Tadahiro Takigawa

Electron‐beam direct writing system EX‐8D employing character projection exposure method

An electron‐beam direct writing system which adopts character projection methods in addition to conventional variable‐shaped beam methods, has been constructed for 0.15 μm class ultra‐large scale integration pattern fabrication. This system is a modified version of our variable‐shaped beam machine. The electron optical system adopts a three stage octapole deflector for a 2 mm field and installs an aperture plate exchange mechanism for character projection. The objective lens system was designed so that the beam resolution is 0.04 μm. An optimization study to write a 1G‐dynamic random access memory pattern with 0.15 μm design rules showed that a preferable character size and number are 2.5 μm and 48, respectively. The writing speed of this system is designed to be 110 s a chip, using the characters for memory cells as well as peripheral circuits. An advanced beam calibration method has been developed for beam current density and for character size, direction, and position. This method effectively adjusts the current density for each character to be the same on wafers. The beam position is accurately corrected by analyzing the obtained beam intensity distribution.

High-Accuracy Proximity Effect Correction for Mask Writing

A high-accuracy proximity effect correction method for high-precision masks has been developed to satisfy current and future requirements. In this paper, we explain the primary features of this method and the theories on which it is based. The developed formula for obtaining the optimum correction dose is expressed in the form of either iterations or an infinite series of functions. The advantage of this formula is that it quickly converges to the sought value, bringing about high-accuracy proximity effect correction with a high calculation speed. A coarse graining method (covering pattern density and representative figure methods) for reducing calculation time is explained. This method has been adopted for an EX-11 series and has been used for mask writing from the 180 nm design rule onward.

Beam induced deposition of an ultraviolet transparent silicon oxide film by focused gallium ion beam

We have deposited a silicon oxide (SiOx) film with a high optical transmittance in the DUV region by a focused ion beam induced deposition technique using a gallium ion beam and a mixture of oxygen and TMCTS(1,3,5,7‐tetramethylcyclotetrasiloxane) as a source gas. The optical transmittance of a 0.3 μm thick film is higher than 90% at the wavelength of 250 nm. The transmittance of the deposited SiOx film depends on both the source gas and ion beam irradiation conditions. A scaling to explain the transmittance along with the ion beam conditions is proposed.

A rewiring technique for integrated circuit operation analysis using a silicon oxide film deposited by a focused ion beam

The silicon oxide deposition technique with a Si focused ion beam was applied to rewiring from the first aluminum line below a wide power line in a 256k CMOS SRAM. The widths of the power line and the first aluminum line were 130 and 1.5 μm, respectively. First, the layer on the first aluminum line was etched for an area of 4.5×4.5 μm by a Ga focused ion beam. Second, silicon oxide was deposited into the hole using a 60‐keV Si2+ focused ion beam with a mixed gas of tetramethoxysilane and oxygen, then, the deposited silicon oxide film was etched for an area of 2×2 μm down to the first aluminum line by the Ga focused ion beam. Last, tungsten was deposited for rewiring from the first aluminum line using the conventional focused ion beam method. The leak current measured between the deposited tungsten and the power line was 1×10−8 A at 5 V which is sufficiently small for operation analyses of semiconductor devices.

Silicon Oxide Film Formation by Focused Ion Beam (FIB)-Assisted Deposition

A silicon oxide film has been formed by means of 60 keV Si2+ focused ion beam (FIB)-assisted deposition. A mixture of tetramethoxysilane (Si(OCH3)4) and oxygen gases was blown onto a sample surface through a 0.2-mm-inner-diameter nozzle. A gold, silicon, and beryllium alloy source was used to produce a Si2+ ion beam in the FIB system. The beam diameter and current were 0.3 µm and 0.1 nA, respectively. The deposited film with 0.1-µm thickness and 0.7-µm width consisted mainly of silicon and oxygen, and contained scarcely any carbon. The relative ratios of silicon to oxygen atomic concentration were 1:2 near the film surface and 1:1 inside the film. The resistivity of the deposited film was 2.5 MΩcm at 5 V, and the breakdown voltage was 40 V. It was found that it would be possible to use the deposited film as an insulator for integrated circuit repair in developing semiconductor devices.

Grain growth of polycrystalline silicon films on SiO2 by cw scanning electron beam annealing

Reduced pressure, chemical vapor deposited polycrystalline silicon films (5000 A thick) over thermally grown SiO2 on (100) silicon wafers are recrystallized by a scanning electron microscope modified electron beam annealing system. On the basis of transmission electron microscope bright‐field images and electron diffraction patterns, large grained polycrystalline silicon films of 20‐μm average grain size are obtained after electron beam annealing. Electron beam current, scanning rate, and annealing repetition are found to be important parameters in the recrystallization. Optimum values for them are from 1.6 to 1.9 mA, from 40 to 80 cm/sec, and from 5 to 10 times, respectively.

Advanced electron-beam writing system EX-11 for next-generation mask fabrication

Toshiba and Toshiba Machine have developed an advanced electron beam writing system EX-11 for next-generation mask fabrication. EX-11 is a 50 kV variable-shaped beam lithography system for manufacturing 4x masks for 0.15 - 0.18 micrometer technology generation. Many breakthroughs were studied and applied to EX-11 to meet future mask-fabrication requirements, such as critical dimension and positioning accuracy. We have verified the accuracy required for 0.15 - 0.18 micrometer generation.

Reduction of long range fogging effect in a high acceleration voltage electron beam mask writing system

We have developed an antireflecting plate with a novel structure to reduce the long-range fogging effect, which is especially serious in mask writing by a high acceleration voltage electron beam mask writing system. This structure is characterized by an array of holes whose axes converge to the beam irradiation position. These holes can efficiently absorb the electrons scattered at a reticle by avoiding the collision of the electrons with the inner walls. The reflectance of this structure is about 30% of that of the plane structure for 30 keV electrons, whereas that of a parallel hole array structure is about 40%. The fogging effect was evaluated in a high accelerating voltage (50 kV) electron beam mask writing system equipped with this new antireflecting plate, and nm-level global uniformity in pattern size was achieved.

Representative Figure Method for Proximity Effect Correction

The representative figure method, which was proposed to greatly reduce the proximity effect correction time, is studied in detail in this paper. The method is shown to have a high correction accuracy (the error is 0.25% for a 50 kV acceleration voltage and a 2 µm small area) and a high speed correction time (which includes the time for preparing representative rectangles; 1/10 compared with the conventional method). The basic idea of the present method is found to be applicable to data compaction. The compaction ratio by this method is (1/2)-(1/100) of that by the conventional method. The representative figure method has a wide range of applications and has many advantages.

Resist heating effect in direct electron beam writing

A new model for the analysis of resist heating is proposed and applied to the direct electron beam writing method using a variably‐shaped electron beam system and a single‐layer resist. From calculations based on the present model and experiment, it was found that the resist heating effect leads to resist ablation, difference in resist sensitivity between single exposure and multiple exposures, and depth dependence and beam size dependence of resist sensitivity. It also explains the degradation of shaped beam stitching accuracy as well as pattern size error. The resist heating effect is more serious for direct writing than the estimation obtained from a conventional model. In order to realize submicron patterns below 0.5 μm with considerable high throughput, the use of a highly sensitive resist is essential for overcoming the resist heating effect.