Here we report on the use of the cavity ring down (CRD) technique for the detection of the silyl radical SiH3 on the broadband (A) over tilde (2)A(1)' <-- (X) over tilde (2)A(1) transition around 215 nm. SiH3 has been detected in a remote Ar-H-2-SiH4 plasma during hydrogenated amorphous silicon (a-Si:H) thin film growth. The measurements demonstrate the capability of CRD to measure small broadband absorptions in the deep UV in the hostile environment of a deposition plasma. The SiH3 absorption shows an expected dependence on the SiH4 precursor flow and correlates well with the a-SiH growth rate. The observed absorptions correspond with SiH3 densities in the range 2-13 x 10(18) m(-3), which is at least two orders of magnitude above the estimated SiH3 detection limit.