Aspects of the present invention provide for a novel photomask for patterning features for an integrated circuit, the photomask including masked features having interior nonprinting windows. In some embodiments, the interior nonprinting window is an alternating phase shifter, while the area surrounding the masked features transmits light unshifted. In other embodiments, the interior nonprinting window transmits light unshifted, while the area surrounding the masked features is an alternating phase shifter. Thus any arrangement of features can be patterned with no phase conflict.

Photomask features with interior nonprinting window using alternating phase shifting

Step and flash imprint lithography (SFIL) is an attractive method for printing sub-100 nm geometries. Relative to other imprinting processes SFIL has the advantage that the template is transparent, thereby facilitating conventional overlay techniques. In addition, the imprint process is performed at low pressures and room temperature, minimizing magnification and distortion errors. The purpose of this work was to investigate alternative methods for defining high resolution SFIL templates and study the limits of the SFIL process. Two methods for fabricating templates were considered. The first method used a very thin (<20 nm) layer of Cr as a hard mask. The second fabrication scheme attempts to address some of the weaknesses associated with a solid glass substrate. Because there is no conductive layer on the final template, scanning electron microscopy (SEM) and defect inspection are compromised. By incorporating a conductive and transparent layer of indium tin oxide on the glass substrate, charging is suppressed during SEM inspection, and the transparent nature of the final template is not affected. Using ZEP-520 as the electron beam imaging resist, features as small as 20 nm were resolved on the templates. Features were also successfully imprinted using both types of templates.

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High resolution templates for step and flash imprint lithography

Photomask plasma etching was thoroughly reviewed over wide topics including history, equipment, etchant, absorbers, phase shifters, thermodynamics, and kinetics. Plasma etch obtained industrial applications for photomask fabrication in the 1990s and presently is a critical fabrication step for the “enabling” photomask industry. Among all types of photomasks (binary, embedded attenuated phase-shift mask, and alternating aperture phase-shift mask), chromium (Cr) containing material etching has been the basis and fundamental for all photomask etches. The main technological challenges for Cr etch occur on (isolated) dark features of a high load photomask due to the etch critical dimension (CD) bias dependence on the local loading. It determines the CD features on the Cr layer, phase shifter MoSi layer, and fused silica (quartz) layer. The CD deviation on pattern layers from the nominal value has been a challenge, especially for the Cr state-of-the-art 65nm node photomasks even though data sizing exists. Induc...

Photomask plasma etching: A review

Improved step and flash imprint lithography templates for nanofabrication

Reactive ion etching (RIE) and inductively coupled plasma (ICP) Cr etch processes have been evaluated for fabrication of extreme ultraviolet lithography masks on 200 mm substrates Experiments were completed to optimize the Cr etch rate, etch rate uniformity, and Cr to resist selectivity for both etch processes The best ICP process was found to have superior etch rate, etch rate uniformity, and comparable Cr to resist selectivity to the RIE process The effect of exposed Cr area on the Cr to resist selectivity, critical dimension (CD) bias, and CD bias 3σ was also investigated A decrease in exposed Cr area from 95% to 7% was found to dramatically increase the Cr etch rate, leading to an increase in the Cr to resist selectivity Lower Cr loading was also found to decrease the CD bias and CD bias 3σ The average CD bias was very high for both processes, with the ICP etch process having a higher CD bias (143 nm) than the RIE process (89 nm) The CD bias uniformity was significantly lower for the ICP proces

Cr absorber etch process for extreme ultraviolet lithography mask fabrication

The use of Plasma Etch in patterning Binary Cr layers for modern reduction reticles has seen dramatic increase in the past two years. The drive towards the 0.25 micrometer and 0.18 micrometer technology has rendered wet etch of Binary Cr inadequate for the demanding gate level designs of most advanced devices. The use of dry etch for these patterns is studied closely through the pattern loading within a mask (Exposed Cr Load). It has been seen that Cr Load strongly affects several plasma etch responses, e.g.: resist selectivity, Cr etch rate, overall CD Uniformity and within- Mask CD Uniformity pattern.

Plasma etch of binary Cr masks: CD uniformity study of photomasks utilizing varying Cr loads

As on-glass line widths shrink and exposure wavelengths approach the physical limitations of optical lithographic printing, the adoption of newer technology such as Off-Axis Illumination and Phase Shift Photomask technologies will substantially expand the operating life of DUV lithographic tools. In this article, the dry etch processes and Inductively Coupled Plasma (ICP) hardware iterations associated with the etch optimization of Levenson-style hard shifters are explored. These Alternating Aperture Hard Shifters currently adopt a single material form, with Levenson-style photomasks making use of a precise removal of quartz material between the Cr lines of a standard Photomask, improving the resolution of the exposed features. This precise quartz removal is performed utilizing dry etch technologies, with the use of high density, de-coupled plasmas such as ICP preferred. We explore Inductively Coupled Plasma shaping techniques along with newer etch processes for these materials, offering a Next Generation ICP Source design. Process conditions are verified and on-mask results are reported.

Plasma etching of quartz for the fabrication of alternating aperture phase Shift photomasks : ETCH rate uniformity study utilizing a Next Generation ICP Source

A mask patterning technology for the 90nm technology node has been developed using the FujifilmARCH resist FEP171 and the state-of-the-art mask making tools SteagHamaTech mask coater ASR5000, Leica 50kV variable shaped e-beam writer SB350, SteagHamaTech developer ASR5000 and UNAXIS Mask Etcher III. A resist resolution of below 100nm dense lines and 150nm contact holes was demonstrated. The line width shrinking due to chrome etching varies between 25nm and 50nm per feature and a corresponding resolution of 125nm dense lines in a 105nm thick chrome absorber has been achieved. The global CD-uniformity with a 3σ of 7.7nm and a total range of 10.8nm met the requirements of the ITRS roadmap. The local uniformity with a 3σ of 3.8nm and a range of 5.6nm offers potential for future application of the Leica SB350. Applying of a new correction method taking electron scattering and process characeristics into account provides a linearity of 6.1nm. In addition, the line width of different featurees was kept in a range up to 12nm when the local pattern density was changed. The composite placement accuracy of 12nm fulfills already the requirements of the 65nm node. A special investigation proved the excellent fogging depression of the SB350.

90-nm mask making processes using the positive tone chemically amplified resist FEP171

As critical dimensions and exposure wavelengths approach the physical limitations of optical lithography, the use of newer techniques such as Phase Shift Photomask Technologies become necessary to extend the viability of DUV lithography tools. Alternating Phase Shift Mask technologies are challenging the capabilities of current quartz dry etch processes; as this phase shift technique is achieved by the precise removal of quartz, the need for ever improving phase shift uniformity across the photomask surface requires extremely uniform quartz etch depth. To this end, a Next Generation ICP (Inductively Coupled Plasma) hardware configuration has been adopted. In this article, the quartz etch parameter space of this new ICP source is explored. Finally, process results including, quartz roughness, sidewall profile, and most importantly quartz etch rate uniformity will be presented.

Investigation of quartz etch rate uniformity for alternating phase-shift mask applications utilizing a next-generation ICP source

As the 193 nm generation of steppers reaches the limit of its capability, alternating aperture phase shift masks (altPSMs) are necessary to extend the lifetime of these tools. The fabrication of a production-worthy altPSM requires that the quartz dry etch satisfy many conditions. The etched quartz features must not only show excellent phase uniformity, but they should have near vertical sidewalls and good etch depth linearity across a wide range of feature sizes. Surface roughness must also be low enough that transmission is unaffected. To this end, Unaxis USA performed a series of quartz photomask dry etch experiments utilizing a Unaxis Mask Etcher III. Etch depth uniformity and etch depth linearity are studied for each experiment. SEM cross-sections of the etched profiles and AFM analysis of surface roughness are also provided. Various models were constructed by IBM that demonstrate the importance of some of the etch responses, and the results from the optimized Unaxis process will be shown.

Chris Constantine

Papers

Plasma etch of binary Cr masks: CD uniformity study of photomasks utilizing varying Cr loads

Plasma etching of quartz for the fabrication of alternating aperture phase Shift photomasks : ETCH rate uniformity study utilizing a Next Generation ICP Source

90-nm mask making processes using the positive tone chemically amplified resist FEP171

Investigation of quartz etch rate uniformity for alternating phase-shift mask applications utilizing a next-generation ICP source

Evalution of 193-nm alternating-aperture phase-shift mask dry etch processes