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The man who helped Stan Lee create 2 legendary Marvel superheroes -- Spider-Man and Doctor Strange -- has died at his home in New York ... TMZ has learned.

Steve Ditko was found unresponsive in his NYC apartment on June 29 when a social worker who hadn't heard from him in a while went to Steve's home to check on him ... according to law enforcement.

The chief medical examiner listed the cause of death as arteriosclerotic and hypertensive cardiovascular disease -- basically, a heart attack brought on by clogged arteries.

Ditko was one of the most acclaimed comic book artists of all time, best known for creating Spider-Man with Lee in 1962, and Doctor Strange a year later. The stories they crafted together inspired the 2016 "Doctor Strange" film starring Benedict Cumberbatch.

While Stan became the face of Marvel and "The Godfather of Comic Books" to the public, Steve remained enigmatic over the years -- refusing to do interviews or make public appearances. He continued to work at a Manhattan studio, though, until at least 2012.

Steve was 90.