Chapter 1 Crystal Growth Wafer Preparation

Polycrystalline semiconductor Process Flow Overview Starting Material Polycrystalline semiconductor Single crystal Wafer Distillation and Reduction Synthesis Crystal growth Crystal growth Grind, saw , polish Grind, saw , polish

Overview Material preparation is the beginning of the process in making an IC chip . The goal for this part of the process is to grow the ingot that will be sliced up into wafers. The wafer is a round solid silicon disc that will have all of the processing performed on it.

CRYSTAL GROWTH 6. Edge Rounding 1. Crystal Growth 7. Lapping 2. Single Crystal Ingot 3. Crystal Trimming and Diameter Grind 4. Flat Grinding 5. Wafer Slicing 6. Edge Rounding 7. Lapping 8. Wafer Etching 9. Polishing 10. Wafer Inspection Slurry Polishing table Polishing head Polysilicon Seed crystal Heater Crucible

Starting Material Preparation The starting material for silicon is relatively pure form of sand (SiO2) called quartzite. This is placed in a furnace with various forms of Carbon. The Silicon treated with HCL at 300° to form tricholosilane.

Starting Material Preparation Fractional Distillation of the liquid removes the unwanted impurities. The purified SiHCL3 is then used in a hydrogen reduction reaction to prepare the electronic-grade silicon ( EGS) . EGS ,a polycrystalline material of high purity is the raw material of single-crystal silicon.

Silicon Crystal Growth from the Melt The Basic Technique , Which is material in liquid form, is Czochralski Technique. The Float –Zone Process also can be used to grow silicon that has lower contamination than normally obtained from CZ Technique. Most popular technique is CZ.

Czochralski Technique Czochralski Process is a Technique in Making Single-Crystal Silicon A Solid Seed Crystal is Rotated and Slowly Extracted from a Pool of Molten Si Requires Careful Control to Give Crystals Desired Purity and Dimensions

CYLINDER OF MONOCRYSTALLINE The Silicon Cylinder is Known as an Ingot Typical Ingot is About 1 or 2 Meters in Length Can be Sliced into Hundreds of Smaller Circular Pieces Called Wafers Each Wafer Yields Hundreds or Thousands of Integrated Circuits

Czochralski Technique Graphite susceptor Sio2 crucible Heating element Crystal Puller Furnace Power supply Rotation mechanism Seed Holder Crystal-pulling mechanism Rotation mechanism Gas Source Ambient control Flow control Exhaust system Control system Control process parameters: Crystal Diameter, pull rate and rotation speed.

Distribution of Dopant In crystal-growth a known amount of Dopant is added to the melt to obtain the desired doping concentration in the growth crystal. At interface, The doping concentration incorporated into the crystal ( solid) is usually different from the doping concentration of the melt ( liquid). The ratio of these two concentration is defined as the equilibrium segregation coefficient: Equilibrium concentration of the dopant in the Solid Equilibrium concentration of the dopant in the Liquid

Distribution of Dopant Note the most values are below 1. As the crystal grows , the melt becomes enriched.

Distribution of Dopant Find the doping concentration in the crystal in CZ technique? The doping concentration in the crystal The initial doping concentration in the crystal The doping concentration in Melt Given point of growth The amount of the dopant remaining in the melt The reduction of the dopant from the melt

Distribution of Dopant

Example

Effective Segregation Coefficient There is a concentration gradient developed at the interface, when .

Effective Segregation Coefficient

Silicon Float-Zone Process RF Gas inlet (inert) Molten zone Traveling RF coil Polycrystalline rod (silicon) Seed crystal Inert gas out Chuck

Silicon Float-Zone Process

Silicon Float-Zone Process

Silicon Float-Zone Process

Silicon Float-Zone Process

Neutron Irradiation

Saturated electron velocity GaAs Advantages Fabrication Cost Noise Breakdown voltage Operating Power Cutoff Frequency Electron mobility Saturated electron velocity Substrate Type High Low GaAs Si These properties make GaAs circuitry ideal for mobile phones, satellite communications, microwave point-to-point links, and radar systems. However, high fabrication costs and high power consumption have made GaAs circuits unable to compete with silicon CMOS circuits in most applications. 

Ga & As Elemental Gallium Native Gallium With 99.9999% purity Bayer Process Zone passes refining Native Arsenic Elemental Arsenic With 99.9999% purity

GaAs Polycrystalline Elemental Gallium With 99.9999% purity Synthesis Process Elemental Gallium With 99.9999% purity

GaAs Crystal-growth The starting materials for the synthesis of polycrystalline gallium arsenide are the elemental , chemically pure gallium and arsenic. The behavior of a combination can be described by phase diagram.

As boiling point Ga boiling point GaAs Crystal-growth

Example Find the fraction of melt that will be solidified? Weight percent scale Weight of the liquid. Weight of the solid (GaAs). Concentration of Dopant in the Liquid. Concentration of Dopant in the Solid. Weight of the Arsenic in the Liquid. Weight of the Arsenic in the solid (GaAs). Tototal Weight of the Arsenic in Ta and Tb.

As boiling point Ga boiling point Example ( cont.)

GaAs Crystal-growth

Bridgman Technique

Equilibrium Segregation Coefficient

Wafer Coding

Packed Wafers

Chapter Overview Raw materials (SiO2) are refined to produce electronic grade silicon with a purity unmatched by any other available material on earth. CZ crystal growth produces structurally perfect Si single crystals which are cut into wafers and polished. Dopants can be incorporated during crystal growth Point, line, and volume (1D, 2D, and 3D) defects can be present in crystals, particularly after high temperature processing. Point defects are "fundamental" and their concentration depends on temperature (exponentially), on doping level and on other processes like ion implantation which can create non-equilibrium transient concentrations of these defects.