Naoyuki Ishiwata

Phase-shifting photolithography applicable to real IC patterns

A phase-shifting technique which simplifies mask fabrication and is applicable to actual IC patterns has been introduced into the i-line positive resist process. It combines edge-contrast enhancement and a chromeless mask. Although the effect of this technique on line and space patterns has turned out to be more restricted than that of the alternating mask technique, it can improve exposure and focus latitude in isolated hole patterning. The authors report on their estimation of the optimum shifter width which maximizes contrast enhancement on lines and spaces as well as on isolated hole patterns. Experimental data is presented to verify the improvements in photolithographic performance of isolated hole patterning due to this technique.

Fabrication of phase-shifting mask

This paper introduces an example for fabricating a phase-shifting mask with a self-aligned process. In this example, a step on the quartz substrate formed by dry etching as a shifter was used. Dry etching was also used for over-etching of chromium (Cr) to form the shifter region. The uniformity and controllability in these etching processes was evaluated. The result has proven the shifter depth accuracy of 15 nm (3(sigma) ) and the shifter width accuracy of 0.07 micrometers (3(sigma) ).

Novel alternating phase-shift mask with improved phase accuracy

We developed an alternating phase shift mask that offers a sufficient phase accuracy for manufacturing sub-0.18 micrometer devices with 248 nm deep-UV lithography. This mask has a Cr/spin-on-glass/quartz structure. Our mask fabrication process utilizes some new techniques which include the use of a SOG shifter with extra thickness, a two step SOG etching, and an additional wet etchign process for phase adjustment. Our process showed a good performance, and a phase controllability of 180 plus or minus 0.7 degrees was achieved. Phase uniformity was less than 3 degrees over a 100 mm square area. It was nearly equal to the uniformity of the SOG thickness. These results prove that the additional etching process is very effective at improving phase accuracy.

Mask pattern designing for phase-shift lithography

The authors have developed a mask pattern designing algorithm for a phase-shift lithography process. This algorithm produces pattern dimension linearity regardless of the pattern size and does not requires any special CAD (computer-aided design) technique. The authors also developed a mask fabrication technique with self-aligned shifters. One can avoid the alignment problem of chromium and shifter pattern by the self-aligning mask process.<<ETX>>

Fabrication process of alternating phase-shift mask for practical use

This paper presents a fabrication process of alternating phase shift mask for actual device production. The most important issue in any practical application of alternating phase shift technology is establishment of a zero-defect mask fabrication process. We developed an altPSM with good phase accuracy suitable for practical use. A total phase accuracy of +/- 1.5 degrees and phase uniformity within 2.0 degrees were achieved by the combination of a Cr/spin-on- glass (SOG)/quartz structure, improvement of shifter thickness uniformity and application of a phase adjustment technique. The disadvantages of using SOG shifters, such as the breeding of defects, were solved by improving both SOG quality and the coating process. Consequently, our proposed process can fabricate altPSM blanks that have the same quality as conventional ones. Moreover, process optimization has reduced the average number of residual shifter defects per mask was to 0.30. Conventional mask inspection systems do not provide sufficient quality assurance in an altPSM, so we had to consider a new inspection technique. The implementation of AIMS simulation for phase measurement, defect classification and printability checks of repaired regions improved the capabilities of mask quality assurance. In addition, we confirmed the effectiveness of printed wafer inspections. Our altPSM achieved satisfactory results in a trial fabrication of actual devices.

Impact of alternating phase-shift mask quality on 100-nm gate lithography

A dual exposure method with an alternating phase shift mask has been proposed for using KrF laser lithography to fabricate 100 nm gate patterns for logic devices. Fine and uniform patterns can be formed and so this process is considered very advantageous in terms of the formation of gate for logic devices. Several factors determine the lithographic performance of the alternating phase shift mask: phase accuracy, amount of undercutting, quartz and chromium defects, and so on. It is thought that these factors need to be strictly controlled. We thus investigated the impact of errors in the fabrication of alternating phase shift masks to determine the quality required for the dual exposure method, focusing on three factors: phase accuracy, amount of undercutting, and defects. A phase error causes CD variation and lateral shift in the defocused condition. Unsuitable undercutting causes lateral shift at the best focus. Shifter and chromium defects cause CD variation and distortion of the gate patterns. Our experimental results showed that these factors do not need to be strictly controlled. We thus propose a fabrication process for alternating phase shift masks to be used in the dual exposure method. Keywords: Alternating phase shift mask, dual exposure, phase accuracy, undercutting, defect

Implementation of attenuated PSMs in DRAM production

We studied the use of attenuated phase shift mask (PSM) in DRAM production. There exists several problems with the use of an attenuated PSM compared to a conventional Cr mask. These include a need to form an opaque region, facilitate reticle alignment with a stepper, and optimize mask bias to prevent side peak printing. First, we investigated the characteristics of checkerboard patterns in achieving an opaque region. We confirmed the feasibility of making a mask to maintain opaqueness. Next we developed a mask fabrication process so to enable reticle alignment in some kinds of steppers by using an additional Cr layer under the attenuated layer. Finally, we tried to implement attenuated PSM in a previous generation stepper. We found that we must pay attention to lens aberration when optimizing mask bias.

Mask data processing technique using GPU for reducing computer cost

The computer cost for mask data processing grows increasingly more expensive every year. However the Graphics Processing Unit (GPU) has evolved dramatically. The GPU which originally was used exclusively for digital image processing has been used in many fields of numerical analysis. We developed mask data processing techniques using GPUs together with distributed processing that allows reduced computer costs as opposed to a distributed processing system using just CPUs. Generally, for best application performance, it is important to reduce conditional branch instructions, to minimize data transfer between the CPU host and the GPU device, and to optimize memory access patterns in the GPU. Hence, in our optical proximity correction (OPC), the light intensity calculation step, that is the most time consuming part of this OPC, is optimized for GPU implementation and the other inefficient steps for GPU are processed by CPUs . Moreover, by fracturing input data and balancing a computational road for each CPU, we have put the powerful distributed computing into practice. Furthermore we have investigated not only the improvement of software performance but also how to best balance computer cost and speed, and we have derived a combination of the CPU hosts and the GPU devices to maximize the processing performance that takes computer cost into account . We have also developed a recovery function that continues OPC processing even if a GPU breaks down during mask data processing for a production. By using the GPUs and distributed processing, we have developed a mask data processing system which reduces computer cost and has high reliability.

Preliminary study of 65-nm-node alternating phase-shift mask fabrication

This paper presents about 65 nm-node alternating phase shift mask (APSM) fabrication. One of issue in fabrication of 65 nm-node APSM is second layer patterning process. As chromium (Cr) pattern CD becomes narrow, tighter edge placement accuracy of second layer resist pattern is required. Therefore higher total overlay accuracy is required in second layer patterning process. To solve this issue, we examined application of 50kV electron beam (EB) vector writing system and chemically amplified resist (CAR) process. Error factors which affect total overlay accuracy were quantified experimentally, and minimum required resist coverage through the shifter etching process was determined. From these results, it was confirmed a second layer patterning process using 50kV EB vector writing system and CAR process had enough performance for 65 nm-node APSM fabrication.

Optimization of alternating phase shift mask structure for ArF laser lithography

An alternating phase shift mask (alt. PSM) must be fabricated in such a way that imbalances in optical intensities are minimized. The mask structure must be optimized to obtain a balanced distribution of optical intensities and this means that the shifter thickness/quartz depth that corresponds to a phase angle of 180 degrees and the correct amount of undercutting should be estimated. There are two key points in the optimization of an alt. PSM. One is to find the optimum structure in terms of reducing the amount of undercutting. Narrower chrome (Cr) line widths are required for ArF laser lithography than for KrF laser lithography, so the undercutting must be restricted to prevent peeling of the Cr patterns, degradation of cleaning durability, and so on. Another key point is to investigate the effect of Cr line widths and pattern pitches on imbalances in the optical intensities. A variety of pattern pitches and Cr line widths are available from actual devices. All patterns, however, have same shifter thickness and amount of undercutting on each mask produced by a given mask fabrication process. It is thus necessary to study the effect on optical intensities of changes in Cr line widths and pattern pitches so that it is possible to optimize mask structures for a variety of patterns. From our simulation and experimental results, we found that an alt. PSM with vertical sidewalls has advantages in terms of reducing the amount of undercutting and is effective in the fabrication of sub 100-nm devices. We also discovered that imbalances in optical intensities vary periodically with Cr line widths. It was found that a structure for an alt. PSM should be optimized for each Cr line widths on these bases.