Silicon photonics
In CPO, which integrates optical components with a switch IC and others in one package, why is it especially important to test optical characteristics at the wafer level before assembly?
- AOptical components stop transmitting light once diced from the wafer, so their characteristics cannot be measured as chips
- BTesting at the wafer level eliminates the need for any final test after assembly
- CIf a defect is found after assembly, good parts, including the expensive switch IC, are wasted as well
Show answer and explanation
Answer: C. If a defect is found after assembly, good parts, including the expensive switch IC, are wasted as well
When multiple chips are integrated into one package, a single defective one makes the whole package defective, wasting the expensive switch IC and other good parts. It is therefore important to use only chips confirmed good before assembly (KGD: Known Good Die). For silicon photonic chips, light is launched from above the wafer with fiber probes via grating couplers, and combined with electrical probes, waveguide loss and modulator and photodetector characteristics are measured automatically. However, final testing after assembly is still indispensable.