Chemical Vapor Deposition (CVD)

Transcription

1 Chemical Vapor Deposition (CVD) source chemical reaction film substrate More conformal deposition vs. PVD t Shown here is 100% conformal deposition ( higher temp has higher surface diffusion) t step 1

2 (a)sio2 gas solid LPCVD Examples SiH 4 O2 SiO2 2 H 2 gas 350oC-500oC (b)psg : phospho silicate glass. P2O5 SiO2 4 PH 3 5O2 2 P2O5 6 H 2 SiH 4 O2 SiO2 2 H 2 350oC-500oC (c)teos :tetraethylene orthosilicate. Si OC2 H 5 4 SiO2 C X H Y OZ 2

3 ( d ) Si 3 N 3 SiH 4 ( e ) Poly 4 NH 3 Si 3 N 4 12 H 2 Si 600 o C SiH 4 Si 2 H 2 ( f )W WF 6 3 H 2 W 6 HF 3

4 CVD Reactors 4

5 CVD Mechanisms reactant surface diffusion 3 stagnant gas layer 4 substrate 1 = Diffusion of reactant to surface 2 = Absorption of reactant to surface 3 = Chemical reaction 4 = Desorption of gas by-products 5 = Outdiffusion of by-product gas 5

6 Example: Poly-Si Deposition 6

7 CVD Growth Rate Model Flux across boundary F1 h G (CG CS ) Flux used in reaction F2 k S CS Normalize to total pressure Y CGAS CALL _ SPECIES F Thickness growth velocity v N PGAS PGAS _ 1 PGAS _ 2... N = atomic density of film 7

8 Diffusion Constant in Gas T3 / 2 D Do P n1 v n2 v v dn Proof: F = 4-4 = 4 ( dx) where = mean free path of gas collision. v dn F=D D= dx 2 kt Since and v T P D T3/2/P

9 Mathematical Model for CVD (Cont.) Transport parameter hg = D/ D = gas diffusion constant = Do T 3/2 / Pressure Surface reaction parameter ks = koexp (-Ea/kT) Balancing F1=F2 Growth velocity CALL _ SPECIES F 1 v Y N (1 / k S 1 / hg ) N CALL_SPECIES = concentration at top of boundary layer N =atomic density of deposited material Limiting cases ks << hg Surface Reaction limited hg << ks Mass Transport limited 9

10 Deposition Rate versus Temp [log scale] Rate 3 2 Mass transport limited R T surface-reaction limited R e EA / kt 1/ T 0 high T low T 10

11 11

12 Boundary Layer Thickness vs. Distance x U S ( x) 1 2 = viscosity, = density, U = velocity *See handout on CVD Kinetics for derivation 12

13 Average Boundary Layer thickness < > 1 L 2 ( x) dx L 0 3 UL Re L UL is called the Reynold Number of the reactor. When Re is small ( < 2000), gas flow is viscous. When Re is large (>2000), gas flow is turbulent which is undesirable. If deposition rate is mass transfer limited, 1 R hg U 13

14 Growth Rate Dependence on Flow Velocity Fixed temp T CALL _ SPECIES F 1 v Y N (1 / k S 1 / hg ) N 14

15 LPCVD Reactors (next page) 15

16 LPCVD: Low Pressure and high gas velocity due to pumping hg Example calculation: P reduces ~1000X from 1 atmosphere to ~1 Torr Velocity of gas flow U increases ~100X due to pumping D 1 P From 760Torr 1Torr D 1000X U 1000 ~ 3X = 100 velocity of gas flow 100X Gas density P hg D X 3 Therefore, LPCVD is more likely to be surface reaction limited 16

17 LPCVD Features (1) R, since hg (2) More conformal deposition, if T is uniform Wafer topography (3) Inter-wafer and intra-wafer thickness uniformity less sensitive to gas flow patterns. (i.e. wafer placement). 17

18 Gas Flow Issues (1) depends on gas flow pattern Furnace tube wafers (2) Mass depletion problem in more less out 18

19 Solutions for mass depletion problem (1) Temperature Ramping along reactor length For reaction - limited regime: R(x) = A exp[-ea/kt] C(x) [where C(x) = SiH4 Conc.] Creating a temperature gradient of C along the tube will give better uniformity. (2) Distributed Feed Reactors 19

20 PECVD Reactors 20

21 Plasma Enhanced CVD Ionized chemical species allows a lower process temperature to be used. Film properties (e.g. mechanical stress) can be tailored by controllable ion bombardment with substrate bias voltage. 21

22 Epitaxial Growth Processing Temperature oC Requires an ultra-clean Si surface prior to epi growth. Requires deposition of Si at very high temperature for perfect crystallinity. 22

23 Strained Si Growth by Heteroepitaxy 23

24 Principle of Atomic Layer Deposition Self-limiting surface reactions of suitable precursor compounds A and B, which form the desired product S in a binary reaction cycle consisting of two sequential half-reactions. For an extensive list of precursors, see Ritala and Leskela, Handook of Thin Film Materials, Vol.1, Chap 2 (2002) 24

25 Timing Sequence : CVD vs ALD 25

26 Spatial ALD Reaction Chamber A Reaction Chamber B The reactor has separate zones exposing the precursors one by one to a substrate that moves underneath the reactor. Gas bearings between the zones eliminate cross-diffusion and therefore the need for purge steps. Half-reaction timescales are ~10 msec, enabling ultrahigh deposition rates while maintaining ALD film quality. Source: PV Society,

27 Possible Spatial ALD Configuration 27

28 Example: ZrO2 ALD 28

29 Step Coverage (Al2O3) 29

30 SUMMARY OF THIN FILM MODULE Vacuum Basics Mean Free Path and Impingement Rate Plasma Basics - A plasma sheath above electrode surface which maintains a voltage drop ( -10 to V) for ion bombardment PVD and CVD Evaporation Vapor Pressure Sputtering Deposition- Sputtering Yield, Co-sputtering PVD Issues thickness uniformity across wafer, Step Coverage ( geometric shadowing, Self Shadowing) CVD Deposition Rate ( hg versus ks effects) CVD Issues Conformal coverage, Mass depletion Plasma Enhanced CVD (PECVD) - qualitative Epitaxial Growth - qualitative Atomic Layer Deposition (ALD) - qualitative 30