Hiromasa Yamanashi

Scanning electron microscope

Provided is a scanning electron microscope equipped with a high-speed and high-precision astigmatism measuring means to be used when both astigmatism generated by an electron-beam column and astigmatism generated from the surroundings of a measuring sample exist. This scanning electron microscope is characterized in controlling an astigmatism corrector (201) with high-speed and high-precision, to correct the astigmatism, by using both a method of obtaining the astigmatism from the qualities of two-dimensional images to be acquired upon changing the intensity of the astigmatism corrector (201), and a method of measuring the astigmatism from the change in the position displacement of an electron beam that occurs when the electron beam is tilted using a tilt deflector (202).

Study of Measurement Condition Optimization in Critical Dimension-Scanning Electron Microscope

The critical-dimension scanning electron microscope (CD-SEM) is an essential tool for semiconductor fabrication process control because of its high resolution and high precision. However, in ArF lithography, the CD of resist changes during CD-SEM measurement due to shrinkage caused by the electron beam irradiation. This shrinkage can be reduced by measurement parameters; however, there is a trade-off relationship between shrinkage and precision. Thus, measuring the CD of an ArF resist pattern precisely with small shrinkage is difficult. The authors propose an optimization method using the Taguchi method. Four measurement parameters were chosen as control factors for an L18 orthogonal array: probe current, acceleration voltage, horizontal length of field-of-view, and number of image acquisitions. As a result, high prediction accuracy was obtained that is smaller than 0.2 nm for shrinkage and 0.1 nm for precision. Moreover, an optimum measurement condition that achieves 0.28 nm shrinkage and 0.37 nm precision was also obtained. Thus, the proposed method was demonstrated as a promising method to optimize CD measurement parameters.

Influence of glass substrate surface roughness on extreme ultraviolet reflectivity of Mo∕Si multilayer

The influence of the surface roughness of a glass substrate on the extreme ultraviolet (EUV) reflectivity of Mo∕Si multilayer deposited on it by ASET’s helicon sputtering system was investigated. Deposition by helicon sputtering was found to produce a smoothing effect on the multilayer. That is, surface roughness with a spatial period below 100nm is smoothed out, but that with a spatial period longer than 100nm is not. The result is that the rms roughness of a Mo∕Si multilayer is slightly smaller than that of the substrate. The EUV reflectivity of the multilayer improved as the rms roughness decreased down to 0.30nm, and than leveled off at about 65% below that value, indicating that the EUV reflectivity is limited by some factor(s) other than surface roughness. However, a relatively high EUV reflectivity of 65% was obtained, even for an rms roughness of about 0.30nm.

Tantalum nitride films for the absorber material of refractive-type EUVL mask

Tantalum nitride (TaxN) films were evaluated for use as the absorber material of masks for extreme ultraviolet lithography (EUVL). TaxN films deposited by DC sputtering using an Ar+N2 gas mixture had a low stress of less than 300 MPa, an amorphous-like structure, and a low deep ultraviolet (DUV) reflectivity. This film provides a DUV contrast of 30% with respect to the Mo/Si multilayer whose top is on Si layer. A TaxN film deposited using a Xe+N2 gas mixture was found to be better in the following ways: the stress was below 100 MPa, the change in stress was below 30 MPa, and the density was more than 1 g/cm3 higher. Furthermore, treating the surface of TaxN film with O2 plasma or sputtering a TaxO film on it using an Ar+O2 gas mixture improved the DUV contrast because the resulting surface has a lower DUV reflectivity than TaxN film. These results indicate that TaxN film is one of the most suitable materials for the absorber of EUVL masks.

RESIST PATTERN FLUCTUATION LIMITS IN EXTREME-ULTRAVIOLET LITHOGRAPHY

Extreme ultraviolet (EUV) projection lithography has been proposed to achieve features as small as L=180 nm to 70 nm for 1 G to 16 G DRAMs. Application will require high‐sensitivity resists and pattern fluctuation control to less than 10% of nominal linewidth. To evaluate low‐dose, resist material, and resist process dependent resist roughness limits in EUV lithography, a roughness model originally by Neureuther and Wilson is extended, and a new model for chemical amplification resists is presented and applied to EUV lithography. Analyses of molecular scale simulation and EUV exposures of novolac negative chemical amplification resists complete the study. For 13 nm exposure wavelengths, 180 nm lithography with positive chain‐scission resists requires at least 0.69 mJ/cm2, which scales to 54.3 mJ/cm2 for 70 nm features, accounting for both intrinsic resist polymer roughness and absorption in 100 nm PMMA. At 4.5 nm exposure, the dose minima are 15.9 mJ/cm2 and 1254 mJ/cm2, for 1 G and 16 G respectively. Nov...

Performance and quality analysis of Mo-Si multilayers deposited by ion beam sputtering and magnetron sputtering

Ion beam sputtering and magnetron sputtering were used to grow Mo-Si multilayer structures to investigate which is more suitable for the fabrication of mask blanks for extreme ultraviolet (EUV) lithography. For ion beam sputtering, the difference between using Ar and Xe as the sputtering gas was also examined. For ion beam sputtering, the peak EUV reflectivity of 40 Mo-Si bilayers was measured to be about 62% at wavelengths in the range of 12-15 nm; while for magnetron sputtering, the value was 65%. A transmission electron microscopy analysis of multilayers deposited by ion beam sputtering revealed an interface layer between the two materials: It had a thickness of 1.5 ± 0.2 nm when Mo was deposited on Si, and a thickness of 0.7 ± 0.2 nm when Si was deposited on Mo. These interface layers were 30-50% thicker than those formed during magnetron sputtering. The mechanism by which interface layers form is discussed based on an ion implantation model.

Smooth low-stress sputtered tantalum and tantalum alloy films for the absorber material of reflective-type EUVL

Tantalum (Ta) and Ta-alloy films were evaluated for use as the absorber material of masks for extreme ultraviolet lithography (EUVL). It was found that Ta film with a stress below 100 MPa, a surface roughness of less than 1 nm rms, a film density of over 14 g/cm3, and a deposition rate of more than 50 nm/min could be obtained by DC sputtering with Ar gas. Experiments on delineating mask patterns in this film by using dry etching revealed that 250-nm line-and-space patterns could be formed. The alloys evaluated were TaGe and TaN. These films were found to have some better properties than Ta film, for example, less stress, a smaller change in stress, and a smoother surface. This is confirmed to be due to the fact that the alloy films are amorphous. Of particular note is that TaN film has a lower deep ultraviolet (DUV) reflectivity than either Ta or TaGe, thus providing higher contrast between the underlying multilayer and the absorber patterns of an EUVL mask during DUV inspection. However, TaN has a lower density than the other two films. So, our current results indicate that using Ta or TaGe for the bulk absorber material and covering that with a thin layer of TaN is a promising way to obtain the film properties required for EUVL mask patterns, including film density and DUV inspection capability.

Ring-Field Extreme Ultraviolet Exposure System Using Aspherical Mirrors

We have designed and started the fabrication of 3-mirror ring-field projection optics for extreme ultraviolet lithography (EUVL) and an experimental exposure system using the projection optics, which enable a large-field (30 mm×20 mm), high-resolution (<100 nm) exposure. EUV exposure experiments will be performed using the EUVL beam line to be constructed at the new synchrotron ring called New Subaru, which is now under construction in Hyogo Prefecture. In this paper the details of the EUV exposure system will be described.

Study of mask process development for EUVL

EUVL mask process of absorber layer dry etching and defect repair were evaluated. TaGeN and Cr were selected for absorber layer and buffer layer, respectively. These absorber layer and buffer layer were coated on 6025 Qz substrate. Two dry etching processes were evaluated for absorber layer etching. One is CF4 gas process and the other is Cl2 gas process. CD uniformity, selectivity, cross section profile and resist damage were evaluated for each process. FIB-GAE and AFM machining were applied for absorber layer repair test. XeF2 gas was used for FIB-GAE. Good selectivity between absorber layer and buffer layer was obtained using XeF2 gas. However, XeF2 gas causes side etching of TaGeN layer. AFM machining repair technique was demonstrated for TaGeN layer repair.

Attenuated phase-shift mask for line patterns in EUV lithography

The printability of isolated line patterns was investigated through simulations. An attenuated phase-shift mask (att-PSM) in combination with annular illumination has the capability of printing 18-nm-long line patterns. The pattern edge contrast of aerial images and the focus latitude become larger as the attenuated reflectance increases in the evaluation region from 4.6 to 14.7%. An att-PSM with annular illumination has another advantage in that it provides a very small difference in line width due to the direction of off-axis incident light. Furthermore, varying the phase shift from 171 to 191° does not produce any significant shift in the focal point on a wafer.