Vacuum Basics. 1. Units. 2. Ideal Gas Law: PV = NkT

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1 Vacuum Basics 1. Units 1 atmosphere = 760 torr = 1.013x105 Pa 1 bar = 105 Pa = 750 torr 1 torr = 1 mm Hg 1 mtorr = 1 micron Hg 1Pa = 7.5 mtorr = 1 newton/m2 1 torr = Pa 2. Ideal Gas Law: PV = NkT k = 1.38E-23 Joules/molecule K = 1.37E-22 atm cm3/k N = # of molecules T = absolute temperature in K [Note] At T = 300 K ; kt = 3.1E-20 torr-liter 1 1

2 3. Dalton s Law of Partial Pressure For mixture of non-reactive gases in a common vessel, each gas exerts its pressure independent of others. Ptotal = P1 + P2 + + PN N total = N1 + N2 + + NN P1V = N 1kT P2V = N 2kT... PN V = N N kt (Total P = Sum of partial pressure) 2

3 4. Average Molecular Velocity Assumes Maxwell-Boltzman Velocity Distribution v = (8kT/ m)1/2 where m = molecular weight of gas molecule 5. Mean Free Path of molecular collision 1 = 2 2 do n where n = molecular density = N/V, d o = molecular diameter [Note] For air at 300 K, = = P( in Pa) P( in torr) with in mm 3

4 6. Impingement Rate, = nv 4 = # of molecules striking unit surface /unit time. = P mt in #/cm2-sec with P in torr, m in amu [Note] For air at 300 K ; (in #/cm2 -sec) = P Example Calculation : Contamination from Residual Vacuum For a residual vacuum of 10-6 torr, = /cm2-sec If each striking molecule sticks to the surface, the equivalent deposition rate of the residual gas is ~ 1/3 of a monolayer of solid per second. 4

5 At 25oC P I M m/min 1 meter! Residual Vacuum Plasma Processing Mean free Path (mm) Time to form a monolayer (sec) Impingment Rate (Molecules/cm2 s) Vacuum Basics (Cont.) CVD Pressure (Torr) 5

6 Thin Film Deposition Physical Methods Chemical Methods Evaporation Sputtering Reactive Sputtering Chemical Vapor Deposition Low Pressure CVD Plasma Enhanced CVD substrate Applications: film Metalization (e.g. Al, TiN, W, silicide) Poly-Si dielectric layers; surface passivation. 6

7 (1) Evaporation Deposition Vacuum 10-5Torr wafer Al film Al vapor Al vapor e Al I hot crucible is water cooled heating boat (e.g. W) Vapor Pressure P P0e H kt P Evaporation Rate (max) 2 mkt m=molecular weight of vapor molecule electron source Log P 1/T 7

8 Vapor Pressure versus Temperature 8

9 Basic Properties of Plasma The bulk of plasma contains equal concentrations of ions and electrons. Electric potential is constant inside bulk of plasma. The voltage drop is mostly across the sheath regions. Plasma used in IC processing is a weak plasma, containing mostly neutral atoms/molecules. Degree of ionization is 10-3 to

10 Source: 10

11 E 0 Drift Flux Diffusion Flux E ~ 0 11

12 (2) Sputtering Deposition Negative Bias ( kv) Al target I Al Ar+ Al Ar+ Al Example: DC plasma Ar plasma Deposited Al film wafer heat substrate to ~ 300oC (optiona12l) Gas Pressure 1-10 m Torr Deposition rate = constant I S ion current sputtering yield 12 12

13 Source: 13

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15 Al Ar Al Al Sputtering Yield S S # of ejected target atoms incoming ion. 0.1 < S < 30 15

16 Sputtering Yield of bombarding ion atomic number For reference only 16

17 The Sputtering Yield with incidence angle 17

18 Sputtering of Compound Targets AxBy Ar+ Aflux Bflux target Film has same composition of target at steady state. Because SA SB, Target surface will acquire a composition Ax By at steady state. 18

19 Proof Target t=t1 AxBy Deposited Film on substrate Ax By AxBy t=t2 AxBy Ax By Bulk target composition Difference between t2 and t1 must be composition deposited on substrate 19

20 Reactive Sputtering Example: Formation of TiN Sputter a Ti target with a nitrogen plasma Note: TiN is a metallic compound with a golden color. It is used frequently in ICs as a metallic conductor. When sandwiched between two thin films, it can also be used as a diffusion barrier to block thin-film reactions Ti Target N2 plasma Ti, N2+ TiN Substrate 20

21 Radio Frequency Plasma Generation For reference only More efficient plasma generation Needed if substrate is electrically insulating 21

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23 Thickness Uniformity with various PVD sources (i) Point-like Source Flux F leaving source surface is independent of (isotropic source) Example: Laser vaporization of a wire tip (ii) Plane-like Surface Source Flux F leaving surface cos Example: E-beam evaporation F surface Emission source F surface Emission source 23

24 Film Thickness Deposition on Wafer Let F = emission flux from source Receiving flux F at a distance r from source = F/ r2 If the receiving surface is making an angle to the F vector Thickness deposited F cos = F cos r source F F with respect / r2 wafer 24

25 Example 1 : Flat Wafer directly on top of Point Source For this special geometry, = wafer -l +l x =0 R0 R Thickness t at x=0 cos / Ro2 = 1 / R o2 Note : =0 at position x=0 point-source (emission flux has no dependence) 25

26 Thickness t at position x= +l cos R cos 0 R 2 l 2 R cos 0 2 l 2 / R2 0 R 3 t x l t x 0 R R 0 x=0 0 2 l2 3 1 l R x=+l 26

27 Thickness Variation of PVD (n=0) (n=1) *For this particular geometry, = N Cheung EE243 S2010 Lec

28 Example 2 : Surafce Source & Spherical Receiving Surface wafer R F r R Emission cos flux F ' 1 cos r2 on wafer F ' cos flux F Receiving plane-source Since cos Thickness 1 cos cos r2 1 cos 2 r2 = (r/2)/r Thickness (1/r2) (r2/4r2) = constant! 28

29 Step Coverage Issues 29

30 Step Coverage Problem with PVD Both evaporation and sputtering have directional fluxes. Flux film step geometrical shadowing film wafer 30

31 self-shadowing t=t1 t=0 step film t=0 t=t1 shadowing distance 31

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33 Methods to Minimize Step Coverage Problems Rotate + Tilt substrate during deposition Elevate substrate temperature (enhance surface diffusion) Use large-area deposition source Sputtering Target 33

34 Lift-off Technique Patterning of deposited layer using directional deposition. Directional Deposition Flux film Photoresist Surface layer treated by chlorobenzene Dip Photoresist in Chlorobenzene to slow down developing rate of surface layer. 34

35 Advantages of Sputtering over Evaporation For multi-component thin films, sputtering gives better composition control using compound targets. Evaporation depends on vapor pressure of various vapor components and is difficult to control. Better lateral thickness uniformity Area of sputtering target can be made much larger than that of an evaporating source. A larger area can be considered as a superposition of many small-area sources. By adding the flux from all the sources, a large area source will provide better lateral film deposition uniformity on wafer. Sputtering Target Superposition of all small-area sources Profile due to one small-area source 35