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8.2 Aluminum Metallization Chapter 8 UEEP2613 Microelectronic Fabrication Metallization Prepared by Dr. Lim Soo King 29 Jul 2012 Chapter 8 ..........................................................................................193 Metallization ....................................................................................193 8.0 Introduction ............................................................................................ 193 8.1 Metal Selection ....................................................................................... 195 8.2 Aluminum Metallization ........................................................................ 196 8.3 Copper Metallization ............................................................................. 198 8.4 Tantalum Deposition .............................................................................. 198 8.5 Characteristics of Metal Thin Film ...................................................... 199 8.5.1 Thickness of Metal Film ................................................................................. 199 8.5.2 Uniformity of Metal Film ............................................................................... 202 8.5.3 Stress of Metal Film ........................................................................................ 203 8.5.4 Reflectivity of Metal Film ............................................................................... 203 8.5.5 Sheet Resistance and Capacitance of Metal Film ........................................ 204 Exercises ........................................................................................................ 210 Bibliography ................................................................................................. 212 - i - Figure 8.1: Cross sectional view of a CMOS integrated circuit showing tungsten via and aluminum/copper interconnection ................................................................. 194 Figure 8.2: Cross sectional view of a CMOS integrated circuit showing copper interconnection ............................................................................................... 195 Figure 8.3: Illustration of junction spiking caused by aluminum diffusion ..................... 197 Figure 8.4: Illustration of electromigration ...................................................................... 197 Figure 8.5: (a) Schematic of stylus profilometer and (b) the profile of thickness ............ 200 Figure 8.6: (a) Schematic of acoustic method for thin film measurement and (b) the change of reflectivity with time ..................................................................... 202 Figure 8.7: The mapping pattern of wafer with (a) point measurement, (b) 9-point measurement, and (c) 49-point measurement ................................................ 202 Figure 8.8: Hillock and crack caused by high stress ........................................................ 203 Figure 8.9: Metal film line................................................................................................ 205 Figure 8.10: Four point probe ............................................................................................. 205 Figure 8.11: Correcton factor versus t/s plot ...................................................................... 207 Figure 8.12: Interconnect metallic structure for RC analysis ............................................. 209 - ii - Chapter 8 Metallization _____________________________________________ 8.0 Introduction A number of conductors such as copper, aluminum, tungsten etc, are used for fabrication of semiconductor devices. Metal with high conductivity is widely used for interconnection forming microelectronic circuit. Metallization is a process of adding a layer of metal on the surface of wafer. Metal such as copper and aluminum are good conductors and they are widely used to make conducting lines to transport electrical power and signal. Miniature metal lines connect million of transistors made on the surface of semiconductor substrate. Metallization must have low resistivity for low power consumption and high integrated circuit speed, smooth surface for high resolution patterning process, high resistance to electro-migration to achieve high device reliability, and low film stress for good adhesion to underlying substrate. Other characteristics are stable mechanical and electrical properties during subsequent processing, good corrosion resistance, and relative receptivity to deposit and etch. It is important to reduce the resistance of the interconnection lines since integrated circuit device speed is closely related with RC constant time, which is proportional to the resistivity of the conductor used to form the metal line. Although copper has lower resistivity than aluminum but technical difficulties such as adhesion, diffusion problem, and difficulties with dry etching etc have hampered copper application in microelectronics for long time. Aluminum has dominated metallization application since beginning of the semiconductor industry, In 1960s and 1970s, pure aluminum or aluminum- silicon alloy were used as metal interconnection materials. By 1980s, when device dimension shrank, one layer of metal interconnect was no longer enough to route all the transistors and multi-layer interconnection became widely used. To increase the pack density, there must be near-vertical contact and via holes, which are too narrow for physical vapor deposition PVD of aluminum alloy to fill the via without voids. Thus, tungsten become a widely used material to fill - 193 - 08 Metallization contact and via holes and serves as the plug to connect different metal layers. Titanium and titanium nitride barrier/adhesion layers are deposited prior to tungsten deposition to prevent tungsten diffusion and peeling. Fig. 8.1 illustrates a cross sectional view of a CMOS integrated circuit with aluminum interconnection and tungsten via plug. Borophosphosilicate glass BPSG is used as the insulating material separating the plugs. Figure 8.1: Cross sectional view of a CMOS integrated circuit showing tungsten via and aluminum/copper interconnection In 1990s, the development of chemical mechanical polishing CMP has open the avenue to use copper for interconnection with damascene or dual damascene process, which gets around the demand of metal etching. Tantalum is used as the barrier layer to prevent copper from diffusion through silicon dioxide. Silicon niride is also used as an etch stop layer for dual damascene dielectric etching process. Fig. 8.2 illustrates a cross sectional view of a CMOS device with copper interconnection. Since most of thin film depositions such as titanium, titanium nitride, tungsten, silicidation have been discussed in previous chapter, we shall not repeat them here. We shall concentrate to provide lecture on aluminum, copper, and tantalum metal depositions. Besides these lectures, we will also be discussing about the characteristics of metal film in terms of its thickness, uniformity, stress, reflectivity, and sheet resistance. Note that FSG is fluorosilicate glass, PSG is phosphorosilicate glass, and USG is undoped silicate glass. - 194 - 08 Metallization Figure 8.2: Cross sectional view of a CMOS integrated circuit showing copper interconnection 8.1 Metal Selection The importance of interconnection metallization has been briefly dicussed in the earlier introductory section, which is controlling the propagation delay by virtue of the resistance of interconnection line. The RC time constant of the line varies with silicon dioxide as the dielectric material follows equation (8.1). 2 Line LLineox RC (8.1) dLine dox where Line is the resistivity of the line material, dLine is the thickness of line, LLine is the length of the line, dox is the thickness of oxide, and ox is the permittivity of oxide. The desired properties of the metallization for integrated circuit are as follows. Low resistivity. Easy to form. Easy to etch for pattern generation. Should be stable in oxidizing ambient; oxidizable. - 195 - 08 Metallization Mechanical stability; good adhersion and low stress. Surface smoothness. Stability throughout processing, including high temperature sinter, dry and wet oxidation, gettering, phosphorous glass (or other material), passiviation, metallization. No reaction with final metal, aluminum. Should not contaminate device, wafer, or working equipment. Good device characteristics and lifetime. For window contact – low contact resistance, minimal junction penetration, low electromigration. 8.2 Aluminum Metallization Aluminum is the most widely used metal in microelectronic industry particular for interconnection and wire bonding. Aluminum is the forth most conductive element with resistivity of 2.65-cm, after silver of resistivity 1.6-cm, copper of resistivity of 1.7-cm, and gold of resistivity of 2.2-cm. Aluminum can be easily dry etched than the other three elements to form tiny metal interconnection lines. Both CVD and PVD processes can be used to to deposit aluminum. PVD aluminum has higher quality and lower resistivity. PVD is a more popular method in microelectronic industry. Thermal evaporation, electron beam evaporation, and plasma sputtering can be used for aluminum PVD. Magnetron sputtering deposition is the most commonly used PVD process for aluminum alloy deposition in advanced fabrication. Aluminum CVD normally is a thermal CVD process wth an aluminum organic compound such as dimethylaluminum hydride DMAH Al((CH3)2H) with aluminum as the precursor. Aluminum interdiffuses
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