Taichi Koizumi

A Novel High-Resolution Scanning Electron Microscope for the Surface Analysis of High-Aspect-Ratio Three-Dimensional Structures

A scanning electron microscope (SEM) with an immersion-type objective lens has been developed. The objective lens generates a strong axial magnetic field of about 0.17 T at a wafer surface and has a very short focal length of about 2.5 mm. The SEM realizes an image resolution of 30 A at 0.7 kV accelerating voltage. The capability to observe the bottom of a trench with a 5.3 µm depth and 0.55 µm width and that of a hole with a 0.46 µm diameter and 1.6 µm depth has been demonstrated at a low accelerating voltage of 0.5~1.5 kV. A good measurement reproduceability of 5 nm/3σ (σ: standard deviation) has been achieved in the measurement of the 0.55 µm SiO2 line pattern on the bottom of a trench of 0.55 µm width and 5.3 µm depth. The SEM has been applied to the observation of 64MDRAM cell patterns with a 0.4 µm design rule.

Quarter micron KrF excimer laser lithography

For quarter-micron KrF excimer laser lithography, a chemically amplified positive resist with high stability and process compatibility has been developed. 0.25- mu m line and space patterns and 0.35- mu m contact hole patterns have been obtained using this resist. The multiple interference effect due to reflection from air and substrate is reduced by using an overcoat film or antireflective coating. The overcoat film is made of a water-soluble polyvinylalcohol derivative. The refractive index of this polymer is 1.3, which is suitable for the resist (index

CD control using latent image for lithography

As a design rule of LSI device gets smaller, critical dimension (CD) control becomes increasingly difficult. This creates a new technology to CD control accurately. The technology utilizes intensity of light diffracted from latent image consisting of periodic patterns in undeveloped photoresist, and its possibility has been reported for several years. We have developed a new method of the CD control by monitoring profile of the latent image using atomic force microscope (AFM). The reduction of CD variation using this method is achieved by controlling development time from the relationship between the profile of the latent image and CD after development. We have tried to apply this method to a resist process in KrF excimer laser lithography and found usefulness of the method for 0.25 micrometers lithography.

Submicron 3-Dimensional Structure Observation by Cyclotron SEM(Scanning Electron Microscope)

Experimental and numerical approaches have been taken to the study of the brightness of scanning electron microscope(SEM) images for bottom surfaces of deep holes and trenches by Cyclotron SEM. Secondary electrons emitted from the bottom are efficiently collected as an image signal by a couple of the following combinational effects. One is an electrostatic converging lens due to accumulated negative charges on the side wall of holes and trenches. The other is a strong magnetic field above the specimen surface generated by an immersion-type objective lens. The former plays the role of reducing the number of electrons which collide with the side wall. The latter binds diverged secondary electrons leaving from the specimen surface and introduces electrons into a detector.

A noncharging direct-write electron beam process for a trilayer resist by ion shower technology

Abstract In compensating the charging effect in electron beam direct writing on trilayer resists, we developed a new process of irradiating the bottom layer resist with a H+ ion shower. Thus the field butting error caused by the charging during electron beam exposure was eliminated. This process requires an extra ion shower irradiation step after the bottom layer coating in the conventional trilayer resist process. The sheet resistance of the bottom layer, 10 13 Ω/□ , was reduced to 10 8 Ω/□ at a dosage of 1 × 1016 H+ ions/cm 2. SIMS analysis showed that the relative carbon concentration of the bottom layer material increased at a higher H+ ion dosage. The reduction of the resistance was caused by the carbonating of the bottom layer resist. As a result, the charge from EB direct writing was easily dissipated through the bottom layer. By using this new trilayer resist process, the field butting error was decreased from 0.5 μm/3σ in a conventional trilayer resist process to less than 0.1 μm/3σ, which is equal to the error for the single-layer resist on the Si substrate. Thus 0.5 μm rule 16MdRAM patterns were successfully obtained by this new direct-writing EB resist process.

An E-Beam Direct Write Process for 16M-Bit DRAMs

For obtaining a very fine wafer pattern below half micron, direct write EB lithography has charging and proximity effect problems. A method of compensating for the charging problem is to use a 40 kV proton shower irradiation process which decreases the bottom layer resistance of the trilayer resist. The charge of the electron beam is dissipated through the bottom-layer resist. As for the proximity effect, we developed a proximity effect correction software system by dosage modification. The theoretical and experimental results showed that in a 2.2-micron-thick trilayer planarizing resist system, a 0.5-micron isolated line, 0.5-micron isolated space, and 0.5-micron contact holes were simultaneously resolved in a half-micron-thick top-layer resist. The resultant half-micron-rule 16M-bit DRAM patterns were successfully obtained on uneven topography of the processed wafer using EB direct write.