Wafer Charging Monitors, Inc.

EOS/ESD Symposium News  

Wafer charging in process tools can degrade GMR head manufacuring yields and reliability, but Charm-2 wafers can detect and help eliminate that charging. That is the main message in an invited paper by Dr. Wes Lukaszek, an expert on wafer charging, presented at the 22nd Annual EOS/ESD Symposium that was held September 26-28, 2000 in Anaheim, California.

Wafer Charging Damage to GMR Head Wafers

The paper -- entitled "Wafer Charging in Process Equipment and its Relationship to GMR Heads Charging Damage" -- points out that possible wafer charging in ion-beam and plasma-based process tools used in the manufacture of GMR read heads can damage the heads during wafer manufacturing. This has become an important consideration, because the construction of these GMR read heads requires extremely thin insulating layers and extremely small feature sizes, which makes them vulnerable to wafer charging in the process chambers of these types of equipment.

Measurement and Control of Wafer Charging

According to the author, although process-induced charging damage is a relatively recent issue for read head makers, charging damage has been a problem for CMOS integrated circuit makers for almost two decades. Moreover, even though read heads and IC's are very different products, they possess some similar structural features that are fabricated using very similar types of equipment. So, the process-induced damage mechanisms should also be similar and, therefore, semiconductor industry expertise in measuring and controlling wafer charging should be applicable to GMR head manufacturing -- and that is good news for head makers.

Current, not Potential, Causes the Damage

The paper reviews the basics of charging damage and explains that current flow, not potential, is the main cause of damage. Damage to thin insulators sandwiched between a conductive substrate and isolated conductive electrodes on the surface of a wafer occurs due to current flow through the insulator, driven by a potential difference between the surface electrode and the substrate. This can happen even when the substrate is electrically floating, because differences in potential between electrodes located in different portions of a wafer can cause current flow from one set of electrodes to the other via the insulator and the substrate. These potential differences can be created by global nonuniformities in plasma density or, in the case of ion-beam equipment, by spatially imperfect neutralization of the ion beam.

Experimental Results and Measurement Tools

The author points out that the charging behavior of ion milling tools used to manufacture GMR heads resembles wafer charging in ion implanters used in the IC industry and presents data to illustrate this type of charging. The paper also presents data on plasma-based processes and explains a localized charging effect called "electron shading" that is associated with hole-in-insulator topographies. In these examples, charging currents are shown to be responsible for charging damage, which emphasizes the need for a charging monitor that can measure current densities in addition to just being able to measure electrostatic potentials. According to the paper, the most convenient and widely used monitor of this kind is the CHARM®-2 monitor, which is implemented as monolithic silicon wafers populated with EEPROM-based potential, charge-flux, and UV sensors.

Application to GMR Heads

The author concludes that the basics of wafer charging and process tool characterization described in his paper should be applicable to GMR head manufacturing due to the similarities it has with IC manufacturing. For this reason, the CHARM®-2 monitor can be employed to detect and help control wafer charging in GMR head manufacturing equipment. However, as it is not possible to fabricate EEPROM-based sensors in GMR wafers, conventional silicon-based CHARM®-2 wafers must be substituted for, or attached to, GMR wafers. Although the absolute magnitude of the response recorded by CHARM®-2 wafers in these situations may be somewhat different than the stress experienced by the GMR structures, the CHARM®-2 sensors still provide a relative measure that makes it possible to monitor and reduce wafer charging through process improvements.

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Lukaszek, W., "Wafer Charging in Process Equipment and its Relationship to GMR Heads Charging Damage," presented at the 22nd Annual EOS/ESD Symposium, September 22-24, 2000, in Anaheim, CA.