More broadly, what the practical maximum a modern silicon chip could be expected to last (assuming it operates at some temperature above 77K, and it has stable power)?

For example, I expect diffusion of dopant materials to over time make it lose it's properties. I believe diffusion rates are roughly exponential with temperature, so one might expect an \$e^{77\alpha}\$ as opposed to \$e^{273\alpha}\$, which is roughly \$e^{200\alpha}\$ times slower.

---

To clarify the applications I have in mind, take the [Clock of the Long Now](http://longnow.org/clock/), which is being designed to last over 10,000 years, but made out of massive mechanical components. Another application would be sending interstellar objects at velocities practically achievable today: it would take about 80,000 years for the voyager probe to reach Alpha Centauri. Radiation shielding isn't that much of a theoretical problem since it's a matter of scaling up the shielding mass surrounding the probe (if you can launch and accelerate 1 probe, you can launch and accelerate 1000 shielding probe-masses at 1000x the cost).