Hitoshi Handa

Process monitoring of chrome dry-etching with RF sensor for reticle production beyond 90-nm node

In reticle production for 90-nm node generation and beyond, quite strict chrome CD control is strongly demanded. For chrome dry-etching process, esetablishment of reliable system for chrome endpoint detection is one of critical issues. In this paper, the effectiveness of radio frequency (RF) sensor as a process monitor for chrome dry-etching was examined. As general endpoint detection system, such as Laser endpoint detection (Laser EPD) and optical emission spectroscopy (OES), the system based on RF sensor shows the ability to get chrome endpoint. Experimental data implied its competence as an endpoint detector for the plates of various chrome loads with enough stability and reliability. Moreover, this sensor has an advantage that plasma impedance observed with the sensor has a correlation with etching performance, such as etching bias and its uniformity. This property is useful, because feedback of the variation of sensor output to process condition is able to play important role in control of reticle CD. As a consequence, the concept of advanced process control based on the RF sensor is proposed.

Process bias control with thin Cr film blanks for 90nm-node reticle fabrication

For today’s advanced reticle production, process bias should be reduced as possible, and be “zero” ideally, because of its negative impacts on CD control and pattern fidelity against minute features. In this paper, blanks with Cr film thinner than 100 nm were examined as promising materials to meet this demand. Results from two aspects, reticle production and lithographic performance, are presented. Dry etching properties such as etching bias, CD Linearity, impact on pattern-pitch were investigated. Thin Cr film blanks showed excellent results in process bias less than 50 % of conventional ones with proper etching conditions. They also showed about 50 %’s decrease of etching bias variations for pattern-pitch with smaller CD error in CD Linearity at the same time. Results of aerial image simulations showed possibilities of them as substitute of conventional binary blanks in ArF lithography. There was no impact of Cr film thinning on depth of focus and optical proximity effect with optimized exposure condition. These experimental results imply that Cr film thickness plays an important role to decide the qualities of reticle CD. Thinner Cr film blanks have capability to realize a 90 nm-node reticle with minimized process bias enough to produce fine OPC features.

Advanced process control of mask dry-etching using RF sensor

Advanced process control (APC) of photomask dry-etching has been studied for strict mean control of both CD and phase angle of phase shift masks (PSMs). Equations to correlate process information with actual etching results have been developed for this purpose. It is showed that plasma reactance measured with RF sensor has noticeable correlation with Cr etching bias, which is affected by Cr load and condition of etching chamber. Simulation of etching bias based on plasma reactance shows the good agreement with the trend of actual etching results. Expectation of process capability index (Cpk) for mean-to-target (MTT) within 5.2nm is about 1.27, corresponding to CD yield more than 99.9%. In case of MoSi based PSMs, monitoring the sensor outputs is also useful to simulate the etching rate of phase shifter. One simple relationship can be also derived as the case of Cr etching bias. Expected phase error is within 1.5degree in almost cases. In actual photomask fabrication, maintenance of the equation for APC is a critical issue to guarantee the high process yield for a long period. It is showed that trend of the plasma reactance gives the meaningful information effective in automatic maintenance of the equations. As a conclusion, it is proved that our APC method is one of the answers to give the highest MTT yield for both CD and phase angle.

Improvement of Cr dry etching characteristics with the MERIE system

Dry etching characteristics of Cr films were investigated and some improvements have ben done with magnetically enhanced reactive ion etching (MERIE) system. Clear field patterns and ark field ones exposed on thin EB resists, whose thickness was less than 300 nm, were etched. Although there had been some difficulties in etching of clear-field pattens with SAL-601, these situations were much improved with an appropriate etching condition of magnetic field. It was found that magnetic field intensity affected Cr etching distributions very much. In marked contrast to the above results, MFI condition showed little contributions to the etching distribution of dark field patterns exposed on ZEP- 7000. It was shown that some waveforms of magnetic field could be effective to improve the etching characteristics for the plate whose etching area was extremely small. Etching characteristics for these extremely varied Cr-loaded are considered through the above etching results. Discussions about more useful dry etching process with MERIE system are also described.

Reticle cleaning process for 130-nm lithography and beyond

Three cleaning methods were examined to check their strong points. Cleaning results were analyzed from the aspects on removal of particles and chemicals. Starlight inspection results showed that conventional wet cleaning based on chemicals, such as H2SO4 and NH4OH, could remain small particles on chrome-oxide (CrOX). DUV irradiation could assist this traditional SC-1 (mixture of NH4OH and H2O2 and HH(subscript 2O) cleaning in removing these sticking particles. Electrolyzed water, contained anode and cathode water, showed same tendency as SC-1 treatment, which could easily attract particles to CrOX surface. Mechanism of particle removal and attraction was considered from the aspect on electrostatic reaction between particles and photomask surface. ArF ((lambda) =193nm) lithography could cloud quartz surface with crystallized substances. Analytical results implied that they had been generated by optical-chemical reaction between ArF light and chemical residue after cleaning. Experimental results showed that DUV treatment before cleaning was effective to prevent reticle surface from chemical contamination. From the above knowledge, suggestion about reticle cleaning process for ArF lithography is described as a conclusion.

Investigations of CD variation in Cr dry etching process

In this report, origins of CD error caused through Cr dry etching were investigated and some process conditions were evaluated for the advanced reticle productions. It is shown that resist patterns of ZEP-7000 written with MEBES-4500 showed a little CD deviation between the sparse and dense regions. These errors could be easily emphasized after Cr dry etching. Some dry etching conditions were examined and slight improvements were confirmed after the addition of etching assist gas and adequate intensity of AC magnetic field of MERIE (Magnetically Enhanced Reactive Ion Etching) system. It is also shown that resist profiles after development play important role in the CD distribution after dry etching for the reticle contained both sparse and dense region on the same plate. With our conventional condition, resist profile of ZEP-7000 showed a gentle slope after development. It is proved that this lower pattern contrast makes the Cr CD difference due to pattern loading much worse. Minimum CD error could be obtained through the process that made resist profile almost vertical. These results imply that total adjustments, not only for dry etching conditions but also for resist process that gives us the highest pattern contrast, are needed to solve the complex issues for the advanced CD control.

Knack for reticle cleaning

Cleaning is one of the most important processes in mask making, because it decides final quality. In cleaning process, it is necessary for reticle cleanliness to not only remove particles from reticle but also prevent adsorption and re-deposition onto reticle. There is the knack for reticle cleaning, and we introduce three keys in this paper. The first key is the rinse after chemical treatment. By the rinse sequence modification, the cleaner was refined and the particle removal ability was improved. The second key is quality control to grasp the situation of cleaner. By the daily check, cleaners abnormal condition is found at an early stage, quick action is taken, and then stable cleaning quality is kept every day. And the third key is proper choice of cleaners. We have adopted pre-cleaning process and selected the adequate cleaner for each cleaning level and improved cleaning yield and quality.