Low Current Analysis for Measuring fA Currents @ Temperature

The Signatone Triaxial Package

The Signatone Low Current System, combined with a suitable parametric instrument such as the HP4156A, allows measurements of current to the femtoamp (1 x 10 - 15 A) range at temperatures between ambient and 300°C for tests which may predict reliability and future performance such as:

  • MOS device sub threshold off currents (click on graph at the right)
  • Oxide voltage and charge-to-break-down testing
  • Hot carrier degradation and impact ionization current

The components of the Triaxial Package are 1) a triaxially driven room temperature chuck or hot chuck, 2) triaxially driven probe tip holders engineered to operate with a 300°C chuck temperature, 3) a suitable electronically shielded dark box, and 4) appropriate cabling.


Triaxial Technology

Triaxial measurements use a 3 conductor system with a driven guard between the source-sense (conductor to be measured) and the signal ground to enable measurements of very low currents. The results are: 1) A low noise floor which is a measure of the effective noise protection of the probe holder or chuck, shielded box, and the HP4156A or equivlaent meter, 2) A high equivalent resistance devinde by 1/Re=dI/dV where Re is the equivalent resistance and dI/dV is the change of current with voltage, and 3) A low residual capacitance defined by I=CrdV/dT. Where I is the forcing current, Cr is the residual capacitance and dV/dT is the change of voltage over time.

Topside Measurements

The Signatone Triaxial Package exhibits a typical noise floor of 2 fA and an effective resistance greater than 1 x 10^ 15 ohms when probing on top of the wafer at chuck temperatures to 300°C on a Semi-Automatic prober. The low nosie floor is obtained by use of proprietary materials in both the probe holder and the hot chuck plus the use of triaxial technology. The sub-threshold measurements of Ids vs. Vg are typical of the measurements that can be made. (click on graph at the top right of the page).

Substrate Current Measurements

Most parameters may be measured through a top-side contact. If the actual curent flowing through the chuck to ground must be measured, the porperties of the chuck become important. The Triaxial Hot Chuck System has a typical effective noise floor of less than 100fA and an effective resistance of approximately 1x10^ 15 ohms @ 200°C. Although this noise floor will affect the measurement of currents that pass through the chuck, the chuck presents a very quiet environment to devices measured through the topside contacts for measurements of 1 or 2 fA.

Typical Noise Floor Topside
Typical Noise Floor - Topside measurements. Note: Actual minimum current resolution is 1 fA.
(Click image for larger view)

Typical Noise Floor
Typical Noise Floor - 8" Triax Chuck @ 200°C. Note: Actual minimum current resolution is 1fA.
(Click image for larger view)


Typical Triaxial Probing System
Typical Triaxial Probing System


Specifications
Electrical Characteristics* Noise Floor Effective Resistance ohms @ Chuck Temp.
Probe tip holder & tip
+/- 2fA
>1 x 10^15
300°C
6" Chuck
<30 fA
>1 x 10^15
Ambient
8" Chuck
<30 fA
>5 x 10^15
Ambient
6" Chuck
<50 fA
>5 x 10^15
200°C
8" Chuck
<50 fA
>5 x 10^15
200°C

Note: Measurements made on an HP4156A under long integration with a 10 second hold time and a 1 second delay time after temperature stabilization and zeroing the SMU.

Residual Capacitance*
6" & 8" both @ Ambient & 200°C.....................4 pf typical

Note: Measured on an HP4156A by recording the voltage change versus time while forcing current with the guard enabled.

Temperature Parameters 6" chuck 8" Chuck
Ambient to 200°C typical
10 min.
16 min.
200°C to Ambient
4 min.
4 min.
Uniformity at 200°C
+/-2°C
+/-3°C
Max Operating Temp.
300°C
300°C


Physical Parameters Diameter Thickness or Length Cable Length
Probe tip holder
0.187"
2.5"
60"
Room Temp. Chuck
6.5"/8.5"
0.625"
60"
High Temp. Chuck
6.5"/8.5"
1.25"
60"

Applications Notes for low current analysis.

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LucasSignatone Corporation, Manufacturers & Producers, Gilroy, CA