Photomask Fabrication Research Articles

Matrix technology of linear–angular measurements

Subject of study. An important technological aspect of fabricating microelectronic elements is the design of high-precision methods for measuring linear and angular displacements. This study proposed a new technique for linear–angular measurements based on using a multi-element mark and obtaining measurement information about the angular and linear shift based on a set of measurements for all elements of the mark image. Method. The image of an optical mark, i.e., an ordered set of simple elements created using high-precision photolithography technology, was recorded using a photodetector matrix of a digital camera used as a metric space for measuring the coordinates of the mark image elements with their subsequent processing. Main results. The main aspects of the proposed measurement technique and the estimation of measurement errors are presented in this study. The results of simulating the measurement process of both linear and angular displacements were considered. The processing of the mark image and a set of computational procedures enabled the determination of both linear and angular displacements of the mark image with high accuracy (linear and angular displacements at the level of a few nanometers and hundredths of an arc second or less, respectively). The main directions for developing the proposed technique of matrix measurements were formulated. Among them, using digital liquid crystal displays to form a measuring mark and developing sensors using a shadow patterns (Fraunhofer diffraction) image of the mark are of particular interest. Practical significance. The proposed techniques for measuring linear and angular displacements can be successfully used to develop high-precision compact length and angle meters, linear–angle sensors, and technological modules for microelectronics in photomask fabrication.

Relationship between blurring factors and interfacial effects in chemically amplified resist processes in photomask fabrication

Chemically amplified resists are an indispensable technology for photomask fabrication. The formation of latent images near interfaces in resist films is strongly affected by low-energy electron dynamics at the interfaces. On the other hand, the latent images are blurred by low-energy electron migration and acid diffusion in latent image formation. In this study, the relationship between blurring factors and interfacial effects was investigated by simulating the latent image formation on the basis of the sensitization and reaction mechanisms of chemically amplified electron beam resists. For low-energy electron dynamics, the interfacial effects on the protected unit distribution was reduced and the chemical gradient slightly increased with decreasing thermalization distance. For acid diffusion, the decrease in exposure pattern width significantly reduced the interfacial effects on the protected unit distribution in the depth direction because the acid diffusion length can be increased without sacrificing resolution.

Theoretical study of interfacial effects on low-energy electron dynamics in chemically amplified resist processes of photomask fabrication

The resist thickness has decreased with the miniaturization of transistors. In chemically amplified electron beam resists used for photomask fabrication, thermalized electrons play an important role in acid generation. The interfacial effects of low-energy electrons strongly affect the latent image formation in thin resist films. In this study, we investigated the relationships between the boundary conditions of low-energy electrons at interfaces and the protected unit distribution in terms of resist thickness, concentrations of photoacid generators and photodecomposable quenchers (PDQs), and sensitivity. The dependences of chemical gradient and optimum PDQ concentration on the boundary conditions at the interfaces for low-energy electrons were clarified. It was found that the PDQ concentration should be optimized, depending on not only the boundary conditions but also the resist film thickness. That is, the resist depth profile can be controlled by the boundary conditions at the interfaces for low-energy electrons.

Changes in molecular weight distribution caused by main-chain scission of electron beam resists

Main-chain-scission-type resists have attracted much attention as the next-generation electron beam (EB) resist used for photomask fabrication. In this study, the gel permeation chromatography (GPC) charts of irradiated EB resists (ZEP series of ZEON) were analyzed by simulation to clarify the changes in the molecular weight distribution caused by the main-chain scission of a resist polymer. In the low-exposure-dose region, the molecular weight distribution obtained by GPC was well reproduced by the simulation using a random-scission model. In the high-exposure-dose region, the calculated molecular weight distribution deviated from the experimental one because of crosslinking and the generation of stable components.

Cell-Imprint Surface Modification by Contact Photolithography-Based Approaches: Direct-Cell Photolithography and Optical Soft Lithography Using PDMS Cell Imprints.

New cell-imprint surface modification techniques based on direct-cell photolithography and optical soft lithography using poly(dimethylsiloxane) (PDMS) cell imprints are presented for enhanced cell-based studies. The core concept of engineering materials for cell-based studies is the material's ability to redesign the physicochemical characteristics of the cellular niche. There is a growing interest in direct molding from cells (cell imprinting). These negative copies of cell surface topographies have been shown to affect cell shape and direct mesenchymal stem cells' differentiation. Analyzing the results is however challenging as cells seeded on these substrates do not always end up in a cell pattern, which leads to decreased effectiveness and biased quantification. To gain control over cell seeding into the patterns and avoid unwanted cell population outside of the patterns, the cell-imprinted surface needs to be modified. From this perspective, the standard optical contact lithography process was modified and cells were introduced to the cleanroom. Direct-cell photolithography was used for a single-step PDMS cell-imprint (chondrocytes as the molding template) surface modification down to single-cell (approximately 5 μm in diameter) resolution. As cells come in a variety of shapes, sizes, and optical profiles, a complementary optical soft lithography-based photomask fabrication technique is also reported. The simplicity of the fabrication process makes this cell-imprint surface modification technique compatible with any adherent cell type and leads to efficient cell-based studies.

Provably Good Max–Min- $m$ -Neighbor-TSP-Based Subfield Scheduling for Electron-Beam Photomask Fabrication

Electron beam lithography (EBL) has been used for high-resolution photomask fabrication; its successive heating process in a certain region, however, may cause critical dimension (CD) distortion. As a result, subfield scheduling, which reorders a sequence of subfields in the writing process, is desirable to avoid the heating problem and thus CD distortion. To consider longer range heat dissipation, this paper models a subfield scheduling problem with blocked region consideration as a constrained max–min <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$m$ </tex-math></inline-formula> -neighbor traveling salesman problem (called constrained <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$m$ </tex-math></inline-formula> -nTSP). To solve the constrained <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$m$ </tex-math></inline-formula> -nTSP which is NP-complete in general, we decompose a constrained <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$m$ </tex-math></inline-formula> -nTSP into subproblems conforming to a special case with points on two parallel lines, solve each of them with a provably good linear-time approximation algorithm, and merge them into a complete scheduling solution. In particular, our algorithm can also minimize the distances between successive subfields to alleviate the throughput degradation of EBL writing due to moving a writing head, while minimizing the heating problem. Average reductions of 10% in the maximum temperature and 14% in the distances between successive subfields over the state-of-the-art work can be achieved.

All‐Solution‐Processed, Scalable, Self‐Cracking Ag Network Transparent Conductor

A scalable and a fully solution‐based method is developed to produce environmentally stable Ag micro/nanowire networks for transparent conductors. By applying a self‐cracking, water‐soluble acrylate copolymer film as a photoresist mask, the need for formal photomask fabrication and costly vacuum and lithographic facilities is obviated. Also, an increase in adhesion and a decrease in roughness of the metal networks is demonstrated by depositing metal into the regions created by the glass etch step. As a result, the networks can potentially be inexpensively scaled to large areas, as well as be flexible after removal from the substrate. They also exhibit record values of figures of merit that have been employed in the literature, offering a possibly excellent replacement for ITO.

Theoretical study on effects of exposure pattern width on line edge roughness and stochastic defect generation in fabrication of 16-nm-half-pitch line-and-space patterns by electron beam lithography

The resolution of electron beam (EB) mask writers has reached a 16 nm half-pitch. The suppression of stochastic phenomena such as line edge roughness (LER) and stochastic defect generation is an important issue in the high-resolution fabrication of photomasks and nanoimprint molds. In this study, the effects of exposure pattern width on LER and stochastic defect generation were theoretically investigated on the basis of the sensitization and reaction mechanisms of chemically amplified EB resists. The exposure pattern width affected stochastic bridging more than stochastic pinching and LER. The optimum pattern widths for the suppression of stochastic bridging in the fabrication of 16-nm-half-pitch line-and-space patterns were 8, 8, 10, and 12 nm for the sensitivities of 300, 240, 180, and 120 µC cm−2, respectively.

Laser technologies in micro-optics. Part 1. Fabrication of diffractive optical elements and photomasks with amplitude transmission

This paper is a review of studies carried out by the staff of the National Research University of Information Technologies, Mechanics and Optics (ITMO University, Saint-Petersburg) and the Institute of Automation and Electrometry of the Siberian Branch of the Russian Academy of Sciences (IAE SB RAS, Novosibirsk) in the field of development of laser engineering processes for the formation of the structure of diffractive optical elements (DOEs) and photomasks with amplitude binary and grayscale transmission. This paper also describes the results of the study of laser thermochemical technology for fabricating chrome DOEs and technologies for the fabrication of grayscale DOEs and photomasks based on the use of amorphous silicon and LDW glass.

Low-cost laser printable photomask: One-step, photoresist-free, fully solution processed high-grade photolithography mask

Owing to the short life cycle of present-day microelectronics, conventional method of photomask fabrication should be replaced in order to improve the resultant agility and productivity of photolithography. To fulfill these requirements, we suggest a fully solution-based one-step fabrication method of a high-grade photomask, without the use of a photoresist, vacuum deposition, and etching process. The photomask is fabricated via the laser-induced instantaneous thermochemical metallization of an optically catalyzed hybrid complex synthesized in-situ from a low-cost particle-free organometallic solution. This reaction yields a masking layer whose high selectivity of less than 1μm, self-generated retroreflective structure, and excellent optical surface are comparable to those of masks fabricated by vacuum depositions. In addition, the complexity of the process is minimized owing to the solution deposition of all the constituent layers. A series of evaluations and the application of this method to an actual photolithography process confirm that this approach constitute a next-generation photomask fabrication method by satisfying both improved agility and productivity of microelectronics manufacturing.

Surface roughness analysis and thermal bonding of microfluidic chips fabricated by CD/DVD manufacturing technology

Micro total analysis system or Lab-on-a-chip, in which microfluidic chips are significant components, has caused wide attention in recent years. CD/DVD manufacturing technology is a highly efficient and low-cost solution to fabricate microfluidic chips. In this paper, microfluidic chips made of polycarbonate (PC) were firstly fabricated by CD/DVD manufacturing technology. The manufacturing process is divided to five main sub-processes which are photomask fabrication, photolithography, electroforming, stamping and thermal bonding. Besides, by repeatedly bonding for three times, the bonding success percentage of the chip could reach about 98 %. The surface roughness of the microstructure in microfluidic chips which has significant influences on thermal bonding success percentage and test results are also detailedly studied. Surface roughness of SU-8 pattern, Ni stamper and cover slips of PC microfluidic chips was measured and analyzed. In the process of electroforming and stamping, the surface roughness is increased. Moreover, influences of cover slips' surface roughness on thermal bonding are analyzed by bonding five groups of cover slips on PC plates with different average surface roughness. The thermal bonding success percentage is obviously reduced with the increase of average surface roughness.

Toward defect-free fabrication of extreme ultraviolet photomasks

Defect-free fabrication of extreme ultraviolet (EUV) masks relies on the appropriate detection of native defects and subsequent strategies for their elimination. Commercial unavailability of actinic mask-blank inspection systems motivates the identification of an optical inspection methodology most suitable for finding relevant EUV blank defects. Studies showed that 193-nm wavelength inspection found the greatest number of printable defects as compared with rival higher-wavelength systems, establishing deep ultraviolet inspections as the blank defectivity baseline for subsequent mitigation strategies. Next, defect avoidance via pattern shifting was explored using representative 7-nm node metal/contact layer designs and 193-nm mask-blank inspection results. It was found that a significant percentage of native defects could be avoided only when the design was limited to active patterns (i.e., layouts without dummy fill). Total pattern-defect overlap remained ≤5 when metal layer blanks were chosen from the top 35% least defective substrates, while the majority of blanks remained suitable for contacts layers due to a lower active pattern density. Finally, nanomachining was used to address remaining native/multilayer defects. Native catastrophic defects were shown to recover 40% to 70% of target critical dimension after nanomachining, demonstrating the enormous potential for compensating multilayer defects.

Viability of pattern shift for defect-free extreme ultraviolet lithography photomasks

Several challenges hinder extreme ultraviolet lithography (EUVL) photomask fabrication and its readiness for high-volume manufacturing (HVM). The lack in availability of pristine defect-free blanks as well as the absence of a robust mask repair technique mandates defect mitigation through pattern shift for the production of defect-free photomasks. By using known defect locations on a blank, the mask design can be intentionally shifted to avoid patterning directly over a defect. The work presented here provides a comprehensive look at pattern shift implementation to intersect EUV HVM for the 7-nm technology node (N7). An empirical error budget to compensate for various measurement errors, based on the latest HVM inspection and write tool capabilities, is first established and then verified postpatterning. The validated error budget is applied to 20 representative EUV blanks and pattern shift is performed using fully functional N7 chip designs that were recently used to fabricate working silicon–germanium devices. Probability of defect-free masks are explored for various N7 photomask levels, including metal, contact, and gate cut layers. From these results, an assessment is made on the current viability of defect-free EUV masks and what is required to construct a complete defect-free EUV mask set.

Characterization for the photomask fabrication based on a high-resolution technique with a non-chemically amplified resist and a post-exposure bake

A high-resolution technique has been developed for the fabrication of photomasks for 10nm logic nodes and beyond. Current mask manufacturing techniques use a chemically amplified resist (CAR) material that has a complex mechanism of acid generation; this complicates the criteria for selecting the polymer and the quencher for industrial purposes. It is therefore important to validate non-CAR materials as alternative solutions for mask fabrication. In this research, we used diluted ZEP520A as a non-CAR material in conjunction with a JBX9000 variable-shaped electron-beam (VSB) lithography tool. Additionally, a post-exposure bake (PEB), routinely used in mask fabrication, was also applied. We investigated the effects of the PEB temperature on the fabrication of masks from non-CAR resists, and we demonstrated the feasibility of using a PEB as a high-resolution technique. The critical dimensions (CDs) for 1:1 line-and-space, isolated space, and isolated line patterns on a diluted ZEP520A resist were measured and showed a shrinkage, an extension effect, and retention of the integrity of the shape after the PEB process, respectively. Furthermore, the line-edge roughness (LER) of the 1:1 line-and-space and isolated space CDs was improved by approximately 40% by optimizing the PEB temperature. We investigated the process characteristics of this PEB annealing effect by examining the hardness of the cured resist with and without exposure to PEB at various temperatures, with the aim of elucidating the underlying mechanism. Optimizing the PEB temperature of the non-CAR increased the resist contrast, annealing the resist and improving the LER. This permitted us to demonstrate an advanced fabrication technique capable of high resolutions of the order of 20nm. The insights gained from the optimization of the PEB process might be useful in advanced methods of fabricating masks of the next generation.

Improvement in photomask critical dimension uniformity using etch selectivity control

We investigated the variation in etch selectivity for chrome and photoresist with respect to the O2 injection ratio during Cl2 process plasma for photomask fabrication. To improve the photomask critical dimension (CD) uniformity, we determined the range of O2 concentration where etch selectivity remains stable. Specifically, we found that the etch selectivity showed small variations from an 8% to 14% O2 concentration. We measured the concentration of chlorine radicals and demonstrated that the relationship between the concentration of chlorine radicals and injected chlorine gas was linear. We also determined the spatial variation of oxygen atoms and chlorine radical concentrations inside the process plasma using a spatially resolvable optical emission spectrometer to confirm plasma uniformity. Under similar spatial distributions conditions for oxygen atoms and chlorine radicals, we expect that the photomask CD uniformity will be affected by the etch selectivity. To prove this concept, we observed the photomask CD uniformity using several O2 concentrations where the variation in selectivity had different slopes. In these experiments, the photomask CD uniformity was estimated to be in the range from 1.26 nm to 1.68 nm. A significant improvement (25% reduction in the CD uniformity) was observed by using proper process conditions related to etch selectivity.

Theoretical study of fabrication of line-and-space patterns with 7 nm quarter-pitch using electron beam lithography with chemically amplified resist process: III. Post exposure baking on quartz substrates

Electron beam (EB) lithography is a key technology for the fabrication of photomasks for ArF immersion and extreme ultraviolet (EUV) lithography and molds for nanoimprint lithography. In this study, the temporal change in the chemical gradient of line-and-space patterns with a 7 nm quarter-pitch (7 nm space width and 21 nm line width) was calculated until it became constant, independently of postexposure baking (PEB) time, to clarify the feasibility of single nano patterning on quartz substrates using EB lithography with chemically amplified resist processes. When the quencher diffusion constant is the same as the acid diffusion constant, the maximum chemical gradient of the line-and-space pattern with a 7 nm quarter-pitch did not differ much from that with a 14 nm half-pitch under the condition described above. Also, from the viewpoint of process control, a low quencher diffusion constant is considered to be preferable for the fabrication of line-and-space patterns with a 7 nm quarter-pitch on quartz substrates.

Fabrication of photomasks consisting microlenses for the production of polymeric microneedle array.

A photomask consisting plano-convex microlenses for the production of polymeric microneedles was fabricated from a microinjection array. The microinjection array was first fabricated using photolithographical approach and subsequently assembled onto a polydimethylsiloxane (PDMS) stamp. Poly (ethylene glycol) diacrylate (PEGDA) solution was loaded into the microinjection stamp. The microinjection stamp was then applied onto a coverslip to dispense the polymer solution, producing liquid microdroplets. They were then irradiated to form plano-convex microlenses. These microlenses were evaluated for their geometric properties and were fabricated into photomasks. The photomask consisting microlenses was used to fabricate polymeric microneedles that were evaluated and tested for skin penetration efficiency.

Fabrication of a high-resolution mask by using variable-shaped electron beam lithography with a non-chemically amplified resist and a post-exposure bake

A high-resolution technique was developed for fabrication of photomasks for 10-nm-technology nodes and beyond. Current mask manufacturing techniques use a chemically amplified resist (CAR) photoresist material that has a complex mechanism of acid generation that obscures the criteria for selecting the polymer, quencher, and crosslinker for industrial purposes. It is therefore important to validate non-CAR materials as alternative solutions for mask fabrication. In this research, diluted ZEP520A was used as a non-CAR material in conjunction with a JBX9000 electron-beam lithography (EBL) tool. Additionally, a post-exposure bake (PEB) is normally used in mask fabrication. The PEB method was also used in this research, and we investigated its temperature dependence. Critical dimensions (CDs) of 1:1 line-and-space, isolated space, and isolated line patterns on a diluted ZEP520A resist were measured and showed CD shrinkage, an extension effect, and retention of the integrity of the shape after the PEB process. A resolution of the order of 20nm was attained and the insights gained from optimization of the PEB process might be usefully applied in advanced methods for fabricating masks of the next generation.

Numerical analysis of the sub-wavelength fabrication of MTMO grayscale photomasks by direct laser writing.

Metal-transparent-metallic-oxide (MTMO) grayscale photomasks fabricated by direct laser writing have been proposed in recent years. The fabrication mechanism is attributed to light-induced melt-oxidization. The temporal-spatial distribution of temperature fields of indium film-glass samples under a laser pulse have been calculated by the Finite-Difference Time-Domain method. The laser action area of the indium film is studied based on the oxidation theories and the absorbed laser power density distribution in molten indium films. The calculated average sub-wavelength fabrication diameter of 302 nm is consistent with the experimental fabrication size under a laser power of 6.0 - 8.0 mW.

Inverse e-beam lithography on photomask for computational lithography

Computational lithography, e.g., inverse lithography technique (ILT) and source mask optimization, is considered necessary for the “extremely low k1” lithography process of sub-20 nm device node. The ideal design of a curvilinear mask for computational lithography requires many changes during photomask fabrication. These range from preparation of the mask data to measurement and inspection. The manufacturability of a photomask for computational lithography is linked to predictable and manageable quality of patterning. Here, we have proposed the use of “inverse e-beam lithography” on photomask for computational lithography, which overcomes the patterning accuracy limits of conventional e-beam lithography. Furthermore, the preferred target design for ILT, a new verification method, and the accuracy required for the mask model are also discussed; with consideration of acceptable writing time (<24 h ) and computing power.