WO2007084071A1

WO2007084071A1 - Micromachined continuous time delay phase shifter

Info

Publication number: WO2007084071A1
Application number: PCT/SE2007/050031
Authority: WO
Inventors: Lars Stenmark
Original assignee: ASTC AEROSPACE AB
Current assignee: ASTC AEROSPACE AB
Priority date: 2006-01-18
Filing date: 2007-01-18
Publication date: 2007-07-26
Anticipated expiration: 2008-07-18

Abstract

The present invention relates to methods and structures for improved high speed radio transmission systems using phased array antennas. According tot the invention as phase shifter with a variable transmission line length is provided. The phase shifter has a microstrip or coplanar transmission line with a movable part (107) and a fixed part (105), wherein the movable part (107) is arranged in parallel and adjacent to the fixed part (105). The movable part (107) is arranged so that an end portion of the movable part (107) overlap an end portion of the fixed part (105) with a gap (115) between the movable part (107) and the fixed part (105), the gap (120) providing a capacitive coupling. By displacing the movable part (105) the length of the overlapping section (120) is varied and consequently the transmission line length of the phase shifter is varied.

Description

Micromachined continuous time delay phase shifter

Technical field of the invention

The present invention relates to methods and structures for improved high speed radio transmission systems using phased array antennas.

Background of the invention

The growing demand for high speed communication and miniaturized systems results in the use of increasing frequencies up to tens of gigahertz and a shift from the traditional disk antenna towards electronically steered antennas. In radio communication system two clear trends result in a need for new design solutions. The first trend is the requirement for high data communication rates leading to the use of higher frequency bands in the radio systems, today normally a few gigahertz (S-band or X-band), but moving quickly towards ten's of GHz (Ka-band). The other trend is the demand for smaller and lighter systems in particular for mobile systems, most pronounced in aerospace and satellite applications. High frequency, high bitrate radio links requires effective antennas with a high gain in the communication between transmitter and receiver. Two antenna types are used in the art, parabolic disk antennas or antennas with electronically steered antenna beam.

In the latter there is a group of antennas in which the relative phases of the signals feeding the antennas are varied in such way that the effective radiation pattern is enhanced in a desired direction and suppressed in undesired directions. Phase shifters are used to vary the phase of the signal. In general phase shifters have a number of switches with digital control, state of the art is 5 bit resolution. When a large number of antenna elements are used for a very narrow beam the resolution requirement on the phase shifter increases. High resolution is however not readily accomplished, particularly not in devices with small overall dimensions. Moreover, conventional phase shifters are lossy at high frequencies. Summary of the invention

Obviously the prior art has drawbacks with regards to being able to accomplish phase shifters with high resolution and low losses, particularly in small dimensions.

The object of the present invention is to overcome the drawbacks of the prior art. This is achieved by the device as defined in claim 1 and the method as defined in claim 17.

The device according to the invention comprises a microstrip or coplanar transmission line with at least one movable part and at least one fixed part, wherein the movable part is arranged in parallel and adjacent to the fixed part. The movable part is arranged so that at least one end portion of the movable part overlap an end portion of the fixed part in an overlapping section with a gap between the movable part and the fixed part, the gap providing a capacitive coupling. The movable part can be displaced so that the length of the overlapping section is varied and consequently the transmission line length of the phase shifter is varied.

Further a phase shifter with continuously variable transmission line length is realised by having a continuously variable movable part.

The movable part may be realised in a first substrate and the fixed part in another substrates. The substrates are then bonded together. The movable part may be formed as a suspended part of the first substrate, in particular the movable part is free- hanging, suspended by a plurality of springs (140) flexible in a direction in which the movable part (107) is displaced, but stiff in the transverse and out of plane directions.

The movable part may be formed on one side of the first substrate, wherein the movable part is U-shaped with two parallel end portions, and each end portion is overlapping separate fixed parts formed on one side of the second substrate.

In addition the movable part can be arranged between a lower fixed part and an upper fixed part, wherein the upper fixed part comprises an input port and the lower fixed part comprises an output port. A linear actuator can be used to move the movable part and the linear actuator may be controlled by a control loop comprising a position sensor connected to the movable part.

The phase shifter of the present invention facilitates the realisation of electronically steered antenna system comprising arrays of phase shifters arranged on arrays of antennas.

A method to fabricate the phase shifters according to the present invention is disclosed and this method comprises the steps of shaping the substrates, forming transmission lines and bonding of the substrates.

Thanks to the invention it is possible to provide true time delay phase shifters.

It is a further advantage of the invention to provide phase shifter arrays in a single device, suitable for being arranged onto antenna arrays.

Embodiments of the invention are defined in the dependent claims. Other objects, advantages and novel features of the invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings and claims.

Brief description of the drawings

Preferred embodiments of the invention will now be described with reference to the accompanying drawings, wherein

Fig. 1 is a schematic cross-sectional view of a micro-strip transmission line sliding across another similar transmission line,

Fig. 2 is a top view of a true time delay phase shifter according to the invention,

Fig. 3 is a top view of a phase shifter according to the invention using a coplanar waveguide as the variable length transmission line,

Fig. 4 is a schematic top view of the suspended sledge with support components, and

Fig. 5 is a cross-section through a three wafer stack with a vertically integrated phase shifter.

Detailed description of embodiments

A highly miniaturized time delay phase shifter with low losses according to the present invention opens new possibilities in electronically steered high frequency antenna systems. The present invention provides a continuous phase shift with high resolution giving the possibility to design antenna arrays with a large number of antenna elements and very narrow fine pointing beam.

At frequencies from around ten Gigahertz and higher, Microsystems Technology (MST), which also is known as micromechanics or Microelectromechanical Systems (MEMS) technology, becomes an excellent technology for realizing RF devices, such as waveguides, switches, antennas, etc. as the device size is in the order of a few mm or smaller, and as a very good dimensional accuracy is inherent in these technologies. The present invention also takes full advantage of the new technology by parallel processing of maybe several hundreds of small movable features on a common substrate. Moreover integration level for a given size is superior conventional technologies, thereby enabling many functionalities in one single integrated device. An accurate true time delay phase shifter with small dimensions and low losses is a very desirable device in electronically steered high frequency antenna systems. The present invention provides such a device using MST processing and in particular multiwafer bonding. When a large number of antenna elements are used in order to obtain a very narrow beam the resolution requirement on the phase shifter increases and a continuously true time delay phase shifter is a preferable solution. The present invention significantly improves the performance of a key component for electronically steered antenna system. It is a low loss micromechanical device for true time delay phase shift with the unique feature of continuous operation over the whole phase shift band, i.e. more than 180°. A typical application is in satellite communication links with high bitrate and low power consumption. Other applications can be found in e.g. instruments using phase comparison for different reasons.

The present invention is a new method to realize a true time delay phase shifter with unique performance regarding low insertion losses, high resolution stable phase shift and small dimensions. The small dimensions makes it possible to produce an electronically steered antenna array at 32 GHz (free space wavelength 9,4 mm) with around 5 mm distance between the center of the antenna elements. As all components, such as power amplifier, phase shifter, and cavity backed antenna, can be accommodated in the size of an antenna element very large arrays with a high gain narrow lobe can be manufactured and a totally integrated miniaturized system is enabled.

The phase change function in the invention is accomplished by a physical change of the length of the transmission line between the power amplifier and the antenna. A U- shaped part of the transmission line is mounted on a small micromachined sledge; the sledge can be moved back and forth to any desired position, preferably measured by an integrated position sensor. Each leg of the movable transmission line is capacitively coupled to a fixed line. The whole assembly reminds of a musical wind instrument, the trombone.

If the antenna array is symmetrical with respect to a central reference antenna element, the number of movable parts can be reduced by 50% by mounting two bended transmission lines opposite each other on the same sledge, when one is increasing its length the opposite transmission line is decreasing its length the same amount.

One embodiment of the present invention is illustrated in Fig. 1. A fixed substrate 101 comprises a ground plane 110 on one side of the substrate 101 and a microstrip transmission line 105 on the other side. Another transmission line 107, which is mounted on a movable substrate 103, hereafter named the sledge 103, is arranged in parallel with and facing the transmission line 105 on the fixed substrate with a gap 115 in between. The gap 115 is preferably below 20 μm. Such a structure with accurate dimensions and gap width is advantageously realized using MST/MEMS processing. Similar structures are not readily made using conventional technology, like fine mechanics, or at least it becomes very cumbersome and costly. The sledge 103 can be moved back and forth along the direction defined by the two parallel microstrip transmission lines 105, 107. At a section 120 the transmission lines are overlapping but not in physical contact with each other. The close distance of the gap 115 makes the capacitive coupling very strong between the two transmission lines 105, 107. Simulation on a 32 GHz system gives a reflection coefficient of less than -27 dB for a continuous 180° phase shift. The fact that the transmission lines 105, 107 have a gap 115 in-between, forming the capacitive coupling, is advantageous since there is no sliding contact, which particularly in space applications, due to a vacuum environment, otherwise could lead to sticking and wear.

The substrates 101, 103, hereinafter also named wafers, are most likely of silicon as silicon is the most common material in the MST/MEMS field. However it may also be e.g. metal sheets, micromachinable glass, polymer or a ceramic material, i.e. materials used as microwave substrates like the ones mentioned but also others such as, but not limited to, SiC, BN, Teflon, etc. Suitable methods for shaping the wafers are, but is not limited to, etching, injection molding, electro discharge machining (EDM), rolling, laser ablation, punching etc. Wafers are bonded using for example fusion bonding, anodic bonding, adhesives, welding, soldering, etc., however not limited to those methods.

A schematic illustration of another embodiment of the present invention used to achieve variable transmission line length with fixed input port 117 and output ports 117 is shown in Fig. 2 (substrates not shown). A U-shaped transmission line 107 with parallel end portions is mounted on the sledge 103. The end portions are overlapping two fixed parallel transmission lines 105, 106 in a section 120, one end portion 105 connected to an input port 117 and the other end portion to an outlet port 118 on the fixed substrate 101. An advantage with this configuration is that the sledge stroke only needs to be half the required transmission line length change, which lower the requirement on an actuator mechanism used for lateral movement. In addition, the fixation of the ports 117, 118 is important when the device is to be connected to another device.

The same concept can be used for other types of transmission lines. In Fig. 3 a coplanar transmission line that is mechanically elongated is schematically illustrated. In this embodiment a fixed substrate 101 has a sledge 103 sliding just above the top surface. The surface has three coplanar transmission lines 105, 106, 107 with the four surrounding ground planes 111, 112. The sledge 103 has the U-shaped coplanar transmission line 107 on the bottom surface. There is no physical contact between the surfaces, but a very strong capacitive coupling both between the conductors 105, 106, 107 and the ground planes 111, 112. If needed, a spring-shaped flexible conductor 109 can provide a DC connection between the two ground planes 111, 112.

The arrangements around the sledge 103 are schematically illustrated presented in Fig. 4. The sledge 103 is a free-etched part of the surrounding suspension substrate 104, preferably made of mono-crystalline silicon, as silicon wafers are the most commonly used material for micromachining, but it can be any suitable, machinable material such as glass, quartz, ceramics, polymers etc. The sledge 103 is suspended by a number of flexible springs or bars 140. The springs 140 are flexible in the movement direction, but as stiff as possible in the transverse and out of plane directions. The purpose for this is that the U-shaped transmission line 107 shall slide very precise above the coupled lines 105,106 for a smooth low loss performance. A lateral linear actuator 130, fixedly mounted in the surrounding wafer 104, is mechanically coupled by a beam 131 to the sledge 103 in order to move it in a controlled way. The actuator 130 can be of shape memory metal, phase change material, piezoelectric or any other suitable type of actuator material that can be integrated in the device. A position sensor 135 may be coupled by a beam 136 to the sledge 103. The sensor 135 can be an optical, magnetic, capacitive sensor or other sensor suitable to be integrated in the wafer stack. The sensor 135 is used to position the sledge 103 to a predetermined position giving a desired phase shift from the true time delay phase shifter.

In contrast to the embodiments illustrated in Fig. 2 and Fig. 3, which both have the bended transmission line in the wafer plane, another embodiment of the present invention has the U-shaped transmission line out of plane, as illustrated in Fig. 5. The phase shifter is accommodated in a three wafer stack 501,504,502. Two continuous through wafer vias 525 and 526, wherein transmission lines extends essentially without changed properties from one side of the wafers to the other side of the wafers, forming inlet 517 and outlet ports 518. The upper wafer 502 carries a straight transmission line 506, which can be microstrip or coplanar transmission line. The lower wafer 501 has a similar arrangement with a transmission line 505. The free- hanging movable sledge 103 is manufactured in the middle suspension wafer 104. The movable sledge 103 has a folded transmission line 507, going from one side to the other through another continuous through wafer via 527. The transmission line 507 on the sledge 103 is partly overlapping the fixed transmission line parts 505,506 an overlapping section 520 as for the in-plane version. Gaps 515, 516 provide capacitive coupling between fixed parts 505,506 and movable part 507 of the transmission line.

MST/MEMS manufacturing methods are used for the present invention, and hence a plurality of phase shifter structures is fabricated in parallel on a single wafer, and arrays of phase shifters may finally be manufactured by bonding a plurality of wafers. Thereby a plurality of phase shifters, i.e. a phase shifter array, may be realized within a single device. This phase shifter array can then be arranged on an antenna array, which results in a compact assembly. An electronically steered antenna system comprising an array of phase shifters, wherein the antenna array is arranged in close proximity to the phase shifter array is realised. The antenna array may be symmetrical with respect to a central reference antenna element is such way that U- shaped movable transmission line parts are arranged in pairs opposite each other on the same movable part of the substrate, so that when one transmission line is increasing its length the opposite transmission line is decreasing its length to the same amount. A fabrication method according to the present invention is exemplified by one possible method to accomplish a phase shifter like the one shown in Fig. 5. The phase shifter comprises three mircromachined substrates 501,504,502 , i.e. three silicon wafers, with a patterned conductive material, such as copper, defining the transmission lines 505,506,507. The movable part 507 of the transmission line is U-shaped, extending along the upper surface of the sledge 503 down through a via to the lower side and back again along the lower side of the sledge 503, and is overlapping the fixed parts 505,506 of the transmission line in the lower and upper substrates 501,502 respectively. Further, the fixed parts 505, 506 of the transmission lines extend through their respective substrates 501,502 to the input port 517 on the upper side and the output port 518 on the lower side of the stack. Shaping of the substrates 501,504,502 are performed using for example photolithography and etching. The conductive material is deposited using conventional deposition technologies used in the MST/ MEMS field such as physical vapour deposition and electroplating and patterning. The silicon wafers 501,504,502 are bonded together using silicon fusion bonding.

While the present invention has been described in connection with what is presently considered to be the most practical and preferred embodiments, it is to be understood that the invention is not to be limited to the disclosed embodiments, on the contrary, is intended to cover various modifications and equivalent arrangements within the appended claims.

Claims

1. A phase shifter with a variable transmission line length characterized by

a microstrip or coplanar transmission line with at least one movable part (107) and at least one fixed part (105), the movable part (107) is arranged in parallel and adjacent to the fixed part (105);

the movable part (107) is arranged so that at least one end portion of the movable part (107) overlap an end portion of the fixed part (105) in an overlapping section (120) with a gap (115) between the movable part (107) and the fixed part (105), the gap (120) providing a capacitive coupling; wherein the movable part (105) can be displaced so that the length of the overlapping section (120) is varied and consequently the transmission line length of the phase shifter is varied.

2. A phase shifter according to claim 1, wherein the movable part (107) is continuously variable giving a continuously variable transmission line length.

3. A phase shifter according to claim 1 or 2, wherein the movable part (107) is formed in a first substrate (103) and the fixed part is formed in a second substrate (101).

4. A phase shifter according to claim 3, wherein the substrates (101, 103) are made of one or a combination of the following materials: semiconductor material, silicon, ceramic, metal, metal alloy, glass or polymer.

5. A phase shifter according to claim 3 or 4, wherein the substrates (101, 103) are shaped using one or a combination of the following technologies: etching, injection molding, electro discharge machining (EDM), rolling, laser ablation, and punching.

6. A phase shifter according to claim 3, wherein the movable part (107) is a free-hanging suspended part of the first substrate (103).

7. A phase shifter according to any of claims 3 or 6, wherein the movable part (107) is suspended by a plurality of springs (140) flexible in a direction in which the movable part (107) is displaced, but stiff in the transverse and out of plane directions.

8. A phase shifter according to any of the preceding claims, wherein the movable part (107) is formed on one side of the first substrate (103), the movable part (107) is U-shaped with two parallel end portions, each end portion is overlapping separate fixed parts (105, 106) formed on one side of the second substrate (101).

9. A phase shifter according to any of the claims 1-7, wherein the movable part (507) is arranged between a lower fixed part (505) and an upper fixed part (506), the upper fixed part (506) comprising an input port (517) and the lower fixed part (505) comprising an output port (518).

10. A phase shifter according to claim 9, wherein the movable part (507) of the first substrate (503) has a transmission line on each side connected to each other in one end by a continuous through substrate via (527).

1 l.A phase shifter according to claim 1, wherein the movable part (107) is displaced by a linear actuator (130).

12.A phase shifter according to claim 11, wherein the linear actuator (130) is controlled by a control loop comprising a position sensor (135) connected to the movable part (107).

13.A phase shifter according to claim 1, characterized in that the movable part (107) is rotated around a point, the fixed parts (105) are shaped as circle segments.

14. Electronically steered antenna system comprising an array of phase shifters according to any of the preceding claims, characterized in that an antenna array is arranged in close proximity to the phase shifter array.

15. Electronically steered antenna system according to claim 14, wherein the antenna array is symmetrical with respect to a central reference antenna element, U-shaped movable transmission line parts (107) are arranged in pairs opposite each other on the same movable part of the substrate (103), so that when one transmission line is increasing its length the opposite transmission line is decreasing its length to the same amount.

16. Phase shifter array comprising an array of phase shifters according to any of the claims 1-13, wherein the individual phase shifters sharing at least one common wafer.

17. Method to fabricate a phase shifter the phase shifter comprising a microstrip or coplanar transmission line with at least one movable part (107) on a first substrate (103) and at least one fixed part (105) on a second substrate (101), the movable part (197) is arranged to overlap the fixed part (105) in a overlapping section (120) with a gap (115) between the movable part (107) and the fixed part (105), comprising the steps of: - shaping the substrates; depositing and patterning a conducting material, the conducting material defining the transmission lines; and bonding the substrates together.

18. Method according to claim 16, further comprising forming through substrate vias in at least one substrate.