Chapter13 :Metallization GILAN UNIVERSITY ELECTRICAL ENGINEERING COLLEGE Device fabrication I: Chapter13 :Metallization By: Moein Moshtaq Ebadghorbandoost Keyvan alireza zad

What is the Metallization? Introduction What is the Metallization? Fabrication of circuits iclude 2 part: 1- the active and passive part. This is called the front end of the line (FEOL). 2- metal systems back end of the line (BEOL)

Objectives List the requirements of a material for use as a chip surface conductor. Draw cross sections of single and a two layer metal schemes. Introduce three materials used in the metallization All about sputtering , evaporation and CVD methods. Describe the principle and operation of vacuum pumps.

Conductors—Single-Level Metal In the MSI era metallization was relatively straightforward : 1 _Wafer with doped region 2_patterning : open contacts contacts 3_layering : conducting layer 10000 to 15000 Å by(sputtering , cvd, evaporation) 4_ patterning: metal mask Leads , Metal lines or interconnects 5_heat treatment ( alloying)

feature of metal system convenient current Connection and adhesion between sio2 and metal Purification Prevent against unwelcome rust A Level without hole

Conductors_ Multilevel Metal Schemes Increasing the chip density Decreasing the available space on the wafer for wiring Multilevel Metallization

Conductors_ Multilevel Metal Schemes Example: creating a CMOS Inverter on the wafer PMOS NMOS thin oxide

Salicide

Silicon nitride

Multilevel Metal Schemes

Conductivity Metal Bulk resistivity (mW*cm) Silver (Ag) 1.6 Copper (Cu) 1.7 Gold (Au) 2.2 Aluminum (Al) 2.8 Tungsten (W) 5.3 Molybdenum (Mo)

Conductors_ Aluminum From an electrical conduction standpoint, aluminum is less conductive than copper and gold. Prior to the development of VLSI-level circuits, the primary metallization material was pure aluminum. resistance Melting point Aluminum 2.7 mWcm 660 ºC Copper 1.7 mWcm 1084 ºC

Conductors_ Aluminum Copper, if used as a direct replacement for aluminum, has a high contact resistance Aluminum emerged as the preferred metal because: 1- has a low enough resistivity (2.7 μΩ-cm) and good current-carrying density. 2-has superior adhesion to silicon dioxide 3- is available in high purity 4-has a naturally low contact resistance with silicon 5- is relatively easy to pattern with conventional photolithography processes. 6- Aluminum sources are purified to 5 to 6 “nines” of purity(99.99999)

Conductors_ Aluminum Problems in Aluminum: one) AL and Si Alloy at 577 C (Start at 455 C the convenient temperature for good junction) But Aluminum spikes punctuate doped junction and make short contact

Conductors_ Aluminum Real photo

Conductors_ Aluminum TiW and TiN are used Barrier Layer (we saw in multilevel schematic) Al with 1 to 2% Si (not reliable) TiW and TiN are used Two Solution ways:

Conductors_ Aluminum Two)Electromigration In longe distance path(vlsi,ulsi),Metal interconnections in ICs are operated in high current densities and under this situation metal movement causes void and metal pileup or hillocks. Atoms are under electrical field and thermal gradiant concurrently and move in itself .

Solution: Al alloy with 0.5 to 4 % Cu or 0.1 to 0.5 % titanium Conductors_ Aluminum Solution: Al alloy with 0.5 to 4 % Cu or 0.1 to 0.5 % titanium Al alloy with Si and Cu can use to solve both problems

Conductors_ Aluminum Overall effectiveness of a metal system is governed by the resistivity, length, thickness, and total contact resistance of all the metal-wafer interconnects. In a simple aluminum system,there are two contacts: the silicon/aluminum interconnect and the aluminum interconnect/ wire. In a ULSI circuit with multilevel metal layers, barrier layers, plug fills, polysilicon gates and conductors, and other intermediate conductive layers, the number of connections becomes very large. The addition of all the individual contact resistances can dominate the conductivity of the metal system.