RIE - Reactive Ion Etching Yannik Junk FYS4310 - Project

(chemical and physical) Why RIE? Desired properties of etching processes: Anisotropy Selectivity Wet etching: Selective, but not anisotropic Dry etching: Plasma etching (chemical) Selective Not anisotropic Ion beam etching (physical) Not selective Anisotropic      Reactive ion etching (chemical and physical) Selective Anisotropic  

Reactive ion etching - RIE Plasmas are utilized for etching Two different types of reactors: Capacitively coupled (CPP) and inductively coupled (ICP) plasma reactor Capacitively coupled plasma reactor: Gas injected between two capacitor plates RF voltage ionizes the gas and generates the plasma High RF frequencies  Electrons are strongly accelerated and hit the electrodes Depletion of the electrons at the upper electrode causes an effective dc bias voltage between the electrodes Ions are accelerated towards the lower electrode

Reactive ion etching - RIE Inductively coupled plasma reactor: Plasma generated by a magnetic field induced by a coil Sample holder coupled to a second RF generator Possible to control the plasma density independently of the dc bias Advantages: Anisotropic etching at higher rates Less damage

Requirements for successful etching Volatile etching products Appropriate masking material Choice of pressure and temperature Choice of plasma power

Usual chemistries: Silicon Chlorine: Advantage of Cl-based RIE: Highly anisotropic due assistance of ion bombardment Undoped Si is etched very slowly in Cl-based chemistries without ion bombardment Highly n-doped Si etches very quickly in Cl2-atmospheres Flourine: Possible etch gases are SF6, CF4, SiF4, NF3, XeF2 and F2 Problem: Isotropic etching  Bosch process

Etching until the passivation layer is gone The Bosch process Etching until the passivation layer is gone Passivating the walls e. g. SF6 e. g. C4F8

Etching of III-V semiconductors Flourides of elements III are not volatile F-based chemistries are not suitable for etching of III-V semiconductors Cl-based chemistries are used Mixtures of Cl- and F-based etchants might be used

Annoying effects in RIE Loading effect: Change of the etch rate with the sample size Caused by a faster depletion of etchants for large samples Strongly dependent on the respective chemistry and the material of the lower electrode If the electrode can be etched, it is contributing to this effect For similar etch rates for different sample sizes, the sample to be etched can be positioned on a large sacrificial sample of the same material

Annoying effects in RIE Lag effect: When etching with a mask, etch rates in large openings are higher than in small openings Caused by differences in diffusion depending on the size

Annoying effects in RIE Micro-masking: Formation of nano-pillars on etched surfaces Can be caused by etching products with low volatility (e. g. InCl3 when etching InP in Cl2-based chemistry Solution: Decreasing the cooling of the sample

Annoying effects in RIE Micro-masking: Different origin: Induced by interplay of substrate and etching chemistry Example: semi-insulating InP etched in Cl2:CH4:H2 Formation of “whiskers” on top of the sample Caused by heating  Polymerization of CH4 Can be remedied by thermal coupling between the sample and the (cooled) electrode

Sources F. Karouta: A practical approach to reactive ion etching. J. Phys. D: Appl. Phys. 47, 233501 (2014) H. Jansen et al.: A survey on the reactive ion etching of silicon in microtechnology. J. Micromech. Microeng. 6, 14-28 (1996) Image sources: https://www.crystec.com/trietche.htm http://www.iue.tuwien.ac.at/phd/ertl/node68.html https://www.tf.uni- kiel.de/matwis/amat/semitech_en/kap_7/backbone/r7_2_2.html J. Karttunen, J. Kiihamäki, S. Franssila: Loading effects in deep etch silicon. Proc. SPIE 4174, Micromachining and Microfabrication Process Technology VI S. A. Campbell: Fabrication engineering at the micro- and nanoscale. Fourth edition, Oxford University Press, 2013