A Basic Review of the Concept of No Free Charges in Insulators

Leo G. Henry

A solid understanding of science is necessary to understand how charges work in insulative materials.

A well-established statement of electrostatic fact is this: Charges do move and can be made to move in conductors, but charges do not and cannot be made to move in insulators. The statement applies to those situations in which both the conductors and the insulators can be charged by direct contact, by direct charging, by tribocharging, or by induction.^1–5 A voltage can be applied directly to a conductor for direct charging, but an insulator cannot be directly charged.

If charges do not and cannot be made to move in insulators, this raises a question: When an insulator becomes charged, are there really extra charges associated with the internal insulator structure? Another question: Did the insulator actually lose charges from the structure? The answer to both questions should be a resounding no. And, why is it that a conductor, when grounded, becomes neutralized, but an insulator retains the charge?^6,7

These questions continue to persist at symposiums, in tutorials, workshops, and panel discussions. Because the answers and explanations are often inconsistent, they inevitably lead to more confusion. This article provides some insight on this subject by reviewing science—physics, chemistry, and materials science—to explain how charges work and don't work in insulators.

The Conductor

It is best to first review what happens to charges in the case of a conductor. Figure 1 shows one of the many representations found in the literature for uncharged metallic conductors.^8,9 In these metallic solids, each atom provides the electric charge in the form of a positive and heavier nucleus, and a cloud of lighter electrons (negative charges) around the nucleus. In general, before the conductor becomes charged, the summation of all the positive and negative charges makes the material neutral as a whole.¹⁰

Figure 1. Uncharged conductor as represented in the literature.

Because very few explanations about the representations and drawings are given in most texts, it is best to regard Figure 1 (the conductor) as a surface or a cross-sectional view of the inside of the material's surface. The representation in this figure is regarded as one that treats the lighter negative charges (electrons) as free charges (i.e., free to drift throughout the entire material). They are considered free because, in theory, the electrons are not tied to any one atom, and because the bonding of the electrons to the atomic nuclei is very weak.¹¹

Figure 2 represents the material after it has been exposed to induction by bringing a positively charged body close to one of the surfaces, say, the left side of the figure. The positively charged rod attracted the electrons that were free to move to the inner surface under the influence of the positive electric field from the charged rod. The nuclei, which are positive, will be repelled, and a chain reaction will occur throughout the material, resulting in a positive charge appearing on the remote (back) surface.

Figure 2. Induced charge separation in a conductor, as represented in the literature.

The electrons on the left (front) surface are bound (held there) because of the force of the externally applied electric field from the charged rod. In some figures found in the open literature, the chain reaction occurring within the material is not shown nor is it explained. So the sense of what is happening to the material as a whole is unclear.

It is quite important to note that during this process no electrons were added to or exchanged within the structure. Only charge separation took place. The material as a whole is still neutral and will only become truly charged if a grounded conductive wire is bonded (touched) to the conductor. If this is done to the back surface, electrons will flow to the conductor from earth ground and the conductor will become negatively charged.

This example, however, illustrates the physics, which indicates that the valence electrons are not bound to any particular atom in the material, but rather are loosely bound (interatomic bonds) in the structure. The electrons, therefore, are free to drift through the entire metallic material. These interatomic metallic bonds in the material are, of course, caused by the electrostatic attraction between the atoms and the electrons. However, if the field (from the charged rod) is removed before grounding the conductor, the material will eventually return to its original state as shown in Figure 1.

The Insulator

With the conductor example in mind, it is easier to review what happens to charges in the case of an insulator. Most insulators in use today (reticle carriers or cassettes, front-opening unified pods (FOUPs) for transporting wafers, standard mechanical interfaces (SMIFs) used for both wafers and reticles, plastic bags, etc.) are polymeric (organic) in nature in that the basic building block (see Figure 3) for the polymeric structure is a combination of carbon and hydrogen atoms joined in chainlike structures. The atoms in the molecular entity share the electrons in the chemical bond.¹² Examples are methane (CH₄), ethane (C₂H₂), ethylene (C₂H₆), and the saturated hydrocarbons (C_nH_n), as shown in Figure 3.

Figure 3. Polymeric hydrocarbon basic building block.

These building blocks are molecular in structure (as opposed to the atomic structures in metals), and it is possible, therefore, to say that the insulator is made up of molecular entities rather than the atomic entities as are found in conductive metallic materials. In Figure 3, the extra electrons on the C atoms at the end of the chain are available for further bonding. These structures are called hydrocarbons.

For the insulator, the electrons are shared by the nuclei of at least two atoms, and therefore, are bound so tightly (covalent bonding) in the molecular structure that there is not enough energy (5–10 eV binding energy) for the electrons to get across the very large energy gap between the occupied (with electrons) valence band and the unoccupied (no electrons) conduction band.¹³

To be more specific, the thermal energy of the electrons is so small (kT = 0.025 eV at ambient conditions) that there is not enough energy for any to reach the conduction band. With no electrons in the conduction band, then there can be no free electrons in the molecular entity. With no free electrons in the material, attaching a piece of grounded conductive wire to the insulator will have no effect.¹⁴

Most of this information can be gleaned from a thorough study of physics, chemistry, and materials science textbooks.^{1,3,12–13,15–17} Much of the confusion stems from the numerous variations for insulators that exist in the literature.^6,12,18–20 Some are quite good,^14,17 but most are inadequate, and in too many cases, completely wrong.

In addition, even when a representation is acceptable, texts often provide only limited explanation as to the nature of what is actually happening inside the material. Understanding the activity inside the material is critical because it contributes to what eventually happens on the surface.

A search of the literature reveals that Figure 4 is used in most cases to represent a charged insulator. This figure is incorrect. If the same theme developed for the conductor (Figures 1 and 2) is used for the insulator shown in Figure 4, then the figure indicates that the negative charges appear to be free to move in the material. As indicated earlier in this article, this is incorrect. There are no free charges in insulators. Figure 4 has also been used to show that the surface is charged locally with different charge polarities, or polarized locally with different charges. However, Figure 4 does not represent polarization because the positive and negative charges in the figure are shown to be separate entities. This is not correct for an insulator.

Figure 4. Charged insulator as represented in the literature.

Recall, however, that the very strong chemical bonding in insulators does not allow the electrons to move, because the electrons are tightly bound to the nuclei in the structure. Insulators, therefore, must be represented differently (from Figure 4) to illustrate what actually happens not only inside the body of the material but also at the inner surfaces.

Uncharged Insulator

If the example above represents a charged insulator, how then is an uncharged insulator represented? It certainly cannot look like Figure 1. Figure 5, however, is a very simple representation because it looks as though the entities are floating around in the material in all possible orientations.

Figure 5. Uncharged insulator showing molecular polar entities.

Figure 5 shows the combination of a positive charge and a negative charge confined to a specific entity (slightly egg shaped for this view). Because molecules are made up of at least two atoms bonded together as one entity, each entity can be called a molecule. These molecular entities are randomly oriented and distributed inside the material. Both positively and negatively charged ends of the molecules also reside on the inner surface of the material. The insulator material as a whole is neutral, and no field will be detected if a fieldmeter is brought within published measurement distance (typically 1 in. or ~2.54 cm) of any of the surfaces.

This figure is a very simplified representation and could be used to represent the molecules in solid water, where the negative charge would be represented by the oxygen atom (it has extra electrons), and the hydrogen atom would represent the positive end of the charge. Could this same picture be used to represent a solid insulator such as sodium chloride?

Because neither the positive nor the negative charges are free to move in the insulator material, these charges must necessarily be tightly bound (chemically) to something. It is beyond the scope of this article to explain how quantum mechanics provides a thorough understanding of the structure of molecules as it relates to the chemical bonding and the structure in solids.¹³

Simply speaking, each entity represents two atoms so strong in their chemical bonding that they function as a single unit, the molecule. In most cases, the electrons are shared unequally between the two atoms, and the electrons spend most of their orbiting time around one atom (electronegativity property) leading to what appears to be charge separation. An example is the C-H covalent bonding found in hydrocarbons. The overlapping electron clouds form the binding energy between the two atoms, but the electrons spend most of their time around the carbon atom for this structure.¹¹

This conclusion simply means that the view shown in Figure 4 is not the best view. Consider Figure 5 as a side view, with F representing the front edge of the front face and R as the rear edge of the rear face. Furthermore, if the figure represents a cutaway or cross-sectional view, it provides a view of the inside of the material. Any one entity (molecule) in Figure 5 can be used to represent the hydrocarbon (Figure 3) where the carbon atoms represent the negative end and the hydrogen atoms represent the positive end (see Figures 6a and 6b). Each molecular entity can have more than one atom in it, which provides some insight into why insulators can develop both a positive and negative charge on the same piece of material.

In Figure 6, a negatively charged (Q ~ = 1-10 µC) rod (plastic or amber resin) was brought close enough to the left edge of the insulator to result in the positive field forcing the negative end of the molecule to be repelled and the positive (nuclei) end to be attracted to the negatively charged rod. In the strictest sense, this is not charge separation. It is polarization, and it creates an an entity called a molecular dipole.^{14,17–18,21} The dipoles align with the field. The field inside the insulator is reduced, but it is does not go to zero. The electrons are not separated completely from the atomic nuclei. None of the electrons is free to move beyond the bonding or field of the nucleus. And, even though the molecular dipoles look as though they are floating about in the material, they are actually bonded to their neighbors by the weaker van der Waals forces, by hydrogen forces, or by additional covalent bonding.

Figure 6. Polarization in insulators: (a) polar molecule, (b) C-H bonding inside the molecule in (a), and (c) the polarized insulator.

When the molecular entities interact with each other during further chemical processing (breaking old bonds and forming new ones), the end result is a polymeric material made up of covalent bonds not only between the atoms, but also between the molecules. These long chains of carbon atoms are both covalently bonded together and covalently bonded to the hydrogen atoms.

During chemical processing, the molecules rotate, bend, kink, coil, and twist in three dimensions, allowing these molecular chains to cross-link and close loops.¹¹ They intertwine and entangle with neighbors, leaving no free electrons. This is the nature of the insulator. For electrostatic discharge (ESD), a good example of an insulator is polyethylene, which is used as the basic structural material for manufacturing ESD bags such as pink poly. If all the hydrogen atoms in the polyethylene structure are replaced with fluorine atoms, the structure becomes polytetrafluoroethylene (PTFE), also known as Teflon, a fluorocarbon and a good insulator.

No Charge Transfer

A practical example of "no charge transfer" or "no charge movement" through an insulator provides further insight. Consider the device-level ESD testing of integrated circuits (ICs) using the charged device model (CDM).

The setup in Figure 7 shows three distinct components: the charging plate (a conductor), the insulative package enclosing the IC, and the device that sits on a conductive paddle, which is directly connected to the conductive lead frame. The lead frame, in turn, is connected via the conductive bond wires to the external conductive leads.

Figure 7. The electrostatic view for ESD device testing.

A high voltage (say, positive) is applied directly to the conductive charge plate. The insulative polymeric package material sitting on this charged conductor actually becomes polarized because of the field from the charged conductor, and the tightly bound electrons (negative charge) in the molecular entities are attracted to the positively charged conductive charge plate.

The oppositely charged (positive nucleus) end of the molecular structure induces a polarized charge in the next molecule. There is no electron transfer. This continues throughout the polymeric insulative package until the polarized molecules at the top induce a charge separation on the conductive paddle, the IC, the lead frame, the bond wires, and the external leads, all of which are directly connected to each other (see Figure 7).

The negative charge on the conductive lead frame leads to a positive charge on the leads farthest away from the direct influence of the insulative package. If any of the device leads are grounded, an air arc discharge occurs, and the resulting ESD event can be displayed in the form of a positive leading-edge waveform when captured on the screen of a single-shot high-bandwidth oscilloscope. Inside the actual IC, the gate oxide (dielectric) structures are where the high fields actually develop. This is where an internal breakdown would occur, causing the IC to fail.

Another example is a production reticle carrier, which uses a clear polymeric material for a cover. This clear plastic can be insulative or, if colored, the plastic can be static dissipative. If it is black, then it is conductive. Most carriers use clear and, therefore, insulative covers. These covers provide no shielding for the sensitive reticle inside because when a charge is induced (not placed), it results in the polarization of the molecular entities in the insulative covers.

The field extends to the inside and, in turn, induces charge separation on the conductive parts of the sensitive reticle. It also induces polarization on the glass structure of the reticle. The insulative cover material becomes charged by the polarization process, even though no charges were transferred to or from the insulative cover. No charges are transferred because the electrons are tightly bound to the nucleus and the whole molecular structure of the insulator.

Conclusion

No charge is transferred from the conductive charge plate to the insulative package of a device, and no charge is transferred from the insulative package to the device paddle. The induction in the conductive charge plate results in a field that polarizes the adjoining insulator. The field from the insulator induces a charge on the predominantly conductive internal IC or device. When a conductive body is exposed to a field, a charge is induced in it, and the conductor will become charged if it is grounded while in the presence of the field. When one of the positive conductive leads of the device is grounded, a discharge occurs where the negative charges (electrons) from the ground neutralize the positive charges on the conductive lead.

When an insulative body is exposed to a field, the molecular structures in the body become polarized, and a field appears on the surface because of charge orientations inside the surface of the insulator. Because there are no free charges in the insulator, a grounded conductive rod, or wire, contacting the polarized insulator will have no effect on the insulator. Therefore, insulative materials should only be discussed from the point of view that no free charges exist because the charges are bound tightly to the nuclei of the atoms that combine to make up the molecular structure. Insulators should not be illustrated as using an atomic entity in which both positive and negative charges are shown as isolated charges. The correct representation is one that contains both the positive and negative charges as one entity representing the molecular nature of the internal structures.

References