Here's a link that explains Comey's action as US Attorney in 2002:
http://www.cbsnews.com/news/feds-close-clinton-clemency-probe/
It doesn't relate to the articles of impeachment against Clinton in 1998, but to an event after. 

Comey's been a loyal Clinton Cleaner for years. 
	
Add to that Comey's brother works for a law firm handling the Clinton foundation.

Comey also kept the Sandy Berger investigation from expanding to include other Clintonistas –
http://www.wnd.com/2016/07/comey-has-long-history-of-clinton-related-cases/