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I.C. Technology Processing Course Trinity College Dublin

I.C. Technology Processing Course Trinity College Dublin. IC Processing. EEE, MTG, TCD. Susceptor (graphite). How Silicon Wafers are produced :. Czrochralski Method. Wafer Slicing and Polishing. Wafer slicing using a diamond tipped saw.

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I.C. Technology Processing Course Trinity College Dublin

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  1. I.C. TechnologyProcessing CourseTrinity College Dublin

  2. IC Processing EEE, MTG, TCD

  3. Susceptor (graphite) HowSiliconWafersareproduced: • Czrochralski Method

  4. Wafer Slicing and Polishing • Wafer slicing using a diamond tipped saw • Wafer is then polished using a diamond polisher and silica slurry paste Top surface now highly polished to electronic grade Wafer slice Silicon ingot

  5. Step 1: Cleaning step • Removal of surface contaminants (metal and organic) • Oxidise the surface using Hydrogen Peroxide and Sulphuric Acid (1:1) • Strip off oxide using Hydrofluoric Acid (10:1) surface contaminants Original Silicon surface New Silicon surface • Contamination free surface ready for processing

  6. 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

  7. Dry Oxidation: • 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 Pure Oxygen Silicon after cleaning

  8. Mobile Charges* Na / K Trapped Charges Fixed Oxide Charges Interface Charges Na Na Na Na Na Na Na Na Na Na K Na Na Na K Na K Na Na Na Na Wet Oxidation: • Similar to Dry Oxidation • Why two methods of growing oxides???? • Oxide Quality Wet oxide Dry Oxide

  9. Rate of growth • Red line represents the growth rate of oxide in Wet Steam • Difference in the consumption of silicon • Difference in the thickness of oxide Dry oxygen growth Wet Steam Growth

  10. Processing temperature 1050 C 10700 C 10500 C 10700 C Field Oxidation: • 5 minutes in Pure O2 • 80 minutes in steam • 5 minutes in pure O2

  11. Photolithography • Develop the Photoresist in 5:1 solution water and Sodium Hydroxide. • Mask is placed onto the wafer • Photoresist is spun on • Wafer is exposed to UV light through the mask • Wafer is then softbaked to evaporate off the solvent • Solution etches the exposed areas only • UV light chemically changes the exposed Photoresist Transparent areas on the mask allows the UV light through Dark areas on the mask does not let the UV light through • Wafer is then hardbaked to chemically change the remaining Photoresist so that it becomes acid resistant • Remove the UV light source and the mask 95 C for 13-15 minutes 1250 C for 15-20 minutes

  12. 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

  13. 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

  14. Predeposition Boron Disc Wafers after first step Boat Diffusion of Boron from the Boron source disk takes place above 9000 C.

  15. 20 10 18 t1 t2 t3 10 conc. (cm ) -3 16 10 14 10 mm Distance into the silicon from the surface Background concentration Time t1:Boron has diffused into the silicon. The surface concentration is at the Solid Solubility of silicon Time t2: Concentration at the surface remains the same but Boron has now diffused deeper into the silicon.

  16. What is happening to the silicon? After time t1, boron is introduced through the open windows of the oxide layer After time t2, more boron is introduced through the windows of the oxide layer t1< t2< t3 Junction depth after predeposition is about 0.2-0.5 mm.

  17. 20 10 18 10 conc. (cm ) t1 -3 16 10 14 10 mm Distance into the silicon from the surface Drive-In (from a limited source) 10500C in a steam environment Predeposition provides the initial state for drive in After t1, surface concentration drops but the impurity diffuses into the silicon After t2, again the surface concentration drops and the impurity diffuses further into the silicon

  18. 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

  19. 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

  20. Na Na Na Na Na Na Na Na Na Na Na Na Na Na Na Na Na Na Na Na Na Na Na Na Na Na Na Na Na Na Na Na Na Na Na Na Na Na Na Na Na Na Na Na Na Na Na Na Na Na Na Na Na Na Na Na Na Na Na Na Na Na Na Na Na Na Na Na Na Na Na Na Na Na Na Na Na Na Na Na Na Na Na Na Na p-type p-type Gate B Gate A 0 Volts 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. depletion layer now at maximum 2 Volts 1.5 Volts 1 Volt 3 Volts 0.5 Volts 0 Volts depletion layer widens depletion layer Conduction layer formed (At elevated temperatures)

  21. How is the sodium concentration reduced? Na+ Na+ Cl_ Na+ Na+ Cl_ Na+ Cl_ Na+ Cl_ Na+ Cl_ Na+ Na+ Cl_ • 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 Sodium Contamination

  22. 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)

  23. Oxide is grown for 60 minutes at 10500C with Tricloroethylene

  24. 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

  25. Metalization Aluminium is evaporated onto the silicon wafer at low pressure

  26. How is the Aluminium evaporated onto the wafer? • 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 4 amps 3 amps • Aluminium gets hot and glows • Aluminium strand melts • Aluminium evaporates and coats the wafer tungsten filament Ceramic Pillars

  27. Final Mask: Patterning • 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

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