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Agilent
Accurate Impedance
Measurement with
Cascade Microtech
Probe System
Application Note 1369-3
1. Introduction
More ICs or circuit modules are used in electronic circuits
to save spaces, more capacitors or inductors, such as
thin dielectric layers and pattern inductors, tend to be
developed on wafer or substrate. These devices usually
have a small capacitance or inductance like pF, nH range.
The Agilent E4991A RF Impedance/ Material Analyzer
and Agilent 4294A Precision Impedance Analyzer have a
wide impedance coverage as well as a good accuracy and
offer an accurate on-wafer or micro-component impedance
measurement solution with probe stations. In this
application note, details of installation and calibration
methods are discussed.
2. 1-Port Impedance Measurement
Application using Probe Station
Table 2-1 shows major 1-port impedance measurement series with a probe station can provide very good
applications. Capacitance or inductance needs to be performances for these applications. Agilent has two
evaluated in these applications. In many cases, the wide impedance analyzers that cover different frequency
impedance coverage and the good accuracy are required ranges. Each solution is discussed from next chapter.
to make these measurements. Agilent impedance analyzer
Table 2-1. 1-Port Impedance Measurement Application using Probe Station
Application Parameter Final product Frequency Measurement requirement
Spiral inductor L, Q RFIC for mobile phone GHz - Low inductance (nH range)
- High Q
IC package C, L IC package GHz - Low inductance (nH range)
- Low capacitance (pF range)
Transistor diode C CMOS FET MHz, GHz - C-V measurement
Pin diode
Transistor/diode for optical
Memory C, D FRAM, DRAM, SRAM MHz, GHz - Low capacitance (pF)
Loop inductors of circuit L, C High-speed digital GHz - Low inductance (nH range)
pattern, stray capacitance - Low capacitance (pF range)
of circuit patterns
Dielectric material C, D Thin film layer, PC board MHz, GHz - Wide impedance range
- Low-loss
Disk head L, Q GMR head, magnetic head MHz, GHz - Low inductance
3. RF Measurement Solution with
Agilent E4991A (1 MHz - 3 GHz)
3.1. Theory
The E4991A employs an RF I-V method as a measure- Measurement Handbook, 2nd Edition" (P/N 5950-3000)
ment technique, which allows us to do accurate and for more details.
wide impedance measurements up to 3 GHz. Figure 3-1
shows the simplified block diagram of the theory. Besides the measurement theory, the E4991A has an
Impedance of a device under test (DUT) is derived unique receiver configuration, in order to increase a
from measured voltage and current values. temperature stability. The E4991A's 2 voltmeters are
switched each other, and the voltage and current data
The current that flows through DUT is calculated from are always measured twice by switching voltmeters
the voltage measurement across a known low value (See figure 3-2). With it, the tracking error of vector voltage
resistor(R). In practice, a low loss transformer is used in measurements are cancelled out. This enables us to
place of the low loss resistor. Refer to "Impedance minimize the temperature drifts of measurement circuits.
Figure 3-1. RF I-V Method Figure 3-2. E4991A's Receiver Configuration
2
Figure 3-3 and 3-4 shows impedance measurement 1 nH inductor over 100 MHz has a very small impedance,
comparisons between the E4991A and a network analyzer. so the wide impedance measurement capability is required.
Figure 3-3 shows the repeatability for 1nH measurements.
Figure 3-4 shows the stability over temperature. The For more details about the comparison between
RF I-V is more repeatable over time and is stable over impedance analyzers and network analyzers, refer
temperature. In general, impedance analyzers can Agilent E4991A application note 1369-2 (P/N5988-0728EN).
provide wider impedance coverage than network analyzers.
Figure 3-3. Repeatability of 1nH Measurement
(a) Inductance Variance (p-p) (b) ESR Variance (p-p)
Figure 3-4. Repeatability Over Temperature
(a) Capacitance (100MHz) (b) ESR (100MHz)
(c) Capacitance (1GHz) (d) ESR (1GHz)
3
3.2. Preparation
Following items are necessary to set up the probe The E4991A option 010 makes easier to establish the
measurement system. system. Detailed installation instruction is also included
1) Agilent E4991A with option 010 Probe Station in the E4991A Operation Manual.
Connection Kit (See table 3-1)
2) Cascade Microtech Probe Station, Probe Head, This combination is carefully evaluated both by Agilent
Impedance Standard Substrate (See table 3-2) Technologies and Cascade Microtech. Cascade Microtech
products listed in table 3-2 need to be purchased from
Cascade Microtech.
Table 3-1. Agilent Products Required for E4991A System
Item Description Remarks
E4991A RF impedance/material analyzer
Option 010 Probe station connection kit It includes:
- Small test head (1 ea.)
- 1 m cable (1 ea.)
- N(m)-SMA(f) adapter (3 ea.)
- 3.5 mm