Plasma etching of Cr films in the fabrication of photomasks .2. A comparison of etch technologies and a first look at defects

The advanced requirements for resolution, critical dimension control, and linearity on photomasks require chromium etch processes, which are highly anisotropic and extremely uniform. They must also allow lithographers to expose the resist at the iso-focal point of the exposure system. A reactive ion etching (RIE) process of chromium films, previously reported, met some of these demands. However, it was reported that system loading effects significantly reduced the etch rate, etch rate uniformity, and CD uniformity. An alternative plasma etch system technology, inductively coupled plasma (ICP), has been investigated, which makes it possible to achieve much higher plasma densities, operate at lower pressures, and control ion energies independently. The combination of these features minimizes loading effects, makes it easier to control selectivity, accelerates etch rates, decreases undercutting, and improves etch uniformity. This allows the maskmaker to transfer the resist image into the chrome with minimal nominal CD shift and degradation of CD uniformity. Furthermore, with little or no undercutting, resolution and CD linearity are greatly improved. Results using the ICP configuration will be presented and compared with the RIE process. Preliminary studies of defects will be presented. Defect densities will be quantified using a plasma etch process and compared to a wet etch process. Sources of defects and possible solutions to defect reduction will be discussed.