Photonics Components Polishing
 
Abstract

Sub Micron Polishing Technology (SÁPTTM) enables full automation of the polish-clean-inspect cycle for significantly increased success rates in manufacturing fiber-optic planar devices.

 
Introduction

SÁPTTM is the preferred methodology in the semiconductor industry in using automated polishing for increased success rates, surface quality and accuracy. The extensive application of this technique is primarily attributed to its high reproducibility and yields.

 

This technique today is also being used by fiber optic component manufacturers to approach higher yields through automation.

 

Sagitta provides the SÁPTTM technology on the Gemini-Pi, the first fully automated multi-station polish and inspection system for photonic devices. Integration and automation of three major steps in a single system will be described:

 

  • Polishing station: Computer control of all the polishing.

  • Cleaning station: Ultrasonic cleaner integrated into the system with temperature control.

  • Inspection station:
  • Batch inspection for future on-line SPC and analysis.

    Requirements

    To polish planar devices such as Arrayed Wave guide Grating, splitters, filters, multiplexeres as well as fiber optics components (V-grooves, fiber tips) to an optical surface finish. General requirements for the polishing finish would be as follows:

    Angular polish accuracy
    Surface quality
    Planarity

      

    0.1°
    Rq < 50A , PV < 300A
    5 fringes @ 550nm light source

     

     
    i. Polishing Process
     
    Process flow

    Standard process should vary between 3 to 4 step process:
    The optimization of the process flow is vital to minimize grain structure deformation in the surface of contact.
    - Rough grinding: Should remove the bulk material and form the required geometry.
    - Planar Polishing (up to 2 stages): Smooth the surface by removing the damages caused during grinding. The depth of defects in this stage is significantly reduced.
    - Final lapping: generates the final surface quality.

     
    Materials

    Polishing materials are chosen according to their compatibility to the device structure: Mechanical properties, chemical compatibility, support of the abrasive as well as hardness and composition of the abrasive bonding for firm abrasives.
    We chose in our polishing process flow firm abrasives during the first polishing stages and loose abrasive in the final lapping with 0.05 micron colloidal silica on soft pad to reach high surface quality.

     
    Sample motion

    The motion of the sample on the polishing wheel is important to reach high surface qualities as well as equal polishing rates across the device. The relative motion of the sample vs. the polishing wheel when both move in circular motion is given by the formula below:

     
    Results

    relative = V polisher - V head

    V relative X = - r sin φ ( ω polisher - ω head )

    V relative Y = r cos φ ( ω polisher - ω head ) + ω polisher R

     

    The ideal relative motion for high surface quality is when the vector of velocity varies between zero to II radians where as for equal polishing rates one velocity throughout the device is necessary (ω polisher = ω head). Optimization between surface quality and equal polishing rates can be done by using  ω polisher « ω head .

     


    Click To Enlarge

     
    ii. Cleaning:

    Avoiding cross contamination, especially before the final lapping stage, is critical to surface quality and overall yields. While some devices may require lower frequencies to protect fragility, SÁPTTM achieves this goal through ultrasonic cleaning generally done in 30-40 kHz, along with detergent media and followed by a rinse and dry cycle. The particles targeted for removal are 1um organic and inorganic based particles, e.g. Silicon, Silicon Oxide and Diamond.

     
    iii. Final Inspection:

    The optical inspection integrated in SÁPTTM detects pre-defined defects, such as scratches and particles in the wave guide area, and characterizes the quality of the cut. Moreover, inspection is used between batches to create statistical process control (SPC) and monitoring, to increased production yields.

     

     
    Results:
     
    Parameters used

    Pressure:
    Linear velocities:
    Abrasives:

      

    40-80 gr/mm²
    300-1000 cm/min
    Diamond films and colloidal silica on soft pad

     
    Samples polished:
     
     
    Click To Enlarge
     
    Click To Enlarge
     
     
    V-Groove polish,
    optical inspection 300X

    Arrayed Wave guide,
    optical inspection 300X

     
    Surface quality:
     

    Location
    Each layer
     
     
    Between layers
    Across sample

      

    Measurement
    Rq < 10A (Rms)
    Ra < 10A (Ave)
    Pv < 100A (Rp-v)
    Step height < 30nm
    Rounding < 1000A

     


    Click To Enlarge

    ASM Surface Roughness Measurement

     

    RegioRp-y

    Rms rough

    Ave rough

    Median Ht

    Mean Ht

    Surface ara

    Volume

    [A]881 A

    5.65 A

    4.50 A

    307 A

    306 A

    9.228µ3

    0.2825µ285