ZAU controller experienced airspace incursion event when he/she confused an airspace boundary.

Narrative

Aircraft X was direct GYY from ZZZ. I descended Aircraft X to 4;000 FT. I had evaluated the route for point outs and believed that the route was west of SBN ATCT airspace. During this time I had a stuck mike on 126.32. I was able to talk to Aircraft X on 126.32 and changed Aircraft X to 127.8 to allow for communication as they descended. I reviewed Aircraft X route again for point outs and decided to call SBN ATCT to be sure. SBN ATCT approved the late point out and made me aware of the fact that I had confused the BEARZ/LOOTH boundary with the SBN ATCT/IKK boundary. The BEARZ/LOOTH boundary is about five to seven miles east of the IKK/SBN ATCT boundary and has similar angles/shapes. My altitude limits for the Radar Display were set to the ground during this period because I also had GRISSOM ATCT airspace at the time. Recommendation; I will now remember that the northwest side of SBN ATCT's airspace is defined from the northwest corner of GRISSOM ATCT airspace then northwest to the C90 boundary.