1964 was the last year for 90% silver in the circulating dime, quarter and half. From 1965 through 1970, halves were 40% silver. Not too many 1970's out there to be found in the wild, since they were only made for proof sets (somebody correct me on this if I'm wrong?). There were some Eisenhower dollars that were 40% silver in proof sets too, but I'm not sure what years. Also, in 1976, the bicentennial halves and quarters were produced in 40% silver for some proof sets as well, and some of the commemorative coins had silver content as well through the 80's. Besides that, from 1971 through 1991, proof sets were just proof-strike versions of the circulating coins--i.e. clads with the copper visible on the edge. Starting in 1992, the Mint began producing these same proof sets, plus a second version each year that had dimes quarters and halves struck from the traditional 90% silver. That would be where your 1997 dime came from. It was never meant for circulation, but it got there anyway
Guys, did I leave anything out?