LOW CURRENT ANALYSIS

for Measuring fA Currents @ Temperature

The Signatone Triaxial Package

The Signatone Tripak System, combined with a suitable parametric instrument such as the HP4156A, allows measurements of current to the femtoamp (1 x 10 -15A) 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.

The Triaxial Chuck is available in standard and temperature chuck configurations. Chucks are easily retrofited to Signatone 6 and 8 inch Probe Stations.

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 top of 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 measurements. Note: Actual minimum current resolution is 1 fA (click on graph for larger image)

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

Typical Triaxial Probing System shown with prober in a dark box. Click image to see a larger photo

Triaxial Probing System Prober equipped with local dry chamber- Click image to see a larger photo

Signatone Triaxial Hot Chuck Performance

Applications Notes for low current analysis.