KEYWORDS: Photoresist materials, Line edge roughness, Optical lithography, Lithography, Diffraction, Nanoimprint lithography, Photoresist developing, Electron beam lithography, Deep ultraviolet, Near field optics
Background: Resolution enhancement combined with multiple patterning enables photolithography to write patterns with both feature size and spacing below the diffraction limit. Continued resolution enhancement at i-line will enable an older generation of lithographic tools to reach resolutions typically achieved using deep UV (DUV).
Aim: A demonstration and deterministic model of large critical dimension enhancement at i-line. In addition to enhanced resolution, the technique must also achieve high repeatability and low line edge roughness (LER), while using commercial resists.
Approach: Overexposing photoresist with high-contrast interference nulls leads to subwavelength critical dimensions. Starting with a theoretical analysis of the technique, we consider limits imposed by optics, linewidth scaling rates, and LER. This analysis shows that low LER and deterministic linewidth control are both readily achievable.
Results: We demonstrate large area, i-line patterning of features with 50-nm linewidth, without the aid of subsequent trim or etch and with LER of 5 nm. Linewidth is shown to scale with dose as predicted from the optical model, independent of photoresist.
Conclusions: These dimensions are similar to what may be achieved using scanning near-field, DUV, or e-beam lithography, yet achieved with far-field near UV exposures over a large area. Deterministic linewidth control and low LER make this process viable for fabrication at length scales well below those typically achieved with i-line tools.
Photoinhibited superressolution (PInSR) lithography is a two--color, one-photon scheme that promises high throughput far-field patterning t deep subwwabvelength scales. Previous work ha shown that the technique susceptible to blurring from active species diffusion, an issue which we have recently overcome with the use of a low-diffiusivity methrylate resist. Here we present out first clear demonstration of superresolution, showing feature spacing 3X better than the 0.2 NA diffraction limit.
Progress in materials for radical initiated, radical inhibited super-resolution lithography is reported. The photochemistry
and optical system is described, with a brief discussion on the theory of operation. A motivation
is presented for developing a new material that may be used as a spinnable photoresist, and qualitative resist
requirements are discussed. Results from FTIR experiments suggest how viscosity and monomer type may affect
resist performance. Finally, focused beam photoinhibition experiments on a novel photoresist are presented.
Scatterometry is a fast, non-destructive critical dimension (CD) optical metrology technique based on the analysis of light scattered from a periodic array of features. With technological advances in manufacturing, semiconductor devices are made in ever shrinking geometries. In recent years, the ability of scatterometry metrology tools to measure these devices at a gage-capable level for parameters such as CD, thickness or profile has become more challenging. The focus of this research is to analyze the acquired diffraction signature and determine an optimum diffraction signature "scan path." An optimized scan path can result in higher precision, reduced development time, smaller pre-generated library databases and faster real-time optimization speeds.
In this work, we will first review several methods for scan path selection and optimization. Our results indicate that the method choice can influence the scan path selection, and that some of the methods are complementary to one another. For example, one method, which we term orthogonal sensitivity, uses intelligent algorithms to select optimal scan path points based on enhancing single parameter sensitivity. While the method works well, it neglects parameter correlation effects. Thus, we will also review a method where correlation effects are considered. Finally, we will calculate and summarize the effectiveness of optimal scan path selection techniques using challenging lithography applications.
In this work we demonstrate the application of a unique type of scatterometer to the measurement of advanced geometry semiconductor devices. Known as a dome scatterometer, the technology is capable of measuring multiple diffraction orders at multiple angles of incidence, thereby providing a means for gathering a large amount of scatterometry data in a short period of time. Dome scatterometers are also capable of measuring light scattered as a function of both theta (zenith) and phi (azimuth) incident angles, and for a variety of polarimetric configurations, all of which provide more information about the scattering structure and contribute to improved sensitivity. A dome scatterometer can also measure a grating structure regardless of its orientation, so that horizontal and vertical structures can be measured without the need for a wafer rotation.
Prior to initiating measurements with the dome scatterometer, we surveyed available laser sources and modeled their expected sensitivity theoretically to determine the best illumination wavelength for the applications we intended to study. Our findings demonstrated that a wavelength around 405nm is suitable for a wide variety of applications, but provides the best improved sensitivity for etch applications. We then modified our dome scatterometry optical system to accommodate a Using a 405nm laser, and performed measurements were performed on several types of grating structures. Examples of the excellent signal-to-noise ratio of dome scatterometry measurements across these applications are provided. Measurement data from applications including patterned photoresist, patterned poly lines and back-end trench interconnect structures will be presented. Comparisons to metrology tools such as AFM and CD-SEM will be made. Precision data will also be summarized, and the extendibility of dome scatterometry will be discussed.
For typical single and double-periodic structures that scatterometry is employed to measure, grating pitch has traditionally been treated as an invariant and well-known parameter. Mask writing processes and lithographic exposure tools are generally regarded to be sophisticated enough to eliminate the possibility of a significantly uncontrolled or unknown grating-pitch. Considering the modern demands in precision and accuracy placed on scatterometry, however, there is value in re-examining this assumption. The factors that can affect grating-pitch variation or inaccuracy include mask writing errors, mask flatness, lithographic magnification errors, focus-height errors, and lens aberrations.
In order to quantify the effects of grating pitch assumptions, several model-based investigations have been performed. Libraries of models were constructed with assumed and invariant grating-pitches. For comparison purposes, an identical second set of libraries was generated assuming a slightly different grating pitch. Those sets were matched such that the pitch difference effects could be investigated by noting the parameter dimension differences in the matched models. We report the estimated severity of errors in parameter dimensions as a result of modeling intentionally mismatched grating pitches. Data from multiple structures that correspond to typical mainstream scatterometry applications will be shown. A summary of the successful model and algorithm-based compensation techniques will follow. While our investigation will include structures that correspond to the current 65 nm technology node, we will also discuss the effect of pitch mismatch for future technology nodes.
Scatterometry is a novel metrology approach for process control that has recently been gaining more momentum in microlithography applications. The method can simultaneously measure Critical Dimension (CD), Side Wall Angle (SWA), and thickness of more than one layer. It analyzes the scattered and diffracted light from a periodic array of lines or holes that represent the surface structure of the measured sample. Scatterometry provides a non-destructive technique offering high precision and stability along with high tool-uptime performance. As such, it offers an excellent approach for real-time high volume production control with significant advantages as compared to traditional technologies such as CD-SEM and Profilometry. As the structure dimension shrinks considerably, producing high precision results becomes more critical. To date, reports on the deployment of scatterometry in real production environment have focused on Front End of Line (FEOL) applications such as STI and Gate. However, Back End of Line (BEOL) process control has not been widely reported. In this work, we will discuss the results of our study specifically for metal trench and contact layer on both patterned and etched wafers for 65nm technology node. We will also report the comparison between Scatterometry results to Critical Dimension Scanning Electron Microscope (CD-SEM) and Atomic Force Microscope (AFM). Finally we will provide a statistical analysis of our scatterometry results including precision, fleet precision, and TMU analysis. In contrast to the relatively simple stacks that comprise a FEOL structure, BEOL layers are typically complex structures with a large number of underlying layers. Generation of simulated scatterometry signatures that constitute a reference library for complex structures can require long computational times and result in large file sizes. To mitigate the computational overhead, it is necessary to intelligently decide which parameters to fix and which to vary. An additional complication is presented due to similarities in the optical properties of BEOL stack materials, which can introduce potential for parameter cross-correlation in the measurement. We will discuss methodologies for optimally selecting parameters to be fixed or varied to minimize these effects.
We report the first two-color 320 x 256 infrared Focal Plane Array (FPA), based on a voltage-tunable InAs/InGaAs/GaAs DWELL structure. The detectors, grown by solid source molecular beam epitaxy (MBE) comprise of a 15-stack asymmetric DWELL structure sandwiched between two highly doped n-GaAs contact layers, grown on a semi-insulating GaAs substrate. The DWELL region consists of a 2.2 monolayer deposition of n-doped InAs quantum dots (QDs) in an In0.15GaAs0.85As well, itself placed in GaAs. The well widths below and above the dots are 50Å and 60Å, respectively. The absorption region asymmetry results in a bias dependent spectral response, with the peak wavelength varying from 5.5 to 10 μm. Using calibrated black body measurements, mid-wavelength and long wavelength specific detectivities (D*) of top-illuminated test pixels at 78K were estimated to be 7.1 x 1010 cmHz1/2/W (Vb= 1.0V) and 2.8 x 1010 cmHz1/2/W (Vb= 2.5V), respectively. Subsequently, a 320 x 256 QDIP FPA array was fabricated on a 30 μm pitch and was hybridized with an Indigo 9705 ROIC. Thermal imaging was successfully carried out at an estimated FPA temperature of 80K, using different optical filters between 3-5 μm, and 8-12 μm, so as to demonstrate two-color operation. The operability of the FPA was greater than 99%, and the noise-equivalent temperature difference was estimated to be less than 100 mK for f#1 (3-5 μm) and f#2 (5-9 μm) optics.