Preventing postback

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C#

I have on apsx page and on button click i am opened one modelpop through ShowModelDialouge in java script but my problem is my aspx page getting post back when that model pop is opening. Here is javascript code:

function showPopup(id,sid) 
{
    var str = "AddSalesOrderDetials.aspx?value=" + id + "&sid=" + sid;
    window.showModalDialog(str, "", 
                           "dialogHeight:175px;dialogWidth:300px;center:yes;", 
                           "Viewstate=false");

}