Hiroshi Hoga

Charge build-up in magnetron-enhanced reactive ion etching

Charge build-up in magnetron-enhanced reactive ion etching (MERIE) was evaluated with metal nitride oxide semiconductor (MNOS) capacitors. In static magnetic field, negative flat band voltage (Vfb) shifts of more than -1.5 V were observed in the area under high-density plasma, and more than 2-V Vfb shifts were observed at the edge of the wafer near the N and S poles. This distributed Vfb shift was considered to result from nonuniform plasma potential caused by secondary electron E×B drift motion. In rotated magnetic field, Vfb shifts were reduced. No significant Vfb shifts were observed when the magnet was rotated at 120 rpm. The Vfb shift reduction in rotated magnetic field was supposed to result from charge neutralization by alternate charge build-up.

X-Ray Mask Distortion Induced in Back-Etching Preceding Subtractive Fabrication: Resist and Absorber Stress Effect

The influence of resist and absorber stress distributions on X-ray mask distortion induced during back-etching preceding subtractive fabrication is analyzed experimentally and simulated. The stress distribution (gradient) in a resist and/or that in an absorber film causes larger pattern displacement rather than the film average stress. Some resists have considerably high stress after coating on a wafer, and this stress also changes during exposure, causing local pattern displacements. However, use of chemically amplified resist systems, in which the reaction after exposure is limited to a small amount acid generation, might solve this problem. Low-stress positive-tone resists should thus be developed.

Effect of synchrotron radiation on electrical characteristics of SiOxNy thin films formed by rapid thermal processing in a N2O ambient

The synchrotron radiation durability of SiOxNy films (about 10 nm) formed by rapid thermal processing in a N2O ambient was studied. No significant difference between the flat‐band voltage of SiOxNy‐metal‐oxide semiconductors (MOS) capacitors patterned by synchrotron radiation lithography (SR‐MOS) and that of capacitors patterned by conventional optical lithography (OP‐MOS) was observed. The midgap interface state density of the SR‐MOS was approximately one order of magnitude larger than that of the OP‐MOS. The differences between the SR‐MOS and the OP‐MOS were nearly independent of the SR dose in the range of 540–2700 mJ/cm2, and were eliminated by annealing in a hydrogen ambient at 400 °C for 30 min.

Optically High Transparent SiN Mask Membrane with Low Stress Deposited by Low Pressure Chemical Vapor Deposition

Optically high transparent and low-stress silicon nitride film for X-ray mask membrane was successfully deposited by low pressure chemical vapor deposition (LPCVD). Deposition of silicon nitride films in a relatively higher temperature range of 900°C-1000°C was investigated as a function of deposition temperature and reactant gas flow (SiH2Cl2 and NH3). Silicon nitride films of low tensile stress in the order of 3-5×108 dyn/cm2 with low refractive index of 2.05 were obtained at 1000°C. Silicon nitride membrane deposited at 1000°C showed optical transparency ~95% (λ=633 nm). Use of such optically high transparent silicon nitride film will be helpful in reducing the alignment signal detection error, which is required for highly efficient mask wafer alignment.

High-precision EB technology with thin EB resist and distortion-free mask holder for x-ray mask fabrication

EB lithography and dry etching technology have been investigated to improve EB pattern resolution and pattern placement accuracy. High selective reactive ion etching of W absorber and thin EB resist process have been developed adopting intermediate Ti mask layer between the EB resist and W. The EB resist pattern resolution on the W absorber covered by thin Ti mask layer was improved by thinning EB resist and it became possible to obtain 0.12 micrometers -pitch line/space pattern with 0.1 micrometers - thick EB resist. Pattern size decrement for the designed pattern size was also effective to improve the resist pattern resolution. The W absorber was etched with very high selectivity above 500 to the Ti mask layer using Cl2 + O2 gas system. Using this RIE technique, it became possible to etch 0.7 micrometers -thick W absorber with 0.05 micrometers -thick EB resist and 0.03 micrometers -thick Ti mask layer. Further, a distortion-free X-ray mask holder for EB writing system has been developed, which is estimated to reduce pattern displacement caused by mask clamping to less than 3 nm within a radius of 10 mm exposure field as far as the clamping distortion was concerned.

Fabrication of reliable x-ray mask using high-temperature deposited SiN membrane by low-pressure chemical vapor deposition system

The SiN membranes were deposited by using high temperature LPCVD system. The SiN films deposited over 1000 degree(s)C showed the suitable properties for X-ray mask, such as well- controlled tensile stress of 5 X 107 Pa, high optical transmittance over 95% and low impurity concentrations. The high optical transmittance of the SiN films deposited over 1000 degree(s)C was related to the high N/Si. The X-ray masks fabricated by using the SiN membranes deposited at 1000 degree(s)C showed the high optical transmittance of about 92% and X-ray durability. The pattern position displacement induced by SR irradiation was simulated using FEM calculation with qualitative agreement.

Improvement in radiation stability of SiN X-ray mask membranes

The synchrotron radiation (SR) stability of silicon nitride (SiN) X-ray mask membranes was successfully improved by low-pressure chemical vapor deposition at a growth temperature of 1000°C. The transmission of the SiN membrane formed at 1000°C exceeded 90% at about 633 nm even after SR absorption with a dose of about 100 MJ/cm3, although those of membranes formed at 800 and 900°C decreased to below 85% after the same SR absorption. In addition, SR-induced pattern displacements for the SiN membrane formed at 1000°C were suppressed to σx=11 nm and σy=10 nm with a dose of 20 MJ/cm3. The N/Si atomic ratio increased, and the H and O concentrations in the SiN decreased with the increase of the growth temperature. The increase ratio of the spin density before and after the SR absorption decreased with the increase of the growth temperature.

Simulation of x-ray mask displacement by absorber and membrane stress

X-ray mask displacements were simulated using finite element method in order to estimate the requirements for the stress and stress distribution of the absorber and membranes. The structure of X-ray mask in this simulation was as follows: the substrate was 3inch φ and 2mm thick Si wafer, the membranes were 2μm thick SiN and SiC, absorber thickness was 0.5μm, and window area was 25mm square. The simulations were focused on the film stress, various absorber patterns, such as half pattern of window, line and space patterns, and the influence of backetch.

Study of SiC x-ray mask distortion induced by backetching receding subtractive fabrication process

In this paper, we report on the evaluation of the SiC X-ray mask distortion induced by the backetching receding fabrication process by experiment and simulation. The window-opening process for the backetching mask induced pattern displacements of about the same level as measurement accuracy. Large pattern displacements normally induced by the Si backetching process are reduced by using a lower-stress membrane and/or a thicker Si substrate. Simulation shows that a larger-diameter substrate also reduces mask distortion. The one-point anodic bonding technique has been developed, which suppresses the pattern displacements in the last stage of bonding to the frame, to within measurement error (20 nm: 3(sigma) ).

Reduction of X-Ray Irradiation-Induced Pattern Displacement of SiN Membranes Using H+ Ion Implantation Technique

We have developed a technique for improving X-ray irradiation stability of silicon nitride (SiN) X-ray mask membrane using H + implantation. This technique can realize the reduction of X-ray irradiation-induced pattern displacement to less than 20 nm after X-ray absorption with a dose of 30 MJ/cm 3 by optimizing the implantation dose to 4 x 10 15 /cm 2 at 150 keV. It is found that the mechanism of the reduction of the displacement is that the stress change after X-ray absorption in the implanted layer (top to 1.1 μm in depth) of SiN film compensates the stress change in the unimplanted layer (1.1 to 2.0 μm (bottom) in depth).