TreeView 无刷新问题

这是我的代码：
         <script language="jscript" type="text/javascript">
        function dopostback() {
            var o = window.event.srcElement;
            if (o.tagName == "INPUT" && o.type == "checkbox") {
                __doPostBack("", "");
            }
        }
</script>
</head>

<body ><form id="form1" runat="server">
  <asp:ScriptManager ID="ScriptManager1" runat="server">
    </asp:ScriptManager>
    <asp:UpdatePanel ID="UpdatePanel1" runat="server">
        <ContentTemplate>
    <div style="float: left; height: 50px; width: 250px">
        <asp:TreeView ID="TreeView1" runat="server" Font-Size="10pt" ForeColor="Black" ShowCheckBoxes="All">
        </asp:TreeView>    </div>
                                             </div>
    <div style="clear: both">
    </div>
    <!--这个层很有用，必须要，否则可能不兼容。-->
     </ContentTemplate>
        <Triggers>
        </Triggers>
    </asp:UpdatePanel>
    </form>后台代码：
    protected void Page_Load(object sender, EventArgs e)
        {
            if (!IsPostBack)
            {
                this.TreeView1.Attributes.Add("onclick", "dopostback()");
                DropDownBind();
                ShowTree();
            }
        }     
        private void ShowTree()
        {
            this.TreeView1.Nodes.Clear();
            TreeNode tn = new TreeNode();
            tn.Text = "西京医院";
            tn.Value = "X-西京医院";
            this.TreeView1.Nodes.Add(tn);            TAreaData areadate = CTAreaFacade.GetAllArea();
            foreach (DataRow dr in areadate.Tables[0].Rows)
            {
                TreeNode tnarea = new TreeNode(dr["AreaName"].ToString());
                tnarea.Value = "A-" + dr["AreaGuid"].ToString().Trim();
                tnarea.Expanded = false;
                tnarea.ShowCheckBox = true;
                tnarea.Checked = false;
                tn.ChildNodes.Add(tnarea);                TBuildData build = CTBuildFacade.GetAllByAreaguid(dr["AreaGuid"].ToString().Trim());
                foreach (DataRow dr2 in build.Tables[0].Rows)
                {
                    TreeNode tnbuild = new TreeNode(dr2["BuildName"].ToString());
                    tnbuild.Value = "B-" + dr2["BuildNo"].ToString().Trim();
                    tnbuild.Expanded = false;
                    tnbuild.ShowCheckBox = true;
                    tnbuild.Checked = false;                    tnarea.ChildNodes.Add(tnbuild);
                    TDoorData tdata = CTDoorFacade.GetAllByBuildNo(dr2["BuildNo"].ToString().Trim());
                    foreach (DataRow dr3 in tdata.Tables[0].Rows)
                    {
                        TreeNode tnDoor = new TreeNode(dr3["DoorName"].ToString());
                        tnDoor.Value = "D-" + dr3["DoorNo"].ToString().Trim();
                        tnDoor.Expanded = false;
                        tnDoor.ShowCheckBox = true;
                        tnDoor.Checked = false;
                        tnbuild.ChildNodes.Add(tnDoor);
                    }
                }
            }
        }问题：
     我选中树形控件的复选框 页面就会闪一下。 我要选中页面不刷新，我该怎么做