Etching and Pattern Transfer (1) OUTLINE J / 3.155J -- Spring Term 2005 Lecture 12 - Etch and Pattern Transfer I (Wet Etch) 1.

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1 6.15JST05.Lecture1-1 1 Etching and Pattern Transer (1) OUTLINE Basic Concepts o Etching Wet Etching Speciic Wet Etches Silicon Silicon Dioxide Aluminum Dry (Plasma) Etch eview o Plasmas eading Assignment: Plummer, Chapter J / 3.155J -- Spring Term 005 Lecture 1 - Etch and Pattern Transer I (Wet Etch) 1 Etching is selective removal o thin ilm(s) resulting in a desired thin ilm(s) pattern The Etch Mask is usually photo-resist or oxide/nitride Multi-layer structures can be etched sequentially using same masking layer Etching can be done in either wet or dry environment Wet etching = liquid etchants Dry etching = gas phase etchants in a plasma. Introduction Plummer, Fig Wet Etch chemical process only Dry Etch chemical and physical (sputtering) process 6.15J / 3.155J -- Spring Term 005 Lecture 1 - Etch and Pattern Transer I (Wet Etch)

2 6.15JST05.Lecture1-1 Basic Concepts Diusion eactants Boundary Layer Diusion eaction ilm substrate Diusion Diusion eaction Products Etching process consists o three steps Mass transport o reactants (through a boundary layer) to the surace to be etched eaction between reactants and the ilm(s) to be etched at the surace Mass transport o reaction products rom the surace through the boundary layer Etching is usually done using liquid phase or gas phase reactants liquid phase (wet) etching reaction products soluble in solvent or gaseous gas phase etching reaction products gaseous / sublimation temperature 6.15J / 3.155J -- Spring Term 005 Lecture 1 - Etch and Pattern Transer I (Wet Etch) 3 Deinition o Terms Plummer, Fig. 10- Etch proiles do not have perect straight walls under the edge o the mask Etching occurs both vertically and laterally Undercutting o photoresist mask and non vertical sidewall Erosion o resist could also occur More lateral etching o resist Etchant could attack substrate leading to changes in proile 6.15J / 3.155J -- Spring Term 005 Lecture 1 - Etch and Pattern Transer I (Wet Etch) 4

3 6.15JST05.Lecture1-1 3 Deinition o Terms Etch Directionality Isotropic Etching Anisotropic Etching Measure o relative etch rates in dierent directions usually vertical vs. lateral Etch rates are same in all directions. It is usually related to chemical processes Highly directional etching with dierent etch rates in dierent directions. It is usually related to physical processes such as ion bombardment and sputtering Plummer, Fig Anisotropic etching is the preerred process Step coverage problems?? 6.15J / 3.155J -- Spring Term 005 Lecture 1 - Etch and Pattern Transer I (Wet Etch) 5 Etch Figures o Merit Etch ate () Etch Uniormity (U) Selectivity(S m, S s ) Anisotropy, A Undercut Substrate Damage ate o ilm removal, typically 1000 A/min. % change in etch rate across a waer, lot, etc atio o the etch rate o various materials e.g. Film to P S m : Film to mask selectivity; S s : Film to substrate selectivity Measure o directionality o the etch. A=1 corresponds to perect anisotropic etch A=0 corresponds to isotropic etch Measure o the lateral extent o the etch per side Physical and/or chemical damage o the substrate. 6.15J / 3.155J -- Spring Term 005 Lecture 1 - Etch and Pattern Transer I (Wet Etch) 6

4 6.15JST05.Lecture1-1 4 Etch Figures o Merit Uniormity U = high high + low low high = maximum etch rate low = minimum etch rate Selectivity S m = S s = m s = ilm etch rate m = mask etch rate s = substrate etch rate Anisotropy A = vertical vertical lateral vertical = vertical etch rate lateral = lateral etch rate 6.15J / 3.155J -- Spring Term 005 Lecture 1 - Etch and Pattern Transer I (Wet Etch) 7 Etch Process Control End point detection usually done by monitoring color o ilm or reaction product species Intererometry to monitor ilm thickness Optical spectroscopy to monitor reaction products Mask dimensions erode during etch especially dry etch Need to account or mask erosion Usually need to each to clear + overetch (typically 5-10%) Plummer, Fig Plummer, Fig J / 3.155J -- Spring Term 005 Lecture 1 - Etch and Pattern Transer I (Wet Etch) 8

5 6.15JST05.Lecture1-1 5 How? Wet Etching Simply place the waer in solution that attacks the ilm to be etched but not the mask (resist). eactants Boundary Layer Diusion P Diusion eaction Film Substrate Diusion Diusion eaction Products Diusion reactive species rom the liquid bulk through the boundary layer to the surace o waer eaction o species at the surace to orm solvable species Diuse reaction products away rom the surace through the boundary layer into the bulk o the liquid Advantages High selectivity because it is based on chemical processes Disadvantages Isotropic, poor process control and particulates 6.15J / 3.155J -- Spring Term 005 Lecture 1 - Etch and Pattern Transer I (Wet Etch) 9 Wet Etching Examples Silicon Dioxide Silicon SiO + 6 HF HSiF6 + HO Si + HNO SiO + HNO SiO + 6HF H SiF + H O 3 6 Preerence is to have it reaction rate controlled because Etch rate can be increased by temperature Good control over reaction rate temperature o a liquid is easy to control Mass transport control will result in non-uniorm etch rate Boundary layer un-even Etchant is stirred to minimize boundary layer and thus make etching reaction rate controlled Etch rate is a unction o temperature, speciic reaction and concentration 6.15J / 3.155J -- Spring Term 005 Lecture 1 - Etch and Pattern Transer I (Wet Etch) 10

6 6.15JST05.Lecture1-1 6 Isotropic Wet Etches Silicon Dioxide Etch is isotropic and easily controlled by dilution o HF in H O Thermal oxide etches at 100 Å/min in 6H O:1HF 300 Å/sec or 1 micron/min in 49% by weight HF Doped/deposited oxide etch aster in HF Selectivity to silicon S s > 100 Buered HF (BHF) or Buered oxide etch (BOE) provides consistent etch rates HF is consumed and the etch rate drops Serious process control issue Solution: HF buered with NH 4 F to maintain HF concentration 6NH 4 F:1HF SiO + 6HF H SiF6 + HO H + SiF6 + HO Etch ate (Ang/min) Silicon Dioxide Etch ate Temperature (C) NH 4 F NH 3 + HF 6.15J / 3.155J -- Spring Term 005 Lecture 1 - Etch and Pattern Transer I (Wet Etch) 11 Isotropic Etches Silicon Nitride Silicon Nitride is etched very slowly by HF solutions at room temperature, or example 0:1 C Etch rate o SiO Å/min Etch rate o Si 3 N Å/min Very good selectivity o oxide to nitride Silicon nitride etches in 49% HF at room temperature at about 500 Å/min Phosphoric acid at 150 C [ C] etches Si 3 N 4 at airly ast rate Etch rate o Si 3 N Å/min Etch SiO 10 Å/min Selectivity o Si 3 N 4 over SiO : S = 10 Selectivity o Si 3 N 4 over Si: S=30 Phosphoric Acid Etch ate 6.15J / 3.155J -- Spring Term 005 Lecture 1 - Etch and Pattern Transer I (Wet Etch) 1

7 6.15JST05.Lecture1-1 7 HF Isotropic Etches Silicon HF Campbell, Fig CH 3 COOH Silicon is etched in two steps by nitric acid and hydroluoric acid mixtures Use oxidant to oxidize silicon to orm silicon dioxide ollowed by HF etch o silicon dioxide Oxidation: HNO 3 -> SiO eduction: HF -> SiF 6 HNO 3 CH 3 COOH Si + HNO SiO + HNO SiO HF H SiF H O HNO 3 Acetic acid (CH 3 COOH) is oten used as diluent instead o water Excess nitric acid results in a lot o silicon dioxide ormation and etch rate becomes limited by HF removal o oxide Polishing etch 6.15J / 3.155J -- Spring Term 005 Lecture 1 - Etch and Pattern Transer I (Wet Etch) 13 Isotropic Etches Silicon This can be broken down into anodic and cathodic reactions and an electrochemical etch can be conigured. Anodic eaction Cathodic eaction Si + h Si + Si( OH) Si + ( OH) Si( OH) SiO + H SiO + 6HF H SiF + H O + + HNO + HNO N O + H O NO 3 4 NO 4 NO NO + h + NO + H HNO J / 3.155J -- Spring Term 005 Lecture 1 - Etch and Pattern Transer I (Wet Etch) 14

8 6.15JST05.Lecture1-1 8 Isotropic Etches Silicon Doping selective etches developed or detecting pn junctions and or etch stops 1HF:3HNO 3 :8CH 3 COOH etches heavily doped silicon (>10 19 cm -3 ) but does not etch lightly doped silicon heavy-doping = 15* light-doping Ethylenediamine-pyrocatechol-water etches lightly doped silicon but does not attack heavily doped p-layers Deect Selective Etch orm etch pits at dislocations, stacking aults and precipitates Deect density observable by optical microscopy ater staining 6.15J / 3.155J -- Spring Term 005 Lecture 1 - Etch and Pattern Transer I (Wet Etch) 15 Anistropic Etch Silicon Oxide Oxide (100) Si (100) KOH : 13.5 IPA : 63H O Orientation selective etch o silicon occur in hydroxide solutions because o the close packing o some orientations relative to other orientations Density o planes :<111> > <110> > <100> (100) < (110) < (100) <100> direction etches aster than <111> direction (100) = 100 (111) It is reaction rate limited 6.15J / 3.155J -- Spring Term 005 Lecture 1 - Etch and Pattern Transer I (Wet Etch) 16

9 6.15JST05.Lecture1-1 9 Isotropic Etch Aluminum 16H 3 PO 4 : H O : 1HNO 3 : 1CH 3 COOH Aluminum etches in water, phosphoric, nitric and acetic acid mixtures Converts Al to Al O 3 with nitric acid (evolves H ) Dissolve Al O 3 in phosphoric acid Gas evolution leading to bubbles Local etch rate goes down where bubble is ormed Non-uniormity 6.15J / 3.155J -- Spring Term 005 Lecture 1 - Etch and Pattern Transer I (Wet Etch) 17 Table o Wet Etches 6.15J / 3.155J -- Spring Term 005 Lecture 1 - Etch and Pattern Transer I (Wet Etch) 18

10 6.15JST05.Lecture Summary o Wet Etches Wet etches are selective isotropic and ast Usually reaction rate limited Advantages Disadvantages Simple & Fast Selective Undercutting Strain at interace can enhance undercutting and lit-o ilm eproducible 6.15J / 3.155J -- Spring Term 005 Lecture 1 - Etch and Pattern Transer I (Wet Etch) 19 Example I Etch 1 micron o SiO on Si with oxide = 4000 Å/min, S s =5 Etch is or 3 min. How much Si is etched? Si = oxide /S s = 160 Å/min Note that Si is exposed only ater oxide is etched completely Time to etch oxide is t oxide 10,000 Ang = 4000 A / min =.5min s Silicon is exposed or t exp =3.0.5 min = 0.5 mins Amount o silicon etched is 160 x 0.5 = 80 Å 6.15J / 3.155J -- Spring Term 005 Lecture 1 - Etch and Pattern Transer I (Wet Etch) 0

11 6.15JST05.Lecture Example II 0.5 micron o oxide layer is etched to achieve equal structure widths and spacings. The etch process has anisotropy o 0.8. I the distance between the mask is 0.35 micron, what structure spacings and widths are obtained? x x W Wm oxide b W = W + b = W + x m x = W W x = W W = x + W = 0.35 m ( 1 A ) [ W + x ( 1 A )] x ( 1 A ) µ m 0.5µ m( 1 0.8) = 0. 55µ m W W Silicon A = 1 b = x ( 1 A ) 6.15J / 3.155J -- Spring Term 005 Lecture 1 - Etch and Pattern Transer I (Wet Etch) 1 lateral vertical = 1 b x Chemical Mechanical Polishing I Campbell, Fig Chemical Mechanical Polishing is used to planarize suraces by removing bumps and micro-protrusions Similar to lens polishing or wood polishing with sandpaper Uses a chemical slurry which contains etchants and abrasives. It essentially combines chemical and physical components o etching Anisotropic because o its tendency to latten suraces Waers on a rotating carrier are brought into contact with a polishing pad on a rotating table 6.15J / 3.155J -- Spring Term 005 Lecture 1 - Etch and Pattern Transer I (Wet Etch)

12 6.15JST05.Lecture1-1 1 Chemical Mechanical Polishing II The polishing rate is a unction o Down pressure Table rotating speed Waer rotation speed Slurry low rate Slurry concentration, particle size and components Polishing pad stiness and graininess Polishing dielectrics, oxides, polysilicon, copper or Shallow trench isolation (STI) & interconnects 4 µm 1 µm 6.15J / 3.155J -- Spring Term 005 Lecture 1 - Etch and Pattern Transer I (Wet Etch) 3 Gas Phase (Plasma) Etching Plasma etching has largely replaced wet etching in IC technology because o the directional etching possible with plasma etch systems Directional etching presence o ionic species in the plasma & the electric ield Systems can be designed so that reactive chemical components or the ionic components dominate Plasma systems use a combination o ionic and reactive chemical species acting in a synergistic manner etch rate that is much aster than the sum o individual etch rates when they are acting alone eactive chemical component o plasma etching oten has high selectivity Ionic component o plasma etching oten has directionality Utilizing both components, directionality could be achieved while maintaining an acceptable selectivity 6.15J / 3.155J -- Spring Term 005 Lecture 1 - Etch and Pattern Transer I (Wet Etch) 4

13 6.15JST05.Lecture eview o DC Plasmas Plasma is partially ionized gas Application o voltage across two electrodes in as gas results in arc and generation o ions and electrons Ions collide with cathode generating secondary e - e - accelerated towards anode gaining energy Energetic e - collide with neutral molecules generating more ions, e - and excited radicals. Campbell, Fig Pressure: 00 mtorr 1 Torr Energy: DC Voltage Source ( 100 V/cm) 6.15J / 3.155J -- Spring Term 005 Lecture 1 - Etch and Pattern Transer I (Wet Etch) 5 eview o F Plasmas Application o F electric ield across two electrodes gas also ionizes atoms/molecules producing positive ions and ree electrons creating a plasma Voltage bias develops between the plasma and electrodes because o the dierence in mobilities (masses) o electrons and ions Plasma is positively biased with respect to the electrodes Plummer, Fig Pressure: 1 mtorr 1 Torr Energy: F MHz 6.15J / 3.155J -- Spring Term 005 Lecture 1 - Etch and Pattern Transer I (Wet Etch) 6

14 6.15JST05.Lecture F Plasma Potential Proile Sheaths orm next to electrodes and voltage drops occur at sheaths corresponding to dark region Electrodes capacitively couple to plasma Ions respond to the average sheath voltage while the electrons respond to instantaneous voltage I electrodes have equal areas, the voltage drop at the sheaths are symmetrical I the electrodes have un-equal areas, the voltage drop between the sheaths and the electrodes are asymmetrical with a much larger voltage drop occuring at the smaller electrode Two capacitors in series Plummer, Fig J / 3.155J -- Spring Term 005 Lecture 1 - Etch and Pattern Transer I (Wet Etch) 7 Plasma eactions For a plasma with inlet low o molecule AB, Plasma processes are: Dissociation e* + AB =A+B+e Atomic Ionization e* + A = A + + e + e Molecular Ionization e* + AB = AB + + e + e Atomic Excitation e* + A = A* + e Molecular Excitation e* + AB = AB* + e Plummer, Fig J / 3.155J -- Spring Term 005 Lecture 1 - Etch and Pattern Transer I (Wet Etch) 8

15 6.15JST05.Lecture Plasma eactions A*, AB*,e* reer to particles whose energy are much above the ground state Molecular ragments and dissociated atoms are very reactive (they are called radicals) Plasma consists o about 1% radicals and 0.01% ions cm -3 neutral species cm -3 ions and electrons cm -3 reactive neutral species Plasma density and ion energy are closely coupled 6.15J / 3.155J -- Spring Term 005 Lecture 1 - Etch and Pattern Transer I (Wet Etch) 9 Summary o Today s Lecture We studied the basic concepts o etching Mass transport to reaction surace eaction at the surace Mass transport o reaction products away rom the surace We deined the metrics o pattern transer Selectivity(ilm/substrate, resist/ilm) Anisotopicity Mask bias, etch bias Non-uniormity + overetches We reviewed DC & F Plasmas Plasma reactions 6.15J / 3.155J -- Spring Term 005 Lecture 1 - Etch and Pattern Transer I (Wet Etch) 30