Joseph G. Garofalo

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...

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.

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.

Focused-ion-beam repair of phase-shift photomasks

Phase-shift photolithography is emerging as an important new technology for sub-half-micron design rule circuits. Unfortunately part of the price paid for the improvements in spatial resolution and process latitude afforded by phase-shift lithography is increased mask defect printing sensitivity. The minimum printable defect size, 0.3 microns (on the mask) for I-line steppers at 0.35 microns, is roughly half that for conventional photomasks. This paper examines the issues associated with extending high resolution focused ion beam mask repair to phase-shift masks.

Automatic proximity correction for 0.35 /spl mu/m I-line photolithography

Recent advances in lithography enhancement techniques such as phase shifting masks (PSMs) and off-axis illumination have raised the possibility of using I-line photolithography for 0.35/spl mu/m generation ICs. It has become clear that in order to achieve the necessary line-width control, optical proximity effects must be taken into account in conjunction with these techniques. In this paper, we describe an automatic approach to optical proximity correction (OPC) that is both effective and fast. The work presented is a joint effort between SEMATECH, AT&T, and Trans Vector Technologies. To our knowledge, this is the first practical approach that can perform viable OPC on a real chip layout.<<ETX>>

Phase-shifting structures for isolated features

The technique for improving optical projection-system resolution by phase-shifting alternate apertures of a periodic grating was introduced in 1982. This halves the frequency content of the image passing through the optics and should therefore double the effective resolution of such patterns. Unfortunately, as feature separation increases, the efficacy of this method diminishes. Previous work applying a similar approach to isolated features involves introducing minute, non-printable, phase-shifted assist slots around the desired feature. The diffraction side-lobes of these slots constructively interfere with the center lobe of the primary aperture. The resolution enhancement afforded be this technique is limited by the printability of the assist slots. This restraint also dictates 1X-size reticle feature dimensions and the employment of high contrast imaging resists. A new approach entails significantly oversizing the desired feature and introducing a phase-shifting region around the periphery. This type of structure affords substantial focus-exposure improvements and may either be fabricated in a single-level, self-aligned scheme or by a two-level exposure with conventional e-beam tools since the phase-shifting regions are on the order of 1 micrometers (reticle dimensions). Extensive modeling of this structure for isolated contact holes and spaces explores the myriad of trade- offs involved in an optimum design. Mask-fabrication tolerances, such as phase-shift uniformity, are also investigated. It is shown that the focus-exposure window enlarges as the overall structure dimensions increase. The degree of enhancement must therefore by weighed against packing density restrictions. Also, the structure suffers, to some degree, from the effect of side-lobes. However, for a given side-lobe intensity, this technique yields enhancements superior to the assist-slot approach. As is typical of phase-shifted systems, performance is improved as the partial coherence ((sigma) ) of the illuminating radiation is reduced. The decrease in throughput sometimes associated with a (sigma) reduction is, in this case, however, mitigated by the oversized aperture that produces twice the illuminating intensity as the corresponding non-phase-shifted feature. Simulated exposure-focus analysis conclude that a 0.45 (lambda) /NA contact hole may be printed with a 15% exposure and +/- .42 K2 unit focus tolerance assuming a +/- 5% CD control. A demonstration mask was patterned with a MEBES III generation reticle writer and exposure-focus latitude predictions for phase-shifting spaces are verified on an I-line, (sigma) equals0.5, 0.45 N.A. stepper.

Aerial image-based design of rim phase-shift masks with annular illumination

A wide variety of lithography enhancement techniques have been introduced in recent years. Each method has certain virtues, such as improving the resolution of tightly packed features or increasing the depth of focus for isolated ones. Normally, these schemes are analyzed for areas in which they work best. However, it is desirable to have a phase-shift method and illumination system which improves the depth of focus for a large variety of patterns. To satisfy both requirements, promising techniques must be biased to obtain the best process improvement. These issues are particularly relevant for masks with random logic. To address the two problems, we have developed an E-D tree based computer aided design system which analyzes phase-shift masks and illumination methods for one-dimensional features and calculates the proper bias for them. Our simulations concentrate on analyzing constant width lines and spaces of varying duty cycle. The results from analyzing the features illustrate that both phase-shifting masks and off-axis illumination have regions of reduced performance, or dead zones, in which the depth of focus is degraded. Examples of dead zones are evident with many types of phase-shift masks, such as attenuating, alternate aperture, and rim, and with illumination systems, such as annular illumination. Combinations of enhancement techniques, however, can reduce the effect.

Fabrication of phase‐shifting masks with shifter overcoat

Phase‐shifting masks have demonstrated the potential to extend the application of existing optical lithography tools by simultaneously enhancing their resolution, depth of focus, and exposure latitude. This paper identifies a processing sequence for the fabrication of phase‐shifting masks using spin‐on‐glass (SOG) as a shifter overcoat. The SOG is applied uniformly, range of 11 nm, across conventional 5‐in. mask substrates. A selective wet etch for patterning the SOG is also demonstrated. Using this processing sequence, alternate aperture, phase‐shifting masks are fabricated, and both focus and exposure latitude are evaluated. Additionally, the role of shifter wall angle and level‐to‐level alignment on the sized rim shifter are explored. It was found that a 45° shifter wall angle resulted in ∼10% degradation in the aerial image of a 0.35‐μm feature as compared with a vertical wall angle, and an alignment error of 0.25 μm on the reticle resulted in pattern placement errors of 0.06 μm. In order to circumven...

Implementing Attenuated Phase Shift Masks for Contacts in Production

It has been shown that the attenuated phase shift mask (PSM) is best suited for imaging dark-field patterns ( e.g., contacts, vias, and metal lines) when positive photoresists are used. This paper presents our experience and technical data in implementing the attenuated PSM on a contact level in production. Specifically, implementation issues such as mask transmission, mask bias, printing linearity, overlay results, opaque border, mask defects and process latitudes are addressed.