gt2n: tighten tap spacing to 2um for denser BPR riser density

Default tapcell.tcl was using `-distance 5` (5 um between taps along
each row), which is loose enough that small designs end up with a
single tap column. The gt2n tap cell does double duty: it is both the
well/body-bias tap (the conventional purpose) and the per-row M1<->BPR
or sole BPR riser for the BSPDN power network -- each tap is the only
hard via from the cell row's BPR rail into the BSPDN power source.
Tap density therefore directly sets per-row BPR resistance and the
worst-case BSPDN IR drop, which is not true in flows where taps only
exist for latchup avoidance.

Drop `-distance` to 2 um. On gt2n/gcd (4-5 um core side, 55 rows):

  -distance     5      2
  tapcells     28    114      (+4x)
  vdd worst  10.9 mV 8.49 mV  (-22%)
  vdd avg    2.62 mV 2.59 mV
  vss worst  7.25 mV 7.35 mV  (essentially same, already well distrib)
  vss avg    2.27 mV 2.27 mV

(IR numbers from analyze_power_grid on 6_final.odb with leakage-only
currents -- absolute drop is small at this design size, but the
worst-case-vdd improvement is what we expect from more BPR risers.)

Detail-route runtime goes up roughly 3x because of the denser obstacle
field, but converges cleanly to 0 DRC + 0 antenna.

Signed-off-by: Matthew Guthaus <mrg@ucsc.edu>

Author: mguthaus

flow/platforms/gt2n/tapcell.tcl

@@ -1,4 +1,10 @@
+# GT2N tap cells (`gt2_6t_tapbspdn_*`, `gt2_6t_tapfspdn_*`) bridge the
+# cell's BPR pin to the backside power network -- they are the only
+# legal M1<->BPR (FSPDN) or sole BPR riser (BSPDN) per row. Tap density
+# directly sets BPR rail resistance and BSPDN IR drop, so a tighter
+# spacing matters more than in a conventional well-tap-only flow. Two
+# microns gives a few tap columns even on small blocks.
 tapcell \
-  -distance 5 \
+  -distance 2 \
   -tapcell_master "$::env(TAP_CELL_NAME)" \
   -endcap_master "$::env(TAP_CELL_NAME)"