No, I don't think so. First: the drain capacitance isn't just a field oxide cap but is composed of several different caps, mainly the bulk-drain-cap Cbd (a junction cap), and the gate-drain-overlap-cap Cgd (a gate oxide cap), s. e.g. Allen/Holberg Chap. 3 .
Only the drain access routing contributes some thick oxide cap, but this usually isn't a dominant part of the total drain capacitance.
Of course the field oxide thickness also depends on the process size.
Field oxide thickness is not really related to FET working
voltage in my experience. The gate oxide is enough to
stand off Vds(max), Vgs(max) and field oxides I've seen
are over 10X thicker.
Part of this is because you don't really want the lowest
level of interconnect (routing poly) to contribute as much
capacitive loading as gate poly does, for no useful upside.
thank you so much .
Excuse me i just want to calculate field oxide thickness approximately . overall is there a formula (approximately) for calculating ?
First: the drain capacitance isn't just a field oxide cap but is composed of several different caps, mainly the bulk-drain-cap Cbd (a junction cap), and the gate-drain-overlap-cap Cgd (a gate oxide cap)
yes,i agree with you and checked it, but i think it's true , because this is for calculating capacitance that unit of it is [F/CM^2] not [F] , on the other hand this capacitance is the oxide capacitance density per unit area so A=1.
how should that fit to any particular process size?
1. the maximum voltage that may be placed on field oxide is not the threshold voltage of a field oxide MOSFET !
Even if your above calculations were correct (I didn't check them), you'd start from a wrong premise.
2. What for do you need the field oxide thickness at all? Your above shown inverter doesn't contain a field oxide transistor but a depletion transistor (I guess it's an NMOS), and such one uses the standard (thin) gate oxide - just a different channel implant. A field oxide NMOSFET would'nt work in this circuit configuration (would always be off).
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BTW: A field oxide thickness of 1.38µm would be that of a very old process: a process size of 1.5 or 2µm had used such a thick field oxide, and such processes we had around 15..20 years ago.
Try the web sites of the "big" semiconductor fabs and foundries, like Intel, IBM, AMD, Samsung, TSMC, UMC, SMIC, ... and see which processes they offer.