Help me understand a CDM ESD protection

Hi all, I have attached a general ESD protection circuitry. The primary ESD is for HBM protection and secondary ESD is for CDM protection. I have the following doubt in CDM.

1. From the figure, i understand that the CDM ESD current will come from the pad and the secondary ESD protection circuit will take that current to vdd or vss. How the CDM current can come from pad? It should be coming from the core, because it is the discharge from the core as far as i understand.

Can somebody please clarify this?

esd cdm clamp

Actually you are right. With CDM the charges are located (and spread out) in the core circuit. The whole circuit is charged to a specific CDM voltage. One pin of the IC will contact a grounded surface and all of the charges leave the IC through the touched pin/pad.

For example: for an IC stressed with negative CDM electrons are spread around in substrate, metal lines, junctions, ... When the IO pad is grounded. Electrons on the metal lines and junctions connected to the grounded pad are 'evacuated' rather quickly. Similarly electrons on the Vdd line will easily flow through the forward biased 'diode up' typically placed between IO and Vdd. Charges in the substrate and Vss lines have no easy path out of the IC. This means that there will be a voltage difference build up between the IO and substrate/Vss and between Vdd lines and substrate/Vss. At some moment in time the voltage reaches a threshold. For succesful CDM protection the clue is to add ESD protection clamps that have a lower threshold than the sensitive elements (e.g. gate oxide) and trigger quickly.

The primary protection in your figure will shunt most of the CDM current. However it may require a higher voltage to trigger into low-ohmic conduction or react slowly thereby damaging the receiver gate oxide. The secondary protection clamp is added to clamp the voltage to a safe value as quickly as possible.
Because the Vdd/Vss capacitance is larger than the IO/Vss capacitance the voltage between IO and Vss builds up faster and fast local protection makes most sense.

esd cdm protection circuits

Hey ESDSolutions,

Thanks for your reply. My doubt is it possible for the substrate to acquire +ve charge and how is that discharged during CDM event...?

Re: esd cdm protection circuits

Yup.....
What happens if substrate acquires positive charge ?