Photolithography (also optical lithography) is a process used in microfabrication to selectively remove parts of a thin film (or the bulk of a substrate). Microfabrication or micromanufacturing are the terms to describe processes of fabrication of miniature structures of Micrometre sizes and smaller It uses light to transfer a geometric pattern from a photomask to a light-sensitive chemical (photoresist, or simply "resist") on the substrate. A photomask is an opaque plate with holes or transparencies that allow light to shine through in a defined pattern Photoresist is a Light -sensitive material used in several industrial processes such as Photolithography and Photoengraving to form a patterned coating A series of chemical treatments then engraves the exposure pattern into the material underneath the photoresist. In a complex integrated circuit (for example, modern CMOS), a wafer will go through the photolithographic cycle up to 50 times. Microchipsjpg|right|thumb|200px|Microchips ( EPROM memory with a transparent window showing the integrated circuit inside Complementary metal–oxide–semiconductor ( CMOS) (pronounced "see-moss" siːmɔːs ˈsiːmɒs is a major class of Integrated circuits CMOS technology
Photolithography shares some fundamental principles with photography, in that the pattern in the etching resist is created by exposing it to light, either using a projected image or an optical mask. Photography (fә'tɒgrәfi or fә'tɑːgrәfi (from Greek φωτο and γραφία is the process and Art of recording pictures by means of capturing This step is like an ultra high precision version of the method used to make printed circuit boards. A printed circuit board, or PCB, is used to mechanically support and electrically connect Electronic components using conductive pathways or traces Subsequent stages in the process have more in common with etching than to lithographic printing. For other uses of etch or etching, see Etching (disambiguation, for the history of the method see Old master prints. Lithography is a method for Printing using a plate or stone with a completely smooth surface Printing is a process for reproducing text and image typically with ink on Paper using a printing press It is used because it affords exact control over the shape and size of the objects it creates, and because it can create patterns over an entire surface simultaneously. Its main disadvantages are that it requires a flat substrate to start with, it is not very effective at creating shapes that are not flat, and it can require extremely clean operating conditions.
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Basic procedure
A single iteration of photolithography combines several steps in sequence. Modern cleanrooms use automated, robotic wafer track systems to coordinate the process. An industrial robot is officially defined by ISO as an automatically controlled reprogrammable multipurpose manipulator programmable in three or more axes. The procedure described here omits some advanced treatments, such as thinning agents or edge-bead removal. [1]
Preparation
The wafer is initially heated to a temperature sufficient to drive off any moisture that may be present on the wafer surface. Wafers that have been in storage must be chemically cleaned to remove contamination. A liquid or gaseous "adhesion promoter", such as hexamethyldisilazane (HMDS), is applied to promote adhesion of the photoresist to the wafer. Liquid is one of the principal States of matter. A liquid is a Fluid that has the particles loose and can freely form a distinct surface at the boundaries of This page is about the physical properties of gas as a state of matter Bis(trimethylsilylamine (also known as hexamethyldisilazane or HMDS is a chemical reagent with the Molecular formula (CH33Si-NH-Si(CH33
Photoresist application
The wafer is covered with photoresist ("PR") by spin coating. Spin coating is a procedure used to apply uniform Thin films to flat substrates In short an excess amount of a solution is placed on the substrate which is then rotated A viscous, liquid solution of photoresist is dispensed onto the wafer, and the wafer is spun rapidly to produce a uniformly thick layer. The spin coating typically runs at 1200 to 4800 rpm for 30 to 60 seconds, and produces a layer between 0. 5 and 2. 5 micrometres thick. A micrometre ( American spelling: micrometer; symbol µm) is one millionth of a Metre, or equivalently one thousandth of a Millimetre
The photoresist-coated wafer is then "soft-baked" or "prebaked" to drive off excess solvent, typically at 90 to 100 °C for 5 to 30 minutes. The Celsius Temperature scale was previously known as the centigrade scale. Sometimes a nitrogen atmosphere is used. Nitrogen (ˈnaɪtɹəʤɪn is a Chemical element that has the symbol N and Atomic number 7 and Atomic weight 14
Exposure and developing
After prebaking, the photoresist is exposed to a pattern of intense light. Optical lithography typically uses ultraviolet light (see below). Ultraviolet ( UV) light is Electromagnetic radiation with a Wavelength shorter than that of Visible light, but longer than X-rays Positive photoresist, the most common type, becomes chemically less stable when exposed; negative photoresist becomes more stable. This chemical change allows some of the photoresist to be removed by a special solution, called "developer" by analogy with photographic developer. In Film developing, photographic developer (or just developer) is a chemical that makes the Latent image on the film or print visible A post-exposure bake is performed before developing, typically to help reduce standing wave phenomena caused by the destructive and constructive interference patterns of the incident light. A standing wave, also known as a stationary wave, is a Wave that remains in a constant position In physics interference is the addition ( superposition) of two or more Waves that result in a new wave pattern
The develop chemistry is delivered on a spinner, much like photoresist. Developers originally often contained sodium hydroxide (NaOH). Sodium hydroxide ( Na[[hydroxide OH]]) also known as Lye, caustic soda and (incorrectly according to IUPAC nomenclature However, sodium is considered an extremely undesirable contaminant in MOSFET fabrication because it degrades the insulating properties of gate oxides. Sodium (ˈsoʊdiəm is an element which has the symbol Na( Latin natrium, from Arabic natrun) atomic number 11 atomic mass 22 The metal–oxide–semiconductor field-effect transistor ( MOSFET, MOS-FET, or MOS FET) is a device used to amplify or switch electronic signals An insulator, also called a Dielectric, is a material that resists the flow of Electric current. Metal-ion-free developers such as tetramethylammonium hydroxide (TMAH) are now used. Tetramethylammonium hydroxide (TMAH or TMAOH is a Quaternary ammonium salt with the molecular formula (CH34NOH
The resulting wafer is then "hard-baked", typically at 120 to 180 °C for 20 to 30 minutes. The hard bake solidifies the remaining photoresist, to make a more durable protecting layer in future ion implantation, wet chemical etching, or plasma etching. Ion implantation is a Materials engineering process by which ions of a material can be implanted into another solid thereby changing the physical properties of the In Microfabrication, wet etching is Chemical etching performed with a Liquid etchant as opposed to a plasma. Plasma etching is a form of Plasma processing used to fabricate integrated circuits
Etching
In the etching step, a liquid ("wet") or plasma ("dry") chemical agent removes the uppermost layer of the substrate in the areas that are not protected by photoresist. Etching is used in Microfabrication to chemically remove layers from the surface of a wafer during manufacturing In Physics and Chemistry, plasma is an Ionized Gas, in which a certain proportion of Electrons are free rather than being bound In semiconductor fabrication, dry etching techniques are generally used, as they can be made anisotropic, in order to avoid significant undercutting of the photoresist pattern. Semiconductor device fabrication is the process used to create chips the Integrated circuits that are present in everyday Electrical and electronic This is essential when the width of the features to be defined is similar to or less than the thickness of the material being etched (ie when the aspect ratio approaches unity). Wet etch processes are generally isotropic in nature, which is often indispensable for microelectromechanical systems, where suspended structures must be "released" from the underlying layer. Microelectromechanical systems ( MEMS) is the technology of the very small and merges at the nano-scale into Nanoelectromechanical systems (NEMS and Nanotechnology
The development of low-defectivity anisotropic dry-etch process has enabled the ever-smaller features defined photolithographically in the resist to be transferred to the substrate material.
Photoresist removal
After a photoresist is no longer needed, it must be removed from the substrate. This usually requires a liquid "resist stripper", which chemically alters the resist so that it no longer adheres to the substrate. Alternatively, photoresist may be removed by a plasma containing oxygen, which oxidizes it. Oxygen (from the Greek roots ὀξύς (oxys (acid literally "sharp" from the taste of acids and -γενής (-genēs (producer literally begetteris the This process is called ashing, and resembles dry etching. In Semiconductor manufacturing plasma ashing is the process of removing the Photoresist from an etched wafer
Exposure ("printing") systems
Exposure systems typically produce an image on the wafer using a photomask. The light shines through the photomask, which blocks it in some areas and lets it pass in others. (Maskless lithography projects a precise beam directly onto the wafer without using a mask, but it is not widely used in commercial processes. In maskless lithography, Radiation used to expose the photosensitive emulsion (or Photoresist) is not projected from or transmitted through a Photomask ) Exposure systems may be classified by the optics that transfer the image from the mask to the wafer.
Contact and proximity
A contact printer, the simplest exposure system, puts a photomask in direct contact with the wafer and exposes it to a uniform light. Contact lithography, also known as contact printing is a form of Photolithography whereby the image to be printed is obtained by illumination of a Photomask in direct A proximity printer puts a small gap between the photomask and wafer. In both cases, the mask covers the entire wafer, and simultaneously patterns every die.
Contact printing is liable to damage both the mask and the wafer, and this was the primary reason it was abandoned for high volume production. Both contact and proximity lithography require the light intensity to be uniform across an entire wafer, and the mask to align precisely to features already on the wafer. As modern processes use increasingly large wafers, these conditions become increasingly difficult.
Research and prototyping processes often use contact lithography, because it uses inexpensive hardware and can achieve high optical resolution. Optical resolution describes the ability of an imaging system to resolve detail in the object that is being imaged The resolution is approximately the square root of the product of the wavelength and the gap distance. Hence, contact printing offers the best resolution, because its gap distance is approximately zero (neglecting the thickness of the photoresist itself). In addition, nanoimprint lithography may revive interest in this familiar technique, especially since the cost of ownership is expected to be low. Nanoimprint lithography is a novel method of fabricating nanometer scale patterns
Projection
- See also: Stepper
Very-large-scale integration lithography uses projection systems. A stepper is a device used in the manufacture of integrated circuits (ICs that is similar in operation to a Slide projector or a photographic Enlarger. Unlike contact or proximity masks, which cover an entire wafer, projection masks (also called "reticles") show only one die. Projection exposure systems (steppers) project the mask onto the wafer many times to create the complete pattern.
Photomasks
The image for the mask originates from a computerized data file. A photomask is an opaque plate with holes or transparencies that allow light to shine through in a defined pattern This data file is converted to a series of polygons and written onto a square fused quartz substrate covered with a layer of chrome using a photolithographic process. Fused quartz and fused silica are types of Glass containing primarily Silica in amorphous (non- Crystalline form Chromium (ˈkroʊmiəm is a Chemical element which has the symbol Cr and Atomic number 24 A beam of electrons is used to expose the pattern defined in the data file and travels over the surface of the substrate in either a vector or raster scan manner. Where the photoresist on the mask is exposed, the chrome can be etched away, leaving a clear path for the light in the stepper/scanner systems to travel through.
Resolution in projection systems
The ability to project a clear image of a small feature onto the wafer is limited by the wavelength of the light that is used, and the ability of the reduction lens system to capture enough diffraction orders from the illuminated mask. In Physics wavelength is the distance between repeating units of a propagating Wave of a given Frequency. Current state-of-the-art photolithography tools use deep ultraviolet (DUV) light with wavelengths of 248 and 193 nm, which allow minimum feature sizes down to 50 nm. Ultraviolet ( UV) light is Electromagnetic radiation with a Wavelength shorter than that of Visible light, but longer than X-rays A nanometre ( American spelling: nanometer, symbol nm) ( Greek: νάνος nanos dwarf; μετρώ metrό count) is a
The minimum feature size that a projection system can print is given approximately by:
where
is the minimum feature size (also called the critical dimension, target design rule). It is also common to write 2 times the half-pitch.
(commonly called k1 factor) is a coefficient that encapsulates process-related factors, and typically equals 0. 4 for production
is the wavelength of light used
is the numerical aperture of the lens as seen from the wafer
According to this equation, minimum feature sizes can be decreased by decreasing the wavelength, and increasing the numerical aperture, i. In Physics wavelength is the distance between repeating units of a propagating Wave of a given Frequency. In Optics, the numerical aperture ( NA) of an optical system is a Dimensionless number that characterizes the range of angles over which the system can accept e. making lenses larger and bringing them closer to the wafer. However, this design method runs into a competing constraint. In modern systems, the depth of focus is also a concern:
Here, 
is another process-related coefficient. Depth of focus is a lens Optics concept that measures the tolerance of placement of the image plane (the Film plane in a camera in relation to the lens The depth of focus restricts the thickness of the photoresist and the depth of the topography on the wafer. Chemical mechanical polishing is often used to flatten topography before high-resolution lithographic steps. Chemical-mechanical planarization or Chemical-mechanical Polishing, commonly abbreviated CMP, is a technique used in Semiconductor fabrication
Light sources
Historically, photolithography has used ultraviolet light from gas-discharge lamps using mercury, sometimes in combination with noble gases such as xenon. Gas discharge lamps are a family of artificial light sources that generate light by sending an Electrical discharge through an ionized gas i Mercury (ˈmɜrkjʊri also called quicksilver or hydrargyrum, is a Chemical element with the symbol Hg ( Latinized hydrargyrum History Noble gas is translated from the German noun de ''Edelgas'' first used in 1898 by Hugo Erdmann to indicate their extremely low level of reactivity Xenon (ˈzɛnɒn or) is a Chemical element represented by the symbol Xe. These lamps produce light across a broad spectrum with several strong peaks in the ultraviolet range. This spectrum is filtered to select a single spectral line, usually the "g-line" (436 nm) or "i-line" (365 nm). A spectral line is a dark or bright line in an otherwise uniform and continuous spectrum, resulting from an excess or deficiency of photons in a narrow frequency range compared
More recently, lithography has moved to "deep ultraviolet", produced by excimer lasers. An excimer laser (sometimes and more correctly called an exciplex laser) is a form of ultraviolet laser which is commonly used in Eye surgery and Semiconductor (In lithography, wavelengths below 300 nm are called "deep UV". ) Krypton fluoride produces a 248-nm spectral line, and argon fluoride a 193-nm line. See also Krypton fluoride laser Krypton difluoride, KrF2 was the first compound of Krypton discovered
Optical lithography can be extended to feature sizes below 50 nm using 193 nm and liquid immersion techniques. Also termed immersion lithography, this enables the use of optics with numerical apertures exceeding 1. Immersion lithography is a Photolithography resolution enhancement technique that replaces the usual air gap between the final lens and the wafer surface with a liquid medium In Optics, the numerical aperture ( NA) of an optical system is a Dimensionless number that characterizes the range of angles over which the system can accept 0. The liquid used is typically ultra-pure, deionised water, which provides for a refractive index above that of the usual air gap between the lens and the wafer surface. The refractive index (or index of Refraction) of a medium is a measure for how much the speed of light (or other waves such as sound waves is reduced inside the medium This is continually circulated to eliminate thermally-induced distortions. Water will only allow NA's of up to ~1. 4, but materials with higher refractive indices will allow the effective NA to be increased further. The refractive index (or index of Refraction) of a medium is a measure for how much the speed of light (or other waves such as sound waves is reduced inside the medium
Tools using 157 nm wavelength DUV in a manner similar to current exposure systems have been developed. These were once targeted to succeed 193 nm at the 65 nm feature size node but have now all but been eliminated by the introduction of immersion lithography. This was due to persistent technical problems with the 157 nm technology and economic considerations that provided strong incentives for the continued use of 193 nm technology. High-index immersion lithography is the newest extension of 193 nm lithography to be considered. In 2006, features less than 30 nm were demonstrated by IBM using this technique[2].
Experimental methods
- See also: Nanolithography
Photolithography has been defeating predictions of its demise for many years. Nanolithography — or Photolithography at the Nanometer scale — refers to the fabrication of nanometer-scale structures, meaning patterns with at For instance, it was predicted that features smaller than 1 micrometre could not be printed optically. Modern techniques already print features with dimensions a fraction of the wavelength of light used - an amazing optical feat. Current research is exploring new tricks in the ultraviolet regime, as well as alternatives to conventional UV, such as electron beam lithography, X-ray lithography, extreme ultraviolet lithography, ion projection lithography, and immersion lithography. Electron beam lithography (often abbreviated as e-beam lithography) is the practice of scanning a beam of Electrons in a patterned fashion across a surface covered X-ray lithography is a next generation Lithography that has been developed for the Semiconductor industry Extreme ultraviolet lithography (also known as EUV or EUVL) is a Next-generation lithography technology using Immersion lithography is a Photolithography resolution enhancement technique that replaces the usual air gap between the final lens and the wafer surface with a liquid medium
Photo Etching Summary
Photo Etching is a form of photochemical milling. It is a process commonly used in the creation of small, intricate parts, such as model cars. This process can be used in almost any industry that creates product out of small, thin metal sheets. The process is completed using the following steps:
1) A sheet of metal is cleaned and prepped for the process by applying a photoresist coating, which makes the metal sensitive to light.
2) The metal sheet is exposed to a UV light source. This is based on an image in a controlling computer, which is to match the end result of the process.
3) By treating the metal in a developing solution, an image appears on the sheet of metal.
4) Processing the metal with an etchant produces the desired design, based on the image on the exposed metal. Common etchants are hydrochloric acid, ammonium persulfate, and ferric chloride.
This process generally creates very exact, high quality cuts with a relatively fast turn around. Compared to other machining options, it is an economical alternative for machining flat parts.
See also
- Types of lithography for microfabrication
- Stereolithography, a macroscale process used to produce three-dimensional shapes
- Foundry (electronics)
References
- ^ Jaeger, Richard C. Nanoimprint lithography is a novel method of fabricating nanometer scale patterns Dip Pen Nanolithography (DPN is a Scanning probe lithography technique where an Atomic force microscope tip is used to transfer molecules to a surface via a In Technology, soft lithography refers to a family of techniques for fabricating or replicating Structures using "elastomeric stamps molds and conformable Stereolithography is a common Rapid manufacturing and Rapid prototyping technology for producing parts with high accuracy and good surface finish In the Microelectronics industry a semiconductor fabrication plant (commonly called a fab) is a factory where devices such as Integrated circuits are manufactured (2002). "Lithography", Introduction to Microelectronic Fabrication. Upper Saddle River: Prentice Hall. ISBN 0-201-44494-7.
- ^ Hand, Aaron. High-Index Lenses Push Immersion Beyond 32 nm.
3. Eli Yablonovitch, Rutger B. Vrijen, <Optical projection lithography at half the Rayleigh resolution limit by two-photon exposure> http://www.ee.ucla.edu/labs/photon/pubs/ey1999oe382.pdf
Advanced Metal Etching, http://www.metaletching.com/?OVRAW=Photo-etching&OVKEY=photo%20etching&OVMTC=standard&OVADID=484267021&OVKWID=1565069521
Model Car Tech, http://modeltech.tripod.com/etching.htm
External links
- Semiconductor Lithography — Overview of lithography
- Optical Lithography Introduction — IBM site with lithography-related articles
- Immersion Lithography Article — Shows how depth-of-focus is increased with immersion lithography
- More about Photolithography ResourcesMore about Photolithography Resources
Dictionary
photolithography
-noun
- A lithographic procedure in which printing plates are made using a photographic process.
- An optical exposure process used in the manufacture of integrated circuits.
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