Abstract:
A method for measuring the impedance of a DUT having a capacitance of less than 1 pF includes applying a voltage or current signal to the DUT, the voltage or current signal including an AC component having a non-zero frequency of less than 1 kHz; monitoring a current or voltage signal, respectively, through the DUT in response to the voltage or current signal; digitizing the voltage signal and the current signal synchronously; and calculating the impedance from the digitized voltage and current signals.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The present invention relates to the measurement of impedances and, in particular, to high impedances at low frequencies. 
         [0002]    The measurement of very high impedances can present difficulties. This is because the impedances involved are so large that the voltage applied to the device under test (DUT) is exceedingly large and/or the resulting current is exceedingly small. For example, using too high of a voltage can result in device breakdown or even arcing. In the case of capacitances, low frequencies exacerbate the problems, because the impedance is inversely related to the frequency. 
         [0003]    Source measure units (SMUs) are well-known in the precision DC electrical measurement field for their ability to very accurately source a DC voltage signal and measure the resulting DC current signal or vice versa. For example, SMUs are available that can selectively source a DC voltage from a microvolt or less to a kilovolt or more and measure a DC current from an attoampere or less to an ampere or more (or vice versa). In DC measurement regimes, this permits the measurement of extremely high impedances (i.e., R=V/I). 
       SUMMARY OF THE INVENTION 
       [0004]    A method for measuring the impedance of a DUT having a capacitance of less than 1 pF includes applying a voltage or current signal to the DUT, the voltage or current signal including an AC component having a non-zero frequency of less than 1 kHz; monitoring a current or voltage signal, respectively, through the DUT in response to the voltage or current signal; digitizing the voltage signal and the current signal synchronously; and calculating the impedance from the digitized voltage and current signals. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0005]      FIG. 1  is a schematic diagram of an example of a measurement configuration for performing an aspect of the invention; 
           [0006]      FIG. 2  is a schematic diagram of another example of a measurement configuration for performing another aspect of the invention; and 
           [0007]      FIG. 3  is a schematic diagram of still another example of a measurement configuration for performing still another aspect of the invention. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0008]    Referring to  FIG. 1 , a measurement configuration  10  includes an example of an idealized source measure unit (SMU)  12  connected to a device under test (DUT)  30 . An adjustable voltage source  14  provides the voltage V 1  to the non-inverting input of an operational amplifier  16 . The feedback loop provided by the resistor  18  (R) forces the inverting input of the operational amplifier  16  to the value of V 1  also. Note that the feedback loop is the source of the current I through the DUT  30  as well. As a result, the voltage drop across the resistor  18  is proportional to the current through the DUT  30  (i.e., V R =IR or I=V R /R). 
         [0009]    The buffer amplifier  20  provides a buffered version of the voltage V 1  (which is the value of the voltage across the DUT  30 ) to the control and measurement section  22  and the buffer amplifier  24  provides a buffered version of the voltage V R  as a scaled (by R) version of the current through the DUT  30  to the control and measurement section  22 . The control and measurement section  22  also controls the desired value V 1  of the adjustable voltage source  14 . 
         [0010]    The control and measurement section  22  includes the ability to measure the values of the voltage V 1  and the current I including digitizing the values. The control and measurement section  22  also controls the desired value V 1  of the adjustable voltage source  14 . 
         [0011]    While essentially a DC device, the SMU  12  does fortuitously include the capability to adjust the value V 1  of the adjustable voltage source  14 . Within the bandwidth constraints of the feedback loops of the SMU  12 , the value of V 1  can be varied periodically by the control and measurement section  22  to produce an AC signal. Typically, the bandwidth limits of the SMU  12  is 1 kHz or less. This allows the SMU  12  to source, for example, a corresponding sinusoidal AC voltage signal of 1 kHz or less. 
         [0012]    To measure the impedance of a DUT  30  having primarily a small capacitance (e.g., 1 pF or less) at these low frequencies, the periodically varying voltage signal V 1  is applied to the DUT  30  and the current signal I through the DUT  30  is monitored. The control and measurement section  22  synchronously digitizes the voltage signal and the current signal in order that the impedance of the DUT  30  may be calculated. This is a complex value that includes, for example, not only the capacitive component but may also a resistive component typical of a non-ideal capacitive device. 
         [0013]    In the case of a 1 kHz frequency, a 1 pF capacitance and a nominally 1 kV voltage signal, the capacitive impedance would be approximately 160 megohms and the current would be on the order of 6 microamps. Considering that the SMU  12  may be capable of measuring attoamps, it can be seen the much lower frequencies and very much higher impedances can be measured without utilizing excessive voltages. 
         [0014]    The control and measurement section  22  can advantageously calculate the impedance from the digitized voltage and current using such techniques as discrete Fourier transforms (DFTs) which are often implemented using fast Fourier transform (FFT) algorithms. 
         [0015]    In real-world operation, the measurement configuration  10  may be somewhat compromised by stray impedances in parallel with the DUT  30  (e.g., SMU output impedance, cable leakage, test and fixture impedances). Referring to  FIG. 2 , improved performance can be obtained with the measurement configuration  10 ′ which uses two SMUs  12 ,  12 ′. 
         [0016]    The SMU  12  applies an AC voltage component to the DUT  30  while the SMU  12 ′ provides a DC bias voltage signal that the AC component rides on top of. This forces the return of all AC signals flowing through the DUT into the SMU  12 ′. Therefore, voltage measured across the DUT and current measured by the SMU  10 ′ can be used as an accurate representation of signals resulting from the DUT  30  impedance. The respective control and measurement sections  22 ,  22 ′ communicate with each other and again, DFTs can be used to make impedance calculations. The voltage across the DUT  30  is V 11 -V 12  and the current though the DUT  30  is V R2 /R 2 . 
         [0017]    The configuration of  FIG. 2  can be extended to provide simultaneous multipin capacitance measurements. For example, referring to  FIG. 3 , the SMU  12  can be used to provide an AC voltage signal component to the DUT  30 ′ while the SMUs  12 ′,  12 ″ and  12 ″′ can each provide a DC bias for a respective test point on the DUT  30 ′ and measure the current through the DUT  30 ′ between the SMU  10  and the respective test point. The respective SMU control and measurement sections are interconnected (A). The respective impedances can then be calculated as above. 
         [0018]    It should be noted that because of the duality of voltage and current, instead of voltages being applied and currents measured, currents may be applied and voltages measured to produce a measurement of the impedance of the DUT. SMUs are particularly useful in such applications because they are designed to interchangeably source voltage and measure current or to source current and measure voltage. 
         [0019]    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.