Photolithography
Photolithography is a process that involves using light to transfer a pattern onto a photoresist layer deposited on a sample, typically a silicon wafer. It is used in the manufacturing of integrated circuits.
The process begins with a photosensitive material, called a photoresist, being applied to the substrate. A photomask that contains the desired pattern is then placed over the photoresist. Light is shone through the photomask, exposing the photoresist in certain areas. The exposed areas undergo a chemical change, making them either soluble or insoluble in a developer solution. After development, the pattern is transferred onto the sample through etching, chemical vapor deposition, physical vapor deposition, plating, or ion implantation processes.
Photolithography processes can be classified according to the type of light used, including ultraviolet lithography, deep ultraviolet lithography, extreme ultraviolet lithography, and X-ray lithography. The wavelength of light used determines the minimum feature size that can be formed in the photoresist. Ultraviolet light is typically used.
Photolithography is the most common method for the semiconductor fabrication of integrated circuits, such as solid-state memories and microprocessors. It can create extremely small patterns, down to a few nanometers in size. It provides precise control of the shape and size of the objects it creates. It can create patterns over an entire wafer in a single step, quickly and with relatively low cost. In complex integrated circuits, a wafer may go through the photolithographic cycle as many as 50 times. It is also an important technique for microfabrication in general, such as the fabrication of microelectromechanical systems. However, photolithography cannot be used to produce masks on surfaces that are not perfectly flat. And, like all chip manufacturing processes, it requires extremely clean operating conditions.
Photolithography is a subclass of microlithography, the general term for processes that generate patterned thin films. Other technologies in this broader class include the use of steerable electron beams, or more rarely, nanoimprinting, interference, magnetic fields, or scanning probes. On a broader level, it may compete with directed self-assembly of micro- and nanostructures.
Photolithography shares some fundamental principles with photography in that the pattern in the photoresist is created by exposing it to light — either directly by projection through a lens, or by illuminating a mask placed directly over the substrate, as in contact printing. The technique can also be seen as a high precision version of the method used to make printed circuit boards. The name originated from a loose analogy with the traditional photographic method of producing plates for lithographic printing on paper; however, subsequent stages in the process have more in common with etching than with traditional lithography.
Conventional photoresists typically consist of three components: resin, sensitizer, and solvent.
Etymology
The root words photo, litho, and 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 in which light plays an essential role.History
In the 1820s, Nicephore Niepce invented a photographic process that used Bitumen of Judea, a 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 resistance to strong acids prolonged its commercial life into the early 20th century.In 1940, Oskar Süß created a positive photoresist by using diazonaphthoquinone, which worked in the opposite manner: the coating was initially insoluble and was rendered soluble where it was exposed to light. In 1954, Louis Plambeck Jr. developed the Dycryl polymeric letterpress plate, which made the platemaking process faster. Development of photoresists used to be carried out in batches of wafers dipped into a bath of developer, but modern process offerings do development one wafer at a time to improve process control.
In 1957 Jules Andrus patented a photolithographic process for semiconductor fabrication, while working at Bell Labs. At the same time Moe Abramson and Stanislaus Danko of the US Army Signal Corps developed a technique for printing circuits.
In 1952, the U.S. military assigned Jay W. Lathrop and James R. Nall at the National Bureau of Standards 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. Inspired by the application of photoresist, a photosensitive liquid used to mark the boundaries of rivet holes in metal aircraft wings, Nall determined that a similar process can be used to protect the germanium in the transistors and even pattern the surface with light. During development, Lathrop and Nall were successful in creating a 2D miniaturized hybrid integrated circuit with transistors using this technique. In 1958, during the IRE Professional Group on Electron Devices conference in Washington, D.C., they presented the first paper to describe the fabrication of transistors using photographic techniques and adopted the term "photolithography" to describe the process, marking the first published use of the term to describe semiconductor device patterning.
Despite the fact that photolithography of electronic components concerns etching metal duplicates, rather than etching stone to produce a "master" as in conventional lithographic printing, Lathrop and Nall chose the term "photolithography" over "photoetching" because the former sounded "high tech." A year after the conference, Lathrop and Nall's patent on photolithography was formally approved on June 9, 1959. Photolithography would later contribute to the development of the first semiconductor ICs as well as the first microchips.
Process
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. The photolithography process is carried out by the wafer track and stepper/scanner, and the wafer track system and the stepper/scanner are installed side by side. Wafer track systems are also known as wafer coater/developer systems, which perform the same functions. Wafer tracks are named after the "tracks" used to carry wafers inside the machine, but modern machines do not use tracks.Cleaning
If organic or inorganic contaminations are present on the wafer surface, they are usually removed by wet chemical treatment, e.g. the RCA clean procedure based on solutions containing hydrogen peroxide. Other solutions made with trichloroethylene, acetone or methanol can also be used to clean.Preparation
The wafer is initially heated to a temperature sufficient to drive off any moisture that may be present on the wafer surface; 150 °C for ten minutes is sufficient. Wafers that have been in storage must be chemically cleaned to remove contamination. A liquid or gaseous "adhesion promoter", such as Bisamine, is 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 pattern. In order to ensure the development of the image, it is best covered and placed over a hot plate and let it dry while stabilizing the temperature at 120 °C.Photoresist application
The wafer is covered with photoresist liquid 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 wafer. In this way, any 'bump' or 'ridge' of resist is removed, leaving a very flat layer. However, viscous films may result in large edge beads which are areas at the edges of the wafer or photomask with increased resist thickness whose planarization has physical limits. Often, Edge bead removal is carried out, usually with a nozzle, to remove this extra resist as it could otherwise cause particulate contamination.Final thickness is also determined by the evaporation of liquid solvents from the resist. For very small, dense features, lower resist thicknesses are needed to overcome collapse effects at high aspect ratios; typical aspect ratios are < 4:1.
The photoresist-coated wafer is then prebaked to drive off excess photoresist solvent, typically at 90 to 100 °C for 30 to 60 seconds on a hotplate. A BARC coating may be applied before the photoresist is applied, to avoid reflections from occurring under the photoresist and to improve the photoresist's performance at smaller semiconductor nodes such as 45 nm and below. Top Anti-Reflectant Coatings also exist. EUV lithography is unique in the sense it allows for the use of photoresists with metal oxides.
Exposure and developing
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 removed 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 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 chemistry is used. This resist is much more sensitive to PEB time, temperature, and delay, as the resist works by creating acid when it is hit by photons, and then undergoes an "exposure" reaction which mostly occurs in the PEB.
The develop chemistry is delivered on a spinner, much like photoresist. Developers originally often contained sodium hydroxide. However, sodium is considered an extremely undesirable contaminant in MOSFET fabrication because it degrades the insulating properties of gate oxides. Metal-ion-free developers such as tetramethylammonium hydroxide are now used. The temperature of the developer might be tightly controlled using jacketed hoses to within 0.2 °C. The nozzle that coats the wafer with developer may influence the amount of developer that is necessary.
The resulting wafer is then "hard-baked" if a non-chemically amplified resist was used, 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.
From preparation until this step, the photolithography procedure has been carried out by two machines: the photolithography stepper or scanner, and the coater/developer. The two machines are usually installed side by side, and are "linked" together.