Silicon photonic ring resonator thermometers have been shown to provide
temperature measurements with a 10 mK accuracy. In this work we identify and
quantify the intrinsic on-chip impairments that may limit further improvement
in temperature measurement accuracy. The impairments arise from optically
induced changes in the waveguide effective index, and from back-reflections and
scattering at defects and interfaces inside the ring cavity and along the path
between light source and detector. These impairments are characterized for 220
x 500 nm Si waveguide rings by experimental measurement in a calibrated
temperature bath and by phenomenological models of ring response. At different
optical power levels both positive and negative light induced resonance shifts
are observed. For a ring with L = 100 um cavity length, the self-heating
induced resonance red shift can alter the temperature reading by 200 mK at 1 mW
incident power, while a small blue shift is observed below 100 uW. The effect
of self-heating is shown to be effectively suppressed by choosing longer ring
cavities. Scattering and back-reflections often produce split and distorted
resonance line shapes. Although these distortions can vary with resonance
order, they are almost completely invariant with temperature for a given
resonance and do not lead to measurement errors in themselves. The effect of
line shape distortions can largely be mitigated by tracking only selected
resonance orders with negligible shape distortion, and by measuring the
resonance minimum wavelength directly, rather than attempting to fit the entire
resonance line shape. The results demonstrate the temperature error due to
these impairments can be limited to below the 3 mK level through appropriate
design choices and measurement procedures