how to avoid page refereshing when click model popup close button

script to model popup hide and show----------->

<script language="javascript" type="text/javascript">
//Function to Hide ModalPopUp
function Hidepopup() {
$find('mdp4').hide();
return false;
}

</script>
<script language="javascript"" type="text/javascript">
function Showpopup() {
$find("mdp4").show();
return false;

}
</script>

panel control inside popup--------->

<asp:Panel ID="Pnlchangepass" runat="server" BorderColor="Black" BorderStyle="Double"
Height="320px" Width="280px" BackColor="AliceBlue">
<table>
<tr>
<td colspan="4" id="line">
<div class="ftitle">
Create New Self-Appraisal</div>
</td>
</tr>
<tr>
<td colspan="3">
<asp:LinkButton ID="linkbtn" runat="server" Text="Close" OnClientClick="Hidepopup();"></asp:LinkButton>
</td>
</tr>
<tr>
<td>
<asp:Label ID="lblstrdate" runat="server" Text="To:"></asp:Label>
</td>
<td>
<asp:TextBox ID="txtstrdate" runat="server"></asp:TextBox>
<asp:CalendarExtender ID="txtstrdate_CalendarExtender" runat="server" TargetControlID="txtstrdate">
</asp:CalendarExtender>
<asp:MaskedEditExtender ID="Masstratdate" runat="server" TargetControlID="txtstrdate"
Mask="99/99/9999" MessageValidatorTip="true" MaskType="Date" DisplayMoney="Left"
AcceptNegative="Left" ErrorTooltipEnabled="true" UserDateFormat="DayMonthYear">
</asp:MaskedEditExtender>
</td>
</tr>
<tr>
<td>
<asp:Label ID="lblenddate" runat="server" Text="From:"></asp:Label>
</td>
<td>
<asp:TextBox ID="txtenddate" runat="server"></asp:TextBox>
<asp:CalendarExtender ID="txtenddate_CalendarExtender" runat="server" TargetControlID="txtenddate">
</asp:CalendarExtender>
<asp:MaskedEditExtender ID="MaskedEditExtender1" runat="server" TargetControlID="txtenddate"
Mask="99/99/9999" MessageValidatorTip="true" MaskType="Date" DisplayMoney="Left"
AcceptNegative="Left" ErrorTooltipEnabled="true" UserDateFormat="DayMonthYear">
</asp:MaskedEditExtender>
</td>
</tr>
<tr>
<td>
<asp:Label ID="lblstutas" runat="server" Text="Active"></asp:Label>
</td>
<td>
<asp:RadioButtonList ID="RblSelf" runat="server">
<asp:ListItem Selected="True">Yes</asp:ListItem>
<asp:ListItem>No</asp:ListItem>
</asp:RadioButtonList>
</td>
</tr>
<tr>
<td colspan="2">
<asp:Button ID="btncreateself" runat="server" Text="Create Self-Appraisal" CssClass="myButton"
OnClick="btncreateself_Click" />
</td>
</tr>
</table>
</asp:Panel>
<asp:ModalPopupExtender ID="ModalPopupExtender1" runat="server" PopupControlID="Pnlchangepass"
BackgroundCssClass="modalBackground1" TargetControlID="createemp" BehaviorID="mdp4">
</asp:ModalPopupExtender>