3.155J/6.152J Nov. 3, Physical Vapor Deposition (PVD): SPUTTER DEPOSITION

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1 Physical Vapor Deposition (PVD): SPUTTER DEPOSITION We saw CVD PECVD Gas phase reactants: P g 1 mtorr to 1 atm. Good step coverage, T > > RT Plasma enhanced surface diffusion without need for elevated T Dry etching Momentum transfer and chemical reaction from plasma to remove surface species We will see evaporation: (another PVD) Evaporate source material, P eq.vap. P g!10 "6 Torr Poor step coverage, alloy fractionation: Δ P vapor Now sputter deposition. Noble or reactive gas P 10 mtorr What is a plasma? 6.152J/3.155J April 6, What is a plasma? A gas of ionized particles, conducting at low freq. Initiate ionization (breakdown) with spark V DC > > V bkdn ions (e.g. Ar + + e ) Self sustaining plasma in V > V bkdn ions < < 1% of atoms. Ionization event P m Torr v Ar + E x ve! V 1 kv What goes on inside a plasma? 6.152J/3.155J April 6,

2 First, what is molecular density at 10 mt? x m 3 Ideal gas: n = P/(k B T) 24 Log[n (#/m 3 )] n = 3.2 x m mt 1 Torr 1 Atm= 0.1 MPa 14 lb/in Log[P (N/m 2 )] Spacing between molecules n 1/3 = 0.15 microns. Is this = λ? 6.152J/3.155J April 6, ! = Spacing between molecules n 1/3 = 0.15 microns. Is this = λ? No! λ >> 0.15 µm k B T 2"d 2 P! Ar " 3 cm (![Ar + ] much less) But 0.1% to 1% of n Ar are ions: What is final velocity of ions? Thermal velocity of Ar, v! 3k B T m Ar 10 3 m/s E field accelerates Ar +, e between collisions. v 2 f = v ax! 2 Eq m " v v e! " 2 #10 7 Ar! 4 "10 5 m/s m/s, 6.152J/3.155J April 6,

3 Be clear abut different velocities: v kt 10 3 m/s v! 3k T B v Ar m m/s Ar v e m/s The plasma is highly conducting due to electrons: J = nqv x J e! = "E = nqv x # nq at 2 Eq $ # nq 2m v Thus J e >> J Ar+! e" # nq2 $ 2m v = ne2 % 2m So we have: lowvel. Ar + high vel e Ionization event v Ar + E x ve! V 1 kv 6.152J/3.155J April 6, Closer look inside plasma Ar + impact on => Lots more electrons, plus sputtered atoms 2 E K ~ v e! Cathode 3 ev 1.5eV v Ar + glow x x E x v e! v 2 f = 2ax Ionization Anode Faraday dark space e s swept out E K e! <1.5eV ~ Cathode dark space, no action e 1.5 < E! ~ K < ~ 3eV e induced optical excitation of Ar visible glow E K > 3eV => ionization, High conductivity plasma 6.152J/3.155J April 6,

4 Inside a plasma (D.C. or sputtering) 3 ev 2 E K ~ v e! 1.5eV glow x Ionization J e!,v e! >>J Ar +,v Ar + " Surfaces in plasma charge negative, attract Ar + repel e Cathode Anode plasma 10 V positive Target relative to Cathode sheath: low ion density + V D.C. Target species Substrate Plasma High conductivity 6.152J/3.155J April 6, Ar + e Which species, e or Ar +, is more likely to dislodge an atom at electrode? Cathode is target, source material P m Torr v Ar + E x ve! V 1 kv At 1 kev, v Ar+ = v e /43 Momentum transfer? p e = mv Sputter removal (etching) occures here P Ar = MV = 1832mv/43 43p e No surprise. so, from ion implantation, most energy transfer when: i.e. incoming particle has mass close to that of target. 4M!E = E 1 M 2 1 ( M 1 + M 2 ) J/3.155J April 6,

5 Sputtering process Ar + impact, momentum transfer at 1) e avalanche and Atomic billiards 2) released target atoms, ions. Elastic energy transfer θ E 2 E 2 ( ) 2 cos 2 " E 2 greatest for M 1 M 2 E 1! 4M 1M 2 M 1 + M 2 E 1 E For e hitting, substrate, M 1 < < M 2 2! 4M 1 E 1 M 2 But e can give up its E K in inelastic collision: 1 2 m v 2 e e! "U Excitation of atom or ion (small) 6.152J/3.155J April 6, Sputtering process: ablation of target Cathode is target, source material P m Torr v Ar + E x ve! V 1 kv Ar + V 1 kv Momentum transfer of Ar + on erodes atoms flux to, substrate. Mostlyneutral source atoms (lots of e s around) Target material () must be conductive or must use RF sputtering (later) 6.152J/3.155J April 6,

6 How plasma results in deposition 1) Ar + accelerated to 5) Flux => deposition at :, some Ar, some impurities 2) Neutral target species () kicked off; 3) Some Ar, Ar + and e also. 4) e may ionize impurities (e.g. O => O ) e + Ar + Ar =Ar + + e + Ar + O Ar V 1 kv 6) Some physical resputtering of film () by Ar 6.152J/3.155J April 6, Sputtering rate of source material in target is key parameter. Typically target atoms released/ar incident Sputtering rates vary little from material to material. Vapor pressure of source NOT important (this differs greatly for different materials). 2) Neutral target species () kicked off. + Ar + Ar =Ar + + e Ar V 1 kv + Ar J/3.155J April 6, cos θ 6) Some physical resputtering of by Ar

7 S = Sputtering yield = ) E S =! 0 n 2 # A " 1 + ln E &, + 2 % (. 4E thresh * + $ E b '. Sputtering yield # atoms, molecules from target # incident ions E 2 >E t E 1 Surface binding energy E b π d 2 # / area # excited in each layer; E t = energy to displace interior atom Random walk to surface; E b = surface binding energy E 2 ( ) 2 cos2 " E 1! 4 M 1 M 2 M 1 + M 2 φ Oring, Fig S Sputter rate depends on angle of incidence, relative masses, kinetic energy J/3.155J April 6, φ 90 Oring, Table 34 ) E S =! 0 n 2 # A " 1 + ln E &, + 2 % (. 4E thresh * + $ E b '. S depends on E 1 A argon = 40 amu S depends on M 1, M 2 A Ti = 48 amu 4M!E = E 1 M 2 1 ( M 1 + M 2 ) J/3.155J April 6,

8 ) E S =! 0 n 2 # A " 1 + ln E &, + 2 % (. 4E thresh * + $ E b '. E 2 Campbell Fig Sputter yield vs. ion energy, normal incidence ln(e 2 ) 6.152J/3.155J April 6, Sputtering miscellany Isotropic flux: cos θ = normal component of flux Large target, small substrate Moving substrates => Good step coverage, more uniform thickness Anisotropic flux: cos n θ: morenarrowly directed at surface (surface roughness) Poor step coverage. Higher pressure => shorter λ,! = better step coverage but more trapped gas k B T 2"d 2 P p λ (cm) 10 mt 2 1 mt J/3.155J April 6,

9 Target composition vs. film composition Sputtering removes outer layer of target; can lead to problem with multicomponent system, but only initially Initial target composition A 3 B If sputter yield A > B Surface composition of target A B A 3 B A 2.5 B A 2 B Deposited film initially richer in A than target. time A B Another approach: Cosputtering => composition control; multiple targets & multiple guns. so can make sample library. A 2 B AB AB J/3.155J April 6, Varieties of sputtering experience D.C. sputter deposition: Only for conducting materials. if DC sputtering were used for insulator. e.g. carbon, charge would accumulate at each electrode and quench plasma within 1 10 micosec. t < 1 micro sec Ar =Ar + + e C C Ar V 1 kv e O What does 1 microsec tell you about ion speed? (>10 6 cm /sec) + Ar + C Therefore, use RF plasma RF sputter system is basically a capacitor with gas dielectric. Energy density as f 6.152J/3.155J April 6,

10 RF plasma sputtering Plasma conducts at low f! p ~ 10 7 s "1 Thus, V = 0 in plasma Target V plasma Substrate Plasma potential still V > 0 due to high e velocity, () charging of surfaces. Potential now symmetric. Mobility of target species (concentration gradient )still => deposition in 1/2 cycle: v! 2eV M " 7 #104 m /s Smaller target => higher field Target V plasma V 1! = A $ 2 # & V 2 " % A 1 m m 1,2 F.C.C. reserves: MHz for sputtering Ar + Ar Substrate Some resputtering of wafer 6.152J/3.155J April 6, Varieties of sputtering experience RF Bias sputtering Negative wafer bias enhances resputtering of film 300 nm 160 nm Target V 1 kv Ohring Fig.324 Resistivity of Ta films vs. substrate bias voltage; (resputter => denser film). Similar results hold for W, Ni. Au, Cr. + Ar + V plasma Ar + + V bias 100 V Substrate Some resputtering of wafer Bias is one more handle for process control. Used in SiO 2 : denser, fewer asperities. Bias affects stress, resistivity, density, dielectric constant 6.152J/3.155J April 6,

11 Film stress in RF sputter deposition Cathode Film Species Ar + V bias Better step coverage 6.152J/3.155J April 6, Varieties of sputtering experience Magnetron sputtering use magnets!" F = q v #!" (! B) + E x q F = mv 2 r, r = mv N qb v z B x F y v y S v y B x F z v z Ions spiral around B field lines S N B N S Cathode target v m/s for electron, r < 1 mm for 0.1 T B field enhances time of e in plasma more ionization, greater Ar + density. E x V + v J/3.155J April 6,

12 Reactive sputter deposition: Mix reactive gas with noble gas (Ar or Ne). analogous to PECVD Ti or TiN target Ar +N 2 TiN so useful for oxides: SiO x, TiO, CrO other nitrides: SiN, FeN, 6.152J/3.155J April 6, TiN formation Campbell Fig Resistivity and composition of reactivity sputtered TiN vs. N 2 flow J/3.155J April 6,

13 Thin film growth details (R < 1) R Rate of arrival Diffusion rate 1) Arrival rate, physical adsorption 3) Chemical reaction 5) Growth Anode 4) Nucleation 6) Bulk diffusion 2) Surface diffusion If R > 1, these processes have reduced probability 6.152J/3.155J April 6, Sputtering metals Can use DC or RF Sputter alloys, compounds Concern about different sputter yield S e.g. Si S = But S does not vary as much as P eq.vap which controls evaporation Sputter target T < < T evap No diffusion, surface enriched in low S components High diffusion, composition change, species distributed over entire source 6.152J/3.155J April 6,

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