Phase Shift Mask Research Articles

Edge effects characterization of phase shift mask

The demand for steadily decreasing dimensions in semiconductor devices is driving the need for increased resolution in optical lithography. The use of phase shift masks (PSMs) is among such resolution enhancement techniques. PSM are well known to show prominent diffraction effects, which cannot be described by the assumption of an infinitely thin mask (Kirchhoff approach) that is used in many commercial photolithography simulators. A correct prediction of sidelobe printability, process window and linearity of an OPC mask requires a rigorous application of diffraction theory. Optical lithography simulation employing a time-domain finite-difference (TDFD) algorithm (TEMPEST) has been used effectively to study the problem of aerial image intensity imbalance through focus with alternating phase shift masks (altPSMs). Using Geometrical Theory of Diffraction (GTD), and the solutions to canonical problems, we obtained a relationship between mask edge and disturbance in an image space. The main objective to develop useful formulations that can be readily applied to diffraction in mask technology. Rigorous analysis of diffraction effect on altPSMs using the GTD approach is discussed and results show that the effect of shifter edge angle is equivalent to that of shifter width.

Alternating phase-shifting mask design for low aberration sensitivity

The aberration present in the lenses of exposure systems can cause placement errors to the images produced by alternating phase-shifting masks (PSMs). In reality, when the aberration signature varies from one lens to another, the magnitude of placement error also varies. It remains a question of how the alternating PSM should be designed, so that the image placement error, on average, can be minimized. To achieve this goal, we are interested in optimizing the phase width of an alternating PSM with a fixed critical dimension (CD). The constraint of the optimization is the mean of root mean square (rms) aberrations for a set of interest of exposure systems. To begin the analysis, the image placement error is expressed as a function of illumination, mask spectrum, and wave aberration. A Monte Carlo technique is then applied to produce random samples of wave aberration and image placement error. This analysis shows that a global minimum of mean image placement error is likely to occur at phase widths between 0.2[/numerical aperture (NA)] and 0.4(/NA). This is further confirmed by analytically considering the expected value of the square of the image placement error. The methodology of finding the optimal phase width is applicable to the design of all alternating PSMs.

Comparison of tilting and piston mirror elements for 65nm node spatial light modulator optical maskless lithography

Spatial light modulator (SLM) maskless lithography, an optical technology that replaces photomasks with an adjustable array of microelectromechanical mirrors can potentially complement conventional, mask-based, lithography. The usefulness of this technology depends on whether it can print features at least as well as conventional tools and can keep pace with the International Technology Roadmap for Semiconductors. In particular, any SLM-based tool developed now must be capable of printing 50nm or smaller gates and 130nm pitch lines and spaces using 193nm light as is being planned for the 65nm half-pitch node. SLM mirrors may be designed to tilt or shift in depth (piston). Aerial image simulations presented here show that 65nm node features formed by piston mirrors have the same process latitude as tilt mirrors with a numerical aperture (NA) of 1.1. In addition, piston mirrors exhibit the added benefit that they may be arranged to mimic alternating aperture phase shift photomasks. Isolated and dense features of 65nm node dimensions can be printed with a reduced NA of 0.90. This form of phase shifting may be required at the 65nm ITRS node both for conventional and maskless lithography applications.

A FinFET and Tri-gate MOSFET's channel structure patterning and its influence on the device performance

A simple and manufacturing worthy method to pattern a very thin channel body of the FinFET/Tri-gate MOSFET is presented. A channel body as thin as 40 nm using the phase-shift mask (PSM) technique with deep UV 248 nm lithography on UV 210 Shipley resist has been achieved, while having an inherently wider Source/Drain (S/D) extension region. The width of extension region was optimized using the Prolitho.3D lithography simulator for the exposure dose and reticle bias conditions. The device simulation of the wider S/D extension region performed using three-dimensional (3-D) Davinci device simulator shows the significant reduction in the S/D parasitic resistance. Furthermore, these patterns on photoresists like UV 210 provides the basis for the easy integration to the mainstream CMOS technology, hence the potential for manufacturability with higher device performance.

The Magnitude of Potential Exposure-Tool-Induced Critical Dimension and Overlay Errors in Double Dipole Lithography for the 65-nm and 45-nm Technology Nodes

Double dipole lithography (DDL, DDL is a trademark of ASML Masktools.) is a viable imaging solution for the 65-nm and 45-nm technology nodes, when using ArF exposure tools. By taking advantage of the extreme off-axis illumination of the dipole, the demonstrated, small critical dimension (CD) can be resolved with a good process window. In this case k1 will be 0.31 when applying formula k1 = (minimum half pitch) × (wavelength, λ)/(numerical aperture, NA), the Rayleigh's resolution equation with minimum half pitch of 80 nm as well as wavelength and NA of 193 nm and 0.75, respectively. The detailed CD measurement data and process window analysis can be seen. The ability of the dipole to resolve this CD, however, applies only to structures that are perpendicular to the orientation of the dipole; i.e., the x-dipole (or horizontal dipole) resolves small, vertical lines and spaces [S. Hsu, N. Corcoran, M. Eurlings, W. Knose, T. Laidig, K. E. Wampler, S. Roy, X. Shi, M. Hsu, J. F. Chen, J. Finders, R. J. Socha and M. Dusa: SPIE 4691 (2002), 476]. The use of the pattern decomposition [S. Hsu, N. Corcoran, M. Eurlings, W. Knose, T. Laidig, K. E. Wampler, S. Roy, X. Shi, M. Hsu, J. F. Chen, J. Finders, R. J. Socha and M. Dusa: SPIE 4691 (2002) 476, S. Hsu, J. F. Chen, N. Cororan, W. Knose, D. J. Van Den Broeke, T. Laidig, K. E. Wampler, X. Shi, M. Hsu, M. Eurlings, J. Finders, T. B. Chiou, R. J. Socha, W. Conley, Y. W. Hsieh, S. Tuan and F. Hsieh: SPIE 5040 (2003) 215] and double exposure of the x-dipole and the y-dipole respectively, make it possible to image an arbitrary device pattern. DDL allows integrated circuit (IC) manufacturers to maintain their roadmaps for shrinking device technology, while extending the use of ArF technology. Compared with other low-k1 imaging solutions, DDL has the advantage of using standard mask technologies, such as binary masks or 6% attenuated phase shift masks (PSMs). Because of the lower cost and faster turn-around time of these masks, DDL has the potential to become the imaging solution of choice for small-volume IC products, such as many application specific IC devices (ASICs). The imaging performance of DDL and the pattern decomposition algorithm are discussed elsewhere [M. Eurlings, E. van Setten, J. A. Torres, M. Dusa, R. Socha, L. Capodieci and J. Finders: SPIE 4404 (2001) 266, S. Hsu, N. Corcoran, M. Eurlings, W. Knose, T. Laidig, K. E. Wampler, S. Roy, X. Shi, M. Hsu, J. F. Chen, J. Finders, R. J. Socha and M. Dusa: SPIE 4691 (2002) 476, S. Hsu, J. F. Chen, N. Cororan, W. Knose, D. J. Van Den Broeke, T. Laidig, K. E. Wampler, X. Shi, M. Hsu, M. Eurlings, J. Finders, T. B. Chiou, R. J. Socha, W. Conley, Y. W. Hsieh, S. Tuan and F. Hsieh: SPIE 5040 (2003) 215]. In this paper, we investigate the CD and overlay (OL) errors caused by exposure tools, such as the illuminator imperfections, for example; these errors include the error caused by the pole intensity imbalance (PIB), aberration induced CD errors, and image placement errors (IPEs). During our research, we carried out extensive simulations of 1-dimensional and 2-dimensional mask-pattern CD errors and IPEs as a function of the PIB, pole size, pole center location, and aberration sensitivity. Simulation results show that the magnitude of the IPE depends on the control of PIB, dipole telecentricity, and the pattern structures. We present an IPE and focus control budget to describe the necessary tool-control requirements, considering the device patterns that satisfy 65-nm and 45-nm technology nodes respectively. The available results show that we can control the sources of the exposure-tool errors, enabling DDL imaging technology to satisfy the requirements of the 65-nm and 45-nm technology nodes.

Optical Properties of Thin Amorphous Silicon Film on a Phase Shift Mask for 157 nm Lithography

We introduced thin amorphous Si (a-Si) films to prevent the degradation of Si–O–N–F films that were fabricated for phase shift masks in 157 nm optical lithography. To investigate the optical properties of thin a-Si films, a spectroscopic ellipsometry (SE) analysis method was used. In SE analysis of thin a-Si films, the optical properties, such as absorbance and optical band gap, were analyzed using the two layer model of effective medium approximation (EMA) (a-Si+SiO)/a-Si/Cauchy model. To verify the validity of SE analysis, Si 2p peak and O 1s peak variations of a-Si films were analyzed by X-ray photoelectron spectroscopy (XPS) respect to sputtering time. By comparing the results of SE and XPS analyses, it was confirmed that SiO could represent the optical properties of an oxidized surface and that EMA(a-Si+SiO)/a-Si/Cauchy model was appropriated.

Complementary double-exposure technique (CODE): a way to print 80- and 65-nm gate levels using a double-exposure binary mask approach

To follow the accelerating ITRS roadmap, microprocessor and DRAM manufacturers are on their way to introduce the alternating phase shift mask (APSM) to be able to print the gate level on sub-130-nm devices. This is done at very high mask costs, long cycle times, and poor guarantees to get defect-free masks. Nakao et al. have proposed a new resolution enhancement technique (RET). They have shown that sub-0.1-µm features could be printed with good process latitudes using a double binary mask printing technique. This solution is very interesting, but is applicable to isolated structures only. To overcome this limitation, we have developed an extension of this technique called complementary double exposure (CODE). It combines Nakao's technique and the use of assist features that are removed during a second subsequent exposure. This new method enables us to print isolated as well as dense features on advanced devices using two binary masks. We describe all the steps required to develop the CODE application. The layout rules generation and the impact of the second mask on the process latitude have been studied. Experimental verification has been done using 193-nm 0.63 and 0.75 numerical aperture (NA) scanners. The improvement brought by quadrupole or annular illuminations combined with CODE has also been evaluated. Finally, the results of the CODE technique, applied to a portion of a real circuit using all the developed rules, are shown.

Total solution in 157 nm lithography for below 65 nm node semiconductor devices

In order to apply 157 nm lithography for the mass production of below 65 nm-node semiconductor devices, several technology modules such as exposure tool, resist material, resist processing and resolution enhancement techniques were developed and integrated. The resolution limits were obtained as 45 nm lines and spaces with an attenuated phase-shifting mask (PSM) and 40 nm lines and spaces with an alternating PSM (Alt-PSM) using a high-numerical-aperture lens and high-transmittance fluorinated polymer resist. Furthermore, 60 nm poly-Si gate patterns were obtained using a hard mask process or a bi-layer process. Combining with the above-mentioned technologies, 65 nm SRAM gates were successfully fabricated. It was confirmed that 157 nm lithography might be reliable for realizing mass-production of below 65-nm node semiconductor devices.

Monte-Carlo-Based Automatic Design of Chromeless Phase Shift Mask

The optical proximity effect correction algorithm (OPERA) program based on the Monte-Carlo method has been used for optical proximity correction (OPC) and phase shift mask (PSM) generation for low-NA projection lithography systems. To apply OPERA to high-NA systems, we adopted an imaging model based on a vector diffraction theory. Also, we developed a new convergence algorithm that does not reduce the degree of freedom and prevents the convergence process from falling into a local minimum. The new program, OPERA-V, is at least 10 times faster than the previous version. OPERA-V not only improves the computation time of traditional OPC, but can also be used as an automatic design tool for creating chromeless PSMs.

Fabry–Pérot structures for attenuated phase-shifting mask application in ArF and F2 lithography

We demonstrate a structure for an attenuated phase-shifting mask (APSM) which is based on a three-layer Fabry–Pérot structure for ArF (193 nm) and F2 (157 nm) excimer laser based lithographies. The APSM structure is composed of a metal/dielectric/metal stack. The optical characteristics of the Fabry–Pérot structure meet the following requirements: 180° phase shift, transmittance in the range of 4%–15%, reflectance of less than 10% at the exposure wavelength, and high inspection contrast at the inspection wavelength. Common dielectric and metal materials, such as chromium, tungsten, amorphous silicon, silicon dioxide, aluminum oxide, and aluminum nitride films, can be used to construct the Fabry–Pérot structure. The top metal layer can also prevent charge accumulation during electron-beam writing.

Electroless deposition in nanotechnology and ULSI

The technology of electroless metallization was developed enabling one to adequately replace Au and Ag with Ni–P or Ni–B alloys in industry and to significantly simplify the metallization process. Other proposed nanotechnologies for the first time allow one to produce photomasks and microdevices with nano-sized adjacent elements of different thicknesses made of various materials by single conventional optical photolithography. These advantages significantly extend functional capabilities of the device and simplify removal of undesirable gases and heat dissipation. The proposed nanotechnologies are much more advantageous and simpler than other expensive and complicated methods such as e-beam, X-ray lithography or production of the devices using light phase shift photomasks. Methods of fabricating the ultra-thin void-free and pore-free electroless coatings on micro-, meso- and nano-sized particles (carbides, borides, nitrides, oxides, diamond, graphite, etc.) are also proposed. These methods allow one to obtain nanostructured composite materials and coatings with the specified properties. The developed technologies have been implemented in industry.

Alternating Phase Shift Mask in Extreme Ultra Violet Lithography

Pattern printability by alternating phase shift mask (alt-PSM) is discussed by simulation. The alt-PSM with an additive structure that provides a completely flat surface structure on a multilayer mask blank is considered. The additive structure is constituted of Si, Ru, TaN and Mo materials that have been generally used for binary masks in extreme ultraviolet lithography so far. The alt-PSM has sufficient capability to resolve an 18 nm line pattern on a wafer due to strong resolution enhancement. Pattern edge contrast of an aerial image by the alt-PSM becomes 1.5 times larger than that by the conventional binary mask. Depth of focus of the alt-PSM increases up to 300 nm that is three times larger than that of the binary mask. The mask error enhanced factor becomes small and changes gradually with defocusing. The alt-PSM enhances pattern edge contrast even for dense lines at a large angle of the off-axis incident light of 7.05 deg.

Design of Phase-Shift Masks in Extreme Ultraviolet Lithography

An attenuated phase-shift mask (att-PSM) and an alternating phase-shift mask (alt-PSM) were designed for extreme ultraviolet lithography (EUVL). For an att-PSM, a bilayer structure is newly designed. The bilayer structure provides appropriate attenuated reflectance by choosing thicknesses and ensures sufficient thickness latitude. For the alt-PSM, the additive structure is newly introduced. The new additive structure generates 180 degrees of phase shift and uniform reflectance simultaneously. The new alt-PSM is constituted of materials of TaN, Ru, Si and Mo that have been already made available in a binary mask. An appropriate combination of materials in the additive structure also gives a flat surface structure. The new PSMs enable the fabrication of smaller gates with lengths of 18 nm or below.

Ultra-thin Cr 2O 3 well-crystallized films for high transmittance APSM in ArF line

Ultra-thin Cr 2O 3 well-crystallized thin films are deposited on UV grade fused silica substrates and Si wafers by using r.f. reactive unbalanced magnetron sputtering from a Cr metal target in an atmosphere of Ar and O 2 at 350 °C. The optical constants of such thin films were found to be a sensitive function of oxygen-to-argon flow rate ratio. At the ratio of 0.2, a Cr 2O 3 well-crystallized thin film with appropriate refractive index and extinction coefficient at a wavelength of 365 nm can be used as attenuated phase-shifting mask (APSM) blank as well as being good for inspection. The simulated thickness range of a Cr 2O 3 well-crystallized thin film was found to be between 28.2 and 30.3 nm. This meets the optical requirements for high transmittance APSM (HT-APSM) with a transmittance of 18–20% at 193 nm for the pattern fabrication and with transmittance less than 50% at 365 nm for the mask inspection. One such Cr 2O 3 well-crystallized thin film that satisfies the optical requirements was fabricated.

Defocusing image to pattern contact holes using attenuated phase shift masks

The patterning of contact holes by selecting out-of-focus image plane (defocus) using attenuated phase shift masks (APSM) has been studied. Defocus is found to enhance the image modulation at low partial coherence for contact holes with negative local average of mask function. Semi-dense holes up to 130nm in 8% APSM have been printed by 0.5μm defocus at a partial coherence of 0.31 using KrF scanner with highest numerical aperture of 0.68. However, these holes were closed with in-focus imaging. Defocus is also found to be beneficial for patterning the pitches that have extensive side lobes with in-focus imaging.

157-nm lithography with high numerical aperture lens for sub-70 nm node

For sub-70 nm semiconductor devices, 157 nm lithography using F 2 lasers is one of the most important technologies. Several candidates for critical components of 157 nm lithography, such as the exposure tool, resist materials and processing have been reported previously [J. Vac. Sci. Technol. B15 (1997) 2112; Microel. Eng. 61–62 (1997) 2112; Jpn. J. Appl. Phys. 41 (2002) 4033]. In this paper, we describe the recent progress of our studies including the use of a micro-stepper with high numerical aperture (NA) lens, phase-shifting masks (PSM), fluoropolymer resists and pattern-transfer processes. As a result, it was confirmed that high NA optics are very effective for improving the resolution limit as well as process margin when combined with phase-shifting masks and high-performance fluoropolymer resists. Moreover, it was found that even higher NA lenses of more than 0.94 NA, in combination with PSM, would be needed for reaching below the 50 nm resolution limit.

Optimizing vacuum ultraviolet attenuated phase shift masking materials

Extensive vacuum ultraviolet (VUV) optical data have been collected for sputter-deposited thin films of masking material compounds for use at 157 nm. The use of VUV optical data and effective media modeling has lead to 27 possible material candidates for further study. These composite material films consist of a silicon dioxide host combined with various oxides and nitrides, generally of refractory metals. By adjusting material stoichiometry through control of deposition parameters (target composition ratios, pressure, power, flow rates, etc.), films have been tailored for specific optical properties. By using extinction coefficient/refractive index relationships for 180° phase shift, 4%–25% transmission, and <15% reflectivity, evaluation of the materials is possible. Additional detailed analysis of three specific composite materials is shown for use as attenuated phase shift masking at 157 nm. In each case, silicon dioxide is the host material within the film and the various metal oxides provide the appropriate absorption. The use of fluoride film etch stop materials is also described as a method to improve etch selectivity.

Customized illumination aperture filter design for through-pitch focus latitude enhancement of deep submicron contact hole printing

A simple graphic analysis technique named the illumination chart method is introduced to aid the customization of the illumination aperture filter for synergistic combination with a high transmission rim-type attenuated phase-shifting mask (PSM) for deep submicron contact hole printing. This graphic method gives direct visualization of the relationship between the interference condition in the pupil and the incident angle of illumination. The working ranges of oblique illuminations with different numbers of diffraction beams taking part in imaging can be easily clarified by this graphic method, which explains the dependence of depth of focus (DOF) on pattern duty. A customized illumination aperture filter (CIF) is synthesized by collecting the effective source elements for every pattern pitch to remedy the inability of the attenuated PSM for dense patterns. To preserve the merits of off-axis illumination to dense patterns and on-axis illuminations to sparse patterns in a single exposure, the illumination chart suggests a zeroth-order-reduction mask design for dense hole pattern. We applied this integrated resolution enhancement technique to 0.17 μm contact hole printing in 248 nm wavelength, 0.55 numerical aperture optics. The experimental results show our CIF illumination not only balances the DOF enhancement throughout the pattern pitches but also suppresses the best focus shift due to spherical aberration.

Investigation of the physical and practical limits of dense-only phase shift lithography for circuit feature definition<xref ref-type="fn" rid="FN1">*</xref>

The rise of low-k1 optical lithography in integrated circuit manufacturing has introduced new questions concerning the physical and practical limits of particular subwavelength resolution-enhanced imaging approaches. For a given application, trade-offs between mask complexity, design cycle time, process latitude and process throughput must be well understood. It has recently been shown that a dense-only phase shifting mask (PSM) approach can be applied to technology nodes approaching the physical limits of strong PSM with no proximity effects. Such an approach offers the benefits of reduced mask complexity and design cycle time, at the expense of decreased process throughput and limited design flexibility. In particular, dense-only methods offer k1<0.3, thus enabling 90 nm node lithography with high-numerical aperture 248 nm exposure systems. We present the results of experiments, simulations, and analysis designed to explore the trade-offs inherent in dense-only phase shift lithography. Gate and contact patterns corresponding to various fully scaled circuits are presented, and the relationship between process complexity and design latitude is discussed. Particular attention is given to approaches for obtaining gate features in both the horizontal and vertical orientation. Since semiconductor investment is dependent on cost amortization, the applicability of these methods is also considered in terms of production volume.

Novel I-Line Phase Shift Mask Technique for Submicron T-Shaped Gate Formation

A novel submicron (<0.2 µm) T-shaped gate technology using a phase shift mask (PSM) technique is developed. The 8% half-tone PSM was selected for the definition of the isolated narrow space. Before lithography, a 2000 Å SiN film was deposited on the wafer. After i-line PSM exposure and reactive ion etching (RIE) of the silicon nitride film, openings less than 0.2 µm wide were formed on the SiN film. To further reduce the dimensions of the openings, an additional 566 Å nitride was then deposited on the wafer and etched back using RIE without any mask. An opening of 0.167 µm was formed on the wafer after the dry etching process. The wafer was then coated with another layer of photoresist to form a lift-off structure. The T-shaped gate with a length of 0.167 µm was obtained using this technique. The novel T-shaped gate technology is a high-throughput process for both silicon and compound semiconductor devices.