Hi Arpit_Soni.
You can try:
<asp:linkbutton id="lnkRegisterInvoice" runat="server" text="Invoice" onclick="window.open('registerinvoice.aspx');return false;" ></asp:linkbutton>
If the above doesn't meet your requirement you could always try the following:
<a href="RegisterInvoice.aspx" target="_blank">Invoice</a>