Voice synthesizer used for the Rabbot's voice on Aqua Teen Hunger Force

Short of this information already existing on the internet or you happening to be on talking terms with the creators, I doubt you're going to be able to find out exactly the process they used. There is no voice actor credited so it's highly likely to be completely synthesized, based on the limited vocabulary of the character it's likely a custom process (certainly custom enough to get a few words into a characters vocabulary and be like "oh, that's enough! 37* different words will do").

Many voice synthesizers already included in the current selection of DAWs (Digital Audio Workstations, like VOCALOID editor in Cubase) available will get you pretty close to the sound shown in the show.

^{* 37 is a guess, but it looks like this character can only say single digit numbers, days of the weeks, etc}