Section 15.5: Materials Aspects of Fabrication of Semiconductor Circuits

1. Section 15.5: Materials Aspects of Fabrication of Semiconductor Circuits

2. Si Over 98% of devices are made of Si Diamond Cubic crystal

3. 1. Metallurgical grade Si (99%) 2. Semiconductor grade Si (ppb) 3. Single crystal growth 4. Wafer 5. Oxidation 6. Photolithography 7. Doping 8. Epitaxial Growth 9. Metallization

4. 1. Metallurgical Grade Si from sand Earth’s crust (about 100km): Mostly Si (26%) and O (49%) !! Reduction of sand by carbon in an electric arc furnace 99% pure: suitable for electronic applications but not for electronic devices

5. 2. Semiconductor Grade Si Step 1 Step 2 Fractional distillation of trichlorosilane to remove impurities Step 3 Chemical Vapour Deposition of electronic grade Si from pure trichlorosilane

6. 3. Single Crystal Growth (A) Czochralski method (CZ) SG Si melted into a silica crucible Si melt does not dissolve silica. Only imprity is that of O. Seed crystal of desired orientation <100> or <111> is inroduced Dipped into the melt (T>Tm), rotated (6-8 rpm) and pulled (50-100 mm hr-1) Crucible rotated in opposite direction (10-12 rpm) and lifted upwards to compensate for the falling level of the melt Dopants introduced into the melt Dia: 200-300 mm, Length 1-2 m

7. 3. Single Crystal Growth (Contd.) (B) Float Zone technique (FZ) No crucible: Lower O contamination Smaller Diameter : 75 mm

8. Si single crystal boule

9. 4. Wafer manufacture Single crystal produced by CZ or FZ is ground to uniform cylindrical shape Crystal orientation is determined by single crystal x-ray diffraction. Important for further processing as the chips are separated along easy cleavage lines Primary flats are ground to indicate orientation and secondary flat to indicate whether the material is n-type or p-type The boule is cut into thin circular wafers using inner diameter slicing or wire saw Past : 200 mm dia; 725 m thickness Current: 300 mm; 775 m Future: 450 mm; 925 m <110> Wafer is etched to remove damaged layer and polished to a mirror finish

10. Wafer manufacture Primary and secondary flats <110>

11. 5. Oxidation Excellent quality of oxide is one of the reasons why Si has been preferred over Ge for semiconductor device Role of Oxide: 1. As a mask against diffusion or ion implantation 2. provide a passive electrical and chemical layer 3. isolate various devices 4. a component of MOS Volume of oxide> volume of Si => compressive stress in the oxide film=> no danger of cracking Two kinds of oxidation: 1. Dry 2. Wet