http://www.state.il.us/court/supreme.../ArtII.htm#237
Illinois Supreme Court Rules
Rule 237. Compelling Appearances of Witnesses at Trial
(a) Service of Subpoenas. Any witness shall respond to any lawful subpoena of which he or she has actual knowledge,
if payment of the fee and mileage has been tendered.
Service of a subpoena by mail may be proved prima facie by a return receipt showing delivery to the witness or his or her authorized agent
by certified or registered mail at least
seven days before the date on which appearance is required and an affidavit showing that the mailing was prepaid and was addressed to the witness, restricted delivery, with a check or money order for the fee and mileage enclosed.