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Low Current Measurement Technologies in 4080 Parametric Test System

Technical Overviews

Keysight 4080 Series¡ Parametric Test Systems

  •  High-resolution SMU
  •  High-resolution pin board
  •  Ultra-low leakage probe card
  •  Optimization of integration time
  •  Comparison of settling time for the ultra low-current model and standard low-current model

Introduction

The trend in semiconductor devices is toward lower cost and lower power consumption with better operating speed and functions. To address this trend, deep sub-micron and low-power consumption processes have been developed, which are evaluated by low-current and low-voltage measuring instruments. For example, DRAM cell evaluation now requires ultra low-current measurement below 100 fA.

Both resistance evaluation of advanced Cu interconnect and matching tests for new analog devices need sub-1 μV measurement capability. Further, the measurement must be fast, especially at production facilities.

To meet these needs, the Keysight Technologies, Inc. 4080 Series Parametric Test System measures low current and low voltage down to the femto-ampere (fA) and micro-volt (μV) levels, without losing test speed. This is the very performance semiconductor process development engineers need.

Semiconductor fabricators will also benefit from the 4080 test system because its high throughput supports an integrated process monitoring environment from development to production.

This technical overview describes how the 4080 attains such high performance by focusing on the following items: high-resolution SMU, high-resolution pin board, ultra-low leakage probe card, optimization of integration time, and comparison of settling time for the 4080 ultra low-current model (ULCM) and standard low-current model (SLCM).

High-resolution SMU

The high-resolution SMU (HRSMU) is designed to achieve ultra low-current measurements. Power coverage of the HRSMU is the same as that of the MPSMU but the lower-current ranges of 100 pA and 10 pA are supported by the HRSMU.

The MPSMU’s lowest measurement range is 1 nA, compared with 10 pA for the HRSMU. In addition, both force and measure resolutions are improved to 1 fA for the HRSMU, allowing the 4080 ULCM to measure low current more precisely than the 4080 SLCM. Accuracy is also improved.

The HRSMU is connected to the drain terminal of the MOSFET and another SMU is connected to the gate terminal. Source and substrate terminals are connected to the GNDU. Drain voltage is set to a constant 2 V, gate voltage is swept from –250 mV to 250 mV in 12.5 mV steps, and drain current is measured for each step in the 10 pA range.

The integration times are 16 power line cycles (PLC). This example shows that the fA level low current is measured in a stable manner.

Low-current measurement to the fA level can be achieved by using the 10 pA range of the HRSMU. Some program statements are necessary to use the 10 pA range.

When limited auto range mode is used, the 10 pA range must be specified as shown in the example at the right.

When auto range mode is used, ranges lower than 1 nA must be enabled by using the Set_rangemode statement.

The minimum current range of the auto range mode is 1 nA (the default setting) in order to maintain compatibility among the 4080 ULCM and SLCM. Note that the lowest available current range for the 4080 SLCM is 1 nA. To enable lower ranges (10 pA and 100 pA) on the 4080 ULCM, a Set_rangemode statement is used, as shown in the example at the right.

The HRSMU enables the low-current measurements as shown in Figure 1. Note that the high-resolution pin board and ultra low leakage probe card, which are discussed in the following sections, contribute to this low-current measurement capability.

High-resolution pin board

The 4080 ULCM uses high-resolution (HR) pin boards, which consist of new relays configured to minimize drift of thermo-electromotive force for the matrix switches. This allows the 4080 ULCM to achieve better low-current and low-voltage measurements than the 4080 SLCM.

This performance improvement is apparent in the digital volt meter (DVM) accuracy of the specifications and Table 2 shows a comparison of the DVM accuracy for the 4080 ULCM an SLCM. Note that the lower range of 0.1 V is supported by the 4080 ULCM and the offset voltage in the accuracy column is 100 μV, while the 4080 SLCM has an offset voltage of 300 μV.

Ultra low leakage probe card

The ultra low leakage probe card, jointly developed by probe card vendors and Keysight, is designed so that force and sense pads for each measurement pin are surrounded three-dimensionally by the guard conductor. This reduces dielectric absorption on the probe card.

Nothing is connected at the probing needles and forced voltage is 100 V. The measurement range is 1 nA and integration time is 16 PLC. Since the dielectric absorption of the ultra low leakage probe card is very small, a current less than 100 fA flows after 100 V is forced.

In general, the current caused by the dielectric absorption is relative to the forced voltage. For example, when the forced voltage is 10 V, the current is less than 10 fA.

Optimization of integration time

New methods for optimizing integration time are supported by the 4080 ULCM. These techniques utilize a new integration mode called SMART and an additional integration time setting. The SMART mode and additional integration time setting can reduce unnecessary integration time to improve measurement throughput.

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