Takashi Soga

NEW DRY SURFACE-IMAGING PROCESS FOR X-RAY LITHOGRAPHY

Ultra-LSIs (ULSIs) now require innovative microfabrication processes to achieve gigabit-scale integration. In order to meet this requirement, a new surface-imaging process involving the dry development of polysilane resist is investigated for use in X-ray lithography from the viewpoints of achieving a dry development process and of improving resolution. Poly(cyclohexylmethylsilane) (PCHMS) is used for the dry-developing surface-imaging layer. We confirm that PCHMS has positive-tone characteristics by ultraviolet (UV) absorption spectroscopy and photon-stimulated ion desorption (PSD) measurements. PSD analyses also clarify that chain side groups of polysilane are desorbed prior to Si–Si backbone desorption, and that heating drastically improves dry-development sensitivity. PCHMS patterns replicated by dry-development can be transferred to the bottom layer resist by O2 reactive-ion etching and, as a result, high-aspect-ratio resist patterns can be replicated. The possibility of applying this process to X-ray projection lithography is also discussed.

Prevention of Resist Pattern Collapse by Resist Heating during Rinsing

A novel resist hardening method is discussed for preventing resist pattern collapse (bending and breaking). In this method, a cross-link-type chemically amplified resist is heated during rinsing. This heating increases the cross-link ratio of the resist, thus hardening the resist pattern before drying. Using this method, the critical aspect ratio of 0.15 μm line and space patterns is improved from 5 to 6.7

Self‐Developing Characteristics of Si Containing Polymers and Their Application to X‐Ray Lithography

The self-development characteristics of Si containing polymers were investigated with the objective of applying these polymers to the surface-imaging process in x-ray lithography. Polymers containing Si atoms in their backbones or substitutents were studied in detail ; UV and Fourier transform infared spectra were used to study their reaction to soft x-rays. The polymers exhibited postive-tone characteristics when exposed to x-rays. Their self-development sensitivity and oxygen reactive ion etching (O 2 -RSB) resistance were measured to estimate the feasibility of their use in lithography. Polysilanes and polystyrene showed low self-development sensitivity while polymethacrylates showed high sensitivity. All of these polymers have O 2 -RIE resistance high enough for application as surface-imaging resists. The films remaining after self-development were characterized by estimating their durability against solvents, their thermal desorption spectra, and their x-ray photoelectron spectra. A polymethacrylate derivative was found to be the most promising of the polymers; it was applied to pattern replication using dry development. The results show that Si containing polymers are fundamentally feasible for the postive-tone dry processes.

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...

Stress‐controlled x‐ray mask absorber using pulse‐current gold plating

The application of pulse‐current gold plating has been studied in the fabrication of x‐ray mask patterns to attain uniform thickness and low stress. Pulse‐current plating with low average current density (iav) has proven effective in improving uniform plating thickness over a wide range of pattern sizes. This result can be explained through the duplex‐diffusion layer model. Stress in the gold plated by low iav becomes compressive. However, this stress can be eliminated by annealing. This annealing effect is attributable to the contraction in volume of plated gold due to the thermal desorption of hydrogen atoms.

Minimization of X-Ray Mask Distortion by Two-Dimensional Finite Element Method Simulation

X-ray mask distortion caused by absorber stress is quantitatively analyzed by using two-dimensional simulation. Simulated results successfully predict the mask pattern distortion in practical mask structures. A square mask window is better than a circular one because the distortion for square windows can be minimized by reducing the pattern length. In addition, it is found that the maximum distortion is virtually the same regardless of the number of LSI chips within the square window mask.

Performance improvement in e-beam reticle writer HL-800M

An advanced e-beam mask-writing system HL-800M has been developed for the 0.25-micrometer rule-devices. To meet the design-rule, the targets of this system specifications are critical dimension (CD) control of 30 nm, positioning accuracy of 40 nm, and throughput over 0.5 plate per hour. To achieve CD control, we judged that it was inevitable to increase the acceleration voltage up to 50 kV for patterns smaller than 2 micrometer. However, for patterns larger than 5 micrometer, the e-beam proximity-effect causes the pattern-width linearity to be worse. To achieve the sufficient linearity, proximity correction on the hardware module of the systems was performed. This hardware module executes proximity effect correction for each patterns over the area on the plate, so that total throughput was improved compared with that of the correction by software. Besides, a noise cancellation module was introduced to reduce the errors in the e-beam shot positions. This module detects the vibration noise caused by with the power-supply frequency and feeds the correction signal back to the e-beam deflectors. For positioning accuracy, in addition to the mirror correction using hardware for the stage interferometer, a new positioning-correction function depending on the coordinates of the system was developed. In the results of the exposure evaluations, CD uniformity on a 6025 plate showed width-deviations of 3 sigma were 31 nm (X) and 18 nm (Y). Pattern-width linearities for various kinds of patterns were within plus or minus 50 nm. Furthermore, the noise cancellation module was made the amplitude of the e-beam vibration reduced from 33 nm to less than 8 nm. For positioning accuracy, evaluation patterns measured by the LMS2020 (Leica) showed sufficient results for our target. For throughput, the average of the writing time per 6-inch plate for ten patterns is shorter than our targeted throughput with a dosage of 4 (mu) C/cm2. The HL-800M system is capable of producing reticles for 0.25-micrometer design-rule.

Negative tone dry development of Si-containing resists by laser ablation

A new negative tone dry development process is proposed. This process consists of area selective oxidation of a Si-containing layer with soft X-ray exposure and removal of the unoxidized areas by KrF laser ablation. Polysilane and Si-containing polystyrene have been investigated as resist polymers. Laser ablation rates of the polymers have been measured. The unexposed areas of a polysilane film have a high ablation rate while exposed areas cannot be ablated. Measurement done in air, O2-, and N2-gas atmospheres show that oxidation of Si atoms suppresses ablation. A bilayer resist using a polysilane film as a surface-imaging layer has been patterned using this dry development process and 0.4-µ m patterns have been replicated. This dry process has the potential to be used in sub-half micrometer lithography.

Reflection masks for soft x-ray projection lithography

A multilayer reflection mask is fabricated for soft x-ray projection lithography at a wavelength of 13 nm. A Mo/Si multilayer is deposited using magnetron sputtering to obtain high reflectivity at near normal incidence. Reactive ion etching in SF6 is applied to form a fine W absorber pattern with a thin SiO2 etch-stop layer. Observation of the resulting pattern profile with a scanning electron microscope shows a smooth reflective surface. Reflectivity measurement using a large- reflective-area sample indicates that the patterning process causes little damage to the multilayer. Projection imaging using a 20:1 Schwarzschild optic confirms that a 0.07-micrometers line-and-space pattern can be printed.

0.1-micrometer scaling by 1:1 synchrotron radiation lithography

Abstract The dimensional accuracy of an X-ray mask and resolution capability of 1:1 SR (synchrotron radiation) lithography for 0.1-μm scaling are studied. The mask-to-mask overlay error is minimized below the measurement accuracy (0.04 μm, 3σ) by controlling the stress of the SiN membrane and the W absorber. 1:1 SR proximity printing is capable of 0.1-μm resolution by adjusting the exposing SR wavelength just over the Si-K edge and reducing the mask-wafer gap to 10 μm.