High aspect ratio (HAR) silicon etch is reviewed, including commonly used terms, history, main applications, different technological methods, critical challenges, and main theories of the technologies. Chronologically, HAR silicon etch has been conducted using wet etch in solution, reactive ion etch (RIE) in low density plasma, single-step etch at cryogenic conditions in inductively coupled plasma (ICP) combined with RIE, time-multiplexed deep silicon etch in ICP-RIE configuration reactor, and single-step etch in high density plasma at room or near room temperature. Key specifications are HAR, high etch rate, good trench sidewall profile with smooth surface, low aspect ratio dependent etch, and low etch loading effects. Till now, time-multiplexed etch process is a popular industrial practice but the intrinsic scalloped profile of a time-multiplexed etch process, resulting from alternating between passivation and etch, poses a challenge. Previously, HAR silicon etch was an application associated primarily with microelectromechanical systems. In recent years, through-silicon-via (TSV) etch applications for three-dimensional integrated circuit stacking technology has spurred research and development of this enabling technology. This potential large scale application requires HAR etch with high and stable throughput, controllable profile and surface properties, and low costs.

High aspect ratio silicon etch: A review

The field of plasma etching is reviewed. Plasma etching, a revolutionary extension of the technique of physical sputtering, was introduced to integrated circuit manufacturing as early as the mid 1960s and more widely in the early 1970s, in an effort to reduce liquid waste disposal in manufacturing and achieve selectivities that were difficult to obtain with wet chemistry. Quickly, the ability to anisotropically etch silicon, aluminum, and silicon dioxide in plasmas became the breakthrough that allowed the features in integrated circuits to continue to shrink over the next 40 years. Some of this early history is reviewed, and a discussion of the evolution in plasma reactor design is included. Some basic principles related to plasma etching such as evaporation rates and Langmuir–Hinshelwood adsorption are introduced. Etching mechanisms of selected materials, silicon, silicon dioxide, and low dielectric-constant materials are discussed in detail. A detailed treatment is presented of applications in current silicon integrated circuit fabrication. Finally, some predictions are offered for future needs and advances in plasma etching for silicon and nonsilicon-based devices.

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Plasma etching: Yesterday, today, and tomorrow

Ionizing radiation has been found to be widely applicable in modifying the structure and properties of polymers, and can be used to tailor the performance of either bulk materials or surfaces. Fifty years of research in polymer radiation chemistry has led to numerous applications of commercial and economic importance, and work remains active in the application of radiation to practical uses involving polymeric materials. This paper provides a survey of radiation-processing methods of industrial interest, ranging from technologies already commercially well established, through innovations in the active R&D stage which show exceptional promise for future commercial use. Radiation-processing technologies are discussed under the following categories: cross-linking of plastics and rubbers, curing of coatings and inks, heat-shrink products, fiber–matrix composites, chain-scission for processing control, surface modification, grafting, hydrogels, sterilization, natural product enhancement, plastics recycling, ceramic precursors, electronic property materials, ion-track membranes and lithography for microdevice production. In addition to new technological innovations utilizing conventional gamma and e-beam sources, a number of promising new applications make use of novel radiation types which include ion beams (heavy ions, light ions, highly focused microscopic beams and high-intensity pulses), soft X-rays which are focused, coherent X-rays (from a synchrotron) and e-beams which undergo scattering to generate patterns.

High-energy radiation and polymers: A review of commercial processes and emerging applications

This Perspective addresses the current state of block copolymer lithography and identifies key challenges and opportunities within the field. Significant strides in experimental and theoretical thin film research have nucleated the transition of block copolymers “from lab to fab”, but outstanding questions remain about the optimal materials, processes, and analytical techniques for first-generation devices and beyond. Particular attention herein is focused on advances and issues related to thermal annealing. Block copolymers are poised to change the traditional lithographic resolution enhancement paradigm from “top-down” to “bottom-up”.

Block Copolymer Lithography

Lithography is a field in which advances proceed at a swift pace. This book was written to address several needs, and the revisions for the second edition were made with those original objectives in mind. Many new topics have been included in this text commensurate with the progress that has taken place during the past few years, and several subjects are discussed in more detail. This book is intended to serve as an introduction to the science of microlithography for people who are unfamiliar with the subject. Topics directly related to the tools used to manufacture integrated circuits are addressed in depth, including such topics as overlay, the stages of exposure, tools, and light sources. This text also contains numerous references for students who want to investigate particular topics in more detail, and they provide the experienced lithographer with lists of references by topic as well. It is expected that the reader of this book will have a foundation in basic physics and chemistry. No topics will require knowledge of mathematics beyond elementary calculus.

Principles of Lithography

Patterning of a layer of material that can be etched with gaseous mixture of oxygen, chlorine, and nitrogen as etchant species, such as a chromium or a chromium-containing compound layer, is accomplished by using a patterned organometallic resist, such as a polymer which contains silicon or germanium. Although gaseous mixtures of chlorine and oxygen etch chromium anisotropically. Some undercut of the chromium is still observed. This undercut is controlled or eliminated by adding nitrogen to the gas mixture. Layers of material that have been patterned in this way can then be used for photolithographic masks or reticles, for X-ray masks, for e-beam masks. or for direct patterning of other, underlying layers in semiconductor integrated circuits or other devices.

Process for dry lithographic etching

The sensitivity and dry etching pattern control of chemically amplified resists used in the production of devices such as electronic devices is substantially enhanced while image quality is improved through use of a specific expedient. In particular, the resist material after coating on the device substrate, is treated, e.g., heated to sufficiently high temperatures, to remove a portion of the protective groups on the polymeric component of the resist. Generally, when removal up to approximately 90% of the protective groups for substituents such as t-butoxycarboxyl is effected through this procedure, sensitivities as good as 10 mJ/cm2 for an X-ray exposure (λ=14Å) have been achieved, and both the image quality and dry etching pattern control are improved.

Device fabrication process

In the last two decades, major advances in fabricating very large scale integration (VLSI) electronic devices have placed increasing demands on microlithography, the technology used to generate today's integrated circuits. In 1970, state-of-the-art devices contained several thousand transistors with minimum features of 10-12 μm. Today, they have several million transistors and minimum features of less than 0.3 μm. Within the next 10-15 years, a new form of lithography will be required that routinely produces features of less than 0.2 μm. Short-wavelength (deep-UV) photolithography and scanning and projection electron-beam and X-ray lithography are the possible alternatives to conventional photolithography. The consensus candidate for the next generation of lithography tools is photolithography using 193 nm light. At this wavelength, the opacity of traditional materials precludes their use, and major research efforts to develop alternative materials are currently underway. Notably, the materials being developed for these short UV wavelengths are demonstrating compatibility with the more advanced electron-beam technologies. Materials properties must be carefully tailored to maximize lithographic performance with minimal sacrifice of other performance attributes, eg adhesion, solubility and RF plasma etching stability.

Radiation chemistry of polymeric materials: novel chemistry and applications for microlithography

The present invention is directed to a process for device fabrication in which a spatially resolved latent image of latent features in an energy sensitive resist material is used to control process parameters. In the present process, an energy sensitive resist material is exposed to radiation using a patternwise or blanket exposure. An image of the latent effects of the exposure is obtained using a near-field imaging technique. This image of the latent effects of the exposure is used to control parameters of the lithographic process such as focus, lamp intensity, exposure dose, exposure time, and post exposure baking by comparing the image so obtained with the desired effects of the exposure and adjusting the relevant lithographic parameter to obtain the desired correlation between the image obtained and the desired effect. The image of the latent effects of exposure are also used to characterize the mask used in the lithographic process or to characterize the lithographic stepper used in the lithographic process.

A process for fabricating a device in which the process is controlled by near-field imaging latent features introduced into energy sensitive resist materials

Electron scattering in thin solid films used for the fabrication of masks for electron projection lithography, e.g., SCALPEL®, is investigated. We have developed an analytical model to calculate electron transmission through the mask membrane and image contrast due to different scattering properties of the patterned area and the membrane. The model utilizes cross sections for electron elastic and inelastic scattering on an atom with exponentially screened Coulomb potential of the nucleus derived in the first Born approximation. The variety and controversy of theoretical and empirical adjustments of the screening parameter are briefly analyzed and attributed to the misinterpretation of experimental data ignoring the effects mostly due to plural scattering of electrons and dense packing of atoms in thin solid films. This model frees us from the computational limitations of Monte Carlo simulations and proves to be effective for the straightforward characterization of various alternative materials for SCALPEL...

http://home.iitk.ac.in/~sbasu/se380/links/e-beam_simul3.pdf

Anthony E. Novembre

Papers

Process for dry lithographic etching

Device fabrication process

Radiation chemistry of polymeric materials: novel chemistry and applications for microlithography

A process for fabricating a device in which the process is controlled by near-field imaging latent features introduced into energy sensitive resist materials

Electron scattering and transmission through SCALPEL masks