I.C. Technology Processing 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 surface New 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 + O 2 = SiO 2 Reaction in Pure Oxygen Si + 2H 2 O = SiO 2 +2H 2 Reaction in Steam

Dry Oxidation: Silicon after cleaning Pure Oxygen First layer of SiO 2 formed Further Oxygen atoms now have to diffuse through the top layer of SiO 2 Oxygen reacts with Silicon to form SiO 2

Wet Oxidation: Similar to Dry Oxidation Why two methods of growing oxides???? Oxide Quality Na K K Wet oxide Na/ K Mobile Charges* Trapped Charges Fixed Oxide Charges Interface Charges K 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 temperature1050C C C 5 minutes in Pure O 2 80 minutes in steam 5 minutes in pure O 2

Photolithography Photoresist is spun on Wafer is then softbaked to evaporate off the solvent 95 C for minutes Mask is placed onto the wafer Wafer is exposed to UV light through the mask Dark areas on the mask does 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 C for 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 Disc Wafers after first step Diffusion of Boron from the Boron source disk takes place above C.

Distance into the silicon from the surface mm -3 conc. (cm ) Background concentration t1t1 Time t 1 : Boron has diffused into the silicon. The surface concentration is at the Solid Solubility of silicon t2t2 Time t 2 :Concentration at the surface remains the same but Boron has now diffused deeper into the silicon. t3t3

What is happening to the silicon? After time t 1, boron is introduced through the open windows of the oxide layer After time t 2, more boron is introduced through the windows of the oxide layer t 1< t 2< t 3 Junction depth after predeposition is about  m.

Drive-In (from a limited source) Distance into the silicon from the surface mm -3 conc. (cm )  C in a steam environment Predeposition provides the initial state for drive in t1t1 After t 1, surface concentration dropsbut the impurity diffuses into the silicon After t 2, again the surface concentration drops and the impurity diffuses further into the silicon

What is happening to the silicon? Initial state after predeposition After t 1 of drive in, the Boron diffuses further into the silicon After t 2 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 p-type 0 Volts Gate A Gate B (At elevated temperatures) 0 Volts 0.5 Volts Na 1 Volt Na depletion layer 1.5 Volts Na depletion layer widens 2 Volts Na depletion layer now at maximum 3 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 + Sodium Contamination 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 C 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

Metalization Aluminium 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 amp 2 amps Aluminium gets hot and glows 3 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