scrap
**Scrap** is the **permanent disposal of semiconductor wafers or lots that are critically defective, unrecoverable through rework, or uneconomical to salvage** — representing the most severe financial outcome in semiconductor manufacturing where all accumulated process value (materials, equipment time, operator labor, overhead) is written off as lost, making scrap rate minimization one of the most direct levers of fab profitability.
**What Drives Scrap Decisions**
Scrap is the disposition of last resort, chosen when:
**Technical Unrecoverability**: The defect mechanism is irreversible — wrong implant species permanently embedded in the crystal, catastrophic contamination that cannot be removed without destroying the structure, physical breakage of the wafer, or yield-killing defect density with no remediation path.
**Margin Exhaustion**: The deviation exceeds not just the specification but the actual device design margin — gate oxide too thin for reliable operation, metal line too narrow to survive electromigration at rated current density. UAI cannot be justified.
**Economic Analysis**: The remaining processing cost exceeds the expected revenue from functional die. A wafer with 10% yield entering a 50-step remaining process flow where each step costs $200 may have negative expected value — scrapping and reallocating capacity to good wafers is more profitable.
**Customer Requirement**: Some customer contracts specify mandatory scrap for certain classes of deviation — particularly in automotive and medical applications where the consequence of a field failure far exceeds the wafer cost.
**Scrap Economics and Value Accumulation**
Scrap cost is not constant — it depends entirely on where in the process the wafer is scrapped:
**Early scrap (bare silicon, thermal oxidation)**: $50–$200 of material value lost. Low financial impact; scrapping marginal wafers early is often correct.
**Mid-process scrap (gate, contact, metal 1)**: $2,000–$8,000 accumulated value. Requires engineer authorization; UAI or rework is preferred if technically justified.
**Late-process scrap (metal 5+, passivation, probe)**: $15,000–$50,000+ accumulated value at advanced nodes. Requires MRB or management authorization; extensive analysis required before scrapping.
**Finished goods scrap (post-probe, packaged)**: Full product value lost plus packaging cost. Typically limited to field-return analysis failures or customer-returned parts.
**Scrap Rate as a KPI**
**Line Yield**: Yield = (Wafers In − Wafers Scrapped) / Wafers In, tracked by process step, module, and overall line. Line yield of 98% means 2% of wafers are scrapped before completing the process.
**Scrap Rate Trending**: SPC-monitored scrap rate by module identifies chronic yield losers. A step consistently scrapping 0.5% of wafers may seem small but represents millions of dollars annually in a high-volume fab.
**Die Yield vs. Line Yield**: Line yield accounts for wafer-level scrap; die yield accounts for die-level functional failures within surviving wafers. Total manufacturing yield = Line Yield × Die Yield — both must be optimized independently.
**Recovery Value**: Scrapped silicon wafers are sold to silicon recyclers who re-polish them into reclaim wafers used for process monitoring and tool qualification, recovering 5–20% of the original wafer cost.
**Scrap** is **the final verdict of failure** — the formal acknowledgment that the accumulated investment in a wafer cannot be recovered, triggering financial write-off, yield accounting, and root cause analysis to ensure the same loss does not recur in the next production run.