Norio Saitou

Electron‐beam cell projection lithography: A new high‐throughput electron‐beam direct‐writing technology using a specially tailored Si aperture

Electron‐beam cell projection lithography realizes a high throughput capability suitable for ultra‐large‐scale integration (ULSI) manufacturing. This method makes it possible to drastically reduce the number of electron‐beam exposure shots by utilizing a specially shaped beam. This shaped beam is created by an Si aperture which forms various shapes, coinciding with the array of each unit cell of an ULSI pattern. The aperture also forms a rectangular shape for conventional variable‐shaped method in order to create random patterns. An aperture made of a single crystal of Si is fabricated using ULSI process techniques. As the reduction ratio of electron optics is larger than that of an optical stepper, pattern size errors caused by aperture inaccuracies can be reduced significantly.

Role of Ion Bombardment in Field Emission Current Instability

The role of ion bombardment in field emission current instability is clarified experimentally by varying the emission current level and pressure over a wide range. It is found that bombarding ions sputter-off adsorbed gas molecules on the emitter surface and this causes field emission current fluctuation. This current fluctuation caused by ion bombardment is larger than that caused by gas molecule migration when the product of the pressure and the emission current is higher than 7×10-12 PaA. The relative fluctuations turn out to be the logarithm of the product of the pressure and the emission current. The ions are mainly generated close to the emitter due to electron collisions with residual gas.

Fast proximity effect correction method using a pattern area density map

This article proposes a new method for proximity effect correction that utilizes newly developed hardware. The correction algorithm modifies the exposure dose for each exposure point by referring to a pattern area density map. The only additional process in this method is virtual exposure to make the map. The virtual exposure is carried out once at the first use of the large‐scale integration circuit pattern and can be processed in only 30 s. The pattern area density map makes it possible to correct the proximity effect from the lower‐level patterns by the new map calculated from the two maps of lower level and exposing level. The usefulness of this method is verified by experiments using model patterns.

Electron‐beam cell‐projection lithography system

An electron‐beam exposure system HL‐800D has been developed for the mass production of both quarter micron large‐scale integrated memories and application specific integrated circuits (ASICs). To achieve a productive level of throughput, the system utilizes a cell‐projection method combined with variable shaped method and a continuously moving stage at variable speed depending on the pattern density. The system is operated at a 50 kV acceleration voltage and a 1 μC/cm2 dosage. Three stage deflectors have been developed to assure high‐speed deflection and highly accurate positioning. A fast pattern controller generates patern data at 200 ns shot‐cycle‐time with the positioning error correction and proximity effect correction. A high‐speed ceramic XY stage and an automatic wafer loder have been developed. The system is operated by a workstation which also provides data conversion. The estimated throughput of the system is 11 wafers/h for 0.3 μm ASICs and 20 wafers/h for quarter micron memories.

Pattern dependent alignment technique for mix-and-match electron-beam lithography with optical lithography

An alignment technique for electron-beam (EB) lithography that corrects the writing pattern to match the pattern-dependent lens distortion of an optical stepper is proposed. In mix-and-match EB and optical lithography, precise measurement of positional distortion caused by the stepper lens is necessary. However, recent research in optical lithography has shown that the lens distortion differs depending on the pattern features. Thus, we have enhanced the measurement method to reflect the pattern-feature dependence of positional distortion in optical lithography. To measure the various types of distortions more effectively, we modified the marks used for distortion and overlay measurement. The measured pattern dependence of the lens distortion was in good agreement with our simulation results and the overlay accuracy in mix-and-match lithography was improved with this method.

Variably shaped electron beam lithography system, EB55: II Electron optics

A variably shaped electron beam exposure system (EB55) with a high exposure rate is developed for direct beam lithography of 0.5 μm patterns. The electron beam column consists of seven magnetic lenses. The projection lens is specially designed to minimize deflection aberations and beam landing angle deviation on the wafer. The column design facilitates a shortened distance between the object and its image plane. Accelerating voltage is 30 kV due to decreased Coulomb interaction and proximity effect. In a shaped beam system, the electron gun should have a wide emission angle, high brightness, and large crossover size. A square rod type LaB6 cathode is developed for this purpose. The shaped beam is vector scanned by electrostatic deflection plates and electromagnetic coils. The scanning area is 2.6 mm square and the maximum beam size is 5.10 μm square. The size change unit is 0.02 μm. A beam current density higher than 10 A/cm2 is obtained with a small beam covergence angle. A shape edge slope smaller than ...

Trajectory analysis of ions formed in the field emitter inter-electrode region

Abstract Ion trajectories are three-dimensionally analyzed to determine the main source of ions striking the emitting area of a field emitter. Computer calculations of the ion trajectories are made for a field emitter in the shape of a sphere radius 1000 A, mounted on an orthogonal cone, and anode radii of 3 cm and 1 cm centered at the emitter. Approximately 10−10 to 10−9fraction of the ions formed at the anode by electron impact desorption reach the emitter tip. Only those residual gas ions generated in the space of a few μm around the tip are able to impinge on the emitting area. Under typical operating conditions of 10−9 Torr, the residual gas ions striking the tip are 10 to 102 times greater than those from the anode, assuming an ion ejection of one ion per 105 electrons, and an anode potential of about 3 kV.

Preparatory Study for the Matrix-Pattern Imaging, EB System

A new method for electron-beam lithography –called matrix-pattern imaging (MPI)– for use as a high-throughput exposure system has been proposed. In MPI, the electron sources are arranged in a matrix so that they produce an electron beam in the shape of a circuit-pattern element; the beam is focused on the target. An evaluation system for measuring the properties of the MIM cathode, a promising candidate for the electron source of an MPI-based system, was constructed. The measured current density (at the cathode) is 2.5 mA/cm2, and the brightness is 1×102 A/cm2/sr (at an accelerating voltage of 50 kV). An image of the cathode was successfully projected onto the target, and delineated on a wafer. Moreover, the composition of the exposure system was optimized so as to increase throughput with an MIM cathode, and the throughput of the resulting MPI system was estimated as one six-inch reticle per hour.

Coulomb effect in cell projection lithography

Beam blurring due to Coulomb effects is very important in cell projection lithography. This article reports on Coulomb effects in cell projection lithography by simulation and experiment, and shows some important results. First, the Coulomb effect in a single cell is substantially uniform in conventional cell projection optics. Second, refocusing is effective in reducing the space charge effect even in Koehler illumination with a Gaussian crossover. Third, the optimum cell size in the case of a large aperture percentage depends on the ratio of the total exposure time in the peripheral circuit to the total settling time in the memory cell mat. In addition, a modified mask technique is proposed to decrease the influence of Coulomb effects on throughput.

Cell projection column for high speed electron‐beam lithography system

The cell projection column for the high throughput electron‐beam lithography system (HL‐800D) has been developed. This column forms a conventional variable shaped beam and five kinds of cell pattern beams with a maximum 5 μm dimension. A new in‐lens three stage deflection system consisting of a magnetic and two electrostatic deflectors provides low distortion and high speed 5 mm square deflection at 50 kV. Beam shaping optics have a two stage deflector for rectangular beam sizing and cell beam selection. The settling times coincide with three stage deflection system to prevent extra overhead time. The beam shaping system has an inspection function for the cell aperture by switching the crossover focusing. Complicated deflection calibration systems are necessary to correct precisely in advance to exposure. To reduce the correction time, a simplified method which separates time dependent terms from whole correction terms, has been developed. This method decreases the total correction time and procedures, be...