Fabrication of High Aspect Ratio Microstructures using Time Multiplexed Reactive Ion Etching
Author
Tang, Anders
Term
4. term
Education
Publication year
2015
Submitted on
2015-08-31
Pages
77
Abstract
Hovedformålet med specialet var at fremstille en gradientindeks-linse (GRIN-linse) ud fra et teoretisk design af Brincker og Karlsen [1]. Vi forsøgte at realisere linsen ved at bruge Bosch-processen, en form for dyb reaktiv ionætsning (DRIE), til at lave meget høje og smalle mikrostrukturer. Processen blev systematisk optimeret i et reaktivt ionætsningssystem med SF6 som ætsgas og C3F8 som passiveringsgas. Efter optimeringen kunne vi fremstille strukturer med lodrette sidevægge og et aspektforhold (dybde/bredde) på 22. Bosch-processen viste høj stabilitet og god kontrol over profilformen. Krom blev brugt som hårdmaske og gav høj selektivitet, dvs. masken blev ætset betydeligt langsommere end materialet under den. Under visse geometrier så vi microtrenching, dvs. små grøfter ved bunden af strukturerne, som gav en ujævn ætsningshastighed. Den vigtigste begrænsning for endnu højere aspektforhold var masketykkelsen. For at komme over 22 kræves yderligere procesoptimering.
The goal of this thesis was to make a gradient‑index (GRIN) lens based on a design by Brincker and Karlsen [1]. We pursued this by using the Bosch process, a type of deep reactive ion etching (DRIE), to create very tall and narrow microstructures. We systematically optimized the process in a reactive ion etching system that used SF6 as the etching gas and C3F8 as the passivation gas. After optimization, we produced structures with vertical sidewalls and an aspect ratio (depth to width) of 22. The Bosch process was stable and offered good control of the etched profile. Chromium served as a hard mask and showed high selectivity, meaning it etched much more slowly than the material underneath. Under certain geometries we observed microtrenching—small grooves at the base of the features—which led to a non‑uniform etch rate. The main factor limiting the aspect ratio was the thickness of the masking layer. Achieving aspect ratios above 22 will require further optimization.
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