Tadahito Matsuda

The Effect of an Organic Base in Chemically Amplified Resist on Patterning Characteristics Using KrF Lithography

A new resist system composed of an SEPR chemically amplified (CA) positive resist and an N-methyl pyrrolidone (NMP) organic base has been developed for KrF excimer laser lithography. Using 0.30-µ m l&s patterns formed with KrF stepper, we studied the effect of contamination from substrate films of plasma chemical vapor deposition silicon dioxide (P-CVD SiO2), low pressure CVD silicon nitride (LP-CVD Si3N4) and reactive sputtered titanium nitride (TiN), and of airborne contamination under the condition of an 8-ppb of ammonia. The results clarify the new resist system reduces the effect of substrate film as well as airborne contamination. The new resist system enables us to form fine patterns on any substrate and attains more than one hour post-exposure delay without overcoat and undercoat films.

Patterning Characteristics of a Chemically-Amplified Negative Resist in Synchrotron Radiation Lithography

To explore the applicability of synchrotron radiation X-ray lithography for fabricating sub-quartermicron devices, we investigate the patterning characteristics of the chemically-amplified negative resist SAL601-ER7. Since these characteristics depend strongly on the conditions of the chemical amplification process, the effects of post-exposure baking and developing conditions on sensitivity and resolution are examined. The resolution-limiting factors are investigated, revealing that pattern collapse during the development process and fog caused by Fresnel diffraction, photo-electron scattering, and acid diffusion in the resist determine the resolution and the maximum aspect ratio of the lines and spaces pattern. Using the model of a swaying beam supported at one end, it is shown that pattern collapse depends on the resist patterns flexural stiffness. Patterning stability, which depends on the delay time between exposure and baking, is also discussed.

XY stage driven by ultrasonic linear motors for the electron-beam x-ray mask writer EB-X3

A new XY stage employing ultrasonic linear motors that is simple, compact, and completely nonmagnetic was developed for an e-beam x-ray mask writer employing step-and-repeat writing. The main performance results are: (1) position fluctuations during writing are less than ±4 nm; (2) the time for a 650 μm step movement is less than 270 ms; (3) the magnetic interference is less than ±1 nT at the e-beam deflection center; and (4) the vibration of the stage when moving at a constant speed is less than ±0.25 m/s2. The new stage is also very easy to maintain. It has been installed in the EB-X3 e-beam x-ray mask writer.

Extendibility of synchrotron radiation lithography to the sub‐100 nm region

This article discusses the resolution of synchrotron radiation lithography in the sub‐100 nm region, taking into consideration the mass production of large‐scale integrated circuits, under attainable conditions for the x‐ray mask, proximity gap, and resist processes. Resolution and exposure latitude for line‐and‐space patterns are markedly improved by using a mask with a contrast of only 2.5. Resolutions of 90, 80, 70, and 60 nm can be achieved with proximity gaps of 30, 20, 15, and 10 μm if a high‐contrast resist and a low‐surface tension developer are used. The latitude will be 10% for pattern sizes as small as 70 nm when the proximity gap is narrower than 15 μm. The effects of mask duty [which is defined to be the ratio of the absorber (line) width to the pattern pitch, i.e., duty cycle] on the optimum exposure dose and mask linearity are also evaluated.

Application of x‐ray lithography with a single‐layer resist process to subquartermicron large scale integrated circuit fabrication

The applicability of synchrotron radiation x‐ray lithography to future ultralarge scale integrated circuit fabrication processes is demonstrated by the test fabrication of subquartermicron bipolar‐ complementary metaloxide semiconductor devices (SRAM, gate arrays, and several test element groups) with a total size of two‐million transistors. Synchrotron radiation lithography is used at four critical levels: gate poly, first metal, via hole, and second metal. Both negative and positive chemically amplified resists are used with a single‐layer resist system to simplify the resist process. An overview of the lithography process is presented with emphasis on patterning and overlay performance.

X-Ray Mask Pattern Accuracy Improvement by Superimposing Multiple Exposures Using Different Field Sizes

A method to improve the accuracy of patterns written by an electron-beam writing machine (e-beam) is evaluated. The method is based on a multiple-exposure method that involves superimposing e-beam exposure patterns with different main-field and sub-field sizes. The main-field and sub-field sizes are varied to eliminate main-field and sub-field stitching on the chip. The stitched areas are overwritten with other exposures and blurred out. When applied to the preparation of X-ray masks for device fabrication, this method improves the e-beam writing pattern placement accuracy of the X-ray masks to 0.04 µm (3σ). Both the field stitching accuracy and the uniformity of the exposed pattern width are also improved to 0.02 µm (3σ).

EB-X3: New electron-beam x-ray mask writer

The EB-X3 is an electron-beam mask writer designed to pattern x-ray masks with minimum feature sizes of 100 nm and below with good beam placement accuracy. The target patterning resolution is 50 nm, and a beam acceleration voltage of 100 kV is used to achieve it. This resolution has already been demonstrated using the previous version of the writer, the EB-X2 (developed last year), which also employs an acceleration voltage of 100 kV. On the other hand, the target placement accuracy for the EB-X3 is less than 10 nm, which is more than three times smaller than that for the EB-X2. To improve the placement accuracy, the resolution of the laser measurement system was changed from λ/128 (=5 nm) to λ/1024 (=0.6 nm), and the address unit was set to 1 nm, which yields a reproducibility of mark detection of 4 nm (3σ) and a calibration accuracy for deflection distortion of 5 nm(|mean|+3σ). In addition, the laser measurement system monitors five axes (X, Y, yaw, and pitch [X,Y]) and the results are fed back to posit...

Improving X-Ray Mask Pattern Placement Accuracy by Correcting Process Distortion in Electron Beam Writing

A new correction method named PAT (previous analysis of distortion and transformation of coordinates) has been developed for improving the pattern placement accuracy of completed X-ray masks. PAT compensates for total-process distortion in the electron beam writing step. In PAT, overall pattern position shifts are estimated using a send-ahead mask in advance of making working masks. Then the pattern position shifts obtained from the send-ahead mask are compensated for during electron beam writing. In an experiment, the pattern placement error of a completed X-ray mask represented by 3σ was reduced to below 0.07 µ m. This confirmed that highly accurate X-ray masks could be fabricated using PAT.

Fabrication of 0.2 μm large scale integrated circuits using synchrotron radiation x‐ray lithography

In the last few years, we have made dramatic improvements in the key components of synchrotron radiation x‐ray lithography including a high‐brightness and compact synchrotron radiation source, high‐accuracy steppers, precise masks, and highly sensitive chemically amplified resists. These advances have been used in the test fabrication of complementary metal–oxide–semiconductor large scale integrated circuits using separation by implanted oxygen technology in the 0.2 μm regime. Synchrotron radiation lithography was used at the active, gate, contact, and four‐level metallization levels or at the gate level only. Fabricated gate array large scale integrated circuits, including a 48×48 bit multiplier, have excellent and fully functional characteristics. This article describes lithographic performance under device fabrication conditions, demonstrating an excellent critical dimension control of 0.02 μm (3σ), an overall overlay better than 0.15 μm (3σ) (including metallization levels), and a throughput of 12 waf...

Mask Contamination Induced by X-Ray Exposure

To investigate the growth of mask contamination induced by X-ray exposure, masks are exposed to X-rays using an X-ray stepper. Independent of the exposure conditions, and in the presence or absence of resist coating, the main component of the contaminant is the salt (NH4)2SO4. On some resist-coated wafers, the contaminant growth is governed by the volume of sulfuric materials which evaporate during exposure. Contamination growth can be suppressed by applying a top coat to the resist, as using a resist that does not evaporate as a sulfuric out-gas. Gases were irradiated by X-rays in order to examine the change in environmental pollutants. In humid air, NO, NO2 and NH3 form rapidly up on X-ray irradiation. SO2 also increases up on X-ray irradiation, and changes into H2SO4. H2SO4 easily combines with NH3 which is produced by X-ray irradiation in humid air, and turns into the salt (NH4)2SO4. Mask coating was introduced to suppress the contaminant growth. The mask coated with TiO2 layer does not show contaminant growth, showing a drastic curtailment of contamination by the photocatalysis effect.