P.B. Ghate

Application of Ti: W barrier metallization for integrated circuits

Abstract Aluminum metallization is the most widely used for contacts and interconnections in integrated circuits. However, the solid state diffusion of aluminum in silicon during contact sintering or high temperature packaging can result in junction shorting or leakage in shallow ( 2 / ti:W/Al film test samples have shown that the resistivity of aluminum increases owing to diffusion of titanium or tungsten into the aluminum. However, the kinetic data show that no more than a 10% increase in the resistivity of the aluminum can be expected in the useful life of a device. High current stress data show that Ti:W/Al interconnections are comparable with those of aluminum films. Auger depth profiling of si/Ti:W/Al samples annealed at 450, 500 and 550°C in N 2 shows no aluminum at the Si-(Ti:W) interface. Application of the PtSi/Ti:W/Al metallization system for large-scale integrated circuits is described.

Improved metallization for surface acoustic wave devices

Surface acoustic wave (SAW) resonators operating at frequencies in the range 100–500 MHz employ lightly coupled high Q (10 000–20 000) acoustical cavities. Resonators fabricated with pure aluminum transducers showed the formation of dendrite-like growths and a degraded electrical response after a relatively short operating time at high power levels. The similarity to thermally induced metal migration led to identifying the problem as stress-induced material migration. Resonator devices fabricated with 2% copper-doped aluminum films showed up to 65 times greater lifetime than the devices fabricated with pure aluminum. The unique amplitude and frequency conditions present in SAW resonators make them useful for evaluating films for their resistance to stress migration.

Metallization in microelectronics

Abstract Continued improvements in the design and fabrication of semiconductor devices have led to the introduction of medium and large scale integration of microcircuits. These advances have placed severe demands on metallization techniques, requiring metal contacts to shallow junction devices and definition of metal interconnections that are closely spaced and of a narrow pattern. Aluminum is the most widely used metal in microelectronics for providing the necessary contacts and interconnections. The contact resistance of the SiAl interface is sensitive to surface preparation, the vacuum ambience of the Al film deposition and contact sintering. Controlled contaminations, such as H2O, CO and O2, have been introduced during the Al film deposition in order to determine their effects on Al/Si contacts. Extensive studies have shown the interdependence of the film deposition parameters, the physical properties of the Al films and the dominant failure mechanisms, such as electromigration, that limit the reliability of integrated circuit interconnections. For high speed very shallow emitter-base junction devices, Al in silicon contact windows can penetrate to the junction during sintering and can cause leakage. Solutions to this problem include the use of Al + Si films, PtSi contacts with a barrier layer (e.g. Ti:W) and Al or Au conductors. Metallization must be suited to all the device processing steps beginning with contact sintering, device packaging, testing and also the operating conditions. Highly complex large scale integrated circuits require multilevel interconnections. A two-level metallization scheme (Al/insulator/Al) and its application are presented. Future trends in metallization processing are also discussed.

ELECTROMIGRATION‐INDUCED FAILURES IN ALUMINUM FILM CONDUCTORS

Electromigration‐induced failures in aluminum film conductors have been studied. It is shown that large‐grain films have a longer mean time to failure (MTF) than the small‐grain films. A further increase in MTF is observed for films with a glass overcoating. This increase can be understood in terms of the observed grain growth which results from the glass overcoating process. Histograms showing the location of failures are interpreted to indicate that effects due to both temperature gradients and microstructural inhomogeneities are important.

Metallization for very-large-scale integrated circuits☆

Abstract Progress in patterning technologies and computer-aided circuit designs have brought us to the threshold of very-large-scale integrated (VLSI) circuits with 100 000 or more devices to be integrated on a silicon chip. In this paper we review thin film applications in the fabrication of contacts and interconnects for VLSI circuits. Device structures suitable for both bipolar and metal/oxide/semiconductor (MOS) VLSI circuit applications tend to have shallow junction depths and contact areas (silicon-metal interfaces) in the 0.2–0.5 μm and 1–2 μm 2 ranges respectively; also some of the circuits require Schottky barrier diodes. Consumption of silicon in the contact windows needs to be minimized with the use of silicide layers for silicon-metal contacts. The formation and use of platinum silicide layers for bipolar applications are reviewed. Our observations indicate that the carbon and oxygen present in Czochralski-grown silicon crystals interfere in platinum silicide formation and affect the electrical characteristics of the contacts. The use of barrier layers in VLSI metallization is illustrated. The interdependence of film microstructure, electromigration-induced failures and VLSI interconnection reliability is examined. The integration of a large number of components on a VLSI chip with a single level of interconnections consumes more chip area. Long interconnection paths adversely affect circuit performance. Multilevel interconnections (conductor/insulator/ conductor) offer an attractive solution to increase the packing density and circuit performance. The application of PtSi/(Ti:W)/(AlCu)/SiO 2 /(Ti:W)/Al film layers in the fabrication of a bipolar VLSI circuit with a minimum feature size of 1.25 μm is illustrated. As the complexity of VLSI circuits continues to grow with micron size device structures, three or more levels of interconnections compatible with shallow junctions on the substrates and complex packaging technologies are required. Areas of concern and desirable features in VLSI metallization are summarized.

SOME OBSERVATIONS ON THE ELECTROMIGRATION IN ALUMINUM FILMS

Thin‐film aluminum resistors with thermally grown SiO2 as the substrate have been subjected to very high current densities of the order 0.5 to 2 × 106 A/cm2. The temperature of the resistor is estimated to be (185 ± 15)°C at high current densities. It is observed that an opening occurs in most of these resistors close to the cathode. Experimental evidence is presented to show that electromigration leads to the observed failures in these thin‐film aluminum resistors.

Electromigration‐Induced Failures in, and Microstructure and Resistivity of, Sputtered Gold Films

Sputtered gold film conductors have been subjected to high current densities in the range of 2.0–3.5×106 A/cm2 in 150 °C air ambience, and data on the mean time to failure (MTF) vs current density J are presented. In this study, a corrosion‐resistant Ti: W–Au metallization system is used. The microstructure and resistivity of the sputtered gold films have been examined and the observed excess resistivity of the sputtered gold films is attributed to gaseous impurities in the film. The average grain size of the sputtered films is 2000 A. The MTF‐vs‐J data are compared with those of aluminum film conductors and an activation energy of 0.90 eV has been estimated for electromigration in sputtered gold films.

Electromigration testing of Ti: W/Al and Ti: W/Al-Cu film conductors

Electromigration-induced failures in metal film interconnections influence the reliability of integrated circuits. For shallow (< 1 μm) junction devices a barrier- metal interconnection system such as Ti: W/Al has been proposed to eliminate contact pitting due to silicon-aluminum reactions. The addition of copper to aluminum films is known to improve the electromigration resistance of aluminum film interconnections. Glass-passivated Ti: W/Al and Ti: W/Al-Cu (1.6 wt.% Cu) film conductors (9 μm wide, 1.14 mm long and 170 nm/800 nm thick) on oxidized silicon substrates were subjected to a current stress of 106 A cm-2 in the temperature range 150–270°C. Mean-time-to-failure data indicate an improvement of approximately a factor of two in electromigration resistance due to the addition of copper. This improvement is smaller than that reported by others. Life test data are consistent with activation energies of 0.61±0.05 and 0.71±0.03 eV for Ti: W/Al and Ti: W/Al-Cu film conductors respectively. Extrapolated mean times to failure are close to 24 and 100 a for Ti: W/Al and Ti: W/Al-Cu films respectively under a current stress of 5 × 105 A cm-2 at 55°C ambience. Projected failure rates at these operating conditions increase very rapidly with time and approach values of 9 × 10-7 and 1 × 10-11 h-1 for Ti: W/Al and Ti: W/Al-Cu film conductors respectively at 100 000 h.

Aluminum alloy metallization for integrated circuits

Abstract Aluminum metallization is most widely used for contacts and interconnections in both bipolar and MOS integrated circuits. Aluminum alloy films, such as AlSi and AlCu films, were introduced to minimize the erosion of silicon from contact windows and to improve the electromigration resistance of interconnections. Recently, magnetron sputter-deposited aluminum, Al2wt.%Cu and Al2wt.% Cu 1wt.%Si films were employed to study the stability and contact resistance of Si-(Al alloy film) contacts on devices with shallow junction depths of the order of 0.35μm. Test structures were used to determine the leakage currents of 100 n + /p + diodes as a function of the storage time (up to 1000 h) at 150°C, and the physical nature of the Si(Al alloy) contacts was examined using scanning electron microscopy. The compatibility of the AlCuSi metallization with the very large scale integrated requirements of interconnection and Si-metal contacts for shallow junction devices is discussed.

Magnetron-sputtered aluminum films for integrated circuit interconnections

Abstract Aluminum films for integrated circuit (IC) contacts and interconnections are routinely deposited by thermal evaporation from resistance-heated, induction-heated and electron-beam-heated sources. Sputtering has not been used until recent advances produced magnetron sputter deposition equipment capable of depositing aluminum films compatible with IC requirements. This paper describes results of a study on magnetron-sputtered aluminum films for IC interconnections. A circular planar magnetron 20.3 cm in diameter was employed for film deposition on silicon substrates 7.6 cm in diameter moving on a circular path above the cathode. Metal coverage was better than 50% for oxides steps with 70° step angles and was equivalent to or better than that observed with other deposition techniques. Aluminum films typically 1 μm thick, having resistivities within 10% of the bulk value of 2.71 μΩ cm, were routinely deposited in this system. Film conductors 12 μm wide and 1.14 mm long, patterned on oxidized silicon substrates and coated with plasma-deposited SiO2 were used for resistivity ratio RR(=ρRT/ρ4.2K) measurements; the RRs lie in the range 35–50. The microstructure of magnetron-sputtered aluminum films is similar to that of conventionally deposited films. MOS capacitor structures fabricated on p-type silicon substrates with sputter-deposited aluminum films showed C-V characteristics comparable with those obtained by using aluminum films deposited from electron-beam-heated or induction-heated sources. Magnetron sputtering appears to produce films as suitable for IC contacts and interconnection applications as any other physical vapor deposition process and offers solutions to problems generated by the requirements of advanced device technology.