Physical Vapor Deposition (PVD): SPUTTER DEPOSITION

Transcription

1 We saw CVD PECVD Physical Vapor Deposition (PVD): SPUTTER DEPOSITION Gas phase reactants: P g 1 mtorr to 1 atm. Good step coverage, T > > RT Plasma enhanced surface diffusion without need for elevated T We will see evaporation: (another PVD) Evaporate source material, P eq.vap. P g 10 6 Torr Poor step coverage, alloy fractionation: P vapor We will see Dry etching Momentum transfer and chemical reaction from plasma to remove surface species Now sputter deposition. Noble or reactive gas P mtorr What is a plasma? Oct. 17, J/3.155J 1

2 What is a plasma? A gas of ionized particles, typically noble gas (e.g. Ar + + e - ) Ionization potential of of Ar Ar ev ev Ionization event - P m Torr cathode v Ar + E x ve anode V 1kV Plasma is only self-sustaining over a range of pressures: typically 1 or 10 mt > P > 100 mt. To understand Why this pressure range?, we need to understand what goes on inside a plasma? Oct. 17, J/3.155J 2

3 First, what is molecular density at 10 mt? 2.5 x m Ideal gas: n = P/(k B T) λ = λ (cm) k B T 2π d 2 P 24 Log[n (#/m 3 )] n = 3.2 x m λ Ar 3 cm ( λ[ar + ] much less) 1 Torr 10 mt 1 Atm= 0.1 MPa 14 lb/in Log[P (N/m 2 )] At 10 mt, molecular spacing n -1/3 = 0.15 microns. Is this = λ? Oct. 17, J/3.155J 3

4 λ = Spacing between molecules n -1/3 = 0.15 microns. k B T 2πd 2 P λ Ar 3 cm ( λ[ar + ] much less) Thermal velocity of Ar, v 3k B T m Ar 10 3 m/s What s final velocity of ions at x = λ? E field accelerates Ar +, e - between collisions. v 2 f = v ax 2 Eq m λ v v e Ar m/s m/s, (only 0.1% to 1% of n Ar are ions): Oct. 17, J/3.155J 4

5 Be clear about different velocities: v kt 10 3 m/s v Ar m/s m/s v e- v 3k B T m Ar So we have: low-vel. Ar + high vel. e - Ionization event - cathode v Ar + E x ve anode V 1kV The plasma is highly conducting due to electrons: J = nqv x Thus J e- >> J Ar+ J e = σe = nqv x nq at 2 nq Eq 2m λ v σ e nq2 2m λ v = ne2 τ 2m Oct. 17, J/3.155J 5

6 Plasma is self-sustaining only for 1 or 10 mt < P < 100 mt. Necessary conditions: Why this pressure range? If pressure is too low, λ is large, too few collisions in plasma to sustain energy 1) λ < L so collisions exchange energy within plasma λ = k B T 2π d 2 P If pressure is too high, λ is small, very little acceleration between collisions 2) E K > ionization potential of Ar mv 2 f = 2ax Eqλ Oct. 17, J/3.155J 6

7 Plasma is self-sustaining only for 1 or 10 mt < P < 100 mt. λ = k B T 2π d 2 P Self-sustained plasma 1 2 mv 2 f Eqλ V L λ(ev) Log[λ (m)] 0-1 1) λ < L 10 cm 100 V 1000 V λ 3 cm: E K 0.3 x V 2) E K > Ar + E K (ev) 2 1 Ne + 22 Ar + 16 Kr mt: λ 3 cm Log[P (N/m 2 )] -2-3 Oct. 17, J/3.155J 7

8 - What about Glow? cathode Ar + impact on cathode => Lots more electrons, plus sputtered atoms anode E K Cathode 2 ~ v e 3 ev 1.5eV v Ar + glow x x E x v e v f 2 = 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 Oct. 17, J/3.155J 8

9 Inside a plasma (D.C. or cathode 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 Cathode anode Anode plasma 10 V positive Target relative to anode Cathode sheath: low ion density + V D.C. Target species Substrate Plasma High conductivity Oct. 17, J/3.155J 9 Ar + e -

10 Which species, e - or Ar +, is more likely to dislodge an atom at electrode? Cathode is target, source material - P m Torr cathode v Ar + E x ve anode V 1kV At 1 kev, v Ar+ = v e /43 Momentum transfer: p e = mv Sputter removal (etching) occurs here P Ar = MV = 1832mv/43 43p e- No surprise. 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 ) 2 Oct. 17, J/3.155J 10

11 Sputtering process Ar + impact, momentum transfer at cathode 1) e - avalanche and Atomic billiards 2) released target atoms, ions. Elastic energy transfer θ E 2 E 2 ( ) 2 cos 2 θ E 2 greatest for M 1 M 2 E 1 4M 1M 2 M 1 + M 2 E 1 For e - hitting anode, substrate, M 1 < < M 2 E 2 E 1 4M 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) Oct. 17, J/3.155J 11

12 Sputtering process: ablation of target Cathode is target, source material - P m Torr cathode v Ar + E x ve anode V 1kV cathode anode - Ar + V 1kV Momentum transfer of Ar + on cathode erodes cathode atoms flux to anode, substrate. Mostly-neutral source atoms (lots of e s around) Target material (cathode) must be conductive or must use RF sputtering (later) Oct. 17, J/3.155J 12

13 How plasma results in deposition 1) Ar + accelerated to cathode 5) Flux => deposition at anode: Al, some Ar, some impurities cathode anode 2) Neutral target species (Al) + Ar + Al kicked off; 3) Some Ar, Ar + and e - also. - Ar =Ar + + e - Al Ar V 1kV e - O - Al 4) e - may ionize impurities (e.g. O => O - ) + Ar + Al 6) Some physical resputtering of film (Al) by Ar Oct. 17, J/3.155J 13

14 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). cathode anode 2) Neutral target species (Al) + Ar + Al kicked off. - Ar =Ar + + e - Al Ar V 1kV + Ar + Al cos θ Al 6) Some physical resputtering of Al by Ar Oct. 17, J/3.155J 14

15 S = Sputtering yield = E S = σ 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 thresh = energy to displace interior atom Random walk to surface; E b = surface binding energy E 2 ( ) 2 cos2 θ E 1 4M 1 M 2 M 1 + M 2 φ Oring, Fig S Sputter rate depends on angle of incidence, relative masses, kinetic energy. Oct. 17, J/3.155J 15 φ 90

16 Figure removed for copyright reasons. Table 3-4 in Ohring, M. The Materials Science of Thin Films. 2nd ed. Burlington, MA: Academic Press, ISBN: Oct. 17, J/3.155J 16

17 Figure removed for copyright reasons. Figure in Campbell, S. The Science and Engineering of Microelectronic Fabrication. 2nd ed. New York, NY: Oxford University Press, ISBN: Sputter yield vs. ion energy, normal incidence Oct. 17, J/3.155J 17

18 Sputtering miscellany Isotropic flux: cos θ = normal component of flux Higher P Anisotropic flux: cos n θ: more-narrowly directed at surface (surface roughness) Poor step coverage. Lower P Large target, small substrate Moving substrates => Good step coverage, more uniform thickness Higher pressure => shorter λ, λ = better step coverage but more trapped gas k B T 2πd 2 P p λ (cm) 10 mt 2 1 mt 20 Oct. 17, J/3.155J 18

19 Target composition vs. film composition Sputtering removes outer layer of target; can lead to problem with multi-component 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 time Deposited film initially richer in A than target; film composition eventually correct. Another approach: Co-sputtering => composition control; multiple targets & multiple guns. Also can make sample library. A B A 2 B AB AB 2 Oct. 17, J/3.155J 19

20 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 mico-sec. cathode anode t < 1 micro sec Ar =Ar + + e - C - C Ar V 1kV 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 Oct. 17, J/3.155J 20

21 RF plasma sputtering Plasma conducts at low f ω 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 => sputtering in 1/2 cycle: v 2eV M 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 re-sputtering of wafer Oct. 17, J/3.155J 21

22 Varieties of sputtering experience RF Bias sputtering Negative wafer bias enhances re-sputtering of film Target V -1 kv + Ar + V plasma Ar + + Some re-sputtering of wafer V bias -100 V Substrate Figure removed for copyright reasons. Figure 3-24 in Ohring, Bias is one more handle for process control. Used in SiO 2 : denser, fewer asperities. Bias affects stress, resistivity, density, dielectric constant Oct. 17, J/3.155J 22

23 Film stress in RF sputter deposition Cathode Film Species Ar + -V bias Better step coverage Oct. 17, J/3.155J 23

24 Varieties of sputtering experience Magnetron sputtering use magnets ur N F = qv v ur ( B)+ E x q F = mv 2 r, r = mv 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 0 Oct. 17, J/3.155J 24

25 Reactive sputter deposition: Mix reactive gas with noble gas (Ar or Ne). analogous to PECVD Ti or TiN target Ar +N 2 TiN Also useful for oxides: other nitrides: SiO x, TiO, CrO SiN, FeN, Oct. 17, J/3.155J 25

26 Figure removed for copyright reasons. Figure in Campbell, Resistivity and composition of reactivity sputtered TiN vs. N 2 flow. Oct. 17, J/3.155J 26

27 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 Oct. 17, J/3.155J 27

28 Sputtering metals Can use DC or RF Sputter alloys, compounds Concern about different sputter yield S e.g. Al 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 Oct. 17, J/3.155J 28

29 Figure removed for copyright reasons. Table 3-7 in Ohring, Oct. 17, J/3.155J 29