Souichi Katagiri

Imaging Characteristics of Multi-Phase-Shifting and Halftone Phase-Shifting Masks

Phase-shifting masks and imaging characteristics are discussed and compared with those of conventional transmission masks. Then, new phase-shifting masks with intermediate values of phase shifting or transmittance are proposed, and their imaging characteristics are investigated. A phase-shifting mask with a 90° phase difference can ease the restrictions on pattern geometries used in phase-shifting technology but does not increase the focus latitude. It is also suggested that a halftone phase-shifting mask is suitable for printing isolated patterns and it gives wider focus latitude than conventional mask technology.

Improved resolution of an i‐line stepper using a phase‐shifting mask

Improved resolution of an available i‐line (365 nm) stepper using a phase‐shifting mask is discussed. The resolution investigated here is not only for periodic lines but also for isolated spaces and hole patterns. To reduce the sizes of isolated space images for printing fine single spaces on a wafer, two additional line apertures with widths smaller than the critical dimension of the stepper lens are placed on each side of the main aperture of the mask. The optical phase of light passing through the main aperture and those through additional apertures are opposite. The additional apertures play a role in reducing the bright feature size to less than the line spread function of the lens. Similarly, printing a fine hole is accomplished by using a main aperture surrounded by four additional apertures. The intensity distribution on the wafer surface is simulated by comparing the images obtained with a phase‐shifting mask and those obtained with a conventional transmission mask. Printing fine patterns are per...

Stabilization of a tungsten ⟨310⟩ cold field emitter

Cold-field-emission current from a tungsten ⟨310⟩ emitter in a scanning electron microscope (SEM) gun, evacuated by an ion pump and a supplementary nonevaporative getter pump, was stabilized. It was verified that the probe current from a local (310) crystal plane exhibits different time variations in comparison to that of total current. As for the probe current under a pressure of 2×10−9 Pa, a stable plateau region—which lasted about 4 h—appeared just after flashing of the emitter. By observing emission patterns, it was verified that these different emission characteristics are originated from the anisotropy of current decay in accordance with crystal planes. With low-temperature “mild flashings” at 700 °C, the plateau region was extended to 12 h, which is long enough for practical SEM application. The superior properties of the plateau region, namely, high current, low noise, and small current variation, will enhance the performance and usability of electron microscopes.

Effect of condenser tilt on projection images produced by a phase-shifting mask

The effects of condenser tilt on the imaging characteristics of optical projection exposure systems for lithography are investigated analytically and experimentally. For the focus dependence of images, the difference between a phase-shifting mask and a conventional transmission mask is examined. Condenser tilt effects are explored by displacing the source images formed on the entrance pupil of a projection lens. Condenser tilt is shown to shift periodic line pattern images when the image plane is out of focus. For fine features near the resolution limit of the projection lens, the direction of image shift that occurs with a phase- shifting mask is shown to be opposite the shift that occurs with a transmission mask. For fine features, condenser tilt is also shown to diminish the image contrast obtained with phase-shifting mask and increase the contrast obtained with transmission mask.

High emission current of 1000 μA at 4 × 10−10 Pa from W⟨310⟩ cold field emitter in electron gun

Total cold field emission (CFE) current has generally been limited to 30 μA because a higher current may initiate a vacuum arc that destroys the emitter. However, high emission current is needed to reduce the operation time in analytical electron microscopy and to further enhance the brightness of CFE. The authors achieved a total emission current of 1000 μA by reducing the pressure around the electron gun to 4 × 10−10 Pa. This high emission current was achieved without initiating a vacuum arc apparently because the number of ions striking the emitter surface was reduced. While a high angular current density of 1670 μA/sr was obtained at 1000 μA, current fluctuation occurred during the initial “plateau region” period of the probe current, and this needs to be investigated. This fluctuation is attributed to surface adsorption of gases emitted by electron stimulated desorption (ESD). Evaluation of the effect of introducing dominant ESD gases, namely, H2 or CO, on the current fluctuation revealed that the ad...

Novel alignment technique for 0.1-μm lithography using the wafer rear surface and canceling tilt effect

A new wafer rear surface alignment (RSA) technique using a rear surface and canceling tilt effect (RECT) for wafer steppers is proposed. RECT alignment can provide the alignment accuracies of less than 0.03 μm (3σ) that are required for 0.1-μm lithography. The accuracy is almost the same as the resolution limit in the alignment sensor; therefore, position detection errors, which are caused by multi-interference in resist films and the asymmetric profile of marks deformed during manufacturing processes, must be decreased until they can be neglected. The concept of wafer RSA has provided a breakthrough to obtain higher overlay accuracy by fabricating alignment marks on the wafer rear surface. RSA, however, has the disadvantage of a position detection error e caused by a thickness between the wafer surface and the rear surface, which strongly depends on the wafer tilt. To overcome this problem, RECT consists of two-beam-illumination optics to cancel the error e. The diffracted beams that are reflected from two separate points on the grating mark include information on both mark positions and light-path difference due to wafer tilt. Thus, the wafer-tilt-induced error can be automatically canceled by processing this information. In an experiment using a six-axis fine positioning stage, it was proven that alignment accuracy can be minimized to as small as 0.015 μm (3σ) for a wafer-tilt angle range of ±50 μrad.

Variable phase-shift mask for deep-submicron optical lithography

A variable phase-shift mask for optical lithography, which gives several kinds of optical phase shifting to light transmitted through the mask apertures, is proposed, and image quality obtained with this mask is investigated. Clear regions with some optical phase shiftings between 0 degree(s) and 180 degree(s) are inserted between the two connected apertures, which are 180 degree(s) out of phase. The role of these inserted regions is to decrease the dip in intensity distribution, which is occasionally needed for producing complicated features. The advantages of this variable phase-shift mask and the effect of wave-front aberrations on imaging characteristics are investigated.

The effect of condenser tilt on optical projection lithography

A basic study on the imaging characteristics of wafer steppers influenced by a condenser tilt is presented. The effects of condenser tilt are explored by displacing the source images formed on the entrance pupil plane of a projection lens. The intensity distribution of the wafer surfaces influenced by this condenser tilt is obtained by the two-dimensional aerial intensity simulation method based on Fourier optics. Periodic line pattern images are shown to shift due to the condenser tilt when the image plane is out of focus. It is also suggested that the degree of these image shifts increases as the focal error increases and the pattern feature sizes increases, and these image shifts for large feature sizes are approximated by geometrical shifts.

Alignment technique using wafer rear surface

A new wafer Rear Surface Alignment (RSA) technique for wafer stepper is proposed. Lithography systems such as wafer steppers are necessary to improve the overlay accuracy corresponding to resolution. For 0.1 ?m lithography, 0.03 ?m (3?) overlay accuracy is required. The RSA has the potential to obtain an overlay accuracy of more than 0.03 ?m (3?), because it is not affected in theory by resist film and asymmetric profile of patterns deformed by various device manufacturing processes. The RSA system, however, is affected by the thickness and tilt of a wafer; the process has been expanded to include the patterning of target marks on the wafer rear surface. These problems arise because the RSA system has not been widely applied to lithography systems. The new RSA called RECT alignment (using REar surface and Canceling Tilt effect) is independent of wafer tilt and dose not require any optics to detect wafer tilt or numerical compensator. The purpose of this paper is to propose the above RSA technique and to validate the RECT alignment optics. Experimental results are as follow: detecting error caused by wafer tilt of 50 ?rad is approximately less than 0.010 ?m. When the conventional sensor is used, on the other hand, the detecting error caused by wafer tilt is more than 0.03 ?m

Surface characterization of Zr/O/W Schottky emitter using AES and TOF-SIMS

The surface of zirconium/oxygen/tungsten (Zr/O/W) Schottky emitters, which are well-known electron sources with high brightness and high stability, was characterized by performing two surface analytical techniques, namely, auger electron spectroscopy (AES) and time-of-flight secondary ion mass spectroscopy (TOF-SIMS) in room temperature. We investigated bonding state of zirconium, oxygen and tungsten on the surface. Based on the measured spectra, a model of Zr/O/W(100) surface was constructed.