Abstract
Improved probing of devices under test that have an impedance mismatch to the test equipment is provided. The probes are connected to the test equipment via a flexible circuit, and the impedance matching is done using lumped elements in the flexible circuit. Such lumped elements can be integral to transmission lines in the flexible circuit.
Claims
1. A probe head for making temporary electrical contact to a device under test, the probe head comprising: an array of probes; and a flexible circuit including at least one impedance matching structure configured to provide an impedance match between a test equipment (TE) impedance and a device under test (DUT) impedance; wherein the array of probes is disposed on the flexible circuit; wherein at least one probe of the probe array is connected to the impedance matching structure such that the probe is configured to probe a device under test at the DUT impedance; wherein the impedance matching structure includes at least one lumped element.
2. The probe head of claim 1, wherein the TE impedance is 50.
3. The probe head of claim 1, wherein the TE impedance is different from 50.
4. The probe head of claim 1, wherein the DUT impedance is not equal to the TE impedance.
5. The probe head of claim 1, wherein the TE impedance is real and the DUT impedance is complex.
6. The probe head of claim 1, wherein the at least one lumped element is integral to a transmission line of the flexible circuit.
7. The probe head of claim 1, wherein the at least one lumped element is configured to provide the impedance match between the TE impedance and the DUT impedance.
8. The probe head of claim 1, wherein a lateral distance between a selected one of the at least one lumped element and a base of a corresponding probe of the array of probes is in a range between 0 microns and 100 microns.
9. The probe head of claim 1, wherein the DUT is selected from the group consisting of: power amplifiers, filters, optical transceivers, low noise amplifiers, and antennas.
10. The probe head of claim 1, wherein the at least one lumped element is selected from the group consisting of: single rectangular stepped impedance structures, double rectangular stepped impedance structures, circular disk stepped impedance structures, interdigital structures, Yagi-Uda structures, single stub structures, double stub structures, excess ground structures, vertically connected structures, and combinations thereof.
11. The probe head of claim 1, wherein an operating frequency of the probe head is 24 GHz or more.
12. The probe head of claim 1, wherein the flexible circuit includes at least one transmission line disposed on or in a thin-film flexible membrane.
Description
BRIEF DESCRIPTION OF THE DRAWINGS
[0029] FIGS. 1A-B show examples of flex circuit technology suitable for use in embodiments of the invention.
[0030] FIG. 2 schematically shows a probe head suitable for use with embodiments of the invention.
[0031] FIG. 3 shows an exemplary flex circuit lumped element impedance matching structure suitable for use in embodiments of the invention.
[0032] FIGS. 4A-I show further examples of flex circuit lumped element impedance matching structures.
DETAILED DESCRIPTION
[0033] FIGS. 1A-B show examples of flexible circuit technology suitable for use in embodiments of the invention. In general, such technology provides a multi-layer structure of flexible dielectric layers (e.g., 102a, 102b, 102c, 102d on FIG. 1A) and metal layers (e. g., 104 a, 104b, 104c on FIG. 1A). Vertical metal vias (e.g., 106a, 106b, 106c) can be used to make electrical connections between structures separated by the dielectric layers. In the example of FIG. 1A, via 106a connects metal layer 104a to probe tip 108, via 106b connects metal layer 104b to metal layer 104a, and via 106c connects metal layer 104c to metal layer 104b. FIG. 1B is similar to the example of FIG. 1A, except that it has fewer layers. In practice, the metal layers in such flexible circuits can be patterned as traces, transmission lines, or the like, with considerable control over the fabricated shapes. Fabrication of such flexible circuit technology is known in the art, so it is not further described here.
[0034] FIG. 2 schematically shows a probe head suitable for use with embodiments of the invention. Here 210 is the device under test, 206 is a flexible circuit as described above, 208 is the array of probe tips, 202 is a substrate (e.g., a printed circuit board (PCB) ), member 204 provides mechanical support for flexible circuit 206 and probe array 208, and 212 schematically shows the core-to-board interface (CBI). Here 204, 206, and 208 are the core of the probe head, and CBI 212 includes the electrical connections between this core and the rest of the probe head (shown schematically here as PCB 202).
[0035] As indicated above, the main idea of this work is to add impedance matching structures to flexible circuit 206. More specifically, an exemplary embodiment of the invention is a probe head for making temporary electrical contact to a device under test, where the probe head includes: [0036] an array of probes (e.g., 208 on FIG. 2); and [0037] a flexible circuit (e.g., 206 on FIG. 2) including at least one impedance matching structure (examples described below) configured to provide an impedance match between a test equipment (TE) impedance and a device under test (DUT) impedance. [0038] The array of probes is disposed on the flexible circuit. At least one probe of the probe array is connected to the impedance matching structure such that the probe is configured to probe the device under test at the DUT impedance, and the impedance matching structure includes at least one lumped element.
[0039] The TE impedance can be 50 or non 50. The DUT impedance is typically not equal to the TE impedance. The TE impedance can be real while the DUT impedance is complex.
[0040] Embodiments of the invention are suitable for probing various kinds of device under test, including but not limited to: power amplifiers, filters, optical transceivers, low noise amplifiers, and antennas.
[0041] Operation of embodiments of the invention can be at any frequency, although the advantages of the present approach become more significant as frequency increases.
[0042] Thus preferred embodiments operate at high frequencies (i.e., 24 GHz or more).
[0043] FIG. 3 shows an exemplary flex circuit lumped element impedance matching structure suitable for use in embodiments of the invention. The left part of FIG. 3 is a bottom view and the right part of FIG. 3 is a corresponding end view. Here 306 is a metal trace that is the signal conductor of a transmission line. Metal traces 302 and 308 are corresponding ground traces of the transmission line. 304 is a lumped element impedance matching structure configured as a single rectangle. The end view on the right part of FIG. 3 shows that all of these traces are part of a bottom metal layer of a flexible circuit 320 as described above. Here 312 is the signal probe tip and 310 and 314 are ground probe tips. 310a, 312a, 314a on FIG. 3 (left) show where these probe tips are relative to the metal traces. In this example, the ground probes and ground traces are connected to buried metal layers 322 and 324 in flexible circuit 320, while the signal probe tip 312 is not connected to those buried metal layers.
[0044] Preferably, the at least one lumped element is integral to a transmission line of the flexible circuit, as in the examples of FIG. 3 and FIGS. 4A-I. The at least one lumped element is preferably configured to provide the impedance match between the test equipment impedance and the device under test impedance.
[0045] The lateral distance between a selected one of the at least one lumped element and a base of a corresponding probe of the array of probes is preferably in a range between 0 microns and 100 microns. The example of FIG. 4B below shows an example where this lateral distance is zero for the signal probe.
[0046] FIGS. 4A-I show further examples of flex circuit lumped element impedance matching structures. Any lumped element suitable for use in impedance matching can be employed, including but not limited to: single rectangular stepped impedance structures (e.g., FIG. 3), double rectangular stepped impedance structures (e. g., FIG. 4A), circular disk stepped impedance structures (e.g., FIG. 4B), interdigital structures (e.g., FIG. 4C), Yagi-Uda structures (e. g., FIG. 4D), single stub structures (e. g., FIG. 4E), double stub structures (e.g., FIG. 4F), excess ground structures (e.g., FIG. 4G), vertically connected structures (e.g., FIG. 4H), and combinations thereof (e.g., FIG. 4I).
[0047] FIG. 4A shows the ground-signal-ground trace pattern for an exemplary double rectangular impedance matching structure including rectangles 402 and 404.
[0048] FIG. 4B shows the ground-signal-ground trace pattern for an exemplary circular disk impedance matching structure including disk 406.
[0049] FIG. 4C shows the ground-signal-ground trace pattern for an exemplary interdigitated impedance matching structure including a signal trace 410 interdigitated with ground traces 408 and 412.
[0050] FIG. 4D shows the ground-signal-ground trace pattern for an exemplary Yagi-Uda matching structure including Yagi-Uda trace 414.
[0051] FIG. 4E shows the ground-signal-ground trace pattern for an exemplary single-stub matching structure including single-stub trace 416.
[0052] FIG. 4F shows the ground-signal-ground trace pattern for an exemplary double-stub matching structure including double-stub trace 418.
[0053] FIG. 4G shows the ground-signal-ground trace pattern for an exemplary excess ground matching structure including excess-ground traces 420 and 422.
[0054] FIG. 4H shows the ground-signal-ground trace pattern for an exemplary vertically connected matching structure including buried metal feature 424 connected to signal trace 306 with a vertical via 426.
[0055] FIG. 4I shows the ground-signal-ground trace pattern for an exemplary composite matching structure. Here the two parts of trace 306 are connected to each other via buried metal 430 and vias 432 and 434. The impedance matching is provided by rectangle 402 and by coupling of ground strip 436 to buried metal 430. Vias 438 and 440 can induce extra capacitance.
