Applied IMO offers ionized sputtering
11/01/1996
Applied IMP offers ionized sputtering
Applied Materials has unveiled an Ion Metal Plasma (IMP) process that utilizes small hardware upgrades to its standard physical vapor deposition (PVD) chamber to achieve high-quality barrier/glue layer step coverage in high aspect ratio 0.25-micron contacts and vias (Fig. 1). The process relies on a medium density plasma Vectra source to ionize most of the metal atoms leaving the sputtering target. Electrostatic coupling then directs the charged ions to the wafer surface.
The target application for the IMP process is sub-half-micron metallization, where increasingly high aspect ratios are driving the development of new deposition technologies. Plain PVD is isotropic, and works fairly well for barrier layer deposition when gaps are not too tight, and vias are not too deep. Current extensions of PVD, long-throw and collimated, have improved directionality but sidewall film thickness and integrity are unresolved issues. Chemical vapor deposition (CVD) has been developed as a PVD replacement for these films, but costs are higher and the process may be more difficult to control. The IMP process may be the best of all worlds.
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Figure 1. An 8:1 aspect ratio hole filled using Applied Materials` IMP ionized sputtering process.
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Figure 2. Schematic diagram of an IMP system
IMP reportedly achieves sidewall coverage that is equivalent to CVD, with only slightly greater cost and complexity than standard PVD. Bottom coverage is 60-80% that of the field for 85? side-wall angles. Since IMP is more efficient at directing film deposition inside deep gaps, far less material is deposited on the field to achieve the same thickness inside a gap. As a result, throughputs are higher, and target life is longer so there should be less downtime.
Structures with 6:1 aspect ratios and 0.25-micron top gaps can be coated with titanium (Ti) and titanium nitride (TiN), resulting in 70% bottom coverage, 10% sidewall coverage, and excellent sidewall integrity. Preliminary tests show that there is a relatively linear relationship between gap opening and bottom coverage. With an 85? sidewall angle, a 0.17-micron gap will result in 68% bottom thickness. Extrapolating these results leads to a predicted 60% bottom thickness for an 85? sidewall and a 0.06-micron gap.
The process occurs at 20-40 mtorr pressure so that the reduced mean-free-path results in more collisions. The kinetic energy from the target is thus converted into thermal energy to assist in ionization. The RF source then creates the 5?1011 to 1?1012 ions/cm3 ion cloud (Fig. 2), which extends to the wafer surface and into the gaps. Applied began work on the process approximately one year ago, after abandoning earlier efforts with more complex high density plasma sources.
For as yet unknown reasons, the deposited TiN layer exhibits different properties from standard PVD TiN. Jaim Nulman, Applied Materials PVD managing director, stated that TEM studies of sidewall film material structure reveal no grain boundaries; the film is allegedly pseudo-amorphous. This crystallographic structure combined with the improved side-wall coverage results in the ability to flow aluminum into 0.25-micron plugs below 380?C, while standard PVD TiN requires over 450?C to achieve the same results.
The company is offering IMP as an upgrade to its Endura and Centura PVD chambers. A source assembly (including the target, magnets, RF feedthrough and matching network), RF source, and system software are all field retrofittable in approximately one week. Applied is also looking to extend the technology to other layers such as liners for copper. - E.K.