Prototyig of III-N LEDs by Laser Direct Writig Techiques Michael Kuzer, Rüdiger Moser, Christia Goßler, Klaus Köhler, Wilfried Pletsche, Ulrich T. Schwarz ad Joachim Wager Frauhofer-Istitut für Agewadte Festkörerhysik (IAF) Tullastraße 72 79108 Freiburg, Germay
Prototyig of III-N LEDs by Laser Direct Writig Techiques Motivatio Exerimetal setu Ablatio threshold Device rocessig Device results Coclusio ad Outlook
Motivatio - Laser materials rocessig High velocity (~3 m/s) Very recise (µm accuracy) Flexible (cuttig, drillig, weldig) Raid rototyig ad serial roductio 20µm huma hair
Exerimetal Setu Self-built system Ultra-short ulse laser» t 10 s» 355 m, 532 m, 1064 m» Pulse eergy u to 35 µj@355 m» Pulse o demad High recisio air bearig stage» Positio reroducibility 0.2 µm» Seed u to 100 mm/s Fixed otics:ω 0 =7 µm ad ω 0 =1.7 µm
Advatages of ultra-short ulse laser t < 1 s: Eergy absortio by electros t < 10 s: Eergy trasfer to hoos t > 10 s: Proagatio i the crystal lattice Lattice temerature well below electro temerature Material is removed with miimal thermal damage to eighbor regios Dirk Wortma Lehrstuhl für Lasertechik ILT cold ablatio
fluece [F] Determiatio of the ablatio threshold Ablatio threshold for s laser ulses @ 355 m Gaussia beam rofile Hole diameter deeds o ulse eergy Determiatio of ablatio threshold ad beam diameter Ablatio of GaN F c F b F a 6µm F d -r a r a -r 0 c -r b r b r c radius [r]
Diameter² [µm²] Ablatio threshold [J/cm²] Ablatio threshold for GaN Determie the ablatio threshold for GaN (LED structure) Ablatio threshold deeds o umber of shots (icubatio effect) 80 70 60 50 40 30 20 10 Exerimetal Data Fit F 1 = 0,64 ± 0,02 J/cm² 1 10 Eergy [µj] 0,8 0,6 0,4 0,2 0,0 Exerimetal Data Fit F if = 0,11 ± 0,01 J/cm² 1 10 100 1000 Number of Shots
Device rocessig Four fudametal stes to realize a fuctioal LED-device 1. Deositio of ohmic -cotact 2. Mesa structure (electrical isolatio + exosure of -GaN) 3. Deositio of ohmic -cotact 4. Mechaical isolatio
-cotact Patterig of molybdeum foil with s laser Shadow mask Smoother sidewalls Better defied structures Smaller feature size Maitai accuracy u to 4 ich metal sahire s - ulse s - ulse 1mm 50µm 50µm solid - liquid - gas solid - gas
-cotact Deositio of ohmic cotacts Defied deositio of ohmic cotacts ad/or bod ads (Ni/Au) Without ay lithograhy stes (for develomet ad rototyig) Feature size dow to 10 µm 200µm 200µm metal sahire 50µm sahire mask sahire metal sahire
Height [m] 215 m -mesa Laser--mesa Lateral LED device defiitio Electrical isolatio ad exosure the -GaN Iteded deth aroud 230 m Ammoia ad KOH treatmet to remove gallium metal sahire ω 0 =7 µm F=1,69 J/cm² ω 0 =1,7 µm F=1,69 J/cm² 300 200 100 0-100 Strai buildu ad relaxatio 10µm 6µm Reductio i ulse eergy by factor 17-200 30 40 50 60 70 80 90 100 X-directio [µm]
-cotact Aliged deositio of -cotacts Evaoratio of ohmic -cotact metallizatio (Ti/Al/Ni/Au) Aligmet marks for multile masks deositio Distace betwee differet features reducible to 10 µm metal sahire Shadow mask Aligmet mark 100µm 300µm
Laser-Trech formatio trech metal sahire Large differece i ablatio threshold for GaN ad sahire @ 355 m Self stoig effect Defied trech deth Ablatio threshold (F if ) @ 355 m ad 10 s: GaN Sahire 0.11 J/cm² << 0.76 J/cm² F=0,68 J/cm² 10µm 1 ass 3 asses 20 asses
Curret [A] Outut Power [mw] Itesity [a.u.] Device results Wafer level characterizatio DC oeratio at room temerature Electrical ad otical results comarable with covetioal rocessed LEDs 10-2 U@ 20mA = 3.75 V 10-4 10-6 10-8 10-10 I @ -5V = 6.8 x 10-7 A -4-2 0 2 4 Voltage [V] 8 7 6 5 4 3 2 1 I @ 40mA = 6.8 mw 0 0,00 0,01 0,02 0,03 0,04 Curret [A] I = 20 ma 350 400 450 Wavelegth [m]
Reverse curret @ -5V [A] Leakage Curret Determie reverse curret @-5 V 1E-5 without aealig with aealig Helful idicator for short circuit ad material degradatio 1E-6 1E-7 Comariso with covetioal dry etched mesa 1E-8 Etched -Mesa Laser -Mesa Aealig of defects Electrical roerties as good as covetioal dry etched LEDs metal sahire
Outut Power [mw] Curret [A] Comariso with covetioal LEDs Comariso with sister wafer Leakage curret ad forward voltage comarable with covetioal LED Outut ower withi the variatio 10-1 10-3 10-5 10-7 10-9 10-11 8 6 4 Covetioal LED Laser Processed LED -4-2 0 2 4 Voltage [V] No evidece for material degradatio due to laser rocessig 2 0 0,00 0,01 0,02 0,03 0,04 Curret [A]
Coclusio ad Outlook Successfully realized GaN-based LEDs without lithograhy stes Demostratio of fudametal stes:» Mesa structures ad treches (direct laser writig)» Cotact metallizatio ( idirect, via shadow masks) LED characteristics comarable with covetioal fabricated referece LEDs Fast ad flexible method for raid rototyig of ew devices (layout ad eitaxial structures) 6µm