Open-loop for waveform acquisition

1. A method for open-loop waveform acquisition, the method comprising:

acquiring an S-curve of an acquisition loop of an electron-beam probe system, the S-curve representing a response of the acquisition loop to changes of potential differences between the acquisition loop and a probe point; and

using the acquired S-curve to calibrate the acquisition loop so that a change in voltage at an output of the acquisition loop is directly proportional in a 1-to-1 ratio to a change in a voltage at the probe point.

2. The method of claim 1 , wherein acquiring the S-curve includes:

varying a voltage difference between the acquisition loop and the probe point; and

measuring an output of the acquisition loop.

3. The method of claim 2 , wherein varying the voltage difference between the acquisition loop and the probe point includes:

varying a voltage at the probe point of while keeping constant a voltage of a filter mesh that is part of the acquisition loop.

4. The method of claim 2 , wherein varying the voltage difference between the acquisition loop and the probe point includes:

varying a voltage of a filter mesh while keeping constant a voltage of the probe point, the filter mesh being part of the acquisition loop.

5. The method of claim 1 , wherein calibrating the acquisition loop includes:

adjusting a filter mesh voltage to a linear portion of the acquired S-curve, the filter mesh being part of the acquisition loop; and

adjusting a gain of the acquisition loop so that a change in voltage at an output of the acquisition loop is directly proportional in a 1to-1 ratio to a change in a voltage at the probe point.

6. The method of claim 1 , wherein calibrating the acquisition loop includes:

performing a linear regression to determine a slope and offset of the acquired S-curve such that a change in a voltage at the probe point is directly proportional in a 1 to 1 ratio to a change in voltage at an output of the acquisition loop.

7. The method of claim 1 , further comprising performing a voltage measurement at a location of interest on a device under test including probing the probe point to the location of interest, wherein a measured voltage swing of the output of the acquisition loop reflects in a 1-to-1 ratio a measured voltage swing of the voltage at the location of interest due to the calibrated acquisition loop.

8. The method of claim 7 , wherein the steps of calibrating and performing a voltage measurement at a location of interest on a device under test are combined.

9. The method of claim 1 , further comprising:

re-calibrating the acquisition loop to reduce errors.

10. The method of claim 1 , wherein using the acquired S-curve to calibrate the acquisition loop includes:

adjusting a voltage of a filter mesh to a linear region in the acquired S-curve, the filter mesh being part of the acquisition loop; and

adjusting a gain of the acquisition loop so that a change in a voltage at the probe point of is directly proportional in a 1-to-1 ratio to a change in voltage at the output of the acquisition loop.

11. A charged-particle-beam probe system for measuring voltage at a location of interest on a device under test, the system comprising:

a voltage source to supply a filter voltage, the filter voltage having been pre-calibrated to a linear region in acquired S-curve data of a device under test so that a change in a voltage at a probe point of the charged-particle-beam probe system is directly proportional in a 1-to-1 ratio to a change in voltage at an output of an integrator;

a filter mesh charged by the filter voltage, and situated between a device under test and a detector;

a beam source to apply to the location of interest a charged-particle-beam pulse at a selected delay t i ;

a detector to detect secondary charged-particles passing through the filter mesh producing a detector current (I SEC );

a current-combining circuit to combine the detector current with a reference current to produce an error signal (V ERROR ); and

the integrator to integrate the error signal over a time period to produce an integrator voltage (V INT ), wherein the integrator voltage (V INT ) is representative of the voltage at the location of interest due to the filter mesh being charged at the pre-calibrated filter voltage.

12. The system of claim 11 , wherein the voltage source comprises a digital-to-analog converter connected to receive the acquired S-curve data from the computing device for supplying the filter voltage.

13. The system of claim 12 , wherein the computing device further selects a working point (V W ) so that the working point is within the linear portion of the S-curve data, and drives the filter mesh to the working point.

14. The system of claim 11 , wherein the integrator comprises a capacitor, and a controllable switch for discharging the capacitor to reset the integrator voltage.

15. An electron-beam probe system, comprising:

a beam source to direct electrons at a probe point on a device under test;

a filter mesh having been pre-calibrated to a voltage within a linear region in acquired S-curve data of a device under test so that a change in a voltage at a probe point of the electron-beam probe system is directly proportional in a 1-to-1 ratio to a change in voltage at an output of an integrator;

a detector to detect a number of electrons that have sufficient energy to overcome a voltage difference between a voltage at the filter mesh and the probe point, the detector producing an output that is proportional to the number of electrons that the detector detects;

a subtractor to receive the output of the detector and a reference input, the subtractor to produce an output that is proportional to the difference between the output of the detector and the reference input;

the integrator to receive the output of the subtractor and produce an output that is linearly proportional to the voltage difference, wherein a measured voltage swing of the output of the integrator reflects a measured voltage swing of the voltage at the probe point due to the filter mesh having been pre-calibrated.

16. The system of claim 15 , wherein the computing device further selects a working point (V W ) so that the working point is within the linear portion of the S-curve, and drives the filter mesh to the working point.

17. A computer program product, tangibly stored on machine-readable medium, for driving a voltage of a filter mesh of an electron-beam probe system, the product including instructions to cause a processor to:

receive an output of an open acquisition loop of a an electron-beam probe system;

retrieve S-curve data that describes changes in output of the acquisition loop in response to changes of a voltage difference between a filter mesh and a probe point on a device under test, the filter mesh being part of the acquisition loop; and

drive the voltage of the filter mesh to a fixed working point (V W ) within a linear region of the S-curve data so that a change in a voltage at a probe point of the electron-beam system is directly proportional in a 1-to-1 ratio to a change in voltage at an output of the acquisition loop.

18. A method for open-loop waveform acquisition using an electron-beam probe system, the method comprising:

acquiring an S-curve of an acquisition loop of an electron-beam probe system, the S-curve representing a response of the acquisition loop to changes of potential differences between a filter mesh and a device under test, the filter mesh being part of the acquisition loop;

calibrating the acquisition loop including adjusting a voltage of the filter mesh to a fixed working point (V W ) within a linear region in the acquired S-curve; and

performing a voltage measurement at a location of interest on a device under test including probing a probe point of the electron-beam probe system to the location of interest; and

determining a voltage swing at the location of interest while maintaining the voltage of the filter mesh at the fixed working point (V W ).

19. The method of claim 18 , wherein calibrating the acquisition loop further includes adjusting a gain of the acquisition loop so that a change in a voltage at a probe point of the electron beam system is directly proportional in a 1-to-1 ratio to a change in voltage at an output of the acquisition loop.