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Countermeasure against Alternating Current (AC) Magnetic Field Interferance for Semiconductor Manufacturing Equipment

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Improvement of SEM image screen measurement accuracy by Active Magnetic Field Canceller
Observation images of SEM based equipment used in semiconductor manufacturing lines were disturbed by the noise, and causing problems with measurement accuracy. For this reason, we installed our magnetic field canceller coil inside the tool as an effective countermeasure against alternating current (AC) magnetic fields generated from inside and outside of the tool. Consequently, we were able to reduce image noise and improve measurement accuracy without changing the tool layout. In this example, although the environmental magnetic field level originally met the permissible value of the tool, the problem was probably caused by internal factors such as tool settings, and then the failure occurred.

Magnetic field impact on SEM

Scanning electron microscope (SEM) irradiates an object with focused electron beam, and observes its image by detecting secondary electrons emitted simultaneously. By placing an object (sample) in the high vacuum and scanning with electron beam, the image of each coordinate is constructed and displayed. Since electron beams are used, tools that require high resolution are susceptible to magnetic fields. In this case, constant line noises were appeared in the SEM image due to the influence of alternating current (AC) magnetic fields generated from inside and outside the tool, which was causing measurement failure.
A schematic diagram of the tool and the magnetic field canceller coil is shown in [Fig. 1].

  • Figure 1: Schematic diagram of the tool and magnetic field canceller coil (side view)

Result by the magnetic field canceller countermeasure

A set of coils was installed inside the tool (see the schematic diagram in [Fig. 1]) to take countermeasures against alternating current (AC) magnetic field.
SEM observation images before and after the countermeasure are shown in [Fig. 2], measurement results are shown in [Table 1], and spectral data are shown in [Fig. 3].
In this example, line noises were appeared in the measurement image due to the magnetic field generated by the power supply system installed inside the tool (inside the frame) and other equipment installed surrounding the tool.
As shown in [Table 1], the magnetic field environment satisfied the permissible value originally, however when the tool started operation, the noise appeared in the measurement image. This indicates that the actual magnetic field sensitivity was higher than the tool specification. (Acceptable level is much tighter.)
Thanks to the effect of active magnetic field canceller implementation, we were able to reduce the magnetic field level to 1/100 or better and improve the measurement image accuracy as shown in [Fig.2].



Figure 2: SEM observation image (before and after magnetic field canceller countermeasures)



Table 1: Magnetic field canceller effect (maximum spectrum component, see Fig. 3 graph)



Figure 3: AC magnetic field spectrum (before and after magnetic field canceller countermeasures)

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