Optical fiber interface for integrated circuit test system

Claims

What is claimed is:

1. A link with an input end and an output end for transmitting a signal from a circuit to be tested to a circuit tester, the link comprising:

a link input end adapted to be connected to a probe card to receive electrical waveform signals directly from a circuit to be tested;

a link output end adapted to be electrically connected to a circuit tester;

an optical fiber having an input end and an output end;

a light source electrically coupled to the link input end to receive electrical waveform signals from a circuit to be tested that appear at the link input end and operating to convert any received electrical waveform signals to light signals the light source being optically coupled to the input end of the optical fiber to transmit the light signals into the input end of the optical fiber, the light source comprising a light emitter having a transfer function l(s);

a photodetector optically coupled to the output end of the optical fiber to receive light signals from the optical fiber and electrically coupled to the link output end to provide a reconstituted electrical waveform signal to the tester through a receiver stage and an equalizer stage providing a transfer function 1/l(s).

2. The link of claim 1 further comprising:

a high-pass filter stage electrically coupled to receive an output of the equalizer stage.

3. The link of claim 1 wherein the equalizer stage comprises:

a differential amplifier in differential mode with a negative feedback function, electrically coupled to receive an output of the receiver stage, whereby the differential amplifier provides a first-order high-pass closed-loop response.

4. The link of claim 1 wherein the light source comprises:

a light emitter; and

a driver stage electrically coupled to the light emitter for presenting the signal to the light emitter.

5. The link of claim 4 further comprising:

a load adapter configured to receive the signal and electrically coupled to transmit the signal to the driver stage.

6. The link of claim 5 further comprising:

a programmable load configured to receive the signal and electrically coupled to transmit the signal to the load adapter.

7. The link of claim 4 further comprising:

a programmable load configured to receive the signal and electrically coupled to transmit the signal to the driver stage.

8. The link of claim 4, wherein the light emitter is a light emitting diode.

9. The link of claim 4, wherein the light emitter is a laser diode.

10. The link of claim 4, wherein the driver stage electrically isolates the signal source from the impedance of the light emitter and provides a good impedance match to the signal source.

11. The link of claim 1, wherein the optical fiber is a multimode 100/140 micron graded-index fiber.

12. The link of claim 1, wherein the optical fiber is less than about five meters in length.

13. A bi-directional interface with a first endpoint and a second endpoint for providing a first path and a second path between the first and second endpoints for transmitting signals between a circuit tester, at the first endpoint, and a connection point proximate to a circuit to be tested, at the second endpoint, the bi-directional interface comprising:

a first link including a first optical fiber to provide a portion of the first path for signals passing between the circuit tester and the circuit to be tested, the first link and the first optical fiber each having an input end and an output end;

a second link including a second optical fiber to provide a portion of the second path for signals passing between the circuit tester and the circuit to be tested, the second link and the second optical fiber each having an input end and an output end;

a first directional gate for coupling both the input end of the first link and the output end of the second link to a common node at the first endpoint; and

a second directional gate for coupling both the input end of the second link and the output end of the first link to a common node at the second endpoint, where

each directional gate senses the presence of a signal transmitted in one direction on one of the links and responds by blocking transmission in the opposite direction on the other one of the links.

14. The bi-directional interface of claim 13 wherein the first link further comprises:

a first light source electrically coupled to the first directional gate and optically coupled to the input end of the first optical fiber to transmit light into the fiber; and

a first photodetector optically coupled to the output end of the first optical fiber to receive light and electrically coupled to the second directional gate; and wherein the second link further comprises:

a second light source electrically coupled to the second directional gate and optically coupled to the input end of the second optical fiber to transmit light into the fiber; and

a second photodetector optically coupled to the output end of the second optical fiber to receive light and electrically coupled to the first directional gate.

15. A method linking a multichannel circuit tester and a circuit to be tested, comprising:

receiving an original electrical waveform signal from the multichannel circuit tester for each of a plurality of connection points on a circuit to be tested;

converting each electrical waveform signal to an optical signal, this step comprising receiving the electrical signal and processing it through a programmable load, a load adapter, and an isolating driver state to drive a light emitter;

transmitting each optical signal over a distinct optical fiber that forms a portion of a channel between the tester and the circuit, each channel corresponding to a connection point on the circuit;

reconstituting each original electrical waveform signal from the corresponding optical signal transmitted over the corresponding channel; and

applying each reconstituted electrical waveform signal to the corresponding connection point on the circuit to be tested.

16. The method of claim 15 wherein the light emitter is a light emitting diode or a laser diode.

17. A bi-directional interface for any one of a plurality of channels of a multichannel tester for testing a circuit, each channel providing a signal path between a node on the circuit and the tester, the bi-directional interface having a first endpoint and a second endpoint for providing a first path and a second path between the first and second endpoints for transmitting signals between a circuit tester, at the first endpoint, and a connection point proximate to a circuit to be tested, at the second endpoint, the bi-directional interface comprising:

a first link including a first optical fiber to provide a portion of the first path for signals originating in the circuit tester and going to the circuit to be tested, the first link and the first optical fiber each having an input end and an output end;

a second link including a distinct second optical fiber to provide a portion of the second path for signals going to the circuit tester and originating in the circuit to be tested, the second link and the second optical fiber each having an input end and an output end;

a first node at the first endpoint that is a common point for signals passing through both the first link and the second link;

a second node at the second endpoint that is a common point for signals passing through both the second link and the first link;

a first directional gate for coupling both the input end of the first link and the output end of the second link to the common node at the first endpoint; and

a second directional gate for coupling both the input end of the second link and the output end of the first link to the common node at the second endpoint, where

each directional gate senses the presence of a signal transmitted in one direction on one of the links and responds by blocking transmission in the opposite direction on the other one of the links.

18. A bi-directional interface with a first endpoint and a second endpoint for providing a first path and a second path between the first and second endpoints for transmitting signals between a circuit tester, at the first endpoint, and a connection point proximate to a circuit to be tested, at the second endpoint, the bi-directional interface comprising:

a first link including a first optical fiber to provide a portion of the first path, the first link and the first optical fiber each having an input end and an output end;

a second link including a second optical fiber to provide a portion of the second path, the second link and the second optical fiber each having an input end and an output end;

a first directional gate for coupling both the input end of the first link and the output end of the second link to the first endpoint; and

a second directional gate for coupling both the input end of the second link and the output end of the first link to the second endpoint;

wherein the first directional gate comprises:

a normally-closed switch disposed between the input end of the first link and the first endpoint;

a normally-open switch disposed between the output end of the second link and the first endpoint;

a sense switch coupled to the output end of the second link and connected, on sensing a signal from the second link, to cause the normally-open switch to close and the normally-closed switch to open; and

a delay line disposed between the output end of the second link and the normally-open switch, in parallel with the sense switch, and connecting the output end of the second link to the first endpoint when the normally-open switch is closed.

19. Apparatus for transmitting an analog signal, comprising:

a light emitting diode, having a specific transfer function l(s) characterizing the light emitting diode;

a photodetector optically coupled to the light emitting diode to receive light emitted by the light emitting diode in accordance with the specific transfer function and to produce a detection output signal; and

a filter electrically connected to process the detection output signal, the filter realizing a transfer function of 1/l(s) reciprocal to the transfer function of the light emitting diode, thereby compensating for specific transfer characteristics of said light emitting diode, the filter comprising an amplifier and a network coupling an output of the amplifier to an input of the amplifier to provide feedback defining a transfer function of 1/l(s).

20. A directionally-exclusive interface providing a point-to-point connection for transmitting signals between a first node and a second node, comprising:

a first optical fiber to provide a portion of a first path for signals traveling from the first node to the second node;

a different second optical fiber to provide a portion of a second path for signals traveling from the second node to the first node;

a first directional gate for coupling both the input end of the first fiber and the output end of the second fiber to the first node; and

a second directional gate for coupling both the input end of the second fiber and the output end of the first fiber to the second node;

where

the first node and the second node are common points for signals passing through the first fiber or the second fiber; and where the first directional gate comprises:

a normally-closed switch disposed between the input end of the first fiber and the first node,

a normally-open switch disposed between the output end of the second fiber and the first node,

a sense switch coupled to the output end of the second fiber and connected, on sensing a signal from the second fiber, to cause the normally-open switch to close and the normally-closed switch to open, and

a delay line disposed between the output end of the second fiber and the normally-open switch, in parallel with the sense switch, and connecting the output end of the second fiber to the first node when the normally-open switch is closed.

21. A link for transmitting a waveform signal from a circuit tester to a circuit to be tested, the link comprising:

a link input end adapted to be connected electrically to a circuit tester to receive original electrical waveform signals from the tester;

a light source electrically coupled to the link input end to receive original electrical waveform signals from the tester that appear at the link input end and operating to convert such original electrical waveform signals to light signals;

an optical fiber having an input end and an output end, the light source being optically coupled to transmit the light signals into the optical fiber input end;

a photodetector optically coupled to receive light signals from the optical fiber output end and circuitry operating to reconstitute the original electrical waveform signals from the received light signals; and

a link output end electrically coupled to the photodetector and adapted to be connected electrically to a circuit to be tested through a probe card of the kind having electrical conductors to meet corresponding connection points on a circuit to be tested, to provide the reconstituted electrical waveform signals directly to the circuit to be tested through the probe card;

wherein:

the light signal is generated by a light emitter having a transfer function l(s); and

an equalizer stage providing a transfer function 1/l(s) is applied in reconstituting the original electrical waveform signals.

22. A method linking a multichannel circuit tester and a circuit to be tested, comprising:

receiving an original electrical waveform signal from the multichannel circuit tester for each of a plurality of connection points on a circuit to be tested;

converting each electrical waveform signal to an optical signal;

transmitting each optical signal over a distinct optical fiber that forms a portion of a channel between the tester and the circuit, each channel corresponding to a connection point on the circuit;

reconstituting each original electrical waveform signal from the corresponding optical signal transmitted over the corresponding channel; and

applying each reconstituted electrical waveform signal to the corresponding connection point on the circuit to be tested;

wherein the step of reconstituting the electrical signal from the optical signal comprises:

receiving the optical signal in a photodetector; and

receiving an electrical output from the photodetector in a receiver stage and processing it through the receiver stage, an equalizer stage, and a high-pass filter stage to produce a reconstituted electrical signal; and

wherein:

the optical signal is generated by a light emitter having a transfer function 1/(s); and

the equalizer stage provides a transfer function 1/1(s).

23. A link for transmitting a signal from a circuit to be tested to a circuit tester, the link comprising:

a probe card of the kind having electrical conductors to meet corresponding connection points on a circuit to be tested;

a link input end connected to the probe card and operable to receive electrical waveform signals generated by the circuit to be tested directly from a connection point on the circuit to be tested through the probe card;

a link output end adapted to be electrically connected to a circuit tester;

an optical fiber having an input end and an output end;

a light source electrically coupled to the link input end to receive electrical waveform signals from a circuit to be tested that appear at the link input end and operating to convert any received electrical waveform signals to light signals, the light source being optically coupled to the input end of optical fiber to transmit the light signals into the input end of the optical fiber;

a photodetector optically coupled to the output end of the optical fiber to receive light signals from the optical fiber and electrically coupled to the link output end through a receiver stage and an equalizer stage to provide a reconstituted electrical waveform signal to the tester.

24. A method linking a multichannel circuit tester and a circuit to be tested, comprising:

receiving an unprocessed circuit electrical waveform signal from each of a first plurality of connection points on a circuit to be tested;

converting each unprocessed electrical waveform signal to a circuit optical signal;

transmitting each circuit optical signal over a distinct optical fiber that forms a portion of a channel of a multichannel signal path between the circuit and the multichannel circuit tester;

reconstituting, each unprocessed circuit electrical waveform signal from the corresponding circuit optical signal;

applying each reconstituted circuit electrical waveform signal to a corresponding node, the node being electrically connected to the multichannel circuit tester and corresponding to the channel over which the corresponding optical signal was transmitted;

receiving an original tester electrical waveform signal from the tester for each of a second plurality of connect ion points on a circuit to be tested, the first and second pluralities of connection points having at least one connection point in common;

converting each tester electrical waveform signal to a tester optical signal;

transmitting each tester optical signal over a distinct optical fiber that forms a portion of a channel between the tester and the circuit, each channel corresponding, to a connection point on the circuit;

reconstituting each original tester electrical waveform signal from the corresponding tester optical signal transmitted over the corresponding channel; and

applying each reconstituted tester electrical waveform signal to the corresponding connection point on the circuit to be tested.

25. A method linking a multichannel circuit tester and a circuit to be tested, comprising:

receiving an original electrical waveform signal from the multichannel circuit tester for each of a plurality of connection points on a circuit to be tested;

converting each electrical waveform signal to an optical signal;

transmitting each optical signal over a distinct optical fiber that forms a portion of a channel between the tester and the circuit, each channel corresponding to a connection point on the circuit;

reconstituting each original electrical waveform signal from the corresponding optical signal transmitted over the corresponding channel;

applying each reconstituted electrical waveform signal to the corresponding connection point on the circuit to be tested;

using equalizer circuitry to cause each waveform signal applied to a connection point on the circuit to be tested to have the original form of the corresponding waveform signal received from the tester.

BACKGROUND OF THE INVENTION

This invention relates generally to systems for testing integrated circuits, and more particularly to the interface between an integrated circuit wafer or chip and a load board.

Conventionally, an integrated circuit test system is connected to the device under test ("DUT") through a load board, which is connected by coaxial cables to a probe card, which is connected to the DUT itself. The probe card is conventionally a printed circuit board with connections for the coaxial cables, fine needles for making contact with the connection points on the DUT, and printed traces on the printed circuit board connecting each cable to a needle.

The quality of test measurements is affected by the nature of the electrical connections, or interface, between the DUT and the test system. It has long been known that long coaxial cables impair the testing of signals, but there has been little success in correcting the problem, despite numerous efforts directed to solving the problem. The bandwidth of coaxial cable is limited. This bandwidth limitation is caused by a high-frequency physical effect called the "internal impedance" of the cable. This effect can cause signal distortion in the cable. Cable-induced signal distortions include mismatch errors between the probe card and cable and between the load board and cable. Cable-induced effects also include power loss and impedance mismatch errors within the cables, dispersion of the transmitted signal, crosstalk, ground loops, and phase distortion.

Nevertheless, flexible and long cables are desired to facilitate movement of the probe card, and to separate spatially the probe card from the load board and the typically large test system. Because the number of channels of an integrated circuit to be tested can number in the hundreds, the bundle of cables can become unwieldy, reducing flexibility and access to the DUT.

SUMMARY OF THE INVENTION

In general, in one aspect, the present invention provides a link based on optical fibers to carry a signal between a test system and a connection point proximate to an integrated circuit device to be tested. The link includes an optical fiber for transmitting the signal, a light source electrically coupled to the link input and optically coupled to the fiber input, and a photodetector optically coupled to the output of the fiber to receive light and electrically coupled to the link output. In other aspects, the invention includes a receiver stage to receive the output of the photodetector, an equalizer stage to receive the output of the receiver stage, a high-pass filter stage to receive the output of the equalizer stage, and a power output stage to receive the output of the equalizer stage or, alternatively, of the high-pass filter stage.

In general, in another aspect, the receiver stage includes a microwave differential amplifier to receive the output of the photodetector, and the equalizer stage includes a differential amplifier in differential mode with a negative feedback function, that receives the output of the receiver stage and provides a first-order high-pass closed-loop response.

In general, in another aspect, the light source includes a light emitter and a driver stage, a load adapter to receive the signal and to transmit the signal to the driver stage. In another aspect, the invention includes a programmable load configured to receive the signal and to transmit the signal to a load adapter or driver stage. In another aspect, the light emitter is a light emitting diode or a laser diode and the driver stage produces a DC bias current of about 60 mA coupled to a modulation current derived from the signal of about 30 mA. In another aspect, the driver stage includes an emitter-follower driver. In another aspect, the driver stage includes an FET (field-effect transistor) source-follower driver.

In general, in another aspect, the photodetector is a photodiode biased in photoconductive mode, and the optical fiber is a multimode 100/140 micron graded-index fiber of less than about five meters in length.

In general, in another aspect, the invention provides bi-directional interface for transmitting signals between a circuit tester and a connection point proximate to a circuit to be tested and includes a first optical fiber link to provide for transmission in one direction and a second optical fiber link to provide for transmission in the other direction, a first directional gate for coupling both the input end of the first link and the output end of the second link to one endpoint of the interface, and a second directional gate for coupling both the input end of the second link and the output end of the first link to the other endpoint of the interface.

In general, in another aspect, the first directional gate includes a normally-closed switch between the input of the first link and the one endpoint of the interface, a normally-open switch between the output of the second link and the one endpoint, a sense switch coupled to the second link and connected, on sensing a signal from the second link, to cause the normally-open switch to close and the normally-closed switch to open, and a delay line between the output of the second link and the normally-open switch, in parallel with the sense switch, and connecting the output of the second link to the one endpoint when the normally-open switch is closed.

Among the advantages of the invention are the following. The invention provides an electrical connection which overcomes the problems of resistive power loss and cross-talk in coaxial cables by substituting a fiber-optic interface system, which provides a bandwidth extending to at least 400 MHz with impedance matching at both ends of this interface, even where the impedances are different at the two ends of the interface. Moreover, the invention eliminates mismatch errors in the interface, reduces the diameter of the cables and cable bundle by a factor of about 2 or 3, provides increased flexibility of cable, and provides cables of any length less than about 100 meters without significant signal dispersion. The invention also provides optical isolation of the signals from each other, from ground, and between the probe card and load board. In particular, source and load are isolated (buffered) from each other, so that mismatch errors between them are reduced, and excellent impedance match to source and load is achieved.

The use of long, light and flexible fiber-optic cables makes it possible to extend the probe card away from the load board, to manipulate the position and angle of the probe card with moderate force, and to make the probe card more accessible.

Other advantages and features will become apparent from the following description and from the claims.

BRIEF DESCRIPTION OF DRAWINGS

The accompanying drawings, which are incorporated in, and constitute a part of, the specification, schematically illustrate specific embodiments of the invention and, together with the general description given above and the detailed description of the embodiments given below, serve to explain the principles of the invention.

FIG. 1A is a block diagram of a bi-directional optical fiber interface.

FIG. 1B is a block diagram of a uni-directional optical fiber interface.

FIG. 2 is a block diagram of a uni-directional optical fiber link.

FIG. 3A is a circuit diagram of a TTL load adapter for use with a 3-transistor driver in the optical fiber link of FIG. 2.

FIG. 3B is a circuit diagram of a TTL load adapter for use with an emitter-follower driver in the optical fiber link of FIG. 2.

FIG. 3C is a circuit diagram of a pr