Beams of electrons and ions are now fairly routinely focused to dimensions in the nanometer range. Since the beams can be used to locally alter material at the point where they are incident on a surface, they represent direct nanofabrication tools. The authors will focus here on direct fabrication rather than lithography, which is indirect in that it uses the intermediary of resist. In the case of both ions and electrons, material addition or removal can be achieved using precursor gases. In addition ions can also alter material by sputtering (milling), by damage, or by implantation. Many material removal and deposition processes employing precursor gases have been developed for numerous practical applications, such as mask repair, circuit restructuring and repair, and sample sectioning. The authors will also discuss structures that are made for research purposes or for demonstration of the processing capabilities. In many cases the minimum dimensions at which these processes can be realized are considerably larger than the beam diameters. The atomic level mechanisms responsible for the precursor gas activation have not been studied in detail in many cases. The authors will review the state of the art and level of understanding of direct ion and electron beam fabrication and point out some of the unsolved problems.

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Gas-assisted focused electron beam and ion beam processing and fabrication

Defects of the multi-layer (ML) mirror on a EUV reticle, so-called ML-defects, are a prime aspect why EUV mask
defectivity is considered a challenge before EUV lithography can be used for the production of future node integrated
circuits. The present paper addresses the possibility to mitigate the printability of these defects by repair. Repair of
natural EUV mask defects is performed using the electron beam based Carl Zeiss MeRiT® repair technology and is
evaluated by wafer printing on the ASML EUV Alpha Demo Tool (ADT) installed at IMEC. Both absorber defects and
ML-defects are included. The success of absorber defect repair (both opaque and clear type) is illustrated. For
compensation repair of ML-defects experimental proof of the technique is reported, with very encouraging results both
for natural pits and bumps. In addition, simulation is used to investigate the limitations of such compensation repair,
inspired by the residual printability found experimentally. As an example it was identified that alignment of the
compensation repair shape with the ML-defect position requires sub-20nm accuracy. The integration of an Atomic Force
Microscope (AFM) into the repair tool has been an important asset to cope with this.

Repair of natural EUV reticle defects

The EUV-photomask is used as mirror and no longer as transmissive device. In order to yield defect-free reticles, repair
capability is required for defects in the absorber and for defects in the mirror. Defects can propagate between the EUV
mask layers, which makes the detection and the repair complex or impossible if conventional methods are used. In this
paper we give an overview of the different defect types. We discuss the EUV repair requirements including SEMinvisible
multilayer defects, and demonstrate e-beam repair performance. The repairs are qualified by SEM, AFM and
through-focus wafer prints. Furthermore a new repair strategy involving in-situ AFM is introduced. Successful repair is
demonstrated on real defects.

Closing the gap for EUV mask repair

In this paper, we present the test results obtained from the first commercial electron beam mask repair tool. Repaired defect sites on chrome-on-glass masks are characterized with 193nm AIMS to quantify the edge placement precision as well as optical transmission loss. The electron beam mask repair tool is essentially based on a scanning electron microscope (SEM), therefore, it can be used for in-situ CD and defect metrology. E-beam for EUV mask defect repair is also discussed. These early results are very encouraging and demonstrate the basic advantages of electron beam mask repair as well as highlight the key challenge of charge control.

Demonstration of damage-free mask repair using electron beam-induced processes

With continuously shrinking feature sizes of integrated circuits, the vast majority of semiconductor companies have become fabless, i.e., chip manufacturing has been outsourced to foundries across the globe. However, by outsourcing critical stages of IC fabrication, the design house puts trust in entities which may have malicious intents. This exposes the design industry to a number of threats, including piracy via unauthorized overproduction and subsequent reselling on the black market. One alleged solution for this problem is logic locking, also known as logic encryption, where the genuine functionality of a chip is “locked” using a key only known to the designer. If a correct key is provided, the design works as intended but with an incorrect key, the circuit produces faulty outputs. As the keys are inserted by the designer after production, an adversarial foundry should not be able to unlock overproduced chips. In this work, we highlight major shortcomings of proposed logic locking schemes. They are primarily due to the absence of a well-defined and realistic attacker model in the current literature. To this end, we characterize physical capabilities of a malicious foundry, especially with respect to invasive attacks. This allows us to derive an attacker model that matches reality, yielding attacks against the foundations of locking schemes beyond the usually employed SAT-based attacks. Our analysis shows that no previously proposed logic locking scheme is able to achieve the intended protection goals against piracy in real-world scenarios. As an important conclusion, we argue that there are strong indications that logic locking will most likely never be secure against a determined malicious foundry.

The End of Logic Locking? A Critical View on the Security of Logic Locking.

High-resolution electron-beam assisted deposition and etching is an enabling technology for current and future generation photo mask repair. NaWoTec in collaboration with LEO Electron Microscopy has developed a mask repair beta tool capable of processing a wide variety of mask types, such as quartz binary masks, phase shift masks, EUV masks, and e-beam projection stencil masks. Specifications currently meet the 65 nm device node requirements, and tool performance is extendible to 45 nm and below. The tool combines LEO's ultra-high resolution Supra SEM platform with NaWoTec's e-beam deposition and etching technology, gas supply and pattern generation hardware, and repair software. It is expected to ship to the first customer in October this year. In this paper, we present the tool platform, its work flow oriented repair software, and associated deposition and etch processes. Unique features are automatic drift compensation, critical edge detection, and arbitrary pattern copy with automatic placement. Repair of clear and opaque programmed defects on Cr, TaN, and MoSi quartz masks, as well as on SiC and Si stencil masks is demonstrated. We show our development roadmap towards a production tool, which will be available by the end of this year.

A novel electron-beam-based photomask repair tool

An electron beam technology for repair of Next Generation Lithography masks is described. Deposition of missing material in clear defects is shown with different material characteristics. Etching of opaque defects is demonstrated. The superiority of the electron beam technology to the well established and widely used focused ion beam techniques is discussed. Electron beam repair avoids the unacceptable transmission loss which is generated by focus ion beam techniques especially for 193 nm and 157 nm lithography by Ga-ion implantation. Shrinking dimensions of printable
defects require higher resolution than ion beams allow, which is, however, obtained routinely with electron beam systems. Specially designed lenses having low aberrations provide outstanding better signal to noise ratio than ion beam systems. Results on deposition and etching of NGL mask relevant materials like TaN, SiC, Mo/Si, and
silicon dioxide is demonstrated. In general 1 keV electrons and a low electron current were used for the etching processes.

Application of electron-beam induced processes to mask repair

The applicability of electron-beam induced chemical reactions to mask repair is investigated. To achieve deposition and chemical etching with a focused electron-beam system, it is required to disperse chemicals in a molecular beam to the area of interest with a well-defined amount of molecules and monolayers per second. For repair of opaque defects the precursor gas reacts with the absorber material of the mask and forms a volatile reaction product, which leaves the surface. In this way the surface atoms are removed layer by layer. For clear defect repair, additional material, which is light absorbing in the UV, is deposited onto the defect area. This material is rendered as a nanocrystalline deposit from metal containing precursors. An experimental electron-beam mask repair system is developed and used to perform exploratory work applicable to photo mask, EUV mask, EPL and LEEPL stencil mask repair. The tool is described and specific repair actions are demonstrated. Platinum deposited features with lateral dimensions down to 20 nm demonstrate the high resolution obtainable with electron beam induced processes, while AFM and AIMS measurements indicate, that specifications for mask repair at the 70 nm device node can be met. In addition, examples of etching quartz and TaN are given.

Ottmar Hoinkis

Papers

A novel electron-beam-based photomask repair tool

Application of electron-beam induced processes to mask repair

Electron-beam Induced Processes and their Applicability to Mask Repair