asp.net imagemapp navigateurl target to new window not tab

While trying to open page in new window, tried using '_blank' , but still it navigate to new tab in most browser,    <asp:ImageMap id="imHome" runat="server" ImageUrl="~/images/sample02.png">                <asp:RectangleHotSpot AlternateText="CreateViewInvoice" Top="10" Left="250" Bottom="80" NavigateUrl="~/CreateInvoice.aspx" Target="_blank" />