Absorber Pattern Research Articles

Electroplated Reflection Masks for Soft X-Ray Projection Lithography

A new process for fabricating reflection masks for soft X-ray projection lithography (SXPL) using an electroplating method was developed. Resist patterns were formed on molybdenum/silicon (Mo/Si) multilayers with the Mo layer on top, and nickel (Ni) absorber patterns were then electroplated on the multilayers using the top Mo layer as the electrode in nickel sulfamate solution. Subsequently, the resist patterns were removed. Using the reflection masks fabricated by the above process, SXPL was performed using 1/32 reduction Schwarzschild optics tuned to the wavelength of 13 nm and the synchrotron radiation source at SORTEC. Line-and-space patterns down to 0.1 µ m were printed in polymethylmethacrylate (PMMA) resist.

Ultra-Fine Pattern Fabrication by Synchrotron Radiation X-Ray Lithography Using a Shifter-Edge Type Phase-Shifting Mask

This paper reports the feasibility of ultra-fine pattern fabrication using an X-ray phase-shifting mask. Using shifter-edge type phase-shifting masks, less-than-100-nm patterns can be printed without preparing thick precise X-ray absorber patterns. However, it is still important that the shifter not be too thick, because thick shifter patterns are also difficult to fabricate. Shifter thickness is decided by the refractive index n of the shifter and a small n value permits a thin shifter. However, materials with a small n value have large absorption, so the latitude in the shifter transmittance and phase-shifting angle was analyzed. It is clarified that the degradation of shifter transparency is not a serious problem, and the tolerance of phase-shifting angle is very large. It is also demonstrated that SR X-rays with a continuous spectrum can be applied. Patterns almost as fine as 50 nm have been printed.

Resolution Enhancement of Hole Patterns in Sychrotron Radiation Lithography

This paper analyzes the influence of Fresnel diffraction on the patterning characteristics of isolated hole patterns in synchrotron radiation lithography by comparing Fresnel diffraction simulation extended to 2-dimensional patterns and pattern replication experiments with mask contrast as a parameter. It is possible to replicate hole patterns down to 0.1 µm at a large proximity gap of 30 µm by keeping the mask contrast at 2.5-7. However, the exposure latitude for fine patterns below 0.1 µm diminishes abruptly. It also becomes clear that the phase-shifting mask which we previously proposed offers the possibility of replicating ultra-fine hole patterns (<0.1 µm) with a large exposure latitude at the 30-µm proximity gap. Moreover, by using an X-ray mask with a circular absorber pattern instead of the conventional square pattern, it should be possible to solve the problem of pattern deformation and improve the exposure latitude.

Low stress and optically transparent chromium oxide layer for x-ray mask making

This article describes the properties of dc-sputtered chromium oxide layers such as internal stress, density, growth rate, and microstructure. Besides these mechanical properties, the composition of the layers and the optical properties such as transmissivity and refractive index were also investigated. Transparent, low-stress, and smooth chromium oxide layers were obtained by optimizing the sputtering conditions: gas pressure, O2 content in the working gas, dc-power density, and annealing temperature. The most critical parameter is the O2 content in the working gas. Using these optimized chromium oxide layers in the mask fabrication process as a masking, as well as an etch stop layer for the W etching, the etching behavior during absorber patterning in a subtractive x-ray mask production process was improved.

Electron cyclotron resonance ion stream etching of tantalum for x-ray mask absorber

Electron cyclotron resonance ion stream etching of Ta film was investigated for preparing x-ray mask absorber patterns. Ta is etched by the system at a high rate and with high selectivity. Using Cl2 as etching gas, the etch rate decreases rapidly with decreasing pattern width below 0.5 μm and large undercutting is observed. The problems are reduced by adding Ar or O2 gas to the Cl2. Etching with a mixture of Cl2 and O2 produces highly accurate Ta absorber patterns for x-ray masks. The pattern width dependence of the etch rate and the undercutting were simulated with a model that takes account of the angular distribution of active species incident on the sample. The experimental results agree well with those calculated assuming that the incidence angles are distributed between −36° and 36°. The addition of O2 or Ar enhances ion assisted etching.

Microfabrication of X-Ray Absorber W Utilizing Al2O3 as an Etching Mask

Microfabrication of W X-ray absorber for X-ray masks was studied. The absorber patterns were fabricated by etching processes using double layer resists which consist of a top resist and second Al2O3 layers. The thickness of the top layer was 0.3 µm to relieve the effect of both forward and backward scattering in electron beam writing. The etching gases for W patterning were SF6 and CHF3. The selectivity between W and Al2O3 was more than 20 by properly adjusting the gas mixture ratio, pressure and etching power. The required thickness of the second Al2O3 layer was only 0.05 µm which could easily be etched using BCl3 gas. Nearly vertical 0.2 µm lines and spaces were fabricated for 0.5 µm thick W absorber.

Stress reduction of gold absorber patterns on x-ray masks

A resonance frequency measurement technique has been used to measure the stress at temperature of plated gold films on x-ray mask membranes over the temperature range of 90–525 K. Room temperature tensile stress increases are observed after above room temperature anneals. Stress relaxations are measured after liquid nitrogen cools. The tensile stress increases and the stress relaxations depend upon the microstructure of the plated films. Focused ion beam imaging shows that grain growth is the dominant mechanism for the tensile stress increases observed for fine grained as-plated gold films.

Fabrication of sub-100nm linewidth x-ray masks and replication using synchrotron radiation at super-ACO

A high resolution x-ray mask technology has been developed based on amorphous silicon carbide covered with stress minimized W and Au patterns. SiC blanks are fabricated from silicon wafers coated with low stress stoechiometric SiC deposited by rf low frequency (110 kHz) glow discharge decomposition of silane and methane (PECVD). Mask absorber pattern fabrication is performed by electron- beam lithography with a single resist layer process. These masks have been tested in the lithography station implemented by our laboratory at Super-ACO using synchrotron radiation in the spectral range 0.5–1.5 nm. Sub-100 nm replication patterns obtained by contact printing in PMMA resist are presented. Results of the evaluation of the x-ray resists SAL 601 and SAL 603 used with the Karl Suss stepper XRS 200 are also described.

Smoothing Roughness of SiC Membrane Surface for X-Ray Masks

We have obtained a smoother surface of SiC membranes than that of as-deposited SiC using etch-back and polishing methods, and investigated the influence of surface roughness of SiC films on internal stress and film structure of X-ray absorber films and optical transmittance of SiC membrane. Surface roughness for the as-deposited SiC surface is 15 nm (Ra), whereas that for etch-backed SiC is 6.9 nm (Ra) and that for polished SiC is 4.3 nm (Ra). Optical transmittance of as-deposited SiC membrane is 48% (λ=633 nm) and increases by a few percent with decrease in the Ra value. Internal stress of sputtered Ta tends to show gentler change with increasing surface roughness of SiC film. Film structure of Ta film is largely influenced by the surface roughness of SiC films at low Ar pressure. Sub-half micron absorber patterns with vertical walls, however, have been obtained even on rough surface SiC.

X-ray masks with large-area 100nm-period gratings for quantum-effect device applications

We report the fabrication and replication of x-ray masks with large-area (∼50 mm2) 100nm-period gratings. Achromatic holographic lithography was used to generate 100nm-period surface gratings in PMMA resist. Subsequent dry processing formed high-aspect-ratio grating lines down to the base of the resist. The x-ray absorber was defined by either: (i) reactive-ion etching low-stress sputter-deposited tungsten, using the resist lines as an etch mask, or (ii) electroplating gold using the resist lines as a mold. The absorber patterns were fabricated on silicon substrates coated with 1 μm-thick polyimide as the membrane material. X-ray masks were formed by etching away the silicon substrate, leaving the x-ray absorber pattern supported by the polyimide membrane. Results of x-ray exposures of PMMA, using the CK line (λ=4.5 nm), are presented.

Novel process using x-ray lithography for T-shaped gate patterns

A fabrication process capable of forming T‐shaped gates useful for high‐performance GaAs field‐effect transistor devices by exposing a single‐layer resist with x rays is proposed in this work. The x‐ray mask used in this method has absorber patterns with T‐shaped cross sections, which are made up of two absorber layers. This method makes good use of the fact that the x‐ray transmittance of materials such as resists and mask absorbers is higher than the ultraviolet/visible light transmittance used in photolithography. In order to confirm the practicality of this method, T‐shaped patterns were formed experimentally using the positive resist XPB and an exposure system with an electron‐excited x‐ray tube (Pd target). The x‐ray mask was fabricated by patterning two layers of W‐Ti alloy absorber on a BN/polyimide membrane using an electron direct writing technology. It is demonstrated that the T‐shaped pattern can be formed easily by using the proposed process.

Investigation of process latitude for quality improvement in x-ray lithography mask fabrication

Statistical methods are applied to improve the quality of fabrication steps in x-ray mask production. A second-order central composite design was used to optimize the bonding procedure in the mask blank preparation. The computer-controlled bonding process delivers 4-in. mask blanks with a flatness in the range of 2–4 μm. Factorial design as an optimizing method was applied to improve the quality of gold absorber patterns. The dependence of surface roughness of pulse plated structures on switch-on time, duty cycle, and current was investigated. Using optimized parameters, an improvement of lateral homogeneity and a decrease of grain size was achieved.

Fabrication and testing of 0.1-μm-linewidth microgap x-ray masks

A technology has been developed for the printing of 0.1-μm-linewidth patterns using a ‘‘microgap,’’ out-of-contact scheme for x-ray nanolithography (as opposed to zero-gap electrostatic contact). Mask-to-wafer gaps of ∼5 μm are maintained by the use of gap-setting aluminum studs fabricated onto the front surface of the mask mesa rim. X-ray mask blanks are fabricated from silicon wafers coated with low-stress, silicon-rich low-pressure chemical vapor deposition SiNx. The resulting 1–2-μm-thick 6×108 dyn/cm2 stress membranes exhibit extreme strength. A novel aluminum-stencil etching procedure—which includes three CF4 RIE and two KOH etching steps—is used to define the mask membrane and ‘‘mesa’’ structure. Mask absorber pattern fabrication is performed by focused-ion-beam lithography (FIBL) and gold electroplating. We present details of the mask fabrication procedure and the results of testing these masks with x rays from a laboratory CuL (λ=1.34 nm) electron bombardment source and a commercial pulsed laser-plasma x-ray source. This marks the first time that FIBL-generated masks have been used with a pulsed laser-plasma source, and also the finest lines that have been printed using a laser plasma. The results of the evaluation of the x-ray resists PMMA, RAY-PF, RAY-PN, and SAL-601 are presented.

Fabrication of X-Ray Mask Using W-CVD for Forming Absorber Pattern

Tungsten chemical vapor deposition (W-CVD) using WF6 and H2 as reactants was applied to forming absorbers of X-ray masks for synchrotron radiation (SR) lithography. For this purpose, the properties of deposited W (CVD-W), such as stress, density and thermal stability, were examined. The stress can be minimized reproducibly to less than 1×108 dyn/cm2 by controlling the flow rate of WF6 at various substrate temperatures. This W film was thermally stable up to 200°C, and the density was 18.5 g/cm3. From these results, CVD-W was found to be suitable as an absorber material. For the formation of absorber patterns, filling SiO2 grooves with stress-reduced CVD-W was examined. The 0.2-µm-wide grooves were filled with CVD-W and absorber patterns were formed by etch-back. The X-ray mask was successfully fabricated by this developed process.

Effects of Photo- and Auger Electron Scattering on Resolution and Linewidth Control in SR Lithography

The effects of photo- and Auger electron scattering on resolution and linewidth control in X-ray lithography are quantitatively evaluated employing resist patterns replicated using a novel contact replication method. In this method, the effects of penumbral shadow and Fresnel diffraction can be neglected because exposure, development and SEM observation are carried out keeping the absorber pattern on the resist. The depth of the undercut of the resist pattern from the absorber pattern edge, which means the range of electron scattering in the lateral direction, increases from zero to the Bethe range as a function of exposure dose, and reaches 0.16 µm in SR lithography with spectra ranging from 0.2 to 1.4 nm. It is shown that the maximum range of photo- and Auger electron scattering does not determine the resolution directly, but rather affects the linewidth control.

X-ray mask technology: Precise etching of sub-half-micron tungsten absorber features

A RIE technique for the fabrication of sub-0.5 μm W absorber patterns has been developed based on a SF 6 /CH 3 /Ar gas system. Upon extensive optimization of particularly CHF 3 content and bias voltage precise pattern with vertical profiles can be etched in 0.8 μm thick W layers on top of a rigid SiC-membrane. Features down to 0.2 μm dimensions have been fabricated with a linewidth homogenity of less than 50 nm. Due to proximity corrected e-beam writing and almost stress free W layers, a pattern placement accuracy of better than 100 nm (3σ) has been obtained for different absorber coverages (Step field 25x25 mm 2 ).

Infrared-measurement of X-ray mask heating during sr-lithography

For the fabrication of ULSI circuits, it is necessary to master sub-half micron design rules, which include an efficient patterning procedure and critical dimensions related overlay requirements. In addition to the alignment accuracy, the overlay budget is determined by the pattern placement precision on X-ray masks. There are two kinds of absorber pattern displacements — those having static and others having dynamic causes. Static displacements are produced by e-beam patterning or during mask fabrication; dynamic distortions can be induced for example by temperature variations. This paper presents experimental results on the temperature rise of X-ray masks in synchrotron radiation lithography. We used an in-situ thermographic system to determine the temperature distribution during the exposure.

An X-ray mask using SiC membrane deposited by ECR plasma CVD

Abstract The ECR plasma CVD system has been improved for depositing SiC films for X-ray mask membranes. This system makes it possible to deposit SiC films with small tensile stress, high optical transparency and high strength at temperatures below 800°C. The SiC film is available for an X-ray mask membrane with 0.1- μ m Ta absorber patterns. Besides the thickness uniformity, the stress uniformity of the membrane is necessary in fabricating a highly accurate X-ray mask.

Pulse plating of quarter micron gold patterns on silicon X-ray masks

In recent X-ray mask fabrication processes, electroplated gold is commonly used for absorber pattern formation. Up to now good results have been reported in the sub micron range down to 0.5 μm by means of DC plating. In order to meet requirements for feature sizes in the quarter micron range, a pulse plating procedure was developed and applied for X-ray mask fabrication. The pulse plated gold shows an improved surface roughness compared to DC plated. Additional results concerning the variation of plating parameters and the properties of deposited gold layers are presented.

Procedure and results of mask fabrication via X-ray lithography

A copying procedure for reproduction of e-beam written X-ray masks has been developed and applied, for the fabrication of optically transparent stepper masks. This process uses normal X-ray lithography to expose an X-ray absorber pattern onto a resist-covered mask blank. In order to achieve high quality X-ray copies, it is necessary to use a copying instrument which avoids mask distortion or pattern displacements due to fixturing. Furthermore, efficient heat dissipation is required to prevent a rise in the temperature of the mask during X-ray exposure. Taking these conditions into account, X-ray copying is a significant mask fabrication technique providing special additional advantages.