Materials Used in Semiconductor Manufacturing

     

Basic Elements Used

       

Silicon, symbol Si, is the most commonly used basic building block of integrated circuits.  Silicon is a semiconductor, which means that its electrical behavior is between that of a conductor and an insulator at room temperature.  With the proper addition of dopant elements, p-n junctions can be formed on silicon.  Useful electronic components and integrated circuits can be built from p-n junctions. Silicon is obtained by heating silicon dioxide (SiO2), or silica, with a reducing agent in a furnace.   Silicon dioxide is the main component of ordinary sand.

   

Aside from being used as semiconductor substrate, silicon is also widely used as dielectric in integrated circuits, usually in the form of silicon dioxide.  Dielectric layers are used to isolate conductive lines and the individual components in the circuit from each other. Polycrystalline silicon, or polysilicon, is also used for making resistors or conductors in integrated circuits. The top glassivation used to mechanically and electrically protect the die is also usually composed of silicon in the form of silicon nitride. Silicon is also widely used in semiconductor packaging, being the main ingredient of plastic encapsulants for integrated circuits.  Silicon is also used in die overcoats.

   

Table 1. Silicon's Basic Properties

Property

Value

Property

Value

Atomic Number

14

Specific Gravity

 2.33

Atomic Group

14 or IVA

Hardness

7 for crystalline silicon

Atomic Weight

28.086

Band Gap/Energy Gap

1.11 eV

Melting Point

1410 deg C

Thermal Conductivity

 1.57 W/cm deg C

Boiling Temp

2355 deg C

Coeff. of Thermal Expansion

 2.63e-6/deg C

       

Aluminum, symbol Al, is a lightweight metal with silvery appearance.  It is the most abundant metallic element on earth. Aluminum is used in many aspects of semiconductor manufacturing.  On the integrated circuit, Al metal lines are commonly used as the main conductor between components, mainly because of its low resistivity (2.7 mohm-cm). As a thin film, it also has good adherence to silicon dioxide. Aluminum is also the metallization used for the bonding and probing pads on the die. When used for IC metallization, Al is usually very lightly doped with other elements such as Si and/or Cu to improve its characteristics and reliability.  In semiconductor assembly, ceramic packages are composed mainly of alumina.  Aluminum is also used for wirebonding integrated circuits in ceramic packages.

       

Table 2. Aluminum's Basic Properties

Property

Value

Property

Value

Atomic Number

13

Melting Point

660 deg C

Atomic Group

13 or IIIA

Boiling Temp

2467 deg C

Atomic Weight

26.9815

Specific Gravity

2.7

Density

2.7 g/cm3

Resistivity

2.7 mohm-cm

       

Gold, symbol Au, is a soft metallic element that is bright yellowish in color.  A good conductor of heat and electricity, it is also the most malleable and ductile of all metals. Gold  is used in many aspects of semiconductor manufacturing, particularly in the assembly or packaging processes.  Its most widespread use is in wirebonding.  Because of gold's excellent conductivity and ductility, it is extensively used in making wires for the connection of the integrated circuit to the leads of the package.  Aside from manufacturability, the ductility of gold wires offers one more advantage when used in plastic-encapsulated devices, i.e.,  it makes the wires resistant to wire breaking during the encapsulation process. Gold is also used as die attach material for the eutectic die attach process, which is commonly used in old hermetic assembly processes.   Gold is also used to cover the die cavity and bonding posts of ceramic packages to protect these from chemical degradation.

        

Table 3. Gold's Basic Properties

Property

Value

Property

Value

Atomic Number

79

Melting Point

1064 deg C

Atomic Group

Transition Elements

Boiling Temp

2808 deg C

Atomic Weight

196.97

Specific Gravity

19.3

      

Silver, symbol Ag, is a shiny metallic element used for ornamental and coinage purposes since the ancient times.  It is next only to gold in terms of malleability and ductility, and is also a good conductor of heat and electricity.  In fact, silver is the best conductor of electricity, better even than copper and gold. Silver, like gold,  is used in many facets of semiconductor manufacturing, again more particularly in the assembly or packaging processes.  Most epoxy die attach materials contain silver fillers for increased thermal and electrical conductivity.  Silver is also used to cover the surfaces of the die pad and bonding fingers of the leadframes of plastic packages to prevent chemical degradation of these areas, which may lead to die attach and bonding problems.

     

Table 4. Silver's Basic Properties

Property

Value

Property

Value

Atomic Number

47

Melting Point

962 deg C

Atomic Group

Transition Elements

Boiling Temp

2212 deg C

Atomic Weight

107.868

Specific Gravity

10.5

      

Copper, symbol Cu, is one of the most widely used metals in the history of mankind, mainly because of  its  many desirable properties.  It is the second best conductor of electricity, next only to silver but better even than gold.  It also is very malleable and ductile, and is also a good conductor of heat. Copper is also widely used in semiconductor assembly.  For instance, most leadframes for plastic packages are composed of copper.   The leadframe is the skeletal support of a plastic package.  Copper, being an excellent conductor, would've been a very good candidate for use in metal lines in an  integrated circuit, but difficulties in the manufacturing of  IC's using copper for metallization resulted in Al being the metal of choice for this purpose.   Recent technological advancements though have already allowed the use of copper as metal lines in semiconductor devices.

     

Table 5. Copper's Basic Properties

Property

Value

Property

Value

Atomic Number

29

Melting Point

1083 deg C

Atomic Group

Transition Elements

Boiling Temp

2567 deg C

Atomic Weight

63.546

Specific Gravity

8.9

     

Other Elements Used

   

Boron, symbol B, atomic number 5, atomic group 13 or IIIA, atomic weight 10.81, is a hard, brittle, semi-metallic element used as a dopant in semiconductor devices.  It is classified as an 'acceptor' when used as dopant for Si devices, given that its atom has one less valence electron compared to the Si atom that it replaces (B has 3 valence electrons while Si has 4).  This results in a vacancy of electron which is known as a 'hole'.  The resulting excess 'holes' make the material 'p-type.'

    

Germanium, symbol Ge, atomic number 32, atomic group 14 or IVA, atomic weight 72.59, is a grayish white semi-metallic element with a melting temperature of 958 deg C. It was actually the original semiconductor material used to fabricate diodes and transistors. However, the low band gap of Ge (0.66 eV) results in relatively large leakage currents in its reverse-biased p-n junctions.

  

Lead, symbol Pb, atomic number 82, atomic group 14 or IVA, atomic weight 207.20, is one of the two major components of solder used in finishing the external leads of the IC package.  The other major component of solder is Tin, or Sn.

  

Phosphorus, symbol P, atomic number 15, atomic group 15 or VA, atomic weight 30.974, is a non-metallic element used as a dopant in semiconductor devices.  It is classified as a 'donor' when used as dopant for Si devices, given that its atom 'donates' or gives up one of its 5 valence electrons when it replaces a Si atom, which only has 4 valence electrons.  The resulting excess electrons make the material 'n-type.'

  

Tin, symbol Sn, atomic number 50, atomic group 14 or IVA, atomic weight 118.69, is one of the two major components of solder used in finishing the external leads of the IC package.  The other major component of solder is Lead, or Pb. 

  

Other Materials Used (alphabetically arranged)

  

Alloy 42.  An alloy used for manufacturing leadframes, usually for ceramic packages such as the CerDIP.  It is composed of about 57.7% Fe, 41% Ni, 0.8% Mn, and 0.5% Co.  Its thermal coefficient of expansion is 4.3 ppm/deg C.

  

Borophosposilicate Glass (BPSG). A compound used as interlayer dielectric in integrated circuits for isolating conductive lines and components from each other.

  

Gallium Arsenide (GaAs). A dark gray crystalline solid composed of gallium and arsenic used as substrate semiconductor material for building integrated circuits.  GaAs has an energy gap of 1.35 eV, which is higher than the 1.11 eV of Si.  Energy gap is the amount of energy needed for an electron to jump from the valence band into the conduction band.  This is the reason why GaAs devices can operate at higher temperatures than Si devices - they are less prone to exhibit thermally-induced noise and leakage.

  

Kovar A vacuum-melted, low-expansion alloy used for manufacturing leadframes, usually for ceramic packages such as the CerDIP.  It is composed of about 53.48% Fe, 29% Ni, 17% Co, 0.2% Si, 0.3% Mn, and 0.02%C.  It melts at 1450 deg C, and has a resistivity of 490 microohm-mm.  

   

Platinum silicide (PtSi).  PtSi found a popular use in silicon integrated circuits as a highly reliable contact metallization between the silicon substrate and the upper metal layers.  PtSi is formed using a self-aligned process, making it  very reproducible and resulting in a very clean interface with silicon. A diffusion barrier metal such as Ti/Pt is often deposited over the silicide before the final metal layer such as Al is placed on top of the barrier metal..

                                                                                               

Some Process Gases and Chemicals Used

in Semiconductor Manufacturing

Acetic Acid

CH3COOH

Acetone

(CH3)2CO

Ammonium Fluoride

NH4F

Ammonium Hydroxide

NH4OH

Buffered Oxide Etchant (BOE)

NH4F, HF

Chrome Etch

KMnO4, Na3PO4

Chromium Trioxide/HF/Water

CrO3, HF, H2O

Chromium Trioxide/HF/Water

CrO3, HF, H2O

Chromium Trioxide/Phos/Water

CrO3, H3PO4, H2O

Glass Etch

NH4F, CH3COOH

Hydrochloric Acid

HCl

Hydrofluoric Acid

HF

Hydrogen Peroxide

H2O2

Isopropyl Alcohol

CH3CHOHCH3

M-Etch

HF, HNO3, CH3COOH

Methyl Alcohol

CH3OH

Methyl Ethyl Ketone

CH3COCH2CH3

Nitric Acid

HNO3

Phosphoric Acid

H3PO4

Poly Etch

HF, HNO3, NH4F

Potassium Hydroxide (Solution, Pellets)

KOH

Pre-Evap Etch

NH4F, (NH4)3PO4

Sodium Hydroxide (Solution, Pellets)

NaOH

Sulfuric Acid

H2SO4

Xylene

C6H4

Polysilicon Polycrystalline silicon, or polysilicon, is used in integrated circuits as a conductor, resistor, or MOS gate metallization.

  

Sichrome (SiCr).  A compound used for fabricating thin film resistors on an integrated circuit.  SiCr resistors are often trimmed to the final resistance values using a laser beam  after deposition.

   

Silicon dioxide (SiO2).  Silicon dioxide is used mainly as an inter-layer dielectric to electrically isolate conductive layers of the integrated cirucit  from each other.  It is also used as a mask or capping layer.  Silicon dioxide may be deposited with dopants, in which case it may serve purposes other than those mentioned above.   

    

Silicon nitride (Si3N4).  Silicon nitride is an amorphous dielectric often used as the topmost or final passivating layer of an integrated circuit, mainly because it offers excellent protection against corrosion and mechanical damage. It's a good barrier against moisture and sodium diffusion, which is why it is highly preventive of corrosion.  It also has a high tensile strength and resistivity. 

     

Spin-On Glass (SOG). A glass compound deposited on the surface of a wafer for planarization of the wafer.

              

Titanium silicide (TiSi2). TiSi2 is the most widely used silicide for the salicide (self-aligned silicide) process, having the lowest resistivity (13-17 mohm-cm) among refractory metal silicides. Its melting point is 1540 deg C.

              

Titanium Tungsten (Ti/W).  A compound used as barrier metal under the Al metal line to prevent the formation of Si nodules on the surface of the metal as a result of interdiffusion between the Si and Al atoms.  The TiW layer also provides additional conduction for electricity.

        

See Also:  Leadframes Die Attach Materials Bonding Wires Mold Compounds;   Marking Inks

Chemicals/Gases;  Hazardous Chemicals IC Manufacturing Semiconductor Eqpt. 

   

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