A highly reliable mask inspection system

An automatic system for inspecting micro mask defects with 1-µm minimum detectable size has been developed. An outline of the system is as follows: The pattern image obtained with a pickup tube is converted into binary video signals which are transferred into two parallel logic circuits for detecting pattern defects. One is based on the pattern-analyzing method, for which one of four algorithms for detecting micro defects is presented in detail. The other is based on the design-pattern data-comparing method, where the data compression scheme and a new idea for avoiding mask alignment errors are adopted. A software system outline, very important in assisting the hardware functions in this system, is also presented. The results of experiments for determining system performance indicate that the system can detect ≥1-µm diameter defects or loss patterns with high probability by complimentary use of the two methods. A 4-in by 4-in mask can be inspected within 100 rain.     

An electron beam maskmaker

An electron beam machine is described, in which the ¼-µm diameter beam is computer controlled to define integrated circuit and other fine patterns at their final size in response to a coordinate data input. Electron sensitive resist is exposed on metallized quartz or glass substrates. Resist development followed by metal etching enables masks to be made, either for subsequent photolithography or, more usually, for use in the electron image projector developed by J. P. Scott. The mask drawing process is entirely automatic and the emphasis is on the rapid generation of complex patterns with high precision. A two-stage deflection system enables rectangular pattern elements to be drawn at a 10-MHz stepping rate and accurately positioned throughout a 2-mm square main deflection field. Patterns are automatically positioned, to an accuracy of ±1/8 µm, relative to an array of markers predeposited on the substrate. The beam is also refocused automatically at the markers. A mechanical stage for the substrate enables 50 × 50-mm arrays of patterns to be built up. A complete mask containing detail as small as 1 µm takes 1-3 h to draw. Finer patterns can be drawn, although more slowly.     

Controlled Deposition of ${rm SiO}_{2}$ Nanoparticles of NIST-Traceable Particle Sizes for Mask Surface Inspection System Characterization

Particulate contamination of masks is a serious challenge in extreme ultraviolet lithography (EUVL) technology due to the unavailability of conventional pellicles. EUVL mask surface inspection tools, operated at low pressure, are used not only for mask contamination control/monitoring but also for mask surface cleaning studies. In EUVL, contaminant particles can be generated during low-pressure stages of integrated circuit (IC) manufacturing and may contaminate the mask critical surface without protective pellicles. It is therefore needed to characterize the EUVL mask surface inspection tools with contaminants commonly seen in vacuum processes. We have developed a method to deposit particles of known material and NIST-traceable sizes on the mask surface for the purpose of calibrating the EUVL mask surface inspection tools. Our method can produce particles with 98% size-uniformity. SiO₂ particles with NIST-traceable sizes of 50 nm, 60 nm, and 70 nm were separately deposited on quartz mask blanks with a controlled deposition spot size and number density, and detected by a Lasertec M1350 mask surface scanner. The results demonstrate high capture efficiencies for 60 and 70 nm SiO₂ particles, and significantly lower capture efficiency for 50 nm SiO₂ particles. The sizing accuracy of Lasertec M1350 deteriorates with decreasing particle size.     

0.4-µm gate-length devices fabricated by contrast-enhanced lithography

This letter describes the fabrication of submicrometer polysilicon-gate MOS devices by an advanced optical process called contrast enhancement. Functional devices having gate lengths as small as 0.4 µm were fabricated with this process. Contrast-enhanced lithography (CEL) allows usable photoresist patterns to be fabricated at smaller dimensions than is possible with conventional resist. The simultaneous replication of mask dimensions for isolated lines at 0.35 µm and above was achieved in this work using a single exposure on an Optimetrix 10:1 DSW system. Contrast enhancement has been applied to the fabrication of n-channel MOS devices having gate lengths from 0.4 to 1.5 µm in steps of 0.1 µm. Long-channel devices were also fabricated. The transconductance of the 0.4-µm devices is 40 mS/mm at Vds= 5 V. Threshold voltages (Vds= 0) are nearly independent of gate length, ranging from 1.21 to 1.31 V over the 7.5- to 0.4-µm range in gate length. The effective mobility for long-channel devices is 430 cm²/V.s.     

Ion-implanted FET for power applications

High-energy ion implantation is coupled with the conventional planar technology to realize a silicon FET for power application. This device known as "Gridistor" is a multichannel FET with a p-type buried as gate. Boron implantation at various energies (600-900 keV) through a metallic mask are used to do a high-doped p-type gate layer, 0.8 µ thick and buried 1 µ below the surface. Since there is no implantation induced defects in the active regions of the device, low annealing temperature can be effectively used. As a consequence, the pattern sharpness is only limited by the definition of the mask. Using ion-etched gold layer as mask, 1 µ wide channels are made in a reproductible way. Few test structures have been made to check the behavior of implantation and planar technologies by measuring their capaci tances, transconductance, and I-V characteristics.     

Fabrication of monolithically Peltier-cooled linear laser diodes using wet chemical etching

The basic structure of a monolithically Peltier-cooled laser (MPCL) diode has been fabricated. The process of forming the structure involves critical masking and etching processes. A reliable etching process that was capable of producing clean mesas 50 µm wide × 300 µm long and of a height greater than 150-µm was developed using buffered hydrofluoric acid. The mask used-with the buffered hydro-fluorice etchant was Shipley AZ1350B photoresist. This combination of the etchant and the mask process gave good results even when etching a grid pattern consisting of an array of 2-µm-wide lines to form mesas 3.74 µm high. It was found that the etch tends to follow the cleaved planes that intersect the     

PSG masks for diffusions in gallium arsenide

This paper describes the use of phosphosilicate glass (PSG) films as an effective mask against zinc and tin diffusions in gallium arsenide. It is shown that films with a high phosphorus pentoxide content (as much as 30 percent by weight) can be used to obtain adequate crack-free masks against these dopants. Effective masking was obtained for diffusion depths (unmasked regions) up to 10 µ.     

Fabrication of submicrometer gold lines using optical lithography and high-growth-rate electroplating

This letter presents a process for fabricating submicrometer gold lines with high aspect ratio and vertical sidewalls using high-growth-rate (133-nm/min) electroplating. A two-resist-layer system, photolithography, angled evaporation, and dry etching techniques are used to fabricate the electroplating mask. Lines less than 0.2 µm wide, with aspect ratios greater than 5, are obtained with the use of this technique.     

Study on critical dimension of printable phase defects using an EUV microscope

We constructed an extreme ultraviolet microscopy (EUVM) system for actinic mask inspection that consists of Schwarzschild optics and an X-ray zooming tube. This system was used to inspect finished extreme ultraviolet lithography (EUVL) masks and Mo/Si glass substrates. A clear EUVM image of a 300-nm-wide pattern on a 6025 glass mask was obtained. The resolution was estimated to be 50 nm or less from this pattern. Programmed phase defects on the glass substrate were also used for inspection. The EUV microscope was able to resolve a programmed pit defect with a width of 40 nm and a depth of 10 nm and also one with a width of 70 nm and a depth of 2 nm. However, a 75-nm-wide 1.5-nm-deep pit defect was not resolved. Thus, in this study, one critical dimension of a pit defect was estimated to be a depth of 2 nm.     

Lateral diffusion of zinc and tin in gallium arsenide

The lateral diffusion of zinc and tin in GaAs, during their diffusion through windows in phosphosilicate glass (PSG) masks, has been measured as a function of oxide composition and oxide thickness. Lateral diffusion of zinc extends up to 20 times the junction depth, whereas the lateral diffusion of tin extends up to 500 times the junction depth, interfacial stress between mask and substrate has been shown to be the driving force for the large lateral diffusion. A method of eliminating the stress gradient by using a thin oxide (1000 Å) in the diffusion windows has resulted in reducing the lateral diffusion to less than 2.5 µm, the resolution of the measurement technique, for diffusion with a junction depth of 1 µm. A new method of measuring the expansion coefficients of thin dielectric layers has been presented. The expansion coefficients of PSG films with 0-30% P2O5content by weight have been measured using this method. Further, stresses in PSG films deposited on GaAS have been measured as a function of oxide composition.     

Applications of the electron beam exposure system

Successful application of high-speed scanning electron lithography machines such as the electron beam exposure system (EBES) requires the development of suitable materials and processes. We describe how electron resists with improved sensitivity have been used to make high-quality photolithographic master masks in which 1-µm lines and spaces are resolved and also to make MOS integrated circuits with improved packing density and performance.     

Reactive sputter etching of silicon with very low mask-material etch rates

Directional etching of deep structures in silicon is often made difficult by a high mask erosion rate. Recent results have given a Si/SiO2etch rate ratio of up to 8 without the undercut problems associated with other selective etches. In this paper a new selectivity mechanism is described which can reproducibly give Si/SiO2etch rate ratios of more than 100 with a nonloading target, and more than about 50 with a loading target. Similar etch ratios are also obtained with masks of MgF2, Al2O3, Al, and Cr. The inherently high Si/SiO2etch rate ratio obtained in Ar/Cl2discharges is here enhanced by causing selective deposition of SiO2onto slowly etched materials. The silicon may be obtained from the target, or, for easier control, from input gases such as SiCl4. The deposition rate is controlled by the oxygen concentration. The results of etching deep grooves in Si are presented. Etch-mask faceting and Si surface decoration appear to limit the attainable etch rate ratios with fine structures; however, 18-µm-deep gratings of 4.5-µm period have been etched in Si.     

A high resolution negative electron resist by image reversal

The use of AZ 1350 family photoresists as negative electron resists is described. Conventional photolithographic technology is used to coat and process the resist, with the exception of an e-beam exposure for patterning. A flood UV exposure is used for image reversal. Using 1.5 µm initial thickness, the exposure threshold for 6 s development in 1 : 1 AZ : H2O developer is 7 µC/cm². The resist contrast under these conditions is 1.3; and the sensitivity is about 25 µC/cm²(70% thickness remaining). Useful resolution on SiO2/Si and Al/SiO2/Si substrates is demonstrated to be at least 0.5 µm, and the resist is shown to mask the plasma etching of Al.     

Dry etching technology for 1-µm VLSI fabrication

A dry etching technology for 1-µm VLSI has been developed. This technology led to successful fabrication of a 1-µm 256-kbit MOS RAM using electon-beam direct writing and molybdenum-polysilicon double-gate structure. Silicon nitride, silicon dioxide, phosphosilicate glass, polysilicon, single-crystal silicon, molybdenum, and aluminum are etched by parallel-plate RF diode reactors. Resist patterns are used as etching masks. The negative resist is CMS and the positive resist is FPM. Plasma polymerization is found to have significant effect on etching selectivity, undercutting, and residue. Directional etching profiles are realized and 1-µm patterns with less than 0.05-µm undercutting are obtained. High etching selectivities are achieved. Methods for preventing and removing contamination as well as damage are established. With these, dry etching proves to bring no adverse effects on device characteristics. Pattern-width fluctuations caused by negative-resist pattern foot are decreased to below 0.1 µm by a new foot trimming technique. Resist step coverage is also clarified.     

Models for characterizing phase-shift defects in optical projectionprinting

An algebraic model is developed for characterizing the printability, inspection, and repair of phase-shift defects in optical projection printing. Phase-shift defects are particularly difficult to characterize because of the many parameters associated with the exposure tool and with the attenuating phase shift mask (PSM) pattern. Furthermore, the parameters change during inspection of the attenuating PSM because the mask is examined under illumination conditions which differ from the exposure illumination. An algebraic model which encompasses this large set of variables is derived by considering the electric fields under the mask to be a combination of the electric fields from the feature and defect. These fields are then combined according to the mutual coherence function for the mask illumination. A notable difficulty is the relative phase shift due to defocus between large and small features. The model is shown to be valid for defects up to 0.35 λ/NA by comparison to SPLAT. Experimental verification is made for defects impacting a 6% transmitting PSM for 0.35-μm features at i-line. The reliability of the model is illustrated by giving rules of thumb for defect printing in attenuating PSM's     

Plasma chemistries for high density plasma etching of SiC

A variety of different plasma chemistries, including SF₆, Cl₂, ICl, and IBr, have been examined for dry etching of 6H-SiC in high ion density plasma tools (inductively coupled plasma and electron cyclotron resonance). Rates up to 4500?-min⁻¹ were obtained for SF₆ plasmas, while much lower rates (≤800?·min⁻¹) were achieved with Cl₂, ICl, and IBr. The F₂-based chemistries have poor selectivity for SiC over photoresist masks (typically 0.4–0.5), but Ni masks are more robust, and allow etch depths ≥10 μm in the SiC. A micromachining process (sequential etch/deposition steps) designed for Si produces relatively low etch rates (<2,000?-min⁻¹) for SiC.     

Development of Real-Time Vision-Based Fabric Inspection System

Quality inspection of textile fabric products is an important problem for fabric manufacturers. This paper presents an automatic vision-based system for the quality control of web textile fabrics. Typical web material is 1–3 m wide and is driven with speeds ranging from 20 to 200 m/min. At present, the quality inspection process is manually performed by experts. However, they cannot detect more than 60% of the overall defects for the fabric if it is moving faster than 30 m/min. To increase the quality and homogeneity of fabrics, an automated visual inspection system is needed for better productivity. Currently, the existing inspection systems are too expensive for small companies. In this paper, a PC-based real-time inspection system is proposed with benefits of low cost and high detection rate. The proposed algorithm showed good results for several types of fabric defects.     

Electrofax®printing with gallium phosphide diodes

Experimental green-light-emitting GaP diodes are shown to be capable of adequately exposing Electrofax^®paper with exposures of 400 µs. The diodes used were prepared from nitrogen-doped epitaxial junction material; the Electrofax paper was sensitized with a rose-bengal dye. Linear arrays of seven diodes spaced 250 µm apart have been constructed and used to print characters on a moving strip of Electrofax paper developed by a reversal liquid toner system. Measured image resolution is better than 40 elements/cm. Data are given on the electrical characteristics of a single diode and an array, and on the dependence of image density and dimensions on diode currents. The observed performance of the diodes indicates the feasibility of making a high-quality printer using light-emitting diodes (LED's) and Electrofax paper.     

A focus mask for color picture tubes using dipolar and quadrupolar lensing

Post-deflection focus masks can increase the electron-beam transmission in a color picture tube over that achievable by conventional shadow masks. Various focus mask structures are considered here and most attention is given to one in which the electrostatic fields in each mask aperture consist of a dipole and a quadrupole component. A theory of the combination of dipolar deflection and quadrupolar focusing is given. Experiments on small masks are described and their quantitative behavior is evaluated. A demountable focus-mask color tube is described which has a 10-in diagonal flat mask. This has been used to display video and the 50-percent transmission of the focus mask has allowed high brightness operation with screen voltage of only 10 kV.     

Null holographic test structures for the measurement of overlay andits statistical variation

Results are presented on the use of null wire segment holograms for the in-line assessment of mask alignment errors in the integrated circuit fabrication process. Process variations are detected by measuring the light intensity generated by a hologram designed to project a null image. To detect alignment errors, the mask for the wire segment hologram (WSH) is distributed between two mask layers. If the two sets of diffracting structures defined by the masks are transferred to the wafer with perfect registration, the result is an area of light cancellation (null) in the image plane. Increased mask misalignment leads to imperfect wavefront cancellation, which is manifested as an increase in light intensity in the null region. In order to characterize misalignment under controlled conditions, the two portions of the holographic test structure were initially recombined into a single structure but with intentional misalignment between the two portions designed into the mask. The technique was then used to characterize the alignment errors between two separate masks with the actual fabricated offsets measured using atomic force microscopy. Initial results indicate the technique is capable of resolving 0.1-μm mask misalignment for a 1-μm minimum feature process