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

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

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

Uses MOS gates Contacts Interconnect

Requirements Uniformity and conformal coating High conductivity High reliability

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

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

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

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: 10-3 -> 10-6 torr

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

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

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

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

Limitations of Evaporation 1. Low melting point of Al 2. Difficult to achieve very large or small thicknesses (typical range = 0.05 - 5 mm) Alternative = sputtering Advantages: Better step coverage Less radiation damage then e-beam Better at producing layers of compound materials

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

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

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

Basic Set-Up

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

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

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.

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

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

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

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.

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

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

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.

Graphical Representation

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.