# Patent

## Title
Cable compensation for pulsed I-V measurements

## Abstract

The errors related to the resistance of test conductors and sense/load resistances for a pulse I-V measurement system are determined by making open circuit and through circuit measurements using a combination of DC and pulse instrument measurements.

## Background
BACKGROUND OF THE INVENTION
The present invention relates to pulsed measurements and, in particular, to the measurement of I-V characteristics for semiconductor devices.
It is well-known to characterize semiconductor devices according to their I-V curves and similar measurements. Historically, such measurements are made with DC signals applied and measured. However, these DC measurements are not always suitable. For example, in many cases, these DC measurements result in significant heating of the devices resulting in measurements with little value.
SUMMARY OF THE INVENTION
A method for calibrating a pulse I-V measurement system for testing a DUT having at least two terminals is disclosed. The system has a first pulse measurement device connectable to an instrument end of a first test conductor; a pulse generator connectable to an instrument end of a second test conductor, the test conductors each having a DUT end connectable to respective DUT terminals; and a second pulse measurement device, one of the pulse generator and the second pulse measurement device being adapted to float on the potential of the other. The method includes shorting the DUT ends of the test conductors together; connecting the pulse generator to the second test conductor; sourcing a first voltage through the test conductors and the load resistance of the first pulse measurement device with the pulse generator; measuring a second voltage across the first pulse measurement device load resistance with the first pulse measurement device; determining the current through the sense resistance of the second pulse measurement device based on the second voltage and the first pulse measurement device load resistance; measuring a third voltage across the sense resistance with the second pulse measurement device; determining the resistance RS of the sense resistance based on the current through the sense resistance and the third voltage; and using RS to correct measurements made on the DUT.



BRIEF DESCRIPTION OF THE DRAWINGS 
FIG. 1 is a schematic diagram of an example pulsed I-V measurement system with a floating pulse generator.
FIG. 2 is a portion of the schematic diagram of FIG. 1 simplified to aid in the understanding of the invention.
FIG. 3 is another portion of the schematic diagram of FIG. 1 simplified to aid in the understanding of the invention.
FIG. 4 is a schematic diagram of an example pulsed I-V measurement system with a floating pulse measurement device.



DESCRIPTION OF THE PREFERRED EMBODIMENTS
Referring to FIG. 1, a pulsed I-V measurement system 10 includes a pulse generator 12 and a source measure unit SMU 14 that are switchably connectable via a cable 16 to a DUT 70. A pulse measurement device 18 is also connected to the cable 16. Another SMU 20 and another pulse generator 22 are switchably connectable via a cable 24 to the DUT 70. The pulse generator 22 floats on another pulse measurement device 26.
The pulse generators 12, 22 provide pulses with desired characteristics including, for example, amplitude, duration, and repetition rate. This includes, for example, the ability to-source DC voltage levels.
SMUs can source a DC voltage and measure a DC current, or vice versa. They are readily available as integral units or they may be implemented with separate voltage/current sources and current/voltage meters. The SMUs 14, 20 allow conventional DC measurements to be made with the same system in addition to pulsed measurements.
The pulse measurement devices 18, 26 measure pulse waveforms. This includes, for example measuring DC voltage levels. The devices may be, for example, oscilloscopes or high speed digitizers.
The cables 16, 24 have a DUT end and an instrument end. They may be, for example, coaxial cables, twin leads, spaced circuit board traces, or other test conductors suitable for pulsed and DC measurements.
The pulse generator 22 floats on the input of the pulse measurement device 26. This means that the pulse generator 22 is not affected by common mode voltages. One result of this is that all current supplied by the pulse generator 22 is proportional to the voltage drop across the sense resistance 28, shown by way of example, within the pulse generator 22. This resistance could also be located, for example, within the pulse measurement device 26 or external to both. In the example shown, it is in parallel with a load resistance within the pulse measurement device 26.
In high precision DC measurements, it is common to use techniques such as Kelvin measurements to remove the effects of potential error sources such as test conductor resistance. In the pulse regime, such techniques are not practical.
The present invention provides a method for removing undesired errors from the system 10.
Referring to FIG. 2, one source of error is uncertainty in the value RS of the sense resistance 28.
To measure RS, the DUT ends of the cables 16, 24 are shorted to each other (commonly, a device called a “through” is substituted for the DUT). The pulse generator 22 is connected to the instrument end of the cable 24. The pulse generator 22 sources a voltage through the cables 16, 24 and the load resistance RL of the pulse measurement device 18. RL may be a known value, or measured as set forth below. The pulse measurement device 18 measures a voltage VS1 across RL.
The current IX through the circuit is then VS1/RL. Typically, the value of the load resistance in the pulse measurement device 26 is such that, effectively, all of the current IX in the circuit passes through the sense resistance 28.
The pulse measurement device 26 measures a voltage VS2 across the sense resistance 28. The resistance RS is then VS2/IX.
Referring again to FIG. 2, another source of error is the resistance of the cables 16, 24. If the cables are identical, it can be assumed that the total resistance of the cables RW is proportional to the sum of the total length of the cable. If the cables are of equal length, then the resistance of each is one-half RW. RW can be determined before or after RS. The following example first finds RW and then RS.
The DUT ends of the cables 16, 24 are shorted to each other. The SMU 14 is connected to the instrument end of the cable 16 and the SMU 20 is connected to the instrument end of the cable 24. The SMU 16 sources a voltage V1 on the cable 16. The SMU 20 sources a voltage V2 the cable 24. If, for example, the voltage V2 is 0 v (it should be noted that this is a virtual ground forced by the SMU not an actual ground), then the SMU 14 itself can then measure the current ID through the cables 16, 24, otherwise the difference in the currents measured by each SMU is the measured current. The resistance RW is then the difference V1−V2 divided by the measured current ID.
Continuing to determine RS:
The SMUs 14, 20 are disconnected. The pulse generator 22 is connected to the instrument end of the cable 24. The pulse generator 22 sources a voltage through the cables 16, 24 and the load resistance RL of the pulse measurement device 18. The pulse measurement device 18 measures a voltage VS1 across RL. The current IX through the circuit is then VS1/RL. The pulse measurement device 26 measures a voltage VS2 across the sense resistance 28. The resistance RS is then VS2/IX.
Referring to FIG. 3, the measurement of RS can be further improved by actually measuring the value RL in the circuit.
In general, it would be desirable to perform this measurement before those described above.
If not already the case, open the DUT ends of the cables 16, 24. Connect the SMU 14 to the instrument end of the cable 16. The SMU 14 sources a voltage V4 through RL. The SMU 14 measures the current I4 through RL. The value of RL is then V4/I4.
The measured values of RL, RW and RS can each be used, for example, to correct measurements made on the DUT by either negating undesired voltage drops or more accurately determining currents based on better knowledge of sense resistance.
It should be understood that, typically, the operation of the system 10 and the performance of the method of the invention will be under the control of a computer or a similar control device.
Referring to FIG. 4, a pulsed I-V measurement system 10′ includes a pulse generator 12 and a source measure unit SMU 14 that are switchably connectable via a cable 16 to a DUT 70. A pulse measurement device 18 is also connected to the cable 16. Another SMU 20 and another pulse generator 22′ are switchably connectable via a cable 24 to the DUT 70. Unlike the system 10, a pulse measurement device 26′ floats on the pulse generator 22′, instead of vice versa.
This means that the pulse measurement device 26′ is not affected by common mode voltages. One result of this is that all current supplied by the pulse generator 22′ is proportional to the voltage drop across the sense resistance 28, shown by way of example, within the pulse generator 22′. This resistance could also be located, for example, within the pulse measurement device 26′ or external to both.
The operation of the system 10′ is essentially the same as the system 10 as described above and the method of removing errors is also essentially the same.
It should be evident that this disclosure is by way of example and that various changes may be made by adding, modifying or eliminating details without departing from the fair scope of the teaching contained in this disclosure. The invention is therefore not limited to particular details of this disclosure except to the extent that the following claims are necessarily so limited.

## Claims

1. A method for calibrating a pulse I-V measurement system for testing a DUT having at least two terminals, said system having a first pulse measurement device connectable to an instrument end of a first test conductor; a pulse generator connectable to an instrument end of a second test conductor, said test conductors each having a DUT end connectable to respective DUT terminals; and a second pulse measurement device, said pulse generator being adapted to float on the potential of said second pulse measurement device, said method comprising:
shorting the DUT ends of said test conductors together;
connecting said pulse generator to said second test conductor;
sourcing a first voltage through said test conductors and the load resistance of said first pulse measurement device with said pulse generator;
measuring a second voltage across said first pulse measurement device load resistance with said first pulse measurement device;
determining the current through the sense resistance of said second pulse measurement device based on said second voltage and said first pulse measurement device load resistance;
measuring a third voltage across said sense resistance with said second pulse measurement device;
determining the resistance RS of said sense resistance based on the current through said sense resistance and the third voltage; and
using RS to correct measurements made on said DUT.

2. A method according to claim 1, further comprising:
opening the DUT ends of said test conductors;
connecting a SMU to said first test conductor;
sourcing a fourth voltage through said first pulse measurement device load resistance with said SMU;
measuring the current through said first pulse measurement device load resistance with said SMU;
determining the resistance RL of said first pulse measurement device load resistance based on said fourth voltage and said current measured through said first pulse measurement device load resistance; and
using RL in determining RS and in correcting measurements made on said DUT.

3. A method for calibrating a pulse I-V measurement system for testing a DUT having at least two terminals, said system having a first SMU and a first pulse measurement device, each connectable to an instrument end of a first test conductor; a second SMU and a pulse generator, each connectable to an instrument end of a second test conductor, said test conductors each having a DUT end connectable to respective DUT terminals; and a second pulse measurement device, said pulse generator being adapted to float on the potential of said second pulse measurement device, said method comprising:
shorting the DUT ends of said test conductors together;
connecting said SMUs to respective test conductors;
sourcing a first voltage on said first test conductor with said first SMU;
sourcing a second voltage on said second test conductor with said second SMU;
measuring the current through said conductors with at least one of said SMUs;
determining the resistance RW of said conductors based on the difference between said first voltage and said second voltage and said measured current;
disconnecting said SMUs;
connecting said pulse generator to said second test conductor;
sourcing a third voltage through said test conductors and the load resistance of said first pulse measurement device with said pulse generator;
measuring a fourth voltage across said first pulse measurement device load resistance with said first pulse measurement device;
determining the current through the sense resistance of said second pulse measurement device based on said fourth voltage and said first pulse measurement device load resistance;
measuring the voltage across said sense resistance with said second pulse measurement device;
determining the resistance RS of said sense resistance based on the current through said sense resistance and the voltage across said sense resistance; and
using RW and RS to correct measurements made on said DUT.

4. A method according to claim 3, further comprising:
opening the DUT ends of said test conductors;
connecting said first SMU to said first test conductor;
sourcing a fifth voltage through said first pulse measurement device load resistance with said first SMU;
measuring the current through said first pulse measurement device load resistance with said first SMU;
determining the resistance RL of said first pulse measurement device load resistance based on said fifth voltage and said current measured through said first pulse measurement device load resistance; and
using RL in determining RS and in correcting measurements made on said DUT.

5. A method for calibrating a pulse I-V measurement system for testing a DUT having at least two terminals, said system having a first pulse measurement device connectable to an instrument end of a first test conductor; a pulse generator connectable to an instrument end of a second test conductor, said test conductors each having a DUT end connectable to respective DUT terminals; and a second pulse measurement device, said second pulse measurement device being adapted to float on the potential of said pulse generator, said method comprising:
shorting the DUT ends of said test conductors together;
connecting said pulse generator to said second test conductor;
sourcing a first voltage through said test conductors and the load resistance of said first pulse measurement device with said pulse generator;
measuring a second voltage across said first pulse measurement device load resistance with said first pulse measurement device;
determining the current through the sense resistance of said second pulse measurement device based on said second voltage and said first pulse measurement device load resistance;
measuring a third voltage across said sense resistance with said second pulse measurement device;
determining the resistance RS of said sense resistance based on the current through said sense resistance and the third voltage; and
using RS to correct measurements made on said DUT.

6. A method according to claim 5, further comprising:
opening the DUT ends of said test conductors;
connecting a SMU to said first test conductor;
sourcing a fourth voltage through said first pulse measurement device load resistance with said SMU;
measuring the current through said first pulse measurement device load resistance with said SMU;
determining the resistance RL of said first pulse measurement device load resistance based on said fourth voltage and said current measured through said first pulse measurement device load resistance; and
using RL in determining RS and in correcting measurements made on said DUT.

7. A method for calibrating a pulse I-V measurement system for testing a DUT having at least two terminals, said system having a first SMU and a first pulse measurement device, each connectable to an instrument end of a first test conductor; a second SMU and a pulse generator, each connectable to an instrument end of a second test conductor, said test conductors each having a DUT end connectable to respective DUT terminals; and a second pulse measurement device, said second pulse measurement device being adapted to float on the potential of said pulse generator device, said method comprising:
shorting the DUT ends of said test conductors together;
connecting said SMUs to respective test conductors;
sourcing a first voltage on said first test conductor with said first SMU;
sourcing a second voltage on said second test conductor with said second SMU;
measuring the current through said conductors with at least one of said SMUs;
determining the resistance RW of said conductors based on the difference between said first voltage and said second voltage and said measured current;
disconnecting said SMUs;
connecting said pulse generator to said second test conductor;
sourcing a third voltage through said test conductors and the load resistance of said first pulse measurement device with said pulse generator;
measuring a fourth voltage across said first pulse measurement device load resistance with said first pulse measurement device;
determining the current through the sense resistance of said second pulse measurement device based on said fourth voltage and said first pulse measurement device load resistance;
measuring the voltage across said sense resistance with said second pulse measurement device;
determining the resistance RS of said sense resistance based on the current through said sense resistance and the voltage across said sense resistance; and
using RW and RS to correct measurements made on said DUT.

8. A method according to claim 7, further comprising:
opening the DUT ends of said test conductors;
connecting said first SMU to said first test conductor;
sourcing a fifth voltage through said first pulse measurement device load resistance with said first SMU;
measuring the current through said first pulse measurement device load resistance with said first SMU;
determining the resistance RL of said first pulse measurement device load resistance based on said fifth voltage and said current measured through said first pulse measurement device load resistance; and
using RL in determining RS and in correcting measurements made on said DUT.