Yaichiro Watakabe

Phase-shift mask pattern accuracy requirements and inspection technology

Computer simulations and i-line phase shift lithography experiments with programmed 5X phase shift reticle defects were used to investigate the effect of opaque and phase-shift layer defects on sub-half-micron lines. Both the simulations and the experiments show that defects in the phase shift layer print larger than corresponding opaque defects, with 0.3-0.4 micrometers defects affecting sub-half-micron critical dimensions by more than the allowable 10%. Inspection of programmed phase shift defects with a prototype mask inspection system confirmed that the system finds the 0.3-0.4 micrometers phase shift defects critical to sub-half-micron lithography.

Characteristics of submicron patterns fabricated by gallium focused-ion-beam sputtering

Cross sections of the patterns fabricated in (100) GaAs by 100‐keV gallium focused ion beam have been studied using a scanning electron microscope (SEM). The probe size of the ion beam is 0.1–0.15 μm at the current of 100 pA. The etched depth becomes saturated at the high dose region (about 5.0×10−6 C/cm) because of the redeposition effect. The pattern profile becomes asymmetric if it is made up of several adjacent lines perpendicular to the beam scanning direction due to the redeposition effect and the increase of sputtering yield for each scan, which is caused by the change of ion beam incident angle. These effects can be eliminated by the use of multiwriting method.

Practical results of photomask repair using focused ion beam technology

Practical results of photomask defect repair using focused ion beam (FIB) technology are presented. From Auger electron analysis, the deposited carbon film for clear defect repair contains 22% gallium, which is an element of the ion beam. Substrate damage produced by ion beam irradiation can be removed incorporating CHF3+O2 reactive ion etching. Printing results using a 5:1 stepper show that the FIB technology can be applied to photomask production.

Attenuated phase-shifting mask with a single-layer absorptive shifter of CrO, CrON, MoSiO, and MoSiON film

Attenuated phase-shifting mask with a single-layer absorptive shifter of CrO, CrON, MoSiO or MoSiON films has been developed. The optical parameter of these films can be controlled by the condition of sputtering deposition. These films satisfy the shifter requirements, both the 180-degrees phase shift and the transmittance between 5 and 20% for i-line. MoSiO and MoSiON films also satisfy the requirement for KrF excimer laser light. Conventional mask processes, such as etching, cleaning, defect inspection and defect repair, can be used for the mask fabrication. Defect-free masks for hole layers of 64 M-bit DRAM are obtained. Using this mask, the focus depth of 0.35-micrometers hole is improved from 0.6 micrometers to 1.5 micrometers for i-line lithography. The printing of 0.2-micrometers hole patterns is achieved by the combination of this mask and KrF excimer laser lithography.

Fabrication of surface acoustic wave devices by using x‐ray lithography

Surface acoustic wave (SAW) devices with 0.6 ∼ 1.7 μm pattern size (fo=287.5∼812.5 MHz) designed as band‐pass filters (BPF) were fabricated by using x‐ray lithography. An x‐ray stepper SX‐5 with a conventional source (Pd terget), high sensitive x‐ray resist chlorinated polymethylstyrene (CPMS) (negative) and EBR‐9 HS (positive), and a low‐distortion x‐ray mask with W–Ti alloy absorber were used. The SAW BPF devices were fabricated by forming Al transducer patterns on an LiNbO3 substrate. The Al film was chemically etched with an acid etchant. The frequency characteristic of the fabricated SAW BPF devices was evaluated. As a result, it is confirmed that the SAW BPF devices satisfied the superior specification, with a center frequency of f0± 2 MHz, an insertion loss of < 15 dB and a band width of 30 MHz. It is thus demonstrated that the x‐ray lithography has the potential for use in the fabrication of submicron SAW devices.

High performance very large scale integrated photomask with a silicide film

A molybdenum silicide (Mo‐silicide) film deposited on a quartz glass substrate offers major advantages as a high performance photomask material for very large scale integrated (VLSI) fabrication. There is no pattern missing due to exfoliation after ultrasonic cleaning with frequency of 28 kHz and 300 W of power, and after being scrubbed over 10 cycles with a high pressure water jet. Reflectivity and optical density of the Mo‐silicide film are not affected by acidic chemicals. Moreover, dry etching can be done at a rate of 50 nm/min; more than five times as fast as etching of a chromium (Cr) mask.

The surface silylating process using chemical amplification resist for electron beam lithography

This paper describes the surface silylating process for electron beam (EB) lithography. Surface reaction and dry development processes have many advantages for electron beam lithography. The original three components resist named SIMPLE (SIlylating resist using chemical aMPlification for Electron beam) is developed for EB silylating process. High sensitivity is attained by using chemical amplification reaction and the optimization of resist contents. The reaction mechanism is examined using Fourier transform infrared spectrometry and Rutherford backscattering spectroscopy. It is found that the silylation rate in the unexposed area is controlled by the reaction rate at the silylation front, and the silylation is suppressed by the limited diffusion rate of the silylating agent in the exposed area. Submicron resist patterns with a thickness of 1.4 μm are successfully fabricated even in a single layer process.

Sputtered W-Ti Film for X-Ray Mask Absorber

Properties of DC-sputtered W-Ti absorber films such as internal stress, density and microstructure were systematically investigated for X-ray mask application. Smooth, stress-free amorphous W-Ti films with a comparatively high density of 16.5-17.0 g/cm3 were obtained by optimizing several conditions: gas pressure, N2 content in the working gas, DC power density and annealing temperature. The reproducibility of the film stress was about ±5×107 Pa. Reproducibility was found to be mainly determined by the degree to which the sputtering tool used could be controlled. Furthermore, it was confirmed that the optimized absorber films offered good stability under stress within 1×107 Pa when stored in an air atmosphere for more than 250 days.

Electron beam pattern inspection system using digital image processing

An electron beam pattern inspection system (EBIS) using digital image processing is presented. A pattern‐to‐data comparison method is used in this system to detect systematic, random and repeating defects of VLSI circuit patterns. The inspection data are generated from the design data, and transferred from a computer aided design (CAD) system via a computer network. In order to obtain the information of mask or wafer patterns, a low‐energy scanning electron microscope, which has the advantage of avoiding damage to the active devices and not charging the surface, is used. Frame memories are installed to store the inspection data and the actual pattern image. An extra image processor preprocesses the pattern image to obtain high signal‐to‐noise ratio and manipulates these images to extract defects. Two algorithms extracting the defects are studied. Spatial differentiation followed by a thresholding operation is a suitable procedure for the signal with drift.

Fine Pattern Etching of W-Ti Absorber for X-Ray Mask with Electron Cyclotron Resonance Discharge Plasmas

The process of etching an amorphous W-Ti absorber using a Cr mask and electron cycrotron resonance discharge plasmas was investigated to fabricate an X-ray mask with a resolution below 0.1 μm. Highly selective and anisotropic etching has been achieved using a mixture of SF 6 , CHF 3 and He gases, by cooling the stage to about -50 o °C and with the control of the plasma conditions to increase ion assist reactions. It was also found that high selectivity (50-150) of W-Ti to Cr or ITO and a small microloading effect were advantageous to uniformly etch patterns of various sises. Moreover, the microfabrication of a smooth W-Ti absorber has been demonstrated for lines and spaces patterns below 0.1 μm and 1-Gbit-class dynamic random access memory patterns