Contamination of chemical amplification resists by airborne basic substances can be minimized by reducing the free volume of resist films by annealing. Good annealing requires heating the resist films above their glass transition temperatures. However, it is not usually possible to bake chemical amplification positive resists based on poly(4-hydroxystyrene) above their glass transition temperatures because thermal deprotection typically occurs below their glass transition temperatures. Use of meta- isomers to lower glass transition temperatures is described first to prove the validity of our annealing concept. Furthermore, our high temperature bake process of a new thermally and hydrolytically stable positive resist (ESCAP) is reported. The annealing effectively reduces uptake of airborne contaminants by resist films because the diffusivity of small molecules in polymer films is an exponential function of the free volume and thus stabilizes the latent images.

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We approached the problem of airborne chemical contamination from the processing side. By applying a very hydrophobic, polyolefinic coating resin, we completely eliminated T-tops. Patterning a resist in a low ammonia concentration also eliminated T-tops and resulted in low line width deviation rates against PED. When we patterned the resist with an ammonia concentration of 0.2μg/m³, the deviation of the line width against PED was within the range 0.56 to 0.80nm/min for line-and-space patterns between 0.30 and 0.35μm. These low deviation rates were considered to be acceptable for manufacturing purposes.

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An environmentally stable resist with high contrast, sensitivity and resolution is presented. Delay stability in excess of 24hr. has been achieved with insignificant change in linewidth. The resist also demonstrates extreme insensitivity to PEB, showing insignificant linewidth changes over a PEB temperature variation between 20°C-100°C. The resist is sensitive to DUV, X-ray and e-beam radiation. Linearity down to 0.25μ m using 0.5NA MSII (248nm) tool has been demonstrated. The resist shows better than 0.8μm DOF and 15% DL at 0.25μm line and space pairs. Resolution down to 75nm line and space pairs has been demonstrated with 100KeV e-beam tool. We have discussed a new approach to achieving an environmentally-stable, chemically-amplified resist. The resist system, KRS, combines the advantages of conventional and chemically-amplified resists. The resist exhibits high contrast, sensitivity and resolution and at the same time is environmentally-robust and extremely insensitive to PEB temperature variation. The resist demonstrated linearity down to 0.25μm with acceptable DOF and DL using 0.5NA MSII stepper. The ultimate resolution is 0.225μm, limited only by the smallest feature on the mask at the current time. The resist attributes make it compatible with current 0.25μm DUV lithography requirements and extendible to future needs with an improved DUV tool set. The resist is also sensitive to e-beam and X-ray radiation. High resolution imaging, 75nm line and space pairs in 0.75μm thick resist, has been demonstrated using 100KeV e-beam tool.

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A new resist system composed of an SEPR chemically amplified (CA) positive resist and an N-methyl pyrrolidone (NMP) organic base has been developed for KrF excimer laser lithography. Using 0.30-μm l&s patterns formed with KrF stepper, we studied the effect of contamination from substrate films of plasma chemical vapor deposited silicon dioxide (P-CVD SiO₂), low pressure CVD silicon nitride (LP-CVD Si₃N₄) and reactive sputtered titanium nitride (TiN), and of airborne contamination under the condition of 8ppb of ammonia. The results clarify the new resist system reduces
the effect of substrate film as well as airborne contamination. The new resist system enables us to form fine patterns on any substrate and attains more than one hour post-exposure delay without overcoat and undercoat films.

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Standard positive tone chemically amplified resists usually suffer from a latent image instability caused by uncontrolled acid diffusion after their exposure. To improve the delay stability, novel resist additives with both high nucleophilicity and radiation sensitivity have been developed. Typical representatives include opium hydroxides, alkoxides, or carboxylates, which act as strong bases in aqueous media. The compounds decompose into neutral fragments upon exposure but preserve their basic character in the unexposed areas. This enables a selective neutralization of the diffusing acid in areas of intended unexposure. When applied to a chemically amplified resist system the shelf life of the resist solution, the film life stability and the delay time stability are significantly improved, while the photospeed is only marginally deteriorated. Enhanced resolution capability, and improved latitudes are additional benefits.

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The effect of two kinds of overcoats on stabilizing the resist during post-exposure delay is evaluated in the presence of water and organic bases. Organic-soluble poly(α-methylstyrene) is more effective in preventing water penetration than poly(acrylic acid), which is a water- soluble overcoat. On the other hand, poly(acrylic acid) is more effective against organic bases such as aniline and N-methyl pyrollidone (NMP).This is probably because the negative charge of the carboxyl groups in poly(acrylic acid) electrically traps aniline and NMP, preventing their penetration to the resist films.

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This paper concerns the acid evaporation caused by the baking treatment of chemically amplified resists. The evaporation of photo- generated acid during the post-exposure-baking (PEB) process was measured as a spectral change of indicator film that consisted of polyvinyl alcohol (PVA) and tetrabromophenol blue sodium salt (TBPB). Using this method, it was observed that the amount of evaporated acid depended on the pre-baking and PEB conditions. A high temperature pre-baking before exposure reduced the amount of evaporated acid. By contrast, a high temperature PEB accelerated the acid evaporation, and the thickness of the insoluble layer increases with increasing PEB temperature. These results suggest that acid evaporation is concerned with the formation of the insoluble layer.

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Current efforts to reduce deep ultraviolet (DUV) photoresist profile abnormalities on silicon nitride (Si₃N₄) and titanium nitride (TiN) are summarized. The nitridic surface of the films is investigated to understand the mechanism that is responsible for severe photoresist footing at the photoresist/substrate interface. Resist footing was compared for various changes in resist processing and substrate pretreatments. For the oxide/Si₃N₄ stack the footing problem was studied using resists processed with different post exp osure bake temperatures, with substrates that had been pretreated and with different thicknesses of an organic barrier coat. ESCA analysis was used to examine the surface of the oxide/Si₃N₄ substrate for contaminants. For TiN substrates, the DUV resist footing problem was studied using different photoresists and TiN film of various stoickhiometries. Multiple characterization techniques were used to examine the TiN film including Auger electron spectroscopy, atomic force microscopy, Rutherford backscattering and reflectivity measurements. Considering these results, and information previously published, it is concluded that there exists detrimental surface and interface states on the TiN substrate that are principally responsible for the footing.

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Activated carbon treated with a hydrogen salt is found to be effective in lowering ammonia concentration in LSI cleanrooms. Carbon block with a honeycomb structure and low pressure loss is utilized. The hydrogen salt contains neither phosphor nor metals. Its ammonia adsorption capacity is about 1000mol/m³ -carbon, This provides a lifetime of around two years in ordinary cleanrooms. A chemically amplified resist, known to be easily degraded by trace alkaline gases, is proved to maintain 0, 25-μm line and space patterns under hydrogen salt treated carbon filtered air, even when the patterns are developed 60 minutes after excimer laser exposure.

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A new chemically amplified Si-containing positive resist designed for ArF excimer laser lithography is described. TMSMMA-OCMA resist (Poly (trimethylsilyl methylmethacrylate-3-oxocyclohexylmethacrylate) with TPS SbF₆) worked as a positive resist when exposed by ArF excimer laser and it resolved 0.19μm Line and Space (L/S) pattern.

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Our approach to the design of positive, single layer resists for 193nm lithography will be discussed. Phenolic resins, the archetype in positive photoresist materials, cannot be used at this wavelength due to optical opacity. Acrylic polymers combine the required optical transparency at 193nm with easily tailored properties. With a design based on methacrylate terpolymers, we developed a high resolution positive resist for 193nm lithography with excellent imaging at both 193 and 248nm. Our recent work has centered on gaining further insight into methacrylate polymer structure/property relationships, improving the imaging performance and finally increasing the etch resistance. Towards that end, we have employed a class of dissolution inhibitors for 193 nm resists that are combined with methacrylate polymers to provide 3-component resists.[1] A family of 5B-steroid dissolution inhibitors that also increase etch resistance will be described. Imaging and etch performance of four versions of our resist will be disclosed. These methac ylate resists show resolution capability below 0.25 micron, etch rates 20% higher than novolak resins, good environmental stability in contrast to traditional DUV resists and dual wavelength (193/248nm) imaging.

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We investigated the etch rates of various polymers etch rates, and found that alicyclic polymers show a good dry etch resistance in spite of the lack of the aromatic rings. On the basis of this finding we proposed a new ArF resist material which has good transparency and good dry etch resistance for use as a single layer resist.. The resist comprises an alicyclic copolymer of adamantyl methacrylate (AdMA) and 3-oxocyclohexyl methaclyrate (OCMA) and a triphenylsulphonium hexafuluoroantimonate as a photoacid generator (PAG). The transmittance of the 1μm thick copolymer is about 70% at ArF wavelength. This high transparency ensures pattern imaging of the resist in ArF lithography. The dry etch resistance of the resist is also high and is comparable to that of Novolak resist. We obtained 0.17μm line and space patterns with this resist, using ArF excimer laser stepper.

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The phenomena involved in positive chemically amplified resists are the result of intricate chemical and physical mechanisms. This complexity is observed from spin-coating to development. The physical properties of the lithographic layer are strongly influenced by the chemistry used. In turn, the deprotection reaction can exhibit different kinetics depending on the physical properties and thermal history of the film. The deprotection reaction generates an expanded matrix that can collapse and densify when the PEB temperature is higher than the transition temperature. This work shows that both deprotection and dissolution rate are not related to exposure dose by simple relationships. This new approach provides a deeper understanding for good control, design and modeling of these systems.

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Depositing materials selectively on small domains like on top of 100 nm Si lines or in 100nm Si trenches is our purpose. Such a technique is not only scientifically important but also it may open up new interesting fields in science and engineering. We fabricated 100nm-wide Si-line/space patterns with the 4th harmonic of Nd:YAG laser (266nm) or with polarized ArF excimer lasers (193nm): first polyimide ripple structures, followed by series of reactive ion etchings. We have successfully deposited amorphous diamond- like-carbon only on top of 100nm-wide Si-lines by photoablation of polymers. We were also successful in depositing pure crystalline diamond over 100nm- wide Si lines. We are successful in fabricating 100nm line/space patterns on silicon using polymer exposures either by Nd:YAG laser at 266nm or polarized ArF excimer laser beam at 193nm, and by subsequent image transfer to silicon substrates by reactive ion etchings. We selectively deposited diamond-like-carbon on top surfaces of 100nm-wide Si lines. We also deposited crystalline diamond on such Si structures. Acknowledgements: The authors like to thank Hong Kong Government Research Grant RGC HKUST 627/94P and HKUST research grant RIG92.93.SC06.

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The current trend of shrinking line width patterns in semiconductor devices, well beyond the 0.25 micrometer barrier, will require new breakthroughs in both lithography processing and resist materials by the end of the decade. Scanning probe microscopy (SPM) offers one potential prospect for bridging the micrometer to nanometer line width barrier. The ultimate in lithography processing, the manipulation of individual atoms, has already been clearly demonstrated using SPM. New resists, such as self assembled monolayer (SAM) films, will also be needed for nanometer-scale fabrication. Recent progress using SPM as a nanolithography tool is reviewed and its prospect as a viable alternative will be discussed.

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Process control in photolithography will be a key requirement for the successful manufacturing of 0.25 micron semiconductor products. A program to study photolithography process control techniques was recently undertaken at Texas Instruments. The target for this program was 0.35 micron technology, and the lithography used a combination of conventional wet developed resists and surface-imaging dry developed resists along with both i-line and deep UV steppers. Advanced process control techniques were applied in order to achieve lithography control without lot send-ahead wafers or test wafers. A notable conclusion was the need for the development of real-time sensor technology, particularly for the stepper exposure process. Lithography for the program demonstration was performed both with conventional i-line processing and with surface imaging and dry develop. Acceptable process control was achieved without lot send-ahead wafers or test wafers by a program of short-loop SPC and regular monitoring of equipment states. The fundamental lithography processes appear to have sufficient short term control in general, but sudden excursions and long term drifts still occur. The goal of fully automated process control can only be met when there are real time sensors and control software on the key steps in the lithography process. The most pressing need is for a sensor that could control the exposure on the stepper. With such a system variations in the incoming wafer state could be compensated, even for short run ASIC lots, to provide an on-target pattern every time.

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Present status and recent advancement of resists in optical, EB and X-ray lithographies are reviewed from the viewpoint of next-generation LSI process engineering. The important issues for advanced resist technology development in these lithographies are discussed. For resists in optical lithographies (KrF and ArF), material optimization is the most critical issue to realize high transparency, high sensitivity and high dry-etching durability. For EB and X-ray resists, sensitivity is the most serious to get higher throughput. In order to be introduced in the mass production line, reproducibility must be improved correspondently to LSI design rule in each generation. Resist improvement and device application scenario are discussed for 256M and 1G DRAM production.

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Progress in VLSI device design and manufacture continues to demand increasingly smaller and more precise device features. While today, almost all commercial devices are made by i- and g-line (365-436nm) photolithography, new lithographic strategies will be required within the next 5-8 years. Concomitant with the development of new lithographic techniques is the development of new resist materials. This paper presents a view of the varied chemistries that are available for the design of resist materials. Particular emphasis is placed on the novel chemistry and processing techniques, such as chemically amplified mechanisms and gas-phase functionalization processes, that have been developed in recent years.

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An overview of the present status of synchrotron radiation-based x-ray lithography (SRL) and its extendibility down to 0.1-micron features and lower are described. The attained performance of SR sources, aligners, and masks is measured with requirements for 1Gb DRAM level. Some of SR sources so far developed can provide enough SR flux for practicable throughput. While several prototype aligners have demonstrated sufficiently high alignment accuracy, the mask pattern placement needs to be further improved in order to fulfill the overlay requirement. The resolution well below 0.1micron is feasible with narrow proximity gap of order of 10micron. Difficulties in fabricating fine pattern masks can be reduced by making use of the phase shift effect. Investigation on the mask membrane deformation during stepping with the narrow proximity gap is now in progress. The present status of SRL and its possibility down to 0.1-micron features have been described. The key issues in 180nm dimensional region are improvement of pattern placement accuracy of masks and development of high throughput aligners. In order to upgrade SRL to a production-viable level, it will be necessary to optimize the whole SRL system. From this point of view, throughput is a good measure, since almost all components of SRL system relate with it. Another important issue is further improvement of the reliability. The SRL system is considered to be cost-effective only when many aligners are operated with single SR source. Thus, extremely high reliability is required of the SR source. This can be accomplished by establishing proper maintenance schemes based on accumulated data on operation, maintenance, and troubles at existing SR facilities routinely operating. Technical problems associated with fine pattern replication of order of 0.1micron have been also discussed. The possible solutions presented will lead to the extendibility over several generations of ULSI.

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