Paper presented at Microscopy and Microanalysis 2001, Long Beach, CA August 2001

*XEI Scientific, 1755 Bayshore Road, Redwood City, CA 94063
**Charles Evans and Associates, 810 Kifer Rd. Sunnyvale, CA 94086

The EVACTRON is a new device (US Patent 6,105,589) designed to remove hydrocarbons from SEM specimens and SEM chambers to prevent contamination artifacts. The device uses a low-powered RF plasma to make oxygen radicals from air. These radicals oxidize hydrocarbons to make CO, H2O, and CO2 that can be removed by the vacuum pump. The use of air as an oxygen source is convenient to the SEM operator but limits the cleaning effectiveness of the system to easily oxidized carbon species. The system does remove high percentages of the usual contamination problems of vacuum pump oil and skin oil hydrocarbons from surfaces in a relatively short time.

The EVACTRON SEM-CLEAN™ system provides a safe method for plasma cleaning inside the SEM. Air as a diluted oxygen gas mixture is chosen as the plasma reactant gas. Air is passed through a low-power RF glow-discharge to create oxygen radicals inside a generator mounted on a specimen chamber port. The generator chamber is subject to the same vacuum as the specimen chamber, and the vacuum pressure and gas flow are controlled by the EVACTRON SEM-CLEAN controller. The radicals are carried out of the plasma into the whole of the specimen chamber by convection. In the chamber they react with all exposed surfaces including the specimen if present. The plasma itself is confined to the generator chamber, which prevents ion and electron bombardment damage to the instrument or specimen.

EVACTRON cleaning using air requires that the RF plasma be operated at low temperature to produce sufficient Oxygen radicals. At higher plasma temperatures Nitrogen Ion production becomes significant and lead to the destruction of the O radicals to produce NO+ ions. The NO+ ion is a low energy species that is stable and has no cleaning ability. By adjusting the operating pressure and plasma temperature the oxygen radical flux is maximized.

  When placed and used on a SEM Chamber, the EVACTRON SEM-CLEAN system can significantly reduce contamination deposits. When used consistently the EVACTRON removes residual hydrocarbons from the chamber to prevent cross-contamination of the specimen. The Specimen may then be cleaned as needed by the EVACTRON inside the SEM to stop scan deposit buildup. Typical cleaning results are shown in Figure 1.

FIG. 1. TWO LOCATIONS OF A SILICON "GRASS" SAMPLE IRRADIATED FOR 10 MINUTES BEFORE (LEFT) AND AFTER (RIGHT) THE USE OF EVACTRON ANTI-CONTAMINATION DEVICE. 50 000X . ANDRAS VLADAR -NIST 1

To test the thoroughness of EVACTRON cleaning the system was placed on the Specimen exchange chamber of an ESCA system equipped with a sputter etch gun which allowed the study of the effects of the EVACTRON cleaning on completely virgin surfaces. Air was used as the oxygen source . The removal of hydrocarbons was tested on various surfaces. The adventitious carbon is usually a mixture of primary hydrocarbon (C-C/C-H) and lesser amounts of oxidized carbon bonding states (C-O, O=C-O). All experiments on these hydrocarbons resulted in partially lowed levels of overall atomic carbon atomic % to a maximum of 75%. The remaining carbon on the surface was oxidized to the extent that majority of the C species left was at a binding energy consistent with O=C-O bonding. Increasing the exposure time and changing the RF power was unsuccessful in removing all of the carbon when using air. On all surfaces analyzed except for sputter cleaning Al and Ti foils, two and three minute exposures to the plasma did not appear to effect the metal/oxide ratios.

Oxidation Studies Using ESCA

Evactron SEM-CLEAN system mounted on Airlock of ESCA system. ESCA system is equipped with a sputter ecth gun to expose fresh surfaces.

Table 1. Atomic % Concentrations of Elements
Carbon Results: The removal of hydrocarbons was tested on Electropolished Stainless Steel Samples (EPSS)

Major points:

1) C levels reduced by ~75%

2) Remaining C is highly oxidized (See Figure 2)

Figure 2

3) Cr shows some evidence of further oxidation after the full 8 minute exposure (see Figure 3).

Figure 3

4) Fe may also show increased oxide/metal after full exposure (Figure 4).

Figure 4

5) The adventitious carbon is usually a mixture of primary hydrocarbon (C-C/C-H) and lesser amounts of oxidized carbon bonding states (C-O, O=C-O).

6) After Evactron treatment the remaining carbon on the surface was oxidized to the extent that majority of the C species left was at a binding energy consistent with O=C-O bonding. Increasing the exposure time and changing the RF power was unsuccessful in removing all of the carbon when using air.

These results show that short exposures of specimens to the Evactron plasma cleaning inside an SEM will remove much of the Hydrocarbons responsible for contamination without significant oxidation of other surfaces and metals. The remaining carbon is highly oxidized and resistant to further reaction. Metals with a native oxide layer show little or no increase in oxidation making the process safe for most SEM applications. Other experiments have shown that 25% Oxygen in Argon is a much more effective cleaning gas that air for total removal of carbon. The EVACTRON device provides the analyst a new tool for removal of hydrocarbon contamination of any source from the SEM chamber and specimen.

References:

1. Andras Vladar, et al. Active Monitoring and Control of Electron Beam Induced Contamination. SPIE Microlithography Conference, ML 4344-015, Santa Clara, CA. February 2001.