Last month, the administrator of Ditko’s estate filed a notice of termination on Spider Man, which first appeared in comic book form in 1962. Under the termination provisions of copyright law, authors or their heirs can reclaim rights once granted to publishers after waiting a statutory set period of time. According to the termination notice, Marvel would have to give up Ditko’s rights to its iconic character in June 2023.
Source: Marvel Suing to Keep Rights to ‘Avengers’ Characters From Copyright Termination