Rohit Grover, February 24, 2001

Standard Process Steps For Ridge Lasers, SOA’s and SSOA’s

IMPORTANT: Check (as often as needed) the sample under microscope in BF, DF and Nomarsky (as relevant) to ensure no solvent residues accumulate on sample surface.

• Cleave off a 13x13 mm² chip of laser/amplifier material. Note the direction of the major flat and mark it gently in a corner with scriber. Dry etching: the ridges will be made parallel to the major flat to allow for a good cleave. If using wet etching to define the ridge, your choice of ridges parallel or perpendicular to major flat is dependent on etch profile desired.
• Degrease by rinsing in TCA, Ace, Meth, IPA, DI H₂O, IPA and blow-dry in N₂.

If high-quality sidewalls are desired for the ridge:

v Soak the sample in OPD4262 for at least 10 min. to remove oxide.

v Coat with 5000 Å of SiO₂ in PECVD (use process 1). Use a witness piece to measure thickness. (5000 Å is enough for up to 3mm deep etch; for deeper etches, you will have to adjust this thickness.)

• PR coating:

v Positive PR

• (In the PR hood) Spin-on HMDS (hexamethyl disilane), adhesion promoter for photoresist, at 3500 rpm for 60 s.
• Wait for 10 s and spin on photoresist OiR908 20HC at 3500 rpm for 60 s.
• Pre-bake PR at 90 ºC (soft bake).

v Negative PR

• (In the PR hood) Spin-on HMDS (hexamethyl disilane), adhesion promoter for photoresist, at 3500 rpm for 60 s.
• Wait for 10 s and spin on photoresist NR8 1500 at 1500 rpm for 60 s.
• Pre-bake PR at 120 ºC (soft bake).

(See standard PR recipe on clean room hood.)

If the ridges mask has alignment keys on it, your first mask level will be the ridge mask, else it will be the standard alignment keys mask.

• Expose pattern in stepper/contact aligner. Align to crystal axis or previously exposed alignment keys. Use focus/exposure for given day. (For large features) positive PR: focus of 248 and exposure of 3.5 s; negative PR: focus of 264 and exposure of 5 s. Depending on how sensitive your sample is, you may need to do a focus-exposure matrix.
• OiR 90820HC: Post-bake at 120 ºC.
  NR8 1500: No post bake.
• Allow the sample to cool down.

v Develop OiR 90820HC in OPD4262 for 60 s with gentle agitation to allow the exposed PR to dissolve. Rinse in DI H₂O for at least 20 s. Blow dry with N₂.

v Develop NR8 1500 in Futurrex RD2 for 2 min. Rinse in DI H₂O for at least 20 s. Blow dry with N₂. Examine under microscope and repeat the develop step for 15 s at a time as necessary.

If SiO₂ was deposited for use as mask for defining ridges:

v Pattern transfer in RIE with SIO2PT.PRC and graphite plate. Use nominal SiO₂ etch rate 325 Å/min. with 10 % over etch to be safe or measure etch rate prior to etching on test piece.

• Etch the pattern into epilayer.

v RIE with aluminum plate

o INPRDG55.PRC if using photoresist mask. Use nominal etch rate 950 Å/min. for InP. Ternary etches at 50 % and quaternary etches at 73 % of this rate. You must cycle this with polymer cleaning (O2SAMPLE.PRC). This is automated in RGMHA55.PRC.

o RG_MHA_O.PRC if using SiO₂ mask. Use nominal etch rate 353 Å/min. This process cycles 5 min. of etching with 3 min. of polymer clean with the time settings of last cycle adjustable for exact depth. The process requires a long chamber preparation, so plan ahead.

v Wet etching

o Useful only if your sample has an etch stop.

o Remove cap layer with H₂SO₄:H₂O₂:H₂O::1:1:8.

o Remove upper clad to etch stop with HCl:H₃PO₄::1:1. You will know etch is complete with the sample stops bubbling. (The bubbles are PH₃.)

• Remove the mask.

v Remove PR mask by 3 min. STRIP in Matrix, followed by a flood exposure in stepper or contact aligner and rinse in developer, Ace, Meth, IPA, DI H₂O, IPA and blow dry with N₂.
Alternately, remove the PR mask by soaking in NMP at 70ºC for 10-15 min. followed by rinse in TCA, Ace, Meth, IPA, DI H₂O, IPA and blow dry with N₂. (When using NMP, cover with inverted watch glass to ensure that most of it stays in the beaker.)

v Remove SiO₂ mask by SIO2PT.PRC in RIE with graphite plate.

Repeat with ridge mask if pattern exposed above was standard alignment keys. You will be aligning to these keys and not the crystal axis from now on.

• Spin on SOG – dielectric.
• Cure SOG in SOG oven with process SOG1. Controller ID 01. Monitor furnace to check if it starts ramping the temperature; if not, reboot the controlling computer and switch off/on the PID module.
• Etch back SOG in RIE with SOGETCH.PRC 1 min. at a time until the top of the ridges is free of SOG. Do not overetch or the devices will short-circuit.
• Pattern the p-side metallization mask in stepper following instructions as before.
• P-side metallization: Ti-Pt-Au. Use 3500 Å of Au to allow easier wire-bonding. (CHA process 1).
• Liftoff in IPA, TCA, Ace, Meth, IPA, DI H₂O, IPA and blow dry.
• Chemo-mechanical polishing of backside in 2.5 - 3 % bromine-methanol. More bromine will turn backside black and less bromine will give a very low polishing rate. Polishing speed 5. Do not use cloth polishing pad if a flat surface is desired.
• N-side metallization: Ni-Ge-Au-Ni-Au. (CHA process 5 or 6).
• Anneal N-side metal to form an ohmic contact. This occurs at 400 ºC – check recipe and monitor the RTA temperature (RTA process alloy.rcp).
• Scribe and cleave.