Melissa Shell

Resolution degradation caused by multispur effect in chemically amplified extreme ultraviolet resists

Extreme ultraviolet (EUV) (92.5 eV) radiation is expected as a next-generation exposure source. When EUV photons enter resists, they are absorbed by resist molecules and photoelectrons are emitted. Photoelectrons with the initial energy of approximately 80 eV further induce ionization and generate ion pairs. Because the inelastic mean free path of these electrons is less than 1 nm, ion pairs are so narrowly distributed that they interact with each other through Coulomb forces. In this study, the details in the interaction among ion pairs were investigated by using a Monte Carlo simulation. It was found that the electric interaction between ion pairs significantly degrade the initial acid distribution. The dependence of resolution degradation and multispur effect on acid generator concentration was clarified. The acid generator loading of >10 wt % is effective for the suppression of resolution degradation.

Theoretical Study on Relationship between Acid Generation Efficiency and Acid Generator Concentration in Chemically Amplified Extreme Ultraviolet Resists

Understanding of the relationship between acid generation efficiency and acid generator concentration is important for the process and material design. In this study, the relationship was clarified by a Monte Carlo simulation. Using the obtained relationship, the dose dependence of acid generation and the depth profile of acid distribution were calculated. It was found that the acid generator concentration of >20 wt % is required within the sensitivity range of 20–30 mJ cm-2 for the fine patterning of chemically amplified extreme ultraviolet resists.

One small step: world's first integrated EUVL process line

The Intel lithography roadmap calls for Extreme Ultraviolet Lithography (EUVL) to be used for the 32 nm node. With the installation of the EUV Micro-Exposure Tool (MET) complete, Intel now has the worlds first integrated EUVL process line including the first commercial EUV exposure tool. This process line will be used to develop the EUV technology, including mask and resist, and to investigate issues such as defect printability. It also provides a test-bed to discover and resolve problems associated with using this novel technology in a fab (not lab) environment. Over 22,000 fields have been exposed, the discharge-produced plasma light source has operated for 50,000,000 pulses, 8 masks have been fabricated, and 8 resists have been characterized. The MET combines high resolution capability with Intels advanced processing facilities to prepare EUVL for high-volume manufacturing (HVM). In this paper we review the MET installation and facilities, novel capabilities of the linked track, data on optics quality and modeled tool capability, and the MET mask fabrication process. We present data on tool performance including printing 45 nm 1/2 pitch lines with 160 nm depth of focus and 27 nm isolated lines. We show tool accuracy and repeatability data, and discuss issues uncovered during installation and use.

Determination of the flare specification and methods to meet the CD control requirements for the 32-nm node using EUVL

The minimum gate CD for the 32 nm node is 15 nm and the CD control requirement on the gate CD is < 2.5 nm 3σ. One of the major concerns for meeting these targets using EUV lithography is flare. Flare degrades the aerial image contrast which decreases the process window, and within-die chrome density variation results in local flare variation which worsens the CD control. Since mirror roughness contributes to flare, mirror polishing needs to be improved so that the Mid Spatial Frequency Roughness (MSFR) will be reduced to < 0.14 nm/mirror for 6 mirror imaging systems. In this paper, we will determine the minimum acceptable flare for the 32 nm node to meet the CD target and control requirements using modeling and present methods to meet them as demonstrated by experiments run on the Engineering Test Stand (ETS). Effectiveness of flare mitigation methods using chrome dummification and negative tone resists are quantified, and the capability of Flare Variation Compensation (FVC) to meet CD control targets are verified experimentally.

Theoretical Study on the Dependence of Acid Distribution on Material Properties of Chemically Amplified Extreme Ultraviolet Resists

In chemically amplified resists used for ionizing radiation such as extreme ultraviolet (EUV) radiation, secondary electrons play an important role in acid generation. In particular, the electron migration after thermalization is closely related to the resolution and acid generation efficiency of the resist. In this study, we investigated the effect of the material properties of the resist on the resolution blur and acid generation efficiency. By using a Monte Carlo simulation, the effects of the effective reaction radius of acid generators, the dielectric constant of the polymer, and the thermalization distance were clarified. For a given thermalization distance, there is a trade-off relationship between the resolution and acid generation efficiency of the resist. However, the thermalization distance should be reduced because its effect on the resolution is greater than that on the efficiency. For the effective reaction radius of the acid generators and the dielectric constant of the polymer, the relationship is not a trade-off one. These two parameters should be increased to improve resist performance in terms of resolution and acid yields.

Comparison of techniques to measure the point spread function due to scatter and flare in EUV lithography systems

The source of flare in EUVL systems is mostly from the mid-spatial frequency roughness (1 /μm - 1 /mm spatial periods) of mirrors. Due to the challenges in polishing mirrors to a small fraction of the wavelength, flare in EUV lithography tools is expected to be greater than flare in current DUV tools. Even though EUV flare is constant across the field, there can be within-die flare variations due to variations in layout density. Hence, it is expected that to meet the CD control requirements for the 32 nm node, Flare Variation Compensation (FVC), akin to Optical Proximity Correction (OPC) would be required. FVC needs the within-die flare level estimated by convolving the Point Spread Function due to scatter (PSFsc) with the mask layout. Thus, accurate knowledge of the system PSFsc is essential for FVC. Experimental results of the Modulation Transfer Function (MTF) technique to estimate flare and the PSFsc of the Engineering Test Stand (ETS) are presented. It was also determined that due to the nature of the PSFsc in EUVL tools a more accurate measure for flare would be to use the 0.5 μm line as opposed to the current 2 μm line standard for measuring flare on DUVL tools.

Intel's EUV resist development

The success of extreme ultraviolet (EUV) lithography depends upon developing resists that meet the patterning requirements for the technology node in which EUV is inserted. This paper presents Intel’s patterning requirements and development strategies for EUV resists. Two of the primary problems for EUV resists are meeting the linewidth roughness (LWR) requirement, and reducing resist absorbance to obtain good sidewall profiles. Benchmarking data shows that none of the current EUV photoresists meet LWR targets. Modeling results for EUV resists show the impact of resist absorbance on sidewall angle and resolution.

Mask substrate requirements and development for extreme ultraviolet lithography (EUVL)

The mask is deemed one of the areas that require significant research and development in EUVL. Silicon wafers will be used for mask substrates for an alpha-class EUVL exposure tool due to their low-defect levels and high quality surface finish. However, silicon has a large coefficient of thermal expansion that leads to unacceptable image distortion due to absorption of EUV light. A low thermal expansion glass or glass-ceramic is likely to be required in order to meet error budgets for the 70 nm node and beyond. Since EUVL masks are used in reflection, they are coated with multilayers prior to patterning. Surface imperfections, such as polishing marks, particles, scratches, or digs, are potential nucleation sites for defects in the multilayer coating, which could result in the printed defects. Therefore we are accelerating developments in the defect reduction and surface finishing of low thermal expansion mask substrates in order to understand long-term issues in controlling printable defects, and to establish the infrastructure for supplying masks. In this paper, we explain the technical requirements for EUVL mask substrates and describe our efforts in establishing a SEMI standard for EUVL masks. We will also report on the early progress of our suppliers in producing low thermal-expansion mask substrates for our development activities.

Use of the in-process bond shear test for predicting gold wire bond failure modes in plastic packages

A strong relationship is found between in-process (post wirebond) bond shear test results on gold ball bonds and post-process, post-board mount wire pull results. The shear test results detect clear differences in shear strength and failure mode between two types of bond pad structures. Significant variation due to the level of ultrasonic power used in bonding was also found. The subsequent wire pull results obtained from units receiving full package assembly and simulated customer board mount agreed with the in-process shear findings of both pad structure and bonding process effects. The results have promising implications for the use of in-process bond shear as a tool for improving ball bond reliability through wirebond process control and bonding parameter optimization.<<ETX>>

Applications of infrared microscopy for bond pad damage detection

The authors quantitatively compare three techniques for their utility in detecting bond pad structural cracking phenomena: wire pull, metal removal, and IR microscopy. The IR microscope (IRM) method was found, statistically and practically, to provide the most information about damage to the aluminum bonding pad and its underlying layers. This study was undertaken with three goals in mind: to identify the types of damage distinguished by the IRM, to demonstrate the utility of using an IRM to characterize bond damage, and to compare IRM performance with existing techniques. Only IRM examination of single level devices is covered. The authors describe the IRM technique and the procedures for the IRM sample preparation and examination. They employ these procedures to quantitatively compare IRM to mechanical wire pull and metal removal for bond pad damage detection.<<ETX>>