Yashesh Shroff

Maskless extreme ultraviolet lithography

Masks have been identified as the high risk, high cost issue for extreme ultraviolet (EUV) lithography. Challenges in EUV mask technology such as providing a pellicle and correcting defects have prompted the search for a maskless technique. Here we describe two approaches in which the mask of a current EUV system is replaced by an array of micron-sized mirrors. Patterns are achieved by modulating individual mirrors to create selected bright and dark spots. In one approach, individual mirrors can be lowered by λ/4 to yield locally dark regions because of destructive interference. In another approach, each mirror is mounted on a cantilever. Selected cantilevers can be tilted such that incident light from those mirrors is out of the pupil of the imaging objective. The wafer is mechanically scanned and the object is electronically scrolled across the array of mirrors in order to build up the required pattern. We have simulated the mechanical properties of the micron-sized mirrors and some aerial images showin...

EUV pellicle development for mask defect control

The absence of a reliable non-removable pellicle is a significant obstacle in the development of EUV lithography. In this paper we present analytical and experimental results of a pellicle concept. The concept is based on the development of an EUV transmissive film supported with a wire-mesh. The form factor of the proposed solution is not different from a standard pellicle application, thus this would not require dramatic tool design changes. Results from developmental studies of two materials, silicon (Si) and ruthenium (Ru), are presented. As expected, Si shows oxidation on both surfaces of the thin film, while the less transmissive Ru has excellent resistance to oxidation. Spectral analysis at EUV wavelengths of pellicle coupons agrees very well with the theoretical analysis.

High transmission pellicles for extreme ultraviolet lithography reticle protection

The authors present the results of a full-field extreme ultraviolet (EUV) pellicle for reticle protection and defect mitigation. Based on novel microelectromechanical systems based fabrication, it comprises a 50 nm Si membrane attached to a wire-grid. Two types of pellicle fabrication techniques are described. The authors present the first actinic results of extreme ultraviolet lithography reticle with pellicle exposed on IMEC Advanced Demo Tool. The impact of different pellicle types on imaging is evaluated as a function of pellicle standoff distance and mesh geometry. A new prototype pellicle has been developed with a measured transmission of 82% in EUV. Actinic exposures are complemented with aerial image modeling, thermal analysis, vacuum cycling, resist outgas tests, and >5 g repeated scan cycle robustness tests.

Fabrication of parallel-plate nanomirror arrays for extreme ultraviolet maskless lithography

A micromirror array for extreme ultraviolet (EUV) maskless lithography was designed and fabricated. The arrays are composed of devices with less than a 350 nm actuation gap and a surface area ranging from 1 μm2 to 20 μm2. The mirror layer is composed of silicon in lieu of the Mo/Si stack used for EUV mirrors in order to debug the process and to simplify the initial fabrication. Germanium was used as a sacrificial material while α-Si acts as a hinge for this parallel-plate design. Silicon migration into germanium was observed, so the thermal budget was restrained to 450 °C for the entire process. Scanning electron microscope images of working devices are provided.

Optical analysis of mirror-based pattern generation

We study mirror based pattern generation systems to provide an understanding of how they can be operated in an analog mode to meet the quasi-continuous sizing and placement requirements of optical lithography. Both tilting mirrors and piston-motion mirrors are examined. The aerial images are compared with those generated by simple binary masks. The effect of grayscaling, used to place and size features, on image quality is measured. Normalized image log slope (NILS) is used as the measure of image quality. Tilting mirrors used in grayscale mode provide image quality comparable to binary masks, and piston mirrors are somewhat better.

Layout decompression chip for maskless lithography

Future maskless lithography systems require data throughputs of the order of tens of terabits per second in order to have comparable performance to today’s mask-based lithography systems. This work presents an approach to overcome the throughput problem by compressing the layout data and decompressing it on the chip that interfaces to the writers. To achieve the required throughput, many decompression paths have to operate in parallel. The concept is demonstrated by designing an interface chip for layout decompression, consisting of a Huffman decoder and a Lempel-Ziv systolic decompressor. The 5.5mm x 2.5mm prototype chip, implemented in a 0.18μm, 1.8V CMOS process is fully functional at 100MHz dissipating 30mW per decompression row. By scaling the chip size up and implementing it in a 65nm technology, the decompressed data throughput required for writing 60 wafers per hour in 45nm technology is feasible.

EUVL alternating phase shift mask

Extreme ultra-violet Lithography (EUVL) alternating phase shift mask (APSM) or other optical enhancement techniques are likely needed for 16nm (half pitch) technology generation and beyond. One possible option is the combination of EUVL and APSM. The fabrication of EUVL APSM is more difficult than either the fabrication of an EUVL binary mask or a conventional optical APSM mask. In the case of EUVL APSM, the phase difference in the two regions (0 and 180-degree phase regions) is created by a phase step in the substrate prior to the multilayer (ML) coating. The step height that induces 180-degree phase mismatch in the ML is determined by [λ/(4cosθ)](2m+1), where m are integers (0, 1, 2,...). In this experiment, we targeted for a step height with m=1. The same mask design also contains the standard binary structures so that the comparison between the EUVL APSM and the EUVL binary mask can be performed under the same illumination and wafer process conditions. The EUVL APSM mask was exposed using Nikons EUV1 scanner in Kumagaya Japan. The wafer level results showed higher dense line resolution for EUVL APSM as compared to that of EUVL binary mask. APSM also showed improved line width roughness (LWR) and depth of focus (DoF) as compared to the best EUVL binary results obtained with C-dipole off-axis illumination (OAI). The wafer CD resolution improvement obtained by APSM in this experiment is partially limited by the resist resolution and the mask phase edge spread during ML deposition. We believe that wafer CD resolution and can further be improved with imaging imbalance compensation mask design and improvements in resist resolution and the phase generation portion of the mask fabrication process. In this paper, we will discuss in detail the mask fabrication process, wafer level data analysis, and our understanding of EUVL APSM related issues.

Image optimization for maskless lithography

This paper discusses image optimization challenges posed by a mirror based pattern generation scheme. We address defocus related image drift encountered with mirror based maskless lithography. While off-grid contacts printed with piston mirrors are most severely affected most other features can be printed with minimum loss of telecentricity. A novel double-piston mirror architecture based on a combination of tilting and piston mirrors is introduced. It operates as a pseudo-tilt mirror but also has the advantage of allowing strong phase-edges due to pure-phase wavefront modulation. Exposure latitude versus depth-of-focus process window curves of typical features show that the new mirror design behaves as well as tilting mirror. An image optimization algorithm is presented that iteratively updates the mirror array phase-map to optimally print dense layout, accounting for inter and intra feature proximity effects.

Flare and lens aberration requirements for EUV lithographic tools

EUV lithographic tools can support the 32 nm MPU manufacturing node and beyond. In order to meet the stringent requirements on CD control and overlay for such technology generations, wavefront error and flare of the EUV exposure systems have to be well controlled. The cross field variations of wavefront errors and flare need to be in the acceptable range in order to improve the common Depth of Focus (DoF) across the field. The impacts of lens aberration and flare to the aerial image at the system level are studied for the 32nm MPU technology node using Intels aerial image simulation tool. The focus control budget of the exposure tools has been estimated. Useable Depth of Focus (UDoF) has been defined, and focus margin between UDoF and focus control budget from the exposure tool has been calculated for various cases. Focus margin has been used to determine the flare and lens aberration requirements for the 32nm MPU node. It is found that <10% intrinsic flare and <0.75nm rms lens aberration are required for the 32nm MPU node. Process window as a measure of individual aberration terms for the 32nm node has been also investigated.

Aerial image improvements on the Intel MET

Since its installment in 2004, Intels extreme ultraviolet (EUV) micro-exposure tool (MET) has demonstrated significant improvements in ultimate resolution capability. Initially capable of printing 45nm half-pitch (HP) lines with a 160nm depth of focus (DOF), it is now capable of printing 22nm HP lines with up to a 275nm DOF and demonstrating modulation down to 18nm HP. Initial improvements in resolution have been chiefly attributable to the maturation of EUV masks and photoresists. Recent improvements that have enabled the 22nm HP imaging with a sizeable process window are largely due to new illumination options that have become available as a result of recent tool upgrades. In particular, the installation of a new nested Wolter collector with an additional outer shell that extended the maximum partial coherence (σ) from 0.55 to 0.68, in conjunction with an updated pupil wheel and apertures, has enabled new rotated quadrapole and on-axis dipole illumination settings with 0.36 inner σ and 0.68 outer σ. Here we present simulated contrast curves alongside the experimental imaging results for the Intel MET using the newly available quadrapole and on-axis dipole illumination settings and discuss our future plans for continued improvements to the Intel MET aerial image.