NCT Controller described TCAS RA event when air carrier inbound to SMF was issued traffic; instructed to continue descent to clear traffic but responded to a TCAS climb; reporter suggesting TCAS response made potential conflict more critical.

Narrative

Air Carrier X was inbound to SMF on the FLUNK arrival. I had an un-targeted VFR at 7;400 ft that was potential traffic for the aircraft. I instructed Air Carrier X to increase rate of descent through 7;000 and called the traffic. Shortly afterward Air Carrier X called SMF in sight. I replied that he needed to continue his rate of descent through 7;000 and leaving 7;000 they were cleared for a visual approach to Runway 16L. About thirty seconds to a minute later; Air Carrier X advised me that they were going to level off at their current altitude. I observed that they were approximately 2 miles southeast of the VFR target about one hundred feet above. I told Air Carrier X negative; continue through 7;000 for the traffic; traffic indicates 7;400. Air Carrier X then reported that they were receiving an RA and climbing back up to 8;000. I replied with a Roger and called traffic again; passing 2 miles north. Air Carrier X reported traffic in sight. I cleared Air Carrier X for a visual approach. Recommendation: No recommendation; however; the actions of the Pilot; even though it was an RA; actually made the situation worse instead of better. If they had continued on the descent; they would have been below the traffic with plenty of room. Leveling off at the altitude of the traffic called doesn't seem like the best of ideas. I did not use any further methods of separation; i.e. vectors; because I observed lateral separation was going to work as well.