Polishing Precision Optics

New polishing systems have been developed for polishing crystals such as Sapphire for Light Emitting Diodes, Silicon Carbide, Calcium Fluoride, Zinc Selenide and Gallium Arsenide. Other applications include polishing Laser Mirrors made out of Beryllium Copper.

Case Studies On Lapping & Polishing 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

Process Breakdown
Stage Plate/cloth type Abrasive type/grade Dispenser settings Process time
1 Kemet Iron with spiral grooves Kemet Liquid Diamond Type K 14 Micron standard 2 second spray every 40 seconds 5 - 10 minutes (Front Surface)
2 Kemet Copper with spiral grooves 6-KDS1488 (water-based) diamond slurries 2 second spray every 40 seconds 10 - 15 minutes
3 Pure Tin 0.75-KDS1438 (water based) diamond slurries 2 second spray every 40 seconds 1.5 - 2 hours

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

Before 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

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

Process Breakdown
Stage Plate/cloth type Abrasive type/grade Process time
1 Lap Cast iron Kemox 0-800s 2o minutes
2 Pol PSU-M Polishing pad Kemox WC 50 minutes

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

Before Polishing Quartz

After 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

Process Breakdown
Stage Plate/cloth type Abrasive type/grade Dispenser settings Process time
1 Kemet XP 3µ Type K STD Diamond slurries 2 seconds spray every 30 seconds 15 minutes
2 ASFL-AW silk cloth 3µ Type K STD Diamond slurries 2 seconds spray every 30 seconds 5 minutes
3 Chem Cloth Col-K CMP Slurry Constant drip feed from peristaltic pump 5 minutes

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

Before Polishing Beryllium Copper

Before Polishing Beryllium Copper

After Polishing Beryllium Copper

After Polishing Beryllium Copper

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

Process Breakdown
Stage Plate/cloth type Abrasive type/grade Process time
1 Lap Cast iron Kemox 080S 2 mins per side
2 Pol ASFL Polishing pad Kemet Liquid diamond Type K 1 Micron 5 Mins per side

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

Before Lapping & Polishing Germanium

Before Lapping germanium

After Lapping & Polishing Germanium

After Lapping germanium

germanium surface finish

PR3 Polishing Plate For Optics

The Kemet PR3 Polishing Plate is a new alternative to using polyurethane pads for polishing precision optics. The plate consists of a thermally stable resin and can be used with cerium oxide, aluminium oxide and diamond slurries.

  • Excellent Flatness produced – below 0.08 micron
  • Improved Edge Exclusion - No fall off at the edges.
  • Produces excellent surface finishes down to 1nm
  • Easy to machine.
  • Ideal for polishing optical glass

Kemet PR3 can be supplied as complete plates in most diameters or as discs for fixing to your machine base plate.

Related Products