He will face current defender Curtis Good, who is on loan at Bradford City, in the final.
Demba Ba, who started in place of Fernando Torres, and Oscar failed to convert Chelsea's best chances, as Swansea showed the same resilience that characterised their first-leg win at Stamford Bridge to book a Wembley appointment with Bradford.
Benitez had opted for Ba up front with his side needing goals, and Fernando Torres not having found the net since December 23.
The Chelsea manager had predicted Swansea would look to sit back and protect their two-goal advantage from the first leg, but the hosts instead took Michael Laudrup's advice to be bold during a sharp opening.
They first had to survive a penalty scare after Ba went down under a challenge from Ben Davies.
Foy opted not to point to the spot although there was contact as Ba looked to step inside the full-back.
While Chelsea complained Swansea broke sharply and it took a superb block from Cesar Azpilicueta to keep out Wayne Routledge's fierce goalbound volley.
Michu, fresh from signing a new four-year contract, was the next to test the Chelsea rearguard from Routledge's reverse pass, but found Petr Cech equal to his angled drive.
Jonathan de Guzman also had a volley blocked, this time by Gary Cahill, as the hosts sought an early goal to take the tie further away from the European champions.
But Chelsea began to assert an authority on proceedings having weathered that flurry, and Swansea dropped progressively deeper as the half wore on.
It took 31 minutes for the Blues to genuinely threaten the Swansea goal, and when they did Oscar failed to take a great chance as Ashley Williams nipped in after a ricochet fell to the Brazilian in the box.
Angel Rangel cleared Gary Cahill's looping header off the line as the pressure increased, while Ba wasted another opportunity late in the half as he scooped a shot over the bar after Davies and Routledge had got in each other's way clearing a corner.
The former Newcastle striker was again off-target with a curling strike from the edge of the area five minutes into the second half, but Laudrup was by now imploring his defenders to get out of their own 18-yard box.
Frank Lampard and Juan Mata linked beautifully on the right, moments after taking part in an awful free-kick routine, but yet again there was a white shirt in the way to block the Spaniard's attempted ball across the six-yard box.
Gerhard Tremmel made a sharp stop to deny a fizzing effort from Hazard as the game entered the final 20 minutes, and Chelsea's hopes were effectively ended when Hazard was then dismissed in unsavoury fashion.
Pablo Hernandez and Nathan Dyer missed chances to secure victory on the night as Cech saved on both occasions, with the stadium still stunned by Hazard's astonishing dismissal.
And Swansea comfortably saw out six minutes of stoppage time to reach Wembley.