Professor N Cheung, U.C. Berkeley

Lecture 18EE143 S06

Nonplanar Metallization

PlanarMetallization

Professor N Cheung, U.C. Berkeley

Lecture 18EE143 S06

“Caps” and “Plugs”

The plug material can be same as interconnect material (e.g. Cu)or different material (e.g. W)

oxide oxide

Professor N Cheung, U.C. Berkeley

Lecture 18EE143 S06

Non-Optimized Planarization example

Professor N Cheung, U.C. Berkeley

Lecture 18EE143 S06

Five Level Metallization for Company CW plugs for contacts and viasW for metal 1, Al/Ti metal 2,3, 4; Al metal 3CMP for all dielectrics.

Good Planarization Example

Professor N Cheung, U.C. Berkeley

Lecture 18EE143 S06

Benefits for Lithography Processes: • Lower Depth-of-Focus requirement• Reduced optical reflection effects on resist profiles • Reduced resist thickness variation over steps

=>

Benefit for Etching Processes:• Reduced over-etch time required due to steps

Benefit for Deposition Processes:• Improved step coverage for

subsequent layer deposition

topview

Surface Planarization

Professor N Cheung, U.C. Berkeley

Lecture 18EE143 S06

β = 1- (final step height/initial step height)

Planarization Factor β

Professor N Cheung, U.C. Berkeley

Lecture 18EE143 S06

• Topography managementmust start at lower layers!

Planarization: A bad example

Professor N Cheung, U.C. Berkeley

Lecture 18EE143 S06

1. Spin-on glass or polyimide

2. Deposit and Etchback

3. Chemical-Mechanical Polishing (CMP)

Planarization Techniques

Professor N Cheung, U.C. Berkeley

Lecture 18EE143 S06

Spin-On Glass (SOG)

Professor N Cheung, U.C. Berkeley

Lecture 18EE143 S06

SOG Annealing

• Cure:• 400-500oC -> inorganic backbone polymer

– exact composition depends on SOG type

• 800-1100oC -> densified glass (inorganic SOGs)• can be performed in N2, O2 or steam

– steam allows densification to occur at lower – temperatures

Professor N Cheung, U.C. Berkeley

Lecture 18EE143 S06

Polymers:

• excellent thermal stability (up to 450oC)

• good dielectric properties (εr=3.3, ρ=1016 Ω-cm)

• superior chemical resistance

Polyimides

Professor N Cheung, U.C. Berkeley

Lecture 18EE143 S06

1. Deposit thick oxide layer (600 - 1000nm)

2. Spin on resist or polymer to planarize surface

3. Etch back with a process that has equal oxide andresist/polymer etch rates(e.g. CF4 + O2 dry etch)

(4. Deposit second oxide layer)

• Simple process, requiring equipment and materialsalready available in the lab

Deposit and Etchback

Professor N Cheung, U.C. Berkeley

Lecture 18EE143 S06

Wafer is polished using a slurry containing• silica abrasives (10-90 nm particle size)• etching agents (e.g. dilute HF)

• Backing film provides elasticity between carrier and wafer

• Polishing pad made of polyure-thane, with 1 mm perforations

– rough surface to hold slurry

Chemical Mechanical Polishing (CMP)

Professor N Cheung, U.C. Berkeley

Lecture 18EE143 S06

CMP Configurations

Rotating waferRotating pad

Rotating WaferLinear track pad

Professor N Cheung, U.C. Berkeley

Lecture 18EE143 S06

CMP Process Control

For reference only

Professor N Cheung, U.C. Berkeley

Lecture 18EE143 S06

Preston Model:

Local Removal rate R = Kp P v

where P = local applied pressure v = relative pad-wafer velocity

Kp = Preston coefficient [unit in pressure-1]function of film hardness, Young’s modulus,

slurry, pad composition and structure

CMP Rate

Professor N Cheung, U.C. Berkeley

Lecture 18EE143 S06

CMP of Selectivity to Si3N4

SiO2 6:1

poly-Si 280:1

W 75:1

Al 40:1

CMP Selectivity

Professor N Cheung, U.C. Berkeley

Lecture 18EE143 S06

Pattern dependenceProblems encountered in CMP

Professor N Cheung, U.C. Berkeley

Lecture 18EE143 S06

• High isolated features polish fast• Increased pressure at corners of features creates rounding

*Note :Y-axis highly magnified !!!

Pattern Dependence of CMP

Professor N Cheung, U.C. Berkeley

Lecture 18EE143 S06

CMP Endpoint Detection - (1) PolisherMotor Current

Professor N Cheung, U.C. Berkeley

Lecture 18EE143 S06

CMP Endpoint Detection - (2) Optical Interference

Professor N Cheung, U.C. Berkeley

Lecture 18EE143 S06

RIE of Cu difficult due to low vapor pressure of by-products=> Cu lines formed by CMP

Cu has to be encapsulatedby a liner (e.g. TiN) to prevent out-diffusion into SiO2 and Si

CMP Application Example

Professor N Cheung, U.C. Berkeley

Lecture 18EE143 S06

1

2

3

4

5

6

ILD = Inter-Level Dielectric

Single Damascene Process

Professor N Cheung, U.C. Berkeley

Lecture 18EE143 S06

Cu

Dual Damascene Cu Metallization with Diffusion Barrier

Professor N Cheung, U.C. Berkeley

Lecture 18EE143 S06

Dual Damascene Process Sequence

Professor N Cheung, U.C. Berkeley

Lecture 18EE143 S06