Which metal layer is estimated by synthesize_ccopt_flexible_htrees command?

Hi,

I want to build a flexible htree and tried to run synthesize_ccopt_flexible_htrees command. It was build but lower level metal (thinner metal) is being used than expected so too many clock buffers were inserted. How to set metal layer to build htree clock? The following commands were used now.

create_ccopt_flexible_htree -name SCLK -effort medium -trunk_cell BUF_X32N_A9PP84TSL_C16 -final_cell BUF_X32N_A9PP84TSL_C16 -sink_grid {4 2} -pin SCLK
synthesize_ccopt_flexible_htrees