Silicon Wafer Fabrication u1r1z

Silicon and Wafer Preparation Lyndon Mark P. Olaguera PS147: Physical Electronics

OBJECTIVES At the end of this lesson, the student should be able to: 1. describe how the raw silicon is refined into semiconductorgrade silicon. 2. explain the crystal structure and growth method for producing mono-crystalline silicon. 3. outline and describe the basic process steps for wafer preparation, starting from a silicon ingot and finishing with a wafer. 4. explain what epitaxy is and why it is important for wafers.

Semiconductor Grade Si Pure silicon—used to minimize micro-defects at the atomic level of the silicon that are detrimental to semiconductor performance. Semiconductor grade Si(SGS)—highly refined silicon >>also known as electronic grade silicon

Semiconductor Grade Si

Source: Semiconductor Manufacturing Technology by Michael Quirk, Julian Seda, 2001

Siemens Process Silicon produced by this process does not have atoms arranged in a desirable crystal order.

Source: Semiconductor Manufacturing Technology by Michael Quirk, Julian Seda, 2001

CRYSTAL STRUCTURE CRYSTAL>>a solid material that has an ordered, repeatable three dimensional pattern over a long range of many atoms.

Source: Semiconductor Manufacturing Technology by Michael Quirk, Julian Seda, 2001

CRYSTAL STRUCTURE Amorphous Materials >>non crystalline solids that lack a repetitive structure and demonstrate structural disorder at the atomic level. E.g. Plastic Source: Semiconductor Manufacturing Technology by Michael Quirk, Julian Seda, 2001

CRYSTAL STRUCTURE  Unit Cell  >>the simplest arrangement of atoms that, when repeated in a three dimensional framework gives the crystal structure.

Source: Semiconductor Manufacturing Technology by Michael Quirk, Julian Seda, 2001

CRYSTAL STRUCTURE

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CRYSTAL STRUCTURE

Source: Semiconductor Manufacturing Technology by Michael Quirk, Julian Seda, 2001

CRYSTAL STRUCTURE Within a crystal structure, unit cells are tightly packed, therefore, share atoms. Sharing of atoms– this is how unit cells build up into a cohesive crystal lattice structure. FCC unit cell—four shared atoms. Silicon crystal—Four shared and Four unshared (four atoms inside the cubic structure) Source: Semiconductor Manufacturing Technology by Michael Quirk, Julian Seda, 2001

POLY-CRYSTAL and MONOCRYSTAL  Poly crystal—the unit cells are not in a regular arrangement.  Silicon produced from the semiconductor grade purification process— poly-silicon.  Mono-crystals—if unit cells are neatly arranged in a three dimensional, repeatable manner.

Source: Semiconductor Manufacturing Technology by Michael Quirk, Julian Seda, 2001.

REQUIREMENTS for Si Wafer Processing

Pure Mono-crystalline WHY?? 1. Repeatable units provide the desirable electrical and mechanical properties

CRYSTAL ORIENTATION

(100)– common for MOS devices, GaAs devices. (111)—bipolar devices (tighter packing density)

Source: Semiconductor Manufacturing Technology by Michael Quirk, Julian Seda, 2001.

MONOCRYSTAL SILICON GROWTH  Crystal Growth—process of converting the polysilicon chunks of SGS into a large monocrystal of Si.  INGOT—grown silicon monocrystal.  Czochralsky Method (CZ)--Most commonly used technique for growing monocrystal ingots.

Source: Semiconductor Manufacturing Technology by Michael Quirk, Julian Seda, 2001.

MONOCRYSTAL SILICON GROWTH  Objectives of the pull process:  Replicate the seed structure while obtaining dopant uniformity  Achieving the correct ingot diameter  Limiting the introduction of impurities

Parameters affecting the growth process: 1. Pull rate 2. Crystal rotation 3. //Temperature

DOPING

Adding impurities to obtain the desired resistivity Resistivity of pure silicon =2.5x10^5 ohm-cm

FLOAT ZONE METHOD  Produces a silicon monoscrystal ingot which significantly lower oxygen content.  Produces ingots with smaller diameters.

WAFER DIAMETERS

WAFER PREPARATION

SHAPING OPERATIONS  End Removal  Diameter Grinding  Wafer Flat or Notch

Source: Semiconductor Manufacturing Technology by Michael Quirk, Julian Seda, 2001.

WAFER IDENTIFYING FLATS

Source: Semiconductor Manufacturing Technology by Michael Quirk, Julian Seda, 2001.

WAFER SLICING

http://www.pveducation.org/node/496

WAFER LAPPING

http://www.microchemicals.com/s/pics/wafer_lapping_polishing.jpg