Richard J. Kasica

Monte Carlo study of high performance resists for SCALPEL nanolithography

Abstract The semiconductor community continues to push the limits of device dimensions by exploring new high-resolution lithography technology. As part of the SCALPEL lithography resist program, our goal is to be able to print sub-100 nm structures at doses that will permit high throughput, reduce wafer heating and still maintain good process latitude. Using 100 KV exposures on a SCALPEL tool, 100 nm structures were printed at exposure dose of 5.8 μC/cm 2 (and 80 nm isolated trenches at 5.4 μC/cm 2 ) in positive resists. In negative resists, isolated 100 nm were printed at a dose of 6.8 μC/cm 2 , and 80 nm structures at 7.2 μC/cm 2 were resolved as well. These results are well below the 10 μC/cm 2 minimum dose requirement for high throughput. Monte Carlo simulations were used as means to understand energy absorption mechanisms of these e-beam optimized resists, DUV and 193 nm resists. Atomic composition was found to factor in improved resist ionization. The resin (or low-Z elements) is found to account for more than 99% of ionization events during exposure.

SCALPEL proof-of-concept system: preliminary lithography results

We have designed, constructed, and are now performing experiments with a proof-of-concept projection electron-beam lithography system based upon the SCALPELR (scattering with angular limitation projection electron-beam lithography) principle. This initial design has enabled us to demonstrate the feasibility of not only the electron optics, but also the scattering mask and resist platform. In this paper we report on some preliminary results which indicate the lithographic potential and benefits of this technology for the production of sub-0.18 micrometer features.

CMOS compatible alignment marks for the SCALPEL proof of lithography tool

SCALPEL alignment marks have been fabricated in a SiO 2 /WSi 2 structure using SCALPEL lithography and plasma processing. The positions of the marks were detected through e-beam resist in the SCALPEL proof of lithography (SPOL) tool by scanning the image of the corresponding mask mark over the wafer mark and detecting the backscattered electron signal. Single scans of line space patterns yielded mark positions that were repeatable within 30 nm 3σ with a dose of 0.4 μC/cm 2 and signal-to-noise of 16 dB. An analysis shows that the measured repeatability is consistent with a random noise limited response. The mark detection repeatability limit, that can be attributed to SPOL machine factors, was measured to be 20 nm 3σ. By using a digitally sequenced mark pattern, the capture range of the mark detection was increased to 13 μm while maintaining 36 nm 3σ precision. The SPOL machine mark detection results are very promising considering that they were measured under electron optical conditions that were not optimized.

Cleaning of SCALPEL next-generation lithography masks using PLASMAX, a revolutionary dry cleaning technology

Due to mechanical and exposure considerations, NGL mask technology lacks the ability to use a pellicle to prevent mask contamination. The PLASMAX (Plasma Mechanical Activation and Extraction of Particle Contamination) process represents a technology, which acts as the functional replacement of a pellicle for the NGL mask. This dry environmentally benign cleaning technology can be directly integrated into the exposure system and serves as an in-situ creative mask cleaning process. Unlike other, more conventional cleaning methods, PLASMAX lifts surface particles from the mask, then suspends, traps and channels these particles down the vacuum port, thus preventing particle redeposition on the mask surface. Originally demonstrated on wafers, this plasma/mechanical cleaning technology has demonstrated its ability to remove particles from the surface of NGL masks such as SCALPEL (Scattering with Angular Limitation in Projection Electron Beam Lithography) masks. PLASMAX uses the combined action of a gentle plasma with simultaneous vibration to clean the mask. Unlike all other methods of mask cleaning, PLASMAX uses no water or hazardous acids, thus reducing the cost of each cleaning step and eliminating the environmental impact of todays aqueous cleaning technologies. Initial work with SCALPEL masks showed them to be highly stable and robust in the PLASMAX environment while yielding cleaning efficiencies of 90% removal of polyester particles 0.8 micron and larger. The PlASMAX technology was proven to be effective in removing particles from the patterned front side and strutted backside of the mask. This paper focuses on the ongoing development of PLASMAX to enhance the cleaning efficiency of SCALPEL masks down to 0.25-micron particles. In addition, the cleaning efficiency of various particle materials will be studied. Sandia National Laboratories is providing software model simulations of the PLASMAX technology to assist in the development effort.

Fabrication and commercialization of scalpel masks

SCALPEL masks have been fabricated for use in the Proof-of- Lithography system and to demonstrate the feasibility of having them produced by a commercial blank manufacturer and optical mask shops. Masks blanks are formed from 100 mm diameter silicon wafers. A 100-150 nm thick SiNx layer is LPCVD deposited onto the wafers followed by magnetron sputter deposition of a thin Cr/W metal layer which is used as the scatterer layer for the mask>the mask is supported by an underlying network of struts which are arranged to be compatible with the step and scan writing strategy of the exposure tool and to provide robustness to the mask. Crystallographic wet etching of the silicon wafer forms membranes and struts. To date over 300 mask blanks have been formed and yield data as a function of the thickness of the silicon nitride membrane has been quantified. Recent developments in the mask blank formation process include the production of blanks by MCNC who serve as a commercial source of SCALPEL mask blanks. They have successfully delivered 36 blanks that exhibit equivalent properties to those produced at Lucent. Mask patterning has been performed at the commercial optical mask shops of PHOTRONICS and DUPONT. In this investigation a MEBES exposure system has been used to write patterns. The resist used is ZEP-520 and development and pattern transfer processes are performed in the STEAG-Hammatech spray/spin processing tool. Metrology is performed using a KMS 310 RT optical microscope. Pattern placement accuracy is measured on the LMS 2020 system without modification. The masks are inspected for defects using the optical based KLA 300 series inspection system in a die to die mode and in transmission. Results to date suggest feasibility of producing SCALPEL masks by a commercial blank supplier and by merchant optical mask shops.

200 mm SCALPEL mask development

SCALPEL is a tue 4X reduction technology that capabilities on high resolution capabilities from electron beam exposure and high throughput capabilities from projection printing. Current mask blank fabrication for SCALPEL technology use widely available 100 mm, crystalline silicon wafers. The use of 100 mm crystalline wafers and a wet, through wafer etch process causes the patterned strut width to increase as the wafer is etched and must be accounted for in the mask blank fabrication process. In the wet etch process, a 100 micrometers wide strut grows to 800 micrometers at the strut-membrane interface. As a consequence, the maximum printable die size due to the wafer size and the decreased amount of open area between each strut is 8 X 8 mm. Additionally, crystal defects in the silicon wafer affect the wet etch process and contribute to mask blank failures. A partial solution for an increased die size is to increase the wafer size used to make the SCALPEL mask blank. A 200 mm wafer is capable of producing large die sizes. This can be further improved by dry etching of the grill structure to form struts with vertical sidewalls. As a result, due sizes of 25 X 25 mm or 16 X 32.5 mm can be produced depending on the grill pattern used. However, use of large wafers and dry etching for mask blank formation has significant issues that must be addressed. Among the issues to be addressed are etch chemistries, etch mask materials, and wafer handling.

Production of SCALPEL masks in a commercial mask facility

The purpose of this paper is to investigate the viability of fabricating SCALPEL masks at a DuPont Photomasks, Inc. commercial mask shop. The MEBES 4500 electron beam exposure system and standard inspection tools were used in SCALPEL manufacture to study the key issues to be overcome and the key components needed to succeed in large-scale manufacture. SCALPEL is a next generation lithography technology being researched and developed at Lucent Technologies as the semiconductor industry moves beyond optical lithography. The SCALPEL tool uses a membrane-type mask for high-resolution patterning on Si wafers. SCALPEL mask manufacturing present new and challenging operations in a commercial mask production facility. The production sequence of SCALPEL masks is not uncommon to the current Cr/Qz environment, but introduces the commercial facility to issues at a different level. SCALPEL mask exposure has been accomplished using MEBES III and an advanced MEBES 4500 e-beam lithography system. Pattern imaging, CD metrology, defect inspection, registration metrology, mask handling, and cleaning operations have been attempted with various levels of success. Data and further development of the processes in the commercial facility, along with the challenges and results of these experiences, are detailed in this presentation.

Techniques to inspect SCALPEL masks

As semiconductor lithography nodes become increasingly difficult to achieve with traditional optical lithography, several new technologies have emerged. SCALPEL (SCattering with Angular Limitation Electron beam Lithography) is at the forefront of the NGL technologies. SCALPEL technology uses an electron beam rather than laser light to produce images on the wafer. The SCALPEL mask is non-traditional in the sense that it is silicon-based instead of glass-based and the patterns are written on a membrane. SCALPEL provides unique challenges for the mask maker as well as the semiconductor manufacturer. In this study, we have demonstrated that the KLA-Tencor 3XX platform is capable of inspecting prototype SCALPEL reticles for pattern defects. The inspections were performed with two light wavelengths: 488 nm and 365 nm. Included are the difficulties faced and a projected roadmap for the inspection tool when SCALPEL enters at the 100 nm technology node.

Overview of SCALPEL mask technology

Scattering with angular limitation projection electron beam lithography is a true 4X reduction technology that capitalizes on high resolution capabilities from electron beam exposure and high throughput capability from projection.

Defect inspection and linewidth measurement of SCALPEL thin membrane masks using optical transmission

The purpose of the study reported here was to determine the range of material parameters and optical conditions necessary for using light to identify and categorize defects and to measure linewidths in SCALPEL masks. A prototype 4X SCALPEL mask with a 150 nm SiNx membrane and 50/10 nm W/Cr scatterer was used for the measurements. Die to die defect inspections were performed using a KLA 300 Series mask inspection system with 488 nm light in transmission. There was sufficient contrast to detect defects within test features with critical dimensions as small as 0.72 micrometer which would make optical defect inspection feasible for the 0.18 micrometer generation of integrated circuit (IC) reticles. Linewidth measurements were performed with the KMS 310RT mask metrology system in transmission on features ranging from 1.04 to 0.32 micrometer and compared to scanning electron microscope (SEM) measurements. The optically measured linewidths were linear in the range 0.4 to 1.04 micrometer which would be suitable for 0.1 micrometer IC reticles. The optical properties of SCALPEL masks constructed with Si3N4 membranes were calculated as a function of wavelength and membrane thickness. The requirements for extending optical inspection capability to smaller feature sizes and other measurement modes are discussed.