restore old routing configuration

diff --git a/gf180mcu/openlane/config.tcl b/gf180mcu/openlane/config.tcl
index d17f8be..f7fc7a4 100644
--- a/gf180mcu/openlane/config.tcl
+++ b/gf180mcu/openlane/config.tcl

@@ -105,8 +105,9 @@
 
 # Routing
 set ::env(METAL_LAYER_NAMES) "Metal1 Metal2 Metal3 Metal4 Metal5"
-set ::env(RT_MIN_LAYER) "Metal1"
+set ::env(RT_MIN_LAYER) "Metal2" ;# stdcells heavily use Metal1 - setting it to Metal1 will cause congestions
 set ::env(RT_MAX_LAYER) "Metal5"
+set ::env(DRT_MIN_LAYER) "Metal1"
 set ::env(GRT_LAYER_ADJUSTMENTS) "0,0,0,0,0"
 
 ## Tracks info