The ITRS roadmap indicates that significant improvements in photomask processing will be necessary to achieve the design goals of 45nm technology node masks. In the past, etch systems were designed to produce an etch signature that was as "flat" as possible to avoid introducing undesirable signatures in the final product. However, as error budgets are shrinking for all tools in the process line, the signatures produced by etch systems are used to compensate for some of the upstream CD issues. Process modifications have been used successfully in this fashion, but frequently process adjustment alone is not sufficient.
CD uniformity results from a complex interaction between the system and the sample. An etch system must be capable of adjusting radial, linear, and loading etch uniformity components to compensate for the specific needs of each sample. The adjustments should also be as independent of process as possible. Towards this end, experiments were conducted with various etch technologies to create specific, controllable etch signatures on demand without the need for hardware changes. CD data collected from binary chrome photomasks was used to verify performance of the uniformity adjustment technologies.
The quartz dry etch is a critical step in the manufacture of Alternating Aperture Phase Shift masks (alt-APSM). In order to maintain uniform phase shift across the mask, the etch depth uniformity has to be strictly controlled. Both the radial and linear components of non-uniformity have to be minimized. The Mask Etcher IV developed at Unaxis USA reduces both the components of non-uniformity using unique hardware adjustments. Using a fluorocarbon based chemistry, etch depth variations between different feature sizes is also minimized. With good etch depth linearity, phase shift does not vary with feature size. To achieve this, etched quartz structures need to have good selectivity to resist / chrome and vertical sidewalls. Etch depth uniformity was measured using an n&k1700 RT and etch depth linearity was measured using an AFM. Etched quartz structure morphologies are observed using a SEM. After preliminary screening experiments, an optimized hardware suite and process conditions that produce good etch depth uniformity, linearity and quartz profiles with vertical sidewalls and minimum microtrenching is determined.
Time division multiplexed (TDM) plasma etch processes have found widespread applications in Micro-Electro-Mechanical Systems (MEMS) device manufacturing. Very often, silicon-on-insulator (SOI) structures are used in MEMS applications with oxide layers used as etch stop/sacrificial layers as well as device function layers. Apart from the conventional requirements for deep silicon etch including high rate, selectivity and sidewall smoothness. SOI structures require finished etches to be free of undercut, commonly referred to as notching, at the silicon/oxide interface. Notching is aggravated due to the aspect ratio dependence (ARDE) effects. The ARDE effects cause structures with different aspect ratio to be etched at different etch rates, and result in the buried oxide layer in bigger features to be exposed while smaller features are still being etched.
At Unaxis USA, we have developed a proprietary technique to eliminate the notch formation while maintaining high etch rate. This technique is integrated into time division multiplexed (TDM) Si etch processes, and is implemented in a single etch process. The conventional "bulk" etch to "finish" etch transition is thus made unnecessary, with the benefit of no end point detection and smooth and uniform etch profile. Etch processes are characterized and notch performance is measured as a function over etch percentage and feature aspect ratio. Using the new SOI etching technique, notching is completely eliminated in aspect ratios up to 9:1 and reduced to well below 100 nm for aspect ratios up to 18:1. Moreover, this new technique has been demonstrated to limit the effect of extensive overetch in increasing notch size.
Alternating Aperture Phase Shift masks (alt-APSM) are being increasingly used to meet present day lithography requirements by providing increased resolution. The quartz dry etch is a critical step in the manufacture of these photomasks. Etch depth linearity, phase uniformity and minimum etched surface roughness are critical factors. To achieve this, etched quartz structures need to have good selectivity to resist / chrome, vertical sidewalls and good etch depth uniformity over the mask area. Using the Mask Etcher IV at Unaxis USA, a series of experiments were performed to study and identify the trends in quartz etching for photomasks. Etch depth uniformity was measured using an n&k1700RT and etch depth linearity from feature sizes ~0.4 micron to ~1.4 micron was measured using an AFM. Cross sections of the ~0.6 micron structure were obtained using a SEM to check for profile and any evidence of micro trenching. After several set-up experiments, an optimized process to minimize etch depth linearity and improve etch depth uniformity was obtained and is presented here.