Process Flow-Chart

1. The process flow-chart is shown on the above figure. First a dielectric layer is deposited and the waveguide/post pattern is transferred to the SiN_x mask. The waveguide width is 0.7mm  (or 0.9mm) below the disk and adiabatically tapers to 3mm for better coupling efficiency to a lensed fiber. The waveguides are dry etched to 1.5mm depth. The etch rate/depth is monitored insitu with laser reflectometry.

2.  Second, the whole structure is flipped over and thermally bonded to another InP transfer wafer. To assure 'perfect' bonding between both wafers a critical step is the sample cleaning to remove particles and organic residue. Multiple step ultrasonic bath/solvents/NH₄OH/DI water treatment is used to remove surface contaminations. The wafers are bonded in H₂ atmosphere at 550C. A sress-free, thermally stable, ohmic interface is obtained without degradation in the material quality. 

3. The third step is the substrate removal. The samples are mechanically polished and the remaining InP from the original substrate is completely removed by selective chemical wet etch (HCl:H₂0 - 3:1), which stops at the InGaAs etch-stop layer. This layer is then removed by H₂SO₄:H₂O₂:H₂O selective etchant. At this point the top disk-cladding layer is exposed for further processing.

4. The most critical step is the disk pattern alignment and etch. Misalignment of the disk mesa with respect to the I/O waveguides, will lead to asymmetric coupling and significant decrease of the transmission contrast ratio. For this purpose a small area (~1mm wide) around the sample edge is etched down to the waveguide pattern, which makes the waveguides, the posts, and the alignment marks defined with the first dry etch visible. The disk mesa is successfully aligned to this alignment mark and dry etched so that 0.1-0.3mm of the InP separation layer was left unetched.