how to hide background from this shader ?

hello. i use this script for my camera to show alpha channel in render texture . but i dont want to show camera background . is there any way . thank you

 var alpha = 1.0;
 
 private var mat : Material;
 
  
 
 function Start ()
 
 {
 
     mat = new Material(
 
         "Shader \"Hidden/Clear Alpha\" {" +
 
         "Properties { _Alpha(\"Alpha\", Float)=1.0 } " +
 
         "SubShader {" +
 
         "    Pass {" +
 
         "        ZTest Always Cull Off ZWrite Off" +
 
         "        ColorMask A" +
 
         "        SetTexture [_Dummy] {" +
 
         "            constantColor(0,0,0,[_Alpha]) combine constant }" +
 
         "    }" +
 
         "}" +
 
         "}"
 
     );
 
 }
 
  
 
 function OnPostRender()
 
 {
 
     GL.PushMatrix();
 
     GL.LoadOrtho();
 
     mat.SetFloat( "_Alpha", alpha );
 
     mat.SetPass(0);
 
     GL.Begin( GL.QUADS );
 
     GL.Vertex3( 0, 0, 0.1 );
 
     GL.Vertex3( 1, 0, 0.1 );
 
     GL.Vertex3( 1, 1, 0.1 );
 
     GL.Vertex3( 0, 1, 0.1 );
 
     GL.End();
 
     GL.PopMatrix();
 
 }