Christian Chovino

193-nm haze contamination: a close relationship between mask and its environment

The integration of 193nm Lithography is close to full production for the 90nm node technology. With the potential of emerging 193nm lithographic resolution down to 65nm, the quality of 193nm reticles including binary, EAPSM and AAPSM must be outstanding so that low K1 factor reticles may be used in production. One area of concern in the IC industry is haze contamination on the mask once the reticle has been exposed to ArF radiation. In this study, haze was found outside of the pellicle and on the quartz side of the mask. Standard through-pell inspections will typically miss the contamination, yet its severity can ultimately affect mask transmission. For this reason, DuPont Photomasks and Cypress joined forces to quickly decipher how it develops. In this investigation, tests were devised which altered conditions such as mask environment, exposure, traditional and advanced cleaning chemistry. This paper describes the relationship between surface and environmental photochemical reactions, the resultant growth, analysis, and how it is controlled.

Architectural choices for EUV lithography masks: patterned absorbers and patterned reflectors

Photoresist patterning experiments on the EUVL Engineering Test Stand using two masks with different types of architecture indicate that etched-multilayer binary masks can provide larger process latitude than standard patterned absorber masks. The trends observed in the experimental data are confirmed by rigorous electromagnetic simulations taking into account the mask structure, the imaging optics characteristics and the illumination conditions.

Influence of environmental components on haze growth

With the use of 193nm lithography, haze growth has increasingly become a critical issue for photomask suppliers and wafer fabs. Recent photomask industry surveys indicate the occurrence rate of haze is 10 times higher on 193nm masks compared to 248nm masks. Additionally, work has been presented that shows strong relationship between environmental conditions around the photomask and the occurrence of haze at 193nm. This underscores the need to better understand the basic mechanisms of haze and the measures such as environmental airborne molecular contamination (AMC) control which can be employed to reduce the occurrence of haze in use. A custom excimer laser test system capable of 193nm and 248nm wavelengths was built to accelerate haze growth and to better understand haze formation mechanisms. Work on materials impact on haze growth, such as pellicles and reticle compacts, as well as preliminary findings on environmental impacts have been presented previously. Results indicate even on pristine surfaces haze can grow when contaminants are present in the storage and use environment. The test system has been upgraded to include tight control on the concentration of specific airborne contaminants of concern. The impact of these contaminants and their relative concentrations will be examined in this paper and are presented to aid the industry in determining the level of environmental control needed over the life of a reticle.

Demonstration of phase-shift masks for extreme-ultraviolet lithography

We report on a method to produce any type of phase-shift masks for EUV lithography. We have successfully fabricated an unattenuated phase-shift mask consisting of phase patterns and confirmed the expected performance of such a mask through resist printing at λ=13.3 nm. Finally actinic metrology reveals that these etched-multilayer masks, left without a capping layer, tend to degrade over time.

EUV mask making: an approach based on the direct patterning of the EUV reflector

Extreme Ultraviolet Lithography (EUVL) is the leading candidate for manufacturing integrated circuits beyond the 45-nm technology node. The masks for EUVL are reflective and significantly different from current transmission masks for deep UV lithography. Many authors have demonstrated the patterning of EUVL masks using different types of absorber stacks that were deposited on top of the multilayer reflector. More recently, a new approach based on the etching of the multilayer reflector in order to define the mask pattern was proposed [2]. Using rigorous electro-magnetic simulations, it was shown that this subtractive approach could provide better process latitude, less H-V bias and smaller image-placement errors compared to the traditional masks based on the additive method. Even though the mask processing shows interesting challenges, this approach might offer immediate advantages over the more traditional patterning technique using the absorber stack, beyond those predicted for lithography imaging. These include the possibility to use optical inspection in transmission mode, which can provide the high-contrast images that are essential for high-sensitivity detection of small defects. In this paper, we present the first results on the patterning of EUVL masks using the direct etching the EUVL multilayer reflector (Mo/Si type) to produce EUV binary masks. In particular, we show how the process parameters can be adjusted to control the pattern sidewall angle. We also present an analysis of the influence of this sidewall angle on lithography imaging, based on lithography simulations. Finally, we show results from the optical inspection of these etched-multilayer binary masks (EMBM).

Study of time dependent 193 nm reticle haze

While significant progress has been made in reducing the occurrence rate of progressive defect growth on photomasks used at 193nm, the issue continues to be a problem for many semiconductor fabs. Increasing evidence from multiple sources indicates that further reduction in haze risk involves closely controlling the storage and exposure environment of the photomask. Further controlled testing is necessary to characterize the impact of environment and individual components on growth. In this way, photomask users, equipment and material providers may be better prepared to ensure the proper storage and use of photomasks in order to reduce the risk of haze growth. In continuation of work previously reported by Toppan Photomasks, advanced test apparatus, recently designed and built, now enables researchers to generate and maintain stable and controlled levels of multiple impurities which potentially effect haze growth. Supported by on-line and off-line analytical methods and instrumentation, new experimental set-up enables accuracy in the testing and validation of the impacts of environmental variables. Different classes of pollutants in multiple combinations have been studied to more precisely characterize environmental sensitivity of varying types of 193 nm reticles. Authors report further on the study of the effect of environmental conditions on severity and rate of haze formation to provide insight into the requirements for reducing or even preventing such conditions.

Manufacturing of the first EUV full-field scanner mask

In the framework of the European EXTUMASK project, the Advanced Mask Technology Center in Dresden (AMTC) has established in close collaboration with the Institute of Microelectronics in Stuttgart (IMS-Chips) an integrated mask process suited to manufacture EUV masks for the first full field EUV scanner, the ASML α-demo tool. The first product resulting from this process is the ASML set-up mask, an EUV mask designed to realize the tool set-up. The integrated process was developed based on dummy EUV blank material received from Schott Lithotec in Meiningen (Germany). These blanks have a TaN-based absorber layer and a SiO2 buffer layer. During process development the e-beam lithographic behaviour as well as the patterning performance of the material were studied and tuned to meet first EUV mask specifications. For production of the ASML set-up mask the new process was applied to a high performance EUV blank from Schott Lithotec. This blank has absorber and buffer layers identical to the dummy blanks but a multilayer is embedded which is deposited on an LTEM substrate. The actinic behaviour of the multilayer and the flatness of the substrate were tuned to match the required mask specifications. In this article we report on the development of the mask manufacturing process and show performance data of produced EUV full field scanner masks. Thereby, special attention is given to the ASML set-up mask.

UV light with oxygen treatment of phase shift photoblank for phase and transmission control: applicable to MxSi(1-x)OyN(y-1)

A method based on UV in air environment to improve the stability of the material of the photoreticles throughout cleans repeated over is suggested in this work. A typical aggressive clean was performed on two different Embedded Shifter materials, 193nm Molybdenum-Silicon-Oxy-Nitride (MoSiON) and 193nm Multilayer Silicon Nitride-Titanium Nitride (SiN-TiN). The variation of phase and transmission of each reticle is reported with the number of cleans. Given the appropriate exposure the phase and the transmission of the treated materials were significantly improved. All treated EAPSMs could stand cleans repeated over.

Contamination control for ArF photo masks

Advanced photolithography tools use 193 nanometer wavelength light for conventional and immersion printing. The increased energy of 193 nm (ArF) light coupled with the higher absorption cross section of most materials has lead to a dramatic increase in the rate of haze formation as compared to previously used lithographic wavelengths (248 KrF and 365 nm i-line systems). It is well known that at this short wavelength photochemical reactions are enhanced leading to progressive defect formation, or haze, on optical surfaces within microlithography tools. Therefore, strict contamination control of the optics environment is needed to avoid cumulative effects. Such measures have been implemented in lithography tools both for the optics and for the reticle during exposure. However, the patterned side of the photomask is the most sensitive element in the litho optical path for haze growth, because it is in focus and small defects will show up as printing defects. Moreover, the reticle life time depends both on rigorous contamination control for expose and transport/storage conditions (both inside and outside of the lithography tool). The litho operating cost depends directly on reticle life time. It is imperative that the industry takes the required measures to improve the airborne molecular contamination levels both in the storage part of the photolithography tool and in devices used to transport reticles outside of the tool to slow down reticle haze Past studies have shown the large effects of humidity and AMC on haze growth during storage and exposure. Therefore, significant improvements in storage and exposure environment have been implemented by many fabs to reduce the frequency of haze failures. It has also been shown that outgassing from materials surrounding the mask can influence or cause haze. It is clear that the reticle must be adequately protected from contamination sources throughout the life cycle of the reticle (both inside and outside of the lithography tool). In this paper we examine improvements in the storage conditions of reticles inside the lithography tool as well as improvements in commercial SMIF pods used in fab storage and automated handling of reticles.

Damage free megasonic resonance cleaning for the 45nm design rule

Scatter bar (SB) breakage presents a mounting confrontation for final cleaning of masks While the industry is strongly dependent on megasonic (MS) energy, MS is hazardous to scatter bars which approach the size of particles which must be removed. The difference in energy needed to remove small particles and the energy needed to remove small features represents a subtle and shrinking domain. Here we observe cleaning effects when the plate is inverted. This gives us a look that at affects which might otherwise remain hidden. We provide evidence of plate resonance effects and constructive interference from internal reflection We assess the ability to clean a plate without direct exposure to the MS beam We adapt a MS bath qualification method for use on spinning plates and use it to assay cavitation activity and uniformity for Upright and Inverted spinning plates. Cavitation activity is recorded in the spalling on a metallic film, which allows quantification by optical reflectance measurements. Value to both cleaning and SB breakage are assessed.