Low Resistivity Metals

In addition to the construction of a multilevel interconnect network, the semiconductor industry also improves the performance of IC chips by incorporating low-resistivity metal wiring such as copper and new dielectric materials with lower k constant (see Section 1.3.1 for details). The added benefit of using low-fc dielectric materials includes a reduction in the crosstalk [29-31] and power dissipation [29-33]. The key challenge for the implementation of low-fc materials is related to their intrinsic weak mechanical properties. Furthermore, in order to achieve a k value below 2.2, practically all materials are made with pores that exacerbate mechanical stability issue [29-33]. This is a particular concern for the CMP community as the operation invariably involves mechanical stress and shear force. In addition, practically all low-fc dielectric materials are hydrophobic in nature. Lfpon exposure to moisture or wetness, the dielectric constant tends to increase. Therefore, unlike silicon-dioxide-based dielectric, the effective k constant may change after CMP. To... 

Devine, M.J. and Sander, L.F., Interatomic Spacing Concept for Solid Lubricant - Metal Systems, Amer. Chem. Soc. Divn. of Pet. Chem., 13, (1968). Przybyszewski, J.S. and Spalvins, T., Friction and Contact Resistance During Sliding in Vacuum of Low Resistivity Metals Lubricated with Sputtered Molybdenum Disulfide Films, NASA TN D-5349, (July, 1969). 

As mentioned in Chapter 1, the present state of CMP is the result of the semiconductor industry s needs to fabricate multilevel interconnections for increasingly complex, dense, and miniaturized devices and circuits. This need is related to improving the performance while adding more devices, functions, etc. to a circuit and chip. This chapter, therefore, discusses the impact of advanced metallization schemes on the performance and cost issues of the ICs. Our discussions start with the impact of reducing feature sizes on performance and the need of various schemes to counter the adverse effect of device shrinkage on the performance of interconnections. An impact of continued device shrinkage on circuit delay is discussed. Then the need of low resistivity metal, low dielectric constant ILD, and planarized surfaces is established leading to the discussion of CMP. Finally various planarization techniques are compared to show why CMP is the process that will satisfy the planarity requirements of the future. 

After the anodization process, most of the time, a stabilization annealing at low temperature (below 500 °C) is performed in an O2 or in a N2 ambient. In some cases, an oxidation annealing is performed at higher temperature. Afterwards, a thin oxide cap layer is generally deposited to seal the pores and to ensure a complete DC isolation between the substrate and the device. Various metal layers can be used for the strips. To reduce the ohmic losses, low-resistivity metals must be used... 

Table 5.9 lists typical electrical-resistivity values of several molded carbons and graphites and selected low-resistivity metals. 

Fig. 9.19. Schematic of a typical drift test . A low-resistance metal stripe is patterned, over a length L, on top of a higher-resistance stripe through which an electric current is applied. As the current shunts through the lower-electrical-resistance metal, current-induced atomic drift leads to the formation of a depleted zone in the wake of electron transport at one end of the stripe and hillocks and extrusions at the other end.

Aluminum, the most common material used for contacts, is easy to use, has low resistivity, and reduces surface Si02 to form interfacial metal-oxide bonds that promote adhesion to the substrate. However, as designs reach submicrometer dimensions, aluminum, Al, has been found to be a poor choice for metallization of contacts and via holes. Al has relatively poor step coverage, which is nonuniform layer thickness when deposited over right-angled geometric features. This leads to keyhole void formation when spaces between features are smaller than 0.7 p.m. New collimated sputtering techniques can extend the lower limit of Al use to 0.5-p.m appHcations. 

The last technique commonly employed to deposit metals for compound semiconductors is electroplating (150). This technique is usually used where very thick metal layers are desired for very low resistance interconnects or for thick wire bond pads. Another common use of this technique is in the formation of air-bridged interconnects (150), which are popular for high speed electronic and optoelectronic circuits. 

Zirconium is used as a containment material for the uranium oxide fuel pellets in nuclear power reactors (see Nuclearreactors). Zirconium is particularly usehil for this appHcation because of its ready availabiUty, good ductiUty, resistance to radiation damage, low thermal-neutron absorption cross section 18 x 10 ° ra (0.18 bams), and excellent corrosion resistance in pressurized hot water up to 350°C. Zirconium is used as an alloy strengthening agent in aluminum and magnesium, and as the burning component in flash bulbs. It is employed as a corrosion-resistant metal in the chemical process industry, and as pressure-vessel material of constmction in the ASME Boiler and Pressure Vessel Codes. 

Ships with nonmetallic hulls frequently have metallic attachments which can be cathodically protected. Here the anodes are screwed onto the timber or plastic hull and electrically connected with low resistance via the interior of the ship to the objects to be protected. The metallic foundation serves for flotation and copper bands. 

Anodes are not attached to the rudder but are situated between the rudder shaft and the ship s wall and connected via a copper strip. The propeller is protected via a slip ring on the shaft. To achieve a low-resistance contact, the divided copper or bronze ring has a rolled silver-bearing surface on which metal graphite brushes slide. The transmission voltage should be below 40 mV. 

The transition resistance between the surface of the metal and the electrolyte with uncoated iron anodes in coke backfill, the transition resistance is usually low. With metals in soil, it can be increased by films of grease, paint, rust or deposits. It contains in addition an electrochemical polarization resistance that depends on the current [see Eq. (2-35)]. 

Some physical properties of the elements are compared in Table 10,2. Germanium forms brittle, grey-white lustrous crystals with the diamond structure it is a metalloid with a similar electrical resistivity to Si at room temperature but with a substantially smaller band gap. Its mp, bp and associated enthalpy changes are also lower than for Si and this trend continues for Sn and Pb which are both very soft, low-melting metals. 

Fig. 1.69 Effect of resistivity of solution on the distribution of corrosion on the more negative metal of a bimetallic couple, (a) Solution of very low resistivity and (b) solution of very high resistivity. Note that when the resisitivity is high the effective areas of the cathodic and anodic metals are confined to the interface between the two metals...

The modern procedure to minimise corrosion losses on underground structures is to use protective coatings between the metal and soil and to apply cathodic protection to the metal structure (see Chapter 11). In this situation, soils influence the operation in a somewhat different manner than is the case with unprotected bare metal. A soil with moderately high salts content (low resistivity) is desirable for the location of the anodes. If the impressed potential is from a sacrificial metal, the effective potential and current available will depend upon soil properties such as pH, soluble salts and moisture present. When rectifiers are used as the source of the cathodic potential, soils of low electrical resistance are desirable for the location of the anode beds. A protective coating free from holidays and of uniformly high insulation value causes the electrical conducting properties of the soil to become of less significance in relation to corrosion rates (Section 15.8). 

All these alloys are characterised by high hardness values and low resistance to impact. In this they are probably more similar to stoneware than to other metals but they are superior to stoneware in thermal conductivity and in their resistance to thermal shock, which, however, is poor compared with that of other metals. Moreover, it is usually easier to make castings of silicon iron than to fabricate required parts from stoneware. 

Perhaps the first practical application of carbonaceous materials in batteries was demonstrated in 1868 by Georges Le-clanche in cells that bear his name [20]. Coarsely ground MnO, was mixed with an equal volume of retort carbon to form the positive electrode. Carbonaceous powdered materials such as acetylene black and graphite are commonly used to enhance the conductivity of electrodes in alkaline batteries. The particle morphology plays a significant role, particularly when carbon blacks are used in batteries as an electrode additive to enhance the electronic conductivity. One of the most common carbon blacks which is used as an additive to enhance the electronic conductivity of electrodes that contain metal oxides is acetylene black. A detailed discussion on the desirable properties of acetylene black in Leclanche cells is provided by Bregazzi [21], A suitable carbon for this application should have characteristics that include (i) low resistivity in the presence of the electrolyte and active electrode material, (ii) absorption and retention of a significant... 

---