2 Detailed Description

Mechanism of contaminant interaction with sensors

The dielectric constant (ε) of a molecule and its polarity are directly proportional to each other. Dielectric constant measures a material's ability to store electrical energy relative to a vacuum. Polarity is the measure of charge separation within a molecule. Higher polarity enables higher charge storage as the molecules align themselves in presence of electric field. Thus, generally higher polar molecules will have higher dielectric constant. Polymers used for capacitive humidity sensors act as dielectric material for the capacitive electrodes. Typically, sensing polymers will have a dielectric constant between 2 and 3. Water vapor primarily interacts with polymer based on its polarity or dielectric constant of 80 at room temperature which varies over temperature. When a water molecule dipole encounters the surface of the polymer, the high polarity of water molecules alters the dielectric constant of the polymer itself. The increasing amount of water vapor increases the overall change in dielectric constant of polymer which is captured by analog signal chain on the IC as capacitance change. The dielectric constants of several common solvents are shown below in Table 2-1:

Table 2-1, taken from: https://depts.washington.edu/eooptic/linkfiles/dielectric_chart%5B1%5D.pdf

Consider bulk contaminations due to gas (for instance, VOCs off-gassed from PCB assembly chemicals such as adhesives or paint) or surface contaminations (for example, from a liquid solvent). If the polarity or dielectric constant of the contaminant is large and interacts with hydroxyl surface states of the polymer, it can alter the dielectric constant or overall capacitance output of the polymer. This will be read as change in %RH output of device despite no change in actual moisture level. This contaminant interaction is a combinatorial effect of following parameters:

• Polarity of contaminants

  Typically, small carbon chain VOCs such as isopropyl alcohol, acetone, ethanol are more polar than long carbon chain VOCs such as butane. Carbon chain molecule's polarity will typically increase with length, and those with functional groups attached (for example, alcohols or ketones) will even more polar. Whereas cyclic organic compounds such as cyclopentane, Benzene, and Toluene are non-polar. Choosing chemicals with less polarity or non-polarity helps to reduce the shift in overall sensor accuracy. Non-polar solvents will typically have dielectric constant even lower than the polymer and thus have a relatively insignificant impact on the RH accuracy shift.

  Even if a potential chemical contamination is non-polar there can be impacts to the accuracy of the sensor. When the exposure concentration is very high and sustained, the polymer can swell due to how much of that chemical has diffused into it. Often the sheer size of the contaminant molecule will occupy a significant amount of free space in the polymer voids. This will further contribute to swelling of the polymer and alter the dielectric constant just through the sheer amount of contaminating molecules there are. Therefore even if a chemical contaminant is non-polar and hence lower risk of affecting the RH accuracy, exposure should be limited as much as possible to protect the RH sensor's accuracy.

• Concentration of contaminants compared to RH levels and the diffusion rate.

  Fickian diffusion is a process where gas particles move from high concentration to low concentration. The diffusion rate of contaminants (liquid or VOC) is a function of its concentration, temperature and affinity towards functional groups of the polymer. In case of the polymer's interaction with moisture or contaminant, it will be a linear function of concentration of moisture/contaminant inside the polymer versus in the ambient at the air-polymer interface. The later stage of this diffusion involves significant reduction in rate of absorption as the material approaches saturation and results in swelling of the polymer.

  Non-polar molecules typically are unable to penetrate the compact polymer chain matrix due to their lack of polarity. Polar molecules will diffuse into the matrix depending on their initial concentration and highly attractive force towards polar molecules of the polymer.

  Figure 2-1 below illustrates the Fickian diffusion. Molecules that are more polar will penetrate deeper into the polymer matrix due to their enhanced diffusion rate. This means that their impact on sensor accuracy will be enhanced, and more challenging to be diffused out of the polymer.

Figure 2-1 Diffusion Properties of Different Levels of Polar Molecules.

• Affinity of contaminants towards the functional groups of the sensing polymer.

  Polar protic solvents interact with polymer functional groups through hydrogen bonding, often increasing the overall dielectric constant of the polymer. Polar protic solvents contain hydroxyl groups which directly compete with water vapor to occupy the polymer since they bond in the same way. This is the main mechanism of moisture interaction with polymer. Thus, alcohol such as ethanol and isopropyl alcohol (IPA) will always have large impact on RH accuracy. Some polar aprotic solvents such as acetone will interact with the sensing polymer by affecting the overall polarity instead of directly competing with water vapor molecules. These will also result in an increase in the dielectric constant of the polymer.