Inductively coupled plasma (ICP) sources are being developed as reactors for high plasma density (1011–1012 cm−3), low-pressure (<10–20 mTorr) etching of semiconductors and metals for microelectronics fabrication. Transformer coupled plasmas (TCPs) are one variant of ICP etching tools which use a flat spiral coil having a rectangular cross section powered at radio frequencies (rf) to produce a dense plasma in a cylindrical plasma chamber. Capacitive rf biasing of the substrate may also be used to independently control ion energies incident on the wafer. The uniformity of generating the plasma and the uniformity of the flux of reactants to the substrate are functions of the geometry and placement of the coil; and of the materials used in the construction of the chamber. In this article, we use results from a two-dimensional model to investigate design issues in TCPs for etching. We parametrize the number of turns and locations of the coil; and material properties of the reactor. We find that at low pressure, designs which produce ionization predominantly at larger radii near the edge of the wafer produce more uniform ion fluxes to the substrate. This results from a ‘‘converging’’ ion flux which compensates for losses to lateral surfaces. Careful attention must be paid to metal structures in the vicinity of the coils which restrict the azimuthal electrical field. This situation results in reduced power deposition at large radii, which can be compensated by over sizing the coil or by using auxiliary solenoidal coils. The plasma and neutral transport, dominated by diffusion, treats the advective flow from the gas inlets and pump port as local sources and sinks which are rapidly volume averaged.
Inductively coupled plasma ͑ICP͒ etching reactors are rapidly becoming the tool of choice for low gas pressure, high plasma density etching of semiconductor materials. Due to their symmetry of excitation, these devices tend to have quite uniform etch rates across the wafer. However, side to side and azimuthal variations in these rates have been observed, and have been attributed to various asymmetries in pumping, reactor structure and coil properties. In this article, a three-dimensional computer model for an ICP etching reactor is reported whose purposes is to investigate these asymmetries. The model system is an ICP reactor powered at 13.56 MHz having flat coils of nested annuli powering Ar/N 2 and Cl 2 plasmas over a 20-cm diam wafer. For demonstration purposes, asymmetries were built into the reactor geometry which include a wafer-load lock bay, wafer clamps, electrical feeds to the coil, and specifics of the coil design. Comparisons are made between computed and experimentally measured ion densities and poly-silicon etch rates in Cl 2 plasmas. We find that the electrical transmission line properties of the coil have a large influence on the uniformity of plasma generation and ion fluxes to the wafer.
In high plasma density ͑[e]Ͼ10 11-10 12 cm Ϫ3 ͒ reactors for materials processing, the sheath thickness is often Ͻ100 s m while the reactor dimensions are 10 s cm. Resolving the sheath in computer models of these devices using reasonable grid resolution is therefore problematic. If the sheath is not resolved, the plasma potential and stochastic electron heating produced by the substrate bias may not be well represented. In this article, we describe a semianalytic model for radio frequency ͑rf͒ biased sheaths which has been integrated into a two-dimensional model for plasma etching reactors. The basis of the sheath model is to track the charging and discharging of the sheath in time, and use a one-dimensional analytical model to obtain the instantaneous sheath voltage drop based on the sheath charge and the plasma conditions at the sheath edge. Results from the integrated model for an inductively coupled plasma etching reactor with powers of 200-800 W and rf bias powers from 50 to 400 W in Ar and Ar/Cl 2 will be discussed. We found that the sheath voltage wave form remains nearly sinusoidal, and that the plasma density, and consequently the ion flux to the surface, scale primarily with inductively coupled power.
Above wafer topography of the substrate, such as wafer clamps, is known to impact adjacent feature profiles during plasma etching of microelectronic devices. The consequences of subwafer topography, such as electrostatic chucks and cooling channels, on feature profiles is less well characterized. To investigate these issues we have developed and integrated a plasma equipment model and a Monte Carlo feature profile model, and applied the integrated model to investigate polysilicon etching in an inductively coupled plasma reactor. We find that, when using low conductivity wafers, subwafer topography reduces the sheath potentials above the wafer which results in lower ion energies incident on the wafer. Etch rates sensitive to ion power are therefore also reduced. Due to the perturbation of the presheath and sheath, subwafer topography can also affect the angular distribution of the ion flux incident on the wafer which then results in asymmetric etch profiles. Superwafer structures perturb both the magnitude and angular distribution of the ion flux due to shadowing at the edge of the wafer. This leads to lower etch rates and asymmetric etch profiles. Inhibitor fluxes can be used to control the etch profile shape but only at relatively low magnitudes of those fluxes.
The filling of deep vias and trenches with metal for interconnect layers in microelectronic devices requires anisotropic deposition techniques to avoid formation of voids. Ionized metal physical vapor deposition ͑IMPVD͒ is a process which is being developed to address this need. In IMPVD, a magnetron sputter deposition source is augmented with a secondary plasma source with the goal of ionizing a large fraction of the metal atoms. Application of a bias to the substrate results in an anisotropic flux of metal ions for deposition. The ion flux also contributes to ''sputter back'' of metal deposits on the lip of the via which could lead to void formation. In this article, we describe and present results from a two-dimensional plasma model for IMPVD using a dc magnetron and an inductively coupled auxiliary ionization source. The scaling of copper IMPVD is discussed as a function of buffer gas pressure, sputter source, and source geometry. We show that the deposition rate of metal on the substrate will be reduced as pressure increases due to the increase in diffusive losses. We also show that the sputtering of the auxiliary coils can be a significant issue in IMPVD systems, which must be addressed in tool design.
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