Minimizing mask-level distortions is critical to ensuring the success of electron projection lithography (EPL) in the sub-65-nm regime. Previous research has demonstrated the importance of controlling the stress in the patterned stencil membranes to minimize image placement distortions. Low-stress, 100-mm diameter EPL mask blanks have been patterned with a layout that simulates the effects of the cross-mask and intra-subfield pattern density gradients found in a realistic circuit design. Extensive IP measurements were made to illustrate how local subfield correction schemes can be used to reduce all mask-level distortions (regardless of pattern type) to less than 15 nm (3s). Combining membrane stress control with the use of repeatable and identical reticle chucking is expected to reduce EPL mask-level distortions to the values that will be needed for the 65-nm design node.
Fabrication of EUVL masks requires formation of both a repair buffer layer and an EUV absorber layer on top of a molybdenum/silicon (Mo/Si) multilayer coated mask blank. Alteration of the Mo/Si multilayer during etch, repair or cleaning of the EUVL mask can be detrimental to the reflectivity and thus the functionality of the final mask. IBM’s Next Generation Lithography (NGL) group has reported on EUVL mask fabrication based on an absorber of low stress chromium (Cr) and a buffer layer of silicon dioxide (SiO<sub>2</sub>). Due to poor etch selectivity between SiO<sub>2</sub> and the underlying silicon capping layer, the finished masks had non-uniform and reduced EUV reflectivity after processing. This led to the development of an alternative absorber stack combination of an absorber layer of low stress TaN<sub>x</sub> on a buffer layer of low stress Cr. This paper describes the improved reflectivity uniformity of this type of mask along with several aspects of mask quality, such as CD control and image placement.
Electron Projection Lithography ( EPL) is a leading candidate for the sub-65 nm lithography regime (1),(2). The development of a low-distortion mask is critical to the success of EPL. EPL has traditionally used either a stencil format mask with a single scatterer layer having the pattern represented by voids in the membrane (3), or a continuous membrane format mask with a patterned scatterer layer supported by an unperforated membrane(4).
Electron projection lithography (EPL) is a promising candidate for the next generation lithography choice. There are several advantages to EPL, such as a large depth of focus and a lower relative mask cost. Significant challenges also face this technology, including the limitation of the membrane format of the mask. One of the obstacles with the membrane format is image placement distortions, which can be very sensitive to the stress of the membrane as well as the pattern density. This paper studies how the stress of various types of films effects image placement distortions, as well as examines the effect of final mask cleans on image placement distortions.
The Next Generation Lithography (NGL) Mask Center of Competency (MCoC) has been developing mask technology to support all of the major next generation lithographies for several years. Cross-cutting process development has been applied to generate progress in both the membrane and reflective mask formats. The mask technology has been developed to early capability stage for all of the mask formats. Proximity x-ray masks, although only for certain niche applications, are a very developed mask format. This information has been used to produce electron beam projection masks, in both continuous membrane and stencil formats, and extreme ultraviolet lithography masks. In this paper, we discuss the status of the lithography technology development and the obstacles that remain between the current early development capability and the availability for manufacturing.
The next generation lithography, either electron or photon based, will be first introduced on critical levels for device manufacture. These levels have different requirements for difficulty of meeting image size uniformity, image placement, and patterning requirements on masks. Membrane masks are needed for electron projection lithography (EPL), and the fabrication of membrane masks generates new requirements such as the need for complementary mask pairs for stencil masks. In this paper, we discuss experiments for fabricating EPL masks for device levels.
The extreme ultraviolet lithography (EUVL) mask differs from its predecessors in many ways. The most significant change is that the EUVL mask is reflective, introducing many new film layers and mask sensitivities. An additional complication is the small linewidths associated with the 45-nm node that is targeted for EUVL mask introduction. This paper concentrates on the physical specifications associated with the 45-nm node EUVL mask. Relative to current masks, the defect levels must be lower and the film quality must be higher. Standard cleans may be incompatible with new mask requirements. To understand the development requirements, the cleaning efficiency, film removal, film roughness, defect levels and film reflectivity are quantified on both EUVL mask film monitors and EUVL masks. Target specifications and measured properties of the 45-nm node masks will be compared.
Fabricating masks for extreme ultraviolet lithography is challenging. The high absorption of most materials at 13.4 nm and the small critical dimension (45 nm) at the target insertion node force many new features, including reflective mask design, new film choices, and stringent defect specifications. Fabrication of these masks requires the formation and patterning of both a repair buffer layer and an EUV absorber layer on top of a molybdenum/silicon multi-layer substrate. IBM and Photronics have been engaged in developing mask processing technology for x-ray, electron beam projection and extreme ultraviolet lithographies at the Next Generation Lithography Mask Center of Competency (NGL-MCoC) within IBM's mask facility at Essex Junction, Vermont. This paper describes recent results of mask fabrication on 6 x 6 x 1/4 inch EUVL substrates (quartz with molybdenum silicon multi-layers) at the MCoC. Masks fabricated with high and low-stress chromium and externally deposited chromium absorber films are compared. In particular, etch characteristics, image size, image placement, line edge roughness, and defect levels are presented and compared. Understanding the influence of the absorber film characteristics on these parameters will enable us to optimize the effectiveness of a given absorber film or to select acceptable alternatives.
Fabrication of masks for EUVL requires the formation and patterning of both repair buffer and EUV absorber layers on top of a molybdenum/silicon multi-layer substrate. Films used for buffer and absorber should have low stress, good uniformity and good etch selectivity to underlying layers. Low stress chromium and tantalum nitride absorber film deposition processes have been developed and characterized on fused silica substrates at the MCoC. Sputtered silicon oxide was used as the buffer layer for work reported in this paper. This paper describes the results of EUVL mask processing at the MCoC, including deposition and etch capabilities of these films. Properties of the low stress chromium and tantalum nitride materials will be discussed, including stoichiometry, stress, uniformity and density. The chromium and tantalum nitride films have been integrated into a mask patterning process with a silicon oxide buffer layer. Etch bias and etch profiles from the two absorber films along with etch selectivities to the underlying silicon oxide layer will be presented. Image size results for both types of absorber layers will be presented, including the improvement in etch bias using the low stress chromium Complete EUVL masks with 160 nm feature sizes have been fabricated with these processes and micrographs of nested lines and elbows will be presented.
Masks for electron projection lithography require the use of thin membrane structures due tot he short scattering range of electrons in solid materials. The two leading mask formats for electron projection lithography are the continuous membrane scatterer mask and the stencil mask. The reduced mechanical stability of the membranes used for electron projection masks relative to conventional optical masks leads to increased levels of process induced image placement distortions. This paper evaluates the image placement distortions due to the pattern transfer processes on the continuous membrane mask format. Image placement was measured from both a cross-mask and intramembrane perspective to evaluate the effects of different patterns, pattern densities and density gradients on the observed image placement and the experimental results obtained were then compared to those predicted by finite element modeling.