Href anchor tag replaces current window when referenced inside BalloonPopupExtender ?

<a href="#" onclick="ShowDetails()" name="moreHlp">Help</a>

<script  type="text/javascript" >
      function ShowDetails() {
          window.open("../Help/details.aspx"  , null, "width=1200,height=600,left=0,modal=yes, alwaysRaised=yes,scrollbars=yes", null);

      }
</script>

I would like to open a new child window instead of a blank window that replaces current ?