The transition of photoresist from KrF photoresist to ArF photoresist poses new challenges for etching process, especially for dielectric etching. In this. Photolithography, also termed optical lithography or UV lithography, is a process used in Inspired by the application of photoresist, a photosensitive liquid used to mark the .. Optical lithography has been extended to feature sizes below 50 nm using the nm ArF excimer laser and liquid immersion techniques. The most serious problem associated with nm lithography using ArF photoresists is the formation of roughness on photoresist film during.
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Photolithographyalso termed optical lithography or UV lithographyis a process used in microfabrication to pattern parts of a thin film or the bulk of a substrate. It uses light to transfer a geometric pattern from a photomask to a light-sensitive chemical ” photoresist “, or simply “resist,” on the substrate. A series of chemical treatments then either engraves the exposure pattern into the material or enables deposition of a new material in the desired pattern upon the material underneath the photo resist.
For example, in complex integrated circuitsa modern CMOS wafer will go through the photolithographic cycle up to 50 times. Photolithography shares some fundamental principles with photography in that the pattern in the etching resist is created by exposing it to lighteither directly without using a mask or with adf projected image using an optical mask.
This procedure is comparable to a high precision version of the method used to make printed circuit boards. Subsequent stages in the process have more in common with etching than with lithographic printing.
It is used because it can create extremely small patterns down to a few tens of nanometers in sizeit affords exact control over the shape and size of the objects it creates, and because it can create patterns over an entire surface cost-effectively.
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 photoreskst require extremely clean operating conditions. Photolithography is the standard method of printed circuit board PCB and microprocessor fabrication. The root words photolithoand graphy all have Greek origins, with the meanings ‘light’, ‘stone’ and ‘writing’ respectively.
As suggested by the name compounded from them, photolithography is a printing method originally based on the use of limestone printing plates in which light plays an essential role.
In the s, Nicephore Niepce invented a photographic process that used Bitumen of Judeaa natural asphalt, as the first photoresist. A thin coating of the bitumen on a sheet of metal, glass or stone became less soluble where it was exposed to light; the unexposed parts could then be rinsed away with a suitable solvent, baring the material beneath, which was then chemically etched in an acid bath to produce a printing plate.
The light-sensitivity of bitumen was very poor and very long exposures were required, but despite the later introduction of more sensitive alternatives, its low cost and superb aff to strong acids prolonged its commercial life into the early 20th century. Inthe U. Lathrop and Photorssist R.
ArF Photoresist Solutions | TOK America
Nall at the National Bureau of Standards later the U. Army Diamond Ordnance Fuze Laboratorywhich eventually merged to form the now-present Army Research Laboratory with the task of finding a way to reduce the size of electronic circuits in order to better fit the necessary circuitry in the limited space available inside a proximity fuze.
A single iteration of photolithography combines several steps in sequence. Modern cleanrooms use automated, robotic wafer track systems to coordinate the process.
The procedure described here omits some advanced treatments, such as thinning agents or edge-bead removal.
If organic or inorganic contaminations are present on the wafer surface, they are usually removed by wet chemical treatment, e. Other solutions made with trichloroethylene, acetone or methanol can also be used to clean. Wafers that have been in storage must be chemically cleaned to remove contamination.
A liquid or gaseous “adhesion promoter”, such as Bis trimethylsilyl amine “hexamethyldisilazane”, HMDSis applied to promote adhesion of the photoresist to the wafer.
The surface layer of silicon dioxide on the wafer reacts with HMDS to form tri-methylated silicon-dioxide, a highly water repellent layer not unlike the layer of wax on a car’s paint. This water repellent layer prevents the aqueous developer from penetrating between the photoresist layer and the wafer’s surface, thus preventing so-called lifting of small photoresist structures in the developing pattern.
The wafer is covered with photoresist by spin coating. Thus, the top layer of resist is quickly ejected from the wafer’s edge while the bottom layer still creeps slowly radially along the pphotoresist.
In this way, any ‘bump’ or ‘ridge’ of resist is removed, leaving a very flat layer. Final thickness is also determined by the evaporation of liquid solvents from the resist. After prebaking, the photoresist is exposed to a pattern of intense light. The exposure to light causes a chemical change that allows some of the photoresist to be photoresiet by a special solution, called “developer” by analogy with photographic developer. Positive photoresist, the most common type, becomes soluble in the developer when exposed; with negative photoresist, unexposed regions are soluble in the developer.
A post-exposure bake PEB is performed before developing, typically to help reduce standing wave phenomena caused by the destructive and constructive interference patterns of the incident light. In deep ultraviolet lithography, chemically amplified resist CAR photoresiat is used.
This process is much more sensitive to PEB time, temperature, and delay, as most of the “exposure” reaction creating acid, making the polymer soluble in the basic developer actually occurs in the PEB.
The develop chemistry is delivered on a spinner, much like photoresist. Developers originally often contained sodium hydroxide NaOH. However, sodium is considered an extremely undesirable contaminant in MOSFET fabrication because it degrades the insulating properties of gate oxides specifically, sodium ions can migrate in and out of the gate, changing the threshold voltage of the phottoresist and making it harder or easier to turn the transistor on over time.
Metal-ion-free developers such agf tetramethylammonium hydroxide TMAH are now used. The hard bake solidifies the remaining photoresist, to make a more durable protecting layer in future ion implantationwet chemical etchingor plasma etching. In etching, a liquid “wet” or plasma “dry” chemical agent removes the uppermost layer of the substrate in the areas that are not protected by photoresist. In semiconductor fabricationdry etching techniques are generally used, as they can be made photoresjstin order to avoid significant undercutting of the photoresist pattern.
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 i. Wet etch processes are generally isotropic in nature, which is often indispensable for microelectromechanical systemswhere suspended structures must be “released” from the underlying layer.
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. 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 phootoresistwhich oxidizes it.
This process is called ashingand resembles dry etching. Use of 1-Methylpyrrolidone NMP solvent for photoresist is another method used to remove an image. Exposure systems typically produce an image on the wafer using a photomask.
The photomask blocks light 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. Exposure systems may be classified by the optics that transfer the image from the mask to the wafer. Photolithography produces better thin film transistor structures than printed electronicsdue to smoother printed layers, less wavy patterns, and more accurate drain-source electrode registration.
A contact printer, the simplest exposure system, puts a photomask in direct contact with the wafer and exposes it to a uniform light. 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 photoresiet processes use increasingly large wafers, these conditions become increasingly difficult.
Research and prototyping processes often use contact or proximity lithography, because it uses inexpensive hardware and can achieve high optical resolution. The resolution in proximity lithography is approximately the square root of the product of the wavelength and the gap distance. Hence, except for projection lithography see aarfcontact 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 ar to be low; however, the shortcomings of contact printing discussed above remain as challenges. Very-large-scale integration VLSI lithography uses projection systems. Unlike contact or proximity masks, which cover an entire wafer, projection masks known as “reticles” show only one die or an array of dies known as a “field”.
Projection exposure systems steppers or scanners project the mask onto the wafer many times to create the complete pattern. The image for the mask originates from a computerized data file. This data file is converted to a series of polygons and written onto a square of fused quartz substrate covered with a layer of chromium using a photolithographic process. A laser beam photoresizt writer or a beam of electrons e-beam writer is used to expose the pattern defined by the data file and travels over the surface photoresish the substrate in either a vector or raster scan manner.
The ability to project a clear image of a small photoresiet onto the wafer is limited by the wavelength of the light that is used, and the ability of the reduction lens system to aff enough diffraction orders from the illuminated mask.
It is also common to write 2 times the half-pitch. The minimum feature size can be reduced by decreasing this coefficient through Computational lithography. According to this equation, minimum feature sizes can be decreased by decreasing the wavelength, and increasing the numerical aperture to achieve a tighter focused beam and a smaller spot size.
However, photpresist design method runs into a competing constraint. In modern systems, the depth of focus is also a concern:. 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.
As light consists of photonsat low doses the image quality ultimately depends on the photon number. Historically, photolithography has used ultraviolet light from gas-discharge lamps using mercurysometimes photoresisf combination with noble gases such as xenon.
These lamps produce light across a broad spectrum with aff strong peaks in the ultraviolet range. This spectrum is filtered to select a single spectral line.
This challenge was overcome when in a pioneering development inexcimer laser lithography was proposed and demonstrated at I. This trend has continued into this decade for even denser chips, with minimum features reaching 10 nanometers in The primary manufacturers of excimer laser light sources in the s were Lambda Physik now part of Coherent, Inc.
Since the mids Cymer Inc. Generally, an excimer laser is designed to operate with a specific gas mixture; therefore, changing wavelength is not a trivial matter, as the method of generating the new wavelength is completely different, and the absorption characteristics of materials change. Furthermore, insulating materials such as silicon dioxidewhen exposed to photons with energy greater than the band gap, release free electrons and holes which subsequently cause adverse charging.
Also termed immersion lithographythis enables the use of optics with numerical apertures exceeding 1. 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 water is continually circulated to eliminate photodesist distortions. Mercury arc lamps are designed to maintain a steady DC current of 50 to Volts, however excimer lasers have a higher resolution.
Excimer lasers are gas-based light systems that are usually filled with inert and halide gases Kr, Ar, Xe, F and Cl that are charged by an electric field. The higher the frequency, the greater the resolution of the image.
In addition to running at a higher frequency, excimer lasers are compatible with more advanced machines than mercury arc lamps are.