FUNDIMENTALS OF PHOTOLITHOGRAPHY

One of the most widely used methods for creating nanoscale circuit components is Photolithography. The word lithography is derived from the Greek words lithos (stone) and graphein (to write) and finds its roots in a process invented by Aloys Senefelder in 1796. By treating a piece of limestone with certain chemicals, Senefelder was then able to transfer an image carved into the stone onto a piece of paper. This was done by coating certain parts of the porous limestone with a water repellant substance. When ink was applied to the stone it would only adhere to the untreated hydrophilic areas, and hence the image carved into the rock could be transferred repeatedly onto paper. Senefelder's technique is still used in some artistry applications today.

As time progressed and technology improved, lithography methods evolved. In the 1820's a French scientist by the name of Nicephore Niepce developed the first photoresist, a component fundamental to photolithography. A photoresist is a substance that undergoes a chemical reaction when it is exposed to light. Niepce's photoresist was a material known as Bitumen of Judea, a kind of naturally occurring asphalt. A sheet of stone, metal, or glass was coated with a thin layer of this bitumen, which became less soluble where it was exposed to light. Areas that were unexposed could then be removed using a solvent, and the resultant exposed areas of the sheet were etched using a chemical bath. After the remaining photoresist was removed, the sheet could then be used as a printing plate.

Photolithography today is in many ways similar to the original process invented by Niepce. In general, modern photolithography involves a procedure with five steps; wafer preparation, application of photoresist, pre-exposure bake, exposure, post-exposure bake, and development.

In photolithography, a wafer is a thin slice of semi conductive material, such as crystalline silicon, that forms the base for the photolithographic process. Before the wafer can be used it must first undergo several steps of cleaning and preparation to remove organic contaminants and prepare it chemically for application of a photoresist.

Figure 1- Two silicon wafers, 12" and 6" respectively, commonly used in photolithography.

https://en.wikipedia.org/wiki/Wafer_(electronics) In general laboratory applications, the wafer is cleaned using a chemical agent, such as acetone, to remove contaminants. This can be done in several ways. One method is to completely submerge the wafer in a chemical bath for ten or so seconds before blow drying it with nitrogen. Another method involves dripping the cleaning agent onto the wafer to create a small puddle, and then rotating the wafer in a spin coater to spread and remove the cleanser. After the cleaning stage is complete, the wafer is baked using an oven or hot plate to remove excess water. This step, called a dehydration bake, helps improve adhesion of the photoresist in the next phase of the photolithographic procedure.

After the wafer has been properly prepared, it is ready for the application of a photoresist. There are two different types of photoresist, positive and negative. A positive resist, when exposed to light, becomes soluble in a photoresist developer. That is, after the resist coated wafer is exposed to electromagnetic radiation (usually deep UV), any portion of the resist that has been touched by the light will be chemically removed, and the finished product will resemble the pattern on the photomask. A negative resist has the exact opposite response so that all portions of the negative resist exposed become insoluble to the developer, and the remaining resist is removed during development. The resultant pattern will be the inverse of the photomask. Figure 1, which includes the steps of exposure and development, shows the difference between positive and negative resists.

Figure 2- Diagram of the basic photolithography process displaying the difference between positive and negative photoresists.

Photoresist can be applied to the wafer in several ways, most of which make use of a device know as a "spin coater". A spin coater is composed of a small drum with a rotating disk inside. The wafer is placed on the disk, where suction from a vacuum holds it in place, and the wafer is rotated at a predetermined speed. In most laboratory applications, resist is added before rotation by applying a few drops of resist to the center of the wafer using a pipette. The spin coater is then activated, and the photoresist is dispersed across the wafer using a two step process. In the first step the wafer is rotated at a speed of about 500 rpm for about 30 seconds. This evenly distributes the resist across the entire surface of the wafer. At this point the spin coater accelerates, and the rotation speed and spin time of the wafer, combined with the viscosity of the photoresist, determine the final thickness of the resist coating. Photoresist coatings are generally 1-6 microns thick, though thicker coatings are sometimes used in commercial applications.

After application of the photoresist, the wafer undergoes a post-apply bake(also termed a "softbake" or "prebake") to Figure 3- Use of spin coater to apply photoresist. stabilize the photoresist

prior to exposure. This step also increases adhesion

between the resist and the wafer, and helps prevent contamination of the sample by airborne particles. The prebake process is often performed using a hotplate. Though temperature and bake time differ slightly depending on the photoresist used, typical prebake conditions require a temperature between 90 and 100 degrees Celsius for a period of 60-90 seconds. A common method used to test if the post- apply bake is complete is performed by touching the edge of the sample with tweezers. If the surface of the wafer is tacky or if a gentle touch by the tweezers leaves a mark on the surface of the sample, the bake process is not yet complete.

Once the prebake is complete, the wafer is ready for alignment and exposure. In most forms of photolithography, the design to be created on the resist coated wafer is dictated using a photomask. A photomask generally consists of a transparent glass sheet on which is printed the pattern to be copied to the wafer surface. Darkened areas of the photomask prevent penetration of light to the resist below, while transparent areas of the mask allow the resist beneath to react chemically. In the alignment stage of photolithography, the photomask is carefully aligned with the resist coated wafer. In some commercial applications the mask is elevated a few nanometers above the surface of the mask so as to prevent long term damage to the mask by chemicals in the resist, but in most laboratory settings the mask is placed in direct contact with the wafer so as to achieve the minimum feature size in the resist pattern.

Figure 4- Chromium photomask

https://upload.wikimedia.org/wikipedia/commons/1/18/Chromium_photomask _%28details%29.JPG

Feature size in the photolithography sample is also limited by the wavelength of light generated by the optical mask aligner. Ultra-violet wavelengths commonly used in many laboratory and commercial applications are g-line(436 nm), h-line(405 nm), i-line(365 nm), and broadband (280-315 nm). It is important to note that some photoresists only react when exposed to a specific wavelength (or range of wavelengths) of light. In this case, a resist that reacts normally under g-line radiation, for example, may not react at all when exposed to i-line wavelengths. Most photoresists, however, are designed to respond to various wavelengths of light, though exposure time will vary. As a general rule, the smaller the wavelength of light used, the smaller the mask components can be. For this reason many technology industries make use of lensing techniques to focus light and decrease feature size.

Figure 5- ABM optical mask aligner during use. Entire mask surface is simultaneously exposed to an even spread of UV light.

http://www.abmusainc.com/index.php/products/product_one

Other forms of photolithography use an electron beam instead of UV light. Because the wavelength of electrons can be 100,000 times smaller than that of visible light, advances in electron beam photolithography have enabled industries to produce photomasks with components less than 10nm in resolution.

Alignment and exposure are performed using an optical mask aligner (see figure 5). The prepared wafer is positioned in the center of the wafer chuck (usually found in the center of the aligner) and the photomask is carefully laid on top of it. Adjustments to the chuck can be made so that the mask maintains firm but gentle contact with the surface of the wafer. Once alignment is complete, the sample is ready for exposure. In photolithography exposure times will vary with a number of factors including the photoresist used, the wavelength of light from the optical mask aligner, and the thickness of the resist. Exposure time may be anywhere from a few seconds to nearly a minute depending on the resist and thickness. Though photoresists instructions will specify the total energy required to successfully expose the resist, trial and error is generally required to find near perfect exposure time in any given laboratory. If exposure time is insufficient, areas of the resist may be only partially exposed, and the developed wafer will contain gaps in the pattern (positive resist) or excess resist in between pattern lines (negative resist) therefore ruining the sample. If exposure time is too great, features of the resist will have diminished size and structural integrity after development. Figure 6 outlines the basic effects of under or over exposure in a typical positive photoresist. Figure 6- Effects of under and over exposure in a positive photoresist.

https://www.halbleiter.org/en/photolithography/exposure/

Once exposure of the sample is complete, the wafer is ready for development. In the development stage, the exposed wafer is submerged in a developer (usually an alkaline solution for positive resists and an organic solution for negative ones) and all soluble portions of the resist are removed. Development time varies depending on the resist, and the process is often temperature sensitive. In addition to controlling the time and temperature of the development process, most developers require agitation in order to function properly. The instructions that come with a photoresist will usually recommend which developer to use, as well as the time needed for development.

Figure 7- Resist pattern after development.

https://www.ncbi.nlm.nih.gov/books/NBK321721/figure/oin_tutorial.F4/

Once development is complete the wafer can be rinsed with distilled water and blow dried with nitrogen. At this point a post development bake is sometimes used to stabilize the remaining resist further before the wafer undergoes etching or layering. Postbaking is usually performed at higher temperatures and longer duration than the other baking procedures, such as 120◦ C for up to twenty minutes. This ensures that the resist is stable enough to withstand the harsh environment associated with etching , or that it adheres to the substrate well enough to act as a foundation for additional layers of resist. Further development of the wafer at this point will vary depending on the intended application of the project. Photolithography has a wide spectrum of uses, and the steps outlined in this paper can be used to lay the foundation for resistors, transistors, inverters, sensors, and more, all of which can be created in the micron or even nanometer scale. In many applications, a thin layer of metal (often referred to as a chrome or film) is applied before application of the resist. After development, areas bare of resist can then be etched chemically or through other means to remove all unprotected areas of the film. When the remaining resist is stripped from the sample, the resultant metallic pattern on the resist resembles exactly the pattern of the resist that remained after development.

Figure 8- Gold and aluminum solar cells.

https://en.wikipedia.org/wiki/Chemistry_of_photolithography#/media/File:Gold_and_aluminum_solar_cell_m anufactured_at_Cal_Poly_Microfab_Clean_Room.jpg