Response.Clear()
        Response.ContentType = "application/csv"
        Response.Charset = ""
        Response.ContentEncoding = Encoding.Unicode
        Response.AddHeader("content-disposition", "attachment; Filename=""Order.csv""")
        'Response.BinaryWrite(UTF8Encoding.Unicode.GetPreamble())
        Response.Write("aaa,bbb,ccc,ddd")
        Response.End()
        

HTML
<asp:linkbutton  ID="Button1" runat="server" OnClick="Button1_Click"  onclientClick="disableWin();" 
>LinkButton</asp:linkbutton>
<div id="div1" class="AjaxLoaderOuter" style="display:none">ddd</div>

script
function disableWin() {
            $("#div1").show()
}