MKE TRACON Controller described a loss of separation event involving an ORD over flight aircraft; noting more structured procedures should be adopted regarding these overflights improving operational standardization.

Narrative

We were landing Runway 25L; departing Runway 19R. I was working East Departure (M). Air Carrier X departed MKE and I saw they were filed out the Squib track (East Gate). So; after I RADAR ID'd the aircraft; I turned it to a 090 heading and climbed it to 13000. The Controller next to me was working East Feeder and Final combined (E & F). The Controller had an aircraft on the downwind for 25L and also an overflight bound for ORD that was being vectored east of MKE on an approximate 160 degree heading to get into the east box for ORD arrivals. This ORD aircraft was Air Carrier Y and they were descending to 080. Normally whenever I climb someone east I always look to the north to see if there are any ORD overflights that may pose a problem for a departure. For some reason this time; I guess I didn't scan well enough and climbed Air Carrier X. As I said before the East Feeder Controller had both of aircraft in approximately the same location at different altitudes; although their data tags overlapped quite a bit. By the time I noticed there were 2 aircraft there (1 MKE arrival & 1 ORD overflight) in the basically the same location; it was too late and Air Carrier X had already climbed through 7000 and the ORD traffic had already descended through 9000. For some reason; I stopped Air Carrier X at 8000 just as a last resort to get some form of separation; even though at that point I knew I would still have an error. I pointed this situation out to the other Controller and he/she turned Air Carrier Y to the southwest and descended him. I also turned Air Carrier X to a southeast heading to try to outrun Air Carrier Y as well. Air Carrier Y got an RA and maneuvered on their own. After that; I climbed Air Carrier X back to the northeast to get him back in the departure gate and climbed him to 13000. At that point; the Air Carrier X aircraft asked me where the aircraft that I had to maneuver them for was; because they never saw Air Carrier Y on the TCAS. I felt a little disturbed by this since this time TCAS was a false sense of security for them. Recommendation; more structured routing's for ORD overflights; maybe some kind of overflight routes or lanes or gates or something. We have numerous ORD overflights daily and there is not one standard practice used by anyone here. Everyone does their own thing. This is not the first time someone has had a deal with ORD overflights because you never really are sure where to look for them. They can mix in quite easily with other MKE inbound traffic. Of course; I can only really blame myself for my lack of ability to see the ORD overflight. If I would've done that better; I wouldn't be sitting in this chair filling out this report.