DISCO Technical Review Feb. 2016

Wafer ESD in dicing saws and the effect of the countermeasures
Engineering R&D Division, Dicer Engineering Department
Abstract
There have been some reports of ESD (electrostatic discharge) occurring when transferring the workpiece
due to static electricity generated when processing and cleaning the workpiece in the dicing saw. In
particular, extremely large amounts of electrostatic charge have been observed in the cleaning and drying
spinners. This review explains the effects of the transferring speed control specification, which is recently
being used with dicing saws in addition to CO2 injectors and ionizers as a countermeasure for ESD.

1. Introduction
In recent years, owing to scaling in the
manufacturing process, electrostatic discharge(ESD) failure in semiconductor devices is being
reported more frequently.
Device manufacturers have also been specifying
more strict control standards for static electricity
every year. In some cases, the amount ofelectrostatic charge is required to be below
several hundreds of volts at a constant level.
Therefore,takingproperelectrostatic
countermeasures in a dicing saw (hereafter,"dicer") is essential.

2.1
Electrostatic countermeasures
The following three measures are mainly takenin the dicer:(1) Installing the CO2 injector(To reduce the resistivity of deionized water)(2) Installing the ionizer(To remove static electricity charged onworkpieces)(3) Suppressing the raising speed of the transferarm*(To reduce changes in capacitance)

2. Static electricity in the dicer
In the dicer, static electricity is mainly generated
by frictional charging, spray charging, orseparation charging.
A workpiece is processed and cleaned in thedicer while it is vacuumed on a chuck table(hereafter, "C/T") or a spinner table (hereinafter,"S/T"). This causes frictional electrification.
Fig. 1 Effect of the CO2 injector and ionizer(under each condition)

Spray electrification also occurs as deionizedwater is sprayed during processing and cleaning.
The workpiece has been charged by processing
and cleaning, and when it is lifted up from eachtable, static electricity appears on the surface,which causes separation charging.
Electric charge is generated mostly at the spinnersection in the dicer. This is a result of frictionalcharging owing to cleaning with deionized waterand drying with dry air, and separation charging
when the workpiece is lifted up from the S/T.
Hereafter, the electrostatic countermeasures andtheir effectiveness will be presented.

Fig. 2 Effect of the CO2 injector and ionizer(time series)

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DISCO Technical Review Feb. 2016

Countermeasures (1) and (2) can be installed as
optional accessories. As shown in Fig. 1 and Fig.
2, the peak voltage can be suppressed toapproximately 6 kV by using both the CO2
injector and ionizer when approximately 18 kV ischarged in the standard specification. Since theeffects of suppressing the charging of both
optional accessories are well known, theverification results of Countermeasure (3) will beexplained in this review.
2.2
Parallel-plate capacitor model
The relationship between a lower speed to raise
the transfer arm and suppression of the charge
amount will be explained using a parallel-plate
capacitor model.

Because of this, if a charge Q can be removed
with the ionizer while the value of this distance dis small, the peak of the charge voltage V can besuppressed at a low level, thereby being able toprevent an ESD failure.
3. Verification results
3.1
Measurement method
When a workpiece is in contact with the table,
an electrically neutralized state is generated on thecontact surface so the voltage on the workpiecesurface is 0 V; therefore, static electricity cannot
be measured. The static electricity can bemeasured only after the workpiece is removed
from the table.

First, the charge amount (V) is expressed withthe following equation:

Q(Equation 1)
V =C
where V is the electrical potential [V], Q is thecharge amount [C], and C is the capacitance [F].

Fig.4 Schematic view of separation charging.

Because of this, an electrical sensor is installedin the transfer arm so that the charge amount canbe measured at the moment when the workpieceis removed from the table (Figs. 4 and 5).

Next, the capacitance is expressed with thefollowing equation from Fig. 3:

Fig. 5 When the sensor is installed
Fig. 3 Parallel plate capacitor model

C=S
ε 0εr (Equation 2)dwhere S is the area [m2], d is the distance [m], ε0is the permittivity in a vacuum [F/m], and εr is therelative permittivity.
From Equation 2, Equation 1 is expressed asshown below:

V=dε 0εr・Ｑ
S(Equation 3)

The following measurement devices are used:
ZJ-SD100 (OMRON)ZJ-SDA11 (OMRON)
3.2
Measurement results
When installing the ionizer and lowering thetransfer arm's vertical axis speed:
Fig. 6 shows the peak voltage measurement
results of the separation charging at the speed of
the transfer arm which picks up the workpiecefrom the S/T (hereafter, "LV axis speed").

In the dicer, since variations of the workpiecearea S and relative permittivity ε are small, thecharge amount V depends on the distance d
between the workpiece and table.

2DISCO Technical Review Feb. 2016

Fig. 6 Spinner section separation charging(depending on the LV axis speed)

When a workpiece is picked up from the S/T at
an LV axis speed of 300 mm/s in the standard
specification, approximately 6 kV or more ofpeak voltage of separation charging is measured(Fig. 6).
When the LV axis speed is decreased, the peakvoltage of the separation charging drops. Whenthe speed is 5 mm/s or less, the peak voltage canbe suppressed to 1 kV or less.
The standby time and position settings for staticelimination were not added to the ionizer
operation in this verification. If this function isused at the same time, the peak voltage of theseparation charging can be further suppressed.
4. Conclusion
A high level of charge, which may cause an ESD
failure at the time of the workpiece transfer, wasmeasured inside the dicer, especially at thespinner section for cleaning and drying.
The measurement results verified that the chargeamount can be suppressed with the use of the CO2
injector and ionizer. In addition, lowering the LV
axis speed is effective as an ESD countermeasure.
*This countermeasure can be introduced into theDFD6341/6362/6560.

References[1] M. Sato, The Institute of Electrostatics Japan,Seidenki Handbook [Static Electricity
Handbook], Ohmsha, Ltd.
[2] K. Asano, T. Fujjita, T. Imadaka, and D.
Kobayashi, Keyence Corporation, Jitsumude
Tsukau Seidenki Taisakuno Riron to Jissen[Theory and Practice of Electrostatic
Countermeasures to be Used in Business],
Japan Industrial Publishing Co., Ltd.

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