This PDF file contains the editorial “Reflection on the Last 17 Years” for JM3 Vol. 3 Issue 03

ArF lithography is approaching its limit past the 90-nm node. F₂ lithography using 157-nm light seems to be a natural extension to the next node. However, several key problems in F₂ lithography are still insurmountable. Thin-film pellicle material cannot withstand more than 10 exposures. Hard pellicle technology is far from being manufacture worthy. Ditto for the F₂ resist systems. Despite great progresses made, the CaF₂ material still suffers from quality and quantity problems. On the other hand, ArF lithography using water immersion between the front lens element and the photoresist effectively reduces the 193-nm wavelength to 135 nm and opens up room for improvement in resolution and depth of focus (DOF). We give a systematic examination of immersion lithography, analyze and evaluate the diffraction, required, and available DOFs in a dry and an immersion system. We also analyze the effects of polarization to dry and immersion imaging. These phenomena are included in simulations to study the imaging of critical layers such as poly, contact, and metal layers for the 65-, 45-, and 32-nm nodes using 193- and 157-nm, dry and immersion systems. The imaging feasibility of 157-nm immersion to the 22-nm node is briefly studied. In addition to the imaging comparison, the impacts and challenges to employ these lithography systems are also covered.

We propose a method for the fabrication of the mirror elements that compose the complex-shaped fly-eye mirrors for use in an extreme ultraviolet lithography (EUVL) system. In this method, silicon blocks are polished to have a spherical shape and smoothness, from which the mirror elements are cut out by wire electrical discharge machining. Thus, this method enables more effective fabrication of the mirror elements than the method proposed previously. To evaluate the effectiveness of the present method, the mirror elements having a flat reflective surface are fabricated as a preliminary experiment. The resultant mirror elements have highly accurate and smooth surfaces, although they have a unique shape compared to that of general optics. Moreover, to construct the fly-eye mirror, we propose a method for arranging the mirror elements, in which mirror elements are fixed to a base plate containing magnets. This allows the accurate arrangement of all the reflective surfaces relative to the base plate. To enable their attraction to the magnets, the bottom of the silicon mirror elements is plated with metal. Experiments confirm the mirror elements are accurately arranged. Thus, we demonstrate that the proposed methods are useful in realizing the fly-eye mirror.

The lithographic performance of the low-energy electron-beam proximity-projection lithography (LEEPL) β-tool is demonstrated in terms of printability and overlay accuracy to establish the feasibility of proximity electron lithography (PEL) for the 65-nm technology node. The CD uniformity of 5.8 nm is achieved for the 1× stencil mask, and the mask patterns are transferred onto chemically amplified resist layers, coupled with a conformal multilayer process with the mask-error enhancement factor of nearly unity. Meanwhile, the overlay accuracy of 27.8 nm is achieved in the context of mix and match with the ArF scanner, and it is also shown that real-time correction for chip magnification, enabled by the use of die-by-die alignment and electron beam, can further reduce the error down to 21.3 nm. On the basis of the printability of programmed defects, it is shown that the most critical challenge to be solved for the application to production is the quality assurance of masks such as defect inspection and repair.

Secondary radiation during LIGA polymethyl-methacrylate (PMMA) resist exposure adversely affects feature definition, sidewall taper, and overall sidewall offset. Additionally, it can degrade the resist adjacent to the substrate, leading to the loss of free-standing features through undercutting during resist development or through mechanical failure of the degraded material. The source of this radiation includes photoelectrons, Auger electrons, fluorescence photons, etc. Sandia's Integrated Tiger Series (ITS), a coupled electron/photon Monte Carlo transport code, is used to compute dose profiles within 1 to 2 μm of the absorber edge and near the interface of the resist with a metallized substrate. The difficulty of submicron resolution requirement was overcome by solving a few local problems, having carefully designed micron-scale geometries. The results for a 10-keV x-ray photons source indicate a 2-μm dose transition region near the absorber edge, resulting from PMMA photoelectrons. This region leads to sidewall offset and to tapered sidewalls following resist development. The results also show a dose boundary layer of around 1 μm near the substrate interface due to electrons emitted from the substrate metallization layer. The maximum dose at the resist bottom under the absorber can be very high and can lead to feature loss during development. This model is also used to investigate resist doses resulting from multilayer substrate.

The advanced requirements of bio-MEMS and MOEMS, i.e., low sidewall surface roughness, submicron critical dimension, and high aspect ratio, necessitate the use of an intermediate mask and a soft x-ray lithography process to fabricate working x-ray masks that are suitable for deep x-ray lithography. Intermediate masks consist of 2 to 2.5-μm gold patterns on membranes/substrates that are highly transparent to x-ray radiation, whereas working masks possess greater than 5 μm of gold patterns. In this work, 1-μm silicon nitride membranes are produced by a low pressure chemical vapor deposition (LPCVD) process on both the front and backside of <100> prime grade wafers and anisotropic wet etch through silicon nitride etch masks. E-beam lithography is used to pattern 0.8- to 3-μm-thick resist layers with submicron resolution. In the case of the 3-μm resist layers, the features are electroplated with approximately 2 μm of gold to form an intermediate mask. The 0.8-μm-thick layers are electroplated with gold up to a thickness of 0.6 µm and form initial masks, which are in turn used in a soft x-ray lithographical process to make intermediate masks. The process of building a high-resolution intermediate x-ray mask, directly by e-beam patterning a 3 μm layer of e-beam resist, followed by gold electroplating, is found to be viable but requires the use of a high energy (>100 keV) e-beam writer. The stability of the resist pattern during soft x-ray lithography (SXRL) by use of an initial mask is found to be problematic. Double-side lithography and gold electroplating, can effectively reduce the aspect ratio of the mask pattern, eliminates the problems associated with the use of an initial mask to fabricate intermediate x-ray masks.

We focus on the problem of obtaining and characterizing the edge roughness of photoresist lines by analyzing top-down scanning electron microscope (SEM) images. An off-line image analysis algorithm detecting the line edge, and an edge roughness characterization scheme, based on scaling analysis, are briefly described. As a result, it is suggested that apart from the rms value of the edge (sigma), two more roughness parameters are needed: the roughness exponent α and the correlation length ζ. These characterize the spatial complexity of the edge and determine the dependence of sigma on the length of the measured edge. Completing our previous work on the dependencies of the roughness parameters (sigma,α,ζ) on various image analysis options, we examine the effect of the type of noise smoothing filter. Then, a comparative study of the roughness parameters of the left and right edges of resist lines is conducted, revealing that the sigma values of the right edges are larger than those of the left edges (due to an imperfect SEM beam alignment), whereas the roughness exponents and the correlation lengths do not show such a trend. Finally, the relation between line width roughness and line edge roughness is thoroughly investigated with interesting conclusions.

We report a problem regarding DUV lithography on topographical substrates and a solution for obtaining desired CD control and resist pattern shape. In our experiment, large footings for a 250-nm resist pattern are observed when the resist pattern is transferred over a polysilicon step pattern of 175 nm in height. This pattern error is not negligible regarding device performance. The exposure tool used is a KrF scanner of NA 0.6. The resist is 500 nm thick with no antireflective coating (ARC). Computer simulation is used to demonstrate the amount of the footing. A nonrigorous diffraction model did not recreate the footing appearance at the poly-Si step. However, a rigorous diffraction model of incident light in a cone recreated the footing amount at the poly-Si step faithfully. In this simulation, optical distribution in the resist over the nonplanar wafer is solved by the finite-difference time-domain (FDTD) method. Optical intensity at the sidewalls of the step differs between the two models. Experimental results as well as simulation results show that larger coherency results in larger footing. In the case of a large coherency, the illumination rays come from various directions to the wafer, and a large shadow area is likely to appear behind the steep step. We also propose a shadow model for simple footing simulation. As a consequence, optical behavior in the vicinity at the steep step has a strong impact on the resist footing.

A hydrogen silisesquioxane (HSQ) bilayer process and a top surface imaging (TSI) process are investigated for application as low-voltage electron beam resist systems. Namatsu, van Delft, and others have reported printing exceptionally small features using high-voltage electron beam exposure of HSQ at high-exposure doses (~2000 μC/cm² at 100 kV). The shallow penetration depth of low-voltage electrons results in greatly reduced dose requirements, and smooth, high-resolution images are generated at 1 kV with an exposure dose of less than 60 μC/cm². HSQ's high silicon content enable it to be used in a bilayer form utilizing reactive ion etching with an oxygen plasma, thus generating high aspect ratio images. TSI has been studied in the past by numerous researchers at low voltages using various TSI schemes. We investigate the use of a chemically amplified TSI resist process based on poly (t-BOC-hydroxystyrene). The effect of base quencher loading in the resist formulation on line edge roughness and resolution is investigated, and is found to have a dramatic influence. High-resolution, high aspect ratio images are printed down to 40 nm, and exhibit only moderate levels of line edge roughness. Furthermore, proximity effects at 1, 2, and 3 kV are examined and compared to simulation.

Several significant technical issues are associated with the current anodized aluminum pellicle frame. These problems will become more acute as shorter wavelengths such as 193 and 157 nm become standard. For example, it is difficult to purge the pellicle space, and the nonporous nature of the aluminum frame can lead to pressure-induced film breakage during transport. In addition, the thermal expansion behavior of aluminum does not match that of the silica substrate, which can lead to pattern distortion or bending of a hard pellicle film. Proposed solutions such as a perforated silica frame or a porous Invar^TM frame may address some of these issues, but a complete solution has not been forthcoming. A porous silica frame technology based on sol-gel processing may eliminate concerns associated with the lack of porosity in the aluminum frame, as well as thermal expansion mismatch problems. Several additional advantages may also be realized, including the removal of organic contaminants from inside the pellicle space, filtration of particulate contaminants, and stability under aggressive cleaning. This approach to a porous silica frame may help enable a cost-effective route to more rapid commercialization of shorter-wavelength microlithographic processes.

Excimer laser ablation is a versatile method to generate 2- or 3-D microstructured devices for mechanics, fluidics, and optics. Particularly, if in 1-D the structure is predefined by a layer design, and the other two dimensions are defined in the layer ablation process by mask projection, very precise structure definition in three dimensions and ablated surfaces with optical quality can be achieved. By patterning optical layers or layer stacks, a variety of applications is possible. Two examples are demonstrated. 1. Dielectric masks (amplitude masks) are fabricated by structuring a highly reflective dielectric layer stack. These masks can be used in high power laser machining systems. 2. Diffractive optical elements (phase elements) are generated by structured ablation of a transparent layer. The performance of these elements in shaping excimer laser beams is demonstrated.

A new design of a bilateral interdigital coplanar waveguide (BI-CPW) phase shifter using tunable barium strontium titanate thin film is presented and analyzed. This new design has shown a phase shift of 203°/cm at 25 GHz. Compared to a normal coplanar waveguide (CPW) type, a two-thirds size BI-CPW phase shifter is realized by interdigital capacitive loading. The phase shifter is fabricated with processes fully compatible with current SiGe technology. In addition, it is shown that operating voltages can be reduced significantly within a practical range by the inclusion of a polysilicon layer as a control electrode.

A microelectromechanical systems (MEMS) isolation amplifier is presented, which uses MEMS switches to realize a "flying capacitor" structure. This isolation amplifier has the advantages of simple circuit structures, smaller dimensions, low cost, excellent electromagnetic compatibility (EMC) performance, easy to make multichannel isolation circuits, integration compatibility with on-chip electronics, etc. The performance of the MEMS switch exhibits 30-V pull-in voltage, 35-μs time delay, and 110-V breakdown voltage. The preliminary performance of the isolation amplifier is also demonstrated. To our knowledge, this work presents the first MEMS isolation amplifier structure.

A layer-by-layer laser ablation process of polymers with a diode-pumped solid state (DPSS) third harmonic Nd:YVO₄ laser with wavelength of 355 nm is developed to fabricate a 3-D microshape. Polymer fabrication using DPSS lasers has some advantages compared with the conventional polymer ablation process using KrF and ArF lasers with 248 and 193 nm wavelengths, such as pumping efficiency and low maintenance fees. This method also makes it possible to fabricate 2-D patterns or 3-D shapes rapidly and cheaply because CAD/CAM software and precision stages only are used without complex projection mask techniques. The photomachinability of polymers is highly influenced by laser wavelength and its own chemical structure. So the optical characteristics of polymers for a 355-nm laser source are investigated experimentally and theoretically. The photophysical and photochemical parameters such as laser fluence, focusing position, and ambient gas are considered to reduce the plume effect, which induces the redeposited debris on the surface of a substrate. Thus, the process parameters are tuned to optimize gaining precision surface shape and quality. This direct photomachining technology using DPSS lasers could be expected to manufacture the prototype of microdevices and be the mold for the laser-LIGA process.

We discuss laser fabrication of microstructures in photoetchable glass ceramics called Foturan (Schott Company, Elmsford, NY). A KrF excimer laser (λ = 248 nm, τ = 25 ns) is used for surface micromachining, and a femtosecond laser (λ = 800 nm, τ = 80 fs) is used for fabricating 3-D structures. Important aspects of the machining, such as depth of machining resulting from different laser processing parameters and threshold laser fluences, are presented. A detailed analysis of the absorption process of both lasers in photoetchable glass ceramics is provided.

We present a novel micromachined passive valve for fluid flow control within microfluidic systems. Forward flow through the valve is in the opposite direction compared to other published designs, and the valve thus has considerable potential for integrated microfluidic devices or systems. In applications such as micropumps, the inlet and outlet valves can be formed on the same surface, removing the need for precision processing on both wafer surfaces. The fabricated valve consists of three major layers, namely: a top bridge with an inlet aperture; a movable plate with four supporting arms; and an underlying substrate with an outlet aperture. The structure and operation of the device is presented, together with a detailed description of the fabrication process. The fabricated valves are characterized for fluid flow under forward and reverse pressures. In the forward direction, a water flow rate of 1920 μl/min is measured at a pressure of 14.1 kPa, and the reverse leakage is less than 8 μl/min.

We describe the design, fabrication, and characterization of a buried polysilicon hot-wire anemometer with an integrated micromachined channel for fluid flow sensing. Hot-wire flow sensor elements and bulk micromachined channels are fabricated separately on device and channel wafers. The direct silicon wafer bonding technique is then used to integrate the flow sensors and channels. This is made possible by fabrication of the polysilicon resistor flow sensing element in a trench. Heavily doped polysilicon connections are also recessed into the device wafer. The attraction of using polysilicon for the flow sensing element and connections is the ability to adjust the resistivity and temperature coefficient of resistance (TCR) by control of the dopant concentration. The effect of doping on both these parameters is characterized in detail to enable selection of the appropriate doping concentration for each region of the device. Light doping is preferred for the resistor element, since this ensures high thermal sensitivity, while heavy doping is used to provide low resistance connections. The materials allow use of standard high-temperature fabrication processes compatible with standard integrated circuit (IC) technology and micromachining techniques, including silicon bonding. This offers the ability to integrate other microfluidic components and electronic control circuits.