Kevin M. Welsh

Design of a bottom antireflective layer for optical lithography

The advent of deep-UV(DUV), chemically amplified, acid catalyzed photoresists as successors to positive diazoquinones photoresists has brought about a new set of process environment concerns directed towards all materials in contact or absorbed by the photoresists. In addition to the application of DUV bottom anti-reflective coatings (BARCs) to suppress optical reflection and subsequent linewidth distortion, we must consider the properties and interaction of the BARC layer with the labile photoacid of the latent image. In this regard, we have examined the physico-chemical aspects of the DUV BARC with regards to acting as a barrier layer to substrate poisoning, and as an optical absorbing layer that does not interact and/or distort the deep-UV profile. Various single component polymeric BARCs were synthesized and examined. Considerations will be discussed of the optical absorbance, the coating quality, dry etch rate, and the impermeability of the BARC layer to photoacid diffusion to fulfill the performance requirements of BARCs for DUV lithography.

Sensitivity enhancers for chemically amplified resists

The addition of phenolic compounds to positive tone chemically amplified resists has increased sensitivity by approximately 2X for Deep UV exposures and up to 5Xfor X-ray imaging. Sensitivity enhancement during e-heam exposures was only 20%. Additives like hydroquinone sensitize various acid generators including triphenyl sulfonium triflate (TPS) and N-tosyloxyphthalimide (PTS) without affecting contrast and image profiles. The sensitization occurs in poly(t-butyloxycarbonyloxystyrene) as well as in base soluble resins. With PTS, the predominant mechanism is believed to involve electron transfer from the excited singlet or triplet state of the additive to the acid generator. For onium salt, direct photolysis plays a significant role in acid generation so that the effect of the additives is not as great as with PTS.

Mechanistic studies on the poly(4-tert-butoxycarbonyloxystyrene)/triphenylsulfonium salt photoinitiation process

Studies on the poly(4-tert-butoxycarbonyloxystyrene)/triphenylsulfonium salt (TBOC resist) photoinitiation process show strong evidence for a dual photoinitiation process. Photolysis studies on the relative quantum yields and also the ratios for in-cage versus cage-escape sulfide show that the TBOC polymer behaves differently from other polymers and likely sensitizes the decomposition of some of the triphenylsulfonium salt. Also the TBOC resist shows photoactivity at 300 nm, where the polymer absorbs but the triphenylsulfonium salt has only very weak absorbance, which suggests that the sensitization process is initiated by the polymer. However the fact that substantial amounts of in-cage products are also formed for photolysis of the TBOC resist, implicates a direct photodecomposition of the triphenylsulfonium salt. Fluorescence quenching studies on TBOC resist films show that the sensitization proceeds by electron transfer from the singlet excited state of the TBOC polymer and that a mainly static quenching mechanism is involved. The photoinitiation of the TBOC cleavage reaction proceeds by a dual initiation pathway which involves both the excited state of the polymer and the excited state of the triphenylsulfonium salt.

Improved reflectivity control of APEX-E positive tone deep-UV photoresist

A study optimizing the actinic absorbance of APEX-E positive deep UV photoresist was performed using a variety of dye additives. The selection of a dye and the optimization of dye content for APEX-E positive photoresist has led to substantial process enhancements in reduction of reflective notching and of thin film interference effects. The usual side effects as found in dyed I- line resists such as significant loss of photospeed, decreased focus latitude and sidewall angle decrease were not apparent with selected conjugated aromatic dyes. The benefit of added absorbance has allowed the direct use of dyed APEX-E to counteract the step interference (notching) problems over the severe topography of CMOS gate level and eliminate the reflective notching of surface strap level in the fabrication of 16 Mb devices. In addition, the depth of focus window was enhanced and process latitude was maintained. Geometries of 250nm were printed, with dyed APEX-E for optical densities ranging from 0.4 to 0.8 per micron with a DUV optical scanner.

TAR processing for CD control in I-line and 248-nm lithography

The combination of dyed photoresist and top antireflection (TAR) coatings was applied to I- line and deep-UV lithography on polysilicon. Optimization of the resist layers absorption and application of the TAR process significantly improves CD control of submicron gate level lithography.

Photochemistry and fluorescence spectroscopy of polymeric materials containing triphenylsulfonium salts

Triphenylsulfonium salts (TPS) have been formulated with polymers to make photosensitive systems for optical and optoelectronic applications. Photolysis of these salts generates strong acid which has been used in cross-linking reactions, deprotection reactions, and depolymerization reactions for photosensitive polymers, photodeformable polymers, and photo-doped conducting polymers. In addition, materials best described as polymeric sulfonium salts have been found to become conducting after photolysis. We have studied the photochemistry of TPS in polymer films and in solution. TPS photodecomposes by a mechanism that gives both in-cage recombination reactions and cage-escape products, and by reaction with solvent or polymer matrix. These products give cage/escape (C/E) ratios which are sensitive to the viscosity, rigidity, and polarity of the environment, and also the excited state from which the photochemistry occurs. Details of the reactivity and C/E ratios from photolysis of TPS salts in solution, have made it possible to determine their reactivity in polymers. In some cases the polymer behaves as a viscous solvent leading to high C/E ratios and in other systems the polymer excited state can sensitize the decomposition of TPS salts to give lower C/E ratios. Fluorescence studies on these polymers and quenching studies with TPS salts have helped to determine which excited states of the polymer and TPS salts are involved, and whether there is static or dynamic quenching in these systems. The photochemistry of model compounds for the polymeric sulfonium salts is also described.