AJAX :: Modal Pop Up Gray Background Not Cover Whole Page?

Question

My problem is that, i have created a Master page and used this master page in page called Main.aspx and and set a iframe in it in iframe i have called a page saying entryscreen.aspx.I have set a modal pop up in entry screen page and when i click a button in entry screen page to show the modal pop up it covers only entry screen page behind the gray backgroun.i want to cover my whole master page behind gray background.