Hiroshi Ohtsuka

Evaluation of repair phase and size tolerance for a phase‐shift mask

The conjugate twin‐shifter method for alternating phase‐shift method provides the flexibility for mask shifter layout, and is applied for both isolated and periodic shifter arrays in conventional i‐line positive resist processes. A new mask structure simplifies shifter pattern delineation and reduces the effects of phase defects. The detail of intermediate phase interference has been analyzed by applying the two‐dimensional shifter array as the phase defect. The results of this analysis provides the optimum repair element array for the new repair method of phase defects. Then the effectiveness of this simple repair method is examined in the i‐line process by using the test arrays that delineated in the XeF2 assisted focused ion beam process.

Phase defect repair method for alternating phase shift masks conjugate twin-shifter method

Phase shift masks (PSMs) enable current optical exposure systems to provide significantly higher resolution in effective depth of focus (DOF). Localized phase errors, or other transparent defects in the mask phase shifter elements, can cause loss of DOF and degradation of image contrast. Transparent defects of this type have prevented PSMs from becoming practical for large-scale production of integrated circuits containing deep submicrometer features. This paper describes a new technique that is useful for repairing Alternating PSMs containing transparent defects in the phase shifter elements.

Conjugate twin-shifter masks with multiple focal planes

A new phase shift lithography method has been developed that allows different integrated circuit (IC) features to be focused in different optical planes, conforming to the IC surface topography. In principle, each pattern in an IC could have its own unique focal plane. Direction and magnitude of each focal shift are determined by the design of the mask phase shifters. This method is applicable for use with conventional opaque mask patterns and unattenuated phase shift patterns. Both types of patterns can be intermixed on the same mask if desired. Characteristics of the Multiple Focal Plane technique have been evaluated experimentally and through mathematical modeling using TCC optical imaging theory. Experiments were conducted with a commercial i-line wafer stepper (N.A.=0.50, ?=0.50) using conventional positive and chemically amplified negative resists. Mask patterns evaluated included dark-field Cr masks, isolated clear-field lines, unattenuated phase-shift patterns. Effects of changes in phase shift are discussed, and practical mask design approaches are recommended.

Fabrication of 0.13-μm device patterns by argon fluoride excimer laser lithography with practical resolution enhancement techniques

This paper presents the formation results of 0.13-µ m device patterns using argon fluoride (ArF) excimer laser lithography that does not incorporate strong resolution enhancement techniques such as levenson type phase-shifting mask or quadrupole illumination. Device patterns of 0.13-µ m can be fabricated by ArF excimer laser lithography when a high performance single-layer photoresist, an anti-reflective layer, an attenuated phase-shifting mask with an off-axis illumination are used. A 0.5-µ m depth-of-focus with a 10.8% exposure latitude can be obtained. Furthermore, 0.12-µ m-rule gate patterns of memory and logic devices can be fabricated. A 1.0-µ m depth-of-focus for a 0.13-µ m pattern will be achieved.

Fabrication of 0.1 µm Patterns Using an Alternating Phase Shift Mask in ArF Excimer Laser Lithography

We demonstrate applications of alternating phase shift mask (Alt-PSM) techniques to ArF excimer laser lithography with a numerical aperture of 0.6 and a coherence factor of 0.3. A 0.10 µm line-&-space (L/S) pattern was fabricated using a single-layer resist and a 0.09 µm L/S pattern was fabricated using a silylation resist. However, the process window was smaller for the silylation resist than for the single-layer resist over the 0.10–0.13 µm L/S range. The maximum depth of focus (DOF) values were approximately 0.3, 0.5, and 0.9 µm for 0.10, 0.11, and 0.13 µm L/S patterns, respectively, for the single-layer resist. From exposure dose-DOF-tree analysis, we estimated the inclusive process margin including the critical dimension difference, the DOF, the dose margin, and the phase error. Assuming that the usable DOF is larger than 0.5 and 0.6 µm for 0.11 and 0.13 µm L/S patterns, respectively, the inclusive process margin for the single-layer resist is poor in comparison with future predictions.

Acid amplification of chemically amplified resists for 193-nm lithography

We analyzed acid amplified positive resists designed for 193 nm lithography. The acid amplified resists are composed of an acid generator, a partially protected alicyclic polymer and an acid amplifier which is designed to produce acid during post- exposure baking. We found that acid amplified resists double the sensitivity. We also found that introducing acid amplified resists improves surface effect and adhesion. The acid amplified resist resolve 0.16 micrometer L&S, whereas conventional chemically amplified resists only resolve 0.2 micrometer L&S.

New method for etching rate and resist profile control in O2RIE

This paper describes a new method for etching rate and resist profile control in O2RIE. The method simply determines the equivalent process conditions for different device layers with various areas of the material to be etched (etchable area) by introducing the ratio of the etchable area to the flow rate defined as S/F parameter. The concept of this method is that the gas composition controlled by S/F parameter determines both the etching rate and the resist profile under the constant energy flux densities of ions and energetic neutrals (ion impact density). S/F parameter is introduced through the extended expressions of Mogabs loading effect theory by applying two important characteristics in O2RIE: (1) The dominant etchant is oxygen molecules. (2) The etching rate is proportional to the ion impact density. The etching rate and the gas composition are expressed as functions of S/F parameter and the ion impact density in the extended expressions. The etching rate and the resist profile have been controlled by applying this method to wafer samples with various etchable areas. Furthermore, a linear relationship between the etching rate and the resist profile is clarified.

Conjugate twin-shifter for the new phase-shift method to high-resolution lithography

This paper describes a new phase shift method for 0.3 micrometers optical lithography. Phase shift lithography provides very high resolution, but current techniques suffer from high contrast levels at the shifter edges and from asymmetric optical intensity profiles. A new method uses conjugate twin-shifters to overcome these problems. This new method provides high resolution for both positive and negative resists and assures symmetric intensity profiles in bright field areas. This method uses two different phase shifts, of (pi) /2 and 3(pi) /2, respectively, placed alternately in the adjacent line pairs. While maintaining the desired (pi) phase shift between adjacent lines, the phase difference between the shifter elements and the mask substrate is reduced to (pi) /2, thus providing optimum resolution while avoiding undesired printing of the shifter edges. Symmetric intensity profiles are obtained by requiring that both shifters provide the same degree of phase shift relative to the substrate; i.e., the phase of the substrate is midway between the phases of the two shifters. The conjugate condition is defined by this phase relationship. The functional characteristics of this new method have been examined both theoretically and experimentally. Experimental results were obtained using a commercially available i-line stepper with 0.50 Numerical Aperture (N.A.) and partial coherence (sigma) -factor 0.50. Mask phase shifters were fabricated of sputtered SiO2 film. Tests were made using novolac-based positive resist.

Parameters Affecting The Ability To Align Aluminum Layers On An Optical Wafer Stepper

Very high registration accuracy is required for 5:1 wafer steppers used in V-LSI production. The image quality of an alignment mark is affected by illumination wavelength, photoresist thickness, light absorption in the photoresist, and by the characteristics of the alignment mark itself. Reflection Modeling and image profile analysis are applied to evaluate the influence of the alignment geometry and image quality on the ability to align the wafer. As a result of this study, ideal image quality is observed by optimization of the step height, and the influence of background noise is studied on rough aluminum surfaces. Degradation of the image profile and edge contrast are observed on rough aluminum surfaces.

ArF excimer laser lithography with bottom antireflective coating

In ArF excimer laser lithography, the bottom antireflective coating (BARC) technique is essential in inhibiting the effect of interference and reflective notching. We investigated the antireflective effect of commercially available organic BARCs, that had originally been designed for KrF and i-line lithography, and also the patterning characteristics of ArF resists with BARCs. The refractive indices of various materials were measured with a spectroscopic ellipsometer. The real part (n) and the imaginary part (k) of the complex refractive index at 193 nm were 1.4 to 1.7 and 0.1 to 0.8 respectively. Almost all the materials had sufficient antireflectivity at 193 nm. We investigated the patterning characteristics of chemically amplified ArF positive resists with suitable BARC materials. The resolution, the depth-of- focus of patterns below 0.16-micrometer lines and spaces, and the exposure latitude were improved and good critical dimensional control over topography was achieved by using BARC. An acceptable profile after gate structure (BARC, W-Si, and Poly-Si) etching could be obtained under the typical etching conditions used for KrF resists.