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Uniformity and Repeatability of Wafer Level Icp-Rie Etching for Semiconductor Lasers

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Uniformity and Repeatability of Wafer Level Icp-Rie Etching for Semiconductor Lasers Riina Ulkuniemi UNIFORMITY AND REPEATABILITY OF WAFER LEVEL ICP-RIE ETCHING FOR SEMICONDUCTOR LASERS Master of Science Thesis Faculty of Engineering and Natural Sciences Examiners: Adj. Prof. Jukka Viheriälä Dr. Lasse Orsila April 2020 i ABSTRACT Riina Ulkuniemi: Uniformity and repeatability of wafer level ICP-RIE etching for semiconductor lasers Master of Science Thesis Tampere University Science and Engineering, MSc April 2020 This thesis advances fabrication of AlInP laser diodes by demonstrating methods to prepare uniform waveguide structures over large areas. Results provide up to 48 % improvement in wave- guide depth uniformity. Moreover, thesis demonstrates strong correlation between efficiency of the laser and waveguide depth correlation highlighting importance of the procedures developed in this work. Methods to improve uniformity, etching profile, uniformity characterization and laser characterization are studied in detail. Work exploits reactive ion etching (RIE) in inductively coupled (ICP) chamber, a plasma etch- ing technique widely used in microfabrication to prepare precision structures, including semicon- ductor laser fabrication. Plasma etching technologies have developed as the requirements for devices have become stricter. In RIE, chemical and physical etching are used simultaneously, which results in a higher etch rate than the sum of both mechanisms separately. Even though a uniform etching result over the wafer is expected of RIE, defects such as RIE lag and microloading still exist. The aim of this thesis was to improve a ICP-RIE etching process for obtaining better uniformity and over a wafer and better repeatability from wafer to wafer. This improves yield of the devices, depressing cost of the devices and thus making them more available to society as whole. The process was developed for 3” AlInP wafers. Three tests were run with a total of five wafers. In the first test, three wafers were processed subsequently, and in the second test, onewafer was processed after stabilizing the conditions of the process chamber by etching four quarters of a wafer. In the third test, one wafer was processed in lower process pressure to obtain better uniformity. In addition, light power-current-voltage (LIV) measurements was done to bars from the edge and the center of the wafer to prove the issue from nonuniform processes. The ridge waveguides (RWG) were also imaged with scanning electron microscope (SEM), and correlations between the LIV results and dimensions measured with SEM were studied. Significant difference in the operation of the bar from the center and the edge ofthewafer was observed. Threshold current had a clear correlation with etch depth and the angle of the RWG. Etch depth was seen to decrease with the three subsequent etchings, and stabilizing the chamber by etching quarters of the wafer resulted in better uniformity within a wafer. The etch depth uniformity improved by decreasing the pressure, but it also affected the profile of the RWG by making it more anisotropic. The results of this thesis can improve performance for existing lasers and enable new type of laser products. Keywords: ICP-RIE, plasma etching, diode laser, repeatability, uniformity, LIV characterization, SEM The originality of this thesis has been checked using the Turnitin OriginalityCheck service. ii TIIVISTELMÄ Riina Ulkuniemi: Kiekkotason ICP-RIE etsauksen tasalaatuisuus ja toistettavuus puolijohdelase- reille Diplomityö Tampereen yliopisto Teknis-luonnontieteellinen, DI Huhtikuu 2020 Tässä diplomityössä esitetään AlInP-laserdiodien valmistusta esittämällä menetelmiä, joiden avulla voidaan valmistaa tasalaatuisia aaltojohderakenteita suurempien kiekkojen yli. Tulokset osoittavat jopa 48 % parannuksen aaltojohteiden etsaussyvyyden tasalaatuisuudessa. Lisäksi työssä todetaan selkeä riippuvuussuhde laserin tehokkuuden ja aaltojohteen etsaussyvyyden vä- lillä, mikä korostaa työssä kehitettyjen toimenpiteiden tärkeyttä. Menetelmiä tasalaatuisuuden, etsausprofiilin, tasalaatuisuuden määrittämisen ja laserin karakterisoinnin parantamiseen käsitel- lään yksityiskohtaisesti. Työssä hyödynnetään reaktiivista ionietsausta (RIE) induktiivisesti kytketyssä plasmakammios- sa (ICP), joka on yleisesti puolijohdeteollisuudessa ja puolijohdelaserien valmistuksessa käytetty plasmaetsaustekniikka. RIE yhdistää kemiallisen ja fysikaalisen etsauksen, joiden yhtäikaisuus tuottaa suuremman etsausnopeuden kuin molempien mekanismien etsausnopeudet yhteensä. Vaikka tasalaatuisuus kiekon yli on vaatimus RIE:n käyttöön, silti yleisiä defektejä prosessissa ovat etsausnopeuserot kuvion koon mukaan (RIE lag) ja etsaussyvyyserot johtuen tiheistä ku- vioista (microloading). Tämän työn tarkoituksena oli parantaa ICP-RIE etsausprosessia paremman tasalaatuisuu- den saavuttamiseksi kiekon sisällä sekä toistettavuuden parantamiseksi kiekkojen välillä. Tämä parantaa komponenttien saantoa pienentäen samalla kuluja, minkä ansiosta komponentit ovat paremmin saatavilla yhteiskunnalle. Prosessia kehitettiin 3” AlInP-kiekoille. Kokonaisuudessaan kolmessa testissä etsattiin yhteensä viisi kiekkoa. Ensimmäisellä kerralla etsattiin kolme kiekkoa peräkkäin, toisella kerralla yksi kiekko, jota ennen prosessikammio stabiloitiin etsaamalla neljä neljänneskiekkoa, ja viimeisellä kerralla yksi kiekko, johon testattiin prosessipaineen alentamis- ta paremman tasalaatuisuuden saavuttamiseksi. Tämän lisäksi valoteho-virta-jännite-mittauksia (LIV) tehtiin laserbaareille sekä kiekon keskeltä että reunasta epätasaisen prosessin vaikutuk- sen tutkimiseksi. Yksi baari kuvattiin pyyhkäisyelektronimikroskoopilla (SEM), jonka perusteella harjanneaaltojohteen dimensioiden ja LIV-mittausten välistä korrelaatiota tutkittiin. Kiekon reunalta ja keskeltä otettujen laserbarien toiminnassa nähtiin huomattava ero. Kynnys- virta korreloi selvästi sekä etsaussyvyyden että harjanteen muodon kulman kanssa. Etsaussy- vyyden nähtiin laskevan kolmen peräkkäisen etsauksen aikana, ja prosessikammion stabilointi etsaamalla kiekon neljänneksiä auttoi etsaussyvyyden tasalaatuisuudessa. Lisäksi etsaussyvyy- den tasalaatuisuus parani prosessipainetta laskemalla, mutta se vaikutti myös harjanteen profiiliin tekemällä siitä anisotrooppisemman. Työn tulokset voivat parantaa valmistettavien laserien toi- mintakykyä, sekä mahdollistaa uudenlaisten laserien kehittämisen. Avainsanat: ICP-RIE, plasmaetsaus, puolijohdelaser, toistettavuus, tasalaatuisuus, LIV karakteri- sointi, SEM Tämän julkaisun alkuperäisyys on tarkastettu Turnitin OriginalityCheck -ohjelmalla. iii PREFACE This thesis was carried out in Photonics Laboratory, Tampere University in collaboration with Modulight. First, I want to show gratitude to my supervisors, Adj. Prof. Jukka Viheriälä and Dr. Lasse Orsila, for sharing their expertise, providing their guidance, and helping me exceeding myself during this project. I also want to thank Dr. Soile Talmila for her advice, insight of semiconductor lasers and encouragement throughout this thesis. I am grateful to Modulight, as I have felt welcome from the very first day to this multi- talented laser family. Special thanks to Seppo Orsila and Dr. Petteri Uusimaa for believ- ing in me and giving me this opportunity. I would like to thank Ville Vilokkinen and Petri Melanen for sharing their neverending knowledge of laser processing and help they pro- vided to me. I am thankful for the whole cleanroom team for helping me get started with laser processing and making the time fly while working. I would also like to thank Mervi Koskinen for her help with the equipment used in this thesis, as well as Ilpo Suominen, Dr. Jari Nikkinen, Antti Saarela, and Dr. Kostiantyn Nechay for discussions on lasers and anything that occurred to mind. In addition, thanks to Prof. Mika Valden for being the first person to see the potential I have and giving guidance to me for the career after studies. Teekkari culture has given me so much during my studies. I want to thank all my peer students at TUT and TAU for sharing the best years in my life so far, pushing through the optics courses, organic chemistry, pedagogical studies, and many others together. I would not have become who I am without Hiukkanen, Tampereen Teekkarit, and LiiKNeu- vosto - thanks to all who I have had honor to work, learn, and have a blast with. You are all irreplaceable. Lastly, I want to thank both of my families, my mom and dad for supporting me and giving me important, yet quite diverse advice throughout my whole life. Thank you, Joni, for the endless support during this spring and keeping my feet on the ground but head up in the clouds. In Tampere, 24th April 2020 Riina Ulkuniemi iv CONTENTS 1 Introduction . 1 2 Semiconductor lasers . 4 2.1 Principle of lasers . 5 2.2 Semiconductor laser structure . 7 2.3 Edge-emitting laser diodes . 10 2.3.1 Transverse mode control . 10 2.3.2 Longitudinal mode control and optical resonators . 11 3 Semiconductor laser fabrication process . 14 3.1 Lithography . 15 3.1.1 Pattern transfer . 15 3.1.2 Photoresists . 17 3.1.3 Lithography defects . 18 3.2 Etching . 19 3.2.1 Etching effects . 19 3.2.2 Wet etching . 20 3.2.3 Dry etching . 23 4 Methods and equipment . 31 4.1 Inductively coupled plasma reactive ion etching . 31 4.1.1 Main process parameters . 32 4.1.2 Equipment . 34 4.2 Scanning electron microscopy . 35 4.3 Contact profilometry . 36 4.4 Laser characterization . 36 4.4.1 LIV characteristics . 36 4.4.2 Far-field . 37 4.5 Process description . 37 5 Results . 40 5.1 LIV characterization and SEM measurements . 40 5.2
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