GMR heads are currently one of the most static sensitive devices that are manufactured today. If they experience a voltage across them of 5-10 V they will draw enough current to destroy them in about 10 nsec. The situation is exacerbated by the fact that it takes very little handling to tribocharge a HSA or an HGA assembly to thousands of volts. Then, any contact with ground of the flex lines or head wiring will destroy a head and result in yield loss.
A complete electrostatic management program can make a dramatic improvement in yield losses due to electrostatic discharge (ESD). This includes a comprehensive program to ground all conductors, elimination of insulators wherever possible, specification of dissipative materials (and grounding them), and finally, the use of air ionization to bleed surface charge off of insulators and ungrounded conductors. The details of fixtures, gloves and of hand tools such as tweezers are critical elements in the electrostatic management program.
Air ionizers are widely used by the disk drive manufacturing industry today. Most use either corona discharge or natural radiation (alpha) techniques to generate air ions. Both are commonly used today and each have certain advantages. Each are discussed below.
The Corona Method
This technique involves the use of high voltage (~5-20 kV) . The voltage is applied to a set of sharp points, an intense electric field is established in the very near (~100 mm.) of the points. This field accelerates free electrons to a sufficiently high energy to allow them to ionize molecules that they collide with. When the voltage on the point is positive, positive ions are repelled into the environment and when the point is negative, negative ions are delivered. Corona ionizers are made with AC voltage and with DC voltage. Each has certain benefits that are discussed below.
The AC ionizer is by far the simplest and therefore the lowest cost to manufacture. It utilizes a step-up transformer to create the high voltage for ion generation. See Figure 1.
Figure 1. Schematic diagram of an AC-type ionizer.
Because the transformer secondary is well isolated from ground, the current drawn from the emitter point(s) during the positive and negative voltage excursions of the emitter (corresponding to positive and negative excursions of the AC power line should be equal. Experience shows that the actual offset voltageas measured with a Charge Plate Monitor (CPM) is typically less than 5 V so indeed, the AC ionizer is self regulating.
Because the AC type ionizer produces the positive and negative ions in sequence from the same emitter point(s), these ions are separated in time by half of the period of the AC power line (i.e. 1/100 or 1/120 sec.). This means that the waves of positive and negative ions are rather close to each other, making loss through recombination a large factor. AC ionizers typically utilize fast airflow velocity to minimize recombination. This is not always desirable in a cleanroom environment.
DC Ionizers use separate emitter points for the positive and negative DC HV power supplies to create the ions. The voltage is applied to separate negative and positive emitter points. In order to provide equal numbers of positive and negative ions from separate sources, DC ionizers need some form of control to maintain this balance. For the demanding requirements of electrostatic management of production lines where GMR heads are handled, the control must be active feedback to account for variations in the environment and for any wear that occurs over the life of the ionizer. A schematic diagram of a DC type ionizer is shown in figure 2. Because of the greater sophistication and control systems that are needed for DC ionizers, the DC ionizing systems are more expensive to manufacture. Owing to the fact that the positive and negative emitters are well separated from each other, recombination is a lesser effect and the DC ionizer can sometimes utilize lower airflow velocity to deliver the ions to the location where the ionization is required.
The Alpha Ionizer Method
The use of ionizing radiation to make ions is the third technique that is employed for electrostatic management. While several forms of ionizing radiation sources are available, only a sources are used for static control..The other forms of source have much longer range and thus require shielding in impractical amounts Most commonly, Po210 is employed because of its properties as an a emitter. It produces a particles with a range of only 3.8 cm. in air and 0.02 mm in aluminum so that virtually none of the a particles are emitted from the ionizing blower used at a workstation either through the air or through the chassis of the alpha source even if he handles the ionizer case extensively. Thus, alpha sources of this sort are regarded as harmless by government organizations in virtually every country. The other aspect of Po210 that makes it an ideal choice is that it decays to Pb206 which is a naturally occurring stable isotope. Unlike Po210, many other sources decay into isotopes that may be active as well.
The reason that the range of a particles is so short is that they move much slower than any other natural type of source and thus couple their energy much more efficiently to the air molecules, resulting in ionization. This makes an alpha type ionizer particularly efficient at creating ions and thus it provides fast discharge times.
The greatest advantage of an alpha ionizer is that is does not employ high voltage to create the ions and thus it does not emit electric fields, either AC or DC. Thus, a ionizers should have a balance voltage of zero, an important attribute as heads become smaller and thus more sensitive.
The disadvantages of the a ionizer systems are that personnel have a concern about sources in their work environment. This disadvantage can be overcome with education of personnel. The device also requires some government paperwork for tracking and regulation, and the replacement of the sources annually. This as not a financial issue but rather a logistical issue as the cost of replacement is comparable to the cost of maintenance required only of corona ionizers.
Balance Voltage Considerations
The objective of an ionizer is to eliminate surface charge on insulators and on isolated/ungrounded conductors. Practically speaking, the action of an ionizer is to drive the voltage on the surface of an object to be a small but non-zero value. Factors that affect this value are the fields emitted from the ionizer and the electric field of the earth.
EOS/ESD 3.1 defines a parameter called the Balance Voltage intended to be a measure of this value. It utilizes a device called the Charge Plate Monitor to measure this parameter. It employs a 150 mm. x 150 mm. plate as the sensor for the voltage. The standard specifies that it should have a capacitance of 20 pF. to ground. This capacitance drastically affects the response time of the device. It has been shown that small objects are driven to the same voltages as the CPM records but the response time of these small objects is much faster than that of the CPM plate. Thus, the HGA can experience effects due to fluctuations that would not be recorded by the CPM.
Measurements have been made by several groups of the residual ESD sensitivity to which HGA assemblies are subjected by AC and DC ionizers by recording the discharge current when an isolated HGA is discharged through a grounded current probe. This measurement is a direct measurement of the ESD threat because it directly measures a CDM (charged device model) discharge of a real product in a real situation. In this investigation, the previous work is reproduced with an increased bandwidth and with shorter probe leads which provide lower inductance and hence are expected to provide shorter and higher amplitude pulses, representing a more realistic view of actual CDM discharges of HGA structures. See Figure 3.
The measurements involved a LeCroy model LC584AL digital oscilloscope with a bandwidth of 1 GHz and a sample rate of 8 Gsamples/sec. And a Tektronix CT-1 current probe with a bandwidth of 1 GHz (composite oscilloscope/probe bandwidth of 707 MHz). A typical CDM waveform obtained by discharging the paddle card of an HGA to ground is shown in Figure 4.
Figure 4. A Typical CDM discharge of a HGA using a 1 GHz oscilloscope and a 1 GHz probe
The signal has a rise time of well under 1 nsec. (~500 psec.) with a ground lead (see Figure 3) of <2 cm. In contrast, a similar signal with a slower (500 MHz bandwidth) oscilloscope and a rather long ground lead (~1 m.) is shown in Figure 5.
Figure 5. A CDM pulse from a HGA as measured with a large series inductance
For the purposes of comparison, we were able to gain access to state-of-the-art equipment to check the validity of our setup. A CDM pulse off of an HGA was recorded with a 2 GHz bandwidth oscilloscope and a 2 GHz bandwidth current probe. The resulting waveform is shown in Figure 6.
Figure 6. CDM pulse off of an HGA captured by a 2 GHz bandwidth oscilloscope with a 2 GHz bandwidth current probe
The rise time of the pulse is 380 psec. Comparing the wave shape in figures 6 and 8 shows little difference (~20% in rise time) so that the oscilloscope/probe combination employed was reasonable for the measurements, providing accurate (~10-20%) conclusions about CDM peak currents and energy dissipation in the head.
Measurements of CDM Discharges for the Three Ionizer Types
An apparatus was designed to allow HGAs to be placed under the influence of an overhead blower. The blower was mounted 36" above a dissipative work surface and any Balance voltage measurements were made 6" (150 mm) above the worksurface as per EOS/ESD 3.1. The heads were discharged by a short (<1 cm.) wire to ground and the resulting current pulse was recorded using a Tektronix CT-1 current probe.
The AC Ionizer
One AC overhead ionizer was used for the test. A CPM recorded the balance voltage as 0 V. Fifty discharges from the HGA to ground were recorded with the HGA on an insulated platform 24" off of the worksurface. Some of the current pulses were positive and others were negative. They were of varying amplitude but in general they were surprisingly large. Fifty discharges are shown in the top trace of Figure 7. Two typical discharges, one positive and one negative are shown in the lower traces. The vertical sensitivity of the traces is 5 mA/division.
Figure 7. Multiple CDM discharges from an HGA under an AC ionizer
As can be seen, the amplitude of the discharges vary dramatically and are of both polarities. Figure 8 shows a histogram of the amplitudes of the events. It shows that two peaks, one corresponding to the positive swing of the ionizer and the other corresponding to the negative swing.
The DC Ionizer
In order to estimate the effects of a DC ionizer on an HGA assembly, discharge measurements were made with Balance Voltages of –20 V, -10 V, +10 V and + 20 V. A histogram of the current amplitudes for each Balance Voltage was generated in order to calculate the mean and the standard deviation of the data set. Two such histograms are shown in Figure 9. In Figure 10, the means and standard deviations are plotted so that the performance of the system can be estimated at 0 V Balance Voltage.
The data in both figures clearly show a variation which is quite extreme and very different from the histogram shown in Figure 8.
Figure 8. Histogram of CDM current amplitudes under an AC ionizer
The variation shows that although the ionizer is balanced to an offset of 0 V or to 10 V or to any other value, the fluctuations in the ionization process are large enough that the ionizer will only be balanced on the average. It is capable of moving off of the zero point by a few volts of CPM voltage even though the CPM is too slow to respond to the variations. Since the ionizer is controlled by a circuit with an averaging time constant , the ionizer can only be expected to remain at its set point when averaged for that length of time. Since a CPM is an inherently slow device, it reads a stable value, and will show zero reading for a balanced ionizer in spite of fluctuations as seen in Figures 9 and 10.
Figure 9. Histogram of CDM discharge pulses for a DC ionizer set to a Balance Voltage of ±10 V
Figure 10. Extrapolating the CDM discharge current amplitudes ot zero balance voltage
In contrast, the AC ionizer shows a two peaked distribution that is typical of a sinusoidal driving signal. Because the signal is varying in time, it will show an amplitude histogram that is determined by the 50/60 cycle driving signal. .This effect is shown in Figure 11. It is an arc cosine distribution. This is the theoretical amplitude distribution based upon a sine wave which is randomly sampled. The dual peaked nature of the distribution looks much like the real data shown in Figure 8.
A similar experiment was done using an alpha ionizer. Again, the HGA was placed at a height of 24" (60 cm) above the work surface with the alpha overhead ionizer at a height of 36". After many attempts to measure a CDM discharge, it was concluded that no such discharge occurs. See Figure 11.
Figure 11. A null signal representing the absence of CDM discharge of an HGA under an alpha ionizer
Discussion and Conclusion
There are many attributes of an ionizer that must be considered in the selection of the appropriate one. These factors have been discussed above. For the purposes of the discussion below, only the ability of the ionizer to provide protection to the delicate heads is considered. The AC ionizer is clearly misleading when interrogated by a CPM. It exhibits considerable voltage swing that is not measured as a result of the limited bandwidth of the instrument. The DC ionizer when properly adjusted produces less of a wander than the AC device. The wander is expected to be a result of the roll off in the control circuit of the ionizer as required to establish a low enough noise level to maintain control of the balance. The Alpha ionizer is clearly the best choice when the issue of RSD protection is the only consideration.
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