Satoru Asai

New double exposure technique without alternating phase-shift mask

The double exposure technique using alternating phase shift mask (alt-PSM) has been proposed and it is well used for the gate layer of the high performance logic devices as strong resolution enhancement technology (RET). This technique has advantage that the fine resist profile is obtained on wafer with extensive process margin. However, this double exposure technique is very expensive because of the alt-PSM cost. This time, the new double exposure technique without alt-PSM is developed for gate layer of 45 nm node logic devices. In this new double exposure method, attenuated phase shift mask (att-PSM) or binary mask (BIM) is used with dipole illumination. It is thought that this new double exposure method is effective for random logic devices which have various pattern pitches by the optimization of dipole illumination condition and pattern placement. Firstly, the optical contrast and depth of focus (DOF) is calculated. From these results, dipole illumination condition is optimized. It is found that DOF of new double exposure method is wider than that of conventional method. In addition, mask pattern is optimized to obtain wide process margin. For dense pattern, mask biasing is effective and optimization of shifter width is effective for isolated pattern. Furthermore, it is found that assist pattern is very effective for isolated pattern. From experimental results, it is proved that new double exposure method have wider process margin than that of conventional one. The strong design for manufacturing (DFM) rule that required the severe line width control is placed at single direction is proposed to realize the new double exposure method. Finally, it is found that the lithographic performance of new double exposure method has same level as conventional method with alt-PSM for gate layer of 45 nm logic devices.

High performance optical lithography using a separated light source

In projection lithography systems with fly’s‐eye elements, a virtual source is created as an array of approximately mutually incoherent point sources. This article describes light amplitude and phase simulations for an image of a mask illuminated by a point source. This article discusses the dependence on the point‐source location for a plane perpendicular to the optical axis. An image projected by a separated light source, rather than a ring‐shaped light source, improves due to interference effects between multiple apertures is shown. Resolution of 0.25 μm equal lines and spaces was improved experimentally at a wavelength of 365 nm and a numerical aperture of 0.54. Also, how an isolated pattern image can be improved theoretically using an optimally separated light source is shown.

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.

New phase-shifting mask with highly transparent SiO2 phase shifters

A phase-shifting mask enables subhalf-micron optical lithography. We propose a new phase-shifting mask with Si02 phase shifters. Si02 phase shifters on a quartz mask substrate have the advantages of low absorption under deep UV and a lack of multiple interference. Si02 phase shifters were fabricated by lift-off of the evaporated Si02 film. The new phaseshifting mask is highly transparent to deep UV and provides a 2 uniformity of phase shift over the full exposure field of a 5X stepper. Improved resolution of 0. 25 im lines and spaces was achieved by using a KrF excimer laser stepper and the new phase shifting mask. We also characterize the image profile projected with a phase-shifting mask because the reproducibility of mask features on a wafer declines when a phase-shifting mask is used. We indicate the importance of the interference between the main and side lobes of diffraction patterns for individual apertures and clarify the mechanism ofreproducibility degradation in optical lithography using a phase-shifting mask. 1.

Correction for local flare effects approximated with double Gaussian profile in ArF lithography

A method has been developed for correcting line width variations due to midrange flare with a scattering range of over a few tens of micrometers (which we call local flare). It is shown that the conventional single Gaussian point spread function (PSF) is not sufficient and that a double Gaussian point spread function is needed to explain the line width variation caused by local flare. The remaining errors after correction are discussed under the assumptions that the mask correction is linear with respect to local flare intensity and is independent of pattern layout considering the order of the local flare correction (LFC) and optical proximity correction (OPC). This simple sizing method can reduce the critical dimension (CD) variation regardless of whether LFC is done before or after OPC. The LFC performance was evaluated using actual 90-nm-node LSI data. A much faster correction time than that of OPC was achieved by introducing the area density map method. The CD variation due to local flare was reduced from 22 to 5 nm.

Improving projection lithography image illumination by using sources far from the optical axis

In a projection lithography system having fly‐eye elements, a virtual source is created as an array of approximately mutually incoherent point sources. This paper describes simulated results of the light amplitude and phase of the mask‐projected image for a point source and discusses the dependence on the point source location on a plane situated perpendicular to the optical axis. We showed that the projected image illuminated by point sources far from the optical axis was improved by the effect of interference between multiple apertures. Resolution of the 0.4 μm lines and spaces was improved theoretically and experimentally at a wavelength of 435.8 nm and a numerical aperture of 0.45.

Experimental and theoretical noise analysis of microwave HEMTs

Low-frequency noise power and high-frequency noise figures in HEMTs (high electron mobility transistors) were measured and compared with calculations based on a one-dimensional noise model to characterize their low-noise properties. It was found that the drain noise current parameter Q in HEMTS is lower than that in GaAs MESFETs. The strong correlation between drain- and induced-gate-noise currents in HEMTs is due to the asymmetric distribution of noise generation along a channel, and the drain noise current is nearly canceled by those induced-gate-noise current. The intrinsic thermal noise from source and gate resistances is about 25% of the total output noise in the 0.25- mu m gate-length HEMT considered. >

Challenging to meet 1nm iso-dense bias (IDB) by controlling laser spectrum

According to the ITRS Roadmap, for 45nm Node (as 65nm Half Pitch), the requirement of Gate CD Control is defined as 2.6nm. One of the most challenging CD errors is Iso-Dense Bias (IDB). Assuming 40% of CD errors are dominated by IDB, IDB should be less than 1nm. In general, the majority of IDB is due to: primarily, exposure tool- related factors such as aberrations, flare, and sigma fluctuation, and secondly, the change in photoresist characteristics. However, due to the rapidly increasing usage of ArF exposure tools, Band Width (BW) characteristics of the laser source is an additional factor whose contribution is becoming more critical. Ideally, BW is monochromatic, thereby not affected by chromatic aberration change. However, in reality, the BW exhibits a shape of spectral distribution with a finite width. This study describes experimental and simulation results for E95%, and how performance of both CDs and Laser is dependent on E95% in order to meet 1nm of IDB towards 45nm Node. -IDB vs. E95% -CD at through pitch vs. E95% -Process Latitude vs. E95% -DOF -EL -Pattern shortening vs. E95%

Practical method of evaluating two-dimensional resist features for lithographic DRC

The optical proximity effect becomes significant near the practical resolution limit of photolithography, depending on the wavelength and numerical aperture of the stepper. Recently, VLSI design rules have almost reached their limits. Larger ICs cannot be designed and be manufactured without using a lithographic DRC (design rule check) tool or an OPC (optical proximity correction) tool. Therefore, it has become necessary to develop a technology which can accurately predict resist features from the designed circuit layout. We studied deviation in both the line width and the length due to proximity effect and investigated the phenomena. Also we developed a technology which can accurately predict the behavior of the proximity effect from an aerial image. This technology is based on a simple threshold model. We optimized the calculations for an aerial image and the threshold of intensity in order to predict deviations in the line width and length. We also considered the profile of an aerial image to predict the critical point. The calculations for an aerial image and threshold which we optimized in this manner can be used to predict 2D patterns.

Super Low-Noise HEMTs with a T-Shaped Gate Structure

The low noise property of HEMTs was studied, based on the noise power generated in the devices. 0.5 µm- and 0.25 µm-gate HEMTs were fabricated, to study the noise power generated in the intrinsic region of the devices. In order to leave the extrinsic noise power out of consideration, a T-shaped gate structure was developed to make equal the gate-resistances of these devices. It was shown that the noise power was independent of frequency and strongly dependent on the gate length. The resultant quarter micron gate HEMTs achieved the noise figures of 1.0 dB at room temperature and 0.5 dB at 100K at 20 GHz.