Sheila Vaidya

Linewidth dependence of electromigration in evaporated Al‐0.5%Cu

The linewidth dependence of electromigration damage has been evaluated for 2.5‐cm‐long, 1–4‐μm‐wide, e‐gun‐evaporated Al‐0.5%Cu conductions. It is observed that the ≲1‐μm lines are much longer lived than the ≳2‐μm lines, reversing the trend at wider widths. These results are rationalized on the basis of the ’’bamboo’’‐type grain structure of narrow lines in contrast to the much more heterogeneous structure of the wider meanders.

Automated optical proximity correction: a rules-based approach

In this work we demonstrate the power, speed, and effectiveness of an automated rules-based approach for performing optical proximity correction. The approach applies to both conventional and phase-shifting mask layouts for optical lithography. Complex imaging, substrate, and process phenomena can be folded into comparatively few rules parameters. Using simple arithmetic, these parameters pre-compensate the layout for the combined proximity effects. The rules consist of edge rules and corner rules for biasing feature edges and for adding sub-resolution assist features. This paper describes an integrated solution that includes rules parameter generation and fast, hierarchical rules application. Experimental results demonstrate improved edge placements and wider process latitude than for non- corrected layouts.

Mask assisted off‐axis illumination technique for random logic

While off‐axis illumination has been demonstrated to improve contrast and depth of focus for low k1, packed line–space (L/S) patterns [S. Asai, I. Hanyu, and K. Hikosaka, J. Vac. Sci. Technol. B 9, 2788 (1991); K. Kamon et al., Jpn. Appl. Phys. 30, 3021 (1991); K. Tounai et al., Proc. SPIE 1674, 753 (1992)], application of this approach to the more discordant patternings associated with random logic levels is suspect. We introduce a conventional mask ‘‘assist’’ feature technique that extends the off‐axis L/S enhancements to more isolated features (both spaces and lines). It will be shown that the effective process window is substantially improved and exhibited proximity effects are mitigated for this technique. Also, a comparison to a phase‐shifting mask solution for the patterning mix expected in random logic layouts is performed. Simulation results are verified on a 0.53 NA, DUV stepper.

Exposure characteristics of alternate aperture phase‐shifting masks fabricated using a subtractive process

Exposure characteristics of an alternate aperture phase‐shifting mask fabricated using a subtractive process will be discussed. The subtractive process, where the phase‐shifted regions are etched into a layer below the chromium, is attractive because it allows for the use of conventional chromium‐on‐quartz blanks, as well as providing more processing flexibility. However, recent results using a subtractive fabrication process have determined that a linewidth variation of ∼0.05 μm exists between features imaged with etched and nonetched regions of the alternate aperture pattern. This article examines some of the potential causes for this linewidth variation, including mask linewidth control, surface roughness, contamination during phase‐shift forming etch step, and sidewall profile and position. Results indicate that the sidewall profile and position are critical parameters in defining the wafer feature size. The impact of phase is also investigated. The wafer feature size depends on the depth of the quart...

Ultraviolet damage properties of various fused silica materials

Five different fused silica types were evaluated for their resistance to UV-induced compaction and color center formation at 193-nm. Real-time monitoring of color- center-induced absorption showed three distinct dependencies of transmission on pulse count. The initial rates of color center formation varied by well over a factor of ten between the materials tested while compaction-induced birefringence rates varied by at most a factor of four. Of the likely candidates for lithographic applications, Corning Excimer Grade 7940 fused silica was the least prone to color center formation while Suprasil 311 showed the lowest compaction rates. The rates of compaction-induced birefringence and color-center-induced absorption from 213-nm radiation were found to increase dramatically under elevated sample temperature conditions. Since a two- photon absorption mechanism is believed to be the catalyst for UV damage to fused silica, two-photon absorption coefficients were characterized at elevated temperatures. The two-photon coefficients at 213-nm for all materials measured including crystalline quartz and under all applied conditions were statistically equivalent, leading to the conclusion that the energy dissipation mechanism, in addition to two-photon absorption, is important to UV damage to fused silica.

Electromigration Resistance of Fine-Line Al for VLSI Applications

The electromigration lifetimes were determined for an as-yet unexplored combination of long lines (up to 3cm.) and narrow linewidths (down to 1 ¿m) of evaporated and magnetron sputter-source deposited Al-Cu-Si films. The lifetimes for the sputtered films were found to be significantly smaller than those for e-beam evaporated films. The latter displayed an unusually large improvement in the lifetime for finer linewidths (1.5 and l¿m). Failure modes were analyzed and correlations made with a new microstructural parameter incorporating the film grain-size, its sigma and the degree of preferred orientation.

Automated layout of mask assist-features for realizing 0.5 k1 ASIC lithography

The virtues of mask-plane assist features for improving imaging performance of generic ASIC layouts in the 0.5k1 realm has been previously proclaimed. In this report we provide experimental verification and introduce a methodology to automatically deploy these features on ASIC layouts.

Degradation of fused silica at 193 nm and 213 nm

Accelerated damage experiments were conducted to better predict the lifetime of fused silica optics under Deep-UV lithographic conditions. Real-time monitoring of UV-induced absorption showed that 193-nm radiation produced color centers four to five times faster that 213-nm radiation. Since the two-photon absorption coefficient at 193-nm is also about five times larger than the 213-nm value, a two-photon initiated damage process was supported. Refractive index changes in irradiated samples extrapolated from stress-induced birefringence distributions and a compaction model correlated with the refractive index changes calculated from interferometry measurements. The rates of UV-induced refractive index changes varied by over a factor of two among five different fused silica types, indicating that fused silica chemistry and processing history effect compaction susceptibility. The compaction rates, when extrapolated down to lithographic intensities, predict that the lifetimes of fused silica optical systems may be limited by UV- damage.

Multiple‐layer blank structure for phase‐shifting mask fabrication

A multiple‐layer blank structure with two shifter layers coated on a quartz substrate is proposed for the fabrication of phase‐shifting masks. The thickness of these shifter layers is such that they induce a 180° phase shift of light compared to air. On top of these shifter layers, an opaque layer is coated. The top shifter layer is patterned during the fabrication of the phase‐shifting mask. The bottom shifter layer is patterned only to repair missing shifter defects. But the main purpose of this bottom layer is to act as an etch layer or give an end point during the repair of the phase defects or during the fabrication of the mask. Using yttrium fluoride etch stop/end point layer (bottom shifter layer), we have demonstrated the feasibility of multiple‐layer blanks. This layer acts as an etch stop during the patterning of the main shifter layer and has a good refractive index match to that of quartz. The phase‐shifting masks made using this multiple‐layer blank structure have shown lithographic performan...

Reduction of ASIC gate-level line-end shortening by mask compensation

One of the most dramatic effects that one encounters when attempting the optical imaging of 0.5 k ASIC gate levels is the truncation or shortening of transistor geometries. This reduces the wafer process latitude and in some cases even eliminates the level-to-level overlay margin. We investigate a number of techniques, including various complexities of mask compensation and modified illumination to mitigate this phenomenon in manners sufficiently general to accommodate ASIC layouts.