AMD Employs ESD Control for Photomasks
Laura Peters, Senior Editor -- Semiconductor International, 6/1/2001
 |
As in most areas of wafer processing, with shrinking device features come increasingly vulnerable reticles because less energy is needed to cause an ESD event. When the reticle is close to a grounded surface, the charge flows to the chromium/ chromium oxide (chrome) mask. Damage can occur even without physical contact and at low levels of electrostatic exposure, resulting in yield loss, process downtime and high reticle replacement costs. If the industry eventually makes a transition to a 9 x 9 in. format from today's 6 x 6 in. standard, ESD reticle damage will become even more severe, as is already occurring with the transition to 300 mm wafers.
Quartz blanks, like plastic carriers and oxide-coated wafers, are excellent insulators, and thus readily charge and hold static electricity. Since these materials cannot be grounded in the fab, ionization is the best method available to neutralize the charge, which can exceed 10 kV. Even though static charge levels held by an object can be low (~10-9 C), they discharge rapidly in a small space (~0.1 µm3). Such an ESD event, often caused by proximity to insulating objects such as minienvironment pods, can melt or vaporize the chrome that defines submicron features on the reticle. The ESD signature looks like chipped or melted chrome on pattern edges or corners where the electric field concentrates. The chrome then separates from the quartz, becoming a potential contaminant for the reticle, pellicle and stepper. Chrome can also migrate from one line to another when the charge between the two differs.
In minor cases, the reticle can be repaired. But, ironically, even the act of transporting a reticle to an inspection station to look for ESD damage increases the static charge level on that reticle. The greatest yield loss results from a killer defect on the reticle that goes undetected. ESD is also an insidious problem because reticles suffer both from repeated, low-level ESD events that cause feature bridging, and from catastrophic events, which vaporize and remove the chrome.
In addition to reticle damage, static charge can cause stepper lock-up, a phenomenon that results from EMI (electromagnetic interference) generated by ESD. The ESD locks up microprocessors inside the tool, causing unpredictable forms of malfunction such as wafers missing the entry port to a chamber and crashing into the side.
The ESD service combines proprietary photomask technology with ESD audit capabilities to determine the cause of ESD damage to photomasks, while also finding ways to reduce static energy. A thorough approach targets the cleanroom, lithography module, inspection modules, reticle stocker areas and photomask supply house.
In the fab, charge levels can be reduced by elevating room humidity levels, if possible. Next, engineers should ground all isolated conductors and static-dissipative insulators including process tools, tables and operator gowns. Then the engineer sizes, specifies and installs ionizers in ceilings, stocker areas and transport modules. Tool lock-up issues can be diagnosed using a digital oscilloscope and a wideband loop antenna inserted inside the tool.
