OPT uses a double sided quadrupole design
to ensure a constant clamping force over
long process times and reliable declamping.

Helium is introduced between the cooled electrode and the substrate to establish an excellent thermal contact.

BACKGROUND
The simplest “monopole” system would look like a capacitor with a high resistance dielectric material separating the plates of the capacitor. In practice the lower plate then is an electrode charged to a high voltage and the dielectric is an insulating layer of material between the HV and the second plate (the wafer). It needs plasma to complete the circuit so there is no clamping when the plasma is turned off . The HV on the electrode induces charge separation in the rear surface of the wafer but it can only do this effectively if the circuit is complete.

Practical systems use more than one electrode, each charged to different voltages to create the charge separation in the wafer without the need for plasma to be present. The simplest of these uses a bipolar electrode arrangement (left pictures). Force is generated from induced charge separation by the proximity of oppositely charged electrodes embedded in a dielectric. Some types need a high voltage to work (thick-film dielectric) and often use ‘D’ shaped electrodes positioned back-to-back. The SEMCO system used in the OPT e-chuck uses a “thin-film” dielectric, which means that the dielectric layer a few 10’s of microns thick only and that the operating voltage is much lower. This particular type uses a series of concentric ring electrodes to optimise clamping efficiency.

The SEMCO e-chuck uses a quadrupole electrode, which is a more advanced form of the bipolar electrode. The need for two high voltage sources originates from the behaviour of dielectrics when exposed to high electric fields for extended periods of time. It is possible to induce a permanent polarization, which ruins the clamping ability of the chuck. The solution is to change the polarity of the applied voltage at regular intervals throughout a process so that any residual polarisation is short lived.

To maintain clamping during the polarity change, a second voltage source is used connected to intermeshed electrodes. This ensures that there is always force acting on some part of the wafer holding it in place. A process could run indefinitely without suffering the effects of permanent dielectric polarization. It helps eliminate residual charge effects that can lead to unwanted clamp force - at the end of a process run, for example. With a system such as this, wafer release occurs within a few seconds of the voltage being removed.

The HV supplies can be controlled directly via the PC software when set in REMOTE mode. The control box forms part of the “Process Page” in the front-end software where all process parameters are defined (left figure). The REMOTE interface allows external setting of HV on/off, set-point and timing information, as well as a readback signal to the controller. The polarity cycling process to prevent permanent polarisation of the e-chuck dielectric involves first removing and then reapplying voltage in such a way that there is always one power supply active during the whole process and therefore ensuring that the wafer is always clamped. The sequence is summarised in the table.

The alternative to the electrostatic clamping is the simple "mechanical" clamping. Both methods can be combined in one design.