Plasma etch challenges for next-generation semiconductor manufacturing

In the photolithography process, a requisite mask layout is printed into a polymer layer. This layer, in turn, is transferred onto underlying inorganic/organic material layers for the fabrication of 3D semiconductors, and for high-volume integrated-chip manufacturing. Moore’s law describes a trend, first observed in 1965, in which the dimension of patterns in these layouts shrinks every two years, doubling the number of transistors on the microchip. Optical lithography has long since reached its physical limit (i.e., printing feature sizes below 40nm), and a number of alternative printing/material deposition schemes have been evaluated for use below this limit (see Figure 1) to maintain the economy of scaling. Among these schemes, plasma etching (which transfers the printed mask layout onto underlying layers by initiating chemical reactions) is employed industrywide. Plasma is partially ionized gas (i.e., which contains gas atoms/molecules, activated radicals, and ions). The dry plasma etching process involves interactions—between radicals and the exposed surface—which lead to the removal/volatilization of the activated/modified layer via energetic ion bombardment. To optimize the etch process, the pressure, gas flow/flow ratios, radio frequency power, and substrate temperature can be modified by adjusting the appropriate tuning knobs. When one of these tuning knobs is adjusted, change is triggered in more than one of the plasma parameters (i.e., the radical flux, ion flux, ion energy, and ion energy distribution). In a continuous plasma-etch process, surface modification (activation) and energetic material removal (desorption) occur concurrently. Concurrence is problematic, however, because changing plasma parameters to improve one aspect of the printed mask transfer may degrade Figure 1. Alternative patterning schemes able to achieve feature sizes of less than 40nm: 193nm immersion lithography combined with selfaligned multiple patterning; extreme UV (EUV) lithography; and directed self-assembly (DSA). Each color represents a different material layer. SADP: Self-aligned double patterning. SAQP: Self-aligned quadruple patterning. SAOP: Self-aligned octuple patterning.1