Dielectric Constant of Hydrocarbon Cleaner

Abstract

The Brookhaven Instruments’ BI-870 is used to accurately measure the dielectric constant of a decanamide cleaner. The dielectric value quickly reveals moisture that would lower the quality of the cleaning liquid.

Introduction

A dielectric material (dielectric for short) is an electrical insulator that can be polarized by an applied electric field. These materials are used in semiconductor and high voltage industry. When a dielectric is placed in an electric field, electric charges do not flow through the material as they do in a conductor, but only slightly shift from their average equilibrium positions causing dielectric polarization.

As the demand for higher speed semiconductor devices increases, low dielectric constant (low-k) films with smaller dielectric constants are required. Low-k films can be damaged by etching and worse, the cleaner may raise the effective dielectric constant of the low-k film. The need for low-k cleaners for this market justifies controlling the dielectric constant of the stripping material. Any hygroscopic low-k stripper will quickly exhibit a higher dielectric constant. In this application note we measure the dielectric constant of a proprietary hydrocarbon/amide stripper.

Furthermore, accurate zeta potential determinations of suspended colloidal particles used for chemical mechanical polishing (CMP) with zeta potential analyzers such as the Brookhaven Instruments’ NanoBrook Omni depend on an accurate knowledge of the dielectric constant of the liquid. For a range of particles of interest a number of different liquids may be used, including mixtures of various proportions creating the need for an accurate measurement of dielectric constant.

While literature values of dielectric constants are suitable when using pure liquids values are not readily available for mixtures of liquids, or for high temperature. There are no generally acceptable methods of calculating values for mixtures from pure component values. For example, 1:1 weighted by volume leads to errors of -8% for Methanol/Water and 2.4% for Dichloromethane/Benzene respectively and weighting by mole fraction leads to errors of 17% and 6% for the two mixtures.

The dielectric constant can be quickly and easily determined for vast variety of liquids, oils and gels using the Brookhaven Instruments BI-870, dielectric constant meter.

Dielectric Constant Calculation

The relative static permittivity, ε_r, can be measured for static electric fields as follows: first the capacitance of a test capacitor,C₀, is measured with vacuum between its plates. Then, using the same capacitor and distance between its plates the capacitance C_x with a dielectric between the plates is measured. The relative dielectric constant can be then calculated as

Material Method

The hydrocarbon/amide cleaner was a clear liquid. Approximately 5 mL of hydrocarbon/amide cleaner was poured into a measurement tube, gently rocked back and forth, and then discarded. The measurement tube was filled with 35 mL of sample and the probe introduced. The probe was gently tapped on the bottom of the tube to ensure no bubbles were present in the sample and between the probe cylinders. The BI-870 probe which was previously rinsed with the cleaner is completely immersed in the cell. The scale and measurement was easily done by selecting the appropriate scale –more on this below- and turning a fine tuner until the indication of a good value was received. The digital display on the front of the BI-870 gives the calibrated value of the dielectric constant of the material measured. In this case, here are some values for the chemical properties of the cleaner studied:

The BI-870 instrument has two selectable sensitivity ranges: 1-20 and 1-200. Absolute accuracy is ± 2% while repeatability and linearity are better than 0.2%. The measurement signal applied to the outer cylinder of the probe is a low-distortion sine wave at a frequency of 10 kHz to avoid polarizing the dielectric.

The amplitude is approximately 7 Volts RMS on the 1-20 range and 0.7 Volts RMS on the 1-200 range. The frequency is crystal-controlled and is, therefore, stable to approximately 1 part in 100,000. The dielectric constant of the liquid sample is determined by measuring the current between the outer and inner cylinders of the probe. With a stable voltage source and precisely known probe parameters, it is possible for the instrument to display the dielectric constant directly. Calibration is done at the factory and is usually not required in the field, but in the case it would be needed, it is simply done using the back panel adjustment with a liquid of known dielectric constant.

After allowing the sample to stabilize for approximately 90 seconds, a dielectric constant reading was taken from the instrument.

Results

Measurement of the cleaner on the BI-870 yielded a dielectric constant value of 16.09. The sample contained no bubbles or significant particulate matter. This cleaner proved to be moisture free.

Reference

1. Atkins, P. W. Physical Chemistry for the Life Sciences. Oxford, UK: Oxford UP, 2006.