The absorbed energy density (eV/cm3) deposited by extreme ultraviolet (EUV) photons and electron beam (EB) high-keV electrons is proposed as a metric for characterizing the sensitivity of EUV resist films. Simulations of energy deposition are used to calculate the energy density as a function of the incident aerial flux (EUV: mJ/cm2, EB: μC/cm2). Monte Carlo calculations for electron exposure are utilized, and a Lambert–Beer model for EUV absorption. The ratio of electron flux to photon flux which results in equivalent energy density is calculated for a typical organic chemically amplified resist film and a typical inorganic metal-oxide film. This ratio can be used to screen EUV resist materials with EB measurements and accelerate advances in EUV resist systems.
In Optical Maskless Lithography, the die pattern to be printed is generated by a contrast device, known as a Spatial Light Modulator. The contrast device consists of a multitude of micro-mirror pixels that are independently actuated. Different physical principles can be utilized to change the optical properties of the pixels. Rasterization in Optical Maskless Lithography is an algorithm that, given the description of a pattern to be printed (e.g. an OPC'd GDS-II or OASIS mask file), computes the necessary states of the contrast device pixels. A Global Optimization rasterization algorithm for Optical Maskless Lithography was recently developed and successfully tested. Utilizing optimization techniques, this algorithm enables contrast devices to match the imaging and placement performance of conventional masks thru focus and dose. The algorithm has been demonstrated for contrast devices based on various light modulation principles, including tilt, phase-step tilt, and piston mirror devices.
This paper enhances the Global Optimization algorithm by significantly improving both computational time and memory requirements. These enhancements enable the algorithm to be implemented on an Optical Maskless Lithography scanner for printing die patterns of full size and complexity. The enhanced method is demonstrated on 130 nm node and 90 nm node SRAM layout test cases to validate the capability of Optical Maskless Lithography to reproduce realistic patterns. Simulations of the dose/focus process window in resist for rasterized patterns are presented, along with the ability of the rasterized images to match the CD and placement error performance of a conventional mask to below the level of process noise. In addition, the rasterization algorithm enhancements are verified experimentally on a calibrated tilt mirror spatial light modulator mounted to a 193 nm aerial image test stand.
Ever-increasing reticle cost makes optical maskless lithography an attractive alternative to mask-based technologies, particularly for low-volume runs such as prototypes, ASIC personalization, and engineering short loops. If the resolution and imaging performance of the optical maskless exposure tool can match or exceed standard reticle based scanners, then one can seamlessly integrate mix-and-match strategies into the manufacturing flow or even go to an all maskless strategy since resists and film stacks are unchanged. We have developed optical maskless analogs for a majority of the reticle based strong phase shifting techniques. These include analogs to binary, attenuated PSM, alternating PSM, CPL + assist features, and vortex reticles. We will present simulation of maskless vs. reticle based lithography of all these techniques, demonstrating how to move off grid, change CD, OPC correct through pitch, and present common feature process windows and CD / image placement error sensitivities that suggest that for certain applications, optical maskless will be superior to reticle based lithography.
Maskless lithography imaging based on SLM tilt mirror architecture requires illumination of light on a non-planar reflective topography. While the actual mirror dimensions can be much larger than the wavelength of light, the spacing between mirrors and the tilt range of interest are on the order of the wavelength. Thus, rigorous electromagnetic solution is required to capture light scattering effects due to the non-planar topography. We combine high NA imaging simulation with rigorous simulation of light scattering from the mirrors to study its effects on 65nm maskless lithography imaging. We vary mirror size, mirror tilt arrangemetn, feature type and illumination settings and compare the rigorous light scattering imagign resutls wtih standard imaging simulations using Kirchoff approximation. While electromagnetic scattering effects are present in the form of lateral standing waves and edge streamers in reflected light near-field intensity, they have negligible effects on SLM imaging for mirror sizes more than 1μm<sup>2</sup>. The effects of mirror tilt arrangement on diffraction orders aer used to study the through-focus behavior of alternating rows arrangement used in the SIGMA maskwriters as well as alternative arrangements. The good imaging properties of the alternating rows arrangement used in the SIGMA maskwriters as well as alternative arrangements. The good imaging properties of the alternating rows arrangement are confirmed and a multipass overlay scheme giving further image fidelity improvements is suggested.
We have been exploring alternating aperture phase-shifting masks for Application Specific Integrated Circuit poly gate CD's below 100 nm. The implementation is dark field altPSM with a complementary bright field binary 'trim' mask. The alternating phase shift approach is attractive because of the potential for improved resolution, increased individual process windows, and reduced developed resist line edge roughening (LER) needed for superior device performance. This can be accomplished with moderately low exposure tool numerical aperture (NA). However, these improvements must justify the increased mask cost, throughput reduction of dual mask, and the necessity of optical and process pattern correction (OPC) of 2 masks. Compared to the single reticle non-strong phase shift approach, the altPSM option potentially has new modes of failure: image intensity imbalance between the 0 and 180 degree phases, phase error sensitivity, and quartz sidewall angle sensitivity, all as a function of feature p9itch. Additionally, the high coherence required to print altPSM sensitivities plus corrections for mask writing inaccuracies, lithography printing inaccuracies, and etch inhomogeneities. Manufacturing with altPSM adds additional CD uniformity requirements across the chip. In this paper we discuss the performance of a 193 nm altPSM dual mask set's printing across the exposure slit of an ASML/950 scanner in the 80 nm regime. We will show how wafer level image placement error varies with focus and pitch across the scanner slit. We will discuss 3 methods of OPC correction as a function of CD and pitch across the scanner slit, their self-agreement, and OPC grid requirements. We will also present OPC corrected common process windows across the slit.
Deep ultraviolet (DUV) bottom anti-reflective coating (BARC)- to-resist compatibility is a key component in process optimization. In addition to the reduction of optical interference effects, BARC's also improve CD uniformity by preventing substrate contamination. However, if the BARC is not compatible with the resist, it can create adverse affects. If the acidity level of the BARC is not tuned to the resist for example, the profiles will foot or undercut, or if the BARC-to-resist developer interactions are not considered, high levels of post-develop defects will most likely occur. Etch selectivity, topography conformality and bowl/drain compatibility are other factors to consider when selecting a BARC. This paper follows the progressions of the leading DUV BARC's for Acetal-based resist systems and addresses the problems that could be encountered with implementing a BARC process. From DUV32 to the topography-conforming DUV42 and finally to the profile-enhancing DUV44, the 248 nm BARC's are continually evolving to resolve the BARC-to-resist compatibility issues.
We present results of a verification study of totally automated optical proximity correction (OPC) for mask redesign to enhance process capability. OPC was performed on an aggressive 0.35 micrometer i-line LSI logic SRAM design using the automated OPC generation code Eoptimask, employing the aerial image simulation code FAIM, both from Vector Technologies, Inc. Three different tests were performed, varying in the aggressiveness and type of corrections made. The key issues addressed in this work are the predictive capability of the aerial image simulation and, particularly, the ability of automatically generated OPC to significantly improve the fidelity of the final printed resist image for different geometries. The results of our study clearly demonstrate the utility of automated OPC based on aerial image simulation. Key experimental results include: two-fold increase of depth of focus latitude; demonstration of the feasibility of full off-axis illumination on the stepper; successful resolution of different feature types (posts, lines and spaces) on the wafer to correct CD at a single common exposure and focus condition. Future research will address detailed issues in reticle manufacture and inspection which are critical for cost-effective large-scale OPC.
This study evaluates the effect of dyes, including photosensitive dyes, on resist performance such as: swing curve reduction, resist dissolution rate, resolution, dose and focus latitude, scumming, etc. The paper demonstrates good correlation between modeling of the dyed resist performance and experimental results.
The advent of deep-UV(DUV), chemically amplified, acid catalyzed photoresists as successors to positive diazoquinones photoresists has brought about a new set of process environment concerns directed towards all materials in contact or absorbed by the photoresists. In addition to the application of DUV bottom anti-reflective coatings (BARCs) to suppress optical reflection and subsequent linewidth distortion, we must consider the properties and interaction of the BARC layer with the labile photoacid of the latent image. In this regard, we have examined the physico-chemical aspects of the DUV BARC with regards to acting as a barrier layer to substrate poisoning, and as an optical absorbing layer that does not interact and/or distort the deep-UV profile. Various single component polymeric BARCs were synthesized and examined. Considerations will be discussed of the optical absorbance, the coating quality, dry etch rate, and the impermeability of the BARC layer to photoacid diffusion to fulfill the performance requirements of BARCs for DUV lithography.
An investigation of the dissolution behavior of an acid catalyzed deep ultraviolet (DUV) positive resist has been completed. The immersion develop dissolution rate as a function of dose and post exposure bake temperature was measured by Perkin Elmer Dissolution Rate Monitor (DRM) for single layer resist on a silicon substrate. A reaction-diffusion model has been built to describe the dependence of development rate on exposure dose and post exposure bake (PEB) time/temperature. A mixed diffusion model has been built to account for catalyst diffusion and quenching. Developed images have been compared with simulated image quality, line width, and process window.
The combination of dyed photoresist and top antireflection (TAR) coatings was applied to I- line and deep-UV lithography on polysilicon. Optimization of the resist layer's absorption and application of the TAR process significantly improves CD control of submicron gate level lithography.
Numerical algorithms employing the ID imaging model and the 2D wave-guide scattering model were implemented to achieve high speed in simulating high NA i-line processes. The CPU consumption and the range of validity of the models used were discussed. The simulator was applied to study the possibility of imaging 0.35/mi lines and spaces(L/S) utilizing lenses of NA=0.55, 0.60 and 0.65 and single layer resist (SLR) processes.
The use of i-line lithography for the 16 to 64 Mbit DRAM device generations calls for increased performance of i-line resists. This paper reports on investigations on novel sensitizers for advanced i-line lithography, starting out with a discussion of general design criteria, then discussing methodology and results of a screening phase, and examining in greater detail a small number of selected candidates for which resolution, exposure latitude, and depth-of-focus data were obtained. Finally, a new advanced resist for i-line lithography, AZR 7500, is presented, and its performance is evaluated in terms of the above criteria as well as thermal flow resistance.
We propose in this paper a new imaging technology for the 64M DRAM, named "CQUEST” (Canon QUadrupole Effect for Stepper Technology). CQUEST is derived from the mathematical analysis of the partial coherence theory1. It can provide almost the same effects with conventional masks as those that result using phase shift masks. Therefore, it is a promising candidate for next generation lithography.
Simulation and some experimental results will be shown to substantiate the above. As shown in the results, the 64M DRAM process can be achieved with the existing i-line technology.