Posted on 28 February 2013

Metallization Processes for Semiconductor Devices

 

 

 

 

 

 

 

Metallization of semiconductors is vital for the connetivity of external conductors, e.g., cables, busbars, etc. The metal plating on the semiconductor is used to create Schottky contacts or Ohmic contacts.

Schottky contacts are metallic semiconductor contacts with a rectifying effect. Only majority charge carriers are involved in this case and there is no storage charge. Schottky diodes are therefore very fast. Upon switching from the forward direction to the reverse direction, only the junction capacitance must be recharged.

Ohmic contacts are metallic semiconductor contacts without rectifying effect. Ohmic contacts are obtained when the silicon is highly doped at the point of contact.

Comparison of different types of diodes

Schottky diodes have no pn-junction and are the fastest diodes for a reverse voltage of up to approximately 100 V. Epitaxial and diffused diodes are pn-diodes, diodes that have a pn-junction.

Doping profile

Figure 1. Doping profiles for different kinds of diodes

Due to the low doping concentration in the n- zone, these diodes are also referred to as pin-diodes (i for intrinsic). Epi diodes have a thin n- zone which makes them very fast. These epi-diodes are generally used up to about 600V.

Chemical deposition of metals

Electroless Deposition of Metals

Electroless Nickel: In an autocatalytic reaction, nickel is deposited on the silicon wafer by immersion in a bath. Before the nickel plating occurs, also electroless, palladium nuclei are formed on the wafer which act as a "jump-start" for the nickel deposition.

Depending on the bath composition, the nickel layers contain 3% to 13% phosphorous or 0.1% to 10% Boron. The electroless deposited layers are hard, corrosion resistant, and have uniform thickness.

After the deposition, the first nickel layer (<1µm) is sintered into the silicon at 635°C to create nickel silicide. This improves the ohmic contact and the adhesive strength. A second nickel layer is finally applied which ranges from 2 µm to 2.5 µm.

Electroless gold with a thickness of 0.05 µm to 0.1µm is the applied to act as a wetting aid during soldering.

A great advantage of electroless cutting is that many wafers can be processed simultaneously and deposed on both sides. High soldering temperatures may, however, result in "black nickel" where there is separation between the nickel layers. The thickness of the nickel layers is also limited to 2.5 µm for the exchange reaction to take place between the layers.

Electrodeposition

In electrodeposition, metal ions are transported in a liquid to the surfaces to be coated. This method can be used to reinforce existing layers.

Electrodeposition is rarely used in power electronics devices.

Physical Vapor Deposition (PVD) and Sputtering

Physical vapor deposition is a process in which a desired film is deposited without a chemical reaction between the materials. Common examples of PVD include evaporation deposition and cathode sputtering.

The mean free path of gas molecules increases greatly at very low pressures. For this reason, evaporation deposition and sputtering processes are carried out in low pressure environments. Vacuum lines with large cross sections are required to create an ultra high vacuum. Many pump principles are used in achieving this vacuum. Some of these include the rotary vane pipe, diffusion pumps, turbo-molecular pump, and a kryopump.

In the vapor phase deposition process, material is vaporized at high temperatures. The vapor is then deposited on the substrate. Only pure elements, no compounds, can be deposited using the vaporization process.

Sputtering systems include DC sputter systems suitable for conductive materials such as metals and semiconductors and AC sputter systems which can also be used for insulators. During the sputtering process, the coating material is atomized by bambarding it with heavy argon ions. The atomized material is then deposited on the substrate material. Many materials can be atomized simultaneously.

Diagram showing vapor deposition and sputtering principles

Figure 2. Vapor deposition and sputtering

Metallization patterning

Metallization patterning, similar to the patterning of other layers, is done through photolithography. The etching solution is selected such that the photoresist is affected very little. For coarse structures, the metal can be deposited through shadow masks and thus applied already having the desired structure.

mask metallized thyristor wafer

Figure 3. Mask metallized thyristor wafer

After metallization patterning, the silicon wafer can be sintered in order to improve ohmic contact and bond strength.

 

For more information, please read:

Masking and Structuring Silicon

Neutron Transmutation Doping of Silicon Rods

Doping Silicon Wafers

 

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