Thin Film Deposition

• Quality – composition, defect density, mechanical and electrical properties

• Uniformity – affect performance (mechanical , electrical)Thinning leads to R

Voids: Trap chemicals lead to cracks (dielectric) large contact resistance and sheet resistance (metallization)

AR (aspect ratio) = h/w with feature size in ICs.

Plummer et al.

Chemical Vapor Deposition

Flat on the susceptor

Cold wall reactor

Methods of Deposition:

Chemical Vapor Deposition (CVD), Physical Vapor Deposition (PVD: evaporation, sputtering)Atmospheric Pressure : APCVD

Cold wall reactors (walls not heated only the susceptor)Low pressure: LPCVD – batch processing.

Hot wall reactor

Plummer et al.

Plasma Enhanced Chemical Vapor Deposition(PECVD)

Used when :

• Low T required (dielectrics on Al, metals) but CVD at decreased T gives increased porosity, poor step coverage.

• Good quality films – energy supplied by plasma increases film density, composition, step coverage for metal decreases but WATCH for damage and by product incorporation.Outgassing , peeling ,

cracking stress.

P 50 mtorr - 5 torr

Plasma: ionized excited molecules, neutrals, fragments, ex. free radicals very reactive reactions @ the Si surface enhanced increase deposition rates

200- 350 °C

13.56 MHz

Ions, electrons, neutrals = bombardment

Plummer et al.

Physical Vapor Deposition (PVD) – no chemical reactions

(except for reactive sputtering)

Evaporation

Advantages:

• Little damage

• Pure layers (high vacuum)

Disadvantages:

• Not for low vapor pressure metals

•No in-situ cleaning

• Poor step coverage

Very low pressure (P < 10 –5 torr) - long mean free path.

• purer – no filaments, only surface of the source melted

• X-rays generated trapped charges in the gate oxides anneal it !

Plummer et al.

Evaporation

Partial Pressure of the source (target)

e

2

1

s2

evap PT

mA1083.5R ⎟

⎠

⎞⎜⎝

⎛×= −

Needed for reasonable v 0.1 - 1m/min

No alloys – partial pressure differences

Use separate sources and e-beam

incident

reactedc F

FS =

Step Coverage Poor :

• Long mean free path (arrival angle not wide = small scattering) and low T (low energy of ad-atoms)

• Sticking coefficient high (@ T) no desorption and readsorption poor step coverage

• Heating can increase Sc but may change film properties (composition, structure)

Sticking coefficient

Plummer et al.

Sputter Deposition

Higher pressures 1 –100 mtorr ( < 10-5 torr in evaporation)

Alloys (TiW, TiN etc)

• good step coverage

• controlled properties

DC Sputtering (for metal)

Conductive

Al, W, Ti

Ar inert gas at low pressure.

No free radicals formed by Ar (ex. O, H ,F as was for PECVD)

Major Technique in Microelectronics for:

Plummer et al.

RF Sputter DepositionDielect

ric

13.6 MHz RF coupled capacitively to plasma

several 100V

wafers

DC sputter cannot be used for dielectrics

secondary e-

plasma extinguished (VZ )

More on the walls charge built-up

potential VP

potential

@ the target ( area)

-

= NON-CONDUCTINGOscillating (with RF) e- ionization yield

pressure

+ magnet e- trajectory Magnetron Sputter Deposition have better ionization yields

deposition rates (10-100X)better film quality (Ar needed)use in DC & RF ( heating of the target since I+ )

large A1 area

A2

A2A1

e- charge

e- d

tenths of volts

faster, smaller

can be used

Plummer et al.

Polysilicon - Very Widely Used in MEMS

columnar structure

As & P deposition rate of poly – Si use doping after poly deposition

B Vpoly - Si

As, P segregate @ the grain boundaries ( B does

not ! )625°C

Low T gives more amorphous layers

Plummer et al.

Various Aspects of Deposition Processes

C. Liu

Etching

DRY ETCH

ANISOTROPIC

Plummer et al.

Etching Profiles

PR Mask

Rounded & sloped PR

Lateral etching chemical

good selectivity

Lateral etching

poor selectivity

Required for scaled down devices

Plummer et al.

Wet Etching – Isotropic Etch

Plummer et al.

Plasma Etching

Parallel plate system

Replace wet processes in VLSI – directional etching, faster, (less) selective but does not degrade PR adhesion as some wet steps do.

MEMS use plasma etching widely (deep etch, highly anisotropic)

• Reactive chemical components

• Ionic components

!

As in CVD & or sputtering (here RF electrode was much smaller and neutral gas Ar)

Plummer et al.

Low pressure1mtorr-1torr

Chemical Etching

Isotropic arrival angle

ISOTROPIC ETCH

Low sticking coefficient

volatile

Free radicals : S

But in practice S is low

(0.01-0.05F - Si)

Physical Etching

Ion bombardmentDegrades selection= sputter etch

Cl+

+ O2 F recombination with CF3

CF4 F etch rates

@ small amounts of O2 but

@ large O2 etch rates decreases+oxidation of Si takes place

Plummer et al.

Ion-Enhanced Etching

Chemical component selectivity

Physical component anisotropy

Etching

Enhancement by ions

volatility of byproducts

Role of ions:

Adsorption, Reaction, Formation of byproducts, and their removal

No plasma Sputtering

Plummer et al.

Polymer formation on all walls but removed at the bottom by bombardment

Anisotropic Etch

Fast formation of the polymer

Slow polymer formation

May contain byproducts of etching, various layers including resist

MEMS call for optimization of cross-reactivity of various materials (layers) and processes

Silicon-Based MEMS Processes

Bulk micromachining (historically the first): silicon substrate is the main active part of the MEMS structures

C. Liu

Oxide etch Or nitride if used as a mask for Si etching

Expose PR

Develop PR

Oxide growth or nitride deposition(if needed)

Wafer bonding

Wafer thinningby Chemical Mechanical Polishing to leave a thin membrane

Make piezoresistors (deposition, patterning, doping) to measure stress (use Wheastone bridge etc.)

Etch silicon

Strip PR

Si etched

Bulk Micromachining

• Fabrication of pressure sensors seen in cross-sections

C. Liu

Membrane made of poly-Si, Si-nitride, or of oxide but also from polymers

Surface Micromachining

C. Liu

Historically - the later process. Relies on the sacrificial layers deposited and etched selectively

etching

LIGA process• Three dimensional metallic and polymer structures 500µm deep (up to 6cm?!) require

deep etching, molding, plating etc.• LIGA=X-ray Lithography, electroplating (galvo) and injection molding (abformung) and

damascene processes are widely used. Now UV-LIGA is used more frequently.

C. Liu

LIGA integration with CMOS via: Post-processing approachPreprocessing approachSide-by-side processing

500-60,000µm

New Materials and Fabrication Processes• Materials: Silicon was the main material but others are also widely

used Polymers as active structures: optical transparency, biocompatibility Polymers as protection and sealing layers High T and corrosive operation conditions (silicon carbide, diamond,

nitrides …) Other semiconductors (optical operation)

• Processes: traditional IC fabrication and other complementary/new processes (for nanoscale dimensions) and/or complementary materials Self assembly New lithography processes (molding, imprints …)