Coulombic and Johnsen–Rahbek are two types of ESC defined by the bulk resistivity of the ceramic dielectric used.
Semiconductor 300mm ESC designs are almost exclusively Coulombic. However some 200mm and 150mm designs were Johnsen–Rahbek and are now being replaced by Coulombic.
The difference governs how much clamping force the ESC makes per volt, how fast the wafer dechucks, and how stable it stays as temperature changes. This article compares both types side by side — the physics that sets each one, the trade-offs, the dielectric materials that map to each type, and a clear rule for choosing between them.
For the full scope of ESC design, materials and applications, see our complete guide to ESCs.
Different dielectric bulk resistivity
Chucking type and chucking voltage are a direct consequence of how resistive each dielectric material is. A dielectric that behaves as a near-perfect insulator, above 10¹⁵ Ω·cm, holds charge on the buried electrode and produces Coulombic clamping. A dielectric which is a high impedance conductor, slightly conductive in the band of roughly 10¹⁰–10¹³ Ω·cm, lets charge migrate through the ESC and produces Johnsen–Rahbek (JR) clamping. The gap between those two resistivity windows is well defined.
How charge behaves for each type
The two ESC types differ because of where the charge ends up. Understanding it is the key to reading the comparison that follows, and it builds directly on the working principle covered in how an ESC works.
- Coulombic — charge stays on the buried electrode because the insulating dielectric will not let it through. The wafer and electrode act as the two plates of a capacitor separated by the full dielectric thickness, and the clamping force builds across that whole thickness.
- Johnsen–Rahbek — charge leaks slowly through the slightly conductive ceramic and accumulates at the ESC surface, directly beneath the wafer. The effective separation is the microscopic gaps between the wafer and the contact points on the ESC surface.
Because electrostatic force rises steeply as the gap shrinks, moving the charge to the surface is what gives JR its defining characteristic: far more force for the same voltage. Everything else in the comparison — voltage, release speed, temperature behaviour — follows from this difference in where the charge sits.
Clamp force per volt and operating voltage
Clamp force scales with the square of the applied voltage and inversely with the square of the gap between the charges. In a Coulombic ESC that gap is the full dielectric thickness, typically a fraction of a millimetre, so generating a useful clamp pressure of 20–80 Torr-equivalent demands a high voltage — often well above 1 kV, within the 600–2,000 V DC range an ESC supply provides. In a JR ESC the effective gap is the microscopic surface roughness, orders of magnitude smaller, so the same clamp pressure is reached at a much lower voltage. JR therefore delivers higher force per volt, and a JR ESC can hit its force target near the bottom of the voltage range while a Coulombic ESC of the same force target sits near the top. How that force is specified and verified against helium pressure is covered in our guide to ESC clamping force.
Release and dechuck speed
Release speed is the mirror image of where the charge sits. A Coulombic ESC stores almost no charge in the ceramic itself, so when the voltage is switched off the field collapses quickly and the wafer releases fast, usually toward the lower end of the < 1–5 s dechuck window. A JR ESC has driven real charge into the ceramic surface, and that charge has to decay back out through the same slightly conductive bulk before the wafer lets go — a slower process that also leaves the ESC more prone to wafer sticking, where trapped charge keeps the wafer stuck after power-off. JR ESCs therefore rely on a controlled dechuck sequence, with a reverse-polarity pulse and plasma-assisted discharge to bleed the stored charge before the lift pins move.
Temperature sensitivity and why JR force shifts
Temperature stability is where the two regimes diverge most sharply in production. A Coulombic ESC is largely insensitive to temperature: its insulating dielectric stays insulating across the operating range, the charge stays on the electrode, and the clamp force barely moves as the wafer heats. A JR ESC behaves very differently, because the resistivity of a slightly conductive ceramic falls as temperature rises. As the ESC heats during processing, the ceramic conducts more freely, charge migrates to the surface faster and in greater quantity, and the clamp force climbs. An ESC tuned for the right force at one temperature can clamp too hard at another, complicating dechuck and risking the wafer. This is the central trade-off of the JR regime: the same surface-charge mechanism that buys high force per volt also ties that force to temperature.
Which dielectric materials map to which regime
Material selection is how an ESC is placed in one regime or the other, because the dielectric’s bulk resistivity is a property of its composition. Three material families cover most production ESCs:
- Alumina (Al₂O₃) — robust, well understood, with a high resistivity that places it with Coulombic ESCs. It is the default choice for fast release and temperature stability.
- Doped or composite alumina — alumina with titania or other additives that lower its bulk resistivity into the 10¹⁰–10¹³ Ω·cm JR window. This is the engineered route to JR behaviour while keeping alumina’s mechanical robustness.
- Aluminium nitride (AlN) — gives better wafer temperature uniformity, and a resistivity that suits JR operation. However high thermal conductivity for even cooling requires precise base-temperature control to keep its clamp force predictable.
Coulombic vs Johnsen–Rahbek: side-by-side comparison
The table below collects the practical differences in one place.
| Property | Coulombic | Johnsen–Rahbek |
|---|---|---|
| Dielectric resistivity | > 10¹⁵ Ω·cm (near-perfect insulator) | 10¹⁰–10¹³ Ω·cm (slightly conductive) |
| Where charge sits | On the buried electrode | At the ESC surface, under the wafer |
| Effective gap | Full dielectric thickness | Microscopic surface contact gap |
| Clamp force per volt | Lower | Higher |
| Operating voltage for a given force | Higher (often > 1 kV) | Lower |
| Release / dechuck speed | Fast | Slower (surface charge must decay) |
| Residual clamping risk | Lower | Higher |
| Temperature sensitivity | Low | Higher (force rises as resistivity falls) |
| Typical dielectric | Alumina (Al₂O₃) | Doped alumina, AlN |
Frequently asked questions
What is the main difference between Coulombic and Johnsen–Rahbek ESCs?
The dielectric resistivity. A Coulombic ESC uses a near-perfect insulator above 10¹⁵ Ω·cm that keeps charge on the buried electrode; a Johnsen–Rahbek ESC uses a slightly conductive ceramic at 10¹⁰–10¹³ Ω·cm that lets charge migrate to the surface, giving much higher force per volt at the cost of slower release and more temperature sensitivity.
Why does Johnsen–Rahbek clamping force change with temperature?
Because the resistivity of slightly conductive JR ceramic falls as it heats. Lower resistivity lets charge migrate to the ESC surface faster and in greater quantity, so the clamp force rises with temperature. Coulombic ESCs do not show this effect, as their insulating dielectric stays insulating across the operating range.
Which clamping regime should I choose?
Choose Coulombic for fast, clean dechuck and stable force in temperature-critical or fast-cycle processes. Choose Johnsen–Rahbek when you need maximum clamping force or must operate at lower voltage. The regime is set by the dielectric material, so it is decided when the ESC is specified rather than adjusted on the tool.