Professor N Cheung, U.C. Berkeley

Lecture 17EE143 F2010

NonplanarMetallization

PlanarMetallization

Professor N Cheung, U.C. Berkeley

Lecture 17EE143 F2010

Metal 1 (copper)

Interlevel dielectric (ILD)

Silicon

Via (tungsten)

Metal 3 (copper)

Metal 5 (copper)

Passivation

Tungsten Plug to Si

Professor N Cheung, U.C. Berkeley

Lecture 17EE143 F2010

“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 17EE143 F2010

Non-Optimized Planarization example

Professor N Cheung, U.C. Berkeley

Lecture 17EE143 F2010

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 17EE143 F2010

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 17EE143 F2010

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

Planarization Factor

Professor N Cheung, U.C. Berkeley

Lecture 17EE143 F2010

• Topographymanagementmust start atlower layers!

Planarization: A bad example

Professor N Cheung, U.C. Berkeley

Lecture 17EE143 F2010

1. Spin-on glass or polyimide

2. Deposit and Etchback

3. Chemical-Mechanical Polishing (CMP)

Planarization Techniques

Professor N Cheung, U.C. Berkeley

Lecture 17EE143 F2010

Spin-On Glass (SOG)

Professor N Cheung, U.C. Berkeley

Lecture 17EE143 F2010

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 17EE143 F2010

Polymers:

• excellent thermal stability (up to 450oC)

• good dielectric properties (r=3.3, =1016 -cm)

• superior chemical resistance

Polyimides

Professor N Cheung, U.C. Berkeley

Lecture 17EE143 F2010

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

2. Spin on resist or polymerto planarize surface

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

(4. Deposit second oxide layer)

• Simple process, requiringequipment and materialsalready available in the lab

Deposit and Etchback

Professor N Cheung, U.C. Berkeley

Lecture 17EE143 F2010

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

• Backing filmprovides elasticitybetween carrierand 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 17EE143 F2010

CMP Configurations

Rotating waferRotating pad

Rotating WaferLinear track pad

Professor N Cheung, U.C. Berkeley

Lecture 17EE143 F2010

Nathan CheungEE243S10 Lec 24

16

Synergism of mechanical polishing andchemical etching give enhanced CMP rate

Moon, PhD thesis, UCB, 2000

Professor N Cheung, U.C. Berkeley

Lecture 17EE143 F2010

Preston Model:

Local Removal rate R = Kp P v

where P = local applied pressurev = 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 17EE143 F2010

Nathan CheungEE243S10 Lec 24

18

CMP Selectivity

Boron Nitride Silicon Nitride

*Boron nitride is a very hard material, next to diamond

Professor N Cheung, U.C. Berkeley

Lecture 17EE143 F2010

Pattern dependenceProblems encountered in CMP

Professor N Cheung, U.C. Berkeley

Lecture 17EE143 F2010

Nathan CheungEE243S10 Lec 24

20

(1) PolisherMotor Current

(2) Optical Interference

CMP Endpoint Detection

Professor N Cheung, U.C. Berkeley

Lecture 17EE143 F2010

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

Cu has to beencapsulatedby a liner (e.g. TiN)to prevent out-diffusioninto SiO2 and Si

CMP Application Example

Professor N Cheung, U.C. Berkeley

Lecture 17EE143 F2010

1

2

3

4

5

6

ILD = Inter-Level Dielectric

Single Damascene Process

Professor N Cheung, U.C. Berkeley

Lecture 17EE143 F2010

Cu

Dual Damascene Cu Metallization with Diffusion Barrier

Professor N Cheung, U.C. Berkeley

Lecture 17EE143 F2010

Dual Damascene Process Sequence

Professor N Cheung, U.C. Berkeley

Lecture 17EE143 F2010

Professor N Cheung, U.C. Berkeley

Lecture 17EE143 F2010

SUMMARY OF METALIZATION AND PLANARIZATION MODULE

•Interconnects, Contacts, and Dielectrics•Electrical resistivity, Contact resistivity, RC time constant•Metal failure mechanisms- Al spiking, Electromigration•Metal Preparation- PVD, CVD,Plating,Metal-Si reaction (Silicides)•Dielectric Preparation - CVD, SOG•Why planarization - Litho, Deposition, and Etch considerations•Planarization techniques - SOG, Etchback, polymers, CMP•Chemical Mechanical Polishing (CMP) - qualitative•Single and Dual Damascene Process