1. ECE/ChE 4752: Microelectronics Processing Laboratory
Metallization
ECE/ChE 4752: Microelectronics Processing Laboratory
Gary S. May
February 26, 2004

2. Outline Introduction Physical Vapor Deposition
Chemical Vapor Deposition
Aluminum Metallization
Copper Metallization

3. Basics Goal: form low-resistance interconnections Types:
Physical vapor deposition (PVD) – evaporation or sputtering
Chemical vapor deposition (CVD) – involves a chemical reaction

4. Uses
MOS gates
Contacts
Interconnect

5. Requirements Uniformity and conformal coating High conductivity
High reliability

6. Outline Introduction Physical Vapor Deposition
Chemical Vapor Deposition
Aluminum Metallization
Copper Metallization

7. Basics Also called “evaporation”
Goal: evaporate metal; condense on wafer surface
Procedure:
Convert metal from solid to vapor phase (melt + evaporate or direct sublimation)
Transport gaseous material to substrate
Condense gaseous material on substrate

8. Evaporation Equipment
Conditions:
High temperature
Low pressure (10-6 – 10-7 torr)

9. Achieving Low Pressure
Evaporation chamber must be “pumped down”
where: P(t) = chamber pressure at time t, P0 = initial pressure, S = pumping speed, Q = rate of outgassing, V = volume of chamber
Pumping apparatus has 2-stages:
1) roughing pump: atm -> 10-3 torr
2) diffusion pump: > 10-6 torr

10. Kinetic Gas Theory Ideal gas law: PV = NavkT
where: k = Boltzmann constant, Nav = Avogadro’s # (6.02 x 1023 molecules/mole), P = pressure, V = volume, T = temperature
Concentration of gas molecules given by:
n = Nav/V = P/kT

11. where: Ns = # molecules/cm2 in the layer
Deposition Rate
Impingement rate of gas molecules hitting surface:
where: P = pressure (N/m2), M = molecular weight (g/mole), T = temperature (oK)
Time to form one monolayer
t = Ns/F
where: Ns = # molecules/cm2 in the layer
molecules/cm2-s

12. where: D0 = deposition rate at center of wafer
Geometric Variation
Deposition rate has radial dependence:
where: D0 = deposition rate at center of wafer

13. Surface Profiometry Used to measure deposited film thickness
Precision = 2 Å

14. Limitations of Evaporation
1. Low melting point of Al
2. Difficult to achieve very large or small thicknesses (typical range = mm)
Alternative = sputtering
Advantages:
Better step coverage
Less radiation damage then e-beam
Better at producing layers of compound materials

15. Sputtering
Source of ions is accelerated toward the target and impinges on its surface

16. Outline Introduction Physical Vapor Deposition
Chemical Vapor Deposition
Aluminum Metallization
Copper Metallization

17. Advantages Conformal coatings Good step coverage
Can coat a large number of wafers at a time
Lower electrical resistivity films than PVD
Allows refractory metal (like W) deposition

18. Basic Set-Up

19. Outline Introduction Physical Vapor Deposition
Chemical Vapor Deposition
Aluminum Metallization
Copper Metallization

20. Properties Can be deposited by PVD or CVD
Al and its alloys have low resistivity (2.7 mW-cm for Al and up to 3.5 mW-cm for alloys)
Adheres well to silicon dioxide
Use with shallow junctions can create problems, such as spiking or eletromigration

21. Eutectic Characteristics
Addition of either component lowers Al-Si system melting point below that of either metal (660 °C for Al and 1412 °C for Si)
Eutectic temperature (577 °C) corresponds to 11.3% Al and 88.7% Si.
Al deposition the temperature must be less than 577 °C.

22. where: r = density, S = solubility of Si, and A = ZL
Solubility of Al in Si
Si dissolves into Al during annealing
After time t, Si diffuses a distance of (Dt)0.5 along Al line from the edge of the contact
Depth to which Si is consumed given by
where: r = density, S = solubility of Si, and A = ZL

23. Junction Spiking
Dissolution of Si take place at only a few points, where spikes are formed
One way to minimize spiking is to add Si to the Al by co-evaporation. Another method is to introduce a barrier metal (such as TiN) between the Al and Si

24. Electromigration
High current densities can cause the transport of mass in metals
Occurs by transfer of momentum from electrons to positive metal ions
Metal ions in some regions pile up and voids form in other regions
Pileup can short-circuit adjacent conductors, whereas voids can result in open circuits

25. Mean Time to Failure
MTF due to electromigration is be related to the current density (J) and activation energy by
Experimentally, Ea = 0.5 eV for aluminum
Electromigration resistance of Al can be increased by alloying with Cu (e.g., A1 with 0.5% Cu), encapsulating the conductor in a dielectric, or incorporating oxygen during deposition.

26. Outline Introduction Physical Vapor Deposition
Chemical Vapor Deposition
Aluminum Metallization
Copper Metallization

27. Motivation
High conductivity wiring and low–dielectric-constant insulators are required to lower RC time delay of interconnect.
Copper has higher conductivity and electromigration resistance than Al.
Cu can be deposited by PVD or CVD,
Downside:
Cu tends to corrode under standard processing conditions
Not amenable to dry etching
Poor adhesion to SiO2

28. Damascene Technology
Trenches for metal lines defined and etched in interlayer dielectric (ILD)
Metal deposition of TaN/Cu (TaN serves as a diffusion barrier to prevent Cu from penetrating the dielectric)
Excess Cu on the surface is removed to obtain a planar structure.

29. Graphical Representation

30. Chemical Mechanical Polishing
Allows global planarization over large and small structures
Advantages:
Reduced defect density
No plasma damage
Consists of moving sample surface against pad that carries slurry between the sample surface and the pad.