Plasma and Dry Micro/Nanotechnologies 8. Etching Lenka Zajíčková Faculty of Science, Masaryk University, Brno & Central European Institute of Technology - CEITEC lenkaz@physics.muni.cz spring semester 2023 Central European Institute of Technology BRNO I CZECH REPUBLIC IP • Etching 8.1 Motivation - Lithography 8.2 Motivation - Etching and Substrate Removal 8.3 Wet Etching 8.4 Dry Etching Plasma & Dry Technologies Etching Lenka Zajíčková 3/19 8.1 Motivation - Lithography process flow Microlithography is a technique that creates microstructures after given geometrical template: ► Lithography is usually applied to shape a thin film =^ deposition of thin film ► Photosensitive material (resist) is coated on the material that should be shaped ► Resist is irradiated through a mask, by projection of UV image or by directed electrons (photolitography or electron lithography) ► Resist development: ► positive resist: soluble in developper at the irradiated places ► negative resits: unsoluble in developper at the irradiated places ► Etching of the film through photoresist pattern ► Rest of the resist is removed Thin film deposition Thin JN Substrate PhoLoiesist coat big & development Mask P bot ores Is Thin film etching Pholoies ist ashing plasmu plasma lithography patterning with positive resist Plasma & Dry Technologies Etchi ng Lenka Zajíčková 4/19 Photolithography - step details ► creation of the mask layout on a computer ► generation of a photomask a sequence of photographic processes (using optical or e-beam pattern generators) that results in a glass plate that exhibits the desired pattern in the form of a thin («100 nm) chromium layer. ► deposition of thin film (discussed later) ► spin-coating of a photoresist (positive or negative) polymeric photosensitive material spun onto the wafer in liquid form (an adhesion promoter such as hexamethyldisilazane, HMDS, is usually used prior to the application of the resist). The spin speed and photoresist viscosity determine the final resist thickness, which is typically between 0.5-2.5 /xim. Due to the better process control that can be achieved for small geometries, the positive resist is most commonly used in VLSI processes. ► soft-baking (5-30 min at 60-100 °C) in order to remove the solvents from the resist and to improve the adhesion. ► mask alignment to the wafer ► exposure of photoresist to a UV source - photoresist is developed in a process similar to the development of photographic films ► hard baking of the resist (improvement of adhesion) 20-30 min at 120-180 °C ► etching of underlying thin film through created pattern on wafer ► removal of the photoresist in acetone or another organic removal solvent Plasma & Dry Technologies Lenka Zajíčková Techniques for Photolithography Three different exposure systems (depending on the separation between the mask and the wafer): 1. contact - better resolution than the proximity technique but constant contact of the mask with the photoresist reduces the process yield and can damage the mask 2. proximity 3. projection - uses a dual-lens optical system to project the mask image onto the wafer =^ one die exposed at a time =^ step and repeat system to completely cover the wafer area. The most popular microfabrication system yielding superior resolutions to the contact and proximity methods. The exposure sources used for photolithography depends on the resolution. ► above 0.25 /iim minimum line width =^ high-pressure mercury lamp (436 nm g-line and 365 nm i-line), between 0.25 and 0.13 /iim =^ deep UV sources such as excimer lasers (248 nm KrF and 193 nm ArF), ► below 0.13/iim regime =^ extensive competition between e-beam, X-ray and extreme UV (EUV) (with a wavelength of 10-14 nm) Plasma & Dry Technologies :ching ka Zajíčková 6/19 8.2 Motivation - Etching and Substrate Removal Plasma & Dry Technologies :ching ka Zajíčková 7/19 Classification of Etching/Sputtering Processes Basic classification: ► wet etching ► dry etching Classification according to the type of process: ► ion sputtering ► chemical etching ► plasma etching Two important properties of etching: ► selectivity - degree to which the etchant can differentiate between the layer to be etched and the masking layer or underlaying material ► directionality - istropic versus anisotropic etching a) Profile for isotropic etch throujjh aphotciesjsl rruisk b) Profile for Jin isotiop ic etch through ü photoresist mask Photoresist SI Photoresist Sill ■.-.Hi Photoresist Photnres ist Silicon Plasma & Dry Technologies Etchi ng Lenka Zajíčková 8/19 Properties of Etching/Sputtering Processes ion sputtering ► purely physical approach, removal by energy transfer ► slow process, no selectivity ► ions are directed by electric field, i.e. anisotropic process chemical etching ► purely chemical processes that requires aggressive chemicals and/or elevated temperature for reaction activation ► can be very fast, selective ► chemical reactions with surface are not directed, i.e. isotropic process plasma etching ► combination of physical and chemical approaches ► directional process a) Piofile for isotropic etch throu.^h a p ha ta res i si musk b) Profile tor unisatrapit etch through a photoresist miisJi Photoresist SI Ph ota res i si Sill ■.-.Hi iliatoresist Photnres ist Silicon Etching Len kaZajickova 9/19 8.3 Wet Etching ► isotropic process (except for crystalline materials) =^ lateral undercut, minimum feature size > 3/iim ► superior selectivity to the masking layer as compared to dry techniques Historically, wet etching techniques preceded the dry ones. Still important for micro/nanofabrication in spite of their less frequent utilization in VLSI technology. Etching of Si02 ► etchant - dilute (6:1, 10:1 or 20:1 by volume) or buffered HF (BHF: HF+NH4F) solutions ► masking materials - photorezist or silicon nitride ► etch rate « 100nm/min in BHF Etching of Si3N4 ► phosphoric acid (H3PO)4 at 140-200 °C ► masking materials - silicon oxide ► not commonly used due to the masking difficulty and nonrepeatable etch rates Etching of metals - Al, Cr, Au various etchants combining acid and base solutions, commercially availble Plasma & Dry Technologies :ching ka Zajíčková 10/19 Wet Chemical Etching Anisotropic and isotropic wet etching of crystalline (Si and GaAs) and amorphous (glass) substrates is an important topic in micro/nanofabrication. The realization of anisotropic wet etching of c-Si is considered to mark the beginning of micromachining and MEMS fabrication. Isotropic etching of c-Si ► HF/HNO3/CH3COOH etchant - "HNA" stands for hydrofluoric acid (HF), nitric acid (HNO3) and acetic acid (CH3COOH). HN03 oxidizes Si, HF dissolves the oxide, CH3COOH prevents the dissociation of HN03 ► masking materials - Si02 for short etch time otherwise Si3N4 ► dopant selectivity - etch rate drops at lower doping concentrations (< 1017 cm-3 n- or p-type), it can be as etch-stop mechanism but it is not widespread due to its difficulty Isotropic etching of glass ► etchant - HF/HNO3 ► masking materials - Cr/Au for shorter time, long etching requires a more robust mask (bonded Si) etching results in rough surfaces, used in fabrication of microfluidic components (mainly channels) Plasma & Dry Technologies Etching Lenka Zajíčková 11/19 Anisotropic etching of c-Si ► three possible anisotropic etchants attacking c-Si along preferred crystallographic directions: ► potassium hydroxide (KOH), ► ethylenediamine pyrocatechol (EDP - a typical formulation consists of ethylenediamine NH2-CH2-CH2-NH2, pyrocatechol C6H4(OH)2, pyrazine C4H4N2 and water) ► tetramethyl ammonium hydroxide (TMAH) ► etch rate « 1 /xm/min at temperature 85-115 °C ► etch rate slowest for (111) planes => used to create beams, membranes and other mechanical and structural components, markedly reduced in heavily (> 5 x 1019cm-3) boron-doped (p++) regions ► etching chemistry is not quite clear: Si oxidation at surface and reaction with with hydroxyl ions (OH~) creates soluble silicon complex (Si02OH2_ ► masking materials - Si02 and Si3N4 (superior for longer etch times) Plasma & Dry Technologies Etching Lenka Zajíčková 12/19 Examples of Si Anisotropic Etching (m) (100) surface orientation 54.74 Silicon (110) surface orientation (111) Silicon fl"Q| [100] SiO, mask t lim / ľšTl • ■ L Plasma & Dry Technologies tching ka Zajíčková 13/19 Types of Directional Dry Etchini ion sputtering (milling) • pressure 0.01 - 0.1 Pa, Ar+ • etch rate few nm/min • poor selectivity (close to 1:1 for most materials) high-pressure plasma etching **.*.-»*,*.*. ^^TVStfwnm^WliiUmtFw' ■ , •.***• >*«*•*« •vWrt9RVWiWwÄWWW-iT«-§v» >*•*•*« »v ES*»*.*■ *»**V»*V»fcV*»V«*»V»*T**»*»*« V»*.*. V. • ■ V. *• *. V« *. V*.*. Wi i *. *.*. •« *.*. •« *, *.*.•• *. *.*. •* - V.•.>>•>>•*« v.*. ■* w. •« *.*.*. *.*.*. t *.*.*. reactive ion etching (RIE) K>*5*!*- ****** '* ** .".*»•. ■«■■•. ••■«■••.' «*»*#*«*■***•*•*• w .*.*.*.*.•.*.*.*.*.*. • pressure 15 - 500 Pa • highly reactive plasma species produce volatile molecules • nonvolatile species are removed by low-energy ions —► directional etching due to passivation of side walls by nonvolatile species • pressure 1 - 10 Pa • reactive species react only with activated atoms of material, activatior achieved by the collision with an incident ion Plasma & Dry Technologies Etching Lenka Zajíčková 14/19 Principles of Plasma Etching - Plasma Chemistry -^ Mil* ťds A:.* k'a>:ii O o in-* o o 1 l>llU,F.... O ll-;-í.íp:p.fj'.'í Pí i . I i (I) CFi+e CFi + 2F +e F 5iF. (4> t<5) F _> 5iF -> 5iF, .flurliuri- CP,1I C:K. CíFi 5=^ CíFí ft I t<" ť" t' F, ft ' P,P; ' P _ ' P a>CF, " CF, -í=^ CFj * _ * CF « * C + F 1. Creation of reactive species within plasma phase by electron-neutral collisions e~ + CF4 CF3 + F + e" 2. Transport of reactive species from plasma to substrate 3. Adsorption of reactive species on surface (physisorption or chemisorption) 4. Surface or volume diffusion of reactants, formation of desorbing species F*+SiFx^SiFx+1 5. Desorption of product species SiF4(S) SiF4(g) 6. Transport of product species into plasma 7. Simultaneous re-deposition of etching products F. Chen, 2003, Lecture notes ..., p. 167 Plasma & Dry Technologies Etching Lenka Zajíčková 15/19 Principles of Plasma Etching - Spontaneous Etching Princip es of P asma Etching - Spontaneous Etching Neutral species from plasma interact with solid surface to form volatile products in the absence of energetic radiation (ion bombardment, UV radtiation) Etching rate follows Arrhenius relationship because it is limited by surface reaction kinetics: Qflux of reactive species, Tsubstrate temperature, k0 preexponential factor, Ea activation energy Typically, Langmuir-Hinschelwood mechanism (reaction between chemisorbed species) - creation of free radicals in plasma eliminates chemical barrier for chemisorption Because of higher activation energy, etching yield by atomic CI is two orders of magnitude lower. It is consistent with high energy barrier for penetration of CI into Si (13 eV) compared to F (1 eV) Plasma & Dry Technologies Etching ka Zajíčková 16/19 Principles of Plasma Etching ntaneous Deposition -200 ! V 1 2 -100 \ Í Leading li addition 4- ■> (i. add* tin < ;F4 C4F„, C,Fft —n-1-1— CF4 I— Etc rang \ Pah/ mcn/a no n -L 1 2 3 Fluorine to carbon ratio highly reactive radicals , e.g. atomary chlor, created in plasma react with surface producing gaseous products > plasma ions bombards the surface (acceleration through plasma sheath) - removal of surface contamination that block etching - contribution to etching kinetics Plasma & Dry Technologies ching ypes of Directional Dry Etching ka Zajíčková 18/19 Types of Directional Dry Etchin< deep reactive ion etching (DRIE) a) ťhíXwicsisl pü.t1eTJiijij; b) Etch step 4 PsSfflvatica asp d) Eitth step fig. B.17a-d DRIE cyclic process: (a) photoresist pattern-Liiß.. (b) etch step. (t) passivation step, and (d) etch step • two step cycle (see fig.) • aspect ratio 30 : 1 • etch rate of Si = 2-3 jim/min Silicon DRIE: etching step - SF6/Ar passivation step - nCF2/Ar (50 nm teflon-like polymer deposited on side walls) Tab Ee a .1 Typica L dr v etdi die i.n Lsti' te.s Si CFJO2. CF2CL->, CFjCL, SFn;0>/CL2, Ch/Hj/GiBtfCCU, C2CIF5/Ü2, Br?. Hilrj/Oi. NFj. CIFi. CCU.CiCUF5, CjOFj/SPi, CjFa/CFjCL. CFjCl/lir2 CF4/H2, CíFů, CjFs, CHFj/Qj CFi/Oi/Hi, CíFó, CjFs, CHF3 OnjiLTjic.1; Ol CF4/O2, SFů/Oi Al BCLí, BOli/Cb, CCU/Cla/BOLj.SiCU/Cla Šilicjdří; CFi/Oi, NFi, SFů/Cb, CF4/CI3 Re tract cries CF4/Q2, NFj/Hí, SFů/Oí GaAs BCLj./'Ar, CL2/Ů2/H2, CCljFj/ťVAr/He. InF CH^H^Czki/Hj. CL2/Ar Au C2CL2F4, CL2, cc1f3 Plasma & Dry Technologies ching ros and Cond of Plasma Etching Pros and Cons of Plasma Etching os an< Most of dry etching applications are plasma based. more anisotropic than chemical etching (smaller undercuts allow smaller lines to be patterned, etching of high-aspect-ratio vertical structures) Dhigher etch rate due to synergy of chemical etching and ion bombardment lower etching selectivity