LinkButton executes HTTP POST operation, you cant change post target here.It is to give the appearance of a HyperLink but the behaviour of a Button.
Instead you can use the asp:HyperLink
control (different from the "<a herf=""><a>"tag)
You can also set the Target property to open in new window.
<asp:HyperLink ID="HyperLink1" Target=_blank NavigateUrl='WebForm1.aspx' runat="server">HyperLink</asp:HyperLink>