Hisatake Sano

Primary evaluation of proximity and resist heating effects observed in high-acceleration voltage e-beam writing for 180-nm-and-beyond rule reticle fabrication

Higher resolution and accuracy are required in e-beam lithography for reticle fabrication for coping with further advances in optical lithography. The trend is to use high acceleration voltage (50 kV) e-beam to improve spatial resolution. However, in the case of high acceleration e-beam writing, a drastic critical dimension (CD) change is caused by a strong proximity effect and a large resist heating effect. The proximity effect is caused by the increase in the back- scattering radius. The back-scattering radius was estimated by two independent observations of the CD variation of a monitor and the thickness variation of a partially developed resist. It is found to be ca. 15 nm. Using the shot time modulation as a proximity correction reduced the proximity effect to a small level: CD error due to the pattern density change remained within 10 nm. On the other hand, the resist heating effect is caused by the change in resist dissolution speed by the temperature rise of the resist. In reducing this effect, multi-pass writing is found to be effective. The range of the CD error of 2 micrometer lines-and-spaces in the writing field has been reduced from 22 nm to 6 nm by changing the writing from one pass to four passes for a conventional resist. Moreover, when a chemically amplified resist (CAR) is exposed through one-pass writing, the range of the CD error is found to be 8 nm. Therefore, the use of the CAR is effective in reducing the resist heating effect. Simulation software ProBEAM/3D and TEMPTATION were used to obtain three- dimensional resist profile and the transient temperature rise of the resist, respectively. Both provided results that agreed well with those by experiment.

Low‐stress tantalum absorbers deposited by sputtering for x‐ray masks

A method for fabricating stable and low‐stress tantalum films, suitable for absorbers of x‐ray masks, by dc‐bias, rf magnetron sputtering is presented. The superiority of Xe over Ar as a working gas is demonstrated. The stress‐vs‐bias‐voltage curve has a plateau, where the film stresses are low and constant over the bias voltage ranging from −15 to 20 V. The plateau stresses vary linearly with both the working Xe pressure and the substrate temperature with slopes of 1.2×1010 dyn/cm2 Pa and 8.6×106 dyn/cm2 deg, respectively. By control of the pressure and the temperature, stresses of (3±2)×108 dyn/cm2 are achieved. The Ta film which is stable under a heat treatment up to 300 °C is obtainable when it is deposited at 220 °C and then annealed at 220 °C for 1 h. The density of the films is (15.0±0.5) g/cm3. By x‐ray diffraction the films are found to be in a mixture of the α‐Ta phase and the β‐Ta phase. Ta patterns of 0.15 μm in width were fabricated.

Manufacturing of half-tone phase-shift masks I: blank

Half-tone phase shift mask (HT-PSM) blanks for i-line (365 nm) and g-line (436 nm) lithography, using chromium composites as a half-tone shifter, are brought into production. A bilayer structure of a 10 - 20 nm thick opaque, conductive chrome layer and a phase-shifting CrON layer is proposed, which can be formed by continuous deposition of the two layers and etched continuously by the process similar to that of the conventional chrome photomask. It shows low visible light transmission of less than 30% so that it can be inspected, and also shows enough conductivity to decay the excess charge during electron beam writing. HT- PSMs made of these blanks can be cleaned by sulfuric acid at 100 degree(s)C and can be used at least up to an irradiation of 1 MJ/cm2, when used for i-line exposure. The specification for the transmission is (target +/- 1)% for any point on any plate, and 0.7% range for any point on one plate, where the target ranges from 6% to 10%. The specification for the phase shift is currently (180+/- 10) degree(s).

Performance of positive-tone chemically amplified resists for next-generation photomask fabrication

Several types of chemically amplified resists (CARs) have been evaluated in comparison with other types of resists. The evaluated CARs were found to have better performance than conventional resists and to satisfy the requirements for next generation reticle fabrication. Especially CARs have high sensitivity and high contrast enough to make photomasks with e-beam writers and have good dry-etch durability. We evaluated the allowance of baking conditions. It was found that it was important to minimize the dependence on prebake temperature as well as on post exposure bake temperature. The influence of airborne contaminants on post exposure delay (PED) stability of CARs was investigated. PED stability depends on resist materials and the ammonia concentration in the process environment. The use of a chemical filter is effective in improving PED stability. In conclusion, CARs can meet the requirements for resists used for next generation reticle fabrication.

Chromium-based attenuated phase shifter for DUV exposure

An attenuated phase shift mask (APSM) for DUV exposure using a chromium fluoride film as an embedded phase shifter is developed. The chromium fluoride film is deposited by the conventional dc reactive sputtering process using gases such as CF4 or SF6 as the fluorine source. This film can be dry-etched by the chlorine chemistry, so that an etch into the quartz plate, which causes phase error, is negligible and high accuracy phase shift control can be achieved. Furthermore, this film shows a transmission of ca. 15% at 248 nm as a single- layered embedded shifter deposited on the quartz plate (100% for air), and therefore, in combination with an opaque chrome layer, various transmissions up to ca. 8% can be obtained without changing the shifter material. Promising properties as a practical shifter material are confirmed, and an exposure test on 2.5 micrometers and 3.0 micrometers hole patterns, using an APSM with a trilayer shifter, where an opaque chrome film is sandwiched by two chromium fluoride films, shows that the focus latitude can be extended compared with conventional chrome masks.

Direct phase measurement in phase-shift masks with a differential heterodyne interferometer

Lasertec has developed a novel phase-shift measurement system 1PM11, which uses a differential heterodyne interferometer. In 1PM11, phase information is converted into a low- frequency heterodyne beat that is easily measurable by an electric current. Two-frequency laser beams which are oscillated from a He-Ne laser (632.8 nm) tube are used. When a 40X objective lens being used, the beam is 2.3 micrometers in diameter and the distance of the two beams is adjustable from 3.0 to 6.0 micrometers . The performance of 1PM11 for three types of phase-shift masks: (1) the shifter of etched quartz, (2) spin-on-glass shifter on etch- stop/quartz, and (3) the attenuating shifter, is reported.

200-mm EPL stencil mask fabrication and metrology

200-mm electron-beam projection lithography (EPL) masks were fabricated starting from stress-controlled silicon-on-insulator (SOI) substrates. The internal stress of the SOI layer is controlled to be ca. 10 MPa by B doping. The blank fabrication process has been established by the Bosch deep trench etch process. EB patterning was done on a JEOL JBX9000MVII with a positive-tone chemically amplified resist of 400-nm thickness. Resist image of 200-nm wide lines-and-spaces pattern was transferred to 2-um thick SOI layer by a shallow trench etching. A dual-mode critical dimension (CD)-SEM was implemented, and used for mask characterization. Preliminary results on uniformity of CD-shift in the dry etching and final CD were reported. 200-mm EPL masks with a gate layer of a system-on-chip device pattern were fabricated.

Performance of i- and g-line phase-shift measurement system MPM-100

The performance of an i-/g-line direct-phase measurement system Lasertec MPM- 100 has been evaluated. The minimum measurable pattern sizes is 2.5 .μm for holes on an 8%-i-line transmittance halftone phase shift masks (HPSMs). The effect of the focus position is not significant for hole pattern of above 3.5 μm. Both short-term repeatability and long-term stability are excellent, being less than 0.5 deg. The effect of the illumination NA has been investigated theoretically and experimentally, and the use of correction factors based on experiment is proposed for estimating effective phase shifts from phase shifts obtained by MPM- 100.

Development of LEEPL 6025 format mask blanks

Low energy electron proximity projection lithography (LEEPL) has three types of mask formats. One of them, LEEPL 6025 square format, is so designed that electron-beam writers for photomasks can accommodate it. LEEPL 6025 square format blanks manufactured by three methods were evaluated in laying stress on cutting and bonding. The starting substrate was a 200-mm silicon-on-insulator wafer composed of a 2-μm-thick silicon layer, a 1-μm-thick SiO2 layer, and a 725-μm-thick base silicon. Membrane wafers were made after dry etching of the backsides of the starting substrates. They were cut and bonded to frames. Ceramic SiC and a Si-base material were used for the frame. Soldering and other thermal methods were employed for bonding. The findings are: 1) No membranes were broken in cutting and bonding, 2) Chipping was observed after cutting, which requires some edge treatment like edge beveling, and 3) The flatness of the membrane wafer was reduced from 87 to 13 μm by bonding. In conclusion the 6025 square format blanks were successfully manufactured by three methods and evaluated in the first time.

Development and evaluation of chromium-based attenuated phase-shift masks for DUV exposure

We had developed an attenuated phase shift mask for DUV exposure (DUV- AttPSM) using a CrFx film as a phase shifting layer. But the durability against DUV irradiation was poor (ca. 0.8% transmittance increase after 37 kJ/cm2). In addition, an exposure test proved that the phase shift angle of the DUV-AttPSM was 170 degrees. We improved the durability against DUV irradiation of our DUV-AttPSM by changing the deposition conditions. The transmittance change after 37 kJ/cm2 irradiation is reduced to 0.15%. The improved film shows acceptable durability against chemicals, and there is no problem in the mask- cleaning process. Furthermore, we evaluated the exposure properties of a DUV-AttPSM with a phase-shift angle of ca. 180 degrees, and the focus latitude for a 0.25 micrometers hole array turns out to be 2.5 micrometers , being 5- times as large as that of a binary mask. In addition, the consecutive deposition stability of the blank was tested, and the DUV-AttPSM proved to be adaptable to mass production.