OK, first, thanks for the reply.
I saw that DS18B20 during my search. But how do you mount it to the heat sink? I saw PT100 probes with an eyelet (I hope that´s translated correctly) with a diameter for M4 screws. That would be perfect because I could mount it under one of the heat sink screws. But I cant´t find a 1-wire probe with an eyelet.
From technical point the 1-wire probe would be perfect. But how do you connect them in series with one wire? You mean parallel, don´t you? So that all VDDs, all GNDs and all DQs are connected?