I.C. TechnologyProcessing Course

Trinity College Dublin

IC Processing

EEE, MTG, TCD

How Silicon Wafers are produced:

• Czrochralski Method

Susceptor(graphite)

Wafer Slicing and Polishing

Silicon ingot

• Wafer slicing using a diamond tipped saw

Wafer slice

• Wafer is then polished using a diamond polisher and silica slurry paste

Top surface now highly polished to electronic grade

Step 1: Cleaning step

• Removal of surface contaminants (metal and organic)

surface contaminants

• Oxidise the surface using Hydrogen Peroxide and Sulphuric Acid (1:1)

Original Silicon surfaceNew Silicon surface

• Strip off oxide using Hydrofluoric Acid (10:1)

• Contamination free surface ready for processing

Field Oxidation• Electrical isolation between devices

• Mask for selective doping areas

• Two methods used in the oxidation of Silicon wafers

• Dry Oxygen Oxidation

• Wet Steam Oxidation

• Si + O2 = SiO2 Reaction in Pure Oxygen

• Si + 2H2O = SiO2 +2H2 Reaction in Steam

Dry Oxidation:

Silicon after cleaning

Pure Oxygen

• First layer of SiO2 formed

• Further Oxygen atoms now have to diffuse through the top layer of SiO2

• Oxygen reacts with Silicon to form SiO2

Wet Oxidation:

• Similar to Dry Oxidation

• Why two methods of growing oxides????

• Oxide Quality

Na

Na Na

Na Na

NaNa

NaKK

Wet oxide

Na / K Mobile Charges*

Trapped Charges

Fixed Oxide ChargesInterface Charges

KNa

Na

Na

Na

NaNa

NaNa

Na Na

Dry Oxide

• Rate of growth

Dry oxygen growth Wet Steam Growth

• Red line represents the growth rate of oxide in Wet Steam• Difference in the consumption of silicon

• Difference in the thickness of oxide

Field Oxidation:

• Processing temperature 1050 C

10500 C 10700 C10700 C

• 5 minutes in Pure O2• 80 minutes in steam

• 5 minutes in pure O2

Photolithography

• Photoresist is spun on

• Wafer is then softbaked to evaporate off the solvent

95 C for 13-15 minutes

• Mask is placed onto the wafer

• Wafer is exposed to UV light through the mask

Dark areas on the maskdoes not let the UV light through

Transparent areas on the mask allows the UV light through

• UV light chemically changes the exposed Photoresist

• Remove the UV light source and the mask

• Develop the Photoresist in 5:1 solution water and Sodium Hydroxide.

• Solution etches the exposed areas only

• Wafer is then hardbaked to chemically change the remaining Photoresist so that it becomes acid resistant

1250 C for 15-20 minutes

Etching• Deal first with chemical Etching

• Buffered Oxide Etch 7:1 used to etch away the exposed areas of the oxide (7 minutes)

• Isotropic Etching - note how the oxide is etched under the protective layer

• Once the window is opened in the oxide, the protective layer is removed using Fuming Nitric Acid

Diffusion• Controlled introduction of impurities into silicon

• Sources:- Solid disk source

- gaseous source

- liquid source

- spin on dopant source

• Two steps involved in the diffusion of impurities

- Predeposition

- Drive in

• Predeposition

Boat

Boron DiscWafers after first step

Diffusion of Boron from the Boron source disk takes placeabove 9000 C.

Distance into the siliconfrom the surface

m

-3conc.(cm )

1014

1016

1018

1020

Backgroundconcentrationt1

Time t1: Boron has diffused into the silicon. The surface concentration is atthe Solid Solubility of silicon

t2

Time t2: Concentration at the surface remains the same but Boron has now diffused deeper into the silicon.

t3

What is happening to the silicon?

After time t1, boron is introduced through the open windowsof the oxide layer

After time t2, more boron is introduced through the windowsof the oxide layer

t1< t2< t3

Junction depth after predeposition is about 0.2-0.5m.

Drive-In (from a limited source)

Distance into the siliconfrom the surface

m

-3conc.(cm )

1014

1016

1018

1020

1050C in a steam environment

Predeposition provides the initial state for drive in

t1

After t1, surface concentration drops but the impurity diffuses into the silicon

After t2, again the surface concentration drops and the impurity diffuses furtherinto the silicon

What is happening to the silicon?

• Initial state after predeposition

• After t1 of drive in, the Boron diffuses further into the silicon

• After t2 of drive in, again the Boron diffuses further into the silicon

Gate Oxidation• Spin on photoresist• Softbake

• Apply mask and UV light• Develop and hardbake

• Etch in BOE for 7 minutes to prepare for Gate Oxidation

• Strip off the Photoresist layer using Fuming Nitric acid

Why the need for a specially grown oxide for the gate?

• Reducing sodium content in oxide.

Why?

• Sodium in a gate oxide alters the threshold voltage.

Na

Na Na

Na Na

NaNaNa Na

Na Na

Na

Na NaNa Na Na

p-typep-type

0 Volts

Gate A Gate B

(At elevated temperatures) 0 Volts0.5 Volts

Na

NaNa

Na Na Na

Na

Na Na

Na Na

Na

Na

NaNa Na Na

1 Volt

Na

Na Na

Na Na Na

Na

Na Na

Na Na

NaNa

NaNa Na Na

depletion layer1.5 Volts

NaNa Na

Na Na NaNa

Na NaNa Na

Na Na NaNa Na Na

depletion layer widens2 Volts

NaNa Na

Na Na NaNa

Na NaNa Na

Na Na NaNa Na Na

depletion layer nowat maximum3 Volts Conduction layer formed

How is the sodium concentration reduced?

•Add a Cl_ ion while growing the oxide, this will react with

the Na+ ion to form a neutral NaCl salt that is electrically inactive.

Oxide

Na+

Na+

Na+

Na+

Na+

Na+

Na+

Na+

Na+

Sodium Contamination

Cl_

Cl_

Cl_

Cl_

Cl_

Cl_

How is the Cl_ added?

• Oxide is grown using pure oxygen with the inclusion of one of the following:

• HCl

• Trichloroethylene

• Trichloroethane

Trichloroethylene is the safest of the three as it is:

• non carcinogenic (unlike Trichloroethane)

• non corroding (unlike HCl)

Oxide is grown for 60 minutes at 10500C with Tricloroethylene

Contact holes• Spin on photoresist• Softbake

• Apply mask and UV light• Develop and hardbake

• Etch in BOE to open the contact holes to the diffused regions

• Strip off the Photoresist layer using Fuming Nitric acid

MetalizationAluminium is evaporated onto the silicon wafer at low pressure

How is the Aluminium evaporated onto the wafer?

Ceramic Pillars

tungsten filament

• Onto this filiment a strand of pure Aluminium is placed

• Wafer is placed on a holder close to the filament

• Once a low pressure is obtained in the chamber, a current is passed through the filament to melt and evaporate the aluminium

1 amp2 amps • Aluminium gets hot and glows3 amps

• Aluminium strand melts

4 amps

• Aluminium evaporates and coats the wafer

• Spin on photoresist• Softbake

• Apply mask and UV light• Develop and hardbake

• Etch in Orthophosphoric Acid to create the metal tracks

• Strip off the Photoresist layer using Fuming Nitric acid

Final Mask: Patterning