{"id":4894,"date":"2026-06-01T17:18:39","date_gmt":"2026-06-01T09:18:39","guid":{"rendered":"https:\/\/www.opticalcutting.com\/?p=4894"},"modified":"2026-06-01T17:18:39","modified_gmt":"2026-06-01T09:18:39","slug":"wafer-double-sided-lapping","status":"publish","type":"post","link":"https:\/\/www.opticalcutting.com\/ko\/wafer-double-sided-lapping\/","title":{"rendered":"\uac8c\ub974\ub9c8\ub284 \uc6e8\uc774\ud37c \uc591\uba74 \ub798\ud551: \uc815\ubc00 IR \uad11\ud559\uc6a9 \ud3c9\ud0c4\ud558\uace0 \ud3c9\ud589\ud55c \ube14\ub7ad\ud06c\ub97c \uc0dd\uc0b0\ud558\ub294 \ubc29\ubc95"},"content":{"rendered":"

After wire saw slicing, a germanium blank has two problems: the surfaces aren’t flat enough, and they aren’t parallel enough. Wire cutting delivers Ra 0.6\u20131.2 \u03bcm surface roughness with TTV (total thickness variation) of 8\u201315 \u03bcm on a \u03a650 mm blank. For many infrared optics applications, that’s not good enough to go straight into spherical grinding<\/a>.<\/p>\n\n\n\n

Germanium wafer double sided lapping solves both problems in a single operation \u2014 improving flatness, parallelism, and surface condition simultaneously on both faces of the blank.<\/p>\n\n\n\n

Where Double Sided Lapping Fits in the Germanium Production Line<\/h2>\n\n\n\n

Lapping sits between cutting\/centering and grinding in the \uc801\uc678\uc120 \uad11\ud559 \ubd80\ud488 \uc81c\uc870<\/a> workflow:<\/p>\n\n\n\n

\ub2e8\uacc4<\/th>\ud504\ub85c\uc138\uc2a4<\/th>\uc7a5\ube44<\/th>Output<\/th><\/tr><\/thead>
1<\/td>\uc724\uacfd \uc808\ub2e8<\/td>Wire saw (SGI 40)<\/td>Shaped preform<\/td><\/tr>
2<\/td>\uc2ac\ub77c\uc774\uc2f1<\/td>Wire saw (SGI 40)<\/td>Flat disc, Ra 0.6\u20131.2 \u03bcm, TTV 8\u201315 \u03bcm<\/td><\/tr>
3<\/td>Centering + chamfer<\/td>\uc13c\ud130\ub9c1 \uba38\uc2e0 (C-120L)<\/td>Round blank, \u2264 5 \u03bcm roundness<\/td><\/tr>
3.5<\/strong><\/td>Double sided lapping<\/strong><\/td>Lapping machine<\/strong><\/td>Flat blank, Ra 0.2\u20130.4 \u03bcm, TTV < 5 \u03bcm<\/strong><\/td><\/tr>
4<\/td>\uad6c\uba74 \uc5f0\uc0ad<\/td>Grinder (G-100\/G-250)<\/td>Curved surfaces, Ra 0.1\u20130.3 \u03bcm<\/td><\/tr>
5<\/td>Polishing + AR coating<\/td>\uc5f0\ub9c8 \uba38\uc2e0<\/td>Ra < 5 nm, finished lens<\/td><\/tr><\/tbody><\/table><\/figure>\n\n\n\n

Not every germanium lens requires a dedicated lapping step. For applications with looser flatness requirements (\u00b10.01 mm), blanks can move directly from centering to grinding. But for precision IR optics \u2014 thermal imaging lenses, FLIR systems, spectroscopy windows \u2014 germanium wafer double sided lapping is the process that ensures the grinding stage starts from a geometrically correct blank.<\/p>\n\n\n\n

\"\ube54\ud380
\ud751\uc5f0, \uad11\ud559 \uc720\ub9ac \ub4f1\uc744 \uc704\ud55c \ub8e8\ud504\ud615 \ub2e4\uc774\uc544\ubaac\ub4dc \uc640\uc774\uc5b4 \ud1b1.<\/figcaption><\/figure>\n\n\n\n

Why Germanium Needs Double Sided Lapping<\/h2>\n\n\n\n

Material Properties That Create Problems<\/h3>\n\n\n\n

Germanium (Ge) has specific material properties that make post-cutting surface condition worse than many other optical materials:<\/p>\n\n\n\n

Crystal structure.<\/strong> Germanium is a single-crystal semiconductor with a diamond cubic lattice. Wire saw cutting generates subsurface damage \u2014 micro-cracks along crystal planes that extend 5\u201315 \u03bcm below the visible surface. Lapping removes this damaged layer in a controlled manner.<\/p>\n\n\n\n

Softness.<\/strong> With a Knoop hardness of approximately 780 (compared to 1,000+ for most optical glasses), germanium is relatively soft. This makes it responsive to lapping but also susceptible to over-removal if process parameters aren’t controlled.<\/p>\n\n\n\n

High material cost.<\/strong> Optical-grade germanium costs $1,800\u2013$2,400 per kilogram. A \u03a650 mm blank weighs approximately 25\u201330 grams, making each blank worth $45\u2013$72 in raw material alone. Every micron of unnecessary material removal during lapping reduces the value available for the final lens. According to Umicore Electro-Optic Materials<\/a>, germanium remains the primary choice for 8\u201314 \u03bcm thermal imaging optics \u2014 meaning demand for precision-processed germanium blanks continues to grow.<\/p>\n\n\n\n

What Lapping Accomplishes<\/h3>\n\n\n\n

Germanium wafer double sided lapping achieves three things simultaneously:<\/p>\n\n\n\n

    \n
  1. Flatness correction<\/strong> \u2014 removes the bow and warp left by wire saw cutting, bringing surface flatness from 10\u201320 \u03bcm down to < 3 \u03bcm across the blank<\/li>\n\n\n\n
  2. Parallelism improvement<\/strong> \u2014 ensures both faces are equidistant, reducing TTV from 8\u201315 \u03bcm (as-cut) to < 5 \u03bcm<\/li>\n\n\n\n
  3. Subsurface damage removal<\/strong> \u2014 the lapping action removes the microcrack layer from wire cutting, creating a clean surface for grinding<\/li>\n<\/ol>\n\n\n\n

    How Germanium Wafer Double Sided Lapping Works<\/h2>\n\n\n\n

    Process Principle<\/h3>\n\n\n\n

    In double sided lapping, the germanium blank is placed in a carrier (typically a thin metal or polymer disc with cutouts) that sits between two flat lapping plates. Both plates rotate \u2014 typically in opposite directions \u2014 while the carrier orbits between them. Diamond abrasive slurry is fed between the plates and the workpiece.<\/p>\n\n\n\n

    The key difference from single sided lapping: both faces are processed simultaneously under equal pressure.<\/strong> This automatically improves parallelism because any high spot on either face receives more contact pressure and is removed preferentially.<\/p>\n\n\n\n

    Critical Process Parameters<\/h3>\n\n\n\n
    \ub9e4\uac1c\ubcc0\uc218<\/th>Typical Range for Germanium<\/th>Effect<\/th><\/tr><\/thead>
    Lapping plate material<\/td>Cast iron (grooved)<\/td>Provides flat reference surface<\/td><\/tr>
    Diamond slurry grit size<\/td>9 \u03bcm \u2192 3 \u03bcm (two-stage)<\/td>Coarser for removal, finer for finish<\/td><\/tr>
    Lapping pressure<\/td>0.5\u20132.0 psi (3.4\u201313.8 kPa)<\/td>Higher = faster removal but more subsurface damage<\/td><\/tr>
    Plate speed<\/td>20\u201360 rpm<\/td>Affects removal rate and uniformity<\/td><\/tr>
    Slurry concentration<\/td>0.5\u20132.0 carat\/liter<\/td>Too low = scratching; too high = waste<\/td><\/tr>
    Material removal target<\/td>15\u201330 \u03bcm per face<\/td>Enough to remove wire saw damage layer<\/td><\/tr><\/tbody><\/table><\/figure>\n\n\n\n

    Two-Stage Lapping Approach<\/h3>\n\n\n\n

    For germanium, a two-stage lapping process produces the best results:<\/p>\n\n\n\n

    Stage 1 \u2014 Coarse lapping (9 \u03bcm diamond)<\/strong><\/p>\n\n\n\n