Nic Cage is half-way back to solvency ... TMZ has learned the actor just took a $6 MILLION chunk out of his MASSIVE tax bill to the federal government.

According to official documents obtained by TMZ ... Cage paid off the $6,257,005 he failed to pay in federal income tax for 2007. 

It's a huge step ... considering Cage fell around $13 million behind in taxes for 2002, 2003, 2004 and 2007. Cage blamed his former business manager for driving him to what he called "financial ruin."

As we previously reported, Cage has been slowly chipping away at the debts from 2002 --2004 ... and if he just knocks out a few more "Ghost Rider" movies, he should be even-steven in no time.