Substrate & Dielectric Loss

Substrate and dielectric-loss outputs explain how material selection and surface participation affect T_1.

#	Parameter	Symbol / Unit	Extraction Method	Typical Q3D Value	Ideal / Optimal	Good Range	Worst Case	Why It Matters	Key Design Note
40	Substrate Bulk Loss Tangent	tan delta_bulk	Q3D dielectric loss tangent input; resonator Q fitting vs power	< 10⁻⁶ (HR-Si, 4K)	< 10⁻⁶	10⁻⁶ – 10⁻⁵	> 10⁻⁴	Bulk dielectric loss sets floor on 1/Q_int from substrate volume; sapphire < 5×10⁻⁷	tan δ improves by 10–100× on cooling from 300K to 4K due to reduced phonon and TLS population
41	Metal-Air Interface Loss (tan delta_MA)	tan delta_MA	Surface participation ratio (SPR) from Q3D E-field + measured Q factor	~10⁻³	< 10⁻³ (passivated Al₂O₃)	10⁻³ – 5×10⁻³	> 10⁻²	TLS loss at metal-air interface is the dominant T₁ source in planar transmon designs	Etching native oxide before Al deposition reduces tan delta_MA by up to 10×; HF vapor clean
42	Substrate-Air Interface Loss (tan delta_SA)	tan delta_SA	SPR analysis from Q3D E-field distribution	~5×10⁻⁴	< 5×10⁻⁴ (HF-etched Si)	5×10⁻⁴ – 5×10⁻³	> 10⁻²	TLS at substrate exposed surface; addressed by passivation, UV ozone clean, or dry etching	Hydrogen-passivated Si surface (HF dip) shows 5× lower tan delta_SA vs untreated Si
43	Metal-Substrate Interface Loss (tan delta_MS)	tan delta_MS	EELS/TEM interface composition + Q3D SPR calculation	~5×10⁻³	< 5×10⁻³	5×10⁻³ – 10⁻²	> 5×10⁻²	TLS at Al–Si or Nb–Si interface; reduced by HF dip substrate prep before metal deposition	Amorphous interfacial SiOx layer of 1–2 nm is the primary TLS host; substrate HF clean removes it
44	Surface Participation Ratio (SPR)	p_MA / ppm	Q3D E-field energy integral on metal-air interface: p = ∫_MA ε|E|²dV / ∫_all ε|E|²dV	5 – 50 ppm	< 5 ppm	5 – 50 ppm	> 200 ppm	p × tan δ contributes directly to 1/Q; minimise by thick metal, wider gap, no sharp corners. For planar transmons, p_MA can reach 100–1000 ppm without geometry optimisation.	1/Q_TLS = Σ p_i × tan δ_i; SPR is the design lever; tan δ is the material lever. <5 ppm ideal is achievable in optimised 3D cavity or large-gap planar designs; planar CPW without optimisation may be 100–1000 ppm.