Yield Enhancement Through Photostabilization

Staff -- Semiconductor International, 9/1/1998

A photostabilization process (known as PhotoKinetics, PK) that enhances post-deep UV lithography features and makes them more robust has been developed by Eaton's (Beverly, Mass.) Fusion Systems Division (FSD) process technology group. The impact of this technology is in better CD control and etch selectivity of deep UV resists.

The semiconductor industry still is transitioning to deep UV photoresist technology with a greater emphasis on increasing the yield parameters associated with deep UV implementation. Two significant parameters challenging deep UV technology are post etch CD control and the control of resist/underlying layer etch selectivity. Currently, the total error budget for across-the-field, wafer, wafer-to-wafer and lot-to-lot is within the ±;25 nm range, emphasizing the need for tighter CD control. Decreasing k1 values for deep UV lithography places further importance on CD control. Deep UV resists however, tend to shrink with isolated and grouped lines and grow with contacts and vias, making CD control more difficult.

Deep UV resists currently do not have the same etch resistance as those of the preceding i-line generation. The thickness of deep UV resists required to withstand etch processes is limited by the lithography process that tries to maintain a 4:1 aspect ratio of thickness to minimum feature size (MFS). While typical ratios are 2:1, improvements are being realized with less aggressive etch processes but at a price, a loss of overall equipment effectiveness. A better approach may be through increasing the productivity of etch tools by improving the robustness of the resist materials at etch.

Fig. 1. Following a poly etch, the photokinetic (PK) process is significantly better at 3.5% shrinkage than are no bake and hardbake pretreatments.

The PK process occurs at the end of the lithography sequence and before etch and implant. The wafer is placed in an atmospheric chamber where proprietary photo-reactive chemistries are introduced in conjunction with photon energy to effectively modify the DUV photresist. Chemical radicals are created due to strong absorption of photon energy. These radicals can be rearranged and recombined to fill voids caused by the acid generated to deblock the DUV polymer. The result is enhanced CD control and selectivity characteristics.

Initial results have shown reduction in shrinkage from a high of 25% being to below 4% (Fig. 1). Additionally, CD control has been improved to 5-50 nm (3s) over hardback after etch, while etch selectivity has been demonstrated to between 2.5:1 and 4:1 (depending on the specific deep UV resist type) over a post develop hardbake process. By opening the possiblity of using more robust etch and implant processes, this technique may improve productivity and increase yields.