FC250 Nano- and microtechnologies chapter 4. Basic Microfabrication Techniques - Etching Lenka Zajíčková Přírodovědecká fakulta & CEITEC, Masarykova univerzita, Brno lenkaz@physics.muni.cz spring semester 2017 Central European Institute of Technology BRNO | CZECH REPUBLIC ^s«mZ^ ^MnZ^ FC250 Nano- and microtechnologies: Len ka Zajickova 2 / 26 utnne - chapter icrofabrication Techniques - Etching • 4.1 Lithography • 4.2 Etching and Substrate Removal 4.2.1 Wet Etching 4.2.2 Dry Etching FC250 Nano-and microtechnologies: 4.1 Lithography Lenka Zajíčková 3/26 4.1 Lithography FC250 Nano- and microtechnologies: 4.1 Lithography ka Zajíčková 4/26 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 Tbl oft Substrate Pholoies ist coat tag & development Photo its ist ashing plasma lithography patterning with positive resist FC250 Nano-and microtechnologies: 4.1 Lithography Lenka Zajíčková 5/26 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 /xrn. 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 FC250 Nano- and microtechnologies: 4.1 Lithography ka Zajíčková 6/26 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/xm 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) FC250 Nano- and microtechnologies: 4.2 Etching and Substrate Removal ka Zajíčková 7/26 4.2 Etching and Substrate Removal FC250 Nano- and microtechnologies: 4.2 Etching and Substrate Removal ka Zajíčková 8/26 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) Pju ti Ic tor isotropic c Ich th roujjh aphotaiesjsl musk b) Prot lie for anisotropic etch through a photoresist mask Photoresist Photoresist Silicon fli-. ■ I. ■■ -_■ --i --I Photo resist Sili FC250 Nano- and microtechnologies: 4.2 Etching and Substrate Removal Lenka Zajíčková 9/26 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 b) \?n\\ ilc for an i >io1 kj p ic etch through a photoresist mask ► directional process a) ťio f j le for isotropic etch throujjh a p ho tu it s ist musk Photoresist Photoresist J Silicon fll-. ■ I. ■■ -_■ -\-\ Photo re*; i s1 Silk 00 FC250 Nano- and microtechnologies: 4.2 Etching and Substrate Removal Lenka Zajíčková 10/26 4.2.1 Wet Chemical 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 FC250 Nano- and micro-technologies: 4.2 Etching and Substrate Removal ka Zajíčková 11/26 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) FC250 Nano- and microtechnologies: 4.2 Etching and Substrate Removal Lenka Zajíčková 12/26 Wet Chemical Etching 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) Examples of Si Anisotropic Etching (in) (100) surface orientation 54.74 Silicon (110) surface orientation (111) Silicon ka Zajíčková 13/26 ft1 io] [1 ■ r\7i F7\ FC250 Nano- and microtechnologies: 4.2 Etching and Substrate Removal Lenka Zajíčková 14/26 4.2.2 Dry Etching Types of Directional Dry Etchini ion sputtering (milling) *t*H ■nB^^H^H*'''4'1 '*'t',,< 1 i *•*■ *• *- *> v» % *■ *I^**th»^*"^W^**Tí^^W'|JW^^^^^,*.*. *.*.%*. *. *. ' .'•*.'■ *»*• •* Vi'iS * * *i **'•'. ***• "i **** '* *^»** S ***• *p **** *p **** '* *p ****' * V.*. *. **V. ** *•■•*• ** V. *. *•*• ■« ***•*• ***•*.-. *» V.*t V. '* **V.*.V.'* *< *.'.' v I' I* I* I' I* I* ? I' I* I" **> I" ? **? I*?**? I* I* I*? y 1*1*1* I* !*!*!* I" I* I* I* I* !*!*> I" I" I"! • 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 ****** *. *»***•*« *»tCJ *i *#-* *♦*•*. ** vi*^raisiraHT]]l^W|A't ***•■•**' . ♦. *»**,t**v*****»v*STfl^^^"rtlfi^^t-****t^í™5t*~**p>**** *•***» *. v* v ■. *. *. *. *. *. *. -. : *.*,-. *. *. *. -. í1--. *. *. *, *. *. *. *. J!í. *. *. *. v*. *. *. V. v*.-.' ■ % "*c *H ** *• *W *# ** *• '* *■* *# "* **** *i *#■ "* % *• "jl V *• *■ N# '* ** *m ' reactive ion etching (RIE) m w ^^^^^m^^M Hli^^^B^^I ^b^^^Hk ■# *»**■»-■ tH^BHľJ'i^H.a^l^l^KMi^BS^l^H^ ****** *********** ** ****** *^m^Ba>">*>a*m^>Vr^>B5^H^>^>V^. ******** **** *****»■ **- *. *. *»*.*, *. *ľW^^Í% fflWIW-fflBW' *********•****** ' *■»*, *« *i *»•» 'j *»*•*•« *•*• ** '•I*. •>*■*•>*■ *» >. -, % * + ; *»*• ■• '* *< ** *i *• '**»■ *.*.*. t *•••". •* *•*."* *••• ^ *, *•*» v****. >**•**. ^ ••**** ***«*«*« *.*»•»•»•,*. *.*•■ W*. *. *. *.% *. *. ***. *. *. *.*. *. *. *.*. *. ***.*. *. *. *.*. *( *. *.*. *. *•'*** ******** ******** ******** *f • 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, activation achieved by the collision with an incident ion FC250 Nano- and microtechnologies: 4.2 Etching and Substrate Removal Lenka Zajíčková 15/26 Principles of Plasma Etching - Plasma Chemistry -^ Mob (ax FinrKtpx O o _ o o 1 Uiífcx,pjO O r Pi Iii?** lit CF^+e" -¥ CFi + 2F +e~ F ■^fLi...-\ SiF, SiF4 + í3> (4) t(5) F-> SiFÄ ->5iF4 surface i'i-^CR^CPi" CR' CP ! I," V t«" F?Pi 1 P,Pi 1 P 1 P a>CFi *=^CF> * CF3 « * CF i== *T * e č CP,]]"" CF^JT*" CF|f£" CiFfl CiF* 5=:: CiFi F= F. Chen, 2003, Lecture notes ..., p. 167 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 FC250 Nano- and microtechnologies: 4.2 Etching and Substrate Removal Lenka Zajíčková 16/26 Adsorption Consider a molecule approaching a surface from the vapor phase. ► A few atomic distances from the surface molecule begins to feel an attraction -interaction with the surface molecules by van der Waals forces (London dispersion forces): ► molecules/atoms without dipole moments (symmetrical or intert) interact due to oscillating dipoles, i.e. induced-dipole interaction ► polar molecules (permanent dipoles) interact more strongly ► The approaching molecule is being attracted into a potential well - accelerates down the curve until it passes the bottom and is repelled by steeply rising potential. Physisorptio ► If enough of the molecule's perpendicular component of momentum is dissipated into the surface the molecule cannot escape the well after being repelled =^ physisorption ► fraction of physisorbed molecules - trapping probability S ► reflected 1 — 5 ► S is different from thermal accomodation coefficient 7 introduced previously, molecule is at least partially accomodated thermally to the surface temperature Ts even when it is reflected ► The physisorbed molecule is mobile on the surface except at cryogenic T - hopping (diffusing) between surface atomic sites. vapor molecule reflection desorption 3 utilization H a incorporation physisorption chemisorption FC250 Nano- and microtechnologies: 4.2 Etching and Substrate Removal Lenka Zajíčková 18/26 Chemisorption During surface diffusion the molecule ► may desorb after a while by gaining enough energy in the tail of the thermal energy distribution. ► may undergo a further interaction consisting of the formation of chemical bonds with the surface atoms, i.e. chemisorption. The chemisorption reaction probability £ is used in the case of chemisorption on a foreign substrate instead of condensation coefficient ac. ► some of adsorbed species eventually escape back into the vapor phase =^ sticking coefficient Sc - fraction of the arriving vapor that remains adsorbed for the duration of the experiment. vapor molecule reflection desorption a * «_, „_^ incorporation physisorption chemisorption FC250 Nano- and microtechnologies: 4.2 Etching and Substrate Removal Lenka Zajíčková 19/26 Chemisorption ► Sticking coefficient Sc has less fundamental meaning than 5 and £ (or ac) that are determined solely by chemistry and energy. Nevertheless, Sc is very useful in thin film deposition - it is the fraction of arriving vapor incorporated into the film (buried before escaping). ► Utilization fraction 77 of a chemical vapor - fraction of molecules utilized for the deposition =^ r\ can approach unity even when Sc is very low. vapor molecule reflection desorption a * „_+ +_^ incorporation physisorption chemisorption s e Consider hypothetical diatomic gas-phase molecules chemisorbing as two Y atoms: dissociative chemisorption Y2(q) adsorbing and then dissociative Lifting atomic Y out of its potential well along curve c results in much higher molar potential energy Ep in the gas phase - roughly the heat of formation, AfH, of 2Y(g) from V2(9)- The curve a represents activated chemisorption - there is an activation energy Ea to be overcome for Y2(g) to become dissociatively chemisorbed. For deeper precursor well, curve b, chemisorption is not activated though there is still a barrier Erb. FC250 Nano- and micro-technologies: 4.2 Etching and Substrate Removal Lenka Zajíčková 21 /26 ► If curve c is followed (Ep is high enough) direct chemisorption (without involving precursor state) =>- Eley-Rideal mechanism, i.e. direct reaction between an incoming species and a surface site ► Reaction among surface species => Langmuir-Hinshelwood mechanisms +400 - of Y2(g) -600 J Erb c precursor physisorption dissociative chemisorption Two ways in which vapor can arrive at surface having Ep > 0: ► Gaseous molecules have their Ep raised by becoming dissociated. ► Solids and liquids have it raised by evaporating. Energy-enhanced deposition processes provide enough energy Ep > Ea ► sputter deposition - arriving species have kinetic energy ~ 1000kJ/mol and Ep > 0. ► plasma-enhanced deposition - vapor molecules are dissociated in plasma FC250 Nano- and microtechnologies: 4.2 Etching and Substrate Removal Lenka Zajíčková 22 / 26 Principles of Plasma Etching - Spontaneous Etching Principles of Plasma 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: JPHBW^PPB Qflux of reactive species, Tsubstrate temperature, k0 ^ y preexponential factor, Ea activation energy Typically, Langmuir-Hinschelwood mechanism (reaction between chemisorbed species) - creation of free radicals in plasma eliminates chemical barrier for chemisorption Neiirral Si 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) FC250 Nano- and micro-technologies: 4.2 Etching and Substrate Removal ka Zajíčková 23/26 Principles of Plasma Etching - £ M ntaneous Deposition 200 •j Í a i 1O0- Lcndng Ii, add*ion 4- ■> O, additun Concept of the carbon/fluorine ratio to help quantify the conditions under which polymer formation occurs. Fluornc In carbon rtfio (FX') o I as phase etching species FC250 Nano- and microtechnologies: 4.2 Etching and Substrate Removal ka Zajíčková 24/26 Principles of Plasma Etching - Effect of Ions chemical sputtering ion-enhanced plasma etching > 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 FC250 Nano- and micro-technologies: Etching and Substrate Removal ka Zajíčková 25/26 ypes of Directional Dry Etching Types of Directional Dry Etchin deep reactive ion etching (DRIE) a) Phrtwrtsiil pu.tirrni.ni; ~i b) Etch step 4 EsfiáwBtuii step d) Etch slep Fíg. B. 17a-d DRI t ty cl if process: (a) photoresist pattern-iag, (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 It S .1 Typicu L úľ y etch cite m Lsti1 ie.s Si CFí/Qj, CPjGi, CFjCL. SFň/0>/CL2, SiFi/02. NFj. CIFj, CCU. CjCLjFs. C2C\Fsf£F^ CjFtJCFsd CF3C1/Br2 CF4/H2, CiFů, CjFs, CHF3/Q2 CF4/Q1/H2, CíEů, CiFfi, CHF3 Grgflnics Al BCli. BCls/Oa, aľU/Cla/Bťľb,SiCJ4/Oa S-ilicJd« CF4/Q1, NFj, SEů/Cb, CF4/CI3 fie i'mctcries CF4/O2, NFj/Hí, SFů/Q; BGj/Ái^ Clj/Oj/Hj, OCLjFj/ťVAr/lfc, Hj, CHt/Hj, CCJHs/Hj InP CH4/H2, CjJWHiCLj/Ar Au C2CL2F4, CL2. CCJFj FC250 Nano- and microtechnologies: 4.2 Etching and Substrate Removal Lenka Zajíčková 26/26 Pros and Cond of Plasma Etching Pros and Cons of Plasma Etching 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) Bhigher etch rate due to synergy of chemical etching and ion bombardment