Soichi Nadahara

The origin of stress in sputter‐deposited tungsten films for x‐ray masks

The mechanism for the cause of stress in a sputter‐deposited tungsten (W) film has been clarified. The tensile stress of the film was calculated using the interatomic forces acting on the grain boundary. The average distance of the grain boundary gaps was determined from the measured film density assuming the film had homogeneous size rectangular grains. The calculated and measured stress values were in good agreement in the high working gas pressure region. The difference between these values in the low working gas pressure region has been able to be explained by the compressive stress due to the peening effect of Ar. The low stress in the high pressure region was obtained by large opened grain boundaries which produced low film density. A low film density causes a low x‐ray stopping power. The film deposited in the low pressure region is suitable as an x‐ray absorber because of its high film density.

Characterization of stress in doped and undoped polycrystalline silicon before and after annealing or oxidation with laser Raman spectroscopy

Stress in polycrystalline silicon (poly-Si) was characterized with laser Raman spectroscopy. Effects of diffusion of phosphorus, annealing and oxidation on stress were especially studied. Relaxation of undirectional stress by annealing and oxidation was observed. Undirectional stress was relaxed by heavy doping of phosphorus which made a grain size larger. Compressive stress increased, however, by oxidation in poly-Si with a smaller grain size.

Protuberance growth at polysilicon surfaces during oxidation

Abstract The change in surface morphology of polysilicon due to oxidation was examined by cross section electron microscopy. The protuberance growth during oxidation at a planarized polysilicon surface, from which the initial surface roughness and bumps were eliminated, was experimentally revealed. It is indicated that the thickness of the protuberance region exceeds that of the polysilicon layer prior to the oxidation, while the top surfaces of the protuberances are oxidized. Its protuberance growth means that excess silicon atoms induced by compressive stress resulted from an intergranular oxidation. We propose a new model of the protuberance growth based on the silicon creep mechanism.

Photoresist stripping using novel sulfuric/ozone process

Sulfuric acid (H/sub 2/SO/sub 4/) and ozone (O/sub 3/) mixture process (SOM) with in-situ concentration monitor for O/sub 3/ and peroxyso-di-sulfuric acid (H/sub 2/S/sub 2/O/sub 8/) was developed Ultraviolet spectrometers with 190-200 nm and 254 nm of wavelength were used to detect H/sub 2/S/sub 2/O/sub 8/ and O/sub 3/ dissolved in SOM, respectively. In order to mix H/sub 2/SO/sub 4/ and O/sub 3/ effectively, the O/sub 3/ gas ejectors were jointed to a quartz bath directly. Using SOM process with UV oxidant monitors and O/sub 3/ gas ejectors, heavily dosed resist and dry etched resist could be removed perfectly without dry ashing process.

The development of batch type low pressure spin dryer for water mark-less wafers

We developed a novel spin dryer, which is equipped with high speed rotation capability and a low pressure N/sub 2/ purging system, to realize a water mark free surface. Water marks which remained on the Si wafer surface were made by chemical reaction of Si, O/sub 2/, and H/sub 2/O during spin drying. In this paper, two experimental approaches: (1) to remove O/sub 2/ from the reaction elements; and (2) to minimize reaction time, were demonstrated. The low pressure, high speed spin dryer with shower rinsing suppressed water marks significantly, i.e. only 500 ppm, compared with that of a conventional one.

Process damage in single-wafer cleaning process

We developed the post CMP cleaning process using cavitation jet in addition to roll sponge cleaning. In this presentation, we discuss the process damage which had been encountered during the cleaning process development using high pressure water. One was erosion and the other was electrostatic charging damage due to high impinging pressure. The tool parameters of cavitation jet were investigated in order to control the cavitation effect for the wafer cleaning process. These process damage were eliminated by control of cavitation power keeping the occurrence of cavity growth and control of the exposure time to the device by cavitation jet. After that, superior cleaning performance was demonstrated by using the combination of cavitation jet and roll sponge cleaner for wafers after CMP.