Christopher Magg

Recent progress in electron-beam resists for advanced mask-making

Resists for advanced mask-making with high-voltage electron-beam writing tools have undergone dramatic changes over the last three decades. From PMMA and the other early chain-scission resists for micron dimensions to the aqueous-base-developable, dry-etchable chemically amplified systems being developed today, careful tuning of the chemistry and processing conditions of these resist systems has allowed the patterning of photomasks of increasing complexity containing increasingly finer features. Most recently, our research efforts have been focused on a low-activation-energy chemically amplified resist based on ketal-protected poly(hydroxystyrene). These ketal resist systems, or KRSs, have undergone a series of optimization and evaluation cycles in order to fine-tune their performance for advanced mask-fabrication applications using the 75-kV IBM EL4+ vector scan e-beam exposure system. The experiments have led to an optimized formulation, KRS-XE, that exhibits superior lithographic performance and has a high level of processing robustness. In addition, we describe advanced formulations of KRS-XE incorporating organometallic species, which have shown superior dry-etch resistance to novolak-based resists in the Cr etch process while maintaining excellent lithographic performance. Finally, current challenges facing the implementation of a chemically amplified resist in the photomask manufacturing process are outlined, along with current approaches being pursued to extend the capabilities of KRS technology.

Simulating the effects of pattern density gradients on electron-beam projection lithography pattern transfer distortions

The development of a low-distortion mask is critical to the success of the sub-0.1 μm lithography technologies. Electron-beam projection lithography (EPL) is one of the potential candidates for next-generation lithography. In order to minimize mask image placement (IP) errors, it is important to understand the factors that induce pattern distortions during mask fabrication and pattern transfer. The fabrication process flows for two EPL mask formats were numerically simulated and experimentally assessed for IP. This study included continuous membranes and stencil membranes for 1 mm ×1 mm and 1 mm ×12 mm window sizes on a 4 in. wafer. Both intramembrane (i.e., within a single window) and intermembrane (i.e., cross-mask) results are reported with excellent correlation between the finite element (FE) data and the experimental measurements. In this article details of the FE simulations are presented; an article by (M. Lercel et al., J. Vac. Sci. Technol. B, these proceedings) describes the corresponding experi...

Patterning-induced image placement distortions on electron beam projection lithography membrane masks

Membrane masks are needed for charged particle lithography and can include both stencil masks and masks with thin continuous membranes. Producing accurate image placement on membrane masks requires careful control of mask shape, pattern writing, and stress control of the mask materials. Pattern density and pattern density gradients also affect image placement (IP) control. This article discusses IP distortions on electron projection lithography masks caused by patterning the imaging layers with low and high density patterns and patterns with large gradients in the density. The process-induced distortion has been found to be largest with the largest vector distortion at the boundary when high pattern density gradients are present. The anisotropic stiffness of the unit cell also affects the process-induced distortion. Qualitatively, the results between continuous membrane and stencil masks show similar characters. The results provide distortion information that could be used to determine the maximum allowab...

Image size control in next generation lithography masks

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.

Next-generation lithography mask development at the NGL Mask Center of Competency

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.

Comparison of substrate curvature and resonant frequency thin film stress mapping techniques

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.

High resolution patterning in chemically amplified resists: the effect of film thickness

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.

Chemically amplified resists for electron-beam projection lithography mask fabrication

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.

CA resist with high sensitivity and sub-100-nm resolution for advanced mask making

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+

Wavelength-dependent spot defects on advanced embedded attenuated phase-shift masks

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+.