PUF in an FPGA

You should have more on-die variation if you stretch the ring oscillator across the device. Or if you want more variation from one ring oscillator to the next on the same device, place the individual ring oscillators far apart. I would expect that any method that has more single-device on-die variation will also increase the device-to-device variation for devices in the same production lot that might tend to be very similar to each other. Usually it is not a good idea to make assignments to combinational nodes, but with the "keep" synthesis attribute the combinational node names should be repeatable from compile to compile so that the assignments will work. In a LUT-based FPGA (as opposed to a p-term CPLD) you could use either LogicLock regions or LAB location assignments to assign individual combinational nodes to widely separated places on the device. Either kind of assignment is OK to stretch out the individual nodes of a single ring oscillator. Locked-origin LogicLock regions are more convenient to place each ring oscillator in a small area with the ring oscillators spread apart from each other; simply assign an instance of the entire ring oscillator block of hierarchy to each region.

--- Quote Start ---

P.S.: I understand, that the intention is to generate an unique device signature. I think this may be difficult based on FPGA delay variations.

... Finally I fear, that the systematic delay variations caused by logic cell and routing topology may be considerably higher than the contribution of random, chip specific variation. Thus the method would fail.

--- Quote End ---

If you want a fixed unique signature for each device as opposed to simply a random number, then my suggestion to spread out the ring oscillator will make FvM's concern even worse. It would be better to place the nodes as compactly as possible to minimize the on-die variation across the individual device. Even with zero on-die variation affecting this circuit, a number that depends on device-to-device variation will also depend on supply voltage and operating temperature. The delays for a given device will vary over the operating conditions. You still won't get a fixed unique signature for each device even with zero single-device on-die variation.