edfas.org ELECTRONIC DEVICE FAILURE ANALYSIS | VOLUME 25 NO. 2 6 fuel efficiency of such a thruster is high. The ion source works by first producing a plasma, usually of chemically inert Ar atoms. This can be done through DC voltages or, more commonly for delayering applications, AC operating at radio frequency, 2 MHz or 13.56 MHz (Fig. 3). With the Ar atoms ionized to become charged Ar+ ions, they can be accelerated by a simple electric field. In practice, the acceleration is given by a circular plate with periodic holes (a grid). Due to the shape of the electric fields around the holes, over 95% of the accelerated ions pass through. A second plate, just behind the first, is grounded. This prevents the ions from being attracted back to the source. In such a gridded ion source the ion energy and the total beam current are decoupled, as the ion energy depends on the voltage applied to the grid while the current depends on the plasma density—via the gas density and RF power. A typical source runs at 200 to 1500 W RF power and 100 to 1500 V on the grids, accelerating the atoms to slightly less than 100 to 1500 eV. For perspective, an average atom at room temperature has 0.025 eV of kinetic energy. Gridded ion sources can be used for sputtering and thin film deposition by directing the ion beam at a target. Ion beam sputtering is preferred for some applications because, as the sputtering is a mainly physical rather than a thermal or chemical process, high temperature, refractory, or alloy material can be deposited. Also, the source and sputter plume are stable over tens of hours and thus it is preferred for the highest precision optical filters such as 100 GHz and 50 GHz DWDM filters for telecommunications. The ability to deposit high density alloys makes it preferred for data storage. In etch applications, where the beam is directed at the substrate, the ion beam can be uniform to a few percent over 300 mm and can etch any material. The same benefits above aid in IC delayering. As a physical process the etch selectivity is relatively low. Between hard and soft materials or different elements the etch rate may vary by a factor of two, compared to several orders of magnitude for chemical etch. ICs commonly contain Ta or W layers which the ion beam has no trouble with. The selectivity can be further tuned or reduced by changing the beam parameters (voltages), incidence angles, and by adding reactive gases to the plasma. The beam is broad, 5 to 20 cm in diameter, to improve the etch uniformity over the chip. The beam current density is greatest in the center and reduced at the edges, so to meet the uniformity requirements much of the beam is discarded and only a single or a few ICs are etched at a time, placed near the center of the ion beam. To achieve near zero selectivity on a complex, patterned IC consisting of many materials, some process development is required. Fortunately, for a typical IC metallization layer consisting of simply a metal and an insulator, a single parameter is all that is required to match the etch rates of the two materials. In practice, the process must be tuned for different chip architectures and different Fig. 3 A schematic of an ion source. Fig. 4 An IC delayering tool available from Denton Vacuum. (a) Shows an overview of the system consisting of a single wafer load lock, the sloped chamber in the center, and a process chamber. (b) Shows the inside of the process chamber. The ion source is to the left and the substrate is on a tilt and rotation stage in the center, with shutters currently closed. (c) Shows a better view of the source on the left, showing the grids themselves (grids from KRI). (a) (b) (c)
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