dissociation.

The catalytically active metal could be palladium (Pd), platinum (Pt), nickel (Ni), cobalt (Co), or alloys of these metals [4].

A layer of aluminium oxide in the nanometer range can be used between the metal and the catalytic layer. With the aluminium oxide layer the thickness of the catalytically active metal layer can be reduced to 1 nm. The aluminium oxide layer prevents the catalytically active metal from diffusing into the metal hydride and prevents the latter from oxidising, which increases the lifetime of the yttrium hydride [9].

The thickness of the yttrium layer in optical switches is typically 100 - 1000 nm [2], but values up to 2000 nm have been mentioned [4]. The switching time between the metallic and semiconducting states change with film thickness and is longer for thicker films. For films of some hundreds of nanometers the switching time is tens of seconds. Films of micrometer thickness can have switching times of several minutes. Using an alloy of a 'rare earth' metal with magnesium in the metal hydride, the switching speed may increase [4] .

The hydrogen can be supplied not only from a gas phase, but also from a liquid electrolyte [6] or a solid state electrolyte [10]. The hydrogen could also be supplied from a hydrogen plasma or from another metal hydride [4]. The hydrogen exchange can be driven by diffusion, be voltage controlled or heat controlled or combinations thereof. An yttrium film could be deposited by conventional methods such as sputtering, vacuum evaporation, chemical vapour deposition, laser ablation, or electroplating. A catalytically active layer could be deposited by the same conventional methods.

The substrate for optical switches would generally be glass or quartz.

DISCLOSURE OF THE INVENTION A method is disclosed for obtaining a device for control and switching of microwave or radio frequency signals. Initially a substrate material having electrical properties adapted for microwave or radio frequency (RF) operation is provided. Typically the substrate may be a thin plate of quartz or similar material, on which for instance an area of a trivalent metal is created. This area is in a preferred embodiment subjected to hydrogen and will act as a metal or as a semiconductor depending on the amount of hydrogen present. The area, for instance, of a trivalent metal may act as a high frequency control device or a switch when applied in a microwave or radio frequency circuitry. Furthermore the device for control and switching of microwave or radio frequency signals is provided with a tool for switching such a trivalent metal area between its states of dihydride conducting metal state and a trihydride semiconducting state.

A typical trivalent metal among the 'rare earth' metals to be used is for instance a deposited layer of yttrium exposed to hydrogen gas or to an electrolyte containing hydrogen to act with the trivalent metal in forming either the dihydride or trihydride state. Also alloys with other lanthanides or 'rare earth' metals can be candidates for a RF or microwave switching layer.

A functional embodiment of the device for control and switching of microwave or radio frequency signals is formed on a substrate for instance provided with strip-lines or corresponding surfaces of gold connecting a trivalent area acting as the switching or controlling element to a suitable input and output terminal. The opposite surface of the substrate is typically having a deposited layer of a well conducting metal, e.g. gold, forming a ground plane. Thus, the device will constitute a control device or switch e.g. for an antenna, a filter, a stub, a capacitance, a resistor or similar microwave or radio frequency component.

SHORT DESCRIPTION OF THE DRAWINGS

The invention, together with further objects and advantages thereof, may best be understood by referring to the following detailed description taken together with the accompanying drawings, in which:

FIG. 1 is a schematic top view of a device for the control or switching of radio frequency and microwave transfer properties;

FIG. 2 is a schematic cross sectional view of the device of FIG. 1;

FIG. 3 is a schematic top view of a simple microwave device for impedance matching or forming a tunable antenna device by means of a switchable metal hydride;

FIG. 4 shows the amplitude of the scattering parameter S21 versus frequency f of the switching device of FIG. 1 with the layer in its metallic phase (thick solid line) and its semiconducting phase (thick dashed line) in comparison with an unbroken transmission line of gold (thin solid line) and a broken transmission line with an air gap of same length as that of the metal hydride (thin dashed line) for experimental frequency range of 10 MHz to 40 GHz;

FIG. 5 shows an enlarged portion of FIG. 4 in a frequency range 20 to 40 GHz; FIG. 6 shows the amplitude of the scattering parameter Sl l versus frequency for the device of FIG. 1 with the layer in its metallic phase (thick solid line) and in its semiconducting phase (thick dashed line) and compared with an unbroken transmission line of gold (thin solid line) and a transmission line with an air gap with a same length as of the metal hydride (thin dashed line) for an experimental frequency range of 10 MHz to 40 GHz;

FIG. 7 shows the amplitude of the scattering parameter S21 in dB versus time in seconds during hydration at a frequency of 5 GHz;

FIG. 8 shows the amplitude of the scattering parameter Sl l versus frequency of the switching device of FIG. 3 with the metal hydride in the metallic state (solid line), in semiconducting phase (dashed- dotted line) and two intermediate phases (dashed line and dotted line); and

FIG. 9 shows the amplitude of the scattering parameter S21 versus frequency of the switching device of FIG. 3 with the metal hydride in the metallic state (solid line), in the semiconducting phase

(dashed-dotted line) and two intermediate phases (dashed line and dotted line) .

DETAILED DESCRIPTION A method and device will be disclosed for control and switching of signal transfer properties in the radio and microwave frequency ranges. It is based on the change between a metallic state and a semiconducting state occuring in metal hydrides when hydrogen is exchanged.

Figure 1 illustrates an embodiment of a device according to the present inventive method. The device can be used for control or switching of radio frequency and microwave signal propagation. The substrate 4 is a plate of quartz 1 mm thick. On top of the substrate there are two strips of gold, 1 and 2 connecting to a central surface portion 3. In the illustrative embodiment the width of the strips is 2.4 mm. The strips both have a length of 23.4 mm and a thickness of 10 μm. In between the two gold strips 1 and 2 there is an yttrium hydride layer 3. The width of the yttrium hydride layer is 2.4 mm, the length is 3.7 mm, and the thickness is 1 μm. The strips 1 and 2 form a transmission line obtaining different properties depending on the state of the yttrium hydride in the short strip 3. On top of the yttrium hydride layer there is a 5 nm cap layer of a catalytic metal, in this case palladium.

Figure 2 displays a cross-sectional view of the switching device in Figure 1. The reference numbers 1, 2, 3, and 4 refer to the same elements as in Figure 1. The bottom 5 of the substrate is a ground plane of gold. The transmission lines were designed to have a characteristic impedance of 50 Ω.

In the measurements a 10 MHz - 40 GHz analyser was connected to the terminal ends of the strips 1 and 2 in Figure 1 and Figure 2.

A slightly more complex device is shown in Figure 3. It is similar to that described in Figure 1 and Figure 2, but the pattern of the metallic layer is not that of a simple transmission line, but rather a pattern that is meant to give a strong frequency dependence in a more narrow frequency interval, compared to the device in Figure 1. Reference numbers 1, 2, 3, and 4, correspond to Figure 1 and Figure 2. The reference numbers 1 and 2 again indicate the terminals where the 10MHz - 40 GHz analyser was connected for the measurement analysis performed.

The amplitude of the scattering parameter S21 (transmission coefficient) of the switching device in Figure 1 is illustrated in Figure 4. When the metal hydride is in its metallic state the device operates as a general transmission line. The transmission decreases at frequencies above 10 GHz. The same behaviour is seen for a transmission line of pure gold, and is consequently not due to specific losses in the metal hydride, but rather in the experimental set-up. Thus, the frequency range of an application would not be limited to the ranges presented here. When the metal hydride is in its semiconducting state the transmission decreases significantly over the entire frequency range. The maxima and minima seen in all the curves are due to the geometry of the sample, rather than material properties.

For clarity a detail of the data presented in Figure 4 is given in Figure 5, in which S21 (transmission coefficient) is given versus frequency in the range 20 - 40 GHz. Although the data are noisy and there is structuring due to geometrical effects, there is a clear difference between the metallic and semiconducting states of the metal hydride. The switching properties resemble those seen as the difference between a pure gold line and a transmission line with an air gap, also illustrated in Figure 5.

The amplitude of the scattering parameter Sl l (reflection coefficient) versus frequency is shown in Figure 6. When the metal hydride is in its metallic state the reflection is almost as low as that seen for the gold transmission line. In its semiconducting state the reflection increases dramatically and is similar to that of a transmission line with an air gap.

The device depicted in Figure 1 and Figure 2 can be switched between the metallic and semiconducting states of the metal hydride and consequently the microwave transmission and reflection can be switched as shown in Figure 4, Figure 5 and Figure 6.

The amplitude of the scattering parameters S21 (transmission coefficient) at a frequency of 5 GHz during the insertion of hydrogen for the hydride is shown in Figure 7, for the device depicted in Figure 1 and Figure 2. At t = Os, the metal hydride is in its metallic state and the microwave signal is transferred normally by the device. For t > 400s, the metal hydride is in its semiconducting state and the microwave signal is no longer transferred.

Figure 7 shows clearly that transmission of microwaves can be controlled and tuned by a device such as that in Figure 1. The switching time between the metallic state and the semiconducting state for this device is approximately 7 minutes.

In Figure 8 the amplitude of the scattering parameter Sl l (reflection coefficient) versus frequency is demonstrated for the device of Figure 3.

The amplitude of the scattering parameter S21 (transmission coefficient) versus frequency for the device in Figure 3 is shown in Figure 9.

The amplitude of scattering parameter Sl l, shown in Figure 8, has a sharp minimum, the position of which can be controlled by the hydrating of the metal hydride in the device depicted in Figure 3. Thus, the device can be used to match the impedance. Furthermore, the device in FIG. 3 will be able to operate as an antenna, with tuneable properties.

In the experiments yttrium hydride was used and the switching was made between a metallic yttrium dihydride state and a semiconducting yttrium trihydride state. However, in such an RF or microwave switching device, any metal, metal hydride, or alloy, which is possible to be switched between a metallic conducting state and an insulating semiconductor state by using for instance hydrogen would be possible to utilise in accordance with the present invention. Such materials are to be found in the lanthanides group in the third group of the periodic table. Typical alloys may, for instance, be using magnesium (Mg). Further trivalent metals from this third group of the periodic table like scandium (Sc), yttrium (Y), lanthanum (La) or alloys of these will be the possible candidates for use according to the present invention. The thickness of the metal hydride is not critical. However, it should not be thinner than 0.1 μm and could for instance be 10 μm thick.

In the described experiment, a palladium cap layer was used for the dissociation of hydrogen. In a switching device, any catalytically active metal - like palladium, platinum, nickel, cobalt, or alloys of these metals - could be used.

The thickness of the catalytic layer is not critical. It could typically be between 2 nm and 25 nm.

In the present experiment illustrated, quartz substrates were used. In a switching device any substrate used for microwave or radio frequency planar transmission lines or microwave integrated circuits should be appropriate.

The thickness and width of the planar conductor and the thickness of the substrate are not specific to the described invention, but could be those used in a specific circuit.

In the conducting layer and the ground plane gold was used in the illustrative experiment. In a switching device according to the present invention any conductor material could be used, e.g. copper, silver, or aluminium.

Microstrip transmission line technology was used in the described experiment. In a device according to the present invention any transmission technology suitable for reasonable planar surfaces could be used, e.g. strip- lines, coplanar wave-guides or slot-lines.

The hydrogen, controlling the switching of the device, in the illustrated experiments was obtained from molecular hydrogen in gas phase. Atomic hydrogen was obtained by dissociation of molecular hydrogen by the catalytical metal of the cap layer. In a switching device hydrogen could be obtained from a liquid or solid state electrolyte or from a hydrogen plasma. A solid state electrolyte should be a proton conducting solid, e.g. zirconium oxide.

The devices that are described above are simple in order to show the new principle as clearly as possible. The radio frequency or microwave tuning or switching properties of the metal hydride could obviously be used in other devices or circuits. It could be used either in single components or as part of RFIC-.s or MMIC:s. Switchable metal hydrides can be used to change the geometry of the metal layer of single components or in RFIC:s or MMIC. Different applications would require different geometries of the switchable metal hydride and the rest of the metal layer that is used.

The frequency range of application using the present invention is not limited to those given in the presented experimental data. The geometries and the properties of the materials used in a specific application set the limits of the frequency range. Physically relevant frequency limits of the present invention for controlling radio frequency or microwave signal transmission properties could be 50 kHz up to 3000 GHz.

One application would be variable attenuators. If a transmission line has a part that is a switchable metal hydride, the attenuation can be continuously changed with the exchange of hydrogen. The frequency range of such a device is expected to be broad.

Another application would be impedance matching. The length of a matching stub could be changed by having one or more elements of metal hydride at the end of the stub or in series along the stub. By having the elements of metal hydride in the metallic or the semiconducting state, the length of the stub could be changed and hence, the impedance of the stub is altered. Trimming could be made in an RFIC or an MMIC by switching the metal hydride between its metallic and semiconducting states.

The invention is suitable for applications in impedance matching of high power circuits like power amplifiers. A metal hydride - being a material rather than an active device - is expected to exhibit low nonlinearity with respect to the RF or microwave signal power level and the signal power loss can be made relatively small.

One application would be phase shifting devices. By forming transmission lines of different lengths, the phase shift of a transmitted or reflected signal can be controlled.

Still an application could be tunable capacitors. By forming one or both sides of a parallel plate capacitor in a radio frequency circuit from metal hydride, the area of the capacitor, and consequently also the capacitance, can be controlled.

Still another application would be tunable resistors. By forming one or both sides of the contacts to a resistive layer in a radio frequency or microwave circuit from metal hydride, the width of the contacts, and consequently also the resistance, can be controlled.

Further applications could be coupling devices. By altering the dimensions of the transmission lines forming the device, the frequency selectivity and coupling ratio can be controlled.

Another typical application could be microwave filters. These can be implemented in planar transmission lines. It can be made using stubs with certain characteristic impedances, or transmission lines with different width and, hence, different characteristic impedance. Other alternatives are coupled line filters or filters with capacitive gaps in the transmission lines. In all these types of filters the characteristic impedance of the elements of the filter can be switched or tuned by using a switchable metal hydride. Metal hydrides could, thus, be used for switching or trimming of microwave filters made of planar transmission lines.

Furthermore one application could be antennas. Switchable metal hydrides could be used in planar or slot antennas to switch between different antenna geometries or for tuning the antenna gain. Another antenna application could be polarisation of antennas. By forming different antenna patterns, the structure will couple to different types of polarisation, either for reception or transmission of radio frequency or microwave signals. Further applications could be phased array antennas. By forming different antenna patterns, specific antenna properties can be achieved that focuses reception or transmission of radio frequency or microwave signals to specific directions.

The particular advantage of the present invention is that the switching and control function can be performed by a non-electric action influencing the hydrogen content of a component operating according to the above disclosed principle. As far as the technique has evolved so far there is not available an instantaneous action of the component, but there are many functions not dependent of an instantaneous action but rather needing an non-electric method for obtaining a switching or control.

It will be understood by those skilled in the art that various modifications and changes may be made to the present invention without departure from the scope thereof, which is defined by the appended claims. REFERENCES

[1] van Gogh et al, Physical Review Letters, vol. 83, pp. 4614 (1999).

[2] R. Griessen et al, U.S Patent No. 5,635,729.

[3] P. Duine, Patent Application WO 00/ 17706. [4] J. Isidorsson et al., Electrochimica Acta, vol. 46, pp. 2179 (2001).

[5] T. J. Richardson, Patent Application WO 02/ 14921

(US 2002/0044717 Al). [6] E. S. Kooij et al, Journal of the Electrochemical Society, Vol. 146, pp. 2990 (1999). [7] J. Isidorsson et al, Applied Physics Letters, vol. 80, pp. 2305,

(2002). [8] J. N. Huiberts et al., Nature, vol. 380, pp. 231 (1996).

[9] van Gogh et al, Applied Physics Letters, vol. 77, pp. 815 (2000).

[10] R. Armitage et al, Applied Physics Letters, vol.75, pp. 1863, (1999). [11] T. J. Richardson et al, Applied Physics Letters, vol. 78, 3047 (2001). [12] Y. Yamamoto et al, Journal of Alloys and Compounds, Vol. 330-

332, pp. 352.

Claims

1. A method for obtaining a device for control and switching of microwave or radio frequency signals, characterised by the steps of providing a substrate material (4) having electrical properties adapted for microwave or radio frequency (RF) operation; creating an area (3) of a thin film of a material which will act as a conductor or as an insulating semiconductor depending on the amount of hydrogen present in the thin film; connecting the area (3) of the thin film in series with at least one terminal (1) of a microwave or radio frequency circuitry; and providing a tool for switching the area (3) of the thin film between states of a conducting metallic state and a semiconducting state by varying the hydrogen content.

2. The method according to claim 1, characterised by the further step of providing the thin film from a trivalent metal such as scandium (Sc), yttrium (Y), lanthanum (La) or alloys of these.

3. The method according to claim 1, ©H araeteϊised j the further step of depositing the thin film as a thin planar conductor to react with hydrogen for creating either a conducting dihydride or an insulating semiconducting trihydride by means of present hydrogen.

4. The method according to claim 1, characterised by the further step of using for a delivery of hydrogen an electrolyte material delivering needed hydrogen in an encapsulation for a switching and controlling function.

5. The method according to claim 1, characterised by the further step of providing the substrate material with a highly conducting ground- plane on a lower side of the substrate opposite to a side carrying the area of thin film used for the switching and controlling function.

6. The method of claim 1, characterised by the further step of using, as the thin film, a material from a group consisting of lanthanides, or an alloy thereof, which can be switched between a conducting, state and an insulating semiconductor state by varying the hydrogen content of the metal film (3).

7. A radio frequency switching and controlling device, characterised in a substrate material (4) having electrical properties adapted for microwave or radio frequency (RF) operation; an area of a thin film (3), which with hydrogen will act as a conductor or as an insulating semiconductor depending on the amount of hydrogen present, the area of the thin film (3) being connected in series with at least one terminal of a microwave or radio frequency circuitry; and an arrangement to by means of hydrogen switch the thin film (3) between a conducting metal state and an insulating semiconductor state.

8. The device according to claim 7, characterised in that the thin film is a 'rare earth' metal or an alloy thereof deposited onto the substrate material (4) for reaction with hydrogen to create a conducting state of the thin film or an insulating semiconductor state of the thin film.

9. The device according to claim 7, characterised in that a substrate (4) is provided with a highly conducting ground- plane (5) on a lower side of the substrate opposite to a side carrying the area of thin film used for the switching and controlling function.

10. The device according to claim 7, characterised in that a substrate (4) is provided with strip-line conductors alternatively micro strip-lines to be used for a microwave signal control function in a GHz frequency region.

11. The device according to claim 7, characterised in that the thin film is a trivalent metal such as scandium (Sc), yttrium (Y), lanthanum (La) or alloys of these.

12. The device according to claim 7, characterised in that a deposited trivalent thin metal film is switched between a conducting dihydride state and a semiconducting trihydride state by means of present hydrogen.

13. The device according to claim 7, ©liaraeteiised an that a circuitry component for microwave and radio frequencies is formed providing a variable parameter value based on a variation of a hydrogen content.

14. The device according to claim 7, characterised in that the thin film is magnesium (Mg) or an alloy of magnesium with a 'rare earth' or transition metal deposited on the substrate material.