A UHF-ECR Plasma Etcher for Insulation Films
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Hitachi Review Vol. 49 (2000), No. 4 211 A UHF-ECR Plasma Etcher for Insulation Films Hironobu Kawahara OVERVIEW: In the rapid evolution of VLSI technology toward smaller Takafumi Tokunaga geometries and increased levels of chip integration, minimum features of µ µ Masayuki Kojima 0.18 m are already being mass produced, and feature sizes of 0.13 m are being developed. At the same time, 300-mm fab lines are being built, and Ken’etsu Yokogawa this is driving a demand for new etching technologies that support smaller tolerances and larger wafer sizes. Insulation layer etching demands good selectivity between mask and substrate and high-aspect vertical processing to deal with increasing aspect ratios and thinner mask thicknesses that go along with shrinking geometries and increasing integration levels. Meanwhile, damascene processes (electroplating for chip interconnects) using a combination of low-κ (low dielectric constant) insulation materials and Cu interconnects are being investigated, and etchers capable of processing these structures will be in great demand. To address these issues, we have developed an insulation layer etcher that uses UHF-ECR (ultra high frequency electron cyclotron resonance) plasma. The unique advantages of this approach are that it enables good control over CF2/F radicals, an important etchant for selectivity and high-aspect openings, and good control over the ratio of ions to CF2, which is important in achieving vertical profiles. This etcher is capable of (1) producing stable and uniform medium-to-high density plasmas over a large area at low-to-medium pressures using UHF wave (450 MHz) ECR, and (2) increasing the doubled-near-surface effect* and good radical ratio control through control over the UHF wave flat antenna and antenna bias. Taken together, these capabilities amount to a powerful plasma process tool able to process fine features below 0.13 µm. INTRODUCTION contaminants, less charging damage, and reduced DEVICE structures and material systems are also running costs, all at the same time. undergoing tremendous changes as integration Hitachi, Ltd. has sought to accommodate these densities increase and feature sizes decrease (see demands with the development of a UHF-ECR (ultra Fig.1). Besides increasing the aspect ratio of self- high frequency electron cyclotron resonance) plasma aligned contact (SAC) and high aspect ratio contact processing tool designated U-622. This article will give (HARC) processes, low dielectric constant materials an overview of the U-622, highlighting the system’s such as SiOF and SiOC and organic films are being primary features, performance, and a number of investigated for interlayer insulation films1). At the process application examples. same time, a damascene process for Cu on-chip metallization is starting to become viable 2), and this KEY FEATURES OF THE UHF-ECR PLASMA is fueling demand for more sophisticated plasma PROCESSING TOOL processing technologies for etching that can readily Insulation layer etching proceeds various radicals adapt to diverse processes. dissociated from fluorocarbon gas coupled with ion- Meanwhile, the scale-up of wafer size to 300-mm diameter not only requires better uniformity across a *Doubled-near-surface effect: The rebombardament of back-to-back wafers larger area, it also mandates lower levels of due to diffusion by products of surface reactions. A UHF-ECR Plasma Etcher for Insulation Films 212 Design rules of semiconductor device development phases 0.13- 0.085- 1.0 µm 0.8 µm 0.5 µm 0.35 µm 0.25 µm 0.18 µm 0.1 µm 0.065 µm M-700 series M-600 series 200-300-mm 150-200-mm UHF-ECR metal, Si multi-chamber metal, Si multi-chamber etching system system system M-300 series 125-200-mm M-500 series 0.13 µm metal, Si etching 150-200-mm metal, Si etching M-200 series 100-150-mm 0.18 - 0.13 µm Si etching 0.35 - 0.25 - 0.18 - 0.13 µm 0.35 - 0.25 - 0.18 - 0.13 µm Chamber of UHF-ECR etching 1.2 - 0.8 - 0.5 µm system U-622 1980 1985 1990 1995 2000 2005 Year UHF-ECR: ultra high frequency electron cyclotron resonance Fig. 1—Road Map Depicting Design Rules and Dry Etching Systems for Different Semiconductor Device Development Phases. UHF-ECR etcher proposed by Hitachi, Ltd. as a powerful plasma process tool capable of processing fine features below 0.13 µm. assisted etching reactions. This means that to achieve Control of the ion ratio to CF2 is also important, for greater etch accuracy, it is essential to impose greater this affects the etching profile. control over dissociation species bombarding the By achieving better control over these parameters, wafer. Among these dissociated species, controlling our UHF-ECR etcher is capable of etching 0.13-µm the ratio of CF2/F radicals affects the selectivity of the and smaller features. Fig. 2 shows a schematic mask and the substrate and high-aspect openness. representation of the etching chamber for insulating layers. Here we will highlight two main features of the UHF-ECR system: UHF Antenna Antenna bias (1) The ECR plasma is formed using 450-MHz UHF Magnetic coil Semi-narrow gap reactor waves. This enables the formation of a medium-to- high density plasma at low-to-medium gas pressure that is required for fine feature processing. Fig. 3 shows measured plasma densities as a function of UHF power at various gas pressures. The figure reveals that densities of 1011 to 1012/cm3 are stably and continuously generated when pressures are held at 0.5 to 3 Pa. In addition, the current density uniformity of ions bombarding the wafer is also less than ±10% across Processing chamber Wafer stage wall the 300-mm wafer (see Fig. 4). RF bias (2) The UHF waves are introduced to the chamber RF: radio frequency using a flat antenna structure. There is a gap of 30 to 100 mm between the flat antenna and the wafer, and Fig. 2—Schematic Representation of Insulating Layer UHF- ECR Etching Chamber. in this semi-narrow gap, the number of collisions Stable, uniform plasma is produced in a semi-narrow gap by between gas molecules and electrons is reduced due UHF waves from a flat antenna coupled with magnetic field. to limited plasma volume, and F atom radicals are Hitachi Review Vol. 49 (2000), No. 4 213 10 Ar 2.7 Pa ) 3 8 2.0 Pa /cm 11 6 1.3 Pa 10 × ECR plasma zone Ions generated 4 0.7 Pa CF2 generated 2 Density ( Diffusion plasma zone F generated 0 0 200 400 600 800 1,000 2,000 UHF power (W) Fig. 3—Measured Plasma Density as a Function of UHF Fig. 5—UHF-ECR Plasma Structure. Power. UHF-ECR plasma is generated under the flat antenna, and Medium-to-high density plasma is stably produced at low-to- diffused over the area below that. The ratio of F atom radicals medium pressure. to CF2 can be controlled by controlling the size of the ECR plasma area and the diffused area. 15 ETCHING PROCESS EXAMPLES ) 3 High-aspect-ratio Hole Processing 10 UHF: 1,500 W 2 In hole processing, balance must be achieved 300 mm: 3.5 mA/cm ±9.3% among CF2, C and F radicals dissociated from 5 ICF (mA/m fluorocarbon gas, and a number of parameters must 0 be improved: high-aspect apertures, vertical profiles, -150 -100 -50 0 50 100 150 and selectivity to masks4). Distance from the center (mm) In the U-622 etcher, the hole aperture and ICF: icon current flux selectivity of mask and substrate is reconciled by : X axis : Y axis reducing the gap and controlling the antenna bias. µ Fig. 4—Measured Current Density Distribution of Ions Processing examples of a 0.1- m hole with an aspect Bombarding Wafer. greater than 20 and a self-aligned contact (SAC) are A uniform plasma is produced over a 300-mm area using the shown in Figs. 6 (a) and (b), respectively. UHF flat antenna structure. Low-κ Material Damascene Processing A number of different materials are being evaluated controlled by excessive dissociation of chemical for use as a low-κ (low dielectric constant) insulation species. What is more, the efficiency of conversion to material including SiOF, SiOC, and organic films. active species is increased by the doubled-near-surface There is thus a ready demand for an etcher that can effect due to rebombardament by products of surface support etching processes tailored to these different reactions between the biased wafer and the biased flat materials. antenna3). This configuration results in an ECR plasma One of the virtues of the UHF-ECR system is that zone beneath the flat antenna of about 30 mm and it is capable of producing stable gas pressures and diffusion plasma zone under the ECR plasma zone. plasma densities over a wide range, so it can be readily The ECR plasma produces ions and relatively high adapted to low-κ damascene process applications. concentrations of CF2 radicals. The diffusion plasma In etching damascene structures, the selectivity of zone produces relatively high concentrations of F the mask and stopper layer and precise critical radicals (see Fig. 5). dimension control are even more exacting; for this Using the gap and antenna bias to independently etching, a good vertical profile and mask/stopper layer control the plasma source and optimizing the ratio of selectivity is obtained by slightly widening the gap CF2 to F radicals, wide process applications — holes, between the antenna and wafer. masks, damascene trenches, vias, and so on — can be Fig. 7 shows SiOF layer damascene structure accommodated. processing examples. Mask losses are minimized with a taper angle of 87.5 degrees.