Test System Capacitance Measurements at Different Wafer to Probe Separations

Abstract

Wafer-level capacitance measurement results often contain significant amounts of error.  Even when proper measurement system setup has been performed, differences in measured values between test systems on the order of a few hundred femto-Farads are commonly seen.  “Proper measurement system setup” in this context includes proper frequency, network model selection, bias, and pin connections, and subtraction of parasitic test system capacitance.  This document identifies the primary reason for these differences as a Z-dependent parasitic test system capacitance.  Specifically, system capacitance measured with probes separated from the wafer is not the same as system capacitance with probes in contact.  The Z-dependence of system capacitance is also a strong function of probe card.  The MZ capacitance measurement method is presented, which takes system capacitance measurements at different wafer to probe separations and extrapolates to determine the system capacitance at contact.  Using this method, measurement offsets between test systems can be reduced significantly.

Problem Description

Accurate capacitance measurement is required for process monitoring and for circuit simulation model creation.  Making accurate capacitance measurements, however, is a challenging task, especially when making measurements on an automated test system with a probe card.  There are many aspects that must be properly considered when configuring the test system.  Bias voltage and frequency must be properly set, appropriate pin connections must be made, an appropriate network model must be selected to extract capacitance from complex impedance, and parasitic test system capacitance must be subtracted from the measurement.

Traditional “2Z” capacitance measurements (Fig. 2a) attempt to measure system capacitance by separating the wafer from the probe tips by a small distance (~10 mils), and measuring capacitance.  Then, the wafer is put in contact with the probe tips, and a contacted measurement is made.  By subtracting the contacted measurement from the system capacitance measurement, the capacitance of the device under test (DUT) is obtained.

The “2Z” capacitance measurement method can result in measurement differences between test systems on the order of a few hundred femto-Farads for the exact same DUT.  This is despite the fact that common capacitance meters, such as the HP4284, have absolute accuracy which is much better than this.  The reason for the measurement differences lies primarily in inaccuracy of system capacitance measurement.  The “2Z” method ignores the fact that system capacitance is a function of wafer to probe separation.  This Z-dependence has been found to be the primary cause of observed system-to-system offsets.

The Z-dep...