Mark A. Hartney

Silylation processes for 193-nm excimer laser lithography

A silylation process for novolac-based resins was developed which results in positive-tone imaging. This process is based on 193nminduced crosslinking followed by a low temperature silylation step. Novolac resin without diazoquinone additives may also be used as positive-tone resists. Typical conditions were exposure to dimethylsilyldimethylainine vapor at 10 Torr for 1 minute at 100 °C. This incorporates silicon in the upperniost 100 to 1000 nn of the film, depending on the resist. Etch selectivities in a 10 rnTorr oxygen reactive ion etching plasma with a bias voltage of -200 V were typically 30:1. Resolution below 0.3 m has been demonstrated with this technique.

Silylation of focused ion beam exposed resists

Silylation processes for lithography involve the selective incorporation of silicon into a polymeric resist, which can then be patterned using an oxygen reactive ion etching plasma. These processes, like other multilayer approaches, have been developed primarily for optical lithography to minimize substrate reflectivity and allow higher resolution. We have extended this technique to exposure with focused beams of Be, Si, Ga, and Au ions with energies between 49 and 240 keV. Conventional focused ion beam exposure of resists relies upon solvent development and requires ion penetration through the entire resist thickness. With a silylation process, however, higher mass or lower energy ions may be used, and the resist thickness is decoupled from the exposure requirements. Resolution of features smaller than 100 nm has been demonstrated.

Comparison of liquid- and vapor-phase silylation processes for 193-nm positive-tone lithography

Liquid- and vapor-phase silylation processes are compared for a 193-nm positive-tone lithographic process using polyvinylphenol as a resist. The liquid-phase process, using a mixture of xylene, hexamethylcyclotrisilazane, and propylene glycol methyl ether acetate, was found to have higher silylation contrast, better sensitivity, and a smaller proximity effect (a decrease in silylation depth for smaller feature sizes). These factors result in a larger exposure latitude, particularly at feature sizes below 0.5 μm. These advantages are greatly offset, however, by the increased chemical costs, which are estimated to be more than 100 times greater than for the vapor-phase process.

Surface imaging resists for 193 nm lithography

193 nm radiation causes direct photocrosslinking of polymer films which is sufficient to generate silylation selectivity under appropriate conditions. A variety of phenolic based polymers and blends with photoactive compounds were studied for their suitability as resists in a 193 nm, positive-tone, silylation process. Meta-cresol novolac resists and polyvinylphenol resists show the best sensitivity for this process. The threshold dose required to restrict diffusion of the silylating agent depends strongly on the composition of the resist. Sensitivities range from 12 mJ/cm2 to over 100 mJ/cm2 for various novolac and polyvinylphenol materials. Wide variations in sensitivity have been found between different synthetic methods for the same resin, different molecular weight characteristics, and different functional modifications.

Small-field stepper for 193-nm lithography process development

A stepper operating at the 193-nm wavelength has been constructed for use in the development of resist processes. The stepper lens has a 4-mm field diameter and a 0.33 NA. The stepper uses an unnarrowed ArF excimer laser as the light source, and uses diffractive lenslet arrays to transform the low divergence excimer beam into a suitable pupil fill. The stepper is routinely used for resist studies and has been used to pattern lines and spaces as small as 0.15 ?m.

Comparison of etching tools for resist pattern transfer

Several etching tools are evaluated for the oxygen-based plasma pattern transfer step in surface imaging and multilayer resist processes. These tools include a conventional parallel-plate reactive ion etcher, a magnetically enhanced reactive ion etcher, an electron cyclotron resonance reactor, and an rf helical resonator (Helicon) reactor. The performance of each tool is examined with respect to etch rate, etch profile, selectivity between the imaging layer and the pattern transfer layer, etch uniformity, etching residue, linewidth uniformity, and process latitude.

Evaluation of phenolic resists for 193 nm surface imaging

A variety of novolac-based polymers and blends with photoactive compounds were studied for their suitability as resists in a 193-nm positive-tone silylation process. The addition of photoactive compound was found to reduce the sensitivity of the resist and to hinder the diffusion of the silylating agent. Pure meta-cresol novolacs and polyvinylphenols, both of which can be polymerized to high (> 10,000) molecular weights show the best sensitivity for this process. Diffusion rates correlate with the molar volume of the silylating agent, although the activation energy does not. Resolution of 0.2-micrometers line-and-space gratings has been achieved with the polyvinylphenol and meta-cresol novolac resins.

Silylation processes for 193-nm lithography using acid-catalyzed resists

A systematic study of acid-catalyzed resist formulations was used to investigate the mechanism for resist behavior in a 193 nm silylation process. Sensitivities for these resists are higher than that of base resins even when processed without their normal post-exposure bake. To investigate this, resist formulations with different combinations of the constituent components of typical acid-catalyzed resists were evaluated. Both liquid-phase and vapor-phase silylation processes were employed and a range of post-exposure bake temperatures between 100 and 140 degree(s)C were used. The improved sensitivity for the acid-catalyzed resists is not due to heating during the vapor-phase silylation process or during the laser pulse. Instead, evidence was found for a direct crosslinking reaction between phenolic resin groups in the presence of acid without a melamine additive.

Resist alternatives for sub-0.35-μm lithography by using highly attenuated radiation

The photoresist processes used for lithography at wavelengths from the deep UY to the soft x ray will need to accommodate the strong resist film absorbance inherent in this wavelength range. Already silylation processes have been demonstrated as manufacturable near-surface-imaged resists in the deep UV. In addition other chemistries are being developed that take advantage of near-surface or at-the-surface imaging. Together these new approaches to imaging will provide not only greater wavelength flexibility but have potential for improved exposure and focus tolerances as well. These attributes will become important as device dimensions are reduced from 0.35 down to 0.1 µm.

Experiment and Simulation of Sub-0.25 microns Resist Processes for 193-NM Lithography

A model was developed to simulate the behavior of near-surface-imaged resist processes, with the emphasis on modeling of resist processes for 193 nm. Silylation, bilayer, and additive resist processes can all be simulated using this model. For the silylation process, the model was found to be in excellent agreement with experimentally observed silylated resist profiles. This model was used in combination with existing programs that calculate aerial images and single-layer resist profiles to predict process margins for 193 nm (0.5 NA) lithography. The results of our simulations for 0.25 micrometers features indicate a depth of focus comparable to the Rayleigh limit (+/- 0.4 micrometers ) for a single-layer resist process and up to two times this value for near-surface-imaged resists. Focus latitudes greater than the Rayleigh limit are predicted for 0.18 micrometers features when using near-surface-imaged resists in conjunction with annular illumination.