Polishing Precision Optics

Polishing Silicon Carbide - Case Study

Test Requirements: To produce best surface finish on 65mmø Silicon Carbide blanks prior to pitch polishing.
Component/Material: Silicon Carbide blanks

Stage 1

Machine Type: Kemet 15 diamond lapping/polishing machine
Lap Plate: Kemet Iron with spiral grooves
Abrasive Type/Grade: Kemet Liquid Diamond Type K 14 Micron standard
Additional Pressure: 3.5Kgs for 1 part

Stage 2

Machine Type: Kemet 15 diamond lapping machine with facing unit
Lap Plate: Kemet Copper with spiral grooves
Abrasive Type/Grade: 6-KDS1488 (water-based)
Additional Pressure: 4 Kg

Stage 3

Machine Type: Kemet 15 diamond lapping machine with facing unit
Lap Plate: Pure Tin
Abrasive Type/Grade: 0.75-KDS1438 (water based)
Additional Pressure: 4 Kg

Results:

Stage 1: - This process removed 10 - 20 µm in 5 - 10 minutes. The surface finish achieved ranged from 0.08 ~ 0.084µm.

Stage 2: - This pre-polish step gives a semi-reflective surface with surface finish of Ra 0.056 ~ 0.060µm.

Stage 3: - The polishing process results in a mirror surface finish of Ra 0.007µm.

After Stage 1 Polishing Silicon Carbide

Conclusion
In order to improve surface finish and flatness, pitch polishing should be used

Flatness was measured on a Zygo and was 0.341 µm (Peak to Valley)

Final Flatness After Polishing Silicon Carbide

Polishing Quartz - Case Study

Test Requirements: To lap and polish 2 samples of Quartz to flatness better than 1 um and Ra better than 1nm
Component/Material: 1 quartz glass tube (OD ~62mm) sample and 1 quartz glass rod (OD ~41mm)
Machine Type: Kemet 15” diamond lapping/polishing machine

The glass was held on the weights using a non-slip film.

Quartz sections were initially cut on Micracut 201 precision saw using diamond cut off wheel and a slow feed rate. This produced a clean cut with minimal chipping

After lapping & polishing, the middle of the part is flat to 1-2 light bands (< I micron).

Before Polishing Quartz

After Polishing Quartz

PR3 Plate Quartz Polishing Test

Work Size: φ100mm×6mmT
Polishing Condition: In-Situ method. The benefit of using the dresser is that it keeps the cutting rate and maintains the PR3 plate flatness.
Plate: 15” PR3 Plate Conditioned by facing unit
rpm: 40rpm　
Slurry: Cerium oxidie (Size 1~2µm 5%/wt)
Flow amount : 5ml /min
Process 1: Pressure 80g/cm2 30 min
Process 2: Pressure 30g/cm2 20 min
Flatness Result: 0.130µm（λ/4.88）

Lapping & Polishing Beryllium Copper - Case Study

Test Requirements: Optical finish, no specific flatness required
Component/Material: Various Beryllium Copper samples

Stage 1

Machine Type: Kemet 15” diamond lapping/polishing machine
Lap Plate: Kemet XP
Abrasive Type/Grade: 3µ Type K STD Diamond slurry
Additional Pressure: 4 Kg

Stage 2

Machine Type: Kemet 15” diamond lapping/polishing machine
Lap Plate: ASFL-AW silk cloth
Abrasive Type/Grade: 3µ Type K STD Diamond slurry
Additional Pressure: 4 Kg

Stage 3

Machine Type: KemCol 15 - Chemical Mechanical Polishing Machine
Lap Plate: Chem Cloth
Abrasive Type/Grade: Col-K
Additional Pressure: 4 Kg

Stage 1:
This is a grading stage to produce a good flat surface and finish. Initially we ran the component on a Kemet XP plate using a 3µ slurry with 4kgs weight for 15 minutes. This produced a good reflective finish with some scratches visible.

Stage 2:
This is a pre-polishing stage, which gives a good reflective finish and prepares the component for final polishing. The component was then run on an ASFL-AW silk cloth using a 3µ type K STD slurry for 5 minutes with 4kgs of weight to produce a 90% blemish free finish.

Stage 3:
Polishing stage, gives a blemish free mirror polish. Finally the component was run on a CHEM cloth using COL-K colloidal silica with 4kgs of weight for 5 minutes. This produced a blemish free mirror polish.

Recommended minimum equipment required to run 1 component

• 1 x Kemet 15” Diamond lapping machine or larger
• 1 x Kemet 15” Col-k Colloidal silicate lapping machine or larger
• 1 x Kemet XP plate
• 1 x Kemet Stainless Steel plate for Col-K
• 1 x ASFL-AW silk cloth
• 1 x Plastic faced control ring
• 3µ Type K STD diamond slurry
• Col-K Colloidal Silicate

Lapping & Polishing Germanium Parts - Case Study

Test Requirements: To lap germanium discs with surface finish Ra<0.01 µm, Rz<0.05 µm;flatness < 1µm; parallel < 3 µm.
Component/Material: 6 x 25mm diameter Germanium

Stage 1

Machine Type: Kemet 15” diamond lapping/polishing machine
Lap Plate: Cast Iron
Abrasive Type/Grade: Kemox 0800S
Additional Pressure: 4 Kg

Stage 2

Machine Type: Kemet 15” diamond lapping/polishing machine
Lap Plate: ASFL polishing cloth
Abrasive Type/Grade: Kemet Liquid diamond Type K 1 Micron standard
Additional Pressure: 4 Kg

Comments:
Flatness required: < 3 µm . Preferably 1 µm
Flatness achieved: 2 light band = 0.67 µm
Ra required Ra: 0.01 µm
Ra achieved: 0.059 µm
Rz required: 0.05 µm
Rz achieved: 0.0253 µm