Ajax Toolkit ModalPopupExtender background issue

I have a Ajax Modalpopupextender control on top of which i have another Ajax Modalpopupcontrolextender. the first ModalPopupExtender control render perfectly fine and background comes as grey as well. But when i use another Modalpopupcontrolextender on top of that, the background of the Modalpopupcontrolextender shows partially on IE Browser. if i use chrome, it shows perfectly fine. 

Any idea, how to control the modalpopupextender window background and its alignment??

I use below css for background:-

modalBackground
{background-color:Gray;opacity: 0.5; }