Junji Miyazaki

Effect of Phase Error on Lithographic Characteristics Using Attenuated Phase-Shifting Mask

An attenuated phase-shifting mask is one of the most useful technologies for sub-half-micron lithography. However, it is necessary to control new parameters such as phase or transmittance when a phase-shifting mask is applied to practical use. We investigated the effect of phase error on lithographic characteristics for a hole pattern using an attenuated phase shifting mask. It is found that a phase error causes a decrease of depth of focus (DOF) and shift of best focus position. It is indicated that this effect depends on several optical parameters such as hole size and wavelength. In the case of the 0.4 µ m hole pattern with i-line stepper, the phase tolerance must be less than 2.7° to control loss of DOF within 0.1 µ m. It is also found that the edge slope effect of the shifter is rather small and the side wall angle of 70° is acceptable.

Attenuated phase-shifting mask in ArF lithography

It was demonstrated that the DOF obtained using the Att-PSM was 0.5 micrometer for 150 nm hole pattern, whereas that obtained using a binary mask was 0.2 micrometer. Durability of the film subjected to ArF laser irradiation is also investigated, and no significant change is observed in both phase and transmittance. It was indicated that the attenuated phase-shifting mask (Att-PSM) is effective and realistic in ArF lithography. Factors in CD control such as mask error factor (MEF), phase and transmittance are discussed. It is important to reduce MEF for hole pattern, since MEF in experiment was over 3 while it is desired to be less than 1.8. Att-PSM with higher transmittance and higher NA are effective to reduce MEF. On the other hand, MEF for line pattern is small enough for use in production. The control of phase and transmittance are also an important issue. It is estimated that phase error of 3 deg and transmittance error of 0.3% are required.

MEASUREMENT METHOD FOR ODD COMPONENT OF ABERRATION FUNCTION UTILIZING ALTERNATING PHASE SHIFT MASK

A novel measurement method for an odd component of an aberration function utilizing an alternating phase shift mask (PSM) is proposed based on optical image calculations. When high coherent illumination is applied, lateral fringe shift in the image, which is formed with lines and spaces(L/S) patterns of alternating PSM, is caused by the phase difference between two beams in interference. The shift distance is simply expressed by a quotient of the phase difference (measured in distance) and sine of the beam incident angle. Thus, by measuring the fringe shift distance by changing the pattern pitch, the odd component of the aberration function can be derived. To measure the fringe shift distance, a test device structure utilizing a modified MOS process is considered. By fabricating the test structure under the appropriate optical conditions and measuring the capacitances of the MOS structure, the fringe shift distance could be measured with high accuracy.

Requirements for reticle and reticle material for 157 nm lithography : Requirements for hard pellicle

Since conventional pellicle material is not transparent to 157nm light, we are developing a thick pellicle made of fused silica. The effect of aberration due to the pellicle on the optics of an exposure tool has been estimated by simulation, and requirements for the pellicle dimensions have been proposed. It was found that a thick pellicle generates spherical aberration, and this has to be corrected in the system optics. It was indicated that the tilt of the pellicle gives an image shift and coma aberration. Sagging of the membrane causes tilting, and the use of a thicker membrane improves the degree of sagging. However, the requirements for tilt angle are then tighter. It was also noted that wedge due to the thickness change generates an image shift and a coma aberration. The effect of wedge is reduced by having a lower pellicle stand-off, but this also increases the printability of particles. We need to consider the total effect of using a thick pellicle as a component of an exposure tool or an inspection tool in order to define the specification of a hard pellicle for 157nm lithography.

ZrSiO: a new and robust material for attenuated phase-shift mask in ArF lithography

We propose zirconium silicon oxide (ZrSiO) film as a powerful candidate for attenuated phase-shift mask (Att-PSM) materials. A bi-layer structure of this material with an absorptive film (AF) and a transparent film (TF) can effectively control the transmittance and phase. We confirmed the durability of the ZrSiO film in ArF laser irradiation. The lifetime with the change in transmittance and phase after irradiation at 30 kJ(DOT)cm-2 is equivalent to a total dose of 3 years in future ArF exposure systems. We investigated the resolution performance of the Att-PSM with ZrSiO film for a 130 nm pattern. The depth of focus with the Att-PSM was larger than that of the binary mask. Therefore, an Att-PSM with ZrSiO is promising for developing a 130-nm-technology node with ArF lithography. Controllability of the critical dimension on the wafer is discussed from the viewpoint of the mask error factor (MEF).

EUV lithography insertion for high volume manufacturing: Status and outlook

We will present the current performance of the state-of-art EUV scanners, and an overview of EUV lithography process infrastructure status. The outlook for high volume manufacturing insertion will be also discussed.

Alternative EUV mask technology to compensate for mask 3D effects

Traditional EUV masks, with absorber on top of the multi-layer (ML) mirror, generally suffer from mask 3D effects: H/V shadowing, best focus shifts through pitch and pattern shifts through focus. These effects reduce the overlapping process window, complicate optical proximity correction and generate overlay errors. With further pitch scaling, these mask 3D effects are expected to become stronger, increasing the need for a compensation strategy. In this study, we have proven by simulations and experiments that alternative mask technologies can lower mask 3D effects and therefore have the potential to improve the imaging of critical EUV layers. We have performed an experimental imaging study of a prototype Etched ML mask, which has recently become available. This prototype alternative mask has only half the ML mirror thickness (20 Mo/Si pairs) and contains no absorber material at all. Instead, the ML mirror is etched away to the substrate at the location of the dark features. For this Etched ML mask, we have compared the imaging performance for mask 3D related effects to that of a standard EUV mask, using wafer exposures at 0.33 NA. Experimental data are compared to the simulated predictions and the benefits and drawbacks of such an alternative mask are shown. Besides the imaging performance, we will also discuss the manufacturability challenges related to the etched ML mask technology.

Impact of mask absorber and quartz over-etch on mask 3D induced best focus shifts

In our study we have investigated the mask 3D impact on best focus (BF) shifts, which may occur on more complex 2D patterns, by looking at simplified line/space test patterns at various pitches. Several test masks were created; with different absorber thicknesses or different quartz etch depths, all containing ASML L/S test patterns. These test patterns consist of 40 and 45nm horizontal lines at multiple pitches (80-315 nm) and 90nm vertical lines. Wafers were exposed on an NXT:1950i, and the critical dimensions (CDs) were measured through focus to get the best focus (Bossung top) for the different features. In this paper we demonstrate that optimizing the mask absorber thickness for 6% att.PSM will reduce feature-to-feature best focus offsets (~25nm smaller BF range measured on L/S test features) and thus improve the Overlapping Depth of Focus. The change in absorber thickness has limited impact on exposure latitude, but will impact the print CDs. Next to the impact of the absorber thickness on best focus shifts we also derived a 1.33 nm/nm sensitivity of the best focus range to etch depth variations for the ASML L/S test features, and show that the over-etch needs to be carefully controlled, as it also impacts the best focus range.

Impact of reticle absorber on the imaging properties in ArFi lithography

In this paper we compare the imaging properties of lithographic test structures formed on test masks with different reticle absorbers for use in1.35 NA immersion lithography. We will look into different aspects like process windows and CD fingerprints. Beyond that we look into the topographic effects caused by the different absorbers, the mask 3D effects. We will study the interaction between the different masks and immersion scanner. Special attention is given towards the correctability of the intrafield CD fingerprint by mask and scanner applying dose corrections.

PSM and thin OMOG reticles aerial imaging metrology comparison study

For sub 20nm features, IC (integrated circuits) designs include an increasing number of features approaching the resolution limits of the scanner compared to the previous generation of IC designs. This trend includes stringent design rules and complex, ever smaller optical proximity correction (OPC) structures. In this regime, a new type of mask, known as opaque MoSi on glass (OMOG), has been introduced to overcome the shortcomings of the well-established phase shift masks (PSM). This paper reviews the fundamental aerial imaging differences between identically designed PSM and thin OMOG masks. The masks were designed for scanner qualification tests and therefore contain large selections of 1D and 2D features, including various biases and OPCs. Aerial critical dimension uniformity (CDU) performance for various features on both masks are reported. Furthermore, special efforts have been made to emphasize the advantages of aerial imaging metrology versus wafer metrology in terms of shortening scanner qualification cycle time.