Kevin D. Cummings

Extreme ultraviolet lithography mask patterning and printability studies with a Ta-based absorber

Extreme ultraviolet (EUV) lithography masks were fabricated using a stack of TaSi or TaSiN (absorber), SiON (repair buffer), and Cr (conductive etch stop) on a Mo/Si multilayer mirror deposited on a Si wafer. High-resolution structures were exposed using a commercial i-line resist, and the pattern was transferred using both electron cyclotron resonance and reactive ion etching with halogen-based gases. Process temperatures to fabricate these reticles were always maintained below 150 °C. EUV properties after patterning were measured using a synchrotron source reflectometer. Completed masks exhibited a negligible shift in the peak wavelength and less than 2% loss in reflectivity due to processing. Qualified masks were exposed with a 10× EUV exposure system. The exposures were made in 80-nm-thick DUV resist and with numerical apertures (NA) of 0.08, 0.088, and 0.1. Resolution down to 70 nm equal lines and spaces was achieved at a NA of 0.1. Line edge roughness in the resist features was 5.5 nm (3σ, one side)...

Uniform low stress oxynitride films for application as hardmasks on x-ray masks

A low stress silicon oxynitride deposition process has been developed in which the average stress level can be tailored by adjusting silane flow in the plasma enhanced chemical vapor deposition reactor. Stress gradients, as might be caused by nonuniform heating or gas distribution, were not found to exist. By volume, the SiON films were found to be approximately 81% silicon dioxide and 19% silicon nitride. Because the films are easily removed in hydrofluoric acid, this composition is ideally suited for use as a hardmask patterning layer on x-ray masks. A reactive ion etch process employing CHF3, O2, and Ar gases has demonstrated selectivity to Shipley SNR 200 resist of better than 3:1. Smooth pattern transfer into TaSi and TaSiN absorber layers of test features as small as 0.1 μm has been achieved using SiON as the hardmask layer. Image placement distortions on the order of 15 nm (3σ) occur from etching the SiON films on 64 Mbit SRAM x-ray test masks.

Optimization of the refractory x‐ray mask fabrication sequence

This article discusses the effects different process flows have on the material and control requirements for refractory (subtractive) x‐ray masks. Our investigation of x‐ray masks shows that a film’s stress uniformity is more critical (for distortion) than the magnitude of the stress. In particular, we find for the wafer sequence the requirement on the membrane film’s stress uniformity is the most critical specification. For the membrane sequence, the requirement is for stress uniformity for films that are removed from the membrane (after pattern formation). We find that SiC reduces the distortions from the film stress gradient, however, the frame structure determines the distortions from a film’s mean stress. Finally, we conclude the best choice of an x‐ray mask flow should be decided on our ability to modify wafer equipment and processes, to control the stress uniformity of the membrane film, to control the stress and uniformity of the resist, hardmask, and absorber films, and finally to handle the lith...

Thermal analysis of an x‐ray mask membrane in a plasma environment

Quantifying the temperatures of a membrane while the mask is in a plasma environment is essential in controlling absorber stress due to deposition, and both etch rate and feature profile due to etching. Temperature gradients across the membrane during deposition lead to nonuniform stress across the absorber resulting in large distortions during pattern transfer [W. Dauksher et al., J. Vac. Sci. Technol. B 13, 3103 (1995)]. This article presents a procedure to obtain the steady state temperature profile of a mask/membrane while in a plasma environment (deposition or etch) subjected to different cooling configurations. Membrane heat fluxes and heat transfer coefficients were determined using a thermal transient technique which compares analytical solutions to experimental results. The steady state temperature profile of the membrane was then obtained by using these fluxes and heat transfer coefficients in a three‐dimensional finite element model. The analysis procedure was demonstrated on a mask subjected t...

Predicting in‐plane distortion from electron‐beam lithography on x‐ray mask membranes

To produce x‐ray masks useable for 0.25 μm lithography and beyond, all sources of mask distortion must be minimized. In order to facilitate the fabrication of high‐quality masks, the phenomenon of changes in resist stress during e‐beam exposure has been studied. Finite element modeling was employed to determine the effects of various geometric and material properties on final image quality. Additionally, writing patterns and multipass exposure were also studied. The results indicate that the stress relief phenomenon can be controlled in a well‐designed system.

Prediction of in-plane distortions due to mask fabrication processes

Abstract In the fabrication of a typical refractory X-ray lithography mask, the pattern transfer process subjects the membrane/pattern to the non-uniform deposition and removal of multiple stressed layers. This removal process can result in unacceptably large pattern distortions in the final mask. This paper presents relationships obtained from finite element (FE) modeling, between pattern distortions and the following mask parameters: membrane material, thickness, size and stress, and the removed film material, thickness and stress. In addition, the sensitivity of the distortions was investigated for stress gradients present in the removed film. Finally, the capability of these models to predict pattern specific distortions (PSD) is demonstrated.

Etch characteristics of an amorphous refractory absorber

An ECR etch process for defining sub-0.25 μm features in a TaSiN absorber layer has been developed. A 2000 A PECVD oxide layer served as a hard mask during the etch. The effect on feature profile has been determined as a function of both rf power and back side temperature. Etch rate uniformity is excellent, with 3σ deviations of less than 4% over the center 40 mm of the substrate. Micro-loading issues associated with the etch process have also been characterized. Minimal line edge roughness is observed, and the feasibility for defining 0.10 μm features has been demonstrated.

Temperature uniformity across an x‐ray mask membrane during resist baking

We have studied temperature uniformity during the post‐exposure bake process across a 40 mm diameter of an x‐ray mask membrane. Membrane temperature was determined by measuring line size as a function of position across the membrane. A two‐dimensional finite element model (FEM) was used to analyze the results and optimize the design of a new bake chuck. The 3σ variation across the diameter of the mask was reduced to 27 nm. The FEM was also used to examine issues associated with resist baking on the ARPA‐NIST X‐ray Mask Standard and the initial results are discussed.

Accelerated radiation damage testing of x-ray mask membrane materials

An accelerated test method and resulting metrology data are presented to show the effects of x- ray radiation on various x-ray mask membrane materials. A focused x-ray beam effectively reduces the radiation time to 1/5 of that required by normal exposure beam flux. Absolute image displacement results determined by this method indicate imperceptible movement for boron-doped silicon and silicon carbide membranes at a total incident dose of 500 KJ/cm2, while image displacement for diamond is 50 nm at 150 KJ/cm2 and silicon nitride is 70 nm at 36 KJ/cm2. Studies of temperature rise during the radiation test and effects of the high flux radiation, i.e., reciprocity tests, demonstrate the validity of this test method.

Extendibility of x‐ray lithography to ⩽130 nm ground rules in complex integrated circuit patterns

Previous experimental and theoretical evidence indicates that x‐ray lithography can be used to pattern ≤180 nm features. In order to be used in manufacturing, x‐ray lithography of complex integrated circuit patterns (i.e., dense two‐dimensional patterns) needs to be demonstrated with a practical proximity gap. However, no large body of experimental evidence exists for the extendibility of x‐ray lithography for complex patterns with ground rules of ≤130 nm at gaps of 10–20 μm. Simulations of image formation and resist dissolution are shown to have good agreement with experimental results. These simulations are then used to predict exposure latitude and gap latitude for printing one‐dimensional 75–125 nm patterns at 10–15 μm gaps. Simulations indicate that at least ±10% exposure dose latitude will exist for simple patterns at these gaps, but significant nested‐to‐isolated linewidth bias will exist. Gaps must be controlled to ±1 μm for ±10% dose latitude. More complex two‐dimensional patterns have been shown...