ONT Local Controller described a TCAS RA event experienced by an air carrier arrival to Runway 26R after unknown VFR traffic information was issued; the reporter suggesting additional Class C airspace be developed to protect visual approach procedures.

Narrative

Air Carrier X was inbound on a visual approach to Runway 26R. The aircraft called inbound approximately 2 miles outside of the Class C airspace for ONT descending out of 3200 ft MSL. I had observed a target 1 mile south of the extended center-line; 6 miles east of the airport indicating 2400 FT MSL northbound. The target was going to cross the path of Air Carrier X. Due to a blind spot approximately 7 miles southeast of ONT; it is not unusual for aircraft targets to pop up after they have attained an altitude the RADAR can pick up. I was not sure if SCT had issued the traffic before communication transfer. When Air Carrier X called inbound on the approach I immediately issued the traffic and then instructed Air Carrier X to not descend below 3000 when the pilot advised he did not have the traffic. I issued a traffic alert. The target was approaching Air Carrier X's 12 o'clock and was less than a mile still indicating 2400 MSL. Air Carrier X reported the traffic in sight. I did not tell Air Carrier X to maintain visual separation I re-issued a visual approach clearance and re-cleared Air Carrier X to land on Runway 26R. The pilot called the Tower later and advised he had received an RA on the target. Recommendation; aircraft conducting an ILS approach into ONT cross the final approach fix; which is approximately 1 mile outside of ONT's Class Charlie Surface Area at the lowest 2800 FT. Air carriers into ONT often are on Visual Approaches which have no restrictions. Aircraft often fly a north-south track at ONT's final approach fix below 2700 FT. which is 1 mile outside of ONT's airspace and below the 2700 FT restriction of outer Class Charlie airspace. Often in the past controllers have requested a corridor extending to the east surface to 3000 FT; so aircraft flying in that area would be required to talk to ONT and we can control the situation. I have controlled at ONT many years and regardless of the numerous times this issue has surfaced; nothing has been done. In my opinion this is an unsafe situation.