Koji Kise

Total evaluation of W-Ti absorber for x-ray mask

W-Ti film was comprehensively investigated for its application as an x-ray mask absorber. A stress-free and amorphous W-Ti film was successfully deposited by dc sputtering with a W-Ti (1 wt%) target using a gas mixture of Ar and N2. To obtain much lower stress, we firstly improve the accuracy of stress measurement up to +/- 1 MPa, and ultra-low stress film was obtained by step-annealing. The stress of the film has been found stable enough to apply to x- ray masks in an air atmosphere and for x-ray exposure. The density, composition, and microstructure of the film were also evaluated by SEM, XD, XPS, and TDS. The film surface was very smooth and the roughness measured by AFM was about 2 nm. The etching properties of the absorber using a Cr mask, ITO stopper and electron cyclotron resonance (ECR) discharge plasmas were also investigated in order to achieve a resolution of below 0.1 micrometers . Highly selective and anisotropic etching has been realized using a mixture of SF6 and CHF3, by cooling the stage to about -50 degree(s)C under controlled plasma conditions. Moreover, the microfabrication of a smooth W-Ti absorber has been demonstrated for lines and spaces patterns of 0.06 micrometers and for 1-Gbit-class dynamic random access memory patterns.

Stress relaxation of EB resist for x-ray mask fabrication

We have investigated the ZEP resist characteristics of stress relaxation and reduction by applying some additives to the ZEP resist. The stress of the resist film was significantly reduced in ZEP resist with (beta) -Carotene. Without an additive, the stress of the ZEP resist film reduced from 30 MPa to 25 MPa as a result of the delay time after resist coating. On the other hand, with the addition of 5 wt% (beta) - Carotene, the stress of the resist film just after coated was almost same to that of the resist film without an additive. The stress, however, decreased to 15 MPa for one month. The stress change caused by deep UV exposure in the (beta) - Carotene additive system became one-third of that in the non- additive system. This system had a similar pattern replication quality to the original ZEP resist. It is considered that this system is useful for improving the image placement accuracy in EB writing on the membrane such as an x-ray mask.

Highly stiff x-ray mask blank with heat resistance and inertness to chemicals

A novel bonding technique with heat resistance and/or inertness to chemicals is required to realize x-ray mask processes in which Si substrate is mounted on support plate at an initial stage. From this standpoint, we have developed a distortion-free bonding method using Au-Si solder to join Si wafer to SiC ceramic support plate which is effective to reduce the placement error due to mask chucking. The change in wafer bow due to bonding was less than 1 μm in spite of comparatively higher bonding temperature of 620°C , and that due to heating at 400°C for lhr is also negligible. It has been also confirmed that both SiC ceramics and Au-Si solder inert to chemicals such as a strong acid used in the back-etch process. Highly stiff x-ray mask blank with heat resistance and inertness to chemicals has been developed by using this method, where back-etched Si wafer with SiC membrane was successfully joined to SiC ceramic support plate. By using this mask blank, x-ray mask process becomes similar to photomask process, and the remained problems for improving the placement accuracy are only precise EB writing and the reproducible deposition of stress-free absorber.

Application of SR lithography to 0.14-um device fabrication

The applicability of synchrotron radiation (SR) lithography to fabricate a giga bit scale dynamic random access memory (DRAM) cell array structure with minimum feature size of 0.14micrometers is demonstrated. Four lithography levels, isolation, transfer gate, bit line and storage node, were exposed by SR lithography. Exposure was carried out at Mitsubishi SR lithography facility using Canon x-ray stepper XFPA with a new negative tone resist and home-made x-ray mask set for each exposure level. These masks were composed of 2micrometers SiC membrane and 0.5micrometers W-Ti absorber. To minimize the mask-induced distortion, we applied the various techniques to the x-ray mask fabrication, changing the mask fabrication process flow, step annealing and electron beam multiple writing, and as the result, the pattern placement accuracy between two exposure level masks was about 50nm. Exposure latitude was about 22 percent for 0.15micrometers line and space pattern at the proximity gap of 30micrometers , and the critical dimension deviation for 0.14micrometers transfer gate pattern was 0.014micrometers at the almost same position in the mask in spite of the replication on the real DRAM topographic structure. The overlay accuracy was about 80nm for 20 X 20mm2 area. These results show SR lithography is the promising technique for giga bit level device fabrication.

A simulation method for streaming electrification by direct current field analysis

A simulation method for evaluating potential and electric field distribution in an oil-filled transformer was developed. Potential distribution around the entrance of the oil duct should be carefully considered in view of the risk of static discharge. A large potential may be built up by negative charge current through highly resistive insulators due to charge separation caused by streaming electrification at the pressboard (PB) surface. At steady state in terms of both fluid dynamics and electrical conduction, a flow of the negative charge can be regarded as a constant current source distributed over the PB surface that flow into grounded conductors through resistive materials such as the solid/liquid insulation of PB and oil. As will be presented in another paper in this conference by the authors, the local streaming current can be obtained from the local shearing stress calculated by a fluid dynamics simulator. Using the wall current distribution calculated from the shearing stress model as a distributed current source, the electric field and potential distribution were calculated by a direct current field simulator.