Phase Shift Mask Research Articles

Fundamental Study of the Mask Topography Effect on the Advanced Phase-Shifting Masks for Next-Generation Lithography

Phase-shifting masks (PSMs) with trench- or mesa-type quartz etched structure are widely known as powerful resolution enhancement technologies for advanced lithography. For advanced PSMs, the feature size on a mask including a phase shifter is beyond the wavelength of the light source, and for such a sub-wavelength phase shifter, the impact of the mask topography effect on the amount of phase shifting and the light transmittance becomes prominent. However, an effective method of measuring the actual phase shifting and transmittance of such a tiny phase shifter has not yet been reported because of the difficulty in measuring the sub-wavelength structure. In this work, we have attempted to measure the actual transmittance and phase shifting using the assembled tiny phase shifter aperture array pattern, and we have successfully revealed the actual transmittance and phase shifting behavior of the tiny phase shifter depending on its feature size and trench depth.

Characteristics of evanescent polarization imaging

A technique for obtaining native (uncrossed illumination/image polarizations) and induced (crossed illumination/image polarization) using a significant amount of evanescent energy is described for a solid immersion lens (SIL) microscope. Characteristics of native and induced polarization images for different object structures and materials are studied in detail. Experiments are conducted with a NA = 1.48 at λ = 550 nm microscope. Near-field images are simulated and analyzed with an RCWT approach. Contrast curve versus object spatial frequency calculations are compared with experimental measurements. Dependencies of contrast versus source polarization angles and air gap for native and induced polarization image profiles are evaluated. By using the relationship between induced polarization and topographical structure, an induced polarization image of an alternating phase shift mask (PSM) is converted into a topographical image, which shows very good agreement with atomic force microscope (AFM) measurement. Images of other material structures include a dielectric grating, chrome-on-glass grating, silicon CPU structure, BD-R and BD-ROM.

Aberration-induced intensity imbalance of alternating phase-shifting mask in lithographic imaging

We discuss the aberration-induced intensity imbalance of alternating phase-shifting mask (Alt-PSM) in lithographic imaging, in contrast to numerous studies on mask-induced intensity imbalance. Based on the Hopkins theory of partial coherent imaging, a linear relationship between the intensity difference of adjacent peaks in an Alt-PSM image and even aberration is established by formulations and verified by numerical results. The application of the linear relationship is briefly discussed.

Competitive Methods for Fabrication of Nanochips and Replacement of Au, Ag and Pd with Nonprecious Metals

Electroless metallization technology which allowed replacing adequately Au and Ag with Ni-P or Ni-B alloys and simplifying significantly the metallization process was developed (1-5). Other proposed patentable nanomethods for the first time allow one to produce nanochips and photomasks with nano-sized adjacent elements by single optical UV photolithography (4). The proposed nanomethods are much more advantageous and simpler than other expensive and complicated methods such as e-beam, X-ray lithography or fabrication of the nanoelements using light phase shift photomasks. The proposed methods of metallization are widely used in electronics, piezoengineering and instrument-making. As a result Au, Ag and Pd were replaced with the alloys of non-precious metals, usage of toxic substances was eliminated. Methods of fabricating the ultra-thin pore free electroless coatings on micro-, meso- and nano-sized particles are proposed.

Dynamic Finite Element Analysis of Failure in Alternating Phase-Shift Masks Caused by Megasonic Cleaning

The mechanical response of alternating phase-shift mask (APSM) microstructures subjected to dynamic pressure loadings relevant to those encountered in megasonic cleaning was analyzed with the finite element method (FEM). A parametric study of the effects of microstructure dimensions, pressure amplitude, and loading frequency on the mask structural integrity was performed for two typical chromium/quartz APSM patterns. Failure due to microfracture and plastic deformation processes which may occur during megasonic cleaning was examined for loading frequencies of 1, 5, and 10 MHz. The FEM results provide insight into possible failure modes and critical microstructure dimensions for instantaneous microstructure damage. Different failure scenarios revealed by the FEM results are in qualitative agreement with experimental observations. The results of this study have direct implications to the design of extreme ultraviolet lithography masks and the optimization of the megasonic cleaning process.

Phase-Shift Photomask Designed by Scalar Diffraction Theory

This work describes the design of a phase shift lithographic photomask for operation in proximity printing mode. The design explores the freedom of the diffraction of light by travelling along the distance between the mask plane and the wafer plane. To this end, a phase and amplitude modulation distributions was generated, which have to be used in the fabrication of the proposed photomask. Then a photomask with test structures containing several geometric features was fabricated. It is able to modulate both phase and amplitude of the incoming wavefront during exposure of a wafer. The lithographic image to be patterning must be projected onto the wafer plane, located at a distance of 50 μm behind the mask. The 50 μm gap is a necessary condition to explore the diffraction of light in order to improve the resolution when compared with traditional binary masks in proximity printing mode.

Highly parallel rigorous simulations of phase-shift masks with a generalized eigen-oscillation spectral element method

We apply a newly developed parallel generalized eigen-oscillation spectral element method (GeSEM) for rigorous simulations of 2-D phase-shift masks (PSMs). The GeSEM combines highly parallel Schwarz-domain decomposition iterations and an eigen-oscillation-based spectral method to model high-frequency oscillatory electromagnetic fields in PSMs with dispersive chrome materials in the PSMs. The performance of the GeSEM has been compared to the popular plane wave-based waveguide method for 2-D masks. The numerical results have clearly demonstrated the GeSEM's advantages in modeling the effects of nonperiodic structures such as optical images near mask edges, and its speedup through parallel implementations, which makes the simulation of a large-scale mask possible in whole-chip mask modeling.

Translational-symmetry alternating phase shifting mask grating mark used in a linear measurement model of lithographic projection lens aberrations

A linear measurement model of lithographic projection lens aberrations is studied numerically based on the Hopkins theory of partially-coherent imaging and positive resist optical lithography (PROLITH) simulation. In this linearity model, the correlation between the mark's structure and its sensitivities to aberrations is analyzed. A method to design a mark with high sensitivity is proved and declared. By use of this method, a translational-symmetry alternating phase shifting mask (Alt-PSM) grating mark is redesigned with all of the even orders, +/-3rd and +/-5th order diffraction light missing. In the evaluation simulation, the measurement accuracies of aberrations prove to be enhanced apparently by use of the redesigned mark instead of the old ones.

Effect of Attenuated Phase Shift Mask Structure on Extreme Ultraviolet Lithography

Optimization of the absorber stack is becoming a critical issue in extreme ultraviolet (EUV) lithography. The attenuated phase shift mask (att-PSM) can be a long-term solution for continued technological improvements. The designed structure is based on the Fabry–Perot structure that consists of a TaN attenuator, an Al2O3 spacer, and a Mo phase shifter. The total thickness of the absorber stack is 52 nm. The characteristics of the designed att-PSM are an EUV reflectivity (9.5% at 13.5 nm) and phase shift effect (180° at 13.5 nm) that maximize the image contrast in EUV exposure, and a low deep ultraviolet (DUV) reflectivity (1.72% at 257 nm) for high efficiency DUV inspection. All the simulations were carried out with the EM-Suite simulation tool. As a result, we determined that the att-PSM shows a higher image contrast, a larger process window, and a lower horizontal–vertical (H–V) CD bias than a binary image mask.

Nano-Photonic & Electronic Structures Pattern and Fabrication

In this paper, techniques developed in the fabrication of nano-structures such as photonic gratings and vertical tube structures are presented for fabrication development that is applicable in the field of MEMS/NEMS technology – particularly for the optical integrated circuits [1] and sensor devices [2]. In such applications, not only is the resolution enhancements important, it is also often imperative that Critical Dimensions Uniformity (CDU) be kept as low as possible - since device performance often times scales directly with structural dimensions and its accuracy [3]. In this regard, techniques such as phase shift masking, off-axis illumination, optical proximity corrections, and also multiple patterning and resist ashing etc needs to be employed. This, not only increases the number and complexity of processing steps; it (at the same time) implicates other specifications such as illumination apertures, mask designs, optical proximity simulations, tool overlay tolerances etc. Coupled with additional non-CMOS requirements of atypical pattern shapes and densities, such as rings, horse-shoe shapes, sharp edges etc, a comprehensive study of the flexibilities involved with micro-lithography needs to be carried out for novel design prototyping. Here, the above described are illustrated, using examples of (i) a photonic gratings structure patterned with sharp edges, and (ii) a vertical-tube switch device structure, which are presented and discussed for their fabrication techniques and measurement results.

Pixel-based simultaneous source and mask optimization for resolution enhancement in optical lithography

Optical proximity correction (OPC) and phase-shifting mask (PSM) are resolution enhancement techniques (RET) used extensively in the semiconductor industry to improve the resolution and pattern fidelity of optical lithography. Traditional RETs, however, fix the source thus limiting the degrees of freedom during the optimization of the mask patterns. To overcome this restriction, a set of simultaneous source and mask optimization (SMO) methods have been developed recently where the resulting source and mask patterns fall well outside the realm of known design forms. This paper focuses on developing computationally efficient, pixel-based, simultaneous source mask optimization methods for both OPC and PSM designs, where the synergy is exploited in the joint optimization of source and mask patterns. The Fourier series expansion model is applied to approximate the partially coherent system as a sum of coherent systems. Cost sensitivity is used to drive the output pattern error in the descent direction. In order to influence the solution patterns to have more desirable manufacturability properties, topological constraints are added to the optimization framework. Several illustrative simulations are presented to demonstrate the effectiveness of the proposed algorithms.

High-Accuracy Measurement of Coma with the Optimized Alternating Phase-shifting Mask with a Specific Phase Width

The correlation between the coma sensitivity of the Alternating Phase-Shifting Mask (Alt-PSM) and the Alt-PSM's structure is studied. It is found that an optimized Alt-PSM whose phase width is two thirds of its pitch has a higher sensitivity to coma than Alt-PSMs with the same pitch and the different phase widths, when the pitch of the Alt-PSM makes only +/-1 and +/-3 orders diffraction light enter the lens pupil. The optimized Alt-PSM is used as a measurement mark to retrieve coma aberration from the projection optics in lithographic tools. In comparison with an ordinary Alt-PSM mark whose phase width is a half of its pitch, the measurement accuracies of Z7 and Z14 have increased by 15% and 31% respectively.

Optimum optics for die-to-wafer-like image mask inspection

The most annoying problem relating to the sensitivity of mask inspection systems is the encountering of false signals arising from nuisance defects. In order to overcome this problem, we have previously proposed a new algorithm for die-to-wafer-like image (D-to-WI) in real time. This paper describes the optimum mask inspection optics for the D-to-WI mask inspection. We examine the optimum mask inspection optics with numerical simulation for various numerical apertures (NAs) and partial coherence factors (σ) in these optics. The simulated result shows that the optimum mask inspection optics for the D-to-WI mask inspection has NA 0.9 and σ = 0.95 for an ArF-6%-phase shift mask (PSM) 260/260 nm line/space pattern on the mask plane, in which the 193 nm-ArF scanner has NA 0.93, σ = 0.92 − 0.92/0.3 annular illumination, reduction factor of ×4, and circular polarization incident light.

Highly selective dry etching of alternating phase-shift mask (PSM) structures for extreme ultraviolet lithography (EUVL) using inductively coupled plasmas (ICP)

Extreme ultraviolet lithography (EUVL) is the most promising candidate for next generation lithography due to its feature size of 32 nm or below. We investigated the etching properties of materials in an alternating, phase-shift mask (PSM) structure for EUVL, including a Ru top capping layer, Mo–Si multilayer (ML) and Ni etch stop layer (ESL), by varying the Cl 2/O 2 and Cl 2/Ar gas flow ratios, and the dc self-bias voltage ( V dc) in inductively coupled plasma (ICP). The Ru layer could be etched effectively in Cl 2/O 2 plasmas and Mo–Si ML could be etched with an infinitely high etch selectivity over Ni ESL in Cl 2/Ar plasmas, even with increasing overetch time.

Coma measurement by use of an alternating phase-shifting mask mark with a specific phase width

The correlation between the coma sensitivity of the alternating phase-shifting mask (Alt-PSM) mark and the mark's structure is studied based on the Hopkins theory of partially coherent imaging and positive resist optical lithography (PROLITH) simulation. It is found that an optimized Alt-PSM mark with its phase width being two-thirds its pitch has a higher sensitivity to coma than Alt-PSM marks with the same pitch and the different phase widths. The pitch of the Alt-PSM mark is also optimized by PROLITH simulation, and the structure of p=1.92lambda/NA and pw=2p/3 proves to be with the highest sensitivity. The optimized Alt-PSM mark is used as a measurement mark to retrieve coma aberration from the projection optics in lithographic tools. In comparison with an ordinary Alt-PSM mark with its phase width being a half its pitch, the measurement accuracies of Z(7) and Z(14) apparently increase.

PSM design for inverse lithography with partially coherent illumination

Phase-shifting masks (PSM) are resolution enhancement techniques (RET) used extensively in the semiconductor industry to improve the resolution and pattern fidelity of optical lithography. Recently, a set of gradient-based PSM optimization methods have been developed to solve for the inverse lithography problem under coherent illumination. Most practical lithography systems, however, use partially coherent illumination due to non-zero width and off-axis light sources, which introduce partial coherence factors that must be accounted for in the optimization of PSMs. This paper thus focuses on developing a framework for gradient-based PSM optimization methods which account for the inherent nonlinearities of partially coherent illumination. In particular, the singular value decomposition (SVD) is used to expand the partially coherent imaging equation by eigenfunctions into a sum of coherent systems (SOCS). The first order coherent approximation corresponding to the largest eigenvalue is used in the PSM optimization. In order to influence the solution patterns to have more desirable manufacturability properties and higher fidelity, a post-processing of the mask pattern based on the 2D discrete cosine transformation (DCT) is introduced. Furthermore, a photoresist tone reversing technique is exploited in the design of PSMs to project extremely sparse patterns.

Electroless Metallization of Nano-Sized Particles, Composites Fabrication

New data on the competitive nanotechnologies for fabrication of fine-grained powder-like particles, films, bulk materials, nanocomposites, devices for microelectronics, nanoelectronics, photonics and photocatalysis are presented. The proposed nanotechnologies for electronics are much more advantageous and simpler than other expensive and complicated methods such as e-beam lithography, X-ray lithography or production of the devices using light phase shift photomasks. The mechanism of sensitization and activation was established. The proposed methods of metallization are widely used in electronics and instrument-making. As a result, Au, Ag and Pd were adequately replaced with the alloys of non-precious metals, the use of toxic substances was eliminated and the technology was significantly simplified.

Design of Hybrid Attenuated Phase-Shift Masks with Indium-Tin-Oxide Absorbers for Extreme Ultraviolet Lithography

Hybrid-type extreme-ultraviolet attenuated phase-shift masks are designed by using an indium tin oxide (ITO) layer in the absorber stack and a Si layer in the (Mo/Si) multilayer stack. The designs provide not only a 180 phase-shift with a low attenuated re ectance ratio ( 90 %) at the 257-nm wavelength. The height di erence between the re ecting multilayer stack and the absorbing stack in the hybrid-type attenuated phase-shift mask designs is reduced enough to minimize the geometric shadow e ect in extreme ultraviolet lithography.

Optimum Biasing for 45 nm Node Chromeless and Attenuated Phase Shift Mask

Resolution enhancement technology (RET) refers to a technique that extends the usable resolution of an imaging system without decreasing the wavelength of light or increasing the numerical aperture (NA) of the imaging tool. Off-axis illumination (OAI) and a phase shift mask (PSM) are essentially accompanied by optical proximity correction (OPC) for semiconductor device manufacturing nowadays. A chromeless PSM is compared with an attenuated PSM (att.PSM) to generate a 45 nm dense line and space pattern. To obtain the best possible resolution, a suitable OPC is required with PSM and the most common application of OPC is the use of bias. An optical system with a high NA, a strong OAI, and a proper polarization can decrease k1 to a value well below 0.3. A chromeless PSM has various advantages over alternating PSM such as the lack of necessity for double exposure, small pattern displacement, and the lack of critical dimension (CD) error caused by intensity imbalance. However, a chromeless PSM has some disadvantages. In the case of a 100% transmittance pure chromeless PSM, there is no shading material that is usually deposited on the line pattern for both att.PSM and alternating PSM to control linewidth. Because there is no shading material for such a chromeless PSM, the required resist CD has to be obtained using phase only and it is difficult to control the resist CD through pitch. As expected, a chromeless PSM needs a smaller dose than an att.PSM to generate a desired 45 nm CD with a 1.0 NA. Our simulation results show that a 10 nm bias is optimum for chromeless PSMs. We demonstrate that chromeless PSM and att.PSM technology can be used to achieve a 45 nm node with optimum biased OPC.

Design freedom gets the brush-off [chip design

Various techniques have been used to push processes this far past the point where diffraction effects kick in. These have included using phase-shift masking to turn diffraction to the designers' advantage, and optical proximity correction (OPC) techniques, such as adding 'ears' to the corners of layout elements so that they are printed as right angles. The problem is that chipmakers are applying ever more Byzantine combinations of these techniques to keep up with shrinking process dimensions, and getting diminishing returns. Device characteristics are varying wildly as more aggressive OPC techniques are pressed into action. The chip equipment industry has been working to solve this for years, but couldn't persuade its customers that moving to 157nm illumination sources was worth the development costs given the marginal advantages it would bring over 193nm light. Unfortunately, the development of EUV lithography is fraught with difficulties. The solution to the 65 nm and 45 nm design for manufacture is new DFM routers. As they are DFM-aware they can run the rule-based checks and ensure that around 80 percent of the DFM problems are avoided. The layout team can then use model- based DFM to take care of the remaining 20 percent.