hv esd layout
DenisMark is right. Many High voltage NMOS devices have serious issues with uniform triggering within multifinger devices as well as single finger!
This problem is most visible when multiple pulses are used during the ESD stress at the same level. E.g. A foundry can claim self protection capabilities reaching 4kV HBM but it is possible that by stressing the device multiple times at 2kV that it will fail. You should certainly ask for information about 'repetitive stress' tests. I would not trust a HV MOS device protection claim without such information!
It is probably better to provide a parallel protection clamp
Some references:
M. Mergens et al, “Analysis of lateral DMOS devices under ESD stress
conditions”, IEEE TED, Nov 2000, pp. 2128
B. Keppens et al, "ESD Protection Solutions for High Voltage Technologies", EOS/ESD symposium 2004
B. Keppens et al., “Contributions to Standardization of Transmission
Line Pulse Testing Methodology”, Proc. EOS/ESD 2001, pp. 461-467.