Exposure of binary masks using ArF (193nm) photolithography processing is a common practice for layers with less critical imaging requirements, but they may experience obstacles for high volume manufacturing; mainly defectivity or requalification cost concerns. The usage of chrome on glass (COG) mask types under ArF wavelength exposure has been shown to lead to chrome migration issues, impacting mask integrity and critical dimensions (CDs) on wafer. While not experiencing the same defectivity concerns as COG, using opaque MoSi on glass (OMOG) as a replacement greatly increases the cost to build the mask but reduces the requalification efforts. We have observed that attenuated phase shift masks (PSMs) built for KrF wavelength (248nm) exposure show the same functional performance as a binary mask under ArF exposure for 45nm node technology. Initial feasibility investigation involved simulating wafer exposure with an OMOG mask and a KrF PSM mask under the same conditions. To demonstrate wafer performance, masks with and without 2nd level processing were built to verify exposure and tool handling capabilities. The application of KrF PSM as a binary mask under ArF photolithography processing for less critical layers of mature technology nodes shows to maintain pattern integrity at a lower total lifetime cost, compared to COG or OMOG, while providing comparable results on wafer.
Focus on Design for Manufacturing (DFM) in semiconductor device design has increased as semiconductor manufacturing technology has become more complex. Many of the techniques developed to improve wafer yield and manufacturability can also be applied to the photomask manufacturing process. For example, for the last several technology nodes, semiconductor manufacturers have known that pattern density and uniformity can have significant impact on wafer processes such as etching and chemical mechanical polishing. Photomask manufacturing can also be impacted by pattern density and its uniformity.
Some of these DFM practices can be beneficial if applied directly to photomask manufacturing while some of them can make photomask manufacturing significantly more difficult. Optical proximity correction (OPC), which involves convoluting the design shape to account for optical, physical and chemical processes, is increasingly required to support advanced lithography; some of the operational parameters of the OPC, such as the fragmentation run length, challenge mask resolution capability, image fidelity, defect inspection, mask repair, and dimensional metrology of photomasks. Sub-resolution assist features (SRAFs), which are utilized to create robust wafer lithography are often the most challenging mask features to create. The size and placement of SRAFs on photomasks are factors that impact photomask manufacturability in terms of image resolution, inspection, and dispositioning criteria. As OPC and other DFM processes become more widely deployed in an effort to make robust wafer manufacturing processes, the photomask maker needs to be involved to evaluate the implications to photomask manufacturing and assist in optimizing these DFM procedures to maximally benefit both the photomask and semiconductor manufacturing processes.
At the challenging ground rules required for 90 nm and 65 nm photomask production, new types of photomask defects are becoming increasingly prevalent. This paper discusses one particular new defect type found on critical 90 nm embedded attenuated phase-shift masks (EAPSMs). These defects had varying transmission characteristics depending on the wavelength used for analysis. Given that photomask inspection wavelength has historically lagged behind lithography wavelength, this type of defect can go undetected and poses a grave risk to wafer lithography yield. Detection and characterization methodologies will be presented along with aerial image analysis and wafer print evaluation results.
The performance of KRS-XE, a low activation energy, chemically amplified resist designed specifically for mask making with electron beam lithography, has been extended in terms of its sensitivity, coated-film stability and etch resistance. By careful manipulation of resist composition, high sensitivity formulations have been generated that will allow exposure doses of less than 10 mC/cm2 with 50 keV electron beam tools. This sensitivity enhancement has been achieved without sacrificing the robust process latitude previously reported for this resist. The performance of this resist can be maintained, even in coated film form, for prolonged periods of time by careful packaging of the coated films. Additionally, formulations with etch resistance versus chlorine/oxygen plasma in excess of that of novolak-based resists have been generated by the incorporation of organometallic additives. The combination of these improvements leads to resist formulations that will allow the high resolution and throughput that is demanded for state-of-the art mask making applications.
Recently, there is a significant interest in using CA resists for electron beam (E-beam) mask making application. CA resists provide superior lithographic performance in comparison to traditional non CA E-beam resists in particular high contrast, resolution, and sensitivity. However, most current CA resists exhibit very large sensitivity to PAB and/or PEB temperatures resulting in significant impact on CD. In addition, image collapse issues associated with high aspect ratio patterning as well as electron scattering effects in low KeV tools necessitate thinner resists. Therefore, there is a need to have a high etch resistant resist system which can withstand the demanding chrome etch process. Previously, we reported on the KRS-XE resist which exhibits dry etch resistance comparable to the best deep UV resist and excellent lithographic performance and bake latitudes. No PEB is needed for this resist. In this paper, we report on an advanced KRS-XE resist formulation which exhibits dry etch resistance surpassing the industry standard, novolak, in the chrome etch process. This new resist also exhibits excellent lithographic performance - 50nm lines/space delineated and requires no PEB. This paper will highlight the lithographic and etch performance of this new resist.
As thin film imaging becomes an accepted means of producing high-resolution microelectronics features, a host of new challenges has emerged. A dose dependence on resist thickness has been observed and systematically measured for chemically amplified resists exposed with 75 keV electron beam radiation. The required dose to print 100nm images increased as the thickness of the film decreased. A physiochemical explanation for this dependence was sought which included exploring thickness-induced variations in thermal characteristics of the resist film. Over the range of film thickness examined, 80-360nm, these parameters were deemed unlikely contributors to this phenomenon. Ultimately the data suggests that the dose variation with thickness may correlate to differences in the population of chemically effective electron with energies in the range of 10 to 100 eV that are responsible for the sensitization of electron beam resists.
Recently, there is significant interest in using CA resists for electron beam (E-Beam) applications including mask making, direct write, and projection printing. CA resists provide superior lithographic performance in comparison to traditional non CA E-beam resists in particular high contrast, resolution, and sensitivity. However, most current CA resists exhibit very large sensitivity to PAB and/or PEB temperatures resulting in significant impact on CD control. In addition, image collapse issues associated with high aspect ratio patterning as well as electron scattering effects in low KeV tools necessitate thinner resists. Therefore, there is a need to have a high etch resistant resist system that can withstand the demanding chrome etch process. Previously, we reported on the KRS-XE resist which exhibits dry etch resistance surpassing the industry standard, novolak, in the chrome etch process. This new resist also exhibits excellent lithographic performance - 75 nm lines/space and 55nm/110 space/lines delineated and requires no PEB. This paper will highlight the lithographic and etch performance of this new resist.
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.
A UV inspection tool has been used to image and inspect Next Generation Lithography (NGL) reticles. Inspection images and simulations have been used to provide feedback to mask makers so that inspectability of NGL masks can be optimized. SCALPEL masks have high optical contrast and look much the same in reflection as conventional chrome on glass masks do in transmission. EPL stencil masks can be imaged well in reflection, but defects below the top surface, in the cutouts, may not be detectable optically. EUV masks that have been made to date tend to have relatively low contrast, with line edge profiles that are complex due to interference effects. Simulation results show that improved EUV inspection images can be obtained with a low reflectivity absorbing layer and proper choice of buffer layer thickness.
Interest in chemically amplified (CA) resist systems has been increasing for advanced mask fabrication due to their superior e-beam sensitivity, contrast and resolution compared to traditional non-CA e-beam resists on high kV (50-100kV) exposure systems. However, most CA resists available are sensitive to temperature variations during the post apply and/or post exposure (PAB/PEB) process steps. This temperature sensitivity can result in poor CD uniformity across the mask. An advanced positive tone CA resist developed at IBM, KRS-XE, has been investigated for use in the fabrication of NGL masks (the continuous membrane and stencil versions of electron projection lithography as well as proximity x-ray masks). KRS-XE is an improved ketal resist system that is robust towards airborne contaminants, is compatible with 0.263N TMAH aqueous developer and has a wide PAB/PEB process latitude. This CA resist has been found to be insensitive with respect to dose and CD over a PAB temperature range of 105°C to 120°C and a PEB temperature range from room temperature to 110°C. Line/space features down to 75nm have been demonstrated in this resist on the IBM EL4+ 75kV vector scan e-beam system. This paper discusses the performance of KRS-XE with respect to CD uniformity on NGL membrane masks and compares the experimentally obtained results from both KRS-XE and a PEB sensitive CA resist (Shipley UVIII™) against the PEB temperature variations predicted from a finite difference model for each mask format.
Recently, there is significant interest in using CA resists for electron beam (E-Beam) applications including mask making, direct write, and projection printing. CA resists provide superior lithographic performance in comparison to traditional non CA E-beam resists in particular high contrast, resolution, and sensitivity. However, most of the commercially available CA resists have the concern of airborne base contaminants and sensitivity to PAB and/or PEB temperatures. In this presentation, we will discuss a new improved ketal resist system referred to as KRS-XE which exhibits excellent lithography, is robust toward airborne base, compatible with 0.263 N TMAH aqueous developer and exhibits a large PAB/PEB latitude. With the combination of a high performance mask making E-beam exposure tool, high kV (75 kV) shaped beam system EL4+ and the KRS-XE resist, we have printed 75 nm lines/space features with excellent profile control at a dose of 13 (mu) C/cm2 at 75 kV. The shaped beam vector scan system used here provides an unique property in resolving small features in lithography and throughput. Overhead in EL4+ limits the systems ability to fully exploit the sensitivity of the new resist for throughput. The EL5 system, currently in the build phase, has sufficiently low overhead that it is projected to print a 4X, 16G, DRAM mask with OPC in under 3 hours with the CA resist. We will discuss the throughput advantages of the next generation EL5 system over the existing EL4+. In addition we will show the resolution of KRS-XE down to 70 nm using the PREVAIL projection printing system.
A positive tone chemically amplified (CA) resist, Shipley UVIIITM, has been investigated for use in the fabrication of scattering electron-beam projection lithography (EPL) masks. Shipley UVIII is a DUV resist that also functions as a high resolution (sub 75 nm) e-beam resist with sensitivities of 12 - 30 (mu) C/cm2 at 75 keV depending upon the bake parameters, and is currently used in the manufacture of advanced x-ray masks with 90 nm feature sizes. This paper discusses the issues associated with the implementation of CARs in EPL mask processing, including the thermal variations and mechanical distortions which can cause nonuniformities during resist processing. The performance of the resist was evaluated based on critical dimension (CD) uniformity across the mask and within individual membranes, image placement (IP) performance within an individual membrane, and image quality. CD uniformity of less than 15 nm 3 (sigma) has been achieved across the mask (approximately 50 X 50 mm area) and less than 10 nm 3 (sigma) has been achieved intramembrane (1.1 X 12.1 mm area), for 400 nm nominal feature sizes in resist. Pattern transfer etch processes for a TaSi scatter layer have also been developed.
Mask image size variation is a major contributor to the total image size budget. To understand the source and contribution of various errors, we have characterized the image size variations on next generation lithography masks. CD control experiments initiated on x-ray masks are now being extended to other NGL technologies through the application of similar patterns, measurement strategy, and error budget partitioning. A systematic measurement methodology has been used to partition the variations into known components. Long-range variations have been found to be the dominant error, and in x- ray masks, are typically membrane edge effects and cross-mask bow. The membrane effects have been shown to be primarily driven by temperature differences during the post-expose bake (PEB) of the chemically amplified resist. To further understand the source of these temperature variations, the x- ray and SCALPEL mask PEB have been modeled through a finite- difference model. Key contributors to controlling bake temperature uniformity have been identified.
A crosscutting issue for Next Generation Lithographies is the ability to monitor and control the uniformity of thin film stresses. Because the global stress fields of thin film layers can introduce distortions in lithographic masks, it is essential that the characteristics of these stress fields be understood and controlled, in order to achieve the high resolution and positioning accuracy required. This paper provides a comparison between resonant frequency and substrate curvature stress mapping techniques. Experiments have been performed using the UW-CMC Rack RFT device and the commercially available Tencor FLX 5510. Measurements across two IBM diagnostic masks identify the magnitude and uniformity of as-deposited SiON film stress. An analysis of the accuracy and limitations of the experimental methods is discussed.
Masks for next generation lithographies present difficult technical processing, challenges for meeting the demanding requirements of future lithography. The Next Generation Lithography Mask Center of Competency is applying the proximity x-ray mask technology developed by the IBM advanced mask facility to all major NGL technologies. Mask fabrication has been demonstrated for proximity x-ray, scattering and stencil electron beam projection, and extreme UV lithographies. The imaging layer for these mask technologies differ significantly from one another and yet present similar processing challenges. This paper discuses the process technology developed at the NGL-MCOC associated with patterning the absorber layers. Issues with chemically amplified resist coating and baking and absorber etching are identified and associated with observed image size variations. The difficulty in the aspect ratio of. the absorber drives much of the processing requirements for the different NGL formats. High-aspect ratios on stencil and proximity x-ray masks require more difficult etch processes than those on lower-aspect ratio absorbers.
Recently, there is significant interest in using CA resist for electron beam (E-beam) applications including mask making, direct write, and projection printing. CA resists provide superior lithographic performance in comparison to traditional non-CA E-beam resist in particular high contrast, resolution, and sensitivity. However, most of the commercially available CA resist have the concern of airborne base contaminants and sensitivity to PAB and/or PEB temperatures. In this presentation, we will discuss a new improved ketal resists system referred to as KRS-XE which exhibits excellent lithography, is robust toward airborne base, compatible with 0.263N TMAH aqueous developer and exhibits excellent lithography, is robust toward airborne base, compatible with 0.263N TMAH aqueous developer and exhibits a large PAB/PEB latitude. With the combination of a high performance mask making E-beam exposure tool, high kV shaped beam system EL4+ and the KRS-XE resist, we have printed 75nm lines/space feature with excellent profile control at a dose of 13(mu) C/cm2 at 75kV. The shaped beam vector scan system used here provides a unique property in resolving small features in lithography and throughput. Overhead in EL4+$ limits the systems ability to fully exploit the sensitivity of the new resist for throughput. The EL5 system has sufficiently low overhead that it is projected to print a 4X, 16G DRAM mask with OPC in under 3 hours with the CA resist. We will discuss the throughput advantages of the next generation EL5 system over the existing EL4+.
Mask fabrication is one of the difficult challenges with all Next Generation Lithography (NGL) technologies. X-ray, e-beam projection, and ion-beam projection lithography all use some form of membrane mask, and extreme ultraviolet (EUV) lithography uses a reflective mask. Despite some differences, the various mask technologies share some common features and present similar fabrication difficulties. Over the past several years, the IBM Advanced Mask Facility (AMF) has focused on the fabrication of x-ray masks. Several key accomplishments have been demonstrated including fabricating masks with critical dimensions (CD) as small as 75 nm, producing line monitor masks in a pilot line mode to evaluate mask yields, and fabricating masks to make working microprocessors with the gate level defined by x-ray lithography. The experience on fabricating 1X x-ray masks is now being applied to the other NGL mask technologies. Progress on membrane and absorber materials can be applied to all the technologies, and patterning with advanced e-beam writing with chemically amplified resists utilizes learning from writing and baking on x-ray membrane masks.
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